solar grid fallback system

in8inite

i live in an area where we have frequent power outages. minimum 8 hrs of power is guaranteed(4 in day and 4 in night). so i need a solar grid fallback system.

these are the solar irradiance figures of my area. measured in kWh/m2/day onto a horizontal surface
Jan Feb Mar Apr May Jun
3.71 4.64 5.73 6.17 6.39 6.04

Jul Aug Sep Oct Nov Dec
5.19 4.78 4.99 4.79 4.07 3.45

i have attached two images of the kind of system i need. these images explain it better than words.

Attachment not found.Attachment not found.

as you can see in the images there is an all in one unit - PCU which does the same thing but i am a little skeptical about it for two reasons

1. these units are not properly reviewed. not enough consumer info about it.
2. these units switch to battery once it is fully charged even when other power source is available and only recharge it after reaching a certain preset level.

so how do i achieve this kind of system if these units(PCU) are not good.

Cariboocoot

Re: solar grid fallback system

Welcome to the forum.

You'll be happy to know I've designed many such "unreliable grid" systems. Usually for people who can't even count on 4 hours of grid power.

Ignore those pre-built systems. They are not likely to be what you need no matter what.

Now on to what you do need. You need a kW hour consumption number. If you don't know how much power you will use in a day you can't plan a system to supply it. You probably don't need solar because you have grid power, even if it is unreliable. As such the PV and charge controller would add to the price but not necessarily to the quality.

Here's what you are looking doing:
1). Supply 'X' kW hours per day to run your loads. You need to know that number as well as the Voltage and frequency (230 VAC 50 Hz, 240 VAC 60 Hz, or 120 VAC 60 Hz) and maximum power at any given time.
2). Having sufficient battery capacity to supply this amount of power.
3). Automatically recharging those batteries whenever the grid is up and running.
4). Option extra charging source such as solar or a generator in case the grid is too undependable.

The basic equipment list would be an inverter-charger capable of supply the proper Voltage, Frequency, and maximum Watts plus a battery bank to supply the daily Watt hours. Simple and effective.

bill von novak

Re: solar grid fallback system

in8inite wrote: »

i live in an area where we have frequent power outages. minimum 8 hrs of power is guaranteed(4 in day and 4 in night). so i need a solar grid fallback system.

Well:

1) You don't need solar, although it's nice to have. What you need for reliable power delivery is a UPS (battery/charger/inverter.) They are similar to but not identical to battery based solar power systems.

2) There are plenty of good UPS-capable inverters out there; the Outback GT line is one of them, and I highly recommend them.

If you want to start cheap:
a) figure out your longest outage and your AVERAGE load during that time. That gives you a number in kilowatt-hours. Multiply that by 2.5 and that's the battery bank size you need.
b) figure out what your PEAK load will be during that time. That gives you a number in kilowatts. That's what your inverter needs to support. It will also indirectly determine your system DC voltage. (More power = 24 or 48 volts)

The above gives you backup power. If you want to add solar later:
c) decide what string voltage you want to operate at. Higher voltages = cheaper wiring and larger arrays; lower voltages = more flexibility with odd panel counts
d) choose an MPPT charge controller that will bring your string voltage to your battery system voltage.

in8inite

Re: solar grid fallback system

my total load is 3.8 kW hours per day.
we have utility power at 230 VAC 50 Hz. i don't understand what do you mean by "maximum power at any given time".

i was thinking of same kind of system you are suggesting. but i will like the option of an extra charging source just to be sure.

Cariboocoot

Re: solar grid fallback system

in8inite wrote: »

my total load is 3.8 kW hours per day.
we have utility power at 230 VAC 50 Hz. i don't understand what do you mean by "maximum power at any given time".

i was thinking of same kind of system you are suggesting. but i will like the option of an extra charging source just to be sure.

Maximum power at any given time is the most loads you'll have switched on at once in Watts. It is important because an inverter can only supply it's rated power: 3.5kW inverter can put out 3500 Watts. If the loads exceed that the inverter will fault. This and product availability will determine which inverters are viable.

