2.9 Supplemental Designing a Solar System -- Technical

Noon Sun  --  the amount of energy possible to gather from the sun at noon is used as a unit in solar equations as there is significantly more energy available at noon than in the morning.  Consequently there are tables of how many noon suns of energy are available in different locations based on weather, topography, etc. . .
Solar Window-- Open space in the sky to receive energy from the sun

Absorption Point- - The highest voltage that the battery system absorbs energy daily.   A common absorption for a 24 Volt system is 29 - 29.6.  Too high of an absorption or too long can contribute to corrosion and water loss.  If you need to add water monthly to your batteries, lower your absorption point.  To get a battery to 100%, it's useful to force in amperage at a higher voltage than the nominal 24 volts of the system.
Bulk/Float Voltage – This is the 'floating' voltage when the batteries are full.  A 24 volt system holds at 26.4 after absorbing for a few hours

Grid tied  -- connected to all the electrical poles and central power plants
Off grid -- often has a battery bank or a generator to offer consistent energy through the night, the exception being  a direct motor load is a  system that only runs when the sun shine (ex. fan or a motor that runs when the sun shines)
Direct motor load runs a motor when the sun is shining (i.e. no storage/battery needed, useful for electricity that can happen 'whenever' or for things that only need to be on during the day, when the sun is shining such as a water pump that pumps water to a holding tank at elevation or a fan or an AC unit that runs during the day)

SOLAR ARRAY
    1.    The more overcast, the less use is a tracker . ..  In general, never use trackers.  Headache on maintenance.
    2.    All of CA, recommended fixed tilt angle is 30 degrees. Generally speaking, the more clouds that happen as you move further and further north offset the change in latitude.
    3.     Go for longer, higher voltage strings.   Unless there is hard shading.  Shading on any part of a string derates all cells on that string.  It used to be that the array voltage needed to match your battery size.  So wiring your battery at the highest possible voltage 48 volts and wiring your panels into strings and paralleling them to 48 volts was the most logical.  Now, we can use MPPTT charge controllers to accept up to 200 volts on battery based systems or 600 volts on grid tied system.  For the simple economy of smaller wire size needed with higher voltages (i.e. less amperage) and less derating when paralleling strings of PV together.
    4.     Buy Used Panels (ideally all the same Voc and Isc).  To test, cover with a blanket, connect positive and negative wires on the same module then face sun/remove blanket to get Isc reading with a clamp on ammeter then reshade entire panel to disconnect.. .  (i.e. don't  short circuit the panel while there is a source/the sun.)
    5.    You can check an old used panel by short circuiting it and then taking a clamp on amp reading.  Open circuit voltage is meaningless.  Connect module when not in sun then fce sun to get reading then disconnect when no sun source.   
    6.    Try to use the same panels in series.  When working with different panels, parallel them together.  Ideally the voltage is as close as possible, consequently, you want the number of cells to be the same.
    7.    Unless constrained by space $/watt is more significant than efficiency of panels since the input fuel (sunshine) is free.
    8.    Physically, the back side of the solar panel is more vulnerable than the glass cell side.
    9.    Open circuit voltage dictates the upper limit of panels in a string.  No residential wiring can exceed 600 volts (grid tie) so when finding the Open circuit voltage . . multiply by the temperature factor and divide into 600 to see how many modules per string is the most you can do in that area with the Voc of that panel.   
    10.    STC = standard test conditions and is based on a relatively low temperature so you will have higher voltage in warmer areas.  
    11.    PV combiner box  -- combines more than 1 string of PV array.
    12.    Horizontal panels will collect more dust and snow.  Have a slight angle even if on a flat roof and you want out of sight.  
    13.    Panels have gone from $5/watt in the 70's to $1/watt today in 2013 -- so a 1 kilowatt array should cost $1000 in panels.

