A large proportion of the barriers relating to DC are a legacy of a bygone era. Ever since the “War of the Currents” in the19th century, the use of electricity – as a provider of light, heat and power – has been standardised on alternating current systems.
Direct current (DC) gave way to the adoption of alternating current (AC) supply principally because of the inefficiencies of long distance transmission. The subsequent development of semiconductor and microprocessor technology and their dependence on DC introduces the need to provide localised transformation and rectification at the point of use.
As customer demand for lower energy costs, driven by rising energy prices, has seen the emergence of white light low voltage LED lighting and the increased adoption of photovoltaic (PV) arrays. In some instances, this has resulted in power being inverted from DC to AC only to later be rectified back to DC at point of use. The prejudices towards DC for power distribution are deep routed, perhaps dating back to Edison, Tesla and Westinghouse. But, with every home and office now hosting several often extremely inefficient switched-mode power supplies or transformer/rectifier units, there has never been a better time to consider rectifying current thinking.
A large proportion of the barriers relating to DC are a legacy of a bygone era. The technology is seen to be dated and dangerous. This low level of utilisation in the built environment has resulted in a lack of standardisation with regard to installation and use. Whilst DC is the power of choice for London Underground or the International Space Station, it is not usually found in domestic or commercial applications, other than pre-inversion or post-rectification. In response to this the IET published a Code of Practice for Low and Extra Low Voltage Direct Current Power Distribution in Buildings. The code seeks to dispel many of the myths about the use of DC and is the first step in encouraging the adoption of this system for designers, specifiers and installers.
There still remain a number of barriers to adoption., namely the availability of suitable design software, plug and socket fittings, appliances that will function on a DC supply; although the development of brushless motors is helping to address this with rotating machines. The drivers for change are more easily understood. There are higher energy efficiencies to be gained by not having large numbers of low efficiency rectifier units. A high efficiency single point of rectification whilst initially a high cost solution will provide payback over it life. In addition, the direct connection of PVs into a DC network removes the costly inverter which typically requires replacement once during the life of the PV array. In commercial building there is the removal of the I2R issues related to transformer rectifier units, thereby providing advantages to the management of air temperature and comfort.
The savings that can be achieved in a commercial environment can be seen by comparing the figures in this DC vs AC UPS cost comparison:
This is based on an assumed a total life cycle cost for 15 years (N-system configuration 45% load factor).
The UK data centre industry in 2016, consumed 3.6 GW hrs of Energy, which cost £252m with an equivalent use of 2.05mt of carbon.
There is a case for cost savings within the domestic and commercial market too, but it requires the DC energy market to mature beyond its current position. Those that engage early will help to drive the market, but there is a general need to educate beyond the early adopters. It is clear that commercial realities will focus decision makers to rectify their current thinking.
By Blane Judd, BEng FCGI CEng FIET FRSA FRI FCIBSE FCIPHE FInstLM
Chair Built Environment Executive