Cogeneration describes the simultaneous production of electricity and thermal energy using a single fuel source. Cogeneration, or cogen for short, also called Combined Heat and Power (CHP), consists of a number of components including a prime mover, generator, heat recovery system, and thermal distribution system. Prime movers of cogen systems include reciprocating engines, steam or gas turbines, micro-turbines, and fuel cells. The prime mover is connected to a generator to produce electrical output. Thermal energy can be used directly or indirectly in applications producing steam, hot water, air heating, and chilled water cooling using absorption. In most cogeneration applications, the exhaust gas from the electrical generation production is sent to heat exchangers to recover the thermal energy from the gas. This thermal energy is then used to heat water. The hot water or steam, in turn, is used to provide thermal energy to heat buildings, or to drive equipment such as absorption or steam turbine driven chillers for cooling or dehumidification. Cogeneration systems are used in dual-purpose power plants, waste heat utilization systems, and some types of centralized heating and cooling plants.
Waste Heat Recovery
Waste heat results from equipment and system inefficiencies as well as thermodynamic limitations of the processes. Adding new equipment or reducing the amount of energy consumed by existing equipment improves the industrial efficiency, and a cost-effective way to improve efficiency is to capture and reuse lost or waste heat that is exhausted from the industrial process. During the power plant operation, 20% to as much as 75% of the energy consumed is lost as waste heat. For the industrial sector, potential sources of waste heat include:
- Conductive, convective, and radiative losses from equipment surfaces and heated product lines
- Hot combustion gases
- Cooling water from furnaces, air compressors, and internal combustion engines
- Lubrication oil
- Intercooling and/or aftercooling of compressed (turbocharged or supercharged) combustion air
- Combustion air preheating
- Boiler feedwater preheating
- Power generation
- Space heating
- Direct drying applications or industrial process heating
- Generation of chilled water using absorption or steam turbine driven chillers
Heat can be reused by employing a Heat Recovery Steam Generator (HRSG), a boiler that uses the heat of exhaust gases instead of a combustion reaction for heat. The HRSG uses combustion exhaust gases to preheat combustion air or feedwater for industrial boilers. By preheating the feedwater, the total amount of energy required to generate saturated or superheated steam is reduced. In addition to its advantages for thermal recovery, waste heat has economic benefits which help reduce capacity requirements for facilities and lead to reductions in cost. This can be achieved by utilizing the combustion exhaust gases to heat building air space, eliminating the need for additional space heating equipment. Other cogeneration systems use the heat in the steam that exits the turbine. Because the exiting steam is still very hot, it must be condensed to complete the Rankine cycle. Using this heat for another process increases the efficiency of the entire plant.
In some applications, particularly with gas turbines, it is possible to further heat the exhaust gases using a supplemental duct burner to generate additional steam output. A duct burner uses an additional fuel supply to combust with the remaining oxygen in the exhaust gas since the original air to fuel combustion ratio in the turbine was significantly greater than the stoichiometric. By using the already hot exhaust gas from a prime mover, the combustion efficiency of the duct burner is higher than normally achievable with a conventional boiler.
Other cogeneration systems use the heat that exits the turbine to generate steam, which then needs to be condensed to complete the Rankine cycle. By using this heat for another process, the efficiency of the entire plant increases, since the latent heat is used beneficially as opposed to being condensed and then rejected to the atmosphere in a combined cycle application.
Because it is difficult and expensive, the majority of the United States electrical generation plants do not reuse waste heat. As a result, the average efficiency of the electrical power production has remained at 36% for all fuel types . Approximately 88% of the total cogeneration production in the United States takes place within the industrial applications that provide power and steam for large industries such as refineries and metal manufacturing. Cogeneration production for commercial and institutional buildings—including campuses, hospitals, and office buildings—constitutes the remaining 12%..
Gas Turbine Cogeneration
Gas turbines burn natural gas, petroleum fuels, biogas, and dual fuel. Gas turbines are well-suited for combined heat and power production, because the high temperature exhaust can be used as thermal energy to generate steam at high temperatures and pressure. Many gas turbine cogeneration systems currently used in the U.S. maximize power production for consumption along the main grid while the steam is supplied to large industrial and commercial facilities.
Steam Turbine Cogeneration
Steam turbines generate electricity from steam produced in a boiler by converting its thermal energy into mechanical energy that is then converted into electrical energy in the generator. The energy is produced by using high pressure steam to rotate the blades of the turbine that turn the generator. In a steam turbine-based combined heat and power system, the steam exiting the turbine is directed to the users of the steam to extract additional thermal energy. Ideal applications for steam turbine-based cogeneration systems include medium- and large-scale industrial facilities, or where there is demand for high thermal loads.
Like central heating and cooling plants, cogeneration systems can burn a variety of fuels. Taking into consideration thermal and electrical loads, the total efficiency of the plant can be as high as 90% efficient. Cogeneration systems also typically require about 40% less energy than operating heat and power systems separately . This saves fuel and increases plant efficiency. Using less fuel has major environmental benefits as well: lowering harmful emissions (of CO2 in particular) with the same level of output.
Combined heat and power plants typically have an overall efficiency of 65-80% or more . Usually, electricity produced by cogeneration plants is used locally, which reduces transmission and distribution losses. Cogeneration offers total energy-saving efficiencies of 15–40% .
