In the spring of 2009, Harvard Real Estate Services (HRES) installed solar thermal collectors at two of our residential properties - 472-474 Broadway and 20-20a Prescott Street. Heat collected by the panels is used to supplement the boilers in heating the buildings’ hot water. In the first two months of operation, the two systems produced over 45,000 kBtu of thermal energy, in spite of useasonably rainy weather and overcast skies.
Over time, the thermal collectors are expected to reduce the energy currently used by boilers to heat water by a total of 30-40%. All told, the 14 solar thermal collectors save up to 28,660 pounds of CO2 per year (13 MTCDE) and over 2,300 therms of natural gas, while providing supplemental hot water for 55 apartments. This energy reduction is in support of Harvard’s goal to reduce greenhouse gas consumption 30% by 2016.
How it Works
The solar thermal system, made by S.O.L.I.D. USA, has been tested and used in countries with colder climates than ours and has proven to be a success.
The entire system involves the sun and three liquids, each transferring its heat to the next in line. Much like an electric stove (electricity heats a coil, the coil heats the pan, and the pan fries the egg that we eat): here the sun heats a delivery liquid, which heats stored water, which finally heats the water that we use from the tap.
First, a water-glycol mixture courses through a series of panels on each roof, collecting and storing heat as it goes – getting as warm as 390 degrees Fahrenheit. The glycol mixture flows from the roof to the basement, where it moves through a plate heat exchanger. There the small amount of very hot glycol mixture is able to heat a large amount of water – 1600 gallons – up to a temperature of 210 F. The glycol mixture returns to the roof to absorb heat again, while the water moves in a closed loop to and from several storage tanks. The storage tanks ensure that heat is available for heating domestic water, even after sunset.
In New England, winter days do not always provide enough sun to bring the domestic hot water to the mandated 118 degrees Fahrenheit, so a backup system was retained. If the stored water isn’t hot enough, the conventional natural gas boiler will kick in and finish the job to ensure a constant water temperature at the tap. The transition from primary to backup systems is so seamless that residents won’t be able to recognize it.
A sophisticated control system monitors set points, flow rates, and other information, and allows remote access via the Internet. Property operations staff will be trained to understand and operate the system.
In a curious twist of fate, the glycol/water pipes from the solar collectors enter the basement through a former coal chute. The room that once stored coal for furnaces, now stores heat from the sun. It has come a long way, from dirty coal to clean, quiet sun powered heat.
The public is invited to track the performance of the system by visiting http://harvard_prescott.heizwerk.at/ and logging in with username "frei" and password "frei".
Thinking Green
HRES is always on the lookout for ways to incorporate sustainability into its capital and operating projects. These two residential buildings have a relatively constant demand for domestic hot water and are tall enough to get unobstructed sun. When a roof replacement project came up, the solar thermal collectors became part of the roof project. The new roofs are a reflective white, which reduce the “heat island” effect caused by large, dark surfaces and have a longer useful life than the previously installed black roofs.
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The entire system involves the sun and three liquids, each transferring its heat to the next in line.