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Bfath

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Posts: 2
Reply with quote  #1 
Hello,

I am looking for some help designing a simple off grid solar system. My mother in law is building a small off grid house on an island in the Pacific Northwest (British Columbia, Canada). I am a red seal journeyman electrician with 15 years experience so the wiring and installation of the system is a no brainer for me, but as far as designing a system to meet her needs is where I am lost. Her budget is modest at best, so cost is definitely a factor though both her and I understand that purchasing a system that meets her needs is of the upmost importance. Here is a little background on her situation:



The house is 800 sq/ft

Location is the PNW so lots of cloud and rain, especially in the winter months

She will live there full time, by herself, and will be very power conscious at all times

Heat is by wood stove, as is her range/oven so no power usage there

She would like the fridge to be DC since it will be the only load to run all the time

All other major loads will be gas powered (hot water tank, etc)

System will include a small backup generator in case of emergency



So with this all being said, her power consumption should be quite low. All lights will be small wattage LED (eg. 6-12w each) and there will be 10-12 lights. Any other power usage will be intermittent (kitchen appliances such as a blender, etc.), as well as charging a cell phone and laptop and that would be about it. Here is where I have some questions.

Should lighting be AC or DC? I was leaning towards AC as to keep costs down with smaller gauge wire and cheaper light fixtures but does this reasoning make sense?

Should there even be DC anything inside the house? She was thinking she might like maybe a DC outlet or two, DC ceiling fan, DC composting toiler, and even the DC fridge. Pros and cons? Again with trying to keep costs down, having both AC and DC runs in the house would require 2 electrical panels (one AC, one DC) as well as the larger gauge wiring for all the DC runs (10 gauge stranded?)

Those are really the only questions I have in regards to the actual wiring of the house itself, its after this that I am a bit confused. Here are my questions to the solar side of the project.



How many solar panels required and what size to meet her needs? I know not all panels are created equal so since she is in a colder wet and cloudy climate what type is best to keep her going even during the dark winter months? What brands are best and most cost effective?

How many batteries and what size etc and wired for 12v, 24v, 48v?

Charge Controller size and type as well as Inverter sizing and type?

Any additional equipment needed or recommended?

I see online a bunch of “all in one” solar kits that include panels, controller, batteries, inverter, etc are these the way to go or is it better and cheaper to purchase specific brands separately? Can anyone recommend a specific site or wholesaler to deal with? (keep in mind we are in Canada so shipping to Canada is a must).



Basically I am asking for help designing the whole system she would need to be comfortable and worry free including electrical specifications and recommended brands and products. Thank you for your time reading through all this and I look forward to all of your replies and help moving forward with this project!



Brad

stmbtwle

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Reply with quote  #2 
Hi Brad and welcome to the forum.

Not going to go into the number of batteries, solar panels etc at this stage but here are some comments:

I would forget the DC fridge.  While attractive, DC appliances are expensive and sometimes hard to find.  AC appliances are a whole lot cheaper, from fridges to light bulbs. Not to mention you'll get a whole lot more choices.  You'll need an inverter of course but the cost of an inverter AND an AC fridge is less than a DC fridge alone.  The inverter also allows you to run light bulbs, power tools, radios/TVs, etc.  Lights need to be LED. They're pricey yes but they last forever and draw a fraction of the power of a conventional bulb.

Generator.  There are a couple options here.  On can go for a standard 120v generator, and use an AC charger to charge your batteries (or an inverter/charger which does both).  IF the appliances are 120v the generator can run them while the inverter/charger charges the batteries.  An alternative is a DC charger made from an alternator and a gas engine (lots of off-grid plans for this). It can charge the batteries directly when necessary, and the inverter will run the appliances as usual.  The latter would be less expensive, the former probably "better" and could be made automatic.  

I would stay away from 12v.  12v stuff is generally made for the marine/RV industry and seems to be expensive for what you get. Also the lower the voltage the bigger (and more expensive) the wires have to be.  120v AC is probably the easiest to work with, and the batteries can be configured for 24, 48 or even 60v. 24v though not the most "efficient" has a lot of advantages, you can find stuff for 24v that you may not be able to find for 48 or 60v (like that toilet or ceiling fan). Once you make your decision you buy the inverter or inverter/charger to suit. Same for the solar charge controller.

