Cogeneration, also known as combined heat and power (cogeneration) or CHP, and total energy, is a very efficient, clean, and reliable approach to generating power and thermal energy from a single fuel source such as natural gas or biomethane. Cogeneration plants recover the “waste heat” that is otherwise discarded from conventional power generation to produce thermal energy.
This energy is used to provide cooling or heating for industrial facilities, district energy systems, and commercial buildings. Through “waste heat recovery,” cogeneration power plants achieve typical effective electric efficiencies of 70% to 90% — a dramatic improvement over the average 33% efficiency of conventional fossil-fueled power plants.
Cogeneration power plants’ superior energy efficiencies significantly reduces air emissions including; carbon emissions and greenhouse gas emissions, as well as emissions ofnitrous oxides, sulfur dioxide, mercury, particulate matter. Carbon dioxide emissions from the burning of fossil fuels is recognized as the leading greenhouse gas associated with climate change.
Cogeneration now produces almost 17% of our nation’s electricity, saves its customers around 50% on their energy expenses, and provides even greater savings to our environment.
Cogeneration, as previously described above, is also known as “combined heat and power” (CHP), cogen, district energy, total energy, and combined cycle, is the simultaneous production of heat (usually in the form of hot water and/or steam) and power, utilizing one primary fuel.
Cogeneration technology is not the latest industry buzz-word being touted as the solution to our nation’s energy woes. Cogeneration is a proven technology that has been around for over 100 years. Our nation’s first commercial power plant was a cogeneration plant that was designed and built by Thomas Edison in 1882 in New York. Primary fuels commonly used in cogeneration include natural gas, oil, diesel fuel, propane, coal, wood, wood-waste and bio-mass. These “primary” fuels are used to make electricity, a “secondary” fuel. This is why electricity, when compared on a btu to btu basis, is typically 3-5 times more expensive than primary fuels such as natural gas.
An example of a cogeneration process would be the automobile in which the primary fuel (gasoline) is burned in an internal combustion engine – this produces both mechanical and electrical energy (cogeneration). These combined energies, derived from the combustion process of the car’s engine, operate the various systems of the automobile, including the drive-train or transmission (mechanical power), lights (electrical power), air conditioning (mechanical and electrical power), and heating of the car’s interior when heat is required to keep the car’s occupants warm. This heat, which is manufactured by the engine during the combustion process, was “captured” from the engine and then re-directed to the passenger compartment.
Due to competitive pressures to cut costs and reduce emissions of air pollutants and greenhouse gasses, owners and operators of industrial and commercial facilities are actively looking for ways to use energy more efficiently. One option is cogeneration, also known as combined heat and power (CHP). Cogeneration/CHP is the simultaneous production of electricity and useful heat from the same fuel or energy. Facilities with cogeneration systems use them to produce their own electricity, and use the unused excess (waste) heat for process steam, hot water heating, space heating, and other thermal needs. They may also use excess process heat to produce steam for electricity production. Cogeneration currently coexists with a regulated industry that is going through major structural changes that may limit or expand its application.
Regulatory Issues
The concept of cogeneration is not new. Early in this century, before there was an extensive network of power lines, many industries had cogeneration plants. As utilities became established and grew, most states began to regulate them in order to limit their pricing power.
The Public Utilities Holding Act of 1935 (PUHCA), together with amendments to the Federal Power Act (also in 1935), were the final steps in protecting utility companies from competition. These laws created vertically integrated utilities with responsibility for the production, transmission, and distribution of power. In exchange for their exclusive franchises (territories) and guaranteed revenues, utilities agreed to government regulation of rates and service. Under these rules, more investments in infrastructure and more sales meant more profits. As the network of power lines grew and electricity from utilities became more economical, industrial facilities bought more of their electricity from utilities. However, many industries still had to generate process heat on-site. The economies of scale that the utilities were able to obtain at that time, as well as the availability of low-priced process heat from cheap oil and gas, removed incentives to retain cogeneration equipment.
In the past three decades, however, the long-term trend of energy prices generally moved upward. Building more and more large power plants no longer provided economies of scale. This was a major factor in the increasing use of cogeneration by commercial and industrial facilities.
