Advantages and usage of DC source in off-grid photovoltaics

In my previous post I mentioned the advantages of a DC PV system. I will go through them in detail to clarify my point. Fist of all I should mention the typical voltage of the DC source we are working with is in the range of 300 to 350V, the value not being precisely fixed (in practice it can vary from 250V to 400V). This is to match approximately the maximum voltage point of the sinusoidal signal of the mains power supply in European countries, as to be able to adapt commercial appliances (made for AC230V) to DC. Such DC voltage is easily achieved connecting the adequate number of solar panels in series (e.g 7 x 48V; 10 x 36 V; 14 x 24V)

Maximum voltage point of an RMS ACV230 sine signal.

 1) A cheaper alternative to photovoltaic installations

This claim is supported by mainly two arguments. The first and most obvious one is that if DC generated by the PV array is directly used to power appliances we remove the need of a costly pure sine inverter. This is even a stronger argument when considering off-grid systems since they typically are in the 5-10 kW range and the price of a durable inverter-charger for such power typically exceeds 2000€. The second argument lies on the fact that with a DC system, energy consumption can be better optimized in such a way as to use energy when it is available, and store it in various different forms allowing downsizing the batteries (this point is developed bellow).
In practice many but not all the appliances can be modified to work on DC, so it is more convenient to install a mixed DC/AC system. In this latter case a pure sine wave inverter will still be needed, however its power output can be scaled down (and so its price) compared to a conventional system since most of the energy demanding work will be performed out of the DC. The AC will be used to power household appliances such as washing machines, dish washers, fridges, induction cooker... or any other appliances not fit for DC (or at least no easily modified to run on DC).

2) A more efficient way to use energy, using it when it is available, and storing it into different forms (electricity, heat, cold, potential energy).

This sounds pretty obvious! doesn't it? Well at low enough latitudes (typically those of the Mediterranean countries in Europe) there are sufficient sunshine days in a row not to be dependent on electricity storage except for illumination and nighttime.  Therefore there is no need for expensive deep cycle high capacity batteries.  All the energy demanding tasks are better carried out in day time and better carried out from the DC source directly as to adjust consumption with available energy, as for example:

-Running 3 phase motors from variable frequency drives allows varying the frequency (and thus the RPMs of the motor) as to adjust it to the available energy (e.g. on a sunny day full speed, on a cloudy day reduced speed)

- Heating up water, feeding the resistance of the boiler with a chopped voltage from the DC source, varying automatically the duty cycle to adapt the current consumption to the available energy (e.g. on a sunny day the water will heat up at a fast rate, while it may take several hours on a cloudy day, though still be warm at the end of the day)
- Controlling cold rooms or walk-in fridges (e.g. in a farm) with a variable frequency drive, adjusting the cooling rate with the available energy.
- Controlling the AC inverter current input and battery charging with a modified high power switched mode power supply, as to adjust it to the available energy.

3)  Automatize consumption

All the above tasks can be carried out in an automated manner. The simplest way to proceed is to modify the appliances in order to have them turning on/off according the DC voltage of the system (we call it the DC BUS) . That is the voltage of the PV array (or the BUS) evolves according to the solar irradiation and the load. The greater the load, the lower it goes following the I-V curve of the array (see figure). therefore, as long as the DC BUS is higher than VMPP, energy is available to use, appliances can then be given priority by setting their turn-on voltage to a higher or lower voltage. The current consumption of the appliance can then be adjusted (as explained above) to maintain the DC BUS to a desired value (in case current is drawn from another point on the BUS, or cloudy weather).

I-V curve of a hypothetical array of solar panel. The voltage of the BUS decreases with load "going up" the I-V curve.The evolution of the value of the BUS at a given solar incident light is proportional to the load. The operation of the appliances is performed according to the turn on threshold with a priority set for each appliance (the lower the threshold, the stronger priority).

For more precise appliances operation, maximum powerpoint trackers can be implemented to the equipement to accurately ajust the load to the available power at a given time.