Cogeneration, also known as total energy or combined heat and power, refers to the use of a single fuel to produce heat and electricity simultaneously. In many cases, the heat produced by engine-driven generators is wasted as exhaust. Harnessing that rejected heat for re-use in boilers and dryers conserves a substantial amount of fuel. Producing both electrical and thermal energy at the same time extracts more energy from the fuel (see Figure 1). That is why cogeneration is one of the most efficient technologies for energy conservation.
Cogeneration has been used for a long time, mostly in Europe. In the United States, cogeneration plants have been coming online gradually. Until 1978, cogeneration plant installations required a complete standby system because the electric utilities, with their exclusive operations, were concerned about losing customers.
The Public Utilities Regulatory and Policy Act (PURPA), passed in 1978, directs utilities to provide standby power and to buy the excess electricity from a cogenerator at a rate equal to the utility's avoided cost. The Act gave cogeneration a genuine boost. Even with resistance from the utilities, the number of cogeneration systems has mushroomed in the past 20 years.
Cogeneration can be implemented in different ways. For diesel generators using natural gas or diesel fuel, absorption chillers capture the energy available in exhaust and the jacket water for heating and cooling. Alternatively, the exhaust heat recovery system of a gas turbine generator can produce steam.
Cogeneration systems are efficient to operate. Located onsite and operating under the direct control of plant personnel, they provide reliable electricity and heat, while remaining independent of problems affecting the electrical grid.
A well-planned cogeneration system with a sound operation and maintenance program is definitely an asset. The owner controls the operation cost, which is substantially less than purchased equivalent energy from the local utilities in the form of electricity and gas.
Not every plant is a viable site for cogeneration. A major consideration is the cost of fuel. Operating economics dictate that a long-term contract for low-cost fuel is essential for the success of a cogeneration system. Once viability is proven, a feasibility study is necessary. It will determine the plant size, footprint, cost and electricity rate.
In general, prime candidates are plants that operate 24/7. The only way to determine the viability is to profile the plant's operation and energy use. The hourly variation of electricity consumption over a typical year of operation shows the electricity profile the cogeneration plant must serve. This profile also reveals the number of days per year that the plant is in operation. In addition to timing, a good understanding of heat use is important. Furthermore, the plant's operations and management's plans for future expansion must be analyzed.
The feasibility study should investigate the choice between internal combustion and turbine generator. In general, internal combustion engines are limited to providing 220º F maximum water temperature. Turbines, with exhaust temperatures of about 1,000º F, offer unlimited applications.
Psychology or economics?
We've observed that a plant manager's level of interest in cogeneration is a function of gas costs. When fuel costs are high, so is the interest in cogeneration. But as soon as costs drop, interest vaporizes. To some plant managers, a cogeneration system seems like a redundant system that they don't need.
In discussions with plant engineers, we've found that many want to replace old boilers with new units before pursuing cogeneration. They believe that boilers are an essential part of the plant, and therefore must be replaced when old. It is not easy to convince them that a cogeneration system will turn the boiler into a standby unit that would work about 400 hours per year. Plant engineers need to understand that cogeneration is the primary source of energy in a plant, and not a redundant system.
In general, plant designers have also shied away from cogeneration. Architects, mechanical and electrical consultants and plant engineers need to be educated in the operational efficiency and economic importance of cogeneration.
With the help of deregulation, feasibility studies offer many strategies for implementing a cogeneration system. For example, consider a plant that uses 60,000 lb./hr. of steam and has an electricity demand in the range of three to five megawatts. It operates 24 hours per day, but only for eight months out of the year. This isn't a viable cogeneration candidate because the cogeneration system would be idle four months every year.
Let's assume that an electricity service provider is interested in selling electricity to this plant and others in the neighborhood. The service provider could partner with the manufacturer and set up an efficient combined-cycle plant onsite to provide steam and electricity to the host. It might optimize the cogeneration system's capacity at 30 megawatts and export the excess electricity. The advantage is it provides efficient energy to a local network and unloads 30 megawatts from the high-voltage transmission lines. Such a project could be sponsored through a partnership among several plants that share electricity and heat. Ideally, the utilities should and could sponsor such cogeneration teamwork. As shown in Figure 2, the electricity cost could be as low as 4.75 cents per kilowatt-hour when the cost of natural gas is three dollars per million BTU.
A cogeneration plant normally operates 95 percent of the time. Even though most downtime is for scheduled maintenance, there are some hours of unplanned outage every year. But this can be offset by the utility. The cost of providing standby power is included as a part of operating expenses. It's a cost-effective way to treat the standby problem.
The recent development of microturbines makes cogeneration more attractive at low power levels. Two common microturbines (28 and 60 KW) use air bearings and don't require oil lubrication. Either unit can be used for many heating or cooling applications; both are ideal for small plants.
Good applications for cogeneration are in cooling and heating. A one-megawatt plant could supply more than 500 tons of air conditioning, sufficient for data reduction centers and large communication systems. The U.S. Department of Energy currently sponsors a development program to provide cooling through absorption chillers by direct use of exhaust heat.
It is important to note that the Environmental Protection Agency is proposing rules to facilitate cogeneration. The rules would amend the Federal Clean Air Act to make it easier and quicker for plants to implement cogeneration.
Figure courtesy Cogeneration Planners, LLC.