Development of a high-performance domestic refrigerator/freezer with R600a utilizing advanced cycle architecture and vapor-injected reciprocating compressor

Domestic refrigerator/freezers account for 6% of all energy consumption around the globe and mainly rely on vapor compression cycles to operate. Researchers have investigated advanced cycle architectures, such as dual-loop cycles and ejector-enhanced cycles to improve their efficiencies. However, the energy saving potential of these advanced cycles often do not justify the additional costs. To further improve efficiency and reduce energy consumption, this work presents and assesses a two-stage vapor-injected cycle for domestic refrigerator/freezer applications. The proposed cycle directs the refrigerant exiting the medium temperature evaporator directly into the injection port of a vapor-injection compressor. The cycle establishes two separate evaporation temperatures to better match the cabinet temperatures of fresh food and freezer compartments. The reduced difference between each cabinet temperature and its evaporation temperature can reduce heat transfer irreversibilities and improve overall system efficiency. A bypass circuit cycle triple-evaporator domestic refrigerator freezer was used as the baseline cycle to investigate the performance improvement of the proposed cycle. A previously validated dynamic model of the baseline cycle was modified to consider a two-stage vapor-injected cycle. The simulation predicted a 12.9% energy consumption reduction with the proposed cycle when utilizing the baseline compressor efficiency for the vapor-injected compressor.

To evaluate additional performance improvements, a detailed mechanistic compressor model of the baseline variable-speed compressor was developed and validated with experimental data. Next, a vapor-injection line was added to the compressor model as an additional flow path to the compression chamber. The injection line is modeled as a tube connected to an opening on the cylinder wall, which is uncovered during the compression stroke. The injection tube is controlled by a fast-acting solenoid valve to separate injection and suction flow for the compressor. Parametric studies were carried out to assess the effects of injection timing and injection port diameter on power consumption and overall isentropic efficiency with respect to the baseline compressor. It was found that vapor injection in the reciprocating compressor can reduce the specific work required by more than 10%. Based on parametric studies, opening time of the fast-acting solenoid valve directly impacts the compressor efficiency and mass flow rate. There exists an optimal cylinder pressure associated with the opening of the solenoid valve that leads to maximum compressor efficiency.

The mechanistic compressor model was integrated into the two-stage system model using a reduced-order model to obtain more realistic and higher fidelity results. The reduced-order model considers the injection thermodynamic state and timing as well as evaporating and condensing temperatures from the system operation. Further, a control scheme based on the two-stage vapor-injected cycle’s available control variables was developed. The vapor-injected reciprocating compressor’s injection timing was varied in a parametric study to study its effect on two-stage system’s performance. As a results, 29.7% energy consumption reduction compared to the baseline cycle was achieved at injection timing of 4.95 radians.

To confirm the design and realistic performance of the designed vapor-injected reciprocating compressor, a prototype was designed and tested. A fast-acting solenoid and in-line check valve are connected and mounted directly on the cylinder wall to reduce dead volume during compression. The solenoid valve is controlled using the crank angle measurement from a Hall-effect sensor detecting a series of specifically placed magnets on the crankshaft. The prototype compressor was tested using a hot-gas-bypass compressor test stand under different suction, discharge and injection pressures. The experimental results show the prototype compressor’s specific work was reduced by up to 25% during vapor-injected operation compared to its single-stage operation with no injection. The results also showed the specific work reduction increased with an increase of injection pressure when condensing and suction pressures were fixed. The experiments also confirmed the compressor is able to separate the suction and injection flow, confirming it’s ability to be used in the two-stage cycle.