heat pump - Efficiency

A device designed to produce effective heating or cooling by moving heat from one place to another. If heating is needed in a building, a refrigerant such as Freon is pumped through a coil which is outside the building. Since the refrigerant is cold, it absorbs heat from the surroundings, such as the ground or the outside air. It then passes through a compressor which increases its temperature and pressure so that it turns into a vapour. The vapour flows through a coil inside the house where the warmth is radiated or blown into the room. The cooler refrigerant flows through a valve which lowers its temperature and pressure so that it liquifies again and is pumped to the outside coil to repeat the cycle. To cool an interior space, the valve reverses the flow so that the refrigerant absorbs heat from the inside and discharges it to the outside. Although the heat pump is an efficient means of heating, the need for heat is usually greatest when the outside temperature is at its lowest. Often, heat pumps which are designed for colder climates need an additional conventional heating system.

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. 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. Conversely, a heat pump requires work to move thermal energy from a cold source to a hot heatsink. 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 amount of 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 heatsink since some of it 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. 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. Thus as with all heat pumps, the energy efficiency (amount of heat moved per unit of input work required) decreases with increasing temperature difference.

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.

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 external energy use for the work input. For example, when an air cooling need can be matched to a water heating load, a single heat pump will serve two useful purposes.

Efficiency

When comparing the performance of heat pumps, it is best to avoid the word "efficiency" which has a very specific thermodynamic definition.

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. 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. 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. 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.

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.

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:

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. Two common types of heat pumps for homes are air-coupled and ground-coupled heat pumps 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;

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. 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.