By: Tom Williams, Editor-in-Chief
Counter to many predictions, the outlook for solar energy growth in the U.S. is fairly bright. Much of the optimism comes from the progress toward getting costs in line with other sources of power. And a good deal of it comes from the use of embedded systems to make installation, configuration and control of such systems straightforward.
The growth of the solar photovoltaic renewable energy industry in the United States is looking to have positive news for the spread of embedded systems. This may seem counterintuitive given the somber news of budget woes and the cutback of government support for development of renewable energies. However, a recent Intersolar conference in San Francisco had a definitely upbeat atmosphere with a number of companies from Europe (primarily from Germany) announcing plans to set up shop in the U.S., and numerous U.S. companies moving aggressively into the residence and small business markets. This development bodes well for the proliferation of inverters, micro inverters, maximizers, gateways and monitoring systems and software—all of which are designed and built around microprocessors and embedded computer intelligence.
There has been a simmering debate in the solar energy industry about whether it is better to concentrate on building vast arrays of solar panels in places like the Mojave desert or rather to concentrate on widely distributed systems of panels on individual rooftops that can connect to the grid and take off power and put it back on the grid as needed. While a number of large plants have been built and other ambitious projects are underway, there has been a surge in the popularity of panels being installed on the roofs of private homes, small businesses and civic buildings such as San Francisco’s Moscone convention center.
In fact, the city of San Francisco has just announced a program called Solar@Work that will offer solar energy systems to businesses in the Bay Area via a group purchase model. The program aims to make it possible for small and mid-sized businesses to pay less for solar energy than they pay for electricity from the grid without local rebates. Estimates are that interested participants will be able to purchase more than two megawatts of solar power over about six months. The city’s stated goal is to be able to meet its electricity needs with 100% renewable energy.
This is partially made feasible by the fact that, according to European Sales VP Bernd Kohlstruck of Enecsys, solar energy is beginning to approach parity with electricity from other sources. In addition, Joe Cunningham, director of operations for CentroSolar, was asked the question of how the industry can be looking so optimistic in the face of apparent disappointing government support. He replied that people are now generally becoming aware that solar power may be more economic. Also, a number of companies are coming up with creative financing/leasing schemes that let people set up solar installations on their homes for no money up front and with terms that let them pay for equipment at rates that come out significantly lower than their monthly energy bill, and at the end let them own the equipment free and clear.
The most basic solar installation is made up of the solar panels, which collect the solar energy and convert it into DC current, and an inverter, which converts the DC to AC and connects to the grid. There must be intelligence to interface to the grid so that the meter can be run forward and in reverse. But for the new systems now being offered to residential users and small to medium businesses, there are a lot more refinements coming into play that involve safety, monitoring, maintenance, ease of installation and use, and efficiency.
For example, the simplest way to set up a solar array has been to connect a string of panels in series, which deliver the cumulative DC current at a constant voltage to the inverter, which then converts it to AC and connects to the grid. The problem with this is that one weakened panel in the string or one blocked by shade or some obstruction like a stray Frisbee can limit output of the entire string. One approach to this has been to use DC-to-Dc converters or “power optimizers” connected to each module to reduce losses due to differences between the modules’ outputs. The output of each module is combined with that of the other modules and the final output sent to the inverter. This keeps one bad module from excessively bringing down the output of the whole string.
Another approach to this is via a patented method from Tigo Energy, which uses units called “maximizers” along with an intelligent Energy Management Maximizer Unit (MMU) to sense the input parameters of each module and with its central processor to calculate the I-V properties of each module. It can then use circuitry consisting of an FET and a capacitor in each maximizer to adjust a nonresistive impedance to match the internal impedance of the preceding module such that a virtual “current tunnel” is created to let each module contribute its maximum output. This method also reduces the heat generated by an underperforming module, which could damage the system (Figure 1). The intelligence of each maximizer unit is also used to read the performance of each panel in the array on the Tigo monitoring system software.
Another method of minimizing the effects of a shaded or malfunctioning panel is the growing use of microinverters. Microinverters attach to each solar panel producing an output of 120V or 240V AC at 50 Hz and are then are connected in parallel to the grid. Thus a low output from one panel simply contributes that low output to the total but does not impair the output of other panels in the array (Figure 2).
One supplier of microinverters, Enecsys, has also built in safety features as well as the ability to monitor the installation. The Enecsys microinverters have two shutdown modes—one thermal in case of overheating and the other that shuts down if the grid is disconnected. This latter mode is important for things like protecting firefighters. If the microinverters were to continue to operate, their AC output would be present on all the lines between the inverters and the disconnect switch with the attendant hazards to firefighters. With shutdown, the only voltage present is the DC voltage between the panels and the inverters.
The Enecsys modules also communicate via a ZigBee wireless connection with a gateway that can connect in the home to the Internet and an in-home monitor (Figure 3). This makes data available to the user in terms of things like carbon footprint, rate savings, etc., and also can be accessed by service personnel for diagnostics and trouble shooting.
While photovoltaic systems probably first came into widespread use by people who wanted to live entirely off the grid, the expected large commercial use will definitely be in grid-tied systems. Still, there are some who will want grid-interactive systems that also supply battery backup. Here again, intelligent embedded systems will be able to support battery backup as well. In systems such as those supplied by Outback Power, the grid-interactive system supplies power while taking extra needed off the grid and selling excess back onto the grid. At the same time, however, it is also maintaining a charge on a bank of batteries that can be brought online and used to produce AC when the grid goes down (Figure 4). Such a system can also incorporate a generator that can be automatically started under preprogrammed conditions.
Outback Power supplies an intelligent system display and controller, the MATE3, that can connect both to the Internet and to a local PC. The MATE3 supports up to one year of data logging and supports system configuration through a configuration wizard. For instance, the system can be set to work with time-of-day power rates and to program inverter and charger operations such as limiting the generator to running at specific times of day. The Internet connection also permits remote monitoring and allows online support personnel to run diagnostics. With the Outback system it is also possible to separate selected circuits for backup and save power by not running less critical circuits.
Of course, such things as monitor gateways depend on data flowing throughout the entire photovoltaic systems from panels to the Internet. Thus all the components require some level of intelligence and connectivity, often wireless, such as ZigBee. The system monitoring, control and diagnostic systems are becoming ever more sophisticated. One of the virtues of embedded control is that it tends to hide very complex matters behind a relatively simple and straightforward user interface. And it is this ability that makes such systems accessible to a broader range of users.
In fact, we are beginning to see some companies, such as miniJoule, a subsidiary of the German company GP Joule, that are about to enter the U.S. market with what can be called a do-it-yourself solar power kit. The kit comes in a box with solar panel, micro inverter, roof mounting bracket and cables. The user simply unpacks the parts, mounts them on the roof and connects the cables—up to the part where they connect to the grid. At that point, a licensed electrician and a permit and inspection from the utility are required. A 185-watt kit will sell for about $1,000 and they can of course be set up in multiples of up to 16 panels per string.
Such kits combined with financial packages that lower effective monthly rates, municipal initiatives like Solar@Work, and other incentives will soon replace things like tax rebates. But above all, the role of embedded systems will continue to drive this market. There are opportunities aplenty for semiconductor companies, software developers and system integrators to expand and exploit this trend.
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