Energy and carbon impact analysis of a solar thermal collector system

Abstract

Much research effort is focusing on the reduction of man's carbon footprint in response to global energy and sustainability initiatives. The prudence of such activities is reliant on the embodied energy (EE) of materials and processes from cradle to gate, which influence overall energy of products and processes from cradle to grave. This paper evaluates the lifecycle energy and carbon intensity of a solar thermal collector with indirect coil heat exchange, for UK applications. The analysis is inclusive of materials, manufacture, transportation, installation and maintenance, and derives energy and carbon payback periods for a typical domestic system suitable for the domestic hot water needs of a three-bedroom home in the UK. The improvement analysis demonstrates how EE and carbon can be reduced through increased use of recycled materials, generating reductions in payback times of up to 10 months.

Keywords:

• life cycle assessment
• embodied carbon
• embodied energy
• payback
• solar thermal collector
• renewable energy

1. Introduction

Heating and domestic hot water (DHW) systems consume far more energy than other household appliances and represent the largest proportion of CO₂ emissions from domestic energy consumption. DHW in the UK was responsible for 125TWh of energy consumption in 2001 and 7.4 Mt of CO₂ emissions (Shorrock and Utley 2003). As part of the effort to reduce energy consumption and carbon emissions resulting from DHW, solar water heating has been widely promoted as one of the most worthwhile energy saving measures; in 2005, 78,470 solar hot water installations were in existence in the UK and 6694 had been installed between 2002 and 2005 with government grant aid (EST 2005). The total installed UK capacity for DHW was reported to be 175 MW_TH by the end of 2006 (Weiss et al. 2008). The UK Heating and Hotwater Industry Council (HHIC) reported a 56% rise in installations of flat plate solar water heating systems from 2007 to 2008, while sales of evacuated tube systems rose by 240% over the same period. Total sales in 2008, from subscribers to the HHIC, grew at 90% from 2007 to almost 40,000 m² (HHIC 2009), despite the economic recession in the latter months of 2008. This paper aims to provide a life cycle energy and carbon analysis of an indirect storage solar thermal collector (STC) system for use in the UK. A number of life cycle assessment (LCA) studies on STC systems have been carried out in the past (Crawford et al. 2003, Ardente et al. 2005a, 2005b, Battisti and Corrado 2005). These assessments are based on non-UK systems and focus on solar collectors with integrated water storage which have largely been replaced in the UK market by systems which provide heat via an indirect coil and connecting pipework. The integrated solar system with storage is limited in its application due to its high mass and increased thermal losses during the cold season, and nocturnally. As a result, these systems are generally more suited to larger buildings, and remain popular in Mediterranean countries where the climate is warmer than the UK (Battisti and Corrado 2005). This paper evaluates the life cycle energy and carbon intensity of a STC with indirect coil heat exchange, which is convenient for domestic use. The chosen system has been widely employed in the UK.

Solar water heating systems heat water using energy from the sun. Solar energy is collected via a panel which is connected by pipes to a hot water storage device. Solar water heating systems use solar energy at the point of use and reduce the need for fossil fuels; they are generally designed to meet 90% of the heating demand in summer, and up to 50% of the heating demand averaged over 12 months. The resulting energy, carbon and cost savings are relatively straightforward to calculate. An inclusive analysis of energy and carbon impact assessment requires a holistic analysis from cradle to grave. Raw material extraction, transportation, manufacture, assembly, services, distribution and installation all consume resources and energy, and generate carbon dioxide. Emissions released as a result of these processes are of considerable importance. LCA is a tool used to assess overall environmental impact; a derivative, life cycle energy analysis (LCEA) focuses on energy as the only measure of impact. LCEA emerged in the late 1970s (Boustead and Hancock 1979) to present a more detailed analysis of energy attributable to products, systems or processes, to enable decision-making strategies concerning energy efficiency and environmental protection. LCEA does not replace LCA but compares and evaluates the initial and recurrent embodied energy (EE) in materials, and energy used during the operational phase, and during recycling and disposal. It is often used to estimate the energy/CO₂ payback period (the time spent for the initial EE cost to be paid back by energy savings during operational and disposal/recycling stages). Life cycle carbon assessment (LCCA) is coupled to LCEA, and relies on prevailing energy structures to convert mega joules of energy into kilograms of carbon. It is required to derive how ‘clean’ the STC really is.

