Solar power - continuing investigation

[David HK]

As part of my continuing study into plans for a solar electric system for my home there are some areas where I would welcome comment and opinion. I have searched this website and can find bits and pieces on the subject, but the search engine is a bit of a pig's ear to use so I may well have missed some previous material.

Back to basics. I mentioned in a previous post that I had purchased a 2 volt 200 milliamp cell to play with. After soldering on contact leads I decided to examine the output voltage - this varied from 1.5 to 1.7+ volts despite bright sunlight. I first thought that what the cell was rated at and what one actually got in the way of volts was the way it was. It turns out not to be so. A few days later I hooked up the cell to a volt meter whilst I worked on something else. The day was blessed with strong sunshine and after an hour or so I checked the reading. The meter was showing the cell output at 2.2 volts. I was astounded and then realised that solar cells need time to warm up in order to produce full output. This is a small detail that never appears to feature in the small print.

Having regard to this information I then pondered on what was best for a solar cell panel. Do I build a panel and assume a 2 volt output and rig for, say, 7 sets of cells for a 14 volt supply, or, 8 cells for 16 volt supply, or do I assume a lesser factor of, say, 1.5 volts per cell and rig the panel for 10 cells to produce 15 volts. The logic to this is that one cannot enjoy 100% pure blue sky all the time and even in cloudy days the cells will happily produce 1.5 volts albeit they are rated at 2 volts. (Forget watts for this part of the enquiry). What are your ideas and views? I understand solar panels now in terms of series and parallel make up, but the foregoing is simply an address about final output voltage.

Back in the home I have documented the voltage and wattage of every single electrical item in the house. The question is where do I pitch my everyday `demand' figure for watts?  Nobody uses every single appliance in the house at the same time. Seasonal changes determine whether an electric blanket is used for a few weeks, or whether an air conditioner is used for several months. It is possible to identify regular use items such as the electric kettle, refrigerator, television, telephone transformer, bedroom lights, hot water heater, computer and so on. Is this how everyone else works it out or does someone have access to a more scientific calculation?

At the end of the day it does not matter if the power supply is from wind, solar or hydro, we are all working at a solution that mitigates the use of a grid supply in favour of renewable sources. Please let me know how you approached the problem.

Regards,

Dave HK

[commanda]

The usual approach, both commercial & diy, is to aim for an open-circuit panel voltage of around 20 volts in full sunshine to charge a 12 volt battery. This gives some leeway to still charge in less than absolute full sunshine. Also, a nominal "12 volt" battery needs close to 15 volts terminal voltage at the peak of an equalisation charge cycle.

I believe cell voltage drops with increasing temperature; or was that current.

Hope this helps.

Amanda

[henjulfox]

Dave,

I've never heard of a solar cell "warming up" to produce full current. I would suspect either the cell wasn't fully vertical to the sun on the first test or the first test was other than mid-day. Even pointed directly at the sun, morning and afternoon sun is weaker based on the fact that the rays must pass through more atmosphere.

Panel manufacturers seem to stick to 36 1/2 volt cells per panel. They feel it is the best compromise.

Easiest way to get your total electricity usage is to look at your electricity bill. That's the bottom line. Be warned - Calculating how many solar panels you would need to produce all your own power leads to sticker shock. Trying to produce power to match your usage during the summer is easier (more sun, less usage) but still expensive as heck.

What I do is have a generator transfer switch next to my breaker panel. Instead of a generator it is fed by my inverter. Then I add solar panels whenever I can ($). During the winter I ran 1 circuit, this summer I shoud be able to run 3, and 1 is the refrigerator.

Good luck,

-Henry

[Volvo farmer]

Agreed with commanda. you want open circuit volts at around 20, and solar cell voltage does drop as temperature increases, that's why MPPT is more advantageous in winter.

