When workers wash their hands at the office or when diners unfold freshly laundered napkins in a restaurant, they’re taking advantage of a modern-day luxury: running hot water. To provide their occupants this amenity while reducing utility costs, building owners can capitalize on a natural energy source: the sun or, more accurately, solar thermal water-heating systems.
The first commercial solar water heater is often credited to Baltimore manufacturer Clarence Kemp, who painted metal water storage tanks dark to absorb the sun’s radiation in 1891. Although he was not the first person to do so, Kemp took the extra step to insulate his rooftop tanks in a glass-framed box to help retain warmth. He called his system the Climax.
Today, the use of solar thermal systems is growing in commercial buildings. In 2011, more than 300,000 square feet of solar collectors were installed, according to the Solar Energy Industries Association. Overall, the solar water heating market grew about 5 percent from 2010 to 2011.
Solar thermal systems typically place solar collectors on a building’s exterior—usually the roof—where the water supply can be heated directly or indirectly by the sun. In direct circulation systems, the potable water is pumped into solar collectors and heated. For indirect circulation systems, which are preferred in four-season climates, an antifreeze fluid such as a mix of water and propylene glycol transfers heat from the collectors to the water system by means of a heat exchanger. In both cases, a conventional water heating system is typically required for backup.
Two types of indirect solar collectors are common in commercial settings. Glazed flat-plate collectors consist of a dark metal or polymer plate, which absorbs solar radiation, topped by a transparent sheet of insulating glass or plastic to retain warmth and slow heat loss. The absorber plate warms up copper pipes filled with the heat-exchange medium that circulates through the collector.
Evacuated-tube collectors use metal pipes insulated within vacuumed-sealed glass tubes that slow heat loss the same way a Thermos does. The pipes warm the fluid by circulating it through the collector. This system converts solar energy into heat more efficiently than flat-plate collectors, and can produce hotter water—up to 350 F, compared to a maximum temperature of about 180 F for a flat-plate collector.
Based on performance, evacuated-tube collectors may seem like the clear winner, but they’re not always the de facto systems. The 367,000-square-foot Gateway Community College in New Haven, Conn., designed by Perkins+Will with mechanical engineering consultant BVH Integrated Services, uses flat-plate collectors. Robert Goodwin, AIA, the project’s design principal, says that the team evaluated which system made the most sense by identifying the hot-water load and its relation to the building type. “For this particular project—a school building—the main demand comes from people washing hands in the restrooms,” he says.
As a result, says Breeze Glazer, an associate and research knowledge manager for sustainable healthcare design at Perkins+Will, “we didn’t need the water to be that hot.” Flat-plate collectors, he says, are also more cost effective and require less maintenance than evacuated tubes.
Gateway’s solar thermal system comprises 40 Viessmann flat-plate solar collectors manifolded together in sets of 10. A closed loop with a 40-percent glycol solution is pumped to two solar hot-water heat exchangers, each with 500 gallons of storage. The solar tanks preheat the domestic hot water, saving more than 4,200 therms annually—about 55,000 pounds of greenhouse gas emissions—over a standalone natural gas–powered system.
Because the college wanted its building to become a showcase of sustainable technologies, the rooftop-mounted system is accessible, Goodwin says. In the classroom, “students can learn about solar thermal systems and how to install and maintain them,” he says. “Then they can go upstairs and see [one] on the roof in action.”
For some projects, owners may prefer concealing the solar collectors. In renovating the six-story Hipolito F. Garcia Federal Building and Courthouse in San Antonio, Texas, design firm Trivers hid both the rooftop solar thermal system, as well as its photovoltaic panels to preserve the aesthetic of the historic 1937 structure. The equipment was placed along an inner ring of the roof around the central lightwell, beyond the pitched tiled roof and pedestrian sightlines.
The 309,550-square-foot project, which earned LEED Platinum certification, uses evacuated tube collectors because of their greater efficiency over flat-plate collectors: between 60 and 80 percent as compared to about 40 percent, says the project’s LEED consultant, Bruce Levitt, executive vice president at William Tao & Associates (WTA). “The more energy we could collect … the more LEED points we could gather,” he says. “Although the higher temperature wasn’t necessary, the higher efficiency was.”
If an existing building is to remain occupied during the integration of a solar thermal system, the design team must phase the construction carefully. At the Garcia building, “we had to keep heat on in the building the entire time, so there were partial heating systems at some phases,” says Joseph Brinkmann, AIA, Trivers’s president. The solar thermal system preheats the water to the extent possible before entering a storage tank that ties into the building’s existing gas-fired heater. If the water surpasses the minimum temperature threshold, the existing system stays off. After the majority of the new system and piping was installed, workers briefly shut down the old system to tie the systems together.
Building-integrated solar thermal systems are another way to harvest the sun’s energy with discretion. The Hybrid Shade, by Los Angeles–based BIO-TECture, performs double duty as a water heater and shading device. “We take advantage of traditional flat-plate collectors,” says Jeff Horowitz, the company’s West Coast sales representative. The indirect closed-loop systems typically use a propylene glycol mix as its heat exchanger. While BIO-TECture offers modular elements, about 80 percent of its business comes from designing custom aluminum shade structures.
The Jonathan Milikowsky Science and Technology Building at the Foote School in New Haven, Conn., features a custom cantilevered trellis that acts as a solar thermal system and shades the building’s west side. Maryann Thompson Architects in Cambridge, Mass., worked with BIO-TECture to design the custom system, which features south-facing trellis blades angled at 45 degrees to capture sunlight.
Currently, solar thermal collectors are more efficient at converting sunlight to heat water than photovoltaic panels are at converting sunlight into electricity. “Solar photovoltaic only approaches at best 15 percent [efficiency],” WTA’s Levitt says. However, building owners can potentially sell excess energy produced by PV panels back to the power grid—an option not available for solar thermal systems.
The falling prices of photovoltaic panels have made another alternate water heating methodology attractive. “With solar hot water, sometimes you don’t have enough sun, sometimes you have too much,” says Tomas Jimenez-Eliaeson, AIA, design director for the architecture firm Little in Charlotte, N.C. “It may be easier to use a photovoltaic array” to generate electricity, which then would power an air-source heat pump to produce hot water, he says.
Little utilized this alternate thermal system when it designed a 26,500-square-foot, net-zero-energy educational facility for the nonprofit organization Learning Lodge at Grandfather Mountain in Linville, N.C. The lodge can send excess electricity back to the power grid for credit, which can help offset the cost of heating water during less sunny months, Jimenez-Eliaeson says.
Nevertheless, Levitt says that solar thermal systems’ greater efficiency over photovoltaics makes them a better choice for most projects, unless they’re in regions with particularly high electricity rates. “It’s a practical renewable technology that could be applied to a wide range of buildings in a cost-effective way.”
Ron Nyren is a freelance architecture and urban design writer based in the San Francisco Bay Area.