August 13th, 2020
In Golm, near Potsdam, in 1999 the Max Planck Society founded a new science campus, comprising three institutes. The energy management system is based on a combined heating-cooling-power system, making use of a geothermal borehole field for storage (Figure 6). This concept includes a cogeneration plant driven by an internal-combustion engine; the excess heat is used for space heating and warm water.
The DHE field contributes to cooling in the summer months, and in the process it stores heat energy. In the winter, it serves as thermal reservoir for a heat pump to supply heat energy to the buildings, and is thereby cooled, causing the temperature in the borehole field to sink below that of the surrounding earth. The field consists of 160 boreholes, each with a depth of 105 m, and it occupies an area of 65 m x 50 m, with an earth volume of about 400,000 m3. Its overall storage capacity is 2.24 MWh, and the input/output power is nominally 538 kW.
The temperature is measured year-round at four of the boreholes. Three of these are within the field, while the fourth gives values from the undisturbed region outside the field for comparison. Each borehole contains four temperature probes at depths of 15 m, 40 m, 70 m, and 100 m, respectively. The measured temperatures during the time period from September 2001 to September 2002 in one of the boreholes within the field are shown in Figure 7. Normally, the natural temperature increases with increasing depth. During charging, the warm heat-transport medium flows downwards and gives up its heat to the storage medium; thus, the temperature gradient in the upper part of the bore-
THE BOREHOLE FIELD IN GOLM
Measured temperatures from September 2001 to September 2002 within the bore-hole field of the Max-Planck Campus at a depth of 15 m, 40 m, 70 m and 100 m.
A HOT-WATER STORAGE SYSTEM
Concrete Thermal insulation Stainless steel inner walls
est, coolest region in the reservoir. This temperature stratification, which normally will already appear as a result of density differences, will be affected by thermal conduction and convection. While thermal conduction within the reservoir cannot be avoided, convection can be held to a minimum. Thus, for example, mechanisms for storing the thermal energy within the layer of the same temperature can be useful. This is especially interesting for solar heat, which can be stored at varying temperatures.
Since the storage reservoirs are usually operated at ambient pressure, the maximum storage temperature lies below 100 °C. In contrast to aquifer storage systems, they have a clear and well-defined storage capacity. The investment costs of these storage systems are estimated to be in the range of 450-120 €/m3 (depending on the overall volume) for concrete reservoirs, and 3000-600 €/m3 (for 0.2-100 m3 volume), or below 100 €/m3 (at volumes larger than 10,000 m3) for steel reservoirs .
Solar-Assisted Local Networks
The German Federal ministries for Commerce and Technology and for the Environment, Nature Protection and Nuclear Safety have subsidized the construction of pilot and demonstration plants for local solar heating networks within the framework of the program ‘SolarThermal2000’ [5-7]. In three of the subsidized pilot plants, in Friedrichshafen, Hamburg, and Hannover, seasonal hot-water reservoirs are integrated into the energy supply system as tank structures. In these solar-assisted local heating networks, use is made of the stored energy to preheat the return flow in the local heating network. If the heat output power or the temperature from the storage reservoir is not sufficient, heat from fossil fuels (gas, oil) or from district heating networks is used to reach the desired temperature.
The largest of these hot-water storage reservoirs is in Friedrichshafen, Germany. It has a height of 20 m and an inner diameter of 32 m, and thus provides a storage volume of 12,000 m3. In the final construction stage, this local heating network will supply heat to 570 dwellings with a heated floor area of nearly 40,000 m2 (Figure 8).
The experience with the pilot projects for solar-assisted local heating with long-term energy storage has thus far yielded a fraction of solar heat in the overall energy consumption of 30 to 35 %. With additional improvements to the systems technology, solar fractions of 50 to 60 % are expected .
Gravel-Water Storage Systems
Gravel-water storage reservoirs are likewise artificial structures. In them, a mixture of gravel and water serves as storage medium, with a gravel concentration of 60-70 vol. %. This mixture is placed into a cavity in the ground which has been lined with a watertight plastic sheet. The maximum storage temperature is limited by the temperature stability of the plastic, and is typically in the range of 80° C. Due to the lower specific heat of the gravel, a gravel-water storage
reservoir requires about 50 % more volume than a pure water storage reservoir for the same storage capacity.
Charging and discharging can be accomplished either by direct water flow through the reservoir or by heat exchange using coiled tubes. The heat-transport medium is thus either the water within the reservoir, or a second medium such as brine or an antifreeze mixture. In gravel-water storage systems, vertical temperature stratification is likewise observed, and it can be enhanced by the charging and discharging processes.
The gravel within the reservoir has two advantages: It supports part of the static load on the container, and thus makes its construction lighter and simpler. In addition, it reduces the free convection of the fluid within the reservoir and thereby the internal losses. The external losses are limited by thermal insulation applied outside the watertight sheet. In the local heating networks in Steinfurth and Chemnitz, which were also subsidized by the program mentioned above, gravel – water storage systems were utilized.