Power and Energy Storage

Vehicle
Image from Bernd Helfert

Once energy has been generated by various means (see this page for some sources of energy used in the Orion's Arm civilisation) it must then either be transmitted directly to the consumer for immediate use, or stored in some form for later consumption. The efficiency of energy and power storage by a particular medium can be measured and this determines how that medium is best used.

The Energy Density of a medium is the amount of energy stored in a medium per unit volume, and the energy per unit mass is the specific energy. For some purposes the specific energy is most important, for instance in rocketry, where the fuel has to be carried on board ship at launch and accelerated with the payload before use. In other cases the energy density is also important, where the size of the energy storage system is an issue, such as in small vehicles and self-powered machinery.

The Power Density is a measure of how much energy can be extracted from a given volume of an energy storage medium in a given time, and the power per unit mass is the specific power. Some energy storage media give out stored energy slowly, so are not useful for applications which require a lot of power quickly. Other energy storage systems can give very high power output; sometimes this output is powerful enough to be used directly as a weapon or for propulsion.

For comparison purposes, the specific energies of various energy storage media listed below are given in megajoules per kilogram, or MJ/kg.



There have been many different systems used to store energy and power throughout the ages:

Fluid Pressure Power Storage

To store energy for use in planetary electrical distribution grids, water can be pumped into high reservoirs and released later when demand is high. This kind of storage is quite widespread in many variations on Gaian or terraformed planets, especially those with suitable craters or other geological structures at various elevations. This form of energy storage (or generation) has drawbacks, as the land flooded by the reservoir can displace populations and flood cultural or ecologically sensitive sites, while the dams themselves can interrupt river migration patterns. Further, dam failures pose a hazard to down-river communities. On the other hand, dams can have many positive effects in addition to electricity, such as flood control, irrigation, and transportation.

On planets with a natural hydrological cycle, hydroelectricity can also be used as a primary electrical source. Natural weather events (evaporation, precipitation, and climatologically active topology) "pump" the water to a higher gravitational potential energy.

Other liquids may be used for liquid power storage on non-Gaian type worlds, depending on temperature and composition; for instance a few Muuh colonies use artificial lakes of methane for power storage or generation.

Another related method of storing energy uses compressed air or other gases, for instance in underwater balloons which can be located under lakes or oceans. The efficiency of storage methods of this kind depend on the efficiency of the various pumping systems and generators used.

Capacitors

Devices that can store electrical charge, in the form of an electrical field which is maintained within the structure of the capacitor. The most powerful hypercapacitors constructed with normal matter can store up to 25 MJ/Kg, but will leak charge over time so are not useful for long term storage. (More details here}.

Chemical Batteries

Chemical batteries store electrical energy in chemical form in cells and when activated will discharge that energy. Primary batteries are not rechargeable, while secondary batteries can be recharged a number of times.Thus, as energy storage devices, the creation costs of primary batteries become part of the overhead of such systems. Secondary batteries have a similar initial cost, but the recharge process adds both duration and cost to their overall lifetimes.

The most efficient chemical batteries have a specific energy of around 10 MJ/kg. Chemical batteries are used in mid-tech societies, but have been generally superceded by superconducting storage loops or other methods in most high-tech societies.

Chemical Fuels

The manufacture of chemical fuels for energy storage require an input of energy during the manufacturing stage, and the energy gained by using the fuel or the battery is always less than the energy used during manufacture, except where the fuel can be manufactured from an existing high energy source.

On a very few worlds reserves of fossil fuels or other combustible materials are easily available, but these represent a primary source of energy rather than an energy storage medium. Fossil fuels are only found on garden worlds which have had periods of abundant life in past eras, which has allowed the formation of kerogen in sedimentary rocks eventually leading to the formation of petroleum and other fossil fuels. Since almost all garden worlds are protected zones, mining and drilling for fossil fuels is rarely permitted.

