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OPTIMAL DESIGN OF CHILLER UNITS AND COLD WATER STORAGES FOR DISTRICT COOLING SYSTEMS
Thorsten Urbaneck, Ulrich Schirmer, Bernd Platzer
Chemnitz University of Technology Faculty of Mechanical Engineering
Department of Technical Thermodynamics 09107 Chemnitz, Germany
Tel.: ++49 371 531 24 63, Fax: ++49 371 525 23 49 [email protected]
[email protected] [email protected]
Ulf Uhlig, Thomas Göschel, Dieter Zimmermann
Utility Company Chemnitz Stock Corporation
Division Networks Department District Heating / District Cooling
P.O.Box 41 14 68 09114 Chemnitz, Germany
Tel.: ++49 371 525 47 40, Fax: ++49 371 525 47 45 [email protected]
[email protected] [email protected]
1 INTRODUCTION The presented results base on the study “feasibility evaluation for empowerment of CHCP by means of cool thermal energy storages in large supplying systems” [1], [2]. The study has been supported by Federal Ministry of Economics and Labour (BMWA), represented by Project Management Organisation Jülich (PTJ). Project partners are Utility Company Chemnitz (SWC) und Chemnitz University of Technology. The goal of this project is the efficiency improvement of combined heat and cool and power cycle (CHCP) by means of Thermal Energy Storage (TES). The central topic of inquiry is the district cooling system of the city of Chemnitz. The examination deals with refitting the system by a TES. The increasing cooling demand because of more air conditioning in city buildings leads to extension of the system (Fig. 2, Fig. 3). SWC are looking for a favourable solution in both ecological and economic terms.
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Figure 1: Distribution of the cooling load, Utility Company Chemnitz, prognosis for the year 2006
Table 1: Distribution of the cooling load, Utility Company Chemnitz, prognosis for the year 2006 with an extreme summer Consumption Load
[GWh/a] [%] [MW] [%] Total 12,1 100 11,1 100 Base 10,9 90 4,1 37 Peak 1,2 10 7,0 63
The usage of Cool TES in Europe especially in Germany is a relative new topic. Therefore the examination of feasi-bility and design is necessary. Fig. 1 and Tab. 1 show a typical distribution of the cooling load and consumption in Germany. The peak load in summer is characterised by high cooling loads and relatively low consumptions. The demonstrated solutions base on the specific examination at Chemnitz district cooling system. But the solutions in chapter 6 are more general and can be adopt to other systems. The central questions are: How does the dimensioning of chillers and storages affect the evaluation marks? What are the ranges for an optimal system dimensioning? As a consequence of these first results the implementation of large TES in connection with better enforcement of CHCP is being striven. 2 DISTRICT COOLING IN THE CITY OF CHEMNITZ For a better understanding the city cooling system will be described (basing on performance in 2003): • Chiller units
- Absorption chiller (AbC), H2O-LiBr, 2*1800 kW, 500 kW, heated by district heating system - Vapour compression chiller (VCC), 3000 kW, 1242 kW
• Network - 4,2 km length of pipelines - Outgoing temperature 5…7 °C, depending on outside temperature - Return temperature about 13 °C
• Consumer - Big stores, office buildings, Opera, University - Proportion of cooling power for air conditioning is about 93 % - Proportion of cooling power for technological cooling is about 7 % - Consumption see Fig. 2 - Over-all connected consumer power see Fig. 3
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Figure 3: Development of the total power of the consumer connections, Utility Company Chemnitz
3 USING OF COOL TES In the meantime the using of cool TES has been favourite because of: • The high load and unload performance with direct water exchange • The high store capacity of the large storages • The option of modular upsizing • The possibility of 5…10 m effective height as the supposition for better development of temperature
stratification • Reduction of heat loss by means of suitable heat insulation • Least investment costs in volume range 100 m3 to about 5000 m3 • Repairs not difficult
• Positive experiences in the field of large hot water storages • Transferability of these results to cool TES The following hot water storage constructions are considered to be usable for large cold water storages: • One or several tanks made of glass fibre reinforced plastic, compound wall with outer reinforced plastic
liners with integrated heat insulation • One or several steel tanks with heat insulation on the outside • One tank or one pit made of reinforced concrete, preferably watertight concrete, with heat insulation on
the outside Because of these performance temperatures of the chillers and the network the storage must be directly integrated into the system without heat exchanger. Currently the version as seen in Fig. 4 “attached storage” will be preferred because of different pressure in storage and network. The district cooling station is to be operated by so called partial-storage load levelling operation strategy: The AbC undertake the base load. The excessive AbC-power is used for storage charging. The several AbC always function under nominal load. The storage covers the peak load (Fig. 1). If the storage is empty the VCC will start. A typical situation is shown in Fig. 5. The storage of the selected system is quite small. It only can cover the peak load of the first half day. The peak load of the second half should be covered by the VCC.
