An Overview of One-Sun Solar Thermal Technology

An Overview of 

One-Sun Solar Thermal Technology 

Jay Burch 

National Renewable Energy Laboratory 

SEET Solar Thermal Seminar 7/26/07

Presentation Outline 

• Solar Basics 

• Technology 

• Collectors 

• Systems 

• Performance 

• Economics and markets 

• TTF tour 

• Future systems 

• Cold-climate thermosiphons 

• Triple play: water heating, space heating, space cooling 


The Sun = Radiant Energy 

Energy in the form of electromagnetic waves 

Invisible 

Travels at “speed of light” away from its source 

Radiant energy can be converted to other forms of 

energy: 

Heat energy: “absorb” the wave 

Electrical energy: photons excite electrons into the conduction band in PV 

Mechanical energy 


Radiant Energy Sources 

Sunlight (6000 o K) 

Q total 

=σT 4 

“Hot” 

Campfire (3000 o K) 

Stove burner (600 o K) 

“Cool” 

Infrared “heat” (330 o K) 


Sunlight: Waves or Photons? 

• As continuous waves: 

λ 

E wave = B X E 

Good model for thermal 

• As discrete photons: 

E photon = hν = hc/λ 

1 

λ 

Good model for photovoltaics 

0 

0 40 

SEET Solar Thermal Seminar 7/26/07 

-1

The Solar Resource 

• Solar constant: 1367 W/m 2 

• World energy use: 400 Quads/yr ~ 4*10 20 J/yr 

⇓ 

Irradiance in 40 minutes = world annual energy use! 

Irradiance is ~ 10 4 times world energy use rate 


The Solar Spectrum 

UV Visible Near Infrared 


Uses of Solar Energy 

• Photovoltaics 

• Thermal electric 

• Building energy 

} 

Yield electricity 

Yields heat/cold (thermal) 

• Other: 

• Industrial processes 

• Desalination 

• Water treatment 


Uses of Solar: 

Photovoltaics 

PV 

Cell 

Electricity 



Concentrating thermal electric 

Concentration 

High-T 

Heat 

Steam 

turbine 

Electricity 


Big electricity-producing plants


Heat production 

Absorber 

Heat transfer fluid 

Thermal end use 

Storage 

• Residential buildings 

• Domestic water heating, space heating, space cooling 

•Other: 

• Commercial buildings 

• Industrial process heat 

• Desalination 

• Water treatment 


Residential Solar Applications 

• Solar water heating 

– Year-round load 

– Most common application 

• Space heating 

– Part-year load 

– High need at low sun 

• Space cooling/dehumidification 

– Emerging hardware: not yet available 

– Challenging 

• Pool heating 

– Swing seasons/summer load 

– Most economical application 


SWH Technology 

Collectors 

Many types 

Focus on more common 


Unglazed Collectors 

Inexpensive unglazed 

polymer collectors 


Photovoltaics as Unglazed Collector 

Triples PV only gain: Q thermal ~ 2*Q electrical 

Air-based 

PV panel/thermal absorber 

Roof 

Air gap/passageway 

Liquid-based 

PV panel/thermal absorber 


Tubes w/ ht trsf fluid (typ. glycol) 


Flat Plate Collector 


Flat Plate Collector Types 

Fin-tube absorber 

Fully-wetted Absorber 

Metal absorbers 

Polymer absorbers 

Sun 

Glazing 


Glazing 

Tube 

Fin/Conductive metal 

Fluid passageways 


Integral-Collector-Storage (ICS) 

Today’s ICS: Serpentine Cu tubes under line pressure in a glazed box 

Gasket 

Glazings 

Water Connection 

Box 

Storage tanks 

Insulation 


Evacuated Tube Collector 


Dewar design 

Evacuated space 

Selective coating 

Out 

In/out tubes 

In 

Glass tube 

Historically expensive; end seal issues 


Collector Performance 

Energy balance: 

