January 12th, 2018

Too much wind power?

7 minute read

by Duncan Oswald

 

Before being welcomed into the magical world of Eunomia, I spent many happy years designing and installing wind turbine and on-site energy management systems in some of the most remote, beautiful (and hostile) environments in the UK.

This is the story of the most recent installation I did, at Talla na Mara, in Horgabost on the Isle of Harris.  It has just won the Best Community Project Award at the 2017 Scottish Green Energy Awards, but I’m particularly proud of it because of how we resolved an unusual problem: generating too much power.

 

Horga-boost

Horgabost is about 10km from the windiest spot in Europe – Frith na Hearadh. At least, that’s the place with Europe’s highest average wind speed, over 10 metres per second according to the NOABL database.  During the 2017 hurricane season in North America, you may well have heard about the Saffir–Simpson scale, which divides hurricanes into five categories based on wind speed. When winds exceed 70 metres per second, a hurricane reaches the highest category.  At Horgabost in the winter, this wind speed is exceeded on average once a fortnight. There is a diary entry in the St. Kilda museum which tells how, following a particularly violent storm, the entire population was deaf for a week. While working on the site, one of the locals told me that crofters have been found asphyxiated because the wind blows so hard you just can’t draw breath. It’s probably an apocryphal tale, but some days it’s easy to believe.

 

Frith na Hearadh

Frith na Hearadh: The actual windiest spot in Europe. Image courtesy of Duncan Oswald.

 

We had previously put up a handful of turbines for community groups across Harris and Lewis.  The site at Horgabost had not been developed because West Harris Trust had planning for a Harbon turbine but changed their minds after experiencing problems with their first one. (For the record, the Harbon turbine is a great piece of kit; they just had a bit of bad luck up the road at Sgarista when the control cabinet blew away.  It had only been bolted to the concrete; now it is bolted down with a brick wall around it.)

Then I met Alex de Broe from Belgian turbine manufacturer XANT at an MCS meeting in London.  XANT had developed a 100kW Class 1a turbine (that’s the toughest) and wanted to test it in some gnarly conditions, so I got in touch with West Harris Trust to see if they might be interested in trialling it.

There was, however, a problem. Until the interconnector to the mainland is commissioned, the need to manage supply and demand means that no new connections above 3.7kW per phase will be allowed in the Western Isles.  The Horgabost site had the last export connection agreement issued before the ban came into effect, but even this was restricted to 50kW. If the project was to go ahead then, in addition to sorting out the finance, access and logistics, we would need to change the planning permission to a different turbine – and design a system that Scottish and Southern Electricity were confident would cope with the excess generation.

An outline business plan for this sort of thing is usually pretty straightforward: you know how windy the site is, what the turbine will generate with that wind, how much you’ll receive for the power and so on.  Having built similar installations, we could make a reasonable estimate of how much it would cost to get the kit to the site, build a road, foundations and all the rest.  With this outline in hand, we put together a joint venture partnership with West Harris Trust, private investors Hilldino and XANT.

 

Wind Turbine

Wind turbine installation at Horgabost. On a clear day you can see St. Kilda. Image courtesy of Duncan Oswald.

 

In this case, though, there was a problem: we had a turbine capable of generating 100kW on a site where it was very likely to do so a lot of the time and could only export 50kW. The only answer was to guarantee that we could use or store up to 50kW at any time.

 

Generating demand

The site has a community hall and six housing association units, with space for six more for private sale, and some campervan hook-ups, which would provide some demand. All electrical loads and the turbine are connected to the mains at a single point, so the demand from the homes and hall would net off against the electricity generated by the turbine. The result would be a net import or export, depending on the balance of generation and use; however, the amount used would vary, depending on occupancy, time of day, weather, activities etc., and therefore wouldn’t guarantee keeping within the 50kW export limit.

Our answer was to use a Coolpower EMMA energy management system connected to a 54kW electric flow heater (comprising 6 individual 3kW elements on each phase). The EMMA system measures generation and demand about a thousand times a second and calculates how much excess is being generated.  If this exceeds 50kW, the system diverts power to the flow heater by precisely the amount of this excess, ensuring that the 50kW export limit is never breached.

 

Circuit diagram

Cunningly devised, fun to build, then rebuild in the space available. Image courtesy of Duncan Oswald.

 

The flow heater is connected via a system of cunningly positioned thermostatic mixing valves to a thermal store comprising ten tonnes of water. The valves ensure that the store is stratified and that it doesn’t overheat; they also supply hot water via a heat main to the hall. Stratification is essential: in a ten tonne thermal store, it is useful to have a tonne of water at 80°C but not so much to have ten tonnes of water at 20°C.  The design return temperature of the heating system in the hall is 45°C, so anything below about 50°C is of no practical benefit in terms of heating.

 

Scientific calculator

The energy management system design was the subject of a separate contract from Scottish Government.  This exercise required the development of a detailed model of renewable energy supply and demand with a one-hour resolution for a sample year. The model was used to calculate the optimum design solution for balancing energy storage on site and export to grid against installation cost and income from sale of electricity and heat. The model was also used to inform the charging structure for electricity and heat used on site to optimise the balance of benefits to the Trust and occupants, while providing an attractive return to the investors.

 

Supply and demand model

Detailed model of supply and demand. Image courtesy of Duncan Oswald.

 

The original design supplied heat directly via heat main to the houses as well as the hall but the Trust didn’t get the £100k grant they needed for the additional heat main. When I re-ran the calculations, the best alternative was electric panel heaters: these aren’t normally the most environmentally-friendly solution but if the electricity is practically unlimited, green and free then they make sense. With the original design, over 95% of the energy demand from the hall and houses was delivered by on-site renewable generation; even without the heat main it remained above 80%.

In the end, the housing association decided to fit their standard solution of individual air-source heat pumps, increasing cost and reducing demand, and therefore reducing use of on-site generation – but it still allowed the system to work, making effective use of all the wind that the weather can throw at it. If you are up that way, it’s worth a visit as a demonstration of how flexibly wind power can be used. I’m sure that its proximity to the Harris Gin Distillery would only be a secondary attraction….

 

Duncan Oswald

 

 

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