ashford bobbin mill hydro feasability study

Ashford Bobbin Mill Hydropower Feasibility

Study

Sam Townsend 941425 REBE Module 3 – Hydropower

Tutor Group – Arthur Butler

January 4th 2009

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Table of Contents 1 Introduction .................................................................................................. 3

2 Site Overview ............................................................................................... 4

3 Resource Analysis ........................................................................................ 5

3.1 Flow Measurements ............................................................................... 5

3.2 Head Measurements .............................................................................. 8

3.3 Grid Connection ..................................................................................... 9

4 Conclusions ................................................................................................ 10

4.1 Site Options .......................................................................................... 10

4.2 Power Output and Energy Capture ...................................................... 11

4.3 Revenue ............................................................................................... 12

4.4 Turbine Options .................................................................................... 13

4.5 Summary .............................................................................................. 15

5 Next Steps .................................................................................................. 15

Appendix A .................................................................................................... 16

Appendix B .................................................................................................... 17

Appendix C .................................................................................................... 18

Appendix D .................................................................................................... 19

Appendix E .................................................................................................... 20

References .................................................................................................... 21

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1 Introduction Electricity generation is the single largest emitter of CO2 in the UK responsible for 1/3

of the total CO2 emitted by all sectors in 2008 (DECC, 2009a).

However hydro electricity generation is zero carbon emitting and as such clearly has

an environmental advantage. Hydro power was once perhaps the principal form of

power supply for the UK’s industrial sector. In the Peak District it was used to provide

kinetic energy to power the many large cotton mills. The same dormant infrastructure

could now be re-incarnated to supply electrical energy to the grid. If existing

infrastructure is re-used, it has a very low embodied energy and barring drought, it is

a reliable source. Economically, the capital cost of re-using existing infrastructure is

low and schemes less than 100kW qualify for grants from the low carbon buildings

program (DECC, 2009b). Socially, it’s a clean and acceptable technology and on a

small scale, visually unobtrusive. Hydro power is an age old proven and reliable

technology that can remain in active service for decades providing a regular income

long after capital investment payback has been achieved.

The aim of this report is investigate the potential for generating electricity using hydro

power at the site of Ashford Bobbin Mill on the river Wye, Derbyshire.

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2 Site Overview

Ashford Bobbin Mill is situated on the southern bank of the river Wye just north of

Ashford on the Water. It was built in the 1870’s as a saw and wood turning mill

providing bobbins to the cotton industry (Bunting, 2006).

A site survey was undertaken on 31st November 2009. The mill compromises a weir

across the Wye; a relatively large approach canal, currently blocked at the weir by a

temporary coffer dam; one small cast iron, breast shot waterwheel which discharges

directly into the river; two larger cast iron, breast shot wheels, mounted on each end

of the mill building; a tailrace comprising a dry stone arched culvert that is in relatively

good condition with no blockages. All the wheels and sluice mechanisms are in poor

condition with heavy corrosion. The culvert issues back into the Wye approximately

200m downstream.

The two mill buildings and large wheels are Grade 2 listed buildings, See Appendix A.

See Figure 1 for a map of the site.

Figure 1 : Ashford Bobbin Mill (1:2500) (Ordnance Survey, 2009a)

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3 Resource Analysis

3.1 Flow Measurements

An Environment Agency gauging station is situated 30m downstream of the mill.

Summary data can be accessed directly from their website. Data has been recorded

from 1965 to 2006 although the weir was decommissioned in 1977 and re-

commissioned in 1994. (Environment Agency, 2009a)

Due to the potential anomalies in the EA data attributed to the weir closure, the

HydrA hydraulic modelling program from the Institute of Hydrology was also

employed to provide an estimate of flow at the site. In order to establish the

catchment area, the map provided by the EA website (Environment Agency, 2009b)

was traced onto a Google terrain map.

Figure 2 – Catchment of the EA gauge at Ashford Bobbin Mill

It was noticed that the catchment diagram provided by the EA covers areas where

the drainage is known not to supply the Wye and conversely does not cover areas

known to supply the Wye upstream of the gauge (see Figure 2). The red shaded area

covers an area where, due to underground drainage, the water supplies the Derwent

via Castleton or Bradwell (Ford, 1977). The blue area covers an approximate

catchment for the Magpie Mine Sough, a mine drainage level that discharges into the

Wye 100m upstream of the mill, which is not included in the EA map. It’s clear that

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the EA catchment has been derived purely from surface watershed, with no regard

for underground drainage.

