Grid-connected PV with batteries, stand-alone power systems and micro-grids

By Susan Neill, engineering director at Global Sustainable Energy Solutions (GSES)

The prospect of being able to disconnect from the electricity grid and avoid ever-increasing electricity bills is attractive to many people.

However, having your own stand-alone solar (or wind) system with battery storage has been a reality for many years: for some consumers this has been their only choice for a power supply, and for others it has been cheaper than a rural grid extension or diesel generator.

When considering the possibility of customers installing stand-alone power systems (SAPS) with the intention of leaving the grid, it is important to realise the two most important aspects of an electricity system: the cost of energy and the reliability of supply. Being able to provide cheap energy from a PV and battery system is relatively easy, however ensuring a reliable supply is challenging, and much more expensive.

When considering a SAPS, or a grid connected PV and battery system, or a mini-grid, one must consider what is the cost of ensuring reliability and how does this compare to the cost of a grid connection?

Because SAPS generate electricity at the point where it is consumed, there are no associated network charges. However, to achieve the reliability demanded by consumers, SAPS systems need to be installed with enough capacity to provide energy during extended periods of low solar radiation and to support any future increased energy use. These factors result in increased capital costs.

This is not the case with a grid-connected solar system with batteries. Having grid connection available means consumers can draw additional energy as required, for example, during times of reduced radiation or a period of increased consumption. The economic viability of a SAPS will therefore be restricted to customers having high network costs, such as those in rural and remote areas.

For customers living in cities and surrounding suburbs, access to the grid provides reliability at a much lower cost.

To design a stand-alone power (PV) system there are many things that must be taken into consideration in order to ensure the system is reliable and meets the needs of the customer, while at the same time minimising capital expenditure. The system, including the solar array, must be capable of delivering power all year round, including any extended periods of bad weather. This requirement greatly increases the size (and therefore cost) of the battery bank, as it must contain enough energy for all consumption during this period.

The design of a SAPS usually includes a generator to provide a regular charge to the battery bank and act as the backup power source. SAPS (including generators) are complex to design and expensive to provide, even with the recent fall in PV and battery prices.

When comparing a SAPS to a grid connected solar system with battery storage, the battery bank for a grid-connected solar system can be sized to meet either a fraction or all of the daily power consumption. The PV array may be sized according to the available roof space, daily consumption, and the average (opposed to minimum) solar radiation.

There will regularly be times when the solar-battery system is not sufficient for the customers’ needs, and additional energy can be drawn from the grid. By sizing the battery bank to only meet, for example, the demand during peak tariffs, allows the system’s performance to compete with the most expensive rate of grid electricity. The fact that these systems are grid connected provides the reliability of supply which customers demand, while reducing the cost of energy by using competitive distributed generation and storage.

However, there will be situations when it is economical to disconnect from the grid. Consider the population density of rural areas compared to the capital cities and their suburbs. Energex and AusGrid (the network providers in Brisbane and Sydney) have 24.4 and 32.5 customers per kilometre of distribution line, compared to Ergon and Essential Energy (operating in rural Queensland and NSW) with densities of 4.3 and 6.8 customers per kilometre of line respectively.

For isolated dwellings in these remote Ergon and Essential network areas, the cheapest energy system would most likely be a SAPS to provide an adequate power supply. The capital and maintenance costs that are required for an extension of the distribution network would exceed the lifetime costs of a SAPS, despite the increased capital costs compared to a grid connected solar and battery system.

Another option available for large sections of the community is to operate from mini-grids. A mini-grid is a viable solution for areas when the population density is high enough to warrant a distribution network, but the area is physically remote from centralised generators. A mini-grid will be acceptable only if it can provide an electricity service that is cheaper than operating and maintaining a much larger and expansive grid, and equally as reliable.

For a micro-grid, the actual generation costs will be higher, however, savings will be made from reducing transmission requirements and associated maintenance costs. For cities and surrounding suburbs that have a much greater electricity demand and high population density, a substantial grid with transmission supply from large generators (i.e. the current grid) will continue to offer a low-cost and highly reliable solution to electricity supply .

As power technologies develop and become more efficient, it is important to be able to adapt the electricity supply system in order to ensure a low-cost and reliable supply of energy to customers. Electricity networks around Australia are now considering their delivery models and associated costs. When considering the impacts distributed storage will have on the electricity system, it is important to realise the cost of reliability and the effect this has on economic models.