Energy and exergy analysis of a novel multiple-effect vapor chamber distillation system for high-salinity wastewater treatment

A novel modular thermally-driven multiple-effect vapor chamber distillation (MVCD) system is presented for compact and portable desalination applications. The MVCD system consists of several vapor chambers connected in series with the condenser section of the upstream vapor chambers serving as the evaporator section of the following effect. A heat transfer model accounting for the major thermal resistances was developed to predict the heat transfer and distilled water production rates. A mass transfer analysis was performed to evaluate the effect of the accumulation of the non-condensable gasses within the chambers. An exergy analysis was also conducted to quantify the efficiency of the system from the viewpoint of the second law of thermodynamics. It was found that for a fixed number of effects, increasing the hot-end temperature increased the distillation rate and decreased the second law efficiency. On the other hand, increasing the number of effects at a fixed hot-end temperature resulted in increased distillation rate and second law efficiency. The increased salinity of the feed water resulted in smaller distillation rates and greater second law efficiency. For all the cases, it was found that sensible heat recovery from the discharging fluids could improve the gained output ratio (GOR) and the second law efficiency by about 10%. Quantitatively, at a hot-end temperature of 70°C, feed water salinity of 35 ppt and recovery ratio of 36%, the MVCD system with six effects and energy recovery from the discharging fluids yielded a GOR of 5.0 and a second law efficiency of 3.8%

Integration of Phase Change Material-Based Storage in Air Distribution Systems to Increase Building Power Flexibility

This paper presents a novel energy storage solution by incorporating phase change material (PCM) in the building supply-air duct to increase a building’s thermal storage capacity. This solution has various advantages compared to PCM-integrated walls including more effective heat transfer (forced convection and greater temperature differentials). During off-peak hours, the system runs at a supply-air temperature below the material’s solidification point to charge the PCM with cooling energy. During on-peak hours, a higher supply-air temperature is utilized so that the stored energy can be discharged into the supply-air. This shifts a portion of the building’s cooling load from the on-peak hours to the off-peak hours. A numerical model for the melting and solidification of PCM in the duct was developed and modified using experimental data. Whole building energy simulations were conducted by coupling the PCM model with EnergyPlus DOE prototypical building model in a Simulink co-simulation platform. Simulations were performed for three cities in different climate zones over a three-month cooling season (June to August), and the PCM storage reduced the on-peak energy consumption by 20-25%. The electricity cost and payback period were determined using current time-of-use electricity rates

Heat Transfer and Thermodynamic Analysis of Heat Pipe-Assisted Latent Heat Thermal Energy Storage Systems for Concentrating Solar Power Applications

Thermal energy storage (TES) is the key advantage of concentrating solar power (CSP) systems. Among various types of TES, latent heat thermal energy storage (LHTES) benefits from large energy density and virtually isothermal operation that can potentially reduce the cost of TES systems significantly. However, most of the phase change materials (PCMs) used in LHTES systems have a prohibitively low thermal conductivity. This work presents a novel approach to substantially improve the heat transfer rates of a LHTES system by incorporating heat pipes (HPs) and/or thermosyphons (TSs) to circumvent the large thermal resistances of the PCMs. The basic design of a HP-assisted LHTES module is presented and a thermal network modeling approach is developed for system level analysis of the thermal response of the module. The model is further extended to predict the thermal performance of a large-scale LHTES system. An exergy analysis is also presented to investigate the second-law efficiency of the LHTES systems and guidelines are provided for the design of LHTES systems to achieve maximum second-law performance accounting for the practical constraints imposed on the operation of solar LHTES systems. The performance of HPs and TSs as the heart of the proposed technology is also studied in detail to ensure that the HPs/TSs are capable of providing the assigned heat transfer load. Numerical analysis of LHTESs is necessary for fully understanding the complex physical phenomena involved, including the solid-liquid and liquid-vapor phase changes and hydrodynamics of the HPs/TSs. To this end, a full numerical simulation of a HP-assisted LHTES for dish-Stirling applications is presented. Conjugate heat transfer effects in a HP-PCM system are analyzed and the effects of HP spacing on the heat transfer response are investigated. The benefits offered by the integration of LHTES with CSP systems, such as damping the temporal variations of solar radiation and shifting the power generation to times of higher demand, are proved

Integral Solution of Two-Region Solid–Liquid Phase Change in Annular Geometries and Application to Phase Change Materials–Air Heat Exchangers

A mathematical model based on the integral method is developed to solve the problem of conduction-controlled solid–liquid phase change in annular geometries with temperature gradients in both phases. The inner and outer boundaries of the annulus were subject to convective, constant temperature or adiabatic boundary conditions. The developed model was validated by comparison with control volume-based computational results using the temperature-transforming phase change model, and an excellent agreement was achieved. The model was used to conduct parametric studies on the effect of annuli geometry, thermophysical properties of the phase change materials (PCM), and thermal boundary conditions on the dynamics of phase change. For an initially liquid PCM, it was found that increasing the radii ratio increased the total solidification time. Also, increasing the Biot number at the cooled (heated) boundary and Stefan number of the solid (liquid) PCM, decreased (increased) the solidification time and resulted in a greater (smaller) solid volume fraction at steady state. The application of the developed method was demonstrated by design and analysis of a PCM–air heat exchanger for HVAC systems. The model can also be easily employed for design and optimization of annular PCM systems for all associated applications in a fraction of time needed for computational simulations