The functional unit (FU) for this study is one STC, one water storage cylinder and one external support system as described in Section 2. A carbon impact analysis is presented which represents a solar thermal system for a three-bedroom dwelling in the UK.

2. The solar water heating system

The active solar water heating system uses solar radiation to directly heat a fluid mixture of non-corrosive antifreeze and water. The heated fluid is used to indirectly preheat stored water in a cylinder. A further direct or indirect heat source (via conventional boiler or immersion heater) is used to raise the water temperature to usable levels. The system consists of three main components:

• solar thermal collector(s);

• water storage cylinder; and

• external support, including a frame used to fasten the solar thermal collector to the roof of the building, and pipework used to connect the solar collector to the water storage cylinder.

Figure 1 shows a typical solar water heating system. The area of the STC is determined by the hot water demand of the dwelling. The International Energy Agency Solar Heating and Cooling Programme (IEA 2004) uses a maximum installed thermal capacity conversion factor of 0.7 kW_TH per m² of STC which also agrees with SSC WG (2007) and ESTIF (2009). Based on this, the number of collectors required for domestic applications is shown in Table 1.

Figure 1 A typical active solar heating system with indirect water storage.

Table 1 Property/system requirements.

Property size	Stored water volume (l)	Solar cylinder size (l)	No. of collectors^a
1 bed/1 shower	90	175	1
2 bed/1 bath/1 shower	90	175	1
3 bed/1 bath/2 showers	90/130	175–215	1–2
4 bed/1 bath/2 showers	130/170	215–255	2–3
4 bed/1 bath/3 showers	170	255–305	3
5 bed/2 bath/2 showers	215	305	3
a Area of each collector, 2.03 m^2.

3. Life cycle carbon analysis

Life cycle carbon analysis (LCCA) encompasses the extraction and processing of raw materials; manufacturing, transportation and distribution; use, reuse, maintenance, recycling and final disposal (Consoli et al. 1993). Figure 2 shows the flow process of LCCA.

Figure 2 LCCA (based on ISO 14040, 2006).

LCA and LCCA have an iterative process of four stages, as illustrated in Figure 3. Appropriate definition of the goals and scope of the assessment is critical to the value of the outcome in terms of data quality and sensitivity analysis. Heavily inclusive studies are costly and time consuming, while under ambitious studies do not articulate sufficient information to stimulate effective carbon counting decisions. The LCCA in this paper considers the EE of materials (STC, associated pipework and cylinder, and supporting structure), manufacturing processes, transportation from cradle to site and installation and maintenance issues. The potential carbon reduction over the life of the STC is also accounted for and a carbon balance and payback period evaluated.

Figure 3 Four stage LCA methodology (based on ISO 14040, 2006).

Inventory data relating to materials, manufacturing and transportation in this study are reported on a single STC basis, while the energy and carbon impact analysis in Section 8 is made on a system basis: a three-bedroom dwelling requiring three collectors of 2.03 m² each, capable of delivering a total of 4.2 kW of heat energy.

