Optimization of high temperature thermal energy storage by the thermocline technology – Opticline

Solar energy conversion and storage by Concentrated Solar Power (CSP) technologies receive an increasing attention because the integrated thermal energy storage (TES) system enhances the reliability, the dispatchability (production of electricity on demand) and reduces the operational cost of CSP plants (increase of annual capacity factor). Currently available options of TES system for CSP applications mainly include two-tank storage and single tank thermocline. Two-tank storage is the most common design and widely used in which hot and cold fluids are located into two separate tanks. In single-tank thermocline storage, both hot and cold fluids are stored within the same tank, but separated by a temperature gradient (stratification) during different operating periods. It is a more cost competitive (about 35% cheaper compared to two-tank storage) and efficient option: space compactness, reduced thermal loss, use of cheap solid materials as fillers and no extra heat exchanger needed between hot and cold tanks.

However, no industrial-scale prototype was built and tested in the world since about 30 years. The main scientific barriers include: (1) lack of detailed data obtained by independent research organization; (2) reduced controllability of the temperature stratification which may be strongly disturbed by maldistribution of the injected inlet fluid flow; (3) non-optimized packing configurations of solid fillers. How to overcome the flow maldistribution problem is actually one major challenge. In fact, the improper design of fluid distributor/collector may cause the flow non-uniformity, local turbulence and recirculation. The mixing of hot and cold fluids and the disturbance of the temperature stratification (increase of the thermocline thickness) will reduce the energy efficiency of the system and the storage capacity.

The general objective of this project is to design and develop a single tank thermocline technology with high energy efficiency and storage capacity by thermal stratification and optimized packing configurations, as a TES system for CSP plants. In fact, this innovative technology is not limited to CSP applications but can be applied in the TES sector in general. The major novelties of the proposal include: (1) systematic studies from the design tools, the modeling, local experiments to the prototypes testing and scaling up guidelines. All these steps were never performed before with the purpose of developing a validated model that can be applied to thermocline tank scaling-up; (2) optimized baffled fluid distributor/collectors to solve the flow maldistribution problem, which was rarely tackled before. The energy efficiency improvement and the storage capacity enhancement by maintaining the undisturbed temperature stratification in the thermocline will be highlighted; (3) use of recycling materials as fillers to reduce the material cost (by a factor of 5 to 10), an idea developed in the framework of a previous ANR project SOLSTOCK, but has never been demonstrated at pilot scale (>100 kWth).

The proposal is thus ambitious. It presents a research of multi-scale nature from the fundamentals (3D modeling, fluid management) via a lab-scale experiments for hydrodynamic study and fluid/solid interactions, to the field testing of pilot-scale prototypes with different heat transfer fluids and global performance evaluation/optimization of the TES system. Finally it includes steps towards the system integration optimization, the scaling-up issue and the commercialization of the thermocline technology for CSP plants.

The project will be realized in collaboration between two academic partners (LTEN-CNRS and PROMES-CNRS) and two industrial partners (Eco-Tech Ceram and ADF PROCESS INDUSTRIES). The consortium presents excellent and complementary experience and expertise. The strategies and methods for operating the project are well established to ensure its good progression.