1、AN-04-7-3 H eat-Act ivated Du a I- F u n ct i o n Absorption Cycle Donald C. Erickson Member ASHRAE G. Anand, Ph.D. Member ASHRAE Icksoo Kyung, Ph.D. ABSTRACT Low-temperature heat is a widely encountered source of energy that is currently used little. Absorption cycles are uniquely capable of upgrad
2、ing low-temperature heat to useful form (refigeration orpower) at high eficiency and low cost. A new dual-function ammonia-water absorption cycle is being developed that accepts heat in the range of 120C to 300C to produce power, refigeration, and/or air conditioning in inter- changeable amounts, de
3、pending on user needs. The cycle builds upon 100+ years of absorption cycle developments and particularly upon 20 years of development of the advanced generator absorber heat exchange (GAX) cycle. The high eficiency is due to the close temperature match between the cycle heat acceptance and the sour
4、ce heat avail- ability. Compared to other recently developed ammonia-water cycles, this cycle can achieve larger temperature glides, which significantly increases the overall energy conversion efi- ciency. Closer approach temperatures are achieved in simpler hardware without using risky components s
5、uch as total evap- orators. Compact equipment, avoidance of vacuum, andfavor- able transport properties yield attractive economics in many applications. This paper presents a description of the dual-function absorption cycle. General background is presented on absorp- tion power cycles, including a
6、comparison to better known power cycles. Several advantageous applications of the dual- function cycle are outlined, including turbine-inlet-cooling plus power in distributed generation, solar or geothermal heated applications, andprocess-coolingplus power in indus- trial applications. The absorptio
7、n cycle can be directly inte- grated with industrial process streams or with prime movers such as turbines, fuel cells, and reciprocating engines. The mix ofpower and refrigeration is easily adjusted to meet seasonal requirements for cooling or power. INTRODUCTION Low-temperature heat is a widely en
8、countered source of energy that is currently used little. Petroleum refineries repre- sent some of the largest, most concentrated sources of anthro- pogenic useful temperature waste heat in existence. In the U.S., the average refinery of 150,000 bpd crude throughput consumes 32 trillion Btu/year to
9、provide process heat (fired heaters, boilers, etc.) and electricity, which is about 9.5% of the heat value of the crude throughput (Energetics 1998). Most of that heat is finally rejected at temperatures too low to be useful. However, approximately 7 trillion Btu/year is rejected in the 130C-500C ra
10、nge where it could potentially be upgraded to useful product. In addition to the refining industry there are many other large industrial sources of waste heat, such as the chemical and forest products industries. A recent report (Energetics 2000) identifies selected chemical sectors, such as ethylen
11、e and ammonia, where more than 80 billion Btu/yr of waste heat is available at the typical plant. Other large sources of low- temperature heat include solar and geothermal power plants and gas turbines. Traditionally, plant operators have regarded the waste heat as merely useful for heat integration
12、. Heat integration transfers low-temperature heat to even lower temperature demands. Because the demands for low-level heat are rela- tively limited, much of the waste heat is presently unused. Recently, waste heat-powered refrigeration is finding new uses in refineries: recovering volatile products
13、 from waste process streams, debottlenecking process units, and improv- ing separation efficiency (Erickson et al. 1998). However, the quantity of low-level waste heat available is well in excess of what would be required to meet all prospective refinery refrig- - - Donald C. Erickson, G Anand, and
14、Icksoo Kyung are with Energy Concepts Co., Annapolis, Md. 02004 ASHRAE. 51 5 eration needs. This leads to consideration of another option- to generate power from the waste heat. Goswami and Xu (1 999) present a dual-function cycle with power production as primary function. However, in most applicati
15、ons, refrigeration is a higher-value product than power. It takes more than just power to produce conventional refrigeration-it takes refrigeration equipment as well. There- fore, the benefits to the user from refrigeration are usually greater than what power alone provides. On the other hand, frequ
16、ently, more waste heat is available than what is required to provide all possible refrigeration needs. Also, the rehger- ation needs may be somewhat seasonal. Therefore, power generation can be considered as an additional benefit. It is particularly advantageous to have a technology that provides re
17、frigeration as well as power, i.e., a dual function. The refrig- eration and power generated from the waste heat can meet internal needs, and excess power can be exported to the grid. The dual-function aspect ensures that the waste heat is fully utilized year-round, always converting it to some usef
