The Micro Hydro

This page is for general technical interest and is not directly relevant to the rental of Sandaig Cottage. It contains a brief description of the Micro Hydro System which has been running at the cottage since 2007.
Knoydart is not connected to the National Grid. Electricity supply to the village of Inverie comes from its own hydro electric turbine, running off a penstock from Loch Bromisaig.

A supply was never run from Inverie to Sandaig bay and so electricity at Sandaig was previously generated solely by diesel generators, as in many remote places. In 2007 we became almost self sufficient in energy by installing a micro hydro system running off the Sandaig burn with the diesel generator being kept as backup.

The Concept

The Sandaig burn runs through our land.
It has a substantial catchment area and we obtained data on the estimated monthly flows in the burn from the hydraulics research station (HRS) Wallingford.

Lines in the graph above indicate the proportion (percentage) of time for which flow will exceed the y-axis value in a given month of the year. (with a 95% confidence level) 
The bottom (light blue) line shows that at least 20l/sec can be expected, 95% of the time, in any month.


The system is a ‘run-of-the-river’ scheme, with water being extracted form the burn (stream) at a small weir.  There is no large dam to form a loch and impound the water, as there is on bigger schemes.  Essentially, the extracted water runs to a settling tanks and then down 800 metres of buried pipe (the penstock) to a turbine, where the electricity is generated at 240 volts alternating current (a.c.). The water then re-enters the burn.  The generated electricity runs in a buried cable to equipment which converts the current to 24 volts direct current for storage in a battery bank.  Inverters are then used to convert the electricity back to 240 volts a.c. (the normal current in UK households) for use in the house.  If the batteries are full, excess electricity is passed to the dump load, which consists of specific radiators in the house.  The scheme is unusual in that a varying amount of water is accepted by the turbine in accordance with the flow in the burn.

The turbine and electrical system was designed and installed by Richard Drover, of Greenearth Energy in Wales. The remainder of the system was designed installed by ourselves with the help of local craftsmen such as Tim Bowyer a resident of Knoydart.

Flash Flood

The key design consideration is that the burns here are subject to ‘flash flood’.  i.e. the flow suddenly increases, to become a torrent, and then quickly decreases to a fairly steady low flow, except that in summer the flow can be small. We have designed the system so that the turbine always accepts a small flow and then valves open to take a larger flow when there is enough water in the burn. 
Water falling on Catchment A, on the sketch (2.4 sq km), flows over the weir. This is all open moorland, with the peat bog acting, to an extent, as a sponge. A report of estimated flow in the Sandaig Burn was made for us by Hydraulics Research Station, who hold the national archives on rainfall.  This is based on statistics of rainfall in the area. They concluded that the mean flow over the weir is generally in the range 200 and 55 litres per second with a mean annual flow of 75 l/s.  For example, there is a flow in excess of 100 litres per second for 50% of the time in March.  Plenty of water.  However in May to August the flow is less than 5 litres per second for 5% of the time so the minimum amount accepted by the turbine is set by this figure. 

Power Generated

The power generated by a hydro system is, very simply:

Power = Head x Flow x g x  Efficiency

where power is measured in 
Watts, head in metres, flow in litres per second, and g is acceleration due to gravity  (9.81 metres per second per second)

The head is the vertical height between the weir, where the water is extracted from the burn, and the turbine.  In this case the ‘gross’ head is 43 metres.  The relatively long distance (800m) between the weir and the turbine is not of relevance to the calculation except in so far as it affects the losses due to friction in the penstock, which are low in this case.  In the calculations the gross head is the actual physical head and the net head is a lesser calculated figure which includes for these friction losses. 

In this case the jets at the turbine are designed to accept a maximum of 13 litres per second.  Assuming an overall efficiency of say 60% (including friction losses in the penstock) the power generated is 43 * 13 * 0.6 * g = 3.3kW

If this is generated continuously it is more than enough for one small house, since the generated electricity is stored in batteries and then released to the house via the inverters as needed.

Weir, Settling Tank and Penstock

There is a small dam or weir, with an intake box. We built the weir in concrete, at a location where there is a small waterfall and good rock foundation. Do please visit this attractive feature.  Go up our track to the point where the burn is very close to it and then walk 50 metres up to the left.  On the downstream face of the weir, water enters via a special grill called a Coanda screen or aqua flow-type, which prevents debris of larger then 3mm size from entering the penstock.  This consists of specially shaped stainless steel bars set close together.

Then once collected it is diverted into a 4” pipe leading to a 1000 litre fibreglass settling or forebay tank, which is in fact salvaged from a fish farm. Inside the tank is a level sensor for automated control of the turbine valves.  The burn takes a right angle turn at this point so that the tank is on a sheltered ledge protected from the flow in the burn, which can be a torrential flood at times. This tank establishes a relatively calm and stable inlet to the ‘penstock’.  The 8” diameter penstock is buried under the moor and consists of HDPE (High Density Polyethylene) which is the blue pipe commonly used for mains water supplies. The pipe is of higher specification than necessary (16 bar design pressure whereas only 5 bar is needed).  We were fortunate to obtain it cheaply because it was surplus to requirements on a housing scheme in lowland Scotland. An image of the pipe being pulled and welded is shown.

