Desalination

At the moment the UK doesn't spend energy on water desalination. But there's talk of creating desalination plants in London. What's the energy cost of turning salt water into drinking water? The least energy-intensive method is reverse osmosis. Take a membrane that lets through only water, put salt water on one side of it, and pressurize the salt water. Water reluctantly oozes through the membrane, producing purer water - reluctantly, because pure water separated from salt has low entropy, and nature prefers high entropy states where everything is mixed up. We must pay high-grade energy to achieve unmixing.

The Island of Jersey has a desalination plant that can produce 6000 m3 of pure water per day (figure 15.10). Including the pumps for bringing the water up from the sea and through a series of filters, the whole plant uses a power of 2 MW. That's an energy cost of 8 kWh per m3 of water produced. At a cost of 8 kWh per m3, a daily water consumption of 160 litres would require 1.3 kWh per day.

Water delivery and removal: 0.4 kWh/d

Figure 15.9. Water delivery: 0.3 kWh/d; sewage processing: 0.1 kWh/d.

Figure 15.10. Part of the reverse-osmosis facility at Jersey Water's desalination plant. The pump in the foreground, right, has a power of 355 kW and shoves seawater at a pressure of 65 bar into 39 spiral-wound membranes in the banks of blue horizontal tubes, left, delivering 1500 m3 per day of clean water. The clean water from this facility has a total energy cost of 8kWh per m3.

Figure 15.10. Part of the reverse-osmosis facility at Jersey Water's desalination plant. The pump in the foreground, right, has a power of 355 kW and shoves seawater at a pressure of 65 bar into 39 spiral-wound membranes in the banks of blue horizontal tubes, left, delivering 1500 m3 per day of clean water. The clean water from this facility has a total energy cost of 8kWh per m3.

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