The most common chemical fuels manufactured for use in an oxygen atmosphere are hydrogen, kerosine, and alcohol. Hydrogen is available in large quantities in jovian type worlds, but it is rarely economic to mine hydrogen and bring it to a world or habitat where it can be used in combustion, because of the cost of extraction from the planet's gravity field and other transport costs. Note that in a hydrogen atmosphere, oxygen can be regarded as a fuel, although technically oxygen is always the oxidizer (electron acceptor), hydrogen is always the fuel (electron donor) in hydrogen-oxygen combustion.

Hydrogen used as a fuel in an oxygen rich atmosphere has a very good combustion energy of 120 MJ/kg, but the amount of energy that can be usefully extracted from a hydrogen storage system is somewhat smaller. The actual work (either mechanical or electrical) produced by burning fuel in a heat engine can be much less (typically a third to a fifth of the heat energy). Hydrogen has a very low density so needs to be compressed or liquefied, which makes it less convenient than some hydrocarbon fuels which are liquid at Earth-like temperatures.

Hydrocarbon fuels when used in an oxygen atmosphere have high combustion energies too, as much as 40MJ/kg. and some mixtures are relatively easy to store and distribute. The discovery of fossil fuels, particularly petroleum, on Old Earth was a great boost to the economy of that planet during the Industrial Age, and contributed to the initial success of human technological civilisation. When hydrocarbon fossil fuels became scarce in the Early Interplanetary Age the result was a severe economic recession.

These fuels, as well as others including solid rocket fuel, hydrazine and metal nanopowders can be used to power vehicles, especially spacecraft (more details here), and many chemical fuels are useful as electricity producers in fuel cells.

Biochemical Storage

In living organisms a series of reactions or a reversible reaction which concentrates energy from the environment into stored chemical energy, which is later released to provide power for living processes. The typical example of this (in terragen biology) is the "Krebs cycle" of adenine triphosphate in the biological cell, generating a molecule that is utilized by other cellular structures for kinetic actions. Sucrose, for example, has a specific energy of 17 MJ/kg when metabolised in an oxygen atmosphere. Such energy-rich compounds can be manufactured artificially to provide energy for living organisms, or grown organically and harvested. Energy rich nutrients are not only used as food for living organisms, but also to power bionano and synano technologies.

Condensed matter storage

Certain forms of condensed matter can be used for energy storage, particularly ultra-dense Rydberg deuterium, a state of matter where the atoms are flattened and arranged in planar groups. When decompressed this kind of matter can release 15 GJ/kg, although the magnetohydrodynamic equipment required for this form of energy storage adds a certain amount of mass to Rydberg matter systems.

Antimatter

Antimatter can be created in large amat farms, or gathered from the space surrounding gas giants and other bodies with other bodies with magnetic fields at least as powerful. Although the processes of production in a modosophont society -even a hi-tech one - are relatively inefficient, the specific energy of antimatter (1.8 x 10e10 MJ/kg when combined with a kilogram of normal matter) is so great that manufacture and use of this material is still widespread. However antimatter is a dangerous material, which will explode on contact with normal matter.

Animatter was the most powerful energy storage medium available during the Interplanetary age and the period of the First Federation, and remained so until the development of magmatter technology by the emerging transapients.

The most common storage medium is the magnetic/electrostatic bottle, capable of storing antimatter as long as it remains powered. Other methods include laser containment and sequestration inside nano- or microscale containment systems. (More details here)

Angular Momentum Storage (Flywheels)

Mechanical energy storage using the rotation of flywheels is limited by the maximum strength of the material of the flywheel and the density of that material. Because of these limitations, flywheels can store
amounts of energy similar to the best chemical fuels, batteries, or capacitors: up to 50 MJ/kg at maximum capacity, 15 to 25 MJ/kg with
a suitable safety margin.

Since the amount of energy stored in a flywheel depends on its volume, while the energy lost to the bearings of the wheel depends on their area, larger flywheels can store energy more efficiently. However small-scale flywheels also have many applications More details here

Torsion Batteries

Mechanical energy can be stored in carbon nanotube devices by twisting them like an ancient clock spring. This mechanical means of energy storage is limited by the same physical constraints as flywheels or superconducting storage devices, so can store similar amounts of energy per kilogram. As the torsion battery unwinds, it can do mechanical work or spin a generator to produce electricity. Because these batteries do not run down like flywheels, they can provide indefinite energy storage. They are also better suited for powering nanoscale devices, such as nanobots. For larger applications, many nanotubes can be combined into larger, stiffer carbon springs.

