- Email: [email protected]
Enhancement of a double-effect absorption cooling system using a vapor recompression absorber
Energy 28 (2003) 1151–1163 www.elsevier.com/locate/energy
Enhancement of a double-effect absorption cooling system using a vapor recompression absorber William M. Worek a,∗, Daniele Ludovisi a, Milton Meckler b a
Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607-7022, USA b Design Build Systems, 10573 West Pico Blvd., Encino, CA 90064, USA Received 27 August 2002
Abstract One of the limitations of aqueous lithium bromide (LiBr/Water) single-effect absorption chillers is their inherently low COP and their inability to take advantage of the availability of high temperature heat to achieve higher COPs. Recent efforts to develop triple or even quadruple-effect absorption chillers appear unlikely to find adequate support in the current deregulated utility marketplace because of their first-cost premiums, corrosion problems associated with high temperature operation, and an oversize footprint. Double-effect cycles, on the other hand, are beginning to capture a significant portion of the heat activated cooling marketplace. This study investigates the effect of adding a Vapor Recompression Absorber (VRA) to a double-effect machine. The VRA, which can be retrofitted into an existing or constructed as part of a new double-effect absorption chiller, is an adiabatic component and uses heat released by condensation to evaporate more refrigerant. This causes an increase in the refrigerant flow rate in the refrigerant circuit, increasing the cooling capacity of the system. Hence, a system utilizing a VRA can achieve higher COPs. This paper presents methods to mathematically characterize the VRA and numerical simulations of a double-effect absorption system employing the VRA unit. The performance enhancement is investigated and the benefits in terms of improving COP and capacity are documented. Also, the impact of design parameters is documented. 2003 Published by Elsevier Ltd.
1. Introduction Recent efforts to develop triple-effect absorption chillers are unlikely to find support in the current deregulated utility marketplace due to first cost pressures. These machines require ∗
Corresponding author. Fax: +1-312-413-0447. E-mail address: [email protected] (W.M. Worek).
0360-5442/03/$ - see front matter 2003 Published by Elsevier Ltd. doi:10.1016/S0360-5442(03)00104-X
1152
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Nomenclature COP coefficient of performance h enthalpy in kJ/kg (if not differently specified) LMTD log mean temperature in K or °C M motor to power the pressure enhancer in the VRA unit m mass flow rate in kg/s N nozzle to spray the LiBr/water solution on the separator wall of the VRA Q rate of heat transfer in kJ/kg (if not differently specified) S separator wall in the VRA unit UA overall heat transfer coefficient in kW/K W rate of work in kW (if not differently specified) x concentration of desiccant in kg of desiccant per kg of solution ω averaging weight Subscripts e evaporator g,high high stage generator vapor—a water vapor going out of the desorption chamber of the VRA unit vapor—d water vapor going into the absorption chamber of the VRA unit
additional desorbers, condensers, solution pumps and heat exchangers at the high temperature levels resulting in first cost premiums and oversized footprints. The high activation temperatures required by these machines exceed 160 °C and accelerate corrosion [1]. Moreover, with additional components, the number of design choices increases and, in turn, increases the difficulty in arriving at an optimized design. Another difficulty in modeling multiple-effect absorption cycles is that the property correlations for working pairs do not encompass the necessary operating temperature range. Often, the water/lithium bromide property routines used for numerical simulations are based on the McNeely’s work [2]. These correlations are valid only up to 175 °C, but usually the temperatures in the high temperature components of a multiple-effect chiller go above this value. Double-effect cycles are significantly more efficient than single-effect machines and they are already commercialized and well established in the marketplace [3]. They are able to take advantage of high temperatures without needing to operate at temperatures as high as in triple- or quadruple-effect machines. This paper investigates methods to further improve the performance of the double-effect absorption chillers by utilizing a new system component, the Vapor Recompression Absorber (VRA) [4].
