Desalination System
Cuts Energy Costs


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More than a billion people lack access to safe drinking water worldwide. Many of them live in low-income villages that cannot support large, expensive desalination plants.  Humidification-dehumidification (HDH) technology would be ideal for these small-scale situations except for two major drawbacks: high thermal energy consumption of 350-550 kWh/m3 and heat transfer rates (HTRs) that are much lower than pure vapor systems. HDH technology is also expensive—about $30/m3 of pure water produced.

To make HDH systems more efficient and affordable, a research team led by MIT mechanical engineering professor John. H. Lienhard V and post-doc student Prakash Narayan, in collaboration with researchers at King Fahd University of Petroleum and Minerals (KFUPM) in Saudi Arabia, has developed an innovative HDH system that dramatically reduces energy costs by up to 80 percent.

Diagram of the humidification-dehumidification system. Image: MIT

 

“We became interested in HDH at the start of our collaboration with KFUPM as a means of providing water to off-grid regions in the developing world,” says Lienhard. “Both the MIT and the KFUPM faculty wanted to develop a technology that might benefit people all over the world.”

This meant designing an HDH solution that was scalable, cost-effective, and easy to implement.

Innovative Engineering

Lienhard’s team accomplished this goal by developing thermal design algorithms for thermodynamic balancing via mass extractions and injections and inventing a bubble column dehumidifier for high-heat and mass-transfer rates in the presence of noncondensable gas.

This clean water system could lead to improved desalination plants for developing countries. Image: David Castro-Olmedo/MIT

 

 

“We used the concepts of thermodynamic balancing developed for heat-and-mass-exchanger (HME) devices and applied them to HDH system design,” says Lienhard. “Detailed algorithms for systems with zero, single, and infinite extractions were developed. Temperature enthalpy diagrams were used to model the systems.”

The bubble column heat (and mass) exchanger substantially improved the heat transfer rate by condensing the vapor-gas mixture in a column of cold liquid, rather than on a cold surface. This reduced the heat-transfer area requirement to a fraction of that for existing HDH systems and brought it very close to pure vapor levels.

To define a thermally balanced state in HME devices, the team defined a nondimensional parameter known as the modified heat capacity rate ratio—the ratio of the maximum change in the total enthalpy rate of the cold stream to that of the hot stream. This parameter enabled designs that minimized the imbalance in local driving temperature and concentration differences.

Thus, by introducing thermal balancing and bubble column dehumidifiers, HDH system performance was improved substantially, making these systems affordable for small-scale applications (< $5/m3).

Future Possibilities

Two innovations the team developed could have major impacts in other fields: thermodynamic balancing of driving forces in simultaneous heat and mass exchange devices and the multi-stage bubble column.

“Until now there has been no clear algorithm to design HME devices for minimal entropy generation,” says Lienhard. “The definition of the modified heat capacity rate ratio enables such design. The idea of using a bubble column for dehumidification in itself is novel and has potential for widespread implications for designing AC and dehumidification equipment. Multi-staging bubble reactors will increase their efficiency to counterflow levels. This can also be applied to any parallel flow heat exchanger.”

MIT’s new HDH system is also well-suited for treating the high volume of “produced” brackish water that comes to the surface during deep oil-and-gas operations. This water, often tapped from a mile or more below the surface, carries high concentrations of salts and minerals and must be decontaminated before it can be reused or discharged into waterways.

The MIT team built a 12-foot-high test unit that has treated barrels of saline water from natural gas wells, producing water that is clean enough to drink. The energy consumption of pure vapor evaporators that currently treat high-salinity water in oil wells and other applications is  about 625 kWh_th/m^3; the MIT system reduces energy consumption to about 125 kWh_th/m^3.

Lienhard hopes their results will make the world a better place.

“When people quote that a billion people lack access to safe drinking water, what they don’t realize is that most of these people live in the developing world and in some community which has a population of around 10,000,” he says. “Small-scale and modular water treatment systems like HDH can fit this niche and help solve the global water problem.”

Mark Crawford is an independent writer.

By introducing thermal balancing and bubble column dehumidifiers, humidification-dehumidification system performance can be improved substantially, making these systems affordable for small-scale applications.

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March 2013

by Mark Crawford, ASME.org