Q out,loss = UA(T avg –T ambient ) 

Q in = Q out 

(steady state) 

T out 

Q in = (τα)I sun 


Q out,to-fluid = mc p (T out –T in ) 

=Q useful 

T in 



Efficiency (η): 

η= Q useful /Q incident 

Q out,loss = UA(T avg –T ambient ) 

T out 

Q in = (τα)I sun 


Q out,to-fluid = mc p (T out –T in ) 

=Q useful 

T in 



(Q useful )/A coll =F r (τα) n I sun –F r U l (T in –T amb ) 

Optical Gain 

Thermal loss 

⇓ 

η=F r (τα) n –F r U l [(T in –T amb )/I sun ] 


Collector Efficiency Equation 

Efficiency vs. Temperature Difference 

0.9 

Efficiency 

0.6 

0.3 

Selective 

Un-glazed 

Non-selective 

Evacuated Tube 

0 

0 0.05 0.1 0.15 

(T_in - T_amb)/I_inc 


Operating parameter 

= ∆T/I sun

Collector Efficiency Equation 

0.9 

Efficiency vs. Temperature Difference 

Efficiency 

0.6 

0.3 

Selective 

Un-glazed 

Non-selective 

Evacuated Tube 

0 

0 20 40 60 80 100 120 

T_in, @ I_sun= 800 W/m2 


Solar Water Heater Certification 

Solar Rating and Certification Corporation 

(SRCC) 

www.solar-rating.org 

Certifies all collectors and systems in U.S. 

Publishes all collector test results: OG100 

Publishes all system performance ratings: OG300 


All SRCC Collectors 

Collector Summary 

9 

Fr Ul [W/m2-C] 

8 

7 

6 

5 

4 

3 

2 

Non-selective 

Selective 

Black Chrome 

Black Nickel 

Black Paint 

Metallic Oxide 

Moderately Selective Black Paint 

Polyester Flat Black Paint 

Selective Coating 

Sputtered Aluminium Nitride 

Sputtered cermet 

Sputtered titanium nitride 

Titianium oxide 

Vapor Deposition Selective Coating 

1 

0 

0 0.2 0.4 0.6 0.8 1 

Fr Ta 

Evacuated tubes 


SWH Technology 

Systems 

Many types 

Focus on more common 


Pool System: Simple 

Collector 

New 

Pool 

Pump, 

Filter 

V 

Existing 


Solar Pool Collectors/System 

Inexpensive unglazed 

polymer collectors 

Inexpensive balance-ofsystem 

(uses pool pump, 

pool is storage,…) 


Solar Pool Heating 

Relatively successful market 

• Inexpensive system 

– Unglazed polymer collectors at 1/10 th glazed cost 

– Very inexpensive “balance of system” (pool = storage, etc.) 

• Good performance 

– Low temperature difference in spring/summer/fall 

– No reflection losses 

Good economics ⇒ Good market 

• < ~5 yr payback, > ~20% return on investment 


Solar Water Heaters (SWH) 

• Successful market in Europe, China 

• Europe: High energy costs, environmentally conscious 

• China: low system costs, no other options in many cases 

• Poor market in the U.S. 

• Robust market (~1B$/yr) during tax credit era 1979-1984 

• Market collapse in 1984 

• Low energy costs + high system cost: 

» 10-50 year paybacks 

» Sales: ~ 6K systems/year 

» 2005 Energy Policy Act: 

Up to $2000 rebate: increased sales? 


