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Monday, April 21, 2014

New Natural Gas Heat Pumps Provide Cool Savings

New technology developed at the U.S. Dept. of Energy’s (DOE) Oak Ridge National Laboratory (ORNL) is yielding an advanced heat pump capable of reducing energy use and eliminating a potential environmental hazard.

Heat pumps actually move heat by means of a refrigeration system that produces a cold surface, and an evaporator, which can pull heat from its surroundings, as long as the surroundings are warmer than the evaporator. Heat pumps can be air-source, ground-source, or water-source, depending on the medium used to derive heat.

A well-designed heat pump, operating within its designated temperature range, can actually move twice as much heat energy — sometime more — than the energy being used to power the system. This gives the impression that it is better than 100-percent energy efficient, which it is not. It is referred to as a coefficient of performance (COP).

Drawbacks of today’s heat pumps and air conditioners, which work the same way, include the fact that they use a great deal of electricity. Many also use CFC-based refrigerants which are very potent greenhouse gases and which, despite best efforts to contain them, often find their way into the atmosphere where they can harm the ozone layer.

Diagram of a phase change heat pump. Credit: Wikimedia Commons.

Diagram of a phase change heat pump. Credit: Wikimedia Commons.

ORNL’s advanced heat pump is capable of reducing the heating energy requirements by 50 percent while eliminating CFCs in the process. The advantage is obvious, considering that heating and cooling accounts for 40 percent of the energy used in buildings.

This new heat pump burns natural gas to compress the refrigerant rather than using mechanical action, as the more conventional vapor-compression system does. This type of refrigeration is called absorption, or heat-activated cooling and heating, and is not new. Some of the earliest refrigerators used an absorption system, going back as far as 1850. These systems are getting renewed attention, in part, because of their potential use as solar air conditioners.

ORNL’s new system is based on a generator-absorber heat exchange (GAX). Rather than using CFCs, it uses a mixture of ammonia and water, which, while not the safest thing around, does not present of threat to the ozone layer or as a greenhouse gas. ORNL co-developed the system with Phillips Engineering and licensed it to Carrier Corp. in 1993. One of the reasons for its high efficiency is the way that heat from the absorber is recycled to the generator tank, which must be done against the direction of buoyant natural convection, though the GAX system manages to do this quite efficiently.

As a heat pump, this system is highly efficient, delivering 40 to 50 percent more heat per unit of energy than conventional systems. However, it is not as efficient in cooling mode. ORNL is currently developing a new “high cool” system, which researchers estimate can improve cooling by 30 percent compared to the GAX system.

Larger buildings use chillers which cool water that is then circulated through heat exchangers. ORNL is also targeting these applications with their “triple-effect” chiller that promises a 40-percent efficiency improvement. Two such systems have already been licensed to Trane and York. The York system is based on a double-condenser coupling (DCC) approach, while the Trane system is a double-loop cycle, triple-effect chiller.

The DOE’s Office of Energy Efficiency and Renewable Energy (EERE) recently awarded an additional $750,000 to ORNL to further develop these new technologies.

With natural gas prices as low as they are today, this technology appears quite attractive. Unfortunately, the prices are not expected to remain at these levels for long. Perhaps the best application would be to continue to develop solar chillers which provide their highest output just when you want it, when the sun is out.

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