PV homerun to parallel loads

NewBeNewBe Registered Users, Users Awaiting Email Confirmation Posts: 4

We have a solar powered heat pump(AC) that requires 4 panels(in series) to supply 100-300 VDC (1100 watts max) for operation. This unit will not operate 100% of time but less than 50% and its variable speed DC compressor(Toshiba/GMCC Rotary) uses varying power dependent on load, normal is 50-75% of max.  (1) We would like to utilize the excess energy the PV panels that is not being used by the DC compressor(Toshiba/GMCC Rotary) while the compressor is running, excess energy (app. 50% of total), to be run to a mppt solar charge controller. (2) we would like to use all (100%) of the PV energy when the DC compressor(Toshiba/GMCC Rotary) is not running, (app. 50% of time) to a mppt solar charge controller.

In our limited ability, we see the (2) situation (simply) being handled by placing appropriate diodes in each parallel run to each load and isolating the lowest priority load (mppt solar charge controller) with a SSR when the compressor is running.

The (1) situation appears most difficult. Using the (2) setup, we would leave the SSR open (both loads having access to PV energy) and utilizing some appropriate resistance device in the low priority line to the mppt solar charge controller, to direct a greater % of current flow to the priority demand, compressor.

Looking for critique and better answers.

Thanks for your commentary/suggestions!

NewBe

Comments

  • EstragonEstragon Registered Users Posts: 3,127 ✭✭✭✭✭
    What would the mppt be loaded with (eg battery bank)?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • NewBeNewBe Registered Users, Users Awaiting Email Confirmation Posts: 4
    48 VDC battery bank
  • NewBeNewBe Registered Users, Users Awaiting Email Confirmation Posts: 4

    My email address appears to be truncated, etc. have not received initial email confirmation, how can I change it online?


  • EstragonEstragon Registered Users Posts: 3,127 ✭✭✭✭✭
    Seems to me the better way to share the array would be to configure it to power the mppt and bank, then boost convert the 48v up to the heat pump voltage.  
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    In general, MPPT controllers do not like to share a single solar array.

    The MPPT controller basically does an "I/V" curve of the array (vary current to "solve" Pmp=Vmp*Imp). The older controllers would simply "rescan" the array every 5-15 minutes and use the "new" Vmp-array number (typically around ~81% or so of Vmp-array-stc) and modulate Imp-array to keep Vmp-array approximately correct (call it +/- 5%-10% or better of Vmp-array-measured).

    If your MPPT controller is using less than Imp-array-at that instant in time, then Vmp-array will drift up toward Voc-array (which is also temperature dependent).

    When you have two MPPT controllers trying to "solve" that equation, and then attempt to control Imp-array and Vmp-array... They tend to get confused.

    So, the intent is to share the single solar array with two different loads. The "optimal" solution would be to have all of the solar energy go to the battery bank (since you already have a battery bank), and then send the power to the various loads as needed and let the battery bank buffer the energy.

    Of course, that gives you more losses (charging battery, DC to AC inverter, etc.) and wear on the battery bank.

    And how much cost would sharing the array save... Say that 50% of the array energy goes to the battery bank, and you choose 10% rate of charge (5% to 13% rate of charge is typical for solar):
    • 550 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge = 4,235 WH of storage
    • 4,235 WH storage / 12 volt battery bank = 353 AH @ 12 volt battery bank
    • 353 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 900 WH per day of stored energy (overnight, cloudy weather)
    And based on hours per day of sun for summer (fixed array, biased towards summer production):
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Wilmington
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 71° angle:
    (Optimal summer settings)

    JanFebMarAprMayJun
    3.17
     
    3.71
     
    4.79
     
    5.61
     
    5.71
     
    5.62
     
    JulAugSepOctNovDec
    5.50
     
    5.03
     
    4.68
     
    4.46
     
    3.63
     
    3.16
     
    Say >5.0 hours of sun per summer day:
    • 550 WH array * 0.52 off grid AC FLA battery system end to end eff * 5.0 hours of sun = 1,430 WH per day "summer average" harvest
    And there is the issue that Batteries are the one thing that is easy to "kill"... If the battery bank is not kept happy, the batteries will not have a very long life. Flooded Cell Lead Acid batteries are cheap ("golf cart" 6 volt @ 200 AH ~$100 each, x4 for a 12 volt @ 400 AH battery bank).

