Solar Thermal Technologies

Solar thermal technologies convert incoming solar radiation into thermal energy for heating or cooling applications. A solar water-heating system is an example of solar thermal technology. Solar energy collectors, in the form of flat plates or evacuated tube collectors, convert solar energy into heat. Heat from the collectors is used to heat the water. In low geographic latitudes, residential hot water temperatures can reach 140 degrees F (60 degrees C) with such solar heating systems. In a similar vein, solar concentrating technologies, using parabolic dish or parabolic trough reflectors, can greatly increase the temperature of a small solar collection area. Heated fluid at much higher temperatures can be used for industrial process heating.

Concentrated solar power (CSP) systems are large-scale, solar thermal energy technologies. CSP systems use large arrays of lenses or mirrors and advanced solar tracking systems to focus a large area of sunlight onto a small area. This concentrated heat is used as the heat source for a conventional power plant to generate electricity.

Solar energy can also be used in the water treatment process. Solar distillation technologies use the thermal energy from sunlight to evaporate saline water. By condensing and collecting the evaporated water, brackish water can be made potable. Similarly, solar water disinfection (SODIS) is a solar energy technology for water pasteurization. By placing polyethylene terephthalate (PET) bottles filled with water in sunlight, the water temperature increases to the pasteurization temperature, thereby killing organic pathogens and resulting in safe drinking water.

Passive solar thermal energy technologies can also be used to offset a portion of the energy required for building heating, ventilation, air conditioning, and lighting systems. Proper building design can allow for solar heating of a thermal mass to store heat. This stored thermal energy is released slowly to regulate, maintain, and offset the building’s heating requirements. Similarly, proper shading techniques, such as the use of deciduous trees and building overhangs, can reduce a building’s heat gain and cooling load.

Passive solar ventilation systems or solar chimneys can be utilized to offset ventilation costs. As the chimney warms when exposed to solar radiation, the temperature of the air inside the chimney increases, creating an updraft that moves cooler air through the building.

Solar Energy Use

Currently, over 40 gigawatts (GW) of power are generated from solar PVs worldwide. Overall, the use of renewable solar energy technologies represents a small fraction of the total energy production worldwide. A primary obstacle to increased solar energy technology deployment is the high initial investment cost. Because of the high availability and relatively low cost of fossil fuel energy sources, the economic rewards for typical solar thermal energy technologies, such as domestic hot water heating, range from approximately 3 to 20 years depending upon the size of the installation and solar energy availability. 

Small scale, solar PV installations currently require from approximately 5 to more than 30 years for the economic gain to be realized. Such large initial capital expenditures and long repayment periods significantly reduce the incentives to implement this technology. As the cost of fossil fuels increases, government incentives for renewable energy technologies increase, and as the conversion efficiencies for solar energy technologies improve, such economic barriers to installation may decrease.