4. Inventory of raw materials and construction

The reliability of LCA studies strictly relates to data availability, characteristics and quality: the quality of LCA results is dependent upon the quality of life cycle inventory (LCI) data (Trusty 2004). The comparability of LCI data on an ‘apples for apples’ basis has been studied at length and contains many potential pitfalls (Menzies et al. 2007). Data relating to EE and carbon in this study have largely been taken from the University of Bath Inventory of Carbon and Energy (ICE) Version 1.6a (Hammond and Jones 2008), which is a collection of secondary data in the public domain. Exceptions to this are noted throughout the text. The ICE database adopts five criteria: preference is given to data which complies with accepted methodologies, i.e. ISO 14040; system boundaries include all processes from cradle to gate; strong preference is given to embodied carbon sources from the UK; preference is given to sources of modern data; and British emission factors are applied to estimate fuel-related carbon. This research paper is strongly UK based, therefore, inventory data which is suitable for use in UK LCA studies were sought. Aside from software and subscription based databases, there is no larger or more inclusive UK specific database.

4.1 Solar thermal collector

The flat plate collector is the most common form of STC in the UK. The main part of the collector is the absorber which is made of two sheets of high quality 0.6 mm stainless steel. A water and anti-freeze mixture irrigates between two pattern stamped sheets, ensuring turbulent flow and improving heat transfer properties. To minimise heat loss, the absorber is insulated with mineral wool and surrounded by reflective aluminium sheets on the reverse side. The glazing cover of the collector is made of 4 mm tempered low-iron glass to reduce reflective losses. The solar glass is fixed into the collector frame with ethylene propylene diene monomer (EPDM) sealant to ensure water tightness of the complete collector.

The dimensions of the STC are 2430 mm (H) × 930 mm (W) × 113 mm (D) with an active surface of 2.03 m². The mass, EE and associated embodied carbon of raw materials used in the STC are detailed in the inventory (Table 2). The EE of EPDM is neglected in this study, as its impact on total EE of the STC, and effect on mass, is less than 0.5%. This follows the 1% cut-off criteria of ISO 14040 (ISO 2006). The mass of EPDM used in the STC is assumed to be around 0.25 kg, based on data from Weir (1998), while the EE of EPDM is reported to be 200 MJ/kg (Hammond and Jones 2008), or 191 MJ/kg ( data, cited in Stacey 2001).

Table 2 Materials inventory for the STC. FU is one STC.

Material	Component	Mass (kg)	EE per kg (MJ)^a	Total EE (MJ)	Embodied carbon per kg (kg CO2)^a	Total embodied carbon (kg CO2)
Stainless steel	Absorber	21.28	51.48	1095.49	6.145	130.77
Low-iron glass	Top cover	22.19	13.50	299.53	0.76	16.86
Aluminium	Frame and back plate	10.59	150.20	1590.62	8.35	88.43
Mineral wool	Insulation	3.36	16.60	55.82	1.20	4.04
Total		57.42		3041.46		240.10
^a Source: Bath University ICE-database (Hammond and Jones 2008).

4.2 Water storage cylinder

The solar water cylinder is made of 0.7 and 1.6 mm gauge copper with indirect copper coils. The solar coil is a 16.5-mm finned copper tube with a heating surface of 1.62 m². The cylinder has 50 mm polyurethane foam insulation and has a capacity of 200 l. The inventory (Table 3) details the mass, EE and embodied carbon of cylinder materials.

Table 3 Materials inventory for the solar cylinder. FU is one STC.

Material	Component	Mass (kg)	EE per kg (MJ)^a	Total EE (MJ)	Embodied carbon per kg (kg CO2)a	Total embodied carbon (kg CO2)
Copper	Solar storage tank	15.05	70.0	1053.50	5.57	83.83
Polyurethane foam	Storage tank insulation	4.11	72.1	296.01	3.00	12.32
Copper	Boiler coil and solar coil	44.58	70.0	3120.77	5.57	248.32
Total		63.74		4470.28		344.47
^a Source: Bath University ICE-database (Hammond and Jones 2008).

4.3 External support

External support for the STC includes 20 m of copper pipework for the heat transfer fluid, and an aluminium sub-frame which supports the STC. The inventory in Table 4 details the mass, EE and embodied carbon of support materials.

Table 4 Materials inventory for the support and external pipework of the STC. FU is one STC.

