1、748 2009 ASHRAEABSTRACT This paper discusses how exergy efficiency can help generate insight into effective and ineffective temperature combinations for heat exchange at near-environmental temperatures. The analysis uses exergy and energy efficien-cies, combined with exergy consumption, warm/cool ex
2、ergy and a dimensionless temperature, to gain insight into the effect of varying temperatures in air-to-air heat exchange at near-environmental temperature. The analysis is performed with a simple model for an air-to-air sensible heat exchanger. The paper presents an example of how the approach can
3、be used as a basis to select exergy efficient temperature combinations when conceiving heat exchange in building ventilation.INTRODUCTIONThis paper is concerned with the effectiveness of temper-ature levels on the exergy efficiency of thermal exergy transfer at near-environmental temperatures.The no
4、tions of heat exchanger heat transfer effectiveness, functional exergy efficiency and exergy consumption are combined with the concept of warm and cool exergy, in order to obtain new insights that may be useful when specifying the operating temperatures of air-to-air sensible heat exchangers used at
5、 near-environmental temperatures. It is possible to define exergy efficiencies in various ways, depending on the significance of various conditions such as sensitivity for changes in a system, applicability in practice, accuracy and accessibility (Sami, 2008; Seme-nyuk, 1990; Sorin and Brodyansky, 1
6、992; Tsatsaronis, 1993 and 2002; Kotas, 2001). Kanoglu, Dincer and Cengel (2008) discuss exergy effi-ciency in heat exchange involving phase change, as well as for other processes. Hepbasli (2008) presents a review on exergy analysis of renewable energy resources. Kanoglu, Dincer and Rosen (2007) pr
7、esent expressions for and examples of exergy analysis for power plants.In the literature, links have been made between sustain-ability, exergy consumption and heat transfer at near-envi-ronmental temperatures, including warm and cool exergy (Shukuya, 1996; Shukuya and Hammache, 2002) and tepi-dology
8、 (Wall, 1990). Links have also been made between exergy resource efficiency and sustainability (Granovskii, Dincer and Rosen, 2008; Swaan Arons et. al., 2004; Connely and Koshland, 2001). Semenyuk (1990) discusses heat exchanger exergetic efficiency as a function of a dimensionless temperature, and
9、indicates domains of technically inexpedient heat exchanger operation. Similarly to the present study, his analysis shows that using hot thermal carriers (above environmental temper-ature) to heat cold thermal carriers (below environmental temperature) is irrational since this heating could be accom
10、-plished by using environmental air. However, his approach is computationally more complex, as it requires knowledge of heat exchanger inlet and outlet temperatures. Moreover, his study focuses on operating temperatures relatively far from the environmental temperature.Wu et al. (2006) use the conce
11、pt of heat transfer effective-ness (ASHRAE, 2000; Holman, 2002) to indicate the relative magnitude of the heat transfer, and perform detailed compar-isons of exergy transfer effectiveness with heat transfer effec-tiveness, for parallel flow, counter-flow and cross-flow heat exchangers operating abov
12、e and below the surrounding Functional Exergy Efficiency at Near-Environmental TemperaturesE.C. Boelman, PhD P. Sakulpipatsin, PhD H.J. van der Kooi, PhDL.C.M. Itard, PhD P.G. LuscuereMember ASHRAEE.C. Boelman and P. Sakulpipatsin are members of the faculty of architecture in the building technology
13、 section, H.J. van der Kooi is an assistant professor in the field of applied thermodynamics and chemical engineering, L.C.M. Itard is a researcher in the field of sustainable buildings and HVAC equipment and leads the group sustainable and healthy building of the Research Institute OTB, P.G. Luscue
14、re is a profes-sor of building services in the Climate Design Group in the faculty of architecture at Delft University of Technology, Delft, the Netherlands. LO-09-070 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transact
15、ions 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 permission.ASHRAE Transactions 749temperature. They analyze variations of exergy transfer effec-tiveness with numb
16、er of transfer units (NTU), with the ratio of the heat capacity of cold fluid to that of hot fluid (Cc/Ch) and with finite pressure drops. They note that there is not an opti-mal combination of NTU and Cc/Chfor maximizing exergy transfer effectiveness. They do not elaborate on the effects of tempera
17、ture variations.Peng et.al. (2007) address the dependence of heat transfer effectiveness on the number of heat transfer units, the heat capacity ratio and the flow pattern. They present an expression for calculating the optimal value of the heat capacity ratio for achieving maximum exergy efficiency
18、, and conclude that this optimum is different for heat exchangers operating above or below environmental temperature. They present results of exergy efficiency with different flow patterns for counterflow, cross flow and parallel flow heat exchangers, all operating below environ-mental temperature.
