Maplins PV clearance

Post by **adam2** » 26 Jun 2008, 13:22

rs wrote:what are people's thoughts on this page describing how to calculate solar requirements?

http://www.greenweld.co.uk/data/How%20t ... ements.pdf

Nicely explained in the figures are accurate?

No blatant errors, but I feel that they are a bit optimistic as to the power produced, especialy they presume that the PV module is correctly aligned, whereas in practice it wont be, or not for long.

The correction figure of 0.85 is certainly a bit optimistic!

For a small emergency lighting scheme though, one installs what can be afforded, and then observes the charge controller to see to what extent the lights can be used.

mobbsey · Post by **mobbsey** » 26 Jun 2008, 22:18

adam2 wrote:
rs wrote:http://www.greenweld.co.uk/data/How%20t ... ements.pdf
Nicely explained in the figures are accurate?
No blatant errors, but I feel that they are a bit optimistic as to the power produced

There's absolutely fatal errors!

Assuming we're talking about deep cycle lead acid batteries if you try and take the whole 17 amp-hour capacity you'll damage the battery. Do it a dozen times and you'll knacker the plates! For deep cycle you never take more than 50% of the battery capacity; for sealed lead acid or gel cells it's 40% to 30%. For car batteries you don't want to take more than 10% or it warps the plates. The only thing you can nearly fully discharge without damage are NiMH and Li-ion batteries, but they're rarely used for PV because of the cost.

I agree the system losses are rather low -- they don't take into account the charge/discharge cycle losses for the battery, which for lead-acid is around 30% to 40% (more if you regularly over-discharge!) -- temperature is an important factor here, because you can pack in more power if the battery is warmer (but it does reduce the operating life -- it's a trade-off, especially in hot countries). The system voltage also has an important effect on losses; if you run at 24 volts or 48 volts instead of 12 volts you considerably reduce the losses through wires, and especially in the charge controller.

The other fluff is the charging rate. This assume the standard insolation value for PV of 1kWh/m2. Take that panel to the Alps and you get 8 times that (because there's less atmosphere at that height); take it to the equator and you get 12 times that (because there's more sunlight). In the UK the average is about 1.25kWh/m2, but that varies from about 0.5 in N. Scotland to 2 in Cornwall.

rs · Post by rs » 27 Jun 2008, 08:41

I have 9 garden lights at 3w apiece. How would you guys calculate what size panel and battery I would need?

The more I look for this on the net the more conflicting information I read so I would much rather hear what folks in the UK who are actually doing it have to say!

Post by **adam2** » 27 Jun 2008, 09:43

I would proceed as follows

Total load is 27 watts (presuming that the rating on the lamps is accurate)

Presuming a 12 volt system, calculate the load in amps, 27 divided by 12 is about 2.3 amps.

How many hours a night are the lamps to be lit for? If all night, then that is about 17 hours in mid winter.

Now calculate the daily ampere/hours required. Mulitply the 17 hours of use by the 2.3 amps, this gives about 40 amp/hours.

Now decide on the number of days backup required, in case the sun does not shine, this is up to the users discretion, perhaps five days might be reasonable?
Now mulitply the daily amp/hour use (40) by the days backup required (5) this gives the ampere hour capacity of the battery, 200 amp/hours*

We now need to size the PV array. Since we have already established that the daily A/H use is 40, we need an array to produce 110% of that per day. The extra 10% is to allow for the fact that of the A/H put in to the battery, about 10% is lost.
We now need to estimate how many hours of "peak sun equivalent" will occur in mid winter, this varies from about one hour to about two hours.

If we assume, perhaps pessimisticly, one hour of peak sun a day, then we need an array able to produce 44 amps.
Since in actual use, PV modules put about one amp into a 12 volt battery for every 20 watts of rated power, this means that we need an array of about 880 watts peak! very expensive!

If we assume, perhaps more optimisticly, two hours of peak sun a day, then the array size can be halved to 440 watts, still quite an investment.

Note that the system has been sized for worse case winter conditions, it would be very over sized for allmost all of the year.
A very much smaller array would suffice for most of the year, in practice an array of perhaps 200 watts could be used, together with charging the battery from grid power, a generator, or a vehicle in the darkest winter months.
It is of course possible to calculate in more detail, but may not be worth it in view of the random changes in the weather etc.

* This assumes 100% discharge which is generally considered unwise, but may be acceptable in this case since the deep discharge should only occur a few times a year, not regularly.

Mitch · Post by **Mitch** » 27 Jun 2008, 10:22

Thanks Adam2 - I can work out more accurately what I can expect! You just confirmed my earlier post regarding cost - 5 x 180 watt panels, ?3,550.00, plus a 200 A/h batt, reg. etc. Total cost of around 4 grand for 9 little 3 watt lights!?!?!

Sure, that is for "worst case" weather wise, and "best case" for ensuring your lights nearly never fail. But is the "reasonable" calc., if you are attempting to get as close to grid supply "convenience" as you can. Mighty, mighty expensive!

Problem is that the sheeple don't have a clue - and are being told daily in MSM that "for around ?8000.00 you can power your 4 bed house, AND sell the excess back to the grid"! (last night on Beeb News 24). If they knew the truth, am sure there would be far less resistance to wind-farms etc.

