Experimental and numerical study of a water-to-water heat pump working with co2

Resumen

Resumen de la tesis: In this thesis, a one-stage transcritical CO2 water-to-water heat pump is numerically and experimentally analyzed in the production of hot water for the applications of space heating and domestic hot water (DHW). Different configurations commonly used in the market are studied and compared for the same water temperatures at the heat sink (evaporator) and heat source (condenser). From now on, since in CO2 transcritical cycles does not exist phase change, the condenser is called gas cooler. Initially, four configurations (C#) are presented, three of them belong to the dry evaporator group, C1 and C2 include liquid receiver, high pressure control and superheating control, and the remaining one, C4, does not use liquid receiver, neither has high pressure control, only superheating control. The difference between C1 and C2 is a bypass that is used in C1, but, when the bypass is closed, the cycle is working in C2. The other configuration, C3, is a flooded evaporator system, with high pressure control, but without superheating since it is a flooded evaporator cycle. One of the most important studies in this thesis is the gas cooler optimal pressure. A numerical model to describe the behavior of a CO2 transcritical heat pump for hot water production is developed and experimentally validated. This model has allowed obtaining relatively simple expressions that can be programmed in a PLC to regulate the pressure in the gas cooler by measuring the temperature of the refrigerant in three points of the cycle. The proposed model is capable of limiting the compressor discharge temperature to 140 °C while maintaining the COP with theoretical deviations of less than 2 % respect to optimal pressure conditions. The liquid receiver pressure, the evaporation temperature, the Internal Heat Exchanger (IHX) efficiency and the superheating degree, have relatively low influences on the system behavior. Furthermore, since the influence of the compressor efficiency is relatively low as well, the model is applicable for a wide range of reciprocating semi-hermetic compressor models, which are the most common in the field of stationary heat pumps. Considering the comparison of the different configurations, one of the first decisions after performing the experimental tests, is letting C4 out of the comparison since this configuration is not able to compete with the other three due to the lack of high pressure control. The second decision after analyzing that the influence of the liquid receiver is negligible showing slightly better performance when the bypass is totally closed and followed by totally opened, the first two configurations were treated as a single configuration, called “C1&C2”. Therefore, the comparisons are mostly performed between “C1&C2” and C3, Furthermore, the influence of varying the heat exchangers (HX) surface is also studied for space heating applications. For the space heating and DHW applications the differences between “C1&C2” and C3 are negligible. However, in the dry evaporator cycles, if the liquid receiver is usually regulated by a gas bypass with the aim of reducing the pressure of the lines that distribute liquid to the evaporators, to allow an adequate pressure regulation range, the liquid receiver should be sized according to the rest of the components and to the usual operating conditions. Varying the HX surfaces showed improvement in most cases numerically studied for space heating application, but there exist optimal surfaces for the different temperature range and this should be considered when designing the heat exchangers, at least for low/medium and high/very high temperatures. http://repositorio.bib.upct.es/dspace/