Solar engineering in all its forms

Why is it so important to accurately predict the characteristics of the solar resource? Given the current embryonic volume of photovoltaic installations and the margin of progression which will happen in the coming years, one can quite frankly pose the question. As solar is rather underfunded in comparison with other energies, why direct the few funds available on studies and in detailed collection of data instead of investing 100% in photovoltaic installations to make up for France’s lag? The Hexagon is indeed far from the European leaders this and the Côte d'Azur, quite counter-intuitively by the way, missed the opportunity to be a driving force.

Between scientific and capitalist logic

For Philippe Blanc, there are two answers to this question. The first element is that to predict the characteristics of the solar resource, there are no associated costs as such to collect the data. The data are extracted from existing data already collected by satellites, including weather. As he himself says: “The sensors of the Meteosat satellites were not initially configured to characterise the solar resource, however, among the indicators already collected, some are relevant to characterise it. Very practically, we use IGN map holdings for France or Google Maps, we retrieve the useful data, and then we process them to provide actionable data to make urban-scale decisions.” This is both timely and realistic. A satellite development costs billions and it is unlikely that such an investment will be made solely to study the solar resource.

The second element is to be sought in capitalist logic. In the solar sector, all the investment is indeed made at once, at the beginning, to build the photovoltaic power plant. Once built, the source of the sun is free, there are few costs except the costs of operation and general maintenance of the facilities. As Philippe Blanc explains: "When there's a project to install a high-capacity photovoltaic power plant, you need to be able to obtain financing for the infrastructure and therefore show the banks that the plant will be profitable. So you need to be fairly precise about the amount of energy the plant will be able to produce, and also about the timing of that production. This 'when' is becoming increasingly important, and on the energy market, some companies are building their model by developing storage solutions to be able to sell when electricity prices are at their highest".

Solar speculation is on the march. Remember that there is a spot market for energy prices (Epex spot) and that in this European electricity market, it is the exchanges that set the spot prices for MWh the day before for the day after, on an intraday basis (i.e. by hourly band), according to estimates that seek to match supply and demand as closely as possible. The idea remains, however, to optimise existing systems, and applied mathematics takes on its full meaning here, because to sell electricity at the right time, it is vital to know your annual production, but also to be able to project it from one day to the next, and then hour by hour. These fine forecasting mechanisms are the new keys to supporting today's transitions.

Between rural and urban logic

Access to land is at the heart of photovoltaic development and a thorny issue in the sector because it affects power plants in natural, agricultural and forestry environments. Building a 10-hectare power plant in a forested area has an impact on both biodiversity and the landscape, even if from a technical point of view the transmission of electricity is not always a problem. Supporters of photovoltaics in these natural environments will see it as a sign of a strong commitment to the energy transition. Its detractors will point out the paradox: why, given the current low level of penetration in urban areas, plant tens of thousands of panels in wooded areas when there are so many opportunities for land that is already developed, unused and cheaper in the city, if only as car park shades or roofs?

For Philippe Blanc, a fervent advocate of urban solutions, developments in recent years have been interesting. On the one hand, at regulatory level, the law of 24 February 2017 explicitly authorises self-consumption on individual and collective housing, which is starting to have the corollary of encouraging private investment in solar. RTE, the electricity transmission system operator, has produced a remarkable piece of work looking ahead to 2020 and 2050 in terms of energy futures. In a country where nuclear power is a religion, this report has shaken people's beliefs by indicating that in the energy mix of the future there is bound to be some solar power, not out of piety but out of necessity. As Philippe Blanc explains "France is currently at 15 gigawatt-peak (GWp), and we're on a major growth curve. By 2050, depending on the scenario, it is estimated that solar energy will account for between 70 and 200 GWp of the French energy mix. If we translate this into a land footprint, 1 gigawatt of photovoltaics corresponds to around 500 hectares of modules, with a footprint of around 1,000 hectares.” The upper range would correspond to less than 0.4% of the surface area of France.

However, shading remains a complex issue in an urban environment, with an immediate impact on the yield of the panels, and a whole area of research within the Observation, Impacts, Energy Centre at Mines Paris - PSL is devoted to developing algorithmic tools that enable the impact of shading to be calculated precisely. A recent simulation was carried out to equip the Parisian site of the École des Mines, in the Luxembourg Gardens. The tool developed in-house is capable of calculating every 5 minutes, for every m² of the site, the precise amount of solar energy that each plot of land receives as radiation. Knowing exactly how much sunshine and shade we are talking about helps to convince developers whether or not to invest in such photovoltaic projects in urban areas. On a more local level, a teaching activity for the civil engineering cycle at Mines Paris - PSL will take place in November 2023 with CASA and Valbonne town council on the role of solar energy in the transition of this area (CASA2040), including photovoltaic self-consumption projects. A number of student engineers will be visiting the Pierre Laffitte campus for three weeks this year to look at this 100% Sophia Antipolis issue.

Increasingly efficient conversion technologies

"Photovoltaic cells currently on the market have an efficiency of around 20%. Efficiency can be as high as 48% for laboratory or space applications. Remember that in 1953, efficiency was just 0.5%. Today's figures are changing very quickly". The curve is therefore exponential. Although interest in solar energy is not new (the first patents date back to the middle of the 19th century), it was not until 1958 that the first autonomous photovoltaic system saw the light of day as part of a satellite project. The OIE Centre at the École des Mines is working on this fine-grained quantification of yield, on several scales (from very local to national).

The composition of the cell will affect the quality of the yield. A single cell, for example, made up of a single type of semiconductor, will only be able to convert a single part of the solar radiation spectrum and will therefore have limited efficiency. An efficiency of 46% requires the superposition of several cells made up of several types of semiconductor. As each semiconductor has a different spectral sensitivity, the sensitivity spectrum of the photovoltaic cell as a whole will be extended. A little semiconductor physics never hurts, and there's no shortage of choice anyway. Highly purified silicon (Si) remains the basic element in the overwhelming majority of photovoltaic cells, and its main advantage is that it is easy to recycle. Alternatives such as cadmium telluride (CdTE) often require rare metals and are of little interest to the industry except for niche uses, for example to equip non-planar or flexible surfaces.

However, one point to be vigilant of in the sector remains the sourcing of photovoltaic cells. In practice China has a virtual monopoly on existing exports. Despite there being not only disadvantages (prices have been divided by ten in a decade), this dependency is worrying. A project is therefore under study in Fos-sur-Mer to build a cell production unit on 60 hectares. Developed and implemented by the French company Carbon, this strategic project would make it possible to secure an independent photovoltaic sector on French territory within two years.

But to fully understand the solar resource, it is important to consider several aspects. Performance, of course, which is intrinsically linked to surface availability. The greater the efficiency for a given amount of energy produced, the less need there will be for photovoltaic cells and therefore ground surfaces to be used. Environmental impact is also a key aspect. If the photovoltaic system itself emits no direct emissions when it is in operation, from a life cycle analysis perspective, everything that revolves around the extraction and processing of its raw materials, its manufacturing processes, transport, installation, maintenance and recycling are all factors, and financial and environmental considerations tend to converge.

If we go back to basics, the most useful energy from solar radiation is thermal energy. As Philippe Blanc puts it so simply: "In the South, it would be inefficient to use electricity to heat water.” Oops! Definitely a word to the wise.