Heat pump energy efficiency
A heat pump is a mechanically simple system that is conceptually difficult to relate to. The boiler plate answer for how geo works is...
“We take heat from the ground and dump it into the house in heating mode and we take heat from the house to dump into the ground in cooling mode. Because the earth has a near constant temperature geo allows us to use less energy in moving this heat than an air source heat pump.”
That doesn’t really answer the question of how the heat pump works but it seems to satisfy the majority of people. Without getting into too much detail, and without omitting so much the explanation is useless, here is basically how a heat pump works.
The Ground Rules
|Heat can only travel from high temperatures (sources) to low temperatures (sinks).|
|Heat can not travel if there is no difference in temperature between the source and the sink.|
|Everything inside the heat pump is in a 'closed' system.|
|In a closed system, the amount of fluid contained is unchanging.|
The Ideal Gas Law
The ideal gas law is the key to understanding the vapor compression cycle (the cycle used by all heat pump systems including your refrigerator and a/c). With this simple equation we can relate pressure to temperature which explains how a compressor and some tubes and one little valve can heat a space without using a flame and cool a space without adding ice.
P · V = n · R · T
pressure The uniform force per unit area exerted on the working fluid.
volume The amount of working fluid in the closed system.
The number of moles of gas which describes the amount of working fluid.
The universal gas constant
temperature The amount of heat in the
T is Proportional to P
Ground Rules #3 and #4 make our life much simpler. The fact that we are operating in a closed system means that the Volume of fluid and the number of moles of fluid are constant.
Since R is also a constant, Pressure and Temperature are the only two terms that can change in our equation and they are directly related by a constant value. In other words, when pressure increases so does temperature.
This is the key to understanding vapor compression cycles. You can change the temperature of a fluid just by changing its pressure.
When pressure increases so does temperature.
Its Temperature Differences That Matter
The purpose of a heat pump is to move heat from where it is, to where we want it. For a heat pump to move heat, there must be a difference in temperature between where the heat is, and where we want it to go.
The heat pump is the middle man in a system containing a high temperature sink, a low temperature source, and the heat pump itself. The heat pump moves energy from the source to the sink using a ‘working’ fluid. The working fluid is inside the heat pump and it is the fluid that the heat pump directly works on. A perfect example of this in action is your home refrigerator.
Your kitchen acts as the sink and is probably around 70°F. The inside of your fridge is the source and probably around 36°F. If Ground Rule #1 is true, then we shouldn’t be able to move heat from inside of the fridge to the kitchen because the kitchen is hotter. This is why we need the fridge’s heat pump and its working fluid.
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