Cogeneration describes the simultaneous production of electricity and thermal energy using a single fuel source. Cogeneration, or cogen for short, consists of a number of components including a prime mover, generator, heat recovery system, and steam 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 is done by using it to heat water. The hot water or steam is used to provide thermal energy to heat buildings or drive equipment such as absorption chillers for cooling or dehumidifiers. 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 occurs from equipment and system inefficiencies as well as thermodynamic limitations of the processes. Improving the industrial efficiency typically incorporates adding new equipment or reducing the amount of energy consumed by existing equipment. The most valuable alternative is the approach to capture and reuse lost or waste heat that is exhausted from the industrial process. During operation, 20-50% 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
Ways of reusing heat includes using combustion exhaust gases to preheat combustion air or feedwater for industrial boilers. This can be done by using a Heat Recovery Steam Generator (HRSG) which is a boiler that uses the heat of exhaustion gases instead of a combustion reaction for heat. By preheating the feedwater, the amount of energy required to superheat the water in the boiler 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 that exits the turbine. This steam is still very hot and needs to be condensed anyway to complete the Rankine cycle. By using this heat for another process the efficiency of the entire plant increases.
The majority of the United States electrical generation plants do not reuse waste heat. As a result, the average efficiency of the electrical power has remained at 34% since the 1960's. The majority of the United States cogen production is found within the industrial applications providing power and steam for large industries such as refineries and metal manufacturing, consuming 88% of the total. Commercial and institutional buildings, including campuses, hospitals, and office buildings, make up the remaining 12% of total cogeneration production.
The University of Montana, Missoula installed a cogeneration unit in 1997. This system was designed to provide an additional 434 kW of power to campus using excess heat from the boilers. Before the cogeneration unit was installed, the Missoula boilers operated at steam pressures of 80-180 psig. Steam was reduced to a pressure of 30 psig before being distributed throughout campus. This drop in pressure before being distributed created an opportunity to install a steam turbine generator. The unit installed reduced 165 psig steam to 30 psig, with a mass flow rate of 24,978 lb/hr. This particular system produced 440 kW of power at full load. As of 2010, the University was making payment on an annual bond for the installation of the unit. After this payment is made, an additional saving of $16,830 is received every year!
Gas Turbine Cogeneration
Gas turbines burn natural gas, petroleum fuels, biogas, and duel fuel. Gas turbines are well suited for combined heat and power production due to the high temperature exhaust which can be used as thermal energy to generate steam at high temperatures and pressure. Much of the current US gas turbine cogen systems maximize the 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 steam energy into shaft power. The energy is produced by high pressure steam that rotates the blades of the turbine which turn the generator. In a steam turbine based combined heat and power system, the steam exiting the turbine is directed to the steam users to extract extra thermal energy. Ideal implementations for steam turbine based cogeneration systems include medium and large scale industrial facilities or where high thermal loads are in demand.
Like central heating and cooling plants, cogeneration systems can burn a variety of fuels. Comparing thermal and electrical loads, cogen systems typically require around 40% less energy than operating separate heat and power systems. This increases the efficiency and reduces fuel consumption. The reduction in fuel consumption is the main environmental benefit leading to fewer emissions with the same level of output; in particular, the amount of CO2 being released is decreased.
Heat output from electrical production for heating and industrial applications generally convert 75-80% of the fuel source to useful energy. Electricity produced by cogeneration plants is normally used locally, cutting down on transmission and distribution losses. Cogeneration offers have total energy saving efficiencies between 15-40%.
The thermal efficiency of cogeneration systems is much higher than other types of plants. This is 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, with the heat energy in process steam, ΔH, (enthalpy of steam leaving the plant - enthalpy of condensate returning to the plant) then divide the sum by the heat added to the plant, Q (coal, nuclear fuel, natural gas, etc.). By adding the electrical production with the change in energy of the process steam, it will give a very high thermal efficiency for the plant, sometimes as high as 80-90%!
