HEAT PUMP A heat pump is a device which applies external work to extract an amount of heat

QC from a cold reservoir and delivers heat QH to a hot reservoir. Thus, heat pumps

move thermal energy in a direction which is opposite to the direction of

spontaneous heat flow.

Summary:-

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.

Working principle:-

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.

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.

Basic Operations

Following are the basic steps that are followed,

Evaporation

Compression

Condensation

Expansion

Evaporation:

The coolant circulating in the heat pump system collects heat from the air, water

or the ground and this process causes the coolant to evaporate and change to a

gaseous state.

Compression:

The heat pump's compressor rapidly compresses the coolant, which is now in a

gaseous state and several degrees warmer.

Condensation:

Using a second heat exchanger, the heat from the heated coolant is transferred to

the water circulating through the radiators. This causes the coolant to drop in

temperature and condense back into a liquid state. The radiators distribute the

heat provided into the heated areas. The chilled water in the heat loop then

travels back into the second heat exchanger, where it is heated again.

Expansion:

By passing through the expansion valve, the coolant travels back to the first heat

exchanger, where it is once again heated.

This cycle keeps repeating itself over and over again.

For any other heat engine, maximum efficiency can

be calculated from the Carnot cycle. Heat Pumps

are usually characterized by a coefficient of

performance.

Coefficient of Performance(COP)

The coefficient of performance (CP) for a heat pump is the ratio of the energy

transferred for heating to the input electric energy used in the process or It is the

number of units of energy delivered to the hot reservoir per unit work input.

So the coefficient is defined by

There is a theoretical maximum CP,

that of the Carnot cycle :

For a refrigerator, however, the

useful quantity is the heat extracted,

QC , not the heat exhausted.

Therefore, the coefficient of

performance of a refrigerator is expressed as

Different sources

The cold source (from which heat is recovered)

The external medium from which heat is drawn is called a cold source. In the heat

pump the refrigerant absorbs heat from the cold source by means of the

evaporator.

The main cold sources are:

Air: outside the space to be heated, typically air from the external environment;

water: from the water table, rivers, and lakes, when in proximity to the spaces to

be heated and at a limited depth below the surface.

ground, into which specific pipes relating to the evaporator are sunk to varying

depths (these pipes constitute a geothermal system).

The hot source (to which heat produced and recovered is transferred)

The air or water to be heated is called a hot source. Heat may be transferred to

the environment through:

- Fan coils, consisting of cabinets in which air is made to circulate through heating

bodies. These may be wall-mounted, ceiling-mounted or built-in models.

- Coils built into the floor, in which hot water circulates. Heating occurs mainly

through radiation and requires a lower water temperature.

- Canalizations, which transfer the heat produced by the heat pump to the

different rooms, by means of suitable air channels and inlet vents.

Different Heat Pump Types

Vapor compression refrigerator

Typical pressure and temperature conditions for such a refrigerator are as below.

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.

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).

Air-source heat pump in cooling mode

Air-source heat pump in heating mode

A packaged air-source heat pump unit

A split air-source heat pump system

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.

A boiler/tower heat pump system

Ground-source (geothermal) heat pumps

A geothermal heat pump system