Thomas Edison’s views on energy supply and sustainability, in addition to supporting DC distribution in the infamous “Battle of Currents” with Tesla and Westinghouse, are strikingly relevant to the policies being made today. resemblance. He favours local generation, which minimises DC transmission losses, and is fascinated by the prospect of harnessing renewable energy sources such as wind, solar and tidal power.
Of course, today, growing concerns about the future of fossil fuels and their impact on the environment are directing policymakers’ attention to renewable energy. Additionally, as technological advances have brought small photovoltaic arrays and wind turbines into the financial reach of businesses, small farms, and some homeowners, local generation—whether connected to the main grid or islanded—has become practical and economically viable . The output of such generators is usually direct current, which must be converted to alternating current of the appropriate frequency and voltage before it can be fed into local line power. The power-conditioning circuits used to achieve this consume a few percent of precious energy. If you can eliminate the need for a DC to AC inverter, you can save energy.
Electricity consumption has increased significantly since the Edison era; especially through the use of more and more devices, such as personal computers, mobile chargers, flat-screen TVs, and small office machines, which are basically DC-powered systems. Additionally, incandescent lamps are being phased out in favor of DC-powered LED replacements. Energy-efficient variable frequency drives in appliances that traditionally contain AC induction motors, such as washing machines or refrigerators, now convert AC line power to DC and then reconstruct the AC waveform at the correct frequency to achieve the desired motor speed. In the long term, the prospect of widespread use of plug-in hybrids or electric vehicles will further shift demand balance towards local DC loads that can run more efficiently from DC power, without the need for AC/DC power at the input. The Electric Power Research Institute (EPRI) estimates that the efficiency gain from using on-site renewable energy directly without converting to AC could be as high as 15%.
However, today’s AC grid represents a huge investment in equipment and technology. In addition, a large number of appliances in use today are designed to be powered by high voltage AC power and need to be adapted to be powered by DC power, or otherwise replaced. Therefore, it is impractical to adopt a revolutionary change in DC power distribution.
Building a DC Microgrid
On the other hand, incremental changes can introduce the benefits of DC distribution in the most accessible areas. Small DC microgrids can be used to augment standard AC line power by powering loads such as LED lighting or other DC appliances with minimal conversion losses. This presents an opportunity for properties such as offices or homes with their own solar or wind micro-generators to efficiently utilize renewable energy sources, thereby reducing utility bills and environmental footprints at the same time.
As today’s distribution grids and appliances become smarter and acquire intelligence to handle demand-side management and balancing, DC microgrids can become an integral part of an overall smart grid strategy by moving away from the main grid when independent power is available. Remove the load and reconnect when independent power is available. Local energy has dried up. Microgrids can also be used to feed in any excess energy from local microgenerators by connecting to the main grid via an inverter.
The University of Bath has implemented what it claims is the UK’s first local DC network to power the campus library. During the six-month experiment, 50 specially adapted personal computers were powered directly from a DC power source, and the LED lighting was also powered by a DC microgrid. In addition, a set of backup batteries, charged directly from the DC microgrid, was installed to power lighting and computers in case the utility supply was interrupted. Because the experiment was designed to study low-voltage DC power distribution within buildings, the energy came from the standard AC grid, rather than the local renewable energy supply. A single AC/DC converter at the front end effectively replaces a single AC/DC power supply in the library computer, eliminating multiple sets of conversion losses and improving overall efficiency. The university claims the library’s total energy consumption has been cut in half, equating to annual savings of around £25,000, and the ambient air temperature in the building has also been reduced.
Standards drive progress
One factor that may hinder the adoption of DC microgrids is the relative lack of standards governing aspects such as operating voltage, wiring, safety, and device interoperability. Standards published by the EMerge Alliance to address this shortcoming include the Occupied Space standard, which specifies 24 V low-voltage DC distribution for powering indoor lighting and small appliances (Figure 1). The consortium has also published data/telecom center standards that specify higher 380 V DC voltages to provide higher power capabilities for high-performance servers. Higher voltages also help reduce copper costs by reducing distribution current. More Emerge Alliance standards are expected, including those that provide loads such as lighting and EV charging points in outdoor spaces.
