Calculating water flow over a solar panel

syiafuku

Hi all, firstly i'm sorry because don't know which thread category should I ask from, but here goes:

I'm doing my final year project about cooling solar panel with water film.basically it's flowing water over the panels. how can I calculate the water flow? does this have got to do with the q= m.c.deltaP? c being the specific water heat capacity?

Bradenbraden

I think you need to elaborate the question. Is the answer as simple as finding the volume of water you have poured over the panel? re-word your question :)

syiafuku

oh sorry :) ok the solar panel will use water for cooling it, by flowing water on to the surface of the solar panel. This water, i.e the water that just flowed over the panels, will then enter a tank, and will be circulated back by a pump to flow again on the solar panel.

so what data should i need to know in order to get that water flowrate?

russ_watters

You need to know the amount of heat being absorbed by the solar panel that you want to dissipate.

Also, I don't see anything in your description about removing the heat from the water - so the water is just going to keep getting warmer and warmer.

syiafuku

ah!you're right.thank you for pointing it! i missed that.what should i do to dissipate the heat from the water? would using a aluminium tank sufficient enough to do that?

gmax137

Try to write out the steps, or make yourself a drawing of the entire system. Mark the places where heat goes in and where it goes out. Then figure out the magnitudes of these, and only then can you start to picture the components themselves. In other words, work from the top down - you can't decide the tank material (eg aluminum) until you know how big it is, and how much/how fast heat must be removed.

Artman

I assume you are trying to cool a photovoltaic panel to make it more efficient. If this is the case, there is a break point at which the cost of moving the water will exceed the benefit of cooling the panel. As an extreme example, 1000 gpm poured over a single panel will certainly cool the panel, but will require a high HP pump, large piping, a large tank to catch the water, but the TD of the entering to leaving water will be low. The cost of a system moving 1000 gpm over a single panel would not be recoverable by efficiency gain of the solar panel in a reasonable amount of time.

A better use of the recovered cooling water would be to heat another process, such as preheat domestic water before it goes to a water heater. Then calculate how many gallons per day could be preheated from say 50 deg F to 80 deg F. Then figure out how many gpm that requires and the temperature difference, account for evaporation, it will have to have a heat exchanger so there will be efficiency losses in that.

berkeman

Also, I don't think you want to flow the water "over" the solar panel. Why would you not want the sunlight to have to go through the water to get to the cells? How else could you route the water? Are there maybe other means of cooling the panels? Where are they mounted? What is the optimum operating temperature of the panels, and how does the efficiency vary about that temperature?

dr dodge

I would also think that running water in a "non sealed" system over the front would lead to deposits being left on the solar panel, slowly decreasing its efficiency, too

I wound think more on the lines of a heat sink on the back to draw the heat away

dr

Artman

Absolutely. If the surface air temperature close to the panels is high, the relative humidity near the panels will be low and evaporation will be fairly high as well, I would think. This is also a factor of the flow rate, if the flow rate is high enough, the panels will be cool enough to reduce evaporation rate, again it is a trade off, it won't be as efficient. Probably the most effective method, at first, would be a fine spray mist allowed to evaporate on the surface. This could bring the panels down to wet bulb temps, but it will leave deposits over time, depending on the hardness of the water used. Some sort of wash down method should be employed.

If rainwater could be collected that is fairly soft and would leave fewer deposits.