How solar power can transform urine into fertilizer

We drain urine away daily without even thinking about it. But what if that waste could be used to produce food, provide power, and provide cleaner sanitation? Engineers have created a new system to do just that – and it’s powered by sunlight.

Fertilizers are costly in much of the world, electricity is unreliable, and clean water is scarce. At the same time, most wastewater goes untreated, leading to severe health and environmental issues.

The new model provides a workable solution, converting a steady stream of waste into something valuable: power and fertilizer, all while treating wastewater to make it safer.

This system uses solar panels to recover nitrogen from urine – without needing a power grid, chemical plants, or fancy equipment.

From urine to crop nutrients

At the heart of the system is a clever trick: pulling ammonia – a form of nitrogen – out of urine and turning it into ammonium sulfate, a type of fertilizer. It does this with a series of chambers and specialized membranes. Solar energy drives ions through the system, separating and capturing ammonia in the process.

Solar panels do more than generate electricity. They also generate heat – typically wasted – that this system captures and uses.

Copper tubing draws heat from the back of the panels and uses it to warm the system. That heat accelerates the process of separation, allowing the gas to become fertilizer more quickly.

There’s an added benefit: when solar panels are cool, they actually generate more electricity. By capturing the heat and putting it to work, the system not only functions at a higher speed – it also produces more energy.

The power of nitrogen in urine

“This project is about turning a waste problem into a resource opportunity,” said William Tarpeh, assistant professor of chemical engineering at the Stanford School of Engineering.

“With this system, we’re capturing nutrients that would otherwise be flushed away or cause environmental damage and turning them into something valuable – fertilizer for crops.”

Why does that matter? Because nitrogen is essential for growing food. It’s the main ingredient in most fertilizers. But producing nitrogen fertilizers the traditional way uses a lot of fossil fuels. It’s also centralized in wealthier countries, making it expensive and harder to access in lower-income areas.

Producing fertilizer where it’s needed

Globally, the nitrogen in human urine equals about 14 percent of annual fertilizer demand. That’s a massive, mostly untapped resource.

“Each person produces enough nitrogen in their urine to fertilize a garden, but much of the world is reliant on expensive imported fertilizers instead,” said Orisa Coombs, the study’s lead author and a Ph.D. student in mechanical engineering.

“You don’t need a giant chemical plant or even a wall socket. With enough sunshine, you can produce fertilizer right where it’s needed, and potentially even store or sell excess electricity.”

Better results with solar heat

In testing, the updated prototype showed significant improvements over earlier designs. By combining solar power and waste heat, power generation increased by nearly 60 percent. Ammonia recovery also became more efficient – up by more than 20 percent.

That’s because about 80 percent of the energy from the sun that strikes solar panels typically gets wasted as heat. This system harnesses that heat, preventing panels from overheating and turning what was once a drawback into part of the solution.

The scientists also constructed a simulation to observe how the system would behave in varrying conditions – changing sunlight, temperature, and electrical configurations.

In places like Uganda, where fertilizer is costly and energy is limited, the system could generate up to $4.13 per kilogram of nitrogen recovered. That’s more than twice the potential earnings in the U.S.

The next step is to build a larger prototype with three times the capacity. It will process more urine, produce fertilizer more quickly, and operate more effectively under strong sunlight.

Clean energy, cleaner water

This technology could also be a major step forward for sanitation. Right now, more than 80 percent of the world’s wastewater isn’t treated. That means nitrogen and other waste ends up in rivers, lakes, and groundwater, harming the environment and public health.

By removing nitrogen from urine before it enters the water system, the device makes what’s left behind safer to discharge or even reuse for things like irrigation.

And it can do that without needing to be plugged in – perfect for communities without access to sewage systems or reliable power.

“We often think of water, food, and energy as completely separate systems, but this is one of those rare cases where engineering innovation can help solve multiple problems at once,” said Coombs. “It’s clean, it’s scalable, and it’s literally powered by the sun.”

The full study was published in the journal Nature Water.

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