18、ul prod- uct. It also ensures that the capital equipment is utilized year- round. THE DUAL-FUNCTION CYCLE A dual-function ammonia-water absorption cycle is being developed, which converts low-temperature waste heat to useful high-value products-power andior refrigeration in interchangeable amounts,
19、depending on needs. The cycle builds upon 100+ years of absorption cycle developments and particularly upon 20 years of development of the advanced generator absorber heat exchange (GAX) cycle. The cycle excels at efficiently converting heat in the range of 120C to 300C to electric power as well as
20、refrigeration. A simplified flow schematic of the dual-function absorp- tion cycle is shown in Figure 1. It consists of a heat-recovery unit, desorber, recuperator, absorber, turbine plus electric generator, condenser, and evaporator. The countercurrent heat and mass transfer processes used in this
21、cycle facilitate an effi- cient heat extraction from sources with large temperature glides, A closed flow loop with a transfer fluid may be used for heat transfer from the waste heat source to the cycle. With this system, waste heat from several sources can be collected to a centralized absorption c
22、ycle. In the power mode, the ammonia vapor drives the turbine, and the resulting low-pressure ammo- nia vapor flows to the absorber, where it is absorbed in the strong (high absorbing capacity, per ASHRAE convention) absorbent solution from the generator. The resulting weak (low absorbing capacity)
23、absorbent is pumped to the recuper- ator and then to the desorber. In the refrigeration mode, the high-pressure ammonia vapor flows to a condenser, and the condensate is reduced in pressure and evaporated in an evap- orator to produce refrigeration. The low-pressure vapor from the evaporator flows t
24、o the absorber to complete the cycle. The Figure 1 schematic flow sheet depicts a “three-pres- sure” absorption cycle, wherein the third (intermediate) pres- sure is present in the recuperator. This flow sheet illustrates the functional relationships between the three blocks labeled “absorber,” “des
25、orber,” and “recuperator.” There are many possible variations of the internal details of each block-one variation is presented later (Figure 3). Although the conventional absorption refrigeration cycle is not well adapted to Co-production of power, three notewor- thy innovative features are incorpor
26、ated in this cycle that substantially improve the economics of recovering low-level waste heat. First is the dual-function aspect-using essentially the same absorption cycle equipment to produce both power and refrigeration interchangeably-and more continuously. A second innovative feature is the re
27、cuperation system, which incorporates the patented “vapor exchange” cycle (Erickson 1992). An intermediate pressure level is provided in the cycle, which has the effect of extracting about 20% more waste heat from high-temperature-glide heat sources, thereby providing more useful product from a give
28、n quantity of waste heat. In other words, the waste heat is cooled to below 9O”C, in contrast to competing processes that cannot extract useful heat from waste heat sources lower than about 150OC. The third innovative feature contributing to the cost- effectiveness of this technology is the applicat
29、ion of recently developed components for the critical heat and mass transfer processes involved in this absorption cycle. When diabatic absorption and desorption of an ammonia-water mixture is conducted in conventional components, unexpectedly high resistance to mass transfer is encountered. This is
30、 largely due to the relative volatility ofthe two fluids, i.e., the concentration gradients that are present in both the vapor and liquid phases. This can cause very low overall transfer coefficients when conventional geometries are used, resulting in very large and costly components. Improved absor
31、bers and desorbers for absorption cycles have been developed, which provide triple the performance of conventional components in cost-effective configurations (Erickson 1998a, 1998b). The turbine cost presents a big advantage for aqua absorp- tion power cycles. The general requirements for the ammon
32、ia I 3 I Figure 1 Flow schematic of the dual-function absorption cycle. 51 6 ASH RAE Transactions: Symposia I turbine are not demanding, i.e., represent routine and conser- vative operating conditions for the backpressure turbine art. The maximum pressure ranges from1 2 to 60 bar, and exhaust pressu
33、re from 1 to 7 bar, with pressure ratios between 5 and 20. The inlet temperature generally does not exceed 260“C, and the exhaust is virtually always superheated, such that there are no issues of dealing with moisture. The ammonia molecule is close enough to steam that the same turbine design works
