Feasibility of off-grid housing under current and future climates

Applied Energy 241 (2019) 196–211

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

Feasibility of off-grid housing under current and future climates ⁎

T

Zhengen Ren , Phillip Paevere, Dong Chen CSIRO Land and Water, Private Bag 10, Clayton South 3169, Victoria, Australia

H I GH L IG H T S

housing in different climates under current and future global warming climates. • Off-grid housing with PV battery only is not economically feasible in heating dominated regions. • Off-grid battery systems hybridized with a petrol generator for off-grid housing become feasible. • PV • Grid-connected net zero energy housing is more attractive than off-grid housing.

A R T I C LE I N FO

A B S T R A C T

Keywords: PV battery system Off-grid housing Residential building simulation Future global warming climate

This study evaluates the feasibility of off-grid operation of fully electric housing under current and future global warming climates through case studies using building simulations. The case studies were carried out for two house types with different sizes and construction materials, under two typical occupancy patterns (occupied full day and evening only), in seven cities chosen to be representative of a broad range of climates. The study examined three operational scenarios: off-grid operation using PV and batteries; off-grid operation with PV, battery and petrol generator; and grid-connected net zero energy operation. Results show that for the houses being powered by electricity only in warm and moderate climates, large PV battery systems are required to achieve off-grid housing using PV and batteries only, and that payback periods are longer than 15.8 years under current and projected future climates considering replacement cost of battery storage, and projected inflation, discount rate and electricity price escalation rate. Off-grid operation with PV and batteries is economically unviable in heating dominated regions. The study shows that when a PV battery system is hybridized with an on-site petrol generator, the payback periods can be reduced significantly, and it may become economically feasible to operate off-grid with PV, battery and generator for both house types under most occupancy scenarios in moderate and warmer climates, but may still not be economically viable in colder climates. The case study also demonstrates that a grid-connected net zero energy home is economically feasible for all seven cities studied, and more economically attractive than a fully off-grid house under both current and projected future climates, even without subsidies for solar feed-in tariffs.

1. Introduction Globally, buildings consume around 40% of the total primary energy supply, and are responsible for 24% of the total greenhouse gas (GHG) emissions [1]. With increasing world population, and improving standards of living, energy consumption in buildings is also increasing. If this trajectory continues, we will be confronted with worsening impacts from climate change and energy shortages into the future. Rooftop solar PV systems provide renewable energy, generated close to the end-consumer households, which avoids transportation losses and the need to make new investments in transmission networks and power stations, whilst also helping to solve the challenges of climate change

⁎

and energy shortages. In Australia, over 15% of households have solar PV power. Excepting some small island nations, that is the highest penetration level in the world [2]. The main factors driving the Australian household PV market are [2]:

• Policy and financial support from state and federal governments. This has historically focused on PV systems less than 10 kW; • Over 70% of Australian dwellings are low-rise detached houses with •

relatively large roof areas, making them suitable for installing PV systems, and reducing overshadowing problems; Most regions in Australia have plenty of sunshine;

Corresponding author. E-mail address: [email protected] (Z. Ren).

https://doi.org/10.1016/j.apenergy.2019.03.068 Received 4 December 2018; Received in revised form 25 February 2019; Accepted 7 March 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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• Australian households have to pay relatively high residential elec•

stochastic approaches are presented in Refs. [19–27] and the deterministic approaches can be refereed to [28–34].

tricity prices by international standards, making PV systems a costeffective way to reduce household electricity bills, and Australia has a relatively high rates of owner-occupied homes, which allows the home owner to fully capture the benefits from investing in a PV system.

1.3. Analytical methods In analytical methods, a simple equation is developed that describes the size of the PV battery system as a function of the system reliability. The main advantage of this method is the calculation of the PV battery system size is very simple while the disadvantage of this method is that it is difficult to find the coefficients of the developed equations as they are location dependent [9]. Markvart et al. [35] developed a sizing approach for a location in UK. The methods developed for some sites in Malaysia are presented in Refs. [36–38]. Bortolini et al. [39] developed a technical and economic model for sizing a PV battery system for a location in Italy. In addition to intuitive, numerical and analytical methods, Khatib et al. [9] also presented artificial intelligence (AI) methods for sizing PV battery systems for off-grid, grid-connected and hybrid PV battery/ wind turbine (diesel) systems. Hybrid methods, which effectively combined two or more above mentioned methods to make use of their advantages, were also reviewed in [9]. In addition to the methods mentioned above, there are some commercial and non-commercial generic software simulation tools for sizing PV battery systems, such as Hybrid Optimization Model for Electric Renewables (HOMER, www.homerenergy.com), Improved Hybrid Optimization by Genetic Algorithms (IHOGA, www.unizar.es/ rdufo/hoga-eng.htm), Transient Systems Simulation Program (TRNSYS, www.trnsys.com), RETScreen (www.retscreen.net), PVSYST (www. pvsyst.com/en/download) and PV.MY [40]. Among the available software tools, HOMER and RETScreen are the most popular tools for PV battery system design [9,41]. The limitations of these tools are detailed in [9,41]. From the literature review and to the best of the authors’ knowledge, most of the studies on PV battery system size optimisation for offgrid housing used historic time series weather data for the PV generation and demand loads are given or predicted by simple approaches using the historic data. Few studies use projected future weather data to investigate potential off-grid housing with PV battery systems under future climatic conditions. Investment costs for technology and PVbattery systems, and solar irradiation and electricity consumption patterns will differ between now and the future, due to global warming and technology development. Given this, it would be valuable to investigate the economic potential for off-grid housing in different climatic regions by modelling PV-battery systems under both current and future climates. This study aims to investigate the feasibility of off-grid housing in different climatic regions in using PV battery systems under current and projected future climates. We employ a numerical (simulation) approach using an integrated tool, which was specially developed by implementing size optimisation algorithms into the AusZEH design tool, which is a comprehensive, validated tool for predicting demand loads and PV battery systems with hourly data through a period of one year [42,43]. Considering interactions between critical parameters (different climate regions, current and future climates, building type and size, and occupancy patterns), the study presented herein uses a novel methodology for quantifying the impacts of these parameters on the economics of using PV battery systems in off-grid housing considering replacement cost of battery storage and influence of inflation rate, discount rate and electricity price escalation rate. This is the first time to present the study of interactions between these critical parameters for off-grid housing with a PV battery system. The study also examines the economic potential of hybridized PV-battery plus generator systems, and PV only systems. This study is presented according to the following structure:

