ONLY COLD WATER?! THE SUCCESS WITH THE FIRST LARGE-SCALE COLD WATER STORE IN GERMANY

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1 ONLY COLD WATER?! THE SUCCESS WITH THE FIRST LARGE-SCALE COLD WATER STORE IN GERMANY Thorsten Urbaneck Chemnitz University of Technology Faculty of Mechanical Engineering Department of Technical Thermodynamics 917 Chemnitz, Germany Tel.: thorsten.urbaneck@mb.tu-chemnitz.de Uwe Barthel, Ulf Uhlig, Thomas Göschel Utility Company Chemnitz Stock Corporation Division Networks Department Operation of Water, Gas, District Heating and Cooling, Waste Water P.O. Box Chemnitz, Germany Tel.: uwe.barthel@swc.de ulf.uhlig@swc.de thomas.goeschel@swc.de Abstract By the storage integration (27) and by the new operational mode of the cold supply (district cooling) of Utility Company Chemnitz, the feasibility of the large-scale cold water storage concept could be verified for boundary conditions in Germany. The first operating results meet the expectations, so that the postulated advantages of this concept could be confirmed by monitoring. The system operation became more flexible due to storage utilization. The operator can choose between many different operating strategies. District Cooling System State of system from 1993 until 2: Many consumers in the city centre of Chemnitz (department stores, office buildings, opera, computer cluster of Chemnitz University of Technology etc.) are supplied with chilled water via the district cooling net. The air conditioning needs approx. 93 % of the annual distribution of cooling energy and the technological cooling approx. 7 %. The annual cooling distribution has doubled itself in the last 1 years. The connections of the customers constantly rose. The central plant (Figure 1) supplies the consumer with the chilled water. This plant is in proximity to the consumers at the edge of the city centre. The absorption chiller units (AbC 1 and AbC 2 since 1993, AbC 3 since 199) are base load chillers and the vapour compression chillers (VVC since 22 and VVC since 2) are peak load chillers. Table 1 combines all important information concerning the system. Storage refitting (27): The ambient air conditions in summer 23 and the increasing consumer connections caused extremely high loads in the net. Utility Company Chemnitz had to increase the cooling capacity of the central plant. This question of the retrofit was examined in the context of a feasibility study (Feasibility evaluation for empowerment of CHCP by means of cool thermal energy storages in large supplying systems [1]). This investigation shows that significant economic, energetic and ecological advantages can be achieved by the retrofit of a large-scale cold water store (conversion by a pilot project [2],

2 community project of Utility Company Chemnitz and Chemnitz University of Technology) [3], [], [], []. The exploitation of waste heat with the existing absorption chiller units is intensified by the storage exertion. The electric power consumption is reducing at the same time. District heating Cooling towers Cold water production Cold water store Peak load chiller units Electrical power supply VCC VCC Discharging Charging AbC 3 Base load chiller units AbC 2 AbC 1 Hydraulic junction Net Figure 1: Division of the chillers (AbC absorption chiller, VCC vapour compression chiller), the hydraulic storage connection and the operational modes charging and discharging Table 1: Description and parameters of district cooling system in Chemnitz Chiller unit Type, Description AbC 1 AbC 2 AbC 3 VCC VVC Absorption chiller, LiBr-H 2 O, single-effect, hot water (12 C) driven generator a, water-cooled absorber and condenser b (Carrier, 1JH-2) Absorption chiller, LiBr-H 2 O, single-effect, hot water (12 C) driven generator a, water-cooled absorber and condenser b (Carrier, 1JH-2) Absorption chiller, LiBr-H 2 O, single-effect, hot water (12 C) driven generator a, water-cooled absorber and condenser b (York, YIA HW-2B1- -A) Vapour compression chiller, turbo compressor, R13a, water-cooled condenser b (York, YK GB FB HF CTE) Vapour compression chiller, screw compressor, R13a, air-cooled condenser b (York, YCAS 121FBYF) Nominal cooling capacity [kw] 1, 1, 3, 1,22 a Use of waste heat of combined heat and power plant in Chemnitz over district heating net, admixture on the hot water side b Recooling system with 1 open evaporative cooling towers, 1.3 kw, 2/37 C, constant flow rate on the side of chilled and recooled water Store Net Cold water storage, /13 C, 3 m³, 33,2 MWh/cycle, aboveground tank, thermally insulated, direct water exchange for charging and discharging (on top and at the bottom) Charging, Discharging, 2 pipes, /13 C,,2 km length of marked-out route Maximum of capacity ca. 2, 17 substations, total capacity on the base of contracts ca. 13,

