North America's natural gas
industry is a dominant player that provides clean energy to
operate power stations, heat homes and buildings, as well as fuel
a range of transportation vehicles. Its supply and distribution
network covers most of the densely populated regions of Canada and
the USA. Environmental considerations have made natural gas a
desirable fuel due to its clean combustion characteristics. The
natural gas industry’s distribution and storage system has
potential to yield downstream energy.
Compressor Station Exhaust Heat:
An example of downstream energy would be the exhaust heat from
gas turbine engines used to pump natural gas at many remote
compressor stations. During summer, the exhaust temperature of a
non-regenerative gas turbine may exceed 300-degrees C or
570-degrees F. This is hot enough to convert water to saturated
steam in a boiler, steam may be used to heat buildings or drive
low-efficiency steam-based power generation equipment. A small
amount of natural gas could be burned to superheat the steam, and
raise both engine thermal efficiency and power output.
Alternatively, thermo-acoustic engines that are presently under
development could generate electricity directly from the turbine
exhaust heat with less complexity and at competitive rates of
thermal efficiency.
During sub-freezing northern winters, the exhaust of a
regenerative gas turbine engine would be near 140-degrees C or
285-degrees F and allow OTEC (Ocean Thermal Energy Conversion)
technology to be adapted for on-land operation. In its traditional
environment floating off shore at tropical locations, OTEC units
generate electricity from a temperature difference of at least
20-degrees C (36-degrees F) between surface ocean water and deep
level ocean water. The temperature difference between the
regenerative turbine exhaust and sub-freezing northern winter air
would enable modified OTEC units to generate electric power for
use in small communities located near compressor stations.
Both the modified OTEC engines as well as thermo-acoustic
engines could also generate power from the exhaust heat of natural
gas powered piston engines that are used at some compressor
stations. Using exhaust heat to generate power could reduce
electric power prices in some remote northern Canadian
communities, where power is usually supplied by diesel-fueled
generators. The remoteness of these communities creates
transportation difficulties that drastically raises the price of
diesel fuel and other goods that have to be transported in from
more southerly locations. If the natural gas companies are willing
to allow exhaust heat from their compressor station engines to be
used to generate electricity, some northern communities could have
lower priced power.
Pressure Drop Energy:
Natural gas is typically transported over long distances
through pipelines varying from 24-inches to 48-inches in diameter,
with gas pressures varying anywhere from 200-psi to 1500-psi.
Local distribution companies typically use pipes of less than
6-inches diameter and line pressures of under 10-psi. A variety of
control valves reduce pressure and flowrate as the natural gas is
transferred from the main line into the local distribution system.
At locations where mainline pressure is high, a variety of engines
can be adapted to receive natural gas at high pressure from the
mainline and generate electric power before releasing the natural
gas at lower pressure into a local distribution system.
Several companies have begun to offer a variety of such engines
than can be installed at transfer points in a natural gas pipeline
system. The engine range includes both continuous-flow and
positive-displacement engines, the latter including a
reciprocating piston engine as well as rotary engines. The
development of ceramic materials such as silicon-nitride, silicon
carbide and boron-nitride has allowed engine piston rings,
cylinder liners and bearings to be made from them. The extremely
low coefficient of friction of these components enables them to
operate without need for oil lubrication, reducing the risk of oil
contaminating the natural gas prior to delivery to customers.
Installing such engines at suitable pipeline transfer points could
provide the gas companies with additional revenue earning
potential.
Energy Storage Potential:
The natural gas industry pioneered the practice of large-scale
storage massive volumes of energy in natural underground
environments. In some locations, the porous and permeable rock in
exhausted deep-level natural gas wells is used for storage. At
other locations, salt beds and salt domes that are located deep
underground were flushed with water before being used to store
natural gas. The top level of salt domes are typically located
between 1500-feet to 6,000-feet below ground surface level and
measure up to 5,000-feet diameter by up to 30,000-feet in height.
