Solar Power

PuffTheMagicDragon

PuffTheMagicDragon

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In the 'Current Observations' part of the weather page that is on the local international airport web site the is a value called "Global Solar Irradiance".
The value is given as "watts per square meter (W/m2)" and a typical value at this time of day - 1400hrs - would be around the 950 level.
Question:- How can I use this value to determine what size of photovoltaic panel I would require to handle two sealed batteries, each having a capacity of 105Ah ?

Thanks for all comments.
 
On the inforemation you have given....

...it is only possible to advise you to obtain the smallest possible panels as you don't seem to have any drain so you just need to cover "natural losses"

It is, you see, the current draw over a 24hour period in Ampere-hours that determines how many ampere-hours you put back (plus a bit for internal losses) and therefore how big a panel array you need.

We run a fridge on our boat in the Med and this takes around 4.5 amps when running at a duty cycle of around eight hours so we need to input 36Ah over a 24 hour period. Our twin "55watt" (= 7.7 amps at 14.3 volts) panels can only be expected to produce this output at peak times around mid morning to late afternoon. If you divide the required 35Ah by the amps out this shows the need for only 4.7hrs of peak charging. Even though it looks as if we're doing fine, all the other loads (Windlass, cabin lighting, TV, radio, Shower pumps etc.) soon eat into the reserve and we just about cope with a couple of engine hours (Sterling regulated) additional charging. Tell us your power requirements and then we can advise more accurately.

Steve Cronin
 
Re: On the inforemation you have given....

[ QUOTE ]
...it is only possible to advise you to obtain the smallest possible panels as you don't seem to have any drain so you just need to cover "natural losses"

We run a fridge on our boat in the Med and this takes around 4.5 amps when running at a duty cycle of around eight hours so we need to input 36Ah over a 24 hour period. Our twin "55watt" (= 7.7 amps at 14.3 volts) panels can only be expected to produce this output at peak times around mid morning to late afternoon.

[/ QUOTE ]

Thanks for your thoughts Steve. I ask because I continue to think about installing a fridge. At the moment the load that I have consists of running lights (@1amp each), GPS, VHF (always on during most of the day, all the time when travelling) and cabin lights in the evening (say 6 Amp hours), CD player for background noise mostly during the evening happy hour. Electric windlass is only operated while the engine is running.
Taking your example of '55 watt panels'. Two of these make '110 watts'. Would I be correct to say that with a Global Solar Irradiance (see original posting) of 950 watts per square metre I would need an area of 950 / 110 square metres of panel? This would be 0,116 m2, i.e. 1158 square centimetres, which seems ridiculously small! How big an area do your '55watt' panels actually cover?
Hence, my original query, how can I relate Global Solar Irradiance to panel area - if such a thing is indeed possible.
 
Re: On the inforemation you have given....

To be a bit more practical than theoretical a 60w panel will give an average of 19/20AH per day on the South Coast in June/July, those figs came from one of the makers who tested them on the IOW some years back. These are averages, some days will be much better, others much worse. Strangely, Med and Caribbean figs were not much better, because the UK enjoys longer hours of daylight in summer (forget winter!).

We have a permanent mounted 75W panel on our stern gantry and a 110W one we put out if needed at anchor, both are aluminium framed ones. We have ammeters on these and our wind genny and can watch what happens with shadows etc and reposition the 110W one accordingly. We run everything we need on a 41 footer at anchor, our fridge is on all the time at normal settings and we occasionally watch TV, have CD/radio on often and so on. Normally we can pretty well be power sufficient for 5/6 days at anchor between moving on or 'normal' use of the engine (never to charge batteries) but we have big batteries. However we often leave after being anchored 5/6days with full batteries and we have on several occasions had to disconnect one/both panels (in August) to prevent overcharging. As we prefer to anchor out of the wind, our wind genny ( big Aero6gen) is only untied if required as an extra and of course if it is windy.

