MPPT , battery , UPS , limiting

Drakyla · October 2018

Hello to everyone ,

I have few different questions .

So lets say i have a solar power and it leads to MPPT and then to battery . Capacity of the batteries and voltage doesn't matter .

Is it possible to limit MPPT amp charge rate ? I mean the one going into the batteries ? (Example: MPPT 150v/85amp to give only 10-15 amp to the batteries no matter how much is incoming from the panels)

Is it possible to limit MPPT for the DOD on the battery ? I mean to not go down to 50% but to stay higher (75% or even 85%) ?

I`m guessing if the batteries are at full charge then solar panels doesn't produce anymore ?

Is it possible MPPT to forward directly the power from the solar panels ? (Example: incoming 17v/40amp , battery full , outgoing power 14v/40 amp)

On the above example the idea is to use MPPT and batteries like UPS .

mcgivor · October 2018

Better quality MPPT controllers can limit charging current

The controller dose not control discharge, a battery monitoring device could be used to achieve this, or low voltage disconnect of the load, inverter for example though this method is less accurate.

When the batteries are fully charged the excess power is available for loads but would be limited to the current limitations programed into the controller, if applicable.

BB. · October 2018

Any charge controller sets the Battery Bus voltage... For example at 14.75 volts (absorb charging) or 13.6 volts (float charging). The controller based on charging current and time will decide which voltage to "hold".

The battery will "accept/allow" the current flow into the battery based on the charging voltage and battery state of charge.

Your DC loads would take the current they need, and this would cause the bank voltage to drop, and the charge controller will output more current to bring the battery bus voltage back up to the set point.

There is the Midnite Family of MPPT controllers that has an optional battery shunt assembly (Whizbang Jr.) that monitors the current going into and out of the battery. The MPPT controller can use the information from the battery shunt to control the maximum current into the battery during charging. The external shunt can also be used to better estimate battery state of charge.

https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html

Other MPPT controllers can limit their output current (battery+load currents), but cannot limit current to the battery itself (no battery shunt monitor). Outback does have a optional system to monitor battery current--It may also be able to limit battery charging current (not sure if that function is implemented or not).

The Midnite Classic (and some other higher end MPPT controllers) do have more charging options to change charging profiles based on state of charge. And there are battery monitors that can be setup to close an alarm contact when State of Charge falls too load... Use alarm contact to turn on warning light and/or shut down your loads.

With solar power, it is sometimes difficult to limit depth of discharge (you can only charge when the sun is shining and/or by turning off loads and/or starting a genset).

UPS system batteries (float service) are a "different" animal vs deep cycle batteries. Float service batteries may only be designed to deep cycle cycle a handful to a 100 times and be replaced (there are float service batteries that are designed for decades of float, and others that are designed to be replaced every two years). Deep cycle batteries generally are not "good" at float only service and many seem to prefer being deep cycled at least once per month (1,000's of cycles).

Shallow cycling lead acid batteries (1,000's of cycles) may accept discharge to 85% state of charge. Deep cycle batteries (1,000's of cycles) to 50% state of charge. Some are designed to go to as low as 20% State of charge (like forklift/traction batteries) and should be quickly recharged (not a good cycle for solar/offgrid systems... Better for forklift applications with utility power to charge at end of shift).

There are other battery chemistries that "care less" about state of charge and storage... One popular battery is the LiFePO4 (lithium Iron Phosphate). Batteries are very happy anywhere between 20% and 80% state of charge--And somewhat left storage life if stored >~90% state of charge.

Right tool for the right job....

-Bill

Drakyla · October 2018

Hello all again ,

I stumbled on some MPPT information , however it doesn't make any sense for me . Dont want to open a new topic so will use this one (it has MPPT in the title anyway) .

Taking for example SmartSolar MPPT 150/35 (12/24/48V-35A) .

Maximum Open circuit voltage ( VoC?) = 150

Nominal PV power at 48V = 2000 (35 Amps) .

Max Pv short circuit = 40A

The thing that confused me is the following : (using 260W/24v panels for example).

When i raise the voltage up to 120-130 for example (4 panels) do i only win less mm2 for the cable ?

I mean total there will be only 8 panels for PV power 2000w (even if i put them at 120-130v/18A system) then on theory the MPPT will lower the voltage and start charging at 50 or so volts and increase the amps .

So do i need 1 MPPT per 8 panels (in this example) or i can raise up to 120-130v and push 30-35A into MPPT (which makes close to 16 panels) ?

Photowhit · October 2018

You can parallel as many strings as you like. Be aware that the charge controller is based on it's output current, without regard to voltage. Some may be able to have much more wattage coming in than is going out, but...

