Small Wind Turbines - The Basics
The wind is a highly unpredictable resource (see Wind resource assessment – the basics) and Small Wind Turbines (SWTs) are remarkably troublesome pieces of equipment. If you have a suitable hydro resource or if your budget allows you to obtain enough PV panels to comfortably meet demand throughout the year, then these will almost certainly provide a more reliable, simpler and/or cheaper energy solution. If not, then wind can offer a third option for rural electrification.
However, it is important to mention that the combination of wind and solar in a hybrid system is much greater than the sum of the parts, as the diversity in power generation sources reduces dependence on battery storage. In addition to this, the main advantage that SWTs offer is the ability to manufacture locally. In a development context, import taxes and international shipping are often very costly and local job creation is highly valued. What is more, mechanical technologies such as wind turbines, require significant amounts of maintenance and by building local capacity to manufacture, local capacity to operate and maintain can also be created.
As the diagrams below suggest, the success of SWTs depends much more on the context in which they are installed than solar PV. As a result, the following decision tree has been designed to guide the reader when deciding whether SWTs are appropriate for the particular local context in which they are working.
Due to the variability in the wind resource, in off-grid applications SWTs are almost always used to charge batteries (although variable loads such as water pumping and grain milling can be used without batteries, as water tanks and buckets of flour can be used as storage in place of batteries.). Small systems can be used to supply electricity to a home, but are much more complicated than a typical solar home system. Solar PV is extremely modular (1 kWh from a 50 W solar panel is not that much more expensive than 1 kWh from a 5 kW solar array), making it ideal for dispersed communities, where each household has a very low power demand and interconnection is not an appropriate solution. In contrast, a 5 kW wind turbine will produce much cheaper power than a 50 W wind turbine. In a development context, where the ability to pay is low, this makes wind more appropriate for higher power applications such as agriculture (irrigation, agricultural processing equipment), small businesses (power tools, fridges/freezers), community services (lighting and computers for schools or health centres, water pumping, etc.) or a micro-grid to supply multiple households.
Wind turbines are normally installed in a hybrid system. Often they are used with diesel/petrol generators because of their dependability, however they can work well with solar PV if the two resources are complementary, i.e. when calm days are often sunny days, the nights are windy, and daily solar generation can fill in for wind during longer calm spells. In locations such as north-west Scotland, the two resources peak in opposite seasons, making a PV-wind hybrid more capable of keeping the batteries full throughout the year than either power source alone. Sumanik-Leary (2013) modelled a household in north-west Scotland and found that a PV-wind hybrid was capable of meeting demand with a net present cost 22% lower than a wind only system and 53% lower than a PV only system). However, in other places, such as many tropical countries with ‘trade winds’ that are in sync with solar energy on a daily and seasonal basis, the benefits of hybrid systems are greatly reduced.
The performance of a wind turbine is very difficult to predict and is highly dependent on the site on which it is installed. In particular, turbulence and shelter from nearby obstructions, the sensitivity of the furling system designed to protect the machine in high winds, cable length and diameter, tower height, seasonal and inter-annual variation in wind resource and air density changes due to altitude (Wind resource assessment – the basics) can all have a major influence on energy yields. In fact, Khennas, Dunnett et al. (2008) suggest that “the best approach may be to make an informed guess and then refine this in light of practical experience.” As a result, due to the high number of variables involved, it is important to check the suitability of a particular site and any energy yield predictions with a wind power expert.
SWTs that range in size from 1–5 m rotor diameter are particularly appropriate for the electrification of remote communities, as going any larger than this and the logistics of transporting the equipment to a remote community (in particular the tall tower and heavy generator), then installing it (digging anchors, lifting the tower/turbine etc.) and performing maintenance (taking it down around once a year) become difficult. The rated power of such machines is likely to range from 100W–2.5 kW and energy yields can vary widely, from 2–430 kWh, depending on the size of the machine and quality of the wind resource at that particular site (the power available in the wind varies with the cube of the wind speed, which means that the site on which a turbine is installed has a huge influence on power production).
Comparing different turbines is difficult, as many factors affect performance. The most commonly used is the rated power, however this can be misleading. The simplest (and often most effective) method is to use the rotor diameter, but if available, standardised predictions of energy yields are the most useful (please see SWTs – the details for more information).
The effect of trees, bushes and other obstacles make the wind resource near the ground turbulent and too low for power production. Typical tower heights range from 6 m (for sites in open areas with good wind resource) to 30 m (for sites with many trees and buildings nearby and poor wind resource). The quality of the wind resource continues to increase with height above ground level: on an open site, doubling tower height typically yields a 40% increase in energy yield (in fact, the increase in energy yield can be much higher on a wooded site if the tower lifts the SWT above the tree line). However, increasing tower height also increases the cost of the tower and the complexity of the installation.
