Oct 15, 2010
Despite geological limitations, SA begins to weigh its geothermal optionsBack
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However, the energy crisis and the drive for renewable-energy generation have sparked new interest in the possibility of generating energy from heat that is readily available from the earth. Technological advances over the past few years also indicate that the use of geothermal energy may be viable in areas like South Africa.
There is, currently, no large-scale geothermal production in South Africa, since coal is abundant and relatively cheap, supplying the largest part of the country’s energy requirements. However, the Renewable Energy Policy Network for the 21st Century, or REN21, ‘Renewables 2010 Global Status Report’ states that, as the geothermal market continues to broaden, a significant acceleration in installations is expected, with advanced technologies enabling the development of geothermal power projects in new countries.
What is Geothermal Energy?
Power generation solutions company HRP Geothermal Power engineering director Andrew Ochse explains that there are three types of heat sources – magmatic, frictional and radioactive. South Africa, predominantly, has radioactive geothermal heat sources. Given high enough temperatures of these heat sources, it is possible to heat water or steam to high enough temperatures to make electricity production possible.
University of the Witwatersrand senior research officer Dr Michael Jones explains that radioactive heat derives from the natural radioactive decay of isotopes of uranium, thorium and potassium, which are disseminated throughout most of the volume of the earth. Radioactive decay contributes about 75% to the earth’s heat budget.
There is also a definite connection between water and geothermal energy generation, a prospect that natural resources management company Touchstone Resources has been exploring. The company is focused on developing energy and water resources for socioeconomic benefit, and CEO David Gadd-Claxton says that water and energy should be seen as a “flux”. Flux is an inherent quality of natural resources that allow them to flow through time and space, in effect recycling naturally.
“Water does not disappear; the amount of water stays exactly the same through the ages. The problem is that, to date, water has been managed as stock. If water is managed as flux and reused, the earth would never run out of water supply. This unlimited water source can be applied to generate unlimited energy,” Gadd-Claxton points out.
Passive or direct use of geothermal energy for heating and cooling is also commercially competitive with conventional energy sources. African Ecosystems MD Cary Praetor explains that, compared with ordinary systems, geothermal technology can save 30% to 60% on monthly domestic energy bills. “Geothermal is the safest, cleanest, most reliable space- conditioning system available,” he notes.
Geothermal energy is an unlimited resource. The lot surrounding a suburban home, office block or hotel contains a reservoir of low- temperature thermal energy, typically ten times that required over an entire heating season. This resource is constantly resupplied by the sun, the surrounding earth and the heat extracted from buildings while cooling it during the summer.
Touchstone Resources supplies a range of heat pump products that take the heat out of a house in summer and store it in the ground. In winter, they take the heat out of the ground again to warm the house. “A heat pump can extract heat out of the air and deliver it into the geyser, increasing its efficiency up to 75%, while solar energy is only 30% efficient,” Gadd-Claxton asserts.
Like other renewable-energy sources, such as solar, wind and hydro, geothermal offers significant potential in terms of climate change mitigation. “Geothermal is 100% indigenous, environment friendly and a technology that has been underestimated for too long,” United Nations Environment Programme (Unep) executive director Achim Steiner said in a recent report on an assessment of geothermal energy prospects, conducted by the body.
The coupling of renewable energies, such as wind, solar and geothermal, with desalination systems holds significant promise for increasing water supplies in water-scarce regions. The Energies journal argues that an effective integration of these technologies will enable countries to tackle water-shortage problems by using a domestic energy source that does not produce air pollution or contribute to the global challenge of climate change. Further, this approach will assist in bypassing the problems of rising fuel prices and decreasing fossil fuel supplies.
South Africa’s Prospects
This was previously not feasible, owing to the cost of drilling but, now, with energy shortages and increased electricity costs, he believes that the finances should become available.
There is some investor interest coming from the mining and industrial sectors, which HRP Geothermal Power has been exploring. Ochse points out that this interest is in large, long-term capital projects with the same magnitude of effort for each megawatt as for coal-fired power plants, but with the advantage that the “fuel” or heat source is free once you get to it.
