Determining Correct Minimum Sun-hours For A Design?

Re: Determining Correct Minimum Sun-hours For A Design?

Quick rules of thumb based on your needs for 500 WH of DC power per day:

• 500 Watt*Hours * 1/12 volt battery bank * 2.5 days of stored power * 1/0.50 max discharge = 208 AH battery bank @ 12 volts

To charge a battery bank, we generally start with a 5% to 13% rate of charge:

• 208 AH * 14.5 volts charging * * 1/0.77 panel+controller deratings * 0.05 rate of charge = 196 watt array minimum
• 208 AH * 14.5 volts charging * * 1/0.77 panel+controller deratings * 0.10 rate of charge = 392 watt array nominal
• 208 AH * 14.5 volts charging * * 1/0.77 panel+controller deratings * 0.13 rate of charge = 509 watt array "cost effective maximum"

Closest place is Lubbock Texas in PV Watts. Fixed array, tilted to latitude (34 degrees from horizontal):

Month    Solar Radiation (kWh/m 2/day)
1      4.78     
2      5.48     
3      6.29     
4      6.40     
5      6.43     
6      6.45     
7      6.00     
8      6.13     
9      5.87     
10      6.27     
11      5.35     
12      4.80     
Year      5.86      

Typically, I would toss the bottom three months and use a generator for backup--But you have pretty sunny winters, so we can use 4.78 hours minimum (average) sum.

500 WH * 1/0.77 panel+controller deratings * 1/4.78 hours of sun = 136 Watt array minimum.

In this case, the optimal range of solar array would be 196 to 509 Watts.

In general, a lightly used system could get away with 5% rate of charge--And a system deeply cycled every day--probably 10% to 13% rate of charge.

But, because of the large amounts of average sun you get, even in winter, you could get away with a smaller than nominal array probably just fine.

In general, a PWM controller will work fine with 400 watts or less of solar panels. And for over 800 watts, a MPPT charge controller usually works better.

Yes, in very hot climates, even Vmp~17.5 volt panels can have significant Vmp depression because of hot weather... So setting up a solar array with > 17.5 Vmp will give you higher voltage to finish charge the batteries even on hot days (and with cool batteries--putting batteries in a basement/under ground "bunker" can help here).

Also, if you are in the ~200+ watt range, a single large format panel may be of interest (cheapest $$$/Watt). Of course, that would require a MPPT controller for optimum power conversion.

Then there is the issue of shipping solar panels--Roughly ~140 watt panels (which are available in 17.5 volt Vmp "12 volt" panels) are about the maximum size you can ship via UPS/Bus/Etc. (still not cheap to ship).

A ~208 AH battery bank would be 2x 6 volt 220 AH battery bank (golf cart batteries) would be a good start--Relatively cheap, pretty rugged, batteries.

Once you have selected the battery bank size, then go back and recalculate the Array Size... Then pick the correct controller.

For example, say you pick a 250 Watt solar array--Roughly 2x 130 Watt panels or one 250 Watt panel.

For a PWM controller, a pair of 130 watt panels (2* $250 each):

Specifications:

Vmp
18.1 Volts


Imp
7.38 Amps


Voc
22.0 Volts


Isc
7.89 Amps


Pmax
130 Watts


Tolerance
+/- 5%


Length
57.7 Inches


Width
26.0 Inches


Depth
1.97 Inches



26.51b(12.0kg)

The current would be ~2*Imp=14.76 amps -- A 15 to 20 amp PWM Controller (check specifications for controller and Imp-Array ratings).

For a 250 Watt panel (1x $265) on MPPT (or two 130 watt panels in series):

Pmax: 250 Watts
Voc: 37.6 Volts
Vmp: 30.5 Volts
Isc: 8.81 Amps
Imp: 8.27 Amps
Weight: 46.7 Pounds
65.94" x 39.41" x 1.22"

250 watts * 0.77 panel+controller derating * 1/14.5 volts charging = 13.3 Amps minimum MPPT controller rating

MorningStar makes a very nice little MPPT 15 amp 12/24 controller that would work well here.

No "right or wrong" answers--Just what works best for you (and $$/performance for your needs).

-Bill

Re: Determining Correct Minimum Sun-hours For A Design?

Hi R.A.,

Lots of questions... Here we go:

KE5YD wrote: »

1. IF I were to go the inverter route, I would determine the useage there, then use the .52 efficiency factor and back-track the design from there?

The typical efficiency factors we use for our rules of thumbs:

• 81% for Solar panel (derate for temperature, dust, wiring losses)
• 95% for GT Inverter or MPPT charge controller
• 80% for flooded cell batteries (can use 90% for AGM)
• 85% for AC inverter
• 0.81*0.95*0.80*0.85 = 0.52 = 52% end to end "typical worst case" system efficiency

You can "adjust the numbers" for your needs... I.e., AGM vs Flooded cell battery, no AC inverter, dropping much of the battery losses if system is mostly used during daylight hours, etc.

Note that the 0.81*0.95=0.77 used for solar panels+MPPT type charge/GT controllers... Also is close enough for use with PWM controller + Panels. The losses between "marketing rating" of solar panels and a PWM controller in a well configured system are for different reasons, but still works out to about the same efficiency number.

2. IS the panel/controller derating factor [.77] determining from something like .85 for the panels and .90 for the controller? .80 for the batteries?

As above, the 0.77 is typical (summer) weather derating for solar panels + Charge Controller (or + Grid Tied inverter if you that type of system). We use this derating to figure out the current available for charging a battery bank (so battery losses are not "accounted for " here).

