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What can wind power provide?

Wind-driven electricity generators are an intermittent source of power and for this reason, they’re most commonly installed as a supplementary power source. Unlike solar energy, wind power may be available throughout the day and night; however, wind is inherently more variable and less predictable than solar energy. The viability of generating sufficient farm-scale wind energy on your property depends on the nature of your wind resource.

For wind turbines, bigger is generally better; however, small-scale wind power can be a viable option for farms that:

• use a significant amount of electrical energy,
• are located in windy areas (e.g. in coastal or upland areas), and
• have the space to erect a wind turbine away from populated areas that has only low-amenity impacts on neighbours.

Assessing the potential for a wind turbine project on your farm is a complex process, in large part due to the influence of topography on the quality of the wind resource and wind characteristics. This paper provides pointers that will assist you in conducting a preliminary assessment of the potential for wind power on your farm.

How does it work?

Small-scale wind power refers to wind turbines that could power a house or farm operations, i.e. turbines with a maximum capacity of 100 kW but usually with a capacity of 1–10 kW. Small-scale turbines can be installed on the ground or mounted on roofs and/or buildings.

A typical wind system consists of a turbine, a tower, a controller, a grid-connected inverter and a meter. The controller ensures that the turbine operates within safe limits and rectifies the varying frequency alternating current (AC) to direct current (DC) power. The DC power is then passed to the inverter, which converts it into AC power of the same voltage and frequency as electricity from the grid.

Details of various types of small-scale turbines are summarised in the table below. Typically, a turbine tower is between 10 and 40 metres high, with a rotor diameter of between two and 20 metres. Winds speeds generally increase with elevation, so maximising turbine hub height is important in developing a financially viable project.

Figure 1: Typical monopole wind turbine with vertical rotation axis and upwind turbine orientation

Stapleton, et al., 2013

There are several types of wind turbines, as shown in Table 1. Figure 1 shows one of the most common set-ups.

Table 1: Types of small scale turbines

Key factors influencing turbine size for an on-site generation project include:

• site base load,
• physical space available and regulatory constraints,
• wind resource and characteristics, and
• potential savings and cost (i.e. financial viability).

Site base load

Request a year’s half-hourly metering data from your retailer. Determine the power demand or load requirement for a typical winter’s day and a typical summer’s day by dividing the kilowatt hour (kWh) consumption data by the hours of the day. Then assess the permanent minimum load your operations require (i.e. the base-load level, as shown in the following figure).

Figure 2: Illustrative demand curve.

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The load shapes for the winter and summer months are likely to be different, depending on the nature of your operations. It is common practice to size any on-site generation to approximately 80 percent of base-load. As wind turbines typically have a lifespan of 20 to 25 years, allowance should be made for future expected load growth.

Physical space and regulatory constraints

A wind turbine must be located where it will be exposed to the longest possible ‘fetch’ (i.e. the distance over which the wind flows uninterrupted). Rough surfaces such as nearby forests or buildings can slow down the wind, while hills and ridges can concentrate and speed up the wind. Small-scale turbines are frequently located near farm buildings, trees and other potential obstructions, however. This requires careful planning if you’re to avoid a turbulent wind resource. Turbulence reduces the efficiency of the rotor and therefore the output of the turbine – sometimes very significantly.

Space constraints may make a few smaller units more viable than one larger unit. However, as a rule of thumb, turbines should not be placed closer than 5 times the diameter of the rotor to avoid significant loss of power (see (MacKay, 2008, p. 265)).

Site selection may also be impacted by NSW Department of Planning and Infrastructure Planning regulations pertaining to small renewable energy proposals, as well as the relevant local council’s Local Environmental Plan (LEP) and / or Development Control Plan (DCP). NSW Planning guidelines ban wind turbines in a 2 km radius from residences unless there is a written agreement with the relevant landowner. Some small wind projects may be exempt, but if development consent is required the local councils may ask for a noise impact and other environmental assessments. Other considerations include nearby natural habitats, important plant or animal species that may be impacted, landscape impacts and preferences of nearby communities. In addition, a buffer from major infrastructure such as public roads may be required.

Wind resource and characteristics

Spatial and temporal variation of wind resource and characteristics are a major challenge when planning a wind project. The choice of method to assess the wind conditions depends on the certainty to which generation must be known.

As a preliminary assessment of whether your site is in a “windy location”, firstly are you in a coastal or upland area? Also refer to the NSW Wind Atlas in the figure below for an indication of whether your farm is located in a high wind speed zone. If you are located in the blue or brown areas the project is unlikely to be financially viable, unless site specific factors are very attractive or unique to the region.

