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I've been looking at a commercial composting toilet for a mobile, off-grid application ("boondocking rv"). I've done a power budget and the toilet is coming out as the biggest user of energy. Power is provided via solar PV panels, and the system needs to work in the winter when there's 3x less sun than summer (at 48 deg N, Montana, USA). Are there any tips or tricks to make the energy consumption smaller, or the supporting electrical system less expensive?

Specs are:

  • 120W heater element, thermostatically controlled, nominally 50% duty cycle
  • 12VDC, 4W fan, continuous

Assuming that the heater's energy is 100% devoted to evaporating liquid, this amounts to 1.5 kWh evaporating about 2.4 L daily. Would insulating help, or are the primary losses due to evaporative cooling (and hence, fixed)? What are the consequences of unplugging the heater at night (and reducing battery size)?

Edit: To clarify, I'm looking for the kinds of tips and tricks which wouldn't violate the warranty on a commercial product or suggestions about alternative commercial products which have different operating principles.

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Options fall in 3 categories:

A: Reduce the amount of water to evaporate.

  • This can be done as easily as running pee to whatever route your greywater uses. Urine is moderately salty (various salts in addition to sodium chloride) and has urea in it from protein breakdown, but is not a health hazard. It does smell, but if mixed with greywater, and discharged a reasonable distance away, it should not be objectionable.

  • If you are in a fixed place for most of this time, dig an outdoor privy. Use the RV internal toilet for night and foul weather use only.

B: Restructure your toilet to use heat from another source. These violate your commercial solution criteria.

  • Reconfigure/rebuild your toilet using principles of the SunnyJohn solar powered composting toilet (http://www.sunnyjohn.com) This uses the heat of the sun directly. Not sure how feasible it would be to do this in an RV.

  • Use a solar water heater panel on the side of the RV to collect thermal heat instead of electricity. For your application this is about three times as efficient, and there are many DIY plans for it. See http://www.builditsolar.com. In addition there are commercial solar heater panels. While panels are commercially available, you would need to put some form of heating coil in the toilet.

  • Add a counterflow heat exchanger to the vent system. This uses the outgoing warm moist air to preheat the makeup air. Potentially this cuts the amount of energy needed in half.

  • Draw the air from a solar air heating panel. By bringing air in at 90 to 120 degrees you are reducing the demand on the heating coil.

C: Generate more electricity.

  • Add more solar cells to your array.

  • Get some cheap aluminum covered panels, and arrange them to concentrate more light on the solar panels. I bet you could triple the output of the panels without crisping them. (A product called fyrestop consists of 1/2" sheets of fused rock wool with foil on one face and white plastic on the other. Sheets are easy to cut. Seal the edges with foil tape.

  • You may want a separate sub system for the toilet. This could run directly off a solar cell. It would mean that the unit's duty cycle would be longer -- run in daytime only.

  • I just edited the question to limit the scope to available commercial solutions. I visited the sunnyjohn site, but it does not seem to be a purchase-able. In addition, the operating principle seems to be massive storage to allow for long rest times (i.e., not mobile). Can you provide a link to a commercial product designed to use thermal energy directly via some sort of heat exchanger? Such a device is likely to be compatible with a wide range of heat sources, not just solar thermal. – Bryce Nordgren Oct 11 '14 at 12:52
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Obviously, the 120 W electrical heater element is where to save the most energy. Not the 4 W fan, which seems ok.

I suppose that a candle or small ethanol or propane burner can be a valid substitute. A candle has 40 - 100 W "thermal power rating". The 120 W electric heater element at 50% duty cycle would have 60 W average rating, so it would fit. If and how this can be safely integrated into a commercial toilet design, I don't know. "It depends".

In any case, electricity for heating is a waste, and as a manufacturer I would have replaced the electrical heating element with an electronically controlled small propane burner.

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