With the recent rapid decline in PV prices, and a similar trend for battery storage, there is a growing interest expressed in public and academic discussions around “living off – grid” by installing PV-battery systems and disconnecting from the electricity network. A study conducted by CSIRO [3] projected that by 2050, around one third of the customers in Australia could leave the grid under some potential future scenarios. The use of battery systems in buildings are not new, starting more than a century ago when lead-acid batteries were used in industrial applications [4]. Deployment of solar PV started more than half century ago [5]. One critical design consideration for off-grid housing is optimal sizing of the PV-battery system, which involves a trade-off among affordability (economic costs) and reliability. Size optimization of PVbattery systems started more than five decades ago, using relatively simple approaches, but since then complex approaches have been developed and used. There have been extensive studies of optimal sizing PV-battery systems for off-grid residential buildings that were reviewed [6–10]. According to a comprehensive review [9], size optimisation can be grouped into intuitive, numerical and analytical methods. 1.1. Intuitive methods The intuitive method involves a simplified calculation of the PV array and battery storage capacities under a given load demand. They use either the lowest monthly average, average annual, or monthly solar radiation as an input to their calculations. The interaction between the system subcomponents or the random nature of solar radiation is not considered in the methods. As mentioned in [8–10], these methods have the disadvantage of over/under sizing PV battery systems resulting in low reliability or/and high costs in system’s capital, operational and maintenance. Various intuitive methods have been developed for PV battery system size optimisation for off-grid housing in different locations, including methods developed by Ahmad [11] for remote houses in Egypt, Bhuiyan and Asgar [12] for a house in Dhaka, Bangladesh, Kaushika and Rai [13] and Chel et al. [14] for some houses in India, Al-salaymeh et al. [15] for residential buildings in Jordan, Mahmoud and Ibrik [16] for houses in rural areas in Palestine, Ghafoor and Munir [17] for a house in the city of Faisalabad, Pakistan, and Okoye et al. [18] for houses in cities in Nigeria. 1.2. Numerical methods In the numerical methods, the system’s energy balance simulations are carried out at each time interval, usually for an hourly or daily time period. Thus, they consider the interaction between the available solar radiation, battery energy storage and the given load demand in sizing the PV battery systems [8–10]. As an advantage, statistical loss of load probability (LLP, defined as the proportion of unsatisfied load demand to total load demand) is applied in numerical methods to consider energy reliability of the PV battery systems. Numerical methods for sizing PV battery systems can be classified as deterministic or stochastic approaches [9]. The deterministic approach uses daily average data of solar radiation in the system design and uncertainty in solar radiation is not considered. On the other hand, the stochastic approach makes use of hourly data to take into account variability in solar radiation and load demand. According to Egido and Lorenzo [6,7], the statistical approach is considered more accurate than the deterministic approach. A range of different numerical methods have been developed. The

(a) Based on the hourly energy consumption data from the AusZEH 197

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(b)

(c)

(d)

(e)

design tool (a whole house energy consumption tool) and hourly energy supply data from the solar PV electricity generation model, the framework and algorithms were developed for size optimization of PV battery system in terms of payback periods (Sections 2.1 and 2.2); PV battery optimal sizing for two house types to operate fully offgrid and as grid-connected net zero energy homes in seven climatic regions (cities) was conducted under current climate and future global warming of 2 °C (Section 3.3); Estimation of simple payback periods of both house types for offgrid and grid-connected net zero energy operation in the seven regions was presented for current and future climates (Section 3.4.1); Considering influence of inflation rate, discount rate and electricity price escalation rate, and replacement costs of battery storage for off-grid operation, and solar PV inverter for grid-connected net zero energy operation, discounted payback period (DPP) of both house types under the different operational modes was conducted for the seven climatic regions under current and future climates (Section 3.4.2); Findings from (b) to (d) are summarized in Section 4

• •

heating and cooling energy load required to achieve thermal comfort; Estimation of annual whole-house energy consumption at hourly or half-hourly time-steps. This capacity was developed by modifying the space heating and cooling modules of AccuRate, then adding three new modules: water and related energy consumption for water heating [48]; lighting; and other appliances; Economic analysis of building design/retrofit [42], including modules to assess the cost-effectiveness of technologies such as insulation, windows, sealing, and shading, major equipment and appliances (such as air conditioning, lighting, refrigerator, washing machine, and TV), and solar PV.

For energy star-rating, Typical Meteorological Year (TMY) weather files with hourly data for a period of one year are available for 69 NatHERS climate zones which covers the entire Australia. The main weather parameters are dry-bulb temperature, absolute humidity, atmospheric pressure, wind speed and direction, cloud cover, direct solar irradiance and diffuse solar irradiance. Weather data from the 1970s to the 2000s were used to compose the TMY files, which are then used as the reference climates for predicting building energy demand and solar PV electricity generation for the current climates. Several studies have demonstrated the efficacy of TMY data in long-term solar system performance assessment [49–53]. Climate changes in a region can be simulated using Global Circulation Models (GCM) [54] based on a global warming scenario (e.g., 2 °C) or the projections by the model for the assessment of greenhouse gas induced climate change. In this study, the GCM of CSIRO MK3.5 [55] was used to project changes in the monthly-mean local ambient temperature, relative humidity and solar radiation of the regions in Australia since 1990. Future hourly weather data are then constructed using ‘morphing’ approach [56] as described in Eqs (1)–(3).

2. Methodology An integrated tool was developed for techno-economic evaluation of off-grid housing design, which includes analysis of both energy demand and supply, sizing PV battery system with targeted LLP, and economic analysis with payback period assessment. The tool is described in more detail below. 2.1. Evaluation of cost-effective approaches for reducing energy demand through design/retrofit of residential buildings and installation of equipment and appliances Considering building envelope, installed equipment and appliances, occupant behaviour and local climate, the AusZEH design tool [42] was developed to predict whole-house, hourly/half hourly energy consumption, over a period of one year based on heating and cooling loads. These are determined using the Chenath engine [44,45]. Both the engine and the AusZEH design tool were successfully validated against electricity meter data from occupied 44 houses in Australia [46]. The AusZEH design tool has three functions:

T = T0 + ΔTm

(1)

RH = RH0 + ΔRHm

(2)

I = (1 + αm) I0

(3)

where T, RH and I are the projected future hourly dry-bulb ambient air temperature (°C), relative humidity (%) and solar radiation intensity (W/m2) respectively; Δ is changes in corresponding weather parameters; αm represents the percentage change of solar radiation (%); subscript 0 is for the reference climate, and m for monthly-mean. Since the 1950, the average temperature of Australia has increased by 0.9 °C, with significant regional variations [54]. For Australian future climate projections, this study assumed a global temperature increase of 2 °C by the end of the century, reflecting the target set out in the Paris Agreement (http://en.wikipedia.org/wiki/Paris_Agreement).

• Determining a house energy “star rating”, which is directly adopted

from AccuRate [47] – a benchmark software tool used in Australia’s the Nationwide House Energy Rate Scheme (NatHERS, www. nathers.gov.au). AccuRate is based on the Chenath engine, uses a graphical user interface (GUI), and assigns individual houses an energy star-rating (on a scale of ten), based on the annual space

DC Bus PV Array

AC Bus Bar

DC/AC Inverter Charge Controller

AC Load

DC Load Battery Bank

Fig. 1. The schematic diagram of PV-battery system for off-grid housing. 198

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zones across Australia, two detached house of different sizes and constructions (House 1 – combination construction, and House 2 – heavyweight construction) were used for simulation in this study. House 1 is a single-storey house with colorbond external walls (steel cladding on 90 mm stud). House 2 is a two-storey house with double brick cavity external walls. The floor construction assumed for both house types is the commonly used concrete slab-on-ground. House 1 (Fig. 3) has four bedrooms, a kitchen/family area, a dining/ lounge area, a laundry, a separated bathroom and toilet. It has a gross floor area of 160 m2, with 140.8 m2 air-conditioned. House 2 (Fig. 4) has four bedrooms, a kitchen/family area, a dining/living area, a rumpus, a study, a TV room, a laundry, a separated bathroom, a toilet and a double garage, with a gross floor area of 267.9 m2 (195.4 m2 airconditioned). In this study, the floor plans for both houses were maintained, while various changes of wall insulation, ceiling insulation, window types, infiltration controls etc. were used to achieve energy star ratings of around 6 stars for both houses under current climates (TMY weather) in the seven cities (Table 1). The houses with 6 stars represent new housing stock built since June 2011 that satisfy current ABCB energy efficiency standard [59]. Both house types are evaluated for all the seven climate zones under both current and future climates.

