Cooling From the Sun?
Photograph by Tim Wimborne, Reuters
For years, the energy world has puzzled over how to harness the sun to make rooms cool.
That may sound counterintuitive to those who think primarily of the uncomfortable heat beating down on roofs and windows as the summer sun reaches its apex. But to those who ponder the sun's tremendous energy, there's a perfect symmetry in the idea of solar cooling. The power of the sun could be used just when it's needed most.
Solar cooling is one of an array of ideas for greener air-conditioning by making better use of the forces of nature. Air-conditioning has transformed summertime living in the developed world, but at a tremendous energy cost. Invented in 1902 by Willis Haviland Carrier, early air-conditioning cooled air by causing it to pass through refrigerant-cooled coils. Conventional air-conditioning is much the same today, guzzling juice from the electric grid to compress gaseous refrigerant back to liquid state in each cooling cycle.
Strain on electricity systems reaches critical mass due to summer AC demand. Heating homes in winter also requires a lot of energy, but there are many warming options—natural gas, oil, wood. Conventional cooling relies on electricity; as a result, peak summer power demand in the United States, where more than 80 percent of households have air-conditioning, is more than 20 percent higher than at the winter high point. (Cooling accounts for 8 percent of U.S. household energy use.) Power companies must have "peaker" plants on hand for the hottest summer afternoons: typically old, inefficient fossil fuel facilities. The hotter it gets, the hotter they run. Talk about counterintuitive.
Although AC traditionally has not been as pervasive in Europe, demand for cooling in the EU is growing, especially in the wake of the devastating 2003 heat wave blamed for 35,000 to 52,000 deaths. So Europe has done some of the most detailed research on the the potential of solar cooling.
(Related: "Heat Wave Due to ‘Exceptionally Strong' Air Mass")
Several companies have recently introduced air conditioners packaged with solar photovoltaic panels, like those being installed on a Sydney, Australia rooftop above.
LG Electronics' solar-hybrid cooling system and Lennox SunSource solar-assisted heating and cooling are two examples. But PV linked to a traditional air conditioner isn't a complete solution. Typical residential central air conditioners draw 2,000 to 5,000 watts of power. The LG system's solar panel can produce only 70 watts of that power, meaning it cuts power demand only slightly.
Lennox says that its system, which includes a 190-watt panel, can slash heating and cooling bills by half. It makes use of its highest efficiency models, the fact that a number of solar panels can be added to the system, and that customers get real-time data on their energy use as part of the package.
But researchers think there are greater advances ahead for cooling with less energy.
(Related: "Quiz: What You Don't Know About Air Conditioning")
—Marianne Lavelle, with Alexandra Arkin and Lauren Biron of Medill News Service
Published August 3, 2011
Photograph courtesy Thermax
This shiny tangle of pipes and ducts at a facility of the power company Industrielle Werke Basel in Basel, Switzerland, is an air-conditioning system that is actually driven by heat. Likewise, heat actually helps to cool banks, hotels, and offices in the Adelgade district of Copenhagen, a T-Mobile data center in Munich and Fumincino Airport in Rome.
The company that engineered all of these systems, Thermax of Pune, India, markets them as one of its "sustainable solutions" for today's environmental concerns, but the technology—absorption chilling—has been in commercial use since the 1920s. Like standard air conditioners, absorption chillers rely on a refrigerant with a low boiling point. When the refrigerant evaporates, it removes heat from the air. Standard air conditioners then change the refrigerant gas back to liquid using an electric compressor. But absorption chillers rely on thermal compression to restart the cycle; they need only heat—no moving parts—to drive the operation.
Absorption chillers are pricey and have what is known as a low performance coefficient; they don't provide a great deal of cooling output for the heat input they need. But absorption chillers turn out to be an efficient solution when they are co-located in facilities that already generate excess heat—as was the case in Thermax's Basel, Copenhagen, Munich, and Rome installations. The systems also don't use hydrofluorocarbon refrigerants, which are potent greenhouse gases, relying instead on water and lithium bromide or ammonia.
(Related: "2010 to Be One of the Hottest Years on Record")
Published August 3, 2011
Dry Air Is Better Air
Photograph courtesy Patrick H. Corkery, National Renewable Energy Laboratory
At the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) in Golden, Colorado, senior mechanical engineer Eric Kozubal shows a part from a system that demonstrates the fundamental truth behind the old saying, "It's not the heat, it's the humidity."
