December 12, 2018
Background
With the observed rapid decline in PV prices and a similar trend for battery storage in recent years, there is a growing interest in off-grid housing in Australia. A study conducted by CSIRO [Future Grid Forum Participants, 2013] projected that by 2050, around one third of Australian households could leave the grid under some potential future scenarios. However, a recent study [Khalilpour and Vassallo, 2015] shows that from an economic perspective, widespread disconnection of housing from the grid is less likely. This study aims to investigate and compare grid-connected net-zero energy housing vs off-grid housing across Australia in terms of payback period, under both current and future climate with 2°C global warming.
Methodology
To achieve an off-grid house with no energy shortage through a whole year, at a time step of one hour, an integrated simulation tool was developed for off-grid housing design [Ren, Chan, Chen and Paevere, 2018]. The tool incorporates a PV battery module into the AusZEH design tool, which has been previously validated against the actual energy consumption data of over one hundred houses [Ren, Chen and James, 2018]. Considering the construction of a building envelop, local climate, occupant behaviour, and installed equipment and appliances, the tool can be used to predict hourly energy consumption throughout a whole year, including energy use for space heating/cooling, lighting, water heating and other appliances. Hourly electricity generation of a PV system is simulated by the HDKR model [Duffie and Beckman, 2004] considering local solar radiation and installed PV configurations. The TMY (Typical Meteorological Year) weather files, based on the weather data of the 1970s and 2000s, are assumed to be reference climates for building simulations for the current climates. The future hourly weather data are constructed using ‘morphing’ approach [Belcher, Hacker and Powell, 2010].
To investigate economic feasibility of grid-connected net zero energy houses vs off-grid housing in different climate zones across Australia (represented by major cities: Darwin, Alice Springs, Brisbane, Sydney, Mildura, Melbourne and Hobart), a single storey house with 6 star under current climate is used to represent new houses built since June 2011 [Ren, Paevere and McNamara, 2012]. To answer the key question for consumers on how long it will take to get their money back on upfront investment, the ‘payback period’ is a simple metric, whereby the initial capital is divided by the bill saving per year due to the investment of PV battery system, to determine the payback time. The capital investment required for net zero energy houses and off-grid houses can be estimated based on the predicted PV and battery sizes and information on the current market prices of PV and battery storage, which is based on the data from www.solarchoice.net.au. In this study, a median price of 1.3 A$/W for solar PV and the average price of 940 A$/kWh for battery plus inverter/charger are applied. The yearly bill savings due to the investment of PV panels and battery systems can be estimated based on the information on residential electricity prices published by Australian Energy Market Commission (www.aemc.gov.au), supply charges (www.wattever.com.au) in the current Australian market, and solar feed-in tariffs (www.solarmarket.com.au).
Results
The simulation was performed for a couple with two children assuming the house is occupied for the full day. To reduce electricity consumption for water heating, a vacuum tube solar hot water system with 4 m² collector area is assumed installed in the houses. Two options for off-grid housing are simulated: PV battery system only and PV battery system hybridized with an on-site petrol generator. The calculated results are shown in the table below.
| City | Current climate | Future climate | ||||
|---|---|---|---|---|---|---|
| Net zero energy | Off-grid with PV battery | Off-grid with PV battery hybridized with petrol generator | Net zero energy | Off-grid with PV battery | Off-grid with PV battery hybridized with petrol generator | |
| Darwin | 3 | 12.8 | 9.6 | 2.9 | 12.8 | 9.2 |
| Alice Springs | 3.5 | 20.7 | 10 | 3.2 | 13.9 | 10.9 |
| Brisbane | 5.9 | 18.2 | 8.6 | 5.5 | 13.9 | 10 |
| Sydney | 5.3 | 22.3 | 9.3 | 5 | 15.7 | 8.9 |
| Mildura | 5.1 | 22.5 | 10.2 | 5 | 19.2 | 8.4 |
| Melbourne | 6.8 | 21.7 | 13.2 | 6.7 | 20.1 | 10.4 |
| Hobart | 8.9 | 48.4 | 21 | 9 | 42.1 | 20.6 |
Results show that in all the seven cities chosen for the case study, large PV battery systems are required, and the payback periods are longer than 12 years for off-grid operation under current and future global warming climates, which is longer than the 10-year battery warranty offered by most providers. However, 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 may become economically feasible for the houses in warmer climates of Darwin, Alice Springs, Brisbane, Sydney and Mildura. However the payback periods are generally over 20 years in the colder climate of Hobart, and over 10 years for Melbourne. This implies that from an economic view, without significant reduction in costs of PV battery systems (more than 50%) off-grid independence with PV battery systems only is not a feasible option for households in Hobart and is marginal for Melbourne. The case study also demonstrates that the grid-connected net zero energy home is economically feasible for all the seven cities and more attractive than the off-grid house under both current and future global warming climates, even without subsidies for solar feed-in tariffs.
References
Belcher, S., Hacker, J.,& Powell, D.(2010). Constructing design weather data for future climates. Building Services Engineering Research and Technology, 26, 49-61.
Duffie, J., & Beckman, W. (2004). Solar engineering of thermal processes, (2nd ed.). New York: John Wiley & Sons.
Future Grid Forum Participants (2013). Change and choice: The future grid forum’s analysis of Australia’s potential electricity pathways to 2050. In Graham, P. (Ed.), CSIRO, Australia.
Khalilpour, R., & Vassallo, A. (2015). Leaving the grid: An ambition or a real choice?. Energy Policy, 82, 207-221.
Ren, Z. Chan, W., Chen, D., & Paevere, P. (2018). A design tool for off-grid housing design in Australia. Ecolibrium, 44-50.
Ren, Z., Chen, D., & James, M. (2018). Evaluation of a whole-house energy simulation tool against measured data. Energy and Buildings, 171,116-130.
Ren, Z., Paevere, P., & McNamara, C. (2012). A local-community-level, physically-based model of end-use energy consumption by Australian housing stock. Energy Policy, 49, 586-596.
