Solar thermal systems

Summary: A system that converts energy from the sun into heat in usable form is called a solar thermal system. With improvements in technology and the increasing cost of conventional fuels, solar thermal systems are attracting attention.

Energy from the sun can be harnessed directly for the creation of either high-temperature steam (greater than 100 degrees Celsius) or low-temperature heat (less than 100 degrees Celsius) for various heat and power applications.

High Temperature Solar Thermal Systems

These systems use mirrors or other reflecting surfaces to concentrate solar radiation. There are three main technologies under high-temperature systems: the central receiver, the parabolic dish system, and the solar line-focusing collector. Solar heat at high temperatures, sufficient to generate electricity, is collected, and electricity is generated using a heat engine. Compared to solar photovoltaic (PV) systems, solar thermal power systems have the following benefits:

• Electricity production is not limited to only sunlight hours, unlike solar PV.

• Effective utilization of the solar spectrum.

• Conversion efficiency is high, and the dusty climatic conditions do not affect power generation.

• There is no need for the storage of large amounts of electricity for usage outside non-sunlight hours via batteries.

Electricity generation from high-temperature solar thermal systems is a reality, but currently there is only a small market for the same. Its future depends a lot on the availability and success of further research and development.

Low Temperature Solar Thermal Systems

These systems collect solar radiation to heat air or water for both domestic and industrial purposes. Low-temperature solar thermal systems are a proven technology and have a mature industry with a wide range of applications. These technologies can be used as passive or active systems.

Energy is collected in passive systems through orientation, materials, and the construction of the collector and does not require the use of pumps or motors. The properties of the collector allow it to absorb, store, and use solar radiation.

Active systems make use of pumps or motors to circulate water or some other heat-absorbing fluid through solar collectors. These collectors are made up of copper tubes bonded to a metal plate, painted black, and encapsulated in an insulated box covered by glass.

Among solar thermal technologies, solar water heating, solar process heat, and solar buildings have progressed far enough to contribute to the energy pool and toward combating climate change. Thus far, solar water heating is the most widely accepted technology worldwide.

Solar Cooking

Utilizing the sun’s heat to cook food is not a new technology; it has been used for centuries. A device that cooks food with heat energy from the sun is called a solar cooker and can be easily manufactured from materials that are readily available. There are certain limitations to the technology; cooking is limited to times when the sun is shining, must take place in an open space, and takes significantly longer than using other means. One of the major developments in solar thermal technology in the 21st century is the introduction of concentrating solar cookers that concentrate the heat on a single focal point. These cookers have been found to be very effective.

Solar Drying

Controlled solar drying is another potential solar-thermal application. Drying is a vital operation in the post-harvest sector, and losses (e.g., grains) during uncontrolled drying are significant. Moreover, open-sun drying can lead to product contamination. Controlled hot-air drying minimizes loss and contamination. With the increasing difficulty of obtaining firewood and the ever-increasing prices of other sources of energy, solar drying systems, in addition to being environmentally sound, can also be more cost-effective.

Solar Air Heating and Cooling

Solar-heated air can be used for space heating during the winter or in cold regions where heating is required for most of the year. Several air-heating collectors have been developed for space heating, which not only conserve fuel but also help reduce the indoor air pollution caused by burning fuels in a living space. Solar cooling can be achieved by passing air over solid desiccants like silica gel or zeolite, drawing moisture from the air, and creating an efficient evaporative cycle. These desiccants can then be regenerated by drying using solar thermal systems.

Solar Homes

In the past, people designed homes to take advantage of the heating power of sunlight, but the availability of relatively cheap electricity in recent centuries has caused most homes to be designed with artificial heating and cooling. Using the sun to heat or cool houses is called passive solar design. In cold climates, homes are designed so that sunlight entering the house is stored by a solid mass through the night. These designs also include well-insulated walls and roofs; heavy curtains can also be used to trap heat.

In passive solar cooling, buildings are designed to slow the rate of heat transfer by using shading insulation, thoughtful landscaping, and removing excess heat with fans, vents, and cross-ventilation.

Solar Water and Pool Heating

Solar water heaters, with their year-round utility, are economically attractive. These designs, using heat exchangers, are widely used by fabricators and can also process hard water. Recently, systems with evacuated tube collectors have found success in the marketplace, as they are less expensive and free from problems like scaling, corrosion, and leaking. However, improvements are still needed to make these tubes durable, to make the selective glass coating more effective, and to ensure that storage tanks are more efficient in extracting heat from the tubes. In solar pool heating, the systems are simple and relatively inexpensive. The pool systems commonly use simple and low-cost unglazed plastic collectors, and the pool itself is the thermal storage container for the system.

Solar

Solar desalination is a technique used to desalinate water using solar energy. The process is called the solar humidification-dehumidification (HDH) process. In this technique, the natural water cycle is mimicked for a shorter time frame by evaporating and condensing water to separate the present substances. The driving force in this process is solar thermal energy, which produces water vapor that is later condensed in a separate chamber to obtain desalinated water.

Bibliography

Duffie, J.A., and W. A. Beckman. Solar Engineering of Thermal Processes. 3rd ed. Hoboken, NJ: Wiley, 2006.

Fields, Spencer. “Solar Thermal Energy: What You Need to Know.” EnergySage, 6 Dec. 2023, www.energysage.com/about-clean-energy/solar/solar-thermal-what-you-need-to-know. Accessed 8 Aug. 2024.

Howell, John R. Solar-Thermal Energy Systems: Analysis and Design. New York: McGraw Hill, 1982.

Ramlow, Bob, and Benjamin Nusz.Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems. Gabriola Island, BC: New Society Publishers, 2010.

“Solar Pool (and Hot Tub) Heating.” Build it Solar, 2015, www.builditsolar.com/Projects/PoolHeating/pool‗heating.htm. Accessed 8 Aug. 2024.