1、2009 ASHRAE 241ABSTRACTResearch on a novel implementation of an Ericsson cycle heat pump for near ambient refrigeration applications was performed. The concept, termed a liquid-flooded Ericsson cooler (LFEC), uses liquid flooding of the compressor and expander to approach isothermal compression and
2、expansion processes. A numerical simulation of the cycle was developed and parametric studies were performed to explore the sensi-tivity of the cycle to changes in various system parameters such as, pressures, heat exchanger effectiveness and working fluids. The goal of the study was to assess the v
3、iability of the tech-nology for vending machine applications. It was found that the target cooling coefficient of performance (COP) of 1.25 could be attained if the adiabatic efficiency of the compressor and expander were 85%. INTRODUCTIONThis paper presents a parametric study of a liquid-flooded Er
4、icsson cooler (LFEC). The LFEC was described in Hugen-roth et al. (2007), where a thermodynamic analysis of the cycle was presented. This analysis assumed ideal gas and constant specific heats for the gas and liquid. The liquid-flooded Eric-sson cooler (LFEC) is a modification of the basic reverse E
5、ric-sson cycle that overcomes the substantial practical difficulties of achieving isothermal compression and expansion processes. In the LFEC, isothermal compression and expan-sion are approached by mixing a nonvolatile liquid with a noncondensable gas during the compression and expansion processes.
6、 The term “flooded” comes from the notion that the compressor and expander are flooded with large quantities of liquid. Liquid mass flow rates may be significantly greater than gas mass flow rates. This is in contrast to oil injection schemes in some types of positive displacement compressors where
7、the principle purpose is to improve sealing of the leak-age paths and the reduction of friction within the compressor, and the oil flow rates represent only about 1% to 5% of the total flow by mass. A practical approach for achieving liquid flood-ing would be to utilize oil as the liquid in combinat
8、ion with compressors/expanders that would tolerate high oil volumes, such as scroll compressors. Scroll compressors are fixed volume ratio machines. This allows them to tolerate liquid flooding since a finite gas volume remains in the discharge pockets when the fluid is ejected through the discharge
9、 port. Off-the-shelf scroll compressors have been shown to operate with reasonable effi-ciency for hundreds of hours under liquid flooded conditions (Hugenroth 2006, Hugenroth, et al. 2008). Hugenroth (2006) contains a detailed discussion of liquid flooded compression theory and practical considerat
10、ions.The motivation of LFEC research is the elimination of HFC refrigerants, which are potent greenhouse gases. Gas cycles, such as the Ericsson cycle, can use environmentally benign working fluids, such as air, argon, xenon, or helium. Replacement of HFC refrigerants with natural working fluids wou
11、ld reduce the direct impact of refrigerant leakage on global warming. However, in order to not increase the indirect global warming impact due to burning of fossil fuels for elec-tricity generation, alternatives to vapor compression systems should have equal or better operating efficiencies.The part
12、icular applications being considered for the LFEC technology were vending machine bottle coolers with a cooling capacity of 380 W. These bottle coolers are free stand-ing units that are approximately the size of full sized refriger-ators used in U.S. homes. They have a single swing-open glass door a
13、nd shelving for holding bottled or canned drinks. While Evaluation of a Novel Liquid-Flooded Ericsson Cycle Cooler for Vending Machine ApplicationsJason Hugenroth, PhD, PE James Braun, PhDAssociate Member ASHRAE Fellow ASHRAEEckhard Groll, PhD Galen King, PhDFellow ASHRAEJason Hugenroth is the owner
14、 of InvenTherm, a research and development consulting company. This research was completed while he was a student at Purdue University, West Lafayette, IN. James Braun, Eckhard Groll, and Galen King are professors in the School of Mechanical Engineering, Purdue University.LO-09-021 2009, American So
15、ciety of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written
16、 permission.242 ASHRAE Transactionsthe bottle cooler was the primary application of interest, the analysis applies to any system operating with the same sink and source temperatures. The goal of the study was to deter-mine the operating parameter values (e.g. pressures, mass flow rates) that minimiz
