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One large solar array for everything or several small dedicated systems for off-grid?  RSS feed

 
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It's difficult to make off-grid solar scalable like adding ice cubes to a drink is scalable. Want it colder? Just add a few more ice cubes. Want more electricity? Just add a few solar panels.

Though it's good advice to buy as big as solar array etc as necessary right up front, it didn't happen that way for me. I have a solar array and battery bank that is reasonably sized for daily usage. Moving forward, I will be gradually using more electricity. Not really from daily stuff but from additional appliances like a distiller and refrigerator.

I'm planning on keeping the existing array as-is and using that for daily usage. Computer, stereo, tool charging, et cetera. And I will add other small dedicated systems to power other devices/appliances. For example, I plan on getting a Sun Danzer refrigerator, and running that off its own battery and solar panel. The same with a water distiller (any recommendations welcome). Using a distiller would drain my existing array, but if it's on its own array, usage is isolated. (I am thinking about an electric distiller despite the inefficiency of electrical heating because for the distiller I would like to have it run every day without lighting/monitoring a propane fire.)

I'm leaning toward several small dedicated systems because:
* It isolates what are essentially closed loop devices. If I drain my batteries from running the stereo or something, I don't want to miss out on being able to run my swamp cooler, for example.
* It's nice to run from DC whenever possible ...without the use of an inverter and without long wire runs.
* It seems simple. Troubleshooting would be simpler also.

I haven't seen much about this approach to off-grid living. Usually everything is hooked to one system, at least from what I have seen. That way sounds very elegant and streamlined. But I will be taking a different path.

What are your thoughts on this?
 
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I would enlarge your system and have one centralized solar system. Every system will require its own charge controller, so there is that expense. Also, it is my experience that solar operates by some alien mathematics formula that obeys its own rules of physics. A larger system, with more stored amps, and hopefully a higher voltage (24 vs 12), will be much more stable and hold a charge better.

It is in fact very easy to add panels and batteries to your system as you go. I started with a 185watt panel, and now have a panel array of around 800watts. I still use the same 1500watt inverter, and the same charge controller. My battery bank doubled, from 2 L16's at 12 volts to 4 L16's at 24 volts. All very easy to add on as you go. I run a sundanzer fridge and freezer direct off the bateries, not using the inverter. Sundanzers are awesome for many reasons, one being that they can use either 12 or 24 volts, and I have run mine on both. Excellent refrigeration units.

Having multiple complete systems, with controllers, fusing, wiring, would be much less simple. Go with one centralized system that can easily be expanded as your needs grow.
 
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It isn't a really easy answer "Always do X" but IT DEPENDS.

Are we talking everything in one small house or spread out across outbuildings?
Do you have SPACE for all the panels in one spot? They start taking up a lot of real estate in a hurry. Running DC over a distance gets to be real money real fast.

The cost of multiple smaller controllers vs. one BIG one is not clear-cut.
There is definite value in building multiple, nearly identical, systems so you have spares. If the controller goes out on the swamp cooler rig, pull the one from the general lighting until you can replace it.
Are most of your loads similar or will one system be 90% of the total load? It is expensive to build a separate system when an existing system has enough capacity or you only need to add a extra battery string and/or panel.
At some point, you have to take a very big price jump to get to the next size of system components.


 
pollinator
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I don't know what is your desired purpose for the water distiller, but you can get a much higher production rate of distilled water if you use vacuum distillation powered by solar thermal, and it can be relatively easily automated. If you're interested, then let me know as I've already worked most of the details.
 
pollinator
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I suppose there are pros and cons, but we have two systems. The big main system runs just about everything. But we have the frig and freezer on their own system. The reason being, if the main inverter crashes, I don't want to lose all the food in the freezer while I am waiting for the replacement inverter to arrive. It hasn't happened to us yet, but I know of several off grid friends who lost freezerfuls of food.

We have two Steca chest units. One is used as a refrigerator, the other as a freezer. Stecas are much like the Sundanzer. They are dc, so the system is simple and basic. If for some reason the charge controller fried, we could run the Stecas using an ac to dc inverter, which we happen to already own.

...Su
www.kaufarmer.blogspot.com
 
Stephen Lloyd
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Marcos Buenijo wrote:I don't know what is your desired purpose for the water distiller, but you can get a much higher production rate of distilled water if you use vacuum distillation powered by solar thermal, and it can be relatively easily automated. If you're interested, then let me know as I've already worked most of the details.


