a design tool for off-grid housing in australia...for off-grid housing, the pv-battery system is not...

AIRAH and IBPSA’s Australasian Building Simulation 2017 Conference, Melbourne, November 15-16.

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A DESIGN TOOL FOR OFF-GRID HOUSING IN AUSTRALIA

ZHENGEN REN PHD

Senior Research Scientist

CSIRO Land and Water Division

Private Bag 10

Clayton South Vic 3169

[email protected]

WANYEE CHAN

Software Engineer


Private Bag 10


[email protected]

DONG CHEN PHD

Principal Research Scientist


Private Bag 10


[email protected]

PHILLIP PAEVERE PHD

Principal Research Scientist


Private Bag 10


[email protected]

ABSTRACT

This study presents an integrated tool for off-grid housing design across Australia. The tool has been

developed by implementing a solar PV battery system model into the existing Australian Zero

Emission House design tool (AusZEH). AusZEH is a tool for designing low/zero carbon housing that

incorporates technologies and measures to reduce whole house energy consumption, and takes into

account local climate, building envelope, equipment and appliances, and occupant behaviour. The

off-grid housing design is evaluated by matching energy supply (predicted by the solar PV battery

model) at one-hour time step with the energy demand (as predicted by AusZEH).

A case study is developed for existing old houses which shows that, with non-electric energy sources

(e.g. natural gas) for the four main end-uses (water heating, space heating, cook-top and oven),

households may operate off the grid at an installation cost of PV-battery of around $47,000 in Sydney

(a balanced heating/cooling region) and $59,500 in Townsville (a tropical region). In Melbourne (a

heating dominated region), the most cost-effective way to go off-grid is to first retrofit the house to

be 6 stars and then install a PV-battery system. The total cost is around $49,500. This tool provides a

cost-effective pathway for off-grid housing design in Australia.


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INTRODUCTION

Over the past 16 years in Australia, electricity prices for households have increased at an average

annual rate of 8% (mainly as a result of rising network charges) while the average inflation rate is

around 2.8% [1]. This has put a financial strain on households across Australia. With the recent rapid

decrease in PV prices and a similar trend for battery storage and increased awareness of

environmental impacts, there is growing interest in public and academic discussions about the

possibility of “leaving the grid” or “living off-grid” by installing PV-battery systems.

Australia has high solar radiation, and around 70% of dwellings are detached [2]. This provides the

opportunity for many households to use rooftop PV with battery systems to live off the grid. The PV

battery systems used for buildings are not new. The industrial application of (lead-acid) batteries

started more than one century ago and the application of PV technology started more than half a

century ago [3]. It is over five decades since the sizing of PV-battery systems was first studied. Initial

studies of PV-battery system sizing were mainly concentrated on rural areas (with no grid

connections) using simple and approximate methods, and often resulted in the systems that were over-

sized or under-sized [4]. Later, Egido and Lorenzo [5] introduced iso-reliability curves by developing

numerous graphs of PV-storage sizes. With computer modelling development, PV-battery sizing

models were improved by using real historical time series data of solar irradiation and loads other

than daily average data [6, 7], or using characteristic equations instead of simple efficiency values for

PV panel, battery and inverters [8]. Some studies were also made to use artifical intelligence

techniques for sizing PV-battery systems [9].

There have been extensive studies of PV-battery systems for residential buildings [1], including some

efforts made to investigate potential PV-battery systems for household electricity self-sufficiency [1,

10-12]. One of the challenges for off-grid housing is affordability. This is dependent upon the capital

and installation cost for PV-battery systems. The capital cost can be reduced significantly by lowering

the household energy consumption through energy efficient building design/construction, installing

high energy efficient equipment and appliances, and by operating the house efficiently. In order to

understand the cost trade-offs at the design stage, this requires an integrated tool for cost-effective

analysis of building design/retrofit options, management of energy demand, and costs and efficiencies

of PV-battery systems. Few studies have combined all of these in an integrated approach, especially

for Australian housing and climates.

This study presents an integrated approach for design and evaluation of the most cost-effective

options for off-grid housing in Australia.

