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Temperature Glide Matching in a Zeotropic Mixture Heat Pump

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Temperature Glide Matching in a Zeotropic Mixture Heat Pump PROCEEDINGS OF ECOS 2017 - THE 30TH INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS JULY 2-JULY 6, 2017, SAN DIEGO, CALIFORNIA, USA Temperature glide matching in a zeotropic mixture heat pump V. Venzika and B. Atakanb a University of Duisburg-Essen, Thermodynamics, Duisburg, Germany, [email protected] b University of Duisburg-Essen, Thermodynamics, Duisburg, Germany, [email protected] Abstract: The use of zeotropic mixtures in heat pumps should lead to a reduction of the exergy losses within the heat exchangers due to the temperature glide (TG) and thus to an improvement of the entire process. In a previous publication (ECOS 2016), the coefficient of performance (COP) as a function of evaporation / condensation temperature, compressor rotation speed and composition of a zeotropic mixture (isobutane / propene) was investigated. Contrary to theoretical analysis, it was shown that the COPs of the mixtures increase only slightly compared to the better performing pure fluid. The temperature change of the secondary fluid in the heat exchangers was relatively small. In the present work, it is experimentally investigated whether a better matching of the temperature glide in the evaporator raises the COP of a zeotropic mixture compared to the better performing pure fluid. The targeted utilization of the TG is achieved by matching the temperature variation of the mixture and the temperature variation of the heat source; this was adjusted by varying the heat capacity flowrate of the source. An isobutane/propene mixture (42:58) with a TG of 8.03 K is investigated as the working fluid, together with investigations of the pure fluids isobutane and propene, all for equal boundary conditions. The experiments were carried out for a fix evaporator inlet temperature and two compressor rotation speeds. The exergy losses of individual components, and for the whole cycle, like exergetic efficiencies and COPs, were determined and were compared. In case of an optimal temperature match between the heat source and the refrigerant in the evaporator, contrary to the expectation, the COP of the mixture decreases by 4.2 % while the second law efficiency of the entire processes increases by 18.15 %. Keywords: Temperature glide matching, refrigerant mixture, vapour compression heat pump 1. Introduction Presently, some well-established refrigerants from the substance group of HFCs (hydrofluorocarbons) like R134a, R404a, R407c and R410a are used in vapour compression based refrigeration cycles in air conditioning and in heat pump units. However, the regulation (No 517/2014) of the European Union [1] prohibits the use of HFCs with a GWP (Global Warning Potential) above 150 in commercial refrigeration devices, coolers and heat pumps from the year 2022. Therefore, it is essential to investigate alternative fluids for vapour compression systems with respect to ecological aspects (zero ozone depletion potential (ODP) and low GWP) besides the general criteria like thermal efficiency and costs. Alkanes represent an appropriate fluid group with good thermodynamic properties and good material compatibility; they are miscible with commonly applied lubricants, easily available, cost-efficient and environmentally friendly. Pure fluids like propane and isobutane are already used in many European refrigerators. Apart from pure fluids, the use of zeotropic mixtures in vapor compression systems is widely discussed (see e.g. [2], [3]). From the theoretical point of view; simple thermodynamic calculations with a constant compressor isentropic efficiency and similar operating conditions show a high potential to improve the COP by 30 % using a zeotropic mixture of isobutene/propene in comparison to propene as an example. The advantage of a zeotropic mixture, in comparison to a pure fluid, is based on the non-isothermal phase change, known as the temperature glide (TG), which offers the possibility to reduce the mean temperature difference between the refrigerant and the secondary fluid in the heat exchanger. This leads in turn to a reduced difference between the thermodynamic mean temperatures of the heat 1 source and refrigerant, and thus, to an improved performance of the entire process with respect to the energetic and exergetic efficiencies. Based on this theoretical approach, several scientists investigated pure HCs (hydrocarbons) and their mixtures, to find an adequate fluid in order to replace CFCs and HFCs. Chang et. al. [4] investigated experimentally the performances and the heat transfer characteristics of four pure fluids (isobutane (R600a), butane (R600), propane (R290) and propene (R1270)) and furthermore of two mixtures composed of R290, R600a and R600 in order to replace R22 in a heat pump. Their results indicate that the COP of the binary mixture R290/R600a (in a ratio of 50:50 by mass) and R290/R600 (in a ratio of 75:25 by mass) increases by 7 and 11 % in comparison to R22, respectively. However, the COP for the