Sounds very enticing. Would love to know more about this.
 
Stephen Lloyd
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R Scott wrote:It isn't a really easy answer "Always do X" but IT DEPENDS.

Are we talking everything in one small house or spread out across outbuildings?
Do you have SPACE for all the panels in one spot? They start taking up a lot of real estate in a hurry. Running DC over a distance gets to be real money real fast.

The cost of multiple smaller controllers vs. one BIG one is not clear-cut.
There is definite value in building multiple, nearly identical, systems so you have spares. If the controller goes out on the swamp cooler rig, pull the one from the general lighting until you can replace it.
Are most of your loads similar or will one system be 90% of the total load? It is expensive to build a separate system when an existing system has enough capacity or you only need to add a extra battery string and/or panel.
At some point, you have to take a very big price jump to get to the next size of system components.




Currently there's just one cabin.

I think probably what makes sense is to have a main system and then smaller systems for dedicated use. I have one pole-mount 1kw string right now, and I could potentially add four more panels to it (someday). For stuff that is DC or farther away (water pump, outdoor shower, maybe refrigerator), it seems good (to me) to have them on their own small system, if possible. This would generally entail a single PV panel and a cheap charge controller and 12v battery.

The more I think about it the more it seems similar to an argument about governance, like whether to give all the political power to a centralized government or to distribute power to distinct states.



 
Stephen Lloyd
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Su Ba wrote:I suppose there are pros and cons, but we have two systems. The big main system runs just about everything. But we have the frig and freezer on their own system. The reason being, if the main inverter crashes, I don't want to lose all the food in the freezer while I am waiting for the replacement inverter to arrive. It hasn't happened to us yet, but I know of several off grid friends who lost freezerfuls of food.

We have two Steca chest units. One is used as a refrigerator, the other as a freezer. Stecas are much like the Sundanzer. They are dc, so the system is simple and basic. If for some reason the charge controller fried, we could run the Stecas using an ac to dc inverter, which we happen to already own.

...Su
www.kaufarmer.blogspot.com


Hadn't heard of the Steca. Would be happy with either it or the Sundanzer. Right now we have a little dorm fridge, and it uses more power than these models. Amazing.


 
R Scott
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Dorm fridges are HIGHLY INEFFICIENT. You can do much better. A regular size fridge uses less power.

One thing I have considered but haven't run the numbers is a keezer. You can google it, but a keezer is a normal chest freezer that has been hacked with an external T stat and used by homebrewers to keep their kegs cool. They seem quite efficient as a fridge and considerably cheaper than a DC chest fridge. Add some additional foam insulation and you should be GTG.

 
Marcos Buenijo
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Stephen Lloyd wrote:
Marcos Buenijo wrote:I don't know what is your desired purpose for the water distiller, but you can get a much higher production rate of distilled water if you use vacuum distillation powered by solar thermal, and it can be relatively easily automated. If you're interested, then let me know as I've already worked most of the details.


Sounds very enticing. Would love to know more about this.


The reason I've considered this is because I recently started a project to develop a micro absorption chiller using lithium bromide as the absorbent. These systems operate at a high vacuum. During my work I've found it to be fairly simple to work with a high vacuum. It's a lot more difficult in my case because I have to get under 1/100 atmosphere. However, a vacuum distillation apparatus for distilling water (or for distilling ethanol) does not require nearly so low a pressure. The required pressure can be easily and very quickly reached using a standard small vacuum pump designed for evacuating small a/c systems. You can also use standard nylon tubing and fittings (including barbed/press on fittings... they hold vacuum fairly well).

I've considered several configurations for a solar powered water distillation apparatus. The system with the best combination of performance and simplicity I've considered would use a small magnetic drive pump to circulate water through a solar thermal array. The pump is thermostatically operated to ensure it operates only when sufficient solar energy is available to provide distillation. The entire water system is under vacuum, and the pump is connected to a small vacuum vessel that is insulated (insulate everything for best performance). The vessel is vented at a high point to collect distillate vapors, the vapors are routed to a condenser, and the condensate drains into a collection tank (tank might serve as condenser). A second magnetic drive pump can then be used to take suction on this vessel through a check valve to dispense water without breaking vacuum. However, there must be a tall water column to provide a boost to the pump... if the entire system were contained on a roof or elevated in some other way with the pump at ground level, then this could provide enough head. Might also provide a tall pipe that can serve as condenser and collection tank, then put a float valve to operate dispensing pump to force water through check valve and into a water vessel in the home. This pump might also be actuated thermostatically since raising level in the pipe would reduce cooling capacity and raise saturation pressure, and this rising temperature would be seen in the insulated vessel where evaporation takes place.

