The heat pumps have a rated output of 400-500 kW heat. The HighLift series SPP 4-106 is designed for heat pumping applications with sink temperatures up to 200°C. As the working medium of the process is a gas and do not undergo any phase changes during the process, the HighLift heat pumps have very few restrictions on source- and sink temperatures. In addition, the process will quickly find a new equilibrium state with changing temperatures, meaning that the process is very robust with respect to e.g. large and sudden changes in inlet temperatures. Many aspects of the design resemble large ship diesel engine designs. The heat pump is a four-cylinder double-acting engine with four gas circuits, with a normal crankcase with crankshaft, oil lubricated bearings and crossheads, connected by two-piece connecting rods. The figure below shows a schematic overview of the heat pump.
Figure 2. Schematic overview of the heat pump configuration. Two pistons move phase shifted to each other (with respect to the crank angle). The two pistons work together to move the gas from one side of the heat pump to the other, and to compress and expand the gas. When most of the gas is in the hot side of the heat pump, the pistons compress the gas and when most of the gas is in the cold side, the pistons expand the gas.
During one revolution, the working medium undergoes state changes that approximates the ideal Stirling cycle. The figures below shows a comparison between the ideal and real Stirling cycles.
Figure 1. The ideal Stirling cycle (a) compared to the real heat pump cycle (b) in p-v diagrams. The real cycle is based on a simulation model of the heat pump with parameters matching the actual parameters of one of the installed heat pumps.
(a) Ideal Stirling cycle in a p-v diagram. State 1?2 isothermal compression, state 2?3 constant volume cooling, state 3?4 isothermal expansion and 4?2 constant volume heating.
(b) Real Stirling cycle based heat pump cycle in a p-v diagram. The motion of the reciprocating pistons approximates the ideal state changes of the working medium.