Heat Pump Mechanics
Although the ground loop that exchanges heat with the earth is the distinctive feature that gives this technology its name, it is the interior heat pump units that transfer heat between the ground loop and the conditioned spaces of a building. The first section on this page explains the basic principles underlying the mechanics of heat pump operation. To show how these principles can be applied to move heat from one place to another, the next section reviews different heat pump types, beginning with an explanation of how a refrigerator works and ending with a description of geothermal heat pump units and their unique features. The last section describes different alternatives for adding a domestic water heating loop to a geothermal heat pump system.
Basic Principles
Heat energy is the collective random motion of molecules, and temperature is a measure of how fast the molecules are moving. Heat energy spontaneously flows “down the temperature scale” from a warmer region to a cooler one, as shown in Figure 1. This is one way of stating the Second Law of Thermodynamics, which also can be be stated as: "The natural tendency of the universe is to become more disorganized." Observe a school classroom over any period of time, and you will see that this is true. The random activities of students will cause loose papers and books to spread into previously organized areas. Likewise, when a group of fast-moving (warmer) molecules is placed next to a group of slow-moving (cooler) molecules, random collisions will cause the slower molecules to speed up and the faster ones to slow down, thus transfering energy from the warmer group to the cooler group.
Latent heat is the energy needed to overcome the molecular interactions that tend to organize matter, such that it can undergo a phase change into a more disorganized state. Solids are more organized than liquids, which in turn are more organized than gases. Consider a block of ice being warmed by an external heat source (Figure 2). Although the ice molecules contain some heat energy, their vibrations are not sufficiently energetic to overcome their interactions that hold them together in a solid structure. As heat energy is absorbed by the ice, its molecules vibrate faster within the solid structure, and its temperature rises. When its temperature reaches 0°C, however, the ice begins to melt, and its temperature stops rising. This is because the molecules now have enough vibrational energy that any additional heat will break down the solid structure into a more loosely connected liquid state. Instead of causing increased molecular vibration within the solid state, all heat energy absorbed by the ice at its 0°C melting point goes into changing its state from solid to liquid until all molecules are in the liquid phase.
Any heat energy absorbed after all the ice has melted can again create faster molecular motions, but now within a liquid structure, and the water temperature begins to rise. Note that the temperature at any given time represents the average speed of random molecular motion; some individual molecules move faster and some move slower. When the faster-moving molecules at the water's surface have enough energy to break free of the cohesive forces that hold them in the liquid structure, they escape into the gas phase. This is the process of evaporation.
As escaping molecules accumulate in the gas phase above the liquid water surface, their chances of randomly striking the surface increase, and when they do collide with the surface, they spontaneously lose heat to the slower-moving molecules remaining in the liquid phase. If they lose enough energy to be "captured" by inter-molecular cohesive forces, they return to the liquid phase. This is the process of condensation.
When the rate of evaporation equals the rate of condensation, the air above the liquid is said to be saturated with vapor, and the pressure exerted by the molecules in the gas phase is called the saturated vapor pressure. If the liquid is contained in a closed volume, and that volume is suddenly compressed (as in a piston pump for example), then the concentration of water vapor molecules will increase. This is only a temporary condition, however, because the increased concentration of vapor molecules causes more of them strike the liquid surface and condense. There is a net flow of molecules back into the liquid until the vapor pressure returns to its original saturated value and evaporation exactly balances condensation. Therefore, the saturated vapor pressure of a liquid depends only on its temperature and not on the volume in which it is contained.
Returning to the graph in Figure 2, we now understand that as water is heated above its melting point, its saturated vapor pressure increases along with its temperature. When the water temperature reaches 100°C, its saturation vapor pressure equals normal atmospheric pressure, and at this point, any additional heat energy causes the internal pressure of water vapor bubbles within the liquid to exceed atmospheric pressure, enabling them to rise to the surface and release their vapor molecules to the gas phase. This is the process of boiling. All heat energy absorbed by the water at its 100°C boiling point goes into expanding the water vapor bubbles, which continue to break the surface until all molecules are in the gas phase.
From the above description, it is evident that a liquid can be boiled at any temperature simply by reducing the applied pressure below its saturated vapor pressure at that temperature. This is exactly what occurs when you press down the spray nozzle of a pressurized "canned air" duster (such as used for cleaning electronic components; see Figure 3).
