Heat Pumps Fully Explained
A heat pump is a device that transfers thermal energy from a source to a sink that is at a higher temperature than the source. Thus, heat pumps move thermal energy in a direction which is opposite to the direction of spontaneous heat flow. The heat pump uses some form of low entropy energy to accomplish the desired transfer of thermal energy from source to sink.
Compressor-driven air conditioners and freezers are examples of heat pumps. However, the term “heat pump” is more general and applies to devices which are used to heat a conditioned-space (i.e., a confined space such as a building), that must be warmer than a cold environment. A heat pump can provide either heating or cooling of a given conditioned-space, depending upon whether the surrounding environment is cooler or warmer than the conditioned-space. When a heat pump is used for heating, it uses the same basic refrigeration-type cycle employed by an air conditioner or a refrigerator, but releasing heat into the conditioned-space rather than into the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground./p>
Heat pumps are used to provide heating because less high-grade (i.e., low-entropy) energy is required for their operation, than appears in the released heat. In general, most of the energy for heating in such systems comes from the external environment, and only a fraction comes from the high-grade energy source. For example, in an electrically powered heat pump, the heat power released to the conditioned environment can be typically two or three times larger than the electrical power consumed, making the system efficiency 200 or 300%, as opposed to the 100% efficiency of a conventional electrical heater, in which all heat is produced from input electrical energy.
Some heat pumps are able to accomplish either conditioned-space cooling or warming, depending on operational setting. Reversible-cycle heat pumps are devices designed to work in either thermal direction, in order to provide heating or cooling of the same conditioned environment. These devices operate by changing which coil is the condenser and which coil is the evaporator, rather than physically turn the device around. Such a function is achieved by a “reversing valve.” In heating and air conditioning (HVAC) applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices that are optimized for high efficiency in both directions of thermal energy transfer.
Heat pumps have the ability to move thermal energy against a thermal gradient at the expense of an external source of power, usually a mechanical compressor.
Heat spontaneously flows in the direction of the gradient of temperature. A heat pump moves heat in the opposite direction. A heat pump absorbs heat from a colder space and releases it to a warmer one. “Heat” is not conserved in this process and it is augmented by the high grade energy expended.
The name heat pump is by analogy with a conventional fluid pump that pumps a fluid against its natural flow from higher to lower elevation.
Heat pumps use a refrigerant as an intermediate fluid to absorb heat where it vaporizes, in the evaporator, and then to release heat where the refrigerant condenses, in the condenser. The refrigerant flows through insulated pipes between the evaporator and the condenser, allowing for efficient thermal energy transfer at relatively long distances.
If the heat pump is reversible, then a reversing valve allows for the flow direction of the refrigerant to be changed.
- In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or ground) indoors, after the fluid’s temperature has been augmented by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler. The refrigerant is then allowed to expand, cool, and absorb heat to reheat to the outdoor temperature in the outside evaporator, and the cycle repeats. This is a standard refrigeration cycle, save that the “cold” side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder.
- In cooling mode the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (which reaches a lower temperature) is the evaporator. This is the familiar mode in which air conditioners operate.
Mechanical heat pumps exploit the physical properties of a volatile evaporating and condensing fluid known as a refrigerant. The heat pump does work on the refrigerant to make it hotter on the side to be warmed, than at the cold side where heat is absorbed.
A simple stylized diagram of a heat pump’s vapor-compression refrigeration cycle:
1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.
The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized vapor is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device also called a metering device. This may be an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils. The refrigerant then returns to the compressor and the cycle is repeated.
In such a system, it is essential that the refrigerant reaches a sufficiently high temperature, when compressed, to release heat through the “hot” heat exchanger (the condenser). Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or else heat cannot flow from the ambient cold region into the fluid in the cold heat exchanger (the evaporator). In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid. Thus, as with all heat pumps, the Coefficient of Performance (amount of thermal energy moved per unit of input work required) decreases with increasing temperature difference.
Insulation is used to reduce the work and energy required to achieve a low enough temperature in the space to be cooled.
To operate in different temperature conditions, different refrigerants are available. Refrigerators, air conditioners, and some heating systems are common applications that use this technology.
Heat is typically transported through engineered heating or cooling systems by using a flowing gas or liquid. Air is sometimes used, but quickly becomes impractical under many circumstances because it requires large ducts to transfer relatively small amounts of heat. In systems using refrigerant, this working fluid can also be used to transport heat a considerable distance, though this can become impractical because of increased risk of expensive refrigerant leakage. When large amounts of heat are to be transported, water is typically used, often supplemented with antifreeze, corrosion inhibitors, and other additives.
When comparing the performance of heat pumps, it is best to avoid the word “efficiency” which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement per work input. Most vapor-compression heat pumps use electrically powered motors for their work input. However, in many vehicle applications, mechanical energy from an internal combustion engine provides the needed work.
When used for heating a building on a mild day, for example 10 °C, a typical air-source heat pump (ASHP) has a COP of 3 to 4, whereas a typical electrical resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place. Note that the heat pump is more efficient on average in hotter climates than cooler ones, so when the weather is much warmer the unit will perform better than average COP. Conversely in cold weather the COP approaches 1. Thus when there is a wide temperature differential between the hot and cold reservoirs, the COP is lower (worse).
When there is a high temperature differential on a cold day, (e.g., when an air-source heat pump is used to heat a house on a very cold winter day of 0 °C), it takes more work to move the same amount of heat to indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump’s performance will approach 1.0 as the outdoor-to-indoor temperature difference increases for colder climates (outside temperature gets colder). This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps.
Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. When it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater rather than to overload an air-source heat pump.
On the other hand, ground-source heat pumps (GSHP) are dependent upon the temperature underground, which is “mild” (typically 10 °C at a depth of more than 1.5m for the UK) all year round. Their year-round COP is therefore normally in the range of 4.0 to 5.0.
The design of the evaporator and condenser heat exchangers is also very important to the overall efficiency of the heat pump. The heat exchange surface areas and the corresponding temperature differential (between the refrigerant and the air stream) directly affect the operating pressures and hence the work the compressor has to do in order to provide the same heating or cooling effect. Generally, the larger the heat exchanger the lower the temperature differential and the more efficient the system becomes.
Heat exchangers are expensive, requiring drilling for some heat-pump types or large spaces to be efficient, and the heat pump industry generally competes on price rather than efficiency. Heat pumps are already at a price disadvantage when it comes to initial investment (not long-term savings) compared to conventional heating solutions like boilers, so the drive towards more efficient heat pumps and air conditioners is often led by legislative measures on minimum efficiency standards.
In cooling mode, a heat pump’s operating performance is described in the US as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/(h·W) (1 BTU/(h·W) = 0.293 W/W). A larger EER number indicates better performance. The manufacturer’s literature should provide both a COP to describe performance in heating mode, and an EER or SEER to describe performance in cooling mode. Actual performance varies, however, and depends on many factors such as installation, temperature differences, site elevation, and maintenance.
Heat pumps are more effective for heating than for cooling an interior space if the temperature differential is held equal. This is because the compressor’s input energy is also converted to useful heat when in heating mode, and is discharged along with the transported heat via the condenser to the interior space. But for cooling, the condenser is normally outdoors, and the compressor’s dissipated work (waste heat) must also be transported to outdoors using more input energy, rather than being put to a useful purpose. For the same reason, opening a food refrigerator or freezer actually heats up the room rather than cooling it because its refrigeration cycle rejects heat to the indoor air. This heat includes the compressor’s dissipated work as well as the heat removed from the inside of the appliance.
Heat pumps are only highly efficient when they distribute produced heat at a low temperature differential, ideally around or below 32 °C (90 °F). Normal steel plate radiators are not practical, because they would need to have four to six times their current size. Underfloor heating is one ideal solution. When wooden floors or carpets would spoil efficiency, wall heaters (plastic pipes covered with a thick layer of chalk) and piped ceilings can be used. These systems have the disadvantage that they are slow starters, and that they would require extensive renovation in existing buildings.
The alternative is a warm air system in which water runs through a ventilator driven water-to-air heat exchanger. Such a setup can either complement slower floor heating during warm up, or it can be a quick and economical way to implement a heat pump system into existing buildings. Oversizing the fans and ductwork can reduce the acoustic noise they produce. To efficiently distribute warm water or air from a heat pump, water pipes or air shafts must have significantly larger diameters than in conventional systems, and underfloor heaters should have much more pipes per square meter.
At National Air Warehouse we carry a selection of the top rated heat pump brand including Trane, Goodman, Coleman, Rheem heat pumps and many more. Most manufacturers have similar build quailty and because we only carry top brands that have excellent reputations, you can trust that you are getting the best quality products for the price.