Solar power - length of solar day - does this make sense

[roland stockham]

Hi all
Finalizing selection of gear for solar installation, does this make sense. It seams that the length of day that the panels produce useful charge for is going to be more important that total energy produced or max output (for me). Seems I need a 6-8hr 'solar day'. OK I have set out what I am thinking below but would also be interested in hearing how many hrs per day you get usable output?

1. My main aim for solar generation is to ensure batteries are fully charged each day rather than to run only on solar power

2. At 95-100% the charge acceptance of my 380a/h (FLA) house bank is about 15-20a (charge acceptance <5% capacity)

3. To get the last 20% of charge (76a/hr) into the battery will take 3-4hr at this rate

4. There will be plenty of power near mid day so assume the batteries are up to 80% by midday

5. I therefore need a setup that will still be producing 15a of charge (plus consumers) 4hr after the peak sun.

6. At 4hr past peak sun a horizontal panel is going to be down to about 50% of its peak output (peak for the day not 'rated' output)

7. Assuming peak for the day is about 90 of rated output (At 0-20deg lat & allowing for losses) this means I need a panel that produces at least 15a @ 40% which would be 37a peak or 37x14.8 = 555w rated output to ensure batteries are fully charged on a reasonable sun day.

[MarkJ]

A bit over my head... but yea, you get plenty in the middle of the day.

2 of my panels swing down pointing aft. Where I am with constant E winds my 2 tilting panels catch the afternoon sun well.

I used to have a small panel forward of the mast, leaning against the mast at 45 degrees to pick up early sun.

So if your array can be tilted you get a great benefit.

Mark

[onestepcsy37]

Can't answer all your questions but we found that the best way to increase solar output was to be able to tilt the panels towards the sun. When cruising you will find you spent most of your time anchored in one place. Being able to reposition the panels two or three times a day will substantially increase the daily amp production.

You can verify this for yourself. Measure amps output while moving a panel around - directly at the sun, slightly angled away, greatly angled away... The difference in output is considerable.

We were very light users on a 37' foot monohull. One 135W Kyocera produced half our daily needs - enough to run the fridge. One more similar panel would have kept everything else running.

[roland stockham]

Good point about tilting panels and easily arranged, will do that.

[dlymn]

Based on home solar, we average 5 hours full rating generation over the entire day. That's at Lat 39. Any shading significantly reduces output. Max output only on clear days even if it's not hot.Clouds inhibit output. The panels are static and so we get better energy output than a panel rocking around. Signficant reduction in energy over winter.
For say each 100w panel I reckon the best you might get in ideal conditions is 500 w/hrs per day. Depending on latitude, I would halve that for calculation purposes. So expect 250 w/hours per day per 100w of panel. I think the worst when estimating.

[StuM]

Assuming you can produce enough to charge to 80% by local noon, You won't need to be producing 15 Amps 4 hours later. Look at the actual current curve during absorption. By that stage, you will be down to accepting a lot less than 15 Amps.

[roland stockham]

Originally Posted by StuM

Assuming you can produce enough to charge to 80% by local noon, You won't need to be producing 15 Amps 4 hours later. Look at the actual current curve during absorption. By that stage, you will be down to accepting a lot less than 15 Amps.

Even at 100% charge a 380a/hr FLA will accept 10-15a @ 14.8v, yes at float charge level 13.2-13.4v it will be under 5a but that means effectively charging has stopped (no chemical conversion in the plates). In fact most references I have seen say that charging is complete when the current at 14.8v drops to 5% of the a/hr rating of the batt (19a for 380a/hr). In practice I find this a bit optimistic and that the specific gravity of the cell will continue to rise until the charge rate is down to about 3%. which is 11.4a for the 380a/hr bank. I periodically check batteries after a week of conditioning on a shore power charger and use that as the 'full charge' point. At 10 years old on the last test the bank still returned to 95% of there new S.G. point for each cell and the bank stored 90%+ of new capacity.

