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Forum Index : Solar : Page about system sizing
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I get asked this alot, so I decided to put a page together about sizing the basic components of a off grid system. Its just the fundamentals. http://www.thebackshed.com/windmill/articles/systemdesign.as p I'm pretty sure I got the maths right, but if someone notices anything wrong please let me know. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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norcold Guru  Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Glenn, In your battery calculations assuming you use 10Kwhrs daily, allowing for the night time period of power consumption when all is going out none in it is very important for long battery life to not allow them to go below 70% capacity(60% if you really want to push) With this in mind you would really need a battery bank around 1000 Ahrs or better. My system and I use 5-6KWhrs /24hr with 1080Ahrs battery bank(2340 watts solar bank) usually have the batteries drop to 85% during winter and 80% during summer this is on days when the batteries have been floating for a few hours during the sunny part of the day. The battery percentage figures are taken from the inverters log which monitors all loads and charging inputs. Batteries are the "wearing" part of the system and do not have the lifespan of the solar panels, solar regulator or the inverter, they always require TLC. One cell goes more than likely the lot should be replaced.(unless that cell was faulty and this showed up early in the banks life) Also it is good practice if possible to have no cells in parallel all in series. This info is taken from my perspective and I feel you will find similar from others who have lived with such a system for a period of time. Batteries are the heart, get the capacity low at the beginning and you will find you will be replacing the lot not just adding a few extra cells. Having a system design section on this forum is tops, I hope my input gets further disscusion in this direction as perhaps I am in error here. We come from the land downunder. Vic |
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| Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462 |
- Have to remember this advice. ( Is it possible to couple such a system with > Ultra/Super-capacitors < instead? ) George |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Georgen, I have no experience with Ultra/Super capacitors thus do not know if they could be utilized. However perhaps there is some one else out there who could enlighten us here. I certainly would go with that idea if it were practical. With the cost of battery banks I would not experiment if there was a possibility of damaging the battery bank. Also a ballpark figure of 5 hours sunlight when sizing solar panel wattage needs is I believe more appropiate. Remember the output quoted on cells is the peak output and unless you have tracking(which I believe is not practical in our situations) the cells only achieve the quoted output when the sun is "beaming" down at right angles to the plane of the cells. Also the 5 hour figure is for those at my latitude(sunny NQ). The further south of this latitude you are the lower that figure would be I believe. I am not saying I only get 5 hours of sunshine, just that it averages out to 5 hours (seasonally)of peak sunshine here in NQ. To further explain my position on tracking, we live in an area that can suffer from high winds and the question of the complexity thus reliability of tracking also must be considered. With the cost of solar panels dropping, better to have a few extra cells and have then fixed. For a 10kWhr system I`d go with at least 3500 watts of panels. When last speaking with the fellow who designed and commisioned my system, I mentioned I have some days where my system is floating the batteries from lunch time on, he replied it would be great to have them doing so from breakfast time on. He made that statement considering what would be great for the health of the battery bank but knowing it is not practically possible. This could be achieved with wind turbines providing you lived where the wind "blows" reliably of a night time. Of possible interest my weather station software tells me today I will have 11hrs 23 mins of daylight, 45 secounds less than yesterday. We come from the land downunder. Vic |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Thanks for the feedback. I've made a few changes to the page. Also mentioned Lithium, which can be discharged much more than lead acid. Supercaps cant really be used in these calculations, but they can help with smaller battery banks. A big super cap can supply large currents for short durations, where a battery supplies less current over a long time. The two work together ( as in CSIRO's UltraBattery http://www.csiro.au/Outcomes/Climate/Reducing-GHG/Ultra-Batt ery.aspx ) to give a battery bank with a higher surge rating. Where this can work well is using an inverter on a battery bank thats a little bit on the small side. When the inverter needs to supply a surge, like a motor, the sudden short duration current draw from the small battery bank will cause it's voltage to drop, and the inverter will shut down. Adding supercaps will help overcome this current surge. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| windlight Guru Joined: 03/03/2007 Location: AustraliaPosts: 331 |
