Hillbilly Heater

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Install of hillbilly heater in 2016 - Seed Eco-Home.

About

A 'hillbilly heater' is black tubing serving as a liquid to liquid or liquid to air heat exchanger. The heater absorbs solar radiation in its working medium, and sends the energy to where it is needed. It is an effective and low cost heat system that is viable wherever there is a greenhouse. Costs are

Formula for Heat Transfer

Thermalcalc.jpg

(source: [1])

Industry Standards

  • Hillbilly heater - 30 sf or 3 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. This is not PEX - need to evaluate PEX for same power if freezing conditions exist. $91 for 300' - not better than 1/2".
  • 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.
  • Furthermore - pumping losses are lower with 3/4" tubing compared to 1/2"
  • 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

  • $45 for 300' of 1/2" poly - [8]
  • $52 for 1/2" PEX - "+PEX+300%27
  • System cost -$20 power supply, $35 pump, $52 for 300' of tubing. Epoxy to waterproof the submersible pump

Cost for Power

  • With an $18/square meter ($72 for 4 sq meters) or $18 per kw captured energy - we are at about $36/kw of useful heat if we assume 50% heat trapping efficiency. 30 kW of this is about $1000 cost.
  • Compare to a 30kW wood burning stove - comparable price. Except in the solar case the energy is free.

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. [9]
  • Verify - [10]

Surprise!

  • Poly is as effective as copper in heat transfer in a simple water heater - [11] - see the Conclusion section.

System Goals

  • Exchange Solar heat to aquarium water.
  • Exchange heat without moisture with house via heat exchanger like [12]

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?

BOM

  • Hosec clamps - 20 - [13]
  • 1" tubing for bends - "+hose
  • 2 GPM submersible pump - [14]
  • Red PEX for freezeproofing in winter?

Reference Links

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