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Development of advanced heat transformers utilizing new working fluids
Development of advanced heat transformers utilizing new working fluids H. B o k e l m a n n and F. Steimle* GEA G m b H , Research and Development Department, Bochum, F R G *Universit~it Essen, Institut f/Jr Angewandte Thermodynamik und Klimatechnik, Essen, F R G
Based on experience from an industrial-scale heat transformer, which is now commercially available, GEA has embarked on a new research and development programme: the development of an advanced heat transformer and investigation into the combination heat transformer/absorption heat pump. Advanced heat transformers are heat transformers utilizing new working fluids and/or a multi-stage absorption plant. A brief report is presented on an experimental research programme at the University of Essen which aims to discover new working fluids for sorption plants. Some of the most important results for heat transformer application are presented in the form of graphs or tables and the suitability of the new working fluids is compared with some well-known working couples. The Industrial requirements for an advanced heat transformer are also considered along with the possibility of meeting these requirements. Finally, the heat transformer is discussed in the context of other heat recovery systems on the basis of two fictitious industrial applications. (Keywords: heat transformers; workingfluids)
Mise au point de transformateurs de chaleur perfectionn de nouveaux fluides actifs
s utilisant
A patir de rexpdrence d'un transformateur de chaleur d'~chelle industrielle, disponible maintenant dans le commerce, GEA a entrepris un nouveau programme de recherche-dOveloppement 'Mise au point d'un transformateur de chaleur perfectionn~ et recherches sur la combinaison transformateur de chaleur-pompe d chaleur c~absorption'. Les transformateurs de chaleur perfectionn~s sont ceux utilisant de nouveaux fluides actifs et/ou des installations dt absorption multi-~tagdes. Le prdsent article rend bridvement compte d'un programme de recherches exp~rimentales effectu~ ti l'universit~ d'Essen dans le but de trouver de nouveaux fluides de travail pour les installations ~J sorption. Quelques uns des r~sultats les plus importants pour rapplication aux transformateurs de chaleur sont pr~sentds sous forme de graphiques ou de tableaux et l'aptitude des nouveaux fluides de travail est comparde d celle de m6langes de travail trds connus. Un autre chapitre traite des besoins de rindustrie pour un transformateur de chaleur perfectionn~ et des possibilities d'y rOpondre. Enfin on compare le transformateur de chaleur d d'autres systdmes de rdcup~ration de chaleur en examinant deux applications industrielles fictives.
(Mots cles: transformateurs de chaleur; fluides de travail)
An energy saving system has been described 1 which was built by GEA for an animal carcass plant in Drrnten. Using a water-LiBr heat transformer this pilot plant provided evidence that a single stage heat transformer (HT) is capable of recovering waste heat on an industrial scale. Nearly 45~o of the waste heat at 100°C was transformed to useful heat at 145°C. Nevertheless, it should be noted that a single stage water-LiBr plant cannot always cope with the requirements of a heat recovery system. Due to the solubility characteristics of the working couple, water-LiBr, the temperature lift between the waste heat and the useful heat is restricted and the useful m a x i m u m temperature is limited because the solution is highly corrosive at high temperatures. However, many inquiries from potential customers have shown that temperature lifts to high operating temperatures are often desirable or demanded. Therefore, the development of an advanced heat transformer suited to these applications is necessary. 0140-7007/86/0100514)9503.00 :. © 1986 Butterworth & Co (Publishers) Ltd and IIR
Advanced heat transformers utilize new working fluids and/or multi-stage sorption plants including combinations of sorption heat pumps and heat transformers. This Paper deals with the search for new working fluids with multi-stage absorption cycles and presents some examples of their application.
New working fluids for heat transformers A brief report is given on an extensive experimental research p r o g r a m m e r performed at the University of Essen with the aim of finding new working fluids for sorption plants. Although this project mainly concerned heat p u m p application, some of the working fluids
"i" The research project was sponsored by: the Commission of the European Commurfities; Arbeitsgemeinschaft fiir Sparsamen und Umweltfreundlichen Energieverbrauch(ASUE); and Deutscher Verein des Gas- und Wasserfaches (DVGW)
Rev. Int. Froid 1986 Vol 9 Janvier
51
Advances heat transformers. H. Bokelmann and F. Steimle suitable for heat p u m p application were also suitable for heat transformers. Within the scope of the research project nearly 150 different systems were subjected to point m e a s u r e m e n t s and the relevant properties (vapour pressure, density, viscosity, thermal stability, v a p o u r liquid equilibria, heat of mixing, heat capacity, etc.) of the most interesting fluid pairs determined. The experimental results were fitted to sets of correlating e q u a t i o n s which, in turn, were used to construct diagrams a n d tables for practical use. This Paper deals with those results which are of interest for heat transformer application. Extensive descriptions of the whole project can be found in References 2-5.
Experimental procedure For this p r o g r a m m e a large n u m b e r of refrigerants a n d a b s o r b e n t s were taken into account. The refrigerants were chosen in accordance with the r e q u i r e m e n t s listed in Table 1. F o r heat transformers the following substances are of special interest: trifluoroethanol (TFE); hexafluoroi s o p r o p a n o l ( H F I P ) ; p e n t a f l u o r o p r o p i o n i c acid (PFPA).
Suitable a b s o r b e n t s were chosen according to their molecular structure giving the possibility of hydrogen b o n d f o r m a t i o n in a d d i t i o n to the requirements of Table 1. D u r i n g the preliminary investigation point m e a s u r e m e n t s of the solubility of the refrigerants were made. Based on these m e a s u r e m e n t s a limited n u m b e r of promising working fluids were chosen for the following further investigations: m e a s u r e m e n t of a line of c o n s t a n t pressure; c o n s t r u c t i o n of provisional equilibrium charts: d e t e r m i n a t i o n of the thermal stability.
Results F o r the a b o v e - m e n t i o n e d refrigerants the following a b s o r b e n t s were f o u n d highly promising: q u i n o l i n e (Chi): tetraethylene glycol dimethyl ether (DTG); ethylpyrrolidone (EP); isoquinoline (Ich); m e t h y l p y r r o l i d o n e (MP); N - m e t h y l p y r r o l i d o n e (NMP); p y r r o l i d o n e (Pyr); tetraethylene glycol (TEG). Table 2 presents the results of the solubility measurements, expressed as mass fractions, at c o n s t a n t pressure of the following systems: T F E - C h i ; T F E - D T G ;
Table I Requirementsfor working fluids Tableau 1 Propriet~s exigees des fluides actifs Requirement
Subject
Effect
High specific enthalpy of evaporation Favourable pressure range
Refrigerant Refrigerant
Desired temperatures attainable Low specific mass flow ratio High difference in boiling points Low pressure difference Low viscosity High density Low specific heat capacity
Type of solution Type of solution Working fluid Refrigerant Solution Solution Solution
High thermal and chemical stability
Refrigerant and solution
Reduction in fluid flow: reduction in pump energy Lower requirements for the apparatus design: reduced weight and costs Realization of the process Reduction in fluid flow; reduction in pump energy No need for rectification Reduction in pump energy Reduction in pump energy Reduction in pump energy Low heat flux in the apparatus of the solution circuit: smaller size and lower costs Realization of the process
Low toxicity Low corrosivness Low inflammability Low price
Refrigerant, absorbent and solution
Responsibilityfor the application and economy
Table 2 Solubilitydata
Tableau 2 Solubilit#s
System TFE Chi TFE DTG TFE EP TFE ICh TFE MP TEE NMP TFE Pyr TEE TEG HFIP-NMP PFPA--DTG
52
Evaporation temperature (C) 0 50 0 50 0 50 0 50 0 50 0 50 0 50 0 50 10 40 0 50
. . 170
.
