Overview

My PhD is on the integration of solar photovoltaic generation, battery storage, electric vehicle charging, and DC distribution within the home.

This can happen with a single DC bus. Other alternatives including using an AC bus (regular system, but could this be as efficient as the DC bus system? Definitely more flexible), and look into multiport convertersDo I really need ground reference on DC bus? Let’s scrap DC loads and just look at integrating solar, batteries, electric vehicle. Where is isolation needed? Where is grounding needed? SolarEdge doesn’t use isolation/grounding for solar panels OR batteries (look into StorEdge).

There are many, many possible converter options:

  • AC-DC converter can be:
    • Non-isolated with non-ground-referenced DC bus (e.g. full bridge, giving ~400V DC bus)
    • Non-isolated with ground-referenced DC bus (e.g. half bridge, giving ~800V DC bus)
    • Isolated with ground-referenced DC bus (e.g. dual active bridge, giving ~400V DC bus)
    • Isolated with non-ground-referenced DC bus (e.g. dual active bridge, giving ~400V DC bus). Referencing will be for the benefit of a different converter (e.g. electric vehicle charger)
  • Solar can be connected to:
    • Fixed 400 or 800V DC bus, with full power processing on each panel. MPPT done in these converters (e.g. SolarEdge Power Optimiser)
    • Variable 400 or 800V DC bus, with full power processing on each panel. MPPT done in these converters (e.g. SolarEdge Power Optimiser)
    • Variable 400 or 800V DC bus, with differential power processing on each panel. MPPT done in the AC-DC converter by adjusting DC bus voltage
    • Directly to mains with AC-DC microinverter on each panel.
  • Batteries can be connected to:
    • Fixed 400 or 800V DC bus, with DC-DC converter to manage charging/discharging
    • Variable 400 or 800V DC bus, with DC-DC converter to manage charging/discharging
    • Variable 400 or 800V DC bus, with the AC-DC converter managing charging/discharging by adjusting DC bus voltage
    • AC mains through AC-DC converter, managing charging discharging.
  • Electric vehicle (EV) charger connected to:
    • Fixed or variable, 400 or 800V, ground-referenced or non-ground-referenced DC bus
    • Directly to AC mains with AC-DC converter (unidirectional over bidirectional, I think)
    • I think the EV charger should always include a DC-DC converter. Connecting directly to the DC bus to manage charging would require the inverter to do the charging, which may change depending on the car? If we had a dedicated DC-DC converter then it could manage all the charging and protocol stuff. If we didn’t use a DC-DC converter, then the EV battery voltage would be directly connected to the PV on the roof for example. Dangerous!
    • I need to think about this. I think it needs grounding, but does it also need isolation? If I already have a ground-reference DC bus can I use this?

Considerations:

