• Energy Research

Abstract Thermal energy storage systems based on Phase change materials (PCM) are an attractive option to bridge the temporal and spatial gap between the energy demand and supply. But, these systems possess poor thermal conductivity causing reduced rate of heat transfer. The objective of the present study is to numerically analyze the melting process in an optimized finned latent heat storage system dispersed with varying volume fraction of Graphene nano plates (GNP). The individual effect of incorporating fins, GNP and a combination of both at different volume fraction has been studied. Effective thermal conductivity of nano-composite PCM has been theoretically evaluated including the effect of aspect ratio, interfacial thermal resistance, anisotropy, non-linear effects as well as concentration for the dispersed GNP. In this work, Dynamic differential scanning calorimetry tests are performed to evaluate the phase change temperature, latent heat and specific heat of the sugar alcohol (d- mannitol). Transient variation of liquid fraction, average temperature and radial/longitudinal temperature differentials are presented which would be useful for designing medium temperature (160–200 °C) storage systems for various applications. Fin height is varied to obtain an optimum fin size such that natural convection currents are not impeded. Various heat transfer models (including natural convection) are analysed using the actual plant data of a double effect solar absorption system at different arrangements of fins and GNP. Effect of Reynolds number and inlet temperature of HTF on the system performance have also been studied. A reduction of 68% in total melting time is observed in finned LHSS with 5% GNP as compared to a conventional system.

This article deals with comparative energy and exergetic analysis for evaluation of natural gas fired combined cycle power plant and solar concentrator aided (feed water heating and low pressure steam generation options) natural gas fired combined cycle power plant. Heat Transfer analysis of Linear Fresnel reflecting solar concentrator (LFRSC) is used to predict the effect of focal distance and width of reflector upon the reflecting surface area. Performance analysis of LFRSC with energetic and exergetic methods and the effect, of concentration ratio and inlet temperature of the fluid is carried out to determine, overall heat loss coefficient of the circular evacuated tube absorber at different receiver temperatures. An instantaneous increase in power generation capacity of about 10% is observed by substituting solar thermal energy for feed water heater and low pressure steam generation. It is observed that the utilization of solar energy for feed water heating and low pressure steam generation is more effective based on exergetic analysis rather than energetic analysis. Furthermore, for a solar aided feed water heating and low pressure steam generation, it is found that the land area requirement is 7 ha/MW for large scale solar thermal storage system to run the plant for 24 h.

In the present paper, integration of a two-stage absorption refrigeration system with a compression refrigeration system is proposed for utilizing low-temperature heat and reducing electric energy consumption. The proposed system is analyzed and compared with vapor compression system from the viewpoint of energy, exergy, environment and economics. The proposed system reduces the electricity consumption by 89.3% and CO2 emission from 112.6 to 12.1 ton/year. The size and cost of the system are determined by designing the heat exchangers. The optimization is also performed with the objective of minimizing the annual cost of plant operation which includes fuel exergy cost, initial investment and maintenance cost and environmental damage cost due to CO2 emission. The annual cost of its operation is 21.6% less than equivalent vapor compression refrigeration system which is further reduced by 18.2% through system optimization.

Abstract A thorough analysis of aqua-ammonia generator-absorber-heat exchanger (GAX) and hybrid GAX (HGAX) absorption refrigeration cycles based on energy and exergy has been carried out in this communication. The coefficient of performance (COP) and exergetic efficiencies are calculated at various operating conditions to study the effect of generator temperature, condenser temperature and evaporator temperature on them. The influence of generator temperature on exergetic efficiency is more pronounced than on COP. The effects of degassing range and approach temperature on first and second law efficiency are also examined. It is observed that the increase in approach temperature from 0 °C to 14 °C causes decrease in COP of GAX cycle by 30% and of HGAX cycles by 40%–45%. Desorber and absorber together accounts for highest exergy destruction.

Abstract Around 97% of the total water available on the earth is saline or brackish and hence, not suitable for direct consumption. There are number of distillation process to convert the saline or brackish water into potable using different techniques, basin type solar still is one of them which is the cheapest, easiest and effective solutions to fulfil the demand of freshwater particularly, for rural and remote areas. However, there are limitations for basin-type solar still which restricts its performance and several (experimental and numerical) studies has discussed the improvements in design and operational strategies. Therefore, this article presents the comprehensive literature survey of solar desalination systems including the design, operational, and environmental characteristics and their impact not only on the performance but also on the quality of water produced from the conventional solar stills. Further, the limitations along with the recommendations for future work, which may enhance the performance and productivity of basin solar still for typical operating conditions are also mentioned.

