Help required to design off grid system

Do you know if these are crystalline solar panels (mono and poly crystalline) or are they thin film panels that folks are seeing rapid degradation on?

Thin film panels will lose a part of their output power in the first few months of installation. Also, thin film types of panels require better "sealing" against oxygen and water vs crystalline panels (not that any leakage is a good think).

-Bill

Many panels have two parallel strings of cells... One string getting a high resistance/failed cell to cell connection would kill 1/2 the output of the panel.

Why high resistance is the next question...

Generally, you have several possible reasons for the failure:

1. Thermal cycling... Glass/Solar Cells expand/contract at one rate. Palastics (backing, glue) and metal (copper strips, cell fingers) expand at a different rate. The differential expansion can cause delamination of metal (poor connections) or hermetic sealing (let oxygen/water into panel).

2. I²R heating of electrical connections. Copper is expensive and blocks sunlight--Not enough copper, then I²R heating of copper connection can cause overheating of copper--connection failure/overheating of backing causing delamination, etc. Look for brown spots in panel (burned areas).

3. Bypass Diodes. Most panels have diodes for every ~10-24 or so series sell. Diodes only conduct if part of panel is in shade (bypass diode lets current bypass dark/high resistance shade cells). Sometimes diodes fail open and the shaded cells get high resistance which causes them to get damaged be high reverse current (typically with panels connected in series for GT / MPPT based controllers or >24 volt battery banks). Or the diodes themselves fail shorted and short out a string of cells. Diodes can commonly fail either open or shorted--Typically over heating and thermal cycling is a major cause of failures (and solar panels have very poor heat sinking for diodes--So diodes can run hot).

And you can have several causes... A water leak allows corrosion to occur (final failure) to delamnation allowed the water to leak in. But delam was caused by over heating or failure of bonding/glue (thermal cycling). Etc.

-Bill

syedbukhari wrote: »

Thanks a lot Bill for your valuable information. I think 60 % of solar panels lies in cell cost and remaining 40 % lies in other materials. As i have told you these panels are available @ 0.35 $/watt which is almost 40 % lower than the price of A grade panel. I am wounder how they are producing these?

In the US, taking product to the garbage dump or recycler can cost a lot of money... If somebody will pay for panels that have no other value to the company--Why not.

Please also clear me about the functioning of solar charge controller.
I have a system of 48 Volt and i want to run 48 V DC load from this system direct from the controller. I want to charge the batteries and run the load from the solar in the day time and after the sun set or in rainy days run the load and charge the batteries from the national grid.

Many charge controllers will not even work if there is no battery bank--They need a minimum amount of voltage to "turn on" (~9 VDC for a 12 volt battery bank is a common value).

Also, solar charge controllers are not really designed to regulate voltage by themselves. They need the battery bank to "buffer" current/voltage for short term (seconds to fractions of a second). They have no filtering/output capacitors designed so that the charge controller can have a "stable" output voltage. Their regulation (if you could get the controller to start) would be just like flipping on and off a light switch very quickly--I.e., a series of zero volt / 60-72 volt voltage spikes. Most loads would be very unhappy with that sort of power.

If you have batteries--I guess I am not sure what you mean by "... i want to run 48 V DC load from this system direct from the controller".

You want to connect the loads directly to the Vbatt output of the controller? Some controllers have a "load" output with Low Voltage Disconnect (LVD).

If the load requirement is 10 amp DC and solar is producing only 5 amp then what will happen
1) will the controller cut off the load due to less amp of solar
2) will it get 5 amp from solar and 5 amp from battery to run the load
3) will it get all 10 amp from the battery to run the load.
​

Assuming "traditional" wiring (solar panels to solar charge controller to battery bank to DC loads), then #2 will be what happens.

-Bill

Yep, still #2. Solar panel + battery will supply the load.

LVD terminals are nothing special. Just turn off the output if battery voltage falls too much.

-Bill

Are you saying you want to put three strings of batteries (each string with matched battery AH capacity) in parallel and charge them as one large battery bank?

Yes--You can do it--However, mixing battery sizes (and age) can be a bit difficult. You really want "matched" sets of battery strings ideally. Mixed size/age of batteries may not last as long and may eventually fail at different times (i.e., String A may fail in in 3 years, String B in 5 years, etc.).

And mixing battery types (flooded cell, AGM, GEL, etc.) in one bank is even more problems. Different battery types really need different charging voltages.

How you set priority for charging--Really depends on the Chargers and AC inverters you choose. And how much you want it to be automatic vs manual control on your part.

Have you picked your hardware brands/models yet.?

-Bill

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syedbukhari wrote: »

Yes i agree with you but can we increase the life of the each string by using reverse diode and a rely. For example when the string of 100 Ah becomes fully charged the rely cut off the charging current coming from the controller or rectifier and reverse diode stop the back ward flow of current to the other string of 150 and 200 AH.

I would not use diodes/relays to control charging/discharging to each string. The complexity, voltage drop of the diodes, and cost of large relays would seem to make the idea impractical.

Lead Acid batteries do vary their voltage based on state of charge and charging/discharging current. So, ideally, a 100 AH string of batteries would allow about 1/2 the current of the 200 AH parallel string in normal operation (i.e., while the 100 AH battery is charging at 10 amps, the 200 AH string is charging at 20 amps, etc.).

Assuming batteries are the same type (all flooded cell, or all AGM, etc.). same age/condition, good wiring practices, etc. And each series string of batteries are as close to identical as you can make them (i.e., each parallel string of batteries is the same battery brand/model/AH rating/identical age and service history/etc.). Do not mix 100 AH and 200 AH batteries in same series string, AGM and Flooded Cell, etc.

The more "different" the batteries are, the less well they will "naturally" share current (i.e., a mix of flooded cell/AGM, brand new vs old batteries, even a mix of different brands of batteries). I would suggest you get a DC Current Clamp DMM (digital multi meter) so you can monitor the current in each battery string during charging/discharging/etc. and ensure that they are sharing properly. If flooded cell, you should monitor specific gravity of the electrolyte too.

Not purchased yet. i am looking for brand and model and hope you will suggest me a better model and brand. Regarding the priority setting i want it fully automatic system and in the system we do not want to use the inverter. We will use rectifier because we need DC out put to run the DC load.

Are you looking for AC battery charger only, or a mix of AC and Solar, etc.?

-Bill

syedbukhari wrote: »

Yes batteries are same type, all are A.G.M batteries of the same company and having the same age. How can we ensure reverse polarity function.

What do you mean by reverse polarity protection? The batteries hold ~12.7 volts or so--And if the battery bus voltage is >12.7 volts, they are charging. If the battery bus is <12.7 volts they discharge.

As long as the batteries are reasonably "matched"--They should discharge and charge together.

Is the reverse polarity protection to allow you to charge each parallel string separately?

There is something you can do for reverse polarity protection--But that is usually done on the battery cell level--Basically, the voltage of each cell is monitored and if one cell goes "near dead", then the string is cut out, a generator is started, or similar. But cell level monitoring is not something done with modern batteries that inter-cell connections sealed below the battery cap (cannot usually access cell connections in smaller/non-industrial batteries 4/6/12 volt types).

I already have a rectifier to charge the batteries from grid so, do not need AC battery charger.

Is this a "dumb" rectifier (i.e., AC transformer and AC to DC rectifier bridge) or an actual AC to DC battery charger with voltage/time set points?

I need a solar charge controller or power management system that can control the power in the system with the following priority:
1) Solar power
2) Grid
3) Batteries
4) D.G set

Many modern AC inverters/chargers will do this... However, you do not need AC power? Just 48 VDC output?

First solar should be used when there is cloudy or low radiation of sun then it should through utility grid power to charge the batteries and to run the DC load, if the grid is not available then the power should be run through batteries and when the battery voltages fell down to a specified limit then it should run the D.G set.

A standard Solar Charge controller will take care of that.

You could use either a Battery Monitor (or two?) set to, for example, turn on an AC charger at 50% state of charge and turn off at 80% state of charge.

And use a second Battery monitor (or voltage based controller) to turn on the Genset (say at ~46 VDC for >10 minutes), then run the genset for 2/4/etc. hours (or a genset controller).

http://www.victronenergy.com/battery-monitors/

I am guessing that you have an AC genset and should use a transfer switch (relay) to switch AC Charger input from AC mains to AC genset if AC mains fail.

Hopefully, somebody can give you some current information on possible Genset Autostart Controller suggestions... I don't really have much information on these guys.

If you can justify a Midnite Classic--I wonder if these can be setup to meet your needs...

Secondly can i connect two 60 AMP MPPT charge controller in parallel to save the wiring cost?

You can parallel multiple charge controllers to the battery bank. I would suggest that you "star connect" the charge controllers (wiring from controller A to battery bus; from controller B to battery bus, etc.).

