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Projektinfo – Detailed information on energy research

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Fin structure of the heat exchanger after partial discharge of the latent heat storage system.
© DLR
Heat storage for solar thermal power plants
Projektinfo 11/2017

Integration of the storage cascade system into a parabolic trough power plant with direct steam generation.
© EnBW
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Night-time electricity from solar thermal power plants

Together with the industrial partner Linde AG, researchers from the German Aerospace Center have further developed a latent heat storage system based on nitrate salts. In combination with a cascade of sensible heat storage units, the goal is to maintain the electricity production of solar thermal power plants at night and at times with low solar irradiation. To this end, the scientists have developed a storage system that can also be used for conventional steam power plants and in

Using heat storage systems, solar thermal power plants can supply electricity around the clock – almost like conventional power plants. Their output can be more easily planned and they can provide balancing capacity for the power grid. The optimised heat management also improves the operating behaviour, reduces the partial load operation and uses the power block more efficiently. The overall economic efficiency therefore also increases..

Solar thermal power plants operate particularly efficiently and cost-effectively when they are operated at high temperatures. This is possible with the direct solar evaporation of water, whereby the superheated steam for the steam turbines is generated in the collector. It reaches temperatures of over 500 °C at a pressure of 120 bar. These steam parameters are almost equivalent to those common in conventional power plants.

“Direct steam generation in solar thermal power plants enables higher efficiency levels. However, coupling with a thermal energy storage system would make the most sense for evening operation. For this purpose, a storage system needs to be developed and optimised,” explains Maike Johnson, manager of the DSG-Store project. DSG-Store stands for “Direct Steam Generation Storage”. The direct steam generation and superheating of the feed water in the heliostat field is already commercially available in parabolic trough, Fresnel and tower power plants. The disadvantage until now, however, has been the lack of thermal energy storage systems. To enable these to be used reliably for direct steam generation, various components need to be further developed, the system structure optimised and operating concepts tested. Within the research project, the German Aerospace Center (DLR) has developed a new generation of thermal energy storage systems in conjunction with Linde AG.

The system developed by the researchers consists of a cascade comprising a latent heat storage unit for the condensation and evaporation and three sensible liquid salt storage tanks for the superheating of the steam. The researchers consider this to be the most promising solution in technical and economic terms. The entire system is designed for use in all solar thermal power plants with direct steam generation, irrespective of the concentrator system used, and therefore suitable for parabolic troughs, linear Fresnel collectors and tower receivers. In addition, the system can be used in both industrial steam processes and conventional steam power plants.

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Storage cascade for different temperatures

To ensure that as much heat energy from the collector is stored as possible, the scientists have developed the concept of a multi-stage storage cascade. This consists of “hot” (527 °C), “warm” (400 °C) and “cold” (306 °C) sensible heat storage units followed by a single latent heat storage unit. These first three storage units absorb the thermal energy in molten salts as sensible heat, i.e. through temperature increases. The molten salt is located in insulated steel tanks whose components and materials are adapted to the temperature level and corrosiveness of the salts. The latent heat storage tank is used as the fourth and last storage unit.

If the solar thermal system provides more steam than the power plant requires, the storage tanks can be charged: the superheated steam enters the first heat exchanger at 550 °C. There, it heats liquid salt that is pumped out of the warm storage tank and is then directed into the hot storage tank. In the same way, the now colder steam in the second heat exchanger brings the molten salt from the cold storage tank to the temperature level of the warm storage tank. Following this step, the steam is cooled down to such an extent that only the condensation energy can be decoupled. Since the condensation takes place at a constant temperature, a sensible heat storage system would only be able to absorb the energy with temperature and therefore exergy losses. This is why the scientists are using a latent heat storage unit optimised for this purpose. It works with salts whose melting temperature corresponds to the temperature of the condensation process. At night and when the sky is overcast, the storage units supply the energy for the steam power plant. In order to generate the steam for the turbines, these are discharged in reverse order. Initially, the latent heat storage unit heats and evaporates the feedwater. The two heat exchangers then heat the steam further using energy from the warm and hot tanks. The molten salts cool down during this process and are pumped into the respectively next colder tank.

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Addresses

Project management and thermal energy storage research
Deutsches Zentrum für Luft- und Raumfahrt (DLR)

Development of the latent heat storage system
Linde AG

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Energy storage systems
Recent reports on research, development and demonstration of energy storage systems

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