Refrigerant content in the solution at various absorption temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 150 140 130 120 110 100 90 80 70 60 50 40 30 0.003 0.045 0.070 0.108 0.152 0.214 0.284 0.365 0.450 0.029 0.037 0.060 0.094 0.132 0.187 0.262 0.355 0.457 0.117 0.132 0.211 0.285 0.352 0.414 0,480 0.540 0.586 0.005 0.062 0.090 0.126 0.175 0.239 0.312 0.390 0.463 0.093 0.153 0.195 0.254 0.309 0.366 0.434 0.505 0.585
0.140 0.225 0.298 0.366 0.423 0.515 0.566 0.634 0.092 0.153 0.200 0.257 0.320 0.379 0.449 0.534 0.617 0.025 0.052 0.066 0.090 0.122 0.166 0.230 0.315 0.410 0.629 0.664 0.605 0.637 0.666 0.697 0.728 0.750 0.781 0.519 0.140 0.258 0.464 0.525 0.601 0.649 0.691 0.727 0.762 0.795
Int. J. Refrig. 1986 Vol 9 January
0.054 0.542 0.052 0.574 0.194 0.667 0.061 0.549 0.144 0.674 0.220 0.700 0.151 0.703 0.043 0.527 0.700 0.811 0.630 0.833
20'C
0.107 0.197 0.287 0.392 0.494 0.091 0.153 0.247 0.369 0.507 0.301 0.385 0.461 0.546 0.109 0.198 0.295 0.396 0.216 0.298 0.390 0.485 0.588 0.328 0.423 0.500 0.572 0.649 0.236 0.323 /).419 0.520 0.627 0.073 0.122 0.203 0.322 (t.'468 0.732 0.766 0.8(/2 0.839 0.846 0.676 0.720 0.755 0.794
Advanced heat transformers: H. 8okelmann and F. Steimle
TFE-EP; TFE-MP; TFE-NMP; TFE-ICh; TFE-Pyr; TFE-TEG a (all at a pressure of 18.8 mbar); and H F I P NMP and PFPA-DTG (at pressures of 90 and 5 mbar, respectively). During these measurements the evaporation temperature was kept constant while the absorption temperature was varied. For each chosen absorption temperature the amount of the refrigerant in the absorbent was determined. With the help of two lines of constant pressure, measured at different evaporation temperatures, it is possible to construct a provisional equilibrium chart (p,t,x-chart) (Figures 1-4). These provisional equilibrium charts were used to determine some key figures for the heat transformer process (e.g. the specific mass flow rate). These are helpful for assessing the new working fluids against well-known working couples. This comparison is presented in Table 3. It refers to the systems TFE-DTG, TFE-NMP, TFE-Pyr, H F I P NMP, PFPA-DTG and the well-known couples ammonia-water and water-LiBr. The Table contains the following values and aspects: r o, enthalpy of evaporation at 0°C; tkr, critical temperature; P2o, vapour pressure of the refrigerant at 20°C; P~oo, vapour pressure of the
,ooo
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0
20
40
I
60 t (*C)
I
I
I
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i
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80 100120140160 200
Figure 3 Provisional equilibrium chart of TFE-Pyr Figure 3 Diagramme de requilibre provisoire TFE-Pyr
o,o
f--/////Z/ff-
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°
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Figure 1 Provisional equilibrium chart of TFE-Chi Figure 1 Diagramme de £~quilibre provisoire TFE-Chi
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Figure 4 Provisional equilibrium chart of H F I P - N M P Figure 4 Diagramme de l'~quilibre provisoire HFIP-NMP
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Figure 2 Provisional equilibrium chart of TFE-DTG Figure 2 Diagramme de I'~quilibre provisoire TFE-DTG
|
200
refrigerant at 100°C; price per kg of solution; toxicity; corrosivness; Ats, boiling point difference; and number of measured data (thermophysical properties). Comparison is made at a generator temperature of 100°C and at a concentration difference between the strong and poor solutions of 5~ at three different sets of working conditions for evaporation temperature, tE, and condensing temperature, tk: 100 and 20°C; 100 and 50°C; 50 and 20°C, respectively. The selection of the second and third working conditions is necessary to allow a comparison with the accepted working couples for which the first working condition is not attainable since with ammonia as refrigerant the pressure reached is unacceptably high and with water-LiBr crystallization problems occur. The following derived values are of great interest and a r e l n c l u d e d in Table 3: concentration of the poor solution; ~,, which should be high to reduce the specific mass flow rate; attainable absorp.tion temperature, ta, which should be high to reach high useful heat temperatures; hypothetical temperature difference in the absorber, Ata, which should be low to increase t,; specific mass flow rate, f , which should be low to decrease the necessary pump energy and the size of heat and mass flow areas. As shown in Table 3, the new working fluids reach higher absorption temperatures than the accepted
Rev. Int. Froid 1986 Vol 9 Janvier
53
Advanced heat transformers." H. Bokelmann and F. Steimle systems. Systems T F E - N M P and TFE-Pyr are especially promising. The thermodynamic properties of the system H F I P N M P are most suitable for utilization in heat pumps. However, the price of the refrigerant is too high for economical application at the moment. PFPA would seem to be suitable as it is available in large amounts but it is too expensive at the moment to be used as a refrigerant in large heat transformers. As the chemical market is subject to fluctuations it is worth pursuing the PFPA systems. Compared with the water-LiBr system the toxicity of the refrigerant TFE must be mentioned as a disadvantage and the different heat and mass transfer behaviour might cause a minor decrease in the coefficient of performance. Irrespective of the above, the advantages of T F E - N M P and TFE-Pyr pairs seem to justify additional investigation.
II
:n
d d ~ d d
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+++11
o
oo
zz~
++
~
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~
=~
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=
Further investigations Some of the detailed measurements of the systems T F E N M P and TFE-Pyr are presented as diagrams. The measured values and the equations can be found in Reference 2. Figure 5 is the equilibrium chart of the system TFE N M P which was established by measuring the vapour pressure of different solutions at approximately constant concentrations over the whole temperature range of interest. In this case nearly 170 measured points are correlated by the least squares method to polynomial equations which are the basis for the p,t,x-charts. This chart for T F E - N M P shows that the solution field of the system permits the realization of heat transformer processes with reasonable temperature lifts. In addition, the chart shows that the difference of boiling points is similar to that determined for ammonia. Rectification may be needed, therefore, but can be avoided by using TFE Pyr which, compared with T F E - N M P , offers a similar solubility behaviour but a higher boiling point difference (Figure 3). For generator/condenser conditions the boiling behaviour of the mixtures can be appreciated from the t, i-chart (Figures 6 and 7), which are based on vapour liquid equilibria measurements. Figure 8 shows the enthalpy-concentration chart of system T F E - N M P . A detailed description of how the chart was constructed is given in Reference 6.
