Schoolboy Physics Question (Watts/Amps/Volts)

[Angele]

I need to do something - the fridge and lights reduce the two new 110 amp domestics to 12.2 v in 48 hours so I end up having to run the engine. I have tried a 60 watt flexible solar panel on a previous boat, but came to the conclusion that it was fit only for keeping batteries topped up on an unoccupied / switched off boat.

I'm like you. All my lights are still halogen - I really ought to get round to changing at least some of them to LED to reduce consumption - and, with the fridge on (even with a bag of ice in there), I doubt a 60W panel would come close to keeping my batteries supplied.

It sounds like Robin has managed it successfully with a pair of panels totalling 195W - so, using his conversion factor of 1/3rd, producing around 65Ah in southern UK waters in the summer months. That should do it for me, but 195W is a lot of panel. By my reckoning that would need something like 1.25m x 1.25m (or equivalent) stuck to the deck. Hmmm! Do I really want that?

 

[Robin]

I'm like you. All my lights are still halogen - I really ought to get round to changing at least some of them to LED to reduce consumption - and, with the fridge on (even with a bag of ice in there), I doubt a 60W panel would come close to keeping my batteries supplied.

It sounds like Robin has managed it successfully with a pair of panels totalling 195W - so, using his conversion factor of 1/3rd, producing around 65Ah in southern UK waters in the summer months. That should do it for me, but 195W is a lot of panel. By my reckoning that would need something like 1.25m x 1.25m (or equivalent) stuck to the deck. Hmmm! Do I really want that?

OK but our set up was 50% fixed and 50% moveable. Our 75W panel was permanently mounted on a stern gantry. We inherited that with the boat but it had a slight design problem because the wind generator (we had an Aerogen 6) and the radar scanner were on the poles built into the gantry and they could put shadows over the panel. The 120W panel we bought for our previous W33 ketch and it lived in the stern cabin when not needed. We adopted the same principle on the Sun Legende, stowing it on a stern cabin bunk and putting it on deck when at anchor more than say a weekend. It had a long cable which meant it could be put anywhere on deck, but in practice it was always on the coachroof to one side of the boom and maybe moved over as the sun moved during the day. We had 3 battery banks, 2 service(one running high users like the fridge and lights, the other the instruments, main plotter, radar and autopilot) and the third for engine plus windlass, and we had sockets in the cockpit to connect the 120W panel to either of the two service batteries, usually to the fridge one. The fixed panel could be switched over between the 2 main banks or switched off. We had two alternators on the engine with smart regulators on both and a 220V 40A smart shorepower charge as well. With our set up of course we would always start off with fully charged batteries and any periods of motoring would also fill them if the engine was needed for propulsion.

So typically on a weekend away if on a buoy or at anchor, only the 75W panel was in use, I wouldn't bother with the wind generator and if there was no sun we had enough battery capacity anyway. At anchor for longer periods then both panels would be in use and these covered (even exceeded) our daily useage and the wind generator still remained tied off most of the time. The windy only came out to play for prolonged cloudy or rainy periods, with wind but little or no sun.

For weekends BTW because we had a large fridge, therefore only half full or less, we would bring a one litre frozen bottle of tonic with us on board Friday to aid the initial cool down, but often this would still be half ice on Sunday night. We also filled some of the unused space under the 2 large fridge baskets with expanded polystyrene cut to fit and covered with cooking foil, cooling 'air space' costs amps!

 

[TQA]

A 12 volt 90 watt panel will have a nominal open circuit voltage of around 17.1 volts so I am afraid the maximum output will be in the order of 5.25 amps not 7.5 amps.

 

[john_morris_uk]

A 12 volt 90 watt panel will have a nominal open circuit voltage of around 17.1 volts so I am afraid the maximum output will be in the order of 5.25 amps not 7.5 amps.

Uh..no - because open circuit volts are meaningless in this context. Its the actual charging volts achieved and the current that goes with them that counts.

So 14.3 volts might be 6.3 amps - equals 90 watts.