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1153
2. Description of the VRA A simplified but quite accurate view of a double-effect absorption system can be obtained by considering it as a three-pressure level device as shown in Fig. 1. This study assumes that pressure drops occur only in throttling valves and pressure increases occur only in pumps. These approximations are quite accurate because other design constraints require the machine to be designed to minimize other pressure drops. Referring to the Fig. 1, the high-stage condenser (C1) and the high-stage generator (G1) operate at the high pressure, the low-stage condenser (C2) and lowstage generator (G2) operate at the intermediate pressure, and the evaporator and the absorber operate at the lowest pressure in the cycle.
Fig. 1. Double-effect water/lithium bromide absorption chiller.
1154
W.M. Worek et al. / Energy 28 (2003) 1151–1163
In Fig. 2, the double-effect system is modified by adding the VRA. We can consider the VRA to be an adiabatic system to which work W is provided by means of a small electric motor M and into which two lithium bromide/water solutions enter (States 11 and 20) and two exit (States 12 and 22) [5]. The VRA unit consists of two separate chambers both at the intermediate pressure level of the cycle. The concentrated high-temperature, high-pressure solution (State 8), from the high-stage generator (G1), is partly delivered to the VRA-heat exchanger (State 9). Upon exiting (State 10), the solution passes through a throttling valve where the pressure is reduced from the high pressure level of the cycle to the intermediate pressure level. The solution then flows into the first chamber of the VRA (State 11). Then it is driven out of that chamber (State 12) at a lower concentration and temperature and it goes into the solution pump that increases the pressure level from the intermediate pressure level to the high pressure level of the cycle. Finally, the solution enters the VRA-heat exchanger (State 13) where it is heated and then it flows into the generator G1 (State 14). Meanwhile, the solution exiting the low-stage generator G2 (State 19) is mixed with the remaining solution exiting the high-stage generator (State 17) to give the solution at State 20. This solution, which is at the intermediate pressure of the cycle and at an intermediate temperature, is then introduced into the other chamber of the VRA. After passing through the VRA, it exits at a higher temperature and concentration, but at the same intermediate pressure (State 22). Therefore, inside the VRA a mass transfer of the refrigerant (water vapor) occurs between the State 11 and State 20, and the VRA solution pump, shown in the Fig. 3, is necessary to effect this mass exchange. The principle on which the VRA is based is the fact that the cooling capacity of the system depends primarily on the amount of refrigerant that is vaporized in the evaporator. By adding the VRA, we are able to retain a portion of the refrigerant that is otherwise underutilized if allowed to flow from the generator G1 to the absorber (i.e., see Figs. 1 and 2). Therefore using the VRA, we are able to send the refrigerant back to the high-stage generator with a diluted LiBr/water solution (State 14). A larger amount of refrigerant is now available in the generator G1 to be desorbed and now more refrigerant can be delivered to the evaporator. This enables an increase in system capacity and COP. The VRA unit doesn’t need other inputs but a minimum amount of power to run a blower (powered by the motor M). Since the VRA is an adiabatic device, the heat transfer from the stream at State 11 to the stream at State 20 promotes the endothermic desorption process of the refrigerant at State 20 and the exothermic absorption process of the refrigerant at State 11. In the Fig. 3, a cross-section of the VRA unit is shown. It consists of two chambers separated by a heat-transfer surface S and of a motorized pressure enhancer. This small pressure enhancer, powered by the electric motor M, connects the two chambers and drives the vapor from the desorption chamber into the absorption chamber. This pressure enhancer is a fan, which causes a slight increase in the pressure of the vapor. Therefore, the wall separating the two chambers can be made using a polymeric material because of the resulting low pressure differential. This should result in the VRA being relatively inexpensive to manufacture. A double-effect absorption system with a VRA unit enables better performance than conventional systems and a potential lower first cost per unit of cooling capacity and smaller footprint than triple- or quadruple-effect absorption chillers. This makes such a systems more suited for the US market. Moreover, the VRA unit can be retrofitted on existing chillers improving their capacity and performance, producing reasonable annual operating cost savings.
