Doerte Laing

Advanced Thermal Energy Storage Technology for Parabolic Trough

The availability of storage capacity plays an important role for the economic success of solar thermal power plants. For todays parabolic trough power plants, sensible heat storage systems with operation temperatures between 300°C and 390°C can be used. A solid media sensible heat storage system is developed and will be tested in a parabolic trough test loop at PSA, Spain. A simulation tool for the analysis of the transient performance of solid media sensible heat storage systems has been implemented. The computed results show the influence of various parameters describing the storage system. While the effects of the storage material properties are limited, the selected geometry of the storage system is important. The evaluation of a storage system demands the analysis of the complete power plant and not only of the storage unit. Then the capacity of the system is defined by the electric work produced by the power plant, during a discharge process of the storage unit. The choice of the operation strategy for the storage system proves to be essential for the economic optimization.

High-Temperature Solid-Media Thermal Energy Storage for Solar Thermal Power Plants

Solid sensible heat storage is an attractive option for high-temperature storage applications regarding investment and maintenance costs. Using concrete as solid storage material is most suitable, as it is easy to handle, the major aggregates are available all over the world, and there are no environmentally critical components. Long-term stability of concrete has been proven in oven experiments and through strength measurements up to 500 °C. Material parameters and storage performance have been validated in a 20-m3 test module with more than 23 months of operation between 200 °C and 400 °C and more than 370 thermal cycles. For an up-scaled concrete storage design with 1100-MWh capacity in a modular setup for a 50 MWel parabolic trough power plant of the ANDASOL-type, about 50 000 m3 of concrete is required and the investment costs are approximately 38 million euro. The simulation of the annual electricity generation of a 50 MWel parabolic trough power plant with a 1100-MWh concrete storage illustrates that such plants can operate in southern Europe delivering about 3500 full load hours annually; about 30% of this electricity would be generated by the storage system. This number will increase further, when improved operation strategies are applied. Approaches for further cost reduction using heat transfer structures with high thermal conductivity inside the concrete are analyzed, leading to a 60% reduction in the number of heat exchanger pipes required. For implementation of the structures, the storage is build up of precast concrete blocks.

Test Results of Concrete Thermal Energy Storage for Parabolic Trough Power Plants

Efficient energy storage is vital to the success of solar thermal power generation and industrial waste heat recovery. A sensible heat storage system using concrete as the storage material has been developed by the German building company Ed. Zublin AG and the German Aerospace Center (DLR). A major focus was the cost reduction in the heat exchanger and the high temperature concrete storage material. For live tests and further improvements, a 20 m 3 solid media storage test module connected to an electrically heated thermal oil loop was built in Stuttgart. The design of the test module and the test results are described in this paper. By the end of November 2008, the second generation solid media storage test module had accumulated five months of operation in the temperature range between 300°C and 400°C and almost 100 thermal cycles with a temperature difference of 40 K. The tests will be continued in 2009.

Solid Media Thermal Storage Development and Analysis of Modular Storage Operation Concepts for Parabolic Trough Power Plants

Cost-effective integrated storage systems are important components for the accelerated market penetration of solarthermal power plants. Besides extended utilization of the power block, the main benefits of storage systems are improved efficiency of components, and facilitated integration into the electrical grids. For parabolic trough power plants using synthetic oil as the heat transfer medium, the application of solid media sensible heat storage is an attractive option in terms of investment and maintenance costs. For commercial oil trough technology, a solid media sensible heat storage system was developed and tested. One focus of the project was the cost reduction of the heat exchanger; the second focus lies in the energetic and exergetic analysis of modular storage operation concepts, including a cost assessment of these concepts. The results show that technically there are various interesting ways to improve storage performance. However, these efforts do not improve the economical aspect. Therefore, the tube register with straight parallel tubes without additional structures to enhance heat transfer has been identified as the best option concerning manufacturing aspects and investment costs. The results of the energetic and exergetic analysis of modular storage integration and operation concepts show a significant potential for economic optimization. An increase of more than 100% in storage capacity or a reduction of more than a factor of 2 in storage size and therefore investment cost for the storage system was calculated. A complete economical analysis, including the additional costs for this concept on the solar field piping and control, still has to be performed.