Basic SWH Classification 

Active 

Has pump, with sensors, 

wires, and controller 

Passive 

No pump, sensors, 

wires, or controller 


SWH Classification: Active 

Active * 

Liquid Heat Trsfr Fluid 

Air as Heat Trsfr Fluid 

Direct Glycol Drainback Front-pass Back-pass Transpired 

* Has pump/fan for fluid 

circulation, with sensors, 

wires, and controller 


SWH Classification 

Passive * 

Integral collectorstorage 

(ICS) 

Irradiated tank of water 

Thermosiphon 

Storage above collector: 

hot water rises 

Other 

Percolation; thermal pump;… 

For these system types, storage is on the roof 

*No pump/fan, sensors, 

wires, or controller 


Further Classification Dimensions 

• Collector 

– Glazings (for flat-plate @ atmospheric pressure) 

– Unglazed 

» Can be used for pool, domestic hot water, and space heating 

» Not common in solar water heaters 

– One glazing (most common) 

– Multiple glazings (rare) 

– Surface coatings 

– Non-selective: high solar absorption and high infrared loss 

– Selective: high solar absorption and low infrared loss 

» Thin IR-transparent coating on IR-reflective substrate 

» IR-reflective elements in paint mixture 

– Evacuated tubes 

– Rare in the US; dominant in China 

– May become common in US? 



• Solar loop heat exchanger 

• Internal/in-the-water 

– Immersed coil 

– Tank-in-tank 

• External to storage 

– Wrap-around coil 

– Side-arm counterflow 

» Only collector side pumped with thermosiphon loop on tank side 

» OR: Both sides of heat exchanger are pumped 

• Storage tank 

• Pressurized 

» potable water in ceramic-lined steel tanks 

• Unpressurized 

» Must have load-side heat exchanger carrying potable water 



• Controller in active systems 

• Active, differential controller 

» Pump on when T collector > (T tank-bottom + ~20 o F) 

• PV-pump 

» DC pump is controlled by PV panel 

• Timer 

• Overheat protection methods 

• None: most common 

• Vent the collector 

• Evaporate fluid from collector 

• Dump storage heat 

» boiling, passive radiator, ground loop, night-time circulation 

• … 


Active System- Direct 

Pressurized potable water 

in the collector loop 

To aux tank 


Active System- Indirect 

Glycol in this loop 

Glycol System 

Glycol provides 

freeze protection 

To aux tank 


Active System - Drainback 

Controller=PV panel 

PV Panel 

To aux tank 

Un-pressurized 

solar storage tank 

Mains inlet 

High-head 

DC pump 

Heat Exchanger 


Another Drainback Design 

Solar 

Sensor 

PEX 

plumbing 

Vented 

Drainback 

Tank with 

Level 

Indicator 


Controller 

Temperature 

Gauges 

Drain Valve 

Circulation 

Module 

FAFCO 

INCOMING 

COLD WATER 

HOME HOT 

WATER 


Collector 

Mounting 

Hardware 


Jacks 

Bypass Valve 

Cold Water 

Shut-Off Valve 

Anti-Scald 

Valve 

Coaxial tank 

adapter 

Tank Temperature 

Sensor 

Storage Tank 

Water Heater 


Passive- Thermosiphon 

Light, hot water rises 

Heavy, cold water sinks 

Convection loop schematic 

Most popular system worldwide: simple, reliable 


Thermosiphon vs. Active 

Thermosiphon 

Active 

Solar tank 

Cold In 

Collector sensor 

Cold In 

Wires 

Hot Out 

Hot Out 

Pump 

• Thermosiphons: 

• Less parts, less cost 

• More reliable 

• ~Equal performance 

Elec. 

tank 

Inside 

Solar tank 

Tank sensor 

Controller 

AC Power 

Extra hardware vs. thermosiphon 


Thermosiphons: Many varieties possible 

Old style: protrusive/ugly? 