1. METHODOLOGY

The modelling tool developed for this study requires two functions: (a) evaluation of the most cost-

effective approch for reducing housing energy demand through design/retrofit of the residential

building and installation of appliances; and (b) sizing the PV-battery system. In order to provide

proper analysis of the most cost-effective options, these analyses require time series data on the

household energy consumption (at least with hourly data over a period of one year), and the

performance characteristics of the PV generation and battery charge/discharge system. Currently

there are no available tools to provide this integrated approach for off-grid housing design/retrofit. In

this study, we used the AusZEH design tool [13, 14, 15] for the building energy assessment analysis.

This produces hourly time-series data on the household energy consumption which is then used to


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determine the required sizing of the PV-battery systems via a modelling module for rooftop PV-

battery charge/discharge [1]. The cost of each option is then calculated based on the combined cost

of measures to improve building energy performance and the installation of PV-battery system.

1.1 Cost-effective analysis for housing design/retrofit

The AusZEH design tool was originally developed for the Australia Zero Emission Home project, to

consider building envelope, installed equipment and appliances, occupant behavior and local climate

in the design of zero-emission houses for Australia [14]. In the tool, the space heating and cooling

loads are simulated by a modified version of AccuRate [16, 17] – which is the benchmark software

for NatHERS (Nationwide House Energy Rating Scheme) in Australia [18]. Based on the annual total

heating and cooling loads for a specific climate, AccuRate assigns a star rating between 0 and 10 to

the residential building. The star band is defined by NatHERS [18]. The higher the star rating, the

more energy-efficient the building. AccuRate has been validated against measured data and was the

BESTEST [19].

Figure 1. Simulation process of the AusZEH for new building design

Figure 2. Simulation process of the AusZEH for existing building retrofit

For new building design, this study adopts a ‘normal’ building simulation process (Figure 1) as

most of current building simulation tools use, which starts from the sources (inputs) to the target

(outputs), and stops after the target is achieved with an iterative process. For an existing building

retrofit, the ‘backward’ simulation process is proposed (Figure 2), which starts from the target

(input) such as energy performance/carbon reductions or budget for the retrofit and output the

cost-effective options required for achieving the target. Using basic information of the existing

building, an initial set of results are obtained after the first run. A number of different simulation

runs are then made until the target is achieved, using various options, which are ranked according

to cost. The outputs include the proposed building components and PV-battery systems associated

with the design target.

In the ‘normal’ process, the building energy performance is the output (result), while in the

‘backward’ process the building energy performance requirement (i.e. a reduction of energy or

Start Source Design

tool Target End Yes

No

o

To alter

Start Target Initial

source Design

tool

Design

finalised End

Yes

No

o

Process optimisation


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carbon emissions) is the input (the target) and the outputs (results) are the proposed most cost-

effective building components and PV-battery systems needed to meet the target.

1.2 Module of PV-battery system for off-grid housing

To investigate the impact of PV-battery system on peak demand and energy consumption, a

rooftop PV-battery model was developed and implemented into the AusZEH design tool [1]. The

model considered 11 different electricity tariffs and two solar feed-in tariffs, and the PV-battery

system was assumed to be connected to the grid. The flow of the PV-battery system was related

to the tariffs. For off-grid housing, the PV-battery system is not connected to the grid and the

logic flow of the PV-battery system’s charging and discharging algorithm is shown in Figure 3.

To achieve no electricity shortage throughout a whole year at any time step (one-hour time-step

used in this study), the size of the PV-battery system is determined by the hourly energy demand

of the household, which can be predicted by the model, based on the Typical Meteorological Year

(TMY) weather data. The output power (W) from a PV array can be estimated as [20]

𝑊 = 𝑓 × 𝑌 ×𝐼𝑔

𝐼𝑠 (1)

where f is the PV derating factor (around 0.75 to 0.8 for modern PV panels [20]), Y 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 1kW/m2 irradiance), Ig is the global solar radiation incidence on the surface

of the PV array, Is the standard amount of radiation used to rate the capacity of the PV array (1

kW/m2).

