Most people with an interest in technology know about the “war of the currents” between Thomas Edison and Nikola Tesla. Edison was a proponent of DC electricity with small generating stations on every city block, while Tesla knew that AC could be generated on a large scale and transmitted over long distances more efficiently. AC became the standard for the power grid, which was fine when most electrical devices at the time - incandescent light bulbs and motors - were capable of running on AC.
Eventually the electronic age brought us a slew of DC-powered devices like radios, televisions, computers, and cell phones. These devices (or their chargers) plug into an AC outlet; the first thing that the AC electricity encounters is a power supply which converts the 120VAC into whatever DC voltage is required - usually in the 5V to 20V range. Unfortunately power supplies are only about 75% efficient, so valuable electricity is converted into heat - not a good thing. It’s estimated that 80% of our electrical devices run on DC, despite being plugged into AC outlets. That’s a lot of wasted energy, not to mention the extra electronics needed to do the conversion.
Maybe someday we’ll switch to a High Voltage DC (HVDC) grid, but that’s a long ways off. The California Energy Commission (CEC) thinks it’s time to reconcile Tesla and Edison’s differences and use the best ideas from both of these great inventors. The CEC is contributing to an experimental AC/DC microgrid using technology from Bosch, a maker of energy management systems, and Imergy, a producer of vanadium flow batteries. Technically it’s being billed as the “Bosch DC Microgrid” since the site is generating DC only. I’m calling it an AC/DC microgrid because it uses both AC and DC ... and it gives me an excuse to show off my geek shirt:
The author, getting his geek on
Microgrids
A microgrid is a self-contained grid that powers a building, neighborhood, or campus. It may or may not be connected to the national grid. Its purpose is twofold: to provide backup power in the event of a grid power loss and to allow an organization to generate some or all of its own power to use locally. In many cases the microgrid will use a photovoltaic (PV) array to generate power and batteries to store excess energy for later use.
Photovoltaic panels turn light into DC electricity, which is usually converted to AC and then put on the grid or used directly in the building. Think about that: we put solar panels on the rooftop, convert the DC to AC so it’s compatible with our wall outlets, plug in a computer, and its power supply converts the AC back to DC. How much energy is wasted due to these conversions? A touch too much, to say the least.
The AC/DC Microgrid
The experimental microgrid will run DC loads directly from DC and still use the traditional AC grid for devices that plug into a wall outlet. The project is intended to serve as a research and demonstration site for AC/DC microgrids. According to Bosch spokesperson Linda Beckmeyer, “The Bosch DC microgrid project will provide a low-cost, highly energy-efficient solution in which the DC microgrid connects rooftop solar PV arrays to energy-efficient DC lighting, DC ventilation and DC energy storage systems on a 380 Volt DC bus to form a DC building grid. The approach allows commercial buildings to become zero-net-energy users in a cost-effective manner.”
The microgrid will be retrofitted into an existing parts distribution center in California. The project is still in the design stage, so many of the numbers and technologies are tentative. I was able to interview engineers from Bosch and Imergy to learn about some of the technology that’s planned. Here’s what I found.
Solar Panels Feed the DC Bus
According to John Saussele, Project Director for the Bosch DC Microgrid Group, the site will generate power with a 100 kW PV array. Rather than sending that DC power through an inverter to run AC devices, the array will feed a 380 Volt DC bus. No new wiring is needed; they’re just repurposing some of the existing AC wiring to run DC. Saussele estimates that bypassing the inverters will allow the microgrid to run up to 10% more efficiently than an equivalent AC microgrid.
Flow Batteries for Storage
On a sunny day the PV array will generate more power than the site uses for its DC loads. Excess energy will be stored in three Imergy flow batteries, which will provide the facility with four hours of storage at full load or twelve hours of storage in emergency mode. If that’s not enough, the site can switch to the AC grid for backup power.
All of the energy routing is controlled by a proprietary Bosch energy management system that balances production, load, and storage for optimal performance using Bosch’s patented mixed microgrid hybrid demand-side management approach.
DC Will Power the HVAC and Lighting
HVAC systems and LED lighting are generally wired directly into a building, and both can run on DC, so these are prime targets for the DC part of the operation. While HVAC systems often use AC motors, it’s more efficient to control them with variable frequency drives (VFDs), which allow fans and pumps to spin at different speeds depending on the need.
Variable Frequency Drives
A typical AC motor spins at a constant speed that’s determined by its input frequency. The motor is either fully on or fully off, which leads to an inefficient use of energy.
A VFD can save up to 40% in energy costs by running a pump or blower at different speeds rather than running on-off cycles at full speed. The VFD is driven by an inverter that’s capable of varying its output frequency. Bosch made the VFD even more efficient by eliminating the rectifier and modifying the inverter to run directly off of the 380 Volt DC bus:
Let There Be Light Emitting Diodes
Interior lighting is provided by energy-efficient LEDs. Unlike many off-the-shelf LED fixtures whose drivers convert AC into DC, these lights are designed to run natively on DC. Saussele told me that a high-end AC driver is 94% efficient at best, where the Bosch DC drivers are 98% efficient. In addition, the AC driver is a known problem child in LED fixtures, often being listed as the cause of premature failures. LEDs that run directly on DC are more efficient and more reliable.
The lighting is controlled by an intelligent system that includes occupancy sensors capable of automatically turning off lights in areas that are unoccupied. The sensors and light controllers will communicate with each other and with the central controller through a network that’s still being designed. Engineers are prototyping various communication methods to determine which works best in this system; they could use wireless or they may go “live wire” and multiplex the communication signal over the DC power bus.
AC/DC Microgrid Advantages
The proposed AC/DC microgrid offers several advantages over a more traditional microgrid approach. The PV system, HVAC, and lighting are all more efficient by generating DC and using it directly. The reduction in electronics (inverters, rectifiers, and LED drivers) increases the overall system reliability. Finally, both Bosch and Imergy claim a lower total cost of ownership over the system’s lifetime.
At this point the project will be a learning opportunity and a proving ground. There are no technological barriers to accomplishing the objective; the only question is whether it’s cost effective. On the surface, I can’t see a reason why it wouldn’t be.
The Future
Tim Hennessy, president of Imergy, predicts more systems that blend AC and DC, as well as more devices that can run on DC natively. Boats, RVs, and many off-grid homes use DC appliances, and some data centers are experimenting with DC servers, eliminating the inefficient AC-to-DC power supplies.
Hey, if Tesla and Edison can peacefully coexist on my shirt, I think AC and DC can get along in the field too.
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