TEScold storagewater tank
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Figure 4: Simplified representation of the integration of the storage in the existing energy supply system of the Utility Company Chemnitz including the main energy fluxes 4 CALCULATION At first the load curves of the last years had been analyzed. The future load by higher connected loads have been generated by a prognosis. A TRNSYS-simulation [3] is used to allow for these loads, the storage [4], the chillers, the hydraulics und the control units and method of district cooling station. All important values will be determined: Temperatures, mass flows etc. Balance values like energy will be gathered hourly. Further values will be determined by means of a separate MATLAB-calculation [5]. These are for instance cost functions. There is an easy way to vary the parameters (see chapter 6). Important assumptions therefore are:
• Supply guarantee • VCC only will be used if neither AbC nor storage can deliver • The dimensioning of the district cooling station (Fig. 4) will be fitted to the peak load of the network • The load curve of 2003 with its very high cool load conduces as reference quantity (Fig.1, Tab. 1)
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Figure 5: Power and temperatures during top-level load time in one year with characteristic on-peak and off-peak behavior for a 1000 m³ cold water storage 5 FIRST GENERAL RESULTS Based on the first results important conclusions regarding the effects of TES exertion can be made: • Benefits of TES use
- Reduction of maximum VCC power - Discharge of the electrical power supply in the peak load time - Decrease of the electricity consumption of VCC - Increase of district heating sales in periods with high waste heat surplus - Performance of the AbC with better COP - Intensification of the night-time operation of the cooling towers - More operational safety of the system in periods without peak load
• Disadvantage of TES use - Increasing electricity consumption for drives (e.g. cooling loop) - Increasing water consumption for cooling towers - In consequence of it increasing demand cost
6 SIMULATION RESULTS UNDER VARIATION OF CHILLER-POWER AND STORAGE
CAPACITY The power of AbC and/or VCC often is given with existing systems. During a new built system however all sizes are freely selectable. If only the storage will be supplemented they are limited freely selectable. A system like shown in Fig. 4 is the base for the following discussion. The results are shown in Figs. 6…18: • The annual cooling supply from the AbC (Fig. 6) rises strongly with the PFAbC because of the increasing
base load covering. The upper limit value is still faster reached with increasing VTES. This influence of the storage is likewise asymptotic. At large storage volumes of approx. VTES > 2000 m3 are hardly still visibly changes. Small storage volumes are therefore energetically already highly effective.
• The annual cooling supply from the VCC (Fig. 7) complementary fits to the cooling supply by AbC. At increasing PFAbC first appears a decrease of medium-load covering and after this the decrease of peak load covering. A complete substitution appears over a wide range (compare with Fig. 12).
• The annual heat amount from storage discharge (Fig. 8) is due to the storage performance und the plant optimization performance. This heat amount directly depends on storage capacity, the amount of AbC-power surplus and the part of network load which not can be covered by AbC. In the range of PFAbC < 0.25 and VTES < 1500 m3 appear strong rises. At PFAbC = 0.25…0.45 it forms a local maximum in shape of a line in direction of growing VTES. The high heat amount is an expression of the great number of charge and discharge cycles. The second maximum at VTES ≈ 2000 m3 and PFAbC = 0.95 is a result of interaction between control strategy and storage behaviour. At about PFAbC = 0.95 the AbC nearly replace the VCC and cover all but the peak load. It exists super proportionally much charging power. But the maximum only appears at the declared range.
