LIFE CYCLE ASSESSMENT FOR COOLING AND HEATING SOURCES OF BUILDING

LIFE CYCLE ASSESSMENT FOR COOLING AND HEATING SOURCES OF BUILDING Lijing Gu, Daojin Gu, Borong Lin, and Yingxin Zhu Department of Building Science, School of Architecture, Tsinghua University, Beijing, China ABSTRACT Design of cooling and heating sources of HVAC system is one of the main parts of green building design, as well as one of key measures to improve building energy efficiency and reduce environmental impact. In this paper, a LCA model for building cooling and heating source system is put forward and an integrated index is given to describe the total environmental impact including energy consumption, resources consumption and pollutant emission through building life, in order to guide the design of building energy system. Firstly, calculation models of environmental load in each phase of building life cycle are proposed. Life cycle environmental impact database of some cooling and heating equipment are established with an investigation of references and stylebooks of several manufacturers. Then, a real building is studied. Life cycle environmental load (LCEL) and economic performance of four cooling and heating sources schemes are analyzed and compared, and the optimal option is proposed. For this special case, the scheme with electrical water-cooled chiller for cooling and gas-fired boiler for heating has lowest LCEL and best economic performance, should be recommended. KEYWORDS LCA, Cooling and heating source, Environmental load, Economic performance INTRODUCTION Design of cooling and heating sources of HVAC system is one of the main parts of green building design, and is important for improving integrated building efficiency. Green means low energy consumption, low resource consumption and less pollution, which has more emphasis on environmental impact of the buildings and their energy systems. However, most practical projects in China at present only take economic performance into account, such as initial cost (IC) and operating cost (OC), when selecting cooling and heating sources. Although sometimes result from economic performance analysis is consistent with that from energy consumption, sometimes not. In recent years more and more research and projects begin to pay attention to energy consumption and pollutant emission. Some researches [1-3] use life cycle assessment (LCA) method to evaluate environmental impact of cooling and heating sources, and give integrated index to describe comprehensive impact of different kinds of pollutant. However, environmental impact of cooling and heating equipment production and transportation is not considered in most researches, which should be one part of life cycle of building energy system. Besides, energy consumption should be taken as one aspect of environmental impact and be evaluated comprehensively together with pollutant emission, but only a few researches [3] do like this. Although some relevant researches [4] in foreign country have been reported, their data and results can not be used directly for China because there are differences of producing technique and production energy consumption between the two countries. In order to guide energy system design for green building, an intact LCA model for building cooling and heating sources in China is put forward in this paper and an integrated index is founded to describe building life cycle environmental impact, including energy consumption, resources consumption and pollutant emission. Corresponding Author: Tel: +8610 62782746, Fax: +8610 62773461 E-mail address: gulj04@tsinghua.edu.cn

Then case study of a real office building in Beijing is carried out. Four design schemes of cooling and heating sources are analyzed and compared, and the best one is found. ASSESSMENT MODELS Life Cycle Assessment (LCA) is an internationally recognized method to evaluate environmental impacts. It is a process to evaluate the environmental burdens associated with a product, process, or activity throughout its life cycle. LCA framework consists of goal and scope definition, life cycle inventory analysis, impact assessment, and interpretation [5]. As for building energy system, its life cycle can be divided into four phases: production of cooling and heating source equipment, maintenance and replacement of the equipment, operation, and recycling and demolishment. Each phase consumes energy and resources and emits pollutant. Based on LCA theory, a LCA database and evaluation system special for buildings and relevant activities in China, Building Environment Load Evaluation System (BELES) [6], has been developed by Department of Building Science of Tsinghua University. BELES uses damage oriented method for Environmental impact assessment (EIA) and uses environmental load (EL) with unit point as an integrated evaluation index. The lower EL is the better environmental performance of building is. Figure 1 shows the procedure of building LCA. With BELES, EL of each phase can be calculated as follows. Figure1. The procedure of building life cycle assessment in BELES Equipment production Equipment producing process includes raw materials exploitation, transportation and processing, various processes of equipment production, and equipment