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Solar Airconditioning: Overview of the Technology

Heat and solar energy go hand in hand. We've all seen installations that provide solar hot water and solar heating. But if we could come up with a sort of “combined-reversible solar-thermal” system that could provide airconditioning in the summer, heating in the winter, and hot water year-round, the circle would be complete. It's possible, and new solar-concentrating collectors and airconditioning technology are making this type of system technically and economically feasible.

Solar Airconditioning: Overview of the Technology

Banyuls sur Mer winery in France uses solar airconditioning to keep their vast (almost 50,000 sq. ft) wine storage facility cool.

By Jean P. Murray
Solar Oregon Board Member

Heat and solar energy go hand in hand. We've all seen installations that provide solar hot water and solar heating. But  if we could come up with a sort of “combined-reversible solar-thermal” system that could provide airconditioning  in the summer, heating in the winter, and hot water year-round, the circle would be complete. It's possible, and new solar-concentrating collectors and airconditioning technology are making this type of system technically and economically feasible. 

Refrigeration Cycles


The classic: vapor-compression

The air conditioner in your window is like your refrigerator: it uses mechanical energy provided by electricity to run a compressor.  The refrigerant circulates, is made to change phase to a vapor, which takes heat out of the space that needs cooling, then is pressurized so it returns to being liquid and it dumps the heat it gained to another space (outdoors, or to the coils behind the fridge). 

Cycles that use heat to cool

Absorption
There are several types of systems that use heat instead of electrical power to provide cooling.  The best known is the absorption chiller, which uses heat from the solar collector to dissociate, by boiling, a solution of lithium-bromide and water. When the solution cools, the recombination of these two constituents absorbs heat to produce a stream of chilled water that can be used by a classic airconditioning system.  The temperature required to power an absorption chiller ranges from about 80°C (176°F) for a single-effect chiller to 140-160°C (220-256°F) for a more-efficient double-effect chiller.  A huge advantage of an absorption cycle is that it can be reversed (like a heat pump) to provide 120-150% efficient heating in the winter, and delivering hot water at about 60°C.

Adsorption
An adsorption chiller uses a heat-cycle that occurs when a liquid is adsorbed and desorbed onto a porous solid.  Water/silica gel systems are in production, but other liquid/solid pairs have been studied.    The temperature range required to power an adsorption chiller is lower; from 60 to 90°C.

Desiccant cooling systems
Cooling of a dry air stream can be done by humidifying the air-an old method used in arid climates and known as a swamp-cooler. A desiccant cooling system uses a clever series of dehumidification, heat exchange and rehumidification to provide a cool stream of air for space cooling. The system requires the lowest temperature solar heat of all the cycles to operate: 50-90°C.

Reversible endothermic/exothermic chemical reactions
Systems that use reversible chemical reactions are still in development, most notably at the CNRS lab where I worked in France. This type of system shows promise to be compact and efficient.

Advantages of Solar Cooling

  • The peak demand for cooling occurs at midday, when the cost of electricity in many markets is priced at its highest. Peaking electricity is generally produced in gas-turbine plants, so its carbon footprint is very high. By contrast, solar energy at this time of day is at its most plentiful, so the demand for cooling matches the source for cooling energy well.
  • Air conditioning uses about 16% of all energy in the US; it is about half of the demand for electricity in sunny places like California.
  • The absorption system uses electricity for pumping and controls, so the electricity consumption is roughly 20X less than for vapor-compression systems.
  • Water/lithium-bromide, the working refrigerant in absorption chillers has zero ozone-depleting potential and zero global warming potential (GWP),  and is considered by the EPA to be environmentally benign. Refrigerants used in vapor-compression systems contribute to global warming:--R-134a  has a GWP  more than 1,000-times that of carbon dioxide. R-410A,  used in newer vapor-compression chillers, has a GWP that is 1,725 that of carbon dioxide.
  • There are virtually no moving parts in the system, thus absorption chillers should have a longer life.

The Solar Collectors

Standard flat-plate collectors have a steep drop in efficiency as their output temperature increases.  They can be used to supply heat efficiently to the desiccant cooling cycle and potentially the lower-T adsorption cycles. Until recently, the only feasible type of collector for the absorption systems has been the evacuated tube and parabolic trough or linear fresnel-type concentrating systems. For cooling very large buildings, site-built linear fresnel and parabolic-trough systems could be used to provide shade as well, but this type of collector is too large for single homes. 

Several companies are developing hybrid systems to provide heat, electricity and light. Two that produce rooftop modules are BrightPhase Energy and Chromasun.  The Chromasun module is designed specifically with the airconditioning application as its market. Their “micro-concentrator” (MCT) unit packages a miniaturized linear fresnel concentrator under a sealed canopy in a collector that can be mounted on a rooftop using the same racking systems as flat-panel solar thermal collectors. It concentrates the sun by a factor of 25 to provide 220°C process heat at 65-50% efficiency(Low T-High-T respectively).  This high temperature is well into an efficient operating input temperature for a double-effect absorption chiller.  By their calculations, with an electricity cost of 18¢/kWhr a solar airconditioning system would have a pay back of 2-4 years.

Conclusion


New developments in high-efficiency and high-temperature solar thermal collectors have resulted in output temperatures in the range that can be used efficiently by high-COP (about 1.4) double-effect absorption chillers. At the same time, absorption chiller maunfacturers have developed smaller units suitable for home applications, and have also introduced reversible units capable of providing hot water year round, and  130-150 percent efficient heating in winter.  Look for a bright future for these systems in the sunniest areas in the US.

 
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