Ref
"The switch from Stage 1 to 2 (in a three step charger) occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate; full charge is reached when the current decreases to 3 to 5 percent of the Ah rating. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to initialize charge termination.

The correct setting of the charge voltage limit is critical and ranges from 2.30V to 2.45V per cell"

http://batteryuniversity.com/learn/a...d_acid_battery

[Rapanui]

The problem is that it's impossible to accurately determine the solar charge you will get on any particular day, the calculation is not deterministic but statistical. Even when you use the most accurate solar irradiance values available, these are just averages which you should achieve over a very long term, with very large variations either way on a daily and hourly basis. If you design to these figures then there is a 50% chance that they will be achieved and 50% they will not.
Not so much of an issue if you are in a very stable sunny climate, but in the temperate zones, there are large daily variations in solar output due to weather. All you can do to improve the odds is to increase the solar generation over the calculated mean value by tilting, more panels etc and/or increase the battery capacity to cover those days below average. However, increasing the battery capacity alone will still mean that they will be undercharged for a siginificant portion of their life, with associated long term battery damage.
My guess is, that if you are a livaboard relying on solar you may need to double up the solar generation capability over the calculated value, particularly in temperate climates.

[motion30]

If you are at anchor, trying to keep up with tilting your panels into the Sun is impractical

[Maine Sail]

Originally Posted by roland stockham

1. My main aim for solar generation is to ensure batteries are fully charged each day rather than to run only on solar power

Depending upon DOD it is possible to get to 100% SOC in one solar day but usually more like 95% SOC day one then to 100% day two..... As batteries age & sulfate the time it takes to get to a true 100% SOC takes longer and two

Originally Posted by roland stockham

2. At 95-100% the charge acceptance of my 380a/h (FLA) house bank is about 15-20a (charge acceptance <5% capacity)

A fully charged 380Ah bank when at 100% will be accepting under 3.8A, if healthy. Battery monitor companies often use 2% but this is not a true full 100% SOC if using the return amps at absorption method. Heck AGM makers insist on 0.3% to 0.5% as 100% SOC. Flooded makers who's batteries charge slower sometimes use 1-2%...

Below is a 400Ah bank after a number of days of float and the voltage returned to absorption.. A 400Ah accepting 0.1A at 14.4V. This is what a healthy bank looks like after floating for a number of days.....

Originally Posted by roland stockham

3. To get the last 20% of charge (76a/hr) into the battery will take 3-4hr at this rate

If you can get the last 20% into the battery in 3-4 hours you'd be doing as good as optimal lab conditions. The last 5% takes the longest...

Originally Posted by roland stockham

4. There will be plenty of power near mid day so assume the batteries are up to 80% by midday

If you really want to get the batteries to 100% SOC in a day it will likely mean fossil fuel to get through bulk then let solar do the rest. This way you have all day to top off the bank.

Originally Posted by roland stockham

5. I therefore need a setup that will still be producing 15a of charge (plus consumers) 4hr after the peak sun.

Again if your 380 Ah bank is still taking 15A in above 95% SOC they are already getting sulfated or your method of counting Ah's and SOC are off...

Originally Posted by roland stockham

6. At 4hr past peak sun a horizontal panel is going to be down to about 50% of its peak output (peak for the day not 'rated' output)

7. Assuming peak for the day is about 90 of rated output (At 0-20deg lat & allowing for losses) this means I need a panel that produces at least 15a @ 40% which would be 37a peak or 37x14.8 = 555w rated output to ensure batteries are fully charged on a reasonable sun day.

If you get through bulk early in the day you may be able to get to 100% or darn close to 100% SOC by the end of the day where the batteries should be taking well under 3-5A.... The biggest hindrance I see on cruising boats is controllers that prematurely float. You can often set up a custom program that holds absorption all day by setting float to .1V lower than absorption.. Use this program when cruising and while actively on-board and revert to an absorption>float profile when leaving the boat unattended.

One day is usually not long enough to get to a true 100% SOC with lead acid batteries, but there are tricks you can use to help this along...

