This article is the first one of a series of five posts dealing with wind & site studies carried out by some of the major wind energy post-war pionneers (Putnam in the US, Golding in the UK, Hütter in Germany and Juul in Denmark). The first four articles will provide highlights of their works. It is my impression (I may be wrong), that these studies are not known among the community of wind & site practitioners, and that they deserve more attention.
As you may already know, Aeolians.net does not deal specifically with wind turbine design (the topic is well covered in various books and essays), but instead focuses on historicising wind & site studies. Therefore, these will be at the core of the articles, and some wind turbine design issues will be left aside (external references will be provided for the curious reader).
The aim of these articles is first of all to provide references to the major wind & site works by these pionneers. This is done either by pointing directly to a scanned version of the document in Aeolians.net’s library, or by referring to the book on worldcat.org (unfortunately, for copyright reasons the books of Putnam and Golding are not freely available on the net).
This series of posts also aims at comparing these pioneer’s studies: what questions do they attempt to answer, and by what means ? how are they linked to wind turbine design ? what results have been obtained, and how are these relevant today ? The last article of the series will compile and combine the comments and reflections of the individual posts.
POWER from the WIND
“POWER from the WIND” is verbatim the title of the 1948 edition of the famous book of Palmer Cosslett Putnam (Putnam, 1948). As you can read from the table of contents below, the book provides a detailed description of both the siting and wind turbine design aspects, as well as general considerations about wind power. A later (mid 1970’s) edition of the book, including some additional information, can be loaned for free on archive.org, just follow this link. Mr. Putnam (1900–1984) has his dedicated Wikipedia page.
As you may already know, the experimental turbine described in this book was a 1.25MW-53m diameter downwind machine, located at the top of a hill named “Grandpa Knob”, in Vermont (USA). See below, it is about 4 hours by car from Boston, Massachussets.
There is already a lot of information about this turbine on the Internet, see for instance Paul Gipe’s website. Long story made short: it did not last long due to 1) a shaft bearing failure after the first 1000 hours of operations, and 2) a blade failure (it broke at the root and flew off). In the following, we will focus on the siting aspects of Putnam’s project.
Putnam provides in Chapter 1 an exhaustive list of the people who contributed to the project, in some way or another. And there are many. I counted at least 21 people involved in the technical design, besides Putnam himself. Among them, scientists like Theodor von Kármán and Sverre Petterssen, all “leading men in each discipline”. This approach to wind turbine development differs, to say the least, from the one of Johannes Juul in Denmark. It did have drawbacks, see for instance what Petterssen wrote about his contribution:
The idea of developing giant wind turbines, rather than conventional
mills with battery storage, had originated with Palmer Cosslett Putnam, a Harvard geologist turned engineer and promoter, who became the leader of the project. Putnam invited me to head a meteorological team to explore wind regimes and develop site selection criteria for blocks of turbines in Vermont, where a utility company had indicated interest.The work branched out in three directions. Professor Wilbur, my colleague at MIT, designed the tower, a complicated structure that could not only carry the turbine but also withstand stresses caused by storm winds and loads of ice. Von Karman designed the wind-catcher, a hundred-foot turbine blade with many sophisticated gadgets. The meteorology the wind regimes, icing storms, and damaging gusts-became my domain. This turned out to be the least explored area and it soon became the crux of the project. Putnam, with his driving energy, was a typical promoter, and believed that money and manpower would solve all problems.
I soon found myself in charge of a think tank of four subconsultants (of whom Rossby was one) and a large field program directed by Professor Karl O. Lange, a German expert on instruments and measurements. The potential buyer of the power was brought into the consultations at the very beginning, and the project was pressed forward with undue speed.
As a result, I soon became too heavily engaged in consultant work. Since some of my not-so-well-off students earned their living by part-time work on the turbine project, there was no easy retreat. Eventually, I made a resolution never again to become involved in industrial consulting work.
OK… well, ahem.
Still, much of what the book contains is interesting and relevant to the genealogy of wind & site studies. And, to my knowledge, the siting aspects of this project have not really been advertised. See, out of thirteen chapters, at least four entire chapters dedicated to wind conditions (chapters II, III, IV and V). I propose to go through some of the main findings in the rest of this post.
