1. What is a high temperature heat pump and how does it work
The name gives more than a little hint; A high temperature heat pump is a heat pump that works at higher temperatures. What does “higher temperature” mean? Well, that depends on who you ask, but a common definition is temperatures higher than around 100°C.
Principle energy flow for the heat pump. The energy from the cooling and the electric power is flowing into the heat pump, and the energy used for heating is flowing out.
The higher temperatures mean that the thermodynamic process and the components of the heat pump must be optimised for the higher temperatures. The HighLift heat pump can have a heat source in the full range (0 to 120°C) and is most efficient when the heat sink is between 100 and 183°C.
A high temperature heat pump works on the same physical principles as a normal heat pump; Heat is taken from a low temperature heat source by cooling it down. The temperature of the heat is raised inside the heat pump. Then the high temperature heat is transferred to a high temperature heat sink by heating it up. The figure below shows the basic principle of a heat pump.
So, the advantage of a heat pump is that you are getting more heat out than electricity used. Also, if you need cooling, you will get both cooling and heating at the same time. The ratio of useful heat to the electric power is a common measure of the efficiency of the heat pump. The ratio is called the Coefficient of Performance of the heat pump, or COP for short.
Simplified heat pump process with main components and connections. The cold and hot heat exchangers are sometimes called evaporator and condenser, respectively. The HighLift heat pump is not evaporating or condensing internally, so this is not correct terminology in this case.
2. The HighLift technology
The technology behind the heat pump is based on the Stirling cycle. The engine uses less than 2 kg of helium (refrigerant gas R-704) as its working medium. Helium makes the heat pumps suitable for any application, as it is an environmentally friendly working medium that is nontoxic, non-flammable and with a global warming potential (GWP) of 0.
The heat pumps have a rated output of 700 kW heat. The HighLift 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.
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.
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.