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Heat pumps in industry
Heat Recovery Systems & CHP Vol. 14, No. I, pp. 51-60, 1994 Printed in Great Britain
HEAT
PUMPS
0890-4332/94 $6.00+ .00 © 1993 Pergamon Press Ltd
IN INDUSTRY
FOUAD AL-MANSOUR and MmA TOMgI(~ Jo~ef Stefan Institute, Jamova 39, P.O. Box 100, 61111 Ljubljana, Slovenia
(Accepted 15 January 1993) Abstract--An analysis of influences of connection ways on the efficiencyof a heat pump in an industrial energy system was made. A new special connection of a heat pump with a heat accumulator system was introduced and is fully discussed with respect to the operating procedure as well as advantages. The dynamics of heat energy consumption influences the economicsand efficiencyof an industrial heat pump. Simulation of a heat pump in an industrial energy system is established in accordance with the dynamic energy consumption and condition of electrical power consumption with respect to the peak power. As a demonstration example of the simulation, data from a chemical plant were used.
1. I N T R O D U C T I O N Low temperature heat losses in some production processes can be upgraded to be useful energy by the use of a heat pump. Besides improving energy efficiency, the investment in and running costs of introducing the heat pump should be taken into consideration. To decide on the integration of a heat pump in industrial internal energy systems, an analysis of the operating method considering characteristics of a heat pump should be established. In the process industry the term "integral processes for energy recovery" is used. The basis of process integration is to identify the minimum utility requirement and the minimum required heat transfer area of any given process by systematic thermodynamic analysis [1, 2]. Introduction of a heat pump based on the optimization of a heat exchange network, using pinch technology and related to the shape of the composite curves have been discussed earlier in refs [2, 3]. In this work the method of introducing heat pumps in industries has been analyzed, using the operational dynamic analysis as described in [4].
2. C R I T E R I A OF A H E A T P U M P I N T E G R A T E D
IN I N D U S T R Y
The basic criteria of introducing a heat pump in an industrial internal energy system is to achieve lower cost heat energy as compared with the conventional system. Practically, this is conditioned by the following [5, 6, 7]: in heat process integration, the source and sink of heat should be in some sufficient rate, - the temperature difference between the heat source and heat sink should be satisfactory, - an electricity-heat price ratio may seem to be low, - the number of running hours per year is high, - a sufficient operational regime with respect to electrical energy consumption. Because of the availability of the waste heat source and the demand for flow temperature heat, the use of a heat pump has been introduced. It is necessary to analyze the influence of changing the heat source and then demand for low temperature heat during the day, the week, the month, and the season. The evaluation of quantities for the positive economical condition of a heat pump proves that the COP (Coefficient Of Performance) and operational time (top) should be high [5, 7]. The usage of the electrically driven compressor heat pump has been presumed; because of that we have to avoid reaching an electricity peak demand. It is also seen that the price of heat produced by using other sources (solid fuel, gas...) should be higher than that of heat produced by a heat pump. -
51
52
F. AL-MANSOURand M. TOM]i(~
LORENZ-CYC LE ARNOT'CYCLE
$
Fig. I. The Lorenz cycle.
3. C O N N E C T I O N
OF
A HEAT
PUMP
Improvement of the operation of a heat pump can be achieved by the series connection of two or more single heat pumps; with this we are close to the Lorenz process (Fig. 1). This system of heat pump connections tends to increase COP. Two different system connections were analyzed, one for connecting two heat pumps while the other is for an infinite number of heat pumps. The theoretical analysis of each system mentioned here, based on the Carnot relation for COP, assumes that the flow media have the same heat capacity. 3.1. Two heat pumps system
In the connection of heat pumps, two different ways are encountered, the first is a parallel flow connection and the second is a counter flow connection. These two systems are analyzed in order to investigate which of them is the best [8]. In both systems mentioned above it is discovered that both are of the same form from the efficiency point of view. In practice, one or the other is chosen in accordance with its real characteristics. The parallel flow system is the best when different types of heat pumps are used, one heat pump "C
kW m: cl:h
8
I
2,*
14:
I,
],
4
1- Heat Pump 2- Hot Water Accumulators 3- Source (Cooling) Water Ac~,_mulators 4- Warm Water Inlet
7
5- Hot Water Oudet 6- Source Water Inlet 7- Source Water Outlet 8..31- Valves
Fig. 2. Heat pump system with heat and cold accumulators.
Heat pumps in industry
53
"•..'
(a)
300
R27o
/q
i ''°
/v"v'~,
/
/
/
"
• 120 _~ ,o >0 0
4
U'l
(b)
'~ 12
6
I
'•°
/ 8
2
\
/
-~
0
4
/
/ •
/
16
10
14
Hot Water VokrM
_
m
24
20 18
22
T;ma[h]
6Hot Water Demand I
300
~27o '~ 240 .
A,/~
~,~ 120
E 90
/ ,"
\/
J
\
/ \
/
\ /
v
\/
0 >.
0
4
0
8
2
12
6
10
Hot Watlr Volume
(c)
16 14
20
18 Time [h]
* Hot Wallr I ~
24 22
I
'c" E. 100 90
6O
1 '°50' ~
40' 30'
B~kl • • m ~LI • m'~ mmmmm~mmmmmm mmmm-mmmmmmmm
20,t
m m m m m m m ~ m m
mmmm
m m m m m
m m m ~
~ m m m
-6 1 0
00
4 2
8 6
12 10
16 14
20
18 Time [hi
24 22
Hot Water Volume ---q4ot Woter dlncmd I
Fig. 3. The dynamics of daily charging of the hot water accumulators and daily consumption of hot water: (a) for a winter day; (b) for an autumn or spring day; (c) for a summer day.
54
F. AL-MANSOUR a n d M. TOM~IC
suitable for low temperature differences (for example a turbo or screw compressor heat pump) and the other is suitable for higher temperature differences (piston compressor). 3.2. Infinite number of heat pumps system
To increase the temperature of heating media, the use of an infinite number of heat pumps is assumed, in this case each heat pump increases the temperature At. It is concluded that the COP as an ideal system is higher than a single heat pump system. It is also concluded that the gradual increase of the temperature of heating media is better than the increase of its temperature in a single stage. With this method the utilization of heat exchangers is improved, where the temperature difference between input and output is low.
