Father and Sons project - help check me

Rusted

New guy, asking a new guy question in the Beginners corner. Holler at me or point me in the right direction if I am in the wrong place

My 10 year old boy is solar crazy, the kid lives and breaths alternative energy. His passion in life is a solar powered bedroom. The light bulbs in his room have been removed and everything electrical that he owns is "off the grid" as much as possible. It has turned out to be a fun father and sons project where my boy is learning about amps, volts, batteries, wire guage, soldering, etc.

What we have so far is a 12V marine battery that is rated at 122 AH. We live in northern Utah (think long summer days, and short winter days). He will have a 12V station of outlets to run lights, and charge the family cell phones, charge a flash light and his nintendo hand held games. He will have an inverter so he has 120V options such as powering a laptop (70 watts) or an alarm clock (probably will buy a 12v alarm clock).

Panels have not been purchased yet, but we will probalby start with a single 60 or 85 watt panel.

The wire run from the solar panels to the controller/battery may end up being ~100 feet long

• With that information will a MPPT be worth the additional cost?
• What controller would you recomend?
• What real world difference will I see between a $200 Morningstar MPPT and a $70 Sunforce PWM controller?
• What guage of wire would you expect to see for that distance?
• Does the 60-85 panel sound like it will fit the needs
• What panels would you recomend for an roof mount solution

Any feedback you may have would make both this father and the son very happy :cool:

waynefromnscanada

Re: Father and Sons project - help check me

Welcome to the form "Rusted"!
First I want to thank you for you're post, and say it's totally awesome that you're 10 year old son is so interested and learning! Just the knowledge alone, that he will gain from this, will put him in a whole different world than the average young person who doesn't have a clue about such things. It's just wonderful, wonderful, wonderful!
Now a few questions:
1) Have you actually measured the power consumption of that lap-top? 70 watts is quite high compared to most that are roughly half that, or even less than half. Some just 20 watts or less.
2) Are you aware that the panel will only produce good power for a rough yearly average of 4 hours per sunny day? And that there are substantial system losses to consider? That means you can usually only expect roughly (for the 80 watt panel) 80X4 minus losses of at least 30% = probably less than 200 watt hours, then consider battery and inverter losses and you're down to roughly half that.
3) That 100 feet mentioned, is that between the controller and battery - - - - is that the one way distance? If so, we have to consider losses in 200 feet of cable.
4) Do you expect to expand in future?
Thanks for you're great, uplifting post. It's ALWAYS uplifting to hear of any young person being interested in "alternative" energy, and to hear of such dedication in a 10 year old it really, really outstanding!
Thanks again for you're post!
Wayne

Rusted

Re: Father and Sons project - help check me

1) We picked up a kill-a-watt and his assignment was to inventory all of the devices that he wants to power or may want to power. Both of my laptops (larger machines) are right at the 65-70 watt size. When we are done with this project and summer is over I will probably find a used or hand-me-down laptop he can use for homework. My mindset was to plan on 70 watt thinking his would be less wattage and it would compensate for some loss. But I see where you are going. Find the real watts and calculate the loss. I will sample some other laptops for true power consumption, but let’s plan on 40 watt.

2) Yes I am aware of the average daylight hours. The charts I looked at put me in the 5 hour zone so i was expecting 80x5 (or 60x5)

3) Wiring option A) is rooftop, down back of house, into garage, up through garage floor, into kids bedroom. I would guess that would be 80' of wire one way (I reported 100 to account for some loss). Option , off roof, into attic, through ceiling, into room would be a much shorter path of ~25' one way. Is there a difference in distance going panels -> controller or distance controller -> battery? If I mount the controller closer to the panels will it help reduce loss?

4) I am open and planning to expanding in the future. His 5th grade science project will be based on solar. If he clings onto this solar project we will keep expanding it.


Thanks for your input. I know enough about this solar project to know I need to ask questions

BB.

Re: Father and Sons project - help check me

In general, you can expect around 52% average conversion efficiency (from solar panel rating to 120 VAC output)... And there can be other significant losses (such as inverter drawing 6-20 watts DC just being "turned on").

Regarding sending power 100'... Several issues. Once is that you need to mount the solar charge controller very close to the battery bank. The controller needs to measure battery voltage "accurately" (ideally, within 0.05 to 0.10 volts for a 12 volt battery).

The other is sending low voltage/high current any distance requires a lot of copper to do that...

