Sunshine and clouds

EARLIER this year, after months of planning, the managers of Europe’s electricity grid were ready for the biggest test so far of an energy system that increasingly relies on renewable power. That event was a partial solar eclipse on March 20th—and the “worst case scenario” was that it would take place on a sunny day, causing a sudden disruption of solar power, creating fluctuations that could damage the grid and anything connected to it.

At the centre of concern was Germany, with some of the world’s biggest solar installations. In 2014, almost 7% of Germany’s electricity was generated by solar panels, although on sunny weekdays the proportion can reach 35% and even 50% at weekends. The passing of the Moon between the Earth and the sun could have been equivalent to the loss of 80 medium-sized European power stations.

As it turned out, although the sun shone in southern Germany and northern Italy, cloudy skies elsewhere limited the eclipse’s impact. Even so, the European grid operators had to greatly increase the backup supplies normally on hand and turn off some energy-intensive processes, such as aluminium smelting. And then it had to cope with the surge in solar generation as the eclipse passed.

An eclipse is an extreme event, but as more homes, offices and factories fit solar panels and become, in effect, mini power plants feeding surplus electricity into the grid, engineers are having to perform a similar balancing act every day. Without careful monitoring, the intermittent nature of solar power—even with the sun passing briefly in and out of clouds—can lead to voltage surges and drops, causing brownouts and power cuts.

Moreover, maintaining a consistent level of power is now vital as modern digital devices, such as computers, televisions and equipment in factories, hospitals and offices, demand a steady voltage and frequency, unlike the clunky electronics of a few decades back.

To protect power quality some countries have placed restrictions on new solar installations. In Hawaii, for instance, sudden swings in electrical output have led to a slowdown in the issuing of solar permits. Australia has also placed constraints on new installations. Even some utilities in all-too-often overcast Britain have limited the amount of solar electricity which can be fed into the grid.

Companies are coming up with ways to maintain power quality. Some of the gear is based on inverters, which are used to convert the variable direct current produced by a solar panel into the alternating current on the grid. Renewable Energy Solutions Australia, for instance, makes a system called VoltLogic which can limit the amount of power exported to the grid from solar panels. This, the company says, allows new solar installations to be automatically approved by a number of Australian utilities.

Some utilities want such solar export-limiting equipment to respond to demand changes on the grid in less than one second, says Fraser Durham, commercial director of Argand Solutions, a British company. His firm produces a control system called GridGEM for companies using solar panels. This constantly analyses an organisation’s power use to determine when to buy electricity or to export any surplus according to a utility’s constraints. The system can also switch equipment on or off, channel any surplus renewable power into batteries and operate “private lines”, which are independent distribution networks which might be set up by a group of neighbouring companies to share locally produced solar power.

Saving some for later

Surplus renewable power could also be stored on the grid with beefier battery systems. Various technologies, from giant lead-acid batteries to pumping compressed air into caverns and letting it out to drive turbines, are being developed (see Technology Quarterly December 6th 2014).

Better batteries will also let businesses and homes store more of the power they generate instead of having to export it. Some of the advanced lithium-ion batteries Tesla will make for its electric cars at a giant factory in Nevada will also be sold as 10kWh and 7kWh storage units for domestic solar installations. Delivery of the first batteries, which will be contained in a unit called Powerwall, are scheduled to begin next year. Multiple Powerwalls can be connected together for sites with greater energy needs.

Instead of being a one-way line to deliver electricity, grids of the future will need to be smarter and more flexible to handle the distributed power systems being created by renewable energy.