Our big 110W panel around noon puts out in practice a measured maximum in bright sunlight of about 7A dropping to 3-4A as time goes by, but still working at maybe 1A early evening. The slightest shadow from a sheet or shroud however will drop the output 25% or more. At one time we had an average 13A going in from both panels and the windy, eat your heart out Chernobyl! We don't have regulators on the solar panels, the fixed one can be switched off and the other one unplugged if needs be but we do monitor battery volts and input amps and the panels are not left on if we leave the boat for any time.
 
solar irradience is largely irrelevant to solar panels as it is the total energy in all parts of the spectrum and not just the bits the solar panels can make use of. As other posters have said, use the power output figure for the panel as a guide. Remember also that the power output will be on the optimistic side, especially if there is the possibility of anything causing a shadow (rigging, spars etc.) and thus reducing the output voltage.
 
Re: On the inforemation you have given....

Thanks for that comprehensive information.
If / when I do install a panel it will be a rigid type and mounted - ideally - on the dinghy davits.
That is why I would like to find out what correlation there is with 'Global Solar Irradiance' and size of panel to get X worth of Amps; if indeed such a correlation actually exists!
 
Panel size...

Here's a picture of them. They are about 48cm by 110 each so your figures look about right except for a factor of 10!

I think that if you want to persue this route then you need to speak to a theoretical physicist, NOT a "Practical Boat Owner". All we know is what works in our individual cases.

Regards

Steve Cronin

IMG_0135.jpg
 
[ QUOTE ]
solar irradience is largely irrelevant to solar panels as it is the total energy in all parts of the spectrum and not just the bits the solar panels can make use of.

[/ QUOTE ]

That's all I wanted to know; The rest I can understand (I think) /forums/images/graemlins/wink.gif

Thanks Ken.

p.s. would you be connected with Glasgow Uni in any way?
 
Re: Panel size...

[ QUOTE ]
They are about 48cm by 110 each so your figures look about right except for a factor of 10!
All we know is what works in our individual cases.




[/ QUOTE ]

That's all I wanted to know. (see my reply to Ken Johnston)

Now that I know what sizes work in Greece I can make meaningful comparisons because of the similarity of location.

Thanks once again, Steve!
 
Re: On the inforemation you have given....

[ QUOTE ]
You may have forgotten to factor in the efficiency which is about 10 to 12%

[/ QUOTE ]

That would sustain the information given by Steve Cronin (practical) and Ken Johnson (theoretical).

Thanks Claudio!
 
Re: On the inforemation you have given....

most reputable solar panel makers will quote an average watt hour per day so that you can work out what you will get from the sun in your area.
 
Re: On the inforemation you have given....

There is a strict relationship between the two:

Electrical output = irradiance x optical efficiency x conversion efficiency x electrical losses

From memory, the peak solar irradiance at "Air Mass 1" (sea level) is about 1200 W/m^2. This gets reduced by cosine of your lattitude + a seasonal correction reflecting the tilt of the earth's axis wrt the sun (probably getting you to the figure you quote).

Some of this irradiance (light + heat +UV) gets reflected off the outside of the solar panel (~8% for a plain glass window). Only part of what gets through can be usefully converted to electricity by the cell.

For a well designed panel, optical losses will be insignificant compared to the conversion efficiency. A few years ago, space quality (silicon) solar cells with super duper coatings, etc. were ~14% efficient [*], with "experimental" gallium arsenide ones making maybe 18%. I believe that tandem cells (thin gallium arsenide over silicon) can now get over 20% efficiency. I would guess that commercial panels using monocrystaline silicon might make 10%, and polycrystaline maybe 8%. "Thin film" panels will probably be 3-5% efficient, so of all the "irradiance" hitting the cell, only 5% might make it into electricity.

BUT

The efficiency figures will be quoted at the most efficient point of the cell's operation, which is probably not where the load you're supplying will be. The "quality" of the cells used to be described by the "fill factor" - how near the cell came to supplying its open circuit voltage at its short circuit current - 100% is ideal. Typically, panels are sized to produce excess voltage, so that there is a specified minimum voltage at a particular current. For example you may be drawing 12V at one amp (12W), but the cells may be generating 18V (internally) at one amp (18W) the balance going to heat up the panel and any regulator you might have attached. In this case, there is a 30% drop in efficiency, on top of everything else due to electrical losses / load matching.

