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Capicators for spikes in a solar pv system

 
pollinator
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I am studying about pv systems now and how to plan for them.  One of the annoying aspects of some of our life conveniences is the spike that comes with starting the electric motors that drive them.  I am thinking especially of a/c, well pumps and washing machines now but I am sure there are other appliances that fit this bill.  Designing a system to cover spikes is more expensive - more panels, more batteries and possibly a larger management system if you are almost borderline.  Would it be simple to put together a capacitative storage system for each appliance to store the electricity needed to start it up?  I have thought about running the a/c, well pump and wm off of its own battery - a battery is a kind of capacitor, but capacitors are low tech and might be cheaper, if possible.  Any ideas?
 
pollinator
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The short and kind of misleading answer is yes, capacitors can be and are used for handling startup surge on beefy motors.

However...

In most cases, people with the sort of loads you are talking about, will be running alternating current loads.

In this case, the limiting factor tends to be the inverter providing said AC, not the batteries or panels.

You need batteries that can handle the current thst the inverter demands, but batteries beefy enough to run the intended load for even a modest amount of time are generally not the bottleneck.

And, you need enough solar or other inputs to keep up with whatever amount of power you use. But, quadrupling your solar panel count won't let the inverter handle a motor with even a slightly larger inrush current..

More panels would, in sunny times, help the *batteries*, if they are marginal for output current. Ie, if you have 100A coming in to a bank that can only output 100A, while this is going on you could output 200A. But, the output current shouldn't be the limiting factor for the battery bank, usually.


There are systems that run all DC loads, but this is much less common, especially when dealing with high wattage items like those listed.

There are also systems that go a step further and omit batteries, but these are much less common still for quite a number of reasons, including such inconveniences as 'nighttime' and 'cloudy days'.


As far as I can see, in all cases it is in theory possible to make it easier to start a load by adding (a) suitable capacitor/s. But, for the most common application involving an inverter, I would speculate that it is likely more practical to buy a more powerful inverter than a bunch of beefy capacitors..
 
pollinator
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AC Loads
If you are running AC loads then the inverters have capacitors built into their circuitry normally with a 2x or more instantaneous output so a 5kw inverter will output 10kw.
Lets say you have a 5kw AC load and the inverter has built in capacitors to do 10kw, adding capacitors to the battery/DC side will not push it to 15kW.
Capacitors will not help with continuous load. 5kw inverter/load needs at least a 5kw battery, actually I recommend 5x that so a 25kw battery = 24V x 1000AH or 48V x 500AH

Lithium battery vs Lead
Lead-Acid batteries like 1/5 or 0.2C discharge rate, but Lithium will give 1C or 5times as much and for instantaneous/startup power it will do 10C aka 50times for lead acid batteries.

Direct DC load from Lead-Acid battery
A lead acid battery like a 1/5 discharge rate, aka a 2400WH battery (24v 100AH) only like to put out 480W (24v & 20AH).
So if 480W motor surges and request 1000W the 2400WH battery can still supply it.
Running a lead-acid battery at 1C continuously is not recommended. So I wouldn't get a 480WH battery for a 480W load.


 
 
S Bengi
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It sounds like you are designing a offgrid system.

Energy Consumption
How much power do you see your system using per day (24hrs)
The avg ongrid household uses 30KWH per day. Lets say you are using 1/5 of that so only 6KWH per day.

Charge Controller
$750

Solar Array
2KW solar array for 8KWH peak production in a lab setting, but 6KWH/day is more realistic, infact 3KW solar array is what I would recommend.
Cost = $0.75/watt

Battery
Battery at least 6kWH but a 3day cushion would be nice so 18KWH.
Lead acid battery only do 50% dischage so 6KWH=12KWH and 18KWH=36KWH
Lithium battery do 80% discharge so 6KWH=7.5KWH and 18KWH=22.5KWH
Cost = $0.50/watt for Lithium and $0.10/watt for lead-acid

Inverter
4.4KW surge to 9KW
You will trip the fuse if you go over this 4.4KW, so lets see what you could turn on concurrently
Laundry Room 1.2KW (iron/washer)
Kitchen 1.2KW (blender/mixer/toaster/microwave/etc)
Garage 1.2KW (sander/drill/saw/etc)
Rest of the House 1.2KW (100lightbulb-LED/20Laptop/6TV/etc)
I think your fuse would blow a few times but then you would figure out what the system can handle concurrently.
Cost = $0.50/watt

You could probably downgrade your entire system by 50%. But your housemates must really love the outdoors*
 
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