Figure 3: NSW Wind Atlas, Sustainable Energy Development Authority (SEDA), 2002

Reproduced from Sustainable Energy Development Authority (SEDA), 2002

Figure 3: NSW Wind Atlas (reproduced from (Sustainable Energy Development Authority (SEDA), 2002))

If you are in a ‘green area’ or a ‘windy location’, you could get an indication of the mean wind speed for your ‘region’ from the Bureau of Meteorology (BoM). The BoM website publishes mean wind speed data taken at 9 am and 3 pm daily. Start by selecting a location close to your farm. Using airport measurement stations typically provides more reliable results, as these areas are open and are generally unobstructed, and because the data collected at these stations is often of higher quality because of its intended end use (health and safety regarding aircraft movements and weather forecasting).[1] This is just a proxy, as on-site conditions will impact wind speed, but it will  provide you with a guide to the local wind resource. To convert the wind speeds provided by BoM in kilometres per hour to metres per second, divide it by 3.6 (1 m/s = 3.6 km/h).

Typically, BoM measurements are taken at 10 metres above ground. Adjusting wind speed to take account of the likely height of your turbine hub is complicated as it depends on a range of factors, such as the roughness of the surrounding terrain and the time of day. As a rule of thumb, however, increase the wind speed estimate by 10 percent every time you double the height of the hub (see (MacKay, 2008), page 266).

Figure 4: Wind shear profile

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If the wind resource in your area and on your specific site looks attractive, you may want to invest in wind monitoring. Wind vanes and a wide range of anemometers are available on the market, but are of varying quality. Moreover, monitoring wind is far from straightforward, as wind is seasonal and can vary significantly over short distances.

For a big project, consider engaging specialist data providers to conduct a feasibility study. They will be able to assist you in determining the wind resource, optimal turbine location, inter-annual variability and ultimately, the business case for investing in a wind turbine. The cost of this service could be anywhere between $5,000 and $15,000 per site.

Figure 5: Typical Weibull distribution.

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If your farm is in a windy location, your next step is to determine the yield potential of the area (i.e. the likely number of kilowatt hours of electricity that would be generated by the wind turbine). Yield potential  is a factor of the turbine’s power output at different wind speeds and the frequency with which the wind is likely to blow at these different speeds. You can get a mean wind speed estimate for your area from the BoM website. However, without detailed wind data, the frequency distribution of the wind at different speeds cannot be known. For a preliminary assessment, you could assume a Weibull distribution of wind speeds across the 8,760 hours available in a year. This distribution, typical for sites with a good wind resource, is illustrated in Figure 5. This illustrated example assumes a mean wind speed measured at hub height of 4.8 m/s.

To determine the likely power output from your project, start by selecting a turbine with a rated power close to the base load requirements of your site. You can always adjust the size of the turbine in later calculations. However, the power output from two turbines with the same rated power can vary significantly. Therefore, the type of turbine you select will determine power output of your project at different wind speeds, as reflected by the unique power curves[2] of different turbines (see Figure 6).

Wind turbines are usually designed to start running at 3 to 4 m/s, also referred to as the cut in speed. Since small turbines are mounted at relatively low heights, the mean hub height wind speeds may be close to their cut-in speeds. The implications are that, for long periods of time, a small turbine may not operate at all. Even if it does visibly spin, it may not generate much electricity.

Figure 6: Illustrative wind turbine power curve.

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In calculating the yield potential, only ‘useful’ wind resource between the cut-in and cut-out speeds must be considered (See Figure 4 – i.e. wind resource at stable speed between and 25 seconds per metre in the example). The ratio of the amount of electricity actually produced in a certain period to the amount of electricity that would have been produced over the same period, had the turbine been generating continuously (i.e. 8,760 hours a year) at its rated power, is referred to as the capacity factor.[3]

By applying the power curve of a selected turbine to the Weibull distribution of wind speeds, an approximate electricity yield can be calculated (see Worked example section, below). Usually, this yield is adjusted downwards by 10 percent to allow for maintenance (availability), electrical losses and other disruptions  to output (loss factor).

If the power curve of the turbine you’re interested in is not readily available during this early exploration stage, you could do a rough calculation of the likely yield by applying the typical capacity factors for small-scale wind turbines. For turbines with a rated power of less than 10 kW, the capacity factor is typically between 10 and 15 percent. This could increase for larger units of 100 kW to between 15 and 20 percent. For large machines in the MW class capacity, factors reach 30 to 40 percent.

The basic ‘ballpark yield’, allowing for a 10 percent loss factor, could be calculated as follows:

Potential savings/cost (financial viability)

‘Ballpark’ net annual electricity generation (kWh) represents the potential electricity savings from a project. It is recommended that your electricity cost savings calculation be based on the variable electricity charge only. Although the project may result in a reduction in peak-demand changes, it is not recommended that you include this in your financial evaluation. A single calm day could result in your peak demand returning to its pre-wind-project profile. This will result in the same demand charges as before.

In addition to electricity savings, consider the potential revenue from certificates under the Renewable Energy Target (RET) Scheme as a co-funding source.

