Using grid tie solar panels off grid

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
I have a 4KW system consisting of 2 sets of 8 panels. I am wondering if these could be used in the event that the grid fails. I know this isn't very likely but we are in an area very near the coast. Most of the homes are at a low level although ours is in a raised area. It's conceivable that the area could lose power for an extended period. Now I believe the output voltage of the panel sets could be quite high but wonder if something could be done with them to provide temporary power.
Any ideas?
 

Bernard

Joined Aug 7, 2008
5,788
I have considered this even to the point of collecting some parts but it just isn't cost effective yet.
Do you own the panels ? Mine 5.5 kW, are leased but in a protracted outage I would manually switch
inverters. Inverter -about 320 V DC input and 2 kW out @ 120 or 240 V 60 HZ.
Another thought was to go 320 V DC to 14 V DC to use with an existing inverter.
 
Last edited:

sagor

Joined Mar 10, 2019
439
If the panels are grid tied via some type of inverters, you can't run those without a source (grid) signal to let them sync to the power line. Without a reference voltage, grid tie inverters cannot run, they can't guess what frequency or voltage to produce.
That said, some people may say if you had a generator with a transfer switch between the grid and the inverters, you could run a generator to simulate the grid voltage/frequency, but that may be dangerous because the inverter tries to "feed" the grid by boosting the voltage higher than the grid to feed power back into the grid. That means the inverter will likely "fry" your generator, which cannot absorb excess power.
Bottom line - you cannot use grid tie inverters to run without the grid being active. Look into a standby generator with automatic transfer switch for you house instead.
 

MisterBill2

Joined Jan 23, 2018
8,793
Certainly there must be a means for a grid-tied solar power system to keep working when the grid fails. Why have one if that is not possible?? So has the TS investigated the actual capability of their system?? I can't imagine a solar power system that is not able to provide power whan the grid fails. Why bother even having it???
 

boostbuck

Joined Oct 5, 2017
42
Certainly there must be a means for a grid-tied solar power system to keep working when the grid fails. .... Why bother even having it???
It's a common misconception, but a grid-tie system won't (must not) run without the grid present, so cannot provide power in the case of grid failure. It purely serves to reduce grid loading and hence power costs.

To use the installed panels of the grid-tied system for a power backup system would require a separate power storage system - ie a battery, and the incompatibilities of the configurations of on- and off- grid power systems mean that sharing components is difficult.

However, most off-grid power systems can be configured to act as a standby for grid failure, as they are by definition intended for operation without the grid.
 

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
I agree with boostbuck and sagor.
As to Bernard I think most off grid take a fairly low input voltage 12 - 48 I think is common. Grid tie are wired in such a way that they output relatively low current as several hundred volts. So I believe you would have to rewire them and then reverse any changes once the grid was back.
One person suggested that grid tie panels would provide as much as 700 volts.
One idea I had,based on this sort of voltage range, was several identical 240 drills wired in series and via a belt to drive one or more car alternators so batteries could be charged. I know it wouldn't be exactly efficient but I suspect it would at least work. Any thoughts about this?
 

MisterBill2

Joined Jan 23, 2018
8,793
Driving alternators with motors is the least efficient way to charge batteries.

And the reality is that grid-tied systems must not feed power back into the grid when it fails. So certainly there must be an adequate means to disconnect from the grid. But as for needing to have the grid on in order to function, that makes no sense at all.
The primary purpose of a solar power system is independence from the power grid. With or without battery backup for times of darkness. Feeding power back into the grid is an extra side benefit.
Those 12 or 48 volt systems are from a much older era, and not representative of what has been available for the past 15 or 20 years.
 

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
I see you are in USA so maybe it's different there. Here a grid tied system is set up so the inverter checks that the grid is operational at the correct voltage and frequency. It then injects A/C at the same frequency and in phase and at a very slightly higher voltage. If the grid isn't running correctly it outputs nothing. For every watt of power I generate from my system, even if I use it myself, I get paid for it. There are no batteries involved and it would be extremely difficult or quite likely impossible to get it to work without the grid.
A quick search shows the off grid systems being sold here right now are 24 to 48 volt battery systems. For example https://www.sunstore.co.uk/product-category/solar-panel-kits/off-grid-solar-systems/ including 3 to 7 KW systems so I don't understand where you think they might not be current. Off grid systems can be modified or pure sine wave systems and here produce an A/C output at around 230 to 240 volts. This cannot be fed into the grid without severe risks and is not allowed.
 

Lo_volt

Joined Apr 3, 2014
189
There are two ways you can do it. You can replace your inverter with one that is a grid tie with a battery backup or you can install a battery backup system, essentially a large UPS.

You'll need the batteries as power outages don't always happen on sunny days and solar, in general, is not always constant.

