Review. Life Cycle Assessment of Renewable Energy Generation Technologies. Yohji Uchiyama. 1. Introduction

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1 TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 2007; 1: Published online in Wiley InterScience ( DOI: /tee Review Life Cycle Assessment of Renewable Energy Generation Technologies Yohji Uchiyama Keywords: renewable energy, generation technology, life cycle assessment, energy balance, carbon-dioxide emission Received 20 January Introduction One of the methods to understand the energy supply system in a comprehensive manner is the Life Cycle Assessment (LCA). LCA is one of the systems analysis methods that are used to deal with various problems created by the current complex industrial society. It is a method to comprehensively analyze various troubles given during the life cycle of a system, from the cradle to the grave, especially those issues affecting the society mainly, such as environmental issues. Clarifying the complex environmental effects on each factor will help take objective measures for decreasing the environmental impact. Life cycle analysis on social infrastructure facilities such as power plants is rather complicated in the ranges of research when compared with other general products. The subjects to be assessed include not only the power generation facility but also the mining of fuels, transformation, transportation and power generation, as well as electric power transmission facilities such as transmitting, transforming and distributing electricity. And on the horizontal system of this entire series of facilities required for electric power generation, the vertical system of from the cradle to the grave should be applied for the analysis. In other words, in each facility the embodied energy and environmental impact should be assessed for construction, operation, maintenance and disposal. 2. Inventory Analysis of Renewable Energy Generation Technology 2.1. Analysis method The analysis method to define the resource consumption and environmental impact of products and technology quantitatively through their life cycle is called the inventory analysis. Inventory University of Tsukuba analysis is the most basic analysis of all the life cycle analyses, and for this reason the largest number of studies and researches has been conducted on it. One of the methods of inventory analysis in power generation technology is the net energy analysis. This is a method to compare the produced amount of electric power with the total of the direct and indirect energy consumed to produce the said amount electric power under the subject power generation technology that is actually measured through its life cycle. The net energy analysis is a method to judge whether the energy supply technology is worth being a producing system, and there exist two types of evaluating methods: the energy balance ratio (produced energy/input energy); and net energy balance (difference between the produced energy and the input energy). The energy balance ratio is the ratio (P/C) of the produced energy by a power plant (P) to the input energy to the plant directly and indirectly (C). On the other hand, the net energy balance means the total net available energy from the plant during its lifetime: in other words, the net energy (P-C) of the produced energy (P) during its lifetime after subtracting the input energy into it for the same period (C). The net energy balance depends on the capacity of a power plant, and becomes larger in proportion to the increase of the plant capacity. Generally, the input energy does not include the fuel for power generation, and therefore the amount of output energy is larger than the input energy. This means, the minimum criteria for a power generation technology applicable to power plant operation is that the energy balance ratio should be greater than 1 and at the same time the net energy balance is greater than Energy balance analysis The energy balance ratio can be explained as the quantity that shows the ratio of the total electric power produced during the working years of the power generation technology to the energy expended in the construction of the power generation facilities and fuel supply facilities as well as the 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

2 LIFE CYCLE ASSESSMENT OF RENEWABLE ENERGY GENERATION TECHNOLOGIES Fig. 1 Energy balance ratio of each power generation technology energy expended for the maintenance necessary to run the plant (except the fuel consumed for production of the output energy). Figure 1 shows the computed results of the energy balance ratio considering factors of conditions in Japan, with regard to fossil fuels (pulverized coal-fired power, oil fired power and LNG burning power), nuclear power and renewable energy (hydropower, photovoltaic power, wind power and biomass). The amounts of electric power from the discussed power generation technologies are the most typical values as current operational plants, and therefore the facility capacity factors are the values calculated by considering the regular maintenance in the case of thermal power and nuclear power plants, and the maximum values obtained in the natural conditions in Japan in the case of renewable energy. It is clearly understood from Fig. 1 that those that have a high energy balance ratio can be listed in the order: hydropower, nuclear power, wind power, oil fired power and pulverized coal-fired power, and in comparison with them, the ratios for the LNG-burning power, which requires a large quantity of energy in its liquefaction process, and the photovoltaic power, for which energy density is low, are rather small. With regard to nuclear power generation, it is possible to remarkably increase its energy balance ratio by shifting the uranium enriching process from the gaseous diffusion method to the centrifugal separation method. The net energy balance means the total electric power that a power generation plant can supply to the consumers during its working lifetime. Figure 2 shows the net energy balance calculated for the same plants for which the energy balance ratios are mentioned above. As can be seen in Fig. 2, nuclear power generation shows the best performance, followed by oil fired power, pulverized coal-fired power and LNG-burning power, but renewable energies such as photovoltaic power generation and wind power generation are inferior. This is because of the fact that the values of the net energy balance are largely affected by the annual facility capacity factor, and the nuclear and thermal power generation sources whose facility capacity is not less than 70% are power sources capable of supplying power on a large scale. On the other hand, if the wind power generation with approximately 25% facility capacity factor and the photovoltaic power generation with approximately 15% facility capacity factor are to compete with the nuclear and thermal power generation in supplying electric power, a large number of power generation plants should be constructed Greenhouse gas emission from renewable energy power generation technology The amount of warming gas emission can be calculated on the basis of the charged amount of energy obtained from the energy balance during the life cycle. The greenhouse gas includes not only the CO 2 generated by the consumption of energy (coal, oil, natural gas and electricity) but also the CO 2 contained in the raw gas of the mined natural gas, CO 2 produced in the chemical process of cement production and methane gas leaking into air when mining coal or natural gas. By translating into CO 2 amount per kilowatt-hour of the total of those gases emitted all through the processes with regard to the power generation from mining fuels, refining, transporting, power generation, disposing waste, etc. during their life cycle, it is Nuclear power Coal fired power Integrated coal gasification combined cycle LNG burning power LNG co-generation power Oil fired power Biomass gas power Hydropower Amount of net power supply Wind power Transmission and distribution loss Indirect loss Photovoltaic power (On house roof) Transforming loss Photovoltaic power (On ground ,000 1,500 2,500 2,500 Amount of electricity power [EJ] Fig. 2 Net energy balance of each power generation technology (1000 MW, life time 30 years) 45 IEEJ Trans 1: (2007)

