jack vegas wrote:Matthew - Oh boy does your last response to Hank Fletcher have my head reeling. Lots of new stuff to think about! The Johnson Su Bioreactor paper is a real find. I haven't read it in depth yet but its bound to keep me awake tonight. Its particularly interesting to me because after getting into this conversation I'm thinking more and more about how to build a larger scale system needed to accommodate larger chunk material like yard waste and wood chips.
jack vegas wrote:Matthew - I like your central wand. One reason is that it would allow the fan to blow air rather than suck. My sucking fan had to work in a humid vapor saturated environment which is not great for an electrical device. There should probably be a dam in the the middle of the wand however to divert all flow through the drum. Without it, air could preferentially just travel in one end and out the other, without much flow through the perforations. Need to think about how to handle the rubbing seal.
jack vegas wrote:A bigger hair dryer may not help much since its important not to overheat the bed beyond about 140 deg-F, otherwise it kills rather than nurtures the bacteria. I picked the one I did since it was powerful enough and also it was DC. Another option might be to use an infrared "reptile heater" bulb installed inside the box. These can be had for about $25 including a a remote thermostat.
jack vegas wrote:Since I was mostly experimenting, I never ran the thing more than 4-5 days without shutting it down. I cleaned it out every time, though I'm guessing it could easily have run a month before clean-out would be needed. I estimated heat by taking the total dry material added over a run and dividing by the hours that were run, estimating 8,000-9,000 Btu/lb of dry material.
jack vegas wrote:I finally broke down and calculated heat soak through the barrel. Can't believe I never did before. Perhaps I did and promptly forgot about it. Anyway, a 55 gallon drum has about 21.6 sqft of surface area. The maximum surface temperature I can imagine is about 140 deg-F and at that point the steel can only radiate about 145 Btu/hr-sqft. So without convection, pure radiation can only dump about 3,130 Btu/hr. Convection with air (or water) is needed to draw more heat out of the system. This is both good and bad. The good news is an un-insulated drum could reach full operating temperature so insulation isn't needed. On the other hand, a fan is needed to be to push enough air over the drum to draw heat away through convection. Both to prevent over-heating and to draw more heat out of the system. It seems my insulated box served as a duct to channel that air. I may have been operating very close to the limits of my system and not have known it! So a good design would probably use the hair dryer to start the system and to protect against sudden cooling and the "box" should be configured to provide a good air cooling jacket to maximize convective heat transfer. Just about any material could be used for this air cooling jacket since it doesn't need to provide insulation. Cardboard and duct tape might be used to build a well formed cooling jacket and a relatively quiet and inexpensive box fan might be used as the convective air source!
corrado de cesare wrote:Hi, maybe you already know this guy's work, but I put a link anyway.
I am willing to build one for my 100% off-grid hut in the Alps, 1100 mt altitude, probably this autumn.
Lif Strand wrote:I'm definitely attracted to low-tech solutions like Hank is experimenting with. For one thing, they are projects I can do myself and that often use materials I might already have. Important because going into town for supplies takes a chunk out of a working day just for the travel time.
I'm off the grid so for me there are no electric bills but also I am limited as to what I can do with electricity to heat my house in the winter (at 7000' altitude the winters are below-zero cold!). My first solution is seal the sources of drafts and insulate, insulate, insulate! That alone will reduce power consumption for heat.
Heating an air-leaky, under-insulated house is a lot like heating the great outdoors. Investing in reducing the need for heat is way less expensive than the cost of heat in the long run.