What are some ways of storing "sun energy" that would be useful for power generation?

[Geof]

Google 'flywheel energy storage' or 'flywheel UPS' and you should find many links. It is being used commercially on a small scale; that is small compared to total consumption of electricity.

[Seraph]

So I didn't dream that one then .

Makes me wonder why this hasn't been scaled up, if its working on a "small" scale already surely the task shouldn't be insurmountable.

Perhaps cost is the limiting factor, I suppose magnetic bearings could become rather pricey pretty quick if they had to support several tonnes of spinning rotor?

[Geof]

Originally Posted by Seraph .....Makes me wonder why this hasn't been scaled up, if its working on a "small" scale already surely the task shouldn't be insurmountable.....

I think one reason could be it simply does not sound sexy enough. You cannot use the prefix 'nano' in your grant application like you can when you are researching batteries and fuel cells.

[darrensmits]

However advanced solar cells become at generating electricity cheaply and efficiently, a major barrier to widespread use of the sun’s energy remains: the need for storage. Cloudy weather and nighttime darkness interrupt solar energy’s availability. At times and locations where sunlight is plentiful, its energy must be captured and stored for use at other times and places.
Many technologies offer mass-storage opportunities. Pumping water or large banks of batteries are proven methods of energy storage, but they face serious problems when scaled up to power-grid proportions. New materials could greatly enhance the effectiveness of capacitors, superconducting magnets, or flyweels, all of which could provide convenient power storage in many applications.Another possible solution to the storage problem would mimic the biological capture of sunshine by photosynthesis in plants, which stores the sun’s energy in the chemical bonds of molecules that can be used as food. The plant’s way of using sunlight to produce food could be duplicated by people to produce fuel.

[MadMax]

One of the most underappreciated aspects of alternative energy analysis is ratio consideration. The viability of every concept is dependent on ratios like:

-gravimetric or volumetric energy density: energy per mass or volume (kJ/kg, kJ/m^3)
-resource energy density: how much energy is imbued per unit of resource
-capital economic production capacity: power per dollar (kW/$)
-thermodynamic efficiencies: how much useful electricity is produced per unit of resource processed
-actual utility factor: actual output under average operating conditions

The first point is pretty much encapsulated in "resource energy density". In the case of falling water or tidal water, a turbine is driven by water moving across a gravimetric pressure head. Basically water at a higher level pushes thru a turbine to reach a lower level. In a water fall (say Niagra falls) the pressure head is across a 50m fall. A kilogram of water at the top of the falls is imbued with 490J of energy (mass * 9.81N/kg * distance). Said kg of water would have an energy density of 490J/kg.

In the case of tidal power, tidal height is typically 6' (call it 1.8m) from low to high tide and you usually get two full periods per day. That means you get 4 crossings from high to low back to high and low. Notionally you should be able to extract energy in both directions if you have a reversible turbine or a turbine for each direction. That means a kg of water moving 1.8m has the potential to do 70J of work per day. I'm not sure what the rate of rainfall is, but that would work out a day production rate for a waterfall.

Anyways, 490J/kg is a whole lot bigger than 70J/kg which is probably why hydroelectrics typically involve waterfalls instead of tidal gadgets.

I'm going to glaze over capital analysis bit because I don't have enough data on the costs of steel, aluminum, or concrete. But it appears that natural gas is bar none the lowest cost method to spin a turbine for a simple reason: there is no heat exchanger needed to boil water into steam. Nat gas turbines burn the fuel and blast the exhaust thru the turbine just as a jet engine would. I can't imagine a simpler way to spin something save hydroelectrics, but a jet engine is a whole lot more dense in terms of space than a waterfall and the civil project supporting the generator.

Note: North America is installing more GJ of capacity of nat gas turbines than any other electrical production methods combined. IRC in 20 yrs over half of NA's nuclear plants will reach end of life. It takes roughly 8 years to build a nuke plant (two presidential mandates) and ground has not been broken on a single plant in decades.

Thermodynamic conversion efficiency: unfortunately every available joule doesn't end up as useful electricity. The zeroth law of thermodynamics rears it's entropic head and ends up defining things like best possible turbine efficiency. IRC, the best turbines can do is around 65% efficiency in driven or driving application. That really is a killer for pump storage methods. You lose 35% going one way and 35% going the other for a net loop eff' of 42%. Throw in generator losses and electrical transmission losses and you chisel away even more.

Actual utility factor is a number often significantly fudged in the alternative energy market. Look for power curves associated with wind turbines and you'll find that they tend to need a steady 18mph wind. Go much higher and they have to be shut down with blades turned into the wind (or they spin out of control). Go a little lower and their output plummets. The kinetic energy in moving masses (air included) is related to the square of velocity. Halve the velocity and the air has 1/4 of the energy. Even worse, wind turbine blades are optimized for narrow velocity regimes. Get below or above these regions and the compounded eff' and avail' energy drops output rapidly.

Meaningful public energy debates are terribly hobbled by a lack of very important vernacular. Most of the math is not very difficult (really more accounting than math), but not being able to compare capital cost per kW capacity makes most discussions irrational.