New Gas-fired Heat Pump
Technologies Help Chill
Greenhouse Effect

By Bill Cabage

"Shut the door! Are you trying to warm the world?"
Everyone's been fussed at for lingering too long in a doorway. In a way, Mom may have been right all along. As we warm and cool the interiors of our buildings, there's a risk we might warm our climate. It all has to do with our use of fossil fuels as an energy source.
Air conditioning and space heating account for 46% of the energy used in U.S. buildings. Air conditioning is the single leading cause of peak electric power demands. Much of this building energy comes from combustion of fossil fuels, a major source of carbon dioxide—a greenhouse gas. Air conditioning and space heating are also sources of ozone-depleting chlorofluorocarbons (CFCs), which are also greenhouse gases. They are called greenhouse gases because they prevent heat energy reflected from the earth's surface from escaping the atmosphere. In theory, that trapped heat could cause global warming, which many scientists believe is already occurring.
Advances in energy conversion technologies offer a solution to this problem. DOE, ORNL, and its industrial partners are focusing on an advanced heat pump technology that would eliminate CFCs and use 50% less energy to heat and cool buildings. Such an energy savings would avert an increase in carbon dioxide and other emissions that would occur from increased power generation by fossil-fueled power plants.

DOE's Office of Building Technologies has selected a technology first patented in 1913 and developed further under DOE sponsorship and ORNL oversight in the 1980s. Called the generator absorber heat-exchange cycle, this absorption heat pump is powered by electricity and natural gas. It does not use a compressor or ozone-depleting CFC refrigerants. Instead, it uses environmentally safe refrigerants that release heat energy when mixed, increasing its thermal efficiency.

Getting cool air by heating something over a flame is no new trick. The principles of heat absorption have been known for a couple of centuries. Although heat pumps have been in use for decades, making efficient ones has been a continuing technical challenge. The single-effect absorption heat pump—consisting of a single loop containing a generator, compressor, evaporator, and condenser—works fine, except it's expensive considering the output relative to the energy going in.

When the energy crises of the 1970s had Americans scrambling to find ways to heat and cool their homes more efficiently, natural gas—cheap and abundant—took on new promise as a fuel. ORNL's Thermally Activated Heat Pump Program set out in 1981 to find ways to maximize the efficiency of gas-fired designs through the Advanced Absorption Cycle Program, which was initially planned by Bob DeVault of the Energy Division.

"For residential use, the generator-absorber heat exchange, or GAX, heat pump has been DOE's number one priority," DeVault says. In the gas-fired GAX, a chemical process substitutes for the motor-driven compressor. The GAX has its advantages: Instead of using ozone-depleting refrigerants common to electric systems, the gas-fired systems use environmentally benign natural refrigerants such as ammonia and plain old water. You can reverse the gas heat pump output to provide air-conditioning: Cooling with gas in the summer would reduce big-city brownout threats during heat waves.

The GAX heat pump's goal is to make heat generated in its process productive. "GAX is more than an acronym; the X is also symbolic of the actual heat exchange process involved," DeVault says. Heat from the absorber is recycled to the generator tank, increasing its efficiency. However, this heat-recycling process is difficult to achieve because the gravity-driven chemical processes are "upside down" from the ideal process needed to enable satisfactory exchange of the heat from the absorber to the generator. "The X in GAX" says DeVault, "is symbolic of inverting the thermal gradient—defying gravity, so to speak."

DeVault issued the first request for proposal for what became GAX in 1982. The system, largely developed by a small business firm, Phillips Engineering, under subcontract to ORNL, was licensed to Carrier Corporation in 1993. According to subcontract manager Patti Adcock, the cost-sharing in the project has been substantial. The GAX, she says, should be making an impact on the residential market in the next few years, primarily in northern markets because its greatest efficiency gain is in heating. A GAX unit is about the size of a conventional heat pump.

A new area, barely out of the dream stage, is called "high cool." It's the next step beyond the GAX for residential and small commercial use. The high-cool goal is a 30% gain over the GAX in cooling. DeVault explained the quest for even greater efficiency: "The GAX is very efficient at heating but only average at cooling, which makes it most attractive to northern markets. In light of that, however, a GAX could heat and cool a house for what a gas furnace operates at now just to heat the house." High cool will build on these efficiency gains.

The GAX technology is aimed at residences. Larger buildings are often cooled with systems called chillers. For the big commercial chillers, which chill water that in turn cools the building, the triple effect appears to be the wave of the future. Single- and double-effect chillers have been sold for years. Double-effect chillers add a second generator and condenser that recycles some of the process heat, increasing efficiency.