To plot the catchment within HydrA, OS grid references describing the included area

are input manually. An approximate catchment area was derived by tracing the EA

catchment, excluding the area known not to feed the Wye, and including the area

that does. By taking a bisecting route along the more detailed outline, any error

should average out along its length. See Figure 3 for the catchment used and

Appendix B for the OS co-ordinates used.

Figure 3 – Catchment (in blue) used for HydrA model.

The EA flow figures are given in Table 1, and a Flow Duration Curve is depicted in

Figure 4.

Catchment area 154 km2

Average annual rainfall 1166 mm

Qmean Average Flow 3.24 m3/s

Q95 Flow exceeded 95% of the time 0.979 m3/s


Table 1 : Data provided by EA for gauging station at Ashford

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Figure 4 – Flow Duration Curve provided by EA for gauging station at Ashford

(Environment Agency, 2009a)

The results given by HydrA are summarised in Table 2 and a flow duration curve

depicted in Figure 5.

Catchment Area 135.62 km2

Average annual rainfall 1212 mm

Qmean Average flow 3.4 m3/s




Table 2 : Flow data provided by Hydra for gauging station at Ashford

It can be seen that the results are comparable. HydrA is using a larger annual rainfall,

which may cancel out the smaller catchment area used. The HydrA mean flow is

greater than the EA, which is probably attributed to the higher annual rainfall used.

However, the Q95 value is lower than that of the EA’s. It is also interesting to note

that the EA website gives a Qmean figure for 2006 of 3.37, which is much closer to

the HydrA figure.

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Figure 5 – Flow Duration Curve provided by HydrA

Since the raw data for either set of the values is unavailable, no determination of

accuracy can performed. However, the values provided from HydrA are interpolated

from an estimated catchment and historic rainfall data, and the EA value based on

actual measurements, making it logical therefore to use the EA values for any

calculations.

Base Flow Index (BFI) is the ratio of Q95:Qmean. The EA good practice guidelines state

that for low head sites and a BFI greater than 0.2, then regardless of depleted reach,

Max design flow can be Qmean, and the ‘Hands off Flow’ (HOF) can be Q95

(Environment Agency, 2009c). Using the EA flow rates provided, a BFI of 0.3 is

achieved. This results in a HOF of 0.979 m3/s and a Qmax of 3.24 m3/s.

3.2 Head Measurements

Head measurements at various points were taken using a tripod mounted Dumpy

Level and Measuring Staff. Readings were taken to ±1mm. The measurements were

taken starting at the tailrace, working up to the weir, and then back again in order to

determine consistency. The difference between the two measurements was

calculated to give a deviation and the mean taken. The measurement at the small

wheel was only taken once due to time constraints on the day, however the result

seems reasonable and in line with what was expected.

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Description of leg Reading 1

(m)

Reading 2

(m)

Deviation

(m/%)

Mean

(m)

Top of Weir to Base of Small Wheelpit 1.530 - - 1.530±0

Top of Weir to Base of Main Wheelpit 2.047 2.138 0.091/4.3 2.093±0.045

Base of Wheelpit to Floor of Tail Race 0.220 0.189 0.031/15.2 0.205±0.015

Table 3 : Head measurements at the site.

Table 3 shows that the largest error was related to the leg ‘tailrace to wheelpit’. This

actually covers a horizontal distance of approximately 180m and so difficulty in

reading the scale over that distance may account for the error. Another reason may

be the soft nature of the ground at both locations. It should also be noted that the

deviation is only 1.5cm. It was felt that the tolerances achieved were acceptable and

for the purposes of this study, values of 2m can be used for ‘weir to main wheelpit’,

and 1.5m for ‘weir to small wheelpit’. See Appendix C for details of all measurements

taken on the day.

3.3 Grid Connection

Details of the nearest Grid connection were obtained from the local District Network

Operator, Central Networks. The nearest substation is located 800m from the site at

Ashford Marble Works and transforms the 11kV supply to Low Voltage for the village.

An underground 11kV line also passes within 500m on top of the hill above the mill.

See Appendix D for a map of the local Grid infrastructure.

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4 Conclusions

4.1 Site Options Since the main mill buildings and wheels are listed, they cannot be considered unless

a complete restoration of the original wheels is to be undertaken.

Option A : A turbine could be placed in an excavated powerhouse in line with the old

tailrace, if a new intake could be constructed between the two mill buildings.

This would give the largest available head at just over 2m. However it would lead to a

relatively long depleted reach of 400m since the tailrace re-enters the river so far

downstream. The civil work involved with excavating the powerhouse and squeezing

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the intake between listed buildings of unknown integrity may prove difficult. This also

has the effect of flow bypassing the EA gauging station. Detailed discussions with the

EA would be needed to ascertain if this is acceptable.

