Solid Sorption Chillers and Heat Pumps

Like liquid absorption systems, solid sorption systems belong to the family of trithermal sorption systems. They are thermotransformers that produce cold or heat from two heat sources: one at high temperature and the other at ambient temperature.

In both cases, the driving force is high-temperature heat. In the case of refrigeration production, the sum of the heat from the hot source and the cold produced is rejected to a heat sink at intermediate temperature (ambient air, subsoil or water). In the case of heat pumps, on the other hand, it is heat production that is valorized, with an amplification effect, since the useful heat is the sum of that supplied by the hot source and that coming from the free heat source (air, subsoil, thermal discharge, etc.). These processes are also suitable for heat or cold storage.

In a context of energy savings and the fight against the greenhouse effect, these solid sorption processes are enjoying renewed interest. The refrigerants used (water, ammonia) have a zero global warming potential (GWP) and therefore make no direct contribution to the greenhouse effect.

At present, these systems are the focus of numerous research and development (R&D) initiatives, resulting in the emergence of innovative products, some of which should find a large market in the near future. The building and transport sectors are among those showing the most interest.

Air conditioning using low-temperature heat (waste heat, solar energy, etc.) is currently one of the strongest points of these systems. Their advantage over liquid absorption systems is that they can operate at lower hot-source temperatures, which is invaluable for air-conditioning from waste heat, and require no mechanical components. As free heat is involved, the low coefficient of performance (COP) is not a major obstacle. On the other hand, weight, volume and cost are currently holding back the development of these systems.

Recent research into innovative adsorbent materials (notably organometallic networks – MOF) is opening up new prospects for more efficient, lighter and less expensive equipment. If these developments come to fruition, there is no doubt that the market for these systems will grow in zero-energy or positive-energy buildings, as well as in transport. In addition, the thermochemical battery concept offers interesting solutions for the optimal management of intermittent renewable energy sources and the recovery of industrial waste heat.

In this article, the various solid sorption systems are analyzed and their operation and performance are discussed. Hermetic systems are the main focus; however, open cycles for air conditioning, dehumidification or thermochemical heat storage...