Robert Pitz-Paal

Porous materials as open volumetric solar receivers: Experimental determination of thermophysical and heat transfer properties

Porous solids like extruded monoliths with parallel channels and thin walls made from various oxide and non-oxide ceramics, ceramic foams and metal structures have been tested in the past with the objective of applying them as open volumetric receivers in concentrated solar radiation. In this application, ambient air flows through the solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. In all cases, to obtain high efficiencies, high absorptivity in the visible and near infrared range has to be combined with a high porosity to create large surfaces for convective heat transfer from the solid absorber to the fluid. However, it can be shown that especially high performance absorbers tend to be sensitive to inhomogeneous flux distributions, which may cause local overheating of the material. In various tests with specific kinds of materials, flow instabilities occurred, which partly leads to hot spots and a sudden destruction of the receiver. To achieve both high efficiencies and reliable operation, an optimised combination of geometrical as well as thermal conductivity and heat transfer parameters has to be selected. A precise knowledge of these quantities for a number of various materials is necessary to estimate the limits for stable flow conditions on the basis of complex numerical simulation programs. Finally, efficiency and performance tests with candidate materials have been carried out. In this paper, the experimental work on a variety of porous materials is reported. The paper will report on methodology and results of thermal conductivity, convective heat transfer coefficient and efficiency measurements of these monolithic materials. It will also present an experimental set-up designed to investigate how the properties of the porous materials affect flow stability. Based on these results, a recommendation for the design of volumetric absorbers will be given.

Trough Integration into Power Plants - a Study on the Performance and Economy of Integrated Solar Combined Cycle Systems

Parabolic trough solar technology has been proven at nine commercial Solar Electric Generating Systems (SEGS) power plants that are operating in the California Mojave desert. These plants utilize steam Rankine cycle power plants, and as a result, most people associate parabolic trough solar technology with steam Rankine cycle power plant technology. Although these plants are clearly optimized for their particular application, other power cycle designs may be appropriate in other situations. Of particular interest is the integration of parabolic trough solar technology with combined cycle power plant technology. This configuration is referred to as integrated solar combined cycle systems (ISCCS). Four potential projects in India, Egypt, Morocco, and Mexico are considering the ISCCS type solar power cycle configurations. The key questions are when is the ISCCS configuration preferred over the SEGS power cycle configuration and how is the ISCCS plant designed to optimize the integration of the solar field and the power cycle. This paper reviews the results of a collaborative effort under the International Energy Agency SolarPACES organization to address these questions and it shows the potential environmental and economic benefits of each configuration.

Experimental and numerical evaluation of the performance and flow stability of different types of open volumetric absorbers under non-homogeneous irradiation

Abstract Numerical results found in the literature predict the capability of volumetric receivers to produce gas outlet temperatures of more than 1000°C. Other approaches found the tendency for an inherent flow instability for volumetric absorbers at high outlet temperatures. Both aspects, which are based on one-dimensional approaches, do not fit the experimental experience available even for semi-commercial volumetric receivers in the case of non-homogeneous flux distribution. Since this is the general case in solar power plants, a new analytical approach which takes into consideration a three-dimensional irradiance distribution and its influence on fluid flow and radial heat transfer is presented in this article. A comparison with experimental data for four totally different volumetric absorber types coincides well with the measured overall thermal efficiency as well as with the local temperature distribution. The model helps to explain how thermal efficiencies and temperature distributions are strongly influenced by non-homogeneous irradiation. Moreover, it demonstrates how flow instabilities are partly prevented by radial heat transfer. Finally, it is shown that a volumetric receiver consisting of sufficiently small absorber modules which are equipped with an additional orifice plate at the rear side will always run under stable flow conditions.

A New Fast Ray Tracing Tool for High-Precision Simulation of Heliostat Fields

A completely new ray tracing software has been developed at the German Aerospace Center. The main purpose of this software is the flux density simulation of heliostat fields with a very high accuracy in a small amount of computation time. The software is primarily designed to process real sun shape distributions and real highly resolved heliostat geometry data, which means a data set of normal vectors of the entire reflecting surface of each heliostat in the field. Specific receiver and secondary concentrator models, as well as models of objects that are shadowing the heliostat field, can be implemented by the user and be linked to the simulation software subsequently. The specific architecture of the software enables the provision of other powerful simulation environments with precise flux density simulation data for the purpose of entire plant simulations. The software was validated through a severe comparison with measured flux density distributions. The simulation results show very good accordance with the measured results.

Materials-Related Aspects of Thermochemical Water and Carbon Dioxide Splitting: A Review

Thermochemical multistep water- and CO2-splitting processes are promising options to face future energy problems. Particularly, the possible incorporation of solar power makes these processes sustainable and environmentally attractive since only water, CO2 and solar power are used; the concentrated solar energy is converted into storable and transportable fuels. One of the major barriers to technological success is the identification of suitable active materials like catalysts and redox materials exhibiting satisfactory durability, reactivity and efficiencies. Moreover, materials play an important role in the construction of key components and for the implementation in commercial solar plants. The most promising thermochemical water- and CO2-splitting processes are being described and discussed with respect to further development and future potential. The main materials-related challenges of those processes are being analyzed. Technical approaches and development progress in terms of solving them are addressed and assessed in this review.