Since the system is under vacuum, then water can be easily replenished by providing a float valve in the distillation vessel that is connected to a water storage tank. This will introduce air to the system and slowly break vacuum. This will in turn raise the saturation pressure of the water in the system which will raise the boiling point. The result will be a steadily rising temperature in the system during operation. The vacuum pump can be thermostatically operated to start when the temperature rises above a certain point. It can restore vacuum in about a minute even for a fairly small pump. Place the suction of the pump at a low point in the system just above the water level as this will get the most air since air is less dense than water vapor (this really works, I tried it and use water vapor to displace virtually all air from a system and was able to freeze water with an absorbent.... could not have been done if air wasn't out... this is particularly interesting because my vacuum pump cannot draw down below a water saturation temp of about 70F, so it must have been the water vapor forcing the air out of the system... however, since you don't need nearly so low a pressure, then it's not so important). NOTE: A single thermostat might operate both the water dispensing pump and the vacuum pump. There is no need to worry about breaking vacuum by overdrawing water using the dispensing pump as it will move the water through a check valve and the pump requires a water head to operate at such a high vacuum, so there will always be a water seal there. Set the thermostat on the circulating pump to a higher set point, or just put it on a timer.

Note that the main benefit of this configuration is that it avoids most of the thermal losses from a solar water heating system as these losses are extreme at higher temperatures. One need only achieve about 20-30F over ambient air temperatures, and the system is self-regulating based on the cooling rate of the condenser. So, it's possible to distill water at 20-30F above air temperature assuming air temperature is above freezing. Limiting the temperature of a solar water heating system to only 30F above ambient will greatly increase efficiency. I fully expect such a system to achieve a yield 3-4 times higher than a system powered by photovoltaics (assuming the same collection area), and even higher with a very good thermal panel.

Note also that anyone working with ethanol production should look into vacuum distillation. It's possible to approach 100% ethanol when under vacuum, and of course much lower temperatures are required. Of course, the big problem with ethanol is having inexpensive fermentable sugars in the first place, but beyond this I find an automated solar vacuum ethanol still to be quite interesting.
 
Stephen Lloyd
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Wow. Yeah sounds really appealing. Is there a diagram somewhere I could reference? I am having a hard time going from project logic to visualizing component assembly. I suppose I don't already know enough about distiller assembly.
Being able to distill water at a lower temperature is brilliant. Why isn't everyone doing this?
 
Marcos Buenijo
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Stephen Lloyd wrote:Wow. Yeah sounds really appealing. Is there a diagram somewhere I could reference? I am having a hard time going from project logic to visualizing component assembly. I suppose I don't already know enough about distiller assembly.
Being able to distill water at a lower temperature is brilliant. Why isn't everyone doing this?


Sorry, no diagrams. It's just something I considered during the course of my other work. In my case I hope to tap the heat in the condenser of a small biomass-fueled absorption chiller for water heating and water distillation. Using another source of heat such as solar will work just as well. It can be assembled in a thousand different ways. It just needs to be air tight, and it needs to have a vessel where the water vapor can readily separate from the hot water (might be a vertical length of pipe). Note that it could also be set up to use thermosiphon thereby doing away with the circulating pump. It would take a lot more care as to it's design, but I think it's worth it.

