Nowadays more and more devices in our household and even more so in the office or the factory are electronic devices that are internally driven by direct current (DC) while energy is delivered by overland lines in form of alternating current (AC).
Solar and Wind energy needs to be stored as it is mostly not produced in the right quantity at the time when you may want to use it. The best form of storage today is to use DC batteries. So my projection is that local DC power distribution (inside a building) and pure DC devices are going to be a major part of a household, hospital, school or factory in the foreseeable future.
While DC is not a good form for long distance transmission as electrolysis is a major problem not easily overcome over long distances and high voltages, on a smaller scale and with lower voltages DC is very manageable and not a problem at all.
There are three major uses for DC on the local level:
- driving DC devices directly without conversion to or from AC makes them much more efficient at least 15% in energy can be saved for those devices as DC devices can typically operate with 98% efficiency while AC adaptor waste at least 15% in the conversion.
- storing energy from Wind or Solar generators in DC batteries or using it directly in form of DC during times of generation.
- load balancing by actually purchasing lower cost AC energy delivered at night and using it in form of DC during daytime periods when the electricity provider’s demand charges are higher.
Perfect candidates for DC system strategies are lighting and computers, both require far less energy compared to 10 years ago. Any home or office lighting can be replaced with super-efficient LED lights that use 1/10th of the energy producing the same or better light.
Leading edge products now support both AC line voltage and 48 volt DC voltage where the systems run with 85% efficiency when used with AC and 98% efficient when used from a 48 volt DC battery source.
When deciding what the best DC voltage is to use around your home or factory we came to the conclusion that it had to be:
- within the internationally accepted Low Voltage Directive (LVD) and hence below 75 volts and
- it also wants to be the highest safe voltage so as to safe money in wiring. When running with half the voltage the current requirement would double and with it the conductor size, doubling the cost of the wiring infrastructure.
Therefore the most practical and safe voltage is 48 volt nominal which happens to be 60 volt fully charged without an electrical load and still be below the 75 volts of the LVD.
When making solar chargers or current drivers for LED lighting a higher voltage is more efficient as switching transistors with a fixed “on” resistance will have lower “on” losses with lower currents vs. the higher currents that would be required for lower voltages.
Typically a 12 volt DC system can only achieve 85% efficiency with today’s transistor technology while a 48 volt DC system can achieve 92..98% depending on tuning and type of electrical user.
Conclusion: When specifying a DC system it would appear best to have at least 48 volt nominal for all the practical reasons above.
