A relatively new desalination process called humidification-dehumidification (HDH) is 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 proposed work, theoretical modeling approach will be 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 Hawaii islands easily accessible to people all around the world, and millions vacationed 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 proposed work will address one of the most critical issues of freshwater supply.

As an island state surrounded by the Pacific Ocean, Hawaii has 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 proposed project will attempt to provide a feasible solution to the freshwater shortage problem in Hawaii by studying a relatively new desalination process called humidification-dehumidification (HDH). As will be discussed later in the proposal, attributes of the HDH seawater desalination process make it very attractive to Hawaii.

Statement of Results or Benefits

Humidification-dehumidification (HDH) is a seawater desalination process that is based on the principle of heat and mass transfer as well as liquid-vapor phase-change. It is a process in which water vapor first diffuses from seawater into dry air through evaporation in a diffusion tower, thus humidifying the air (humidification). The water vapor is then carried away by the air to a condenser where water vapor is condensed out from the humidified air through condensation to produce freshwater (dehumidification).

Established conventional desalination processes of MED, MSF, and RO are very energy intensive and not suitable to remote islands like Hawaii. In addition, conventional processes often operate under extreme temperature or pressure conditions, i.e., MED requires high temperature, MSF low pressure (vacuum condition), and PO high pressure. HDH, on the other hand, operates under near ambient pressure and low temperature (<90 °C). Because of the low temperature requirement, HDH 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. The solar energy and low-grade thermal energy sources are often otherwise wasted by simply rejecting into the ambience. While energy sources such as electricity and high-grade hydrocarbon-based thermal energy could be expensive because of our 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.

The proposed project will focus on small scale decentralized HDH water desalination systems driven by solar energy that are intended to meet the freshwater need of multiple families or a small community. Figure 1 shows a simplified schematic diagram of the proposed DHD water desalination system. The system is composed of three main fluid circulation subsystems denoted as seawater, air/vapor, and freshwater. In the seawater subsystem, seawater is circulated using a seawater pump (a). Solar radiation heats the seawater in a seawater solar heater (b). The heated seawater is then sprayed into the top of a diffusion tower (c) and is in direct contact with dry air. A portion of the seawater evaporates and thus humidifies the air. The purpose of heating the seawater using solar energy before it enters the diffusion tower is to increase fresh water production rate. As both the rate of water evaporation and the humidity ratio of the exiting air increase with increasing temperature, higher seawater temperature leads to greater fresh water production. The brine not evaporated in the diffusion tower will be collected at the bottom of the diffusion tower and discharged into the seawater reservoir (d).

In the air/vapor subsystem, cold dry air is pumped into the bottom of the diffusion tower (c). While flowing upward, the air is heated and humidified by the seawater though an evaporation process. The warm and humid air leaving the diffusion tower is drawn into a direct contact condenser (e), where it is in direct contact with the cold freshwater. As a result of a heat and mass transfer process, the air temperature is reduced, and the water vapor is condensed out from the air into the freshwater. The low humidity air is directed back to the diffusion tower and used repeatedly.