High-end computer servers and workstations often employ a data storage system using a Redundant Array of Independent Disks (RAID) array, which delivers a combination of fault-tolerance and increased performance. Ten K Solar, a photovoltaic company that hails from "sunny" Minnesota, took a page from the RAID designer's handbook and applied the RAID principle to solar designs. With eight years of experience and over six hundred installations around the world, Ten K's Redundant Array of Integrated Solar (RAIS) has a pretty impressive record, and data to back up its claims. What's unique about their technology?

Module Integrated Electronics

RAIS uses Ten K Solar's panels with proprietary Module integrated Electronics. These intelligent panels perform maximum power point tracking (MPPT) individually, assuring that each panel is performing at its peak efficiency at irradiance levels as low as 100 watts per square meter (very cloudy) and as high as 1000 watts per square meter (direct sun).

Unlike traditional panels, in which all cells are wired in series, a Ten K module uses an interconnection method that allows any cell to be bypassed if it's underperforming due to shade or excessive wear. This video explains what it does. As for how … well, that's proprietary information.

Redundant Array of Integrated Solar

Conventional PV systems come in two main configurations, depending on what kind of inverters they use: string inverters or microinverters. An array that uses string inverters will arrange the PV panels in series to produce a very high DC voltage - possibly reaching 600 to 1000 volts - and send that voltage to a central "string" inverter. A microinverter system includes a small inverter on each panel with their outputs wired in parallel, so the voltages never exceed grid-level. Each has its own benefits and tradeoffs.

Redundant Topology

The image below shows the topology of a Ten K system. Notice that the panels are connected to a DC bus whose voltage never exceeds 60V. This makes the system automatically compliant with the National Electric Code (NEC) voltage limits. By contrast, when designing a string inverter array, engineers must ensure that the string voltage is high enough to meet the inverter's startup voltage, while never exceeding the NEC maximum voltage. These calculations must be thoroughly documented in order to pass inspection.   

While traditional microinverter-based systems have an inverter attached to each panel, the RAIS topology uses non-dedicated microinverters. Under low-light conditions, the array output voltage will be relatively low, which triggers just a few microinverters to turn on. As the conditions brighten and the array voltage increases, additional microinverters will engage as needed. This reduces wear on the microinverters, and allows them to operate near their peak efficiency. Since its inverters aren't dedicated to a panel, one inverter can fail without affecting the total production. An intelligent algorithm rotates the order in which the inverters turn on every day, which spreads the wear evenly across all microinverters.

Panel Configuration

Ten K's systems are designed for flat roofs, and they offer two configurations to maximize roof utilization and solar energy production. The Duo PV System arranges the modules in zigzag configuration, with south-facing panels tilted at 25 degrees and the north-facing panels at 16 degrees. The 25 degree tilt optimizes the system for maximum summer production in most temperate climates in the northern hemisphere. But why have panels that face north? Well, in the summer, the sun rises in the northeast and sets in the northwest; during the morning and evening hours, south-facing panels are pretty unproductive. This is especially true as one approaches the equator. The north-facing panels convert this otherwise wasted sunlight into electricity.

It may seem like overkill to buy PV panels that face north since they'll never produce power during peak sun hours, but I think the Duo system is intended for locations with latitudes south of 30 degrees, where the extra cost can be justified.

For those of us in northern temperate climates, the Reflect PV System appears more appropriate. The Reflect PV System replaces the north-facing PV panels with inexpensive reflectors, delivering more sunlight to the south-facing panels. In some conditions, the panels produce more than their rated values because they're getting more light than they would under standard test conditions.

Safety

RAIS appears to be an inherently safe PV system from several perspectives. As mentioned earlier, the DC bus voltage is 60V or less. Inside the module, voltages are limited to 18V, which prevents internal arcing that, under fault conditions, could cause the module to "halt and catch fire."

One of the big dangers of PV systems is that a panel that's receiving light is always considered "live," which exposes firefighters and other rescue personnel to the danger of electrocution. Ten K modules never exceed 16V internally, and its output is switched so that when a panel is disconnected from the array, its output is zero, even when the module is exposed to light. The modules are designed with a fail-safe architecture, so the output is disabled unless the panel's internal control system turns it on. The module data sheets actually list the short circuit current and open circuit voltage as 0A and 0V, respectively.

For systems that include batteries, the DC voltage bus acts as a built-in charge controller. Solar power that's not being used locally is directed to the batteries. When the batteries are fully charged, the extra power goes to the grid. If for any reason the power doesn't go to the grid, the intelligent panels will turn off their outputs, so a "dump load" isn't required. Ten K claims that the systems are "universally compatible," but they only list three battery companies - Samsung, LG Chem, and Electrovaya - all of whom sell Li-ion energy storage systems.

Results

Ten K Solar has installations around the world, and the company has done comparison studies at various US locations (shown on the map below) covering a wide variety of conditions. The company's findings have been presented to the National Renewable Energy Laboratory (NREL) and are published on Ten K's website.

For comparison purposes, Ten K examined normalized data from conventional PV systems in the same area as a Ten K installation. Depending on the location, Ten K systems have outperformed conventional solar arrays by 10% to 38%.

For what it's worth, here's one customer's testimony:

Like almost every company that's doing something unique in renewable energy, Ten K Solar doesn't list its prices, so I can't give you a cost comparison. I'm almost certain that the smart modules are more expensive than conventional panels. On the other hand, the integrated systems will reduce design and installation time, and the built-in safety mechanisms should decrease permitting and inspection costs. Solar panels and associated hardware prices continue to drop, but the "soft costs" still account for a big chunk of the total price tag of a PV system. That's why I expect solar companies to create more integrated systems like these.  

Images and videos courtesy of Ten K Solar

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