Technology description

Two thermodynamic cycles are used within STEPS to obtain a heat pump. These cycles and basic component configuration are described below.

Multi-stage reverse Rankine cycle

A reverse Rankine cycle heat pump works with the same operating principle as a conventional refrigerator. The basic layout and the thermodynamic cycle are depicted below. The working medium of the heat pump is evaporated at low temperature using waste heat (4-1). The vapor is compressed by a compressor to a higher pressure and temperature (1-2). Condensation of the vapor takes place in the condenser at high temperature (2-3), thereby providing useful process heat. The liquid is then expanded through an expansion valve to be returned to the evaporator (3-4). The working medium is selected based on the operating temperatures of the heat pump cycle.

 


The STEPS project develops a multistage Rankine cycle. This allows for theoption to supply waste heat at more than one temperature level, still achieving high temperature lifts and good efficiency. This concept is presented below. The heat pump is intended to operate with waste heat temperatures in the range of 60-100
°C and process heat temperatures between 120-150°C.. The chosen working medium for the cycle will be pentane due to it providing high cycle performance and thermodynamic suitability in this temperature range. As for the one-stage cycle, the basic layout and TS diagram are presented next.


Thermoacoustic cycle

The working principle of a thermoacoustic heat pump is based on the Stirling thermodynamic cycle. A Stirling cycle is a thermodynamic cycle in which a gaseous working medium (usually Helium) is successively compressed, displaced, expanded and displaced again to the original position. A heat pump can be constructed by expanding the gas at low temperature and compressing it at high temperature. The timing in a thermoacoustic systems is controlled by a traveling acoustic wave, as opposed to Stirling systems which uses a piston and displacer. Both systems use a regenerator to separate the low and high temperature side of the heat pump. The main components of the system and the thermodynamic cycle are presented below. These are the piston compressor generating the acoustic power, the hot and cold heat exchangers and regenerator located inside the pressure vessel. Main part of the pressure vessel is the acoustic resonator.

 


The use of a gas as the working medium allows for a wide range of operating conditions, since condensation/evaporation will not occur. Therefore, a thermoacoustic heat pump can be applied in a variety of applications and under high temperature lifts.

Objective in STEPS is to demonstrate this flexibly of the thermoacoustic cycle for cases in which the waste heat source contains sensible heat and with high degree of waste heat recovered. This implies strong cooling of the waste heat flow (temperature glide). Common heat pumps will show a low efficiency (low COP) for such a case. A single thermoacoustic heat pump can effectively handle such temperature glides in the waste heat flow. A single thermoacoustic heat pump will operate as a number of parallel heat pumps, each operating under a different temperature lift.