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.     

发展我国小水电问题的探讨

小水电作为清洁可再生资源 ,充分开发利用已受到世界各国的重视。我国小水电的发展与成就举世瞩目 ,本文拟在我国小水电发展政策回顾的基础上 ,分析我国影响小水电发展的相关因素 ,提出进一步发展我国小水电的政策建议。     

Technology parameter for thermal power stations

The technology parameter which represents the ratio of fractional change of an investment to fractional change in cost of electrical energy has been computed for thermal power stations. The value lies below ten which signifies that the is good scope for additional investment (bigger power stations).     

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.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

Binary dual-flashing geothermal power plants

A new type of geothermal power plant is proposed and analysed, which combines the advantages of the binary and the dual-flashing units. This combination avoids some of the shortcomings of conventional binary plants. At their optimum conditions these binary dual-flashing plants may produce up to 40% more work than a conventional binary optimized unit. Any fluid with suitable thermodynamic properties may be used as secondary fluid. For this study ammonia, freon-12 and isobutane have been used in the secondary Rankine cycle. Two parameters are optimized in the binary dual-flashing plant, namely the temperature of evaporation and the dryness fraction in the evaporator. The optimization procedure and the governing equations for the operation of the new plants are described in the paper.     

A binary-flashing geothermal power plant

A novel type of binary geothermal installation is analyzed. It would increase the work obtained by more than 20%. This installation utilizes partial evaporation of the secondary fluid and flashing of the remaining liquid. Freon, ammonia and isobutane are examined as secondary fluids for the new installation. They all yield superior thermodynamic performance with respect to a conventional binary unit because the binary-flashing installation optimizes two parameters rather than one only. It also appears that the new installation is economically advantageous over existing installations.     

Investment prospects for geothermal power in El Salvador''s electricity market

A mixed-integer optimizing programming model was created to simulate capacity expansion for the electricity market in El Salvador. Various demand scenarios were constructed, under which capacity expansion alternatives were tested. Results showed that possible geothermal projects were able to meet the growing energy needs of El Salvador, while yielding relatively low prices for the end-user. A best case projection for 2020 showed an increased proportion of geothermal generation in the energy mix by 6% compared to the present mix.

Much of the current generating plants and planned capacity are distanced from the load center, San Salvador. In order to meet the country's increasing demand, it was found that generating capacity investment should be accompanied by transmission upgrades. Even when current conditions were simulated, transmission congestion appeared to be present. Results from some expansion scenarios showed that transmission congestion increased nodal prices despite the addition of further generating capacity.     

A scenario for geothermal electric power development in Imperial Valley

The growth of geothermal electric power operations in Imperial Valley, California is projected over the next 40 yr. With commercial power forecast to become available in the 1980s, the scenario considers three subsequent growth rates of 40, 100 and 250 megawatts (MW) per year. These growth rates, along with estimates of the total resource size, result in a maximum level of electric power production ranging from 1000 to 8000 MW to be attained in the 2010 to 2020 time period. Power plant siting constraints are developed and used to make siting patterns for the 400 through the 8000 MW level of power production. Two geothermal technologies are included in the scenario: flashed steam systems which can produce their own cooling water from the geothermal steam condensate and which emit noncondensable gases to the atmosphere; and high pressure, confined flow systems which inject all the geothermal fluid back into the ground. An analysis of the scenario is made with regard to well drilling and power-plant construction rates, land use, cooling water requirements, and hydrogen sulfide emissions.     

Dry cooling towers as condensers for geothermal power plants

The aim of this paper is to present scaling laws for a dry natural draft cooling tower by modelling the heat exchanger and the tower supports as a porous medium. Porous medium modelling of the tube bundles that allows a vigorous theoretical analysis of the problem is adopted. Scale analysis is used as the theoretical tool to study the problem of turbulent free convection through the heat exchanger bundles and along the cooling tower chimney. Results are then compared with full numerical simulation of the problem to observe splendid agreement.     

