Grid and design consequences of the latest solar PV technology

Energy professionals will know that the reason for the competitiveness is related to a dramatic reduction in installation costs, as highlighted by the International Renewable Energy Agency (IRENA) in its Renewable Power Generation Costs in 2019 report, which says: “costs for electricity from utility-scale solar photovoltaics (PV) fell 82% between 2010 and 2019.”

While these cost reductions have been driven mainly by economies of scale, mass production, collaboration between module manufactures and suppliers, support schemes and competition in mainly the Chinese PV industry, this paper will discuss the nature of the technology advances and the impact of these upgrades on the utility-scale system designs and associated requirements for development such as grid connection, finance, divestment and management of assets.

This development in PV technology has been achieved by both thin film modules based on cadmium telluride cells as manufactured by First Solar, which takes up around 5% of the overall market, as well as for silicon-based PV modules.

In this paper the technical developments related to silicon-based high-power modules will be discussed, covering: increased diameter of Czochralski monocrystals, replacement of wire saw cutting abrasive from silicon carbide to diamonds, opening of the cell backside to enable bifacial modules, replacing backsheets with glass in bifacial applications, stepping up quality assurance through third party inspection at module factories, introducing inverters with higher current carrying capability per maximum power point tracking (MPPT) and ensuring compliance to very elaborate power quality requirements at the point of grid connection.

Silicon-based solar cells

When it comes to the solar cell, the most significant technical advances relate to the silicon substrate, which is a key factor for most the solar market, i.e., the market covered silicon based solar cell.

For years both casted multicrystalline and Czochralski (CZ) grown monocrystalline ingots have been cut into wafers by use of slurry-based wire saws and used as substrates for the cells processing.

Within a very short period of a few years, the casting method has been abandoned and all new PV products are now based on monocrystalline wafers – and they are cut with by diamond coated wires, which ensure less material loss than previous slurry-based wire saws. More importantly, the diameter of the monocrystal has also increased significantly, from previous cells typically 200-223mm, while today either 250 or 300mm dominates, corresponding to full square wafers with side lengths of either 182mm or 210mm.

Since the current generation under illumination of a solar cell is proportional to the area of the cell, these two wafer sizes impact the current rating of the solar module, which is an important design criterion when the electrical design is made. At least when the mainstream p-type PERC cell architecture is considered, the cell current for a 210mm cell will be around 1.3 times higher than that of a corresponding 182mm cell.

Large-format PV modules

The size of a standard module mostly depends on the quantity of full cells encapsulated into the laminate, which often is chosen to be either 72 or 78. The number of cell units might be doubled by using half-cells to reduce the internal resistive losses, but two typical large-format modules sizes would be the 182mm wafer based 540Wp series with dimensions 2256 x 1133mm² (LONGi version) or the 210mm wafer based 600Wp series from Trina, Risen and others, which measures 2172 x 1303mm².

While the 540Wp product will generate a module current close to 13A, the 600Wp module will generate more than 17A. Correspondingly to the large current, the voltage of these modules also differ such that the highest power modules have the lowest voltage – around 41V – whereas the 540Wp module will provide a maximum power point (mpp) voltage close to 50V. Both the large physical size and module current have implications when assessing the system designs for these new products.

Bifaciality and glass-glass module versions

A very important feature within these new large-format modules is the option to open the backside of both the cell and laminate for light penetration into the cell semiconductor junction. For p-type modules the efficiency of light conversion received on the back will be around 70% whereas n-type modules might see 85% or higher backside efficiency relative to the front. In most cases the energy gain from bifacial modules exceeds the limited extra price to be paid for this feature, but the financial value of this gain depends on how well the modelling of this extra gain has been verified and validated in the scientific community, since the bankability of this gain highly depends on this status.

Transparency of the backside of the module can be obtained by both transparent backsheets and glass. Whereas the backsheet can ensure an overall lower weight of the module – which may be of importance to certain roof projects – the glass-glass laminate panels benefit from a stronger and stiffer module with better protection of the cells from moisture ingress and mechanical stress.

Module quality assurance

Utility scale projects tend to assign up to 40% of the total Capex on PV modules and it’s of high importance that the quality, durability and performance of this key component can be guaranteed towards the financing bank and ultimate asset owner. Only a few years ago, it was sufficient to ensure that the product was type-certified against IEC 61215 and other relevant standards, that the supplier bankability could be confirmed based on its ranking on the Bloomberg listing of Tier 1 suppliers, and that the warranty terms did cover both replacement and shipping costs and a warranty re-assurance was in place.

Today it’s common to also conduct third party audits at the factory sites, not only prior to supplier selection but also during the manufacturing campaign and after the actual production in a pre-shipment inspection. In order to ensure that such inspections provide value to the buyer, it’s necessary to include the detailed inspection scope, acceptance criteria and inspector mandate into the contract. This part of the contract is also where technical concerns related to the new large-format modules must be addressed by defining inspection procedures dedicated a mitigation of uncertainty in quality and performance, which a first-mover needs to address for any new product that cannot provide long-term field testing and where yet unknown failure modes cannot be excluded since this new product is based on several underlying process changes from crystal growth to encapsulation as described above.