This is only under the most beneficial circumstances though.

 

[VicS]

FWIW the power rating of solar panels is given at the voltage at which the maximum power is delivered. It varies a bit ( around 15 IIRC) but it should be stated in the technical bumph.

 

[TQA]

Uh..no - because open circuit volts are meaningless in this context. Its the actual charging volts achieved and the current that goes with them that counts.

So 14.3 volts might be 6.3 amps - equals 90 watts.

This is only under the most beneficial circumstances though.

I am afraid that this is not the case with most manufacturers. I was mistaken in using the term open circuit but the figure of approx 17 volts for the wattage is correc tEG Kyocera They use 17.4 as the voltage for the wattage calculation and find the peak amps output and multiply that by 17.4 to give the watts.

The spec for one panel

Kyocera Solar KC85TS Solar Panel
General Panel Information: The Kyocera KC85TS is a multicrystalline panel intended for all general use applications. It is suitable for battery charging, RV, Marine, and grid-tie applications.
Basic data at Standard Test Conditions. For detailed information and complete specifications, see the manufacturers specification sheet.
Rated Power 87.3 Watts
Peak Power Voltage (Vmpp) 17.4 Volts
Peak Power Amps (Impp) 5.02 Amps
Open Circuit Voltage (Voc) 21.7 Volts
Short Circuit Amps (Isc) 5.34 Amps
Power Tolerance (percent) +10%, -5%
Manufacturers Warranty 20 Years
Dimensions (inches) 25.7" x 39.6"
Weight (pounds) 24 pounds

Peak amps 5.02 .for charging purposes with this 85 watt nominal panel.

My setup is three 130 watt panels giving 390 total. The most I ever see is 22 amps with the sun directly overhead on a clear day in the Caribbean. This also gives approx 17 volts working backwards.

 

[whipper_snapper]

Energy cannot be created or destroyed it just changes form. If the photo-electric process cannot dispatch energy in the form of electrical current where does the energy go?

Imagine a water tank 10' above ground.

Water flows from it, through a pipe, and generates electrickery by turning a water wheel at ground level which is attached to a dynamo. We have power generation.

Now close a tap on the pipe stopping the flow.

What happens to the power that was coming out of the dynamo??! Where does it all go ?!

Obviously nowhere, it remains as potential energy sitting in the water tank. If you measure the pressure at the bottom of the pipe, you can see a pressure which is equivalent to a solar panel open circuit voltage.

Not perfect, but a pretty close analogy I think.

 

[jonjo]

Not perfect, but a pretty close analogy I think.

No I think you are confusing potential energy v. energy. Your elevated tank is a good analogy for a battery but not a solar panel. A photo/electric cell continues to produce energy in the form of local circuits within a cell even when the output from the panel is cut, so the local currents within the cell turn to heat. Or that is my interpretation of the main techno post above.

 

[pteron]

Now close a tap on the pipe stopping the flow.

The only issue with your otherwise fine analagy is that with a solar panel 'closing the tap' is equivalent to covering it up.

If you don't cover it, then the sunlight generated electron/hole pairs are still created, and as I said earlier they then cause an internal current to flow in the photo diode.

 

[Andrew_Fanner]

I'd love to fit these things in sensible quantities, I really would. They would satisfy my basic "its a sci-fi gadget" urge, keep the batteries happy on a trot mooring and all the associated goodness.

BUT, they remain flippin' expensive for a decent output and in the UK you deon't get a decent output that often without adding yet more panels.

As an aside, and being no more electrically minded than most, how would you charge a 24 volt battery system? I assume you are forced immediately to twice the number of panels than someone on an equivalent 12 volt system?

 

[pteron]

BUT, they remain flippin' expensive for a decent output and in the UK you deon't get a decent output that often without adding yet more panels.

They are getting cheaper all the time - so much so that with the current [!] feed in tarrifs it looks like you can make money after 5 years if you put them on your house.

