“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Saturday, October 27, 2012

Rocket City Rednecks redux

Photo courtesy space.com
In an earlier post, I discussed an episode of a series running on the National Geographic Channel called "Rocket City Rednecks" whose concept is that a bunch of exceptionally intelligent but eccentric Southerners take on projects that "big science" is too slow and ponderous to handle.

In the earlier post I discussed the Rednecks' design of a paddlewheel generator to use the energy in the flow of a river past their anchored boat to power some appliances on the boat. I concluded that, as shown, the system was implausible (though, admittedly, it could have been scaled up in such a way as to actually do what the Rednecks were claiming).

The episode I'd like to discuss here is called "Trailer Power" wherein the Rednecks purport to take Rog's (one of the Rednecks) trailer off grid, in large part due to wishing to avoid both the cost of electricity for cooling and the safety hazards of spraying their roof with water as they were doing as the episode started. As it happens, my intention is to build a house in the desert outside of the Los Angeles basin, and to take my house off grid. Thus, I watched the episode with interest.

They came up with a system consisting of three separate energy sources. One is a "gasifier" (really, as best I can tell, a wood gas generator) in which they intend to burn heat without oxidizing refuse, wood, etc. to generate flammable gasses that are then used as fuel input to an internal combustion engine driven generator. The next is to use ethyl alcohol to power a small internal combustion engine driving an alternator. For this, they did mention that they'd need to build a system for producing the ethanol but, whether or not they actually did this, it was not shown.

The third (intended to supply power for a cooling system that works by misting water onto the roof of Rog's trailer) wound up being powered by an exercise bicycle driving an alternator to charge a car battery which, in turn, powers a pump. The pump sends the water to a barrel reservoir mounted in a tree above the trailer roof, and then gravity drives the water through the hose system laying on the roof to supply the mist. (As an aside, the misting capability of the hose was produced by repeatedly shooting a garden hose with birdshot from a shotgun).

Are any of these systems practical in any way? Without a doubt, a wood gas generator system is capable of providing fuel for an internal combustion engine, FEMA even has a document describing the fabrication of such a system. I'll give this a plausible (though I'm not at all sure the the gasifier they built actually worked).

And, of course, it's true that alcohol can be used as an energy source for an internal combustion engine. The devil is in the details, however - where will the alcohol come from? In the episode as shown, they simply buy it. But electricity is cheaper, on an "effective joule for effective joule" basis, than pure ethanol. And home brew production of biofuel ethanol is fairly problematic, and this was simply left out of the show.

Finally, using leg power to lift water via a system of exercise bicycle -> alternator -> battery -> pump is very inefficient. However, not a lot of water is needed, so let's proceed in my usual fashion of estimation. In a perfect system, the evaporative cooling provided by the mist would by provided by a replacement rate of the water precisely equalling the rate of evaporation. This is the flow that would need to be achieved by the system (though, of course, intermittent operation between setpoints would certainly be the likely mode of operation).

I'm going to propose that the factors involved in the rate of replacement of water needed are: temperature, relative humidity, and roof area. It's much more complex than this of course, since a thorough analysis would incorporate the roof temperature which, in turn, would be a function of ambient temperature, emissivity, insolation, specific heat, etc. For the level of analysis appropriate here, we won't need this depth.

I'm going to use an article from the University of Michigan for an evaporation rate, and use an estimate of 8.5 meters X 17 meters or 144.5 m^2 for the roof area, and some ad hoc estimates for temperature and relative humidity. I will settle on an estimate of 5*10^(-4) grams/(cm^2*sec) or 5*10^(-3) kg/(m^2*sec) for the needed rate of water replacement.

Thus, in an hour, Rog will need to provide energy sufficient to, through the alternator, battery charging system, battery, and pump efficiencies, supply 5*10^(-3)*144.5*3600=2,601 kilograms of water. It looked like the storage barrel was something like 8 meters high, so this amounts to 2600*9.8*8=203,840 joules of work. Doing this in 3600 seconds equates to 56.6 watts.

A human can deliver this for quite a while, but I estimate that the alternator's efficiency might be 80%, the charging system a similar 80%, the battery 95%, and the pump perhaps 50%. Thus, Rog must input 56.6/(.8*.8*.95*.5) watts or 186 watts. Suffice it to say that only periodic activation of the cooling system will be possible if my estimates are in the ballpark (Rog is second from left in the photo above).

And finally, why do I care? I'm quite passionate about energy and sustainability and seeing it trivialized in such a way is troubling to me.

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