- Email: [email protected]
The thermal effects of some control logics used in GDHS
Applied Thermal Engineering 27 (2007) 1495–1500 www.elsevier.com/locate/apthermeng
The thermal effects of some control logics used in GDHS Serhan Ku¨c¸u¨ka _ Mechanical Engineering Department, University of Dokuz Eylu¨l, 35100 Izmir, Turkey Received 18 January 2006; accepted 28 September 2006 Available online 15 November 2006
Abstract The temperature of the water returning from the network affects greatly the efficiency of a geothermal district-heating system (GDHS). The temperature of the returning water depends on whether there is a heat exchanger between network flow and indoor circulation. The return temperature also depends on outdoor temperature and logic of the indoor temperature control system. In this paper, four control logics are defined depending on whether indoor circulation is separated from network circulation or not. Return temperature and circulation rate of network flow are calculated for these control logics. The results show that the flow rate of the network flow and annual consumption of the geothermal fluid could be decreased about 10% or over by using optimum control logic in district heating systems. 2006 Elsevier Ltd. All rights reserved. Keywords: District heating; Geothermal heating; Balc¸ova GDHS
1. Introduction The utilizable part of the heat extracted from a geothermal source is limited by the return temperature of the network flow as well as the flow rate and production temperature of the geothermal fluid. To obtain maximum heating capacity, the return temperature should be at a minimum level. Thus, the utilized heat increases and, the flow rate of the circulation water and the pump power decrease. Some schemes and general rules which effect on the performance of a geothermal district heating system (GDHS) were introduced in the literature [1,2]. There are a few applications assisted by heat pumps. However, in the most of these systems, geothermal heat is transferred into fresh water through a heat exchanger. Thus, the heating system has two circuits: Geothermal loop and user network. However, some of these systems have also secondary heat exchangers through which the heat is transferred into indoor circulation water. Thus, the heating system has three circuits separated by heat exchangers: Geothermal
E-mail address: [email protected] 1359-4311/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.09.025
loop, user network, and indoor circuit. Other systems in which the geothermal fluid flows in the network or directly through the indoor heaters are out of the scope of this study. Because of the secondary heat exchangers, the investment cost and return-temperature of the circulation water are higher in three-circuit systems than that in two-circuit ones. However, hydraulic balance can be managed better in a three-circuit system. On the other hand, there are only a few articles in literature about how control logic and heat exchanger installation affect thermal behavior of a district heating system. Karlsson and Ragnarsson [3] have described a two-circuit system in which geothermal fluid flows in the user network and fresh water circulates in the indoor circuit. The thermal behaviors of the radiators and exchangers used in the system were analyzed, and the return temperature of the geothermal fluid was carried out depending on the supply temperature of the indoor circuit at various outdoor temperatures. The results were also compared with a direct heating system in which geothermal fluid is circulated through heating radiators. Chuanshan [4] has analyzed how the heat capacity changes as the flow rate of the water circulating in user
1496
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
Nomenclature C DH E Q N Tbase Tin Tout Tr
specific heat (kJ/kg C) sum of the degree-hours throughout a year (C h) annual enegy consumption (kJ or kW h) heat load (kW) sum of the hours at which provided heating base temperature (C) indoor temperature (C) outdoor temperature (C) return temperature of indoor circuit (C)
network is increased. When the circulation rate is increased over an optimum value, the supply temperature of the user network decreases because of the limited capacity of the exchanger extracting heat from the geothermal fluid. The heat radiated from the hot water radiators also decreases depending on this decreasing of the supply temperature of the network. Chuanshan [4] has carried out the optimum flow rates of the network circulation and geothermal fluid depending on the outdoor temperature for a geothermal district-heating system. The heat transferred from a network is also used to heat domestic water. Zsebik and Sitku [5] have discussed the effects of the heat exchangers connections on the return temperature of the network. The connections of the exchangers might be in various orders-parallel, serial, and various combinations of these-among the user network, the indoor circle, and the domestic hot water system in a substation. The return temperature of the network fluid also depends on the control logic, which is used to keep indoor temperature at a desired value. The indoor temperature is controlled by changing the temperature and flow rate of the indoor-circulation water, and these changes affect the return-temperature of the user network. In this article the flow rate and return-temperature of the user network have been analyzed depending on the outdoor temperature and the control logic in two- or three-circuit systems. 2. Heat load and capacity of hot water radiators
Trn Ts Tsn V_ q DTlm
return temperature of network circuit (C) supply temperature of indoor circuit (C) supply temperature of network circuit (C) flow rate (m3/s) specific density (kg/m3) logarithmic mean temperature difference (LMTD) (C)
Subscript 0 calculated under design conditions
2.2. Capacity of hot water radiators The heat capacity of a hot water radiator depends on the logarithmic mean temperature difference (LMTD, C) and is estimated at changing values of LMTD as given in Eq. (2). The value of LMTD between the indoor space and the water circulating through the radiator is calculated according to Eq. (3). Q ¼ Q0 ðDT lm =DT lm;0 Þn Ts Tr i DT lm ¼ h in ln TT sr T T in
ð2Þ ð3Þ
where Ts and Tr are the supply- and return-temperatures (C) of the water respectively. DTlm,0 is the value of LMTD that is calculated under design conditions. Gretarsson et al. [6] have investigated the heat capacity of a radiator out of its design temperature and flow rate. Numerical results show that Eq. (2) is valid when the ratio of actual flow rate to the design flow rate is between 0.05 and 2. Exponent n has been calculated to be 1.21 for plate type hot water radiators when supply temperature is 80 C. On the other hand, exponent n also changes with the type of radiator, and has been assumed to be 1.3 in this paper as the German Standard DIN 4703 gives: Q ¼ Q0 ðDT lm =DT lm;0 Þ
1:3
ð4Þ
The heat transferred from a radiator is also equal the enthalpy change of the water passing through the radiator and the difference between the supply and return temperatures of the circulating water is calculated according to Eq. (5).
