Thermal energy storage in concentrated solar power (CSP) plants plays a key role to assure energy dispatchability. The NEWCLINE project was conceived with the objective of improving significantly the thermal energy storage of both parabolic troughs (PTC) and central receivers (CR) in CSP plants. The research was focused on thermocline systems based on novel structured filler refractories and the possibility of using encapsulated phase change materials (PCMs) in strategic parts of the tank.
The consortium was formed by a perfect balance of industry and research centres. Empresarios Agrupados Internacional (EAI), a relevant Spanish engineering company with a long experience in the design of commercial CSP plants, was in charge of all engineering issues related to the mechanical design and construction of the thermocline tanks, together with the economic analysis of their integration in the whole CSP plant. The German company Kraftblock (KB) was responsible for the structured material development, a critical aspect in the design of the storage system. The UPC, a large technical university in Spain, contributed as project coordinator, as well as in activities related to the computational and experimental studies needed to optimize the designs. DLR, the well-known German research centre, developed experiments in an outstanding large pilot-scale facility of 4 MWh of thermal capacity, together with material testing and model validation studies. Finally, the Swiss research centre SPF contributed to the thermal analysis of the whole CSP plant, considering the thermocline energy storage systems developed in the project.
An important part of the work was oriented to the development of specific computational codes for the simulation of the structured thermocline systems using the developed filler material and with the possibility of implementing layers of encapsulated PCM in strategic parts of the tank (basically in its bottom and top regions). Different filler materials and manufacturing processes were proposed and tested, looking for good compatibility with the molten salt and low cost, using recycling materials from the steel and aluminium industry. Different compatibility tests were carried out by DLR, KB and UPC, and two selected mixtures, together with a commercial refractory material, were tested in the pilot scale unit at DLR under several charging/discharging conditions. The different mathematical models developed in the project (from solid/fluid 1D/1D to 3D/3D CFD codes) were carefully validated against experimental data generated at the DLR pilot scale plant, and data published in the technical literature considering packed beds and also encapsulated PCMs.
A rigorous analysis of the tank design from an engineering point of view was carried out by EAI, considering both mechanical and civil issues for the two plants (tank design, materials, support structure of the filler material, distribution flow rings, pumps, slip plates, foundation, etc.). A careful analysis was carried out to define the constructability and technical challenges of the proposed structured thermocline tank.
The integration of the tank in the two defined plants, PTC and CR, was examined by SPF from a thermal point of view by means of dynamic simulations in pytrnsys/TRNSYS, and also by EAI from an economic point of view in terms of CAPEX, OPEX and LCOE. It was concluded that the design of the thermocline tanks in its current form is competitive for PTC plants, with a reduction of the LCOE with respect to the standard two-tank systems of about 10%. However, more effort is needed for its implementation in CR plants, where no clear advantages have been observed. For both plants, future activities will be mainly focused on the reduction of the filler material cost, the associated costs related to tank construction and refractory and encapsulated PCM material arrangement in the tank, together with more detailed thermo-mechanic simulations.