The ATES Project at Stockholm Arlanda Airport - Technical Design and Environmental Assessment

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1 The ATES Project at Stockholm Arlanda Airport - Technical Design and Environmental Assessment O. Andersson SWECO Environment AB Box 286, Malmö, Sweden Tel: olof.andersson@sweco.se ABSTRACT At Stockholm Arlanda airport, an Aquifer Thermal Energy Storage (ATES) for supply of natural heating and cooling of the airport is under construction. For the seasonal storage of heat and cold a section of an esker is used. With a number of high capacity wells the system can provide the airport with low temperature thermal energy to be used for air conditioning, pre heating of ventilation air and as a source for snow melting at the airport gates. The system is expected to reduce the emissions of environmentally harmful gases to a large extent. 1. INTRODUCTION Stockholm Arlanda airport is situated close to Stockholmsåsen (the Stockholm esker), an extensive glacial formation from the last ice age, se figure 1. Eskers form in general excellent conditions for large scale ATES systems, by having high permeability and favorable boundary conditions. At Arlanda, up sticking impermeable rock ridges is cutting the esker into several separate hydraulic sections. The deepest section forms a natural trough that is remarkably suitable for an ATES application. With eleven high capacity wells (5 cold and 6 warm wells) and a total maximum flow of 720 m 3 /h, the system can provide the airport with heat and cold at capacity of at least 10 MW. In terms of thermal energy, up to 20 GWh annually is expected to be produced. The storage temperature on the warm side is expected to be around 20 o C while the average temperature on the cold side is some 5 o C. All energy stored in the system will be natural cold and heat. The project, that is administrated by LFV (the Swedish Civil Aviation Administration), started in early Since then it has been developed in several steps with more and more extensive site investigations. In parallel a far-reaching permit procedure has been carried out, ending with a conditioned permit for realization in early After that the project has been detailed design and the wells been drilled and tested. It is expected to have the system put into operation in the early summer In this paper the ATES system is principally presented as well as the site investigations needed to develop the project. Furthermore, the legal procedure and the conditions for the permit are briefly given.

2 2. THE SYSTEM IN OVERVIEW The ATES system consists of two groups of wells, forming a warm group in the southern part of the aquifer, and a cold group in the northern part. The wells are connected to pipe system ending at the distribution centre where heat and cold from the ATES system is transferred over to a distribution pipe through a large plate heat exchanger. The geographical system lay-out is shown in figure 1 (SWECO 2007). Figure 1: The Långåsen esker (green color) and the locations for the ATES wells and piping. In the distribution centre (Bef. kylcentral) where are a few remaining chillers that will be used for peak shaving of cold, if required. The system is also connected to the lake Halmsjön that may be used for dumping of surplus heat from the warm side of the aquifer, and hence for surplus storage of cold in the winter. The lake was earlier used for dumping of a large amount of condenser heat from the chillers that are now being replaced. The cold side will be charged with natural cold that are mainly derived from out-door air. This is achieved by distributing heat from the warm side of the aquifer to the ventilation systems and to the gates of the airport. At the gates there are ground heating systems for the purpose to keep the gates free of snow. Currently district heating is used for this function. It has been shown that most of the energy needed for gate heating can be replaced by ATES heat at a temperature of +20 o C (Persson 2007). The return temperature from pre heating of ventilation air and gate warming will be in the range of 3-5 o C. In the summer season the flow of the ATES system will be reversed and the stored cold will be utilized for air conditioning. The return temperature to the warm side will in most cases be in the order of o C. However, by using the ground heating systems at the gates as solar collectors,

3 the temperature can be increased to a maximum of approx. 30 o C. In average it is expected to have a storing temperature of approx. 20 o C. As shown in figure 1, there are five cold wells and six warm wells connected to the main distribution central. The mains and the connecting pipes are made of plastic (PE) and are not insulated. They are placed at a frost safe depth of approx. 1,5 m. The dimension of the main pipes is mm and the well connection pipes mm. The total piping length is approx m. The wells are screened and of the type lost filter completion. These types of wells are developed by air lifting, or hydro-jetting, in order to wash out fines and accumulate coarse particles around the wells screen (natural development), see figure 2 (SWECO 2007). Steel Packer with swelling rubber Continuous slotted screen. Stainless steel Slot size 0,5-2 mm Bottom plate with a lifting hook Figure 2: The principal well design (lost filter) and illustration of a natural developed screen Depending on the local hydro-geological condition the well depth varies between m. For the same reason the capacity also varies (30-60 l/s) and so does the dimensions ( mm). The wells are equipped with submersible pumps and with doublets of re-infiltration pipes. To regulate the production flow, the well pumps are all frequency controlled. For controlling and monitoring reasons, the wells are equipped with temperature and pressure sensors. Back valves in the riser pipe and regulating valves in the return pipes will keep the water under hydraulic pressure at all times. Together with air tight well caps, this solution will minimize problems with clogging and corrosion.