I think people don't generally consider this (even the DIY, off grid minded folks) because working with a vacuum seems daunting, and it sure is an awful lot of trouble for a few gallons of potable water each day. However, my recent work shows me that it's quite simple. Just keep everything air tight. As long as vessels/tubing/piping are small, then the forces are low and it's quite easy to contain. Also, people generally don't have a grasp on basic physics, and there are some subtleties here that many would miss. For example, recognizing that boiling temperature follows the pressure makes for a simple control system using thermostats. It's also unfortunate that the low temperatures would not provide pasteurization. However, a system can also be had to provide pasteurization automatically, and it can use very little energy by making use of heat regeneration. In my case I will be tapping the furnace exhaust of my chiller system to pasteurize water, then tapping the condenser for water heating and water distillation. It's all automated. While work intensive to build, it is fundamentally simple. Of course, that doesn't make it any easier to build, ;-(.



 
pollinator
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One thing I would definitely have separate is most configurations of a water pump. The most common is a DC, direct solar system pumping water into an elevated storage tank, either in the landscape or on a tower, from which water is fed to points of use by gravity if at all possible. This eliminates the need of batteries on that system (the elevated water itself forms a kind of energy storage), and the complication of a pressure tank. If there isn't enough pressure for gravity flow to everything, then perhaps a smaller pump and pressure tank can feed off the larger tank for these use points. I am thinking here that in most climates, a garden/orchard/homestead of any size but the very smallest will use at least ten times the water for irrigation alone than all other uses of water on that homestead combined. That being the case it might be worthwhile to design one water system for the irrigation and another for everything else.....benefitting additionally from the fact that the irrigation water need not be of potable quality (i.e. it can be sourced from surface water or rain catchment, reserving the well for the house....)
 
Stephen Lloyd
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Marcos Buenijo wrote:
Stephen Lloyd wrote:Wow. Yeah sounds really appealing. Is there a diagram somewhere I could reference? I am having a hard time going from project logic to visualizing component assembly. I suppose I don't already know enough about distiller assembly.
Being able to distill water at a lower temperature is brilliant. Why isn't everyone doing this?


Sorry, no diagrams. It's just something I considered during the course of my other work. In my case I hope to tap the heat in the condenser of a small biomass-fueled absorption chiller for water heating and water distillation. Using another source of heat such as solar will work just as well. It can be assembled in a thousand different ways. It just needs to be air tight, and it needs to have a vessel where the water vapor can readily separate from the hot water (might be a vertical length of pipe). Note that it could also be set up to use thermosiphon thereby doing away with the circulating pump. It would take a lot more care as to it's design, but I think it's worth it.

I think people don't generally consider this (even the DIY, off grid minded folks) because working with a vacuum seems daunting, and it sure is an awful lot of trouble for a few gallons of potable water each day. However, my recent work shows me that it's quite simple. Just keep everything air tight. As long as vessels/tubing/piping are small, then the forces are low and it's quite easy to contain. Also, people generally don't have a grasp on basic physics, and there are some subtleties here that many would miss. For example, recognizing that boiling temperature follows the pressure makes for a simple control system using thermostats. It's also unfortunate that the low temperatures would not provide pasteurization. However, a system can also be had to provide pasteurization automatically, and it can use very little energy by making use of heat regeneration. In my case I will be tapping the furnace exhaust of my chiller system to pasteurize water, then tapping the condenser for water heating and water distillation. It's all automated. While work intensive to build, it is fundamentally simple. Of course, that doesn't make it any easier to build, ;-(.




I'd love to see such a thing in action. Then I think I could figure it out.
 
Stephen Lloyd
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Alder Burns wrote:One thing I would definitely have separate is most configurations of a water pump. The most common is a DC, direct solar system pumping water into an elevated storage tank, either in the landscape or on a tower, from which water is fed to points of use by gravity if at all possible. This eliminates the need of batteries on that system (the elevated water itself forms a kind of energy storage), and the complication of a pressure tank. If there isn't enough pressure for gravity flow to everything, then perhaps a smaller pump and pressure tank can feed off the larger tank for these use points. I am thinking here that in most climates, a garden/orchard/homestead of any size but the very smallest will use at least ten times the water for irrigation alone than all other uses of water on that homestead combined. That being the case it might be worthwhile to design one water system for the irrigation and another for everything else.....benefitting additionally from the fact that the irrigation water need not be of potable quality (i.e. it can be sourced from surface water or rain catchment, reserving the well for the house....)


You've described my situation perfectly. I have 2 solar panels powering my well pump, and the water runs through 1/2" poly tubing to a wooden tank about 25 feet higher up the ridge than the cabin. PSI from gravity is sufficient for faucet/sink usage, but to get the irrigation system to click on, I had to add a separate 12v dc pump and 20 gallon pressure tank. This is on a timer so that it comes on when the irrigation timer is triggered and for the rest of the day is off. This irrigation pump has its own solar panel and battery.

