Three years ago, Greentech Media covered the launch of an experiment in California to see if ice-making air conditioners could help solve a tricky problem for solar power.
The goal was to see how well sun and ice could work together to help shift solar PV‘s peak power production, which takes place in the mid-afternoon, to play a role in providing power during the “evening peaks” that come after 5 p.m. or so. That’s when people start getting off work, heading home and turning on their lights, TVs, electric appliances and, on hot summer evenings, their air conditioners.
Another thing that people do after work, of course, is go shopping. Malls, department stores and other retail centers also have an evening peak to deal with, as they start turning on their lights and cranking up their air conditioners to keep buildings cool. Heat, unlike sunlight, keeps gathering throughout the day, and tends to peak in the late afternoon. That means that those rooftop AC units are cranking their hardest right when solar power is already fast on its way to dropping off to its evening zero point.
Armed with a $1.475 million grant from the California Public Utilities Commission, thermal energy storage startup Ice Energy set out in 2010 to test the capabilities of solar energy shifting across a variety of settings in California.
But while batteries can store solar-generated electricity for use at later times, Ice Energy’s rooftop units use that solar output to make ice, then use that ice later in the day to drastically reduce the amount of electricity those AC units need -- a procedure that’s known as "thermal energy storage." While thermal energy storage has been a part of district energy systems for decades, scaling it down to rooftop size is a newer development.
Earlier this month, I checked in with Greg Miller, Ice Energy’s executive vice president of market development, to see how well sun and ice have worked together for green energy storage and power shifting.
It turns out that these projects take a long time to get going, and end up changing quite a bit in the process. Eventually, Ice Energy ended up teaming up with PV project developer SunEdison on a rooftop that already had solar panels installed, rather than starting from scratch on empty commercial rooftops. That led them to the solar-equipped Kohl’s department store in Redding, Calif., where Ice Energy is already working with the city’s municipal utility on a larger-scale thermal energy storage project.
But while that project aims to help Redding’s utility lower air conditioning power demand to make up for the planned shutdown of a nearby coal-fired power plant, this project ties Ice Energy’s ice-making AC units, SunEdison's panels and Kohl’s building energy management system into a single unit, he said.
“Just this last month, we were able to start collecting the data on the Ice Bear unit performance,” Miller said, referring to Ice Energy’s brand name for its thermal storage units. “We’re monitoring the main load of the building, and measuring the power production of the solar system," while SunEdison is overlaying the functionality of the solar and storage on top of the building load.
In other words, the project is centered around giving Kohl’s -- a recognized leader in green energy technologies for its department stores -- a way to use its solar power not only to generate carbon-free energy, but to help mitigate the costs of buying power during peak periods. That’s something that more and more commercial building owners that have invested in rooftop solar are starting to focus on, he said.
“What I think some of these companies are finding out is that it’s not truly meeting the best economic need -- it’s just displacing kilowatt-hours,” he said. “They’re not able to deliver energy during the peak, and so they haven’t really solved the peak problem for the customer.”
Compared to batteries, thermal energy storage is fairly cost-competitive, though not overwhelmingly so. Ice Energy estimates the capital costs of its energy storage systems at about $2,070 per kilowatt, which falls below the costs of today’s grid-scale lithium-ion and sodium-sulfur battery systems, but is higher than some of the cheaper advanced lead-acid or flow battery technologies out there.
At the same time, adding thermal storage to air conditioning units is certainly a lot less complex than installing and operating electricity storage systems, and fits in nicely with one of the chief peak load culprits in hot climates.
Both forms of energy storage are competing against the chief source of peak power generation today: natural gas-fired peaker plants. While these are certainly cheaper, particularly in the current regime of low natural gas prices, they also represent major utility investments, and certainly can’t be applied in the distributed fashion that both batteries and Ice Energy’s rooftop units can.
Indeed, by providing a role in lowering air conditioning power use, while also shifting solar power’s predictable daily peaks a few hours later in the day, thermal energy storage could play a nice two-for-one role, Miller noted. California, which is proposing a groundbreaking energy storage mandate to help the state reach its aggressive renewable energy goals, is a natural testing ground for this kind of application, he said.
Miller said that the months of August and September should provide lots of critical data on just how well the sun-and-ice combination on Kohl’s Redding department store rooftop works to solve these dual goals. In fact, one of the only problems he sees for the project is that, because the store’s roof was already mostly filled up with the solar panels that make up its 304-kilowatt solar array, there wasn’t enough room to put more than 35 kilowatts of ice-making AC units on it.
“I would have sized the solar down, and put more Ice Bears on,” he said.