Soe sources for energy could be:
1) hydraulics
2)flywheel energy storage
3)nuclear fission(though extremely rarely used except in chinese rovers)
4) radioisotopic thermoelectric generators.[RTG]
There is small extract of an industrial pipe line robot power requirement report but we can use this in getting an idea of RTGs which I think is the best alternative:
RADIO-ISOTOPIC THERMOELECTRIC GENERATORS (RTGs):
RTGs enjoy the distinct advantage of long and uninterrupted service life over the
other options. Strontium-90 and plutonium-238 are two radioisotopes, with half- lives of
28 years and ~87 years, respectively, that are used at present for RTGs. These long halflives
are the bases for the chief advantage of RTGs. RTGs are powered by the decay heat
of these long- lasting radioisotopes. The decay heat generates power by sustaining a large
temperature gradient across a thermocouple junction. The long half- lives of radioisotopes
result in a practically flat discharge curve for an extended period though strictly speaking
power output of RTGs is a function of time. (Since thermal energy of RTGs is the result
of its radioactivity, as time proceeds radioactivity declines and so does thermal energy
output.) RTGs have no moving parts and has been found to be a highly reliable source.
In the USA most of the RTG applications have been in space missions to outer
planets, with at least one case of military-related remote, unattended monitoring
application in Burnt Mountains of Alaska. Soviet Union (now Russia) has apparently
made extensive use of RTGs on the earth as well as some in their space missions.
In spite of this track record, public acceptance of their use in natural gas
transmission pipelines is a huge question mark. Recent news items of discarded RTG
units in the Republic of Georgia causing radiation-related sicknesses in hunters who
accidentally found them further dim prospects of public acceptance. Potential does exist
for these RTG units to be stolen from the pipelines by terrorists and used to make “dirty
nuclear weapon” even though radioisotopic material in RTG is not nuclear weapons
grade material.
Operating temperature of RTG’s hot junction is 1,300 K and at the cold junction
about 500-600 K, with efficiency of less than 10%. Though radioisotopic material in a
RTG is always well shielded, more than 90% of heat, which is not converted into
electricity, has to be dissipated. In natural gas pipeline, the convective heat transfer
provided by the gas flow itself may be inadequate to ensure that the gas or the pipe
material does not get excessively hot. This is a particular cause of concern if the
possibility exists that robot may remain stationary for some time at a particular location –
during repair work, for example. Any localized heating could cause weakening of the
pipe material or natural gas pressure build-up. For this reason heat transfer considerations
need to be analyzed quantitatively and if necessary, provide a cooling fan. The energy for
the fan would have to be supplied by RTG.
One serious drawback of RTGs is their relatively low power densities (power to
mass ratio) though their energy densities are very high. For example, the RTG used in the
Galileo spacecraft weighed 56 kg of which 11 kg was plutonium dioxide and it generated
~285 watt electric power. (The excess mass is made of iridium, lead, or tungsten and is
for the purpose of radiation shielding to reduce it to about 10 millirem/hour at a distance
of 1 meter.) This corresponds to about 6% thermal efficiency and power density of ~5
W/kg. (The RTGs located in Burnt Mountains of Alaska have even lower power
densities, as low as 0.053 Watt/kg, due to the large mass of the lead shielding used for
these land-based units.)
Another drawback of RTGs is that once fabricated they can not be turned off.
They are also reportedly difficult to handle because of the precautions that must be taken
to due the high temperature and radiation effects. To install or remove them from
pipelines would require specially trained operators and special equipment.
Since all RTGs are manufactured for the military or space missions, i.e., for
government use and they are typically custom- manufactured, their cost information is not
readily available. Pu-238 is extremely expensive and fabrication of RTG is very complex
and dangerous – almost certainly involving robotic technology. An estimate put forth by
the Atomic Energy Insights is that the price of a 50-watt plutonium-238 powered RTG
unit would be close to a million dollars. Another Internet source reports $3,000/Watt
thermal for Pu-238, which leads to an estimated price of $1,500,000 for a 50-watt RTG
unit, assuming 10% efficiency. For strontium-90, reported cost is $250/Watt thermal,
corresponding to $125,000 for a 50-watt RTG unit, assuming 10% efficiency. (However,
it has been suggested that strontium-90 RTGs would have lower efficiency because they
operate at lower temperature.)
Since the RTGs for robots in pipelines would be fabricated for and used by
private companies, their fabrication, ownership, and use would be subject to the Nuclear
Regulatory Commission (NRC) regulations. The interstate pipelines may be subjected to
additional regulations involving jurisdictional aspects of different states, as NRC has
authorized some states to manage some regulations on its behalf. Similarly, the intrastate
pipelines located in states that have NRC authorized regulatory authority will have to
obtain licenses from that state’s regulatory authority.
The manufacturer/distributor of RTGs would need to obtain a license for
possessing strontium-90, a byproduct material, and/or plutonium-238, a special nuclear
material, as well as distributing RTG devices. (Each model of RTG device will have to
have separate distribution license.) NRC reviews the safety aspects of fabrication and
conducts device review prior to issuing registration certificate to the manufacturer of
RTGs. Each operating company that employs RTG powered robot(s) in its pipeline(s)
also needs to obtain NRC license for receiving and possessing RTGs. This license is
applicable for all the RTG units owned by the company, i.e., no separate license for each
RTG unit owned by a company is required. This license also requires prior end use
review by NRC. There are various application fees as well as annual renewal fees, which
would add to the cost of robot operation.
More in-depth analysis of RTG viability as a power source for a robot in a natural
gas pipeline will have to incorporate failure mode analysis. Any feasibility study could
include as a first step experiments to ensure adequacy of locomotion and maneuverability
of a robot powered by RTG by loading on a robot excess mass equal to that of RTG but
powered by an external power source through electric wires.
The high cost of RTG, its low energy density, extreme handling difficulties, major
regulatory requirements, and anticipated public resistance to it may make RTG usage as a
power source in a natural gas transmission pipelines highly unlikely in spite of its long
service life.
tHE SCORING POINT OF AN RTG IS ITS LONG LIFE AND ALSO RELIABILITY.
Are you sure? This action cannot be undone.
