I recently purchased a small 600 square foot cabin that is wired for 12 volt dc. It has eight small florescent lights, a car radio and I also plug in a 19 inch 12 volt TV occasionally. In addition I purchased a used refrigerator Nova Kool model 3800 24volt DC for this cabin.
I am currently carrying two 6 volt golf cart batteries back and forth (for recharging) when I stay at the cabin on the weekends. I would like to make this process easier and am considering purchasing a Suntech 175W 24V Solar Panel, a charge controller, two more golf cart batteries and a power converter 24 volt DC to 12 volt DC. I plan to keep the lighting and power outlets at 12 volt DC and have 24 volt DC to power the fridge.
Am I on the right track? Do you know if Suntech solar panels are good quality or not? Can you recommend a charge controller and power converter for this application? Any other advice you might have for me?
Thanks,
Bob Kruckenberg
Bob,
The solar module you are considering is a 24 volt module, and cannot be connected to a 12 volt battery system unless you purchase a $400+ MPPT solar charge controller that allows a higher voltage solar array to charge a lower voltage battery. If you use a standard charge controller, it will force the solar module to operate at half its normal voltage which cut its watts output in half.
Although I do not buy or install any solar hardware made in China for more reasons than I can discuss here, I will say that SunTech is one of the oldest and best respected solar manufacturers in China, but I was not pleased with workmanship on the few that I have purchased.
If you stay with a 24 volt battery to match the 24 volt solar module, you will need four (4) golf cart batteries, and I think you will find the lower cost voltage converters to be very light-duty for this application. We have had several fail when loaded near their advertised ratings.
Since this is a small cabin, I would keep the batteries and all wiring 12 volts DC as you can find almost anything in an RV or boating supply store that will operate on 12 volts DC. You cannot find much of anything to run directly from 24 volts DC unless you use the voltage converter, but then if it fails you lose everything.
I own land on a river and want something simple that will pump a small amount from the river to the water trough on the other side of a fence about 50 yards from the river and at an elevation of about 10 feet. Nothing fancy. Simple and small is fine.
Can you suggest something? Many thanks,
David Mayfield
David,
Check out this article from a past issue that covers this topic:
I have 2 40w panels wired in parallel for my camper. Can I connect a 20w panel in parallel with them?
I have been told I can only use panels of the same wattage. Is this correct? If so, can you explain the reason?
Thanks for your help.
Yours,
Rodger Newgreen
Roger,
Although it’s always best to match all solar modules in any array for the best performance, it’s possible to mismatch wattages in smaller systems under certain conditions.
The key is not the “wattage” of each module, its the “voltage”. You want all modules to have the same nameplate voltage ratings under open, short, and peak power. If the voltages match and they are all wired in parallel, the current for each will be based on each module wattage.
If wired in series, you may have problems if the smaller wattage module cannot handle the amp current passing through from the larger modules. There is also the possibility that the lower wattage module in the circuit will “draw down” the other modules so and reduce their output, much like what happens when you install one old battery in series with two new batteries.
Good Luck,
Jeff Yago
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I am an electrical engineer and received this project to design an off grid PV system ( I think also known as a stand alone system since the power grid is not involved). I have never done anything similar in my classes that had to do with solar panels and PV systems in general.
I would appreciate any guidance that you can give me regarding the basic steps or important calculations to calculate in order to get started with this designing. I have attached the project question to this email.
Thanks for your help, I just need guidance to get started since I have never ever heard of PV systems before in my life.
I appreciate your help
James
James,
As noted on this web site, we cannot provide specific design information on any solar project due to limited time available to answer each question on a free site, and with any design there could be critical information we do not have that could affect the advice given. However, I always try to provide a direction for our readers when possible.
First, any off-grid system starts with identifying what electric loads are to be powered, and for how many hours per day. Your list of this equipment has some serious problems which need to be addressed before even starting to complete a solar design. For example, you list an electric stove which is almost never powered from a solar system due to high load, but it is OK to power a microwave oven since they only operate a few minutes.
You list running power tools each day, but make no mention of a well pump which we would expect to have on an off-grid rural installation.
You list a refrigerator as a constant 24 hour per day load, but refrigerators and freezers cycle on and off all day, so you need to find the daily kW-hour usage for your average room temperature to use and not an instantaneous watt value.
