Manage_GeoCity - Development of a method for the coordinated management of geothermal energy in urban areas

Starting point/Motivation

Due to the numerous heat sources in urban areas (e.g. underground construction) ground water temperatures and subsurface temperatures increase („subsurface heat island"). In some cases this temperature increase has negative impacts on the ground water quality. On the other hand these relatively high subsurface temperatures offer a potential for the use of shallow geothermal energy for heating and cooling. If this heat source is used properly (e.g. heat extraction for heating purposes), a cooling effect of the underground can be achieved. Especially in urban areas the uncoordinated use of underground heat by a lot of small installations could lead to mutual interferences. The energy use might become inefficient and unsustainable. Some cities already discuss the prohibition of the energetic use of groundwater for cooling purposes.

This situation can be improved by a coordinated use and management of different geothermal heat sources, taking into account the thermal preload of the subsurface and framework conditions of water management. Furthermore a coordinated management of geothermal heat sources leads to an efficient and sustainable use of shallow geothermal energy from the subsurface of urban areas.

Contents and Objectives

The aim of the project was to develop a methodology for the coordinated use and management of shallow geothermal energy for heating and cooling as well as seasonal storage in urban areas. Using case studies in the model region Graz the following aspects are considered: ground water flow, different geologic conditions, heating and cooling demand, heat input from solar collectors and industrial waste heat, and the possibilities of seasonal heat storage in the subsurface.

Methods

The development of the methodology was based on the urban areas of the model region Graz, due to the existing data basis for the subsurface. Subsurface favorable geothermal areas for the use of geothermal energy with and without groundwater were identified.

Case studies within these subsurface favorable geothermal areas were selected. For these case studies the heating and cooling demand were analyzed. The energy demand and the subsurface energy potential were compared and the possibilities of seasonal storage investigated. The case studies were techno-economically and environmentally evaluated and for selected areas extrapolated.

Simulation was performed for these selected areas by use of an existing instationary ground water flow model combined with a heat model. The influence on subsurface temperatures by heat extraction and storage was analyzed and the influence on the subsurface favorable geothermal areas investigated. Considering the current management of geothermal and water resources, optimization measures and utilization concepts were developed.

Based on the results of the model region Graz the methodology for the coordinated use and management of shallow geothermal energy was developed. The methodology comprises a procedure for an improved implementation and optimized utilization of shallow geothermal energy projects. Its basic system is flexible enough to enable a transfer of the methodology to other urban areas.

In the project advisory board the methodology, assumptions and results were discussed and presented to relevant stakeholders (e.g. city planners, energy utilities, administrative authorities).

Results

The overall aim of the project was the development of the methodology for a coordinated use and management of shallow geothermal energy for heating and cooling, which is the basis for future use and management plans for cities and urban regions. The methodology is divided into 6 sections: (1) New energy system – starting point, (2) subsurface characteristics, (3) System definition, (4) System dimensioning, (5) Evaluation, (6) Optimization and Implementation.

The work on three case examples in the model region Graz showed, that basic data, which is needed to determine heating and cooling demand of urban areas, are available from different sources. However, data quality varies.

The simulation results on two case studies using groundwater as geothermal source show, that the groundwater in subsurface urban heat islands can be cooled downed by using heat pumps for heating. The simulation results on the case study with bore hole heat exchangers show that the spatial influence on subsurface temperature is limited. Temperature changes are insignificant 50 meters away from the bore hole heat exchanger field. Hydrochemical modelling showed that temperature changes in the range of ΔT ≤10°C lead to very minor changes in the hydro chemistry. However, the maximum permitted temperature range should account for the physical, chemical and biological state of the affected ground water.

Results from the environmental analysis show, that the type of electricity generation strongly influences total greenhouse gas emissions of geothermal heat pump systems. In the scenarios were the Austrian electricity (including a high share of renewables) was used a greenhouse gas emissions reduction from 75% to 85% is reached compared to other heating systems with fossil energy. If the electricity is generated by a combined cycle natural gas power plant the reduction is less or even no reduction can be achieved.
The results on the economic evaluation (calculation of yearly heat generation costs) of the case studies show, that compared to the heat generation in large scale natural gas boilers (40 €/MWh) an optimization of the energy concept is needed. These refers to the reduction of investment costs (amount and depth of bore hole heat exchangers, cheap heat sources for loading the subsurface, detailed data for the design of the heat pumps) and the temperature level needed in the building. The temperature level of the heat supply in the building (approx.. 85 to 95°C for injection into the district heating grid and existing buildings, approx.. 35°C for new buildings) significantly influence the efficiency of the heat pump and the electricity costs.

Prospects / Suggestions for future research

Future research questions are the development of a central „energy demand database" as basis for spatial energy planning, implementation of heat input from subsurface buildings in ground water modelling, investigation of new ground water formation in urban areas and accompanying research work at underground seasonal storage projects (e.g. monitoring underground temperatures and calibrating simulation models).

Die results on the case studies are the basis for future demonstration activities. The next step is the investigation of additional possibilities for the technical system design and its optimization.