Many of today’s most promising sustainable solutions have been decades, sometimes centuries, in the making. The world’s first windmill for electricity was built in Cleveland, Ohio, in 1888, though wind machines can be traced back as early as 200 B.C. in Persia. Solar technologies were first developed in the 1860s, but the early enthusiasm waned due to the increasing availability of coal.
Another sustainable solution that has existed in the USA for more than 150 years, and is poised to expand in the 21st century, is district energy.
The concept of district energy dates back to the Roman Empire, where it was used to deliver heat to structures. By 1853, at least two district energy networks were built in the USA, and by the 1900s, the first commercial network was built in New York, and the underlying concepts were heavily embraced throughout Europe. In the USA today, district energy solutions are being deployed to increase the sustainability of modern buildings, institutions, and mixed-use developments. (see Colleges Turn Waste into Useful Products).
A district energy network provides the thermal energy requirements for dense clusters of buildings, helping to meet the diverse energy demands of a wide variety of occupants, and providing an opportunity to maximize efficiency and minimize fuel consumption. District energy networks often go hand-in-hand with cogeneration technology, or combined heat and power (CHP), which is the simultaneous production of heat and electricity using a single volume of fuel.
Cogeneration captures and recycles the waste heat that is expelled during the production of electrical energy. While the best electrical production technologies typically achieve less than 50% efficiency, district energy and cogeneration can create overall system efficiencies of more than 80%. District energy networks are a perfect complement to cogeneration because they provide a local heat sink for the thermal energy produced.
Although the basic concepts behind district energy are mature, the technology that produces the energy evolves continuously. One example of this is a condensing economizer, which improves overall cogeneration efficiency and reduces water consumption of the district energy system. In order to understand how the economizer can improve efficiencies, it is necessary to first take a look at how district energy and cogeneration systems work.
District Energy: Heating & Cooling
District energy is the distribution of a heating or cooling medium to a local assembly of end users. The system typically includes a central energy plant, or several small ones, connected by distribution pipes to the buildings served, delivering steam and hot or chilled water to buildings. This medium is then used for heating, indoor air quality, and air conditioning. This centralized generation of a heating or cooling medium lowers area emissions and improves system efficiencies. Furthermore, as individual buildings do not have to generate their own heating or cooling, district energy systems energy may lower building costs, resulting in significant environmental and economic benefits.
District cooling networks produce and distribute chilled water. District heating networks transport hot water (ranging from 150°F (65°C) to 400°F (204.4°C)) or steam (365°F (185°C) for district heating and light commercial use). Steam systems are able to transfer large amounts of energy that can be used for heating or other industrial processes.
Today, district energy systems are typically found in large urban areas and are a perfect fit for campus or building complexes such as commercial facilities, hospitals, universities and hotels. When an individual facility is connected to a district energy network, it avoids the capital costs associated with buying and installing individual boilers and chillers, as well as the costs associated with hiring staff to maintain equipment.
Furthermore, by being connected to the network, valuable space that would have been utilized to house boilers and chillers may be used more productively.
Cogeneration
Most forms of electrical production begin with a thermal process that generates heat. The heat produced is then converted to a mechanical motion which is used to drive an electrical generator. However, not all the energy in the fuel can be converted to useful mechanical energy. As a result, a large amount of low grade, relatively low temperature waste heat is produced.
Cogeneration uses processes to recapture the heat that would otherwise be rejected to the ambient environment, and channels it into a common medium like steam or hot water. This common medium is then delivered for use in industrial processes or used to power a chilling system in the case of trigeneration (systems that simultaneously produce electricity, heating and cooling).
A History of District Energy
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- District energy has been in use for hundreds of years. Some of the oldest forms employed geothermal hot springs to heat villages or town centers;
- In the U.S., district energy systems were first built in 1853 and, by the 1870s, factories and institutions began to centralize their steam boilers on a large scale, and many new boiler plants were built. The first commercially successful district energy network was started in New York in 1877;
- Today, district heating is widely used in Europe, but in the U.S. it is isolated primarily to dense population centers and large campus environments (e.g., industrial sites, hospitals, airports, universities, mixed-use developments);
- In the U.S. alone, there are now approximately 540 district energy systems (330 colleges and universities, 85 downtown utilities, and 123 Hospitals).
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Cogeneration technologies:
- Cogeneration systems range from units of 1.5 kW in capacity (small enough to power a home) to hundreds of megawatt s, powering and heating entire cities;
- Small residential systems of the order of 1-5 kW are almost exclusively some form of reciprocating engine or sterling engine;
- Up to 5 MW, cogeneration systems are typically composed of reciprocating engines, gas turbines, conventional boilers, biomass boilers, waste fuel boilers or fuel cells;
- Larger cogeneration systems are often powered by gas turbines;
- Cogeneration technologies have been economically adapted to a variety of fuels: coal, natural gas, oil and biomass.
When project developers look to implement sustainable technologies, costs are obviously a large factor in the decision making process. A new cogeneration system can range US$1000-US$2000/kW of electrical generation.
However, retrofitting a cogeneration system into an existing system is a larger investment, with costs of the order of US$3000-US$5000/kW of electrical generation. Cogeneration systems can be implemented to power district energy networks, or can be implemented for a single building.
Tulsa, Oklahoma, is a prime example of a city that has taken advantage of a small cogeneration system (actually a trigeneration system, in that it produces three forms of energy).
Approximately 30 customers in Tulsa’s central business district receive centrally-produced steam and chilled water from the local district energy network. The trigeneration facility, built in 1992, has a 520 kW induction motor/generator that is connected to a 1.2 MW combustion turbine and an ammonia compressor for generation of chilled water. The trigeneration system is capable of producing up to 2000 tons of peak chilling and 4800 pounds per hour of steam production, which supplements steam from conventional boilers that is delivered into the network.
