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Life cycle assessment of residential heating and cooling systems in four regions in the United States
Energy and Buildings 40 (2008) 503–513 www.elsevier.com/locate/enbuild
Life cycle assessment of residential heating and cooling systems in four regions in the United States Viral P. Shah, David Col Debella, Robert J. Ries * Department of Civil and Environmental Engineering, Benedum Hall Room 949, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 9 March 2007; received in revised form 6 April 2007; accepted 13 April 2007
Abstract Heating and cooling systems consume the most energy and are the largest source of emissions in the entire life cycle of a house. This study compares the life cycle impacts of three residential heating and cooling systems—warm-air furnace and air-conditioner, hot water boiler and airconditioner, and air–air heat pump over a 35-year study period. Simulation and life cycle assessment studies of the systems at four locations in the United States, namely Minnesota, Oregon, Pennsylvania and Texas determine the effect of regional variations in climate, energy mix, and the standard building characteristics on the systems’ environmental impacts. In Minnesota, Pennsylvania, and Texas, the heat pump has the highest impacts whereas in Oregon the heat pump has the lowest impacts. A second scenario shows that substitution by high-efficiency equipment reduces the impacts of all systems but does not affect the order of relative performance by region. Another scenario examined the replacement of coal-generated electricity by renewable generation in regional grids. In order to reduce the impacts of the heat pump system to the lowest of the three systems, renewable sources would have to replace 42% of electricity generation in Minnesota, 15% in Pennsylvania, and 38% in Texas. # 2007 Elsevier B.V. All rights reserved. Keywords: Life cycle assessment; Residential; Heating; Cooling; Heat pump; Regional effects
1. Introduction The Residential Energy Consumption Survey conducted by the U.S. Department of Energy (US DOE) [1] estimates that almost every house in the U.S. has some form of space heating. In addition, nearly 76% of the homes use air-conditioning systems, indicating that a majority of the households use both heating and cooling systems during the year. The natural gas powered central warm-air furnace system is the most popular, having nearly a 42% share of the total number of heating systems. About 70% of the households make use of central air-conditioning systems run by a conventional external condenser or a heat pump. Previous studies have shown that the major proportion of the environmental impact of a residential building is due to the energy consumption for space heating and cooling [2,3]. Heating and cooling systems differ in their source of energy, the type of appliance, and the distribution system; hence, they also vary in their environmental impacts. Prek [4] has studied the
* Corresponding author. Tel.: +1 412 624 9548; fax: +1 412 624 0135. E-mail address: [email protected] (R.J. Ries). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.04.004
environmental impacts of three alternative distribution systems for hydronic heating or cooling of a single family house. The study focuses on the material production phase of the systems’ components and excludes the impacts of operational energy consumption. It is seen that the environmental impacts are strongly related to the materials used in the systems. A study by Heikkila¨ [5] compares the life cycle environmental impacts of two air-conditioning systems for an office building in Sweden. The difference in the form and source of the energy dominates the relative environmental effects of the systems. Hence, a careful selection of the type of conventional heating and cooling systems can considerably enhance the environmental performance of a residential building. This study applies a life cycle assessment (LCA) framework [6] to evaluate three heating and cooling systems which differ in their source of energy and the type of system. The systems studied are: (a) central natural gas furnace heating and conventional central air-conditioning, (b) natural gas powered hydronic heating and conventional central air-conditioning, and (c) electric air–air heat pump for heating as well as cooling. The study is intended to present a comparison of the three systems in terms of their life cycle environmental impacts and assist home
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owners and property developers in making informed decisions at the time of new construction or renovation. A hypothetical case study is developed wherein the three alternative systems are installed in a new single family residential house and operated over a life of 35 years. Since the design and performance of the heating and cooling systems depends largely on the regional climate, the three systems are studied for the same house at four locations having different climatic conditions: Minnesota, Oregon, Pennsylvania, and Texas. These regions were chosen for their representative range of climate. The regional differences in the fuel mix for electricity generation and consequently their emission rates are also considered while evaluating the systems. The second portion of the study focuses on two scenarios: (i) replacing the systems with currently available high-efficiency systems of the same types and (ii) increasing the percentage of renewable energy sources in the electricity mix. 2. Methodology and data sources The analysis is performed using a process based approach wherein the life cycle of the systems is divided into distinct phases: extraction of raw materials, manufacturing and transportation of the system components, operation and disposal. The life cycle impact of the entire system is the combined impact of these sub-phases of the system. SimaPro 5.0 [7] is the software used to model the systems’ life cycles. SimaPro 5.0 facilitates the use of several process databases which contain the material and energy inputs and outputs for the basic materials and processes which are used as building blocks to model higher level processes. The Franklin USA 98 [8] and the ETH-ESU 96 [9] databases reflect average practice in the USA and Europe, respectively. The unit processes described in these databases are used to model the systems. The material constituents of the system components are determined from the manufacturers’ literature wherever applicable. However, in the absence of a detailed material description from the manufacturer, the proportion and the quantity of the basic materials is based on judgment and experience. The transportation distances are from the manufacturing location to the house. The operating energy consumption of the systems is calculated using the Home Energy Saver (HES) [10] web interface to the DOE-2 building energy simulation software developed by the US DOE. The house is simulated based on several parameters such as area, spatial orientation, building envelope, and climate. The annual energy consumption for heating and cooling is calculated, and energy consumption is considered to remain constant across the
entire 35-year study period. The life cycle impact assessment is performed using the Impact 2002+ [11] method which characterizes the inventory results via 14 midpoint categories into four damage categories. 3. House characteristics The house selected for the case study measures 181 m2 (1950 ft2) in livable area. It is occupied by a family of two adults and two children. It is an L-shaped house, has two stories above the ground and a partial unconditioned, unfinished basement and a single car parking garage. The house is assumed to be surrounded by trees and other houses on three sides. Further shading is provided by roof eaves extending 1 ft on each face of the house. The glazing area is about 18% of the gross opaque wall area. An unconditioned attic acts as the ceiling of the second story. The walls above the concrete basement walls are wood framed with brick veneer finish. The building envelope materials and insulation level of a typical house vary with the climate. For instance, a ‘‘typical’’ house in a cold northeastern climate has higher wall insulation level compared to the insulation in the hot southern climate. Thus, selecting the same insulation values for the house in all the four locations would not truly capture the difference in the common practice in these places. The insulation levels for the four locations are calculated based on the recommendations of the International Energy Conservation Code (IECC) [12]. Table 1 summarizes the insulation levels adopted for modeling the buildings at the selected locations. The entire living area is conditioned at all times of the day throughout the year. The thermostat settings are fixed at 27 8C (81 8F) and 18 8C (64 8F) during the cooling and heating seasons, respectively. 4. Central appliances and distribution systems The central furnace and the boiler are a source of heating only as opposed to the heat pump which is used for heating as well as cooling. To maintain equivalence between the systems, the furnace and the boiler are coupled with a conventional airconditioning system for the analysis. The components of the three systems are given in Table 2. 4.1. Warm-air furnace The warm-air furnace uses natural gas as its primary source of energy and electricity as the secondary source. The burner assembly of the furnace uses an electrical spark to ignite a mixture of natural gas and air. The air, warmed from the
Table 1 Building envelope insulation levels at various locations Building Component 2
2
Glazing U-factor, W/K m (Btu/h 8F ft ) Glazing solar heat gain coefficient Floor insulation, K m2/W (h 8F ft2/Btu) Ceiling insulation, K m2/W (h 8F ft2/Btu) Wall insulation, K m2/W (h 8F ft2/Btu)
Minnesota
Oregon
Pennsylvania
Texas
2.0 (0.35) 0.55 5.3 (30) 8.7 (49) 3.7 (21)
2.6 (0.45) 0.55 3.4 (19) 5.3 (30) 2.3 (13)
2.3 (0.40) 0.55 5.3 (30) 6.7 (38) 3.2 (18)
3.7 (0.65) 0.40 3.4 (19) 5.3 (30) 2.3 (13)
V.P. Shah et al. / Energy and Buildings 40 (2008) 503–513 Table 2 The components of the three heating and cooling systems
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transported a distance of 800 km (500 mi) in a 28-tonnes capacity truck from the manufacturer’s factory to the local distribution centers.
