The potential of geothermal energy

[摘要] Geothermal energy is heat extracted from the earth. This heat is extracted by circulating a fluid through the reservoir to bring the heat to the surface where it can be used to generate electricity or as heat in direct use applications. The most commonly-exploited geothermal resource used for power generation globally are convective hydrothermal systems that are found in high heat flow and volcanic regions associated with tectonic plate boundaries (e.g. west coast of the USA, New Zealand, Indonesia). These resources have high geothermal gradients (high temperatures at relatively shallow depths) and have significant volumes of fluid (steam and hot water) that can be extracted from the reservoir, bringing heat to the surface. These resources are not found in Australia’s intraplate tectonic setting, and the Australian geothermal industry is targeting two new concepts of geothermal reservoirs.The first are enhanced geothermal systems (EGS), which uses resources that are deeper in the crust. These resources are in crystalline rocks, heated by radioactive decay with this heat trapped in the reservoir by insulating sediments. The rocks need to be fractured to allow a working fluid to be circulated from the surface, through the reservoir where it is heated, and back to the surface where the heat can be used. The second are hot sedimentary aquifers (HSA). These consist of hot water in highly-permeable rocks, typically in sedimentary basins. Australia has substantial EGS and HSA resources, but these technologies are in the development phase.The key ingredients for geothermal energy production can be summarised by this equation:where cp is the specific heat of the working fluid; F is the flow rate from the production well; ΔT is the sensible heat that can be extracted from the fluid produced by the production hole (Treservoir – Trejection); η is the efficiency with which the heat energy can be used, and P is the parasitic losses. The goal in geothermal systems development is to optimise as many of these parameters as possible to increase electrical output relative to the capital costs of developing the geothermal energy resource and surface plant. This approach improves the economics, since the capital cost is by far the largest component to the levelised cost of electricity (LCOE) of geothermal systems.In EGS systems, by drilling deeper wells ΔT increases. However, the cost of drilling can be as much as 80% of the capital cost of a plant, and increases with depth. In addition, P for pumping increases in deeper wells and it is more difficult to achieve a high flow-rate (F) ; therefore, tradeoffs need to be made. In HSA systems ΔT tends to be lower. Therefore, targets for increasing the output and lowering the cost are increasing the flow-rate, reducing parasitic losses and improving the efficiency of energy conversion. Research targets for technical and economic development of these technologies are improved exploration and resource/reservoir characterisation, lower-cost drilling techniques, reservoir engineering (obtaining the required fluid flow) and more efficient power conversion. Currently, fluid flow in the reservoir is the biggest technical challenge facing EGS developers. Other issues can also impact the development of geothermal in Australia, such as social and environmental risks, surrounding groundwater use and induced seismic events from drilling; competing use of resources as geothermal resources can be located near coal seam gas, groundwater and sites for CO2 sequestration; and financial risks given the large scale, relatively high capital costs and high number of uncertainties. Geothermal energy’s largest contribution to the world’s energy market is through direct use applications such as process heat, thermal desalination and heating and cooling (via adsorption chillers) of buildings. Direct use applications can access low temperature HSA, EGS or convective hydrothermal systems and have the advantage of not nee

[发布日期] 2011-03-23 [发布机构] CSIRO

[效力级别] Renewable Power and Energy Systems Engineering (excl. Solar Cells) [学科分类] 地球科学(综合)