You list a solar array output of almost 5 kWhs per day per kW installed which is highly doubtful. This may be possible for peak summer sunny days, but this is not realistic for an average output per day. We typically see solar arrays producing only 50 to 70% of their “nameplate” ratings except for very brief periods of extreme cold sunny days, and blue sky noon summers days with mild temperatures. For off-grid designs, this is a critical factor since you have no grid to fall back on.
Your battery bank requirements are not realistic and must have come from somebody reading a book. For example, your daily energy loads total 8.9 kWh and you want 3 days of no-solar battery time with 50% discharge. This means the battery bank would need have a capacity of 53.4 kWh, which would require four (4) deep cycle 1200 amp-hr batteries @ 12 volts each in series, which would have a combined weight of 4,640 pounds and would cost over $10,000 just for the batteries, plus a pot load of shipping and un-loading costs.
Finally, there are serious electrical design considerations when designing any solar power system, and this is even more serious for an off-grid system. The National Electric Code has an entire Section 690 that addresses wire and fuse sizing for solar arrays and I strongly suggest that you either obtain the services of a solar designer that has experience with these systems or plan on taking several courses on the subject first.
Good luck, and be sure to include a good fire extinguisher with your installation if you do not follow my advice.
Jeff Yago
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Posted in Design, Solar | Comments Off on Designing an off-grid PV system
Thanks for all the great information you provide BHM readers. I’m planning to install a small PV system using the instructions in your article in issue #116. This will be a very small system and used primarily for back-up during a black out. I have a Uni-Solar US-32 panel and Sunsaver 10 amp controller. I’m installing the panel on a carport roof, (I live in Massachusetts), running the wires to connect to one or two 12-volt deep-cycle marine batteries in my attached garage. I’ll be using the 2-pole fused disconnect and correct wire size for the panel and ground wires you recommend.
I have three questions.
1. What can I use as a bracket to mount the panel to the roof? I’ve seen small RV racks for sale and wondered if there is an alternative mounting bracket that can be purchased at a hardware store.
2. The wiring diagram in your article shows the load wires going to two 12 volt lights, hooked-up in parallel. I plan on wiring one light for the garage and continuing the wires about 30 feet to be run to the inside of the house. Those wires will be hooked up to a wall plate with a 12-volt cigarette plug receptacle mounted on the wall. I want to use the plug for a 12-volt TV or any other 12-volt appliance/charger that works using the auto style plug. What do you think of this arrangement and do you have any suggestions or recommendations.
3. Do I need to vent the batteries outside if they are in an un-insulated garage? I plan on putting the batteries in an insulated box for the winter and wondered if I can drill a hole into the top of the box and let the batteries vent into the garage.
Thanks for the help.
Ernie Smith
Ernie,
I can give you some general answers, but since we would not know everything about a specific installation, we cannot be too specific.
Although most solar dealers offer an “approved” angle “foot” to attach any solar module to a roof, I have taken 1-1/2 X 1-1/2 aluminum angle and installed the leg standing up to the side of the solar module and the flat leg bolted into the roof framing. Keep in mind that if you do not use the pre-drilled mounting holes on the back of the module you will void the warranty, so you might try a combination of two short lengths of angle back to back to make a “Z” shape. This will give you a way to utilize the bolt holes on the back of the module.
As long as your loads are 12 volt DC and wired in parallel, they do not all need to be at the same location. However, I would increase the wire size to the more distant load as there is 10 times the voltage drop at 12 volts DC that there is at 120 volts AC.
Finally, a few RV or golf cart batteries out in a garage will not give off enough gas to cause a problem due to the large space, but don’t locate them next to a gas fired hot water heater. Hydrogen gas is only explosive when highly concentrated like in a small sealed up closet or battery box. If you do place the batteries in an insulated box, you will need a vent at the top. For larger battery systems we use a 1-1/2″ PVC pipe vented outside, with screening to prevent insects from entering. The pipe needs to slope uphill since gas rises, but you will need something to keep out the rain.
I want to start buying the pump and pipes to supply my cabin, however I do not know what to buy at this time. Let me explain my situation.
