How can a micro hydro turbine work in a mill renovation?

Peter Downs uses a micro hydro turbine on the river running alongside his property to part drive a water source heat pump delivering underfloor heating in the main house. This large converted mill building from 1778 is now a guest house/B&B. In this video he explains how it works and how it has led to having zero net energy costs.

Micro hydro system

This micro hydro turbine is a siphonic device that generates electricity by passing water over a vaned propeller that then drives a generator.

Micro hydro performance

Maximum output is 4.5 – 5kW and 2-3kW is produced most of the year round. The turbine is sized to draw 450 litres per second and at this size it should be fine to have on 90% of the year. The turbine’s efficiency and output depends largely on rainfall or lack of rainfall. It is switched off for 6 weeks in Autumn because too many leaves clog the weed screen. In summer an overabundance of reeds in the river can reduce the flow and also effect output.

Micro hydro components

There are a number of components in addition to the propeller and generator including a motorised air valve and air blower.

How the turbine works

The air valve shuts, the air blower switches on pumping air out from inside the cylindrical turbine housing. This creates the suction inside which draws water up into the turbine. At a certain point the water starts falling over through the draft tube out of the turbine. This establishes the siphonic effect so that water continues to be pulled through the turbine and over the propeller which turns a fan belt which turns the generator.

Micro hydro maintenance

The micro hydro turbine does not require very regular maintenance. You need to grease the bearings and make sure the oil reservoir is kept reasonably full. Occasionally you also need to get inside the turbine and clear leaves and weeds other debris from the struts that support the turbine. That said most of the components apart from the generator and the propeller on this micro hydro turbine have been replaced at some stage.

Protecting fish

A fine mesh screen was installed to prevent fish from getting caught in the turbine but this collects all sorts of debris other than fish. If Peter were to do it again, he’d consider a hydrodynamic screw system which wouldn’t require the screen, would do away with the cleaning and might work better over a variety of river conditions.

Permissions for micro hydro

Approval from the Environment Agency is required for carrying out any works on the river banks. They were encouraging and quite helpful in this case. They requested that a water vole survey be carried out so action to avoid any water vole burrows could be taken.

The turbine had to be registered as a generating station with Ofgem which was quite a drawn out procedure with a long application form. Then there were some regulatory requirements to do with connecting the turbine to the grid. They needed to know the spec of the turbine to ensure it was a suitable feed for the national grid.

Feed-in tariffs for micro hydro

Thanks to feed in tariffs 9p is earned per unit generated by the turbine. When used, the generated electricity also saves importing grid electricity as a cost of some 13-14p a unit (green tariff). When exporting 3p per unit is earned on top of the payment for generating green electricity.

Return on Investment

The turbine provides all the electricity for the house in summer plus some for export to the grid, but in winter it is providing electricity for a water source heat pump which draws between 8 – 12kW. The turbine, at its best, provides 4- 5kW so the difference must be imported in winter. However, at the end of the year it works out that electricity and heating is being provided at no cost. Energy bills are negative, so the turbine has worked out well in terms of financial returns.


Peter Down’s SuperHome has undergone extensive eco renovation and includes solar thermal panels, replica double glazed sash and casement windows, and extensive insulation to loft, roof and floor. It is one of the homes in the SuperHomes network of older homes that have been renovated by their owners to reduce carbon emissions by at least 60%. Most SuperHomes host public open days in September.

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