2.1. The change in heat load with outdoor temperature
Q ¼ V_ qCðT s T r Þ
The heat load for a building changes linearly with the difference between indoor and outdoor temperatures:
where V_ is the flow rate (m3/s), q is the density (kg/m3), and C is the specific heat (kJ/kg C) of the circulation water respectively.
Q ¼ Q0 ðT in T out Þ=ðT in T out;0 Þ
ð5Þ
ð1Þ
where Q is the heat load (kW) and Tin and Tout are indoor and outdoor temperatures (C). Design heat load, Q0, is calculated under design conditions, and Tout,0 is the design outdoor temperature (C).
2.3. The temperature change through a secondary heat exchanger The secondary heat exchangers are used in a threecircuit system to separate the user network and the indoor
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
circulation. The LMDT between the network flow and the indoor circulation is calculated using Eq. (6) in which Tsn and Trn are the supply- and return-temperatures (C) of the user network respectively. If the overall heat transfer coefficient has a constant value at altering heat loads, the ratio of the transferred heat to the design load changes linearly with the LMTD (Eq. (7)). DT lm;hex ¼
ðT sn T s Þ ðT rn T r Þ h i s ln TT snsn T T r
ð7Þ
In actual fact, the overall heat transfer coefficient changes at partial loads with changing flow rate. However, the LMDT has a small value especially in the plate type heat exchangers and the deviation of the actual return temperature than estimated one is usually negligible according to the temperature change of the network flow throughout the exchanger. 3. Indoor temperature control In hot water heating systems running with conventional fuels, several control logics are used to hold the indoor temperature at a predetermined value while outdoor temperature is changing. One commonly logic is to regulate the temperature of the supply water depending on the outdoor temperature, and the other is to decrease the flow rate of the hot water passing through each radiator using a thermostatic valve triggered by the indoor temperature. In heating systems using by geothermal energy, it is also important to decrease the outgoing temperature of the indoor and network circuits. In the following sections, the flow rate and return-temperature of the network flow in two- or three-circuit systems were analyzed depending on the control logic. Case A: Indoor temperature control in two-circuit systems by changing the flow rate. The flow rate through a radiator is regulated using a thermostatic valve activated by the indoor temperature (Fig. 1). Heat load depends on the outdoor temperature
Indoor
Tv User network
Q Hot water radiator
Geothermal loop
Indoor
Temperature sensor
Temperature sensor
User network
Q Hot water radiator
ð6Þ
Q ¼ Q0 ðDT lm;hex =DT lm;hex;0 Þ
Outdoor
Outdoor
1497
Main heat exchanger
Fig. 1. Basic diagram of a two-circuit system with flow rate control via thermostatic valve.
Geothermal loop
Main heat exchanger
Three-way valve
Fig. 2. Basic diagram of a two-circuit system with supply temperature control via three-way valve.
as shown in Eq. (1), and the LMTD between the hot water and indoor space is calculated using Eq. (4). Supply temperature of the indoor circulation is the same as the network temperature, which is supposed to set at a constant value throughout heating season. The return temperature is calculated according to Eq. (3), and the flow rate through each radiator is determined by using Eq. (5). Case B: Indoor temperature control in two-circuit systems by changing the supply temperature of circulating water. The temperature of the indoor space is controlled with changing the temperature of the supply water. The water coming into the radiators is mixed with some part of the return water via a three-way valve (Fig. 2). Thus, the supply-temperature of the indoor circuit is changed depending on the outdoor temperature although the supply-temperature of the user network is constant. Flow rate passing through each radiator does not change throughout the heating season. Depending on the heat load, the LMTD between the radiator and indoor space is calculated using Eq. (4). To determine the supply- and return-temperatures of the indoor circuit, Eqs. (3) and (5) should be solved simultaneously using iterative methods. Case C: Indoor temperature control in three-circuit systems by changing the flow rate The difference between case C and case A is that indoor circuit is separated from the user network by a secondary heat exchanger as shown in Fig. 3. In this case, the supply-temperature of the indoor circuit is held at a predetermined value, and the rate of the network flow passing through the secondary exchanger is regulated using a thermostatic valve. Other thermostatic valves mounted on the radiators control the flow rate passing through each radiator in order to keep indoor temperature at a certain level. The LMTD between the hot water radiator and indoor space is calculated according to Eq. (4), and the returntemperature of the indoor circulation is calculated by Eq. (3). On the other hand, to estimate the value of the LMTD in the secondary heat exchanger, Eq. (7) should be used. When the supply temperature of the network flow is prescribed, the return temperature of the network flow is
1498
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500 Outdoor
Indoor
Indoor circulation
Tv Q Hot water radiator
User network
Geothermal loop
Main heat exchanger
Secondary heat exchanger Tv
Fig. 3. Basic diagram of a three-circuit system with flow rate control via thermostatic valve.
