Reiner Buck

Concentrating solar power plants are considered an important contributor to renewable energy supply due to their high efficiency and cost-effective storage. Solar tower systems using solid particles as a heat transfer and storage medium, in combination with concentrating photovoltaics (CPV), promise to reduce the levelized cost of energy. This study evaluates the cost potential from hybridizing solar power plants with CPV modules integrated into the radiation shield of a particle receiver, to capture spillage radiation that is otherwise lost. Simulation results for a commercial configuration case show that the annual electricity production could be increased by up to 12%, with levelized cost of electricity from the CPV part ranging from 0.02 to 0.024 €/kWh. The electric components of the CPV subsystem have by far the highest single component cost contribution, the cooling water loop adds a minor contribution. Further improvement options exist and are discussed briefly. A demonstration test in a real receiver application is currently under preparation in DLR’s solar tower test facility. Six CPV modules will be integrated into the radiation shield of a particle receiver. Solar testing will validate the performance and also provide information about potential dust contamination problems.

The next generation of central receivers is expected to reach high outlet temperatures of 800 °C and above, maintain stable operation and react to a varying incoming flux magnitude caused by hourly and seasonal variations, endure high-temperature operation, and remain cost-effective. Particle-based central receivers are being considered due to their high-temperature durability and favorable thermal properties of particle materials such as bauxite particles. This paper describes a new particle-based central receiver concept and provides an initial exploration of its performance in comparison with the existing CentRec® technology. Discrete Element Method (DEM) simulations demonstrated that a stable falling and sliding particle film can be achieved inside the SlideRec. The residence time of particle flow within the SlideRec is estimated to be higher than a falling particle receiver and similar to that of an obstructed-flow receiver. The results of the thermal model (incorporating reflective, radiative, convective and conductive heat losses) indicate that the SlideRec demonstrates a higher thermal efficiency than the CentRec® under an incoming aperture flux of between 0.1 MW/m2 and 1.7 MW/m2. The concept therefore is promising and is recommended for further experimental exploration.

The centrifugal solar particle receiver (CentRec) is a promising design compared to other particle receiver concepts because it allows for an active adjustment of particle residence time and particle outlet temperature by adjusting the rotational speed of the drum and particle mass flow rate. A Discrete Element Method (DEM) tool is utilized to model the particle flow in CentRec. However, the numerical modeling of the particle flow in large scale receiver is computationally infeasible because of excessive number of particles in the simulation. Thus, in this study, a scale down approach is developed and validated to be used to estimate the particle film characteristics in large scale receivers. The particle velocity profile and film thickness distribution are employed to compare different receiver sizes.

In this work, two different methods for measuring the efficiency of central receivers are analyzed by the case of the High Performance Molten Salt II Project (HPMS-II): the continuous power-on method, and the semi-analytical method. The main difference between the two methods is the procedure to calculate the thermal losses of the receiver: on the one hand, the continuous power-on method calculates the thermal losses from the measurement of the absorbed power by the molten salt for different measured incident powers on the receiver. Here, it is assumed that the thermal losses are independent of the incident power if the molten salt temperature is kept constant. On the other hand, the semi-analytical method calculates the thermal losses as the sum of convective and radiative losses, calculated directly from the Newton and Stefan-Boltzmann equations by measuring the temperature of the tube surface, the ambient temperature, and the wind speed. Therefore, the calculation of the thermal losses is independent from one method to another. The procedure of applying these methods during the experimental test campaign of the HPMS-II receiver is detailed in this paper. Additionally, an uncertainty analysis of both methods is conducted to determine the uncertainty expected for the receiver efficiency measurements.

A numerical model of the CentRec® receiver has been developed and validated using the measurement data collected during the experimental test campaign of the centrifugal particle system at the solar tower Jülich. The model has been used to calculate the thermo-optical efficiency of a scaled-up 20 MWth receiver for various receiver geometries. A cost function has been deduced and was used to perform a techno-economic study on an levelized cost of heat (LCOH) basis of the CentRec® receiver concept. Attractive LCOH as low as 0.0209 €/kWhth for a system with thermal storage, or as low as 0.0150 €/kWhth for the LCOH without storage, is predicted. This study has shown that the optimal configuration from an LCOH perspective for a 20 MWth centrifugal particle receiver reaches specific receiver costs of 35 €/kWth. Hereby, the costs of the receiver can be reduced by 60% compared to the original configuration.