International Journal of Refrigeration 30 (2007) 3e10 www.elsevier.com/locate/ijrefrig Natural refrigerant-based subcritical and transcritical cycles for high temperature heating J. Sarkar, Souvik Bhattacharyya*, M. Ram Gopal Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India Received 11 April 2005; received in revised form 31 January 2006; accepted 13 March 2006 Available online 5 June 2006 Abstract Theoretical analyses of subcritical/transcritical heat pumps using four natural refrigerants, carbon dioxide, ammonia, pro- pane and isobutane have been carried out for high temperature heating applications at different heating outlet temperatures and heat sources using computer models. The compressor discharge pressures have been optimized for transcritical and subcrit- ical (with near critical operation of condenser) cycles. Results show that for subcritical heat pumps, use of subcooling is efficient for heating applications with a gliding temperature. Results also show that although propane yields better coefficient of perfor- mance (COP) in low temperature heating applications, ammonia performs the best in high temperature heating applications. Finally, design aspects of major components of all the four heat pumps for high temperature heating have been discussed, par- ticularly with reference to suitability of available lubricants to the newly evolved operating conditions. Ó 2006 Elsevier Ltd and IIR. All rights reserved. Keywords: Heat pump; Carbon dioxide; Ammonia; Propane; Isobutane; Research; Thermodynamic cycle; Transcritical cycle; Performance; COP Cycles souscritiques et transcritiques utilisant des frigorige`nes naturels dans les applications a` tempe´rature e´leve´e Mots cle´s : Pompe a` chaleur ; Dioxyde de carbone ; Ammoniac ; Propane ; Isobutane ; Recherche ; Cycle thermodynamique ; Cycle transcritique ; Performance ; COP 1. Introduction negative impact on the environment, researchers have pro- posed various alternative synthetic refrigerants of zero In the last few decades, several studies have been carried ODP such as R143, R152, E143 and E245 [1]. However, out to find suitable refrigerants for heat pumps applicable to these refrigerants have higher GWP. Hence, environmen- high temperature heating. Previously, R115 was used for tally benign natural refrigerants such as carbon dioxide, am- high temperature process heat applications, but due to its monia, propane, butane, isobutane and propylene [2] appear to be better alternatives for heat pump applications. Ammo- * Corresponding author. Tel.: þ91 3222 282904; fax: þ91 3222 nia is a very old refrigerant, which is generally used up to 255303. a temperature of about 70 C. Carbon dioxide has been re- E-mail address: [email protected] (S. Bhattacharyya). vived as a potential refrigerant for heat pump applications, 0140-7007/$35.00 Ó 2006 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2006.03.008 4 J. Sarkar et al. / International Journal of Refrigeration 30 (2007) 3e10 Nomenclature AT temperature approach (K) Subscripts COP coefficient of performance 1e6 state points of refrigerant f friction factor c refrigerant outlet in condenser/gas cooler h specific enthalpy (kJ kgÿ1) ci fluid inlet to condenser/gas cooler Nu0 Nusselt number at bulk property co fluid outlet from condenser/gas cooler P pressure (MPa) crit critical property Pr Prandtl number dis compressor discharge Rc compressor pressure ratio e evaporator Re Reynolds number opt optimum t temperature ( C) sv saturated vapour 3 Vc volumetric capacity (MJ/m ) which can be used up to 120 C [3]. Although most of the heat pumps for high temperature heating have been common process heat applications in pulp, paper, chemical, discussed with a special reference to whether currently avail- plastics, agricultural, textile, plaster and food industries re- able lubricants would be suitable for use in the new systems quire heating up to 140 C [1], some food drying and petro- having difficult operating conditions. chemical processes require temperatures above 200 C. Because no such heat pump system is available for these 2. Optimum discharge pressures high temperature heating applications, electrical heater or heat transformer is used in these applications (e.g. in milk Performance analyses of a carbon dioxide based tran- powder production, air is heated up to 70e80 Cbya scritical cycle showed that there exists an optimum com- heat pump and then by an electrical heater up to the re- pressor discharge pressure, where the cycle yields the quired temperature), which yields a COP less than or equal maximum COP. The optimum pressure depends on evapora- to 1.0. In this study subcritical and transcritical heat pumps tion temperature, cooler exit temperature, internal heat using four natural