I000 500
o~
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e~
==
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60
8 0 I00 120140160 2 0 0
t (°C}
Figure 5 Equilibrium chart of TFE-NMP Figure 5 Diagramme de lYquilibre TFE-NMP
54
Int. J. Refrig. 1 9 8 6 Vol 9 January
Advanced heat transformers." H. Bokelmann and F. Steimle 260 0.0
0.1
0.2
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4(. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 C Figure 6 t, ~ diagramfor TFE-NMP Figure 6 Diagramme t, ~ pour TFE-NMP
0.0 0.8 0.9
1.0
0.1 0.2
0.3
0.4
0.5
0.6 0.7
0.8 0.9
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Figure 7 Preliminaryt, ~ diagramfor TFE-Pyr Figure 7 Diagrammepr~liminaire t, ~ pour TFE-Pyr
1200[ I100 r"
Requirements for advanced cycle heat transformers Another paper s gives an extensive theoretical description of advanced cycles. It is important that a heat recovery plant is economically attractive. The GEA research and development project 'Advanced Heat Transformers' is strictly related to these requirements which reflect industrial needs.
I000 900
80C 70C
B
Achievement of a higher useful temperature
Figure 9 shows typical design parameters of a single stage water-LiBr heat transformer. The indicated absorption temperature of 150°C for a generator/evaporator temperature of 90°C cannot be exceeded with this kind of apparatus. Nevertheless, higher useful temperatures can be reached by using new working fluids as mentioned above. Figure 10, for example, presents the characteristics of the T F E - D T G (E181) system. The first promising results could be achieved with a T F E - D T G heat transformer test plant in the GEA research laboratories. Figure 11 indicates that even higher useful heat temperatures can be reached with other absorbents for refrigerant TFE. With Pyr as absorbent a maximum temperature of 2OO°C and a temperature lift of 100K would seem to be realistic. However, practical experience is needed prior to giving guarantees for attainable temperatures or temperature lifts.
600: zoo*c
400 3
0
0
~
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.6
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8 Enthalpy--concentration chart of T F E - N M P . Condensing line; B, construction line; C, evaporation line
A,
¢
Figure
Figure 8 Diagrammeenthalpie-concentration de TFE-NMP
Rev. Int. Froid 1986 Vol 9 Janvier
55
Advanced heat transformers. H. Bokelmann and F. Steimle
0.7---
2.6 . . . . . . . . . . . . . . . . . .
~/to
zu ......
/
?
i
' 0.07 . . . . . . . . . I
0.07 t - -
tab
I tz u
40%
9OoC
tab 20°C
tN
150°(2
Figure9 Design characteristics of a water-LiBr heat transformer. V, Evaporator: K, condenser; A, absorber; G, generator; 0~N,usable heat flow at tN; Qzu, heat input at tzu; Qab, waste heat output at tab Figure 9 Caracteristiques de la conception d'un transformateur de
tzu
tN
lO0°C =E25°C Figure II Designcharacteristics of a TFE Pyr heat transformer. Key as in Figure 9 Figure 11 Caract&istiques de lu conception d'un trunsfi~rmoteur de chaleur d TFE-Pyr. RepUtes comme Figure 9
chaleur & eau-LiBr. V, Evaporateur; K, condenseur; A, absorbeur," G, 9enbrateur: 0N, chaleur utile ~i tN,' Q=., chaleur introduite (t t=.," 0,b, chaleur perdue extraite d t,,b .
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tab 20°C I
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fzu
65°C
fN
125°C
Figure 10 Design characteristics of a TFE DTG heat transformer.
Figure12 Singlestage TFE Pyr heat transformer for a temperature lift from 70 to 110~C. Key as in Figure 9 Figure 12 TransJbrmateurde chaleur d TFE-Pyr rnono~tay~ pour une Jl&,ation de tempdruture de 70 d I IO'~C. RepJres comme Figure 9
Key as in Figure 9 Figure 10 Caractkristiques de la conception d'un trunsjbrmuteur de chaleur d TFE-DTG. Repgres comme Figure 9
Comparison of different heat recovery systems
Achievement o f higher temperature liJis
Different heat recovery systems are c o m p a r e d below on the basis of two hypothetical applications.
tab 38°C
tzu
100%
fN
172°C
A temperature lift from 70 to l l 0 ° C is typical of the requirements of the food processing industry. Such temperature increases c a n n o t be realized economically with a single stage water-LiBr plant. This requirement can be met with an advanced heat transformer utilizing new working fluids (Figure 12). If the use of these systems is not acceptable because of their toxicity an alternative may be an advanced water-LiBr heat transformer (see Fiyure 13, combination heat transformer/absorption heat pump). This plant can be interconnected either by using external heat transfer circuits or by connecting the inner circuits 8. However, the absorption heat p u m p necessitates the input of highenergy heat despite the heat transformer, and this fact must not be overlooked.
56
Int. J. Refrig. 1986 Vol 9 January
First example (Figure 14)
This example refers to a thermal process proceeding between 150 and 100°C. Heating steam at 200°C serves as the drive energy. To simplify matters we assume the process is non-dissipative. The temperature scale helps to demonstrate the different a m o u n t of exergy of identical energy flows. Figure 14a shows the simple application without heat recovery. 100% high-grade energy is fed to the process and the same quantity of energy is emitted to the atmosphere as waste heat and always at a relatively high temperature level. Figure 14b shows that ~ 8~/o of the above waste heat flow can be recovered by using a simple heat exchanger. The recovered heat has a temperature of
Advanced heat transformers." H. Bokelmann and F. Steimle 80°C and can be considered profitable only if heat is needed for heating purposes at that temperature. However, 100~o of high-grade energy must also be supplied to the process in this case. Now it is possible to reduce this energy by use of a heat transformer (Fioure 14c). The assumption of a heat transformer with an efficiency of 0.4 is conservative as plants already built and those in operation realize efficiencies of > 0.45. When operating a heat transformer with this efficiency the primary energy demand is cut to
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60%0. An identical heat flow is transmitted to the atmosphere by the heat transformer at a low temperature which is virtually useless for other applications. Fioure 14d shows the energy flows in a plant with a high temperature absorption-type heat p u m p and heat exchanger. To produce I00% drive energy for the process only 67% of high-grade energy is needed by the absorption heat pump. Of the process waste heat 33°/0 is utilized in the absorption heat p u m p and 67°/0 is supplied to the heat exchanger. 54% is then available as useful low temperature heat. Figures 14e and f show two further complex cycles. Alternative e achieves the largest saving of high-grade energy. Only 46% of the energy is supplied at 200°C and there is no useful low temperature heat. With alternative f, 72% of the process heat can be utilized as useful low temperature heat at 90°C. Unlike alternative e, where the heat p u m p and heat transformer are operating independently, in alternative f they are interconnected. This results in a heat recovery system which emits a minimum of non-used energy to the atmosphere. However, 80~o of high-grade energy is required for alternative f , and so it can be recommended only when low temperature heat at 90°C is useful.
