Overall objectives and importance for society
The Next-CSP project aims at developing and testing a new generation (Gen3) of Concentrating Solar Power (CSP) plant using particle suspensions as heat transfer and storage medium. Thus, the concept provides the same benefits as molten salt (direct thermal energy storage, TES) with the capacity to operate at higher temperature, 700 °C and more. The Next-CSP complete system is composed of all the components of a CSP plant, including a heliostat field, a solar receiver, a heat storage system, a particle-to-working fluid heat exchanger and a gas turbine. The gas turbine of the pilot plant features a supplementary firing. The solar receiver developed at pilot scale (2.5 MWth) uses the fluidized particle-in-tube technology, an indirect particle-heating concept.
This particle-CSP technology can contribute to the security of renewable electricity supply to the grid offering a dispatchable production capacity thanks to cheap TES. Moreover, the efficiency of the power cycle can be improved by 15-20% with respect to cycles suited to the temperatures allowed by current central receiver technology, thus reducing the cost of electricity.
Issues addressed during the project
- Choice of particles with respect to physical, thermal, mechanical and health properties.
- Design and manufacturing of the Next-CSP prototype components.
- Assembly of all the components installed atop the Themis tower (an already existing 5 MWth central receiver solar facility) accounting for space limitation.
- Implementation of the control instrumentation including a drone equipped with an IR camera.
- Management of the flow of particles: as particles are not properly speaking a fluid, specific requirements must be met in terms of pressure balance and component geometry.
- Testing the solar receiver without overheating the metallic absorber tubes.
- Scaling-up issues: heliostat field performance, maximum size of the solar receiver, particle conveying and associated thermal losses and cost, heat exchangers, advanced power block.
- Electricity cost (LCOE) of a 150 MWe plant operating in peaker mode.
- Environmental impact of the technology by comparison with current molten salt CSP towers.
The main conclusions
The solar receiver technology was validated successfully at the MW scale by conducting tests at the Themis solar tower, where the complete particle loop was operated in a closed circuit. Manufacturing of such a complex particulate system was a technological challenge in terms of production and integration. Upscaling of the fluidized particle-in-tube solar receiver accounting for tube height limitation concluded that the maximum single unit power is approximately 50 MWth with an efficiency in the 80-85% range. Therefore, this finding imposed a multi-tower option for large commercial scale power plants (typically 150 MWe) operating in peaker mode. This option included 6 to 8 solar towers sharing the same storage system and power block, the particles heated in each tower being conveyed horizontally between the towers and the particle hoppers. Hot particles conveying was identified as a challenge that leads to heat losses and additional CAPEX and OPEX costs, affecting the final cost of electricity.
An innovative multistage fluidized bed heat exchanger was manufactured, implemented and validated. The assessment of advanced cycles to be integrated in the commercial plant revealed that combined cycles are most probably not the best solution, due to the very high temperatures needed and the cost and bulkiness of the heat exchangers. Steam or sCO2 cycles could offer good efficiencies at more moderate temperatures, with positive impacts on the efficiency and cost of the whole plant. In particular, USC steam cycles can allow for a higher storage density and are less penalized by high cooling temperatures (CSP plants are generally built in desert areas).
The cost reduction allowed by particle heat storage offers a real opportunity: multi-day storage can be envisioned.