Outdoor
Indoor Temperature sensor
Temperature sensor
Indoor circulation
User network
Q Hot water radiator
Secondary heat exchanger Tv
Geothermal loop
Main heat exchanger
total number is around 20. The production temperature changes from 100 C through 140 C depending on the well. The geothermal fluids exploited from the different wells are collected by a pipeline and directed to a plate type heat exchanger installed in the heat center to heat the water circulating in the user network. The average temperatures of the supply- and return-water circulating in the user network are 85 C and 55 C respectively. Indoor circulation is common for all flats in a building and each building benefiting from the system is connected to the network via a secondary heat exchanger. Today, the system is being expanded and the heating capacity of it is increased. 4.1. Annual heat load of district-heating system Different methods are defined in literature to estimate the variation of the heat load depending on the outdoor temperatures. This article uses the degree-hour method suggested by Durmayaz and Kadıog˘lu [8] as in Eq. (8). Annual heat consumption of a system is related to the sum of degree-hours (DH) throughout the heating season and calculated according to the Eq. (9). In these equations Tbase is the base temperature below which the outdoor temperature causes a heat load and N is the number of hours at which the heating is provided. XN DH ¼ ðT in T out Þj ; for T out < T base ð8Þ j¼1 E¼
calculated by using Eq. (6) depending on the numerical value of the LMTD. Case D: Indoor temperature control in three-circuit systems by changing the supply temperature of indoor circulation A thermostatic valve located on the return line of the network flow regulates the supply-temperature of the indoor circuit depending on the outdoor temperature (Fig. 4) and the circulation rate of the indoor circuit is hold at a constant value throughout the season. The supply- and return-temperatures of the indoor circuit are calculated as in case B, though the LMTD in the secondary heat exchanger and the return-temperature of the network flow are calculated as in case C. 4. Numerical results for Balc¸ova gdhs The return temperatures of the network and geothermal flows and annual consumption of the geothermal fluid were estimated at Balc¸ova GDHS for each case of the control _ logic. This system is served over 10 000 residences in Izmir, and heat demand reached 73 MW at peaking time in 2002 [7]. The geothermal fluid is exploited from the deep wells (550–1100 m) and shallow wells (50–150 m), of which the
ð9Þ
Annual heat load has been calculated for Balc¸ova district heating system by using these equations. Tin and Tbase have been adopted to be 20 C and 15 C, respectively, and Tout was obtained from the meteorological data of the average _ year of Izmir (Balc¸ova), 1993 [9]. According to this data, N is 3615 h and DH is 39470 C h in Balc¸ova. The distribution of the temperature difference between indoor space and ambient is shown in Fig. 5. On the other hand, Tout,0, the design outdoor temperature, is 0 C, and Q0, the heat 25 Temperature difference for heating demand,˚C
Fig. 4. Basic diagram of a three-circuit system with supply temperature control via secondary heat exchanger.
DH Q0 ðT in T out;0 Þ
20
15
10
5
0 0
1000
2000 hours
3000
4000
_ Fig. 5. Annual distribution of the heating degree-hours in Balc¸ova, Izmir, in 1993.
4.2. Temperatures and flow rates of circuits
Water circulation temperature,˚C
In current application in Balc¸ova district-heating system, the return temperature of the network circulation is limited to a maximum 50 C with thermostatic valves installed on the secondary exchangers. But, the heat demands of the users change throughout the heating season and the return-temperature should change depending on the outdoor temperature. In the following section, a study has been carried out on what the flow rates and temperatures in the user network of Balc¸ova GDHS would be depending on the control systems defined in cases A, B, C, and D. The supply temperature of the network is kept at 85 C throughout the season. The indoor is 20 C, and the outdoor temperature at design point is 0 C as given previously. The hot and cold fluids are in counter flow in the secondary exchangers, and the temperature difference between them is supposed to be 5 C throughout the exchanger for the projected heat load. Case A: The supply temperature of the network equals the supply-temperature of the radiators and is 85 C. The return temperature should be as low as possible and has been selected to be 40 C under design conditions. At the design point, the LMTD between the indoor circulation and indoor space is 38.2 C. When the outdoor temperature increases, a thermostatic valve decreases the flow rate passing through each radiator to keep indoor temperature at the desired point. Case B: To regulate the LMTD between the water passing through the radiator and indoor space, some part of the return flow is mixed to the supply flow. The supply and return temperatures of the indoor circuit are plotted in Fig. 6 for cases A and B. The flow return from the indoor circuit also is return of the network. Case C: The supply temperature of the indoor circulation is lower than the supply temperature of the network flow and is kept at 80 C throughout the season. The return
1499
100 supply temperature (network)
80
supply temperature (indoor)
60 40
return temperature (network)
20
return temperature (indoor)
0 0.00
5.00
10.00
15.00
Outdoor temperature, ˚C
Fig. 7. Supply and return water temperatures for case C.
Water circulation temperature,˚C
load at this temperature, is 73 MW in Balc¸ova geothermal district-heating system. E, the annual heat consumption of the system, is calculated 144066 MW h or 518.6·106 MJ.
