AQUIFER THERMAL ENERGY STORAGE IN NEUBRANDENBURG MONITORING THROUGHOUT THREE YEARS OF REGULAR OPERATION F. Kabus, M. Wolfgramm Geothermie Neubrandenburg GmbH Seestrasse 7A, 17033 Neubrandenburg
[email protected] A. Seibt Boden Wasser Gesundheit GbR Seestrasse 7A, 17033 Neubrandenburg U. Richlak, H. Beuster Neubrandenburger Stadtwerke GmbH John-Schehr-Straße 1, 17033 Neubrandenburg
ABSTRACT Since 2005, the Neubrandenburg aquifer thermal energy store (ATES) which is installed at a depth of approx. 1,200 m is in regular operation. This presentation focuses on the energy balances of the three cycles of operation completed so far without any major problems. The store was charged with 14,300 MWh in 2005/06 and 12,800 MWh in 2007/08, and 6,500 or 5,900 MWh were discharged with a recovery coefficient of 46 %, respectively. The previous 50 % share of the peak-load boilers in the heat supply decreased to 0.2 % by 2007/2008. Generally, the materials and equipment applied in the thermal water loop proved their suitability, with very few exceptions. Numerous full analyses of samples from both wells were assessed within the framework of the accompanying geochemical and microbiological monitoring programme. The most important results are presented here. 1.
INTRODUCTION
An aquifer thermal energy store was installed in Neubrandenburg by 2004 based on the technical equipment of an existing geothermal heating plant. Two of the four geothermal wells were re-installed and modified for the surface systems. 100 m³ of thermal water can be produced from or injected in each well per hour. A 77 MWel gas- and steam-turbine driven cogeneration plant (GSCP) serves as the heat source for charging at a temperature of 80 °C and with a capacity of up to 4 MW in summer. A low-temperature district heat supply system with a connected load of 12 MW forms the heat sink for the heat discharged in winter. Parameters of the detailed design are given, among others, by Kabus, Richlak, Beuster (2006). 2.
OPERATING RESULTS
Since the spring of 2005, the ATES is operated (except for a failure of the submersible pump in 2006/07) practically without any store-related problems – controlled solely by the surplus heat/heat demand situation and interrupted by the annual plant inspection in June. Meanwhile, three full annual cycles are completed. The following two figures show the temperatures at the well heads or the charging and discharging capacities, respectively.
Figure 1: Temperatures measured at the heads of the warm well Gt N 1/86 and the cold well Gt N 4/86
Figure 2: Heat capacities measured in the phases of charging and discharging
When comparing the full cycles in 2005/06 and 2007/08, the different quality of the behaviour of the discharging temperatures becomes evident (cf. Figure 1). Assuming approximately the same charging before (it was even considerably higher in 2005), the „run-in“ process of the store becomes clear. In 2007/08, the store temperature decreases at practically the same production (210,000 and 245,000 m³) with a clearly lower gradient (1.90 K/100,000 m³ compared to 2.85 K/100,000 m³). The losses in the previous years caused a warming-up of the underground near-store zone. As a consequence, the discharging capacity dropped clearly in the winter of 2006. Even on days with a high heat load the discharging capacity could not be increased inspite of operation at maximum flow rate. The limit was given by the outdoor-temperature driven feeding flow temperature in the network. But, in the winter of 2007/08 it becomes evident that the store is able without any problems even after longer operation to react to high load demands. Still in March 3.0 MW are possible, whereas 2.5 MW could not be exceeded in 2005/06. Surely, the above statement implies another component which plays, however, a subordinated role only compared to 2006 and 2008 – namely the continuous decrease of the return flow temperature in the heating network caused by technical measures in the consumer system. Finally it was possible to decrease the specific water quantity to be produced per MWh of discharged heat from 41 m³/MWh in 2005 to 35 m³/MWh in 2007/08, thus reducing also the relative power demand when discharging from approx. 2.5 % to approx. 2.1 %. Figure 3 shows the situation giving grounds to think about the installation of an ATES. Approx. 50 % of the heat could be supplied to the „Rostocker Strasse“ residential area from the GSCP via a connection of the high- and low-temperature district heat supply systems in Neubrandenburg. The remaining 50 % (= approx. 9,000 MWh/a) had to be generated by the peakload boilers. 50.3%
GSCP boilers
49.7%
Figure 3: Percentages of the individual heat producers in the coverage of the demand within the „Rostocker Strasse“ district heat supply system – initial situation when starting to design the heat store Figure 4 and Figure 5 illustrate the effect expected from the ATES. Basically, the consumption of fossil fuels should be reduced from 50 % to 2.5 % by the seasonal shifting of 12,000 MWh of surplus heat arising from the GSCP and the feeding of approx. 8,800 MWh thereof (minus store losses) into the supply system.
