Photovoltaics: Distributed Generation or Energy Efficiency?
In the past, Photovoltaic Solar Panels have been a key part of a distributed generation program. It seems obvious, doesn't it? New technologies, though, are enabling solar electric panels to become a part of energy efficiency programs. This distinction can have a significant impact on the funding of, and the future of solar installations. The typical solar electric system consists of solar panels which create Direct Current (DC) electricity and an inverter which changes the DC to Alternating Current (AC) to be compatible with the grid. When the solar panels generate more electricity than the building is using, unused electricity is sent back to the grid. Utilities usually pay for this electricity through 'net-metering' programs. In effect, this system uses the grid as a place to 'store' unused electricity.
It turns out that this 'storage' of unused electricity is quite expensive. The cost of the inverter and its maintenance is a factor, as is the efficiency losses of the inverter. In addition, the net metering programs are expensive themselves and may not be sustainable for utilities in the future.
Inverting DC electricity to grid-compatible AC electricity is complex and expensive. To be compatible with the grid, the AC produced must meet strict requirements and the inverter itself must be capable of shutting down instantly in the event of a power failure. This regulation, called 'anti-islanding' protects linemen who might be working on a downed power line but also shuts off the whole solar electric system when you need it the most; during a power failure. Typical inverters consume up to 15% of the solar power generated and carry warranties of only five years, a quarter of the estimated life of the solar system.
Net metering is the great advantage of an inverter because it allows a building owner to sell back unused power. Systems can be designed so that, over the course of a year, the electric bill 'nets out' to zero. But is net-metering sustainable? Is it fair to the utilities to mandate net-metering? In effect, we're telling the utilities that they have to buy their own product from their customers at retail. Could a grocery store survive if it had to buy vegetables from local gardeners at the retail price?
Many utilities have gone to a more reasonable 'avoided cost' structure. This means that, if you generate electricity and send it back to the grid, the utility will credit you whatever it costs them to generate electricity, or wholesale cost. It's as if the grocer were paying you for your vegetables whatever they pay the farms. True, this sounds fair, but frankly, as a gardener, it would make more sense for me to eat my own broccoli then sell it to the grocer at half of what I'll need to buy it back for later.
The first point here is that storage is expensive. The most effective way to deal with power you generate is to avoid storage altogether and use it all, where and when it is generated. This means that an optimal solar electricity system will never generate more power than will be used. The challenge with this is that building electricity usage changes throughout the day, as does the availability of sun (except in California where it's always sunny).
The solution, at least for most commercial office and retail buildings, is lighting. In most offices and almost all large retail establishments, the fluorescent lighting is 'on' all day, every day, and often uses as much as 60% of the total building's electricity. The optimal solar system, then, provides just enough electricity to power the fluorescent lights.
The second point here involves the fluorescent lights themselves, and a fact that few realize. Each fluorescent light ballast contains a small, rather inefficient, AC to DC converter. This means that the fluorescent light itself is a DC device and can be powered directly from the solar cells without an inverter. If we can do away with the inverter (which is unnecessary anyway because we're not trying to put AC power back into the grid), we can avoid inverter losses, maintenance costs, and complexity. And because we don't have to shut the system down to comply with anti-islanding laws, we can keep the lights on during a power failure!
The concept is called 'Direct Coupling' of DC generation to the load. Here's how a system works. It uses power where, when, and how (DC) it is generated: DC power from the solar panels is sent through a 'power router' directly to DC fluorescent ballasts in the lighting. When there isn't enough solar power being generated, the power router takes electricity from the AC grid, converts it to DC, and adds it to whatever is being produced by the solar panels. The power router takes all the electricity from the solar panels and whatever else is needed from the grid to keep the lights operating during the daytime, on cloudy days, and at night. If the grid fails, then power from the solar panels and, optionally, batteries, is used to keep the lights on.
A system designed like this is less expensive initially because the solar array tends to be a little smaller. It's ideal for retail use because it keeps the lights on (and customers in the store) during a power failure. Utilities tend to support the idea because it doesn't involve complex and expensive bi-directional interconnection to the grid; to them it's an energy efficiency measure, not energy generation. It's more efficient during the day because all of the solar energy gets used and it's at least as efficient at night because the centralized AC to DC conversion in the power router is better than a similar conversion at each fluorescent ballast.
It may be that the best way to design a photovoltaic system in a commercial building is to direct couple the lighting load. This system will have lower up-front costs, be more efficient, keep customers in the store during a power failure, and save the occupant the largest portion of his electrical expenses.
A graphic demonstration of this technology can be found at www.DirectCoupling.com and at Nextek Power System's website at Direct Coupling Demo.
Mark Robinson is VP Sales & Marketing of Nextek Power Systems. Formerly, he was involved in the design and service of inverters for solar systems. He is a licensed master electrician and a LEED accredited professional.