Water circulation temperature, ˚C
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
100 80
supply temperature (network) supply temperature (indoor)
60
return temperature (network) 40 return temperature (indoor) 20 0 0
5 10 Outdoor temperature,˚C
15
Fig. 8. Supply and return water temperatures for case D.
temperature of the indoor circulation is 42.4 C under the design conditions, thus the LMTD between the indoor circulation and indoor space is same as in case A (38.2 C). For various outdoor temperatures, the return temperatures of the network and indoor circuits are plotted in Fig. 7. Case D: Depending on the outdoor temperatures, the supply temperature of the indoor circulation is regulated via a thermostatic valve installed in the network line going out from the exchanger. The supply- and return-temperatures of the network and indoor circuits are shown in Fig. 8 for various outdoor temperatures. However, return temperature of the network circuit is very close to indoor circuit’s for small loads of heating. 4.3. Total flow rate and annual consumption of geothermal fluid
100 Case A
80 Case B
60 40
Case B (return)
20
Case A (return)
0 0
5
10
15
Outdoor temperature,˚C
Fig. 6. Supply and return water temperatures at indoor circulation for cases A and B.
The flow rate in the user network depends on the temperature difference between the supply- and return-flows and the heat load of the district. The flow rate has been calculated 1397 m3/h at the design temperature in cases A and B, and 1673 m3/h in cases C and D, in which secondary exchangers are used. The heat load and the flow rate in the network are given in Table 1 for various outdoor temperatures. The consumption of the geothermal fluid is also calculated for changing heat loads. The average temperature of the geothermal fluid exploited from different wells is accepted to be 115 C, and the return temperature is
1500
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
Table 1 Heat load of Balc¸ova district and flow rate in the network Tout (C)
Q (MW)
Flow rate of network circuit (m3/h) Case A
Case B
Case C
Case D
0.00 3.00 6.00 9.00 12.00 15.00
73 62.05 51.10 40.15 29.20 18.25
1397 1060 797 581 402 244
1397 1140 899 676 468 277
1673 1190 851 605 408 245
1673 1213 934 695 478 281
Table 2 Heat load of Balc¸ova district and consumption of the geothermal fluid Tout (C)
Q (MW)
Consumption of the geothermal fluid (m3/h) Case A
Case B
Case C
Case D
0.00 3.00 6.00 9.00 12.00 15.00
73 62.05 51.10 40.15 29.20 18.25
850 673 523 391 275 168
850 704 565 431 304 183
944 723 545 401 278 169
944 732 578 439 308 185
0 C, the heat load of the district has been estimated to be the same as at the design point. The total of the flow rates of the network circulation throughout the year and the annual consumption of the geothermal fluid are given in Table 3. The results show that the annual consumption of the geothermal fluid in case D is around 11% higher than in case A. 5. Conclusions The consumption of the geothermal fluid and the flow rate circulating in a district heating system are affected directly by control logics used for regulating the indoortemperature. The results show that, in the systems in which the flow rate passing through each radiator is controlled by a thermostatic valve, the consumption of the geothermal fluid is lower than that in the systems in which supply temperature is changed depending on the outdoor temperature. The numerical results also show that the annual consumption of the geothermal fluid in three-circuit systems is higher than that in two-circuit systems, and in Balc¸ova GDHS this difference is around 11%. References
Table 3 Total flow rate in the user network and annual consumption of the geothermal fluid
Total flow rate in the network (m3/year) Annual consumption of the geothermal fluid (m3/year)
Case A
Case B
Case C
Case D
2 188 202
2 464 909
2 329 315
2 570 415
1 445 020
1 562 561
1 502 095
1 602 964
1 C over the return temperature of the network fluid. Depending on these assumptions, the consumption of the geothermal fluid is calculated as 850 m3/h for a two-circuit system, and 944 m3/h for a three-circuit system when outdoor temperature is 0 C (Table 2). The flow rate of the network circulation and the consumption of the geothermal fluid have been calculated for each hour throughout the heating season. These calculations have been done for 1993, and for the hours at which outdoor temperature below than the design temperature,
[1] R. Harrison, The design and economics of european geothermal heating installations, Geothermics 23 (1) (1994) 61–71. [2] A. Piatti, C. Piemonte, E. Szego¨, Planning of Geothermal District Heating Systems, Kluwer Academic Publishers, Dordrecht, 1992. [3] T. Karlsson, A. Ragnarsson, Use of very low temperature geothermal water in radiator heating systems, in: Proceedings of World Geothermal Congress, Florence, Italy, 1995, pp. 2193–2201. [4] D. Chuanshan, Thermal analysis of indirect geothermal district heating systems, Geothermics 26 (3) (1997) 351–364. [5] A. Zsebik, Gy. Sitku, Heat exchanger connection in substations – a tool of decreasing return temperature in district heating networks, Energy Engineering 98 (5) (2001) 20–29. [6] S. Gretarsson, P. Valdimarsson, V. Jonsson, Heat transfer modelling of a plate radiator for district heating applications, International Journal of Energy Research 15 (1991) 301–315. [7] C. C ¸ anakc¸ı, Balc¸ova geothermal district heating system, Cooling World (in Turkish) 21 (2003) 33–39. [8] A. Durmayaz, M. Kadıog˘lu, Heating energy requirements and fuel consumptions in the biggest city centers of Turkey, Energy Conversion and Management 44 (7) (2003) 1177–1192. [9] O. S ß en, L. S ß aylan, H. Toros, Meteorologic data of Turkey, Project Report of TTMD (Turkish Association of Air Conditioning Engineering), 2000.
