Case Study:Site: New Mills, DerbyshireTurbine: Archimedes screwPower: 63kWHead: 3.0m

Western Renewable Energy (WRE) was brought in by MannPower Consulting (MPC) after the feasibility stage. WRE and MPC carried out detailed design, leading the WRE planning and carrying out the construction for the client. WRE also designed and installed the fish-pass element for the Environment Agency.

WRE were first consulted in January 2007 about the engineering feasibility of the scheme, and despite a range of obstacles, a detailed plan and costing was developed. Major issues to be worked around included:

• the existence of a storm water outfall across the site, which needed to be integrated into the new works, but without affecting the discharge during construction.

• Archaeological issues which required an Archaeologist on site at all times during the excavations, and a large amount of detail to be recorded.

• Restricted site space in a rocky gorge.• Very limited access, restricted to 7.5t on site, and no crane access.• Large volume of spoil for removal, all requiring double-handling and off site disposal.• Additionally the site was very public and in an attractive park area.

Work physically started in March, replacing the storm water discharge, which had to run through the intake area

After the diversion was complete, archaeological

excavation by WRE, under the direction of staff from

Archaeological Research Services continued for a

month. The extensive remains on site were documented and

artefacts retained, as part of the planning permission

Once through the level of archaeology, another 3m of Gritstone bedrock was removed to provide the correct tailrace depth

The floor pour for the turbine and fish pass then occurred

The restricted access to site required all the walls to be cast

in one day, when the pump, pipelines and manpower was

available, and access could be arranged for the procession of

concrete lorries, so all shuttering for the structure was set up to

be carried out in one pour

With concrete complete, the complicated phase, unique to the Torrs project was started. Because weight on site was limited to 7.5t, no crane could be brought it, and indeed the turbine itself exceeded this weight. Firstly wooden ramps were installed

The wooden ramp provided a continuous slope from ground level to the bottom of the slope. Because of tight space restrictions, the screw had to be brought in sideways and lowered onto the ramp

With the main road through New Mills closed, and hundreds looking on, the screw was lowered into the gorge, to

avoid the weight restrictions on the normal access route.

The turbine landed on a purpose built trolley, with steerable castors,

which was rigidly coupled to two mini diggers to pull it into position at

the top of the ramp

The turbine was crabbed out sideways, over the concrete channel and onto the temporary wooden structure. From here it was then suspended at one end to allow the removal of part of the trolley, and part of the wooden temporary structure

With the temporary wooden structure changed, the

supported end was lowered until it was at 22 degrees, but at the top of the slope.

From here electric winches under the wooden structure

were gradually relaeased, allowing the turbine to slide into position on a carefully located central rail running

down the slope. This positioned the turbine in the

approximately correct position, and allowed

completion of the concrete works

The top slab was then cast, and the turbine lowered onto it, after it had cured

While the concrete was going off, other jobs commenced,

including craning excavators into the river to excavate the

tailrace and adapt the river bed

The fish pass was installed at this point, to allow migration of fish past the weir for the first time in over two centuries

Transmission and generation equipment, as well as sluices and other ancillary equipment was then installed

Finally the intake was excavated and the temporary storm outfall

was removed, to leave the system looking much as the

original artists impression had envisaged

The final step was the installation of the guarding and fencing, as the system is located in a public park.

The system was commissioned and handed over in September 2008, and by the end of the year had generated almost 100,000 kWh.

a casestudy...Archimedes’ screw: CopleyHydropower GeneratorThe Archimedes’ screw – used for over 2000 years as a pump – is nowbecoming a popular technology choice for low-head hydropowergenerators. Future Energy Yorkshire (FEY) has carried out a study onthe relative costs of an Archimedes’ screw and the more commonKaplan turbine for a small scale hydropower site.