New style: sleek/aesthetic; “like a skylight” 


Solco/Australian TSiphon 

2/3 cost reduction 

Cost ~ $400 US 

Rotomolded PE Body, 

PMMA glazing 


ICS System: simple 

No pump, no 

controller, no 

separate tank 

Mains inlet 

Conventional DHW tank 

End use 


ICS Systems 


Unpressurized ICS 

Immersed heat exchanger 

Glazing(s) 

Insulation 

Supply/Return Piping 

Thin-walled polymer 

vessel of water 


Unpressurized ICS: DEG/SE 

Thermoformed 

acrylic glazing 

Cu Load-side Hx 

PE tank 


DEG/SunEarth ICS 

Glazed version 

Unglazed version 


Field Installations 

Migrant housing 

camp in CA 

Glazed units 

Unglazed units 


Invisible Collector 

Shingles + 

Deck 

Ht trsfr fluid tubes 

Insulation 

Conductive fin 


Evacuated Tube Collector 


Dewar design 

Evacuated space 

Selective coating 

Out 

In/out tubes 

In 

Glass tube 

Historically expensive; end seal issues 


Variations with Evacuated Tubes 

• Passive thermosiphon 

• Tubes directly enter storage tank, 

thermosiphon loop each tube 

• Active system 

• Fin-tube heat pipe transfers 

heat to condensor in manifold 


Performance Basics 

• System efficiency ≡ η sys = Q sav /Q inc 

• Q inc ~ 3-6 kWh/m 2 /day, 100-200 kWh/ft 2 /year 

• Solar Fraction ≡ f sol = Q sav /Q fuel,no-solar 

• η sys ≈ constant ≈ 0.4 for active system 

1.00 

0.80 

0.60 

Active Solar Water Heating 

(216 locations in the continental U.S.) 

Solar Fraction 

Efficiency 

1.00 

0.80 

0.60 

0.40 

0.40 

0.20 

0.20 

0.00 

0.00 

0.0 5.0 10.0 15.0 20.0 25.0 30.0 

Annual-Average Ambient Temperature ( o C) 


Racked/Space Heating System 

UGGHH! 


SDHW: Flush-mount 

Neat!! 


SDHW: Wide Orientation Range is OK 

90%-100% 

80%-90% 

70%-80% 

60%-70% 

50%-60% 

Collector Orientation Factor 

Solar Water Heating 

(Lat=40 o ) 

-90 

-60 

-30 

0 

30 

60 

90 

Azimuth 

90 

75 

60 

45 

30 

15 

0 

Tilt 


U.S. Sales by Year 

$1 billion/yr @ peak 


U.S. Collector Area by Year 

20 Million ft 2 /yr @ peak 


Market Penetration 

Maximum Market Penetration vs. Payback 

(avg of 3) 

Penetration (fraction) 

1 

0.75 

0.5 

0.25 

0 

Target: 4 yrs (50%) to 7 yrs (10%) 

0 2 4 6 8 10 

Simple Payback (years) 


Cost Goals for SDHW System 

Cost of 40 ft2 System vs Cost-of-elec for spec payback 

(Incidence = 5.4 kWh/m2/day; Denver) 

Cost_system [$] 

5000 

4000 

3000 

2000 

1000 

0 

Target 

0 5 10 15 20 

Payback 

4 

7 

10 

13 

16 

Cost_electricity [c/kWh] 

1. Choose mkt. penetration payback (PB line) 

2. For site cost of fuel, read Cost_System @ PB line (dotted lines) 


SWH and Cost 

• Present systems: 

– Cost is a barrier 

– New construction: $3000 to $5000 

– Retrofit: $4500 to $10,000 

– Need rebates to bring net cost to below $2000 

• Future system: 

– Cost reduction efforts: bring price under $2000 


Polymer Collector 

First Cost 

Eliminate one pump 

Integrated valve pkg 

Integrated piping 

$3,500 

$3,000 

$2,500 

$2,000 

$1,500 

$1,000 

Cold Climate Systems: 

Cost Reduction Potential 

Cost & Cost-of-Savings/ Glycol 

[COSE = (Cost)/(Savings)] 

BOS Variations 

1st Cost 

COSE 

Collector Variations 

Base case 

One pump 

Polymer tank + hx 

Integrated piping 

Valve package 

Non-selective mtl-gls 

Polymer selective 

Polymer non-selective 

Polymer unglazed 

12 

10 

8 

6 

4 

COSE [c/kWh] 

Film-lined storage 

Heat exchanger 

Retainer ring 

Submersible pump 

Polymer film liner 

Insulation 

Sheet metal cylinder 

Rigid foam base 

Polymer heat exch. 


