In recent years, a lot of research has been directed at attacking some of the traditional shortcomings associated with gas-powered cooling appliances designed for the lower-capacity end of the cooling market, residential and light commercial. One company in particular, Rocky Research, Boulder City, Nev., reports significant progress in discovering the solutions that will make gas cooling more feasible, and is close to commercializing a 5-ton absorption chiller. A 5-ton absorption heat pump is also in the works.

Gas-fired absorption air-conditioning systems, which were briefly popular in the 1960s, use ammonia as the refrigerant and water as the absorbent. In such systems, ammonia is boiled out of the water then condensed in an outdoor coil. The refrigerant is then expanded and evaporates in the evaporator at low pressure, producing the cooling effect. The ammonia is then reabsorbed into the water.

Ammonia is a much more effective refrigerant than fluorocarbons. In addition, ammonia has no ozone-depletion potential and no global-warming potential, and is not harmful to the atmosphere if released in the environment. Ammonia/water absorption systems also have fewer moving parts than vapor-compression systems and exhibit long life. All of that sounds good except for the fact that, in the past, these gas-fired systems have exhibited very low levels of efficiency.

Over the past decade or so, many researchers have been trying to improve the efficiency of ammonia/water absorptions systems, most notably by trying to capture and reuse the heat that is released when the ammonia is reabsorbed into the water. This approach, described as generator-absorber heat exchange, or simply GAX, has been shown to raise efficiency levels, and some GAX units have already emerged on the market.

The new chiller/heat pump technology developed at Rocky Research also utilizes a GAX cycle, but the GAX aspect represents only part of the solution, according Uwe Rockenfeller, president and CEO, who lists a number of innovations contributing to the technology’s progress.

“One, is that we achieve high-efficiency vapor separation. Our generator has a special construction that allows us to get a high vapor purity despite having very low ammonia concentrations. For example, on the hot end of the generator, ammonia concentration in water is on the order of 3 percent to 5 percent. At the other end, we have 99.8 percent ammonia. So it is a very extreme distillation process.

“In the absorber, we have a very unique heat-transfer surface enhancement that provides high heat transfer, and good surface wetting at part-load conditions, so it is amenable to accepting variable-flow rates.”

“Then we have a low-emission, variable-capacity combustion process, unlike the other products on the market that are single-speed, on/off. The variable-capacity is made possible by a multi-stepped gas burner and by a new means of actively controlling the refrigerant. Instead of employing a traditional orifice pack, which will not function optimally under part-load or off-design ambient temperature conditions, we use a pulsing, thermal expansion valve that allows for refrigerant flow control over a wide range of capacities and temperatures.”

Those improvements put together can deliver an absorption chiller with much higher efficiencies, but Rocky Research, which has backing from the Department of Energy and a consortium of energy companies for the project, had much bigger game in its sights. For all of the stakeholders, the ultimate goal was a gas-fired, ammonia/water absorption heat pump. To take the technology to that level required another key innovation, an efficient, positive-return, solution pump.

“The solution pumps on existing absorption systems are designed so that the diaphragm depends on the pressure of the solution being

higher than the ambient pressure,” Rockenfeller says. “Taking into account friction losses, you’re looking at 30 psi to 35 psi as the lowest possible operating pressure for the solution, which limits your operating solution temperature to somewhere in the mid-30s. Together, this means that an absorption heat pump using this conventional type of solution pump would not work below the range of 45?F outdoor, which is where your heating load hours typically begin.

“So we developed a solution pump with a positive return, which gives us a high delta-P, allowing us to go as low as 8 psia to 10 psia. We tested it in all six of the ASHRAE-defined climate regions and it worked fine in all, which means that it worked at -22?F.”

The new solution pump is also smaller and significantly more energy efficient than the conventional pump.

The first gas-fired absorption units built were dedicated 5-ton chillers. The goal was to achieve a COP (Coefficient Of Performance) of 0.7 at the 95?F ambient temperature rating point for chillers. The goal was surpassed slightly, with the units hitting 0.71 COP. But that’s not the biggest part of the story.

