The Future Potential for On-Site Small-scale Power Generation
Historically, power conversion began with on-site installations such as water wheels located on rivers to drive machinery, or windmills pumping water or grinding grain into flour. By the early19th century, on-site steam powered engines had been introduced to drive machinery. By the late 19th century, the development of steam turbines, electrical generators and motors led to the introduction of large-scale off-site power generation. The development of AC power transmission allowed electricity to be transmitted over longer distances to distant customers. As the 20th century began, big power stations could provide power to factories and buildings at higher efficiency and at a lower cost than labor-intensive on-site thermal power production.
As the 20th century closed, advances had been made in small-site power generation technology that included the development of new technologies. One such technology was small-site solar thermal power generation using steam that became cost competitive with fossil-fuel power generation by 1990. The development of high-efficiency mini- and micro- water turbines allowed small-site hydroelectric power generation to become economically viable in serving small, local markets. Energy consumption could be reduced in buildings receiving small-site hydroelectric power when they were heated or cooled by heat-pumped low-grade geothermal energy.
Advances in the electronics industry led to the development of photovoltaic (PV) cells. Ongoing research has reduced the cost of solar PV cells that convert energy at 9% efficiency, while more costly premium solar PV cells that convert 27% of solar energy to electric power are now starting to appear on the market. Future research promises to raise efficiency to 36% while reducing the initial capital costs of such technology over the long-term future. Advances in storage battery technology and the appearance of efficient lighting technology has added to the appeal of on-site solar-electric power generation for domestic and commercial use.
New generation Stirling engines can convert over 30% of concentrated solar thermal energy aimed at them into electric power. Recent technological developments that involve thermo-acoustic engines and converters hold the promise of converting up to 40% of high-grade thermal energy into electric power. At temperatures over 200-degrees C, thermo-acoustic engines are pressurized tubes that convert heat into standing sound waves that activate the piston of a linear alternator to produce electric power. These engines are projected to develop up to 100-Kw of power while the largest Stirling engines in existence presently developed up to 60-Kw of power.
Advances in high-temperature optical materials have seen Fresnel lenses and optical fiber lines being made from alumina. Thermally insulated alumina optical fibers could be developed to transmit concentrated solar thermal energy directly into thermal energy storage chambers during daylight hours. As night begins, the stored thermal energy would be used to energize a battery of thermo-acoustic engines or Stirling engines to produce useable power. Research into thermal energy storage technology has revealed that some eutectic metal-oxide compounds can store large quantities of heat with little deterioration after 100,000 repeated full reheat and deep drain cycles.
An ore called diaspore (O=Al-O-H) melts at 450-degrees-C and can be mixed with alumina (O=Al-O-Al=O) which absorbs 458-Btu/lb of heat to melt at 2045-degrees-C. The resulting eutectic metallic-oxide compound would melt at under 400-degrees-C while contained inside corrosion-resistant cylinders made from either silicon carbide or silicon nitride. After sundown, the compound would release over 500-Btu/lb of heat at over 300-degrees C and energise thermo-acoustic and/or Stirling engines for several hours, or raise steam for use in steam engines. On overcast days, these on-site externally heated engines would be energised by combusting natural gas, gasified biomass, gasified solid fuel or a low-cost liquid fuel.
At locations where such fuel would regularly be burnt to produce power, new small-scale on-site thermal power technologies that can operate on a compound cycle may be used. Solid-oxide fuel cells operate at high enough temperatures that allow the hydrogen they need to operate to be separated from liquid hydrocarbon fuels or from natural gas. The remaining heat rejected by the fuel cell could energise either thermo-acoustic or Stirling engines, which in turn could reject enough heat to heat buildings during winter or to drive absorption-refrigeration air-conditioners during summer. The thermal efficiency of a small-scale compound-cycle could exceed 50% to produce power. When this system is expanded into a cogeneration system where a building is heated or cooled by the reject energy, overall thermal efficiency could exceed 80%.
The same high efficiency could be realised for a compound-cycle involving a small-scale steam engine running on ultra-critical steam. Enginion from Germany recently developed and tested a small-scale steam engine of 100-Kw output while running on ultra-critical steam. It delivered a thermal efficiency comparable to that of a large power station. Enough reject heat was available to have energised a thermo-acoustic or Stirling engine. Steam engines have greater fuel flexibility than fuel cells and could be more widely used in on-site power generation. The combined fuel cell and thermo-acoustic engine system does have the advantage of only one moving part; that being the activation piston of the linear alternator. This low complexity system could offer high reliability and a long service life at a competitive capital cost.
The operation of automated on-site compound-cycle/co-generation small-site power installations could potentially become cost competitive against multi-megawatt commercial power production. As demand for electric power increases as power prices rise, the feasibility of installing such on-site small-scale power technologies would become more attractive. Small-site power installations could supply internal markets that include commercial tenants renting space in office buildings or a campus of such buildings that are located on a single commercial property. Residential tenants of high-rise apartment buildings where their rent includes heating, cooling and power could also become indirect customers of a small-scale on-site co-generation system.
Small-scale power conversion technologies are presently being developed to convert low-grade geothermal energy into electric power during winter months. Such technology would use refrigerants such as R-34 in engines using scroll compressors to produce power from a temperature difference of 20-degrees C (58-degrees F). Low-grade geothermal heat could be sourced from and stored in converted salt domes that are located deep underground (see Energy Pulse article 1082) as well as in the deep underground porous rock of exhausted natural gas wells. Several thousand such wells exist in Western Canada and the USA where natural gas exploration prevailed.
Ground water that seeped into these dry wells over several years has been measured at over 25-degrees C (over 80-degrees F). During cold northern winters, the temperature difference between the ground water and the winter air could vary from 20-degrees C to as much as 60-degrees C (140-degrees F). During sub-freezing winters, small-scale on-site power installations could produce power and supply heat in commercial buildings during business hours. During the overnight hours, geothermal energy may pumped into on-site thermal storage chambers containing eutectic metal-oxides that melt between 40-degrees C and 60-degrees C. This stored heat could then provide additional daytime power and heating in commercial buildings during winter.
Ongoing research and development into cost-competitive, automated small-scale on-site power generation technologies could begin to reverse the trend toward mega-power stations that began during the latter 19th century. As the 21st century progresses, a proliferation of cost-competitive and efficient small-scale on-site power stations could appear and supply electric power to internal markets located on a single extended private property, as well as to external markets. The higher efficiency of small-scale compound-cycle/co-generation installations could realise sufficient cost savings over purchasing regulated commercial power, to justify investment in such technology. Potential for using new-generation small-scale on-site power generation technology exists worldwide, in climates ranging from tropical to sub-arctic.