80%
.
AWP ~ /
0.250 x,.~,~
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= 108%
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65
I
90 t (°C)
I;~0
Second e x a m p l e (Figure 15) The second example is a little more complex. It is assumed that an industrial facility has two engineering processes, one requiring a temperature lift between 100 and 150°C and the other between 70 and 110°C. Each process has an energy demand of 35% of the total energy consumption of the facility. Another 2 0 ~ is needed for space heating and another 10% for the hot water supply. This application is shown in Figure 15a, without a heat recovery system. Fioure 15b presents a simple and inexpensive method of heat recovery. The demand for high-grade energy can be
150
Figure 13 Combination ofa H20-LiBr heat transformerwith a H20-
LiBr heat pump for the temperature lift from 70 to I10°C. AWP, Absorption heat pump; WT, heat transformer. Rest of key as in Figure 9 Figure 13 Combinaison d'un transformateur de chaleur a HeO-LiBr avec une pompe ~tchaleur ~tH 20-LiBr pour l¥1~vation de la temperature de 10 ~ 110°C. A WP, Pompe (t chaleur fi absorption; WT, transformateur de ehaleur. Le reste des rep~res eomme Figure 9
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100%
60%
46%
67=/=
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=C,'~ I¢
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• f I Figure 14 Representationof the primary energy savingsof the first example. P, Process; WAT, heat exchanger; NTN, useful heat at low temperature; WT, heat transformer; AWP, absorption heat pump. Steam temperature, t = 200°C
Figure 14 Representation des ~conomies d'~nergie primaire du premier exemple. P, Processus; WA T, echangeur de chaleur : NTN, chaleur utlie tJ basse temperature; WT, transformateur de chaleur; A WP, pompe d chaleur d absorption. Temperature de la vapeur, t = 200'C
Rev. Int. Froid 1986 Vol 9 Janvier
57
Advanced heat transformers. H. Bokelmann and F. Steimle L__
~oo%
so%
70%
~2 5 %
52.5%
=o%
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150
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Figure 15 Representation of the primary energy savings for the second example. PI, Process I; PII, process II: WAT, heat exchanger; H, space heating; BW, hot water supply; WT, heat transformer; AWT, absorption heat pump. Steam temperature, t=200 C Figure 15 ReprJsentation des #conomies dYnergie primaire pour le second exemple. PI, Processus I: PII, proeessus 1/; WH T, #chan qeur de chaleur; tt, chaf[u,qe du Iocak B W, #mrniture d'eau chaude; WT, transfi)rmuteur de chaleur; A WT, pompe ~i chuleur ~i absorption. Temp#rature de la vapeur, t = 200C
reduced to 70°/by using two heat exchangers. In this case the waste heat flows are used for water heating. However, the waste heat potential is greater than the demand for low temperature useful heat. Therefore, a large amount of the high temperature waste heat is rejected unused, into the atmosphere. Figure 15c shoes a heat recovery system which is more efficient but also more expensive. With the help of two heat transformers it is possible to reduce the demand for high-grade energy to 52.5%. In this example the efficiency of heat transformers is assumed to be 0.5, which is realistic because of the relatively lower temperature lifts. Figure 15d shows a further decrease in energy demand. Through the utilization of a high temperature absorption heat pump, a heat transformer and two heat exchangers, the high-grade energy consumption can be cut to 47%. In this example the heat pump utilizes part of the waste heat of the first process for producing useful heat to be supplied to both processes. Figure 15e shows a costly method of heat recovery which entails a reduction of the preliminary energy demand to 37};. This is possible with the help of two heat transformers and two heat pumps. The whole system is presented schematically in Fiqure 16.
Conclusions The decision in favour of a heat recovery system is not only a question of thermodynamics but also of the economical possibilities. Therefore, the profitability of the proposed technical solution has to be evaluated for each option, in accordance with the specific application and the operating reliability.
58
Int. J. Refrig. 1986 Vol 9 January
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3s.e./o~
Figure 16 Combination of two heat transformers with two heat absorption pumps. Temperatures are approximate Figure 16 Combinaison de deux transformateurs de chaleur avec deux pompes • chaleur ~i absorption. Les temp#ratures sont approximatives
Present heat transformers comprising single stage water LiBr units are safe and profitable. The pay back periods for MW-plants operated full-time are ~ 2.5 years. The advanced heat transformers presented here are still the subject of research and development. GEA has proposed a project ' Development of an Advanced Heat Transformer and Investigation on the Combination Heat Transformer/Absorption Heat Pump' to the EEC Research and Development Programme. For this project
Advanced heat transformers." H. Bokelmann and F. Steimle G E A is i n t e n d i n g to use the n e w w o r k i n g fluids in a test plant.
References 1 Suhr, L., Paikert, P., Bokelmann, H. Operational Experience with an Industrial Scale Heat Transformer, International Workshop Heat Transformation and Storage, Ispra (9-10 October 1985) 2 Bokelmann, H., Ehmke, H. J. Arbeitsstoffsysteme fflr eine Sorptionsw/irmepumpe(Working Fluids for a Sorption Heat Pump), final report of the EEC-sponsored research programme EE-A4-03 IDCB (1985) 3 Bokelmann, H. Auswahl, Messung thermophysikalischer Eigenschaften und Beurteilung der Eignung yon Niederdruck Stoffsystemen f/Jr Absorptionsw/irmepumpen Forschungsbericht des Deutschen Kd'lte- und Klimatechnischen Vereins (1984) 12
4 Ehmke, H. J. Stoffsysteme fiir Absorptionsw/irmepumpen Experimentelle Bestimmung thermophysikalischer Eigenschaften yon L6sungen der K/iltemittel Methylamin, Ammoniak und Monochlordifluormethan (R22) Forschungsbericht des Deutschen Kiilte- und Klimatechnischen Vereins (1984) 13 5 Bokelmann, H., Ehmke, H. J., Steimle, F. Presentation of New Working Fluids for Absorption Heat Pumps, Absorption Experts Meeting 1985, Paris, France (2(~22 March 1985) 6 Bokelmann, H., Ehmke, H. J. Erstellung von Enthalpiekonzentrationsdiagrammen fiir Stoffsysteme ffir Absorptionswfirmepumpen Ki Klima Kdlte Heizung 13 (1985) 6 214-244 7 Nowaczyk, U. Thermophysikalische Eigenschaften yon Stoffsystemen fiir Absorptionsw/irmepumpen Diplomarbeit Universitysit/it Essen, FRG (1985) 8 Alefeld, G., Ziegler, F. Advanced Cycles for the Working Pair H20/LiBr, ASHRAE meeting, Honolulu, USA (June 1980)
Rev. Int. Froid 1986 Vol 9 Janvier
59
Based on experience from an industrial-scale heat transformer, which is now commercially available, GEA has embarked on a new research and development programme: the development of an advanced heat transformer and investigation into the combination heat transformer/absorption heat pump. Advanced heat transformers are heat transformers utilizing new working fluids and/or a multi-stage absorption plant. A brief report is presented on an experimental research programme at the University of Essen which aims to discover new working fluids for sorption plants. Some of the most important results for heat transformer application are presented in the form of graphs or tables and the suitability of the new working fluids is compared with some well-known working couples. The Industrial requirements for an advanced heat transformer are also considered along with the possibility of meeting these requirements. Finally, the heat transformer is discussed in the context of other heat recovery systems on the basis of two fictitious industrial applications. (Keywords: heat transformers; workingfluids)
Mise au point de transformateurs de chaleur perfectionn de nouveaux fluides actifs
s utilisant
A patir de rexpdrence d'un transformateur de chaleur d'~chelle industrielle, disponible maintenant dans le commerce, GEA a entrepris un nouveau programme de recherche-dOveloppement 'Mise au point d'un transformateur de chaleur perfectionn~ et recherches sur la combinaison transformateur de chaleur-pompe d chaleur c~absorption'. Les transformateurs de chaleur perfectionn~s sont ceux utilisant de nouveaux fluides actifs et/ou des installations dt absorption multi-~tagdes. Le prdsent article rend bridvement compte d'un programme de recherches exp~rimentales effectu~ ti l'universit~ d'Essen dans le but de trouver de nouveaux fluides de travail pour les installations ~J sorption. Quelques uns des r~sultats les plus importants pour rapplication aux transformateurs de chaleur sont pr~sentds sous forme de graphiques ou de tableaux et l'aptitude des nouveaux fluides de travail est comparde d celle de m6langes de travail trds connus. Un autre chapitre traite des besoins de rindustrie pour un transformateur de chaleur perfectionn~ et des possibilities d'y rOpondre. Enfin on compare le transformateur de chaleur d d'autres systdmes de rdcup~ration de chaleur en examinant deux applications industrielles fictives.