Material	Component	Mass (kg)	EE per kg (MJ)^a	Total EE (MJ)	Embodied carbon per kg (kg CO2)^a	Total embodied carbon (kg CO2)
Aluminium	Support of solar collector	4.16	70.0	624.83	5.57	34.74
Copper	Pipework for heat transfer fluid	19.44	72.1	1360.80	3.00	108.28
Total		23.6		1985.63		143.02
a Source: Bath University ICE-database (Hammond and Jones 2008).

5. Inventory of manufacturing

Primary research to evaluate the EE and carbon of the manufacturing stage was not conducted by the authors. Ardente et al. (2005a, 2005b) calculated the EE of a STC with water tank and support, based on a cradle to grave analysis in Italy. The boundary conditions and LCA scope were similar to those set within this research paper. Based on Ardente et al. (2005b), the ratio of EE of materials to EE of manufacture was used to interpolate the primary energy demand for the manufacture of a single flat plate type collector, cylinder and support system as shown in Table 5. The embodied carbon of manufacture, based on the UK energy structure (DEFRA 2008), was estimated from this, and is shown in Table 5.

Table 5 EE and carbon from the manufacturing phase. FU is one STC.

Component	Primary EE (MJ)	End use EE (MJ)^a	Embodied carbon (kg CO2)^b
Solar collector	176.40	72.32	8.64
Solar cylinder	425.92	74.63	20.88
Support	51.63	21.17	2.53
Total	653.95		32.05
a Based on average fuel efficiency of 41%.
b Based on DEFRA fuel conversion factor of 0.43 kg CO2 per kWh  electricity.

6. Inventory of transportation

The environmental impact associated with transport is a function of mass and distance transported, and is measured in tkm (energy or CO₂ associated with transporting 1000 kg of goods over a 1-km route). The STC selected is manufactured in , Switzerland and is transported by road and sea on pallets. Pallets loaded with STCs are stacked into an articulated lorry (180 panels – 60 kg/panel). Spare container capacity is filled with goods from other companies and ensures 100% space utilisation.

Transportation is broken down into three stages:

• Stage 1: from Sierre, Switzerland to Calais seaport, France by road.

• Stage 2: from Calais seaport, France to Dover seaport, UK by sea.

• Stage 3: from Dover seaport to London by road.

EE consumption and CO₂ emissions from various transport means are summarised in Table 6. The total EE and carbon for transporting one STC is calculated as shown in Table 7.

Table 6 EE consumption and CO₂ emissions for various transport means.

Transport method	Energy use	CO2 emission
HGV 50 km	0.19 MJ/kg	0.906 kg CO^2/km
3.5 t Commercial van	4.30 MJ/km	0.272 kg CO^2/km
Average car	3.27 MJ/km	0.204 kg CO^2/km
Sea	0.0003 MJ/km/kg	0.007 kg/tkm
Note: sources Weir (1998) and DEFRA (2008).

Table 7 EE and carbon from transport. FU is one STC.

	Distance (km)	tkm	EE (MJ)	Embodied carbon (kg CO2)
Stage 1	873	52.4	199.00	4.39
Stage 2	40	2.4	0.72	0.02
Stage 3	123	7.38	28.04	0.62
Total			227.8	5.03

7. Inventory of installation and maintenance

7.1 Installation

The installation consists of transportation of the STC from a London warehouse to the user via commercial courier, and its subsequent installation. A representative single-trip delivery distance in kilometres was calculated, based on the population weighed average distance between London and all major UK cities (Table 8), inclusive of a 20-km average travel distance within the London area itself. The representative single-trip delivery distance is 135.8 km.

Table 8 Distance between London and UK major cities.

Destination	Distance (km)
Edinburgh	660.6
Aberdeen	864.2
Glasgow	648.4
Newcastle	463.5
Liverpool	340.2
Manchester	321.7
Cardiff	243.4
Birmingham	195.2
Oxford	99.9
Bristol	190.8
Brighton	86.7
London^a	20
Population weighted average distance (km)	135.8 km
a Distance travelled within London area included into population weighted average distance.