4. HEAT PUMP SYSTEM WITH HEAT (AND/OR COLD) ACCUMULATORS Heat pump operation depends on the need for heating and cooling in an industrial unit, at the same time it has to be adjusted to electric power sale conditions. This is difficult to achieve without heat (and/or cold) accumulators in the system. As a rule, an inclusion of heat accumulators in the energy system with heat pumps is justified. Such a combined heat pump with accumulators enables certain independence of heat pump operation and preserves the balance of the whole system. Besides this, thermodynamic improvements are achieved if the heat scheme is suitable and the process is properly controlled [9]. A microcomputer control system should assure the most satisfactory operation possible of the equipment itself and its inclusion in an industrial internal energy system [7]. The scheme of a system provided and discussed for heat supply with heat pumps is presented in Fig. 2. The basic supposition for effectiveness of the system discussed is that it is possible to interconnect
Table 1. Typical simulation results. For a heat pump compressor electric power 1050 kW and efficiency 0.6; for the accumulator of hot water (HR) of volume Vh = 250 m 3 and for source water (CR) Vc = 250 m 3 Initiation
Duration HP
HR
CR
Date
h:min
h:min
h:min
h:min
DQc (m 3)
DQh (m 3)
20/1/85 20/1/85 20/I/85 20/1/85 21/1/85 21/1/85 21/1/85 21/1/85 21/1/85 21 / 1/85 22/1/85 22/I/85 22/I/85 22/1/85 22/1/85 22/1/85 23/1/85 23/1/85 23/1/85 23/1/85 23/I/85 23/I/85
0:15 8:45 16:45 20:15 3:30 5:0 6:30 8:45 16:15 20:15 3:15 4:45 6:15 8:45 16:0 20:15 3:0 4:30 6:0 8:45 15:30 20:15
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
1095.8 1305.0 84.3 1178.1 203.9 203.9 42.1 1219.8 168.6 I 132.8 203.9 203.9 84.3 1175.2 210.7 1093.2 203.9 203.9 126.4 1094.1 291.9 674.3
2240.3 2671.1 172.3 2412.6 430.8 430,8 86.2 2498.8 344.7 2326.4 430.8 430.8 172,3 2412.6 430.8 2240.3 430.8 430.8 258.5 2240.3 603. I 1378.6
Simulated operation statistics 4 days No. of days 18.11 h/day Avg. daily operation 19097 kWh/day Avg. daily consumption el. energy 16341 kWh/day Avg. consumption during higher rate 2756 kWh/day Avg. consumption during lower rate 3050 m3/day Avg. daily hot water production 69579 kWh/day Avg. daily heat energy production 3.66 COP Accumulator: (1) Hot: Vt=250.OOm ~, Tt=70.O°C, TI=50.O°C. (2) Cool: Vh=250iOOm 3, Th = 19.5c, Ti = 26.5c.
Heat pumps in industry
55
heat consumers and sources reasonably with a system for distribution of low temperature heat and also with a pipe system for waste heat collection. A combined system with heat pumps and heat accumulators is considerably more convenient than a system without accumulators, especially if accumulation reservoirs are connected in a special way where the operation is described as follows [9]: The solution of the hot or cold accumulation problem with an improved thermodynamic efficiency enables filling of a hot or cold accumulator which is represented by one or more accumulators with working media (for example, water) at the lowest temperature difference at the heat source or heat sink (for example, among suitable parts of a heat engine and media flow). For the resolution it is characteristic that media for heat (cold) storage are taken at a certain distance from the accumulator top or the warmest part of the accumulator series (in the case of more accumulators connected successively) or at the accumulator bottom or the coolest place in an accumulator series, and the heated medium is returned at the same height or proportionally small vertical distance from the consumption site. The medium is heated at the heat source for a relatively small temperature difference. For the system execution it is characteristic that pipes for water (or other fluids) leaving and returning which are destined for water circulation to the hot or cold source (for example, to the heat exchange of a heat pump), are connected to an accumulator or accumulators at a relatively small vertical distance, as for example, pipes with a valve (10) and a valve (11) (see Fig. 2). According to the stratification appearing in accumulators, it is useful to foresee more pairs of connections, as for example, one more pair of pipes with valves (12) and (13). Another system characteristic, which can be presented besides the one mentioned above, is that more accumulators are planned for heat or cold storage. They are connected in parallel, and they are equipped with pipes and valves which enable hot or cold water consumption for use in an individual accumulator where other accumulators are excluded from the consumption circuit. All accumulators are equipped with at least one pair of pipes for the circuit to the hot or cold source so that it is possible to renew the heat or cool store in an accumulator which is excluded from the consumption circuit. According to the scheme in a certain phase of the operation, valves (8) and (14) are closed and valves (12) and (13) and valves from (16) to (19), and valves from (9) to (11) and valve (15) are open. The mode of operation is as follows. At the beginning of hot water accumulator filling, for example, if we decide to fill the left accumulator, valves (8) and (14) are closed, the accumulator being excluded, hot water consumption from the circuit and the mixing being reduced (in the meantime it is possible to empty the right accumulator so that valves (9) and (15) are open). The heat to the top part of the left accumulator is conveyed from the heat pump system by valves (10) and (11) being open, but the rest of the hot water accumulator valves from (12) to (19) are closed. The water temperature in the upper part of an accumulator is gradually increased by passing water several times through the heat exchanger of the heat pump system. Due to changes in the working medium density with the temperature (hot media regularly having a lower density), it is gradually heated above the consumption level or where the working medium returns and the lower part stays cooler. Regarding the necessary hot water temperature and the state in the second accumulator after the water in the upper part of the left accumulator reaches the required temperature, either filling of the second accumulator is started or filling of the left accumulator is continued to reach the full capacity. The entire capacity of the left accumulator for heat storage is reached by valves (10) and (11) being closed and valves (12) and (13) being open. Gradually, with water circulation through the heat pump exchanger and gentle mixing in an accumulator, all water in the accumulator reaches the required temperature. Analogically to the process of the hot water accumulator filling is the filling of a cold water accumulator so that the initial or partial filling is done by means of pipes and valves at the bottom of the accumulators (valves (24) and (25) are open for the left cold accumulator). The entire capacity is reached by means of pipes and valves being connected at the top of the accumulators (valves (22) and (23) are open).
