Engineering Solutions: Can you have too many solar photovoltaics in Hawaii?

By Tom Gibson

In the state of Hawaii, rooftop solar panels are popping up everywhere. On the island of Oahu, where most of the state’s population lives, roughly 12 percent of utility customers have rooftop solar, compared to an average of 0.5 percent in the mainland United States.

It only makes sense because the state has a combination of high energy prices, abundant sunshine, and federal and state tax credits available for going solar. Hawaii gets most of its electricity from burning natural gas and oil, resulting in costs as high as 35 cents per kilowatt-hour. Compare that to the 10 cents typically found on the mainland.

The fact the state is split into several small islands only complicates matters, according to Ben York, Ph.D., an electrical engineer with the Electric Power Research Institute. “One of the challenges unique to Hawaii is each island is its own independent grid, so there are no neighbors to lean on. All the time, you have to balance what’s available on that island.”

But while converting to renewable energy is a natural and commendable response to this situation, the popularity of solar panels is actually causing problems. Reza Ghorbani, a professor in the Mechanical Engineering Department at the University of Hawaii at Manoa and director of renewable energy design there, says, “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 (PV) 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 explains. 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 high PV. Power can go back through the nearby substation transformer and cause a rise in the voltage.

Renewables are intermittent
You can blame the intermittent nature of renewable energy sources like wind and solar power for this predicament, as they present several challenges to utilities. Ghorbani says, “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 Company (HECO), the main utility in the state, has put a moratorium on granting permits for new solar panel installations.

Peter Jansson, an electrical engineering professor at Bucknell University whose research focuses on PV system optimization and the smart grid, works as a consultant in the solar business and has followed developments in the Aloha State. He states, “We need to know how the system will respond with power flowing in the opposite direction when different faults occur, 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 solar PV systems are connected to the grid to provide electricity through a utility at night or when the sun doesn’t shine. The grid serves as a battery, as you pull power off it when you need more than your solar system can generate, and you send power to it when you generate an access. This leads to the concept of net metering, a major driver of increased solar use. For any power used from the grid, you pay the normal retail rate, and you receive that rate for any power fed back to it, up to the amount you use. Net metering serves as an incentive to install a system that matches the power you use, a so-called net-zero system.

Once you reach 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. This 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 — utilities use this all the time. Power electronics devices like the inverters used in solar installations to convert direct current generated to alternating current for the grid have no momentum and don’t add to the stiffness. (Some people use the term “inertia” to describe this.)

But there is an answer to this. John Berdner serves as senior director of regulatory and policy strategy at Enphase Energy, headquartered in Petaluma, CA. A major solar installer, Enphase has about 60 percent of the PV generation market in Hawaii. He explains how inverters there have become stiffer, in a way. “Regulations have required the inverters to trip offline aggressively if anything goes wrong on the grid. 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.

Improving inverters
As Berdner reports, at Enphase, “We remotely reprogrammed inverters to the new ride-through settings, as we can monitor and communicate with those systems. We updated the settings on roughly 800,000 inverters in the field.” Enphase has a built-in, two-way data-over-powerline link they use to monitor every one of its microinverters. Their 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.
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 should 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 factor that stands to improve the situation is the smart grid we often hear about. 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.” It involves many sensors, devices that can predict when there will be a problem on the grid. All those things will come into play when you introduce solar and other renewables, so the operators can monitor the condition of the grid at any given time. “The smart grid will play a critical role moving toward more renewables on the grid.”

Other solutions appear on the horizon as well. An obvious one is energy storage, mainly meaning batteries. Berdner thinks we can ultimately achieve 100 percent on grids with this. “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?” He adds, “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.”

But not everyone sees batteries as the primary answer. Jansson says, “There are two or three ways to solve this without storage. 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. As Berdner relates, “We’re going to see more coordinated energy management, both of loads and storage and on-site generation.”

Working through the Internet
Ghorbani reports that his group at the University of Hawaii is working on several measures. One is a low-cost technology to connect all the major loads like AC and water heaters through the Internet. “We are dealing with demand response technology, low-cost inverters, and communication technologies. The next thing we’re doing is developing prediction algorithms that an predict power for the next 20 minutes at a substation that has a lot of solar PV in the feeders.”

Hawaii is actually not the first area to deal with the problem of overabundant solar PV systems feeding the electrical grid.Sun-drenched areas like California, Arizona, and Australia have gone down this road. But probably the best role model is a country that sees far less sun: Germany, a nation known for its extensive use of solar PV.

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. They are, as we are, looking for a more intelligent solution to strengthen the grid, better ways to incorporate more renewables.” Jansson reports, “Germany seems pretty happy; 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.”

A chance to prove this came several months ago, when Germany experienced a 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.

Over all, 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. The problems experienced in Hawaii could soon be seen all across the country as the penetration of rooftop solar continues to increase. “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 predicts.

Hawaii recently enacted a law that will require all the state’s electricity to come from renewable energy sources no later than 2045, making it the first state in the nation to adopt a 100-percent-renewable requirement. The law also has an interim requirement of at least 30 percent renewable electricity by 2020. As Ghorbani puts it, “This is a great opportunity for the state … and a challenge.”

Based in Milton, PA, Tom Gibson, P.E. is editor and publisher of Progressive Engineer, an online magazine and information source (www.ProgressiveEngineer.com).