Small Cabin System

Re: Small Cabin System

The same batteries arranged as 12 volts or 24 volts is still the same amount of power... The big advantage with 24 volts is the reduction in current (1/2 the current vs 12 volts) and the additional voltage drop (2 volts vs 1 volt for typical 24/12 volt systems) for wiring losses makes higher voltage systems easier to wire and sometimes less costly (for a charge controller like the very nice MorningStar 15 amp MPPT controller), they are rated for a maximum charging current:

• 14.5 volts * 15 amps = 217.5 watts into battery bank
• 29 volts * 15 amps = 435 watts into battery bank

So, the same controller can manage ~2x the number of panels/wattage with a higher voltage battery bank.

Looking at your system components:

• 2- sharp ND-230QCJ
• morningstar sunsaver Mppt 24v
• 4 -T105 Batteries to make 2- 24V banks (actually makes 1x series string 24 volt bank)
• magnum MM-AE inverter

For the panels:

Maximum power (Pmax) 230 W
Open-circuit voltage (Voc) 36.9 V
Maximum Power Voltage (Vpm) 29.3 V
Short-circuit current (Isc) 8.45 A
Maximum Power Current (Ipm) 7.85 A
Module efficiency 14.1 %

First, realize that solar power does not generate near as much energy as most people assume. And that most people use much more power than they realize.

I normally suggest that people start by defining their loads (peak wattage, average wattage, and Watt*Hours or Amp*Hours per day needed), where the system will be used/installed, and what months/seasons it will be used (plus weekend or full weeks).

At this point, lets assume that the 4x T105 6v 225 AH batteries is your starting point. What would the "ideal system" look like and how would it perform.

First rule of thumb is 5% to 13% rate of charge with 10% being a healthy nominal rate of charge:

• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.13 Rate of charge = 1,186 watts of solar panel "cost effective maximum"
• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.10 Rate of charge = 848 watts of solar panel "nominal"
• 225 AH * 29 charge volts * 1/0.77 solar+controller derating * 1/0.05 Rate of charge = 424 watts of solar panel "minimum"

Next, what is the amount of power such a system could output. Assuming Delta Rat means you hang out around the Sacromento Delta, using PV Watts for Sacramento, 1,000 Watts of panels (1 kW is minimum software takes) and 0.52 system derating:

"Station Identification"
"City:","Sacramento"
"State:","California"
"Lat (deg N):", 38.52
"Long (deg W):", 121.50
"Elev (m): ", 8
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 38.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:","12.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.75, 41, 5.12
2, 4.23, 57, 7.12
3, 5.27, 79, 9.88
4, 6.33, 90, 11.25
5, 6.83, 97, 12.12
6, 6.93, 93, 11.62
7, 7.23, 99, 12.38
8, 7.30, 101, 12.62
9, 6.96, 95, 11.88
10, 5.67, 82, 10.25
11, 3.68, 52, 6.50
12, 2.71, 39, 4.88
"Year", 5.50, 927, 115.88

So, 1kW of solar panels in Sacramento will generate around 39kWH to 101 kWH per month (fixed array) depending on the month. I use the 9 month estimate as the minimum sized system that will run without backup genset--In this case, February at 57 kWH per month or:

• 57,000 WH / 28 days per Feb = 2,036 WH nominal per day for February (+/- 10-20% depending on weather)

If you choose a 424 watt array (minimum size) then the math would look like:

• 2,036 WH per Feb day * 0.424 kW array = 863 WH per day

If your there 12 months of the year or only 3 month for summer, you can pick the numbers that make sense for your usage. Note, don't plan on using 100% of your power every day--You should only plan on using 50-75% of your daily power unless you "micro manage" your power usage and make up with a genst (more fuel).

How much power would a 225 AH 24 volt battery bank support? Normally, we assume that a battery bank will support 1-3 days of no sun and 50% maximum discharge in normal use. For yours, I will assume 2 days and 50% maximum discharge.

• 225 AH * 24 volt * 1/2 days of no sun * 50% maximum discharge = 1,350 WH per day
• 225 AH * 1/2 days of no sun * 50% maximum discharge = 56 AH 24 volt per day

Again, just a starting point using rules of thumbs and other assumptions.

Sizing of the inverter... Normally, a flooded cell battery bank is rated for ~C/8 for continuous current discharge maximum:

• 225 AH * 24 volts * 0.85 inverter eff * 1/8 rate of discharge = 574 watt inverter average output

And the maximum surge is typically around C/4:

• 225 AH * 24 volts * 0.85 inv eff * 1/4 = 1,148 watts maximum surge load

So, typically, I would suggest a maximum of ~500 watt inverter with 1,000 watt maximum surge rating.

Next, looking at the solar panel you have choosen with the MorningStar 15 amp MPPT charge controller... There is a problem running a 24 volt battery bank. A single panel with Vmp~29.6 volts is too low for a 24 volt battery bank (need ~35-37 volts or so Vmp). And two panels in series to too high of voltage for a MorningStar controller on a cold day (will over voltage the controller).

Your choices (with hardware given) is to put the panels in parallel and use a 12 volt battery bank (really too big of array for a 15 amp @ 12 volt MPPT controller). Or choose a higher voltage controller (MorningStar TS MPPT series at 45 or 60 amp output, or similar from other vendors).

I will stop here for the moment... There are probably more than enough numbers and assumptions that I have made. Some feedback from you will help us in designing a system that will meet your needs.

-Bill

Re: Small Cabin System

Using PV Watts for Pheonix Az, 1kW (1,000 watts) of solar array, first is fixed mount with "defaults", second is 2 axis tracking. Note I have used 0.52 system derating for off grid AC power:

"Station Identification"
"City:","Phoenix"
"State:","Arizona"
"Lat (deg N):", 33.43
"Long (deg W):", 112.02
"Elev (m): ", 339
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 33.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.09, 75, 6.38
2, 6.06, 81, 6.88
3, 6.61, 94, 7.99
4, 7.54, 102, 8.67
5, 7.53, 103, 8.76
6, 7.28, 93, 7.91
7, 7.13, 94, 7.99
8, 7.17, 95, 8.07
9, 7.15, 92, 7.82
10, 6.75, 94, 7.99
11, 5.59, 78, 6.63
12, 4.88, 73, 6.21
"Year", 6.57, 1074, 91.29

And 2 axis tracking:

"Station Identification"
"City:","Phoenix"
"State:","Arizona"
"Lat (deg N):", 33.43
"Long (deg W):", 112.02
"Elev (m): ", 339
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: 2-Axis Tracking"
"Array Tilt:","N/A"
"Array Azimuth:","N/A"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 6.40, 95, 8.07
2, 7.76, 105, 8.93
3, 8.65, 125, 10.62
4, 10.57, 146, 12.41
5, 11.39, 159, 13.52
6, 11.44, 149, 12.67
7, 10.39, 140, 11.90
8, 10.05, 137, 11.64
9, 9.61, 126, 10.71
10, 8.83, 125, 10.62
11, 7.17, 101, 8.59
12, 6.33, 94, 7.99
"Year", 9.05, 1503, 127.76

First column is Month, second is "average hours of noon time sun per day", third is average kWH per month, last is $$ worth of power per month based on 8.5 cents per kWH (cheap utility rates).

-Bill