Photovoltaic Progress

by Don Loweburg

Home Power 120 / August & September 2007

PV Progress

Over the last few decades, two main forces have been driving the advancement of the photovoltaic (PV) industry. Technical improvements, such as module efficiency and inverter design, have expanded the market beyond off-grid users. Policy improvements, such as streamlining utility interconnection standards, and the growing availability of financial incentives for grid-tied systems, have made using the technology more economically feasible for a wider variety of applications.

Equipment Evolution
In 1987, a typical PV module produced from 35 to 50 watts (W) and cost about $10 per watt, retail. Module efficiency ranged from 10 to 12 percent. And although output rating tolerance typically fell within the range of plus/minus 10 percent, field-measured values generally fell outside (on the low side) of these limits. Some modules of this vintage also tended to yellow or brown after a few years in the sun, a condition caused by deterioration of the encapsulant used to seal out moisture. A standard module warranty at this time was only ten years. Despite this, many of these early model modules are still generating electricity after more than twenty years of service. 
In two decades, the per-rated-watt cost of modules has decreased by half, while the average module size has increased three- to fourfold, falling within the 175 to 200 W range. Module efficiency has also been on the upswing, averaging about 15 percent; one manufacturer (SunPower) reports a module efficiency approaching 17 percent. Typical output tolerance has narrowed to the plus/minus 5 percent range, and at least one manufacturer (Evergreen) holds the lower limit to minus 2 percent. Improved encapsulation means that module discoloration is a problem of the past, and most modules now come with a 25-year warranty. These advances have been the result of constant, incremental improvements in cell and module manufacturing, such as improved antireflective surfaces, more efficient use of silicon, better regulation of the cell manufacturing process, and tighter quality-control measures.
Developments in PV power conversion equipment have also taken place on a parallel track. Twenty years ago, inverters, which convert DC produced by a PV module (or stored in a battery bank) to AC, were in their infancy. Trace Engineering was one of the first companies to offer a 1,500 W inverter and to make higher-efficiency, improved waveform (modified square wave) units intended strictly for off-grid applications. These inverters enabled the operation of many conventional AC appliances, materially improving the ease of off-grid living. These inverters offered efficiencies in the mid-80 percent range and carried two-year warranties.
Off-grid inverter technology today has evolved with improved reliability, waveform quality, and efficiency. Now, off-grid inverters are available that have true sine-wave output (the same as, or often better than, commercial power) and high power capacities. Many modern inverters can hit the 85 to 90 percent average efficiency mark. And stacking options allow multiple inverter configurations for both 120 and 240 VAC applications. Some contemporary battery-based inverters also allow for synchronization with the grid. 
Inverters intended for grid connection (without battery backup) also have improved significantly. Early grid-connected inverters typically operated at 48 VDC nominal and had efficiencies in the 80 percent range. Efficiency of modern grid-tie inverters is about 95 percent, due primarily to the use of high-voltage PV inputs. Most grid-tie inverters for residential applications now have onboard output metering, meeting requirements of many programs that provide performance-based incentives. Other improvements include outputs for computer logging and remote monitoring, which allow users to keep tabs on their systems even if they are off-site. Today, most inverter manufacturers back their products with up to a ten-year warranty.

Policy Drives PV
Though a product’s technical performance, reliability, and efficiency are important, government incentives and the liberalization of utility interconnection policies have played a vital role in fostering the global surge in PV use. 
In 1978, the Public Utility Regulatory Policies Act (PURPA) was enacted to encourage more energy-efficient and environmentally friendly commercial energy production, and independent generators in the United States were finally allowed to interconnect with the grid. PURPA allowed small-scale electricity cogenerators to sell power to the utility to which they were connected, but usually at an “avoided cost” rate. These cogenerators often used biomass or even hydrocarbon fuel to generate electricity and heat for their own needs, displacing power purchased from the utility. Although dispatching or selling power was generally of secondary importance to many of these producers, PURPA did open the door for independent renewable energy (RE) generation, such as the large-scale wind farms that began interconnecting during the 1980s. 
But without targeting a price for renewably generated electricity, PURPA has only played a support role for the installation of grid-tied PV systems, and has not done much to stimulate demand in the United States. In contrast, European electricity feed-in laws that permit the interconnection of renewable electricity sources and specify tariffs—the high price paid per kilowatt-hour (KWH) of electricity generated—have led to the rapid development of RE resources there. 
Here in the United States, individual states have been responsible for establishing net metering policies over the years. Net metering allows customers to receive credit for excess KWH produced when solar output exceeds demand. Minnesota was the first state to establish a net metering policy in 1983, and other states have slowly followed suit. 
California’s net metering law wasn’t enacted until 1996, and originally only applied to residential solar-electric systems. But the state’s combination of net metering and a strong rebate program, launched in 1998, has made it a PV leader in the United States. The California rebate program has recently been extended to 2017 and modified to provide production incentives (dollars per KWH) for systems 20 KW or greater, while maintaining an up-front rebate for smaller systems. 
Although net metering laws and incentives have resulted in an upsurge in the number of PV systems installed in the United States, PV system installation in Europe and Japan (where national laws opened the grid to RE generation) has left the United States in third position. In 2006, PV installations in Japan totaled 883 megawatts (MW). Europe added 611 MW and the United States trailed, distantly, at 137 MW.

Big Mama Calls the Shots
A third driver to this story—beyond technological advances and policy changes—is climate change, coupled with the increasing cost of all carbon-based fuels and the energy they provide. This is becoming a fundamental stimulus for RE technologies, as the far-reaching impact of using fossil fuel comes to light, and these nonrenewable resources become more difficult to obtain and increasingly expensive. Clearly, PV will continue to be an important part of the future as we continue to seek innovative, nonpolluting ways to meet our energy demands. 

In two decades, the per-rated-watt cost of modules has decreased 
by half, while the average module size has increased three- to fourfold.

home power 120 / august & september 2007