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Photo: Hallowell

How Do Heat Pumps Pull Heat or Cool From Air?

The technology in an air source heat pump is similar to the standard kitchen refrigerator. Using a simple refrigeration cycle, refrigerators remove heat from food and drinks and reject it into the kitchen. This is why the coils on the back of a refrigerator will feel warm. This process of moving heat is achieved by taking advantage of the energy stored and released when a refrigerant changes from a liquid to a gas.

Basically, a heat pump can move heat into or out of a home. In the summer, it acts like a standard air conditioner and moves heat from inside the home to the outdoors. In the winter, it does exactly the opposite; it captures heat from the outdoors and moves it into
a home.

But how does it get heat from cold outside air?
Heat is molecular motion in the air. The temperature at which molecular motion stops, also known as Absolute Zero, is -459˚F. So, even at -30˚F, there is plenty of heat in the air to take advantage of.

The properties of the refrigerant used in a heat pump are such that it evaporates and condenses—changes from a gas to a liquid—at much lower temperatures than water. In terms of the refrigeration cycle, this means that these phase changes can be used to store and transport this heat energy into a home.

A heat pump looks just like a central air conditioner, and most of the components are the same. On the inside of the home, there’s an air handler attached to ductwork. On the outside of the home, an outdoor unit houses a fan to draw air through refrigerant coils. Running between the outdoor unit and the air handler is a pair of copper pipes called a “line set.” These are the pipes through which the refrigerant travels between the outdoor unit and the air handler.

Taking Heat From the Ground or the Air
Heat pumps also function as air conditioners and have been around for decades, but the fall-off in performance below 30˚F limited their usage typically to residential applications in moderate climates. With the long track record of home usage, it’s not surprising to find that one of the first industries to take advantage of the Acadia in a large-scale application is residential.

Photo: Hallowell Acadia units atop TD Banknorth offices

United ordered 1800 Acadias as a cornerstone of their plan for energy-efficient, environmentally conscious dwellings. And, because the systems use only electricity to heat and cool, United saves on costly infrastructure construction. Moreover, the electric units allow United the flexibility to install a distributed energy solution to save residents from rising utility rates.

“The question we asked ourselves was: could we go to an all-electric system and avoid the cost of gas infrastructure because the gas company wanted to charge us to replace or add additional gas lines?” recalls Gafford.

When he and United’s engineers looked at their options, they found the standard split system heat pump which involves a heating unit inside a house and a condensing unit on the outside, or the better performance, but more costly extreme: geothermal heat pumps. Geothermal units use the constant temperature of the ground to effectively and efficiently heat and cool an environment. But they require trenching, digging, or drilling, and the site has to be appropriate for a geothermal system. Often, the expense of installing such a system becomes prohibitive.

United contracted for 1,800 systems over four years. “We have 450 already installed,” says Gafford. “All of the residents are sold on the unit and we haven’t had any complaints about the way the system works. It’s quieter, and it has a steady airflow rather than a surge, and all of the operating properties are excellent.”

A Closer Look at the Technology
The breakthrough in efficiency comes from a unique approach, and it’s well worth a closer look. The design starts with a primary compressor identical to ones used in present day heat pumps, but then adds a booster compressor and a sub-cooling economizer. The Hallowell engineering team describes the process as also including “…a means for tapping a portion of the condensed refrigerant liquid leaving the heating condenser and evaporating it within the economizer for the purpose of significantly sub-cooling the still-warm, liquid refrigerant, before it is supplied to the air-source coil evaporator.

“It’s a sub-cooling economizer, and there are no moving parts,” adds Duane Hallowell, president of Hallowell. “It’s brought into the system only during heating performance, and it furthers the sub-cooling of liquid refrigerant going to the outdoor coil, which furthers the heat absorption by lowering the outdoor coil’s temperature and creating a wider gap between the two coils. And that gives more capacity for absorption.”

