FAVOURITES- 28 August 2009 – Small scale wind energy has recently gained a lot of interest, particularly in the urban context, due to the general drive for sustainability and the appeal of the technology. But there are limits to what it can do.
Wind energy is one way of generating energy at the point of consumption. It can make a meaningful contribution where the wind resource is adequate, and it requires a detailed study of the site to maximise the return on a significant investment. Although the design phase makes use of advanced engineering techniques, a simple preliminary calculation is worth undertaking to assess the scope of a proposed wind installation at your site.
Consider a commercial building in a location where the wind resource is adequate. The building has 20 storeys, each 1500 sq m in area, and its energy consumption is 150 kWh a sq m, yearly. If one were to develop wind energy at the site, what would a reasonable objective be for the amount of energy that can be generated in this project?
Let’s start with the wind turbines. Except for a few radically novel designs where buildings are designed around wind turbines, it is assumed here that the rotor of a wind turbine suitable for a building installation can be at most five metres tall, or wide, or whatever the largest dimension of the building is. Such a machine would typically be commercialised as having a power rating of approximately six kW.
One key performance indicator of any wind installation is the capacity factor, which is the ratio between the mean power output and the rated power capacity of the installation. For reference, the capacity factor of commercial wind farms in Australia generally range between 20 per cent and 40 per cent.
In the case of building-mounted wind turbines, we suggest the capacity factor of a good to outstanding installation can range between 10 per cent and 30 per cent, depending on the conditions and the quality of the development. For illustration, a single 6 kW turbine operating at a capacity factor of 20 per cent will produce a mean power of 1.2 kW, which corresponds to slightly less than 38 000 kWh, or 38 MWh a year.
Back to our office building, which has a mean power usage of 514 kW. It would take a 500 kW wind turbine, operating at a capacity factor of 30 per cent to supply enough energy to offset 30 per cent of the building’s consumption. In terms of scale, the rotor diameter of a typical 500 kW wind turbine is around 45m. This is equivalent to more than five floor-areas of typical solar photovoltaic panels rated at 100 W a sq m.
Let’s take a more reasonable approach, based on the limitations of the building size and roof area: assume an installation where nine 6 kW machines are installed along the predominant windward perimeter of the building, spaced approximately 10 metres apart. If this installation was designed to perform with a capacity factor of 20 per cent, it would generate around 2 per cent of the building’s energy needs.
It should be noted that increasing the number of wind turbines (say, loading the other side of the building with turbines) also increases wake interference between the turbines, which will reduce the overall capacity factor. Taking the same approach to solar, an energy yield of around 3% of the office building’s demand could be expected with a photovoltaic installation covering 50 per cent of the building’s roof area.
Clearly, these two technologies are not mutually exclusive, and the actual solution should involve a combination of both where possible. As an example, installing nine wind turbines and covering half of the roof surface with photovoltaic panels could be expected to supply approximately 5 per cent of the building’s energy needs.
So at a first estimate, our generic 20-storey office building could be expected to generate around 5 per cent of total energy demand on-site, based on reasonable roof-top wind turbine and solar installations. These figures were derived from a simple study of power density for consumption and generation.
The power demand of the building was evaluated per square metre of total internal floor area, while the renewable energy generation was calculated from the total area of solar panels exposed to available solar power, and from the wind power available to the total swept area of wind turbines.
Before the design phase, even these simple calculations can provide, to within an order of magnitude, a reasonable estimate of wind turbine performance.
A recent surge of interest in urban on-site generation has led to some high profile investments into building-mounted wind turbines in Australian cities. In the press surrounding these announcements, some promoters have claimed that the wind turbines would offset a large percentage of the buildings’ energy demands.
This article provides a basic estimate of what may be reasonably expected from a wind turbine installation on the roof of a typical office building. A building-mounted wind turbine installation is a significant investment, and should be undertaken with an understanding of expected returns. At this nascent stage in the building-mounted wind turbine industry, the disappointing outcome of an over-optimistic installation should not precipitate unwarranted conclusions about otherwise promising technology.Damien Leclercq is managing director and Shannon Mason is project engineer of Cyclopic Energy Cyclopic Energy specialises in wind resource studies, including the micro-siting and installation design of urban and building-mounted wind turbines.