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**golfmenorca** · 08-31-2012, 11:11 PM

Regarding the first one, I always wondered why mechanical storage wasn't considered. Something that pushed water uphill, or compresses air, etc and can release it later would surely be cheaper than a battery to produce and install and increase in capacity? Maybe even almost as efficient as a battery in terms of the energy recovered.

Has it ever been seriously considered?

Yes, it is done commercially in large scale systems and it works very well. There are numerous pumped storage systems around the world.

On a smaller scale, there are some problems. The first is that you need two tanks, or perhaps dams. My 300 watt solar pump will hoist up around 15000 litres of water per day against about 15 metres of head. That might represent 1.5 kwh. From this, I might be able to get 1 kWh back from a turbine. So lets multiply this by 3 as a minimum to provide 3kWh per day (which is about what I use). Then allow another factor of say 5 to allow for dull days. So now I need two tanks of size 15,000 * 15 = 225,000 litres. That matches those large water tanks that you often see in rural homes. The cost however is going to be perhaps $25k (I think). That is before you buy the turbine and pump that would be needed. By comparison, my batteries cost $9000. The life of the tanks might exceed that of the batteries I suppose. I am not sure how long the tanks would last.

The second issue is following the load demand. Turbines are not instantaneous. So you would need to use a battery bank to allow you to follow the load demand. A small battery bank perhaps, but batteries none the less. And of course an inverter.

08-31-2012, 11:11 PM	#7
golfmenorca Join Date Oct 2005 Posts 449 Senior Member	Regarding the first one, I always wondered why mechanical storage wasn't considered. Something that pushed water uphill, or compresses air, etc and can release it later would surely be cheaper than a battery to produce and install and increase in capacity? Maybe even almost as efficient as a battery in terms of the energy recovered. Has it ever been seriously considered? Yes, it is done commercially in large scale systems and it works very well. There are numerous pumped storage systems around the world. On a smaller scale, there are some problems. The first is that you need two tanks, or perhaps dams. My 300 watt solar pump will hoist up around 15000 litres of water per day against about 15 metres of head. That might represent 1.5 kwh. From this, I might be able to get 1 kWh back from a turbine. So lets multiply this by 3 as a minimum to provide 3kWh per day (which is about what I use). Then allow another factor of say 5 to allow for dull days. So now I need two tanks of size 15,000 * 15 = 225,000 litres. That matches those large water tanks that you often see in rural homes. The cost however is going to be perhaps $25k (I think). That is before you buy the turbine and pump that would be needed. By comparison, my batteries cost $9000. The life of the tanks might exceed that of the batteries I suppose. I am not sure how long the tanks would last. The second issue is following the load demand. Turbines are not instantaneous. So you would need to use a battery bank to allow you to follow the load demand. A small battery bank perhaps, but batteries none the less. And of course an inverter.
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