August 13th, 2020
In the construction and the energy management of the new government buildings along the River Spree in Berlin, future – oriented, environmentally responsible and exemplary energy concepts were required . The energy-supply system of the Berlin parliament buildings therefore contains, along with components for combined heat-cooling-power production, two aquifer storage reservoirs (Figure 2). The heat storage reservoir increases the contribution of the combined heat and power plant to the overall energy supply, even when the system is power-directed. The cooling reservoir permits the use of the low winter temperatures for air conditioning in the summer months.
The storage region is a sandstone stratum at a depth of 285 to 315 m which contains salty water (brine). Two boreholes access this aquifer. It is charged as a rule with water at a maximum of 70° C, and discharged at 65-30° C. In operation, a volume flow rate of up to 100 m3/h can be extracted or injected. The maximum charging power is about 4.4 MW Heat from the discharge supplies the low-temperature sectors of the various building heating systems via direct heat exchange. Additional cooling of the water (down to a minimum of 20° C) can be carried out using absorption heat pumps as needed; their installed cooling power is ca. 2 MW.
This project is unique and has been accompanied by extensive scientific monitoring. Among other things, numerical models of the substrata have been constructed within the framework of research projects; they allow the temperature evolution in the storage media to be predicted [3, 4]. Figure 3 shows as an example the temperature at the wellhead of the warm borehole during the time period from June 2003 to December 2005. This period includes almost
three charging and discharging cycles. The decreasing output temperature during discharging is characteristic of aquifer storage. It results from the losses described above, due to thermal conduction and convection at the perimeter of the warm storage volume.
At a considerably shallower depth of 50 m, under the bend of the Spree, another aquifer storage system was developed. It is used mainly for cooling of the buildings. In operation, fresh ground water is cooled in winter to 5° C. This is carried out essentially at outside temperatures below 0° C in dry cooling towers by heat exchange with the surroundings . In the summer, this cooling reservoir supports the air conditioning systems via direct heat exchange.
In Neubrandenburg, a city in the German state of Mecklenburg-Vorpommern, a major portion of the buildings are connected to a central 200-MW district heating network. Its base load is carried by a combined-cycle gas and steam turbine power plant with 77 MW of electric output power and 90 MW thermal power. Since heat supplies in Neubran – denburg in the summer are essentially used only for providing warm water, the thermal power consumption is relatively low. As a rule, it is considerably less than the heat which is produced even when electric power is being generated at minimum load. The difference of up to 20 MW was in the past simply released to the environment through a cooling tower. Today, part of this ‘waste heat’ is fed into an aquifer storage system and used for heating in the winter in a portion of the district heating network which operates at a relatively low temperature level. The thermal power in this network is 12 MW at 80° C input temperature and 45° C return temperature.
The aquifer storage system consists of a cool and a warm borehole spaced ca. 1300 m apart. Both boreholes tap the operating stratum at a depth of about 1200 m, and they can extract or inject 100 m3/h of thermal water. The natural temperature of the stratum at this depth is about 55° C .
In summer, cooler water is pumped out at 40 to 50° C and is stored underground at a temperature of 80° C. In win-
Excess heat and its use on the 5th and 6th of November 2005 in the Neubrandenburg aquifer reservoir.
ca. 46 mm.(1.8 in)..
Cross sections of typical geothermal downhole heat exchangers .
ter, the flow direction is reversed; water is now extracted from the warm borehole. The output temperatures are between 80° C and 65° C, depending on the time until the water is extracted.
According to plan, it is expected that in the months April to September, a quantity of thermal energy equivalent to 12,000 MWh can be stored in the reservoir. In the winter, 8,800 MWh of this can be recovered through direct heat exchange at a thermal power of 4.0 to 2.9 MW.
The Neubrandenburg aquifer storage system has now been operating for four regular annual cycles. Figure 4 shows as an example the operating range of the reservoir during charging.