Solar Thermal Energy
When solar energy is mentioned, most people think of photovoltaic energy systems that are mounted on the rooftops of homes sprinkled throughout their community. What isn’t in the forefront of most people’s thinking is solar thermal energy. Solar thermal energy (STE) is the technology of harnessing solar energy for the purpose of creating thermal energy (heat). It’s been around for hundreds, and even thousands of years. The government continues to give out tax incentives to encourage individuals to install a home solar system.
One of the advantages photovoltaic has over solar thermal technology is that it produces electricity that can be used for a variety of applications. Solar thermal energy is primarily used to heat water. Another disadvantage of solar thermal energy systems is there aren’t as many uses for hot water in the summer time as there are in the winter. The days are also shorter during the months hot water is needed the most. Shorter days also equate to less solar energy being absorbed by the solar panels.
Solar energy has been used for decades for hot water solar systems. If the system is for heating a swimming pool, in many areas of the world, you can get an additional two to four months of swimming in as a result. Solar hot water systems are cost effective, environmentally friendly, and for the most part maintenance free. They can now be installed on your roof to blend in with your home’s architecture. Of the 1.5 million homes and businesses throughout the country that have solar hot water systems, surveys have indicated that more than 94 percent of those owners consider their investment to be a good one. SolarHot is a manufacturer of these systems that boasts of having the highest rating of any freeze-protected solar hot water systems that use less than 95 square feet of collector area. Their OG-300 system uses only 64 square feet of collector area. The Solar Energy Factor (SEF) rating they received was 86%, far above many of their competitors.
Most solar hot water systems with flat plate solar collectors are installed facing south to south-east on a rooftop. Did you ever wonder why a shallow lake is normally warmer than a lake with deep water? That is because the sunlight heats the lake bottom in the shallow areas. This in turn, heats the rest of the water. This is nature’s way of using solar heat. The sun can also be used in this same way to heat water that is used in buildings or swimming pools.
When hot water systems are used for buildings, there are normally two main parts. The first is the solar collector, which is normally called a flat-plate collector that is mounted on the roof. This collector consists of a thin, flat, rectangular box with a transparent cover that is facing the sun. Through this thin box run small tubes which carry the fluid. That fluid is either water or some other fluid, such as an antifreeze solution. These thin tubes are attached to an absorber plate which is black in color to absorb the heat. At the heat builds up inside the collector, the fluid that is passing through it is also heated. The second part of this kind of system is a storage tank that holds the hot liquid. This container is usually larger than a regular water heater and is very well insulated. Systems that use something other than water have a coil of tubing inside the tank. As the fluid passes through this tubing, it heats the water.
There are two types of solar water heating systems. The active type of solar hot water heating system relies on a pump to move the liquid between the collector and the storage tank, while the passive system relies on gravity to circulate the water.
With a swimming pool system, the pool’s filter pump is utilized to pump the water through the solar collector. That solar collector is normally made of black plastic or rubber tubing. The pool is what stores the water in this case.
The United States Energy Information Administration classifies solar thermal collectors in three separate categories. Those categories are low-temperature collectors, medium-temperature collectors, and high-temperature collectors. Low-temperature collectors are generally used to heat swimming pools and use flat plate technology. Medium-temperature collectors are also normally flat plates, but are used to heat water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses, and are generally used for electric power production.
STE is quite different from photovoltaic energy, which converts solar energy directly into electricity. They are also much more efficient. Another use for solar thermal energy is to power smelting furnaces. A solar furnace is a structure that uses an array of curved mirrors that acts as a parabolic reflector. This concentrates light onto a focal point somewhat like a magnifying glass does using sunlight to start a fire. The primary difference is the smelting furnaces use reflected light which has a small focal point. The smelting furnace in Odeillo, France reaches a temperature of 3,500 degrees. The curved mirror for this furnace is as tall as an eight story building. The focal point is about the size of a cooking pot. This system has been in operation since the 1970s. The salt flats, located in the Western United States use solar thermal power to evaporate the water, which allows them to retrieve the salt. There is a similar solar oven located in Parkent in Uzbekistan which melts down aluminum. This smelting oven is called the “physics-sun.” The biggest smelting oven in the world is found in Rehovot on the terrain of the Weizmann Institute of Science in Israel. It is used for scientific experiments. There is one other big solar furnace called the Solar Two Furnace located near Barstow in California (U.S.A.).
Solar Thermal Energy For Electrical Production
Believe it or not, but solar thermal energy is actually more efficient than the traditional photovoltaic cells when creating electricity from sunlight. Photovoltaic energy converts photons of the sun directly into electrons. Solar thermal energy uses heat from the sun to power a turbine, which in turn powers a generator. This generator creates electricity in a similar to coal or nuclear fired plants. One of the biggest advantages of solar thermal energy over photovoltaic is that reserve energy can be stored as heat much easier than it is to store electricity. Instead of needing large banks of expensive batteries, thermal energy is stored in heated sand or heated liquid. This energy can be stored overnight. As a result, you have the ability to create a supply of electricity around the clock, 24 hours a day.
Most of the large scale solar thermal plants are now being constructed overseas in such places as Australia or Spain. There are also some large-scale solar thermal energy projects being constructed in the United States.
A New Solar Hybrid Breakthrough
Now it’s possible to have a solar panel that produces both photovoltaic energy and thermal energy at the same time. There is a team of researchers from MIT and Boston College that have developed a hybrid flat panel that is capable of doing both. These panels are eight times more efficient than the previously developed solar thermoelectric generators and are more cost effective. With this new technology, solar thermal energy could become a much more valuable investment. The energy output has increased significantly without much of an increase in cost.
The team used nanotechnology to combine solar absorbers that are spectrally-selective with a high performance thermoelectric material. This new product actually produces both electricity and hot water. It is touted that this system will give a much faster payback on the investment. They are saying it could shorten the time by more than 30%.
The problem most PV panels have is a drop-off of efficiency when the solar panel heats up. A photovoltaic panel loses about one percent of its efficiency for every 3.5 degrees Fahrenheit temperature increase. Solimpeks, a solar panel manufacturer, has also addressed this problem with this hybrid technology. This is how it works. Their panels address this problem by using water to absorb this excess heat. This keeps the panels cooler which equates to keeping the production at a higher level. The heated water is then used to supply hot water to the building. With this technology, it has been shown that there is a 20 percent improvement over similar sized electric-only PV arrays. What’s better yet is that traditional PV systems don’t provide any hot water. Now that’s a nice added benefit. Another benefit to this hybrid system is it extends the life of the panels, since heat is the one factor that can shorten their life-span.