PV Water, Why micro inverters?

m151
m151 Registered Users Posts: 39 ✭✭
I was reading about sun bandits hot water system and I see that they use micro inverters. I'm curious wouldn't it be easier and less expense if they used DC with panels connected in series? Especially if it was used as a tempering tank. What is the purpose of the micro inverters, perhaps for their control systems?

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    First of all, DC is horrible to switch and fuse.  

    Secondly (or maybe primarily) the NEC is moving toward requiring module level disconnect.  That means the fire department needs to be able to make sure (from the ground) that there are no high voltages on the roof.  Microinverters are perfect for that... if you cut off (from the ground) the AC that they synchronize to, they stop production.

    --vtMaps


    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    m151 said:
    I was reading about sun bandits hot water system and I see that they use micro inverters. I'm curious wouldn't it be easier and less expense if they used DC with panels connected in series? Especially if it was used as a tempering tank. What is the purpose of the micro inverters, perhaps for their control systems?
    The primary issue, leaving aside the whole question of economics of PV powering resistive heating for domestic hot water, is that a resistive element will draw a fixed amount of current at any particular voltage while solar panels deliver a varying amount of current at a relatively constant voltage.
    Putting panels in series gives you a voltage closer to the nominal voltage of the element, but does not deal at all with the problem of varying current.
    So to effectively use PV to drive a resistive heating element there would have to be an MPPT device to adapt the varying characteristics of the panels to a fixed resistance heater. More precisely, the device would not deliver a constant voltage as an MPPT CC does, instead it would have to deliver a varying voltage set specifically to cause the fixed resistance to use the full amount of power available from the panel.
    For example:
    If the heater element is designed to deliver 1000 watts when fed with 100V, it has a resistance of 10 ohms. (P = (V**2)/R)
    If the panels are only producing 500 watts, the element would have to be fed with 74 volts instead. ((74**2)/10 = 500).

    It is less expensive to use "off the shelf" microinverters to get a fixed voltage AC from the panels than to use a unique MPPT circuit to get a fixed voltage DC. And in any case, a fixed voltage is NOT what is needed.
    But off the shelf micros will not work without a  grid present, so the Solar Bandit would have to be using a customized micro for off-grid use (possible since the dedicated heating element is never connected to the grid and the shared heating element is used with a transfer switch.)

    The Solar Bandit will produce next to no power during the off solar noon hours or during cloudy weather. It appears that the SB attempts to deal crudely with this issue by using two separate heating elements which can be switched on and off individually depending on the available power.

    My person preference is for a heat pump type domestic water heater, since that produce about three times the heat for a given input of electrical energy.  The same heat pump would then reduce the cost of water heated using grid power when the sun is down.
    There is still the problem of how to deal with the varying power output from the panels, and even the inverter and conventional heat pump does not do this.


    SMA SB 3000, old BP panels.