Upgrading residential/building PV system with DC coupling

 In many Solar PV systems, we throw a significant amount of energy away in the conversion process from DC to AC. Because it is not possible to convert more energy exceeding AC rated power of the inverter, although the DC parts are generating significantly more energy. Especially in Japan, due to the domestic regulation of interconnection voltage upon the scale of AC output power, we have advantages to install significantly larger  DC capacity compared to the rated AC power, if the AC rated power is just below the limit of lower voltage interconnection. We can make our PV plant grid connected at 200V if the AC power is under 50kW, and 6600V if the AC power is under 2000kW(2MW) respectively. What I am going to emphasize is that we have a huge capacity of DC unused power everywhere in Japan, and probably something similar in many other countries around the world.  

To utilize such excess energy, it is possible to install DC based load and/or storage devices directly connected to the DC part. We call such architecture as DC-DC coupling from now on.

With DC-DC coupling architecture, we can utilize the excess energy efficiently before cutting off at the inverter. In other words, we can use or store the excess energy from the DC part.  To do this effectively, it is advantageous to set common DC busbar voltage over the system.

In addition to the possibility of utilizing excess energy, there are some more advantages of DC-DC coupling. The first thing is that we can use energy more efficiently without passing the unnecessary conversion process of DC-AC-DC power conversion. (Many of the devices are actually driven by DC power.) The second point is that we can configure a stable power supply system coworking with PV and batteries without having complicated sequences, just by setting the maximum DC voltage for each component.

Since it is not easy to differentiate the attraction of a battery system only by the difference of the battery itself, it is not at all promising to launch a battery business with conventional AC-coupling architecture without having other advantages. DC-DC coupling might be one of the most promising entry routes.