    Of course, you could use LiFePO4 type batteries (don't care about charging current, actual hours of charging current, and state of charge as much as FLA do)--But much more expensive (and typically need a Battery Monitoring System).

    I worry that the battery bank will either be damaged from improper/non-optimum charging and that the "cost" of electricity (for a typical MPPT system, ~$2.00 per kWH (GC batteries last ~3-5 years typically) is a good starting estimate)--And the amount of battery energy per day is variable based on the A/C system usage (even more complexity to energy management).

    I don't believe that diodes are needed to isolate your two loads (A/C and Battery Charging), and using an SSR to cycle the solar array voltage to the MPPT charge controller (SSR "ON" when "A/C" compressor is off)--Can sort of work.

    But one of the "advantages" of "inverter compressor" A/C systems is that they are variable load. Ideally, your compressor is running 100% of the time at 50% (plus or minus) of rated power. So the SSR does not really work well in this case.

    Another possibility is--If you have the ability--To program the MPPT Vmp voltage for both A/C and Charger... Set the charger Vmp manually (or with an offset) X volts over the A/C system... As the array Vmp drifts up (A/C uses less power), the MPPT solar charge controller will draw more energy.

    Another cost issue is that "High Voltage" MPPT solar charge controllers tend to be much more expensive (>Vmp-std ~ 100 VDC rating). For example, you would need a ~40 amp (minimum) Solar Charge controller for a 550 Watt @ 12 volt raise:

    https://www.solar-electric.com/search/?q=schneider+mppt

    https://www.solar-electric.com/xaxwmp80amp6.html (high "600 Vmax" voltage 80 amp MPPT controller ~$1,100)
    https://www.solar-electric.com/xaxwmp60amps.html (mid-voltage "140 Vmax" 60 amp MPPT controller ~$500)
    https://www.solar-electric.com/c40.html ("low voltage" PWM charge controller 40 amps ~$110)

    https://www.solar-electric.com/morningstar-ts-mppt-600v-48-db-solar-charge-controller.html (60 amp 600 Volt MPPT ~$1,600)

    Anyway--My guess is that "saving" 550 Watts of solar (at less $1 per watt for larger panels that "Need" MPPT controllers" or ~$2 per Watt for 140 Watt panels that can work with either PWM or MPPT controllers)...

    I don't think this will be cost effective... It will cost you more to harvest the excess power from the 1,100 Watt array than to simply build a second 550 Watt array "lower voltage" + Charger + Battery bank. And both the battery and end user will be "happier".

    Anyway, just a starting point for discussion. Questions or corrections to my assumptions/guesses?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • NewBeNewBe Registered Users, Users Awaiting Email Confirmation Posts: 4

    thanks Bill, you put a lot time your response.

    The heat pump does actually power down the compressor when not cooling or heating which is about >50% of time. We have three of these mini Splits to replace a 3 ton carrier unit. They run off of AC 240V or DC 100-300V and merge the two sources via the internal inverter to DC compressor to provide full power to unit. We just feel that 1200 watts of PV energy(4 panels) dedicated to each(3) units is wasteful on the order of maybe 50%. Why not use the most efficient path PV to Heat Pump internal inverter and salvage the balance of unused energy for charging my batteries. Since these heat pump inverter compressors run seamlessly (not on or off switching) with varying % of AC vs. DC we should be able to make this work on a demand basis.

    Some MPPT controllers are controllable via software and can be turned on or off, perhaps that is a way to go. However, that does not still speak to the loss of app. 1500 watts (50%) of total PV energy feeding the heat pumps running at moderate demand (most of the time).

    I guess the greatest argument I have for direct connect of the PV energy to the MC4's on the heat pump, is the simplicity of that, eliminating battery cycling/replacement relative to that use and downtime due to other hardware required to convert DC to AC, etc. during the day light hours.

  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    Yep... Simple and reliable. Complexity is the bane of many solutions.

    I had to design a large redundant computer system (voice mail system for telephone companies). I would try to use the simplest solutions to meet the requirement.

    There were still lots of single points of failure--But nobody really cared as most of the system was redundant.

    We even put in a very simple switch that would disable auto-shutdown (over temp, other issues) and the tech publications group refused to document the switch (jumper). Had a system that would not start because of the failsafe--And they mechanically exchanged a $1,000,000 system because it needed to be UP and they could not debug it or let it run without the auto shutdown (not in manual). Turned out to be cable where one pin was not fully snapped in during cable manufacturing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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