Exergy Analysis of a Double-effect Parallel-flow Commercial Steam Absorption Chiller

MATERIALS AND METHODS

A computer code for the simulation of a double-effect absorption chiller has been written using Matlab software. The model is based on mass and energy balances, heat transfer equations and equation of state. For thermal properties of LiBr solutions and steam used in the calculations are obtained from Herold et al. (1996), Chua et al. (2000), Talbi and Agnew (2000) and (Kreith and Goswami, 2007). The initial conditions consist of steam inlet pressure, cooling and chilled water inlet temperatures and flow rates and ambient conditions.

By applying mass balance, energy balance, heat transfer equations and equation of state for the LiBr-H₂O solution for each component of double effect steam absorption chiller, the following set of equations are obtained.

The following assumptions were employed to properly represent the absorption chiller cycle:

Mass balance: For each chiller component, the steady state total mass balance equation for refrigerant, chilled water, cooling water and lithium bromide can be expressed as:

(1)

where, m is the mass flow rate and in and out are the stream entering and leaving each of the chiller components.

The mass balance of LiBr-H₂O streams:

(2)

where, x is the concentration of LiBr solution.

Energy balance: The energy balance for each chiller components is expressed as:

(3)

For the absorption process in the absorber:

(4)

Heat transfer equations: The log mean temperature difference method (LMTD) is used to evaluate the product of overall heat transfer coefficient U and heat transfer surface area A:

(5)

The log mean temperature difference is expressed as:

(6)

Exergy analysis: The exergy method, known as the second law analysis, calculates the exergy loss caused by irreversibility which is an important thermodynamic property that evaluates the useful work that, can be produced by a substance or the amount of work needed to complete a process. The exergy analysis is a useful tool for thermodynamic analysis of energy-conversion systems.

The exergy content of a pure substance is generally given by:

(7)

The exergy of the solution is calculated by:

(8)

The exergy loss in each component under steady state condition and neglecting the potential and kinetic energies is calculated by:

(9)

The first term on the right-hand side is the sum of the exergy input; the second is the sum of exergy output while the third term is the exergy of heat Q which is transferred at constant temperature T. The last term is mechanical work transfer to or from the system.

The total exergy loss (Δψ_T) of the system is the sum of exergy loss in each component:

(10)

The exergetic efficiency τ is given by:

(11)

RESULTS AND DISCUSSION

The mathematical equations that govern the operation of the steam absorption chiller are developed. A computer program which is written in MATLAB environment was developed. Simulation was performed in order to get the various state points properties such as, temperature, concentration, mass flow rate, entropy and exergy. Table 1 shows the properties at each state point at the design condition. The exergy flow has a maximum value at state point 22 which is the input heat source (saturation steam) and followed by the state point 7 where the strong solution left the HTG. The exergy loss and exergetic efficiency are calculated based on the previous state points properties.

The exergy destructions for various components are plotted versus heat load starting from minimum heat load till full load (20-100%). The high temperature generator has the greatest exergy destruction followed by the absorber and high temperature solution heat exchanger as shown in Fig. 2.

Fig. 2:	Exergy loss under various load conditions, HTG: High temperature generator, LTG: Low temperature generator, CON: Condenser, EVP: Evaporator, ABS: Absorber, HTX: High temperature solution heat exchanger, LTX: Low temperature solution heat exchanger

Table 1:	Properties of state point values of double-effect steam absorption chiller at design condition

Fig. 3:	Exergy efficiency and COP under various load conditions

The exergy destruction in the first two is due to heat of mixing and separation in the absorber and first generator, respectively. For high temperature solution heat exchanger, the exergy loss is due to large terminals temperature difference. For single and series flow double effect lithium bromide-water absorption systems, it is found that the highest exergy loss occurs in the absorber (Kaushik and Arora, 2009; Gebreslassie et al., 2010). For double effect parallel flow chillers, (Gebreslassie et al., 2010) also found that the highest exergy loss occurs in the high temperature generator which well matches our results.

Figure 3 shows the exergetic efficiency (EX.eff) and COP under different load conditions. The exergetic efficiency decreases with increasing of the load factor while the COP increasing with the load factor till reached its highest value. This behaviour is mainly due to the increase in the heat input to the system which leads to increase the cooling effect in the evaporator and finally it increases the COP. But due to the rapid increase in heat input, the heat transferred in all components of the system is increased. The aforementioned increase in heat input result in an increase in irreversibility and thus reduces the exergetic efficiency. This result is in a very good agreement with the research work done by Sencan et al. (2005).

Figure 4 presents the variation of exergy loss of the main component in the system for different heat load. It is obvious from this figure that the exergy loss is increasing with the increasing of the load factor and it is increasing with load factor for all components. The first generator has the greatest exergy loss that, increasing with load factor. After 60 % load the increase in exergy loss is lower than that for load less than 60%. The absorber has the second higher exergy loss and its increases proportionally with load factor.

Fig. 4:	Total exergy loss under various load conditions, HTG: High temperature generator, LTG: Low temperature generator, CON: Condenser, EVP: Evaporator, ABS: Absorber, HTX: High temperature solution heat exchanger, LTX: Low temperature solution heat exchanger

Fig. 5:	Total exergy loss and exergetic efficiency under various load conditions

The total exergy loss is increasing with the load factor while the exergetic efficiency is decreasing as illustrated in Fig. 5. This manner is mainly due to increasing the heat input with respect to the load factor which leads to increasing the heat transferred in all the system components. Therefore, the increase in heat transfer results in an increase in the exergy loss in all components. Hence, it decreases the exergetic efficiency.

CONCLUSIONS

The model that represents the operation of the steam absorption chiller is developed. The properties of refrigerant and solution at each state point are calculated (temperature, enthalpy, concentration, flow rate, entropy and exergy). The exergy destruction in different system components is evaluated. The first generator has the greatest exergy destruction followed by the absorber and high temperature solution heat exchanger. Further, the performance of the system is evaluated by means of Coefficient of Performance (COP) and exergetic efficiency (second law efficiency). This model can be used as a tool for thermo-economic analysis of the absorption system.

ACKNOWLEDGMENT

The authors would like to acknowledge Universiti Teknologi PETRONAS for providing the financial support and facilities.