You can determine whether your project qualifies under the RET Scheme for Small-scale Technology Certificates (STCs) and Large-scale Generation Certificates (LGCs) by referring to the Clean Energy Regulators website (Australian Government, 2014). Wind systems of <10 kW are eligible for STCs and those of >10kW are considered ‘large-scale’ and qualify for LGCs. Both certificates act as revenue support for a project and typically trade in the range of $30 to $40 each[4].

Table 2 provides useful guidelines for the high-level financial viability assessment. If the high-level business case looks attractive, approach the market for quotations before developing a detailed business case.

Table 2: Planning assumptions for small-scale turbines (<100kW capacity).

Worked example

A farm has a base load of 15 kW, taking account of seasonal variation in electricity use. The farm is located in an area with a Weibull distribution of wind speeds typical of a good site,
as illustrated in Table 3 (Columns A and B). The farmer has sourced the power curve for a wind turbine with a rated power of 15 kW (Column C). The electricity production (column D) for each ‘wind speed bin’ can be derived by applying the following formula:

Table 3: Example wind-speed distribution of site and calculated electricity production from wind turbine.

Calculate the sum of the electricity produced at each bin speed to derive an estimate of the electricity this turbine will generate: namely, 22,759 kWh per annum. The 20,483 kWh net annual generation of electricity represents a capacity factor of 15.6 percent (i.e. 20,483 kWh/(15 kW x 365 x 24). Assuming a variable electricity charge of $250/MWh, this represents annual savings of:

=20.5 MWh x $250/MWh≅$5,000 per annum

Assuming that the project qualifies for LGCs, which have been trading recently at ~$38/MWh, the annual revenue from government incentives could be in the region of $800 per annum, reducing total annual electricity cost by approximately $5,800.

At an up-front cost of approximately 15 kW x $5,000 = $75,000, the simple payback on this project will be 13 years. This calculation does not account for future energy price increases and assumes no financing costs. It is further assumed that 100 percent of the wind energy generated will be used to displace farm energy use.

Evaluating quotes: performance parameters

There is no formal wind-turbine product approval system in Australia, and new models are entering the market on an ongoing basis. As a starting point, however, Appendix B of the NSW Small Wind Turbine Consumer Guide provides a list of common small-scale wind turbine models, along with distributor details. When approaching the market for a quote, it is recommended that in addition to client references and warranties, you request information on the following parameters to ensure comparability among quotes from different distributors:

1. rated power in kilowatts (kW) at rated wind speed (typically, this ranges between eight and  13.9 metres per second, depending on the model),
2. hub height options,
3. diameter of the rotor blades,
4. power curves (or at a minimum, cut-in and cut-out speeds),
5. voltages available (DC and AC),
6. sound power level (dB(A)),
7. weight (in kilograms, including mounting), and
8. supplier willingness to manage paperwork associated with government grants and permitting.

Further information

NSW Farmers wind farm guide for host landowners - Hosting a large turbine for a commercial wind power company is a different proposition to farm-scale wind. This guide was developed by NSW Farmers in 2012.

Farm-scale wind power in the USA - Farm-scale wind power generation is more strongly established in the USA. This website provides useful general information.

Clean Energy Council (Australia): Wind best practice guidelines - The Clean Energy Council’s 2013 Best Practice Guidelines for the development and implementation of wind energy projects in Australia are detailed on its website.

Renewable Energy Calculator for nursery owners (Australia) - The NGIA Renewable Energy Calculator has been developed as a simple-to-use tool for the assessment of solar and wind energy options for nursery owners. If you enter 12 months of electricity billing data, the calculator can determine the feasibility of replacing some or all of your business’ energy requirements with a grid-connected small generation unit (SGU).

Dairy farm case: an example of the use of wind turbines (UK) - A Devon dairy farmer has installed two 11kW turbines at a cost of £100,000, with an expected payback on investment within five years. (Note: the article, in error, used kW instead of kWh as the unit for expressing electricity consumption.)

Horticulture cases: examples of the use of wind turbine (USA) - A cranberry farm in the USA installed 35 kW and 40 kW turbines to take control of its electricity cost. Each turbine produces 85,000 to 100,000 kWh annually.

A 40-acre (16.18-hectare) vineyard installed a 10-kilowatt wind turbine and nearly halved its annual electricity bill.

Eco Whisper Turbine - This novel Australian wind turbine design makes use of several blades to reduce noise and increase efficiency. Its design is suitable for smaller farm-scale applications.

Australian Government: guide to residential wind systems

References

Australian Government, 2013. Climate Data Online. [Online]

Australian Government, 2014. About the SRES. [Online]

Enhar Sustainable Energy Solutions, 2011. NSW Small Wind Turbine Consumer Guide. [Online]

MacKay, D. J., 2008. Sustainable Energy — without the hot air. [Online]

NSW Government, 2007. State Environmental Planning Policy (Infrastructure) 2007. [Online]

Stapleton, G. et al., 2013. Wind Systems. [Online]

Sustainable Energy Development Authority (SEDA), 2002. The New South Wales Wind Atlas. [Online]

This content was produced with assistance from Energetics www.energetics.com.au