"The next big step is the triple-effect chiller, which takes efficiency one stage further," DeVault says. Although the increased sophistication and complexity of the triple-effect system will require a cost premium of 25% or more (competitive pricing is also a goal of these programs), the triple-effect chiller's goal is a 40 to 50% gain in efficiency. "Because a triple-effect system runs at higher temperatures, it requires new chemistry and new materials to resist the corrosion that accompanies high temperatures. Many of the technologies to deal with those challenges have been invented at ORNL."

All of the large U.S. heating and air conditioning companies are participating in the national partnership to develop the triple-effect chiller. One triple-effect technology invented at ORNL has been licensed to the Trane Company for marketing and development. A second, different ORNL technology is being developed by York International under a cost-shared subcontract with DOE.

The Efficiency and Renewables Research Section in ORNL's Energy Division has been developing the required new technologies in step with these projects. An absorption simulation computer modeling program will take much of the time and tedium out of modeling new heating and cooling schemes. "Before, you had to write them out from scratch," DeVault says. "Now you can use this model to change a system or operating condition by pulling equations out of a data bank. It also features a graphical interface." Gary Grossman initially wrote the program and Steve Fischer is continuing the work. "Our sponsor is really proud of it, and it's not even finished yet."

Abdi Zaltash and Delmar Fraysier, both of Energy Division, are investigating advanced absorption fluids that work better than water with ammonia or lithium bromide. Much of the magic in cooling with gas lies in better refrigerant and absorbent combinations. "One issue for large commercial chillers is developing additives that accelerate heat and mass transfer," DeVault says. "Also, with the triple effect, corrosion caused by the high temperatures is a factor."

Bill Miller of the Energy Division has investigated lithium bromide and water falling-film absorption systems for several years. His original research, which featured a vertical column, concentrated on quantifying heat and mass transfer that occurred in a falling-film absorber. Although falling-film absorption systems have been used for decades, most designs ignored mass transfer—data for heat- and mass-transfer modeling were not available. The Gas Research Institute, the sponsor, was impressed enough with ORNL's work to fund studies toward industrial applications—double- and triple-effect chillers. As a result, the tall vertical absorption column, designed to be compact enough for residential use, took on a more conventional horizontal layout.

That work has been successful; Jerry Atchley and Miller have characterized 12 different tube surfaces for a falling-film absorber system. Two of the surfaces, donated by Wolverine Tube, Inc., improve performance comparable to alcohol additives. "Alcohol in small amounts will double performance, but it is difficult to control in these triple-effect, high-temperature systems," Miller says. "Advanced surfaces would eliminate design problems with alcohol and would improve system reliability."

Why has the Thermally Activated Heat Pump Program become so heavily involved with the HVAC industry? As DeVault explains, one reason is the Energy Policy Act of 1992, which authorizes labs to work with industry and requires programs to be cost-shared, "which industry has been willing to do." Working with industry adds the aspects of demonstration and commercialization to R&D.

Another reason could be global competitiveness. Japanese makers now dominate the world market for large commercial absorption chillers. Although the double-effect chillers were pioneered in the United States, the U.S. makers were slow to manufacture them, and now most of the double-effect chillers made by U.S. companies are licensed Japanese technologies. The triple-effect chiller could give the United States a jump in the global market.

The environmental benefits are important. More efficiency means reduced energy consumption and related emissions. Natural gas is our "cleanest" fossil fuel, and instead of using ozone-depleting refrigerants, absorption chillers use natural refrigerants—ammonia or water. In most cases, especially compared with natural gas furnaces or coal-fired electric generating plants, they will substantially reduce carbon dioxide and nitrous oxide emissions to the atmosphere.

Defying Gravity: How the GAX works

A flame from the natural gas burner heats a sealed pot containing a mixture of refrigerant and absorbent solution such as ammonia and water. The refrigerant is boiled out. Because the refrigerant—the ammonia—is in an enclosed chamber, heating also raises its pressure. The high-pressure ammonia vapor is then condensed, extracting heat from the refrigerant. The condensed refrigerant travels to the low-pressure evaporator, where the liquid refrigerant picks heat up from the environment—the cooling effect—and is turned once again into vapor, except now at low pressure and temperature. At the same time, the absorbent (water) from the generator, after the refrigerant is boiled out, travels to another heat exchanger called the absorber, which is at low pressure. The refrigerant vapor from the evaporator is next recombined with the water in the absorber. This recombining of the ammonia refrigerant and the water absorbent involves a chemical reaction that produces heat. This heat is removed from the absorber to increase GAX's thermal efficiency, and the now cool low-pressure mixture is pumped back to the generator, completing the process.

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