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Estimating Hydropower’s Contribution
to the Control of Greenhouse Gas Emiions
ABSTRACT One of the environmental effects of hydropower operation that should be evaluated in licensing decisions is the general benefit to air quality.Hydropower’s contribution to the reduction of greenhouse gas(GHG)emiions is an increasingly important component of these air quality benefits.The Oak Ridge Competitive Electricity Dispatch(ORCED)computer model is one method that can be used to quantify these benefits.ORCED provides a relatively simple method that is applicable and cost-effective and that has been succefully applied in other GHG studies.ORCED can be used to calculate a region-specific value of the carbon intensity factor(CIF, kg carbon/MWh)that would be aociated with likely replacement power(i.e., a regionally representative mix of coal, gas, and other energy sources).The project’s plant factor and operational mode(e.g, baseload versus peaking)can also be incorporated in the CIF calculation.The resulting parameter can then be multiplied by the energy output of the hydropower project that is being analyzed to estimate a CO2 emiion value that is avoided by the project’s operation.Valuing Energy Production Hydropower’s contribution to GHG emiion control is related to avoided emiions(i.e.emiions that would occur if hydroelectricity had to be replaced by another foil-fueled energy source).The estimation of an appropriate value for avoided emiions is complicated, because there is not a single equation to calculate the emiions that are not produced at hydropower projects.The characteristics of avoided emiions depend on the type of power that is displaced by hydropower generation.If a kilowatt-hour were not generated at the hydro plant, what plant would have generated it? The answer depends on a range of factors: the time of day, the plants already on the system, the plants available, their variable costs, the type of fuel they use, their efficiencies, even the transmiion loes and constraints.These factors are regionally and seasonally variable.GHG Measurement Units A common unit for measuring GHG is metric tons of carbon.Although carbon is largely emitted as carbon dioxide gas(CO2)when it is burned, small percentages are also emitted as carbon monoxide(CO)and methane(CH4), which eventually convert to CO2 in the atmosphere.Other GHGs are ozone(O3)and nitrous oxide(N2O).The atmospheric warming effect of GHGs other than CO2 can be represented relative to the effect of CO2.The exact relationship between the gases is complicated by factors such as the wavelength of radiation absorbed, decomposition of gases in the atmosphere, and other atmospheric chemical reactions that can increase or decrease greenhouse gas effects.The warming value will change over time as the other gases are converted to CO2 or otherwise removed from the atmosphere.In general, CH4 is 56 times more potent than CO2 over a twenty year period, but over 100 years this difference drops to 21 times, and over 500 years it drops to 7 times the effect of CO2(EIA 1999b).Nitrogen oxide(NOx)is not a greenhouse gas on its own, but it can combine with CO to promote the formation of ozone.However, the greenhouse impact of power plant NOx emiions compared to CO2 emiions is several orders of magnitude le.Estimation Methods Different levels of modeling complexity can be used in estimating GHG benefits.The simplest approach is to make an a priori aumption of the type of power that would replace hydropower production and then use a representative carbon intensity factor to