Harnessing the power of the sun

Figure 1. A ground view of São Gonçalo solar PV plant in Brazil.

Umberto Magrini, Head of Engineering and Construction at Enel Green Power, discusses the latest innovations driving the current solar revolution and why solar photovoltaics are set to be a long-lasting success story within the renewables industry.

Solar photovoltaics (PV) are leading the energy transition through a significant and fast evolution of the technology’s main components, availability of more operating data, and reliable production modelling tools. The field of solar generation is also rich with innovations that allow the technology to become increasingly widespread.

PV module

The continued implementation of silicon-based cell enhancements, such as passivated emitter rear contact (PERC) and half-cell interconnection module technology, has paved the way for higher average module efficiencies. The use of larger wafers (182 mm and 210 mm) has resulted in higher module power classes of 500 W and above, as well as a larger size area. Improvements in all fields result in an increase of module area efficiency, with today’s mainstream p-type mono-Si based modules reaching efficiencies of 21%, and increasing to 22.2 - 22.5% within the next five years.

The PV industry is currently in the final phase of p-type dominance, characterised by the use of p-type multi and p-type mono substrates. From 2024, however, all signs point to a dramatic shift to n-type substrates, with n-type based modules

including Heterojunction (HJT) providing the highest power modules with today’s efficiencies of approximately 21.5%, which will increase to approximately 23% within the next 10 years. Looking to the next PV module generation, the mass production of Si-based Tandem cells and modules is expected by around 2025, starting with module efficiencies of 22.5% with a high margin for improvement (up to 28 - 30% of efficiency).

In the current solar scenario, most modules are monofacial, but the share of bifacial modules is expected to grow to approximately 55% in the coming years. In 2016, Enel first began using bifacial modules, starting operations at its La Silla solar plant in Chile. With this pilot project, Enel deeply analysed the advantages of the bifacial technology and customised the mathematical model for energy estimation and the geometrical aspects of the design. Following that experience, Enel started using bifacial modules in large PV plants such as São Gonçalo in Brazil and Magdalena II in Mexico. For Enel today, with few exceptions, the bifacial module is the preferred solution for guaranteeing the lowest levelised cost of energy (LCOE).

Inverters

Today, inverters and their control system are the real brains of solar PV arrays, representing a key tool for efficient solar power plant operation and management as well as grid services. The size of these inverters is continuously increasing, especially for ultra-large utility scale plants, while at the same time producers of string inverters are also offering higher power solutions, with the largest reaching up to 350 kW to compete in the field of large scale power plants.

Interest in string inverters continues to grow in utility scale applications. While central inverters are, and will remain, popular in the industry, string technology has become

increasingly appealing for Enel over the past few years. Utility scale systems larger than 20 MWdc are still typically suited for central inverters but there have been instances where large scale projects – even as large as 100 MWdc – have considered string. Enel, after a small pilot in Brazil, immediately implemented string inverters in large PV plants such as Magdalena II. Today, centralised and string inverters guarantee very similar LCOE values, so the specific characteristics of the project determine which type of inverter is best suited.

From a material point of view, Silicon carbide (SiC)-based semiconductors, although still not available at commercial level, are highly promising. SiC products offer higher power density and are more lightweight than traditional inverters. The thermal behaviour of SiC inverters means they do not have to reduce output to avoid overheating but can feed power at ambient temperatures over 50˚C. Less overheating translates to smaller fans for cooling inverters, which helps to reduce the total weight.

The cost is currently the main issue with SiC inverters. While studies have shown that SiC inverters have lower LCOE and higher improved system efficiency, the solar industry has a strong focus on lowest initial cost, making, at least for the moment, SiC inverters less competitive than traditional ones.

Micro inverters and solar optimisers, which are widely used in small scale rooftop applications, could see broader implementations in the near future, even for large scale applications.

Grid forming

Inverters provide the interface between the grid and energy sources such as solar modules, wind turbines, and energy storage. When there is a large disturbance or outage on the grid, conventional inverters will shut off power to these energy sources and wait for a signal from the rest of the grid that the disturbance has settled and it is safe to restart, also known as grid-following. As wind and solar account for increasing shares of the overall electricity supply, it is becoming impractical to depend on the rest of the grid to manage disturbances. Gridforming inverters are an emerging technology that allow solar and other inverter-based energy sources to restart the grid independently.

The new roadmaps highlight recent innovations in gridforming inverter technology. They identify the challenges for researchers and operators of the small isolated grids or microgrids where this technology could be piloted. In the short-term, research opportunities exist for creating new grid-forming hardware, software, and controls; redesigning regulatory and technical standards; and developing advanced modelling techniques. Building on these, the authors envision a future where grid-forming inverters are integrated into electric grids of steadily increasing size and complexity over the next 10 - 30 years.