2.2. Sizing PV-battery system Fig. 1 shows the logic flow of a PV-battery system’s charging and discharging algorithm for off-grid housing operation. To achieve no electricity shortages throughout a whole year, at a time step of one hour, the size of the PV-battery system is determined by the hourly energy demand of the household. This demand can be predicted by the model, based on the Typical Meteorological Year (TMY) weather data for current weather data, and under global warming described above for future weather data. The output power (W) from a PV array can be estimated as [43]

W=f×Y×

Ig (4)

Is

where f is the PV derating factor (around 0.75 to 0.8 for modern PV panels [43]), Y is the peak capacity of the PV array (the amount of power it would produce under standard test conditions of a panel with 25 °C and 1 kW/m2 irradiance), Ig is the global solar radiation incidence on the surface of the PV array (calculated using the HDKR model [57] when data on direct solar irradiance and diffuse solar irradiance are available), and Is is the standard irradiance used to rate the capacity of the PV array (1 kW/m2). With the predicted hourly data on housing energy demand, and electricity generation of a 1 kW solar PV panel, a platform was developed using Microsoft Excel for optimal sizing of the PV battery system following the logic flow of charging and discharging described above. For each scenario a LLP must be defined. For instance, LLP = 0 for 100% off-grid without energy shortage at any time-step. LLP can be expressed as [21],

LLP =

t = 8760

DE (t )

t = 8760

P (t )Δt

∑t = 1 ∑t = 1

3.3. Simulation results for houses 1 and 2 in the seven climate regions under current and future changing climates In this study, whole house energy consumption was predicted by the AusZHE design tool, which was validated against the actual measured energy consumption of 44 households in Australia [46] over more than one year. The simulated electricity consumption of present study (Houses 1 and 2) presented following are within the range of the morning data. The simulation was conducted using two typical occupancy scenarios for a couple with two children: (1) occupied for the full day; and (2) occupied in evening only. To reduce electricity consumption for water heating, a vacuum tube solar hot water system with 4 m2 collector area is assumed to be installed in both houses. Natural gas or other non-electric appliances used for four main enduses (space heating, water heating, cooking tops and oven) was evaluated in our previous study [43] for sizing PV-battery systems for offgrid housing. With significant increase in price of natural gas and reducing price and increasing performance of heat pumps, in this study we considered it timely to examine the case for houses powered by electricity only, which are becoming more common in Australia (some new housing developments are choosing not to connect gas at all). The predicted annual electricity consumption for House 1 is shown in Fig. 5 for occupied all day, and in Fig. 6 for occupied in the evening only. The corresponding results for House 2 are shown in Figs. 7 and 8 respectively. Based on the space heating and cooling energy requirements under current climate conditions, the seven climate zones defined by ABCB (Table 1) can be grouped into three typical regions: heating dominated (Hobart, Melbourne and Mildura), cooling dominated (Darwin) and heating/cooling balanced (Alice Springs, Brisbane and Sydney). With a global warming of 2 °C, it is calculated that Alice Springs, Brisbane and Sydney become cooling dominated regions, while Melbourne and Mildura become heating/cooling balanced regions. Under the 2 °C warming scenario, the largest energy consumption increases occur in Darwin, of around 30% for House 1 and 33% for House 2. The largest decreases are in Hobart, around 10% for House 1, and 12% for House 2. In cooling dominated regions, the energy consumption of the households is dominated by space cooling and appliances, while in heating dominated regions, the energy consumption is dominated by space heating and appliances. In all regions, energy consumption by appliances contributes a significant portion of the total energy consumption, most prominently in heating/cooling balanced regions. The results indicate that different occupancy patterns have a greater

(5)

where DE(t) is the deficit energy which is defined as the incapacity of the PV battery system to supply power to the load at a specific time period, P(t) is the load demand at the same time period, and Δt is the time period for both terms (in this study Δt = 1 h). An iterative approach is applied for sizing the PV battery system and the procedures of the method is illustrated as the depicted flowchart in Fig. 2. 3. Case study 3.1. Climate zones and representative cities used in the case study The performance of PV systems depends largely upon the local meteorological and environmental conditions such as solar radiation and ambient temperature, and the energy demand for space heating and cooling is also related to the local weather. To evaluate climatic impacts on the feasibility of off-grid housing, households under different climate regions should be considered. The Köppen climate classification divides climates into five main climate groups (A-tropical, B-dry, C-temperate, D-continental and Epolar). Australia has a varied climate, and eight National Construction Code (NCC) climate zones are defined by the Australian Building Codes Board (ABCB, www.abcb.gov.au). There are very few residential buildings in NCC climate zone 8 (Alpine). Consequently, this case study focuses on the remaining seven NCC climate zones (zones 1–7). One main city is chosen in each to be representative of the corresponding NCC climate zone (Table 1). Table 1 also lists the corresponding Köppen climate groups and NatHERS climate sub-zone numbers for each cities. 3.2. Descriptions of the residential buildings In Australia, around 70% of the dwellings are detached houses [58]. To investigate the feasibility of off-grid houses in different climate 199

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Start

Define building and location Calculate demand load and obtain the meteorological data for PV panel Set PV module and battery specifications, define the LLP Set range of PV size = Min. Size: Step: Max. Size Set range of battery capacity = Min. Capacity: Step: Max. Capacity Calculate PV battery system energy output Calculate energy difference: Ed = Epv battery - Eload No Store in energy deficit rally

Ed >0

Yes

Store in energy excess rally

Calculate LLP Max. battery capacity

Calculate battery sizing ratio (BS)

No

Yes Calculate PV sizing ratio (PS) No

Max. PV size Yes Desired LLP Yes Possible configurations array (BS, PS, LLP) Select the optimum PV battery configuration using the proper economic parameter End Fig. 2. The integrated tool work flowchart.

Table 1 Main climate zones and representative cities used in case study. NCC climate zone

Description

Representative city

Köppen climate group

Climate sub-zones defined in NatHERS

Zone Zone Zone Zone Zone Zone Zone

Hot humid summer, warm winter Warm humid summer, mild winter Hot dry summer, warm winter Hot dry summer, cool winter Warm temperate Mild temperate Cool temperate

Darwin Brisbane Alice Springs Mildura Sydney Melbourne Hobart

A C B B C C C

1 10 6 27 17 21 26

1 2 3 4 5 6 7

200

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Fig. 3. The floor plan of House 1.

Ground Ňoor

First floor

5m Fig. 4. The floor plan of House 2.