The CO₂ emissions from a 3.5-t commercial van due to the single-trip delivery was calculated to be 36.94 kg CO₂. The corresponding EE consumption was 583.94 MJ, based on Table 6.

Installation procedures mainly include fastening the support of the STC to the roof, fixing the collector to the support, and connecting the collector to the solar cylinder. Data of energy and carbon use relating to installation procedures are difficult to quantify. According to Ardente et al. (2005b), consumption of low voltage electricity regarding the drilling operations during the fastening is estimated to be 0.5 MJ. The corresponding CO₂ emission is 0.06 kg CO₂ (UK electricity CO₂ emissions are 0.43 kg CO₂/kWh; DEFRA 2008). Energy and carbon statistics published by the UK Department for Environmental, Food and Rural Affairs (DEFRA) are based on current UK energy structures. DEFRA (2008) publish two conversion factors for the carbon intensity of electricity: the 5-year rolling average (0.52 kg CO₂/kWh) is used to calculate emission reductions from activities that bring about short-term electricity savings; while the long-term marginal factor (0.43 kg CO₂/kWh) assumes that avoided electricity will displace future electricity generating plant. The long-term marginal factor has been used, based on the long-term investment nature of solar thermal systems.

7.2 Maintenance

Manufacturing data suggests an average useful life of 20 years. In the absence of rare external damage, such as broken glass, the STC does not require frequent maintenance. General maintenance includes replacement of seals and anti-freeze fluid every 5 years. During a 20-year life cycle, four maintenance operations are performed by manufacturer personnel. Each operation is made by London based maintenance technicians making return journeys by an average size car. The single travel distance was 135.8 km which represents a population averaged distance from London to UK major cities, as explained in Section 7.1. Table 9 shows the total EE consumption and CO₂ emission generated from these installation and maintenance procedures.

Table 9 EE and carbon associated with installation and maintenance. FU is one STC.

	EE (MJ)	Embodied carbon (kg CO2)
Installation	584.44	36.94
Maintenance	888.13	55.41

8. Carbon impact analysis for a STC system appropriate to a three-bedroom home

Based on the foregoing analysis, the total carbon emissions for 6.09 m² of STC, suitable for a three-bedroom dwelling, over the lifetime of the STC (material, manufacture, transport, installation and maintenance) are summarised in Table 10.

Table 10 Energy and CO₂ emissions from different stages in the life of a STC system for a three-bedroom dwelling incorporating three STCs.

	EE (MJ)	Percentage	CO2 emissions (kg)	Percentage
Installation and Maintenance	1472.57	7.33	92.35	6.46
Transport	683.40	3.40	15.09	0.35
Manufacture	1110.01	5.52	54.39	3.81
Materials	16,829.95	83.75	1277.27	89.38
Total	20,095.93	100	1439.10	100

For a three-bedroom family home the typical DHW demand is 3054 kWh/year, based on an average daily draw of 200 l, consisting of 28 daily short draws (1 l), 12 daily medium draws (6 l), two daily showers (40 l in 5 min) and one weekly bath (140 l) at 45°C (Jordan and Vajen 2001). The corresponding CO₂ emissions equate to 580 kg/year, based on the calorific value of natural gas. Three collectors are required to meet the needs of a three-bedroom home. STC systems can supply up to 50% of the energy required to supply DHW needs, although there are claims that up to 70% requirement can be met: the efficiency varies with the angle of the sun, with spring and summer seasons offering the best returns. Table 11 shows the total EE and carbon for this system size, and energy and carbon payback periods based on a range of supply estimates (30–70%).

Table 11 EE and carbon payback periods (in years) for DHW savings of 30–70%: STC system for three-bedroom dwelling incorporating three STCs.