(Mots cles: transformateurs de chaleur; fluides de travail)
An energy saving system has been described 1 which was built by GEA for an animal carcass plant in Drrnten. Using a water-LiBr heat transformer this pilot plant provided evidence that a single stage heat transformer (HT) is capable of recovering waste heat on an industrial scale. Nearly 45~o of the waste heat at 100°C was transformed to useful heat at 145°C. Nevertheless, it should be noted that a single stage water-LiBr plant cannot always cope with the requirements of a heat recovery system. Due to the solubility characteristics of the working couple, water-LiBr, the temperature lift between the waste heat and the useful heat is restricted and the useful m a x i m u m temperature is limited because the solution is highly corrosive at high temperatures. However, many inquiries from potential customers have shown that temperature lifts to high operating temperatures are often desirable or demanded. Therefore, the development of an advanced heat transformer suited to these applications is necessary. 0140-7007/86/0100514)9503.00 :. © 1986 Butterworth & Co (Publishers) Ltd and IIR
Advanced heat transformers utilize new working fluids and/or multi-stage sorption plants including combinations of sorption heat pumps and heat transformers. This Paper deals with the search for new working fluids with multi-stage absorption cycles and presents some examples of their application.
New working fluids for heat transformers A brief report is given on an extensive experimental research p r o g r a m m e r performed at the University of Essen with the aim of finding new working fluids for sorption plants. Although this project mainly concerned heat p u m p application, some of the working fluids
"i" The research project was sponsored by: the Commission of the European Commurfities; Arbeitsgemeinschaft fiir Sparsamen und Umweltfreundlichen Energieverbrauch(ASUE); and Deutscher Verein des Gas- und Wasserfaches (DVGW)
Rev. Int. Froid 1986 Vol 9 Janvier
51
Advances heat transformers. H. Bokelmann and F. Steimle suitable for heat p u m p application were also suitable for heat transformers. Within the scope of the research project nearly 150 different systems were subjected to point m e a s u r e m e n t s and the relevant properties (vapour pressure, density, viscosity, thermal stability, v a p o u r liquid equilibria, heat of mixing, heat capacity, etc.) of the most interesting fluid pairs determined. The experimental results were fitted to sets of correlating e q u a t i o n s which, in turn, were used to construct diagrams a n d tables for practical use. This Paper deals with those results which are of interest for heat transformer application. Extensive descriptions of the whole project can be found in References 2-5.
Experimental procedure For this p r o g r a m m e a large n u m b e r of refrigerants a n d a b s o r b e n t s were taken into account. The refrigerants were chosen in accordance with the r e q u i r e m e n t s listed in Table 1. F o r heat transformers the following substances are of special interest: trifluoroethanol (TFE); hexafluoroi s o p r o p a n o l ( H F I P ) ; p e n t a f l u o r o p r o p i o n i c acid (PFPA).
Suitable a b s o r b e n t s were chosen according to their molecular structure giving the possibility of hydrogen b o n d f o r m a t i o n in a d d i t i o n to the requirements of Table 1. D u r i n g the preliminary investigation point m e a s u r e m e n t s of the solubility of the refrigerants were made. Based on these m e a s u r e m e n t s a limited n u m b e r of promising working fluids were chosen for the following further investigations: m e a s u r e m e n t of a line of c o n s t a n t pressure; c o n s t r u c t i o n of provisional equilibrium charts: d e t e r m i n a t i o n of the thermal stability.
Results F o r the a b o v e - m e n t i o n e d refrigerants the following a b s o r b e n t s were f o u n d highly promising: q u i n o l i n e (Chi): tetraethylene glycol dimethyl ether (DTG); ethylpyrrolidone (EP); isoquinoline (Ich); m e t h y l p y r r o l i d o n e (MP); N - m e t h y l p y r r o l i d o n e (NMP); p y r r o l i d o n e (Pyr); tetraethylene glycol (TEG). Table 2 presents the results of the solubility measurements, expressed as mass fractions, at c o n s t a n t pressure of the following systems: T F E - C h i ; T F E - D T G ;
Table I Requirementsfor working fluids Tableau 1 Propriet~s exigees des fluides actifs Requirement
Subject
Effect
High specific enthalpy of evaporation Favourable pressure range
Refrigerant Refrigerant
Desired temperatures attainable Low specific mass flow ratio High difference in boiling points Low pressure difference Low viscosity High density Low specific heat capacity
Type of solution Type of solution Working fluid Refrigerant Solution Solution Solution
High thermal and chemical stability
Refrigerant and solution
Reduction in fluid flow: reduction in pump energy Lower requirements for the apparatus design: reduced weight and costs Realization of the process Reduction in fluid flow; reduction in pump energy No need for rectification Reduction in pump energy Reduction in pump energy Reduction in pump energy Low heat flux in the apparatus of the solution circuit: smaller size and lower costs Realization of the process
Low toxicity Low corrosivness Low inflammability Low price
Refrigerant, absorbent and solution
Responsibilityfor the application and economy
Table 2 Solubilitydata
Tableau 2 Solubilit#s
System TFE Chi TFE DTG TFE EP TFE ICh TFE MP TEE NMP TFE Pyr TEE TEG HFIP-NMP PFPA--DTG
52
Evaporation temperature (C) 0 50 0 50 0 50 0 50 0 50 0 50 0 50 0 50 10 40 0 50
. . 170
.
