This form of energy has been harnessed from Roman times for small milling operations on coastal sites. No part of the UK is further than 70 miles from tidal water. The gravitational energy from the Sun and Moon move sea water up and down in a regular, predictable and constant pattern. Thus Britain is well placed to take advantage of this inexhaustible energy source. To do this either the flow of the tide must harnessed as it moves round these islands [Ref.1]. Or the sea must be channelled to flow from a high tide level to a low tide level, which is the approach of this paper. This involves creating "ponds" in the walls of which equipment is sited to generate energy from the flow. As the ponds will both fill and empty, the equipment must be capable of bi-directional flow. The equipment must also be effective under conditions of flows below it's maximum capability, and have a high conversion of flow energy to mechanical or electrical energy.

WATER ENGINES

These conditions are met by a Water Engine. The operation is that of two weighted floats being alternately raised and lowered by water entering the chamber underneath them, and then draining out of it. The flow is controlled by flap valves. Flow can be in either direction, as may be controlled by the valve programme.

The floats are linked to two sets of hydraulic rams, so that the force of the floats rising and falling is converted to hydraulic oil (or water) pressure. This pressure stream can then be used to power machinery, including electricity generation equipment, heat pumps, and other rotating equipment.

The mechanism is essentially a pressure intensifier. In that the low pressure of a few feet of water is converted into 3-4000 psi hydraulic pressure. The operating range for single units is from 1ft to 10ft head of water, and is thus suitable for large flow, low head, installations in rivers, and for tidal power collection using "ponds". Higher heads can be handled by "cascade" installations of two or more units in series. Though reverse flow is thereby inhibited.

In the 1980's two machines were built and reported on by ETSU [Refs.2,3,4,] but since then only two small test machines have been built. he mechanisms are simple and robust, and in volume production should be comparable in cost with other hydropower equipment. Maintenance should be simple, and given good construction parameters, the equipment should have a long life. For example all parts in contact with sea water could be made of fibreglass or other non corroding materials.

A major cost however is the construction of the ponds. Three approaches can be considered for Tidal energy collection. The estuary approach , the Shoreline approach  and the open sea approach . Both the last two envisage additional energy income being generated from Wind and Wave energy and from fish farming. The open sea approach is similar to that being pioneered by Tidal Electric off Cornwall, but using Water Engines, and adding the additional income generating items mentioned above. For estuary and inshore installations the hydraulic power could be piped ashore, have hydraulic accumulators included for some energy storage to help iron out demand peaks and troughs, and the driven items, heat pumps, generators or other machinery mounted well away from sea water.

HYDRAULIC POWER APPLICATIONS

The Estuary and Shoreline approach benefits from the possibility that initially all power developed by Water Engines would be collected by an hydraulic main, and taken on shore. Where a central generating or other energy using facility could be set up, well away from the sea. Given suitable materials of construction the Water Engines could just act as pumps, delivering sea water under high pressure into the hydraulic main. In a similar manner to the London Hydraulic Power Company, which at its height in 1930 supplied 8000 machines with power through 186 miles of pipes.[Ref.5]  Or Bristol's Avonmouth Docks, which were originally powered by hydraulics. [Ref.6] There are of course many inland applications for water engines, in locations with heads of 3m and below. But sadly while Eire has surveyed such sites, [Ref.7], the UK has only done surveys down to heads of 3m. Not below. [Ref.8]

HEAT PUMPING

A major potential application for Water Engines is to drive heat pumps. The major energy advantage is that while electricity generation may give 60-65% of the Tidal Energy as usable power, a direct driven heat pump, which excludes electrical machinery, "adds" to the energy output to the extent that for every 100 units of hydro energy available, up to perhaps 250 units of heat energy can be delivered by a heat pump system.  The "extra" energy coming from cooling the sea.

References

1/. Bryden IG, Tidal Power Systems, The Encyclopedia of Energy, pub. Elsevier Oxford, March 2004, ISBN 0-12-176480-X

2/. ETSU Contractor Report, No. SSH 4065, 1898, The AUR Water Engine,

3/. ETSU Contractor Report.No. 4063, Vols 1,2 & 3 1989, Small-scale Hydroelectric Generation Potential in the UK

4/. Reid, A U, Draft Notes on the development of Hydropower and River Flow Control.  (Technical director AUR Hydropower Ltd  1996, private communication)

5/. Donnachie, The Hydraulic Power Company,  Lambeth & Southwark Archeological Society, November 1979.

6/. Scott R P, Letter in the Professional Engineer for 15 August 2001

7/.Department of Energy, Dublin, Ireland, Small-Scale Hydro-Electric Potential of Ireland, October 1985.

8/.  ETSU Contractor Report SSH 4063 P1 & 2 Small Scale Hydroelectric Generation Potential in the UK, Salford Civil Engineering Limited, 1989.

Article extracted from Marine Scientist, N0.9 Q4 2004 page 42, by A F Stobart BSc.Chem.Eng See www.grunweb.org.uk and http://myweb.tiscali.co.uk/aferrand  HYDRO section