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 2. Double-effect water/lithium bromide absorption chiller employing a VRA unit.
1155
1156
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 3. Enlarged detailed diagram of the Vapor Recompression Absorber Unit.
The solution at State 11 is sprayed in absorption chamber over the inner side SI of the heat transfer surface S. Meanwhile the solution at State 20 goes into the desorption chamber of the VRA and then is sprayed over the outer side SO of the heat transfer surfaces by the VRA solution pump (Fig. 3). Refrigerant falling film evaporation occurs in this last chamber of the VRA because of the temperature difference between the two chambers. Therefore, as water vapor is evaporated in the desorption chamber, the remaining concentrated solution is collected underneath the unit (in the sump shown in Fig. 3) and then it is sent out of the VRA at a higher temperature and concentration and at intermediate pressure (State 22). The evaporated refrigerant vapor, at intermediate pressure, is drawn through the pressure enhancer and sent in the absorption chamber where it is forced into the inner surface SI and is absorbed by the sprayed concentrated solution at State 11. The solution coming out the absorption chamber is at an intermediate pressure level and at lower concentration and temperature (State 12). It is then sent to the solution pump to increase its pressure to the high pressure level of the cycle and from there it is sent to the VRA heat exchanger to be warmed up and finally sent again to the generator G1 (State 14). As it is shown in Fig. 3, there is a “falling film evaporation” side surface SO of the separator wall S between the two chambers. This is accomplished by a solution distribution system consisting of a weir or a series of nozzles for recirculating low temperature solution from the sump. In practice, large surface areas SO and SI are employed and they can be accomplished in various ways such as by using flat sheets of heat transfer material, tubular or concentric walls as shown. The circular configuration of the VRA is compact and highly advantageous for the pressure
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1157
enhancer adaptation and for the solution sump as well. Moreover, it provides a large heat transfer surface area that increases the heat transfer rate, increasing the production of refrigerant. Finally, it should be pointed out that the VRA operates essentially as a first-stage absorber/heat pump and so it works in a double-effect absorption system as a third stage. This allows doubleeffect chillers using the VRA to operate basically at the same pressures and temperatures as a conventional double-effect chillers, but with the advantage that a greater amount of the refrigerant is available in the evaporator and so higher COPs can be achieved. 3. Modeling of the VRA As was shown in the previous section, the VRA consists of two chambers and a pressure enhancer connects them. In the absorption chamber an absorption process occurs and in the desorption chamber a desorption process occurs. Fig. 4 shows how the VRA is modeled. We don’t show there the VRA-heat exchanger, throttling valve and solution pump (Fig. 3), because they are conventional devices and so already known, and moreover we don’t consider the VRA solution pump because it is useful only for recirculating the solution of the sump and we neglect the pressure drops due to operation of the spray nozzles. Referring to the Fig. 4, the solution at State 20 enters the desorption chamber and a more concentrated solution (State 22) and water vapor (vapor—d) exit the chamber. The vapor—d state enters the pressure enhancer at intermediate pressure and exits at a pressure slightly higher. However, we neglect the small pressure increase given by the pressure enhancer and we assume that
Fig. 4. VRA unit seen as a parallel flow heat exchanger.
