Energy and exergy analyses of a solar desalination plant for Karachi Pakistan

Thermoelectric (TE) generation is becoming a valuable and promising research direction. Many researchers have carried out system analysis and performance optimization of thermoelectric technologies based on the generalized thermoelectric energy balance equations. However, it is assumed that TE legs have no heat exchange with the ambient except at the junctions of the hot and cold ends where heat flows in and out. Based on basic thermoelectric effects and fundamental theories of heat transfer, a detailed derivation of the revised generalized thermoelectric energy equations considering convective heat transfer between TE legs and the ambient has been carried out. Irreversible heat transfer processes have been analyzed by employing energy analysis based on the first law of thermodynamics and exergy analysis based on the second law of thermodynamics. The results show that convective heat transfer leads to a decrease in both energy and exergy efficiencies: the rate and magnitude of the decrease in exergy efficiency are greater than those of the decrease in energy efficiency. The exergy efficiency is relatively high despite the low energy efficiency in operation, revealing the features and advantages of thermoelectric generators (TEGs) in low‐grade energy utilization. For TEG efficient operation, the load resistance value should match the system's internal resistance, or at least be greater than that, to avoid a sharp drop in power output and efficiencies. In an attempt at theoretical analysis, the concept of entransy was first introduced into thermoelectric analysis, yielding two concise relational equations which reflect the intrinsic link between Carnot cycle efficiency, energy efficiency, exergy efficiency, and entransy flow transfer efficiency. The entransy analysis based on the index of entransy flow transfer efficiency, together with energy analysis and exergy analysis, may be a novel and valuable guideline for the operation and optimization of TEGs, which needs to be further investigated.

Comparison between energy and exergy efficiencies in a weir type cascade solar still

With rapid increase in human population, the demand of fresh water has been increased significantly and the world will encounter enormous water crisis by 2040. The sea contains nearly 97% of available water on earth. To fulfill the demand of fresh water, desalination of sea water can be best solution. As desalination process can be performed by using low grade energy, the use of solar energy for desalination is very popular. The most popular solar thermal desalination system is conventional solar still but its productivity is very low. In the present study, a new desalination system based on evacuated tube collectors (ETC) is developed and its performance is compared with conventional solar still experimentally. The present system consists of 15 evacuated tubes, an insulated storage tank, a heat exchanger, a condenser, and a thermic fluid as working medium. The performance of both systems is compared on the basis of daily productivity, energy and exergy efficiency, and economic viability. It is observed that the ETC‐based desalination system performed better as compared to the solar still. The average energy and exergy efficiency, productivity and produced fresh water cost of desalination systems based on ETC and conventional solar still are 43.74%, 9.52%, 4.2 L/m2/day, 0.022 $/L and 19.23%, 1.4%, 1.975 L/m2/day, 0.024 $/L respectively. Further, the pay‐back time for ETC based desalination system is also less (104 days) as compared to solar still (113 days).

In this paper, a new thermodynamic model for photothermal solar radiation conversion into mechanical through a heat engines is proposed. The developed equations allow for the energy and exergy contents of solar radiation to be found, as well as the energy and exergy efficiencies corresponding to concentration type solar-thermal heat engines operating under a range of conditions. The calculation method remains accurate to other published models when their assumed conditions are imposed to the newly developed model. The heat flux absorbed by the receiver (which is assumed to be a grey body and is placed in the focal point of the solar concentrator) depends on the hemispherical absorptivity and emissivity, concentration ratio and receiver temperature. The model is used to conduct a parametric study regarding the energy and exergy efficiencies of the system for assessing its performance. The use of a selective grey body receiver (having a reduced emissivity and a high absorptivity) for enhancing the conversion efficiency is also studied. If the absorptivity approaches one and the emissivity is low enough the photothermal conversion efficiency becomes superior to the known black body receiver limit of 0.853. It is found that in the limit of receiver emissivity tending to zero and absorptivity lending to one, the present model gives the exergy content of solar radiation because the work generated reaches its maximum. In this situation the energy efficiency approaches the exergy efficiency at 1-ITTIN0/TINS where TS and T0 are the sun and ambient temperatures, respectively. The influence of the ambient temperature on the exergy and energy efficiencies becomes apparent, with effects of up to 15%, particularly for high absorptivity and low emissivity. The heat transfer conductances at sink and source of the heat engine have a considerable impact on the efficiency of solar energy conversion. The present model is developed in line with actual power system operations for better practical acceptance. In addition, some irreversibility parameters (absorptivity, emissivity, heat transfer conductivity, etc.) are studied and discussed to evaluate the possible photothermal solar radiation conversion systems and assess their energy and exergy efficiencies.