3,000 2,500 2,000 heat [MWh/month]
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Figure 4: Shares of the individual heat producers in the coverage of the demand within the „Rostocker Strasse“ district heat supply system upon integration of the aquifer store – designed figures by months
47.7%
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Figure 5: Percentages of the individual heat producers in the coverage of the demand within the „Rostocker Strasse“ district heat supply system upon integration of the aquifer store – designed figures Before naming the store effects achieved practically, the changes of the frame conditions need to be highlighted which came into force since the beginning of the design work. In the first line, this refers to the heat sales in the supplied area. Referred to the average demand of 18,125 MWh/a (basis of the design) determined over a period of several years, the demand decreased in parallel with the commissioning of the ATES in 2004. Basically, this is caused by energy saving measures taken by the consumers. For the above reasons, the reduction amounts to approx. 13 %. Moreover, a block-type cogeneration plant working on sewage gas from the adjacent waste water treatment plant was integrated in the base load. This reduces the sales potential from the aquifer store by approximately another 15 %.
Assuming a power-driven operation of the GSCP, i.e., it was not subjected to basic changes in the course of the years, and that the savings concerning the heat consumption mentioned before for the „Rostocker Strasse“ network occurred also in the centralised district heat supply system, it becomes clear that the limit after which the peak-load equipment should be switched on (and here is the positive potential of the reservoir) will be shifted towards lower temperatures. This leads to the fact that a situation will be achieved earlier in spring leading to switchingover to the charging mode of operation. In autumn, the moment of switching-off of the charging mode of operation will be later, respectively. According to the design, the discharging and charging periods would last 6 months each (Figure 4). In reality, the charging period has been lasting approx. 7.5 months and the discharging period 4.5 months since 2004. Assuming an even distribution of the quantities of the charged and discharged heat throughout the months concerned (based on the designed figures), solely on this temporal basis the charging of 15,000 MWh and the discharging of only 6,500 MWh appears to be realistic. This would correspond to a store coefficient of 43 %. With an advance view to the following presentation of the results, this is very close to reality. Actually, the above capacity limit of the heat supply from the power plant is achieved at outdoor temperatures around 0 °C. However, a phase of numerous and unpredictable - in terms of time - alterations between excess heat and demand on additional heating capacity starts here. The aquifer store which is very inert in the starting-up and shutting-down processes cannot balance these variations. For this reason, a simple mode of operation was fixed: Above a daily mean temperature of 5 °C which can be predicted for three days the store is operated generally in the charging mode and below respectively predicted 2 °C in the discharging mode. It remains switched-off in between. Also this design of the switching processes for which less time was planned initially, leads to another reduction of the phase of discharging, but here also of the phase of charging. At times, the return flow temperatures in the district heat supply system exceed clearly the designed maximum 45 °C in the heating period. Basically, this has to do with the function of the house service station in times of sanitary hot water preparation. The heat capacity integratable from the store is reduced in this way at times by up to 40 %. Here major improvements could be achieved while working on this topic, but 40 °C – as designed initially as minimum value – could not be achieved. The above aspects lead to the following figure. A major quantity of charged heat in summer is compared to a minor quantity of heat discharged in winter. The heat recovery coefficients achieve values of 46 %. The deviation from the designed value of 72 % is to be assigned mainly to the alterations of the demand/surplus situation at the site described above. Assuming here an unchanged balance, model calculations result in a value of approx. 65 %. The remaining difference has to be assigned to the increased return flow temperatures in the heating network.
3,000 2,500 2,000 heat [MWh/month]
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Figure 6: Shares of the individual heat producers in the coverage of the demand within the „Rostocker Strasse“ district heat supply system - values measured in 2007/08 by months Finally it was possible to achieve the target set in the energetic field: the reduction or even undercutting of the additional use of fossil fuels for the heating of the „Rostocker Strasse“ district heat supply system from approx. 50 % to approx. 2.5 % which is illustrated by the following figure: 42.6% 0.2%
GSCP CP discharging boilers 34.5%
22.7%
Figure 7: Percentages of the individual heat producers in the coverage of the demand within the „Rostocker Strasse“ district heat supply system – values measured in 2007/08 3. GEOCHEMICAL MONITORING 3.1 Sampling Approximately 30 water samples were taken and analysed over a period of three years with 14 samples being represented by full analyses. Seven samples, respectively, have to be assigned to the charging and the discharging cycles. In addition, 22 solid samples obtained from 15 samplings were analysed by means of the scanning electron microscope (SEM) and EDX.
3.2 Assessment of the Water Analyses The obtained parameters were assessed with due consideration of the mode of operation. It became clear that the zinc concentration increases continuously during operation (from 10 µg/L to more than 200 µg/L). This increase is independent of the mode of operation and should be caused by mobilisation of solids from technical components (pumps, surface valves and fittings). A geologically induced mobilisation can be excluded. The concentrations of arsenic, lead and copper increase generally when the water is cooling down and decrease when heating up. Iron and manganese show exactly opposite behaviour compared to nonferrous heavy metals. Other parameters (pH, concentrations of phosphate, hydrogen carbonate, potassium, boron and silicium) are strongly tied to the mode of operation (summer or winter operation). 3.3 Assessment of the Solids The formations of solids in the deep geothermal reservoir in Neubrandenburg were described comprehensively by Wolfgramm & Seibt (2006). Iron sulphides, carbonates and detrital particles from the store rock could be proven. Further investigations showed increasing concentrations of copper, nickel and other heavy metal sulphides. The origin of the heavy metals which precipitated now instead of iron is not quite clear. Again, mainly technical components of the plant might be the source. Another important observation results from the simultaneous analysis of the residues in the filters directly after production and immediately before the injection of the waters. Carbonates and heavy metal sulphide scales could be proven in the filters arranged after the respective production well (summer and winter operation). From here, the water was transported through the surface part of the plant, i.e., a pipe system with a total length of more than 1 km. In the downstream filters, major quantities of fine-grained heavy metal sulphides could be observed. Obviously, these scales were formed within the surface part of the plant or „grew“ there at least. The heavy metal sulphide scales are characterised by a shelly structure. The iron sulphides which accumulated closely to the borehole wall are covered by copper and nickel sulphides which grew there subsequently which may be due to microbial activity and corrosion. More investigations will follow. E.g., sulphate-reducing and fermenting bacteria were proven in correlation with the season (Würdemann et al. 2008). 4. Increase of the Charging Temperature It was to be determined along with and by the geochemical monitoring whether charging of the deep store is possible at temperatures > 80 °C. For that, additional solution tests of the store rocks were done in the laboratory as well as geochmical modelings. Problems are caused mainly by SiO2 which is undersaturated when heating up and implies the risk of scaling when cooling down. Regarding the operation of the store, it means that SiO2 should dissolve on the warm side of the ATES. When injecting the water on the cold side, amorphous SiO2 should precipitate from the oversaturated fluid. The geochemical modelings showed that a mobilisation of SiO2 is mainly restricted to the dissolution of clay minerals. As their content is rather low in the rock, the risk of a deterioration of the injectivity on the cold side does not have be assessed as that drastically. Also the laboratory tests and the water analyses showed a slight increase of the SiO2 concentration only after heating-up of the waters. Thus, an increase of the injection temperature up to 90 °C is possible. However, it should be stepwise and accompanied by a geochemical monitoring programme.
5. SUMMARY The ATES implemented under the research project at the site of Neubrandenburg works very satisfactorily basically for three periods now and is accompanied by a geochemical, geomicrobiological, energetic and process engineering routine monitoring programme. Meanwhile, there were • • • • • • •
achieved the predicted store efficiency with due consideration of the real surplus/demand scenario, optimised the charging and discharging cycles within the context of the gas and steam cogeneration plant, adapted the behaviour of the consuming district heat supply system to the requirements by an ATES, in particular concerning the return flow temperatures, adapted the technical design (directional switching-over, filtration, filling of the system with protective gas, etc.) and the selection of the materials in some elements to the results of the accompanying investigations, optimised the automation equipment and sensor engineering for the thermal water loop, the secondary store integration and the connection between the cogeneration plant and the store, improved and validated the models for the advance calculation of the thermic and hydraulic states in the store in a way allowing for the prognosis of both the long- and short-term effects of different modes of operation with high accuracy, proven by the accompanying solid, water and gas analyses as well as microbial investigations – independent of minor trends towards seasonally varying CaCO3 contents and iron(II)-ion concentrations as well as changes of the microbial settlement, that the overall system is characterised by safe and stable operation under the existing conditions in the thermal water and at temperatures of 80 °C.
ACKNOWLEDGMENT The work was supported by the Federal Ministry of Economics of Germany (project no. 0329838B) which the authors gratefully acknowledge. The authors are responsible for the content of this publication. REFERENCES Kabus, F.; Richlak, U.; Beuster, H. (2006): Saisonale Speicherung von Überschusswärme aus einem Heizkraftwerk in einen Aquifer in Neubrandenburg. Statusseminar „Thermische Energiespeicherung“, 2.- 3.11.2006, Freiburg, pp. 143-152 Seibt, P.; Kabus, F. (2006): Energiespeicherung in Aquiferen – von der Idee zur erfolgreichen Projektrealisierung. 9. Geothermische Fachtagung, 15.-17.11.2006, Karlsruhe Würdemann, H., Lerm, S., Alawi, M., Vetter, A., Mangelsdorf, K., Seibt, A., Wolfgramm, M., Vieth, A. (2008): Betriebssicherheit der geothermischen Nutzung von Aquiferen unter besonderer Berücksichtigung mikrobiologischer Aktivität und Partikelumlagerungen – Erste Ergebnisse des Screenings an Wärme- und Kältespeichern. Der Geothermiekongress 2008, 11.-13.11.2008, Karlsruhe, pp. 434-439 Wolfgramm, M., Seibt, A. (2006): Geochemisches Monitoring des geothermalen Tiefenspeichers in Neubrandenburg, 9. Geothermische Fachtagung, 15.-17.11.2006, Karlsruhe, pp. 388397
