NREL Releases Report on “Urban Turbines”

By Mick Sagrillo

urban turbines

Wind turbines on the roof of a building.

Late in 2016, the National Renewable Energy Laboratory (NREL) released a long-awaited report titled Deployment of Wind Turbines in the Built Environment: Risks, Lessons, and Recommended Practices. This followed a presentation by NREL’s Ian Baring-Gould of the findings at the 2016 Small Wind Conference. While the report certainly breaks the news gently, as NREL is prone to do, it makes it perfectly clear that what are termed Built Environment Wind Turbines (BEWTs) are not a great idea…depending on the project objectives.

BEWTs include a wide array of conventional horizontal axis turbines and vertical axis turbines (VAWTs) plus hybrids of these two, building integrated turbines, and all manner of new inventions, most of which are touted by their purveyors for their ability to take advantage the low winds and turbulence found in the urban or built environment. The built environment includes cities and suburban areas, as well as pretty much anywhere that one would install a turbine that does not meet the small wind industry best practices (i.e., on a tower so that the rotor is at least 30 feet above obstacles within 500’ or above the mature tree line in the area, whichever is higher).

The report draws its conclusions from six case studies that were installed with various project-specific objectives as well as the Warwick Wind Trials in the UK.

For those of us in the small wind industry, these conclusions are not news. However, for those contemplating designing, specifying, purchasing, installing, or offering grants for BEWTs, there may be some eye-opening takeaways including:

• “None of the case study projects met their energy production estimates.” The NREL report documents performance, something which is nearly impossible to get from either urban turbine manufacturers or the owners of such installations. One installation reported a capacity factor (CF – the amount of energy generated versus potential output based on rated output of the turbine) of 7%, actually a high number for such an installation. This was for a set of four Skystream wind turbines on 45-foot towers atop a 23 story building, a highly unusual installation to say the least. At the other end of the spectrum was the more usual performance for an urban turbine: a 0.04% CF. If you were to calculate payback for such an installation, it would be in the thousands of years. Without including maintenance costs.

