HEAT PUMP
Hydronic heat is also compatible with Heat Pumps. The Coefficient of Performance or COP of a heat pump is based on input/output temperature and can be as high as 5 in a properly designed system. This means that every horsepower-hr (or BTU) shaft horsepower into the heat pump compressor, results in 5 BTU's of heat into the home. This compares to an electric room heater or electric baseboard where one energy unit into the heater results in one energy unit out. A more expressive way to phrase it – the heat pump is 500% more efficient, compared to the electric heater at only 100%. These numbers are built into the Fuel Cost Spreadsheet and can be changed to reflect your situation and pricing. Where natural gas is not available, or if summer cooling is desired, heat pumps can be the heat source of choice.
WHAT IS A HEAT PUMP?
What is a heat pump? Think of your refrigerator; it is using the shaft energy of the compressor working on a refrigerant to pull heat out of the refrigerator box and exhausting that heat into the kitchen. The heat pump uses this same concept, moving heat from a colder source to a warmer source. In this case, the colder source is the ground (ground loop) or outside air and the warmer source is the house. The refrigerant is “flashed” to vapor across the Thermal Expansion Valve. The refrigerant is now colder than the outside source temperature and therefore picks up heat in the Evaporator. In the example shown to the right, this would be Ground-Source water loop, approximately 50F in the northwest.
Or, if an Air-Source heat pump refrigerant can pick up heat as low as 20F depending on the refrigerant used. The refrigerant is then compressed via mechanical work in the Compressor. This heats the refrigerant, which now heats the hydronic water in the Condenser. This in turn cools the refrigerant back to a liquid. And then the cycle repeats itself as the refrigerant flashes back to a vapor across the Thermal Expansion Valve. |
This is the heating mode. A reversing valve within the heat pump can change this to a cooling mode for use through an air handler system (not shown).
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The slide, show below, shows the heat pump in more detail, and the calculation of the COP in standard units. It should be noted that detracting from the heat-pump efficiency is the energy required to pump the ground loop fluid. Without proper sizing here, it is possible to negate much of the energy saved by the heat pump in terms of pumping horsepower.
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It can be shown that the closer the temperature into the heat pump to the temperature out of the heat pump the more efficient the heat pump in terms of mechanical energy necessary at the compressor shaft. This is known as COP or Coefficient of Performance and is shown graphically in the graph below:
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In this example if we keep the Entering Load Water Temperature (ELWT or water heating the building) at 80F, we would achieve a COP of 5.5 for a Ground Source Water Temperature of 45F. This is a real example and can be accomplished in a well-insulated building with low resistant floor covering, such as, tile or hardwood, and tubing/pumps sized for low Delta T across the floor. Perhaps the COP would be slightly higher, since the ground temperature in the Pacific NW is in the low 50F degree range.
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In our designs, we typically do not use resistive heat except as a backup heat source. It is our view, that electrical power is too valuable as work energy to be degraded to just heat. Even electric water heaters can benefit by pre-heating the entering water with boiler or heat pump. In fact, there are now all-electric water heaters on the market that use a built-in heat pump. Expensive and not recommended but this is the wave of the future. (Note: See the "Wikipedia Power Generation" under our More...Other Resources tab for additional energy discussion.)
Some heat pumps have an add-on feature, known as a Desuperheater, that can preheat domestic hot water using the efficiencies of the heat pump. This device is installed on the hot stream, between the Compressor and the Condenser, and can preheat water into the 120F range. This lessens the capacity of the heat pump for space heating when in the heating mode, as the Desuperheater has “first call” on the heat that is generated because it is ahead of the Condenser. But this is only when hot water is demanded. In cooling mode there is no lessening of capacity, in fact it could be termed “free hot water” as the heat-pump is rejecting heat, and this heat will move to the Evaporator and then outside (ground-source or air) if not used for domestic hot water.
Note: For clarification, many ground-source heat-pump (GSHP) manufacturers refer to their product as “geothermal”. For the purist, geothermal is subterranean heating from the earth interior such as a hot springs. The energy captured by the ground-source heat pump is “top-down” energy basically from the sun. The GSHP just takes advantage of the fact that earth temperatures below the frost line are relatively stable, summer and winter. “Ground Source” or “Earth Coupled” is the more accurate term.
Some heat pumps have an add-on feature, known as a Desuperheater, that can preheat domestic hot water using the efficiencies of the heat pump. This device is installed on the hot stream, between the Compressor and the Condenser, and can preheat water into the 120F range. This lessens the capacity of the heat pump for space heating when in the heating mode, as the Desuperheater has “first call” on the heat that is generated because it is ahead of the Condenser. But this is only when hot water is demanded. In cooling mode there is no lessening of capacity, in fact it could be termed “free hot water” as the heat-pump is rejecting heat, and this heat will move to the Evaporator and then outside (ground-source or air) if not used for domestic hot water.
Note: For clarification, many ground-source heat-pump (GSHP) manufacturers refer to their product as “geothermal”. For the purist, geothermal is subterranean heating from the earth interior such as a hot springs. The energy captured by the ground-source heat pump is “top-down” energy basically from the sun. The GSHP just takes advantage of the fact that earth temperatures below the frost line are relatively stable, summer and winter. “Ground Source” or “Earth Coupled” is the more accurate term.
FOR FURTHER INFORMATION
For more information of Ground-Source/Geothermal Heat Pumps, Wikipedia has a good overall discussion. See our More...Other Resources tab.
Residential Renewable Energy Tax Credit
For information, there is a 30% federal tax credit for ground-source heat pump installations through 2016, with a possibility of extension at a lesser level. For more information on this and other incentives visit the dsireusa.org website operated by the N.C. Clean Energy Technology Center at N.C. State University, funded by the U.S. Department of Energy. (Note: See "Federal and State Tax Incentives" under our More...Other Resources tab.)
One final note about ground-source heat pumps, it is extremely important to have a good design regarding the ground loops. This is a specialized field and needs to be designed by either the heat-pump manufacturer or an accredited designer. HHD does not design these systems but can work with your designer on Pumping and Fluid Mechanics. The heat pump is dependent on good heat transfer between the fluid and the ground for maximum efficiency. There are many variables to consider in the design, type of soil, dry or wet, etc. In general it is better to error on the side of more tubing in the ground than too little. But also needs to be balanced with regards to pump horsepower as mentioned above.
Residential Renewable Energy Tax Credit
For information, there is a 30% federal tax credit for ground-source heat pump installations through 2016, with a possibility of extension at a lesser level. For more information on this and other incentives visit the dsireusa.org website operated by the N.C. Clean Energy Technology Center at N.C. State University, funded by the U.S. Department of Energy. (Note: See "Federal and State Tax Incentives" under our More...Other Resources tab.)
One final note about ground-source heat pumps, it is extremely important to have a good design regarding the ground loops. This is a specialized field and needs to be designed by either the heat-pump manufacturer or an accredited designer. HHD does not design these systems but can work with your designer on Pumping and Fluid Mechanics. The heat pump is dependent on good heat transfer between the fluid and the ground for maximum efficiency. There are many variables to consider in the design, type of soil, dry or wet, etc. In general it is better to error on the side of more tubing in the ground than too little. But also needs to be balanced with regards to pump horsepower as mentioned above.