Solar panel for cruising

[charles_reed]

Cruising in the Med, I've found a fridge essential, life has been miserable during the occasions the power rectifier has packed in.
The critical factor is how efficient the fridge might be. That's a combination of a number of factors:-

efficiency of the cooling unit - if an expensive Danfoss unit a draw of 5 amps on start and 4 when running for 10% of the time, if a cheap Peltier unit the same draw for 100% of the time
effectiveness of insulation - I'd not reckon much on the exiguous insulation on a caravan unit or one which loses all its cold air when you open the door
How often you open the unit and whether you put warm food into it.

As the OP has not even hinted at the fridge type/use most of the comments posted are wild guesses.

 

[vyv_cox]

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:

My stern arch, along with those of many we see in the Med, are constructed almost entirely from inch stainless tubing and standard fittings, mostly Ts and elbows. I had the brackets holding the panels made by a local welder, cost me £35. I was fortunate that I could support it with the radar pole but it is perfectly possible without that.

This is the Mark 1 version, the aft poles now go to the transom with braces between the uprights. This means that we are no longer able to tilt the panels, which we found to be unnecessary.

 

[Cardo]

We have 300W solar panels on our boat here in the Med. Due to the design of the boat, we were limited as to where we could fit the panels, so we have one on top of the bimini, and two on the sides at the stern that can swing up and down.
The biggest factor we've found is shading, this will kill the output of our panels. In full daylight with no shading, we usually get around 13a charge from the panels. As soon as one of the panels is slightly shaded, the output from that panel will drop to around 1a, if we're lucky. All three of our panels can become even slightly shaded depending on where the sun is coming from. So, depending on what the wind is doing whilst we're swinging at anchor, we have found that we either have absolutely no problems with power, keeping our front loading fridge going, laptops, etc. and even running the watermaker for an hour or two; or we see the batteries dropping further and further as the panels struggle to even keep the fridge going.
We do have a relatively large battery bank - 500Ah for domestics that does mean we can cope with little input for a few days before we start getting itchy. Our next purchase will be a proper alternator charger so we can put in a decent amount of juice when we're motoring in and out of anchorages.

 

[Boo2]

Boat is a 34ft wooden ketch, going cruising.

Wooden ketch ? That's not your Victor 34 then ?

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...

In the following A is current in amps, W is power in watts and V is battery volts:

If you take the wattage of the panel and divide by 13 you will get the maximum charge current. You might expect around 80% of that in practice. To find out how much charge it will put back into the battery you need to multipy the 80% figure by the number of hours of strong daylight.

Eg :

100W / 13V = 7.6A (absolute max)
7.6A * 0.8 = 6.0A (average current supplied by panel)

6.0A * 6h = 36 Ah in an English summer day

To get your current consumption in AH per day you need to do the same thing: obtain the average current from either the spec plate on the equipment or the manufacturers data sheet or by measurement. If they provide a power rating divide by 12 (not 13) to convert W to A. Then multiply the current by the number of hours it will be switched on in a typical day to get its consumption in AH. Do that for all items of electrical equipment and add them up to get the total Ah. IF that is more than you are putting in (quite likely living aboard with a 100W panel) then the difference tells you how much you will be running the engine.

Hth,

Boo2

 

[oldvarnish]

.

Would like to fit solar, to lessen the need to run the engine to charge batteries on passage/at anchor

All the above is good advice, but the OP is only trying to reduce his engine use (as above). So what he plans is a good step on the way. He clearly likes a simple boat. We shouldn't try and talk him out of that noble aim.

 

[vyv_cox]

All the above is good advice, but the OP is only trying to reduce his engine use (as above). So what he plans is a good step on the way. He clearly likes a simple boat. We shouldn't try and talk him out of that noble aim.

True enough, but by adding a little more solar power and maybe another battery, engine use can be reduced to zero. No more complexity involved, mine is a very simple system.

 

[William_H]

Wooden ketch ? That's not your Victor 34 then ?


In the following A is current in amps, W is power in watts and V is battery volts:

If you take the wattage of the panel and divide by 13 you will get the maximum charge current. You might expect around 80% of that in practice. To find out how much charge it will put back into the battery you need to multipy the 80% figure by the number of hours of strong daylight.

Eg :

100W / 13V = 7.6A (absolute max)
7.6A * 0.8 = 6.0A (average current supplied by panel)

6.0A * 6h = 36 Ah in an English summer day

To get your current consumption in AH per day you need to do the same thing: obtain the average current from either the spec plate on the equipment or the manufacturers data sheet or by measurement. If they provide a power rating divide by 12 (not 13) to convert W to A. Then multiply the current by the number of hours it will be switched on in a typical day to get its consumption in AH. Do that for all items of electrical equipment and add them up to get the total Ah. IF that is more than you are putting in (quite likely living aboard with a 100W panel) then the difference tells you how much you will be running the engine.

Hth,

Boo2

You have converted solar power by assuming 13v into 100w. The correct figure is 20v so 5amps max. Only by using an MPPT controller can you relate the output ie battery voltage to the watts to get more than 5 amps however this must be tempered by an inefficiency so only about 90% of power. So pretty close to 5 amps anyway.
Solar manufacturers rate their panels at max volts (20) multiplied by max current 5 amps to get 100watts. sorry about that olewill

 

[Gerry]

More encouragement from me to do without a fridge. If you want to keep things simple this is the way to go. We have been cruising for 12 years, six of those without refrigeration. Canning meats, butter, fruit and veg means you can have good produce in store for the times when you can't buy fresh. Those cold beers can be achieved by wrapping them in a wet towel and letting evaporation perform it's magic.

