Low-grade waste heat driven desalination technology

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1 Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 Ó A. Christ et al., Publishe by EDP Sciences, 214 DOI: 1.151/smo/2137 Available online at: ARTICLE OPEN ACCESS Low-grae waste heat riven esalination technology Alexaner Christ 1,2, Xiaolin Wang 3, Klaus Regenauer-Lieb 2, an Hui Tong Chua 1,4,* 1 School of Mechanical an Chemical Engineering, The University of Western Australia, 35 Stirling Hwy, Crawley WA 69, Australia 2 School of Earth an Environment, The University of Western Australia, 35 Stirling Hwy, Crawley WA 69, Australia 3 School of Engineering, The University of Tasmania, Hobart TAS 1, Australia 4 School of Environmental Science an Engineering, Taiyuan University of Technology, Taiyuan, Shanxi Province, P.R. China Receive 27 August 212 / Accepte 13 November 212 / Publishe online 4 February 214 Abstract Low-grae heat riven multi-effect istillation (MED) esalination is a very promising environmentally frienly, low emission technology. Many countries, such as Australia, are water short an conventional esalination technology is energy intensive. If a primary fossil fuel source is use, then esalination will significantly contribute to carbon ioxie emission. Low-grae waste heat from process plants an power plants generate minimal aitional carbon ioxie. This source of energy is typically abunant at a temperature aroun 65 C, which ovetails with MED technology. In this paper, we report on a new MED technology that couples perfectly with low grae waste heat to give at least a 25% freshwater yiel improvement compare with conventional MED esign. Typical applications an their expecte improvement will also be reporte. Key wors: Distillation, MED, Waste heat, Low-grae heat, Sensible heat, Evaporator. Nomenclature BPE Boiling point elevation, C c p Specific heat capacity, kj/kgk L Latent heat, kj/kg _m Mass flow rate, kg/s _Q Heat transfer rate, kw T ;# Temperature, C S Salinity, kg/kg SDM Single effect istillation moule Subscripts evap Evaporator i Effect or conenser input f Fee h Heating meium hy Hyrostatic hea s Saturate o Effect or conenser outlet v Vapour n Number of effect 1 Hotter sie of heat exchanger 2 Coler sie of heat exchanger z Final conenser * huitong.chua@uwa.eu.au 1 Introuction In many countries scarcity of freshwater is a serious issue. Globally this problem is getting worse as populations grow, lifestyles in eveloping countries improve, an climate change is leaing to locally istorte weather patterns. Accoring to the Worl Health Organization almost one fifth of the worl s population is living in areas where water is physically scarce [1]. Even in areas with significant rainfall an freshwater resources, the water eman for househols, agriculture an inustry can be a serious issue, particularly in regions with a high population ensity. Inustrial esalination of seawater or process water is one of the solutions to assist in alleviating this problem. Technically there are several possible ways to turn seawater into freshwater. However, the major technologies currently applie on an inustrial scale can be split in two categories [2, 3]: (1) thermal processes incluing multi-stage flash istillation (MSF), multieffect istillation (MED), thermal vapour compression (TVC) an mechanical vapour compression processes (MVC) an (2) membrane processes incluing reverse osmosis (RO). In general, the overall energy consumption of all the esalination processes is very high, an consequently energetic optimization is one of the key challenges in the fiel of esalination. This can be aresse by two ifferent approaches: (1) optimization of the esalination technology in orer to minimize the overall consumption of energy an This is an Open Access article istribute uner the terms of the Creative Commons Attribution License ( which permits unrestricte use, istribution, an reprouction in any meium, provie the original work is properly cite.

