خنک کننده غیر متمرکز در شبکه های گرمایش منطقه ای: سیستم شبیه سازی و مطالعه پارامتریک

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.

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 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], [2], [3] and [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] and [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 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 conducted to further investigate an impact of the system when a new chiller and other parameters change. 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] and [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 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.

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 centralized 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. 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 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.