17、e the required efficiency of various compo-nents in the system while meeting the specified COP and capacity requirements.A schematic of a LFEC is shown in Figure 1. Two arrangements are shown. The arrangement in Figure 1a is being called a Type 1 liquid-flooded Ericsson cooler (LFEC1). The LFEC1 is
18、how the concept was originally conceived. Hugenroth et al. (2008) reported on the experi-mental work of a system using this arrangement. However, it was later found that the COP is greater for the arrangement shown in Figure 2b. This is being called a Type 2 liquid-flooded Ericsson cooler (LFEC2). T
19、herefore, the LFEC2 is the focus of this study. In Figure 1b, the solid lines correspond to liquid flows while the dashed lines are gas flows. A solid line next to a dashed line indicates a liquid gas mixture. The liquid and the gas are separate substances (e.g. oil and nitrogen). No phase change oc
20、curs for the fluids in the system. The components to the left of the regenerator are on the hot side. (i.e. temperatures are at or above ambient) while the components on the right side are on the cold side (i.e. temper-atures are below ambient). The system operates as follows: Starting at state poin
21、t 1, a low pressure high temperature gas flows into the hot side mixer where it mixes with liquid coming from state point 9. The gas and liquid are different substances and are assumed noncondensable and nonvolatile, respectively. The liquid and gas mixture (state point 2) enters the compressor wher
22、e they are compressed simultaneously. The liquid absorbs much of the heat of compression, such that the temperature Figure 1 (a) Schematic of Type 1 liquid flooded Ericsson cooler (LFEC1). (b) Schematic of Type 2 liquid flooded Ericsson cooler (LFEC2).ASHRAE Transactions 243of the fluid mixture at s
23、tate point 3 is much lower than it would be for a dry compression process. This occurs because the thermal capacitance of the liquid is greater than that of the gas, and intimate thermal contact between the liquid and gas is achieved. For sufficient liquid flooding the process is nearly isothermal.
24、The fluid mixture then enters the hot side heat exchanger where heat is rejected from the system. The fluid flow at state point 10 enters the hot side separator where the liquid and gas streams are separated. The high pressure gas stream (state point 4) enters the regen-erator where heat is rejected
25、 to the low pressure stream. The gas exits the separator as a cold high pressure stream (state point 5). The gas then enters the cold side mixer where it mixes with the high pressure liquid stream (state point 13). This mixture is discharged from the mixer (state point 6) before entering the expande
26、r. The fluid mixture exits the expander as a low pressure low temperature liquid and gas mixture. The temperature of the fluid is higher than it would be for a dry expansion process. The liquid and gas mixture enters the cold side heat exchanger where heat is absorbed from the refrigerated space. Th
27、e fluid stream at state point 12 enters the cold side separator where the liquid and gas are separated. The low pressure low temperature gas at state point 8 then enters the regenerator where it absorbs heat from the high pressure gas stream. The path just described, traced by the dashed line, is re
28、ferred to as the gas loop. The path traced by the solid line on the hot side of the system is referred to as the hot liquid loop, and the path traced by the solid line on the cold side of the system is referred to as the cold liquid loop. Completing the hot liquid loop starting at state point 14, th
29、e liquid exits the hot side separator at high pressure. It passes through a hydraulic motor to lower its pressure, recovering shaft work during the process. The liquid exiting the hydraulic motor (state point 9) enters the hot side mixer, as described previously. Similarly, on the cold side of the s
30、ystem the liquid exiting the cold separator (state point 11) is at low pressure. A pump is used to increase the pressure of the liquid to the high side system pressure (state point 13).The reason for using a liquid gas mixture is that, in prin-ciple, the work required to compress the gas is substant
31、ially reduced because of the higher specific volume that results from the liquid absorbing the heat of compression. Simi-larly, an increase in expander work output will occur, in theory. When the rotating machinery in the system has an adiabatic efficiency of 100% and the heat exchangers have effect
32、iveness values of 100%, then the COP of the system approaches the Carnot COP as the liquid flooding rate increases Hugenroth, 2006. In addition, the gas loop processes are identical to the Ericsson refrigeration cycle.The LFEC Type 1 and 2 are novel system concepts developed by this papers authors.
