What is solar cooling all about?

Compared to other solar-energy applications, solar cooling is a relatively new, but growing technology. Many projects which incorporate it are for the purposes of demonstration only, but a growing number of systems are being implemented all over the world for conventional use. In order to give an insight into this innovative technology, detailed information about the possible technical applications of solar cooling systems is provided in this section.

Passive solar-cooling, based on bioclimatic strategies, such as sun protection using natural screening devices, or increased cooling by using ponds or water basins on the roof or close to the external walls, is widely applied and should be the first step to take in cooling a building. Such measures are easier and less costly to implement. They decrease the need for additional cooling and therefore, for additional energy demand and investment. Sufficient insulation of the building will decrease the need for cooling, as well as for heating.

If the outcome of these measures is not sufficient in itself, a solar-assisted cooling system may be an intelligent solution. With these systems, solar heat is used to drive the cooling process for air conditioning in buildings. Instead of using electricity, free solar thermal energy is used for cooling through a thermal-chemical sorption process

Benefits

The main benefit of solar cooling is that, in general, levels of solar radiation are highest when climatisation is most needed: the sunnier the day, the more energy is produced for cooling. As the application uses a renewable-energy source, it offers environmental benefits i.e. a reduction in conventional energy use, as well as lower levels of harmful emissions.

Additionally, although a chemical process is adopted, the refrigerants that are used (water, salts, silica gel, lithium bromide, and lithium chloride) are harmless, and the chemicals do not come into contact with the air. Furthermore, as opposed to those used in many electrically driven cooling systems, the materials used for solar cooling do not have a relevant global warming potential (GWP).

Another aspect, which is becoming increasingly relevant, is the lowering of demand on grid electricity in hot regions. The use of solar-thermal energy reduces the need for electrical energy, especially at midday during summer, which is a peak time for electricity use.

The benefit for the user is in the reduced need for electricity, with a respective reduction in energy bills. In many Western countries the “peak load” for electricity that can be partially substituted by solar driven systems is very expensive, while the solar energy itself is free. As energy costs are predicted to rise in the future, this cost aspect could become one of the most significant factors in the growth of solar cooling.

Technical issues

Various technical solutions are possible, depending on factors such as the type of building, its function, and the existing infrastructure. In principle, two different cooling technologies are available: closed cooling systems and open systems for dehumidification and/or cooling. In addition to using solar energy, both systems can also use waste heat from, for example, combined heat and power (CHP) plants to power or regenerate the system.

Closed cooling systems are based on the thermo-chemical process of sorption. A liquid or gaseous substance is either attached to a solid, porous material (adsorption) or is taken in by a liquid material (absorption). Globally, absorption chillers are the most widespread. A thermal compression of the refrigerant is achieved by using a liquid refrigerant or sorbent solution and a heat source. This process replaces the electricity consumption of a mechanical compressor.

Open or desiccant cooling systems, on the other hand, are able to reduce the humidity, which means that the air only seems to be cooler, yet comfort levels are significantly increased. Desiccant systems are often used in combination with evaporative cooling, leading to air dehumidification by a desiccant liquid or solid material. These systems are ‘open’ in the sense that the refrigerant is taken out of the system after having provided the cooling effect, and is replaced by a new refrigerant in an open-ended loop. As there is direct contact with the atmosphere, the refrigerant is always water.

The technical performance of thermally driven chillers is given in COP, the thermal Co-efficient Of Performance. It is defined as the fraction of heat discarded from the chilled water cycle (‘delivered cold’) and the required driving heat. Typical ranges of COP for closed cycles are 0.5 to 0.7 for adsorption chillers, and 0.6 to 0.75 for absorption chillers. The COP range for open cycles typically lies between 0.5 and >1. The generated waste heat either has to be diverted to a re-cooling tower, or is forwarded to a heat storage system; this could be, for example, a swimming pool where the waste heat is used for water heating.

To operate the solar assisted cooling systems, the solar thermal collector systems have to reach certain temperatures. For thermally driven chillers, the driving temperature is mainly between 60°C and 80°C, and for desiccant cooling systems, the driving temperature is from 55°C to 90°C.

Obstacles

Although a large potential market for solar cooling exists, the current high investment costs present a significant barrier to broad implementation. Compared to conventional cooling systems, the upfront costs are around 2 to 2.5 times higher.

A solar-assisted cooling system is quite complex and includes solar collectors, the cooling device, and the control technology. Therefore, ongoing technical maintenance is necessary and can present a challenge.

Most devices are still large scale, both in terms of their application and physical size. This makes adapting the technology problematic, especially for detached houses. However, smaller appliances are in development and some are already on the market.

Replicability

Despite the cost factors, it is accepted that there is great potential for solar cooling due to the basic benefits that it offers. Additionally, greater standardisation will, in time, result in cost reduction. Any solar assisted cooling system has to be adapted to the local climatic conditions: in some regions dehumidification is of great importance and a desiccant cooling system might be the best alternative. In other regions where the cooling need is moderate, small adsorption or absorption systems might be sufficient.

Part 3 of this series will appear in the next issue of the RACA Journal. ◊