Sensitivity study of thermal performance characteristics based on optical parameters for direct steam generation in parabolic trough collectors
Solar parabolic trough collector is one of the most potential Concentrated Solar Power (CSP) technologies with high dispatchability. The performance of the Parabolic Trough Collector (PTC) is significantly influenced by the distribution of radiant flux around the line focus receiver. In this paper, an accurate estimation of radiant flux around the line focus receivers of different parabolic trough collectors of varying aperture widths from 5 to 7.5 m has been studied. Effects of total internal reflection in the glass tube, glass tube absorptivity, receiver reflectivity, limb-darkening effect, surface errors and anti-reflective coating have been considered. A direct steam generation model (recirculation mode) has been developed and the effect of geometric errors in the collector on its optical and thermal performance characteristics has been discussed. The thermal efficiency of the direct steam generation collector is influenced by the optical characteristics of the collector and hence the thermal efficiency at different optical errors has been evaluated for the fore-mentioned commercial collectors under the influence of declination. The thermal efficiency is more sensitive to optical errors at low insolation levels and vice versa. For a change in tracking from 0 to 10 mrad, for PTC7.3 configuration, thermal efficiency drops by 0.58% at 1000 W/m2, and 2.02% at 300 W/m2. The thermal efficiency is also more sensitive to optical errors at higher values of declination. The collector of low geometric concentration ratio has a low sensitivity and low thermal efficiency and vice versa. Hence, the overall efficiency of different collectors at different optical errors has to be studied so as to obtain the appropriate collector configuration for specified optical errors and location of installation. Based on the analysis, graphical results which could aid in the selection of best collectors based on the latitude location, DNI and optical errors has been developed. The evaluation of collectors has also been extended towards power plant characteristics. The nominal power has been set to 50 MW. The location of Jodhpur in India has been selected and the total power generated per unit area has been studied for different collectors, for different optical errors. Based on the graphical results developed, appropriate collectors may be chosen for those locations based on their associated optical errors.
Experimental performance investigations of an elliptical hyperbolic non-imaging solar concentrator with trapezoidal surface receiver for process heat applications
The use of non-imaging collectors has a wide scope in process heat applications based on its performance and economics. An experimental investigation of trapezoidal/concave cavity surface receiver (TSR) for non-imaging solar concentrating collector is carried out in this paper. The implementation of surface/coil receiver instead of tubular receiver for non-imaging collector is presented in this paper. A TSR with helical coil is developed for non – imaging concentrating collector, Elliptical Hyperbolic Collector (EHC). Experiments are carried to estimate the thermal performance of the system under various operating conditions such as two operating modes: series and parallel modes of operation of the collector, two circulation modes: passive and active modes. The stagnation temperature of the trapezoidal/concave cavity surface receiver is measured to be 118 °C on a clear sunny day in October and 102 °C on a cloudy day in February. The daily performance tests are performed under different operating conditions. Based on the experimental study, for the flow rate of 0.03 kg/min and 0.5 kg/min, the fluid outlet temperature is estimated to be 87 °C at 768 W/m2 and 49 °C at 908 W/m2 respectively. The corresponding instantaneous efficiency was calculated to be 9% and 40% respectively. The numerical model is developed to predict the temperature of the fluid along the receiver. The pressure drop across a receiver is estimated to be 9.3 kPa for a flow rate of 0.5 kg/min. Exergy analysis of the system is carried out and it ranges between 10 and 20%. The costs involved in fabricating the EHC system are compared to that of a non-imaging concentrating collector (CPC) of same aperture area. An economic analysis of the system is also carried out to study the feasibility of the system based on the life cycle savings method by estimating the annual solar savings from the EHC system. The present system can be a suitable option for low and medium temperature process heat applications.
Experimental performance investigation of tilted solar still with basin and wick for distillate quality and enviro-economic aspects
Tilted wick type and stepped solar stills are well known for their increased distillate yield compared to basin type stills. In this study experiments were conducted on tilted solar still with basin and tilted solar still with wick to assess their performance, distillate quality, environmental benefits and economic feasibility. Distillate quality of both the tested units was superior. Annual average distillate yield of tilted solar still with basin was nearly 19.76% higher than that of the unit with wick. Maximum distillate yield of 4.99 L/d and 4.54 L/d was noticed for tilted solar still with basin and wick, respectively during April. Yearly average thermal and exergy efficiency was around 41.06% and 3.06% for the unit with basin and 33.83% and 2.88% for the unit with wick. Energy payback time of the unit with basin was around 2.80 yrs and it can prevent 17.65 tons of CO2emission during 20 yrs of life time. Distillate production cost was around 0.026 USD/L (Rs. 1.74/L) and 0.046 USD/L (Rs. 3.08/L) for an interest rate of 5% and 12%, respectively. Tilted solar still with basin has superior performance compared to that of unit with wick and can produce 21.76 L of distillate/USD invested on it.
