Marginal Column

Modern steam engines

November 2012

Solar thermal energy is a promising way to generate power. But how do the various technologies work?

 
 

Our greatest source of renewable energy is the sun. In sunny regions solar thermal power stations currently rank as the most efficient generators of electricity. Areas that have little or no other use, like deserts, offer more than enough space for high-capacity installations with large surface areas. Unlike photovoltaic plants, it is relatively easy to store the heat en­ergy generated by solar thermal power stations, in liquid salt for instance. This means that energy is available as needed, even at night or in cloudy weather. However, the challenge here is finding sufficient water to clean the surfaces of the mirrors and to feed the steam turbine.

This is why dry cooling technology, somewhat less efficient, is frequently used for this particular application. The functional principle behind solar thermal technology is as old as the hills and brilliantly simple. A reflector focuses solar radiation on a defined point or line to heat up a heat transfer medium (thermal oil, liquid salt, air or water). Yet there are considerable differences in how this prin­ciple is applied, differences which have an effect on usage, performance and efficiency.

Parabolic throughs


 
Illustration | Bosch Rexroth AG / design hoch drei GmbH & Co. KG

How they work:

Miles of collectors made up of individual curved mirrors reflect solar radiation onto an absorber tube located in the focal line. This tube contains a medium, usually heat-transfer oil, which is heated to about 400 degrees Celsius. Steam is generated in a heat exchanger, ultimately driving a turbine in the same way as steam in conventional power stations. This turbine then sets a generator in motion.

Advantages:

This leading technology has now been successfully applied for 20 years. The experience gained over the years has increased its reliability. Ongoing further development and the resulting drops in investment costs reflect a high level of maturity and prevalence. Higher process temperatures (liquid salt up to 550 degrees Celsius or direct vaporization up to 500 degrees Celsius) and thus better efficiency levels can be achieved by using other heat transfer media.

Disadvantages:

Level surfaces are required to set up parabolic mirrors. Heat transfer oil and liquid salt both need heat exchangers. The absorber tube also swings, which requires additional pipe fittings and increases the number parts subject to wear and tear.

Efficiency factor:

About 16 percent on an annual average

Solar power towers


 
Illustration | Bosch Rexroth AG / design hoch drei GmbH & Co. KG

How they work:

Individual mirrors, so-called heliostats, focus the sun’s rays on a point in a tower, the so-called receiver, where temperatures of up to 1,300 degrees Celsius can be generated. Under normal working conditions this is limited to about 500 to 600 degrees Celsius depending on the heat transfer medium used (air, liquid salt or water). Heat exchangers then turn this energy into steam, which is converted into electricity in a conventional power generator. This technology is currently making strong progress. A high-performance photovoltaic cell that can directly transform concentrated sunlight into power may also be used instead of the thermal receiver.

Advantages:

As each heliostat is a stand-alone unit, the location does not need to be on a level field. Solar radiation from the entire heliostat field is focused on a single point, meaning means that higher temperatures and thus improved efficiency factors can be achieved.

Disadvantages:

This technology is not yet fully developed. There is still no ideal solution for the material in the receiver, which is exposed to high temperatures.

Efficiency factor:

Up to 20 percent on annual average

Linear Fresnel Reflectors


 
Illustration | Bosch Rexroth AG / design hoch drei GmbH & Co. KG

How they work:

Here, as is the case with the parabolic trough, clusters of long and narrow mirrors reflect solar radiation onto a permanently installed absorber tube. The mirrors are all aimed at one single focal line. There is no requirement for swivel fittings, which are prone to wear and tear.

Advantages:

The mirrors all have the same curvature and this means they are cheaper to manufacture. Additionally, the mirrors are erected about two meters above the ground and are thus less vulnerable to the wind.

Disadvantages:

Flat and level surfaces are a necessity. They are less efficient due to lower operating temperatures. However, there have been positive developments which permit higher temperatures with corresponding increases in efficiency. Thermal transfer media are used (refer to parabolic trough). No sufficient long-term empirical values.

Efficiency factor:

13 percent on annual average.

Parabolic dishes


 
Illustration | Bosch Rexroth AG / design hoch drei GmbH & Co. KG

How they work:

A completely self-contained plant: An individual mirror with a parabolic curvature focuses solar radiation onto one point. There energy can be transformed either by a photovoltaic cell or a Stirling engine.

Advantages:

A stand-alone unit like this can be set up wherever desired, with no major requirements for the ground at the site. The dish is suitable for smaller projects such as municipal power generation. Using Stirling engines or photovoltaic cells does away with the need for cooling water. This concept can reach temperatures of up to 750 degrees Celsius and offers a high efficiency factor.

Disadvantages:

This technology is still in the experimental stage so production costs are high and there is a lack of long-term experience. Only a very limited number of manufacturers is capable of producing reliable Stirling engines. Storing electric current in the form of heat is difficult.

Efficiency factor:

Up to 25 percent on annual average.