Superconducting Storage Devices

A superconducting storage device made of normal matter can carry as much as 50 MJ/kg. can store electrical energy indefinitely, and are capable of lossless energy transmission. Note that safety considerations generally limit the effective specific energy of such devices to about 25MJ/kg.

Superconducting storage devices take the form of superconductive solenoids of various kinds, and particularly useful are the type known as 'structure batteries'. When a structure battery is in use, it inflates magnetically; the supercurrent provides the magnetic pressure to keep the structure under tension. High tensile strength materials are required to provide backing and support when the superconductor is inflated in this manner. Since all supercurrent is at the surface, a thin layer of superconductor can be surrounded by carbon nanotube windings or graphene sheets for strength.

Such superconductor storage devices inflate themselves when energy is added to them, and conveniently deflate and fold away when not required. If too much electrical energy is drained from the "structure battery" for other applications, the structure collapses.

Magmatter Storage Devices

Physical storage devices such as superconducting solenoids, capacitors, and flywheels typically have much greater energy storage capacity when constructed out of magmatter. This is due to its increased bond strength — magcarbon buckyfibre has 4.7x 10e8 the strength to mass ratio of normal matter. Since strength limits the specific energy of normal matter energy storage devices to about 15 to 25 MJ/kg with suitable safety factors, this means magmatter storage devices han hold about 4.5 x 10e9 MJ/kg to 1.2 x 10e10 MJ/kg.

Unfortunately, magmatter interacts destructively with normal matter. As such, magmatter designs may not contact normal matter and should have significant safety features in their design. These safety features typically reduce the actual storage capacity somewhat.

Small scale devices can use very small magmatter energy storage units, allowing them to operate continuously for long periods. For instance, there are known examples of this technology that store enough power in microscopic volumes to break down a cubic meter of the most refractory normal matter known.

Dangers of Catastrophic Failure

Some forms of energy storage such as certain kinds of chemical battery or biochemical nutrient are safe even when severely damaged. Other storage media can be very dangerous. Compressed liquified fuels can expand rapidly if their tanks are punctured, leading to the possibility of a boiling liquid expanding vapor explosion, or BLEVE.

Antimatter is extraordinarily hazardous if the containment systems fail, and amat traps can be used as improvised explosive weapons with only minor modifications.

Superconducting loops also fail catastrophically if cracked or broken. The EMF generated will force the circulating current across the air gap, creating a plasma arc discharge. The plasma arc will erode the material in contact with it, increasing the size of the gap and increasing the rate of energy loss. Within milliseconds, a large portion of the device will have disintegrated into high pressure plasma hotter than the surface of the sun and fine grit. As the structure of the containment hull fails, the magnetic pressure acting on the current will cause the container to rupture, producing more cracks and plasma arcs. The resulting explosion of the high pressure plasma coupled with the magnetic pressure drives the final disassembly of the device. This explosion will have about 3 to 5 times the explosive power of an equivalent mass of TNT (or 8 times if the solenoid is energized up to its maximum (unsafe) capacity).

Ultracapacitors will provide similar effects with regard to the plasma arc discharge, resulting in explosive disassembly, although they lack the magnetic pressure (the plates of the capacitor are being squeezed together, rather than pushed apart).

Nano-flywheels and torsion batteries explode into grit and nanotube fluff if damaged, the resulting a blast wave of comparable magnitude but without the arc flash. Note that carbon nanotube can present a significant health risk if inhaled, although treatments are available for most levels of exposure.

Magmatter storage systems fail with a considerably greater release of energy, and magmatter-powered swarm-bots for example may detonate in a chain reaction if exposed to sufficient stress.

 
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Development Notes
Text by John B, Luke Campbell and Steve Bowers
additional comments by AI Vin, Craig Higgs and Ares Johnson
Initially published on 16 September 2004.