Energy is absorbed by the earth from the sun throughout the year and stored in large quantities. The air at the surface then absorbs this solar energy from the ground. As the energy in the air is constantly be replenished, it provides a constant, renewable energy source that an air-to-water heat pump can use to heat a building.
Specific heat capacity
The specific heat capacity of a substance is the amount of heat energy it requires to raise the temperature of 1kg of the substance by 1oC. For water this is 4.2 kJ kg-1 K-1 and for air at normal conditions it is 1.0 kJ kg-1 K-1. Note that this does not change if you are heating the substance from 10oC to 11oC or from 90oC to 91oC.
Conversely, 4.2kJ of heat must be given out to cool 1kg of water down by 1oC. If we look at the example below we see that all a air-to-water heat pump is really doing is moving heat energy from one body of water to another.
1) Both bodies of air and water start at the same temperature (N.B. 1m3 of water = 1000kg and 1m3 of air = 1.2kg)
2) The heat pump cools the larger body of air by approx. 4oC. This gives out 4,800kJ of energy!!! (1kJ x 1200kg x 4oC)
3) The heat pump then uses this energy to heat the smaller body of water. As water has a much higher specific heat capacity however this only heats the water by 1.1oC (4800kJ ÷ 1000kg ÷ 4.2kJ)
From this example you can see that by cooling a large body of air by a few degrees, it is possible to heat a small body of water. This is the main principal behind an air source heat pump. A large volume of air is pumped through one side of the heat pump while a small volume of water is heated as it passes through the other.
However there is one obvious problem with the above example. We all know that heat travels from the hotter body to the cooler one until both temperatures are the same e.g. a hot cup of tea cools down to room temperature. Therefore how is the heat pump moving energy from two bodies that start at the same temperature?
The answer lies in the common household fridge and the refrigeration cycle.
Latent Heat
To explain latent heat we use a simple experiment that many people will have carried out in the early years of secondary school. Ice is placed in a beaker with a thermometer and is heated using a constant heat source. Then the temperature is recorded a set time intervals and a graph plotted. An example of the apparatus is shown below.
Obviously you will find that the ice melts at 0oC and boils at 100oC. If however you were to perform this experiment under strict conditions your results would look something like this.
At point A, C and E the temperature is rising, as we would expect when we add heat. However at points B and D, the temperature is constant, even though we are heating the substance the whole time. However as we know point B and D are the melting and boiling point of water, the energy must be being used to change the state of the water (i.e. solid à liquid and liquid à gas). This is called the ‘latent heat’ and it is the energy required to overcome the molecular forces between the particles so the substance can change state.
If we now look at the change of state from liquid to gas (evaporation/vaporisation), we will get a better idea of how a heat pump works.
The specific latent heat of vaporization is defined as ‘the amount of heat required to convert a unit mass of a liquid into the vapour without a change in temperature’.
For water at its normal boiling point of 100 ºC, the latent specific latent heat of vaporization is 2260 kJ.kg-1. This means that to convert 1 kg of water at 100ºC to 1 kg of steam at 100ºC, 2260 kJ of heat must be absorbed by the water. Conversely, when 1 kg of steam at 100ºC condenses to give 1 kg of water at 100 ºC, 2260 kJ of heat will be released to the surroundings.
Very simply
When something evaporates, heat is taken in e.g.
o You feel cold when you get out of a swimming pool. The water evaporating off your skin takes extra heat from your body and makes you feel cold even though the pool hall is hotter than a normal room
o You sweat when you’re warm. The sweat evaporates and takes heat away from your body, cooling you down.
When something condenses, heat is given out e.g.
o When you get steam from a kettle on your hand, it burns so badly. The steam condenses on your hand and gives out extra heat.
These two processes are the foundation of refrigeration and heat pump technologies.
Refrigeration Cycle
The above diagram shows the refrigeration cycle inside a air-to-water heat pump. Instead of water however, the heat pump uses a refrigerant gas with a very low boiling point (-40oC and lower, depending on the refrigerant used). This allows it to evaporate very easily at low temperatures.
The refrigeration cycle is made up of 4 steps
1) The refrigerant is allowed to evaporate in the evaporator. This draws heat from the air into the refrigerant
2) The refrigerant is compressed using a mechanical motor. This increases the energy per unit volume and allows the heat pump to produce higher output temperatures. This is the largest electrical input into the heat pump.
3) The compressed gas is allowed to condense in a condenser. This gives out the heat to the heating water. The more compressed the gas, the higher temperatures can be reached
4) The gas is allowed to uncompress as it goes through an expansion valve and the cycle starts again
To achieve maximum efficiency, the temperature of the heat source must be as high as possible and the flow temperature on the heating side must be as low as possible. In this way the compressor will not have to compress the refrigerant gas as much and less electricity will be used. This again highlights the importance of the design of the geothermal heat source and the heat distribution system.
With the correct design of the heating system from heat source to heat distribution, Mitsubishi Ecodan air-to-water heat pumps can achieve a co-efficient of performance (COP) of more than 4 (defined for an air temperature of 7ºC and a heating side flow temperature of 35ºC), i.e. for every 1kW of electrical power used in the compression stage, there is 4kW of heat output, 3kW of which comes free from the air.