Experimental investigation of the Recuperative Two-Phase heat pump Cycle
The Recuperative Two-Phase Cycle (RTPC) is an approach developed at TU Dresden in recent years to increase the efficiency of thermodynamic cycles. The main difference to the state of the art is the use of an asymmetric working fluid, which enables a liquid two-phase recuperation with minimal entropy production, as the capacity flows on both sides of the internal heat exchanger are equalized by a phase change on the lowpressure side. The efficiency improvement in the heat pump mode of the cycle is achieved by reducing the isenthalpic throttling losses by subcooling the working fluid to a temperature close to the evaporation temperature. The design of a test bench is presented, with which the heat pump cycle is realized for the first time. The set-up consists of five main components and uses the asymmetric mixture propane-dodecane. A conventional high-speed rotary compressor is used; however, it is operated without oil and the compression takes place in the two-phase region. A particular challenge is to determine the circulating and local composition of the asymmetric working fluid. A specific solvent-based method is described for taking small samples from the liquid and the two-phase flow, which can be analysed in a gas chromatograph. The experimental results of transient ramp-ups and various stationary operating points of the heat pump process are shown. A maximum temperature lift of 33 K between heat source and heat sink is achieved, and the pre-calculated behaviour of the cycle could be found. Observations on the operation of the process are discussed, the influence of the compressor speed and the source temperatures is highlighted, and the significance of the composition of the working fluid is analysed. Finally, observations on the compressor operability are discussed and necessary follow-up investigations for the quantification of the observed phenomenon are derived.