calculate the relevant carbon emiions.Average carbon intensity factors range from 266kg C/MWh for a coal-fired steam electric plant to 90 kg C/MWh for an advanced gas combined cycle plant(EIA 1999a and 1999b).For example, if the lo of a hydro facility would be replaced by increased coal-fired production at 33 percent efficiency then carbon emiions would increase 266 kg per MWh.If the facility had a capacity of five MW and a plant factor of 50%, replacement with coal-fired production would generate about 5,800 tons of carbon per year.This is equivalent to 4,500 vehicles.Conclusions The production of hydroelectricity is aociated with significant reductions in the nation’s GHG emiions, although the specific amount of this benefit is difficult to measure directly.ORCED provides a relatively simple method to estimate the GHG benefits of hydropower.If more precise answers are required, then other large-load flow or capacity expansion models may need to be used.However, for most cases ORCED provides a mechanism to get results without the high cost or long time of these large models.ORCED can be used to calculate a region-specific value of the carbon intensity factor that would be aociated with likely replacement power, and that factor can be adjusted to account for the hydropower projects plant factor.The resulting carbon intensity factor can then be multiplied by the energy output of the hydropower project that is being analyzed to produce a CO2 savings that is aociated with the project’s operation.This GHG emiion can then be converted to an equivalent value the number of cars needed to produce the same emiion or to some other common measure, to put this savings in a more easily understood measure.This analytical approach can be adapted to evaluating alternative plant operations, such as shifts from peaking to baseload.The model also generates the marginal cost of power for a given region, allowing the user to determine the economic impact of the generation.估算水电的贡献的控制温室气体排放
摘要
水电的操作应在许可决定的环境影响评价的一般利益之一，是空气质量。水电的贡献，是温室气体（GHG）的排放量减少是这些空气质量效益越来越重要的组成部分。Oak Ridge竞争性电力调度（ORCED）计算机模型是一种方法，可以用来量化这些利益。ORCED提供了一个相对简单的方法是可行和符合成本效益，并且已成功地应用在其他温室气体的研究。ORCED可以用来计算一个地区的碳强度因子的特定值（CIF，公斤碳/兆瓦时），将与可能的替换电源（即煤、气区域代表性的组合，和其他能源的来源）有关。该项目的植物因子和运作模式（如基荷与峰值）也可纳入到CIF计算。由此产生的参数，然后可以乘上水电项目正在分析估计二氧化碳排放值，是由该项目的运作避免能量的输出。
重视能源生产
水电的贡献，与控制温室气体排放有关，避免排放量（即排放量会发生变化，如果水电必须由另一个化石燃料能源替代）。为避免排放量估计一个适当的值是复杂的，因为没有一个公式来计算未在水电项目产生的排放量。对避免的排放量的特性取决于对能源的类型，是由水电发电流失。如果一千瓦小时未在水力发电厂产生的，哪种植物会产生呢？答案取决于一系列因素：一天的时间，植物已在系统中，提供的生产基地，他们使用的燃料类型，它们的效率，甚至是传输损耗和可变成本的限制。这些因素是区域和季节的变量。
温室气体计量单位
用于测量温室气体常用的单位是公吨碳。虽然主要的碳排放是二氧化碳气体（CO2），当它燃烧，而一氧化碳（CO）和甲烷（CH4）占很小的比例，最终转化为二氧化碳排放在大气中和其他温室气体的臭氧（O3）和氧化亚氮（N2O）。其他的温室气体等使大气变暖的影响可以表示相对于二氧化碳的效果。气体之间的确切关系有复杂的因素，如吸收波长的辐射，大气中的气体分解，和其他气体的化学反应，可以增加或减少温室气体的影响。气候变暖的价值将随时间而改变为其他气体，转化为二氧化碳或以其他方式存在于大气中。一般情况下，甲烷是56倍以上，过去20年间二氧化碳更主要，但这种差异在100年下降到21倍，超过500年就降到了7倍的二氧化碳的影响（环境影响评估1999b）。氮氧化物（NOx）的温室气体是不针对自己的，但它可以与CO结合，促进臭氧的形成。然而，与电厂氮氧化物排放量相比，二氧化碳温室气体排放的影响是少几个数量级
估计方法
由于建模的复杂程度不同，可用作估算温室气体排放的好处。最简单的方法是使一个假想的电源类型，以取代水电生产，然后使用具有代表性的碳强度因子计算碳排放有关的假设。平均碳强度因子范围从266公斤的C /兆瓦的燃煤蒸汽电热设备厂到90公斤先进的燃气联合循环电厂的C /兆瓦时（环评1999a和1999b）。例如，如果一个水电设施损失将通过增加燃煤改为生产效率33%的碳排放量将增加到每兆瓦时266公斤。如果工厂有五兆瓦和50％植物因子，与燃煤生产替代将会每年产生约5,800吨二氧化碳。这相当于4500辆的排放量。
结论
水电的生产使全国的温室气体排放量大幅减少，但这样做的收益的具体数额是难以直接测量。ORCED提供了一个相对简单的方法来估算温室气体在水电生产中的好处。如果需要更精确的答案，然而更大流量或负载容量扩充模型可能需要使用。然而，在大多数情况下ORCED提供一种机制来获得无高成本或为这些大型模型花很长一段时间的结果。ORCED可用于计算的碳强度的因素，就是可能与更换电源相关区域的具体值，这个因素可以调整，以考虑水电工程机组的使用率。由此产生的碳排放强度因子然后乘上水电项目的能源输出，它是正在分析以产生一个与该项目的操作有关的二氧化碳排放。这种温室气体排放可以被转换为等值的需要产生同样的排放，或其他一些常见的措施，或放在更加容易理解衡量这一储蓄。这种分析方法可以适用于其他植物评估业务，如从峰值到基荷变化。该模型还生成一个特定区域电力边际成本，使用户能够确定产生的经济影响。
