Foto: Wohnhaus der SolarCity

Evaluation of solar-thermal energy stores using a uniform and marketable system of characteristic numbers

Comparison of solar-thermal energy stores
A clear and uniform system of characteristic numbers enables a cost-saving comparison of the efficiency of solar-thermal energy stores of different technologies for the first time and gives important impulses for the market.

Short Description




Thermal energy storage tanks represent one of the most important components of solar thermal systems providing domestic hot water and space heating. The successful realization of the concepts of passive and low energy houses is highly coupled with an efficient use of solar energy, where the storage tank plays a crucial role. Therefore, the efficiency gain of thermal storage tanks is currently a hot research topic, with the objective to enable the monovalent operation of solar systems through further development of storage materials and storage technologies. The improvement of thermal stratification in water stores, the application of latent storage materials and the development of sorption storage tanks contribute to achieve this objective.

The evaluation of the design and the operating behavior of such stores represent a significant criterion to compare and assess different storage technologies. Therefore, a uniform and clear system of characteristic numbers which is accepted on the market is necessary. Although, there are already various concepts and measurement protocols available for geometrical, energetic and fluidic characteristic values, no uniform system has become established yet. The proposed systems lack due to the complexity of the characteristic numbers, and due the cost and time consuming measurement protocols. Particularly, no comparison between different storage technologies, like stratified water stores, latent stores or sorption stores, is possible at present. Hence, it is absolutely necessary to have a possibility for comparison of solar thermal stores to assess the progresses in research and technological development on the one hand, and to give important impulses for the thermal store market, on the other hand.

The aim of the project is to develop a uniform and transparent system of characteristic numbers for thermal energy storage tanks and to validate this system with measurements on different storage tank technologies. The most important requirements on the system of characteristic numbers concern the applicability to all kinds of thermal storage technologies presently used or investigated (water, latent and sorption storage tanks), the simplicity of determination of the characteristic numbers through input and output measurements, and transparency. These features will ensure acceptance from the market and manufactures which is the most important point.

Based on existing knowledge a new system of characteristic numbers consisting of already defined and new characteristic numbers will be elaborated. The new system should primarily describe the input/output behavior of storage tanks using exergy efficiency factors. Thus, it will be possible to characterize the benefit of the store for the user without entering into constructional details of the store design. Therefore, the store will be considered as a dynamic system which will be parameterized and identified with a few characteristic numbers. The new elaborated characteristic numbers will be tested and adapted using the simulation program TRNSYS. Afterwards, measurements on a test plant will be performed for three different types of storage tanks. Depending on the experience gained through the measurements, the system of characteristic numbers will be improved and adapted. Finally, a guideline describing the new system of characteristic numbers and the corresponding instructions for measurements will be compiled. During the whole project manufactures are involved through interviews and discussion in workshops. The tested water storage tanks will be provided by manufactures free of charge.

The gained knowledge associated with the new system of characteristic numbers will be used as a basis to integrate the Austrian views into the European discussion process concerning the standardization of store tests and to bring a new impulse into this discussion through the developed method. This will be achieved in close cooperation with the national standardization committee (FNA 173). Furthermore, an extensive cooperation with distinguished European institutions working in the field of storage is foreseen.

As a result, a marketable, clear and uniform system of characteristic numbers will be available to evaluate and compare different thermal storage tanks. The new system will be characterized by transparency, uniformity, and by time and cost saving measurements in order to determine the characteristic numbers. It enables, for the first time, a comparison between different storage technologies. Additionally, first demonstrative measurement results for a stratified water storage tank, a latent and a sorption storage tank, as well as a comparison between these different technologies will be presented. The results will be presented at national and international conferences, and disseminated through the national organization Austria Solar and the European solar thermal industry association ESTIF.

Project Partners


Dieter Gottwald
arsenal research

Project or cooperation partner

  • Dagmar Jähnig
    Feldgasse 19
    A-8200 Gleisdorf
    Tel.: +43 (0) 3112 5886-28
  • Wolfgang Streicher
    ITW TU Graz
    Inffeldgasse 25/B
    A-8010 Graz
    Tel.: +43 (0) 316 873-7306
  • Arnold Teufel
    Teufel & Schwarz GmbH
    Achenweg 3
    A-6353 Kitzbühel
    Tel.: +43 (0) 5358-3939
  • Arthur Sief
    Siko Energiesysteme Ges.m.b.H & Co KG
    Rotholzerweg 14
    A-6200 Jenbach
    Tel.: +43 (0) 5244-64466
  • Gerald Jungreithmayr
    Solution Solartechnik GmbH
    Hauptstraße 27
    A-4624 Sattledt
    Tel.: +43 (0) 7244-20280
  • Werner Pink
    Pink GmbH - Energie- und Speichertechnik
    Bahnhofstraße 22
    A-8665 Langenwang
    Tel.: +43 (0) 3854-36663

Contact Address

Dieter Gottwald
arsenal research
Giefinggasse 2
A-1210 Wien
Tel.: +43 (0) 50 550 - 6413
Fax: +43 (0) 50 550 - 6390

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