The search for ‘potable water’ to quench global thirst remains a pressing concern throughout many regions of the world including arid and semi-arid regions. Although most of the Earth’s surface is covered by oceans, the challenge of providing the world’s inhabitants with fresh or potable water seems to be insurmountable. According to the WHO report, at least one billion people do not have access to clean and fresh water. On the other hand, as individual incomes increase, it is likely that cooling demand will increase proportionally. From the environmental and sustainability perspectives, it is a requirement to develop innovative thermodynamic cycles which produce cooling and potable water at a lower specific energy input. This project introduces adsorption cycles powered by solar energy or waste heat at temperatures range 60º C to 85º C for the production of both desalination and cooling effects. An artist’s impression of the large scale, four-bed adsorption desalination cum cooling (ADC) plant is shown in Figure 1.

The ADC plant comprises an evaporator, a condenser and two or more adsorber/desorber heat exchangers. The adsorption cycle utilises adsorbent-adsorbate processes to produce fresh water at the condenser and cooling effect in the evaporator. These effects are achieved by the amalgamation of ‘adsorption triggered evaporation’ and ‘desorption resulted condensation’. When the unsaturated adsorbent (silica gel) is exposed to vapour in the evaporator, the uptake is quickened by the high affinity of the water molecules to the pores of the silica gel. Similarly, when the same adsorbent is heated thermally, the water vapor molecules are expelled to the cooler surfaces of the condenser tubes, producing high grade potable water while the heat of condensation is removed by the cooling water.

This ADC process offers the lowest specific energy consumption ever reported for a desalination plant. As no additional fossil fuel is burned and only waste heat sources between 60º C and 85º C are used, the cycle mitigates global warming. The advantages of the ADC cycles are; (i) The adsorption cycle has no major moving parts rendering low maintenance, (ii) It utilises an environmentally benign working pair such as silica gel and water, (iii) The rates of corrosion and fouling of the evaporator tubes are low, as the saline solution evaporates below 30º C, (iv) The heat-driven desorption process thwarts any ‘bio-contamination’ that is known to plague existing membrane pores, and (v) The cycle helps to reduce global warming with the re-utilisation of waste heat.

Figure 1. An artist’s impression of a large-scale adsorption desalination cum cooling system.

The life cycle cost of an ADC is the lowest, about US$0.478 per m³, as compared with published data for other desalination methods, as shown in Figure 2. This is a 30-year life cycle calculation where both the capital and operational costs are accounted for. The specific energy consumption for the production of potable water of ADC is 1.38 kWh/m³- about twice that of the thermodynamic limit of 0.73 kWh/m³. Owing to the robust desalting process in the evaporator, the performance (total useful effects to heat input) and the recovery (amount of distilled water from unit volume of seawater input) ratios for ADC plant are 1.2 and 0.75, respectively. In the ADC plant, the energy cost contributes to about 40% of the total cost. Figure 3 shows the production capacities of cooling and water at two evaporator inlet temperatures: With 10^o C for the chilled water, the cooling is suitable for air-conditioning or dehumidification whilst the higher evaporator temperature is suitable for process cooling and higher water production. The ADC cycles have been successfully commercialized through business licensing and Table 1 highlights some of the plants that have been commissioned and installed around the world.

ADC plant configure for latent cooling application	ADC plant optimized for desalination

Country	Function type	Plant size
Singapore	Desalination cum cooling	1.NUS (experiment prototype 5 Rtons) 2.Pfizer Singapore (10 Rton) 3.Tampanies Grande (2 units of 45 Rton)
Saudi Arabia	Desalination cum cooling	KAUST (Lab prototype, advanced cycle- 5 Rton)
Korea	Desalination	Jeju National University under the WCU programme (Lab prototype (2 Rton)
Poland	Cooling (tapping low temperature waste heat at 65°C to 70°C)	Wroclaw Science park building – project by NET (2 units of 25 Rton under negotiation)

Table 1. AD plants commissioned around the world

The research is funded by the King Abdullah University of Science and Technology (KAUST) of Saudi Arabia and a business licence has been issued to Aik Moh Paints & Chemical Pte Ltd, Singapore, by the Industry Liaison Office of National University of Singapore. Several large commercial and experimental units of ADC plants have been designed for applications in Singapore, South Korea, Saudi Arabia, and Poland. The pictorial view of the ADC experimental prototype in NUS and the team members are shown in Figure 3.

 

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About the IES Prestigious Engineering Achievement Awards (PEAA)

The IES Prestigious Engineering Achievement Awards (PEAA) are given annually to an organization or persons responsible for an outstanding engineering project in Singapore. The PEAA aims to recognize the outstanding achievements of the engineers who have attained engineering achievements which demonstrate outstanding engineering skills and have made a significant contribution to the engineering progress and the quality of life in Singapore.

The most outstanding PEAA project will be awarded the ASEAN Outstanding Engineering Achievement Award during the annual Conference of the ASEAN Federation of Engineering Organisations (CAFEO).

The criterion for the PEAA is as follows:

1. Contribution to the well being of people and communities;
2. Resourcefulness in planning and in the solution of design problems,
3. Pioneering in use of materials and methods;
4. Innovations in planning, design and construction; and
5. Unusual aspects and aesthetic values.