56
F. AL-MANSOUR and M. TOMgl(~
Construction of a system with one accumulator alone or two accumulators is possible, one on a hot and one on a cool side. With placing and regular switching over of at least two accumulators on a hot or cold, side, the possibility of mixing and accompanying thermodynamic loss is minimized and the creation of an optimum temperature profile in an accumulator, which is being filled, is made possible. Reduction of thermodynamic losses is reached especially at the point of heat receipt because media at the transition through or past the heat source are heated only for a proportionally small temperature difference. The use of innovation is reasonable also in the case when the basic aim and method of system operation is not heat storage, but simultaneous use of heat. In that case it is possible to use the construction in Fig. 2, but the accumulators are proportionally smaller and valve switch overs are more frequent. In that case, with a system of temporary storage, the thermodynamic efficiency of a heat engine is improved. The heat stored is used so that the hotter medium is taken from the top (or at the level with a suitable temperature) and returned to the bottom or at the level which suits the cooled (used) medium temperature. (In the case of cold storage, the role of consumption and returning is reversed.) In the case when filling (storage) and heat (cold) consumption occur simultaneously, the thermodynamic optimum method of filling is disturbed due to mixing. In that case it is possible to use two accumulators (accumulator systems) where heat (cold water) is pumped from the first while the second one is simultaneously filled. In this way the possibility of mixing and thermodynamic loss is minimized and creation of the optimum temperature profile in an accumulator, which is being filled, is made possible. The system, having one or both of the characteristics described, enables the improvement of thermodynamic efficiency of heat engines which use hot or cold sources irrespective of whether hot or cold water is stored or used immediately. 5. E F F I C I E N C Y OF A H E A T P U M P H E A T SYSTEM W I T H A C C U M U L A T O R S When heat pump efficiency in an accumulator system is discussed, a continuing increase of hot accumulator temperature from a certain temperature, Tj, to the temperature Tc is considered. For the media (water) temperature increases for d T the formula for the COP is: COP = Od___~_~
( I)
dw'
where dQ is the necessary (obtained) energy or condenser capacity. dw =
CdT
(2)
COP"
dw is the invested energy power. C = rhcp is the capacity of the media flow, its mass flow is rn. The average COP of a heat pump for a gradual temperature increase of working media from TI to Tc is calculated from an equation: COP = -Q - = C(L-
T,)
w fr~Cdr COP
C O P - - ( T o - T~)
(3)
(4)
fr, ~COP dT The real heat pump COP which is working at evaporation temperature, Te, and condensation temperature, T, is calculated from the relation:
1
To
COP = -1 - -T q¢ q¢ is compressor efficiency.
(5)
Heat pumps in industry
57
This is put in the equation (4): (re-
cop =
T~)
(6)
1 frC(l - - ~ ) d T ~lc Jr, and we get: COP--
(To- T~)
t/c.
(7)
(To - T~) - Tc In It is considered that the heat source temperature is constant, i.e. Te = const. If temperature differences are also considered in a condenser (ATe) and an evaporator (ATe), then COP is calculated as follows: (To- T,) COP =
Tc + Arc r/c" (8) (Tc - T~) - (Te - ATe)In - TI + a T c The supposition that evaporating temperature is constant, is not totally justified in the case when the filling of a hot and cold accumulator is carried out simultaneously. In this case, the logarithmic medium value of temperature between heat source temperature (Ta) and the desired temperature of cooling media (To) is taken for Te:
Ta- To
Tc = - -
ln~ To
(9)
In the case when temperature is not increased gradually, the COP is calculated using the formula: 1 ro _ Are~c.
COP= 1
(lO)
Tc+AT¢
6. C O M P U T E R PACKAGE FOR I N D U S T R I A L HEAT PUMP OPERATION
ANALYSIS (TCRTH) A complex system involving heating and cooling needs during the day, electrical energy prices, costs of hot and cold recovery from a heat pump and from other alternative systems and the way of linking heat pump operation and other parts of the internal energy system cannot be readily analyzed other than by a computer. A computer package described below takes into consideration all the factors which influence heat pump operation as part of the internal energy system where a heat pump system with a heat (cold) accumulator is considered. Working media for heat storage are very responsive to heat storage. For the cost of hot working media, water is considered but the computer package can be used for other media so that basic characteristics have to be entered in a database. In practice, the use of any other mediums is not expected despite the fact that other chemical compounds have been investigated. Economy of storage greatly depends on the price of the working medium (and reservoir), which practically excludes use of media more expensive than water [10].
7. SIMULATION OF HEAT PUMP OPERATION
7.1. Dynamics of heat and cool consumption The dynamics of heat and cold consumption is considered in the simulation on the basis of daily diagrams of heat and cold consumption. According to the condition set, that heat pump operation does not cause a peak in electric power
58
F, AL-MANSOURand M. TOM~I~
consumption, a daily diagram of electric power consumption from the utilities and power demand limit is considered. 7.2. Heat produced and electric power consumption The heat energy, produced by a heat pump, is calculated by means of a formula: Q = wCOP.
(11)
COP is calculated by formulae (8) or (9), depending on whether the temperature increase is gradual or it occurs in one pass through a heat pump condenser. A heat pump heats the hot water accumulator from the temperature T~ to the temperature T and at the same time cools the water accumulator heat source (or cools reservoir-cooled water), from the temperature To to Te. The quantity of heat recovered by a heat pump is expressed by a quantity of heated water in a hot accumulator and cold produced by a quantity of cooled water in a cold accumulator. AQH = w • COPH" At
(12)
AQR = w • COPR" At
(13)
AQx = AVH "PM" Cpa" ATH
(14)
AQR = AVR" PR" Cp" ATR
(15)
COPR = COPH -- 1
(16)
ATH = T -
TI
(17)
ATR-- T o - Te.
(18)
w is input power to compressor, V is water volume, p is average water density, Cp is average specific heat. Indexes H and R indicate hot and cold side. Volumes of heated and cooled water in the accumulators in the time At from previous equations are calculated: AV. =
w . COPH
At
(19)
w • COPR _ A VR = p~. ~ ~---TRAt.
(20)
p.. CpH. AT.
7.3. State in accumulators The state in hot and cold water accumulators is characterized by volumes of heated and cooled water, respectively, and it is established for each time interval according to the consumption and production of heat and cool, respectively: VH,i = VH.i - I "~ A VH. i - - UH, i
(21 )
VR.i ~- VR,i - I -~- A YR, i - - UR. i .
(22)
v,.i and Vs., are volume flows of heat and cool water consumption in the interval i. 7.4. Conditions of heat pump operation A heat pump connection in the simulation is conditioned by heat and/or cooling demand, accumulator filling by the electric power consumption peak. A heat pump always operates so that it does not cause a peak in electric power consumption. Electric power availability up to the limitation is the first condition for heat pump operation. When electric power is available, a heat pump is regularly switched on. It is shut down according to the state in the accumulators and the season or operating regime: (i) in summer it is shut down when a cold accumulator is full and a hot accumulator is not empty; (ii) in winter it is shut down when a hot accumulator is full and a cold accumulator is not empty; (iii) in a transitional regime it is shut down when one of the accumulators is full.
