Book - School of Science and Technology

662 Running costsAfter allowance has been made for typical combustion inefficiency (75% for gas andoil; 65% for coal) the proportion of primary energy actually remaining for use in buildingsystems is approximately as follows:Electricity 26%Coal 63%Natural gas and oil 69%The order of these figures must be borne in mind when considering the merits ofconservation measures and those of alternative sources of energy.Various methods are available for making energy consumption calculations, rangingfrom manual estimates through to detailed thermal modelling using computer simulationtechniques. It is generally accepted that heating, hot water service and ventilation systemsmay be assessed with reasonable accuracy using manual methods, but that air-conditioningsystems require calculation by some more refined form of analysis.The various methods available may be used with reasonable confidence to comparedesign solutions. Use of the results to forecast actual energy use must be approached,however, with caution. There are numerous factors which may affect such a forecastadversely, such as the relevance of the weather data assumed, the method of operation orcontrol of the systems, how the occupants use the space ± such as the hours of use andlevel of internal heat gain and the quality of the maintenance (or lack of it). All theseaspects will have a marked effect on actual energy use.Heating systems ± Part L regulationsThe edition of Part L of the Building Regulations for implementation in 2002 introducesnew requirements for minimum efficiencies for heating plant at both full and part load.The main driver for setting building energy efficiency standards is to limit carbonemissions as part of the UK's climate change commitments. Consequently, the performanceof the heating plant is as important as the envelope insulation standards. Becausethe driver is carbon emissions, the efficiency of a heating system is assessed in terms ofcarbon intensity. This is a measure of the carbon emitted per useful kWh of heat output.Where there are multiple pieces of heat generating equipment, the carbon intensityof the heating plant is the rating weighted average of the individual elements, and isgiven by:" c ˆ 1 P RX R:Cf tWhere" c ˆ the carbon intensity of the heating system (kgC/kWh of useful heat).R ˆ the rated output of an individual element of heat raising plant (kW). t ˆ the gross thermal efficiency of that element of heat raising plant (kWh of heat perkWh of delivered fuel).C f ˆ the carbon emission factor of the fuel supplying that element of heat raising plant(kg of carbon emitted per kWh of delivered fuel consumed) as given in Table 23.2.It can be seen that there is a significant variation in the emission factors for the differentfuels. It is not practical to expect the same carbon intensity irrespective of fuels, and so