Abstract Growing energy demands and environmental degradation with uncontrolled exploitation of fossil fuels have compelled the world to look for the alternatives. In this context, biogas is a promising candidate, which can easily be utilized in IC engines for vehicular as well as decentralized power generation applications. Primary constituents of raw biogas are methane (CH4) that defines its heating value, and carbon dioxide (CO2) that acts like a diluent. This dilution effect reduces the flame speed and heating value of biogas, eventually deteriorating the engine performances. Present article focuses on experimental evaluation and quantification of these variations of the engine performance. Three compositions of biogas: BG93, BG84 and BG75 (containing 93%, 84% and 75% of CH4 by volume respectively) were studied on a small CI engine in dual fuel mode. Moreover, to evaluate individual process inefficiencies, exergy analysis based on second-law of thermodynamics is implemented. Exergy balances for different compositions of biogas are presented. Biogas dual fuel operation showed 80–90% diesel substitution at lower engine loads. At higher loads, total irreversibility of the engine was increased from 59.56% for diesel operation to 61.44%, 64.18% and 64.64% for BG93, BG84 and BG75 biogas compositions respectively. Furthermore, combustion irreversibility was found to be decreasing with higher CO2 concentrations in biogas. BG93 showed comparable results to that of diesel operation with 26.9% and 27.4% second-law efficiencies respectively.

Abstract Desalinated water, the final product of a solar distillation system, is useful for drinking purpose, community services, industry and agriculture on a small scale. It is expensive and may be considered as an industrial product. In the present communication, it has been tried to collect information about the ongoing research activities in the field of solar distillation system with the aim to enhance productivity and efficiency through an effective thermodynamic tool i.e. energy and exergy analysis, especially of the solar stills, similar to its wide application in complex thermal systems such as steam or gas turbine, boiler and cogeneration systems. Thermodynamic models for the energy and exergy analysis have been presented based on the fundamental heat transfer correlations in literatures for the simple basin type solar stills. Energy efficiency and productivity of the conventional solar stills is found to be low in the range of 20–46% and less than 6 L/m 2 /day, respectively, for most cases, even under optimized operating conditions. The exergetic efficiencies are estimated to be between 19% and 26% for a triple effect system, 17–20% for a double effect system, and less than 5% for a single effect system. Productivity increases significantly by the use of integrated solar stills with better efficiency. The overall energy and exergy efficiency of the integrated systems rises up to 62% and 8.5%, respectively, using single effect solar stills. An attempt has also been made to review works on economic and thermo-economic analysis of solar stills. The cost of desalination through solar stills is reported in the range of US$0.014 to 0.237/L. It decreases further with increase in efficiency. It is observed that integrated solar desalination systems and technologies will be better choice than the conventional solar distillation systems for rural as well as urban areas blessed with sufficient sunshine.

In this investigation, a prototype model of the solar adsorption refrigeration system was constructed and its performance was evaluated with different mass ratios of adsorbents in the laboratory for possible field application. The main components of the solar-driven cooling system were vacuum tube collector, adsorption bed, condenser, evaporator, chilling chamber, and temperature data logger. The experimental study was conducted to analyze the performance of solar cooling unit using different mass ratios of activated carbon–methanol from 0.250 to 2.50. A minimum cooling temperature of 12.2 °C was obtained with the manually prepared activated carbon–methanol with mass ratio of 1.00. The cooling coefficient of performance and specific cooling potential was also evaluated from performance calculations.

The transient thermal behavior of an exoreversible thermodynamic model of an annular thermoelectric cooler has been studied by one-dimensional unsteady-state heat transfer analysis. Unlike the flat plate geometry of thermoelectric coolers, which have equal heat transfer area at their hot and cold sides, an annular thermoelectric cooler has a higher heat transfer area at its hot side than its cold side. The temperature variations with time at the hot and cold sides of an annular thermoelectric cooler have been predicted for different cooling loads, current flow, variable thermocouple length and with different heat transfer coefficients at its hot side. Finally, the transient thermal behavior of an annular thermoelectric cooler has been compared with a flat plate thermoelectric cooler. It was found that, for typical operating conditions with zero cooling load, the annular thermoelectric cooler can maintain a 2.3-K lower temperature than the flat thermoelectric cooler. It was also found that when the cooling load is 0.055 W/cm2 with the hot side heat transfer coefficient of 0.010 W/cm2 K, the coefficient of performance of the annular thermoelectric cooler is 2.32% higher than the flat thermoelectric cooler.