System
Central appliance
Distribution
Furnace and AC
Warm-air furnace Air-conditioner Fan coil unit
Ductwork
Boiler and AC
Hot water boiler Air-conditioner Fan coil unit
Ductwork, pipes and radiators
Heat pump
Heat pump Fan coil unit
Ductwork
4.2. Hot water boiler
combustion of the natural gas, rises up to the heat exchanger where the heat is transferred to the air flowing through the duct network in the house. The combustion by-products rise up through a masonry chimney and escape to the atmosphere. The furnace chosen for the current analysis is the 58DLA model manufactured by Carrier Corporation [13]. The annual fuel utilization efficiency (AFUE) of the furnace is 80%. AFUE is the average of the ratio of the heat generated to the fuel energy consumed during an entire year. It includes all heat losses due to seasonal variations and occupant temperature control. The basic material composition of the furnace and its estimated lifetime is given in Table 3. The materials mentioned in Table 3 form the bulk of the furnace’s weight. Other materials which constitute a smaller fraction of the appliance weight are not considered for the analysis. The furnace is assumed to be Table 3 Material composition and estimated life for the central appliances and distribution components Appliance
Basic material
Weight (kg)
Estimated life (year)
Furnace
Steel Galvanized steel Aluminum Copper
46 18 9 3
20
Boiler
Cast iron Galvanized steel Copper
145 18 1
35
Air-conditioner
Steel Galvanized steel Copper Aluminum R-22 (Refrigerant)
78 35 17 17 6
20
Heat pump
Steel Galvanized steel Copper R-22 (Refrigerant)
101 32 17 7
20
Fan coil unit
Steel Galvanized steel Copper
48 26 2
25
Duct
Galvanized steel Fiberglass
265 140
35
Pipes and radiators
Steel Copper Fiberglass
415 82 202
35
The boiler is also powered by natural gas. The operation of the boiler is similar to the furnace except that the combustion energy is utilized in heating water through a heat exchanger which is then distributed through a network of pipes throughout the house. A boiler manufactured by Burnham is used for the study [14]. The boiler, too, operates at 80% AFUE. To maintain equivalence, the hot water from the boiler is used only for space heating and not for other domestic purposes. Table 3 summarizes the major material components and the estimated lifetime of the boiler. The longer lifetime of the boiler is primarily due to the durability of the cast iron components. The boiler is transported a distance of 400 km (250 mi) in a 28-tonnes truck. 4.3. Air-conditioner The air-conditioner studied is manufactured by Carrier Corporation [15] and marketed under the model number 24ACR3. The condenser unit is located outside the house. Two refrigerant lines run from the condenser unit to the fan coil unit located in the basement of the house where the heat exchanger is located. The cool refrigerant absorbs the heat from the air flowing through the duct work in the house. The seasonal energy efficiency ratio (SEER) of the unit is 13. The major material constituents and the expected life are given in Table 3. It is assumed that half the total refrigerant charge escapes during the lifetime of the condenser and is replaced. The unit is transported a distance of 800 km (500 mi). 4.4. Heat pump The heat pump is similar to an air-conditioner, except for the provision for reversal of the refrigeration cycle. Thus the heat pump is used for cooling as well as heating. The heat pump used in the study is the E1RA90 model manufactured by York [16]. The heating seasonal performance factor (HSPF) of the heat pump is 7.5. The HSPF is the ratio of the total thermal output (in British Thermal Units) to the electricity (in Watt-hour) consumed during a normal heating season. The SEER for the heat pump is 10. Heat pumps, generally, have a lower efficiency compared to air-conditioners because of their reversibility. Table 3 summarizes the major materials and estimated life of the heat pump. Similar to the air-conditioner, the refrigerant is assumed to escape during operation and maintenance. However, since the heat pump operates throughout the year, as opposed to the air-conditioner, it is assumed that the full charge of the refrigerant escapes during its entire life. 4.5. Fan coil unit The fan coil unit is located inside the house and contains the interior heat exchanger. The refrigerant from the air-conditioner
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Table 4 Regional climate overview for the four locations [18] Location
Heating degree days (base 65 8F)
Cooling degree days (base 65 8F)
Minnesota Oregon Pennsylvania Texas
7876 4400 5829 1648
699 390 726 2974
insulation material are located 650 km (400 mi) and 800 km (500 mi) away, respectively. Baseboard radiators, which are interspersed with the pipe circuit, are the means of heat exchange between the hot water and the room and air in the house. These are manufactured about 975 km (600 mi) from the house. Table 3 gives the basic materials and lifetimes of these components. 5. Overview of the locations
or the heat pump circulates through the heat exchanger, cooling the internal air flowing through the ducts. The FA4C model manufactured and marketed by Carrier Corporation [17] is chosen for the study. The material breakdown and the average life of the fan coil unit are given in Table 3. It is estimated that the unit is transported 800 km (500 mi). 4.6. Ducts The duct network emanates from the fan coil unit located in the basement and spreads horizontally outwards, along the ceiling, on both the floors. These duct both hot and cold air except for the hydronic heating system, where heat is distributed by piped water. The ducts are made of 0.76 mm (22 gauge) galvanized steel sheets and are insulated with a 50 mm (2 in.) fiber glass layer. The material quantities are given in Table 3. 4.7. Piping system and baseboard radiators The hot water from the boiler is carried via a pipe circuit running throughout the house. The pipes are copper tubes of 38 mm (1–1/2 in.) diameter and are insulated with 20 mm (0.8 in.) thick fiberglass. The manufacturers for the pipes and
The three systems are simulated at four different locations – Minnesota, Oregon, Pennsylvania, and Texas – to capture the difference in performance based on regional variability. The parameters which are varied are: building characteristics, regional climate, and electricity mix. The difference between typical construction in each of the states, for the purpose of this study, is manifested primarily in the levels of insulation of the building envelope. Table 4 shows the differences in the regional climates for these states based on the 30-year average heating and cooling degree days. The regional climatic conditions are based on the data collected at a single station in each state and is not the statewide average. Minnesota and Texas represent two extremes, predominantly requiring heating and cooling, respectively. Oregon and Pennsylvania require some amount of heating as well as cooling. The third significant difference between the four locations is in the energy mix used for electricity generation, that is, the annual average proportion of the different fuel sources for a unit of electricity. The energy mix for the states is summarized in Table 5. Fossil fuels are the primary source of electricity, except for Oregon which generates most of its electricity from hydropower. The energy mix plays a significant role in determining the performance of the systems as the environmental impact of a unit of electricity
Table 5 Electricity generation energy mixes by type for the four locations [19] Fuel source
Minnesota
Oregon
Pennsylvania
Texas
Coal (%) Natural gas (%) Nuclear (%) Petroleum and other gases (%) Hydropower and other renewables (%)
65 3 25 2 5
7 25 0 1 67
55 5 35 2 3
38 48 10 2 2
Table 6 Lifetime fuel consumption for the three systems at the four locations Location
Mode
Furnace and AC Natural gas (kWh)
Boiler and AC Electricity (kWh)
Natural gas (kWh)
Heat Pump Electricity (kWh)
Electricity (kWh)
Minnesota
Heat Cool
703,200
4,950 26,850
820,400
39,650 26,850
417,200 34,900
Oregon
Heat Cool
351,600
2,050 6,200
395,500
26,550 6,200
116,300 9,400
Pennsylvania
Heat Cool
527,400
3,900 29,800
542,000
29,250 29,800
214,800 39,600
Texas
Heat Cool
190,500
3,200 112,700
162,250
13,100 105,200
52,100 148,400
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Fig. 1. Illustration of the Impact 2002+ methodology [11].