Heat pumps in industry
59
7.5. Basic data input Necessary basic input data are: (i) daily diagram of consumption of low temperature hot and cold water: average 15-min volume flow; (ii) the time when the start-up of a heat pump is permitted according to electric power consumption; (iii) volumes of both accumulators, hot and cold water; (iv) average specific heat and density of water in both accumulators; (v) temperature range in both accumulators; and (vi) capacity and efficiency of the heat pump compressor. 8. D E S C R I P T I O N OF A COMPUTER PACKAGE TCAR A computer package analyzes hot (cold) water consumption and the state in a hot and cold accumulator considering optimum heat pump operation in accordance with conditions explained above and the characteristics of heat pump operation and heat consumption. Results of the analysis are presented in two forms. In the first table the necessary data and the results are presented. 8.1. Data about the simulation course initiation of a heat pump (day, month, year, hour and minute) duration of heat pump operation (hour and minute) duration of hot water accumulator filling (hour, minute) duration of cold water accumulator filling (hour and minute) quantity of heated water in a hot accumulator (m 3) - quantity of cooled water in a cold accumulator (m3). -
-
-
-
-
8.2. General results - number of days analyzed - average daily heat pump operation (h/day) - average daily electric power consumption in the time of higher tariff and the tariff period (kWh/day) average daily quantity of heated and cooled water produced (m3/day) - average daily heat cooling produced (kWh/day) - the highest and the lowest value of COP. In the second table the daily quantity of hot and cold water in the accumulators is given in detail and the necessary quantity of hot and cold water for the average of 15 min. -
9. EXAMPLE Our aim is the evaluation of heat pump integration in an industrial energy system. In a chemical industrial plant using steam and hot water, there is a large loss of hot water at 26.5°C. All the data were obtained from a study of the plant [11]. The consumption of hot water is used for industrial purposes and for heating. The heat energy required for heating is calculated in accordance with the meteorological data for the plant location and the installed capacity for the heating system for each section of the plant. In hard winter time the heating system is enhanced by an auxiliary heating source (steam) to cover the required load, taking into consideration that two accumulators for hot water are available. For the selection of heat pump capacity and the accumulator volume, we made an analysis for a winter day temperature of -1.2°C and for a spring or autumn day temperature of 5°C. In summer time the heat is required only for industrial purposes. The operation of a heat pump is limited in the peak time. Typical simulation results are given in Table 1. The dynamics of daily charging of the accumulator with hot water and the daily consumption of hot water are illustrated in Fig. 3. The result of the analysis shows that for covering the demand of heat energy and recovery of
60
F. AL-MANSOURand M. TOM~I~
waste heat: waste heat water a c c u m u l a t o r o f v o l u m e o f 150 m 3, h o t water a c c u m u l a t o r for heating o f 200 m 3, a n d for industrial p u r p o s e s o f 50 m 3, a n d a heat p u m p with an electrical m o t o r o f 3 x 350 k W are needed. 10. C O N C L U S I O N S T h e w a y o f c o n n e c t i n g h e a t p u m p s in an internal industrial system influences the efficiency o f the whole system. T h e c o n n e c t i o n o f h e a t energy a c c u m u l a t o r s with h e a t p u m p s p r o v i d e s flexibility which is convenient for c h a n g i n g the o p e r a t i o n a l c o n d i t i o n o f h e a t p u m p s a n d c o n d i t i o n s o f electrical p o w e r c o n s u m p t i o n to a v o i d increase in p o w e r peak. The s i m u l a t i o n m e t h o d i n t r o d u c e d in this w o r k where the d y n a m i c s o f c o n s u m p t i o n a n d the d y n a m i c s o f h e a t source availability are t a k e n into c o n s i d e r a t i o n in c o n n e c t i o n with the electrical p o w e r c o n s u m p t i o n c o n d i t i o n to a v o i d the increase o f p e a k p o w e r a n d o p e r a t i o n d u r i n g lower rate has a higher efficiency t h a n the c o n v e n t i o n a l m e t h o d . This s i m u l a t i o n m e t h o d has been successfully used before [4]. REFERENCES 1. U. F. Kenney, Energy Conservation in the Process Industries. Academic Press, New York (1984). 2. B. Frhanieh and B. Sunden, Analysis of an existing heat exchanger network and effects of heat pump installation. Heat Recovery Systems & CHP 10, 285-296 (1990). 3. E. Walline, P. A. Franck and T. Berntsson, Heat pumps in industrial processes--an optimization methodology. Heat Recovery Systems & CHP 10, 437-490 (1990). 4. M. Tomgi6 and F. AI-Mansour, Optimal operation of cogeneration plants in industry. Energy 14, 483-490 (1989). 5. Commission of the European Communities, Industrial Heat Pump; Energy Conservation in Industry, Vol. !, Combustion and Heat Recovery. VDI Verlag, Diisseldorf (1984). 6. F. Morer, and H. Schnitzer, Heat Pumps in Industry. Elsevier, Netherlands (1985). 7. M. Tomgic, F. A1-Mansour and T. Blatnik, Vklju~itev toplotnih 6rpalk v interni (industrijski) energetski sistem kot pogoj gospodarnosti. RAVE Portoro~ (1985). 8. T. Blatnik, F. AI-Mansour and M. Tomgi~, Zvi~anja grelnega gtevila sistema s topiotnimi 6rpalkami s serijsko vezavo agregatov, 16. Kongres o grejanju, hladenju i klimatizaciji, Beograd, 13-15 November (1985). 9. M. Tom~i~ and F. A1-Mansour, Sistem za shranjevanje toplote ali hladu z izboljganim termodinamskim izkoristkom, patentna prijava IJS, Ljubljana (1986). 10. M. Tom[i6, Z. Marin[ek and C. Physicos, lzbolj~anje izrabe energije s shranjevanjem in avtomatizacijo obratovanja, Nova Proizvodnja, ~t., 5 June (1982). 11. A. Ahmed, F. AI-Mansour, B. Selan, R. Tavzes and A. Todorovi~, Idejni projekt za izrabo nizkotemperaturnih sekundarnih toplot z uvedbo toplotnih ~rpalk v DO KRKA Novo Mesto, IJS-DP/RE-205, Ljubljana (1986).
