New central receiver system concepts

Short description

Combined cycle plants, i.e. the coupling of topping Brayton cycles with bottoming Rankine steam cycles, are the state-of-the-art solution for providing high thermal-to-electric conversion efficiencies that reach values around 58%. Thus, an obvious step in engineering is the application of combined cycles in solar power generation (Solar Combined Cycles -SCC), i.e. the application of solar-driven Brayton cycles combined with bottoming Rankine steam cycles driven by the relatively hot exhaust gas stream of the topping Brayton cycle.

Although the concept of solar combined cycles has at least been discussed since the 1990’s, it is still far away from commercialization. On the one hand, high temperatures as well as frequent transients due to varying solar direct normal irradiance require advanced and innovative materials for the solar receiver. On the other hand, due to operational aspects of the solar-driven combined cycle and up-scaling issues of central receiver heliostat fields (atmospheric attenuation and spillage), a considerable amount of research work is left in order to introduce this technology at commercial level.

Developments at the Solar Thermal Energy Department at CENER address the latter two issues by means of an advanced decoupled solar combined cycle configuration, applying multiple small-scale central receiver (Brayton cycle) units with air as working fluid, and one single centralized bottoming Rankine steam cycle that is decoupled from the topping Brayton cycles via the thermal energy storage (TES) system. This means that the exhaust gases of the Brayton cycles charge the TES system and the subsequent Rankine cycle is powered via discharging the TES system. This decoupled multi-tower approach features substantial advantages regarding operational aspects and solar field efficiency. In particular, the decoupling via the TES system enables a solar-irradiation-independent operation of the centralized Rankine cycle, avoiding frequent transients and inefficient partial loads; it is either running at full nominal load or not at all.

On the other hand, the modular multi-tower concept implies relatively small and thus efficient solar fields, due to lower atmospheric attenuation and lower spillage losses.

J. García-Barberena, A. Monreal, A. Mutuberria, M. Sánchez, Towards Cost-competitive Solar Towers – Energy Cost Reductions based on Decoupled Solar Combined Cycles (DSCC), Energy Procedia, 49 (2014) 1350-1360.

Main technical features

The receiver technology of the innovative plant concept is based on open volumetric air receivers, which have proven to be very robust in operation and easy to operate. The big advantage of air as HTF is that there is no temperature limitation, nor degradation over time.

The coupling with turbine (pressurized air stream) is planned to be done via a regenerative heat exchanger that is charged under atmospheric conditions and discharged pressurized.

The most cost-effective option for the centralized TES is the thermocline packed-bed (rocks or ceramics) technology, thus avoiding bulky and very costly air-to-molten-salt heat exchangers between topping gas turbine and centralized TES.

The bottoming Rankine steam cycle can then be powered by conventional state-of-the-art heat recovery steam generators (HRSG).

Innovative aspects

Currently, ceramic SiC foam is being studied as volumetric absorber, showing better thermal efficiencies than ceramic honeycomb absorbers.

The topping gas turbine is planned to operate in solar-only mode, without the additional combustion of conventional fuels.

Applications

The power plant concept is currently being evaluated in the framework of the H2020 research project CAPTure “Competitive SolAr Power Towers” (http://capture-solar-energy.eu/ ).

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    Contact person details
    Details
    Showcase ID
    JP.EERA.316:New central receiver system concepts
    Joint Programme / IRP / ECRIA
    IRP STAGE-STE
    Development stage
    Development phase – Laboratory tested
    IPR / IPR Status
    Trade secret (not applicable)
    Exploitation status
    N/A
    Technology keyword(s)
    Central Receiver Systems,Central receivers modelling,High concentration point focussing systems