2 2 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 Figure 1. Schematic of a basic multi-effect-istillation system [4]. (2) utilization of energy sources with reuce cost an environmental impact, like renewable energies or waste heat. This paper presents the optimization of the MED technology combining both approaches for low-grae heat applications. In general, thermal riven systems consume more energy than RO, but whereas the latter requires electrical power (which represents energy of a high quality), thermal systems can be operate with a broa range of heat sources. This makes them an optimal choice to utilise low-grae heat, incluing waste heat. However, current MED technologies for waste heat extraction are so far mainly focuse on low pressure steam. In contrast the novel MED esign presente in this paper is particularly tailore for the use of sensible low-grae heat sources. Especially in the range of 65 1 C a vast number of these sources are available, which cannot be optimally use by current technologies. Examples inclue cooling water from inustrial processes, mining sites, power generation, an renewable energy sources like geothermal energy. In most of these cases the energy in this temperature range is currently unuse, rejecte to the environment, even causing aitional expenses for cooling facilities. The propose novel MED esign allows to harvest this promising sources in an economic an environment frienly way. This is achieve by a simulation base, comprehensive optimization approach consiering the iniviual nature of the lowgrae heat source; for example geothermal heat. 2 Working principle of a MED plant A basic MED plant (Figure 1) comprises a set of evaporators, calle effects, an a final conenser. The motive steam or hot water enters the heat exchanger of the first effect as the heating meium. Feewater is applie on the opposite sie of the heat exchanger. As the energy is transferre from the hotter heating meium to the coler fee, the feewater warms up to its boiling temperature an a portion of it evaporates subsequently. The generate vapour passes through emisters an is conense in the secon effect, where the release latent heat of the conensation is utilize to evaporate more feewater. This process is repeate in several effects. To maintain the heat flow, each subsequent effect operates at iminishing temperature an pressure. To conense the vapour generate in the last effect a final conenser coole by seawater is use. By using parts of the heate up cooling water for the fee stream, a portion of the heat is recovere. The remaining portion of cooling water is rejecte back to the sea together with the prouce brine. The generate conensate in each effect consists of boiler grae freshwater an can be use for the esire purpose. Base on the basic MED, several esign variations are possible: for example, thermal vapour compression to increase efficiency, treatment of contaminate water rather than saline water an optimization to minimize waste water flow. 3 Numerical analysis of the MED system The main focus of this article is to show the new esign an its improvement over the conventional systems rather than the simulation moel itself. Therefore only the basics of the moel are presente below. The moel is evelope base on the steay state mass an energy balances couple with the heat transfer equations for each iniviual effect together with the ratio between the mass of feewater to that of prouce freshwater. When only sensible heat is use as the heating meium, the energy balance for the first effect (evaporator) can be expresse as: _Q evap;i ¼ _m h;i c p;h ðt 11i T 11o Þ ¼ _m 1;f c p;f ðt 12o T 12i Þþ _m 1v L 12 ; ð1þ c p;h an c p;f are the constant pressure specific heat capacities of the heating meium an the fee, respectively; L 12 is the latent heat of evaporation, which epens on the boiling temperature an can be calculate by the vapour saturation temperature [5]: L ¼ 2499:5698 2:24864 # vs 1: # vs 2 ; ð2þ

