commentThe Lowdown on Mobile Photovoltaic Power Generators
Mobile photovoltaic (PV) generators consist of PV panels, charge controllers, lead-acid batteries, and inverters mounted on easy-to-tow trailers. They offer an environmentally responsible, silent alternative to the diesel or gasoline generators typically used for construction jobsites, lighting, or other off-grid power requirements.
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Mobile PV generators use components similar to those found on stand-alone (non-grid-connected) PV systems, but are configured for the harsh demands placed on portable generators. The PV panels are designed to handle rough conditions on a jobsite and are mounted on a trailer to minimize travel damage. Mobile Solar Power, Mobile Green Power, and Energy Acumen make enclosed trailers that protect electronic components from the elements, while Pure Power Distribution and SolaRover opt for an open-trailer design.
Since solar access and intensity vary and construction equipment often draws a lot of power, mobile PV generators require batteries that are capable of being discharged repeatedly. Most common today are deep-cycle, lead-acid batteries, though if costs come down on other options, such as lithium-ion and nickel-zinc, we may see those as well. A charge controller monitors the batteries and adjusts the current to maximize charge and prolong the life of these batteries.
“When the battery is at a low state of charge, the charge controllers we use will increase their charge voltage; this allows the battery to be charged more rapidly and efficiently,” according to Travis Semmes, founder and president of Mobile Solar Power. His company uses either OutBack or Blue Sky Energy charge controllers to maintain its 24-volt lead-acid battery banks, which are capable of storing between 9.3 and 45 kWh (390 and 1880 amp-hours) of power at a 20-hour discharge rate, depending on the configuration.
The direct current (DC) generated by the PV panels and stored in the batteries has to be converted to 120V or 240V alternating current (AC) by an inverter before use. These inverters are rated for continuous power and surge power and, along with the PV array and batteries, should be sized appropriately for the job. “Construction sites tend to run large loads for short periods of time,” said Semmes, “so they may need larger inverter outputs but smaller battery sizes.” Motors draw significant surge power at start-up but require much less continuous power while running. To size mobile PV generators appropriately, manufacturers can help perform the complicated load calculations necessary based on equipment power demands, power output from the solar array, amount of battery backup, and amount of inverter power required.
It is important to purchase inverters that create pure sine wave power—which is as good as, or better than, that obtained from utilities—rather than modified sine wave power that could interfere with sensitive electronics. Pure sine wave inverters from OutBack and Xantex are found on most manufacturers’ mobile PV units, including those from Mobile Solar Power, Pure Power Distribution, and Energy Acumen. These inverters can also recharge the battery banks rapidly using power from another source, such as the grid or a diesel generator run for a short time, during high-demand use or prolonged periods without sun. Most mobile PV generator companies offer hybrid models that incorporate propane or diesel backup generators, and biodiesel options are available through Mobile Green Power, Energy Acumen, and Pure Power Distribution.
If sized appropriately, a mobile PV generator can serve as a cost-effective replacement for a fossil-fuel-fired model. Most mobile PV generators range from $10,000 to $100,000, though much larger custom models are available from SolaRover and Pure Power Distribution. Leasing options are available, but with few maintenance costs, no fuel expenses, and a possible 30 percent federal tax credit, these generators could be considered a bargain in the long run.
Tom Weinberg, superintendent for Frank Schipper Construction, has been using Mobile Solar Power’s MS 225 as the primary power source on the 18-room Serenity House hospice project in Santa Barbara, California. Because the site is in a residential neighborhood, the PV generator’s silence and lack of fumes are key assets. The PV generator is running job trailers, computers, lighting, heaters, skill saws, grinders, compressors, and hand tools for about 25 workers, mainly for site work. Once construction of the main building begins, “it would not be able to keep up with the full capacity of the job,” he confided. “But on a small job like this, the generator will carry us through until power is installed.” Weinberg calculates that the unit will pay for itself in about two years. “We are totally impressed with it. The only problem is people take more time looking at it than they do my building.”
Copyright 2010 by BuildingGreen, LLC
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