The Public Utilities Regulatory Policies Act of 1978 (PURPA) provided further encouragement for developers of cogeneration plants. Section 210 requires utilities to purchase excess electricity generated by “qualifying facilities” (QFs) and to provide backup power at a reasonable cost. QFs included plants that used renewable resources and/or cogeneration technologies to produce electricity. PURPA cogenerators must use at least 5% of their thermal output for process or space heating (10% for facilities that burn oil or natural gas). In many cases, this forced independent cogenerators to accept very low rates for their steam production in order to become a qualifying facility under PURPA. Another problem is the rate at which utilities purchase a cogenerator’s excess power production.
Most states set the price at “avoided cost,” or the cost to the utility of producing that extra power. Utilities with excess power generation capacity are often allowed to have extremely low avoided costs. This practice has created artificial barriers to cogeneration as well as to independent power generators.
The Energy Policy Act of 1992 (EPAct) tried to create a more competitive marketplace for electricity generation. It created a new class of power generators known as Exempt Wholesale Generators (EWGs). These are exempt from PUHCA regulation and can sell power competitively to wholesale customers. A cogeneration facility can be (but does not have to be) a QF under PURPA and an EWG under EPAct. This happens when the facility is in the exclusive business of wholesale power sales, and makes no retail power sales to its “steam host” (customer).
Cogeneration Technologies
A typical cogeneration system consists of an engine, steam turbine, or combustion turbine that drives an electrical generator. A waste heat exchanger recovers waste heat from the engine and/or exhaust gas to produce hot water or steam. Cogeneration produces a given amount of electric power and process heat with 10% to 30% less fuel than it takes to produce the electricity and process heat separately.
There are two main types of cogeneration techniques: “Topping Cycle” plants, and “Bottoming Cycle” plants.
A topping cycle plant generates electricity or mechanical power first. Facilities that generate electrical power may produce the electricity for their own use, and then sell any excess power to a utility. There are four types of topping cycle cogeneration systems. The first type burns fuel in a gas turbine or diesel engine to produce electrical or mechanical power. The exhaust provides process heat, or goes to a heat recovery boiler to create steam to drive a secondary steam turbine. This is a combined-cycle topping system. The second type of system burns fuel (any type) to produce high-pressure steam that then passes through a steam turbine to produce power. The exhaust provides low-pressure process steam. This is a steam-turbine topping system. A third type burns a fuel such as natural gas, diesel, wood, gasified coal, or landfill gas. The hot water from the engine jacket cooling system flows to a heat recovery boiler, where it is converted to process steam and hot water for space heating. The fourth type is a gas-turbine topping system. A natural gas turbine drives a generator. The exhaust gas goes to a heat recovery boiler that makes process steam and process heat. A topping cycle cogeneration plant always uses some additional fuel, beyond what is needed for manufacturing, so there is an operating cost associated with the power production.
Bottoming cycle plants are much less common than topping cycle plants. These plants exist in heavy industries such as glass or metals manufacturing where very high temperature furnaces are used. A waste heat recovery boiler recaptures waste heat from a manufacturing heating process. This waste heat is then used to produce steam that drives a steam turbine to produce electricity. Since fuel is burned first in the production process, no extra fuel is required to produce electricity.
An emerging technology that has cogeneration possibilities is the fuel cell. A fuel cell is a device that converts hydrogen to electricity without combustion. Heat is also produced. Most fuel cells use natural gas (composed mainly of methane) as the source of hydrogen. The first commercial availability of fuel cell technology was the phosphoric acid fuel cell, which has been on the market for a few years. Molten carbonate fuel cells are also in widespread use. There are about 150 now installed and operating in the United States. Most are in cogeneration mode, but newer installations are in trigeneration mode, where air-conditioning is also generated in addition to heat and power.
Cogeneration Applications
Cogeneration systems have been designed and built for many different applications. Large-scale systems can be built on-site at a plant, or off-site. Off-site plants need to be close enough to a steam customer (or municipal steam loop) to cover the cost of a steam pipeline. Industrial or commercial facility owners can operate the plants, or a utility or a non-utility generator (NUG) may own and operate them. Manufacturers use 90% of all cogeneration systems. Some industries and waste incinerator operators who own their own equipment realize sizable profits with cogeneration.
