It’s possible to use local resources to maintain adequate air temperatures underground. CREDIT: STANTEC
If you’ve ever been in an underground mine, you’ve felt the heat.
Underground mines can get very hot.
Why? There are three main factors: Geothermal heat in surrounding rock, heat from diesel or electrical equipment operating underground, and auto-compression of ventilation air – which warms as it’s pushed deeper into mines. Things get hot.
So, it’s important to keep underground mines cool for workers. No one appreciates working in an oven. Here’s some background on how mines are generally cooled: Typically, chilled water is cooled in a large refrigeration chiller. Then, this chilled water is sprayed in a large chamber onto the incoming ventilation air, which cools the air. Alternatively, the chilled water can be pumped in a heat exchanger, and the ventilation air cools as it’s forced into the heat exchanger. The cooled air can then be used to ventilate the mine.
Unfortunately, these refrigeration chillers can be expensive to operate, especially in remote areas, where the cost of electricity is at a premium. Due to their reliance on electricity, these systems can be a big contributor to the overall carbon emissions in the operation of a mine.
If you’re looking for a new solution to your mine heat problem, I have some ideas. Here are three innovative ways that you can keep your mine cool – all involving different sources or forms of water. It’s possible to cool your mine in a sustainable way that’s also more affordable.
- Lake water cooling
Now let’s look at some alternate ways to cool your mine. If you’re considering this first innovative system – lake water cooling – a fun fact about water: it’s heaviest at 4ºC. If a lake is deep enough, you can find cool water at the bottom – even if the lake is frozen over. To avoid contamination, you can use a closed loop heat exchanger. This consists of an array of pipes at the bottom of the lake that transfers cooling to the mine’s cooling heat exchanger. So, as the water gets pumped around and into a heat exchanger, the mine’s ventilation air gets cooled down.
If this concept sounds familiar, you may have heard about it through the work of Enwave Energy Corp. in Canada. Enwave uses this technology for both district heating and cooling in Toronto, via cold water from pipes in Lake Ontario.
Scotiabank Arena, home of the Toronto Maple Leafs and Toronto Raptors, utilizes Enwave’s cooling system.
Here’s the thing: Lake water cooling has not yet been proven for mining. Why? Well, the high capital cost of the system could be a reason, since it’s surprisingly expensive compared to refrigeration cooling – especially when you consider that the cooling system is mainly a bunch of pipes at the bottom of a lake.
Thankfully, the operating cost of the system is very low, so this concept could be an attractive solution for owners of remote mine sites dealing with carbon taxes or high electricity prices.
- Seasonal thermal storage (aka ice stopes)
A schematic of an ice stope that was used in the Frood-Stobie mine in Ontario until 2017. This system was used for heating the mine, but it could also be used for cooling. CREDIT: KIM TRAPANI/STANTEC
Here’s a second alternative to cooling mines. Seasonal thermal storage refers to a system that transfers cooling from one season to another. It’s a fascinating idea, but it’s only applicable for countries with subarctic climates. Canadian mine owners can consider this system. Wouldn’t it be great to use all that winter snow to cool our mines in the summer?
A hospital in Sundsvall, Sweden, successfully tested this concept in 1999. Hospital workers stored winter snow in a pit, using sawdust to insulate the snow from the sun. The next summer, they used stored snow to meet all their cooling demands. Their biggest challenge? Rainwater melting the snow.
In a mine, you can use underground excavations to store the snow – away from rainfall and solar radiation. Melt the snow as required in summer by spraying the cooling water onto it. Then, you use this chilled water to cool the mine ventilation air.
Interestingly, you can take this system a step further and use it to heat the ventilation air as well. You spray warm mine return water onto the cold intake air before it’s introduced underground. When the water meets with the cold air, the air warms up and the water changes state into ice, which deposits at the chamber and can be stored for cooling later in the year.
In a mine, the old working areas that have been mined out are typically called ‘stopes.’ So, you can call this cooling (and heating) system an ‘ice stope.’ Historically, two Canadian mines have used ice stopes, but only for winter heating. Miners in Quebec used the system in Rouyn-Noranda from 1942 until the mine closed in 1976. Ontario miners used it at Frood-Stobie, which operated from 1955 until 2017.
If the cost of the excavation is excluded, the capital and operating costs of an ice stope are significantly lower than conventional refrigeration cooling. Could this make sense for your mine?
- Hot ground fissure water
Here’s another technique: Use hot ground fissure water for cooling. Certain mines experience very high ground water temperature, which requires constant cooling so that miners can do their jobs. For example, Resolution mine in Arizona deals with water temperatures that reach 80ºC, making it extremely challenging to mine.
For these sorts of mines, you can use absorption chillers to provide cooling. Absorption chillers use heat instead of power – like in a conventional refrigeration system – to produce cooling. Since heat is freely available, the only power required is to pump the cooling fluids in the absorption chiller.
When you use ground water, cooling can be produced underground, right where people are working. This reduces the losses from moving cooling from surface to underground.
We don’t fully understand the economics and performance of this cooling system in mining. But my team is currently looking into its limitations and benefits, and I’m excited to learn more.
These three innovative cooling systems use local resources to maintain adequate air temperatures underground. All three systems have minimal operating costs, because the energy comes from either cool water at the bottom of a lake, snow (which is plentiful in subarctic climates), or hot fissure water that gets pumped to a treatment plant.
When considering cooling options for your underground mine, it’s important to be resourceful. Use the energy sources available on a mine site. Through creative thinking, you can find ways to cool your mine affordably and sustainably.
Kim Trapani is a ventilation engineer at Stantec’s Sudbury offices.