The solar air conditioning immediately seems a very attractive idea when you think of using solar heat to counteract that heat. How does that become possible? The concept however is not new, as it dates back to 1960s and 1970s, when several cooling and refrigeration cycles were investigated and proposed. In general, the use of solar cooling was intended for two main purposes:
1. Refrigeration for food storage and
2. Space cooling (air conditioning)
Furthermore it was realized that such systems as flat-plate solar collectors can be use for heating in the winter and cooling in the summer, thus significantly increasing system usability and efficiency.
The main idea behind this concept is that the physicochemical cycle that is able to transfer thermal energy from one location to another is driven by an external heat source (solar or non-solar). Another important idea here is that removal of heat from any part of the system essentially equals cooling. There are a number of different cooling cycles based on somewhat different principles. The following three are mentioned by Duffie and Beckman (2013) in connection with solar thermal systems:
- Absorption cycles
- Desiccant cycles
- Solar mechanical cycles
All of these cycles may have some technical variations depending on specific conditions and load.
Absorption cooling technology is considered in more detail here. The cooling effect is based on the evaporative cooling of a refrigerant. Because the vaporization process requires energy input, when it happens it takes heat from the system, thus making the remaining fluid colder.
Desiccant cooling is based on cycling dehumidification-humidification processes. It uses some hygroscopic substances or materials for dehumidification, and those materials are regenerated in the cycle by applying solar heat.
Solar mechanical cycles attempt to combine the solar-powered mechanical work cycles (such as Rankine) with conventional air conditioning systems. The solar-driven part of the system is not actually the chiller, but the engine that produces energy for operating the air conditioner.
At over 50 years of history of commercial systems, why haven't solar absorption chillers simply taken off and taken over the air conditioning market? What about solar desiccant cooling systems? If the theory and proof of concept have been shown to work, the only thing that holds back a technology from taking off is the cost. In this case, for most locations, electricity generated from fossil fuels is currently less costly than the cost of a solar thermal collector, storage tank, and other system components required for cooling with solar thermal energy. Even when combined with solar heating for both domestic hot water as well as space heating (as shown in Figure 15.3.1 of the D&B text), where cost savings through shared system components are expected, the payback period is often too long to justify with current low fossil fuel energy costs as the alternative. In certain applications, however, such systems are indeed the lowest cost alternative, and, as time goes on, these dual-purpose (heating and cooling) systems will become more competitive.
Since cooling is quite expensive, reducing cooling loads via methods of passive building design and insulation are important for overall efficiency. For example, summer solar gains in the building can be minimized by using windows with low transmittance in the infrared part of spectrum. Also various shading designs (internal and external) can be planned to manipulate direct solar gains. Creating passive ventilation loops that take advantage of natural air flows and prevailing winds at a locale can also prevent excessive indoor heating. Finally some building designs may use the ground as heat sink ("earth tempering"). All these methods are discussed in more detail in Passive Cooling Handbook (US DOE, 1980) and should certainly be considered in building designs in warm climates as a way of reducing cooling loads.