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Some background on my knowledge of solar power systems:
I have worked in the telecommunications industry for ~38 years and managed solar powered installations providing 24/7 power for telecommunications systems, mobile radio installations and 240VAC for test equipment in remote locations.
Our home has a 1500Wp stand alone power system, using a 48VDC 550Amp Hour battery. This system provides >90% of the summer power and >50% of the winter power needs for our home. We also have a 1000Wp grid connect power system which covers the other 10% in summer and "banks" some money in the power bill for winter, hopefully giving us >95% effective electricity self sufficiency. Because we need 3 phase power fo water pumping, we remain connected to the grid. It also allows us to switch back to the grid to allow maintenance or when our batteries are low.
Some basic Solar panel theory:
Operating voltage (also known as Maximum Power Point voltage) the voltage at which the panel puts out its maximum power - this depends on temperature Higher temp means lower MPP Voltage.
Operating Current (MPP current) the current at which the panel puts out its maximum power.
Usually MPPV * MPPI = Panel rated power (Maximum Power Point - MPP).
The only way to realise this power from a solar panel is to hold the panel at an internal temperature of 20 or 25°C (depending on laboratory specs) AND use a Maximum Power Point Tracking (MPPT) regulator or inverter.
We cannot control the panel internal temperature, but we can operate the panel using a MPPT device.
Most Grid connect inverters are MPPT Inverters and adjust their output to maximise the power form the panels.
Regulators:
With a battery backup system, the panel could just be connected to the battery, but the MPPV will never be realised, and without charge regulation, the battery will be gassing to destruction.
Simple regulators will also not realise the MPP of the panels as they can only switch (on or off) the voltage applied to the battery.
A Maximum Power Point Tracking charger/regulator is highly desirable.
Mine cost around $1000 but it is good for 60A @ up to 60V (for boosting our 48V battery) This would be overkill and too expensive for ~250W of panels.
The maximum voltage which can be connected to my regulator/charger is ~120VDC, which is the open circuit voltage of 3, 24V (nominal) panels when open circuited in COLD climates. I have arranged my solar panels in groups of 3, panels in series. My panels have a 25°V MPPV of around 30VDC.Thus the MPPV of the strings is up to 90V. This drops significantly on HOT days like we have has this year.
This is too high to connect directly to the batteries, but by putting 3 panes in series, the current in the panel strings is lower than with only 2 panels in series.
The MPPT regulator/charger converts the panel voltage to the best voltage for the battery state of charge, but it cannot increase the voltage above the panel voltage.
In operation, it recharges the BULK of the charge at the Float voltage, then increases the voltage above the Float voltage, to recover the maximum charge the battery can hold, then drops the voltage back to Float to prevent overcharging the battery.
Float is the only part of this process which does not occur at the Maximum Power Point (for the temperature of the panels) because the battery is fully charged.
If the battery has been discharged excessively, it may be prudent to BOOST the battery for a period to equalise the charge in all cells of the battery. (Hydrogen gas is released from vented batteries during the boost phase. Boost should not be used with sealed cells)
There should be smaller MPPT charger/regulators, which perform the same functions, available at a lower price than ours cost. If your budget doesn't extend to a MPPT charger/regulator, there are good non-MPPT regulators made by Plasmatronics, and other companies, which were the industry standard before MPPT units became available. These chargers also allow multi stage charging, and manual boost functions, but cannot match the efficiency of using a MPPT unit and require that the panel voltages be a couple of volts greater then the battery voltage.
An option available on many regulators is low battery disconnect. This can be set to disconnect the load and protect the battery from excessive discharging, shortening its life.
For your panels, the MPPV is 16.5V which is adequate to charge (@13.6V) and even Boost (@15.0V) a 12V battery using a non-MPPT charger/regulator.
Cables:
In a 12V system, the panels should be connected, using separate cables for each to the regulator, where they are paralleled, using capable of carrying greater than the rated current of each panel.
For a 24V system, panels would be connected so a 2 strings of 2 panels in series. A separate cable from each string to the regulator where they are paralleled. The cable used should be capable of carrying the current of one panel (current is the same as 12v, voltage is doubled)
The cable from the charger to the battery should be capable of carrying the regulator charge current, or the load whichever is greater, preferably double that current capacity or more. This cable should be fused at the battery end, to prevent damage if the cable short circuits near the regulator, or regulator failure. This fuse should be chosen for the current capacity of the battery cables.
Load cables can be connected to the battery terminals of the regulator and should be fused there, to prevent damage if the cable is short circuited at the load end. This fuse should be chosen for the current capacity of the load cables.
Batteries:
The cheapest batteries are usually Lead Acid automotive style batteries, which are fine, but unless specifically designed as "Deep Cycle" the lead acid batteries lose lifespan if they are discharged deeply (>50% discharge). Even deep cycle batteries have a reduced life if deep discharges occur too often.
We only allow our lbattery bank to discharge to 50%, in a worst case scenario, preferring to discharge to 75% of capacity remaining, in normal operation.
Your system:
So, back to your "free" panels (remembering that you could be charged for theft by finding). If you can use 24VDC instead of 12VDC, the battery current for the same power consumption will be halved, meaning for a similar voltage drop in your load cables, you need a lighter wire, or for the same sized wire (as used in 12V installations), you lose less voltage in the wiring for a 24V system. For the same power rating, each battery in your battery bank can be smaller.
Your choice of 12V or 24V system is more dependant on the voltage of the devices available for you to use, than the power efficiency of your system.
As can be seen, there are quite a few considerations in planning and installing such a system.
Support:
I am happy to offer advice on any aspect of your system to maximise the usefulness and reliability of it. I am happy to show our system to any forum member who wishes to consider a similar, or much smaller system for their use.
Apologies for the length of this post, but there is a lot which can be considered in solar power applications.,
Tony.
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Backyard Aquaponics