HEAT
PUMPS
0890-4332/94 $6.00+ .00 © 1993 Pergamon Press Ltd
IN INDUSTRY
FOUAD AL-MANSOUR and MmA TOMgI(~ Jo~ef Stefan Institute, Jamova 39, P.O. Box 100, 61111 Ljubljana, Slovenia
(Accepted 15 January 1993) Abstract--An analysis of influences of connection ways on the efficiencyof a heat pump in an industrial energy system was made. A new special connection of a heat pump with a heat accumulator system was introduced and is fully discussed with respect to the operating procedure as well as advantages. The dynamics of heat energy consumption influences the economicsand efficiencyof an industrial heat pump. Simulation of a heat pump in an industrial energy system is established in accordance with the dynamic energy consumption and condition of electrical power consumption with respect to the peak power. As a demonstration example of the simulation, data from a chemical plant were used.
1. I N T R O D U C T I O N Low temperature heat losses in some production processes can be upgraded to be useful energy by the use of a heat pump. Besides improving energy efficiency, the investment in and running costs of introducing the heat pump should be taken into consideration. To decide on the integration of a heat pump in industrial internal energy systems, an analysis of the operating method considering characteristics of a heat pump should be established. In the process industry the term "integral processes for energy recovery" is used. The basis of process integration is to identify the minimum utility requirement and the minimum required heat transfer area of any given process by systematic thermodynamic analysis [1, 2]. Introduction of a heat pump based on the optimization of a heat exchange network, using pinch technology and related to the shape of the composite curves have been discussed earlier in refs [2, 3]. In this work the method of introducing heat pumps in industries has been analyzed, using the operational dynamic analysis as described in [4].
2. C R I T E R I A OF A H E A T P U M P I N T E G R A T E D
IN I N D U S T R Y
The basic criteria of introducing a heat pump in an industrial internal energy system is to achieve lower cost heat energy as compared with the conventional system. Practically, this is conditioned by the following [5, 6, 7]: in heat process integration, the source and sink of heat should be in some sufficient rate, - the temperature difference between the heat source and heat sink should be satisfactory, - an electricity-heat price ratio may seem to be low, - the number of running hours per year is high, - a sufficient operational regime with respect to electrical energy consumption. Because of the availability of the waste heat source and the demand for flow temperature heat, the use of a heat pump has been introduced. It is necessary to analyze the influence of changing the heat source and then demand for low temperature heat during the day, the week, the month, and the season. The evaluation of quantities for the positive economical condition of a heat pump proves that the COP (Coefficient Of Performance) and operational time (top) should be high [5, 7]. The usage of the electrically driven compressor heat pump has been presumed; because of that we have to avoid reaching an electricity peak demand. It is also seen that the price of heat produced by using other sources (solid fuel, gas...) should be higher than that of heat produced by a heat pump. -
51
52
F. AL-MANSOURand M. TOM]i(~
LORENZ-CYC LE ARNOT'CYCLE
$
Fig. I. The Lorenz cycle.
3. C O N N E C T I O N
OF
A HEAT
PUMP
Improvement of the operation of a heat pump can be achieved by the series connection of two or more single heat pumps; with this we are close to the Lorenz process (Fig. 1). This system of heat pump connections tends to increase COP. Two different system connections were analyzed, one for connecting two heat pumps while the other is for an infinite number of heat pumps. The theoretical analysis of each system mentioned here, based on the Carnot relation for COP, assumes that the flow media have the same heat capacity. 3.1. Two heat pumps system
In the connection of heat pumps, two different ways are encountered, the first is a parallel flow connection and the second is a counter flow connection. These two systems are analyzed in order to investigate which of them is the best [8]. In both systems mentioned above it is discovered that both are of the same form from the efficiency point of view. In practice, one or the other is chosen in accordance with its real characteristics. The parallel flow system is the best when different types of heat pumps are used, one heat pump "C
kW m: cl:h
8
I
2,*
14:
I,
],
4
1- Heat Pump 2- Hot Water Accumulators 3- Source (Cooling) Water Ac~,_mulators 4- Warm Water Inlet
7
5- Hot Water Oudet 6- Source Water Inlet 7- Source Water Outlet 8..31- Valves
Fig. 2. Heat pump system with heat and cold accumulators.
Heat pumps in industry
53
"•..'
(a)
300
R27o
/q
i ''°
/v"v'~,
/
/
/
"
• 120 _~ ,o >0 0
4
U'l
(b)
'~ 12
6
I
'•°
/ 8
2
\
/
-~
0
4
/
/ •
/
16
10
14
Hot Water VokrM
_
m
24
20 18
22
T;ma[h]
6Hot Water Demand I
300
~27o '~ 240 .
A,/~
~,~ 120
E 90
/ ,"
\/
J
\
/ \
/
\ /
v
\/
0 >.
0
4
0
8
2
12
6
10
Hot Watlr Volume
(c)
16 14
20
18 Time [h]
* Hot Wallr I ~
24 22
I
'c" E. 100 90
6O
1 '°50' ~
40' 30'
B~kl • • m ~LI • m'~ mmmmm~mmmmmm mmmm-mmmmmmmm
20,t
m m m m m m m ~ m m
mmmm
m m m m m
m m m ~
~ m m m
-6 1 0
00
4 2
8 6
12 10
16 14
20
18 Time [hi
24 22
Hot Water Volume ---q4ot Woter dlncmd I
Fig. 3. The dynamics of daily charging of the hot water accumulators and daily consumption of hot water: (a) for a winter day; (b) for an autumn or spring day; (c) for a summer day.
54
F. AL-MANSOUR a n d M. TOM~IC
suitable for low temperature differences (for example a turbo or screw compressor heat pump) and the other is suitable for higher temperature differences (piston compressor). 3.2. Infinite number of heat pumps system
To increase the temperature of heating media, the use of an infinite number of heat pumps is assumed, in this case each heat pump increases the temperature At. It is concluded that the COP as an ideal system is higher than a single heat pump system. It is also concluded that the gradual increase of the temperature of heating media is better than the increase of its temperature in a single stage. With this method the utilization of heat exchangers is improved, where the temperature difference between input and output is low.
