Energy efficiency of buildings - Nordic Folkecenter for Renewable ...

Energy efficiency of buildings 

Alain Jolly, 

engineering student 

Ecole Polytechnique de l’Université de Nantes, France 

August 2006 

and renewable energy 

page 1

Index 

INTRODUCTION 5 

THE PASSIVE HOUSE CONCEPT 7 

SOLUTIONS THAT MAKE A BUILDING ENERGY-EFFICIENT 9 

I- HEATING AND COOLING 9 

1.1 INSULATION AND MINIMIZATION OF HEAT LOSSES 9 

1.2 OPTIMIZATION OF THE NATURAL GAINS 18 

II- HOUSEHOLD APPLIANCES 27 

2.1 WHITE GOODS 27 

2.2 BROWN GOODS 28 

2.3 OFFICE APPLIANCES 29 

2.4 LIGHTING 29 

REMAINING ENERGY NEEDS 33 

I. SPACE AND WATER HEATING 33 

1.1 SOLAR WATER HEATERS 33 

1.2 HEAT PUMPS 35 

1.3 BIOMASS (WOOD) HEATING 38 

II. ELECTRICITY 42 

2.1 SOLAR PHOTOVOLTAIC POWER 42 

2.2 WIND POWER 44 

CONCLUSION 47 

APPENDIX 1 51 

APPENDIX 2 53 

APPENDIX 3 55 

SOURCES 57 

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Introduction 

Every time we switch on a light, a computer, the central heating, or even eat a 

hot meal we use energy. Everything we consume or use – our homes, their contents, 

our cars and the roads we travel on, the clothes we wear, and the food we eat – 

requires energy to produce and package, to distribute to shops or front doors, to 

operate, and then to get rid of. We rarely consider where this energy comes from or 

how much of it we use – or how much we truly need. 

Whether in the form of gasoline to fuel a car or uranium to generate electricity, 

the energy required to support our economies and lifestyles provides tremendous 

convenience and benefits. But it also exacts enormous costs on human health, 

ecosystems, and even security. Energy consumption affects everything from a 

nation's foreign debt (due to fuel imports) to the stability of the Middle East. From the 

air we breathe to the water we drink, our energy use affects the health of current and 

future generations. Inefficient and unsustainable use of fossil fuels is altering the 

global climate by releasing carbon dioxide (CO2), which is one of the main gases that 

contribute to global warming, into the atmosphere. And as we seek out more-remote 

sources of fuel, we endanger the culture and way of life of indigenous peoples from 

the Amazon to the Arctic. 

Fossil fuels represented 88 % of the world energy demand in 2000. This is a 

real threat to today's climate. It doesn't just mean warmer summers and milder 

winters: global climate change is responsible for the increasing occurrence of floods, 

storms and droughts around the world. This would have a catastrophic effect on the 

Earth, with widespread melting of glaciers and ice-sheets, and a highly probable rise 

in sea level that could lead to the inundation of countries such as the Netherlands 

and Bangladesh. Latest scientific concern is focused on melting ice lowering salinity 

in the North Atlantic Ocean, which could lead to the reversal of the "Great Atlantic 

Conveyor" - better known as the Gulf Stream. If this were to happen we could 

witness temperatures in NW Europe falling by as much as 10°C, despite 

temperatures elsewhere in the world rising. 

The last report returned by the Intergovernmental Panel on Climate Change 

(IPCC) has confirmed that the average temperature of the Earth's atmosphere will 

rise from between 1,5 to 6°C by the end of the cent ury. The actual value will depend 

primarily on the importance and the date of application of the public policies 

implemented by the countries consuming the most energy. 

This warming has already caused climatic disturbances with extremely serious 

human repercussions of increasing importance to the future. To limit these dangers 

and ensure the sustainable development of our societies it is now acknowledged that 

it will be necessary to cut in half the world emissions of gases contributing to the 

greenhouse effect by 2050. 

Nuclear energy is the second world energy source with 7 % of the supply. The 

nuclear industry has presented itself as part of the solution to global warming 

because it does not emit any CO2. But nuclear power creates serious long-term 

security issues in the form of risks of proliferation, severe nuclear accidents, and 

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vulnerability to terrorism. Furthermore it produces radioactive waste at every stage of 

the nuclear fuel cycle, from uranium mining and reactors to reprocessing irradiated 

nuclear fuel. Despite several decades of research, no nation has yet solved the 

problem of what to do with this material. Much of this nuclear waste will remain 

hazardous for thousands or even millions of years, leaving a poisonous legacy to 

future generations. 

Furthermore we must anticipate the future running out of fossil fuels, which are 

currently the main energy sources in the world. The global economy is highly 

dependent on these fuels that we consume faster and faster. This situation will lead 

us to a major crisis if we don't change our lifestyle and it is already responsible for 

tensions and wars. 

Worldwide, people use about a third of all this energy in buildings – for heating, 

cooling, cooking, lighting and running appliances. It is one of the most important 

sectors, indeed the most important, for energy consumption in the industrialised 

countries. Building-related energy demand is rising rapidly, particularly within our 

homes. And potential savings in existing buildings are enormous. It has been shown 

in practice that in the developed countries energy consumption in existing buildings 

can be reduced by a factor of 10! About new buildings, it has been shown that the 

consumption can be reduced by 75 % compared to the reference standards, at low 

extra cost! Improvements in the design and construction of buildings could yield 

significant energy savings. According to energy analyst Donald Aitken, "Buildings 

remain the most underrated aspect of energy economics, and the most unexploited 

opportunity for improving efficiency". It is particularly important to take account of this 

aspect because the cleanest energy is the one we don't use. 

Besides, considering the current and probably lasting increase of the energy 

cost, energy efficiency becomes also significantly important to save money. 

Therefore, it is really necessary to reduce our energy consumption, especially at 

home and in the buildings where we work. However, energy efficiency does not 

require a compromise in occupant comfort, using higher efficiency makes it possible 

to increase comfort while reducing energy consumption. 

This document will first present the passive house concept, and then it will give 

information about the different ways and solutions to make an energy-efficient 

building. These solutions are currently the most widely used because they are well 

known and easy to implement. Finally the last part is about the renewable energies 

we can use to fulfil the residual needs, such as electricity for appliances and heat. 

This report deals mainly with central-European buildings, that is to say that they need 

substantial heating in winter and moderate cooling in summer. 

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The passive house concept 

Living in a house which has very low energy consumption and which is 

moreover environment-friendly and energy sufficient is not utopian, it is real. Passive 

houses are such houses. A passive house is a building in which a comfortable 

interior climate can be maintained without active heating and cooling systems 

(Adamson 1987 and Feist 1988). The house heats and cools itself, hence "passive". 

The term "passive house" is now accepted as referring to a construction 

standard. For European passive construction prerequisite to this capability is an 

annual heating requirement that is less than 15 kWh/(m²a) (4755 Btu/ft²/yr), not to be 

attained at the cost of an increase in use of energy for other purposes (e.g., 

electricity). Furthermore, the combined primary energy consumption of living area of 

a European passive house may not exceed 120 kWh/(m²a) (38039 Btu/ft²/yr) for 

heat, hot water and household electricity. 

Comparison of energy ratings of homes (Source: CEPHEUS) 

With this as a starting point, additional energy requirements may be completely 

covered using renewable energy sources. 

The passive house standard can be met using a variety of technologies, designs 

and materials. It is the result of the constant developments that have been made to 

the low-energy house concept: "Passive" building and technical measures ensure 

that very little heat is lost and that heat gains are optimally used, thus resulting in 

improvements to thermal comfort and a reduction in the energy requirements. 

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The passive house provides us with the opportunity to reach extremely low 

levels of energy consumption by employing high-quality, cost-efficient measures to 

general building components - such measures are in turn an advantage to the 

ecology and economy sector. The energy consumption has been divided by 10 for 

existing buildings converted into passive houses (Ludwigshafen AkkP 24 and 

Nuremberg Schulze Darup CEPHEUS demonstration projects). 

Most of the following solutions for building's energy efficiency are inspired by 

those which are used in passive houses. 

Passive house in Törwang, Germany (Source: Lechner Holzbau) 

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Solutions that make a building 

energy-efficient 

The energy consumption of the current average European home is about 280 

kWh/m² per year and is essentially due to heating (80 %), water heating (10 %) and 

household electricity (10 %). For an office building, we should besides consider 

cooling. Consequently, heating is the main field on which we must make an effort. 

That's why this part principally deals with the ways to reduce the heating energy 

consumption. However the household consumption is also treated whereas the 

remaining energy needs are considered in the next part. 

I- Heating and cooling 

The main principles that we must apply to maintain a feeling of wellbeing in a 

building are super insulation to minimize the influence from outside and optimization 

of the passive natural (mainly solar) heat gains. These principles are based on heat 

transfer properties and the energy balance. Read appendix 1 and 2 for a better 

comprehension of how the following solutions work. 

1.1 Insulation and minimization of heat losses 

In winter, the aim of insulation is to prevent heat from leaving rather than to 

prevent the cold from entering. Indeed, the walls of an un-insulated dwelling do not 

retain the heat which passes through without heating the space. In summer, the 

insulation prevents heat from entering. The principle of the insulation is to use 

materials which have a thermal conductivity as low as possible in order to create a 

barrier between outside and inside, between the heat and the cold. 