best mixture is, compared to the better pure fluid (propane), only slightly better. Park and Jung [5] carried out a further experimental investigation, aiming to replace R22 by a mixture. In their work, the performance of a heat pump charged with R170/R290 was studied for five different mass fractions of R170 (between 2 and 10 %). It is shown that the cooling and heating rates increase with increasing mass fraction of ethane while the COP of the mixture decreases continuously. Compared to R22, the mixture with a mass fraction of 4 % ethane achieved a similar COP as well as similar cooling/heating rates and is therefore identified as a long-term alternative for R22. Aiming to find an R22 replacement for residential air conditioning applications, Park et. al. [6] investigated the thermodynamic performance of two pure fluids (R290 and R1270) and seven mixtures composed of R1270, R290, R152a and dimethyl ether (RE170) experimentally. Here, the best COP was reached by the ternary mixture composed of 45:40:15 by mass of R1270/290/RE170. However, in comparison to the best performing pure fluid, propane, the ternary mixture enhanced the COP only by 3.5 %. Richardson and Butterworth [7] intended to replace R12 in a hermetic vapor compression system and conducted experiments with R290 and R290/R600a mixtures. Regarding a composition of 50% propane by mass, the COP was improved and a similar saturation characteristic was scored. Based on those results, it is stated that the mixture can be used in an unmodified R12 system. Similar approaches for domestic refrigerators regarding propane and isobutane mixtures were conducted by Refs. [8] and [9]. Further references also with respect to HFC, HFC/HC and azeotropic mixtures can be found in the review of Mohanraj [3]. Many researchers also reported [4], [10], [11] that the heat transfer coefficients of zeotropic mixtures decrease. Summarizing, the reported experimental investigations show that the benefit of mixtures with respect to the COP is often only marginal, and therefore, contradicting the basic theoretical approach while the reason for the discrepancy remains unclear. However, if the experiments are reviewed, it is noted that it was not yet considered to optimize the targeted use of the mixture, meaning with heat sources with considerable change in temperature and also varying this temperature change. In zeotropic fluid mixtures further complications may occur: because of different vapour and liquid flow rates, composition shifts can occur [12]. It is conceivable that besides the TG and thus decreasing exergy loss in the heat exchangers, other components (e.g. the compressor) or properties may be negatively influenced by the use of a zeotropic mixture. Up to now, experimental investigations with respect to the influence on single components with respect to the temperature change of the heat source are lacking. Aiming to improve this situation, we investigated the influence of three different heat capacity flowrates of the secondary fluid within the evaporator on the complete cycle and on each component, for a mixture and for pure compounds, respectively. The change in heat capacity flowrate leads to better or worse matching of the temperature glide of the mixture and the temperature variation of the heat source; their influence is investigated. It is assumed that sometimes the reduction of exergy losses in the evaporator may be overcompensated by larger exergy losses in other parts. Isobutane/propene mixture (42:58) with a TG of 8.03 K is investigated as working fluid, together with investigations of the pure fluids isobutane and propene, all for equal boundary conditions. All experiments were carried out for an evaporator inlet temperature of 0°C and two compressor rotation speeds (1050 min-1 / 2100 min-1). The exergy losses of individual components, and performance factors for the whole cycle, like exergetic efficiencies and COPs, were determined and are compared. 2 2. Experiments 2.1 Experimental setup and measurement equipment The experimental setup with the main components of the heat pump is schematically shown in Fig. 1; it contains a compressor, an expansion valve, a condenser, an evaporator, liquid tank reservoir and further auxiliary valves in order to be able to control the mass flow of the heat source and sink. It was already described previously [14], more details were given there. For later referencing, the states are numbered in Fig. 1. heat source out PC controlled p T p M p evaporator FI p 5 sight glass T 1 expansion valve T p M compressor T rotameter motor heat source in heat sink out 2 p T liquid tank reservoir p CF p condenser 3 p T T 4 p T sample point sampling device T heat sink in suction line T temperature discharge gas line p pressure discharge liquid line CF Coriolis flow meter FI frequency inverter Fig. 1: Schematic flowchart of the experimental rig. A semi-hermetic reciprocating compressor was used which was specially developed for hydrocarbons such as isobutane, propane and propene. It has two cylinders, a displacement volume of 5.4 m³/h and a maximum electrical power intake of 2.2 kW.
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