Gold major AngloGold Ashanti has installed an energy efficient cooling system at its Mponeng mine that will see the mine reduce its energy demand by as much as 10 MW of electricity during peak times and is expected to bring about a yearly electricity savings of about R2-million, which is about 2% of Mponeng's total yearly cost of consumption. A similar system is to be installed at its Moab mine this year.

Deep mines use a significant percentage of their overall electrical power for cooling and refrigeration. Mponeng is the world's deepest mine, with temperatures reaching up to 54,5 ºC at 3,8 km below surface. Cooling systems are important to keep temperatures in underground working areas below the 27,5 ºC threshold, which is considered safe for employees to work in.

Water is the primary cooling medium used at most mines. It is cooled in a refrigeration plant situated on the surface or underground and then distributed to various locations throughout the mine. A significant disadvantage is that the plant is situated at a distance from the rock face resulting in the water heating up as soon as it leaves the chillers, thereby adding to the energy and cost requirements.

To significantly reduce operating costs and improve energy efficiencies, AngloGold Ashanti looked at alternatives to its traditional cooling system, which consisted of a large refrigeration system with dams, cooling towers and five 11-MW conventional refrigeration plants, which cool service water from 10 ºC to 3 ºC for circulation inside the mine.

As a result, the mine installed three underground generator/turbines using gravitational energy released by the water falling to the bottom of the mine as part of its underground cooling system. The gravitational energy drives two 1,8-MW generators and a single 2,8-MW generator 24 hours a day, seven days a week, which equates to a total of 32 GWh of electricity being generated a month. This equates to a 2,3% saving of Mponeng's total electricity consumption.

Meanwhile, AngloGold Ashanti, in partnership with desalination technology firm Israeli Desalination Engineering (IDE), which developed vacuum ice technology, researched the viability of cooling the mine with ice. In 1992, the mine commissioned its first 1 520 kWh pilot vacuum ice plant. The mine currently has a 76 MW installed refrigeration capacity, circulating 49-million litres of chilled water through its water-cooling system. Six IDE-built 3-MWR vacuum ice plants produce 4 200 t of slurry ice daily.

The precooled feed water is vacuumed to produce ice by lowering the freezing point of the water in a saline solution. Together with the service water, the ice is used to help keep the underground temperature below 27,5 ºC. The use of vacuum ice technology has had the effect of reducing operating costs significantly while reducing the water required fivefold.

Mponeng mine stores its ice in ice dams at different levels of the mine, readily available to cool the water closer to site instead of first pumping it to surface. Although ice plants are slightly more expensive than conventional refrigeration plants, the initial capital expenditure is offset by lower pumping requirements associated with the reduced volumes for the same refrigeration capacity.

Ice technology is being extensively used at Mponeng mine, which has embarked on a new ice thermal storage system to produce and store enough ice to reduce the load on the refrigeration machines during peak electricity times. The mine has built a new 45-million litre ice storage dam and has installed a number of ice coil banks inside the dam as part of a demand side management initiative.

As part of this initiative, a subzero water/brine mixture is circulated through a refrigeration plant and ice coil banks during off-peak times to produce ice. During peak times, the ice is melted again by pushing water over the ice banks providing a ready supply of cooled water for mine circulation. The system can be 'recharged' during cheaper electricity tariff periods to reduce the load on the other refrigeration plants during periods of high demand.

The ice-thermal storage facility is an important part of AngloGold Ashanti's broader energy management strategy.