3. IF there is a 10-30% capture increase for mppt over pwm; Is this normally taken into account when computing panel wattage?

You might see a true 10-15%+ increase in solar panel output in very cold weather on clear days (below freezing weather). Remember that MPPT controllers are "constant power" Devices with a ~5% loss. And solar panel Vmp voltage rises around 0.3% per degree C for every degree C below 25C panel "Cell" temperature.

Remember that cell temperature for a solar panel may be 20-35 C above ambient temperature (standard test condition is 25C "flash" test -- Bright light for a few seconds to check panel output but not raise its temperature). But for panels that are spending hours under a hot sun, assuming a 20-35C rise of cell temperature is only practical. NOCT (normal operating cell temperature) is usually defined as 45C (25C ambient + 20C rise).

Note that there is a slight increase in Imp current as cell temperature rises--But it is ~1/5 to 1/10 the temperature sensitivity as Vmp -- So normally it is ignored.

The ~0.81 panel derating accounts for temperature derating, dust, and some panel aging. I try to be conservative--You don't want the system to only work well on the first day of install in Winter--You want it working well 10 years down the road on hot summer days too.

If you get within 10% of predicted performance (based on our estimates here)--you are doing well... Between variability in weather (clouds, temperature, viability, humidity) and measurement errors (shunts, meters, etc.)--Expecting anything closer is not really possible without using lab equipment.

4. This one concerns me the most...I have seen in a number of posts, people using or wanting to use the Morningstar 15 mppt controller with panel wattage [G.T.] greater than 200 watts for a 12v system. Is there something I need to know? Such as normal or induced loss between the panels and controller, or maybe an upgraded controller version?

We can "over panel" MPPT charge controllers (and grid tied inverters) for two reasons. Good quality US/European designed MPPT charge controllers monitor their output current (or power) and will not exceed their design limits (buck mode switching power supply can control output current and voltage). To the point that sometimes, people will by a small MPPT solar charge controller and down convert a 24 to 48 volt battery bank ("unlimited" current) and charge a smaller auxiliary 12 volt battery bank (to run their HAM gear).

The other reason is for "optimum" cost of hardware... If you assume that most of the time you will only get ~77% of the solar panel+charge controller for a typical installation--We can over size the array by 1/0.77 -- And for only very few hours per year will the controller "limit" its output current (very cool/clear days around noon).

You can even over array with more solar panels--It just will mean that the "small" MPPT charge controller will spend more hours a year in output current limit mode... My math:

15 amps * 14.5 volts charging * 1/0.77 panel+controller deratings = 282 Watt array "cost effective maximum" for an MPPT controller

Note, you cannot do this "up rated solar array" with PWM controllers--They do not have the capability to modulate their output current (they are simply On/Off controllers).

Make sense?

-Bill

Re: Determining Correct Minimum Sun-hours For A Design?

The Rogue 3048 is actually Marc's third generation controller (his other two were 12/24 volt units)... Have not read anyone here that have used it yet (just came out a month or two ago)--But his first two generation units were very solid performers with lots of built-in functions (networking, logging, etc.) that others charged extra for.

Regarding cell tempertature, put your had on a black car in full sun--Much hotter than ambient... How much is that temperature increase for a solar panel?...

"it depends"... The highest (worst case numbers) I have seen is 35C (Xantrex used this number in their calculations). In standard conditions, NOCT (normal operating cell temperature) is usually 45C on panel specs.

This website has a NOCT calculator (no wind):

http://pvcdrom.pveducation.org/MODULE/NOCT.htm

And here is NOCT definition:

A PV module will be typically rated at 25 °C under 1 kW/m². However, when operating in the field, they typically operate at higher temperatures and at somewhat lower insolation conditions. In order to determine the power output of the solar cell, it is important to determine the expected operating temperature of the PV module. The Nominal Operating Cell Temperature (NOCT) is defined as the temperature reached by open circuited cells in a module under the conditions as listed below:
Irradiance on cell surface = 800 W/m²
Air Temperature = 20°C
Wind Velocity = 1 m/s
Mounting = open back side.

Note that mounting a solar panel hard against the roof can increase cell temperatures by ~10C.

-Bill

Re: Determining Correct Minimum Sun-hours For A Design?

You can also get those buck/boost converters intended to be installed in cars to hold the voltage at ~13.8 volts and run that from your off grid battery bank...

But then, you may as well look at going AC inverter + 12 VDC system supply...

-Bill

Re: Determining Correct Minimum Sun-hours For A Design?

Do you want significant 120 VAC for anything else--Or do you just need "12 volts DC" for your ham shack loads?

You can program many of these MPPT controller for a 14 volt battery bank (7 cells in series)--And acheive the "night time" voltage goals.

The problem can be that you are looking at ~14.5 to 15.5 volts for charging a 12 volt bank or 16.9 to 18.1 volts charging a 14 volt (7 cell) battery bank... And you may fry the rig from over voltage.

Personally, if I only needed 12 VDC--I would look at the buck/boost 12 VDC converter you see in the Ham mags.

And I would still get a MorningStar 300 Watt TSW 12 VDC AC inverter to run other loads (computer, lights, other 120 VAC friendly equipment). This inverter has both "sleep" mode (looking for >6 watts of 120 VAC load) and remote on/off with 12 volt signal line--Very easy to keep battery power usage to a minimum (throw an AC transfer switch in and you have a manual or automatic UPS system too). Using 120 VAC gets rid of a lot of ground loop issues in a 12 VDC system and with RF current floating around because most AC power systems have ~1,800 VAC of AC Input isolation.

-Bill