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Hawaii's Solar-Grid Landscape and the 'Nessie Curve'

Jeff St. John's picture
Greentech Media
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  • Feb 15, 2014
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If California grid planners are worried about the famous “Duck Curve,” representing the future disruptions that distributed solar PV systems will have on the state’s grid, they should check out Hawaii’s “Nessie Curve.”

That’s the term that Hawaii’s utility and grid planners have adopted for the real-world grid disruptions they’re seeing today from the island state’s growing solar PV generation mix. They’ve dubbed it the Nessie Curve after the nickname for the mythic Scottish lake-dwelling dinosaur, the Loch Ness Monster.

Unlike California’s Duck Curve, Hawaii’s curve is happening today, not in the future. Also, unlike California’s projection of reduced, but not erased, demand due to solar PV generation, Hawaii’s curve actually drops “underwater” during peak solar times of the day.

In other words, Hawaii is already seeing enough solar coming onto parts of its grid to start feeding back power onto certain distribution circuits on sunny days, and to drive system-wide demand curves below zero on certain peak days.

This graph is from a presentation made by Dora Nakafuji, director of renewable energy planning for Hawaii Electric Co. (HECO), at last month’s DistribuTECH conference in San Antonio, Texas. This chart represents typical circuits on the island of Oahu, where lucrative incentives have led to a huge increase in distributed solar PV. As of the most recent tally, 10 percent of the island’s customers have rooftop PV, which equates to 29,558 systems with a cumulative nameplate generating capacity of 221 megawatts.

“It’s starting to look like this Loch Ness Monster,” Nakafuji said, pointing to the mid-day sag in residential energy demand, as rooftop solar PV energy supply exceeds the energy demand on those circuits, then the steep curve upward as solar fades away and late afternoon demand increases.

That’s made Hawaii a vanguard in testing the boundaries of how to manage such large penetrations of customer-owned generation, which is outside the utility’s control, and largely unmonitored by utilities and grid operators to manage the balance of energy supply and demand. In a controversial move, HECO has put new interconnection requirements in place for even small-scale rooftop solar PV systems, which has slammed the brakes on new projects and drawn the ire of the solar industry.

As Nakafuji pointed out, one of the clearest representations of this disruption can be seen in the data that tracks daily demand from homes, businesses and other energy users on distribution circuits. Because distributed solar PV causes that demand to drop during sunny mid-afternoon hours and then fades away in late afternoon and evening, it’s giving HECO a much more challenging situation in terms of turning down its oil-fired generators when solar is at its peak, then ramping them up much faster than it’s used to when solar power availability declines.

The radical effect that distributed solar has had on Hawaii’s load curve is demonstrated by the following graph, tracking changes from 2010 to 2013 on average 46kV transformer loading. On one peak day in August, the system experienced a backfeed condition, not just on some individual circuits, but across the system as a whole, she noted.

That’s the same challenge that California grid operators believe may start occurring on that state’s much larger and more integrated grid in coming years, as represented by the well-known “Duck Curve” from California Independent System Operator (CAISO). But while California is facing this effect in the future, Hawaii it facing it today — and it’s doing so with an island system with far fewer generation resources to tap to manage it.

This becomes a serious problem for managing the addition of new solar systems on heavily impacted circuits, Nakafuji noted. “We’ve noticed that 16 percent of the customers on the feeders who have the PV are already pushing the daytime minimum load up to over 100 percent. That means it’s back-feeding on our system. That becomes a concern,” she said, because circuits that send power back up to distribution transformers and substations cause all sorts of technical and operational challenges, with attendant costs.

Even getting the data to understand what’s happening on Hawaii’s grid has been a challenge, Nakafuji noted. “One of the things about renewable integration is, we really need locational data,” she said. As this map of Oahu’s distribution circuits indicates, certain parts of the island are far more affected by solar PV penetration than others.

“We’ve been deploying sensors out in the field to correlate” information pulled in from existing SCADA systems, she said, since “this was information the traditional system didn’t look at.” HECO is also planning to deploy smart meters, which could add more data to help guide day-to-day grid management decisions, she said.

Then there’s the essential problem of relying on a solar PV fleet for the island’s energy needs, she said — “What do we do when those renewables aren’t there?” HECO is investing lots of money in weather forecasting systems and distributed energy analysis and management technology, as part of a program funded by the Department of Energy’s SunShot grant initiative (PDF).

HECO is also installing grid-scale battery systems, incorporating demand response, and taking other steps to manage this solar challenge. Given that these island problems are expected to become challenges in many more regions in the future, it’s worth keeping an eye on how Hawaii manages them. 