Inverter reliability

As the solar PV industry matures and asset owners focus more on total system lifetime cost – and not just initial costs – inverter reliability becomes increasingly important.

Inverters require more maintenance activities within a solar plant than any other system component because they are expected to operate over a wider set of environmental and electrical conditions for longer periods of time. Furthermore, inverters contain hundreds of internal components, operational subsystems, and circuits.

Meanwhile, models constantly change as do operational requirements, which is evident from grid code updates and local jurisdictional requirements. Quality control, laboratory and factory testing, as well as controlled field testing, are the best alternatives to long-term field exposure for evaluating product reliability and performance.

Tracker

Figure 2. An aerial view of São Gonçalo solar PV plant in Brazil. PV support structures are split between fixed and trackers, with the latter being predominantly horizontal single axis supports.

Trackers increase energy production by 15 - 25% compared to fixed, with an additional increase in production expected with the implementation of a backtracking control algorithm for irregular terrains and a diffuse radiation algorithm considering bifacial applications. Wind tunnel testing has become a key driver for tracker design optimisation and cost reduction, while the market is split between 1 Portrait (P) and 2P configurations based on the project’s constraints. As longer rows might be the answer for further optimisation, multiple slew drives are expected to introduce stiffness and ensure tracker reliability.

Database and data reliability

The collection of solar data plays a prominent role in the design, financing, and energy forecasting of PV systems, with plants requiring reliable information about solar and climatic data, including their variability over time.

Several properties are carefully investigated during the scouting of potential PV sites, such as grid access and land availability, but solar radiation is without question one of the first

selection criteria that has a huge and direct impact on energy production. Variability of the solar resource occurs not only seasonally, but it may change from year to year (inter-annual variability). This is one of the reasons behind the success of satellite-derived models with respect to the ground measurements. Satellite-derived models generally own more than 10 - 20 years of high-quality solar radiation data all around the world, with a good level of accuracy thanks to strong validation of satellite-based models and several solar data providers present in the market.

Physical and mathematical model of a PV plant

The physical and mathematical models widely used in the development phase have been developed by several research centres within the PV sector and are fully validated. In addition, most of the input data used are from external data providers, such as solar resource information, and are certified by them. The rest of the input for the design is selected by using reliable data coming from suppliers of the main equipment and engineering department experiences matured over more than 10 years.

The best simulation of a standard large scale PV plant can be carried out by using reliable mathematical and physical models that can calculate the losses related to the interaction between the intrinsic characteristics of the main components of the PV plant and the environment data. These models also allow for the addition of new and challenging levels of complexity considering innovative solutions such as bifacial modules, floating plants, multi-orientation layout, as well as mixed equipment configurations. Further improvements can be obtained by choosing different objective functions, which can also capture the evolution of market energy prices as well as the generated revenues and their added value.

Operation and maintenance future development

To guarantee the profitability and the performance of a huge portfolio, efficient and effective operation and maintenance (O&M) is required. To do this, Enel is building its O&M solar strategy on three pillars: robotisation, data driven maintenance, and automation.

Some examples of the applications already implemented include: the use of drones to identify panel and string failures; analysis of the strings’ I-V curves to identify faults by machine learning approach; and optimisation of the soiling management, with automated monitoring and scheduling of cleaning activities.

These are just the first steps towards an automated solar O&M model. The industry as a whole is further developing analytics solutions for predictive maintenance approach. On top of that, more sensors in the field will be necessary upstream of the inverter itself: applying specific sensors in the field on PV trackers and inverters will provide more and more precise information in order to identify the inefficiencies affecting PV plants at component level, anticipating the maintenance activities and therefore avoiding having to fix the failures only after they have happened.

Figure 3. La Silla solar PV plant in Chile.

Figure 4. Magdalena II solar PV plant in Mexico.

Conclusions

Solar PV has, since its inception, repeatedly surprised scientists and engineers, overcoming all growth expectations and demonstrating an outstanding capacity to break all previous technological taboos. Without question, solar technology has reached a level of maturity that makes it one of the cheapest sources of energy available at industrial scale in the world.

The unique characteristics of being produced for direct conversion with no mechanical or thermal energy in the middle, modularity, which allows it to be easily utilised in a portable device as well as in a ground mounted giga plant, and ubiquitous presence, mean that a long story of success can, without a doubt, be predicted for solar PV towards a fully sustainable global energy footprint.