Advances in Science and Technology Vol. 74

Abstract: Consideration of technologies for the use of concentrated solar power (CSP) leads to the conclusion that there is substantially more energy in the sun’s heat than there is in its light. At present, solar-thermal energy conversion and storage systems using CSP have the shortcomings of the use of high pressures and potential problems with corrosion. In the development of new materials and designs, two of the key issues of consideration are the: (a) thermal properties of the materials and (b) heat transfer within the system. Most current technologies utilise convective heat transfer of liquids but there are none that use conductive heat transfer with solid-state systems. The present work introduces such a system in the form of highly dense and aligned self-assembled graphite, which can be heated in air, provided the hot face temperature is at a temperature sufficiently low to avoid the onset of oxidation. Modelling of a small domestic-scale system, which has no competition in the marketplace, consisting of: (a) 4 m diameter concentrator, (b) block of graphite weighing ~160 kg, and (c) electricity generation system demonstrates that, in only 90 min and at ≤420°C, sufficient heat can be stored to supply 25% more than is required for a typical 24 h, domestic, electricity usage cycle.

Abstract: Increasing energy prices and shortage of fossil fuels lead to a growing interest in alternative energy sources. In combination with energy storage systems the generation of solar process heat can be provided independent from the weather leading for example to a cost efficient stabilization of power output. For this application latent heat storage units with phase change materials (PCMs) can be designed to store solar process heat within a narrow temperature interval utilizing the high storage density of the different PCMs. This is achieved using the latent heat of melting in the melting / solidification process, or the latent heat of re-crystallization in a solid / solid phase transition. However, this advantage can only be used in technical applications if the heat transfer in the PCM is sufficiently high. As most pure PCMs exhibit a low thermal conductivity (about 1 W/(m•K) or less), methods to improve heat transfer in PCMs have been under investigation for decades. The heat transfer in a PCM can be increased by addition of highly thermal conductive materials. Due to its superior properties - high thermal conductivity, good processability, and chemical inertness - graphite has distinct advantages for this purpose. Depending on the requirements of the respective application, various routes to combine PCM and graphite are used. For example, besides the fabrication of PCM/graphite composite materials, the increase of heat exchanger surface by highly thermal conductive graphite plates is a favorable method for large scale applications, in particular. Effective thermal conductivities up to 30 W/(m•K) have been realized. This paper gives an overview of actual and potential applications of PCM/graphite heat storage systems focusing on storage of solar heat for high temperature applications such as process heat generation and solar thermal power plants.

Abstract: The Solar-Institute Jülich (SIJ) has initiated the construction of the first and only German solar tower power plant and is now involved in the accompanying research. The power plant for experimental and demonstration purposes in the town of Jülich started supplying electric energy in the beginning of 2008. The central receiver plant features as central innovation an open volumetric receiver, consisting of porous ceramic elements that simultaneously absorb the concentrated sunlight and transfer the heat to ambient air passing through the pores so that an average temperature of 680°C is reached. The subsequent steam cycle generates up to 1.5 MWe. A main field of research at the SIJ is the optimization of the absorber structures. To analyze the capability of new absorber specimens a special test facility was developed and set up in the laboratory. A high-performance near-infrared radiator offers for single test samples a variable and repeatable beam with a power of up to 320 kW/m² peak. The temperatures achieved on the absorber surface can reach more than 1000°C. To suck ambient air through the open absorber - like on the tower - it is mounted on a special blower system. An overview about the test facility and some recent results will be presented.

Abstract: 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.

Abstract: For building integration, concentrating photovoltaic systems (CPV) can offer a host of advantages over conventional flat panel devices, the most notable being: a higher electrical conversion efficiency in the PV cells, better use of space, ease of recycling of constituent materials, and reduced use of toxic products involved in the PV cells’ production process. However, the viability of building-integrated concentrating PV systems (BICPV) is dependent on their ability to offer a comparative economic advantage over flat panel photovoltaic technologies whose market prices are decreasing from day to day (<1.8 € / Wp) and which offer other advantages such as ease of replacement of structural elements. A comparative analysis is presented of the main existing CPV systems’ suitability for use in buildings, in which the different challenges specific to integration of each system are discussed. The systems are categorised by type of concentration technology and concentration factor. Two further sets of BICPV systems are proposed, one refractive and one reflective, which we consider well adapted for use in buildings in that they are cost and space efficient, structurally practical and conserve architectural harmony.

