Scholarly article on topic 'Environmental impacts of microgeneration: Integrating solar PV, Stirling engine CHP and battery storage'

Environmental impacts of microgeneration: Integrating solar PV, Stirling engine CHP and battery storage Academic research paper on "Environmental engineering"

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Abstract of research paper on Environmental engineering, author of scientific article — Paul Balcombe, Dan Rigby, Adisa Azapagic

Abstract A rapid increase in household solar PV uptake has caused concerns regarding intermittent exports of electricity to the grid and related balancing problems. A microgeneration system combining solar PV, combined heat and power plant (CHP) and battery storage could potentially mitigate these problems whilst improving household energy self-sufficiency. This research examines if this could also lead to lower environmental impacts compared to conventional supply of electricity and heat. Life cycle assessment has been carried out for these purposes simulating daily and seasonal energy demand of different household types. The results suggest that the impacts are reduced by 35–100% compared to electricity from the grid and heat from gas boilers. The exception is depletion of elements which is 42 times higher owing to the antimony used for battery manufacture. There is a large variation in impacts with household energy demand, with higher consumption resulting in a far greater reduction in impacts compared to the conventional supply. CHP inefficiency caused by user maloperation can decrease the environmental benefits of the system significantly; for example, the global warming potential increases by 17%. This highlights the need for consumer information and training to ensure maximum environmental benefits of microgeneration. Appropriate battery sizing is essential with the 10–20kWh batteries providing greatest environmental benefits. However, any reduction in impacts from battery storage is heavily dependent on the assumptions for system credits for electricity export to the grid. Effective management of the battery operation is also required to maximise the battery lifetime: a reduction from 10 to five years increases depletion of elements by 45% and acidification by 32%. Increasing the recycling of metals from 0% to 100% reduces the impacts from 46% to 179%. If 90% of antimony is recycled, the depletion of elements is reduced by three times compared to the use of virgin antimony. However, this impact is still 12 times higher than for the conventional system owing to the use of other metals in the system.

Academic research paper on topic "Environmental impacts of microgeneration: Integrating solar PV, Stirling engine CHP and battery storage"

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Applied Energy

journal homepage: www.elsevier.com/locate/apenergy

Environmental impacts of microgeneration: Integrating solar PV, Stirling engine CHP and battery storage

Paul Balcombe a,b,c, Dan Rigbyb, Adisa Azapagic a,Q*

a School of Chemical Engineering and Analytical Science, The University of Manchester, M13 9PL, UK b School of Social Sciences, The University of Manchester, M13 9PL, UK c Sustainable Consumption Institute, The University of Manchester, M13 9PL, UK

HIGHLIGHTS

• LCA of a household system with solar PV, Stirling engine and battery storage.

• Environmental impacts are 35-100% lower than from grid electricity and gas boiler.

• However, depletion of elements is 42 times higher due to the antimony in batteries.

• Environmental savings greater for large households due to a higher energy demand.

• Inefficient Stirling engine operation by the user increases impacts up to 67%.

ARTICLE INFO

ABSTRACT

Article history:

Received 28 July 2014

Received in revised form 12 November 2014

Accepted 19 November 2014

Keywords: Microgeneration Solar PV

Stirling engine CHP Battery storage Life cycle assessment Environmental impacts

A rapid increase in household solar PV uptake has caused concerns regarding intermittent exports of electricity to the grid and related balancing problems. A microgeneration system combining solar PV, combined heat and power plant (CHP) and battery storage could potentially mitigate these problems whilst improving household energy self-sufficiency. This research examines if this could also lead to lower environmental impacts compared to conventional supply of electricity and heat. Life cycle assessment has been carried out for these purposes simulating daily and seasonal energy demand of different household types. The results suggest that the impacts are reduced by 35-100% compared to electricity from the grid and heat from gas boilers. The exception is depletion of elements which is 42 times higher owing to the antimony used for battery manufacture. There is a large variation in impacts with household energy demand, with higher consumption resulting in a far greater reduction in impacts compared to the conventional supply. CHP inefficiency caused by user maloperation can decrease the environmental benefits of the system significantly; for example, the global warming potential increases by 17%. This highlights the need for consumer information and training to ensure maximum environmental benefits of microgeneration. Appropriate battery sizing is essential with the 10-20 kWh batteries providing greatest environmental benefits. However, any reduction in impacts from battery storage is heavily dependent on the assumptions for system credits for electricity export to the grid. Effective management of the battery operation is also required to maximise the battery lifetime: a reduction from 10 to five years increases depletion of elements by 45% and acidification by 32%. Increasing the recycling of metals from 0% to 100% reduces the impacts from 46% to 179%. If 90% of antimony is recycled, the depletion of elements is reduced by three times compared to the use of virgin antimony. However, this impact is still 12 times higher than for the conventional system owing to the use of other metals in the system. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://

creativecommons.org/licenses/by/3.0/).

* Corresponding author at: School of Chemical Engineering and Analytical Science, The University of Manchester, M13 9PL, UK. Tel.: +44 0161 306 4363. E-mail address: adisa.azapagic@manchester.ac.uk (A. Azapagic).

1. Introduction

The uptake of solar PV has been growing rapidly over the past few years, driven largely by the need to reduce greenhouse gas emissions from energy generation. By the end of 2013, the global installed capacity of solar PV reached 138 GW, with 37 GW added in 2013 alone, a 35% increase on the previous year [1]. However,

http://dx.doi.org/10.1016/j.apenergy.2014.11.034 0306-2619/® 2014 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).

this has led to concerns related to intermittency of supply and grid balancing. For example, the UK National Grid warned that a penetration of PV higher than 10 GW (equivalent to the uptake by 10% of UK households) will exacerbate problems with grid balancing and that the uncontrolled exporting of PV electricity would make it unreliable, requiring rapid ramping up and down of load-following generators such as coal and gas plants [2]. This may also necessitate the installation of additional load-following plants to run at reduced capacity, in order to meet the greater variation in supply [2]. Or, more likely, once new capacity is installed, older plants with lower efficiency and higher environmental impacts will be used at a reduced capacity [3,4]. The construction of more capacity alongside lower-efficiency operation would result in higher environmental (and economic) impacts [5] which are typically not accounted for when considering intermittent renewable energy generation.

One of the solutions proposed for dealing with these grid issues is coupling solar PV with battery storage which could potentially help to reduce uncontrolled exports and prevent the balancing and ramping problems [6-8]. Battery storage would also help to improve household self-sufficiency of energy supply, an important motivation for installing microgeneration technologies [9], by allowing them to use electricity when needed rather than when generated. A recent study [10] demonstrated that coupling solar PV and battery storage with a Stirling engine combined heat and power plant (SECHP) would help further towards improving self-sufficiency of supply. Like a standard gas boiler, SECHP is fuelled by natural gas to provide heat and co-generate electricity. Its daily electricity generation profile is likely to match household electricity demand more closely than solar PV, as the system only generates electricity when there is a heat demand, that is, when the residents are likely to be at home. However, as the system is heat-led, this applies only in the winter months so that the PV is still needed during summer. Additionally, the SECHP efficiency, and consequently its environmental impacts, depend greatly on how the system is operated [11,12]. As it needs time and fuel to reach the operating temperature (~500 °C), frequent switching on and off reduces its efficiency [11,13,14] - there is currently little information on how the environmental impacts are affected by its operation. Furthermore, it has been suggested that SECHP is only suitable for large households with higher energy demands [11], but the effect of different demand profiles on environmental impacts has not been investigated yet.

