Harvest Efficiency revisited

Re: Harvest Efficiency revisited

It depends on the system/needs/usage... Some of the factors we use as rules of thumbs assuming sea level with typically non-winter conditions:

• 0.81 solar panel Marketing rating derating
• 0.95 MPPT controller derating
• 0.77 MPPT or PWM charge controller+solar panel derating
• 0.80 typical flooded cell lead acid battery derating
• 0.90 typical AGM lead acid battery derating
• 0.85 typical AC inverter derating

So, given X hours of sun per day, the deratings would look like:

• 0.81 panel * 0.95 MPPT controller * 0.80 battery * 0.85 AC inverter eff = 0.52 typical Off Grid end to end system eff

4 hours of sun * 0.52 OG sys eff * 1,000 Watt array = 2,080 Watt*Hours per day

A Grid Tied (no battery) system will be around 0.77 typical efficiency (solar panel marketing to power to the AC main panel).

Notice that "array potential" should have hours of sun in it... So, the typical array will "see" around 4-6 hours of "noon time equivalent" sun per day--Or 1/6 to 1/4 (0.16 to 0.25) array availability factor.

If you want to compare this to wind turbines where small wind maybe 5% to 15% of name plate and industrial wind farms may be 25% of name plate--local weather/seasonal conditions will vary.

And, you have to take shading (buildings, trees, chimneys, etc.) into account for solar panel shading (solar electric panels perform very poorly when even partially shaded).

Compare this with a genset/utility power station which will run close to 100% availability for days/weeks/months at a time. However, efficiency still depends on loading (lightly loaded power systems waste more "fuel" than those running steady loads near capacity).

What kind of harvest efficiency number are you looking for, and what are you comparing?

-Bill

Re: Harvest Efficiency revisited

Two issues (that I see).

First, MPPT charge controller only "harvest more" than derated solar panel values in extremely cold weather.

If you live in southern Canada, you probably would see ~10-15% more than a PWM controller (well designed system) in winter (average peak increase).

There is being 35% more efficient... If efficiency of the average MPPT controller is 95% efficient, that is 5% of loss.

0.05 loss * 0.35 improvement = 0.0175 "improvement"

Or

96.75% controller efficiency. This is better, but not a huge improvement for the end user. For the engineer, that is 35% less heat inside the controller that needs to be removed--This is a big help for him (smaller heat sinks/fans, lower operating temperatures for longer life, etc.).

And there are some MPPT algorithms that are better than others... You might see 10-15% difference in overall harvest between a "bad" and a "good" algorithm... (some of the older MPPT controllers did not really actually adjust for optimum Vmp*Imp -- They simply did something like 0.8 * Voc measured. It works, just not the best.

Problem is, it is difficult to get better than 10% accuracy without setting up laboratory test gear and reference panels... $10-$20k minimum.

Good MPPT controllers are going to cost more than with the cheaper ones... Reading reviews and asking questions is usually going to get you "close enough" to optimum to make some good decisions.

Paying 10% more for a controller that gives you 10% more power--May or may not make sense (i.e., how much does 10% more solar panels+mounting cost vs the increased controller cost).

-Bill

Re: Harvest Efficiency revisited

Technically speaking, all of the US based MPPT charge controllers that I know of are "buck mode" converters--They take higher voltage and down convert it to lower voltage.

The only boost converter solar charger company that I am aware of is Genasun.

http://genasun.com/all-products/solar-charge-controllers/for-lead/gvb-8a-pb-solar-boost-controller/

I am sure there are others--I am not in the solar business and do not watch the industry.

More or less, the "optimum" Vmp voltage for a 12 volt battery bank is ~17.5 to around 18.6 volts. When you add Vmp suppression because of hot cells under direct sun, wiring and controller voltage drop, that Vmp voltage panel will fully/correctly recharge a lead acid battery under most conditions.

If you use a PWM controller with Vmp over that ~18 volts for a lead acid battery, yes, the panel/system efficieny will drop:

PWM eff for 12 volt battery bank ~ Vmp-optimum / Vmp-actual where Vmp-actual is >~17.5 volts

For a 100 volt Vmp-array on a 12 volt battery bank:

Eff ~ 18 volts / 100 volts = ~0.18 eff

For Vmp~18 volt panels on a 12 volt battery bank, there is very little efficiency improvement with a MPPT controller over a PWM controller for ~9 months of the year.

For very cold months in the deep north, you can see around a ~10-15% improvement due to Vmp increase in sub zero weather.

For High Vmp-array arrays--Yes, MPPT controllers are usually the only cost effective solution for charging batteries. However, it is a trade off (cheaper GT panels vs more expensive MPPT controllers).

-Bill