Transition in Action, Totnes 2030, an Energy Descent Action Plan

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Hydropower

Hydropower is the capture of energy in flowing water and can be captured in the following ways:

Micro-hydro – the capture of energy from rainwater runoff flowing in rivers
Tidal lagoon – the capture of energy by the release of water impounded at high tide
Marine current turbines – the capture of energy from tidal streams in the open sea
Wave power – the capture of energy from the movement of water on the surface of the open sea.

N.b. under EU rules, energy generated from offshore resources such as wave and tidal current cannot count towards local RE targets.

Micro-hydro

Turning moving water into energy is a very useful technology. Historically, this area had a number of mills that were used to power industries, mostly those to do with the production of fabrics or paper. The DARE report also speculated that as a consequence of climate change, there may be a need to construct additional reservoirs to retain excess winter rains to supply summer time water needs, this could provide additional micro-hydro potential, however no additional assumptions for power in the calculated capture were made on this basis at this stage. So what is the potential energy that could be generated from hydro?

In the mid 1980’s a resource assessment was carried out for micro-hydro across the UK¹, known as the Salford Study. Part 2 of this study lists all sites identified as having a potential with an installed capacity of 25kW or greater (25kW was then considered the smallest size hydro that would be economic to develop). Part 3 lists the rejected sites (i.e. under 25kW). The DARE report researched this report and identified the following sites:

Salford Study Results for Totnes and District Potential hydro-electricity sites - feasible
ETSU No.	River	Site	Head (m)	Installed capacity kW	Annual Energy Capture MWh
46004	Dart	Staverton Mills	2.5	68	441
46005	Dart	Town Mills	2.4	90	430
46006	Dart	Swallowfields Weir	2.9	196	887
46007	Avon	Diptford Manor	3.2	43	221
46009	Avon	Lydia Bridge	6	-49	25*
			Totals	446	2,201

Notes

* Assessed by Salford Study as having an installed capacity of 49kW and an annual energy capture potential of 247 MWh; however the recently installed turbine has provided a lower actual capture of 25MWh/y

Then there was another list of sites that were felt by the study to be unfeasible, for the reasons given.

Salford Study Results for Totnes and District Potential hydro-electricity sites - unfeasible
River	Site	Reason for rejection
Harbourne	Bow Mill	May be suitable for small scheme
Harbourne	Beenleigh Manor	Operational
Harbourne	Crowdy Mill	Operational
Harbourne	Old Mill	May be suitable for small scheme
Harbourne	Harbertonford	May be suitable for small scheme
Avon	Curtisknowle	Prior use
Garra	Garra Mill	Power under 50kW
Dart (trib.)	Stoke Gabriel	No potential

The DARE report did not investigate these sites, and since the power output is site specific, site flow data has not been calculated for each location. However, it concluded that “it would appear there could be merit in a further study of micro-hydro power at a number of sites” in the area.

We estimate that the annual potential energy capture from the T & D ETSU sites = 2,201 MWh/y

Domestic micro-hydro

On the basis that as energy shortages encourage householders to explore all possible sources of renewable energy that may be available to them, and taking into account the favourable topography in T & D with many small streams, a very rough estimation below is made on the basis that 0.1% of all households in T & D may be viable for micro-hydro for electricity use on site. Potential Annual Energy Capture per domestic micro-hydro scheme = 10 MWh/y (DARE). In 2006 DARE conducted a feasibility study on the disused Rattery Corn Mill. They estimated a potential net output of 16MWh/y. Further small mills creating power for direct use in processes such as traditional flour mills, may also enjoy a renaissance, and while this may provide additional benefit in the future, there is no further speculation or assumption here.

Estimated potential Annual Energy Capture in T & D
= 94.8 MWh/y

Tidal lagoons

The DARE report suggests there may have been several old tidal mills sites in South Devon, however none still operate and all would only generate small amounts of power. A possible constraint is the availability of possible sites and the volume of water a lagoon may hold. A further possible site with a tidal lagoon identified by the DARE report is on the River Dart at Stoke Gabriel. There has been no investigation of this site, so no assumption of potential output is included here.

There may also be scope to capture energy from tidal barrages across all major rivers if climate change results in rising sea levels². A Thames barrier style device to hold back tides and prevent flooding of low-lying areas would allow energy generation on falling tides. The DARE report makes no further assumptions or estimated energy potential from this speculative and uncertain long-term possible energy source in T & D. There is a potential of using Tidal Pools with Pumping as an energy storage system to moderate renewable energy supplies in the grid³.

The River Dart at Sharpham, below Totnes

Many years ago the riverbank was canalised to create a narrow but deep stream suitable for boats to navigate up to Totnes. The bank created has recently breached and at high tide water inundates the reed beds to the side of this channel. Unless the breach is regulated, the entire length of riverbank may fail and this channel will silt up. However the flow in and out of the reed bed could offer an opportunity to generate power. An early assessment of the Sharpham site indicates that it may be possible to install a 64 kW turbine, which could produce up to 64,000 kWh of energy annually.

The DARE report also suggests there may be potential for additional capture on the R. Dart at Stoke Gabriel.

Estimated potential Annual Energy Capture in T & D
= 64 MWh/y

Marine Current Turbines

Marine current turbines are similar to wind turbines, but they function under water. Water is 700 times denser than air, so the energy contained in a moving body of water is far greater than air, against this, tidal currents move more slowly. Turbine blades need to withstand the pressure of water and algal growths. The constraints on marine current turbines is the availability of seabed with a high current flow and located close enough to land to allow a shore connection to the grid.

As this is an offshore technology, any energy capture would be fed directly into the grid and would not be included in any T & D local energy targets. The DARE report suggests that Start Point of South Devon could provide a suitable site with adequate current speeds and access to the shore, where 10 turbines together could produce a total of 3,570 MWh/y. It is possible that once the technology is fully established, unit costs may come down, although in the first instance, other areas with a better energy density are likely to be developed. Once the infrastructure has been put in place, South Devon may benefit as installers seeks additional installations to give greater return on their investments. For more information about the potential for marine current in the South Hams area the DARE report should be referred to. The national potential for this technology is estimated at 9kWh/d per person by Prof. Mackay⁴ who goes on to review the greater possible resource, which could be tapped, based on bottom friction.

(For information only – assessed resource based on per capita share by population in Totnes):

T&D share of estimated national potential
of off-shore marine current farms
= 7,839 MWh/y

Wave Power

There are a wide variety of wave energy designs from the ‘Pelamis’, which comprises a string of sealed floating tubes with a pump at each joint to the oscillating column where water enters the bottom of the vertical tube and compresses trapped air to drive a turbine. The more recent Oyster designed for waters around 12m deep can produce 600kW; these devices may also be used to drive reverse-osmosis desalination plants⁵. Any marine device will need to be strong enough to cope with extreme weather and demonstrate an economic return. The constraints of wave energy are the availability of suitable locations to site devices and bring a power connection to the national grid. A suitable site might be an installation (in addition to marine current devices) at Start Point. There is scope for many devices off the North Cornish coast. Wave energy devices are still very much under development and costs remain high. This energy option is unlikely to contribute to T&D’s renewable energy mix in the short term, and as an offshore technology, energy capture will not be countable towards local energy targets. . The national potential for this technology is estimated at 4kWh/ per capita/per day⁶.