Basic Heat Pump Theory

A heat pump transfers thermal energy from a heat source to a heat sink. However, as it is commercially applied, a heat pump can alter the flow of refrigerant so as to change the heat sink into a heat source. This is basic heat pump theory. In other words, the evaporator becomes the condenser and the condenser becomes the evaporator. Some heat pumps are sold as a heating only unit, but most are manufactured with a cooling option as well.

The diagrams below show a vapor compression system employing a reversing valve in both a heating mode and a cooling mode.

Heat Pump - Cooling Mode

heat pump theory cooling

Heat Pump - Heating Mode

heat pump theory heating

Heat pumps are considered to be quite energy efficient if it has a thermally-advantageous heat source. For example, geothermal heat pumps are quite popular due to its ability to tap into one of the most thermally advantageous heat sources, the earth.

When analyzing heat pump theory, one does well to consider the coefficient of performance (COP). The coefficient of performance calculation needs to be adjusted due to the fact that the main objective of a heat pump is the heating portion of the vapor compression system.

Based upon the first law of thermodynamics, the following equation can be derived using the control system above.

Qo = Qi + Wc

The coefficient of performance (COP) for a vapor compression heat pump is:

COP = Qo/Wc

or, applying the first law equation

COP = (Qi/Wc) + 1

The equations above help us to see that the COP of heat pumps is never less than 1. The useful heat is always greater than the amount of work put into the system. So the electrical energy that is used to drive the compressor is an efficient source of mechanical work that converts into heat

In comparing heat pump heating to traditional heating methods one must be familiar with the ambient temperature averages and ranges that exist. Typically cold or mild climates do not use air source heat pumps due the low COP during mild days that require heat.The equation below is the COP for an air conditioning system.

COP = Qi/Wc

Do you see the difference? A heat pump (used for heating) always makes best use of electrical energy.

So Is It Really a Heat Pump?

One versed in classical thermodynamics understands that heat cannot be transferred from a cold space into a warm space unless some form of mechanical work is accomplished. This is also true in heat pump theory. Therefore, heat is not truly pumped, but it is converted into mechanical work to be re-converted into thermal energy.

How Does It Work?

Most heat pumps utilize the vapor-compression refrigeration cycle . Therefore, when studying heat pump theory, most discussions surround the vapor compression cycle.

The cycle begins in the compressor (e.g. reciprocating, rotary, scroll) where low pressure, low temperature gas is compressed to a high pressure, high temperature super-heated vapor.

This vapor flows into a heat exchanger (condenser) that allows this high pressure, high temperature refrigerant to reject heat causing the refrigerant to condense into a saturated liquid.

The liquid moves across an expansion device (e.g. capillary tube, orifice, valve, turbine) into a second heat exchanger (evaporator).

Inside this heat exchanger, heat is absorbed by the refrigerant. This absorbed heat causes the liquid refrigerant to boil off into a completely vaporized state.

.....The cycle then starts all over.

A great paper was published by Arif Hepbasli at Ege University in Turkey. It discusses heat pump theory for geothermal applications.

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