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Decentralized cooling in district heating network: System simulation and parametric study
Applied Energy 92 (2012) 175–184
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Decentralized cooling in district heating network: System simulation and parametric study Seksan Udomsri a,⇑, Chris Bales b, Andrew R. Martin a, Viktoria Martin a a b
Division of Heat and Power Technology, Department of Energy Technology, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden Solar Energy Research Center (SERC), Högskolan Dalarna, 781 88 Borlänge, Sweden
a r t i c l e
i n f o
Article history: Received 17 December 2010 Received in revised form 4 October 2011 Accepted 6 October 2011 Available online 29 November 2011 Keywords: Thermally driven chiller Decentralized cooling District heating network Simulation study TRNSYS
a b s t r a c t This paper presents system simulation and parametric study of the demonstration system of decentralized cooling in district heating network. The monitoring results obtained from the demonstration were calibrated and used for parametric studies in order to find improved system design and control. This study concentrates on system simulation studies that aim to: reduce the electricity consumption, to improve the thermal COP’s and capacity if possible; and to study how the system would perform with different boundary conditions such as climate and load. The internal pumps inside the thermally driven chiller (TDC) have been removed in the new version TDC and implemented in this study to increase the electrical COP. Results show that replacement of the fourth with the fifth generation TDC increases the system electrical COP from 2.64 to 5.27. The results obtained from parametric studies show that the electrical and thermal COP’s, with new realistic boundary conditions, increased from 2.74 to 5.53 and 0.48 to 0.52, respectively for the 4th generation TDC and from 5.01 to 7.46 and 0.33 to 0.43, respectively for the 5th generation TDC. Additionally the delivered cold increased from 2320 to 8670 and 2080 to 7740 kWh for the 4th and 5th generation TDC’s, respectively. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background and objectives A demonstration system (Subproject 1b: SP1b), was one of 11 demonstration systems installed and monitored within the EU-PolySMART project in Sweden. The aim was to design and develop the best system configuration for the combination of district heating and distributed absorption chillers [1]. PolySMART stands for POLYgeneration with advanced Small and Medium Scale thermally driven Air-Condition and Refrigeration Technology. The overall PolySMART project aims were to develop a set of technical solutions for a new market segment of polygeneration, in particular the market for small and medium scale tri-generation systems (combined production of electric power, heat and cooling). The key components of these systems always include the combined heat and power (CHP) plant along with the thermally driven chiller (TDC). There are different approaches of composing the CHP with TDC [1]; (i) centralized CHP and centralized TDC in combination with a heating and cooling network, (ii) centralized CHP and decentralized TDC in combination with a heating network, and (iii) decentralized CHP and decentralized TDC both on the demand side. Each approach can ⇑ Corresponding author. Tel.: +46 8 790 6140; fax: +46 8 20 41 61. E-mail address: [email protected] (S. Udomsri). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.10.009
be applied under different conditions depending upon the source of generation, consumption, and applications [1]. However, this article reports on a combination of centralized CHP and decentralized TDC in district heating network. District heating can be described as rational and environmentally friendly method to heat residential and commercial buildings etc. District heating is very common heating method and available throughout Sweden. The system is an example of distributed cooling with centralized combined heat and power (CHP), where the driving heat is delivered via the district heating network. Since the demand for cooling has increased tremendously around the world during the past decades [1–4], the conventional compression chillers share more than 15% of worldwide electricity energy consumption [5]. An absorption chiller is an excellent example of thermally driven cooling technology where the low temperature heat can be utilized for cooling production [6,7]. As part of the project, the aim of subproject 1b was to demonstrate the use of the ClimateWell chiller in distributed cooling with centralized CHP in order to develop the best system configuration for the TDC using a particular form of chemical heat pump [8]. The monitoring results and system calibration of the PolySMART demonstration system (SP1b) has been reported in [8]: monitoring results and calibration of simulation model. The main objective of this work was to calibrate and analyse the monitoring results obtained from the demonstration system and validate against a dynamic simulation model using TRaNsient SYstem Simulation
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Nomenclature CHP COP Cp E Eff HEP LHV _ m P PE Q RH SP T TCA TDC Tim V dpi
Combined Heating and Power Coefficient of Performance specific heat capacity electricity consumption efficiency high efficiency pump lower heating value of a fuel mass flow power Primary Energy thermal energy flow Relative Humidity subproject temperature Thermo-Chemical Accumulator thermally driven chiller time volume flow pressure drop in each circuit
Subscript Air air BOP balance of plant
program (TRNSYS) [9]. The calibration of the base case was made in three stages and the energy performance figures were within 4% of the measured values. Although a number of weaknesses have been found, it is fair enough to state that the TDC has worked reliably during the whole cooling season that was monitored. The major fault during the cooling season was due to an external electrical component (relay). However the electrical COP for the complete system was relatively low and in fact was lower than that of the main compression chiller. This was due to relatively large electrical energy use in the pumps, the TDC itself as well as the fan for the heat rejection unit (dry cooler). Other principle conclusions from the monitoring period were that the system worked well but at lower capacity than the nominal capacity of the TDC due to the boundary conditions for the system. This also resulted in lower than nominal thermal COP values. For the complete monitoring period during the summer of 2008, the thermal and electrical COPs for the TDC were only 0.41 and 2.1, respectively and the highest values were 0.50 and 4.6, respectively during the hottest period. Additionally, the system COP’s were found to be significantly lower than those for the TDC itself. There are a number of other causes of the relatively low thermal and electrical COP values of the entire system such as: (i) Heat exchanger in the driving circuit, causing extra heat losses in the driving circuit of the TDC, (ii) heat exchanger in the chilled water delivery circuit, causing heat gains in the delivery circuit both through normal gains through the insulation and components but also due to thermal energy from the two pumps used, and (iii) the driving temperature available from the district heating network is lower than ideal for the TDC. It is on average 75–80 °C, whereas 80–90 °C would be more ideal. (iv) The operating times of the whole system are relatively short. (v) The TDC cannot deliver at full power with the normal operating conditions of the system. The main objectives of this simulation study were to: reduce the electricity consumption, and if possible to improve the thermal COP and capacity at the same time; and to study how the system would perform with different boundary conditions such as climate and load. Studies in terms of the replacement of high efficiency pumps, new TDC version and variations in boundary conditions were con-
Cc Cdn Dc Dh el Fl HEP Hr Meas OA Opt Pump Rc Rl Sim Sys Tdc Th ti
chilling circuit of TDC cold distribution system for end-use N driving circuit of TDC district heating electricity feed line (considering as the one leaving the heat source, and thus the hottest line) high efficiency pump heat rejection system measurement outside ambient optimization pump recooling circuit of TDC return line (considering as the one returning to the heat source, and thus, the coldest line) simulation system thermally driven chiller thermal energy time
ducted to further investigate an impact of the system when a new chiller and other parameters change. 1.2. Methodology and the base case A chemical heat pump or Thermo-Chemical Accumulator (TCA) has been employed and installed in this project as a TDC unit. It has been developed and is sold by a Swedish company ClimateWell AB [10]. It is a three-phase absorption chillers/heat pump that is capable of storing energy internally with high energy density in the form of crystallized salt (LiCl) with water as refrigerant [8,10]. The demonstration system SP1b has been modeled in TRNSYS and calibrated against monitored data; from subsystem level towards a complete system level. TRNSYS is a transient systems simulation program with a modular structure. The TRNSYS library contains many of the components commonly found in thermal and electrical energy systems [9]. Component routines are also included to handle input of weather data or other time-dependent forcing functions and output of simulation results [9]. The modular nature of TRNSYS allows the program tremendous flexibility and facilitates the addition to the program of mathematical models that is not included in the standard library [9]. In order to find improved system design and control, parametric studies have been conducted using TRNSYS. The parameters studied in this work have been derived from different working groups and partners in the project [1], which considers together all those issues relative to the design, commissioning, operation, maintenance, monitoring and evaluation of demonstration plants. The calibration of the base case was made in three stages: (i) estimation of parameters based on manufacturer data and dimensions of the system; (ii) calibration of each circuit (pipes and heat exchangers) separately using steady state data points; (iii) and finally calibration of the complete model in terms of thermal and electrical energy as well as running times, for a five day time series of data with one minute average data values [8]. In the final stage complete system model was calibrated against a five day dynamic measurement sequence from a hot period [8]. The main criteria for calibration were the thermal and electrical energies of the whole
S. Udomsri et al. / Applied Energy 92 (2012) 175–184
system, and the resulting simulated energy quantities for all circuits were within 4% of the measured values. The parameters that were varied in order to gain a good fit were the control parameters for TDC, electrical power of components, UA-values for the TDC heat exchangers and losses from the internal stores. Finally the complete cooling system was changed to use the weather data (Meteonorm) for the location of Borlänge, Sweden. A validation check on the system was done for the same 5-day measurement period used for the calibration, with weather in the (Meteonorm) weather data file [8]. The resulting system showed a good agreement and it was defined as the base case for this study. The first study was to change all four pumps in external circuits to high-efficiency pumps, resulting in a new HEP base case (high efficiency pumps). Using this new base as starting point a number of parametric studies were performed for the 4th generation TDC [10], used in the demonstration system and available commercially until 2009. Finally a new version of the system was created using the latest version of the ClimateWell chiller (5th Generation) [11], available commercially since 2009 – 5G base case. For this the basic control parameters were adjusted to give reasonable performance for the boundary conditions of the demonstration system described in [8]. This essentially consisted of optimizing the state of charge for the swap between charge/discharge. This is dependent on the driving temperature from the district heat. This model was then used for a range of parametric studies. 1.3. Calculation of electrical and thermal COP The thermal COP for both system and TDC unit can be expressed simply through standard equation. Note that the values mentioned below are based on energies as charge/discharge cycles are independent and instantaneous values are not relevant.