1158
W.M. Worek et al. / Energy 28 (2003) 1151–1163
the vapor exits at an intermediate pressure level (vapor—a, Fig. 4). Then the vapor enters the absorption chamber where it is absorbed by the solution entering at State 11. Therefore, a more diluted solution (State 12) than the solution at State 11 leaves the VRA unit and comes back to the high stage generator G1 (Fig. 2). Only a thin wall separates the two chambers and so a heat transfer occurs from the absorption to the desorption chamber. Since the VRA is adiabatic, this heat transfer from one chamber to the other is the only heat that activates the absorption and desorption processes. 3.1. VRA analysis The numerical system simulation of the VRA performed in this study is based on the assumptions stated above. To completely describe the VRA by equations we have to describe separately the three principal components that constituted the VRA: the desorption chamber, the pressure enhancer and the absorption chamber. Each component is treated as a control volume with their respective inputs and outputs, and the First Law Analysis is applied for each. The VRA must be described also as a single device. This requires that we need linking equations to express the VRA as a single device. 3.1.1. Component models 3.1.1.1. Desorption chamber The pressure level in this chamber is maintained at the intermediate pressure level of the double-effect cycle. By adding the thermal energy to the entering stream 20, a vapor stream, vapor—d, is generated while any remaining liquid leaves as stream 22. It is assumed that the vapor leaving the desorber is in equilibrium with the incoming liquid stream. However, the validity of this assumption depends strongly on the design of the actual heat exchange surface and flow regime. The entering solution may be at the saturated state or at vapor-liquid phase and in this last case it is allowed to reach an equilibrium state (State 21), that is the saturation state. The governing equations for this component are: m20 ⫽ mvapord ⫹ m22
Total mass balance Absorbent mass balance Energy balance
m20x20 ⫽ m22x22
mvapordhvapord ⫹ m22h22⫺m20h20⫺QVRA ⫽ 0
(1) (2) (3)
Equilibrium Condition (State 21): The equilibrium temperature (State 21) in the desorption chamber is given by the temperature of saturated LiBr/Water solution at the intermediate pressure and at the concentration of the liquid phase of the entering solution (State 20). 3.1.1.2. Pressure enhancer Compressors, as well as pumps and fans are devices used to increase pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. Because we employ this component only for circulating the water vapor from the desorption chamber into the absorption chamber, we neglect the pressure increase given by this component and then it results that the pressure inside the absorption chamber is at intermediate level and the work required is zero. The governing equations for this component are:
W.M. Worek et al. / Energy 28 (2003) 1151–1163
hvapora ⫽ hvapord
Energy balance
1159
(4)
3.1.1.3. Absorption chamber As was stated above, the pressure level in this chamber is assumed to be at the intermediate pressure level of the cycle. By extracting the thermal energy from the entering stream 11 and from the vapor—a, the vapor stream vapor—a is absorbed by the solution at State 11, and the diluted solution (State 12) exits the chamber, that is the VRA. Because the VRA is an adiabatic system the heat extracted from this chamber is provided in the desorption chamber to activate the desorption process. The governing equations for this component are: Total mass balance Absorbent mass balance
m11 ⫹ mvapora ⫽ m12
(5)
m11x11 ⫽ m12x12
(6)
m11h11 ⫹ mvaporahvapora⫺m12h12⫺QVRA ⫽ 0
Energy balance
(7)
where QVRA is the same as in the Eq. (3) and mvapora = mvapord. 3.1.2. Linking equations To complete the simulation of the VRA unit, connections between the desorption chamber, the pressure enhancer and the absorption chamber need to be further described. These connections represent the conductive heat transfer through the VRA internal separator wall and the effect of the pressure enhancer assembly on the refrigerant temperature. Typically, the heat transfer rate is described as QVRA ⫽ (UA)VRALMTDVRA
(8)