Guyed towers have more failure points and a larger footprint, but are much cheaper than either lattice or monopole towers. They are also much easier to raise and lower, requiring either a team of people or a rope winch, as opposed to a specialist crane. As a result, they have become the standard for development projects, where cost, transportability and maintainability take priority over aesthetics and land use. As a result, only tilt-up guyed towers are discussed in this book. Piggott’s A Wind Turbine Recipe Book gives a step-bystep guide to building such a tower, whilst Piggott’s (2000) Windpower Workshop gives a more in-depth discussion on designing tilt-up guyed towers.
The economics of wind power depend primarily on four things:
- The size of the turbine.
- The wind resource.
- Where and how it is manufactured, assembled and installed.
- How it is maintained.
Figure 29 shows that bigger turbines generate cheaper power and that imported turbines are generally much more expensive than locally manufactured machines. However it is the wind resource itself that has the most dramatic effect on the unit cost of electricity produced by an SWT due to the cubic relationship between power production and wind speed.
Figure 30 gives a rough approximation of the cost breakdown of the initial purchase costs of a typical small wind power system, showing that the wind turbine itself is actually only a small fraction of the total cost. Please note that this does not include operation and maintenance costs, which are very high for SWTs and depend on a multitude of different factors, including level of training offered to end-users, supply chain length for spare parts and the prevalence of environmental hazards (e.g. lightning).
The following key terms are often used when describing wind turbine performance:
For more on the theory of wind power, see:
- Paul Gipe’s (1999 and 2014) text books, which can be found on his
free online resource, Wind-Works.org.
- The Danish Wind Energy Association’s (2014) guide to wind
power, Wind Know-How.
Local manufacture vs. importation
One of the biggest decisions to be made after choosing wind power is whether to import a wind turbine or whether to manufacture one locally. In the following section the terms ‘imported’ and ‘commercial’ SWT are used interchangeably to refer to a massproduced machine from a high-tech factory that is sold on the free market. Whilst such manufacturing facilities have been successfully employed to manufacture SWTs in emerging economies such
as India, China and Argentina, they are rarely available in less developed countries. As a result, there is normally a choice between importing such technology or starting to manufacture it locally.
Piggott’s A Wind Turbine Recipe Book gives a step by step guide for manufacturing a range of SWTs from 1.2–4.2 m rotor diameter using only basic tools and techniques and readily available materials. Blades are carved from wood, the main bearing is the wheel hub of a scrap car and much of the steelwork can be made using scrap pieces of metal. Piggott designed the machines to provide power for his home community of Scoraig in Scotland and over the last 30 years, he has continually refined the design to improve reliability, lower costs and create a machine that is well respected across the globe for its durability, simplicity and adaptability. As a result, the design is now used in development projects in over 20 countries around the world, such as the Clean Energy Initiative in Mozambique shown below.
When importing SWTs, you certainly get what you pay for, as very cheap machines will often have lifetimes in weeks rather than years (Sumanik-Leary, 2013). In contrast, the best quality imported SWTs are likely to be more efficient, more reliable and have a longer lifespan than locally manufactured SWTs. However, a failure will eventually occur and the following key issues will then arise:
- Will there be sufficient local technical knowledge to perform the repair?
- How long will it take/how much will it cost to send a spare part from overseas?
Imported technology will most likely be subject to import taxes of 10–20% (although many less developed countries now offer tax exemptions for renewable energy products and components, they are often restricted to solar PV or certified products, however, regardless of the cost, clearing goods through customs often involves significant bureaucracy and requires a lot of patience) and shipping charges of a similar amount, pushing up an already high initial purchase cost even further. What is more, the analysis below shows that commercial technology is often optimised for best performance at rated power (which is more marketable) rather than in low winds (which is more useful for battery charging systems, as calm periods can last up to a week and the batteries are often filled quickly during periods of high winds). The comparison below shows that whilst the commercial machine (Bergey XL.1) produces much more energy in the windiest month, November, it is actually the locally manufactured SWT (Piggott 3N) that produces more in the least windy month, September, i.e. when the power is needed most.