19、Their study does not directly address the effect of temperature variations.Hirs (2003) defines the thermodynamic performance of a heat exchanger as being determined by the input exergy of the stream that is cooled and the output exergy of the stream that is heated, and the difference between these t
20、wo as being the exergy loss due to loss in quality of the transferred heat. He defines the thermodynamic efficiency as the ratio between output and input exergy, and omits exergy losses due to the flow resistance to enable a direct comparison between the thermodynamic efficiency and the heat exchang
21、er heat transfer effectiveness. Hirs defines two ratios Rcand Rhof temperatures at the heat exchanger inlets and the environmental temperature (Tc,in/Teand Th,in/Te). For a coun-tercurrent heat exchanger with equal thermal capacities of hot and cold fluid, he plotted heat exchanger effectiveness aga
22、inst thermodynamic efficiency and found that at higher values of the temperature ratio, Rc 1, the thermodynamic efficiency is clearly above the effectiveness. At lower values of the temperature ratio, RcCh)mass flow for hot air: 1 kgs-1(Cc = Chand Cc Ch) and 10 kgs-1(Cc Th,in Tc,inand the heat excha
23、nge process can be regarded as relatively far from environmental conditions.Figure 3 illustrates how the dimensionless temperature can express different temperature combinations of Tc,inand Th,in, for a given environmental air temperature Te= 10C.For , Tc,inis equal to Te. In practice, this could co
24、rrespond e.g. to a heat exchanger taking up environmental air at the cold heat exchanger inlet in order to (pre) heat it for use in balanced ventilation systems. For , Tc,inis above Te. This could be the case for ventilation air being pre-heated (e.g. by a sun room or in buried air ducts) above Tebe
25、fore reaching the heat exchanger inlet. For , Teis between Tc,inand Th,in. This could be the case of heat exchange between air from a freezer and from an office, but is unlikely to occur in space heating applications. For , Teis above Tc,inand Th,in. This could be the case upon heat exchange between
26、 two cold air streams and is also unlikely to be the case in space heating applications.Heat Exchange at Near-Environmental TemperaturesFor the purposes of this paper, environmental tempera-tures are based on typical ranges applicable to HVAC systems in heating applications (Table 2). Heat exchanger
27、 operating temperatures are taken over a somewhat broader range than in usual HVAC applications, in order to enable the temperature combinations required to obtain dimensionless temperatures covering the relatively broad range of . A number of heat exchanger inlet air temperature combinations are de
28、fined, within the ranges given in Table 2. Air temperatures at the heat exchanger outlets are then determined, as simple functions of inlet air temperatures, exchanger heat transfer effectiveness and heat capacity ratios.The equivalent temperatures (engel and Robert, 2001) of the cold air Teq,cand t
29、he hot air Teq,hare calculated from the air temperatures at the heat exchanger inlets Tinand outlets Tout.Table 2. Temperature Domains253.15 K Te 293.15 K10K Th,inTc,in 130K0 5293.15 K Th,in 373.15 K283.15 K Tc,in 363.15 KT T Figure 2 Simplified diagram of the thermal exergy profiles in a counter-fl
30、ow sensible heat exchanger (environmental air temperature and operating temperatures and above ).TeThTcTeT Th,inTeTh,inTc,in-=T T T 4 T 40 T 1 Chwe have(7)(8)In the specific case of balanced heat capacity ratios (Cc= Ch), equations 5 and 7 become(9)(10)Functional Exergy EfficiencyThe functional exer
31、gy efficiency of the heat exchanger can be defined as a ratio of all product outputs Exproductto all source inputs Exsource, as shown in equation 11 (Woudstra, 2002).(11)When the goal is to increase the thermal exergy of the cold air by exergy transfer from the hot air, the cold air thermal exergy i
32、ncrease Excis taken as the net product output, and the absolute value of the hot air thermal exergy decrease |Exh| is considered as the net source input. (12)Figure 3 Examples of dimensionless temperatures in relation to and for .Tc,inTh,inTe10=TeqToutTinlnToutTin-=QQmax-CcTc,outTc,in()CminTh,inTc,i
33、n-ChTh,inTh,out()CminTh,inTc,in-= =Tc,outTc,inCminCc-Th,inTc,in()+=Th,outTh,inCcCh-Tc,outTc,in()=Th,outTh,inCminCh-Th,inTc,in()=Tc,outTc,inChCc-Th,inTh,out()+=Tc,outTc,in Th,inTc,in()+=Th,outTh,in Th,inTc,in()=ffExproductExsource-=fExcExh-Exc,outExc,inExh,inExh,out-=ASHRAE Transactions 753By assumin