As for writing down figures for all appliances power consumption... I think you can come up with an inaccurate consumption figure by using the rating plate. A coffee maker uses a lot more power brewing than it does just keeping the coffee warm. If my laptop is idle, it draws 2 watts, full tilt computing uses more like 20. I think it's far more accurate to get a kill-a-watt and measure your actual usage over a matter of weeks. If you're truly planning a solar electric system for the house, things like electric water heaters and air conditioners are off the list unless you've got 50-70 large to throw down.

I'm about 30-60 days from getting off the grid myself. I did it kind of backwards. Everyone says to determine your usage and build the system accordingly. I just built the system I could afford and figure I will adjust my usage accordingly. I'm using propane for the refrigerator, hot water, and heat (along with wood). Coffee brewing will be done in an old fashioned percolater, all clocks will be wind-up or battery, all lights will be CFLs, etc. The only thing I really need electricity for is lights, to pump hot water through the floor in the winter and to pump domestic water to the faucets out of the cistern. If it turns out I have more power than I'm using, I'm sure I can find some stuff to plug in.

Take my advice with a grain of salt, because I haven't lived with it yet. My panels are in the air, the inverter is mounted, the batteries are in their box.... It's an exciting time and by this time next year, I should be able to provide more hands-on experience.

[David HK]

Thank you gentlemen for some interesting contributions. The thing I forgot to mention is that I live in Hong Kong which is pretty well described as vertical city of high rises. My own home is a semi detached in a residential complex near the border with China proper.

I do not know of any house in Hong Kong that has a chimney! Most Hong Kong residents spend their money to keep cool in the summer whilst (I suspect) many members using this website spend their money to keep warm in the winter. Such is the difference at different latitudes of the globe.

The note about working things out from the electric bill etc is that I have done all that. My gut feeling is that I will end up with several columns defined as - always on 24hrs; used every day - spring, summer autumn and winter variations; frequent and regular use; infrequent and irregular use; rare; very rare use.

How I envy you people that live out in the sticks. My dream home has always been to live on a south facing knoll with a landscape view of 30 plus miles; perfect view of the satellite arc; permanent wind supply; a stream for a hydro system; solar voltaic; breakfast window facing east for the rising sun; living room facing west for the setting sun etc. If you know this place called paradise let me know quick!

Regards,

Dave HK

[Volvo farmer]

Wow, I didn't realize you were trying to accomplish this dream in such an urban setting. I grew up in Phoenix, AZ, USA. we had summertime temps at 110F/80F day/night for days on end in June and July. My dream was always to move underground in that climate. But in a dense urban environment, this might be impossible. Would there be some sort of low-tech way to keep yourself cool that wouldn't take too much electricity? Surely people lived at your latitude before electricity, what did they do? I actually found it easier to live in 110F heat without energy assistance than 15F cold, though we had low humidity and a tropical atmosphere might be less accommodating.  

[BT Humble]

My dream home has always been to live on a south facing knoll with a landscape view of 30 plus miles; perfect view of the satellite arc; permanent wind supply; a stream for a hydro system; solar voltaic; breakfast window facing east for the rising sun; living room facing west for the setting sun etc. If you know this place called paradise let me know quick!

I can manage all that except for the stream and the 30+ miles view - would 10km be enough?  

(Also, I'm in the Southern hemisphere so North is the sunny side).

BTH

[David HK]

BTH

Thanks for the very pleasant photographs. Not a car in sight, no girlie bars, no nothing. Moving from a city to your location would be painful on the ears.  Thanks anyway, it brightens up the article no end.

Dave HK

[willib]

wow , beautiful

[geoffd]

Hi Dave

We live totally off grid.  What I did to calculate our usage was to total up the time * the wattage per day.  So for example.  1 x 11w lamp for 6 hours = 66 watt/hours.  We did this for each applicance we used and averaged out things that we only used once every couple of days.  That gave me an maximum figure of 3kw/hrs per day.  You then total all the machine wattages of the machine that will be on at the same time.  E.g. 1 x computer @ 40 watts + 5 x lights = 55w + 1 x vacuum cleaner @ 1kw, this figure gives you the minimum size for your inverter (if you use one).

