Rooftop solar cells can also be an advantage for water conservation

DURHAM, NC – Rooftop solar cells that generate electricity not only save carbon emissions from global warming, but also save a significant amount of water, say a pair of Duke University researchers who did the calculations.

A given family can save an average of 16,200 liters of water per year by installing solar panels on their roofs, they found. In some states, such as California, this savings can increase to 53,000 gallons, which equates to 60 percent of the average U.S. domestic water consumption.

You won’t see the savings on your home water bill, but they’re still important.

This is because energy consumption is closely linked to water consumption. Power generation in the United States consumes nearly the same amount of water as the agricultural sector. But that figure doesn’t include the additional water used to produce fossil fuels in the first place, nor to manage coal ash waste.

“To generate electricity for the grid, we must mine and burn coal, fractionate and pump natural gas and cool nuclear power plants, all of which result in high volumes of water that is continually lost,” said Avner Vengosh, Duke’s distinguished environmental quality professor. University at the Nicholas School of the Environment and co-author of a new article that will appear on September 19 in Science of the Total Environment.

“However, with the solar cell, it’s a one-time consumption of a much smaller volume of water for manufacturing,” Vengosh said. “And then, once installed, there is no longer any water consumption resulting from that for the next 25 years of intended use.”

Currently, over 70 percent of the world’s solar panels are produced in China, so the consumption of water to generate solar energy occurs overseas.

Co-author Erika Weinthal, professor of environmental policy at Nicholas School, said that in order to understand the broader impacts on water from solar panel manufacturing, it is crucial to examine the entire supply chain around the world.

“From a contamination standpoint, solar cells have a huge potential for environmental damage,” Vengosh said. “It contains heavy metals, some of which are very toxic, and therefore could have an impact on the surrounding environment where production takes place.”

But after that, the solar water consumption is zero.

Previous studies have attempted to evaluate the amount of water used for the different stages of energy production, typically expressed as the volume of water for a given energy such as liter or gallon per gigajoule. In the new study, the authors combined the energy sources that are used to generate electricity for the residential sector in the contiguous United States and translated that into the volume of water consumption in each state.

After evaluating statewide water use for the residential sector, the new study calculated the virtual water use of individual homes in 48 states. These calculations estimate that the total amount of water consumed to power the residential sector in the United States is 2.6 trillion gallons.

Conversion to solar in homes reduces the use of the electricity grid and therefore also the volume of water. In some states, such as the southwestern United States, individual household water savings can reach up to 1000 percent by installing rooftop solar panels.

These water use calculations are a follow up to a recent Vengosh and Weinthal book published earlier this year on the intersection of energy and water quality that provides a detailed baseline for water consumption of various fossil fuel sources.

During the close of the pandemic, the authors had decided to add solar panels to the house they share. While in the beginning the main motivation was to save carbon emissions, after a while they realized that water can also be saved.

“So this article is really a product of the desire to decarbonise our personal life,” Weinthal said. “I teach global environmental policy and I teach the Paris Agreement and I always try to get students to connect what is happening at the interstate level with what we can do with our agency forms.”

Today, photovoltaic solar cells represent about 1.5% of the national electricity supply. This represents a saving of 99 billion gallons of water per year, the authors estimate, which is equivalent to about four days of California’s total water consumption. But as the solar percentage increases, the savings will also increase.

Their paper also compares water consumption for energy by state, as each state uses different energy sources to generate electricity and has different usage patterns and number of homes.

The New England states, for example, show huge payouts for converting to solar because they don’t use much electricity for air conditioning and tend to heat their homes with oil, not electricity, Vengosh said. Arizona and California, states that have water shortages and lots of sunny days, would also be big winners.

In their book, Weinthal and Vengosh showed that the use of water is only part of the puzzle. Contamination of water resources due to coal mining, fracking and disposal of coal ash further reduces the availability of water and, therefore, the water footprint from fossil fuel mining is much larger than we think .

“We also sought to quantify the effects on water quality, such as the amount of water contaminated by an event such as an oil spill or the chronic disposal of coal ash into water resources,” Vengosh said. “So if you know the volume of the oil spill, you can translate it into the volume of water that is contaminated and lost.”

The important thing is to assess the total cost of any technology, Weinthal said. “We need to move towards renewable energy, solar and wind, but in doing so, we need to recognize that for any form of energy infrastructure, even if renewable, there may be costs when it comes to water. We need to think about the water footprint for any form of energy production, “she said.

Researchers have already turned their attention to the use of water and the environmental effects of mining lithium, a key metal for next-generation batteries.

QUOTE: “Reductions in water consumption from the domestic solar installation in the United States”, Avner Vengosh, Erika Weinthal. Total environmental science, 17 September 2022. DOI: 10.1016 / j.scitotenv.2022.158738

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