Say you want to send 80 watts 100' (one way trip) with a maximum of 3% voltage drop (too much drop, and you will have problems reaching the ~15 volts required to recharge your battery bank):

• 17.5 volts Vmp * 0.03 drop = 0.525 volt maximum recommended drop
• 80 watt panel / 17.5 volt Vmp = 4.57 Amp Imp
• Voltage Drop Calc for 100' of wire, 4.57 amps, 0.5 volt max drop => 6 awg wire @ 0.4 volt drop

And, if you want more panels (day double to 160 watts), you are looking at 3 or 2 awg wiring to maintain the 0.5 volt maximum drop.

If you want to use a MPPT charge controller, you will need to make the Vmp-array a higher voltage (2 or more panels in series, or use a >100 watt "higher Vmp" panel). There are many options, but all of them are going to add to the expense of the system (more panels, more expensive MPPT controller, if you get Vmp~100 VDC then you will need a $500-$600 or so charge controller, etc.).

And, you should think about seasonal variations... Assuming you are near Salt Lake City, UT, and using PV Watts, a fixed array tilted to Latitude would give you:

Month    Solar Radiation (kWh/m 2/day)
1      3.32     
2      4.50     
3      5.17     
4      5.60     
5      6.50     
6      6.47     
7      6.86     
8      6.96     
9      6.45     
10      5.61     
11      3.97     
12      2.73     
Year      5.35

Toss the lowest three months, then you can expect an average minimum of ~4.50 hours of sun per day, assuming you are using an inverter which gives around 52% average efficiency:

• 80 watts * 4.50 hours of sun * 0.52 eff = 187 Watt*Hours per day

If you wanted to power 70 watts of loads, then, roughly you would get:

• 187 WH per day / 70 watt load = 2.67 hours of use for "average" February day

A 122 AH @ 12 volt battery with 5% to 13% rule of thumb for designing the Solar Array, the recommended array size (rough numbers, going over/under a bit is not going to hurt anything):

• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.05 rate of charge = 115 watts minimum
• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.10 rate of charge = 230 watts "healthy nominal"
• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.13 rate of charge = 299 watts "rough maximum cost effective"

-Bill

BB.

Re: Father and Sons project - help check me

Also, you guys should read through this thread--Lots of information:

Working Thread for Solar Beginner Post/FAQ

Including this post:

Emergency Power

Basically a very long thread that starts from the beginning with a few vague requirements through design and assembly for a "portable" solar RE off-grid power box.

And here is another example by Mike90045 called the Solar Monolith:





Update pictures/information here.

-Bill

mike95490

Re: Father and Sons project - help check me

I'd strongly suggest the battery be NOT in the bedroom, but located somewhere, so that when a conductor gets dropped accross the teminals, the house does not burn dowm. 10 yr old, it's very likely to happen in the next 6 years. Fuse the 12V wire going to the room from the battery.

http://features.boats.com/boat-content/2011/01/a-short-story-about-battery-shorts/

I think this wrench is now worth more as a reminder for me, and a lesson for my kids if they ever say,
“But Dad, it’s just a 12 volt system, right?”

Rusted

Re: Father and Sons project - help check me

mike90045 wrote: »

I'd strongly suggest the battery be NOT in the bedroom, but located somewhere, so that when a conductor gets dropped accross the teminals, the house does not burn dowm. 10 yr old, it's very likely to happen in the next 6 years. Fuse the 12V wire going to the room from the battery.....

The battery is sitting on shelves in the garage that is located directly under his room. 5 amp fuses feed the room so I will get a report if anything stray goes on

Rusted

Re: Father and Sons project - help check me

BB. wrote: »

In general, you can expect around 52% average conversion efficiency (from solar panel rating to 120 VAC output)... And there can be other significant losses (such as inverter drawing 6-20 watts DC just being "turned on").



Regarding sending power 100'... Several issues. Once is that you need to mount the solar charge controller very close to the battery bank. The controller needs to measure battery voltage "accurately" (ideally, within 0.05 to 0.10 volts for a 12 volt battery).



The other is sending low voltage/high current any distance requires a lot of copper to do that...

Say you want to send 80 watts 100' (one way trip) with a maximum of 3% voltage drop (too much drop, and you will have problems reaching the ~15 volts required to recharge your battery bank):

• 17.5 volts Vmp * 0.03 drop = 0.525 volt maximum recommended drop
• 80 watt panel / 17.5 volt Vmp = 4.57 Amp Imp
• Voltage Drop Calc for 100' of wire, 4.57 amps, 0.5 volt max drop => 6 awg wire @ 0.4 volt drop

And, if you want more panels (day double to 160 watts), you are looking at 3 or 2 awg wiring to maintain the 0.5 volt maximum drop.




If you want to use a MPPT charge controller, you will need to make the Vmp-array a higher voltage (2 or more panels in series, or use a >100 watt "higher Vmp" panel). There are many options, but all of them are going to add to the expense of the system (more panels, more expensive MPPT controller, if you get Vmp~100 VDC then you will need a $500-$600 or so charge controller, etc.).