generated from coal plant differs significantly from a unit of electricity generated by a hydropower plant. 6. Operational energy consumption The operational fuel consumption for a life of 35 years is calculated by simulating the house for a period of 1 year using HES [10]. The annual energy consumption is assumed to remain constant for the entire life span of the systems. The lifetime energy consumption for the three systems at the four locations is presented in Table 6. As expected, the operational
energy consumption is a function of the regional climate. It is also a function of the type of heating and cooling system and their efficiencies. For example, the electricity usage for the boiler is higher than the furnace, and the heat pump consumes more electricity than the air-conditioner because of lower efficiency. 7. Assumptions and limitations It is important to consider the assumptions and limitations of the study when interpreting the results.
Fig. 2. A comparison of the impacts associated with manufacturing the systems and the associated infrastructure using the Impact 2002+ method.
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a. The study considers a specific set of building characteristics and operating conditions, which significantly influence the environmental impacts of the systems. However, the variation in the relative impacts of the systems would be smaller.
b. The manufacture of the appliance’s components and their transportation are not considered in the study. The transportation of the finished appliance from the manufacturer’s factory is the only transportation considered in the study.
Fig. 3. (a) A comparison of the life cycle impacts in terms of four midpoint categories in Minnesota. (Results are normalized to the worst case.) (b) A comparison of the life cycle impacts in terms of four midpoint categories in Oregon. (Results are normalized to the worst case.) (c) A comparison of the life cycle impacts in terms of four midpoint categories in Pennsylvania. (Results are normalized to the worst case.) (d) A comparison of the life cycle impacts in terms of four midpoint categories in Texas. (Results are normalized to the worst case.)
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509
Fig. 3. (Continued ).
c. The installation of the systems at the site, the material waste resulting during installation, and the impacts associated with labor are not considered in the LCA. d. Although the design of the systems changes with location and the heating and cooling load, the system infrastructure has been kept constant for all the locations. e. The study estimates the lifetime fuel consumption from the annual consumption, calculated using historical climate data, without factoring in any loss of system efficiency. f. It is assumed that an appliance, at the end of its life, is replaced with another appliance of the same efficiency and design. Thus, it does not consider the effect of any technological advances in the system designs. g. The electricity generation mix for all the locations is considered to remain the same throughout the 35 years, to illustrate a ‘business as usual’ scenario. h. It is assumed that 90% of the materials by mass are recycled and the remaining 10% are disposed in a landfill. 8. Life cycle impact assessment Life cycle impact assessment (LCIA) [20] is the phase of the LCA where the results of the inventory are assessed based on the life cycle resource inputs and emission outputs. The typical steps involved in a LCIA are: (i) definition of the impact categories based on issues of environmental concerns, (ii) classification of the inventory into these impact categories based on their potential environmental effects, and (iii) characterization of the inventory within each category using equivalence factors based on the severity of their environmental effects. Additionally, normalization and weighting of the impact categories may be used to reflect disparities in the relative magnitude and importance of the categories. Since LCIA results demonstrate the performance of the systems in terms of environmental impact categories, instead of mass emissions or energy requirements, they are extremely useful when comparing systems. The Impact 2002+ LCIA method [11] is used to evaluate and compare the impacts of the three heating and cooling systems. This method links the results of the inventory to four damage
categories – human health, ecosystem quality, climate change, and resources – via 14 impact categories, as illustrated in Fig. 1. The scores in the damage categories are further normalized based on the overall extent of impacts to one person in 1 year in Western Europe. This brings equivalence to the impacts in the four damage categories and thus makes them comparable to each other. 9. Results and interpretation The impact assessment is divided into two parts: the impacts due to the system infrastructure, i.e., raw materials for the appliances and the associated distribution system and terminals including maintenance over the 35-year study period, and the impacts for the operational energy use over the 35-year study period. The impacts associated with only the system infrastructure are presented in Fig. 2 in terms of human health, climate change, resource use, and ecosystem quality damage categories in Impact 2002+. Even though the boiler is assumed to function for the entire study period without replacement, the boiler and AC system have the highest impact in the all the damage categories. This is primarily because of the high impacts due to the manufacturing of the metals used in the system. The human health impacts are high as a network of copper pipe and steel radiators are used for heat distribution in addition to the ductwork required by the AC system. The impacts due to the heat pump are the least in all four categories, since a single appliance fulfils both the heating and cooling functions. The life cycle impacts over the study period including energy use for the three systems at the four locations are presented in Fig. 3 and Fig. 4(a–d). Fig. 3 shows the relative life cycle impact for the three systems in terms of respiratory inorganics, aquatic ecotoxicity, global warming, and nonrenewable energy midpoint categories in Impact 2002+. The majority of the impact in the respiratory organics category is from SOx and NOx which is released during the extraction and distribution of natural gas for the furnace and boiler systems, or from coal-generated electricity for the heat pump system. The impact in the aquatic ecotoxicity category is mainly from the oil
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Fig. 4. (a) A comparison of the life cycle impacts in terms of the four damage categories in Minnesota. (b) A comparison of the life cycle impacts in terms of the four damage categories in Oregon. (c) A comparison of the life cycle impacts in terms of the four damage categories in Pennsylvania. (d) A comparison of the life cycle impacts in terms of the four damage categories in Texas.
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511
Fig. 4. (Continued ).
emissions during natural gas manufacturing and due to dispersion of metallic ions during manufacturing of the system appliances. Thus, the impacts in this category are higher for the boiler and furnace systems than the heat pump system. The extraction and combustion of fossil fuels for energy is the single largest source of impact in the global warming and the nonrenewable energy categories. The allocation of impacts in the damage categories is shown in Fig. 4(a–d). For Minnesota, Pennsylvania, and Texas, the heat pump has the highest life cycle impact, and in Oregon, the boiler and AC system has the largest impact. This is primarily because of the difference in the electrical energy generation mix in the states. Oregon derives two-thirds of its energy from hydropower which has considerably lower impacts in emissions and resource use compared to fossil fuel powered plants. The impacts for the heat pump in the ecosystem quality and resource categories in Pennsylvania are relatively lower than Minnesota
and Texas because the electrical energy generation mix is over one-third nuclear power. The impact for the boiler and AC system is larger than the furnace and AC system due to the higher electricity requirement for distributing water for heating. Oregon has the lowest annual combined heating and cooling load among the four states and therefore it has the lowest impacts in all the damage categories across all systems. The impacts for the boiler and AC and the furnace and AC systems are almost equal in Texas since it is cooling load dominant. The impact for the heat pump system in the climate change and resources category is only 3–8% higher in Pennsylvania than in Texas, even though the combined heating and cooling primary energy consumption in Pennsylvania is about 27% higher. This can be attributed to the percentage of nuclear energy in the electricity mix in Pennsylvania versus a similar percentage of natural gas generation in Texas. Minnesota has the highest impacts overall for all three systems because of its higher
Fig. 5. A comparison of the life cycle impacts of the high efficiency and base systems at the four locations. (The bars indicate the score of the base case and the numbers indicate the percentage change from the base case.)
