Flexibility will be key to a large-scale rollout of solar power

The cost of solar power has declined by a factor of five in less than ten years, to an average of 6.8ct/kWh. This form of renewable energy is now cost-competitive and can be generated just about anywhere – and it’s poised to play a pivotal role in the transition to a carbon-neutral society. However, one challenge still needs to be overcome: how to incorporate solar power effectively into power grids, especially low-power ones. Jordan Holweger, a PhD student at EPFL’s Photovoltaics and Thin Film Electronics Laboratory, looked at how power utilities can promote flexibility in solar-power systems so that such systems can be rolled out on a large scale in low-voltage grids.

The main problem with incorporating locally generated solar power into power grids is the intermittent nature of this form of energy. On an average fall day, solar irradiance can jump by a factor of five as soon as the sun comes out, leading some solar-panel-equipped homes, which until then had been consuming energy, to suddenly start injecting it into the grid. These swings, nearly impossible to predict, can lead to power overloads, push grid voltage above their limits and provoke other grid disturbances. To help power utilities prevent these risks, Holweger looked into how flexibility could be used to balance the amount of solar power generated by a given system with the amount it consumes. He only considered distributed generation systems – as opposed to large photovoltaic plants – whose power is injected mainly into low-voltage grids.

Two possible levers

The first lever Holweger identified for improving system flexibility and mitigating the impact on power grids relates to consumer behavior. That is, how can financial incentives be used to encourage consumers to change their habits? Holweger conducted an experiment with around 600 households in the Bernese Jura, where they were offered a lower electricity rate between 11am and 3pm or based on weather forecasts. None of the homes in the experiment were equipped with solar panels. “The response was fairly muted,” says Holweger. “Households tended to increase their electricity use when the rate was lower but didn’t really reduce their use when the rate was higher. That’s probably because electricity prices in Switzerland are too low for small changes in them to influence consumer behavior. The savings are on the order of just a few francs.”

Jordan Holweger defended his thesis last July. ©Alain Herzog/EPFL

He also measured how consumers alter their habits after they install solar panels. Here, the response was more marked with a significant shift in behavior, most likely triggered by the combination of cost savings and a sense of moral imperative. These households have a strong incentive to consume the solar energy they produce in order to cut their electricity bill. “But this lever isn’t strong enough to really affect power grids,” says Holweger.

The second lever Holweger identified relates to technical measures like batteries, heat pumps, electric heaters and systems for curtailing solar-power generation. He found that here too, electricity rates are an important factor. Only rates that offer a real incentive will prompt consumers to purchase a battery, for example. “To achieve genuine flexibility, utilities need to introduce time-of-use rates and install smart meters in order to make solar panels an attractive investment,” he says. One approach utilities could use, if they want to avoid the cost of increasing grid capacity, is to take advantage of flexibility in the various components of solar-power systems. For instance, if overproduction becomes a problem, they could charge batteries or switch off the solar panels’ inverters so they stop supplying energy.

Will all that be enough? “Yes, for a long time,” replies Holweger. “But once the solar-power penetration rate reaches 70%, utilities will need to upgrade their low-voltage grids. This won’t be an issue in cities where solar power capacity is limited. But in the countryside, rooftops can be covered with solar panels. By using the technical flexibility measures we outlined, utilities can accommodate a penetration rate twice that high before having to invest in upgrades. But that means they’ll have to get consumers’ consent to remotely control their solar-power installations – which raises the issue of whether consumers would go for that.”