3 Y. UCHIYAMA possible to compare the environmental impact of various systems. In the Table I, the basic CO 2 unit calculated for the power generation system is shown for each separate category such as at the manufacturing facility, operation and maintenance, combustion and methane leakage. The values in the Table are the total CO 2 amount emitted during the 30-year lifetime of plants divided by the total amount of the generated electric power during the same period. The contribution to global warming by power generation is considered to be in proportion to the size of basic CO 2 unit. If we look at the results shown in the Table, hydropower is the smallest, followed by nuclear, geothermal, other natural energy and thermal power. Especially noticeable is thermal power generation, for which the effect is considerably larger than the nuclear and natural energy sources. It is because the amount of CO 2 directly emitted by combustion of fuels on power generation is very much larger than the amount of the indirect emissions, such as facilities and operation or leakage of methane. When looking at thermal generation alone, the basic unit of CO 2 emission becomes smaller in the order: coal, oil, LNG. Their ratio for power generation alone is 100: 76: 56, but when comparison is made on the amounts including facilities and operation as well as methane leakage, the ratio becomes 100: 74: 66. In this case, the oil-fired power gains a slightly higher position and the LNG-burning power loses. This is explained by the fact that the energy consumed for mining and liquefying the natural gas is large and the amount of CO 2 contained in the raw natural gas is also large. The amount of CO 2 generated in liquefying as well as contained in the raw natural gas is as large as 25% of the CO 2 generated from the fuel on power generation. Nuclear power generation does not emit CO 2 from its fuel, which results in less global warming, and the value is about one thirtieth of that from an LNGburning power generation. Nuclear power generation has a complicated fuel cycle, and construction of the plant requires considerable amount of materials and energy. However, if the emission related to the construction is distributed equally over the working years and translated into per-plant equivalent emission, the value is not so large. Compared with the CO 2 emission caused by the construction work, that emitted in relation to the operational energy necessary for fuel cycling is larger, and especially the amount of CO 2 generated by the large consumption of electricity used for uranium enrichment is tremendous. Renewable energy sources are power generation system that can also slow down global warming. The basic emission unit in photovoltaic power generation is larger than that from nuclear or hydropower generation, but is considerably smaller than that from thermal power generation. In particular, if the photovoltaic power system is installed on the rooftops of dwelling houses, the basic emission unit can be brought down by half of that of electricity business facility which requires its own stands and foundation that add to the burden of cost. 3. Conclusion The evaluation of power generation technology in which renewable energy is utilized is strongly affected by the characteristics of the energy itself. The characteristics of the renewable energy include low energy density, uneven regional distribution and temporal fluctuation. These characteristics are different depending on the types of renewable energy and the conditions of the site, and therefore it is necessary to clarify these conditional factors before conducting the inventory analysis. It may be safe to say that from the results of the inventory analysis conducted up to now, the characteristics of the power generation technology applicable to the renewable energy are as given below Assessment on facility (kw) In the case of photovoltaic power generation or wind power generation, the lower the energy density becomes, the more materials are required for construction of the facility for electricity Table I. Basic unit of CO 2 emission for each power generation system (Unit : g-c/kwh) Power generation system Manufacture of facility Maintenance Combustion Methane leakage Total Coal fired power Oil fired power LNG burning power Nuclear power generation Hydropower generation Geothermal power generation Wind power generation Photovoltaic power (dwelling house) Photovoltaic power (on the ground) IEEJ Trans 1: (2007)

