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HONG KONG — Daryl Ng was taking a long shower, as he does every morning. And his wife, as she does every morning, scolded him for needlessly using so much water.

“How can I justify the amount of time I’m taking?” Ng recalled thinking to himself. And while looking at the water running down the drain, he got an idea.

Why not harness that wasted water running down the drain? With Hong Kong’s high concentration of skyscrapers, could gravity generate a considerable amount of electricity?

Fortunately, Ng is the grandson of the founder of Sino Land, a Hong Kong property developer, and one of the company’s executive directors. So he set up a prototype system in Olympian City, one of the company’s shopping malls in Hong Kong.

The system uses excess pressure in the water system to spin a Swiss-built turbine, which generates electricity to power the lights in a backroom. Ng is also installing turbines in the company’s newest apartment development, a multi-tower project up to 48 stories tall called The Avenue in Hong Kong’s Wan Chai neighborhood, that should be able to power the lighting in entrances, elevator lobbies and other public areas of the building.

There are many reasons the novel idea might not work. Small-scale systems cannot easily generate enough power to justify their cost to large developers. The price per kilowatt-hour of generating power can be five times as high as simply buying it from the grid. And factors like simple geography — will water be flowing far enough down? — can derail plans for turbines built into municipal water infrastructure.

Nonetheless, the developer has pitched the concept to the governments of Beijing, Hong Kong and Singapore. He said the feedback from Hong Kong in particular had been very positive, with the city’s new director of water supplies, Enoch Lam, expressing interest.

Hong Kong is one of the most densely populated cities in the world, with about 23,000 people per square mile in its most crowded district. About 40 percent of the city’s territory is devoted to parkland, and Hong Kong crowds people into office towers and residential apartment blocks that often top 50 stories.

“All these buildings are energy-sucking monsters,” said Claude Touikan, a Hong Kong-based director at the architecture and engineering firm Benoy, which is not involved with Ng’s project. “Of course there is potential.”

But putting hydroelectric systems into new buildings is expensive. Ng has hired Arup, the engineering and project management company, to design a “plug and play” turbine system that would be relatively cheap and easy to fit into a building’s infrastructure. He has also asked the company to find a way to charge his electric car, a Renault Fluence, using the water in his office building while he is at work.

Hong Kong’s water services department said it was pleased to see private developers taking the initiative. After five years of work, the department has also installed a turbine generator at the city’s water-treatment works in the Tuen Mun neighborhood that started operation in July and should generate 1.5 gigawatt-hours of electricity per year. That would save 10 percent of the plant’s electricity bill and cut carbon-dioxide emissions by 2,000 tons per year, the department said. It is working on installing a second generator in the same plant by 2015. The city’s drainage services department is also exploring the system.

Arup estimates that turbines in the public-water system in Hong Kong could power 6,000 households, or a population of about 24,000, which both the company and Ng concede is a tiny fraction of a city of 7 million. While Ng is developing systems that could serve his buildings, he said he would be willing to turn over the technology “at cost, or potentially for free” to the government as well.

“I don’t care if people copy this idea if it can help the planet,” Ng said. “We have to think outside the box so we can maintain our modern consumer lifestyle.”

This type of hydropower is used on a small scale in places like Mühlau, Austria, where the drinking water network provides 34 gigawatt-hours of electricity a year, and Rino di Sonico, Italy, where a reservoir network generates 14 gigawatt-hours a year, or enough to power 3,100 households. But in-building systems are rare.

“Today we don’t have a product that is commercially available, so someone has to take ownership and understand the product,” said Ravi Krishnaswamy, the vice president of the energy and environment practice for Asia at Frost & Sullivan, the market-research company. “You can power a few light bulbs. The challenge is going to be commercial viability and the scale.”

Experts say it makes no sense to install generators in pipes if they create more resistance than is offset by the amount of power they create. Arup’s engineers said turbines could be installed in water pipes to harness the water falling from reservoirs to water treatment sites for city-size systems, as well as in gravity-driven wastewater systems. This would regulate pressure and flow as well as generate electricity.

In skyscrapers, the water in pipes is put under high pressure to ensure consistent supply throughout the structure. The high pressure on the lower floors is released with valves, providing an opportunity to harness otherwise wasted energy by attaching a generator. For now, the amount of power generated would be nowhere close to the building’s total energy consumption.

“In buildings, it’s quite straightforward — as long as it’s tall it will be viable,” said Vincent Cheng, the director of building sustainability at Arup. In Hong Kong, the biggest advantages come in “our buildings, which are tall.”

In Beijing, many of the reservoirs are much higher than the city, meaning they have more potential for providing energy, Cheng added.

Cheng, who leads a team of about 100 people in Asia working on building sustainability for Arup, said his company could develop an easily replicable building-level system using water power that would be easy to deploy. The concept simply needs more time for testing and development, he said.

“If it proves to be reliable in generating electricity and there’s no disruption in operating the building, the cost of this technology will be reduced, and it will help to make it popular,” Cheng said. “If we can test it for two to three years and it is reliable, the market will catch up very soon.”