Solar panel for cruising

[jellyellie]

Hello all

Boat is a 34ft wooden ketch, going cruising.

Very limited electronics onboard - VHF, depth sounder, simple GPS (not a plotter), a few interior lights (swapping the bulbs to LED), car stereo & 6x9 speakers (can do without). Also just buying a 12v 50w chest-style fridge/freezer.

Two batteries, one 180ah for domestics and one 110ah starter battery.

Would like to fit solar, to lessen the need to run the engine to charge batteries on passage/at anchor, and to keep the fridge ticking along in hot weather.

Only one 100W solar panel will fit on the boat (across the top of the davits).

Can somebody break down the maths/physics and give a likely idea of how far off the mark this setup will be? I've done some reading and basic calculations, have a general idea but would be good to get some more advice...

 

[oldvarnish]

Not certain you need any maths. You're only seeking to reduce engine use, and a 100W panel will certainly do that - on a bright day - especially with such a low load.
Congratulations on keeping things simple.

 

[Hadenough]

We had a 100w panel on our last boat a 30' ketch. It was more than adequate in summer months and kept up pretty much with our 4A compresser fridge. You probably realise but be carefull with your fridge spec, some of the stand alone units are absorption fridges with an 8A constant load.

 

[jellyellie]

Thanks for the quick replies guys!

@oldvarnish, thanks for your words... that is certainly our attitude, nice to receive some reassurance. we are definitely aiming to keep it simple.

@Hadenough good to hear it worked for you. what battery bank did you run it off? and yep, we're going for a 4A compressor fridge - getting hold of the one we want means having to import the damn thing from China!

 

[Hadenough]

Had 2 x 95AH domestics and an 85AH engine battery. Charge was split 90% domestic 10% engine through a regulator. Our load was much the same as yours by the sound of it.

 

[VicS]

Hello all

Also just buying a 12v 50w chest-style fridge/freezer.

Only one 100W solar panel will fit on the boat

Can somebody break down the maths/physics and give a likely idea of how far off the mark this setup will be? I've done some reading and basic calculations, have a general idea but would be good to get some more advice...

Depends on the type and spec of the fridge....... basically how much of the time it will actually be running and hence its average power consumption over say 24 hours

Worst case scenario is a Peltier element type of fridge.

The average power consumption of a decent compressor driven fridge however will be much lower, with a decent chance that your solar panel will come close to supplying at least the needs of the fridge.

Need to know the full specification details of the fridge to do any meaningful calculations.

 

[ghostlymoron]

Do you need to run the fridge continuously. We have a small coolbox connected to its own battery and run off a small solar panel. It switches off after a while when the voltage drops but, provided we don't open it too often, the contents remain cool and fresh until the battery re-charges. This obviously wouldn't be satisfactory for keeping frozen food frozen but its OK for milk, butter, cheese and bacon for a week.

 

[macd]

As VicS said, much will depend on fridge efficiency, mainly how well it's insulated. A 100W panel will certainly contribute very usefully to your needs. If you're installing a pre-made fridge box, do try to add extra insulation if there's room. And if the natural airflow over the compressor etc is a bit stodgy, a 3 inch computer fan draws almost no power and works wonders. Go for a rigid panel, if you can: far more bangs for your buck and lasts for aeons, unlike that self-recycling semi-flexible rubbish.

 

[TQA]

Try and devise a mounting system that allows you to tilt the panel to follow the sun. This about doubles the ah hour input you get.

At anchor it can be as simple as bungying it to the boom on your mizzen. Watch for the topping lift though, it's shadow will drop the input a lot.

The temptation is to screw it down somewhere convenient at the cost of much lower total input.

If you have not already bought your Chinese unit check out the Adler Barbour [Super] Cold machine kit. It is suitable for DIY installation.
See http://www.dometic.com/f8b47925-b677-4de7-a942-bedee33b961a.fodoc

I have one and it keeps a large fridge really cold in the tropics, makes ice and keeps frozen stuff frozen on about 50 a/h a day.

 

[laika]

No cop for passage but if you want more power than what you can install on your davits but don't have the deck space don't overlook the suitcase-type folding panels I just bought a 100W set (2x50w) for ~£200. I'll do a review of them when I have some time but in short:
- they're pretty large (510x695x35mm allegedly). They do 150 watt ones but I don't think I've got anywhere to stow them. There's also smaller ones
- hinges and catches look cr&ppy and will corrode, but I'm prepared for that.
- Includes a regulator which I was going to cut out of the loop and feed them into a bigger mppt regulator when I fit more solar
- seems to output 5.5 amps facing the sun with no shade (uk south coast sunny early afternoon). drops to 3-ish if I don't move it to follow the sun. Drops to 1-2 if next door's mast shadow falls across it
- currently has crocodile clips, but I intend replacing that with a dri-plug connecting to a socket in the cockpit for ease of deployment.

As TQA said, seems pointing at the sun is key to optimising performance. Something you can move around on deck is helpful there but does make you get up from your mai tai every so often. But do see "size" above: they are a bit hefty. And you need somewhere to store them below.

 

[vyv_cox]

Sorry to be negative but you have very little hope of running your proposed setup if you run your fridge full time. Assuming a compressor type fridge like ours it will consume around 4 amps for about a third of the time. Our solar panel is 125 watts, used in the Mediterranean for the past nearly ten years. With 2 x 110 Ah batteries their voltage fell to about 12.0 overnight, a recipe for very short life. On uprating to 3 x 110 Ah the voltage falls to about 12.4 volts overnight, far more healthy. These figures are in summer but we see a significant fall in autumn. A panel as small as ours would be highly insufficient in winter.