Energy and exergy analysis of hydrogen production by solid oxide steam electrolyzer plant

Assessment of energy utilization in Iran’s industrial sector using energy and exergy analysis method

Safe drinking water for the coastal areas of Bangladesh has become a big challenge. Arsenic adulteration and salinity intrusion in surface water body has accelerated the scarcity of water in the coastal region. As situation ameliorating and also investment for water-borne diseases is decreasing, it becomes the major threat for a third-world country like Bangladesh. There are lots of alternatives for water supply but there are also a huge number of constraints. Most of the traditional dug wells (DW), ring wells (RW) and alternative pond sand filters (PSF) are now inoperative due to shortage of fresh surface water body and also adequate maintenance. Except a few, most of the shallow tube wells (STW) and deep tube wells (DTW) in coastal areas face arsenic (As) contamination. There could be a blended solution for these problems based on existing situation, constraint, hydrogeology and individual’s economy. Different kinds of filters, reverse osmosis (RO), solar desalination plants (low-cost and small scale) and fuel-powered desalination plants (high-cost and large scale), etc., would be a good solution for mitigation of these problems. Solar PSF and rain water harvesting (RWH) might be an effective solution for some areas, respectively where fresh water and rainfall is abundant. Keywords: Arsenic contamination, salinity intrusion, climate change, pond sand filter, rain water harvesting, desalination, natural disaster

Transient simulation of a solar-based hydrogen production plant with evacuated tube collector and ejector refrigeration system

Sustainable, inexhaustible, economical, and clean energy has become a vital prerequisite to replace fossil fuel sources for power production. In such a context, countries like Pakistan, which are heavily skewed towards fossil fuel-fired plants, are diverting attention to install more and more indigenous renewable energy sources projects such as solar-photovoltaic and wind turbine power plants. In order to harness the maximum energy of wind turbines, it is crucial to factually and precisely assess system performance, which is traditionally inferred by energy analysis (first law analysis). Nevertheless, this analysis only computes the nominal power generation output and ignores the effect of meteorological variables that can lead to some serious errors during the energy planning phase. Consequently, this case study presents both the energy and exergy analysis of a wind farm located in Gharo town of Thatta District along the coastline of the Indus Delta. Energy analysis is carried out to quantify energy efficiency, while exergy analysis computes exergy efficiency by taking into account the effect of pressure, temperature, and wind speed. Comparisons of both efficiencies are provided, and the result substantiates that exergy efficiency turns out to be lower than energy efficiency. However, exergy is a more viable index due to the inclusion of exergy destruction, and in comparison to the energy indicator, it presents the actual performance of a thermodynamic system. The monthly energy and exergy efficiency of the general electric wind turbines are maximum in July having values of 0.5 and 0.41, respectively.

Thermodynamic assessment of photovoltaic systems

Gasification is a thermo-chemical reaction which converts biomass into fuel gases in a reactor. The efficiency of conversion depends on the effective working of the gasifier. The first step in the conversion process is the selection of a suitable feedstock capable of generating more gaseous fuels. This paper analyses the performance of different biomasses during gasification through energy and exergy analysis. A quasi-equilibrium model is developed to simulate and compare the feasibility of different biomass materials as gasifier feedstock. Parametric studies are conducted to analyze the effect of temperature, steam to biomass ratio and equivalence ratio on energy and exergy efficiencies. Of the biomasses considered, sawdust has the highest energy and exergy efficiencies and lowest irreversibility. At a gasification temperature of 1000 K, the steam to biomass ratio of unity and the equivalence ratio of 0.25, the energy efficiency, exergy efficiency and irreversibility of sawdust are 35.62%, 36.98% and 10.62 MJ/kg, respectively. It is also inferred that the biomass with lower ash content and higher carbon content contributes to maximum energy and exergy efficiencies.

