Imported electricity from solar thermal power plants can gain importance in Germany in the long term

Author: Dr. Klaus Hassmann, Energy Technology Cluster (as of October 2017)

Initial situation:

Power plants in which solar radiation is used to generate solar thermal electricity have existed for decades. In the 1980s, the author was part of a working group commissioned by the Ministry of Research to assess whether or not the Federal Republic should continue its involvement in development work at the solar thermal test field in Almeria, southern Spain - the "yes" vote was unanimous. It is again and again enriching to google on the newest conditions starting from the realizations at that time.

It was always clear, in Germany itself makes for lack of sufficient solar irradiation the solar-thermal generation of current no sense - in addition one must move into the sun belt of the earth. For Germany, North Africa with its solar conditions was a preferred destination. Building the power plants there and transporting the electricity to Germany using high-voltage direct current technology seemed promising. Studies judged this concept to be technically and economically attractive. This was true at a time when there was no political/religious turmoil in the North African countries.

It is worth keeping an eye on the technology; even if the North African desert areas fail as sites in the long term: Also in the south of Europe the solar irradiation is sufficient for the thermal current harvest, e.g., in Spain. By the time electricity from this technology can be made available in large quantities in the European grid and thus also in Germany, the energy/electricity transition in Europe will have made significant progress. Should there then still be bottlenecks in the power supply here in Germany, this power source could contribute to covering demand. The first voices are being raised that predict a significant increase in electricity consumption as a result of the sector coupling of electricity, heat and mobility. More reliable indications are expected from the results of coupled calculation models for the three consumption sectors.

Technical concepts

In principle, a distinction is made between two types of solar power plants, the farm and the tower.

Solar farm: 

In the farm, parabolic trough collectors connected in parallel concentrate the solar radiation on an absorber tube guided in the focal line; these are trough-shaped curved mirrors. The tube contains a heat transfer medium, usually a thermal oil. Water vapor can also be used. The direct water-steam cycle has economic, but also thermodynamic advantages, since at a maximum of 500 °C, process temperatures about 100 °C higher than those of thermal oil can be achieved. The heat transfer medium is heated in the trough collectors. Thermal oil as the heat transfer medium releases its heat in a heat exchanger to a water-steam cycle and is then returned to the mirror system. With steam as the heat transfer medium, the heat exchanger is not required. The steam generated is used to produce electricity via the steam turbine/generator components. The waste heat is released in a condenser and fed back to the heat exchanger as feed water. For such a "desert power plant", infrastructure similar to that of a fossil power plant must be available; above all, condenser cooling is essential; water is necessary, or dry cooling as an alternative in case of water shortage. The amount of water needed to clean the mirrors should not be underestimated - sand, airborne, is often a nuisance in such regions. For better solar harvesting, the mirrors are designed uniaxially rotatable and follow the sun's path. 

 Solar tower:

In the solar tower, the solar radiation is concentrated via numerous individual mirrors (heliostats in technical jargon) mounted on the ground, tracking the sun, onto a central absorber (receiver) mounted on the top of the tower. Due to the concentration of solar radiation, high temperatures are generated there which, for material reasons, must be limited to a level of around 1300 °C for technically sensible use. Suitable heat transfer media include liquid nitrate salt, steam or hot air. As already described for the farm, the tower can be used to generate electricity either via the classic power plant components steam turbine/generator or, with air as the storage medium, with a gas turbine. In the latter case, the combustion chamber is the absorber installed on the tower; the hot air charged in the  gas turbine compressor is fed into the receiver, where it is solar-heated and expanded in the gas turbine. The water problem at the cold end of the process described for the trough and the cleaning issue also apply to the tower. Tower technology can also be used to generate process heat at a very high temperature level.

Development work in Germany

Tower and farm technology are being developed for export to countries with high solar radiation; the federal government is also providing funding for this purpose. The goals are market-ready components and systems at the lowest possible cost.  

In Germany, the German Aerospace Center (DLR) operates a solar thermal experimental power plant as a pilot plant and reference for commercial plants at suitable locations. The Fraunhofer Institute for Solar Energy Systems (ISE) is also involved in the development of this technology, as are other companies and institutes. The diversity of those involved and the technical content is surprising for a technology whose large-scale application is not possible in Germany.

Solar power plants in operation worldwide (source: Wikipedia "Solar thermal power plant")

Tab.1 shows a detailed list of plants in operation worldwide; their commissioning took place in or after 2007. The list refers to locations larger than 10 MW, where more than one plant may well be in operation. To date, the USA and Spain in particular have multiple sites; presumably more than the 3 countries listed are operating/testing the technology at one site each. The solar farm clearly dominates over the solar tower. In Spain, the so-called Fresnel technology (the name is derived from the Fresnel lens, which has no curvature and which, arranged in strips, focuses the solar radiation in this case on the heat transfer medium, steam) is also being tested on a commercial scale. The table also shows that, as might be expected, the U.S. is well ahead of Spain (30% and 68 MW, respectively) in solar thermal power generation relative to global installed capacity (61%) and also in average capacity per site (280 MW). The full load operating hours per year vary strongly - depending upon location of the country in the sun belt of the earth and availability of the plants this value lies between 2000 and 3000 hours and thus clearly (around the factor 2 to 3) over photovoltaics in Germany.

In total per year solarthermisch with an installed achievement of 3700 MW 7,2 TWh river are produced. In terms of solar utilization, solar thermal is in competition with photovoltaics (PV). Depending on the operating temperature/heat carrier and system technology, the electrical efficiencies of the farm and tower are between 14 and 30 for the tower, and between 10 and 23% for the farm; PV averages between 14 and 19%. Fraunhofer ISE has published data on land consumption, among other things, under "Current facts on photovoltaics in Germany"; this is also an important parameter for an evaluation. The PV module area for 40 GW already installed in Germany is 300 km². For solar farms and towers, only values for a single smaller farm installation were found; the specific land requirement in GW/km² is not significantly different from the PV value. The author would have expected a much higher value for solar thermal.

Tab 1: Solar thermal power plants worldwide at sites larger than 10 MW

 Average power sites USA: 282 MW; Share power worldwide: 61%

Average power sites Spain: 68 MW; Share of power worldwide: 30%

Share of power worldwide: 4%

Share of power worldwide: 2.5%

Share of power worldwide: 2.5%

Outlook

Solar thermal power plants are a proven technology with technical and economic development potential; the federal government would be well advised to continue promoting development to further expand expertise at home. In Southern Europe (someday also in a peaceful and democratic North Africa?) electricity could then be generated at marketable costs with the cooperation of Germany. From the European interconnected grid, Germany would also be able to profit from this technology.