Energy is an indispensable part of human life. To meet the ever-increasing energy demands and mitigation of its adverse production effects, the new technological advancements and shifting towards the ...renewable energy resources (RERs) has become imminent. Solar energy is the most prominent among RERs, where solar thermal collectors utilizes more effective working wavelength range compared to photovoltaic cells. Nanofluids are engineered working fluids for enhancing collector performance. Synthesized carbon nanofluids usually have higher thermal conductivities than metals and metal oxides, leading to higher collector performance.
This review presents the outcomes of experimental investigations conducted with carbon based nanofluids in non-concentrating collectors and discusses its effects on thermophysical properties.
From review, it can be inferred that the carbonaceous nanofluids improved the performance of solar collector systems. It was noted that the thermal performance and stability of nanofluid was affected by several factors such as preparation method, working temperature, concentration and dimension of nanoparticles etc. Likewise, the performance enhancements of collectors reported were mutually inconsistent and repeatability of the data was difficult to produce. There are also lack of studies on stability of nanofluids after utilization, carbon allotropes except nanotubes and graphene, and base fluids except water and glycols.
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•Carbon nanofluids provides highest collector performance enhancement than other nanofluids.•Carbon nanotubes and graphene derivates are most investigated nanomaterials for FPSC and ETSC.•Experimental examinations of carbon nanofluids in FPSC and ETSC still lacks long term stability.•Further investigation with other carbon allotropes, effect of morphology are still needed.
•Thermal analysis helps to choose the best performance enhancement technique.•Performance augmentation methods try to minimize thermal losses from collectors.•Use of nanofluids significantly enhances ...the thermal performance of collectors.•Integration of collectors with PCMs can both increase efficiency and save energy.
Solar energy as a plentiful and environment-friendly source of energy has an acceptable potential in nearly most of the regions around the world. Thermal technologies are commonly used to provide heat requirements of different domestic, agricultural, residential, and industrial applications from the sun. This paper reviews thermal performance enhancement techniques of the most widely-used low-temperature solar collectors (LTSCs) including flat-plate collectors (FPCs), evacuated tube collectors (ETCs), and compound parabolic concentrators (CPCs) by introducing challenges and discussing future research potentials. In this regard, energy analysis of each collector type along with the latest advancements to boost the heat collection capability of the LTSCs reported in the previous studies is presented. The discussed methods in this study broadly cover structural modifications, absorber coatings, integration with reflectors, using alternative working fluids including nanofluids, and employing thermal energy storage (TES) systems. This comprehensive review is reflecting the level of technical maturity of each type of LTSCs and is expected to serve scientists, engineers, and developers with the latest achievements in this technology.
Nanofluids as a new generation of fluids have attracted a lot of research works. There is growing evidence that the nanofluids are more suitable for various heat transfer applications than the ...conventional fluids. One of the most important parameters in the thermal behavior of such fluids is the type of nanoparticles dispersed in the base fluid. The present review paper is devoted to provide an overview on the distinct types of particles used in nanofluid research with emphasis on their application in solar collectors. The impact of nanofluids on heat transfer under different dispersive particles is explained for both numerical and experimental researches. Works concerning the application of nanofluids in solar collectors are singled out and analyzed. The nanoparticles are classified into metallic and non-metallic and the various configurations of solar collectors are considered. The review indicates that the flat plate solar collectors are the most investigated among the considered configurations. It is also shown that Al2O3-based nanofluids have been widely considered in these collectors compared to other nanofluids. In addition, the non-metallic based nanofluids can be more useful for the efficiency enhancement of solar collectors compared to metal based nanofluids. Some negative aspects related to nanofluids are also pointed out such as the thermo-physical instability and the reduced efficiency at high nanoparticle volume fraction. Finally, the general conclusions are drawn and future directions for research are proposed. It is noted that the present review concerns the nanofluids based on a single nanoparticle type. Hybrid nanofluids are subject to a future review.
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•Flat plate solar collectors are the most investigated among other configurations.•Al2O3-based nanofluids have been widely considered in solar collectors.•The results of Newtonian and non-Newtonian nanofluids are compared and discussed.•Non-metallic based nanofluids can be more useful for the efficiency of SC.
