Operation of Solar and Waste-heat Powered Adsorption Desalination

Introduction Quenching global thirst by adsorption desalination is a practical and inexpensive method of desalinating the saline and brackish water to produce fresh water for agriculture irrigation, industrial and building, and potable applications. The AD cycle consumes the lowest specific energy per unit volume of product water, and we have devised an advanced AD cycle to achieve less than 1.5 kWh/m3, achieved in the recent patent of the authors.. As compared with other desalination methods, the AD cycle has the unique advantages, namely (i) the utilization of renewable (such as solar) or waste heat sources at temperatures below 80C, (ii) low corrosion and fouling rates on the tube materials due to the evaporation of saline water at low temperature (typically below 30C), (iii) it has no major moving parts which renders low maintenance cost and (iv) the adsorbent is silica gel which is made of environmentally available material, i.e., sand. In addition, the adsorption cycle offers two important benefits that are not found to the existing desalination technologies; namely, (i) a two-prong phenomenal barrier to eliminate ‘‘bio-contamination’’ during the water generation process as compared with existing membrane methods, and (ii) the reduction in global warming due to the utilization of low-temperature renewable or waste heat which otherwise would have been purged or dissipated into the atmosphere. The performance of an AD cycle is measured by specific daily water production (SDWP) and the performance ratio (PR). Preliminary tests in our laboratory indicates SDWP ranges from 8 to 28 m3/tonne of silica gel whilst the PR value could be as high as 1.5 with a through-put ratio of 75% of the saline input. , , , , Figure 1. An Overview of Solar Powered Adsorption Desalination (AD) Project: A pilot plant W.G. Chun 1 , K. Chen 2 , K. C. Ng 3 1 Department of Nuclear & Energy Engineering, Jeju National University, Jeju, Republic of Korea 2 Department of Mechanical Engineering, University of Utah, Salt Lake City, Utah, USA 3 Department of Mechanical & Production Engineering, National University of Singapore, Singapore Operation of Solar and Waste-heat Powered Adsorption Desalination Design and Construction of a Pilot Plant Under the CNU-NUS Partnership programme, the team will jointly conduct advanced research in the area of solar-powered adsorption desalination (AD) that produces dual useful effects, using the low temperature thermal energy source at 65 to 80o C from solar collectors. Solar energy is collected using high thermal efficiency collectors of greater 55%, and the AD pilot plant is designed to have a nominal useful effects (i) 2.5 m3 per day of potable water and (ii) a cooling capacity up to 18 kW. Technical Overview A lab-scaled prototype adsorption desalination (AD) plant, that employs low temperature waste heat to operate the AD Cycle, has been successfully tested in NUS. Based on the proprietary patent of authors, it mimics the processes occurring naturally in the ambient except that it is now performed in a single-component environment and operating at a much faster pace. The generation of water from the sea-water is performed using a two-step process, namely evaporation and condensation as shown schematically in Figure 2. The key advantages of the AD over the conventional desalination methods are: • The AD cycle employs low temperature heat sources at 65 to 85C, available in abundance at no cost from either renewable sources such as solar thermal or industrial processes which, otherwise, would have been purged into the ambient; • It has no major moving parts other than the water pumps and valves. This implies low maintenance and operational costs for the AD cycle; • It employs environment-friendly adsorbent + adsorbate pair, namely the silica gel-water, which has been proven to have a lifespan of 1.3 to 1.5 million cycles or about 10 years of operation in the case of an adsorption chiller. Silica gel is one of the most reliable and cost-effective adsorbent available at less than US$6-8 per kg; • The AD cycle incorporates internal heat regeneration and heat recovery during the pre-heating and adsorption processes, enabling the boiling process (1st desalting process) at the evaporator where the boiling temperature is optimized at 25 to 30C. Tube fouling from the saline solution is significantly reduced at these low boiling temperatures. • Heat input to the adsorbent during desorption of vapour is conducted at 85C whilst the desorbed vapour condenses in a water-cooled condenser. Being a heat driven process, the AD cycle has robust features to cope with the changing demands of the volumetric through-put of the potable water or the fluctuations in the quality of saline solution feed to the plant. Figure 2. The schematics of a batch-operated Adsorption Desalination (AD) plant Figure 1. A changeable evaporator module to be installed at the cold end of a water vapor adsorption system Table 1. Energy costs comparison for various methods of desalination Figure 2. Different evaporator modules integrated into a multi-function outlet