Drakyla said:

Taking for example SmartSolar MPPT 150/35 (12/24/48V-35A) .
Maximum Open circuit voltage ( VoC?) = 150
Nominal PV power at 48V = 2000 (35 Amps) .
Max Pv short circuit = 40A

This information is that the SmartSolar (Victron Energy) controller can put out 35amps at 12, 24, or 48 volts.

Amps x volts = watts. So roughly 35amps x 48 volts = 1680 watts and more likely charging at about 58 volts for 35 x 58 = 2030.

You might keep in mind that solar panels only put out about 75% of their rates wattage in most situations and provide about 33% more wattage in bot equations.

Drakyla said:
When i raise the voltage up to 120-130 for example (4 panels) do i only win less mm2 for the cable ?

Cable sizing will vary by voltage drop/system losses. If that is what you are asking. This will vary with the volts/amps and distance traveled. You should likely shoot for less than 2-3% losses.

https://www.calculator.net/voltage-drop-calculator.html

Victron makes a spread sheet to help you calculate the size of array for your system, I've never used it, but information on it and their charge controllers is here;

https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/

Drakyla · October 2018

Photowhit said:

You might keep in mind that solar panels only put out about 75% of their rates wattage in most situations and provide about 33% more wattage in bot equations.

You mean that 270 panel will produce close to 210 wats only ? And then i put 15% more loses average in my calculations ?

I`m calculating that with batteries , MPPT and what not i can break even in 12-13 years when counting only 5 hours per day (or panel wats * 5 = total power output for the whole day) . I`m aware that sometimes they will provide more (example 250 wp panel i consider to be providing 1250 watts /day ) .

However if they only provide 75% and then i lose another 15% total .. i dont think i can ever break even with batteries ...

littleharbor2 · October 2018

As long as you have batteries in the calculation you can never break even. Remember you will be replacing batteries every 3 - 5 years, possibly longer if higher end batteries are used and meticulously maintained. If they are sitting on float and your system isn't being used then you aren't generating any daily use power so the "Payback" really doesn't apply.
In the end, your cost per kwh. will always be higher than the power company's power.

mcgivor · October 2018

@Drakyla said
I`m calculating that with batteries , MPPT and what not i can break even in 12-13 years when counting only 5 hours per day (or panel wats * 5 = total power output for the whole day) . I`m aware that sometimes they will provide more (example 250 wp panel i consider to be providing 1250 watts /day ) .

Only 5 hours per day? That would be an optimistic figure in most locations, in reality there is actually a smaller window where peak sun is available, either side of the window will be less in terms of maximum production, seasonal changes will influence this dramatically. Are these figures calculated using tracking or fixed array, if fixed a recallculation would be required which may dissapoint, sorry for breaking the bad news.

Drakyla · October 2018

I haven't used tracking or any other tool . I`ve based my speculations on the fact that at 8 am the sun start shining on the panels (on the place where they will be ) and keep on shining up to 18:00 . So from 12 hours should be possible to get 1250w from 250w panel (still i`m considering total output , not only the 5 hours. I mean 3 hours each for 250 and rest of the hours each by 100 .. or something like that) .

@littleharbor2 i`ll be using minimum batteries . I have constant load so it wont be issue . Trying to figure out the exact number of MPPT i need and then i`ll figure out the mm2 cable and minimum number of batteries i need

BB. · October 2018

Constant loads are actually "the problem" for solar power... What has proven to be a good match with the highly variable solar power is water pumping. The modern pumps (with variable frequency drives--Basically DC solar to variable frequency 3 phase AC inverters that drive the 3 phase pump motors). Simply, your pumping is based on available sun. Great for irrigation and pump to storage... Not so good if you want to have water 24x7 at your home (pumping to an elevated storage tank helps here--Your energy storage is the water tank instead of battery bank).

You have not told us what your loads are or how many Watt*Hours (kWH) per day you need (that I recall). Sounds like trying to do bit coin mining (constant power usage). Anyway, just to give you an idea. 3,300 WH per day (a medium size solar power system) that would give a small/energy efficient home a "near normal" electrical existence (Refrigerator, Washing machine, well pump, Laptop computer, LED lighting)... A very quick "nominal" sizing calculation would be, first the battery bank (you want power at night and during bad weather):

3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 647 AH @ 24 volt battery bank

Then there is sizing the solar array... You have to size it to give the battery bank a proper amount of current (assuming flooded cell lead acid deep cycle storage batteries) and sizing the solar array based on hours of available sun per day by season. Note that suggested rate of charge is ~5% to 13% typically for solar... For full time off grid, 10%+ is highly recommended:

647 AH * 29 volts charging * 1/0.77 solar panel+charge controller deratings * 0.10 rate of charge = 2,737 Watt array minimum (10% rate of charge)