With hourly weather data, AccuRate contains 69 weather files linked to 69 climate zones across

Australia. The weather files include global solar irradiance on horizontal plane (W/m2), diffuse solar

irradiance on horizontal plane (W/m2) and direct solar irradiance on a plane normal to the beam

(W/m2). With these data, Ig can be calculated using the HDKR model [21].


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Figure 3. Logic flow of PV-battery system charging and discharging for off-grid house

2. CASE STUDY

2.1 Descriptions of the residential buildings

As mentioned above, around 70% of Australian dwellings are detached. In this study, a detached

house with 4 bedrooms is chosen (Figure 4), which is one of the eight sample houses previously used

for energy rating accreditation by NatHERS. It has a gross area of 160m2 that is equivalent to the

typical average floor area of national detached houses built before 1990 [22]. The house is a single

storey house with concrete slab-on-ground floor, colourbond external wall (steel cladding on 90mm

stud), colourbond roof and aluminium single-glazed widows. The roof size is around 230m2.

In this study, the house floor plan is kept constant while various changes of insulation, window type,

air leaking gaps and cracks, etc. are made to achieve 6 stars in three typical climate zones in Australia

(Melbourne – heating dominated, Townsville – cooling dominated and Sydney – balanced heating

and cooling). The 6-star houses are selected to represent new housing stock that satisfies current

energy standards [22]. The houses are assumed to be occupied by a couple with two school children

(i.e. unoccupied during school hours). As natural gas (or LPG gas) can be available to homes in these

three cities, natural gas or other non-electric appliances are assuemed to be used for four main end-

uses (space heating, water heating, cooktops and oven).

The following case studies are conducted for two scenarios of building thermal performance: (a)

under current conditions and (b) building retrofited to 6 stars.

Off-grid house with solar PV-battery system

Do the solar

panels provide

enough power

to the house?

No Does the battery

have power?

DC/AC Inverter

Yes

Get power from

Diesel generator

No

Yes Is the battery fully

charged

No

Charge the battery

To heat water

for later use

Yes


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Figure 4. The house floor plan

2.2 Options of solar PV-battery systems for existing houses to stand off the grid

Under baseline conditions of the houses (with aluminium single-glazed windows and no

insulation to the building construction), the houses are predicted to be 0.6 star, 0.5 star and 3.7

star in Melbourne, Sydney and Townsville, respectively. Based on TMY weather data, the

predicted annual electricity consumption and electricity generation of 1kW solar PV are shown

in Figures 5 and 6 respectively. It can be seen that electricity for space cooling accounts for less

than 10% of the total electricity consumption in Melbourne and Sydney, while it is over one third

in Townsville. In Melbourne, in winter the solar PV generates less than half the electricity as it

does in summer. In Sydney the electricity generated in winter is less than two thirds of the

summer amount. In Townsville, there are no significant differences between the seasonal

generation. So for off-grid housing, winter will be critical for Melbourne and Sydney, and in

Townsville summer will be.

5 m


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Figure 5. Annual electricity consumption of the households in Melbourne, Sydney and Townsville

Figure 6. Annual electricity generated from 1kW PV (facing north and tilt angle of 10°) in

Melbourne, Sydney and Townsville

Based on the hourly data of the predicted electricity consumption of a household and electricity

generation from its rooftop solar PV, the sizes of the battery for the household to operate

completely off the grid can be determined using the model, with the target to achieve no electricity

shortage through a whole year. Table 1 shows the predicted results with battery cycle limited to

70 % depth of discharge (to preserve battery life). It can be seen that the sizes of battery required

drop significantly with increase in the size of PV up to 10kW. After 10kW in Melbourne and

Sydney, and 15kW in Townsville, the installation of larger PV makes only a minor difference to

the required battery size. With installation of 10kW PV that requires a roof size of around 80m2,

the sizes of battery for off-grid housing are 31kWh, 18kWh and 30kWh in Melbourne, Sydney

and Townsville respectively.

