From the perspective of life cycle assessment (LCA), the development, construction, and operation of all kinds of new energy power generation technologies release greenhouse gas (GHG) emissions. This sparks concerns about the low-carbon nature of the new energy power generation technologies. Based on national and international literature review, this paper estimates and compares the GHG emission factors of traditional thermal power generation and new energy power generation technologies in China with the LCA approach. The GHG mitigation potential of new energy power generation technologies as substitution for traditional thermal power generation in China was evaluated, according to the objectives of new energy power generation of the national development planning. The results show that the GHG emission factors of new energy power generation are much lower than that of traditional thermal power generation even with LCA accounting, and the GHG mitigation potential of new energy substitution is huge.
life cycle assessment ; greenhouse gas mitigation ; new energy power generation
The power industry is the largest source of greenhouse gas (GHG) emissions in China, which is determined by the overwhelming coal-fired power generation in the energy structure. The emission mitigation potential of structural adjustment is supposed to be great. At the end of 2009, the Chinese Government released the target that the CO2 emissions per unit of GDP in 2020 will decrease 40%–45% of that in 2005, which will greatly promote the structural adjustment in the power industry. Developing new energy power generation is one of the most important approaches to achieve this goal. The new energy power industry will enter a rapid development period in the next few years.
Much attention has been paid to the GHG mitigation in the processes of power generation. New energy power generation technologies are often considered to be zero-carbon, such as nuclear, wind, and solar photovoltaic. However, from the perspective of life cycle assessment (LCA), GHGs are emitted in the process of construction, operation and decommissioning of new energy power generation facilities, including raw material exploitation, equipment manufacture, transportation, sales and facilities decommissioning and disposal. It sparks our concerns and doubt about the low-carbon nature of new energy power generation technologies. It is necessary to analyze and compare the GHG emissions potential of different new energy power generation technologies by LCA methodology and to clarify the facts.
The calculation of life cycle GHG emissions of new energy power generation technologies is based on the GHG emission factors from intensive literature review. Many researches have been carried out on the GHG emission factors of new energy power generation. The CO2 and N2 O emission factors of coal-fired power were measured based on on line monitoring data, and were compared with the IPCC default factors [ Wu et al. , 2010 ]. Shi et al.  analyzed the GHG emission inventory of power industry in China and established the methodological framework for compiling the GHG emission inventories of the power industry in line with China’s reality. By summarizing multiple research results and using the system dynamics method, an energy carbon source emission simulation model was constructed [ Wang , 2006 ].
Some domestic and foreign researches have focused on the life cycle GHG emission factors of new energy power generation. Ma  calculated and compared GHG emission factors of thermal power, nuclear power, and hydropower system using LCA methodology. Spadaro et al.  estimated life cycle GHG emissions of hydropower projects. GHG emissions of wind power in the life cycle process of materials manufacturing, construction and operation was reported by Zou and Ma  . Liu et al.  reported the life cycle GHG emissions of straw-fired power generation system. Niles et al.  reported life cycle GHG emissions of solar photovoltaic in Switzerland.
In this paper, we analyze and compare the GHG emission mitigation potential of different new energy power generation technologies in China, based on the GHG emission factors analysis and the national new energy development planning from 2008 to 2020.
LCA is a methodology to evaluate the environmental load of a product, process or activity, from raw material exploitation to product manufacture, transportation, sale, use, recovering, conservation, recycling and final disposal. This methodology can be used to evaluate life cycle GHG emissions of various power generation technologies, including the emissions of construction, operation and other processes [ Fan et al. , 2007 ].
LCA has been widely used in various industries. The system boundary identification has great influence on the GHG emission factors estimation in the LCA process, so a consistent system boundary must be used to guarantee the comparability of evaluation results [ Duan and Cheng , 2008 ]. The LCA system boundaries of different power generation technologies investigated in this paper are as follows.
(1) For thermal power generation, coal-fired, oil-fired, and gas-fired power generations are included. GHG emission factors and emission mitigation potential of the combustion process of thermal power generation are comprehensively estimated for all three types of thermal power generation. While the GHG emissions of other processes in the thermal power life cycle are calculated only for the coal-fired power generation. Those are the emissions from coal exploration, washing, transport, and spontaneous combustion, building material production, such as steel and cement, the facilities installation process, the fuel combustion process, and the power plant decommissioning process.