Percentage of total DHW requirement supplied by STC	EE payback (years)^a	Embodied carbon payback (years)
30	6.1	8.2
40	4.6	6.2
50	3.7	4.9
60	3.0	4.1
70	2.6	3.5
a Total EE/carbon of STC system is 20,095.93 MJ/1439.10 kg CO2.

9. Sensitivity analysis

The ICE database (Hammond and Jones 2008) is the most comprehensive LCA dataset in the UK, adopting a cradle to gate analysis and boundaries/scope acceptable to the ISO 14040 methodology. The LCI data presented are not generated from primary research, but gathered from secondary sources. Hammond and Jones present upper and lower margins for data gathered and is summarised here.

EE values for stainless steel are highly dependent upon the grade of steel used. Average data from the Institute of Stainless Steel Forum have been used, but values of 11–81.8 MJ/kg also exist. The EE of aluminium is based on a worldwide recycle content of 33%, and is reported to vary by ± 20%. The EE of copper is highly dependent on the grade of the ore used and can vary from 45 to 153 MJ/kg. The EE of mineral wool is reported to vary by ± 40%, while the EE of polyurethane foam is slightly ambiguous as regards its feedstock energy. This could lead to an EE variation of ± 30%. On a system basis of three STCs, suitable for a three-bedroom dwelling, this results in a total LCA EE varying from 14.0 to 30.1 GJ (compared to 20.1 GJ as calculated in the study). The LCI of materials represents between 76.8 and 89.1% of the total system EE. Ardente et al. (2005a) found that input materials implied about 70–80% of environmental impacts, while energy consumption varied from 8 to 15 GJ. The system considered in this study is larger than Ardente et al. yet the implications on energy payback are similar. Ardente et al. found that payback periods were still less than 4 years for an integrated system based in Italy. At worst, the EE payback in this study was found to be 3.9–9.1 years, and at best 1.8–4.3 years. These payback periods are considered acceptable, given the average 20 year life of the STC system.

10. Potential for impact reduction

The fourth stage in the LCA process is Improvement Analysis (ISO 14040 2006): interpreting and identifying the potential for impact reduction. Table 10 indicates that the largest proportion of both embodied energy and embodied carbon is attributed to the materials required to construct the STC, storage tank and associated pipework. Use of recycled metals in place of virgin materials would yield significant benefits: the use of recycled aluminium for the frame and back plate and support structure of the STC (total mass, 4.75 kg) would generate an overall reduction of 1816.7 MJ (9.04%) embodied energy, and 98.3 kg (6.83%) embodied carbon, based on data from Hammond and Jones (2008) and detailed in Table 12; while the use of recycled copper for the solar and secondary coils, storage tank and pipework (total mass, 79.07 kg) would yield a 1581.6 MJ (7.87%) embodied energy saving, and 167.8 kg (11.66%) embodied carbon saving, based on data from Hammond and Jones (2008).

Table 12 Materials inventory for the STC with recycled aluminium and copper, based on the FU of one STC.

Material	Component	Mass (kg)	EE (MJ)	Percentage of energy reduction	Embodied carbon (kg CO2)	Percentage of carbon reduction
Recycled aluminium	STC frame and back plate	10.59	285.93	6.49	17.90	4.90
Recycled copper	Solar storage tank	15.05	752.5	1.50	59.90	4.58
Recycled copper	Boiler coil and solar coil	44.58	2229	4.44	177.43	4.93
Recycled aluminium	Support of solar collector	4.16	112.32	2.55	7.03	1.93
Recycled copper	Pipework for heat transfer fluid	19.44	972	1.93	77.37	2.15
Notes: aluminium: primary EE 150.2 MJ/kg: EC 8.35 kg CO2/kg, recycled EE 27 MJ/kg: EC 1.69 kg CO2/kg (Hammond and Jones 2008); copper: primary EE 70 MJ/kg: EC 5.57 kg CO2/kg, recycled EE 50 MJ/kg: EC 1.69 kg CO2/kg (Hammond and Jones 2008).

