Bifacial PV Systems: Page 4 of 5
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FIELD APPLICATION AND PERFORMANCE
Data from initial test beds and performance simulations—some of which are summarized later in this article—suggest many potential applications for bifacial PV systems. These include most conventional applications such as flat roofs and free fields, where installers deploy monofacial PV modules today, as well as niche applications such as building-integrated PV (BIPV) carports and awnings, where they typically deployed early bifacial modules. Back-side power collection also rewrites the rules that apply to traditional PV system design and performance, which could enable new markets and business models.
Sandia test results. Sandia National Laboratories recently published a report (see Resources) documenting the side-by-side test results for Prism Solar bifacial modules in comparison to reference monofacial modules. Sandia installed modules at the test bed in five orientations over two surfaces at its New Mexico Regional Test Center. Data collected over a 6-month period (between February 15 and August 15, 2016) indicated that the bifacial modules were outproducing the monofacial devices by anywhere from 18% to 136%, depending on the orientation and ground cover. Figure 3 provides the average daily power output curve for each test condition.
The report’s authors draw some interesting conclusions from these data. First, they note that bifacial gains vary throughout the day, depending on the angle of the sun or whether conditions are clear or cloudy. The impacts of sun angle are somewhat intuitive when you consider that the sun is closest to the horizon early in the morning and late in the afternoon, which not only decreases the available incident energy but also increases the amount of reflected light. As a result, the percentage of the instantaneous power output resulting from the bifacial contribution is highest at these times, and the bifacial gains are relatively lower at or around solar noon. The impacts of direct versus diffuse irradiance are similar. During cloudy conditions, the incident energy is relatively low, which increases the percentage of bifacial gain due to reflected light. Under sunny conditions, by comparison, the bifacial contribution is higher in absolute terms (back-side power) but lower in relative terms (percentage of bifacial gain).
The authors also note that bifacial modules are relatively insensitive to changes in array azimuth. As you rotate a bifacial array east or west of true south, the bifacial boost increases, effectively offsetting some of the losses that a monofacial array experiences in non-optimal orientations. As a result, “west-facing bifacial modules tilted at 15° produced a similar amount of energy as south-facing, 15°-tilted bifacial modules and surpassed the energy production of all of the monofacial orientations considered.” Not only did the west-facing, 15°-tilted bifacial array outperform the optimally oriented monofacial arrays, tilted at 15° and 30°, but also the west-facing, vertically oriented (90° tilt) bifacial array “outperformed monofacial modules at any orientation.”
Not surprisingly, the bifacial gains were also greatest in a west-facing, vertically oriented application, which creates an effective collection area for bifacial modules literally double that of monofacial modules. As a result, the bifacial power curve in this application has two peaks, one in the morning and one in the afternoon, whereas the equivalent monofacial power curve has one peak only. An east-west facing array is also effective at shifting solar power production later into the afternoon, when electric demand is often greatest. This configuration is likely well suited to take advantage of certain time-of-use rate structures and could provide additional value to utility operators. (The downside of an east-west vertical orientation is its high susceptibility to horizon shading losses.)
BARRIERS TO ADOPTION
On the one hand, bifacial PV arrays require specialized modules and mounting systems, as compared to conventional PV systems, which invariably increases up-front system costs. On the other, side-by-side field tests, such as those Sandia conducted, clearly reveal a bifacial energy boost. It is entirely possible, therefore, that bifacial PV systems could provide the best value, in terms of LCOE or return on investment, in certain applications. Making that case and taking it to investors, however, remains a barrier to widespread market adoption.
Macroeconomic conditions. In the short term, the low costs for conventional monofacial PV modules represent one of the biggest challenges to the commercialization of bifacial products. Module prices are at an all-time low, largely due to downward price pressure caused by global oversupply. As a result, many manufacturers are operating at low to negative operating margins, which hinders investment in new manufacturing tools and product lines.
The authors of the EPRI report note: “It is financially difficult to sustainably grow manufacturing capacity of existing products, let alone a more innovative concept such as bifacial PV modules. This issue is exacerbated by the more expensive manufacturing tooling and processes required to produce bifacial modules today. The high capital expense and low returns on cell and module production is a bottleneck for adoption by manufacturers.”