Entropy Generation Analysis of Desalination Technologies

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies. Entropy generation analysis, and specifically, Second Law efficiency, is an important tool for illustrating the influence of irreversibilities within a system on the required energy input. When defining Second Law efficiency, the useful exergy output of the system must be properly defined. For desalination systems, this is the minimum least work of separation required to extract a unit of water from a feed stream of a given salinity. In order to evaluate the Second Law efficiency, entropy generation mechanisms present in a wide range of desalination processes are analyzed. In particular, entropy generated in the run down to equilibrium of discharge streams must be considered. Physical models are applied to estimate the magnitude of entropy generation by component and individual processes. These formulations are applied to calculate the total entropy generation in several desalination systems including multiple effect distillation, multistage flash, membrane distillation, mechanical vapor compression, reverse osmosis, and humidification-dehumidification. Within each technology, the relative importance of each source of entropy generation is discussed in order to determine which should be the target of entropy generation minimization. As given here, the correct application of Second Law efficiency shows which systems operate closest to the reversible limit and helps to indicate which systems have the greatest potential for improvement.King Fahd University of Petroleum and MineralsCenter for Clean Water and Clean Energy at MI

HELIUM AS A CARRIER GAS IN HUMIDIFICATION DEHUMIDIFICATION DESALINATION SYSTEMS HME Heat and Mass Exchanger MED Multi-effect Distillation MSF Multi-stage Flash RO Reverse Osmosis

ABSTRACT A promising technology for small scale seawater desalination is the humidification dehumidification (HDH) system. This technology has been widely investigated in recent years. Since existing HDH systems have very high specific energy consumption, the authors have previously invented several ways to increase the energy efficiency of these systems. Even for these relatively higher efficiency systems the dehumidifier is expected to be large, owing to the large thermal resistance associated with the presence of non-condensable carrier gas (air) in the system. In this manuscript, we demonstrate that changing the carrier gas from air to helium a potential solution to this problem. In addition, the energy performance of a brine heated HDH system using helium relative to those using air is analysed in detail through well established on-design models for the components in the system. Symbols c p specific heat capacity at constant pressure (J/kg·K) D h hydraulic diameter (m) h specific enthalpy (J/kg) h cgm specific enthalpy of moist carrier gas (J/kg of dry carrier gas) h fg specific enthalpy of vaporization (J/kg) HCR modified heat capacity rate ratio (-) Ja Jakob number (-) k thermal conductivity (W/m.K) M molar mass (kg/kmol) m r mass flow rate ratio (-) m mass flow rate (kg/s) Nu Nusselt number (-) p partial pressure (Pa) P absolute pressure (Pa) ∆P pressure drop (Pa) Q heat transfer rate (W) Re Reynolds number (-) RR recovery ratio (%) s specific entropy (J/kg·K) 1 NOMENCLATURE Acronyms GOR Gained Outpu

Optimal concentration and temperatures of solar thermal power plants

Using simple, finite-time, thermodynamic models of solar thermal power plants, the existence of an optimal solar receiver temperature has previously been demonstrated in literature. Scant attention has been paid, however, to the presence of an optimal level of solar concentration at which the conversion of incident sunlight to electricity (solar-to-electric efficiency) is maximized. This paper addresses that gap. The paper evaluates the impact, on the design of Rankine-cycle solar-trough and solar-tower power plants, of the existence of an optimal receiver temperature and an optimal level of solar concentration. Mathematical descriptions are derived describing the solar-to-electric efficiency of an idealized solar thermal plant in terms of its receiver temperature, ambient temperature, the receiver irradiance (radiation striking unit receiver area), solar receiver surface to working fluid conductance, condenser conductance, solar collector efficiency, convective loss coefficients and radiative loss coefficients. Using values from the literature appropriate to direct-steam and molten-salt plants, curves of optimal solar receiver temperature, and optimal solar-to-electric conversion efficiency, are generated as a function of receiver irradiance. The analysis shows that, as the thermal resistance of the solar receiver and condenser increases, the optimal receiver temperature increases whilst the optimal receiver irradiance decreases. The optimal level of receiver irradiance, for solar thermal plants employing a service fluid of molten salts, is found to occur within a range of values achievable using current solar tower technologies. The tradeoffs (in terms of solar-to-electric efficiency) involved in using molten salts rather than direct steam in the case of solar towers and solar troughs are investigated. The optimal receiver temperatures calculated with the model suggest the use of sub-critical Rankine cycles for solar trough plants, but super-critical Rankine cycles for solar tower plants, if the objective is to maximize solar-to-electric efficiencyOther funderUS Department of State for funding through the Science and Technology PhD programDeposited by bulk importkpw7/11/1

The benefits of hybridising electrodialysis with reverse osmosis

A cost analysis reveals that hybridisation of electrodialysis with reverse osmosis is only justified if the cost of water from the reverse osmosis unit is less than 40% of that from a stand-alone electrodialysis system. In such cases the additional reverse osmosis costs justify the electrodialysis cost savings brought about by shifting salt removal to higher salinity, where current densities are higher and equipment costs lower. Furthermore, the analysis suggests that a simple hybrid configuration is more cost effective than a recirculated hybrid, a simple hybrid being one where the reverse osmosis concentrate is fed to the electrodialysis stack and the products from both units are blended, and a recirculated being one hybrid involving recirculation of the electrodialysis product back to the reverse osmosis unit. The underlying rationale is that simple hybridisation shifts salt removal away from the lowest salinity zone of operation, where salt removal is most expensive. Further shifts in the salinity at which salt is removed, brought about by recirculation, do not justify the associated increased costs of reverse osmosis.Hugh Hampton Young Memorial FellowshipCenter for Clean Water and Clean Energy at MIT and KFUPM (Project R15-CW-11