(2) For hydropower generation, the CH4 emissions of the reservoir, building material production processes, such as steel and cement, the equipment installation process, the external energy input emissions during the period of drought and maintenance, and the power plant decommissioning process are included.
(3) For nuclear power generation, it includes the material production of steel, cement, copper and aluminum, the project building process, the uranium mining and smelting process, the spent fuel treatment process, the transport process, and the power plant decommissioning process.
(4) For wind power generation, it includes the material production, the wind turbine production and transportation, the wind farms construction and related machinery, concrete, iron and steel, and the power plant decommissioning process.
(5) For biomass power generation, it includes the processes of planting, harvesting, and transportation, the manufacture of equipment, the building material production, the fuel combustion process, and the equipment recycling and scrapping process. The CO2 absorption in the growth and emission in the combustion process of crops are excluded.
(6) For solar photovoltaic power generation, it includes the polycrystalline silicon smelting process, the solar photovoltaic modules production process, the transportation process, the equipment installation process, the operation and maintenance process, and the equipment recycling and scrapping process.
Only the direct emissions of the production process is included when evaluating the GHG emissions of building materials, such as steel, cement, aluminum, and silicon, for any power generation technology.
LCA is a systematic and complex evaluation methodology. LCA valuation of the GHG emissions of any power generation technologies is time consuming. In this paper, emphasis is put on the evaluation of life cycle GHG emissions mitigation potential of new energy power generation substitution for traditional thermal power generation, literature reviews are quoted to form the basis of emission factors. Emission factors with consistent system boundary and less disputation are chosen for the calculation, in order to reduce uncertainty of the analysis.
In this paper, the emission factors of the power generation technologies are compared with those in the reference year of 2008. For some quoted values that are not reported for the year 2008, their emission factors are converted to be comparable with the 2008 level. This is done in consideration with the technological and efficiency improvement rates to ensure the comparability of the data. In this paper, the GHG emission factors and mitigation amounts are converted to the CO2 equivalent according to their global warming potential (GWP) [ IPCC , 2006 ].
The default emission factors of fuel combustion of the IPCC Guidelines for National Greenhouse Gas Inventories [ IPCC , 2006 ] and the fuel consumption data of the China Energy Statistical Yearbook 2009 [ NBSC , 2010 ] are used to calculate the total emissions of thermal power generation. This is divided by the total thermal power generation to obtain the comprehensive GHG emission factor of the combustion process, here 1,036.8 g (kW h)–1 , and the variation interval, here 931.6–1,189.8 g (kW h)–1 .
According to the researches [ Ma, 2002 , Di et al., 2005 , Spadaro et al., 2000 , Zheng, 2001 and Tremblay and Schetagne, 2006 ] and the combustion emission factors estimated above, the life cycle emission factor of thermal power generation is set as 1,188.8 g (kW h)–1 , and the variation range of GHG emission factor is estimated at 1,083.7–1,341.9 g (kW h)–1 .
There are various sources of GHG emissions in the reservoirs. The emission quantity is affected by many factors, including the soil texture, age of reservoir, water depth, water level, and other reservoir characteristics, as well as climate, water pH, vegetation, clean-up of the reservoir area. There are lots of disputes and great differences in observation and monitoring results on reservoir GHG emissions. Some scholars believe that the carbon source and carbon sink function of reservoirs will reach a balance eventually, so that the net emissions in reservoirs are zero [ Gagnon and van de Vate , 1997 ].
In this paper, the emission factor of clean development mechanism (CDM) methodology is used to calculate the GHG emissions of hydropower generation [ UNFCCC , 2010 ]. Only the incremental GHG emissions from increase in water area caused by the hydropower projects’ dam construction are considered. The drainage type of hydropower stations is excluded from the calculation. According to the power density and power generation of typical hydropower stations in China and the research results of reservoirs in the Northern Hemisphere [ Gagnon and van de Vate , 1997 ], the GHG emission factor of hydropower reservoirs is estimated to be 15.0–23.6 g (kW h)–1 .