3 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 3 where # vs the vapour saturation temperature in C. The vapour saturation temperature is a function of the pressure in the evaporator vapour space. It is less than the boiling temperature T 12 by the boiling point elevation: T 12 ¼ T vs þ BPE þ T hy ; ð3þ DT hy is the boiling point rise causes by the hyrostatic hea in the effect. This term is negligible in horizontal falling film evaporators. The boiling point elevation BPE reflects the increase in the boiling temperature ue to the salts issolve in the water. It can be calculate base on the following empirical relation [6, 7]: BPE ¼ AS 2 þ BS; A ¼ 4: # 2 þ 2: # þ 17:95; B ¼ 1: # 2 þ 5: # þ 6:56; ð4þ where S is the salinity in kg/kg an the boiling temperature in C. This formula is vali for S.12 kg/kg an 2 C. Since the vapour generate in the first effect acts uring conensation as heat source for the subsequent effect, the energy balance for the nth effect can be expresse as: _Q n;i ¼ _m n 1;v L n;1 ¼ _m n;f c p;f ðt n;1o T n;1i Þþ _m nv L n2 : ð5þ It must be mentione that in real processes aitional effects lea to a further reuce vapour temperature besies the boiling point elevation. These inclue non-equilibrium effects, temperature losses, as well as pressure relate rops cause by the emister an friction losses. However, these are typically very small an their effect on the vapour temperature rop can be neglecte for a simple moel. Consequently the input temperature of the vapour can be assume as the boiling temperature from the vapour outlet of the upstream effect. By far the largest amount of the initial input energy is carrie over in the form of latent heat from effect to effect. Only a small amount of sensible heat is carrie out with the brine an freshwater streams leaving each effect. Therefore the effects o owngrae the energy, but consume only a small portion. Consequently the prouction in each subsequent effect is only slightly reuce. On the other han, this requires a rejection of the major part of energy in the final conenser by cooling with seawater. The energy balance for the final conenser can be calculate: _Q z¼n;i ¼ _m n 1;v L z;1 ¼ _m z;2i c pf ðt z;2o T z;2i Þ; ð6þ whereas _m z;2i is the cooling water stream an T z,2i the cooling water input temperature, which is in typical applications the sea water temperature. Since a portion of the cooling water output stream is use as fee, the cooling water output temperature T z,2o etermines the fee water temperatures for each effect (T n,2i ). In an iealize system, T n,2i = T z,2o can be assume. Typically, the major optimization aim is to maximize freshwater prouction from a given initial heat input. This can be expresse as: X Z 1 n¼1 _m n;v ¼ max! ð7þ Fresh Water Prouction [kg/s] Bounary conitions are forme by the laws of thermoynamics, with aitional technical, economic, an operational constraints. These inclue for example minimal temperature ifferences in the heat exchanger an environmental compliance. The moel is calibrate against the plant performance ata of a commercially available Alfa Laval single-effect istillation unit [8]. Also a moifie simulation version incorporating the new esign presente below has been evelope. 4 Conventional MED systems Input Temperature Heat Source[ C] In most of the current applications, the fuel costs to operate the system are proportional to the energy input, _Q evap;i. These cost can be of a irect or inirect nature, for example costs for the fuel to heat the system irectly or prouction losses of electrical energy from power plants ue to the extraction of low pressure steam from the Rankine cycle. Because of the high energy eman, the associate fuel costs are major rivers for the costs of esalination, in particular in the backgroun of increasing energy costs. Consequently, the major optimization aim is traitionally minimal energy consumption per unit freshwater. Figures 2 an 3 isplay the results of the simulation software base on an example stanar MED, in this case riven by a liqui heat source. For comparison reasons the heat input is kept constant. The significant impact of the number of effects on the freshwater generation is shown in Figure 2 an consequently the impact on the specific energy consumption is shown in Figure 3. This is ue to the fact that most of the energy is carrie over via the latent heat of the vapour, whereas only a small amount is actually rejecte from the process by the brine Number of Effects Figure 2. Depenency of the freshwater prouction on the heat source input temperature an the number of effects, shown for an example stanar MED.

4 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 Specific Energy Consumption [kwh th /m 3 freshwater] 3 3 2 2 1 1 an freshwater stream.

