Smart Buildings

Edison’s Dream Reawakened: Direct Current may Edge Out Alternating Current in Smart Buildings

Electricity in power grids and buildings is distributed as alternating current (AC) today. This is due to a historical decision based on AC’s better capability for transmitting power over long distances. This decision, taken in the late 19th century, lives on in the 21st century as a disadvantageous requirement for a high number of AC/DC (direct current) rectifiers and DC/AC inverters in modern smart buildings. In 1903, as a last-ditch effort to maintain DC as the standard for distributing electricity around the United States, Thomas Edison presided over a notorious event meant in part to demonstrate the danger of alternating current: the electrocution of Topsy, a circus elephant deemed a threat to humans, by a 6,600-volt AC charge. Edison’s stunt was pure fear mongering (DC being equally dangerous at high voltage), and it failed, hence our grid today is primarily AC. However, the current combination of AC and DC reduces energy efficiency, increases investment cost […]

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Electricity in power grids and buildings is distributed as alternating current (AC) today. This is due to a historical decision based on AC’s better capability for transmitting power over long distances.

This decision, taken in the late 19th century, lives on in the 21st century as a disadvantageous requirement for a high number of AC/DC (direct current) rectifiers and DC/AC inverters in modern smart buildings.

Edison

In 1903, as a last-ditch effort to maintain DC as the standard for distributing electricity around the United States, Thomas Edison presided over a notorious event meant in part to demonstrate the danger of alternating current: the electrocution of Topsy, a circus elephant deemed a threat to humans, by a 6,600-volt AC charge. Edison’s stunt was pure fear mongering (DC being equally dangerous at high voltage), and it failed, hence our grid today is primarily AC.

However, the current combination of AC and DC reduces energy efficiency, increases investment cost and total cost of ownership due to power losses in the inverters and rectifiers. As more renewable electricity generators like solar panels and wind turbines producing DC come online, DC power systems can ease their integration into the grid. Currently, DC power has to be inverted to AC before it is fed into smart buildings or the grid. Almost exactly 120 years after it lost the grid battle against AC, DC could finally be making a comeback.

A heated debate continues about the advantages and disadvantages of DC. The majority of progress in developing DC based technologies has occurred at either the high-voltage (more than 1,000V) or low-voltage (less than 100V) level. Since microgrids and building-scale nanogrids typically operate at medium voltage (roughly 380V–400V), much work needs to be done to bridge this voltage innovation gap.

Another challenge facing DC distribution networks lies with the need for standards and open grid architectures that can help integrate the increasing diversity of resources. Yet, there is momentum at the distribution level of electricity service to diversify power offerings and pursue hybrid solutions that incorporate a growing proportion of DC.

However, the market for direct current (DC) distribution networks is not a single, cohesive market. Rather, it encompasses several disparate opportunities; telecommunications towers, data centres, grid-tied commercial buildings, and off-grid military networks, that revolve around different market assumptions, dynamics, and drivers. The industry is currently focused on medium-voltage DC distribution networks, systems that are mostly concentrated on the data centre market segment, but which can also, and increasingly, apply to commercial buildings.

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Lawrence Berkeley National Laboratory researchers Vagelis Vossos, Karina Garbesi, and Hongxia Shen investigated the potential savings of DC power distribution in net-metered residences with on-site photovoltaics (PV) by modelling the net power draw of the ‘DC house' with respect to today's typical net-metered AC-house configuration. Both houses were assumed to have identical DC-internal loads based on an analysis of 32 electricity end uses, all of which were found to be DC compatible.

Model comparisons were run for 14 representative cities across the United States, using hourly, simulated PV-system output and residential loads. The modelling tested the effects of climate, load shifting, and battery storage, as well as considered partial load conditions. A sensitivity analysis determined how future changes in power system component efficiencies might affect potential energy savings.

Results showed that net-metered PV residences without storage could save 5% of their total electricity load by using DC internal appliances, and those with battery storage could save 14% of their total load. The residence without battery storage would achieve only a modest savings because the time of peak PV production (midday) does not coincide with the peak residential load (late afternoon/early evening). However, residential PV systems incorporating battery storage could achieve much higher savings because the system can both save and use the generated power in DC form.

The project also found that DC energy savings are sensitive to power system and appliance conversion efficiencies, but that they are not significantly influenced by climate.

In the EU meanwhile a new project envisions taking an important step to reach Europe’s ambitious climate and energy policy goals by developing power distribution systems with highest efficiency and best integration of renewable energy sources. The Direct Current Components + Grid (DCC+G) project found that using direct current would save energy and support smart grid infrastructure as well as smart building integration.

DC would surely be the net winner in terms of higher overall efficiency and reduced energy consumption, offering higher reliability than comparable AC due to the greater ability to tie in renewables and storage, easier interconnection to a grid, and reduced cost due to the opportunity for smaller renewables.

We will be discussing "DC in Smart Buildings" with Mike Hook, Executive Director at LMG Building Intelligence, during our next Webinar on Tuesday 25th August. Amongst other things we will be discussing the IET's Code of Practice for Low and Extra Low Voltage Direct Current Power Distribution in Buildings.

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