The thermal effects of some control logics used in GDHS Serhan Ku¨c¸u¨ka _ Mechanical Engineering Department, University of Dokuz Eylu¨l, 35100 Izmir, Turkey Received 18 January 2006; accepted 28 September 2006 Available online 15 November 2006
Abstract The temperature of the water returning from the network affects greatly the efficiency of a geothermal district-heating system (GDHS). The temperature of the returning water depends on whether there is a heat exchanger between network flow and indoor circulation. The return temperature also depends on outdoor temperature and logic of the indoor temperature control system. In this paper, four control logics are defined depending on whether indoor circulation is separated from network circulation or not. Return temperature and circulation rate of network flow are calculated for these control logics. The results show that the flow rate of the network flow and annual consumption of the geothermal fluid could be decreased about 10% or over by using optimum control logic in district heating systems. 2006 Elsevier Ltd. All rights reserved. Keywords: District heating; Geothermal heating; Balc¸ova GDHS
1. Introduction The utilizable part of the heat extracted from a geothermal source is limited by the return temperature of the network flow as well as the flow rate and production temperature of the geothermal fluid. To obtain maximum heating capacity, the return temperature should be at a minimum level. Thus, the utilized heat increases and, the flow rate of the circulation water and the pump power decrease. Some schemes and general rules which effect on the performance of a geothermal district heating system (GDHS) were introduced in the literature [1,2]. There are a few applications assisted by heat pumps. However, in the most of these systems, geothermal heat is transferred into fresh water through a heat exchanger. Thus, the heating system has two circuits: Geothermal loop and user network. However, some of these systems have also secondary heat exchangers through which the heat is transferred into indoor circulation water. Thus, the heating system has three circuits separated by heat exchangers: Geothermal
E-mail address: [email protected] 1359-4311/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2006.09.025
loop, user network, and indoor circuit. Other systems in which the geothermal fluid flows in the network or directly through the indoor heaters are out of the scope of this study. Because of the secondary heat exchangers, the investment cost and return-temperature of the circulation water are higher in three-circuit systems than that in two-circuit ones. However, hydraulic balance can be managed better in a three-circuit system. On the other hand, there are only a few articles in literature about how control logic and heat exchanger installation affect thermal behavior of a district heating system. Karlsson and Ragnarsson [3] have described a two-circuit system in which geothermal fluid flows in the user network and fresh water circulates in the indoor circuit. The thermal behaviors of the radiators and exchangers used in the system were analyzed, and the return temperature of the geothermal fluid was carried out depending on the supply temperature of the indoor circuit at various outdoor temperatures. The results were also compared with a direct heating system in which geothermal fluid is circulated through heating radiators. Chuanshan [4] has analyzed how the heat capacity changes as the flow rate of the water circulating in user
1496
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
Nomenclature C DH E Q N Tbase Tin Tout Tr
specific heat (kJ/kg C) sum of the degree-hours throughout a year (C h) annual enegy consumption (kJ or kW h) heat load (kW) sum of the hours at which provided heating base temperature (C) indoor temperature (C) outdoor temperature (C) return temperature of indoor circuit (C)
network is increased. When the circulation rate is increased over an optimum value, the supply temperature of the user network decreases because of the limited capacity of the exchanger extracting heat from the geothermal fluid. The heat radiated from the hot water radiators also decreases depending on this decreasing of the supply temperature of the network. Chuanshan [4] has carried out the optimum flow rates of the network circulation and geothermal fluid depending on the outdoor temperature for a geothermal district-heating system. The heat transferred from a network is also used to heat domestic water. Zsebik and Sitku [5] have discussed the effects of the heat exchangers connections on the return temperature of the network. The connections of the exchangers might be in various orders-parallel, serial, and various combinations of these-among the user network, the indoor circle, and the domestic hot water system in a substation. The return temperature of the network fluid also depends on the control logic, which is used to keep indoor temperature at a desired value. The indoor temperature is controlled by changing the temperature and flow rate of the indoor-circulation water, and these changes affect the return-temperature of the user network. In this article the flow rate and return-temperature of the user network have been analyzed depending on the outdoor temperature and the control logic in two- or three-circuit systems. 2. Heat load and capacity of hot water radiators
Trn Ts Tsn V_ q DTlm
return temperature of network circuit (C) supply temperature of indoor circuit (C) supply temperature of network circuit (C) flow rate (m3/s) specific density (kg/m3) logarithmic mean temperature difference (LMTD) (C)
Subscript 0 calculated under design conditions
2.2. Capacity of hot water radiators The heat capacity of a hot water radiator depends on the logarithmic mean temperature difference (LMTD, C) and is estimated at changing values of LMTD as given in Eq. (2). The value of LMTD between the indoor space and the water circulating through the radiator is calculated according to Eq. (3). Q ¼ Q0 ðDT lm =DT lm;0 Þn Ts Tr i DT lm ¼ h in ln TT sr T T in
ð2Þ ð3Þ
where Ts and Tr are the supply- and return-temperatures (C) of the water respectively. DTlm,0 is the value of LMTD that is calculated under design conditions. Gretarsson et al. [6] have investigated the heat capacity of a radiator out of its design temperature and flow rate. Numerical results show that Eq. (2) is valid when the ratio of actual flow rate to the design flow rate is between 0.05 and 2. Exponent n has been calculated to be 1.21 for plate type hot water radiators when supply temperature is 80 C. On the other hand, exponent n also changes with the type of radiator, and has been assumed to be 1.3 in this paper as the German Standard DIN 4703 gives: Q ¼ Q0 ðDT lm =DT lm;0 Þ
1:3
ð4Þ
The heat transferred from a radiator is also equal the enthalpy change of the water passing through the radiator and the difference between the supply and return temperatures of the circulating water is calculated according to Eq. (5).