4 4 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 Specific Energy Consumption [kwh th /m 3 freshwater] an freshwater stream. As it can also be shown, the heat source temperature itself has only minor influence on the prouction of at a given number of effects, provie the heat input is constant. Nonetheless the temperature profile of the heat source is a significant parameter of the system esign since it etermines the top en of the available temperature interval an is therefore of material importance for the maximal possible number of place effects. The lower en of the temperature interval is efine by the cooling water temperature, usually the seawater temperature. The interval can be split by the number of effects, whereas the minimal temperature ifference over each effect is primarily limite by economic consierations to C [9, 1]. Most current MEDs are steam riven, where the isothermal nature of the conensation allows the brine temperature in the hottest effect, the so-calle top brine temperature, to approach the heat source temperature. A ifferent case applies if only sensible heat is use an the heating meium unergoes a temperature change: restricte by the secon law of thermoynamic, the top brine temperature is epening on this temperature graient (pinch-point-analysis). As a result, any increase of the applie temperature ifference of the heating meium is reucing the available temperature range to place effects, an therefore the efficiency of the system. In this context it must be mentione that in MED systems without aitional pretreatment like nanofiltration (NF) the top brine temperature is limite by scaling issues to 65 C [11]. 5 Novel MED esign Input Temperature Heat Source [ C] For true waste heat sources an most renewable energies the irect relationship between the energy input _Q evap;i an fuel costs oes not apply. This inclues all situations where the Number of Effects Figure 3. Depenency of the specific energy consumption per m 3 freshwater on the heat source input temperature an the number of effects, shown for an example stanar MED. energy costs are not a function of the actual amount of energy use. A typical example is geothermal energy: costs come from the well construction, the energy eman of the prouction pumps, etc. to make the hot grounwater available at the surface, but these costs are totally inepenent from the energy that is finally extracte from that water. The same applies for several waste heat applications: the energy content of hot water from 65 to 1 C is too low for most thermoynamic processes. It has therefore to be rejecte unuse to the environment, causing aitional expenses for cooling facilities. In consequence, this low-grae heat can frequently be available at neutral cost. As a result, the efficiency oes not solely epen on the minimal energy requirement per unit freshwater anymore, rather the capability to extract as much energy as possible. Hence a paraigm shift is require. Instea of an optimization of the esalination unit alone, the entire low grae heat system with its iniviual characteristics must be consiere. Instea of the minimal energy consumption of the esalination unit, the key objective is to maximize freshwater yiel from the available low grae heat source. Figure 4 shows the novel MED esign incorporating this optimization approach. The hot water is first introuce into the first effect heat exchanger as the riving source for the primary MED plant. The hot water leaving the heat exchanger is still very hot. Therefore this water is pumpe into a single effect istillation moule (SDM) where a portion of vapour is generate. This generate vapour is juiciously fe into an appropriate effect within the primary MED plant. Obviously, the size an seawater fee rate of the following effects have to be increase accoringly to cater for the boost in capacity. It is obvious that this process substantially boosts the freshwater yiel an results in significantly increase exploitation of the low grae heat source. Alternative options consisting of two inepenent conventional MED in series coul reach to a similar result but woul be far more expensive. 6 Expecte improvements To etermine the expecte improvements for particular process conitions, the simulation moel is use. Figures 5 an 6 epict the improvement of the propose esign applie respectively to a four- an a six-effect istillation plant at ifferent heat source an cooling water temperatures. This reflects typical operating ranges for commercially available multi-effect istillation plants at a heat source temperature between 75 an C. The corresponing operating conitions are liste in Table 1. As it can be shown, the propose esign improves the freshwater yiel by 25 6% epening on the heat source an cooling water temperatures. It is observe from the figures that the improvement using the SDM esign ecreases with the heat source temperature. With higher heat source temperature, a higher temperature rop in the first effect of the primary plant can be realise. Relate to the total energy input, the proportion of recovere energy is therefore lower than that for a lower temperature heat source an hence the improvement to freshwater yiel iminishes.

