Unconventional ‘Hot Rock’ Geothermal Energy

Natural radioactivity keeps the solid earth hot and provides the driving force for dynamic geological processes such as plate techtonics, earthquates and volcanoes.  Power can be generating by converting the Earths heat from “hot rocks” using super-hot water and steam from several kilometres below the ground surface.

According to Internatioanl Energy Agency 2012′s Energy Technology Perspectives, investment in geothermal energy for heat and power has potential to reduce CO2 by 17% by 2050.

How does it work?

2.1 Types of geothermal systems

Geothermal Hot Rocks Works. Geodynamics, presented by the Australian Geothermal Energy Association (AGEA). http://www.agea.org.au

Types of geothermal systems

1. Conventional – Volcanic, geothermal well, hot springs or steam vent. These require natural convective hydrothermal resources. These geothermal power systems exploited only resources where naturally occurring heat, water, and rock permeability are sufficient to allow energy extraction

2. Unconventional – Hot dry rocks. Enhanced Geothermal Systems (EGS) is a new type of geothermal power that does not require natural convective hydrothermal resources. EGS technologies enhance and/or create geothermal resources in this hot dry rock (HDR) through ‘hydraulic stimulation’. The extraction of heat from hot rocks is achieved by pumping water into the rocks at depth using an injection well, and subsequently withdrawing it from a production well at a much higher temperature after it has flowed under pressure through fractures in the hot rocks.

Types of geothermal steam plants: 

2.2 Binary Cycle Power Plant Example1. Dry Steam Power Plant : The first is the dry steam power plant which is used to generate power directly from the steam generated inside the earth. In this case, we do not need additional heating boilers and boiler fuel, as steam or water vapour fill the wells through rock catcher and directly rotates the turbine, which activates a generator to produce electricity. This type of power plant is not common since natural hydrothermal reservoirs dry steam are very rare.

2. Flash Steam Power Plant : The most common type of geothermal power plant, flash steam plants use waters at temperatures greater than 360F. As this hot water flows up through wells in the ground, it is collected in a flash tank where drop in pressure causes the liquid to boil into steam. The steam is separated from the liquid which is then used to run turbines which in turn generate power. The condensed steam is returned to the reservoir.

3. Binary Steam Power Plant : This type of plant uses high temperature geothermal water to heat another fluid which has a lower boiling point than water. This fluid vaporizes to steam, drives the turbines, then condenses to liquid to begin the cycle again. The water, which never comes into direct contact with the working fluid, is then injected back into the ground to be reheated. Since the most resources are with lower temperature the binary steam power plants are more common.

Current developments and trends

Baseload power generation is still under utilised globally
• United States has the highest geothermal electric capacity producing 3086 MW in 2010 an 77 plants (accounting for 0.3% of national production)

• Phillipines is ranked second with just less than 2000MW in 2010 and accounts for 27% of its national electricity production.

• Indonesia is third highest geothermal electric capacity with 1197 MW installed capacity in 2010 which accounts for –3.7% of its national production.

In global terms, 24 countries are currently generating geothermal energy, with a 2007 capacity of 9732MW producing over 50,000 GWhs every year. Significant producers are the USA, Iceland, Italy, New Zealand and Japan. The majority of this generation comes from hot springs associated with volcanic activity. As many as 46 countries could be generating geothermal power by 2010. Several new hot rock projects are already underway with France currently hosting the world’s largest geothermal plant.

2.3 Geothermal Australia

The geothermal map of Australia produced by Google and Hot Dry Rocks. Image courtesy of HDR.

Geothermal potential in Australia

According to Geoscience Australia, “hot rocks within five kilometres of the earth’s surface contain energy sufficient to deliver 2.6 million years worth of energy to Australia, based on the nation’s total current energy supply. If just one per cent of this energy could be tapped, it would be the equivalent of 26,000 times Australia’s annual power consumption” (Clean Energy Australia: 2012).

This map shows the geothermal potential over the entire country of Australia. According to an analysis by Hot Dry Rocks, a geothermal consulting group, and Google.org, just 2% of Australia’s geothermal potential could generate ten times more electricity than its total coal and gas electricity production today. Using today’s technology, geothermal energy could produce 395,092 MW. (November 2011) 

Geothermal gradient is  is the rate of change of temperature () with depth (), in the earth. Units of measurement are °F/100 ft or °C/km. Average geothermal gradient is 16°C. To fulfill the world energy demand with geothermal would be unrealistic as it would be too deep. However, geothermal energy will be major part of Australia’s energy mix.