Sumanik-Leary, Piggott et al. (2013) modelled two hybrid PV-wind household energy systems: one using a locally manufactured SWT (Piggott 3N) and the other using an imported SWT (Bergey XL.1). The table below shows the key variables in this analysis, which found that whilst the ability of the two systems to fill the batteries and meet demand was similar, the Piggott 3N was able to offer a lower cost solution than the Bergey XL.1. Providing that sufficient local technical knowledge and access to the relevant tools and spare parts are available, then locally manufactured technology has the potential to offer savings of over 20% during the lifespan of the system. What is more, the graph below shows that the majority of the costs of locally manufactured technology are distributed throughout the lifespan of the energy system, lowering the barrier of high upfront capital costs that often inhibits the uptake of renewable energy technologies.
Khennas, Dunnett et al. (2008) state that manufacturing SWTs locally not only has the potential to boost the local economy and build local capacity, but it can also help create a resilient energy system through the creation of a strong supply chain for spare parts (accompanied by trained local tradesmen to perform repairs). In addition, by involving community members in the construction and installation phases, local manufacture can increase the likelihood of successful knowledge transfer to the end-user. This is necessary to make productive use of the energy and to ensure reliable operation through the carrying out of proper operation and maintenance procedures (Sumanik-Leary, 2013). However, the greatest advantage of local manufacture is often the flexibility to adapt the technology to the local context and provide an appropriate energy solution based on factors such as the
local availability of skills and materials, wind resource and energy demand.
Unfortunately, the risk with local manufacture is that lack of skills, knowledge and quality standards will result in the production of unreliable, low quality equipment that will fail to meet the expectations of the end-user and undermine the reputation of the technology as a whole. The unpredictable availability of raw and reclaimed materials of a consistent quality can also significantly weaken the supply chain and hinder both manufacturing and maintenance operations. There is also a significant hurdle to overcome in terms of scale, as it is difficult to achieve the necessary quality standards and offer the required maintenance services when producing just a few machines. Not only are poor quality or poorly maintained machines potentially dangerous, they can create a negative perception of the technology and inhibit future growth in the market, as occurred in both Nicaragua (Marandin et al., 2013) and Kenya (Vanheule, 2012).
Ultimately, the decision as to whether to manufacture SWTs locally or to import must be made independently for each local context as there are many place-specific factors that influence the decision, for example:
- The availability of maintenance services for imported technology in that particular country and more importantly, that particular region.
- The import taxes in that particular country.
- The shipping costs to get the equipment into that particular country and then into that particular region.
- The value placed on local job creation , as local manufacture shifts a significant portion of the value chain into the country and can even shift it into rural areas.
- The wind regime in that particular region – locally manufactured technology can be better adapted to low wind regions (4–5 m/s annual mean wind speed).
- The planned scale of manufacture – at low volume, costs are high and it is difficult to ensure the necessary quality with local manufacture.
- The external environment – corrosive environments (heat, humidity, salinity etc.) push up the cost of locally manufactured technology (metal parts may need to be galvanized, stainless steel bolts used instead of cheaper mild steel etc.), whereas some commercial turbines are designed specifically for marine environments and have these features as standard. Lightning, sand/dust and other hazards can have similar effects.
- The capacity and willingness of communities to perform maintenance – locally manufactured machines require more preventative maintenance (greasing bearings, repainting blades etc.) and regularly sending engineers out to remote communities quickly becomes expensive.
Vertical axis (VAWTs), rooftop mounting and
Fortunately the decision between HAWTs (horizontal axis wind turbines) and VAWTs (vertical axis wind turbines) is much clearer cut, with HAWTs more efficient, more reliable, cheaper and better understood than VAWTs. Rooftop turbines are subject to high levels of turbulence and a diminished wind resource. They are more likely to annoy/endanger the occupants and cause structural damage (either through vibration or falling parts) and are less likely to be maintained if it is difficult to access them. Shrouded turbines add extra weight to a tower and ‘cheat’ the theoretical limit for power production by forcing air onto the blades, artificially raising the efficiency. Simply building a turbine with slightly larger blades is a safer, easier and more cost effective solution.
For more information, see Paul Gipe’s free online resource, Wind-Works.org.
Site selection and installation
Installing an SWT is difficult and potentially dangerous and should not be undertaken without reading either Piggott’s (2013) Wind Turbine Recipe Book, Piggott’s (2000) Windpower Workshop, Little and Corbyn’s (2008) EWB/SIBAT Technical Guides and/or the installation instructions of the commercial machine you have purchased. The following section outlines the main steps in the site selection and installation of a Piggott turbine, but is likely to be very similar for commercial/imported machines:
- Site selection – section 2.2.3: Wind resource – the basics, describes the procedure for finding a site with a good wind resource and then estimating its magnitude. Selecting a site for an SWT is a balancing act between selecting the site with the best wind resource to maximise energy yield, selecting the site with the best potential for anchoring and selecting the site closest to the enduser to minimise power cable length 1.
