Grid tie system in 2014

BB. wrote: »

Mike welcome to the forum and the Alice through the Looking Glass world of AC inverters.

What used to be a (relatively) simple subject has been turned into a maze of options because of electrical engineers and marketeers.

There are two basic classes of AC inverters... The Grid Tied units that take power from the solar panels and "inject" that into your home's electrical panels and help reduce your billed power (save money in areas with high electric power costs). And Off Grid units which take power from a battery bank and generate a separate AC power--Generally used for off grid homes and emergency backup power.

To make things a bit more complex. There are "hybrid AC Inverters" with battery banks that can work both as a GT inverter and pump "excess" charging current back into the grid, and if the grid fails, the inverter flips over to AC off grid mode and supplies power to local AC loads from the battery bank (and solar panels).

And even more complex, SMA has created a pure GT AC inverter (no batteries) that during the middle of a sunny day can supply ~1,200 watts to an auxiliary AC power outlet that can be used for emergency power during the middle of a sunny day. When the sun is not in the middle of the sky (or when clouds pass overhead), the aux AC power "goes away".

The Radian, I believe, is the typical Hybrid AC inverter. It needs a battery bank to provide AC power--Either to the grid or for emergency backup/off grid power operation.

By the way, the rule of thumb for designing a off grid/hybrid AC power system is to have ~100 AH @ 48 volts minimum per 1kW of AC loads and/or solar array. The battery bank has to be large enough to "buffer" the AC loads and DC ripple current when operating in Grid Tied mode.

It can be difficult/costly to "grow" a solar power system... If you are doing this for emergency/backup power, then I would do things in this order (as money becomes available)--assuming the end point is a hybrid AC system capable of GT and OG operation:

1. AC Generator + Fuel supply for emergency operation (Naturl Gas and Propane are great fuels for long term operation/storage. Diesel is nice but smelly and gasoline is available everywhere--but both can have issues if stored more than ~a few months to a year).
2. AC Hybrid Inverter + Battery Bank (full size battery bank). Recharge from AC power/Genset during power outages
3. Build out solar array (you can add solar panels in several stages).

On the battery bank, you can start with "cheap" batteries and learn how to operate your system and figure out how many batteries you really need... And when the first set "goes bad" in ~3-5 years (or you "murder" them with poor maintenance and/or oops by leaving loads on when you go on vacation), you can then replace with your "dream set".

In general, batteries are the weak point of off grid/hybrid power systems. They are easy to damage (over/under charging, running dead when somebody leaves too many loads on, poor maintenance, well pump runs water into a broken pipe, etc.) and age even when operated correctly (3-5 years for "cheap batteries", perhaps 10-15+ years for expensive batteries).

And, with Grid Tied systems, your utility needs to allow GT solar systems (not all do)--And some utilities are overhauling their rate plans to "punish" GT solar folks and even those customers that work hard on conservation...

Old days of $4.50 per month connection fee and $0.20 per kWH rates are being replaced with $40-$100 per month connection charges and $0.05 per hour load rates (or even $0.025 per kWH paid for GT solar generated power)...

In general, solar GT power can be "break even" or even save money. Off Grid/Hybrid solar will never save money unless you do not have (cost effective) grid power available. Of course, emergency power can be worth every dime if you are hit by storm/earthquake and our out of power for weeks to months at a time.

But remember, in major metropolitan areas, the cities are shutting down municipal water because they don't have power for pumping and/or sewer backups/flooding because of lack of lift pumps/sewage treatment during power emergencies. Figuring out your emergency plans can be a nightmare.

-Bill

BB. wrote: »

And to add a bit more complexity...

Many (few, some, most?) Off Grid AC sine wave inverters are capable of being "back fed" power and actually capable of recharging their battery bank directly.

However--Many (and older) AC sine wave OG inverters do not have any way of regulating the battery state of charge and can be "over charged" unless other methods of power control are employed (turn off GT inverter when battery bank is full, add "dump/shunt" loads and controller to bleed off excess charging current, etc.).

SMA, Schneider/Xantrex, and Magnum (at a minimum, Outback?) have some newer OG Inverters that do have methods of controller battery charge when "AC Coupled" to a GT inverter (providing charging current). Usually this is done by varying the AC frequency (i.e., move from 60 Hz to 59/61 Hz and "knock" the GT inverter off line).

SMA (Sunny Island series) has a really interesting method of varying the frequency to "throttle" the GT inverter (when configured for "micro grid"). Unfortunately, in the US/Canada, SMA solutions tend to be really pricey.

The choice between solar array -> solar charge controller -> battery bank vs solar array -> GT inverter -> AC sine wave inverter -> battery bank...

I am not sure what is the "best answer"... I would tend towards the traditional DC Solar Charge Controller and not bother with the GT AC Coupled solution unless there are some extenuating circumstances.

In any case, I would highly suggest doing a paper design with several solutions and figure out the cost / benefit ratios for your needs. There are few black & white answers in solar design (or engineering). Everything is trade offs.

-Bill