5 ms·
First, the water is lost - because evaporation over the large area of the reservoir only increases with its area, which increases with its level Second, the sc
by lstodd 8y ago
First, the water is lost - because evaporation over the large area of the reservoir only increases with its area, which increases with its level
Second, the scheme decreases flow, and that means level of the downstream. This means negative effects on ecosystems downstream.
Third, large hydroelectric dams require subordinate downstream reservoirs to tackle exactly the second problem - keep the downstream flow of the system (of the two+ dams) at least at some acceptable level to hide the variations in the main dam's
outflow which are due to daily/weekly/monthly/yearly fluctuations in inflow and power requirements.
Fourth - hydroelectric turbines require some amount of backpressure to operate. If the backpressure (that is the level of water downstream of the dam) gets too low, they can't operate unless risking damage. (that's actually the original reason behind the subordinate downstream dams)
So the project just does not make any sense. Where are they going to get the water?
- hyperpape 8y ago> “the scheme decreases flow” Can you spell this out in more detail? I would have thought that in the near future, it would just time shift flow from daytime (when solar production is high) to nighttime, but perhaps my mental model is over simplified?
- lstodd 8y agoThe most obvious explanation is that for an unit of power (not energy, mind) one has to spend a unit of flow (which is unit of volume per unit of time) falling over a unit of height (which the dam is for). Now if you pump that unit of flow back up, total downstream loses that same unit of flow. Second order effect is that when you pump water back up into the reservoir, it is evaporating at an increased rate. Determining the evoparation increase is not trivial, since the reservoir surface area increases much faster than linear compared to its level at the dam. If they didn't, the dam would've been that much higher, because evaporative losses of the future reservoir are a major factor in the design of the dam.
- hyperpape 8y agoI'm only asking about the first order effect (your second point, not your first). My question was: if you're letting that water back out every night, don't you get the unit of flow back then? It's delayed by up to 12 hours, but is that a really big effect?
- lstodd 8y agoYou have to keep enough water somewhere downstream of the dam for the night so that you have something to pump up during the day. If you don't, there's either not enough water to pump up, or severe level fluctuations. If you do, it requires another dam, evapo losses, etc, etc.
- jessriedel 8y agoEvaporative losses are still likely to condense in the Colorado River drainage basin, no?
- jcranmer 8y agoI expect it would condense in the Rio Grande or Mississippi basins instead, for the most part. Arizona isn't exactly known for its high rainfall.