According to Clean Energy Australia report 2011,  0.002% of total clean energy generation is from geothermal energy. Australia sits in 24th place with 1.1MW installed capacity. Geothermal has the potential to play a major role in decarbonising Australia’s electricity supply by providing emissions free and reliable power generation. Modelling by ROAM and SKM MMA for the Department of Treasury shows that geothermal could account for 13% to 23% of Australia’s electricity needs in 2050.

What are the advantages?

  • When designed and managed well, geothermal systems are a clean, abundant, and reliable source of renewable energy.
  • Conventional geothermal energy plants typically achieving much higher load factors compared to typical load factors for hydro and wind power plants.
  • Does not consume any fuel or produce significant carbon dioxide emissions.

What are the disadvantages?

Three key elements that must be satisfied before a viable geothermal prospect can move commercially. These are:

  • Quantum of energy: temperature and flow rate. Flow rate needs to increase by 3 to 4 times for geothermal to reach commercial potential
  • Cost of production: exploration and development drilling
  • Proximity to markets: transmission grid for geothermal energy, heat drying needs of domestic and industrial customers etc


  • It is not sustainable as heat is being mined. As demonstrated in data of the Ohsaki Geothermal Plant in Japan
  • Gas release (CO2, H2S, CH3 and CH4). Approx 400kg CO2/MWh
  • Dissolved chemicals (arsenic, mercury, boron, antimony, salt)
  • The gradual caving in or sinking of an area of land (subsidence)
  • Hard to predict permeability and makes it hard to calculate the resource
  • Freshwater use is very low
  • Induced seismicity
  • Not cost competitive compared to coal and gas
  • Stimulation has been banded due to the need for fracking system

What are the issues to economically developed?

According to International Energy Agency 2012′s Energy Technology Perspectives, global investment needs in geothermal in the world by 2020 is $104 billion USD, by 2030 is $155 billion USD and $1,004 billion USD in both geothermal power plants.

LCOE of geothermal flash plants will be drop between the range of $55-$80 USD/MWh in 2010 to $50-$72 USD/MWh in 2050. LCOE of geothermal binary plants will be drop between the range of $60-$110 USD/MWh in 2010 to $50-$90 USD/MWh in 2050.

  • Cost competitive in many cases (currently low electricity prices in Australia);
  • Financial risks of exploration phase. Expensive upfront investment in market and risky compared to mature technologies like wind power;
  • High cost of drilling

The levelised cost of generating geothermal energy depends on the origin of the resources invested and the way they are secured, as well as the amount of the initial capital investment. In the case of geothermal development, in some countries such as the United States, debt lenders (typically charging interest rates from 6% to 8%) usually require 25% of the resource capacity to be proven before lending money, so the early phases of the project, which have a higher risk of failure, often have to be financed by equity at higher interest rates (Hance, 2005). The average capital structure of geothermal power projects is composed of 55%-70% debt and 45%-30% equity. Large utilities building their own plants, either with their own available balance sheet or with cash flows, have a different cost structure from other investors when combining equity and loans to finance plants. Different financing schemes can have significant consequences on the costs of generating electricity and the expected rates of return on investment. Costs can be reduced by the availability of a long-term energy supply contract from a creditworthy off taker.

What are the technical issues to development?

  • Pioneering technology that needs improved geothermal resource assessment to accelerate geothermal development by developing publicly available databases for developing geothermal tools for identifying hot rock and hydrothermal resources
  • Improve accessing and engineering the resource by developing cheaper drilling technologies and improving hard rock and high temperature/high pressure drilling and down hole instrumentation and well monitoring
  • Develop EGS pilot plants in different geologic environments, develop simulation techniques and decision tools for reservoir monitoring, improving management, long term production and scale up to realise 50-200+ MW plants

What are the institutional and political challenges?

  • Lack of awareness of resources and applications,
  • Lack of appropriate legislation,
  • Complex permit procedure,
  • Shortage of qualified workers.

What are the social and environmental challenges?

  • Health, safety and environmental concerns,
  • Public opposition due to visual and odour related impacts

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