Also, the warmer the cells get, the less efficient they are. (Cos thermally generated holes/electrons compete to mop up the optically generated ones).

[*] My info is probably out of date, and related to Air Mass Zero (i.e. space) irradiance which favours silicon cells due to the relatively greater amount of "blue" light when compared to AM1 (cos the atmosphere scatters it and makes the sky look blue- innit?)

Additional, useless info - the spectral information in common use for the Sun was collated by a Mr. Moon.

Andy

Anyone still awake.......?
 
Re: On the inforemation you have given....

Solar panels are very usefull if you are on a remote or swing mooring and do not have any other charging form. They are also usefull if you do not have shore power and are away from the boat for many weeks such that batteries need a little boosting to over come natural discharge.
If you hope to provide power for living on the boat especially using a fridge you are up for a lot of solar panel, read money, and inconvenience.

The most effective way to keep batteries up is with shore power or regular engine/generator running. (or even taking battery home for a charge) If these are not possible then you resort to solar or wind generator.

Solarpanels get in the way are easily damaged and don't last very many years. You need a lot for a little current.

I am trying to say that while it is fashionable /cool to say we run on solar power. Like wind generator the cost and inconvenience are a large price to pay. Or put another way if you must have solar then the location and mounting arrangements (and cost) will dictate what size you fit.

All this negative from one who has relied totally on solar for power for the last 18 years on my boat. Cos it is on a swing mooring and I leave the outboard home. I keep a small panel attached to the top of the boom cover by bungees and remove it when I go sailing. It does keep a small battery charged for nav lights and radio. But I don't love it and I have gone through 4 panels. It is however better than toting the battery home for a charge. olewill
 
Re: On the inforemation you have given....

Ole Will's comment serve to remind us that there are different solutions and indeed different requirements depending on boat usage, location, cruising plans and of course boat size and expectations.

IMO small flexible panels are very limited in their usefulness, the convenience of small size and portability means lower output and reliability - but they will keep a small battery topped up on a mooring.

Sure it is more efficient to recharge by shorepower, generator or main engine. The assumption there however is that the boat is day sailed or if away from home will routinely run a generator with it's attendant extra cost, weight, space, fuel and noise or worse still run the main engine which is very ineficient use and even noisier. We have 2 alternators and smart regulators on our boat and I still hate running the engine for charging, I prefer to save the fuel too for when we cannot sail and are forced to motor. I also hate to be anchored next to boats that DO run generators or main engines for hours for charging!

On a larger boat the situation is a little easier. Firstly although they probably use more power daily, they have bigger batteries too which can store excess energy when it is available, ie if they have to motor they can at least store a bit more of the by-product electrical power in the bigger batteries. Secondly they have more space for things like solar panels and wind generators to be fitted permanently or even to store temporary ones. In our case we have a 75W fixed solar panel on a stern gantry that also has the mounting for a powerful wind generator, plus have space below to store a fixed frame rigid 110W solar panel for additional use at anchor. A lot of power available 'free' but yes costly initially, in our case we inherited some and added some, it wasn't a one time big bill. However our setup would not fit easily on say a 24 footer and the initial costs with added structural work would probably buy a small older boat! Some of the purchase cost of course can be offset by savings made by not needing to be in a visitor berth plugged in, sailing only from marina to marina.
 
Re: On the inforemation you have given....

[ QUOTE ]
There is a strict relationship between the two:

Electrical output = irradiance x optical efficiency x conversion efficiency x electrical losses

Andy

Anyone still awake.......?

[/ QUOTE ]

Yes, Andy, I am still awake. (Actually I just got out of bed /forums/images/graemlins/wink.gif)

The information that you gave makes sense, although I do not quite follow the part "this gets reduced by cosine of your lattitude". The closer that one gets to latitude zero, the closer would cos(lat) approach unity....unless, of course, you meant 'multiplied' by cos(lat).

[ QUOTE ]
Also, the warmer the cells get, the less efficient they are. (Cos thermally generated holes/electrons compete to mop up the optically generated ones).

