Demonstrating the pump-as-turbine as a creative means of energy recovery

The design and installation of two hydropower demonstration sites showcases the leading-edge research on Pumps-as-turbines and is one of the key deliverables of the Dŵr Uisce project. A pumps-as-turbine, in short PAT, is a hydraulic pump operating in reverse mode as a turbine, producing energy rather than consuming it. Pumps are mass produced all over the world and are well suited to this creative application. The main advantages of a PAT over a conventional hydro turbine include lower cost, compact dimensions, short delivery time, reduced installation cost, ease of maintenance and unrestricted availability of spare parts. In practice, for the same power output a PAT can cost up to 1/10th of a bespoke hydro turbine.

PAT technology can be applied either to recover energy from pressurized water networks, or as generators for conventional small-scale hydropower. In order to explore these applications, the Dŵr Uisce project team decided to build one pilot installation in a water distribution network in Ireland and the second one in a small river hydropower scheme in Wales. Despite the fact that both installations feature a similar PAT with power output of around 4 kW, the operating contexts are radically different.

The Irish installation is located at the inlet of the water treatment works serving the Blackstairs Group Water scheme, a rural network in the south-eastern corner of the country. Since the raw water source is located at a higher elevation than the treatment facility, the significant overpressure was being dissipated by a water jet splashing into the raw water storage tank. Potential energy was wasted while the treatment plant drew costly energy from the national grid. In a creative initiative, which had the support of the local community and the plant operator, a PAT was designed and installed in a bypass of the valve regulating the inflow to the tank. The regulatory aspects were straightforward, since the only requirement was to request permission from the operator of the electricity network - Electricity Supply Board (ESB) - to connect the PAT in parallel to the grid. Now in operation, the PAT is offsetting the electricity consumption of the treatment works by 20-25%.

In contrast, the Welsh installation consists of a micro hydro scheme to supply the historical farmhouse of Ty Mawr Wybrnant. The PAT is fed by diverting a small flow from the Afon Wybrnant, a river near the town of Betws-y-Coed. When compared to the installation at Blackstairs, more significant civil works were needed to build the intake weir, lay 300 m of buried pipeline and to erect the shed housing the turbine. In addition, applications had to be filed to obtain planning permission and a licence for water abstraction. While the licence would have allowed for the installation of a 20kW micro hydro scheme, it was decided to limit the power output to 3.68 kW, sufficient to supply the adjacent historical farmhouse. In addition, the grid connection procedure was simplified by using a G98 type-approved inverter of the kind normally used for residential solar photovoltaic (PV).

The lessons learned during the process are significant. We learn how to make a better choice of the electrical equipment which controls the operation of the PAT. For example, the first inverter installed at Ty Mawr Wybrnant did not work and had to be swapped for another one of different kind. We now also able to identify ideal locations for a PAT installation.

To sum up, both demonstration sites prove the feasibility of PAT technology for small scale hydropower in different operating contexts. With reliable power output of around 4 kW, both sites are expected to be economically viable with a return of investment between 5 and 8 years. More importantly, they are environmentally viable.