Optimization opportunities for vapour-compression cycles
Vapour-compression cycles are used to move heat from a cooler to a warmer place by adding work via a compressor. The performance of the cycles is generally characterised using the ratio of heat removed (cooling) or heat added (heating) to the energy added (in the form of electricity or compressor work), a.k.a. COP (coefficient of performance).
To determine the COP of the system, thermal loads (the amount of cooling or heating) and energy users should be defined.
In case of refrigeration, the heat is being removed (from evironment) at evaporators which is in form of air coils, heat exchangers, or chillers while for heat pump applications the heat is rejected from the cycle at condensers. Condensers and evaporators use energy to run fan/pump on the secondary loop, such as atmosphere air or water. The largest energy user in the system is the compressor since the compressor is the main driving force of the system.
Inefficiency of performance can be caused by various sources: excessive compressor lift, poor part-load performance, defrost controls, unnecessary refrigeration loads, and auxiliary component efficiencies.
One measure for reducing the pressure lift is increasing suction pressure (or decreasing discharge pressure). Compressor's energy consumption is proportional to the pressure differential between suction and discharge. The suction pressure is determined by the evaporation temperature at the evaporator, and the discharge pressure is determined by the condensing temperature at the condenser. The temperatures are usually fixed set points. To increase/decrease the suction/discharge pressure, the temperature set points should be flexible, and a proper control system needs to be implemented. Another measure for increasing the suction pressure is reducing the suction pressure drop through the evaporator or suction line. For the condensing pressure (dischage pressure), allowing the condensing pressure (temperature) to float with ambient conditions could decrease the discharge pressure. This can be achieved by operating fans more frequently. However, the trade-off between energy consumptions of a fan and a compressor needs to be made, also the fan will have more aggressive control actions.
A vapour compression system (refrigeration or heat pump) operate at part-load most of the time as the system is designed to meet the highest thermal load. Thus, in order to maximise the efficiency, equipment and operating strategies play an important role. For reciprocating compressors, cylinder unloading and compressor staging (multi-compressions) can improve the part-load efficiency. On the other hand, screw compressors wth slide valve for capacity control have poor part-load performance. In multi-compressor systems, screw compressors should be base-loaded (i.e. running full capacity all the time) for better efficiency. Variable frequency drives can improve the part-load efficiency of screw compressors.
Proper defrost controls can improve the system efficiency. Defrosting is required to remove/prevent ice build-up on the evaporator coil surface as the ice layer will inhibits heat transfer between the refrigerant and the air. Defrosting is necessary process, but needs to be carefully controlled as most defrost methods increase cooling loads. Also, the ice build-up depends on many factors, such as humidity, infiltration rates, and product, etc. The objective of better defrost control will be minimizing defrosting frequency and duration.
Refrigered space or cold storages should be reviewed for reducing unnecessary loads. Insulation levels, operating temperatures, and loading/unloading frequency need to be reviewed and monitored. Heat infiltration can be reduced by installing strip curtains or fast roll doors. Fluorescent and metal halide lighting introduce parasitic heat loads, so they need be replaced with LED. Also, occupancy sensors can be used to minimize the impact of lighting. In large refrigerated spaces, the air inside tends to settle and form temperature stratification within the space, which may cause wrong temperature readings. Evaporator hoods or destratification fans can be used to equalise temperatures.
Auxiliary components such as fans are recommended having VFD, and also, compressor heat recovery can be considered for improving the system efficiency.