By Jason Deign of CSP Today | Published January 05th, 2015: A Spanish team has uncovered a radical new encapsulated phase-change material (PCM) storage system after creating a novel modelling tool for CSP.
The multi-layered solid-PCM (MLSPCM) system has a theoretical thermal capacity, measured as energy delivered over energy stored, of more than 74 %, while requiring only 32 % of the molten salt needed for traditional two-tank systems.
MLSPCM combines two storage methodologies that are being investigated for their cost-reduction potential in CSP. One, encapsulated PCM, involves using phase changes to improve the efficiency of storage processes, reducing the volume of the tank required.
The other, a thermocline system, again aims to reduce the cost of traditional two-tank storage by using cheap filler materials such as quartzite rocks or granite to separate hot and cold heat transfer fluids in a single tank.
In MLSPCM, two thin layers of PCM sandwich a thermocline layer made up of filler materials. The thermocline is designed in such a way that it keeps the top layer of PCM within an optimum temperature range for discharging, say around 550 ºC.
The bottom layer, meanwhile, is kept within its optimum charging range, which may be in the region of 300 ºC. This improves the efficiency of the PCMs while at the same time greatly reducing the total volume of PCM required.
Researchers say this concept reduces the degradation of the thermocline, improves the accumulation capacity of the system, and allows for longer discharge cycles and more stable discharge temperatures, all for a cost not much above that of traditional thermoclines.
MLSPCM was first described at SolarPACES 2013 by a team from the Heat and Mass Transfer Technological Centre of the Polytechnic University of Catalonia (Universitat Politècnica de Catalunya or UPC in Catalan), but its performance results were not uncovered till a year later.
PCM layers
“The key aspect of this new concept is the inclusion of PCM layers at both ends of the tank, whose fusion temperatures are conveniently chosen to be inside the ‘admissible’ temperature ranges for the outlet of both charge and discharge processes,” says the team in a paper.
It was uncovered as part of modelling work carried out using a tool called LTES (for Latent Thermal Energy Storage), which is being developed for commercial launch in 2015 by TermoFluids, a UPC spinoff.
LTES and STES (for Sensible Thermal Energy Storage), a separate modelling tool for two-tank systems, were created as work packages within Tesconsol, a project consortium between UPC, the KTH Royal Institute of Technology in Sweden, Total, Gas Natural Fenosa and Tecnalia.
Both tools allow system developers to model the performance of different types of thermal energy storage within a virtual environment that almost perfectly mimics real life.
“The philosophy of our group is to go in depth into the analysis of fluid dynamics problems, heat transfer and mechanical problems such as tension and deformation, in thermal systems in general,” says Prof Carlos-David Pérez-Segarra of the UPC.
“In particular, we’ve always been interested in the whole subject of taking advantage of solar energy, and particularly CSP. It’s important for us to develop advanced simulation codes that can faithfully reproduce specific phenomena.”
LTES and STES use tried-and-tested mathematical formulae to provide a complete description of the processes that take place inside a CSP storage tank. For STES, says Pérez-Segarra: “These simulation codes have been validated through experimental results.”
Beyond helping to uncover new storage systems, the tools also allow developers to improve the efficiency of existing designs, for example by revealing how losses can be avoided. Nor are the results restricted to improving efficiency.
Safety and reliability
In some cases, they can also be used to help improve safety and reliability. In a separate SolarPACES 2014 paper, for example, the team used LTES to help overcome a traditional thermocline tank problem called thermal ratcheting.
This is the tendency of filler materials to pack themselves in the bottom of a tank after repeated heating and cooling cycles.
It can put a major strain on the walls of the tank, but with LTES the UPC team was able to predict the maximum stresses that could be expected, based on a simulation of a thermocline storage system attached to Acciona’s Solar One plant in Boulder, Nevada.
“You can test different tank configurations for different work conditions,” says Pérez-Segarra. “You can put your data in and do a simulation based on any parameter, from tank materials to different types of fluid with different operating temperatures, viscosities and so on.”
Tesconsol is the only CSP project currently supported by the European Institute of Innovation & Technology’s InnoEnergy Knowledge and Innovation Community (KIC), a private sector body that uses public funding to help start-ups with major potential.
KIC InnoEnergy Iberia managing director Mikel Lasa says: “It’s a project that has been going on for four years to provide optimisation tools for energy storage tanks.”
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