Predicting subsidence at Wairakei and Ohaaki geothermal fields, New Zealand

A finite-element model coupling compaction and fluid flow processes in porous materials has been applied to the subsidence bowls at Wairakei and Ohaaki geothermal fields to provide a basis for predicting subsidence. Most of the subsidence is due to slow drainage of relatively impermeable (0.05–0.3 mD), compressible (15–45 kbar⁻¹) mudstone at less than 300 m depth. Maximum subsidence rates at both fields peaked at close to 500 mm/year, before declining to between 200 and 300 mm/year today. However, it took over 20 years for maximum subsidence rates to start to decrease at Wairakei, compared to 8 years at Ohaaki. This difference is due to the relatively rapid stabilisation of pressure beneath the compacting mudstone at Ohaaki compared to that at Wairakei. Predictions of future subsidence at both fields are made assuming that the pressure beneath the mudstone remains constant. At Wairakei, the present total maximum subsidence of 14 m is predicted to increase to 20 ± 2 m by the year 2050. At Ohaaki, the short history of subsidence makes predictions less certain, and the present maximum subsidence of 2.5 m is predicted to be 3–4 m by the year 2006.     

Controls on stibnite precipitation at two New Zealand geothermal power stations

Stibnite (Sb₂S₃) forms in the heat-exchanger units of several New Zealand binary geothermal power stations. By analysing aqueous samples collected from two representative plants, it was determined that stibnite forms at a rate of 8.7 and 15.8 kg/day at the Rotokawa and Ngawha power stations, respectively. These results were compared to theoretical predictions of stibnite solubility. It was shown that pH change is the principal cause of stibnite deposition at Rotokawa, while at Ngawha the effect of temperature decrease is more significant. Antimony was not detected in vapour-line samples, suggesting that transport is completely within the aqueous phase.     

世界地热发电现状

地热发电技术经过几十年的发展已经取得了很大的进展,目前欧美一些发达国家正在积极探索地热发电新技术,但是我国的地热发电经过第一次开发之后基本上处于停滞状态,主要是受到资源和技术两方面的限制,今后的地热事业应该将解决腐蚀、结垢、回灌作为发展重点和方向。     

Small-medium size geothermal power plants for electricity generation

This paper describes the main methods of utilization of geothermal resources for electric energy production. A thorough review is also given of the general criteria adopted by ANSALDO in their design of new geothermal electric units. The objective of this design is to achieve a more rational exploitation of the source of the primary power. Along with the working characteristics of the units, the paper also describes the provisions taken to eliminate or reduce the problems arising when utilizing geothermal fluids.     

SmartGrid: Future networks for New Zealand power systems incorporating distributed generation

The concept of intelligent electricity grids, which primarily involves the integration of new information and communication technologies with power transmission lines and distribution cables, is being actively explored in the European Union and the United States. Both developments share common technological developmental goals but also differ distinctly towards the role of distributed generation for their future electrical energy security. This paper looks at options that could find relevance to New Zealand (NZ), in the context of its aspiration of achieving 90% renewable energy electricity generation portfolio by 2025. It also identifies developments in technical standardization and industry investments that facilitate a pathway towards an intelligent or smart grid development for NZ. Some areas where policy can support research in NZ being a “fast adapter” to future grid development are also listed.This paper will help policy makers quickly review developments surrounding SmartGrid and also identify its potential to support NZ Energy Strategy in the electricity infrastructure. This paper will also help researchers and power system stakeholders for identifying international standardization, projects and potential partners in the area of future grid technologies.     

Evaluation of tube enhancement for geothermal power generation heat exchangers

Some geothermal waters are relatively clean, so that the use of enhanced surface heat exchangers is possible. This is the basic premise of the present work where trade-offs using enhanced surfaces in binary fluid power generation heat exchangers have been evaluated. Effects of the heat transfer performance and required pumping power resulting from the use of axially finned tubes (included are externally, internally, and externally and internally finned configurations in a variety of dimensions) are compared with smooth-tube designs. The trade-offs indicate where enhanced surfaces may be cost effective.     

Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand

Poorly permeable basement rocks commonly occur in geothermal regions around the world, and the Quaternary Taupo Volcanic Zone (TVZ) of New Zealand is no exception. Production from basement terrane requires detailed knowledge of its geological and geophysical parameters, as shown by the history of Kawerau and Ohaaki, the only geothermal fields in the TVZ where Mesozoic Torlesse terrane greywacke (litharenite) basement is commonly penetrated at drilled depths of 1–2.5 km. In both fields the basement is step-faulted down into the TVZ. Although hot and hydrothermally altered, the greywackes have little permeability. Some production wells feed from elusive basement faults at Kawerau, but rarely at Ohaaki. Greywackes at Ohaaki are of “granite-rhyolite” provenance, and have more interbedded argillite than the “andesite-dacite” derived Kawerau greywackes. In consequence, the Kawerau basement may sustain brittle fracture at higher temperatures and depths than the more ductile Ohaaki basement, allowing convective circulation of higher enthalpy fluids into permeable Quaternary aquifers.     