3 Utilization of central cooling plant This article refers to the operation of the central cooling plant after start-up in the year 27 (compare with [7]). No high loads arise due to low ambient temperatures and a natural cooling of the buildings at night (Figure 2, Figure 3). Typical maximum net loads in the range of to 1 MW were not reached in August 27. The recooling of the chiller units is not subject to extreme weather situations. The supply of the net took place with and without store nearly completely by means of the absorption chiller units (Figure, Figure ). The store is used for peak load shifting and for the optimization of chiller operation. A primary goal of the project (effective use of waste heat and displacement of the electric power consumption for the vapour compression chiller units) has been achieved (Table 2). The utilization of the compression chiller units took place only for maintenance work or similar measures. In addition, the absorption chiller units are operated occasionally without store operation. The capacity adaptation to the net load is made by a higher admixture at the hydraulic junction (conventional approach) Figure 2: Ambient temperature, hourly mean values ( ) All chiller units Net load Capacity [kwh] Figure 3: Total capacity of all chiller units and the net loads, hourly mean values ( ) Capacity [kwh] AbC 1 AbC 2 AbC 3 VCC VCC Figure : Utilization of chiller units, hourly mean values ( )

4 Capacity [kwh] Charging Discharing Figure : Utilization of store (absolute values), hourly mean values ( ) The difference between loading and unloading the store (energy and volume) is very small (Table 2). This lies in the range of a partial storage charge and/or water filling. Both values for the use of the store almost match. The store possesses low losses on the base of this balance and/or the storage operation is efficient. That is an important condition for the optimization of the chiller operation. The storage losses must be clearly under the gain of an optimized cold water production. Table 2: Balance of central cooling plant ( ) Store Energy [kwh] Fraction [%] Cold water [m³] Fraction [%] Charging , Discharging Net Net Chiller AbC , 31.,3 AbC , ,9 AbC , , VCC 1.7, 3,1 VCC.1,1 1.1,2 All AbC , , 97, Storage behaviour A good function mode of store is the condition for an optimal system operation. Therefore results of measurement from the summer operation and winter operation in 27 are shown ( , Figure, Figure 7, Figure, Figure 9, []). Clear differences are noticeable between the summer and winter operation. The summer operation was characterized by producing and holding a relatively high volume of cold water. The thermocline was exchanged rarely. Accordingly the size of thermocline increased (reduction of the maximum temperature gradient of on approx. 2 K/m). This behaviour is uncritical, since this volume was not needed (no maximum net loads). Only small store losses arose upward. A cyclic loading and unloading took place in the winter operation (only base load). Distinct temperature gradients over the entire store height can thereby be proven. However the temperature difference over the storage height was reduced. From a practical point of view a supply less than C could always be realized. From the analysis of the temperature gradients the following findings can be presented: The system for the loading and unloading generates termoclines with a height of approx. a meter or lower. The storage volume can be used from technical point of view effectively

5 by the separation of the cold and warm zone. A system and/or a store operation with relatively constant temperatures are therefore possible even with very low or very high stages of charge. The thermocline remains for a long time. Thus the store does not have to be unloaded immediately after the loading. Also after several days the optimal use of the chilled water is still possible. The thermocline was taken in most cases above. This is more favourable concerning the system operation. The highest temperature gradients result from the upper facility to the loading in connection with high net return temperatures. 1 1 at the bottom mean on top Difference of temperature [K] Figure : Temperatures in the lowest and highest layer, average store temperature, temperature difference between the highest and lowest layer 1 static energetic analysis State of charge [%] 2 dynamic energetic analysis State of charge [%] 1 2 volumetric analysis Figure 7: State of charge, different methods of determination (see Definitions a, b, c)

6 Intermediate layer Figure : Temperature gradient inside the store in dependence on time and height ( layers for measurement of temperature, increasing layer number with increasing height) of store Temperatur gradient [K/m] Height above ground [m] 12 1 Height above ground [m] Figure 9: Vertical distribution of store temperature (thermal stratification) at hourly intervals, left: operation in summertime ( ); right: operation in wintertime ( ) System interactions The adherence to planned temperature levels (/13 C) is very important for a good system and/or a storage operation. That concerns in particular the effective storage capacity, the efficiency of the cooling and the efficiency of the water transportation. The store is to adapt for this reason flow rate in all modes of operation (peak load shifting, operation with optimal chiller utilization). The absorption chiller units can be operated roughly then with full load. Figure 1 shows the real temperatures in dependence of the rate of system utilization. The return temperatures of the net is falling (e.g. in the winter operation) and varying increasingly with decreasing loads. The flow rate fluctuations in the net, which are produced in this case by two substations, cause variations in temperature at the hydraulic junction, which become visible within the range of low loads. The control tries to adjust the fluctuations. The chiller units can keep the desired temperatures from. to. C well. The supply temperature is somewhat lower during the storage charging. The store can supply the temperature of C very well, regardless of the capacity.

