Absorption power cycle with two pumped absorbers

Details Absorption power cycle with two pumped absorbers Absorption power cycle with two pumped absorbers

2000-06-06

2001-08-07

Nguyen, Hoang (Department: 3748)

Power plants

Motive fluid energized by externally applied heat

Process of power production or system operation

C060S668000

Reexamination Certificate

active

06269644

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING THE FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not applicable.
TECHNICAL FIELD
This invention relates to a method and apparatus for converting thermal energy to mechanical energy utilizing an absorption power cycle. The mechanical energy may further be applied to a variety of useful ends: generating electricity, compressing a vapor, pumping a liquid, or propelling a vehicle or conveyance.
BACKGROUND OF THE INVENTION
Absorption power cycles have been known and practiced for over one hundred years. These cycles are comprised of a circulating absorbent liquid and a condensable working fluid. Vapor phase working fluid is desorbed from the absorbent at high temperature and pressure, then expanded to produce work, and then reabsorbed at low pressure and temperature. Thermal energy is input to the cycle at the high-pressure desorber (also termed generator), and rejected from the low-pressure absorber.
An early example of this cycle was the “soda engine” used to power locomotives and streetcars in Germany in the late 1800s (U.S. Pat. No. 340,718 and 124,594). H
2
O was the working fluid, and aqueous NaOH was the usual absorbent. Similar cycles were built in Japan in the 1970s, powering a tricycle, a golf cart, and a pickup truck, and called “concentration difference engines” (U.S. Pat. No. 4,122,680).
An early absorption power cycle using NH
3
as working fluid was described by Sellew and Koeneman, and used ZnCl
2
as absorbent. More recent absorption power cycles based on the NH
3
—H
2
O pair are disclosed in U.S. Pat. Nos. 3,505,810; 4,307,572; and 5,953,918. An aqua ammonia refrigeration cycle wherein the absorption cycle power is used to pump cycle liquid is disclosed in U.S. Pat. No. 2,408,802.
Another type of power cycle which bears certain similarities to the absorption power cycle is referred to as the “Kalina” cycle (U.S. Pat. Nos. 4,489,563; 4,548,043; 6,058,695; and others). This type of power cycle also uses a multi-component working fluid such as ammonia-water. It differs most prominently from absorption power cycles in that there is no circulating liquid absorbent—the working fluid is entirely evaporated at high pressure in lieu of being desorbed. This necessitates various changes in the lower pressure sections of the cycle as well, e.g., using condensers in lieu of absorbers.
The absorption working pairs used in power cycles can be categorized according to whether the absorbent is volatile or non-volatile. Volatile absorbents will have appreciable presence in the vapor phase as well as the liquid phase, and accordingly the manner in which all mass transfers (latent heat exchanges) are conducted assumes overriding importance. That is, a completely different result is obtained from co-current mass exchange vs. counter-current mass exchange. Ammonia-water is an example of a working pair with volatile absorbent. Note that the Kalina cycles are inherently restricted to volatile absorbents, so as to allow complete evaporation.
Absorption power cycles have the characteristic that the absorbent increases in temperature as more vapor is desorbed from it. Thus it is possible to supply heat of desorption over the corresponding temperature range. For heat sources which are characterized by having a temperature glide (e. g., sensible cooling of a fluid such as combustion exhaust gases or geothermal brine, or condensation of a multi-component vapor), this provides a thermodynamic advantage. More of the source heat can be transferred into the desorbing fluid with reduced loss in availability, and hence more work can be derived from the cycle.
Nevertheless, prior art absorption power cycles have been limited in the degree to which they can match heat source temperature glide, thus limiting their useful work production, by a variety of cycle-specific factors.
First, many cycles produce a low purity ammonia vapor, about 85% purity or lower. In order to avoid excessive moisture formation during expansion, the vapor must be superheated to well above peak desorption temperature. Superheat causes a major variation in the temperature glide, unless several costly stages of reheat are additionally incorporated.
Second, the liquid desorption step itself, although occurring over a wide temperature range, is also relatively non-linear, with much more heating required at the cold end than at the hot end (for reversible desorption).
Third, the temperature glide of desorption is a function of how it is conducted. With co-current desorption all the way to complete vapor (complete evaporation), the glide is limited to the difference between bubble point temperature and dew point temperature. With co-current partial desorption, the glide is even more severely restricted.
Fourth, the low temperature end of the desorption step is usually so warm that there is appreciable useful thermal energy remaining in the source heat even after counter-current heating of the desorbing fluid.
Fifth, the absorption heat rejection is also quite non-linear, requiring higher flowrates of cooling fluid compared to more linear heat rejection scenarios.
Those cycles which entail evaporating the working pair completely to vapor have the problem that trace dissolved solids will become very concentrated and corrosive, and will likely form scale in hot sections of the evaporator. The extreme variation in wetting makes heat transfer very difficult.
It is one object of this invention to overcome the above limitations of the prior art absorption power cycles, so as to achieve a closer match to the temperature glide of the heat source, and hence a more efficient cycle, but in practical and economic equipment.
BRIEF SUMMARY OF THE INVENTION
The above and additional objects are achieved by providing method and apparatus for producing power from thermal energy comprised of an absorption power cycle comprised of: a working pair with a volatile sorbent, such as ammonia-water; a high-pressure generator with temperature glide; a work-expander for vapor from said high-pressure generator; and two spaced-apart liquid feeds to said generator; one pumped from a first absorber; and another at a different concentration pumped from a second absorber. The first absorber is at the low-pressure of the expanded vapor, and is externally cooled. The vapor from the high-pressure generator is at a purity of at least about 90%, and preferably about 95%. The second absorber may be either at low pressure also, in which case it is internally cooled by desorbing liquid; or at intermediate pressure, in which case it is also cooled by external cooling. When using ammonia-water as absorbent, the absorber yielding higher ammonia content absorbent is the one which is pumped to the lower temperature inlet end of the high-pressure generator; and the absorbent from the other absorber, having lower ammonia concentration, is supplied to a mid-section of the high-pressure generator.
Achieving a close match between the temperature glides of the heat source and the cycle heat input, and ultimately a high cycle efficiency, requires a variety of measures or features. The combination of features appropriate will vary with the heat source characteristics: starting temperature, linearity of cool down glide, restrictions on final temperature, heat quantity, type of fluid (liquid, gas, condensable vapor, etc.), and pressure. Some features, such as those disclosed above (two pumped absorbers and vapor purity above 90%) are always desirable; others disclosed below may only apply in certain circumstances.
The 90% purity limitation on the vapor being expanded is related to the allowable wetness (percentage liquid) at the turbine exhaust. Most turbines have a limitation on the order of 7 to 10% maximum liquid at the exhaust, to prevent damage. For peak desorber temperatures less than about 140° C., it is possible to have co-current desorption in conjunction with counter-current heat exchange, followed by vapor-liquid separation, thereby directly yielding vapo

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