Upgrading industrial waste heat with a thermochemical heat pump

This page describes the application of a thermochemical heat pump for upgrading industrial waste heat. A general description of the working principle of a thermochemical heat pump can be found elsewhere. A heat pump with a temperature lift of 50-100ºC is necessary in order to upgrade industrial waste heat. A thermochemical heat pump is theoretically able to do this. There are two applications in this field.

• Generic upgrading of industrial waste heat to a general utility like medium pressure steam.
• Process specific upgrading of waste heat across the pinch temperature.

The system operates at three temperature levels for both applications. These temperature levels are the waste heat temperature, the ambient temperature and the temperature of the upgraded heat. The system consists of two reactors, each containing a different salt. For this specific system use is made of lithium chloride as low temperature salt (LTS) and magnesium chloride as the high temperature salt (HTS). Ammonia vapour is exchanged between these two salts. Industrial waste heat is used to free the ammonia from the LTS. The ammonia flows, driven by the pressure difference between the two reactors, to the HTS and reacts with the HTS. This exothermic reaction delivers heat at high temperature. During the regeneration step the ambient temperature cools the LTS and the waste heat heats the HTS. The ammonia vapour flows back to the LTS under these conditions. The scheme below shows the implementation of such a system in an industrial process. Both the LTS and HTS reactor vessel are build in twofold in order to achieve a continuous system. A switching control system determines whether the above pair of reactor vessel are loading (regenerating) or discharging. The other vessels are running in the reverse process.

The picture below shows a prototype of a test installation of this system. This installation concerns a batch system with only two reactor vessels.

The goal of the present activities is to obtain the most efficient thermo-chemical reactions by increasing our knowledge on the thermodynamics and kinetics of the individual reactions as well as optimisation of heat and mass transfer in the salt/vapour reactors. In the first few years’ period the technical performance of a system with a thermal power of 1-5 kW will be proven, after which it will be scaled up to about 100 kW. On the long term field tests with a 1 MW system are foreseen.