Calculations and practical experiments carried out in several projects 

demonstrated that in climates comparable to central Europe, emphasizing the 

insulation is more efficient than emphasizing active or passive solar energy use. Note 

that it is however recommended to combine both of these strategies. 

Thermal wellbeing is not only depending on the air temperature, but also on wall 

temperature, the air circulation and on personal factors. The wellbeing decreases 

when a difference of more than 2°C exists between t he walls and the ambient air. 

The larger the difference in temperature between the walls and the ceiling of a room, 

the more discomfort we feel, due to the "cold radiation" of the walls. Therefore, a 

heated room at 19°C but with the walls at 15°C will be more comfortable than a room 

at 22°C but with the walls at 12°C. Thus insulation has to be designed to minimize 

the heat flux but it has also to keep the inside wall at a suitable temperature. 

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1.1.1 An insulated shell 

The first rule to apply for the implementation of the insulation is that the whole 

exterior shell of the building must be insulated. Only one non-insulated part can lead 

to heavy heat losses. 

Many efficient insulation materials are now available such as cellulose, mineral 

wool, fibreglass, polystyrene, containing more or less recycled material, or cotton, 

sheep's wool, straw, hemp, which are natural materials. Other solutions exist; using 

cavity bricks or by leaving a layer of air between exterior and interior walls to act as 

insulation. 

With regard to another significant parameter for the insulation of the envelope, 

the glazed fenestration, very efficient materials are available for the glazing as well 

as the frames. This point will be dealt with in a special section about glazings. 

1.1.2 The thermal bridges 

One of the weak points of the heat insulation is the "thermal bridge". A thermal 

bridge is a portion of a structure whose high thermal conductivity lowers the overall 

thermal insulation of the structure. Indeed heat will flow the easiest path from the 

heated space to the outside - the path with the least resistance. This significantly 

increases heat loss. Due to the increased heat transfer across the thermal bridge, the 

surfaces on the warm side of the bridge (the internal side in winter) become cooler 

and vice versa, reducing 

the occupants' wellbeing. In 

the worst cases this can 

result in high humidity in 

parts of the construction. 

The main thermal bridges 

are caused by the 

presence of a conductive 

material crossing the 

insulation (metal framework 

in concrete or metal frame 

of a window) or by the 

walls' geometry (when the 

interior surface is smaller 

than the corresponding 

exterior surface, so more 

heat is transferred than in 

Location of the main thermal bridges (source: Passivhaus Institut) 

other areas). Examples 

are: 

• the lintel above a window 

• aluminium framing around a window 

• concrete or steel beams linking inside to outside temperatures 

• filling of air cavity with rubble 

• corners and junctions of walls and floors 

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junction of a wall with an 

inside partition 

corners 

Examples of thermal bridges 

window 

There are different kinds of thermal bridges which depend on the kind of 

insulation (internal, external or cavity brick wall). The better way to avoid thermal 

bridges is generally with external insulation, but this solution is not always feasible 

and is not enough to get rid of all the thermal bridges. Several techniques exist 

appropriate to each case. It is important to consider this aspect when designing a 

new building or renovating an old one. 

With regard to the windows and the doors, it is necessary to avoid metallic 

frames. They should have an insulated wooden or plastic frame to eliminate the 

thermal bridges. 

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This simulation shows the typical thermal bridges in yellow : corners, windows and window lintels. 

The left-hand diagram shows the high heat flow caused by the lintel, the frame of the window and the single 

glazing. On the right-hand simulation you can see the consequences in terms of temperature. 

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1.1.3 Glazings 

The insulating envelope of a 

building includes the windows. It means 

that the glazings too have to minimize 

the heat transfers. Single glazings were 

the most common glazings at the 

beginning of the 1970's, their U-value 

was some 5.5 W/(m²K). The heat losses 

through 1 m² of a single glazing window 

were equivalent to 60 L of heating oil 

per year in Central European climate. 

Cross section of a thermal comfort window 

(source: Passivhaus Institut) 

U-Value 

A measure of air-to-air heat transmission 

(loss or gain) due to thermal conductance 

and the difference in indoor and outdoor 

temperatures. As the U-Value decreases, 

so does the amount of heat that is 

transferred through the glazing material. 

The lower the U-Value, the more restrictive 

the wall (or the fenestration product) is to 

heat transfer (Reciprocal of the R-Value). 

The performance of the double 

glazings is much better (the heat loss 

coefficient is reduced to some 2.8 

W/(m²K)). There is an air gap enclosed 

between two panes, allowing almost half 

of the losses to be saved compared to 

single glazing. The interior surface 

temperature will not be lower than 7.5 °C 

even during extraordinary cold periods (- 

15°C). There will not be any frost pattern 

– but the surface is still uncomfortably 

cold and will show condensation, because 

the temperature is far lower than the dew 

point. The introduction of razor-thin metal 

coatings inside the inter-pane space is the 

most important success - called "low-e"layer 

for the low thermal emissivity. These 

coatings reduce the thermal radiation 

between the inner and the outer pane by 

a factor of 5 to 20. Even more, the gas in 

the gap is changed from air to noble 

gasses (Argon or Krypton), which have a 

far lower heat conductivity. A commonly 

used wooden or plastics window using a 

standard spacer and a standard double 

pane low-e glazing has a U–value 

between 1.3 and 1.7 W/(m²K), this is 

another reduction in heat loss by a factor of two when compared to the old double 

pane window. Now the average interior surface temperature will be some 13°C 

during heavy frost periods. 

The breakthrough for energy efficient construction is the availability of triplepane 

low-e glazing. If one adds two gaps between panes with one low-e-coating in 

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A thermal comfort window with an Uw-value 

lower than 0.8 W/(m²K) assures a high 

thermal comfort. 

each gap and noble gas filling U-values 

between 0.5 and 0.8 W/(m²K) result. This 

requires that not only the glazing, but the whole 

window should have such a quality, including an 

insulating window frame and insulating spacers. 

The result is a "thermal comfort window". Using 

such a window the annual heat loss is reduced 

to no more than 8 L heating oil by each square 

meter of window area – this is a factor of eight 

compared to the initial value. But the 

advantages of the thermal comfort window are 

not only reduced heat loss, but also increased 

thermal comfort. Even during heavy frost 

periods the interior surface temperature will not 

fall below 17°C. The consequence is that no 

"cold radiation" can be perceived from such a 

window nor is an uncomfortable cold air layer 

possible. 

Different models of thermal comfort windows (source: Passivhaus Institut) 

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1.1.4 Airtightness 

The external envelope of a building should be as airtight as possible - this is true 

for conventional as well as for energy saving houses. It is the only means to avoid 

damage caused by condensation of moist, room temperature air penetrating the 

construction (see the figure on the right hand side). Such damage not only occurs in 

cold climates; in hot and humid climates the problem can occur from airflows from the 

outside to the inside. The cause is the same in both cases: a leaky building envelope. 

Drafts in living spaces are not tolerated by occupants any more, therefore a very 

airtight construction is essential to fulfil modern thermal comfort expectations. 

Air tightness should not be mistaken for insulation. Both qualities are essential 

characteristics of a high quality building envelope, but in most cases both have to be 

achieved independently: 

• A well insulated construction is not necessarily airtight. Air can easily pass 

through insulation made from coconut, mineral or glass wool. These materials 

have excellent insulation properties, but are not airtight. 

• On the other hand an airtight construction is not necessarily well insulated: 

e.g. a single aluminium foil can achieve excellent air tightness, but has no 

relevant insulation properties. 

Air tightness is an important, 

but not the most important 

requirement for energy efficient 

buildings (contrary to the 

impression given by some 

popular publications). 

Furthermore, achieving air 

tightness should not be mistaken 

with the function of a "vapour 

barrier". The latter is a diffusion 

tight layer, for example an oiled 

paper is airtight but allows 

moisture vapour to pass through. 

Conventional room plastering 

(gypsum or lime plaster, cement 

plaster or reinforced clay plaster) 

is sufficiently airtight, but allows 

vapour diffusion. 

(source: Passivhaus Institut) 

Infiltration can not guarantee good indoor air quality. Houses built in Germany 

after 1985, for example, are so airtight that infiltration alone is inadequate to assure 

acceptable indoor air quality. Yet, these houses are still at risk regarding moisture 

damage to the construction from moist room air exfiltration. A greater level of air 

tightness is needed and these houses must be considered as "untight", their n50-air 

leakage varies between 4 and 10 h -1 . The consequences are draft-discomfort and 

moisture damage to the construction. The construction is too leaky to avoid 

page 15

exfiltration caused damages - but too tight for sufficient infiltration to maintain room 

air quality. 

The new 2001 German building code ("EnEV" Energy Saving Standard) for the 

first time addresses the air tightness of new constructions. Without a ventilation 

system the n50-airchange-values have to be less than 3 h -1 , with ventilation systems 

1.5 h -1 . From the experience in low energy houses, tighter construction (lower n50) 

leakages are recommended. In practice values between 0.2 und 0.6 h -1 have been 

measured in passive houses. 

Air tightness does not depend on a massive or light weight construction. 