Similarly, it is unclear how the impacts associated with the production and use of batteries would affect the environmental performance of an integrated PV-SECHP-battery system in comparison with the conventional supply of electricity from the grid and heat from a boiler. Although the environmental impacts of batteries have been reported previously [e.g. 15-18], only a few studies considered their use with solar PV [17,19-21], finding the impacts to be unfavourable compared to solar PV only [17,19] or highly dependent on the battery capacity [20]. Several researchers also considered the impacts of individual technologies such as solar PV [e.g. 22-24] and SECHP [25,26], but none investigated the impacts associated with the integrated system comprising all three technologies and considering its dynamic operation with respect to daily and seasonal energy demand.

Therefore, the aim of this research is to evaluate the life cycle environmental impacts of such a system installed in a household and compare it to conventional electricity and heat supply. For these purposes, demand profiles in different household types have been simulated [10], considering different SECHP operating efficiencies as well as battery capacities. The impacts have been estimated using the life cycle assessment (LCA) methodology detailed in the next section. The results are presented and discussed in Section 3 with the conclusions drawn in Section 4.

2. Methodology

The LCA study follows the ISO 14040/14044 methodology [27,28]. The following sections define the goal and scope of the study together with the data and assumptions.

2.1. Goal and scope

The main goal of the study is to determine the environmental impacts associated with an integrated solar PV, SECHP and battery storage system installed in a household and compare it to the impacts from a conventional supply of electricity from the grid and heat from a domestic boiler. A further goal is to determine the effect on environmental impacts of the following parameters:

• variation in the daily and seasonal electricity and gas demand in

different households;

• efficiency of CHP operation related to the way it is operated by

the user; and

• different battery capacity.

As shown in Fig. 1, the scope of the study is from cradle to grave, comprising the following life cycle stages:

• extraction and processing of raw materials;

• manufacture of solar PV, SECHP and battery;

• installation of the system in a household;

• operation and maintenance over the lifetime of the system;

• waste disposal and recycling at the end of life of system components; and

• all transportation.

These stages and the individual technologies are described in turn below assuming the system to be installed in a household in the UK. The functional unit is defined as the annual heat and electricity demand of a household.

2.1.1. Solar PV manufacture

A multi-crystalline silicon panel mounted on a slanted roof is considered to represent a typical UK installation [29,30]. An average panel size of 3 kWp is assumed; the effect of different panel sizes (1.1-4 kWp), based on the available roof-space of different households, is explored later in the paper as part of the sensitivity analysis. The inventory data for the 3 kWp panel are shown in Table 1. Life cycle inventory data for the manufacture of solar PV have been sourced from the Ecoinvent database V2.2 [31], assuming that the panels are produced in Europe.

2.1.2. SECHP manufacture

The type of the SECHP considered in the study is Baxi Ecogen, the only system accredited by the UK Microgeneration Certification Scheme [32]. It is manufactured in the UK with a capacity of 1 kWe and 6.4 kWth. The inventory data are given in Table 2 and have been obtained mainly from the manufacturer [33], with missing data sourced from Ecoinvent [31].

The SECHP requires an auxiliary burner of 18 kW to supplement the heat generation; this is included in the system boundary. However, the ancillary household heating components, such as pipework, radiators and hot water tank, are not considered as these are also required for gas boiler with which the system is being compared.

2.1.3. Battery manufacture

A lead-acid battery is chosen for consideration here as the most common type [15]. The average battery capacity, defined as the

Fig. 1. The life cycle diagram of the household microgeneration system comprising solar PV, SECHP and battery storage [T: transport].

Table 1

Inventory data for the manufacture of a 3 kWp solar PV, by component [22,31]

Energy and materials Value Unit Energy and materials Value Unit

PV panel manufacture (22.8 m2) Inverter manufacture (2.5 kW)

Electricity, medium voltage 387 MJ Electricity, medium voltage 76.3 MJ

Heat, natural gas 123 MJ Steel, low alloy 9.8 kg

PV cell 21.3 m2 Copper 5.5 kg

Solar glass 230 kg Corrugated board 2.5 kg

Water 485 l Aluminium 1.4 kg

Aluminium alloy 60 kg Inductor 0.35 kg

Corrugated board 25 kg Capacitor, film, through-hole mounting 0.34 kg

Ethylvinylacetate foil 22.8 kg Polystyrene foam 0.3 kg

Polyethylene terephthalate 8.5 kg Capacitor, electrolyte type 0.26 kg

Glass fibre reinforced plastic 4.3 kg Connector 0.24 kg

Silicone 2.8 kg Printed wiring board 0.22 m2

Copper 2.6 kg Polyethylene 0.06 kg

Polyvinylfluoride film 2.5 kg Diode 0.047 kg

Acetone 0.3 kg Transistor 0.038 kg

Brazing solder 0.2 kg Integrated circuit 0.028 kg

Propanol 0.19 kg Capacitor, Tantalum 0.023 kg

Methanol 0.049 kg Polyvinylchloride 0.01 kg

Vinyl acetate 0.037 kg Styrene-acrylonitrile 0.01 kg

Lubricating oil 0.037 kg Resistor 0.005 kg

Nickel 0.0037 kg

Electric installation Mounting frame manufacture (23.5 m2)

High density polyethylene 17.61 kg Aluminium 64.6 kg

Copper 14.7 kg Steel, low alloy 34.2 kg

Polyvinylchloride 2.13 kg Corrugated board 3.04 kg

Steel, low alloy 0.86 kg Polystyrene 0.16 kg

Nylon 0.23 kg High density polyethylene 0.03 kg

Polycarbonate 0.2 kg

Zinc 0.04 kg

Brass 0.02 kg

Epoxy resin 0.002 kg

maximum quantity of electricity stored, is assumed at 6 kWh with a range of other battery sizes (2-40 kWh) considered within the sensitivity analysis. The inventory data are given in Table 2 for the average-size battery, assumed to be produced in the UK. A

charge controller would normally be required to operate the battery efficiently, but no data were found so that this component is not included in the study. Average composition of the battery cell was taken from Sullivan and Gaines [15], based on the composition

Table 2

Inventory data for the manufacture of SECHP (left) and battery (right), by component [15,31,33].

Energy and materials Value Unit Energy and materials Value Unit

SECHP manufacture Battery manufacture (6 kWh)

Electricity, medium voltage 147 MJ Electricity, medium voltage 1318.2 MJ

Electricity, low voltage 32 MJ Heat, natural gas 709.8 MJ

Heat from natural gas 295 MJ Lead 107.64 kg

Heat from light fuel oil 66 MJ Water 28.08 l

Water 94 l Sulphuric acid 17.16 kg

Iron 33 kg Polypropylene 6.24 kg

Steel 30 kg Glass fibre 6.24 kg

Chromium steel 6.8 kg Antimony 1.56 kg

Copper 0.99 kg Inverter manufacture (2.5 kW)

Aluminium 0.53 kg Electricity, medium voltage 76.3 MJ

Tin 0.057 kg Steel, low alloy 9.8 kg

Lead 0.026 kg Copper 5.51 kg

Nickel 0.013 kg Corrugated board 2.5 kg

Zinc 0.0088 kg Aluminium 1.4 kg

High density polyethylene 2.8 kg Inductor 0.351 kg

Polyvinylchloride 0.26 kg Capacitor 0.341 kg

Ceramic tiles 0.11 kg Polystyrene foam 0.3 kg

Rock wool 2.6 kg Capacitor 0.256 kg

Auxiliary boiler manufacture Connector 0.237 kg

Electricity, medium voltage 74 MJ Printed wiring board 0.2246 m2

Heat from natural gas 119 MJ Polyethylene 0.06 kg

Heat from light fuel oil 63 MJ Diode 0.047 kg

Water 46 l Transistor 0.038 kg

Steel 29 kg Integrated circuit 0.028 kg

Aluminium 1.9 kg Capacitor 0.023 kg

Chromium steel 1.3 kg Polyvinylchloride 0.01 kg

Brazing solder 1.0 kg Styrene-acrylonitrile 0.01 kg

Copper 0.77 kg Resistor 0.005 kg

Brass 0.013 kg Wiring

Rock wool 2.0 kg Copper 20 kg

Alkyd paint 0.32 kg Polyvinylchloride 10 kg

Corrugated board 1.3 kg

High density polyethylene 0.23 kg

of lead-acid batteries for vehicles (w/w): lead 69%, water 18%, sulphuric acid 11%, polypropylene 4%, glass fibre 4% and antimony 1%. The energy consumption for the manufacture is assumed at 13 MJ/ kg battery cell, of which 65% is electricity and the rest heat from natural gas [34]; the weight of the battery is 156 kg [35,36].