Abstract: Concentrating photovoltaic (CPV) systems based on III-V semiconductors entered the photovoltaic market recently. Concentrix Solar deploys the FLATCON® CPV module technology originally developed at the Fraunhofer Institute for Solar Energy Systems ISE. This paper gives a short introduction in design considerations of FLATCON CPV module and provides detailed field data of the system technology over up to two years are presented as well as field experiences with the operation of the systems. FLATCON modules are based on III-V triple junction cells, a Fresnel lens array, and a glass cover and bottom plate. In 2008, Concentrix installed a single tracker system and two power plants in Spain. A system solar-to-grid AC energy efficiency of 21% over the whole period of two years with maximum system AC power efficiency values of 23% for a single tracker system are shown. Similar systems installed in 2009 and equipped with the current FLATCON module CX-75 achieved system AC power efficiencies of 25%. The effects of spectral variations of the solar irradiation on power plant performance are discussed.

Abstract: In an previous research project a new type of greenhouse with an integrated concentrated photovoltaic system (CPV) was developed which has an integrated filter for reflecting the near infrared radiation (NIR) to the greenhouse and exploiting this radiation in a solar energy system. The performance of the system was promising. In this study further optimalisations of the CPV system are made to avoid the large construction for solar tracting. Hereto all parts will be integrated into the greenhouse. The NIR-reflector material is carried out as a NIR-reflective lamllea system and the CPV–module is mounted into the ridge. In this paper the results of the optimization process of the CPV system based on NIR reflecting lamellae is presented. The optimization process is based on a maximal total annual electricity production and is performed with a ray tracing model and actual radiation data. Results show that the optimization of the lamellae greenhouse can be seen from a theoretical and a practical point of view. Theoretically, the number of lamellae for the investigated concept must be high (>100) and focus with a generic focal length of 3.5 m and glazing bars must be avoided. Then the maximal annual electricity output can be over 26 kWh/m². In practice, mechanical restrictions, plant conditions and costs will determine the implementation. The proposed CPV-system has positive side-effects like reducing the heat load (and need for cooling) during summer and blocking of the direct radiation which can be harmful for some crops. With this, the feasibility of the system depends greatly on local conditions which require a tailor-made economical analysis.

Abstract: High-temperature thermochemical processes efficiently convert concentrated solar energy into storable and transportable fuels. In the long run, H2O/CO2-splitting thermochemical cycles based on metal oxide redox reactions are developed to produce H2 and CO, which can be further processed to synthetic liquid fuels. In a transition period, carbonaceous feedstocks (fossil fuels, biomass, C-containing wastes) are solar-upgraded and transformed into valuable fuels via reforming, gasification and decomposition processes. The most promising solar thermochemical processes are discussed and the latest technological developments are summarized.

Abstract: Concentrating Solar Power Technology (CSP) is nowadays growing mainly due to the technical and economic success of the first projects and to the stable green pricing or support mechanisms that bridges the initial gap in electricity costs (i.e. feed-in tariffs). Future growth will depend on a successful cost reduction and on a strong effort in R&D to optimize the potential for technical improvement [1]. Testing of new materials, components and systems is still of key importance to drive research and innovation improvements to a commercial stage. Receiver manufacturers are investing in R&D in order to improve performances and reduce costs, while project developers are demanding standards to help them evaluate satisfactorily the risks and the benefits of introducing new developments in commercial power plants. The Solar Thermal Energy Department, of the National Renewable Energy Centre (CENER) and the Applied Optics Department of the Universidad de Zaragoza (UZ) have joined efforts to develop a characterization equipment able to measure as far as possible most of the receiver optical and thermal properties. In this paper the testing facility developed by CENER-UZ is described technically. The methodology for optical and thermal characterization of solar receivers for parabolic trough collectors is explained and the preliminary results are presented and discussed in detail.