transportation from manufactory to building site. Environmental impact of all these processes will be considered. Life cycle EIA needs inventory data which expressed detailed input and output amount of substances, but corresponding measuring and statistical work about cooling and heating equipment production has not been carried out in China. To solve the problem of lack of inventory data, embodied energy model is founded in this paper. Embodied energy is the total energy used in all processes from raw materials exploitation to the end of production and energy used in transportation. In this data model, embodied energy is used as life cycle energy consumption of product; pollutant emission from embodied energy using process in China is used as life cycle pollutant emission of product; and raw materials of product is used as life cycle resources consumption of it. Because there are no embodied energy data of cooling and heating sources equipment in China, data of Japan [7] are adopted after a revision based on an investigation at Beijing. Considering that energy consumption of production and transportation in China are usually higher than that in Japan or other developed countries, equation (1) is used to revise the data. EE = η EE + η δ EE (1) China 1 p 2 t Where, EE p is the production embodied energy per unit product in Japan, MJ; η 1 is the ratio of production embodied energy per unit product in China over that in Japan; EE t is the transportation embodied energy per unit product in Japan, MJ; η 2 is the ratio of transportation embodied energy per

unit product in China over that in Japan; δ is the ratio of average transportation distance in China over that in Japan. According to the known energy consumption data [8] of main product in China and corresponding data in Japan, energy consumption ratio of each product can be worked out, and take the average of them as η 1, then η 1 is equal to 1.393. Similarly, η 2 is equal to 1.678 according to the oil consumption of transportation. Railway is the main means of transportation in China. The average transportation distance of goods is 767km [8] in China and 442km in Japan [9], so δ is equal to 1.735. Then the revised embodied energy data for product in China can be calculated with equation (2). EE = 1.393 EE + 2.912 EE (2) China f t The uppermost raw metal materials of cooling and heating equipment are steel, copper, aluminum and the uppermost nonmetal materials are refrigerant and plastic. Each one takes up different proportion in different kinds of equipment, but the exact data are hard to get. Therefore the amounts of these main materials are estimated based on data from equipment manufacturer and relevant references. Maintenance and replacement The environmental impact in this phase is mainly caused by producing new component and equipment for maintenance and replacement. Annual maintenance ratio is used to analyze EL of Maintenance. The Life-span of cooling and heating equipment is always shorter than that of building, so replacement is needed. Times of replacement can be worked out with equation (3). Where, T r is times of replacement; L is the life-span of building, year; l is the life-span of equipment, year; INT is a mathematic function to calculate the biggest integer smaller than the value. Tr = INT[ L/ l] (3) Then EL of this phase can be calculated with equation (4). Where, EL r is EL of maintenance and replacement phase, point; EL p is EL of production phase, point; t m is annual maintenance ratio, %. EL = EL ( t L+ T ) (4) r p m r Operation The environmental impact in operation phase mainly comes from energy use, such as use of electricity, burning of coal, oil and gas. Firstly, hourly cooling and heating load of the building are simulated by a software DeST (Designer s Simulation Toolkits) developed by Department of Building Science of Tsinghua University. Then the consumption of each kind of energy is estimated based on building load and equipment performance parameters. Finally, operation EL can be worked out with BELES. Recycling and demolishment Some waste materials can be recycled after use. So recycling ratio is considered and the reduced EL by recycling is counted into EL of production phase. Waste materials that can not be recycled will be disposed with landfill, combustion or direct emission. CASE STUDY Case introduction The study case is a 30-storeyed office building in Beijing with total area 49445m 2. The first four floors are annex. All-glass curtain wall is used for building facade. The thermal parameters of envelope comply with local building design standard. Heat transfer coefficient K of glass curtain wall is 1.6 W/(m 2 K), Shading coefficient SC is 0.45. Cooling and heating load of it are simulated with DeST, and the results are shown in Table 1. It can be seen, heating load of this building is less than cooling load, especially total value. This is because large amount of indoor heat gain of this office building reduces heating load and increased the cooling load. Besides, the large scale glass curtain wall with relatively lower K increases more solar gain but not too much heat conduction loss through windows in winter. However, in summer, more solar gain is a main reason for high cooling load.