Refrigerant content in the solution at various absorption temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 150 140 130 120 110 100 90 80 70 60 50 40 30 0.003 0.045 0.070 0.108 0.152 0.214 0.284 0.365 0.450 0.029 0.037 0.060 0.094 0.132 0.187 0.262 0.355 0.457 0.117 0.132 0.211 0.285 0.352 0.414 0,480 0.540 0.586 0.005 0.062 0.090 0.126 0.175 0.239 0.312 0.390 0.463 0.093 0.153 0.195 0.254 0.309 0.366 0.434 0.505 0.585
0.140 0.225 0.298 0.366 0.423 0.515 0.566 0.634 0.092 0.153 0.200 0.257 0.320 0.379 0.449 0.534 0.617 0.025 0.052 0.066 0.090 0.122 0.166 0.230 0.315 0.410 0.629 0.664 0.605 0.637 0.666 0.697 0.728 0.750 0.781 0.519 0.140 0.258 0.464 0.525 0.601 0.649 0.691 0.727 0.762 0.795
Int. J. Refrig. 1986 Vol 9 January
0.054 0.542 0.052 0.574 0.194 0.667 0.061 0.549 0.144 0.674 0.220 0.700 0.151 0.703 0.043 0.527 0.700 0.811 0.630 0.833
20'C
0.107 0.197 0.287 0.392 0.494 0.091 0.153 0.247 0.369 0.507 0.301 0.385 0.461 0.546 0.109 0.198 0.295 0.396 0.216 0.298 0.390 0.485 0.588 0.328 0.423 0.500 0.572 0.649 0.236 0.323 /).419 0.520 0.627 0.073 0.122 0.203 0.322 (t.'468 0.732 0.766 0.8(/2 0.839 0.846 0.676 0.720 0.755 0.794
Advanced heat transformers: H. 8okelmann and F. Steimle
TFE-EP; TFE-MP; TFE-NMP; TFE-ICh; TFE-Pyr; TFE-TEG a (all at a pressure of 18.8 mbar); and H F I P NMP and PFPA-DTG (at pressures of 90 and 5 mbar, respectively). During these measurements the evaporation temperature was kept constant while the absorption temperature was varied. For each chosen absorption temperature the amount of the refrigerant in the absorbent was determined. With the help of two lines of constant pressure, measured at different evaporation temperatures, it is possible to construct a provisional equilibrium chart (p,t,x-chart) (Figures 1-4). These provisional equilibrium charts were used to determine some key figures for the heat transformer process (e.g. the specific mass flow rate). These are helpful for assessing the new working fluids against well-known working couples. This comparison is presented in Table 3. It refers to the systems TFE-DTG, TFE-NMP, TFE-Pyr, H F I P NMP, PFPA-DTG and the well-known couples ammonia-water and water-LiBr. The Table contains the following values and aspects: r o, enthalpy of evaporation at 0°C; tkr, critical temperature; P2o, vapour pressure of the refrigerant at 20°C; P~oo, vapour pressure of the
,ooo
1000
20 10 5 2
I -20
I
0
20
40
I
60 t (*C)
I
I
I
I
i
I i i i i i i i
80 100120140160 200
Figure 3 Provisional equilibrium chart of TFE-Pyr Figure 3 Diagramme de requilibre provisoire TFE-Pyr
o,o
f--/////Z/ff-
I00
_
/ zo
°
C
"~0
'
0
20
40 60 t (*C)
80
100120140160 200
Figure 1 Provisional equilibrium chart of TFE-Chi Figure 1 Diagramme de £~quilibre provisoire TFE-Chi
,//~/
1000 500
~
I
-20
0
20
20
40
60 t(*C)
80 I00 120140160 200
Figure 4 Provisional equilibrium chart of H F I P - N M P Figure 4 Diagramme de l'~quilibre provisoire HFIP-NMP
I0
-20
0
40 60 t (*C)
80
"i
I I II
100120140160
Figure 2 Provisional equilibrium chart of TFE-DTG Figure 2 Diagramme de I'~quilibre provisoire TFE-DTG
|
200
refrigerant at 100°C; price per kg of solution; toxicity; corrosivness; Ats, boiling point difference; and number of measured data (thermophysical properties). Comparison is made at a generator temperature of 100°C and at a concentration difference between the strong and poor solutions of 5~ at three different sets of working conditions for evaporation temperature, tE, and condensing temperature, tk: 100 and 20°C; 100 and 50°C; 50 and 20°C, respectively. The selection of the second and third working conditions is necessary to allow a comparison with the accepted working couples for which the first working condition is not attainable since with ammonia as refrigerant the pressure reached is unacceptably high and with water-LiBr crystallization problems occur. The following derived values are of great interest and a r e l n c l u d e d in Table 3: concentration of the poor solution; ~,, which should be high to reduce the specific mass flow rate; attainable absorp.tion temperature, ta, which should be high to reach high useful heat temperatures; hypothetical temperature difference in the absorber, Ata, which should be low to increase t,; specific mass flow rate, f , which should be low to decrease the necessary pump energy and the size of heat and mass flow areas. As shown in Table 3, the new working fluids reach higher absorption temperatures than the accepted
Rev. Int. Froid 1986 Vol 9 Janvier
53
Advanced heat transformers." H. Bokelmann and F. Steimle systems. Systems T F E - N M P and TFE-Pyr are especially promising. The thermodynamic properties of the system H F I P N M P are most suitable for utilization in heat pumps. However, the price of the refrigerant is too high for economical application at the moment. PFPA would seem to be suitable as it is available in large amounts but it is too expensive at the moment to be used as a refrigerant in large heat transformers. As the chemical market is subject to fluctuations it is worth pursuing the PFPA systems. Compared with the water-LiBr system the toxicity of the refrigerant TFE must be mentioned as a disadvantage and the different heat and mass transfer behaviour might cause a minor decrease in the coefficient of performance. Irrespective of the above, the advantages of T F E - N M P and TFE-Pyr pairs seem to justify additional investigation.
II
:n
d d ~ d d
d
I
+++11
o
oo
zz~
++
~
-~
~
=~
F-
=
Further investigations Some of the detailed measurements of the systems T F E N M P and TFE-Pyr are presented as diagrams. The measured values and the equations can be found in Reference 2. Figure 5 is the equilibrium chart of the system TFE N M P which was established by measuring the vapour pressure of different solutions at approximately constant concentrations over the whole temperature range of interest. In this case nearly 170 measured points are correlated by the least squares method to polynomial equations which are the basis for the p,t,x-charts. This chart for T F E - N M P shows that the solution field of the system permits the realization of heat transformer processes with reasonable temperature lifts. In addition, the chart shows that the difference of boiling points is similar to that determined for ammonia. Rectification may be needed, therefore, but can be avoided by using TFE Pyr which, compared with T F E - N M P , offers a similar solubility behaviour but a higher boiling point difference (Figure 3). For generator/condenser conditions the boiling behaviour of the mixtures can be appreciated from the t, i-chart (Figures 6 and 7), which are based on vapour liquid equilibria measurements. Figure 8 shows the enthalpy-concentration chart of system T F E - N M P . A detailed description of how the chart was constructed is given in Reference 6.