Another large-scale application of cogeneration is for district heating and cooling. Many colleges, hospitals, office buildings and even cities, that have extensive district heating and cooling systems, have at their core, a cogeneration or trigeneration power plant. The University of Florida has a 42 Megawatt (MW) gas turbine cogeneration plant, built in partnership with the Florida Power Corporation. Some large cogeneration facilities were built primarily to produce power. They produce only enough steam to meet the requirements for qualified facilities under PURPA. If no steam host is nearby, one can be built. For example, there are large (80 MW) plants operating under PURPA that have large greenhouses as “steam hosts.” The greenhouses operate without losing money only because their steam heat is virtually free of charge. These types of plants are candidates to become EWGs in the new regulatory environment.
Many utilities have formed subsidiaries to own and operate cogeneration plants. These subsidiaries are successful due to the operation and maintenance experience that the utilities bring to them. They also usually have a long-term sales contract lined up before the plant is built. One example is a 300 MW plant that is owned and operated by a subsidiary co-owned by a utility and an oil company. The utility feeds the power directly into its grid. The oil company uses the steam to increase production from its nearby oil wells.
Cogeneration Applications
Cogeneration systems are also available to small-scale users of electricity. Small-scale packaged or “modular” systems are being manufactured for commercial and light industrial applications. Modular cogeneration systems are compact, and can be manufactured economically. These systems, ranging in size from 20 kilowatts (kW) to 650 kW produce electricity and hot water from engine waste heat. It is usually best to size the systems to meet the hot water needs of a building. Thus, the best applications are for buildings such as hospitals or restaurants that have a year-round need for hot water or steam. They can be operated continuously or only during peak load hours to reduce peak demand charges, although continuous operation usually has the quickest payback period.
Environmental Issues
While cogeneration provides several environmental benefits by making use of waste heat and waste products, air pollution is a concern any time fossil fuels or biomass are burned. The major regulated pollutants include particulates, sulfur dioxide (SO2), and nitrous oxides (NOx). Water quality, while a lesser concern, can also be a problem. New cogeneration plants are subject to an Environmental Protection Agency (EPA) permit process designed to meet National Ambient Air Quality Standards (NAAQS). Many states have stricter regulations than the EPA. This can add significantly to the initial cost of some cogeneration facilities located in urban areas.
Some cogeneration systems, such as diesel engines, do not capture as much waste heat as other systems. Others may not be able to use all the thermal energy that they produce because of their location. They are therefore less efficient, and the corresponding environmental benefits are less than they could be. The environmental impacts of air and water pollution and waste disposal are very site-specific for cogeneration. This is a problem for some cogeneration plants because the special equipment (water treatment, air scrubbers, etc.) required to meet environmental regulations adds to the cost of the project. If, on the other hand, pollution control equipment is required for the primary industrial or commercial process anyway, cogeneration can be economically attractive.
Even the environmental groups are on the cogeneration bandwagon. Since its’ founding, the Sierra Club has supported total energy (cogeneration). See the Sierra Club’s statement on energy policy.
Future Market Development
Several factors will affect the growth of cogeneration activities. They include the initial cost of buying and bringing a cogeneration system on-line, maintenance costs, and environmental control requirements. Some electric utilities do not need additional electricity. They may have excess generation capacity or a stable customer base. This leads to lower “avoided cost” rates, which reduces the viability of cogeneration projects that rely heavily on power sales to utilities.
The restructuring of the electric power generation and distribution industry that is currently underway in many states, makes it more attractive for developers to become independent power producers and to build “electricity only” power plants, instead of cogeneration plants. There has also been a great deal of pressure from utility and industrial special interests to repeal or amend PURPA. If they are successful, it could be difficult for new cogeneration projects to get off the ground. Barring that development, improved technology and cooperation among industries, businesses, utilities, and financiers should provide impetus to the continued development of both cogeneration projects and independent power production projects.
One significant impetus for cogeneration is the issue of global climate change from global warming caused by the greenhouse effect, of which fossil fuel combustion is a major contributor.
Cogeneration is the environmentally-friendly, economically-sensible way to produce power, simultaneously saving significant amounts of money and also dramatically reducing total greenhouse gas emissions.
