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New Zealand Engineering 1999 July Energy
Geothermal Energy - the Forgotten Renewable The development of geothermal energy is part of New Zealand’s technological expertise. Pioneering efforts at Wairakei during the 1950s, followed by several decades of refinement and innovation, gave rise to New Zealand’s international reputation in the field. Since that exciting start Wairakei power station has gone on to become one of the most reliable power stations in the country, quietly producing base load electricity, and New Zealand scientists and engineers have developed a steady export business for geothermal know-how. But it has not all been plain sailing and may not be in the future. The acceptance of geothermal energy utilisation as a reliable and proven technology means it has become mundane. After decades of operation in dozens of countries, improvements are mostly incremental rather than spectacular breakthroughs. But geothermal technology does not promise to deliver in the next 10 or 20 years, it delivers reliable heat and power right now. Like any energy source geothermal heat and power has shortcomings and risks, it is not right for every situation, and is probably not renewable when used intensively. Careful judgment is required before any development, and conservation issues must be reconciled. Despite this, it offers a number of advantages in New Zealand’s energy mix which make compelling reasons to use more geothermal energy in the future, for electricity generation and as a direct source of heat. Geothermal power plants are particularly well-suited to providing base load power, exceeding even hydro stations in their availability with factors over 90% commonly achievable. Base-load operation combined with high availability and immunity to climatic variations, means that geothermal annual generation from a given installed capacity is high. Geothermal resources presently provide about 4% of New Zealand’s installed electricity generating capacity, but 6.4 % of the annual generation. These figures are expected to increase to 5% and 7.5% respectively when the Mokai plant is commissioned in 1999. Deregulation of power generation and growing participation of Maori in joint venture projects with energy companies has seen an increase in geothermal generation in recent years. Scope for new projects is wide with substantial (over 1500 MWe) untapped high temperature geothermal resources available. Energy consumption continues to rise in New Zealand as the economy diversifies with a gradual shift of emphasis from sheep and dairy farming. Much of the recent demand has been met by fossil fuel, and no one needs reminding that there are more cars on the road than a decade ago. The country’s output of CO2 has risen as a consequence and, although partly offset by an increase in forestry and a decrease in methane production due to lower stock numbers, the net greenhouse gas emission from New Zealand is increasing. This poses something of a dilemma. On one hand the desirability of economic growth is widely accepted; on the other, New Zealand has made an international commitment, in an agreement known as the Kyoto Protocol, to reduce greenhouse gas emission to 1990 levels over the period 2008-2012. Many would argue that the jury is still out on the link between man-made CO2 and climate change. Nevertheless, the New Zealand Government has opted for a cautious approach; taking the view that it is better to be safe than sorry. Reducing CO2 emissions without severely impacting on the economy is the hard part. This aim is also at odds with the notion that economic forces should dictate future energy policy, which appears to be the current political view. Low worldwide oil prices and long-term commercial agreements that hold down natural gas prices provide little incentive for any conservation measures, let alone a switch to alternatives aimed primarily at lower CO2 production. Appropriate development of New Zealand’s extensive geothermal resources could make a significant contribution towards Kyoto Protocol commitments. The CO2 emissions from geothermal power stations are much lower than from fossil-fuelled stations. To put this in perspective, generating 1500 MW of base load power with fossil fuel would emit between 5 and 10 million tonnes of CO2 per year, depending on fuel type, compared to less than one million tonnes of CO2 from geothermal power. By 2008 New Zealand is expected to be 34 million tonnes over its greenhouse gas emission target level, if we continue with "business as usual". Increasing the number of geothermal power stations would also reduce dependence on hydro power, with its consequent supply security risk in dry years, which has led to additional use of fossil-fuel plants at times of shortage. The amount of CO2 in geothermal fluids varies greatly from field to field. Interestingly, New Zealand’s two largest developments (just 30 km apart) represent extreme ends of the spectrum, even when compared to other plants around the world. The Ohaaki development taps high gas sources (2.5%) whereas gas at Wairakei is very low (0.15%). Most New Zealand fields have a gas content somewhat less than 1%. The final CO2 emission level depends on the technology used, but for a typical geothermal power plant tapping a source fluid with, say, 1% gas in separated steam, the emission of CO2 would be about one-fifth to one-tenth of the amount produced per MWe in a fossil fuel plant. Geothermal plants also have sulphur emissions, which are less than coal or oil fired plants, but higher than natural gas stations. However, they do not emit nitrous oxides, since there is no combustion process. Even greater savings in CO2 emission are possible when geothermal energy is used directly as a source of heat, substituting for a fossil fuel. Data published by EECA indicates that 39% of all industrial energy use in New Zealand is for process heat under 300�C, which could be readily substituted with geothermal where the demand is in suitable locations. The best local example of geothermal direct heat use on an industrial scale is the Tasman Pulp and Paper mill in Kawerau. Geothermal fluids are used to generate clean process steam for paper drying, as a source of heat in evaporators, for timber drying, as well as for electricity production. Geothermal condensate is then reused as feed water in power and liquor recovery boilers. Many other smaller scale operations are scattered about the central North Island, but industrial use of geothermal heat is not extensive. To attract future attention perhaps geothermal must offer something more attractive. The great, as yet undelivered, promise is the mineral riches offered by the fluids. Many useful industrial minerals are currently reinjected back into the ground with the geothermal water. The most abundant of these is silica, which in the right form is a valuable industrial material. Silica deposition in geothermal equipment is currently a major constraint on energy extraction, and considerable effort is being devoted to inhibit silica deposition and to extract silica as a marketable product. Reducing silica scaling would reduce greenhouse gas emissions only slightly, but it would open the door to extraction of other minerals such as boron and lithium, and could make new and existing geothermal projects significantly more attractive in financial terms. Geothermal energy offers heat and power with a considerable advantage over fossil fuels in terms of greenhouse gas emissions. The technology is well proven and reliable on the scale required and recent developments have shown that it competes in the current energy market. It might not be very glamorous, but geothermal energy could make an important contribution to New Zealand’s objectives in the medium term. Michael Dunstall is a lecturer, Geothermal Institute, University of Auckland |
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