Future Solar Products

Cariboocoot
Cariboocoot Banned Posts: 17,615 ✭✭✭
in General Solar Power Topics #1
As I listen to my wife complain for the umpteenth time about the energy we're wasting when the batteries reach 'Float' and I explain for the umpteenth time that nothing is being wasted, it's just not being 'harvested', I get to thinking about this persistent annoyance of not being able to harvest that power you know is there.

It occurs to me that the technology must be reaching the point where an MPPT CC could be built with primary and secondary outputs. Theoretically, it could tell how much additional power potential is present at the PV's when the batteries only need a few Amps to keep them Absorbing or Floating. The additional power could then be diverted to the secondary output to charge a back-up bank.

As far as I know, there is currently (no pun intended) no such unit on the market. The existing methods of utilizing the 'excess' energy are somewhat imprecise, as the amount of power available from the PV's inevitably varies and any additional 'diversion' load is either a fixed resistance such as a water heater or has further variables such as a battery needing charging. To measure the actual potential of the PV's (in the same manner as the MPPT's already do) and give priority output to one function while diverting only the available extra to the other would seem the most economic method of increasing efficiency.

From existing technology, the only method I could think of to achieve this would be imprecise; using an AUX function to connect an additional MPPT to the PV's. An all-in-one unit would be better.

Morningstar makes a "dual battery" CC (http://store.solar-electric.com/modubachco25.html), which is similar to what I'm talking but not exactly (it is PWM, not MPPT), and it is limited to 25 Amps @ 12V.

Another thing; any chance we'll be seeing 120 VDC Inverters come into common use? Sure there's trouble with handling high DC Voltage, but there's also trouble handling high DC current.

I'm probably not explaining myself well, but a discussion on "things we'd like to see" couldn't hurt. :D
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Future Solar Products

    Probably would be to get a second PWM charge controller and set it for diversion mode and use it to, for example, pump water into your storage tank. Or get a second MPPT controller to charge a lower voltage battery bank... But if you have that much excess power anyway--you probably would have made your primary bank just that much larger (or--if not enough power in winter to charge the secondary battery bank--you would be wasting fuel on the genset to keep that bank reasonably alive and useful).

    I think-the biggest issue is that people do not have many optional loads loads that they can power when the bank is full and the sun is still up... Fix that problem (well pumping, water heating, space heating, etc. -- at least you will have someplace useful for that power to go)...

    Problem is if that load is not in sync with your seasons (i.e., hot dry summer, lots of pumping loads, A/C run in hot sunny weather; vs some sort of electric space heating--want the "extra" electricity when the sun is not shining very much)....

    It is the old "do we over buy on solar panels" or do we "use more fuel for the genset" (run genset 3 months of the year or no months of the year)... No right or wrong answers--just choices.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: Future Solar Products

    Actually the Bluesky series of MPPT controllers have an aux load output. When the CC begins to go into absorb and it ramps down the amperage, it can trigger a load controller that either charge a second battery bank,or control a load.

    I have toyed with the idea of having it trigger a second inverter to power my LP fridge when I have excess PV. I tried it and it worked fine, except that the cost for me made no sense. I was getting about 2 hours of excess sun, so having it run the fridge for those two hours. translated to about $.02 worth of propane saved. (That figures a duty cycle on the propane of ~25%) So the $300 cost of a new inverter, plus a couple of extra watts of PV would pay off,,,,never!

    I do in fact use it to keep a 12 volt utility battery floated.

    T
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #4
    Re: Future Solar Products

    I think the answer lies with better monitoring and control systems, rather than more features in the charge controller. Such a system would consist of current sensors, an interface to read battery monitors and/or charge controllers and relays to turn optional loads on or off. Oh, and it should integrate with weather stations or with Internet based weather data. Then have all decisions made by a central low power computer.

    So essentially, the system would know the state of your batteries, how much your loads are using AND the short-term forecast for solar input. Once it knows that, and it can control optional loads, you could write complex rules for managing power, e.g.
    • If the 7kW genset is on, but the charger is only using 3kW then turn on the water heater
    • If the batteries have reached 60% DoD, but it's 6pm and from past usage, the systems knows that they'll reach 50% DoD in the middle of the night, then turn on the genset now
    • if the water tank needs a top up and the batteries are at 60% DoD and it will be overcast for the next few hours, then turn on the genset
    • If the batteries are charged and there will be sun for the next 30 minutes, hten turn the immersion heater on
    • etc.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: Future Solar Products

    Stephen makes a good point.

    One very good way to make a system run better is to "load shift" For example I wait until the house batteries have gone to absorb, then I pump water, plug in the lap tops, charge the drill batteries etc.

    Tonu
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Future Solar Products

    The problem with interfacing with weather stations is; I have never, ever had an accurate forecast for my area. In twenty years. The past week they've been predicting snow: we've had bright sunshine. Climate here is very localized (it can snow on one side of the lake while the sun shines on the other, and it's a small lake).

    I was just aiming for the idea of a CC that 'knows' what amount of 'excess' power is available (per sweep) and could then efficiently utilize it. You could hit 'Float' but only have another 30 Watts 'available', in which case there's no sens in applying it to a 1500 Watt water heater, having the charge drop out of Float, turning off the load, starting all over again ...

    But Stephendv is on to a good thing; it would definitely need additional programming capacity so you could set your particulars for controlled diversion.

    Next year I may try it with a PWM triggered off AUX at Float to charge up some 'extra' batteries. It's a pity we can't store our extra Summer sunshine for Winter.
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