Geothermal use of pit water for local heating supply

Settlement summary

Project status	Realisation
Location of network	Zwickau urban region, campus of Westsächsische Hochschule Zwickau (WHZ)
Location of local community	Stadt Zwickau, 08056 Zwickau, Landkreis Zwickau, Sachsen
Settlement in figures	Area: 102,6 km²; inhabitants: 93.517
Developer, organizer	Public Enterprise Saxon Property and Construction Management, Subsidiary Zwickau
Network operator	Zwickauer Energieversorgung (ZEV)
Generation	Geothermal energy
Heating system	low temperature heating in a new laboratory building – currently fed using district heating, via a decentral heat pump in future
Network length	approx. 0.25 km
Heat customer, cooling energy customer	Westsächsische Hochschule Zwickau (WHZ)
Project themes	New buildings, Decentralised solutions, Heat and cold tranfer, Local heating and cooling networks, Renewable energey sources

Project description

The former Zwickau mining region was the most important hard coal mining region in Saxony. During the active mining phase, which lasted several centuries, numerous underground voids with an estimated volume of up to 40 million m³ were excavated. When active mining ceased in 1978, the up to 1,200-m deep pit structure was flooded in the 1980s and 1990s. After the production of hard coal, the voids, in particular in the area of the planned drilling sites, were filled using a variety of backfilling technologies and materials. The residual void volume in the pit structure of the Zwickau region is estimated to be approx. 5 million m³.

In the studied area, of the original 2.5 million m³ of void, a residual volume of approx. 250,000 m³ remained after backfilling and subsidence. The flooding of the pit structure and the associated rise of the flooding water level caused an uncontrolled mixture of deep water from the Carboniferous rocks with layers close to the surface. The pressure potential of the flood water is near the upper surface layer. In hollows, deep water leaks out at the mine surface via diffuse water exits.

Project targets and tasks

The project is intended to research the as yet largely unknown reservoir properties of the existing mine structure. They form the basis for an economic use of the geothermal potential and in future will provide thermal energy for the new building of Laboratory I and for further planned new buildings for Westsächsische Hochschule Zwickau (WHZ). Under corresponding conditions, there is an additional option of geothermal use for further locations in the city centre of Zwickau and the southern suburbs. The advantage of this test site is a close connection of possible consumers with the energy reservoir directly under the city. There are no comparable uses as yet.

For a realistic estimation of the usability of the geothermal potential, information on hydraulic parameters such as the specific confined groundwater level, the permeability of the remaining voids in the Carboniferous rocks and on continuous inflow quantities and temperatures is required. This data can only be determined via hydraulic tests in the vicinity of the planned development level. Investigative drilling into the former track level is required for this. An appropriate drilling point was already planned taking the geological and hydrogeological conditions as part of the EnEff:Wärme project “Fernwärmeversorgung unter Nutzung der Wärme aus Grubenwässern“ (district heat supply using the heat from pit water) by Westsächsische Hochschule Zwickau (WHZ).

Work plan

• Geological and hydrological studies to determine the pit water and to investigate the mine abandoned in the 1970s: Findings for detailed planning of the project will be provided and conclusions for further development of pit water as an energy source in the city of Zwickau will be drawn. The research borehole will be extended to a breakage zone of the mine structure. Studies on the pump conditions and the properties of the remaining pit water will be carried out. Positive results from drilling and hydrogeological tests can provide a starting point for less complex development technologies, which can also be used in the inner city area.
• Use and field tests of plastic heat exchangers adapted to the chemical properties of the pit water which is available as a heat source: plastic heat exchangers can use materials with a variety of configurable properties in terms of chemical and mechanical resistance. They are durable and less expensive to produce than heat exchangers made of valuable metals. The heat exchangers will be developed as part of the project funded by the German Federal Ministry of Economics and Technology “Energieeffiziente Fernwärmeversorgung unter Nutzung der Wärme aus Grubenwässern und anderen natürlichen Wärmequellen” (energy-efficient district heating supply using heat from pit water and other natural heat sources) and tested in the laboratory. The pilot project advances and completes methodical planning approaches and tools for heat exchanger design.
• Development of a system for utilising and distributing heat at a low temperature level for building air conditioning: In the development of piping systems for a local heating network at a low temperature level, the use of suitable and low-cost pipe materials, minimal thermal losses via thermal insulation and cost-effective, mechanical laying are the primary concerns.
• Design of an operating system for the future heat distribution system and strategies for process control of systems with multiple sources and hollows.
• Optimisation of the operation according to exergetic and economic criteria.

Planning and implementation is to be performed according to a work schedule which is coordinated with the WHZ research project. This allows findings from the hydrogeological studies to influence system development.

Preparatory planning, implementation

In comparable research projects like Heerlen in the Netherlands, titanium plate heat exchangers are used to withdraw heat from the pit water and subsequently feed it back into the pit structure via an injection well. The heat is fed to the consumer in distribution lines with different temperatures. By contrast, the design for Zwickau focuses on plastic heat exchangers and heat distribution at a low temperature level. Decentral heat pumps are used to create the currently required temperature level. The heat network can be used as a source and a sink.

The regional energy supply company Zwickauer Energieversorgung (ZEV) will operate the energy supply system including the heating network which will be created in the pilot project. The supply network is designed as a two-pipe system of underground pipelines – as “cold district heating“ (temperatures for flow and return of 23/10 °C).

In order to prepare the planning, a study was commissioned on the energy-related and ecological evaluation for thermal use of pit water in district heating supply to the city of Zwickau. In this study, the energy consulting company SEF reveals that the operating variant with a 150-kW heat pump for supplying heat to a new laboratory building for WHZ using the pit water has the lowest annual CO2 emission compared to other studied variants. The second key finding is that the operation of a local heating line at a low temperature level for the supply of the laboratory is economical if the pit water transfer is implemented as an interface of the balancing circuit.

Energy characteristics

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