Reference
1. K.C. Ng, X.L. Wang, L.Z. Gao, A. Chakraborty, B.B. Saha, S. Koyama, A. Akisawa and T. Kashiwagi, Apparatus and Method for Desalination, Singapore Patent Application No: 200503029-1, filed date: 12th May 2005, PCT No: WO 2006/121414 A1.
2. H.T. Chua, K.C. Ng, A. Malek, T. Kashiwagi, A. Akisawa, and B.B. Saha, Regenerative Adsorption Process and Multi-Reactor Regenerative Adsorption Chiller, USPO No: 6,490,875, 2002.
3. Seawater Desalination in California, Chapter 1: Energy Use Section, published by California Coastal Commission, ( http://www.coastal.ca.gov/index.html ).
4. G. Ehrenman, From Sea to Sink, Mechanical Engineering, Vol. 126, No.10, pp. 38-43, 2004.
N.H. Afgan, M. Darwish and M.G. Carvalho, Sustainability Assessment of Desalination Plants for Water Production, Desalination, Vol. 124, No. 1-3, pp. 19-31, 1999.
Acknowledgements
This work is supported by the grant (No.R33-2008-000-10166-00) of the World Class University (WCU) programme of the Korea Science & Engineering Foundation.
Table 1 compares the energy cost of water production between the AD cycle and the conventional methods, such as the MSF, MED, and RO. From experiments, the initial design of AD cycle gives an energy consumption at 14 kWh per m3 or equivalent to US$0.25 per m3 whilst the highest production cost is associated with the MSF at US$0.647. We are of the opinion that with advanced design of the AD cycle, the specific energy consumption could be further reduced to 1.5 kWh/m3. All data shown in the table is extracted from the report of Seawater Desalination in California, California Coastal Commission, Chapter 1: Energy Use section, http://www.coastal.ca.gov/index.html, [3]. Natural gas is assumed to cost US$5 per mmBTU (adopted from Singapore’s natural gas prices in 2005) whilst the conversion efficiencies of power plants (ηc) and the boilers are 38% and 80%, respectively. A multi-function outlet at the cold-end of a water-vapor adsorption system The most common application of the cooling effect for present water-vapor adsorption systems is the production of chilled water. (The system was first developed in Japan and the large quantity of chilled water was used in brewing) A multi-function outlet design is proposed in this patent application to more effectively and energy-efficiently utilize the cooling effect of the water-vapor adsorption system. The proposed outlet consists of several evaporator modules for different cooling applications such as de-humidification and air conditioning, chilled water production, and refrigeration. Heat exchangers and water injectors of different designs are housed in module cases which are thermally insulated rigid vessels. The designs and sizes of these heat exchangers may be quite different. For air conditioning, fans and fins or other heat transfer enhancement designs and techniques should be applied to the air side since the thermal resistance of the water side is restively low (The water will undergo phase change during evaporation, and the heat transfer rate of convection heat transfer with phase change is typically one- to two- order of magnitude higher than that without phase change). On the contrary for refrigeration applications, phase change takes place on both the water and the refrigerant side. Finned tubes with multiple tube passes are recommend for the evaporator module for refrigeration applications. The evaporator modules can be sold separately, or integrated into a complete outlet unit for automatic control and production of the desired cooling effect(s). The designs of these 2 options are illustrated in Figures. 1 and 2. For the first option the evaporator assembly of the adsorption system must be designed for easy module change. Each evaporator module has 4 adapters for the connections of fluid flows to or from the water tank, the reactor(s), and the vacuum pump as depicted in Figure 1. Early AD research in NUS Solar-powered AD plant in CNU CNU CNU Method of Desalination Thermal energy consumed kWh/m 3 (A) Electric energy kWh/m 3 consumed (B) Primary fuel input kWh/m 3 C= (A/ η b +B/ η c ) Energy cost of water US$ per m 3 =5* Multi-stage Flash (MSF) 19.4 5.2 37.9 0.647 Multi-effect Distillation (MED) 16.4 3.8 30.5 0.520 Vapour C ompression (VC) - 11.1 29.2 0.497 Reverse Osmosis (RO) – single pass - 8.2 21.5 0.366 Reverse Osmosis (RO) – double pass - 9.0 23.7 0.403 Adsorption D esalination Energy from waste heat (< 85  C) 5.6 * Author’s data 14.6 0.25 New adsorption cycle as proposed in this project Energy from waste heat (< 85  C) 1.4-1.8 3.7 – 4.7 0.06 – 0.08

Operation of Solar and Waste-heat Powered Adsorption Desalination

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Operation of Solar and Waste-heat Powered Adsorption Desalination — Document Transcript

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