Types of Solar Thermal Systems

There are quite a few different types of Solar Thermal systems available. Now with all of these different types of systems how do you figure out what systems is best for your heating needs. In some cases the climate can play a big factor in deciding what systems are appropriate. Lets cover the different types of systems first.


Non-Glazed Collectors:

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These types of collectors are typically used in pool heating applications. It is fairly easy to tell these types of collectors apart from others. They are typically made out of black plastic like material. They can come in many shapes and sizes. These types of panels are the least expensive and can have a 3 to 5 year payback if replacing a natural gas heater. Non-glazed collectors can not be used during freezing conditions.



Glazed Flat Plate Collectors:

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When talking about solar thermal, Flat Plat Collectors are what people tend to be most familiar with. A glazed flat plate collector consists of a shallow rectangular box with
transparent glass covering a flat black plate. The black plate is attached to a series of parallel tubes or one serpentine tube through which air, water, or other heat transfer fluids pass. As the sunlight passes through the collector’s glazing, it strikes an absorbing material. This material converts the sunlight into heat, and the glazing prevents the heat from escaping. These systems can be used for heating the domestic hot water, hot tub's, radiant floors, etc...



Glazed Evacuated Tube Collectors:

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An evacuated tube collector consists of parallel rows of glass tubes connected to a header pipe. Each tube has the air removed from it to eliminate heat loss through convection and radiation. The evacuated tubes are used in the same type of applications as flat plate collectors; however, they can achieve higher temperatures and are well-suited to commercial and industrial heating applications. They tend to be more efficient in cloudy climates and in low ambient temperatures due to the insulating quality of the vacuum. There are two types of evacuated tube systems, direct flow and heat pipe evacuated tubes.

Direct Flow: These consist of a group of glass tubes inside each of which is a flat or curved aluminum fin attached to a metal (usually copper) or glass absorber pipe. The fin is covered with a selective coating that absorbs solar radiation well but inhibits radiative heat loss. The heat transfer fluid is water and circulates through the pipes, one for inlet fluid and the other for outlet fluid. Very similar to how flat plate collectors operate.

Heat Pipe: These consist of a metal (copper) heat pipe, to which is attached a black copper absorber plate, inside a vacuum-sealed solar tube. The heat pipe is hollow and the space inside, like that of the solar tube, is evacuated. The reason for evacuating the heat pipe, however, is not insulation but to promote a change of state of the liquid it contains. Inside the heat pipe is a small quantity of liquid, such as alcohol or purified water plus special additives. The vacuum enables the liquid to boil (i.e. turn from liquid to vapor) at a much lower temperature than it would at normal atmospheric pressure. When solar radiation falls the surface of the absorber, the liquid within the heat tube quickly turns to hot vapor rises to the top of the pipe. Water, or glycol, flows through a manifold and picks up the heat, while the fluid in the heat pipe condenses and flows back down the tube for the process to be repeated.

Concentrating Solar Power Collectors:

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Unlike other solar thermal systems these systems are designed to produce power. Electricity generation is their primary purpose but depending upon the application they can provide both heat and power. Concentrating solar power use mirrors to reflect and concentrate sunlight onto receivers that collect the solar energy and convert it to heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine driving a generator. Smaller CSP systems can be located directly where the power is needed. Single dish/engine systems can produce 3 to 25 kilowatts of power and are well suited for such

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distributed applications. Larger, utility-scale CSP applications provide hundreds of megawatts of electricity for the power grid.