Help with solar power and biolet composting toilet

Re: Help with solar power and biolet composting toilet

Hi Rich,

Simple question, not a simple answer.... One way of approaching the problem is exactly how you asked it...

1. Where do you live? If in the US, find the city nearest one listed in the link below:

http://rredc.nrel.gov/solar/pubs/redbook/

2. Usually, the sun's energy is less in winter, and in your case, you will need the heater most in winter... So, assume that you get roughly 2-3 kWhrs per day in winter (on average) and that you need 2.5 kWhrs per day to run the system, and that charging the batteries, running the inverter (assuming 110/220 VAC) your overall efficiency is 80% charging, 80% inverter, 80% solar panel fudge factor derating from factory specs:

Solar Panel Rating (winter 2 kWhrs per day northern latitudes or "2 hours of sun per day":

SPr = 2.5 kWhrs per day (load) / (2hours of sun per day) * 0.8 * 0.8 * 0.8 (available sun * all of the efficiencies)
SPr = 2.44 kW or 2,440 watts of panels (not a small system)...

Battery size required; Assume 3 days of storage and don't use power below 50% of battery capacity:

Battery = 2.5 kWhrs (daily power) * 1/0.50 (battery margin) * 3 days (of storage) * 1/0.80 (inverter eff) = 18.75 kWhrs of battery

Or, in amp*hours at 12 volts (or pick your voltage) (again at 3 days of power storage and 50% battery margin):

Battery Amp*Hours = 18,750 watthours / 12 volts = 1,562.5 Amp*Hours (at 12 volts) of storage battery.

The average car sized storage battery is about 100 amp*hours plus or minus--so somewhere around the equivalent of 16 car sized storage batteries (or 21 batteries of the 75 amp*hour one you have).

The above numbers are probably within +/- 50% of what you would need for your application (depending on your location, the temperature required for running the composting toilet, etc.).

Now, for the real discussion... That toilet as described uses quite a bit of power. If I try, I can get my 3 bedroom suburban home (just south of San Francisco) down to about 6 kWhrs per day (4 people, washer/drier, fridge, 2 computers, laser printers, lighting, work at home/home school, natural gas hot water/drier).

The composting toilet represents almost 50% of my power usage. Also, depending on the type of inverter you will purchase, it may have some standby losses too that would need to be added.

My first question, is, I am guessing, that you already have Grid Tied power (electric utility service) for your place? If so, it would not make much sense to take this unit off-grid. Instead, look into Grid Tied Solar Power where you would connect the solar panels to a Grid Tie Inverter and dump the power directly into your AC Mains... And, if you have a large enough system (and small enough loads), you will actually turn your meter backwards.

The details of Grid Tied systems vary quite a bit state by state (and even more so, country by country). So, knowing more details about where you live will help somebody here with those questions.

Regarding the toilet itself, the 25 watt fan running continuously is a pretty large fan--if you can change it to a smaller fan and run it on DC--that would be a help (the best thing to do with solar power is to squeeze every bit of conservation first before you through, very expensive, solar dollars at a problem).

The 25 watt mixer is not a bad load for solar at all... It can be very easily run with a small solar panel and battery/inverter system (switching the inverter on when running the mixer or changing to a DC motor would save quite a big of "standby" power loads)... In this case, the standby load of an inverter would probably be more than the daily load of the mixer.

Lastly, the 225 watt heater is a big, variable load, that would be better served by either reducing its need (like adding insulation, passive solar heating to the tank) or even adding active solar hot water or hot air to replace the electrical elements. Other solutions like propane/gas or some other heating source would be another solution would probably be better than solar electric. Using solar electricity is like using a Lexus for a shopping cart in Kmart... It may work, but it is a huge waste of money to use solar electricity (and batteries) to generate heat.

I will stop here--because I am just guessing at your location, needs, and other stuff. You could easily need 1/2 of the 2.44 kWhr panels (and 1/2 the batteries) that I suggested in the first calculations, but you could also need more... It all depends on your loads, location and habits.