A parametric study of the effect of reference state on the energy and exergy efficiencies of geothermal district heating systems is presented. In this regard, the work consists of two parts: a modeling study covering energy and exergy analysis and a case study covering the actual system data taken from the Salihli Geothermal District Heating System (SGDHS) in Manisa, Turkey. General energy and exergy analysis of the geothermal district heating systems is introduced along with some thermodynamic performance evaluation parameters. This analysis is then applied to the SGDHS using actual thermodynamic data for its performance evaluation in terms of energy and exergy efficiencies. In addition, a parametric study on the effect of varying dead state properties on the energy and exergy efficiencies of the system that has been conducted to find optimum performance and operating conditions is explained. Two parametric expressions of energy and exergy efficiencies were developed as a function of the reference temperature. Both energy and exergy flow diagrams illustrate and compare results under different conditions. It has been observed that the exergy destructions in the system particularly take place as the exergy of the fluid lost in the heat exchanger, the natural direct discharge of the system (pipeline losses), and the pumps, which account for 31.17%, 8.98%, and 4.27% of the total exergy input to the SGDHS, respectively. For the actual system that is presented, the system energy and exergy efficiencies vary between 0.53 and 0.73 and 0.58 and 0.59, respectively.

In this study, the analysis of energy and exergy of a horizontal axis wind turbine based on blade element momentum (BEM) theory is presented. The computations are validated against wind tunnel data measured in the MEXICO wind turbine experiment. Blade roughness as one of the important environmental parameters is considered in the computations. Results show that the blade element momentum (BEM) theory has good ability to predict the energy and exergy efficiencies. The computation of energy and exergy exhibits that with the increasing the roughness from 0 mm to 0.5 mm, 2324 W of the output power is reduced. Roughness of 0.5 mm at the wind speed of 16 m/s reduced exergy and energy efficiencies 5.75% and 5.83%, respectively. It is also found that the roughness in the first four months of the operation has a more negative effect on the wind turbine performance.

Heat exchangers are one of the main equipment used in food industry because of their convenience to transfer energy to both auxiliary facilities and various food products. In food industry, there are several reasons for heat transfer such as pre-heating, pasteurizing and sterilizing in which heat exchangers require high amount of energy. On the other hand, as being a unique quality assurance unit heat exchangers should be cleaned easily and extensively. Having high operating costs due to energy consumption and requiring high investment cost due to ensure a reliable hygienic design make heat transfer units an expensive and energy-consuming unit. Therefore, developing new approaches to generate energy and transferring it hygienically with minimum loses will be an opportunity for the food industry. With the view of developing new equipment for industry, induction-driven heating system was investigated in this study and energy and exergy efficiencies were calculated and compared with conventional heat exchanger system. Selected food system was the tomato paste sterilization/pasteurization which is a part of tomato paste production line. After assumptions and theoretical calculations for both conventional application and inductive heating, it was found that inductive heating system has 95.00% energy efficiency and 46.56% second law efficiency while the conventional heating system with electric boiler has 75.43% energy efficiency and 16.63% exergy efficiency. As a consequence, inductive method was found more beneficial compared to a commercial method having higher energy and exergy efficiencies.

In this study, a comprehensive thermodynamic investigation through energy and exergy analyses is conducted to assess the performance of an industrial chips drying process and study how its operating conditions and efficiency can be improved further. In this regard, energy and exergy efficiencies are evaluated with the actual thermodynamic data available, as obtained from the factory, in Turkey. Energy and exergy efficiencies of the drum drying system (DDS) are found as 34.07% and 4.39%, respectively. The analysis results show that exergy efficiency is less than energy efficiency. The main reason of this low exergy efficiency for this drying process is high exergy destruction, as 41.5% of input exergy value. Energy can be recovered via an economizer from hot moist air leaving from the system. If stack gas temperature decreases from 130 to 90°C, regain energy and exergy values are to be 51 976 and 8162 kW, respectively. These recovered potentials can be used for district heating system in winter season and for district cooling system in summer season by using absorption cooling system. Energy and exergy efficiency values can be increased to 93.15 and 43.08%, respectively, by incorporating a heat exchanger into the system. Copyright The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org, Oxford University Press.