Push and pull

Waves represent a vast but largely untapped energy resource. E&T looks at a novel wave machine designed to put wave energy, literally, on tap.

There's something faintly anachronistic about the word 'inventor'. It conjures up a vision of an obsessive individual, beavering away beyond the reaches of the academic-industrial mainstream in search of that elusive 'eureka' moment that will change the world. A natural enough approach to innovation at the dawn of the industrial age, when academia and industry were necessarily in short supply, but surely totally out of place in the high-tech, big-money, team-based environment that characterises innovation at the start of the 21st century. If that's what you think, then maybe Alvin Smith, inventor, from Dartmouth in the UK, will change your mind.

Practicality is central to the inventor stereotype, and Smith has practicality by the truck load. Born in 1946, he left school shortly before his 15th birthday to work in his father's garage business - his father had been teaching him to weld and operate a lathe since the age of nine. By the time he came to get married at 21, he'd built his own bungalow, helped by a younger cousin and a book on elementary building construction. In parallel, he'd constructed a series of hovercraft - culminating in a nine by fifteen foot monster powered by two Volkswagen Beetle engines - and had been more or less solely responsible for running the garage business, following his father's severe stroke shortly after he left school. "My philosophy," he explains, "was to do everything as soon as possible".

Then, in September 1997, he saw a creek-side house for sale on the west bank of the Dart estuary. It was crying out for some serious building work, had abundant space for a workshop, and a view onto the creek you could die for. "I decided it was time for a life change," he says, and at the age of 51 he sold the garage business and moved to Dartmouth.

This wasn't an early retirement. He found work with local builders, and spent the bulk of his spare time doing up his house. However, living by the sea makes you very conscious of the power of waves, and, being of an inventive turn of mind, in his more contemplative moments, Smith began to think about how this abundant energy source might be harvested.

Compressed-air pump

His first idea was to use the movement of the waves to compress air. This isn't quite as unlikely as it sounds. In Germany and the US, low-cost, off-peak electricity is used to compress air to around 1000psi in the caverns formed from old salt mines, which is then used to generate electricity at times of peak demand. You can think of it as air-based pumped storage. Unfortunately, the conversion losses associated with compressed-air energy storage are high, around 50 per cent, so Smith soon moved on to the much more promising prospect of designing a pump that would produce high-pressure water.

There's nothing especially new about the idea of using wave action to pump water. Imagine a large bicycle pump, with the base of the pump tethered to the seabed by a flexible chain, and the pump handle - it needs to be buoyant - floating on the surface of the sea. As an added refinement, there's a sub-sea floatation collar fitted around the top of the pump cylinder, to keep the pump upright.

Now, picture the pump in the compressed position, with the piston at the bottom of the cylinder, and the cylinder full of water. Along comes a wave, the pump handle is forced upwards by the movement of the sea, and the pressurised water above the rising piston is forced out of the pump via an outlet valve. As the piston rises, water is drawn into the space under the piston, ready for the down stroke.

Now the wave falls, and the pump handle, which is weighted with ballast, starts to move downwards. It's at this point that the drawback to this design becomes apparent. On the up stroke the chain connecting the pump to the seabed is in tension - preventing the pump cylinder moving upwards. On the down stroke the chain becomes slack, so there is nothing to prevent the pump cylinder moving downwards as the pump handle starts its descent. It's the equivalent of trying to use a bicycle pump with one hand - you can't do it. All things considered, it's not end of the world - water is pressurised on the up stroke, and a one-way pump is better than no pump. But, undeterred, Smith wanted to design a two-way pump.

Alvin Smith's invention

It was in September 2006 that he realised how it could be done. "I was sitting in my boat, out in the creek, when it suddenly came to me," he recalls. "You increase the volume of the sub-sea float so that the upward displacement force is greater than the force of the descending piston. With hindsight it seems blatantly obvious, but nobody had done it. It was quite amazing - a 'eureka' moment."

