Included among its ambitious targets to increase the use of clean energy, Ireland hopes to achieve a renewable contribution of 40% to gross electricity consumption by 2020. Although much of this is expected to be sourced from wind, contributions from other sources, including hydropower, will be important. Parts of the country have abundant rainfall and in the past many small towns and mills relied on hydropower to provide electricity.

Over the last few years, SLR Consulting has worked with two Irish property owners to obtain planning permission to have a former hydropower scheme reinstated and upgraded to modern standards. This had been in place along the Mill River in Buncrana, Donegal from 1905 until the mid-1980s.

Much of the original site infrastructure, including the weir, mill race and pipelines, remain in place and will be re-used as part of the proposed development.

SLR was initially tasked with taking river flow measurements over a year-long monitoring period to help establish the technical and economic viability of the scheme. Having established that the scheme was feasible, our Dublin-based team then prepared pre-planning documentation and began informal consultations with planning authorities, statutory consultees and other stakeholders.

The consultation process was followed by the preliminary design of the scheme and preparation of a detailed Environmental Impact Statement. This focussed in particular on hydrological, ecological, visual and architectural heritage impacts.

Proposed scheme
The Mill River between the existing weir and the former mill is designated as a ‘Category 2’ River as the existing (natural) river channel over this stretch includes a series of modified barriers (waterfalls) which permits some fish movement to take place. The proposed hydropower scheme is therefore required to ensure that some compensation flow is maintained over the depleted section of the existing channel in order to limit any potential adverse impact on the aquatic environment, the existing fishery and the amenity value of the river.

Reinstating the scheme will entail making structural repairs to (but not altering the design of) the existing fixed weir across the Mill River. Water from the impounded lake behind the weir will be abstracted via a new sluice gate and intake channel with a wash out channel and spillway to return excess water to the river.

Over 210m of new twin 600-800mm diameter low friction pipes will be installed and buried close to the alignment of the existing open channel section of the mill race, between the intake and the existing cast iron pipes. At that point, the pipes will be slip lined into the old cast iron pipes which run for a further 120m to a site along the river bank, adjacent to the old mill building (a protected structure). The pipes will then be intercepted and connected to a new length of single larger diameter pipe placed in an excavated trench which leads down to a new turbine house at the river bank.

The small turbine house will be approximately 7m long, 6m wide and 6m high. It will be constructed on a rock outcrop at the edge of the river and will have a water outlet below river level.

Turbine choice
In developing the scheme to planning approval stage, SLR Consulting worked closely with New Mills Engineering, a turbine manufacturer based in Carrickfergus, Co. Antrim in Northern Ireland, to identify the most appropriate turbine to install at the site. Having reviewed the flow duration curve for the Mill River and taken account of available flow volumes, a full Kaplan turbine was proposed for the scheme in order to accommodate high variations in flows on the river and ensure the maximum efficiency over the flow range. This will produce a very flat efficiency curve and allow a high unit speed for direct coupling of the generator to the turbine.

Given the low head, a steel semi spiral casing will be used, with the generator close coupled and mounted on the turbine casing. The runner hub is directly coupled to the generator shaft, with the runner blade operating rod running through the generator to the hydraulic blade servo motor at the top of the generator. The turbine guide vanes are also fully regulated. A modified inclined elbow draft tube returns the water to the river, having removed the last of the energy.

A buried electrical cable from the turbine house will be connected to a newly installed transformer at the site. It is anticipated that the electricity generated by the turbine can be fed directly from this point to nearby offices or alternatively to the local electricity distribution network, operated by ESB Networks.

Planning permission for the proposed hydropower scheme was initially granted by Donegal County Council in 2012, but following a third party appeal to the national planning appeals board (An Bord Pleanála) and submission of additional information and clarifications in respect of the scheme by SLR Consulting, planning approval was finally secured earlier this year. Having obtained full planning permission, SLR Consulting and the scheme promoters are now beginning the process of moving the project forward to the development phase.

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