[thedoc (OP)]

Energy sponge technologies soak up excess any energy from the the grid, and then “wring” it back out when less is available...
Read a transcript of the interview by clicking here
or [chapter podcast=1000689 track=14.05.13/Naked_Scientists_Show_14.05.13_1002276.mp3] Listen to it now[/chapter] or [download as MP3]

[jccc]

Read half way, don't believe it.

My way will be use the electricity to produce h2 and o2 and store. When needed, fuel cell it.

[alancalverd]

Small problem caused by Carnot here: the efficiency of a heat engine depends on the temperature differential so you can't wring the last drops out of the sponge.

I like the idea of gas storage, with a preference for storing a mixture of H2 and O2 from AC electrolysis, but there's no reason why we can't put pure hydrogen into the existing gas grid: before 1963 everyone used "town gas" which was about 40% hydrogen.

But the whole idea of wind or tide energy is a farce. If we covered the entire UK in windmills, we could just about produce the electricity we currently use. And that only accounts for 30% of all the energy we use. 

[syhprum]

Untold millions died as the result of the trivial three mile island nuclear accident not directly of course but due to the destruction of the nuclear power generation industry that resulted.
This led to the continuing use of fossil fuels with the attendant pollution, wars and industrial accidents that this entails

[evan_au]

hydroelectric storage ...they're not terribly efficient?

I didn't think the show was very clear about the relative efficiencies of hydroelectric & thermal storage.
By an unfortunate sudden change of topic, the show gave the impression that thermal storage would be 75% efficient.

As I understand it:

• Pumped Hydro can be 75% efficient
  
• Thermal storage is unlikely to exceed 30% efficiency, unless very high temperatures are used
  
• Thermal storage may reach 50% efficiency if "waste" heat (or cold) from the thermal storage plant were used to provide winter heat and/or summer cooling.
  

I would be interested to know what is the claimed efficiency of the gravel heat storage system, and its planned operating temperatures.

[alancalverd]

Untold millions died as the result of the trivial three mile island nuclear accident not directly of course but due to the destruction of the nuclear power generation industry that resulted.

Hear, hear! 200 coal miners this week in Turkey alone.

[chris]

Evan, you're quite right; unfortunately he side-tracking himself and didn't complete the answer regarding their expected efficiency measures. However, I spoke with Isentopic's Jonathan Howes a few years back (when they were at a much earlier stage) and he quoted 75-80% efficiency.

Here's the link to that interview:

http://www.thenakedscientists.com/HTML/content/interviews/interview/1501/

Chris

[alancalverd]

The theoretical maximum efficiency of a heat engine is 1-T_cold/T_hot.

Now Tc = 300K (ambient) so to get 75% efficiency you need Th = 1200K, around the maximum exhaust gas temperature of an internal combustion engine. Quite difficult to contain and insulate, let alone use as a working fluid.

Interestingly, the claimed energy density (joules per kilogram) of the "production" machine is about 1/1000th of the energy density of diesel fuel. It might be more economical to store spare energy as a chemical reaction, say using electricity to convert CO2 and water into oil,  rather than heat.

The idea of using residual heat as heat is cheating! If you convert electricity into heat directly you get 100% efficiency, but the object is to store energy in a useful (i.e. multifunctional) form, not to use it for heating! 

[evan_au]

using electricity to convert CO2 and water into oil

I forsee a Nobel Prize for the first person to achieve this with > 10% efficiency!

[CliffordK]

There may be two types of energy efficiency. 
Instantaneous generation/consumption, and standby efficiency.

A heat based energy storage system might have significant losses over time.

An interesting type of UPS is the flywheel energy storage, which can have quite high efficiency, as well as essentially instantaneous switching from charging/maintenance to generation.

As I've mentioned a few times, I think the most efficient energy storage is a hydroelectric generator that can have variable output.  For example, here in Oregon, we have Lookout Point reservoir which has a large drop, and variable output, followed by Dexter Reservoir which is designed to capture the surges from Lookout Point, and to maintain an average water flow in the river.  One should be able to couple such generators to a grid with variable input such as provided by solar or wind.  I don't know how quickly the current hydro plants can react to changing energy demands, but there isn't any reason why at least some of the generators can't be fairly quick acting.
Of course, something like hydro should be sized to allow for peak demand which may give it too much capacity for normal days.  The "grid", of course, allows movement of power from one place to another.

Energy rates based on demand can also encourage power hog companies such as aluminum refineries to work during off-peak hours, somewhat leveling out the diurnal demand.  Potentially EV's could be designed to charge during off-peak hours.

[alancalverd]

using electricity to convert CO2 and water into oil

I forsee a Nobel Prize for the first person to achieve this with > 10% efficiency!

Plants do it, using solar energy directly, without intermediate conversion to electricity. The fascination with electricity is misleading: it accounts for only about 30% of energy use and can be generated wherever and whenever it is needed from almost any primary source except wind and sun. The key to a good life is to invest in nuclear power plant and reduce the population to the point where demand for solid and liquid fuel can be sustained by biomass.

Nevertheless the challenge of a Nobel prize is accepted! 10% efficiency is probably adequate if the primary source is both cheap (or heavily subsidised) and unreliable. I'm off to the shed right now!