I don't know if Tesla is selling them in the UK, but they make a device they call a Powerwall. It's just a neatly packaged 13.5 kWH Lithium Ion battery backup system. It's usually installed to power the whole house. It can also be connected directly to your existing inverter so that solar energy can be used to charge it. Not cheap either.

Alternatively, the cheaper path is to purchase a backup generator and install a transfer switch.
 

MisterBill2

Joined Jan 23, 2018
8,793
OK, here in the USA people have solar power systems as an alternative to the grid. all of them will provide power if the grid fails, and many of them also have battery backup for when there is no sun.
Is there some rule that demands that the system be unable to work without the grid active?? Here, the only reason to own a solar power system is to not need to depend on the grid at all times. Many of the systems do sell excess power to the utilities, but that is very much a secondary benefit. Independent power is the motivator.
So worst case you could buy a USA interface and connect it to your existing solar array, and have power whenever you needed it, in addition to selling the excess to the utility company.

You may also be able to have a small inverter to substitute for the grid connection, powering that small inverter from a storage battery charged by either a mains powered charger or the solar power system. You would need to install a transfer switch to assure that you never send power into the inactive grid. That is a vitally important function indeed. Transfer switches do cost a bit but they are mandatory and very important.

And I am wondering where it is that folks are not allowed to have power independent from the utility grid. That does not seem right at all. If the issue is assuring proper disconnection when the grid is off, it seems that the lawmakers have not been aware of the options that have been available for the past 35 years.
 
Last edited:

sagor

Joined Mar 10, 2019
439
Most grid tie systems are not allowed to run without a valid grid signal (voltage and phase). That is to prevent electrocuting a hydro line worker when they have to do maintenance and cut power off to do so. Imagine the liability if your grid tie ran and electrocuted a worker when he had the power shut off... In fact, my grid tie system has extra libaility insurance on my home policy to cover any such accident, should the grid tie inverter somehow fail and actually try to send power out to the grid when it is not supposed to.
Grid tie inverters run at around 350-370VDC input (most of them), and most systems are NOT allowed to have battery backups in those instances. That is for the same reason, to prevent any electricity from feeding into the grid if the grid is turned off.
Now, you could have an inverter system at home that is fed by the grid, with battery backup charged by solar. That is a different animal so to speak. It would act the same as a fail-over backup generator. When hydro goes out, it kicks in to provide electricity to the house only, not the grid. Most home systems like those use 24-48VDC battery storage, and the solar panels are configured that way. That is a lot different from 370V that most grid tie systems run at. The solar panel configuration is totally different between those two types of systems.
Bottom line, don't try to mix those two different systems, they are totally different.
 

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
Back to the idea I had,based on this sort of voltage range, was several identical 240 drills wired in series and via a belt to drive one or more car alternators so batteries could be charged. I know it wouldn't be exactly efficient but I suspect it would at least work.
As a simple, cheap temporary system despite it's inefficiency does anyone think it would work? I'm not sure whether the 240volt A/C drills would run on D/C or not.
 

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
We can have completely off grid systems such as those I showed the link to. I don't know if we can have grid-tie with battery backup here but I'm sure it would be much more expensive than my idea for a temporary solution. The solar panels can easily be disconnected via a built in switch and unplugged form the inverter. Also the grid can equally be disconnected via a built in switch.
 

MisterBill2

Joined Jan 23, 2018
8,793
Back to the idea I had,based on this sort of voltage range, was several identical 240 drills wired in series and via a belt to drive one or more car alternators so batteries could be charged. I know it wouldn't be exactly efficient but I suspect it would at least work.
As a simple, cheap temporary system despite it's inefficiency does anyone think it would work? I'm not sure whether the 240volt A/C drills would run on D/C or not.
There are 240 volt powered battery chargers available commercially. They are frequently used in marinas to charge the battery systems on boats. And they are both fully automated and quite efficient. And way more quiet than drill motors spinning alternators. In addition, switcher mode power supplies to convert from almost any voltage to almost any other voltage can be designed and built.
 

MisterBill2

Joined Jan 23, 2018
8,793
NO! I do not think that spinning an automotive alternator with a number of drill motors is a viable concept. First, because the torque and speed requirements of an alternator are typically a few thousand RPM, while a drill motor might deliver 800 RPM, or less. Coupling multiple drill motors to a single belt would not allow adequate belt contact to deliver useful power, if the arrangement could even be assembled.
 

Thread Starter

Norman Elliott

Joined Jan 29, 2016
9
Some interesting comments which I appreciate. My corded drill has a 2 speed gearbox for 800/3000 rpm full speeds. However it's only 600 watts so it wouldn't provide that much power to and alternator. So if both 75% efficiency and with losses in the system maybe 300 watts or so. Perhaps 2 sets of these feeding a battery bank would allow a short period of a decent amount of power each day. Still far better than no power for weeks on end and no need for diesel for a generator.
 
Top