Glenn as far as batteries go most manufacturers quote the 100 hour rate, which is not applicable to Re use, what we need to understand is the 10 hour rate simply because nights are not 100 hours long. My 1320 A/h batteries (48V) are realy 930 A/h when used in Re, therefore I have a theoretical capacity of 46 Kwh full to flat or 13.8 KWh to 70% charged. Allan "I like this place and willingly could waste my time in it" - (Act II, Scene IV). |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Thanks Allan Is there a "rule of thumb" conversion between the 10 hour and 100 hour rating, or would someone need to get the datasheet for the battery to be sure. What I'm hoping to do with that page is give the basics for calculating the bare minium, and then make a few recomendations above that for a more versatile system. Might need to fudge the figures a bit and hope it doens't lead anyone up the garden path. I found that formular on a web site ages ago, it might be outdated and maybe there is a better way of working out the minimum battery capacity? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| windlight Guru Joined: 03/03/2007 Location: AustraliaPosts: 331 |
Glenn, I don't know of any particular formula for de-rating battery amp hour, my 70% came from the makers handbook, looking around at others 70-75% seems to be the go. For your purpose I would use 70%, I feel the most important point is awareness of the reduced energy that is available when drawn off in a relative short time. Battery type, construction and age all contribute to that figure, not something one would want to wade too deeply into for your purpose. Allan "I like this place and willingly could waste my time in it" - (Act II, Scene IV). |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
OK added some more detail to that page. I found a 20 hour rating, and looks like it was what I was using for the trojans anyway. The 100 hour rating is 250Ah compared to 225Ah for 20 hours. Good to know. Also found this page, lots of good info. http://www.windsun.com/Batteries/Battery_FAQ.htm Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Battery Cell SG readings for NQ 1.26=100% 1.25=90% 1.24=80% 1.23=70% Start generator charging 1.22=60% Do not go any lower These Specific Gravity readings are for the Exide wet lead-acid and apparently take into consideration our NQ temperatures. They tally approx with the Inverters logging. Frequent SG readings are neccessary to ensure the balancing of the cells amongst other reasons. To ensure the cells are balanced each 2 months it is important to equalise the cells using the inbuilt function in solar regulators. Just a few titbits given to me by the fellow that commissioned my setup. My cells are 1080a/hrs -100hr 685a/hrs -10hr We come from the land downunder. Vic |
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panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1101 |
Hi Guys, Tracking is nice if you have the space, money or time (and money) to build your own however there are times when a tracking system is not possible/pratical. To that end I played around with an Excel spreadsheet that calculates the possible input from panels at different angles on different roof panels. There was a need to spread the solar panel input over the daylight period with peaks at breakfast, lunch, afternoon and evening. By having multiple panels at multiple angles the input can be optimised for the entire day with static panels. The variable parameters in the calculations are all adjustable for your own circumstances as explained in the readme section. Also I have graphed the data to better demonstate the power out of the panel system. Not saying the assumptions on amount of power (efficiency) are fully correct but they should be indicative (anyway, they are a user variable if you want to change them). Hope it is of use to someone. Doug. 2013-01-18_003852_Solar_Array_Configurator.zip ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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| norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
panky, very comprehensive, you have put a lot into the data. I am going to spend a lot of time studying your results. Whilst if I need more output I`ll just go with more panels. My house supply panels are facing due north, but I have 6X80w panels facing NNNW (thats the way I built the pump shed definately not intentional) on a bore pump. I reckoned if they didn`t perform I`d tilt them towards north, they have performed well, with a suspision they perform even better. My reading of your data helps reinforce that suspision, ie a westerly bias from north is advantageous. We come from the land downunder. Vic |
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| panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1101 |
Thanks Vic, My idea was that where you did not have the option of a tracking system, a method of simulating output from a number of panels at different tilt and elevation angles would allow you to determine an optimal power configuration for your specific circumstances. I would be very happy to discuss the mechanics and methodology of the spreadsheet with anyone interested. Cheers, Doug. ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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