Linear Fresnel Collector Receiver: Heat Loss and Temperatures

For design and component specification of a Linear Fresnel Collector (LFC) cavity receiver, the prediction of temperature distribution and heat loss is of great importance. In this paper we present a sensitivity analysis for a range of geometry and material parameters. For the LFC receiver analysis we use two models developed at Fraunhofer ISE. One is a detailed model, combining the spatial distribution of reflected radiation via ray tracing with detailed convective simulations through computational fluid dynamics. The second one is a fast algorithm based on a thermal resistance model. It is applying a similar methodology as the well-known model for vacuum absorber, enhancing an absorber tube model by parameters describing the influence of the secondary mirror and cover glass. The thermal resistance model is described in detail. Obtained results indicate a significant effect of the secondary mirror temperature on heat loss for specific geometries

Extended Heat Loss and Temperature Analysis of Three Linear Fresnel Receiver Designs

Heat loss prediction models for parabolic trough receivers do not consider the thermal effect of a secondary mirror. As an extension a Thermal Resistance Model (TRM) has been developed at Fraunhofer ISE for the prediction of the heat loss of three different Linear Fresnel Collector (LFC) receiver configurations. In previous investigations we have found the energy balance of a LFC receiver to be strongly influenced by the amount of solar radiation absorbed by the secondary mirror. This absorption provokes an increase of temperature of the secondary mirror and hence a decrease in the total amount of heat loss of a LFC. The size of this effect depends on the receiver geometry and diverse ambient parameters. Investigated parameters are wind velocity, ambient temperature and Direct Normal Irradiance (DNI). This dependency and its effect on heat loss and secondary mirror temperatures are analyzed for three different LFC receiver configurations. As the radiation absorbed by the secondary mirror is affected by the aperture area of the LFC, studies are performed for small-scale and for large-scale collectors

Trombe walls with nanoporous aerogel insulation applied to UK housing refurbishments

There is an opportunity to improve the efficiency of passive Trombe walls and active solar air collectors by replacing their conventional glass covers with lightweight polycarbonate panels filled with nanoporous aerogel insulation. This study investigates the thermal performance, energy savings, and financial payback period of passive Aerogel Trombe walls applied to the existing UK housing stock. Using parametric modeling, a series of design guidance tables have been generated, providing estimates of the energy savings and overheating risk associated with applying areas of Trombe wall to four different house types across the UK built to six notional construction standards. Calculated energy savings range from 183 kWh/m2/year for an 8 m2 system retrofitted to a solid walled detached house to 62 kWh/m2/year for a 32 m2 system retrofitted to a super insulated flat. Predicted energy savings from Trombe walls up to 24 m2 are found to exceed the energy savings from external insulation across all house types and constructions. Small areas of Trombe wall can provide a useful energy contribution without creating a significant overheating risk. If larger areas are to be installed, then detailed calculations would be recommended to assess and mitigate potential overheating issues.The EPSRC, Brunel University, and Buro Happold Lt

Optimization problems of transparently insulated systems

The optimization of a honeycomb transparent insulation material (TIM) combined with a selectively coated integrated storage collector (ISC) is discussed in this paper. In order to find the optimum material, characterized by the heat loss coefficient Lambda (T,D) and solar transmittance tau (Phi, phi;D), a theoretical model of the honeycomb absorber system for heat transport and solar transmission will be used. The modelling of the ISC itself is solved by a rather simple short-term simulation of representative days of each month (see SCHMIDT et.al./1/ for a description of reals ISC's). The optimization aim is to change parameters of the TIM in order to find the optimum balance between solar gains and heat losses, which results in a maximum yearly output or a minimum auxiliary energy demand for a one square metre collector. Two cases will be considered; The effect of two different scattering mechanisms in determing the combination of mass, mass distribution and both aspect ratios for rec tangular honeycombs

A learning curve for solar thermal power

Photovoltaics started its success story by predicting the cost degression depending on cumulated installed capacity. This so-called learning curve was published and used for predictions for PV modules first, then predictions of system cost decrease also were developed. This approach is less sensitive to political decisions and changing market situations than predictions on the time axis. Cost degression due to innovation, use of scaling effects, improved project management, standardised procedures including the search for better sites and optimization of project size are learning effects which can only be utilised when projects are developed. Therefore a presentation of CAPEX versus cumulated installed capacity is proposed in order to show the possible future advancement of the technology to politics and market. However from a wide range of publications on cost for CSP it is difficult to derive a learning curve. A logical cost structure for direct and indirect capital expenditure is needed as the basis for further analysis. Using derived reference cost for typical power plant configurations predictions of future cost have been derived. Only on the basis of that cost structure and the learning curve levelised cost of electricity for solar thermal power plants should be calculated for individual projects with different capacity factors in various locations

Parametrising a transparent insulation wall system in a simulation program - studies for a convective system

A characterisation method for TI wall systems was developed. Existing numerical building simulation programs can easily be extended by using this method e.g. as a subroutine. Any given transparent insulation system is treated as a "black box" system mounted on a house wall. In a similar way to TI materials, it can be described by a systematic energy gain factor gsubsys and a systematic u-value usubsys. A ventilated TI wall system was parameterised and thus included in a building simulation program. The results show that the complex convective heat transfer can be described with the parameters introduced above. Here gsubsys as well as usubsys are dependent on the wall temperature, the ambient temperature and the solar irradiation

Total energy transmission of transparent insulation material

Transparent insulation materials for solar energy applications are classified and described in this paper. An experimental set-up with an integration sphere for the whole global spectrum range having a large entrance port is capable of measuring angle-dependent directional-hemispherical transmittances for planar samples and non-planar structures with acceptable accuracy up to about 70 degrees. Especially for structures such as honeycombs or sun-protecting glasses, which may absorb in the solar range, the total energy transmission is markedly higher than the solar transmission because of additional heat flux towards the absorber. A measuring device, currently in development, is presented