Can you take the heat? – Geothermal energy in mining

In 2013, there are less than 20 documented examples of operational geothermal systems on mine sites worldwide. This is surprising, since on remote mine sites, where fuels may have to be shipped in over great distances, heating and cooling from low-enthalpy geothermal sources may have a significant advantage in operational cost over conventional energy sources. A review of factors affecting the feasibility of geothermal systems on mining projects has been undertaken, and has identified the possible configurations of geothermal systems suitable for the exploration, operational and closure phases of mine development. The geothermal opportunities associated with abandoned or legacy mines are also discussed. The potential categories of heat reservoirs associated with mine sites are: natural ground; backfilled workings; mine waste; dewatering pumping; and flooded workings/pit lakes. The potentially lower operational costs for heating and cooling must be offset against the capital cost of a geothermal system. The focus for mine operators should therefore be on identifying at feasibility stage those projects where conditions are favourable for geothermal systems, the potential risks are understood, the economics are likely to be beneficial, and geothermal systems can be established while minimising additional capital costs

Water from abandoned mines as a heat source: practical experiences of open- and closed-loop strategies, United Kingdom

Pilot heat pump systems have been installed at two former collieries in Yorkshire/Derbyshire, England, to extract heat from mine water. The installations represent three fundamental configurations of heat exchanger. At Caphouse Colliery, mine water is pumped through a heat exchanger coupled to a heat pump and then discharged to waste (an open-loop heat exchange system). The system performs with high thermal efficiency, but the drawbacks are: (1) it can only be operated when mine water is being actively pumped from the colliery shaft for the purposes of regional water-level management, and (2) the fact that the water is partially oxygenated means that iron oxyhydroxide precipitation occurs, necessitating regular removal of filters for cleaning. At Markham Colliery, near Bolsover, a small amount of mine water is pumped from depth in a flooded shaft, circulated through a heat exchanger coupled to a heat pump and then returned to the same mine shaft at a slightly different depth (a standing column arrangement). This system’s fundamental thermal efficiency is negatively impacted by the electrical power required to run the shaft submersible pump, but clogging issues are not significant. In the third system, at Caphouse, a heat exchanger is submerged in a mine water treatment pond (a closed-loop system). This can be run at any time, irrespective of mine pumping regime, and being a closed-loop system, is not susceptible to clogging issues

Ground energy systems: delivering the potential

Ground energy systems are increasingly being considered as an alternative to traditional heating and cooling systems as a way to reduce carbon emissions, control energy costs and improve the environmental performance of buildings. These systems use the ground and groundwater beneath a site as a heat source or sink, interacting through boreholes or pipes that exchange heat with the ground. This paper reviews the principles underpinning the systems and describes the two main types of system (open loop and closed loop). Four potential barriers to wider scale application of ground energy systems are highlighted. These are thermal interference between neighbouring systems in densely developed urban areas, increasing environmental regulation of below-ground elements, capital cost and the need to ensure that systems are sustainable in the long term. If the full potential of ground energy systems is to be realised, it is important that input from geotechnical and geological specialists is used to ensure that the below-ground elements are designed using appropriate design tools and site-specific data. It is also important that appropriate post-occupancy monitoring is in place to provide feedback to designers on the long-term performance of these systems.<br/

Performance of ejectors in construction dewatering systems

Construction dewatering in fine soils can be difficult, becausewell yieldsare often low and it is usually necessary to apply a vacuum to assist drainage. Ejector systems are therefore ideally suited to groundwater control in fine soils and have been used increasingly in the UK in recent years. Although their efficiency is low, this is not usually an issue when the flow rate of groundwater is small...<br/

Steady state performance of construction dewatering systems in fine soils

The applicability of conventional theory to construction dewatering systems in fine soils is uncertain for a number of reasons. This Paper uses case records from 30 dewatering systems in fine soils to assess the suitability of various methods of analysis of conditions at the steady state. Guidelines are proposed for the selection of soil permeability from such data as are likely to be available at the design stage. The use of these guidelines, together with analytical techniques appropriate to the field boundary conditions, gives calculated flow rates which are generally within a factor of 3 of the recorded values. The field data suggest that dewatering systems in fine soils should not be designed to operate with average hydraulic gradients at entry into the wells in excess of 4 for well-points, or 10 for ejectors. It is shown that whereas the effectiveness of a dewatering system in a fine soil should not normally be affected significantly by a variation of a factor of 3 in the average soil permeability, an unexpected close source of recharge will seriously impair the performance

Construction dewatering in fine soils: some problems and solutions

Ground Panel Paper 10195 water level in fine soils are often unstable, but can be stabilized by reducing porewater pressures in the surrounding soil. Construction dewatering is a technique for local reduction of pore pressures on a temporary basis by pumping from an array of wells..

Time-drawdown behaviour of construction dewatering systems in fine soils

In the design of a construction dewatering system in a fine-grained soil it is necessary to consider not only the steady state flow rate that must be pumped, but also the time it will take to achieve a given drawdown. This may be weeks because the soil will usually remain saturated, and the mechanism of pore-water pressure reduction is consolidation rather than dewatering per se. If the response time is not taken into account in programming an excavation, the cost implications could be considerable. In this Paper approximate methods based on the assumption that the isochrones are parabolic are developed for the estimation of time-drawdown relationships for dewatering systems in fine soils. The parabolic isochrone analyses are compared with exact solutions where these exist, and with field data from five case studies. The parabolic isochrone approximation is shown to be suitable for flow to lines of wells, and to rings of wells, provided that the distance of influence remains small compared with the radius of the ring