2.1.4. Installation and operation of the PV-SECHP-battery system

For the installation, only transport of the energy units to the household has been considered as detailed in Section 2.1.6.

To obtain energy generation profiles, the system operation was simulated for 30 real households with differing electricity and heat demands over the course of a year, considering demand in 5-min intervals at different times of the day and year. Energy demand according to the size of the house has also been considered for three main types of house in the UK: detached, semi-detached and terraced. The simulation data used to estimate the impacts are summarised in Table 3 with further details given in Supplementary material; a detailed description of the simulation and the results is available in Balcombe et al. [10]. In the base case, average figures across all 30 households have been used for LCA modelling. The influence on the impacts of the variability of household demand is explored in the sensitivity analysis in the latter parts of the paper.

In the simulation model [10], the PV-SECHP-battery system is assumed to be operated to maximise household self-sufficiency of energy supply, as follows. The SECHP unit supplies all the heat demand (Table 3), also co-generating electricity to meet a proportion of the demand, together with the solar PV. When the total electricity generation from the SECHP and solar PV exceeds demand, residual electricity is stored in the battery. If the battery is at full capacity, the residual electricity is exported to the grid and the system credited for the equivalent avoided impacts. When

the total electricity generation from SECHP and solar PV is lower than the demand, it is supplemented by electricity from the battery. If there is insufficient capacity in the battery, the electricity shortfall is met by grid imports (Table 3). The average UK electricity grid mix used for LCA modelling is given in Table 4.

Based on the simulation and the actual PV generation data [10], the efficiency of solar PV is estimated at 13.2% and average annual generation at 924 kWh/kWp yr. This compares to the UK average solar PV performance of 840 kWh/kWyr [38]. Using the same simulation model, the average efficiency of SECHP operation is found to be 94.7%, including start-ups and shut-downs. The influence on the impacts of lower and higher efficiencies (72.9-96.5%) is explored in the sensitivity analysis.

To generate heat and electricity, SECHP uses 1.19 MJ of natural gas per MJ heat output and 0.05 kWh of electricity per kWh electricity generated [39]. It is assumed to be serviced annually, with steel parts being replaced at the rate of 1% per year [31]). The battery is also serviced yearly to top-up the evaporated water (50% of the original amount). Its average efficiency over the lifetime is assumed at 80% [15,34,40].

The operational lifetime of the solar panels is assumed at 30 years and 10 years for the SECHP and the battery [41]. The inverter for both the PV and battery has the lifetime of 11 years [42,43] and the wiring 30 years.

2.1.5. Waste disposal and recycling

At the end of life, metal components are assumed to be recycled according to the global recycling rates as follows: aluminium 91%, copper 41%, iron and steel 62% and lead 94% [44-48]. The system has been credited for displacing the equivalent quantity of virgin material used to manufacture the components (see Tables 1 and 2 for the quantities). Battery cells are recycled at 100% as this is

Table 3

Household annual energy demand and generation by different components of the system, also showing the imports and exports of electricity.

Minimum (kWh/yr)

Average (kWh/yr)

Maximum (kWh/yr)

Energy demand

Electricity

Energy generation Solar PVa (electricity) SECHP (electricity) SECHP (heat) Battery storage

Electricity imported from the grid Electricity exported to the grid

1491 6321

692 715 6321 401 218 36

3265 14,716

2772b 1477 14,716 797 982 1965

6276 23,339

4557 2946 23,339 958 2882 3433

a Solar PV capacity: 1.1-4 kWp (average: 3 kWp).

b Average annual generation per household: (2772 kWhyr_1)/(3 kWp) = 924 kWh/kWp yr.

Table 4

UK electricity mix in 2013 [37].

Source

Contribution (%)

Coal 37

Gas 28

Nuclear 19

Onshore wind and solar PV 6a

Bioenergy 5

Offshore wind 3

Hydro (natural flow) 1

Total 100

a Only aggregated data are available. It is assumed that 90% is from wind and 10% from solar PV.

required by UK law [49]. For every 1000 kg of waste batteries, 650 kg of secondary lead (94% of lead input) and 71 kg of sulphuric acid (65%) is recovered [50]; the system has been credited for both. All other battery components are assumed to be landfilled. The recycling of antimony is not considered because of lack of data but its potential effect on the impacts is discussed in Section 3.3.6. The quantity of tin and nickel is very small (<0.1%) so that their end-of-life management has not been considered. Plastics are incinerated with energy recovery (and system credits) while rock wool and ceramics are landfilled [31].

2.1.6. Transport

The assumptions made for transport of raw materials and components as well as maintenance and recycling are summarised in Table 5. The life cycle inventory data for transport have been sourced from Ecoinvent [31].

2.1.7. Conventional system

Most UK households (>99%) use electricity from the grid [51,52] and heat from natural gas boilers (83%) [53] so that these options are considered for comparison with the PV-SECHP-battery system. The UK electricity grid mix used to estimate the LCA impacts is given in Table 4; the life cycle inventory data for the individual electricity sources have been sourced from Ecoinvent. The grid infrastructure is included in the system boundary.

The life cycle of the gas boiler is outlined in Fig. 2 and the inventory data are detailed in Table 6. As shown in the figure, all the stages from 'cradle to grave' are considered, from extraction of raw materials and fuels, construction, operation and maintenance of the boiler to end-of-life waste management. The life cycle inventory data are sourced from Ecoinvent, adapted for the UK energy mix to reflect the fact that the boiler is manufactured in the UK. A condensing boiler with an efficiency of 90% [54] and the lifetime at 15 years is assumed. At the end of life, the individual

components are either recycled or landfilled, following the same assumptions for metals, plastics, rock wool and ceramics as for the PV-SECHP-battery system (see Section 2.1.5).

3. Results

GaBi v6 [55] has been used to model the system and the CML 2001 method (April 2013 update) [56,57] to estimate the impacts. The following impact categories are considered: abiotic resource depletion elements (ADP elements), abiotic resource depletion fossil (ADP fossil), acidification potential (AP), eutrophication potential (EP), fresh water aquatic ecotoxicity potential (FAETP), global warming potential (GWP), human toxicity potential (HTP), marine aquatic ecotoxicity potential (MAETP), ozone depletion potential (ODP), photochemical oxidation creation potential (POCP) and terrestrial ecotoxicity potential (TETP).

In the following sections, first the environmental impacts associated with the PV-SECHP-battery system are presented and compared with the conventional grid electricity-gas boiler system. This is followed by a discussion and comparison with the literature of individual results for the solar PV, SECHP and battery. The effect on impacts of different household types and energy demand is described in Section 3.3, followed by an investigation of the influence of SECHP operation efficiency. The effects of different battery sizes are discussed subsequently, followed by different battery and SECHP lifespans as well as metal recycling rates.