According to the reviews [ Spadaro et al., 2000 , Zheng, 2001 , Ma, 2002 , Zou et al., 2004 and Tremblay and Schetagne, 2006 ], the life cycle GHG emission factor of hydropower projects’ dam construction varies at 3.5–8.0 g (kW h)–1 .
Based on the analysis above, the life cycle emission factor of hydropower generation is set from 18.5 to 31.6 g (kW h)–1 .
According to Ma , Spadaro et al. , Zheng , and Tremblay and Schetagne , the life cycle GHG emission factor of nuclear power generation is 7.0–13.0 g (kW h)–1 .
In general, no GHGs are emitted from the operation process of wind power generation. According to Zou et al.  , Schleisner  , and Spadaro et al. , and using parameters of inland wind power plants, the life cycle GHG emission factor of wind power generation is set from 6.0 to 9.0 g (kW h)–1 .
LCA methodology is used to evaluate GHG emissions of power generation of direct straw combustion. According to Lin et al. , Feng and Ma  , and Liu et al.  , the interval of the life cycle GHG emission factor for biomass power generation is 210.0–260.0 g (kW h)–1 .
Solar photovoltaic power generation has not been commercially applied in China, and few researchs have been done on its LCA emissions. There are only few domestic reports on life cycle evaluation of solar photovoltaic modules. According to the estimation reported by Asakura et al.  and Niels et al.  , the life cycle GHG emission factor of solar photovoltaic power generation varies from 20.0 to 40.0 g (kW h)–1 .
According to the aforementioned emission factors, the emission factors of new energy power generation technologies, namely hydropower, nuclear power, wind power, solar photovoltaic power, and biomass power, are much lower than thermal power generation. Among the new energy power generation technologies, biomass power generation is the least preferable technology with its highest emission factor.
The new energy power generation technologies are undoubtedly with low-carbon emissions, though far from absolute zero carbon emissions.
Based on the analysis of life cycle GHG emissions, the GHG emission mitigation potential of new energy power generation substitution for traditional thermal power generation is evaluated.
According to the NDRC [2007a] , the capacity of hydropower, wind power, biomass power, and solar photovoltaic power in China will reach 300 million kW, 30 million kW, 30 million kW, 1.8 million kW, respectively by 2020. According to the NDRC [2007b] , the capacity of nuclear power in China will reach 40 million kW by 2020.
Actually, new energy power generation has been developed quickly in China in recent years with the planned objectives laging behind the actual growth rate. The new Development Plan of New Energy Industry [ CEC , 2010a ] has been approved and the development objectives of new energy have been revised. The objectives of power generation capacity in hydropower, nuclear power, wind power, biomass power, and solar photovoltaic power by 2020 have been improved to 380 million kW, 86 million kW, 150 million kW, 30 million kW, 20 million kW, respectively. However, the objectives of power generation capacity in nuclear power may be adjusted because of the concern on nuclear safety induced by the radioactive spill accident of Fukushima Nuclear Power Plant in Japan. But nuclear capacity reduction is unlikely to happen due to the vast international pressure of carbon emissions reduction. So, 86 million kW is still used as nuclear power capacity objective in this paper.
The new energy power generation is nearly zerocarbon if only emissions in the operational process are considered. According to the planned development objectives, the GHG emission mitigation potential for new energy power generation substitution for traditional thermal power generation by 2020, with the reference year of 2008, is calculated and shown in Table 1 . The static annual GHG mitigation potential is 1,484–1,895 Mt.
