Ciclo kalina gy ANGELRAMBO 03, 2010 12 pagos Pergamon Energy VOI. 21, NO. l, pp. 21-27, 1996 copyright C 1996 Elsevier Science Ltd Printed in Great Britain. Al rights reserved 0360-5442/96 SI ABSORPTION POWER CYCLES O. M. IBRAHIM-‘$ an tDepartment of Mec University of Rhode and Solar Energy La Madison, WI 53706, PACE 1 12 S»ipe nd. W pplied Mechanics, n, RI 02881-0805 isconsin-Madison, h 1995) Abstrac t – – W e present a thermodynamic analysis of the Maloney and Robertson and the Kalina absorption power cycles.

The maximum power for specified external conditions is identified and used as reference to evaluate the performance ofthese two absorption power cycles. The evaluation focuses on power cycles operating with Iow temperature heat sources such as geothermal heat, solar energy or waste heat. INTRODUCTION Many of the recent proposals for alternative power cycles employ a non-azeotropic binary mixture in a Rankine or absorption- type power cycle. The motivation for using mixtures is that heat transfer can occur at variable temperature while at a constant of an absorption cycle.

Their conclusion showed that the absorption power cycle has no thermodynamic advantage ver the Rankine cycle. More recently, Kalina 2 proposed an absorption power cycle using ammonia-water. In apparent contradiction to Maloney and Robertson’s conclusion, Kalina shows that his cycle has a thermal efficiency which is higher than comparable steam power cycles. Kalina 3 and Kalina and Leibowitz4-7 explained the basic advantages of what has become known as the Kalina cycle technology.

El -Sayed and Tribus 8 compared the Rankine and Kalina cycles theoretically when both cycles are used as a bottoming cycle with the same thermal boundary conditions. They conducted a first and second aw thermodynamic analysis and concluded that the Kalina cycle can have 10-30% higher thermal efficiency than an equivalent Rankine cycle. A recent publication by Stecco and Desideri 9 presents the results of an analytical study showing both thermodynamic and practical advantages for the Kalina cycle compared to a Rankine cycle using the exhaust of a gas turbine as energy source.

Marston m developed a computer model of the cycle analyzed by EI-Sayed and Tribus. 8 The results of this model show good agreement with published results of El-Sayed and Tribus. s ThlS paper provides a detailed evaluation of the Kalina and Maloney and Robertson absorption power cycles and a comparison of their performance with the maximum power (M P) cycle. A methodology for providing an engineering evaluation of absorption power cycles 2 OF V power (M P) cycle. A methodology for providing an engineering evaluation of absorption power cycles is described.

The evaluation focuses on power cycles operating with Iow temperature heat sources such as geothermal heat, solar energy or waste heat. A case study is considered in which the heat source for the power cycle is a fluid stream with an inlet emperature of 455 K and a thermal-capacitance rate of 10 kW/ K. The sink temperature is 286 K. Heat transfer to and from the power cycle occurs through heat exchangers which are described with traditional heat-exchanger relations. In case study, the ratio of the hot-side to cold-side heat-exchanger conductances is 1. 0, i. e. LIA. UAL in all cycles.

Similar results are obtained for other heat-exchanger conductance ratios. The turbines and pumps are modeled as reversible adiabatic processes. STO whom all correspondence should be addressed. 21 22 O. M. Ibrahim and S. A. Klein MAXIMUM POWER (MP) CYCLEMODEL The best cycle that Will result in the upper limit of the maximum power for specified external conditions has been studied in Refs. 11-13. he purpose of this section is to identify the MP cycle for given heat-source and sink streams and heat- exchanger characteristics. The MP cycle is necessarily irreversible since zero power results from perfectly reversible cycles.

The thermodynamic irreversibilities occurring in real power cycles are primarily the result of heat-transfer processes. To account for these irreversibilities, the MP cycle is modeled as an in V heat-transfer processes. To account for these irreversibilities, the MP cycle is modeled as an Internally-reversible power cycle coupled to heat source and sink streams through conventional counterflow heat exchangers. An MP cycle model is found by recognising that an internally reversible thermodynamic cycle can be broken into a sequence of Carnot cycles (Fig. ) having the same total heat interactions with the heat source and heat sink and the same power output as the original cycle. As the number of cycles in sequence increases, the performance and shape of such a sequence approaches the performance and shape of the MP cycle. When the sequential Carnot cycles are coupled to a heat source and sink with finite thermal-capacitance rates, the power from the N-Carnot cycles is given by where TH. in,i and TL. i„iare respectively the specified source and sink inlet temperatures for a Carnot cycle in the sequence, correspondingly EH. ?? and ELi are the effectivenesses of the hot- side and cold-side heat exchangers of each cycle, and Th,—and Tl,iare the high and Iow temperatures of a Carnot cycle in the sequence, respectively. he shape of the MP cycle is determined by maximizing if’ with respect to Thj and i 1 to N subject o the entropy balance constraints CH EH CL EL (TI . An analytical solution is not apparent for this optimisation problem. However, th 40F problem. However, the MP cycle with a finite thermal-capacitance rate heat source and heat sink can be described numerically.

