System Sizing Method

Re: System Sizing Method

Hi All, I am new to solar electricity. I am trying to understand system sizing techniques, for stand alone solar system. Anyone can shed light on system sizing method which to look out for, I'd be very glad.

Phoe Tar,

In addition to understanding your power requirements, you need to establish your average daily energy requirements (kWh/day). For example, if you check your last home electricity bill and find that you’re using, say, 270 kWh/month of electrical energy, then your average net daily requirement is ~9 kWh.

You then need to understand that typical off-grid system efficiency is ~60%. So, to meet a net energy requirement of 9 kWh/day, a system will need to generate ~15 kWh/day.

You also need to know how much insolation you receive in your intended location. For example, a south facing PV array tilted up at latitude (~35 degrees) in Albuquerque, NM, will collect the equivalent of five hours per day of “full” Sun in December, the month it receives the lowest average daily insolation, and assuming a clear year ‘round “view” to the east, south and west. This information suggests that you’d need a PV array rated at 15 kWh/day / 5 hrs/day = 3 kW STC for this location.

You can find out more about insolation for your intended U.S. location from this site: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html

In addition to a PV array of this size, you'd also need a charge controller, a battery bank rated at ~50 kWh to 60 kWh (to support 9 kWh/day for three days without dischargeing below 50% SOC), an inverter or two, and miscellaneous cabling, breakers, disconnects, enclosures, etc.

This basic information will give you an initial sense of the scale and scope of your project.

HTH,
Jim / crewzer

Re: System Sizing Method

OK... You should be able to find local solar irradiation / insolation data for your location on the Internet. Good luck with your project in Burma / Myanmar.

Regards,
Jim / crewzer

Re: System Sizing Method

Kyaw,

Knowing your average daily energy requirement and you local insolation gets you off to a good start in designing your system. My July 17 post above includes some of the fundamentals of a “simple” method.

For example, if your daily energy requirement is 4.2 kWh, your overall system efficiency is 60%, and your local minimum average insolation is 5 hours/day of “full” Sun, then you’ll theoretically need an array rated at (4.2 kWh/day / 60%) / 5 hrs/day = 1.4 kW (STC). As a practical matter, something in the range of 1.5 kW to 1.6 kW would probably be more dependable.

Assuming 90% inverter efficiency, your 4.2 kWh/day net energy use will draw ~4.7 kWh/day from the battery bank. Allowing for three days of autonomy and not letting the bank’s state-of-charge drop below 50%, you’ll need a bank rated at 4.7 kWh/day x 3 days / 50% = ~28 kWh.

Targeting a minimum charge current of between 4% and 5% of battery bank capacity, a PV array of ~1.6 kW and a battery bank of ~24 kWh (~1,000 Ah x 24 V nominal) should be a workable balance. The array-power to battery-bank-energy ratio would be 1.6:24, or 1:15. My system’s ratio is 725:9600, or ~1:13, and it works fine as long as I limit loads while the controller is in bulk (MPPT) charging mode.

Knowing the power requirements for your individual loads is important as well, because the inverter(s) will have to be size accordingly. I’m having a little trouble with this issue, but my thinking is that you might want to buy two sine wave inverters (230 VAC, 50 Hz?). One big one (2 kW to 3 kW) could handle the rice cooker, wash machine, and/or well pump as needed and be turned off the rest of the time to save on standby power loss. Another smaller inverter (~1,000 W) could handle all of the other loads and run 24 hours/day.

http://www.outbackpower.com/Export.htm
http://www.exeltech.com/ex_root/ex_products/ex_inverters/ex_xp/ex_xp_specs.htm

Alternately, I suspect the 3 kW inverter could handle just about any combination of your loads, but it would operate fairly inefficiently when lightly loaded. Either way, I think you’re looking at 24 V system for the batteries and inverter, and perhaps 48 V.

HTH,
Jim / crewzer