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8000 6000 4000 2000 0

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Fig. 5. Annual electricity consumption of households in the seven cities when the house 1 is occupied full day under current (left) and future global warming (right) climates.

was assumed. Using the calculated annual energy consumption, and the electricity generated per 1 kW of PV installed, the sizes of the solar PV arrays required to produce a ‘net-zero’ energy usage outcome can be estimated. The results are shown in Tables 2 and 3 for Houses 1 and 2 respectively. As expected, to achieve net-zero energy home, the PV arrays required for House 2 are larger than for House 1, and the PV arrays required for houses occupied for the full day are larger than for houses which are occupied in the evening only. Under the 2 °C warming scenario, the PV array sizes required for both houses to achieve net zero energy increase in Darwin and Alice Springs, and decrease in the heating dominated regions (Hobart, Melbourne and Mildura). For the heating/cooling balanced regions (Brisbane and Sydney), required PV array sizes increase, except for House 1 in Sydney, where the size remains unchanged. The roof areas of Houses 1 and 2 are around 230 m2 and 260 m2 respectively. The area of 1.5 kW of solar PV panels is around 10 m2. Considering the orientation of the roof for solar PV installation, in this study the largest potential solar PV array is assumed to be 20 kW, which requires around 200 m2, given that the PV panels should be 30 cm away from the roof edge. To investigate the feasibility of off-grid housing with rooftop PV and battery systems, we use the tool described in Section 2 to evaluate the PV array sizes between the smallest (i.e., the PV array sizes required for zero energy houses, refer to Tables 2 and 3) and the maximum size of 20 kW. With the defined range of the PV sizes,

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impact on energy consumption for space heating/cooling and appliances than for water heating and lighting. The greatest difference between the two occupancy scenarios occurs in Darwin, of around 21% for House 1, and 15% for House 2. Note that in this study, line drying of clothes was applied for off-grid or net zero energy homes considering the households are more likely to utilise this to save energy in Australian climates. Energy consumption for water heating is dominated by personal hygiene (shower and hand washing). For clothes washing machines and dishwashing machines with their own water heating systems, energy consumption for the water heating was accounted for in the appliances. Drinking water kettle energy consumption was also accounted for in the appliances [42]. Fig. 9 shows annual electricity generation from a 1 kW solar PV system for the seven cities, under both current and future global warming climates. Alice Springs has the largest solar electricity generation, while Hobart has the smallest. Under the 2 °C warming scenario, minor increases in PV electricity generation occur in all cities studied, ranging from a 1.6% increase in Hobart up to a 3% increase in Alice Springs. Note that Australia is in the southern hemisphere, so PV panels are chosen to face north. In Australia, standard roof pitches are usually 15° or 22.5°. Most PV panels are installed at whatever angle the roof happens to be titled at. This is because the additional cost of tilt frames is always greater than cost of adding a solar panel or two. In this study 15°

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Fig. 6. Annual electricity consumption of households in the seven cities when the house 1 is occupied evening only under current (left) and future global warming (right) climates. 202

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12000 10000

8000 6000 4000 2000 0

oolinŐ >iŐhƟnŐ

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Fig. 7. Annual electricity consumption of households in the seven cities when the house2 is occupied full day under current (left) and future global warming (right) climates. 2000.0

oolinŐ

12000

Hot water

>iŐhƟnŐ

10000

Appliances

4000 2000 0

Future

1600.0 1400.0 1200.0 1000.0 800.0 600.0 400.0 200.0 0.0 Darwin

Alice Springs

Brisbane

Sydney

Mildura

Melbourne

Hobart

Fig. 9. Annual electricity generation of 1 kW solar PV in the seven cities under current and future global warming climates.

systems are not operated when the houses are not occupied (consequently, the heat accumulated inside the house during the day will need to be removed in addition to the heat loads when the occupants come back the house in the evening). Further research is required for buildings being operated with smart technologies, such as to reduce PV battery system size, how long the windows should be closed and the air conditioners be turned on before the occupants back to their homes. Based on information on current prices of solar PV systems and batteries in the Australian market [60], a median price of 1.3 A$/W for solar PV (including supply and installation of inverter and all required

Electricity consumpƟon (kWh/annual)

Electricity consumpƟon (kWh/annual)

HeaƟnŐ

6000

Current

1800.0

14000

14000

8000

Electricity generaƟon (kWh/annual)

the range of battery capacities will be determined (i.e., maximum battery capacity is needed with the smallest PV size, and minimum battery capacity is required with the largest PV size). The results for PV battery systems are shown in Tables 4 and 5 for House 1 occupied full day and evening only respectively, while Tables 6 and 7 show corresponding results for House 2. For this study, battery charge/discharge efficiency is assumed to be 85%, and the results for PV size smaller than 10 kW are not shown as the associated battery sizes are too large to be economical. As expected, the required size of PV battery system is larger for House 2 to get off the grid than for House 1. Under current and future climates for both houses, Hobart needs the largest PV battery systems to get off-grid due to its low PV electricity generation (refer to Fig. 9) and high heating energy demand (refer to Figs. 4–7). Brisbane requires the smallest systems due to its relatively high PV electricity generation (refer to Fig. 9) and low heating/cooling energy demand (refer to Figs. 5–8). Under the 2 °C warming scenario, Darwin becomes harder to get off-grid due to a large increase in cooling requirement, while the sizes of PV battery systems required for Hobart and Melbourne drop significantly due reductions in the heating energy demand. It is interesting to see that the sizes of PV battery systems in some houses are larger for houses that are occupied evening only than for houses are occupied all day. This is due to the larger amount of energy required to heat or cool the houses when the occupants return to their homes in the evenings under assumption that space heating or cooling

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Fig. 8. Annual electricity consumption of households in the seven cities when the house 2 is occupied evening only under current (left) and future global warming (right) climates. 203

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Table 2 PV sizes (kW) of net zero energy homes in the seven cities for House 1 under current and future global warming climates. City

Current climate

Table 6 The predicted sizes of PV battery systems for off-grid housing in the seven cities under current and future global warming climates when House 2 is occupied full day.

Future global warming climate

Climate Full day occupied Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

4.6 2.9 3.2 3.6 3.2 5.1 5.7

Evening only

3.7 2.7 2.9 3.3 2.9 4.6 5.3

Full day occupied 5.9 3.2 3.5 3.3 3.2 4.4 5.0

PV size (kW) Battery size (kWh) for each city

4.6 2.8 3.1 3.0 2.9 4.0 4.6

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Current climate Full day occupied

Evening only

5.5 3.5 3.6 4.1 3.6 5.8 6.5

4.7 3.1 3.2 3.8 3.3 5.3 6.0

Future global warming climate

Climate

Full day occupied

Evening only

7.2 4.2 4.3 3.8 4.0 5.1 5.6

6.4 3.8 3.7 3.5 3.6 4.6 5.2

PV size (kW) Battery size (kWh) for each city

PV size (kW) Battery size (kWh) for each city

Current

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

10 23.5 26 23 113.5 43.5 693.5 1092

Future global warming 15 14 21 15 39 25 147.5 421

20 13.5 18 11.5 33 17.5 32.5 79.5

10 61.5 15.5 16 34 29.5 324.5 787

15 24 15 12.5 27.5 12 32.5 206

20 17 14.5 12.5 22 10 23 57

Table 5 The predicted sizes of PV battery systems for off-grid housing in the seven cities under current and future global warming climates when House 1 is occupied evening only. Climate PV size (kW) Battery size (kWh) for each city

