- Email: [email protected]
Solar versus fossil fuels
olar
versus
f.O S A Net Energy Analysis of Domestic Solar Hot Water Systems in Australia t is c o m m o n l y assumed that solar hot water systems save energy and reduce greenhouse gas emissions compared to conventional electric and gas hot water systems. Very rarely has the life-cycle energy requirements (including the embodied energy of manufacture) of solar hot water systems been analysed. The extent to which solar hot water systems can save energy compared to conventional electric or gas hot water systems can be shown through a comparative net energy analysis. This method determines the 'energy payback period', including consideration of the difference
I
Manufacture, installation
I
Operation
1
Energy, goods, and services
Energy
1 Figure 1: Conceptual model for life cycle energy of a hot water system NB '... ' indicates an upstream requirement for goods and services and associated embodied energy, energy is in primary terms, other life cyclephases such as decommissioning have been ignored for the purposes of this study.
38
in operational energy savings and energy embodied in the devices relative to a base case. Dr Robert Crawford, Deakin University, Australia presents the results of a net energy analysis that compared solar and conventional hot water systems for a southern (Melbourne) and a northern (Brisbane) Australian climate.
Introduction Energy consumption in Australia is steadily increasing, as a result of population growth and increasing standards of living (Bush et al., 1997). This trend is producing an increasing demand on our dwindling resources, and on the environment. The operation of residential buildings in Australia accounts for around 2.4% of the greenhouse gas emissions from energy and up to 28% of these emissions are attributable to the operation of hot water systems. (Harrington et al., 1999). Thus, there is a need to reduce the energy used by these devices. The aim of this study was to determine whether solar hot water systems pay back in energy terms within 10 years, relative to conventional systems, for typical Australian climates and usage patterns. The five hot water systems compared were: electric-boosted solar, gas-boosted solar, electric storage, gas storage and a gas instantaneous system. All five systems were selected to provide equivalent hot water supply to a typical four-person household. June 2OOl REFOC<~$ www.re-focus.net
Life cycle analysis of hot water systems The life-cycle energy analysis combines both the operational energy and the embodied energy of the hot water systems, as shown in Figure 1. The life-cycle energy of each hot water system after x years is made up of the embodied energy of the hot water system, and the operational primary energy consumption of the system over x years. For the purpose of this study and the comparison between systems, the period of the life-cycle energy results is 10 years (i.e., the typical warrantee period). Since the aim of the study was to evaluate savings over 10 years, life cycle energy associated with maintenance, refurbishment and decommissioning of the systems was ignored. Operational energy is the energy consumed in the actual running of a hot water system, for example, electricity or gas. Solar energy is counted as 'free' in this study, since it does not require any fossil fuels from the earth or result in any greenhouse gas emissions on the earth. The energy consumed by a solar hot water system is dependent on a number of factors: the climate; the general type and size of system; the source of energy used; the consumption patterns of the users; and the system's efficiency. Average figures were obtained from utilities and solar hot water system manufacturers. These delivered energy figures (that is, the energy supplied at the point of use) for each hot
Q
FEATURE
- SOLAR
VERSUS
FOSSIL
system in parallel with the solar hot water system in this case.
water system were multiplied by the primary energy factors for the appropriate fuels, to relate back to fossil fuels in the earth. The energy consumed in the manufacture of hot water systems is also of importance in a net energy analysis. This energy is commonly referred to as embodied energy of manufacture, and includes the energy of assembly, and the energy embodied in the input of goods and services and transportation at all phases, back to mining of raw materials. Two Australian locations were chosen: Melbourne (latitude 37.8°S) and Brisbane (latitude 27.5%). Their difference in climate has a significant effect on the solar fraction and therefore the operational energy consumption of the hot water systems. The average annual solar fractions for the electric-boosted solar hot water system used were 80% for Brisbane and 62% for Melbourne. The solar fractions for the gas-boosted system used were 65% for Brisbane and 50% for Melbourne. The reduction was due to the inefficiencies of the use of a gas instantaneous hot water
Life cycle energy
The annual operational energy consumption of each of the five hot water systems for both Melbourne and Brisbane is shown in Figure 2. Operational energy in Melbourne was generally one third more than for Brisbane, except for the gas-boosted solar hot water system in the colder climate, which required almost double the operational energy. Figure 3 shows the life-cycle energy analysis of each hot water system over a 10-year period for Melbourne and Brisbane. In both of these locations, the embodied energy was relatively insignificant. The life-cycle energy consumption of each hot water system consists of the initial embodied energy of each of the hot water systems together with their annual operational energy requirements. In Melbourne (see Figure 3), although electric storage hot water systems have the lowest embodied energy, they have an extremely high lifecycle energy usage. Although the gas- and electric-boosted solar hot water systems have a relatively high embodied energy when compared to the other three systems, - 25[n n D_MMelbourne• Brisbane ] the energy payback 20 - - - period for both systems when compared to a gas instantaneous g 1 system is 3 years and 5 years respectively. N This is shown by the N cross-over points of each of these systems ~o in Figure 3. In Brisbane (see Figure 3), the total life-cycle energy consumption of all of the Figure 2: Annual operational energy o f hot water systems by location (GJ) hot water systems is N B Energy is in 'primary' terms so as to enable valid comparisons.
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0
FUELS
significantly reduced when compared to the same systems installed in Melbourne (see Figure 3). The operational energy consumption of gas- and electric-boosted solar hot water systems over a 10 year period is considerably lower than for any other system, due mainly to the higher solar fraction in this climate. The energy payback periods for both of these hot water systems in this climate are reduced to around 2 years when compared to gas storage and instantaneous hot water systems, compared to the case for Melbourne.
Conclusions Despite the study being performed over a relatively short period (ten years), it was evident, in both southern and northern Australian locations, that the embodied energy component of the life-cycle energy was relatively small. It should be noted that this embodied energy component (being almost totally fossil fuel based) would become more important for very low usage situations, or if the energy efficiency of the systems is increased, or the auxiliary energy supply is from a renewable source. Despite the slightly longer energy payback periods in Melbourne, compared to Brisbane, solar hot water systems are still a viable alternative to the conventional hot water systems as they provide a net energy saving within 10 years. The financial payback period, however, may be longer than the energy payback period, despite current government subsidies, indicating perhaps that the subsidies should be increased.
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Brisbane
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10
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Figure 3: Life cycle primary energy o f hot water systems. N B 'EE' indicates the manufacturing embodied energy consumed prior to operation. H W S = hot water system, E = electric, G = gas, I = instantaneous, S = solar.
June 2001 REFOCUS
www.re-focus.net
Contact: Robert H. Crawford, Postgraduate Research Student, Built Environment Research Group, School of Architecture and Building, Deakin University, Geelong Waterfront Campus, Geelong 3217, Australia. Tel: +61 3 5227 8353; Fax: +61 3 5227 8303; e-mail: [email protected]; www.ab.deakin.edu.au/berg
References Bush, S., Harris, J. and Trieu, L. H. (1997) Australian Energy Consumption and Production: Historical Trends and Projections to 2009-10, ABARE Research Report, Canberra, Report 97.2. Harrington, L., Foster, R., Wilkenfeld, G., Treloar, G.J., Lee, T. and Ellis, M. (1999) Baseline Study of Greenhouse Gas Emissions from the Australian Residential Building Sector to 2010, for the Australian Greenhouse Office, Canberra, February, 157.
versus
f.O S A Net Energy Analysis of Domestic Solar Hot Water Systems in Australia t is c o m m o n l y assumed that solar hot water systems save energy and reduce greenhouse gas emissions compared to conventional electric and gas hot water systems. Very rarely has the life-cycle energy requirements (including the embodied energy of manufacture) of solar hot water systems been analysed. The extent to which solar hot water systems can save energy compared to conventional electric or gas hot water systems can be shown through a comparative net energy analysis. This method determines the 'energy payback period', including consideration of the difference
I
Manufacture, installation
I
Operation
1
Energy, goods, and services
Energy
1 Figure 1: Conceptual model for life cycle energy of a hot water system NB '... ' indicates an upstream requirement for goods and services and associated embodied energy, energy is in primary terms, other life cyclephases such as decommissioning have been ignored for the purposes of this study.
38
in operational energy savings and energy embodied in the devices relative to a base case. Dr Robert Crawford, Deakin University, Australia presents the results of a net energy analysis that compared solar and conventional hot water systems for a southern (Melbourne) and a northern (Brisbane) Australian climate.
Introduction Energy consumption in Australia is steadily increasing, as a result of population growth and increasing standards of living (Bush et al., 1997). This trend is producing an increasing demand on our dwindling resources, and on the environment. The operation of residential buildings in Australia accounts for around 2.4% of the greenhouse gas emissions from energy and up to 28% of these emissions are attributable to the operation of hot water systems. (Harrington et al., 1999). Thus, there is a need to reduce the energy used by these devices. The aim of this study was to determine whether solar hot water systems pay back in energy terms within 10 years, relative to conventional systems, for typical Australian climates and usage patterns. The five hot water systems compared were: electric-boosted solar, gas-boosted solar, electric storage, gas storage and a gas instantaneous system. All five systems were selected to provide equivalent hot water supply to a typical four-person household. June 2OOl REFOC<~$ www.re-focus.net
Life cycle analysis of hot water systems The life-cycle energy analysis combines both the operational energy and the embodied energy of the hot water systems, as shown in Figure 1. The life-cycle energy of each hot water system after x years is made up of the embodied energy of the hot water system, and the operational primary energy consumption of the system over x years. For the purpose of this study and the comparison between systems, the period of the life-cycle energy results is 10 years (i.e., the typical warrantee period). Since the aim of the study was to evaluate savings over 10 years, life cycle energy associated with maintenance, refurbishment and decommissioning of the systems was ignored. Operational energy is the energy consumed in the actual running of a hot water system, for example, electricity or gas. Solar energy is counted as 'free' in this study, since it does not require any fossil fuels from the earth or result in any greenhouse gas emissions on the earth. The energy consumed by a solar hot water system is dependent on a number of factors: the climate; the general type and size of system; the source of energy used; the consumption patterns of the users; and the system's efficiency. Average figures were obtained from utilities and solar hot water system manufacturers. These delivered energy figures (that is, the energy supplied at the point of use) for each hot
Q
FEATURE
- SOLAR
VERSUS
FOSSIL
system in parallel with the solar hot water system in this case.
