This article discusses why Hawaii needs to find new ways to handle the influx of solar power. In Hawaii, rooftop solar panels are popping up everywhere. However, the solar boom has also created some complications, including sudden changes or disruptions in the system or problems at the distribution side, when there are lots of PV connected at the rooftops. Electrical loads in homes fluctuate all the time as power-hungry appliances like air conditioners and water heaters turn on and off, resulting in a high probability of low load and too much power. Power can go back through the nearby substation transformer and cause a rise in the voltage. That the state is split into several small islands further complicates matters. Another challenge unique to Hawaii is that each island in Hawaii is its own independent grid, and there are no neighbors to depend on for power.
You don’t need a lot of sunshine to make solar power work—gray Germany is a leader in making electricity from photovoltaic panels—but it sure helps. In sunny Hawaii, rooftop solar panels are popping up everywhere. For example, on the island of Oahu, where most of the state's population lives, roughly 12 percent of utility customers have rooftop solar.
The average in the U.S. mainland is only 0.5 percent.
Factors other than sunshine help make solar power attractive in Hawaii. The state has high energy prices: compared to the mainland average of roughly 10 cents per kilowatt-hour, the retail price for electricity in Hawaii can run as high as 35 cents per kWh. Most of Hawaii's electricity is produced from burning natural gas and oil, and those resources must be shipped in.
In addition, homeowners can receive both federal and state tax credits for going solar.
But the solar boom has created some complications. According to Reza Ghorbani, a professor in the mechanical engineering department at the University of Hawaii at Manoa and director of renewable energy design there, “A couple of problems happen. One is sudden changes or disruptions in the system.” For example, if you lose a generation source in the grid, the system frequency can drop below a threshold, causing solar photovoltaic systems to disconnect. “You lose all the PVs on the island, then it causes a blackout.”
“Another problem they face is at the distribution side, when you have lots of PV connected at the rooftops,” Ghorbani said. Electrical loads in homes fluctuate all the time as power-hungry appliances like air conditioners and water heaters turn on and off, resulting in a high probability of low load and too much power. Power can go back through the nearby substation transformer and cause a rise in the voltage.
That the state is split into several small islands further complicates matters, according to Ben York, an electrical engineer with the Electric Power Research Institute in Knoxville, Tenn.“One of the challenges unique to Hawaii is each island is its own independent grid,” York said, “so there are no neighbors to lean on. All the time, you have to balance what's available on that island.”
So in spite of its insolation, Hawaii may not be an ideal place to try to depend on solar power. But the state is going to have to find a way to make it work: the state recently enacted a law that will require all its electricity to come from renewable energy sources no later than 2045.
Even today, the intermittent nature of renewable energy sources like wind and solar power is presenting several challenges to utilities. Ghorbani said, “One of the main concerns of utilities in Hawaii is lack of understanding of what is really going on with the grid. Dealing with all these issues is really challenging.”
As a result, Hawaiian Electric Co., or HECO, the main utility in the state, has put a moratorium on granting permits for new solar panel installations.
Peter Jansson has followed these developments. Jansson is an electrical engineering professor at Bucknell University whose research focuses on photovoltaic system optimization and the smart grid and who works as a consultant in the solar business.
“We need to know how the system will respond with power flowing in the opposite direction when different faults occur,” Jansson said, “just to make sure the public is safe and the system is still reliable. The grid wasn’t designed for that, and since you’re running an experiment, they’d like it to be a slow, controlled one where they can learn step by step.”
Most residential PV systems are connected to the grid. The utility connection provides electricity whenever the solar panels don’t produce enough to cover the home demand; the PV system might not be large enough or maybe the sky is overcast or dark. The grid serves as a battery of sorts, as the home pulls power off it when it needs more than its solar system can generate and sends power to it when the PV system generates an excess.
That give and take is the basis for the concept of net metering, a major driver of increased solar use. The utility charges the normal retail rate for power drawn from the grid and pays homeowners for the electricity their PV systems feed back into it, at least up to the point where the feed in equals the draw out. Net metering serves as an incentive for homeowners to install residential systems big enough to create a so-called net-zero system.
With only a fraction of a percent of residences feeding power to the grid, net metering works fine. But at a certain threshold, where power comes extensively from distributed power generators like solar panels, the system doesn’t have a lot of what Jansson calls “stiffness.”
Stiffness comes from the interconnection of many rotating synchronous machines generating at the same frequency. If there's a fault or changes in loads or generation, the grid can respond and not just shut down. (Some people use the term “inertia” to describe this.) Utilities do this all the time.
Power electronics devices such as the inverters used in solar installations have no momentum and don’t add to the stiffness.
But John Berdner, senior director of regulatory and policy strategy at Enphase Energy in Petaluma, Calif., says there is a solution to the stiffness issue. A major solar installer, Enphase has about 60 percent of the PV generation market in Hawaii.
According to Berdner, inverters in Hawaii have become stiffer, in a way.
“Regulations have required the inverters to trip offline aggressively if anything goes wrong on the grid,” Berdner said. “If you have a little bit of PV, you just want it to get out of the way. But if you start doing lots of renewables, then you need to rethink your strategy when something goes amiss on the grid.”
In Hawaii, regulators have added something called a ride-through requirement, which stipulates that during a voltage or frequency disturbance, the inverter stays online for a certain time, instead of tripping offline.
