Power from grid tie system when grid down?

micahv · December 2010

I am looking at a 5kw install for a grid tie system but with the requirement for power to a few devices when the grid is down. The client doesn't want batteries at this time and is aware when grid is down power will only be available when the sun is shinning. He basically just wants the fridge, water pump(240V), and a couple of outlets to be powered during grid outage and during the day.

Any suggestions on basically directly coupling an inverter to PVs? This can be done with a toggle/transfer switch of some sort. I imagine one catch is the high voltage from the PV output circuit to drive a stand-alone inverter. Use a DC to DC voltage converter?

In the future batteries will be added to the system. What are good Bimodal options for Inverters? Can we use these now with out inverter?

One option we are looking at is Sunny Boy and Island but this seems very expensive and also redundant in that we are basically paying for 2 inverters? and also the Xantrex XW4548...I don't think either of these will work with grid-down and no batteries.

The gist is we want a grid tied system that can power some devices when the grid is down with out batteries, and at some time in the future add batteries. We have suggested maybe adding a very small bank of batteries to allow the use of a bimodal inverter.

Thanks a lot!

mike95490 · December 2010

Re: Power from grid tie system when grid down?

You HAVE to have batteries to act as a buffer. Loads fluctuate. Solar fluctuates. Batteries even it all out. Even 4, 12V batteries will work. Not well, but barely. and when the sun goes down, you can run a light or 2 at night (not a fridge though)

Coming soon, Xantrex will have a hi volt DC charge controller for batteries!

micahv · December 2010

Re: Power from grid tie system when grid down?

What I am wondering that since Voltage out from a panel is fairly consistent with varying irradience until we get a low enough level and then there is a sharp drop with only current varying with irradience. So if I am using say a 24V panel(or maybe 300V string converted down to 24) and plug this into a basic inverter will it not behave the same as if on batteries? When voltage falls the inverter shuts off, and when when it rises it comes back on line? Just as it does with low battery voltage? Granted running at lower voltage will start to have to worry about losses and higher currents, but this will probably only be for a part of the array.

mike90045 wrote: »

You HAVE to have batteries to act as a buffer. Loads fluctuate. Solar fluctuates. Batteries even it all out. Even 4, 12V batteries will work. Not well, but barely. and when the sun goes down, you can run a light or 2 at night (not a fridge though)

Coming soon, Xantrex will have a hi volt DC charge controller for batteries!

niel · December 2010

Re: Power from grid tie system when grid down?

that might be true if under no load for the consistent voltage, but with a load the pv voltage does drop. pvs as a source are not that consistent to use like a battery for even a cloud would send everything plummeting. if we could get away with not needing batteries we certainly would have done so already.

micahv · December 2010

Re: Power from grid tie system when grid down?

There are directly coupled applications where no batteries are used, i.e. water pumping. This would basically be a directly coupled system when the grid is down. cloud, night what ever we will cease generating power and in our case stop pumping water and the fridge will stop.

How much does Voltage vary? I would think it would be between Vmp and Voc until we get to very low light levels. What do you mean that with a load the pv voltage drops? From looking at I-V curves the voltage seems to be pretty constant with current fluctuating with irradience. If I have a 225W, 30V 7.5A panel and hook a load up that consume 225 watts I would expect the voltage measured to still be around 30V? if it dropped by half my current would need to exceed the limits the panel to produce the required power?

niel wrote: »

that might be true if under no load for the consistent voltage, but with a load the pv voltage does drop. pvs as a source are not that consistent to use like a battery for even a cloud would send everything plummeting. if we could get away with not needing batteries we certainly would have done so already.

nsaspook · December 2010

Re: Power from grid tie system when grid down?

micahv wrote: »

There are directly coupled applications where no batteries are used, i.e. water pumping. This would basically be a directly coupled system when the grid is down. cloud, night what ever we will cease generating power and in our case stop pumping water and the fridge will stop.

How much does Voltage vary? I would think it would be between Vmp and Voc until we get to very low light levels. What do you mean that with a load the pv voltage drops? From looking at I-V curves the voltage seems to be pretty constant with current fluctuating with irradience. If I have a 225W, 30V 7.5A panel and hook a load up that consume 225 watts I would expect the voltage measured to still be around 30V? if it dropped by half my current would need to exceed the limits the panel to produce the required power?

Think you need to look at this chart for an idea of what will happen. You can see the current a panel can supply without dropping the voltage to a very low level as light levels fall.

Cariboocoot · December 2010

Re: Power from grid tie system when grid down?

Unlike batteries. PV's are a current source not a Voltage source. Their Voltage varies greatly according to the available light and attached loads. For instance, a panel will hit its Voc instantly the minute enough photons fall on it; even moonlight will do it. But it won't produce enough current in moonlight to do anything. In full, direct sun it will put out its Vmp @ Imp, but otherwise you have great fluctuations in total available power.

The water pumping scenario you mention is a totally different circumstance. It utilizes a linear current amplifier to produce a fairly stable power, but still variable Voltage, output. Since it is driving a DC motor which can run at different Voltages (and therefor different efficiencies) it isn't a problem; there's no call for steady output. It might pump 0.5 gpm or 3.5 gpm, depending on what power is available.

An inverter requires a fairly consistent Voltage input, within its operating range. Grid-tie inverters have a wide operating range and vary their output Wattage accordingly. Since there is "someplace to go" for any amount of power they can produce (either household load or sell to grid) this also isn't a problem.

Off-grid or back-up inverter function is different in that it relies on a fairly stable Voltage source and draws current according to output needs. Without batteries, there is nothing to stabilize the Voltage against the varying current draw.

In short: grid-tie plus no grid equals no power; off-grid plus no batteries equals no power. There's no way around this.

If you want back-up for power outages, a generator is usually the most cost-effective solution.

BB. · December 2010

Re: Power from grid tie system when grid down?

As Marc and others have said, Solar panels are NOT solar batteries...

You are used to how a battery works... Holds around 12 volts from -50 amps to +50 amps.

Solar Panels are a current source... They will output (for example) 7 amps in full noon time sun... Clouds, time of day, birds, dirt, shade, all can reduce the available output current.

Regarding voltage for solar panels--they will output 0-7 amps of current around Vmp--But it is up to the load to adjust its current flow to the optimal voltage... Very few loads do this (linear current boosters, MPPT type inverters/charge controllers, are the common ones around here).

To give you a made-up example... You have a 200 watt panel with Vmp=17.8 volts and Imp=11.2 amps.

And you want to run a 100 watt TV from an AC inverter (ignoring the fact that a typical off-grid AC inverter uses 10.5-15.0 volts and will shut down at other input voltages).

Itv=100 watts * 1/0.85 inverter eff * 1/17.8 volts = 6.6 amps at 17.8 volts

Say a cloud goes in front of the sun, and your panel starts reducing its available current output (approaching 6.6 amps)... As the panel's output voltage falls, the inverter needs to draw more power (more current) because it is a constant power device... Say the voltage falls to 12 volts:

Itv=100 watts * 1/0.85 inverter eff * 1/12 volts = 9.8 amps at 12 volts

Notice that the input DC current requirement for inverter now went up as the available solar energy was going down... The increased current requirement will further pull down the solar panel's output voltage and quickly bring the output to near Zero volts. The inverter will have to shut down, the solar input voltage will rise again, and the inverter will attempt to restart.

Needless to say--most people would not be very happy with the available AC power from such a setup.

Remember that Power = Voltage * Current ... And if one falls, the other must increase for constant power. And no matter how much current you have, if the voltage is near zero volts, the available solar panel output power is near zero watts.

-Bill

Power from grid tie system when grid down?

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