Solar cooling is a system that converts heat from the sun into cooling that can be used for refrigeration and air conditioning. A solar cooling system collects solar power and uses it in a thermally driven cooling process which is in turn used to decrease and control the temperature for purposes like generating chilled water or conditioning air for a building.
There are many different cooling cycle techniques using various different principals to function. Three of the most popular techniques include:
- absorption cycles
- desiccant cycles
- solar mechanical cycles
How solar cooling works
Regardless of the technique being used, a solar cooling system typically includes three core components:
- A solar collector, such as a solar panel, which is used to convert solar radiation into heat or mechanical work.
- A refrigeration or air conditioning plant that is used to produce the cooling.
- A heat sink that collects any rejected heat and radiates it away from the system.
While techniques used to achieve solar cooling vary, the end goal remains the same: utilize an external heat source, like a solar panel, to collect ambient temperature and then use that heat with a refrigerant to create pressure within a closed loop of refrigerant, thus enabling the solar cooling system to work.
A refrigerant is a substance or mixture that absorbs heat from the environment and can create refrigeration or air conditioning if it is combined with the other necessary components, like compressors and evaporators. In most cooling cycles, the refrigerant will transition from the liquid phase to the gas phase and then back again to achieve its cooling purpose.
In absorption cycles, the cooling process relies on the evaporative cooling of a refrigerant. Since vaporization requires energy input, the process takes heat from the system, leaving the remaining fluid cooler than before. Absorption cycles complete pressurization by dissolving a refrigerant in an absorbent, or something that soaks up liquid easily, instead of using a mechanical compressor.
Absorption cooling cycles possess four specific, major components: an absorber, a generator, a condenser and an evaporator. The evaporator is, essentially, the refrigeration or air conditioning plant used in all cooling systems since it is where the cooling occurs.
In an absorption cycle, the cooling process progresses as follows:
- The absorber holds an absorbent-refrigerant mixture that is delivered to the generator through a liquid pump.
- The generator takes the absorbent-refrigerant mixture and heats it up using the external solar energy that has been collected through a source such as a solar panel. The solution starts to boil in reaction to the heat, turning water into vapor which flows to the condenser.
- The condenser liquefies the water vapor, rejecting heat in the process which is collected by the heat sink. The new liquid condensate is then directed towards the evaporator through an expansion valve.
- Finally, evaporation of the refrigerant at low pressure causes the evaporator to absorb the heat from the cooled space, creating the cooling effect.
At the end, the vaporized refrigerant returns to the absorber and the cycle repeats. Solar power is responsible for driving this cycle.
Desiccant cooling systems rely on cycling dehumidification-humidification processes. It uses substances and materials that readily attract water from their surroundings for dehumidification. These materials are known as desiccants. The desiccants are regenerated in the cycle by applying solar power.
Desiccant cooling systems can operate with both liquid and solid desiccants. The desiccant cooling process progresses as follows:
- Desiccants absorb the water vapor and remove the moisture from the process air in the dehumidification, or absorber, unit. A transfer results from the difference in vapor pressure, thus releasing heat due to the condensation of water and creating a heat exchange.
- The air is then introduced into the space or into an evaporative cooler for further cooling while the diluted desiccant is sent to the regenerator. However, before the diluted desiccant can enter the regenerator, it must pass through a liquid-liquid heat exchanger and a heating coil in order to raise its temperature.
- Once in the regenerator, the heated, diluted desiccant is exposed to regenerative air, causing moisture to transfer from the diluted solution to the air. This transfer is due to the created difference in vapor pressure.
- Next, the resulting, more concentrated desiccant passes through the liquid-liquid heat exchanger once again as well as a cooling coil and then moves back into the dehumidification unit, allowing the cycle to repeat.
The third technique, solar mechanical cycles, works very differently from the absorption and desiccant cycles. Instead of creating an entirely new system, solar mechanical cycles attempt to combine solar powered mechanics with conventional cooling systems. In this cycle, solar power is used to fuel the actual engine that produces the energy used to operate the entire cooling system instead fueling the absorption chiller, like it does in both the absorption and desiccant cycles.
Applications of solar cooling
Solar cooling is primarily intended for two main purposes: refrigerating food storage and space cooling, or air conditioning. Solar cooling can be seen in vehicles like RVs and campers which use the system for refrigeration. Vapor absorption refrigeration systems, which are used in industries where extremely low process temperatures are required as well as large thermal capabilities, also display the use of solar cooling.
Perhaps the most beneficial application of solar cooling is its ability to provide cooling systems to countries that otherwise would not be able to handle the total electric and energy cost and burden required by conventional cooling systems. Solar cooling greatly reduces the amount of energy required to refrigerate necessities such as vaccines and agricultural products, which, in turn, creates cost savings and benefits the environment by using renewable energy and reducing the use of ozone depleting materials.
Challenges of solar cooling
While solar cooling has been applied in various industrial settings, domestic cooling systems often are not economical. Domestic systems' expected high cost and low efficiency have been a major obstacle in their wider domestic application.
Furthermore, while the long term operating costs of the system are less than those accumulated by conventional cooling systems, the initial investment cost is much higher due to the smaller supply and expensive prices of system components like the solar collector and storage tanks.