And then there is sizing the array based on your daily load and amount of sun. Lots of guesses here, but fixed array and no shading (trees, buildings, power lines, deep in valley):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Plovdiv (Bulgaria)
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 48° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.92	3.37	3.98	4.27	4.60	5.08
Jul	Aug	Sep	Oct	Nov	Dec
5.35	5.35	4.76	3.70	2.79	2.47

Say you need a genset for poor weather in winter... So "toss" the bottom three months and use February as your break even month (3.37 hours of sun per day):

3,300 WH per day * 1/0.52 off grid AC system end to end efficiency * 1/3.37 hours of sun (Feb) = 1,883 Watt array (Feb "break even")

Energy usage is a highly personal set of choices... The above is just an example of what the math could look like with a generic set of guesses (basically an off grid home, very energy efficient, full time off grid + genset in Bulgaria).

Batteries last 3-5 years (if "golf cart" type)... Better batteries last 5-7 years. And Forklift batteries can last 15+ years. If you have a "mistake" and take your battery bank dead--The bank may only last a few weeks.

Electronics (charge controllers, AC inverters, etc.)--Have a ~10+ year life and can fail in as few as 5 years.

So, you need to have a bank account to be your own utility. Do not take out loans/use your credit card for these systems. It is too easy for something bad to happen (such as a nearby lightning strike or a wind/hail storm--If issues in your region) an leave you with debt and no power.

-Bill

Drakyla · October 2018

@BB. i never specified a load because it doesn't matter .

3,300 WH per day * 1/0.52 off grid AC system end to end efficiency * 1/3.37 hours of sun (Feb) = 1,883 Watt array (Feb "break even")

Your math showing that 3300 Wh array (close to 13 250wp panels ) will only produce 1800 watts for the whole day ?!

Considering "Measured in kWh/m2/day onto a solar panel set at a 48° angle from vertical:
(For best year-round performance)" and single panel is close to 2 m2 i don't get how the stats for Feb are 3.37 Kwh/m2/day and on your calculations you put it down to 73wh (not Kwh) / m2 /day.

Taken from the webpage you posted " You can multiply this irradiance figure by the wattage of your photovoltaic panels to give you an average daily amount of energy you can expect to generate with your system, measured in watt-hours."

If i`m not mistaking then it should be 3300*3.3 and then calculate the loses.

I think in the calculations i can skip few things . As for the shading/trees and such .. i wrote above that the panels have direct sun from 8am to 6pm (during summer is from 8 am up to 8-9 pm).

I just don't understand why you write 3300 WH per day ? How many panels your calculating with that ?

Estragon · October 2018

I think the 3300wh number is just an example number for consumption (not array size), which is used to estimate the size of system needed to supply that power.

Drakyla · October 2018

@Estragon that make more sense ... however is not what i`m looking for . I`m mostly interested how much power i can pull from the panels or more specific .. can i pull out 1250w total a day from a single 250wp panel . This is the best close example for the information i need .

And if someone knows i need direct answer for this : On Mppt 150/50 for example .. if it says it can charge and output at the same time ... do i need to worry if on the specs it also says "Max PV array 2500 wats" or i can aim for 120v/30amps as example (which is way over 2500 watts) ?

I know its some examples on top of my head , just trying to figure things out before hand .

Regards

Estragon · October 2018

You may occasionally be able to get 1250wh in a day from a 250w panel, but not on average.

On average, the estimated max potential will be roughly 75% of STC rating x average daily insolation for a given month in your location. A 250w panel x 75% is 187w. In June, that might give on average ~950w. Some days may be higher, but most places aren't sunny every day. In gloomy days, production can be near zero.

The 75% number is largely a result of the sun heating the panel. STC ratings are based on moderate ambient temps, but actual panel temps in the sun are normally much higher. In actual use, a cold, breezy, high altitude location may see something close to STC rating, but most locations won't.

BB. · October 2018

As Estragon said, that was an example of 3,300 WH per day (3.3 kWH per day or ~100 kWH per month) of energy (an amount, like 10 gallons of gasoline--Not 10 gallons per hour of gasoline being pumped into a tank).

In the example, to generate 3,300 WH per day with an off grid system (battery bank+AC inverter) you need ~1,883 Watt array (in February)... Notice that I am using an end to end derating factor of 52% of solar panel "marketing" Pmp. When you add all of the deratings together, you get only about 1/2 of the panel's rating into an AC voltage load (computer, lights, water pump, etc.)--If you are using a battery bank.

If you were to use a 3 phase water pump and no battery bank (i.e., converting from sun, on a warm summer day), you would have a derating factor of ~77% (roughly 81% derating of solar panel and 5% losses of VFD). That is also about the best if you have an off grid solar system and want solar panels -> DC loads (not using battery bank to buffer, not using AC inverter). In power systems, there are a lot of losses that you just cannot get around.

