|Active Solar Cooling and Refrigeration|
|Active Solar Cooling and Refrigeration
It is possible to use solar thermal energy or solar electricity to operate or power a cooling appliance or a refrigerator. The following is a brief description of "active" solar cooling and refrigeration technologies. Active solar energy systems use a mechanical or electrical device to transfer solar energy absorbed in a solar collector to another component in the "system." It is possible to also cool a building or structure by using the natural processes of solar heat transfer (conduction, convection, and radiation). This is often referred to as "passive solar cooling," and is primarily an architectural technique. This brief focuses on active solar cooling systems. The American Solar Energy Society (ASES, see Source List below) is one source of information on passive solar cooling techniques.
Absorption Cooling and Refrigeration
Absorption cooling is the first and oldest form of air conditioning and refrigeration. An absorption air conditioner or refrigerator does not use an electric compressor to mechanically pressurize the refrigerant. Instead, the absorption device uses a heat source, such as natural gas or a large solar collector, to evaporate the already-pressurized refrigerant from an absorbent/refrigerant mixture. This takes place in a device called the vapor generator. Although absorption coolers require electricity for pumping the refrigerant, the amount is small compared to that consumed by a compressor in a conventional electric air conditioner or refrigerator. When used with solar thermal energy systems, absorption coolers must be adapted to operate at the normal working temperatures for solar collectors: 180¡ã to 250¡ãF (82¡ã to 121¡ãC). It is also possible to produce ice with a solar powered absorption device, which can be used for cooling or refrigeration.
Desiccant cooling systems make the air seem cooler by removing most of its moisture. In these systems, the hot, humid outdoor air passes through a rotating, water-absorbing wheel. The wheel absorbs most of the incoming air's moisture. This "desiccates" (heats and dries) the air. The heated air then passes through a rotating heat exchanger wheel, which transfers the heat to the exhaust side of the system. At the same time, the dried air passes through an evaporative cooler, further reducing its temperature. The heated exhaust air continues through an additional heat source (e.g., a solar heat exchanger), raising its temperature to the point that the exhaust air evaporates the moisture collected by the desiccant wheel. The moisture is then discharged outdoors. The various system components require electricity to operate, but they use less than a conventional air conditioner. Most desiccant cooling systems are intended for large applications, such as supermarkets and warehouses. They are also ideal for humid climates.
Electric evaporative coolers, also known as adiabatic or "swamp" coolers, have been common for many years in hot, dry climates. As the outside air passes through a fine mist of water, it gives up much of its heat through evaporation. In direct evaporative systems, the evaporation process humidifies the air. In indirect evaporative systems, the evaporation process is isolated from the air stream, and uses a heat exchanger to cool the air. It is possible to design a solar photovoltaic (PV) array to provide some or all of the electricity to operate the unit.
Heat Engine/Vapor Compression Cooling (Rankine-Cycle)
The Rankine-cycle cooling process uses a vapor compression cycle similar to that of a conventional air conditioner. Solar collectors heat the working fluid, which has a very low vaporization point. The working fluid then drives a Rankine-cycle heat engine. This technology, however, is mainly experimental, and is not used often because it needs a large system size to do any meaningful amount of cooling.
Photovoltaic (PV)-Powered Heat Pumps, Air Conditioners, and Refrigerators
PV cells/modules can power devices such as evaporative coolers, heat pumps, and refrigerators. In most cases, you need an inverter to change the low-voltage, direct current (DC) produced by the PV array into the higher-voltage, alternating current (AC) that powers most heat pumps, air conditioners, and refrigerators. There is, however, a U.S. company that has designed a solar powered heat pump for both cooling and heating. The heat pump has a DC motor that operates the heat pump compressor. The motor is powered by a PV array and by electricity from the utility. While in the cooling cycle, the heat pump can also preheat domestic water. Systems for vaccine refrigeration are becoming widely used in remote areas of developing countries.
Articles and Conference Papers
"Alternative Energy Sources to the Rescue," G. Wright, Western HVACR News, (21:3), March 2001.
ASES Solar 2002, Conference Proceedings, available from ASES (see Source List below). Contains 10 papers related to solar cooling and refrigeration. Contact ASES regarding purchase price.
"Autonomous Solar Cooling Unit Begins Operating in German Office Building," Ed., Solar & Renewable Energy Outlook, (27:4) pp. 159-160, July 15, 2001.
"Demonstration of a New ICPC Design With a Double-Effect Absorption Chiller in an Office Building in Sacramento California," W. Duff, et al., ASES Solar 99 Conference, Portland, Maine, June 12-16, 1999; pp. 175-179. Available from ASES (see Source List below).
"Experimental Evaluation of a Solar PV Refrigerator with Thermoelectric, Stirling, and Vapor Compression Heat Pumps," M, Ewert, et al, ASES Solar 98 Conference, Albuquerque, New Mexico, June 14-17, 1998; pp. 59-65. Available from ASES (see Source List below).
