Testing efficiency of off-grid systems - is there an app for it?

PortuGal

Hi

There are many people in rural areas who have hacked together low budget systems. Where I am in Portugal, the only solar companies nearby refuse to troubleshoot these systems because it is time-consuming and there is very little profit to be made. We were discussing the other day the need for an app where you can enter all details of your system, using a multimeter for test results, to see where the inefficiencies are and make appropriate corrections/purchases.

Does anybody know if anything like this exists? A trawl around the net has not found much on the app stores etc. 

mvas

Instead of looking for an APP, use an Excel spreadsheet. There are many PV design spreadsheets that allow you to enter the system specs, and then it computes the theoretical volts, amps, watts, kWhrs, amp-hours, voltage drop vs wire size, battery runtime, days of autonomy, etc for the Solar Panels, Battery Bank and Loads. Collecting accurate and meaningful data will be time consuming.

BB.

A very useful debugging tool (and good for understanding your system) is a Current Clamp DMM (Digital Multi-Meter). Here are a couple (US Amazon):

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (cheap--Good enough for debugging smaller systems)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (example of a mid-priced CCDMM)

Current clamp meters are great... Just need to put the clamp over one wire and you can measure AC or DC current (There are AC only clamp meters and they are great--But for our systems we need AC+DC Current Clamp function--Watch out as the AC only DMM does measure DC voltage but not DC current).

For debugging a solar power system--It is not a trivial excercise. For Grid Tied / Utility Interactive, the output power (Wattage) to the utility grid is 100% of available input Wattage from the array--Basically the sun and any "problems" with the array/system show up pretty easily.

For Off Grid power systems, the solar charge controller (battery charge controller) modulates the available power from the solar array to the battery bank--So a nice/bright sunny afternoon, the charge controller could be cutting back on battery charging current because the battery is full (or the battery/wiring could be having issues too).

The debugging methods are pretty variable--What is it you are testing (working system, uninstalled solar panels, etc.) and configuration (such as 2 or more parallel solar panel strings and such).

For example... You have a few (identical) used solar panels you want to test before placing into your system.

• Place solar panels on ground facing full sun. Measure open circuit voltage with DMM (Voc)
• Use a DMM or Current Clamp DMM to measure short circuit current--With multiple panels, in identical sun, all should have almost identical short circuit (Isc) current.
• Last test if you have "iffy panels"--For Vmp~18 volt panels--Connect one to a 12 volt battery with panel in full sun--Should have about 75% of Isc (test above) charging battery. Make sure solar panel is plugged with correct polarity to battery (+ to +, - to -). If you connect plus to minus, you can destroy the panel (basically short circuit the battery and fry the panel).

If you have an array with 2 or more parallel strings of panels... You can connect the Current Clamp to one string, then the other. Ideally, all strings should have very similar currents.

You can do other tests--If you have 3 or more parallel strings and a circuit breaker combiner box... You can turn off one string at time and look at the charge controller LCD display and see how much the current drops for each string--Again, each string turned off should drop about the same current.

Where things get confusing is trying to figure out if the battery bank is being charged correctly and if the solar array is outputing "full power" or not... The solar charge controller has several modes... Bulk (full current with battery voltage below voltage set point--roughly below 80% state of charge). Absorb (declining charging current with battery at set point--80-99% state of charge). Float (charge controller holds set point at 100% state of charge and supplies any current for DC loads).

This is confusing because the charger could be in "bulk" (full power/current to battery). Or it could be in Float (just enough current to hold bank at float voltage).

Using a Clamp Meter and DMM function while your system is running normally allows you to get a good understanding of how the system works. Then if you problems later, you can see how the system behaves differently (when broken).

Usually it comes down to bad connections (dirty, corroded, loose) or a "failing component" (solar panel, charge controller, battery bank, inverter, etc.). And you use the Current Clamp/DMM to see what is happening. Failure in current flow, voltage, etc....

Another good tool to have is a hydrometer for measuring the specific gravity of flooded cell lead acid batteries.

https://www.solar-electric.com/midnite-solar-battery-hydrometer.html
https://www.amazon.com/OTC-4619-Professional-Battery-Hydrometer/dp/B0050SFVHO

There are similar tests that can be run on your system--Depending on how it is designed...

-Bill

Graham Parkinson

"Efficiency" is an important concept for assessing a PV system, but efficiency can be defined in many ways. 

Perhaps the most useful way to check system efficiency is by looking at annual solar input (historically predicted insolation, factored for location, panel type, panel type etc.) versus annual power produced and or consumed (as logged by your system).   

There are several websites that use historical records of insolation, climate data and details of your system that you enter to predict your theoretical maximum solar power available.  Europe provides PVGIS and the USA has PVWatts.  PVGIS is here:

https://photovoltaic-software.com/pv-softwares-calculators/online-free-photovoltaic-software/pvgis

If your system was 100% efficient, "predicted input power" would equal "consumed power" however all of the inefficiencies compound (solar panel efficiency of about 20%, losses due to shading (usually much more severe than people think), losses in wiring (typically 5% to 10%), losses in controller (a few % to maybe 15% depending on whether you have an MPPT or PWM controller for instance), parasitic always on system idle loads (sometimes adding up to considerable amounts, enough to flatten battery banks), losses in charging and discharging batteries (from 10% to 50%) and then finally - losses in inverters (maybe 2% to 10%).

If your system logs input and output data (i.e. via the Victron Bluetooth app or similar) then assessing efficiency is much easier.  If not, you have to go the route that BB suggests component by component by measuring current and voltage (Amps times Volts = power in Watts.  A watt for an hour = 1 Watt-hr which is a measure of energy). 

 It will be worth it though to track down just where the power is going in your system which will help you make improvements.

The concept of "efficiency" is also complicated by the type of battery and size of battery you have. That will control whether or not energy harvested will be stored and available for use.   Having a way to use energy produced right away during the day is more efficient that storing the energy for use later.

Photowhit

I think people look for high efficiency in off grid systems are somewhat confused. Off Grid systems MUST be wasteful to be healthy!

You can use the extra energy produced once your batteries are full to do other things, heat water or add to heating of the home, but on normal days you should expect the batteries to become fully charged early in the day so that on days and weeks of overcast skies they receive enough charge to not be degraded.

Solar panels are relatively cheap today, better to have a bit to much to avoid running a generator ever. In 20+ years of living off grid I've never used alternat sources of power.