3.1. Environmental impacts of the PV-SECHP-battery system

The environmental impacts associated with the system are compared to the conventional system (grid electricity and heat from gas boiler) in Fig. 3. The results for the PV-SECHP-battery system include electricity imports and exports. The system has been credited for the latter for avoiding the impacts by not using the equivalent amount of electricity from the grid. The UK electricity mix has been assumed for the credits (see Section 2.1.4). The system credits are shown in Fig. 4, also indicating the contribution of each system component to the total impacts.

Overall, the microgeneration system has significantly (35100%) lower impacts than the conventional energy supply for nine out of 11 categories. The greatest difference is found for the TETP which is negative for the PV-SECHP-battery system (-0.09 kg DCB eq./yr) because of the system credits for electricity exports. By comparison, this impact for the conventional system is equal to 36.6 kg DCB eq./yr; all other toxicity-related as well as other impacts are also much lower for the microgeneration system. For example, the AP is 13 times lower and the GWP by 40%. The ODP is approximately the same for both systems. However, depletion

Table 5

Transport assumptions for the SECHP, solar PV and battery systems.

System Stage Transport mode Distance (km)

Solar PV Materials (panels) Freight rail 600

Lorry (>161) 100

Materials (wiring and controllers) Lorry (20-28 t) 100

Freight rail 200

Materials (inverter) Transoceanic freight ship 2000

Lorry (20-28 t) 100

Freight rail 200

Materials (mounting frame) Freight rail 200

Lorry (>161) 50

Van (<3.51) 100

Manufacture Lorry (>161) 500

Installation Van (<3.51) 100

SECHP Materials Freight rail 200

Lorry (>161) 200

Manufacture Lorry (>161) 200

Installation Passenger car 200

Maintenance Passenger car 200

Battery Materials Lorry (>161) 200

Materials (inverter) Transoceanic freight ship 2000

Freight rail 200

Lorry (>161) 100

Manufacture Lorry (>161) 200

Maintenance Passenger car 200

Metals recycling Sorting Freight rail 200

Lorry (>161) 100

Recycling Freight rail 200

Lorry (>161) 100

Fig. 2. The life cycle of a natural gas boiler.

of elements is 42 times higher for the microgeneration system as discussed below.

As shown in Fig. 4, the system component contributing most to the impacts is the SECHP, particularly for the ADP fossil (82%), GWP (78%), ODP (67%) and POCP (52%). The battery is a major contributor only to ADP elements (85%) while the contribution of solar PV is relatively small across the impacts, ranging from 3% for ADP fossil to 27% for the HTP.

Abiotic resource depletion (elements): The reason for a 42-fold increase in this impact for the PV-SECHP-battery compared to the conventional system (0.18 kg Sb eq./yr vs 4 g Sb eq./yr) is the use of antimony in the batteries (1% of the cell weight) which contributes 80% to the total depletion of elements. Solar PV is the next main contributor with 14% (Fig. 4), owing to the use of silver within the metallisation paste coating on the solar cells. The avoided impact from exports of household-generated electricity to the grid is minimal (-1%).

Table 6

Inventory data for a condensing gas boiler.

Energy/materials Value Unit

Manufacture

Natural gas 472 MJ

Electricity, medium voltage 294 MJ

Light fuel oil 249 MJ

Water 182 l

Steel low alloy 115 kg

Aluminium 7.5 kg

Chromium steel 5 kg

Corrugated board 5 kg

Brazing solder 4 kg

Copper 3.03 kg

Alkyd paint 1.25 kg

High density polyethylene 0.9 kg

Brass 0.05 kg

Installation

Transport (van,<3.5 t) 200 km

Operation

Natural gas 1.1 MJ/MJ heat

Abiotic resource depletion (fossil): This impact (44.6 GJ/yr with the credits for electricity exports or 58.2 GJ/yr without the credits) is largely caused by natural gas extraction, used for the SECHP operation (44 GJ/yr). A certain amount of coal and gas is also depleted (7.54 GJ/yr) through the use of electricity imported from the grid by the household but this is made up by the credit to the system from the electricity export which saves twice as much fossil resources (13.6 GJ/yr). Overall, the PV-SECHP-battery system reduces this impact by 35% relative to the conventional system because of the reduction in electricity imports.

Acidification potential: The total impact is 0.735 kg SO2 eq./yr including the avoided impact from electricity exports. As mentioned earlier, this is 13 times lower than for the conventional system (9.4 kg SO2 eq.). Although electricity imports contribute 2.6 kg SO2 eq./yr, the impact is reduced by 4.6 kg/yr SO2 eq. through electricity exports owing to the avoidance of SO2 and NOx emissions. SECHP adds 1.3 kg SO2 eq./yr while solar PV and the battery each

ADP elements x 0.01 (kg Sb eq/yr)

ADP AP x 0.1 EP x 0.1 FAETP x GWP x HTP x MAETP ODP x POCP x TETP

fossil (kg SO2 (kg PO4 10 (kg 100 (kg 100 (kg x 100 (t 10 (mg 0.01 (kg (kg DCB

(GJ/yr) eq./yr) eq./yr) DCB CO2 DCB DCB R11 C2H4 eq./yr)

eq./yr) eq./yr) eq./yr) eq./yr) eq./yr) eq./yr)

Fig. 3. Environmental impacts of the PV-SECHP-battery system in comparison with the grid electricity and gas boiler. [All impacts correspond to the average household annual electricity and heat demand. Credits for electricity exports are included, assuming the UK electricity grid. Some impacts have been scaled to fit. To obtain the original value, multiply the value in the graph with the factor shown against relevant impact. ADP elements: abiotic depletion of elements; ADP fossil: abiotic depletion of fossil fuels; AP: acidification potential; EP: eutrophication potential; FAETP: fresh water aquatic ecotoxicity potential; GWP: global warming potential; HTP: human toxicity potential; MAETP: marine aquatic ecotoxicity potential; ODP: ozone layer depletion potential; POCP: photochemical ozone creation potential; TETP: terrestrial ecotoxicity potential.]

■ Electricity imports □ Solar PV □ Battery a SECHP

Electricity exports

-60% —

ADP ADP AP (kg EP (kg FAETP GWP (kg HTP (kg MAETP (t ODP (mg POCP TETP (kg elements fossil SO2 eq.) PO4 eq.) (kg DCB CO2 eq.) DCB eq.) DCB eq.) R11 eq.) (kg C2H4 DCB eq.) (kg Sb (GJ) eq.) eq.)

Fig. 4. The contribution to environmental impacts of solar PV, SECHP, battery and electricity imports and exports.

contribute approximately 0.7 kg SO2 eq./yr, primarily from NOx and SO2 emissions associated with steel, copper, aluminium, lead and silicon production.

Eutrophication potential: Similar to the AP, electricity generation from coal dominates this impact estimated at 0.9 kg PO4 eq./yr with the export credits (Fig. 3). Phosphate emissions to fresh water from coal generation for electricity imports cause 0.97 kg PO4 eq. but the system saves 1.7 kg PO4 eq./yr through electricity exports. As indicated in Fig. 4, the remainder of the impact comes from the manufacture of solar PV (0.53 kg PO4 eq./yr), battery (0.5 kg PO4 eq./yr) and SECHP (0.66 kg PO4 eq./yr). This is due to coal electricity used for their manufacture as well as phosphate leaching from the disposal of sulphide tailings in the beneficiation process of lead, copper, antimony, zinc, silver and nickel. In total, the microgeneration system reduces the EP by a factor of four relative to the conventional energy supply.

