HRES installs solar arrays on buildings

Harvard students can do a lot of things, but hovering five stories in the air is not one of them.

That’s what you’d have to do to see the latest Harvard Real Estate Services (HRES) sustainability project: 14 solar arrays on the rooftops of two old apartment buildings just east of Harvard Yard.

Lined up facing south, the solar collectors will use the power of the sun to make hot water for dishes, showers, and laundry.

Last month, the flat tablelike collectors — each weighing about 750 pounds — were hoisted onto the roofs at 20-20A Prescott St. (where there are 39 apartments) and at 472-474 Broadway (16 apartments). They’re in place now, angled at a fixed 45 degrees and anchored into steel I-beams.

From the sidewalk, you’d have to strain to see just “edges and corners” of the silvery blue solar collectors, said Justin Stratman. He is HRES assistant director of property operations for residential real estate.

No hovering necessary for Stratman, who has a key to the rooftop door at the Prescott Street building. On a recent gray afternoon, he showed the solar array to a visitor. The collectors, shimmering and shining and in a neat line, were warm to the touch.

Inside a protective layer of glass, thin overlapping aluminum fins in each collector gather in the sun’s heat. A pump the size of a coffee cup transfers solar heat to loops of copper tubing.

By the end of May, that tubing will be insulated and primed with a glycol-water mixture designed to circulate hot water.

It’s simple, and has no moving parts except for the pumps. “That’s one of the appeals,” said Stratman. “Just sunlight.”

Solar-heated hot water will loop through a heat exchanger in the basement, get stored in massive basement tanks, and supplement the building’s conventional hot water system.

On the rooftop, water-glycol temperatures can reach 390 degrees Fahrenheit. In the basement tanks, hot water hovers at the boiling point. Shuttled to the domestic water supply, it’s moderated to a workable 118 degrees.

At the faucet, “tenants won’t recognize the difference,” said Bjorn Storz, who is the sustainability program engineer at HRES.

The solar thermal project, operational by the end of May, should supply up to 40 percent of the hot water needs of both buildings. It is also expected to reduce natural gas consumption by the same percentage, and knock up to 6 percent off carbon emissions.

“That’s really the idea — to support the University’s greenhouse gas emissions goals,” said Steven C. Nason, HRES director of residential real estate.

Last year, Harvard pledged to reduce such global warming emissions 30 percent by 2016.

The solar-thermal water systems are made by Solid Energy in Austria, a key European supplier of solar technologies. The company installed solar cooling and hot water systems at the 2008 Summer Olympics in China. Until now, its U.S. projects have all been in sun-rich Arizona and California.

On the rooftop at Prescott Street, the distant Boston skyline looks like a stack of toys. Cambridge is a carpet of rooftops.

Atop the Prescott and Broadway buildings, the rooftops are a brilliant white. (Such “high albedo” — highly reflective — roofs scatter sunlight and keep buildings cooler.)

Both buildings needed new roofs, said Nason, and that opened the way to adding in a solar thermal pilot project.

Performance will be monitored closely for a year, and that will help determine the future of such solar thermal installations. (Sunlight intensity audits have already been done at most of the apartment buildings in the HRES portfolio.)

“These are nice little pilots,” said Nason of the Prescott and Broadway buildings. Both are relatively old — about 80 years — and both are of a modest size, like a lot of HRES properties.

HRES manages about a quarter of all Harvard-owned real estate, including 2,900 apartment units in 71 buildings or complexes.

In the past two years, it opened two new LEED Gold buildings, at 5 Cowperthwaite and 10 Akron streets. Another, at 2 Grant St., was fully renovated in 2008 to LEED Platinum standards.

LEED, a U.S. green building measure, stands for Leadership in Energy and Environmental Design. Its rating system is based on precious metals; ranking first and second are platinum and gold.

But sustainability projects don’t have to involve new buildings or large-scale efforts, said Nason. They can be part of modest investments, like the new roofs at the Prescott and Broadway properties.

“We’re working on the overall portfolio — existing, new, and renovated — to make our buildings more efficient,” he said.

Rooftop solar thermal systems have some technical limitations. Roofs have to be fully exposed to the sun and strong enough to handle the extra weight of the solar arrays.

Inside, buildings have to be roomy enough for mechanicals, including large hot water storage tanks. (At Prescott Street there are four 240-gallon tanks, each the size of a small car.)

Prescott Street is a one-stop history lesson in heating technology. In a few weeks, pipes will carry solar-heated water from the rooftop along the path of an old chimney.

They’ll enter the basement through an old coal chute, and deposit hot water in tanks where coal once stood in heaps. All this will happen a few feet away from the current (and conventional) gas-fired system.

It is, said Nason, “a wonderful coincidence.”