Life Cycle Assessment Estimation for Eco-Management of Co-Generation Systems

Transcription

1 Seizo Kato Naoki Maruyama Yasuki Nikai Hidekazu Takai Anugerah Widiyanto Department of Mechanical Engineering, Mie University, Tsu, Mie , Japan Life Cycle Assessment Estimation for Eco-Management of Co-Generation Systems A LCA (life cycle assessment) scheme for any industrial activity system is introduced to estimate the quantitative load on the environment with the aid of the NETS (numerical environment total standard) method proposed by the authors as a numerical measure. Two kinds of environmental loads respecting fossil fuel depletion as input resources to the system and global warming due to CO 2 emission as output are taken into account in the present eco-criterion, in which the total eco-load (EcL) value is calculated from the summation of respective environmental load factors on the whole process in a life cycle of the system. This NETS method is applied to eco-management co-generation systems, in which a computer-aided output navigator proceeds the LCA estimation with ICON and Q&A communication. An operation scheme most friendly to the environment with a minimum EcL value, i.e., an eco-operation scheme, is derived from the optimization theory. DOI: / Keywords: Life Cycle Assessment, Fossil Fuel Depletion, Global Warming, Co- Generation System, Environmental Load Introduction Any industrial activity, including power generation systems, inevitably results in some load on the environment such as the depletion of fossil fuel resources or global warming due to exhaust gases. The industrial world, therefore, has been analyzing the action program of environmental priority strategies for sustainable development and demanding the international standardization for environmental performance. ISO The International Organization for Standardization accepted the growing trend and established in 1993 the technical committee on environmental management TC207 consisting of six subcommittees and one working group. Thus, the ISO series are planning to be enacted. However, an objective way of estimating the total ecoload EcL value on the environment throughout the entire life cycle from cradle to grave in an industrial activity has not been developed yet. As a consequence, a numerical measure taking the LCA concept into account needs to be introduced. In particular, as co-generation systems are to be operated most favorably to fuel economy because of their tight connection with the global environment and energy issue, the numerical LCA estimation may contribute to eco ecology-oriented management. The purpose of this paper is first to propose a NETS numerical environment total standard method which gives us numerical impact values on the environment as an objective measure. The NETS value is specified on the basis of the maximum sufferable value for the human race, as mentioned in the next section. The total impact value to the environment, i.e., eco-load value, is calculated from the total summation of respective environment load factors on the whole process in a life cycle. First, an industrial activity system, like a co-generation system, is composed of many elemental processes. Second, this NETS method is applied to cogeneration systems for their eco-management from the LCA viewpoint, and a friendly operation scheme with a minimum EcL value, i.e., so-called here eco-operation scheme, is derived from the optimization theory. Contributed by the Fuels and Combustion Technologies Division and presented at the 2nd International Symposium on Advanced Energy Conversion Systems and Related Technologies, Nagoya, Japan, December 1 3, 1998, of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. Manuscript received by the FACT Division March 15, 2000; revised manuscript received October 30, Associate Editor: C. Saltiel. The whole NETS calculation of the elemental load factor (LF i ), the total eco-load value, and their minimization are easily performed by software programmed in this work. An arbitrary co-generation system is constructed by selecting elemental ICONs, and various kinds of specification items are numerically given in Q&A styles regarding hourly demands of space heating and cooling, hot water supply, electricity, and the input resources such as fossil fuel depletion factors like purchase electricity, natural gas, oil, coal, etc. Then, the total EcL value respecting the fossil fuel depletion and the CO 2 global warming due to the cogeneration operation is promptly calculated and graphically displayed. The cost estimation can be made and economical efficiency can be achieved. NETS Method Respecting Fossil-Fuel Depletion and CO 2 Global Warming NETS Outline. The objective definition of the environmental load value of any industrial activity has not been established yet. To interpret the numerical results of the environmental load in the LCA estimation, an overall integrated expression is necessary. The environmental load is numerically calculated from the environmental load value ELV idea proposed originally in the EPS environmental priority strategy project of the Swedish Environmental Research Institute 1 in The EPS project defined the standard value as 100 ELU/person from the taxation on the people in order to keep a present livelihood. Since the value is applied to the environmental loads respecting the fossil fuel depletion, the procedure to calculate the logical value is not shown clearly. Thus, we have revised it if necessary and developed an algorithm for the ELV calculation. The NETS evaluation method proposed here gives us accurate numerical values of the environmental load in which the values change according to the maximum sufferable value for the human race. The NETS value is numerically calculated from the following expressions: n EcL i 1 LF i x i NETS (1) LF i AL i i P i (2) Journal of Energy Resources Technology Copyright 2001 by ASME MARCH 2001, Vol. 123 Õ 15