2.1. The change in heat load with outdoor temperature
Q ¼ V_ qCðT s T r Þ
The heat load for a building changes linearly with the difference between indoor and outdoor temperatures:
where V_ is the flow rate (m3/s), q is the density (kg/m3), and C is the specific heat (kJ/kg C) of the circulation water respectively.
Q ¼ Q0 ðT in T out Þ=ðT in T out;0 Þ
ð5Þ
ð1Þ
where Q is the heat load (kW) and Tin and Tout are indoor and outdoor temperatures (C). Design heat load, Q0, is calculated under design conditions, and Tout,0 is the design outdoor temperature (C).
2.3. The temperature change through a secondary heat exchanger The secondary heat exchangers are used in a threecircuit system to separate the user network and the indoor
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
circulation. The LMDT between the network flow and the indoor circulation is calculated using Eq. (6) in which Tsn and Trn are the supply- and return-temperatures (C) of the user network respectively. If the overall heat transfer coefficient has a constant value at altering heat loads, the ratio of the transferred heat to the design load changes linearly with the LMTD (Eq. (7)). DT lm;hex ¼
ðT sn T s Þ ðT rn T r Þ h i s ln TT snsn T T r
ð7Þ
In actual fact, the overall heat transfer coefficient changes at partial loads with changing flow rate. However, the LMDT has a small value especially in the plate type heat exchangers and the deviation of the actual return temperature than estimated one is usually negligible according to the temperature change of the network flow throughout the exchanger. 3. Indoor temperature control In hot water heating systems running with conventional fuels, several control logics are used to hold the indoor temperature at a predetermined value while outdoor temperature is changing. One commonly logic is to regulate the temperature of the supply water depending on the outdoor temperature, and the other is to decrease the flow rate of the hot water passing through each radiator using a thermostatic valve triggered by the indoor temperature. In heating systems using by geothermal energy, it is also important to decrease the outgoing temperature of the indoor and network circuits. In the following sections, the flow rate and return-temperature of the network flow in two- or three-circuit systems were analyzed depending on the control logic. Case A: Indoor temperature control in two-circuit systems by changing the flow rate. The flow rate through a radiator is regulated using a thermostatic valve activated by the indoor temperature (Fig. 1). Heat load depends on the outdoor temperature
Indoor
Tv User network
Q Hot water radiator
Geothermal loop
Indoor
Temperature sensor
Temperature sensor
User network
Q Hot water radiator
ð6Þ
Q ¼ Q0 ðDT lm;hex =DT lm;hex;0 Þ
Outdoor
Outdoor
1497
Main heat exchanger
Fig. 1. Basic diagram of a two-circuit system with flow rate control via thermostatic valve.
Geothermal loop
Main heat exchanger
Three-way valve
Fig. 2. Basic diagram of a two-circuit system with supply temperature control via three-way valve.
as shown in Eq. (1), and the LMTD between the hot water and indoor space is calculated using Eq. (4). Supply temperature of the indoor circulation is the same as the network temperature, which is supposed to set at a constant value throughout heating season. The return temperature is calculated according to Eq. (3), and the flow rate through each radiator is determined by using Eq. (5). Case B: Indoor temperature control in two-circuit systems by changing the supply temperature of circulating water. The temperature of the indoor space is controlled with changing the temperature of the supply water. The water coming into the radiators is mixed with some part of the return water via a three-way valve (Fig. 2). Thus, the supply-temperature of the indoor circuit is changed depending on the outdoor temperature although the supply-temperature of the user network is constant. Flow rate passing through each radiator does not change throughout the heating season. Depending on the heat load, the LMTD between the radiator and indoor space is calculated using Eq. (4). To determine the supply- and return-temperatures of the indoor circuit, Eqs. (3) and (5) should be solved simultaneously using iterative methods. Case C: Indoor temperature control in three-circuit systems by changing the flow rate The difference between case C and case A is that indoor circuit is separated from the user network by a secondary heat exchanger as shown in Fig. 3. In this case, the supply-temperature of the indoor circuit is held at a predetermined value, and the rate of the network flow passing through the secondary exchanger is regulated using a thermostatic valve. Other thermostatic valves mounted on the radiators control the flow rate passing through each radiator in order to keep indoor temperature at a certain level. The LMTD between the hot water radiator and indoor space is calculated according to Eq. (4), and the returntemperature of the indoor circulation is calculated by Eq. (3). On the other hand, to estimate the value of the LMTD in the secondary heat exchanger, Eq. (7) should be used. When the supply temperature of the network flow is prescribed, the return temperature of the network flow is
1498
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500 Outdoor
Indoor
Indoor circulation
Tv Q Hot water radiator
User network
Geothermal loop
Main heat exchanger
Secondary heat exchanger Tv
Fig. 3. Basic diagram of a three-circuit system with flow rate control via thermostatic valve.
