I've had this idea kicking around for a while. An awful lot of our systems are designed around the modern mindset of power on demand, that we must have what we want, when we want it, and in the amounts that we want it.
A lot of times this doesn't actually make a lot of sense. My car has a pretty big battery, and it spends a lot of time time sitting in my car port on sunny days. Why can't I just harvest that energy and accumulate it in the car, as and when it's available? It makes no sense to accumulate the solar energy into a separate, large battery just so I can rapidly transfer it to the large battery in my car.
Here's my design - I'll post again after it's built.
1. Requirements
EV's are designed to charge rapidly from a DC charger (400V, 800V or more), or much more slowly from an AC charger (240V). They mostly come with a portable charger that plugs into a normal household outlet and can charge the car at 2.4kW, which is a slow but useful rate.
My carport is about 6m x 3.5m, which if the sun is directly overhead could mean as much as 4kW of electrical power.
So if I design the system to be able to charge the car at 2.4kW I should be able to minimise costs, using the portable charger I already have been using for the last 6 months, and making use of the carport roof which will minimise the complexity (cost) of the installation.
I don't need the car to charge on demand, just harvest energy when it's available, so ideally I have no battery or just a small battery, again to minimise cost.
The system should be completely independent of the grid, again to minimise installation and administrative complexity.
2. Design logic
The idea behind the design is to use a relatively small battery as a system stabiliser. At the beginning of the day the battery is depleted. The inverter is plugged into the charger and the car, but is not making 240V AC because it has detected low system battery voltage. The sun comes up, the solar starts producing power, which feeds into the system battery. The MPPT is sized to produce a maximum of 45A (~2.4kW), the battery is sized to be able to take this safely although it will rarely if ever need to do this.
Early in the morning, the battery charges at whatever rate the MPPT can deliver power. This is anticipated to take up to several hours depending on the weather and the initial battery level (determined by the programmable low battery cutoff in the inverter).
When the battery reaches a "full" state (to be programmed in the inverter) the inverter switches on, the charger negotiates with the car and resumes the charging session. By this time the solar input has increased. The inverter is drawing power depending on what the portable car charger is set to (2.4, 1.9 or 1.4kW). The power comes from whatever the solar panels are suppling, plus whatever needs to come from the system battery if there is a shortfall. The solar charge controller is over-paneled close to its limit, with 3.5kW of solar panels under ideal conditions, allowing it to operate at or close to its full power output for a longer period during the day.
The car battery charges, the system battery remains full or is slowly discharging depending on system settings and the sun. If the sun is interrupted by clouds, power is drawn proportionally more from the system battery. The system battery can charge the car it full power, by itself, for an hour or two depending on settings.
Later in the day when the sun is lower, the system battery depletes faster, eventually causing the inverter to trip off on the low system battery state. The residual solar power for the remainder of the day partly recharges the battery but this will not be enough to bring the inverter back on before sunset.
I'm using the (MPPT) solar charge controller as the power limiting factor in the design. The Victron MPPT units are specified by a maximum input voltage and a maximum output current. The 150V, 45A unit has a 45A output, which when feeding a lithium battery at 54V results in around 2.4kW of output - about enough to feed the car charger at full power. This unit must be connected to a battery at all times, it cannot take solar input without a place to send the output.
3. Equipment selection and sizing
4. Issues and mitigation
High string voltage
The open circuit voltage for these panels under standard conditions of 1000 W/m2 irradiance and 25oC is 34.92V with a 3% error. Worse case, that's 35.45V. A string of 4 panels gives us 141.7V, close to the MPPT's limit of 150V. Goulburn, NSW Australia (where I'm located) can reach -10oC in the early morning. The stated temperature coefficient of voltage for these panels is -0.25% per oC. So on the coldest morning, the string voltage could be as high as 154.1V, which would almost certainly destroy the charge controller.
However, that assumes a standard test condition irradiance of 1000 W/m2, which I think is very unlikely given that this temperature is reached at dawn on winter mornings, and the panels will actually be oriented about 5 degrees away from the equator, due to the slope of the roof. Solar panels are known to have a log dependence of open circuit voltage on irradiance. This is shown for the NMOT test condition of 800 W/m2 for these panels, where open circuit voltage is 33.44V (+3% = 34.44V). Downrating the NMOT condition to -10oC gives a string open circuit voltage of 148.1V.
What I take away from this is that to blow up the MPPT I would have to have irradiance on the panels of more than 800 W/m2 on a morning of -10oC, which I'm willing to bet an MPPT charge controller, is impossible at this location.