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Table 7 Electricity generation energy mixes by type such that the impact of the heat pump is the lowest among the three systems Fuel source
Minnesota
Pennsylvania
Texas
Coal (%) Natural gas (%) Nuclear (%) Petroleum and other gases (%) Hydropower and other renewables (%)
23 3 25 2 47
40 5 35 2 18
0 48 10 2 40
heating loads and its dependence on coal for electricity. Thus, both the regional climate and the electrical energy generation mix have significant effects on the total impacts associated with the systems and are also important factors in determining which system has the least environmental impact for a given region. 10. Scenario analysis The efficiency of the appliances has increased over the recent past due to advances in technology. In this scenario, the appliances in the base case are substituted with higher efficiency appliances. The AFUE of the high-efficiency furnace and boiler increases to 94%, the SEER of the AC and the heat pump to 18 and 14, respectively, and the HSPF of heat pump to 9. These represent the typical high-end products currently available from manufacturers. The efficiency of these appliances is higher than the minimum Energy Star criteria, which are 90% and 85% AFUE for the furnace and boiler, respectively; 13 SEER for the AC and heat pump; and 8 for the HSPF. The life cycle impacts due to high-efficiency appliances are presented in Fig. 5 as a single Impact 2002+ score. The single score is the summation of the impacts in all the four Impact 2002+ damage categories. The decrease in the impacts is in the range of 10–21%. The largest relative decrease is observed in the furnace and AC system in Texas because of the significant increase in the cooling efficiency from the base case. In all four locations the change in the impact of the furnace and AC is greater than that of the boiler and AC since the higher efficiency appliance does not affect the operating electricity consumption, which is higher for the boiler. However, the three systems continue to follow the same order of preference at all locations, and as in the base case, the impacts of the heat pump system are higher than the impacts of the other systems, except in Oregon. The second scenario is aimed at determining the efficacy of replacing coal electricity generation with renewable energy for decreasing the environmental impacts. Studies have shown that Minnesota, Pennsylvania, and Texas have potential wind energy resources for utility-scale electricity generation [21]. For this scenario, renewable wind energy was substituted for coal electricity generation until the all-electric heat pump option had the lowest environmental impact. The combination of electrical energy generation types by state that would reduce the impacts of the heat pump to make it the lowest among the three systems is given in Table 7. Approximately, 64, 27, and 100% of the current coal-generated electricity in Minnesota, Pennsylvania, and Texas should be substituted by wind energy. This would require installation of wind energy generating
capacity of 2400, 3700 and 17,000 MW in the three states, respectively, and require a wind farm land area of about 350, 550 and 2500 km2, respectively. 11. Conclusion Three residential heating and cooling systems – furnace and AC, boiler and AC, and air–air heat pump – were compared using life cycle assessment. The three systems were studied at four locations – Minnesota, Oregon, Pennsylvania, and Texas – representing different climatic conditions and electricity generation mix. The aim of the study was to understand their relative environmental performance in a regional context. The boiler and AC system have the largest impacts associated with the appliances and distribution systems. However, the impact of the operational energy consumption is dominant over the entire study period. The heat pump has the maximum impacts in regions where a high proportion of the electricity is derived from fossil fuels, and this applies in heating as well as cooling climates. The furnace and the AC system perform the best in these regions. In Oregon, where a large share of electricity is generated from hydropower, the heat pump exhibits the lowest impacts. Scenario analysis substituting currently available high-efficiency appliances can result in up to about a 20% reduction in impacts, without any change in the order of preference in all climates. In order to minimize the impacts of the heat pump, it was found that from 15–40% of the current electric grid generation capacity should be substituted with renewable energy sources. The current percentage of renewable energy generation in the average US electricity grid is less than 10%, including hydropower, wind, solar photovoltaic, and others. Increasing this percentage would require additional infrastructure investment and possibly lead to increased cost for electricity. Before reaching a conclusion, it is crucial to understand the details of the assessment methodology and the assumptions made during the study. In addition to environmental impact, additional criteria of cost, comfort, fuel availability, constructability, and maintenance should be considered. References [1] U.S. Department of Energy, Energy Information Administration, Residential Energy Consumption Survey 2001, http://www.eia.doe.gov/emeu/ recs/contents.html, 2001 (retrieved December 15, 2006). [2] G. Keolian, S. Blanchard, P. Reppe, Life cycle energy, costs and strategies for improving a single family house, Journal of Industrial Ecology 4 (2) (2000) 135–157. [3] L. Ochoa, R. Ries, H.S. Matthews, C. Hendrickson, Life cycle assessment of residential buildings, in: American Society of Civil Engineers Construction Research Congress, San Diego, CA, April, 2005. [4] M. Prek, Environmental impact and life cycle assessment of heating and air conditioning systems: a simplified case study, Energy and Buildings 36 (10) (2004) 1021–1027. [5] K. Heikkila¨, Environmental impact assessment using a weighting method for alternative air-conditioning systems, Building and Environment 39 (10) (2004) 1133–1140. [6] ANSI/ISO, Environmental Management – Life Cycle Assessment – Principles and Framework (ISO 14040-1997), NSF International, Ann Arbor, MI, 1997.
V.P. Shah et al. / Energy and Buildings 40 (2008) 503–513 [7] M. Goedkoop, M. Oele, SimaPro 5.0 Reference Manual, Pre Consultants, Netherlands, 2001. [8] Franklin Associates Ltd., The Franklin US LCI library, USA, 1998. [9] R. Frischknecht, N. Jungbluth, SimaPro Database Manual, The ETH-ESU 96 Libraries, Report Version: 2.1, ETH-ESU, Uster, CH, 2004. [10] Lawrence Berkeley National Laboratory, The Home Energy Saver, http:// hes.lbl.gov, 2006 (accessed on December 1, 2006). [11] O. Jolliet, M. Margni, R. Charles, S. Humbert, J. Payet, G. Rebitzer, Impact 2002+: a new life cycle impact assessment methodology, International Journal of LCA 8 (6) (2003) 324–330. [12] International Code Council, International Energy Conservation Code 2000, Washington, DC, 2000. [13] Carrier Corporation, 58DLA/DLX comfort 80 deluxe 4-way multipoise induced-combustion gas furnace, http://www.xpedio.carrier.com/idc/ groups/public/documents/techlit/58dl-5pd.pdf, 2006 (retrieved on January 13, 2007). [14] Burnham, Series 2 Gas-Fired Hot Water Boiler, No. PL81401171000-6/ 02-60Mc, Burnham Corporation, Lancaster, PA, 2002. [15] Carrier Corporation, 24ACR3 comfort 13 series R-22 air conditioner 1-1/2 to 5 nominal tons (size 18 to 60), http://www.xpedio.carrier.com/idc/ groups/public/documents/techlit/24acr3-2pd.pdf, 2006 (retrieved on January 13, 2007).