When the spray nozzle is up, the can's internal pressure equals the saturated vapor pressure of the propellant liquid. When the spray nozzle is depressed, the can's interior becomes open to the atmosphere, and its internal pressure drops as propellant vapor is expelled. This reduces the pressure over the liquid propellant, causing it to boil. The propellant vapor absorbs its latent heat of vaporization from the liquid phase, which in turn absorbs heat from the surrounding can metal, and the can begins to feel quite cold. In fact, if the nozzle is held down too long, the hand holding the can will be in danger of frostbite!
A final basic principle underlying heat pump mechanics is that when a gas is compressed, the number of molecular collisions increases due to the increased concentration of molecules, and its temperature rises. This is sometimes referred to as Gay-Lussac's Law, which is a special case of the Ideal Gas Law, and it can be easily demonstrated with a deflated bicycle tire and hand pump. As the tire is inflated and its internal pressure rises, even greater external pressure is needed to force air into the tire. Increased compression is required with each additional stroke, and the temperature of the pumped air volume increases according to Gay-Lussac's Law. After several strokes this creates a significant temperature gradient, and heat spontaneously flows from the compressed air to the surrounding pump body, which begins to feel quite warm.
Different Heat Pump Types
A heat pump is a device that uses mechanical energy to pump heat “up the temperature scale” from a cooler region to a warmer one. It does so by changing the pressure of a working fluid called a refrigerant. Commonly used refrigerants are distinguished by the fact that over a relatively moderate range of elevated pressures, they vaporize and condense at temperatures that can drive spontaneous heat flow in the direction needed to preserve food, chill water, or maintain interior building comfort. This is illustrated for several different types of heat pump described below.
Vapor compression refrigerator
The familiar kitchen refrigerator is perhaps the simplest example of a heat pump and uses all the basic principles described above to move heat out of its cold box. It has four main components (Figure 4):
- Compressor, which increases the pressure of the refrigerant vapor, pushing it through the system, and increasing the vapor's temperature above that of the surrounding kitchen.
- Condenser, usually behind the refrigerator, where the refrigerant vapor condenses to a liquid.
- Expansion valve, which causes a sudden drop in refrigerant pressure, causing it to boil; also called a "metering" valve, since it passes only as much liquid as can be completely vaporized in the evaporator.
- Evaporator, where the latent heat of refrigerant vaporization is absorbed from the cold box.
Typical pressure and temperature conditions for such a refrigerator are illustrated in Figure 5, below.
For kitchen refrigerators, the compressor typically operates at a compression ratio in the range of 4 to 5, depending on what refrigerant is used. For example, if R134a (tetra-fluoroethane) is used, an evaporator pressure of 1.6 atm is required to achieve a vaporizing temperature of 5°F, while a condenser pressure of 7.6 atm is required to achieve a condensing temperature of 85°F. On the other hand, if R717 (ammonia) is used, the required evaporator and condenser pressures for the same phase change temperatures are 2.3 atm and 11.5 atm, respectively.
The refrigerant's vaporizing temperature (and associated pressure in the evaporator) is chosen such that it is significantly colder than the target temperature of the compartment to be refrigerated. This creates a temperature gradient for spontaneous heat flow to the evaporator, vaporizing the refrigerant, which thereby gains latent heat from the cold compartment. Likewise, the condensing temperature (and asociated pressure in the condenser) is chosen such that it is warmer than room temperature. This creates a temperature gradient for spontaneous heat flow away from the condenser, causing the refrigerant to liquify and thereby lose the latent heat that it had gained from the cold compartment. In this way, heat is pumped out of the refrigerator.
Water chillers, air conditioners, and dehumidifiers
Water chillers range in size from small drinking water coolers to large central chiller plants that use an insulated piping loop to distribute chilled water to hydronic fan-coil units throughout a commercial or institutional building. Water chillers use a vapor-compression refrigeration cycle nearly identical to that illustrated above, except that the refrigerator's cold air compartment is replaced by a water tank or reservoir.