Existing passive houses using masonry, timber, prefabricated, lost frame with 

concrete and steel bearing structure have achieved this superior level of air 

tightness. Sören Peper, a scientist at the Passive House Institute, has proved with a 

systematic field study that n50 leakage rates between 0.2 and 0.6 h -1 can reproducibly 

be achieved today. Careful design and accurate workmanship are the prerequisites 

to success. Construction details needed to achieve tightness are available for all 

important joint and envelope penetration situations. For timber constructions in most 

cases wooden composite boards are used (taped at the joints), in masonry 

construction a continuous inside plastering is sufficient. It is important, that the 

airtight envelope is continuous, without interruptions. This must be designed for, with 

particular attention to joints. 

(source: Passivhauss Institut) 

A key principle is maintaining "an 

undisturbed, airtight envelope", which 

can be recognised by the "rule of the 

red line" (see the section on the right 

hand side). Of course details are 

important, but an envelope can be air 

tight only if it consists of a uniform, in 

tact, airtight enclosure wrapping 

around the whole house volume. The 

first step is that each element of the 

envelope which is to achieve air 

tightness must be specified (e.g. the 

particle board or OSB in a roof 

construction). The second step is 

determining how the airtight layers of 

elements will be connected to assure 

lasting air tightness. 

With such an airtight envelope it is necessary to use a ventilation system to 

regulate the room air humidity. This system can also permit to reduce the heat 

losses. 

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1.1.5 Ventilation with heat recovery 

Excellent indoor air quality is indispensable, but this can only be achieved if 

stale air is exchanged with fresh outdoor air at regular intervals. Ventilation will work 

accurately only if polluted air is removed constantly out of kitchen, bathrooms, and all 

other rooms with significant air pollution. Fresh air has to be supplied to the living 

room, children’s room, sleeping rooms, and workrooms to substitute the removed air. 

The system will supply exactly as much fresh air as is needed for comfort and for 

good indoor air quality; only outdoor air will be supplied – no re-circulated air. This 

will lead to a high level of indoor air quality. 

What has been discussed so far could be satisfied by using a simple exhaust fan 

ventilation system, where the air is supplied through direct vents in external walls. 

These vents allow fresh (cold) air to enter the room at the required rate; however, the 

heat losses caused by such a system are much too high. 

Thus the ventilation installation must be combined with an efficient heat 

recovery system, this allows evacuating polluted air while keeping the heat inside the 

building. Heat from the exhaust air is recovered and applied to the supply air by a 

heat exchanger, the air flows are not mixed in the process. State of the art ventilation 

systems may have heat recovery rates of 75% to more than 95%. Of course this only 

works with counter flow heat exchangers and very energy efficient ventilators (using 

so called EC-motors with extraordinarily high efficiency). With this technology the 

recovered heat is 8 to 15-times higher than the electricity needed. 

The scheme of a comfortable ventilation system. Stale air (brown) is removed permanently from the rooms 

with the highest air pollution. Fresh air (green) is supplied to the living rooms. (Section from the Passive 

House estate at Hannover Kronsberg, design by Rasch & Grenz. Source: Passivhaus Institut) 

page 17

This is how a counter flow heat exchanger works: The warm air (red, extract air) flows through a channel 

and delivers heat to the plates. This air will leave the exchanger cooled (orange, then called exhaust air). On 

the opposite side of the exchanger plates the fresh air (blue) flows in separate channels. This air will absorb 

the heat and it will leave the exchanger with a higher temperature (but still unpolluted), then called supply air 

(green). The counter flow principle allows for almost 100% recovery of the temperature difference, if the 

exchanger is long enough. In practise, systems with 75% to 95% are available. (source: Passivhauss Institut) 

Notes : 

• This ventilation system with heat recovery is useless if the building's envelope 

is not well airtight. 

• In winter, in a well designed building, the temperature is supposed to be lower 

than outside. As a consequence instead of being heated the inlet air is cooled 

by the heat exchanger. This system helps to keep a comfortable inside 

temperature throughout the year, in winter as well as in summer. 

• In North America it is common to have air based heating and cooling systems 

(that's why it is called "air conditioning"). But the systems used in America are 

almost all just recirculating indoor air at a very high rate (but the air is not 

"changed", it is just recirculated). The system discussed here is something 

very different; it only replaces the indoor air at a very low rate with external air 

to maintain a good indoor air quality, there is no recirculated air. The airflows 

are much lower, there is almost no noise and no draft at all. Well, the use of 

such a system might be very similar to that which Americans are used to - but 

much more comfortable. 

1.2 Optimization of the natural gains 

The second main rule to follow in order to improve the energy efficiency of a 

building is to facilitate the natural energy gains (or to avoid them if the aim is to keep 

the rooms cool). The building design must be appropriate to seasonal needs. In 

temperate zones this mainly means favouring winter comfort (without deteriorating 

summer comfort). Such a building uses solar energy available as light or heat so that 

it consumes the least energy possible for a similar comfort. It must take into account 

the location, the aspect and the room arrangement in order to achieve these 

following functions: 

• harness the solar radiation 

• store the energy 

• distribute the heat inside 

• regulate the heat 

• avoid losses due to wind 

page 18

Besides solar energy, some heat can be collected from the ground thanks to a 

ground heat exchanger. 

But these measures must not lead to overheating the building in summer. That 

is why solar gain limitations must also be allowed in the warm season. In addition, a 

passive cooling system such as a geothermal heat exchanger can be added. 

This part is divided in two sections: the first one about heating and the second 

one about cooling. But in temperate climate, both of these functions have to be 

combined to ensure winter and summer wellbeing. 

1.2.1 Winter heating 

Most of the heat needed in winter to offset the remaining losses can be provided 

by the sun. Heating with solar energy is easy: just let the sun shine in through the 

windows. The natural properties of glass let sunlight through but trap long-wave heat 

radiation, keeping the house warm (the greenhouse effect). The challenge often is to 

properly size the south-facing glass to balance heat gain and heat loss properties 

without overheating. 

Passive solar heating requires careful application of two main design principles: 

orientation of the glazings and room layout. Other simple systems can be added such 

as a greenhouse and thermal masses. This part will also deal with the appreciable 

amount of heat which can be gained from the ground, thanks to a geothermal heat 

exchanger. 

Building's shape and orientation for collection and distribution 

The difference between a passive solar building and a conventional building is 

design. And the key is designing a passive solar building to best take advantage of 

the local climate. Besides the considerations about heat loss that have been 

discussed in the first part, elements of design include window location and the 

building's shape. 

The first component of a passive solar heating system is the collector, 

consisting of glazings. But increasing the glass area can increase building energy 

loss, especially if simple pane windows are used and if they face north. Because the 

sun’s position in the sky changes throughout the day and from one season to 

another, window orientation has a strong bearing on solar heat gain. Solar radiation 

has to be trapped by the greenhouse action of correctly oriented (south facing) 

windows exposed to full sun. The south side of the home must be oriented to within 

30 degrees of due south to receive about 90 percent of the optimal winter solar heat 

gain. Compared to a southern orientation, the south-south-east or south-south-west 

reduces by 5% the solar contributions; and south-east and south-west decrease 

them by 15%. As for the west and east orientations, up to 45% of the solar 

contribution is lost. North facing glazings are not advisable because they don't 

page 19

contribute to the solar radiation trapping. Thus 

they just favour heat loss as they are less 

insulating than walls. 

The heat trapped by the greenhouse 

effect has to be diffused to the main rooms. 

This diffusion is easier by orienting the house 

with the long axis running east/west (and, 

especially in cold climate, if the house is 

densely built-up). In addition, the main rooms 

(living room, kitchen, offices and rooms used in 

the day time, e.g. children bedrooms) must be 

located on the south side to take advantage of 

the solar energy. The rooms that need less 

heating should preferably be located by the 

north side. Lastly, the least used rooms must 

be situated on the north side in order to create 

a buffer zone increasing the insulation effect for 

the main rooms. The garage, the wash-house, 

the staircases, the corridors, etc are ideal 

buffer zones. To diffuse the heat a strictly 

passive design will use the three natural heat 

transfer modes – conduction, convection, and radiation – exclusively. Radiation is the 

most comfortable way of heat diffusion, it is performed by floors and walls, which can 

also be used to stock the heat. 

Example of rooms arrangement 

Example of rooms arrangement (source: ADEME) 

(source: National Renewable Energy 

Laboratory) 

page 20

Thermal masses 

Thermal mass, or materials used to store heat, is an integral part of most 

passive solar design. Materials such as concrete, masonry or wallboard absorb heat 

provided by the sun during the day and slowly release it by radiation as temperatures 

drop. This dampens the effects of outside air temperature changes and moderates 

indoor temperatures. The thermal masses can be walls or floors made of appropriate 

materials. These materials have a high density (i.e. little trapped air) and a high heat 

capacity, that is to say that a lot of heat energy is required to change their 

temperature. For instance; concrete, sandstone, baked clay and compressed earth 

are good thermal masses. Water has also a very high thermal capacity but it is more 

complicated to use. The more massive the thermal mass is, the more heat it can 

store. Conversely, light materials are bad thermal masses, as well as insulating 

materials and AAC (autoclaved aerated concrete) blocks. 

The surfaces of these masonry floors and walls are typically a dark colour 

because dark colours usually absorb more heat than light colours. 