Option B : The smaller independent wheel and associated building are not listed, so

redeveloping this position can be considered. See Figure 6.

Figure 6 – Option B site. (Ordnance Survey, 2009a)

It has a smaller head of 1.5m but the depleted reach is shorter at 70m and the civil

work involved in creating a new intake looks to be straight forward. This also avoids

flow bypassing the EA gauging station and is therefore the preferred option.

4.2 Power Output and Energy Capture

Power output is given by

𝑃𝑃 = 𝑄𝑄 ∗ 𝐻𝐻 ∗ 𝛾𝛾 ∗ 𝑒𝑒

Where P = power in kW

Q = design flow in m3/s

H = Head in m

𝛾𝛾 = specific weight of water (9.81kN/m3)

e = overall efficiency of system. (Langley et al., 2004)

If the site of the small wheel is used, then a head of 1.5m is available. Using a design

flow 2m3/s and an overall efficiency factor of 0.5, then 14.7kW of power could be

obtained from the site.

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Looking at the Flow Duration Curve provided by the EA (See Figure 7), the Capacity

Factor (CF) for a river flow rate of 3m3/s, i.e. that would be required for a design flow

rate of 2m3/s in the turbine after the HOF has been considered, is approximately 42%

(yellow line). This means the turbine could only operate at full capacity 42% of the

year.

Energy per annum = P * CF * 8760

Therefore annual generated energy is 14.7 * .42 * 8760 = 54084 kWh/annum

≈ 54 MWh/annum

If a design flow of 1m3/s is considered, the peak power output is reduced to 7.35kW

but the capacity factor is increased to 64% (red line). This results in an annual

generated energy of 7.35 * .64 * 8760 = 41207 kWh/annum

≈ 41.2 MWh/annum

Figure 7 – EA Flow Duration Curve with Capacity Factors

4.3 Revenue

In 2010, Feed in Tarrifs will be available to allow the sale of electricity by micro

generation to the grid. For small producers of electricity, these will replace the

Renewable Obligation Certificates. The prices quoted by Feed-in Tariffs Ltd for

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hydro are 17p/kWh for schemes less that 10kW and 12p/kWh for schemes 10-100kw.

(Feed-in Tariffs Limited, 2009)

Using the figures for energy capture above, the 14.7kW - 2m3/s option would

generate 54084 * .12 = £6480/annum

≈ £6500/annum

Interestingly, the 7.35kW 1m3/s option would qualify for the higher feed in tariff and

therefore generate 41207 * .17 = £7005/annum

≈ £7000/annum

Given that the lower flow/power option would probably have a lower capital cost, it

would be more cost effective to go with this option. It should be noted that a fairly

cautious efficiency factor of 50% was applied to the initial power calculations. It is

possible that the overall efficiency could well be higher which would increase the

amount of energy captured and hence the revenue returned.

4.4 Turbine Options

The choice of turbine is dictated mainly by the head and the flow. In this case we are

dealing with a high flow and low head. By using the ‘Specific Speed’ equation, turbine

suitability can be determined.

𝑁𝑁𝑁𝑁 =𝑛𝑛 ∗ (𝑃𝑃 ∗ 1.4)0.5

𝐻𝐻1.25

Where Ns = specific speed

n = shaft speed

P = required power output (kW)

H = head (Langley et al., 2004)

The figure achieved can then be compared to the range of specific speeds provided

in Table 4.

Pelton 12-30

Turgo 20-70

Crossflow 20-80

Francis 80-400

Propeller/Kaplan 340-1000

Table 4 : Specific Speed Ranges (Langley et al., 2004)

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Using a Head of 1.5m, power at 14.7 kW and a shaft speed of 1500rpm (the usual

RPM of generators generating 240v at 50Hz) we get a Specific Speed of 4099. If the

lower power of 7.35kW is used, a Specific Speed of 2899 is achieved. These lie well

outside the range of any turbine. However, if a belt drive is used with a ratio of 4:1 or

5:1, the generator rpm and thus the specific speed can be reduced, putting it firmly in

the range of a Propeller or Kaplan turbine.

A Siphonic turbine (see Figure 8) would be most suited to the small wheel house site.

Appendix E contains details of such a turbine installed at nearby Borrowash Mill,

Derbyshire. A Propeller turbine has fixed pitch blades and a poor efficiency at part

load however the more costly Kaplan turbine allows the pitch of the blades to be

adjusted to improve part load efficiency.

Figure 8 – Diagram of a Siphonic turbine. (Mragowo, 2009)

Another solution suited to low head, high flow situations is an Archimedes screw.

