Opportunities for shallow heat management via subsurface stirring

In many sectors of urban and industrial activity waste heat is released to the atmosphere or hydrosphere. This energy release is wasteful both through the loss of potentially useable heat but also through the consumption of water during evaporation. Sustainable energy solutions will provide options to recycle heat and to minimise water consumption. Lowtemperature geothermal reservoirs offer substantial capacity to accept and store waste heat. Typical openloop subsurface heat rejection or storage applications rely on arrangements of injection and extraction bores that pass heatcarrying fluid through the local porous medium, often in steady pumping modes. This abstract presents a new technology for enhancing heat transfer rates in the subsurface by using timedependent pumping modes. Appropriate selection of pumping modes can induce useful flow characteristics, ranging from kinematic isolation of reservoir zones to chaotic mixing where thermal energy transfer is maximised. Theoretical aspects of the subsurface stirring approach are discussed, and comments on the transferability of this method to the geothermal context are made.