Air-conditioning the Desert
Boosting heat pump efficiency reduces power demand and consumer cost
Over the last decade the population of Las Vegas, Nevada, ballooned by nearly a third. At the peak of the surge, 5000 families were moving to town every month. As it turns out, the population boom has been a bust for local electric power plants that rely on scarce water resources for their cooling towers in this arid part of the country. To help reduce the demand for electricity, several local utility companies encouraged customers to supplement their electricity usage with natural gas appliances. One of these companies, Southwest Gas, went so far as to design its own natural gas-powered heat pump. Gas heat pumps are a particularly attractive option in the Las Vegas area because they not only use 85 percent less electricity than electric units, but they also enable customers to avoid premium charges for electricity use during peak-demand hours.
When Ed Vineyard's group in ORNL's Energy and Transportation Science Division heard about Southwest Gas's plan to lighten the load on the Las Vegas electric grid, the unusual nature of the project captured their attention. "Utilities aren't normally in the business of designing heat pumps," Vineyard says, "but Southwest Gas had a prototype for small commercial buildings they wanted to have evaluated, so we asked if they wanted us to do some testing."
An efficient solution
In Las Vegas in the summer, a heat pump's primary job is the same as that of an air conditioner: to pull warm air out of a building and through a heat exchanger, cool it down and pump it back inside. The more efficiently this happens, the less it costs to do it. That's where ORNL's test facilities come in. ORNL is nationally respected for its expertise in testing and evaluating heat pumps and other HVAC equipment, and laboratory researchers have worked with several of the major manufacturers to evaluate and improve the efficiency of their units. The laboratory tests heat pumps in two large environmental chambers. The units are evaluated at industry standard temperature and humidity levels for both heating and cooling. By measuring the temperature of the room while a heat pump is running, researchers can calculate the heating and cooling capacity and efficiency of the unit.
When Southwest Gas sent its initial prototype to Oak Ridge, Vineyard's group ran a battery of efficiency tests on it and found that that its performance has plenty of room for improvement. "As it turns out," Vineyard recalls, "the heat exchangers weren't functioning efficiently." To do a thorough study of the problem, ORNL and Southwest Gas sought and received funding from the Department of Energy to develop a more efficient prototype of a gas heat pump.
During the course of the collaboration, the laboratory and Southwest Gas shared design responsibilities. Southwest Gas built the prototypes, and ORNL created the computer models and did the testing and analysis of the unit.
Through several rounds of testing and analysis, Vineyard's group paid particular attention to the efficiency of the unit's heat exchanger, making a series of measurements using sensitive thermal cameras. These studies found that uneven airflow across the heat exchanger caused a 10- to 15-degree temperature difference between the warmest and coolest parts of the device.
Building a better heat pump
As the U.S. and other nations approach this technological crossroads, ORNL is finding more opportunities to apply its experience with nuclear technologies in several key areas.
Normally heat pump airflow problems can be addressed by rearranging components inside the unit, allowing more room for the air to circulate. However, because this heat pump was a rooftop unit, both size and weight were concerns. "We wanted to have an efficient airflow path and the most compact system possible," Vineyard recalls. "Generally, those parameters are at odds with one another." Eventually, using fluid dynamics modeling to build computer simulations of the air flow paths within the unit, the research team developed a configuration that greatly increased the efficiency of the heat exchanger within the available space. "When we do fluid dynamics modeling," he explains, "we build the model in pieces. For example, we model the air flow through the heat exchangers, through the radiator for the gas engine and through the exhaust fans separately. Then we put them together to build a model of airflow through the entire unit." Several computer models and prototypes later, the result of this process was a unit in which the temperature variations across the heat exchanger were reduced to 1 or 2 degrees Fahrenheit.
Vineyard says that the improved prototype developed by Southwest Gas and ORNL is about 25% more efficient than the original unit. The cost savings provided by this increase in efficiency suggest the unit could pay for itself in about 3 years, depending on the price of natural gas and electricity.
Not long after this highly efficient unit was completed, the technology behind it was licensed and brought to market by Intellichoice Energy under the name NextAire. The design and performance of the NextAire Packaged Gas Heat Pump was impressive enough to earn ORNL, Southwest Gas and Intellichoice Energy a 2011 R&D 100 Award from R&D Magazine. R&D 100 Awards honor the 100 most outstanding advances in technology for the year and are chosen by an expert panel of independent judges and the magazine's editors.
The successful conclusion of the laboratory's original collaboration with Southwest Gas won't be the end of the partnership between the organizations. The research team is currently working on improving the overall efficiency of a smaller, residential gas heat pump by incorporating water heating and using engine heat to improve overall system efficiency. In addition to heating and cooling, plans for the residential prototype include adding an alternator and storage batteries to enable the system to generate and store electricity during off-peak hours.
Vineyard notes that while working with a gas heat pump enables designers to build in a lot of extra functionality, the primary goal of the collaboration is still to design a system that offers a significant improvement over the state of the art and is competitive in the commercial heat pump market. "That's a big challenge," he says. "Once that's taken care of, we'll think about adding features to it."—Jim Pearce