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[16] York, Technical Guide: Split-System Heat Pumps, 10 SEER, No. 036-21317-001 Rev. A (0702), York International Group, York, PA, 2002 . [17] Carrier Corporation, FY4A, FA4C base series fan coil sizes 1.5–5 ton (018 through 060), http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/fa4c-3pd.pdf, 2006 (retrieved on January 13, 2007). [18] National Oceanic and Atmospheric Administration, US Department of Commerce, Monthly station climate summaries (CLIM 20), http:// cdo.ncdc.noaa.gov/cgi-bin/climatenormals/climatenormals.pl, 2007 (retrieved on January 14, 2007). [19] U.S. Department of Energy, Energy Information Administration, State electricity profiles 2004, DOE/EIA-0629 (2004), http://www.eia.doe.gov/ cneaf/electricity/st_profiles/sep2004.pdf, 2006 (retrieved January 14, 2007). [20] ANSI/ISO, Environmental Management – Life Cycle Assessment – Life Cycle Impact Assessment (ISO14042-2000), NSF International, Ann Arbor, MI, 2000. [21] U.S. Department of Energy, National Renewable Energy Laboratory, Wind energy resource atlas of the United States, DOE/CH-10093-4, http://rredc.nrel.gov/wind/pubs/atlas/atlas_index.html, 1986 (retrieved January 22, 2007).
Life cycle assessment of residential heating and cooling systems in four regions in the United States Viral P. Shah, David Col Debella, Robert J. Ries * Department of Civil and Environmental Engineering, Benedum Hall Room 949, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 9 March 2007; received in revised form 6 April 2007; accepted 13 April 2007
Abstract Heating and cooling systems consume the most energy and are the largest source of emissions in the entire life cycle of a house. This study compares the life cycle impacts of three residential heating and cooling systems—warm-air furnace and air-conditioner, hot water boiler and airconditioner, and air–air heat pump over a 35-year study period. Simulation and life cycle assessment studies of the systems at four locations in the United States, namely Minnesota, Oregon, Pennsylvania and Texas determine the effect of regional variations in climate, energy mix, and the standard building characteristics on the systems’ environmental impacts. In Minnesota, Pennsylvania, and Texas, the heat pump has the highest impacts whereas in Oregon the heat pump has the lowest impacts. A second scenario shows that substitution by high-efficiency equipment reduces the impacts of all systems but does not affect the order of relative performance by region. Another scenario examined the replacement of coal-generated electricity by renewable generation in regional grids. In order to reduce the impacts of the heat pump system to the lowest of the three systems, renewable sources would have to replace 42% of electricity generation in Minnesota, 15% in Pennsylvania, and 38% in Texas. # 2007 Elsevier B.V. All rights reserved. Keywords: Life cycle assessment; Residential; Heating; Cooling; Heat pump; Regional effects
1. Introduction The Residential Energy Consumption Survey conducted by the U.S. Department of Energy (US DOE) [1] estimates that almost every house in the U.S. has some form of space heating. In addition, nearly 76% of the homes use air-conditioning systems, indicating that a majority of the households use both heating and cooling systems during the year. The natural gas powered central warm-air furnace system is the most popular, having nearly a 42% share of the total number of heating systems. About 70% of the households make use of central air-conditioning systems run by a conventional external condenser or a heat pump. Previous studies have shown that the major proportion of the environmental impact of a residential building is due to the energy consumption for space heating and cooling [2,3]. Heating and cooling systems differ in their source of energy, the type of appliance, and the distribution system; hence, they also vary in their environmental impacts. Prek [4] has studied the
* Corresponding author. Tel.: +1 412 624 9548; fax: +1 412 624 0135. E-mail address: [email protected] (R.J. Ries). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.04.004
environmental impacts of three alternative distribution systems for hydronic heating or cooling of a single family house. The study focuses on the material production phase of the systems’ components and excludes the impacts of operational energy consumption. It is seen that the environmental impacts are strongly related to the materials used in the systems. A study by Heikkila¨ [5] compares the life cycle environmental impacts of two air-conditioning systems for an office building in Sweden. The difference in the form and source of the energy dominates the relative environmental effects of the systems. Hence, a careful selection of the type of conventional heating and cooling systems can considerably enhance the environmental performance of a residential building. This study applies a life cycle assessment (LCA) framework [6] to evaluate three heating and cooling systems which differ in their source of energy and the type of system. The systems studied are: (a) central natural gas furnace heating and conventional central air-conditioning, (b) natural gas powered hydronic heating and conventional central air-conditioning, and (c) electric air–air heat pump for heating as well as cooling. The study is intended to present a comparison of the three systems in terms of their life cycle environmental impacts and assist home
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owners and property developers in making informed decisions at the time of new construction or renovation. A hypothetical case study is developed wherein the three alternative systems are installed in a new single family residential house and operated over a life of 35 years. Since the design and performance of the heating and cooling systems depends largely on the regional climate, the three systems are studied for the same house at four locations having different climatic conditions: Minnesota, Oregon, Pennsylvania, and Texas. These regions were chosen for their representative range of climate. The regional differences in the fuel mix for electricity generation and consequently their emission rates are also considered while evaluating the systems. The second portion of the study focuses on two scenarios: (i) replacing the systems with currently available high-efficiency systems of the same types and (ii) increasing the percentage of renewable energy sources in the electricity mix. 2. Methodology and data sources The analysis is performed using a process based approach wherein the life cycle of the systems is divided into distinct phases: extraction of raw materials, manufacturing and transportation of the system components, operation and disposal. The life cycle impact of the entire system is the combined impact of these sub-phases of the system. SimaPro 5.0 [7] is the software used to model the systems’ life cycles. SimaPro 5.0 facilitates the use of several process databases which contain the material and energy inputs and outputs for the basic materials and processes which are used as building blocks to model higher level processes. The Franklin USA 98 [8] and the ETH-ESU 96 [9] databases reflect average practice in the USA and Europe, respectively. The unit processes described in these databases are used to model the systems. The material constituents of the system components are determined from the manufacturers’ literature wherever applicable. However, in the absence of a detailed material description from the manufacturer, the proportion and the quantity of the basic materials is based on judgment and experience. The transportation distances are from the manufacturing location to the house. The operating energy consumption of the systems is calculated using the Home Energy Saver (HES) [10] web interface to the DOE-2 building energy simulation software developed by the US DOE. The house is simulated based on several parameters such as area, spatial orientation, building envelope, and climate. The annual energy consumption for heating and cooling is calculated, and energy consumption is considered to remain constant across the