Cost ~$12

Key Reliability Issues 

• Installation problems 

• Leaks in roof 

• Leaks in piping 

• Sloppy installation 

• Controller set wrong 

• … 

• Hardware problems 

• Pump, sensor, controller fails 

• Storage tanks fails 


Installation problems 






Installation Training 

North American Board of Certified Energy 

Practitioners 

(NABCEP) 

• Administers tests for certification 

• Photovoltaics 

• Solar thermal 

• Many jobs (100,000s) coming 

• Qualified/certified installers needed badly 


SWH Show-and-Tell 

Let’s go see a few SWH articles in the TTF 


Future Solar Thermal 

• Cold-climate thermosiphons 

• Address space heating and cooling also 


Pipe Freeze Problem 

Collector 

Storage 

Pipes in attic can freeze 

Supply piping 

Mains Inlet 

Return piping 

House load 

Aux 

Tank 


Pipe Freeze Probability 

Probability is for at least one 

freeze in twenty years 

Safe to install passive 

systems 


Passive Systems and Pipe Freeze 

Passive systems: 

– Currently: pipe freeze limits markets to mild climates 

• Cu piping with insulation 

– To extend market, must freeze protect piping 

• Circulate warm water through the piping 

– Heat from auxiliary tank or interior air heat exchanger 

• Circulate warm interior air in duct around piping 

• Circulate mains water through piping with Freeze Protection Valve (FPV) 

– To prevent catastrophe when freeze protection fails: 

• Must have freezable piping ⇒ failure/freeze is minor inconvenience 


Freeze Protection System 

Two Levels of protection: 

1. Primary Freeze Protection 

Keeps pipes from freezing (until it fails) 

Freeze protection valve 

Natural convection loops;… 

2. Fail-safe Backup: Freeze-tolerant piping 


Is there a Freeze-tolerant Piping? 

• Polybutylene: 

• PEX? 

• Great freeze tolerance (withstood >700 freeze-thaw cycles) 

• Withdrawn from market ~1992 

Probable reason: Brass/copper fittings attack the material 

• Some manufactures/web sites claim “freeze tolerance” 

• No documentation 

Goal: determine freeze-tolerance of PEX 


Freeze-thaw Test 

Freezer 

PEX 

Result: 2 kinds of 

PEX are freezetolerant 

Over 500 cycles of 

freeze-thaw done 


Freeze Protection Valves (FPV) 

~ $60 

~$100-$200 


FPV Protecting Piping 

Collector 

Storage 

Freeze Protection 

Valve located HERE! 

Return piping 

House load 

To drain 

Supply piping 

Mains Inlet 

Aux 

Tank 


Dole FPV/35 0 F Setpoint 

Annual Flow (gallons) through Dole FP-35 Freeze Prevention Valve 

Using Air Freezing Index Correlation with y-intercept Equal to Zero 

flow=0 

0

Passive Systems Market Extension? 

Safe/Non-wasteful areas= 

Freeze Protection Valve + 

Insulated copper pipe 

freezable piping 

Limited by Pipe Freeze 

Limited by water consumption 

BUT: collector/store freeze? 

Untested, other affects? 

Market Uptake/Transformation? 

Cold Climate Thermosiphons? 


