The Homemade Heat Pump Manifesto

superlen · 11-30-15, 08:34 AM

FYI,

Here's what led me to this.

The thermal conductivity of PE(X) is approximately 0.41-0.46 W/(m K), copper is a whopping 28. (no surprise)

And here is a table I found that is recommendations of trench length vs. the Thermal Conductivity of your dirt. As you might expect, if you have good dirt, you can have a shorter trench, but all we really care about for this discussion is that the range of thermal conductivity shown. I'm assuming that this covers 90% of the installations. Note that ever the worst soil is still almost exactly equal to the conductivity of our plastic pipe, and in most cases the dirt is twice as good or better at pulling the heat away from the pipe, once the heat gets *INTO* the pipe. Therefore anything we can do to more easily get the heat from the water to the pipe (turbulent for instance) can easily be absorbed by the ground and we can reap the benefits.

Now lets think about another scenario. We could *GREATLY* increase the heat xfer from the water to the pipe by making the pipe Copper instead of pex. (Let's totally ignore the potential pitfalls and massive cost of doing this for now). If we did this, then turbulent flow is not going to be as useful, as once you have the heat into the copper, it still flows slowly into the ground. A smaller loop field could be in order to reduce the under utilized surface area, but then you might need to bump back up turbulent....see how it all relates. The loop field of a geothermal has one simple task, transfer heat. But is an entire system with several variables, and they all need to work in sync for us to get the greatest efficiency out of them. Lots to think about. Luckily, others before us have done some thinking & established at least a rough outline of what has been shown to work. I think one can start there and then push the boundaries some to see what happens.

Len

11-30-15, 08:34 AM	#11
superlen Helper EcoRenovator Join Date: Nov 2015 Location: NW Arkansas Posts: 37 Thanks: 5 Thanked 7 Times in 7 Posts	FYI, Here's what led me to this. The thermal conductivity of PE(X) is approximately 0.41-0.46 W/(m K), copper is a whopping 28. (no surprise) And here is a table I found that is recommendations of trench length vs. the Thermal Conductivity of your dirt. As you might expect, if you have good dirt, you can have a shorter trench, but all we really care about for this discussion is that the range of thermal conductivity shown. I'm assuming that this covers 90% of the installations. Note that ever the worst soil is still almost exactly equal to the conductivity of our plastic pipe, and in most cases the dirt is twice as good or better at pulling the heat away from the pipe, once the heat gets INTO the pipe. Therefore anything we can do to more easily get the heat from the water to the pipe (turbulent for instance) can easily be absorbed by the ground and we can reap the benefits. Now lets think about another scenario. We could GREATLY increase the heat xfer from the water to the pipe by making the pipe Copper instead of pex. (Let's totally ignore the potential pitfalls and massive cost of doing this for now). If we did this, then turbulent flow is not going to be as useful, as once you have the heat into the copper, it still flows slowly into the ground. A smaller loop field could be in order to reduce the under utilized surface area, but then you might need to bump back up turbulent....see how it all relates. The loop field of a geothermal has one simple task, transfer heat. But is an entire system with several variables, and they all need to work in sync for us to get the greatest efficiency out of them. Lots to think about. Luckily, others before us have done some thinking & established at least a rough outline of what has been shown to work. I think one can start there and then push the boundaries some to see what happens. Len