Q Cdn Q CHP
COPth;system ¼
COPth;Tdc ¼
Q TdcCc Q TdcDc
ð1Þ
ð2Þ
QCdn is cold produced by the system (measured), QCHP is the driving heat from district heating system (measured), QTdcCc is cold produced by the TDC system (measured) and QTdcDc is the heat supplied to the TDC system (after heat exchanger). A calculation of the electrical COP of the TDC circuit takes into account the pressure drop in the heat exchangers. These include the electrical consumption in TDC alone plus electric consumption to overcome the internal pressure drop of the machine in the internal hydraulic circuits.
COPel;Tdc ¼
Q TdcCc P3
ETdc þ
i¼1 Edpi
_ i Dpi m
g
ti
The electrical COP of the system (COPel,sys) includes the electric requirements for the TDC (ETdc), the three hydraulic circuits (Ecircuit) and the heat rejection unit (EHr). It is based on the cold provided to the distribution system (QCdn) and thus includes possible cold losses. But it excludes the distribution system as this depends on the application and is not an inherent component of the cold production system. However, a general formula is given below:
COPel;sys ¼
ETdc þ
Q Pn Cdn i¼1 Ecircuit;i þ EHr
ð5Þ
2. Modeling approach for system simulation 2.1. Modeling approach The system was modeled in TRNSYS using standard components for the heat exchangers and dry cooler, and with a specially developed gray box model for the chemical heat pump (TDC). The load and district heat supply were not modeled explicitly, rather were derived either as constant values or as a correlation based on the monitored data from the system. The system supplies cold at maximum available capacity, but the cooling system is only turned on when the ambient temperatures is above the balance temperature of 13 °C and during the hours of 06–17 on office days. These are the same conditions as for the monitored system described in [8]. The study used detailed dynamic modeling with TRNSYS in order to attain results for the whole cooling season for different boundary and operating conditions. The basic dynamics of the system are accounted for with thermal masses for pipes as well as chemical storage in the TDC. Two different versions of the TDC were simulated, a fourth generation ClimateWell and a fifth generation available commercially from 2009. The validation of the 4th generation TDC model has been made and reported by Bales and Ayadi [12]. The results show that the model does not predict very accurately for the first 5–10 min of a 4–8 h charge/discharge period, while averages for normal length cycles are realistic. The validation of the 5th generation model showed similar weaknesses [13] for the start of charge and discharge. As the demonstration plant provides pre-cooling for a much bigger chiller, the TDC is always operating at the maximum capacity for the current boundary conditions. Therefore no modeling of the building, auxiliary chiller or cold distribution was made. Instead the temperature of the fluid returning from the air handling unit was modeled with a correlation based on measured data. Similarly, the supply temperature from the district heating network was modeled using a constant supply temperature, the average for the complete cooling season.
ð3Þ 2.2. Matrix of cases
ETdc is the electricity consumption of TDC alone (measured), Edpi is estimated electricity consumption due to the internal pressure drop of the TDC in the three hydraulic circuits (estimated) and Edpi is estimated according to:
Edpi ¼
177
ð4Þ
_ i is the flow rate of circuit i; i = driving circuit, heat rejection cirm cuit, cooling circuit (measured) (m3/s), Dpi is the pressure drops in circuit i at each flow rate (from manufacturers’ data sheet) (Pa), g is the electric efficiency of standard pump and the figure of 0.3 was used in this study for comparison (Why/Wel). The ti is the observation/evaluation time when the circuit i is active. This time was derived from the operation time of the respective pump (measured) [1].
The matrix of cases has the base case system defined in Section 1.2 as the starting point. The following parameters have been derived from experiences of the partners in the project, which considers all those issues relative to the design, commissioning, operation, maintenance, monitoring and evaluation of this particular system [1]. Some of them are specific for the TDC machines used in subproject 1b whereas others are more general and apply also to other subprojects. Two versions were created, one for the 4th generation TDC and one for the 5th generation, and similar cases were studied for both base cases. The first study was to change to high-efficiency pumps, resulting in a new HEP base case (high efficiency pumps). Using this new HEP base as starting point a number all other parametric studies were performed. For the 4th generation TDC, the following parametric studies were performed:
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a. Use of high efficient pumps. b. Variation of flow in the driving, recooling and cooling circuits. c. Set temperature for return from the recooling circuit (THrRl,set) using variable fan speed in recooler. d. Flow temperature from the cooling distribution circuit (TCdnFl). e. Available driving temperature from the district heating system (TDhFl). f. Balance temperature for the cooling load, above which cooling is required (TOAbalance). g. Possible operation at any time of the day, all days of the week (24/7 operation) instead of being limited to office hours as in the monitored system. h. Climate (Borlänge, Stockholm, Gothenburg, Copenhagen, Berlin, Madrid). A new version of the system was also created using the latest version of the ClimateWell chiller (5th Generation) – 5G base case. Using the base case with the 5th generation TDC, the following parametric studies were performed: a. Variation of flow in the driving, recooling and cooling circuits. b. Set temperature for return from the recooling circuit (THrRl,set). c. Flow temperature from the cooling distribution circuit (TCdnFl) using variable fan speed in recooler. d. Available driving temperature from the district heating system (TDhFl).
e. Balance temperature for the cooling load, above which cooling is required (TOAbalance). f. Possible operation at time of the day, all days of the week (24/7 operation) instead of being limited to office hours as in the monitored system. g. Climate (Borlänge, Stockholm, Gothenburg, Copenhagen, Berlin, Madrid). 3. Description and modeling of system components The complete TRNSYS system model is shown in Fig. 1, not including the output components. Details of the calibration of parameter values were given in [8] and Table 1 describes boundary condition and system component. Table 1 presents description and modeling of system components for the base case system. Detailed model description can be found in [9], unless otherwise stated. Table 2 shows the identified values (Ppump), values for high efficiency pumps, Grundfos Magma (Ppump,HEP), together with the flow rates and pressure drops for the whole circuit (dPtot) as well as the TDC itself (dPTdc). Additionally the nominal electrical power required to overcome the pressure drop in the TDC is given based on a nominal efficiency of 0.3 (PTdc). It was assumed that 50% of the pump power was converted to heat and transferred to the fluid circuit. The electrical power of the high efficient pumps was estimated also for the system variations with the 5th generation ClimateWell machine by assuming the same flows and pressure drops in the external part of the circuit and recalculating the total pressure drop with the new TDC. This resulted in lowers pressure drops and thus lowers power in the pumps (See Table 3).
Fig. 1. TRNSYS studio representation of base case system model with main subsystems marked.
