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Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus.

Commercial Heat Driven Chillers in Polysun: Modeling and Validation A. Witzig1, B. Lacoste1, A. Wolf2, O. Hallstrom3, T. Tschan4, M. Brünig4, L. Leppin5, P. Gantenbein5 1

Vela Solaris AG Stadthausstr. 125, 8400 Winterthur, Switzerland, 2 Studio Wolf , Via Assisana, 40, 06087 Perugia, Italy 3 ClimateWell HQ, Instrumentvägen 20, 12653 Hägersten, Sweden 4 Ernst Schweizer AG, Metallbau, Bahnhofstrasse 1, 8908 Hedingen, Switzerland 5 SPF Institut für Solartechnik, Hochschule für Technik HSR, Oberseestrasse 10, 8640 Rapperswil, Switzerland [email protected], , +41 55 220 71 00 www.polysunsoftware.com

Abstract This work presents the integration of commercial heat driven absorption and adsorption chillers into the simulation tool Polysun. The modelling procedure is discussed for three different systems all existing as hardware installations and one partially with measurement results available for validation. It is shown which simplifications in the simulation template can be made compared to the physical installation to achieve more robust simulation setups suitable for the dissemination of heat driven chillers in commercial applications.

Heat Driven Chiller Model in Polysun The Implementation of heat driven chillers in Polysun has been presented in [1] and [2] and is based on a full set of experimental parameters directly representing steady state measurements of the chiller. For general heat driven chillers, the three temperature levels of the inflow temperatures into the chillers are used with given design flow rates as supporting points. The resulting cooling power and COP values are listed in a table together with these temperature triples. The list is stored in the Polysun component database. During model execution in the simulation process, the simulation kernel interpolates between the measured parameter values to evaluate the cooling power and COP in every operation state of the machine. The physical dynamics is accounted for with a built in system inertia (thermal mass) and is subject to a validation procedure as described in example 1 of this paper. The interpolation scheme has been improved in the latest release of Polysun now applying the scattered data algorithm [3] to allow high flexibility in the choice of measurement points in a multi-dimensional matrix.

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Vela Solaris

Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus. It is important to see that a chiller model based on measurements results fully covers all processes inside the heat driven chillers and allows integrating of the chillers into the hydraulic pipe system in the modular manner that is inherent to the Polysun software. In consequence, self-consistent dynamic system simulation is fast and robust and allows estimating the overall performance in the cooling period of the year. In particular, the combination of free cooling and the double usage of the solar collectors for the hot water production or even heating and thermal driving a cooling machine can be simulated. As a result, an integral system efficiency calculation becomes possible including a profitability (cost) analysis. In the following three solar thermal assisted system examples implemented in Polysun are presented.

System 1: ClimateWell with Applications in Commercial Buildings In the first example, the ClimateWell absorption chiller system is studied in the application area of commercial buildings. Specifically, a hospital in the United Arab Emirates is under investigation. In this case it is a typical system where the entire roof of a building is covered with solar collectors to be used both for heat driven chiller(s) as well as for the production of the hot water used in the hotel. The ClimateWell chillers support a conventional compression chiller installation to replace as much fossil fuel as possible with solar energy. In the Polysun simulation, steady state laboratory measurements have been used to represent the ClimateWell machine in the component database. In a second part of the project, the dynamics (out of steady state) of the ClimateWell chiller was addressed. For this, the modular concept of the simulation tool Polysun was used to introduce an additional storage tank next to the steady state implemented machine.

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It has been found that a single storage tank on the primary (high temperature level) side of the machine is adequate for representing the dynamic behaviour of the real machine. Comparison with measurements (switch-on and switch-off) has been used to validate the model, resulting in a hot water storage tank size of 500 litres. Additionally, TRNSYS simulation results [4] are well suited for this validation process. It is important to see that the TRNSYS simulation setups published and available internally at ClimateWell reflect the internal physical processes in a more rigorous manner. Therefore, these models are better suited for the machine

Polysun Solar Cooling

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Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus. optimization at the site of the chiller manufacturer. In contrast, Polysun allows the analysis of the system around the chiller including the solar collectors, heat rejection and the controller strategies. In the practical application such as the chosen example for a hospital, several ClimateWell chillers are typically used in parallel. In Polysun, this can be accounted for with a special component called “flow multiplier”. This is a handy way to avoid too much components in one simulation setup and long simulation times. Flow multipliers typically come in pairs; on the one side of the flow multiplier, there is the (normal) (designed) flow rate and energy flow. On the other side of the flow multiplier, flow rate and energy flow are divided by a certain number or multiplied, respectively. In the Polysun system setup (Figure 1), it can be seen that all tubes to or from the combined ClimateWell chiller and hot water storage component are designed by flow multipliers. The number of the chiller& hot water storage component used inside them can then be set in one single variable (number) allowing a very simple usage of the system in “Polysun Professional” version.

Figure 1: Polysun layout for the ClimateWell system for hospital application. Hot water is used for the heat driven chiller as well as for heating, sanitary hot water and laundry application. Several ClimateWell cooling machines support an already existing cooling system to reduce fuel costs and CO2 emission.