The amount of passive solar (sometimes called the passive solar fraction) 

depends on the area of glazing and the amount of thermal mass. The glazing area 

determines how much solar heat can be collected. And the amount of thermal mass 

determines how much of that heat can be stored. It is possible to undersize the 

thermal mass, which results in the house overheating. There is a diminishing return 

on over sizing thermal mass, but excess mass will not hurt the performance. The 

ideal ratio of thermal mass to glazing varies by climate. 

Another important thing to remember is that the thermal mass must be insulated 

from the outside temperature. If it's not the collected solar heat can drain away 

rapidly, especially when thermal mass is directly connected to the ground, or in 

contact with outside air whose temperature is lower than the desired temperature of 

the mass. 

In winter avoid coverings such as carpet that inhibit thermal mass absorption 

and transfer. 

An example of solar collector : the conservatory (or sunspace) 

A conservatory is a habitable glazed 

room. It is a heat source for the building and 

also constitutes a pleasant and luminous 

place directly heated by the sun. In order to 

ensure as much comfortable as possible 

while still providing heat to the rest of the 

home several criteria must be taken into 

account. Indeed, although it collects the heat 

in winter the conservatory has to remain a 

pleasant living place all the year and still not 

to be overheated in summer. Most 

A sunspace within a home in Minden, 

Nevada (source: Donald Aitken, NREL) 

page 21

homeowners and builders also separate the sunspace from the home with doors 

and/or windows so that home comfort isn’t overly affected by the sunspace’s 

temperature variations. The interface between the conservatory and the housing is 

important: at night it transmits to the rooms the heat collected during the day. It has 

to act as a thermal mass; the choice of materials is very important. It is also possible 

to transfer the heat to the ventilation system with heat recovery. 

Thus a conservatory has to: 

• collect the sun radiation in winter (it has to be sheltered from it in summer) 

• stock the heat 

• transfer efficiently the heat toward the housing 

Principle on which a conservatory works 

Orientation: The best orientation is facing South, but South-West and South- 

East are good too. Full West orientation is particularly bad because the sunshine 

comes frontally to the glazings in the afternoon, the hottest moment of the day, 

causing overheat. 

Colours: On inner walls dark and warm colours are better as they absorb the 

solar energy more efficiently. Brown, maroon and ochre are suitable colours. White 

has to be avoided because it is a reflective colour which would return the solar 

radiation outside. 

Shape: Conservatories can have various shapes, which have to fit the needs 

and possibilities in each case, as well as the climate (see below the main shapes). 

An opaque roof is recommended in warm and sunny climates. 

page 22

Different positions and shapes for a conservatory (source: ADEME) 

For a multi-storeyed building it can be relevant to build a multi-storeyed or at 

least a high-ceilinged conservatory to facilitate the heating of the upper floors. 

However, this may need a careful design (in case of more than two levels) to avoid 

overheating the last floors. 

A different view of the same sunspace (source: Donald Aitken, NREL) 

page 23

In summer: A conservatory thermally well designed must be sheltered from the sun 

in summer. Openings are recommended in the high part as well as external blinds. 

Drawing heat from the ground with a ground heat exchanger 

An additional opportunity to collect natural heat is to use a simple system called 

ground (or geothermal) heat exchanger connected to the heat recovery ventilation 

system. The ground during winter has a higher temperature than outdoor air, and 

during the summer a lower temperature than outdoor air. The deeper you are the 

steadier the temperature (at 2 m depth the temperature is almost constant and 

around 10 to 18 °C according to the climate). There fore it is possible to preheat fresh 

air in an earth 

buried duct in 

winter, or to cool it 

in summer. This 

can be done 

directly with air 

ducts in the 

ground, or 

indirectly with brine 

circulating in earth 

buried pipes and 

heating or cooling 

the air with a 

water-to-air heat 

exchanger. 

The pipes 

Ground heat exchanger connected to the ventilation system with heat recovery 

generally used are 

less than 200 mm 

in diameter to facilitate the heat transfer and are made of polyethylene. The duct 

must be buried at least 1.20 m deep and it is not profitable to go beyond 1.80 m. The 

longer the covered distance is the closer one gets to the earth’s air temperature. It is 

recommended to be more than 50 m long to reach a good impact. Finally, the circuit 

has to be buried far enough from the building to avoid subtracting heat from the 

building. 

Wind protection 

When the wind blows on a building, it increases the convective heat transfer 

between it and the atmosphere. In Europe the wind is usually stronger in winter thus 

it is better to shield the houses from it and limit heat losses. Trees, fences, or 

geographical features can be used as windbreaks. 

Evergreen trees and shrubs planted to the prevailing wind direction (usually 

north and west) are the most common type of windbreak. Trees, bushes, and shrubs 

page 24

are often planted together to block or impede wind from ground level to the treetops. 

Or, evergreen trees combined with a wall, fence, or earth berm (natural or man-made 

walls or raised areas of soil) can deflect or lift the wind over the home. Be careful not 

to plant evergreens too close to your home’s south side as this would prevent the 

absorption of warmth from the winter sun. 

A windbreak will reduce wind speed for a distance of as much as 30 times the 

windbreak’s height. But for maximum protection, plant your windbreak at a distance 

from your home of two to five times the mature height of the trees. 

Properly selected and placed evergreen trees and 

shrubs can shelter the home from winter winds 

and reduce heating costs. (source: NREL) 

1.2.2 Summer "cooling" 

Overhangs 

Fixed protections on a high school in Cavaillon, 

France 

If south winds are a problem in the winter, 

plant evergreens far enough away to lift winds 

without shading the home. (source: NREL) 

It makes little sense to save energy on 

winter heating just to spend it on summer 

cooling. So in most climates, a good building 

design must provide summer comfort as 

well. Here the issue is not exactly to cool the 

building but to limit the natural heat gains. 

Insulation, air tightness and ventilation are 

as important as in winter to maintain the 

inner temperature. A geothermal heat 

exchanger can cool the in-going air during 

the summer while providing heat in winter. 

The main improvement which can be 

done is to shelter the south-facing glazings 

from the summer sun. The solar heat must 

be blocked by architectural masks, a roof 

overhang, slatted canopies or other 

devices such as awnings, shutters, and 

trellises. These outside protections are 

much more efficient than inside 

protections. 

The advantage of architectural 

masks, overhangs and slatted canopies is 

that they are totally passive (there is no 

page 25

need to put them on or off). They have to be sized so that they allow the winter 

sunshine and avoid summer sunshine to reach the windows. 

Note: Architectural masks are parts of a building (balconies, loggias, etc) which 

create shadows on other parts of this building, usually glazings. 

The shadows created by these 

systems depend on several parameters: 

• the time 

• the season 

• the shape and the size of the 

glazing 

• the shape and the size of the mask 

or overhang 

• the position of the mask or 

overhang regard to the glazing 

Their length has to be calculated so 

that they allow winter sun and cut summer 

sun, this depends on the latitude, the 

orientation and the window size. 

Adjustable canopies allow the 

appropriate protection for the changing 

conditions (sun's height along the day and 

sunlight), outside Venetian blinds can be 

adjusted as well. 

Another solution is to use 

vegetation to provide shade for the 

building. To block solar heat in the 

summer but let much of it in during the 

winter, use deciduous trees. Deciduous 

trees with high, spreading crowns (i.e., 

leaves and branches) can be planted to 

the south of your home to provide 

maximum summertime roof and 

windows shading. Trees with crowns 

lower to the ground are more 

appropriate to the west, where shade is 

needed from lower afternoon sun 

angles. 

(source: NREL) 

Deciduous shading trees to the south and west and 

wind breaking evergreens to the prevailing winter 

winds direction. (Source: NREL) 

page 26

II- Household appliances 

Once a building is built following the principles mentioned up to now in the first 

parts, most of its energy consumption is due to the household appliances (fridge, 

lighting, dishwasher, TV, etc). Following an important series of measurements one 

could show that it is possible to reduce their consumption by 40% to 45% in housing. 

It corresponds to an energy saving in 1 200 to 1 800 kWh/an (Source Cabinet 

SIDLER). 

2.1 White goods 

Distribution of the Household goods energy consumption (Source: ADEME) 

First of all these devices have to be chosen according to the needs. A big 

appliance consumes more than a small one. Thus it is important to adjust the size to 

the family's needs. 

1 person 100 to 150 L 

2 or 3 persons 150 to 250 L 

3 or 4 persons 250 to 350 L 

More than 4 persons 350 to 500 L 

Fridge size 

Be aware that a US fridge consumes 3 times as much as a normal one. 

Most of the energy consumed by a washing machine or a dishwasher is used to 

heat the water via an electrical element. It is better to connect the machine to hot 

water heated via renewable energy (see next chapter, Remaining energy needs). 

page 27

Average energy consumption according to appliances 

(Source: Enertech 2002) 

A recent device consumes much less 

electricity than an old one but different appliances 

from the same generation can have very different 

efficiencies. According to an EU Directive most 

white goods and light bulb packaging must have an 

EU Energy Label clearly displayed when offered 

for sale or rent. The energy efficiency of the 

appliance is rated in terms of a set of energy 

efficiency classes from A to G on the label, A being 

the most energy efficient, G the least efficient. The 

most efficient fridges and freezers can now be 

identified by new ‘A+’ and ‘A++’ markings on the 

Example EU energy label 

large black arrow appearing against the green ‘A’ arrow. By law, this label must be 

displayed on all new household products of the following types: 

• Refrigerators, freezers and fridge-freezer combinations 

• Washing machines 

• Electric tumble dryers 

• Combined washer-dryers 

• Dishwashers 

• Lamps 

• Electric ovens 

• Air conditioners 

2.2 Brown goods 

It is more difficult to know the efficiency of audiovisual equipment because they 

don't have any energy label. However it is possible in most cases to reduce their 

consumption quite easily. Usually these appliances are not turned off but only put on 

standby. This consumes useless electricity. If care is not taken the TV, the video tape 

page 28

ecorder, the DVD player, the hi-fi system, the TV decoder, the amplifier, etc, remain 

on standby mode permanently and finally consume more "off" than on. For instance, 

a video tape recorder (which is rarely used) uses more than 90 % of its annual 

electric consumption when it does not run! A well-equipped family can consume 

anywhere from 100 kWh to more than 800 kWh per year only for appliances on 

standby mode! Thus it is recommended to turn them off. 