2 where EcL denotes the eco-load value or the total load value on the environment of any industrial process through its life cycle, LF i, the elemental load factor on environment, and x i, the amount of input resource or output emission at the ith subprocess of the whole process. Thus, since the process of interest consists of many elemental subprocesses, the respective LF i plays essential roles for this environmental life cycle assessment. LF i is calculated from Eq. 2, where P i is the amount of quantitative measurements such as fossil fuel reserves, CO 2 emissions, etc., based on forecasts regarding the earth s carrying capacity. AL i is the absolute load value which the earth can sustain, and i is the weighting factor at the ith subprocess. EcL is expressed in the numerical environment total standard NETS ; it is numerically defined so that 100 NETS stands for the maximum permissible load for one-person survival. With such NETS values, we can evaluate quantitatively the loads of any industrial activity from the global viewpoint. The total eco-load value is calculated from the total summation of the elemental load factor through the whole processes in a life cycle. Table 2 NETS values of load factor LF i respecting global warming NETS Evaluations Respecting Fossil Fuel Depletion and Global Warming. The basic idea of the NETS evaluation respecting fossil fuel depletion is as follows. If we continue every year to consume oil, natural gas, coal, uranium, etc., at the same current rate until their depletions of proved reserves, then we can put the absolute load value as the earth s maximum carrying capacity AL i NETS, which is the product of 100 NETS/person defined in the foregoing and the world population of person. AL i relates different types of global environmental load such as fossil fuel depletion and global warming problems. For example, the value of LF oil for oil is estimated using the proved oil reserve, P oil ton, as follows: LF oil NETS NETS/ton (3) ton This result means that if we consume 1 ton of crude oil, then this industrial activity brings the environmental load of LF oil 4.2 NETS/ton in terms of the oil depletion. Table 1 lists the NETS values of various natural energy resources for depletion, in which the environmental load index ELI values estimated by the EPS project 1 are also given for reference. In regard to the NETS value for the global warming due to CO 2 emission, we may base its criterion on the COP Kyoto Conference agreement that if we continue to emit CO 2 of ton, the level in 1997, the global temperature will rise by 2 3 C within 100 yr. On such a basis, we can evaluate the environmental load as follows: NETS LF CO ton/yr 100 yr NETS/ton Table 1 NETS values of load factor LF i respecting fossil fuel depletion, in comparison with EPS estimations (4) Since there are many kinds of greenhouse gases other than CO 2, we estimate their LF i values using the quantities of global warming potential 2 GWP i which are expressed in numerical values relative to the standard GWP CO2 1 for CO 2. Table 2 lists the NETS values of typical greenhouse gases. NETS Estimation for Power Generation in Japan Purchase Electricity. First of all, we start to calculate the EcL and LF i values respecting fossil fuel depletion due to commercial electricity generation in Japan. Table 3 lists the NETS results of EcL and LF for the Japanese purchase electricity in The superscript and the subscript F.D., i stand for the purchase electricity and fossil fuel depletion, respectively, and i denotes oil, coal, natural gas, and nuclear power stations, respectively. The lowest row, total, indicates the totally integrated value for all power stations. The EcL F.D.,i values for the respective fuels are calculated by the following equation: EcL F.D.,i LF i G i (5) where G i is the consumption for the respective fossil fuels. Then the total EcL is calculated as follows: EcL F.D.,total i EcL F.D.,i The total electricity generation reaches E total MWh/yr in 1997 and E F.D.,total MWh/yr is generated by the fossil fuel fired stations. Therefore, the partial LF F.D.,i values are easily obtainable for any kind of fossil fuel power stations using a component ratio r i to the whole electricity generation. LF F.D.,i EcL F.D.,i (7) E total r i Finally, the total load factor of fossil fuel depletion is obtained by the following equation: (6) Table 3 EcL and LF respecting fossil fuel depletion due to electricity generation in Japan 16 Õ Vol. 123, MARCH 2001 Transactions of the ASME

