Tuesday, 2 January 2018

Assessing the potential for low-enthalpy geothermal energy generation in South Africa.

South Africa is the largest producer of greenhouse gasses on the African continent, and one of the largest producers worldwide. This is largely due to the country's dependence on coal to produce energy, with over 80% of electricity in South Africa being produced from coal burning power stations. Aware of the problems associated with greenhouse gas production, the nation has set itself the ambitious target of generating 40% of its energy needs from renewable sources, such as wind, solar, or hydroelectric plants, by 2030. This target is made harder by rising demand for electricity, with a rising population and many populations denied electricity under the former apartheid regime now wanting to be connected. Geothermal energy, energy generated by geological processes, is not usually considered as a viable option in South Africa, as the nation lies far from any active volcanic field, the most obvious source of geothermal energy, but the country could still potentially generate some of its power from low-energy geothermal sources, which rely on temperature gradients within deep rocks, rather than active heat eruption at the surface.

In a paper published in the South African Journal of Science on 29 December 2017, Taufeeq Dhansay of the South African Council for Geoscience and the Africa Earth Observatory Network at Nelson Mandela University, Chiedza Musekiwa, Thakane Ntholi, Luc Chevallier, and Doug Cole, also of the South African Council for Geoscience, and Maarten de Wit, also of the Africa Earth Observatory Network at Nelson Mandela University, present the results of a study into the viability of low enthalpy geothermal energy production in South Africa.

Low enthalpy geothermal energy production relies on temperature differences within geological formations rather than direct heating by magma or hydrothermal sources, and can produce useful amounts of electricity at temperature gradients as low as 40°C per km. This does not necessarily require the presence of plutonic intrusions from the Earth's interior, in many cases the presence of radiogenic elements such as uranium within the rock strata produces sufficient heating for low enthalpy geothermal plants to operate..

Schematic illustration of a binary fluid enhanced geothermal system related to surrounding fracture-controlled geological features. Dhansay et al. (2017).

Much of South Africa is underlain by the Kaapvaal Craton, an ancient subcontinental mass of lithospheric material that averages 40-50 km in thickness, but is 250 km deep at its thickest. This ancient mass at first seems to have rather poor potential for geothermal energy production of any kind, but the craton is in fact made up of several large blocks of more ancient material that were fused together during the Archean Eon (4.0-2.5 billion years ago). Thus, while the majority of the Kaapvaal Craton is highly thermally stable, something which has discouraged investigation of the potential for geothermal energy in the area, it is cross-cut by a number of ancient geological sutures, with rather different properties.

These sutures are the remains of ancient orogenic belts, bands of volcanic activity similar to that seem around the Pacific Rim today. The geological process associated with such structures tend to concentrate heavier elements within certain strata. This has led to the high concentrations of gold and other precious metals that drive the South African mineral industry, and also higher concentrations of radiogenic metals such as uranium, which at high enough densities can raise the temperatures of the deposits that host them.

Within the Kapvaal Craton these include the Cape Granite Suite (part of the Cape Fold Belt, which formed during the assemblage of the supercontinent of Gondwana) which contains uranium concentrations of up to 34 parts per million, the Namaqua-Natal Belt that formed during the assemblage of the supercontinent of Rodinia, and has uranium concentrations of between 10 and 54 parts per million, the Thabazimbi-Murchison Lineament, which marks the boundary between the ancient Kaapvaal Greenstone Belt and the slightly younger Limpopo Mobile Belt, with uranium concentrations of up to 30 parts per million, and traces of older Archean granite-gneisses around Mombela and Johannesburg, with uranium concentrations of 20-28 parts per million.

Overview of the major tectonic structures and zones across South Africa with the locations of significant earthquake focal mechanisms and inferred structures related to these events. Locations of the various data sources used within this study (e.g. hot springs and temperature measurement points) and high heat producing plutonic rocks are also highlighted. Note that the Namaqua-Natal Belt probably continues beneath the Cape Fold Belt as far as the offshore Agulhas Fracture Zone. Dhansay et al. (2017).

Dhansay et al. obtained measurements of temperature in South African rocks at depths of between 2 and 5 km from previous studies, as well as inferring deeper temperatures from hot springs at the surface, and the presence of deep faults from Earthquake data, and used this data to build up a map of temperature gradients across the country.

Graphical overview of the calculated geothermal gradients across South Africa. Map includes major tectonic contacts and structures, seismic activity and earthquake focal mechanisms and hot spring locations. Dhansay et al. (2017).

Using this data Dhansay et al. were able to identify a number of regions with geothermal temperature gradients high enough to support potential low-enthalpy geothermal energy generation, most notable along the the Colesberg, Thabazimbi-Murchison and Makonjwa Lineaments, ancient tectonic zones associated with early stages of the assemblage of the Kaapvaal Craton, which are also associated with a number of recent seismic events, and a number of hot springs, which may be due to reactivation of ancient faults with a northeast to southwest orientation.

(a) Potentially viable low-enthalpy geothermal investigation regions (1–5); based on (b) high heat producing plutonic rocks and overlying volcanosedimentary rocks; and (c) approximate groundwater yield. (d) Regional seismicity. Dhansay et al. (2017).

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