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Technical Editor Margery Conner's PowerSource streams the latest developments in electronic power design and related technologies. Follow Margery on Twitter at: http://twitter.com/margeryc.

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Friday, June 19, 2009

Metrics make a difference in evaluating solar and auto efficiency

Jun 19 2009 11:20AM | Permalink |Comments (11) |

TI’s alternative energy blog, TInergy, has a mild rant on the common-but-mostly-useless $/W metric often used to evaluate the cost-effectiveness of solar power systems. Common wisdom says that when solar panels reach $1/W they will reach cost parity with grid electricity. However, $/W for the solar panel doesn’t taking into account variations in cell efficiency, inverter efficiency, control electronics, and installation costs. In addition, and perhaps more importantly, we all get our electricity bills with rates listed in $/kWh, which can vary widely (in addition to being tiered); $.10/KWh is a good number. $/kWh is the real rate we use to evaluate whether solar power is cost-effective, and it requires knowledge of the efficiencies of each piece of a solar installation.

Of course, it’s natural for TI to take that position, since it’s starting to look more and more like a system company in the solar arena. Solar panel companies will continue to quote $/W because that’s what they control, power inverters vendors will continue to brag about their power in/power out percentage efficiency because that’s what they control, and installers will continue to quote their end price assuming government subsidies because … well, because that’s currently the only way their numbers look competitive to grid power. It’s good to see TI taking a leadership position in encouraging the use of the right metric to understand the overall economies of solar.

Here’s another example of using the right metric to better understand auto fuel efficiency: Replace the common miles-per-gallon used in the US with gallons/100 miles, often used in Europe (only in liters.) From this month’s Popular Mechanics:

“…which is better: Replacing an 18 mpg car with a 28 mpg ride, or going from 34 mpg to 50 mpg? Researchers say that drivers find it easier to get the right answer when efficiency is expressed as gallons per 100 miles (g/100m). 18 mpg versus 28 mpg becomes 5.5g/100m versus 3.6 g/100m, for a savings of 2 gallons every 100 miles. Going from 34 mpg to 50 mpg is the same as switching from 2.9 g/100m to 2 g/100m – only half as big a gain.”

Related entries in: Power Sources/Controllers | Solar/Photovoltaics |

Reader Comments

at 6/19/2009 2:27:08 PM, Jonathan said:
Hear, hear. But, be careful what you say, the eco-ists will drum you out of town for even suggesting that PV solar might not be such a good idea. I think generating electricity using Sol is a lousy idea (at this time) unless you do not have access to the grid. Space is a great application for PV. I'm all for using Sol to heat our water and homes though, that's a good use for that roof top space.

at 6/19/2009 3:02:38 PM, John L. said:
Which is better? .
Stupid question... what defines "better"? becomes my next question.
To get the answer they are apparently looking for, the question should be: " which offers the most improvement".
I don't think the answer is obvious with their question.
Which is why the answer isn't always answered correctly (by their definition).

efficiency..
or
amount of change/improvement...

Two difference questions... two different answers.

I doubt the general public takes note of the difference... the real source of mis-understanding.

I doubt most pollsters of the public know how to correctly present the question without confusing the issue.

Yet I see the general public screw this up all the time.

Do you think the general public uses percentage correctly? .. I am amazed at the mis-use of percentage change vs level of improvement in articles I read in magazines and newspapers... and I doubt a metric change will eliminate the problem.. it will just change where the mis-use/mis-understanding happens.

This week in newspaper: " there was a 150% improvement in average pricing of xxxxx .... xxxx changed from $100 to $150"..." typical screw up.
obviously the price change was an increase of 50%.. not 150%. But so much of the populace will not notice the error.

Because the general pubic has trouble with this simple math.. we have "lies , damn lies and Statistics" being used to justify anything.

at 6/19/2009 3:44:27 PM, mikeS said:
Of course solar is not cost effective yet. It is a relatively young technology for terrestial energy production. How many people 15 years ago would have thought a disk drive would be 80 bucks for 300G when back then they were 1000 bucks for 2G. As engineers become adept at making solar for power the price will fall. Oh, and I have a friend in Hawaii paying 40c/KWh. So power pricing is very region specific.

at 6/19/2009 4:10:07 PM, NoOracleHere said:
I find it encouraging that we're beginning to quibble about the details. Back when solar was $10/W, nobody cared what the inverter cost was because nobody could afford it anyway. When we start quibbling about the details, I know we're getting close.

at 6/19/2009 4:14:32 PM, Jim Jarvis said:
Clearly, we need a systems efficiency approach to a figure of merit for PV Solar. But those who believe that it's only a matter of time before things reach parity with fossil fuel need to revisit their physics.

The Quantum Efficiency of Silicon, or the various III-V mixes being used simply don't allow for another factor of 3 or 4x improvement. And right now, PV solar costs between 4x and 5x that of fossil fuel generation. There's a significant gap.