Cogeneration systems have much higher thermal efficiency than other types of plants due to the reduction of waste heat. The cogeneration plant thermal efficiency equation is shown in the sidebar. In this equation you add the electric energy produced, E, to the heat energy in process steam, ΔH, (enthalpy of steam leaving the plant minus enthalpy of condensate returning to the plant) then divide the sum by the amount of heat added to the plant, Qin (coal, nuclear fuel, natural gas, etc.). Adding the electrical production with the change in energy of the process steam results in a very high thermal efficiency—sometimes as high as 90% for the plant.
In addition to its advantages for thermal recovery, reusing waste heat has economic benefits which can help to reduce capacity requirements for facilities which can lead to reductions in building and construction costs. This can be achieved by utilizing the combustion exhaust gases to heat building air space, eliminating the need for additional space heating equipment.
In 2009, cogeneration accounted for slightly over 8% of the world's electrical generation capacity . Cogeneration uses single fuel sources such as natural gas, fuel oil, biomass, biogass, coal, or waste heat. The countries that use cogeneration the most are in Europe with 47.4%..
The higher efficiency of cogeneration systems helps countries reduce fuel demands and meet greenhouse emission reduction targets. According to the IEA (International Energy Agency), by 2030, cogeneration could help reduce the greenhouse gas emissions by 10% .
With the increasing demand for electricity worldwide, higher capacities translate into more opportunities to save energy and money by modifying oversized existing systems or adding newer equipment. In addition to saving energy, proper sizing can reduce noise, lower the cost of equipment, reduce maintenance needs, and improve reliability. Cogeneration facilities that use a diverse group of technologies are more environmentally friendly.
Cogeneration at MNSU
In 1995, Minnesota State University looked into using a cogeneration cycle that used excess heat from the boilers to generate an additional 434 kW to the campus. After the system was installed on campus, the impact of this steam reduction was felt quickly. Because insufficient amounts of steam had been passing through the main building pressure regulating valves (PRVs), all of the available steam was used for showers, leaving no steam to heat the buildings! This system posed problems in the academic buildings as well. For instance, after receiving a report that the university greenhouse was very cold, one of the maintenance engineers discovered that no steam was coming out of the associated valve. Assuming that the valve was faulty, he removed the valve and examined the pipe that supplied the steam. Only a few drops of water trickled out. All of the energy from the steam had already been depleted in the Trafton building, which preceded the green house along the steam line. With all of its available energy removed, the steam condensed and was consumed in the utility plant before it could reach the greenhouse.
The amount of steam admitted to each building is controlled with between two and five PRVs depending on the size of the building. These valves block the 150 psig steam from entering if the pressure on the opposite side of the valve is higher than 10 psig. As more heat is required in the building, the air handlers admit more steam flow. When the flow rate increases, the steam pressure on the building side drops. The resulting pressure imbalance on the PRVs causes them to open further and allow more steam to enter the building's heating system. The valve continues to stay open, or opens further, until the pressure on the building side of the valve returns to 10 psig.
When a fluid flows through an obstruction such as a valve, there is a loss of pressure. This loss of pressure must balance the pressure difference across the obstruction. Therefore, the mass flow rate of steam through a PRV is related to the pressure differential across the valve as well as the valve size. With only 50 psig of backpressure from the cogeneration unit, the flow rate of steam which could pass through the building PRVs—even when they were fully open—was only 1/3 of the required amount for the building. For example, a building that required 18,000 lb/hr of steam was only receiving 6000 lb/hr due to the decreased driving force of the inlet pressure.
To handle these changes in demand, the cogeneration system was designed so that a control valve regulated the steam flow through the turbine unit up to a maximum of 40,000 lb/hr. At that point, any additional steam bypassed the unit through a 6'-diameter pipe. Before being sent to campus, the bypassed steam and the 40,000 lb/hr steam flow from the turbine needed to be recombined. However, because the turbine steam was at 50 psig the bypassing steam needed to be throttled down to 50 psig prior to mixing. At this this limited pressure, the flow rate of steam which could get through the building was only 1/3 of the required amount to heat the building.
As well as causing steam supply problems, the cogeneration system became less cost-effective over time. At the time the cogeneration unit was installed the campus did not have stand-by generators. When the stand-by generators were installed, the university, when asked, could be disconnected from the electric company in order to reduce the strain on the local power grid. In return for this arrangement the local utility reduced the price of electricity for the university from the variable residential rate to a constant $0.045 per kWh. At the time that the cogeneration unit was running, the electric rates varied according to the season. Furthermore, during the years after the cogeneration installation, the price of natural gas used to fuel the boiler increased from $2.53 to $10.25 per Mcf (1000 cubic feet). (One Mcf of natural gas produces 779 lbs of steam.) With the cost of fuel higher and the price of electricity lower, the cogeneration unit was no longer cost-effective, so the university decided to stop using the steam turbine and dismantle the cogeneration system.
|What are the basics of cogeneration? This EPA article is here to help.|
|How can a cogeneration plant increase efficiency, reduce cost, and reduce emissions? Read this ASME article about the University of Connecticut and see what cogeneration can do.|
|Want to know the benefits of a HRSG and its design? Click here.|
|Want to know more about using cogeneration for on-site energy distribution? Click here.|
|Want to learn about an unconventional cogeneration plant? Click here.|
|The federal internal tax credit for CHP recognizes 65% as the low threshold required in order to qualify.|
|Cogeneration Thermal Efficiency Equation|
|Though the system at MSU is a cogeneration system, it works off of a backpressure turbine rather than the typical PRVs to reduce pressure. Typically, a PRV is used to reduce the steam pressure in a cogeneration system whereas with this system, a backpressure turbine is used to lower the steam pressure instead.|
|Want more information on combined heat and power technologies? Read this Department of Energy page.|