Solar thermal collector
A solar thermal collector is a crucial component of solar heating systems designed to capture and convert sunlight into usable energy, primarily for heating water. These systems can operate through passive or active heating; passive heating relies on natural sunlight to increase temperature, while active heating employs collectors to intentionally harness solar energy using fluids for heat storage. Historically, the concept of solar energy utilization dates back thousands of years, with significant advancements occurring over the past few centuries, including developments by Swiss scientist Horace de Saussure and American inventor Clarence Kemp.
There are two main types of solar thermal collectors: non-concentrating and concentrating. Non-concentrating collectors, such as flat-plate collectors, are commonly used for applications like heating buildings and pools, while concentrating collectors can achieve higher temperatures by focusing sunlight on a smaller absorber. The effectiveness of these systems has been recognized worldwide, leading to increased adoption, particularly in response to energy resource shortages and environmental concerns. According to the International Energy Agency, millions of households globally now utilize solar thermal technologies, with ambitious goals set for future installations to support sustainable energy practices.
Solar thermal collector
A solar thermal collector is the part of a solar heating system that captures the sun's radiation and converts it to energy. Passive heating occurs when the sun's rays naturally raise the temperature of an item or a space; for example, sunlight coming through the glass walls of a greenhouse heats the air inside. Active heating means that the sun's energy is intentionally captured using a solar thermal collector and a fluid to hold the energy as heat.
Solar Energy Background
Humankind has long harnessed the sun's radiation to generate light and heat. As early as the seventh century BCE, people realized that magnifying glasses could concentrate the sun's energy enough to burn, and mirrors and other highly reflective surfaces were used for centuries to light torches during religious ceremonies. The Romans used passive solar heating in their bathhouses and other structures, and many cultures have used passive heat to make their dwellings more comfortable.
In 1767, Swiss scientist Horace de Saussure created the first solar collector using an insulated box and glass panels. Saussure's "hot boxes" were used by African adventurers to cook food in the early 1800s. In 1891, American inventor Clarence Kemp invented the first commercial solar water heater. Kemp's device combined a metal tank with the principles of Saussure's hot boxes; prior to that, water heaters consisted of a metal tank painted black then filled with water that heated in the available sun.
American William J. Bailey invented the first two-part solar collector system in 1908. The Day and Night heater had a collector that was outside and gathered the sun's energy, and a separate tank inside the house to store the heated water. Bailey's collector, which consisted of an insulated box filled with copper coils, became the prototype for the design used in many modern solar collectors.
Components of Solar Thermal Heating
The solar thermal collector is the first step in gathering solar energy. As the sun's rays shine on the collector, fluid or air is circulated either through or past the collector by fans or pumps. The heated fluid or air is either sent directly to the room or rooms it is heating or to a storage system for later use. A system of pipes or ducts, along with circulating pumps or fans, moves the heated air or fluid to its destination. In a water heating system, a tank stores the water to be used as needed.
Each collector has two parts. The part directly exposed to the sun's radiation is the collector area. The collector transfers the sun's energy to the absorber. Absorbers are often black in color because black collects and holds heat more efficiently, and they are usually made of metal or plastic. In some cases, they may be treated with a special coating to increase absorbency.
The two basic types of solar collectors are nonconcentrating and concentrating. Non-concentrating collectors have collectors and absorbers that are roughly the same size. Flat-plate collectors are the most common type of nonconcentrating collectors and are used for generating heat for buildings and swimming pools. While the design may vary, flat-plate collectors share at least four characteristics: they have a flat area that gathers the sun's radiation, they include a transparent cover to reduce the loss of heat from the absorber, the collector encases tubes filled with fluid or air to carry the heat away from the absorber, and the absorber is insulated to further prevent heat loss. Nonconcentrating collectors work best to generate temperatures of less than 200 degrees Fahrenheit, or 93.33 degrees Celsius.
Concentrating collectors have a collector area that is many times larger than the absorber. This allows the collected solar radiation to be concentrated on the absorber, generating greater temperatures. By their nature, concentrating collectors intensify the amount of heat collected from the sun; they are used when higher temperatures are desired.
While flat-panel collectors are the most common type, solar collectors also come in other forms. These include:
Evacuated tube models that use an absorber at the end of a glass tube system that is evacuated, or emptied of air, and pressure proof.
Tube reflector types, in which a reflective surface is curved in a half-barrel shape around a tube that contains the absorber and transfer fluid.
Point reflector types, in which a reflective half-sphere or parabolic dish concentrates the sun's radiation in a small absorber.
Solar Collector Statistics
Shortages of resources in Japan after the Second World War caused increased interest in solar hot water heating in that country; about 4,000,000 units similar to Kemp's Climax water heater were in use there by the end of the 1960s. In the 1970s, the worldwide petroleum embargo's effect on the price of heating fuel and growing concern about the environment led to an increased interest in using active solar heating systems to provide energy for commercial structures, schools, and homes around the world.
According to the International Energy Agency (IEA), 250 million dwellings throughout the world used solar thermal technologies for water heating. The agency set a goal in its "Net Zero Emissions by 2050 Scenario" report of reaching 400 million dwellings by 2030, which meant 290 million new solar thermal systems needed to be installed before 2030. This number took into account solar thermal systems that would need to be replaced.
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