The embodied energy of recycled copper and aluminium use in place of virgin materials, related to the material inventory, are shown in Table 12. The total reduction from recycled material use sums to 3398.3 MJ energy and 266.1 kg carbon, reducing the energy and carbon payback periods, based on a 50% hot water provision, from 3.7 to 3.0 years and 4.9 to 4.0 years, respectively.

The potential for further energy and carbon reduction may be found in product/process design, and in the mass of materials used. Changes to these inputs may require some optimisation analysis to assess the impact, if appropriate, on the efficiency of the STC, which may positively or negatively influence the payback periods for both energy and carbon. LCA was developed to be an iterative process; when one round is completed, another begins. By targeting the largest or most profligate areas of the lifecycle in each iteration, the LCA process which follows will reveal new data, and will support new product, process or lifecycle change decisions.

Although of lesser impact in LCA terms, the transportation of finished products and of maintenance provision could be improved. Road haulage is significantly more energy and carbon intensive than rail freight (UK average CO₂ emissions per kilometre for HGVs is 132 g/tonne goods transported, while rail freight is a factor of six lower, at 21 g/tonne; DEFRA 2008).

11. Conclusions

This paper has evaluated the LCEA and LCCA of a STC manufactured in Sierre, Switzerland, and transported to the UK for use in domestic properties. A case study of a STC system to suit an average three-bedroom property was conducted. It revealed energy payback periods of between 2.6 and 6.1 years, and carbon paybacks of between 3.5 and 8.2 years. These values are dependent upon the hot water yield of the STC system (30–70% of total requirement), which is a strong function of latitude and exposure. Crawford et al. (2005) found an emissions payback period for both gas and electricity boosted indirect STC systems with storage, of around 2.5 years. This research was conducted for a 4-m² indirect storage (300 l) STC system in Brisbane, Australia. In Melbourne, the emissions payback periods were found to be 6 months and 2.5 years for electricity and gas boosted systems, respectively. Battisti and Crawford (2005) found emissions payback periods of 5–19 months for an integrated solar system in Rome, Italy. The FU for this study is reduced due to the omission of the copper cylinder and associated pipework. Ardente et al. (2005a, 2005b) found energy and emissions payback periods of around 2 years each for an integrated system with roof support, in Palermo, Italy. These findings correlate well with the findings of this paper, given the climatic differences between the UK and Europe, or Australia.

Assuming a 50% hot water energy supply from the STC system, the analysis revealed energy and carbon payback periods of 3.7 and 4.9 years, respectively. Use of recycled copper and aluminium in the manufacture of the STC reduces these values to 3.0 and 4.0 years, respectively.

Use of further recycling/product redesign, process/manufacturing changes and alternative transport means could further reduce the impact which STCs have over their assumed life cycle of 20 years. From an energy and carbon perspective, careful optimisation is necessary to ensure that any further investment in the life cycle of the STC is prudent and contributes to the lowering of payback periods.

This study has evaluated the energy and carbon payback periods associated with the use of a STC system typical of a three-bedroom dwelling in the UK. The results are particular to the UK energy structure, and for a STC system which is popular in the UK marketplace. The study has avoided making assumptions about the average annual yield of hot water from a system for a number of reasons. The UK climate is highly variable from the north of Scotland to the south of England, and from its west coast to its east in terms of rainshadow effects. Additionally, the hours of available sunshine are variable with latitude. Instead a range of outputs are offered, and more and less conservative estimates are made to illustrate the potential differences in paybacks due to lesser and greater hot water yields. An interesting addition to the study would be an analysis of the life cycle cost implications, as in Crawford et al. (2005). The authors are keen to explore this avenue of research also, and to compare the financial investment of an individual or organisation, compared to the investment which the world makes in terms of spent resources and emissions generated.

Additional information

Notes on contributors

Y. Roderick

1. 1. [email protected]

Notes

^1. [email protected]