Solar Power and Scale Factor

There was a post recently about the possibility of a solar powered cargo boat (which I can't find at the moment) but it lead me to this question:

Should a solar powered vessel be big or small?

The plan area available for panels is a function of the scale squared, but the displacement is a function of the scale cubed, and the waterline length speed formula is a function of the square root of the waterline length. But large cargo vessels have several waves along the hull, so they don't strictly comply with that formula. So which wins, big or small?

 

do you mean the propulsion was solar powered?

 

As I understand it these may also influence things:

A high Cp, which many cargo vessels have, is said to (apparently still assuming fine ends) benefit a higher cruising speed.

As you point out a larger ship may be less likely to cruise at the upper end of its "hull speed" ... which brings up:

Froude number, used as the basis for comparing similar hulls of different size, at the same Fn.

Since, as you point out, the available area for solar panels varies with the square, but displacement with the cube (therefore power consumption), one might think that big solar ships are doomed ... but power used at a given speed (Fn) does not progress linearly either.

So it seems to be a question of which increases faster: area available for solar panels or power requirements for a presumably low Fn ... also keeping in mind that a lower Cp is beneficial for efficient cruising at lower "hull speed".

Finally, a slow ship represents more time schlepping from place to place for a given amount of cargo which affects operating costs in other ways (like payroll). Additionally some cargos may be too time sensitive, some customers less willing to have more cargos at sea for their on time manufacturing processes ... that sort of thing.

 

Was this the thread?

http://www.boatdesign.net/forums/boat-design/solar-maximum-31633.html


My only real question is really how useful is this attempt at going green? It seems like an aweful lot of complexity to avoid going back to sail driven transport when the results of solar power appear to result in a boat that's slower than it's sailing equivalent for a LOT more complexity and expense.

Seems counter productive to me on a huge scale.

 

Solar Power

Bigger is more economic, a fundamental property. This was well known for the last 200 years, when coaling stations limited the economics of steamships.

However the well stated question of sails vs solar power trumps all of this discussion. A sailing ship, with relatively very low cost compared to solar, would win the race against solar powered ships every time, and for any size vessel. Sail power would satisfy the environmentalists just as well.

 

You can build a huge sail ship using modern materials, you just need to design it to handle the forces that sails introduce. There is no reason why a panamax or other canalmax can't be propelled by sail and use solar to drive auxiliary power. None...we just haven't been looking there.

 

This is really a silly conversation. There isn't enough energy density in solar radiation to power a cargo ship, not even close. Maybe to run the lights and reefers, but propulsion demands are massive compared to the best possible solar production.

But just to run the numbers.... if it works on big ships.

The Emma Maersk is one of the largest cargo ships so let's start there. With an installed power of 111,000kw, let's assume 80% is dedicated to propulsion, and she cruises at 75% of that. So she is using 66,600kw or power for propulsion, or roughly 1,600,000kwh/day.

Solar radiation density is almost exactly 1kw/m^2, and EM has a surface area of 22,232m^2 (is she was square, not boat shaped). Let's give her 6 hours a day of solar production (high, but reasonable). So under a best case scenario she could generate 133,392kwh of power or about 8% of her current power demands.

Even if you slowed her down a lot the power draw would far outstrip her production capability.


Second problem is you need to store all the power produced during the few hours a day that solar is generating. So to store 1,600,000kwh of power you would need how many batteries? 1.6m/1.4 (kWh/battery)= 1.14m batteries at a weight of just 50lbs per battery that's 57,142,000lbs or 28,500tons. Given her GWT of 156,000tons, one day of batteries works out to be about 20% of her total weight.

So all you need to do to get this to work wouldbe reduce her power demand by 92% and increase her GWT by 20%. Sounds simple enough.


And yes I know you would never use 50lb batteries for this type of installation, you would use the largest 2v traction batteries you could find. But energy density doesn't change that much with size. Now how you water and service over a million batteries I have no idea, but that is left as an exercise for the solar installer.