refrigerants, ammonia, carbon dioxide, exchanger effectiveness and compressor isentropic propane and isobutane have been proposed for high temper- efficiency. However, the effect of the last two parameters ature heating applications. Barring a few individual defi- is found to be negligible compared to others [4]. Similar be- ciencies, such as toxicity and flammability of ammonia, haviour can be observed for other refrigerant-based tran- flammability of hydrocarbons, and high pressure and low scritical cycles also. It is very interesting to note that theoretical COP of CO2, these natural refrigerants offer var- subcritical cycles also yield optimum discharge pressure ious advantages when compared to synthetic refrigerants. when the condenser is operated near the critical point Even though the theoretical COP of a simple CO2 cycle is (Table 1). For the subcritical cycles, if the temperature of relatively low, the COP of an actual CO2 system could be the refrigerant at condenser exit is in subcooled region, an considerably higher due to high compressor efficiency and optimum condensation pressure is found to exist, which excellent transport properties of CO2. During the last few yields maximum COP. Fig. 1 shows the Peh diagram with years there have been many studies, which show that the several constant temperature lines. Due to the curvy nature COP of a real CO2 cycle is higher than that of cycles using of the isotherms in the super-critical region, such cycles conventional working fluids. This indicates that actual sys- tend to have an optimum gas cooler pressure [4]. If we lower tem’s COP depends very much on component and system the gas cooler exit temperature in subcooled region, this na- design. However, this aspect cannot be revealed in a rela- ture of the isotherms may continue up to some extent and the tively simple theoretical study such as the one presented existence of optimum condensation pressure is observed; in this paper. however, eventually the isotherms become vertical and there In the present study, the compressor discharge pressure is no optimum point in the subcooled region. In that case, of transcritical cycles have been optimized and then perfor- the saturated liquid point becomes the optimum. Thus, to mance analyses of heat pumps based on four natural refrig- erants (carbon dioxide, ammonia, propane and isobutane) Table 1 have been carried out for heating applications at different Critical properties of natural refrigerants studied heating outlet temperatures. Two heat sources: a low temper- Refrigerant Carbon Ammonia Propane Isobutane ature source (e.g. ambient air or ground water) and a high dioxide temperature source (e.g. power plant condenser) have been t ( C) 31.06 132.25 96.70 134.70 considered for the analyses. Finally, design related issues crit P (MPa) 7.377 11.33 4.248 3.640 of the major components of all the four refrigerants based crit J. Sarkar et al. / International Journal of Refrigeration 30 (2007) 3e10 5 5 9 120 ºC 130 ºC 140 ºC te = 0 ºC 4.5 Curvature of 8 P isotherms te = 60 ºC dis,opt 7 4 6 Isobutane 3.5 5 Propane 3 100 oC 4 Ammonia 2.5 3 Pressure (MPa) COP increment (%) 2 2 1 1.5 0 02468101214 1 K 400 500 600 700 800 tcrit tc ( ) Specific enthalpy (kJ/kg) Fig. 2. COP improvement through discharge pressure optimization. Fig. 1. Peh diagram of isobutane. with higher increment of COP value due to the divergent na- summarize, the central concept is that for subcritical cycles, ture of compression line in superheated region. So, for most with near critical operation of condenser, an optimum refrig- of the refrigerants, this behaviour may be observed signifi- erant outlet condition in the condenser exists in subcooled cantly till about 5 K below the critical temperature. region near critical point, and with far critical operation, A detailed thermodynamic analysis has yielded optimum the optimum point shifts to saturated liquid line for a given discharge pressures for carbon dioxide, ammonia, propane refrigerant outlet temperature. and isobutane based transcritical cycles. Computer codes Employing the latest available literature on the thermo- for cycle simulations have been developed incorporating, physical properties of carbon dioxide [5], a comprehensive in turn, property code for each refrigerant. The discharge property code ‘CO2PROP’ has been developed [4]. Based pressure has been optimized numerically. It is observed on the seminal works of Haar and Gallagher [6], and that variation of the optimum compressor discharge pressure Younglove and Ely. [7], additional property codes have with cooler exit temperature and evaporation temperature of been developed to estimate thermodynamic properties of the transcritical ammonia cycle shows similar behaviour