entire 35-year study period. The life cycle impact assessment is performed using the Impact 2002+ [11] method which characterizes the inventory results via 14 midpoint categories into four damage categories. 3. House characteristics The house selected for the case study measures 181 m2 (1950 ft2) in livable area. It is occupied by a family of two adults and two children. It is an L-shaped house, has two stories above the ground and a partial unconditioned, unfinished basement and a single car parking garage. The house is assumed to be surrounded by trees and other houses on three sides. Further shading is provided by roof eaves extending 1 ft on each face of the house. The glazing area is about 18% of the gross opaque wall area. An unconditioned attic acts as the ceiling of the second story. The walls above the concrete basement walls are wood framed with brick veneer finish. The building envelope materials and insulation level of a typical house vary with the climate. For instance, a ‘‘typical’’ house in a cold northeastern climate has higher wall insulation level compared to the insulation in the hot southern climate. Thus, selecting the same insulation values for the house in all the four locations would not truly capture the difference in the common practice in these places. The insulation levels for the four locations are calculated based on the recommendations of the International Energy Conservation Code (IECC) [12]. Table 1 summarizes the insulation levels adopted for modeling the buildings at the selected locations. The entire living area is conditioned at all times of the day throughout the year. The thermostat settings are fixed at 27 8C (81 8F) and 18 8C (64 8F) during the cooling and heating seasons, respectively. 4. Central appliances and distribution systems The central furnace and the boiler are a source of heating only as opposed to the heat pump which is used for heating as well as cooling. To maintain equivalence between the systems, the furnace and the boiler are coupled with a conventional airconditioning system for the analysis. The components of the three systems are given in Table 2. 4.1. Warm-air furnace The warm-air furnace uses natural gas as its primary source of energy and electricity as the secondary source. The burner assembly of the furnace uses an electrical spark to ignite a mixture of natural gas and air. The air, warmed from the
Table 1 Building envelope insulation levels at various locations Building Component 2
2
Glazing U-factor, W/K m (Btu/h 8F ft ) Glazing solar heat gain coefficient Floor insulation, K m2/W (h 8F ft2/Btu) Ceiling insulation, K m2/W (h 8F ft2/Btu) Wall insulation, K m2/W (h 8F ft2/Btu)
Minnesota
Oregon
Pennsylvania
Texas
2.0 (0.35) 0.55 5.3 (30) 8.7 (49) 3.7 (21)
2.6 (0.45) 0.55 3.4 (19) 5.3 (30) 2.3 (13)
2.3 (0.40) 0.55 5.3 (30) 6.7 (38) 3.2 (18)
3.7 (0.65) 0.40 3.4 (19) 5.3 (30) 2.3 (13)
V.P. Shah et al. / Energy and Buildings 40 (2008) 503–513 Table 2 The components of the three heating and cooling systems
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transported a distance of 800 km (500 mi) in a 28-tonnes capacity truck from the manufacturer’s factory to the local distribution centers.
System
Central appliance
Distribution
Furnace and AC
Warm-air furnace Air-conditioner Fan coil unit
Ductwork
Boiler and AC
Hot water boiler Air-conditioner Fan coil unit
Ductwork, pipes and radiators
Heat pump
Heat pump Fan coil unit
Ductwork
4.2. Hot water boiler
combustion of the natural gas, rises up to the heat exchanger where the heat is transferred to the air flowing through the duct network in the house. The combustion by-products rise up through a masonry chimney and escape to the atmosphere. The furnace chosen for the current analysis is the 58DLA model manufactured by Carrier Corporation [13]. The annual fuel utilization efficiency (AFUE) of the furnace is 80%. AFUE is the average of the ratio of the heat generated to the fuel energy consumed during an entire year. It includes all heat losses due to seasonal variations and occupant temperature control. The basic material composition of the furnace and its estimated lifetime is given in Table 3. The materials mentioned in Table 3 form the bulk of the furnace’s weight. Other materials which constitute a smaller fraction of the appliance weight are not considered for the analysis. The furnace is assumed to be Table 3 Material composition and estimated life for the central appliances and distribution components Appliance
Basic material
Weight (kg)
Estimated life (year)
Furnace
Steel Galvanized steel Aluminum Copper
46 18 9 3
20
Boiler
Cast iron Galvanized steel Copper
145 18 1
35
Air-conditioner
Steel Galvanized steel Copper Aluminum R-22 (Refrigerant)
78 35 17 17 6
20
Heat pump
Steel Galvanized steel Copper R-22 (Refrigerant)
101 32 17 7
20
Fan coil unit
Steel Galvanized steel Copper
48 26 2
25
Duct
Galvanized steel Fiberglass
265 140
35
Pipes and radiators
Steel Copper Fiberglass
415 82 202
35
The boiler is also powered by natural gas. The operation of the boiler is similar to the furnace except that the combustion energy is utilized in heating water through a heat exchanger which is then distributed through a network of pipes throughout the house. A boiler manufactured by Burnham is used for the study [14]. The boiler, too, operates at 80% AFUE. To maintain equivalence, the hot water from the boiler is used only for space heating and not for other domestic purposes. Table 3 summarizes the major material components and the estimated lifetime of the boiler. The longer lifetime of the boiler is primarily due to the durability of the cast iron components. The boiler is transported a distance of 400 km (250 mi) in a 28-tonnes truck. 4.3. Air-conditioner The air-conditioner studied is manufactured by Carrier Corporation [15] and marketed under the model number 24ACR3. The condenser unit is located outside the house. Two refrigerant lines run from the condenser unit to the fan coil unit located in the basement of the house where the heat exchanger is located. The cool refrigerant absorbs the heat from the air flowing through the duct work in the house. The seasonal energy efficiency ratio (SEER) of the unit is 13. The major material constituents and the expected life are given in Table 3. It is assumed that half the total refrigerant charge escapes during the lifetime of the condenser and is replaced. The unit is transported a distance of 800 km (500 mi). 4.4. Heat pump The heat pump is similar to an air-conditioner, except for the provision for reversal of the refrigeration cycle. Thus the heat pump is used for cooling as well as heating. The heat pump used in the study is the E1RA90 model manufactured by York [16]. The heating seasonal performance factor (HSPF) of the heat pump is 7.5. The HSPF is the ratio of the total thermal output (in British Thermal Units) to the electricity (in Watt-hour) consumed during a normal heating season. The SEER for the heat pump is 10. Heat pumps, generally, have a lower efficiency compared to air-conditioners because of their reversibility. Table 3 summarizes the major materials and estimated life of the heat pump. Similar to the air-conditioner, the refrigerant is assumed to escape during operation and maintenance. However, since the heat pump operates throughout the year, as opposed to the air-conditioner, it is assumed that the full charge of the refrigerant escapes during its entire life. 4.5. Fan coil unit The fan coil unit is located inside the house and contains the interior heat exchanger. The refrigerant from the air-conditioner
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Table 4 Regional climate overview for the four locations [18] Location
Heating degree days (base 65 8F)
Cooling degree days (base 65 8F)
Minnesota Oregon Pennsylvania Texas
7876 4400 5829 1648
699 390 726 2974
insulation material are located 650 km (400 mi) and 800 km (500 mi) away, respectively. Baseboard radiators, which are interspersed with the pipe circuit, are the means of heat exchange between the hot water and the room and air in the house. These are manufactured about 975 km (600 mi) from the house. Table 3 gives the basic materials and lifetimes of these components. 5. Overview of the locations
or the heat pump circulates through the heat exchanger, cooling the internal air flowing through the ducts. The FA4C model manufactured and marketed by Carrier Corporation [17] is chosen for the study. The material breakdown and the average life of the fan coil unit are given in Table 3. It is estimated that the unit is transported 800 km (500 mi). 4.6. Ducts The duct network emanates from the fan coil unit located in the basement and spreads horizontally outwards, along the ceiling, on both the floors. These duct both hot and cold air except for the hydronic heating system, where heat is distributed by piped water. The ducts are made of 0.76 mm (22 gauge) galvanized steel sheets and are insulated with a 50 mm (2 in.) fiber glass layer. The material quantities are given in Table 3. 4.7. Piping system and baseboard radiators The hot water from the boiler is carried via a pipe circuit running throughout the house. The pipes are copper tubes of 38 mm (1–1/2 in.) diameter and are insulated with 20 mm (0.8 in.) thick fiberglass. The manufacturers for the pipes and