Development of PCM Storage for Process Heat and Power Generation

The increased interest in solar thermal systems using steam as a working medium either for power generation or process heat applications gave rise to a growing demand for latent heat storage units. Essential for the development of cost-effective latent heat storage systems is the achievement of a sufficient power level in spite of the characteristic low thermal diffusivities of latent heat storage materials. The sandwich concept using fins made either from graphite or aluminum has been identified as the most promising option for latent heat storage systems. The feasibility of this approach has been demonstrated by three prototypes using graphite and one prototype using aluminum fins. The prototype with aluminum fins was filled with sodium nitrate and was operated for more than 4000 h without degradation of power. The volume specific average power density is in the range 10-25 kW/m 3 , so it is proven that the major problem of phase change material (PCM) storage of low heat transfer rates has been overcome and high-temperature PCM storage with large capacity factor is possible.

Development of a Thermal Energy Storage System for Parabolic Trough Power Plants With Direct Steam Generation

For future parabolic trough plants direct steam generation in the absorber pipes is a promising option for reducing the costs of solar thermal power generation. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge. A three-part storage system is proposed where a phase change material (PCM) storage will be deployed for the two-phase evaporation, while concrete storage will be used for storing sensible heat, i.e. for preheating of water and superheating of steam. A pinch analysis helps to recognize interface constraints imposed by the solar field and the power block and describes a way to dimension the latent and sensible components. Laboratory test results of a PCM test module with approx. 140 kg NaNO3, applying the sandwich concept for enhancement of heat transfer, are presented, proving the expected capacity and power density. The concrete storage material for sensible heat was improved to allow the operation up to 500 °C for direct steam generation. A storage system with a total storage capacity of approx. 1 MWh is described, combining a PCM module and a concrete module, which will be tested in 2009 under real steam conditions around 100 bar.

Latent Heat Storage Systems for Solar Thermal Power Plants and Process Heat Applications

Solar thermal systems using absorber evaporating steam directly require isothermal energy storage. The application of latent heat storage systems is an option to fulfill this demand. This concept has been demonstrated mainly for low temperature heating and refrigeration applications, the experience for the power level and temperature range characteristic of solar process heat and solar thermal power plants is limited. Cost effective implementation of the latent heat storage concept demands low cost phase change materials (PCMs). These PCMs usually show low thermal conductivity limiting the power density during the charging/discharging process. This paper describes various approaches, which have been investigated to overcome these limitations. Based on fundamental PCM-research and laboratory-scale experiments, the sandwich concept has been identified to show the highest potential. The sandwich concept has been demonstrated successfully for three different storage units ranging from 2 kW to 100 kW at melting temperatures of 145°C and 225°C.

Overview of PCMs for concentrated solar power in the temperature range 200 to 350 °C

Thermal energy storage is an essential advantage of solar thermal power plants. The present paper focuses on latent heat storage using a phase change material (PCM). The paper lists literature and gives the current status of PCM work in the temperature range 200 to 350 °C. The system KNO3-NaNO3 is discussed in detail in terms of their thermo-physical properties in the liquid and solid phase. A comparison of literature data and own measurements for the density, heat capacity, thermal diffusivity and thermal conductivity is presented. Measurement results with the following methods are discussed: helium pycnometer, differential scanning calorimeter (DSC) and laser flash. Missing data of the thermal diffusivity and thermal conductivity are partly supplemented. Consistent thermo-physical properties in the liquid phase are presented.

Hybrid Dish/Stirling systems: Combustor and heat pipe receiver development

A hybrid sodium heat pipe receiver has been developed within the project HYHPIRE, funded 50% by the European Commission. The hybrid receiver was designed for the SBP/LCS 10-kWel dish/Stirling system with the SOLO-161 Stirling engine. Design of the heat pipe receiver and combustion system are described in this paper The system has been tested successfully in all operation modes. Results and experience from the lab tests in combustion-only mode, as well as results from demonstration testing in the dish in solar-only, gas-only, and hybrid mode on the Plataforma Solar de Almeria (PSA) in Spain, are reported.

20 – High-Temperature Molten Salts for Solar Power Application

Solar thermal power plants are a key technology for electricity generation from renewable energy resources. Thermal energy storage (TES) systems correct the mismatch between the solar supply and the power demand. TES makes it possible to meet the intermediate load profile with dispatchable power, a benefit that has a high value to power utilities and that gives concentrating solar power (CSP) technology an edge over photovoltaic and wind power. Hence, TES is a key technology for solar thermal energy utilization with growing present and future importance. This chapter focuses on material aspects of alkali nitrate salts. They include thermal properties, thermal decomposition processes, steel corrosion issues, and phase diagrams of multicomponent salt systems. In addition, two CSP applications using molten nitrate salts as sensible and latent TES are discussed.