Sometimes we buy block ice if it's available, it lasts 5-6 days in our icebox and extends fresh supplies. We don't rely on it though!

We run a low energy boat, a wind generator supplies almost all our needs- no chart plotter etc. Use oil lamps. Still on the diesel that we put in the tank over twelve months ago!

Simplicity gives you tremendous freedom. Do think again as to whether you really need that fridge.

 

[ghostlymoron]

A lot of the advice given so far has been from power hungry mediteranean cruisers where fridges and aircon are the rule. Jelly doesn't say where she is planning to cruise but it may not be the med.

 

[Hadenough]

Jellyellie
Blimey, I'm bet you are glad you asked now!

 

[vyv_cox]

A lot of the advice given so far has been from power hungry mediteranean cruisers where fridges and aircon are the rule. Jelly doesn't say where she is planning to cruise but it may not be the med.

True, but I automatically assume going somewhere warmer. Arctic cruising definitely not for me, but it does overcome the need for a fridge.

 

[sailorman]

True, but I automatically assume going somewhere warmer. Arctic cruising definitely not for me, but it does overcome the need for a fridge.

A Fridge is more reliable then an Ebbersplutter

 

[C08]

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.

I had an 80W solar panel 3-5 amps most days south coast uk and 1-2 amps when overcast-so impressed I bought another. I think the Peltier Coolbox is a problem. I use a Waeco compressor coolbox and this uses about 5 amps at 20% duty rate and one panel easily copes with that. I also have a Rutland 913 and much as I like it, one 80w panel produces more power on average than one 913.

Panels are obviously very different as my panels are on the sprayhood top and are often shaded by the boom and show only proportionate reduction in power when shaded!

A mix of wind and solar is perhaps the ideal way to go plus good sized battery banks to store the energy.

 

[vyv_cox]

A Fridge is more reliable then an Ebbersplutter

Perhaps so. My fridge has run constantly for six months of each year since May 2004. Never given a moment's trouble other than minor fouling of the seawater cooling system. My Eberspacher was used heavily for about three years prior to heading south but has only been used rarely since then. That makes it about 13 years old, still runs fine.

 

[sailorman]

Perhaps so. My fridge has run constantly for six months of each year since May 2004. Never given a moment's trouble other than minor fouling of the seawater cooling system. My Eberspacher was used heavily for about three years prior to heading south but has only been used rarely since then. That makes it about 13 years old, still runs fine.

Nothing wears out if you dont use

 

[CAPTAIN FANTASTIC]

I had an 80W solar panel 3-5 amps most days south coast uk and 1-2 amps when overcast-so impressed I bought another. I think the Peltier Coolbox is a problem. I use a Waeco compressor coolbox and this uses about 5 amps at 20% duty rate and one panel easily copes with that. I also have a Rutland 913 and much as I like it, one 80w panel produces more power on average than one 913.

Panels are obviously very different as my panels are on the sprayhood top and are often shaded by the boom and show only proportionate reduction in power when shaded!

A mix of wind and solar is perhaps the ideal way to go plus good sized battery banks to store the energy.

Perhaps it is time for me to invest in a solar panel too.

 

[VicS]

Solar manufacturers rate their panels at max volts (20) multiplied by max current 5 amps to get 100watts. sorry about that olewill

I thought the power rating was the maximum power output obtainable (P_max) and that the terminal volts (V_mpp) at which this occurs is usually in the 16 - 17 volt area.

Current at P_max (I_mpp) X V_mpp = P_max

Open circuit volts V_oc is uaually around 20 volts or so but thats is wilth no current flow so the power output at that point is zero.

 

[Pete7]

Perhaps it is time for me to invest in a solar panel too.

Certainly sitting in Alderney last year for 5 days and leaving with full batteries convinced me so we sold the honda genny. The lack of hot water was a minor inconvenience and 240v for the phone charges comes from an inverter. Prices have come down considerably over the last few years as the market is awash with them.

 

[Boo2]

You have converted solar power by assuming 13v into 100w. The correct figure is 20v so 5amps max. Only by using an MPPT controller can you relate the output ie battery voltage to the watts to get more than 5 amps however this must be tempered by an inefficiency so only about 90% of power. So pretty close to 5 amps anyway.
Solar manufacturers rate their panels at max volts (20) multiplied by max current 5 amps to get 100watts. sorry about that olewill

I don't really understand your figures ? The power the manufacturers quote is the maximum the panels will deliver. I was trying to avoid being too technical but the 80% factor was intended to allow for the fact that the battery charging voltage is not the optimum for the panel. I think 80% is quite a reasonable relative efficiency figure for modern panels when charging a battery.

Boo2

 

[noelex]

I thought the power rating was the maximum power output obtainable (P_max) and that the terminal volts (V_mpp) at which this occurs is usually in the 16 - 17 volt area.

Current at P_max (I_mpp) X V_mpp = P_max

Open circuit volts V_oc is uaually around 20 volts or so but thats is wilth no current flow so the power output at that point is zero.

+1.

The only thing I would add is that the Vmp is specified under the standard test conditions which is a cell temperature that is not achievable in practice under conditions where the panel will be producing near maximum power.

So the real Vmp will be 1-1.5v lower than specified on the panel.