A Feasibility Study into Tidal Mills as a

Source of PowerRhys Hobbs 15071588

MSc. Renewable Energy and Resource Management

Introduction- UK tidal ResourceType Location Indicative annual energy [TWh/year] Indicative maximum power [GW]

Tidal stream

England 34 11

Wales 28 9.5

Scotland 32 11

Northern Ireland 1 0.5

Total 95 32

Tidal range: barrage

England 57 27

Wales 23 8

Scotland 16 10

Total 96 45

Tidal range: lagoon

England 14 8

Wales 7 3.5

Scotland 4 2.5

Total 25 14

The U. K’s tidal resource, per region and type of tidal device. Source: The Crown Estate

Introduction – Tide Mills A tide mill is a simple water mill that

harnesses the power of the rising and falling tide , A mill pond is created by forming a dam across an appropriate area, whether that is a tidal inlet, or a part of a river estuary.

When the tide rises, the water enters the mill pond through a gate and channel known as a penstock. This gate then closes when the tide beings to drop, storing a large amount of water in the mill pond, gathering potential energy. When the tide drops to a certain level, the gate is re-opened and the water is released and subsequently turns the water wheel.

Diagrams showing how the tide mill work when the tide falls (Minchinton, 1979)

Aims and ObjectivesThe overarching aim for this dissertation project is: To complete and evaluate a feasibility study into tidal mills as

a source of renewable energy for the U.K Objectives: To calculate the power output of the tidal mills at spring, neap and

regular tides To analyse the various types of tidal technology and whether they are

suitable for use in tidal mills To critically evaluate the potential environmental impacts of the

scheme To examine the feasibility of the technology of the scheme To analyse the potential economic feasibility of the small scale tidal

scheme

Methodology – Tide data

Various tidal data points. Source: www.willyweather.co.uk

Methodology – Tide MillsTide Mill Location Current Status

Thorrington Tide Mill Thorrington, Essex Operational

Eling Tide Mill Eling, Hampshire  Operational

Three Mills Tide Mill Bromley-by-Bow, London Restored, not operational

Woodbridge Tide Mill Woodbridge, Suffolk  Restored, not operational

Tide Mill Newhaven, East Sussex  Demolished - Sluice only

Carew Mill Carew, Pembrokeshire Restored, not operational

Pembroke Tide Mill Pembroke, Pembrokeshire Demolished - Mill pond and Sluice only

Methodology - Technology As all of the mills in the study

have been refurbished and repurposed or derelict, the turbine needed for the generation of electricity has to be chosen, as it will affect the overall power output of the turbine.

As tidal mills work on a similar level to hydro-electric power (pumped storage) then there are a number of available options in regards to which turbine will be most applicable for the mills themselves.

Diagram of a Kaplan turbine. Source: (Charlier, 1982)

Diagram of a Francis turbine source: (Charlier, 1982)

Methodology - Calculations Change in mill pond over

time:

Volumetric flow rate:

Power output:

Trash Rack Losses Equation

Head losses calculation

Methodology - Calculations Kaplan Turbine costs CK = 15000 × (Q × H) 0.68 (£)

Feed in Tariffs Export Tariffs

Hydro generating station with total installed capacity of less than 100kW

p/kwh1 Apr to 30 Jun 2016 7.68

1 Jul to 30 Sep 2016 7.66

1 Oct to 31 Dec 2016 7.65

1 Jan to 31 Mar 2017 7.63

Feed in Tariff rates for normal hydro, source: (Ofgem.gov.uk, 2016)

Results - Thorrington  Spring Neap Normal

Total  Losses (m)

0.029818553 0.000319585 0.000381925

  SPRING NEAP NORMAL

Total £ for generation £2511.10 £1097.32 £5336.63

  Spring Neap Normal 

Total £ for Export £1609.59 £703.37 £3420.74

Total Power Output:

111.5KW

Capacity Factor:19.5%

Results - ElingTotal Power Output:

73.7KW

Capacity Factor: 21%

  Spring Neap NormalTotal Losses 0.015278012 0.000157241 0.077412627

  SPRING NEAP NORMALTotal £ for generation £1513.94 £631.78 £4506.41

  Spring Neap Normal Total £ for Export £828.12 £345.58 £2,464.99

Results – Bromley By BowTotal Power Output:

48.02KW

Capacity Factor: 16%

Total Losses Spring Neap Normal 

  0.001327291 2.23464E-05 2.84E-05

  SPRING NEAP NORMALTotal £ for generation £845.26 £561.18 £2898.56

  Spring Neap Normal 

Total £ for Export £462.36 £306.97 £1,585.51

Results - WoodbridgeTotal Power Output:

7.87KW

Capacity Factor: 23%

Total Losses Spring Neap Normal   0.000354741 4.61164E-06 6.88E-06

  SPRING NEAP NORMALTotal £ for generation £151.58 £78.56 £477.07

  Spring Neap Normal Total £ for Export £82.91 £42.97 £260.96

Results - NewhavenTotal Power Output:

106.85KW

Capacity Factor: 21%

Total Losses Spring Neap Normal 

  0.016131975 0.000175263 0.000273

  SPRING NEAP NORMALTotal £ for generation £2,220.47 £975.54 £6,180.63

  Spring Neap Normal Total £ for Export £1,214.59 £533.62 £3,380.79

Results - CarewTotal Power Output:

60.61KW

Capacity Factor: 14%

Total Losses Spring Neap Normal 

  0.027767721 7.5754E-05 0.000253

  SPRING NEAP NORMALTotal £ for generation £1801.04 £201.09 £2735.92

  Spring Neap Normal Total £ for Export £985.16 £110.00 £1,496.54

Results - PembrokeTotal Power Output:

160.97KW

Capacity Factor: 14%

Total Losses Spring Neap Normal 

  0.237035131 0.000646538 0.002162

  SPRING NEAP NORMALTotal £ for generation £4,554.98 £586.90 £7,977.68

  Spring Neap Normal Total £ for Export £2,491.56 £321.03 £4,363.77

Discussion – Technological Feasibility

In terms of feasibility, Woodbridge (7KW) would be the least feasible purely on the basis that it produces the least amount of power.

For hydro technologies, the capacity factor usually ranges between 20 and 50% over a monthly period.

Although the tide is regular, the water level in the mill pond is simply not high enough to generate a substantial amount of power.

Discussion – Economic Feasibility Without utilising both of these tariffs, the mills simply wouldn’t

generate enough money to be sustainable.

The Kaplan turbine costs vary greatly per site, the general pattern was if a site had a large head and fast volumetric flow rate, then the cost of the turbine went up significantly.

Discussion – Economic Feasibility

Apart from Pembroke and Newhaven, all of the tide mills are fully restored and have been refitted for different purposes, but the key features such as the penstock and trash rack are still in place.

This means that in order for these Mills to be fitted for electricity generation, they simply need to be refurbished to accommodate the turbine and generator. This is relatively cheap compared to Pembroke and Newhaven, which would have to be built rather than re-fitted.

Discussion – Environmental impacts

Before commissioning any renewable energy project, an environmental impact assessment has to be carried out in accordance with EU regulations. Assessors evaluate the site for potential impact areas and draw up a detailed report on their findings that can be used in tailoring the project to negate as much of these impacts as possible.

Some impacts include: Potential flooding Turbine damage due to fish and debris

Assumptions

The same method of calculation was used for the tidal mills as a previous method for hydroelectric power plants.

The turbine efficiency was assumed to stay constant through the four hours of generation.

The length, diameter and material of the penstock was kept the same for all the sites, based on research. As all of the sites fell into a small range of head sizes, the same length and diameter was used for each one

Limitations By presuming all of the penstock lengths are the same, some

sites may have a greater head loss than they should get, as different sizes, even if by millimetres could make the penstock more efficient.

Although several of the mills had websites, there was very little data available in order to get accurate measurements of the mill pond, and google earth had to be used in order to get area data.

By averaging the tide ranges that are not influenced by the spring or neap tides, the normal tide range does not take into account the regular fluctuations of the tide.

Further Study

Accurate tidal readings for the year can be taken at each mill location, which means that each high tide and low tide can be analysed precisely and give coverage for the whole year in terms of power and money generated.

The mill ponds can be properly surveyed, whether it is by bathymetry or simple measuring techniques, to give an accurate reading of the mill pond level and the mill pond base height above sea level

Conclusions

To summarise these three aspects (technological, economical and environmental), tidal mills are feasible to a certain extent as a source of renewable power, however, this power can only be classed as micro scale, and they are better for providing local communities with power, rather than generating power for the grid

References Minchinton, W. (1979). Early Tide Mills: Some Problems. Technology and

Culture, 20(4), p.777. UK Wave and Tidal Key Resource Areas Project. (2012). 1st ed. [ebook]

London: The Crown Estate. Available at: https://www.thecrownestate.co.uk/media/5476/uk-wave-and-tidal-key-resource-areas-project.pdf [Accessed 9 Sep. 2016].

Charlier, R. (1982). Tidal energy. New York: Van Nostrand Reinhold. Tides.willyweather.co.uk. (2016). Tide Times - United Kingdom -

WillyWeather. [online] Available at: http://tides.willyweather.co.uk/ [Accessed 1 Sep. 2016].

Any Questions?