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Thermochemical Energy Storage for Concentrated Solar Power Plants

Final Report Summary - TCSPOWER (Thermochemical Energy Storage for Concentrated Solar Power Plants)

Executive Summary:
Within the TCS Power project, thermochemical energy storages operating at high temperatures were investigated and developed for a direct integration into advanced CSP plants. Since one of the main advantages of CSP plants - dispatchable renewable electricity – depends strongly on cost-effective and efficient thermal energy storages, the project focussed on two different basic reactions for energy storage that proposed low cost storage capacities: On the one side manganese oxide reacting reversibly with oxygen at temperatures above 700 °C was addressed. Besides the good availability of the raw materials, efficient integration strategies into high temperature solar towers propose synergies since the heat transfer fluid (air) can be directly used as carrier for the reaction gas (oxygen). On the other side, the research focused on the reversible reaction of quick lime (calcium oxide) with water vapour. This material is already widely applied in several processes and therefore - already at present - available in large industrial scale. Due to its working temperature range between around 400 and 600 °C and potential synergies the reaction system was investigated as thermochemical energy storage for steam cycle based power blocks.
One of the main objectives of the project was an integrated approach for the development of the thermochemical energy storage. Starting with work package one, all three aspects of thermochemical energy storage were taken into account. These are the intrinsic material properties and their potential improvements as well as the requirements from the CSP plant that pose clear limitations and boundaries on integrational and operational aspects of thermochemical energy storages. The third aspect is related to the thermochemical reactor – the actual heat exchanger where the reaction takes place needs to fit to the requirement of the CSP plant as well as to the respective material properties.
Due to the specifications of the power plant on one side and the material properties on the other side, both thermochemical storage approaches focussed on the development of a reactor design that allows for a movement of the material through a reaction zone. Based on this concept, the reactor is designed to reach the required power level whereas the capacity can be easily increased by adding additional cheap tanks to store the required amount of reaction material. According to this overall project goal the improvement of the material mainly addressed the possibility to move reactive solids through the reaction zone. In case of the manganese oxide this was achieved by granulation of the reaction powder whereas in case of the calcium oxide, the enhancement of the flowability was addressed by the addition of nano-structured spacer particles to the reactive powder. The reactor development work packages consequently addressed a fluidized bed of reactive manganese oxide granules as well as a counter-current moving bed concept for the calcium oxide system. The first one was developed and brought into operation in lab-scale (100 Wh) whereas the latter one reached a pilot scale level of 10 kW power and 100 kWh capacity (~ 300 kg of material).
The process evaluation work package started with different integration concepts for the two storages taking the power block requirements as well as the available thermal power from the solar field at different sites into account. Based on a performance evaluation model and an up-scaling strategy for the two developed reactor concepts a techno-economic evaluation was done. It could be shown that the concept of separated capacity is clearly favourable for large storage quantities. However, future work should address the reduction of the overall complexity of the reactor. An important finding is that even if material modifications increase the cost for the storage material, it can be clearly beneficial if at the same time the reactor and system complexity can be reduced.

Project Context and Objectives:
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Project Results:
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Potential Impact:
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List of Websites:
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