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Saturday, February 26, 2011

A floating breakwater - but....

Where I worked we grew fish in floating cages and we needed to protect them from the waves that weakened the structure and made work difficult. We found a plan for building a floating breakwater out of discarded tires and built up a breakwater large enough to protect the rafts. It was very effective but.... we never did get the fastening system perfected so there was a fair bit of maintenance needed. If someone managed to improve on the fastening, these breakwaters would be a very practical prospect. I'll try to describe them for anyone who wants to have a try.

To make up a module you must start with 9 tires of the same size. (a larger module of 16 tires can be made but if you are using fairly large tires, these can be pretty heavy to shift around). Start by standing three tires on the ground (in the same orientation that they would be in on your car). Put them in a line with the treads touching. Put two tires in the same orientation on either side of the two. They overlap like bricks in a wall. Put a single tire on the outside of the two. Fasten them together (I will go into the fastening below). You now have a diamond shaped unit of 9 tires. If you start with 4, 3-3, 2-2, 1-1, you will produce a unit of 16 tires. Push these units into the water and fasten them together end to end and side to side. You are aiming for a breakwater that is at least as wide as the length of the wave you are wanting to absorb. The way they work is as follows.

The water in a wave doesn't move forward. If you were to suspend something in the water and watch it as a wave goes by, you would see that it moves in a circular path. This floating breakwater connects the rotation of a peak with the rotation of an adjacent trough. They are moving in opposite directions. What happens is that this motion is turned into turbulence and the wave is absorbed.  The shorter the wave , the more efficient the system so you remove all the chop and short waves and are left with swell with a wave length which is longer than the width of your tire breakwater.

Flotation is provided by the air trapped in the top of the tire. As long as there is some wave action, the turbulence stirs air into the water and it gets trapped in the tire. A very long period of calm weather can result in the breakwater sinking but it is easily raised by going down with SCUBA gear and letting some air into the tires. A single diamond is quite enough to float a couple of people and once the tires are joined in an array, there is plenty of flotation for anything you want to put on the unit.

For anchoring, there are a number of reasonably inexpensive solutions. Simple 45 gallon drums filled with concrete with a large steel eye at both ends is a good solution. They are joined with lengths of chain, one behind the other. They dig into soft bottoms pretty well and as long as you have a good length of chain at the anchor end of the anchor line, they give good anchoring. You can  use polyprop rope for much of the connection to the breakwater. The principle is that the chain, where it meets the first drum in the series, should never be at an angle to the bottom. You need enough chain so that in the worst conditions, it is parallel to the bottom where it connects to the first drum.  The leading edge of the drums digs in and provides very solid anchoring.

Actually, while I have been typing this, a thought just came to me. I bet you could get quite an effect from a vertical wave barrier. Simply take some of your diamonds and drill small holes in the top of each tire so they won't float. Fasten them under floating diamonds. The principle here is slightly different although the same at its core. It depends on the fact that for every ninth of a wave length you go down into the sea, the circle of action of a particle decreases by half. Take a wave of 1 metre high and 9 metres long. At the surface, a particle is describing a circle with a diameter of 1 metre. 1 metre below the surface the particle is describing a circle with a diameter of half a metre and at 5 metres, the circle is one thirty second of a metre. In essence, the deep diamonds should anchor the tires and stop them going up and down with the surface waves. The surface waves should break over the unit and change the wave energy into turbulence. Something to try.

So how did we fasten the tires together. We got old conveyor belts and cut them into palm width strips. We then got a couple of winches and pulled the tires together tightly enough to distort the tires. The straps were measured and cut with some overlap and 4 holes in a square pattern was drilled in each one. Brass bolts and brass plates were used to fasten the ends of the belts together. When the winches were released, the belts were nice and tight. The weak spot was where the bolts went through the conveyor belt. Our fastening was lasting a few weeks to a few months but as the barrier got bigger, the amount of maintenance got pretty time consuming.  Possibly in a commercial system one could use the same eye and pin system that is used to join conveyor belts together.

We were always amazed how much buoyancy these units had. A single 9 tire unit was plenty to float a person or two.  You could make a Kontiki with this sort of raft.

Something else we noticed was the amount of sea life they attracted. They were great protection for little fish and the big fish came along to see if they could get the wee ones.. Lots of organisms settled on them as well.

If you have need of such a breakwater, have a go and let me know how you ended up fastening them together. That is the key to this system.  If you are really adventurous, haul one of these into the deep ocean, build a hut on the top and go drifting around the world

Sunday, February 6, 2011

Wind energy's no good

At least that is what I read in an anti-wind farm article in our local news paper.  The reason given was that wind energy has to be used as it is produced and can not be stored.  Fortunately this is not true. 

So let's examine this question of how to get the maximum benefit from the energy from a wind farm; of how you store wind energy. 
Most countries generate some of their electricity from Hydro.  Here in New Zealand we are one of a very few nations which generates more than 50%  of our power  from Hydro.  Combined with our geothermal energy we have 70% of our energy from renewable sources.  Hydro is the perfect partner for wind farms.  Every unit (kWh) of electricity which is generated by wind leaves water in the dams to be used instantly when the wind isn't blowing.  Energy already stored is left in place to be used as needed.   Now, if you are on your toes, you will immediately ask what happens if the dams are full and the wind is blowing.   You see the problem.  I'll get to that in a minute.