Another example, which is much larger in scale, is the Grays Ferry cogeneration facility located in Philadelphia, Pennsylvania. The facility has been in operation since 1998 and provides steam to the city’s district energy system. It consists of a large frame, combined cycle gas turbine, producing 170 MW of power and 1.5 million pounds per hour of steam. The steam is produced by recovering heat from the turbine exhaust in a Heat Recovery Steam Generator (HRSG). With supplementary duct firing, the HRSG can produce 711,000 pounds per hour. The balance of the steam is produced in a 735,000 pounds per hour high-pressure superheat boiler.
Optimizing District Energy Efficiency: Condensing Economizer
To optimize the efficiency of district energy, new technologies and new combinations of existing technologies are continually employed. One of the best technologies available today is the condensing economizer, an innovative application of existing technology that is mutually beneficial to both cogeneration and steam distribution systems.
A History of Cogeneration
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- Cogeneration systems have been in use for over a century, but the technology within the systems has evolved;
- The earliest powerhouses typically employed some form of cogeneration;
- In the mid-20th Century, cogeneration was marginalized by centralized electrical generation plants burning low-cost fuel, and by large hydroelectric facilities;
- Cogeneration has been in revival in the U.S. since the 1970s as a result of incentives and transmission constraints.
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A condensing economizer increases the volume of energy that can be extracted from the thermal energy processes of a district’s cogeneration system. As the combustion process creates waste gases and exhaust, they must be expelled. These exhaust gases have historically been maintained at elevated temperatures (230-350°F/110-176.7°C) to ensure that the water vapor in the exhaust gas (a product of combustion) does not condense and corrode the exhaust ducting.
All combustion processes create some volume of water during combustion. This water vapor in the exhaust gas contains approximately 15% of the energy released during combustion in the form of latent energy, which is the energy required to vaporize a liquid. Inserting a condensing economizer into the process captures the latent energy that would otherwise have been lost.
In a typical gas-fired combustion turbine application, a condensing economizer cools the exhaust gases to approximately 185°F (85°C), causing much of the water vapor in the exhaust to condense. The latent heat of vaporization is absorbed into the condensing economizer and conversely heats the cooling medium circulating through the economizer to approximately 180°F (82.2°C). The cooling medium may then be efficiently heated to steam for use in a steam district heating network, or circulated in a hot water district heating network.
Furthermore, condensing economizers may cool the exhaust gas to temperatures below 100°F (37.8°C), depending on the application. Materials resistant to corrosion are installed following the condensing economizer to deal with the moist, corrosive exhaust gas.
The Benefits
The installation of a condensing economizer has many benefits, including the ability to allow more energy to be extracted from the process. This materially increases the cycle’s efficiency by 5-10%. With a condensing economizer, cogeneration systems connected to a district heating system may achieve efficiencies exceeding 90%.
A typical natural gas boiler converting to a condensing economizer will mitigate approximately 500 tons of CO2 per year for every MMBTU of energy recovered per hour.
The condensing economizer works best on steam systems that have a high degree of water consumption (make-up), such as many steam district heating systems. The technology creates usable water during the cooling process, which otherwise would have been exhausted out of the stack, but can now be collected and used for plant processes. In a water constrained environment, this can be a significant advantage and cost-cutting measure.
Further, in most steam distribution networks, a steady source of fresh water make-up is a requirement.
However, with a condensing economizer, a portion of the make-up that would otherwise come from some other fresh water source may now be supplemented with water that has been collected and treated from the condensing economizer.
Condensing Economizer Case Study
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The Kent County District Heating and Cooling System in Grand Rapids, Michigan, was purchased in 2008 by Veolia Energy North America. In an effort to make continuous operational improvements to further drive economic growth for the city, increase environmental benefits, and increase energy efficiency, the Grand Rapids system implemented a condensing economizer. The sophisticated heat recovery system was designed and built by Progressive AE and ensured competitive steam rates for customers served by the Grand Rapids district energy facility.
The technology was installed on 120,000 pounds per hour nominal steam production and took three months to install. Once installed, it achieved an approximately 6% efficiency increase and reduced CO2 emissions by the equivalent of 1000 cars. The condensate produced by the economizer is used to supplement the existing make-up water supply. |
For example, installing an economizer as part of an 180,000 pound-per-hour district heating boiler can produce more than five million gallons of water per year.
Feasibility and Payback Period
Ideally, a condensing economizer and corrosion resistant exhaust duct may be installed for all combustion technologies, including natural gas, oil, biomass, and coal. The technology works best on applications that require large amounts of heated water and consume fresh water, as most steam district heating systems do. In terms of return on investment, the payback for implementing this technology is typically less than two years.
It is important for developers to identify sustainable solutions that ensure a successful project today, and also enable performance improvements in the future. More and more developers and building operators are recognizing the unique benefits of district energy, cogeneration, and related technologies.
These proven solutions increase energy efficiency, reduce overall CO2 emissions, and lower costs associated with construction, fuel and operations. Enhancing district energy and cogeneration through the application of technologies such as the condensing economizer increases the commercial viability of these sustainable solutions, which are available today.
About the author:
Kevin Hagerty is a Project Manager at Veolia Energy North America. For the last decade, Hagerty has worked in the power industry as a plant manager, emerging technology developer, and power engineer.
The author wishes to thank ConDex Systems Inc, the company behind the condensing economizer, as a source of the technical data in this article.
Renewable Energy Focus U.S. issue 2, September/October 2010.