- Transportation – Getting the components to the installation site is often very challenging, as many components are bulky and/or heavy. Towers are usually designed to be dismantled into sections of around 6 m and the tail and individual blades should detach from the generator. However, the logistics of transporting these components is still challenging, especially if they have to be carried long distances by hand over difficult terrain.
- Foundations – As shown in Figure 109, the base of the tower, four guy anchors and a lifting anchor will need to be secured to the ground. On rocky ground, these points can simply be bolted to the rock, otherwise an anchor/base must be created using concrete or deadmen (heavy items buried in the ground).
- Electrical installation – An armoured power cable (or flexible cable protected by conduit) runs from the base of the tower to the battery bank. The wind turbine will require a charge controller capable of operating in diversion load control mode, as when the batteries are full, the turbine must not be left to run free, as it would reach dangerously high rotational speeds. A brake switch is also required to stop the turbine when raising/
lowering the tower, in high winds or when a fault has occurred. An electrical brake switch is the simplest and most reliable method, as it simply shorts the power cables and stops the wind turbine by maximising current (and therefore braking torque). A lightning protection system is also recommended in locations
with frequent lightning strikes (see Figure 176).
- Assembly – After the foundations have been built, the tower and guys should be assembled and erected without the heavy turbine on top in order to adjust the guys to the correct length and verify that everything is working as expected. Then the turbine can be reassembled and placed on top of the tower. Next, the power cable can be threaded down the centre of the tower and connected to the underground cable running to the powerhouse. After the brake has been switched on, the tower can then be raised using the procedure shown below.
Operation and maintenance (O&M)
Wind turbines are troublesome pieces of machinery and are likely to face a major problem at least once per year. It is not a question of whether or not they will break, but more of who is able to put them back together when they do. As a result, the technology can only work successfully if access to the relevant maintenance services are available (in much the same way that cars and bicycles could not be successful without garages/repair shops). This maintenance infrastructure must be planned for at the beginning of any development project using SWTs: end-users or a community technician must either be trained to perform maintenance themselves or be made aware of somewhere in their local area where they can get access to maintenance services. In most places around the world, these maintenance services are not available, in which case, a service centre near to the installation sites will need to be established. The more remote the installation site, the more important it becomes to have technical knowledge, spare parts and tools available in the community itself, else the cost of travelling (in terms of both time and money) each time there is a failure will make the technology prohibitively expensive.
Sumanik-Leary et al. (2013) estimated annual O&M costs for locally manufactured SWTs at around 15% of the initial capital costs. However, this is highly dependent on the price of labour (community technicians cost less per hour than engineers from the city) and the distance that the person has to travel to reach the installation site and/or obtain spare parts (if community technicians already have spare parts on hand, these costs are zero, however if an engineer from the city has to obtain the spare part from elsewhere and then travel to the community, costs will be very high). The better established the technology is at the local level, the lower O&M costs will be, the quicker repairs will happen and the less likely failures are to occur in the first place 1. Figure 111 shows that the net present cost of operating and maintaining an SWT is likely to exceed the initial capital cost, even when an effective community training program has been implemented.
Figure 112 shows the community technicians in the Nicaraguan village of Cuajinicuil inspecting the turbine after hearing a rubbing noise when the turbine was spinning. The noise was found to originate from the rotor rubbing on the stator and a simple adjustment of the spacing between the two prevented the potentially costly failure of both components. The organisations responsible for installing this wind turbine, blueEnergy and AsoFenix, invested significant time and effort into transferring knowledge to these community technicians by running specialist training courses and inviting them to participate in not just the installation of the wind turbine, but also in its manufacture. The local manufacture of SWTs offers the opportunity for community members to take part in the construction of the machine that will be installed in their community. This not only increases the sense of ownership of the resulting technology, but also greatly improves the transfer of knowledge to the community technicians. This practical, rather than theoretical approach to learning is much more likely to succeed with people who may have had little formal education, but have highly developed practical abilities, such as subsistence farmers (Sumanik-Leary, 2013).
- Khennas, S., Dunnett, S., and Piggott, H. (2008) Small Wind Systems
for Rural Energy Services. Rugby: Practical Action Publishing
- Sumanik-Leary, J. (2013) Small Wind Turbines for decentralised rural
electrification: case studies in Peru, Nicaragua and Scotland. Doctor of
Philosophy, The University of Sheffield
- Sumanik-Leary, J., Piggott, H., Howell, R. & While, A. (2013) Locally
manufactured Small Wind Turbines – how do they compare to commercial
machines? 9th PhD Seminar on Wind Energy in Europe. Uppsala
University Campus Gotland, Sweden