34、g that the heat exchanger is well insulated and that airflow effects are neglected (Table 1), thermal energy is assumed to be completely transferred from the hot air to the cold air (Qh,out= Qc,in). Equation 12 can be rewritten as a func-tion of the temperatures (Tc,in, Th,in, Tc,out, and Te) and th
35、e total heat capacities of the air streams (Chand Cc), as shown in equation 13.(13)A similar expression, as a function of heat capacity ratio (Ch/ Cc), can be written as:(14)The functional exergy efficiency can also be calculated as a function of dimensionless temperature and exchanger heat transfer
36、 effectiveness (ASHRAE, 2000; Holman, 2002; Wu et al., 2006), without the need to first determine outlet temperatures.For CcChwe have(16)For balanced heat capacity ratios (Ch = Cc), equations 15 and 16 simplify to (17)RESULTSPrevious papers (Boelman et.al., 2008; Boelman and Sakulpipatsin, 2005) dis
37、cussed the temperature sensitivity of functional exergy efficiency regarding heat exchange at near-environmental temperatures, for balanced heat capacity ratios. Hirs (2003) related thermodynamic efficiency to thermal effectiveness for heat exchangers, for balanced heat capacity ratios, and noted th
38、e importance of selecting appropriate temperature combinations to avoid exergy losses below envi-ronmental temperature.The present paper discusses how temperature combina-tions (expressed as a dimensionless temperature ) can affect the functional exergy efficiency of a simplified heat exchanger used
39、 for heating applications, and presents guide-lines for selecting temperature combinations likely to be more effective from the viewpoint of thermal exergy.Dimensionless Temperature and Functional Exergy Efficiency for Heating ApplicationsFigure 4 schematically illustrates the relationship between i
40、nlet air temperatures Th,in, Tc,inand environmental air temperatures Te. This relationship is expressed in terms of the dimensionless temperature defined in equation (2), for temperature combinations corresponding to 0 |Exh|. From an exergy viewpoint this heat exchange could have been effective if t
41、he desired prod-uct had been cooled air, although another definition would have been needed for . Heating below Teis considered out of scope and will not be discussed further in this paper.Dimensionless temperatures between ca. 0.35 and 0.65 correspond to heating across the environmental temper-atur
42、e Te. For example when the hot air enters the heat exchanger at Th,in Teand exits at Th,inTe. On the cold air side, the loss in cool exergy is bigger than the gain in warm exergy, which results in Exc Chthan for Cc 0Exc = 0Teq,c= TeExc TeTeq,c TeExh 0, Teq,h TeExh 0+Teq,hTe+Exh= 0Teq,h= TeExh 0Teq,h
43、 TeExc|Exh|heating above Te(effective range for heating)heating across Te(ineffective range, not recommended)heating below Te(out of scope)Th,in100.00 40.00 25.00 22.00 20.50 19.78 19.00 17.55 15.29 15.28 14.50 10.00 -50.00Th,out89.50 29.50 14.50 11.50 10.00 9.28 8.50 7.05 4.79 4.78 4.00 -0.50 -60.5
44、0Teq,h94.73 34.72 19.72 16.72 15.22 14.50 13.72 12.27 10.01 10.00 9.22 4.72 -55.29Tc,in85.00 25.00 10.00 7.00 5.50 4.78 4.00 2.55 0.29 0.28 -0.50 -5.00 -65.00Tc,out95.50 35.50 20.50 17.50 16.00 15.28 14.50 13.05 10.79 10.78 10.00 5.50 -54.50Teq,c90.22 30.22 15.22 12.22 10.72 10.00 9.22 7.77 5.51 5.5
45、0 4.72 0.22 -59.79Exh,in11.909 1.493 0.386 0.249 0.191 0.166 0.141 0.099 0.049 0.049 0.036 0.000 7.464Exh,out9.479 0.645 0.036 0.004 0.000 0.001 0.004 0.016 0.049 0.049 0.065 0.201 10.6272.430 0.847 0.350 0.245 0.191 0.165 0.137 0.084 0.000 0.000 0.029 0.201 3.163Exc,in8.509 0.386 0.000 0.016 0.036
46、0.049 0.065 0.100 0.171 0.172 0.201 0.414 12.191Exc,out10.839 1.089 0.191 0.098 0.063 0.049 0.036 0.016 0.001 0.001 0.000 0.036 8.739Exc2.330 0.703 0.191 0.082 0.027 0.000 -0.029 -0.084 -0.170 -0.171 -0.201 -0.378 -3.452Notes:1. The unit of the temperatures (Te, Th,in, Th,out, Teq,h, Tc,in, Tc,outan
47、d Teq,c) is degree Celsius (oC). 2. The unit of exergy values (Exh,in, Exh,out, Exh, Exc,in, Exc,outand Exc) is kilojoules (kJ).3. Dimensionless temperature ( ) and functional exergy efficiency ( ) are dimensionless.4. The numbers highlighted in bold indicate Tinand Teq(equation 3) equal to Te.5. Ex
48、ergy differences (Exh, Exc) correspond to the differences between air exergy at the inlets and outlets.6. Functional exergy efficiencies are calculated using equation 17.T ffExhT ffffffT 1fT 1.5fff756 ASHRAE Transactionsfunctional exergy efficiency clearly shows less sensitivity to , particularly for the two upper and two lower values of considered. Also, the lines do not converge towards = 1 as they do for balanced heat capacity rates. Figure 8 shows another similar plot of ver