5 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 5 Figure 4. Schematic of the avance multi-effect-istillation with single-effect-moule booster [4]. Freshwater prouction [m 3 /ay] Heat source temperature [ C] Projecte boost of freshwater yiel (%) Figure 5. The efficacy of the novel MED esign applie to a foureffect istillation plant. s, freshwater yiel in a stanar four-effect istillation plant at a cooling water temperature of 25 C; h, freshwater yiel of a stanar four-effect istillation plant at a cooling water temperature of 32 C. e, freshwater yiel an improvement by using our propose esign at 25 C;, freshwater yiel an improvement by using the propose esign at 32 C [4]. Freshwater prouction [m 3 /ay] Heat source temperature [ C] Figure 6. The efficacy of the propose scheme as applie to a sixeffect istillation plant. s, freshwater yiel in a stanar four-effect istillation plant at a cooling water temperature of 25 C; h, freshwater yiel of a stanar four-effect istillation plant at a cooling water temperature of 32 C. e, freshwater yiel an improvement by using our propose esign at 25 C;, freshwater yiel an improvement by using our propose esign at 32 C [4] Projecte boost of freshwater yiel (%) For comparison reasons, the influence of the cooling water temperature is also shown. It is notable that for a fixe number of effects the boost ecreases with increasing cooling water temperature. At a lower cooling water temperature, the whole MED plant can operate at a relatively lower temperature in each of the effects an hence the vapour generate in the single seawater istillation moule can be fe into a higher effect, thereby generating more freshwater through the various effects ownstream. For example when the cooling water temperature is 32 C, the vapour generate in the SDM can only be fe into the last effect of a four effect istillation plant an the fourth effect of a six effect istillation plant. However when the cooling water temperature is 25 C, the whole MED plant operates at a relatively lower temperature in each effect. The vapour generate in the SDM can now be fe into the thir effect of the four effect istillation plant an the thir effect of the six effect

6 6 A. Christ et al.: Int. J. Simul. Multisci. Des. Optim. 214, 5, A2 Table 1. The operating conitions for the four an six effect istillation plants. T 11i T 11o _Q evap;i T sm,i T sm,o _Q sm;i m_ h C C kw C C kw kg/s MED without SDM MED with SDM Cooling temperature, T z,i 25 C/32 C Salt concentration 35, ppm istillation plant. The recovere energy is then use to generate more freshwater. Therefore a greater improvement exists at lower cooling temperatures. 7 Possible applications Possible applications of the novel esign inclue all cases where sensible (waste) heat is available. Economical operation can typically be achieve from a heat source input temperatures of ~ 65 C or greater. This inclues, but is not limite to: However, the final improvement is in particular a function of the proportion of the energy input in the basic MED versus the input in the SDM part. Typical applications commonly allow a minimal improvement of at least 25%, with values ranging up to 65% or more. Further simulation results are publishe in [11]. Acknowlegements. We gratefully acknowlege the financial support from the Western Australian Geothermal Centre of Excellence an the National Centre of Excellence in Desalination Australia, which is fune by the Australian Government through the National Urban Water an Desalination Plan. Geothermal systems; Waste heat from power generation; Waste heat from inustrial processes; Waste heat from oil an mining sites; Inustrial launries. It is anticipate that some existing MED can be retrofitte with the new system. This applies to small an misize units in particular with plate type heat exchangers, where the shell esign allows an increase in heat exchanger surface for the effects after the booster injection. 8 Conclusion An avance esign paraigm for geothermal an waste heat riven MED plants is propose. Combine with true waste heat sources, an improvement of the freshwater yiel by 25% to 6% over stanar MED can be expecte. This is achieve by a cross isciplinary approach also consiering the iniviual nature of the heat source. The improve process allows a cost effective, environmental frienly esalination with a minimal carbon footprint by the use of broaly available sensible waste heat in the range of ~65 1 C. In general, the actual expecte improvement over a stanar MED is very case epenent. For an optimal result, the specific nature of the present heat source must be taken into account. References 1. Worl Health Organization facts about water scarcity El Saliby I. 29. Desalination plants in Australia, review an facts. Desalination, 247(1-3), Miller JE. 23. Review of water resources an esalination technologies. Sania National Laboratories: Albuquerque. 4. Wang X Low grae heat riven multi-effect istillation technology. International Journal of Heat an Mass Transfer, 54(25-26), El-Dessouky H Steay-state analysis of the multiple effect evaporation esalination process. Chemical Engineering & Technology, 21(5), Bromley LA Thermoynamic properties of sea salt solutions. AIChE Journal, 2(2), Sharqawy MH, John VJH, Zubair SM Thermophysical properties of seawater: a review of existing correlations an ata. Desalination an Water Treatment, 44(1-3), Alfa Laval Marine & Diesel Prouct Catalogue. 23. Desalt JWP-16-C Series. Lun, Sween: Alfa Laval. 9. Ophir A, Lokiec F. 25. Avance MED process for most economical sea water esalination. Desalination, 182(1-3), Awerbuch L Unerstaning of thermal istillation processes. IDA Acaemy: Singapore. 11. Awerbuch L Unerstaning of hybri systems an ual purpose power esalination. IDA Acaemy: Singapore. Cite this article as: Christ A, Wang X, Regenauer-Lieb K & Chua H: Low-grae waste heat riven esalination technology. Int. J. Simul. Multisci. Des. Optim., 214, 5, A2.

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