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BackgroundThe Archimedes’ screw is well suited to low-head sites andis reportedly less harmful to fish. A few dozen are in operationin Germany and the first examples are now appearing inthe UK. However, low-head hydropower is developing veryslowly in the UK despite there being thousands of potentialsites at river weirs across the country. It is often hard tojustify the capital expenditure of installing a modernhydropower scheme on such sites given the relatively lowrevenue from electricity sales, so this study compared thecapital costs of the two systems for a specific low-headhydro site.

Description of ProjectFEY commissioned a study to compare the costs of installinga Kaplan turbine and an Archimedes’ screw at the potential(and former) hydro site at Sterne Mill in Copley, near Halifax.One hydropower consultant was asked to design a systemusing the Archimedes’ screw while a second consultantwas asked to design a scheme with a similar annual energyoutput but using a Kaplan turbine. Each consultant wasalso asked to cost the equipment and its installation butexcluding the civil engineering costs. The two designs weregiven to an independent civil engineering firm whichcalculated cost estimates for the civil works.

ResultsThe two scheme designs did not arrive at exactly the sameannual energy output, but nevertheless the study showeda strong cost advantage for the Archimedes’ screw. For anenergy output of about 15% more, the Archimedes’ screwcost about 10% less. So in terms of capital cost in £ perMWh per year, the Archimedes’ screw was 22% cheaperthan the Kaplan turbine. The results are summarised in thetable below.

The study also demonstrated how much differentassumptions can affect the civil engineering costs. If thespoil from the construction had to be landfilled, for example,this would add from 30% to 40% to the costs above forboth schemes.

The costs for a hydro scheme are highly site dependentso it cannot be assumed that the Archimedes’ screw willalways be the cheaper option, but this study shows thatfor this site there is a significant cost advantage.

A full report of the results of this study will be published byFEY. It is hoped that a hydro system will be installed aspart of a major property development around the weir.

t 0113 237 8436e email@fey.org.uk www.fey.org.uk

If you would like more information on the Archimedes’ screwproject or how we can help your company please contact:

Jo Adlardjo.adlard@fey.org.ukDirect line: 0113 237 8426

OV1693

Annual energy outputInstalled capacityCapital costCapital cost (per MWh/yr)Return on investment(IRR, 20yrs)

Kaplanturbine

390 MWh95 kW

£414000£1061

5%

Archimedes’screw

448 MWh124 kW£371000

£8289%

So in terms of capital cost ...the Archimedes' screw was22% cheaper than the Kaplanturbine.

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Copyright © 2000 Fluent Inc. EX150 • Page 1 of 2

A P P L I C A T I O N B R I E F S F R O M F L U E N T

A 3D CFD model is used tosimulate the operation of anArchimedes screw, a simplemechanical device invented by theGreek mathematician Archimedes(287-212 BC) for raising water (orother material). Used forirrigation in ancient times, itconsists of a helical element, orscrew, that rotates inside a close-fitting cylinder. The axis of thedevice is inclined at some angleabove the horizontal. One end isplaced in a lower reservoir ofwater and as the screw turns, asmall quantity of water is scoopedup and trapped in a cavitybetween the screw and outercylinder. As the rotationcontinues, this scoop is lifted andadditional scoops are trapped andlifted as well. At the top of thedevice, the scoops of water arereleased into an upper reservoir.In modern pump classification,the Archimedes screw is a positivedisplacement (PD) pump.

The geometry of an Archimedesscrew is governed by certainexternal parameters (its outerradius, length, and slope) andcertain internal parameters (itsinner radius, number of blades,and the pitch of the blades). Theexternal parameters are usuallydetermined by the location of the

EX150

lower reservoir and does notefficiently lift it.

A simple, single-blade screw ismodeled in this simulation. Thescrew is inclined from thehorizontal at 45 degrees and isrotated at a constant speed of 4rad/s (76 rpm). To simulate themotion of the screw, a rotatingmesh zone of 105,000 tetrahedralcells (derived from the surfacemesh shown in Figure 1) is used.A fine grid is used in this regionso that sloshing of water may becaptured in the CFD model.Outside the rotating zone is astationary zone containing 70,000hexahedral cells. This zoneencompasses the upper and lower

screw and how much material isto be lifted. The internalparameters, however, are free tobe chosen to optimize theperformance of the screw. Theoptimal parameters formaximizing the volume of waterlifted in one turn of the screw aregiven in a paper by Rorres (2000).

