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To Manage the Grid, a Vermont Utility Hooks Up to a Hospital's Icebox

As distributed energy resources gain traction, virtual power plants could add value for consumers and utilities alike

4 min read
Photograph of money behind a layer of ice.
Photo: iStockphoto

It’s not too far-fetched to think of a day when the ice that cools your favorite fizzy drink also puts money back in your pocket.

Think of it this way: Those ice cubes tinkling merrily in your glass are a form of energy storage, albeit on a really micro-scale. Your refrigerator’s freezing unit transforms water to ice. Ice in a large enough volume could potentially offset a portion of your electricity demand to run air conditioning units on a hot summer afternoon. By tapping into financial incentives offered by your local utility, that ice-chilled beverage could be a money-maker.

Admittedly, the notion of using cold drinks as retirement income is a bit out there. But utility Green Mountain Power has launched an 18-month pilot program to demonstrate how an ice storage facility at a Vermont hospital can work as a distributed energy resource. If it’s successful, the hospital’s large-scale ice maker, combined with other load resources that the utility hopes to enlist in the pilot, could reduce peak electricity demand and create a revenue stream.

“The pilot aims to demonstrate that storage capacity is all over,” said Jeff Monder, who leads new product development for the utility. He sees potential to tap energy sources related not just to ice but also to a commercial building’s internal environment and to heating and cooling systems that form part of many industry operations.

The pilot is intended to test the idea that “on any circuit, we can come up with a strategy to balance generation and load and start to monetize the customer’s participation based on performance.”

Green Mountain Power hopes to tap controllable loads and use them to manage grid fluctuations

In a filing with Vermont regulators in December, the utility explained that as electricity demand moves up and down through the day, generating resources must constantly be adjusted to keep the grid in balance. 

Now that distributed energy resources (DERs) like rooftop solar and battery storage are gaining traction, Green Mountain Power said it can tap those controllable loads as another tool to manage grid fluctuations in general and for peak demand days in particular.

The new pilot project rests on the idea that large amounts of flexible loads exist at commercial and industrial sites, including thermal energy storage resources. In its filing, the utility said that those resources can be “reliably, and flexibly leveraged” to reduce peak energy demand or at least shift it to more desirable times of day. 

The ice storage equipment is located at the Brattleboro Retreat Hospital, a mental health and addiction treatment facility. The equipment originally was put in place to enable the hospital to take part in a demand response program. That program has long since expired, leaving the ice facility as something of a stranded asset.

In practice, electric chillers would run during relatively low-cost off-peak hours (most likely at night) to freeze water to make ice. Then, when air conditioning load starts to rise on hot afternoons, the hospital’s HVAC system could be dialed back and the stored ice would be used for cooling. As the hospital reduces its electricity load, that reduction would be offered to the utility as a demand-response source. Doing so helps to reduce the utility’s peak electricity load and enables the hospital to be paid for its conservation effort. 

In its filing, Green Mountain Power said it plans to control the hospital’s ice-making facility as a demand response resource. In turn, it will credit the hospital for off-peak demand charges. A share of the benefits also will help lower the cost of service for other Green Mountain Power customers.

The pilot hopes to show that while the hospital’s ice energy system is one example of a controllable resource, it is not the only thermal storage asset that can be aggregated. The utility’s regulatory filing said that essentially any device that can store British thermal units, “including the shell of a building,” may be able to take part in the program.

Green Mountain Power is not alone in looking for novel ways to tap the potential of distributed energy resources.

In Belgium, a business unit of energy provider Centrica manages a grid-scale battery energy storage project that supports a 32-megawatt virtual power plant and offers primary reserve and frequency regulation services to the country’s grid.

“The pilot aims to demonstrate that storage capacity is all over.”

The project makes use of an 18-MW Tesla Powerpack battery along with a mix of demand response assets from a gaggle of commercial and industrial consumers. Software provided by Centrica manages and controls the energy consumption of equipment at the customer sites. As it aggregates load, the energy manager uses the virtual power plant it has created to offer ancillary services to the grid operator. Centrica claims the arrangement can yield a revenue stream for the battery that is around 1.5 times higher than if it was monetized on a standalone basis.

In the Green Mountain Power pilot program, the utility is working with Efficiency Vermont, which formed in 2000 as one of the country’s first energy efficiency utilities. As a first step, Efficiency Vermont visits a customer’s site to perform metering and verification as well as to collect baseline operating data on flexible assets. Then, Green Mountain Power and Efficiency Vermont will huddle to decide on control options to how best to automate responses for the assets.

During the pilot period, Green Mountain Power will send curtailment notices by 3 pm the day before it expects peak demand to occur. Those curtailment periods will typically last 3 or 4 hours and likely will occur after 4 pm (that’s when summertime air conditioning loads are at their highest.) 

During the curtailment period, participating companies would agree to cut their targeted electric loads either using automated controls or by manual setbacks. Efficiency Vermont will measure and verify the performance to arrive at a kilowatt reduction estimate.

Monetary credits are based on how well the participating facility trimmed its flexible load. If a participant cut its load by 100 kW to meet a forecasted peak demand day, then a US $660 credit would appear on the customer’s bill. Perfect performance in 2019 (in other words, reducing demand by 100 kW in 12 out of 12 monthly peaks) would earn a credit of $7,920.

Green Mountain Power's pilot for now focuses only on a single day per month for peak demand. In reality, peak demand occurs more frequently and could be managed and compensated on an intra-day basis or even more frequently. For example, demand from a variety of source could be bundled together and used as a resource to meet so-called “ancillary services,” which are designed to support grid functions to ensure reliability.

In addition to the hospital, Green Mountain Power hopes to attract another nine facilities to the pilot program, which extends into mid-2020. Monder said that targeted facilities include a lumber mill kiln, a government office building, a digester at a wastewater treatment facility, and a compressor to make ice. That’s ice for skating, at least for now, and not for cool drinks.

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