The three systems are simulated at four different locations – Minnesota, Oregon, Pennsylvania, and Texas – to capture the difference in performance based on regional variability. The parameters which are varied are: building characteristics, regional climate, and electricity mix. The difference between typical construction in each of the states, for the purpose of this study, is manifested primarily in the levels of insulation of the building envelope. Table 4 shows the differences in the regional climates for these states based on the 30-year average heating and cooling degree days. The regional climatic conditions are based on the data collected at a single station in each state and is not the statewide average. Minnesota and Texas represent two extremes, predominantly requiring heating and cooling, respectively. Oregon and Pennsylvania require some amount of heating as well as cooling. The third significant difference between the four locations is in the energy mix used for electricity generation, that is, the annual average proportion of the different fuel sources for a unit of electricity. The energy mix for the states is summarized in Table 5. Fossil fuels are the primary source of electricity, except for Oregon which generates most of its electricity from hydropower. The energy mix plays a significant role in determining the performance of the systems as the environmental impact of a unit of electricity
Table 5 Electricity generation energy mixes by type for the four locations [19] Fuel source
Minnesota
Oregon
Pennsylvania
Texas
Coal (%) Natural gas (%) Nuclear (%) Petroleum and other gases (%) Hydropower and other renewables (%)
65 3 25 2 5
7 25 0 1 67
55 5 35 2 3
38 48 10 2 2
Table 6 Lifetime fuel consumption for the three systems at the four locations Location
Mode
Furnace and AC Natural gas (kWh)
Boiler and AC Electricity (kWh)
Natural gas (kWh)
Heat Pump Electricity (kWh)
Electricity (kWh)
Minnesota
Heat Cool
703,200
4,950 26,850
820,400
39,650 26,850
417,200 34,900
Oregon
Heat Cool
351,600
2,050 6,200
395,500
26,550 6,200
116,300 9,400
Pennsylvania
Heat Cool
527,400
3,900 29,800
542,000
29,250 29,800
214,800 39,600
Texas
Heat Cool
190,500
3,200 112,700
162,250
13,100 105,200
52,100 148,400
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507
Fig. 1. Illustration of the Impact 2002+ methodology [11].
generated from coal plant differs significantly from a unit of electricity generated by a hydropower plant. 6. Operational energy consumption The operational fuel consumption for a life of 35 years is calculated by simulating the house for a period of 1 year using HES [10]. The annual energy consumption is assumed to remain constant for the entire life span of the systems. The lifetime energy consumption for the three systems at the four locations is presented in Table 6. As expected, the operational
energy consumption is a function of the regional climate. It is also a function of the type of heating and cooling system and their efficiencies. For example, the electricity usage for the boiler is higher than the furnace, and the heat pump consumes more electricity than the air-conditioner because of lower efficiency. 7. Assumptions and limitations It is important to consider the assumptions and limitations of the study when interpreting the results.
Fig. 2. A comparison of the impacts associated with manufacturing the systems and the associated infrastructure using the Impact 2002+ method.
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a. The study considers a specific set of building characteristics and operating conditions, which significantly influence the environmental impacts of the systems. However, the variation in the relative impacts of the systems would be smaller.
b. The manufacture of the appliance’s components and their transportation are not considered in the study. The transportation of the finished appliance from the manufacturer’s factory is the only transportation considered in the study.
Fig. 3. (a) A comparison of the life cycle impacts in terms of four midpoint categories in Minnesota. (Results are normalized to the worst case.) (b) A comparison of the life cycle impacts in terms of four midpoint categories in Oregon. (Results are normalized to the worst case.) (c) A comparison of the life cycle impacts in terms of four midpoint categories in Pennsylvania. (Results are normalized to the worst case.) (d) A comparison of the life cycle impacts in terms of four midpoint categories in Texas. (Results are normalized to the worst case.)
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509
Fig. 3. (Continued ).
c. The installation of the systems at the site, the material waste resulting during installation, and the impacts associated with labor are not considered in the LCA. d. Although the design of the systems changes with location and the heating and cooling load, the system infrastructure has been kept constant for all the locations. e. The study estimates the lifetime fuel consumption from the annual consumption, calculated using historical climate data, without factoring in any loss of system efficiency. f. It is assumed that an appliance, at the end of its life, is replaced with another appliance of the same efficiency and design. Thus, it does not consider the effect of any technological advances in the system designs. g. The electricity generation mix for all the locations is considered to remain the same throughout the 35 years, to illustrate a ‘business as usual’ scenario. h. It is assumed that 90% of the materials by mass are recycled and the remaining 10% are disposed in a landfill. 8. Life cycle impact assessment Life cycle impact assessment (LCIA) [20] is the phase of the LCA where the results of the inventory are assessed based on the life cycle resource inputs and emission outputs. The typical steps involved in a LCIA are: (i) definition of the impact categories based on issues of environmental concerns, (ii) classification of the inventory into these impact categories based on their potential environmental effects, and (iii) characterization of the inventory within each category using equivalence factors based on the severity of their environmental effects. Additionally, normalization and weighting of the impact categories may be used to reflect disparities in the relative magnitude and importance of the categories. Since LCIA results demonstrate the performance of the systems in terms of environmental impact categories, instead of mass emissions or energy requirements, they are extremely useful when comparing systems. The Impact 2002+ LCIA method [11] is used to evaluate and compare the impacts of the three heating and cooling systems. This method links the results of the inventory to four damage
categories – human health, ecosystem quality, climate change, and resources – via 14 impact categories, as illustrated in Fig. 1. The scores in the damage categories are further normalized based on the overall extent of impacts to one person in 1 year in Western Europe. This brings equivalence to the impacts in the four damage categories and thus makes them comparable to each other. 9. Results and interpretation The impact assessment is divided into two parts: the impacts due to the system infrastructure, i.e., raw materials for the appliances and the associated distribution system and terminals including maintenance over the 35-year study period, and the impacts for the operational energy use over the 35-year study period. The impacts associated with only the system infrastructure are presented in Fig. 2 in terms of human health, climate change, resource use, and ecosystem quality damage categories in Impact 2002+. Even though the boiler is assumed to function for the entire study period without replacement, the boiler and AC system have the highest impact in the all the damage categories. This is primarily because of the high impacts due to the manufacturing of the metals used in the system. The human health impacts are high as a network of copper pipe and steel radiators are used for heat distribution in addition to the ductwork required by the AC system. The impacts due to the heat pump are the least in all four categories, since a single appliance fulfils both the heating and cooling functions. The life cycle impacts over the study period including energy use for the three systems at the four locations are presented in Fig. 3 and Fig. 4(a–d). Fig. 3 shows the relative life cycle impact for the three systems in terms of respiratory inorganics, aquatic ecotoxicity, global warming, and nonrenewable energy midpoint categories in Impact 2002+. The majority of the impact in the respiratory organics category is from SOx and NOx which is released during the extraction and distribution of natural gas for the furnace and boiler systems, or from coal-generated electricity for the heat pump system. The impact in the aquatic ecotoxicity category is mainly from the oil
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Fig. 4. (a) A comparison of the life cycle impacts in terms of the four damage categories in Minnesota. (b) A comparison of the life cycle impacts in terms of the four damage categories in Oregon. (c) A comparison of the life cycle impacts in terms of the four damage categories in Pennsylvania. (d) A comparison of the life cycle impacts in terms of the four damage categories in Texas.
