Impact of field design and location on the techno-economic performance of fixed-tilt and single-axis tracked bifacial photovoltaic power plants

In the design phase of the photovoltaic field for a bifacial photovoltaic power plant (B-PV), the influence of installation parameters on both the energetic and economic performance must be considered, which makes determining the cost-optimal field design a challenge. Although some studies have dealt with this topic, many questions remain unanswered. Therefore, this work investigated the site-dependent impact of the installation parameters row spacing, module elevation, tilt angle and soil reflectivity of a fixed-tilt and a single-axis tracked B-PV with an east–west and north south-axis on the energy yield, the levelized cost of electricity (LCOE) and the bifacial gain. Based on the results, the magnitude of the influence of an installation parameter on the energy yield and LCOE could be quantified for all three system designs. However, three findings are particularly noteworthy: 1. in the case of the fixed-tilt design, the relative energy yield gain caused by a larger row spacing increases with increasing latitude; 2. depending on PV field’s configuration, soil brightening measures can significantly increase the energy yield of all three system designs, practically independent of location, and at the same time reduce the LCOE; 3. the choice of a too high module elevation can lead to small yield losses. Finally, the simulation model used was validated with the Swiss BIFOROT test array. In summary, it can be said that the complex interactions of installation parameters must be thoroughly investigated in order to avoid energy yield losses and unnecessarily high LCOE

Simulating the energy yield of a bifacial photovoltaic power plant

Bifacial photovoltaics (bifacial PV) offer higher energy yields as compared to monofacial PV. The development of appropriate models for simulating the energy yield of bifacial PV power plants is a major topic in both research and industry. In particular, the adequate calculation of the energy yield from ground-reflected irradiance (GRI) is challenging. The purpose of this work is to investigate the currently available energy yield models and suggest areas for improvement. A new model with the proposed enhancements is used to investigate the behaviour of bifacial PV power plants in more detail. The model calculates the absorbed irradiation originating from eight irradiance contributions for the front and rear of each cell string: DNI, DHI, GRI from DHI (GRIDHI) and GRI from DNI (GRIDNI). The model was tested using a defined case study power plant. The breakdown of absorbed irradiation (subscript “ab”) into its contributions revealed that while in summer months GRIDNI-ab-rear is significantly larger than GRIDHI-ab-rear, both are roughly the same in winter months. Furthermore, for the calculation of GRI the common simplification of infinitely long module rows was avoided by implementing an algorithm for the view factor calculation for a three-dimensional space. This procedure allowed for the assessment of impact of the ground size on the annual energy yield. In a sensitivity analysis, it has been shown that the extension of the relevant ground area resulted in an asymptotical increase of the energy yield. Additionally, the impact of ground shadows on the power plant's performance was quantified. The presence of ground shadows reduced the annual electricity generation by almost 4%, compared to a hypothetical scenario where no ground shadows existed. Finally, five different ground surfaces and the resulting bifacial gains were analysed. The results show that while dry asphalt (12% reflectivity) gave less than 6% of bifacial gain related to generated electricity (BGel), the use of a white membrane (70%) would result in 29% of BGel

Towards solar power supply for copper production in Chile: assessment of global warming potential using a life cycle approach