Cogeneration Technologies
Cogeneration technologies are conventional power generation systems with the means to make use of the energy remaining in exhaust gases, cooling systems, or other energy waste stream.
Typical cogeneration prime movers include:
Combustion turbines
Reciprocating engines
Boilers with steam turbines
Microturbines
Fuel cells
Cogeneration Benefits
Cogeneration offers energy, environmental, and economic benefits, including:
Saving money
By improving efficiency, cogeneration systems can reduce fuel costs associated with providing heat and electricity to a facility.
Improving power reliability and electric grid reliability.
Cogeneration systems are located at the point of energy use. They provide high-quality and reliable power and heat locally to the energy user, and they also help reduce congestion on the electric grid by removing or reducing load. In this way, cogeneration systems effectively assist or support the electric grid, providing enhanced reliability in electricity transmission and distribution.
Reducing environmental impact
Because of its improved efficiency in fuel conversion, cogeneration reduces the amount of fuel burned for a given energy output and reduces the corresponding emissions of pollutants and greenhouse gases.
Conserving limited resources of fossil fuels
Because cogeneration requires less fuel for a given energy output, the use of cogeneration reduces the demand on our limited natural resources—including coal, natural gas, and oil—and improves our nation’s energy security.
Where Can cogeneration Be Used?
Cogeneration installations are most likely to be economically viable at locations where the following characteristics exist:
* Coincident demand for electricity and thermal energy (i.e., steam, heating, or cooling) during most of the year.
* Access to fuels, including natural gas, biomass, and/or by-product fuels.
The following are typical markets for cogeneration:
Energy-intensive industries, including the chemical, refining, forest products, food, and pharmaceutical sectors.
District energy systems that distribute heat or chilled water to a network of buildings. Such systems show the greatest promise in downtown areas, industrial parks, college campuses, military bases, and other large institutional facilities.
High power reliability/quality applications, such as Internet or telecommunications data centers requiring high-quality, reliable power and substantial cooling capacity.
Institutional markets, including hospitals, hotels, and convention centers where large year-round demands exist for electricity, heating, and cooling.
Abandoned industrial sites, or brownfields, where cogeneration-based systems can provide the energy infrastructure for “power parks,” facilitating economic redevelopment of underutilized properties.
Commercial buildings—as building-scale cogeneration technologies become better integrated and increasingly cost-effective, this market offers large potential for new applications.
A small sample of successful businesses now using cogeneration include:
Agriculture, apartment buildings, auto/car dealerships, casinos, cold storage facilities, communications sites, convenience stores, credit card processing facilities, customer service centers, dairies, fabrication plants, feed yards, foundries, golf courses, government buildings, commercial greenhouses and nurseries, grocery stores, hospitals, hotels, ice skating rinks, industrial parks, ISP’s, landfills, laundries/laundromats, malls, manufacturing plants, military bases and installations, motels, nursing homes, oil & gas leases, office buildings, paper & pulp, parking garages, printing companies, processing plants, radio stations, resorts, restaurants, retail stores, retirement homes, schools, server farms, shopping centers, sports complexes, steel manufacturing, supermarkets, television stations, universities, warehouses, waste treatment facilities and wineries.
The U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency Supports Cogeneration
Because the average efficiency of the fossil-fueled power plants in the U.S. is around 30-33% and has remained virtually unchanged since the 1930′s. This means that two-thirds of the energy in the fuel is lost as heat. Cogeneration systems recycle this waste heat and convert it to useful energy and achieve effective electrical efficiencies of 70% to 80%. This improvement reduces emissions of sulfur dioxide, nitrous oxide, mercury, particulate matter, and carbon dioxide, the leading greenhouse gas associated with climate change. In addition to reducing air pollution, cogeneration conserves our limited fossil fuel resources, thereby increasing our nation’s energy self-sufficiency.
The Advantages of Cogeneration and Trigeneration
Owners of commercial buildings and commercial businesses are increasingly seeking ways to use energy more efficiently. This is a direct result of dramatically increasing electric rates, decreased power reliability (blackouts, brownouts, rolling blackouts, and other power interruptions), as well as competitive and economic pressures to cut expenses, increase air quality, and reduce emissions of air pollutants and greenhouse gases.
