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More than 30 international research and pilot seasonal thermal energy stores (TESs) were realized within the past 30 years. Experiences with operation of these systems show that TES are technically feasible and work well. Seasonal storage of solar thermal energy or of waste heat from heat and power cogeneration plants can significantly contribute to substitute fossil fuels in future energy systems. However, performance with respect to thermal losses and lifetime has to be enhanced, while construction costs have to be further reduced. This paper gives an overview about the state-of-the-art of seasonal thermal energy storage with the focus on tank and pit TES construction. Aspects of TES modeling are given. Based on modeled and measured data, the influence of construction type, system configuration, and boundary conditions on thermal losses of large-scale TES is identified. The focus is on large-scale applications with tank and pit thermal energy stores and on recent investigations on suitable materials and constructions. Furthermore, experiences with the operation of these systems with respect to storage performance are discussed. [DOI: 10.1115/1.3197842]

A new method for characterization of stratified thermal energy stores (TES) that integrates both the first law and the second law concerns is presented here. The first law concern is incorporated into a quantity called energy response factor and the second law concern into an entropy generation ratio. A product of these two quantities is at the heart of the TES efficiency definitions. This approach removes the overemphasis of the existing methods either on the first or the second law of thermodynamics which often biases the characterization results. The information about the evolution of the temperature field of the system in time is the prerequisite of the new method. It may be obtained from experiments or from suitable numerical simulations. The current method can be easily integrated into computational fluid dynamic (CFD) simulations and thus facilitate CFD-based design analysis. As an example of such CFD-integrated analysis, a large-scale hot water seasonal heat store is numerically studied to identify the effects of aspect ratio, containment shape, internal structures, and containment size on their efficiency. The results suggest the effectiveness of the new method in deriving useful design insights. Keywords. Hot water seasonal heat store; Computational fluid dynamics (CFD) model; First law of thermodynamics; Second law of thermodynamics; Stratified thermal energy store (TES) characterization; Entropy generation ratio; Energy response factor; Aspect ratio; Containment shape; Internal structures; Containment size

In April 2001 the new European Standard EN 12975:2000: 'Thermal solar systems and components--Solar Collectors' was established. With the publication of this European standard all national standards, related to the same topic, have to be withdrawn by the nations of the European Community. Now only one standard for testing solar collectors is valid throughout Europe. This European Standard specifies test methods for validating the durability, reliability and safety requirements for liquid heating collectors. The standard also includes two alternative test methods for the thermal performance characterization for liquid heating collectors. Apart from the well-known test method under steady-state conditions according to ISO 9806-1, ISO 9806-3 and ASHRAE 93-77 the EN 12975 permits a quasi-dynamic test method for the thermal performance characterization of solar thermal collectors. This paper presents the improved approach to outdoor performance testing of solar thermal collectors under quasi-dynamic test conditions. The test requirements and collector theory are closely connected to those long agreed on for steady-state testing, as described in the ISO and ASHRAE standards mentioned above. The most important effects for the all day performance of the collector are taken into account. The test method covers most collector designs on the market today (except ICS type). Only some correction terms are added to the basic collector models of the present steady-state test methods. Still this limited change will allow test data to be collected and used from whole days. An important fact is that the collector model used for the parameter identification is written so that the error in collector output power is minimized. Therefore an accurate long-term prediction of the collector performance can be an integral part of the test method, where the same collector model and parameters are used for both testing and prediction.