Table1 Result of building load simulation item unit value heating load maximum kw 4355 cooling load maximum kw 6811 heating load maximum of unit building area W/m 2 88.1 cooling load maximum of unit building area W/m 2 137.8 total heating load in winter kwh 566624 total cooling load in summer kwh 3608489 Four schemes of cooling and heating sources are compared in aspect of life cycle environmental load (LCEL). Equipment size of each scheme is listed in Table 2. Because cooled water system of each scheme is almost same, only equipment of cooling and heating sources and cooling water system are included in comparison. The life-span of building is assumed 50 years. Table2 Equipment size and parameters of each scheme No. scheme Model and size Rating cooling (kw) Rating heating (kw) amount 1 Chiller: CVGF 500-430-1030-31-T700A-T700A 2285 3 electric water-cooled chiller for Cooling tower: LSH-500 3 cooling, Cooling water pump: IGZ300-380B 3 gas-fired boiler for heating Gas-fired boiler: WN-200 2300 2 2 Chiller: CVGF 500-430-1030-31-T700A-T700A 2285 3 electric water-cooled chiller for Cooling tower: LSH-500 3 cooling, Cooling water pump: IGZ300-380B 3 oil-fired boiler for heating Oil-fired boiler: WN-200 2300 2 3 Absorption chiller/heater: BZ 175 2326 1791 3 LiBr vapour absorption Cooling tower: LSH-700 3 chiller/heater Cooling water pump: IGZ300-380A 3 4 electric air-cooled chiller for cooling, city centralized heating net for heating Air-cooled chiller: LSBLGF1140M 1139 6 Environmental load analysis Firstly embodied energy of cooling and heating equipment are calculated and the results are shown in Table 3, as well as the average life-span of equipment, annual maintenance ratio and replacement times. It can be seen the embodied energy of three kinds of chillers and cooling tower are totally same, because the data are average statistical value of the whole industry. There should be difference between different chillers, but the average value is adopted in this paper because of lack of detailed data. Then resources consumption are worked out according to references [2-4] and corresponding stylebooks of manufacturers. Since detailed data can not be obtained, only main materials are considered. Resources consumption for initial construction of each scheme is shown in Table 4. Based on building load simulation results and equipment efficiency, energy consumption of each scheme is calculated. The amounts of operating equipment, including chiller, cooling tower and pump, are calculated hourly as well as COP of chillers according to stylebooks. Fans of cooling tower and pumps are assumed to operate at rating condition all the time. Efficiency of gas/oil-fired boiler is assumed 0.89. Heat of heating net comes from coal-fired thermal power plant and peak-shaving

oil-fired boiler. Thermal efficiency of thermal power plant is 53.7%, power efficiency 17.5%, and heating net efficiency 0.9. Annual energy consumption of each scheme is shown in Table 5. Table 3 Embodied energy and life-span for cooling and heating equipment EE p in EE t in EE p in EE t in EE china life-span [7] [10] t m Japan Japan China China MJ/kg MJ/kg MJ/kg MJ/kg MJ/kg years % water-cooled centrifugal chiller 77.4 6.3 107.9 18.3 126.2 20 2% 2 LiBr vapour absorption chiller/heater 77.4 6.3 107.9 18.3 126.2 20 2% 2 Air-cooled chiller 77.4 6.3 107.9 18.3 126.2 15 2% 3 gas/oil-fired boiler 33.3 2.4 46.4 7.1 53.4 15 2% 3 cooling tower 77.4 6.3 107.9 18.3 126.2 15 2% 3 cooling water pump 61.7 4.1 86.0 11.8 97.8 15 2% 3 T r Table 4 Resources consumption of each scheme total cast steel copper aluminum Refrigerant * LiBr No. equipment weight iron solution PVC kg kg kg kg kg kg kg kg water-cooled chiller 32529 24397 6506 435 1191 1 cooling tower 13380 6690 6690 cooling water pump 4650 4650 4650 gas-fired boiler 12840 12840 water-cooled chiller 32529 24397 6506 435 1191 2 cooling tower 13380 6690 4650 6690 cooling water pump 4650 4650 oil-fired boiler 12840 12840 LiBr vapour absorption 88800 37464 29436 21900 chiller/heater 3 cooling tower 16380 8190 8190 cooling water pump 5175 5175 5175 4 Air-cooled chiller 59400 10080 23760 23760 1800 * R134a is the refrigerant for water-cooled chiller, R22 for air-cooled chiller Table 5 Annual energy consumption of each scheme summer winter scheme electricity cool from gas heat from gas Heat form oil heat from coal No. kwh MJ MJ MJ MJ 1 837660 2291962 2 837660 2291962 3 255792 8381990 1932649 4 1104417 138677 1916423 Different material has different recycling ratio. In demolishment phase, it is assumed that 90% of the steel will be recycled at the end of equipment life, and 50% for cast iron, 40% for copper, 95% for aluminum, 10% for PVC. However, no recycling is assumed for refrigerant and annual leak ratio 0.5% is considered. Landfill is the common method for most solid waste, combustion for PVC waste, and direct emission for refrigerant (R134a and R22). The amount of waste materials in building life-span of each scheme is shown in Table 6.

