Concentrated solar power plants (CSP) combined with thermal energy storage (TES) offers the benefit to provide continuous electricity production by renewable energy feed. There are several TES technologies to be implemented, being the thermochemical energy storage the less studied and the most attractive since its volumetric energy density is 5 and 10 times higher than latent and sensible TES, respectively. Thermochemical energy storage technology is based on reversible chemical reactions, also named thermochemical materials (TCM). One of the main challenges of TCM is to achieve a proper reversibility of the reactions, which in practical conditions leads to lower efficiencies than the theoretically expected. A new concept based on changing from reversible TCM reactions towards TCM consecutive reactions aims to eliminate reversibility problems and therefore improve the overall efficiency. Consecutive TCM reactions can either be based in one cycle, where reactants are needed to feed the reaction, or two coupled cycles which offer the possibility to work without any extra mass reactants input. The plausibility of the implementation of both concepts in CSP is detailed in this paper and case studies are described for each one.

Renewable energies are main players to ensure the long-term energy supply. Solar power plants with thermal energy storage (TES) are one of the available renewable technologies which have more potential. Nowadays, there are still several aspects in the design and operation of these power plants which need to be improved, such as the correct operation of some specific instrumentation, the compatibility between TES materials and storage tanks materials, and operational process strategies. This paper presents the acquired experience during the design, start-up, and operation of a kWh scale pilot experimental facility built at the University of Lleida (Spain) together with Abengoa Research (Spain) in 2008. The versatility of this facility has allowed simulating real working conditions and therefore testing different TES systems, TES materials, solar power plant components, and operational strategies focused on TES for temperatures up to 400 ºC. In the present paper, the authors show the lessons learnt at pilot and present the main problems and limitations encountered, and give advices of this experimental set-up to extrapolate the data to real plant, to provide solutions to technical problems and reduce the cost of commercial plants.

a b s t r a c tPhase change materials (PCM) are able to store thermal energy when becoming liquids and to release it when freezing. Recently the use of PCM materials for thermal energy storage (TES) at high temperature for Concentrated Solar Power (CSP) technology has been widely studied. One of the main investigated problems is the improvement of their low thermal conductivity. This paper looks at the current state of research in the particular field of thermal conductivity enhancement (TCE) mechanisms of PCM to be used as TES. This work considers a numerical approach to evaluate the performance of a group of TCE solutions composed by particular configurations of two of the principal TCE systems found on the literature: finned pipes and conductive foams. The cases are compared against a single PCM case, used as reference. Three different grades of graphite foams have been studied, presenting a charge time 100 times lower than the reference case for the same capacity. For fins two materials are analyzed: carbon steel and aluminum. The charge times of fin cases are from 3 to 15 times faster, depending on the amount and type of material employed. The internal mechanisms are analyzed to understand the results and locate possible improvement.

Phase change materials (PCM) is one of the most interesting solutions to be used in thermal energy storage (TES) systems for direct steam generation (DSG) thermosolar facilities. Properties such as high energy density and energy storing/delivery at constant temperature bring PCM based systems in excellent candidates for DSG facility storage units. Accordingly, LiOH-KOH peritectic mixture, with a melting point of 315 ºC and an enthalpy change of 535 kJ/kg, has been reported as attractive solution for the saturated storage module in DSG plants. A steam-PCM heat exchanger is the critical component to carry out the thermal transference between both substances. Although materials selection to be applied for steam applications is well known, lack of knowledge is detected in the field of high temperature hydroxides corrosion. Therefore, three metallic materials, A516 Gr70 carbon steel, A316L stainless steel and Inconel 625 Ni-base alloy, have been evaluated to determine their corrosion performance after hydroxides exposure. While A516 Gr70 was discarded for this application due to high corrosion rates, A316L and Inconel 625 displayed good corrosion resistance after 2640 hours. Finally, A316L stainless steel was selected as potential candidate for the construction of the steam-PCM heat exchanger considering cost and thermal efficiency optimization.

A387 Gr 91 Cr-Mo alloy corrosion compatibility with LiOH-KOH mixture was evaluated at 315 ºC (Test#1) and 360 ºC (Test#2). This alloy is studied for a techno-economical optimization of a steam-PCM heat exchanger proposed in the thermal energy storage (TES) system of a direct steam generation (DSG) facility. The corrosion damage was analyzed with visual inspection, optical microscopy, SEM, EDS and XRD, and corrosion rates were also calculated. A387 Gr 91 alloy generates protective oxide layers over base metal. Localized corrosion damages were not detected with these test conditions, while XRD and EDS profiles showed CrO and LiFeO 2 as main corrosion products generated by this alloy. On the other hand, corrosion rates decrease with exposure time showing the passivation of the alloy. A387 Gr 91 corrosion performance was compared with A316L stainless steel, which was preliminary proposed by the authors in previous studies. In addition to corrosion performance, parameters such as cost, thermal properties, and mechanical properties are discussed. In conclusion, the use of A387 Gr 91 instead to A316L alloy for the construction of the steam-PCM heat exchanger involves the techno-economical optimization of the equipment.