The Kyoto Protocol, while not ratified in the United States, continues to be a major driver in much of the rest of the world. In the United States, “ecogeneration” is becoming a preferred method to produce a company’s or facility’s power and energy requirements.
Ecogeneration defines the optimization of economic and ecological benefits in the power generation process. Ecogeneration produces huge savings for our environment through the reduction, or even elimination, of pollution associated with power and energy production. Additionally, ecogeneration appeals to our customers’ economic bottom line by providing them with significant fuel and electrical savings.
Energy technologies that fall under ecogeneration include: wind, solar, geothermal, hydrogen fuel, hydrogen fuel cells, soybean diesel fuels, ocean/tidal power, waste to energy/waste to fuel and waste to watts, combined cycle, district energy, cogeneration, trigeneration, and even quadgeneration power plants.
There are two major ecogeneration initiatives and technologies that we will discuss in this article — cogeneration and the newer technology, trigeneration. Trigeneration is one of the most attractive options, and is even more efficient and economically rewarding than its cousin, cogeneration.
Cogeneration, also known as combined heat and power (CHP), is the simultaneous production of electricity and useful heat, usually in the form of either hot water or steam, from one primary fuel, such as natural gas. While not necessarily defined correctly, cogeneration has also been referred to as district energy, total energy, combined cycle, and simply cogen.
Cogeneration has been mostly a technology used in the utilities and industrial marketplace.
Trigeneration, as the name implies, refers to three energies, and is defined as the simultaneous production of heat and power, just like cogeneration, except trigeneration takes cogeneration one step further by also producing chilled water for air conditioning or process use with the addition of absorption or adsorption chillers. Trigeneration, also referred to as CHCP (combined heating, cooling and power), BCHP (building cooling, heating and power) and integrated energy systems, permits even greater operational flexibility at businesses with demand for energy in the form of heating and cooling. Just as a cogeneration power plant captures and makes use of the waste heat, absorption or adsorption chillers capture the waste (or rejected) heat and produce chilled water.
Trigeneration systems are found in commercial applications typically where there is a need for air conditioning or chilled water by the customer.
When a trigeneration power system is installed on-site, that is, where the electrical and thermal energy is needed by the customer so that the electrical energy does not have to be transported hundreds of miles away, and the thermal energy is fully utilized, system efficiencies can reach and surpass 90 percent.
On-site trigeneration plants are much more efficient, economically sound, and environmentally friendly than typical central power plants. Because of this, customers’ energy expenses are significantly lower, and the associated pollution is also much less than if the customer had an energy system supplied with electricity from the grid, along with water heaters and boiler systems on-site. Trigeneration’s superior efficiencies surpass even the latest state-of-the-art combined cycle cogeneration power plants by up to 50 percent. Coupled with a four-pipe system, hot water/steam and chilled water can be produced simultaneously for circulation throughout the building or campus (which would be referred to as a district energy system).
And size is not an impediment, since trigeneration systems can be installed, for example, in small commercial settings, such as restaurants, hotels, schools, office buildings, and shopping centers, to large applications such as petrochemical plants, refineries, and in a city’s downtown area, providing the energy requirements for multiple buildings. And it will still provide system efficiencies of 90 percent.
History Of Cogeneration Technology
Many people know that Thomas Edison built the first commercial power plant. However, most people do not know that Edison’s first commercial power plant known as the “Pearl Street Station,” built in 1882 in Lower Manhattan, New York, was also a cogeneration power plant!
Because cogeneration and trigeneration continue to be the most efficient method of generating electrical and thermal energy, in terms of energy output, the U.S. Department of Energy (DOE) has called for the doubling of electrical power generated from cogeneration power plants — from the existing 46 GW (one gigawatt = 1,000 MW) to 92 GW by the year 2010. When this goal is reached, cogeneration will represent about 14 percent of the total U.S. generating capacity of electricity. The American Council for an Energy-Efficient Economy (ACEEE) estimates that an additional 95 GW of cogeneration capacity could be added between 2010-2020, resulting in 29 percent of total U.S. electric power generation being produced through cogeneration. Europe is also dramatically increasing the number of cogeneration power plants over the next decade.
