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Geothermal - How it Works

Common Misconceptions:

  1. Geothermal is NOT the tubing that runs in the floor. Tubing that runs in or under a floor system is called radiant floor heat or radiant floor warming. It is a method for distributing heat (it cannot, in our climate be used for distributing cooling), not a method of producing heat. Can geothermal be used to generate the heat that can be subsequently distributed through a radiant floor system – YES.
  2. Geothermal DOES NOT mean that we circulate air through pipes in the ground and that the ground heats this air to about 55 Degrees and the air is then brought into the home and then only needs to be heated the additional 20 or so degrees to warm the house.
  3. Geothermal DOES NOT mean that we circulate water through pipes in the ground and that the ground heats this water to about 55 Degrees and the water is then brought into the home and then only needs to be heated the additional 20 or so degrees to warm the house.
  4. In this case, Geothermal DOES NOT mean that we drill deep enough into the Earth’s core to be able to extract heat from the core – much the way a volcano produces heat.

What is Geothermal?

Put quite simply geothermal is a method for heating and cooling a structure using the constant ground temperature. In reality the Earth is the world’s largest solar collector and at depths of roughly 5 feet below grade the Earth has stored enough energy to maintain about a 50 degree temperature ( in our area of Pennsylvania) year round. Geothermal heating and cooling utilizes a ‘ground source’ heat pump to either extract heat from the ground during the winter or reject heat into the ground during the summer.

One of the simplest comparison’s for a geothermal unit is, it is like your refrigerator – in the summer you are standing inside of the refrigerator, and in the winter you are standing in front of the refrigerator - where you usually feel the warm air coming out at the bottom.

How Does Geothermal Work

A ground source heat pump – or geothermal system- is nothing more than a series of interconnecting loops that work on the principle of the 2nd law of thermodynamics –‘Heat moves to Cold”

Ground Loop - This ‘loop’ consists of piping buried in the ground in some manner. It can be vertically (bore holes), or horizontally (slinky loops), or even thrown in the bottom of a pond. A Water and Anti-freeze solution is circulated through the piping, through the use of typically 1 or 2 small circulator pumps. The fluid that passes through this piping is either much colder than the ground temperature in the winter (as the heat has just been removed the fluid inside the geothermal unit) or much warmer than the ground temperature in the summer (as the heat has just been added to the fluid inside the geothermal unit). If the fluid is colder than the ground then the heat from the ground is transferred into the fluid, if the fluid is much warmer than the ground then the heat from the fluid is transferred into the ground. Ground loops may be installed in either a pressurized or a non-pressurized configuration.

NOTE: Open Loop Systems and Direct Exchange Systems see below.

Refrigerant Loop - The fluid that has been circulated through the ground is now brought into the structure and into the geothermal unit. The fluid is now run through what is called a coaxial coil – this is nothing more than a tube inside of a tube. While the fluid that has been circulated through the ground is passing through one of the tubes, refrigerant (technically now R410a, generically ‘freon’) is passed through the other tube. Again the principle of the second law of the thermodynamics is utilized. The refrigerant is at either a much lower temperature (having just had its heat energy removed) than the fluid and id therefore able to absorb the heat from the fluid, or is at a much greater temperature than the fluid (having just had heat energy added to it) and is therefore able to reject its heat into the fluid.

To make it simple, this refrigerant essentially has a very low boiling point, and therefore even when heated to just the 55 degree of so fluid coming in from having circulated through the ground, it is able to turn into a vapor. Once the refrigerant has changed states it is run through a compressor and then becomes a ‘hot gas’

Distribution Loop – Once the ground has heated the fluid, the fluid has heated the refrigerant, the refrigerant has changed state and been compressed the refrigerant is then passed through a heat exchanger of some sort. If this is going into forced air distribution then it is passed through an air coil, that looks much like the radiator in your car. If it is a water distribution system then it is passed through another coaxial coil heat exchanger. In the forced air system a blower is used to pull air from the house, and run it over the air coil, thereby absorbing heat from the coil or if the coil is colder than the air passing over it , the coil and subsequently the refrigerant absorbs heat from the air, then the ‘conditioned’ air is blown throughout the house. The same principle applies to the water to water unit applications.