You really do not want to save money on the wiring--The charge controllers need to accurately measure battery voltage for optimum charging (something like ~0.04 to 0.20 maximum voltage drop for a 48 volt battery bank wiring to charge controller).

-Bill

syedbukhari wrote: »

Sorry it was reverse current protection. I am not sure weather the current can be reversed from one string to another battery string as the energy flow from higher to lower concentration. Please clear me about this.

In general, when batteries are put in parallel, they should all charge/discharge in sync with each other. There should not be any "circulating current" between a "higher voltage" battery string and a "lower voltage" string--Unless there is a failed battery (shorted battery cell).

Get a DC Current Clamp DMM so you can check the charging/discharging currents between strings (i.e., do a quick current check once a month when checking specific gravity/water levels in all cells). If you see something "wrong" (string not taking current, or taking too much current), then you need to do further research to look for bad wiring/failed battery/etc...

There are "isolation" relays and diode systems... However, these are usually used for things like Recreational Vehicles. You have a "house bank" and a vehicle battery bank. The isolation relay keeps the two banks "separate" when camping--So if the house bank is run "dead", you still have a good vehicle bank (or generator bank) so you can start the engine. In an RV, then the motor/generator is running, the two banks are tied together with the relay so that both banks can be charged with the genset/vehicle motor.

it is a dumb rectifier

This could be an issue.... It is possible to over charge the battery bank if somebody forgets to shut down the genset and/or if the rectifier has a very high charging voltage.

I like to design a system so that it is "goof resistance"--You don't want to have some normal operation (running the Diesel Genset) turn into over charging/damaging the battery bank. If you have some backup system (4-6 hour timer, monitor battery bank voltage, etc.) to shut down the DG / turn offf the AC rectifier input to protect the battery bank/electronics from over charging/over voltage.

Making for a highly automated system gets very complex (and expensive) very quickly. The automation has not only to 'work right' for the battery bank, it has to monitor the genset too (fails to start, runs low on fuel, engine oil pressure, coolant temperature, generator rpm too high or too low, etc.)... Lots of weird conditions that can cause problems.

Yes i need 48 VDC out put so i need solar charge controller that can manage the power source as i required. So, please suggest the right one.

Midnite, Outback, MorningStar solar charge controllers all have good reputation. I would also look at what you can get locally (and local support, parts and repairs). What is simple to get in the US may not be a good choice for you. Victron makes some good controllers too (Europe). I am not in the business--So, I am not the best person to ask.

In any case, look at the manuals and figure out what functions are important to you.

Do you any updated price of this battery monitor?

The BMV-702 is available from US Amazon for ~USD$225 ...

Some of the meters (I thinkt he BMV=702 does) have a data bus you can connect to a small computer--You could log/setup furhter logic to control your various energy sources/loads as needed.

Yes i think it is better in my solution. But please clarify how will it cut off grid supply at day time when the plenty of sun power is available for running the DC load and for charging the batteries, how will it through the grid power when there will be cloudy and low radiation days. As it can run the genset , can it also stop the genset through the relay?

As an example, you could program the Battery monitor to turn the alarm "on" at 50% state of charge and turn "off" at 80% SOC. Connect that to your DG autostart controls or to an AC transfer switch (relay) for the battery charger.

-Bill

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syedbukhari wrote: »

Can i also use the midnite or outback charge controller relay for the genset or AC power on and off function?
Can i use solar charge controller relay to start and stop the genset and at the same time use the relay of battery monitor as a secondary source to switch off and on the AC power.[/code]

I do not know. I would do the same thing as you. Read the manuals, go to their forums, etc. Midnite has a nice forum here:

http://midnitesolar.com/Forum/

MorningStar also sells relays that can be connected to their digital bus... Might also be an option (I really do not know).

Outback also has an AGS (automatic generator start) system (or two) also:

http://outbackpower.force.com/articles/Troubleshooting/AGS-not-automatically-starting

For example if i program the battery monitor to turn the alarm "on" at 60 % state of charge and turn off at 90 % SOC and connect it to an AC transfer relay for battery charging and program the solar charge controller relay to turn the alarm "on" at 50 % SOC and stop at 80 % SOC and connect it to auto start DG controller.

By doing this i think we can control the functioning of AC power and genset very effectively. If the grid power will be available at 60 % SOC relay will through the AC power to charge the batteries and if the AC power will not available the battery will fell down to 50 % and second relay will work and start the genset.

Please correct me if my approach is wrong or impractical and also update if i can buy a charge controller with built in double relay and dual DC out put source.

Seems reasonable.

-Bill