What Is Heat Extraction?
While it may be difficult to explain the technology of extracting heat from air, water or ground, the concept is easier to comprehend once one understands the principles of heat extraction and heat exchange. Consider the following:
- All matter contains heat. Zero degrees Kelvin/Rankine (minus 273 degrees Celsius/minus 460 degrees Farenheit) is absolute zero. This is a hypothetical, but fairly well substantiated, theory. There is nowhere in the universe where absolute zero exists. Temperatures in outer space have been found to be approximately three degrees Kelvin, which supports the theories developed by scientists.
- Cold is the absence of heat. Cold exists only in relative terms, and plays no part in scientific theory. While we all verbalize such expressions as “It is cold out”, to be technically correct we should say “the heat level outside is ten degrees farenheit” (which, admittedly, is pretty cold).
- Heat always flows from higher temperature matter to lower temperature matter by conduction (from molecule to molecule), by convection (air currents) and by radiation (electro-magnetic waves).
- Heat can be moved or “extracted” from one source and delivered to another by various means such as “heat exchangers”.
What Is A Heat Pump?
A heat pump, as the name suggests, is a device that “pumps” heat from one location to another. The most popular heat pump is the air-source type (air-to-air), which operates in two basic modes:
- As an air-conditioner, a heat pump’s indoor coil (heat exchanger) extracts heat from the interior of a structure and pumps it to the coil in the unit outside where it is discharged to the air outside (hence the term air-to-air heat pump) and
- As a heating device the heat pump’s out door coil (heat exchanger) extracts heat from the air outside and pumps it indoors where it is discharged to the air inside.
The problem in comprehending such technology is that it is difficult to understand how heat extracted from,say, ten degree air (or water) can heat anything. This is where the unit’s compressor and the “phase-change” physical properties of the refrigerant come into play: the compressor boosts the extracted heat to a much higher temperature gas which gives up its heat as it condenses to a liquid in the condensing coil and is distributed to the structure by the fan or blower in the air-handler.
What Is Geothermal Heat Pump Heating & Cooling?
Differences between air-source and geothermal heat pumps
As with air-to-air heat extraction technology, geothermal (ground water/ground source) technology utilizes a type of heat pump known as a geothermal heat pump. This type of geothermal heat pump device extracts its heat from water rather than from air. While the principles are fundamentally similar, the methodology varies in that water is pumped through a special type of heat exchanger and is either “chilled” by the evaporating refrigerant (in the heating mode) or heated by the condensing refrigerant (in the cooling mode).
Why Is A Geothermal Heat Pump Better?
Water stores tremendous quantities of heat. In nature, few substances have a higher specific heat (one BTU per pound) than does water, making it an ideal heat storage medium for both natural and man-made phenomena.
Air, on the other hand has a very low specific heat (.018 BTU per cubic foot). There is 3472 times more heat stored in a cubic foot of water (62.5 BTU per degree F) as in a cubic foot of air . In other words it would be necessary to move 3472 cubic feet of air through a heat exchanger in an air-to-air heat pump in order to expose that heat exchanger to the same quantity of heat stored in a cubic foot of water (7 1/2 gallons) that is moved thru a geothermal heat pump.
|This cube represents one cubic foot of water||This cube represents 3472 cubic feet of air|
Furthermore, Enviroteq geothermal heat pump units have such low resistance to water flow (pressure drop) that about two cubic feet per minute (15 gallons) is the average water flow through an Enviroteq geothermal heat pump utilizing only about 235 watts of energy compared to well over 1000 watts to move nearly 7000 cfm of air through the air-source heat pump.
While these differences are significant, there is more: the heat transfer characteristics of water make it superior to air. Conduction is more rapid, more complete, and more efficient a heat transfer phenomenon than convection. A ground-water heat pump extracting heat from water at freezing is approximately equal in performance to that of an air-source heat pump extracting heat from 60 degree air.
What are Open Loops and Closed Loops?
An open loop is a loop established between a water source and a discharge area in which the water is collected and pumped to a GWHP then discharged to its original source or to another location. The piping for such configuration is open at both ends and the water is utilized only once.
Examples of such loops are: systems operating off wells wherein water is pumped from a supply well, through the unit and discharged to a return well; open systems operating from such surface water sources as ponds, lakes, streams, etc, where the source water is pumped to the unit and returned to the source.
Open loops have the advantage of higher equipment performance since the source water is used only once and then discharged, but have two significant disadvantages:
- water quality needs to be carefully analyzed and treated if such corrosives as sulfur, iron, or manganese are present , if pH is low, or if there are abrasives in it
- the costs of pumping water through an open loop are usually somewhat higher than those associated with circulating water through a closed loop
A closed loop is one in which both ends of the loop’s piping are closed. The water or other fluid is recirculated over and over and no new water is introduced to the loop. The heat is transferred thru the walls of the piping to or from the source, which could be ground, ground water, or surface water. As heat is extracted from the water in the loop the temperature of the loop falls and the heat from the source flows toward the loop.
In closed loop operation water quality is not an issue because corrosives become rapidly “spent” or used up and corrosion caused by poor water quality is quickly curtailed. The wire-to-water efficiencies of circulators used in closed loop operation are very high and the costs of pumping the water are lower as compared to open loops. System efficiencies are somewhat lower in closed loop operation, but given the lower pumping costs associated with this method, economics sometimes, but not always favor this approach. Installed costs, however, are higher and need to be considered if the consumer already has a well or other water source.
Types Of Closed Loops
While there are several loop configurations used in closed loop operation, generally two types of closed loops are utilized by the industry – vertical and horizontal.
In vertical loop installation, deep holes are bored into the ground and pipes with U-bends are inserted into the holes, the holes are grouted, the piping loops are manifolded together, brought into the structure and closed. The argument for this type of ground-loop heat exchanger is that because the piping is in the deeper ground – unaffected by surface temperatures – performance will be higher. Generally, installed costs are higher than with a horizontal loop.
In horizontal loop installation, trenches are dug, usually by a backhoe or other trenching device, in some form of horizontal configuration. Various configurations of piping are installed in the trenches. A larger number of horizontal loop designs have been tried and utilized successfully by the industry. While installed costs have been lower, horizontial loops have been thought to be less efficient than vertical loops because of the effect of air temperatures near the surface of the ground.
Resistance To Heat Transfer
Two significant factors need to be considered when designing and sizing a ground-loop: 1) Resistance of the heat source to heat transfer eg. ground, pond, lake, etc. and 2) Resistance of the pipe to heat tansfer.
Of the two factors, pipe resistance is the dominant one. But, while little control can be exercised over source resistance, a great deal of influence can be exercised by the designer over the pipe resistance. Plastic pipes are generally poor conductors as compared with metal. Increasing the ratio of pipe surface area to trench length yields significant gains in loop performance.
“SLINKY(TM)” Loops work!
The SlinkyTM ground loop, developed by the International Ground Source Heat Pump Association (IGSHPA) represents a good compromise between performance and installed costs.
While perhaps not quite as efficient as appropriately sized vertical ground loops, it represents an improvement over other horizontal loop configurations and is less expensive to install.
Massive quantities of pipe – 700′ to 1,000′ per ton of unit capacity – are utilized in the SlinkyTM configuration.
IGSHPA’s design utilizes 1,000 feet of pipe in an 80′ trench per ton of unit capacity.
At Sunteq we have developed a configuration that utilizes 840 to 930 feet of pipe in an average ratio of 100′ trench per ton of unit capacity. Accordingly, there is about a 22% overall performance improvement of the 80′ trenched SLINKYTM over the other horizontal loop styles — two pipe, four pipe, and extended slinky.