2.3 Office appliances 

Office computing (computers, peripherals, printers, modems, phones, fax, etc) 

has a smaller consumption than white goods. Nevertheless the use of these devices 

is increasing so that their share in the total energy consumption increases. These 

appliances are also generally left on standby mode. 

It is recommended to use Energy Star labelled appliances. The 

Energy Star label indicates that the device is energy efficient in use 

as well as on standby. The Energy Star data base (www.euenergystar.org) 

allows you to find the most powerful models in terms 

of energy efficiency. 

Advice: 

• Laptops consume 50 to 80 % less energy than desktops 

• Flat liquid crystal screens consume 60 % less energy than an ordinary screen 

• Ink jet printers consume 5 to 10 W in use and don't need pre-heating as do 

laser printers (200 to 300 W) 

• Combined appliances (for instance printer + scanner) consume less energy 

than the appliances themselves together. 

2.4 Lighting 

2.4.1 Natural light 

The most ecological light comes from the sun. Although it is only available at 

daytime, it is better to use it as much as possible thanks to a well thought out home 

arrangement and light ducts. 

Home arrangement advice: 

• Use light colours inside, especially for ceilings, as they reflect the light. 

• Place the furniture so that it avoids shade on tables and desks. 

• Place the work-tops in front of windows. 

Light ducts: 

In a building designed for passive solar heating, the rooms located to the north 

receive much less natural light because most of the glazings are south-facing. 

However, it is possible to lead the sunlight to these rooms via light ducts. 

page 29

The light duct is a simple system which allows the flow of sunlight into a dark 

place thanks to a reflective film covering its inner wall. A breakthrough in this 

technology is a new film with a reflective factor of 99 %. The system includes a dome 

(sunlight collector) located on the roof, the duct itself consisting in a rigid pipe of 250 

to 650 mm in diameter according to the geographic location and the amount of light 

needed, and a diffuser located in the room to be lighted. 

Principle of the light duct 

(Source: ADEME) 

2.4.2 Artificial light 

In Europe, electric lighting can 

account for 400 to 600 kWh/year in 

housing. This consumption can 

easily be divided in two, particularly 

using energy efficient lamps. There 

are two main kinds of lamps 

available: incandescent lamps 

(including ordinary bulbs and 

halogens) and fluorescent lamps 

(including tubes and energy saving 

bulbs). Besides, another kind of 

lamp, using LEDs, is appearing. 

Let's first consider the two 

main categories: 

Incandescent bulbs consist of 

two electrodes with a tungsten 

filament attached between them. 

When the light is switched on 

Components of a light duct (Source: ADEME) 

Lamps comparison (Source: S. Bedel and T. Salomon, La 

maison des négawatts, Edition Terre Vivante) 

page 30

electricity flows through and heats up the filament until it glows, they produce a lot of 

heat (95 % of their energy consumption) and little light (5 %). In a fluorescent lamp, a 

gas is encased within a glass tube coated with a layer of phosphor. When electricity 

passes through the gas it emits ultraviolet rays which cause the phosphor coating to 

glow, this is more energy efficient because most of the energy is turned into light (80 

%) instead of heat (20 %). Among these lamps energy saving bulbs are particularly 

interesting because they are compact and their caps are similar to the ordinary ones'. 

For the same amount of light a 18 W energy saving bulb consumes 5 times less 

energy than a 100 W ordinary bulb and lasts 10 times as long (10 000 hours). While 

energy saving bulbs cost more initially the electricity that they save easily recovers 

this cost over their life span. 

As for white goods, the energy label is compulsory for domestic light bulbs. 

Classes A and B 

Class D 

Classes E and F 

LED lamps 

Energy savers fall in to these 

categories. They are the most 

efficient type of light bulb and 

use up to 80% less energy than 

standard light bulbs. 

Mains voltage halogen bulbs 

usually fall into this category. 

Standard incandescent light 

bulbs are the least efficient 

alternatives. 

Nowadays a new kind of energy saving lamp is 

appearing: LED (light-emitting diode) lamps. Already used 

for a long time for traffic lights and electronics panels they 

are moving to home lighting with their new white colour. 

They are composed of several super-bright LED with a very 

high life span (100 times as long as an ordinary bulb, i.e. 50 

000 to 100 000 hours!). In addition there is no glass or 

filament which makes LED lamps particularly durable. Their 

main drawback is that they are still too expansive. 

page 31

Remaining energy needs 

Despite the huge amount of energy which is possible to save in a building 

thanks to all the previously mentioned measures, energy is still necessary for 

different uses. It is needed for heating domestic water, for back-up heating, for light 

and electric appliances. In order to limit the building's impact on the environment it is 

preferable to use renewable energy with this aim in view. 

I. Space and water heating 

Hot water is needed both for space heating and for domestic use (shower, cooking, etc). It can 

be provided by a solar water heater, by a heat pump, by geothermal energy, by a boiler using 

biomass as a fuel or by an electric heater (see next part, Electricity). 

1.1 Solar water heaters 

Solar hot water heaters use the sun to heat either water or a heat-transfer fluid 

in collectors. There are passive systems (based on the thermo-siphon principle, the 

water flows upward when heated in the panel) and active systems (using a pump to 

circulate the fluid). Sometimes the plumbing from a solar heater connects to a 

house's existing water heater, which stays inactive as long as the water coming in is 

hot or hotter than the temperature setting on the indoor water heater. When it falls 

below this temperature the home's water heater can kick in to make up the 

difference. 

Most systems use a roof-mounted solar collector, so you’ll need a south-facing 

roof with good solar exposure. 

The most common collector for solar hot water is the flat plate collector. It is a 

rectangular box with a transparent cover (to raise the temperature by greenhouse 

effect). Small tubes run through the box and carry fluid, either water or other fluid, 

such as an antifreeze solution. The tubes attach to a black absorber plate, as heat 

builds up in the collector it heats the fluid passing through the tubes. The hot water or 

liquid goes to a storage tank. If the fluid is not hot water, water is heated by passing it 

through a tube inside the storage tank full of hot fluid. 

Evacuated tube collectors consist of rows of parallel transparent glass tubes, each 

containing an absorber and covered with a selective coating. Sunlight enters the 

tube, strikes the absorber and heats the liquid flowing through the absorber. These 

collectors are manufactured with a vacuum between the tubes, which helps them 

achieve extremely high temperatures (80 to 180°C, s o they are appropriate for 

commercial and industrial uses). 

page 33

Evacuated tube collectors Flat plate collectors 

Direct systems use a pump to circulate potable water from the water storage 

tank through one or more collectors and back into the tank. In indirect systems, a 

heat exchanger heats a fluid that circulates in tubes through the water storage tank, 

transferring the heat from the fluid to the potable water. 

Solar water heaters allow savings up to 70 % of the energy needed to heat 

water. A typical system for a four-person home in a sunny region consists of a tank of 

150 to 300 L and 3 to 4 m² of solar collector panels. 

Solar water heater (active system) with back-up heater (Source: ADEME) 

page 34

1.2 Heat pumps 

Heat flows naturally from a higher to a lower temperature. Heat pumps, 

however, are able to force the heat flow in the other direction. Although we may not 

know it, heat pumps are very familiar to us - fridges and air conditioners are two 

examples. A heat pump is a machine which moves heat from a low temperature 

reservoir to a higher temperature reservoir under supply of work (see appendix 3). 

Even at temperatures we consider to be cold, air, ground and water contain useful 

heat that's continuously replenished by the sun (thus they are renewable energy 

sources). Heat pumps can transfer heat from these natural heat sources in the 

surroundings to a building. Heat pumps can also be used for cooling; heat is then 

transferred in the opposite direction, from the application that is cooled to 

surroundings at a higher temperature. Sometimes the excess heat from cooling is 

used to meet a simultaneous heat demand. 

1.2.1 The vapour compression heat pump 

The great majority of heat pumps work on the principle of the vapour 

compression cycle. The main components in such a heat pump system are: 

• the evaporator - (e.g. the squiggly thing in the cold part of your fridge) takes 

the heat from the heat source; 

• the compressor - (this is what makes the noise in a fridge) moves the 

refrigerant round the heat pump and compresses the gaseous refrigerant to 

the temperature needed for the heat distribution circuit; 

• the condenser - (the hot part at the back of your fridge) gives up heat to a hot 

water tank which feeds the distribution system. 

• the expansion valve expands the high-pressure working fluid to the evaporator 

pressure and temperature. 

These components are connected to form 

a closed circuit, as shown in the figure righthand. 

