DESCRIPTION OF THE HEAT PUMP, SPP HIGHLIFT 4-106-series
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 400-500 kW heat, and are basically double-acting piston compressors with integrated heat exchangers, mounted on a skid, and connected by flanges to the external energy systems.
The SPP HighLift engine series SPP 4-106 is designed for heat pumping applications with sink temperatures up to 200°C. Many aspects of the design resemble large ship diesel engine designs. A detailed description of the process components is given by the others in the paper [Høeg A., et. al 2009]. 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 process components, i.e. cylinders and heat exchanger components, differ from the ones found in diesel engines.
Separate process cylinders with double-acting pistons, are bolted onto the crosshead liners, and the pistons are connected to the crossheads with piston rod, retained by hydraulically tightened nuts. The cylinder volumes are connected by heat exchanger packages, bolted on to the sides of the process cylinders. Linear gas seals around the piston rods prevent gas and oil leakage between the gas circuits and the engine block. The engine/heat pump’s gas circuits are pressurized with helium. The figure below shows photos of two commercial installations of the heat pump.
(a) Installation at the dairy plant
(b) Installation at the dairy plant TINE Frya
Figure 1: Photos of two commercial installations of the HighLift heat pump
The heat pumps have separate subsystems for oil lubrication and cooling, working medium handling and control, diagnostics and logging.
The thermodynamic process and implementation
The heat pump is divided into a hot- and a cold side. There is a heat storage placed between these two sides. When the gas is in the hot side of the engine it is being compressed, resulting in a higher gas temperature. Heat is then transferred to the hot water (sink) at a high temperature. Then the gas is moved to the cold side of the engine heat is transferred to the heat storage. When the gas is in the cold side it is expanded, resulting in an even lower temperature. Heat is then transferred to the gas from the cold water (source) at a low temperature. When the gas is moved back to the hot side, it is heated by the heat storage. The figure below shows an infrared image of one of the heat exchanger packages on the engine during operation.
The cycle sketched above thus consists of heating and cooling of the working medium (a gas with no phase changes), as well as compression and expansion. The real Stirling cycle will have an elliptical shape in a pressure-volume diagram, of which an example is given in the figure below.
Figure 3: The thermodynamic process. Heat is entering the heat pump at a low temperature when the pressure in the heat pump is low (1) and heat is rejected at a high temperature when the pressure is high (3). The heat storage stores the heat when the working medium is cooled (4) and gives it back to the process when the working medium is heated (2).
Heat transfer from the cold source and the hot sink is taking place when the gas is respectively expanded and compressed at an almost constant temperature (isothermal process). Heat transfer to and from the internal heat storage takes place when the volume is approximately constant (isochoric prFigure 3: The thermodynamic process. Heat is entering the heat pump at a low temperature when the pressure in the heat pump is low (1) and heat is rejected at a high temperature when the pressure is high (3). The heat storage stores the heat when the working medium is cooled (4) and gives it back to the process when the working medium is heated (2).ocess). The internal heat storage - or regenerator - is the key to making the process feasible. The regenerator makes the temperature variations of the gas in the heat exchangers small, and thus considerably increasing the efficiency of the process.
As the working medium of the process is a gas and do not undergo any phase changes during the process - it is a single-phase 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.