• The annual district heat consumption (Fig. 9) corresponds with power supply of AbC. • The annual electricity consumption (Fig. 10) strongly rises with increasing power supply by the VCC (in
this case at increasing PFAbC). With decreasing PFAbC appears asymptotic approach of the limit. • The annual water consumption (Fig. 11) mainly corresponds with power supply from AbC because of the
high specific water consumption. But also the influence of the VCC takes place. • One of the most interesting curves is shown in Fig. 12 with the maximum power demand of VCC. At first
there appears with increasing PFAbC up to about PFAbC ≈ 0.3 a linear substitution of VCC power from the base load range and from the medium load range. At continues increasing AbC power quota in connection with sufficiently storage capacity comes a rapid substitution of VCC quota. At the diagram this is the range between PFAbC ≈ 0.8 with VTES ≈ 500 m3 and PFAbC ≈ 0.4 with VTES ≈ 8000 m3 . It appears more rapidly on increasing storage capacity.
• Maximum storage discharge power (Fig. 13) shows further interesting results: The line like maximum is at PFAbC ≈ 0.2 and starts at VTES ≈ 4000 m3. How already mentioned a sufficient storage capacity, charge heat amount and demand are presuppositions for effective storage performance. The strong drop in direction of small PFAbC signals an insufficient charge power. The moderate drop in direction of large PFAbC indicates the growing network load covering by the AbC. This figure especially shows the concurrent situation between AbC-power and storage discharge power. At low VTES and medium PFAbC the function is not really continuous.
• The whole capital outlays (Fig. 14) are composed of the partial investments for chillers, storages, storage supplies and buildings. All cost functions will overlay. The line like minimum which is to be seen has been generated by the strong reduction of VCC-power (compare Fig. 12). At small amounts of VTES we find a relative constant function. Reason is the compensation of several cost functions. At the range of small amounts of PFAbC we find a continuous rise of partial investments for storage. The inclined plane at high PFAbC is caused by partial investments of AbC and storage.
• The several partial investments have substantial influence on annuity of whole capital outlays (Fig. 15). The slight displacement of the function profile compared with capital outlays is caused by different technical utilization periods.
• The total operation costs (Fig. 16) directly correspond with partial investments. The function plane is compared to the whole capital outlays more flat because of several operation expense factor. The storage itself shows smaller running costs.
• The total demand costs (Fig. 17) consist of costs of district heat energy, electricity and water consumption (compare Fig. 4). Most influence have district heat energy and water consumption. Therefore the shape of this function essentially differs from the annuity function ore the operation expense function.
• The annual overall costs (Fig. 18) consist of annuity, operation expense and demand cost. The previously shown functions will overlay. The very distinct line like minimum is caused by the substituted VCC-power. At very small amounts of PFAbC and VTES there occurs a further minimum. This means a supply only by the VCC without storage. This effect is caused by low demand cost. The function has an arc like curve shape at small amounts of VTES in the range of PFAbC = 0.0…0.8. The mostly effective storage performance (e.g. PFAbC ≈ 0.4 and VTES = 2000 m3) doesn’t exactly fit the annual overall cost minimum. But this minimum is not far. So it could be reached by ingenious fitting the parameters.
