Solar Thermal TECHNOLOGY Eric Buchanan Renewable Energy Scientist West Central Research and Outreach Center Wcroc.cfans.umn.edu
Solar Energy History Solar Thermal 700 BC Chinese use burning mirrors 1760 Saussure uses solar hot box 1891 First DHW patent 1909 Separate panel & storage 4000 sold in 10 yrs (CA) until nat. gas discovered in 20 s Used in 1/2 of houses in FL (1941) until cheap electricity Boom & bust cycle repeats itself in Japan, Israel and Australia due to cheap fossil fuels
Sunlight to Heat Solar thermal systems heat air or a fluid to provide space conditioning or domestic hot water (DHW) Active systems use a pump or fan to move fluids, passive systems use gravity and convection Direct systems heat the fluid that is consumed, indirect systems heat a working fluid and use a heat exchanger
Sunlight to Heat Active, indirect systems are typically used in a cold climate 2 types of freeze protection Antifreeze 50% Propylene glycol (food grade) & water o Freezes at -29 deg F, boils at 222 deg F o Limited life ~ 5 years, annual ph check Drain back When pump stops, fluid drains from panels into a storage tank o Allows use of plain water o Can also prevent overheating
Sunlight to Heat Solar thermal panels Flat plate panels Simple, proven design Very robust More effective at lower fluid temperatures Evacuated tube panels Elegant design Passive tracking More efficient at higher fluid temperatures
Sunlight to Heat Collector efficiency unglazed flat plate collector Unglazed collectors are used for pool heating Efficiency calculations are based on collector gross area Penalizes evacuated tubes because of spaces between tubes The SRCC provides test data for thermal panels www.solar-rating.org More sunlight Lower fluid temp Higher air temp Less sunlight Higher fluid temp Lower air temp
Sunlight to Heat Domestic Hot Water (DHW) systems One Tank Or Two?
Sunlight to Heat Domestic Hot Water (DHW) systems Can provide 50% to 75% of hot water needs A back up system is needed Can t be 100% solar Must deal with excess heat Heat dump or drain back system A boiler can be added to provide solar space heating
Sunlight to Heat Heating air Fan turns on when panel temperature is higher than room temperature www.rreal.org
Sunlight to Heat Solar fraction is the percentage of load provided by solar energy It is not practical to have a 100% solar fraction A back-up system is required Doubling the system size does NOT double the solar fraction (24% to 37%) Must deal with excess summer heat o Heat dump or drain back system o Increase summer load Swimming pool Seasonal storage Air conditioning?? www.caleffi.us
Solar Thermal History Solar cooling 1858 Absorption cooling invented by F. Carre 1878 Auguste Mouchout makes ice at the Universal Exhibition in Paris 1929 First Icy Ball patent 1930 Einstein refrigerator (Electrolux) Absorption cooling has been used commercially since the early 20 th century Used in RV s and tractor trailers
Solar Thermal Basics Solar air conditioning Absorption chillers water and salt (LiBr) or ammonia and water Adsorption chillers water and silica gel or zeolite
Solar Thermal Basics Peak air conditioning loads typically occur when solar resources are high High water temps needed (190 deg F) Evacuated tube collectors Need cooling tower for heat rejection Requires chemical water treatment Residential sizes (<5 ton) are rare Chillers are more efficient in larger size
WCROC System Details LEED silver office building addition/remodel completed in 2010 Honeywell control and monitoring system Primary HVAC system is 15 ground source heat pumps Horizontal and vertical closed loop system Addition space is 4,700 ft 2 (434 m 2 ) HVAC design loads: 10 tons (35 kw) cooling 15 tons (53 kw) heating
WCROC System Details Solar thermal system 3 heating/cooling coils inserted ahead of heat pumps About 2100 ft 2 (200 m 2 ) of evacuated tube collector area 40 Solar Panels Plus, SPP-30 panels Heat transfer fluid is 50/50 mix of water and propylene glycol System volume is 600 gallons (2270 liters) including 300 gallons (1135 liters) of storage Slant fin tubing for heat dissipation
Heating Mode Solar heated water is directed through a mixing valve to the fan coils and/or the storage tank Mixing valve is controlled by temperature drop across coils When coil load is low, hot water is directed to storage tank
Heating Mode Results SRCC Comparison Insolation = 793 Btu/ft 2 /day = 2.50 kwh/m 2 /day (Cloudy Day = 1000 Btu/ft 2 /day) Ave fluid temp = 150 F (66 C) Outdoor temp = 45 F (7 C) Evacuated Tubes: Panel output = 12.8 kbtu (13.5 MJ) SRCC table* 8.8 kbtu (9.3 MJ) *Extrapolated
Cooling Mode Solar heated water is directed to an absorption chiller Chilled water is directed through a mixing valve to the fan coils and/or storage Mixing valve is controlled by coil exit temperature When coil load is low, chilled water is directed to the storage tank
Cooling Mode Results SRCC Comparison Insolation = 1049 Btu/ft 2 /day = 3.32 kwh/m 2 /day (Cloudy Day = 1000 Btu/ft 2 /day) Ave fluid temp = 180 F (82 C) Outdoor temp = 45 F (7 C) Chilled water temp = 46 F (8 C) Evacuated Tubes: Panel output = 17.2 kbtu (18.1 MJ) SRCC table 11.2 kbtu (11.8 MJ)
Cooling Mode Solar air conditioning equipment Yazaki SH-10 absorption chiller Refrigerant is water, absorbent is lithium bromide Operating range: 158 F (70 C) to 203 F (95 C) Nominal input: 190 F (88 C) at 38 gpm (144 lpm) Chiller can be reversed to operate in heating mode Amcot ST-50 cooling tower Requires chemical treatment of water Variable Frequency Drive (VFD) for solar pump Helps low light heat collection
U.S. Air Conditioning
A/C Conclusions Is solar air conditioning the future? Only about 1000 units in operation worldwide Planners and installers have little experience Small, unstable market creates tough business plan On the plus side Technology works! Improvements are in the works Solar A/C decreases grid load at peak times Source: IEA SHC Task 38 - October 2011
Germany had 6 times as much solar in 2011 as in 2007 Peak electricity price dropped 40% - EVERYBODY pays less!! 2008 2012
The Solar Thermal Industry China and Europe account for more than 90% of the world SHW market! It is mostly residential Source: International Energy Agency Total collectors in use as of 2009 Total collectors sold in 2009 China is driving the collector market towards evacuated tubes
What is China doing?
UMM RFC Pool 32 Solar Skies flat plate collectors Solar heated water is pumped through a heat exchanger Pool pump sends pool water through the same heat exchanger http://146.57.12.91/db/um_morris_web_sup/graphics/pe_center/solar_hx
UMM RFC Pool - JUN 2012
Solar Thermal TECHNOLOGY Eric Buchanan Renewable Energy Scientist West Central Research and Outreach Center Wcroc.cfans.umn.edu