In vapour-compression cycles (VCC) and more specifically in heat pumps, mechanical work is supplied to the compressor for increasing the pressure of the evaporated refrigerant. Heat is rejected in the condenser (effective heating at heating mode), for the condensation of the refrigerant, which is then expanded in a throttle valve (no power produced), in order to decrease its pressure, resulting to a two-phase mixture. The heat supply to the evaporator (effective cooling at cooling mode) evaporates further the refrigerant.The aim of the EXP-HEAT project is to develop a dedicated component and to integrate it in heat pump units. This component is actually a positive displacement liquid expander with capacity of around 1-2 kW and will replace the throttle valve used in vapour-compression units. Its purpose is to recover the energy of the high-pressure liquid refrigerant, by producing power and assisting in the compression process, reducing the compression work by up to 10-15%. At the same time the heat removed is increased by around 5%, since the expansion of the liquid refrigerant is not isenthalpic. The resulting increase of the Coefficient Of Performance (COP) is expected to reach even 20% at specific operating conditions and for both heating/cooling modes, significantly improving the performance of a heat pump, without major interventions and keeping the additional cost low, in order to ensure the cost-effectiveness of this concept.In order to achieve these targets, the developed expander will be extensively tested under a wide range of operating conditions, in order afterwards to be delivered and integrated in a re-designed heat pump with a capacity of 10-20 kW, and also used for the proper retrofitting of an already installed unit of the same capacity.