Substructure solutions

Bifacial modules do request new substructure design, to ensure the shading from the structure on the backside of the module is kept to an acceptably low level. Larger panels may also lead to larger areas per fixed tilt table or tracker, which translates into renewed focus on statics. Whereas the design criteria will be mostly unchanged, recent incidents with torsional galloping – where panels under high wind loads have blown modules off trackers – will need to be considered. For this reason, technical advisors (TA) will have an increased awareness of the static calculations, to ensure that they are fully updated with respect to the new format.

At the inverter level

The capability requirements of the inverter for conversion of dc-power to ac will be more demanding when a 1,500V string (typical maximum voltage criterion) is equipped with the new large-format modules that may carry up to either 33 modules at of 600Wp format or 27 modules of 540Wp per string, at typical string lengths. Not only does the overall dc/ac power ratio impose design restrictions to the electrical design, but also the fact that the maximum power point tracker, which needs to manage power from several input strings, often has a maximum current capability per MPPT close to 25A, which invalidates the option to have two high current strings (each of 17A excluding the bifacial uplift) managed by a single MPPT. In order to load the inverter with a reasonable dc capacity above the inverter rated power, often new inverter models are required that are capable of managing more than one high-current string per MPPT.

Not only have the new PV panel specifications introduced new and more demanding inverter features and upgrades, new requirements for generators have also recently been implemented within the European Union, which will place more focus on the inverter and the inverter manufacturers expertise in the area of grid-compliance.

Grid connection regulation

For many years, the typical residential or commercial PV system could be connected to the electrical grid with few formalities and grid compliance documentation in the form of type-test certification of the inverters. Such systems demonstrated limited capability to deliver grid support on request and often it was only possible to offer remote control in terms of an ‘ON’ and ‘OFF’ status requested by the utility. Today the technical requirements for generators have increased significantly, and in some countries like Denmark and Ireland the transmission system operator (TSO) takes action to ensure conformance to these requirements, not only based on type-testing certificates but through detailed analysis and verification of the provided grid-simulation models they cover. This is provided using, for example, PSCAD (power systems computer-aided design), which defines the component characteristics in the electrical system, and RMS (transient analysis) modelling.

Since the national implementation of the European regulation (2016/631) on Requirements for Generators is still fairly new, the electrical design based on the new large-format modules is also new. And since new inverter models have just been introduced to allow for more current to be managed per MPPTs, it’s obvious that this process of grid-simulation model compliance validation is not only time-consuming but also represents a high risk of delays. So even if it may be possible to obtain an Energisation Operational Notification, or EON, whereby the new PV park can start to test the electrical equipment installed, the more interesting Interim Operational Notification (ION) status that allows the project to feed energy into the grid for the first couple of years, cannot be received before these grid-simulation models have been approved.

Since the exact and detailed requirements to the inverter sub-model documentation may not be published and available by the TSO, but still may exceed the requirements given in the commonly referenced VDE AR-N 4110/4120/4130 and EN 50549-2 certificates, it can be quite troublesome to ensure timely compliance to these grid connection conditions.

Grid-access and energy storage

Many utility-scale projects are being connected to the grid these days and the pipeline of potential projects are booming. Once a municipality has accepted the plans for a project and a final investment decision has been made, it’s much faster for the project developer to execute the detailed engineering, procurement and construction of the project, than for the utility to ensure availability of sufficient grid connection capacity, including necessary enforcement of the shallow and deep grid.

For the utility or TSO, the decisions on grid maintenance/upgrade and strategic enforcement must relate to (mandatory) long-term grid plans covering a five to ten-year development horizon, and it’s often difficult to adapt such plans to capacity requests that may be developed from a green-field screening into ready-to-build status in a just a few years.

For projects where full connection capacity may not be available until several years after the PV project can be realised, it may be an option to grid-connect with export capacity that is permanently or temporarily limited. This might also reduce the connection costs in case the connection charges are high and structured as a capacity-based connection charge.

In such a situation it will be relevant to consider establishment of a battery or other energy storage system to capture the otherwise curtailed energy. Similar considerations may evolve from grid connection agreement requirements addressing allowed export capacity ramp-rates and other minimum export capability requirements. Whereas it may not be too difficult to analyse the energy storage business case related to curtailment, and even a revenue stream related to down-regulation and arbitrage provided that the future baseload electricity price and volatility can be assessed in the specific bidding zone, it’s much more difficult to assess the amount and duration of revenues that may be collected in the ancillary service market.

Grid connection and energy storage decisions are becoming more demanding and critical, but this may relate more to the changed operational conditions for the grid-owner when more intermittent energy sources and higher demands are being connected to the grid, rather than the latest technology developments directly within the PV field.