Turning the fridge off overnight is very risky in warm climates, particularly if you intent to use the freezer section. Bacteria love such a regime.

Our Peltier cooler will easily flatten the two 110 Ah batteries in our motorsailer overnight.

 

[pcatterall]

Turn the question around a bit..... Our 2x 40 watt panels were producing around 4 amps max in full sun in the med this was with no shading and the panels at the best angle. (Probably about 30 AH in the day) Your usage can be calculated so you should be able to see if you have enough power. The NASA BM2 will be useful in monitoring inputs and outputs. You need to get a feel for what your panel realistically will provide. Poor angle to sun will detract and any shade may significantly detract.

 

[ribrage]

Another question

Do you actually need a fridge ? we walk into town mostly every other day, fresh veg and fruit from markets much cheaper and nicer that way.

i managed without a fridge for 7 years , fitted one as I thought i would use it and leave the damn thing off as it just consumes to many amps

i run 2 x 100 solars for the 24 volt system and 2 x 36 solars for the 12 volt system , the 24 volt is only really running the fridge as lights and nab are 12 volt, 2 x100 wont stay on top of consumption so i leave it off and use the fridge as a normal stowage for food, not missing what i never had , just the £1500 it cost to fit the fridge !

 

[TimBennet]

Another question

Do you actually need a fridge ? we walk into town mostly every other day, fresh veg and fruit from markets much cheaper and nicer that way.

i managed without a fridge for 7 years ,

So did we. Your fridge will dominate your power management life. Without everything is a doddle.

 

[[3889]]

So did we. Your fridge will dominate your power management life. Without everything is a doddle.

Bought my first boat with fridge in May and have to agree. With an 80W panel I seem to spend far more time in marinas and missing out on anchoring. Never had a problem before with a bag of ice from the supermarket in the coolbox every couple of days.

 

[CAPTAIN FANTASTIC]

I have 4 x 110 amps for domestic and 1 x 110 for engine start. When I use the Pelrier coolbox which is 40 watts, drawing 3.5 amps continuously, the domestic batteries at the end of the day drop down to just over 12 volts; my Rutland 913 wind generator keeps everything running and charges up the batteries perfectly but can not keep up when the coolbox is operating.

Would, in addition to the wind generator, a 100 watt solar panel help in the South English coast? I am not sure; perhaps not. So for the time being, I use the coolbox, most of the time switched off with just a few bags of ice which lasts about 3 days.

 

[Pete7]

Agree with Vyv you are going to slowly run out of power. We have 125w of solar maxing at 7.5 AH at mid day that will keep us topped up at anchor quite happily with a daily usage between 30 - 40 AH a day, with fridge (not freezer), lights and laptops etc. However, this is mid summer UK. At sea or during the winter you will have lights, VHF, auto helm perhaps and shading from the sails. In the med summer the fridge will take much more power.

I think you need to double the size of the panels. Appreciate you are on a budget so stainless steel A frames at the stern are probably out. However, if looks are not important then how about a frame of galvanised steel to mount them on:

http://www.robinsonpolytunnels.co.uk/acatalog/Fruit_Cage_Parts.html

 

[oldvarnish]

Your fridge will dominate your power management life. Without everything is a doddle.

This is VERY true

 

[William_H]

Well you have concluded that 100w is all you have room for. IMHO actual size you cna fit is the determining factor anyway unless you go for temporary or portable panels as well.
The fridge will typically draw 25 amp hours in a day. This will depend on the ambient temp of the day and the temp you set the thermostat at pluss the amounf of warm food you put in or the amount you open it. Plus as said the efficiency of the box.
The solar panel if you are lucky might supply something like that depending on time of year and panel orientation. Maybe more maybe less.
All you can do is watch a volt meter on the batteries. If it is looking low then you will need to run the engine as well. A smart charger will be useful here to minimise engine charging times. In any case I would suggest you fit an amp meter so you cna monitor just how ell the solar panel is doing and mor eimportantly how ell the engine charging is doing. At some point in engine charge the charge current will fall to a point where it is not worth continuing. Stop and give it some more later. I suspect it will be best to engine charge when solar panel is not charging.
I think your other current loads will be small compared to the fridge. A VHF radio might use 10 amp hour per day if left on full time. LED lights probably around 1 amp hour per day. good luck olewill

 

[charles_reed]

Sorry to be negative but you have very little hope of running your proposed setup if you run your fridge full time. Assuming a compressor type fridge like ours it will consume around 4 amps for about a third of the time. Our solar panel is 125 watts, used in the Mediterranean for the past nearly ten years. With 2 x 110 Ah batteries their voltage fell to about 12.0 overnight, a recipe for very short life. On uprating to 3 x 110 Ah the voltage falls to about 12.4 volts overnight, far more healthy. These figures are in summer but we see a significant fall in autumn. A panel as small as ours would be highly insufficient in winter.

Turning the fridge off overnight is very risky in warm climates, particularly if you intent to use the freezer section. Bacteria love such a regime.

Our Peltier cooler will easily flatten the two 110 Ah batteries in our motorsailer overnight.

I do agree with Vyv. I've gradually built up to 328 watts of PV panel in fixed mounts, I can now, during June/July/August hang at anchor as long as I like and have the system volts in 215ah domestic batteries not drop below 12.2v. In April and September I can only be at anchor for about 1 week.
My fridge and computer each take about the same number of AH, the PV panels are wired to produce a nominal 24v through an MPPT controller. The original 100w of panel that I had just couldn't cope.
Music and audio-books are played through the computer sound-card - that uses about 40% of the power of the car-radio-cassette-CD player.
Generally @ 30-40N so more insolation than UK waters.