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S. Udomsri et al. / Applied Energy 92 (2012) 175–184 Table 1 Details of the system components of the base case system. System component
Description
Centralized CHP model
Simplified model: Type 5 with constant inlet temperatures and flow on hot (district heat) side Municipal waste fired CHP plant produces electricity and heat that is distributed via district heating system. The TDC system is connected to district heating via a heat exchanger The district heat supply is not modeled explicitly, rather a constant temperature of 77.7 °C, the measured average for the monitoring period, is supplied to the heat exchanger in driving circuit. The flow rate on the district heat side is also constant with the measured flow of 941 kg/h during charging of the TDC. At other times it is zero. The UA-value for the heat exchanger, modeled as a counter flow heat exchanger of Type 5, was identified to be 5850 W/K based on monitored data
TDC and cold storage
Detailed model: Types 215 and 216 for 4th generation TDC [12] and Types 825 and 826 for 5th generation [13] The TDC is modeled as a gray box model using two different TRNSYS components: Type 215 for the so called barrel containing reactor, evaporator/condenser, and storages for water and LiCl solution; and Type 216 for the switching unit/controller. In a complete TDC there are two barrels and a switching unit, and this is reflected in the TDC model. The models are described in more detail by Bales and Ayadi [12], who also show parameter values and the validation results based on lab measurements. All parameter values used in this study are the same apart from the UA-values for heat transfer for charge/discharge and for losses to ambient. These were adjusted in order to give good agreement with the monitoring data from SP1b. The TDC has cold storage as an integral part of each barrel, and no other storage is included in the system model. The 5th generation TDC was simulated using parameter values derived from lab testing [13]
Pipes and cold distribution
Simplified model: Two pipes (Type 31) in each circuit (flow and return) together with a counter flow heat exchanger (Type 5) with constant UA-value Two pipes are included in each circuit. The size and approximate UA-value for losses were estimated based on the data for the SP1b system, but UA-values were adjusted in order to give good agreement with the monitored data. This was done for each circuit independently using data from quasi steady state operation. The size (thus dynamics) was not adjusted based on monitored data The cooling distribution system is designed for 12 °C supply temperature and is in operation between 06:00 and 17:00 weekdays. The studied system is an addition to this existing system and thus it worked with maximum capacity. Cold distribution was not modeled explicitly, rather the flow (FCdn) was a constant 1116 kg/h during operation (when TOA was above 13 °C during 06:00–17:00 on office days) and the return temperature from the cold distribution (TCdnFl) used the following correlation derived from monitored data. T CdnFl ¼ ð13:0 0:5 LTðT OA ; 20:0Þ ð20 T OA Þ=7 þ GEðT OA ; 20:0Þ ðT OA 20ÞÞ
Heat rejection
Detailed model: Type 52 for dry cooler The heat rejection for the system is with a dry cooler (Flexcoil VTHD) with design capacity of 25 kW at 27 °C ambient temperature with 35/30 °C fluid temperatures at a flow rate of 0.5 kg/s. The dry cooler fan (560 W) is controlled on/off to maintain a return temperature of 26 °C. Type 52 was used for modeling the dry cooler. Parameters were derived based on the manufacturer’s data and then adjusted to fit with the monitored data together with the heat loss coefficient of the connecting pipes
PID controller
Detailed model: Type 23 A PID regulator of Type 23 was used to maintain a constant return water temperature of 27 °C from the dry cooler by varying the air mass flow. This approximated the action of the real on/off control at 26/28 °C
Components for system control
Forcing functions: Detailed model: Type 14 Two forcing functions together with an equation were used to determine when cooling was possible. These limited cooling to be able to occur during the period 06:00–17:00 on office days (no holidays were implemented as the cooling system was on during the whole summer). Additionally cooling is only turned on when the outside ambient temperature is above 13 °C
Electrical model: TDC, Pumps and fans
Detailed model: No specific types. Implemented in equations and as parameters in existing models The power of the pumps was estimated based on the nominal powers, the measured flows, the pump curves and the measured electrical energy use, which was for all pumps together and not individually. These values were then applied as the pump power in each of the pumps of Type 3 in the system model
Table 2 Pressure drops and pump power for the three circuits coupled to the 4th generation TDC. Circuit
Flow (kg/h)
dPtot (kPa)
dPTdc (kPa)
Electrical power and pressure drop for circuits using 4th generation ClimateWell Dc 732 40 26 Rc 1560 70 38 Cc 1360 60 30
Ppump (W)
Ppump,HEP (W)
PTdc (W)
90 190 145
41 99 76
18 55 57
Ppump (W)
Ppump,HEP (W)
PTdc (W)
– – –
21 57 76
4 18 38
Table 3 Pressure drops and pump power for the three circuits coupled to the 5th generation TDC. Circuit
Flow (kg/h)
dPtot (kPa)
dPTdc (kPa)
Electrical power and pressure drop for circuits using 5th generation ClimateWell Dc 732 20 6 Rc 1560 45 13 Cc 1360 60 30
The power use of the 4th generation TDC is calculated according to the following equation, based on monitored data:
_ TdcCc ; 0:0Þ ½W PTdc ¼ 153 þ 30 GTðm
ð6Þ
For the 5th generation TDC, the power is a constant 18 W (manufacturer’s data [11]).
For the dry cooler, the monitored electrical use as well as inlet and outlet conditions for the water loop and the inlet conditions for the air were used in a model of the recooling circuit. A PID controller was used to control the air mass flow with the set temperature set to give the monitored return temperature. The results were then used to derive the following
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correlation of the fan power dependent on the air mass flow (see Table 1).
_ air 4:01 106 m _ 2air ½W; PHr ¼ 37:7 þ 0:112 m based on air flow in kg=hr:
ð7Þ
4. Results 4.1. Base case system Table 4 shows the main energy performance figures for the base case system. This is the system that has been calibrated against the monitored data and then been simulated with the same boundary conditions, but for the whole cooling season, defined as being from mid-May to mid-September (3240–6216 h), and with the weather data for Borlänge available with TRNSYS. The figures for COP for the whole season are better than those derived from the monitoring data for the following reasons: The monitored system had a relatively long period when no cold was delivered due to an electrical fault in a relay, however the TDC was charged during this period and additionally used significant amounts of electricity for the internal pumps. A fault in the internal controller for the TDC limited the operation of the TDC at certain times, often at the start of the day, resulting in reduced cooling output. In all the following figures, the base case value is shown with a vertical dashed gray line. 4.2. Improved electrical COP The pumps in the base case model were replaced with high efficiency Grundfos Magna pumps. The electrical power for these was determined using the pump characteristics and the pressure drop in the circuits. This resulted in a 49% decrease in electricity use for the pumps and 24% increase in COPel,sys, see Table 5. The table also shows the results for the 5th generation TDC, for which COPel,sys is 5.27, roughly double that for the 4th generation TDC. The electrical COP is much higher due to the very much reduced power consumption of the TDC and also a reduced pump energy, due to reduced running times as well as reduced pressure drops. The fan energy is more or less the same. The thermal COP is substantially lower as is the delivered cold. Fig. 2 shows the variation of the system electrical COP with the set temperature for the fan speed control of return temperature from dry cooler. It shows that for the 4th generation TDC the COPel,sys decreases steadily above a set temperature of 25 °C (base case
Fig. 2. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying set temperatures for the fan speed control of the return temperature from the dry cooler. Base case is for 27 °C. Top 4th generation TDC, bottom 5th generation TDC.
27 °C) while it is essentially the same below this level. The delivered cold however, decreases steadily over the whole range as does the electrical energy for the dry cooler fan. The pump energy (EPump) increases with increased set temperature due to the lower charging powers and thus longer running times for the pumps. The pattern for the 5th generation machine is different. Here there is a maximum COPel,sys at 27 °C due to the fact that both the delivered cold energy and fan energy decrease with increasing return temperature from the dry cooler, but at different rates. The cold energy is lower for the 5th generation and is more dependent on the return temperature from the dry cooler. At a return temperature of 22 °C, it delivers 10% less energy and at 30 °C 23% less energy than the 4th generation TDC. The results of the 4th generation TDC also show that optimizing the operating conditions of the dry cooler fan, by reducing the set point for the return temperature from
Table 4 Summary of main performance figures for the base case system, which has the same system and boundary conditions as the SP1b monitored system, but weather data from TRNSYS. COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568
0.447
4.56
2.13
4434
1982
345
414
163
Table 5 Summary of main performance figures for the base case system with high efficiency pumps together with the relevant improvement in performance figure due to the change to the new pumps. The last line shows the values for the 5th generation TDC with high efficiency pumps.
4th G 5th G
COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568 0% 0.378
0.447 0% 0.297
4.56 0% 22.97
2.64 +24% 5.27
4434 0% 5641
1982 0% 1677
174 49% 106
414 0% 48
163 0% 165
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Table 6 Summary of main performance figures for the systems optimized in terms of electricity use compared to the base case system. The optimized systems use high efficient pumps and a return temperature from the dry cooler of 24 °C. The percentage improvement compared to the base case is given as well.
Base case 4th gen, opt 5th gen, opt
COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568 0.594 5% 0.402 29%
0.445 0.483 9% 0.327 26%
4.57 5.38 18% 27.94 512%
2.13 2.74 29% 5.01 135%
4415 4803 9% 6360 44%
1965 2320 18% 2081 6%
345 155 55% 105 69%
413 413 0% 47 89%
164 279 70% 263 60%
the dry cooler from 27 to 24 °C, does increase the electrical COP but only by 4% (relative). Fig. 3 shows the variation of the system electrical COP with different flow rates in the three circuits. It shows that for the 4th generation TDC, the system electrical COP decreases with flow rate in all the three circuits. For both the recooling and cooling circuits, decreasing to 75% of the monitored flow results in small differences in COP as well as delivered cooling energy and total electrical energy used. However, at half the flow rate of the base case the delivered cold is significantly reduced as is the electrical COP despite less electricity use, because the delivered cooling is much reduced. For the driving circuit the delivered cooling is hardly affected by a change in flow, showing that charge and discharge are essentially independent of one another. The electrical COP is much reduced at lower flow rates due to increased electrical use of the Dc pump during the significantly longer running times. The results of the 5th generation TDC are similar to the results of the 4th generation TDC. However electrical COP of the 5th generation is higher than the 4th generation. Table 6 compares the base case system with the systems with 4th and 5th generation TDC’s optimized for reduced electrical use, i.e. with high efficient pumps and a set temperature for the re-
Fig. 3. Delivered cold (brown) and electrical energy use (green) as well as COPel,sys (blue) plotted for relative flow rate in the three circuits: thick line (recooling), thin line (cooling) and dashed line (driving). The same flow was used on both sides of the heat exchangers. Top 4th generation TDC, bottom 5th generation TDC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
turn from the dry cooler of 24 °C. This is not the exact maximum for the electrical COP of the 5th generation TDC, but is a compromise between electrical and thermal COP as well as delivered cold. For the 4th generation TDC it was possible to increase both the thermal and electrical COP at the same time as increasing the delivered cold. The 5th generation TDC has nearly twice the system electrical COP compared to the optimized 4th generation system, but it has 26–29% lower thermal COP and delivers slightly less cold. 4.3. Variation of boundary conditions: load, driving temperature and climate Fig. 4 shows how the performance figures vary with the driving temperature for the TDC (supply temperature from the district heating network, TDhFl). For the 4th generation TDC the cold delivered and COPel,sys increase steadily up to a temperature of 85 °C but then increase only slowly above this temperature. For the 5th generation TDC the increase continues over the whole range for cold delivered but COPel,sys stabilizes above 100 °C. The 5th generation TDC requires a higher temperature to fully utilize the chemical storage and is designed for these higher temperatures. Above a supply temperature of 90 °C, the 5th generation TDC delivers more cold than the 4th generation TDC, together with a higher COPel,sys. However the COPth,sys is much lower. Fig. 5 shows how the performance figures vary with the base temperature for the return from the cooling distribution loop (TCdnFl). The actual return temperature depends on the outside ambient temperature, and will be higher for ambient temperatures above 20 °C (see Table 1 for the equation). Increased return temperatures represent improved heat exchange to the cooled space. Both the 4th and 5th generation TDC’s react in the same way, with improved cold delivered and electrical COP for increasing return temperatures, an improvement of 35–50% for the temperature range shown. Fig. 6 shows how the performance figures vary with the cooling balance temperature, the outside ambient temperature above which cooling is required. These simulations are for different boundary conditions than for SP1b and the base case: the cooling can be on at any time of the day (24/7) as long as the ambient temperature is greater than the balance temperature. This results in much greater delivered cold (nearly double). The 4th generation TDC produces much more cold the lower the balance temperature while for the 5th generation the produced cold reaches a more or less stable level when the balance temperature is below 12 °C. Similar trends are seen for the electrical COP. As with the results for operation limited to the hours of 06–17, the produced cold is much greater for the 4th generation while the electrical COP is much greater for the 5th generation TDC. Fig. 7 shows how the key performance figures vary with different load conditions. There are three cases, giving longer operating hours in each case. In the third case, there are also better operating conditions for the TDC: 1. Base case with optimized electrical performance, as shown in Table 6 (591 operating hours).
