1 State-of-the-art Heat Transfer Fluids for Parabolic Trough 2 Collector 3 Yathin Krishna1, M.Faizal1, R. Saidur2,3, K.C.Ng5, Navid Aslfattahi4 4 1School of Engineering, Taylor’s University, Lakeside campus, 47500 Selangor Malaysia. 5 2Research Centre for Nano-Materials and Energy Technology (RCNMET), School of Science and Technology, 6 Sunway University, No. 5, Jalan University, Bandar Sunway, Petaling Jaya, 47500 Selangor Darul Ehsan, 7 Malaysia. 8 3Department of Engineering, Lancaster University, LA1 4YW, UK. 9 4Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, 10 Malaysia. 11 5Department of Mechanical Engineering, Materials and Manufacturing Engineering, The University of 12 Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia. 13 Contents 14 Abstract ................................................................................................................................ 2 15 1. Introduction ................................................................................................................... 4 16 2. Concentrating solar power plants ................................................................................... 6 17 2.1. Influence of thermophysical properties of HTF on Parabolic trough efficiency ...... 15 18 2.2. Energy and Exergy Modelling ............................................................................... 15 19 2.3. Thermophysical properties required for HTF in PTC ............................................. 19 20 3. Recent developments in heat transfer enhancement ...................................................... 22 21 3.1. Water/steam as heat transfer fluid ......................................................................... 24 22 3.2. Synthetic thermal oil as heat transfer fluid ............................................................. 25 23 3.3. Molten Salts as heat transfer fluid ......................................................................... 26 24 3.4. Nanofluids as heat transfer fluid ............................................................................ 40 25 3.4.1. Properties and preparation ................................................................................. 42 26 3.4.2. Progress in obtaining stable nanofluid................................................................ 49 27 3.4.3. Water-based Nanofluids .................................................................................... 54 28 3.4.4. Ethylene glycol-based Nanofluids...................................................................... 56 29 3.4.5. Synthetic Oil-based Nanofluids ......................................................................... 57 30 3.4.6. Molten salt based Nanofluids ............................................................................. 59 31 3.4.7. Hybrid nanofluids .............................................................................................. 64 32 3.5. Gaseous Heat transfer fluids .................................................................................. 68 33 3.6. Liquid Metals as the heat transfer fluid .................................................................. 72 34 3.7. Ionic Liquids ......................................................................................................... 74 35 3.8. Vegetable oil ......................................................................................................... 76 36 4. Prospects of parabolic trough collector ......................................................................... 77 37 5. Conclusion ................................................................................................................... 77 38 References .......................................................................................................................... 81 39 40 1 41 Abstract 42 Solar thermal energy conversion is gaining more attention among researchers due to the 43 recent development in nanofluids and molten salt technology. Among various solar 44 collectors, parabolic trough collector has received significant attention from researchers due 45 to their operating temperature range (60-240 °C) feasible for power generation. Parabolic 46 trough collector is currently having a higher number of installations compared to other 47 concentrated solar power technology around the globe. Most of the conventional heat transfer 48 fluid used in PTC has poor heat transfer and light to heat conversion properties. Therefore, it 49 is advantageous to enhance the thermophysical properties of heat transfer fluid to improve the 50 overall efficiency of the system. Well-engineered nano-enhanced heat transfer fluid is 51 advantageous because a very low mass fraction of nanoparticles bring considerable 52 enhancement in thermophysical properties. This paper focuses on the most recent 53 advancement in heat transfer fluids, their preparation, and stability issues when doped with 54 nanoparticles. Various heat transfer fluids currently used in parabolic trough collectors and 55 the nano-enhanced heat transfer fluids having the properties better than conventional heat 56 transfer fluids are compared and their preparation methods and properties are discussed. 