Overview | Technology | Applications | Strengths/Limitations | Related Links

Overview

Steam turbines and Stirling engines both work by converting external heat into more useful energy. For steam engines, the heat source boils water, creating either low- or high-pressure steam, which is then used to rotate large turbines that generate electricity. The steam-powered turbines work in a continuous cycle. Stirling engines, on the other hand, work because of the differences of temperatures. Gases that are contained within a Stirling engine (typically air, helium, or hydrogen) are heated and cooled, which causes the gas to expand and contract. The expansion of the gas pushes one or more pistons, and the contraction of the gas allows the engine to reset so that it can go through another cycle. Since both engines utilize external heat sources, they are often categorized as heat engines.

Steam turbines tend to use some form of combustible fuel - coal, natural gas, biomass, or other combustible fuel - for the external heat source. Some notable exceptions include nuclear energy and geothermal heat. Where steam turbines utilize waste heat from another industrial process, they can be considered zero emission generators. Coal-fired burners, typical in many large central station electricity generation facilities, tend to be the most polluting. There are no commercial DE technologies that use coal as a fuel source.

The external heat source for Stirling engines can also be generated from a variety of fuel sources. Unlike steam turbines, Stirling engines operate on heat differences, so technically they can operate on very little heat. This allows Stirling engines to employ external heat sources, such as solar energy, that are not practical for steam turbines. The emissions profile of Stirling engines depends on the fuel source, ranging from zero emissions if using solar energy to moderate emissions if using fossil fuels.

Like other DE technologies, heat engines work best in specific applications that take in the unique characteristics of the technologies. The following section describes both technologies in more detail and examines the applications that are likely to be the best fit for each technology.

Technologies

Steam turbines are one of the oldest and most versatile prime-mover technologies used to drive a generator or mechanical machinery. According to the U.S. Department of Energy, steam turbines account for nearly three-quarters of the total electric generation in the United States today (80% of fossil fuel-driven electric generation and all nuclear plants use steam turbines). NOx emissions from modern steam turbines vary, depending on the fuel used, from 0-2 pounds per MWh. The energy in high-pressure steam from a boiler or other source spins a steam turbine, and the turbine spins the shaft of a generator. All "steam-electric" generating plants use this basic process-whether powered by coal, natural gas, oil, nuclear, or geothermal energy. Thus, the steam turbine is not inherently tied to fossil fuel combustion. A steam turbine consists of a stationary set of blades (called nozzles) and a moving set of adjacent blades (called buckets or rotor blades) installed within a casing. The two sets of blades work together enabling the steam to turn the turbine's shaft and the connected load.

Steam turbines convert steam pressure energy into blade velocity energy as the steam passes through the blades. Additionally, steam turbines can be used to reduce steam pressure for process steam through either a backpressure steam turbine or through extraction at one of the stages of the steam turbine. Two main types of steam turbines are used for central station (condensing turbine) or distributed (backpressure, non-condensing turbine) generation. Modern large condensing steam turbine plants have efficiencies approaching 40-45%; however, efficiencies of smaller backpressure turbines can range from 15-35%. There are several types of steam turbines including;

1. Condensing Steam Turbines- Condensing steam turbines are used in most large power plants. Used steam goes from the turbine into a condenser or series of condensers to ensure that the maximum amount of heat can be extracted. This is the most efficient steam turbine cycle, but is also more expensive due to the requirement for a condenser. If there is a need for feed water pre-heating, it will be accomplished by extracting steam from the steam turbine casing. The pressure of the steam depends on the stage of the turbine where steam is extracted. By restricting (regulating) the extraction, the operator has control over the process, and can allow more steam to be used for thermal applications, or can allow more steam to flow through the turbine in order to generate additional electricity during periods of low thermal demand. For DE applications, there is usually only one extraction point.

2. Non-Condensing (Backpressure) Steam Turbines- Backpressure systems function by using high pressure steam to drive a turbine, leaving lower pressure steam or hot water that can be used for other processes. In these turbines, condensing occurs during the downstream process that drives the cycle. Manufacturers have lowered costs dramatically on modular backpressure, steam-turbine generators in the 1 megawatt (MW) range. These generators, which convert normally wasted energy into valuable electricity, are being applied mainly in institutional or industrial settings where high-pressure steam is available and low-pressure steam is needed for process or space heating. These smaller designs are less efficient, but their lower cost allows them to be a more viable option.

Figure 1: Steam Turbine

The ability of steam turbines to capture external energy typically requires the purchase and integration of additional equipment. These costs must be understood and considered when determining the economics for steam turbines. For combined heat and power (CHP) plants, the waste heat is obtained at no cost, but a heat recovery steam generator (HRSG) and associated equipment are still required to convert the hot exhaust gases to useful steam.

Robert Stirling, a Scottish priest, developed the Stirling engine in 1816. These engines recover heat from an external heat source and convert it into shaft rotation, creating mechanical energy that can be used to drive a generator. Considerable refinements to Stirling engines are being made so that commercialization can occur in the near future and reliability problems can be resolved. Stirling engines have also been restricted due to technical and economics considerations to a few Kilowatts or less in size. In the past 10 years, technological breakthroughs such as the "free-piston" Stirling have greatly increased reliability, with one free-piston Stirling engine operating more than 50,000 hours of continuous operation.