[/ QUOTE ]

Now this is what appears to be the quandary. Air temperature in summer is in the mid 30's. Deck temperature gets to be such that walking barefooted becomes a virtual impossibility - and we are only on Latitude 36. Panel temperature would be even higher (darker colour).

All this would seem to infer that:
(1) the closer that one gets to the equator, the more sunshine one is likely to be exposed to and, therefore, more 'photovoltaic' conversion, but, on the other hand,
(2) the closer one gets to the equator the higher the temperature of the cells and, therefore, the lesser the output.

Which would mean that what is gained on the swings is lost on the roundabout!

In deference to the other posters, I agree that the bottom line would always be the pragmatic approach: 'How much area for my needs'. It's just that I have always wished to know the 'why' of how something is decided.

Thanks for your explanation and also for everyone's contribution. I hope that my question was not too boring, in which case please accept my apologies.
 
I'm with Steve Cronin on this. One can dig very deeply into the theory of solar energy generation and it is very interesting but there are some very basic issues.
How much power do you need NOW and in the forseeable future and how much mounting space do you have?

If you want a fridge, and in the Med summer why wouldn't you? Further I don't buy the idea of switching fridges off overnight, they still have to recover lost capacity when turned on again and there is the real risk of food spoilage as the temperature rises with the power off.

We live aboard a 13.6m (44') monohull during the Med summer and I have had 2x54W panels for 3yrs. They are great and despite a bit of shadowing when sailing they supply enough to cover the fridge demands and often more. The next greatest demand on our boat is the computer! Not for navigation, for e-mail, ship's diary for family back home and for weather. We can typically stay at anchor for 3-4 days without using the engine to recharge.

For the coming season I am installing 2 more panels so that I am even less reliant on engine/shore power for my batteries. I did a lot of research on solar panels, efficiency, susceptability to shadowing, size, weight etc. However in the end the choice was made on availabilty of space for mounting. I will install an arch/extension to my bimini at the stern similar to Steve's and although the panels I am buying are physically larger for similar output than my "best selection" they fit on the arch and are of a type which is less susceptible to shadowing and cloud. Some panels will reduce their output by up to 75% with the smallest shadow on them - even a rope. This can negate all of the "best case performance" theories.

Take a good look at where you can mount the panels then go for as large/many as your budget can handle considering your near term power requirements. The practical matters will outweigh the absolute theory in the end.

Rgds
Ray
 
[ QUOTE ]
I'm with Steve Cronin on this. ......there are some very basic issues.

If you want a fridge, and in the Med summer why wouldn't you? Further I don't buy the idea of switching fridges off overnight, they still have to recover lost capacity when turned on again and there is the real risk of food spoilage as the temperature rises with the power off.

.......The practical matters will outweigh the absolute theory in the end.

Rgds
Ray

[/ QUOTE ]

I totally agree. My query was really on two fronts, one being a natural, personal, inclination towards learning more background to why we choose one avenue rather than another. Hence, the 'theory' part.

The second - arguably the most important - was to gain first-hand information about area and output.

Steve's reply and photo gave a practical indication of what to expect in Greece. Your experience, and I notice that you are in Italy, also illustrates panel size versus actual performance and for that I thank you all. In either case you are living with the 'shadow' factor as well as the 'temperature' situation and your opinions reflect real life.

Better than any manufacturers' claims would.

At some stage you made a decision to choose one type over another. You mention, for example, an impressive figure of how much the output deteriorates when there is even a small shadow. How does one get this information? Surely not from sales literature?

Regarding space and placement, mine would be over the dinghy davits, which are joined at the outboard ends by a stainless tube that is 1 metre long , so, I would guess that a realistic usable size would be up to 100 x 50cms or perhaps 100 x 75. Either way, it will be a rigid panel / s.

From what I can gather, this should be sufficient to run a small fridge (say 20-25 litres) while sailing or at anchor.

Is there anything else that I should be aware of?

Thanks once again to everyone.
 
Re: On the inforemation you have given....

[ QUOTE ]
..unless, of course, you meant 'multiplied' by cos(lat).

[/ QUOTE ]

Yep you're correct, sorry.

[ QUOTE ]
What is gained on the swings is lost on the roundabout!

