. you are here->home->Mechanical Engineering->Solar energy Labs->Parabolic Trough -Flow Rate . . Parabolic Trough -Flow Rate . . Objective: To measure the Outlet and Inlet Temperatures and Flow rate of the Parabolic trough collector as a function of flow rate variation. To plot the Inlet and Outlet temperatures and Flow rate versus Time. To calculate the Efficiency of the compound parabolic concentrator. Introduction The optical principle of a reflecting parabola is that all rays of light parallel to its axis are reflected to a point.  A parabolic trough is simply a linear translation of a two-dimensional parabolic reflector where, as a result of the linear translation, the focal point becomes a line.  These are often called line-focus concentrators.  A parabolic dish (paraboloid), on the other hand, is formed by rotating the parabola about its axis; the focus remains a point and are often called point-focus concentrators.  If a receiver is mounted at the focus of a parabolic reflector, the reflected light will be absorbed and converted into heat (or directly into electricity as with a concentrating photovoltaic collector).  These two principal functions, reflection to a point or a line, and subsequent absorption by a receiver, constitute the basic functions of a parabolic concentrating collector.  The engineering task is to construct hardware that efficiently exploits these characteristics for the useful production of thermal or electrical energy.  The resulting hardware is termed the collector subsystem.   The structure has a steel frame with body made of fibreglass. The receiver is a steel tube, with black chrome coating to reduce radiation losses. The reflector is Alanod reflectors, which has the structure as shown below (courtsey : Alanod website) The solar reflectance of the Alanod sheet is given below (Alanod website) Generally, a collector consisits of a receiver or absorber that converts sunlight to heat and transfers it to water in the tube(s). The collector is insulated on the back to keep the heat transfer losses down. Though, there still exist radiation and convection losses. The following modelling equations are generic, and can be modified to specific Flat Plate or Parabolic collector set up. See Ref 2. for further details. Energy Balance of Solar Collector Though the incoming solar radiant power is 100% availability, the glass cover (if used) has a reflectance-absorptance-transmittance characteristic, which loses 10% of available energy. About 8% loss through reflection, and another 2% through absorption. So 90% solar radiation is transmitted through to the receiver or absorber. The sum of reflectance-absorptance-transmittance must be 1. If the glass cover is in thermal equilibrium,then absorptance $\alpha$ = emittance $\epsilon$ $\rho +\alpha +\tau = 1$ Out of 90% transmittance, 8% energy is lost due to absorber reflection, 6% loss due to absorber emission, and up to 13% in convection losses. Conduction losses through the back of the receiver ot absorber is 3%. This leaves net available solar radiation for transfer to heat transfer fluid at 60% of initially available total solar radiation. $\phi_{e} = \phi _{e} \left ( \alpha +\rho +\tau \right )$ The absorber or receiver (flat plate or parabolic) must have high absorptance in the wavelength range below 2 micron, as the main solar spectrum is in this range. the absorber can heat upto 350K, and this emittance spectrum is higher than 2 micron. Since $\alpha = \epsilon$, the absorptance should be as low as possible above 2 micron to limit radiation losses. Collector Performance: The collector converts solar irradiance, E, to absorber surface ($^{A}c$) heat. Power ouput $_{Qout} = \tau E A_{c} - Q_{ref} - Q_{conv} - Q_{rad}$ is reduced by losses due to reflection, convection and radiation. Collector Power Output: We combine the following terms and substitute, $Q_{conv}+Q_{rad}=Q_{RC}$ $Q_{ref}=\tau \rho E A_{c}$ $Q_{out}=\tau E A_{c} \left ( 1-\rho \right ) - Q_{RC}$ For absorber, $\alpha = 1-\rho$ So, $Q_{out}=\tau \alpha EA_{c}-Q_{RC} = \eta _{o}EA_{c}-Q_{RC}$ where the product of transmittance and absorptance is replaced as the Optical Efficiency of the absorber or collector. Losses: $Q_{RC}=a1A_{c}\left ( T_{c}-T_{a}} \right )+a2A_{c}\left ( T_{c}-T_{a} \right )^{2} = a A_{c}\left ( T_{c}-T_{a} \right )$ a1 and a2 are loss coefficients. Evacuated tube collector loss coefficients arer much lower than Flat plate collectors. The loss coeffiecients are experimentally determined. For a Flat Plate Collector, optical efficiency is 0.7-0.8, a1(W/m^2K) = 2.5-3.8, a2(W/m^2K^2) = 0.007-0.23. Efficiency:  $\eta _{c}= \frac{Q_{out}}{E A_{c}} = \eta _{o} - \frac{Q_{RC}}{E A_{c}}$ $\eta _{c}= \eta _{o} - a1(T_{c}-T_{a})+a2(T_{c}-T_{a}))/2E$ Collector Flow Rate: $Q_{out}=\hat{m}C_{p}(T_{out}-T_{in})$ $\hat{m}= Q_{out}/C_{p}(T_{out}-T_{in})$ The collector flow rate should be high so that efficiency does not go down from high collector temperatures. On the other hand, flow rate should not be too high, in order to maintain the energy transfer demands within acceptable range. ${m}'=\eta _{o}E-a1(T_{c}-T_{a})-a2(T_{c}-T_{a})^{2}/ C_{p}(T_{out}-T_{in})$ This is the mass flow rate rewritten as a area flux, measured in kg/m^2 hr. We have a gear and drive mechanism with Bonfigioli gear set with a BLDC motor assembly. The solar azhimuthal and longitudinal position of the specific day at the specific time is calculated using a solar position algorithm and the trough is rotated and positioned to face the sun,automatically. The user can manually pick the angle at which the sun is faced and the trough will get remotely positioned to the angle chosen by the user. Water at room temperature is fed to the receiver through the flexible pipe. It gets heated up and the hot water exits from the other side of the receiver pipe.   Please enable JavaScript to view the comments powered by Disqus. Cite this Simulator:vlab.amrita.edu,. (2015). Parabolic Trough -Flow Rate. Retrieved 23 August 2017, from vlab.amrita.edu/?sub=77&brch=298&sim=1746&cnt=1 ..... ..... .....
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