Fresh water aquatic ecotoxicity potential: The coal electricity is again a large contributor to FAETP, estimated at 174.2 kg DCB eq./yr of which 159.2 kg DCB eq./yr is from imported electricity. This is due to discharges of heavy metals to fresh water associated with the coal life cycle. Heavy-metal emissions from the battery's

life cycle contribute 118 kg DCB eq./yr, from solar PV 114 kg DCB eq./yr and from SECHP 69 kg DCB eq./yr. However, the system is also credited for the avoided impact of 286 kg DCB eq./yr for exporting the electricity. Overall, the FAETP is three times lower than for the conventional system.

Global warming potential: CO2 emissions from the combustion of natural gas in the SECHP contribute 78% of the GWP of 2967 kg CO2 eq./yr. The remainder is from combustion of coal and natural gas during generation of grid electricity. The electricity exports save 1137 kg CO2 eq./yr but more than half of this saving is lost through the imports (633 kg CO2 eq./yr). Nevertheless, the GWP is still 41% lower than for the conventional system.

Human toxicity potential: The HTP is contributed almost equally by the emissions of heavy metals associated with life cycles of grid electricity (249 kg DCB eq./yr), battery (290 kg DCB eq./yr), SECHP (212 kg DCB eq./yr) and solar PV (281 kg DCB eq./yr). However, their total impact is almost halved trough the electricity exports (448 kg DCB eq./yr) to yield the overall HTP of 585 kg DCB eq./yr. This represents a 40% reduction relative to the conventional energy supply.

Marine aquatic ecotoxicity potential: This impact is reduced significantly because of the electricity exports: from 16201 without

system credits to 250 t DBC eq./yr with the credits. This is 11 times lower than for the conventional system. The main contributors to the MAETP are emissions of HF to air (397 t DCB eq./yr) and beryllium to fresh water (169 t DCB eq.) from the life cycle of grid electricity as well as heavy metal emissions to fresh water from the life cycles of solar PV (345 t DCB eq.), battery (337 t DCB eq.) and SECHP (175 t DCB eq.).

Ozone depletion potential: The life cycle of natural gas is the main cause of the ODP, estimated at 0.16 g R11 eq. for the PV-SECHP-battery system. Specifically, emissions of Halon 1211, used for natural gas compressor station coolant and as a fire retardant in natural gas pipelines [58], causes approximately 68% of the impact. The remainder is mainly from halogenated emissions during the production of tetrafluoroethylene used in PV cell manufacture. As can be seen from Fig. 3, both systems have a similar ODP -although the amount of natural gas used in the SECHP is slightly higher than in the gas boiler (1.19 vs 1.1 MJ/MJ heat), system credits for the electricity exports reduce the impact from the microgeneration system to make it almost equal to that of the conventional energy supply.

Photochemical oxidation creation potential: The total POCP is estimated at 0.49 kg C2H4 eq./yr, the majority of which is due to hydrocarbon emissions from natural gas used in the SECHP (0.4 kg C2H4 eq./yr) with the rest being from the life cycle of grid electricity (0.15 C2H4 eq./yr) and the production of the PV cells (0.13 C2H4 eq./yr). The credits for electricity exports save 0.28 kg C2H4 eq./ yr, so that the overall impact is 41% lower than for the conventional system.

Terrestrial ecotoxicity potential: As mentioned earlier, this impact is negative (-0.09 kg DCB eq./yr) because of the avoided grid electricity imports. This compares very favourably with 36.6 kg DCB eq./yr for the conventional system. The TETP originates from chromium emissions to soil from the electricity distribution network (10.4 kg DCB eq. for electricity imports, -18.7 kg DCB eq. for exports). Another major contributor is the chromium emission to air from the production of steel for the SECHP unit (4.8 kg DCB eq.).

3.2. Comparison of results with literature

No other studies have investigated an integrated PV-SECHP-battery system, so that comparison of results at the system level is not possible. Instead, the results obtained for the individual technologies comprising the system are compared to those found in the literature.

3.2.1. Solar PV

Only one other LCA study was found in the literature for the same type of solar PV (multi-crystalline silicon panel mounted on a slanted roof) for UK conditions [22]; these results are compared to the current study in Fig. 5. As indicated, per kWh of electricity generated, the impacts estimated in the present study are on average 25% lower, ranging from 16% lower ADP elements to 32% lower MAETP. These differences are mainly due to the different assumptions in the two studies. For example, the annual electricity generation in the current work is estimated at 924 kWh/kWp yr based on the household simulation data (see Section 2.1.4 and Table 3); in the study by Stamford and Azapagic [22], the assumed generation of 750 kWh/kWp yr is 20% lower [22]. Furthermore, the assumed lifespans are different: 30 years for the PV panel and 11 years for the inverter [42,43] in this study as opposed to 35 and 15 years, respectively in Stamford and Azapagic [22]. Finally, the current work assumes the use of virgin materials for the PV manufacture and credits the system for their recycling at the end of life, whereas the other study assumes recycled materials in the inputs but no credits for recycling.

3.2.2. SECHP

Two studies estimated the environmental impacts associated with SECHP, one based in the UK [22] and another in Germany [25,26]. While the former considered all the LCA impacts as the current study, the latter only reported the results for the GWP and AP.

The results are compared to those estimated by Greening [25] in Fig. 6 for the functional unit of 1 kWh of electricity generated; the credits for heat generation are not considered. The capacity of SECHP in both studies is 1 kWe. As can be seen in the figure, the average relative difference in the results is 35%, ranging from 6% difference for the GWP to 75% for the ODP. This is due to the different system boundaries and the assumptions made in the two studies. First, unlike this study, Greening did not considered the influence of the varying household demand and the way in which the unit is operated. Further, the mass of the materials in the SECHP unit is lower in this study, 115 kg compared to 175 kg in Greening [25], resulting in lower impacts from the manufacture of materials. Greening's study also included a 200 l water tank, mainly consisting of glass-reinforced plastic (GRP) and steel (80 kg each). This was not considered in the present work as the same water tank is required for the gas boiler within the conventional energy system. On the other hand, an auxiliary boiler unit within the SECHP system has been considered here but not by Greening. Moreover, the Greening study also included 40 kg of steel pipework that was excluded from the system boundaries here. Finally, Greening considered the UK grid electricity mix of 2009 while the present study is based on the 2013 mix with a higher proportion of coal (37% vs 28%) and lower contribution from gas (28% vs 45%) electricity. Nevertheless, despite these numerous differences in the assumptions, the results still fall within the same order of magnitude.

The study by Pehnt [26] considered a slightly smaller SECHP unit than here (0.8 kWe vs 1 kWe), based in Germany. The author credited the system for heat generation, so to enable a comparison, the results obtained in the current study have been recalculated to include the heat credits. Despite the difference in the geographical location, the results in Pehnt [26] and here are relatively close, respectively: 0.5 vs 0.2 kg CO2 eq./kWh for the GWP and 0.3 vs 0.25 g SO2 eq./kWh for the AP.

3.2.3. Battery

The life cycle impacts of the battery estimated here are shown in Fig. 7, expressed per 1 kg of the battery cell, as in many other LCA studies [15]. However, it is not possible to compare the results directly with the literature because of the methodological differences with the existing studies. For example, Sullivan and Gaines [15] reviewed 12 existing LCA studies of batteries but reported only life cycle air emissions rather than the impacts. Therefore, to enable comparison, Fig. 8 shows the life cycle emissions estimated in the present study together with the data in Sullivan and Gaines [15]. It can be seen that all values are within the range reported in these studies, with the exception of the carbon monoxide which is 2.5 times greater. This may be due to the assumed ratio of virgin and secondary lead used for batteries: the CO emissions in this study are mainly caused by secondary lead (72%). The primary lead production process emits less CO [58] but Sullivan and Gaines [15] do not specify the percentage of lead assumed in different studies that they reviewed so that it is not possible to discuss this difference in more detail.