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Fig. 4. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying supply temperatures from the district heating network (TDhFl). Base case is for 77.7 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 6. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying outside ambient temperatures at which the cooling system is turned on (balance temperature). Base case is for 13 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 5. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying base return temperatures from the cold distribution loop (TCdnFl). Base case is for 13 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 7. Delivered cold (QCdn), thermal and electrical COP for the system (COPth,sys and COPel,sys) for three different cases. Top 4th generation TDC, bottom 5th generation TDC.
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tralized CHP. The system simulation model was calibrated in a three stage process against the monitored data. The final calibration stage showed very good agreement with the monitored data for a five day period in terms of both thermal and electrical energy quantities as well as running times of the pumps. However, the performance figures for this base case system for the complete cooling season of mid-May to mid-September were significantly better than those for the monitoring data. This was attributed to longer periods when the monitored system was not in operation and due to a control parameter that hindered cold delivery at certain times. The simulations showed that the electrical COP of the system can be improved by the following measures: Installation of high efficiency pumps: increase from 2.13 to 2.64. Reduction of the set point for the return temperature of the dry cooler from 27 to 24 °C: 2.64 to 2.74.
Fig. 8. Delivered cold (QCdn), thermal and electrical COP for the system (COPth,sys and COPel,sys) for several different climates, with good operating conditions for the TDC. The simulation time was adapted to the cooling season for each climate. Top 4th generation TDC, bottom 5th generation TDC.
2. As 1, but with cold delivery possible at any stage during the day, every day (24/7) (1431 operating hours). 3. As 2, but with the temperature from the district heating network (TDhFl) at 90 °C instead of 77.7 °C, the return from the cold supply based on 14 °C instead of 13 °C, and for a balance temperature of 10 °C (1814 operating hours). The results show that the boundary conditions make a very large difference in the performance of the system. For both 4th and 5th generation TDC’s the delivered cooling and COP increase from case 1 to 3. The 4th generation TDC has a better thermal COP and delivered cold but worse electrical COP, compared to the 5th generation, but the relative differences are smaller for case 3. Fig. 8 shows how the key performance figures vary with climate, using the same operating conditions as case 3 for Fig. 7. The results show that the thermal COP is very similar for all climates but that the electrical COP is worse for the hotter climates. This is due to higher electrical use in the dry cooler fan. In these climates, and especially Madrid a cooling tower would be more appropriate. As expected the delivered cold is greater for the hotter climates. The cooling season was adapted for each climate, varying from 15/5–15/9 in the Swedish climate to 1/4–31/10 in Madrid. 5. Discussion and conclusions This simulation study has been based on the boundary conditions for the SP1b demonstration plant of the PolySMART project with distributed cooling supplied via district heating network from a cen-
Reducing the flow rate in the external circuits caused a reduction in the electrical COP. Replacement of the 4th generation TDC with the 5th generation TDC that has lower pressure drops and very little power consumption internally resulted in an increase of the system electrical COP from 2.64 to 5.27. However, this also resulted in a reduced thermal COP from 0.45 to 0.30. As the reference system is a compression chiller with a COP of 2.7 [14], all systems that have an electrical COP of less than 2.7 have a higher electricity consumption than the reference system. The best case with 4th generation TDC and the boundary condition for the SP1b system had an electrical COP of 2.74. A number of parametric studies were carried out to determine the performance for a range of different boundary conditions. These showed that: The 4th generation TDC has better thermal COP but worse electrical COP than the 5th generation TDC in all the studied cases. Increased operation time due to reduced cooling balance temperature and allowing cooling to be supplied at any time of the day, every day (24/7), leads to a significant increase in delivered cold as well as improved electrical and thermal COP’s. The electrical COP increases if the return temperature from the cooling distribution has a higher temperature. This effect is more pronounced for the 5th generation TDC, for which the thermal COP also increases. In contrast the 4th generation machine has a thermal COP that is more or less independent of this temperature level. Increased driving temperature increases significantly the electrical and thermal COP of the 5th generation TDC as well as delivered cold. There is only a small increase for the 4th generation TDC. For the following realistic boundary conditions (base case in parentheses), the electrical and thermal COP’s increased from 2.74 to 5.53 and 0.483 to 0.522, respectively for the 4th generation TDC and from 5.01 to 7.46 and 0.327 to 0.432, respectively for the 5th generation TDC. Additionally the delivered cold increased from 2320 to 8670 and 2080 to 7740 kWh for the 4th and 5th generation TDC’s, respectively. – Driving temperature of 90 °C (77.7 °C), cooling balance temperature of 10 °C (13 °C), return from cooling distribution of 14 °C (13 °C) and with possible operation 24/7 (office hours from 06–17). Finally it was shown that the thermal COP does not vary with climate but that the electrical COP is lower for hotter climates due to increased use of the dry cooler fan. The delivered cold however, is much greater for the hotter climates. It was also shown that for the more appropriate boundary conditions, the differences in the
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performance figures for the 4th and 5th generation TDC’s was much smaller and that for both of them the electrical COP was significantly higher than that of the base case system. With this system improvement especially electrical COP, it is apparent to conclude that there is a potential to employ decentralized cooling in district heating network supplied by heat from cogeneration. Therefore, expanding cogeneration towards trigeneration can certainly augment the energy supply from cogeneration for summer months in Europe. Utilizing waste heat for cooling production could further reduce the high demand for electricity from compression chillers, while improving overall efficiency of cogeneration plants and reducing the environmental impact in parallel. Here in the summer the cogeneration unit generates electricity and heat where the heat can be distributed in terms of steam and district heating in the district heating network that can be employed further in decentralized cooling systems. It is a very attractive approach where the cooling is provided on the demand side and consumers can deploy the system themselves to produce cooling by using heat from existing district heating networks. The main obstacle of this approach could be the price for the whole system, especially for absorption chiller. Further developments and improved system COP could partly solve this problem. It is however necessarily for the future work to find improved system performance both thermal and electrical COP’s using different absorption chillers; giving an opportunity to introduce different absorption chillers to compare with existing chiller. Beyond this study, it would be of great interest to see if the results obtained from this research and demonstration could be applied (with some modification) to tropical locations, where the largest cooling demand (year-round cooling) has always been found. It would be of great interest to see a comparison with continuous full load where cooling needs all year-round, yielding the results on the maximum possible cold production. Experiences learned from this demonstration will be used to modify and design a proper or better system in Europe as well as in the tropical locations. Hence, there is a distinct possibility of suggesting pathways for significant greenhouse gas reductions and possibly reducing the impact of climate change on developing countries in the decades to come.
Acknowledgments The authors wish to convey their sincere appreciation to European Commission (EC) for financial support of this project through FP6-2004-TREN-3 (Contract No. 019988), and similarly to the Swedish Energy Agency for funding via project: P22374-1. References [1] PolySMART’s EU-Integrated Project. Project No. 019988, FP6-2004-TREN-3. POLYgeneration with advanced Small and Medium scale thermally driven Airconditioning and Refrigeration Technology (PolySMART);. [2] IEA International Energy Agency. Available from [accessed 01.04.10]. [3] Capital cooling. District cooling – a proven technique for enhanced business and improved environment; 2005. [accessed 15.01.05]. [4] Sivak M. Potential energy demand for cooling in the 50 largest metropolitan areas of the world: implications for developing countries. Energy Policy 2009;37:1382–4. [5] Lucas L. IIR news. Int J Refrig 1998;21:87–8. [6] Lindmark S. The role of absorption cooling for reaching sustainable energy systems. Licentiate Thesis, Division of energy processes. Sweden: Royal Institute of Technology (KTH); 2005. [7] Udomsri S, Martin A, Fransson T. Possibilities for various energy applications from municipal solid waste incineration in Bangkok and Hanoi: combined heat, cooling and power generation (CHCP) in Southeast Asia. In: Proceeding of the 5th international conference on combustion, incineration/pyrolysis and emission control (i-CIPEC). Chiang-Mai, Thailand; 2008. [8] Udomsri S, Bales C, Martin AR, Martin V. Decentralised cooling in district heating network: monitoring results and calibration of simulation model. Energy Build 2011. doi:10.1016/j.enbuild.2011.08.00. [9] TRNSYS. TRNSYS coordinator. WI 53706, USA: Solar Energy Laboratory, University of Wisconsin-Madison, 1500 Engineering Drive, 1303 Engineering Research Building Madison. [10] ClimateWell AB. Product description –ClimateWellTM 10. ; [accessed 05.05.10]. [11] ClimateWell AB. ClimateWell SolarChiller – design guidelines for solar cooling. ; [accessed 06.05.10]. [12] Bales C, Ayadi O. Modelling of commercial absorption heat pump with integral storage. In: Proceedings effstock 2009, CD-rom. Stockholm, Sweden: Swedvac; 2009. [13] Bales C. Component models, WP5: D5.3 PolySMART final report. Högskolan Dalarna, 781 88 Borlänge, Sweden: Solar Energy Research Center (SERC); 2010. [14] STEM. STEM rapport: Förbättrad energistatistik för lokaler –’’Stegvis STIL’’ Rapport för år 1 – Inventeringar av kontor och förvaltningsbyggnader. Sweden: Eskilstuna; 2007. .
