Are small wind turbines a good investment?

Are small wind turbines a good investment?

Chronicle: Electricity production from small wind turbines is often much lower than expected.

  • Helic R.C

    PhD student, UiS

  • Trond-Ola Hagbo

    PhD student, UiS

  • Knut Eric T. Gilgarhus

    Associate Professor, UIS

  • Anders R. Mueller

    Associate Professor, UIS

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This is a discussion post. The post was written by an external contributor, and quality assured by Aftenbladet's debate department. Opinions and analyzes are the author's own.

On 30 December 2023, Aftenbladet published an article about farmers' experiences with agricultural windmills. Some are satisfied, but many are disappointed with the actual energy production.

We are looking at wind and energy systems in the project Future Energy Center In the user interface. We regularly receive inquiries from people who want to install their own wind turbines on their property. Sometimes farmers, sometimes people, think about setting up small wind turbines on their rooftops in an urban area. Many companies market small wind turbines as a cheap way to produce their own electricity. In fact, it is unfortunate that many people find that their electricity production is much lower than expected. It's not really a surprise to us.

Proceedings authors (former): PhD researchers Helic R.  Ceci, Trond-Ola Hagbo, and Associate Professors Knut Erik T.  Gilgarhos and Anders R.  Müller, all at the University of Stavanger.

Wind physics

Wind turbines produce electricity by converting kinetic energy in the wind into electrical energy. More wind means more energy produced. What many people don't realize is how sensitive energy production is to small, local changes in wind speed. The formula for wind energy is a function of wind speed raised to the third power. What does this mean in practice? Yes, if we went from a wind speed of 5 meters per second (m/s) to 10 m/s, this would give an eight-fold increase in the energy available in the wind!

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The fact that small changes in wind speed have a large impact on energy production is important when we evaluate the profitability of small wind turbines. There are large local variations in wind conditions, especially in urban areas and generally complex, close-to-ground terrain. Large wind turbines naturally operate at higher elevations above the ground, and here the winds are generally stronger, compared to lower elevations where buildings and landscapes are more exposed to the wind. Wind turbine manufacturers also report the effect under absolutely ideal conditions, so actual production is often lower.

Things other than altitude and wind speed also mean that large wind turbines are generally more cost effective than small ones. One point is that the “sweep area”, i.e. the area from which the turbine “harvests” energy, increases directly with blade length. In practice, this means that doubling the blade length quadruples the power output, as the area of ​​the circle increases directly with the radius.

The final point is that medium and large wind turbines generally have a more optimized design and better control system than small turbines. We can see this in action if we look at data from actual wind turbines.

Large wind turbines often have an output capacity of 4 MW or more. A 4.3 MW turbine with a rotor diameter of 130 metres, if placed in a windy location in Jiren, could produce electricity equivalent to the electricity consumption of about 1,000 households.

In comparison, a small wind turbine with a capacity of 10 kilowatts will have a rotor diameter of about 10 metres. Such a turbine will only produce electricity for up to 1-2 households. Hence we see that increasing the rotor diameter by 13 times can increase the power produced by 1000 times.

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The economy is in turmoil

So, can one say something general about the profitability and efficiency of small wind turbines based on this? Yes, the rule of thumb is that the higher a wind turbine is above the ground, and the longer the blades, the more cost effective it is to produce energy. This is also an important point to consider when we look at natural interventions related to wind turbines. Yes, small wind turbines often have a smaller visual and environmental footprint than large ones, but if you need several hundred small turbines to produce the same amount of energy as one large turbine, the calculation becomes very different. Therefore we also see that development in recent decades is moving towards larger wind turbines.

In some special cases, small wind turbines can be profitable. In areas with strong and stable winds, it will be easier to obtain profitability even in small and medium-sized wind turbines. On rooftops or elevations exposed to winds, smaller wind turbines can generate renewable energy at a reasonable cost. Especially if the wind turbines are relatively cheap and do not require significant interference with the roof or nature.

Larger wind turbines in the same location are likely to produce more cost-effective energy.

We fully understand the desire to have our own energy and local ownership. But medium and large wind turbines are often too expensive for a single farm or homeowner. In Denmark, it is common for several neighbors and farmers to join together in so-called windmill syndicates to purchase and operate large wind turbines.

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Maybe we can learn something from this in Norway? What if we designed policy so that it would be easier to achieve cooperation between neighbors, and thus local ownership of wind turbines as well? Hence we can avoid small inefficient wind turbines on each strand.

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Hanisi Anenih

Hanisi Anenih

"Web specialist. Lifelong zombie maven. Coffee ninja. Hipster-friendly analyst."

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