Problem: An energy storage facility uses pump hydro to capitalize on the low cost of off-peak electricity of 2 cents per KWH.
Given: 400GPM of 60 degree water is pumped from a low reservoir to a high reservoir at a thousand feet higher elevation thro...
Approach: This problem is just asking us to calculate the cost of the nighttime part of that operation, the part where they pump the wate...
Key formula: formula for water horsepower, this is pumping water, is Q times delta H, where delta H is the head added by the pump
Calc: Assuming the off-peak operation lasts for 8 hours per night, what is the cost of electricity for one night?
Calc: They're doing that for 2 cents per KWH.
Result: Best answer choice is C.
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Student questions asked in live office hours about this problem
OH 46: HVAC: Fluids-23
Q: I went straight for ΔP = (P1−P2)/γ and also forgot to multiply the friction loss rate from the tables by the pipe length—can you review my work?
A: Going straight for P1−P2/γ misses the pump head-added term and doesn't represent the full Bernoulli equation—that's the same conceptual error that came up in the adjacent problems. Also, the steel pipe friction tables give head loss per 100 feet of pipe, so you always need to multiply that value by the actual pipe length. These are two distinct mistakes, so fixing just one won't get you to the right answer.