Solar goes to the grid

At PV Powered in Bend, Ore., mobile solar carts test modules and MPPT technologies as part of the SEGIS program.

With the push to expand the amount of energy produced from renewable sources and the maturing of the solar energy industry, utilities are now beginning to add power from multi-megawatt photovoltaic projects into the utility grid. To make these installations valuable and cost-competitive with other energy sources at a utility scale, solar technology must continue to get less expensive and perform better.

The U.S. Government has noted the challenges and, in 2008, the Dept. of Energy commissioned the $24 million Solar Energy Grid Integration System (SEGIS) project as part of the Solar America Initiative (SAI). Its task is to encourage the development of new technology and products that help put solar photovoltaic (PV) systems into American utility grids.

Now five SEGIS teams across the country are working on various aspects of the project. PV Powered, Inc., maker of solar power inverters in Bend, Ore., leads one of them. Its work focuses on improving system economics through optimized energy harvest, reliability, and efficiency, developing technologies for utility-scale monitoring and control system integration and researching ways of reducing the impact of weather changes on solar power output. The hope is that SEGIS will bring advances that will lead to less downtime and more predictable operation, improving the overall economic equation for solar power plants.

There are many factors involved in the efficiency of energy harvest, but one of the most important is MPPT, or Maximum Power Point Tracking. This refers to how the PV electronics adjust the voltage/current ratio delivered by a solar array to maximize power as the output from the array changes. The array output might change substantially as clouds go by, the cells get dirty, and so forth.

Tests of this system will take place on a wide variety of configurations. These will range from high fill-factor technologies like concentrated solar or mono-crystalline silicon to low fill-factor thin film technologies, including copper indium gallium arsenide and cadmium telluride.

A 100-kW array set up by the Oregon Dept. of Transportation as the country’s first solar highway project will test developments coming from the SEGIS team. The 100-kW inverter is visible at right.

Large-scale solar power plants are typically modular arrangements. They may include several arrays, inverters, and other components configured somewhat in parallel. The idea is to prevent the whole system from shutting down because of a single-point failure or through a propagating fault. Instead, when a single subsystem fails, there is only a fractional loss in power output.

In systems of this nature, it is tough to make accurate reliability predictions. Reliability data can, however, be collected on the different subsystems (power inverters, modules, and tracking devices) and combined for a system-level analysis. Designers can use this information to project preventive maintenance schedules, budgets for spare parts, estimates of downtime, and the PV plant's overall financial viability. These projections are generally quite accurate. The resulting subsystem downtime estimates can be used to calculate system-level availability, power loss, and energy harvest.

Designers need to understand the installation environment to estimate the reliability of a PV system. PV systems are unlike hydroelectric, nuclear, and coal- or gas-fired power equipment because they aren't typically located in controlled surroundings such as a building. PV systems outdoors see environmental stresses such as humidity, dust, and so forth, as well as temperature extremes.

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