•Double wall tube and PCM are the keys of the developed compact solar collector.•The compact solar collector has a favorable performance compared to FSWHS.•Fins enhance the heat transfer ...characteristics during the simultaneous operation.•Fins assist in keeping the PCM profile almost uniform.
Space and weight requirements coupled with time delay between energy production and consumption represent great barriers toward further deployment of typical solar water heating systems in existing buildings with limited space. Therefore, a new compact U-pipe evacuated tube solar collector (ETC) integrated with paraffin wax (ALEX WAX 600) for energy storage is presented in the current study. The ALEX WAX 600 is an organic chemical-based phase change material (PCM) having an average melting temperature of 60°C and a thermal conductivity of 0.21W/mK. The key issue of the developed system is the elimination of entire systems components by storing the energy in the evacuated tube itself through the usage of paraffin wax. Due to the low thermal conductivity of paraffin wax, heat transfer plate (fin) with an area of 0.1251m2 is integrated in the proposed system. The present study investigates the developed system under two configurations; un-finned and finned U-pipe evacuated tube solar collector side by side with a typical forced recirculation solar water heating system (FSWHS) under the same operation and weather conditions. The operation of the solar water heating systems is studied during the on-demand operation under a simultaneous operation and a real water consumption profile. The results clarify the favorable performance of the developed compact solar water over the typical FSWHS during different operation scenarios and weather conditions due to their low thermal inertia. Furthermore, the utilization of fin in the developed system has a substantial effect on improving the heat transfer characteristics of the PCM and enhances the overall system stability. During simultaneous operation tests, the total effective energy discharged for the un-finned system is higher than FSWHS by 35.8% under clear day weather conditions. However, the finned system is higher than FSWHS by 47.7%. The simultaneous long-term predictions based on regression modeling show that the average annual efficiency is 71.8%, 85.7% and 40.5% for the un-finned, finned and FSWHS systems, respectively. During real water consumption profile tests, the daily system efficiency is, 33%, 26% and 20% for the un-finned, finned and FSWHS systems.
Among different sources of renewable energy, solar energy is widely used almost exclusively because of its ease of availability and its lowest environmental effects. The most commonly used solar ...collectors are the flat plate solar collectors (FPSCs). However, they are less powerful (low capacity to convert solar energy to thermal energy). It is possible to classify the use of nanofluid on FPSCs as an efficient way to boost the solar collectors’ performance. In this paper, studies on metal oxides, non-metal oxides, solid metals, semiconductor nanomaterials, carbon nanostructured, and nanocomposite nanofluids used as heat transfer fluids (HTFs) within FPSCs are examined sequentially. Various parameters affecting the FPSC’s thermal efficiency, such as nanoparticle type, nanoparticle concentration, nanoparticle size/shape, solar radiance, and mass flow rate, are extensively analyzed. Studies have also compared various types of single nanofluids or mixture nanofluids with FPSCs under the same operating conditions. It is found that the use of carbon-based nanofluids compared to metal oxides of nanofluids under the same conditions has resulted in a major improvement in the energetic and exergetic performance of the FPSC. Furthermore, the reviewed research revealed that there is a tremendous opportunity to achieve the commercial application of carbon-based nanofluid FPSC. The obstacles and opportunities for further study are also highlighted.
•Single-layer graphene and GO nanofluids were designed for solar thermal application.•Uniform temperature distribution is achieved in nanofluids based on the RI-DASC mode.•The photothermal conversion ...efficiency of SLG is 49.13% at 100 ppm in the RI-DASC mode.•The photothermal conversion efficiency of GO reaches 46.26% at 100 ppm in the RI-DASC mode.
