Aquifer Reservoirs for Energy Management – Reichstag and Neubrandenburg

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 [2]. 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

Подпись: FIG. 4Подпись: ■ Charging the reservoir ■ Direct heating — Excess heat from the CCGT plant — I-I.ii | I.MI requirements image180Подпись: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 [1]. 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 [10].

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

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Excess heat and its use on the 5th and 6th of November 2005 in the Neubrandenburg aquifer reservoir.

GEOTHERMAL PROBES

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ca. 46 mm.(1.8 in)..

Coaxial

probe

Cross sections of typical geothermal downhole heat exchangers [9].

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.