Figure 6: Annual cold energy supply of all AbC as function of PFAbC and VTES
Figure 7: Annual cold energy supply of all VCC as function of PFAbC and VTES
Figure 8: Annual energy supply of TES discharge as function of PFAbC and VTES
Figure 9: Annual consumption of waste heat (delivery over the long-distance net) as function of PFAbC and VTES
Figure 10: Annual consumption of electric power as function of PFAbC and VTES
Figure 11: Annual consumption of water as function of PFAbC and VTES
Figure 12: Maximum of requisited VCC power as function of PFAbC and VTES
Figure 13: Maximum of requisited TES discharge power as function of PFAbC and VTES
Figure 14: Total investment cost as function of PFAbC and VTES
Figure 15: Annuity of all units as function of PFAbC and VTES
Figure 16: Total operation cost as function of PFAbC and VTES
Figure 17: Total demand cost as function of PFAbC and VTES
Figure 18: Annual overall costs as function of PFAbC and VTES
7 CONCLUSIONS The study shows with reference to storage application in large systems • That it is feasible • That it can win out with respect to economy • That it has additional ecological and energetical advantages This storage technology furthermore has a future because of • Increasing cooling demand • The technical solution is transferable on existing systems • The question of heat excess will stay present at nearly all kinds of electricity generation • Minimization of electrical peak load • Improvement of sales conditions of electricity
The case studies were the first steps. In addition a sensitivity analysis was necessary. This first steps are a good base for additional investigation to system design and performance. Regarding a further research following aspects are important: • Establish a pilot project for performance experience and technical development, • Consolidation of theoretical studies, • Transfer of knowledge and technology from storage temperature range up to 95 °C into the range
0…20 °C, • Variation of storage integration into the system, • Comparison between nominal and actual value behaviour, • Optimization of performance. ACKNOWLEDGEMENT The study was financially supported with resources of the Federal Ministry of Economics and Labour under the sign of promotion 0327357A. The authors would like to thank the Project Management Organisation Jülich for support. LIST OF SHORTCUTS AbC Absorption Chiller VCC Vapour Compression Chiller CHCP Combined Heat and Cool and Power Cycle TES Thermal Energy Storage
DEFINITIONS AbC-net power fraction: The PFAbC considers the maximum power of all AbC in relation to the maximum of net load. Thus the potential power of the necessary converters for the CHCP can be described.
net
nAbCAbC
Q
QPF
max,
,max,
&
&∑= (1)
Annual overall cost: This value is the sum of the annuity, total operation cost and total demand cost. The advantage is that the capital cost will be divided over the technical operation time. Annuity: The capital cost take in to consideration over the annuity. The rate of interest and the time of technical utilization have influence on the annuity. Total demand cost: The partial sum of all consumptions (energy and material). Total investment cost: This capital investment is the sum of all necessary partial expenditures (technical units and building without area). Total operation cost: The cost for operation preparedness of the plant summarized in the group of expense (man power, maintenance, insurance etc.). VCC-net power fraction: Similarly to the PFAbC the characteristic number for VCC is defined. This power fraction is less important here because of the special consideration of CHCP .
net
nVCCVCC
Q
QPF
max,
,max,
&
&∑= (2)
Volume of storage / capacity of storage: With VTES the storage capacity is described. The usable volume of the tank or pit given here with the unit cubic meter water equivalent. REFERENCES [1] Urbaneck, T.; Schirmer, U.; Platzer, B.; Uhlig, U.; Zimmermann, D.; Göschel T.: Trends in the combined
usage of chiller units and cold water storages for district cooling systems. Joint Workshop (Energy Conservation Through Energy Storage (ECES), District Heating and Cooling (DHC)): The use of cold storage in district cooling systems, Berlin, 2005
[2] 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. Abschlussbericht BMWA-Forschungsvorhaben, Identifikation 0327357A, Chemnitz: Stadtwerke Chemnitz, TU Chemnitz, 2006. - ISBN 3-00-015770-0
[3] Klein, S. A.; et. al.: TRNSYS – A transient system simulation program. Solar Energy Laboratory, University of Wisconsin—Madison, Madison (USA), 1994.
[4] Drück, H.; Pauschinger, T.: MULTIPORT Store Model for TRNSYS. Institut für Thermodynamik und Wärmetechnik, Universität Stuttgart, 1997.
[5] The MathWorks, Inc.: MATLAB. Version R14, 1984-2005.
