Wave Powered Desalination Tasks.
Material cost data
Choose acceptable cost numbers for:
Steel sheet per kilogram for various thicknesses, say 6 to 14 mm.
Weld by length for the same thicknesses. There could be a sharp rise for thicker plate.
Foam concrete insulation material which can be applied by pumping. It would be very good if it could contribute some structural strength, presented an alkaline environment for the steel but did not take up water. Some materials can get very hot as they cure.
Surface finishes which can take double strength brine at 100C (or more?) but not release anything.
Pipes for internal hot water and brine.
Pipes for the run to shore and ballast or trenching to keep them on the sea bed.
Elastomeric sheet for steam valves.
Elastomeric sheet for loop pump valves.
Precision machined parts by kilogram such as the SKF bearings and the big hydraulic rams.
Post-tensioned, slip-formed concrete for the legs and suction anchor.
Continue Mathcad worksheet
Change to new cost parameters as above.
Extend the worksheet to include the change of water properties, especially viscosity with temperature, in the liquid-to-liquid heat exchanger.
Improve demister pressure drop predictions.
Add scatter diagram lookup for a likely wave site (Canaries, Greek islands, California, Oman?) to allow estimate of annual output.
Double-check algebra.
Optimize dimensions for new climate, corrected algebra and cost data.
Steam heat exchanger test rig
Confirm the non-scaling properties, heat transfer coefficient and life of Maxwell Davidson’s new carbon PVDF sheet at the expected heat flow rate and brine strength.
The same rig can be used for trials with a demister to measure pressure drop and contamination passage. It can contain material samples for hot life tests. A convenient design might be an inverted Tee-piece with the bar of the Tee taking a demister column, the cross arm flanges holding two mesh plastic specimens. A small pipe can connect the centre compartment to a header tank with double strength brine and ways to adjust the level. Bolted to the cross arms will be two short lengths of flanged pipe with closed ends can contain a heat transfer fluid from the Dowtherm or Xceltherm range. Dowtherm T has a low vapour pressure and can go up to 288C. The two heater compartments will contain electrical immersion heaters which may have to be derated and will be driven by a temperature regulator. All three compartments will need temperature sensors and stirring rotors. The central stirrer should splash the brine against the walls. Evaporated brine can run back to the tank through water cooled condenser. We can do lots of useful life tests if the system is kept just simmering. The whole system should be well lagged with glass fibre or rockwool. If this is good enough we can use the electrical voltage and current to measure the heat transfer.
Loop pump
We build a spoked wheel from, say Dexion, about 3 metres in diameter. We mount it on a car back axle bearing. It should have a water tank made from an oil drum at the centre. We fix transparent flexible hose in one or more loops. We fit test valves also with transparent bodies. We can drive the pump by hand to convince dubious investors of by a crank from an induction motor with variable speed control. We measure pressure drops through valves
Steam valves.
These might be made from a folded flap of high temperature elastomeric sheet.
We use CFX to predict pressure drop as a function of flow rate and opening.
We check flexure life at the working temperature and multiples of the expected bending strain using and an electric motor drive.
We then drive valves with a reciprocating air pump. Density of the air is too high but it should be easy to calculate coefficients which can be applied to steam.
Tank model.
We will learn a lot by building a model which is driven by waves and pumps air rather than steam through a suitable adjustable resistance to represent the evaporator. We measure duck angles and compartment pressures for a wide range of wave inputs. We measure anchor forces in extreme ones with the option to loll to protect the device. The Edinburgh tank is 1.2 metres deep so a 40 metre water depth would be 1/30 scale. The central wave period is 1 second with reasonable size waves from 0.5 to 1.4 seconds. Time goes with the square root of scale so periods increase by 5.77. This means that our tank periods map to 2.88 to 8 seconds. This is enough for moderate tropical conditions but not open Atlantic ones. By focussing waves we can produce the equivalent of a 10 metre plunging breaker.
Water pendulum test rig.
It will be difficult to observe the motions of the water pendulum in a tank model because of its rather small size and interference from the waves. We should build a separate rig about one metre diameter with a glass side, a central partition and resistive load to represent the pressure drop across the heat exchanger. It could use the same bearings and drive as the loop pump. With air rather than condensing steam and volumes which are too small, the compartments will appear too stiff. This can be partly corrected with external air spaces. We need to know if the pumping chamber will develop internal sloshing which would need suppressors.
Designer water project.
Establish cost centres and profitability of present bottled water industry.
Get advice on range of water additives thought beneficial by the medical profession.
Open negotiations with existing operator for bulk purchase.
Drawings and land-based demonstrator.
Modify assembly drawings. Do detail drawings of all components
Apply all lessons learned on small rigs to an electric-drive full-diameter reduced-width unit on land.
SHS 11 November 2004