Maui Electric Co. is testing a unique solution to over-generation from rooftop solar, and has installed a device on the island of Moloka'i that allows it to waste a little electricity — but in exchange, install more rooftop capacity.
At the heart of the solution is a resistive dynamic load bank developed with the Hawaii Natural Energy Institute (HNEI) at the University of Hawaii, which helps maintain grid reliability by keeping the island's diesel generation online. While the Moloka'i grid is unique — low energy demand and high solar penetration — the over-generation issue is not.
Utilities with a lot of renewable energy on their grids can face challenges in balancing supply and demand, and at times may need to curtail production. It's a situation they typically try to avoid, as it means less emissions-free energy on the grid while driving up costs for generators.
The size of the problem as well as the solutions will vary, depending on the utility and grid.
Overgeneration throughout the US
The largest example of overgeneration in the United States is CAISO's Western Imbalance Market, which in the first quarter of this year was able to deliver 52,254 MWh of surplus renewable energy that otherwise would have been curtailed. And CAISO says the market is growing, generating double the energy savings it did a year before.
In Texas, utilities for years have given away free energy during times of high wind and low demand. Arizona Public Service developed "reverse demand response" to balance system load with excess renewable generation.
Putting excess generation into energy storage is an effective and widespread solution, but while the cost of batteries has been falling they are not yet economical. A recent report from Minnesota suggests intentionally overbuilding solar generation, rather than purchasing more expensive batteries, could make more sense.
The report on the state's solar potential concluded wind and solar power could serve 70% of Minnesota's electrical load in 2050, but "additional capacity coupled with energy curtailment is considerably less expensive than, and a viable alternative to, long-term or seasonal storage in a high renewables future."
On Moloka'i, the utility has not found an alternative use for the potential overproduction and so is looking to waste a small amount.
"While no one likes to discard excess renewable energy, it is not economically feasible to use a battery to store the few megawatt-hours of excess energy produced per year," HNEI said in a research brief on the Moloka'i project. "Using something inexpensive like the load bank enables over a gigawatt-hour of additional renewable energy on the system while discarding only a few megawatt-hours of excess energy."
Dynamic load on Moloka'i
Moloka'i has high levels of rooftop solar, which keeps its daytime system demand modest. Power system demand can dip as low as 3.3 MW, and the island has approximately 2.3 MW of rooftop solar to meet most of the system’s demand during the day, according to HNEI.
"This makes Moloka'i the perfect location to demonstrate advanced solutions for reliable operations of grids, in a state with some of the highest penetration levels of distributed solar generation in the world," HNEI Director Rick Rocheleau said in a statement.
The problem with the island's grid is that adding more rooftop systems could push the utility’s diesel generation below its minimum reliable operating levels during times of high solar production. To address the issue, MECO put a hold on new rooftop solar on the island in 2015.
But now, with the new load bank installed, MECO will likely be able to accept an additional 725 kW of rooftop PV. Utility officials say the work with HNEI can be a model for adding more clean energy for some utilities.
"Such partnerships with HNEI are critical as we continue to work together to integrate more renewable energy on small stand-alone systems like on Moloka'i," MECO Director of System Operations Chris Reynolds said.
The new load bank is designed to complement a 2 MW battery energy storage system commissioned at the utility’s Pala'au power plant in 2016, also develop in conjunction with HNEI. Overall, the projects are part of MECO's efforts to reach 100% clean energy in Moloka'i.