Stability analysis of the thermocline thermal energy storage system during high flow rates for solar process heating applications
The thermal energy storage system is a pivotal system for solar thermal plants for improving reliability. The stability in the thermocline is more significant to clarify and improve the performance of thermal energy storage tank which legitimately shows the quality of the thermocline. In this stability analysis investigation, the modern engineering energy storage material concrete was used as a filler material for high-temperature thermal energy storage applications as a result of the intrinsic properties. A comprehensive laminar and k-ε turbulent flow energy transport model accounts for the heat transfer fluid and filler material with adiabatic and non-adiabatic conditions using LTNE (Local Thermal Non-Equilibrium model). The axial, radial, and diagonal temperature differences were identified which was used to calculate the stability of the thermocline. A thermal energy storage tank size of 1 m height and 0.250 m diameter with a 0.030 m size of filler material packed with an average porosity of 0.3 for the storage capacity of 150 kWh/m3 is used for solar process heating applications considered for the present study. The thermocline stabilities are performed with Reynolds numbers, Re varied from 1 to 3000. It is found that the Re = 1 provides better stability in the axial and radial direction as well as diagonal than other Reynolds number. It is observed that Re = 1 provides superior discharging efficiency for nearly 5.84 hrs which is highly suitable for solar process heating applications and the discharging efficiency consistently drops, when Re increases from 1 to 3000. The wall condition of the tank and velocity of the heat transfer fluid is highly disturbing the thermocline in the radial direction and it creates a ‘spike’ profile in the axial direction. Based on the newly introduced stability scale, the effective length of packing and timing to achieve stability is identified for H/D = 4. From that result, the top and bottom layer of the thermocline tank porosity is also found which is used to decide the porosity of the packed bed distributors. The identified porosity for the top and bottom distributors in the ɛ = 0.3 thermal energy storage tank is less than 0.3 is more suitable for provide the uniform flow in the tank.
Numerical modeling and performance assessment of elongated compound parabolic concentrator based LCPVT system
In this work, a non-imaging low concentrating 2.5X Compound Parabolic Concentrator (CPC) truncated to 1.7X has been explored. CPCs inherently form non-uniform distribution of flux on the PVT module which has been mitigated by the integration of optimized homogenizer referred to as Elongated CPC (ECPC). The study involves detailed optical, thermal, and electrical modeling of the ECPC based Low Concentrating Photovoltaic Thermal (LCPVT) system. Optical simulations provide insight into the flux distribution on the PVT panel surface, which is further coupled with a thermal and electrical model for precise prediction of the performance of the system. These models are validated experimentally with a 315 Wp solar panel integrated with ECPC based LCPVT system. Performance evaluation of the system has shown peak thermal efficiency of ∼40% at ΔT of 16 °C, peak electrical efficiency of 12%, and an overall peak exergy efficiency of 15% at 38 Liters per hour (LPH) flow rate. A comparison of outlet water temperature results obtained from the numerical model and experiments has shown an excellent match with a relative error of 4%. Results also show that the increase in mass flow rate from 22 LPH to 38 LPH improves the electrical efficiency by 3% however a drop in ΔT of 2–3 °C is observed.
4-E (Energy-Exergy-Environment-Economic) analyses of integrated solar powered jaggery production plant with different pan configurations
Conventional jaggery making process utilizes the bagasse for boiling of sugar cane juice which releases pollutants into the atmosphere and high particulate matter from these emissions causes air pollution. In this article, solar powered jaggery industry with freeze pre-concentration is proposed with conventional and modified heating pans. The system performance, environmental impacts and economic feasibility were assessed by carrying out 4E (Energy-Exergy-Environment-Economic) analyses using the developed mathematical model. These systems were designed to produce 300 kg of jaggery per day when operated for 7.5 h in 3 batches with average solar direct normal irradation of 662 W/m2 and 343 °C. These systems are integrated with auxiliary heating for uninterrupted production in the absence of sunlight. These systems can mitigate nearly 2015.95 to 3062.15 tons of CO2 emission during its 25 years of lifespan under 300 clear days of operation each year. Jaggery produced by this technique is rich in its colour and completely safe for human consumption as no artificial clarificants are used. Amount invested in these systems can be recovered in a span of 12.03 to 13.45 years for jaggery selling price of USD.0.514/kg or INR.36/kg.
A review of solar energy driven desalination technologies
Water plays an important role in all our day to day activities and its consumption is increasing day by day because of increased living standards of mankind. Some regions of the globe are under severe stress due to water scarcity and pollution. The fresh water needs of mankind can be only satisfied if saline water which is available in plenty is converted to potable water by desalination. Desalination industry has shown increased threats of CO2 emissions and severe environmental impacts. Desalination industry can be made sustainable if they are integrated with renewable energy and if proper brine disposal methods are followed. In this review different desalination units integrated with renewable energy with special emphasis given to solar energy is discussed. The problems associated with desalination units and their remedies have been presented. Apart from this some novel methods of desalination process has also been explained. This review will allow the researchers to choose appropriate desalination technology for further development.