Figure 1: The Occupied Space standard envisages low-voltage DC power distribution at the ceiling and is suitable for the infeed of field-generated DC.
While low voltage DC distribution is ideal for powering LED lighting, there are no mechanical, safety or protection standards for low voltage DC plugs, cables or input sockets that allow low voltage DC powered equipment to be marketed or easy to use. If DC microgrids are employed, they may first be used only to power loads such as lighting, and may need to be installed in parallel with AC indoor distribution at standard AC single-phase voltage.
A plan outlined in the International Journal of Engineering Technology and Physical Issues (IJTPE) proposes interconnecting AC and DC grids to create a hybrid network (Figure 2) that can be powered by utility power or local solar or wind micro Generator power supply. The inverter handles the transfer of energy from the DC to the AC side of the grid, while the AC/DC converter enables the utility power source to power the DC bus when needed when the solar array stops generating electricity.
Figure 2: Hybrid AC-DC grid interconnected by inverters and converters.
Energy storage options
Due to inconsistent renewable energy sources, backup power sources may be required to help stabilize energy flow and provide emergency power. Grid-tied storage can be a battery array, as shown in Figure 2, also used in the University of Bath experiments, but other types of storage, such as flywheels or supercapacitor arrays, could also be considered.
Supercapacitors have very high power density and rapid discharge capability, which can quickly respond to short-term high power demands. Charge times are also much shorter than batteries, and other benefits include longer cycle life, higher reliability, lower maintenance, superior temperature stability, and a wider operating temperature range. Volumetric efficiency improved by thousands of times compared to common electrolytic capacitors, enabling engineers to design compact and lightweight storage for anything from board-level pulse or hold circuits containing a single supercapacitor to large supercapacitors suitable for microgrid backup or other uses applications such as group UPS or electric propulsion.
AVX has supercapacitors such as the SCC series, which have a nominal 2.7 V voltage and are available in a variety of sizes up to 165 mm x 60 mm with capacitance values ranging from 1 F to the SCCR12B105SRB up to 3500 F. In an array of multiple ultracapacitors, a device manager IC such as the Texas Instruments BQ33100 can provide control, active balancing to prevent overvoltage, health monitoring, and protection for up to five series-connected ultracapacitors. When used with the BQ24640 charger IC in Figure 3, the IC sends information to the system over the SMBus connection, such as the state of charge of an individual device.
Figure 3: Charging and monitoring of small supercapacitor energy storage.
To build a large capacitor bank for microgrid backup, the number of supercapacitors required depends on the required energy and output voltage. In their paper “Ultracapacitor Energy Storage Applications in Microgrid Systems”, Gorakhnath, Venkateshwarlu and Tukaram used the following equation to calculate the supply of 86 W/h to provide 86 W/h for three 400 V AC phases via DC/AC inverters The number of units required for each power supply is up to 7 kW.
where W ini = energy required for the application; C c = capacity of each supercapacitor cell; U Max = maximum voltage of the supercapacitor; d = voltage-to-discharge ratio
Again, the number of cells required to meet the voltage requirements is given by:
where V Min = minimum voltage of the group
Maxwell Technologies has a range of off-the-shelf supercapacitor banks for a variety of backup applications, with total capacitances up to 500 F and voltages ranging from 16 V to 75 V. Among them, the BMOD0165 P048 has an operating voltage of 48 V DC, a rated capacitance of 165 F, and can store 53 W/h of energy.
In a home or office environment, it makes sense to directly use locally generated low voltage (eg 24 V) power to power loads such as LED lighting. By minimizing DC/DC conversion and avoiding inverters, local microgrids can make better use of renewable energy to save on utility bills and reduce greenhouse gas emissions. Parallel or hybrid AC/DC networks are feasible and may help encourage the integration of low-voltage DC microgrids with existing infrastructure. These can be powered using energy from the main utility power source or on-site micro-generators and backed by batteries or high-power sources such as supercapacitor banks.