The most general definition of solar air conditioning refers to any air conditioning (cooling) system that uses/ manipulates solar energy to provide active or passive cooling of a structure. This can be done through many methods including solar thermal energy and photovoltaic conversion (sunlight to electricity) or simply arranging fans for ventilation.
In 2007, the U.S. Energy Independence and Security Act provided funding for innovative solar air conditioning research and development programs and has resulted in a wide number and variety of new technologies that actually use active solar thermal power to be developed. In some cases these newer approaches have been “installed” in real applications for feasibility-demonstration and in cases for actual commercial use.
Solar Powered Air Conditioners
PV electrically supplied compressor air conditioning/heating systems are often improperly included in a discussion concerning solar air conditioners. However, a more correct definition of a solar a/c unit is an apparatus dependent entirely on the sun’s thermal energy to produce a cooling system without significant need of electricity.
This concept does not include evaporative cooling and the use of solar chimneys and other approaches. Nor does it include so-called passive solar methods involving building design to lower cooling costs such as ventilation and insulation improvements.
Absorption/Adsorption Cooling Methods
Most solar air conditioner systems are based on a closed loop absorption or adsorption cooling method and employ thermal solar collectors to provide energy to drive adsorption or absorption chillers. Closed loop absorption/adsorption chillers have been in use for many years.
Systems involved in most common technologies for solar thermal closed-loop absorption cooling (phase change refrigerants) include ammonia/water, water/lithium bromide, and water/lithium chloride. Those for adsorption/evaporation refrigerants include water on silica gel or water on zeolites and methanol on activated carbon.
Historically, these heat driven technologies were known and in commercial practice as early as the late 1900s. At that time, an ammonia/water phase-change refrigerant solution was in use and remains in use, but many modern applications now use a water/lithium bromide mixture.
Very simply described, the system utilizes an expansion/condensation tube where one end of the tube (pipe) is heated and the other end then cools to as low as ice making temperatures. Absorption chillers remain the more common method.
Absorption Chillers Versus Compressor Air Conditioning
The most notable difference between an absorption chiller and compressor chiller system is the way the refrigerant is recycled from a gas back into a liquid. An absorption refrigerator effects this change by a method that needs only heat whereas a compressor chiller system requires mechanical work for the compression cycle and ultimately electrically driven pumps.
Another significant difference between the two designs is that compressor refrigerators typically use an HCFC or HFC as a refrigerant as opposed to the water based refrigerants used by absorption refrigerators.
Powering an Absorption Chiller
Despite the popularity of compressor driven refrigerators, absorption refrigerators remain a popular alternative to regular compressor refrigerators because they produce less noise and vibration and usually require lower costs and no need for an electrical supply.
Originally, natural gas was burned as a heat source and today many RVs use propane to power absorption chiller refrigerators. Industrially, many absorption chiller systems are driven with “waste” heat from various industrial processes. However, the heat required for absorption refrigeration has always come from sources that ultimately involve the consumption of fossil fuels as an immediate combustion source or in electrical generation.
Power from Solar Thermal Energy
A solar cooling system employs solar thermal collectors to provide the heat to drive the an absorption chiller. An active solar cooling drives an absorption chiller (fundamentally an air-source heat pump) with solar-heated water or other thermal transfer fluid that is used to collect solar heat and not by electricity or other fuel sources.
Unfortunately, much like early solar PV panels, thermal collection can be inefficient and iterative absorption cooling cycles are often used (up to three) to improve efficiency of the process. It has been determined that water temperatures of at least 190 ºF (88 ºC) are required for efficient absorption chiller operation. Unfortunately, inexpensive available flat-plate solar thermal collectors provide only about 160 ºF (71 ºC) water temperatures thus requiring iterative processes.
Solar Thermal Collectors
These realities have led to efforts to maximize the efficiency of solar collection materials and physical design to obtain higher sustained water temperatures. Some recent applications have involved projects such as the Lisbon headquarters of Caixa Geral de Depositos producing 545 kW of cooling power with solar collectors covering 1579 sq m (17,000 sq ft) and a similar installation on the Olympic Sailing Village in Qingdao/China.
These and other projects have shown that flat plate solar collectors can be made to ultimately produce temperatures over 200 ºF (93 ºC) and be cost efficient. In addition, Sopogy, Inc introduced the SopoFlare technology designed specifically for rooftop installations with trough like collectors intended for steam creation, solar thermal cooling and air conditioning, dehumidification and even desalinization.
A similar parabolic trough solar collector has been developed by the The Energy Concepts Company for the Intermittent Solar Ammonia-water Absorption Cycle (ISAAC) Solar Icemaker with no fuel or electric input and a unit in operation that can produce 150 lbs of ice per day.
Solar Air Conditioner Components
In general, commercially available solar air conditioning installations will consist of the following components. In most cases a majority of the components will be self contained and not obvious to the eye except for inlet outlet connections and probably an electronic controller/monitoring unit with digital readouts and some system control units to occasionally modify settings.
The components present in a system will vary as to whether the unit is intended to operate as a reversible system. Meaning it is switch able between being a solar cooling unit to a solar heating unit for winter months. Other options are whether it is also a hot water supply year round. Clearly these decisions will depend on individual needs.
Vacuum tube collector
Here is probably the greatest variety of designs available. Collectors can range from absorbent tubes with parabolic trough reflectors or vacuum tubes and pipes through which heat transfer fluid is circulated. Two common heat transfer fluids are mineral oil and ethylene glycol.
Solar heat exchanger
If present, the heat exchanger transfers the heat from the collector fluid to water. Alternatively, the collector fluid directly heats the chiller externally to drive the system.
Solar chiller and Closed Cooling Tower
This is the heart of the system where the heated refrigerant, under essentially vacuum conditions produced by the heated adsorbing solution, evaporates and cools. The process relies on the absorbent to create a near vacuum inside the system. In this vacuum, the refrigerant evaporates at a very low temperature that removes heat from the refrigerant producing the cooling effect. This process takes place at a temperature lower than the absorbent fluids evaporation temperature at normal pressure.
Other equipment can include a heat recovery system and hot water storage tank for a hot water supply (optional), a chilled water storage tank, a heat exchanger for winter (optional), a control system, pumps, valves, and other devices. These small pumps and the control system will require minimal electrical power and at least 98% of the energy is supplied by the solar thermal energy.
Residential and Small Business Systems
At present there are a few companies that provide smaller solar air conditioner systems that are intended for small business application and could be applied to certain residential applications. However, for the foreseeable future home systems for the average home size and needs will require substantial cost improvements in collection and equipment, as was true in the early development of Solar PV systems. Hopefully, as it has with PV systems, costs will drop and demand will increase to help in this economic and scientific process.
H.R. 6–110th Congress: Energy Independence and Security Act of 2007. (2007). In GovTrack.us (database of federal legislation). Retrieved September 6, 2011, from http://www.govtrack.us/congress/bill.xpd?bill=h110-6
George O. G. Löf (1993). Active solar systems. MIT Press. p. 682. ISBN 9780262121675.
Nathan Olivarez-Giles (2009-08-20). “Using solar heat to power air conditioning”. Los Angeles Times.