# Sun as water pump in underpressure water power circle

### Introduction

A water storage power plant pumps water up to a reservoir when there is more power available than needed and lets the water flow down when power is needed.

For pumping water up, electricity is needed.

### Concept / Idea

I'd like to create a water storage power plant, where the electric pump is replaced by the sun (see also image below):

• The sun would evaporate water on the ground.
• The water would rise up to a reservoir (as gas).
• The water can cool down at the reservoir and becomes a fluid again.
• The water is ready to flow down to produce energy.
• The whole circle would take place within an vacuum (or near-vacuum) environment so water boils by the sun alone.

### Question

• Is something like this feasible? Where are the pitfalls?
• What is the professional terminology for this concept? (it's difficult to search without one)
• Are there any publications/researches regarding this concept? (hopefully some, that usual human beings can understand)
• Check out the water mountain aka Dinorwig. – Solar Mike Mar 14 at 10:03
• Why not compare the energy needed to evaporate water to the amount that could be pumped with a motor/pump combination. Then see which would deliver more for the input? – Solar Mike Mar 14 at 10:04
• Solar Mike: This question aims more for "can you build a closed vacuum chamber that size and keep it vacuumed?", "Does the evaporation principle actually work 'that easy'?", "Would water probably condense within the pipe going up?" I'd say, if the principle works, input/output is simply a matter of scaling up. – DarkTrick Mar 14 at 10:10
• Solar Mike: Not sure, how Dinorwig helps as a reference here. It's a traditional storage power plant(?) – DarkTrick Mar 14 at 10:25
• Here's an article that might get you close. Have to buy it to read it though. "Hydropower Generation by a Self-Circulating Solar Thermosyphon | ASME Power Conference | ASME Digital Collection" asmedigitalcollection.asme.org/POWER/proceedings-abstract/… – ericnutsch Mar 15 at 16:40

I have seen this design a couple times before. Not sure what it is called. If I were to name it I would probably call it a "2 phase thermosiphon gravity turbine generator". It would definitely work, but there are several reasons why it is not in use.

1. The primary reason is that there is a lot of infrastructure to build for a very low efficiency. The solar energy might be "free" but the materials and equipment to collect it is not, thus a high efficiency is desired to optimize the energy produced per equipment cost. It is much cheaper to produce electricity directly from photovoltaics or produce steam and send it though a turbine. Even considering the efficiency penalty of different storage methods you come out way ahead.
2. The upper limit of the efficiency of any thermodynamic system is governed by Carnot's theorem. The difference in temperature determines the possible maximum efficiency of that system. For an optimistic 40C water temp in the sun and 10C in the ground you get 1-(10+273)/(40+273) or 9.5% efficiency maximum, not considering any other losses.
3. A pelton wheel turbine is a good choice, but will only be 50% efficient. With this loss and piping on top of carnot, you can probably plan on a 2% real world efficiency.
4. Vacuums are harder to contain than pressure. This would add even more cost because the lines would have to be larger and the thicker walled. You could select a different working fluid like ammonia that has a lower boiling point, but that comes with additional hazards.
• I don't quite understand why/how Carnot takes effect here. I should learn more on the theorem. However the rest is clear. Thank you very much! – DarkTrick Mar 14 at 13:54
• Carnot's law is applicable to any thermal machine. It gives the theoretical maximum efficiency given a machine's medium minimum and maximum temperature. – NMech Mar 14 at 17:28
• Analyzing this machine as a thermodynamic cirlce would be a cool homework for a student (of any age, with or without degree) – mart Mar 15 at 8:43
• For reference purpose: I found a Solar Updraft tower is a tiny bit similar. I couldn't find any other research using you keywords (no blaming, just informing :) ) – DarkTrick Mar 15 at 13:52
• Yeah I couldn't find any examples of it either or I would have linked them. I signed an NDA for a similar system that used ammonia back in 2009; so it was at least an undocumented system at that time. It never turned into a product for the above reasons. So since you're the first person to document it you can name it whatever you like. Pretty rare opportunity on the internet these days ;-) If I find any examples or names in the future I'll post them here. – ericnutsch Mar 15 at 16:33

I suggest that this is a needlessly complex way to store solar energy.

The typical objective of a water energy storage system is to pump the water up with excess energy during low demand. This system is just using sunlight to move the water (you've just described rain, really). Almost certainly it would be better to just produce electricity from the sun and then pump the water. That way you can get the raw power from the system during the afternoon without the rest of this system.

This reminds me of attempts to make Stirling engines for large energy production. The energy density just isn't high enough for the capital investment.