Disused coal mines could help decarbonise our heating. Here’s how

A disused coal mine in Herten, western Germany, in 2013. Image: Getty.

Fossil fuels currently dominate the production of electricity and heat. Although renewable energy accounts for around a quarter of electricity produced in the UK, the production of central heating is dominated by natural gas, which supplies around 70 per cent of UK heat demand (the UK has been a net importer of gas since 2004).

There are fewer low carbon alternatives for heat production than there are for electricity. Solar hot water and biomass are the two main touted alternatives. Solar hot water is usually produced at a domestic level and requires access to a south facing roof. Biomass can be used as a heat source but may be constrained by availability and the transportation of fuel. And so it is unclear how future heat demands could be met from low carbon sources.

Geothermal heat is one solution that offers a low carbon, secure and continuous energy source. Classic geothermal regions such as Iceland and New Zealand capitalise on their volcanic landscapes by capturing the steam and hot fluids produced as a result of tectonic activity. Geothermal fluids in the UK are over 100°C and hot enough to drive turbines, produce electricity and also supply heat. Also, geothermal fluids may issue naturally at the surface as hot springs and geysers, avoiding the need to drill to access them.

Of course, the UK is not characterised by such tectonic activity. But we believe that abandoned deep mines contain good geothermal potential.

Geothermal potential

The deeper you drill into the Earth, the warmer it gets. Geologists call this the Earth’s “geothermal gradient”, driven by heat produced at the Earth’s core that radiates towards the crust. In non-tectonic areas, temperatures increase on average by around 25°C per kilometre. This means we can predict what temperature may be encountered at any specific depth.

But extracting geothermal energy from these warm depths is only possible if water is present and is able to flow from the rock. Heat and water flow are essential for extracting geothermal energy.

Abandoned coal mines, therefore, seem incredibly promising due to their networks of flooded galleries and shafts lying at depths of up to several hundred metres below the surface. One can be almost certain that the water flow necessary for deep geothermal wells will be found in these flooded underground voids. The risk of not finding flowing water underground can inhibit deep geothermal developments elsewhere.

Vast volumes (over 15bn tonnes) of coal have been extracted from deep mines in the UK over the last century. To put this into context, if this extracted coal were spread over the UK land surface, this would result in a five cm deep layer of coal across the country. Today, UK coal production from deep mining has declined to almost zero and the nation celebrated its first coal free day of power generation in April 2017.

Eco-friendly coal mines

Think about this. The volume of coal extracted compares to an equivalent void volume underground. On this basis (once allowing for subsidence), we estimate that the abandoned mines of the UK contain around 2bn cubic metres of water at temperatures which are constantly around 12-16°C, and in some instances higher still. If heat corresponding to a 4°C temperature drop was removed from this volume, around 38,500TJ of heat could be liberated. This conservative estimate could provide enough heat for around 650,000 homes nationally.

Clearly, you wouldn’t want to take a bath or heat your home with water at these low temperatures, but using a heat pump, the water temperature could be upgraded to more useful temperatures of 40-50°C.

A heat pump takes energy from a source such as water within an abandoned mine and “lifts” it to a more useful temperature. You can think of it working like a fridge: if you put food at room temperature in a fridge, after a while it will be cooled to the temperature of the fridge. The heat removed from the food is lost from the back of the fridge, which is why this area feels warm. The radiators in a home are effectively the same as the back of the fridge. A heat pump uses electricity to boost the temperature but for every kW of electrical power used, the heat pump will produce three to four kW of heat. This is why heat pumps are a low carbon alternative to gas boilers.

Next steps

So we know that the UK has sufficient potential for geothermal heat production in its extensive mines. The next consideration, then, is proximity to the heat demand. Given the low temperatures involved, the heat source needs to be close to the end user to minimise losses. Many UK towns and cities grew due to their coal reserves, meaning that centres of heat demand and areas of abandoned mines often coincide, making them ideal targets. The UK government’s fifth carbon budget sets out plans to decarbonise heat by stating that one in 20 homes should be connected to a heat network by 2030. This is an ambitious challenge but abandoned mines could make a significant contribution here.

Minewater district heating schemes have already been successfully developed at several locations. One early example was developed at the Ropak packaging plant in Springhill, Nova Scotia, in 1998. A minewater and heat pump system that uses minewater at 18°C provides heating and cooling for the 13,500-square-metre site leading to huge savings in avoided fuel-oil costs. And at Heerlen in the Netherlands, a larger scheme has been operating since 2008, supplying heat to 500,000 square metres of commercial and residential buildings. Areas planned for new development in former mining areas make ideal targets as they provide an opportunity to incorporate the necessary above ground infrastructure.

But if coal mines are to decarbonise heat, we need to deploy these systems in more places. Is this possible? We think so. The fact that many coalfields are overlain by urban centres means that there is certainly good potential for many former mining areas. Although abandoned mines provide a lower temperature resource than deeper geothermal sources, they are systems known to flow copious quantities of relatively warm water and provide a readymade subsurface store of heat.

The ConversationThere is a delightful irony that the legacy of the dirtiest of fuels, coal, now has the potential to deliver a low carbon energy future.

