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The floating solar PV array

Floatovoltaics: A Winery's Solution to becoming Net-Zero

5 April 2017

When Greg Allen found out that the owner of the winery he worked at wanted to be a net-zero 100% renewable energy facility, he realized that he wasn’t going to get out of doing engineering work just because he abandoned that career path years before. A biomechanical engineer turned oenologist, Allen set out to design a solar PV array to offset the winery’s energy usage. However, he ran into a bit of a dilemma. “Obviously, the land on a winery is extremely valuable,” Allen told Stanford Energy Club members who visited the winery. So how can one build a solar array without using much land? Well, one solution would be to put it on water instead. This is how the Far Niente Winery in Napa Valley came to build the first grid-connected floating solar panel array in the world.


To offset the 800,000 kWh per year the winery required, a system with 2296 panels was constructed. Unfortunately, the pond the floating array was built on wasn’t quite large enough to hold all the necessary panels, so 1200 of the 2296 panels were placed on land. Nevertheless, the floating system was impressive yet cleverly simple. 18-inch ribbed pipe filled with Styrofoam formed many pontoons the PVs sat on and four concrete mounts allowed the array to rise and fall up to 18 feet to account for changes in water level, while also keeping the array from rotating or translating. Since the winery’s electricity load is only during a specific time of year, the billing method they arranged was effectively an annual bill instead of a monthly bill. Despite not receiving compensation if the panels produce more energy than Far Niente consumes in a year, their $110,000 annual electricity bill was brought down to $0 and the system will pay itself off in three years.


The floating solar PV array


While the system was operational when we saw it, there were many unforeseen challenges that Allen encountered along the way. “The first day when it was all ready to be turned on, we flipped the switch and BOOM, the inverter blew up,” said Allen. Fortunately, nobody was hurt and it was a manufacturer’s error and not his own wiring, but on day one it was a real disappointment. Allen took what he had learned and improved the situation by installing six inverters to replace the one central inverter. Even before that setback, Allen had to build transmission lines to connect to the central energy meter to avoid “wheeling,” the practice of offsetting one energy meter’s usage by generating at a different meter. Now wheeling is legal, but the unnecessary transmission lines (not to mention a new transformer on his neighbor’s lawn) were already in place.


On the bright side, having a floating solar array also had some benefits. The pond water is used to spray the grapes to prevent freezing, which can hurt the plants. Often this process can use 100,000 gallons per hour, so the water in the pond is valuable. Having solar panels floating on the pond prevents evaporation and growth of algae, keeping that valuable water clean and in the pond. The pond is also a big thermal mass that has less variation compared to air temperatures. The cool temperatures keep the solar panels about five degrees cooler than the panels on land, unless it’s a very windy day which would cause the land panels to be cooler. A cooler solar panel has a higher efficiency, so the panels and pond are both happy.


Image of wine barrels


After leaving the floating array with smiles on our faces, our smiles got a bit bigger when we toured the rest of the winery and got to try some wine. Massive tanks, thousands of smooth, oak barrels, and caves that stretched longer than a football field made the cool internals of the winery feel ancient and filled with tradition. And outside, with the rolling hills of vines and beautiful landscape – even if the wine wasn’t delicious, our palates could not be spoiled. But the wine was delicious; a refreshing chardonnay followed by a cabernet and a dulce that we will never forget. And to top it all off, we got to see the owner’s old car collection. It was the engineer’s dream come true. So at Far Niente, come for the solar panels, stay for the wine… or just come for the wine if that’s your thing.


The gallery of old, classic cars


Charlie is the Editor in Chief of the Stanford Energy Journal. He is pursuing a master's degree at Stanford in Civil and Environmental Engineering with an emphasis on Atmosphere and Energy. In his studies, he is focusing in on the development of renewable energy and stability of the grid through energy storage and optimization of energy systems.