Geothermal energy can be extracted from an aquifer, where the groundwater is used as heat exchange medium while heat and cold are stored in the surrounding material in the aquifer and to some extent in the groundwater. Application of aquifer storage for the use of geothermal energy is mainly used in large scale facilities and is limited to sites with suitable aquifers in the form of ridges, sandstone and limestone aquifers. Löwenströmska hospital in the municipality of Upplands Väsby, north of Stockholm, is located nearby the northern part of the Stockholm esker. This means that it can be profitable and environmentally beneficial for the hospital to examine the possibilities of aquifer storage in the esker material next to its property. The purpose of this master thesis has been to investigate if geothermal energy storage with a seasonal storage of heat and cold can be applied within Löwenströmska hospital’s property area using groundwater modeling. A hydraulic groundwater model was constructed in MODFLOW based on a simplified conceptual model of the groundwater system. The hydraulic groundwater model was calibrated and validated against observed groundwater levels before and after a pumping test. The hydraulic groundwater model was then used to implement a fictitious geothermal energy storage with MT3DMS. MT3DMS is a modular function used with MODFLOW, which can be modified to simulate heat transport. The result shows that the geothermal energy storage can store seasonal heating and cooling of about 4 GWh, which covers 85 % of the hospital’s heating demand with an assumed SP-factor of 4, and the entire cooling demand. To cover 50 % of the peak heating power it was calculated that a flow of 63 l/s was needed, and according to the model this is possible. The geothermal energy storage does not need to be completely in energy balance, since the aquifer is recharged with its natural groundwater. The location of the wells influences which flows that are needed to create energy balance. A too close placement of the wells leads to a thermal breakthrough. The hydraulic conductivity of the esker material affects the amount of energy that can be stored. A higher hydraulic conductivity provides greater energy losses and a lower hydraulic conductivity favors the energy storage but gives a greater influence area. A number of assumptions have been made in the model construction of the hydrogeological model and further investigation of the geological and hydrogeological conditions are desirable to improve the model.

<p>When a combined heat and power plant produces heat and power it often faces a deficit of heat load during the summer or other periods of time. This heat is often unnecessarily cooled away or the power production has to be reduced or shut off. If it is possible to store heat from periods with low heat demand to periods with high heat demand one can get many benefits. Among these benefits are: increased power production, decreased operation with partial load, uniformly distributed load.</p><p>To be able to store heat in situations like this long-term thermal heat storages are needed. In this thesis five different types of stores are presented: rock cavern storage, tank storage, pit water storage, borehole storage and aquifer storage. In this thesis the principles of the different storages is presented and experiences from operation in Sweden, Germany and Denmark are also presented.</p><p>The thesis contains a calculation of costs for the types of thermal heat storages that are suitable for use in a combined heat and power plant. To be able to function in a combined heat and power plant, a long-term thermal heat storage must be able to handle a high charge and discharge output. Storages that can meet these demands use water as store medium.</p><p>The conclusion is:</p><p>Pit storages are interesting if the capacity is below 20 000 m^3.</p><p>For capacities between 20 000 to 50 000 m^3, tank storages are most suitable.</p><p>Rock cavern storages are interesting if the capacity is larger than 100 000 m^3.</p><p>For capacities between 50 000 to 100 000 m^3, either rock cavern storages or connected tank storages are appropriate.</p>

When a combined heat and power plant produces heat and power it often faces a deficit of heat load during the summer or other periods of time. This heat is often unnecessarily cooled away or the power production has to be reduced or shut off. If it is possible to store heat from periods with low heat demand to periods with high heat demand one can get many benefits. Among these benefits are: increased power production, decreased operation with partial load, uniformly distributed load. To be able to store heat in situations like this long-term thermal heat storages are needed. In this thesis five different types of stores are presented: rock cavern storage, tank storage, pit water storage, borehole storage and aquifer storage. In this thesis the principles of the different storages is presented and experiences from operation in Sweden, Germany and Denmark are also presented. The thesis contains a calculation of costs for the types of thermal heat storages that are suitable for use in a combined heat and power plant. To be able to function in a combined heat and power plant, a long-term thermal heat storage must be able to handle a high charge and discharge output. Storages that can meet these demands use water as store medium. The conclusion is: Pit storages are interesting if the capacity is below 20 000 m^3. For capacities between 20 000 to 50 000 m^3, tank storages are most suitable. Rock cavern storages are interesting if the capacity is larger than 100 000 m^3. For capacities between 50 000 to 100 000 m^3, either rock cavern storages or connected tank storages are appropriate.