Fifty years of geothermal power generation at Wairakei

The challenges and changes that have occurred over the last 50 years of remarkable service from the Wairakei Geothermal Power Project are reviewed. The project was initially constructed during the 1953–1963 period. Plant changes including the decommissioning of the high-pressure turbine generators, the installation of a 3.5-MW intermediate-low pressure steam turbine at the Wairakei Power Station in 1996, the commissioning of the 55 MW Poihipi Power Station in 1997, the 14 MW binary power plant at the Wairakei Power Station in 2005, and a proposed new station to be constructed in the Te Mihi area in 2011–2016 are briefly discussed. Also reviewed are steamfield aspects including steam separation processes, a pilot scheme that was designed to carry hot geothermal water some distance before flash steam generation by pressure reduction, steam production from vapor-dominated regions in the Wairakei reservoir, geothermal water injection, and cascade and direct heat uses. Finally, various aspects of the Wairakei development that have contributed to its success are described. It is anticipated that the geothermal resource will be producing beyond 2028 at generation levels 50% above the current (2008) level.     

Thermodynamic analysis and optimization of various power cycles for a geothermal resource

In this study, geothermal resources in Kutahya-Simav region having geothermal water at a temperature suitable for power generation is considered. The study is aimed to yield the method of the most effective use of the geothermal resource and a rational thermodynamic comparison of various cycles for a given resource. Maximum first law efficiencies vary between 6.9 to 10.6% while the second law efficiencies vary between 38.5 to 59.3% depending on the cycle considered. The maximum power output, the first law, and the second law efficiencies are obtained for Kalina cycle followed by combined cycle and binary cycle.     

Efficiency improvement for geothermal power generation to meet summer peak demand

Geothermal power is an important part of New Zealand's renewable electricity supply due to its attractive cost and reliability. Modular type binary cycle plants have been imported and installed in various geothermal fields in New Zealand, with plans for further expansion. Power output of these plants deteriorates in the summer because plant efficiency depends directly on the geothermal resource and the ambient temperature. As these plants normally use air-cooled condensers, incorporating a water-augmented air-cooled system could improve the power output in summer thereby matching the peak air-conditioning demand. In this work, power generation for the Rotokawa plant was characterized using a similar plant performance and local weather. The improved performance was modelled for retrofit with a wet-cooling system. Maximum generation increase on the hottest day could be 6.8%. The average gain in power over the summer, November–February, was 1.5%, and the average gain for the whole year was 1%. With current binary unit generation capacity at the Rotokawa plant of 35 MW, investment in a water-augmented air-cooled system could provide 2 MW of peak generation on the hottest days. This investment in efficiency is found to compare favourably to other supply options such as solar PV, wind or gas.     

Radon measurements at three New Zealand geothermal areas

Radon-222 measurements ranging from 0·10 to 62 nCi l⁻¹CO₂ are reported for geothermal areas at Wairakei, Broadlands and Ngawha, New Zealand. They suggest that for Wairakei the origin of the ²²²Rn is deep, follows the carbon dioxide, and there is little contribution from local ground water. The reverse is true at Ngawha, and Broadlands is intermediate. Comparison of radon and carbon dioxide levels gives some information about permeability in a given field. If a negative correlation is found, permeability may be predicted to be low.     

Numerical studies on power generation from co-produced geothermal resources in oil fields and change in reservoir temperature

The effects of injection rate and the temperature of injected (or re-injected) water on reservoir temperature during power generation by utilizing hot fluids co-produced from oil and gas field were studied using a numerical simulation approach. The chosen target reservoir was LB oil reservoir from Huabei oil field. The reservoir temperature was about 120 °C. It has been found that there was significant temperature decline if the water injection rate was greater than a specific value and the temperature of injected water was less than a specific value. Also studied were the effect of water injection rate on oil production and water cut in LB oil reservoir. The results demonstrated that the oil production increased with the water injection rate, which is reasonable and would be helpful to conduct the power generation project in LB oil reservoir from the economic point of view.     

