The Not So Magical Geothermal Heat Pump

Written by Virginia DeVit on Tuesday, 08 May 2012. Posted in Geothermal

Glance into the Process and History

Every modern day house has a heat pump, whether they realize it or not.  In fact Americans use this item every day, it is the refrigerator.  Stand barefoot in front of your refrigerator, you will notice warmth on your toes, but when you open the refrigerator the inside is quite cold.  A geothermal heat pump works on the same principle.  In fact it also shares many of the same components; the compressor, the condenser, the evaporator, and the expansion valve. 

 This heat pump began is history in the 11th century Persia as a distillation technology to produce essential oils by Ibn Sina.  In 1755 that technology was used as artificial refrigeration to create ice during the summertime by Dr. William Cullen, an Englishman.  And again in 1805, by an American inventor named Oliver Evan; invented, but never built, the more recognized form of the vapor-compression refrigeration machine.  However, it wasn’t until 1851 that Dr John Gorrie, the “father of refrigeration”, was granted patent no. 8080 for a machine that makes ice using compressed and cooled air. 

The most recent representation of this technology was the air source heat pump.  This is an outdoor unit that transfers the heating potential from the outside air and transports it inside using refrigerant.  In the Adirondack and Albany regions of New York, winter air has an average temperature of 21.4oF and the year round temperature ranges between -25oF to 100oF.  In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency.  This inherent problem would eventually lead to using the consistent temperature of the ground with a geothermal heat pump.  A ground-source heat pump uses the shallow ground or ground water (typically starting at 45–54 °F) as a source of heat, thus taking advantage of its seasonally moderate temperatures. In contrast, an air-source heat pump draws heat from the colder outside air or tries to force more heat into already hot air and thus requires more energy.

 In this system, ‘heat’ absorbed into the liquid refrigerant causes it to evaporate and become a gas.  This heat-laden gas is pumped out of the evaporator by a compressor.  The compressor squeezes the gas into a small volume and raises its pressure and temperature.  The high pressure gas then flows into a series of tubing called a condenser and is cooled and condensed back into a liquid by passing either water or air over the tubing.  This cooled liquid flows through an expansion valve, where the liquids pressure returns to normal and then back to the beginning again in the evaporator.  The cycle is purely physical, not chemical, and heat energy is neither created nor destroyed. 


Geothermal, heat pump, diagram

How does the system switches between heating and cooling?  The simple explanation is that the evaporator and the condenser are actually very similar.  Just like we are all humans with different jobs, the evaporator and the condenser are both heat exchangers with different jobs.  The main difference comes from the direction of refrigerant flow (or in other words, the direction of the heat transfer).  Therefore, when you want to switch between a heating season and a cooling season, all that changes is the flow direction of the refrigerant by the means of a reversing valve.  The result is the evaporator coil becomes the condenser coil and the condenser coil becomes the evaporator coil. 

cooling with a heat pump

The compressor serves two functions and is the “magic” of the cycle.  First, it regulates pressure in the evaporator by withdrawing refrigerant vapors when pressure (or temperature) is higher than desired.  Second, it compresses the gas and in so doing, adds energy or heat content.  Compressing gas increases the gas’ temperature and decreases its specific volume.  There are many different types of compressors, including reciprocating, scroll, centrifugal, and rotary.  The function of compressing the heat-laden refrigerant gas into a high temperature gas is the same for all types of compressors.

A contributing factor for a compressor’s function is that the volume of a gas varies with the container.  A gas automatically fills any container it is put into, regardless of the containers size.  Neither a solid nor a liquid reacts this way.  A container for solids or liquids can be partially full, but a gas container is always full.  That is why when measuring the volume of a gas it is also important to know the pressure of the gas.   So a greater amount of gas within a specific volume means the pressure is higher. 

This pressure to volume ratio also has the added benefit of affecting the temperature of the gas.  For instance, if you have a container filled with gas and you add heat to the container, the pressure within the container will increase, without the addition of any more gas.  This principle is known as the “Charles Law”. Which specifies that, IF the volume remains the same, the absolute pressure of a gas varies at the same rate as the absolute temperature varies.  However the very nature of a compressor is that the volume changes, which changes the pressure of the gas which changes the temperature of the gas.  This is explained in a combination of Charles’ Law, Boyle’s Law and Gay-Lussac’s Law on temperature (T), volume (V) and pressure (P) relationships and results in the following relationship formula.

Temperature, pressure, volume relationship

The expansion valve does the work of the compressor in the opposite direction.  Unlike the compressor, the expansion valve does not have a major affect on temperature of the refrigerant, since it is working with liquid state of the refrigerant and not the vapor, or gas, state of the refrigerant.  The expansion valves main job is controlling the flow of the refrigerant liquid into the evaporator coil.  In order for the higher temperature fluid to cool, the flow must be limited into the evaporator to keep the pressure low and allow expansion back into the gas phase.

All of these components put together in a series, with the use of the state changes and the pressure changes, explain how the 45oF ground can result in a warm 70oF air in the home.  That is why geothermal heating and cooling is not so magical!

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