But now we know we're dealing with the European power standard of 230 VAC 50 Hz, and 3.8 kW hours per day. So if we convert that to DC and add inverter consumption we get about 4.7 kW hours - or just about 100 Amp hours on a 48 Volt system. Very easy to deal with. Since the grid will be available approximately 1/3 of the day you could reasonably size the battery bank for "50% DOD" or about 200 Amp hours @ 48 Volts. What sort of batteries you can get where you are will be a factor too. If we were planning this for the North American market it would be a simple string of GC2's giving 220 Amp hours @ 48 Volts.

Not difficult to recharge such a battery bank from solar either. Basically you'd only need around 686 Watts of array and a small MPPT controller (I'm using lowest charge rate here as there will be some grid charging).

in8inite

Re: solar grid fallback system

bill von novak wrote: »

Well:

1) You don't need solar, although it's nice to have. What you need for reliable power delivery is a UPS (battery/charger/inverter.) They are similar to but not identical to battery based solar power systems.

2) There are plenty of good UPS-capable inverters out there; the Outback GT line is one of them, and I highly recommend them.

If you want to start cheap:
a) figure out your longest outage and your AVERAGE load during that time. That gives you a number in kilowatt-hours. Multiply that by 2.5 and that's the battery bank size you need.
b) figure out what your PEAK load will be during that time. That gives you a number in kilowatts. That's what your inverter needs to support. It will also indirectly determine your system DC voltage. (More power = 24 or 48 volts)

The above gives you backup power. If you want to add solar later:
c) decide what string voltage you want to operate at. Higher voltages = cheaper wiring and larger arrays; lower voltages = more flexibility with odd panel counts
d) choose an MPPT charge controller that will bring your string voltage to your battery system voltage.

yes i understand what you are suggesting
lets examine a case here
suppose i have 500 va inverter charging a 24v battery bank @ 240 Ahr. now i want to add solar and achieve the system depicted in the images of my first post. i.e. falling back on grid when there is no solar power and then the battery. how should i go about that?

in8inite

Re: solar grid fallback system

Cariboocoot wrote: »

Maximum power at any given time is the most loads you'll have switched on at once in Watts. It is important because an inverter can only supply it's rated power: 3.5kW inverter can put out 3500 Watts. If the loads exceed that the inverter will fault. This and product availability will determine which inverters are viable.

But now we know we're dealing with the European power standard of 230 VAC 50 Hz, and 3.8 kW hours per day. So if we convert that to DC and add inverter consumption we get about 4.7 kW hours - or just about 100 Amp hours on a 48 Volt system. Very easy to deal with. Since the grid will be available approximately 1/3 of the day you could reasonably size the battery bank for "50% DOD" or about 200 Amp hours @ 48 Volts. What sort of batteries you can get where you are will be a factor too. If we were planning this for the North American market it would be a simple string of GC2's giving 220 Amp hours @ 48 Volts.

Not difficult to recharge such a battery bank from solar either. Basically you'd only need around 686 Watts of array and a small MPPT controller (I'm using lowest charge rate here as there will be some grid charging).

maximum power at any given time will be 500 W.
i understand the calculation.
now my question is how do i wire up such a hybrid system to utilize both solar and grid with battery as last resort.

bill von novak

Re: solar grid fallback system

in8inite wrote: »

suppose i have 500 va inverter charging a 24v battery bank @ 240 Ahr. now i want to add solar and achieve the system depicted in the images of my first post. i.e. falling back on grid when there is no solar power and then the battery. how should i go about that?

You might have one of the more common misconceptions, which is that you can "steer" the power from one place to another. (Graphics like the ones in your original post makes it look like the system is busily switching power from some input to some output.) In general, that happens inherently due to very basic electrical maxims (Kirchoff's Law etc) rather than explicitly.

So if you have a system that is charger -> battery -> inverter, then any power you add from solar will decrease power required from the charger, and the input from the grid will decrease. I don't know of any 500 VA charger/inverter/transfer switches (other than some cheezy Meanwell ones) so you'd have to do those two functions separately.

Cariboocoot

Re: solar grid fallback system

in8inite wrote: »

maximum power at any given time will be 500 W.

Are you sure? A refrigerator, for example, draws <200 Watts running but has a start-up surge demand over 1000 Watts. It is important to know if there will be any such surge demands as well.

now my question is how do i wire up such a hybrid system to utilize both solar and grid with battery as last resort.

