An improved model for multiple effect distillation

Lienhard

2013

102

118

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies driven by different combinations of heat, work, and chemical energy. This paper develops a consistent basis for comparing the energy consumption of such technologies using Second Law efficiency. The Second Law efficiency for a chemical separation process is defined in terms of the useful exergy output, which is the minimum least work of separation required to extract a unit of product from a feed stream of a given composition. For a desalination process, this is the minimum least work of separation for producing one kilogram of product water from feed of a given salinity. While definitions in terms of work and heat input have been proposed before, this work generalizes the Second Law efficiency to allow for systems that operate on a combination of energy inputs, including fuel. The generalized equation is then evaluated through a parametric study considering work input, heat inputs at various temperatures, and various chemical fuel inputs. Further, since most modern, large-scale desalination plants operate in cogeneration schemes, a methodology for correctly evaluating Second Law efficiency for the desalination plant based on primary energy inputs is demonstrated. It is shown that, from a strictly energetic point of view and based on currently available technology, cogeneration using electricity to power a reverse osmosis system is energetically superior to thermal systems such as multiple effect distillation and multistage flash distillation, despite the very low grade heat input normally applied in those systems.

Section: Desalination Powered By Heatmentioning

confidence: 99%

“…• C [44,45,50]. MSF systems typically have performance ratios from about 6 to 9 and operate with steam around 100…”

Section: Desalination Powered By Heatmentioning

confidence: 99%

Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes

Lienhard

2013

102

118

“…A detailed numerical model for multiple effect distillation (MED) was created by Mistry et al [10], based upon more basic models in [9,23,24]. A schematic diagram of a typical forward feed MED system is shown in Figure 9.…”

Section: Multiple Effect Distillationmentioning

confidence: 99%

“…Properties are evaluated using IAPWS 1995 Formulation [26]. Unlike many MED models in the literature, the model by Mistry et al [10] relies on the simultaneous equation solver, EES.…”

Section: Multiple Effect Distillationmentioning

confidence: 99%

Entropy Generation of Desalination Powered by Variable Temperature Waste Heat

Warsinger

Nayar

et al. 2015

Powering desalination by waste heat is often proposed to mitigate energy consumption and environmental impact; however, thorough technology comparisons are lacking in the literature. This work numerically models the efficiency of six representative desalination technologies powered by waste heat at 50, 70, 90, and 120• C, where applicable. Entropy generation and Second Law efficiency analysis are applied for the systems and their components. The technologies considered are thermal desalination by multistage flash (MSF), multiple effect distillation (MED), multistage vacuum membrane distillation (MSVMD), humidification-dehumidification (HDH), and organic Rankine cycles (ORCs) paired with mechanical technologies of reverse osmosis (RO) and mechanical vapor compression (MVC). The most efficient technology was RO, followed by MED. Performances among MSF, MSVMD, and MVC were similar but the relative performance varied with waste heat temperature or system size. Entropy generation in thermal technologies increases at lower waste heat temperatures largely in the feed or brine portions of the various heat exchangers used. This occurs largely because lower temperatures reduce recovery, increasing the relative flow rates of feed and brine. However, HDH (without extractions) had the reverse trend, only being competitive at lower temperatures. For the mechanical technologies, the energy efficiency only varies with temperature because of the significant losses from the ORC.Entropy 2015, 17 7531

“…Both are distillation methods in which the overall energy requirements are reduced through the use of energy recovery in each stage or effect of the system [21]. Cost information for representative 100,000 m 3 /d MSF and MED plants is provided by [22][23][24][25][26].…”

Section: Multistage Flash and Multiple Effect Distillationmentioning

confidence: 99%

An Economics-Based Second Law Efficiency

Lienhard

2013

Second Law efficiency is a useful parameter for characterizing the energy requirements of a system in relation to the limits of performance prescribed by the Laws of Thermodynamics. However, since energy costs typically represent less than 50% of the overall cost of product for many large-scale plants (and, in particular, for desalination plants), it is useful to have a parameter that can characterize both energetic and economic effects. In this paper, an economics-based Second Law efficiency is defined by analogy to the exergetic Second Law efficiency and is applied to several desalination systems. It is defined as the ratio of the minimum cost of producing a product divided by the actual cost of production. The minimum cost of producing the product is equal to the cost of the primary source of energy times the minimum amount of energy required, as governed by the Second Law. The analogy is used to show that thermodynamic irreversibilities can be assigned costs and compared directly to non-energetic costs, such as capital expenses, labor and other operating costs. The economics-based Second Law efficiency identifies costly sources of irreversibility and places these irreversibilities in context with the overall system costs. These principles are illustrated through three case studies. First, a simple analysis of multistage flash and multiple effect distillation systems is performed using available data. Second, a complete energetic and economic model of a reverse osmosis plant is developed to show how economic costs are influenced by energetics. Third, a complete energetic and economic model of a solar powered direct contact membrane distillation system is developed to illustrate the true costs associated with so-called free energy sources.Keywords: Second Law efficiency; irreversibilities; economics; desalination; cogeneration; least work of separation; cost