After seeing images of the melting Greenland Icecap on television this weekend my mother asked me if it would be economical to collect that water and deliver it to drought stricken regions. After my posting on bottled water (AskPablo: Exotic Bottled Water) many of you know that I am generally not a fan of shipping drinking water over great distances. But could this be a more environmentally friendly solution than desalination (AskPablo: Desalination and the Water-Energy Relationship)? Read on to find out…
Both sea and land-based ice are melting at a staggering rate. This week it was announced that the fabled Northwest Passage is ice-free for the first time in recorded maritime history, a situation which will sadly become more common over the next 40 years. In fact the North Pole will be ice-free during the summer months by 2050. While the melting of sea-ice does not contribute to sea-level rise, the melting of land-based ice does. Currently, sea levels are rising at about 3.5 cm per decade, almost double the rate during the previous decade. We can assume that this rate will continue to accelerate as we reach various tipping points. If we stopped all anthropogenic emissions today, the stability of the climate would continue to weaken for at least another 20 years before it begins to recover.
Since the excess melt-water draining into the ocean dilutes its salinity and has potentially negative effects on the North Atlantic thermohaline circulation it would not pose any environmental risk to divert some of this water. Since single-hull supertankers are being replaced by more robust double-hull ships there is a surplus of ships that would be otherwise destined for salvage that could be retrofit for a new life as water tankers.
In a previous article we learned that desalination requires around 4,000 kWh/Acre-foot, or 3.24 kWh per 1000 liters. If the electricity for the desalination plant comes from a coal-fired power plant the emissions per MWh are around 2000 lbs (AskPablo: Coal Fired Power Plants), or 0.89 kg/kWh. This means that every 1000 liters of desalinated water result in 2.9 kg of CO2 emissions.
Since the emissions from shipping Greenland water are dependent on distance we can figure out the distance at which it makes more sense to desalinate. To determine the shipping emissions I turn to a 2006 report by CE Delft, entitled “Greenhouse Gas Emissions for Shipping and Implementation Guidance for the Marine Fuel Sulphur Directive.” This document provides emissions factors for various types of vessels. The emissions factor for a supertanker is 8.0 g of CO2 per ton-nautical mile (taking into account the empty return-trip), or 4.32 g of CO2 per ton-km. Since one ton of water is 1000 liter we know that shipping emissions are 4.32 g per km for each 1000 liters.
So the break-even point is where 2.9 kg is equal to 4.32 g multiplied by the shipping distance. The result is 671 km, which would barely get you to Iceland or Northern Canada. So sorry mom, even with a highly efficient supertanker it is more efficient to desalinate seawater.