Elsevier

Desalination

Volume 249, Issue 2, 15 December 2009, Pages 602-608
Desalination

Humidification–dehumidification desalination system driven by geothermal energy

https://doi.org/10.1016/j.desal.2008.12.053Get rights and content

Abstract

The work presented in this paper focuses on desalinating sea water system using a humidification–dehumidification process as it is supplied with water heated by geothermal energy as clean and renewable natural resources of energy. Computer simulation of the behavior under various working conditions of the desalination system was carried out to predict the variations of key output. Such variables include the ratio of sea water mass flow rate related to air mass flow rate, cooling water temperature difference across the condenser, geothermal source inlet temperatures to the heat exchanger and the amount of produced distilled water. To validate the computer program, a comparison between the experimental and theoretical results was conducted, and a good agreement had been obtained. The result showed that, the optimum value of the ratio between sea water mass flow rate to air mass flow rate was found to be in the range of 1.5 to 2.5. Improvement in the fresh water productivity at the optimum ratio of sea water mass flow rate to the air flow rate was observed by increasing both the geothermal source inlet temperature and the cooling water temperature difference across the condenser.

Introduction

Earth is a water-rich planet, which is fortunate because water is key to man's progress. It is essential for agricultural and industrial growth and is required to support growing urban populations.

Most of the countries in the Middle East are arid, and they are facing great challenges due to limited water resources and aridity. Desalination seems to be the most suitable solution. Desalination of sea water and brackish water has been given high priority as a source of water for domestic, industrial and agricultural applications in this region [1]. Standard desalination techniques such as multi-stage flash, multi-effect, vapor compression and reverse osmosis are expensive technologies especially when driven by conventional energy sources. Additionally, the geothermal energy is environmentally advantageous energy source which produces far less air pollution than fossil-fuel sources. The life of a geothermal resource may be prolonged by re-injecting the waste fluid which is the most common method of disposal.

Geothermal energy as one of the renewable energy resources represents an alternative source of energy, which provide many advantages compared to conventional energy produced from fossil fuel [2]. Most desalination thermal process is performed at relatively low temperature. Consequently low enthalpy geothermal energy is gradually emerging as successful renewable energy source of producing fresh water with great advantages [3], [4].

Akpinara and Hepbasli [5] studied the exergetic performance of the geothermal heat pump systems installed in Turkey based on the actual operation data. They found that the geothermal thermal application is suitable for the developing countries as an available source of energy. This is due to their higher energy utilization efficiencies than those of both conventional heating and cooling systems.

Manologloua et al. [6] presented the socio-economic impacts of a geothermal desalination plant on the island of Milos (Greece), which suffers from lack of water.

Several comparative studies on the different renewable energy sources that are used for desalination of brackish and sea water are published [7], [8], [9]. Tzen and Morris [7] and Rodriguez [8] found that geothermal energy is suitable for different desalination processes at reasonable cost wherever a proper geothermal source is available. One of the main advantages is that no energy storage is required.

Many workers have studied the humidification–dehumidification desalination system (HDDS) for different low temperature energy sources (solar, geothermal, PV systems, etc.). Bourouni et al. [10] presented and analyzed the operation and performance of different HDDS plants worldwide. They recommended that HDDS installations can be used for the low temperature part of classical distillers, this is to avoid effects of vacuum in which distillers have to function.

A new sea water solar HDDS was described by Chafik [11]. Nafey et al. [12], [13] investigated theoretically and experimentally the solar HDDS system. The results showed that the productivity of the unit is strongly influenced by the air flow rate, cooling water flow rate and total solar energy incident through the day. Recently, Orfi et al. [14], [15] proved theoretically that the daily production of fresh water by solar HDDS system depends on the ratio between the salt water and the air mass flow rates.

More recently, Yamal and Solmus [16] designed a theoretical model to simulate the solar HDDS system based on the idea of closed water and open air cycles. They found that the system productivity increased by about 8%. Four configurations are analyzed for the air humidification–dehumidification water desalination system by Eettouney [17]. Al-Enezi et al. [18] experimentally evaluated the desalination process characteristics as a function of the flow rate of the water and air streams, the temperature of the water stream and the temperature of the cooling water.

The previous studies of the HDDS focused on the use of solar energy as renewable source to drive the desalinating sea water system. This is because energy cost is one of the most important elements in determining the cost of water production from desalination plants.

The present work concerns with the use of the geothermal energy, at low enthalpy, as a heat source for the HDDS. Also included are design guide lines for the proposed desalinating sea water system.

Section snippets

Geothermal desalination system

The selection of the appropriate renewable energy/desalination technology depends on a number of factors. These include, plant size, feed water salinity, remoteness, availability of grid electricity, technical infrastructure and the type and potential of the local renewable energy resource. Among the several possible combinations of desalination and renewable energy technologies, some seem to be more promising in terms of techno-economic feasibility than others. However, their applicability

Experimental apparatus and measurement techniques

The main objective of the experimental test facility is to validate the different simulated heat and mass transfer processes in the simulation model. The humidification–dehumidification desalination system with geothermal heat source test facility is shown in Fig. 1.

The system operates at atmospheric pressure in which air is used as a carrier for vapor. The hot water is injected to the top of the humidifier tower equipped with packed bed to increase the contact surface and therefore improve the

Theoretical model description

The mathematical model was developed according to the energy and mass balance equations for each process in the cycle. It mainly includes heat exchanger energy balance, humidification process, dehumidification process and the system boundary conditions. The system is assumed to be adiabatic (no heat losses). The system is also working under the atmospheric pressure. It is assumed to be working under steady state conditions. The sea water properties are assumed to be equal to the fresh water

Simulation model and its validation

A computer simulation program based on the energy and mass balance equations mentioned above has been developed by means of VISUAL BASIC programming language. The simulation program is then used to investigate the different effects of the system working conditions and the temperature of the geothermal source on the productivity of the system. In this simulation program, energy and mass balance equations and boundary conditions are solved simultaneously using the analytical and iteration methods.

Results and discussion

Various theoretical works have been carried out to assess the performance of utilizing the geothermal energy for the purpose of sea water desalination. In the present work the effect of the different working parameters on the productivity of the considered desalinating sea water system has been investigated. The design of the plant is shown in Fig. 7. The air is heated in an air-preheater using the geothermal energy source after passing the heat exchanger. The sea water at inlet to plant is

Conclusions

The present wok was carried out to evaluate the performance of sea water desalination system powered by the geothermal energy as a renewable energy that is available with a great level worldwide. The following can be concluded:

  • The utility of the low enthalpy geothermal energy that is available in the temperature range up to 100 °C, is found to be a good candidate to drive sea water desalination system to convert sea water to fresh water.

  • The fresh water productivity increases by increasing the

References (20)

There are more references available in the full text version of this article.

Cited by (0)

1

Tel.: +966 55 3856918; fax: +966 6 3802992.

View full text