Solar energy technologies are a promising option to lower the greenhouse gas emissions of energy generation. Using solar technologies in energy-intensive industries located in arid climate zones is an attractive alternative for that purpose. In this work, the environmental benefit of integrating solar energy in the Chilean copper industry is explored in respect of global warming potential (GWP). A new life cycle assessment model for copper cathodes production in Chile and the integration of three solar technologies was developed. The GWP of the production of copper cathodes was calculated considering local representative conditions for climate, energy mix, and energy demand of the industry. It was computed at 6.0 tCO(2eq)/t Cu2 for a pyrometallurgical process (P-Cu) and 4.9 tCO(2eq)/t Cu for a hydrometallurgical process (H-Cu). Further contributions of this paper are the consideration of the decline in ore grade (i.e. copper content in the mineral) and the interconnection of Chile's two main power grids as sensitivities to the baseline. The interconnection of the power grids causes a GWP-reduction of 22% for P-Cu and 37% for H-Cu. In parallel, the expected lower ore grade by 2020 would increase the GWP of copper production by 10% for P-Cu and 4% for H-Cu. If the electricity that is currently taken from the grid is exclusively fed by solar technologies, the reduction on the GWP of copper production would be up to 63% and 76% for P-Cu and H-Cu processes. These numbers do not represent the upper bound for the reduction on the GWP of copper production that can be achieved with solar technologies because the substitution of on- site fossil fuel combustion with solar energy is another interesting mitigation option, which was not considered in this study. In order to achieve even less carbon-intensive production processes, an improved understanding of the copper's industry energy flows and profiles is needed. This would allow to assess the integration of further solar energy technologies and conceive the future of solar copper mining.Chilean Council of Scientific and Technological Research through the Solar Energy Research Center SERC-Chile CONICYT/FONDAP/15110019 Solar Mining project [Program for International Cooperation/CONICYT-BMBF] 20140019 FCFM grant University of Chil

Bifacial Photovoltaic Modules and Systems: Experience and Results from International Research and Pilot Applications:Report IEA-PVPS T13-14:2021