A volatile liquid, known as the working 

fluid or refrigerant, circulates through the four 

components. In the evaporator the 

temperature of the liquid working fluid is kept 

lower than the temperature of the heat source, 

causing heat to flow from the heat source to 

the liquid, and the working fluid evaporates. 

Vapour from the evaporator is compressed to 

a higher pressure and temperature. The hot 

vapour then enters the condenser, where it 

condenses and gives off useful heat. Finally, 

the high-pressure working fluid is expanded to 

the evaporator pressure and temperature in 

The vapour compression heat pump 

(Source: Heat Pump Center, IEA) 

the expansion valve. The working fluid is returned to its original state and once again 

enters the evaporator. The compressor is usually driven by an electric motor. 

page 35

The evaporator can easily be turned into a condenser and vice versa, so that 

the heat pump works in the other way. The hot side and the cold side are then 

switched around by use of a reversing valve which switches the direction of 

refrigerant through the cycle. 

1.2.2 The absorption heat pump 

The other kind of heat pump is the absorption heat pump. Although mainly used 

in industrial or commercial settings, absorption coolers are now commercially 

available for large residential homes, and absorption heaters are under development. 

Absorption heat pumps are thermally driven, which means that heat rather than 

mechanical energy is supplied to drive the cycle. This system utilises the ability of 

liquids or salts to absorb the vapour of the working fluid. The most common working 

pairs for absorption systems are: 

• water (working fluid) and lithium bromide (absorbent); and 

• ammonia (working fluid) and water (absorbent). 

Residential absorption heat pumps use 

an ammonia-water absorption cycle to 

provide heating and cooling. As in a 

standard heat pump the refrigerant (in this 

case, ammonia) is condensed in one coil to 

release its heat; its pressure is then reduced 

and the refrigerant is evaporated to absorb 

heat. If the system absorbs heat from the 

interior of your home, it provides cooling; if it 

releases heat to the interior of your home, it 

provides heating. But absorption heat 

pumps are not reversible as the vapour 

compression heat pumps are. 

The absorption heat pump (Source: Heat 

Pump Center, IEA) 

The difference in absorption heat 

pumps is that the evaporated ammonia is not pumped up in pressure in a 

compressor, but is instead absorbed into water. A relatively low-power pump can 

then pump the solution up to a higher pressure. The problem then is removing the 

ammonia from the water, and that's where the heat source comes in. The heat 

essentially boils the ammonia out of the water, starting the cycle again. 

The advantage of absorption systems is that they can make use of any heat 

source, because of this they can make use of solar energy, geothermal hot water, or 

other heat sources. 

page 36

1.2.3 The heat sources 

In cooling use the heat source is of course the inside of the building. In heating 

mode, the main heat sources used are ambient air, ground water and ground (they 

are heat sinks when operating in cooling mode). 

• Ambient air is free and widely available, and it is the most common heat 

source for heat pumps. Air-source heat pumps, however, achieve on average 

10-30% lower seasonal performance factor (SPF) than water-source heat 

pumps. This is mainly due to the rapid fall in capacity and performance with 

decreasing outdoor temperature, the relatively high temperature difference in 

the evaporator and the energy needed for defrosting the evaporator and to 

operate the fans. 

• Ground water is available with stable temperatures (4-10°C) in many regions. 

Open or closed systems are used to tap into this heat source. In open systems 

the ground water is pumped up, cooled and then re-injected in a separate well 

or returned to surface water. A major disadvantage of ground water heat 

pumps is the cost of installing the heat source. Additionally, local regulations 

may impose severe constraints regarding interference with the water table and 

the possibility of soil pollution. 

• Ground-source systems are used for residential and commercial 

applications and have similar advantages as (ground) water-source systems, 

i.e. they have relatively high annual temperatures. Heat is extracted from 

pipes laid horizontally or vertically in the soil (horizontal/vertical ground coils). 

The thermal capacity of the soil varies with the moisture content and the 

climatic conditions. Due to the extraction of heat from the soil, the soil 

temperature will fall during the heating season. In cold regions most of the 

energy is extracted as latent heat when the soil freezes. However, in summer 

the sun will raise the ground temperature, and complete temperature recovery 

may be possible. 

Higher efficiencies are achieved 

with geothermal (ground-source or 

water-source) heat pumps, they can 

be used in more extreme climatic 

conditions than air-source heat pumps, 

and customer satisfaction with the 

systems is very high. The ground is 

nowadays the most used source for 

residential application. There are four 

basic types of closed ground loop 

systems: vertical, horizontal (1 to 2 m 

deep), slinky and pond/lake. Which 

one of these is best depends on the 

climate, soil conditions, available land, 

and local installation costs at the site. 

page 37

All of these approaches can be used for residential and commercial building 

applications. 

1.3 Biomass (wood) heating 

Residential vapour compression ground heat pump 

Biomass is a renewable energy resource provided by plants and animals. It 

includes the carbonaceous waste of various human and natural activities. It is 

derived from numerous sources, including the by-products from the timber industry, 

agricultural crops, raw material from the forest, major parts of household waste and 

wood. 

Biomass does not add carbon dioxide to the atmosphere as it absorbs the same 

amount of carbon in growing as it releases when consumed as a fuel. Even allowing 

for emissions of fossil carbon dioxide in planting, harvesting, processing and 

transporting the fuel, replacing fossil fuel with wood fuel will typically reduce net CO2 

emissions by over 90%. 

Wood is the most used biomass fuel for space heating; it has been used for 

millenniums. Nowadays it is a very versatile fuel and can be burned in many different 

forms and in a number of different appliances. It can be burned to heat one or more 

rooms, the whole house, to produce hot water and to cook on or a combination of all 

or some of these. Today you can choose from a new generation of wood burning 

appliances that are cleaner burning, more efficient, and powerful enough to heat 

modern homes and even supply district heating. At present domestic heating with 

wood is the most efficient and most competitive way of using biomass for energy. 

Several different technical concepts are available for domestic heating, including 

masonry heaters, stoves and boilers. Improvements in building insulation are leading 

to significant reductions in fuel demand. This reduces the handling effort and the 

page 38

equired storage space for wood fuels - which used to be major disadvantages of 

heating with wood. 

1.3.1 Masonry heaters 

An ordinary fireplace is more a decorative object than a heating appliance. To 

produce heat for one or more rooms logs can be burned on an open fire, these look 

nice but tend to have low efficiencies - about 80-85% of the heat that goes up the 

chimney. On the other hand masonry heaters, also known as "Russian," "Siberian," 

and "Finnish" fireplaces, commonly reach a combustion efficiency of 90%. They take 

advantage of a big thermal mass to absorb and later release the heat from the fire. 

Masonry heaters include a firebox, a large masonry mass (such as bricks or 

stones), and long twisting smoke channels that run through the masonry mass. Their 

fireboxes are lined with firebrick, refractory concrete, or similar materials that can 

handle temperatures over 1,000°C. 

Example masonry heater 

1.3.2 Wood boilers 

A small hot fire built once or twice a 

day releases heated gases into the long 

masonry heat tunnels. The masonry absorbs 

the heat and then slowly releases it into the 

house over a period of 12–20 hours. A wide 

variety of masonry heater designs and styles 

are available. Larger models resemble 

conventional fireplaces and may cover an 

entire wall. Smaller models take up about as 

much space as a wood or pellet stove. They 

cost from 5,000 to 15 000 € but avoid a 

central heating system investment provided 

the house is built following the previous 

recommendations in first part. 

The development from the multi-fuel boiler of the past to today's modern 

logwood boilers represents a quantum leap in terms of convenience as well as 

efficiency. Furthermore, thanks to the development of fuels from logwood via 

woodchips to pellets, a pourable wood-based fuel is now available which can be 

transported like a liquid such as oil: wood (in the form of pellets) is supplied by tank 

lorry and pumped into the pellet tank by means of a fuel hose. Thanks to their 

automatic ignition systems these boilers are now fully automatic, thus they represent 

a wood-based heating system providing the same level of convenience as fuel oil 

(except for periodical ash disposal, which can also be taken care of by the chimney 

sweep). They can be purchased new or the necessary adaptation equipment 

retrofitted to an existing coal or oil boiler. If a central heating system is currently 

running on oil or LPG or if rooms are currently heated with electricity, then using 

page 39

wood to run the central heating system is often a cheaper option although the initial 

cost of the boiler will be more than a conventional one. 

• Logwood boilers 

Example modern logwood boiler 

(Source: R&D in Austria, Ministry of 

Science and Transport) 

In a new generation logwood boiler the 

combustion of wood takes place in a two-stage 

process: gasification in the first stage and high 

temperature combustion in a specially designed 

chamber in the second stage. 

In order to ensure clean combustion, most 

producers nowadays use lambda sensors to 

measure the remaining oxygen in the flue gas so 

that an optimal quantity of secondary air for the 

combustion process can be supplied via 

adjustable valves. 

The boiler is generally operated at full 

power and heats up a hot water storage tank for 

continuous heat retrieval. The storage tank can 

serve as a solar boiler in summer if thermal solar 

collectors are installed. The latest models of 

logwood boilers do not need a storage tank as they can run at 30% of full power 

without significantly higher emissions than at full power. 

The average price of a unit including storage tank for a single family house is 

about 6000 €. Logwood costs about 25 €/MWh. It can be stored outdoors. 

