Heat pump
Encyclopedia : H : HE : HEA : Heat pump
A heat pump is a machine which moves heat from a low temperature reservoir to a higher temperature reservoir under supply of work.
Common examples are:
- gas compression heat pumps
- phase change heat pumps
- thermoelectric heat pumps that use the Peltier effect
- geothermal exchange heat pumps
- vortex tubes
Operation
Heat pumps are realized through several physical effects, but they are classified depending on their applications (driving energy, source and sink of heat, or a heat pump which is basically a refrigeration machine). Refrigerators, air conditioners, and some heating systems are all common applications of heat pumps.An easy way to imagine how a heat pump works is to imagine the heat in a given space - say the volume of a football (or soccerball). The air within the volume of the ball has say 100 units of heat. This air is then compressed to the size of ping pong ball (table tennis ball); it still contains the same 100 units of heat, but the heat is much more concentrated and thus the average heat per volume unit is much higher. The ping pong volume of heat is then moved from the heat source area to the target area that has a lower per volume concentration of heat. Since the heat of the ping pong ball volume is now a higher concentration than the surrounding heat, the heat is given off until the ping pong ball volume heat reaches the same concentration of heat as the surrounding area. The ping pong ball volume is then moved outside the target area back to the heat source area and allowed to expand. In expanding the volume of refrigerant in the ping-pong ball is expanded to the size of the football, and the heat energy per unit volume is now well below the 100 units enabling the expanded refrigerant to absorb heat from the surrounding area. The compressor unit creates the pressure difference which causes this cycle to endlessly repeat as long as the heat pump system is running.
Efficiency
When comparing the performance of heat pumps, it is best to avoid the word "efficiency", as it has many different meanings. The term coefficient of performance, or COP (or sometimes CP), is used to describe the ratio of heat output to electrical energy input. When used for heating on a mild day, a typical heat pump has a COP of three to four, whereas a typical resistive electric heater has a COP of one. That is, one joule of electrical energy will cause a conventional heater to give off one joule of warmth, while under ideal conditions, one joule of electrical energy can cause a heat pump to move more than one joule of heat from a cooler place to a warmer place. Sometimes this is expressed as an efficiency value greater than 100%, as in the statement, "XYZ brand heat pumps operate at up to 400% efficiency!" This is not quite accurate, since the work does not make heat, but moves existing heat "upstream". This does not violate the second law of thermodynamics, because it takes less work to move the heat than to make the heat. Note that when there is a wide temperature differential, i.e. when heating a house on a very cold winter day, it takes more work to move heat indoors, and it is possible that a heat pump's COP would be below 1 in such a case. In other words, when it's extremely cold outside, it's better to just make new heat indoors using a conventional heater, than to try to take it from outdoors using an air-sourced heat pump.Heat pumps are typically somewhat more efficient for heating than for cooling. This is because the machinery always wastes some energy as heat. For a heating application, this waste heat can be captured and added to the warm side, but for cooling, at best it can be mostly excluded from the cold side. That's why opening your refrigerator on a hot day will actually heat up your kitchen. Not only does it not cool your kitchen, since the heat taken from the ice box is ejected into the room, but there is actually more heat coming out the warm side than going in the cold side. An open refrigerator is essentially a very complicated electric heater. For this reason, we use a different formula for computing the COP of a cooling appliance or a heating appliance. For cooling, we're not interested in how much heat is ejected out the back of the refrigerator, we want to know how much heat is taken out of the ice box.
Here's how to compute the COP for a heat pump in a heating or cooling application:
- [COP_} = \frac}} \leq \frac}}}-T_}} = \frac}}]
- [COP_} = \frac}} \leq \frac}}}-T_}}]
Commercial heat pump technologies are currently in a stage of rapid improvement: the COP for commercially available heat pumps has risen in the last 5 years from 3 to 4 and even (in a few cases) 5. As a result heat pumps are becoming popular choices for home-heating. Two common types of heat pumps for home heating are air-source and ground-source heat pumps depending on whether heat is transferred from the air or from the ground.
For an air-source heat pump its COP is limited by its need to pump the heat into the house from outside - and so they work less well in very cold climates where there is less heat density outside to pump in. Typically the COP decreases markedly once outside temperatures go below around -5 or -10 degrees Celsius (23 to 14 Farenheit).
Those buying an air-source heat pump should look closely at the heat pump's COP, at what outside temperature range that COP is effective for, at the cost of installation of the pump, at how much heat it can pump, and at the noise generated.
Because a ground-source heat pump draws heat from the ground (usually from groundwater), which below a depth of about 8 feet is at a relatively constant temperature year round, its COP is higher than for an air-source heat pump and its COP is constant year round. The penalty for this improved performance is that a ground-source heat pump is significantly more expensive to install than an air-source heat pump.
Heat pumps are also becoming more commonly used to heat swimming pools and to heat water for household use.
See also
External links
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