Batteries may be expensive but modelling shows how residential storage can be a powerful force in steadying a grid in transition.

Depending who you ask, solar is our salvation or on the verge of inflicting terrible damage on our energy system. The problem is that it generates in an inverted U shape, peaking at midday. Homeowners who have installed PV systems to cut their bills may not appreciate that often they are way too large for their daytime needs. All that surplus energy is simply sent to the grid. It then becomes a problem for networks to deal with.

It’s too late to change things because more than 2.5 million systems have been installed around the country – with more being rigged up every day.

“If you want your government to lose the next election, just take away rooftop solar,” says Paul McCormick, emeritus professor in the mechanical engineering faculty at the University of Western Australia. 

“There is so much solar on the roofs that it out-produces the conventional generation [in parts of the country]. The problem is [solar] goes to zero very quickly at 4 o’clock. You cannot start up a coal-fired power station that quickly.”

A solution to levelling out residential load, McCormick says, is to rollout batteries.

McCormick has modelled the effect of rapid and widespread adoption of residential batteries in Western Australia and found an opportunity exists to effectively make the state’s 360,000-odd solar households disappear. Currently, they export about 1.1GW at noon, he says, but the addition of a 20kW battery would have the effect of making the cohort of PV owners totally self-sufficient.

Level out load

By adding a 10kWh battery to every home PV system in his state, McCormick’s modelling shows exports would be reduced by more than 60%, from 17.5kWh to 7.5kWh. Self-consumption levels would rise from 38% to 82%.

On clear days in the state, rooftop solar supplies about 1,100MW power at midday – equal to about 45% of peak demand. With 10kWh of storage in each solar household, exported solar would fall from 57.2GWh to 7.5GWh, an 87% decrease.

That energy would be used after 3pm, during the evening peak. The daily ramp up needed to meet afternoon load would be reduced from about 1,050MW to 460MW, a 56% reduction.

A 20kW battery sounds extreme, when 10kW is about standard for the minority of new systems that include a battery, but it illustrates the potential for storage as prices fall. Plenty of people will tell you a battery isn’t worth it, with payback periods about equal to 10-year warranties, but McCormick sees it differently. “The economics depend on how much you have to pay for your energy and how much the government is willing to pay you for your energy. In Western Australia now it’s quite good. I think that economics argument is falling away.”

Besides, if networks are buying batteries themselves, isn’t that a good indication that the investment makes sense? McCormick says there is very little price difference between big and small batteries. With residential batteries, however, there is no need for additional network infrastructure and maintenance.

OK, let’s say batteries are installed at such a scale that daytime demand – the belly of the duck – rises back to a pre-solar level. What would be the effect on tariffs? “I would think the tariffs would go in favour of all that because it just released the grid of a lot of work and energy and it would slow down the depreciation of the assets of the grid because you’re not putting that much current through it anymore,” McCormick says.

Will there be a point when batteries become compulsory in home PV systems? “I think that would be a great idea … and they could make it economically viable so no-one could complain.”

Out of phase

McCormick’s modelling shows only 38% of available solar from a 4kW system is used in the home on a clear day, when rooftop solar is the single largest generator in Western Australia. “Household energy supply and demand are out of phase,” he says. “That’s the real problem with renewables now.”

In the days before solar the power companies had a very regular load – the industry load – so they could predict demand. Today, the load probably hasn’t changed that much but the supply of energy has changed drastically. “You could say the big mistake was to allow people to put rooftop solar on in the first place,” McCormick tells EcoGeneration.

Many reports on the use of residential batteries and their effect on networks points out that they often are fully charged before noon, so that peak exports are no lower in areas where solar systems include storage. But there is a simple solution, says McCormick: owners can limit the current going into a battery. “Instead of allowing the battery to be charged as rapidly as the size of the array, you can simply put a throttle on it,” he says, “so it’s still charging when you need it to be charged, towards the end of the afternoon.”

By lowering the current into the battery it would not take the full power in the morning. “It shifts excess current you might be generating in the morning to the grid, and the grid doesn’t have much problem in the morning taking that, because the radiation levels are lower.”

The battery will then be charging away at 3 or 4 o’clock, which will help lessen the afternoon ramp.

Further modelling will look at the difference between summer and winter. “Winter is a different problem because the batteries are sitting around relatively unused because radiation levels are lower.” 

Forward, not backward

Household solar is a giant in the Australian energy system and tempering it with storage would avoid regressive strategies such as limiting and controlling rooftop and utility-scale PV generation.

Utility-scale batteries such as Neoen’s Hornsdale Power Reserve aren’t doing much to address the issue of drooping demand. “Large batteries are there just to patch up the grid; very little of the battery’s capacity is used for anything useful. They just sit there 50% charged – day-in, day-out – in case they’re needed. My view of batteries is they are for storing energy – energy to be used for something useful,” says McCormick, who specialises in modelling of energy systems at UWA.

Community-scale batteries, such as those deployed by Western Power, are treating the symptom and not the cause, he says. “I don’t see anything that would favour the usage of a community battery over a household battery. With the community battery you then have to rewire everything. The nice thing about household batteries is there is no infrastructure cost. What you buy is what you get – and the price generally includes installation. That has a great effect on the grid in that it takes away all the high currents and low currents that they’re worried about.

“Since the ‘duck’ problem originates on the rooftops of individual households, it makes sense to solve the problem there, behind the meter.”