First, some more background. Another traditional and significant downside to gas-fired absorption systems when compared to vapor compression technology is the issue of cycling losses. Every time a gas unit shuts down, the entire solution in the system cools off. So when the unit restarts, the solution has to be reheated before the system begins cooling again. This wastes a lot of energy, and it can also irritate the consumer, who may have to wait 5 to 10 min. after restart for the unit to begin producing cool air again.

“The cycling losses can cause up to a 30 percent decline in efficiency,” Rockenfeller says. “So even if you have a unit with a 0.7 COP, that could translate into a 0.48 or 0.49 seasonal energy efficiency rating (SEER) due to cycling losses. That’s why we developed a system with variable capacity, allowing us to run it a part-load capacity without frequent on/off cycling. So when you get to the part-load point of 85?F, instead of our COP dropping from 0.7 to 0.5, our variable-capacity unit actually goes up to 0.84. That’s the real breakthrough.”

Rockenfeller is even more excited about the 5-ton gas-fired absorption heat pump being tested.

“Since we are able to pump heat at lower ambient temperatures, we can provide a heating COP of 1.44 at the 47?F rating point. That is total gas efficiency, not internal cycle efficiency. But more important is the unit’s ability to provide heat pumping down to -22?F. That is obviously extremely significant in light of the temperature operating range for existing electric heat pumps.

“Another issue is capacity. A 5-ton, 13 SEER electric heat pump can deliver roughly 53,000 BTUs at 47?F. Its balance point, where supplemental electric resistance heating has to kick in, is somewhere between 32?F and 35?F, at which point the heat pump is typically only delivering about 30,000 BTUs. Depending on the model, that particular electric heat pump quits pumping heat altogether between 22?F and 24?F, which puts the heating burden solely on the electric heater. That’s why it doesn’t make sense to use electric heat pumps in cold climates.

“By contrast, our heat pump delivers 108,000 BTUs at 17?F, that’s 6? lower than where the electric heat pump has already ceased to provide any heat at all. Our heat pump’s balance point, where supplemental heating kicks in, is approximately 0?F. That means you could use this heat pump throughout the country, with two exceptions, Alaska and a narrow belt of the country that’s north of Minneapolis. That’s the big news.”

It’s also important to understand, notes Rockenfeller, that heat pump capacity nomenclature is based on comparison with vapor-compression air conditioners.

“Our gas heat pump delivers twice the heating capacity of its cooling capacity. So when we say it’s a 5-ton unit, we’re talking about its cooling capacity. For heating purposes, it is equivalent to a 10-ton heating system.”

Rockenfeller says that an independently performed, operating-cost study showed that, if the gas heat pump replaced both the gas furnace and vapor-compression air conditioner typically found in a Chicago-area home, the homeowner would save roughly $1,000 a year in combined heating/ cooling operating costs, based on current Chicago-area utility rates.

Another potentially interesting application for the new technology may be indoor swimming pools, particularly institutional installations. Swimming pools at such locations consume enormous amounts of energy because the proprietor is using two different types of equipment, one to heat the pool water, and another to dehumidify the air around the pool to keep it comfortable for those sitting around it.

“The gas heat pump can produce 45?F chilled water for pool dehumidification,” Rockenfeller says. “At the same time, we can put a hydronic heat exchanger on the heat rejection side and pump that heat into the pool water. So we can replace two appliances with one. From the pool heating standpoint alone, this would cut energy costs in half. On the dehumidification side, it would cut energy costs by 80 percent. So this product would provide a huge advantage in this market.”

While Rocky Research developed the technology for the gas-fired chiller and heat pump, several commercial versions will be marketed by Ambian Climate Technologies, LLC, which was formed by a consortium of Southern utilities specifically for that purpose. The assembly and packaging will be contracted to a unit of Dectron Inter-nationale based in Niagara Falls, N.Y.

Rockenfeller predicts the 5-ton gas-fired chiller unit will be in limited commercial production by the middle of 2004, and that the 5-ton gas-fired heat pump should be in production in 2005. At some point after that, the plan will include the production of 3-ton versions of both appliances.

If you have read this article, enter 157.