Ideal PV array voltage

Re: Ideal PV array voltage

The minimum charging voltage for a 12 volt battery is around 14.4 volts (15+ volts for equalization). Allow for 1-2 volts drop across the controller and another bit for wire drop. Plus add for the fact that hot solar panel's drop their output voltage--and you are pretty near needing ~17+ volts minimum from the solar panel to properly charge a 12 volt battery bank. The loaded "optimum power voltage" from a solar panel:

• Pmp=Power at max power point
• Vmp=Voltage maximum power
• Imp=Current maximum power
• Pmp=Vmp*Imp

For a 24 volt battery bank, 2x the battery voltage (~28.8 volts + 3 volts for drops > 31.8 volts Vmp). And so on...

Now, things get a bit more complex. There are simple PWM controllers (Pulse Width Modulation)... They are nothing more than a controller flipping on and off a switch from a few times a second to >10,000 times a second. Same as you turning on and off a switch to charge a battery with the "optimum" current by changing the duty cycle -- The more "on time" (% on), the more average current.

A PWM controller works by simply having Vmp>Vbatt-charging + Vdrops

Notice, that the higher Vmp (say Vmp=100 volts), the battery really does not see more than Vbatt-charging -- so there is a loss of energy when solar panel Vmp>>Vbatt-charging when using PWM controllers.

But they are cheap and with the right Vmp setup--pretty efficient.

There is a second major class of charge controllers called MPPT solar charge controllers (Maximum Power Point Tracking). These guys are really "switching power supplies" that use inductors to efficiently down convert from High Voltage / Low current (from the solar panels) to Low Voltage / High Current (to the battery bank)... More or less similar to an AC power transformer.

Much more expensive, a bit more wasteful of power (more electronics to such energy to operate) but works very efficiently over a wide range of input voltages (unlike PWM controllers).

For example... Say you had a solar array with 5x 50 watt solar panels with a Vmp rating of 17.5 volts:

• 50 watts / 17.5 volts = 2.86 amps Imp
• PWM: 5x 2.86 amps * 14.4 volts batt charging = 206 watts into the battery bank

Now do a High Voltage array (5 panels in series). Since the current of 5 panels in series is still 2.86 amps:

• PWM high voltage array: 2.86 amps * 14.4 volts = 41 watts with same array in series

A PWM must have the Vmp ~ Vbatt-charge + 3 volts or so to be efficient.

However, a MPPT type controller can "transform or converter" energy... So it would take:

• MPPT Low voltage: (5x * 2.86 amps) * 17.5 volts / 14.4 Vbatt = 17.4 amps charging battery
• MPPT High voltage: (5x 17.5 volts) * 2.86 amps / 14.4 Vbatt = 17.4 amps charging battery

So, ignoring losses, the MPPT converter can convert more energy into the battery bank with either a low voltage or high voltage setup.

Now--There are real losses and limitations. For example, Varray low still has wiring drop (which I ignored) and needs heavy wiring to transmit the ~14.3 amps of Solar Array Current.

But, with MPPT, I can make a high voltage array where the solar wiring only has to pass 2.86 amps at current (but at 87.5 volts). This allows me the option of using less expensive (smaller gauge) copper wire from the array to the charge controller, and/or to put the solar panels a much longer distance away from the battery bank/charge controller/battery shed...

For many people, the higher voltage / lower input current from the solar array and the ability to have a longer distance between the array and the battery shed is well worth the extra cost of the controller.

There are some more "gotcha's" in there... For example, the higher Vmp voltage tends to be less efficient for the MPPT controller to down convert. Also, there is a maximum Voc (Voltage open circuit) from solar panel which ranges from ~30 volts to 150 volts for MPPT controllers.

At least for a start--It will probably make more sense if you describe the setup you would like and we do a paper design--After you do it a couple times, it will make more sense.

-Bill

Re: Ideal PV array voltage

Actually, you may want to look at powering everything using 120 VAC and an efficient inverter (or several, one smaller TSW for electronics. A large cheap MSW for pump, vacuum cleaner,etc.).

!2 volts is difficult to send very far, need very heavy gauge copper wire for your power runs (your 120 watt device takes 1 amp at 120 VAC and 10 amps at 12 VDC--Plus a 4 volt drop at 120 VAC is no big issue--a 4 volt drop at 12 VDC is a non-working device). 24 volt appliances are around (made for trucks and boats). 48 VDC are pretty rare (and probably expensive).

And, it depends on how much you want/can afford to spend for off-grid power.

You can probably power your home with 500 watts of solar panels--but you would have to dump the hair drier and use the other stuff minimally. Some lights, laptop computer, modem of some sort.

In the end, it really does work out to something like $1-$2 per kWhr for off grid power (including battery bank replacement every 4-8 years or so, capital costs, hw repairs, generator fuel cost, etc.)...

So--when you look at something like a hair drier. Throw a Kill-a-Watt meter on it and see what your daily usage would be... It is very possible that the $0.40 per day power usage (vs 0.$04 per day at home) is well worth keeping the spouse happy.

In the beginning, perhaps a generator and a smaller battery bank+inverter for quiet evenings/nights while you are building/living there part time. Get a used RV propane powered refrigerator. Once you are about ready to live there full time, look at a larger solar off-grid system and a good Energy Star Refrigerator or chest freezer conversion.

The following year, then you can measure your real loads and better plan out your system.

In the end, we can make some guesses based on, for example, using 2kWHours per day and no generator usage for ~9 months of the year (where will your home be--look up local solar irradiation).

For lots of interesting ideas and other sources for information, plus even an example or two for planning an off-grid system--take a look at this thread (random information about Solar RE):

Working Thread for Solar Beginner Post/FAQ

-Bill

PS: Unless you are pressed for "roof space" for your solar panels--Check out pricing just plain Solar PV + Solar Thermal panels separately.

Solar Thermal panels are so efficient (compared to solar PV), that you will need only something like 40 sqft of solar thermal panels per person for much of your hot water needs.

Solar thermal heating in winter--Don't know--probably depends on your location. In any case, the above thread also has some links to DIY solar thermal projects too (solar thermal is easier/more practical for DIY vs solar PV/Electric systems).