Geothermal power plant. Picture: THINKSTOCK
Geothermal power plant. Picture: THINKSTOCK

A hidden, almost limitless supply of energy could be swirling around, directly beneath our feet. Local geologist Taufeeq Dhansay and a group of five fellow scientists from the Council of Geoscience and Nelson Mandela University believe that the government is overlooking significant volumes of buried Earth heat, which could be tapped to generate renewable energy.

Writing in the latest volume of the South African Journal of Science, Dhansay notes that SA does not have active volcanoes or evidence of recent volcanic activity. Yet he believes a recent heat-mapping study suggests there is still enough heat and warmth beneath the land that could be used to generate steam and electricity, making use of recent technological advances in the field of low-enthalpy geothermal energy harvesting.

In this harvesting, geothermal fluids are pumped deep underground at high pressure to absorb the Earth’s buried heat and returned to a heat-exchange mechanism on the surface to generate steam and electricity before the cooled fluids are recirculated underground to collect more heat.

SA's geothermal blocks

"The results of this study suggest that despite geothermal resources not being part of SA’s renewable energy mix, the country does have some potential for harnessing low-enthalpy geothermal energy. We therefore recommend that SA seriously considers geothermal energy as another renewable option," Dhansay says.

For centuries, people have made rudimentary use of geothermal energy — by bathing in hot springs, for example. But it was Italy that began the first experiments to tap this heat and convert it to electricity at a new plant in Lardarello in 1904. More geothermal wells were dug in Japan in 1919, followed by the more ambitious The Geysers project in California.

The International Geothermal Association says that several nations are making increasing use of this resource to meet their electricity needs, including the Philippines (about 27%), Kenya (12%), Costa Rica (11%) and El Salvador (about 4%).

But while tapping heat from the Earth is often presented as a relatively benign technology, there could be some serious catches. Quite apart from high costs of exploration and development, Dhansay and his colleagues acknowledge that there could be negative environmental effects — including pollution of scarce groundwater, subsiding land and an increased risk of mini-earthquakes.

Newer technologies known as "enhanced" geothermal systems rely on hydraulic fracturing (fracking) to stimulate the movement of geothermal fluids and groundwater at depths of more than 5km.

Less than a decade ago, a major geothermal project under the Swiss city of Basel was shut down after an expert group linked a 3.4 magnitude earthquake on December 8 2006 to geothermal fluid injection.

An official report by the Seismic Risk Analysis Expert Group found that the earthquake caused property damage of at least 7-million Swiss francs ($7.1m). The group also recommended that a through evaluation of seismic risks for all future geothermal projects in Switzerland be done after finding that geothermal reservoirs can worsen natural earthquake activity by changing subsurface geological stresses.

"In all likelihood, property damage of 40-million francs is to be expected in case of continued development of [this] geothermal reservoir. This comprises minor structural damages, which we expect to occur in large numbers due to the high population density.

"There is a 15% probability that damages will even exceed 600-million francs in an extreme case. During the projected facility’s operational period of 30 years, the most probable property damage is set at 6-million francs per year."

Similar concerns also emerged with an enhanced geothermal project in Landau, Germany, while significant fluid losses associated with hydraulic fracking occurred in enhanced geothermal systems plants in the UK and Japan.

The International Geothermal Association acknowledges that sucking up large quantities of fluid from geothermal reservoirs can cause land subsidence. "This is an irreversible phenomenon, but by no means catastrophic as it is a slow process distributed over vast areas. Over a number of years, the lowering of the land surface could reach detectable levels, in some cases of the order of a few tens of centimetres and even metres, and should be monitored systematically, as it could damage the stability of the geothermal buildings and any private homes in the neighbourhood. In many cases, subsidence can be prevented or reduced  by reinjecting the geothermal waste waters."

Another significant concern is the pollution of rivers and groundwater. The International Geothermal Association accepts that dumping hot water into rivers or lakes could harm fish and the smaller organisms they rely on for food.

"For example, a mere 2°C-3°C increase in the temperature of a body of water as a result of discharging the waste water from a utilisation plant could damage the ecosystem. The plant and animal organisms that are most sensitive to temperature variations could gradually disappear, leaving a fish species without its food source."

SA may therefore have a realistic chance of developing geothermal energy, but will still need additional research and development

More worrying, perhaps, is the potential for water pollution from radioactive and toxic substances such as mercury, uranium and arsenic buried deep below the surface.

In the latest South African study, Dhansay and his colleagues note that local rocks associated with high heat flow signatures often contain high concentrations of radiogenic elements such as uranium and potassium. In fact, it is these rocks, which are rich in heat-producing elements, that release heat during the decay of radiogenic elements. Nevertheless, the study has identified five regions that could be considered promising areas for geothermal energy projects.

They include parts of the Cape mountains north-east of Cape Town; the Southern Karoo region from Oudtshoorn to Cradock; a large belt of land north of Durban; the Thabazimbi-Murchison belt north of Johannesburg and the northern Limpopo region near Musina.

"SA may therefore have a realistic chance of developing geothermal energy, but will still need additional research and development including new temperature measurements and structural, hydrogeological and economic investigations."

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