Solar panels at a drinking water plant can do more than reduce the electricity bill. It’s also about adding resiliency to water production, says Michael Harris, P.E., energy coordinator for Middletown, Connecticut.

The city recently installed a solar array at one of its two water plants. “Resiliency is all about making the system capable of bouncing back more quickly and easily in the case of strong storms or other emergencies,” Harris says.

The city entered a power purchase agreement with Greenskies Renewable Energy, a Clean Focus company with East Coast headquarters in Middletown. Greenskies Renewable Energy owns, operates and maintains the solar array. The city’s Bacon Water Treatment Plant buys the electricity for less than it would pay the local electric utility.

Long-term savings

The solar array went online in May 2018. The Bacon plant, built in the 1970s, is the smaller of the city’s water plants. The system draws from a wellfield and from a complex of reservoirs to serve a bit less than half the city’s 50,000 population. Average demand is about 4 mgd.

The 714-panel solar array is expected to produce 261,653 kWh of electricity per year, about 75 percent of the plant’s usage. The PPA will save the city an average of about $9,000 a year for 20 years. Much of the savings stem from the avoidance of anticipated electricity price inflation.

While the difference between the average cost of electricity and the PPA contract price appears to support significant savings, these will be tempered to the extent that demand charges will still apply. This, of course, is related to how sunny it may be on any given day of the week or by how much electric demand the plant may require in excess of the solar system’s capacity. The savings are significant, but not the whole reason for the project.

“The solar system is intended to add to the resiliency of the plant and the overall system,” Harris says. “Providing drinking water is an important function of the city, and edifying the system is something we need to think about as weather events become more pronounced, as the potential grows for energy or fossil fuel interruptions, and with the likelihood of greater volatility in the reliability of the grid.

“We feel this is an important coupling of an essential resource that our citizens need — which is drinking water — with solar power, but it’s only a first step. A couple of steps need to be taken to get the most out of it.

“As it stands now, it is largely symbolic because it’s a grid-tied solar system. If the grid goes down, the system doesn’t work. That’s true for most solar systems. If anybody puts a solar system on their house, unless they spend a whole bunch more money, it’s not going to work when the power is out.”

Energy storage

Battery-based energy storage would be required to make the solar power system grid-independent. “Battery storage is really starting to come along,” Harris says. “Technically and financially, it’s just becoming viable.” For now, in a power outage, a diesel generator would keep the Bacon plant operating.

“As long as you can keep the truck coming in to fill the fuel tank, the emergency generator can keep the system running, and drinking water will be available,” Harris says. “We had that before we had the solar system, and we have it now. The way to couple it all together is with battery storage.”

A battery system would enable the plant to store some of the solar energy to use later. “We could reduce, minimize or even eliminate use of the generator,” Harris says. “Or we could take a generator that would operate for a week on a tank of fuel and maybe operate it for two weeks, or a month, or two months, depending upon how the system is designed and how sunny it is.”

Flattening demand

Another advantage is that the solar system can flatten demand and so limit electric utility demand charges. The water department has already done that in recent years by installing variable-frequency drives on motors in the water plants, according to Brian Robillard, assistant chief engineer: “We didn’t track the costs, but we believe that we achieved reasonable savings.”

Stanley Chin, Greenskies Renewable Energy president and CEO, says the company has installed solar arrays at several water plants around the country and is seeking more such projects. “Installing solar is essentially prepaying for electricity,” Chin says. Water plants and wastewater treatment plants are good prospects for PPAs because they generally have good credit and good long-term prospects and so are attractive to lenders.

Greenskies Renewable Energy has solar projects at five water plants and has three more under construction. The company has more than 340 solar projects in the United States. It also operates renewable energy projects in Taiwan and China.