Summary - Fluid passing in the loop in the ground extracts heat from or rejects heat into the ground, refrigerant loop extracts heat from or rejects heat into the fluid in the ground loop, compressor section, changes state of refrigerant, distribution loop extracts heat from or rejects heat into the refrigerant loop, and then sends the heat out to the house or is extracting the heat from the house.

Efficiency – Because geothermal works on the second law of thermodynamics, and it is primarily a method for ‘moving’ heat, not a method of ‘creating’ heat (like a burning process), it is extremely efficient. Basically for every $ 1.00 you put into operating the pumps and compressor on a geothermal unit you will get somewhere between $ 4.00 and $ 5.00 worth of heating capacity out of the unit.

 

Types Of Loop Fields:

  1. Vertical Bore Holes – Typically a 4 – 6” diameter hole that is drilled approximately 150 ft deep, usually containing ¾” or 1” HDPE pipe, that has a factory installed U-bend configuration at the bottom. The pipe is installed in the hole, and the hole is then filled back up with a ‘bentonite’ grout to provide good heat transfer between the fluid in the pipe and ground surrounding it. Bore holes are typically installed at depths of 150ft per ton on a 15 ft square grid.
  2. Slinky Loops - A slinky loop looks like a giant ‘slinky’ that has been stretched out and then is laid over flat on its side. Using either ¾” or 1” dia. HDPE pipe, roughly 750 ft of pipe is installed in 150 ft of trench. Trenches are dug approximately 3 ft wide x 150 ft long by 5 ft deep and the coil is laid flat on the bottom of the trench. 1 trench = 1 ton of system. The trenches need to have a minimum of 10 ft between them, but do not all need to be a straight line, they may curve around trees or go around corners.
  3. Pond Loops – A pond loop typically consists of about 300 – 400 ft of ¾” or 1” HDPE pipe that is laid out in a ‘slinky’ configuration then ‘rolled’ up into a ‘ball’. Ideally the pond itself should be at least ½ acre in surface area and a minimum of 8ft deep. One ‘slinky’ ball equals one ton of system. NOTE: There are other loop configurations and methods for installation in ponds, like ‘hyper-loops’ and ‘slim-jims’. While these have their place in larger residences and commercial structures, usually they are not as cost effective in small residential applications. Also note that the pond needs to be in relative proximity to the home.
  4. Open Loops – An open loop system is where water is extracted from the ground, typically from a well, or a spring, or an artesian well, the water is run through the geothermal units coaxial heat exchanger and the the water is discharged down a drain or another well or into a pond, etc. While these systems are highly efficient, as the incoming water temperature never changes, they are also extremely troublesome. (2) things have to be there for a ‘good’ open loop system:
    1. Very good water quality – all equipment manufacturer’s have water quality specifications that must be met not only for the unit to function to it’s fullest capacity but also not to void the warranty on the unit.
    2. Flow Rates – because an open loop system, essentially extracts water from the ground uses and the dumps it out, it does not immediately replenish itself. Therefore an open loop system must be able to maintain a flow rate of 1.5 gallons per minute per ton of system 24 hours a day, 7 days a week, 365 days a year, etc. So, in the middle of August, when this ‘well’ is being used for your domestic water, your laundry, and your dishwasher, not to mention possible adding water to your pool, it has to have 1.5 gallons per minute still in ‘reserve’ or your air conditioning system will shut down.

NOTE: Western Pennsylvania Geothermal does NOT recommend the use of open loop systems in our area.

  1. Direct Exchange – in a direct exchange system, rather than having a coaxial heat exchanger and the transfer of heat energy that take places between the refrigerant and the fluid, the refrigerant tubing is directly buried in the ground and the transfer occurs directly between the refrigerant and the ground. While this is a very efficient method, as it eliminates one transfer, and does take up less ground space, there are 2 issues with this type of installation:
    1. a. The piping that is buried in the ground is copper. You have hundreds of feet of copper tubing buried as opposed to thousands of feet of plastic pipe. The copper piping is not as durable as the plastic pipe. (NOTE: the HDPE plastic pipe comes with a 50 year manufacturer’s warranty and a 200 year life expectancy)
    2. b. The substance that is passing through the buried pipe is refrigerant, R410a, or ‘freon’. This is regulated by the EPA, and fines are levied if it is discharged into the atmosphere, would you want it in your yard?

 

 
 

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