3 Table 4 EcL and LF respecting CO 2 global warming due to electricity generation in Japan LF F.D.,total EcL F.D.,total (8) E F.D.,total CO 2 Global Warming. Next, we calculate also the purchase electricity load factor value respecting global warming due to CO 2 emission in Japan as of Table 4 lists the LF CO2,i and EcL CO2,i, where the superscript and the subscript CO2 indicates the purchase electricity and global warming due to CO 2 emissions, respectively. LF CO2,i is calculated as follows: LF Co2,i LF CO2 g c,i (9) where g c,i is the incidence of CO 2 emission from the fuels. In Table 4, g c,nuclear does not have null value, because the auxiliary power units that are driven by the fossil fuels are running in order to maintain the plant. EcL CO2,i is also calculated as follows: EcL CO2,i LF CO2 E total r i (10) Finally, EcL CO2,total is calculated as follows: EcL CO2,total i EcL CO2,i (11) From comparison of Tables 3 and 4, LF CO2,i respecting global warming due to CO 2 emissions from power plants in Japan have the same digit numbers as LF F.D.,i respecting the depletion of fossil fuel power plants, except for the nuclear power station. Eco-Operation Scheme of Co-Generation Systems We have attempted to apply the aforementioned NETS environmental evaluation method to the co-generation systems, because any operation of power generation systems links directly to the depletion of natural resources like fossil fuels and also to global warming due to CO 2 emissions. We have constructed in this work an algorithm and software to minimize the environmental loads of fossil fuel depletion and CO 2 global warming in currently operating co-generation energy systems. Co-Generation Model. Figure 1 shows the co-generation system employed here, where the symbol CU denotes a cogeneration unit consisting of gas turbines GT, wasted heat recovery boilers BW, and steam turbines ST. BA, RE, RS, and Fig. 2 Typical pattern of hourly energy demands a in a weekday in summer, b in a weekday in winter HE symbolize an auxiliary boiler, an electric turbo refrigerator, a steam absorption refrigerator, and a heat exchanger, respectively. The gas turbines are selected here as the main electricity generation machines, because exhaust flue gas has high potential to be recovered in boilers and to be converted into electricity by steam turbines as combined cycle units or to supply hot water to the community. Consequently, the total efficiency of a co-generation system becomes higher. Their flexibility for fuel selection is also advantageous. The energy flows of electricity, steam, hot water, cold water, and fuel are shown by the distinctive lines in the figure. The energy and mass balances of electricity, fuel, steam, hot water for heating, hot water for warm water supply, and cold water for cooling derive many balance equations for the respective machines composing the entire co-generation unit. LCA Optimization for Eco-Operation. The algorithms applied here as the first step are the well-known simplex and branchbound methods to seek optimized solutions of objective functions in linear programming. In this work, all the elemental machines composing the co-generation unit to be estimated are assumed to have a linear function between the output y i generated electricity, steam, etc. and input x i fossil fuel, etc., as expressed in Eq. 12 Fig. 1 Co-generation system employed in this study y i a x i b (12) where a and b are constants denoting the performance of each machine, and is a one-zero variable to express ON-OFF of the machine; 1 when the machine is operated and 0 when stopped. Journal of Energy Resources Technology MARCH 2001, Vol. 123 Õ 17

4 Fig. 4 Hourly EcL changing of the co-generation system Fig. 3 Typical ICON display to set up co-generation unit of interest a system configuration and data input, b system layout Equation 12 should be formulated for all the components shown in Fig. 1 so well as to express its performance in advance, and the equations thus obtained are formulated in the computer program. The energy balance conditions for each energy demand also have to be formulated for the respective components, by taking into account the hourly energy demand in a district consisting of eight offices and four hotels. Figure 2 shows a typical pattern of the hourly energy demands for electricity, space cooling, space heating, and hot water supply on a a weekday in summer, and b a weekday in winter at the district area. The space cooling in summer and space heating in winter, particularly the space heating at the beginning of business hours in winter have distinctive features. Such data are also put into computer as a database. The branch and bound method is used here to obtain with high efficiency the optimum solutions of objective functions, including the two environmental loads under many constraint conditions. The solution scheme is programmed so as to obtain the optimum solutions in very short time. Figure 3 shows a typical display to set up the co-generation unit to be estimated, which we can easily operate using ICON and Q&A communications. Figure 3 a shows an example of the input field, where the number of machines installed in the system and their performances are fed. Since various kinds of numerical values required are asked on the display in a Q&A style, their inputs are easily completed and confirmed before calculations. Figure 3 b shows an example of the system layout of interest constructed in this software. Figure 4 shows the hourly changing EcL value optimized by the combined simplex and branch-bound method. These curves correspond to the results on the weekdays in summer and winter shown in Figs. 2 a and b, respectively. The hourly optimal operation scheme for each component is also obtained to minimize the total EcL value. The evaluation conditions are listed in Table 5. The electricity demand is supplied by the gas turbine generator and purchase electricity, then the space heating, hot water and space cooling are mostly supplied by the waste heat from the co-generation unit consisting of gas turbines, heat recovery boilers, and steam turbines. On the other hand, the electricity demand is supplied only from the purchase electricity and the heat energies are supplied by the fossil fuel and the purchase electricity. The LNG is used for both the systems as a fuel. Figure 4 also includes the EcL value evaluated using only purchase electricity in order to compare with the data from the cogeneration system proposed in this work. The symbols and represent the EcL values estimated from the environmentally and the economically optimized operation of the co-generation unit, respectively, and are lower compared with the value evaluated from the ordinary method, given by the symbol. The analogous profiles are shown for the EcL in winter. In both the cases, the electricity generated by co-generation units is much friendlier to the environment than purchase electricity, particularly for the energy demand during business hours. The EcL value integrated Table 5 Electricity and heat supplies by the co-generation system and ordinary method 18 Õ Vol. 123, MARCH 2001 Transactions of the ASME