4 LIFE CYCLE ASSESSMENT OF RENEWABLE ENERGY GENERATION TECHNOLOGIES transformation, which means that not only is the input energy per kilowatt output larger but the efficiency of transformation into electricity is also low. To produce the same amount of electricity as that produced with the power generation technology that has a higher energy density, a much larger facility is required and therefore the construction cost becomes higher. Hydropower and geothermal power generation have better characteristics as power generation technology in terms of input energy amount per kilowatt than the photovoltaic and wind power generation, which is also a renewable energy source. The issues to be solved are the small number of suitable locations for construction of the power plants with high-output, high-capacity operating rates, and the long distance to be covered by the transmission lines to bring the generated power to consumers. The biomass system has high energy density, though not as high as fossil fuels, and therefore it can decrease the amount of energy per kilowatt to be charged in the power generation facility. However, the resources are dispersed over many locations, which creates some problems in collecting and transporting the fuel Assessment on facility usage With regard to the power generation facility, if the facility capacity factor becomes higher, the amount of generated power increases. For this reason, it improves not only the energy balance but also the economics of production. Photovoltaic and wind power generation completely depend on nature, and their annual facility capacity factor depends on their site conditions, and therefore, in Japan their sites are limited to only a small number of places where the factor can be not less than 30%. If the facility capacity factor is low, the quantity of the electric power that can be supplied to the society becomes small, which may result in low performance of the net energy balance and its economics as well. In comparison with this, the facility capacity factor of the power generation technology using stored energy such as hydropower, geothermal power and biomass power reaches approximately 40 to 70%. If the energy supplied to raise the facility capacity factor is sufficiently high, their energy balance and economics can be improved Assessment of reliability Renewable energy sources such as photovoltaic power, wind power, wave power, etc. are not only low in energy density but are also intermittent energy sources. The intermittent energy is supplied depending on nature itself. Photovoltaic energy system cannot generate power at nighttime owing to the unavailability of sunshine, and its output decreases substantially on cloudy or rainy days even during daytime. Wind power systems cannot generate electricity without wind. Fluctuations of output power due to meteorological condition lead to less reliability of the power supply. If the electricity generated by wind power and photovoltaic power, which are intermittent, are connected to the transmission or distribution lines, the voltage and frequency in the traditional electricity lines will be severely affected. If the fluctuation of the electricity from the new power generation sources is controlled at a level not more than 10% of that of traditional lines, the fluctuation can be compensated through the traditional lines, but if not, a new system will be required to stabilize such fluctuating voltage and frequency. Energy must be supplied in accordance with the demand. Especially for the reason that the electricity cannot be stored, the facility capacity should be secured to meet the maximum load demanded by customers. If electricity cannot be supplied when customers need it, the existence value of the power plant surely drops. If the electricity is generated intermittently, the power supply cannot always satisfy customers needs, which also decreases the value of the power plant. This type of imbalance should be compensated by capable thermal power generation or nuclear power generation. Photovoltaic power generation and wind power generation are so-called parasitic electric power sources that cannot supply stable electric power by themselves. There is a proposal to use storage batteries to improve the reliability, but while storing the electricity from the photovoltaic power generation and wind power generation, the charging and discharging loss would amount up to approximately 30% of the electric power, which worsens the economics in addition to the storage battery cost. To solve this difficulty, it may be effective to use the cost-effective power at nighttime and offpeak time from the thermal power or nuclear power generation in combination with storage batteries. If it is possible to utilize such redundant power for filling the supply shortage caused by the power fluctuation of photovoltaic and wind power generation, the uneconomical part of the photovoltaic power generation can be compensated, and the large scale thermal power plant and nuclear power plant can operate steadily even in the off-peak time. The parasitic power sources, namely, photovoltaic and wind power, require a support system helped by the large-scale electric power resource. And finally, if a co-operating system of both power resources becomes available, the drawbacks of each system will be compensated by the other, and it will be able to prevent global warming and serve as a stable energy supply source. 47 IEEJ Trans 1: (2007)

5 Y. UCHIYAMA Yohji Uchiyama completed the doctoral degree in Nuclear Engineering at the Graduate School of Science and Engineering, Tokyo Institute of Technology, in 1981 (Doctor of Engineering), and in the same year joined the Central Research Institute of Electric Power Industry as Technical Assessment Group Leader and Senior Researcher, from 2000 he was Professor at the Institute of Engineering Mechanics and Systems, University of Tsukuba. He is engaged in the research of energy technology and assessment of policies. Since 2004, he has been Professor of the Graduate School of System and Information Engineering, University of Tsukuba. 48 IEEJ Trans 1: (2007)