Wolf-Dieter Steinmann

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.

Latent Heat Storage for Solar Steam Systems

Solar thermal systems, including direct steam generation in the absorbers, require isothermal energy storage systems. One option to fulfil this requirement is the application of phase change materials (PCMs) to absorb or release energy. The implementation of cost-effective storage systems demands the compensation of the low thermal heat conductivity that is characteristic for the candidate materials for PCM. Solar steam generation for power plants requires latent heat storage systems for a saturation temperature range between 200°C and 320°C. This paper describes the basic concepts investigated and first results of research activities aiming at the demonstration of a storage system using steam provided by parabolic trough collectors.

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.

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.

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.

Maximum Temperature Difference in Horizontal and Tilted Absorber pipes with Direct Steam Generation

The direct steam generation (DSG) in parabolic trough collectors is a promising option for the im provement of the reliable parabolic trough technology for solar thermal electricity generation. The DISS (DIrect Solar Steam) test facility at the Plataforma Solar de Almeria is a full-scale solar steam generator consisting of parabolic trough collectors, used to investigate the DSG process under real operating conditions. According to theory and results from lab-scale experiments it was expected that the tilt of the absorber tubes can improve the thermohydraulic behavior of the DSG process significantly. Within the DISS project a test campaign has been performed to investigate the influence of tilt. The evaluation of the test data showed, that the horizontal absorber tube guarantees sufficient cooling confirmed by low temperature differences even at very high steam qualities.

Solarthermal parabolic trough power plants with integrated storage capacity

Parabolic trough power plants use concentrated direct insulation for steam generation in a Rankine cycle. The fundamentals for direct steam generation in parabolic troughs have been developed in recent years, the feasibility was successfully demonstrated at a solar powered test facility reaching steam temperatures of 400°C at a maximum pressure of 100 bar. Important components for the accelerated market penetration of solarthermal power plants are cost-effective local storage systems. Main benefits of storage systems are: extended utilisation of the power block, improved efficiency of components and facilitated integration into the electricity system. Within the R&D project WESPE, a test facility for solid sensible heat storage is developed and connected to a solar collector loop. A simulation tool for the analysis of the transient performance of the storage system integrated into the power plant was implemented. The results show a significant potential for economic optimisation by applying advanced operation strategies based on an improved adaptation of the storage unit to the characteristics of solar collectors and power cycle.

Thermal energy storage systems for concentrating solar power (CSP) plants

Abstract: The integration of thermal energy storage systems enables concentrating solar power (CSP) plants to provide dispatchable electricity. The adaptation of storage systems both to the solar energy receiver system and the power cycle of the plant is essential. Three different physical processes can be applied for energy storage: sensible heat storage in solid or liquid media, latent heat storage using phase change material and thermochemical energy storage. This chapter gives an overview of the various technical concepts developed for thermal energy storage in CSP plants and describes their states of development, their potentials for use and their performances.

Modeling and Design of Direct Solar Steam Generating Collector Fields

The direct steam generation (DSG) is an attractive option regarding the economic improvement of parabolic trough technology for solar thermal electricity generation in the multi megawatt range. According to [1] and [2] a 10% reduction of the LEC is expected compared to conventional SEGS like parabolic trough power plants. The European DISS project has proven the feasibility of the DSG process under real solar conditions at pressures up to 100 bar and temperatures up to 400°C in more than 4000 operation hours [3]. In a next step the detailed engineering for a pre-commercial DSG solar thermal power plant will be performed. This detailed engineering of the collector field requires the consideration of the occurring thermohydraulic phenomena and their influence on the stability of the absorber tubes. A design tool has been developed at DLR calculating all relevant process parameters including pressure drop, temperature field and stress in the absorber tubes. The models implemented in this design tool have been validated in detail at the DISS test facility under real DSG conditions for pressures between 30 and 100 bar and inner diameters between 50 and 85 mm. The models have been implemented into a MATLAB® program to allow for a first quick determination of critical process conditions. Once critical process conditions have been identified the FEM package ANSYS® is used for a detailed investigation. This article summarises the models used and shows the design procedure for a DSG collector field. The design program has proven to be a reliable tool for the detailed design of DSG collector fields.Copyright