Charlotte Adams and Jon Gluyas, Durham University.

This article was originally published on The Conversation. Read the original article.


The IPPC report on the melting ice caps makes for terrifying reading

A Greeland iceberg, 2007. Image: Getty.

Earlier this year, the Intergovernmental Panel on Climate Change (IPCC) – the UN body responsible for communicating the science of climate breakdown – released its long-awaited Special Report on the Ocean and Cryosphere in a Changing Climate.

Based on almost 7,000 peer-reviewed research articles, the report is a cutting-edge crash course in how human-caused climate breakdown is changing our ice and oceans and what it means for humanity and the living planet. In a nutshell, the news isn’t good.

Cryosphere in decline

Most of us rarely come into contact with the cryosphere, but it is a critical part of our climate system. The term refers to the frozen parts of our planet – the great ice sheets of Greenland and Antarctica, the icebergs that break off and drift in the oceans, the glaciers on our high mountain ranges, our winter snow, the ice on lakes and the polar oceans, and the frozen ground in much of the Arctic landscape called permafrost.

The cryosphere is shrinking. Snow cover is reducing, glaciers and ice sheets are melting and permafrost is thawing. We’ve known this for most of my 25-year career, but the report highlights that melting is accelerating, with potentially disastrous consequences for humanity and marine and high mountain ecosystems.

At the moment, we’re on track to lose more than half of all the permafrost by the end of the century. Thousands of roads and buildings sit on this frozen soil – and their foundations are slowly transitioning to mud. Permafrost also stores almost twice the amount of carbon as is present in the atmosphere. While increased plant growth may be able to offset some of the release of carbon from newly thawed soils, much will be released to the atmosphere, significantly accelerating the pace of global heating.

Sea ice is declining rapidly, and an ice-free Arctic ocean will become a regular summer occurrence as things stand. Indigenous peoples who live in the Arctic are already having to change how they hunt and travel, and some coastal communities are already planning for relocation. Populations of seals, walruses, polar bears, whales and other mammals and sea birds who depend on the ice may crash if sea ice is regularly absent. And as water in its bright-white solid form is much more effective at reflecting heat from the sun, its rapid loss is also accelerating global heating.

Glaciers are also melting. If emissions continue on their current trajectory, smaller glaciers will shrink by more than 80 per cent by the end of the century. This retreat will place increasing strain on the hundreds of millions of people globally who rely on glaciers for water, agriculture, and power. Dangerous landslides, avalanches, rockfalls and floods will become increasingly normal in mountain areas.

Rising oceans, rising problems

All this melting ice means that sea levels are rising. While seas rose globally by around 15cm during the 20th century, they’re now rising more than twice as fast –- and this rate is accelerating.

Thanks to research from myself and others, we now better understand how Antarctica and Greenland’s ice sheets interact with the oceans. As a result, the latest report has upgraded its long-term estimates for how much sea level is expected to rise. Uncertainties still remain, but we’re headed for a rise of between 60 and 110cm by 2100.

Of course, sea level isn’t static. Intense rainfall and cyclones – themselves exacerbated by climate breakdown – can cause water to surge metres above the normal level. The IPCC’s report is very clear: these extreme storm surges we used to expect once per century will now be expected every year by mid-century. In addition to rapidly curbing emissions, we must invest millions to protect at-risk coastal and low-lying areas from flooding and loss of life.

Ocean ecosystems

Up to now, the ocean has taken up more than 90 per cent of the excess heat in the global climate system. Warming to date has already reduced the mixing between water layers and, as a consequence, has reduced the supply of oxygen and nutrients for marine life. By 2100 the ocean will take up five to seven times more heat than it has done in the past 50 years if we don’t change our emissions trajectory. Marine heatwaves are also projected to be more intense, last longer and occur 50 times more often. To top it off, the ocean is becoming more acidic as it continues to absorb a proportion of the carbon dioxide we emit.

Collectively, these pressures place marine life across the globe under unprecedented threat. Some species may move to new waters, but others less able to adapt will decline or even die out. This could cause major problems for communities that depend on local seafood. As it stands, coral reefs – beautiful ecosystems that support thousands of species – will be nearly totally wiped out by the end of the century.

Between the lines

While the document makes some striking statements, it is actually relatively conservative with its conclusions – perhaps because it had to be approved by the 195 nations that ratify the IPCC’s reports. Right now, I would expect that sea level rise and ice melt will occur faster than the report predicts. Ten years ago, I might have said the opposite. But the latest science is painting an increasingly grave picture for the future of our oceans and cryosphere – particularly if we carry on with “business as usual”.

The difference between 1.5°C and 2°C of heating is especially important for the icy poles, which warm much faster than the global average. At 1.5°C of warming, the probability of an ice-free September in the Arctic ocean is one in 100. But at 2°C, we’d expect to see this happening about one-third of the time. Rising sea levels, ocean warming and acidification, melting glaciers, and permafrost also will also happen faster – and with it, the risks to humanity and the living planet increase. It’s up to us and the leaders we choose to stem the rising tide of climate and ecological breakdown.

Mark Brandon, Professor of Polar Oceanography, The Open University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.