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In a world where most grid electricity energy is from solar (with wind, hydro etc only supplying small amounts), you would need to balance out the load somehow. One idea I've had is to use "reverse peakers". Our current grid uses power plants that are relatively cheap by capacity (watts) but expensive per watt-hour to run, like diesel, to briefly balance load spikes.

In this world, electricity demand that exceeds current supply is brought from other continents by superconductive power lines. However, this only has about 10% of peak generation capacity, so electrically intensive industries are done during the day. Most heat related energy needs, especially residential space heating is still done with natural gas.

However to manage local excesses of energy, they would like time their use desalination plants to "reverse peak", to soak up excess energy.

What would the energy density of this practice be like? How would it compare to normal hydro electric ponds energy density, assuming hydro storage has normal loss inefficiencies but desalination does not. This is because it's not being used to generate more electricity, it's an "offset" equal to the amount of energy it took to create, that would have been drawn from the electrical grid. Update: As used here, energy density is the total energy(watt-hours or equivalent) per unit of volume (Assume non hand-wavuim desalination for this portion, i.e. currently realistic).

Bonus: If this society uses solar powered desalination to provide the bulk of its agricultural water, given hand-wavium solar panels that are ten times cheaper, but not more efficient, and ×10 energy inefficiency nano-prefix desalination, would it feel more like a utopia or a dystopia? (Yay! Bread-basket Sahara or Boo! Corn is $20 a pound)

  • Why do we desalinate? — World population of 30 Billion+, can survive without it but more comfortable with it!
  • Why do we mostly rely on solar? — Because fusion sucks, nuclear is "scary", fossil fuels are dirty and the other power sources I mentioned aren't available everywhere.
  • Why isn't there any grid level storage? — I am not aware of any good grid level storage solutions. That link to the superconductive based storage may change my mind, however.
  • How do people deal with cloudy days? — Continental grid can match local variances easily. Capacity is designed to meet needs in subpar conditions anyway.
  • Do we have any major issues (e.g. food shortage?) Lets add in the 30+ billion population for this too. Can we meet our food needs, presumably by lifestyle change or irrigating more farmland than before? How about luxuries like beef and alcohol?
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  • $\begingroup$ How are you looking to measure energy density? As written, I'm not entirely sure if it lines up with anything I've seen called energy density. Usually energy density is something you pay attention to with a energy source, like gasoline. That being said, some businesses (not desalinization afaik) do actually do this. Aluminum foundaries, for example, consume a great deal of power, and getting off-peak rates can be a rather big deal! $\endgroup$
    – Cort Ammon
    Commented Oct 19, 2015 at 22:14
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    $\begingroup$ Molten salt could act as short-term energy storage, but storing enough salt to make it worthwhile could be an issue. $\endgroup$
    – Ignacio Vazquez-Abrams
    Commented Oct 19, 2015 at 22:30
  • $\begingroup$ Odd choice given that space and water heating are perhaps what solar is best at since it does not require a high grade energy source. $\endgroup$
    – Gary Walker
    Commented Oct 20, 2015 at 7:25
  • $\begingroup$ @GaryWalker Even in winter, when thermal energy needs are highest? $\endgroup$
    – wedstrom
    Commented Oct 20, 2015 at 15:04
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    $\begingroup$ "World population of 30 Billion+, can survive without it" I really doubt that. $\endgroup$
    – RonJohn
    Commented Sep 22, 2017 at 22:10

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However to manage local excesses of energy, they would like time their use desalination plants to "reverse peak", to soak up excess energy.

This sounds a little convoluted? I believe you're trying to say that you have a civilisation with ample solar power and has a major desalination industry. During times of peak load, this industry ramps down - as it prefers to run on cheap, off-peak energy.

However, from this point the impacts on your civilisation depend entirely on the many factors that got it to this point, as well as external factors. These factors are not addressed in the Question, so it would be pure speculation.