Remember too, we are trying to give real world "sort of conservative" estimates of solar energy harvest. If you live on a mountain peak above the snow line, yes, the the derating would be closer to 100% (no derating, 100% of array Pmp rating to your DC loads). But most of us do not live there.

Between weather (clouds, rain, haze, humidity, smog) and atmosphere (sun 30 degrees above horizon going through much more "air"), etc... You just do not harvest near what the "marketing" numbers imply (infer?).

And, sizing a system, loads do matter.... Yes, paper wise, I can run any numbers, however when it comes time to pick equipment--We really need to know.

It is like asking what is the best vehicle for you... Knowing nothing about your needs, it could run anywhere from a moped to a "B Train" semi truck. Or perhaps you are are an island--And boats are what are needed and all this talk of wheeled vehicles is a waste of everyone's time. You have land where you can install a relatively large solar array?...

Are you in/around Plovdiv Bulgaria? It looks like a beautiful area (5,000 years of habitation--Oldest city in Europe?).

https://www.youtube.com/watch?v=l3ke2ez-IFM

-Bill

Estragon · October 2018

IMHO 120v@30a (3600w) might be pushing it a bit. The controller will limit output to 50a, but it can be hard on the controller. The 2500w limit seems a bit low for a 50a controller (50a@50v charging=2500w) though.

Photowhit · October 2018

Drakyla said:

I just don't understand why you write 3300 WH per day ? How many panels your calculating with that ?

I believe it's just an arbitrary(made up) number. He was trying to give some idea about total system losses.

In battery based system, not only do you have to worry about the Normal Operating Cell Temperature (NOCT)value that I was describing. More information here;
https://thegrid.rexel.com/en-us/forums/renewable-and-energy-efficiency/f/forum/674/what-is-the-difference-between-stc-and-noct-in-photovoltaic-module-specifications

Basically panels are rated on Standard Test Conditions, which entail perfect orientation to the sun, and cold panels. Panel voltage drops when they warm up.

In addition to these losses, there are losses when converting DC (battery current) to AC for use. Most inverters are about 90% efficient at best, but there is a range from about 80-96% peak efficiency. Most are lucky to get 90%.

Batteries also take a chunk of energy to use. The input and output of current is roughly equal, but you must have higher voltage from the input source. Also lead acid batteries accept less current as they approach full...

Roughly;
Flooded Lead acid batteries loss 15-20% in the storage
AGM low 5-10%
Lithium Iron Phosphate loss about 5%
Nickel Iron loss about 35%

Also charge controllers aren't 100 % efficient. 95-99% for MPPT type with a greater amount usually with higher conversion.

Also OFF Grid systems must completely charge the battery a couple times a week to maintain a reasonable battery life.

Bill/BB likes to use 52% as total system losses in off grid systems, 50% is a nice round number for me. If you were calculating from the stand point of minimum system to supply the energy use when compared to a grid tied system, I like to say a minimum array 3 times larger is required. This is likely low if the energy use doesn't match the energy production profile closely.

Drakyla · October 2018

@Estragon i`m just taking example from the MPPT specs . Above in one of the first posts i wrote the exact specs on SmartSolar MPPT 150/35 (which limits me to 2000 w array on the panels if i`m reading it correctly .

@BB. I lived in Plovdiv/Bulgaria for around 33 years ... still have a house there (which is rare due to over population and crowding) .Other than too many buildings and people and cars .. its nice town (especially if you visit it as vacation) .

The location i`m planing to put the panels is 20 km away from Plovdiv , but the weather is almost the same (-2 Celsius as temperature and much less pollution from cars/factory and such) .

As for the loads ... i`m still planing to use the batteries as jump point only . In other words to keep them almost full charged at all times and figure out a way to take directly power from the MPPT without going into battery first and then from there thru the mppt and to the inverter . I`m aware that inverters lose 10% of the energy (since computers , video cards , asics and such for coin mining all operate on 12v .. soon or later i`ll figure out how to remove/lower that 10% loss) .

My theory for now is that if lets say array provide 3300 watts / hour at max peak to put 2500 watts load on it (yes i know i`ll lose 800 watts if the batteries are full) . During the day those watts will lower with hours from 3300 .. to 3000 .. then to 2500 .. and then to 2000 or less . At this point the mppt will start taking from the batteries . Unless mppt have a setting to shutdown the load if battery goes below 80% .. then human intervention will be needed to pull out the plug . After the load is down (lets say there is still some small sun left) all of the energy will go back to charging the batteries from 80 to 100% .If they dont succeed on the next day when it start producing again 3300 watts .. then on 2500 watts load the rest should go at the batteries until they reach 100% .. and this complete 1 day of power production/usage .