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Decentralized cooling in district heating network: System simulation and parametric study Seksan Udomsri a,⇑, Chris Bales b, Andrew R. Martin a, Viktoria Martin a a b
Division of Heat and Power Technology, Department of Energy Technology, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden Solar Energy Research Center (SERC), Högskolan Dalarna, 781 88 Borlänge, Sweden
a r t i c l e
i n f o
Article history: Received 17 December 2010 Received in revised form 4 October 2011 Accepted 6 October 2011 Available online 29 November 2011 Keywords: Thermally driven chiller Decentralized cooling District heating network Simulation study TRNSYS
a b s t r a c t This paper presents system simulation and parametric study of the demonstration system of decentralized cooling in district heating network. The monitoring results obtained from the demonstration were calibrated and used for parametric studies in order to find improved system design and control. This study concentrates on system simulation studies that aim to: reduce the electricity consumption, to improve the thermal COP’s and capacity if possible; and to study how the system would perform with different boundary conditions such as climate and load. The internal pumps inside the thermally driven chiller (TDC) have been removed in the new version TDC and implemented in this study to increase the electrical COP. Results show that replacement of the fourth with the fifth generation TDC increases the system electrical COP from 2.64 to 5.27. The results obtained from parametric studies show that the electrical and thermal COP’s, with new realistic boundary conditions, increased from 2.74 to 5.53 and 0.48 to 0.52, respectively for the 4th generation TDC and from 5.01 to 7.46 and 0.33 to 0.43, respectively for the 5th generation TDC. Additionally the delivered cold increased from 2320 to 8670 and 2080 to 7740 kWh for the 4th and 5th generation TDC’s, respectively. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction 1.1. Background and objectives A demonstration system (Subproject 1b: SP1b), was one of 11 demonstration systems installed and monitored within the EU-PolySMART project in Sweden. The aim was to design and develop the best system configuration for the combination of district heating and distributed absorption chillers [1]. PolySMART stands for POLYgeneration with advanced Small and Medium Scale thermally driven Air-Condition and Refrigeration Technology. The overall PolySMART project aims were to develop a set of technical solutions for a new market segment of polygeneration, in particular the market for small and medium scale tri-generation systems (combined production of electric power, heat and cooling). The key components of these systems always include the combined heat and power (CHP) plant along with the thermally driven chiller (TDC). There are different approaches of composing the CHP with TDC [1]; (i) centralized CHP and centralized TDC in combination with a heating and cooling network, (ii) centralized CHP and decentralized TDC in combination with a heating network, and (iii) decentralized CHP and decentralized TDC both on the demand side. Each approach can ⇑ Corresponding author. Tel.: +46 8 790 6140; fax: +46 8 20 41 61. E-mail address: [email protected] (S. Udomsri). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.10.009
be applied under different conditions depending upon the source of generation, consumption, and applications [1]. However, this article reports on a combination of centralized CHP and decentralized TDC in district heating network. District heating can be described as rational and environmentally friendly method to heat residential and commercial buildings etc. District heating is very common heating method and available throughout Sweden. The system is an example of distributed cooling with centralized combined heat and power (CHP), where the driving heat is delivered via the district heating network. Since the demand for cooling has increased tremendously around the world during the past decades [1–4], the conventional compression chillers share more than 15% of worldwide electricity energy consumption [5]. An absorption chiller is an excellent example of thermally driven cooling technology where the low temperature heat can be utilized for cooling production [6,7]. As part of the project, the aim of subproject 1b was to demonstrate the use of the ClimateWell chiller in distributed cooling with centralized CHP in order to develop the best system configuration for the TDC using a particular form of chemical heat pump [8]. The monitoring results and system calibration of the PolySMART demonstration system (SP1b) has been reported in [8]: monitoring results and calibration of simulation model. The main objective of this work was to calibrate and analyse the monitoring results obtained from the demonstration system and validate against a dynamic simulation model using TRaNsient SYstem Simulation
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Nomenclature CHP COP Cp E Eff HEP LHV _ m P PE Q RH SP T TCA TDC Tim V dpi
Combined Heating and Power Coefficient of Performance specific heat capacity electricity consumption efficiency high efficiency pump lower heating value of a fuel mass flow power Primary Energy thermal energy flow Relative Humidity subproject temperature Thermo-Chemical Accumulator thermally driven chiller time volume flow pressure drop in each circuit
Subscript Air air BOP balance of plant
program (TRNSYS) [9]. The calibration of the base case was made in three stages and the energy performance figures were within 4% of the measured values. Although a number of weaknesses have been found, it is fair enough to state that the TDC has worked reliably during the whole cooling season that was monitored. The major fault during the cooling season was due to an external electrical component (relay). However the electrical COP for the complete system was relatively low and in fact was lower than that of the main compression chiller. This was due to relatively large electrical energy use in the pumps, the TDC itself as well as the fan for the heat rejection unit (dry cooler). Other principle conclusions from the monitoring period were that the system worked well but at lower capacity than the nominal capacity of the TDC due to the boundary conditions for the system. This also resulted in lower than nominal thermal COP values. For the complete monitoring period during the summer of 2008, the thermal and electrical COPs for the TDC were only 0.41 and 2.1, respectively and the highest values were 0.50 and 4.6, respectively during the hottest period. Additionally, the system COP’s were found to be significantly lower than those for the TDC itself. There are a number of other causes of the relatively low thermal and electrical COP values of the entire system such as: (i) Heat exchanger in the driving circuit, causing extra heat losses in the driving circuit of the TDC, (ii) heat exchanger in the chilled water delivery circuit, causing heat gains in the delivery circuit both through normal gains through the insulation and components but also due to thermal energy from the two pumps used, and (iii) the driving temperature available from the district heating network is lower than ideal for the TDC. It is on average 75–80 °C, whereas 80–90 °C would be more ideal. (iv) The operating times of the whole system are relatively short. (v) The TDC cannot deliver at full power with the normal operating conditions of the system. The main objectives of this simulation study were to: reduce the electricity consumption, and if possible to improve the thermal COP and capacity at the same time; and to study how the system would perform with different boundary conditions such as climate and load. Studies in terms of the replacement of high efficiency pumps, new TDC version and variations in boundary conditions were con-
Cc Cdn Dc Dh el Fl HEP Hr Meas OA Opt Pump Rc Rl Sim Sys Tdc Th ti
chilling circuit of TDC cold distribution system for end-use N driving circuit of TDC district heating electricity feed line (considering as the one leaving the heat source, and thus the hottest line) high efficiency pump heat rejection system measurement outside ambient optimization pump recooling circuit of TDC return line (considering as the one returning to the heat source, and thus, the coldest line) simulation system thermally driven chiller thermal energy time
ducted to further investigate an impact of the system when a new chiller and other parameters change. 1.2. Methodology and the base case A chemical heat pump or Thermo-Chemical Accumulator (TCA) has been employed and installed in this project as a TDC unit. It has been developed and is sold by a Swedish company ClimateWell AB [10]. It is a three-phase absorption chillers/heat pump that is capable of storing energy internally with high energy density in the form of crystallized salt (LiCl) with water as refrigerant [8,10]. The demonstration system SP1b has been modeled in TRNSYS and calibrated against monitored data; from subsystem level towards a complete system level. TRNSYS is a transient systems simulation program with a modular structure. The TRNSYS library contains many of the components commonly found in thermal and electrical energy systems [9]. Component routines are also included to handle input of weather data or other time-dependent forcing functions and output of simulation results [9]. The modular nature of TRNSYS allows the program tremendous flexibility and facilitates the addition to the program of mathematical models that is not included in the standard library [9]. In order to find improved system design and control, parametric studies have been conducted using TRNSYS. The parameters studied in this work have been derived from different working groups and partners in the project [1], which considers together all those issues relative to the design, commissioning, operation, maintenance, monitoring and evaluation of demonstration plants. The calibration of the base case was made in three stages: (i) estimation of parameters based on manufacturer data and dimensions of the system; (ii) calibration of each circuit (pipes and heat exchangers) separately using steady state data points; (iii) and finally calibration of the complete model in terms of thermal and electrical energy as well as running times, for a five day time series of data with one minute average data values [8]. In the final stage complete system model was calibrated against a five day dynamic measurement sequence from a hot period [8]. The main criteria for calibration were the thermal and electrical energies of the whole
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system, and the resulting simulated energy quantities for all circuits were within 4% of the measured values. The parameters that were varied in order to gain a good fit were the control parameters for TDC, electrical power of components, UA-values for the TDC heat exchangers and losses from the internal stores. Finally the complete cooling system was changed to use the weather data (Meteonorm) for the location of Borlänge, Sweden. A validation check on the system was done for the same 5-day measurement period used for the calibration, with weather in the (Meteonorm) weather data file [8]. The resulting system showed a good agreement and it was defined as the base case for this study. The first study was to change all four pumps in external circuits to high-efficiency pumps, resulting in a new HEP base case (high efficiency pumps). Using this new base as starting point a number of parametric studies were performed for the 4th generation TDC [10], used in the demonstration system and available commercially until 2009. Finally a new version of the system was created using the latest version of the ClimateWell chiller (5th Generation) [11], available commercially since 2009 – 5G base case. For this the basic control parameters were adjusted to give reasonable performance for the boundary conditions of the demonstration system described in [8]. This essentially consisted of optimizing the state of charge for the swap between charge/discharge. This is dependent on the driving temperature from the district heat. This model was then used for a range of parametric studies. 1.3. Calculation of electrical and thermal COP The thermal COP for both system and TDC unit can be expressed simply through standard equation. Note that the values mentioned below are based on energies as charge/discharge cycles are independent and instantaneous values are not relevant.