Current

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

10 23.5 23 21.5 51.5 36.5 552.5 911

Future global warming 15 19 20 12.5 35 20.5 79 284

20 19 17 12 35 11.5 30 64

10 37.5 21 17.5 30 15.5 214.5 645

15 22.5 20 16 22 15 27.5 125.5

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

10 57 33 30.5 342 87 1250 1811

15 18.5 28.5 22.5 70 63 548 1015

20 17.5 25 17 51.5 50 154 438.5

10 396 60.5 30.5 63.5 37.5 547 1223

15 60 42 19.5 40.5 20.5 112 557

20 32 25 18 30 16 40 145

Current

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

10 69 34 29 219 73.5 1037 1557

Future global warming 15 53 32 20 53 53.5 410 827

20 43 32 16 46 41 74 300

10 133 64 29.5 53.5 34 434 1056

15 46 47.5 27 36 30 50 418

20 37 41.5 27 35.5 30 37 86

net-zero energy housing can also be estimated based on the PV sizes (refer to Tables 2–3) and this is shown in Table 10. The results show that Hobart is the most expensive city to get offgrid, for both housing types, for both occupancy scenarios, and under both current and future climates. Under the current climate, Darwin and Brisbane are the cheapest cities to get off-grid for both housing types when the houses are being occupied for the full day. Under the 2 °C warming scenario, Brisbane is the cheapest city to get off-grid for both house types, except under all day occupation, when Alice Springs is slightly cheaper than Brisbane for House 1. In general, leaving the grid requires relatively large PV and battery systems, and the installation costs of the systems for off-grid housing are much higher than netzero energy homes in all the seven cities under both current and future global warming climates (refer to Tables 8–10). In addition, a PV battery systems have a significant amount of unused PV generation to curtail. For instance, 3.2 kW PV can meet the electricity requirement of House 1 in Brisbane under current climate when the house is occupied full day (i.e., net-zero energy achieved with 3.2 kW PV, see Table 2), however the 15 kW/15kWh PV battery system for the off-grid housing (Table 8) will curtail 76% of its PV output (with the system energy efficiency considered). To reduce the initial investment in off-grid housing, one economic option is to integrate an on-site generator (diesel or petrol) into the PV battery system. This will be discussed in detail in the next section.

Table 4 The predicted sizes of PV battery systems for off-grid housing in the seven cities under current and future global warming climates when House 1 is occupied full day. Climate

Future global warming

Table 7 The predicted sizes of PV battery systems for off-grid housing in the seven cities under current and future global warming climates when House 2 is occupied evening only.

Table 3 PV sizes (kW) of net zero energy homes in the seven cities for House 2 under current and future global warming climates. City

Current

Evening only

20 22.5 20 15.5 21 15 21 51

3.4. Payback periods for net zero energy homes and off-grid housing To assess whether the investment in solar PV and battery systems for net zero homes (grid-connected) and off-grid housing is worth-while economically, the ‘payback period’ is a simple way to answer people’s question on how long it will take to get their money back on upfront investment. The actual payback period is complicated and impacted by a number of factors that we may not know in advance, including (but not limited to) the future prices of electricity and solar PV battery system, changes in electricity usage patterns over time, and the performance and the service life of the PV battery systems over time. To keep things simple and manage the number of variables, the most

cabling, safety and mounting equipment), and an average price of 940 A$/kWh for installed battery plus inverter/charger were used for the economic analysis in this study. Based on these figures, and the results of projected PV battery sizes (shown in Tables 4–7) required for off-grid housing, the cheapest options and indicative prices (in A$) for off-grid housing in the seven cities have been estimated for House 1 and 2, and are shown in Tables 8 and 9. The indicative prices of PV installation for 204

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Table 8 The economic options of PV battery systems and indicative costs of off-grid housing for House 1 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

PV/battery

Cost ($)

PV/battery

Cost ($)

PV/battery

Cost ($)

PV/battery

Cost ($)

15 kW/14 kWh 10 kW/26 kWh 15 kW/15 kWh 15 kW/39 kWh 20 kW/17.5 kWh 20 kW/32.5 kWh 20 kW/79.5 kWh

32,660 37,440 33,600 56,160 42,450 56,550 100,730

20 kW/17 kWh 10 kW/15.5 kWh 10 kW/16 kWh 10 kW/34 kWh 15 kW/12 kWh 20 kW/23 kWh 20 kW/57 kWh

41,980 27,570 28,040 44,960 30,780 47,620 79,580

15 kW/19 kWh 10 kW/23 kWh 15 kW/12.5 kWh 15 kW/35 kWh 20 kW/11.5 kWh 20 kW/30 kWh 20 kW/64 kWh

37,360 34,620 31,250 52,400 36,810 54,200 86,160

15 kW/22.5 kWh 10 kW/21 kWh 10 kW/17.5 kWh 15 kW/22 kWh 15 kW/15 kWh 15 kW/27.5 kWh 20 kW/51 kWh

40,650 32,740 29,450 40,180 33,600 45,350 73,940

Table 9 The economic options of PV battery systems and indicative costs of off-grid housing for House 2 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

PV/battery

Cost ($)

PV/battery

Cost ($)

PV/battery

Cost ($)

PV/battery

Cost ($)

15 kW/19 kWh 10 kW/33 kWh 15 kW/22.5 kWh 20 kW/51.5 kWh 20 kW/50 kWh 20 kW/154 kWh 20 kW/438.5 kWh

37,360 44,020 40,650 74,410 73,000 170,760 438,190

20 kW/32 kWh 20 kW/26 kWh 15 kW/19.5 kWh 20 kW/30 kWh 15 kW/20.5 kWh 20 kW/40 kWh 20 kW/145 kWh

56,080 50,440 37,830 54,200 38,770 63,600 162,300

15 kW/29 kWh 10 kW/34 kWh 15 kW/20 kWh 20 kW/46 kWh 20 kW/41 kWh 20 kW/74 kWh 20 kW/300 kWh

46,760 44,960 38,300 69,240 64,540 95,560 308,000

20 kW/37 kWh 15 kW/47.5 kWh 10 kW/29.5 kWh 15 kW/36 kWh 10 kW/34 kWh 20 kW/37 kWh 20 kW/86 kWh

60,780 64,150 40,730 53,340 44,960 60,780 106,840

Table 10 Costs (A$) of net zero energy homes for Houses 1 and 2 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

5980 3770 4160 4680 4160 6630 7410

7150 4550 4680 5330 4680 7540 8450

7670 4160 4550 4290 4160 5720 6500

9360 5460 4160 4940 5200 6630 7280

4810 3510 3770 4290 3770 5980 6890

6110 4030 5590 4940 4290 6890 7800

5980 3640 4030 3900 3770 5200 5980

8320 4940 4810 4550 4680 5980 6890

common form of payback calculation is the “simple payback” (www. evergen.com.au), which is equal to the upfront cost divided by the savings in the first year due to the installation of the system. It assumes we will get the same savings year-on-year (i.e., the money value and electricity prices varying with time are ignored) and the costs of the system replacement aren’t take into account. In practice, to consider the influence of inflation, discount rate and electricity price escalation rate, the discounted payback period is applied. In this study both the simple and discounted payback periods are calculated, which are detailed in the following subsections.

Table 11 Residential electricity prices [61] and supply charge [62] for FY2018-19 (including GST) and solar feed-in tariffs [63].