However, these tend to require a long ‘footprint’ which may not be available at this

site.

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4.5 Summary

In summary, if a Siphonic Propeller turbine was installed at the small wheel house

site, with a flow of 1m3/s and a power output of 7.35kW, 41.2MWh/annum could be

generated. Under a Feed In Tariff, this would create a return of ≈ £7000/annum.

5 Next Steps

The EA would need to be consulted in order to obtain the required abstraction

licences. Discussions with them are likely to include effects on the local flora and

fauna, and acceptable flow regimes.

The local District Network Operator, Central Networks, would need to be consulted

about the practicalities and costs of obtaining a grid connection at either the

substation in Ashford or the nearby 11kV line. As the site is within a National Park,

this cable would need to be buried.

The Peak District National Park would need to be consulted and Planning Permission

obtained.

If the above steps pose no ‘show stoppers’, then a more detailed design and costing

of the Electro/Mechanical equipment can be undertaken.

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Appendix A

Details of Grade II Listings. (Peak Park Joint Planning Board, 1984)

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Appendix B

OS Grid Co-ordinates used to plot Catchment in HydrA

Eastings Northings

418 238 369 705

417 473 371 080

418 575 371 617

418 100 373 167

419 400 373 642

419 225 376 667

418 050 377 867

414 925 377 917

409 675 380 043

408 385 378 463

406 210 378 013

406 110 376 363

403 210 375 188

403 635 374 238

403 023 373 825

402 383 371 020

405 383 369 030

406 445 369 813

410 580 367 973

411 768 370 795

415 023 370 417

415 533 368 275

416 913 367 762

417 865 367 687

418 238 369 705

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Appendix C Details of all measurements taken on Site Visit.

Backsight Foresight Notes Delta h Total Deviation Mean Value %

Weir to Small Wheel1.860 3.390 House to wheel2 -1.530 -1.530

Weir to Wheel1 Wheelpit3.548 0.943 wheel - sm house 2.6051.300 1.858 sm house to wier -0.558

2.0473.480 2.737 wier to house2 0.7430.515 3.396 house2 to wheel -2.881

-2.138 0.091 2.093 4.3

Wheelpit to Tail race4.050 3.830 tail race to wheel1 0.2203.396 1.295 wheel to path 2.1011.000 3.290 path to tail race -2.290

-0.189 0.031 0.205 15.2

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Appendix D

Central Networks Map of Grid cables in the area and points of potential connection

(Ordnance Survey, 2009b)

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Appendix E

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References

Bunting, J. (2006) Bygone Industries of the Peak, Sheffield, Wlidtrack Publishing Ltd. DECC (2009a) “Energy and emissions projections - Department of Energy and

Climate Change.” Carbon Dioxide Emissions Tables. [online] http://www.decc.gov.uk/media/viewfile.ashx?filepath=statistics/projections/1_20090812111722_e_@@_tablea.xls&filetype=4 (Accessed December 10, 2009).

DECC (2009b) “Small scale hydro - Low Carbon Buildings Programme.” Low Carbon

Buildings Programme. [online] http://www.lowcarbonbuildings.org.uk/micro/hydro/ (Accessed December 10, 2009).

Environment Agency (2009a) “Gauge Station 28023 - Wye at Ashford.” [online]

http://www.nwl.ac.uk/ih/nrfa/station_summaries/028/023.html (Accessed December 5, 2009).

Environment Agency (2009b) “Catchemnt Elevation - Wye at Ashford.” [online]

http://www.nwl.ac.uk/ih/nrfa/spatialinfo/Elevation/elevation028023.html (Accessed December 5, 2009).

Environment Agency (2009c) “Good practice guidelines annex to the hydropower

handbook.” [online] http://www.environment-agency.gov.uk/static/documents/Business/Low_Head_Hydropower_August_2009.pdf (Accessed December 11, 2009).

Feed-in Tariffs Limited (2009) “F I Tariff eligible technologies.” [online]

http://www.fitariffs.co.uk/Technologies.html (Accessed December 14, 2009). Ford, T. D. (1977) Limestone and Caves of the Peak District, Norwich, Geo

Abstracts. Langley, B. and Curtis, D. (2004) Going with the Flow, Centre For Alternative

Technology. Mragowo (2009) “siphonic propeller turbine.” [online]

http://www.wtw.mragowo.pl/images/ofe07.jpg (Accessed December 17, 2009). Ordnance Survey (2009a) “Ashford Bobin Mill.” Ordnance Survey (2009b) “Ashford Area Grid .” Peak Park Joint Planning Board (1984) “Ashford Bobbin Mill Listing Details.”

ashford bobbin mill hydro feasability study

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