The following weekend, he went down to his workshop, and in two hours, with the help of a plastic dustbin, a few lengths of plastic tubing, a steel rod, and four snooker balls (for the inlet and outlet valves), he'd made a model of his design.

Getting the model into the sea for testing without being observed proved a bit of a challenge. The first attempt was abandoned when a friend spotted Smith in his boat, complete with the pump hidden in a plastic bag, and invited himself over for a cup of tea. Smith was forced to moor his boat, with the pump still on board, overnight. "I couldn't sleep," he says, "if someone had stolen the boat, they'd have got my invention".

The next day he finally got the pump into the water - after first upping anchor to avoid an apparent observer, and then waiting for ten canoeists to pass. "We put it in, and it pumped. I was absolutely elated. So we pulled it up and came home."

Wave energy and tides

There's a second crucial feature to Smith's design. In the waters around Dartmouth the maximum wave height is about 4m, but the difference between high and low tide can be as much as 5.5m. The maximum wave height determines the length of the connecting rod, but extending the length of the rod to accommodate the movement of the tide would increase the required length to the point where the bending forces would be unacceptably high.

To get around this problem, the pump, with its piston and valve assembly, is mounted on the top of a 6m cylinder. The buoyancy of this cylinder can be controlled by moving water in and out and, as the tide rises and falls, the cylinder moves up and down, within a second outer cylinder. It's this outer cylinder that's tethered to the sea bed and is fitted with the sub-sea floatation collar. The outer cylinder thus remains in a fixed position, relative to the seabed, but the inner buoyant cylinder moves up and down with the tide so that the distance between the pump and the surface of the sea is always matched to the length of the connecting rod.

The use of a variable-height, buoyant cylinder also plays a vital role in ensuring survivability in high seas - a critical consideration for all wave machines. In moderately rough seas, the pump is protected from lateral stresses, as it is free to sway from side to side on the chain connecting it to the seabed. However, when severe storms threaten, the buoyant cylinder can be flooded, pulling the pump and the upper float below the waves, and out of harm's way.

Inspired by the success of his plastic model, Smith, helped by financial support from some local business angels, built a prototype pump, now christened 'Searaser', with an 83mm piston. This prototype machine underwent sea trials in the spring of this year, pumping 483m3 of water, at 75psi, in the space of 93 hours - 50 per cent more than had been expected.

Searaser sea trials

To date, the bulk of wave-energy machines have been designed to generate electricity directly. This seems a fairly obvious approach, but design complexity and cost are necessarily high, and there's the constant challenge of keeping seawater out. Electricity generation will also be a prime role for Searaser, but now the generator will be shore-based, via a Pelton water turbine.

There are two generation options: direct generation, or via a pumped-storage system. Other possible applications include producing fresh water from seawater using reverse osmosis, and producing large volumes of flowing seawater for shore-based shellfish farming. Reverse osmosis requires water pressures of up to 1,000psi - easily realised by the simple expedient of reducing the cross-section of the piston.

Development effort is currently focused on finalising the design specification for a 324mm Searaser - the smallest of an anticipated family of four machines - with sea trials of this machine planned for early 2009.

These are, of course, very early days, and the viability of Searaser will only become clear on the basis of extended operating experience across the full range of sea conditions. But the indications are that the design will produce useful amounts of storable (using pumped storage) energy, and could be a particularly attractive option for island communities, where energy costs tend to be high, and drinking water is frequently in short supply.

On the basis of the 93-hour sea trail for the prototype device, it's estimated that a 600mm Searaser (expected to be the dominant production design) would support a continuous electrical output of 61kw - assuming the device is linked to a pump storage system with the upper reservoir sized to permit water flow 24/7.

Looked at another way, this means 17 Searasers will produce more than 1MW, or some 217,000 machines would generate enough electricity to meet the UK entire domestic electricity demand. Not a bad result for a lazy afternoon up the creek.

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