The self generation society01 October 2005

There is a growing movement towards generating electricity from wind and water locally, with rooftop wind turbines, small hydroelectric plants and solar photovoltaic cells.

The government has set UK goals of obtaining 10% of electricity from renewables by 2010 and 20% of the expected 370TWh consumption by 2020. The current figure is around 3% of 350TWh. While some advocate covering the British landscape and coastal areas with wind turbines, others are urging caution.

Look at any group of wind turbines and it is noticeable that about a fifth of them are not turning. The reasons are varied, but include bearing failure, lightning strike damage to blades and lightning strike damage initiating failure in gearboxes. It is difficult enough to repair a wind turbine on land, but it is even more difficult to repair a large turbine offshore, with blades up to 70m long and mechanical components weighing several tonnes, 100m off the ground.

One solution of increasing interest, according to Dr Keith Melton, director of technology at the New and Renewable Energy Centre (NaREC) in Blyth, is to have roof-mounted wind turbines. On a house, these can be accessed by ladder in the same way as a satellite dish or, on a factory, lifted down for repair using a mobile crane. NaREC intends to have five different wind turbines under test on its roof at any one time to evaluate different designs.

More importantly, it has also set up a laboratory to investigate the best methods of integrating alternative energy-generating systems with a mains powered network. The idea is to combine input from different systems with the mains supply in a way that won't produce interference to other users. On a sailing yacht, it is not uncommon to combine a solar panel and a wind turbine to provide a supply of energy, regardless of weather - fine and sunny with light winds, or stormy and windy with no sun. This can be done in a small standalone system by producing direct current flows that pass through diodes to charge a battery, and taking current from the battery terminals. In a domestic or factory environment, the power system is 230 V 50Hz AC. Any power supplied through alternative sources has to be kept in step with the mains supply through synchronised phase inverters, unless the mains is first turned off before plugging in the alternative supply. It is envisaged that, in future, all users will keep both mains and their own generated supply connected at the same time. Surplus power will be sold back to the grid through their supply meters, which will run backwards at a lower rate to represent power sold at a lower price than power bought.

At present, most people with a wind or solar generator either have a standalone power system not connected to the National Grid or use the wind or solar generator to charge batteries, with surplus power directed to storage heaters. Inverters which allow connection to mains supplies include Windy Boy and Sunny Boy, made by SMA Technologie AG, of Niestetal, Germany, a company which was spun out of Kassel University in 1981. Power ratings are from 700W to 5.5 kW and efficiencies are up to 96%.

Successfully integrating locally produced micro generated power with the National Grid on a large scale is not a trivial problem. NaREC's EnergyLINK laboratory is studying this very problem and links real alternative energy sources to real generators, and to both simulated output and physical devices. The facility is equipped with six Control Techniques Unidrive SP inverters. A bank of five drives, with fully regenerative front ends, are used to provide comprehensive testing of motors and generators coupled in different configurations, to validate machine responses, compared with their mathematical models. The system can introduce faults and short circuits to monitor system response and can simulate common situations, such as an offshore DC generator coupled to an on-shore converter.

In the same laboratory, a unique test rig provides information on the characteristics of linear generators used in wave-power generation schemes. This £250,000 project employs a 15kW servo motor driven by a 20kW Unidrive SP programme to simulate any kind of wave motion. The generator uses rare earth magnets encased in carbon fibres and produces about 10kW. The project is under the control of Durham University's School of Engineering, specifically the New and Renewable Energy Group.

Lightning tests of wind turbine components, along with other high-voltage components, are undertaken at the nearby Clothier Laboratory at Hebburn, opened by Reyrolle in 1970. It was acquired by NaREC in September. When visited by Plant Engineer in July, the team was delivering some impressive simulated lightning strokes at 1.2MV, and said they could increase this to 4MV if required.

An assessment of the current range of commercially available small-scale power generation technologies can be found in a DTI document, 'Microgeneration strategy and low carbon buildings programme: consultation'.

According to the document, there were 8164kWp (the 'p' stands for peak) of photovoltaic generating power installed in the UK in 2004. Average cost is around £6,300 per kWp. A 2kWp system would generate about 1500kWh/year, which could amount to a saving of £100 per year and a carbon saving of about 0.65 tonnes CO2/year. Payback time on an average 2kWp household system is estimated to be about 120 years, based on current prices of photovoltaics and electricity. However, technical development around the globe is intense. Efficiencies are increasing, while costs are dropping. The fundamental material from which silicon photovoltaics are made is purified sand, which is not likely to be in short supply, and technologies are being developed that would allow power-generating coatings to be sprayed on to ordinary window panes.