4 3. AQUIFER SHAPE AND GEOMETRY Originally, the esker has formed a m wide ridge with relatively steep slopes. However, today this form is only remaining for a shorter distance east of the lake. On other places the esker has been leveled during the construction of the airport. In the project 38 slim steel pipes has been driven down to the bedrock for sampling and spottily hydraulic mapping. Included in the site investigations is also a radar survey in order to map upsticking bedrock bodies. An essential part of the site investigations have been two long term interference pumping tests for estimation of the aquifer hydraulic properties and boundary conditions, and for water chemical analyses. All gathered data has later been the used for technical design and for the permit application. As can be seen from the long section, figure 3, the esker is cut by ribs of the underlying bedrock. These ribs are tight and separate the esker into several hydraulic systems, of which the lowest situated, is used for the ATES plant. This hydraulic system is partly controlled by the lake Halmsjön that has a slight hydraulic contact with the esker. Else the esker is drained to the east, marked by a wet land area with organic soils at the side of the esker. A high level of the bedrock in the middle of the esker will be of favor by forming a natural boundary between the warm and cold side of the aquifer. Figure 3: Long section of the aquifer used for storage (SWECO 2007). It has also been established that the bedrock surface below the esker is eroded and form a minor cigar-shaped depression beneath the esker, clearly shown on the cross sections in figure 4. This is of cause in favor for the project since the bedrock seems to be practically water tight. It is also a favor that the slopes of the esker often are dressed with fine grained sediments (silt and clay) or occasionally with peat deposits. This will prevent larger leakages when the ground water level is increased (at reinjection), but will also decreases the environmental impact on the surroundings.

5 South Nort Middle Figure 4: Typical cross sections at the south, middle and north part of the aquifer of the aquifer (SWECO 2007). By using several conceptual cross sections, the volume available for storage has been estimated to 3, 2 million m 3. From this theoretical gross volume it has been estimated that approx. 2 million will be thermally active with temperatures that are feasible for heating and cooling. However, by overloading this volume may be increased to some extent. 4. AQUIFER PROPERTIES The ground water level in the esker is practically flat and coincidence with the level in the lake. That level is controlled at the lake out-let and stays almost the same the year around. Hence no natural flow disturbing the storage function is at hand. In the north the groundwater level is approx. 7-8 m below the surface, while in the south, the distance is somewhat smaller, around 4-5 m. The smaller distance in the south is a limiting factor for the uplift of the groundwater level during infiltration. For this reason there are six wells in the south in order to spread the infiltration points over a larger area. Mainly based of evaluation of pumping tests the hydraulic properties of the aquifer have been estimated, see table 1.

6 Table 1: Average hydraulic conductivity (k) and the equivalent specific storage coefficient (S/B) for different parts of esker aquifer (SWECO 2007) Part of aquifer k (m/s) S/B (m -1 ) Northern Middle Southern 2, , In the northern part, the fine grained sediments towards east has a estimated k-value in the order of 5x10-4 m/s, while the boundary towards the lake in west is estimated to be ten times less. In rest part of the esker the k-values of the side sediments has been estimated to 2 x10-5 m/s. These k-values indicate that the esker boundaries will only marginally disturb the storage function. During the pumping tests the temperature of extracted groundwater was continuously measured to be practically constant at 8,0 o C. The heat capacity of an esker is strongly related to the porosity. The porosity has not been a subject for analyses, but by experience a value of 30 % has been used, indicating a heat capacity in the order of 0.8 kwh/m 3 x o C. The thermal conductivity of the esker, especially the overlaying dry sand and gravel, will mainly affect the energy losses from the storage. The value used in the project is 0,5 W/m x o C for dry sand and 1,5 for the wet fine grained sediments at the sides of the esker. Water chemistry is directly related to different kind of potential corrosion and clogging problems that may disturb the operation of an ATES plant (Andersson 1992). During the pumping tests several samples were taken and analyzed. The results points at a sweet water that in general is slightly oxidized all over the area. The oxidized environment is mainly indicated by a low content of dissolved iron and manganese and that nitrogen is in the form of nitrate. However, there is a significant difference between north and south concerning the content of dissolved carbonates. In the north the hardness is 16 o dh. In the south this figure is 24, and caused by a higher content of magnesia. Also the content of sulphate is much higher in the south, 150 mg/l, compared to 30 in the north. The content of organic material is low, but showed slightly increased values with time during both the pumping tests. Based on the current chemical and physical status of the water no severe problems with corrosion, clogging by iron or manganese precipitates, or scaling are expected. 5. SIMULATION OF THE OPERATION In order to study the draw down and up-lift in the aquifer and its surroundings the operation has been simulated by a 3D hydraulic model (MODFLOW). The simulations has been carried out for the maximum flow rates in both directions (the summer and winter modes). The summer situation at a flow of 270 m 3 /h (200 l/s) is principally illustrated in figure 5. At this mode the maximum draw down around the cold wells will be approx. 2,5 m, while the up-lift cone around the warm wells would be practically the same. The simulations are done till stationary conditions have been reached in the aquifer.

7 Figure 5: Result of hydraulic simulation of the summer mode. The spreading of heat and cold in the aquifer is principally illustrated (SWECO 2006). The simulation for the winter mode with pumping from the warm wells and injection in the cold wells show that the draw down around the warm wells will be in the order of 3,5 m, while the uplift cone around the cold wells corresponds to the draw down at discharge. The difference between draw down and uplift values in the south reflects the narrow trough shape of the esker here. The draw down and uplift cones follow the elongated boundaries of the aquifer and it was clearly shown that both the sides and the rock ridges act as negative hydraulic boundaries that limits the influence area substantially. It was also shown that Lake Halmsjön has a limited influence on the hydraulics, probably due to fine grained sediments at the bottom of the lake. 6. PERMIT PROCEDURE According the Environmental Act from year 2000, projects of this kind need a permit given by the Environmental Court. The Act also demands an early consultation with local authorities prior to the final permit application. The main documents required for the permit procedure are a technical system description (TB), showing the locations and type of wells etc., and an environmental assessment report (MKB) describing the potential environmental impacts on the surroundings, all environmental and health aspects included. The influence area is defined as an area within the changes of the groundwater level is larger than 0,3 m. Landowners within this area are regarded as being stakeholders that have the right to resist a permit. The court decides weather or not the resistant is relevant and may cause any damages. If so the applicant is forced to pay for such damages. For the Arlanda ATES project the early consultation with local authorities and stakeholders started in spring The final application to environmental court was sent in early summer 2007 and the permit was given in early autumn In the permit it is concluded that the

8 environmental impact is expected to be limited and that the project is not in violation of other interests within the influence area. It is also said that the value of the project (from a national economic point of view) is larger than the costs for any damages the project may cause. In order to trace unforeseen damages and to check if the applicant follows the conditions given in the application, the court is prescribing a monitoring program for number of years. In this case this program covers, flow rates, groundwater levels, temperatures, water chemistry and affects on the flora east of esker. The results are to be presented once a year to the local environmental authority, which also decides on duration and changes over time. 6. CONCLUSION The Arlanda ATES project has been developed in several steps over quite a long period of time. The main reason for this procedure is the parallel handling of technical and permits related items. This procedure recommended for any large scale Swedish ATES project. The duration may vary, but normally it takes approx. 2 years from starting a project to have it realized and put into operation. In order to have a proper technical design and thorough environmental assessment analyses made in the Arlanda case, extensive hydro-geological site investigations had to be carried out. This is something that is relevant for any equal project in Sweden. Specific for ATES projects, the water chemistry related to operational problems (corrosion and clogging) is of uttermost importance to consider. In the Arlanda case, the water quality seems to be favorable. However, long term problems can not be excluded. The predicted environmental impact is another important issue to consider in an early stage. In the Arlanda case the esker showed to have a favorable geometry that seems to limit the impacts to the surroundings. Instead, the project is judged to be environmental favorable due to the drastic cut harmful emissions it provides. REFERENCES SWECO (2007). Aquifer Storage at Långåsen, Arlanda Stockholm Airport. Permit Application. SWECO, Malmö, May 11, 2007 (in Swedish). Persson, P. (2007). Analyses of ground heating systems at Arlanda Airport. Optimization regarding the planned Aquifer Storage. Thesis for Master of Science. Lund Institute of Technology, Dept. of Technical Geology. June 2007 (in Swedish with summery in English). Andersson, O. (1992). Scaling and Corrosion. Annex VI Environmental and chemical aspects of thermal energy storage in aquifers. Swedish Council for Building Research. Document D12:1992.