Bifacial photovoltaic cells, modules, and systems are rapidly overtaking the market share of monofacial PV technologies. This is happening due to new cell designs that have replaced opaque, monolithic back surface foil contacts with isolated contacts, which allow light to reach the cell from the rear side. Minor adjustments to cell processing steps have resulted in bifacial solar cells with rear side efficiencies from >60% to over 90% of the front side. Bifacial cells now come in many varieties (e.g., PERC+, n-PERT, HIT, etc.) and many cell lines have converted to producing bifacial cells. P-type solar cell limitations are driving the PV industry’s attention toward high efficiency ntype solar cells, including n-PERT solar cells, which are promising for two reasons:• Their process sequence calls for machinery that is generally compatible with currentsolar cell production lines.• The n-PERT cell concept permits very high bifaciality, up to 95% Today, busbar-less heterojunction (HJT) cells fabricated in a pilot-line on mass production equipment can reach efficiencies greater than 24%. With its high efficiency potential and lean manufacturing process flow, HJT cell technology is expected to gain greater global photovoltaic market share in the coming years. Even multijunction designs for bifacial cells are being considered. A multijunction bifacial cell based on a perovskite top cell and silicon HJT bottom cell appears promising. Bifacial cells have valuable applications in both monofacial and bifacial modules. Placing bifacial cells in a monofacial package with white back encapsulant or a reflective backsheet results in a significant boost to front-side module rating and several companies are investigating this application. However, most bifacial cells end up in bifacial double-glass modules (or bifacial modules with a transparent polymer backsheet). Rating and safety standards are actively being updated to account for differences in the behavior and performance of these modules. A new IEC Technical Specification was released in 2019 (IEC TS 60904-1-2) that guides the measurement of the electrical characteristics of bifacial modules. Additional product certification requirements for bifacial PV modules are mainly related to the higher operating currents of these modules and the associated potential safety issues. As bifacial modules have been deployed in the field, several bifacial-specific degradation issues have been discovered and are actively being researched. Light and elevated temperature induced degradation (LeTID) can specifically affect PERC cells if a stabilization process during cell manufacturing is not followed. The addition of isolated metal contacts on the rear side of bifacial cells may expedite hydrogen induced degradation processes. Potential induced degradation (PID) results from the migration of ions within the module package. When there is a potential gradient in the module, sodium ions from the glass can migrate to the cell surface and interfere with cell operation at stacking faults. A buildup of ions can also lead to surface passivation loss which results in degraded performance. Use of polyolefin encapsulants largely prevents PID. Double-glass bifacial modules using EVA encapsulant can be more susceptible to PID due to the increased availability of sodium ions from the glass. Bifacial cell and module innovations have led to new optimized bifacial system designs. The reflectivity of the ground (albedo) is one the most important site characteristics influencing bifacial PV performance. Sites that experience significant snowfall typically benefit from bifacial PV because of the increased albedo during these periods. Bifacial PV performance advantage is expressed as “bifacial gain”, which is the additional fraction of total energy that a bifacial PV system will produce compared with a monofacial system of the same orientation and size. Bifacial gain increases with albedo, diffuse fraction, array height, row spacing, and space between modules. The light received on the rear side of the array is much more nonuniform than light hitting the front. This nonuniformity leads to some electrical mismatch within each module and also can affect strings of modules depending on the configuration. Another characteristic of bifacial arrays is that they operate at higher DC current levels than monofacial arrays; therefore, system designers may need to adjust calculations for wire, fuse, andinverter sizing. International electrical design and safety codes are actively being reviewed to account for bifacial PV technologies.Bifacial systems come in many forms. Many are nearly identical to monofacial designs such as fixed-tilt and single-axis trackers. Performance gains of bifacial over monofacial for these system designs vary depending on site conditions and system design details. Ground reflectance or albedo and the bifaciality of the modules are generally the most important factors. Bifacial modules on single-axis tracker fields over typical natural ground covers (albedo = 0.2 to 0.3) generally see bifacial gains less than 10%. These values increase significantly when the ground is covered with snow. Other system designs, such as east-west (E-W) verticallyorientated arrays, are especially suited to bifacial PV technologies and offer some unique advantages such as a wider period of power generation that better matches typical load profiles, very low soiling rates, and such designs leave much of the land available for other uses, such as livestock. In addition, vertical bifacial PV has performance advantage at high latitudes due to the large variation in solar azimuth angle during the summer. In all cases, bifacial modules near the edge of rows will receive an extra amount of light due to the fact that there are fewer nearby modules and structures that shade the nearby ground. Such “edgeeffects” can be especially important for smaller arrays or arrays that are separated from one another. For example, elevated parking structures, fixed-tilt arrays on flat white roofs, and vertical sound barriers all benefit from the additional energy available near the edge of the array. Despite this benefit, economies of scale are also important. A recently published global analysis of bifacial PV economics determined that bifacial PV installed on single-axis trackers resulted in the lowest levelized cost of electricity for the vast majority of potential PV sites on the planet (93% of the Earth’s land area) [1]. A survey of field performance measurements from 27 different bifacial PV test systems compared bifacial gains with an array of design and site parameters and found that none of theparameters alone correlated well with the bifacial gain. A major limitation of small bifacial research systems is that their performance is dominated by “edge effects” or the increased light that reaches the back of the array due to the lack of adjacent modules and rows that for large systems result in less light reaching the array. Therefore, one should not expect the same performance measured on a small system when planning for a larger system. Instead comprehensive performance models are required to understand these relationships. These models differ primarily in how they calculate the amount of light that reaches the rear side of the array. There are two main types of bifacial models: (1) models based on view factors and (2) models that use ray-tracing. View factor models are less numerically expensive and generally assume infinitely long rows due to their two-dimensional formulations. As such view factor models are unable to represent detailed geometries. For detailed evaluations, raytracing models are recommended despite the computational challenges. A bifacial PV modelling comparison was organized to evaluate the state of the art of bifacial PV performance models. Four hypothetical system designs and two designs based on field measurements were defined and the necessary input parameters and weather files were provided to volunteers from 13 different research and commercial entities, each with their own bifacial PV performance model. These models are described in detail in this report. Thecomparison showed that the current bifacial models result in a range of results, with some models being unable to simulate all of the scenarios. The resulting predicted bifacial gains varied by as much as a factor of two. This exercise demonstrated the value of defining standard test cases to verify and validate bifacial performance models. The last section of this report provides a summary of eleven bifacial field test sites around the world along with examples of field results. Many of these sites include a variety of bifacial test arrays with different orientations, designs, and site conditions. Many test labs are experimenting with enhancing albedo using white rocks or reflective cloths. These tests have been instrumental in validating performance models and better understanding the important role of albedo in bifacial performance. Measured bifacial gains from fixed tilt sites from sites in theUS and France demonstrate how bifacial gains vary with season due to the changing sun path, with the highest gains in the summer when the solar elevation reaches it maximum. In the winter, the lower solar elevation angles result in more of the ground being covered in shadows and less light reaches the rear side of the array