Concentrated solar power

Solar thermal power systems offers opportunities for local manufacturers of power plant components

by Ivan Fenyves, BHR

From the numerous types of commercially viable renewable power generation technologies, solar power and certain variants of it are the most exciting for sunny in general and for the local manufacturers of conventional thermal mechanical/ thermal equipment (like pressure vessels, heat exchangers, general and specialised piping) in particular.

There are currently two basic types of product palette of the local power station because they include piping, pressure vessels, commercially viable solar power stations mechanical/thermal component industry. heat exchangers and conventional steam available: photovoltaic (PV) power stations turbine based power station modules. Several Photovoltaic plants are the least expensive and concentrating solar power (CSP). variants of CSP systems are commercially variants – in terms of investment costs of solar power, they can be installed relatively available: Parabolic trough system and Photovoltaic (PV) power stations quickly but the main disadvantage is that they compact linear fresnel reflector (CLER) can only produce the rated output power in systems. Working with very large solar collectors the hours of full sunshine with (currently) no (laminate panels) consisting of a multiple Parabolic trough system economical option for storage of electricity for array of individual low voltage DC solar the dawn/dusk, cloudy, rainy or night periods. Constructed as a long parabolic mirror with a cells that convert the sun’s radiation energy Dewar tube (similar to a conventional vacuum to electricity. These systems need either Concentrating solar power (CSP) flask) running its length at the focal point. components for storage of the DC energy or The sunlight is reflected by the mirror and an DC/AC power converter (inverter) to output The various commercially available concentrated on the tube. The heat transfer AC, which can be fed directly into the national technologies of this system are of particular fluid (usually oil) is heated up in the tubes and grid. This article will not discuss this system in and prime interest to the above mentioned goes through a heat exchanger to generate detail, because of its limited relevance in the local SA group of equipment manufacturers, steam which in turn will drive a steam turbine. This is the technology used in most of the presently operational solar power stations. A typical view is shown in Fig. 1. Stations of this type are already operational in USA, Spain and other countries. The usual maximum electrical output of a field is 30 – 65 MW.

In South Africa the first large scale parabolic trough CSP installation (KaXu Solar One at Pofadder/Northern Cape) owned by a consortium of Abengoa (Spain) and CMI (Belgium) will have 100 MW rated electrical output and three hours of thermal storage capacity.

Another station in South Africa, using this technology which is in project phase is Bokpoort CSP (Groblershoop, Northern Cape), a 50 MW, parabolic trough type, similar to KaXu, but with 9,5 hours of storage, so that the plant will be able to supply power around the clock. 40% of ownership will be of the Saudi-Arabia based ACWA Power International Group and also portions of IDC and local communities.

Compact linear fresnel reflector (CLFR) systems

These use long, thin segments of mirrors to focus sunlight onto a fixed absorber located at a common focal point of the reflectors. These mirrors are capable of concentrating the sun’s energy to approximately 30 times Fig. 1: Parabolic trough type CSP. its normal intensity. This concentrated energy

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sliding pressure where the storage medium is steam or pressurized hot water. Some systems use fossil energy to extend the operating hours of the steam plant in periods with no or reduced intensity of sun radiation.

The main problems of CSP solar power installations are:

 High investment costs of the station and of the high voltage transmission lines from the remote sites to the centers of power consumption

 Large ground area is required to accommodate the mirrors

 Keeping the mirrors clean under desert conditions is difficult

 Water shortage in most of the sunny but arid (desertlike) areas

Fig. 2: Compact Linear Fresnel Reflector (CLFR) type CSP. Another interesting CSP design is the Stirling dish. This is similar to the above systems, but uses a parabolic dish that follows the Sun’s movement along the ecliptic to heat up the working fluid and with a proprietary Stirling engine to drive a generator. This solution provides the highest efficiency among the CSP systems. However, Stirling engines are currently not produced by any local SA company.

Future trends

Besides the constant strive to improve the economy of the existing systems, in the leading industrial states there is an extensive activity of development and research of totally new, high-tech methods of solar energy production. For example: thin layer sandwich panels using very special rare metal combinations, nanotechnology and similar Fig. 3: Solar Power Tower (heliostat) CSP. technologies are under investigation. It is interesting to note that researchers are looking deeply into the secrets of nature, like the thin is transferred through the absorber into some installation is shown on Fig. 3. In South Africa layer structure of butterfly wings. thermal fluid, like molten salt or oil. The fluid the first large scale solar power tower CSP has to be suitable to maintain its liquid state installation ( at Upington/Northern Conclusion at very high temperatures. The fluid then goes Cape), owned by a consortium of Abengoa through a heat exchanger to generate steam (Spain) and CMI (Belgium) will have 50 MW Solar power generation has an important role which in turn will drive a steam turbine. A rated electrical output, 2 h thermal storage to play in the mix of renewable energy systems typical arrangement is shown on Fig. 2. Plants using molten salt and will use a superheated of South Africa. The first large scale plants are of this type are offered by Areva and already steam turbine with air cooling. already approved and under development. in operation in USA. Manufacturers of conventional thermal power The combined investment value of Khi and station mechanical/thermal equipment KaXu is R10-billion. Solar power tower have a great potential in participating in the Systems using molten salt (40% potassium local fabrication and installation of the plant An array of flat, movable mirrors arranged nitrate and 60% sodium nitrate) can be components, particularly for CSP systems, in a circular pattern around a central power combined with thermal storage vessels so that such as large scale high-tech piping systems, tower (also called heliostat) reflect the sunrays the steam production can be continued for pressure vessels, heat exchangers and to the collector tower. The solar energy will be 2 – 4 h without sunlight. The steam temperature conventional steam turbine cycle modules transferred in the tower to molten salts which could be as high as 500°C. that form essential parts of this technology. have a high thermal capacity. The hot liquid salt will generate steam in a heat exchanger There are also systems that use conventional Contact Ivan Fenyves, BHR, Tel 012 380-5337, which drives the steam turbine. A typical thermal storage systems with constant or [email protected] ܀

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