33、As stated, the motivation of the research was the elimination of HFC refrigerants, which are potent global warming gases. Additional, infor-mation about LFEC systems can be found in the literature shown in the References section. CYCLE MODEL DEVELOPMENTA numerical model was developed to simulate the
34、 LFEC2. Several improvements were made over the model developed for the LFEC1 analysis that was presented in Hugenroth et al. (2007). These include:The impact of pressure drop on cycle performanceModeling of gas in the liquid line due to incomplete separationModeling of liquid in the gas line due to
35、 incomplete separationReal gas flooded compression and expansion modelingReal gas modeling of the heat exchangers and regeneratorAllowing for temperature dependence of liquid specific heat and specific volumeWhile the effect of small liquid droplets of oil in the gas loop was modeled, the effect of
36、oil vapor in the gas loop was neglected. Oil vapor in the gas loop was estimated to be a maximum of 0.3% of the total flow in the gas loop (Hugen-roth 2006).Real Gas Liquid Flooded Compression and ExpansionReal gas flooded compression and expansion were modeled by employing a control volume analysis
37、 assuming negligible kinetic and potential energy changes and steady state operation. It was further assumed that the flow was homogeneous (the liquid and gas move through the control volume at the same speed), and the oil and gas are in thermal equilibrium. A mathematical description of the cycle s
38、imu-lation including flooded compression and expansion processes is given in the Appendix. Modeling of the Remaining Components in the System The remaining components in the LFEC system model include hot and cold side heat exchangers, a regenerator, hot and cold side mixers, hot and cold side separa
39、tors, a hydrau-lic motor, and a pump. Effectiveness models were used for the heat exchangers. The temperature of the liquid and gas exiting the separators is assumed to be equal to the fluid inlet temperature. The mixer model implements an adiabatic mixing process. The pump and hydraulic motor were
40、modeled in the same manner as the compressor and expander, respectively, to account for gas in the liquid loops due to incomplete separation in the separators.For all components in the system, excluding the rotating machinery, pressure drops are accounted for by specifying a pressure drop across eac
41、h component in the system. This 244 ASHRAE Transactionssimple approach was used so that the sensitivity of the cycle performance to pressure drop could be investigated. Fluid PropertiesHighly accurate real gas equations of state were used for all of the gaseous fluids considered (Klein, 2006). The o
42、il used in the modeling and the experimental work (Hugenroth et al. 2008) was 60 SUS alkyl-benzene oil. Specific heat data were obtained for the oil using differential scanning calorimetry and a curve fit for the data is given in the Appendix. Fluid CarryoverIn practice, the separators used in a rea
43、l system will not perform perfect separation. Therefore, gas remains in the liquid loop flows while liquid is entrained in the gas loop flow. The presence of a secondary fluid in the primary fluid flows will result in different properties for the combined flow. The LFEC2 simulation calculates the ch
44、ange in the fluid mixture properties due to fluid carryover using a model that is presented in the Appendix.Performance ParametersPerformance parameters for the model are presented in this section. The cooling capacity ( ) is(1)where is the enthalpy and the subscript refers to the state points shown
45、 in Figure 1b.The heat rejected ( ) is(2)The cooling coefficient of performance (COP) is defined as(3)The second law efficiency is defined as(4)The entropy generation rate is(5)The heat transfer rate term ( ) was zero except for the hot and cold heat exchangers, and Ticorresponded to either the ambi
46、ent or cold space temperature.Parametric StudiesThree parametric studies were performed to study impacts of different system parameters and operating condi-tions on design requirements for a fixed cooling capacity. The first two studies focused on the bottle cooler (i.e. vending machine) application
47、 that was the motivation for the research program. This application required a COP of 1.25 with a cool-ing capacity of 380 W (1297 Btu/hr). The ambient and cold space temperatures were 32.2C (90F) and 2C (35.6F), respectively. In order to attain a COP of 1.25 the rotating machinery in the system (i.
48、e. compressor, expander, pump, hydraulic motor) must operate at some minimum adiabatic efficiency. This minimum efficiency will vary depending on system pressure drops, fluid carryover, heat exchanger effec-tiveness, and system operating pressures. The first parametric study investigated the sensiti
49、vity of the required rotating machinery efficiency to changes in the other system parame-ters. Table 1 summarizes the input parameters used for the study. For each case, the values for the compressor and expander and the compressor pressure ratio were optimized. QinQinH12H7=HQoutQoutH10H3=COPQinWnet-=IICOPCOPcarnot-=SgenSQiTi-=QiCratioTable 1. Inputs for Sensitivity StudyCase Figurem,preghxxc, xeycP2,kPa (psia)P,kPa (psia)1 2 n/a 0.81-0.9 0.85 0.9 0.01 0.01 500 (72.5) 5 (0.73)2 3 n/a 0.85 0.81-0.9 0.9 0.01 0.01 500 (