Enphase has a built-in, two-way data-over-powerline link it uses to monitor every one of its microinverters. The company's cloud-based systems communicate with each of its panel-level devices every five minutes. A microinverter is attached to a dedicated panel, while a string inverter serves several panels and usually sits inside a building.
“We remotely reprogrammed inverters to the new ride-through settings, as we can monitor and communicate with those systems,” Berdner said. “We updated the settings on roughly 800,000 inverters in the field.”
Enphase is continuing to work with the inverter industry and utilities like HECO to add more capability to inverters. Within the next couple of years, they could have a host of advanced inverters that will be programmed to help keep the grid stable and allow it to accommodate much higher levels of renewable energy than currently possible.
Another suite of technologies that promise to improve the integration of solar power is the so-called smart grid. According to Clay Perry, senior media relations manager at EPRI, “The smart grid is a lot of things. You’re looking at resiliency. You’re looking at making the grid more robust and more reactive so you have two-way flow.”
The smart grid enables system operators to monitor the condition of the grid at any given time. Sensors embedded throughout the service area link to devices that can predict when a problem will strike the grid. In some versions, the utility can take steps to shed load, such as turning off residential customers’ appliances, or to tap stored electricity to compensate for the intermittency of renewables.
With solar and other renewables adding instability to the system, Perry said, “The smart grid will play a critical role moving toward more renewables on the grid.”
Some experts see energy storage, mainly batteries, as particularly important. Enough stored energy that can be dispatched immediately can significantly increase the amount of renewables the grid can support. Berdner thinks we can ultimately achieve a grid powered by 100 percent renewable energy through storage.
“You have to have a continuous supply of power, so you’re probably going to have lots of storage, whether it be electrical chemical storage like batteries or hydro or something else like fuel cells. Who knows?” Berdner said, “Batteries are starting to happen, and as prices come down, they’ll become more and more common. I happen to like lithium iron phosphate as the current battery technology. It seems to be a good compromise. But I’m chemistry agnostic. It's an area with lot of advances occurring, so we shouldn’t get too attached to a specific chemistry at the moment.”
Not everyone sees batteries as the primary answer.
“There are two or three ways to solve this without storage,” Jansson said. “Batteries are only about 80 percent efficient.” Building automation and load control together form one of the alternatives, and they may work in conjunction with storage. Building automation is currently common in commercial buildings but not seen much in residential.
According to Johanna Mathieu, assistant professor in the Department of Electrical Engineering and Computer Science at the University of Michigan in Ann Arbor, the concept of load control (also called demand response) will take advantage of power-hungry appliances, specifically those that store thermal energy, such as refrigerators, furnaces, water heaters, and air conditioners. Control systems can turn these devices on and off within the ranges of their thermostats to match short-term fluctuations in the available electricity from renewable generators.
That is a departure from conventional grid operation, where supply generation is varied to meet fluctuating demand.
To achieve this will require widespread communications and sensing infrastructure. Mathieu said that some of her colleagues have participated in a joint effort with HECO, the U.S. Department of Energy, and the Lawrence Berkeley National Laboratory on the Hawaiian Electric Company Demand Response Roadmap Project to address these issues.
Ghorbani's group at the University of Hawaii is working on several measures. One is a low-cost technology to connect all the major loads such as air conditioners and water heaters via the Internet.
“We are dealing with demand response technology, low-cost inverters, and communication technologies,” Ghorbani said. “The next thing we’re doing is developing prediction algorithms that can predict power for the next 20 minutes at a substation that has a lot of solar PV in the feeders.”
As it moves toward its renewable energy goal, hawaii can look at other places that have had to deal with the challenge of solar PV systems overtaxing the electrical grid. Some lessons can be learned from sun-drenched areas such as California, Arizona, and Australia, but the best role model is a country that sees far less sun: Germany, a nation known for its extensive use of solar PV.
Because of its location in Europe, Germany can harness both strong winds in its north and modest insolation in its south—occasionally at the same time.
“They’ve had some days well over 50 percent penetration by renewables and maybe even days almost 100 percent, like in the spring and the loads were low and the PV was going,” Jansson said.
As a pioneer, Germany has often had to take the only available option to solve problems, which may not have always been the cheapest option, according to Ben York at EPRI.
“A lot of times, it was just reinforcing a feeder, adding more copper and iron,” York said. “They are, as we are, looking for a more intelligent solution to strengthen the grid, better ways to incorporate more renewables.”
A different sort of challenge occurred this past spring when Germany experienced a partial solar eclipse. System operators had to import power from neighboring countries and do a lot of work to balance the system. They made it through without any issues, partly because they could model what the eclipse might do and plan around it.
Whether an island state like Hawaii could compensate so well remains to be seen.
The challenges that Hawaii is facing now in integrating renewables could be a vision of the future throughout the United States. Home solar is rapidly spreading across the United States, even while demand for electricity is softening. There are now about 600,000 installed PV systems, and the number is expected to reach 3.3 million by 2020, according to the Solar Energy Industries Association.
“Hawaii is going to be a great test bed for where the United States is going to end up in terms of public policy,” Jansson said.
And that future is rapidly approaching. In addition to the mandate for a completely renewables-based electricity system by 2045, Hawaii state law also has an interim requirement of at least 30 percent renewable electricity by 2020.
“This is a great opportunity for the state,” Ghorbani said, “and a challenge.”