Wind barriers optimization for minimizing collector mirror soiling in a parabolic trough collector plant

Wind barriers, according to their sizes and shapes, can effectively control, shift and even modify the airflow field in their downstream. These structures can accelerate the wind flow over the mirror field and move the airborne particles away from the mirrors. For example, in concentrated solar power plants it is highly desirable to engineer the system in a way that fewer particles are deposited within the solar field, in particular, onto mirror surfaces. Therefore, design optimization of dust barriers could significantly impact the mirror soiling and favourably reduce the cleaning water consumption of a solar power plant. This study focuses on the optimization of a solid wind barrier around a parabolic trough collector plant for their protection against dust soiling. The presented simulation results show that an optimum solid wind barrier is able to direct large amount of particles (in this study, more than 86%) to pass over the solar field with very small fraction (around 0.8%) being deposited on the mirrors. In addition, it was found that the barrier wall is more effective in deflecting the larger particles from the solar field.http://www.elsevier.com/locate/apenergy2019-09-01hj2018Mechanical and Aeronautical Engineerin

Reducing the optical end losses of a linear Fresnel reflector using novel techniques

The objective of this work is the investigation of some alternative ideas for enhancing the optical performance of a linear Fresnel reflector (LFR) with North-South orientation. More specifically, the examined methods aim to reduce the optical end losses which are crucial for the short LFR plants in particular during the winter period. The first studied idea is the extension of the receiver after the concentrator which is able to enhance the yearly mean incident angle modifier up to 50.3%. The second examined idea is the displacement of the receiver in order to eliminate the non-illuminated area at the beginning of the receiver. This idea proved that the mean yearly incident angle modifier can be enhanced up it 20.2% for a displacement equal to 20% of the concentrator length. The third examined idea is the hybrid design with the extended and displayed receiver in combined. This idea leads to intermediates enhancements compared to the previous cases but its advantage is the lower investment cost compared to the simple receiver extension case. The analysis is performed with a developed optical model in SolidWorks Flow Simulation and the yearly evaluation has been done for the location of Athens (Greece).Bodossaki Foundationhttp://www.elsevier.com/locate/solener2020-07-01hj2020Mechanical and Aeronautical Engineerin

A novel computational approach to combine the optical and thermal modelling of Linear Fresnel Collectors using the finite volume method

A computational approach is presented, which uses the finite volume (FV) method in the Computational Fluid Dynamics (CFD) solver ANSYS Fluent to conduct the ray tracing required to quantify the optical performance of a line concentration Concentrated Solar Power (CSP) receiver, as well as the conjugate heat transfer modelling required to estimate the thermal efficiency of such a receiver. A Linear Fresnel Collector (LFC) implementation is used to illustrate the approach. It is shown that the Discrete Ordinates method can provide an accurate solution to the Radiative Transfer Equation (RTE) if the shortcomings of its solution are resolved appropriately in the FV CFD solver. The shortcomings are due to false scattering and the so-called ray effect inherent in the FV solution. The approach is first evaluated for a 2-D test case involving oblique collimated radiation and then for a more complex 2-D LFC optical domain based on the FRESDEMO project. For the latter, results are compared with and validated against those obtained with the Monte Carlo ray tracer, SolTrace. The outcome of the FV ray tracing in the LFC optical domain is mapped as a non-uniform heat flux distribution in the 3-D cavity receiver domain and this distribution is included in the FV conjugate heat transfer CFD model as a volumetric source. The result of this latter model is the determination of the heat transferred to the heat transfer fluid running in the collector tubes, thereby providing an estimation of the overall thermal efficiency. To evaluate the effectiveness of the phased approach in terms of accuracy and computational cost, the novel 2-D:3-D phased approach is compared with results of a fully integrated, but expensive 3-D optical and thermal model. It is shown that the less expensive model provides similar results and hence a large cost saving. The novel approach also provides the benefit of working in one simulation environment, i.e. ANSYS Workbench, where optimisation studies can be carried out to maximise the performance of linear CSP reflector layout and receiver configurations

Simulation-based optimisation of a linear Fresnel collector mirror field and receiver for optical, thermal and economic performance

Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisation tools is one of the current topics of solar thermal researchers. Of these technologies, Linear Fresnel collectors (LFCs) are the least developed. Therefore, there is plenty of room for the optimisation of this technology. One of the goals of this paper, in addition to the optimisation of an LFC plant, is introducing an applicable optimisation procedure that can be applied for any type of CSP plant. This paper focuses on harvesting maximum solar energy (maximising plant optical efficiency), as well as minimising plant thermal heat loss (maximising plant thermal efficiency), and plant cost (the economic optimisation of the plant), which leads to the generation of cheaper solar electricity from an LFC plant with a fixed power plant cycle (The performance optimisation of this study is based on the plant performance throughout an imaginary summer day). A multi-tube cavity receiver is considered in this study since there is plenty of room for its optimization. For the receiver, optimal cavity shape, tube bundle arrangement, tube numbers, cavity mounting height and insulation thickness are considered, while for the mirror field, the number of mirrors, mirror width, mirror gaps and mirror focal length are considered to achieve the optimisation goals. A multi-stage optimisation process is followed. Firstly, optical (using SolTrace), thermal (using a view area approach) and economic performance are combined in a multi-objective genetic algorithm as incorporated in ANSYS DesignXplorer (DX). This leads to an optimal LFC with a variable focal length for each mirror. After determining a fixed optimal focal length for all the mirrors, a Computational Fluid Dynamics (CFD) approach is used to optimise the thermal insulation of the cavity receiver for minimal heat loss and minimal insulation material. The process is automated through the use of ANSYS Workbench and Excel (coding with Visual Basic for Application (VBA) and LK Scripting in SolTrace). The view area approach provides an inexpensive way of calculating radiation heat loss from the receiver that is shown in the subsequent CFD analysis to be dominating the heat transfer loss mechanisms. The optimised receiver is evaluated at different LFC plant tube temperatures to assess its performance.The University of Pretoria (South Africa) and the South African National Research Foundation (DST-NRF Solar Spoke).http://www.elsevier.com/locate/solener2018-09-01hj2017Mechanical and Aeronautical Engineerin

Thermal Stability and Performance Testing of Oil based CuO Nanofluids for Solar Thermal Applications

For solar thermal systems, nanofluids have been proposed as working fluids due to their enhanced optical and thermal properties. However, nanoparticles may agglomerate over time, heating and thermal cycles. Even though pristine nanofluids have be proven to enhance performance in low temperature application, it is still unclear if nanofluids can meet the reliability requirements of solar thermal applications. To investigate this, the present study conducted experiments with several formulations of oil‐based CuO nanofluids in terms of their maximum operational temperature and their stability upon cyclic heating. In the samples tested, the maximum temperature ranged from 80oC to 150 22 oC and the number of heating cycles of ranged from 5 to 45, with heating times of between 5 to 60 minutes. The results showed that heating temperature, heating cycles, and heating time all exacerbated agglomeration of the samples. Following these experiments, orthogonal experiments were designed to improve the preparation process and the resultant thermal‐impulse stability. Thermal properties of these samples were characterized and thermal performance in an ‘on‐sun’ linear Fresnel solar collector was measured. All tests revealed that thermal performance of solar collecting system could be enhanced with nanofluids, but that thermal stability still needs to be further improved for industrial applications

Minimising mirror soiling of a PTC plant by an optimum wind barrier design

This study presents a simulation-based optimisation of a solid wind barrier surrounding a parabolic trough collector plant to minimise dust soiling on the plant mirror facets. The results of the presented simulation show that designing and constructing a barrier is one of the effective approaches to mitigate airborne particles and, consequently, to significantly reduce water consumption in solar plants for mirror cleaning. It is shown that an optimum solid wind barrier could deflect more than 86% of the particles that pass over the solar field with a small fraction of around 0.8% of airborne particles being deposited on the mirrors.https://aip.scitation.org/journal/apc2020-07-26am2020Mechanical and Aeronautical Engineerin

Applications of Heat Transfer Enhancement Techniques: A State-of-the-Art Review

The fundamentals of heat transfer and its applications, the classification of heat transfer technology and different heat transfer techniques, and the needs for augmentation and its benefits and the different combinations of two or more inserts and integral roughness elements for heat transfer augmentation purpose have been introduced and discussed in this chapter. It is shown that most of the compound techniques performed better than the individual inserts for heat transfer enhancement. This chapter has also been dedicated to understanding the basic concepts of vortex generators for heat transfer enhancement in plate-fin heat exchangers. The performance of transverse, longitudinal, and wing-type vortex generators has been discussed as well

Energy recovery from domestic radiators using a compact composite metal Foam/PCM latent heat storage

With the increasing demand for energy consumption in domestic buildings and consequent CO2 emission, there is a need to provide proper products to reduce energy loss. Domestic radiators for space heating can be improved by using a Compact Latent Heat Storage (CLHS) unit mounted on the wall side surface in order to offer energy saving and peak-shaving. The unit offers the potential to save otherwise wasted energy from the back surface of the radiator to the walls in the charging mode of the energy storage system. When the heating system is turned off, the CLHS unit discharges the stored heat towards the room to provide a uniform temperature on the surface of the radiator. An aluminium foam embedded inside the bulk Phase Change Material (PCM) can modify the heat storage/retrieval rate. A PCM is selected depending on the radiator's surface temperature, which is almost equal to the hot water temperature delivered to the radiator. Different metal foam porosities are examined and compared with the PCM-only alternative (i.e. without metal foam enhancement). The results show the porous-PCM CLHS alternative provides an almost constant temperature during the discharging process equal to 54 °C. However, for the PCM-only alternative, the temperature of the surface reduces continuously. Using the porous medium results in a shorter melting time, about 95% of what is needed for the PCM-only alternative. Increasing the metal foam porosity results in shorter charging/discharging time; however, since the surface temperature of the porous-PCM unit is almost constant for different metal foam porosities, a system with higher porosity (97%) is desirable