Q Cdn Q CHP
COPth;system ¼
COPth;Tdc ¼
Q TdcCc Q TdcDc
ð1Þ
ð2Þ
QCdn is cold produced by the system (measured), QCHP is the driving heat from district heating system (measured), QTdcCc is cold produced by the TDC system (measured) and QTdcDc is the heat supplied to the TDC system (after heat exchanger). A calculation of the electrical COP of the TDC circuit takes into account the pressure drop in the heat exchangers. These include the electrical consumption in TDC alone plus electric consumption to overcome the internal pressure drop of the machine in the internal hydraulic circuits.
COPel;Tdc ¼
Q TdcCc P3
ETdc þ
i¼1 Edpi
_ i Dpi m
g
ti
The electrical COP of the system (COPel,sys) includes the electric requirements for the TDC (ETdc), the three hydraulic circuits (Ecircuit) and the heat rejection unit (EHr). It is based on the cold provided to the distribution system (QCdn) and thus includes possible cold losses. But it excludes the distribution system as this depends on the application and is not an inherent component of the cold production system. However, a general formula is given below:
COPel;sys ¼
ETdc þ
Q Pn Cdn i¼1 Ecircuit;i þ EHr
ð5Þ
2. Modeling approach for system simulation 2.1. Modeling approach The system was modeled in TRNSYS using standard components for the heat exchangers and dry cooler, and with a specially developed gray box model for the chemical heat pump (TDC). The load and district heat supply were not modeled explicitly, rather were derived either as constant values or as a correlation based on the monitored data from the system. The system supplies cold at maximum available capacity, but the cooling system is only turned on when the ambient temperatures is above the balance temperature of 13 °C and during the hours of 06–17 on office days. These are the same conditions as for the monitored system described in [8]. The study used detailed dynamic modeling with TRNSYS in order to attain results for the whole cooling season for different boundary and operating conditions. The basic dynamics of the system are accounted for with thermal masses for pipes as well as chemical storage in the TDC. Two different versions of the TDC were simulated, a fourth generation ClimateWell and a fifth generation available commercially from 2009. The validation of the 4th generation TDC model has been made and reported by Bales and Ayadi [12]. The results show that the model does not predict very accurately for the first 5–10 min of a 4–8 h charge/discharge period, while averages for normal length cycles are realistic. The validation of the 5th generation model showed similar weaknesses [13] for the start of charge and discharge. As the demonstration plant provides pre-cooling for a much bigger chiller, the TDC is always operating at the maximum capacity for the current boundary conditions. Therefore no modeling of the building, auxiliary chiller or cold distribution was made. Instead the temperature of the fluid returning from the air handling unit was modeled with a correlation based on measured data. Similarly, the supply temperature from the district heating network was modeled using a constant supply temperature, the average for the complete cooling season.
ð3Þ 2.2. Matrix of cases
ETdc is the electricity consumption of TDC alone (measured), Edpi is estimated electricity consumption due to the internal pressure drop of the TDC in the three hydraulic circuits (estimated) and Edpi is estimated according to:
Edpi ¼
177
ð4Þ
_ i is the flow rate of circuit i; i = driving circuit, heat rejection cirm cuit, cooling circuit (measured) (m3/s), Dpi is the pressure drops in circuit i at each flow rate (from manufacturers’ data sheet) (Pa), g is the electric efficiency of standard pump and the figure of 0.3 was used in this study for comparison (Why/Wel). The ti is the observation/evaluation time when the circuit i is active. This time was derived from the operation time of the respective pump (measured) [1].
The matrix of cases has the base case system defined in Section 1.2 as the starting point. The following parameters have been derived from experiences of the partners in the project, which considers all those issues relative to the design, commissioning, operation, maintenance, monitoring and evaluation of this particular system [1]. Some of them are specific for the TDC machines used in subproject 1b whereas others are more general and apply also to other subprojects. Two versions were created, one for the 4th generation TDC and one for the 5th generation, and similar cases were studied for both base cases. The first study was to change to high-efficiency pumps, resulting in a new HEP base case (high efficiency pumps). Using this new HEP base as starting point a number all other parametric studies were performed. For the 4th generation TDC, the following parametric studies were performed:
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a. Use of high efficient pumps. b. Variation of flow in the driving, recooling and cooling circuits. c. Set temperature for return from the recooling circuit (THrRl,set) using variable fan speed in recooler. d. Flow temperature from the cooling distribution circuit (TCdnFl). e. Available driving temperature from the district heating system (TDhFl). f. Balance temperature for the cooling load, above which cooling is required (TOAbalance). g. Possible operation at any time of the day, all days of the week (24/7 operation) instead of being limited to office hours as in the monitored system. h. Climate (Borlänge, Stockholm, Gothenburg, Copenhagen, Berlin, Madrid). A new version of the system was also created using the latest version of the ClimateWell chiller (5th Generation) – 5G base case. Using the base case with the 5th generation TDC, the following parametric studies were performed: a. Variation of flow in the driving, recooling and cooling circuits. b. Set temperature for return from the recooling circuit (THrRl,set). c. Flow temperature from the cooling distribution circuit (TCdnFl) using variable fan speed in recooler. d. Available driving temperature from the district heating system (TDhFl).
e. Balance temperature for the cooling load, above which cooling is required (TOAbalance). f. Possible operation at time of the day, all days of the week (24/7 operation) instead of being limited to office hours as in the monitored system. g. Climate (Borlänge, Stockholm, Gothenburg, Copenhagen, Berlin, Madrid). 3. Description and modeling of system components The complete TRNSYS system model is shown in Fig. 1, not including the output components. Details of the calibration of parameter values were given in [8] and Table 1 describes boundary condition and system component. Table 1 presents description and modeling of system components for the base case system. Detailed model description can be found in [9], unless otherwise stated. Table 2 shows the identified values (Ppump), values for high efficiency pumps, Grundfos Magma (Ppump,HEP), together with the flow rates and pressure drops for the whole circuit (dPtot) as well as the TDC itself (dPTdc). Additionally the nominal electrical power required to overcome the pressure drop in the TDC is given based on a nominal efficiency of 0.3 (PTdc). It was assumed that 50% of the pump power was converted to heat and transferred to the fluid circuit. The electrical power of the high efficient pumps was estimated also for the system variations with the 5th generation ClimateWell machine by assuming the same flows and pressure drops in the external part of the circuit and recalculating the total pressure drop with the new TDC. This resulted in lowers pressure drops and thus lowers power in the pumps (See Table 3).
Fig. 1. TRNSYS studio representation of base case system model with main subsystems marked.