It's really quite simple. There is the AC wiring which looks like this:

GRID ---> INVERTER CHARGER ---> LOADS
When the grid is active load power will be drawn from that and the inverter will charge the batteries. When the grid goes down the inverter will supply the loads, drawing power from the batteries.

Then there is the DC wiring which looks like this:

SOLAR PANELS ---> CHARGE CONTROLLER ---> BATTERIES <---> INVERTER CHARGER

stephendv

Re: solar grid fallback system

Victron do a 800W inverter/charger with built in transfer switch: http://www.victronenergy.com/inverters-chargers/multiplus-12v-24v-48v-800va-3kva/
It would be setup as 'coot described above.
It will do almost what you want, except the power from the inverter will always come from the grid if the grid is connected (not from solar).

in8inite

Re: solar grid fallback system

Cariboocoot wrote: »

Are you sure? A refrigerator, for example, draws <200 Watts running but has a start-up surge demand over 1000 Watts. It is important to know if there will be any such surge demands as well.

i know that. there won't be any.

in8inite

Re: solar grid fallback system

Cariboocoot wrote: »

It's really quite simple. There is the AC wiring which looks like this:

GRID ---> INVERTER CHARGER ---> LOADS
When the grid is active load power will be drawn from that and the inverter will charge the batteries. When the grid goes down the inverter will supply the loads, drawing power from the batteries.

Then there is the DC wiring which looks like this:

SOLAR PANELS ---> CHARGE CONTROLLER ---> BATTERIES <---> INVERTER CHARGER

if i am getting this right, then the system should look like the image below
Attachment not found.
correct me if i am getting this wrong

stephendv

Re: solar grid fallback system

in8inite wrote: »

if i am getting this right, then the system should look like the image below

Exactly right. As you can see all the magic happens in the inverter/charger, they typically have a built in transfer switch which can switch from grid to batteries in under 20ms (check specs), so that you won't notice the switch over. Depending on the make model of the inverter/charger, they may include internal programmable relays which you can use to modify the behaviour. E.g., by default most will always use grid power if it's connected, but you could override this by putting a relay between the grid and the inverter which you can then program so that if the batteries are above a certain voltage (or above a certain state of charge), then use battery power, not grid. Switch back to grid when the batteries dip below X volts, etc.

The extent and ease with which you can program this type of behaviour varies considerably between different brands and models.

ButchDeal

Re: solar grid fallback system

in8inite wrote: »

if i am getting this right, then the system should look like the image below
Attachment not found.
correct me if i am getting this wrong

Have you looked at an OutBack Flexpower one or Flexpower two system. They do exactly what you are needing. I have the Flexpower One in my home.
It acts like a large 3kW UPS with solar charging and mine is programmed for net metering, feeds excess power back to the grid when available. It has a setting to avoid doing that, and basically just use the grid for backup.
We use a lot more power than you but I have it set up on just emergency panel with just the things that need power all the time, fridge, freezer, well, electronics.

You could do the same and maybe get by with a smaller system.

http://www.outbackpower.com/outback-products/integrated-systems/item/flexpower-one

South Africa

Re: solar grid fallback system

There are 2 other options:

a) Investigate off the shelf inline (online more expensive but much better option) UPS's that can charge the same batteries that a solar system would ideally use. Me, I would look at 48v, 100ah or bigger bank so that I can add as many batteries as I need in case of power failure. It is probably the 'cheapest' option with the least amount of tech knowhow required. Do NOT go for offline UPS's, they will NOT last. Online UPS is also much more robust the any other types due to the fact that can handle a very unfavourable grid supply for extended periods, and it will protect your equipment more.

b) And if your mind is set to go solar with grid as backup and battery backup if the grid is unavailable, and you do not yet want to spend on a OutBack FLEXPower One / Two, I have built a switch to switch between utilities / solar. Am now in the process of refining it to include that if their is no grid and no sun, to use the batteries (which are only charged via solar). This system of mine runs all the lights 100% from the batteries, it is just the extra loads that are switched at night / insufficient solar power to not use the batteries unless it has to. Yes, I can make it all 100% off grid with grid as backup, but the ROI is not worth it as grid power is cheaper than batteries.

But if you can, because a and b above are not what you want to do, the OutBack FLEXPower One / Two option is the best ito plug and play and functionality.