The indicative prices for the installation of solar PV and battery are shown in Table 2 (note that

the prices do not include the regulator/management system and invertor). Table 3 lists the costs

of the potentially most cost-effective options for solar PV-battery system for off-grid housing in

0

1000

2000

3000

4000

5000

Melbourne Sydney Townsville

Ele

ctri

city

consu

mpti

on (

kW

h)

Location

Appliance Lighting Cooling

-200

300

800

1300

1800


Ele

ctri

city

gen

erat

ion (

kW

h)

Location

spring summer autumn winter


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the three cities. Among the three cities, Sydney is the cheapest city for the household to get off

the grid and Melbourne is the most expensive one.

Item City Solar PV size (kW)

5 10 15 20 25

Battery

size

(kWh)

Melbourne 149 31 30 29 29

Sydney 39 18 17 16 15

Townsville 209 30 22 21 20

Table 1. Combinations of PV plus battery storage for off-grid housing in the three cities

Item City Solar PV size (kW) Battery size (kWh)

5 10 10 20

Installation

price ($)

Melbourne 6,435 13,433 20,000 36,650

Sydney 6,097 13,061 20,000 36,650

Townsville 5,153 13,280 20,000 36,650

Table 2. Indicative average costs of installation of solar PV in the three cities (adopted from

Solar Choice, http://www.solarchoice.net.au, accessed 07/06/2017)

City Solar PV

size (kW)

Battery size

(kWh)

Cost of installing PV-battery

system ($)

Melbourne 10 31 71,883

Sydney 10 18 47,061

Townsville 10 30 68,280

15 22 59,430

Table 3. Indicated costs of installing solar PV-battery system for off-grid housing in the three

cities

2.3 Options of solar PV-battery systems for 6-stars houses to stand off the grid

The model was run to evaluate the most cost-effective options for improving building energy

performance. To retrofit the house to be 6 stars in Melbourne, the options and the related

indicative costs are listed in Table 4. To achieve 6 stars in Sydney, the only difference to the

Melbourne case, is that the external wall insulation is required to be R1.5. To retrofit the house to

be 6 stars in Townsville, with no changes to windows and external wall of the existing house, the

options are to insulate the ceiling with R1.0 batts and seal exhaust fans, and shade the windows

on the outdoors with shade cloth.

http://www.solarchoice.net.au/


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Item Indicative prices

($)

Sealing exhaust fans and winows and doors 560

Ceiling isulation with R6.0 batts 1144

Window outdoor covering with shade cloth 2030

External wall insulation with R3.0 batts 6798

Windows replaced with Timber double-glazed argon low-e windows 6807

Total 17,339

Table 4. Options of retrofitting the house to be 6 stars in Melbourne and the indicative prices

With the building energy performance improvement, the energy consumption for space cooling

is reduced, which is shown in Figure 7. It can be seen that there is a significant reduction for space

cooling energy in Melbourne. In Sydney the cooling energy is reduced more than half.

Figure 7. Annual electricity reductions of space cooling from existing houses to 6-star houses

in the three cities

With the improvement of building energy performance, the sizes of PV-battery systems for off-

grid housing in the three cities are shown in Table 5. There is a significant reduction in the battery

sizes required in Melbourne due to the decrease in cooling energy requirement (Figure 7). Table

6 presents the indicative costs of the potential cost-effective options of PV-battery system for 6-

star housing to stand completely off the grid in the three cities. With the house retrofitted to 6

stars, the cost of solar PV-battery system for the off-grid household drops significantly in

Melboure (from around $72,000 to below $50,000), while it has minor impact on the costs of

Sydney and Townsville.