34、for both. Usually the only modification required is to change the labyrinth seal to a mechanical seal andor delete any copper- containing materials. The absorption power cycle uses a low- cost backpressure turbine. COMPARISON WITH CONVENTIONAL POWER CYCLES Since the refrigeration portion of an absor
35、ption cycle flow sheet is well known and extensively documented in the liter- ature, this paper will focus on the much less known absorption power cycle portion. First, a review of similar power cycles is presented as background to highlight the relative merits of the absorption power cycle. Then th
36、e specific cycle flow sheet chosen for analysis is presented. There are competing cycles for conversion of low- temperature waste heat to useful power. The conventional approach, such as the steam Rankine cycle, uses a closed cycle, wherein the circulating fluid undergoes phase change and the power-
37、producing step involves vapor expansion through a turbine. Other types of Rankine cycles are also known: the organic Rankine cycle (Meador 1983) and the binary fluid Rankine cycle (Maloney and Robertson 1953; Kalina and Leibowitz 1987). The specific focus of this power cycle comparison is on four pa
38、rticular cycles and their ability to convert glide heat in the range of 120C to 300C to power. The four cycles are steam Rankine, ammonia Rankine, ammo- nia-water binary Rankine, and ammonia-water absorption. Steam Rankine Cycle The steam Rankine cycle enjoys a domineering position in the power prod
39、uction application. Two reasons can be cited for this: the ready availability of water, and its excellent ther- modynamic and transport properties. For heat input above 350“C, these advantages are decisive. However, with lower temperature heat input, disadvantages begin to appear with this cycle, wh
40、ich provide the opportunity for the other cycles. In the low-temperature range, the boiling pressure of steam is on the order of one to five atmospheres. Any lower boiling pressure would require much larger flow passages and bulkier, costlier equipment to mitigate serious pressure drop penalties. Th
41、is effectively excludes cycles having multiple boiling pressures (the normal approach to achieving better glide matching). Similarly, the heat rejection is at deep vacuum. Much design effort is required to make the turbine final stages, exit passages, and condenser inlet passages very large to ensur
42、e low pressure drop. It has been pointed out (Angelino et al. 1999) that this condenser design condition (typically 37C and 6 a absolute pressure) prevents steam plants from taking much advantage of colder-than-design conditions. It also makes air cooling difficult compared to using cooling water. V
43、acuum operation allows air in-leakage, making de-aeration necessary, which adds to the thermal losses. Any tabulation of the loss mechanisms of multi-pres- sure steam plants is dominated by the low-pressure compo- nents. Another limitation of the steam Rankine cycle originates from the nature of the
44、 heat rejection curve. Most heat sources originating from fuel combustion have a temperature glide- as more heat is extracted from the combustion gas, the temper- ature decreases. The heat input to a steam Rankine cycle includes three modes: sensible heating of liquid water (steep temperature glide)
45、, constant temperature boiling (no temper- ature glide), and sensible superheating of the steam (very steep temperature glide). That heat input sequence does not match well with the linear temperature glide characteristic of combustion gas. Regions of large temperature difference heat transfer are u
46、navoidable, which leads to entropy production and loss of efficiency. Heat rejection from the steam Rankine cycle also poses a problem. The deep-vacuum condensation occurs at a fixed temperature. However, the cooling fluid (cooling water or air) experiences a temperature glide as it picks up heat. O
47、nce again, regions of high temperature difference are unavoidable. Superheat can play a key role in phase-change cycles. The polar fluids, i.e., water and ammonia, tend toward wetness as they are work-expanded. The moisture formation degrades turbine efficiency. Thus, vapor superheat is always advis
48、able and becomes essential for high-pressure ratio expansions. This superheat requirement can be a major thermodynamic penalty when it must be done above the boiling (vapor gener- ation) temperature. Organic Rankine cycles (ORC) enjoy a comparative advantage with regard to superheat. The common orga
49、nic working fluids tend toward dryness during work expansion; hence, vapor superheat is not necessary. On the other hand, this causes the ORC turbine exhaust vapor to be highly superheated, which is a major loss mechanism in that type of cycle: In summary, the four interrelated factors of constant pres- sure boiling, vacuum, pressure loss, and cost make steam plants not very effective in the low-temperature regime. Ammonia Rankine Cycle Ammonia is the only other working fluid that comes close to water with regard to favorable thermodynamic and transport properties. Over the past cen