• Woodchip boilers 

Woodchip storage (Source: ADEME) 

The advantage of woodchip boilers is their 

automatic feeding system for fuel which allows for 

full automatic operation and similar userfriendliness 

as oil or gas fired boilers. State of the 

art woodchip boilers are equipped with 

continuous power control and do not need a heat 

storage tank. A disadvantage of woodchips is 

their relatively large space requirement for indoor 

storage. Due to possible variations in fuel 

humidity, woodchip boilers need advanced 

electronic control equipment. The possible 

variation of woodchip size and humidity requires 

rather robust (and expensive) feeding and control 

mechanisms. Thus woodchip boilers are 

significantly more expensive but also more 

page 40

comfortable than logwood boilers. They are a good solution when heat requirements 

are so high that the effort of logwood handling is unacceptable. The average price of 

a unit for a single family house is 11 000 €. Woodchips cost 20 - 25 €/kWh. 

• Pellet boilers 

Wood pellets are a homogeneous fuel with high energy 

density (similar to high quality coal) which allows for simpler 

and cheaper boilers. They are usually made of highly 

compressed waste sawdust to a consistent size (usually about 

2cm long with a diameter of 6 to 8mm). No chemical additives 

are needed, the natural lignin of the wood itself serving as a 

binder, although sometimes small quantities of maize starch 

are added as well. 

Pellet boiler 

As with logs, pellets can be used to fuel especially designed 

for the purpose. Wood pellet boilers are fully automatic and 

almost as convenient as using gas or oil. They are well suited to 

meet variable load demands and can be operated on a timer. In 

addition they operate at high efficiencies of around 90%. 

Being a very dense fuel pellets require less storage space 

than logs or chips. The best solution for a pellet boiler, allowing 

for easy and convenient pellet handling, is to install a pellet store 

that is designed to receive pellets delivered by bulk tanker (in 

which case the pellets are blown into the silo). The pellet store 

can either be built outside the house or inside. The alternative 

system for the storage of pellets is to buy them by the bag. 

The price of pellet boilers is about 7500 €. In Austria (one of the most advanced 

countries for wood heating), pellets cost about 30 €/MWh. Recently boilers have 

been designed that can be fuelled both with pellets and with logwood. 

1.3.3 Stoves 

Wood pellets 

Stoves can be used to provide comfortable and cheap additional heating to one 

or several rooms. They are now high efficient and some of them can be feed with 

pellets (around 2 500 €). Some models are fit with a water heater and a 

storage tank. 

page 41

II. Electricity 

To meet the remaining needs in electricity for light and household appliances a 

building can also produce its own renewable energy. Thus it can finally be totally 

neutral toward environment for its energy needs. The most widely used energy 

sources for this application are solar and wind power. Indeed they are available 

almost in every part of the world. 

Some photovoltaic (PV) modules and windmills are designed for this use, these 

energy sources can be used either separately or combined in a hybrid system. If you 

are fortunate enough to have a stream running through your property, you might be 

able to generate hydropower, but we will not consider this exceptional case here. 

2.1 Solar photovoltaic power 

The photovoltaic process converts solar energy directly into electricity. A PV cell 

consists of two or more thin layers of semi-conducting material, most commonly 

silicon. When the silicon is exposed to light electrical charges are generated and this 

can be conducted away by metal contacts as direct current (DC). The electrical 

output from a single cell is small, so multiple cells are connected together and 

encapsulated (usually behind glass) to form a module (also referred to as a "panel"). 

The PV module is the principle building block of a 

PV system and any number of modules can be 

connected together to give the desired electrical 

output. 

From cell to array (Source: Sharp) 

Photovoltaic modules can be placed on 

almost any building surface which receives 

sunshine for most of the day. Roofs are the usual 

location for PV systems on houses but photovoltaic 

modules can also be placed on facades, 

conservatory or atrium roofs, sun shades, etc. 

The surface on which the PV array is mounted should receive as much light as 

possible. The three issues which affect how much light a surface receives are: 

• Orientation: Due south is the best possible orientation. If the PV is to be 

mounted on a vertical façade the orientation should preferably be between 

South East and South West. If the PV is to be mounted at a tilt a wider range 

of orientations will still give a reasonable energy yield. North facing 

orientations should be avoided. 

• Tilt: A tilted array will receive more light than a vertical array. The ideal angle 

depends on the latitude. In Europe the optimal tilt angle ranges from 30° to 60° 

for a south facing array. Shallower tilt angles are better for east or west facing 

arrays. 

page 42

• Shadowing: Shadows cast by tall trees and neighbouring buildings must also 

be considered. Even minor shading can result in significant loss of energy. 

The area required for mounting a PV array depends on the output power desired and 

the type of module used. Broadly, an energy efficient house needs an area of no 

more than 10 m². 

There are various ways in which a PV array can be mounted on a building. The 

various options offer different appearances and vary in cost. The commonest way of 

mounting an array on a house is to place it on the roof either with modules mounted 

in a frame above the existing roof tiles or integrated into the roof. If the array is to be 

integrated into the roof PV roof tiles may be used instead of modules. Several 

companies have started integrating 

PV products into building materials such as tiles, shingles that look like 

traditional asphalt shingles or glass for windows and skylights that generates 

electricity. 

Balance-of-system: 

We can think of a complete photovoltaic (PV) energy system as composed of 

three subsystems: 

• On the power-generation side, a subsystem of PV devices (cells, modules, 

arrays) converts sunlight to direct-current (DC) electricity. 

• On the power-use side, the subsystem consists mainly of the load, which is 

the application of the PV electricity. 

• Between these two, we need a third subsystem that enables the PV-generated 

electricity to be properly applied to the load. This third subsystem is often 

called the "balance-of-system," or BOS. 

Most PV modules deliver direct current (DC) 

electricity at 12 or 24 V, whereas most 

common household appliances run off 

alternating current (AC) at 230 V. The BOS 

typically consists of structures for mounting 

the PV arrays or modules and powerconditioning 

equipment that adjusts and 

converts the DC electricity to the proper form 

and magnitude required by an alternatingcurrent 

(AC) load. The BOS can also include 

storage devices, such as batteries, so PVgenerated 

electricity can be used during 

cloudy days or at night. 

In grid-connected systems, the main 

additional equipment needed is an inverter 

BOS for a stand-alone system (a) and for a 

grid-connected system (b) (Source: NREL) 

page 43

that makes the panel output electrically compatible with the utility grid. No batteries 

are needed except if you prefer using your own electricity as much as possible 

instead of selling it. 

In a stand-alone system, batteries and a charge controller are additionally 

needed. 

A typical price for a grid connected, building integrated PV system is between 

9€ and 10 € per Wp. 

2.2 Wind power 

Home wind turbines consist of a rotor, a 

generator mounted on a frame, and (usually) a 

tail. With the spinning blades, the rotor captures 

the kinetic energy of the wind and converts it 

into rotary motion to drive the generator. The 

best indication of how much energy a turbine 

will produce is the diameter of the rotor, which 

determines its “swept area,” or the quantity of 

wind intercepted by the turbine. The tail keeps 

the turbine facing into the wind. 

A 1.5-kW wind turbine can meet the needs 

of a home requiring 300 kWh per month, for a 

location with a 6 m/s annual average wind 

speed. The manufacturer will provide you with 

the expected annual energy output of the 

turbine as a function of annual average wind 

speed and elevation at the site. Most turbines 

Example small windmill (400 W) 

have automatic speed-governing systems to 

keep the rotor from spinning out of control in 

very high winds. This information, along with your local wind speed distribution and 

your energy budget is sufficient to allow you to select the turbine size. 

You can have varied wind resources within the same property. If you live in hilly 

terrain, take care in selecting the installation site. If you site your wind turbine on the 

top or on the windy side of a hill, for example, you will have more access to prevailing 

winds than in a gully or on the 

leeward (sheltered) side of a hill 

on the same property. 

Because wind speeds 

increase with height in flat 

terrain, the turbine is mounted 

on a tower. Generally speaking, 

the higher the tower, the more 

energy the wind system can 

produce. The tower also raises 

Obstruction of the wind by a building or tree of height H 

page 44

the turbine above the air turbulence that can exist 

close to the ground (see diagram o). There are 

two basic types of towers: self-supporting (free 

standing) and guyed. Most home wind power 

systems use a guyed tower. Guyed-lattice towers 

are the least expensive option. They consist of a 

simple, inexpensive framework of metal strips 

supported by guy cables and earth anchors. 

However, because the guy radius must be onehalf 

to three-quarters of the tower height, guyedlattice 

towers require enough space to 

accommodate them. 

As for PV systems, wind-generated 

electricity needs to be properly applied to the load 

thanks to a balance-of-system. As well as solar 

cells, small wind turbines generate direct current 

This system produces alternating 

current for home use (Source: NREL) 

which has to be store in batteries or/and converted into alternative current. It can be 

a grid-connected or stand-alone system, hybrid or not. The situation is exactly the 

same as for PV (see balance-of-system in the previous part about photovoltaic). 

page 45

Conclusion 

The great majority of existing buildings – for housing as well as for offices – 

waste the energy even when they give a high comfort level. However, we saw that it 

is quite feasible to reduce their energy consumption by 70 % and even by 90 % in 

some cases, and increase their comfort at the same time. This could lead to a 

reduction of energy consumption in industrialized countries energy consumption by 

20 % ! 

