Hybrid storage and heat exchanger device with PCM – EUROPA

The intermittence of renewable energies generally requires the establishment of storage facilities because the resource is rarely in tune with the needs. For thermal solar applications applied to buildings, storage solutions with a small footprint, simple to implement and at a reasonable cost will certainly contribute to the development of this market, which is currently experiencing a significant slowdown. The EUROPA project is thus targeting ultra-compact heat storage for the production of hot water, which is an important consumer item in new residential buildings. French manufacturers are clearly behind in this market given the absence of a product available in the form of a thermal battery adapted to the solar which requires the presence of two exchangers. Compactness is of great interest both financially (land price), the feasibility of a solar project in renovation (often little space available) and for the reduction of thermal losses. Other possible applications of the concept are numerous. In photovoltaics, electrical storage solutions that are still very expensive make it interesting to store overproduction in the form of heat by means of a simple electrical resistance or a heat pump.

The aim of the EUROPA project is therefore to develop an innovative concept for storage and heat exchange, which is both modular, ultra-compact and of limited cost. Compared to a conventional solar hot water tank, better energy efficiency is expected with the new concept thanks to storage with phase change materials (PCM). Indeed, the rise in temperature is rapid over the sensible heat range since the melting temperature corresponds to the set-point temperature of DHW ; this reduces the use of auxiliary heater. On the other hand, the compactness makes it possible to reduce the thermal losses. Each storage module couples a honeycomb panel containing the PCM with two separate planar heat exchangers located on both sides of the cavity. These exchangers ensure the charge (storage + direct transfer) and the discharge (destocking + direct transfer) simultaneously or not. The interest of the honeycomb is to improve both the equivalent conductivity of the PCM and to intensify the direct transfers between the two exchangers in case of simultaneous charge and discharge. These direct exchanges have a real interest in improving the energy efficiency of the solar system. Furthermore, there is little research on the simultaneous charge and discharge of storage with PCM. To our knowledge there are no scientific studies on the intensification of these direct exchanges by means of fins between the 2 exchangers. That is why it is one of the main scientific objectives of the project, which will focus on a configuration of fins with honeycomb containing PCM.

We will establish experimentally the empiric laws involving dimensionless numbers related to the geometric characteristics of the honeycomb, the type of PCM... They will make it possible to determine the temporal variation of the liquid fraction, as well as the efficiency of the direct exchange when a module is subjected to a charge and / or discharge. These laws make it possible to design the modules according to the requirements related to storage and direct transfer between the two exchangers. The dynamic model of a single module and then of an elementary storage composed of several modules will be developed. After experimental validation, the dynamic model will be used for the multicriteria optimization of the design of a prototype operating in a solar thermal system of a residential building. The optimal solution chosen will correspond to the prototype experimentally tested at INES.