• Energy Research

Solar Tower Technology is a promising way to generate sustainable electricity from concentrated solar radiation. In one of the most effective variants of this technology, a so called volumetric air receiver is used to convert concentrated radiation into heat. This component consists of a high temperature resistant cellular material which absorbs radiation and transfers the heat to an air flow which is fed from the ambient and from recirculated air. It is called volumetric, because the radiation may penetrate into the “volume” of the receiver through the open, permeable cells of the material. In this way a larger amount of heat transfer surface supports the solid to gaseous heat transfer in comparison to a tubular closed receiver. Finally the heated air is directed to the steam generator of a conventional steam turbine system. In this study an advanced cellular metal honeycomb structure has been designed, manufactured and tested for use as an open volumetric receiver. It consists of winded pairs of flat and corrugated metal foils. The technology is based on a one which has been primarily developed for the treatment of combustion engine exhaust gases. A number of variations of the pure linear honeycomb structure have been introduced to increase local turbulence and radial flow. Firstly, a set of samples has been tested in laboratory scale experiments to determine effective properties and the solar-to-thermal efficiency. After that, results have been compared with theoretical predictions. Finally, the three most promising materials have been used for a 500 kW test on the research platform of the Solar Tower Julich. Air outlet temperatures of more than 800 °C have been achieved with efficiencies of about 80%, which is about 5% more than the state-of-the-art technology, which is currently used at the main receiver of the Solar Tower. Next to this, lifetime models will be developed to increase the overall reliability of the technology.

Abstract Solar towers with volumetric air receiver technology are a promising technology for continuous or demand oriented electricity supply from a sustainable resource with competitive costs. The absorber elements used for this receiver technology are permeable ceramic honeycombs, which are heated up by concentrated solar radiation. Afterwards the heat is being transferred to air, which finally feeds the steam generator of a Rankine-Cycle. Resulting from the intensive solar flux of up to 800 kW/m2, the absorber modules are exposed to severe thermal loads. Maximum temperatures of more than 1000 °C are reached. Furthermore, during daily start-up and shut down procedures and additionally during cloud transitions the absorbers undergo transient heat loads, which might reduce their lifetime. The objective of the present study is to quantify the effect of typical thermal loads and to calculate the resulting mechanical stresses inside the absorber modules. Two types of absorbers have been compared, a conventional coarse honeycomb and an advanced one with a higher cellularity offering an improved efficiency potential. The mechanical strength of the cellular material is used to find out whether the computed thermomechanical stresses are critical or not. Finally, experimental thermo-shock tests have been carried out to validate the numerical models. It could be shown that under normal operation the thermomechanical stresses remain in a range markedly below the fracture stress in both absorber variants. As a consequence, limits are given to what extent the material should be thermally loaded during operation.

Tubular Si-infiltrated SiCf/SiC composites composed of an inner cellular ceramic and an outer dense Ceramic Matrix Composite (CMC) skin have been fabricated by the electrophoretic deposition of matrix phases followed by Si-infiltration for pre-feasibility testing in solar receiver applications. The tubes have been considered to be used as high temperature receiver components for the solar operation of a gas turbine or a combined cycle with temperatures up to 1300 °C and typical pressures of more than 6 bar. The cellular structure inside the tube has been introduced for the improvement of heat transfer from the irradiated outer surface of the tube to the working fluid inside. Heat transfer and permeability characteristics of the composite samples have been determined experimentally as effective properties. These properties have been used in numerical models to predict the performance of such kind of components in gas turbine service conditions. It could be demonstrated, that the heat transfer rate in a tube with a porous in-lay could be increased to approximately four times compared to the rate of an empty tube of the same size. The results of the study give reason for further experimental testing in service environments.

Abstract Hydrogen production by solar-driven 2-step reduction–oxidation cycles has been successfully demonstrated in recent years. While the demonstrated efficiencies were quite low (up to 5.25% solar-to-fuel efficiency), there is a large potential for much higher efficiencies. This paper presents a detailed parametric investigation of a large scale (>100 kW) volumetric solar receiver-reactor, performed using a finite volume method model. The reactor temperature profile and extent of reduction are investigated by evaluating the effects of heat recovery, sweep gas mass flow, radiation flux and porosity. The current reactor concept is shown to have a temperature gradient across the absorber of 500–700 K, leading to a mean ceria reduction extent of 0.004–0.012, thus limiting the amount of hydrogen that can be generated to the order of several milligrams per cycle or less. It is also shown that heat recovery can reduce the temperature gradient across the absorber to 350 K, thus increasing the hydrogen generation significantly, up to four orders of magnitude higher than with no heat recovery (up to 5.25 g/cycle). The solar heat flux on the receiver can also significantly increase the hydrogen production and reduce the reduction step duration. Thus, the optimization of both design and operation of large volumetric reactors can increase the hydrogen generation rate significantly.

This paper reports on the numerical analysis of a volumetric solar receiver-reactor for hydrogen production, using the 2-step reduction–oxidation cycle. A detailed parametric sweep covering hundreds of various parameter combinations is performed for a large solar reactor, using a transient physical model. We generate performance maps which are currently cost prohibitive via experimental or high–fidelity simulation studies. The following performance metrics are evaluated: solar to fuel efficiency, hydrogen yield, conversion extent and specific hydrogen yield. We show that the relations between the different performance metrics are complex, leading to different optimal points depending on the metric pursued. The daily hydrogen yield for a single reactor varied between 0.89 kg for an absorber thickness of 30 mm, and up to 1.04 kg for a 60 mm thick receiver, with solar to fuel efficiency values of 3.84% and 3.81% respectively. For a case with 45 mm thick receiver, an intermediate hydrogen yield of 0.94 kg is calculated, while exhibiting the highest efficiency (4.05%). The efficiency can be further increased to 5.86% by using a simple heat recovery system, and reach an upper limit of 21.16% with a more sophisticated heat recovery method. Renewable Energy, 179 ISSN:0960-1481 ISSN:1879-0682