4 ms·

I am curious how much of an effect evaporation has on the efficiency of these systems. You spend energy pumping the water uphill, then it literally vanishes an

I am curious how much of an effect evaporation has on the efficiency of these systems. You spend energy pumping the water uphill, then it literally vanishes and that energy is wasted. I would imagine that in a hot climate like the one Hoover Dam is in, evaporation would be a non-trivial issue should they go through with the retrofit.

ema 8y ago
I assume evaporation is gonna be below an inch a day in even the most hot and dry climates. Considering the total height of the water column it's negligible.
- allannienhuis 8y ago
  I'm not disagreeing, but wouldn't you need to compare the total volume lost to evaporation to the total volume of water pumped (== cost) to have a sense for whether it's a significant factor? Its hard to build up a casual sense for how much water is lost to evaporation in most lakes we have experience with because the lake's level is based on geography (level of the outlet) when there's a source of water coming in to replenish it. It could be a larger factor than we think. That said, I expect that the efficiency losses in the pumping and generation steps would dwarf evaporation. Any storage system has some losses; the ability to store larger volumes makes up for efficiency disadvantages compared to things like batteries might have.
  - seanalltogether 8y ago
    I wonder if evaporation is offset entirely by water that naturally flows into the reservoir. The graphic on the page seems to indicate that there is always a small amount of water flowing down through the system regardless of whether any uphill pumping occurs, which would indicate there's always a surplus of water.
JoeAltmaier 8y ago
It also rains?
ed_balls 8y ago
I doubt it's a problem for Austria, but it's trivial to solve https://www.youtube.com/watch?v=mEmwJzQjwyw https://www.youtube.com/watch?v=mEmwJzQjwyw
tzs 8y ago
I think that is backwards. In hot climates, like where the Hoover Dam is, I'd expect that evaporation would have no almost no effect on efficiency. It is cold climates where it might be an issue. The water flowing into the top reservoir of these systems from higher up in the mountains should be able to make up for free for the water lost to evaporation. (If there is not sufficient flow to allow that, then you are going to have a hard time keeping a full reservoir there in the first place). Evaporation should only be a problem when a system does not have free water from a higher source. I'd expect the most common reason for that would be in cold climates where for part of the year the water that would run down and replenish evaporation is stuck as snow or ice up the mountain. When you pump some water back to the higher reservoir from the lower reservoir it will raise the water level in the higher reservoir, but probably won't raise the evaporation rate significantly, for the evaporation rate is a function of the surface area, and if the walls are steep at the water level the surface area won't change much. The only evaporation that should be counted against the water you pumped up is the extra evaporation due to that minor change in surface area.

ema 8y ago

I assume evaporation is gonna be below an inch a day in even the most hot and dry climates. Considering the total height of the water column it's negligible.

allannienhuis 8y ago
I'm not disagreeing, but wouldn't you need to compare the total volume lost to evaporation to the total volume of water pumped (== cost) to have a sense for whether it's a significant factor? Its hard to build up a casual sense for how much water is lost to evaporation in most lakes we have experience with because the lake's level is based on geography (level of the outlet) when there's a source of water coming in to replenish it. It could be a larger factor than we think. That said, I expect that the efficiency losses in the pumping and generation steps would dwarf evaporation. Any storage system has some losses; the ability to store larger volumes makes up for efficiency disadvantages compared to things like batteries might have.
- seanalltogether 8y ago
  I wonder if evaporation is offset entirely by water that naturally flows into the reservoir. The graphic on the page seems to indicate that there is always a small amount of water flowing down through the system regardless of whether any uphill pumping occurs, which would indicate there's always a surplus of water.

JoeAltmaier 8y ago

It also rains?

ed_balls 8y ago

I doubt it's a problem for Austria, but it's trivial to solve https://www.youtube.com/watch?v=mEmwJzQjwyw https://www.youtube.com/watch?v=mEmwJzQjwyw

tzs 8y ago

I think that is backwards. In hot climates, like where the Hoover Dam is, I'd expect that evaporation would have no almost no effect on efficiency. It is cold climates where it might be an issue. The water flowing into the top reservoir of these systems from higher up in the mountains should be able to make up for free for the water lost to evaporation. (If there is not sufficient flow to allow that, then you are going to have a hard time keeping a full reservoir there in the first place). Evaporation should only be a problem when a system does not have free water from a higher source. I'd expect the most common reason for that would be in cold climates where for part of the year the water that would run down and replenish evaporation is stuck as snow or ice up the mountain. When you pump some water back to the higher reservoir from the lower reservoir it will raise the water level in the higher reservoir, but probably won't raise the evaporation rate significantly, for the evaporation rate is a function of the surface area, and if the walls are steep at the water level the surface area won't change much. The only evaporation that should be counted against the water you pumped up is the extra evaporation due to that minor change in surface area.