V.P. Shah et al. / Energy and Buildings 40 (2008) 503–513
511
Fig. 4. (Continued ).
emissions during natural gas manufacturing and due to dispersion of metallic ions during manufacturing of the system appliances. Thus, the impacts in this category are higher for the boiler and furnace systems than the heat pump system. The extraction and combustion of fossil fuels for energy is the single largest source of impact in the global warming and the nonrenewable energy categories. The allocation of impacts in the damage categories is shown in Fig. 4(a–d). For Minnesota, Pennsylvania, and Texas, the heat pump has the highest life cycle impact, and in Oregon, the boiler and AC system has the largest impact. This is primarily because of the difference in the electrical energy generation mix in the states. Oregon derives two-thirds of its energy from hydropower which has considerably lower impacts in emissions and resource use compared to fossil fuel powered plants. The impacts for the heat pump in the ecosystem quality and resource categories in Pennsylvania are relatively lower than Minnesota
and Texas because the electrical energy generation mix is over one-third nuclear power. The impact for the boiler and AC system is larger than the furnace and AC system due to the higher electricity requirement for distributing water for heating. Oregon has the lowest annual combined heating and cooling load among the four states and therefore it has the lowest impacts in all the damage categories across all systems. The impacts for the boiler and AC and the furnace and AC systems are almost equal in Texas since it is cooling load dominant. The impact for the heat pump system in the climate change and resources category is only 3–8% higher in Pennsylvania than in Texas, even though the combined heating and cooling primary energy consumption in Pennsylvania is about 27% higher. This can be attributed to the percentage of nuclear energy in the electricity mix in Pennsylvania versus a similar percentage of natural gas generation in Texas. Minnesota has the highest impacts overall for all three systems because of its higher
Fig. 5. A comparison of the life cycle impacts of the high efficiency and base systems at the four locations. (The bars indicate the score of the base case and the numbers indicate the percentage change from the base case.)
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Table 7 Electricity generation energy mixes by type such that the impact of the heat pump is the lowest among the three systems Fuel source
Minnesota
Pennsylvania
Texas
Coal (%) Natural gas (%) Nuclear (%) Petroleum and other gases (%) Hydropower and other renewables (%)
23 3 25 2 47
40 5 35 2 18
0 48 10 2 40
heating loads and its dependence on coal for electricity. Thus, both the regional climate and the electrical energy generation mix have significant effects on the total impacts associated with the systems and are also important factors in determining which system has the least environmental impact for a given region. 10. Scenario analysis The efficiency of the appliances has increased over the recent past due to advances in technology. In this scenario, the appliances in the base case are substituted with higher efficiency appliances. The AFUE of the high-efficiency furnace and boiler increases to 94%, the SEER of the AC and the heat pump to 18 and 14, respectively, and the HSPF of heat pump to 9. These represent the typical high-end products currently available from manufacturers. The efficiency of these appliances is higher than the minimum Energy Star criteria, which are 90% and 85% AFUE for the furnace and boiler, respectively; 13 SEER for the AC and heat pump; and 8 for the HSPF. The life cycle impacts due to high-efficiency appliances are presented in Fig. 5 as a single Impact 2002+ score. The single score is the summation of the impacts in all the four Impact 2002+ damage categories. The decrease in the impacts is in the range of 10–21%. The largest relative decrease is observed in the furnace and AC system in Texas because of the significant increase in the cooling efficiency from the base case. In all four locations the change in the impact of the furnace and AC is greater than that of the boiler and AC since the higher efficiency appliance does not affect the operating electricity consumption, which is higher for the boiler. However, the three systems continue to follow the same order of preference at all locations, and as in the base case, the impacts of the heat pump system are higher than the impacts of the other systems, except in Oregon. The second scenario is aimed at determining the efficacy of replacing coal electricity generation with renewable energy for decreasing the environmental impacts. Studies have shown that Minnesota, Pennsylvania, and Texas have potential wind energy resources for utility-scale electricity generation [21]. For this scenario, renewable wind energy was substituted for coal electricity generation until the all-electric heat pump option had the lowest environmental impact. The combination of electrical energy generation types by state that would reduce the impacts of the heat pump to make it the lowest among the three systems is given in Table 7. Approximately, 64, 27, and 100% of the current coal-generated electricity in Minnesota, Pennsylvania, and Texas should be substituted by wind energy. This would require installation of wind energy generating
capacity of 2400, 3700 and 17,000 MW in the three states, respectively, and require a wind farm land area of about 350, 550 and 2500 km2, respectively. 11. Conclusion Three residential heating and cooling systems – furnace and AC, boiler and AC, and air–air heat pump – were compared using life cycle assessment. The three systems were studied at four locations – Minnesota, Oregon, Pennsylvania, and Texas – representing different climatic conditions and electricity generation mix. The aim of the study was to understand their relative environmental performance in a regional context. The boiler and AC system have the largest impacts associated with the appliances and distribution systems. However, the impact of the operational energy consumption is dominant over the entire study period. The heat pump has the maximum impacts in regions where a high proportion of the electricity is derived from fossil fuels, and this applies in heating as well as cooling climates. The furnace and the AC system perform the best in these regions. In Oregon, where a large share of electricity is generated from hydropower, the heat pump exhibits the lowest impacts. Scenario analysis substituting currently available high-efficiency appliances can result in up to about a 20% reduction in impacts, without any change in the order of preference in all climates. In order to minimize the impacts of the heat pump, it was found that from 15–40% of the current electric grid generation capacity should be substituted with renewable energy sources. The current percentage of renewable energy generation in the average US electricity grid is less than 10%, including hydropower, wind, solar photovoltaic, and others. Increasing this percentage would require additional infrastructure investment and possibly lead to increased cost for electricity. Before reaching a conclusion, it is crucial to understand the details of the assessment methodology and the assumptions made during the study. In addition to environmental impact, additional criteria of cost, comfort, fuel availability, constructability, and maintenance should be considered. References [1] U.S. Department of Energy, Energy Information Administration, Residential Energy Consumption Survey 2001, http://www.eia.doe.gov/emeu/ recs/contents.html, 2001 (retrieved December 15, 2006). [2] G. Keolian, S. Blanchard, P. Reppe, Life cycle energy, costs and strategies for improving a single family house, Journal of Industrial Ecology 4 (2) (2000) 135–157. [3] L. Ochoa, R. Ries, H.S. Matthews, C. Hendrickson, Life cycle assessment of residential buildings, in: American Society of Civil Engineers Construction Research Congress, San Diego, CA, April, 2005. [4] M. Prek, Environmental impact and life cycle assessment of heating and air conditioning systems: a simplified case study, Energy and Buildings 36 (10) (2004) 1021–1027. [5] K. Heikkila¨, Environmental impact assessment using a weighting method for alternative air-conditioning systems, Building and Environment 39 (10) (2004) 1133–1140. [6] ANSI/ISO, Environmental Management – Life Cycle Assessment – Principles and Framework (ISO 14040-1997), NSF International, Ann Arbor, MI, 1997.
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