SCHOOL OF MECHANICAL ENGINEERING SEMINAR Monday, July 2, 2012 at 15:00 Wolfson Building of Mechanical Engineering, Room 206

M.Sc. student of Prof. Avi Kribus, School of Mechanical Engineering, Tel-Aviv University

Current solar thermal power plants operate at moderate temperatures, usually around 400°C, leading to low conversion efficiencies and consequently to a high and uncompetitive cost of the produced electricity. Hence, increasing the temperature and efficiency of energy conversion of these plants is vital. The talk will center on volumetric receiver, which is considered for use in high-temperature, high performance solar tower power plants. The receiver usually consists of a high temperature (usually ceramic) porous absorber that absorbs the concentrated solar radiation provided by a heliostat field. Gas (usually air) is streamed through the absorber parallel to incident radiation, accepting heat from the absorber by convection. The hot gas later can be utilized directly for electricity production in a gas turbine, or to produce high-temperature superheated steam via a heat exchanger for a steam turbine.

The main problem with the current volumetric receiver technology is that high temperatures are achieved at the aperture of the absorber, leading to high radiation losses and low receiver efficiency. Therefore, choosing the appropriate structure and material properties of the volumetric absorber is crucial. Our purpose is to examine the effect of thermal, geometrical and optical absorber properties on the receiver efficiency, and to suggest the most suitable properties combination so that radiation losses could be reduced. The optimal absorber behavior is obtained when its temperature near the aperture remains relatively low, minimizing the loss of energy by thermal emission, while the desired temperature is still achieved downstream near the exit end of the absorber.

We found that the highest efficiency is obtained with the combination of high porosity, small pore size, and low thermal conductivity. High porosity materials allow the incident radiation to penetrate deep into the bulk and be absorbed gradually. Due to this phenomenon the maximal temperature is reached inside the bulk, causing a temperature gradient in the solid phase. The low thermal conductivity helps to reduce conductive heat flow toward the aperture and keeps the front side temperature relatively low. The small pore size is beneficial to increase the area for convection and keep the fluid temperature close to the solid absorber temperature .

Furthermore, we found that using an absorber material with spectrally selective optical properties also makes a contribution to increase the efficiency. The material should have high absorptivity for short wave radiation (visible and near IR, as found in solar radiation), and low emissivity for long-wave radiation beyond the solar spectrum. The best cutoff wavelength between the two spectral regions depends on the incident flux and the temperature.

High-performance solar volumetric absorbers

Michael Grijnevitch