And, you should think about seasonal variations... Assuming you are near Salt Lake City, UT, and using PV Watts, a fixed array tilted to Latitude would give you:

Month    Solar Radiation (kWh/m 2/day)
1      3.32     
2      4.50     
3      5.17     
4      5.60     
5      6.50     
6      6.47     
7      6.86     
8      6.96     
9      6.45     
10      5.61     
11      3.97     
12      2.73     
Year      5.35

Toss the lowest three months, then you can expect an average minimum of ~4.50 hours of sun per day, assuming you are using an inverter which gives around 52% average efficiency:

• 80 watts * 4.50 hours of sun * 0.52 eff = 187 Watt*Hours per day

If you wanted to power 70 watts of loads, then, roughly you would get:

• 187 WH per day / 70 watt load = 2.67 hours of use for "average" February day

A 122 AH @ 12 volt battery with 5% to 13% rule of thumb for designing the Solar Array, the recommended array size (rough numbers, going over/under a bit is not going to hurt anything):

• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.05 rate of charge = 115 watts minimum
• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.10 rate of charge = 230 watts "healthy nominal"
• 122 AH * 14.5 volts battery charging * 1/0.77 panel+charger deratings * 0.13 rate of charge = 299 watts "rough maximum cost effective"

-Bill

--edit--
inline quotes did not work as I expected. I need to give responses at the bottom of the post


Concerning 120v loss
We know and expect to see loss on the 120v side. We are converting everything we can to 12v. I did not expect to hear 20 watts of overhead. Sounds like we need to measure that loss and plan to install a switch that feeds the inverter

concerning 2 or 3 AWG
So you are saying that I need to first purchase stock in the copper insustry, then purchase 2 guage wiring. I checked the price and I don't want to pay $2 per foot for 2 leads of wire go span that distance. Once part I did not understand..... If I mount my controller close to the panels, can I get away with smaller wire between the controller and battery? I wanted the controller close to him so he could "see it" but not for the cost of heavy wiring. Thank you for your comments there, you just saved me a huge headache of thinking my controller was not working.

concerning MPPT
Again a good heads up. I am wanting to keep things simple, so a single panel with a simple controller sounds like the best solution.


I need to chew threw your response a few more times and re-think and set different expectations for this 5th grader. I really appreciate your time with answers. You are saving me from learning things the hard way.

BB.

Re: Father and Sons project - help check me

Yes, short/heavy cables between charge controller and battery bank (usually a few feet with "normal" size wiring--even then, you are not talking about a lot of money for copper).

Sending power at low voltages and long distances is a killer... And very dependent on the amount of power you want to send... Notice the 6 awg for 80 watts and the 3-2 awg for ~160 watts...

But, this is a "soft number", everything is variable in solar (including solar array output voltages, battery voltages, controller losses). The rules of thumb usually work well--You can try something else as a paper design, then walk about through the details. You can use smaller awg wire, but the amount of power to the battery bank may be reduced too. And, as you see, there are already a lot of deratings here.

The inverter losses--depends on what inverter and what size of inverter... Smaller inverters tend towards 6 watts of losses (plus other losses with increased loading). Larger inverters (which you probably won't use here), have higher idle power losses--and yes, for small systems, most inverters need some sort of on/off switch.

About the only think you can "damage" here is your battery. If it is is over discharged, under charged over time, or even over charged--They will not last long. Running a battery flat can kill it in a day.

-Bill

Rusted

Re: Father and Sons project - help check me

Continued thanks for the continued responses. I run a few forums so I can appreciate the value of quality feedback. Thank you

The morningstar controllers seem to be the standard to compare against. For what I am looking what is the right size

http://www.morningstarcorp.com/en/sun-saver

Do I understand this correctly? 180 watt panel @ ~17V = ~10.5 A

So if I start with or want to grow to 180 watts of panel I will need the SS-20L-12V model? And I can use the 20 amp model up to ~240 watts of panel before I add a second controller?

Did I read that correctly?

--edit--

Do most people mount these on the roof with the panels in a weather proof box?

Rusted

Re: Father and Sons project - help check me

Continuing to add parts. The inverter showed up today. We picked a Cobra PCI 880. With the swich in the off position it has zero phantom draw. With the swtich in the On position it draws .35 mA, so that is better than expected but is still a 4 watt draw. I have not tested for efficiency but I am expecting to see that ~50% loss.

BB. wrote: »

In general, you can expect around 52% average conversion efficiency (from solar panel rating to 120 VAC output)... And there can be other significant losses (such as inverter drawing 6-20 watts DC just being "turned on").

