Notes to remember in this instructable:1. My country does not permit grid tie. I cannot provide technical help on grid tie questions.
2. The entire installation I performed myself. In your case you will either need to be competent in electrical installations or hire an experienced electrical professional to do it.
3. I have fulfilled code (NFPA 70, TTS-171 Part 1) and power utility requirements.
4. Owning a power harvesting system means you are responsible for it. You can’t expect to always call someone should there be issues during operation. Take control!5. This is technical only instructable. I shall not answer any questions on cost or savings seen since commissioning.
Now let’s continue on how I implemented my off grid power system. Remember when it comes to solar, the BIGGER the BETTER!
Step 1: Calculations!
Sizing of cables (AC and DC) is very important to have as low a voltage drop as possible. For my home I used 2% maximum voltage drop from any supply Breaker to the furthest load. You can use a more tighter requirement and its no problem; your cables will have to be sized larger. The attached chart is an example I used however remember PVC insulated cables dont carry as high an amperage as XLPE. Also there are other types of insulators and temperature ratings for them all. Consult your cables’ datasheets or simply use the worst case scenario charts. Also stick with copper conductors please.
For simplicity I refer to the AC ampacity chart to indicate what wire sizes to use. Please note that DC ampacity for the same cable is higher due to the fact there is no SKIN EFFECT for DC. Take for example the wiring in cars. For headlamps, alternator, starter etc. notice the cross sectional area difference for the equivalent amperage on an AC system. Using the AC chart for DC cable sizing means you will have a lower voltage drop which is good.Breakers:
Molded case breakers are typically permitted to run a maximum continuous current of 80% their nameplate rating (unless its a 100% rated Breaker). Breakers for DC are differently rated than AC Breakers. Should you wish to use an AC rated Breaker for DC service, the general rule is the maximum DC voltage is 1/5 the AC voltage rating on the Breaker. Exceeding that limit means the breaker will not properly extinguish the arc during opening operations. Always check the cutsheet for the Breakers you select to verify operation at DC. Also dont oversize your Breakers else you defeat your protection (eg. 15A breaker for a 2 amp load).
I used a rule of 4 hours usable sunlight to size my panels. The energy demand for a 24 hour period needs to be determined. You can either use the utility energy meter and average your usage during the week or use standalone meter. The power utility meter is the easiest method.
For example if you use 1000Watt-hours of energy for a 24 hour period, then the panels you need will be (1000/4 =) 500Watt of panels.
You can assume 6 hours or more of usable sunlight to carry your cost down but I prefer more power to provide for my needs.
The Watt-hour demand for a 24 hour period is the minimum size the battery bank should be. Battery watt hour is volts x amp-hours.
Step 2: Installing the Solar Panels.
The optimum angle for installation varies around the world. For me it’s 15degree from horizontal so mounting directly on the roof worked out great for me. I have panels facing the east and west to capture the sun from dawn to dusk.It is a good idea to install the panels where you can access them for cleaning but for the most part they are maintenance free and have long service life.
If you have alot of shade from trees then you need to address this first or have an alternate location to harvest sunlight from dawn till dusk.I made a penetration in my roof’s sheeting and ran the power cable through it. I put strain relief and lots of silicone on the penetration. The panels are actually installed over that penetration so water ingress is not a problem.
Step 3: The Solar Charge Controller.
I originally had a morningstar MPPT 60Amp controller but it failed after a month in service. I just threw it in the garbage, angrily, warranty be damned. I have in service since mid 2013, an Outback MPPT FM80. I firmly trust this controller and will recommend it anyday. I also recommend MPPT type rather than PWM to get more energy into the battery bank.You can have multiple charge controllers into the same battery bank. Each controller will need to have its own set of solar panels.
The outback FM80 can accept up to 150VDC so I wired 4 of my panels in series. I have a parallel connection of two of those strings and this is fed into the outback. My outback is currently maxed in this configuration. Always know the maximum open circuit voltage for your panels to know how many can be wired in series to your mppt charge controller. Never exceed the maximum input voltage to the mppt (you know what will happen!).
Using an MPPT controller means you can use smaller cables to deliver the power into the controller.
It is important to have a Breaker before the input to the outback for protection and maintenance.
Here is a free piece of timeless advice; the breaker to the battery bank needs to be turned off last and also it has the be the first one to turn on. Why? If solar power is present on the input without battery control power, your expensive charge controller will go bye bye! Trust me I know!
Step 4: The battery bank.
I used lithium iron phosphate (Lifepo4) to store the harvested energy. It’s a 25.6v 160amp hour bank.
The details on this bank is covered in my instructable below:
Generally the higher the battery bank voltage the better once you can get a compatible inverter.
It’s critical to have short circuit protection for each battery or battery string. A circuit breaker is a good means of doing this. Also for maintenance or upgrades, throwing a breaker off will facilitate this. Don’t ever wire up a battery bank without protection!
Also people, please don’t use SLI batteries. These are shallow cycle and used only to start engines. You Want deep cycle batteries. Lifepo4 batteries are actually suited for both uses but it’s up to you to decide what Your budget can provide. Lead acids are cheaper up front but don’t have good lifespan, safety, environmental impact or discharge characteristics as Lifepo4.
For more detail: Solar Powering My Home!