Energy storage - briefing document
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The energy storage
problem

 


briefing document

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Energy storage is part of a series of briefing documents on the problems of power consumption, posed by the steady depletion of fossil fuels and most particularly of pumpable oil.
One of a grouping of documents on global concerns at abelard.org.
on energy on global warming
On housing and making living systems ecological
sustainable futures briefing documents
Tectonics: tectonic plates - floating on the surface of a cauldron

Index

the storage problem
molten salt storage
fuel formation
battery and fuel cell storage
pumped storage
other

 


 

 

 


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The storage problem

If energy is not wanted immediately, for instance to switch on a light, some means of storage of energy required for later use has to be achieved. A battery, a dam, a gallon of petrol, a hydrogen fuel cell, a log for the fire, or radioactive sources, are all means of storing power/energy.

Learn to think clearly about the difference between generating and storing power.

  • A power station, or a growing tree, are means of generating usable power.
  • A log is a store of energy, but a power station is not. The power station uses means such as oil or uranium to store energy prior to using it in the generating process.
  • For completeness, an engine is a device that converts energy from one form to another. Thus, a car engine converts petrol into moving along the road, while a tree converts sunlight into logs.

Even insulation your house and increasing efficiency of processes, thus countering waste, has similarities to storage.

All conversion to energy involves inefficiencies. Energy may be converted to storage or to produce work. Thus, energy is involved in manufacturing a battery or fuel cell, energy is used to charge up the battery. After that, the energy in the battery may be used to drive a car, or to run a laptop computer.

Oil, in the form of gas (petrol) or diesel, may be used to run a car, or the oil may be used to drive electricity generators in a power station. The electricity may then used to charge a battery, and the battery then may be used to drive a car. At each stage, energy is lost through inefficiencies. These inefficiencies contribute to making storage a bottle-neck issue in cleaning and modernising energy production.

 

molten salt storage

'Power Tower' where molten salts are heated, surrounded by heliostat panels. Image: SolarReserve
'Power Tower' where molten salt are heated, surrounded by heliostat panels.
Image:
SolarReserve

SolarReserve will build power plants designed as Solar Power Towers. This configuration captures and focuses the sun's thermal energy with thousands of tracking mirrors (called heliostats) in a two square mile field. A tower resides in the center of the heliostat field. The heliostats focus concentrated sunlight on a receiver which sits on top of the tower. Within the receiver, the concentrated sunlight heats molten salt to over 1000 degrees Fahrenheit. The heated molten salt then flows into a thermal storage tank where it is stored, maintaining 98% thermal efficiency, and eventually pumped to a steam generator. The steam drives a standard turbine to generate electricity.”

Using molten salts to store energy collected from the sun. Image: SolarReserve
Using molten salt to store energy collected from the sun. Image: SolarReserve
Click to go to larger version at Solar Reserve - page 1 of 8-page .pdf

further details

  • Area of heliostats [mirror] field: approximately two square miles or 1,200 acres
  • Height of power tower: approx. 600 feet
  • Power generated: between 100-600 megawatts, depending on configuration.
    [1MWatt supplies approximately 1,000 U.S. households.]
  • Molten salt: mixture of sodium and potassium nitrate. (When solid, this mixture is used as garden fertiliser.)

Salt energy storage is more efficient with tower solar units than with parabolic units.

Length of parabolic mirror with oil-filled tube (purple in sun ray diagram). Photo credit: Michael Kanellos/CNET News.com
Length of parabolic mirror with oil-filled tube (purple in sun ray diagram).
In this photo the mirror is facing the ground for inspection, and is reflecting the ground and people.
photo credit: Michael Kanellos/CNET News.com

With parabolic mirrors, molten salt is run through the tube, instead of oil. Thus, there are much longer runs of piping over a whole field of parabolic mirrors to be heated, rather than the sun’s heat being directed on a central tower and there absorbed by the molten salt. So, for parabolic mirror systems, more piping, molten salt and insulation is required, making this storage method more applicable to the tower configuration.

fuel formation

Hydrogen and methanol are fuels created as a means of storing energy effectively.

As well as producing hydrogen by electrolysis, the gas can be made by high-temperature disassociation. The heat generated by a solar power plant could be used for making hydrogen, as well as for driving steam turbines or, perhaps, space heating.

Dual use nuclear power—electricity and hydrogen from heat process

By running a nuclear power station at higher temperatures, using the heat directly to split hydrogen from water, 40 – 50% efficiency can be achieved from a nuclear power plant.

Beyond Oil and Gas: The Methanol Economy

 

battery and fuel cell storage

Fuel cells and battery-powered vehicles

 

pumped storage

With pumped storage, water is pumped to the top of, say, a mountain during off-peak hours, then dropped during peak hours to turn turbines, and so generate electricity.(The turbines that generate the electricity are used in reverse to pump the water back up the feed pipe.) This generation method is very expensive.
Examples: Dinorwic/Dinorwig power station, Llanberis, Snowdonia, North Wales;
Raccoon Mountain Pumped Storage Plant, Tennessee Valley Authority, U.S.A.

Pumped storage power station schematic. Image: tva.gov
Pumped storage power station schematic. Image: tva.gov

 

other

Other, more esoteric methods include flywheels (also used for smoothing power output), capacitors (including super-capacitors - experimental), and even underground gas compression is under sporadic discussion.

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