Circuit configurations of large heat pumps
To increase the efficiency of compression heat pumps and optimize system operation, various circuit configuration variants have been developed that are now widely available on the market. The choice of the appropriate circuit configuration variant allows for optimal adjustment to specific temperature and performance requirements, which improves the efficiency of overall operations.
While more complex circuit configuration concepts that integrate additional components to increase the COP involve higher investment costs, they also offer the potential for significant efficiency improvements. Therefore, it is important to weigh the possible benefits against the additional costs. As operating time increases, the lower operating costs can offset the initial additional investments, allowing for the economic viability of an circuit configuration variant to be assessed based on the planned operating hours.
A classic compression heat pump consists, in its simplest form, of an evaporator, compressor, condenser, and expansion valve, and operates in a counterclockwise thermodynamic cycle. First, the refrigerant evaporates in the evaporator by absorbing heat from a heat source. Then, it is compressed in the compressor, which increases pressure and temperature. In the condenser, the refrigerant releases energy to a heat sink and condenses. Finally, the refrigerant is expanded in the expansion valve and returned to its initial state before the process begins again.
By expanding a single-stage cycle with an internal heat exchanger (IHX), the efficiency of the system can be further increased. After evaporation, the gaseous refrigerant is superheated in the IHX, which prevents potential liquid slugging in the compressor and enhances operational safety. At the same time, the liquid refrigerant is subcooled after exiting the condenser, which also contributes to a higher overall efficiency of the system. These measures optimize the entire heat transfer process and increase the performance of the heat pump.
In two-stage or multi-stage compression with intermediate cooling, there are two common configurations, both aimed at increasing efficiency, particularly at higher temperature lifts, by dividing the compression process into multiple stages.
Multi-stage cycle with economizer
In this configuration, compression is performed using sequentially arranged compressors. This reduces the pressure ratio in each compressor, which enhances energy efficiency. During intermediate compression, superheated vapor from the economizer is fed into the process. A portion of the liquefied refrigerant is expanded to a medium pressure level, passed through the economizer, and then supplied to the second compressor. The increase in efficiency results from lower exergy losses, as only a portion of the refrigerant is expanded to the low-pressure level. This approach is widely used in the industry and has been successfully implemented in commercial high-temperature heat pumps.
Multi-stage cycle with flash tank
Similar to the economizer, this cycle also performs compression in two stages with two compressors and injects vapor in the intermediate process. However, this cycle uses a flash tank (separator) instead of a closed economizer. In this case, saturated vapor is injected instead of superheated vapor, allowing for a different form of intermediate cooling.
In a heat pump equipped with an ejector, the expansion process occurs polytropically. By throttling the working fluid flow after the condenser, a compression of the suction flow after the evaporator takes place. This leads to an increase in efficiency, as the pressure lift that the compressor must achieve is reduced due to this pre-compression. In a flash tank, phase separation of the refrigerant occurs, with the exiting saturated vapor being passed through an IHX to achieve superheating and avoid liquid slugging in the compressor. However, it should be noted that the use of an ejector is not equally suitable for every refrigerant.
By adding additional evaporators or condensers, the system can be optimized for different temperature levels at both the heat sources and heat sinks.
Additional evaporators (boosters)
An additional evaporator allows for the utilization of heat sources at different temperature levels. This configuration can be combined with an economizer.
Additional condenser
In this configuration, the condensation process is divided between two condensers operating at different pressure conditions. This offers a significant advantage for processes with a high temperature difference between the inlet and outlet temperatures of the secondary heat transfer fluid at the heat exchanger.
In a heat pump cascade, two circuits are connected in series to efficiently bridge larger temperature lifts. The first circuit compresses the refrigerant to a medium temperature level. The condenser of the first circuit serves as the evaporator for the second circuit. The second circuit compresses the refrigerant to the required temperature level for the heat sink. By splitting into two circuits, compressors and refrigerants can be optimally matched to their respective temperature ranges, resulting in a higher COP. However, there may be losses due to gradients in the heat exchanger between the two heat pumps.
By expanding the refrigerant in a turbine (expander) to drive the compressor instead of using an expansion valve, the external drive power of the compressor can be reduced, potentially leading to an increase in efficiency. The use of expanders in large heat pumps particularly depends on the specific properties of the refrigerant and the operating conditions of the system.