  • Single AC-DC interface, with PV, batteries, EV charger connected to the DC bus (either via DC-DC converter or directly)
    • Would I rather connect PV, batteries, or EV charger directly to the bus? PV is most likely to be present, followed by batteries and then EV charger.
    • If PV or batteries are to be connected directly to the bus, consider the possible voltage range of the bus. (What is the MPPT DC bus voltage range with differential power processing? What is the DC bus voltage range when charging/discharging batteries?). This voltage range needs to be considered when looking at other converters on the bus, as this will place requirements on their input voltages.
    • Three options (ignoring EV battery directly connected to bus):
      1. Fixed bus voltage, PV power optimiser, DC-DC battery converter
      2. Variable bus voltage, PV delta power processing, DC-DC battery converter
      3. Variable bus voltage, PV power optimiser, battery connected directly to bus
    • Consider the number of power-processing stages, assuming DC-DC converters on each sub-system. Assume we’re only charging from PV and discharging into both AC loads and EV (need to determine a ratio of how much power goes to each):
      1. Twice when charging (once by PV power optimiser, once by DC-DC battery), twice when discharging (DC-DC battery, and either DC-AC converter or DC-DC to EV) = 4x total
      2. 1.5 when charging (estimate 0.5 in delta power processing, once by DC-DC battery), twice when discharging (DC-DC battery, and either DC-AC converter or DC-DC to EV) = 3.5x total
      3. Once when charging (PV power optimiser), once when discharging (C-AC converter or DC-DC to EV) = 2x total
    • Where is ground-referencing necessary? Is it only in the EV charger? Would there be significant leakage currents with common mode voltage present on a large DC network with high capacitance to ground?
  • AC-DC interface on each sub-system (PV, batteries, EV charger)
    • This is potentially more expandable as the AC-DC interface doesn’t need to be sized for everything (e.g. EV), but can instead be sized per sub-system.
    • Consider the number of power-processing stages. Assume we’re only charging from PV and discharging into both AC loads and EV (need to determine a ratio of how much power goes to each)
      • 2.5 when charging (estimate 0.5 in delta power processing, once DC-AC solar converter, once AC-DC battery converter) and once when discharging into AC loads (DC-AC battery), and twice when discharging into EV (DC-AC battery then AC-DC EV) = 3.5x for AC loads and 4.5x for EV
  • To decide on the best solution, we need to look at a few factors:
    • Efficiency of the various converter options (isolated vs. non-isolated, half-bridge vs. full bridge, power optimiser vs. delta processing. etc)
    • How much power produced by the solar goes to the batteries vs. directly to the grid or AC loads
    • How much power supplied by the batteries goes to AC loads vs. to the EV
    • These numbers allow the calculation of a weighted efficiency.
    • Perhaps also loosen assumptions, to say how much of the batteries power comes from solar vs. from the grid (if used for utility service purposes)
    • Once these numbers have been established,  a weighted efficiency can be calculated
    • As previously mentioned, it is also necessary to see the impact on other converter input requirements from letting the DC bus voltage be variable in order to allow delta power processing or elimination of the battery DC-DC converter.
    • Also consider the storage technology (lithium ion vs lead acid) if connecting storage directly to DC bus.
    • Also consider expandability of solar/batteries if connecting directly to the DC bus and making the inverter do MPPT/charge-discharge management. Other strings/batteries would need to be connected through DC-DC converter.
    • Expandability in terms of PV power, battery capacity, battery power. For example, if interested in increasing capacity without increasing power perhaps a single DC-DC converter could be used to connect multiple batteries in parallel, with only 1 battery switched in at a time? Check out SolarEdge StorEdge!
    • Also consider having solar/batteries on DC bus, but EV charger connected to AC mains via dedicated converter. Why? Firstly, all EV power would need to be processed twice rather than once (AC-DC then DC-DC), and the AC-DC converter would need to be sized accordingly. If we wanted to add a second EV charger, that might require AC-DC converter upgrade. Consider the impact this would have on the number of power processing stages and weighted efficiency discussed earlier. We also need to factor in how much the EV is charged from solar/batteries vs. from the grid. Power supplied from the batteries would need to be processed 2-3 times depending on if a DC-DC converter is present, vs. just once from the grid.
    • 7 total options!
      1. Everything AC-DC connected
      2. Everything on DC bus with DC-DC converters
      3. Everything on DC bus with delta processing
      4. Everything on DC bus with no battery converter
      5. Same as 2-4 but with EV connected directly to AC via AC-DC converter.

Grid Interface

Need for grounding on DC bus (or maybe not?). Options are 50Hz transformer, high-frequency transformer (e.g. dual active bridge, or other bi-directional isolated AC-DC converter), and half-bridge. Currently using half bridge, but investigate dual active bridge. Wynand Louis Malan mentioned efficiency of 98% using GaN devices, dual active bridge soft switched resonant converter with high frequency transformer. This would allow a smaller unipolar DC bus (maybe 400V) which is significantly better than split DC bus which requires balancing.

Power devices: IGBTs (could switch 5-40kHz, or more), MOSFETs, GaN, SiC devices. Choice depends on topology (because of the bus voltage), etc.

Solar Converters

MPPT through one of:

  • Optimizer
  • Partially processed (delta)
  • Some combination (e.g. switch from Optimizer to Delta when balance becomes even)
  • Isolated secondary bus

Domestic Energy Storage

Tesla Powerwall, 350-450V DC connection, with charging/discharging controlled by communications port.

Electric Vehicle Charging

EMotorWerks. Four main options:

  • AC to the car, using standard J1772 connector, controlled by EVSE controller. What is the efficiency of the onboard rectifier?
  • DC to the car, using standard J1772 connector (I’m not sure if this is possible. Some places seem to suggest it is, though perhaps some cars have the standard J1772 pins hardwired to the onboard rectifier so the pins are only used for AC charging).
  • DC to the car, using the CCS J1772 connector
  • DC to the car, using a CHAdeMO charger. CHAdeMO is being phased out though, so CCS J1772 is probably the best option! (If there is even a sufficient advantage over using the car’s onboard rectifier, e.g. efficiency, power capacity, though an increased power capacity also affects the power capacity of the grid interface converter).

Does the EV charger DC-DC converter connect to the DC bus or battery terminal directly?

Wider-Scope

Module-Level Power Electronics (MLPE): optimizer vs. microinverter: Optimizer is considerably better (see: http://mcelectrical.com.au/blog/SolarEdge-optimisers-vs-Enphase-Micro-Inverters/)

Need to assess partially processed (delta) vs. fully processed (optimizer) which either use DC bus to do MPPT or do MPPT in the full power processed stage with constant (controllable) DC voltage output. I wonder if it’s possible to have a combination, with partially processed within say 350-450V then switch to fully processed outside this range.