I000 500
o~
I00 ,a¢
"7
5C
E
e~
==
-20
0
20
40
60
8 0 I00 120140160 2 0 0
t (°C}
Figure 5 Equilibrium chart of TFE-NMP Figure 5 Diagramme de lYquilibre TFE-NMP
54
Int. J. Refrig. 1 9 8 6 Vol 9 January
Advanced heat transformers." H. Bokelmann and F. Steimle 260 0.0
0.1
0.2
0.3
0.4
0.5 0.6
0.7 0.8
0.9 1.0,.,,.,,.,,
240 220 200 ~
~24
180 160 260 240'
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60
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40
,-: 140
16(
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~... ,ll
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4(. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 C Figure 6 t, ~ diagramfor TFE-NMP Figure 6 Diagramme t, ~ pour TFE-NMP
0.0 0.8 0.9
1.0
0.1 0.2
0.3
0.4
0.5
0.6 0.7
0.8 0.9
1.0
Figure 7 Preliminaryt, ~ diagramfor TFE-Pyr Figure 7 Diagrammepr~liminaire t, ~ pour TFE-Pyr
1200[ I100 r"
Requirements for advanced cycle heat transformers Another paper s gives an extensive theoretical description of advanced cycles. It is important that a heat recovery plant is economically attractive. The GEA research and development project 'Advanced Heat Transformers' is strictly related to these requirements which reflect industrial needs.
I000 900
80C 70C
B
Achievement of a higher useful temperature
Figure 9 shows typical design parameters of a single stage water-LiBr heat transformer. The indicated absorption temperature of 150°C for a generator/evaporator temperature of 90°C cannot be exceeded with this kind of apparatus. Nevertheless, higher useful temperatures can be reached by using new working fluids as mentioned above. Figure 10, for example, presents the characteristics of the T F E - D T G (E181) system. The first promising results could be achieved with a T F E - D T G heat transformer test plant in the GEA research laboratories. Figure 11 indicates that even higher useful heat temperatures can be reached with other absorbents for refrigerant TFE. With Pyr as absorbent a maximum temperature of 2OO°C and a temperature lift of 100K would seem to be realistic. However, practical experience is needed prior to giving guarantees for attainable temperatures or temperature lifts.
600: zoo*c
400 3
0
0
~
2O0 I00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.6
0.9
I;0
8 Enthalpy--concentration chart of T F E - N M P . Condensing line; B, construction line; C, evaporation line
A,
¢
Figure
Figure 8 Diagrammeenthalpie-concentration de TFE-NMP
Rev. Int. Froid 1986 Vol 9 Janvier
55
Advanced heat transformers. H. Bokelmann and F. Steimle
0.7---
2.6 . . . . . . . . . . . . . . . . . .
~/to
zu ......
/
?
i
' 0.07 . . . . . . . . . I
0.07 t - -
tab
I tz u
40%
9OoC
tab 20°C
tN
150°(2
Figure9 Design characteristics of a water-LiBr heat transformer. V, Evaporator: K, condenser; A, absorber; G, generator; 0~N,usable heat flow at tN; Qzu, heat input at tzu; Qab, waste heat output at tab Figure 9 Caracteristiques de la conception d'un transformateur de
tzu
tN
lO0°C =E25°C Figure II Designcharacteristics of a TFE Pyr heat transformer. Key as in Figure 9 Figure 11 Caract&istiques de lu conception d'un trunsfi~rmoteur de chaleur d TFE-Pyr. RepUtes comme Figure 9
chaleur & eau-LiBr. V, Evaporateur; K, condenseur; A, absorbeur," G, 9enbrateur: 0N, chaleur utile ~i tN,' Q=., chaleur introduite (t t=.," 0,b, chaleur perdue extraite d t,,b .
2.6 . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
~ ...... 'Oz L.. t0' l
Q. 0 J~
Q,
t°o !
tab 20°C I
I
I°,. !
fzu
65°C
fN
125°C
Figure 10 Design characteristics of a TFE DTG heat transformer.
Figure12 Singlestage TFE Pyr heat transformer for a temperature lift from 70 to 110~C. Key as in Figure 9 Figure 12 TransJbrmateurde chaleur d TFE-Pyr rnono~tay~ pour une Jl&,ation de tempdruture de 70 d I IO'~C. RepJres comme Figure 9
Key as in Figure 9 Figure 10 Caractkristiques de la conception d'un trunsjbrmuteur de chaleur d TFE-DTG. Repgres comme Figure 9
Comparison of different heat recovery systems
Achievement o f higher temperature liJis
Different heat recovery systems are c o m p a r e d below on the basis of two hypothetical applications.
tab 38°C
tzu
100%
fN
172°C
A temperature lift from 70 to l l 0 ° C is typical of the requirements of the food processing industry. Such temperature increases c a n n o t be realized economically with a single stage water-LiBr plant. This requirement can be met with an advanced heat transformer utilizing new working fluids (Figure 12). If the use of these systems is not acceptable because of their toxicity an alternative may be an advanced water-LiBr heat transformer (see Fiyure 13, combination heat transformer/absorption heat pump). This plant can be interconnected either by using external heat transfer circuits or by connecting the inner circuits 8. However, the absorption heat p u m p necessitates the input of highenergy heat despite the heat transformer, and this fact must not be overlooked.
56
Int. J. Refrig. 1986 Vol 9 January
First example (Figure 14)
This example refers to a thermal process proceeding between 150 and 100°C. Heating steam at 200°C serves as the drive energy. To simplify matters we assume the process is non-dissipative. The temperature scale helps to demonstrate the different a m o u n t of exergy of identical energy flows. Figure 14a shows the simple application without heat recovery. 100% high-grade energy is fed to the process and the same quantity of energy is emitted to the atmosphere as waste heat and always at a relatively high temperature level. Figure 14b shows that ~ 8~/o of the above waste heat flow can be recovered by using a simple heat exchanger. The recovered heat has a temperature of
Advanced heat transformers." H. Bokelmann and F. Steimle 80°C and can be considered profitable only if heat is needed for heating purposes at that temperature. However, 100~o of high-grade energy must also be supplied to the process in this case. Now it is possible to reduce this energy by use of a heat transformer (Fioure 14c). The assumption of a heat transformer with an efficiency of 0.4 is conservative as plants already built and those in operation realize efficiencies of > 0.45. When operating a heat transformer with this efficiency the primary energy demand is cut to
°zo I
0.701
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
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.
.
.
.
60%0. An identical heat flow is transmitted to the atmosphere by the heat transformer at a low temperature which is virtually useless for other applications. Fioure 14d shows the energy flows in a plant with a high temperature absorption-type heat p u m p and heat exchanger. To produce I00% drive energy for the process only 67% of high-grade energy is needed by the absorption heat pump. Of the process waste heat 33°/0 is utilized in the absorption heat p u m p and 67°/0 is supplied to the heat exchanger. 54% is then available as useful low temperature heat. Figures 14e and f show two further complex cycles. Alternative e achieves the largest saving of high-grade energy. Only 46% of the energy is supplied at 200°C and there is no useful low temperature heat. With alternative f, 72% of the process heat can be utilized as useful low temperature heat at 90°C. Unlike alternative e, where the heat p u m p and heat transformer are operating independently, in alternative f they are interconnected. This results in a heat recovery system which emits a minimum of non-used energy to the atmosphere. However, 80~o of high-grade energy is required for alternative f , and so it can be recommended only when low temperature heat at 90°C is useful.
