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In the previous assignment, you determined the total area required for a solar farm at your
sites to produce 50 MW of electricity per year. Your employer recently heard about a potential
competitor who is designing a similar solar farm, they are designing a cooling system to
reduce the operating temperature of the solar panels. Cooling water is pumped over the top
surface of the solar panel, flowing over the surface as a “free film” of liquid. The cooling
water is available from nearby river, with an average temperature of 22°C, and a pump is used
to supply a mass flowrate of 1.2 kg/s water to each panel. The surface temperature of the solar
panel is to be maintained at ≤ 30°C. The dimensions of the solar panel are unchanged from
your Assignment II.

Investigate the rate of evaporation from the free-film cooling system

A. Draw a clear diagram of the mass balance for the free-film cooling system over the
solar panel, including all related assumptions. (5 marks)

B. The location of the solar farm proposed for this assignment is the same as your
Assignment II. Using the Bureau of Meteorology, identify key data to be used for
mass balance calculations; you will need to collect data to represent both mid-
summer (hottest time of the year) and mid-winter (coolest time of the year) so that
you can perform your calculations with both extremes in mind. Carefully reference
your data source and include any assumptions made. (8 marks)

For questions C – E below, show hand calculations for one condition (i.e. mid-summer or
mid-winter), and include an Excel spreadsheet showing the solution for both mid-summer
and mid-winter conditions. From a discussion with your employer, it is not clear whether or
not this evaporation problem is better analysed as a convective problem or a stagnant film
diffusion problem.

C. Estimate the convective mass transfer coefficients, km, for the free-film cooling
system for a solar panel at your site, assuming either forced or natural convection,
and determine the evaporation rate based on convective analysis. (14 marks)

D. Estimate the evaporation rate for the free-film cooling system assuming stagnant film
diffusion. The frame holding the solar panels is used to help direct the flow of water,
such that there is a stagnant air film above the water of ~1 cm. Based on your answers
to C and D, which approach makes more sense? (12 marks)

E. Will the evaporative mass transfer rate affect the outlet rate of water from the panel
surface? (4 marks)

F. The day-to-day variations in ambient conditions might dramatically impact the
evaporation rate for this system. Use graphs to show the effect of varying wind
velocity and relative humidity on the evaporative rate. Graphs can be produced in
Excel, but provide explanations to describe the results. (10 marks)

G. Calculate the evaporative heat rate (W) associated with evaporation of water from
the free-film. Explain in words how this would affect the energy balance and power
generation capacity for the solar farm, in comparison to an alternative approach of
back-surface panel cooling whereby cooling jacket was installed at the back of the
panel. Would you recommend jacketed cooling system or free-film approach?

Explain your answer in terms of fundamental heat and mass transfer concepts. Limit
your answer to a maximum of 150 words. (7 marks)

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