There has always been one problem with solar and wind energy. It boils down to two words: darkness and calm.

Solar panels are great electricity sources, so long as the sun shines. The same is true of wind turbines, so long as the wind blows. But at night and when the air is still, those renewable sources go dead, and conventional power plants have to take over.

Renewable energy advocates have long said: If only the electricity solar and wind systems produce could be stored and saved for later. For years, there have been ways to store energy from generated electricity. One way is to use the electricity to spin a heavy flywheel, which becomes an energy source when the electricity is shut off. Another is to use an electric chiller to make ice at night and then use the ice to cool water for air conditioning during the day.

Unfortunately, those methods have limited applications, and they’re not especially cost-effective; they require large capital investments that can take a long time to pay back. But now, as evidenced by the Sustainable Operations feature in this issue of Treatment Plant Operator, there’s a third option.

Just common sense?

In one word — batteries. One might reasonably ask: Why didn’t someone think of this before? Well, many people did think of it. It’s just that batteries weren’t sophisticated or cheap enough to store electric energy affordably and in meaningful amounts. Now, that appears to be changing.

The Napa (California) Sanitation District is using banks of lithium-ion batteries designed and built by Tesla to make more efficient use of electricity from the wastewater treatment plant’s cogeneration system and an array of solar panels. Simply stated, the batteries store energy generated at times when utility power is cheap and discharge it when utility power is expensive.

Batteries still have a fairly long way to go, but the technology keeps advancing. Lithium-ion batteries aren’t the only game in town for large-scale energy systems. For example, flow batteries store and deliver energy not by way of solid electrodes, but through two chemical components dissolved in liquids. Iron-air rechargeable batteries produce electricity through the interaction of iron oxide (rust) and air.

Completing the picture

Now, consider the possibilities at a wastewater treatment plant where cost-effective, battery-based storage is combined with digester-gas-fueled cogeneration, a gas- or diesel-driven standby generator, solar and wind power (for which treatment plants often have ample space), and sophisticated software that can deploy each resource to maximum economic advantage.

At times of low utility rates, the generating sources could charge the batteries. At times of high utility rates, the batteries could discharge, saving expensive purchased kilowatt-hours. Further, with battery storage, solar and wind power essentially cease to be intermittent energy sources; the power they generate can be used all day long.

And that’s not all. Imagine a setting where the local utility allows interconnections with major customers and pays a feed-in tariff, and where market power prices fluctuate by the hour based on demand. Now the energy management software, communicating with the power markets, can follow the market prices.

Suppose a standby generator can produce electricity for 6 cents per kilowatt-hour, but the market will buy the power for 12 cents. It then becomes profitable to start that generator and feed power to the utility grid. The right software and systems can enable it all to happen automatically.

Exploring all options

A system like this adds meaning to the phrase “water resource recovery facility.” Now to this mix of resources, add co-digestion, feeding food waste or FOG to the digesters. Suddenly, status as a net-zero energy facility seems well within reach, and cost-effective at that. These are exciting times for plant managers and operators looking to get the most from their facilities. Battery-based storage is another tool in the progressive energy manager’s kit.