Last 30th April, the Sub-directorate of Electrical Energy, which reports to the Directorate General for Energy Policy and Mines and therefore to the Secretary of State for Energy, responded to a query from the Regional Ministry of Sustainable Development of the Junta de Comunidades de Castilla-La Mancha concerning the "installed power to be considered in photovoltaic installations using two-faceted photovoltaic modules".

Before dealing with the response referred to above, a brief introduction to basic concepts that may help to understand the background that led the Regional Ministry of Sustainable Development of the Junta de Comunidades de Castilla-La Mancha to put the consultation to the Ministry.

Bifacial modules are photovoltaic panels that produce energy on both sides of the panel due to the installation of photovoltaic cells on both the front and the back. Their introduction into the market was initially going to mean an evolution or even a revolution because they have some clear advantages when compared to the single-face modules traditionally used by the industry.

Among these advantages, one can be mentioned which, following Royal Decree-Law 23/2020 of 23rd June, which approved measures in the field of energy and other areas for economic recovery, is becoming key in the sector: most experts argue that plants installing two-faceted modules will require less land. And given the difficulties that land acquisition currently poses for promoters, anything that reduces efforts and costs in this phase will be a point that adds value to their projects; on top of that, bifacial modules can be a solution for projects such as self-consumption on rooftops where space is limited but high albedo can be counted on.

Of course, there are also disadvantages, such as the price of bifacial photovoltaic modules as opposed to single-facial ones: the former have a price that, according to recent data, can be between 1.50% and 2% higher than traditional single-facial ones. This disadvantage would be reduced if, as sources in the sector also say, efficiency gains of between 3.5% and 6% would be made.

Having said this, the doubt of the promoters that motivated the consultation of the Regional Ministry of Sustainable Development of the Castilla-La Mancha Regional Government arises: How should the installed power of a plant that uses bifacial photovoltaic modules be measured?

The answer must be based on Article 3 of Royal Decree 413/2014, of 6 June, which regulates the production of electrical energy from renewable energy sources, cogeneration and waste:

"The installed power shall correspond to the maximum active power that can be achieved by a production unit and shall be determined by the lowest power of those specified on the rating plates of the engine, turbine or alternator units installed in series or, where appropriate, when the installation is configured by several engines, turbines or alternators in parallel, it shall be the lowest of the sums of the powers of the rating plates of the engines, turbines or alternators that are in parallel.

In the case of photovoltaic installations, the installed power will be the sum of the maximum unit powers of the photovoltaic modules that make up the installation, measured under standard conditions in accordance with the corresponding UNE standard".

Based on this norm, the response of the Ministry for Ecological Transition and the Demographic Challenge was that "the installed power would result from the sum of the maximum power of both sides".

This response has generated surprise and unease in the sector. The reason for this is essentially technical and is based on the fact that the "UNE standard" referred to in Royal Decree 413/2014 does not exist: there is currently only one UNE standard that refers to the measurement of photovoltaic modules. This UNE standard is UNE-EN IEC 61853-3:2018, referred to in the Resolution of 9 January 2019, issued by the Directorate General for Industry and Small and Medium-sized Enterprises, publishing the list of European standards that have been ratified during the month of December 2018 as Spanish standards.

The measurement of nominal power for two-sided photovoltaic modules is detailed in the IEC TS 60904-1-2:2019 standard: reading this standard leads to the clear conclusion that nominal power cannot be understood to be the result of the sum of the power of both sides as the Ministry has responded.

It should be borne in mind that the guarantees of the manufacturers of these modules with respect to the rear face are far from being those given for the front face. It is not difficult to understand that a module manufacturer does not wish to, and cannot, give performance guarantees for a product whose performance will be conditioned by factors over which he has no control, such as the design of the installation in which it is located or the albedo. Focusing on the first point (the design of the installation), we will find that within the same plant there will be two-stage modules with different power and others (from the same manufacturer and the same series) with different behaviour and, therefore, different power.

This being the case, the Ministry's response makes it difficult to install this type of component, as the administration is considering a measurement system that is not real and which involves estimating the power of the modules above their value.

The impact of this interpretation means a delay in the implementation of this technology as:

• It can lead to projects that were previously below the 50-megawatt limit being exceeded and consequently having to be restarted.
• The guarantees to be deposited must be greater.

It is to be hoped that the proposed Royal Decree on access and connection to the transmission and distribution networks will regulate this issue in a more reasonable manner, since maintaining the current interpretation would put a brake on the implementation of this technology and, given that greater performance is achieved with less use of space, this is a solution that should be favoured.

You can see the full article in the Energetica magazine.