When most people think of solar power, they think of photovoltaic panels, not solar thermal energy. Photovoltaic solar power, however, relies on only the ultraviolet part of the light spectrum, missing out on the massive amounts of energy to be gained by harvesting the sun’s heat. A distinct source of power, the applications of solar thermal power include the generation of electricity, the heating of water, the cooking of food, and even the growing of plants. Humans have been relying on the sun’s heat for thousands of years, of course, but modern technology has increased the energy we can generate from the sun’s infrared radiation many times over. Read on for everything you need to know about thermal energy, from the many applications, to the science behind the process, to the types of systems available, to the ultimate advantages and disadvantages of the technology.
The History of Solar Thermal Energy
Technically speaking, humans have been harvesting the sun’s heat for as long as our species has existed. Modern technology, however, has greatly increased the efficiency of collection, along with the number of ways we can use solar thermal power.
One of the most important innovations in the modern history of solar thermal energy was the invention of the solar thermal collector. The first solar collector was invented by Augustin Mouchot, a French inventor who initially presented his innovation at the 1878 Universal Exhibition in Paris. The collector was part of a thermal cooling engine Mouchot used to make ice cream. Later, the same man used his solar collector technology to create the world’s first solar thermal engine, a device which converted the sun’s thermal energy into mechanical energy via steam generation, much as solar thermal power plants still function today.
The first true installation of solar thermal equipment meant to generate energy occurred almost thirty years later, around 1910. Frank Shuman, an American engineer working in the Sahara desert, created a solar thermal energy system that collected sunlight in order to produce a solar powered steam engine. Although the project was abandoned, it had far reaching consequences for the technologies of the modern day.
Interest in solar thermal persisted at a moderate level throughout most of the twentieth century. In 1939, for example, engineers at MIT built a ‘solar house’ completely powered by solar thermal energy. Interest in solar thermal technology picked up as methods were refined in the mid-1990s. Since that time, in fact, over 125 large solar thermal heating plants have been constructed across Europe, each with over 500 square meters (about 5400 square feet) of solar collectors.
Solar thermal energy has had a large resurgence in recent years, as it proves a very promising source of renewable energy, with a wide variety of uses and applications. Until recently, in fact, solar thermal was used over photovoltaic solar for many purposes due to the increased efficiency of solar thermal collection.
Uses of Solar Thermal Energy
Solar thermal energy can be used in a variety of ways. In addition to providing heat, solar thermal energy can be used to heat water, control climates, cook food, and even provide large scale power to municipal power grids.
What is Solar Thermal Energy?
Solar thermal energy is solar energy captured in the form of heat, and then used for other applications such as heating water (like with these solar sun rings and solar sun squares) or producing electricity. Solar thermal energy is distinct from traditional solar energy which relies on photovoltaic panels to capture the sun’s ultraviolet radiation and convert that light energy into a difference in electric potential and, ultimately, an electric current.
Solar thermal energy used for heating relies on the collection and distribution of the sun’s heat, usually via circulated fluids. Solar thermal energy used to generate electricity, on the other hand, relies on concentrated solar power (also known as CSP). These systems use groups of mirrors to concentrate solar energy on a central collector. This, in turn, produces temperatures high enough to generate steam. Similar to other means of generating electricity, this steam then drives a turbine, which generates electricity. Concentrated solar power systems work best in desert locations where space and sunlight are abundant.
Solar thermal energy has several advantages over traditional power sources. First and foremost, solar thermal energy is clean and renewable, using a free form of fuel available the world over. Most solar thermal energy systems are also relatively low maintenance, as they use simple technologies and, sometimes, even manage to function with no moving parts.
Types of Solar Thermal Energy Systems
As mentioned above, solar thermal energy can be divided into two main categories based on how the thermal energy is collected and used. The first type, solar thermal energy used to generate electricity, typically involves large power plants that rely on concentrated solar power.
The second type, solar thermal energy used to heat and transfer materials, can be further subdivided into two groups: active and passive. Both types collect free solar heat energy using solar thermal collectors, but the two types differ depending on how the heat is then transferred from the collector.
Passive solar thermal energy systems rely on either gravity and existing fluid pressure or natural circulation of hot and cold fluid to provide pressure and heat transfer. Thus, they rely on free solar heat to transfer water rather than external energy sources. In addition to their self-sufficiency, passive solar thermal systems are usually cheaper than active systems. On the flip side, however, passive systems are also less efficient. Some examples of passive solar thermal energy systems include passive solar water heaters, solar ovens, and even time-tested technologies such as greenhouses and laundry lines.