The river which I plan to pull my water from is about 75 yds away from my cabin. The problem is that for the first 55 yards its flat then the water has to travel up hill about 30 ft.
What pump should I buy?
Should I have a double tank system where one is located at the bottom of cliff then pull it up from there and get a second pump to raise it up to pressure tank?
Should I buy the pump from Harbor Freight which has a 2 inch outlet and cost 349.00 which the website says it can lift water 197 ft.?
Should I put a booster pump at the bottom of cliff, or is a solar submersible pump which cost 740.00 powerful enough to do this, or is a 1hp 230 volt submersible pump the only alternative?
Please help.
Thank you,
Arnulfo
Arnulfo,
Read the article I wrote several years ago on this exact same issue. I suggest you consider a smaller version of this design.
I’m looking to link some wireless networks via direct links with larger antenna arrays (constructed from home “dish” antennas). A particular link does not have line of sight (small hill in the way). I wish to place a wireless router as an access point on top of that knoll, but will need solar power to operate it. I’m looking at 12V/2A for the router. Of course, I’m attempting to keep size to a minimum on both antenna and solar unit. Any suggestions on required solar gear?
Thanks,
John
John,
I have had several articles describing how to size and install a small solar power system including the article on a solar cabin in the current issue that is on the newsstand in a week or two.
Although it should not take much to power a small unit like you are talking about with solar, I will give you a word of caution. Most DC powered telephone answering machines, and some computer network equipment like routers utilize either a “floating” ground or a reversed positive ground system. You would not normally notice this because this equipment typically uses one of those plug-in AC transformers that convert the 120 VAC from the wall outlet to 5, 9, or 12 volts DC as required by the small electronic devices. However, we have had major problems with “hum” or communication line noise when we tried to power this DC equipment directly from a DC source without the external plug-in power supply, since the ground was either reversed or the equipment had isolation filtering provided by the transformer.
Although your solar power system charging a DC battery might easily have the capacity to power this equipment, you need to verify that the equipment will work when powered directly, or you will be forced to add a 120 VAC inverter to supply the power and that involves added battery losses and much higher cost. It’s best to keep it simple, but some equipment just will not work without the isolation function of the plug in power supply. Check first.
Good Luck,
Jeff Yago
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Posted in Batteries, DC, Solar | Comments Off on Solar powered wireless routers
Here is my problem. My house must sit on an old artisian (sp?) well or something. My sump pit is situated in the north east corner of the basement. There is an old (red) clay tile that leads out of my pit to the “outside”.
Even in dry times, there is steady stream of water running into my sump pit from outside. My basement is only 24′ x 36′, but if the sump pump gets unplugged, my basement will fill with 3″ to 4″ of water in less than 24 hours.
My sump pump runs, on average, 2 times per hour for around 5 minutes each time, 24/7. I have not calculated how many gallons it is but I know it is killing my electric bill.
I thought, solar water pump. The less the sump pump runs, the better off my electric bill. Also, I thought it could be collect in a “rain barrel” for summer time watering needs and drain to the ditch or current dry well in the winter months.
Any help/suggestions would be helpful?
Thank You,
Arnie
Arnie,
Sounds like you need to start building an ark.
Although this does seem like a very unusual amount of water infiltration, based on your estimate of 5 minutes run time every hour of day and night, and my estimate that this is a 1/4 HP sump pump, this still only adds up to $1.62 per month at $0.09 per kwh. Either you are under-estimating the actual run time, or more likely, you have far more energy using appliances and lights than you think. For example, an older model refrigerator/freezer can represent up to one-quarter of a home’s electric bill if we don’t include air conditioning costs.
The problem with using a solar powered pump is it only pumps on a sunny day and only during daylight hours unless you have a battery bank, and you need a pump that can run 24/7 regardless of the weather or time of day. This means you would need a solar array, battery bank, solar charger, and DC pump, and this would most likely cost around $3000 for the size pump we are talking about.
I would check with a foundation water-proof company as it may be possible to install a French drain around your exterior basement walls and re-direct most of this water away from your basement and get rid of a large amount of this water before it can seep into your basement walls and floor.
Good luck,
Jeff Yago
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Posted in Solar, Sump pump | Comments Off on Sump Pump Help
