Comparative evaluation of R1234yf, R1234ze(E) and R450A as alternatives to R134a in a variable speed reciprocating compressor

Highlights

• Correlations based on dimensionless parameters are proposed for compressor efficiencies.

• The Buckingham π-theorem was applied to compressor efficiencies.

• An energetic comparison between R134a, R1234yf, R450A and R1234ze(E) is developed.

• R1234yf, R450A and R1234ze(E) volumetric efficiencies are lower than R134a.

Abstract

A comparative energetic evaluation of R1234yf, R1234ze(E) and R450A as alternatives to R134a in a variable speed compressor is carried out. A compressor model based on dimensionless numbers was obtained using the Buckingham π-theorem, which was validated with experimental data; showing that the prediction error of the model is lower than ±10% and ±2 K for temperature. The experimental data were obtained by testing R134a, R1234yf, R1234ze(E) and R450A for a wide range of operating conditions. Results obtained with the validated model, show that the dimensionless approach provides a similar estimation of energy parameters compared with the experimental results, such as power consumption, refrigerant mass flow rate, cooling capacity, COP, discharge temperature and compressor efficiencies for each refrigerant tested using the dimensionless approach proposed. The comparative evaluation of the compressor predictions shows a reduction in the cooling capacity obtained with R1234yf, R450A and R1234ze(E), in comparison with R134a. Also, COP values for R1234yf, R450A, and R1234ze(E) are lower than those obtained from R134a. Finally, results shows that the dimensionless correlation compressor model can be used to predict the performance of other reciprocating compressors, at similar operating conditions for a wide range of compressor rotation speed, with a reasonable accuracy.

Introduction

Hydrofluorocarbons (HFCs), as non-ozone depletion substances, covered almost all refrigeration and air conditioning applications replacing chlorofluorocarbons [1]. Nowadays, HFCs are used all over the world (except in some developing countries) being the most relevant R134a, R404A, R407C and R410A. Even though the most relevant greenhouse gas is CO₂ due to fossil fuels burning, HFCs also have significant contribution in global warming [2]. If HFCs were phased out, as proposed under the Montreal Protocol, global warming would prevent up to 0.5 K by the end of the century [3].

To prevent climate change, HFCs with high global warming potential (GWP) are being reduced due to national and communitarian environmental policies [4]. R134a have a GWP value of 1300 and it is one of the most used HFCs in medium-temperature refrigeration and air conditioning applications (i.e. chillers, mobile air conditioning, stationary refrigeration, etc.) [5].

Besides natural alternatives (hydrocarbons, carbon dioxide and ammonia) [6], hydrofluoroolefins (HFO) and mixtures of them with HFCs have very-low GWP, being options to replace HFCs [7]. R1234yf and R1234ze(E) are today the most promising HFOs alternatives. Both fluids are cataloged as low-flammable fluids (A2L) by ASHRAE and their GWP values are 1 and 4, respectively. Studies have revealed low performance for R1234yf [8] and considerable low cooling capacity for R1234ze(E) [9] in retrofit substitutions.

Mixtures formed by R1234yf or R1234ze(E) with HFCs are also an option to take into account when good performance is desire and acceptable values of GWP [10]. Among the different mixtures commercialized, R450A can be used in light retrofit substitutions in R134a systems [11], [12], obtaining a similar energy efficiency with a few system modifications [13].

Thermophysical properties of HFOs and their mixtures have been recently defined in the literature and these data can be used to design properly system components (compressor, evaporator, condenser, expansion device, etc.) [14]. Compressor designing has a great influence on the net performance of the vapor compression system, hence on the CO₂ equivalent emissions [15].

Reciprocating compressors are used in several refrigeration and air conditioning applications, for the majority of refrigerants in the market (except R600a and R717). Table 1 shows the applications where the reciprocating compressor is used [16].

According to Table 1, there are several refrigeration systems operating in the world, therefore, experimental and computational evaluations are essential to improve the accuracy of predictions and reducing development time of new refrigerants and their systems.

Reciprocating compressor modeling can be divided into three categories: (i) empirical or map-based models, which describe capacity, energy consumption and discharge temperature, are determined by polynomial equations that fit experimental data of the compressor. ARI Standard 540 [17] recommends the use of third-degree-equations of 10 coefficients in the calculation of power input, mass flow rate of refrigerant and compressor efficiency. (ii) Semi-empirical models are generally based on simple thermodynamic correlations which fits experimental data [18], [19], [20]. (iii) The fundamental models or white box models are used to study details of the compressor design such as valve flows, cylinder heat transfer, cylinder-piston leakage, bearing losses, among many others [21], [22], requiring large amount of data.

Some investigations have addressed the modeling of reciprocating compressors for refrigeration systems, for example Winandy et al. [19] proposed a simplified steady-state compressor model. Their model needs seven parameters to compute the mass flow rate, mechanical power, exhaust temperature, and ambient losses. Navarro et al. [23] developed a model which predicts compressor and volumetric efficiencies in terms of ten parameters. Pérez-Segarra et al. [24] presented a detailed analysis of thermodynamic efficiencies used to characterize hermetic compressors. Negrao et al. [18] presented a semi-empirical model to predict the transient mass flow rate and the power of a domestic refrigeration compressor. Li [25] developed a detailed analysis of semi-empirical methods to calculate mass flow rate, shaft power and discharge temperature for three types of variable speed compressors: reciprocating, scroll and piston rotary. The proposed methods are an integration of physical-based models for constant speed compressor and the physical characteristics of volumetric efficiency and isentropic efficiency between different speeds.

Because of this, a model based on dimensionless volumetric, isentropic and overall efficiencies for variable speed reciprocating compressor is presented. This model characterizes the compressor efficiencies in terms of certain groups of dimensionless parameters. Specific data from the compressor are required to determine the values of the dimensionless parameters. When these parameters are known, the model can be applied to obtain information of the compressor behavior using low GWP refrigerants R1234yf, R1234ze(E) and R450A. Therefore, the aim of this paper is to verify the capabilities of the aforementioned model to analyze the performance of a variable speed reciprocating compressor using R1234yf, R1234ze(E) and R450A as working fluids. Finally, the relative predicted differences, when those refrigerants replaced R134a in refrigeration systems, are examined through COP, power consumption, discharge temperature and cooling capacity.