And the historical basis and success of cogeneration has been the foundational basis for expanding the efficiencies of cogeneration to trigeneration and even quadgeneration, with each new increase in energies recovered resulting in higher efficiencies and lower fuel/energy costs and fewer related emissions.
President Bush’s National Energy Plan
In the United States, President George W. Bush’s National Energy Plan recognizes the efficiency of cogeneration technologies — and it plays an important role in meeting national energy objectives and maintaining comfort and safety in commercial and office buildings. Released in May 2001, the president’s National Energy Plan states:
A family of technologies known as combined heat and power (CHP) can achieve efficiencies of 80 percent or more. In addition to environmental benefits, cogeneration projects offer efficiency and cost savings in a variety of settings, including industrial boilers, energy systems, and small building scale applications. At industrial facilities alone, there is potential for an additional 124,000 MW of efficient power from gas-fired cogeneration, which could result in annual emissions reductions of 614,000 tons of NOx emissions and 44 million tons of carbon equivalent. Cogeneration is also one of a group of clean, highly reliable, distributed energy technologies that reduce the amount of electricity lost in transmission while eliminating the need to construct expensive power lines to transmit power from large central power plants.
Since the 1930s approximately two-thirds of all the fuel used to make electricity in the U.S. is generally wasted by central power plants in the form of unused thermal energy in the electrical generation process. While there have been impressive energy efficiency gains in other sectors of the economy since the oil price shocks of the 1970s, the average efficiency of power generation in this country has remained around 27 to 35 percent for nearly 70 years. The use of cogeneration and trigeneration can significantly improve that efficiency.
Pollution Associated With Inefficient Power Plants
Currently, power plants in the U.S. have been cited for producing two-thirds of its annual sulphur dioxide emissions, one-quarter of the nitrogen oxide emissions, one-third of mercury emissions, and one-third of carbon dioxide emissions. These resulting pollutants produce serious environmental and health consequences, including:
The U.S. Environmental Protection Agency (EPA) understands that resolving these problems must start with pollution prevention, which equates to using fewer energy resources to produce goods and services. The National Energy Plan includes four specific recommendations to promote CHP, three of which were directed to EPA for action:
Clean On-Site Power For Commercial And Industrial Customers
Distributed generation locates smaller and more efficient power plants where the power and thermal energy is actually needed. These on-site power systems are also called “inside the fence” power systems and are designed and engineered to maximize the customer’s power and energy requirements.
The DOE’s Energy Information Administration (EIA) recently sponsored a study to estimate the potential of cogeneration installations in the U.S. According to their study, there are 1,431,805 buildings in the United States that are suitable for on-site cogeneration power systems (most of these are actually better suited for trigeneration) requiring a capacity of 77,281 MW. At an average of $1 million per MW, this translates into a $77,281,000,000 market opportunity. That’s over $77 billion in the U.S. alone. Trigeneration would be an even greater market opportunity as this study focused on applications where thermal energy load was in the form of steam or hot water, and does not take into consideration use of thermal technologies, such as absorption/adsorption chillers or desiccant dehumidification, as part of the potential for the building’s thermal load.
When absorption/adsorption chillers are added to a cogeneration system, it is now referred to as a trigeneration system. Therefore, the total market potential in the study could be significantly higher than the 77,281 MW when considering the opportunity for trigeneration applications. The study also estimates the total existing capacity of cogeneration installations in the U.S. to be only about 4,930 MW, and that over 70 percent of the existing facilities are under 1 MW and are powered by small reciprocating engines.
Even quadgeneration is a possibility, taking trigeneration one further step, producing four energies from one process. By extracting most, if not all, of the available heat from the power/energy generation process, end users obtain the most efficient, optimized energy system. But the efficiency gains are wasted if the recovered waste heat is not put to work or the existing boilers or water heaters displaced, reduced, or eliminate entirely. This is why it is absolutely critical that a thorough and complete feasibility study is done to determine a properly sized on-site energy system, and that conventional systems are either eliminated, compensated for, or integrated into the new energy system.
It should go without saying, but if the facility that installs a trigeneration system does not replace or reduce other systems, there can be a net loss of efficiency. If the facility does not offset the net efficiency gains of the new trigeneration system by reducing, displacing, or eliminating the existing water heaters/boilers load, then the facility will not have an optimized installation and therefore will not profit to the extent it could have had the feasibility and design studies been properly conducted.
