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This is a homework exercise I got:

A renewable energy facility has to be built immediately under the Golden Gate Bridge. Estimate the time-averaged power that can, at the resource level, be generated by such a facility. The answer needs to be expressed in GigaWatts.

I got till so far:

Potential energy content of the water in the basin at high tide = ½ × area × density × gravitational acceleration × tidal range squared

= 0.5 x (1280 x 27.4)m x 1000 kg/m^3 x 9.8 x 10^2

= 1.7185 x 10^(10) J

total energy potential per day = Energy for a single high tide × 2

= 3.437 x 10^(10) J

mean power generation potential = Energy generation potential / time in 1 day

= y J/ 86400 s = 397807.4 W = 0.0003978074 GW

Is this correct?

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    As a former teacher, I will say the following: When we ask you to express an answer in certain units you can bet that the "right" answer looks relatively tidy in those units — e.g. if we ask for Watts, then the answer may be 1W, 10W, 100W but will not be over 1000W — because we would have asked for it in kW if that were the case. If 0.000397GW was the right answer, we would have asked for it in MW instead (0.397) or kW (397) — both of those are neater. On that basis alone, your answer is wrong. Hint: "energy content of water in the BASIN" ... but you used the area of the BRIDGE. – Tim Mar 8 '18 at 18:53
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    @user3489850 I agree with everything Tim wrote. And one more thing: in your equation, you can see that the tidal range is squared. So it's not linear. That means that you can't just take the linear average of high tide and low tide, and put it into the equation. You need to either integrate, or take several values in the range from low tide to high tide (and 14 could be a good number - have a think as to why), put them into the equation one at a time, and average the results. – EnergyNumbers Mar 9 '18 at 7:06

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