Outdoor
Indoor Temperature sensor
Temperature sensor
Indoor circulation
User network
Q Hot water radiator
Secondary heat exchanger Tv
Geothermal loop
Main heat exchanger
total number is around 20. The production temperature changes from 100 C through 140 C depending on the well. The geothermal fluids exploited from the different wells are collected by a pipeline and directed to a plate type heat exchanger installed in the heat center to heat the water circulating in the user network. The average temperatures of the supply- and return-water circulating in the user network are 85 C and 55 C respectively. Indoor circulation is common for all flats in a building and each building benefiting from the system is connected to the network via a secondary heat exchanger. Today, the system is being expanded and the heating capacity of it is increased. 4.1. Annual heat load of district-heating system Different methods are defined in literature to estimate the variation of the heat load depending on the outdoor temperatures. This article uses the degree-hour method suggested by Durmayaz and Kadıog˘lu [8] as in Eq. (8). Annual heat consumption of a system is related to the sum of degree-hours (DH) throughout the heating season and calculated according to the Eq. (9). In these equations Tbase is the base temperature below which the outdoor temperature causes a heat load and N is the number of hours at which the heating is provided. XN DH ¼ ðT in T out Þj ; for T out < T base ð8Þ j¼1 E¼
calculated by using Eq. (6) depending on the numerical value of the LMTD. Case D: Indoor temperature control in three-circuit systems by changing the supply temperature of indoor circulation A thermostatic valve located on the return line of the network flow regulates the supply-temperature of the indoor circuit depending on the outdoor temperature (Fig. 4) and the circulation rate of the indoor circuit is hold at a constant value throughout the season. The supply- and return-temperatures of the indoor circuit are calculated as in case B, though the LMTD in the secondary heat exchanger and the return-temperature of the network flow are calculated as in case C. 4. Numerical results for Balc¸ova gdhs The return temperatures of the network and geothermal flows and annual consumption of the geothermal fluid were estimated at Balc¸ova GDHS for each case of the control _ logic. This system is served over 10 000 residences in Izmir, and heat demand reached 73 MW at peaking time in 2002 [7]. The geothermal fluid is exploited from the deep wells (550–1100 m) and shallow wells (50–150 m), of which the
ð9Þ
Annual heat load has been calculated for Balc¸ova district heating system by using these equations. Tin and Tbase have been adopted to be 20 C and 15 C, respectively, and Tout was obtained from the meteorological data of the average _ year of Izmir (Balc¸ova), 1993 [9]. According to this data, N is 3615 h and DH is 39470 C h in Balc¸ova. The distribution of the temperature difference between indoor space and ambient is shown in Fig. 5. On the other hand, Tout,0, the design outdoor temperature, is 0 C, and Q0, the heat 25 Temperature difference for heating demand,˚C
Fig. 4. Basic diagram of a three-circuit system with supply temperature control via secondary heat exchanger.
DH Q0 ðT in T out;0 Þ
20
15
10
5
0 0
1000
2000 hours
3000
4000
_ Fig. 5. Annual distribution of the heating degree-hours in Balc¸ova, Izmir, in 1993.
4.2. Temperatures and flow rates of circuits
Water circulation temperature,˚C
In current application in Balc¸ova district-heating system, the return temperature of the network circulation is limited to a maximum 50 C with thermostatic valves installed on the secondary exchangers. But, the heat demands of the users change throughout the heating season and the return-temperature should change depending on the outdoor temperature. In the following section, a study has been carried out on what the flow rates and temperatures in the user network of Balc¸ova GDHS would be depending on the control systems defined in cases A, B, C, and D. The supply temperature of the network is kept at 85 C throughout the season. The indoor is 20 C, and the outdoor temperature at design point is 0 C as given previously. The hot and cold fluids are in counter flow in the secondary exchangers, and the temperature difference between them is supposed to be 5 C throughout the exchanger for the projected heat load. Case A: The supply temperature of the network equals the supply-temperature of the radiators and is 85 C. The return temperature should be as low as possible and has been selected to be 40 C under design conditions. At the design point, the LMTD between the indoor circulation and indoor space is 38.2 C. When the outdoor temperature increases, a thermostatic valve decreases the flow rate passing through each radiator to keep indoor temperature at the desired point. Case B: To regulate the LMTD between the water passing through the radiator and indoor space, some part of the return flow is mixed to the supply flow. The supply and return temperatures of the indoor circuit are plotted in Fig. 6 for cases A and B. The flow return from the indoor circuit also is return of the network. Case C: The supply temperature of the indoor circulation is lower than the supply temperature of the network flow and is kept at 80 C throughout the season. The return
1499
100 supply temperature (network)
80
supply temperature (indoor)
60 40
return temperature (network)
20
return temperature (indoor)
0 0.00
5.00
10.00
15.00
Outdoor temperature, ˚C
Fig. 7. Supply and return water temperatures for case C.
Water circulation temperature,˚C
load at this temperature, is 73 MW in Balc¸ova geothermal district-heating system. E, the annual heat consumption of the system, is calculated 144066 MW h or 518.6·106 MJ.