[/ QUOTE ]

Pretty much, but you gain more than you lose. A well designed, high quality panel will minimise the heating, provided you don't bolt it to something which gets hot anyway.

Also in deference to others, I don't decry the pragmatic approach - cover whatever space you've got with the best panels you can afford!

Me? I've got a 20 quid boat jumble panel that I leave in the cockpit when Pippin is on her mooring. This serves to keep the battery charged enough to run the VHF, depth/log, GPS & autohelm for the weekend use we require. We also have the outboard wired for charging, but nothing much happens until it's screaming its nuts off. Without the panel, we had to charge the battery at home occasionally. With the panel - no problem.

YMMV

Andy
 
Hi Wally

My first 2 panels were Sunware 54/1 SRM2 (Shadow Resistant Model). I chose them because of size/power, stainless steel back which allows up to 3% bend and shadow resistance. I had expected to screw them to the coachroof so the bend capability and the ability to walk on them was useful and I liked the shadow resistance because I expected boom/sail shadows. However they are quite expensive in a $/W comparison.

After receiving the panels and trying to find good mounting positions on the coachroof I realised none would really be convenient or practical so decided to mount them on the top of the sprayhood as I did not have the time nor facilities to build an arch over the stern. Size was good, weight a potential problem so I re-inforced the frame a little but shading was still a potential problem. The SRM units are electrically divided into 2 areas, like having 2 panels side by side on the one mounting. This means one half of the panel can be shaded without affecting the second half. This works well and can be readily seen by watching the current (I have battery monitor and the regulator also shows current into the battery) as you gradually shade the panel. As I have one panel each side of the centreline I usually have 1 or 1.5 panels in the sun when sailing and when at anchor/moored I will sometimes move the boom away to ensure the maximum exposure. Some people go to great lengths to change the angle of the panels as the sun moves but my view is that it is a lot of fiddling around which can be best avoided by adding more fixed panel capacity and so capturing more energy when the sun is at it brightest overhead and conforms to the KISS principle.

I see you are in Malta. That is where my boat is now, at MIYY on the hard. We go to it June 6th. I wanted to add more panels when I arrived in Malta last year. I had decided to buy Kyocera 70W panels on the basis of cost/size/output but could not find a source in Malta in the limited time I had before leaving for Sydney. In fact I was surprised to find that there was very little interest from the chandleries in solar. This time I planned to put them on the top of the bimini or on an extension aft of the bimini much like that shown by Steve Cronin. More space and less shading. I purchased 2x Unisolar US64 panels which are longer than the Kyocera but ironically will be simpler to mount because I don’t need to make additional supports if I put them on the bimini. They are slightly lower power but I believe they will be better in part shade.

You are right to question shade performance claims. Sunware supported their claims with figures and a diagram in their catalogue (take a look at their site www.sunware.de/download/Download.aspx?Language=2&Contents=Kataloge) which has useful information even if you don’t choose their products. As for Uni-Solar they claim that their panels are shade resistant because they have bypass diodes across each cell but I was further swayed by findings in a PBO test of solar panels in Oct05 which found their FLX32 was the least affected by shadows of 6 manufacturers. This report also showed that a rope diagonally across a panel reduced the output by as much as 57%. I will know for sure in 3weeks!

On a final note. Watch out for claims of power output (Watts), my view is that the maximum current (I max) is more important because some panels will be rated at 70W and deliver Imax of 3.8A at 18V whilst another may be 70W at 16.5V delivering 4.2A. So the 4.2A panel is better because the regulator reduces the voltage to 13.5 (approx) for battery charging so you get no benefit from the higher voltage. There is the potential for argument on this point if one talks about the voltage out as the sun energy fades but I would still go for the higher current version for the best overall power generation.

The question of the fridge is also BIG. It is very dependent on compressor size and good insulation. Insulation is the key. The compressor should cycle on for only 25-35% of an hour. Then if the comprssor draws 5A (may be less for a smaller fridge) then you will use approx 36Ah per day.

If you will be in Malta from June 6-18, we could compare notes. I’d be happy to show you my system if you have not already committed by then.
Rgds
Ray
 
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