Trigeneration Takes Lead Over Cogeneration Due To Superior Efficiency
A trigeneration system consists of a cogeneration plant, and either absorption or adsorption chillers that produce chilled water by making use of some of the waste heat recovered from the cogeneration power plant.
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While cooling can be provided by electric-driven compression chillers, low quality heat (i.e., low temperature, low pressure) that is not used by the cogeneration power plant can be used to drive the absorption or adsorption chillers so that the overall primary energy consumption is reduced.
Trigeneration power plants with absorption and/or adsorption chillers have gained acceptance due to their capability of not only integrating with cogeneration systems but also because they can operate with industrial waste heat streams that can be fairly substantial. The benefit of power generation with absorption or adsorption cooling can be realized through the following example that compares it with a power generation system with conventional electric-driven compression systems.
Assume in this example a factory needs 1 MW of electricity and 500 refrigeration tons (RT). (Defintion: A refrigeration ton or RT is defined as the transfer of heat at the rate of 3.52 kW, which is roughly the rate of cooling obtained by melting ice at the rate of one ton per day.)
Let us first consider the gas turbine that generates electricity required for the processes as well as the conventional electric-driven compression chiller. With an electricity demand of 0.65 kW/RT, the compression chiller needs 325 kW of electricity to obtain 500 RT of cooling. Therefore, a total of 1,325 kW of electricity must be provided to this factory. If the gas turbine has an efficiency of 30 percent, primary energy consumption would be 4,417 kW.
However, a trigeneration system with absorption or adsorption chillers can provide the same energy service (power and cooling) by consuming only 3,333 kW of primary energy.
In this example, the trigeneration power plant saves about 24.54 percent of the primary energy needed compared to the cogeneration power plant with electric-driven compression chillers. Since many industries and commercial buildings can use combined power and heating/cooling, trigeneration systems have a high potential for industrial and commercial applications. (The above example is courtesy of ASHRAE.)
Trigeneration, when compared to combined-cycle cogeneration, can be up to 50 percent more efficient, further reducing operating costs, fuel expenses, and environmental pollutants.
Trigeneration systems for commercial buildings are very profitable investments for building owners. A new trigeneration system can pay for itself in as little as two years, depending on local electric rates, natural gas (or other fuel) costs, and the load profile of the building. Trigeneration systems help not only the building owner, but also benefit society in a number of ways, including:
Improved Power Reliability
Economic losses due to power outages in the U.S. have cost American businesses billions of dollars. The following table shows the economic impact of power outages on some industries.
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As we all know, power outages and rolling blackouts are occurring more frequently than ever before. And they are not happening only in California; many other states have experienced similar problems. These problems primarily occur when demand for power exceeds its supply, for example, on hot days when power demand for cooling systems increases significantly. Similar situations occur on very cold days when demand for heating becomes very high. There may also be local areas that are more prone to power outages because the demand for power exceeds the ability of the local distribution line to provide the energy. Other times, weather-related storms knock down power lines and substation transformers.
Cogeneration and trigeneration systems give commercial and industrial end users their own reliable power supply to keep equipment and facilities operating. Plus, they help reduce the load on the power grid and local area lines and, thus, help improve the local community’s power reliability.
Improved Indoor Environments
Also of increasing interest is the issue of indoor air quality. In order to prevent the growth of mold, mildew, and bacteria, it is important to keep humidity in the indoor air to below 60 percent. Cogeneration and trigeneration systems for buildings can help improve indoor air quality by supporting the use of a desiccant dehumidification system to dry the air. Desiccant systems use a material that directly removes moisture from the air, then use heat, such as that provided by the exhaust gases of the cogeneration/trigeneration equipment, to regenerate the desiccant. This provides a very energy efficient and cost effective method of dehumidifying indoor air, rather that using an air conditioner to “over cool” the air to remove humidity.
Summary: Advantages Of On-Site Cogeneration And Trigeneration
Cogeneration produces a given amount of electric power and heat with 20 to 30 percent less fuel than it takes to produce the electricity and heat separately. Trigeneration produces chilled water in addition to electric power and heat with approximately 50 percent less fuel than it takes to produce electricity, heat, and chilled water separately.