As for the land .. i have prepared close to 1500 sq. m clear of any shade at the moment (with plans to expand it to 2500 at least or 5000sq.m if possible) and i`m pretty sure i can fit close to 200 panels or more there (its 60m wide and 25m long)

Estragon · October 2018

Have you considered grid-tied solar?
Depending on the regulatory regime in your location, this could be a much more cost-effective way to use solar.

Drakyla · October 2018

@Estragoni have considered it .. however i don't have that much experience yet installing it and running it . So i need to start from somewhere and build up/expand over and over again until i have enough info and put application for grid-tied . Thats the reason to ask so many questions to have full info .

Since if i have lets say 15 modules each working separately and each module containing 10-20 panels .. when i apply for permit it will be much easier to combine those modules into 1 huge array and lead it into the grid than to build from scratch 200-300 panel array and then apply for permit (and no one said it will be good conditions on that permit since they are monopolists here). They can offer me anything they want .. from twice the normal power rate .. to 50% of it or less .. and you dont have other choice .

BB. · October 2018

The problem is that Off Grid solar costs around $1-$2+ per kWH (all up costs, including maintenance for 20 years). The more sun you have (good solar/hours of sun) and more of your energy potential harvest you use (typically, with off grid solar, you can, at best use about 50% to 75% of the sun's harvest potential vs actual energy used--Without driving yourself nuts to optimize power usage every day).

Or, to produce the same amount of usable energy, an Off Grid system costs about 4x as much as an "equivalent" Grid Tied system to install (batteries, charge controllers, backup genset, etc. are simply more than a simple solar panels+GT inverter+possible upgrade to main panel and new utility meter).

And what makes/breaks the return on investment for GT solar is, as you say, what the local regulations are at any particular point in time. In the USA, it started with GT Solar getting a large amount of subsides from utilities and the non-solar customers. Today, that imbalance is being "corrected"... And in some states/locations, new GT Solar is effectively outlawed (or subsidies have been removed and it is no longer economical).

For PC's, you can get DC input ATX type power supplies:

https://www.powerstream.com/DC-PC-48V.htm

And if you could (software wise) match the number of co-processors (power consumption) vs amount of solar harvest at any particular point in time--That would do a pretty good job of optimizing your solar harvest/hardware costs (and minimize battery usage).

You can play with the numbers and see how things actually happen (the solar day is usually a 20 year long term average, and the "day"
that is choosen is a "real" day that follows the long term average):

https://pvwatts.nrel.gov/pvwatts.php

Here is Plovdiv, and the hourly data for a 4 kW array (with 14% deratring default and 20 degree default tilt--Play with the numbers that make sense for you--I have no idea why the chose those numbers). And you download an hourly CSV file to put in a spreadsheet to see how it all works out for you (time of day, available power):

Location and Station Identification
Requested Location	plovdiv bulgaria
Weather Data Source	(INTL) PLOVDIV, BULGARIA 0.8 mi
Latitude	42.13° N
Longitude	24.75° E
PV System Specifications (Residential)
DC System Size	4 kW
Module Type	Standard
Array Type	Fixed (open rack)
Array Tilt	20°
Array Azimuth	180°
System Losses	14.08%
Inverter Efficiency	96%
DC to AC Size Ratio	1.2

Header:

Month Day Hour Beam Irradiance (W/m^2) Diffuse Irradiance (W/m^2) Ambient Temperature (C) Wind Speed (m/s) Plane of Array Irradiance (W/m^2) Cell Temperature (C) DC Array Output (W) AC System Output (W)

Today (or yesterday just about for you), October 13th, typical available energy harvest:

"10","13","0","0","0","14","1","0","14","0","0"

"10","13","1","0","0","14.2","0","0","14.2","0","0"

"10","13","2","0","0","14","1","0","14","0","0"

"10","13","3","0","0","14","2","0","14","0","0"

"10","13","4","0","0","13.5","0","0","13.5","0","0"

"10","13","5","0","0","14","2","0","14","0","0"

"10","13","6","0","2","15","2","1.889","13.086","6.856","0"

"10","13","7","0","39","14","0","37.009","11.66","135.166","112.243"

"10","13","8","0","115","14.5","1.5","112.712","15.858","404.013","375.58"

"10","13","9","10","184","15","3","188.969","18.194","669.811","635.272"

"10","13","10","15","236","15.8","4","247.141","20.149","868.504","828.971"

"10","13","11","16","265","17","3","278.765","22.724","968.193","926.016"

"10","13","12","61","272","19","4","337.47","25.528","1156.534","1109.11"

"10","13","13","25","269","19","2","290.737","26.164","993.429","950.568"

"10","13","14","11","222","18","8","230.089","20.711","806.354","768.422"

"10","13","15","11","145","18","5","149.502","20.009","524.95","493.821"

"10","13","16","0","56","16.4","8","53.791","16.348","192.386","168.346"

"10","13","17","0","6","13","10","5.673","12.234","20.668","0"

"10","13","18","0","0","12","7","0","12","0","0"

"10","13","19","0","0","12.6","8","0","12.6","0","0"

"10","13","20","0","0","12","11","0","12","0","0"

"10","13","21","0","0","12","11","0","12","0","0"

"10","13","22","0","0","12.2","9","0","12.2","0","0"

"10","13","23","0","0","12","9","0","12","0","0"

Second to last column is DC energy harvest for that hour.... Barely 1,156 Watts at 12:00 to 13:00 out of a 4 kW array tilted to "summer" harvest... From looking at the numbers, do you get a lot of high cloud cover during this time of year (looks sort of like overcast skies in this data)?

