Heat pump

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A diagram of a simple heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.
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A diagram of a simple heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

A heat pump is a machine or device that moves heat from one location to another via work. Most often heat pump technology is applied to moving heat from a low temperature heat source to a higher temperature heat sink.[1] Common examples are:


Contents

[edit] Operation

The term 'heat pump' is a slight misnomer; heat is not 'pumped', but instead is 'moved' by these devices. According to the second law of thermodynamics heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this.[2] Heat pumps differ in how they apply this work to move heat, but they can essentially be thought of as heat engines operating in reverse. A heat engine allows energy to flow from a hot 'source' to a cold heat 'sink', extracting a fraction of it as work in the process. Conversely, a heat pump requires work to move thermal energy from a cold source to a warmer heat sink. Since the heat pump uses a certain amount of work to move the heat, the amount of energy deposited at the hot side is greater than the energy taken from the cold side by an amount equal to the work required. Conversely, for a heat engine, the amount of energy taken from the hot side is greater than the amount of energy deposited in the cold heat sink since some of the heat has been converted to work.

One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant. 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 gas 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 like an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. This device then passes the low pressure, barely liquid (saturated liquid) refrigerant to another heat exchanger, the evaporator where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.

In such a system it is essential that the refrigerant reach a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heat sink. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid. 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 consequentially more energy is needed to compress the fluid. Thus as with all heat pumps, the energy efficiency (amount of heat moved per unit of input work required) decreases with increasing temperature difference. Due to the needed variations in temperatures and pressures, many different refrigerants are available.

Refrigerators, air conditioners, and some heating systems are common applications that use this technology.

In HVAC applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. The reversing valve switches the direction of refrigerant through the cycle and therefore the heat pump may deliver either heating or cooling to a building. Because the two heat exchangers, the condenser and evaporator, must swap functions, they are optimized to perform adequately in both modes. As such, the efficiency of a reversible heat pump is typically slightly less than two separately-optimized machines.

In plumbing applications, a heat pump is sometimes used to heat or preheat water for swimming pools or domestic water heaters.

In somewhat rare applications, both the heat extraction and addition capabilities of a single heat pump can be useful, and typically results in very effective use of the input energy. For example, when an air cooling need can be matched to a water heating load, a single heat pump can serve two useful purposes. Unfortunately, these situations are rare because the demand profiles for heating and cooling are often significantly different.

[edit] Refrigerants

Until the 1990s, the common refrigerant were often chlorofluorocarbons such as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture was discontinued in 1995 due to the damage that CFC's cause to the ozone layer if released into the atmosphere. One widely-adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Other substances such as liquid ammonia, or occasionally the less corrosive but flammable propane or butane, can also be used.

[edit] Efficiency

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 is used to describe the ratio of useful heat movement to work input. Most vapor-compression heat pumps utilize electrically powered motors for their work input. However, in most vehicle applications shaft work, via their internal combustion engines, provide the needed work.

When used for heating a building on a mild day, a typical heat pump has a COP of three to four, whereas a typical electric resistance heater has a COP of 1.0. That is, one Joule of electrical energy will cause a resistance heater to produce 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. Sometimes this is inappropriately 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 instead moves existing heat "upstream".

Note that when there is a wide temperature differential, e.g., when heating a house on a very cold winter day, it takes more work to move the same amount of heat indoors as compare to 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. This typically occurs around 20°F or so outdoor temperature. 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. In other words, when it's extremely cold outside, it's simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-coupled heat pump.

In cooling mode a heat pump's operating performance is described as its energy efficiency ratio (EER) or seasonal energy efficiency ratio (SEER), and both measures have units of BTU/h*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 installation, temperature differences, site elevation, and maintenance.

Heat pumps are more effective for heating than for cooling if the temperature difference is held equal. This is because the compressor's input energy is largely converted to useful heat when in heating mode, and is discharged along with the moved heat via the condenser. But for cooling, the condenser is normally outdoors, and the compressor's dissipated work is rejected rather than put to a useful purpose.

For the same reason, opening your food refrigerator or freezer heats up your kitchen 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.

The COP for a heat pump in a heating or cooling application, with steady-state operation, is:


COP_{\mathrm{heating}} = \frac{\Delta Q_{\mathrm{hot}}}{\Delta A} \leq \frac{T_{\mathrm{hot}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}} = \frac{1}{\eta_{\mathrm{carnotcycle}}}


COP_{\mathrm{cooling}} = \frac{\Delta Q_{\mathrm{cool}}}{\Delta A} \leq \frac{T_{\mathrm{cool}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}


Where Qcool is the amount of heat extracted from a cold reservoir at temperature Tcool, and Qhot is the amount of heat delivered to a hot reservoir at temperature Thot.

[edit] Air or Ground Heat Source/Sink?

Commercial heat pump technologies are currently in a stage of rapid improvement; the COPs for commercially available heat pumps have risen in the last five years from three to four, and even in a few cases to five. As a result heat pumps are becoming popular choices for home-heating as well as cooling, especially in areas with less-severe winters. Two common types of heat pumps for homes are air-coupled and ground-coupled heat pumps (geothermal heating), depending on whether heat is transferred between the indoor air and the outdoor air or the ground.

For an air-coupled heat pump in heating mode the COP is limited by the need to move the heat into the house from outside; they don't work well at times in cold climates. Typically the COP decreases markedly once outside temperatures fall below around -5 or -10°C (23 to 14°F).

Those buying an air-coupled 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 move, and at the noise generated.

Because a ground-coupled heat pump, in heating mode, draws heat from the ground or groundwater, which below a depth of about eight feet is at a relatively constant temperature year 'round, its COP is often higher, on average, than for an air-coupled heat pump. Its COP is more constant year 'round. The tradeoff for this improved performance is that a ground-coupled heat pump is usually more complicated due to the need for wells or buried coils, and thus is also usually much more expensive to install than an air-coupled heat pump.

[edit] See also

[edit] External links

[edit] References

  1. ^ The Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
  2. ^ Fundamentals of Engineering Thermodynamics, by Howell and Buckius, McGraw-Hill, New York, 1987
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