5 Fig. 5 Running cost evaluation over the whole day may be reduced in comparison with the case without the optimization. Consequently, it can be concluded that the NETS estimation and its optimization described in the present paper contribute well to reducing the environmental loads respecting fossil fuel depletion and CO 2 global warming due to electricity generation, and also to propose the eco-operation scheme of cogeneration energy systems. Figure 5 shows the running cost during the environmentally and economically optimized operation of the co-generation unit compared with the cost of purchase electricity. Judging from the running cost viewpoint, the energy supply generated by co-generation systems is more economical than the cases of the energy demand supplied by only the purchase electricity. This figure also shows that the environmentally optimized results have low values compared with those of the economically optimized; the reason is considered a computing error. In this case, we set the prices of purchase electricity at 17 yen/kwh and natural gas at 30 yen/m 3 for gas turbine operations. The price of purchase electricity includes the cost of facility investment of a power plant. So, the total cost for the energy demands will be reduced with the real fossil fuel prices. The co-generation systems employed in this paper give advantageous methods from environmentally and economically ecooperation viewpoints. The other evaluation methods are desired in order to confirm the validity of this procedure. Figure 6 shows the environmentally and the economically ecooperation schemes of gas turbines, for instance, on a weekday in summer and in winter. The three gas turbines are incorporated in this co-generation system. These schemes presented in this work Fig. 6 a Operating ratio of gas turbines in summer, b operating ratio of gas turbines in winter Journal of Energy Resources Technology MARCH 2001, Vol. 123 Õ 19

6 are no more than an example. They depend strongly on the energy demand and the estimation of LF. These results also show that the EcL for the construction of a power plant and/or co-generation system should be included in the NETS estimation. Concluding Remarks In this paper, we have introduced the LCA scheme with the aid of the NETS as a numerical measure to estimate the quantitative load of any industrial activity on environment. The two kinds of environmental loads respecting the fossil fuel depletion and CO 2 global warming due to the electricity generation from power stations in Japan have been chosen as the criteria. This NETS scheme has been applied to co-generation energy systems because of their direct connection with energy and environmental issues. The computer-programmed algorithm and software using the combined simplex and branch-bound technique give us the numerical EcL in the unit of NETS and its optimization. The EcL optimization has been calculated corresponding to the hourly energy demands for electricity, space cooling, space heating, and hot water from the community consisting of four office buildings and eight hotels. As a result, the NETS scheme constructed here is found to be an attractive and powerful tool to estimate quantitatively the LCA environmental loads of any industrial activity like co-generation energy systems, and also to propose the ecooperation scheme for the industrial activity of interest; although, further improvements are required for more successful estimation. References 1 Steen, B., and Ryding, S. O., 1992, The EPS Enviro-Accounting Method; An Application of Environmental Accounting Principles for Evaluation and Valuation of Environmental Impact in Product Design, Swedish Environmental Research Institute IVL Report, Sweden. 2 Houghton, J. T., Calleander, B. A., and Varney, S. K., 1992, Climate Change 1992, The Supplementary Report to the IPCC Scientific Assessment, Cambridge University Press, UK. 20 Õ Vol. 123, MARCH 2001 Transactions of the ASME