All passive solar thermal energy systems rely on design components to circulate heat naturally. However, they can also be further subdivided into low, medium, and high temperature systems. Low temperature applications (<100 degrees celsius) include hot water or space heating. Medium-temperature (100-250 degrees celsius) applications are not as common. One example is a solar oven, which uses a specially shaped mirror to focus the sun’s rays on a central cooking pot. The third and final subdivision, high temperature (250 degrees celsius>) solar systems, typically use large groups of mirrors to concentrate sunlight onto a central collector. An example of these systems include concentrated solar power plants.
Unlike passive solar thermal energy systems, active systems use electric pumps to circulate fluid or air after it is heated in solar thermal collectors. For example, in direct active solar water heating systems, city water flows into an insulated storage tank. A pump then draws water from the storage tank, pumps it to the solar collector, and then returns the warmed water to the storage tank. Some systems also pass the water through a booster or backup heater before storing it. The whole system is controlled by automated sensors attached to the pump; whenever the water in the collector is warmer than the water in the water in the tank, the pump turns on.
Overall, the difference between passive active solar thermal systems boils down to how the energy is transferred. If the heat energy absorbed through the sun is moved on the power of the heated material’s thermodynamic properties, the system is likely passive. If, however, outside energy is required to make the system function, the system is likely active. Understanding this difference is crucial to understanding the different possibilities and applications of solar thermal energy.
Types of Solar Collectors
No solar thermal energy could exist without a solar thermal collector to harvest the sun’s heats. Solar thermal collectors, however, can come in a wide variety of shapes and sizes. The most common type of solar collector is a flat-plate collector. These devices consist of a flat plate made of a dark, heat absorbing material. Within the collector, air or fluid circulates to collect the heat absorbed by the dark material. Outside the plate, there is usually a transparent, protective layer which shields the sunlight-absorbing material while still allowing sunlight to pass.
Another type of solar thermal collector is the evacuated-tube collector. These collectors are designed to surround the heat-absorbing material with a glass tube. The space between the glass tubing and the absorbing material is a high vacuum, which allows the collector to resist atmospheric pressure. This insulates the absorber, greatly reducing the heat lost through convection and conduction, thus increasing the thermal efficiency of the collector. Fluid or air then flows through the tubes, absorbing the heat and carrying it elsewhere in the system.
There is debate about flat-plate versus evacuated tube collectors.While evacuated tube collectors absorb and retain heat more efficiently, they also take up more space with less absorbent surface area than flat-plate collectors, leading some to question their ultimate advantage. Evacuated flat plate collectors combine the features and benefits of flat plate and evacuated tube collectors.
Many passive solar heaters rely on air rather than fluid to transfer captured heat use a simpler sort of collector that consists simply of an unglazed absorber plate that captures the sun’s heat and transfers it to the surrounding air, providing heat for applications within the home.
Solar thermal power plants rely on yet another type of solar thermal collector sometimes known as a solar concentrator. These specially shaped mirrors can come in a variety of forms. The three most common types are rows of curved mirrors known as parabolic troughs, large, curved mirrors known as parabolic dishes, and large numbers of mirrors known as heliostats surrounding a central collector tower. All three of these mirror types bend the sun’s rays, concentrating them on some sort of absorber, be it a central tower, an absorber tube, or in the case of parabolic dishes, a stirling engine which generates electricity through the rapid expansion and compression of air.
Advantages of Solar Thermal Energy
The main advantage of solar thermal energy is that it relies on a power source that is abundant, clean, renewable, and free. Once proper infrastructure is installed, it can provide large amounts of low-cost energy for a wide variety of applications, like solar kettles.
Disadvantages of Solar Thermal Energy
Unfortunately, solar thermal energy does have a few notable disadvantages. The first is that sunlight is an intermittent power source, and can only be collected during daylight hours under favorable weather conditions.
Additionally, although the fuel is cheap once the facility is built, solar thermal power plants require a relatively large up-front capital investment.
Furthermore, large-scale thermal power plants require large, open, sunny spaces like desert. At the same time, steam generation requires large amounts of water–difficult to come by and maintain in remote desert areas. Finally, because these power plants must typically be placed in out-of-the-way locals, efficiently transporting the generated electricity across long distances can prove difficult.
Overall, however, solar thermal energy is an excellent, clean alternative to traditional methods of electricity generation, so long as favorable conditions and appropriate investment are first obtained.
In conclusion, solar thermal energy is making waves in a variety of ways, changing the face of renewable energy. A lesser-known alternative to photovoltaic solar energy capture and generation, solar thermal energy has the advantage of harnessing a portion of the sun’s energy not accessed by photovoltaic technology. Solar thermal power plants and other technologies are already providing renewable energy solutions across the world; likely, these exciting innovations will only multiply in the years to come.