Key factors include:

  • Why do we desalinate?
  • Why do we mostly rely on solar?
  • Why isn't there any grid level storage?
  • How do people deal with cloudy days?
  • Do we have any major issues (e.g. food shortage?)

Lastly, I'm not entirely sure the question about 'energy density' makes any sense - are you asking how much water could be desalinated per kWh? In which case a trivial google returns 3-5.5kWh/m^3 for Reverse Osmosis. As a fun factoid:

Supplying all domestic water by sea water desalination would increase the United States' energy consumption by around 10%, about the amount of energy used by domestic refrigerators.

Based on the answers to some of my questions earth has a 30 Billion+ population. This means the world is drastically different from the one we know today - on one hand they have greatly different problems and on the other they must have solved a lot of those problems to get 30Bn+ people.

So on one hand, I have a hard time believing that they don't have grid level storage, nuclear energy, and significant other renewables. On the other hand, I can easily imagine large solar component and excess energy being used to desalinate water (at around ~1kWh/m^3).

I'd also expect that these people are experts in waste recycling and intensive farming - hydroponics, algae farms and 'growing meat' are modern day forerunners. They likely will have replicator like technology!

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    $\begingroup$ If your energy source is cheap enough, it generally doesn't matter how much of it you have to use. Desalinization might be the easiest way to obtain fresh water in some places. One caveat I thought of was that a tremendous increase in the use of electricity could generate a lot of waste heat. $\endgroup$
    – Howard Miller
    Commented Oct 19, 2015 at 22:35
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    $\begingroup$ It would balance out with the solar power that was converted into electricity rather than heat, so globally at least it would not cause change. It could intensify urban heat islands, at worst. $\endgroup$
    – wedstrom
    Commented Oct 19, 2015 at 22:43
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    $\begingroup$ +1 for putting the energy use of desalination in context with the fun factoid. Seriously I suspect the main issue here is that since water is and would be cheap per volume transporting it beyond coastal areas might be uneconomical. Then again, if you convert the coasts of Kalahari or Atacama into rice paddies, you can use lots of water and any long distance transport will be by the wind. $\endgroup$
    – Ville Niemi
    Commented Oct 20, 2015 at 0:00
  • $\begingroup$ I have a reverse osmosis system that doesn't need any power source other than the drop in water pressure relative to the regular tap. (The energy used to pump up the water tower is spent whether I need the water to come out with pressure to wash the roof, or fill a drinking glass) $\endgroup$
    – JDługosz
    Commented Oct 21, 2015 at 17:51
  • $\begingroup$ @HowardMiller, worldbuilding.stackexchange.com/questions/25414/… -- relates to large amounts of waste heat, you might enjoy, $\endgroup$
    – Gary Walker
    Commented Oct 21, 2015 at 20:59
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Practical superconductivity would change everything. If your superconductive material could withstand extremely high magnetic fields, (which tend to force conductors to stop superconducting), energy could simply be stored in a ring shaped superconductor. Energy is added and removed using wire coils wrapped around the ring. Such rings could be buried under each facility and house, saving up excess energy generated during the day and returning it during the night. There'd probably still be some kind of power network so that excess energy could be distributed. If you were short of power in one place it could be brought in from somewhere else. Super small rings could be used to power electronic devices for years instead of hours or days. One day you might buy an electric car that will be fully fueled when you receive it and never need refueling.

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Wikipedia has already sorted this out for you with a nice overview of energy consumption for the numerous methods of desalination. In a nutshell: assume 5 to 10 kWh/m³ for a more realistic approach. The article also claims that more than half of the total cost of desalination comes directly from energy cost with a total cost in the range of 0.45 to 1.00 Dollar/m³.

Say Megapolis with 10.000.000 inhabitants and 4.5 m³/inhabitant/day (taken from 1630 m³/year per-capita for the US - world record! - this ammount obviously includes domestic, industrial and agricultural use) would result in 450 GWh per day (1.6 PJ). Quite a number and if you can find a spot to store large amounts of water this could be a part of demand-side-management.

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