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Solar Thermal Water Heater Efficiency

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Hey all, new to this.

I am working on a project involving the efficiency of a solar water heater. The basic set-up is a two-loop system. One loop has the solar absorber, in my case a Flat-Plate glazed collector, a centrifugal pump, and a heat exchanger (counter-flow brazed plate) interfacing with second loop. The second loop is an insulated water tank, pump, and same heat exchanger. 

My question is whether I am using the correct efficiency equation for my solar absorber.
I am using this equation
n = Qu / Ac*It
Qu is the energy output (in Watts)
Ac is the collector area (in m^2)
It is the solar irradiance (In W/m^2)

I have further broken the Qu down to
Qu = Ac*Fr*[Ipl - Upl(Tin - Tair) 

With Fr being the Heat removal Factor
Ipl being the Radiation Absorbed by the plate (in W/m^2)
Ul being a combination heat transfer coefficient that takes into account frontal, perimetral, and rear losses.
Tin being the Solar Collector Inlet Temperature (in C)
Tair being the ambient air temperature (in C)

And furthermore am using the following equations:
Ipl = It * (alpha)(tau)
alpha being the absortivity of the flat plate absorber
tau being the transmittivity of the plastic above the plate

Fr = G*cp*(Tout-Tin) / (Ipl - Upl(Tin-Tair))
G being mass flowrate per absorber area (g/s*m^2)
cp being specific heat capacity of fluid (water) (in J/g*C)
Tout being Solar Collector Outlet Temperature (in C)

This has worked so far with my data, but as time elapses, my solar collector efficiency tends to approach 100%, which seems incorrect. I am wondering if I am using the incorrect equation for my setup or not taking into account something. My solar collector delta T tends to keep going up(but i dont think it should push efficiency up). No real steady state due to consistently changing irradiance.

I am an electrical engineering student primarily, so I have never formally taken a mechanical engineering university course.

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  • 11 months later...

let me try to give a suggestion. Your installation is more complex than your equation which you following. So many heat losses during operation especially when water outlet from solar heater into heat exchanger. About delta t (deviation temp) between outlet solar heater and inlet feeder tanks, you must notice when fluid has deviation temp. It also release heat energy so enthalpy must have deviation too. Correct me if I wrong.

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