If you are interested in converting to Solar Electric power, and you have an existing 120 VAC installation, a great way to find your usage is to purchase a Kill-A-Watt meter for $25-$35:

http://www.amazon.com/P3-International-Kill-Electricity-Monitor/dp/B00009MDBU

Knowing your daily and monthly loads (in kWatt*hours or 1,000x Watt*Hours is the conversion) will take a great part of the guess work out of sizing your solar electric system, and also help you to conserve power in you day to day living.

-Bill

Re: Help with solar power and biolet composting toilet

Rich,

I cruised by the Biolet web site and see that it requires a minimum temperature of 64F to operate... Can you, through solar (passive or active) keep the room/supply air at that minimum temperature? Is there an air inlet where you can run some black painted PVC pipe (under glass in wooden frame, if needed) to warm the supply air? A simple mixer (warm/cold) to prevent overheating on hot days would probably almost eliminate the need for a heater except on the coldest days.

If there is no inlet pipe, just suppling heated air into the toilet room from you solar hot air panel (weather stripping the interior door to isolate the rest of the cabin) may be enough to keep it warm and happy. Especially since you are running part time. The constantly running fan (if it is exhausting room air) is not a great thing for keeping the cabin interior temperature comfortable (cold days, pull in cold air, hot days, pull in hot air to room).

Also, it needs positive airflow for composting. It appears that they have a #30 unit (no heater, no fans) where they simply install a 12 volt @ 0.6 amp fan to force more air through the unit for more capacity... If you can on the exhaust riser (create negative pressure in unit and no air leaks in-building) install a similar fan connected to a small solar panel (10-20 watt unit--don't bother with a battery if you are happy with 5-6 hours of generator operation today), you would probably be pretty happy with the results.

Doing the two items above would probably allow you to use the 75 amp*hour battery and generator for "mixing" flushes during the times the generator is not running. The 25 watt fan and mixer (without the heater) should work fine with the battery for about 18 hours between charges (25 watt / 12 volts = 2 amps ... 75 amp * 1/2 (50% battery reserve) / 2 amps = 18.75 hours).

If you wanted to run just the fan (and mixer) 24 hours per day from solar using an inverter and solar panels (and not the heater), it is not near as bad... Probably just 4-6x 75 amp*hour batteries (for 3 days of reserve power), and 200-400 watts of panels (with appropriate low-loss inverter) would work well (use a controller to prevent over-discharging of of batteries)... But a simple 12volt fan and 1 panel hacked into the exhaust pipe would be so much cheaper and easier to install and operate.

And if you have issues with temperature, only turn on the heater when the generator is running.

-Bill

Re: Help with solar power and biolet composting toilet

Rich,

For Passive Solar I was, more or less, talking about a south facing room with good sun and a window (and shade?) that you could open an close depending on heat gain or loss requirements.

The active solar could be of several types... From some black painted PFC or metal duct pipe against the wall to building a glass top box with wood sides and an air vent top and bottom to the room (bottom lets in cool room air, top exhausts warmed air back to room). You could place a piece of sheet metal painted black, spaced evenly between the glass and bottom, to gather the heat better and transfer it to the air stream. And, if needed, you could add a solar powered fan for forcing more air (make up for small piping, long runs, or if convective flow is not possible).

There are companies that make solar hot air heat exchangers for home heating. From what I have read, a properly designed system can use 40-80 sq. feet of collector to heat enough hot water for a family of 2-4 people... If possible, just build one out of 4x8 exterior grade plywood and some scrap glass and see how it works for keeping the small room warm.

Since it is probably a warm place in the summer, you would mostly need this for the winter. You may be able to simply mount this vertically on a south facing wall (if you are lucky enough to have one nearby) and get all the warm air you need for much of the winter. Summer you will probably need to close the vents.

Regarding the liquid wastes, I saw that one model actually had a drain for to draw off the liquids... Perhaps you can figure a way to do that for yours (or just run the heater when you have the generator running would probably be enough).

Good Luck!
-Bill