Numerical evaluation of surface condensers for geothermal power plants

The transport and partitioning of gases in four surface condensers for geothermal power plants has been modelled numerically. A vent condenser between the main condenser and the first stage gas ejectors improves hydrogen sulphide partitioning, particularly if the condensate from it, and from the inter- and aftercondensers, is recirculated to the main condenser tube bundles. Regardless of steam composition, hydrogen sulphide emissions may be largely eliminated by a Stretford Unit combined with a suitable surface condenser. However, if the steam contains ammonia it may be necessary to add sulphur dioxide to neutralize part of the ammonia. In no case is hydrogen peroxide needed to largely eliminate hydrogen sulphide air pollution.     

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。     

Unusual applications of small geothermal resources

The purpose of this paper is to draw attention to possible applications of small geothermal resources which are relatively little known. The applications will be grouped into three categories, namely, (a) applications based on the content of geothermal resources, (b) applications based on the location of geothermal resources and (c) various small-scale applications.     

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.     

Optimum size of thermal power stations

It is well known that the price per unit of generated electrical energy decreases with increasing size of the generator, which implies longer transmission lines and hence larger transmission losses. Considering both these facts, the optimum size of a thermal power station has been obtained by using life-cycle costing analysis; the demand is proposed to be met by a base load generator and a peak load generator. The dependence of the investment ratio (the ratio of present worth of net income to the capital investment) on relevant parameters has been studied. It is seen that there exist optimum sizes of base load generator and peak load generator of a power station, for a given load density. The effect of electricity price, coal price and escalation rates on the optimum sizes has also been investigated. The analysis has been made for constant demand as well as for growing demand. The effect of the ratio of base load to peak load on the economics has also been investigated. The cost data from a recent study in India have been used.     

Construction costs of nuclear power stations

Assuming that nuclear power will increasingly supply the base-load of electricity generation in the industrialised world, the author identifies areas where techniques and practices can be improved to control costs. He cites lack of standardisation and over-long licensing times as major factors in the recent rapid escalation of nuclear power costs and suggests that the adoption of standard reactor designs, the encouragement of a vigorous and competitive European nuclear industry and the streamlining of licensing procedures could improve matters.     

Operating policies for wind-pumped storage hybrid power stations in island grids

Pumped storage is today viewed as the most suitable storage technology for achieving high wind penetration levels in multi-megawatt-sized autonomous island grids, where the technical constraints introduced by the conventional generating units impose limitations on the output of wind farms. In this study, an operating policy is proposed for hybrid wind-hydro power stations (HPSs) in island grids, to increase wind penetration levels, while at the same time minimising the impact on the conventional generation system and ensuring the viability of the HPS investment. The proposed operating strategy is applied to three different autonomous island systems using a dedicated logistic model, to evaluate the effect of the HPS on the overall operation and economics of the island systems and to assess the feasibility of HPS investments.     

An upstream reboiler design for removal of noncondensable gases from geothermal steam for Kizildere geothermal power plant, Turkey

Kizildere geothermal power plant, Turkey, has an installed capacity of 20.4 MW_e. The field contains a high level of noncondensable gases (NCGs), changing from well to well, in amounts as high as 10–20% (with an average of 13% at the inlet of the turbine) by weight of steam. This amount of NCGs is being extracted from the condenser by gas compressors that consume about 17% of the total power production of the plant.An upstream reboiler process could be adopted to remove the NCGs from geothermal steam before they enter the turbine. Upstream reboilers therefore provide a cleaner and less corrosive steam supply to the turbine and condenser, increasing power generation performance for very high NCG contents.In this paper, upstream reboiler systems are investigated as an alternative to conventional gas extraction systems for Kizildere geothermal power plant. A vertical tube type reboiler has been designed and it is found that, as NCG content increases, the condensation heat transfer coefficient reduces steeply. It is concluded that vertical tube type reboilers are not efficient for fields that contain high levels of NCG (>15% by weight of steam). It is recommended that the use of direct contact reboilers be further investigated for this application.     

Energy analysis of nuclear power stations

Responding to the controversy stirred by the initial papers analysing nuclear power, the author has refined the original analysis, correcting its ommissions, and aims to document the factual basis on which energy analysis of nuclear power is based. He concludes that differences in opinion on this application of energy analysis are due not simply to differences in data or the adoption of different analytical conventions, but rest upon fundamentally conflicting views of consumer behaviour and how energy is used.