Simple and compact low-temperature power cycle

Details Simple and compact low-temperature power cycle Simple and compact low-temperature power cycle

2002-12-09

2004-06-22

Richter, Sheldon J. (Department: 3748)

Power plants

Motive fluid energized by externally applied heat

Power system involving change of state

C060S647000, C060S651000, C060S671000, C060S676000

Reexamination Certificate

active

06751959

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is a thermodynamic power cycle in which the working fluid (also called motive fluid) is energized by externally applied heat. Within that field, the present invention is a process in which the working fluid in the course of power production reaches a pressure and a temperature above that at which its vapor and liquid have the same density (i.e., a fluid state that is above its critical pressure and temperature, also called supercritical conditions). The present invention is also a process in which the working fluid is other than water or steam. Ammonia is the working fluid of choice for most applications contemplated for the present invention, but other fluid types which, like ammonia, have boiling points below 32° F. at a pressure of one atmosphere, absolute, may also be selected for reasons of obtaining greater efficiency, operability, or economics.
2. Description of the Prior Art
Many industrial processes have flowing streams of liquids, solids, or gases that contain heat which must be exhausted to the environment or removed in some way to facilitate proper operation of the process. Typically, the process designer for these industrial processes will use heat exchange devices to capture the heat and recycle it back into the process via other process streams. Often, however, there are not streams suitable to capture and recycle this heat, because they are either already too high in temperature or they contain insufficient mass flow. Any heat which cannot be recycled into the process is typically referred to as waste heat. Most often waste heat is simply discharged to the environment, either directly as an exhaust stream, or indirectly via a cooling medium, such as cooling water.
One method of utilizing waste heat is to raise steam in a boiler to drive a turbine, a known method well recognized by practitioners of the art known as the Rankine cycle. The steam-based Rankine cycle, however, is only economic when it is applied to heat source streams that are relatively high in temperature (generally 600° F. or higher) or are large in overall heat content. In other words, high thermal efficiency or significantly large scale is generally needed to make the Rankine cycle economic. A major reason for this is that efficient removal of waste heat from a process stream requires boiling water at multiple pressures/temperatures to capture heat at multiple temperature levels as the heat source stream is cooled. This complexity is costly from standpoints of both equipment cost and operating labor. Overall, the steam-based Rankine cycle is either too expensive or too inefficient or some combination of the two to be applied to streams of small flow rate and/or low-temperature.
Some process developers have substituted other working fluids for steam in the Rankine cycle to obtain greater compatibility with heat source streams of low or moderate temperature. Typically, an organic fluid such as propane is used. Although improved over steam, organic cycles present the same fundamental inadequacies of the Rankine cycle described above.
Accordingly, there is a need for a relatively simple, low-cost, and relatively efficient method of capturing and utilizing waste heat from process streams that are low in temperature or low in overall heat content.
The advantages of using supercritical conditions in a power cycle have been recognized for many years. For example, a 1927 patent (U.S. Pat. No. 1,632,575, Jun. 14, 1927, Abendroth) describes a system for generating power from supercritical steam. Even then the inventor, Abendroth, did not claim supercritical steam power generation as the invention, rather he claimed a variation of it.
Abendroth, '575, above, highlighted the advantages of supercritical steam generation when he stated, “The advantage of this process resides in the fact that a separation of steam and liquid of equal temperatures but of different physical properties cannot take place at any point in the process. In this way the dangers are eliminated which are caused by the well-known ebullition or boiling phenomena.” In other words, a heated fluid does not boil when it is at a pressure above critical, instead the fluid simply transitions from liquid to vapor as its temperature rises through the critical temperature. Indeed, the properties of the liquid and the vapor are identical at the critical temperature. And, although the dangers of boiling are well-understood and easily controlled in today's power plants, boiling requires specialized equipment to separate the liquid phase from the vapor phase. Under supercritical conditions, in which no such separation takes place, the equipment is simplified. Moreover, as will be explained later in detail, supercritical operation can have thermal efficiency advantages over boiling operation. Generally, with most working fluids, multiple pressure boiling stages are needed to achieve the same thermal efficiency that supercritical operation can achieve in one stage.
A disadvantage of the supercritical steam cycle is that the heat source must be above 705° F., the critical temperature of water. This eliminates many moderate and low temperature heat sources as potential applications for the cycle. A supercritical ammonia cycle, however, is applicable to these heat sources because of the relatively low critical temperature of ammonia, 270° F.
The use of ammonia as a working fluid is also known. In U.S. Pat. No. 781,481, Jan. 31, 1905, Windhausen, Jr., a basic form of Rankine cycle is described in which ammonia is the working fluid. The patent covers generally all pure working fluids in which their normal boiling temperature is less than 32° F. These “low boiling” fluids are, in general, a good match for Rankine cycle applications in which the heat source temperature and/or the condensing temperature is relatively low. Since Windhausen's patent was first issued in 1905, there have been variations of using ammonia and other low boiling liquids to capture low temperature heat. Many of these were patented during the late 1970's and early 1980's at the height of the energy crisis in the U.S. Exemplary of these is an invention to convert natural heat sources (solar, geothermal, etc.) to power (U.S. Pat. No. 4,100,744, Jul. 18, 1978, DeMunari), an invention to produce power from low temperature heat sources in a petroleum refinery (U.S. Pat. No. 4,109,469, Aug. 29, 1978, Carson), an invention to exploit natural temperature differences on the earth such as a mountain top and a desert valley (U.S. Pat. No. 3,953,971, May 4, 1976, Parker), and an invention that is another variation of the use of natural heat sources (U.S. Pat. No. 4,192,145, Mar. 11, 1980, Tanaka).
In 1969, William L. Minto discussed the value of low boiling compounds as working fluids in his patent of a Low Entropy Engine (U.S. Pat. No. 3,479,817, Nov. 25, 1969, Minto). Minto's engine was in essence a basic Rankine cycle with certain low boiling compounds, mainly halogenated hydrocarbon compounds such as carbon tetrachloride, as the universe of working fluids from which to select. (One example of an acceptable working fluid for Minto's invention, chlorodifluoromethane, is specifically cited as a potential selection of working fluid for our invention.) Minto's engine vaporizes the working fluid by ordinary means of boiling at subcritical pressure. In his patent, Minto stated that low boiling compounds could endow a power cycle with certain characteristics when he stated that an “object of the present invention is to provide an improved [working fluid] . . . characterized by its efficiency, simplicity, [and] compactness . . . ”. Minto did not recognize, however, that the use of supercritical operating conditions could further increase thermal efficiency of the process and enhance simplicity by eliminating boiling and the attendant boiler equipment.
Only one patent was discovered which mentioned the concept of using ammonia as a working fluid with supercritical ope

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