“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.

Indicated gas mileage

My Lexus CT200h has an indicator that I reset at each fill up that shows an indicated fuel efficiency (in miles/gallon or m.p.g.). I've noted that it consistently (and, now that I've analyzed it, ALWAYS) reads higher than the mileage that I calculate by dividing miles travelled on the odometer versus gallons logged at the pump during fill up.

I thought that I'd have a look at the data and found that every single one of the 56 fill ups I've performed since acquiring the vehicle exhibit this phenomenon. I ran a linear regression between the calculated and indicated fuel efficiency and found the best fit line to have a calculated mileage about 2.6 m.p.g. below the indicated mileage for each fill up. I noted the same error in my previous vehicles (the ones that had such a display), though I never took the time to plot and analyze the data so I can't say if there were no exceptions as I now can with the Lexus.

I don't have an explanation, it doesn't seem credible that it's intentional in order to mislead the driver - the calculation is simply too easy.  Nor can I come up with a theory that attributes the discrepancy to my driving habits. Further, I'm not sure how the indicated m.p.g. is calculated, though I assume that it uses the odometer and integrates a gasoline flow rate. In any case, it seems to exaggerate my actual fuel economy by about 5%. It's a good thing I keep the actual data!

The chart below shows the data points, a line showing where the points should fall, and the best fit line for where they actually fall. The horizontal axis is indicated m.p.g., the vertical is actual (that is, calculated from miles driven and gallons added) m.p.g. Note that every point falls below the upper line, which represents where they'd fall if the indicated and calculated mileages were equal. The vertical distance between the two lines represents the difference between the best fit line for indicated mileages versus a perfect fit between calculated and indicated mileages. In other words, it represents the error in the indicated m.p.g.

Saturday, October 20, 2012

A wind blows in London

As anyone regularly reading my blog knows, my enthusiasm for renewable energy and energy efficiency is deep and wide. I'm in the midst of starting an entire division in my Company devoted to efforts in the energy space. I'm a voracious reader of renewable energy and energy efficiency articles, blogs, and literature. And, of course, my Company is positioned in the materials testing and built environment testing arena. Thus, it was with some interest that I subscribed to a free trade magazine entitled "Building Test News," whose tag line is "The industry publication for the testing, research and certification of building materials and applications" and whose thrust, in the first issue at least, seems to be energy related.

In this, the premier issue, one of the main articles is entitled ""Peak demand" and is about a London project called "One St George Wharf." A large part of the article is devoted to a vertical axis wind turbine (VAWT) atop the 181 meter, 49 story residential tower. The VAWT was designed by Matilda's Planet, a firm specializing in green energy harvesting, energy efficiency, and clean tech. A two page pdf describing the VAWT (from the company's point of view) can be read here. It's anticipated that the VAWT will power the tower's common areas. Given my skepticism regarding the VAWT installation atop the Hess Tower in Houston, what am I to make of this installation? Let's dig in!

According to the brochure linked above, the turbine is rated at 12 kW (kilowatts - a unit of power and a "rate" of energy production) and is anticipated to provide approximately 35,000 kWh (kilowatt hours - a unit of energy produced) in the course of a year, depending on wind conditions. This is a capacity factor of

This is a pretty high number for a VAWT in an urban environment, but they do say "depending on wind conditions." In Mythbusters style I'll give them a "plausible."

OK, next they give the dimensions of the turbine as 10 meters high by 6 meters in diameter, yielding an intercepted area of 60m^2. Let's take a wind velocity of s m/s^2 and look at a calculation similar to the one I detailed in the "What can the wind do?" post. This will yield an available power in the wind of 36*s^3 watts. Now, the Betz' law  shows that, at the very best efficiency, only 59% of the power in the wind can be turned into electrical energy, yielding an absolute maximum of 0.59*36=21.2*s^3 watts. A typical VAWT might actually achieve 30% but we'll be generous and assume that Dr. Tony Mewburn-Crook, the VAWT's primary designer, came up with innovations that allow 35% of the available energy to be captured. This would yield a power of 0.35*36*s^3 or 12.6*s^3 watts.

The literature states that the turbine is rated at 12 kW, so this implies that it achieves its rated power at a wind speed of

This is 22 m.p.h., not unreasonable at all. Also, it would not be surprising to find winds of such speed on a regular basis at the top of a 181 meter (almost 600') tall building in London. Again, quite plausible.

It's stated that the VAWT will power the "common areas" in this residential structure. In the post about the Hess Tower, linked above, I used 15 kWh/ft^2 per year as a measure of energy use in an office building. I would expect a residential tower to use less light, less frequently and the common areas less still. I'll go with 3 kWh/ft^2 per year. This estimate means that the VAWT could provide the energy to light 35,000 kWh/(3 kWh/ft^2)=11,700 ft^2. I assume that, in a tower, the common areas are first floor lobby, elevator lobbies, some outdoor areas, and a variety of other miscellaneous areas. For 49 stories, 11,700 ft^2 yields an average of 239 ft^2/floor. I suspect the actual number is significantly higher on most floors and much higher on the ground floor. I'm not buying that the VAWT can light the common areas.

Finally, let's look at the economics. The brochure states that "Approx payback under Feed in Tariff based on current prototype electric outputs 20 yrs." Hmm. 20 years*35,000 kWh/yr yields 700,000 kWh. Feed in tariffs (FITs) are monies paid under contract to supply (typically renewably sourced) electricity. As best I can determine from the University of Google (see here for example), the FIT in England for a generator of this size would be about 24p (pence)/kWh. At current exchange rates this is about $0.39. I find this rather shocking, in that I only pay a bit over a dime for electricity at our house. In any case, this seems to imply that the VAWT costs on the order of 700,000 kWh*$0.39/kWh or $273,000.

Now, payback periods are a poor way to calculate the economic desirability of an investment when they exceed a couple of years, since they fail to recognize the time value of money. But suppose that I receive a cash flow of 35,000 kWh*$0.39/kWh or $13,650/year. If I use a rational discount rate of, say, 8%/year and carry it over 10 years (a VERY long period for this type of analysis), the net present value (NPV) of the cash flows is just under $92,000. Even using a ridiculously low cost of capital of 3%, the NPV is about $116,400.

Conclusion: I don't think ludicrous claims are being made for what this turbine will provide in terms of electrical energy, but I don't think it will light the common areas and I certainly think that the reasons for including it in the project are mostly non-economic. I wouldn't call it greenwashing, but these types of projects really amount to a distraction.