When we got our original estimate and realised we could not afford a system that big, we then worked on replacing power hungry applicances with power saving ones etc, and it is amazing how much you can save. E.g. using a laptop instead of a desktop computer, using CFL lights instead of incandesant.  It is worth doing this exercise just to see how much you can save on your normal electric bill.

Cheers

Geoff

[wpowokal]

Dave I suspect your power requirements are such that it would make more sence to power some circuits off Re initally and build on that, ie leave the AC on the mains.

allan down under

[nothing to lose]

 For many people I agree with that if they can do so. That is kinda what I am doing myself.

Nice thing about it is if you see your hitting times the RE is not quite making it you can always plug in a grid powered charger or if you set up grid outlets and inverter outlets (well marked) next to each other you can just plug into which ever one you want whenever. For the outlets and circuits I just pulled the wire out of the breaker box and put the proper amp plug on it, plug into either system as needed. Not a big deal for me.

[TomW]

NTL;

For the outlets and circuits I just pulled the wire out of the breaker box and put the proper amp plug on it, plug into either system as needed. Not a big deal for me.

Exactly what I have been doing right along. Never a chance of backfeeding the grid. Although here pretty soon I will set up my Outback properly to handle that switching rather than as a stand alone inverter as it is now.

Cheers.

TomW

[nothing to lose]

 Some things I get my own ideas for, like a Ni-cad powered E-canoe

 Many of my ideas come from the forum though.

Might have been from you awhile back where I got the idea from for the outlets. I don't remember now.

It does work well though.

[kenl]

 I agree with some of the comments above. I moved to Georgia and was greeted by sticker shock when I started getting my monthly electric bill. At the peak it was running $260 a month. What did I do? Got on the internet like you did and started searching for ways to decrease the cost. That's how I found this web site.

 Before you start figuring how much power you need to produce take the time to see where you can conserve. 1st thing I did was change out all the 60w lamps in my house for CF's. 39x60w=2340w old lamps 39x14w=546w. Hummm seems like a good investment. Lets check the fridge. Wife has it set to 9 the highest setting, what happens when I turn it down to 4 1/2. Well my trusty Kill a Watt meter told me I went from a 1.2 kwh per day to 3/4 kwh. Maybe I should check the hot water heater. Why is it wired for 120v when the name plate says 220v only. Is that why I don't have much hot water? Let's rewire it for 220v. Does it need to be set to 130f? Lets try 110f. Why heat up water only to cool it back down mixing it with cold water. I'm still trying to determine if cloths really get cleaner with warm water. Can't tell a differance when I only use cold. Net electric bill $60 a month for the last 3 months. 1800kwh before, 560kwh now.

 Check you windows for infultration. Are they single or double pane? How much insulation do you have in the walls and ceilings? Even things like the color of your roof can make a big differance. Light for southern latitudes dark for northern.

 How old is you fridge, ac, heater, microwave? Over ten years might want to replace them. How big of a fridge do you really need? How about the microwave? Could you get buy with a lower wattage one?

 You can see your panel cost shrinking and shrinking. It's the little things that make a huge differance I found.

[BT Humble]

I held a bit of a party there on Saturday night - it'll be the last time I'll see it until December (currently living 750km away).  There are some pictures here:

http://gallery.smidsy.com.au/v/unaugral06/

WARNING - some of the pictures show some minor vulgarity from a bloke who looks and dresses remarkably like me, but he is, in fact, err, my adopted twin brother! ;-)

This one is my favourite - I'm mowing a track through the long grass for young Blake to ride his minibike on.  He was so impressed he let me have a go - I had my knees up around my ears!

http://gallery.smidsy.com.au/v/unaugral06/NC0S3888-01.jpg.html

BTH

(Some people just make a spectacle of themselves when they get drunk!)

[ghurd]

(At least they don't remember it!)

G-

[BT Humble]

(Provided nobody brings a camera).

BTH

[ghurd]

(all the new cell phone have cameras.

Everybody has a camera.)

We are busted.