Another study [16] estimated the impacts but used the ReCiPe instead of the CML method applied here to estimate the impacts. Therefore, the only impact that can be compared between the two studies is the GWP as the methodology for its estimation is the same in both methods. There, the GWP is estimated at 0.9 kg CO2 eq./kg battery, compared to 2.55 kg CO2 eq. here. It is not

ADP ADP AP x 0.1 EP x 0.01 FAETP x GWP x HTP x 0.1 MAETP x ODP x POCP x TETP x elements fossil x (g SO2 (g PO4 10 (g 10 (g (kg DCB 100 (kg 0.01 (mg 0.01 (g 0.1 (g (mg Sb 0.1 eq./kWh) eq./kWh) DCB CO2 eq./kWh) DCB R11 C2H4 DCB eq./kWh) (MJ/kWh) eq./kWh) eq./kWh) eq./kWh) eq./kWh) eq./kWh) eq./kWh)

Fig. 5. Comparison with literature of environmental impacts of solar PV. [*The results reported in Stamford and Azapagic are for the global mix of solar PV technologies but here only the results for multi-crystalline silicon panels are shown to enable comparison with the current study which considers this type of panels.].

60 50 40 30 20 10 0

ADP ADP fossil AP x 0.1 EP x 0.01 FAETP (t GWP x HTP x MAETP x ODP x POCP x TETP x

elements (MJ/kWh) (g SO2 (g PO4 DCB 0.1 (kg 0.01 (kg 10 (kg 0.01 (mg 0.01 (g 0.1 (g

x 0.1 (mg eq./kWh) eq./kWh) eq./kWh) CO2 DCB DCB R11 C2H4 DCB

Sb eq./kWh) eq./kWh) eq./kWh) eq./kWh) eq./kWh) eq./kWh)

eq./kWh)

Fig. 6. Comparison with literature of environmental impacts of SECHP. [SECHP capacity: 1 kWe. All impacts expressed per kWh electricity generated. The credits for heat generation are not considered.].

10-, 9.32 9 8 7 6 5 4 3 2 1 0

ADP ADP fossil AP x 0.01 EP x 0.01 FAETP GWP (kg HTP (kg MAETP (t ODP x 0.1 POCP (g TETP x elements x 10 (kg SO2 (kg PO4 (kg DCB CO2 DCB eq. DCB (mg R11 C2H2 0.01 (kg (g Sb (MJ/kg) eq. kg) eq./kg) eq./kg) eq./kg) kg) eq./kg) eq./kg) eq./kg) DCB eq./kg) eq./kg)

Fig. 7. Environmental impacts of the battery cell estimated in this study. [Functional unit: 1 kg of battery. System boundary: from cradle to grave.]

— 22

—Sullivan and Gaines (high) [15] ♦ This study

—Sullivan and Gaines (low) [15]

1.65 0.7

— 11

♦ 5.5

15.0 14.9

-P 8.7 f9 6.2

2.4 1.1

NMVOC x CO (g/kg) NOx (g/kg) PM (g/kg) SOx (g/kg) CH4 (g/kg) N2O x 0.01 CO2 (kg/kg) 0.1 (g/kg) (g/kg)

Fig. 8. Comparison with literature of selective emissions from the life cycle of battery. [The functional unit: 1 kg of battery cell. System boundaries: from cradle to gate.]

possible to discern the reasons for this difference owing to a lack of detail in the other study. For example, the energy used for battery manufacture is not specified so that it is not known what assumptions were made. Additionally, the impacts associated with antimony extraction and production were not considered which could have affected the results.

3.3. Sensitivity analysis

To investigate the effect of various assumptions on environmental impacts, a sensitivity analysis has been carried out for the following parameters:

• variation in demand and generation profiles of different households;

• efficiency of SECHP and its lifespan;

• battery capacity and lifespan;

• metal recycling rates; and

• antimony recycling.

These are discussed in turn below.

3.3.1. Variation in demand and generation profiles

As mentioned in Section 2.1.4, the energy demand profiles of 30 real households have been simulated in 5-min intervals over a period of one year with the estimated ranges given in Table 3. These data have been used to estimate the environmental impacts for each household. For these purposes, the following parameters have been considered in the simulation model:

• different type of dwelling: detached (DH), semi-detached (SDH) and terraced (TH) house;

• different capacity of solar PV (1.1-4 kWp) and the corresponding sizes of the panel and the mounting frame;

• electricity generation by each solar PV panel;

• heat and electricity generation by SECHP;

• the electricity imported and exported by each household.

The results are compared for each type of dwelling in Fig. 9. On average, detached houses have the highest impacts because of the highest energy demand. However, there is a great deal of variation in the impacts across the different households owing to the large difference in the amount of electricity exported, ranging from 36 to 3433 kWh/yr. In the Since the detached households export the most electricity owing to the larger roof area and the related PV capacities as well as greater exports from SECHP generation, they receive larger credits for the avoided impacts, in the best case leading to negative values for the AP, EP, FAETP, HTP, MAETP and TETP. There is also a large variation in the ADP fossil, GWP, ODP and POCP owing to the variation in heat demand and the fact that the SECHP operation dominates these impacts. Depletion of elements varies much less because it is caused by the variation in the solar PV size, which is comparatively small (1.1-4 kWp).

Compared to the conventional energy system, the largest average reduction across all the impacts (73%) is found for the dwellings with the highest energy demand, i.e. the detached houses (Fig. 10). The reduction of the semi-detached and terraced houses is 58% and 32%, respectively. The exception is again depletion of elements which is higher for microgeneration than for the conventional system for all the household types.

3.3.2. Efficiency of SECHP operation

Two modes of SECHP operation have been simulated:

• an inefficient operation where the SECHP is turned on whenever there is a heat demand throughout the day; and

• an efficient operation where the system is only turned on twice

per day for a more prolonged period.

Fig. 11 shows that the efficient SECHP operation reduces the depletion of fossil fuels and the GWP by 17%, ODP by 12% and POCP by 11% compared to the inefficient operation because these impacts are caused by the combustion of natural gas during SECHP operation. All other impacts are also reduced but to a lesser extent as they are largely due to the materials used to manufacture the equipment or the quantity of electricity imported and exported.

3.3.3. Battery capacity

To determine the effect of the battery size on the environmental impacts, the following battery capacities have been considered: 2, 4, 6, 10, 20 and 40 kWh. The impacts are also compared to the case where there is no battery storage in the system. The results in Fig. 12a indicate that, when the system is credited for electricity exports, the total impacts from the microgeneration system are lower than for the conventional system for all battery capacities. The only exceptions to this are the ADP elements and ODP. For the case with no battery, some impacts are negative, notably the AP, MAETP and TETP. However, every increase in battery capacity leads to higher impacts. The reason for this is partly the assumptions made with respect to crediting the electricity exports. Whilst the battery reduces electricity imports and their associated environmental impacts, it also reduces the avoided burden associated with exporting excess electricity. Furthermore, owing to the inherent round-trip inefficiency of battery storage, assumed at 80% [15,34,40], the reduction in exports is always greater than the reduction in imports. This means that for every 1 kWh battery charge which would otherwise be exported without a battery, only 0.8 kWh is discharged to offset the imports.

