By Geoff James
Residential customers wanting to manage their power are likely to be the fastest-growing market for energy storage, and especially grid-connected batteries. These customers have a big appetite for storage and understanding why will be important for developing the right storage products, services, and owner/user experiences.
Backup power seems an obvious motivation for customers. After all, the largest battery installations in the world exist to supply emergency power to secure facilities and data centres. Residential owners of energy storage will also expect their lights to stay on when the rest of their street is blacked out. But mass-market storage systems will have different capabilities to large specialised installations, and the energy storage industry should set realistic expectations so that early adopters of residential storage are not disappointed.
Grid safety and energy storage
Safety is everyone’s business and is the main concern of distribution grid operators. Grid connection standards vary significantly between countries and states, but one universal requirement is that when there is a problem with the grid, or when part of the grid is de-energised for maintenance, customer energy sources including batteries must be disconnected from it. This protects line workers from being electrocuted by wires they thought were inactive, and prevents fallen wires remaining live and endangering the public after storms and accidents.
The technology that implements this safety requirement is called anti-islanding protection because it prevents “islands” of power remaining when power is no longer being delivered by the grid. It must be implemented by all power conversion systems approved for mass-market sale and grid connection. This includes battery systems as well as solar PV generation. A combination of voltage, frequency, and impedance presented by the grid is typically used to detect when grid power is no longer present.
How hard can this be?
Well, it becomes problematic when the penetration of local generation is so high that it can supply the total load in a local grid region, such as a single distribution feeder. This is a real prospect in some present-day grids – this will be discussed in a later article.
Limits of battery capacity
In most situations, at some additional expense, a battery system can incorporate switching so that the residence can be safely re-energised after the grid power is interrupted. This means backup power is provided, only not continuously, and perhaps to a limited number of circuits.
This brings up another difficulty with backup power—supplying the normal household load at times of peak usage can drain a battery quickly. Even a large residential-scale battery of 10 kWh capacity would last only about 2-3 hours during the evening peak period of many all-electric houses. This assumes the battery is fully charged at the time of grid interruption, and that it can be fully discharged, which is not desirable for most technologies. The power conversion system may also not have the kW capacity to supply peak load.
The conservative approach to backup power is to energise a selection of critical circuits so that the battery capacity can support a longer period without grid power. Many customers would be delighted if their lights, internet, entertainment, and main kitchen appliances continued to operate during a blackout. This would require modest additional wiring at the switchboard. At present, there is no universal approach to battery connection for residential customers, and standards and accreditation bodies are engaged in the task of documenting recommended wiring systems and their benefits.
Recognising multiple benefits of storage
Backup power is only one of several key motivating factors for residential energy storage. Equally important, home battery storage allows solar energy to be valued at the retail price of grid-delivered energy.
It is by riding the huge wave of rooftop solar PV generation that residential energy storage is showing signs of a coming surge. Customers still need the grid because solar PV panels are a variable energy source. Their output is predictably zero at night, and during the day solar panels produce less than their rated output when the sun is low in the sky, obscured by cloud, eclipsed or shadowed, or if its rays are not normally incident on the panels.
When PV panels are producing less than household demand, a customer has to buy energy at retail prices from the grid. When they are producing more, the surplus is delivered into the grid and usually earns much less than the retail price. Even if it presently earns more due to generous government incentive schemes, such as feed-in tariffs, many customers are rolling off these in the next few years.
With batteries to store the surplus, customers can use most or all of the energy they produce, effectively valuing solar energy at the retail price that would otherwise apply to their energy consumption. This creates a strong incentive for solar customers to buy energy storage.
Next week, we look at how customer energy storage can help towards a greener grid and earn revenue at the same time. The Energy Storage World Forum in Rome next month will include a Residential Energy Storage Forum where experts will address these issues in depth.
Dr Geoff James writes regularly for Dufresne Research and producers of the annual Energy Storage World Forum (Europe and Australia) where more than 30 Utilities/TSO/DSOs gather each year. He is a consultant in clean energy with 22 years of experience with CSIRO.
If you want to know more about this and other topics directly from end users of energy storage technologies join us at one of these annual events: The Energy Storage World Forum (Grid Scale Applications), or The Residential Energy Storage Forum, or one of our Training Courses.