Photovoltaic conversion: from the cell to systems

The scientific observation that our modern development based on the consumption of fossil fuels is profoundly disrupting several planetary balances, particularly biodiversity and climate, points to the need for a transition to a more sustainable development model. Confronted with the climate emergency that has now gained consensus since the Paris Agreement adopted in December 2015, humanity has high expectations of an announced "energy transition". In this context, the realization of a long-standing utopia could provide a lasting response to these challenges: to rebuild, like the food chains of life on Earth, the energy supply of human societies on the only low-entropy energy source outside the Earth: the Sun.

Today's technologies make it possible to take advantage of this remarkable thermodynamic situation, and several renewable energy sources of solar origin with very high potential are called upon: solar radiation, wind power on land and at sea, hydraulic power, biomass, etc. Photovoltaic electricity directly exploits the most important of these energy potentials: solar radiation. It has now logically taken the lead in terms of new installed capacity. It can supply electricity to large public power grids as well as to micro-grids and isolated sites. When combined with electrochemical storage - batteries or green hydrogen and power-to-gas, which are also the focus of major global programs - it can compensate for daily intermittency and seasonal variations in sunlight, or indirectly power autonomous or mobile systems. Its versatility and qualities give it enormous potential for a wide range of applications. But, like any technology, it is not without impact on our environment: in order to "densify" the diffuse energy of solar radiation, it requires large collector surfaces and the mobilization of significant mineral resources, which are themselves exhaustible if not recycled.

In this series of two articles, we begin with an analysis of the energy transition, including the thermodynamic situation of the Sun-Earth system; this serves as a framework to which we can explicitly relate the specific physical properties of photovoltaics presented below. Next, the properties of solar radiation and the principles of photo-voltaic conversion are described, along with the technological devices that enable their implementation, from the elementary photovoltaic cell to the modular, multi-cell generator. This modularity, combined with the power electronics that drive photovoltaic electricity, leads to a wide variety of structures, first for generators, then for systems, from a few watts to several megawatts, installed in dense urban and industrial areas as well as in rural and isolated areas. In this second part, we detail this diversity of photovoltaic systems and propose a...