It is also interesting to notice that the effect of the battery size on the impacts changes when the credits for electricity exports are excluded. As shown in Fig. 12b, the impacts generally reduce with increasing battery capacity, until it reaches 10 kWh, after which they start to increase. This is because smaller batteries are unable to store enough energy to significantly reduce the electricity imports, whereas larger batteries are over-sized such that their additional capacity is not utilised for large periods the year. However, there are some exceptions to this trend. The EP, FAETP, HTP and POCP increase slightly when a 2 kWh battery is added to the system, compared to the case where the battery is not used. This is because, for small battery sizes, the reduction in imports is not enough to counter the impacts from the additional components required for the battery (e.g. inverter, copper wiring). A further exception is depletion of elements which increases with the battery size because a reduction in electricity imports has a negligible effect on this impact, whereas an increase in battery size and, therefore, the impacts from its manufacture, has a significant effect. Overall, all the impacts but ADP elements and ODP from the microgeneration system are still lower than for the conventional energy supply, regardless of the battery size; this was also the case when the system was credited for the electricity exports, as discussed above.

3.3.4. Battery and SECHP lifespans

Battery cell lifetime varies widely depending on its application and operation [59,60]. Much less is known about the lifespan of SECHP as it is a relatively immature technology with less than 500 installations in the UK [61]. Therefore, this section considers how the impacts may be affected if the lifetime of both technologies is varied between 5 and 15 years [59,60,62] compared to 10 years considered in the base case.

Fig. 13 indicates that if the battery cells only last for five instead of 10 years, all impacts go up, with most significant increases

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со.—.—!

□ DH 0SDH

□ TH

100 80 60 40 20 0 -20 -40 -60

ADP ADP AP x 10 EP x 10 FAETP x GWP x 10 HTP x MAETP x ODP x POCP x TETP (kg elements fossil (GJ) (kg SO2 (kg PO4 0.1 (kg (t CO2 0.01 (kg 0.01 (t 100 (g 100 (kg DCB eq.) x 100 (kg eq.) eq.) DCB eq.) eq.) DCB eq.) DCB eq.)R11 eq.) C2H4 eq.)

Sb eq.)

Fig. 9. Environmental impacts for the PV-SECHP-battery system, showing the variation in impacts for different dwelling types. [DH: detached house; SDH: semi-detached house; TH: terraced house.].

200% -1

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100% -

50% ■

-50% -

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X X X X X X X X x X x x X X x x X x x X H H H H H H H H H H H H H

Q D i- D D i- D D i- D D i- D D T D D T D D T D D T D D T D D T D D T

m m m m m m m m m m m

ADP ADP AP EP FAETP GWP HTP MAETP ODP POCP TETP

elements fossil

Fig. 10. The reduction in environmental impacts when replacing the conventional energy supply by the PV-SECHP-battery system, also showing the variation in impacts for different dwelling types. [DH: detached house; SDH: semi-detached house; TH: terraced house. Conventional system: grid electricity and natural gas boiler. The relative difference relates to the annual household energy demand.].

100-, ^ □ Efficient operation of SECHP

0 Inefficient operation of SECHP

80-1 ®

CD ^ M

60 40 -20 -

ADP ADP AP x 0.1 EP x 0.1 FAETP x GWP x HTP x MAETP x ODP x 10 POCP x TETP (kg elements fossil (kg SO2 (kg PO4 10 (kg 100 (kg 100 (kg 100 (t (mg R11 0.01 (kg DCB eq.) x 0.01 (kg (GJ) eq.) eq.) DCB eq.)CO2 eq.) DCB eq.) DCB eq.) eq.) C2H4 Sb eq.) eq.)

Fig. 11. Effect on the environmental impacts of the efficiency of SECHP operation. [Inefficient operation: SECHP turned on several times during the day; efficient operation: SECHP turned on only two times during the day. Conventional system: grid electricity and natural gas boiler.].

noticed for depletion of elements (45%), AP (32%), MAETP (34%) and TETP (three times). Increasing the battery lifespan to 15 years results in a more moderate improvement in the impacts: ADP elements by 27%, AP by 15%, MAETP by 17% and TETP by two times. However, for all the lifetimes the impacts still remain significantly lower than for the conventional system, with the exception of the ADP elements, as before.

The effect of the SECHP's lifetime is smaller than for the battery but affects the same impacts as the battery lifespan. As shown in Fig. 14, reducing the lifespan from 10 to 5 years reduces the AP, FAETP and HTP by 17% and MAETP by 22%; however, the reduction in TETP is much more significant (37 times). Increasing the lifespan from 10 to 15 years yields small improvements, around 7% for the above impacts except for the TETP which is 14 times lower. Again,

100 80 60 40 20 0 -20

.jrfff

□ 0 kWh

□ 4 kWh

□ 10 kWh

□ 40 kWh

Ш 2 kWh

□ 6 kWh (base case)

□ 20 kWh

■ Conventional system

ADP elements x 0.01 (kg Sb eq./yr)

ADP fossil (GJ/yr)

AP x 0.1 (kg SO2 eq./yr)

EP x 0.1 (kg PO4 eq./yr)

FAETP x 10 (kg DCB eq/yr)

GWP x 100 (kg CO2 eq./yr)

HTP x 100 (kg DCB eq./yr)

MAETP x ODP x 10 POCP x TETP (kg 100 (t (mg R11 0.01 (kg DCB DCB eq./yr) C2H4 eq./yr) eq./yr) eq./yr)

(a) With system credits for electricity exports to the grid

100 80 60 40 20

ADP ADP

elements fossil x 0.01 (kg (GJ/yr) Sb eq./yr)

AP x 0.1 (kg SO2 eq./yr)

EP x 0.1 (kg PO4 eq./yr)

FAETP x 10 (kg DCB eq/yr)

GWP x 100 (kg CO2 eq./yr)

HTP x 100 (kg DCB eq./yr)

MAETP x ODP x 10 POCP x TETP (kg 100 (t (mg R11 0.01 (kg DCB DCB eq./yr) C2H4 eq./yr) eq./yr) eq./yr)

(b) Without system credits for electricity exports to the grid

Fig. 12. Effect on the impacts of different battery capacities. [The base case considered in the rest of the paper assumes a 6 kWh capacity.]

120 100 ■ 80 -60 -40 -20 0 -20

■ Battery lifespan:

□ Battery lifespan:

□ Battery lifespan:

□ Battery lifespan:

5 years 8 years

10 years (base case) 15 years 0

ADP ADP AP x 0.1 EP x 0.1 FAETP x GWP x HTP x 10 MAETP x ODP x 10 POCP x TETP (kg

elements fossil (kg SO2 (kg PO4 10 (kg 100 (kg (kg DCB 100 (t (mg R11 0.01 (kg DCB

x 0.01 (kg (GJ/yr) eq./yr) eq./yr) DCB CO2 eq./yr) DCB eq./yr) C2H4 eq./yr)

Sb eq./yr) eq./yr) eq./yr) eq./yr) eq./yr)

Fig. 13. Effect on the impacts of different battery lifespans.

as for the battery, the impacts from the microgeneration system remain significantly lower than from the conventional energy supply, regardless of the SECHP lifetime (with the exception of ADP elements, as in the base case).

3.3.5. Metal recycling rates

In this study, global recycling rates of metals have been assumed at the end of life of the system components (see Section 2.1.5). However, it is not known if and at what rate they will be recycled in the future. Therefore, this section examines the effect on the total impacts for two extreme cases: no recycling and 100% recycling of metals. The results in Fig. 15 show that the most affected impacts are the toxicity-related categories as well as

acidification and eutrophication. Increasing the recycling rate from no recycling to 100% recycling reduces these impacts from 46% for the EP to 179% for the TETP, increasing the relative difference between the microgeneration and conventional system in favour of the former. Depletion of elements and fossil fuels is unaffected by the recycling rates of metals because these impacts are dominated by the extraction of antimony and natural gas. The effect of recycling the former is examined next.