Direct absorption solar collectors (DASC) are extremely attractive in solar energy utilization. In this paper, starting from these two aspects, graphene-based nanofluids, including single-layer graphene (SLG) and graphene oxide (GO), are prepared to enhance solar absorption and photothermal conversion performance. The influence of nanofluids' concentration and two different irradiation modes: traditional DASC and reverse radiation DASC (RI-DASC) on the photothermal conversion performance of graphene-based nanofluids has been studied. The addition of a small amount of SLG or GO significantly improves the photothermal conversion efficiency of base fluid, and it increases along with the increase of the concentration. Furthermore, the RI-DASC mode has a more uniform thermal field distribution and the higher photothermal conversion efficiency than the DASC mode. In DASC mode, the photothermal conversion efficiency of pure water is 17.00%. By changing the irradiation mode, the introduction of GO and SLG nanofluids (100 ppm) increased the photothermal conversion efficiency by about 172% and 189%, reaching 46.26% and 49.13%. According to the photothermal-thermoelectric conversion experiment, the positive correlation between the output power of the TE module and the heating state of the nanofluid makes it possible to adjust the nanofluid in real-time. This work presents a feasible way to enhance solar energy absorption and improve the photothermal conversion efficiency of nanofluids for DASC.
•This review presents effect of nanofluids in solar Evacuated Tube Solar Collectors.•Recent studies on this type of solar collector are summarized.•Highest thermal enhancement belongs to heat pipe ...model and SWCNT-based nanofluids.•Nanoparticles concentrations are challenges of using nanofluid in ETSCs.
This review presents impact of nanofluids in solar evacuated tube solar collectors (ETSCs). Recent works on this type of solar collector are summarized. The first part depicts the significance of choosing ETSCs for solar domestic hot water in addition to classification of these collectors and application of each categories. In second part, usage of nanofluids is studied in considered system and through this pervasive review, some issues such as, various kinds of nanofluids, size of nanofluids, volume fraction, and nanofluid application in every kind of ETSCs on the heat transfer augmentation have been analyzed. This state-of-the-art review shows that most of nanofluids in ETSCs are water-based with nanoparticles of TiO2, CuO, and Al2O3. Other types of nanoparticles including CeO2, WO3, Ag, CeO2, GNP, and Cu were less considered up to now. Moreover, 40, 34, and 26% of nanofluids were used with size between 1–25 nm, 25–50 nm, and 50–100 nm, respectively. Furthermore, highest thermal improvement belongs to heat pipe type solar collector with SWCNT-based nanofluids by 93.43% at ṁ = 0.025 kg/s. Besides, nanoparticles concentrations are challenges of using nanofluid in ETSCs. This parameter has marginally affected on performance of ETSCs. So, the optimum amount of this parameter for each type of nanofluids will suggest for future works.
The present work analyzes the performance of unshielded receiver tube integrated solar parabolic trough collector where Al.sub.2O.sub.3/deionized (DI) water nanofluid of low concentrations was used ...as heat transfer fluid (HTF) element. Nanofluid is synthesized at various volume fractions starting from 0.2 to 1.0% with surfactant-free condition, by ultrasonic technique. Several researchers investigated the performance of higher nanofluid concentrations (1.0-5.0%) with and without surfactants on parabolic trough solar collector. The outdoor experiments are conducted for two HTF flow rates of 0.010 kg s.sup.-1 and 0.015 kg s.sup.-1. When the nanofluid is subjected as HTF, the DI water acted as a base fluid. While DI water is allowed to flow through the absorber, it performs both as HTF and heat storage fluid. The synthesized nanofluid at various volume fractions is allowed to flow through the receiver for the purpose of analyzing the thermal performance and compare the results with DI water. The collector efficiency increases with the mass flow rate as well as the concentration of nanofluid. For 0.015 kg s.sup.-1, the maximum efficiency was calculated as 59.13% (hourly) and 58.68% (average).
•A methodology for the outdoor testing of hybrid PVT collectors is presented.•Glazed and unglazed collectors featuring c-Si and thin-film PV modules are compared.•Steady state and dynamic thermal ...performance of the PVT collectors are characterised.•Thermal and electrical energy yields over diurnal operating periods are assessed.•Experimental performance is compared to model simulations under intermittent cloud conditions.
Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of ∼8 min is measured for a commercial PVT module, compared to <2 min for a flat plate solar-thermal collector. It is also concluded that the use of a lumped, first-order dynamic model to represent the thermal mass of the PVT collector is not appropriate under certain irradiation regimes and may lead to inaccurate predictions of the system performance. This paper outlines a procedure for the testing and characterisation of solar collectors, provides valuable steady-state and dynamic performance characterisation data for various PVT collector designs, and also provides a framework for the application of this data in a system model to provide annual performance predictions in a range of geographical settings.
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