ABSORPTIONPOWERCYCLES The Maloney and Robertson -and the Kalina2 absorption power cycles are considered in the following analysis. Ammonia-water mixtures are used as the working medium. The performance of both cycles is investigated and compared to that of the MP cycle. Heat sink Entropy Fig. 1 . A thermodynamiccycle broken into a sequenceof Carnot cycles. Absorption power cycles 23 • 6 Flash Tarik Superheater Turbine – N Heater & Boiler 8 119 s OF V basic solution (state 1). The basic solution is then pumped to a high pressure and then h » t e d and partially boiled before entering the flash tank (state 5) to complete the cycle.

Hot and cold fluids are used as the heat sour—’e and sink, respectively. The cycle is shown in temperature-enthalpy plane in Fig. 3. The thick sol, ine represents the basic solution, while the thin salid, and thick dashed lines represent the strong somtion, and the weak solution, respectively. The separation and mixing processes are shown With a thin dashed line. 450 425 400 – 375 [-350 3-f.. 325 / 12 08 » Basicsolution (x–0. 3) – Strong solution in ammonia (x=O. 5) 300 . Weak solution in ammonia (x—O. 1) 1,2 . or mixing processes 275 E . 0 500 1000 1500 2000 2500 1-1 C- -r 10,11 h (kJ&g) .

Seperation . -500 Fig. 3. The Maloney and Robertson cycle in a T-h plane. EGY ZI/I-C 24 O. M. Ibrahim and S. A. Klein The Kalina absorption power cycle The Kalina cycle is shown in Fig. 4. The Kalina cycle has a slightly different system configura Maloney and Robertson cycle. he strong vapor sol from the distillation the distillation unit to operate at a pressure Iower than the boiler pressure. The liquid mixture from this condenser is pumped to the boiler pressure (state 9), then heated, boiled, and superheated befare it enters a turbine (state 12).

Energy is recovered from the turbine exhaust to heat and partially boil the basic solution befare it is flashed. The turbine exhaust (state 14) is then reunited with the weak solution from the distillation unit (state 16). The weak solution is used to absorb the rich vapor in ammonia to regenerate the basic solution (state 1). The basic solution is pumped to an intermediate pressure and then Split into two streams; about 80% passes to a flash tank after it recovers energy from the turbine exhaust, while the other 20% (state 3) bypasses the flash tank to be mixed with the strong vapor solution (state 6).

The combined stream (7) is cooled and condensed in the second condenser (state 8). In Fig. 5, the cycle is shown in temperature-enthalpy plane. he thick solid line represents the basic solution, while the thin solid line, and thick dashed line represent the strong solution, and the weak solution, respectively. The separation and mixing processes are shown With thin dashed lines. COMPARISON OF ABSORPTIONCYCLES The thermodynamic performance of the Maloney and Robertson and Kalina cycles is compared for different heat-exchanger sizes and thermal-capacitance rates.

The effect of the ratio of thermal- capacitance rates on the maximum power for the Maloney and Robertson and Kalina cycles thermal-capacitance rates on the maximum power for the Maloney and Robertson and Kalina cycles is shown in Fig. 6 for NTUH = 10. Similar behavior can be shown for other values of NTI_JH. The curve labeled maximum power cycle gives the pper limit for the povver output of any cycle for specified heat- exchanger sizes 21 -20 18 19.

Pump 2 — Condenser -7 Turbine Flash Tank 13 15 Recuperatorl 4 14 1] Pump IX’Z»r-[ Absorber-C0ndenser — D r— 1— – Basic solution » m m -W component Strone solutio the more volatile volatile component • Hot thermal-capacitance-rateratios on the power outputs of absorption power cycles. and external streams conditlons. This upper limit was determined as described in Ref. 11. As the ratio of the thermal-capacitance rates increases, the power output increases rapidly first and then levels off for thermal-capacitance- ate ratios greater than 5.

A typical power plant operates with a thermal-capacitance-rate ratio between 5 and 10. At a very high thermal-capacitance-rate ratio (e. g. , CLICH 20), the Kalina cycle produces about 80% of the maximum power and the Maloney and Robertson cycle about 70% ofthe maximum power. The effect of heat-exchanger sizes on the power output of the two cycles is shown in Fig. 7 for CLICH = 5. The power output of the MP cycle and the Kalina cycle increases as the hot-side heatexchanger size increases. The power output of the Maloney and Robertson cycle is greater than the power output of the

Kalina cycle for NTUH less than 5, but as NTUH increases, the power output of the Kalina cycle surpasses that ofthe Maloney and Robertson cycle. Atvery large heat-exchanger conductances (e. g. , NTUH — 15), the Kalina cycle produces about 90% of the maximum power and the Maloney and Robertson cycle about 70% of the maximum power. The Maloney and Robertson and Kalina cycles are compared with the MP cycle in a plane in Figs. 8 and 9. In order to compare the heat-transfer processes of absorption power cycles to the MP cycle, the Maloney and Robertson and Kalina cycles are shown w

Maloney and Robertson and Kalina cycles are shown without the internal processes, i. e. the heat recovery, mixing and separation processes. In both cycles, the heat is added and rejected at variable temperature. The Maloney and Robertson cycle has a higher pinch point at the boiler and its heattransfer processes do not match the MP cycle, particularly at the condenser (process 12-1, Fig. 8). On the other hand, the heat-transfer processes of the Kalina cycle match the MP cycle more closely. 26 300 250 200 150 edei— = 5 50 Maloney & Robertson cycle, [19531 liiiili* 5 NTUH 10 15 2