4. HEAT PUMP SYSTEM WITH HEAT (AND/OR COLD) ACCUMULATORS Heat pump operation depends on the need for heating and cooling in an industrial unit, at the same time it has to be adjusted to electric power sale conditions. This is difficult to achieve without heat (and/or cold) accumulators in the system. As a rule, an inclusion of heat accumulators in the energy system with heat pumps is justified. Such a combined heat pump with accumulators enables certain independence of heat pump operation and preserves the balance of the whole system. Besides this, thermodynamic improvements are achieved if the heat scheme is suitable and the process is properly controlled [9]. A microcomputer control system should assure the most satisfactory operation possible of the equipment itself and its inclusion in an industrial internal energy system [7]. The scheme of a system provided and discussed for heat supply with heat pumps is presented in Fig. 2. The basic supposition for effectiveness of the system discussed is that it is possible to interconnect
Table 1. Typical simulation results. For a heat pump compressor electric power 1050 kW and efficiency 0.6; for the accumulator of hot water (HR) of volume Vh = 250 m 3 and for source water (CR) Vc = 250 m 3 Initiation
Duration HP
HR
CR
Date
h:min
h:min
h:min
h:min
DQc (m 3)
DQh (m 3)
20/1/85 20/1/85 20/I/85 20/1/85 21/1/85 21/1/85 21/1/85 21/1/85 21/1/85 21 / 1/85 22/1/85 22/I/85 22/I/85 22/1/85 22/1/85 22/1/85 23/1/85 23/1/85 23/1/85 23/1/85 23/I/85 23/I/85
0:15 8:45 16:45 20:15 3:30 5:0 6:30 8:45 16:15 20:15 3:15 4:45 6:15 8:45 16:0 20:15 3:0 4:30 6:0 8:45 15:30 20:15
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
6:30 7:45 0:30 7:0 1:15 1:15 0:15 7:15 1:0 6:45 1:15 1:15 0:30 7:0 1:15 6:30 1:15 1:15 0:45 6:30 1:45 4:0
1095.8 1305.0 84.3 1178.1 203.9 203.9 42.1 1219.8 168.6 I 132.8 203.9 203.9 84.3 1175.2 210.7 1093.2 203.9 203.9 126.4 1094.1 291.9 674.3
2240.3 2671.1 172.3 2412.6 430.8 430,8 86.2 2498.8 344.7 2326.4 430.8 430.8 172,3 2412.6 430.8 2240.3 430.8 430.8 258.5 2240.3 603. I 1378.6
Simulated operation statistics 4 days No. of days 18.11 h/day Avg. daily operation 19097 kWh/day Avg. daily consumption el. energy 16341 kWh/day Avg. consumption during higher rate 2756 kWh/day Avg. consumption during lower rate 3050 m3/day Avg. daily hot water production 69579 kWh/day Avg. daily heat energy production 3.66 COP Accumulator: (1) Hot: Vt=250.OOm ~, Tt=70.O°C, TI=50.O°C. (2) Cool: Vh=250iOOm 3, Th = 19.5c, Ti = 26.5c.
Heat pumps in industry
55
heat consumers and sources reasonably with a system for distribution of low temperature heat and also with a pipe system for waste heat collection. A combined system with heat pumps and heat accumulators is considerably more convenient than a system without accumulators, especially if accumulation reservoirs are connected in a special way where the operation is described as follows [9]: The solution of the hot or cold accumulation problem with an improved thermodynamic efficiency enables filling of a hot or cold accumulator which is represented by one or more accumulators with working media (for example, water) at the lowest temperature difference at the heat source or heat sink (for example, among suitable parts of a heat engine and media flow). For the resolution it is characteristic that media for heat (cold) storage are taken at a certain distance from the accumulator top or the warmest part of the accumulator series (in the case of more accumulators connected successively) or at the accumulator bottom or the coolest place in an accumulator series, and the heated medium is returned at the same height or proportionally small vertical distance from the consumption site. The medium is heated at the heat source for a relatively small temperature difference. For the system execution it is characteristic that pipes for water (or other fluids) leaving and returning which are destined for water circulation to the hot or cold source (for example, to the heat exchange of a heat pump), are connected to an accumulator or accumulators at a relatively small vertical distance, as for example, pipes with a valve (10) and a valve (11) (see Fig. 2). According to the stratification appearing in accumulators, it is useful to foresee more pairs of connections, as for example, one more pair of pipes with valves (12) and (13). Another system characteristic, which can be presented besides the one mentioned above, is that more accumulators are planned for heat or cold storage. They are connected in parallel, and they are equipped with pipes and valves which enable hot or cold water consumption for use in an individual accumulator where other accumulators are excluded from the consumption circuit. All accumulators are equipped with at least one pair of pipes for the circuit to the hot or cold source so that it is possible to renew the heat or cool store in an accumulator which is excluded from the consumption circuit. According to the scheme in a certain phase of the operation, valves (8) and (14) are closed and valves (12) and (13) and valves from (16) to (19), and valves from (9) to (11) and valve (15) are open. The mode of operation is as follows. At the beginning of hot water accumulator filling, for example, if we decide to fill the left accumulator, valves (8) and (14) are closed, the accumulator being excluded, hot water consumption from the circuit and the mixing being reduced (in the meantime it is possible to empty the right accumulator so that valves (9) and (15) are open). The heat to the top part of the left accumulator is conveyed from the heat pump system by valves (10) and (11) being open, but the rest of the hot water accumulator valves from (12) to (19) are closed. The water temperature in the upper part of an accumulator is gradually increased by passing water several times through the heat exchanger of the heat pump system. Due to changes in the working medium density with the temperature (hot media regularly having a lower density), it is gradually heated above the consumption level or where the working medium returns and the lower part stays cooler. Regarding the necessary hot water temperature and the state in the second accumulator after the water in the upper part of the left accumulator reaches the required temperature, either filling of the second accumulator is started or filling of the left accumulator is continued to reach the full capacity. The entire capacity of the left accumulator for heat storage is reached by valves (10) and (11) being closed and valves (12) and (13) being open. Gradually, with water circulation through the heat pump exchanger and gentle mixing in an accumulator, all water in the accumulator reaches the required temperature. Analogically to the process of the hot water accumulator filling is the filling of a cold water accumulator so that the initial or partial filling is done by means of pipes and valves at the bottom of the accumulators (valves (24) and (25) are open for the left cold accumulator). The entire capacity is reached by means of pipes and valves being connected at the top of the accumulators (valves (22) and (23) are open).
