Vapour quality determination for heat pumps using two-phase suction

Abstract

For residential heat pumps, R32 becomes one of the most widely used refrigerant, thanks to an environmental impact much lower than R410A. However, with a classic suction superheat control, R32 induces higher discharge temperatures. Two-phase suction is thus generally allowed in rotary compressors to lower the discharge temperature. In theory, this process can also improve the heat pump performances. However, using two-phase suction for real-time performance optimization implies to identify and control the suction vapour quality. In this study, an iterative method using the compressor energy balance and the compressor efficiencies is presented to determine the suction vapour quality and refrigerant mass flow rate without using a flowmeter. An R32 air-to-air heat pump is then experimentally tested. The reference values are indirectly measured through the mass flow rate measurement and the compressor energy balance. A correlation is thus presented, giving the vapour quality from the operating parameters. If the volumetric or global compressor efficiencies are known, the proposed iterative method allows to calculate the vapour quality with a 0.4 % accuracy. Moreover, this method gives the refrigerant mass flow rate and heating capacity with a 1 % accuracy. The sensitivity of the method is evaluated considering a 10 % error on the volumetric or global efficiency values. In this case, the suction vapour quality values are kept within 2.5 % of the reference measurement. In addition of allowing a precise control of two-phase suction, this method can also make the in-situ performance measurement of these heat pumps possible.

Introduction

Thanks to their high efficiency, heat pumps play a major role in greenhouse gas emission and energy consumption reduction in buildings in Europe. In order to reduce the impact of the refrigerant on climate change, the fluids with the highest global warming potentials (GWP) will progressively be phased out, as a consequence of the F-Gas regulation (EU) No 2014/517 (EU 2014). Therefore, in particular for residential heat pumps, R410A is being replaced with R32, which has a 3-time lower GWP.

R32 can operate under the same temperature conditions than R410A without any major change in the thermodynamic cycle structure and can also improve the heat pump performances. (Xu et al., 2013) experimentally demonstrated that the COP improvement using R32 instead of R410A on the same single-stage heat pump was between 2 and 9 %. However, controlling the expansion valve on suction superheat in the same way would lead to much higher discharge temperatures that could damage the compressor. (Xu et al., 2013) showed that the discharge temperature using R32 was significantly higher than when using R410A in every operating condition, up to + 34°C for an outdoor temperature of - 18°C.

Thus, several patents were published with new control strategies. Using liquid injection cycles would allow to reduce the discharge temperature, but it would increase the manufacturing cost of the heat pump. Taira et al. (2003) recommended a method using the electronic expansion valve (EEV) to control directly the discharge temperature, and Yamada (2017) suggested to use the EEV to control the discharge superheat value, depending on the compressor frequency and the compression ratio. For both solutions, it leads to two-phase fluid at the compressor suction port in most operating conditions, which reduces significantly the discharge temperature. Taira et al. (2003) states that this wet compression cannot harm rotary-type compressors (rolling piston) as long as the suction vapour quality is higher than 0.75. Nowadays, two-phase suction is widely used in order to lower the discharge temperature in R32 air conditioners and heat pumps using a rotary compressor.

The effects of wet compression on the heat pump performances have been studied, but the effect of the discharge temperature control of heat pumps using two-phase suction on vapour quality is unknown. For example, Seong et al. (2017) showed that this process can improve the heat pump performances by lowering the compression ratio and increasing the isentropic efficiency. However, for that purpose, the suction vapour quality should be controlled to an optimal value depending on the operating conditions. In order to optimise the performances of heat pumps using two-phase suction, the real-time estimation of the suction vapour quality on an operating heat pump is necessary. (Huang et al., 2019) presented a method based on Quasi Saturation Isentropic Compression in order to control the discharge temperature. With this method, the refrigerant can also be diphasic at the compressor suction, so the suction vapour quality was estimated using a fixed compressor global efficiency value. This method allows to compare the suction quality values on the same compressor under different operating conditions, but as the method precision is not established by the authors, it is not known if it is accurate enough for wet compression optimisation. (Endoh, 2015) presented a method to determine the suction vapour quality from the mass flow rate measurement and the compressor energy balance. It gives very good results, with a 0.5 % accuracy. However, using a mass flow meter on an operating heat pump is not always possible and can be expensive, so this method is not adapted for a real-time in-situ optimisation purpose.

When the fluid is in two-phase state at the compressor suction port, making the hypothesis that it is saturated leads the compressor energy balance to highly overestimate the mass flow rate. On the contrary, making this assumption to calculate the mass flow rate using the volumetric efficiency or the global efficiency gives underestimated results. The real mass flow rate value can thus be bounded using the compressor energy balance and the volumetric or the global efficiency with a vapour quality of 1. This paper presents two similar methods using iterations on suction vapour quality to make these calculations converge, allowing to obtain the suction vapour quality and the refrigerant mass flow rate without using a flow meter. An experimental study of an air-to-air heat pump using R32 in climatic chambers is then presented. The first objective is to measure the suction vapour quality from the mass flow rate measurement to obtain a correlation as a function of the operating conditions. The second one is to experimentally validate the iterative method, using the compressor energy balance and the volumetric or global efficiency of the compressor, and to compare the results with the reference measurement. The results and the main uncertainty factors of the method are discussed.