Development Steps for Parabolic Trough Solar Power Technologies With Maximum Impact on Cost Reduction

Besides continuous implementation of concentrating solar power plants (CSP) in Europe, which stipulate cost reduction by mass production effects, further R&D activities are necessary to achieve the cost competitiveness to fossil power generation. The European Concentrated Solar Thermal Roadmap (ECOSTAR) study that was conducted by European research institutes in the field of CSP intends to stipulate the direction for R&D activities in the context of cost reduction. This paper gives an overview about the methodology and the results for one of the seven different CSP system concepts that are currently under promotion worldwide and considered within ECOSTAR. The technology presented here is the parabolic trough with direct steam generation (DSG), which may be considered as an evolution of the existing parabolic systems with thermal oil as heat transfer fluid. The methodology is explained using this exemplary system, and the technical improvements are evaluated according to their cost-reduction potential using a common approach, based on an annual performance model. Research priorities are given based on the results. The simultaneous implementation of three measures is required in order to achieve the cost-reduction target: Technical improvement by R&D, upscaling of the unit size, and mass production of the equipment.

Assessment of Solar Power Tower Driven Ultrasupercritical Steam Cycles Applying Tubular Central Receivers With Varied Heat Transfer Media

For clean and efficient electric power generation, the combination of solar power towers (SPTs) with ultrasupercritical steam cycle power plants could be the next development step. The methodology of the European concentrated solar thermal roadmap study was used to predict the annual performance and the cost reduction potential of this option applying tubular receivers with various appropriate high temperature heat transfer media (HTM). For the assessment, an analytical model of the heat transfer in a parametric 360 deg cylindrical and tubular central receiver was developed to examine the receivers efficiency characteristics. The receivers efficiency characteristics, which are based on different irradiation levels relative to the receivers design point, are, then, used to interpolate the receivers thermal efficiency in an hourly based annual calculation of one typical year that is defined by hourly based real measurements of the direct normal irradiance and the ambient temperature. Applying appropriate cost assumptions from literature, the levelized electricity costs (LEC) were estimated for each considered SPT concept and compared with the reference case, which is a scale-up of the state of the art molten salt concept. The power level of all compared concepts and the reference case is 50 MWel. The sensitivity of the specific cost assumptions for the LEC was evaluated for each concept variation. No detailed evaluation was done for the thermal storage but comparable costs were assumed for all cases. The results indicate a significant cost reduction potential of up to 15% LEC reduction in the liquid metal HTM processes. Due to annual performance based parametric studies of the number of receiver panels and storage capacity, the results also indicate the optimal values of these parameters concerning minimal LEC.

Technologies and trends in solar power and fuels

Solar radiation is the largest indigenous energy resource worldwide. It will gain a significantly more relevant role in covering the energy demand of many countries when national fuel reserves fall short and when demand increases as is expected within the next 10 years. If solar energy is transformed into heat by concentrating and absorbing the radiation, energy can be stored easily. Thermal energy from mirror fields that focus solar radiation not only is able to generate electricity but also can be used to generate storable heat, to desalinate salt water or to synthesise fuels from water and carbon dioxide to store, transport or use them on-site. The application of concentrated solar radiation as a primary energy source can help to decarbonise electricity generation and many other sectors to keep the chance of staying within the 2 °C goal for limiting the effects of global warming. The aim of the present contribution is to give an overview on the state-of-the art of technologies for solar thermal power production and fuel production and to describe the status and outlook of commercial projects and perspectives of market development.

Comparative assessment of solar concentrator materials

Abstract This paper reports results from long-term durability tests of reflector materials to be used for solar concentrating systems. The studies have been conducted under the auspices of an IEA–SolarPACES collaboration between the National Renewable Energy Laboratory (NREL, USA), the Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT, Spain) and Deutsches Zentrum fur Luft- und Raumfahrt (DLR, Germany). In this co-operative effort, accelerated ageing tests as well as outdoor exposures at a number of test sites having various climatic conditions have been carried out since 1995. In addition to materials already in use at solar power stations, newer materials offering the chance of a significant cost reduction in solar electricity and process heat generation are being investigated. Comparative optical tests are carried out to assess the efficiency as a function of exposure/service time in a solar concentrator. Among the materials showing promise for long-term outdoor applications are various silvered glass mirrors, a silvered polymer film, and an anodized sheet aluminium having an additional protective polymer coating. In addition to durability tests of reflector material samples, practical results are also reported for experiences with field applications of silvered thin glass and anodized sheet aluminium mirrors.

Concentrating Solar Power in Europe, the Middle East and North Africa: A Review of Development Issues and Potential to 2050

This paper summarizes the findings of a study undertaken by the European Academies Science Advisory Council to evaluate the development challenges of concentrating solar power (CSP) and its consequent potential to contribute to low carbon electricity systems in Europe, the Middle East and North Africa (the MENA region) to 2050. The study reviewed the current status and prospective developments of the four main CSP technology families, and identified prospective technical developments, quantifying anticipated efficiency improvements and cost reductions. Similarly, developments in thermal energy storage were evaluated, and the role and value of CSP storage in electricity systems were examined. A key conclusion was that as the share of intermittent renewables in an electricity system increases, so does the value of thermal energy storage in CSP plants. Looking ahead, the study concludes that CSP should be cost competitive with fossil-fired power generation at some point in the 2020s provided that commercial deployment continues at an increasing rate, and through support mechanisms that incentivise technology development. Incentive schemes should reflect the real value of electricity to the system, and should ensure sufficient transparency of cost data that learning rates can be monitored. Key factors which will determine CSPs contribution in Europe and the MENA region over the period to 2050 are generating costs, physical constraints on construction of new plants and transmission, and considerations of security of supply. The study makes recommendations to European and MENA region policy makers on how the associated issues should be addressed.