Glazed Systems 

A natural end-use progression for solar: 

Domestic Water Heating 

Domestic Water Heating + Space Heating 

(Combi-systems) 

Domestic Water Heating + Space Heating + Space Cooling 

(Triple-play systems) 


100% Solar HVAC 

Another natural progression: 

Solar System + Standard HVAC 

(solar is an “extra”) 

100% Solar 

Solar “does it all” 

Eliminate conventional furnace, air conditioner, water heater 

Implies annual storage 


Solar Progressions: Quantitative 

End Uses Addressed 

Area 

m 2 

Storage 

m 3 

Load 

GJ 


Fraction 

Water heating (WH) 6 0.3 16 0.6 

WH + Space Heating (Htg) (Combi-system) 20 1.5 40 0.3 

High sf combi-sys, w/ H2O annual store 1 40 70 40 0.9 

2 

100% Solar WH/Htg/Clg, w/ Des. TCHP 30 14 50 1 

1. From (Sillman 1981) 

2. Desiccant thermochemical heat pump storage; estimated. 

Boston, MA climate 

Note on units: 

Multiply m 2 times 110 to get ~ ft 2 

Divide volume in m 3 by 4 to get ~ kGal 

1 GJ ≅ 1 MMBtu 


Solar Combi System Schematic 

Compared to Solar Water Heaters: 

•Larger Collector Area 

•Large Storage Tank 

•More complex controls 


Combi-System/Diurnal Storage 

1= Excess Consumption: load that cannot be met by solar 

2 = Potentially Usable Solar 

3 = Excess Solar 

Irradiation 

Load 

SF max = 2 /( 2 + 1) 

1 1 

3 

Overheating 

Potential 

2 

2 


Architectural/aesthetic issues 

Low Winter Sun ⇒ High Tilt for Space Heating? 

Ugly? 


Building-integrated Combi-systems 


Combi-System: High Solar Fraction 

Need large capacity, 

low-loss storage 

Irradiation 

Load 

SF max = (A col *H day )/(Load) 

1 

Store Excess 

Summer Heat for 

Use in Winter 

3 

1 

2 

2 


High Solar Fraction ⇒Annual Heat Storage 

Heat storage types: 

– Sensible: need ~5-30 kGal (w/ efficient house) 

– Costly, bulky, “formidable” 

– Thermochemical heat pump/liquid desiccants 

– High energy density possible (1/6 th that of sensible) 

– Can use flat-plates at ~160 o F 

– Integrates well with solar-driven cooling in summer 


Annual Storage 

1500 

Solar Fraction Contours vs (Vstor, Acol) 

0.8, super-ins 

0.9, super-insul 

Acoll [ft2] 

1000 

500 

Unrealistically large 

0.8, good passive 

0.9, good passive 

0.8, ~ '81 code 

0.9, ~ '81 code 

Reasonable sizes? 

0 

0 10000 20000 30000 40000 

Vstor [gal] 

Boston, MA 

Flat Plate Collectors 

From Sillman 1981 “Trade-off…in Annual Storage Solar Heating” 

2-tank System 


Desiccant Heat Pump Storage 

• Solar heat in summer: 

• Evaporate water out of weak desiccant (1250 Btu/lb) 

• Makes strong desiccant 

– For cooling, and store for winter heat 

• Desiccant heat in winter 

• Condense water vapor onto strong desiccant (1250 Btu/lb) 

• Makes weak desiccant 


Adsorption/Desorption Desiccant Storage Cycle 

Q desorption,charge 

(from solar) 

Summer 

Charging 

Q condensation,charge 

Weak 

desiccant 

Water vapor, condensed 

Ground 

Storage 

Strong 

desiccant 

Water vapor, “re-created” 

Ground 

Storage 

Q condensation,discharge 

(to load) 

High Temperature Processes 

Winter 

Discharging 


Q desorption, discharge 

Low Temperature Processes

Future Residential Solar HVAC 

Beyond SWH to “triple-play” 

100% solar triple play, with desiccant heat pump? 

(Stay tuned!) 


Thank you for your Attention 

• Just skimmed the surface 

• Much more to learn 

• Viewgraphs/references available to sign-up 

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