Anyway, at 8:00, you have typically 404 Watt*Hours minimum available through 16:00 with 524 Watts*hours available from a 4kW array...

And the harvest is around 6,391 Watt*Hours from 8:00 to 16:00 hours (again from the 4kW array).

All assuming "typical" weather, a fixed 4 kWatt array, and 14% DC power losses (wiring, dirt on solar panels, mild day (14-19C).

And, on a very cool/bright winter day, you might be able to get close to your 4kWatt harvest from a 4 kWatt array (you have to figure out your computer configuration... Number of co-processors vs available energy at any time during the day/season (probably not a good idea to configure 4kW of computing that is only available from a 4 kWatt for a few hours a year).

And, if your "core" harvest is ~9:00 to ~14:00 (5 hours) to support a minimum of 800 Watts... How does that compare to "not harvesting" during the other 19 hours in a day ("free" solar energy, but there are still other costs for computer hardware, etc.).

So--If you can reasonably "throttle" your system's energy usage from ~400 Watts (or a bit less) to 1,000 Watts (or even >3,000 Watts during summer), you can get around 8 hours of "useful" computation on a typical October day without using the battery bank (the batteries are acting like a giant capacitor bank for you--They will consume around 0.1% of your harvest (float current) to a worst case of 1.0-2.0% of their rated Amp*Hour capacity--Something like:

647 AH * 0.001 = 0.65 Amps nominal
0.65 amps * 29 volt battery bus during charging = 18.85 Watts nominal float energy

As the batteries get older (especially if they are flooded cell lead acid/traction batteries), worst case could get to the 1% to 2% float current range (10-20x as much power vs 0.1%). At 2% or worse, you should replace the bank--There is a danger of overheating and fire.

Lots of different ways to model this... More or less, I am trying to show you what data is available (hourly harvest estimate) and how you could build out an economic model based on "minimizing" battery usage).

Of course, if you maximize battery usage, then you could run 24 hours per day (but more losses in power system... I.e., smaller computer loads 24 hours per day vs larger computer loads 5 hours per day)...

Your thoughts?

-Bill

Drakyla · October 2018

BB. said:

And what makes/breaks the return on investment for GT solar is, as you say, what the local regulations are at any particular point in time. In the USA, it started with GT Solar getting a large amount of subsides from utilities and the non-solar customers. Today, that imbalance is being "corrected"... And in some states/locations, new GT Solar is effectively outlawed (or subsidies have been removed and it is no longer economical).

Something similar is/was happening in Bulgaria . At start there was tons of government subsidies and what not for solar energy . 1 Kwh was brought out from the energy company at x4 rates than normal costumer pay for 1 kwh to the grid . And of course now we (all Bulgarians) have a tax on our energy bill which goes into fixing that issue since there is already contracts and such .

Right now i think there is no more bonuses for GT solar ...and if your lucky you may get price that match the actual paying price .

However like i pointed out its not a sure thing . You fill a form and in 1 month they offer you terms (which of course you don't have to accept , but .....) .

Played with the link you gave me . However i picked

Array Tilt	45°
Array Azimuth	135°

Total for the year is Annual 3.62 solar radiance 4,172 KWH at 4 Kwp system . Which makes it close to 2.5 times the output (total output/days in year/system power) .

Unfortunately this is twice lower that i need to be possible to break even (i need 5x times to do this in 10 years or so) .

I still hope that there is GT inverter that i can use for direct power . I mean without injecting power into the grid , but to use from the grid .

In example: On 4 kwp solar array with the above calculations ... even in the best case scenario there shouldn't be possible to harvest more than 4Kwh at any moment . So to put a 4 kwp array into a GT inverter and to put a 5 Kwh load into it and use all of the solar power and rest from the grid .

I`m looking at https://en.wikipedia.org/wiki/Grid-tie_inverter and this catch my eye "Properly configured, a grid tie inverter enables a home owner to use an alternative power generation system like solar or wind power without extensive rewiring and without batteries. If the alternative power being produced is insufficient, the deficit will be sourced from the electricity grid."

Which is exactly what i need probably . If that's possible that is .

