Every day we hear reports and read articles about distributed energy resources (DER). Much of what we hear and read about, however, is extensively focused on wind and solar. Other DER technologies are also addressed, but to a much lesser degree. This article by guest author, Harry Valentine, shows that there are several other technologies at various stages of development poised to contribute to the evolving DER solution set. These alternative technologies – though perhaps not as high-profile as others – are nonetheless emerging and promise to help solve the energy challenges we are facing, even as formidable issues such as load growth, environmental complexities and cost concerns loom large. And, while some seem driven toward finding a singular solution, it is increasingly evident that the final solution set will likely be every bit as diverse as the challenges.
The origins of using energy other than human or animal power to perform work can be traced back over 4000 years when the first known waterwheels appeared. At that time sailors learned to harness the power of the prevailing winds by installing crude sails on trading boats. Hero of Alexandria used the power of steam to rotate a large drum. The power of river water and wind power formed the basis of decentralized mechanical power generation that drove the grinding wheels to convert grain into flour. Steam power eventually joined river power and wind power to drive the production machinery of the industrial revolution.
Mega-power installations evolved from small-scale power generation and realized benefits from the economy of scale. Fewer men could produce more kilowatts at a large power installation than at a smaller installation. Many small-scale or decentralized power plants closed during the latter 19th century and early 20th century as mega-power plants flourished. However, decentralized or distributed generation did not quite disappear and included small, privately-owned hydroelectric power plants that served local communities in many parts of the world.
Development of Distributed Generation Technology
Despite the proliferation of large-scale centralized power generation, entrepreneurs and inventors continually sought ways to improve small-scale power generation. The same hydraulic turbines are used in small-site hydroelectric power dams served equally well in the hydraulic torque converters of automotive automatic transmissions, including in units used in some diesel-hydraulic railway locomotives. Steam turbines were developed from hydraulic turbines and formed the basis of ocean ship propulsion during the early decades of the 20th century and continue to be used in nuclear powered military ships and submarines.
Manufacturers connected to the commercial transportation industry developed a range of reciprocating compression-ignition engines to power trucks, buses, locomotives and ships. Diesel engines developed for commercial road and railway application were used in certain marine vessels. Some marine and railway diesel-electric propulsion systems were eventually to burn fuels such as natural gas and propane.
During the late 1980’s and early 1990’s technical breakthroughs increased both the thermal efficiency and service life of diesel engines of 250kW to 500kW output.
The early turbine jet engines were developed from steam turbines and in turn formed the basis of gas turbine engines. Following World War II several companies sought to develop gas turbine engines for applications in locomotive propulsion, ship propulsion and even motor vehicle propulsion. The Allison division of General Motors began testing a gas turbine engine in a bus in 1947 and continued doing so right into the
1970’s. Gas turbine powered locomotives pulled trains the USA, the UK and several other nations. Several of the world’s navies tested and operated various designs of boats powered by gas turbine engines.
Beginning during the early 1960’s, NASA literally commandeered the research into improving externally heated piston engines or Stirling-cycle engines to provide needed onboard power in manned spacecraft using intense solar thermal heat as its source of energy.
During the period between the mid-1960’s and the early 1980’s there were several attempts to develop large-scale externally heated piston engines; however, the research and testing indicted that the Stirling engine was in fact a small-scale power technology.
Potential for Improved Distributed Generation Technology
The ongoing research and development that has been undertaken into power conversion technology for mainly transportation purposes can be refined at relatively little or no cost to provide service in distributed generation applications. Ongoing design and refinement of hydraulic turbines has resulted in the development of ultra-low-head turbines that can operate at high efficiency on rivers and streams. Diesel-electric systems, steam turbine electric systems and gas turbine electric systems that were originally designed for railway or marine propulsion can be readily adapted to decentralized power generation.
During the latter 20th century there was ongoing development of new materials with improved mechanical and thermal properties that could be used in various power generation applications. Ceramics such as silicon nitride and silicon carbide provide excellent thermal properties at elevated temperatures and were applied to large-scale commercial piston and turbine engines. While turbine blades made of silicon nitride could improve the efficiency of large-scale turbine engines, entire turbine wheels of small turbine engines could be made from the same material to improve thermal efficiency and extend service life.
Modern Water Power
A low head turbine built by Zotloeterer or Austria can deliver some 150kW at 70% conversion efficiency while operating over a head difference of 0.7m or 2 feet, 4 inches. Another design from Canada can offer 1MW at a nominal efficiency of 84% over a head of 3.5m or 11feet, 6 inches and can produce power over a head of 1.2m or 4 feet. Examples of such technology can be applied on rivers and streams around the world to serve small communities at remote locations. In some cases local power generation would ease strain on strained power grids.
Several companies have developed technology that based on the classical waterwheel that can extract power from the kinetic energy of rivers and tidal currents without constructing a dam. Free-flow kinetic turbines are currently been tested and demonstrated on numerous streams, rivers
and tidal straits.
The larges turbine is that is currently being developed will yield a peak output of 2MW while many smaller turbines deliver 250kW. Several companies offer a range of technologies that can convert the power of ocean waves to electric power with ocean wave power conversion technologies currently being demonstrated around the British Isles and the Iberian Peninsula.
Modern Small Steam Power
Development of steam power technology has given rise to newer large-scale power stations that operate using ultra-critical steam at over 4000-psia at temperatures well in excess of 1200°F. Companies such as Enginion in Germany (now Amovis) and Cyclone Power in the USA have developed diminutive steam piston engines that also operate using ultra-critical steam and can deliver the thermal efficiency of a diesel engine. These engines receive superheated steam at a pressure of 4000-psia at 1200°F and can exhaust saturated steam at much lower temperature and pressure.