water system were multiplied by the primary energy factors for the appropriate fuels, to relate back to fossil fuels in the earth. The energy consumed in the manufacture of hot water systems is also of importance in a net energy analysis. This energy is commonly referred to as embodied energy of manufacture, and includes the energy of assembly, and the energy embodied in the input of goods and services and transportation at all phases, back to mining of raw materials. Two Australian locations were chosen: Melbourne (latitude 37.8°S) and Brisbane (latitude 27.5%). Their difference in climate has a significant effect on the solar fraction and therefore the operational energy consumption of the hot water systems. The average annual solar fractions for the electric-boosted solar hot water system used were 80% for Brisbane and 62% for Melbourne. The solar fractions for the gas-boosted system used were 65% for Brisbane and 50% for Melbourne. The reduction was due to the inefficiencies of the use of a gas instantaneous hot water
Life cycle energy
The annual operational energy consumption of each of the five hot water systems for both Melbourne and Brisbane is shown in Figure 2. Operational energy in Melbourne was generally one third more than for Brisbane, except for the gas-boosted solar hot water system in the colder climate, which required almost double the operational energy. Figure 3 shows the life-cycle energy analysis of each hot water system over a 10-year period for Melbourne and Brisbane. In both of these locations, the embodied energy was relatively insignificant. The life-cycle energy consumption of each hot water system consists of the initial embodied energy of each of the hot water systems together with their annual operational energy requirements. In Melbourne (see Figure 3), although electric storage hot water systems have the lowest embodied energy, they have an extremely high lifecycle energy usage. Although the gas- and electric-boosted solar hot water systems have a relatively high embodied energy when compared to the other three systems, - 25[n n D_MMelbourne• Brisbane ] the energy payback 20 - - - period for both systems when compared to a gas instantaneous g 1 system is 3 years and 5 years respectively. N This is shown by the N cross-over points of each of these systems ~o in Figure 3. In Brisbane (see Figure 3), the total life-cycle energy consumption of all of the Figure 2: Annual operational energy o f hot water systems by location (GJ) hot water systems is N B Energy is in 'primary' terms so as to enable valid comparisons.
~
0
FUELS
significantly reduced when compared to the same systems installed in Melbourne (see Figure 3). The operational energy consumption of gas- and electric-boosted solar hot water systems over a 10 year period is considerably lower than for any other system, due mainly to the higher solar fraction in this climate. The energy payback periods for both of these hot water systems in this climate are reduced to around 2 years when compared to gas storage and instantaneous hot water systems, compared to the case for Melbourne.
Conclusions Despite the study being performed over a relatively short period (ten years), it was evident, in both southern and northern Australian locations, that the embodied energy component of the life-cycle energy was relatively small. It should be noted that this embodied energy component (being almost totally fossil fuel based) would become more important for very low usage situations, or if the energy efficiency of the systems is increased, or the auxiliary energy supply is from a renewable source. Despite the slightly longer energy payback periods in Melbourne, compared to Brisbane, solar hot water systems are still a viable alternative to the conventional hot water systems as they provide a net energy saving within 10 years. The financial payback period, however, may be longer than the energy payback period, despite current government subsidies, indicating perhaps that the subsidies should be increased.
"E
E
•E
CO
--
Melbourne 800
.
EHWS
==
/ /
$
:~:
o o
400
o (j)
200
/-'/
0EE 1
Years
]
600
ooo
--G- SEHWS 400 -C~ SGHWS
Brisbane
800
/
2
3
, :~..-
4
5
6
~
7
'
8
9
10
Years
Figure 3: Life cycle primary energy o f hot water systems. N B 'EE' indicates the manufacturing embodied energy consumed prior to operation. H W S = hot water system, E = electric, G = gas, I = instantaneous, S = solar.
June 2001 REFOCUS
www.re-focus.net
Contact: Robert H. Crawford, Postgraduate Research Student, Built Environment Research Group, School of Architecture and Building, Deakin University, Geelong Waterfront Campus, Geelong 3217, Australia. Tel: +61 3 5227 8353; Fax: +61 3 5227 8303; e-mail: [email protected]; www.ab.deakin.edu.au/berg
References Bush, S., Harris, J. and Trieu, L. H. (1997) Australian Energy Consumption and Production: Historical Trends and Projections to 2009-10, ABARE Research Report, Canberra, Report 97.2. Harrington, L., Foster, R., Wilkenfeld, G., Treloar, G.J., Lee, T. and Ellis, M. (1999) Baseline Study of Greenhouse Gas Emissions from the Australian Residential Building Sector to 2010, for the Australian Greenhouse Office, Canberra, February, 157.