Comparative Assessment of Operation Thermal Characteristics of Different Types of Refrigeration/Heat Pump Systems for Space Cooling/Heating

The electrification of the thermal sector through refrigeration/heat pumps for space cooling and heating is a fundamental part of the energy transition. In this paper, dynamic models of different types of refrigeration/heat pump systems (air-air, water-water, air-water, water-air refrigeration/heat pumps) are developed and applied to estimate and compare their operation thermal characteristics. The main purpose of the paper is to show the benefits of water/water refrigeration/heat pumps and to emphasize their growing importance in the context of the 5th generation district heating and cooling systems (centralized/decentralized refrigeration/heat pumps). The dynamic models are developed in Matlab and the thermodynamic properties of refrigerants are calculated using Coolprop. Using these models, the influence of the time variable heat sources and heat sinks, which depend on variable outside climate conditions and indoor temperature conditions, on the performance characteristics is investigated. Moreover, on/off control and variable speed control are analyzed as control strategies for demand side management to respond to intermittent renewable energy generation, taking advantage of the thermal inertia of the building and energy storage. The results show the COP dependence on the type of refrigeration/heat pump, temperature lift, outside climate conditions, inside temperature conditions, compressor efficiency, heat transfer conditions in the evaporator and the condenser, refrigerant thermodynamic properties, and thermodynamic cycle characteristics. The water-water refrigeration/heat pumps can reach the highest COP (COP>7). They have higher operational flexibility because the water can also be a medium for heating and cooling storage. The COP of air-air refrigeration/heat pumps strongly depends on outside climate conditions. For extreme winter climate conditions, when the heating demand is large, the COP is low (COP<3) and the heating capacity of the air-air refrigeration/heat pump decreases. However, water accessibility is a constraint for the wider application of water-water refrigeration/heat pumps. This can be overcome with the 5th generation of district heating and cooling. The COP of the decentralized plants is high because the evaporation temperature is high (heat pump) and the condensing temperature is low (refrigeration system). Also, the COP of the central plant is high because the temperature lift is low. The high COP and wide operational flexibility make water-water refrigeration/heat pumps an asset in the energy transition and their wider applicability is facilitated by the development of the 5th generation district heating and cooling systems.