By mounting panels differently and routing wires different I should be able to keep my the distance between the panels and the battery around 25 feet. With roof mount panels are people mouting the controllers close to the panels in a weatherproof box? Or am I better to mount the controller indoors and closer to the battery?

BB.

Re: Father and Sons project - help check me

Rusted wrote: »

With the switch in the On position it draws .35 mA, so that is better than expected but is still a 4 watt draw.

One typo--That is probably 0.35 amp (350 mAmp).

I have not tested for efficiency but I am expecting to see that ~50% loss.

Yea--it is the "overall" lose... From solar panels that really do not produce STC (Standard Test Condition) power levels (in full sun, solar panels get real hot, and their Vmp drops significantly). The rest of the losses, such as battery and inverter really depend on how you "run them"... Batteries tend to be efficient when charging from 20-80% state of charge, and not very efficient from 90-100% SOC.

Similar with inverters--Very light loads and very heavy loads they tend to be less efficient.

By mounting panels differently and routing wires different I should be able to keep my the distance between the panels and the battery around 25 feet. With roof mount panels are people mounting the controllers close to the panels in a weatherproof box? Or am I better to mount the controller indoors and closer to the battery?

Always keep the wires short between the battery bank and the chargers (and loads/inverters). For a charge controller, it needs to measure the battery voltage "accurately" for proper/fastest charging.

Also, unless you are using a Remote Battery Temperature Sensor, the Charge Controller and the Battery Bank should be in the same "room". There is usually a "thermometer" in the charge controller to perform temperature compensation in the output voltage for the battery bank (hot batteries need lower charging voltages).

Always lots to learn. Most of the time, we learn more from our mistakes.

-Bill

kellylipp

Re: Father and Sons project - help check me

I put a small system on an RV that sounds pretty similar to what you are trying to do. I had two panels and used the Tristar PWM 45 amp charge controller. As has been mentioned many times the run from the panels to the CC can be longer and lighter gauge with the key being shortening the run between the CC and battery. I had about 10 feet from the roof junction box where the two panel cables merged to one #8 downwire. From the CC to the batteries I had about four feet of #2/0. At 25' with a single panel think about #6 from your local electrical house and right away check how much #4 costs and what it looks like. Might not cost too much more and may be easy enough to work with. The problem is the heavier the wire the harder to work. If it's a straight shot down then no worries really. It's my opinion that #6 will be acceptable at 25'.

If you want to private email me you can and I'll happily send you a paper I wrote about the system I installed on the RV. There are a bunch of photos that are relevant for your installation and might give you some ideas about how to do this.

This is a really cool project! Hi to your son!

Kelly

Rusted

Re: Father and Sons project - help check me

BB. wrote: »

One typo--That is probably 0.35 amp (350 mAmp).

You are absolutly correct. I did mis-type and the measurement is 350 mAmp


I had been reading through the responds and in my mind I needed the heavy wire between the panels and the CC. Not sure why I mis-read that so many times. Thanks to Kellylipp for being the 3rd person to tell me the answer before I finally caught on

Mounting the controller close to the battery is simple. I can come off the roof, down about 25 feet and into the controller fairly easy. The controller can sit next to the battery without any problems. So that wil help me on the copper purchases.

Next question...
It seems that the ebay solar panels are some of the least expensive. I also see good ratings from the ebay sellers. I worry about the "get what you pay for" expereince. Are there any ebay sellers who are recomended? What other sources do you know of for 'value' panels?

BB.

Re: Father and Sons project - help check me

I have given up on trying to keep track of "Good and Bad" panel mfg... You can always double check with our host (or whomever you trust locally that sells/services a lot of solar PV systems).

Personally, I have avoided EBay as it seems to be a large fencing operation (in at least some cases)--But your money, your choices.

There have been other places that you can get good prices--but frequently, those are from bankrupt manufacturers and/or miss labeled or other issues (overruns/blemishes/Non-UL Listed/etc.).

I tend towards glass/crystalline cell panels (as opposed to plastic/thin film types) if I want them to last more than 10 years. If it was for a 5 year life system, thin film can be an interesting option.

Even then, I purchased from a major manufacturer with a history--And they still ended up replacing my entire array after ~6 years of use (no damage, just design/mfg. problem).

Solar is not cheap... Panels+batteries+electronics all have limited life and risk (hail/wind damage for panels, batteries that get taken dead by accident, electronics that are prone to failure after 10+ years, etc.)... Just make sure you are not spending money you don't have.

I know it is not your situation--But too many people want to go "off grid solar" to save money... For the most part, grid energy is "cheap" and spending money/time on conservation will have a better return on your money.

-Bill "stepping off soap box now" B. :roll:

-Bill