What we have to do is just to apply the passive house principles (walls and 

glazings insulation, airtightness and ventilation with heat recovery). The technology is 

available and affordable. Then we have to attentively choose the energy-efficient 

household equipment. 

Of course, new buildings have also to respect these energy-efficiency golden 

rules in addition to adopting a solar passive design, which is basically as easy to 

implement as an ordinary design. Doing so, we simply use the sun to directly meet a 

great part of our energy needs. 

Huge savings are within reach but the problem is now to make people aware of 

this. 

Then the low remaining energy needs for hot water and household appliances 

can be met by self-produced renewable energy. They have currently a considerable 

renewed interest and reach henceforth outstanding performances thanks to recent 

technical advances. 

It is finally attainable to reach such a situation where buildings produce more 

energy than they use, that is to say giving them an additional function: producing 

energy. The significant area devoted to housing and offices could in this way 

contribute to the industry energy supply. 

The predicted depletion of fossil fuels may lead human activity to be based on a 

diversified, renewable and decentralized energy system. This new way to produce 

electricity could ideally be a part of this new pattern and contribute to the energy 

resources securing in the sustainable development context. 

Each citizen and each company could thus become responsible for a part of the 

society energy supply. 

page 47

Appendix 

page 49

Appendix 1 

Using energy balances to meet energy 

efficiency 

All energy is conserved - no energy gets lost. However, energy can escape out 

of the region, where the energy service is utilized. This is what we call an "energy 

loss", although the energy only moved to another place and may have changed its 

form. 

Already this introduction shows that energy balances only make sense within a 

well defined region with a well defined boundary. The boundary of the region is called 

the envelope. 

In the case of heating or air conditioning the region of interest is the "heated or 

conditioned space". More precise: it is the volume in the building, which is 

conditioned to comfortable thermal conditions. In most cases it is convenient, to 

include "passively heated" parts, as long as the balance envelope will be simplified. 

Generally the envelope should be chosen by pragmatic considerations: For a building 

it is convenient, to choose the envelope at the external surface of the insulated 

external building shell. 

The task for heating or air conditioning now is just to keep the temperature 

inside of the envelope constant. 

Let's have a look at a heat flow going inside out through the envelope; it may be 

hot air moving out through a window. Such a "heat loss" would reduce the "inner 

energy" inside the volume; and that would cause the temperature drop inside of the 

building. Just that should be avoided in order to keep the conditions comfortable. 

Therefore the energy flow to the outside has to be compensated: Another heat flow 

has to be created, going outside in, to keep the level of the temperature. 

This is an important insight: the need for heating is always only a reaction on 

heat losses. Because of the law of energy conservation a building will stay well 

conditioned - as long as there are no heat losses. It is a pit, that the physical 

mechanisms by which hotter systems transfer heat to a cooler environment are quite 

numerous and efficient. If we do not "isolate" the hotter system (by insulation), in 

general a lot of heat will get lost by heat conduction, convective heat transfer and 

radiation. "Heating" is always the substitution of energy losses - therefore "heating" 

can be reduced to an arbitrary low amount by effectively avoiding losses. 

There is some luck when looking at the heating task: there are some free "heat 

gains" too: For example the solar radiation through the window panes (so called 

passive solar energy) and the energy of the electricity supply, which is converted to 

page 51

"internal heat sources" in the building. This adds to the heat radiated from persons 

inside the building. This energy is as well transferred through the envelope into the 

house - at any time, when the persons enter the building or nourishments are 

delivered. 

Under the simplified 

conditions given here, it is simple 

to give the energy balance of the 

building: 

The sum of the heat losses 

equals 

the sum of the heat gains. 

It is quite simple to calculate 

the heat losses (depending on the 

insulation). The internal heat 

sources and the passive solar 

energy can be estimated as well. 

Therefore, on the basis of an 

energy balance, the heating energy 

required can be calculated. 

There is only one minor problem: the amount of the solar gains which can not 

be utilised has to be determined. But even for this, there are well validated simplified 

formula given e.g. in the European norm EN 832. For practise, these methods have 

been integrated into simulation tools such as the "Passive House design Package 

(PHPP)" for instance. 

Author: Dr. Wolfgang Feist (Passivhauss Institut) 

Heat losses (transmission and ventilation losses) exit the 

building through the envelope. Heat gains enter the building 

through the same envelope. Using the law of energy 

conservation, the sum of the gains equals the sum of the 

losses as long as the Inner Energy does not change. 

page 52

Appendix 2 

Heat-Movement Physics 

(applied to passive solar home) 

To understand how passive solar design works, you first need to understand 

how heat moves. 

As a fundamental law, heat moves from warmer materials to cooler ones until 

there is no longer a temperature difference between the two. A passive solar building 

makes use of this law through three heat movement mechanisms—conduction, 

convection, and radiation—to distribute heat throughout the living space. 

Conduction is the way heat moves through materials, travelling from molecule to 

molecule. Heat causes molecules close to the heat source to vibrate vigorously, and 

these vibrations spread to neighbouring molecules, thus transferring heat energy. For 

example, a spoon placed into a hot cup of coffee conducts heat through its handle 

and into the hand that grasps it. 

Convection is the way heat circulates through liquids and gases. Lighter, 

warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises 

because it is lighter than cold air, which sinks. This is why warmer air accumulates on 

the second floor of a house, while the basement stays cool. Some passive solar 

homes use air convection to carry solar heat from a south wall into the building’s 

interior. 

Radiant heat moves through the air from warmer objects to cooler ones. There 

are two types of radiation important to passive solar design: solar radiation and 

infrared radiation. When radiation strikes an object, it is absorbed, reflected, or 

transmitted, depending on certain properties of that object. 

Opaque objects absorb 40 to 95 percent of incoming solar radiation from the 

sun, depending on their colour—darker colours typically absorb a greater percentage 

than lighter colours. This is why solar-absorber surfaces tend to be dark coloured. 

Bright white materials or objects reflect 80 to 98 percent of incoming solar energy. 

Inside a home, infrared radiation occurs when warmed surfaces radiate heat 

towards cooler surfaces. For example, your body can radiate infrared heat to a cold 

surface, possibly causing you discomfort. These surfaces can include walls, 

windows, or ceilings in the home. 

Clear glass transmits 80 to 90 percent of solar radiation, absorbing or reflecting 

only 10 to 20 percent. After solar radiation is transmitted through the glass and 

absorbed by the home, it is radiated again from the interior surfaces as infrared 

radiation. Although glass allows solar radiation to pass through, it absorbs the 

page 53

infrared radiation. The glass then radiates part of that heat back to the home’s 

interior. In this way, glass traps solar heat entering the home. 

The term thermal capacitance refers to the ability of materials to store heat, and 

thermal mass refers to the materials that store heat. Thermal mass stores heat by 

changing its temperature. This can be from a warm room or by converting direct solar 

radiation into heat. The more thermal mass, the more heat can be stored for each 

degree rise in temperature. Masonry materials, like concrete, stones, brick, and tile, 

are commonly used as thermal mass in passive solar homes. Water also has been 

successfully used. 

page 54

Appendix 3 

How to move heat from a low temperature 

place to a higher temperature place by 

vapour compression 

Heat pumps are realized through several physical effects, but they are classified 

depending on their applications (driving energy, source and sink of heat, or a heat 

pump which is basically a refrigeration machine). Refrigerators, air conditioners, and 

some heating systems are all common applications of heat pumps. 

An easy way to imagine how a heat pump works is to imagine the heat in a 

given space - say the volume of a football (or soccer ball). The air within the volume 

of the ball has, say, 100 units of heat. This air is then compressed to the size of a 

ping pong ball (table tennis ball); it still contains the same 100 units of heat, but the 

heat is much more concentrated and thus the average heat per volume unit is much 

higher. In other words the temperature of the air in the ball will have increased. Now 

the walls of the ping pong ball will become hotter, and therefore heat will start to flow 

out of it faster than before. To transfer this heat somewhere else, we can move the 

ping pong ball to a cooling area, and the ball will gradually adjust its temperature to 

match it. By the time the temperature has equalized, it may have transferred 50 units 

of heat to the cooling area. After the ball has cooled a bit, move it back to the source 

area and allow it to expand. Since it has lost a lot of heat, once it expands the 

temperature will be lower than it was at the start of the whole process. Therefore the 

ball will be now cooler and can absorb energy to cool the surrounding input area. 

The compressor/pump unit creates the pressure difference which causes this 

cycle (the ball expanding and contracting) to endlessly repeat as long as the heat 

pump system is running. 

page 55

Sources 

The Worldwatch Institute, "State of the world 2004", W.W. Norton & Company 

S. Bedel and T. Salomon, "La maison des négawatts", Edition Terre Vivante 

Passivhauss Institut, www.passivhaus-institut.de 

HESPUL, www.hespul.org 

Energy Saving Trust, www.est.org.uk 

Heat Pump Center (International Energy Agency), www.heatpumpcentre.org 

British Photovoltaic Association, www.pv-uk.org.uk 

ADEME (French Environment and Energy Management Agency), www.ademe.fr 

National Renewable Energy Laboratory (US Department of Energy), www.nrel.gov 

Energytech (Austrian Energy Agency), energytech.at 

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