Since Edison's day Alternating Current (AC) and Direct Current (DC) have co-existed by necessity: AC to make the trip from the generating plant and DC to power electronic loads. This has resulted in billions of electrical compromises in the form of the ubiquitous power supply, or a rectifier that must stand in front of DC loads to convert AC to DC. As Arthur Rosenfeld, California Energy Commissioner calls them, our Energy Vampires.
But now, many buildings are generating power of their own, usually Direct Current energy. Is this wasteful back-and-forth conversion really necessary? Just as the automobile industry has advanced to the hybrid car, buildings can use multiple sources of power to achieve dramatic increases in efficiency.
Edison and Westinghouse fought the AC/DC battles around the turn of the century. AC won because Tesla's transformer allowed AC voltage to be boosted for easy transmission from Niagara Falls into the city. Edison's DC network required unpopular 'backyard' DC generation stations every few miles. But where AC won in the transmission, it's DC that is now used inside almost all of our devices. You see, only DC can be precisely regulated to get the exact voltages we need for sensitive electronics. So our current building electrical systems are fed with AC that is converted to DC at every fluorescent ballast, computer system power supply, phone system, and other electronic device.
This model works fine until we bring back Edison's original idea and buildings begin generating power of their own usually DC power such as solar, fuel cell, and wind. The inefficiencies involved with inverting to AC, matching grid frequencies, and protecting linemen from hazards are all avoidable by creating a Hybrid Electrical System.
Inefficiencies in an inverted solar system:
The inverter model of the traditional solar system has several flaws:
- Inverter Efficiency. Rated inverter efficiencies rated between 90% and 95%, Actual field efficiencies are even less. Many inverters consume power at night. Several models do not turn on in low light conditions.
- Anti-Islanding For the protection of utility line workers, inverters are required to shut down in the event of grid failure. This means that, for most solar systems, there is no energy production during a power failure (when we need it the most).
- Net Metering. Power sent back into the grid is not always repurchased at full cost. Sending excess power back into a sometimes overburdened grid may not be the best way to manage the resource. Net-Metering, as a business practice for utilities, is not sustainable and is likely further erode the value of power sent back to the grid. Net-Metering agreements and the meters that they require can be expensive.
- Reconversion losses. Now that we've suffered the losses of inverting, additional losses are incurred converting back to DC in the electronic devices like fluorescent ballasts, computers, and more.
The Hybrid Solution The theory is simple: In a building that produces DC power of its own, use the DC power for the DC devices and use the AC power of the grid for everything else. If more DC power is needed then is available, take some grid power, convert it to DC, and use it to supplement the local source.
- Efficiency gains come from the fact the locally generated DC power is never converted and AC power from the grid is only converted when necessary.
- The system is more reliable because there are redundant sources of power. It is not necessary to shut down a DC system during a grid failure like it is with an AC system.
- The system is simpler because no net-metering or utility interconnection agreements are necessary. The utility cannot even 'see' the system and, in most cases, does not even need to be notified.
Drawbacks of the Hybrid Solution The only drawback of the hybrid system is that there is no efficient provision to store excess electricity. It cannot be sent back into the grid and re-purchased later and storing excess power in batteries can be too expensive to justify.
The solution is to identify base DC loads that will always be on when the system is generating. If solar panels are the local DC source, then the local DC loads need to be on all day every day. An ideal example of this is commercial fluorescent lighting.
Example of a Hybrid Lighting System In this example (which can be seenlive at http://www.DirectCoupling.com) solar panels are connected to DC ballasts in the lighting.
Daytime: The solar power from the panels is sent directly to the lighting, with no conversion, at nearly 100% efficiency. Wiring losses are the only significant losses. The system is designed so that, at full sun, about 90% of the power needed for the lighting is supplied by the panels. The additional 10% is taken from the grid, converted to DC at the NPS1000 power gateway.
Clouds: When clouds reduce the PV production, more power is taken is taken from the grid and converted to DC. The system is using all available power from the panels (the least expensive source) and using the grid as the backup.
Night: When there is no solar power available, all the power is taken from the grid and converted to DC. As we discussed previously, a typical AC lighting system takes AC power from the grid and it is converted to DC at every ballast. In this DC system the conversion is handled centrally. The number of conversions is not increased.
Power Failure: In the event of a grid outage, the lighting system continues to be powered by the solar panels and, if needed by optional batteries. In a traditional inverter based solar system the inverter is required to shut down, shutting off the lights.
Other suitable DC loads in commercial buildings include telephone systems, motor controllers, computer server systems, and more. Current installations include grocery stores, offices, big-box retailers, and, most recently, a Frito Lay Distribution center in Rochester, New York.
Whole Foods, Berkeley
This 30k system powers the lighting and was installed with Powerlight Photovoltaics. One of the primary benefits of this system, besides increased efficiency, is the reliability aspect. Power failures are extremely expensive for grocery stores, not because of the freezers, but because 200 people with shopping carts full of frozen food abandon the carts and leave the store. This creates an expensive emergency for the store as the staff need to scramble around, reshelving the food by opening coolers which should really stay closed. This, as well as the lost sales cost the average small grocery store over $8,000 per five minute failure. Shortly after the installation, Whole foods experienced a brief power failure. The lights were powered by the solar panels and did not shut off. Customers remained in the store.
Frito Lay, Rochester
One of the other benefits of low voltage DC ballasts is the ease at which they can be controlled. Each ballast has a phone wire-type connector which can be used to provide DC power to, and a light switch for an occupancy sensor. This reduces the installation cost of an occupancy sensor for $200.00 each to $75.00 each.
Target Stores, El Cajon, CA
This system uses the Nextek system for part of the store, and an inverter for the rest. This allows us to monitor each of the systems and compare the efficiency of both. Initial readings illustrate that the Nextek System is providing over 20% more power than the inverter based system.
The most efficient way to utilize locally generated power is to consume it all, where, when, and how it is generated. We can accomplish this by identifying DC devices in a building and powering them with the locally generated energy and use the grid as a backup.
Mr. Mark Robinson, LEED VP Sales and Marketing Nextek Power Systems