Hillbilly Heater

Formula for Heat Transfer

(source: [1])

Industry Standards

• Hillbilly heater - 30 sf or 2 sq meters per 500' roll of 1/2" tubing if tubing has .7" OD - $62 - [2](note: special order, but 100' sections are in stock). OD is .712".

• Compare to 3/4" tubing - $72 for 500' roll- [3], 1" OD. Based on surface area, the 3/4" option is about 20% cheaper, and less work to install. 3/4" should be used, as it saves 30% on installation length for the same power.
• Thus: Hillbilly tubing is 30 square feet or about 3 kW of heat capture per 500' coil. 500 feet *12 inches per foot *.7 in (square inches) = 30 square feet. For 3/4", that is 500*12/144 = 40 sf, or 4 sq meters. 4 kW per 500' coil.

• Amazon - 4 sf - $124 - [4]
• Evacuated tube - 5 sf - $179 + $230 ship - [5]
• Evacuated - $670 - 15 sf - [6]

DIY

• $250 cost for 4x8 foot finned collector with CPVC, which works comparably to finned aluminum collector - or $8/sf - [7]

Calculations

Area

Current greenhouse has 16' long rafters, for a 512 square foot area. Solar income on that area is 50 kW. 30% of this would be 15kW.

• 1500 feet of 1/2" tubing combines to 90 square feet - under 20%.

Conductivity

• Conductivity of polyethylene is about 0.3 W/mK. [8]
• Verify - [9]

Surprise!

• Poly is as effective as copper in heat transfer in a simple water heater - [10] - see the Conclusion section.

System Goals

• Exchange Solar heat to aquarium water.
• Exchange heat without moisture with house via heat exchanger like [11]

Efficiency

Heat transfer is limited by the thermal conductivity of the pipe (PEH ~0.5 W/m K good for plastics).

Is high-pressure system ideal or is a low-pressure gravity system possible using black gutter(s) etc.?

Can water be mixed directly or is thermal mixing a bigger problem?

Can temperature be managed by pump control?

Reference Links

https://www.youtube.com/watch?v=0csXsRzrq2Y&t=5s Marcin Aquaponic Green House walkthrough