Inter-Seasonal Solar Heat Storage Process - 2 – PROSSIS2

This concept has been previously studied under the PROSSIS ANR STOCK-E project. However, several points still have to be tackled before the process can reach the market stage, and these subjects will be dealt under the present proposal:
The chosen absorption couple (LiBr/H2O) presents interesting thermodynamic parameters but is toot expensive to be used at a wide scale. The study of innovative absorption couples and their characterization will be performed to solve this issue.
The crystallization of the solution inside the storage tank has proven to be necessary to increase the storage density, and thus reduce the storage tanks size and cost: this crystallization has to be reversible and controlled. The crystallization kinetics and the forms of the crystals will be measured to optimize the shape of the solution storage.
The process control has to be improved, both at a process level and at the global solar system level. At the process level, the aim is to reduce the electric consumption of the internal pumps and improve the storage density by a better control of the process parameters. At the larger level, the aim is to combine this long term storage with the other components of a solar thermal system (solar collectors, short-term water storage, etc) to optimize its global energy efficiency.
The heat exchangers chosen are based on the falling film technology, to cope with the constraints of the system functioning. However, the heat exchangers were designed as shell-and-tube and this design could be improved, to reduce further the size of the reactor and improve the heat and mass transfers, with specific plate heat exchangers design. New exchangers will thus be designed, characterised at the plate level and the global exchanger level, to improve the power provided by the process and its compactness.

The heat exchangers have been designed, from existing corrugated plates surface-treated to increase wettability. The distribution of the liquid film on the corrugated exchanger surface has been studied experimentally. An absorption model has been developed and simulations obtained are generally satisfactory and consistent with experimental results.
Regarding the overall process, a nodal model was developed using Matlab to predict and understand the behavior of the system. The power expected through the simulation for the prototype varies from 1 to 8kW, depending on operating conditions.
For the characterization of sorption couples, steam pressure measurements of saline solutions were performed. Dühring diagrams of the various salt / water couples were built. Storage capacity, efficiency and salt and water masses necessary for the storage were calculated for the different studied couples. Potassium formate is as a good compromise and was therefore chosen for further study.
A crystallization study in agitated vessel was carried out with potassium formate. KCOOH solubility was measured from 5 to 90 °C. The boundary of the nucleation metastable zone was also measured experimentally. This solution has a wide metastable zone, so the use of a nucleation medium is considered for future manipulations as a crystallization driver.

An important experimental work is still expected in the coming months: for the overall system, designed exchangers and the new couple sorption will be tested in a prototype at the LOCIE lab. The crystallization bench of the LAGEP lab was also upgraded and will be tested in the coming months. After these experiments, the exchangers will also be finely characterized on the MAEVA test bench of the CEA.

An article published in Solar Energy, a book chapter and 5 international communications were produced, as well as 9 national congress participations. These productions, most of them involving several partners, highlight the strong partnership in the project. They cover both the development of the overall process and analysis to caracterise an innovative sorption couple.

This project aims at developing a solar heat storage process to cover the needs of a building. The process under study is based on the absorption principle. It can store heat over different time lengths and could go up to an inter-seasonal storage, as heat is stored as a chemical potential and this storage type is not subject to heat losses. This concept has been previously studied under the PROSSIS ANR STOCK-E project. A dynamic model has been developed and a prototype has been built to demonstrate the feasibility of this process. However, several points still have to be tackled before the process can reach the market stage, and these subjects will be dealt under the present proposal:
- This technology should be introduced at the most efficient level on the market: a market study and case studies will be performed, and process specifications will be delivered by our industrial partner.
- The chosen absorption couple (LiBr/H2O) presents interesting thermodynamic parameters but is toot expensive to be used at a wide scale. The study of innovative absorption couples and their characterization will be performed to solve this issue.
- he cristallisation of the solution inside the storage tank has proven to be necessary to increase the storage density, and thus reduce the storage tanks size and cost: this cristallisation has to be reversible and controlled. The crystallization kinetics and the forms of the crystals will be measured to optimize the shape of the solution storage.
- The process control has to be improved, both at a process level and at the global solar system level. At the process level, the aim is to reduce the electric consumption of the internal pumps and improve the storage density by a better control of the process parameters. At the larger level, the aim is to combine this long term storage with the other components of a solar thermal system (solar collectors, short-term water storage, etc) to optimize its global energy efficiency.
- The heat exchangers chosen are based on the falling film technology, to cope with the constraints of the system functioning. However, the heat exchangers were designed as shell-and-tube and this design could be improved, to reduce further the size of the reactor and improve the heat and mass transfers, with specific plate heat exchangers design. New exchangers will thus be designed, caracterised at the plate level and the global exchanger level, to improve the power provided by the process and its compactness.