Comparison of Prime Mover Characteristics

Once all of the required data for a potential prime mover application is gathered, various prime mover system options can be compared. Once the field of options has been limited by preliminary screening, more detailed analyses are performed on remaining options that are close in economic and environmental performance.

During the preliminary screening phase, certain generalizations may be applied to different categories of prime movers (i.e., reciprocating engines, gas turbines and steam turbines) and to types within these categories, such as condensing or non-condensing steam turbines, single- or multi-shaft gas turbines, and Otto- or Diesel-cycle reciprocating engines. It is important, however, to maintain a broad perspective at the outset so as not to eliminate a prime mover class or type that could, surprisingly, show better economic performance than would be indicated by preconceived generalizations.

Comparison of Combustion Gas Turbines and Reciprocating Engines

Generally, when comparing reciprocating engines and combustion gas turbines, gas turbine economic performance will improve under the following conditions:

• Power production maintained continuously at full load

• High-temperature thermal energy required by on-site processes

• A high-capacity system

Conversely, reciprocating engine economic performance tends to improve relative to gas turbine performance when operation is not continuous or loads vary, low-temperature thermal energy is needed, and as capacity requirements decrease.

In most cases, reciprocating engines offer higher full-

and part-load simple-cycle thermal efficiencies than do gas turbines. Reciprocating engines operate on a batch-type combustion cycle, which permits higher peak temperatures and, therefore, promotes high cycle efficiencies. Gas turbines, which are characterized by continuous combustion, have component temperature limitations that reduce cycle efficiency.

Reciprocating engines also tend to be more efficient over a broader range of load and ambient conditions than gas turbines. Due to the relatively high compressor power requirement of a gas turbine, even very small changes away from full-load design conditions produces degraded efficiency in both the compressor and turbine sections. For the same reason, gas turbines are more sensitive to changes in ambient air conditions than reciprocating engines.

In small capacities, both capital cost and thermal efficiency tend to favor reciprocating engines. As equipment capacities increase, these differences tend to decrease. Gas turbines are available in capacities of up to several 100,000 hp (kW), while the largest reciprocating engines are about 75,000 hp (56 MW). Stationary applications featuring reciprocating engines of capacities greater than 30,000 hp (22 MW) are not common, though multiple smaller capacity engines may also be applied in large capacity applications.

Gas turbines offer greater power density than reciprocating engines, since they are physically smaller and lighter per unit of power output. This can result in easier installation. However, when considering total system size, inclusive of heat recovery units and other auxiliary components, systems featuring gas turbines will not necessarily be smaller than systems featuring reciprocating engines.

All recoverable thermal energy from gas turbines is in the form of high-temperature exhaust that can be used to raise high-pressure steam. The high oxygen content of gas turbine exhaust also allows for supplementary duct firing, providing added production of high-pressure steam at very high efficiency. In contrast, only a portion of the recoverable energy from reciprocating engines is in the form of high-temperature exhaust. The remaining portion is low-temperature heat, recoverable from engine coolant systems. In cases where all or most of a facility's thermal energy requirements is in the form of high-pressure steam, with limited application for lower temperature recovered energy, gas turbines have an advantage. Reciprocating engines have an advantage where low-temperature recoverable energy in the form of hot water or low-pressure steam can be effectively used. Also, while most combined-cycle applications include gas turbines with heat recovery steam generation, reciprocating engines may also be effectively used.

Gas turbines generally offer lower maintenance cost requirements and greater reliability than reciprocating engines. In practice, this will depend on the type of application and specific units being compared. Maintenance costs for smaller capacity, high-speed reciprocating engines may be significantly greater than with gas turbines. However, with decreased engine operating speeds and increased capacity, the difference tends to diminish. Very large capacity low-speed reciprocating engines may be equally reliable, or, in some cases, more reliable than comparable capacity gas turbines.