As a second choice to deep-level porous and permeable rock,
natural gas companies will seek salt domes that are located close
to main interstate and intrastate pipelines that serve major
population centres.
The top level of salt beds would typically be located below
500-ft underground and may measure up to 1,000-feet in height.
Salt domes are generally preferred to salt beds for use in natural
gas storage. This preference allows available salt beds to be used
for other purposes, including seasonal thermal energy storage. On
rare occasions and in a few locations, deep-level porous and
permeable rock that could be saturated with water could become
available for such a purpose. If the ambient temperature of a
water-moistened salt bed is near 20-degrees C (58-degrees F), it
could serve as a heat sink during summer months into which heat
rejected from air conditioners, building cooling systems, even
low-grade heat rejected from engines may be deposited.
During cold winter months, this energy could be retrieved and
used to heat an entire campus of large, high-rise buildings. A
favourable salt to water ratio could allow some change of phase
change to occur, thus raising the overall seasonal thermal energy
storage capacity of this subterranean geothermal reservoir. A
sufficient temperature difference between this reservoir and
sub-freezing winter air could allow modified OTEC technology to
generate electric power. This type of deep-level low-grade
geothermal energy storage using salt beds or permeable rock could
offer economic benefit to regions that undergo extremes between
winter and summer temperatures. These regions would include the
northern USA, southern Canada, northern and central Europe,
southern Russia, northern China and Latin America south of the
45th parallel.
In such regions, it may be possible to reduce summer time air
conditioner energy demands by installing water piping systems to
cool the roofs of large industrial buildings, warehouses, large
department stores and shopping centres. The build up of heat
inside large-roofed buildings is the result of the hot summer sun
invariably shining directly on their extensive roofs. The heat
collected by these summer-time roof-cooling systems may be
transferred into deep-level geothermal reservoirs for retrieval
during winter months.
Alternatively, a sufficient difference in temperature may exist
between the roof cooling water and the reservoir to enable
modified OTEC technology to generate electric power at inland
locations. Just before the onset of winter, the roof piping
systems would need to be drained of water to prevent the pipes
from bursting due to the expansion of freezing water. At coastal
locations where no low-grade geothermal energy storage is
available, a body of water may be used as a heat sink during hot
summer weather. Roof water piping systems connected to
subterranean heat source/sinks (or suitable bodies of water) may
actually incur lower overall capital costs than equivalently
large-scale solar PV installations that produce electricity at
comparable levels of energy efficiency.
The size and thermal capacity of deep-level reservoirs may
enable them to serve the seasonal thermal energy needs of entire
industrial or commercial districts. It is possible that large salt
domes could still remain undiscovered deep underground, below a
few large northern cities. In such cases, they may not easily
become available to a natural gas company for use as storage. They
could be processed into a giant subterranean heat sinks and
low-grade geothermal energy storage reservoirs. During cold
northern winters, the ambient temperature and sheer thermal
capacity of such reservoirs may be sufficient to support the
large-scale operation of modified OTEC technology that generates
electricity.
Population centres do exist in desert regions with sweltering
temperatures during the day and sub-freezing temperatures at
night. These centres are often sustained by a supply of fresh
water that is either piped in from distant locales or that is
taken from nearby rivers or streams. Subterranean salt beds that
exist near such locations could be moistened to serve some of the
local energy needs. A low-enough ambient temperature could enable
it to be used as the heat sink for building cooling and either
modified OTEC technology or other solar thermal power technology.
After sundown and after ambient air temperatures drop to near
freezing, the latent heat in the salt bed may be sufficient to
enable modified OTEC technology to generate electric power
overnight, until sunrise.
Conclusions:
Over the long term future, using low-grade geothermal
reservoirs to heat and cool buildings as well as to generate
electric power wherever possible, could become common place in
regions that undergo seasonal extremes in weather and temperature.
Engine technologies that are fueled by energy that is presently
rejected by heat engines could also become more commonplace in the
future of the energy industry. |