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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.
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.