Comparison of Steam Turbines with Gas Turbines and Reciprocating Engines

Relatively low capital and maintenance costs and high net thermal efficiencies characterize steam turbines. However, comparisons with reciprocating engines and gas turbines will vary widely depending on applications and the characteristics of the host facility. Generally, it is not economical to install a complete steam generation and distribution system in a facility solely for the purpose of applying steam turbine technology. However, if a central high-pressure steam generation system is required or is already in place, there are a variety of applications that tend to favor steam turbines over reciprocating engines and gas turbines. A classic example is a facility that requires high-pressure steam to serve a portion of the total load and low-pressure steam to serve the balance of the load. The steam turbine can serve as a pressure-reducing station, producing power as the steam pressure is dropped. If the facility has a low-pressure steam distribution system in place, a back-pressure or extraction turbine can be applied, perhaps far more cost-effectively than any other prime mover option.

Other applications that favor steam turbines involve heat recovery from process applications and, of course, combined-cycle applications. Condensing steam turbine applications that do not utilize recovered thermal energy are usually less efficient than other prime mover alternatives. In addition, these applications usually require more costly turbines than do back-pressure applications and involve additional auxiliary equipment, such as surface condensers and cooling towers. Still, they may prove cost-effective for certain applications. Steam turbine-driven chillers, for example, are designed around excess boiler capacity in non-heating season months. Incremental maintenance costs are low (for an existing steam system) and project capital costs may be lower than with the other prime mover options.

A potential advantage of steam turbines is that the cost per unit of fuel may be lower because boilers have more fuel choice flexibility than do most reciprocating engines and gas turbines. Another potential advantage of steam turbines is that if a steam generation system is already in place, environmental permitting requirements may be avoided. This would depend on the prevailing local environmental regulations and whether or not the facility has taken restrictions on boiler operation.

In summary, the preceding generalizations should be applied with caution. They are by no means hard and fast rules and are subject to debate within the energy industry. While many of these generalizations may often prove true, there are many variables involved in the prime mover selection process. A site-specific investigation of all available options is required to determine the best strategy for meeting facility energy requirements. This involves consideration of prevailing fuel and electricity purchase options, environmental regulations, and the characteristics of alternative systems with respect to power and thermal load profiles.

Comparison of Alternative Prime Mover Options

In addition to the three main types of prime movers discussed, four other power-producing technologies warrant consideration in the selection process under certain circumstances: wind turbines, water turbines, photovoltaic cells, and fuel cells. Wind, water, and photovoltaic systems rely exclusively on renewable resources. While fuel cells currently rely on traditional fossil fuel energy sources, they are being promoted in today's market as "green technologies," along with the other three renewable sources, due to their nearly air emissions-free operation. Public policy support, in the form of financial incentives, the ability to avoid air permitting and emissions control costs, and other strategic advantages have elevated each of these technologies to a market position that merits consideration in the power generation source selection process.

Wind and water (hydro) turbines are traditional prime movers in every sense of the term and, in fact, predate, by centuries, reciprocating engines and gas and steam turbines. Driven by wind or water instead of combustion gases or steam, these prime movers produce rotational power in much the same way as a gas or steam turbine. Their distin guishing characteristic is that they rely on no-cost and emission-free renewable resources as their energy sources. Their ability to effectively operate on an essentially no-cost energy source, with only moderately higher initial capital costs, allows them to be quite market-competitive in many circumstances. However, applications are limited to locations in which this free energy source is available at sufficient levels to cost-effectively produce power.

It may be somewhat misleading to refer to hydropower as an alternative energy source, since it provides nearly 20% of the world's electricity. In some countries, such as Canada or New Zealand, hydropower is the major source of electricity and in the United States it provides more than 10% of the nation's electricity and is the dominant electricity source in the Northwest. However, where once it was a fundamental tenant of industry (initially for mechanical power production and later for electricity production), today hydropower applications are greatly limited for individual commercial, industrial, and institutional (CI&I) facilities. Instead most applications reside in the domain of major utilities, regional power authorities, and independent power producers (IPPs) that locate facilities at remote sites and feed electricity to the grid.

