Trends in geothermal power generation is technology has some promise as being proven to be a clean, renewable resource providing energy around the world for centuries in various forms of hot springs. Keeping special areas with signs like hot springs aside, the heat of the earth is available for everyone everywhere.
Modern use of geothermal energy include electricity production, heat source applications for industrial purposes, and commercial as well as residential HVAC purposes through geothermal heat pumps.
Trends in geothermal power generation shows that plants use geo-fluids extracted by drilling wells into a geothermal reservoir. Such plants pose three main challenges in exploiting geothermal energy for power generation:
- High cost and risk of exploration and drilling of a well (around USD 10 million per well)
- Low temperature (typically in the range of 80 – 300 degree C)
- Disposal or re-injection of toxic brine that comes out of geothermal reservoir
Whenever high temperature super-heated steam is directly available from geothermal wells it can be used with steam turbines for power generation. But this is not the case with low temperature geothermal reservoirs. Low temperature geo-fluids require use of Organic Rankine Cycle (ORC) turbines through heat exchange mechanisms. This adds to the cost of geothermal power plant as compared to those using steam-turbines, in addition to the cost of wells. However, the high cost of drilling a well can be avoided by selecting abandoned oil wells which have depleted hydrocarbon reserves.
Trends in Geothermal Power Generation since 1989
US Department of Energy (DOE) test operated such a plant in 1989 demonstrating depleted reservoir conversion to geo-pressurized thermal power plant as part of its geo-pressured-geothermal energy program. The program aimed to utilize the heat brought to surface in the form of produced hot water (thermal energy), burning any entrained hydrocarbons on site for generating electricity (chemical energy) and high well head pressure (mechanical energy) to generate electricity. Pleasant Bayou in Brazoria County in Texas was chosen as the site for the power plant.
The plant generated electricity from the geo-fluid and separated the natural gas to test the production of electricity from combustion in an on-site hybrid power system.
The binary power plant with a design output of 905 KW (541 KW from ORC turbine, 650 KW from gas engine and subtracting an operational load of 286 KW). The plant operated at only 10,000 bbl of water per day with small volumes of gas flow.
Bottom hole temperature was given as 154 degrees C, with a maximum brine T of 136 degree C. The overall plant availability was 97.5%, at par with many other geothermal plants.
BP Statistical Review 2016 reported total consumption of coal, natural gas, oil, nuclear, hydro-power and renewables as 13147.3 MTOE in year 2015 to produce electricity. The renewable sources including geothermal power generation contributed 364.9 MTOE (on the basis of thermal equivalence assuming 38% conversion efficiency in modern thermal power station) which is less than 3%.
The representative of Enel Green Power Innovation Department has following views on the future of geothermal power plants:
“Renewable sources can interact between each other in order to fully exploit the characteristics of the single technologies and to use Balance of Plant to increase utilization factor,”
Hybrid among Trends in Geothermal Power Generation
Enel Green Power has taken lead where 33 MW Stillwater geothermal power station in Nevada was commissioned in 2011, got paired with 26 MW of photovoltaic facility in 2014 and another 17 MW CSP (Concentrated Solar Power) facility in 2016. The triple hybrid power plant has been reported by National Renewable Energy Laboratory to achieve 5% reduction in the levelized cost of energy (LCOE).
Like solar energy, the resource is indefinitely available with demonstrated potential of these trends in geothermal power generation via hybrid power systems as reliable source of green energy which is now receiving the attention of engineers, technologists and investors in proportion to the benefits that it will deliver.