3.3.6. Antimony recycling

As shown in Section 3.1, antimony used within the battery cell contributes 80% to the total depletion of elements from the microgeneration system, which is 42 times higher than for the

100 и

■ SECHP lifespan

□ SECHP lifespan

□ SECHP lifespan: 15 years

□ Conventional system

5 years со

10 years (base case) со

CO^f^CO

юююю

ADP ADP

elements fossil x 0.01 (kg (GJ/yr) Sb eq./yr)

AP x 0.1 EP x 0.1 FAETP x GWP x

(kg SO2 (kg PO4 10 (kg 100 (kg eq./yr) eq./yr) DCB CO2

eq./yr) eq./yr)

HTP x MAETP x ODP x 10 POCP x TETP x

100 (kg 100 (t (mg R11 0.01 (kg 0.1 (kg

DCB DCB eq./yr) C2H4 DCB

eq./yr) eq./yr) eq./yr) eq./yr)

Fig. 14. Effect on the impacts of different SECHP lifespans.

Fig. 15. Effect on the impacts of different recycling rates of metals used to manufacture the microgeneration system.

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■ Global recycling rates (base case) m QNo recycling

□ 100% recycling

□ Conventional system

CNCOCN ^

ADP ADP AP x 0.1 EP x 0.1 FAETP x GWP x HTP x MAETP x ODP x 10 POCP x TETP x

elements fossil (kg SO2 (kg PO4 10 (kg 100 (kg 100 (kg 100 (t (mg R11 0.01 (kg 0.1 (kg

x 0.01 (kg (GJ/yr) eq./yr) eq./yr) DCB CO2 DCB DCB eq./yr) C2H4 DCB

Sb eq./yr) eq./yr) eq./yr) eq./yr) eq./yr) eq./yr) eq./yr)

elements fossil x (kg SO2 (kg PO4 0.1 (kg 0.01 (kg 0.01 (kg 0.01 (t 0.1 (mg 10 (kg DCB x 100 (kg 0.1 eq./yr) eq./yr) DCB CO2 DCB DCB R11 C2H4 eq./yr) Sb eq./yr) (GJ/yr) eq./yr) eq./yr) eq./yr) eq./yr) eq./yr) eq./yr)

Fig. 16. Effect on the impacts of recycling of antimony used in batteries.

conventional energy system. This is due to the assumption that virgin antimony is used for the manufacture of batteries (1.56 kg per 6 kWh battery; see Table 2) since no data were available to suggest that recycled antimony is used to manufacture batteries. However, it has been reported [63] that around 90% of antimony is recovered during the battery recycling process; indeed, the main source of secondary antimony is from battery recycling [58]. Therefore, this section considers the effect of 90% recycled antimony being used to manufacture batteries instead of using it elsewhere.

The results in Fig. 16 show that the depletion of elements and TETP for the whole microgeneration system are three and two

times lower, respectively, compared to the system using only virgin antimony. Some other impacts are also reduced, including the EP by 12%, FAETP by 16% and MAETP by 31%. However, depletion of elements is still 12 times higher than for the conventional system owing to the contribution from the remaining antimony that is not recycled (10%) as well as other metals used within the system. Even if antimony was completely eliminated from the batteries which is envisaged to occur by 2020 [63], the depletion of elements from the use of silver in the metallisation paste for solar PV panels is still five times higher than for the conventional system.

4. Conclusions

This study has estimated the life cycle environmental impacts of a household microgeneration system comprising solar PV, SECHP and battery storage, generating heat and electricity. The results have been compared to conventional electricity supply from the grid and heat from a natural gas boiler. Overall, the microgeneration system provides significant improvements in all environmental impacts compared to the conventional energy supply, ranging from 35% for depletion of fossil fuels to 100% for terrestrial ecotoxicity. The exception to this is depletion of elements which is 42 times higher, caused largely (85%) by the antimony used in batteries.

Natural gas used for SECHP is the main contributor to depletion of fossil fuels and global warming (80%) as well as ozone layer depletion (62%) and creation of photochemical oxidants (44%). Electricity generation from coal also contributes significantly to marine ecotoxicity and acidification (40%), eutrophication (29%) and fresh water ecotoxicity (26%).

The results show a large variation in environmental impacts across households with different energy demand. The system studied is particularly suited for detached households with typically higher demand, where there are significant reductions in impacts; for example, acidification is reduced by 104%, eutrophication by 88% and global warming by 53% compared to the conventional system. The system installed in the smallest (terraced) households also has lower impacts than the conventional supply, but the reduction in impacts is much smaller; for example, acidification is reduced by 62% and global warming by 35%. However, these reductions in impacts are predicated on the system credits for the exported electricity. Since the contribution of coal in the UK grid electricity is currently high and is expected to go down, the benefit of the microgeneration system over the conventional supply will be reduced, particularly for households with lower energy demand.

The environmental impacts are also affected by the way in which the SECHP system is operated. In particular, global warming is reduced by 17% if the system is operated more efficiently, depletion of ozone layer and fossil fuels by 12% and 17%, respectively, and creation of photochemical oxidants by 11%. This highlights the need for providing information and appropriate training for consumers to maximise the environmental benefits of the microgeneration system. There is also a financial gain associated with higher operational efficiencies, which benefits the consumer.

The results show that improvements associated with adding battery storage are sensitive to the system credits for electricity exports. When credits are included, the addition of any battery storage leads to higher environmental impacts owing to the inherent round-trip inefficiency of battery storage: the quantity of avoided electricity imports is always lower than the avoided exports. When the credits for electricity exports are excluded, the battery performs favourably for all impacts, with the exception of depletion of elements. However, the greatest environmental benefits occur for mid-sized batteries: the addition of a small (2 kWh) battery does not reduce the imports enough to offset the impacts of the battery manufacture. Likewise, battery capacities above 20 kWh provide little extra benefit. Thus, the correct sizing of battery storage is essential, not only for the environmental impacts but also for costs reasons.

The lifespan of both battery cells and SECHP has a large effect on environmental impacts: a decrease in SECHP lifespan from 10 to five years results in an increase in acidification, fresh water and human toxicity (all by 17%) as well as marine aquatic ecotoxicity (22%) and terrestrial ecotoxicity (37 times). Likewise, a decrease in lifespan of battery cells from 10 to five years results in increases

for all impacts, including depletion of elements (45%), acidification (32%) and terrestrial ecotoxicity (three times). Therefore, using effective control systems to maximise the battery cell lifespan would increase the environmental benefits from the microgeneration system. Nevertheless, even for the lowest lifetimes, all impacts are still lower than for the conventional system, except for depletion of elements.

Increasing metal recycling rates from zero to 100% reduces a number of impacts, including acidification (by 56%), eutrophication (46%), fresh water ecotoxicity (60%), human toxicity (58%), marine (91%) and terrestrial ecotoxicity (179%). If 90% of recycled antimony is used in batteries, the depletion of elements and TETP are three and two times lower, respectively, compared to the system using only virgin antimony. However, the depletion of elements is still 12 times higher than for the conventional system because of the use of other metals in the system. Even if the use of antimony was eliminated altogether, this category would still be five times greater because of the materials used for the solar PV.

Acknowledgements

This work has been funded by the Sustainable Consumption Institute at the University of Manchester and UK Engineering and Physical Sciences Research Council, EPSRC (Grant No. EP/ K011820/1). The authors gratefully acknowledge this funding. We are also grateful to Dr. Laurence Stamford and Dr. Harish Jesw-ani for their advice and help with data collection.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2014. 11.034.

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