He and his colleagues at NREL are working on a new generation of air-conditioning technology called dessicant-enhanced evaporative air-conditioning, or DEVap. It could cut the energy needed for cooling by 40 percent or more in humid climates, and by 80 to 90 percent in areas like the U.S. desert Southwest, where the air is already dry, Kozubal says. That greatly reduces demand on the electric grid; the only electricity the system needs is for the fans to move the air.
The secret is a liquid dessicant—essentially, a very strong salt solution—to pull moisture from the air using very little energy. (In contrast, electric dehumidifiers are typically very energy-intensive.)
Once the air is dried out, it can be chilled through evaporative cooling—the simple principle that water cools the surrounding air significantly when it turns from liquid into vapor.
Some households in arid climates already use so-called swamp coolers, but they are difficult to maintain as care must be taken that they don't develop mold. But NREL's model would work in humid as well as dry climates. And mold doesn't develop because in this indirect evaporative system the air stream stays dry, with the wet side of the system sealed off and used only as a "heat-dumping mechanism," in Kozubal's words.
Such an approach could truly revolutionize air-conditioning. Because of its lower energy use, and the fact that it is thermally driven—not mechanically driven—the idea of running the system on solar power is much more viable than with conventional high-wattage AC. "Dessicants are really made for solar cooling," says Kozubal. Also, being able to control humidity in such an energy-efficient manner would allow homeowners to strive for much tighter and well-insulated structures without worrying about mold and "sick building" problems. "If we need to push the envelope in building efficiency, we really have to go to a different air-conditioning technology," says Kozubal.
After the successful testing of their small prototype DEVap system, the NREL scientists are now working on building a larger scale prototype to demonstrate the commercial viability of the approach.
(Related: "Seeking to Cool Air Conditioning Costs")
Published August 3, 2011
A Freeze on Energy Demand
Photograph courtesy Ice Energy
Put summer air-conditioning demand on ice—that's the approach of a Windsor, Colorado, company that is working with power companies from California to Ontario.
Ice Energy has developed thermal storage tanks that attach to the standard rooftop air-conditioning systems on commercial buildings. At night, when temperatures cool and demand on the electricity system is low, the units, called Ice Bears, draw grid power to freeze 450 gallons (1,703 liters) of water. Ice Bear's giant block of ice (shown amid the coils above) works as a battery to store that cheap, efficient nighttime energy.
During the day, when temperatures and demands on the electricity grid rise, the Ice Bear takes over the work of the electric motor-driven compressor in the building's AC unit. Instead of using electricity to reject heat by compressing and condensing refrigerant gas back into its liquid state, the Ice Bear does the same job with ice. (Here's Ice Bear's video on how it works.) Furthermore, because the heat inside the building is being moved into a tank of ice that was generated during the relatively cooler evening hours, the system does not have to work as hard as an AC unit compressor exposed to the noon sun, and that can save energy. For a five-ton compressor, Ice Energy guarantees six hours of work from the Ice Bear. "It's a big animal. It's a big battery," says Brian Parsonnet, vice president, chief technology officer and a founder of Ice Energy.
Because utilities can avoid the need to run inefficient peaking plants during the day, and instead use Ice Bears to shift AC demand to run off more efficient power plants at night, the systems save 30 to 40 percent of the generator source fuel. "The net gain to the utility is dramatic," says Parsonnet.
By using Ice Bear units to permanently shift AC unit energy, utilities can avoid or defer capital expenditures. At about $1.5 million per megawatt installed, Ice Bear units cost about the same as conventional generation and transmission today, Ice Energy says.
In Redding, California, north of Sacramento, new U.S. Social Security Administration offices opened last fall with 11 Ice Bear units on the roof. Each is paired with a five-ton, high-efficiency Trane AC unit, the first that the giant cooling company shipped "ice-ready" from the factory.
Glendale, California, just outside Los Angeles, used a portion of its economic stimulus funding to replace 80 aging, inefficient HVAC units on its city buildings with new high-efficiency units and Ice Bears. The city estimates that average annual energy consumption has dropped more than 386,000 kilowatt-hours per site in the initial phase of the project.
In eastern Ontario, Canada, the utilities Toronto-Hydro and Veridian Connections are installing 12 Ice Bear units in a pilot project to reduce peak demand. The project is funded by a grant from the Ontario Power Authority's Conservation Fund program.
(Related: "Earth at Farthest Distance From Sun—Why the Heat Wave?")
Published August 3, 2011
Photograph courtesy Harvard University
A cooling method used by the ancient Romans—circulation of cold water—has found a modern home at Harvard University. The hydronic air-conditioning system used at Harvard's operational services facility at 46 Blackstone Street in Cambridge, Massachusetts, is one of the energy-efficiency innovations that has helped win the building the U.S. Green Building Council's top-ranked Platinum LEED status.