Still, in certain limited cases, facilities located on or near rivers can effectively apply hydropower systems for mechanical drive or electricity production applications. Today, standardized "micro-turbine" systems are available for applications ranging from a few MW down to a few kW. These can be applied in "run-of-the-river" type systems that use the power in river water as it passes through the plant without causing an appreciable change in the river flow. These systems can be built on small dams that impound very little water or, in some cases, do not even require a reservoir or dam. Hence, given unique access to a flowing river, hydropower merits consideration as a prime mover system of choice for a CI&I facility.

While a far smaller contributor to the world energy mix (well below 1%) than hydropower, wind turbines, or wind-energy conversion systems (WECSs) as they are now commonly referred, have had a similar historic evolution. These systems were once more common-place for mechanical service applications for pumping or grain production processes; WECSs are also now most commonly found in utility or IPP remote site applications that feed electricity to the grid. Today, several thousand MW of capacity are being installed each year, largely in Europe and the United States, mostly in large wind-farm installations and located where their tall towers are unobtrusive and accessible to steady wind flow at sufficient speed and frequency to be cost-effective.

While these grid-connected systems require wind speed of about 12 or 13 miles per hour (5.4 to 5.8 m/s) for operation, smaller mechanical drive and non-grid connected electric generator applications can effectively operate at wind speed as low as 7 or 8 miles per hour (3.1 to 3.6 m/s). Given their energy cost- and emission-free characteristics, WESCs can be life cycle cost-competitive in today's market, with the appropriate location for siting and steady minimum wind speed availability. In fact, in areas where it is difficult and costly to permit new combustion sources, such as California, WESCs may demonstrate superior economic performance over traditional fuel/steam-driven prime mover systems. They can be particularly attractive for remote locations for which grid connection is either costly or inaccessible. In these cases, they can be used in mixed (hybrid) system applications with other prime mover systems (e.g., Diesel engines) and/or various forms of storage (e.g., batteries or pumped water storage) to provide a steady, reliable power source.

Photovoltaic and fuel cell systems are uniquely distinct from traditional prime movers. Unlike conventional power generation systems that operate on power cycles, these technologies rely on photoelectric or electrochemical reactions to produce direct current (dc) electricity, which, through an inverter, can be converted to alternating current (ac) electricity. Compared with hydropower and even with wind power, these electric power generation technologies have experienced extremely small market penetration to date and, except in very specialized market niches, have not been considered cost-competitive as power producing alternatives due primarily to very high initial capital costs. These costs can be partially offset by elimination of air-permitting and emissions controls costs, which can be substantial in areas with the strictest air permitting limitations (e.g., Severe and Extreme Non-attainment areas) and, in many cases, government-sponsored financial incentives. Hence, they do merit consideration for strategic application under a limited set of circumstances.

While still extremely capital cost intensive, the production costs of photovoltaics have continued to decrease while their electric generation efficiency has increased. For maximum effectiveness and associated financial return, they must be sited in locations with continuous or near continuous sunlight, such as the Southwestern United States. In areas with limited or high-cost access to the electric grid, photovoltaics have proven moderately cost-effective, though usually as part of a mixed or hybrid system, typically with a Diesel engine backup, to provide for high reliability and full load-tracking capability.

Fuel cell costs have also continued to decrease due to increased production and technology advancements, while increasing in thermal efficiency. Because they are not power-cycle limited, it is believed that over time, thermal efficiency will surpass that of traditional combustion and steam-driven prime movers. Capital and long-term maintenance costs (largely the periodic cost of stack replacement) remain significant obstacles to wide-scale market penetration. They have found somewhat of a niche market in what is referred to as "clean power" applications, where the avoidance of power conditioners, uninterrupted power supply (UPS)

systems, and standby generators can offset much of the first-cost premium.

While each of these alternative technologies is limited for wide-scale application in CI&I facilities, each offers advantages, which, under the proper conditions, can make them life cycle cost competitive. Moreover, increasing environmental control costs and energy costs, and continual advances in design and production effectiveness, provide the expectation that these "green" technologies will continue to penetrate the energy market place at an increasing rate.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment