Untangling the Grid-Tied Knot

The conventional Solar Energy system includes photovoltaic panels connected to the grid through an inverter. Even with all its flaws including inverter complexity, inverter losses, and an often daunting inter-tie approval process, it has been considered the best solution. Now, a not-so-new technology called “Direct Coupling” is gaining acceptance and popularity with the promise of increased efficiency, simplicity, and a design concept that is, by nature, sustainable.

RE Insider, August 11, 2003 The Inverter: As far as I can tell, inverters must be designed by brilliant mad-scientists. They need to be big, smart, and fast. Because they are such high-powered devices, they need to be built with thick copper to withstand the heat. Thousands of watts course through these beasts’ coils, threatening to burst into arc at the slightest provocation. They need to be smart too, because they are connected to the grid, and need to react with it. Utility grid characteristics need to completely understood and matched by the device so that the electricity produced will be compatible to it. They need to respond in microseconds in order to get UL approval for anti-islanding. An inverter has to shut down and stay down instantly in the event of a power outage so that its deadly current is not fed into the load-side of the line into the unsuspecting hands of working utility linemen. Imagine an amalgam of Mr. Wizard and The Terminator. Inverters are complex, expensive, and are a large part of the losses, the wasted PV, inherent in a conventional system. Losses occur in the conversion of the solar panel’s DC to the grid’s AC. There are additional losses inherent in the amount of power it takes to run an inverter, and in the PV power not used during low-sunlight times before the inverter even turns on. Although most inverters claim efficiency greater than 95 percent, most solar system installers estimate total panel-to-grid losses to be between 15 and 30 percent. Just because it’s renewable energy doesn’t mean we should waste it. Net Metering: Another serious concern with the inverter-based system is net metering. Net metering refers to the fact that utilities can be made to buy any power you make at either the same cost they sell it to you for, or the same price they buy it for. Although it is currently one of the key reasons why people buy distributed generation systems, there are inherent flaws in the concept of net metering. In principle, utilities don’t like inverters and the net metering they imply. Why would a utility want to pay the same price to buy power as they do to sell it? This is definitely not a sustainable business model — and the utilities, although they may seem like “the other guys” are publicly owned. They belong to us. Personally, I prefer my businesses to sell product for a few dollars more than they buy it for. As a customer, if the utility is paying me wholesale rates and charging me retail then, frankly, I’d rather use all the energy myself or, perhaps, sell it to my neighbors. I’ve invested in all these expensive solar panels, why do I want to sell their power at a discount? As businesses and homeowners generate more and more of their own electricity, utility companies still need to build the plants and capacity necessary to service these customers if their systems stop generating power. This requires that they build and maintain generating plants, then shut them off. Although net metering is known by all to be a “good thing” it is not great business and not completely sustainable. Direct Coupling: So what’s the alternative? It’s called Direct Coupling. Imagine, for a moment, a retail or commercial two-story building. It has a 10,000 square foot flat roof and is in a sunny climate. They keep most of their fluorescent lights on all day, seven days a week. About 15 to 20 percent of their electric bill is for lighting. In this case, a $20,000 photovoltaic array on the roof will provide most or all of the power needed to run the lighting. The building would never need to put power into the grid. It would use all the power it produced, when and where it was produced. No net metering needed and no grid-interconnect approvals needed. They’ll run out of roof before they run out of lighting load! Next, let’s attack the losses. If we’re not going to connect to the grid, why do we need to convert the DC from the solar panels into AC at all? Fluorescent lights run just as well, possibly better on DC! In fact, most modern-day equipment runs on DC but, like florescent light ballasts, have a built-in AC to DC converter (not an “inverter” a converter. Changing AC to DC is very easy and efficient). So here’s an example of a Direct Coupled system: – Solar panels on the roof are wired into 54 volt, 1000-watt arrays. – Each array is wired 50 to 100 feet to the system’s power supply. Remember, when you’re wiring DC, especially low voltage DC, wire runs need to be kept short. – Ballasts in the standard T-8 fluorescent fixtures are changed to high-efficiency, 54 VDC, electronic ballasts. – A few small changes are made in the existing lighting wiring so that the fixtures are “clustered” into 1000-watt groups. Next, we connect the grid to the system’s power supply so that when there is not enough sun, the grid AC is converted to DC (remember, this is the easy way), and the DC from the panels is supplemented from the grid. There are some significant advantages to this system: – No DC to AC conversion with 15 – 25 percent losses. – More efficient electronic ballasts. – Easy to individually control each ballast with occupancy sensors. – No utility interconnection. – Every watt produced by the PV goes to the lighting. – No net-metering considerations. And there’s more. The entire system is running is 54 VDC. You’ve heard that number before, haven’t you? It’s the optimal float voltage of sealed lead-acid batteries. That means that if you connect a small battery pack to the system, you have a built-in backup system. The ballasts are electronic. Each one is equipped with what looks like a telephone wire. This means that we can control each ballast individually or in groups with low-voltage switches, occupancy sensors, or a building management simply system by running telephone wire. The Direct Coupling model only works when there is a constant daytime DC load, such as commercial lighting. If the lights are shut off, the energy produced by the panel is unused and, therefore wasted. Direct Coupling is a great solution for a specific type of application. It saves money, is more efficient, and does not depend on local net-metering initiatives. In addition, it does not require grid-interconnection approval. Its ability to easily accommodate backup batteries and low-voltage controls make Direct Coupling a brilliant idea. About the Author: Mark Robinson is Vice President of Sales and Marketing of Nextek Power Systems in Long Island, NY. He is based in New York and California. Formerly, his consulting firm The Energy Grid, provided inverter service and technology support and computer services to solar energy companies nationwide. Robinson was the Director of Customer Service and Information systems at Advanced Energy, manufacturer of the GC-1000 and MultiMode Inverters.