Air conditioner coils operate at a much higher set of temperatures than refrigerator coils, but otherwise use the same vapor-compression cycle. Instead of the heat source being a cold box maintained at 35-40°F, the heat source is an air-conditioned room maintained at a comfortable shirt-sleeve temperature of 70-75°F. On the high-pressure side of the cycle, the heat to be rejected is pumped outdoors, where the heat sink design temperature can be as high as 100-105°F.
A dehumidifier removes moisture from room-temperature air by passing it over an evaporator coil. The moisture condenses out of the room air onto the cold surface of the coil and then drains into a tray that must be periodically emptied. The drier room air is then discharged over a condenser coil and out the front of the dehumidifer, which warms it by several degrees, further reducing its relative humidity. In this cycle, air at or near room temperature is both the heat source (for the evaporator) and heat sink (for the condenser).
Air-source heat pumps
Heat pumps use the same vapor-compression cycle as the refrigeration systems described above, but they have additional components that enable them to pump heat in either direction, such that the same equipment unit can provide cooling or heating. The two added components that make heat pumps fully reversible are:
- A 4-way reversing valve, which can reverse the direction of flow around the refrigerant loop while maintaining the same direction of flow through the compressor (the next two diagrams show how this is done).
- A bi-directional expansion valve, which is able to meter the flow of liquid refrigerant in either direction (for purposes of illustration, the diagrams below show two expansion valves and two bypass valves, but modern heat pumps often incorporate all of this functionality in a single valve).
As shown in Figure 6, an air-source heat pump in cooling mode acts identically to an air conditioner. Refrigerant vapor exits the compressor at a temperature in the range of 120-140°F, which is warmer than the outside air temperature. As a result, it spontaneously loses heat when it enters the outdoor coil, causing it to condense. The refrigerant leaves the condenser as a liquid, still under high pressure. The expansion valve lets through just as much refrigerant liquid as can be completely vaporized by the indoor coil. The pressure drop through the expansion valve vaporizes some refrigerant and lowers its temperature to 40-50°F. As a result, it spontaneously gains more heat, which vaporizes the rest of the refrigerant liquid. The low-pressure refrigerant vapor leaves the indoor coil, goes through a U-bend in the reversing valve, and returns to the compressor, where the cycle begins again.
As shown in Figure 7, the reversing valve can be switched to heating mode such that the high-pressure output of the compressor is directed toward the indoor coil, which now acts as a condenser where the refrigerant gives up its latent heat to the room. It is then expanded in the reverse direction (compare with Figure 6) and vaporized in the outdoor coil, where it gains latent heat from the outside air. The refrigerant vapor then goes through a U-bend now on the other side of the reversing valve, and returns to the compressor where the cycle begins again.
To summarize, the indoor and outdoor coils are where the refrigerant changes phase, gaining or losing latent heat through evaporating and condensing. The compressor drives the refrigerant around the loop and creates the high-pressure and high-temperature conditions that enable it to condense as a liquid. The expansion valve meters the flow of liquid refrigerant from the high-pressure/temperature side to the low-pressure/temperature side of the loop, such that it all will be vaporized in whichever coil is acting as the evaporator. The reversing valve determines which coil is on the ""high side" (condensing) or "low side" (evaporating) of the loop. Following the natural tendency of spontaneous heat flow, the high side loses heat to its surrounding environment, and the low side gains heat from its surrounding environment.
An air-source heat pump can be installed either as a "packaged" unit, where both coils are in a weatherproof enclosure, typically mounted on flat roofs (Figure 8), where they supply warmed or cooled air to the rooms immediately below them, or as a "split" system, where the indoor coil is contained within the building's air handling system (Figure 9). Packaged units are more common in commercial and institutional buildings, while split systems are more common in residential applications.
Packaged terminal heat pumps also can be installed as "through the-wall" units, where they are fitted into a sleeve that passes through an exterior building wall. These typically are noisier than rooftop packaged units, since the compressor and fan are located in the room; they are commonly found in hotels and motels.
During the winter heating season, air-source heat pumps cease to be effective when the outside air temperature falls below 25-35°F. To handle such conditions, they are supplied with a supplemental heating system - usually electric resistance strips to further warm the building supply air after it leaves the indoor coil (Figure 9).
During the heating season, moisture in the air outside may freeze on the outdoor coil if its surface temperature drops below 32°F. Therefore, when outside temperatures fall below about 40°F the heat pump will periodically enter a defrost cycle, during which the reversing valve intermittenly sends hot refrigerant through the outdoor coils for periods lasting anywhere from two to ten minutes. During a defrost cycle, the electric heater is used to warm the indoor supply air, but its temperature still may fall below skin temperature, causing a "cold blow" sensation. This is not a problem with geothermal heat pump units, which do not require defrost cycles even in cold weather, due to the stable ground loop temperature.
Water-source heat pumps
Water is a much more efficient heat energy transfer medium than air, due to its much higher specific heat. Using pumps or fans with comparable efficiencies, it takes four times less energy to move a given quantity of heat with water than with air. Furthermore, due to the higher density of water, a piping conduit takes up less space than an air duct with the same heat moving capacity. Therefore water is the preferred heat distribution medium for large, multi-story buildings.
Conventional water-source heat pumps use a fossil-fuel-fired boiler as a heat source during the winter and an evaporative cooling tower to reject heat during the summer. This is sometimes referred to as a boiler/tower system; it is also known as a "California system", since this concept is thought to have originated in that state. The water loop temperature is maintained between 60 and 90°F. When the loop temperature falls below 60°F, the boiler adds heat, and when the loop temperature exceeds 90°F, the cooling tower rejects heat.
As shown in Figure 10, there is a common water loop connected to all the heat pump units distributed throughout the building, which themselves can have a variety of configurations, including horizontal, vertical, or console. The water loop can be integrated with the sprinkler system to reduce cost.
Ground-source (geothermal) heat pumps
As shown in Figure 11, a GHP heat pump system is a water-loop system where the boiler and cooling tower have been replaced with a buried earth heat exchanger (see the Ground Loops page for more information about various ground loop configurations). Heat is absorbed from or rejected to the ground, and the external energy needed to operate a boiler or cooling tower is eliminated.
Conventional water-source heat pumps are designed to operate in the relatively narrow temperature range of 60 to 90 °F and will not perform adequately in a GHP system unless their range is extended. This can be accomplished by replacing the fixed expansion device of a conventional unit with a thermostatic expansion valve. Some manufacturers also add specially designed compressors, heat exchanger coils, and controls.
The mechanics of how heat pumps move heat “uphill,” from a cooler region to a warmer one, have been explained above. Heat pumps must work harder to move heat up a steeper temperature gradient, and if the gradient is too steep a heat pump will not work at all.
As explained on the Earth Temperature and Site Geology page, during the times of the year when heating and cooling are required, ground temperatures are nearer room temperature than outdoor air temperatures. Thus during the summer cooling season, the ground is cooler than the air, and the ground loop is able to condense the refrigerant at lower temperatures (Figure 12), which means that less compressor power is needed to pressurize the refrigerant vapor.
Likewise during the winter heating season, the ground is warmer than the air, and the ground loop is able to evaporate the refrigerant at higher temperatures (Figure 13), which again means that less compressor power is needed, since the pressure drop through the expansion valve can be less and still vaporize the refrigerant liquid.
The water/refrigerant heat exchanger, refrigerant/air heat exchanger, and all refrigerant loop components shown in Figures 12 and 13 are contained in a single enclosure, which can be mounted horizontally (above a dropped ceiling, as shown in Figure 14), vertically (in a utility closet, as shown in Figure 15), or as a console (against the wall or under a window, as shown in Figure 16). Maintenance experience in schools with GHP systems suggests that horizontal units should NOT be mounted above the dropped ceiling in classrooms, because this makes access difficult, adding considerable time (and labor cost) to routine servicing of the heat pump units. Vertical and console units are easier to service and should be used if possible. When horizontal units must be used, they should be mounted above hallways adjacent to the rooms they serve.
Having concluded our review of different heat pump types, it is clear that GHP systems are very flexible. They can be easily subdivided or expanded to fit building remodeling or additions. They also can save money by allowing building managers to isolate and shut down unoccupied areas of the building, an important consideration for many school programming scenarios.
GHPs offer the energy- and space-saving features of a building interior water distribution loop, but without the cooling tower or boiler of a conventional water-source heat pump system.
There is no visible outdoor equipment exposed to weather deterioration or vandalism. The loop recirculating pumps can be installed in one or more small mechanical rooms, while the heat pump units can be installed above dropped ceilings, in utility closets, or as console units against classroom walls.
Finally, GHP systems can incorporate a domestic water heating loop for relatively little additional cost(in dollars or energy usage), as explained in the last section on this page.
GHP refrigerant fluids
The refrigerant working fluid used in geothermal heat pumps is R22, which also is the most popular refrigerant for packaged air conditioners and air-source heat pumps. R22 was developed in the early 1900s as a substitute for R12. It can produce more cooling capacity from the same size compressor with no significant power penalty. Because R-22 contains some chlorine, it is classified as an HCFC (hydro-chloro-fluoro-carbon).
The ozone depletion potential of R22 is only 5.5% compared with the depletion potential of R11 and R12 (CFC-11 and CFC-12), which are the two refrigerants thought to be most responsible for atmospheric ozone layer depletion.
The 1990 Clean Air Act Amendments call for R22 production (not use) to be phased out by the year 2030. This could be extended at a later date if continuing atmospheric tests so indicate. Regardless, R22 will be available over the useful life of any geothermal heat pumps installed within the next two decades.
A long-term substitute for R22 is R410a, a chlorine-free blend of two HFC refrigerants (R32 and R125), which is safe and easy to use, with no ozone-depletion potential. R410a has up to 6% greater refrigeration capacity than R22 and its Energy Efficiency Ratio (EER) is 5-6% higher, offering improved performance in addition to its environmental benefits.
It is important to note, however, that R410a is not a "drop-in" replacement for R22. Its operating pressures are nearly 50% higher than R22, requiring a redesign of the compressor as well as other components. In addition, there are compatibility issues with the lubricants, cleaners and other fluids used in the heat pump manufacturing process.
A leading geothermal heat pump manufacturer, WaterFurnace of Fort Wayne, Indiana, introduced an R410a GHP unit with its Premier E Series in 2002. The company plans to have its entire GHP product line converted to R410a by 2006.
Domestic Water Heating Alternatives
Water heating can be provided more efficiently with vapor compression technology than with electric resistance or fossil-fuel-fired water heaters. Coupling GHPs with vapor compression water heating has attractive economic benefits, and such hot-water recovery systems can supplement or replace conventional water-heating systems. When the GHP system is in cooling mode, hot-water recovery increases the system's space cooling capacity, possibly reducing the length of the ground loop in cooling-dominated GHP projects, or minimizing the need for supplemental heat rejection in hybrid systems. Even in the heating mode, a GHP can still provide hot water more efficiently and less expensively than other systems. There are three alternatives for combining GHPs with vapor compression water heating: Desuperheaters, Dedicated Heat Pump Water Heaters, and Multi-Function, Full-Condensing Water Heaters.
De-superheaters
A de-superheater is a relatively small refrigerant/water heat exchanger at the compressor outlet (see Figures 12 and 13). It transfers excess heat from the compressed gas to a water line that circulates water to a building's hot water storage tank. Placing the de-superheater coil between the compressor outlet and reversing valve enables water heating when the GHP is in either cooling or heating mode. They heat water with 5 to 15% of the energy that would otherwise be given up by the system's condenser. Note, however, that de-superheaters provide water heating only when the GHP system to which they are attached is operating. Backup water heating may be needed when space heating or cooling are not required, as may occur in the spring and fall.
Dedicated heat pump water heaters
Dedicated heat pump water heaters are heat pumps designed solely to heat water. They can be used with any type of building space heating and cooling system, and provide on-demand, high-efficiency water heating year-round. As with space conditioning heat pumps, water heating heat pumps can be either air-source or water-source. Water-units are particularly adaptable to GHP systems, where the ground-loop is the heat source for the water-heating heat pump.
Multi-function, full-condensing water heating systems
Multi-function, full-condensing water heating systems incorporate separate refrigerant condensing coils that use a building's potable water loop as a latent heat sink, in addition to having the usual heat pump coils for space conditioning, which use the ground loop as a heat source or sink. These systems can operate in several modes: space cooling only, space cooling plus water heating, space heating only, and water heating only.