I also have an outdoor shower (on-demand propane) which needs its own pressure pump for the same reason (insufficient PSI for the on-demand thing to work). We switch this on when we need to take a shower and then turn it off when we're done. This shower system has its own little solar panel and battery. I also hooked up some low-voltage LEDs for showering at night.

It didn't make sense to run lots of wire from the main solar array, and I would have needed to adapt my solar array's 24v to the other equipment's 12v. Anyway I seem to enjoy that each system is independent.

 
Stephen Lloyd
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Stephen Lloyd wrote:
Alder Burns wrote:One thing I would definitely have separate is most configurations of a water pump. The most common is a DC, direct solar system pumping water into an elevated storage tank, either in the landscape or on a tower, from which water is fed to points of use by gravity if at all possible. This eliminates the need of batteries on that system (the elevated water itself forms a kind of energy storage), and the complication of a pressure tank. If there isn't enough pressure for gravity flow to everything, then perhaps a smaller pump and pressure tank can feed off the larger tank for these use points. I am thinking here that in most climates, a garden/orchard/homestead of any size but the very smallest will use at least ten times the water for irrigation alone than all other uses of water on that homestead combined. That being the case it might be worthwhile to design one water system for the irrigation and another for everything else.....benefitting additionally from the fact that the irrigation water need not be of potable quality (i.e. it can be sourced from surface water or rain catchment, reserving the well for the house....)


You've described my situation perfectly. I have 2 solar panels powering my well pump, and the water runs through 1/2" poly tubing to a wooden tank about 25 feet higher up the ridge than the cabin. PSI from gravity is sufficient for faucet/sink usage, but to get the irrigation system to click on, I had to add a separate 12v dc pump and 20 gallon pressure tank. This is on a timer so that it comes on when the irrigation timer is triggered and for the rest of the day is off. This irrigation pump has its own solar panel and battery.

I also have an outdoor shower (on-demand propane) which needs its own pressure pump for the same reason (insufficient PSI for the on-demand thing to work). We switch this on when we need to take a shower and then turn it off when we're done. This shower system has its own little solar panel and battery. I also hooked up some low-voltage LEDs for showering at night.

It didn't make sense to run lots of wire from the main solar array, and I would have needed to adapt my solar array's 24v to the other equipment's 12v. Anyway I seem to enjoy that each system is independent.




Also, as you mentioned, in the future (as soon as I can afford it) I would like to add a separate water tank solely for potable use. I would run the line from the well pump into the potable tank, and then have an overflow from the potable tank that ran into the existing wooden tank. The float switch is in the wooden tank, and so both tanks would fill before the pump shut off.

 
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Is there an easy, cheap way to shunt power from a main array to a secondary?

Say I have two arrays (battery+panels) one for main house power and a secondary for crucial power needs, ie, 12v pump and fridge. In event of the main power failing, I'll just light candles and not listen to music. But should the secondary (crucial) array fail, I'd like to be able to flip a switch and draw off the mains batteries. Preferably without causing a house fire.

Basically, I'd like a one-way battery shunt that will maintain voltage in the crucial batteries, even in event of failure, while not drawing power into the mains if I play the music too loud and drain the main batteries.

And dumping excess power to the secondary when the mains are full (every day)
 
R Scott
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You can do that two ways I know of:

1. with a charge controller between the main and critical batteries. Set it up like the main is the solar array and the critical load is the battery. Like Steve did on thesurvivalpodcast www.solar1234.com and battery1234.com has the info. That is DC all the way so you have to size your cables accordingly.

2. with an inverter on the main bank running a charger on the critical bank. That has some power losses but is running a normal AC extension cord so it will be more efficient over a long distance.

Either way is not that efficient because you lose 30% of your energy in charging the batteries.

It may be simpler to just carry the batteries over to the critical load and hook them up with jumper cables. But that is not easy.
 
Stephen Lloyd
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That is a very interesting setup. I hadn't thought about having a back up battery bank. The scenario you mention is a realistic one for me as well. Having a spare smaller battery bank is a neat if maybe unusual idea.
 
Just put the cards in their christmas stocking and PRESTO! They get it now! It's like you're the harry potter of permaculture. richsoil.com/cards
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