June 2, 2015 § 2 Comments
A century or so ago Tesla and Westinghouse beat Edison in the war of electricity transmission and AC became our way of life. In an odd modern twist, the first, and most famous electric car is named after Tesla, but runs on DC current. Most electronics run on DC, but AC continues as the transmission medium, dooming us to the ubiquitous “brick” converting to DC for our phone charging, computers and so on. The DC worm is turning. In some measure this is due to fact that the output of solar panels is in the DC mode, as is that of back up batteries. Organizations such as the EMerge Alliance are making some inroads in commercial buildings with a proposed 24 V wiring standard. But curiously the lead for the resurgence of DC usage in homes may well be from India.
wtih apologies to the Australian rock group
Power shortages are a way of life in most developing nations. Consumers who can afford it have back up devices which are inherently inefficient. The rest simply do without for several hours at a time often each day. Most governments respond with more power plants, which in many countries are coal fired, with attendant effects on public health and climate change. The Indian Institute of Technology, Madras (IITM), has initiated the Uninterrupted DC (UDC) program. This is an innovative scheme to provide continuous power even during the intervals of shortage. This is accomplished through some changes in the grid system at a sub-station level, combined with households using energy-efficient DC devices. Widespread acceptance of this concept will require some equipment to be redesigned. But many other common devices such as computers and cell phone chargers, as well as energy efficient LED lights already operate on DC. DC powered fans are already available. Large scale adoption will improve consumer experience through uninterrupted service and reduced costs and have a net positive impact on the environment.
India is poised for rapid economic growth. This growth brings with it increased requirement for electric power at the industrial and consumer levels. Chronic power shortages especially at peak intervals have to be managed. Industrial consumers rely on diesel powered back up power, which has its own issues with particulate matter emissions. Private consumers have two choices. Those that can afford to install inverters in each home which charge batteries for use during the outage. AC power is converted to DC for storage and then reverted to AC for running devices. Each of these steps has an associated loss. Furthermore, when the power comes back on, each of these systems charges up for the next time, creating a surge on the grid. The UDC system is targeted at providing limited service continuously while at the same time reducing the overall energy consumption. In essence this is an aspect of Demand Side Management. It fits with the overall direction from the International Energy Agency that any reasonable carbon emission targets in 2050 can only be met by using 50% less. India and China are routinely cited as major contributors to atmospheric carbon due in part to reliance on coal for power. Program such as UDC could lead the way to mitigating the environmental impact of coal for power. Uninterrupted DC (UDC) technology is so named by its inventors, to emphasize that it delivers a useful quantity of power in uninterrupted (24×7) mode, and in DC form, incentivizing use of efficient DC appliances. Devices powered by DC can be 50% or more efficient than their AC counterparts. Use of such devices and the systems to enable these are central to the concept of UDC. In low to moderate income households the critical devices for continuous operations are lights, fans and either cell phone chargers or LED televisions. A home that typically uses 1 kW of AC peak power, could get by with 100 W of DC with somewhat reduced functionality.
The UDPM is a new device at the spot of the current meter and is the heart of the UDC system. It incorporates the existing AC meter and adds capability to split the incoming power into a DC 48 V line and a conventional AC 230 V line. The house is rewired to accommodate a few low voltage lines to run the low voltage devices. In a peak demand period the sub-station will send 10% of the normal electricity to each home instead of turning it off, as is the current practice. The UDPM at the home will utilize it solely for the 48 V service. During the period of the brownout the sub-station steps down the power to 4.2 kV from the normal 11 kV. The UDPM detects this voltage drop, cuts the AC output, and limits the 48V DC output to, say, 100W. This robust signaling is another innovative feature of the system. Importantly, during normal operation, both home circuits are in use, but the DC output is always limited to the brownout level of 100W. This allows for the utilization of the low power DC devices all the time and not solely during the brownouts. The consequential lowering in the power bill is a positive for the homeowner, and the continuous use incentivizes the manufacturer.
Fit with Solar Energy: While the initial focus of UDC is reasonably moderate income homeowners, the middle and upper-middle class segment could also be addressed through the addition of solar energy. This source is DC power to begin with and is artificially converted to AC for conventional appliances. This can still be allowed while a significant portion could be used in the DC mode. Typical solar outputs are 12 V and four together add up to 48 V. Perhaps this is why IITM chose that particular voltage, not to mention that 48V has been the standard DC voltage for telecom equipment worldwide. 12 V is also the output of standard lead-acid storage batteries. Ultimately one could expect even compressors for refrigerators to go the DC mode. Air conditioning would be next, but for the drier parts of India air coolers using water function quite well and those components are DC amenable.
Conclusions: UDC is an elegant addition to the Demand Side Management arsenal. It generally falls in the category of technology solutions although a small element of behavioral change exists. Utilities will undoubtedly welcome this development. Since the changes have to be at the sub-station level, the conversion could be staged community by community. IITM reports that pilots have already found word of mouth spread of the demand. An innovative business model may be necessary to pay for the modifications in the homes. Widespread use of this technology is certain to reduce the overall national burden on the power sector. Countries could justifiably claim advances in GHG mitigation.