Child of Atlantis

Intro

  This relational summary is brought to you by the Sun. Literally as I type I hear the soft hum of my new 1000W inverter as it shifts the 12 volt direct current from the photoelectric panels on the roof of my porch into the 110 volt alternating current which my laptop craves. Rooftop solar energy is listed as #10 in the "Drawdown" list of things to do to reverse global warming with an estimated 24.6 Gigaton anticipated reduction in CO2 by 2050 as it grows from 0.4 % global electric generation to an anticipated 7.0% contribution. How the authors of the drawdown list came about these numbers I neither know or care. Rather than babbling on about how great solar photovoltaic cells are and how putting them on the roofs of reed huts floating in lake Titicaca will save the world and enrich the lives of some undoubtably interesting people, I decided to do my own thing. Throughout this semester I have made it my project to learn about and put together a small, off grid, DIY solar system.  What follows is a record of my efforts to catch the Sun and make my own power. Some of you may be inspired and follow my lead. Others may point out the obvious errors I will make along the way. Enjoy...

The starter setup

Let me add a preface to this relational summary now: I am not an electrician, nor do I pretend to have a particularly deep understanding of the mysterious magic that is electricity.  While I will be including an excel worksheet I developed to aid me in my design calculations it is by no means a substitute for professional knowledge from an electrician, solar installer, or electrical engineer.  In fact it may not even be particularly accurate. Working in spreadsheets I endeavored to streamline some of the basic math that is used in sizing a solar system, so that a layman like myself could successfully navigate the assembly of their own off grid system using easily accessible components found online.  There are websites and apps available for free which do this, but I found them to be annoying, so I set out to make my own. This has been a big learning experience; prior to this my biggest electrical project was installing a new stereo in my Jeep. For better or for worse this is my spreadsheet for solar calculations.

I will link to the products I purchased and try to explain I chose them and what I like and dislike about each item.  Pretty much every part of my current system was bought on amazon.com with free prime shipping, accept for the battery which I will talk more on later.  

To start out I purchased a pair of 100 Watt solar panels from HSQT.  The panels were priced on sale as a "lightning deal"as so the price was very reasonable.  Since I now had two solar panels sitting in my living room I had to figure out what else I needed to set them up.  There are numerous websites and forums dedicated to the DIY amature solar electrician and I visited many of them. I also consulted my old physics text book for a basic refresher in electrical circuitry.  I decided to wire them together in parallel to keep the voltage low and simple at 12V. My calculations are all based on a 6.1 hours peak solar exposure per day (National Renewable Energy Laboratory)  

To connect the panels I bought a set of parallel adapters and a 30 foot, 10 AWG cable.  The solar panels, adapters and cable use a type of male/female clip connecter called MC4 which seems to be a standard in the solar industry.  The MC4 clips were new to me but I soon found that they made wiring the panels together foolproof. I opted to purchase the premade adapters and wires because at project this scale It would not have saved me any money to build them myself.  I bought a longer than necessary 10 AWG cable because I figured I could trim off the running ends to make the other cables I needed for the build.

I ran the cable from the panels into my HSQT 30A Charge Controller which I chose because it could handle a little extra amperage should I decide to expand my panel set in the future, has a heat sink designed into the backing and it has the option to run a negative ground.  It was the most aesthetically pleasing one in my price range which I felt was important since it would likely be the part visible once my system is fully installed.

Now a little about my design objectives.  It is my intention to one day run my entire home of the grid using solar techniques like those I am learning here, but I am not yet at a point life where that is practical  Instead my goal was to create a small backup power system to use in case of power failures caused hurricanes and the like. Ideally I would like to be able to run my refrigerator, a fan, some lights and a few other luxury items such as my laptop and fish tank pump, but as I got more involved in calculating the size of the system I would need it became apparent that this was more difficult than I anticipated.   

