Overall system efficiency and performance of a solar powered system

bikiranguha

Hi all,

I am looking for some information about what is the current overall efficiency of the energy conversion and storage part of a solar-powered system (inverter, batteries, connections, charge controllers). Also, any breakdown of the efficiency for each of the components would be highly appreciated.

How much is the expected no. of cycles available from deep-cycle Valve Regulated Lead Batteries?

I am looking for all this info for my research project, so any mention of links where I can get updated info about all these things would be great.

Thanks.

BB.

Re: Overall system efficiency and performance of a solar powered system

Welcome to the forum Bikiranguha,

Your first stop is learning all about batteries. They are both the "heart" and weak spot of most solar power systems:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

The general rules of thumbs for efficiency work out to:

~81% derating for solar panels (warm summer days--Below freezing, you can do better)
~95% for charge controller derating
~80% for flooded cell batteries (typically 80-90%--depending on how you cycle them)
~90% for AGM/Sealed/GEL batteries (typically 90-98%--avoiding high voltage charging/equalization gains efficiency)
~85% for typical AC Inverter
~80% for typical battery charger efficiency (can be >90% for some units)
~67% Power Factor for typical battery charger (can be >95% for power factor corrected chargers)

So, for a typical off grid system with flooded cell batteries and "conservative" efficiencies:

• 1,000 watt array * 0.81 panel derating * 0.95 charge controller derate * 0.80 batt eff * 0.85 inverter eff * 4 hours of typical sun = 2,093 Watt*Hours of 120 VAC energy per day (minimum for ~9 months of year in much of north America) per 1,000 watt of solar panels

Those deratings are typically "good enough" to plan and build out a typical off grid power system where you (mostly) charge during the day and use power in evenings/mornings.

If you have specialized needs, you may need to take other factors into account. Most AC inverters use around 6 to 40+ watts just being "turned on" (tare losses, the larger the inverter, the more losses).

Also note that batteries have self discharge (from a few percent per month for AGM to as much as 2% per day for old forklift batteries). And batteries ususually have minimum charging limits (again another rule of thumb)--Where "extra large" battery banks are not usually worth the extra battery and solar power cost (storing enough power for 2-3 days of "no sun" is usually the cost effective limit). Use a genset (and/or reduce power usage) during poor weather conditions.

Cycling life from a battery--Ends up being (usually) from 500 cycles to several thousand cycles (recommend cycling to 75% to 50% state of charge typically, and never below 20% state of charge). However, the actual cost of stored/cycled power, is actually quite flat once you cycle deeper than 20% (and do not take battery bank dead). I.e., a small battery bank may cycle life 3 years, and a 2x larger battery bank may cycle 6.6 years (2.2x longer)... Sometimes it is not worth the costs of a large battery bank (more panels, more batteries, more batteries to replace). And if you have an "oops" (left well pump turned on during vacation), you may kill your very expensive battery bank.

Other issues too... Besides cycle life, there is aging life. Lead Acid batteries are defined at 77F/25C. For every 18F/10C increase in battery bank temperature, aging life is 1/2. (and cold batteries will age slower. -10C, battery will last ~2x longer).

I like to work with a "real project" that means something too you (i.e., is this a small cabin, large home, EV charging station, in Hawaii or Alaska, etc.). Going through the design steps is usually easier to see the overall picture... Then if you have specific questions, we can talk about those (why flooded cell or AGM/VRLA, etc.)...

We try for "balanced designs/systems".... If you take any one element to the extreme, it frequently causes issues that have to be addressed in other areas.

-Bill

vtmaps

Re: Overall system efficiency and performance of a solar powered system

BB. wrote: »

The general rules of thumbs for efficiency work out to:

~81% derating for solar panels (warm summer days--Below freezing, you can do better)
~95% for charge controller derating
~80% for flooded cell batteries (typically 80-90%--depending on how you cycle them)
~90% for AGM/Sealed/GEL batteries (typically 90-98%--avoiding high voltage charging/equalization gains efficiency)
~85% for typical AC Inverter
~80% for typical battery charger efficiency (can be >90% for some units)
~67% Power Factor for typical battery charger (can be >95% for power factor corrected chargers)

I have my doubts about the 67% power factor derating. You certainly need to size your wiring, fuses, and generator for the 'apparent' power (VA), but only the 'real' power (watts) used by the battery charger matters to the overall efficiency of the system.

--vtMaps

bikiranguha

Re: Overall system efficiency and performance of a solar powered system

Thanks for your detailed reply, BB.

I am actually trying to write a proposal on trying to optimize the performance for a solar-powered car-charging station. There is a small 300W pv standalone system to power a computer, and its currently not being used. Initially I plan to use the equipments there to build a mini-prototype. However, this time, I also plan to make the system grid-tied by proposing a 300W 3 phase inverter. Additional equipments, like those to monitor and implement algorithms so that optimization is possible, will have to be purchased. However, I don't know exactly which equipments to propose and how much they will cost. I think the control equipments are already here, like microprocessors and matlab.

I would like to know your opinion about what sort of equipments I would need, and how much they cost. The proposal can go upto $10000 max.

solar_dave

Re: Overall system efficiency and performance of a solar powered system

You need to know about your loads first off. If your grid tie then it is as simple as what the car charger draws times the hours of use per day to determine your break even point. From that you can determine the amount of solar required to back feed the grid to get a net zero cost. Of course you will need to know the net metering policy of the utility you are back feeding.

For instance my Chevy Volt draw 3.5 kw X about 4 hours for a full recharge from full discharge which equals about 13.5 - 14 kWh. Assuming a full charge every day I would need to generate about 420 kWh a month. Here in Phoenix where the conditions are pretty excellent, I would need about 4Kw of STC rated panels to cover me all the months of the year (with a slight short fall in Dec and Jan). I would have 150% of my needs in the best months of April and May.

In reality we don't drive enough to require a full charge every day and historically for 2 Chevy Volts we use roughly ~250 - ~375 kWh a month (8 to 12 kWh a day average).

If you intend to have this station recharge multiple cars per day you will need to know if they can take advantage of the 6.6 kW charge rate option offered by some manufacturers and also scale the time accordingly to a daily consumption figure. Much depends on the charging station capacity and the amount of time it is in use.

BTW a typical EREV charging station runs from on the low end about $500 all the way to about $1750.