Geothermal heat transfer

[steve9491]

First, I am a non-engineer. I am building a geothermal heating & cooling system for my garage (kind of an experiment). System description: 24 - 4" (inside dia) PVC pipes connected horizontially in two rows of 12 ea, buried 5' below ground surface (volume = 82 cu ft). The plan: use a small fan to pull ambient air from outside, down thru the buried pipes, and then (when cooled to the ground constant temperature (60 degrees), pulled by the fan into the garage. I need to know how long ambient hot air takes to cool to constant temperature below the ground surface (rate of cooling). e.g. surface air temp 90 degrees, underground constant temperature of 60 degrees...how long must air sit in pipes to go from 90 to 60? Current plan is to have the fan on a timer (run for the time it takes to move 82 cu ft of air to the garage - I have a damper flap to close the pipes after the run cycle). Then for winter, the opposite...how long to raise the ambient temperature (say 20 degrees) to the constant 60 degrees underground?

 

[MintJulep]

Well, almost forever. But it will take less time to cool or heat the air in your pipes to almost 60.

So let's just for arguments sake say it will take and hour.

Your system will provide a whopping 45 BTH/hr cooling or heating effect.

About equivalent to letting an ice cube melt every hour.

 

[EnergyProfessional]

not sure if you just cool your garage... but mold is an issue with such air-cooling. The air you bring in will get below or near dewpoint.

It even is an issue with solar thermal storage that store heat with air in bricks. for cooling i imagine it being even worse.

Even if it would work well, the loop needs to be deep down vertically to have much surface for heat to travel. Otherwise your ground warms up over time and doesn't cool anymore. Geothermal wells are 300'deep (and still use a refrigeration cycle to cool/heat in a usable manner)

 

[willard3]

You will fill this system with condensate and mold quickly if you run it during hot/humid weather.

 

[walkes]

i believe there are a number of buildings on the west coast of Canada using that type of system to pre-treat ventilation air (earth tubes).

For sizing a geothermal system you should know your soil conductivity, conductivity of the piping, flow rate etc.

 

[tsgrue]

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Try some W3 searches for "earth air heat exchanger" and you'll get lots of hits. These are commonly used in many parts of the world...

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

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