3.4.1. Simple payback periods for off-grid housing and net zero energy homes Using information on residential electricity prices published by Australian Energy Market Commission [61] and supply charges [62] in the current Australian market (see Table 11), electricity bill savings can be estimated for off-grid housing (Table 12), based on the predicted annual electricity consumption (refer to Figs. 4–7). Bill savings for the net-zero energy home (Table 13) can also be estimated based on the electricity price, solar feed-in tariffs [63] (Table 11) and the predicted amount of the electricity purchased from or fed into the grid, which is

City

Electricity price (c/ kWh)

Daily supply charge ($/day)

Solar feed-in tariff (c/kWh)

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

29.17 29.17 28.6 34.05 31.68 34.05 24.64

0.56 0.56 1.26 1.43 1.02 1.28 1.05

26.88 26.88 11 11 11.6 11 8.9

determined by the predicted household hourly energy consumption and PV battery system size (see Fig. 1 for the electricity flowchart). Note that in Table 11, the median or average values are chosen when the price varies between retailers. The electricity prices (including taxes) were averaged values from most of the electricity retailers of each state [61]. The prices are flat as shown in [61] (i.e., no difference between day and night), which are chosen for this case study to simplify payback period analysis. 205

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Table 12 Bill savings (A$/year) of off-grid Houses 1 and 2 for FY2018-19 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

2552.5 1812.6 1849.7 2491.2 1905.3 2601.9 2079.6

3034.9 2099.5 2028.5 2750.3 2085.1 2888.0 2307.0

3276.7 1984.8 2012.0 2347.5 1956.7 2371.9 1889.2

3971.3 2558.4 2371.2 2642.4 2319.6 2654.2 2064.6

2074.1 1659.4 1725.3 2306.1 1775.4 2405.0 1938.9

2619.6 1924.6 1878.3 2564.1 1942.3 2686.1 2155.6

2624.3 1769.6 1833.1 2186.2 1810.1 2213.1 1771.0

3527.3 2317.8 2121.1 2465.8 2122.9 2469.2 1940.0

offgridenergy.com.au). The petrol price is assumed at A$1.35/L. Note that this study shows that further increasing the electricity offset by the generator cannot reduce the payback periods, given the prices of electricity sourced from the generator are around A$ 0.8–0.9/ kWh, much higher than electricity purchased from the grid (Table 11). The optimized options and their corresponding payback periods are shown in Tables 17 and 18 for Houses 1 and 2 respectively. With a PV battery system hybridized with a petrol generator, the required PV battery size is reduced, yielding a significant drop in the payback period. For House 1 occupied all day, the simple payback periods in Darwin and Brisbane drop to less than 10 years under both the current climate and 2 °C warming scenario. The payback periods are also shorter than 9 years for Sydney and Mildura under the 2 °C warming scenario. For House 1 occupied in the evening only, Mildura has the shortest simple payback periods (less than 11 years) under both current and the 2 °C warming scenario. For House 2 occupied all day, the simple payback periods are shorter than 10 years for Brisbane under current climate and for Mildura under future global warming climate. For House 2 with evening occupancy, under both current climate and future global warming climate all the payback periods in the seven cities are longer than 11 years. For both Houses 1 and 2, the simple payback periods are all over 20 years in Hobart for both occupancy scenarios under current and future climates.

Based on the predicted costs of the cheapest options (Tables 8 and 9) and bill savings (Table 12), the shortest payback periods are estimated for off-grid housing, and shown in Table 14. Simple payback periods for net zero energy housing, presented in Table 15, are based on the predicted costs (Table 10) and bill savings (Table 13). With relatively low cost PV systems and high feed-in tariffs, the simple payback periods of net zero energy homes can be as short as 3 years in Darwin for houses that are occupied for the full day. As expected, the simple payback periods of the houses occupied in the evening only increased significantly (more than twice as much for some houses) than those being occupied all day, due to large amount of surplus PV output fed into the grid at relatively low feed-in tariffs compared to the electricity prices (Table 11). With a 25 year warranty on nearly all solar panels being sold in Australia from 2012 onwards (www.coffssolarenergy.com.au), net-zero energy houses with solar PV are economically attractive for full-day occupancy houses, even in Hobart (the city with the longest payback period, approximately 10 years). Due to the relatively high cost of batteries, the simple payback periods for off-grid housing in all seven cities, under both current and projected future climates, are more than 12 years, which is longer than the 10 year warranty offered by most battery companies in the current market (note that the “Powerwall” is advertised by Tesla as having a lifetime of 15 years and 5000 cycles [64]). From an economic view, they are therefore less attractive. As mentioned above, one option to reduce the capital cost of off-grid housing is to integrate a generator with the PV battery system. In Australia, on average, petrol is 10% cheaper than diesel. Diesel engines emit more noxious gases and CO2 per litre of fuel than petrol-powered engines. Considering the generators are run at home site and fuel cost, in this study the PV battery system of an off-grid house is assumed to be hybridized with a petrol generator, which is assumed to be able to provide up to 10% of the total energy consumption of the house when the PV battery system cannot deliver (in this study, three scenarios, 1%, 5% and 10% of generator provided energy are evaluated). Basic information on petrol generators is given in Table 16 (www.

3.4.2. Discounted payback periods for off-grid housing and net zero energy homes To consider cost changes year-on-year, a classical investment calculation for PV battery installation is the Net Present Value of cash flows (NPV), which is calculated under influence of inflation, discount rate and electricity price increase [65], n

NPV =

cash flowi

∑ (1 + real i=1

interest )i

− cash flow0

(6)

cash flowi = electricity cost withoutinstallation, i − electricity cost withinstallation, i (7)

Table 13 Bill savings (A$/year) of net zero energy Houses 1 and 2 for FY2018-19 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

1964.4 1068.4 707.4 926.2 791.2 973.6 835.2

2282.3 1337.8 847.8 1025.8 931.1 1075.3 885.6

2058 1293 829.4 852.9 828 855.5 724

3013.9 1736 1070.5 991.8 1031.1 1007.4 772.2

590.6 496.7 385.3 452.8 442.1 474.3 438.3

670.1 559.2 444.1 468.5 496.7 489 435.8

688.6 520.1 410.4 430.1 447.8 440.2 395.9

817.3 595 462.8 455.8 491 470 402.3

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Table 14 Simple payback periods (year) of off-grid Houses 1 and 2 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

real interest =

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

12.8 20.7 18.2 22.5 22.3 21.7 48.4

12.3 21.0 20.0 27.1 35.0 59.1 189.9

12.8 13.9 13.9 19.2 15.7 20.1 42.1

14.1 19.7 16.0 20.5 16.7 24 78.6

18.0 20.9 18.1 22.7 20.7 22.5 44.4

17.9 23.4 20.4 27.0 33.2 35.6 142.9

15.5 18.5 16.1 18.4 18.6 20.5 41.8

17.2 27.7 19.2 21.6 21.2 24.6 55.1

1 + discount rate −1 1 + inflation

Table 16 Costs of petrol generators and fuel consumption for electricity generation.

