Solar power system help.

[jdp]

I'm new here, so I hope I'm doing this right. I have some questions about a solar power system setup. I've read through alot of the post on this site and seen that some of the people have experience working with solar power. So first off I live in central nebraska,  my dad just bought some land along the Platte river and I'm looking to build a cabin there. I have some plans for a small 12x18 cabin. It has a shed style roof with a loft and a 4x9 bathroom built off the side. My question/problem is, is it a bad idea to run a small window AC using solar power? I know it can be done, but is the solar setup that I would need to do this gonna be super expensive? It gets pretty warm here, but the biggest deal is the humidity. It wouldn't need to run all day, just evenings and nights to cool the cabin down and get rid of the humidity. I would like to know what you guys think, or maybe some other ideas to cool the cabin down.

Thanks,
Jeff

[MountainDon]

Window mount A/C's are the least efficient type. Running one on solar can be done; but the question is for how long? From how many batteries? How many watts worth of PV modules?  A split mini A/C is much more efficient. The equipment does cost more though.   

Building carefully with great insulation would be of paramount importance.  Moving sources of heat outside helps; outdoor kitchen and getting rid of the heat from any refrigerator is important to keeping cool in a small place.

[hhbartlett]

Don's right, take the money you would have to spend on extra batteries, bigger inverter, more panels and instead put it into more insulation. Being that it's a shed roof there are ways to passively cool using solar alone (not solar electricity) simply by the design of the building. I am no expert on this, but I know the information is out there. Try this for starters:

http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm

[Dave Sparks]

In my opinion the only way to do this offgrid is with a mini-split heat pump, a tracked array, and at elevation probably over 2500 feet so it cools off at night. Running any cooling or heating (heatpump) at night is not easily done (I have and it is expensve) offgrid.

The strategy is by running a small 9000btu or a 1 ton max heat pump at 8am until sundown 7pm you can keep 1800 square feet or so at 74F
The loading by the compressor can be kept at 400 watts if the programmed differential from setpoint to actual room temp is kept low. Most HVAC people will tell you all kinds of reasons why this is crazy. I just will say they end up after seeing this work, pretty amazed. I have about 10 folks doing this now.

As you grow your solar other options are very favorable. If you do this whatever the extra cost is of the mini-split, buy the heating function option. A heat pump is a very efficient way to heat and cool and is the perfect way to use excess energy offgrid!

In a place that does not cool at night the only way to do this is with a generator or something really esoteric. Good Luck!

[NM_Shooter]

Is this going to be a permanent living structure for you or just a fishing camp?

The amount of energy required to start a compressor on an AC unit is huge.  You would be spending a lot of money on your batteries and panels and inverter to do this.  If this is only a weekend-once-in-awhile sort of place, I'd be tempted to buy an inexpensive generator to cool the thing down for a couple of hours and then coast through the night.

Ditto on lots of insulation.  Light bulbs are also a huge source of heat, so watch what you use. 

Just curious..... how close are you to the river and what is the river temperature?

[Squirl]

All great advice here.  I second Shooter.  If it is a weekend cabin, the cost/benefit of a mini-split might not be worth it.  Good insulation, and thermal mass inside helps keep temperatures even.  A generator with A/C should be able to get a small place like that very cold quickly and the insulated thermal mass will help keep it there.

If concerned with heat, avoid south facing windows, or put awnings over them to reduce solar gain.

[alex trent]

I run my AC in the bedroom for an hour before I go in and cut it off one hour later.  Works fine..a bit sticky by morning but I just get up and open up the windows.  So the advice about cooling it and then coasting will likely work, with a generator.  We have 88 degrees and 70% humidity at bedtime and its been 94 F in the day and unfortunately I do not have shade on the roof, but do on the walls.

I have 12,000 BTUs in a 18x 18 room with 12 foot cielings with fair insulation.  Gets cold in a hurry. Use a fan too.

Seems this is a lot better than all the complicated solar stuff to be cool at night.  I also find beer works great.

[jdp]

Thanks for all the advise. After reading through all the replies I realized there was alot of stuff I left out. I only live 28 miles from the land so I'll probably be there every weekend if not more. I'm probably just gonna go with the generator idea, it would cost way to much to go all solar. The AC won't run all day just at night to cool it down a bit and get the humidity out of the air. On the land there is a real thick wooded area, I think I gonna build the cabin in there to keep it in the shade. So my next question is, how far away can I put the solar panels before I have to worry about voltage drop? Am I better off having the battery bank next to the panels or can I keep them by the cabin?

NM_SHOOTER: It is right up against the river. Actually land on either side goes to the middle of the river so we own half the river for the length of the property. Its kind of a weird deal cause if you canoe down the river its fine cause the water is public. But when the river is dry and someone is four-wheeling they are trespassing. As for the river temp. I've never checked I guess.