Item City Solar PV size (kW)

5 10 15 20 25

Battery

size

(kWh)

Melbourne 17 9 8 8 8

Sydney 39 10 9 9 9

Townsville 145 29 19 18 17

Table 5 Combinations of solar PV-battery systems for the 6-star houses to stand off the grid in

the three cities

281.8166.1

1580.7

13.582.7

1518.4

0

500

1000

1500


Ele

ctri

city

consu

mpti

on

(kW

h/a

nnual

)

Location

Current 6-star


10

City PV size

(kW)

Battery size

(kWh)

Cost of PV -

battery ($)

Cost of

house

retrofilt ($)

Total

cost

($)

Melbourne 5 17 37,435 17,399 54,834

10 9 31,933 49,332

Sydney 5 39 76,097 16,839 92,936

10 10 33,061 49,900

Townsville 10 29 67,680 3,134 70,814

15 19 57,630 60,448

Table 6. Indicative costs of PV-battery system for the 6-star houses to stand off the grid in the

three cities

If the measures applied in Melbourne (Table 4) are used to retrofit the houses in the other two

cities, the houses are upgraded to be 6.9 stars and 7.9 stars in Sydney and Townsville, respectively.

With these upgrades, there are no changes in the sizes of PV-battery system needed for off-grid

households in Sydney (i.e. the same as shown in Table 5). The changes in Townsville are shown

in Table 7. It can be seen the total costs for off-grid households in Townsville changes slightly

and the sizes of PV-battery system required decreases due to reduced cooling energy requirement

with the improvement from 6 stars to 7.9 stars.

PV size (kW) Battery size (kWh) Cost of PV-battery (A$) Total cost (A$)

5 53 96,803 114,202

10 23 59,345 76,744

15 14 49,295 66,694

Table 7. The sizes and costs of PV-battery system for the 7.9 star houses to stand off the grid in

Townsville

CONCLUSION

This study presents an integrated approach to evaluate the most cost-effective options for off-grid

housing in Australia. Within the approach, a ‘normal’ forward design process, starting from

source (inputs) to the target (outputs), was adopted for new building design, while a ‘backward’

design process, starting from the target to the outputs (building components and associated PV-

battery), was adopted for assessing the most cost-effective retrofit options. With non-electric

resource (such as natural gas) for the main four end-uses (space heating, water heating, cooktop

and oven), the case study using the model with ‘backward’ process shows that for a couple with

two school children off-grid living in old houses in Melbourne, Sydney and Townsville, the most

cost-effective options are to install 10kW solar PV with 18kWh battery at a cost of around $47,000

in Sydney, and install 15kW solar PV with 22kWh battery at a cost of $59,400 in Townsville. The

most cost-effective options in Melbourne are to retrofit the house to be 6 stars first and then install

10kW solar PV with 9kWh battery at a cost of $49,300.

For practical applications, further study is required to evaluate more house types, climate zones,

and household types, their occupant behaviour impacts on the energy consumption and payback

period etc. Considering off-grid living with PV-battery systems comes at quite a high cost (above

$47,000 in the three cities), efforts should be made to investigate an alternative of smaller PV-

battery system plus a diesel generator to meet electricity demand when the battery is discharged.


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REFERENCES

1. Ren, Z., Grozev, G. and Higgins, A.; “Modelling Impact of PV Battery Systems on Energy

Consumption and Bill Savings of Australian Houses under Alterative Tariff Structure”,

Reneable Energy, Vol. 89, pp 317-330 (2016).

2. Murray, G. and Dale, H.; 2015; “Composition of Australia’s housing”, HIA Economics

Research Note. Available from https://hia.com.au (accessed on 28/05/2017).

3. Khalilpour, R. and Vassallo, A.; “Leaving the Grid: An Ambition or a Real Choice”, Energy

Policy, Vol. 82, pp 207-221 (2015).

4. Gordon, J.; “Optimal Sizing of Stand-Alone Photovoltaic Solar Power Systems”, Solar Cells,

Vol. 20, pp295-313 (1987).

5. Egido, M. and Lorenzo, E.; “The Sizing of Stand Alone PV Systems: A Review and a New

Porposed Method”, Solar Energy Materials and Solar Cells, Vol. 26, pp51-69 (1992).

6. Fragaki, A. and Markvart, T.; “Stand-Alone PV System Design: Results Using a New Sizing

Approach”, Renewable Energy, Vol. 33, pp162-167 (2008).