Optical thermal and economic optimisation of a linear Fresnel collector

Solar energy is one of a very few low-carbon energy technologies with the enormous potential to grow to a large scale. Currently, solar power is generated via the photovoltaic (PV) and concentrating solar power (CSP) technologies. The ability of CSPs to scale up renewable energy at the utility level, as well as to store energy for electrical power generation even under circumstances when the sun is not available (after sunset or on a cloudy day), makes this technology an attractive option for sustainable clean energy. The levelised electricity cost (LEC) of CSP with thermal storage was about 0.16-0.196 Euro/kWh in 2013 (Kost et al., 2013). However, lowering LEC and harvesting more solar energy from CSPs in future motivate researchers to work harder towards the optimisation of such plants. The situation tempts people and governments to invest more in this ultimate clean source of energy while shifting the energy consumption statistics of their societies from fossil fuels to solar energy. Usually, researchers just concentrate on the optimisation of technical aspects of CSP plants (thermal and/or optical optimisation). However, the technical optimisation of a plant while disregarding economic goals cannot produce a fruitful design and in some cases may lead to an increase in the expenses of the plant, which could result in an increase in the generated electrical power price. The study focused on a comprehensive optimisation of one of the main CSP technology types, the linear Fresnel collector (LFC). In the study, the entire LFC solar domain was considered in an optimisation process to maximise the harvested solar heat flux throughout an imaginary summer day (optical goal), and to minimise cavity receiver heat losses (thermal goal) as well as minimising the manufacturing cost of the plant (economic goal). To illustrate the optimisation process, an LFC was considered with 12 design parameters influencing three objectives, and a unique combination of the parameters was found, which optimised the performance. In this regard, different engineering tools and approaches were introduced in the study, e.g., for the calculation of thermal goals, Computational Fluid Dynamics (CFD) and view area approaches were suggested, and for tackling optical goals, CFD and Monte-Carlo based ray-tracing approaches were introduced. The applicability of the introduced methods for the optimisation process was discussed through case study simulations. The study showed that for the intensive optimisation process of an LFC plant, using the Monte Carlo-based ray-tracing as high fidelity approach for the optical optimisation objective, and view area as a low fidelity approach for the thermal optimisation objective, made more sense due to the saving in computational cost without sacrificing accuracy, in comparison with other combinations of the suggested approaches. The study approaches can be developed for the optimisation of other CSP technologies after some modification and manipulation. The techniques provide alternative options for future researchers to choose the best approach in tackling the optimisation of a CSP plant regarding the nature of optimisation, computational cost and accuracy of the process.Thesis (PhD)--University of Pretoria, 2017.Mechanical and Aeronautical EngineeringPhDUnrestricte

Integrated solar-driven hydrogen generation by pyrolysis and electrolysis coupled with carbon capture and Rankine cycle

Reduction of carbon emissions from conventional gray Hydrogen (H2) production is a promising option in moving towards much greener H2 generation. To minimise carbon emissions and improve plants’ efficiencies of conventional gray H2 production, this study focused on process simulation of hybrid CSP, catalytic Methane (CH4) and biomass pyrolysis and Water (H2O) electrolysis plants with 1000°C HTF output temperature. This integrated system differs from current pyrolysis and electrolysis technologies for H2 production because of the involvement of CSP as a thermal energy source; the use of part of recovered heat from the reactor to power downstream units including thermolysis of Sulphuric Acid (H2SO4) and steam generation for both H2O electrolysis and Rankine cycle; the use of H2O as a reaction media and carbon looping to promote biomass decomposition; anodic oxidation of SO2 in AEC to promote hydrogen evolution reaction. In that regard, CSP systems were modelled and simulated in SAM and MATLAB software. The output result of the simulated CSP system got exported to the Simulink to feed simulated CH4 and biomass pyrolysis coupled with TES and Rankine cycle from Aspen plus. In addition, simulated thermal disassociation of H2SO4, electrolysis of H2O with SOEC and AEC from Aspen plus was also exported to the Simulink to feed the CSP system. Both integrated systems were fed with CH4 as the working fluid of the solar furnace. About $1.7/kg is estimated to be a H2 selling price for simulated pyrolysis of CH4 and biomass plant which is cheaper than SMR with a CCS system. While between 4.6 - 10.48 is also estimated to be a H2 selling price for another simulated CH4 pyrolysis and H2O electrolysis. Just like existing CSP systems for electricity generation, both simulated hybrid systems generate electricity for up to 200 minutes in the absence of the Sun. Similar to SMR with a CCS system, CO2 by-product from biomass pyrolysis was captured. Due to coking issues related to catalytic pyrolysis, noncatalytic pyrolysis of CH4 was investigated. Results of the research work show that a return on investment within a period of 6 years is possible with the adoption of these new innovative technologies while reducing carbon footprints in H2 generation plants