A little more on the property. Its 160+/- acres, 40 acres farm ground, 90 acres pasture, and 30+/- acres trees/river. My dad and a landlord of ours (we farm) bought this property together. They bought it cheap, so the plan is to do some improvements to the farm ground then sell that off. They will just about get all their money back after the farm ground is sold and still have 120+/- acres. Our landlord just wants it in the fall/winter for hunting and we want it in the summer for canoeing and four-wheeling.

Thanks for all the help.

Jeff

[MountainDon]

So my next question is, how far away can I put the solar panels before I have to worry about voltage drop? Am I better off having the battery bank next to the panels or can I keep them by the cabin?

The real question is how much wire can you afford?   


We have about 325 feet of wire between the PV modules and the charge controller/batteries/inverter. With the series connected panels we used the AWG#2 copper provides for a very low voltage drop.

When I ran costs on wire for DC from our modules up the hill like we ended up using, vs running AC up the hill from the inverter and batteries etc located at the PV modules, it was a wash on wire costs. That was because I was planning on a 3500 watt AC output inverter so I could run tools, etc. That many amps at 120 VAC worked out to about the same size wire for lower amperage medium voltage DC. Your case could be different. There are a number of voltage drop calculators online. Google and look at a few. I found I liked some better than others for ease of use and variability of inputs.

[NM_Shooter]

The key to running the feed from solar panels a long way is to put your panels in series instead of parallel.  The thief that you are trying to thwart is resistive loss, and you cheat him by decreasing the current that is running in the wire.  You calculate your needs based on power for your system.... power is calculated by current x current x resistance.  It is also calculated by Voltage x current. 

This next part is a math example only.  12V panels frequently output 28 or more volts in open circuit, so keep that in mind.

If you needed 480W, and you had your panels set up in 12V (parallel) mode, you would have to run 40 Amps to your controller.  If you stacked your panels in series so that two of them made 24V, you are now only pushing 20 Amps.

So now think about your wire.  Again, I am going to use made up numbers for this example.

If the total resistive drop of your wire is .2 Ohms, in the first example you are losing 320W just in the wire alone!  (40 x 40 x .2).  So your deliverable power is wasted by the wire.

Running the panels in series means higher voltage, less current, so less loss in the wire : 20 x 20 x .2 = 80W. 

Power is also calculated by V^2 / R.  You save power loss by the square of the voltage.  The example above shows that by doubling the voltage, we cut the wire power loss by 4.

Higher voltage complicates things a little.  You have to make sure that your controller can handle the higher voltage.  Higher DC voltage also needs better insulators, and better protection from the weather.  Put all joints together with a dielectric grease, and protect them from moisture.  If they get wet and are close, they are more susceptible to cathodic reaction issues.

All this noise boils down to the following... keep your resistance low and your voltage high.  This is why major power lines carry voltages approaching 1/2 million Volts.   

You could also put your inverter closer to your panels and make that transition to 120VAC close to the panels and run an AC line to your cabin, but that has its own issues.  Safety for one.  So ask lots of questions here. 

Good luck.

[NM_Shooter]

BTW, I was curious about the water temp because I was thinking about using the river in a heat exchanger.

Here in NM, we use a lot of evaporative cooling because our air is mostly dry.  There is a trick to get the cooling capability of a swamp cooler even lower, and that is to pre-cool the air before you evaporate. 

Master Cool used to make a heat exchanger that you put on the front of the cooler that circulated cold water to pre-cool the air.   It was expensive, and I thought that a salvage aluminum radiator or two from a vehicle might have done the same thing.  I was curious if that stream was a tailwater stream, and had cold enough temps.  If you were close enough you might use a little solar pump and fan to somewhat cool your cabin with the cold water from the river. 

A bit on the baling-wire-hokey side, but I think it might work. 

[Squirl]

Shooter touched on it. Playing with longer wire runs, almost necessitates the need for an MPPT charger to convert the higher voltage wire run to get back down the voltage of the battery bank.

Another good thought is a DIY hydro system might work also work.  Even though they usually produce low wattage, they run 24 hours a day.  It adds up and can usually be very cheap per watt.

[considerations]

On a more basic note. Spend a little time determining which direction the breeze comes from most often.  A cabin design which includes a window or door facing that breeze, and an unobstructed path through the cabin to a window or screen door on the opposite side can go a long way to keeping the cabin cool.  Many of the big plantation houses I have seen in the South are rooms designed around a central breezeway (great hall) that take advantage of this.  Even though I live in a cool climate where A/C is not justified, I built in a "breeze path", and on those few days when temps run into the 80's and 90's, it works. I use it in the afternoon, when the closed up cabin reaches the outside temp and opening it up to the breeze makes for a shady and cool(er) retreat.  I know its not enough for your purposes, but it can help.

[Dave Sparks]

If you have the time to situate and design the house to cooling the inside (excellent idea), you should also situate the roof correctly to cool the attic (if there is one). That afternoon breeze will move air thru the attic if the soffitts are on the right sides and also be a a very nice choice for keeping cool.

The one thing I really like about the mini-splits is they are a no brainer for existing offgrid homes that have cooling/design problems. They really were not an option before 5 years ago.