POINT-FOCUS CONCENTRATING SOLAR THERMAL TECHNOLOGIES
Head: Antonio Ávila
The activity of the High Solar Concentrating Group (HSCG) of the Solar Concentrating Systems Unit is mainly focused in Solar Central Receiver systems, but also deals with activities related with other focal systems allowing high solar flux such as Dish-Stirling or Solar Furnaces. The commercial deployment of solar power thermal central receiver plants (STE-CR) started timidly in Spain with the inauguration of PS10 (2007) and PS20 (2009) and GEMASOLAR (2010). At international level there are a renovated interest for Central Receiver technology, with some operating plans, several under construction and numerous in project phase (mainly in USA and South Africa).
Until the start-up of PS10, in full commercial exploitation since March 2007, the “training curve” of the solar thermal central receiver was based on the test of more than 10 experimental central receiver installations over the world, and a large variety of components (heliostats, receivers, storage devices).
The accumulated experience has served to demonstrate the technical feasibility of the concept and its capacity to operate with high solar incident fluxes (typically between 200 and 1.000 kW/m2), allowing working at high temperatures (between 250ºC and 1100ºC) and integrating the technology in more efficient cycles, going from Rankine cycles with saturated water steam to their integration in Brayton cycles with gas turbines. Also, they have demonstrated that can easily operate in hybrid mode in a large number of options and have the potential to generate electricity with high capacity factors by means of thermal storage: currently considering systems exceed 6000 equivalent hours of annual operation. Predictions on the efficiency values of the systems, from solar to electricity, are 20-23% in design point and 15-17% annual.
Differently to the technological homogeneity currently observed in the deployment of the Parabolic Trough collectors technology, Central Receiver power plants offer larger diversity of design options, with less accumulated experience in the implementation of each typology or components. A reduction of the cost and the perception of the risk associated to the less commercial experience of this technology results therefore, essential to the expansion of commercial applications. In particular, the accumulation of experience and developments has contributed to confirm the feasibility, durability and cost reduction capability of these STE-CR technologies.
The objective of reducing the electricity production cost is leading the R+D efforts, on the one hand to improve the existing design options and, on the other hand, to demonstrate the feasibility of new design options, such as:
- The election of heat-transfer fluid and the receiver operation temperature (choosing between water-steam, nitrate salts or air)
- Optimizing the size of the plant (choosing between one tower solar fields or multi-towers plants, that add modularity in the construction) associated to thermal storage and/or hybridation solutions to make them more manageable
- The compromise efficiency/cost of the solar field, choosing between two opposite tendencies:
- bigger heliostats, around 100m2 and more,
- good optical quality and less specific cost of the tracking devices of small heliostat, about 10 m2 with better use of the land and possibilities to operate them in groups.
Aware of the different options in competition, without having clear decisive criteria for the election, the HSCG, besides participating in the first commercial demonstration projects of TCR, maintains permanently a R&D line focused in the technological development of components and systems aiming to generate information that helps to reduce the uncertainties and analyze the technical feasibility of the different options.