Thermal Bin Part I

Submitted by Barbara on Wed, 02/27/2013 - 20:08

We know how hard it is squeak by on a sustainable level of Natural Gas.  This is becoming more and more important with the advent of Fracking

Winter heating is now only a fraction of our NG consumption (~40 therms/year) mostly thanks to Airkrete.  We need to work on modifying our habits to use the solar cooker more, but our highest aspiration is to find another way to heat our water.  Last summer we went with out/limited hot water for 2 months and we learned many little tricks to get us by.  We also decided that we like our hot water and we assume that many of you do to.

We got a quote for solar hot water that was $8000.  Thats a big bill for how much we could potentially save (maybe $100 per year?).  We wanted to do something low tech that other people could replicate at a low cost.  For a few years we have been thinking about a methane digester.  Methane digester/Anaerobic digesters are similar to composters/aerobic digester in that they use bacteria to break down waist and convert it fertilizer and create heat as a by product.  Composters work in an oxygen rich environment.  AD work in an oxygen deprived environment and use a different more aggressive set of microbes. 

Why an AD?  There is a range of published information that suggests more nutrients are retrained in the fertilizer though the AD process.  AD produces methane which can be used as a fuel source.  AD can more effectively handle a wider range of stuff such as poop and meat scraps.  AD break down materials into fertilizer much faster than a composter and so it can put out more fertilizer over time.

Draw backs to an AD system include: ordor problems, methane vented into the atmosphere traps 20x more heat than CO2, methane is a flammable gas and storage of methane can result in hazardous conditions.  AD are more picky about operating temperature.   

The design challenges were to eliminate the need to store and pressurize the methane/bio gas so that a simpler system can be used without safety risks and to create a thermally stable environment without having to generate heat from external resources.

We have finally come up with a design that we feel is worthy of a test model. The basics of the Thermal Bin are this.  It is a four fold symbiotic system. 

  • Take a large volume of compost and run PEX tubing though it.  An active compost pile should be 135-160 degrees.  The PEX tubing holds about 16 gallons of water at any given time.  This part of the system is essentially a batch heater.  As long as we draw only 16 gallons at a time (one load of laundry, 1 short shower or running the dishwasher once), the tubing in the compost should supply enough hot water to refill the hot water heater without lowering the internal temperature.
  • Also buried in the compost is an AD.   This will help to hold the AD at a constant & elevated temperature (122-140 degrees).  The AD in turn will generate fertilizer and methane.
  • The methane that is produced by the AD will then be routed under the water heater where it is immediately burned off by the pilot light eliminating the need for storage.  The heat of the NG pilot plus flares of methane should be enough to hold the tank at a high temperature (130 degrees is required for sanitizing) and to supply additional water for baths and the occasional 18 gallon shower. 
  • At the end we will have a large pile of compost to use in the garden.

Next up, our test model.  What worked and what didnt work with assembly.