As an aside, and being no more electrically minded than most, how would you charge a 24 volt battery system? I assume you are forced immediately to twice the number of panels than someone on an equivalent 12 volt system?

You can either stack them or use a controller called an MPPT controller which runs the panels at their optimum voltage/current and outputs whatever voltage you need for charging the baterries. Personally, I'd go for one of these as it will get about 25% more power out of the panels.

 

[Angele]

I'd love to fit these things in sensible quantities, I really would. They would satisfy my basic "its a sci-fi gadget" urge, keep the batteries happy on a trot mooring and all the associated goodness.

BUT, they remain flippin' expensive for a decent output and in the UK you deon't get a decent output that often without adding yet more panels.

As an aside, and being no more electrically minded than most, how would you charge a 24 volt battery system? I assume you are forced immediately to twice the number of panels than someone on an equivalent 12 volt system?

Me too but, from the website link in one of the early posts, it looks like a 70W panel costs about £450. I would need a least two of these to get an acceptable 45Ah/day in summer - half this would not be enough. So, including regulator and wiring etc, that is more than a grand to become energy self sufficient. I'm afraid I'll continue to take the environmentally unfriendly and noisy option of running the engine to recharge the batteries. It'll be a awful lot cheaper.

... and therein lies the problem. This is a micro example of the reason why the planet continues to burn fossil fuel when environmentally friendly alternatives exist. It is simply much cheaper to do so even at today's diesel prices.

For a 24V system I understand you simply connect two panels in series (although not all panels will allow you to do this). Yes, you will produce half the Amp Hours of a 12V system, but doesn't a 10W lightbulb at 24V consume half the current of a 10W bulb at 12V?

....Edit: or, as Pteron says, use an MPPT regulator: here

 

[Angele]

They are getting cheaper all the time - so much so that with the current [!] feed in tarrifs it looks like you can make money after 5 years if you put them on your house.

I agree that, with the "feed-in tariffs" introduced earlier this year by the last Government, it starts to make financial sense to fit panels to your home. You are paid to generate electricity even if you use it yourself.

However, until they pay an equivalent subsidy for energy you generate on your boat, the economics of using solar panels on board looks pretty poor value.

Your 5 year break-even assumption on home generation does look a bit optimistic given that grants that used to be available to subsidise the cost of installation are no longer available (effectively they have been replaced by feed-in tariffs). I did some fag packet calculations earlier this year:

Installation cost for a 3 kW (240V) peak supply system is around £15,000;
A well sited 3kW installation should be capable of producing a little under 3MWh over the course of a year;
The “Feed-in Tariff”, which is payable even if you use your own electricity, is 41.3p per unit for the first two years;
Adding to that the unit price for the electricity that you no longer need to purchase takes you to about 50p per unit;
For 3MWh that works out at about £1,500;
So, a 10% initial running yield (or a 10 year break even);
In year three the tariff drops in real terms, but it appears to be subject to indexation.

In short, it pays for itself, but very slowly, and you are reliant on them lasting at least 15 years to get a barely respectable return (i.e. more than if you had left the £15k in the bank). I understand you should be able to get a 25 year warranty, but you are taking a risk that the company giving the warranty goes bust before you make a claim.

 

[pteron]

I used the data on the midsummer site. http://midsummerenergy.co.uk/grid-connect-solar/solar-feed-in-tariff-calculator.html#nogo

and clicked on the middle sized installation. It shows profit in year 6.

 

[savageseadog]

The only issue with your otherwise fine analagy is that with a solar panel 'closing the tap' is equivalent to covering it up.

If you don't cover it, then the sunlight generated electron/hole pairs are still created, and as I said earlier they then cause an internal current to flow in the photo diode.

I'm not sure if "flow" is the correct word. The electrons and/or holes will be generated but recombine in the lattice. If 50W of energy could be generated with a certain level of sunshine over the area of panel then the holes and electrons will recombine but the potential difference will be much lower therefore the I x V power calculation will be much lower than if the panel were connected. I would think it more probable that a good proportion of light energy will simply reflect or be absorbed as heat energy in the panel. Time to break out Schroedinger's wave equation.

