By: Bruce Nagy
In the never-ending game of making buildings energy efficient, minimizing cooling load is among the biggest challenges. Modern cooling systems tend to be powered by electricity and more electrification is leading many to re-visit the age-old problem of peak loads and peak electricity charges. Ice-based thermal energy storage (TES) is not new, but it is enjoying resurging interest because it offers efficient time-of-use shifts of electrical consumption along.
When mechanical engineer Ed Fowler of WalterFedy, an integrated engineering, architecture and construction firm in Kitchener, Ont., designed the HVAC system for a new 36,000 square foot (3252 sq. metre) building at the University of Waterloo, Waterloo, Ont., he reduced electricity charges and peak load by specifying a 52-ton chiller, rather than a 90-ton chiller.
To make it work he added a Calmac TES system consisting of two 120-ton/hour ice making tanks (420 kWh), a completion module and some pumps. The smaller chiller saved more than enough to pay for the TES equipment.
Ontario’s Save On Energy program provided some of the funding, which meant that savings have to be monitored and verified. WalterFedy’s Colin Umbach analyzed system performance in August 2015 when outdoor temperatures exceeded design conditions. Ninety-seven percent of the cooling system power use was successfully shifted to off-peak hours from 7 p.m. to 7 a.m., he reported. Power usage stayed under 6.4 kW during on-peak hours (11 a.m. to 5 p.m.). The school operates twelve months of the year (Cooperative education, ESL, summer school) and serves thousands of students.
How it works
The chiller sends -6C (22ºF) glycol into the tanks at night, where it helps make ice for up to 10 hours. As it melts during the day, glycol circulates from ice to air handler coils. Fans then distribute the cooling though ductwork, while glycol absorbs heat and returns to the ice for more cooling. The Trane chiller controls manage the system. “Cooler glycol produced by the ice means lower supply air temperatures, so fans and ductwork are smaller,” said Fowler. “Also, the chiller is not maxing out, so it requires less maintenance.”
“Savings depend on the rates you are paying,” added Calmac marketing vice president Paul Valenta. “In Ontario you’re paying 41 percent more to use energy in the day than at night. And you could be hit with peak usage charges. The biggest contributor to peak demand is usually AC, even in northern climates.” Customer payback for new construction is typically under three years and, with most old chiller replacements, less than seven.
Peak demand in remote areas
Peak demand charges exist, theoretically, because of the need for peak demand generating stations. Whereas baseload stations run continuously, “peaker plants” are fired up a small percentage of the time when demand outstrips baseload capacity, usually on hot summer afternoons when air conditioning everywhere is running full tilt. According to the International Energy Agency, about 30 gigawatts of peak capacity are added each year around the world to keep pace with population growth and increasing demand.
Because blackouts are economically and politically untenable in the western world, expensive peaker plants are thrown up quickly, costing about $1.8M U.S. per megawatt, usually fuelled by natural gas. In addition, delivering more power is problematic for utilities, because “we’re trying to operate a 21st century economy using a 20th century grid that we can’t afford to upgrade,” said Valenta.
The problem becomes more pronounced in remote areas, where miles of old power lines are serving communities that are gradually increasing their loads.
Remote communities desperately want new businesses and new developments that can bring jobs, consumer traffic and general economic stimulus. But with bigger loads either the new entity or the local utility has to pay for generation and transmission upgrades.
Demand side solutions
Boothbay is a beautiful Maine resort area with a population of just over 2,000 residents, competing with Long Island, Cape Cod, Martha’s Vineyard and so on. More restaurants, bars, hotels and increasing tourism had driven up the peak load, outstripping the local utility’s infrastructure capacity.
Rather than do the expensive transmission upgrade, Central Maine Power contracted with a consultant to install about 30 Ice Bear thermal energy storage units along with other system elements at a cost of about $6 million, saving about $12 million.
“In Boothbay they wanted the new businesses, but they were going to have to do an $18-million transmission line upgrade,” said Ice Energy CEO Mike Hopkins. “We worked with the customers, consultants and the utility. In the end it was solved with a combination of Ice Bears, solar, chemical batteries, generators and demand response programs.”
An Ice Bear is a smaller thermal energy storage system, providing 20 to 30 tons of ice for cooling by freezing 1703 litres (450 gallons) of water overnight. The first Ice Bear, the Model 30, worked in tandem with outdoor condensing units. The new Model 20 combines both functions. During peak hours, the Model 20 delivers up to four hours of cooling, using just five percent of the power that is usually needed.
Meanwhile in California, the City of Redding is trying to avoid building a peaker plant. “Instead they buy more TES each year,” says Hopkins. “I don’t think they’re ever going to build that plant. It’s cheaper for everyone to just make ice at night. Ice Bear storage is about half the cost of a lithium ion battery.”
Residential ice systems
A residential scale product that will be launched soon combines ice energy with a heat pump, reports Ed Lohrenz of GeoOptimize Inc. in Winnipeg. He says this could mean a 2.5-ton air conditioning unit could provide about four tons of cooling.
Lorenz says that ice-based TES meets a utility’s need more cost effectively than any form of electricity storage currently available and does it on the demand side, eliminating transmission challenges. Also, because there’s little or no cycle degradation, like with a battery, equipment life is longer. And the system extends the life of condensing equipment too.
He worked on an IKEA store in Centennial, Colorado that combined 350 tons of cooling equipment with TES to provide 530 tons of cooling. Again, the TES investment was erased by the capacity savings. In this case it was a geothermal heat pump system that cost about $280,000 less than it would have without TES.
“In some designs, when the heat pumps are producing ice at night, you can also heat the building and/or domestic hot water with recaptured heat energy from the ice-making,” said Lohrenz.
Bruce Nagy is a Toronto-based freelance writer that reports on green technologies and solutions.