By Daniel Barandalla, Solar Advisory Lead EMEA and Latin America, UL
Bifacial solar modules are able to produce energy on both sides of the module with the back-side contribution made by capturing the light reflected off the ground surface. This technology results in an overall increase in energy generation of the asset, but also calls for unique financing.
Bifacial technology is forecasted to grow exponentially, with an anticipated market share of around 60% by 2029 according to the 2019 International Technology Roadmap for Photovoltaic (ITRPV) report. Cost reductions and increased energy yield are key drivers for the adoption of bifacials.
Technology selection for a solar farm plays an important role when one has project bankability in mind. By performing energy modelling and assisting clients through the lifecycle of bifacial projects in various regions, UL has developed a strong understanding of some of the key design considerations and best practices required to increase confidence in bifacial projects and reduce uncertainty.
The transition towards bifacial silicon-based technologies is now a reality in most markets globally as, when diligently designed and with the correct equipment specification for the site, projects can expect enhanced profitability. Cost reductions and increased energy yields are driving the adoption of bifacial technology, with a 60% market share expected by 2029.
Additionally, the energy yield (depending on several factors) could increase by anywhere between three and 10% with minor cost impact on the overall investment.
Current monocrystalline manufacturing processes are relatively easy to adapt to the manufacturing of bifacial products and are therefore able to support the rapid growth of this technology in the current market context.
As bifacial products introduce a new technological concept with high-efficiency modules (mono PERC, PERT, half cut or shingle cells), there is a limited track record globally on its performance and durability in utility-scale projects. Care, therefore, needs to be taken when assessing the viability of a project using bifacials.
Some critical aspects to take into account during the feasibility stage are summarised in this article.
Solar resource and site conditions
Albedo is a direct contributor to back-side irradiance and bifacial energy production. For bifacial projects, when monitoring the energy resource the following is recommended:
• Ground conditions. These should be maintained on a regular basis representing future asset conditions.
• Mounting. Downward facing albedometer should have an unobstructed field of vision. Proper setup is required avoiding reflection of nearby structural elements.
• Height. Albedometers must be mounted to avoid shading impact on the upward-facing instrument. Can be mitigated by placing the sensor on the sunward (usually southern) side of other measurement equipment.
• Shadow mitigation. Surrounding structures should be sufficiently far away to prevent them from casting shadows over the sensors.
Although the optimal design is situation-dependent (considering PPA rate, land availability, climatology, etc.), evaluations performed confirmed the following general trends:
• Bifacial projects tend to have lower DC-AC ratios to accommodate back-side energy contribution and to keep the inverter limitation loss within a reasonable/optimal range.
• For bifacial projects, the tracker/mounting structure height derives in higher production given the high back-side radiation captured. A properly designed bifacial project may have an energy gain of 0.5-1.0% depending on the climate and PV configuration. Final design must consider the offsetting factor of increased mounting structure costs.
• Lower GCRs (ground cover ratios) allow for capturing better back-side radiation. A 3% reduction in the GCR can result in an increase of between 1.0-1.5% on the net energy production, depending on the project location and other design considerations.
Several design differences are common between monofacial and bifacial systems. The bifacial advantage (approximately 3-10%) is often realized as a combination of DC capacity cost reduction and energy gain.
Energy modelling approaches
Energy modelling software packages are evolving in their ability to model bifacial energy with increasing complexity. PVsyst, a commonly used modelling software, currently supports:
• Detailed light modelling, including back-side irradiance modelling and front-side three-dimensional shading losses.
• Less detailed loss modelling, such as:
– no input fields for back-side soiling or back-side condition-based ‘mismatch’ contributors (irregular shading and soiling); and
– back-side shading that relies on a two-dimensional, simple generic factor for mounting structure impact.
In future, bifacial energy modelling uncertainty can be reduced by more complex simulation and loss models and field performance data to calibrate model assumptions.
Technology and module performance
Bifacial module technology should verify performance to conform to IEC TS 60904-1-2 (measurement of i-v characteristics of bifacial PV devices). Despite not being a standard yet (rather a Technical Specification, TS), it is in widespread use and will be referenced in updates to performance and safety standards.
In addition, IEC 61215 and IEC 61730 updates are in draft mode at the time of writing this article including bifacial related tests.
All affected bifacial tests are defined in 61215, and referenced in 61730. Those include definition of the bifacial nameplate irradiance (BNPI) used to assess performance levels before and after stress tests (UL/IEC 61215) and the bifacial stress irradiance (BSI), which is a reference condition for stress tests simulating higher rear-side contribution to total current.BSI does not address all possible field scenarios, but is expected to cover typical installations that could result in high current generation over short periods.
Since new silicon-based technology concepts are being used for bifacial modules (mono PERC, poly PERT, half-cut cells or shingle cells), UL proposes a series of actions to ensure module reliability and durability:
• Safety: Verify compliance as per UL 1703 / IEC 61730.
• Quality: Factory surveillance to verify conformance with agreed BOM for the modules and best industry practices for module inspection and QA.
• Performance: Verify conformance as per IEC 61215 and any other relevant standards as applicable (ammonia corrosion or salt mist). Proper characterization of bifaciality factor.
• Independent testing: Considering the lack of definition of a standard procedure for bifaciality factor and total peak power capacity, independent laboratory is recommended.
• Durability: Representative batch testing from manufacturing line for accelerated lifetime testing (TC, HF, DH, DML, UV).
• Reliability: Degradation modelling and pre-shipment evaluation. Testing to prove LeTID suppression techniques and independent in-process batch testing.
All the aspects highlighted above should be looked at in detail on a project-specific basis through a proper due diligence of the project so all the risks associated with an investment deploying bifacial technology are properly addressed and the right set of mitigations are put in place.
Following best industry practices for characterisation of GHI, DHI and albedo can mitigate the risk of a less accurate energy modelling for this type of system, so the energy projection for the project can be looked at with more certainty. Operations and maintenance (O&M) procedures and operating expense (OPEX) projections should also be addressed to allow for maximum return on the bifacial investment. ESI