To date, the burning of coal is responsible for 25% of total annual global greenhouse gas emissions. Still, coal mines may after all contribute to a sustainable future.
Geothermal regions such as Iceland and New Zealand (photo) have been capitalizing their volcanic landscapes for many years by using hot fluids for warming and bathing. The earth is an inexhaustible source of heat, providing the presence of water to be heated. Hot fluids are relatively exceptional and many cities have been built in the vicinity of coal mines.
A green opportunity?
After the closure of many coal mines in the last decades, its shafts and galleries have been filled with water. The temperature of this water has adapted to the environment: the deeper, the warmer, ranging from 10oC just under the surface to 30oC at a depth of 700 meters.
Some scientists believe that this water has significant potential to contribute to our need for sustainable energy, thus providing the old mines with a green image. The Durham Energy Institute started experiments in several places. In the United Kingdom alone, 15 billion tons of coal have been extracted in the last century, giving room for two billion cubic meters of water at temperatures of 12-16°C. This is the equivalent of 38,500 terajoules of heat, enough to supply the need for warm water of 650,000 homes in the UK, under the assumption that the underground water keeps its temperature.
A study done by researchers of McGill University in Montreal, Canada, estimates that each kilometer of mine galleries has a power of 150 kW. Interesting, too, is a study of the potential contribution of flooded mines to the production of energy by deploying a modelling approach. A relevant but somewhat disappointing finding in this study is that the temperature of the water will decline 7 – 8°C over a period of 50 years, given that the whole underground water reservoir will circulate within one year.
The Ropak packaging plant in Springhill, Nova Scotia, has successfully been deploying mine water since 1998. Mine water at a temperature of 18°C is pumped from a flooded coal mine at a rate of 4 liters per second. It then passes through a heat pump system before reinjection into another separate (but linked) mine shaft takes place (see picture below). As a consequence, it is a completely closed loop system. Compared to conventional systems, the company yearly saves C$ 45,000 or the equivalent of about 600,000 kWh.
Wells providing Ropac Company with warm water – Centre for the Analysis and Dissemination of Demonstrated Energy Technologies
Another case study is available from Asturias (north-west Spain) where a hospital and a university building are heated successfully using coalmine water.
However, the most interesting and innovative field experiment is located in the Netherlands, in the municipality of Heerlen.
The evolution of the use of water from former coal mines in Heerlen (the Netherlands)
About 15 years ago, the city of Heerlen started a pilot to investigate the value of the water in an abandoned coal mine as a geothermal source for heating and cooling of buildings. A few years later, a company to exploit this resource was founded: Mijnwater B.V.
Five wells were drilled: two for warm water, two for cool water and one for the return of water (figure). A three-pipe distribution network of seven kilometers, the so-called backbone, provided the buildings and houses with energy.
The warm- and cold-water buffers of Mijnwater te Heerlen. Figure from: Weg van gas: Kansen voor de nieuwe concepten LageTemperatuurAardwarmte en Mijnwater, CE Delft 2018.
It soon became clear that the water that was pumped back into the mine influenced both the temperature of the hot water and the cold water, conforming the predictions of the aforementioned Polish modelling study.
This challenged the company to revise its concept into a new one: Mijnwater 2.0. The main principles of this renewed concept are:
- Instead of delivering lukewarm mine water, the exchange of energy between buildings deploying a low-temperature network became the main driver of the project;
- The flooded mine will be deployed as a warm and cold storage buffer for water upon request;
- In each of the connected buildings, the water will be heated up or cooled down all the way to the geothermal temperature before it is brought back to the corresponding hot or cold part of the mine water reservoir. Therefore, all wells become bidirectional;
- The return pipe (orange pipe in the figure) will no longer be needed and will be deployed for the supply and disposal of additional hot or cold mine water, as to enlarge the hydraulic mine water capacity;
- The distribution system will be layered. At first, buildings within the same cluster will exchange surpluses of warm and cold water. Next, clusters will mutually exchange their surpluses. At last, the backbone will provide the clusters with water from the underground buggers (figure below);
- The control over the heat and cold supply at any time will be fully automated; the provision of hot and cold water is completely demand-oriented;
Clusters and backbone – Figure from: Weg van gas: Kansen voor de nieuwe concepten Lage Temperatuur Aardwarmte en Mijnwater, CE Delft 2018.
At this moment, hundreds of households and about ten companies are served by the network. Contracts have already been signed for the addition of another 1000 households.
Both individual and collective heat pumps are deployed to increase the temperature of the water from 25oC to 60oC. This enables heating of modestly isolated houses without implementing major changes in their heating system. This is a significant added value to all-electric solutions, which require extensive isolation of houses, as heat pumps that use air deliver low temperature water of 30°C.
The evolution of the last 10 years changed the system from the direct deployment of water from a coal mine, into a ‘smart thermal grid’ deploying excess heat and cold of participating companies. The mine water functions as buffers of 28oC and 16oC. Because of its decreased use, the temperature of the mine water will be constant over years.
The extension of the Mijnwater-concept with low temperature geothermal heat
The concept of Mijnwater 2.0 has been studied extensively by CE Delft. The conclusion was that about 4,6 millions of households in the Netherlands, as well as many companies can be served by a concept like this, if it is combined with the deployment of low temperature geothermal heat.
The video below demonstrates the harvesting of low temperature geothermal heath.
The deployment of low temperature geothermal heat (extracted between 250 – 1250 meters below surface) is a welcome supplement to the better-known extraction of geothermal heat from depth between 1250 – 4000 meters, which requires much more complex and expensive drilling. The drilling of low temperature geothermal heat is further simplified by so-called horizontal drilling. The advantage of this is that drilling of puts usually happens at the same place and that the water output is increased (figure below).
A comparison between standard geothermal drilling and drilling for low temperature geothermal sources – Drawing: Visser & Smit Hanab.
Smart thermal grids (Mijnwater 2.0) together with the extraction of low temperature geothermal heat have four characteristics:
- Heat and cold exchange between users within and between clusters;
- Connecting clusters with buffers of warm and cold water (like mines or any other underground reservoirs of isolated layers);
- Extracting low temperature geothermal heat within the clusters;
- Advanced software that controls demand and supply of energy.
The attractivity of this concept is fourfold as well:
- Its scalability
Individual clusters might be starting points, connecting backbones might follow.
- Availability of heat sources
The proven availability of low temperature heat sources in the underground in contrast to other geothermal sources (below 1250 meters) and the relative ease of their extraction
- No more natural gas
Renouncing the use of natural gas, deploying heat pumps in modestly isolated houses and deploying existing heating systems, thus lowering costs
The deployment of buffers to store surpluses of energy (for example electricity after having cooled or heated water).
What about coal mines?
Straightforward use of water from former coal mines is risky, due to the possibility that the water temperature will drop in case of increase of usage. Former coal mines in the vicinity of places with substantial demand can be used as a buffer in smart thermal grids storing both warm and cool water. However, the value of the concept of smart thermal grids goes beyond the areas where coal was mined in the past, given natural or artificial storage opportunities.
 Zbigniew MaJolepszy: Modelling of geothermal resources within abandoned coalmines, Upper ˜˜Silesia, Poland. Faculty of Earth Sciences, University of Silesia.
*) Source of header: Hot springs – Photo: Michael Bower (Pexels)
**) This article was brought to you by Professor Herman van den Bosch, Professor at Open University of The Netherlands.