Thermodynamic optimisation of multi effect distillation driven by sensible heat sources Alexander Christ a,b , Klaus Regenauer-Lieb b,c , Hui Tong Chua a, ⁎ a School of Mechanical and Chemical Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia b School of Earth and Environment, The University of Western Australia, 35 Stirling Hwy, Crawley WA 6009, Australia c CSIRO Exploration and Mining, P.O. Box 1130, Bentley, WA 6102, Australia HIGHLIGHTS • We couple multi effect distillation systems with sensible low-grade heat sources. • Gain output ratio and performance ratio are incongruent with sensible heat sources. • We posit a novel waste heat performance ratio (PR WH ). • Using PR WH raises freshwater yield by 40% for multi effect distillation systems. abstract article info Article history: Received 13 March 2013 Received in revised form 6 December 2013 Accepted 11 December 2013 Available online 30 January 2014 Keywords: Distillation MED Waste heat Low-grade heat Evaporator While the multi effect distillation desalination industry has made definite strides in terms of improving efficiency, the potential of low-grade heat sources and renewable energies remains overlooked. Although the basic principle remains the same, the nature of sensible heat sources requires different optimisation approaches than conven- tional steam driven systems. We hold that the conventional performance measure is incongruent with such ap- plications and posit a new benchmark. We demonstrate this with the optimisation of multi effect distillation systems, so that freshwater yield can be improved by up to 40% for both conventional and advanced systems. This modus operandi significantly stretches the viability of multi effect distillation, and rejuvenates the potential of waste heat streams. © 2013 Elsevier B.V. All rights reserved. 1. Introduction By rendering the vast sources of saline water accessible, desalination is a key component toward a sustainable water supply for the ever in- creasing global population. Heretofore over 16,000 desalination plants have been commissioned globally, providing an online capacity of over 74.8 million m 3 /day [1] or effectively over 10 l of freshwater per human/day worldwide [2]. Despite being a remarkable amount, this is just a fraction of the overall water consumptions and desalination still has a lot to contribute. One major reason is the energy intensive nature of desalination and the heavy reliance on electricity or fossil fuels. This not only makes de- salination exclusive to relatively affluent countries, but also contributes much to environmental impacts [3,4]. Consequently, heightening desalination efficiency as well as exploiting hitherto untapped, preferably renewable and sustainable, energy sources remain a holy grail. A repertoire of desalination technol- ogies has since been developed, with new technologies still emerging [5]. There are two major categories of separation processes used in desa- lination, namely (1) processes based on a phase change of the feed liq- uid, and (2) processes without a phase change. The first category mimics the natural water cycle, where pristine water vapour is formed during the evaporation. This category includes, among others, multi ef- fect distillation (MED), multi stage flash distillation (MSF), thermal va- pour compression (TVC), mechanical vapour compression (MVC), and membrane distillation (MD). The second category includes desalination processes achieved by other separation methods, with the diffusion based reverse osmosis (RO) being the most dominant. While phase change based methods are exhibiting generally higher energy requirements, their main advantage is that they embrace a broad range of thermal energy, particularly low-grade heat. Electricity is only used sparingly as mainly pumping power. This is strategic when we consider waste heat applications (e.g. [6–8]) and renewable energies [9–12], such as geothermal energy [13–15], at temperatures below 100 °C. Low-grade heat is a parasitic Desalination 336 (2014) 160–167 ⁎ Corresponding author. E-mail address: huitong.chua@uwa.edu.au (H.T. Chua). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.12.006 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal