Abstract:World-wide water scarcity, especially in the developing world, indicates a pressing need to develop inexpensive, decentralized small-scale desalination technologies which use renewable resources of energy. This paper provides a comprehensive review of the stateof-the-art in one of the most promising of these technologies, solar-driven humidification-dehumidification (HDH) desalination. Previous studies have investigated many different variations on the HDH cycle. In this paper, performance parameters which ena… Show more
“…A water heater between the humidifier and dehumidifier provides the heat input to the system. This particular embodiment of HDH is known as a closed air, open water (CAOW) cycle; there are several others that have been studied in detail [1,7], but will not be discussed further here.…”
The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.
“…A water heater between the humidifier and dehumidifier provides the heat input to the system. This particular embodiment of HDH is known as a closed air, open water (CAOW) cycle; there are several others that have been studied in detail [1,7], but will not be discussed further here.…”
The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vapor/noncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.
“…From the literature cited above and a previous literature review [1], we have found no study which systematically attempts to improve and/or optimize the cycle performance by modifying the cycle itself. Hence, the objective of this paper is to analyze the thermodynamic performance of various HDH cycles in literature and to propose novel high-performance variations on those cycles.…”
Humidification dehumidification desalination (HDH) is a promising technology for small-scale water production applications. There are several embodiments of this technology which have been investigated by researchers around the world. However, from a previous literature review [1], we have found no study which carried out a detailed thermodynamic analysis in order to improve and/or optimize the system performance. In this paper, we analyze the thermodynamic performance of various HDH cycles by way of a theoretical cycle analysis. In addition, we propose novel high-performance variations on those cycles. These high-performance cycles include multi-extraction, multi-pressure and thermal vapor compression cycles. It is predicted that the systems based on these novel cycles will have gained-output ratio in excess of 7 and will outperform existing HDH systems.
“…Solar thermal technologies, i.e., the conversion of the sunlight to thermal energy, are being developed for many applications such as power generation, domestic water heating, desalination, and other industrial processes. [1][2][3][4][5][6][7] Steam and vapor generation is often desired in these applications, but the dilute solar flux (1000 W/m 2 ) does not provide enough power per unit area of the absorber to reach the required high temperatures and to compensate for the large latent heat of water vaporization. Optical…”
Abstract:Harvesting solar energy as heat has many applications, such as power generation, residential water heating, desalination, distillation and wastewater treatment. However, the solar flux is diffuse, and often requires optical concentration, a costly component, to generate high temperatures needed for some of these applications. Here we demonstrate a floating solar receiver capable of generating 100°C steam under ambient air conditions without optical concentration. The high temperatures are achieved by using thermal concentration and heat localization, which reduce the convective, conductive, and radiative heat losses. This demonstration of a low-cost and scalable solar vapor generator holds the promise of significantly expanding the application domain and reducing the cost of solar thermal systems.
Keywords: solar thermal, steam generation, thermal concentration, water treatmentThe sun is a promising and abundant source of renewable energy that can potentially solve many of society's challenges. Solar thermal technologies, i.e., the conversion of the sunlight to thermal energy, are being developed for many applications such as power generation, domestic water heating, desalination, and other industrial processes. [1][2][3][4][5][6][7] Steam and vapor generation is often desired in these applications, but the dilute solar flux (1000 W/m 2 ) does not provide enough power per unit area of the absorber to reach the required high temperatures and to compensate for the large latent heat of water vaporization. Optical
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