Groundwater is potentially a useful source for storing and providing thermal energy to the built environment. In a nordic context, aquifer thermal energy storage, (ATES) has not been subject to a wider extent of research concerning environmental impact. This thesis intends to study the impact on groundwater chemistry from an ATES that has been operational since 2016 and is located in the northern part of Stockholm, on a glaciofluvial deposit called the Stockholm esker. Analysis of groundwater sampling included a period of 9 months prior to ATES operation as well as a 7 month period after operation and sampling was conducted in a group of wells in vicinity of the installation and within the system as ATES operation began. Means of evaluation constituted a statistical approach which included Kruskal-Wallis test by ranks, to compare the ATES wells with the wells in the surroundings and principal component analysis, (PCA), to study the chemical parameters that could be related to ATES. In addition, a geophysical survey comprising 2D-resistivity and induced polarization, (IP) was done to elucidate whether the origin of high salinity could be traced to nearby possible sources. The analysis was based on foremost the cycle of cold energy storage. The results showed large variations in redox potential, particularly at the cold wells which likely was due to the mixing of groundwater considering the different depths of groundwater being abstracted/injected from different redox zones. Arsenic, which has shown to be sensitive to high temperatures in other research showed a decrease in concentration compared to surrounding wells. There were found to be a lower specific conductivity and total hardness at the ATES well compared to their vicinity. That indicates that they are less subject to salinization and that no accumulation has occurred to date.  It is evident that the environmental impact from ATES is governed by the pre-conditions in  soil- and groundwater. / Grundvatten har förutsättningen att utgöra en värdefull resurs för att lagra och förse byggnader med termisk energi. I en nordisk kontext har termisk energilagring i akviferer, (ATES)  inte varit föremål för någon bredare forskning angående miljöpåverkan. Denna uppsats syftar till att studera kemisk grundvattenpåverkan från ett ATES som togs i drift 2016 i norra Stockholm, i en isälvsavlagring vid namn Stockholmsåsen. Analysen omfattar grundvattenprovtagning 9 månader före ATES driften samt 7 månader efter driftstart och provtagningen genomfördes i ett antal brunnar i närheten av installationen samt i ATES systemet då driften startade. Utvärderingsmetoden bestod av ett statistiskt tillvägagångssätt och omfattade Kruskal-Wallis test by ranks, för att jämföra ATES brunnarna med omgivande brunnar och principal component analysis, (PCA), för att studera kemiska parametrar som kan kopplas till ATES. I tillägg genomfördes en geofysisk undersökning som omfattar 2D-resistivitet samt inducerad polarisation, (IP) för att klarlägga huruvida källan till den höga saliniteten kunde spåras. Analysen baseras på främst på cykeln då kyld energi lagras. Resultaten visar stor variation i redoxpotential, i synnerhet vid de kalla brunnarna vilket sannolikt beror på omblandning av grundvatten med tanke på en differens i djup som grundvattnet infiltrerar/pumpas från med tillhörande skillnad i redox zon. Arsenik vilket har visat sig känsligt för höga temperaturer i annan forskning visade minskade koncentrationer jämfört med omgivande brunnar. ATES brunnarna uppvisade även lägre specifik konduktivitet och totalhårdhet i jämförelse. Det pekar mot att brunnarna är mindre utsatta för salinitet och att ingen ackumulering har skett till dags dato. Det framgår tydligt att miljömässig påverkan från ATES styrs av grundförutsättningarna i mark och grundvatten.