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S. Udomsri et al. / Applied Energy 92 (2012) 175–184 Table 1 Details of the system components of the base case system. System component
Description
Centralized CHP model
Simplified model: Type 5 with constant inlet temperatures and flow on hot (district heat) side Municipal waste fired CHP plant produces electricity and heat that is distributed via district heating system. The TDC system is connected to district heating via a heat exchanger The district heat supply is not modeled explicitly, rather a constant temperature of 77.7 °C, the measured average for the monitoring period, is supplied to the heat exchanger in driving circuit. The flow rate on the district heat side is also constant with the measured flow of 941 kg/h during charging of the TDC. At other times it is zero. The UA-value for the heat exchanger, modeled as a counter flow heat exchanger of Type 5, was identified to be 5850 W/K based on monitored data
TDC and cold storage
Detailed model: Types 215 and 216 for 4th generation TDC [12] and Types 825 and 826 for 5th generation [13] The TDC is modeled as a gray box model using two different TRNSYS components: Type 215 for the so called barrel containing reactor, evaporator/condenser, and storages for water and LiCl solution; and Type 216 for the switching unit/controller. In a complete TDC there are two barrels and a switching unit, and this is reflected in the TDC model. The models are described in more detail by Bales and Ayadi [12], who also show parameter values and the validation results based on lab measurements. All parameter values used in this study are the same apart from the UA-values for heat transfer for charge/discharge and for losses to ambient. These were adjusted in order to give good agreement with the monitoring data from SP1b. The TDC has cold storage as an integral part of each barrel, and no other storage is included in the system model. The 5th generation TDC was simulated using parameter values derived from lab testing [13]
Pipes and cold distribution
Simplified model: Two pipes (Type 31) in each circuit (flow and return) together with a counter flow heat exchanger (Type 5) with constant UA-value Two pipes are included in each circuit. The size and approximate UA-value for losses were estimated based on the data for the SP1b system, but UA-values were adjusted in order to give good agreement with the monitored data. This was done for each circuit independently using data from quasi steady state operation. The size (thus dynamics) was not adjusted based on monitored data The cooling distribution system is designed for 12 °C supply temperature and is in operation between 06:00 and 17:00 weekdays. The studied system is an addition to this existing system and thus it worked with maximum capacity. Cold distribution was not modeled explicitly, rather the flow (FCdn) was a constant 1116 kg/h during operation (when TOA was above 13 °C during 06:00–17:00 on office days) and the return temperature from the cold distribution (TCdnFl) used the following correlation derived from monitored data. T CdnFl ¼ ð13:0 0:5 LTðT OA ; 20:0Þ ð20 T OA Þ=7 þ GEðT OA ; 20:0Þ ðT OA 20ÞÞ
Heat rejection
Detailed model: Type 52 for dry cooler The heat rejection for the system is with a dry cooler (Flexcoil VTHD) with design capacity of 25 kW at 27 °C ambient temperature with 35/30 °C fluid temperatures at a flow rate of 0.5 kg/s. The dry cooler fan (560 W) is controlled on/off to maintain a return temperature of 26 °C. Type 52 was used for modeling the dry cooler. Parameters were derived based on the manufacturer’s data and then adjusted to fit with the monitored data together with the heat loss coefficient of the connecting pipes
PID controller
Detailed model: Type 23 A PID regulator of Type 23 was used to maintain a constant return water temperature of 27 °C from the dry cooler by varying the air mass flow. This approximated the action of the real on/off control at 26/28 °C
Components for system control
Forcing functions: Detailed model: Type 14 Two forcing functions together with an equation were used to determine when cooling was possible. These limited cooling to be able to occur during the period 06:00–17:00 on office days (no holidays were implemented as the cooling system was on during the whole summer). Additionally cooling is only turned on when the outside ambient temperature is above 13 °C
Electrical model: TDC, Pumps and fans
Detailed model: No specific types. Implemented in equations and as parameters in existing models The power of the pumps was estimated based on the nominal powers, the measured flows, the pump curves and the measured electrical energy use, which was for all pumps together and not individually. These values were then applied as the pump power in each of the pumps of Type 3 in the system model
Table 2 Pressure drops and pump power for the three circuits coupled to the 4th generation TDC. Circuit
Flow (kg/h)
dPtot (kPa)
dPTdc (kPa)
Electrical power and pressure drop for circuits using 4th generation ClimateWell Dc 732 40 26 Rc 1560 70 38 Cc 1360 60 30
Ppump (W)
Ppump,HEP (W)
PTdc (W)
90 190 145
41 99 76
18 55 57
Ppump (W)
Ppump,HEP (W)
PTdc (W)
– – –
21 57 76
4 18 38
Table 3 Pressure drops and pump power for the three circuits coupled to the 5th generation TDC. Circuit
Flow (kg/h)
dPtot (kPa)
dPTdc (kPa)
Electrical power and pressure drop for circuits using 5th generation ClimateWell Dc 732 20 6 Rc 1560 45 13 Cc 1360 60 30
The power use of the 4th generation TDC is calculated according to the following equation, based on monitored data:
_ TdcCc ; 0:0Þ ½W PTdc ¼ 153 þ 30 GTðm
ð6Þ
For the 5th generation TDC, the power is a constant 18 W (manufacturer’s data [11]).
For the dry cooler, the monitored electrical use as well as inlet and outlet conditions for the water loop and the inlet conditions for the air were used in a model of the recooling circuit. A PID controller was used to control the air mass flow with the set temperature set to give the monitored return temperature. The results were then used to derive the following
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correlation of the fan power dependent on the air mass flow (see Table 1).
_ air 4:01 106 m _ 2air ½W; PHr ¼ 37:7 þ 0:112 m based on air flow in kg=hr:
ð7Þ
4. Results 4.1. Base case system Table 4 shows the main energy performance figures for the base case system. This is the system that has been calibrated against the monitored data and then been simulated with the same boundary conditions, but for the whole cooling season, defined as being from mid-May to mid-September (3240–6216 h), and with the weather data for Borlänge available with TRNSYS. The figures for COP for the whole season are better than those derived from the monitoring data for the following reasons: The monitored system had a relatively long period when no cold was delivered due to an electrical fault in a relay, however the TDC was charged during this period and additionally used significant amounts of electricity for the internal pumps. A fault in the internal controller for the TDC limited the operation of the TDC at certain times, often at the start of the day, resulting in reduced cooling output. In all the following figures, the base case value is shown with a vertical dashed gray line. 4.2. Improved electrical COP The pumps in the base case model were replaced with high efficiency Grundfos Magna pumps. The electrical power for these was determined using the pump characteristics and the pressure drop in the circuits. This resulted in a 49% decrease in electricity use for the pumps and 24% increase in COPel,sys, see Table 5. The table also shows the results for the 5th generation TDC, for which COPel,sys is 5.27, roughly double that for the 4th generation TDC. The electrical COP is much higher due to the very much reduced power consumption of the TDC and also a reduced pump energy, due to reduced running times as well as reduced pressure drops. The fan energy is more or less the same. The thermal COP is substantially lower as is the delivered cold. Fig. 2 shows the variation of the system electrical COP with the set temperature for the fan speed control of return temperature from dry cooler. It shows that for the 4th generation TDC the COPel,sys decreases steadily above a set temperature of 25 °C (base case
Fig. 2. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying set temperatures for the fan speed control of the return temperature from the dry cooler. Base case is for 27 °C. Top 4th generation TDC, bottom 5th generation TDC.
27 °C) while it is essentially the same below this level. The delivered cold however, decreases steadily over the whole range as does the electrical energy for the dry cooler fan. The pump energy (EPump) increases with increased set temperature due to the lower charging powers and thus longer running times for the pumps. The pattern for the 5th generation machine is different. Here there is a maximum COPel,sys at 27 °C due to the fact that both the delivered cold energy and fan energy decrease with increasing return temperature from the dry cooler, but at different rates. The cold energy is lower for the 5th generation and is more dependent on the return temperature from the dry cooler. At a return temperature of 22 °C, it delivers 10% less energy and at 30 °C 23% less energy than the 4th generation TDC. The results of the 4th generation TDC also show that optimizing the operating conditions of the dry cooler fan, by reducing the set point for the return temperature from
Table 4 Summary of main performance figures for the base case system, which has the same system and boundary conditions as the SP1b monitored system, but weather data from TRNSYS. COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568
0.447
4.56
2.13
4434
1982
345
414
163
Table 5 Summary of main performance figures for the base case system with high efficiency pumps together with the relevant improvement in performance figure due to the change to the new pumps. The last line shows the values for the 5th generation TDC with high efficiency pumps.
4th G 5th G
COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568 0% 0.378
0.447 0% 0.297
4.56 0% 22.97
2.64 +24% 5.27
4434 0% 5641
1982 0% 1677
174 49% 106
414 0% 48
163 0% 165
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Table 6 Summary of main performance figures for the systems optimized in terms of electricity use compared to the base case system. The optimized systems use high efficient pumps and a return temperature from the dry cooler of 24 °C. The percentage improvement compared to the base case is given as well.