"The Field Test of a Photovoltaic Heat Pump," A. Lowenstein, ASES Solar 98 Conference, Albuquerque, New Mexico, June 14-17, 1998; pp. 67-79. Available from ASES (see Source List below).
"Home Air Conditioning on a Renewable Energy System," E. Sheldon, Home Power, (No. 81) pp. 10-16, February/March 2001.
"A Hybrid Solar Absorption Air Conditioning System," J. Bergquam, Solar Today, (7:4) July/August 1993, pp. 23-25.
"Modeling of a Photovoltaic Powered Refrigeration System," G. Furler, et al, ASES Solar 94 Conference, San Jose, CA, June 27-30, 1994. Available from ASES (see Source List below).
"Multi-Pressure Absorption Cycles in Solar Refrigeration: A Technical and Economical Study," S. Alizadeh, Solar Energy, (69:1) pp. 37-44, 2000.
"A New Desiccant Evaporative Cooling Cycle for Solar Air Conditioning and Hot Water Heating," J. Archibald, ASES 2001 Solar Forum/Conference, Washington, DC, April 21-25, 2001. Available from ASES (see Source List below).
"Performance of a Combined Solar Desiccant Filtration Air-Conditioning System," T. Jekel, et al, ASES Solar 94 Conference, San Jose, CA, June 27-30, 1994. Available from ASES (see Source List below).
"PV Chilled (Tractor) Trailer Quietly Saving Emissions," CADDET UK National Team, , (No. 4/00) pp. 10-12, December 2000.
"," G. Wright, Western HVACR News, (21:11), November 2001.
"A Simple Economic Model for Solar Cooling and the Potential for the ICPC Collector," J. O'Gallagher and R. Winston, ASES Solar 98 Conference, Albuquerque, New Mexico, June 14-17, 1998; pp. 401-406. Available from ASES (see Source List below).
"A Solar-Assisted Desiccant Cooling: Case Study," K. Miller and M. West, , (8:4) July/August 1994, pp. 15-16.
"Solar Energy Assists Desiccant Air Conditioning," R. Franklin, , (4:95), November 1995, pp. 26-28.
"Solar Ice," S. Vanek, , (53), June/July 1996, pp. 20-23.
"Solar Icemakers in Maruata, Mexico," D. Erickson, , (8:4) July/August 1994, pp. 21-23.
"A Solar Powered Vaccine Storage Refrigerator That Can Be Powered By a Single-Truck Battery," L. Schlussler, ASES Solar 99 Conference, Portland, Maine, June 12-16, 1999; pp. 33-37. Available from ASES (see Source List below).
State of the Art of Active Solar Cooling, J. Mitchell, University of Wisconsin-Madison, 1986. Available from the , University of Wisconsin at Madison, 1500 Johnson Drive, Madison, WI 53706. 13 pp.
Books and Reports
Advances in Open Cycle Solid Desiccant Cooling, T. Penney and I. McClain-Cross, Solar Energy Research Institute, 1985. Available from NTIS (see Source List below). 16 pp. NTIS Order No. DE85008797.
Analysis of Advanced Solar Hybrid Desiccant Cooling Systems for Buildings, D. Schlepp and K. Schultz, Solar Energy Research Institute, 1984. Available from NTIS (see Source List below). 56 pp. NTIS Order No. DE85000540.
Development of a Single-Family Absorption Chiller for Use in a Solar Heating and Cooling System, R. Reimann and W. Biermann, Carrier Corporation, 1984. Available from NTIS (see Source List below). 201 pp. NTIS Order No. DE85003217.
Engineering Principles and Concepts for Active Solar Systems, B. Hunn, et al. (eds.), Solar Energy Research Institute, 1987. Available from NTIS (see Source List below). 312 pp. NTIS Order No. DE87001181.
Evaporatively Cooled Chiller for Solar Air Conditioning Systems Design and Field Test, R. Merrick and J. Murray, Arkla Industries, 1984. Available from NTIS (see Source List below). 57 pp. NTIS Order No. DE84014154.
High Performance Dehumidifying Solar Desiccant Cooling Systems, D. Schlepp and K. Schultz, Solar Energy Research Institute, 1983. Available from NTIS (see Source List below). 10 pp. NTIS Order No. DE83011964.
Open Cycle Lithium Chloride System, T. Lenz, et al., Colorado State University, 1983. Available from NTIS (see Source List below). 57 pp. NTIS Order No. DE84005831.
Potential of Solar Cooling Systems for Peak Demand Reduction, A. Pesaran, National Renewable Energy Laboratory (NREL), 1994. Available from NTIS (see Source List below). 12 pp. NTIS Order No. DE95000255.
Principals of Absorption Systems Machines, Solar Air-Conditioning and Refrigeration, A. Sayigh and J. McVeigh (eds.), Pergamon Press, 1992. Out of print.