We welded the individual 12m lengths together using an electro-fusion machine powered by a 6 kW generator, which we hauled across the moor on a boat trailer! (Shown is a 4" pipe welded in place).The route of the penstock is predominantly across peat moor, with part of it passing through new plantations. The penstock was buried by using a digger to dig a trench alongside the pre-welded pipe, then rolling the pipe into it and back-filing. One year after this, the line of the trench is barely visible.

There is an air bleed point at a low point 60 metres from the settling tank i.e. 10 metres above the track to the fank (sheep pen). If the penstock is ever emptied then the plug at this point has to be removed form the pipe to void the air.

Turbine and Valves

The turbine is of the Turgo type in which up to three jets of water are directed via three nozzles, at an angle, onto a wheel or ‘runner’ consisting of small plastic buckets mounted on a vertical axis. Above the wheel is a Crompton Induction motor, which is rated to 3.6 kW at 50 Hz a.c.

The turbine, valves and control box are in a hut which is in a dell 250m away from house on the Sandaig burn, so that the turbine cannot be heard, although it is anyway very quiet.

Flow from the 8” penstock passes into a simple 6” dia. steel manifold with three 2” BSP outlets leading to 2” diameter pipes, with a valve on each of the pipes. There is also a 1.25” vent pipe, valve and pressure gauge. To see these, remove the doors on the hut by unscrewing the stainless steel butterfly valves.

The three valves are on the left with the red manual valve furthest away from you, or upstream.
    • Jet on valve No.3 which is permanently on generates 0.8 kW
    • Jet on valve No. 2 (middle hose) generates about1.5KW
    • Jet on valve No. 1 (closest hose) generates about 1KW
Thus total real output producible is 3.3KW .  The jet velocity is 28 m/s

Valves 2 and 1 have remotely operated actuators to enable them to be opened or closed automatically, in accordance with the amount of water present in the settling tank. This is monitored by a pressure sensor which measures the depth of water in the tank. Signals from the sensor are relayed down a (800m long) cable, to the control box in the turbine, which then opens or closes the two valves accordingly. This part of the electronics was specially designed for us.

The main point is that the system must always have sufficient water at the settling tank to match the consumption of the turbine. The necessary adjustment to valves must be made before the level in the stream gets too low. Insufficient water will result in zero power output. The sensor tells valve No. 2 and then if necessary No. 1 to close if the level in the settling tank falls.  After a set interval (about 6 hours) the valve automatically opens again and the sensor then sees once again if sufficient water remains in the settling tank to merit keeping these two valves open.  If not, they close again.  The 6 hour interval is to avoid ‘hunting’ i.e. repeated opening and closing

The control box is powered by a power supply which is plugged into a normal 13 amp socket in the hut.  If power is on, there should be readings on the control box.

The main reason for accessing the turbine hut is to close down the turbine in the event that the turbine and the control box in the porch in the house become ‘de-synchronised’.  When this happens the speed of the turbine increases to a whine and this may damage the bearings of the turbine.

To close down the turbine turn the handle of the red upstream valve clockwise.  If the two other valves are open then close them manually using a spanner on each actuator.

Electricity Distribution

An armoured  power cable leads from the turbine house, approximately 265metres, to the porch, to the boxes to right of the porch window, (see photo).  The dials indicate the incoming electricity.  A metre shows the generated power and a further metre shows power flowing to the neighbour’s house where battery chargers convert the 240V a.c. into 24Vd.c. and store this in his battery banks.

The electricity passes, via the white box beneath the metres as follows:

a)         via a cable to the battery chargers in the boatshed, shown between the two inverters, which are used to charge the battery bank in the generator shed.   The capacity of the top (SIP) charger is 1.6kW.

b)         via another cable to our neighbour’s at Sandaig House, to charge their batteries in the same way .  Also 1.6kW capacity.

c)         any surplus, not used for a) and b), goes to heat the oil fitted radiators in the porch of the house.  These are permanently connected and must remain fully on, because all generated electricity has to be used up.  This is called a `dump load`.

When there is demand from the houses, the stored electricity is ‘inverted’ by two Outback (shown) inverters in the boathouse, back to 240 volt a.c. which is the normal domestic supply.

The battery bank supplies electricity to the normal domestic fuse box in the porch (and thus to the house) via the inverters  which transform 24volts dc to 240 volts ac.

Check the water levels in the battery every month. The plates must always be submerged. Top up only with de-ionised water.  

Diesel Generation

There is a Lister diesel engine linked to a 7 kW alternator with Autostart, which is a back-up to the hydro.  This should start if the demand from our house exceeds a total of 2 kW.

Try to avoid using large loads such as heaters for prolonged periods of time. Use of such loads will result in the inverter starting the diesel generator.

Measuring the power generation