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Competitive SolAr Power Towers – CAPTure

Periodic Reporting for period 3 - CAPTure (Competitive SolAr Power Towers – CAPTure)

Reporting period: 2018-03-01 to 2020-07-31

The world's power demand is currently of about 15 terawatts. Most of it is provided by burning fossil fuels, which has a severe impact on the environment and on the world’s climate. Given the fact that the sun provides roughly 85 petawatts, available for terrestrial solar collectors, makes solar-energy-based technologies a highly promising research area. Especially the concentrated solar power (CSP) sector, also known as solar thermal electricity (STE), with the potential of storing thermal energy in very cost effective manner, is expected to play a key role regarding the integration of large fractions of renewable energy in the power grid. Energy storage will be the key for a sustainable future.

The principal aim of the CAPTure project is to bring the central receiver technology to the next level, introducing the combined cycle technology to concentrated solar power. In particular, the CAPTure project tends to combine highest thermal-to-electric conversion efficiencies (typically known from modern combined-cycle plants that are in the order of magnitude of 58%), with the concentrated solar power technology with the highest cost-reduction potential and also best potential for advanced solar combined cycles (the solar power tower technology).
Project partners developed 3 key components to achieve higher conversion efficiency: a solar receiver, a regenerative heat exchanger and a solar field (mirrors and trackers). The mirrors concentrate the sun onto the solar receiver that heats atmospheric air to high temperatures (1000 °C). Heat is exchanged between the pressurised air flow and the atmospheric air flow from the solar receiver by the regenerative heat exchange system. A hot air turbine then converts heat into electric energy. CAPTure’s most significant achievement was the successful design, production and commissioning of a 300-kWth prototype installed at a solar research facility in southern Spain. It contains all three components, as well as pipes and valves. In addition, the CAPTure team designed a downsized, sun-tracking mirror or heliostat that has been optimised for cost-effective mass production.

The CAPTure concept has been optimized techno-economically and benchmarked. Unfortunately, the simulation results are not as expected. The combined cycle configuration has despite its higher solar-to-electric conversion efficiency a higher LCOE than the simpler and less efficient Rankine single cycle plant concept. The gain in electricity yield is not enough to outweigh the higher investment costs of the more complex combined cycle plant layout. The combined cycle configuration seems to be competitive only at smaller power classes. The combined cycle seems to be only attractive for very small power tower plants (below 5 MWe), which is also in line with the up-scaling constraint given by the CAPTure regenerative system’s valves. Although gas turbines can be scaled down quite well having a reasonable performance at small power classes, this is not the case for Rankine steam cycles. Hence, when thinking of very small (i.e. “micro”) combined cycles, the application of the organic Rankine cycle (ORC) as bottoming power cycle should be considered. This concept could be attractive for small and modular CSP central receiver plants for “electricity islands”, i.e. small remote grids, where electricity price is very high. In this context, it should be noted that this outcome is very much in line with the objectives of the H2020 research project POLYPHEM.
PROGRESS BEYOND THE STATE OF THE ART:

The development and application of solar combined cycles allows to boost conversion efficiencies of solar thermal power plants. The CAPTure concept, represents here clearly a huge breakthrough
regarding state-of-the-art technology. Solar-to-electric peak efficiencies of up to 30% are possible, and this in dispatchable form due to the high-temperature thermal energy storage system – CAPTure’s dispatchable solar combined cycles. Unfortunately, at this point in time, the proposed power plant layout is not profitable for large-scale, high-capacity deployment; however, it will be very beneficial for a range of uses. The developed components are highly valuable for several applications. In particular, the solar receiver and regenerative system can be used for high-efficiency solar-heat integration in several processes. The regenerative system can work as a means of high-temperature thermal-energy storage, cost-effective heat exchanger between atmospheric and pressurised air circuits. It can also be used as a chemical reactor for either thermochemical energy storage with high-energy density or other types of high-temperature reactors.


IMPACT:

- Replicability: The CAPTure consortium includes two companies with expertise in high temperature materials (both ceramic and metallic), companies with expertise in receivers and turbo-compressor development, companies with expertise in high volume manufacturing (especially in drive systems), companies specialized in flow control solutions at different temperatures and finally reference Engineering, Procurement and Construction (EPC) partners and promoters like EDF and FLAG ensuring strong industrial links with major European players and easily deployable technologies for quick market transfer.

- Socio-economics: The CAPTure project strengthens the European industrial technology base since key and more costly components (not only main heliostat components like drives, but also receivers, turbines, high temperature valves, etc.) of the system will be produced in Europe, implying a quite important demand of different employee skills, representing an intense integration of European enterprises and thereby contributing to creating growth and jobs in Europe.

- Environment: The CAPTure project will allow reducing the total amount of water consumed. On one hand due to the implementation of a combined cycle which demands less water per kWh produced, and on the other hand, because of increased efficiency which leads to an equivalent solar field size reduction for a given power and hence the need of water for cleaning can be reduced.

- Market Transformation: One of the key aspects of CSP tower power plants is the integration of thermal energy storage systems which make them a much more reliable and predictable source of electricity compared to other renewable energy sources. Utilizing stored thermal energy to operate a conventional synchronous generator, CSP plants with thermal energy storage can support power and provide ancillary services including voltage support, frequency response, regulation and spinning reserves, and ramping reserves – services that would otherwise be provided, at least in part, by conventional fossil-fuel generation.

- Policy: Concentrated solar power with its key advantage of thermal energy storage and/or hybridisation in combination with an abundant availability of solar energy, forms the obvious solution for providing clean, renewable, 100% safe and dispatchable electricity all at once. Hence, developing highly efficient and thus cost competitive solar thermal power plants is one of Europe’s strategic goals. The central receiver technology addressed in CAPTure is highly suitable for Europe’s “sun belt” (e.g. Spain, Italy and Greece) with the potential of providing a considerable amount of renewable and clean electric energy in the near future.