Coal is plentiful. We know how to control the effluent gases, even though we're not doing a super job of doing it, right now. Even $150/bbl oil won't make PV Solar a stunning economic investment, absent government subsidies.

When you add up cell efficiencies and inverter efficiencies, it makes the idea of PV-Steam solar stills look extremely interesting. And it justifies the cost of the transmission lines and real estate, to locations where they pay off. Rotating steam turbines are well proven and highly efficient.

There is a forecast which says we will demand 40% more electricity in 10 years than today. The present economy may slow that growth, but we don't have that much capacity online at present. We must build plants. The question is, what, and at what environmental, social, and economic cost.

at 6/19/2009 7:29:43 PM, jeeshenlee said:
Having a proper measurement unit to evaluate Solar Power "System"'s efficiency is necessary to overcome the hype.

at 6/20/2009 7:07:40 AM, GPM said:
John L.:

The original Science article described by Popular Mechanics did ask the question clearly. Here's the original citation:

Larrick, R. P., & Soll, J. B. (2008). The MPG illusion. Science, 320, 1593-1594.

Try googling "mpg illusion" for links that will take you to a free link.

at 6/20/2009 2:35:55 PM, ScottsAI said:
Lets keep changing how things are measured to keep people totally confused. Today people do know 28 mpg ride is cheaper than 18 mpg ride. Change to 5.5g/100m they will have to wonder if that is grams per 100 meters or what?

Problem comparing the cost of grid vs solar is comparing a monthly fee to the purchase of a system. One is a fixed up front cost vs the other has an unknown fee every month forever.
Armed with the knowledge $1/w competes with grid, you understand as least its costing more than grid power based only on that cost. Compare solar cost by $/kwhr is about as meaningless as you can get. What assumptions on solar hours were used? Time value of money? Grid $/kwhr cost increasing over the same time period? Are the assumptions based on the life time of the panel or electronic or what?
I see tremendous opportunity to snow customers on a system, in fine print claim the output is based on the mountain tops of Colorado, yet the system is sold in Wisconsin. Claim system is good for 50 years on cost bases with 25 years warranty on panels, not including installation cost in the $/kwhr number etc. Very bad idea.

Calculate the "present value cost" of the grid power using the same time period as the solar system cost, state interest rated used, assumptions on power increase cost. Now you have two "today" cost to compare basing your choices on. (Make sure you include meter reading fees!)

Homes are built off grid, people are shocked at the cost to bring grid power into their home with the ongoing monthly power cost. Comparing all the cost up front allows for a simple comparison of the choices. Easily make changes in assumptions seeing how the two cost then compare.

at 6/22/2009 10:46:12 AM, Gator said:
Solar water heaters are one of the big answers, just some pipes and glass, Photovoltaics? Only if the Goverment can continue to subsitize them. At this rate they should be able to keep subsidizing for at least 6 more months before they are totally broke. If you are off grid then OK. On grid forgetaboutit. Coal is the answer as soon as we design a good scrubber. Until then Nuclear.

at 6/30/2009 1:56:06 PM, DaveW said:
$/W is simple and useful. Payback time in years is approximately ($/W) / ($/KWH). For $1/W and $0.13/KWH, the payback time is 7.7 years. This is an OK payback time if the system is good for 10+ years.

These numbers are for Silicon Valley (San Jose), one of the largest markets for solar in the US. It receives 1100 hours of full sun equivalent sunlight. Multiply this by ~90% system efficiency and you get 1000 full sun hours/year. I have observed these efficiencies.

PG&E rates for Tier 2 (101%+ of minimum baseline = typical use) is $0.131 per KWH. Tier 1 (minimum lifeline) is $0.115. Tier 3 (131%+) is $0.259. Tier 4 (200%+) is $0.379, and Tier 5 (300%+) is $0.441. Higher rates = quicker payback and/or 7.7 year payback at higher $/W.

Solar looks interesting even today, with panels available at $3.50/W in low quantity. (I bought one.) If you are in Tier 5, your payback period will be 8 years. If you buy just enough panels to get you out of Tier 5, the economics get even interesting.

The panel cost is still the largest fraction of the system cost. As its cost goes down, volume will tend to drive down the inverter cost, the next largest.

So $/Watt is not a bad place to start. It gives you a way to calculate the ROI payback time. If the payback time gets down to less than 3 years, it is a no-brainer. Even at 10 years, it is an OK investment.

at 11/4/2009 4:38:09 PM, seacrow said:
“… 5.5g/100m versus 3.6 g/100m, for a savings of 2 gallons every 100 miles. ... 34 mpg to 50 mpg is the same as switching from 2.9 g/100m to 2 g/100m – only half as big a gain.”

What moronic math & conclusions! The first case is a 34% reduction in gas consumption per mile, the second is a 31% cut. So the reduction is about the same on a percentage basis, but what counts is the charge at the pump.

OTOH going from 18 to 50 MPG means a 64% reduction in gas consumed and 2/3 fewer stops on long trips. Sounds like a good deal to me. Whether the hybrid-plant cost premium is worth it almost beside the point.

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