COMPARISON OF HIGH TEMPERATURE HEAT PUMPS ON 4-TH GENERATION REFRIGERANTS

Transcription

1 COMPARISON OF HIGH TEMPERATURE HEAT PUMPS ON 4-TH GENERATION REFRIGERANTS Ildar A. Sultanguzin, Alina A. Potapova, Alexandre V. Govorin, Andrey V. Albul Industrial Thermal Engineering Systems Department Moscow Power Engineering Institute (Technical University) Krasnokazarmennaya street, 14, Moscow, Russian Federation ; Abstract The present work is devoted to the use of a two-stage heat pump with 17 MW heat capacity with the R-134a refrigerant for central heating systems. An energy and ecological efficiency comparison of new refrigerants, based on fluorinated propylene, that have less effect on the global warming, with R-134a refrigerant is made. The comparison is made on a high-temperature heat pump, that uses the heat of sewage waters. KEYWORDS Fluorinated propylene, refrigerant, global warming potential, thermodynamic cycle, vapor compression heat pump, twostage centrifugal compressor, district heating system, high temperature heat pump, sewage. INTRODUCTION In the world the biggest vapor-compression heat pumps have heat capacity up to 30 MW with two-stage radial compressors [1, 2]. A heat pump station with six heat pumps with a total capacity of 180 MW is built and functioning in Stockholm (Sweden) for the city heat supply. The sea water is used as a heat source, which at winter period has a temperature of С [1]. In Helsinki (Finland) and Oslo (Norway) the heat pumps operate on sewage water [2]. In the summer period they produce both the heat for hot water supply and cold for air conditioning needs of big trade and business centers. The use of heat pumps of large capacities is most effective in large cities, where heavy heat and cold loads are demanded for a long period, where waste recycling (particularly heat waste of sewage waters) is a big problem. In Russia, where the main source of heat are centralized heat supply systems, the use of hightemperature heat pumps with ecological 4th generation refrigerants could have great significance. SEWAGE WATER HEAT PUMP WITH R-134a REFRIGERANT A two-stage heat pump with 17 MW heat capacity shall be regarded, which works on R-134a refrigerant (1,1,1,2-tetrafluoroethane CH 2 F-CF 3 ) with critical temperature С and pressure bar, that does not affect the ozone layer, but has a global warming potential GWP = 1340 compared to CO 2. The sewage waters are use as a heat source. Fig. 1 presents a heat pump setup with the temperature level in the evaporator at 3.5 and 90.1 С in the condenser. The results of calculation of the heat pump [3] showed that energy consumption of the compressor is N e = 7075 kw. The transformation ratio is defined by equation 1: Q cond N e kw 7075 kw (1)

2 For cooling M 5 Sewage T=111ºC T=52ºC 11 2 st. Compressor T=16ºC к ад 2 T=88ºC For heating 75% 1 st. 1 T=52ºC T=4.0ºC Evaporator P=0.333MPa 10 5 Condenser P=3.25MPa 3 Subcooler 4 Intermediate pressure vessel P=1.19MPa T=3.5ºC T=90.1ºC 9 T=10ºC 6 7 T=45.8ºC T=88ºC 8 T=50ºC 0% Fig. 1. Heat balance diagram of a high-temperature heat pump with sewage water as a heat source In the summer period the cooled water is supplied to the condenser instead of sewage waters, thus the heat pump can also produce cold for air conditioning systems with cooling capacity at Q evap = kw. Fig. 2 displays the heat processes diagram based on the calculation results (pressure P enthalpy H). The diagram on fig. 2 shows that the two-stage scheme with intermediate pressure vessel allows to heat the system water most simply and with high reliability [4]. The intermediate vessel works as a phase separator at intermediate pressure after receiving vapor-fluid (flow 8 on Figs. 1 and 2) and over-heated steam (flow 2), and is the simplest way of creating a two-stage scheme (without the risk of fluid going to the second stage of the compressor with flow 3). Additional efficiency upgrading is achieved by subcooling the refrigerant in the supercooler (process 6, 7) since the transferred heat in condenser is raised (5, 6) and the flow rate of refrigerant is constant. The transformation rate μ = 2.4 occurred not to be big enough, but it has to be considered, that the large span between the temperatures of refrigerant in the condenser and evaporater was intentionally chosen (2): Δt = t cond t evap = = 86.6 С. (2)

