Inexpensive Home Building

Cut through the jargon and nonsense of home building and house construction by starting from zero dollars and trying to figure best-value bang-for-your-buck when choosing construction methods or construction materials. My research might answer some of your questions and at other times perhaps you have the knowledge or experience to post the answers to my questions and thereby help others too. The goal is an affordable and sustainable home for all.

Monday, February 19, 2007

Myth of Photo-Voltaic Solar Power?

I felt that this part of a previous post deserved promotion to its own post:

The Myth of Photo-Voltaic Solar Power?

Solar power provides about 1,000W (1kW) per square meter at the equator's sea level. However, typical photo-voltaic (PV) solar panels are only 15% efficient so a square meter of solar panels provides only 150W. Further, even compact fluorescent lights (CFL) are only 20% efficient so 150W of CFLs provide only 30W of light. Incandescent light bulbs are only 5% efficient so 150W of incandescent light bulbs provide about 8w of light—less than 1% of the solar power that hit the solar panels.

So, the expensive “solar” system is 97-99% inefficient at providing electric light.

(For a similar reason, PV solar-electric water-heating is less efficient than passive solar water-heating which puts the water tank in direct sunlight.)

Then consider that the sky is not always clear and sunny. Solar panels will generate less in winter and more energy in summer because of the number of daylight hours. However, even a seemingly clear day might have particles that reduce available power to 80%, moderate clouds can reduce available power to 33% and heavy clouds can reduce it to 5% (a 45W panel would provide about 2W).

What do you get at the end of the day? The Southwest USA accumulates the equivalent of about 5 full sun hours per day. However, much of the US population should expect only about 3 hours or less of full sun equivalent per day (at 3 hours, a 45W solar panel would provide 135Wh per day, which might power a medium TV for 1 hour per day) . Northern Michigan should expect only 1 full sun hour per day (a 45W panel would provide 45Wh per day, which might power a 60W light bulb for about a half-hour per day after accounting for system inefficiencies).

Backup Logic

Backup power should be independent of what it is replacing, or have an inverse relationship to it. The obvious disadvantage of solar-electric lighting as a backup to natural solar lighting is that you are trying to squeeze more energy from the very thing that is disappearing on you.


At 1:16 PM, Anonymous Anonymous said...

You have some interesting info but I will have to differ with your math & figures.As a user of solar power on my RV, I have done alot of trial & error and I currently have only a 5w panel that receives an average of 4 hours of sunlight per day to charge 1- 80 Amp/hr deep cycle battery and it is more than enough to power flourescent lights, (4@ 6v, 0.4a)about 4 hr per night, my water pump (12v, 2a) for about 1 hr per day to keep the system pressurized. I also power the blower in my furnace when needed and still have some power left over to run my 13in color TV (75w 1.6 Amps)for about 1.5 hrs. It seems you have done some math and paperwork but I ask have you ever actually tried it out first hand.
Also, I have less than $100 in the whole PV/Battery/lights/wiring & fuse block. You have a great post and I thank you for some interesting info.

At 8:27 PM, Blogger J at IHB and HFF said...

Hello. I am shopping before I buy and trying to square all the conflicting information out there. On a 12V 12.5Ah per day, your 80Ah battery can carry you through the better part of a week with no solar input (though you do not want to empty the battery). Still, it seems like you are generating 20Wh per day but using at least 150Wh per day and the 5W panel should lose the race by the end of the week. One possible factor is that devices often draw less power than the label indicates (In my “Real Power” post, I had a “45W” fan that drew only 15W, so it helps to measure actual load during use). On the other hand, you can lose 20% or more power to “system losses” (battery storage, drain, etc.). Even 24-hour sun could not keep up with the provided figures so maybe we overlooked something (charging the battery off the vehicle engine while driving could explain it). If someone were designing your system, I think that they would say the 5W panel is inadequate even in normal conditions without any other supplemental input. You can try an online solar-power-system calculator to see its recommendation. If I am all wrong, I would like to know.

Thank you and please post again.

At 8:36 PM, Blogger J at IHB and HFF said...

I used 75W for the TV but 1.6A * 12V is only 19W. Even the lower figure means maybe 60Wh per day for everything, triple the panel's 20Wh input as described (5W * 4h).

At 1:37 PM, Anonymous Anonymous said...

My Mistake, On the TV you are correct, my memory did not serve me well there. And for the others you are also correct as I would like to clarify my post. My battery sometimes does start to run out of steam towards the end of the week, But there are many variables involved that save me. I may be out to eat, or working late or visiting friends or many volunteer activities 1-4 nights a week that conserves alot of my power. I must say that if I was home 24/7 or 12/7 I would run out of battery and be in bad shape! My only point is that is is possible to live on less than the "math" may show or even the manufactures recommendations. I did my own fuzzy math to get started 5 yrs ago and have done alot of trial and error since. Some good things I've done that may help another reader are if I may, Designed my little system so the battery is very very close to the water pump lights, blower motor etc to reduce resitance and voltage drop, also the leads from my 5w pv on the roof is only about 4' long to the battery. all my connections are very clean, tight and or soldered. You have done alot of great math. I hope you have good luck with your system, with much less problems than i had getting started.

At 8:46 PM, Blogger J at IHB and HFF said...

I appreciate the follow-up "from the field." The short wires with good connections are smart. I was trying to convey realistic expectations, so your comments about daily 29Wh of TV (19W * 1.5h) versus the more common 1,000Wh of TV (roughly 150W * 7h) help.

Stop by anytime.

At 10:36 PM, Blogger J at IHB and HFF said...

To clarify, my original post was trying to convey realistic expectations to others. I agree with your main point that 'mileage may vary.'


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