"Folks who come to this building always comment on how quiet this building is," said Jeffrey Smith, director of facilities maintenance operations at Harvard. "They're not even sure why—but it's the absence of mechanical noise."
Hydronic cooling works through convection. Chilled water (typically between 45° and 50°F (7.2° to 10°C) circulates through pipes affixed to walls or incorporated into ceilings. Air flows past the tubes, sinking down the walls as it cools. A tray, usually formed by the casing around the pipes, catches condensation and returns it to the water source. The Harvard building relies on 170 hydronic valances to cool and heat the 40,000-square-foot office space.
The system is quiet because it needs no fan to blow air past the pipes. According to the U.S. Department of Energy's Lawrence Berkeley National Laboratory, hydronic systems can fit in 20 percent of the space required for a traditional ducted air-conditioning system.
A system that can be affixed to a wall, such as that made by Edwards Valance, will save $183 per unit per year compared with fan coil systems, according to the company's website. Installation can cost roughly $2.50 per square foot, but depends on the type of building, contractors' fees, and location, according to Edwards Valance spokeswoman Pat Colvin.
"Initially they might be a little bit more than a standard fan coil" or the space units known as Packaged Terminal Air Conditioning systems, she said. "But in actuality that money will be returned to you in three years."
Published August 3, 2011
Air-Conditioning Goes Underground
Photograph by M. Timothy O'Keefe, Alamy
The earliest home of America's first president has tapped into the Earth for cooling.
In June, the propane-based heating and cooling system at the George Washington Birthplace National Monument (above), known formerly as Popes Creek Plantation near Fredericksburg, Virginia, was replaced with a geothermal system. In addition to replacing fossil fuel with a renewable source—the constant temperature of the Earth, the system is silent, which is important to maintaining the building's atmosphere.
So far it has been “a huge success,” says Dick Dretsch, chief of cultural natural resources and maintenance at the park. “It works terrifically.”
Geothermal heat pumps circulate water or other liquids in a continuous loop through pipes buried underground at depths of 10 to 300 feet (3 to 91 meters), according to the Geothermal Energy Association. Underground, it's 55°F (12.7°C), regardless of the season. During warm weather, heat is sucked out of the building and circulated underground to be cooled before returning to the building. During cold weather, the underground temperature is warmer than the outdoors, so that relative warmth is drawn out of the Earth and distributed through the house to cut energy needed for heating.
According to the U.S. Environmental Protection Agency, the pumps use 25 to 50 percent less electricity than conventional heating or cooling systems, and can reduce energy consumption and carbon emissions as well. The EPA says geothermal heat pumps also are very effective in humid areas because they can keep relative indoor humidity at about 50 percent.
Payback periods vary. Doug Dougherty, president and chief executive of the Geothermal Exchange Organization, just had an $18,000 geothermal system installed in his own home in Springfield, Illinois. Although he says that's twice the cost of traditional air-conditioning, he expects it will pay for itself in four to five years, cutting his annual energy bill by two-thirds.
Published August 3, 2011
Drawing In the Cool
Photograph by Justin Kase, Alamy
Ordinary fans make you feel cooler by moving air around, but they don’t cool the air. Whole-house fans operate on an entirely different concept. Installed in the attic or on the ceiling between the attic and living space, whole-house fans are designed to pull large volumes of warm, stale air up and out of the house while drawing fresh, cooler air in from outside.
The best way to use a whole-house fan, according to Quiet Cool Manufacturing, which makes Quiet Cool Whole House Fans, is by turning off the air-conditioning in the evening when the outside temperature is below 85°F (29°C), and then turning on the fan.
Whole-house fans cost between $430 and $1,550, according to R.E. Williams Contractor, which distributes fans.
Savings and payback period depend on the house and the climate, says Mary Driscoll, an R.E. Williams representative. "(A fan) definitely saves you money," she says. "Our fans are more energy-efficient than a traditional air-conditioning unit."
Noise can be an issue with whole house fans—especially, as the EPA notes, if they are improperly installed. In general, according to the EPA, a large-capacity fan running at a low speed is quieter than a small fan running at a high speed. The agency recommends installing all fans with rubber or felt gaskets to lessen the noise, and setting multi-speed fans to lower speeds.
R.E. Williams's fans have "noiseable" ratings, Driscoll said.
Of course, whole-house fans are effective only when the outdoor temperature is lower than the indoor temperature. But in climates where the temperature drops at night, a whole-house fan can save 50 to 90 percent of the cost of air-conditioning.
Published August 3, 2011
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