(8)

where cash flow0 is the investment of PV battery system and cash flowi/ (1 + real interest)i is the present cash flow value of year i. To account for the influence of inflation, discount rate and electricity price escalation rate, the discounted payback period (n) can be determined using Eq. (6) (i.e., NPV = 0). The lifetime of inverters installed in residential buildings is around 10 years (www.ilume.be). On average, the inverter should be replaced once during the lifetime of solar panels. The costs of extended warranty to 15 years and 20 years for 5–8 kW solar PV inverters are around A$750 and A$1160 respectively (www.sma-australia.com.au). As mentioned previously, the general range of a solar PV battery’s useful lifespan is between 5 and 15 years. A10 year warranty is offered by most battery companies in the current market. So the batteries should be replaced once during the lifetime of solar panels. It is expected that battery prices will go down in the future [66,67]. By 2030 the prices of battery storage will decline to about half the current price [67]. In this study, half current price of battery storage system (i.e., 470 A$/kWh) is assumed for the replacement cost of the installed battery plus inverter/charger after 10 year warranty. There is little information on the battery storage O&M (operations and maintenance) costs and the O&M costs are relatively low to the capital cost (e.g., the O&M cost is A$300 every 5 years for Li-ion battery storage [67]). Solar panels generally require very little maintenance since they have no moving parts. With a tilt angle of 10 degrees or greater solar panels are self-cleaning. If a general cleaning is required, a standard garden hose can be used to wash the face of the panels during either the early morning or in the evening. In this study we assumed that no cost will occur in O&M for solar panels and battery storage. With an average annual inflation rate of 2.3% (www.rateinflation. com) and a 6.5% discount rate assumed, based on the past ten years [68] and a 4.4% average annual electricity price increase [69] in Australia, the discounted payback period of each scenario can be calculated based on the electricity saving with the PV panels and battery

Generator size

Price (A$), GST incl.

Fuel consumption (L/kWh)

2.8 kVA 6.5 kVA 8 kVA

490.0 850.0 990.0

0.63 0.60 0.60

installation and the investment (initial capital cost plus the replacement cost). The discount payback periods of net zero energy House 1 and 2 in the seven cities are shown in Table 19. It can be seen that the differences between discounted payback period (Table 19) and simple payback period (Table 15) are smaller (less than 0.1 years) when the replacement of the inverter is not required (i.e., the payback period is shorter than 10 years). That implies the combined influences of money value (the inflation rate and the discount rate) and the electricity price escalation rate on the differences between these two payback periods are marginal. After ten years the replacement of the inverters are needed and the differences between these two payback periods become greater. The largest difference occurred in Hobart for House 2 occupied evening only and they are 1.4 years for both under current and future climates. Table 20 presents the discounted payback periods of off-grid Houses 1 and 2 in the seven cities with the PV battery systems (see Tables 8 and 9). The discounted payback periods in Table 20 are all longer in comparison with the corresponding simple payback periods in Table 14. For both houses in Mildura, Melbourne and Hobart under current and future climates, the discounted payback periods are longer than 25 years (represented by ‘–’ in the tables) and off-grid housing in these cities is economically unviable. Darwin has the shortest payback periods (only the payback period of House 2 occupied evening under current climate is longer than 25 years). For House 1 in Brisbane under current and future climates the payback periods are shorter than 25 years for both occupied full day and occupied evening only. For both houses in Sydney occupied full day they are economically unviable under current climate.

Table 15 Simple payback periods (year) of net zero energy Houses 1 and 2 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

3.0 3.5 5.9 5.1 5.3 6.8 8.9

3.1 3.4 5.5 5.2 5.0 7.0 9.5

2.9 3.2 5.5 5.0 5.0 6.7 9.0

3.1 3.2 5.2 5.0 5.0 6.6 9.4

8.1 7.1 9.8 9.5 8.5 14.2 18.4

9.1 7.2 9.4 10.5 8.6 16.5 20.6

8.7 7.00 9.8 9.1 8.4 13.6 18.0

10.2 8.3 10.4 10.0 9.5 14.4 20.0

207

3.0 kVA, 8 kW/13.5 kWh 3.5 kVA, 4 kW/11 kWh 2.5 kVA, 6 kW/8 kWh 3.5 kVA, 8 kW/10 kWh 3 kVA, 6 kW/7.5 kWh 3 kVA, 15 kW/11.5 kWh 3.5 kVA, 15 kW/12 kWh

9.6 10.2 9.7 10.2 10.3 13.2 21

4.0 kVA, 8 kW/16 kWh 3.5 kVA, 6 kW/8 kWh 3 kVA, 6 kW/7.5 kWh 3 kVA, 6 kW/8 kWh 2.5 kVA, 5 kW/7 kWh 4 kVA, 8 kW/9.5 kWh 3.5 kVA, 10 kW/15.5 kWh

9.2 10.9 10.0 8.4 8.9 10.4 20.6

3.3 3.5 2.5 3.5 2.5 3.5 3.5

kVA, kVA, kVA, kVA, kVA, kVA, kVA,

6 kW/16 kWh 4 kW/12 kWh 6 kW/9.5 kWh 6 kW/12.5 kWh 5 kW/9.5 kWh 10 kW/13 kWh 15 kW/19 kWh

Generator, PV/battery

Generator, PV/battery

Generator, PV/battery

Payback (year)

Current climate

Future global warming

Current climate Payback (year)

Evening occupied only

Full day occupied

13.1 11.8 11.2 10.8 11.4 13.4 22.6

Payback (year) 3.5 kVA, 8 kW/19 kWh 3 kVA, 6 kW/15 kWh 3 kVA, 6 kW/12 kWh 2.5 kVA, 6 kW/13.5 kWh 2 kVA, 6 kW/8.5 kWh 3 kVA, 8 kW/10.5 kWh 3 kVA, 10 kW/14 kWh

Generator, PV/battery

Future global warming

12.8 14.6 12.1 10.6 11.7 11.7 20.8

Payback (year)

208

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

City

5.5 kVA, 8 kW/17.0 kWh 5.5 kVA, 6 kW/10 kWh 3 kVA, 6 kW/8.5 kWh 4 kVA, 8 kW/13.5 kWh 4 kVA, 6 kW/8.5 kWh 4 kVA, 15 kW/13 kWh 4.5 kVA, 20 kW/15.5 kWh

10.3 11.6 9.1 10.7 10.1 14.0 24.6

5.5 kVA, 10 kW/20.5 kWh 6.5 KVA, 8 kW/15 kWh 3.5 kVA, 6 kW/10.5 kWh 5.5 kVA, 8 kW/10 kWh 5.5 kVA, 8 kW/7.5 kWh 3.5 kVA, 10 kW/10.5 kWh 4 kVA, 15 kW/16 kWh

9.6 13.3 10.1 9.7 10.1 11.2 23.3

5.5 kVA, 8 kW/23.5 kWh 3.5 kVA, 6 kW/14.5 kWh 3 kVA, 6 kW/11 kWh 4.5 kVA, 8 kW/15 kWh 4 KVA, 5 kW/10.5 kWh 4.5 kVA, 15 kW/15.5 kWh 4.5 kVA, 15 kW/20.5 kWh

Generator, PV/battery

Generator, PV/battery

Generator, PV/battery

Payback (year)

Current climate

Future global warming

Current climate Payback (year)

Evening occupied only

Full day occupied

14.6 15.4 11.2 11.6 11.1 16.1 25.5

Payback (year)

6.5 kVA, 10 kW/29 kWh 6.5 kVA, 8 kW/24.5 kWh 4.5 kVA, 6 kW/16.5 kWh 7.5 kVA, 8 kW/13 kWh 4 KVA, 6 kW/13.5 kWh 3.5 kVA, 10 kW/13 kWh 4 kVA, 15 kW/16 kWh

Generator, PV/battery

Future global warming

13.4 19.8 12.8 11.7 12.7 13.1 24.8

Payback (year)

Table 18 The economic options and their simple payback periods of hybrid PV battery system with a petrol generator of off-grid housing for House 2 in the seven cities under current and future global warming climates.

