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Parabolic Trough - Angle
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Objective:

  • To measure the Outlet and Inlet Temperatures of the Parabolic trough collector as a function of angle of incidence of solar radiation.
  • To plot the Inlet and Outlet temperatures versus Time.
  • To calculate the Efficiency of the compound parabolic concentrator.

                 

 

Introduction and theory:

                The sun is considered as the major source of energy on the planet earth because of the vast amounts of energy that are made freely available. If harnessed by modern technologies we can use it in different ways. One of the easiest ways is using photovoltaic cells, which will directly convert solar energy to electricity. A much safer and practical way of harnessing the solar energy is to use the solar energy to heat up water. By heating continuously in a loop, it will make steam to rotate the electricity - producing turbine. We can make use of the latter technology by doing simple arrangements.

              The basic arrangement consists of one solar collector (mirrors or parabolic dishes), that collects the heat from sun and one glass tube containing a heat transfer fluid (mineral oil, synthetic oil, molten salt etc.). In the case of parabolic solar collector set up, it contains one parabolic shaped mirror as the heat collector and one glass tube containing black painted pipe carrying heat transfer fluid on the focus of the parabolic mirror. In order to collect maximum heat reflected from the parabolic mirror, it must cover the entire length of the parabolic reflector. If the parabolic mirror tracks the sun on one linear axis from north to south during the day we can collect maximum radiation from the sun so that the fluid is easily getting heated. This is called tracking. We can do the tracking by means of electronic circuits using microcontrollers or data acquisition systems.



               The variation of insolation (Solar intensity) during the day time is shown in Fig 1. In the figure we can see that solar intensity at particular region will vary depending on time. We will get maximum heat only at a particular angle of incidence. In order to get optimum performance collector must be aligned on the azimuth of the location. The longitude of the location determines the azimuth (Difference between magnetic south and true south).To get the maximum efficiency, the suggested installation angle for the parabolic collector is latitude of the location plus 15 degree.

 

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 ( Ac ) 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}= {Q_{out}}/{E A_{c}} = \eta _{o} - {Q_{RC}}/{E A_{c}}}

\eta _{c}= \eta _{o} - {a1(T_{c}-T_{a})+a2(T_{c}-T_{a})/{2}}{E}}]

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.

Cite this Simulator:

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