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High Temparature concentrated solar thermal power plan with particle receiver and direct thermal storage

Periodic Reporting for period 3 - NEXT-CSP (High Temparature concentrated solar thermal power plan with particle receiver and direct thermal storage)

Reporting period: 2019-10-01 to 2021-07-31

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.
Solar process development

A complete particle-CSP pilot plant using the fluidized particle-in-tube solar receiver concept was designed, manufactured and tested. Olivine particles with 60 μm mean diameter were used. The pilot system is composed of a 2.5 MWth solar receiver, a fluidized hot store hopper, a particle-to-compressed air heat exchanger, a bucket elevator and a cold store bin. Forty 3 m long vertical tubes constitute the absorber of the solar receiver. The hot store hopper can store 3 tons of particles. The heat exchanger is composed of 1400 horizontal tubes immersed in a 6-stage cross-flow fluidized bed. The cold store can contain 4.5 tons of particles. All the components are assembled on a metallic frame atop the Themis tower. The prototype is instrumented with 128 K-type thermocouples, 43 differential pressure sensors, 7 pressure sensors, 10 valves and 10 flowmeters.

Solar reactor testing

The test campaign was performed in the following experimental conditions: Particle mass flow rate, 0.6-3.5 kg/s; Psolar, 550-850 kW; resulting in a particle temperature gain in the range of 100-400 K and a 40-60% thermal efficiency.

Scaling up and solar process integration

The upscaling work for a 150 MWe utility-scale peaker plant to be implemented in Ouarzazate includes the solar field layout, the solar receiver modeling, the particle storage and conveying, the heat exchanger design and the power cycle. The plant considered is a peaker with a capacity factor of about 17%. In comparison to a base load plant with a 63% capacity factor, its LCOE is 46% higher, but the value of the power generated can be more than 3 times higher. The estimated 2030 LCOE is 102 €/MWh.

The main contribution to the environmental impact of a Next-CSP based power plant is in the manufacturing and construction phase. However, the analysis concludes that the concept results in improving the overall environmental impact compared to traditional CSP systems.

Exploitation and dissemination

The consortium has identified a list of KER and most of the project partners have projects for further research and commercialization. However, intermediate steps are necessary for the industrial development of the Next-CSP concept (i.e. operation of a demo-scale system).
The fluidized particle-in-tube solar receiver concept was demonstrated for the first time at the MW-scale. With respect to other particle-CSP technologies, the Next-CSP prototype is the unique integrated system including all the components of a CSP plant: solar receiver, storage vessels, particle vertical conveying, particle heat exchanger and turbine (hybrid gas turbine).

Experiments with a 50 mm I.D. tube revealed that high heat transfer rates (>800 W/m²K) are achieved in receiver tubes up to 8 m in height when in-bed bubble rupture promoters are included in the receiver tubes to avoid slugging hydrodynamics.

Flexibility of CSP with built-in storage provides value to the grid. Particle TES reveals to be significantly cheaper than current molten salt technology. Compared to a PV farm equipped with batteries, a particle driven CSP tower is a cheaper solution (with a 2030 horizon) to shift all the energy collected during the day and generate electricity during 4 evening peak hours.
Solar receiver tubes without the cavity
Installation of the solar receiver tubes and the cold store
Next-CSP in operation
2.5 MWth particle solar receiver during on-sun test campaign
Particle bucket elevator, hot store and particle-to-pressurized air heat exchanger installed
Solar receiver and drone equiped with IR camera
Particle pilot loop components assembled at Themis solar tower
Hybrid GT connected to the particle heat exchanger
Lifting of the solar receiver particle dispenser atop the Themis tower