Table 6 Amount of waste building materials of each scheme scheme common solid waste refrigerant PVC No. kg kg kg 1 46850 3871 30105 2 46850 3871 30105 3 102665 36855 4 82260 7650 With all the data above, EL of each phase and life cycle can be calculated with BELES, the results are shown in Table 7 and Figure 1. It indicates scheme 4 has the highest LCEL which is obviously higher than other schemes because of its highest EL of operation and higher EL of production, maintenance and replacement. Scheme 1, 2 and 3 have almost same LCEL and scheme 1 has the lowest. Scheme 3 has the lowest EL of operation but highest EL of production, maintenance and replacement which make its LCEL higher than that of scheme 1 and 2. But scheme 3 has lowest EL of demolishment because no R134a or R22 refrigerant which causes more EL than other building material waste is used in this scheme and EL of LiBr solution is not considered for lack of data. Table 7 EL of each phase of each scheme (unit: point/m 2 ) scheme No. production maintenance and replacement Operation demolishment life cycle 1 0.239 0.817 12.426 0.050 13.531 2 0.239 0.817 12.494 0.050 13.600 3 0.472 1.506 11.910 0.006 13.895 4 0.256 1.024 16.098 0.116 17.495 18 EL of unit floor area (point/m 2 ) 16 14 12 10 8 6 4 2 0 1 2 3 4 production maintenance and replacement operation demolishment Figure 2. EL of each scheme Compare EL of each phase, EL distribution of each scheme is show in Figure 3. It can be seen, for scheme 1, 2 and 4, operation EL takes up more than 90% of the LCEL, production EL takes up less than 2%, maintenance and replacement EL takes up 6%, and demolishment EL takes up 0.4~0.7%. For scheme 3, operation EL takes up 85.7% of the LCEL, production EL takes up 3.4%, maintenance and replacement EL takes up 10.8%, and demolishment EL takes up only 0.05%. Since operation EL accounts for most of LCEL, further analysis is made to it. Figure 4 shows operation EL distribution of summer and winter. It can be seen EL of summer takes up about 85% of the total EL of operation. Scheme 3 has lowest EL both in summer and winter because natural gas is used all the

year. Scheme 4 has highest EL both in summer and winter because COP of air-cooled chiller is lower than that of water-cooled chiller in summer and coal is used for most time in winter. Scheme 2 has a little higher EL in winter than scheme 1 because EL of oil use for unit heating load is a little higher than that of gas use. 100% 0.4% 0.4% 0.05% 0.7% 90% 80% 70% 60% 50% 91.8% 91.9% 85.7% 92.0% 40% 30% 20% 10% 6.0% 6.0% 10.8% 5.9% 0% 1.8% 1.8% 3.4% 1.5% 1 2 3 4 production maintenance and replacement operation demolishment Figure 3. EL distribution of each scheme annual operating EL of unit floor area (point/(m 2 a)) 0.350 EL of summer EL of winter 0.300 0.250 0.200 0.150 0.100 0.050 0.000 1 2 3 4 EL of winter 0.039 0.040 0.033 0.045 EL of summer 0.210 0.210 0.206 0.277 Figure 4. Operation EL distribution Economic performance analysis Economic performance of each scheme, including initial cost (IC) and operating cost (OC) per year, is also analyzed. Equipment IC and annual OC are listed in Table 8. Where, electricity cost is calculated hourly according to time-differentiated price in Beijing [11], natural gas price is 1.9 RMB/m 3, and oil price is 2.4 RMB/kg. It indicates scheme 1 has lowest IC and OC, but scheme 4 has highest IC and OC because heating with city heating net is charged according to heating area in Beijing. However, as it is analyzed above, heating load of this building is lower than the average level of the same kind of building, so heating by boiler has better economic performance. Scheme 2 has the same IC with scheme 1 and a little higher OC. Scheme 3 has higher IC but lower OC than scheme 1 because natural gas use in summer saves lots of electricity cost. But the payback time (comparing with scheme 1) of scheme 3 is 42 years, longer than the equipment life-span. When the benefit of CO 2 emission reductions (ERs) (80RMB/t CO 2 ) in operation phase is considered (take CO 2 emission amount of scheme 1 as baseline), the conclusions are still same, only payback time of scheme 3 becomes slightly shorter. The results with the benefit of CO 2 ERs are also listed in Table 8.

