Pinch point optimized semi-cascade heat pump (PiPoVP)

A semi-2-stage heat pump with ammonia as one of the refrigerant possibilities has been developed. It is designed to be able to produce hot water at temperatures above 100 °C, which is normally difficult to produce by heat pumps today. The project examines the possibilities and boundaries of the proposed concept.

The most promising designs are developed and calculations of the expected performance are carried out in order to estimate the expected efficiency. A laboratory model of the heat pump is built and tested using isobutane as the refrigerant. There are other heat pump solutions that can produce hot water at temperatures above 100 °C, but these solutions typically have a limited distribution and a relative high cost. In cases where the purpose is to heat water by more than approximately 25 K, and the pressure ratio is too large for a 1-stage solution, then this solution will be advantageous for this application. In cases where the pressure ratio is sufficiently low, a 1-stage system can fulfil the requirements. However, the developed semi-2-stage heat pump will still be more effective, but also more expensive. The required temperature difference in the cascade chiller may in some cases reduce the advantages.

The concept may also be applied for utilization of surplus heat from cooling plants, where it is wanted to recover more energy than achieved from the pressure gas alone. The objective of the project is to develop a concept for a novel type of cascade heat pump, together with companies from home and abroad, which will have a large potential for application in particular industrial and district heating systems.

The focus will be on the ammonia and isobutane refrigerants, which both are natural refrigerants, because the technology can also be applied outside the industry, but also because the technology can be used in connection with heat recovery from the refrigeration systems in general.

The heat pump performance has been simulated, to optimize the "intermediate pressure". An experimental model is designed and built using isobutane for both the low and high pressure stage, to demonstrate the functionality and efficiency of the concept. By using isobutane, it is possible to reduce the size of the plant and apply a high temperature due to its high critical point and low working pressure. This refrigerant is also chosen because it fits well as a smart solution with ammonia at the low-pressure stage and isobutane at the high-pressure stage.

The project will contribute to recover much of the energy that otherwise will be wasted and instead use it internally in the industrial processes.