|Technology||Power capacity in 2008 (MW)||Operation hours in 2008 (h)||Planned power capacity in 2020 (GW)||Increase in power capacity from 2008 to 2020 (GW)||Increase in power generation from 2008 to 2020 (109 kW h)||GHG emission mitigation contribution for new energy power generation substitution for traditional thermal power generation|
|Life cycle emission||Operation process emission|
|Annual mitigation amount (Mt)||Mitigation proportion (%)||Annual mitigation amount (Mt)||Mitigation proportion (%)|
|Solar photovoltaic power||150.0||1,267||20||19.8||25.1||27–33||1.6||23–30||1.6|
Notes: 1) Increase in power generation is calculated by increase in power capacity multiplied by the operation hours. The number of operation hours is calculated according to the data of power capacity and power generation in 2008. 2) Power generation capacities in 2008 of hydropower, nuclear power, and wind power are quoted from CEC [2010b] . 3) Power capacities in 2008 of biomass power and solar photovoltaic power are quoted from CRES 
If using the life cycle GHG emission factors with the reference year of 2008 and based on the planned development objectives, the static annual GHG mitigation potential is projected to be 1,690–2,084 Mt. It can be seen that the GHG emission mitigation potential rises by 10% to 14% if the LCA methodology is incorporated.
In Table 1 , it can be seen that developing new energy power generation has great GHG emission mitigation effects compared with developing traditional thermal power generation. Nuclear power and hydropower have the largest mitigation potential, and perform a fundamental role in structural adjustment mitigation in the power generation industry. This partly explains why China is unlikely to reduce its nuclear power generation. Wind power, biomass power, and solar photovoltaic power contribute to considerable amounts in mitigation, and are important complements to GHG emission mitigation in power generation.
There exist large CH4 and CO2 emissions from coal mining and spontaneous combustion processes. This enlarges the difference of life cycle GHG emissions between traditional coal-fired thermal power generation and the new energy power generation (except the biomass power generation).
(1) Methodology selection. Due to the limitation of statistic data availability, this paper uses the “Tier 1” method introduced by IPCC , which is relatively inaccurate than “Tier 2” and “Tier 3” to calculate GHG emissions from various sources. There are still great disputes about whether the construction of reservoirs will cause GHG (CH4 ) emissions. The CDM emission factor of reservoirs used in this paper is also in a dynamic revision process, whose accuracy remains to be validated. The life cycle system boundary definition is also rough and could lead to some uncertainty.
(2) Selection of emission factors and calorific value. The GHG emission factors of fossil fuel combustion depend on the carbon content and the combustion rate. Incomplete combustion will leave some carbon in dust and ashes, which may lower the GHG emission factors. Generally speaking, the default GHG emission factors of oil and gas are relatively accurate, but that of coal may change in several percentages because of the different combustion conditions. Changes in fuel calorific value will also bring uncertainty.
(3) Activity data selection. The statistical data used in this paper may inherit systematic and random errors. The process of data reporting and correction also increases the uncertainty. In addition, the energy efficiency of producing, exploration, and transporting processes of steel, cement and other raw materials will gradually increase, and the GHG emissions in these processes will decline gradually in future.
(4) Quotation of secondary data. Many published results are quoted in this paper, of which the baseline period, and boundary definition vary. Inevitable errors still exist, though, calibration and verification of data have been done very carefully.
From the perspective of LCA, the GHG emission factors of new energy power generation, such as hydropower, nuclear power, wind power, biomass power, and solar photovoltaic power, are much lower than traditional thermal power generation. However, new energy power generations are low-carbon, but not absolutely zero-carbon technologies. The GHG emission factors of nuclear power and wind power are the lowest two, while the hydropower and solar photovoltaic power are moderate and biomass power is relatively high.
The estimated GHG emission mitigation for most of the new energy power generation based on LCA are more attractive. Adjusting the power structure and increasing the proportion of new energy power generation will greatly contribute to the GHG emission mitigation in China. Of course, the mitigation potential depends on both the emission factors and planned development objectives.
The LCA methodology can also be used to evaluate the environmental impacts of new energy power generation, for example, the ecological and immigrant influence of hydropower station, the radioactive material spill risk of nuclear power, the influence of wind mill on birds, the silicon tetrachloride from polysilicon producing process [ Liu et al. , 2011 ]. In general, in the making of mitigation policies and planning of new energy development, LCA should play a more important role in evaluating the GHG emission mitigation potential and environmental impacts.
This study is supported by the China Sustainable Energy Program, Energy Foundation (No. G-0911-11642), and Environmental Protection Industry of Commonweal Project “Research on Co-control Policies and Demonstration of Air Pollution and Greenhouse Gas Emissions of Key Industries” (No. 201009051).