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

City

Table 17 The economic options and their simple payback periods of hybrid PV battery system with a petrol generator of off-grid housing for House 1 in the seven cities under current and future global warming climates.

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Table 19 Discounted payback periods (year) of net zero energy Houses 1 and 2 in the seven cities under current and future global warming climates. City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

3.0 3.5 5.9 5.0 5.3 6.8 8.9

3.1 3.4 5.5 5.2 5.0 7.0 9.6

2.9 3.2 5.5 5.0 5.0 6.7 9.0

3.1 3.1 5.2 5.0 5.0 6.6 9.5

8.2 7.1 10.0 9.6 8.6 14.6 19.4

9.2 7.2 9.4 10.7 8.7 17.1 22.0

8.7 7.0 9.9 9.1 8.5 13.9 19.0

10.3 8.3 10.5 10.1 9.6 14.8 21.4

electricity generation is low. Considering the 25 year warranty of PV systems, the 10 year warranty on batteries typically offered by most providers, replacement cost of battery storage, and money value (inflation rate and discount rate) and electricity price escalation rate, offgrid housing using PV battery systems only, are economically unviable in Mildura, Melbourne and Hobart for both houses under current and future climates. Darwin has the shortest payback periods, based on the profiles of household electricity consumption, costs of PV battery systems, and prices of electricity used in this study. However, PV battery systems hybridized with an on-site generator facilitate significantly smaller and cheaper PV battery capacity requirements. The payback periods drop significantly if the generator is allowed to offset up to 10% of the total electricity consumption that the PV battery system cannot meet (in most cases only 5% is required). Only in Hobart, off-grid housing with PV battery system with the petrol generator is economically unviable for both houses under current and future climates. When the houses are occupied evening only, the payback periods are longer and the economic benefits are smaller. In general, larger houses (House 2) have longer payback periods than smaller size houses (House 1). The case study also demonstrates that from an economic view the grid-connected net zero energy home is more economically attractive than an off-grid configuration under both current and future global warming climates - even without subsidy for solar feed-in tariffs. In Darwin, with relatively high solar feed-in tariff, the payback periods can be as short as around 3 years when the houses are occupied full day. Even for the longest payback city (Hobart – which has the lowest price of electricity and solar feed-in tariff), the payback periods are shorter than 10 years and 22 years for both house types being occupied full day and evening only respectively. The comparisons between simple payback period and discounted payback period show the influence of money value (inflation rate and discount rate) and electricity price escalation rate (used in this study) on payback period is marginal. The replacement costs of battery storage and solar PV inverter dominated the difference between these two payback periods for off-grid housing and net zero energy home.

However, they are economically viable under future global warming climate. The smallest difference between these two payback periods (see Tables 20 and 14) is greater than 3 years for both houses in all the seven cities under current and future climates. The discounted payback periods of off-grid Houses 1 and 2 with the hybrid PV battery system with the petrol generator (see Tables 17 and 18) in the seven cities are shown in Table 21. The differences between the simple and discounted payback periods are minimal (less than 0.1 years) when the replacement of the batteries are not required (i.e., the payback period is shorter than 10 years). After 10 years when the replacement of batteries is required, the differences between these two payback periods become greater. The smallest difference between the two payback periods is greater than 2.0 years for both houses in the seven cities under current and future climates. Even with the hybrid PV battery system with the petrol generator, the payback periods are longer than the lifespan of the system (25 years) in Hobart for both houses under current and future global warming climates. That implies off-grid housing in Hobart is economically unviable with the economical parameters used in this study. 4. Discussions and conclusions In this study, the economic feasibility of taking house off-grid is investigated. The analyses cover two new 6-star energy rated house types of different construction and sizes, under both current climate conditions and a 2 °C future warming scenario. Both house types are evaluated for two typical occupancy profiles (occupied all day, and occupied in the evening only), for seven different Australian climate zones, represented by seven major cities. For comparison purposes the simple payback period and discounted payback period are estimated. The study indicates that large PV battery systems are required, and the simple payback periods and discounted payback periods are longer than 12 years and 15.8 years respectively for off-grid housing in all cases. Going off-grid is hardest in cooler regions (Hobart and Melbourne), where demand for heating energy is high during winter when PV

Table 20 Discounted payback periods (year) of off-grid House 1 and 2 in the seven cities under current and future global warming climates (‘–’ presents the payback period greater than the PV battery system lifespan of 25 years). City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

15.9 – 23.8 – – – –

15.8 – – – – – –

15.8 18.4 18.6 – 19.7 – –

18.8 – 21.1 – 22.4 – –

25.0 – 23.2 – – – –

– – – – – – –

20.8 – 22.0 – 24.4 – –

24.1 – – – – – –

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Table 21 Discounted payback periods (year) of hybrid PV battery system with a petrol generator of off-grid housing for Houses 1 and 2 in the seven cities under current and future global warming climates (‘–’ presents the payback period greater than the PV battery system lifespan of 25 years). City

Full day occupied

Evening occupied only

Current climate

Darwin Alice Springs Brisbane Mildura Sydney Melbourne Hobart

Future global warming

Current climate

Future global warming

House 1

House 2

House 1

House 2

House 1

House 2

House 1

House 2

9.7 13.7 9.8 12.8 13.0 16.1 –

13.7 15.1 9.2 13.9 12.8 17.4 –

9.3 13.8 12.5 8.4 8.9 13.0 –

9.7 17.8 13.1 9.8 12.2 13.8 –

18.0 22.6 14.5 14.3 15.0 17.3 –

20.7 21.5 14.8 15.4 14.7 20.7 –

17.5 24.7 16.1 14.2 14.9 15.0 –

18.8 – 17.7 15.2 17.2 16.8 –

In summary, with the relatively high electricity price and significantly drops in cost of solar PV in recent years, from an economic view it is worth investing in the grid-connected net zero energy homes for households across Australia. With further declines in the cost of PV battery system (especially for battery storage as many market analysts predict) and flat-lining or increasing electricity prices due to more rooftop PVs being installed, households may take further steps, and move to fully off-grid configurations, and avoid the risks of high future electricity costs, as economic benefits will be apparent in most of the cities (except Hobart) if initial capital costs of PV and batteries are reduced. It is noted that the results of sizing PV battery system for off-grid housing are determined upon building design, operation, installed PV battery and local climate. Although the conclusions are drawn from the case study for Australia under the proposed PV battery prices and electricity tariffs in this study, the general findings should be applicable to other regions with similar climates (three of five the Köppen climate groups investigated in this study), construction types and operations, PV battery prices and electricity tariffs. It is also noted that the case study was conducted for houses powered by electricity only. Further research is required if non-electric energy sources (such as natural gas) are available for the houses.

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