The conventional Solar Energy system includes photovoltaic panels connected to the grid through an inverter. Despite all its flaws, it has been considered the best solution, including inverter complexity, losses, and an often daunting inter-tie approval process. 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.
– Mark C. Robinson
RE Insider, August 11, 2003
Brilliant mad scientists must design inverters, according to my understanding. Inverters must be big, bright, and fast because they are high-powered devices that must withstand heat by being built with thick copper. These devices have coils through which thousands of watts of power flow, and they are in danger of bursting into an arc at any time. Moreover, they must be intelligent since they are connected to the grid and must react accordingly.
To make the electricity produced compatible with the utility grid, the device must wholly understand and match the grid’s characteristics. The inverter must react in microseconds to obtain UL approval for anti-islanding. In a power outage, it must shut down instantly to prevent the deadly current from being fed into the line’s load side, where utility linemen could be working unsuspectingly. An amalgam of Mr. Wizard and The Terminator comes to mind.
Inverters are complex and costly, and a conventional system has inherent PV losses. Failures arise when converting the solar panel’s DC to the grid’s AC. Additional losses occur when running the inverter, and the PV power is not used before the inverter switches on during low-sunlight periods. Although most inverters claim efficiency greater than 95 percent, most solar system installers estimate total panel-to-grid losses between 15 and 30 percent. Just because it’s renewable energy doesn’t mean we should waste it.
Net metering, which refers to utilities buying any power a customer generates at the same price they sell, is one of the key reasons people buy distributed generation systems. However, the concept of net metering has inherent flaws that are a severe concern with inverter-based systems.
Inverters and the net metering they imply are not liked by utilities in principle. Paying the same price to buy power as they do to sell it is not a sustainable business model. Utilities are publicly owned and belong to the people. Businesses should sell products for a few dollars more than they buy them.
As a customer, receiving wholesale rates from the utility is not beneficial while being charged retail. Instead, customers may use all the energy themselves or sell it to their neighbors. Customers have invested in expensive solar panels and would not want to sell the power they generate at a discount.
Moreover, as businesses and homeowners generate more electricity, utility companies still need to build plants and maintain generating capacity to service these customers when their systems stop generating power. This requires a significant investment, and offering net metering as a business model is not ultimately sustainable.
“What’s the alternative? It’s called Direct Coupling,” said the speaker.
Imagine a retail or commercial two-story building with a 10,000-square-foot flat roof in a sunny climate. The structure keeps most of its fluorescent lights on all day, seven days a week, and about 15 to 20 percent of its electric bill is for lighting. In this case, a $20,000 photovoltaic array on the roof will provide most or all the power needed to run the lighting, eliminating the need to put energy into the grid. The building will use all the control it produces, when and where it is made, without requiring net metering or grid-interconnect approvals. The building will run out of roof space before it runs out of lighting load.
Next, let’s attack the losses. Why convert the DC from the solar panels into AC if the building is not connected to the grid? Fluorescent lights run just as well, possibly better, on DC. Most modern-day equipment runs on DC and, like fluorescent light ballasts, has a built-in AC to DC converter (not an “inverter,” a converter). Changing AC to DC is 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. Wire runs must be kept short when wiring DC, especially low-voltage DC.
– 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 runs on 54 VDC, which you may have heard before as the optimal float voltage of sealed lead-acid batteries. This means that if you connect a small battery pack to the system, you have a built-in backup system,” stated the speaker.
The ballasts are electronic and come equipped with what looks like a telephone wire. This feature allows us to control each ballast individually or in groups with low-voltage switches, occupancy sensors, or a building management system by running a telephone wire.
Direct Coupling is ideal for applications with a constant daytime DC load, such as commercial lighting. However, if the lights are shut off, the energy produced by the panel goes unused and is therefore wasted.
Direct Coupling saves money, is more efficient, and does not depend on local net-metering initiatives. Furthermore, it does not require grid-interconnection approval. Its ability to easily accommodate backup batteries and low-voltage controls makes Direct Coupling a brilliant idea for a specific type of application.