When Thomas Edison pioneered the sale and delivery of electric power, he envisioned small, local power generators delivering electricity to a limited number of customers using direct current (DC). DC worked well with light bulbs and the electric motors of the time, while no practical alternating current (AC) motor was available.
In the “current wars” of the day, however, AC power advanced by Nicola Tesla and George Westinghouse came to dominate electricity generation and transmission almost exclusively. AC could be transmitted more efficiently over long distances at high voltage while using lower current. AC could also easily be stepped down to lower voltages for use in homes and factories when necessary. Thus, AC could take better advantage of larger economies of scale enjoyed by large power plants.
Yet DC power may be making a comeback. Today, with more efficient small, renewable energy sources available, distributed generation using private renewable energy systems is seen by many as a new energy paradigm. Photo-voltaic cells generating solar power, for example, lend themselves to distributed generation, since they can be sited in arrays on rooftops or in the back 40, taking advantage of the sun’s unlimited power and supplying a substantial portion of the site owner’s energy needs.
Distributed generation takes advantage of DC’s strengths versus AC. With AC, some power is lost as heat generated by resistance in the conducting wires. This was less significant when large power plants generating AC power were the most efficient and least expensive game in town and customer-sited generation was not an option. With today’s more efficient renewable energy systems, opportunities for customer savings on electricity have increased. DC also lends itself to battery storage, for later use if the regular energy supply is interrupted.
Many electricity users seeking to reduce their costs or to “go green” are concerned with the generation and use of electricity on their side of the meter, not with selling AC power back to the local utility over the grid. There may not be much excess to sell, and the price the local utility is permitted to offer may not be attractive. The real goal is reducing reliance on the grid. DC systems powered by PV cells, for example, can achieve that goal with greater efficiency, through the local generation and use of power over a customer-owned “microgrid.”
A microgrid is simply an independent system that supplies power for a defined physical entity, such as a store, office building or factory. It can accept power from all kinds of energy sources. The owner can still be connected to the local utility grid and will generally still need that source of AC power. However, when needed, the microgrid will generate and store DC power efficiently, which is what most electronic devices require. If the DC supply should exceed demand, the excess can be converted to AC and sold back to the grid when that makes economic sense.
According to the Electric Power Research Group, electronic data centers powered by DC microgrids are 20% more efficient than the best AC systems, and up to 30% more efficient than most AC systems. This not only translates into cost savings for the consumer, but also helps the local utility since that power need not be delivered over the grid. This also helps the power utility meet generation capacity challenges. Furthermore, by eliminating some of the efficiency losses inherent in AC systems, the grid’s overall efficiency is improved to the benefit of all ratepayers.
Today, many of the solar arrays you see are DC microgrids being operated to serve on-site loads efficiently, usually in a single building or complex. Several companies specialize in microgrid design and construction. This sector should expand as the demand for efficient green energy increases.