For solar thermal power plants with parabolic trough collectors, the most common concept is to use thermal oil as heat transfer medium in the solar field circuit. Due to the maximum operating temperature of the thermal oil of about 395°C, the live steam temperature is limited to about 377°C and generates an efficiency of up to 40% at a steam pressure of 100 bar.
A fundamental advantage of solar thermal power plants is the integration of a thermal energy storage. It allows decoupling the solar energy from the electricity generation. The electricity can be fed into the grid according to the demand at day and night. In the thermal storages, the cost-efficient molten salt is used as heat transfer medium instead of thermal oil. The Institute of Solar Research is working on technology concepts in order to use the molten salt not only as storage medium, but also as heat transfer fluid in the solar field, which would entail several technical and economic advantages.
Increase of system efficiency
Using salt instead of thermal oil as heat transfer and storage medium increases the usable temperature level to at least 565°C, which pushes the power block efficiencies over 45 %. In addition, the energy density in the thermal storage increases due to the higher temperatures resulting in a smaller and more cost-efficient storage.
Strict separation of the gained solar energy and the electricity generation
The molten salt circulating as heat transfer medium in the solar field is used directly as storage medium, as well as for the heat transfer to the power block. This leads to complete decoupling of thesolar field from the power block, in other words of solar irradiation from electricity generation, because no thermal oil circulates directly between solar field and power block (compare illustrations above and below). The storage tanks separate the two systems strictly, which prevents any effect on the electricity generation in case of strong short-term variations of the irradiation due to passing clouds, for example.
Reduction of the levelized electricity costs
Since more than ten years, studies by different institutes have enlightened the economic aspect of using molten salt in parabolic troughs. All results show that molten salt systems may reduce the levelized electricity costs up to 25 % in comparison to existing plants based on thermal oil. This leads to levelized electricity costs below 10 €-Ct/kWh.
The structure of a plant which could lower the levelized electricity costs significantly is shown in the figure below.
In total, the system only uses two heat transfer media: molten salt in the solar and storage parts, and water/steam in the power block. In the steam generator, the thermal power is transferred from salt to water/steam. The generator in the power block (on the upper right hand side in the picture) supplies generated electricity to the connected grid. In contrast to thermal oil as heat transfer fluid, no heat exchanger is needed by the storage.
Selection of an appropriate salt mixture
A technical challenge regarding the operation of parabolic trough collectors with molten salt as heat transfer medium is the high solidification temperature of more than 140°C. Freezing of the salt inside the solar field must be prevented in any case. The solidification temperature of a salt mixture is dependent on the salt composition. Though, the use of a mixture with a solidification temperature as low as possible is not always optimal for the system in total since these mixtures often also come with limited upper process temperature. In addition to the mentioned safety aspects, the choice of salt also depends on the location of the power plant, as well as the implemented technology. Because of the technological development to higher concentration factors, the importance of the upper process temperature is increasing.
Experiences with molten salt technology
The use of molten salt as heat transfer medium was already demonstrated in the project “Solar Two” in solar power towers. The first commercial solar power tower with molten salt, Gemasolar, was built in 2009. Further commercial solar power towers with this technology are currently being constructed.
Despite the promising study results regarding the potential for cost reductions, molten salt in parabolic trough systems is commercially implemented only in small-scaled plants. This is mainly due to the fact that some solutions for technical challenges coming with the usage of molten salt still have to be proven:
Besides the identification of error prone components, the experiences from the “Solar Two” tower project have also provided clues for solving of the challenges mentioned above. The objective of the ongoing research is to elaborate solutions for the existing problems, and also to disprove unwarranted doubts in an objective and transparent way.