System 2: Absorption Cooling The second system under investigation is a LiBr-H2O absorption cooling machine installed at the Hochschule für Technik Rapperswil as it is studied already for several years by the Institute for Solar Technologies SPF [5]. The system uses flat plate collectors and a 1000 litre hot water storage tank, as displayed in the Polysun Polysun Solar Cooling

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Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus. schematic in Figure 2. The Absorption cooling machine has a nominal cooling power of 10 kW and is used in combination with a traditional air conditioning system for the climatisation of a university laboratory building. For the comparison between measurement and simulation, the local weather data has been used for the Polysun simulation. The overall energy consumption, solar collector yield as well as the flow rates and temperatures in the pipes and in several layers of the storage tank have been compared. Intermediate results show that very good agreement is achieved in the comparison of the energy flows. Even the temperature and flow rates show a fair coincidence. In conclusion, the Polysun simulation can be used for the optimization of control strategies and for the prediction of the COP and the overall performance of several variants of the system.

Figure 2: Polysun system schematic for the LiBr-H2O absorption chiller at SPF.

Figure 3: Comparison between measurement and simulation. For a representative period of time (1 month in the year 2011), Polysun simulations predict the electrical energy use of all auxiliary consumers (such as the pumps and fans). Polysun Solar Cooling

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Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus.

System 3: Server Room Cooling Solar assisted server room cooling is a promising new application area for heat driven chillers [6]. A demonstrator system with high performance flat plate collectors and a Sortech adsorption cooling machine has been built by Ernst Schweizer AG, Metallbau and is now ready for investigation in a joint project [7, 8]. Free cooling is combined with solar assisted cooling to optimize overall system performance and to reduce electric energy consumption in the year round operation.

Figure 4: Polysun layout for a server room cooling. The bypass of the adsorption cooling machine is used for the heat rejection in the free cooling mode which is ambient air temperature controlled.

Outlook Simulation of solar thermal assisted chillers has been shown to be robust enough to be the basis for the evaluation of system variants and controller strategies. Using predictive and physics based numerical models for the optimization improves the system performance and result in more robust solar thermal driven cooling systems. The Polysun systems – based on real built cooling systems – discussed in this paper are relatively complex and the handling of the highest user level “Polysun Designer” is recommended for experts. However, all these systems can be saved for the usage in the user level “Polysun Professional” or even made available in the browser version “Polysun Online” where it is available for the use case of a sales engineer. In this setup, simulation is not used to invent topologically new variants of the system but to adjust the demand side (cooling and heating) as well as the geographical location of the planned implementation of the cooling system. Polysun adjusts the weather data [10] including ambient temperature, air humidity and wind speed and calculates the system performance as a basis for the profitability (cost) analysis.

Polysun Solar Cooling

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Vela Solaris

Published in the proceedings of the 4th International Conference Solar AirConditiongin organized by OTTI, 12 to 14 October 2011, Larnaca, Cyprus.

Conclusion The potential of heat driven chillers is often unrecognized or underestimated because the double usage of the solar collectors cannot easily be calculated. In the same system, high performance solar collectors can be used for hot water production (all seasons), space heating ( in the heating season) and as a driving source for heat driven chillers ( in the cooling season). With a simulation tool at hand that accounts for all the relevant system components and effects and system conditions (configurations), it is possible to recognize the situations where heat driven chillers outperform conventional cooling technology and to optimize systems depending on the cooling and heating demand. Furthermore, it promotes trust in green chiller technology and is a good means for communication in the sales process.

Acknowledgements This work has been financially supported by the Swiss Commission for Technology and Innovation KTI under the Projekt-Nr. 11680.1 PFIW-IW.

References [1]

S. Rezaei, A. Witzig, M. Pfeiffer, B. Lacoste, A. Wolf: Modeling and Analyzing Solar cooling Systems in Polysun, Proceedings of the 3rd international conference Solar Air-Conditioning by Otti, Palermo, Italy, October 2009 [2] A. Witzig, S. Rezaei, S. Geisshüsler, P. Brönner: Solares Kühlen in Polysun, Proceedings of the 20th Symposium Thermische Solarenergie, Bad Staffelstein, Germany, May 2010. [3] W. Press, S. Teukolsky, W. Vetterling, B. Flannery: Numerical Recipes. Cambridge University Press, third edition 2007. [4] C. Bales, O. Ayadi: Modelling of a Commercial Absorption Heat Pump with Internal Storage, presented in the proceedings of the 11th International Conference on Energy Storage, Stockholm, June 2009. [5] P. Gantenbein, R. Helfenberger, E. Frank: Wärmeabwurf aus einer solarthermisch getriebenen LiBr-H2O Absorptionskältemaschine durch gepulstes Besprühen eines Trockenkühlers mit Wasser, presented in the proceedings of the 10th Symposium Thermische Solaranlagen, Bad Staffelstein, Germany, May 2010. [6] G. I. Meijer: Cooling Energy-Hungry Data Centers, Science, volume 328, No. 5976, Pages 318-319. April 2010. [7] K. Knecht: Konzept und Integration einer Sorptionskühlmaschine in das Kühlsystem eines Rechenzentrums mit Hochleistungs-Mikroprozessoren, Bachelor Thesis at SPF, Hochschule für Technik Rapperswil HSR, June 2010, Rapperswil, Switzerland. [8] M. Brünig: Using Flat Plate Collectors for Active Solar Cooling of Computer Server Rooms, Proceedings of the 4th international Conference Solar Air-Conditioning, Larnaka, Cyprus, October 2011. [9] J. Marti, A. Witzig, P. Brönner: Systemauslegung für Geothermie und Free Cooling, Proceedings of the Otti Anwenderforum Oberflächennahe Geothermie, Linz, Austria, April 2010 [10] J. Remund et. al.: Wettersimulation mit Meteonorm. www.meteonorm.com.

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