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Nathan Wilson's picture
Nathan Wilson on Feb 15, 2014

Hmm, everyone is eager to have solar PVs on their roof, but the batteries are not selling nearly as well.  Hopefully Hawaii will help spread the word that PV is very limited without batteries.

Bas Gresnigt's picture
Bas Gresnigt on Feb 15, 2014

Hawaii authorities should consult in Germany.

German scenario studies indicate no problem with up to 40-50% of all electricity produced with renewable.
With now ~24% of all consumed electricity produced by renewable, their grid has superior reliability:
Average customer connection has a total outage time of ~15min/year.

That reliability increased from 30min./year down ~8years ago, towards 15min/year down now, when the share of wind+solar became substantial.
Main reasons more distributed generation by thousands of small units.
So a sudden outage of a 1MW wind turbine of a 10KW solar installation does not affect the grid much, which is totally different with a break down of a 900MW plant.

Leo Klisch's picture
Leo Klisch on Feb 15, 2014

I don’t know how much Hawaian infrastructure and real estate is at risk from rising sea levels but I would think they would be very concerned about CO2 – enough to push the limit on grid PV by investing in better distribution and accepting substantial hybrid diesel that would include batteries,super capacitors,etc. for quick response to variable generation. They mite accept nuclear generation but not politically likely. The only other option I see is some of the floating off shore wind that is being piloted now.

Clayton Handleman's picture
Clayton Handleman on Feb 15, 2014

Lots of mountains.  I wonder why they aren’t making more use of pumped storage.

Even without pumped storage, at $30 / MWhr and a 30% federal tax credit, gets them pretty close to cost effective PV with battery backup.

donough shanahan's picture
donough shanahan on Feb 18, 2014

Pity that the German grid regulator has come out several times then saying that the grid is becoming less stable and that specifcially due to renewable surges, more interventions were required to keep the grid stable.

Further stop conflating Germany renewables with solar, the thrust of the article here. Around 45% of German renewable electricity is in the form of biomass and hydro. ~60% of renewable energy is biomass. The total investment for biomass was estimated at around 2.5 billion. Solar providing 8% of the total renewable energy share has cost over 11 billion in investments according the AGEE stat. What to make of this? Any comment thus on  which is the better valued technology (PS the subsidy case is worse for solar again)?

Bas Gresnigt's picture
Bas Gresnigt on Feb 18, 2014

Look at the figures regarding German electricity generation (1990-2013). :
http://www.ag-energiebilanzen.de/
Wind: 7.9%, Solar: 4.5%: Together 13.4%.
Biomass: 6.8%, Hydro: 3.4%, Waste: 0.8%: Together 11.0%.

Then look into the expansion plans (e.g. VDE or Agora). Nearly only solar+wind.
Not strange as the Germans want to keep their forests, etc.

…German grid regulator has come out several times then saying that the grid is becoming less stable and that specifcially due to renewable surges…
Of course. They want to minimize further delay with the grid upgrades, so they put political pressure up. And they succeeded as last summer German parliament installed a special law in order to minimize delays due to NIMBY etc.

Important as those delays caused that the FiT’s are adapted such that the max. installation rate of wind+solar is ~5-6GW/year now. So they are sure to keep their excellent grid reliability. With the grid upgrades they can increase the installation rate.

These grid stability remarks are published in English by Bloomberg etc. totally out of their connection. Apperently in order to rise fear or so (may be in order to prevent USA follows Germany).
How important is it?
In the Netherlands it got no attention at all.
Stronger. We are installing two new high power interconnections with the German grid, so we can import more of their cheap, reliable electricity.
We will sell part of it to UK, as in UK wholesale prices are even higher than in Netherlands.

Paul O's picture
Paul O on Feb 18, 2014

I went ahead and checked this story out. What has struck me is that there is out there a new Cookie-Cutter  announcement saying that  Kuhaku Wind Farm no longer uses batteries, but it now …

“Instead of batteries, Kahuku Wind now uses new technology that allows continuous voltage regulation. The company says this technology improves the grid’s voltage stability.”

I haven’t been able to track down any mention of what this wonderful new technology is.

Joris van Dorp's picture
Joris van Dorp on Feb 19, 2014

In other words: it’s a big flywheel. Flywheel are great for many things except bulk energy storage. It is storage that the Hawaiian’s need.

I wonder if they are contemplating deep ocean compressed air storage (pumping air into deep undersea balloons as a means of storing energy) like some of the carribean island nations are doing, or so I’ve heard. Such systems can store lots of energy and are supposed to be cheaper than batteries, but they are not cheap and also not efficient. eff= 50% or so?

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