Category Archives: EuroSun2008-11

Determination of the maximum usable hot water volume

In order to evaluate the system behaviour under different draw off volumes and to determine the maximum usable hot water over a whole year, several simulations have been carried with a collector tilt angle of 42° under the climatic conditions of Rome. It has been assumed that hot water is only needed in the evening in between 6.00 — 8.00 pm. The system is included into an existing hot water system consisting e. g. of a gas boiler for hot water and room heating or a simple electronic hot water boiler.

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An automatic tempering valve at the hot water outlet of the storage prevents the users from scalding themselves. A selector valve automatically takes the hot water from the backup heating if the hot water temperature turns below 45°C. Figure 6 shows a schematic drawing of the system.

The daily draw off pattern was varied in the range of 120l/d.480l/d. As a first result of these variations it can be seen that the hot water draw off out of the 180l storage is limited to about 35,000l/a respectively the draw off gets saturated at about 240l/d, because of the climatic

conditions. With a draw off profile of only 120l/d, an annual solar hot water fraction of about 66% can be achieved. In terms of saved energy this equals a reduction of about 95l of fuel oil. With a hot water demand of 180l/d, it is still possible to cover nearly 50% of the annual hot water demand or an equivalent of 125l of fuel oil with only one collector of about 2m2 and a 180l storage tank.

4. Conclusion

The paper presents the implementation of a double mantle heat exchanger into the simulation environment CARNOT and first simulation results calculated with the new storage type.

The developed double mantle heat exchanger storage is based on the TRNSYS double mantle heat exchanger storage but was enhanced by the possibility of including user settable material and geometric data.

Concerning the system simulation, some “to dos” remain, like the validation of a modified “thermosiphonic pump”, which is able to calculate the fact of reverse thermosiphoning e. g. at night. In order to improve the system and to identify other main influencing variables, further simulation runs have to be carried out, e. g. with regard to overheating.

References

[1] S. Gurtner, F.-D. Treikauskas, W. Zorner: „Prufstand fur Thermosiphonanlage am Kompetenzzentrum Solartechnik an der Fachhochschule Ingolstadt“, 16. Symposium Thermische Solarenergie, Kloster Banz / Staffelstein (Germany), May 2006, p. 118 — 122.

[2] S. Brandmayr, W. Zorner: “Thermosiphon Systems: Market, State-of-the-Art and Trends“, 3rd European Solar Thermal Energy Conference (estec2007), Freiburg (Germany), June 2007, p. 182 — 188.

[3] N. N.: “Matlab/Simulink user manuals”, The Mathworks Inc., http://www. mathworks. com, Natick (USA), 2002.

[4] B. Hafner, J. Plettner, C. Wemhoner: “CARNOT Blockset: Conventional And Renewable eNergy systems OpTimization Blockset — User’s Guide”, Solar-Institut Julich, Aachen University of Applied Sciences (Germany), 1999.

[5] A. Carillo Andres, J. M. Cejudo Lopez: “TRNSYS model of a thermosiphon solar domestic water heater with a horizontal store and mantle heat exchanger”, Solar Energy, Vol. 72/2 (2002), p. 89 — 98.

[6] G. L. Morrison, D. B.J. Ranatunga: “Thermosyphon circulation in solar collectors”, Solar Energy, Vol. 24 (1980), p. 191 — 198.

Application of Sensitivity Analysis to Parameters of Large Solar Water Heating Systems

O. Kusyy*, K. Vajen, U. Jordan

Kassel University, Institute of Thermal Engineering, Kassel, Germany
* Corresponding Author, solar@uni-kassel. de
Abstract

In this paper the application of sensitivity analysis to the investigation of solar water heating systems is considered. Two global sensitivity analysis methods are described and applied to different solar heating systems. The first one is the Morris method that only ranks parameters by importance and the second one is the Fourier amplitude sensitivity test (FAST) that quantifies the influence of the parameters on the target functions. The both methods were implemented into the GenOpt (Generic Optimization) software and coupled with the TRNSYS simulation program.

Keywords: Sensitivity analysis, Fourier amplitude sensitivity test, Morris method

1. Introduction

In recent years, many middle to large solar heating systems were installed all around Europe and especially in Germany. A proper design of such systems is decisive for their functionality. Underdimensioning or poor selection of design parameters as well as the control strategy could lead to an overall poor efficiency of the systems. During the designing process, the advanced numerical optimization methods should be used in order to find the optimal parameter values that provide the best efficiency of the system. Considering that the target functions depend on a high number of optimization parameters and that the global optimization algorithms require large number of system simulations, the task of optimization turns to be very computationally expensive. In order to decrease the number of optimization parameters and, thus, make the optimization faster, the sensitivity analysis of parameters could be used prior to optimization. Only the most influential parameters are then selected for optimization. Another straightforward application of the sensitivity analysis is analysis of uncertainties, that is, how uncertainties in parameters influence the uncertainty of the target function. Here two sensitivity analysis methods are described and applied to the analysis of two solar heating systems. For the first system the influence of the operation parameters on the cost function is investigated by the qualitative Morris method. A more comprehensive Fourier amplitude sensitivity test is applied to the investigation of the influence of the design parameters on the solar fractional savings function of the second system. In this paper only the exemplary examples of applications of the both methods are considered. The methodological application of the methods is planned but not yet realized.