57 Enhancement of thermophysical properties of molten salts by doping nanoparticles and their 58 enhancement in thermal stability at high temperature, the possibility of using mono and 59 hybrid nanofluid, ionic liquids, gaseous heat transfer fluid, and vegetable oil as the heat 60 transfer fluid in parabolic trough collectors are the key highlights of this review. 61 Keywords: Parabolic trough collectors, Heat transfer fluids, Nanofluids, Renewable energy. 2 62 Abbreviations RGO Reduced graphene oxide CNT Carbon nanotube SEGS Solar energy generating system CPV Concentrated photovoltaic SEM Scanning electron microscope CLFR Compact Linear Fresnel reflector STA Simultaneous thermal analysis CSP Concentrated Solar Power SWCNT Single wall carbon nanotube DASC Direct Absorption Solar Collector TEM Transmission electron microscopy DI Deionized water TES Thermal energy storage DLS Dynamic light scattering TGA Thermogravimetric analysis DNI Direct normal irradiation UV-Vis Ultra Violet-Visible spectrophotometer DOE Department of Energy Greek Symbols DSC Differential scanning ηth Thermal efficiency calorimetry λ Thermal conductivity, W/mK DSG Direct steam generation µ Fluid dynamic viscosity, Pa.s EG Ethylene glycol ρ Density, kg/m3 EEW Electrical explosive wire φ Volume fraction of nanoparticles, (%) EO Engine oil ɛ Emittance FPSC Flat plate solar collector GHG Greenhouse gas Nomenclature 2 GE Graphene Aarea Aperture area, m 2 GNP Graphene nanoparticles Ar,outer Receiver outer area, m 2 GO Graphene oxide Ar,inner Receiver inner area, m HCE Heat collecting Elements cp Specific heat capacity, kJ/KgK HDH Humidification and Dr,in Receiver inner diameter, m 2 Dehumidification Gbeam Solar beam radiation, W/m 2 HTF Heat transfer fluid h Convective heat transfer coefficient, W/m K IEA International Energy Agency k Thermal conductivity, W/mK LFR Linear Fresnel reflector Nu Nusselt number,- MSBNF Molten salt based nanofluid Pr Prandtl number - MWCNT Multi-wall carbon nanotube Re Reynolds number - PCM Phase Change Material Qsolar Total available solar radiation, W PNC Plasma nano colloid T Temperature ⁰C or K PSD Particle size distribution PTC Parabolic trough collectors PV Photovoltaic 63 3 64 1. Introduction 65 The energy consumption in 2018 is twice the rate of energy consumption since 2010. Figure 66 1 shows the annual energy demand growth from 2010 to 2018. Due to increased energy 67 consumption, the CO2 emission rose to a 33.1G tonne in 2018, which is highest in history, as 68 shown in Figure 2. Therefore, there is a need to scale up renewable energy production to meet 69 the energy demands of the rapidly growing economy of the world. Also, due to changing 70 weather conditions resulting from GHG (greenhouse gas) emissions, there is an increasing 71 demand of power for heating and cooling equipment in many parts of the globe [1]. Using 72 renewable and sustainable energy sources, the impact on the environment can be reversed by 73 reducing emissions caused by GHG and fulfill the demands of energy guzzlers. Therefore, 74 there is a need for energy production in a gigawatt power scale to meet the energy demands. 75 Solar, wind, bio-energy, hydropower is well-established power sources that can meet energy 76 demands if the efficiency of the system further investigated and improved [2]. In one hour, 77 energy from sun radiating on earth is 410×1015 J, which is more than the total energy 78 consumed worldwide in 2001[3]. Therefore, solar energy is a promising alternative for 79 current challenges to fulfill energy demands. 80 There are three distinct technological approaches to utilize solar energy: solar photovoltaic, 81 solar thermal, and solar fuel technologies [4]. Currently, PV (photovoltaic) and solar thermal 82 are the leading technologies to utilize solar energy. PV directly converts solar radiation to 83 electricity [5] but, the efficiency of PV decreases with an increase in temperature, production 84 of power during overcast day is not possible; these are the significant challenges in PV solar 85 cells [6]. The solar thermal system employs mirrors or lenses to focus the sun’s radiation on 86 an absorber. HTF (Heat transfer fluid) is made to pass through the absorber tube, which 87 converts water to steam and then the