7 A more stable system performance can be achieved by the suggested mode of operation. The following points are particularly important: fast state of readiness (less then 2 min), fast compensation of fluctuating load changes and a function even within the range of low loads. The experiences and/or the measurements support the thesis. Net Chiller units Store Supply Return 2 Flow rate [m³/h] Supply Return 2 Flow rate [m³/h] Exchange at the bottom Exchange on top 2 Flow rate [m³/h] Figure 1: System temperatures depending on flow rates, hourly mean values ( ) Coolant water, supply Hot water, supply Cooling Cold water, return Trend c) Coolant water, return Hot water, return Trend c) Trend b) Trend a) Capacity [kw] Recooling.7 Cold water, supply Trend c) Capacity [kw] Figure 11: Efficiency of chiller unit AbC 1, coefficient of performance (COP) dependent on inlet temperatures, outlet temperatures, capacity and operation of store (red: charging; green: discharging; blue: without store operation), -hour mean values ( )

8 The coefficient of performance (COP) of the absorption chillers (without auxiliary energy) could be increased in comparison to the previous operation (part load:.; full load:.9) to.. (Figure 11). The efficient full load operation is very important, because then low values for the specific consumption of electric power (pump drives) and water (open evaporation cooling towers) can be achieved. Higher COPs (up to.7) are possible only in the partial load operation at low average temperatures in the recooling heat exchanger (trend b) and at a little higher temperatures in the cold water heat exchanger (trend a). The cold water temperatures in the return however deviate then from the planning value (13 C). The chiller units have a higher flow rate in comparison to the net. It comes to a higher admixture at the hydraulic junction, which is not desirable. COPs under. (trend c) are to be led back to a chiller operation with temporary active Cycle-Guard-System (bypass between evaporator and absorber, level-driven valve) in this period. The investigations and optimizations still persist. Definitions a Statically energetic state of charge: The average temperature of the store is determined with temperature sensors (equidistant distribution over the height) and set into relation to the temperatures of design ( C corresponds to a state charge of 1 % and 13 C a state of charge of %). b Dynamically energetic state of charge: The average store temperature is set into relation to the minimum and maximum store temperature. These are measured in the lower and upper storage layer. Thus operating conditions are to be better considered in relation to design. With approximately full loading or full unloading the relationship does not supply accurate results. c Volumetric state of charge: The temperature distribution in the store is utilized for the analysis of the usable volume (cold zone). A limit temperature (border of unloading) must be defined, which amounts to in this case 7. C. This size is suitable from technical view very well for the determination of the charge. Acknowledgement The study was financially supported by resources of the Federal Ministry of Economics and Technology under the sign of promotion 32737B/C. The authors would like to thank the Project Management Organisation Jülich for support. The responsibility for the content of this release is borne by the authors. References [1] Urbaneck, T.; Uhlig, U.; Platzer, B.; Schirmer, U.; Göschel T.; Zimmermann, D.: Machbarkeitsuntersuchung zur Stärkung der Kraft-Wärme-Kälte-Kopplung durch den Einsatz von Kältespeichern in großen Versorgungssystemen. Final report, Project of Federal Ministry of Economics and Labour, Identification 32737A, Chemnitz: Utility Company Chemnitz, Chemnitz University of Technology, 2. - ISBN [2] Urbaneck, T.: Project website, [3] Urbaneck, T.; Schirmer, U.; Platzer, B.; Uhlig, U.; Göschel T.; Zimmermann, D.: Optimierung der Kraft- Wärme-Kälte-Kopplung mit Kältespeichern. EuroHeat&Power, VWEW Energieverlag 3. Jg. (2) Heft 11 S. -7. ISSN 99-1X-D979F [] Urbaneck, T.; Schirmer, U.; Platzer, B.; Uhlig, U.; Göschel T.; Zimmermann, D.: Absorptionskältemaschinen und Kaltwasser-Speicher - Eine Analyse zur Kurzzeit-Speicherung. ki Luftund Kältetechnik Hüthig 1. Jg. (2) Heft 12 S ISSN 9-9 [] Urbaneck, T.; Schirmer, U.; Platzer, B.; Uhlig, U.; Göschel, T.; Zimmermann, D.: Optimal design of chiller units and cold water storages for district cooling. Ecostock, 1th International Conference on Thermal Energy Storage, Stockton (USA, New Jersey), Richard Stockton College of New Jersey, 2. [] Urbaneck, T.; Schirmer, U.; Platzer, B.; Barthel, U.; Uhlig, U.; Zimmermann, D.; Göschel, T.: Kurzzeitige Kältespeicherung Optimierung der Energieversorgung durch den Einsatz großer Kaltwasserspeicher. BWK - Das Energie-Fachmagazin, Verein Deutscher Ingenieure (Hrsg.), Düsseldorf, Springer-VDI-Verlag, 9. Jg., -27, S ISSN X [7] Urbaneck, T.; Uhlig, U.; Göschel, T.; Baumgart, G.; Fiedler, G.: Operational Experiences with a Large- Scale Cold Storage Tank District Cooling Network in Chemnitz. EuroHeat&Power, English Edition VWEW Energieverlag Vol. (2) Heft 1 S ISSN 99-1X [] Urbaneck, T.; Uhlig, U.: Kaltwasserspeicher mit Schichtungsbetrieb Analyse des Speicherverhaltens. ki Luft- und Kältetechnik Hüthig. Jg. (2) Heft 7- S ISSN 9-9