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Storage not as green as first impressions, says solar study

A report landed in our inbox this week that caused rather a lot of consternation, as it claimed that adding storage to a solar-equipped home would raise its carbon emissions. We spent a lot of time unpacking it and trying to wrap our heads around the mathematics involved, in which the Cockrell School of Engineering’s findings suggest that environmental advocates should be piling the pressure on utility generation portfolios, rather than advocating for home-based battery storage.

When we first read the report, it didn’t make sense – prompting a whole heap of questions fired off to its authors Robert Fares and Michael Webber. At first glance, it doesn’t look logical – that adding storage to a home would increase the amount of electricity you have to send to the home, and therefore its emissions. After all, storage is the carbon-friendly future, right?

Admitting that the discussion of the two models in the paper was a bit confusing, Fares clarified a number of points we had enquired about. The model was based on usage and production data pulled from 99 homes in Texas, and represents homes that consume more electricity than they generate – which is the case for almost all solar-equipped homes.

Two types of storage behaviors were set; “target zero,” which tries to reduce the power draw from the grid to zero at all times, by discharging the stored solar panel production – and never topping the batteries up with power from the grid. So in this model, the solar production never leaves the home, instead of being sent back to the utility – acting as a green source of electricity that the utility can distribute.

Without that solar, the utility’s energy mix becomes more carbon-heavy – meaning that the electricity it ends up sending to one of these ‘target zero’ homes has a higher carbon footprint. As Fares said, “energy from the grid is drawn to make up for that loss, and with the current fossil grid mix, emissions increase too.”

That dynamic is why the study didn’t make sense to us when we first met it, and the problem of higher emissions would be reduced by a greener energy mix from the utility. So if emissions are the top priority, that’s where the effort should be spent – convincing the utility to diversify and invest in renewables, rather than installing batteries.

At some point, once utilities reach very high renewable penetration, the inefficiency argument could also be brought to bear on storage, with Fares noting that the model shows that solar is a cleaner option than solar plus storage – and can improve as the utility energy mix gets cleaner. He adds that solar with storage is still a cleaner option than no solar at all, and that storage will help with the curtailment of renewable energy sources, removing the current dynamic when their output has to be slashed in order to not overload the grid – although we’re still a while away from that scenario being commonplace.

The second storage behavior in the model was “minimize power,” which can store power from the grid but aims to minimize that draw as much as possible by discharging from the battery – which the report says is the primary value proposition offered to residential customers, based on solar self-sufficiency, utility independence, and less sensitivity to grid outages.

The gist of the model is that the inherent inefficiency of battery storage means that if you fill batteries with electricity from a carbon-heavy source, you need to supply more of that electricity to the home to account for that wasted in the storage process – converting it from AC to DC, and then back to AC for use in the home. Currently, storage wastes between 8-14% of all the electricity put into it.

But the crux of the problem is that the home is dependent on the utility’s choice of electricity generation – meaning that if the utilities were a more environmentally friendly source of electricity, i.e. renewables, then the inefficiency of storage emissions wouldn’t be an issue, as they would be coming from a carbon-neutral source (after the renewables are installed, that is, as the hardware and installation have an inherent carbon footprint to consider).

As Webber put it, “the research demonstrates batteries aren’t inherently a panacea to the grid’s challenges. They offer some useful performance benefits, but are only one part of a suite of solutions that are needed. Plugging a high-tech battery into a low-tech (dirty) grid isn’t necessarily a good thing; a better option would be to plug in a high-tech battery to a high-tech (cleaner) grid). Now that would be something.”

So, the model suggests that storage is ahead of its time, somewhat. Fares notes that utilities are more likely to favor grid-scale storage, as they can more directly operate it to meet their particular objective – such as reducing a peak demand or offsetting a transmission or distribution investment.

He added that “the advantage of customer storage, from the utility perspective, is that the customer might cover a significant portion of the cost, and allow the utility to control the battery during critical peak-demand periods in exchange for a small subsidy (i.e. demand-response). ‘Renting’ customer batteries in this way might end up more cost-effective for the utility.”

Fares closed by saying that cleaning up the electric grid is a huge priority, and that if the target is to lower emissions, then large-scale wind and solar are generally more cost effective than rooftop solar. Rounding out his explanation, with great patience given the number of questions we fired off, Fares concluded that in today’s utility dynamic, if you don’t need storage, you shouldn’t add it for environmental purposes.

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