[Bulawayo]

Yonks ago we made our array tilt - however, after about a week of reorientating the panels several times a day it all wore a bit skinny. You also need to be paying attention and if in tidal waters might have to reorientate yet again. You also have to reorientate as the sun goes down ready for the morning and also appreciate that the boat may also lay differently in the morning. The early morning and late afternoon benefits are not that significant and if reliant on those few amps I believe you are better investing the money for the tilting frame into an additional panel (if you have the space) and having everything flat.
It might all be very well on a holiday boat but its a bit of a pain on a cruising boat and gives varying outputs.

Originally Posted by onestepcsy37

Can't answer all your questions but we found that the best way to increase solar output was to be able to tilt the panels towards the sun. When cruising you will find you spent most of your time anchored in one place. Being able to reposition the panels two or three times a day will substantially increase the daily amp production.

You can verify this for yourself. Measure amps output while moving a panel around - directly at the sun, slightly angled away, greatly angled away... The difference in output is considerable.

We were very light users on a 37' foot monohull. One 135W Kyocera produced half our daily needs - enough to run the fridge. One more similar panel would have kept everything else running.

[Utahsailor]

Our experience:

840 AHr Kyocera panels on hard Bimini, dual Morningstar controllers, 1100 AHr T105 battery bank, 310 A dual Balmar alternators and Balmar battery monitor.

After installation of battery bank last winter and mid May launch, I was delighted to see batteries at 100% every clear day. (One cool day at noon, over 90 amps were flowing into the battery bank.)

This January in Fernandina Beach FL, I was disappointed in how little charge was provided to the batteries from the solar panels.

The difference: Launched near the longest day of the year, current in Penobscot Bay happened to align the boat on our mooring so the panels had unobstructed sun exposure during all tide cycles and June was a very sunny month. This winter, shortest day of the year, current at Florida mooring obscures panels by mast & rigging a large part of each day and cloudy winter weather.

We have been content with a daily charge to 85-90% (running the engine until the charging current falls below 50 A early in the morning, then depending on the panels) and a trip to shore power weekly for a full overnight charge and equalization fairly often.

[Joel E Felt]

Jesus I'm getting scared, im starting to understand some of this stuff. My little "etch-a-sketch" size panel works for the 70 a/hr bank. Small time I know, but it's all in the tilt.
Joel E. Felt

[onestepcsy37]

What Utahsailor says. Don't obsess over 100% charges, they aren't necessary.

For us, we keep our power requirements low and then put up all the solar and batteries we have room for. Biggest draw is the fridge, maybe 40 amps a day. 40 more amps will run everything else. Our six golf cart batteries have 675 amp hours, so maybe 320 is usable, enough for four days of no charging.

Two Kyocera 135's will return that on a good day, especially because we tilt the panels. Once early AM, once maybe 10 am, once maybe 3 pm.

Occasionally the batteries will go under 60% when there's been days of little sun (not often in Florida/Bahamas) and then we use the Honda 1000. Hooked up to a smart 35amp charger it brings them back up to 90% or better for maybe a quart or so of gasoline.

So, install all the panels and batteries you guess you need, use heavy wiring to cut down on line loss, a good controller, and cut back on your electrical needs. We have an inverter but rarely use it - only to run the mini shop vac. Otherwise it's shut down.

After a year or so of real life experience you can rethink your system.

[barnakiel]

Many places it is btw 4 and 6 full solar hours. There are tables and chartlets online.

The problem with most solar installations is you must size them midway. They are often 'not enough' in the morning and an overkill by late afternoon. And so you will look for an alternative load (say a watermaker).

New style batteries (lithium and the family) minimize the morning blues (you simply place less batt capacity per same solar juice factory) BUT you still want some sort of late hours alternative. Again, watermaker, if offshore.

Then again there will be huge day to day variations: say from 15% to 95% of the rated ability. And if there are shady 3 days in a row, you end up starting up the donkey.

Etc.