I bought this house with the free space with panels in mind .. and i`ll put them on place.

As for dust on them , since they will be close to me i can easy clean them each month or so .

mcgivor · October 2018

@Drakyla read this pdf, also look into SMA they have solutions and are willing to discuss particular requirements as needed.

BB. · October 2018

If you run your "5 kW" computer system 24x7:4

5 kW * 24 hours per day * 365 days per year = 43,800 kWH per year energy used
3.62 hours of sun per day(year avg) * 4 kW panels * 0.77 panel+controller derate * 365 days per year = 4,070 kWH per year (solar)
4,070 kWH per year solar / 43,800 kWH per year used = 0.093 = ~9.3% of energy generated for solar used vs total power consumption

So, if you want to run the bitcoin miner 24x7 with GT solar that never backfeeds the grid (load > GT Solar), you will only support ~10% of your yearly energy consumption with solar (offset by GT solar).

This is the big issue with solar power... No storage, you have to either install less solar than you can use (avoiding utility meter "turning backwards" to use grid as "storage"), or install batteries to store energy for the other 90% of "off solar peak" energy usage.

So, it can be done, but at ~10% of the total energy consumption, the "Free" solar power is just a bit more than a drop in the bucket of 24x7 energy usage.... Or you run the system at 10% or a bit less of rated capabilities to modulate power usage (10x more computer hardware costs to "use GT solar only").

Just not very much return on investment (at today's costs of systems and co-processors).

-Bill

Drakyla · October 2018

@BB. i think your stuck in a way of thinking "full off-grid mode" . Right now i`m using roughly 16 Kwh non-stop (with few machines turned off due to low or even income with the power bill). In full power i easy use 20 Kwh/hour (20 kW * 24 hours per day * 365 days per year = 175 200 kWH / year) .

My math is little different . I`m not calculating how much of the total solar power i can use to support the whole mining .. i`m calculating in a way to use the whole solar incoming power .. and that incoming power to be used to pay off the solar panels .. and then after they are paid off all of the remaining energy will be the real profit (yes i know i`ll not be milliner with solar and such .. but every little drop in the bucket helps

).

3.62 hours of sun per day(year avg) * 4 kW panels * 0.77 panel+controller derate * 365 days per year = 4,070 kWH per year (solar)

At my grid power rate 4070 kWH of day power is 4070*0.216st (i think they will raise it soon with few %) = 879.12 lv ( 1$ is 1.69lv , but i`ll keep my units to keep it easy for me) .

4 Kw solar panels (15 275wp ones) i can buy for 230lv each right now (bulk order of minimum 30 pcs) . So total this is 230*15= 3450 lv (yes i know there is wires and GT inverter to be included into calculations) . So in 4 years ill break even on the panels only .. in another 3 years i`ll easy break even on the wires , GT and the frames .. and then everything is bonus . Which makes yearly return of 14% (in our country everywhere the year rate for deposits is 0.6% .. annually i mean) . For me it seems like good investment since i have what to do the energy after all .

Would appreciate info/links to a GT which can do the thing needed (thinking logically : especially the ones that can be set to never put energy into the grid at GT software) .

Here is a video on the future panel place i was talking about : https://youtu.be/mVOI98KkgJM

@mcgivor i`ll start reading it now

Thank you

BB. · October 2018

Whatever math you use that supports you needs and budget--I am all for that. I am just trying to get a starting point. Remember that solar panels are 20 year devices (hopefully 25-40 years--But I would not bet my life savings on that). And that GT inverters last around 5-10+ years between replacements. Hail, wind storms, lightning, rocks thrown by lawn mowers, etc. all can affect your return.

Racking used to be quoted at something like USD $1 per Watt (when panels were close to USD $10 per Watt)... With panels less than USD $1 per Watt, racking and stuff has to be a bit more reasonable pricing.

At this point (that I know of--And I am not in the solar business, so I do not keep up on this stuff), only a few Hybrid Inverters (inverters that use a battery bank and can do GT, Off Grid, or various forms of grid and generator support). One brand pretty much "leaks" ~2 AAC (amperes AC) from the grid before they kick in the solar+battery support for local hardware.

It has been an ongoing question folks keep asking for... A GT Inverter that will never feed back into the grid (to not violate local utility and sometimes government rules and regulations). I have not seen that yet.

