The CSP2 project proposes to use dense gas-particle suspensions -DPS- (approximately 30-40% of solid) in tubes as alternative heat transfer fluid (HTF) in order to extend working temperature and to decrease environmental impact of standard HTF used in concentrating solar power (CSP) plants. The assembly of these tubes will constitute the solar absorber (receiver) placed at the top of a central receiver CSP system. The radiation absorbed by these tubes submitted to the concentrated solar radiation is transmitted by conduction, convection and radiation towards the suspension, which warms up by contact with the hot walls. This suspension circulates easily between the inlet and the outlet of the solar receiver and transports the absorbed energy towards the energy storage. Therefore this HTF enables to dissociate the production of electrical or thermal energy from the solar energy collection (as currently done in molten salts CSP plants, and cannot be done with other HTF such as gas or steam). The proposed HTF is totally safe; it permits higher possible operating temperature (700°C) and does not risk solidification at low temperature. The project includes fundamental studies on particle hydrodynamics and heat transfer in small diameter tubes, construction and testing of a laboratory 150 kW solar pilot and process scaling up to different sizes.
In the frame of the project, the influence of the various operating hydrodynamic parameters was studied. The behavior of particles in the vertical tube was pointed by tracking technique (PEPT as Positron Emission Particle Tracking). There exists a significant downward particle flow near the tube wall. Three models were developed in parallel: 1/ A simplified engineering model for the preliminary design of 57 MWth and 290 MWth solar receivers. 2/ A detailed two-phase Euler-Euler model for dense gas- particle systems to investigate the heat transfer in slowly rising bubbling fluidized beds used as HTM for CSP plants. 3/ A detailed heat and mass transfer model to predict the flow and heat transfer behavior of the DPS inside sun rays-impinged tubes.
The Consortium implemented successfully on-sun the pilot solar loop in the CNRS 1MW solar furnace. One hundred hours of on-sun experiments were run totally, with experiments lasting up to 8 hours continuously and solid flow rate in the range 0.660-1.760 T/h. Particle outlet temperature up to 700°C (500°C in average) was obtained, and calculated cavity thermal yield was up to 90%.
Two reference cases were considered for scale-up studies: 57 MWth and 290 MWth. The simplified model was used for integration and annual performance studies of various thermodynamic cycles. It shows the potential for plant efficiency increase of Brayton s-CO2 and combined cycles. The scale up and economic assessment was developed for a 10 MWe CSP plant, operating with Rankine cycle, based on typical meteorological year in Ouarzazate 2014. The plant construction total cost and O&M cost were estimated. The environmental impact assessment showed such a power plant suffers very limited powder-related environmental effects, no explosion risk or fire hazards, and very low equipment erosion because of the low gas velocities. Together with both expected improved thermal efficiency and reduced parasitics, the CSP2 concept can be considered as environmentally friendly.