56
F. AL-MANSOUR and M. TOMgl(~
Construction of a system with one accumulator alone or two accumulators is possible, one on a hot and one on a cool side. With placing and regular switching over of at least two accumulators on a hot or cold, side, the possibility of mixing and accompanying thermodynamic loss is minimized and the creation of an optimum temperature profile in an accumulator, which is being filled, is made possible. Reduction of thermodynamic losses is reached especially at the point of heat receipt because media at the transition through or past the heat source are heated only for a proportionally small temperature difference. The use of innovation is reasonable also in the case when the basic aim and method of system operation is not heat storage, but simultaneous use of heat. In that case it is possible to use the construction in Fig. 2, but the accumulators are proportionally smaller and valve switch overs are more frequent. In that case, with a system of temporary storage, the thermodynamic efficiency of a heat engine is improved. The heat stored is used so that the hotter medium is taken from the top (or at the level with a suitable temperature) and returned to the bottom or at the level which suits the cooled (used) medium temperature. (In the case of cold storage, the role of consumption and returning is reversed.) In the case when filling (storage) and heat (cold) consumption occur simultaneously, the thermodynamic optimum method of filling is disturbed due to mixing. In that case it is possible to use two accumulators (accumulator systems) where heat (cold water) is pumped from the first while the second one is simultaneously filled. In this way the possibility of mixing and thermodynamic loss is minimized and creation of the optimum temperature profile in an accumulator, which is being filled, is made possible. The system, having one or both of the characteristics described, enables the improvement of thermodynamic efficiency of heat engines which use hot or cold sources irrespective of whether hot or cold water is stored or used immediately. 5. E F F I C I E N C Y OF A H E A T P U M P H E A T SYSTEM W I T H A C C U M U L A T O R S When heat pump efficiency in an accumulator system is discussed, a continuing increase of hot accumulator temperature from a certain temperature, Tj, to the temperature Tc is considered. For the media (water) temperature increases for d T the formula for the COP is: COP = Od___~_~
( I)
dw'
where dQ is the necessary (obtained) energy or condenser capacity. dw =
CdT
(2)
COP"
dw is the invested energy power. C = rhcp is the capacity of the media flow, its mass flow is rn. The average COP of a heat pump for a gradual temperature increase of working media from TI to Tc is calculated from an equation: COP = -Q - = C(L-
T,)
w fr~Cdr COP
C O P - - ( T o - T~)
(3)
(4)
fr, ~COP dT The real heat pump COP which is working at evaporation temperature, Te, and condensation temperature, T, is calculated from the relation:
1
To
COP = -1 - -T q¢ q¢ is compressor efficiency.
(5)
Heat pumps in industry
57
This is put in the equation (4): (re-
cop =
T~)
(6)
1 frC(l - - ~ ) d T ~lc Jr, and we get: COP--
(To- T~)
t/c.
(7)
(To - T~) - Tc In It is considered that the heat source temperature is constant, i.e. Te = const. If temperature differences are also considered in a condenser (ATe) and an evaporator (ATe), then COP is calculated as follows: (To- T,) COP =
Tc + Arc r/c" (8) (Tc - T~) - (Te - ATe)In - TI + a T c The supposition that evaporating temperature is constant, is not totally justified in the case when the filling of a hot and cold accumulator is carried out simultaneously. In this case, the logarithmic medium value of temperature between heat source temperature (Ta) and the desired temperature of cooling media (To) is taken for Te:
Ta- To
Tc = - -
ln~ To
(9)
In the case when temperature is not increased gradually, the COP is calculated using the formula: 1 ro _ Are~c.
COP= 1
(lO)
Tc+AT¢
6. C O M P U T E R PACKAGE FOR I N D U S T R I A L HEAT PUMP OPERATION
ANALYSIS (TCRTH) A complex system involving heating and cooling needs during the day, electrical energy prices, costs of hot and cold recovery from a heat pump and from other alternative systems and the way of linking heat pump operation and other parts of the internal energy system cannot be readily analyzed other than by a computer. A computer package described below takes into consideration all the factors which influence heat pump operation as part of the internal energy system where a heat pump system with a heat (cold) accumulator is considered. Working media for heat storage are very responsive to heat storage. For the cost of hot working media, water is considered but the computer package can be used for other media so that basic characteristics have to be entered in a database. In practice, the use of any other mediums is not expected despite the fact that other chemical compounds have been investigated. Economy of storage greatly depends on the price of the working medium (and reservoir), which practically excludes use of media more expensive than water [10].
7. SIMULATION OF HEAT PUMP OPERATION
7.1. Dynamics of heat and cool consumption The dynamics of heat and cold consumption is considered in the simulation on the basis of daily diagrams of heat and cold consumption. According to the condition set, that heat pump operation does not cause a peak in electric power
58
F, AL-MANSOURand M. TOM~I~
consumption, a daily diagram of electric power consumption from the utilities and power demand limit is considered. 7.2. Heat produced and electric power consumption The heat energy, produced by a heat pump, is calculated by means of a formula: Q = wCOP.
(11)
COP is calculated by formulae (8) or (9), depending on whether the temperature increase is gradual or it occurs in one pass through a heat pump condenser. A heat pump heats the hot water accumulator from the temperature T~ to the temperature T and at the same time cools the water accumulator heat source (or cools reservoir-cooled water), from the temperature To to Te. The quantity of heat recovered by a heat pump is expressed by a quantity of heated water in a hot accumulator and cold produced by a quantity of cooled water in a cold accumulator. AQH = w • COPH" At
(12)
AQR = w • COPR" At
(13)
AQx = AVH "PM" Cpa" ATH
(14)
AQR = AVR" PR" Cp" ATR
(15)
COPR = COPH -- 1
(16)
ATH = T -
TI
(17)
ATR-- T o - Te.
(18)
w is input power to compressor, V is water volume, p is average water density, Cp is average specific heat. Indexes H and R indicate hot and cold side. Volumes of heated and cooled water in the accumulators in the time At from previous equations are calculated: AV. =
w . COPH
At
(19)
w • COPR _ A VR = p~. ~ ~---TRAt.
(20)
p.. CpH. AT.
7.3. State in accumulators The state in hot and cold water accumulators is characterized by volumes of heated and cooled water, respectively, and it is established for each time interval according to the consumption and production of heat and cool, respectively: VH,i = VH.i - I "~ A VH. i - - UH, i
(21 )
VR.i ~- VR,i - I -~- A YR, i - - UR. i .
(22)
v,.i and Vs., are volume flows of heat and cool water consumption in the interval i. 7.4. Conditions of heat pump operation A heat pump connection in the simulation is conditioned by heat and/or cooling demand, accumulator filling by the electric power consumption peak. A heat pump always operates so that it does not cause a peak in electric power consumption. Electric power availability up to the limitation is the first condition for heat pump operation. When electric power is available, a heat pump is regularly switched on. It is shut down according to the state in the accumulators and the season or operating regime: (i) in summer it is shut down when a cold accumulator is full and a hot accumulator is not empty; (ii) in winter it is shut down when a hot accumulator is full and a cold accumulator is not empty; (iii) in a transitional regime it is shut down when one of the accumulators is full.