3 Pressure, kpa VIII Minsk International Seminar Heat Pipes, Heat Pumps, Refrigerators, Power Sources, t = 100 C t = 120 C t = 140 C t = 160 C t = 80 C t = 60 C t = 40 C t = 20 C 10 9 t = 0 C 1 t = -22 C Enthalpy Н, kj/kg Fig. 2. P-H diagram of heat processes of two-stage heat pump with R-134a refrigerant on sewage water CHARACTHERISTICS OF THE HIGH-TEMPERATURE HEAT PUMP WITH DIFFERENT REFRIGERANTS The goal of this work is to compare the energy and ecological efficiency of new refrigerants based on fluorinated propylene, that have little impact on the global warming, with R-134a. The comparison of energy efficiency of different refrigerants is a difficult multiple-factor task, in which their positive properties are compared with their negative ones. Table 1 shows the comparison of characteristics of high-temperature heat pumps with new ecological refrigerants and R-134a. Table 1 uses initial data from different sources: R-1234yf [5, 6], R-1234ze(E) [7], R-1243zf [8], R-1234ye(E) [9]. During the recalculation of stages of compressor with R-134a for another refrigerant a method displayed in [10] was used. The positive properties of new refrigerants are higher critical temperature (apart from R1234yf), bigger flow rate in both compressor stages, bigger transformation ratios, coefficient of compressibility in the 1 st compressor stage closer to 1, which moves them closer to the ideal gas. But there are significant negative properties of refrigerants based on fluorinated propylene: less specific heating power in the condenser Δh cond, less volume heat capacity (VHC) and cold capacity (VCC), less heat pump capacity Q cond with same design characteristics. Table 1. Comparison of characteristics of high-temperature heat pump with new environmentally refrigerants Refrigerant R-134a R-1243zf R-1243ze(E) R-1243ye(E) R-1234yf Global Warming Potential (GWP) Molar weight μ (kg/kmol) Acentrical factor, ω

4 Boiling point at atmospheric pressure Т b, K Critical temperature Т с, K Critical pressure Р с, bar Heat capacity C p, kj/(kmol К) τ = T c / T c R-134a AF = ω / ω R-134a CP = C p / C p R-134a Heat pump Condenser heat capacity Q cond, kw Evaporator heat Q evap, kw Power consumption N e, kw Coefficient of performance COP, μ Lower cycle flow G L, kg/s Higher cycle flow G H, kg/s G H /G L Spercific heat consumption in condenser Δh cond, kj/kg Volume heat capacity in condenser VHC, kj/m Specific volume ν 1, m³/kg Volume flow in stage 1 V 1, m³/s Specific heat consumption in evaporator Δh evap, kj/kg Volume cold capacity VCC, kj/m Volume flow in stage 2 V 2, m³/s compressor stage Coefficient of gas compressibility Z Adiabatic k-value k Sonic speed a, m/s Gas speed u 2, m/s Mach number Mu = u 2 / a Pressure ratio ε Rotation rate, 1/с Density proportion, k v1 = ν 1 / ν compressor stage Gas speed u 2, m/s Mach number Mu Pressure ratio ε Density proportion k v2 = ν 11 / ν

5 ANALYSIS OF TABLE OF COMPARISON OF ENERGETICAL EFFICIANCY OF DIFFERENT REFRIGERANTS IN HEAT PUMPS The main parameters that affect the properties of refrigerants are: T. Relative critical temperature τ of R-1234ze(E) is the largest, next, decreasingly, are c T cr 134a R-1234ye(E), R-1243zf. High temperature is an advantage of refrigerants in question. AF = ω / ω R-134a. The acentrical factor of molecules of refrigerants in hand is less, than of R-134a, which «squeezes» their P-H characteristic (lessens the specific heat of evaporation) and is a disadvantage. The closest to R-134a is R-1243zf. CP C p C p R. Specific molar heat capacity CP is a characteristic of vapor-liquid compound. The 134a raise of CP leads to a raise of decline of the top in the critical point of P-H characteristic and moves it to the right. All refrigerants in question have CP larger than R-134a, which as a disadvantage. Refrigerant R-1243zf is closest to R-134a. The analysis cannot be narrowed only by three examined parameters. In reality the efficiency of the heat pump is affected by lost of other factors. The more heat is transferred in the condenser Q cond, the more effective is the refrigerant. This parameters is one of the main when it comes to choosing the recommended refrigerant. R-1243zf has maximum Q cond. The amount of heat of R-1234ye(E) and R-1234ze(E) is significantly less. The amount of heat of every kg of the cooled refrigerant, transferred to water in the condenser Δh cond = =h 5 h 6 (Fig. 2) of refrigerants in question is less than R-134a has. That is a disadvantage. The volume heat capacity of the top cycle (with the second stage of the compressor) VHC of refrigerants in hand is less than R-134a has. That is their substantial disadvantage. The decrease of volume cold capacity of the lower cycle (with the first stage of compressor) VCC compared to R-134a is a disadvantage of refrigerants in hand. The specific volume ν 1 of refrigerants in question is bigger than R-134a has. That is a disadvantage. The volume flow in the first stage of the compressor is equal V 1(R-1234ze(E)) = 0,0786 m³/kg, which negatively affects the work of the compressor. The volume flow V 1 = G L /v 1 of refrigerants in hand in the first stage of compressor is a bit more than R-134a has (except for R-1234ys). That is and advantage. The raise of the volume flow is explained by the raise of the volume proportion of refrigerants in hand compare to R-134a. This leads to a bigger mass flow ratio of the working fluid. The volume flow V 11 = G H v 11 in the second stage of compressor of refrigerant R-1243zf is bigger than the same for other refrigerants. That is and advantage. The mass flow ratio of the lower stage of the heat pump G L = G H (h 4 h 7 ))/(h 4 h 9 ) of refrigerants in hand is less than R-134a has. That is a disadvantage, especially for R-1234yf. The mass flow ratio in the lower stage of the heat pump of refrigerants in hand is less than R-134a has. That is a disadvantage, especially for R-1234yf. The mass flow ratio in the upper stage of the heat pump G H = (Q cond )/(h 5 h 6 ) allows to raise the flow of the working fluid in the upper cycle. For R-1243zf this parameter is significantly larger compared to other refrigerants. This is the substantial advantage when it comes to choosing a refrigerant. For R-1234yf it does not give a significant advantage due to a small value of Δh cond. The power consumption for both stages of compressor N e for refrigerant R-1234ze(E) are the lowest, then (ascending) R-1234ye(E), R-134a, R-1243zf. The less the power consumption, the cheaper the heat pump is to run. Coefficient of transformation of heat COP, φ is maximal for R-134a, for R-1234yf it is minimal, which is a disadvantage. COP is a ratio between heating capacity and consumed power. The coefficient of compressibility of gas Z is taken from the main parameters table. The closer the value is to 1, the closer the refrigerant is to the ideal gas by it s properties. The refrigerants in hand are closer to 1, than R-134a, which is their advantage, The energy consumption is proportional to the adiabatic k-value. The decrease of this index of refrigerants in hand compared to R-134a is their disadvantage. Sonic speed. R-1243zf has a maximal sonic speed, which lowers the Mach number with same gas speed in the compressor and less possibility of hit. The Mach number is defined as the ratio of the speed of body,