Performance improvement of a desiccant based cooling system by mitigation of non-uniform illumination on the coupled low concentrating photovoltaic thermal units
A Low Concentrating Photovoltaic Thermal system typically employs compound parabolic concentrator to focus sunlight and enhance the quality of both thermal and electrical energy extracted. One of the major issues during this process is the introduction of non-uniform illumination on the photovoltaic panels which can cause hot-spots and significantly reduce both the reliability and the electrical output from this system. This non-uniform illumination can be mitigated by integrating homogenizers which are typically linear extensions to the compound parabolic concentrators profile also referred to as elongated compound parabolic concentrators. In this work, the performance of a 2.5× Elongated Compound Parabolic Concentrator truncated to 1.7× and connected to a desiccant based cooling system has been explored. For a detailed analysis of the system, a coupled 3-D optical, electrical, thermal and process efficiency model has been developed. A full-scale prototype of the modelled system is also fabricated using a 380-Watt peak photovoltaic panel. Experiments conducted on the developed system showed a peak outlet water temperature of 56 °C at a mass flowrate of 24 L per hour. Comparative studies between compound parabolic concentrators and elongated compound parabolic concentrators based low concentrating photovoltaic thermal system is also presented to showcase the overall improvement in the process efficiency due to the mitigation of non-uniformity. Using a 400 mm length of the homogenizer the spatial non-uniformity factor was found to drop from 0.5 to 0.29 under normal incidence angle and results in a rise of 12% in the electrical output when compared to a compound parabolic concentrators-based system. The coefficient of performance of the desiccant-based air-cooling system is found to increase by 50% when coupled with two series-connected elongated compound parabolic concentrators based low concentrating photovoltaic thermal system. The improvement in coefficient of performance is mainly because of thermal and electrical energy savings from the developed system amounting to 352 kWhe/year and 665 kWhth/year, respectively. Further, the mitigation of non-uniform illumination showed a performance improvement of 5% in the coefficient of performance of the air-cooling system compared to a compound parabolic concentrators-based system.
Energy-environment-economic investigations on evacuated active multiple stage series flow solar distillation unit for potable water production
Multi stage solar distillation units are well known for their higher distillate productivity and are capable of fulfilling the potable water requirements of families in remote, rural, and coastal regions. In this article, active multiple stage series flow solar distillation unit has been proposed for desalting saline water and its performance, environmental benefits and economic feasibility were assessed by carrying out 3E (Energy-Environment-Economic) analyses using the developed mathematical model. Better performance was observed for the distillation unit with five distillation stages, two solar collectors connected in parallel configuration and saline water mass flow rate of 135.0 kg/d and 75.0 kg/d, during summer and winter seasons. Low-pressure operation in combination with evaporative cooling of condenser of last stage has enhanced the annual average daily distillate productivity from 12.60 kg/d to 48.80 kg/d. Distillate production was found to drop from 48.80 kg/d to 38.90 kg/d with the increase in salt concentration of saline water from 0 wt% to 10 wt%. Energy payback time of the unit desalting saline water with 5 wt% salt concentration was within 1.0 yr and the unit can mitigate nearly 221.80 tons of CO2 emission, 1594.73 kg of SO2 emission and 651.37 kg of NO emission from Indian coal based power plants during its 20 yrs lifespan under 250 clear day operation. Increased salt concentration in saline water reduces the emission mitigation potential and increases the energy payback time of the proposed distillation unit. Distillate production cost was increased by 16.0% for every 5 wt% increase in salt concentration. Amount invested in the unit can be regained with in 3.56 yr irrespective of interest rate for distilled water selling price of 0.06 USD/L (Rs. 3.91/L).
Design and analysis of dense array CPV receiver for square parabolic dish system with CPC array as secondary concentrator
In this paper, a two-stage square parabolic concentrating photovoltaic (CPV) receiver dish with an overall geometric concentration ratio of 500 suns is designed to provide a uniform intensity distribution on high-efficiency triple-junction solar CPV cell module. The system comprises of a square parabolic dish with an aperture area of 9 m2 as a primary concentrator and an array of the compound parabolic concentrator integrated optical homogenizer of 0.27 × 0.27 m2 as a secondary concentrator in tandem with the dish. The CPV module consists of an array of 12 × 12 triple junction CPV cells with each cell connected in parallel or series combination and integrated CPC homogenizer dedicated to each cell. The homogenizer length is selected based on the peak to average ratio of concentrated flux with the aim to maximize the optical and electrical performance of the CPV system. Monte Carlo ray-tracing model is used to predict the flux distribution. The predicted solar flux distribution on individual CPV cells are used as input to determine the electrical performances of the CPV module with three different cell interconnections. For optimized homogenizer length of 0.005 m and receiver height of 3.7 m, the maximum optical and CPV module efficiencies are obtained as 68.30% and 32.03% respectively. A year-round electrical power output of developed CPV system is 2.19 MWh which is up to 33.54% higher as compared to conventional CPV system. The proposed novel geometric design could accommodate the bypass diode for each cell, effectively reducing the current mismatch effects.