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, just over 8% of the world's electrical generation capacity used cogeneration. Natural gas accounts for 33% of the total cogeneration capacity, where coal makes up 36%, oils 5%, and biomass which accounts for 6%. Regions containing the most cogeneration are in Europe. Denmark is the global leader in cogeneration with 52% of its electricity produced in 2003. Germany's cogeneration production reached 13% of its electricity produced in 2005, which is expected to reach 57% in the years to come. The United States production is relatively small with a total of 8%, even though the country leads the world in total electrical capacity at 84,707 MW.
Due to its higher efficiency, cogeneration systems help countries reduce the fuel demand and meet greenhouse emission reduction targets. According to the IEA (International Energy Agency), cogeneration could help reduce the greenhouse gas emissions by 10% by 2030.
With the increasing demand for electricity, higher capacities translate to more opportunity for savings. Many existing systems are oversized and current systems can be modified or newer equipment can be added. Apart from the energy savings, proper sizing can reduce noise, lower the cost of equipment and performance, reduce maintenance and improve reliability. With cogeneration facilities using a diverse group of technologies, it makes its impact environmentally friendly.
Cogeneration at MNSU
In 1995 Minnesota State University decided to look into the idea of a cogeneration cycle for campus. This system was designed to provide an additional 434 kW of power to campus using excess heat from the boilers. The impact of this steam reduction was felt quickly. Without enough total steam getting through the main building pressure regulating valves (PRVs), all of the entering steam was being used for showers. This left no steam for heating the buildings! This was also an issue throughout the academic buildings. For instance, one of the maintenance engineers received a call from the green house reporting that it was very cold. Upon investigation, the engineer discovered no steam coming out of the associated valve. Assuming that the valve was faulty, he took out the valve and looked at the pipe supplying the steam. All that came out was a few drops of water. All of the energy from the steam had already been used in the Trafton building, which preceded the green house on the steam line. With all the useful energy removed, the steam condensed and was sent back to the utility plant before reaching the green house.
The first problem with the cogeneration system was not getting enough steam into the buildings during the winter. The valves installed in the buildings on campus function with a pressure of 150 psig at the inlet to 10 psig at the outlet. Valve manufacturers often provide graphical means of relating valve size, pressure differential, and flow rate. Valves are needed to provide the necessary flow rate of 18,000 lb/hr of steam during the winter with the pressures of 150 psig at the inlet to 10psig at the outlet. With an inlet pressure decrease to 50 psig due to the cogeneration unit, the flow rate received by the building would only be 8,185 lb/hr. A 5' valve would need to be installed to supply steam at 18,700 lb/hr with an inlet pressure of 50 psig. The pipe size leading up to this valve (across campus in the tunnels) would also need to be increased to 5'. However, the price of this kind of installation was determined to be more than the amount that would be saved from successfully running the unit.
The amount of steam admitted to each building is controlled with two to five PRVs. These valves block the 150 psig steam from entering if the pressure on the opposite side of the valve is above 10 psig. As more heat is required in the building, the air handlers admit more steam flow. With this flow rate increase, the steam pressure on the building side drops. The resulting pressure imbalance on the PRVs causes them to open farther 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 and the valve size. With only 50 psig of backpressure from the cogeneration unit, the flow rate of steam which could get through the building PRVs, even when completely open was only 1/3 of the required amount for the building. For example, a building that needed 18,000 lb/hr of steam was only receiving 6,000 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. Once this point was reached, 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 from the turbine needed to be recombined. However, since the turbine steam was at 50 psig this meant that the bypassing steam needed to be throttled down to 50 psig prior to mixing. With 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.
In addition to the steam supply problems the economics of the system changed over time. At the time the cogeneration unit was installed the campus did not have stand-by generators. When these were installed it allowed the campus to be disconnected from the electric company when asked in order to reduce demand. In exchange for this option the local utility reduced the price of electricity from the varying residential rate to a constant $0.045. Originally the electric rates varied according to season when the cogeneration unit was running. Also, in 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 higher fuel cost and lower electricity cost, the cogeneration unit was no longer as economical to run. The decision was made to stop using the steam turbine and dismantle the cogeneration system.