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Solar Thermal Energy

11:25 AM, Sun 20 Apr 2008 .. Posted in Solar .. 0 comments .. Link

Solar Thermal towers collect the heat from the sun in a completely different way than by using photovoltaics (solar panels). A solar panel converts the sun directly into electricity, whereas this method concentrates the sun's rays onto an area that collects the heat. That heat boils a liquid (such as water) and produces steam, which is then converted into electricity. This may seem like an added step in the process, but in actuality it is more efficient in large-scale production. Heat can be stored more easily than the electricity produced by panels, hence solar thermal towers can provide a more even energy source for a longer time.

Higher operating temperatures allow for different technologies to be used. One proposed method utilizes liquid fluoride salts, using multi-stage turbine systems that allows the plant to use higher-temperature dry heat exchangers for its thermal exhaust. This reduces the plant's water use, which comes in handy if it's situated in the desert... where large solar plants are the most practical. With the use of a back-up system for the rare emergencies, this method of obtaining power becomes very attainable.

This is a very promising way to produce power to the masses, in my view. Have a look at Wikipedia's list of Solar Thermal Power Stations from around the world.

Now, there are several different designs when it comes to solar thermal power stations. Most people have, at one stage or another, seen or heard about the solar farms that use masses of mirrors surrounding a central tower structure. These Power Tower designs use mirrors (called heliostats) to focus the sun's rays on a collector at the top of the tower. But to do this each mirror needs to be fitted with a motor to track the path of the sun, maximising the heat collected. This increases the cost, and decreases the attraction of this type of solar tower.

Parabolic trough power plants use a system of curved troughs, which reflect the sun's rays onto a receiver positioned above the trough. As the position of the sun changes, the whole trough tilts so that the focus remains on the receiver. However, if the troughs are positioned parallel to the sun, it does not require adjustment of the mirrors, as the light is simply concentrated on another part of the receiver. So in this way, the trough design takes out the added cost of a tracking system.

Another design uses a large, reflective, parabolic dish (similar in shape to a satellite dish). It focuses all the sunlight up onto to a single point above the dish, where a receiver captures the heat. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System. These engines require no fuel, creating a rotational kinetic energy that can be converted to electricity using an electric generator. Of all these technologies the solar dish/stirling engine has the highest energy efficiency. A single solar dish installed at Sandia National Laboratories National Solar Thermal Test Facility produces as much as 25 kW of electricity, with a conversion efficiency of 30%, compared to around the 20% of the parabolic systems.

A linear Fresnel reflector power plant uses a series of long, narrow mirrors to focus light onto one or more linear receivers above the mirrors, much like the parabolic system. The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are much simpler structurally. When suitable aiming strategies are used this can allow a denser packing of mirrors, thus producing more and/or saving valuable land space when needed. Another advantage of this design is that lenses are cheaper than mirrors. Furthermore, if a material is chosen that has some flexibility, a less rigid frame is required to deal with wind instability. It does bear mentioning however, that no full-scale thermal systems using Fresnel lenses are known to be in operation, although products incorporating Fresnel lenses in conjunction with photovoltaic cells are already available.

Below are a few more interesting links for you to peruse on this subject...

CSIRO - Solar Thermal Energy Research.

Triple Pundit article - Solar Thermal Electricity: Catching the Eye of Utility Companies.

Clean Technica article - Solar Thermal Electricity: Can it Replace Coal, Gas and Oil?
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