6 that moves in an environment, to the sonic speed in that environment. For home compressors the maximum Mach number is about 1.4. The raise of Mu above this level can lead to lowering the efficiency coefficient of stages of compressor. Rotary velocity of gas on the exit of a stage of compressor u 2 = 3.14(wheel size)(number of revolutions). The raise of this parameter for the most of refrigerants in hand compared to R-134a is an advantage. The limiting factor for most of new refrigerants is the Mach number in the first stage of compressor, for R-1234yf in the second stage. Number of revolutions per minute influences to the speed u 2, the bigger value, the bigger speed we can achieve. R-1243zf has a maximum allowable rotary velocity. The raise of ratio of density of gas on the exit of the stage of compressor to the density of gas on the entrance k y of the gases in hand compared to R-134a brings to raise of volume flow, which is a positive factor. The results of calculations show, that the best alternative for R-134a is R-1243zf. It has the maximum heat pump capacity Q cond mainly because the maximum sonic speed, that compensates the volume heat capacity, it is widely used to produce polymers. It has some drawbacks, e.g. R-1243zf is flammable. It can be assumed, that it s influence on the global warming is low (GWP = 3). The second recommended refrigerant is R-1234ye(E). It can be assumed, that by influence on the global warming it will be close to R-1234yf which has GWP = 4. But his Q cond is significantly less. The third by energy efficiency is the R-1234ze(E) refrigerant. The refrigerant R-1234ys cannot be recommended for use in high-temperature heat pumps because of low critical temperature. References 1. Bailer P., Pietrucha U. Disrtict heating and district cooling with large centrifugal chiller heat pumps // Proc. 10 th Intern. Sympos. on District Heating and Cooling, Hanover, Germany. 3 5 September Unitop 50FY heat pump/chiller units simultaneously generate 90 MW heat energy and 60 MW chilled water // 3. Sultanguzin I. A., Potapova A. A. High-temperature high power heat pumps for a heat supply // Novosti Teplosnabzheniya (News of a Heat Supply) No. 10. Pp (in Russian). 4. Potapova A. A., Sultanguzin I. A. Use of heat pumps in the heat supply system of a factory and a city // Metallurgist Vol. 54. Nos Pp Leck T. J. Evaluation of HFO-1234yf as a Potential Replacement for R-134a in Refrigeration Applications // Proc. 3 rd IIR Conf. on Thermophysical Properties and Transfer Processes of Refrigerants, Boulder, CO, Paper Akasaka R., Tanaka K., Higashi Y. Thermodynamic property modeling for 2,3,3,3-tetrafluoropropene (HFO-1234yf) // Int. J. of Refrigeration Vol. 33. Pp Akasaka R. An application of the extended corresponding states model to thermodynamic property calculations for trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) // Int. J. of Refrigeration Vol. 33. Pp Brown J. S., Zilio C., Cavallini A. Thermodynamic Properties of Eight Fluorinated Olefins // Int. J. of Refrigeration Vol. 32. Pp Zernov V. S., Kogan V. B., Lyubetsky S. G., Duntov F. I. Equilibrium liquid vapor in an ethylene trifluorinepropylene system // Zhurnal Prikladnoi Himii (J. of an Applied Chemistry) No. 3. Pp (in Russian). 10. Sarevski M. N. Influence of the new refrigerant thermodynamic properties on some refrigerating turbo compressor characteristics // Int. J. of Refrigeration Vol. 19. Pp