“Wind is not wind” (Chapter 2, 1939)
After summarising what is known about the wind conditions at the global scale, the second chapter of the book provides a list of the wind conditions that were considered relevant for structural design and for the estimation of the turbine production, see below. These were consequently tested, and from the results of these tests some research topics were identified and investigated further, by making more detailed field- and theoretical studies (see Chapters 3 and 4, 1940-1945).
The seven topics outlined above are considered from a high-level perspective only, as desktop studies. The data available come from the US weather bureau, and earlier works. For instance, see below the mean wind speed profiles at Boston (Figure 9, curve B) and Burlington (Figure 10). The curve B from Figure 9 was used for estimating the mean wind speed at the turbine location at hub height, and Putnam explains that it led to a large overestimation (their guess was 24 mph, later the measured value was close to 17 mph). It is unclear whether curve B was derived by Putnam’s team, or if it was provided by Brooks (I doubt it). Definitively, the approach was not sound, as it mixed two different things: the wind speed profile at a given location (properly represented by the measurements in Figure 10), and the difference in surface mean wind speed between plain and hill tops. Assuming that the mean wind speed was 40% larger at Grandpa Knob compared with the Blue Hill Observatory (already quite windy) was a pretty wild guess. It just shows how little was known about surface wind conditions at the time for this project team.
Another interesting question is the quantification of speed-up on top of hills, a matter of concern as well for Golding in the UK in the 1950’s. See below some schematic information available to the project team in 1939, based on works by Von Kármán and Petterssen. On this basis. Grandpa Knob was selected as it was thought to lead to a speed-up of 20% compared to the “free-air” (geostrophic) wind speed.
Other aspects (the effect of the turbulence on the time-averaged power values, icing, air density) were investigated in this preliminary phase. Then started the site prospecting phase and the Grandpa Knob hill was selected based on a number of criteria (including steepness and roughness as mentioned above). A met mast was erected, and the first measurements were not that encouraging see below how the team ran into an issue which many have to deal with, still today:
Followed much efforts to model the wind conditions above hills using wind tunnel measurements, but these proved to be unconclusive. As a result, the second phase of the wind & site studies was launched.
Wind in the mountains of New England (Chapters 3 and 4, 1940-1945)
The measurement campaign which followed was very extensive, see below a description of the measurement locations. For most of the sites, measurements were taken at different heights above the ground; at some sites more than a year of measurements was gathered.
Initially, some 4-cups Robinson-type anemometers were used, but due to icing, they were replaced by heated anemometers. Calibration tests in wind tunnels were used to quantify the difference between the two types. At the summit of Grandpa Knob, a gigantic 57m met mast was erected and nicknamed “Christmas tree”. Wow. Anectodically, and to put in parallel with what was done in the late 1970’s in California, windiness was assess using trees and local vegetation.
The data analysis (still preliminary at the time the book was written) revealed interesting but deceiving trends, compared to the original assumptions (see Chapter 2). Some attempts to quantify the effect of local roughness (trees) on the wind profile proved to be relatively informative.
Care was taken to assess the minimum length of a time series so that its mean is representative of the long-term (in this case 5-year) value. The book also includes considerations about monthly variations of the hypothetical power ouput of a turbine.
How to find it (the Wind) (Chapter 5)
In the last chapter dedicated to wind & site studies, some useful tips are provided for future studies. These includes, besides site visits and the support from a meteorologist, and unsurprisingly: a proper mounting of anemometry.
Summary
We have explored bits of the wind & site studies carried out during the Smith-Putnam wind turbine prototype project, highlighting that these took considerable efforts and time, and were well integrated in the project. The next posts in this series will shed some light on similar works by Juul (in Denmark), Hütter (DE) and Golding (in the UK). The book from Putnam contains of course much information about his turbine, its design and testing. Exploring the relationship between wind & site studies and turbine design is a topic which in my opinion is easier to treat when comparing together the works from Putnam, Juul, Golding and Hütter. This will be the topic of the last post in this series.
Comments/suggestions are welcome, thanks for reading this post.
Rémi