Stirling engines operate by converting differences in temperatures into electrical output. An external heat source is typically used to create the temperature differential, but theoretically a Stirling engine could run on chemical processes that produce cold instead of heat, such as the sublimation of dry ice or the boiling of liquid nitrogen. Stirling engines are indifferent to the fuel used to create the heat. This allows great fuel flexibility such as most variations of fossil, biomass fuel and waste gases.

The engines work on three main principles: (1) the engines contain a fixed amount of gas, sealed so there is no leakage; (2) when that fixed amount of gas is heated, the pressure will increase; and (3) when pressure on that fixed amount of gas increases, the temperature will also increase.

A typical configuration uses a Stirling engine closely coupled to a burner with a heat exchanger. The basic concept involves heating gas to push one or more cylinders and then cooling it back down to repeat the cycle. One example of a Stirling design is summarized in the following four steps:

1. Gas in a cylinder is heated;

2. As the gas heats, it expands, pushing a piston downward in the cylinder and creating mechanical energy;

3. The gas in the cylinder is cooled;

4. As the gas cools it contracts, allowing the piston to move upward in the cylinder

The piston is where the power of the engine can be measured. The piston connects to a generator to produce alternating current power at any desired voltage. There are several types of Stirling engines including the following designs:

1. Kinematic- These versions of Stirling engines have a crankshaft and flywheel. Crankshafts require penetrating the engine housing to turn the generator, which requires seals. These seals have been an Achilles heel in terms of insurmountable reliability and engine lifetime.

2. Free-Piston- These versions of Stirling engines show substantial promise in that they do not have crankshafts or flywheels, meaning that many of the structural problems that have plagued Stirling engines in the past are minimized. The two pistons in a free-piston system are mounted allowing axial, but not radial movement.

Stirling engines have been demonstrated at power levels up to 150kW, but the new free piston flexure-bearing versions are best suited to power levels below 10kW. Units at this level would be well suited to residential and small business applications. These types of Stirling engines are especially simple devices that, unlike most prime movers, require little additional hardware to produce 50 or 60 Hz alternating current. The natural vibration frequency when a Stirling engine operates is near 60 Hz.

Figure 2 Stirling Engine

The Stirling engine requires heat and the associated combustion system, comprising a heater head and air preheat systems, tends to be relatively sophisticated. Recent data shows NOx emissions of less than 10 ppm from an uncontrolled Stirling generator.

Several European utilities are demonstrating this technology for residential applications. In order to keep the cost low, Europeans have developed a very simple system with minimal heat exchangers and thus low electric efficiency. In higher value applications, electric efficiencies of more than 40 % and system efficiencies of more than 95 % have been achieved.

Commercialization is estimated to occur sometime in 2003 and the Stirling micro-CHP packages are targeted to cost $2,500/kW (thermal + electric) compared to $1,200/kW or less for combustion turbine and IC CHP packages. Theoretically, Stirling engines should have very high availability and reliability. Service intervals of between 3,500 and 5,000 hours (equivalent to over one year's economic operation) are expected with a product lifetime of more than six years' continuous operation. In the newest designs, previous vibration and reliability problems have been addressed by mounting the piston that produces the power on flexure bearings that avoid rubbing, while the tight clearance seals that are self-centering have no gas bearings or other ports. New designs with moving-iron linear alternators avoid mechanical stresses associated with other alternators and are easier to thermally integrate, reducing required burner temperature-another traditional drawback.

Applications

Steam turbines are widely used in utility central power stations and industrial process plants. Stirling engines are pre-commercial, but are expected to contribute to residential and small business needs.
When steam turbines convert boiler fired steam into useful work, this is called a bottoming cycle. These installations are inexpensive and have low to medium efficiencies when compared to CHP and other advanced DE technologies. The condensing steam turbine is best fit for this application. If process steam is required, removal of steam at an extraction point on the casing allows a supply 15 to 150 psig steam at low to high volumes with a higher overall system efficiency.

In the case of peak shaving, backpressure steam turbines are used due to their lower capital costs and the fact that the steam has process uses when not being diverted to the backpressure steam turbine for peak electric production.

Figure 3: Steam Turbine Integrated Combined Cycle Package

Courtesy Solar Turbines

Combined cycle plants typically utilize condensing steam turbines where the overall cycle produces a highly reliable and highly efficient electrical production system. Figure 3 above reflects the addition of a low cost backpressure steam turbine to drive a generator. This allows using excess steam to generate additional electricity often during summer months when less steam is needed while electricity usage is higher due to electric air conditioning.

Strengths/Limitations

Related Links

These links provide supplemental information on steam turbines and Stirling Engines. The vendor links will provide direct access to the Websites for manufacturers of steam engines. Contact the manufacturer to obtain product specifications. The general distributed energy links found in related links enable you to search other leading DE Websites for reference on steam engines.