“We would generally target a 50-MW or more installed capacity geothermal plant, as the financials make sense at this size, with the risk-weighted capital cost amounting to about R1,45-billion,” he says.
HRP Geothermal Power says it has discovered that an Organic Rankine Cycle (ORC) power plant is the preferred tech- nology for South African application, as it allows for lower-temperature heat sources, in the 100 °C to 150 °C temperature range.
The ORC is unlike conventional Rankine Cycles, which use water or steam as a working fluid, as it uses an advanced refrigerant as the working fluid. This allows the cycle to generate high-pressure ‘steam’ from lower-quality heat to drive its turbine and generate power. This also means that ORCs can operate between smaller temperature differentials than traditional Rankine Cycles.
Jones says that South Africa is far removed from active plate boundaries and heat flows to the surface predominantly by conduction. Thermal gradients vary from as low as 8 °C/km to as much as 40 °C/km. “These values for heat flux and thermal gradient are considerably lower than those experienced in geothermal areas, but the heat is there if one goes deep enough – it is a matter of extracting it at economically viable rates,” he explains.
Challenges and Limitations
“There seems to be resistance from local mechanical engineers to venture into geo- thermal. A possible reason for this may be that they do not have the relevant knowledge and, therefore, see it as a threat,” he says.
Ochse agrees that there are limited geo- thermal skills within South Africa, but points out that there are numerous geology experts who can assist in the process. Harnessing geological knowledge, HRP Geothermal Power has worked with local geothermal experts as well as experts from the US, Australia and New Zealand to assess the viability of geothermal in South Africa.
Jones, being a local geology expert, notes that geothermal energy is economically and tech-nologically difficult to extract. Much of the equipment needed is sourced internationally, as the expertise is not available locally. HRP Geothermal Power would like to shift some of this capacity to local manufacturing, if warranted by sufficient geothermal energy gener- ation.
Local power utility Eskom senior process engineer Gary Dysel says that Eskom, in its drive to reduce the country’s energy footprint, welcomes energy- reducing technologies. The company met with African Ecosystems, part of sustainable energy company Geothermal Energy Systems, about three years ago to discuss possibilities, where Praetor presented the significantly improved coefficient performance values of geothermal systems.
“The improved performance attracted us to the product. We assigned about 15 energy advisers across the country to distribute knowledge and offer advice on the use of geothermal energy,” he notes.
The role of an energy adviser is to assist the industrial, commercial, domestic and agriculture sectors on the most efficient way of using energy. Dysel says that the advisers are linked to the demand-side management (DSM) process of project funding and present companies with energy saving projects for evaluation and funding. They are under significant pressure to reduce energy use by 1 074 MW in the next three years.
He adds that Eskom has, to date, not put through any DSM-funded geothermal projects, but is aware that interest in geothermal energy has increased globally and that Praetor has been successful on numerous projects.
“I do believe that, as we enter an energy crisis, all forms of energy savings will find a place in the market and that the price of electricity will dictate the lengths to which the consumer will go to make the necessary savings,” he asserts.
Coal vs Geothermal
The Energies journal explains that, if the electricity that drives the geothermal heat pump can be produced from a renewable- energy source like hydropower or geothermal energy, the emission savings will increase to 100%. The total CO2 emissions reduction potential of geothermal heat pumps has been estimated to be 1,2-billion tons a year, or about 6% of global emissions.
Coal-fired power plants produce about 25 times as much CO2 and sulphur dioxide (SO2) emissions for each megawatt hour as geothermal power plants, which emit about 120 g/kWh. However, in a geothermal power plant, hydrogen sulphide (H2S) also needs to be routinely treated and converted into elemental sulphur, since about 0,8 kg of H2S may be produced for each megawatt hour of electricity generated. The Energies journal argues that this is still significantly better than oil-fired power plants and natural- gas-fired plants, which produce 814 kg and 550 kg of H2S for each megawatt hour respectively.
