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

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

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.

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