Next Generation of Seasonal Gravel-Water Thermal Energy Store - Design and Operating Results from Eggenstein-Leopoldshafen, Germany

Oct 29, 2026

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Seasonal thermal energy storage enables central solar heating plants to reach high solar fractions while balancing summer heat supply with winter demand. A latest-generation gravel-water thermal energy store (GWTES) built in 2007 and operating since 2009 in Eggenstein-Leopoldshafen is described from both design and performance perspectives. The system uses 4,500 m³ of gravel with vertical wells, enhanced insulation protection via HDPE liner chambers, and a 60 kW heat pump to enable discharging down to ~10 °C, supporting solar fractions up to 40% and substantial primary energy savings.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

Introduction

The Gravel-Water Energy Store in Eggenstein-Leopoldshafen

Next Generation of Seasonal Gravel-Water Thermal Energy Store – Design and Operating Results from Eggenstein-Leopoldshafen, Germany Roman Marx, Dan Bauer, Harald Drueck University of Stuttgart, Institute of Thermodynamics and Thermal Engineering (ITW), Research and Testing Centre for Thermal Solar Systems (TZS), Pfaffenwaldring 6, 70550 Stuttgart, Germany, Phone: +49 711 685 63229, Fax: +49 711 685 63503, [e-mail:](e-mail: marx@itw.uni-stuttgart.de)[ ](e-mail: marx@itw.uni-stuttgart.de)[marx@itw.uni-stuttgart.de](e-mail: marx@itw.uni-stuttgart.de) 1. Introduction Seasonal thermal energy storage is an integral component for central solar heating plants to achieve high solar fractions. For economical and ecological reasons the systems must be designed very carefully. One focus of the design is the dimensioning of the seasonal thermal energy store as well its construction. In Eggenstein-Leopoldshafen located in the south-west of Germany the latest generation of a gravel-water thermal energy store (GWTES) was built in 2007 and is in operation since 2009 [1], [2] and [3] . The store is integrated into Germany’s first central solar heating plant with seasonal thermal energy storage (CSHPSS) realised with existing buildings (Figure 1) . After a major refurbishment of the buildings and the heating network 1 600 m² flat plate collectors (FC) have been installed. The store has a volume of 4 500 m³ . Discharging of the seasonal GWTES down to temperatures of around 10 °C is facilitated by the use of a 60 kWth heat pump [4] . Thus, the usable thermal capacity of the GWTES can be increased by more than 40 % compared to discharging from 80 °C to the network return temperature of 40 °C. For backup heating two 600 kW gas boilers and a 30 m³ hot water buffer store are available. The system is designed to achieve a solar fraction up to 40 % of the total heat demand. According to calculations, primary energy savings of 65 % may be achieved due to the refurbishment of the buildings and the integration of the CSHPSS. This corresponds to an annual reduction of CO2 emissions of 390 t. Figure 1. CSHPSS with 1 600 m² flat plate collectors and seasonal GWTES in Eggenstein-Leopoldshafen (Germany) 2. The Gravel-Water Energy Store in Eggenstein-Leopoldshafen The GWTES closes the gap caused by the seasonal mismatch between high solar thermal supply in summer and high heat demand in the winter period. With regard to the construction of the seasonal thermal energy store in EggensteinLeopoldshafen several boundary conditions had to be considered. As the ground water level is only 7.5 m below the ground surface level, the store had to be constructed in such a way that the thermal insulation is protected from penetration of ground water. Unrestricted accessibility and trafficability was required by the customer due to the location within the school yard. Therefore a totally safe construction had to be selected even in the case of a total failure of the store liner. Hence, the concept with a GWTES was favoured over a hot water thermal energy store. The geometry of the store consists of two truncated cones, see Figure 2. Two thirds of the volume of the store are located below ground surface level. It is filled with washed 16 – 32 mm gravel to a height of 2.5 m. In the remaining volume, up to the ground surface level, the excavated gravel/sand is refilled in order to reduce construction costs. The upper third of the store is formed as a truncated cone with washed 16 – 32 mm gravel. Charging and discharging of the GWTES is realised directly by two vertical wells. The deep well is embedded into the bottom gravel layer, the shallow well into the top gravel layer. Special attention was given on the thermal insulation type, thickness and protection. The full surface of the store even the bottom is thermally insulated. Expanded glass granules at the bottom truncated cone and foam glass gravel at the top truncated cone of the store were employed as i

FAQs

What is the role of seasonal thermal energy storage in solar heating plants?

Seasonal thermal energy storage balances the seasonal mismatch between high solar thermal supply in summer and high heat demand in winter.

What are the key design boundary conditions for the GWTES in Eggenstein-Leopoldshafen?

The store must protect thermal insulation from ground water penetration and ensure safe accessibility and trafficability within a school yard, motivating a robust construction concept.

How does the GWTES configuration improve operational performance and usable capacity?

Using a 60 kWth heat pump allows discharge to ~10 °C, increasing usable thermal capacity by more than 40% versus discharging from 80 °C to the network return temperature.

Oct 29, 2026

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