Turn on the wind

Windmills, the ancestors of modern wind turbines, are believed to have originated in ancient Persia and the oldest known windmills are in Afghanistan. The first documented windmill in Europe was found in Normandy in 1180. The most common type of modern wind turbine has three blades, although a multitude of turbine forms has been invented that are claimed to be more efficient, such as the helical blades of the Wind Wandler launched in Germany earlier this year at the Hannover Fair.

The DTI document states that a typical small-scale system costs between £2,500 to £5,000 per kWe installed (the 'e' stands for electric). A wind turbine of 6kWp capacity, sufficient for the electricity requirements of two or three typical UK households, will cost about £20,000 and generate about 10,000kWh per year. This might amount to savings of about £700 per year and equate to a carbon saving of 4.3 tonnes CO2/year. Payback time would therefore be around 29 years.

The cheapest wind turbines that have been brought to our attention are made by a company in St Petersburg, Russia, and generate 5kWp using directly-driven permanent magnet generators. According to its agent Pavel Tatarintsev, cost is between ?6,000 and ?7,000 (£4,100 to £4,800) and maintenance is effectively zero, because of the direct drive. On that basis, payback time would only be eight years. If the Chinese electrical industry turns its attention to this particular market opportunity, costs could drop even farther.

Turn on the rivers and streams

The first horizontal-shaft undershot water mill was introduced by the Roman architect and engineer
Vitruvius in 27 BC. The ancient Iraqis probably used water power even earlier to irrigate the Hanging Gardens of Babylon, but there is no record of how their machines worked.

In the UK, there is presently a total installed capacity of 100MW at 120 small hydro sites, each with an installed capacity of less than 5MW. This is apart from larger scale operations.

One planned addition to the list is the £1m, 200kW four-turbine plant to provide about a third of the electricity consumption of Windsor Castle.

Of more relevance to this study is the untapped availability of the many river sites previously used for water mills, with potential for only a few kW each, but presently mostly unused. These include the site opposite Franks Hall, where this article is being prepared, which is said to have been used to provide power to the house in the days before mains electricity was available.

Engineering physicist Alan Pengelly tells us that he built, tested and used his own 2kW hydroelectric generator over a period of some years until his water source became problematic.

He says that his turbine was an 8-inch diameter propeller turbine, "with one stator propeller that spun the stream and a second, similar but reverse pitch rotor that extracted the angular momentum and straightened the flow to discharge." Pengelly made the turbine himself and based it on an 8-inch standard mild steel bend.

"I designed the propellers based on data from the old mill stream: 1/3 cubic metre/second normal, nonflood flow, dropping 3.5 m. I made single blade patterns and had the propellers cast by a local bronze foundry before finishing. Efficiency overall was just over 50%, measured by electric output divided by the stream potential at the turbine inflow.

"At the inflow weir, the four 4-inch header pipes could be covered by simple flap doors, operated via a rope arrangement. A mesh gate just upstream was then rotated 180 deg to let the stream backwash debris over the closed gates. That took about a minute and we did it once a day. The generator behaved very well despite being flooded a few times and being dried out in front of a fire. The water was my neighbour's and the engineering was mine so we split the power by feeding immersion heaters in our domestic hot water systems. The system worked impeccably for ten years until the mill changed hands.
Also the stream, which was pumped through a sand quarry, became unreliable."

Pengelly says the cost is difficult to assess: "I spent many hours on design and fabrication, but the casting, the elbow and the bearings and seals cost around £150 and the pipework and concrete perhaps £50. I guess that a standard, manufacturing engineered turbine could be built for about £1000 to £1500 today after formal design specification."

At this point, it's worth mentioning that heat pumps, mostly based on ground source heat, are also a big growth area, as are solar hot water heaters, biomass heating systems and combined heat and power, where waste heat from electricity generation is used to heat buildings instead of being wasted to atmosphere.

Funding is available from a variety of sources to finance demonstration projects. These include Clear Skies, which ends in March 2006, activities led by local and regional government such as Solar for London, and the European Union's Concerto.

SOE

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