Water circulation temperature, ˚C
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
100 80
supply temperature (network) supply temperature (indoor)
60
return temperature (network) 40 return temperature (indoor) 20 0 0
5 10 Outdoor temperature,˚C
15
Fig. 8. Supply and return water temperatures for case D.
temperature of the indoor circulation is 42.4 C under the design conditions, thus the LMTD between the indoor circulation and indoor space is same as in case A (38.2 C). For various outdoor temperatures, the return temperatures of the network and indoor circuits are plotted in Fig. 7. Case D: Depending on the outdoor temperatures, the supply temperature of the indoor circulation is regulated via a thermostatic valve installed in the network line going out from the exchanger. The supply- and return-temperatures of the network and indoor circuits are shown in Fig. 8 for various outdoor temperatures. However, return temperature of the network circuit is very close to indoor circuit’s for small loads of heating. 4.3. Total flow rate and annual consumption of geothermal fluid
100 Case A
80 Case B
60 40
Case B (return)
20
Case A (return)
0 0
5
10
15
Outdoor temperature,˚C
Fig. 6. Supply and return water temperatures at indoor circulation for cases A and B.
The flow rate in the user network depends on the temperature difference between the supply- and return-flows and the heat load of the district. The flow rate has been calculated 1397 m3/h at the design temperature in cases A and B, and 1673 m3/h in cases C and D, in which secondary exchangers are used. The heat load and the flow rate in the network are given in Table 1 for various outdoor temperatures. The consumption of the geothermal fluid is also calculated for changing heat loads. The average temperature of the geothermal fluid exploited from different wells is accepted to be 115 C, and the return temperature is
1500
S. Ku¨c¸u¨ka / Applied Thermal Engineering 27 (2007) 1495–1500
Table 1 Heat load of Balc¸ova district and flow rate in the network Tout (C)
Q (MW)
Flow rate of network circuit (m3/h) Case A
Case B
Case C
Case D
0.00 3.00 6.00 9.00 12.00 15.00
73 62.05 51.10 40.15 29.20 18.25
1397 1060 797 581 402 244
1397 1140 899 676 468 277
1673 1190 851 605 408 245
1673 1213 934 695 478 281
Table 2 Heat load of Balc¸ova district and consumption of the geothermal fluid Tout (C)
Q (MW)
Consumption of the geothermal fluid (m3/h) Case A
Case B
Case C
Case D
0.00 3.00 6.00 9.00 12.00 15.00
73 62.05 51.10 40.15 29.20 18.25
850 673 523 391 275 168
850 704 565 431 304 183
944 723 545 401 278 169
944 732 578 439 308 185
0 C, the heat load of the district has been estimated to be the same as at the design point. The total of the flow rates of the network circulation throughout the year and the annual consumption of the geothermal fluid are given in Table 3. The results show that the annual consumption of the geothermal fluid in case D is around 11% higher than in case A. 5. Conclusions The consumption of the geothermal fluid and the flow rate circulating in a district heating system are affected directly by control logics used for regulating the indoortemperature. The results show that, in the systems in which the flow rate passing through each radiator is controlled by a thermostatic valve, the consumption of the geothermal fluid is lower than that in the systems in which supply temperature is changed depending on the outdoor temperature. The numerical results also show that the annual consumption of the geothermal fluid in three-circuit systems is higher than that in two-circuit systems, and in Balc¸ova GDHS this difference is around 11%. References
Table 3 Total flow rate in the user network and annual consumption of the geothermal fluid
Total flow rate in the network (m3/year) Annual consumption of the geothermal fluid (m3/year)
Case A
Case B
Case C
Case D
2 188 202
2 464 909
2 329 315
2 570 415
1 445 020
1 562 561
1 502 095
1 602 964
1 C over the return temperature of the network fluid. Depending on these assumptions, the consumption of the geothermal fluid is calculated as 850 m3/h for a two-circuit system, and 944 m3/h for a three-circuit system when outdoor temperature is 0 C (Table 2). The flow rate of the network circulation and the consumption of the geothermal fluid have been calculated for each hour throughout the heating season. These calculations have been done for 1993, and for the hours at which outdoor temperature below than the design temperature,
[1] R. Harrison, The design and economics of european geothermal heating installations, Geothermics 23 (1) (1994) 61–71. [2] A. Piatti, C. Piemonte, E. Szego¨, Planning of Geothermal District Heating Systems, Kluwer Academic Publishers, Dordrecht, 1992. [3] T. Karlsson, A. Ragnarsson, Use of very low temperature geothermal water in radiator heating systems, in: Proceedings of World Geothermal Congress, Florence, Italy, 1995, pp. 2193–2201. [4] D. Chuanshan, Thermal analysis of indirect geothermal district heating systems, Geothermics 26 (3) (1997) 351–364. [5] A. Zsebik, Gy. Sitku, Heat exchanger connection in substations – a tool of decreasing return temperature in district heating networks, Energy Engineering 98 (5) (2001) 20–29. [6] S. Gretarsson, P. Valdimarsson, V. Jonsson, Heat transfer modelling of a plate radiator for district heating applications, International Journal of Energy Research 15 (1991) 301–315. [7] C. C ¸ anakc¸ı, Balc¸ova geothermal district heating system, Cooling World (in Turkish) 21 (2003) 33–39. [8] A. Durmayaz, M. Kadıog˘lu, Heating energy requirements and fuel consumptions in the biggest city centers of Turkey, Energy Conversion and Management 44 (7) (2003) 1177–1192. [9] O. S ß en, L. S ß aylan, H. Toros, Meteorologic data of Turkey, Project Report of TTMD (Turkish Association of Air Conditioning Engineering), 2000.