The Key to Efficiency
In the Acadia’s typical heating cycle, only 50% of the primary displacement level is activated until the outdoor ambient temperature drops to 42˚F. At this temperature, 100% of the primary displacement is now activated. No additional heating capacity can normally be brought online until the outdoor ambient further drops to 30˚F, even if the second step of the indoor thermostat calls for more heat. This design prevents the system from supplying more capacity than is really needed. If it were supplied, it would come about at a low efficiency level, because the condenser would operate at an unnecessarily high pressure, and the evaporator would operate at an unnecessarily low pressure.

According to Hallowell, good performance was always a hallmark of heat pump technology—as long as the outdoor temperature didn’t drop below 30˚F for extended periods. To combat this problem, the Acadia is equipped with Opti-Cycle technology, so that when 30˚F is reached, the Acadia’s booster comes on. Then, if the second step calls, the economizer operation will begin until the temperature drops further to 16˚F. Finally, if the thermostat calls for more heat, the first stage of resistive heat provided in the indoor air handler is also allowed in addition to the booster/economizer.

Commercial Systems Gaining Traction
So, if that kind of heating can save money in a residence, could it work equally as well in a commercial setting? Not surprisingly, Hallowell conditions over 10,000 square feet of office space with ACADIA systems at its facility in Bangor. TD Banknorth, a banking and financial services company headquartered in Portland, ME, is currently installing ACADIA split system equipment in all of their new buildings, along with other equipment designed to reduce carbon footprint and decrease heating and cooling expenses. The first was installed in their Epping, NH branch. Other commercial locations include gas stations, convenience stores, coffee houses, cafes, and restaurants. Both commercial and residential units follow the same installation procedures as standard heat pump systems.

In the near term, Hallowell will be releasing 3-ton and 4-ton rooftop contained systems. These units will offer greater than 14 SEER (Seasonal Energy Efficiency Ratio), and greater than 9.5 HSPF (Heating Seasonal Performance Factor) values. In addition to indoor and outdoor ECM (electronically commutated motor) fans, these units are designed to be very easy to install, service, and operate. They will offer a direct replacement of existing rooftop units, as well as alternatives for spaces where split systems are not a viable option. The new models are expected to debut in 2009.

Larger commercial equipment development is also underway for long-term release. The company is targeting 5-ton, 10-ton-and-larger equipment, paired with communicating intelligent controls for maximizing not only system efficiency, but building efficiency. “Commercial spaces often offer the fastest returns simply because their energy consumption is much greater than those of residential spaces,” explains Hallowell. “In many cases, ROIs have been calculated to be completed in less than five years. Of course, utility rates, considered alternate equipment, and actual energy usage play heavily into this value. Very often, when a business is still in the payment period, the actual savings will be greater than the annual debt service on the equipment. In this case, the business owners will have a higher cash flow while paying the equipment off.”

Gafford agrees, and notes that the Acadia saves roughly 30% of the energy consumption over a standard new system. That’s a critical factor in these homes, since all are a minimum of three bedrooms with average square footage of 1,900 square feet (heated living space). “We need to speak about how to make every resident in the United States more efficient, and that starts with a good heating and cooling system, but the heart and brain of the system has to be the resident,” says Gafford. “At McGuire, we have advanced metering technology, so they can look on the Internet and see how much energy they are using.”

Based on the current usage of the 450 installed units, Gafford expects the savings to generate a payback in five years. “People get hung up on payback, but what they should really be looking at is ‘how much money can I save by decreasing my electric bills,’” he adds. “That’s an easier comparison and makes more sense to people than telling them they have to wait five years before they get their money back.”

The savings on utility bills are also a factor in United’s future plans for distributed energy, because the company will be managing the fort’s family barracks for a period of 50 years, and Gafford says United wants to keep utility costs under control.

“These houses in New Jersey are very good, and, eventually, we’ll look at having a generator backup or distributed power system in the event that electricity rates from the grid become outrageous,” he says. “Now that we have Hallowell products, we can look at making these communities totally self sufficient.”

And finally, there’s a health benefit. “When you’re dealing with all electric systems, you’re not filling your lungs with hydrocarbons,” notes Gafford. “Any kind of system that burns oil or gas creates humidity and puts more contaminants into the air.”