Table 8 Economic performance analysis payback benefit of CO 2 annual OC with payback IC annual OC Scheme No. time ERs benefit of CO 2 ERs time 10 4 RMB 10 4 RMB/year year 10 4 RMB/year 10 4 RMB/year year 1 698.1 93.9 0.0 93.9 2 698.1 94.8 -- 0.3 95.1 -- 3 1286.3 79.9 42.1-0.3 79.6 41.7 4 881.9 226.4 -- 3.7 230.1 -- CONCLUSIONS In this paper, a LCA model for building cooling and heating source system in China is put forward and an integrated index is given to describe environmental impact through building life cycle. With an investigation of references and equipment stylebooks, life cycle EIA database of some building cooling and heating equipment is established. Especially, embodied energy model and revision method are proposed to solve the problem of lack of equipment inventory data. Annual maintenance ratio and recycling ratio are considered in maintenance and demolishment phases. Then four schemes of cooling and heating sources of a real office building in Beijing are analyzed and compared with this LCA model. The results indicate scheme 1, electric water-cooled chiller for cooling and gas-fired boiler for heating, has the lowest LCEL. Further analysis of economic performance both with and without the benefit of CO 2 ERs also indicates scheme 1 is the best one. So it should be recommended. Scheme 3, using LiBr absorption chiller/heater, has lowest operation EL, but its highest production EL makes its LCEL higher than that of scheme 1. Similarly, it has higher IC and lower OC than scheme 1, but the payback time is too long. Although all the conclusions are based on this special case, the LCA model is suitable for all cases in China. In our further research, more types of cooling and heating sources will be analyzed, such as building cooling heating and power generation (BCHP) system. REFERENCES 1. MY Lin, SS Zhang, and Y Chen (2004) Life cycle assessment of environmental impact of air conditioning cold and heat source. Heating Ventilating & Air-conditioning, Vol. 34 (7) p93-96. 2. Y Zheng and TZ Zhang (2002) Life cycle assessment for water source heat pump system. Environment Engineering, Vol. 20 (6) p76-78. 3. JQ Deng, WL Li, and ZX Zhang (2006) Gas engine driven heat pump system life cycle assessment. Journal of Xi an Jiao Tong University, Vol. 40 (11) p1258-1262. 4. M.Prek (2004) Environmental impact and life cycle assessment of heating and air conditioning systems, a simplified case study. Energy and Building, Vol. 34 p1021-1027. 5. ISO (1997) ISO 14040. Life Cycle Assessment --Principles and Framework. 6. DJ Gu (2006) Building life cycle environmental load evaluation. Ph.D. Thesis, Tsinghua University (China). 7. Institute for Building Environment and Energy Conservation (IBEC) (2004) Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) Manual 1: Green Design Tool, published China building industry Press. 8. National Bureau of Statistics of China (2002) China Statistical Yearbook 2002, published by China Statistic Press. 9. Japan Ship & Ocean Foundation (2000) A Report on Research Concerning the Reduction of CO2 Emission from Vessels, Tokyo. 10. Architecture committee in Japan (2003) Building LCA index. 11. Anonymity. Electricity price in Beijing http://www.bjdsm.com/newweb/article.