The battery thread...some facts please

While I do not think that electric engine, solar panels , and batteries alone are viable for a cruiser, if the intended use of the boat is to daysail from anchorage to anchorage and essentially live off grid and on the hook, then given enough real estate for the panels and a lightweight easily driven hull( hulls) as well as an efficient sail plan, for those willing to pay the premium for the perceived benefits I think its worth looking into. A Torqeedo cruise 4 is 40lbs. 4 26 v li batteries 220 lbs. 3000 watts solar panels 380lbs. total 540 lbs. if we instead use a ten horsepower electric start 4 stroke 120lbs ( with fuel tank ) a honda generater 50 lbs, 2 li batteries 110 lbs and 20 gallons fuel 120 lbs , total 400 lbs we save 140 pounds. The honda will need to be run 2 hours a day if the boat has refrigerator freezer and all the cruising goodies. In a month it will burn 15 gallons leaving 5 gallons available for the out board. I am probably wrong but this would seem to indicate that in some circumstances ( a month cruise or more) the solar powered sail electric boat has a greater powered range at the same speed for the same weight( 4 hours a day or so). As to the perceived benefits I see them to be, less maintenance no fuel, noise, fumes, grease or oil.

 

quote " I read a blog of a totally electric propulsion boat and he highlights some other factors that he believes are more relevant in the overall system."

And these were ....????/

 

Good stuff everyone...

There is some good stuff here, thanks to all for your input.

Two questions for discussion purposes.

We speak of energy density which seems a useful number and tool for discussion. what energy density would batteries have to get to before they would be classed as good/adequate for sailboat propulsion. lets use a 10 tonne disp sail boat as a model for discussion. That would need say a 30¬40hp diesel conventional motor to be classed as suitably powered. (we can change the numbers if necessary. So what battery energy density would be sufficient to see this capable of moving to an electric propulsion system?

The second question is being a devils advocate. From an overall propulsion systems viewpoint is energy density as important as other factors or is it the most important factor? Just to give an example, I read a blog of a totally electric propulsion boat and he highlights some other factors that he believes are more relevant in the overall system.

Thanks for the good civilised conversation so far...lets maintain this example of how battery threads can work

Thanks
Mick

 

Civilized conversation;
1. Your 10 tonne boat should not need 30-40hp.
2. Even a 20 hp electric motor is huge, heavy and power hungry.
3. A 20 hp electric motor, DC or AC? is going to require a speed control of some kind and many, many batteries.
4. So the question is, how long do you want power for? 1 hour, 4 hours, 1 day, 4days
5. I will make a guess that your boat is not big enough to hold enough batteries for 4 hours.
6. Oh solar panels, if you where to cover your whole top side of your boat with panels, it would take you a week for your 4 hour charge.
7. Yet 10 gallons of diesels might last 10 hours or more.

Someone else can do the numbers, but remember when someone says you need 200 amps that is per hour. And if you are charging at 20 amp, that is also per hour. So you need 10 hours to charge for one hour of us. And when a battery is 1000amp hours, even on the best batteries, which allow 50% drain, that mean you can only get 500amps, that 2 1/2 hours of use.

 

Ok, I have been looking at the numbers a bit, and using 30kw a day is a pretty extreme amount, close to what an average house uses. Most boats don't need nearly this much power, but since the solar will need to drive the engine this one might. To generate this much power from solar panels you will need to generate 6kw an hour for the roughly 5 hours of solar generation a day.

The closest installed system I could find to do this was a 25kw a day array, using 36 seperate pv panelsSo we are close enough for comparison sake. This installed array is 35 foot long, by 17 foot wide, with panels butted up to each other. This size would be impossible on almost all boats, and at 40lbs per panel you are looking at 1400lbs, all of which would have to be pretty high on the boat you could have stability issues.

Now storing all that power is an even bigger problem. I always screw up battery sizing, so double check this part. My numbers indicate you will need something like 60 100amp batteries in order to store a usable 30kw a day. At a weight of around 60lbs per battery you will have 3,600lbs of batteries when you set off. But remember you will have no reserves, a cloudy day wrecks your energy budget.

Total installed system looks something like
36panels 1400lbs
60batteries 3,600lbs
3 inverters 120lbs

Total installed weight 5,120lbs

Now in diesel... Assuming the same weight allowance

Fischer panda 6500generator 350 lbs
Fuel 4770lbs, at 7.49lbs/gallon=636 gallons

This generator burns .48 gallons/hour at maximum generation. And will need to be run ~4.5 hours a day, using roughly 2.25 gallons a day. This means you could have 300 days of fuel on board for the same weight as the electrical panels plus batteries.

Now if we do it by cost... The 5kw array from earlier cost $38,000 installed. We still have to buy the batteries though. With this much charging, sealed are defiantly the way to go, so figure $150 per battery. So we have another $9,000 in batteries to buy. For a total installed cost of $47,000.

The 6.5kw panda is $14,000 installed, and let's assume another $2,000 for the fuel tank. So an all up cost of $16,000, which leaves us $31,000 to buy fuel. I am paying about $3 a gallon for diesel... So we could fill up our 635 gallon tank for $1,900, (roughly a year of fuel) invest the remaining $29,000 in low yield bonds and generate another $1,500 by the time we need to fill up.

Assuming your usage rate, over 10 years we have burned something like 6,600 gallons of diesel, for a total fuel bill of $18,000, But still have $30,000 in investments. Over the same time span figure we will have to replace the batteries twice, so you have spent an additional $18,000 in batteries, but that has to come out of your pocket. So the solar system will have a life span cost of $50,000 more than the diesel option. Assuming everything is done in 2011 prices, and no compound interest. When you add these numbers in the dollar figures actually help the diesel more. Also I have ignored slow degradation of the pv cells.