I decided to base my design around running the refrigerator since I would hate to lose the valuable food items like fresh produce, frozen fish and wild game  I read the label within to discover that it claims to use 7.20 Amps Max (115V AC). While not drawing nearly as much power as the Air Conditioner system a refrigerator is still one of the biggest power hogs in the house.  There are several brands of extremely efficient fridges on the market especially geared towards off grid solar use, but mine is not one of them; it is just the cheapest, most basic Magic Chef unit my landlords could find. When I plugged the 7.2 amp power requirements into my calculator I found that I would not be able to run my fridge for very long on any reasonably sized battery system.  Luckily refrigerators themselves are in a way a type of battery that stores "coldness" rather than energy. This means the fridge not need to be running the entire time; instead it should spend the majority of its time off. I adjusted my calculations to assume the refrigerator would need to run only 20 minutes out of each hour.

At this point I figured I could start my battery collection with a single 100 amp sealed lead acid deep cycle battery (I would strongly discourage buying from this vendor in the future since my battery came with a lot of cosmetic damage on it and the seller would make me pay shipping plus 20% restocking fee) which should give me approximately 2 hours of refrigeration after the sun goes down.  I will likely expand my battery bank in the future but for starters this should work. I based the calculations on a 50% maximum discharge, this is important because going lower can damage the lifespan of the battery. As more batteries are added to the system I can reduce the level of discharge to further protect my investment.  

To complete my system I ordered a 1000W power inverter.  This should have really been my first purchase and I recommend that everyone have at least a power inverter in their kit for emergencies since if all else fails in the solar system I can use the inverter in my vehicle as a 1000 watt backup generator.  I chose a somewhat larger inverter than I needed so I can run multiple loads beyond just the refrigerator. Throughout my estimations I used a 85% inversion efficiency based on the advertisement. I chose a modified sine wave inverter to save money and because I would not be running anything that required a pure sine wave inverter.  The inverter came with heavier cables with alligator clips to attach directly to the battery terminals, but since I wanted to control the level of discharge and monitor the load amperage through my charge controller I made another set from the 10 AWG cable I used to connect the panels.

Test

When I first hooked the system up It seemed to run smoothly until I connected my refrigerator.  When tested under load the was fridge drawing way more than the 7.2 amps listed on the label; it drew 18 amps and ran for over 40 minutes before I disconnected it and switched back to grid power.  This first test was using the alligator clip cables that came with the inverter. On my second test the fridge drew 30-40 amps before causing an error on my charge controller which cut of the load.  Naturally I am at a loss as to what to do here. It looks like it's back to the drawing board/spreadsheet.

What Went Wrong?

It may be that my makeshift 10 gauge wiring between the battery, charge controller and inverter are too narrow support the current demands of the fridge.  I ordered some beefier 4 gauge battery cables put on instead that should hopefully help the problems.

Another issue could be my running the inverter load through the charge controller.  Most off grid setups I studied online have the inverter connected directly to the battery banks.  As it turns out that this a basic rookie mistake (Northern Arizona Wind & Sun Solar Forum).  I will have to rewire the inverter to my battery when my new cables arrive in a day or so.  I will need to find another way to measure the current being drawn by my inverter. My multimeter may work but it is only rated to 10 amps max and my anticipated current is nearly 16-18 amps.  I will also lose the charge controller's ability to shut off the load at a predetermined battery discharge which is unfortunate because my battery system is not yet large enough to operate for long periods without fully discharging.  This also makes me wonder what I can use the 12V outputs on my charge controller for if not an inverter. The controller seems to have lots of timing option for automatic lighting, but so far that is all I can come up with.

It is possible as well that my power inverter and the load of the refrigerator are simply too large for the 100 amp sized battery which I have connected.  I don't seem to have a problem running my laptop a light and a small fan right now but I the fridge is another story. I have been reading differing "rules of thumb" regarding how to size an inverter and battery combination.  Vanner shows a 1A DC:10W AC conversion ratio for sizing, which is what I used for the original design assuming I would be adding more panels and batteries in the near future; but on Solar Panel Talk forums "SunKing" suggests 1A DC:3W AC ratio be adhered to.  

For now it seems like I need to continue improving my calculation spreadsheet and using smaller loads on my system until I get this sorted out.