Solar Energy: Beyond Efficiency

Click to EnlargeBy Kirk Hasserjian, Applied Materials

While few would argue the need for renewable energy to reduce our reliance on fossil fuels, the biggest obstacle for widespread deployment remains this: How do we optimize renewable energy production to ensure that it is cost-competitive to grid power?

Even as countless technological advancements in recent years have made utility-scale solar photovoltaics (PV) power generation more viable, one unlikely barrier to further rapid deployment exists outside the scope of engineers and researchers. It’s an accounting problem.

Determining costs is the key to establishing when peak parity and eventually grid parity will be reached. Traditionally, solar panel efficiency has been the primary means of calculating the total annual cost of the energy produced. It’s easy to understand why: efficiency is a key and well-known cost metric because a field installation assembled from less efficient modules will require more modules and more area, thus incurring larger installation costs.

But for utility-scale installations, calculating solar panel efficiency alone results in an incomplete cost analysis. In its simplest terms, electricity cost is total costs divided by the amount of the electricity produced. Thus, to best determine the relative performance and total cost of solar installations, one must assess each of these four independent factors: panel architecture, panel size, energy yield and energy efficiency. All are important contributors to electricity’s final cost.

In fact, in many places around the world when calculating cost based on this broader set of factors, solar PV is already price competitive (using natural gas as the industry benchmark) when measuring cost during peak power, the time when electricity is the most expensive to produce.

For instance, in the U.S., power generated by a solar farm has reached an “inflection cost” of $3.50/Wp installed (where PV electricity intersects with grid costs) in Hawaii, California New York and Texas; especially with natural gas prices trending upward.

Following is an explanation of those lesser-known but vital cost metrics.

Panel architecture: Solar panel module architecture can have a profound effect on overall installation costs through factors such as panel size, ease of installation and crew experience. That’s because balance of system (BoS) costs (such as wiring and installation) account for as much as half the cost of a typical solar field and most operations involved in solar field construction take place on a per-panel basis. With these issues in mind, modules designed to reduce installation and BoS expenditures have a keen impact on total cost.

Panel size: Though module architecture can make installation operations more efficient, even greater savings can accrue based on panel size. Panels come in a wide range of sizes, with the largest being 5.7m2. If this size is used in an installation and compared to other standard sizes for a 1 MW field, the differences can be dramatic. Assuming tandem cells with 8.5 percent efficiency, 2,057 of these 5.7m2 panels would be required. The same field would require 13,228 CdTe panels, assuming 0.7m2 per panel at 10.5 percent efficiency, or 3,760 crystalline silicon panels (1.9m2 at 14.0 percent efficiency). For this type of construction, the larger the panel size the less the cost of labor and time.

Energy yield: Energy yield, another factor in overall cost, reflects the differing operating characteristics and efficiencies of various solar technologies. While efficiency is measured at standard illumination and operating temperature, panels in the real world see varying conditions from the change in light intensity through the day and seasonal temperature changes. Research has shown that efficiency alone does not accurately predict energy yield.

For example, all solar panels are less efficient at higher temperatures since heat raises the cells’ internal resistance. However, different technologies have different temperature coefficients. Crystalline silicon cells have a significantly higher temperature coefficient than thin film, making weather an important parameter to consider when selecting a technology to maximize energy harvest per megawatt hour. In a warm climate like Los Angeles, for example, the difference in temperature coefficients allows thin film panels to deliver 5 to 10 percent more electricity (MWh/year) per nameplate megawatt installed. Another consideration is the amount of sun. While crystalline silicon cells are most effective in direct sunlight, thin film silicon cells can continue to generate electricity in cloudy weather and other diffuse light conditions.

When analyzing these four variables holistically, one must realize the following distinctions: efficiency, panel size and panel architecture define the total installation costs per megawatt, while energy yield defines the amount of electricity harvested from the field.

As these examples make clear—and especially when comparing panels of varying architectures and cell technologies—efficiency alone does not tell the whole story; this is especially true when considering specific applications, like utility-scale or rooftop power generation. Also, some panel designs are inherently more cost-effective and some cell technologies offer inherently greater energy yield.

Ultimately, the best solar panels for utility power generation are those which deliver the most electricity at the lowest cost. Ensuring that the total cost is calculated, based on the metrics described here, will result in an analysis that more accurately reflects the true value of solar fields.

Author: Kirk Hasserjian is vice president and general manager of Applied Materials’ SunFab thin film solar business. He holds a Bachelor’s degree from the University of San Francisco and a Master’s degree in Chemical Engineering from Stanford University.


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