Base case 4th gen, opt 5th gen, opt
COPth,Tdc (–)
COPth,sys (–)
COPel,Tdc (–)
COPel,sys (–)
QDh (kW h)
QCdn (kW h)
EPump (kW h)
ETdc (kW h)
EHr (kW h)
0.568 0.594 5% 0.402 29%
0.445 0.483 9% 0.327 26%
4.57 5.38 18% 27.94 512%
2.13 2.74 29% 5.01 135%
4415 4803 9% 6360 44%
1965 2320 18% 2081 6%
345 155 55% 105 69%
413 413 0% 47 89%
164 279 70% 263 60%
the dry cooler from 27 to 24 °C, does increase the electrical COP but only by 4% (relative). Fig. 3 shows the variation of the system electrical COP with different flow rates in the three circuits. It shows that for the 4th generation TDC, the system electrical COP decreases with flow rate in all the three circuits. For both the recooling and cooling circuits, decreasing to 75% of the monitored flow results in small differences in COP as well as delivered cooling energy and total electrical energy used. However, at half the flow rate of the base case the delivered cold is significantly reduced as is the electrical COP despite less electricity use, because the delivered cooling is much reduced. For the driving circuit the delivered cooling is hardly affected by a change in flow, showing that charge and discharge are essentially independent of one another. The electrical COP is much reduced at lower flow rates due to increased electrical use of the Dc pump during the significantly longer running times. The results of the 5th generation TDC are similar to the results of the 4th generation TDC. However electrical COP of the 5th generation is higher than the 4th generation. Table 6 compares the base case system with the systems with 4th and 5th generation TDC’s optimized for reduced electrical use, i.e. with high efficient pumps and a set temperature for the re-
Fig. 3. Delivered cold (brown) and electrical energy use (green) as well as COPel,sys (blue) plotted for relative flow rate in the three circuits: thick line (recooling), thin line (cooling) and dashed line (driving). The same flow was used on both sides of the heat exchangers. Top 4th generation TDC, bottom 5th generation TDC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
turn from the dry cooler of 24 °C. This is not the exact maximum for the electrical COP of the 5th generation TDC, but is a compromise between electrical and thermal COP as well as delivered cold. For the 4th generation TDC it was possible to increase both the thermal and electrical COP at the same time as increasing the delivered cold. The 5th generation TDC has nearly twice the system electrical COP compared to the optimized 4th generation system, but it has 26–29% lower thermal COP and delivers slightly less cold. 4.3. Variation of boundary conditions: load, driving temperature and climate Fig. 4 shows how the performance figures vary with the driving temperature for the TDC (supply temperature from the district heating network, TDhFl). For the 4th generation TDC the cold delivered and COPel,sys increase steadily up to a temperature of 85 °C but then increase only slowly above this temperature. For the 5th generation TDC the increase continues over the whole range for cold delivered but COPel,sys stabilizes above 100 °C. The 5th generation TDC requires a higher temperature to fully utilize the chemical storage and is designed for these higher temperatures. Above a supply temperature of 90 °C, the 5th generation TDC delivers more cold than the 4th generation TDC, together with a higher COPel,sys. However the COPth,sys is much lower. Fig. 5 shows how the performance figures vary with the base temperature for the return from the cooling distribution loop (TCdnFl). The actual return temperature depends on the outside ambient temperature, and will be higher for ambient temperatures above 20 °C (see Table 1 for the equation). Increased return temperatures represent improved heat exchange to the cooled space. Both the 4th and 5th generation TDC’s react in the same way, with improved cold delivered and electrical COP for increasing return temperatures, an improvement of 35–50% for the temperature range shown. Fig. 6 shows how the performance figures vary with the cooling balance temperature, the outside ambient temperature above which cooling is required. These simulations are for different boundary conditions than for SP1b and the base case: the cooling can be on at any time of the day (24/7) as long as the ambient temperature is greater than the balance temperature. This results in much greater delivered cold (nearly double). The 4th generation TDC produces much more cold the lower the balance temperature while for the 5th generation the produced cold reaches a more or less stable level when the balance temperature is below 12 °C. Similar trends are seen for the electrical COP. As with the results for operation limited to the hours of 06–17, the produced cold is much greater for the 4th generation while the electrical COP is much greater for the 5th generation TDC. Fig. 7 shows how the key performance figures vary with different load conditions. There are three cases, giving longer operating hours in each case. In the third case, there are also better operating conditions for the TDC: 1. Base case with optimized electrical performance, as shown in Table 6 (591 operating hours).
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Fig. 4. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying supply temperatures from the district heating network (TDhFl). Base case is for 77.7 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 6. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying outside ambient temperatures at which the cooling system is turned on (balance temperature). Base case is for 13 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 5. Delivered cold (QCdn) and electrical energy use as well as COPel,sys plotted for varying base return temperatures from the cold distribution loop (TCdnFl). Base case is for 13 °C. Top 4th generation TDC, bottom 5th generation TDC.
Fig. 7. Delivered cold (QCdn), thermal and electrical COP for the system (COPth,sys and COPel,sys) for three different cases. Top 4th generation TDC, bottom 5th generation TDC.
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tralized CHP. The system simulation model was calibrated in a three stage process against the monitored data. The final calibration stage showed very good agreement with the monitored data for a five day period in terms of both thermal and electrical energy quantities as well as running times of the pumps. However, the performance figures for this base case system for the complete cooling season of mid-May to mid-September were significantly better than those for the monitoring data. This was attributed to longer periods when the monitored system was not in operation and due to a control parameter that hindered cold delivery at certain times. The simulations showed that the electrical COP of the system can be improved by the following measures: Installation of high efficiency pumps: increase from 2.13 to 2.64. Reduction of the set point for the return temperature of the dry cooler from 27 to 24 °C: 2.64 to 2.74.
Fig. 8. Delivered cold (QCdn), thermal and electrical COP for the system (COPth,sys and COPel,sys) for several different climates, with good operating conditions for the TDC. The simulation time was adapted to the cooling season for each climate. Top 4th generation TDC, bottom 5th generation TDC.
2. As 1, but with cold delivery possible at any stage during the day, every day (24/7) (1431 operating hours). 3. As 2, but with the temperature from the district heating network (TDhFl) at 90 °C instead of 77.7 °C, the return from the cold supply based on 14 °C instead of 13 °C, and for a balance temperature of 10 °C (1814 operating hours). The results show that the boundary conditions make a very large difference in the performance of the system. For both 4th and 5th generation TDC’s the delivered cooling and COP increase from case 1 to 3. The 4th generation TDC has a better thermal COP and delivered cold but worse electrical COP, compared to the 5th generation, but the relative differences are smaller for case 3. Fig. 8 shows how the key performance figures vary with climate, using the same operating conditions as case 3 for Fig. 7. The results show that the thermal COP is very similar for all climates but that the electrical COP is worse for the hotter climates. This is due to higher electrical use in the dry cooler fan. In these climates, and especially Madrid a cooling tower would be more appropriate. As expected the delivered cold is greater for the hotter climates. The cooling season was adapted for each climate, varying from 15/5–15/9 in the Swedish climate to 1/4–31/10 in Madrid. 5. Discussion and conclusions This simulation study has been based on the boundary conditions for the SP1b demonstration plant of the PolySMART project with distributed cooling supplied via district heating network from a cen-
Reducing the flow rate in the external circuits caused a reduction in the electrical COP. Replacement of the 4th generation TDC with the 5th generation TDC that has lower pressure drops and very little power consumption internally resulted in an increase of the system electrical COP from 2.64 to 5.27. However, this also resulted in a reduced thermal COP from 0.45 to 0.30. As the reference system is a compression chiller with a COP of 2.7 [14], all systems that have an electrical COP of less than 2.7 have a higher electricity consumption than the reference system. The best case with 4th generation TDC and the boundary condition for the SP1b system had an electrical COP of 2.74. A number of parametric studies were carried out to determine the performance for a range of different boundary conditions. These showed that: The 4th generation TDC has better thermal COP but worse electrical COP than the 5th generation TDC in all the studied cases. Increased operation time due to reduced cooling balance temperature and allowing cooling to be supplied at any time of the day, every day (24/7), leads to a significant increase in delivered cold as well as improved electrical and thermal COP’s. The electrical COP increases if the return temperature from the cooling distribution has a higher temperature. This effect is more pronounced for the 5th generation TDC, for which the thermal COP also increases. In contrast the 4th generation machine has a thermal COP that is more or less independent of this temperature level. Increased driving temperature increases significantly the electrical and thermal COP of the 5th generation TDC as well as delivered cold. There is only a small increase for the 4th generation TDC. For the following realistic boundary conditions (base case in parentheses), the electrical and thermal COP’s increased from 2.74 to 5.53 and 0.483 to 0.522, respectively for the 4th generation TDC and from 5.01 to 7.46 and 0.327 to 0.432, respectively for the 5th generation TDC. Additionally the delivered cold increased from 2320 to 8670 and 2080 to 7740 kWh for the 4th and 5th generation TDC’s, respectively. – Driving temperature of 90 °C (77.7 °C), cooling balance temperature of 10 °C (13 °C), return from cooling distribution of 14 °C (13 °C) and with possible operation 24/7 (office hours from 06–17). Finally it was shown that the thermal COP does not vary with climate but that the electrical COP is lower for hotter climates due to increased use of the dry cooler fan. The delivered cold however, is much greater for the hotter climates. It was also shown that for the more appropriate boundary conditions, the differences in the
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performance figures for the 4th and 5th generation TDC’s was much smaller and that for both of them the electrical COP was significantly higher than that of the base case system. With this system improvement especially electrical COP, it is apparent to conclude that there is a potential to employ decentralized cooling in district heating network supplied by heat from cogeneration. Therefore, expanding cogeneration towards trigeneration can certainly augment the energy supply from cogeneration for summer months in Europe. Utilizing waste heat for cooling production could further reduce the high demand for electricity from compression chillers, while improving overall efficiency of cogeneration plants and reducing the environmental impact in parallel. Here in the summer the cogeneration unit generates electricity and heat where the heat can be distributed in terms of steam and district heating in the district heating network that can be employed further in decentralized cooling systems. It is a very attractive approach where the cooling is provided on the demand side and consumers can deploy the system themselves to produce cooling by using heat from existing district heating networks. The main obstacle of this approach could be the price for the whole system, especially for absorption chiller. Further developments and improved system COP could partly solve this problem. It is however necessarily for the future work to find improved system performance both thermal and electrical COP’s using different absorption chillers; giving an opportunity to introduce different absorption chillers to compare with existing chiller. Beyond this study, it would be of great interest to see if the results obtained from this research and demonstration could be applied (with some modification) to tropical locations, where the largest cooling demand (year-round cooling) has always been found. It would be of great interest to see a comparison with continuous full load where cooling needs all year-round, yielding the results on the maximum possible cold production. Experiences learned from this demonstration will be used to modify and design a proper or better system in Europe as well as in the tropical locations. Hence, there is a distinct possibility of suggesting pathways for significant greenhouse gas reductions and possibly reducing the impact of climate change on developing countries in the decades to come.
Acknowledgments The authors wish to convey their sincere appreciation to European Commission (EC) for financial support of this project through FP6-2004-TREN-3 (Contract No. 019988), and similarly to the Swedish Energy Agency for funding via project: P22374-1. References [1] PolySMART’s EU-Integrated Project. Project No. 019988, FP6-2004-TREN-3. POLYgeneration with advanced Small and Medium scale thermally driven Airconditioning and Refrigeration Technology (PolySMART);