Because there's no question that the core focus of smart grids is end users, they have to be able to decide how much to use based on the information they receive. Are they in a period of low-carbon power production or not? Are their batteries charged or depleted? How much will the photovoltaic panels produce today, or even the next hour?
"We have to be able to say precisely, on a scale of one to 24 hours, what's going to happen in terms of solar irradiance in the next five to 60 minutes. This lets people make energy choices, notably about how much to use," explains Valérick Cassagne, photovoltaic rooftop & BIPV manager at Total. The company can supply such ultra-precise information because of its dual role as the consortium's photovoltaic solutions supplier and power production forecaster. Consumer choices can be automated or manual, such as adjusting the temperature of air conditioning or the settings of power-guzzling equipment. For example, the Sequana office tower4 turned off its air conditioning at 4 p.m. for a few hours one very hot day when the grid was stressed and the power generated was high-carbon because of a spike in usage. The temperature in the offices only crept up by 0.2 degrees Celsius. This was imperceptible to end users, while offering much bigger environmental and financial benefits. In this case, quality forecasting allowed the building manager to cool the infrastructure during "green power"(low carbon intensity) hours, to strike the best balance with "black" (high carbon intensity) hours.
Although one hour is a very short period of time, it's nonetheless a serious challenge. And there are more complications. Forecasting must be done where the sun's rays are being captured. Meaning on the roof of the building on which the photovoltaic panels are installed. Valérick Cassagne confirms that: "It's difficult, because we're talking about a timescale of around one minute. In photovoltaics, a cloud passing overhead lowers the panels' energy production by 30 to 50 percent — immediately." Batteries are installed to buffer such inevitable production fluctuations. But they're a significant expense and have to be sized accurately. "We have to find the right balance between spending on batteries and quality forecasting. The more accurate the forecasting, the smaller — and cheaper — the battery," says Valérick Cassagne. "In a way, we watch the clouds go by," he adds with a smile.
Ever more precise
Today weather data are culled from traditional — and sometimes free — websites such as Météo France. "But they aren't precise enough in terms of time windows," notes the project manager. "We're turning to specialized service providers such as Reuniwatt, a Reunion-based start-up whose data is much more detailed." But precision comes at a cost: Around 4,000 euros ($4,472) a year for a site that generates roughly 5,000 euros in photovoltaic production revenue over the same period. Not happening! "We're looking for solutions priced at around 100 euros a year, which are far more compatible with a market in which a 50 kW site requires an investment of 100,000 euros that pays for itself in 25 years and creates about 4,000 euros a year in value." Detailed weather forecasters should probably scale their services to districts or cities, rather than buildings, and create business models with that in mind. The ball is in their court. Especially, says Valérick Cassagne, as: "We have installed instruments that can validate forecasts on EFB's rooftops. They send us back data on what actually happened, enabling us to correlate predictions and actual events and gauge the quality of forecasts."
Compared to "conventional" power plants, the production sites supplying smart grids may seem tiny. But unit by unit, rooftop by rooftop, the city is undergoing a metamorphosis. "There is no technological revolution per se," notes Valérick Cassagne. "We have to bundle a wide range of different technologies and make them compatible with one another. Cities change slowly. If you figure that 1 percent of the housing stock is replaced or renovated each year, it will take another 25 or 30 years to see significant changes."
In the meantime, Total's Renewable Energies Division continues to scope out the situation. It wants to better understand the challenges of its partners, for whom data integration was "a key issue we couldn't afford to underestimate. We had to give a shared protocol and language to an assortment of information systems. Each partner provided its expertise and together, we succeeded," comments Valérick Cassagne. His parting comment: "Let's not forget that the visible part of smart grids, the user part, has to be simple and accessible, while the submerged part, the technology, functions independently."