CHARGE CONTROLLER
    1.    Maximum Power Point Tracking Charge Controller can receive 200 volts (battery based) and 600 volts grid tie and transform to 12 volts. Go for longer strings of panels.  Make sure you don't exceed the Maximum amperage or voltage.
    2.    MPPTCC successfully boost array energy output by 10-20% (especially effective in the winter) by periodically  checking all the possible voltages and selecting the IV ratio that yields the highest Wattage.  Occasionally there can be an error when there is a little bubble in an IV curve due to shading.
    3.    No charge controller in grid tied -- the inverter becomes the brains

INVERTER
    1.    WATTAGE Inverters are often sold as a four digit number.  The first two numbers represent the power / number of kilowatts your inverter can supply.  This corresponds to the number of loads you can run simultaneously. The final two digits are the voltage that your batteries or panels are sending into the inverter.  Grid tie inverters are smarter now and can accept a wider range of voltage and still put out 120.  If you have batteries, then you need to wire your battery bank to the same voltage as your inverter.     
    2.    SIZE -- same wattage as the array for grid tie.  The inverter only needs to be able to handle the invoking wattage.  Any loads you run above the size of your inverter will be supplemented by the grid.  Most battery based homes probably would like in the range of 3000-5000 watt inverter.  If you want air conditioning or have lots of power tools or like to run your dishwasher, electric dryer, Air conditioning and high power blender simultaneously, lean more toward 7000-8000 watts.  Add up all the loads that you could simultaneously run, and get an inverter that can supply that.  Cost wise, with each 1000 watts you add you are probably adding $750 -1000 in larger inverter/larger breakers/bigger wire.    Sizing the inverter dictates how much management/awareness of simultaneous loads you are willing to have vs. the convenience of I want energy whenever I want.    Ultimately, if you run too many things at once,  a breaker will trip but no harm or damage will occur.
    4.    Microinverters basically only if there is a lot of shade or you need to use different orientations/tilt angles on your array. This separates the amperage per string rather than create equilibrium when the strings are paralleled
    5.    Low standy losses 3-15 watts acceptable in standby mode.  (Bigger inverter maybe has a 20 W standby rather than 15W)
    6.    High surge capacity  (many motors will surge 2x and motors attached to a compressor possibly up to 5 -6x their continuous load).   Get a soft start compressor rather than a significantly bigger inverter to handle this.  A variable frequency drive compressor (mini-splits) has less of a surge  -- good for battery based air conditioning.
    7.    Controls the set points for absorption bulk and float for grid tie systems (charge controller does this on battery systems.
    8.    Controls diversion controls -- what to do when you have too much energy (switch to a water pump that pumps water uphill to a cistern or run something nonconsequential)
    9.    Needs to be able to absorb all of the power that PV makes

BATTERIES
See the Downloadable article on Batteries.
Batteries are finicky and are the weakest link (shortest shelf life in the solar system with the most maintenance).
Ideally, your batteries can take in much of the available solar energy that your array receives (or you have an oversized array).  and they can get you through the night (or days of no sun) without discharging regularly more than 50%.  

WIRE
    1.    Higher voltage (with lower amperage) saves money on wiring.
    2.    Home scale grid tie inverters and smart charge controllers these days can receive up to 600 volts.  It used to be that DC usually ran at 12, 24 or 48 volts; grid tie can receive up to 600 Volts (UL maximum) and MPPTCC are able now to receive upwards of 200 volts for battery systems and transform to the 12,  24 or 48 volts that the batteries want.
    3.    Half he voltage needs 4 x as much copper wire so a 24 volt system needs 4 x the wire as a 48 volt system and a 12 volt system needs 16 x as much.
    4.    It is not true to say that AC is easier to transport far distances than DC.  It is more accurate to say that high voltage is easier/cheaper to transport far distances than low voltage (i.e. 12 volt DC).  Higher voltage means lower amperage which means less line loss on smaller wire.  
    5.    Line Loss is an aspect of all electrical.  We lose some energy as heat when it flows through a conductor.  A voltage drop indicator helps assess what gauge wire we should lose to minimize our line loss.  Wire size is our most direct control over line loss and you can do the math as to whether to buy bigger wire or accept more energy loss in the line/wire.The higher the voltage (i.e. lower amperage) and the shorter the run, you are already dealing with smaller wire so don't accept above 1-2% line loss and when you are dealing with high voltages, %ages become more significant. on these high voltages.

Overcurrent protection need to be 1.25 bigger than I SC (short circuit amperage) for current flowing through.  The breaker also needs to be rated for open circuit voltage.  If a motor has a surge amperage, hopefully the breaker will only flip if it surges too long but double the max amps for seconds may not flip a breaker.  
Breakers and Fuses are thermal devices that melt or bimetal strip that boing when heated too much too long which means that for a 20 amp breaker -
2000 amps will break in milliseconds and 22 amps will break in 10 minutes. . . .