Heat pumps in industry
59
7.5. Basic data input Necessary basic input data are: (i) daily diagram of consumption of low temperature hot and cold water: average 15-min volume flow; (ii) the time when the start-up of a heat pump is permitted according to electric power consumption; (iii) volumes of both accumulators, hot and cold water; (iv) average specific heat and density of water in both accumulators; (v) temperature range in both accumulators; and (vi) capacity and efficiency of the heat pump compressor. 8. D E S C R I P T I O N OF A COMPUTER PACKAGE TCAR A computer package analyzes hot (cold) water consumption and the state in a hot and cold accumulator considering optimum heat pump operation in accordance with conditions explained above and the characteristics of heat pump operation and heat consumption. Results of the analysis are presented in two forms. In the first table the necessary data and the results are presented. 8.1. Data about the simulation course initiation of a heat pump (day, month, year, hour and minute) duration of heat pump operation (hour and minute) duration of hot water accumulator filling (hour, minute) duration of cold water accumulator filling (hour and minute) quantity of heated water in a hot accumulator (m 3) - quantity of cooled water in a cold accumulator (m3). -
-
-
-
-
8.2. General results - number of days analyzed - average daily heat pump operation (h/day) - average daily electric power consumption in the time of higher tariff and the tariff period (kWh/day) average daily quantity of heated and cooled water produced (m3/day) - average daily heat cooling produced (kWh/day) - the highest and the lowest value of COP. In the second table the daily quantity of hot and cold water in the accumulators is given in detail and the necessary quantity of hot and cold water for the average of 15 min. -
9. EXAMPLE Our aim is the evaluation of heat pump integration in an industrial energy system. In a chemical industrial plant using steam and hot water, there is a large loss of hot water at 26.5°C. All the data were obtained from a study of the plant [11]. The consumption of hot water is used for industrial purposes and for heating. The heat energy required for heating is calculated in accordance with the meteorological data for the plant location and the installed capacity for the heating system for each section of the plant. In hard winter time the heating system is enhanced by an auxiliary heating source (steam) to cover the required load, taking into consideration that two accumulators for hot water are available. For the selection of heat pump capacity and the accumulator volume, we made an analysis for a winter day temperature of -1.2°C and for a spring or autumn day temperature of 5°C. In summer time the heat is required only for industrial purposes. The operation of a heat pump is limited in the peak time. Typical simulation results are given in Table 1. The dynamics of daily charging of the accumulator with hot water and the daily consumption of hot water are illustrated in Fig. 3. The result of the analysis shows that for covering the demand of heat energy and recovery of
60
F. AL-MANSOURand M. TOM~I~
waste heat: waste heat water a c c u m u l a t o r o f v o l u m e o f 150 m 3, h o t water a c c u m u l a t o r for heating o f 200 m 3, a n d for industrial p u r p o s e s o f 50 m 3, a n d a heat p u m p with an electrical m o t o r o f 3 x 350 k W are needed. 10. C O N C L U S I O N S T h e w a y o f c o n n e c t i n g h e a t p u m p s in an internal industrial system influences the efficiency o f the whole system. T h e c o n n e c t i o n o f h e a t energy a c c u m u l a t o r s with h e a t p u m p s p r o v i d e s flexibility which is convenient for c h a n g i n g the o p e r a t i o n a l c o n d i t i o n o f h e a t p u m p s a n d c o n d i t i o n s o f electrical p o w e r c o n s u m p t i o n to a v o i d increase in p o w e r peak. The s i m u l a t i o n m e t h o d i n t r o d u c e d in this w o r k where the d y n a m i c s o f c o n s u m p t i o n a n d the d y n a m i c s o f h e a t source availability are t a k e n into c o n s i d e r a t i o n in c o n n e c t i o n with the electrical p o w e r c o n s u m p t i o n c o n d i t i o n to a v o i d the increase o f p e a k p o w e r a n d o p e r a t i o n d u r i n g lower rate has a higher efficiency t h a n the c o n v e n t i o n a l m e t h o d . This s i m u l a t i o n m e t h o d has been successfully used before [4]. REFERENCES 1. U. F. Kenney, Energy Conservation in the Process Industries. Academic Press, New York (1984). 2. B. Frhanieh and B. Sunden, Analysis of an existing heat exchanger network and effects of heat pump installation. Heat Recovery Systems & CHP 10, 285-296 (1990). 3. E. Walline, P. A. Franck and T. Berntsson, Heat pumps in industrial processes--an optimization methodology. Heat Recovery Systems & CHP 10, 437-490 (1990). 4. M. Tomgi6 and F. AI-Mansour, Optimal operation of cogeneration plants in industry. Energy 14, 483-490 (1989). 5. Commission of the European Communities, Industrial Heat Pump; Energy Conservation in Industry, Vol. !, Combustion and Heat Recovery. VDI Verlag, Diisseldorf (1984). 6. F. Morer, and H. Schnitzer, Heat Pumps in Industry. Elsevier, Netherlands (1985). 7. M. Tomgic, F. A1-Mansour and T. Blatnik, Vklju~itev toplotnih 6rpalk v interni (industrijski) energetski sistem kot pogoj gospodarnosti. RAVE Portoro~ (1985). 8. T. Blatnik, F. AI-Mansour and M. Tomgi~, Zvi~anja grelnega gtevila sistema s topiotnimi 6rpalkami s serijsko vezavo agregatov, 16. Kongres o grejanju, hladenju i klimatizaciji, Beograd, 13-15 November (1985). 9. M. Tom~i~ and F. A1-Mansour, Sistem za shranjevanje toplote ali hladu z izboljganim termodinamskim izkoristkom, patentna prijava IJS, Ljubljana (1986). 10. M. Tom[i6, Z. Marin[ek and C. Physicos, lzbolj~anje izrabe energije s shranjevanjem in avtomatizacijo obratovanja, Nova Proizvodnja, ~t., 5 June (1982). 11. A. Ahmed, F. AI-Mansour, B. Selan, R. Tavzes and A. Todorovi~, Idejni projekt za izrabo nizkotemperaturnih sekundarnih toplot z uvedbo toplotnih ~rpalk v DO KRKA Novo Mesto, IJS-DP/RE-205, Ljubljana (1986).