To understand your Utility Meter... There are (more or less) three to four different operating modes--What you have depends on your utility's rules/supplied meters:

Old power meters: They run "forward" as your loads consume power. Original meter designs would run backwards if energy is "fed backwards" through the meter (or the meter was installed "upside down" for xx days per month--A common method to steal electricity).
Option One Meter: Meter runs forwards as normal. Will not turn backwards. Many newer meters will do this
Option Two Meter: Meter runs forwards as normal. Power fed "backwards" through meter, it will still turn forward. This is a "revenue guard meter". Even if user turns meter upside down in socket, it still runs forwards. Turns out that if you install GT solar and run with this meter, if you do feed power backwards to the grid, the utility will charge your for the 'contribution'.
Option Three Meter: Much more complex logging (for commercial, time of use, gt solar)... Can record forward and backward power feeds in different "registers". Some meters are Time of Use (different times, different registers--Charge more in the afternoon, etc.).

Newer "electronic" power meters do have the capabilities to report "strange behaviours"--Meter power cut/unplugged. Meter fed power backwards. Meters see "strange" power factor (may be GT solar "helping" local customer loads, etc. What your utiilty may do with this information, I do not know.

We get a fair number of "warnings" in our area about unapproved GT solar and utility power... But I have not read anything about prosecutions... I cannot predict what will happen in the future.

So, if you do not install a Hybrid AC inverter system (more expensive, need local batteries)--I have not seen any off the shelf pure GT Solar systems where you can install a bunch of (now) relatively cheap solar panels and GT inverters to do "guerrilla" solar installs (systems that in theory meet the utility requirements to not feed power back into the grid).

There are some very good inverter/solar power systems manufacturers in your area of the world (SMA and others out of Germany). Many devices these days have communications buses that can be used (and many times are used) to manage the output power of GT inverters.

To make an interface module that feeds power amounts and directions back to the utilty and use that information to vary the output of GT inverters--Not a very difficult task these days. I would not be surprised if somebody has already done this (either at a large manufacturer, or some hobbyist with a Raspberry Pi or similar processor)--But finding them and getting long term support will be an issue. And what happens if the system "crashes" and does feed power back to the utility (what will be their response). Or if there is a failure, and your solar system shuts down--Will you get support 5+ years down the road to keep your farm going...

-Bill

Drakyla · October 2018

So far i have found 2 models that on theory support never to feed back to the grid and if i`m reading correctly they don't need a battery bank. One of them was explained in the file that @mcgivor gave me .. which proved to be the tech specs on the inverter (just from different webpage) : https://solar.schneider-electric.com/product/conext-xw-hybrid-inverter/

Note on the tech specs was that the option is valid for the XW+ model only (not for clear XW) on page 16th bottom.

However the price i can find for this hybrid inverter is 5000+$ which doesn't make any sense for that kind of AC output power .

The second one that catch my eye (and have local supplier for it) is : http://solar.huawei.com/eu/products

For some reason they don't have separate webpage for their products , but i`m talking about the SUN2000-33KTL-A

(Edit: just now looked and there are even lower ones with less output . Will look for the prices on them at the local retailer)

Specs in the file below.

On the support page it says it has a option to disable grid sell .

I`m guessing that Huawei made that option too with AC generator in mind (like Schneider) and probably can be used with the grid itself .

But that's just guessing .. nothing more .

As for the utility meter , In Plovdiv all of the meters can be read remotely so i`m guessing they are Example 3.

In this place i think they are Example 1 or 2 (i have and the old version , but its 3 phased and don't want to play with it) .

However since i have load that will always be over the produce rate , it should be a problem (on theory) .

Your thoughts ?

Regards

mike95490 · October 2018

Owning an XW, I'm pretty sure a battery bank is required. And you get a LOT of functionality & reliability for your $5K

Drakyla · October 2018

mike95490 said:

Owning an XW, I'm pretty sure a battery bank is required. And you get a LOT of functionality & reliability for your $5K

As long it can operate with something like 48v 100A battery bank (and still control 20-40 panels) its fine , but i think soon will call that Huawei hybrid inverter and ask them a question or 2 about it . On their webpage (on bulgarian) it clearly says it wont feed back power to the grid , however it doesn't say does it work with batteries or not .

littleharbor2 · October 2018

It should be easy enough to figure out by looking at the specs. If there is any spec referring to low voltage disconnect or maximum Vdc. being in the 42- 60 volt range you are dealing with a battery based inverter.

Drakyla · October 2018

I send them a message .. will post when i receive a reply .

However i think the second one i looked into on their webpage is exactly this model ( https://www.alibaba.com/product-detail/-MG-Solar-power-inverter-5000w_50040579165.html?spm=a2700.7724857.normalList.21.1eca16b7TiO07Z ) . I like where it says "Supports batteryless operation" . However it doesn't say for option to block grid feeding . Anyhow will wait a reply from my supplier .. then will ask and this alibaba seller .

MPPT , battery , UPS , limiting

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Plovdiv (Bulgaria)
Average Solar Insolation figures

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MPPT , battery , UPS , limiting

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Plovdiv (Bulgaria)Average Solar Insolation figures

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Plovdiv (Bulgaria)
Average Solar Insolation figures