Mitch · Post by **Mitch** » 27 Jun 2008, 10:34

Quick question Adam, is there any loose relation between depth of discharge and voltage,(lead -acid)? My inverter kicks out at 10 volts, (12v system), and I have about a volt drop over the cabling/connections when hauling 3kw, (about 300 amps I would guess). Since I haven't got the panels yet, and re-charge when I find a mains source, I regularly run the batt's down till the inverter kicks out - down to about 11v accross the batt's. They then come back up to about 11.8/12v and I just leave them with no load till the next recharge. Could this be classed as 100% discharge? 80%? what's your view? (I DO know that I am killing them though).

Post by **adam2** » 27 Jun 2008, 10:50

Mitch wrote:Thanks Adam2 - I can work out more accurately what I can expect! You just confirmed my earlier post regarding cost - 5 x 180 watt panels, ?3,550.00, plus a 200 A/h batt, reg. etc. Total cost of around 4 grand for 9 little 3 watt lights!?!?!

Sure, that is for "worst case" weather wise, and "best case" for ensuring your lights nearly never fail. But is the "reasonable" calc., if you are attempting to get as close to grid supply "convenience" as you can. Mighty, mighty expensive!

Problem is that the sheeple don't have a clue - and are being told daily in MSM that "for around ?8000.00 you can power your 4 bed house, AND sell the excess back to the grid"! (last night on Beeb News 24). If they knew the truth, am sure there would be far less resistance to wind-farms etc.

?8,000 spent on a grid tied PV system could well power an economical house, I suspect that they refer to a system costing ?16,000 but with a 50% grant.

Remember that with a grid tied system every watt produced (apart from small losses in the invertor) is either used in the home or exported to the grid. No battery losses, and the PV modules will allways work at the most efficient voltage.

A system sized for outdoor lighting such as the one disscussed is about the least efficient you can get, though still justified in some cases.
Remember that the array has to be sized for very little sunlight in winter, which is just when the demand is greatest.
In middsummer, with only about 6 hours of full darkness, and perhaps 7 hours of full sunlight, most of the power is wasted.

On strictly economic grounds, grid tied is the way to go. BUT its no good if you fear grid failure, or if no grid connection is available.

Mitch · Post by **Mitch** » 27 Jun 2008, 11:03

Ah, yes - and all the BIG energy stuff is gas or oil fired, makes more sense now, if you only consider the 10-15% energy consumption derived from electricity. Being P.O. aware - I keep considering "total" energy consumption/provision - hence my complaints of "mis-leading advertising".

rs · Post by rs » 27 Jun 2008, 11:05

Thanks for that Adam, it just goes to show it's not just a case of chucking a couple of panels on the roof and expecting to put Sellafield out of business

Mitch wrote:Problem is that the sheeple don't have a clue - and are being told daily in MSM that "for around ?8000.00 you can power your 4 bed house, AND sell the excess back to the grid"! (last night on Beeb News 24). If they knew the truth, am sure there would be far less resistance to wind-farms etc.

It's worrying isn't it. My expectations are a lot lower, just to be able to provide some basic needs in a powerdown scenario, but even so, it could be a problem keeping batteries in good health or even getting hold of new ones.

Post by **adam2** » 27 Jun 2008, 11:19

Lead acid batteries are quite low tech, and if required could be localy manufactured in a lower energy world.

Lead acid batteries were in very general use a hundred years ago, before the oil age, and are not going to be un-invented
(yes I know that oil had been discovered then, but it was not yet a major energy source)

A very crude lead acid battery can be home made, and would be far more durable than modern ones, though very, very bulky and heavy for the capacity.
Should last at least 50 years though!

Mitch · Post by **Mitch** » 27 Jun 2008, 11:39

Very interesting Adam2 - the electrodes should be fairly simple, wouldn't even neccesarily have to be lead, but what could one use as acid? Fruit juice? Would it work straight off, or would one have to distill/refine it in some way for it to be useable? I definitely feel that skills in creating home-made batteries would be very sdvantageous post P.O.

RenewableCandy · Post by **RenewableCandy** » 27 Jun 2008, 12:55

Looks as if people have been making batteries of one sort or another for quite some time!

Vortex · Post by **Vortex** » 27 Jun 2008, 13:51

Just ordered a few dinky PVs and one 60-watt from Maplins.

(Their stocks are dropping fast, so if you want a 60 W unit be fairly prompt)

rs · Post by rs » 29 Jun 2008, 21:31

Been using my 12w panels for a week or so now. One of them is exhibiting some strange behaviour though.

At the moment I have 3 hooked up each to their own controller and 7ah battery. 2 seem to work quite well, generating from early in the morning right up to dusk. Voltage across the panel seems to be around 12-14v. However the third panel runs the same for a little while then jumps up to 24-25v and the controller cuts out.

Any ideas?

Mitch · Post by **Mitch** » 01 Jul 2008, 17:38

rs wrote:Been using my 12w panels for a week or so now. One of them is exhibiting some strange behaviour though.

At the moment I have 3 hooked up each to their own controller and 7ah battery. 2 seem to work quite well, generating from early in the morning right up to dusk. Voltage across the panel seems to be around 12-14v. However the third panel runs the same for a little while then jumps up to 24-25v and the controller cuts out.

Any ideas?

Check the battery voltage - sounds like the panel is fine, maybe the regulator is switching off due to the battery being fully charged? Could also be a minor fault with the regulator, or dodgy wiring between the reg. and batt? Adam2 should be able to help us out a bit more here