 

[Andrew_Fanner]

I am afraid that where home installation and turning a profit are concerned I see the concept as snake oil pure and simple. What's more, £15k is more than almost everyone can casually lay aside in the hope that it will create a return in several years time. I am also very wary of any investment where the payback comes after the next General Election and the chance of restrospective legislation.

All of which is a pity because coating the roof in these things is basically a good idea in principle.

 

[whipper_snapper]

The only issue with your otherwise fine analagy is that with a solar panel 'closing the tap' is equivalent to covering it up.

If you don't cover it, then the sunlight generated electron/hole pairs are still created, and as I said earlier they then cause an internal current to flow in the photo diode.

OK, you could view it that way! That is the trouble with analogies!

I thought of closing the tap as breaking the circuit, not covering the panel.

The light is still pushing those electrons but they have nowhere to go so no current flows through the wire.

The water still has potential energy and thus pressure in the pipe by virtue of its height and the panel still has an open voltage by virtue of the sunlight.

 

[pteron]

I'm not sure if "flow" is the correct word. The electrons and/or holes will be generated but recombine in the lattice. If 50W of energy could be generated with a certain level of sunshine over the area of panel then the holes and electrons will recombine but the potential difference will be much lower therefore the I x V power calculation will be much lower than if the panel were connected. I would think it more probable that a good proportion of light energy will simply reflect or be absorbed as heat energy in the panel. Time to break out Schroedinger's wave equation.

Not really. The electron/hole pairs will be generated at a rate that depends only on the amount of light falling on the photodiodes. Whether you have a circuit connected or not is irrelevent.

The potential difference is higher with no connection. When you connect a circuit, some of the free electrons are passed out of the n type material and around that circuit. This removal of charge from the diode causes the voltage across it to drop which causes a reduction in the diffusion current in the diode.

And no cats.

 

[snowleopard]

Dragging the discussion back to ground level for a moment, is anyone seeing a drop in PV panel prices as a result of the feed-in tariff initiative?

 

[Angele]

I used the data on the midsummer site. http://midsummerenergy.co.uk/grid-connect-solar/solar-feed-in-tariff-calculator.html#nogo

and clicked on the middle sized installation. It shows profit in year 6.

Hmm!

The middle sized installation link appears not to be working at the mo, so I tried the small one. The assumptions it uses are similar to those in my previous post (e.g. cost of installation per kWh of output, number of units generated over a year per kW, feed-in tariff, etc) and yet it comes up with a much more favourable result.

The results are not displaying properly (in that things are not appearing in the correct column) but I think that what they are saying is that (for the small installation) immediately after it is installed you have paid £8,404 but it is only worth £3,753. So you have on day one, in effect, pi$$ed £4,651 down the drain (being the installation cost and 25% of the cost of the panels and inverter).

After a year things are not quite so bad. You have earned 12 months worth of income (around £660), so you are now only £3,990 out of pocket.

Another 12 months on and you have earned another £660, but the value of the equipment has fallen by another £150. so, you have made a "profit" in year of £510, bring your cumulative loss to £3,480 and so on.

All these calculations assume that at the end of a given year you sell up and realise a proportion of your original investment. (How you would ever be able to demonstrate that is questionable, but I'll give them the benefit of the doubt). so, what this is saying is that, after 9 years (in the case of the small installation) you will break even if at that point you sell your house and get back some of your original investment.

Or, put another way, if you sell your house before year 9, you will not get back what you paid for the equipment originally, let alone anything for lost income in the meantime. But the purchaser of your property should be quids in.

Is it sounding so good now?

This is a long term "investment" and really only works well if:

a) You think you will stay put for several years; and
b) You don't have a better idea for investing your cash and its just going to be stashed under the matress..

Also, as pointed out by Andrew Fanner, few people have £15k lying around to spend. If you have to borrow the money, and pay interest, then the numbers look even worse.

Hey ho!