7. Lorenzo, E. and Navarte, L.; “On the Usefulness of Stand-Alone PV Sizing Methods”,

Progress in Photovoltaics, Vol.8, pp391-401 (2000).

8. Peippo, K. and Lund, P.; “Optimal Sizing of Grid-Connected PV-Systems for Different

Climates and Array Orientations: a Simulation Study”, Solar Energy Materials and Solar Cells,

Vol. 35, pp445-451 (1994).

9. Mellit, A., Kalogirou, S., Hontoria, L. and Shaari, S.; “Artifical Intelligence Techniques for

Sizing Photovoltaic Systems: A Review”, Renewable and Sustainable Energy Reviews, Vol. 13,

pp406-419 (2008).

10. Castillo-Cagigal, M., Caamaño-Martín, E., Matallanas, E., Masa-Bote, D., Gutiérrez, A.,

Monasterio-Huelin, F. and Jiménez-Leube, J.; “PV Self-Consumption Optimization with

Storage and Active DSM for the Residential Sector”, Solar Energy, Vol. 85, pp2338-2348

(2011).

11. Colmenar-Santos, A., Campíἢez-Romero, S., Pérez-Molina, C. and Castro-Gil, M.;

“Profitability Analysis of Grid-Connected Photovoltaic Facilities for Household Electricity Self-

Sufficiency”, Energy Policy, Vol. 51, pp749-764 (2012).

12. Zeh, A. and Witzmann, R.; “Operational Strategies for Battery Storage Systems in Low-

Voltage Distribution Grids to Limit the Feed-in Power of Roof-Mounted Solar Power Systems,

Energy Procedia, Vol. 46, pp114-123 (2014).

13. Ren, Z., Foliente, G., Chan, W., Chen, D. and Syme, M.; “AusZEH Design: Software for

Low-Emission and Zero-Emission House Design in Australia”, Paper presented at the 12th

Conference of International Building Performance Simulation Association, Sydney, 14-16

November, 2011.

14. Ren, Z., Foliente, G., Chan, W., Chen, D., Ambrose, M. and Paevere, P.; “A Model for

Predicting Household End-Use Energy Consumption and Greenhouse Gas Emissions in

Australia”, International Journal of Sustainable Building Technology and Urban Development,

Vol. 4, pp 210-228 (2013).

https://hia.com.au/


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15. Ren, Z., Chan, W., Wang, X., Anticev, J., Cook, S. and Chen, D.; “An Integrated Approach

to Modelling end-use Energy and Water Consumption of Australian Households”, Sustainable

Cities and Society, Vol. 26, pp344-353 (2016).

16. Delsante, A.; “Is the New Generation Building Energy Rating Software up to the Task? – A

Review of AccuRate”, Paper presented at ABCB Conference ‘Building Australia’s Future

2005’, Surfers Paradise, 11-15 September, 2005.

17. Ren, Z. and Chen, D.; “Enhanced Air Flow Modelling for AccuRate – A Nationwide House

Energy Rating Tool in Australia”, Building and Environment, Vol. 45, pp1276–1286 (2010).

18. NatHERS; “Nationwide House Energy Rating Scheme (NatHERS)- Software Accreditation

Protocol”, Available from http://www.nathers.goc.au (accessed on 29/05/2017).

19. Delsante, A.; “A Validation of the AccuRate Simulation Engine Using BESTEST”, Report for

Australia greenhouse Off, CMIT-2004-152.

20. Ren, Z., Paevere, P., Grozev, G., Egan, S., Anticev, J.; “Assessment of end-use electricity

consumption and peak demand by Townsville housing stock”, Energy Policy, Vol. 61, pp 888-893

(2013).

21. Duffie, J. and Beckman, W.; Solar Engineering of Thermal Processes, 2nd ed., John Wiley &

Sons, New York (2004).

22. Ren, Z., Chen, D. and Wang, X.; “Climate change adaptation pathways for Australian

residential buildings”, Building and Environment, Vol.46, pp2398-2412 (2011).

http://www.nathers.goc.au/

a design tool for off-grid housing in australia...for off-grid housing, the pv-battery system is not...

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