As more material is lifted by thescrew, more effort is required toturn it; this puts a practical limiton how long (or wide) such a tubecan be. Nagel (1968) observedthat the rotational velocity of ascrew, in revolutions per minute,should not be more than 50/D2/3,where D is the diameter of theouter cylinder in meters. If thescrew is rotated much faster,turbulence and sloshing preventthe cavities from being filled.The screw churns the water in the

Archimedes ScrewA CFD simulation of an Archimedes screw is presented in this example. The VOF and slidingmesh models in FLUENT work together to capture the transient lifting of water from a lower toan upper reservoir by the rotating screw.

Figure 1: The surfacemesh for the rotating zone,composed of the screwand the cylindrical slidingmesh boundary

Figure 2: At time zero, the fluid is at rest in thelower reservoir

Copyright © 2000 Fluent Inc. EX150 • Page 2 of 2

reservoirs. A non-conformal gridinterface links the rotating andstatic zones.

Initially (Figure 2), the lowerreservoir contains a finite volumeof water, and the upper reservoiris empty. At this time, the wateris at rest. Figure 3 shows thescrew in action raising water fromthe lower reservoir to the upperreservoir. During this process, thewater level in the lower reservoirdecreases. After about 8 seconds(Figure 3a), the first scoop ofwater (a tiny amount, owing tothe initial orientation of thescrew) begins to emerge from theupper end of the screw. Abouttwo seconds later (Figure 3b), thesecond scoop of water begins toemerge. After another revolutionof the screw (Figure 3c), the thirdscoop of water empties into theupper reservoir. (The details ofthe flow in the upper reservoir arenot accurately captured by thismodel because a coarse grid isused in this region.)

The equilibrium water volume isdefined as the maximum volumeof water that can be held in eachchamber between the threadswhen the screw is stationary.

During the early stage ofoperation, when the water levelin the lower reservoir is high, thescrew scoops up more than theequilibrium water volume in eachrevolution. As the water is raisedwith each turn of the screw, theexcess water cascades down to thenext chamber. This cascadingeffect can be seen near the bottomof the screw in Figure 3. Whenthe water level in the lowerreservoir reaches a certain criticallevel, the efficiency of the screwbegins to drop, because the screwscoops up less than theequilibrium volume with eachrevolution.

The Archimedes screw is still inuse today. The underlyingprinciple of the device is appliedin machines used for drainage,irrigation, oil well extraction, foodprocessing, and in high-speedtools. It can also be applied forhandling light, loose materialssuch as grain, sand, and ashes inmachines known as screwconveyors, direct descendants ofthe Archimedes screw. While theArchimedes screw lifts fluidsbecause of its inclined position,the screw conveyor propelsgranular materials through the

pushing action of its rotatingblades. Most screw conveyers inuse today have a single blade,while modern Archimedes screwstypically have two or threeblades. For screw conveyors orfor the Archimedes screw itself,FLUENT is an excellent tool forsimulating the mechanics of theprocess at work. The VOF andsliding mesh models workingtogether can capture the essentialphysics of devices of this type.This is a powerful combinationthat can be applied to other freesurface tracking applications thatinvolve rotating parts.

References:

1. Rorres, C. (2000), The turn ofthe screw: optimal design of anArchimedes Screw, ASCE Journalof Hydraulic Engineering, Vol.126, no. 1, pp. 72-80, January,2000.

2. Nagel, G. (1968), Archimedeanscrew pump handbook, Preparedfor Ritz-Atro Pumpwerksbau GMBH Roding, Nurnberg,Germany.

Figure 3 : The lifting of water by the rotating screw is illustrated after 8 seconds (a), 10 seconds (b), and 13 seconds (c)