December 1997

Direct Exchange Thermal Systems:
The Wave Of The Future

Copper Applications in Health & Environment

By Ralph Willett

Introduction | Heat Pump Systems | Test Program | Summary


Heat pumps! While the term seems relatively new, Nicholas Carnot proposed the concept in 1824. While heat normally flows from hot to cold, Carnot reasoned that a mechanical device could be built to reverse the process and "pump" heat from a cooler region to a warmer region. This is known today as the refrigeration cycle. Today's air heat pumps work on this principle, pumping cool outside air to a device that extracts some of the air's thermal energy for distribution to your home. Even on cold days, 0° F, outside air contains almost 90% of the heat it contains at 70° F.

Heat pumps are generally thought of as very large assemblies mounted on building tops that are used to heat and cool the facility by pumping large quantities of outdoor air through a heat exchanger and, ultimately, through a refrigeration cycle into or out of the building depending on the season. These are called air-to-air heat pumps and have been around since the 1930�s. Equipment costs are high and in northern states with very cold outside temperatures, stress on the refrigeration equipment caused many failures.

As heat pump design work continued, another form of heat pumping was developed, the geothermal heat pump. That is, a heat pump whose source of energy is the earth. Geothermal heating and cooling operates by harnessing the earth�s thermal energy content below the ground surface for heating or using the relatively cool earth to dissipate heat for cooling. That energy is transferred to or from the home to either heat or cool the interior. Perhaps a simpler term for these systems would be ground source heat pump. Five to six feet under ground, the earth's temperature is relatively constant. The earth is a huge storage plant for the sun's energy, absorbing approximately 47% of that available - 500 times more energy than man uses every year.

Still, until recently, extensive development and use of this type of heating and cooling has been slow. This is despite the fact such a system can:

  • Save 30-60 percent on annual heating and cooling costs compared to conventional systems.
  • Provide constant comfort with no hot or cold spots.
  • Have no flame, flue or odors.
  • Be environmentally friendly, emitting no carbon dioxide.
  • Take up no more interior space than a washing machine.

Part of the answer is that installation costs can be somewhat higher than those of traditional systems. A fully installed geothermal heat pump in a new 2,500-square-foot home can cost as much as $16,000 with traditional systems a few thousand less. However, focusing on the operational costs of a traditional heating and air conditioning system for a new home, significant savings can be realized. Conventional heat, cooling and hot water cost approximately $2,500 per year to operate. The geothermal heat pump�s operating costs should only be about $900 per year. The savings in operating costs can quickly pay back the extra first cost of the heat pump system. The heat pump saves more than just operating dollars. That's why present predictions by the Department of Energy are that geothermal installations will increase tenfold by the year 2001, largely as the result of dwindling crude oil sources and mankind's rapid fouling of the earth's atmosphere.

Heat Pump Systems top

The rest of this article focuses on just two potentially dominant heat pump designs and how they operate. The Copper Development Association�s role in surveying the use of copper tubing in one of those systems, and their inherent advantages, will also be discussed.

Both geothermal heat pumps to be discussed are identified by the acronym GCHP or ground coupled heat pump. Both GCHP systems use a reversible vapor compression cycle linked to a closed heat exchanger buried in the ground. The most widely used system, the one that has received the most development and funds to date, is the water-to-air heat pump. This system circulates water or a water/antifreeze solution to a refrigerant heat exchanger and a buried thermoplastic piping network. This system is called a closed loop water system. Briefly, closed loop water systems transfer heat from the ground to the water circulating in the pipes, which are usually made of polyethylene. The heated water then transfers its heat to a refrigerant, freon for instance, and the refrigerant heat is transferred to the inside air by a heat exchanger. After the energy transfer, the water is recirculated to begin its cycle over again.

There is an alternative to this whereby groundwater, obtained from a nearby lake or well, is used in the system and, after the energy transfer, the water is returned to the original source. This is unceremoniously called a "pump and dump" system. The availability of such a design is dependent on the chemistry and quantities of water at the location and local environmental laws. The operation is identical to the closed loop system although the efficiency of using a water source is better than a ground-source system.

An important feature of the closed loop system is the underground array of polyethylene piping. Typically, wide, long trenches are dug and lengths of pipes are laid in place. This is called a horizontal GCHP and requires considerable land area for installation. Extreme care must be taken with the numbers of pipes installed and their proximity to each other. The horizontal installation is probably the least expensive approach and many trained installers available. The disadvantage is seasonal variation in the thermal properties of the ground and, thus, somewhat lower efficiencies compared to the vertical GCHP. In that installation, smaller diameter, high density, polyethylene pipes are inserted in vertically drilled holes. This takes up considerably less land area but is more expensive to install. However, it has advantages, such as more stable ground thermal conditions and, generally, higher efficiencies.

The second GCHP system to be discussed, and one that is known to a smaller segment of the population, at first glance, appears similar to the water to air system. However, instead of the pipes being filled with water or water/anti-freeze, they are filled with refrigerant, and the pipes are copper instead of polyethylene. This is a direct exchange system. (For ease of discussion, we will refer to this system as a refrigerant-to-air system.) In this system the heat transfer mechanism is as follows: Heat in the ground is absorbed by a refrigerant and released by transfer from an inside air handler and heating coil. Note the fewer steps in the refrigerant-to-air system compared to the water-to-air system. Since the refrigerant is already in the ground tubing system, one less heat exchanger and one less pump are required. This has an immediate 40% saving on installation costs of refrigerant-to-air equipment compared to water-to-air systems. The main problem with the refrigerant-to-air system is that it's a late starter because initial industry and governmental funding and testing were devoted to the water-to-air systems. Hence, data to determine how well the refrigerant-to-air system compares are scarce and, even more important, no test standards have been established to verify the performance. This is where Professor Stan Johnson and the Copper Development Association come into the picture.

Test Program top

Professor Johnson is on the staff of the University of Tennessee, Knoxville. The university, under Professor Johnson's direction, is currently monitoring two residences that are being heated and cooled using the refrigerant-to-air system. Included in the system design is heating of the household hot water. These are not the first homes to be so equipped. It is estimated that approximately 5,000 homes in the United States employ refrigerant-to-air heating and cooling systems. The importance of the Tennessee site is the rigorous data and documentation planned. The Copper Development Association is closely following this work as a result of the potential impact on the use of copper tube, from 1,000 to 2,000 feet of tube per installation, depending on diameter. To date, data for only one heating and cooling season have been captured at the University of Tennessee site. While still thus limited, some preliminary heat pump efficiency numbers have been calculated for the system. The COP, Coefficient of Performance, is a ratio of the useful heating or cooling provided by a system compared to the energy consumed by the system in delivering the heating or cooling.

Installation of tubing at the University of Tennessee heatpump site. Installation of tubing at the University of Tennessee heatpump site.
Installation of Tubing at the University of Tennessee Heat Pump Site

COP = rate of useful energy output, BTU/rate of energy input, BTU. The higher the COP value, the more efficient the system. Initial tests compared the COP of the refrigerant-to-air system to the COP of an air-to-air system.

The results:

SystemWinter Cop ValueSummer Cop Value
Air-to-Air 1.9 to 2.1 2.2 to 2.3
Refrigerant-to-Air 2.5 3.7

A quick comparison suggests the potential superiority of the refrigerant-to-air system compared to the air-to-air system. These are impressive numbers, but what about refrigerant-to-air compared to water-to-air? In the one documented study of these two systems, refrigerant-to-air systems COP's averaged 25% higher than the water-to-air systems. The University of Alaska conducted this brief study in the winter of 1995-1996. In their study, three refrigerant-to-air systems were compared to four water-to-air systems. Future studies will be more meaningful and may show bigger advantages to the refrigerant-to-air systems if and when the amount of money and study that has been devoted to the water-to-air system, is applied to the refrigerant-to-air system. Adding to the complexity of the comparison is the fact that there are no established USA uniform test standards for performance measurement.

Additional information obtained from one of the residences tested at the University of Tennessee compared refrigerant-to-air heating and hot water costs to those costs incurred using the originally installed conventional (non heat pump) system. When these costs were compared, a reduction of 60% in the electrical cost for heating energy was achieved.

Significant advances have been made that deal with early problems associated with both the copper tubing and the heat exchanger and pump units in the refrigerant-to-air systems. There are a few soils that react to corrode copper. All problems encountered to date in such corrosive soils have been successfully defeated by cathodic protection of the copper tubes, or, by coating the tubes with a protective film. Early problems with lubrication have been eliminated through the use of loop design improvements and refrigerant flow controls.

The testing program at the University of Tennessee, is just underway and holds the potential for much useful information. The goals of the University of Tennessee's program will help generate this information as it:

  1. Provides a facility to test refrigerant-to-air heat pump systems according to the Canadian Standard C748, currently the only existing national standard for testing direct exchange ground coupled heat pump systems.
  2. Carries out tests on a consistent basis, formulating improvements and striving to develop a U. S.-based standard for testing direct exchange ground source heat pumps.
  3. Carries out long-term performance testing of multiple direct exchange systems and gains information needed to improve designs.
  4. Publishes results for dissemination to the technical community for evaluation as well as to the general public as an aid in choosing the best installation possible.
  5. Offers training sessions for direct exchange installers to insure that the latest technologies are consistently used in all installations.

Summary top

In summary, it is clear that a ground coupled heat pump design combining the use of refrigerant filled copper tubing can offer significant improvements compared to other heat pump systems. These are:
  • Vastly improved heat transfer characteristics provided by the copper tube.
  • Improvements in the Coefficient of Performance compared to existing heat pump technologies.
  • Development and publication of a United States standard of performance for ground source heat pump systems.
  • Improvements in ground source heat pump system designs accomplished using copper and the testing program as a guide.
  • Reductions to homeowners of 30% to 60%, or more, in the cost of heating, cooling and hot water.
  • An installation cost saving of 40% compared to water-to-air systems.

Under the direction of Professor Stan Johnson at the University of Tennessee, and with the sponsorship of the Copper Development Association, a long term testing program aimed at system evaluation and comparison to existing technologies is well underway. There are several system benefits that must be carefully examined, and verified. These data and documentation is expected to provide the foundation on which to fully develop the potential of refrigerant-to-air systems.

Ralph Willett
Willett Technical Services
Bloomingdale, IL 60108

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