  • “Accurately assessing the wind resource in the built environment is perhaps the single most challenging element of a BEWT project….The current tools used for modeling the resource of small wind projects can have very high uncertainty in complex terrain. BEWTs are considered to be located in extremely complex terrain, and there is a demonstrated over-prediction of energy outcomes in the urban environment.” The report goes on to recommend complex—read: expensive—computational fluid dynamics (CFD) modeling as a possible means of understanding the wind resource at the proposed installation. “The challenge is that even the advanced models don’t capture the full array of flow physics and are expensive to run, potentially costing more than the wind turbine itself.” More reliable might be three-dimensional wind measurements at the exact location of the proposed installation for at least a year. Such measurements would need to include determination of turbulence intensity, which affects not only energy generation but also turbine loading and ultimately operations and maintenance (O&M) and longevity. Experience reveals that, amazingly, most urban turbine installations never do a site assessment, let alone three-dimensional documentation of the wind resource and turbulence at the proposed site followed by a CFD analysis, which could easily exceed the cost of the equipment. Yet even if you do monitor and model, there’s no guarantee that this is the wind resource that is actually “out there.” As such, there’s no reliable way to predict performance.
  • Although certified turbines should be selected for all small and distributed wind projects, current national and international turbine standards do not reflect wind conditions often seen in the built environment. This should be a heads up for anyone interested in a BEWT installation. I don’t know of a public benefits program or granting agency in the country that will fund a small wind turbine that is not certified to American Wind Energy Association (AWEA) 9.1 or International Electrotechnical Commission (IEC) Standards. Since current certification does not cover urban or built environments, in all likelihood such an installation will not be eligible for grant dollars. And in all likelihood, nor will it be covered by the manufacturer’s warranty should the manufacturer know the details of the installation. “Because installations in the built environment are currently not common, it is important to establish with the manufacturer that they are willing to warranty and support the wind generator in the environment where it will be installed.”
  • Logistics of getting equipment and workers onto the roof and providing for safety can be a significant additional expense…The potential additional complexities of performing maintenance on BEWTs can lead to cumber- Copyright © 2014 American Solar Energy Society. All rights reserved. SOLAR TODAY SPRING 2017 57 some and expensive practices. Additionally, increased turbulence levels common in built-environment areas may result in additional maintenance requirements and decreased turbine reliability.
  • The reference here is to codes, insurance, equipment requirements, access to buildings or sites , and all of the other liability issues that can complicate an installation and routine maintenance, especially when pedestrians are present. Compound this with the additional maintenance requirements due to the turbulence caused by the built environment and you may as well hire a full time crew to babysit the installation. Experience shows that the building’s maintenance personnel are not interested in the responsibility of trying to keep the installation running. All too often, the result of trying to keep a BEWT running is a non-functional turbine.
  • If a wind system is being added to an existing building that was not designed for it, the structural reinforcements required to distribute the load to the existing structure can often exceed the cost of the wind generators.Unless the wind turbine’s loads were designed into the building at the building’s conception, doing so after the fact is another very costly and timeconsuming endeavor. The structural engineering required to understand the dynamic loading given difficult-to-quantify wind loads and turbulence would keep any PE up at night, and his or her professional liability insurance carrier salivating. The inspections and scrutiny for a building permit from an agency that might be capable of such a task will add months to the installation, as it should. Oh, now add the required additional materials and labor to the installation cost.
  • Another issue is the vibration caused by the rotating blades. While the vibration will likely become audible as it is transferred to the building, it is nothing compared to the impact that the vibration has on a structure itself. All manner of structures have been known to self-destruct under the continuous vibrations posed by the turbine rotor, something accounted for in the design of any standard wind turbine tower and foundation. So, what positives made BEWT installations worth their cost and effort in the report? These included:
  • LEED (Leadership in Energy and Environment Design) certification creditsTwo of the case study sites specifically mentioned LEED as a reason for their installation. The awarding of LEED certification points to BEWT installations has been a years-long disagreement between the Distributed Wind Energy Association and the US Green Building Council. The small wind industry has gotten nowhere in persuading the USGBC to upgrade its criteria for wind installations on and around buildings.
  • Marketing or public relations value of a commercial buildingMost in the small wind industry cynically scoff at this as greenwashing. The NREL report unintentionally explains why: “In some cases, siting turbines for appearance will result in a project that has virtually no energy production value.” Footnote: “It is worth noting that if the ultimate goal of a project is to demonstrate a dedication to green energy or sustainability, turbines may be installed in a location that maximizes visibility over production. This placement can lead to turbines that spend a large amount of time motionless and do not produce much energy. The end result could be a perception that wind turbines do not work, enforcing the opposite impression than was desired.” As stated in the NREL report, while they may be seen, BEWT turbines don’t deliver as promised.
  • Education and outreach Only one installation cited using the multiple turbine installations as a test lab “providing data and experience for the general public and industry professionals”: the Boston Museum of Science. This is actually highly unusual as most project owners are not thrilled with exposing themselves for what will likely be perceived as a poor investment due to poor, if any, energy generation.Let’s do a little exercise for the non-believers. Four Urban Green Energy (UGE) Eddy GT VAWTs (rated at 1 kW each) at one site together produced enough energy in March 2015 to keep one 60-watt light bulb lit for just under two hours (out of a total possible 774 hours). Doing the math on payback, and assuming it was an average month: –60 watts / hour x 2 hours = 120 watt hours / month = 0.12 kWh / month generated –0.12 kWh / month x 12 months = 1.44 kWh / year (theoretical generation over one year) –Assuming $0.12 / kWh as the value of a kilowatt hour, then $0.12 x 1.44 kWh / year = $0.17 as value of electricity / year (yup, you read that right) –This cost of this installation of four UGE Eddy turbines was $100,000. Again, doing the math: –$100,000 / $0.17 in electricity / year = 588,235 year payback. For the mathematically challenged, that’s over half a million years. Without adding maintenance costs.

When added together, it’s understandable why the expenses due to engineering, permitting, and liability versus a difficult to quantify (at best) or non-existent (more likely) wind resource, simply do not make BEWTs cost effective on an energy production basis, especially when compared to installing a properly cited certified wind turbine on a tower suitable for the site.

This is an incredibly valuable report! If you want to do something positive with renewable energy in an urban or suburban built environment, put up PV instead, something that will actually generate energy in that environment. And this is coming from a wind guy!

About the author:
Mick Sagrillo (msagrillo@ has powered his family’s home with all manner of wind turbines for the past 35 years.

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