A 100% renewable electricity generation system for New Zealand utilising hydro,wind, geothermal and biomass resources

The New Zealand electricity generation system is dominated by hydro generation at approximately 60% of installed capacity between 2005 and 2007, augmented with approximately 32% fossil-fuelled generation, plus minor contributions from geothermal, wind and biomass resources. In order to explore the potential for a 100% renewable electricity generation system with substantially increased levels of wind penetration, fossil-fuelled electricity production was removed from an historic 3-year data set, and replaced by modelled electricity production from wind, geothermal and additional peaking options. Generation mixes comprising 53–60% hydro, 22–25% wind, 12–14% geothermal, 1% biomass and 0–12% additional peaking generation were found to be feasible on an energy and power basis, whilst maintaining net hydro storage. Wind capacity credits ranged from 47% to 105% depending upon the incorporation of demand management, and the manner of operation of the hydro system. Wind spillage was minimised, however, a degree of residual spillage was considered to be an inevitable part of incorporating non-dispatchable generation into a stand-alone grid system. Load shifting was shown to have considerable advantages over installation of new peaking plant. Application of the approach applied in this research to countries with different energy resource mixes is discussed, and options for further research are outlined.     

Optimal design of binary cycle power plants for water-dominated,medium-temperature geothermal fields

Exploitation of lower temperature, water-dominated geothermal fields is analyzed, and a methodology for optimizing geothermal binary plants is discussed. The geothermal fluid inlet temperatures considered are in the 110–160 °C range, while the return temperature of the brine is assumed to be between 70 and 100 °C. The analysis shows that the brine specific consumption, ranging from 20 to 120 kg s⁻¹ for each net MW produced, and the efficiency of the plants, ranging from 20% to 45% in terms of Second Law efficiency, are dictated mainly by the combination of the brine inlet temperature, the brine rejection temperature and the energy conversion cycle being used. For given operating conditions and with correct matching between working fluid and energy conversion cycle, it is possible to obtain very similar performances in a number of different cases. It is shown that optimization of the plant can yield improvements of up to 30–40% in terms of reduction of brine specific consumption compared to conventional design.     

An estimation of the enhanced geothermal systems potential for the Iberian Peninsula

An estimation of the Enhanced Geothermal System's theoretical technical potential for the Iberian Peninsula is presented in this work. As a first step, the temperature at different depths (from 3500 m to 9500 m, in 1000 m steps) has been estimated from existing heat flow, temperature at 1000 m and temperature at 2000 m depth data. From the obtained temperature-at-depth data, an evaluation of the available heat stored for each 1 km thick layer between 3 and 10 km depth, under some limiting hypotheses, has been made. Results are presented as the net electrical power that could be installed, considering that the available thermal energy stored is extracted during a 30 year project life. The results are presented globally for the Iberian Peninsula and separately for Portugal (continental Portugal), Spain (continental Spain plus the Balearic Islands) and for each one of the administrative regions included in the study. Nearly 6% of the surface of the Iberian Peninsula, at a depth of 3500 m has a temperature higher than 150 °C. This surface increases to more than 50% at 5500 m depth, and more than 90% at 7500 m depth. The Enhanced Geothermal System's theoretical technical potential in the Iberian Peninsula, up to a 10 km depth (3 km–10 km) and for temperatures above 150 °C, expressed as potential installed electrical power, is as high as 700 GW_e, which is more than 5 times today's total electricity capacity installed in the Iberian Peninsula (renewable, conventional thermal and nuclear).     

An improved global wind resource estimate for integrated assessment models

This paper summarizes initial steps to improving the robustness and accuracy of global renewable resource and techno-economic assessments for use in integrated assessment models. We outline a method to construct country-level wind resource supply curves, delineated by resource quality and other parameters. Using mesoscale reanalysis data, we generate estimates for wind quality, both terrestrial and offshore, across the globe. Because not all land or water area is suitable for development, appropriate database layers provide exclusions to reduce the total resource to its technical potential. We expand upon estimates from related studies by: using a globally consistent data source of uniquely detailed wind speed characterizations; assuming a non-constant coefficient of performance for adjusting power curves for altitude; categorizing the distance from resource sites to the electric power grid; and characterizing offshore exclusions on the basis of sea ice concentrations. The product, then, is technical potential by country, classified by resource quality as determined by net capacity factor. Additional classifications dimensions are available, including distance to transmission networks for terrestrial wind and distance to shore and water depth for offshore. We estimate the total global wind generation potential of 560 PWh for terrestrial wind with 90% of resource classified as low-to-mid quality, and 315 PWh for offshore wind with 67% classified as mid-to-high quality. These estimates are based on 3.5 MW composite wind turbines with 90 m hub heights, 0.95 availability, 90% array efficiency, and 5 MW/km² deployment density in non-excluded areas. We compare the underlying technical assumption and results with other global assessments.     

2030年希腊地热发电将达100兆瓦

近日,希腊PV杂志报道,希腊政府发文为今后十年国家的能源和气候政策开辟了道路,同时概述了到2030年促进可再生能源发展的重大计划。该计划要求到2030年,可再生能源在国家最终能源消费中的占比达到35%。希腊最初的目标是31%。到2030年,将有约61%的电力来自可再生能源发电,43%的供暖和制冷来自可再生能源,约19%的交通需求由可再生能源满足。