Another advantage of geothermal plants is low freshwater use. The plants use about 20 ℓ of freshwater for each megawatt hour, while a coal plant uses 1 370 ℓ/MWh.
Geothermal power plants generally consist of small modular plants under 100 MWe, compared with coal or nuclear plants of around 1 000 MWe. Further, a geothermal facility normally uses 400 m2 of land for each gigawatt hour, compared with a coal facility which uses almost ten times that area for each gigawatt hour and a wind farm, which uses three times the area for the same power generation. However, sub- sidence and induced seismicity, such as earthquakes, are two land use challenges that must be considered when withdrawing fluids from the ground.
Neither of these potential problems is associated with direct-use projects, as the fluid use is minimal. Further, using geo- thermal resources eliminates the mining, processing and transporting required for electricity generation from fossil fuel and nuclear resources.
Exploring Other Potential Sources
GX Energie director Michael Seeger says that the heat in local gold and platinum mines poses a significant challenge to mining houses. The cooling requirements of a deep-level gold mine producing 200 000 t/m of ore are in excess of 100 MW. The cost of providing ventilation is 50% of operational costs.
“Mining companies are planning to mine below 4 000 m, requiring the existing cooling infrastructure to be expanded. There is a cubic relationship between the quantity of the air to be delivered and the power required to move it, which is generated using coal-based power plants, emitting extensive greenhouse gases,” he notes.
Gadd-Claxton adds that geothermal energy and significant amounts of water collect at the bottom of gold mines. The dirty water could be cleaned by using the available geothermal heat to distill the water. This technology is being applied in Australia.
“Although there are no rifts in the tectonic plates in South Africa, the country has enough deep-level mines and hot rocks to generate geothermal energy. But it will be expensive, costing $50-million to $100-million to implement, which is about the same price as that for a power station,” he adds.
The technology for such exploration and implementation is already available and being used in other countries, such as the US and Israel. Gadd-Claxton points out that neither the idea nor the technology is new but that, because energy in South Africa was always cheap, geothermal energy generation was not considered.
Unep’s assessment showed that South Africa is relatively well endowed with 87 thermal springs with temperatures ranging from 25 °C to 67,5 °C documented. Of the 87 thermal springs, 29 have been developed for direct use, mainly as family leisure and recreational resorts, using the water for health or spa purposes.
However, University of Stellenbosch economic geology professor Abraham Rozendaal says that, although South Africa has warm water sources, there is no steam rising from the ground and the water’s temperature is only 50 °C to 65 °C, which limits the prospects for geothermal energy generation.
“It is possible to generate energy, if we can drill holes of 500 m and use that warm water. But then the geothermal gradient and the capacity of the water’s flow must be adequate and the water source must be geographically located close to the consumer to prevent heat loss,” he explains.
University of South Africa (Unisa) researcher Ernest Tshibalo argues that, with new technology, such as the binary system, hot springs can generate electricity from 74 °C. In a binary system, the heat is used to evaporate a low-boiling-point fluid, which drives a turbine.
Tshibalo explains that a binary power generation system uses two kinds of fluids, namely geothermal fluids and low-boiling-point fluids, like R1-134a, a refrigerant found in many air conditioning systems. “My perception is that drilling into the crust in hot springs, such as at Brandvlei, Bela Bela, Tshipise and Siloam, can lead to temperatures above 74 °C,” he notes.
Gadd-Claxton and his partner at Touch-stone Resources, Dr Anthony Turton, have explored new ways of using water to generate energy. The company is convinced that community-based geothermal projects can be developed soon if commercial partners come on board.
Although securing funds for such projects will be challenging, Gadd-Claxton notes that, if a good, feasible idea is combined with competent, enthusiastic people, the project is sure to get support and invest- ment.
Jones notes that, in July 2009, a group of scientists and engineers met at the AfricaArray yearly meeting, at the University of the Witwatersrand, to discuss the way forward in researching geothermal energy in South Africa and surrounding countries. Representatives included people from the Council for Geoscience, diamond company De Beers, mineral research organisation Mintek and Unisa.
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