Geothermal systems - a brief history of the use of geothermal resources

Geothermal (Geo = Earth; thermal = heat) energy stems from the Earth’s outward heat-flux, which originates from the planets internal heat – a heat leftover from the planets creation, as well as from the decay of radioactive isotopes in its interior. Geothermal systems are regions in the Earth’s crust where this flux and the associated energy storage are abnormally great. These systems are often manifested in flowing hot water, and/or steam at the Earth’s surface, but substantial resources are also “hidden” in the Earth’s crust and are only manifested in locally elevated heat-flow. Geothermal resources constitute the only renewable energy resource not originating in the Sun.

Geothermal energy has been harnessed for thousands of years worldwide. Early use centered around bathing, washing, and cooking in or around natural warm springs. Good examples of countries with a long history of geothermal use are China and Japan. In Europe, ruins of baths that date back to the days of the Roman Empire, can be found from England in the north to Syria in the south. Fast forward to the 19th century and we see a rapid growth in the commercial utilization of geothermal energy world-wide.

Since the late 1970’s various systems of classification of geothermal systems have been proposed. Classification may be based on temperature of the fluid, or its energy content. Alternatively one can classify systems based broadly on their geological setting or play type, mimicking the classification of systems from economic geology. A simple but useful classification was developed in Iceland where low temperature systems were defined as systems with temperature below 150 °C at 1 km depth (or a gradient dt/dZ ≤ 150 °C/km) and high temperature systems were defined as systems with a temperature exceeding 200 °C at 1 km depth in the crust (or a gradient dt/dZ ≥ 200 °C/km). This simple classification worked well for the geothermal fields in Iceland. To extend it to the rest of the world intermediate temperature system may be defined as systems falling between hi- and low-temperature systems and this class represented by many systems hosted in sedimentary basins.  

The use of geothermal energy for space heating was pioneered in Iceland during the last century. The volcanic island has ample geothermal resources with fluids of optimal temperature and often benign chemistry to be suitable for direct use in space heating. Ólafsfjördur was the first village in Iceland to be entirely heated by geothermal water, Reykjavik followed shortly thereafter. Before that, there were some examples of farmsteads using local warm springs for both space heating and cultivation of vegetables in greenhouses. In Iceland the first oil crisis had the effect of forcing authorities to double their efforts to reduce the dependence on imported hydrocarbon fuels. Presently the energy markets both in Iceland and internationally is again resulting in efforts to replace space heating by use of electricity (from hydro or geothermal) with direct use of geothermal waters.

Electricity production from geothermal started in Italy well over a century ago. There, the Larderello geothermal field was harnessed as early as 1913.  Since the pioneering work in Italy, a number of countries have followed suit and started to use relatively high enthalpy geothermal resources for electricity production. The afore mentioned Larderello field is in many respects unique in that the reservoir is superheated steam. Most other systems, used for power generation with what is often referred to as the conventional method, rely on wells where fluid flashes and a mixture of water and steam is brought to the surface. At the surface the steam is separated from water and then later non-condensable gases are separated from the steam before the steam is passed through turbines that generate electrical power.   

Another method of generating electrical power involves the Organic Rankine cycle or ORC. It uses the Rankine cycle but with the addition of an organic working fluid for the cycle. This method is often suitable for geothermal systems of intermediate temperature such as those often encountered in sedimentary basins.  

A special type of systems, are the so called Enhanced Geothermal Systems or EGS, sometimes also referred to as Hot-Dry rock systems. In these systems a working fluid is pumped into a hot dry rock and in some the fluid is used to enhance existing permeability by hydrofracturing. The fluid is then extracted via another well and used for power generation.

During the last century geothermal utilization has increased and diversified greatly. It now encompasses a multitude of different uses, both electricity generation and direct use of the thermal energy, such as for space heating, greenhouse heating, food drying, industrial uses, bathing and washing, and much more. During the last decade installed capacity for geothermal electricity generation has grown significantly, as has the direct heat utilization, including utilization of shallow resources through ground-source heat-pumps. The increase in electricity generation can mainly be attributed to countries like USA, Indonesia, Turkey and Kenya. The installation of ground source heat pumps has grown drastically in countries like China, USA , Sweden and Switzerland. Other direct heat utilization is led by countries like China, Turkey, Japan and Iceland. This exemplifies the worldwide distribution of different grades of geothermal resources.

To advance geothermal development in decades to come “unconventional” resources need to come into play. This involves the application of the afore mentioned EGS-technology to hot but poorly permeable parts of the Earth’s crust, a technology still in the development stage. Superheated or even supercritical resources in the deep roots of volcanic geothermal systems are being studied through experimental deep drilling. They will hopefully be realized in the next decade or two. Finally, mid-ocean ridges host resources that have a vast potential as they are situated on the Earth’s plate boundaries, where terrestrial heat flow is at a maximum.

In addition to the development as well as propagation of the various modes of direct geothermal utilization, increasing emphasis should be placed on improved energy efficiency, both in direct use as well as electricity generation. Cascaded utilization is a utilization mode aiming for this. In addition to the benefit of greater energy supply, this will also affect the cost of geothermal energy leading in most cases to a reduction of energy costs.