A smallish (7cu ft) AC fridge draws about 1kwh/day here in Florida (I have one and run it off solar), probably less where it's cooler.  Maybe another kwh/day for "other stuff".  A GC2 "golf cart" battery holds about 1/2 kwh at 50% usage. They're easy to come by, manageable, and not all that expensive.  There are better batteries out there that hold more, and will cost and WEIGH more (you're going to have to transport these critters by boat I guess and maybe haul them up a hill). Your call.  You can do the math and figure out how many days you're likely to be without power and how many batteries you'll need.  

Solar array: That's for someone else to advise.  There is lots of software that can help you estimate how much you're going to need for a given average output in your area.  I'd want to make sure my system was expandable though, cause in my experience is you'll eventually want more than you initially installed.

Kits: my guess you'll probably save money (and aggravation) with a kit if you find one that meets your requirements.

I'm sure others will come in with more help.  Good luck with your "project" and please keep us posted.






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Willie, Tampa Bay
sundug

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Reply with quote  #3 
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stmbtwle

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Reply with quote  #4 
Very interesting (and I think useful) site!

A thought, Brad, would be to install the batteries, generator, and inverter/charger first, you'll need them all anyway. This will get her up and running, she can run the generator a few hours during the day and let the batteries carry the load at night. It will also give you an idea of how much power she actually is using, and you can then size the solar array accordingly. It can be installed later, as it's basically a charging system for the batteries.  It will also spread the cost out a bit.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Rick H Parker

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Reply with quote  #5 
Have you considered a gas powered refrigerator/fridge? You could salvage one out of a RV. Then you would have the option of powering it off gas or electric. Depending on the boiling temperature of the refrigerant, it is possible to convert one to power off solar thermal.

If you list the name of the island we can look up the solar resources for that location. That will quantify what you have to work with in terms of solar energy.

Make a list of all the electrical loads that she will have. Do a guesstimate of the weekly time for each. Use the time and the power rating to calculate the weekly energy requirements. That will that would produce a reasonable quantification of energy requirements and peak power.

"How many batteries and what size etc and wired for 12v, 24v, 48v?"

Depends on the charge controller and inverter.  You already know lower current means less copper.
Another advanage of higher voltage and lower current is, battery efficiency goes up as the current goes down because there is less power loss due to the bulk resistance of the battery. This is why a battery's capacity rating is higher at the C20 rate then the C5 rate of discharge.

Kits are for people that do not have a electrical/electronics background. You could assembly your own for less and it would be tailored to her needs.





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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
stmbtwle

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Reply with quote  #6 
Not sure the resistance in the batteries will change that much. Suppose I have 4 12v batteries in series (48v) and I draw 1000w through the appropriate inverter. The current is about 21 amps, and they're in series so the current is still 21a at 12v through each battery. If those same batteries are in parallel (12v) with the same 1000w load, the total current is now about 84 amps. But the batteries in parallel are sharing the load so each still only sees 21 amps at 12v, the same as before. So the internal resistance,"C" rating, amp hours, and the discharge time for each battery should remain the same.

Of course if you try to go cheap and only use ONE battery, it has to carry the full 84a load by itself and won't last long.

The number of batteries required depends more on the total KWH needed than on the voltage used. Regardless of how they're configured, the more batteries, the merrier.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Bfath

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Reply with quote  #7 
Thank you all for the info! Lots to think about and many options to explore. The island is off of Vancouver Island, British Columbia and is called Denman Island. I am going to go through all of this information you have all so graciously supplied more in depth and get back to you all with questions and comments. Thanks so much again!
Rick H Parker

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Reply with quote  #8 
"Not sure the resistance in the batteries will change that much."

The resistance does not change at all, the power loss does.
The relationship between power, current and resistance is exponential.


P= I ^2 R .... If you decrease the current by a factor of 4 the power loss due to bulk resistance will decrease by a factor of 16.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
stmbtwle

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Reply with quote  #9 
We're not changing the current (per battery), it remains the same as long as the number of batteries does.

So if the resistance R doesn't change, and the current I doesn't change, neither will the power loss P. I^2 R = P. If the number of batteries remains the same, so will the total power loss from them.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Rick H Parker

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Reply with quote  #10 
"We're not changing the current (per battery), it remains the same as long as the number of batteries does."

For a given power rate, if you change the voltage the current must change proportionally.  P=VI.

Current is passive it is drawn by the load.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
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