Photo/IllutrationA solar power plant on the coastal industrial zone in Oita in 2014 (Asahi Shimbun file photo)

FUKUOKA--The sun could be setting on renewable energy in Kyushu, as solar power plant operators face the prospect of being forced to suspend their successful operations to make way for resurgent nuclear power.

Kyushu Electric Power Co. is considering using its right to order solar power operators to stop production temporarily, possibly, next month because of a surplus of power resulting from the resumption of operations at the utility’s four nuclear reactors on the main island of Kyushu. Unlike renewables, nuclear energy has the status as one of Japan's top priority power sources.

The order would be the first of its kind on a main island in the nation if the utility proceeds with it.

Solar power generation is thriving in Kyushu partly because the region, which is situated in the south in the nation, enjoys more sunlight hours and less snow than others.

Buoyed by government subsidies to promote renewables in the wake of the 2011 Fukushima nuclear disaster, solar quickly spread in Kyushu.

At some points during the year, solar power has provided the vast majority of energy used in Kyushu. For example, it accounted for more than 80 percent of electricity used in the region as of 1 p.m. on April 29 during the Golden Week holidays.

Now, the amount of power generated by solar energy that Kyushu Electric buys from operators under the feed-in-tariff system is growing at a pace of 50 megawatts a month on average.

But the spread of renewables’ growth has a drawback, according to utilities.

A power supply in excess of demand causes fluctuations in electric frequency that could result in power outages across a broader area.

Fukuoka-based Kyushu Electric has tried to coordinate supply and demand by curtailing the operation of thermal power plants and producing power at night with water pumped up during the day by solar energy.

When such coordination becomes difficult, electric power companies can issue a directive for solar power operators to halt their operations temporarily under a rule set by the government.

Kyushu Electric has already turned to this option in remote islands such as Ikishima island, Nagasaki Prefecture, and Tanegashima island, Kagoshima Prefecture.

The option of output control looms large in spring and autumn, the two seasons when demand for power to use heaters and air conditioners remains low, but solar power output soars due to clear weather on many days.

When factories and company offices are closed on holidays in those seasons, a need for output control becomes more likely.

“We may issue a directive for output control this autumn,” a Kyushu Electric official said.

When the company decides on output control of solar energy, based on the projected power demands by taking into account data on weather and other factors, it is supposed to notify solar energy operators via e-mail by the evening before.

Kyushu Electric is now capable of supplying more power through conventional sources with the restart of all its four nuclear reactors--two units at its Sendai nuclear plant in Kagoshima Prefecture and another two at its Genkai plant in Saga Prefecture.

Renewables are the first energy sources to be subjected to output control under the government rule.

Nuclear energy is the last, because the government designates it a “base load power source,” alongside hydraulic power and geothermal power. The government says those sources are relatively inexpensive to produce power and their output is stable.

Experts say one solution to fully utilize renewable energy output is to strengthen tie-ups between regional utilities so that a surplus of power in one region can be used elsewhere.

Projects to reinforce the capacity of power lines between utilities are getting under way for that purpose.

But electric power companies on the receiving end are not eager to receive power from other utilities, because it means their own power stations may then experience drops in the rates of their output.

“We need to draw up rules to coordinate between utilities, including how to share the financial cost, and to optimize the use of renewables beyond regional boundaries,” said Yukari Takamura, professor of environmental law at Nagoya University, who is well-versed in power generation systems.

(This article was written by Yuji Yamashita and Rintaro Sakurai.)