In addition to saving water for future use, wind displaces the use of coal. In New Zealand we have one large coal fired power station.  Every kWh generated by wind leaves approximately 300g of coal in the ground* (This is  energy which is already stored).  A single 3mw wind turbine operating at a site that has a 35% capacity factor, leaves approximately 2,700 tons per year of this  non-renewable resource in the ground to be used by future generations of Kiwis. A typical 30 turbine wind farm over a 25 year life leaves over 2 million tons of coal in the ground and reduces our (expensive under Kyoto) Carbon dioxide production by over 6 million tons**. Hopefully, because of our foresight in building wind farms,  our kids won't have to burn this valuable resource but will use it as an industrial feed stock.  Thankfully, our grandfathers showed  foresight by building hydro-electric dams which is why our generation is in pretty good shape right now.  If we tried to build these Hydroelectric dams today it would take a decade just to get through the consent process.

*I have used a very conservative figure here which is for the most modern, coal-fired power stations.  The average world wide figure is above 400g of coal per kWh.  (look at paragraph 7 in the linked article)

**12g of coal plus 32g of Oxygen yields 44g of Carbon dioxide.  Hence roughly 3 2/3 tons of Carbon dioxide is produced for every ton of coal burnt

Wind also displaces the use of natural gas.  Like coal, this is a far to valuable resource to be burnt.  In the past New Zealand has used natural gas* to produce value-added methanol.  Both coal and gas can be used as the starting point for the production of a mind boggling range of value added products including fertilizers, parmacuticals, plastics, fine chemicals and so forth.  However looking at natural gas as a source of energy rather than as an industrial feed stock, every kWh of wind energy produced leaves either coal or gas energy unused (stored).

*Note that in 2012, new technology has produced a glut of natural gas which some commentators say will continue for a few decades.  Any bets on what will happen to our energy consumption with the advent of this cheap energy. 

Further energy storing opportunities come from demand balancing of our power grid (as opposed to the present, predominantly, supply balancing system).  In New Zealand we already have a crude form of demand balancing.  Many of us heat water when 'electricity-is-available' rather than 'on-demand'.  This is done by the clock rather than when  excess power is actually available. Some of us also have heat storage devices on the same circuit.   In the future, we will have the choice of operating washing machines, turning on heat storage devices, doing our cooking and so forth with cheaper 'when-available' electricity rather than more expensive "on-demand" electricity.  The critical development is the introduction of "Smart Grids" which you have been hearing so much about.  There are two levels of such applications.  If you have set your dish washer to come on when the signal for third level electricity is sent, (use now, level 1, level 2, level 3 etc. - each one a bit cheaper than the previous one), you don't want it to go off if there is a switch to level 2.  A heat storage device such as a hot water tank or space heater is a different story.  It can switch on and off instantly and as often as needed to balance the supply.  Devices that use power which can be switched on and off at any time would likely get a lower rate than devices such as dish washers.

Smart grids, of course, explain what happens when the wind is blowing but the dams are full.  At present, we would have to let the water flow over the spillway rather than generating electricity with it.  With smart grids and smart devices, the electricity company can send a lower priority signal, triggering a bunch of devices to come on line.  More electricity is used, making a profit for the company and saving money for the user and water (energy) is not wasted.

And finally, electric cars will begin to appear on our roads this year.  Not only can they be charged, for the most part, when electricity is available and hence cheaper, but there is much talk about using them as peak shaving devices to send electricity back into the grid when demand is high.  Charging 'when-available' will reduce driving costs (and use energy when the Nor-wester is blowing).  Giving back power 'when-needed' will generate a small but much appreciated revenue for the electric car owner.  It will also eliminate the need to borrow money to build an expensive peak shaving power station.

In summary, wind energy is stored in a number of ways.

#For hydro dams it is in the form of water in the dam not used and ready to go through the generators on demand. 
#For fossil fuels it is stored in fuel not used and available far into the future (very very long term storage).
#For the home owner energy is stored in the form of a linen closet of washing or a cupboard of dishes, washed when electricity was cheapest
#For the electric car owner it is stored in a battery charged when electricity was least expensive.

We have not even touched on the use of batteries which will come into their own as the technology improves.

In none of the above have we talked about methods the industry uses such as pumped storage.  That is another story.

Saying that wind farms (or any other form of renewable energy) are not effective because electricity can not be stored shows an insularity bordering on hermitage. Saying that we must have batteries to store the energy is true as far as it goes but there are many other ways of storing energy from intermittent sources.

Post Script
Since writing this blog, a Ted Talk has come out by a professor from MIT.  He is doing work on the construction of very cheap, very large batteries to store megawatt quantities of energy in units the size of shipping containers.  Thus another element is added to smart grids to make the generation of renewable intermittent energy more flexible.