The exhaust steam may be suitable for numerous industrial applications as well as to heat buildings, drive bottom-cycle engines that operate on low-grade heat or energize absorption cooling technology. Small-scale steam power would appeal to companies that process organic agricultural material or wood products and that have much waste organic material that may serve as fuel for the steam engines. Such engines could find application at ethanol plants and paper plants.
Steam engines may also operate on concentrated solar thermal energy as well as on small-scale nuclear power. Toshiba has developed a micro nuclear reactor of 200kW that uses lithium-6 as its fuel source while Hyperion Power Systems has developed a reactor of 25MW output that uses uranium-nitride as fuel. Modern high-efficiency steam engines may operate in combined-cycle operation in rural communities where suitable waste organic material is readily available.
Efficient Gas Micro Turbines
There has been improvement in the efficiency, fuel flexibility and durability small-scale gas turbine engines of up to 300kW output. Combustion chambers and entire turbine wheels are now made from silicon-nitride that can maintain constant mechanical properties up to 1400°C or 2550°F. Improvements in the recuperative heat exchanger that recaptures exhaust heat and puts it to productive use has raised thermal efficiency to the levels of large-scale gas turbine engines. Wilson Turbines of Massachusetts offers a small-scale gas turbine engine of 300kW that offers the thermal efficiency of large units of some 30,000kW.
Gas micro-turbines fueled by natural gas have been installed in the basements of many large office towers to provide back-up power or co-generative power. The exhaust heat can provide interior heating to the building or energize absorption cooling technology. Over the long term the cost savings from reduced electrical and/or natural gas consumption allows for full cost recovery of the micro-turbine engine. The fuel flexibility of the micro-turbine engines makes them especially attractive where a variety of fuels with high solvent properties are available and that could otherwise destroy the lubrication used in piston engines.
Externally Heated Air-based Engines
There is much development in air-based engines that convert heat into electric power and include Stirling-cycle engines, thermo-acoustic engine and air turbine engines. Stirling Energy Systems has plans to develop a farm of solar-heated Stirling-cycle engines that will provide electric power to California’s Pacific Gas and Electric Company. There is scope for private companies and individual owners to generate electric power using Stirling-cycle engines using concentrated solar thermal energy or the exhaust heat of various internal-combustion engines.
Thermo-acoustic engines convert heat to low-frequency standing sound waves that resonate inside a pressure chamber and drive a linear alternator of 50kW to 200kW maximum output. The conversion efficiency can exceed 40% and the desired source of fuel would be concentrated solar power. Thermo-acoustic engines can operate as bottom-cycle engines and convert the exhaust heat of many types of internal-combustion and external-combustion engines into useable power.
Modern open-cycle and closed-cycle air turbine engines can deliver much higher efficiency than their predecessors of an earlier generation due to improvements in the design of heat exchangers and air heaters, as well as in improvements in the material from which such components are made. Air turbine engines need a heater temperature of some 1000°C or 1800°F in order to operate efficiently from a variety of heat sources that include concentrated solar thermal energy or high-temperature nuclear energy. The range of power output can vary from some 10kW to over 50MW.
Solid-State Engines
The attractiveness of solid-state engines is the absence of moving parts. Solar photovoltaic cells and thermo-electric panels are the most common form of solid-state engines. There is ongoing research intended to raise the efficiency and lower the cost for both the photovoltaic and thermo-electric versions of the engine. Johnson Electro-Mechanical Systems of Texas is one of several groups seeking to raise the efficiency of thermo-electric engines from 5% or 6% to over 30%.
Some experimental photovoltaic cells have converted energy at well over 30% efficiency. The cost per kilowatt is gradually declining for solar PV cells as the efficiency slowly improves. Recent breakthroughs include thin film technology, silicon-free solar photovoltaic cells, photovoltaic roofing tiles, photovoltaic siding for buildings and photovoltaic windows. Breakthroughs that lower the cost and improve the conversion efficiency of solid-state power technology would attract customers after the economy improves.
Wind Power
Ongoing developments in the aeronautical field and in the development of innovative designs of kites along with advances in mass-production technology form the basis upon which to develop cost competitive wind power technology. Several companies offer vertical-axis wind turbines that can be fitted on to the roofs of buildings. Other developments revolve around the ongoing development of airborne wind turbines by groups such as Magenn, Skywindpower and Makani Power whose technology carries airborne electrical generation equipment. The greater energy in winds at higher elevation can provide more power at more competitive costs.
Competing designs combine ground-based electrical generation equipment with various forms or airborne technologies that include wings and kites. A research group based at Delft University in the Netherlands has developed a LadderMill (Insert: Laddermill) that involves a series of kites that form the rungs of a giant ladder. A design from Italy and New Zealand proposes to coordinate the drag of kites via multiple control lines to drive a vertical drive shaft connected to generation equipment (Insert: Kite-Driven-Wheel). Various wind power technologies are well suited to serving localized markets through distributed generation.
Conclusions
Distributed or decentralized generation is a power generation option awaiting application on a mass scale. Most of the expense of developing the technology was focused on other applications, except that the technology could easily and be adapted to distributed generation at low cost. An increased demand for electric power could see and increased number of smaller power plants supplying that electric power.
Advances in the efficiency, reliability and durability along with low cost make distributed generation a competitive option. Multiple small power stations can be monitored and managed remotely using computer control and modern telecommunications technology. The technology has perhaps unexpectedly advanced to the point where it challenges the economy of scale of mega-scale power stations.
About the Author
Harry Valentine holds a degree in engineering and has worked for several years in energy and transportation research organizations. He undertakes transportation and energy-related research for several clients and publishes internationally on commercial transportation energy matters as well as other energy related issues.
