The primary process used to generate electricity is the combustion of a fossil fuel to heat air. This high temperature air, or thermal energy, is then used to heat a liquid power generation medium (typically water) in a boiler to create a gas (steam) that is expanded across a steam turbine that drives an electrical generator. The measure of the thermal energy is the British Thermal Unit (BTU). Other sources of energy used to heat air and/or water to generate electricity in this manner include: heat from nuclear reactions; heat from the exhaust of gas turbines; heat from the combustion of refuse or other combustible materials in incinerators; and others.
Steam turbine systems used to generate power are generally closed loop systems in which pressurized water is vaporized in a boiler or heat exchanger; expanded in the steam turbine where the pressure levels are reduced as power is generated; condensed back to water in a condenser or cooler; and pumped back to pressure and returned to the boiler to repeat the cycle. In the process of making steam in this closed loop system, there are two major sources of wasted energy. The first is the waste heat exiting the boiler in the form of high temperature flue gas (typically heated air) due to the inherent design and thermodynamic characteristics of the water to steam conversion process that prevents using all the useful thermal energy (heat) in the flue gas. The second is the latent heat of vaporization or the amount of energy required to convert water to steam that is dissipated to the atmosphere during the process of condensing the steam back to water.
In the first instance of wasted heat, the boiler heat source must provide thermal energy (in the form of high temperature flue gas) not only to deliver 1000 BTU/LB to convert water to steam but also sufficient thermal energy to superheat the steam to high enough energy levels to provide sufficient excess energy to drive a steam turbine to generate power. The thermodynamic requirements of the steam cycle limit the temperature differential available to produce superheated steam to the difference between the original heat source temperature and approximately 400 to 500° F. This results in wasted heated flue gas exiting the boiler at temperatures of about 400 to 500° F. Although a portion of the energy in the flue gas exhaust may be recaptured by, for example, using it to heat the power plant, using it to pre-heat the boiler water, or by other known means, the amount of useful energy recovered is limited.
In the second instance of wasted heat, the energy in the form of heat required to change the state of a liquid to a gas is controlled by the thermodynamic characteristics of the liquid. The pressure and associated temperature at which a fluid begins to become a vapor is defined as the vapor pressure of the fluid. For a given liquid there is a specific range of pressures and temperatures at which the liquid becomes a vapor. The BTUs required to change a liquid to a gas at the vapor pressure is defined as the “heat of vaporization”. The heat of vaporization for water is approximately 1000 BTU/LB. At the vapor pressure at which water turns to steam, the amount of energy resident in the vapor is only that amount required to maintain a vaporous state and is defined as the “latent heat of vaporization”. At the vapor pressure point, if the vapor is cooled in a condenser or the pressure is reduced through an expansion process, the vapor will change states back to a liquid by discharging the latent heat of vaporization, or 1000 BTU/LB to the environment as an increase in thermal energy or temperature of the cooling medium. As such, little, if any, useful energy can be extracted from a vapor that only contains the latent heat of vaporization because such vapor will immediately condense upon expansion in a turbine, causing dramatic inefficiencies and possibly damaging the turbine. The physical phenomenon of the heat of vaporization causes waste heat in conventional power generation cycles because this amount of heat must be imparted into the liquid water before it changes into a useful gaseous state but this heat can not be extracted as useful energy. Upon cooling the medium back to a liquid so that it can be pumped to the desired pressure, this latent heat is discharged without being recaptured in the form of useful energy. Thus, the thermal energy discharged to the atmosphere through the cooling medium that returns the water to the liquid state is waste heat.
Converting heat to useful power and developing power in a more efficient manner from the combustion of fossil fuels are of paramount importance as fuel costs rise and energy sources are depleted. In addition, the negative effects on the environment caused by pollution generated from the combustion of fossil fuels dictates that power plants be designed to reduce the pollutants generated per unit of energy produced. These factors create a need to improve power plant efficiency and recover energy from waste heat generated by power plants, waste heat from various manufacturing processes, and thermal energy from renewable energy sources.
Various methods and processes are used to improve the efficiency of power systems that convert fossil fuels to usable energy. These efficiency-enhancing systems include gas turbine combined cycle plants, cogeneration plants and waste heat recovery systems. Cogeneration and combined cycle systems generate useful energy from the waste heat of gas turbine exhausts or other fossil fuel heat sources, including low grade heating value fuel sources, by using the heat of combustion to generate steam. In systems that use water as the primary power generation medium, the temperature of the heat source (typically flue gas heated by combusting fossil fuels) must be high enough to vaporize the water to create steam in a heat exchanger (boiler). The resulting steam is expanded in a steam turbine to produce power. Steam boilers are generally limited to recovering the thermal energy associated with the differential temperature between the initial temperature of the heat source and about 500° F. or higher because this is the temperature required to achieve efficient thermal energy transfer to water to produce steam. Further, the available heat for transferring energy to the steam is limited by the temperature differential restrictions imposed by the vapor pressure versus temperature characteristics of steam, and using a heat source with a temperature close to about 500° F. can lead to inefficient and minimal steam production. In a typical steam power generation system, the low temperature (˜500° F.) exhaust of the heat source exiting the boiler can be used to pre-heat the boiler feed water using a separate heat exchanger. However, only a limited amount of the heat in the discharge air is recoverable, and this heat is generally restricted to the temperature differential between the ˜500° F. discharge temperature of the heat source exhaust and about 300° F. or above due to the vapor pressure and temperature characteristics of water. Using the exhaust heat to pre-heat the boiler feed water in this manner increases the overall efficiency of the system, and may provide about a 10% increase in efficiency in some cases.
Some cogeneration and combined cycle systems also envision incorporating an Organic Rankine Cycle (ORC) system in combination with the steam turbine system to capture additional power output from the low temperature exhaust stream of the heat source as it exits the boiler. Methods are known in the prior art that utilize an ORC cycle to generate useful power. Typical methods are disclosed, for example, in U.S. Pat. Nos. 5,570,579 and 5,664,414, which are incorporated herein by reference. These prior art systems use a conventional ORC medium such as normal pentane, iso-pentane, toluene, fluorinated hydrocarbons and other refrigerants. These conventional ORC media have pressure and temperature limitations and can not sustain high temperatures due to their respective auto ignition temperatures and vapor pressure versus temperature characteristics. For example, prior art ORC systems that utilize refrigerants or toluene are restricted to operation with heated water since the ORC medium can not absorb energy at elevated temperatures. Other prior art ORC methods require an ORC medium with a vapor pressure near atmospheric pressure to be efficient. Other prior art systems are restricted to a specific power output range while others require spraying a fluid ORC medium into the heat exchanger for efficient operation. These limitations reduce their effectiveness and efficiency thereby restricting the circumstances under which they can be employed, and limiting the useful energy output that may be obtained from them.
In addition, although the majority of energy is generated using closed loop systems (i.e., systems in which the power generation medium, such as water/steam is constantly recirculated) such as those described above, other methods of generating power have been created to take advantage of open loop power sources that require constant replenishment of the power generation medium. For example, where the pressure of light hydrocarbon supplies in petrochemical plants or on gas pipelines must be reduced before being sent to consumers, it is known to generate useful power by expanding the high pressure gas in an expansion turbine that operates an electrical generator, pump or compressor, rather then reducing the gas pressure in a valve where no energy is recovered. Examples of this type of technology are provided in U.S. Pat. Nos. 4,711,093 and 4,677,827, which are incorporated herein by reference. These systems are open loop systems that require constant replenishment of the power generation medium, and depend on the pressure level of the process design.