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PROJECT TITLE:

Experimental Study of Humidification-Dehumidification (HDH) Seawater Desalination Driven by Solar Energy


PRINCIPAL INVESTIGATOR:

Dr. Qu Weilin, Mechanical Engineering, University of Hawaii at Manoa (UH)

SPONSOR:

USGS

PROJECT PERIOD:

03/01/08 - 02/28/09

ABSTRACT:

A relatively new desalination process called humidification-dehumidification (HDH) was proposed as a feasible solution to the freshwater shortage problem in Hawaii. HDH operates under near ambient pressure and low temperature and can be driven by renewable solar energy as well as a variety of low-grade (low-temperature) thermal energy sources, including low pressure condensing steam from a steam power plant, waste heat from a combustion engine, and waste heat from an oil refinery. While energy sources such as electricity and high-grade hydrocarbon-based thermal energy could be expensive because of Hawaiis geographical location, there are abundant solar radiation and low-grade thermal energy sources around the islands. Therefore, HDH water desalination driven by renewable solar energy or low-grade waste heat may constitute a viable future supplemental water source to Hawaii that is both cost-effective and energy-efficient. In the research project, theoretical modeling approach was used to develop a fundamental understanding of the various thermal/fluid transport processes in small-scale HDH water desalination systems driven by solar energy.

Statement of Regional or State Water Problem:

The state of Hawaii has witnessed unprecedented economy development and population growth in the past decades. Modern air travel made the beautiful Hawaiian islands easily accessible to people all around the world, and millions vacation in Hawaii every year. The state's recent effort to diversify its economy also led to a booming high-tech industry. While the development is necessary to move the state forward, such a rapid growth starts to put strains on the islands' limited resources. The state is increasingly facing the challenge of maintaining sustainable development under the constraints of essential resources such as land, energy, and freshwater. The project addresses one of the most critical issues of freshwater supply.

As an island state surrounded by the Pacific Ocean, Hawaii has an abundance of seawater. On the other hand, Hawaii's freshwater sources are far from unlimited. In Hawaii, the principal freshwater supply has been the ground water in basal aquifers. Freshwater floats atop of seawater in the form of a freshwater lens. The two zones are separated by a transition zone that contains brackish water. The salinity of water in the three zones can be measured by its total dissolved solids (TDS) concentration. Freshwater has a TDS concentration of less than 500 mg/L, while seawater 25,000 mg/L or more. Brackish water's TDS concentration is usually between 1000 and 10,000 mg/L, and is unsuitable for direct domestic consumption.

As a result of the past economy and population growth, freshwater demand in Hawaii is gradually approaching the sustainable ground freshwater yield of the basal aquifers. Overpumping of ground freshwater will lead to an expansion of the transition zone and cause more freshwater to become brackish water. It is therefore of paramount importance to identify and develop alternative freshwater sources to supplement the existing sources in order to avoid future shortage of freshwater supply that will adversely affect the state's development. Among many different options, seawater desalination seems the most viable.

Desalination is a process in which dissolved minerals are removed from seawater, and has a long history of being an effect means to provide freshwater to people living in the coastal areas. Desalination Processes that are technologically mature and widely used include multistage effect distillation (MED), multistage flash distillation (MSF), and reverse osmosis (RO). MED and MSF are based on liquid-vapor phase-change principle, where seawater is first evaporated to water vapor at either atmospheric pressure (MED) or reduced pressure (MSF) and the vapor is later condensed to freshwater. RO, on the other hand, is based on membrane technology. Seawater is forced to flow through a membrane by a high-pressure pump. The membrane only allows freshwater to pass while filtering out the dissolved minerals.

The main drawback of the aforementioned MED, MSF and RO processes is that they are very energy intensive. Although the three processes are considered the most cost-effective in regions such as the Middle East where energy supply is abundant, they are not suitable to Hawaii because of the high energy price in the state. The main source of energy supply is the raw oil shipped to the state from the Southeast Asia.

The project provides a feasible solution to the freshwater shortage problem in Hawaii by studying a relatively new desalination process called humidification-dehumidification (HDH). Attributes of the HDH seawater desalination process make it very attractive to Hawaii.

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