 

And that's at 100% conversion efficiency. To get solar to the prop where it is comparable to the diesel figure, divide by about six. So we are back to the 1% power that we tend to keep coming up with, because we can't quite get to 2% yet. If you look at the batteries, lead batts tend to weigh about 100 times what diesel does for equal power and energy. Wound out, Emma could burn 20+ tons of bunker fuel per hour. You'd need about 2000 tons of convention lead batteries to run her flat out for 1 hour. Her Wartsila weighed 2300 tons. (I'm using some slightly different WAGs than Stumble.)

 

Well and this is why i wonder if the answer isnt just return to sail power. i know the arguments about why not, but most of those are categorized as expectations. instead of planning cargo trips in say one trip of stuff per month, send 4 ships staggered 1 per week over the course of that same month. the ships can be smaller and cheaper to run than that mega ship, and employ more people

 

Not too long ago I joked about posting pictures of old cable laying ships and captioning them that they are early electric ships either laying out or reeling in their extension cords as they cross the Atlantic.

I wonder how many gullible fish such a joke would reel in?

 

I almost wish that I hadn't bothered. For those telling me that solar power is useless, stop telling me.

Scot Mcpherson - it doesn't exist, it's just a question. And no, it's not that thread.

Rurudyne - exactly! you understand my question and that the answer is not obvious.

All the rest - I give up. I didn't ask the question you seem to want to answer.

 

I'll go out in a limb here and say a solar powered vessel should be small. Tiny in fact.

 

If your question was about scalability, then yes a larger boat is less suitable for solar power than a large one. But I don't think solar propulsion is suitable for any boat, it just gets less so the larger you go.

Making a toy car out of marzipan is silly and won't work but you could probably get it built. Making a cargo ship out of marzipan wouldn't get off the layout table.

 

It amazes me how the arguments and assertions are the same -completely lacking creativity. The question is not weather solar should replace oil as the fuel of boats or ships, the question is what opportunities are there in the dramatic drop in solar panel cost? What threats are there to oil supply, cost, and taxation? How does solar compare to wind on various routes?

When I consider boats (personal use) it seems obvious that a combination of solar and wind is attractive with the potential to eliminate diesel -maybe it could have an oversize propane tank and small generator backup. For commercial ships there are monstrous ships that don't need to travel at 14+k. The drilling and oil service ships that just need engines to maintain position. I also wonder about replacing supertankers with super barges that would be collected under a giant solar power plant deck, constantly shuttling across the doldrums of the equator.

The present cost of oil is not sustainable. The tax exemption is crazy and it looks to me like it won't survive the current economic and political cycle. When oil goes up again China will push solar and the creative drought will end.

 

Its not the cost of solar, it's its efficiency. For a bass boat, solar can be great for propulsion. For a recreational sailboat, auxiliary power by solar could be an option under the right circumstances (either a small sailboat that only needs say less than 10 horses to move her along for an hour or a larger boat not needing more than 15-30 minutes of auxiliary power in a day for instance). Anything larger than that or needing sustained and powerful propulsion just isn't feasible, When you do scale it up, not only is it a power output problem, but it's costs way too much to store it so it's available when you need it.

Internal combustion engines can run off of a lot of different fuels, they don't have to all be fossil fuels, and if you rely on sail for 99% of the journey and only need auxiliary power at ports, you don't need to store a huge amount of fuel. We could even go back to wood powered steam for auxiliary power, wood gasification, or Biogas Methane, biodiesel, hydrogen, methanol, whatever...it wouldn't matter because you don't need huge amounts and therefore storage of the fuel doesn't become a limiting option.

Look at cars for example...forget solar for a moment but just consider electric cars...they have effective ranges of 200 miles, and they're done until you fill up again, and filling up isn't dumping 10 gallons of gas in the tank, you need to charge the battery which takes time. Solar takes even longer to charge, so an electric car totally covered is solar panels might get an off the cuff increase of 10% range, but that's not much of a dent in the overall amount of time the car needs to be sitting next to a charging station.

Its not about a lack of creative use of the solar product, it's that the solar product is functionally useless for propulsion of significant tonnage. This arena is the wrong arena for photovoltaics.

You get a high amount of solar harvesting from using traditional sail, it's a direct use of a side effect of solar power on our planet.