80%
.
AWP ~ /
0.250 x,.~,~
~--Io0% ~o ~ / : . . . . ,~,n,W'a(
• ,~
....
= 108%
0.074 40
65
I
90 t (°C)
I;~0
Second e x a m p l e (Figure 15) The second example is a little more complex. It is assumed that an industrial facility has two engineering processes, one requiring a temperature lift between 100 and 150°C and the other between 70 and 110°C. Each process has an energy demand of 35% of the total energy consumption of the facility. Another 2 0 ~ is needed for space heating and another 10% for the hot water supply. This application is shown in Figure 15a, without a heat recovery system. Fioure 15b presents a simple and inexpensive method of heat recovery. The demand for high-grade energy can be
150
Figure 13 Combination ofa H20-LiBr heat transformerwith a H20-
LiBr heat pump for the temperature lift from 70 to I10°C. AWP, Absorption heat pump; WT, heat transformer. Rest of key as in Figure 9 Figure 13 Combinaison d'un transformateur de chaleur a HeO-LiBr avec une pompe ~tchaleur ~tH 20-LiBr pour l¥1~vation de la temperature de 10 ~ 110°C. A WP, Pompe (t chaleur fi absorption; WT, transformateur de ehaleur. Le reste des rep~res eomme Figure 9
I~1~%
100%
60%
46%
67=/=
100%
i
80%
]
69%
=C,'~ I¢
II
1i 7 ¢ I 31 %
100%
P i
-gAWP-
e%~,
z*/.
54%
46%jj
6O o
b
I
d
• f I Figure 14 Representationof the primary energy savingsof the first example. P, Process; WAT, heat exchanger; NTN, useful heat at low temperature; WT, heat transformer; AWP, absorption heat pump. Steam temperature, t = 200°C
Figure 14 Representation des ~conomies d'~nergie primaire du premier exemple. P, Processus; WA T, echangeur de chaleur : NTN, chaleur utlie tJ basse temperature; WT, transformateur de chaleur; A WP, pompe d chaleur d absorption. Temperature de la vapeur, t = 200'C
Rev. Int. Froid 1986 Vol 9 Janvier
57
Advanced heat transformers. H. Bokelmann and F. Steimle L__
~oo%
so%
70%
~2 5 %
52.5%
=o%
47% l
150
zl%
37%
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~%
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~
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o
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Figure 15 Representation of the primary energy savings for the second example. PI, Process I; PII, process II: WAT, heat exchanger; H, space heating; BW, hot water supply; WT, heat transformer; AWT, absorption heat pump. Steam temperature, t=200 C Figure 15 ReprJsentation des #conomies dYnergie primaire pour le second exemple. PI, Processus I: PII, proeessus 1/; WH T, #chan qeur de chaleur; tt, chaf[u,qe du Iocak B W, #mrniture d'eau chaude; WT, transfi)rmuteur de chaleur; A WT, pompe ~i chuleur ~i absorption. Temp#rature de la vapeur, t = 200C
reduced to 70°/by using two heat exchangers. In this case the waste heat flows are used for water heating. However, the waste heat potential is greater than the demand for low temperature useful heat. Therefore, a large amount of the high temperature waste heat is rejected unused, into the atmosphere. Figure 15c shoes a heat recovery system which is more efficient but also more expensive. With the help of two heat transformers it is possible to reduce the demand for high-grade energy to 52.5%. In this example the efficiency of heat transformers is assumed to be 0.5, which is realistic because of the relatively lower temperature lifts. Figure 15d shows a further decrease in energy demand. Through the utilization of a high temperature absorption heat pump, a heat transformer and two heat exchangers, the high-grade energy consumption can be cut to 47%. In this example the heat pump utilizes part of the waste heat of the first process for producing useful heat to be supplied to both processes. Figure 15e shows a costly method of heat recovery which entails a reduction of the preliminary energy demand to 37};. This is possible with the help of two heat transformers and two heat pumps. The whole system is presented schematically in Fiqure 16.
Conclusions The decision in favour of a heat recovery system is not only a question of thermodynamics but also of the economical possibilities. Therefore, the profitability of the proposed technical solution has to be evaluated for each option, in accordance with the specific application and the operating reliability.
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Int. J. Refrig. 1986 Vol 9 January
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Present heat transformers comprising single stage water LiBr units are safe and profitable. The pay back periods for MW-plants operated full-time are ~ 2.5 years. The advanced heat transformers presented here are still the subject of research and development. GEA has proposed a project ' Development of an Advanced Heat Transformer and Investigation on the Combination Heat Transformer/Absorption Heat Pump' to the EEC Research and Development Programme. For this project
Advanced heat transformers." H. Bokelmann and F. Steimle G E A is i n t e n d i n g to use the n e w w o r k i n g fluids in a test plant.
References 1 Suhr, L., Paikert, P., Bokelmann, H. Operational Experience with an Industrial Scale Heat Transformer, International Workshop Heat Transformation and Storage, Ispra (9-10 October 1985) 2 Bokelmann, H., Ehmke, H. J. Arbeitsstoffsysteme fflr eine Sorptionsw/irmepumpe(Working Fluids for a Sorption Heat Pump), final report of the EEC-sponsored research programme EE-A4-03 IDCB (1985) 3 Bokelmann, H. Auswahl, Messung thermophysikalischer Eigenschaften und Beurteilung der Eignung yon Niederdruck Stoffsystemen f/Jr Absorptionsw/irmepumpen Forschungsbericht des Deutschen Kd'lte- und Klimatechnischen Vereins (1984) 12
4 Ehmke, H. J. Stoffsysteme fiir Absorptionsw/irmepumpen Experimentelle Bestimmung thermophysikalischer Eigenschaften yon L6sungen der K/iltemittel Methylamin, Ammoniak und Monochlordifluormethan (R22) Forschungsbericht des Deutschen Kiilte- und Klimatechnischen Vereins (1984) 13 5 Bokelmann, H., Ehmke, H. J., Steimle, F. Presentation of New Working Fluids for Absorption Heat Pumps, Absorption Experts Meeting 1985, Paris, France (2(~22 March 1985) 6 Bokelmann, H., Ehmke, H. J. Erstellung von Enthalpiekonzentrationsdiagrammen fiir Stoffsysteme ffir Absorptionswfirmepumpen Ki Klima Kdlte Heizung 13 (1985) 6 214-244 7 Nowaczyk, U. Thermophysikalische Eigenschaften yon Stoffsystemen fiir Absorptionsw/irmepumpen Diplomarbeit Universitysit/it Essen, FRG (1985) 8 Alefeld, G., Ziegler, F. Advanced Cycles for the Working Pair H20/LiBr, ASHRAE meeting, Honolulu, USA (June 1980)
Rev. Int. Froid 1986 Vol 9 Janvier
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