In addition we have to figure out a place to put a solar array that has a larger foot print than the boat... And it has to be high up so it is never shaded to get close to these production numbers.

In short solar just doesn't work, it is more expensive, to large, requires the entire boat be designed around it, and we have just specced out the minimum size panels here, without concern for shading, clouds, or efficiency loss.

 

Hello guys, very nice discussion. Just one thing regarding units, if you don't mind.

When you say "using 30 kW" you are talking about the power and there's no need to add "a day", or "an hour". If you use 30 kW, it means you are doing it in that moment, it is the instantaneous power you need to run your boat in that precise moment. It is just like when you say "my boat needs a 30 kW (or 30 HP) engine to move at 8 kts". Your boat doesn't need 30 HP a day to move at that speed. Just needs 30 HP of power from an engine to make it move at 8 kts in that precise moment.

different stuff is if you want to talk about the overall energy which you need during one day, or one hour. In that case you can say that you need 30 kWh (kilo watt hours), which can be obtained by running a 10 kW generator for 3 hours, or a 30 kW generator for 1 hour. So 1 kWh (or kVAh) is the power in kW multiplied by the number of hours it is running. A 30 kW generator running for, say, 8 hours will deliver 240 kWh of energy.

Sorry if I am being too pedantic, I just feel that if you use a correct language, you will be able to better understand the technical specifications of various batteries, solar pannels, inverters etc. I believe that many errors in calculations are due to this mixing-up between power and energy units.

Cheers, and please keep up posting the good info.

 

I have a pretty specific systems question to go through (like the successful HVAC system thread I started).

The question pertains to installing a huge solar array and large battery bank to support the 3-4 ton air conditioner (with 23 SEER or higher if possible). I just want to look through the possibility of it, with the dream of getting away from the generator, which in my case, will likely be gasoline powered, since I will have gasoline for outboards already. (Boat uses outboards for propulsion)

I want to go through the numbers and post my results here, but I am not sure if it belongs in the "battery thread." Should I start a new one?

Much of my thought process has to do with LiFePO4 batteries, but there is also solar sizing and weight to consider. Lastly, cost is a consideration.

Should I do that here, or start a new thread?

PS: I think Daiquiri is talking to me here. Looking back, I used 30KW improperly in describing my daily power budget. Sorry about that. I'll do this in a much more clear and concise way on the next post - either here or in a new thread.

 

Cat I would start a dedicated thread.

 

Ok, I will bow out of the battery thread with this stuff and start a new one. Thanks.

Here is the link: http://www.boatdesign.net/forums/bo...ditioner-sun-holy-grail-42158.html#post536596

There is a detailed analysis of LiFePO4 batteries in there.

 

Found it... http://biankablog.blogspot.com.au/2008/05/going-electric.html

 

Mydaupin and stumble have put up some fairly straight numbers here. Fairly direct and o the point. Thanks.

What this and dacquiri,s KWH post makes me think of is the time of usage. Maybe this is the critical point for me...what is the min, ave and max length of time you could use the hybrid system for. SO let me throw something out there.
For going off and on the mooring, dock, etc there may be an argument that hybrid might be possible if not financial, but for motoring against a storm or making long passage then hybrid is saddled with a low energy density and high initial cost handicaps.

How does that sound to the readers?

Mick

 

That sounds right to me.

 

Exactly as it is (unfortunately).

Cheers

 

As I understand it the OP intended solar cells to replace engines as propulsion, that is something we haven't really addressed. Cat on the other hand is attempting to get rid of a diesel generator to run ships system (including AC), that is a marginal, but possible goal with existing technology.

The panel and battery combination I outlines would provide something like 25kw/day of usable power, which might be enough for house loads on a charter boat, and assuming enough battery storage would allow for constant usage. But there is no margin really for bad luck. A couple of cloudy days and you are back at the dock for electricity, since the consumption budget and the production budget are pretty close to identacle. If production falls even a moderate amount you just don't have the juice to keep going.


A system designed for propulsion on the other hand is a totally different story. In fact it is such a different story I am just going to roughly cover the numbers, since the exacts don't really matter.

My beneteau 381 has an installed 42hp westerbeak diesel. That 42hp converted to kw is 31kw/hour. Now ignoring efficiency loss, and sizing issues, and whatnot, the solar system needs to be roughly 24 times larger than it is to directly replace the installed engine. So our new arrangement is as follows

864 solar panels weighing in at 33,600lbs
1,440 batteries at 87,840 lbs
73 inverters at 2,880 lbs

For a net weight of 124,320lbs, this replaces a 300lbs engine, and another 320lbs of fuel onboard. Note that the boats current displacement is around 15,000lbs. This system also has a foot print of 14,875 square foot, roughly equal to a square 122foot a side, or roughly 1/4 acres in size.

The installed cost... A whopping $1,128,000.

More exact calculations could be done, this is just a rough assumption, but even if you assume I am off by 50% it is still impossible by huge margins. The only way to use solar power and batteries to drive a boat is if you design the solar system first, then maximize the boat underneath it, and just accept whatever performance you get, as well as ignore the cost for it.

 

Is 25kw/day of usable power intended to mean an average of 25kw over 24 hours which would be 25kw x 24 hours or 600 kw-hours per day, or is it intended to mean 25kw-hours per day.

The difference between the two is very large, a factor of 24. 25kw is 208 amps at 120 volts which is very large. My guess is the "hour" is being left off of 25 kw-hours / day.