The main concept of the Exp-Heat project is to replace the throttle/expansion valve used in common vapour-compression units (Figure 1) with an expansion machine. Its purpose is to recover energy from the high-pressure liquid (condensed) refrigerant with the use of the expander and provide it to the compressor, reducing its electricity consumption. According to some preliminary calculations, the overall COP of the system can be increased in both cooling and heating mode by 15-20%. The expansion machine can either be integrated in new heat pump design (Figure 1a) or retrofitted to existing units (Figure 1b).
Figure. 1. Vapour-compression cycle using an expander (in cooling mode): a) preliminary design, b) retrofitting design.
The concept of recovering the high pressure of the liquid refrigerant has been previously investigated by various researchers. Focus is given mainly on transcritical configurations using CO2, where the condensation pressure is sufficiently high (exceeding 100 bar) and there is a great margin of energy recovery. However, most of the research has been carried out on several different technologies (screw, vane or rotary expanders), but none has been carried out on hydraulic piston and reciprocating type expanders, which seem instead the most feasible technology for this application, at least for small-scale applications. Those expanders have efficiencies in the magnitude of 70%, higher than other technologies in this range, especially at partial loads. Apart from that, most of the proposed technologies are considered for medium size units (at least 20-30 kW cooling load). Also, in most of the cases a CO2 VCC (Vapour Compression Cycle) unit was considered, whereas very few works exist with HFC refrigerants, which are the most common ones.
Reciprocating and hydraulic piston expanders seem to be well suited for energy recovery in heat pump units using HFCs as working fluid. However, there is no prior research carried out in this field. The expander’s operating conditions include a pressure difference of around 15-25 bar, temperature ranging from -10 oC up to 60 oC, mass flow rate from 0.05 kg/s up to 0.2 kg/s, while the volume flow rate shows a large variation: at the inlet is around 5-10 l/min (saturated liquid), while at the outlet is around 50-150 l/min (two-phase mixture), strongly depending on the mass flow rate, evaporation temperature and the working fluid. This large range of conditions makes the successful development of such an expander a very demanding task.
Some research aspects of high interest and importance include the cavitation effects, which have to be avoided, the heat transfer due to the expansion of the liquid refrigerant and the expansion efficiency at both low and high loads. Moreover, the design of a heat pump unit is also demanding due to these new un-exploited settings. Slightly different configuration (e.g. different recuperation design, multiple compression stages, etc.), adjusted control and use of suitable components (e.g. heat exchangers, compressors) may prove necessary for the achievement of optimal efficiency.
A prototype reciprocating or hydraulic piston expander with a power capacity of 1-2 kW will be designed, simulated, constructed and experimentally tested within the Exp-HEAT project. Its performance will be measured in a dedicated test-rig installed in the laboratory, while identifying its mechanical, volumetric and isentropic efficiencies, with a final task to optimize its performance. After its optimization, an isentropic efficiency of 65-70% is expected to be achieved. Special attention will also be given on its reliable and efficient performance during its variable load operation, together with the proper selection of materials to withstand the operational pressure/temperature range. Also, care will be given on decreasing the heat transfer from the refrigerant to the expander inner walls, matching the operational speed range of compressor/expander, and also the elimination of leakages and over/under-expansion.
Subsequently, the developed expander will be integrated in a re-designed heat pump unit and retrofitted to an existing commercial unit, which will be further tested in the lab for their overall performance evaluation, An important aspect which will be investigated is the capability for direct coupling of the expander with the compressor and the electric motor, using a common shaft, preferably without the use of a gearbox, or even using a belt/chain adjusting the transmission ratio as well, in order also to minimize the cavitation effect (might appear at high speed, causing instability to some expander designs).
Concerning the cost of this combined concept, it should be emphasized that since a fraction of the required compression work is provided by the expander, the size of the main compressor and motor are reduced (lower capacity), decreasing their cost. On the other hand, taking into consideration the additional cost introduced with the use of an expander, it is expected that the hardware cost of the improved end-product will have just a small increase, ensuring its cost-effectiveness short return of investment. It should be however mentioned, that in case a heat pump unit is retrofitted using the developed expander by replacing the throttle valve, the compression/motor will be probably kept the same, in order not to increase the additional costs. Nevertheless, the retrofitting cost can be compensated at short time (low pay-back-period), since the performance will be increased, leading to lower operational energy cost.