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Evaluation of Existing Subsurface Cooling Systems Worldwide and Development of Efficient Cooling Methods and Systems Based on Renewable Energy Sources
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As relatively hot mineral deposits are being developed to considerable depth, the issue of mitigating heat will require substantial cooling capacity in some cases. For deep deposits to be mined at increasing depths, mechanical cooling systems used to maintain an acceptable underground climate will most surely become increasingly complex and expensive, which most certainly preclude practical solutions without fresh innovation. In most subsurface operations, bulk air-cooling methods, the main source of potential cooling energy comes from cold-water applications. The cold water can come from ice (stowed or transported), in situ reservoirs or refrigerated water chillers in either low or high-pressure systems. The main issue with ice is the service temperature is limited to freezing point of water, at 0 °C. In respect to cooling capacity considerations, it is important to highlight that most industrial applications of refrigerants are capable of maintaining the liquid phase state far below 0 °C. This dissertation primarily investigates the practical use of low-pressure, chilled propylene glycol, which has a freezing point of -68 °C, in a closed circuit as the source of cooling the intake air for deep and hot mines. Air-cooling power is available by means of passing intake airflow across plate heat exchangers through which low pressure chilled propylene glycol is pumped. A key factor to the design and operation of an efficient and sustainable cooling system is that the tanks of propylene glycol are chilled during favorable atmospheric conditions when the tanks can be exposed to the cold of the night. Planning for sustainable cooling potential requires protected space to successfully maintain chilled propylene glycol in storage until needed. Stored propylene glycol should be judiciously cared for, safely kept from the ill effects of heat, in order to remain useful for bulk air-cooling. Maximizing the chilled temperature of the propylene glycol reservoir tanks means that atmospheric conditions will be monitored closely during the chilling phase. Storage facilities would maintain restricted airflow while minimizing possibilities for heat transfer during the storage phase. Secondly, this dissertation investigates the use of ice accumulated in underground access tunnels as the source of renewable air-cooling. The current market trend in profitable mining operations involves continued mining activity below a viable mineral deposit which was initially developed with an open pit. Due to the scale of this type of operation, a popular current trend from the surface mining perspective is to use very large haul trucks bringing ore up a long ramp system which daylights in the open pit. This type of ramp mine access drift is driven into the most stable strata surrounding the open pit. This mining arrangement is the proposed application for bulk air cooling via stowed ice reservoirs. The proposed ice storage arrangement would work successfully in any lower latitude mining operations that experience a cold winter season. Snow is produced under favorable atmospheric conditions using commercial snow making equipment typically found at ski resorts. Regulators with air locks would be installed on either end of each gallery to maintain minimum velocities during snow and ice accumulation. Using compressed air and water, snow is blown, during near-freezing conditions across snow fences arranged in an array aligned with the airflow. Snow fences are employed to maximize the snow deposition potential. The system will use wastewater from the mine and recycle water during operation as the ice melts into storage sumps. Water quality monitoring will be required since nucleation agents will need to be added to the reservoir water before spraying application. In order to maintain the highest level of safety, continual monitoring of ground conditions will be maintained during the operation and maintenance of the renewable cooling ice storage facility. Additional geotechnical measures will be necessary to ensure pit portal integrity due to the addition of the stowed ice reservoirs. Sustainable cooling systems could reduce or eliminate the need for mechanical cooling if installed with favorable conditions. Successful installations will prioritize environmental management to favor long time span usage of sustainable cooling systems.