An experimental investigation on the effect of ferrofluids on the efficiency of novel parabolic trough solar collector under laminar flow conditions Mustafa Alsaady, Yuying Yan, Rabah Boukhanouf Faculty of Engineering University of Nottingham, UK
Introduction The demand for modern energy services is increasing rapidly. The growth of the global energy demand is projected to rise by about 35% from 2010 to 2040. Without using the energy efficiently the demand was estimated to grow by 140 %. China and India together are expected to account half of the growth. Industry accounts for 30% of primary energy usage and 50% of electricity demand. Solar energy has the potential to meet a significant portion of the world s energy demand Solar Energy is sufficient, clean renewable source, and it is available in both direct and indirect forms.
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Solar Energy Two Types: Photovoltaic and Thermal Photovoltaic technology directly converts sunlight into electricity. Solar thermal technology harnesses its heat. Solar Thermal Collectors classified as low-, medium-, or high temperature. High temperature collectors use mirrors and lenses to concentrate the sunlight (concentrated Solar Power)
Concentrated Solar Power (CSP) Concentrated Solar Power (CSP) systems have advantage of easier energy storage compared to photovoltaic (PV) systems. CSP technologies include the parabolic trough systems, parabolic dish systems, solar tower systems and linear Fresnel systems. The most commercially and technically developed CSP technology is the parabolic trough technology. The successful operation of the first solar electricity generation system (SEGS) in California s Desert was the reason for the technical and commercial success of parabolic trough systems.
Parabolic Trough Parabolic trough is effective for industrial process heat application. Almost all industrial process requires heat in temperature ranges that can be provided by a solar thermal system. Parabolic trough can also provide an alternative to cooling processes in sectors. However, The cost of energy generated by Parabolic trough system is higher than conventional energy sources
Introduction Researchers, attempt to improve the solar collector efficiency to ease the high solar energy costs. One of the methods to reduce cost is increasing the heat transfer performance of parabolic trough systems. Heat transfer can be enhanced by increasing the thermal conductivity of the heat transfer fluid (HTF) Suspending nano-sized metal particles in common fluids (nanofluids) can significantly increase the thermal conductivity
Ferrofluids A Special branch of nanofluids is Ferrofluid. Ferrofluids also known as magnetic nanofluids (MNF) are dispersed colloidal ferromagnetic nanoparticles in fluids. Due to the magnetic property of solute particle, the thermo-physical characteristics of ferrofluids properties such as thermal conductivity and viscosity can be effectively enhanced and controlled by external magnetic field.
Ferrofluids Parabolic Trough Collector (FPTC) The main difference between the conventional solar parabolic trough collector and FPTC is the magnet or electromagnet found in FPTC design. The parabolic trough collector (PTC) used in the study consisted of a sheet of reflective mirror with parabolic shape, absorber tube, and electromagnets. 2 types of absorbers was used a) non-direct absorber (black coated copper tube) b) direct absorber (Glass tube). No envelope was used in this experiment.
Ferrofluids Parabolic Trough Collector (FPTC) Non-direct Absorber Direct Absorber
Ferrofluids Parabolic Trough Collector (FPTC) Three electromagnets are placed in the entrance, middle, and the outlet of the absorber. The electromagnet is air core solenoid with an inner diameter close to the outer diameter of the absorber tube, allowing the absorber tube to enter into the electromagnet. The electromagnet generates a uniform magnetic field when a current is passing through the coils. Adjustable DC power supply is used to control the current applied to the electromagnet with maximum current of 5A.
FPTC Experimental Setup An experimental setup was developed based on BS EN ISO 9806:2013 Solar energy. Solar thermal collectors. Test methods to study the performance of the proposed ferrofluids parabolic trough. The experiment was carried out with indoor solar simulator. The experimental set up consisted of water tank, pipeline system, solar simulator, water pump, parabolic trough, heat exchanger, chiller, chiller pump, and measurement system. The temperatures in the inlet, outlet and atmosphere were measured by non-magnetic type T thermocouples.
FPTC Experimental Setup Parameters Values Length of the receiver 500 mm Width of the aperture 250 mm outer Diameter of the absorber 15 mm Solar irradiance 1000 W/m 2 Mass flow 0.02 kg/s Magnetic field intensity 3.14, 6.28,10.47mT A pyranometer was used to investigate the solar radiation. The maximum unevenness error percentage was about 12.3%, which is in a good agreement within the permissible limits of 15% provided by British standards for testing a solar simulator. The solar simulator turned on and kept for 30 minutes before taking any readings. A steady state method was used to calculate the parabolic trough efficiency, according to BS EN ISO 9806:2013 standard η = ሶ Q U I D A C
Results FPT efficiency non-direct absorption in the absence of external magnetic field
Results FPT efficiency non-direct absorption in the presence of external magnetic field
Explanation to the Enhancement of Thermal Conductivity in Ferrofluids The enhancement of thermal conductivity is attributed to the improved conduction of heat during the formation of chain-like particle assembly under the magnetic field. They observed the presence of a maximum value of thermal conductivity of fluid as increasing the flied intensity. The followed decrease was explained by the Zippering of chains (clumping of chain). The enhancement can reach to 300%. Philip, J., P.D. Shima, and B. Raj, Evidence for enhanced thermal conduction through percolating structures in nanofluids. Nanotechnology, 2008. 19(30): p. 305706.
Results FPT efficiency non-direct absorption in the absence of external magnetic field
Results FPT efficiency non-direct absorption in the presence of external magnetic field. ± signs describe the orientation of magnetic field to the flow direction. + sign the magnetic field is parallel to the flow direction. - sign the magnetic field is opposite to the flow direction
Results FPT efficiency direct absorption in the absence of external magnetic field
Results FPT efficiency of direct absorption in the absence of external magnetic field
Results FPT efficiency direct absorption in the Presence of external magnetic field
The Potential of FPT In a conventional parabolic trough systems, the absorber tubes are supported at the focal line by support brackets that run from the collector to the absorber tube. There is a support bracket at each end of every absorber (about every 4 m of absorber length). Those brackets are a source of heat loss. On average the brackets can contribute to 1.5 % of the total heat loss depending on the operating and ambient temperature. In FPT the magnets can be installed on the brackets, which will increase the efficiency of parabolic trough.
Future Work Developing a ferrofluids suitable for heat transfer applications. (No surfactant & stable) Environmental and Economic Analyses. Numerically investigate the performance of FPTC using Lattice Boltzmann method. Build a pilot plant of FPTC with thermal storage at the University of Jeddah.
Conclusion The thermal efficiency of the solar collector enhanced when nanoparticles are added to the base fluid. In the presence of external magnetic field the thermal efficiency increased further. This parabolic trough benefits from the magnetic and optical property of ferrofluids which enhance the thermal efficiency of the collector. In a direct absorption the optical path is important factor in absorption. Therefore the diameter of the absorber is important factor. Increasing the diameter will increase the absorption rate. Utilizing ferrofluids in concentrating solar collectors has environmental benefits and causes improving heat transfer, consequently reducing the needed heat transfer area of collectors. These advantages could be achieved with little change on the system.