where (UA)VRA is the product of the overall heat transfer coefficient U times the area A available for the heat transfer process in the VRA, and LMTDVRA is the log mean temperature difference of the streams in the VRA unit. The VRA unit can be modeled as a parallel flow heat exchanger device. Fig. 4 shows a schematic of the VRA used for modeling. The dotted line encloses the control volume of the equivalent parallel flow heat exchanger and the two dashed lines represents the two flows passing through this heat exchanger. We could consider as entering streams the solution 11 and 20, and as exiting streams, respectively, the solution 12 and 22. By representing the VRA in this way, the expression for the LMTDVRA would be the following: LMTDVRA ⫽
(T12⫺T22)⫺(T11⫺T20) T12⫺T22 ln T11⫺T20
冉
冊
(9)
Here we are neglecting the flow vapor—d coming out the desorption chamber and the flow vapor—a coming into the absorption chamber. Because the temperatures of the streams vapor— d and vapor—a are different respectively from the temperatures of the streams 22 and 11, we need to decide to use Tvapordinstead of T22 or to use Tvaporainstead of T11. To avoid this difficulty, we calculate for both these two pairs of streams the mean temperature,
1160
W.M. Worek et al. / Energy 28 (2003) 1151–1163
weighted according the amount of energy that they carry. In particular, the equations to calculate the mean temperature are the following: Weight22
w22 ⫽
Weightvapord
wvd ⫽
Weight11
w11 ⫽
Weightvapora
wva ⫽
m22h22 mvapordhvapord ⫹ m22h22 mvapordhvapord mvapordhvapord ⫹ m22h22 m11h11 mvaporahvapora ⫹ m11h11 mvaporahvapora mvaporahvapora ⫹ m11h11
(10) (11) (12) (13)
Mean Temperature between stream 22 and vapord T22/vapord ⫽ w22T22 ⫹ wvdTvapord
(14)
Mean Temperature between stream 11 and vapora T11/vapora ⫽ w11T11 ⫹ wvaTvapora
(15)
With these definitions we can redefine more properly the log mean temperature to use in the Eq. (10) as follows: LMTDVRA ⫽
(T12⫺T22/vapord)⫺(T11/vapora⫺T20)
冉
ln
T12⫺T22/vapord
冊
(16)
T11/vapora⫺T20
This formula, taking into account the mean temperature of the mixed stream 11 and vapor— a and of the separated stream 22 and vapor—d, results to be more precisely than the formula (11) in describing the heat transfer process inside the VRA unit. However, further experimental investigations should be addressed in finding a more fundamental definition to determine the LMTDVRA. 4. Performance of the cycle employing the VRA unit 4.1. Variable definitions The COP for the absorption system in Fig. 1 is defined as follows: COP ⫽
m20(h21⫺h21) Qe ⫽ Qg,high m22(h22⫺h23)
(17)
while the COP for the VRA enhanced double-effect absorption system shown in Fig. 2 is defined as follows:
W.M. Worek et al. / Energy 28 (2003) 1151–1163
COP ⫽
Qe m30(h31⫺h32) ⫽ Qg,high m32(h32⫺h33)
1161
(18)
The energy term “W” defined earlier for the pressure enhancer, which would normally have been added to the denominator of the Eq. (18), is negligible and therefore ignored. 4.2. Simulation results discussion Using the design and operating conditions given in Table 1, for a reverse series flow doubleeffect cycle with and without a VRA unit, the performance of the two systems were compared. The value of the parameter UAVRA is basically a measure of the size of the VRA. Fig. 5 shows the variation of the coefficient of performance (COP) of the chiller using a VRA unit (Fig. 2) as compared with the performance of the same chiller without a VRA (Fig. 1). It is possible to see that the COP of the system with the VRA is 6.5% higher that the system without, and it slightly increases when increasing values of the UAVRA. Furthermore the capacity of the system with the VRA also increases by as much as 7.5% as compared to the system without VRA (Fig. 6) and it increases when the UAVRA increases. Figs. 7 and 8 show that the benefit of using the VRA unit consists of increasing the mass flow rate of the refrigerant and of increasing the temperature of the solution going into the high stage generator (State 7, Fig. 2). This reduces the heat input. These two effects improve the performance of the chiller.
Table 1 Inputs for the reverse series flow double-effect cycle model with and without VRA unit Heat transfer characteristics (UA) Absorber 6.06 kW/K Condensers 2 17.74 kW/K Condensers 1 and Generator 2 3.37 kW/K Evaporator 11.84 kW/K Generators 1 8.42 kW/K Solution Heat Exchangers 1 & 2 2.01 kW/K VRA Unit Heat Exchanger 0.5 kW/K Mass flow rates Cooling water to the absorber and then the condenser 2 3.62 kg/s Heat source generator 1 3.12 kg/s Chilled water from the evaporator 2.25 kg/s Weak solution from absorber 0.45 kg/s Operating temperatures Heat source temperatures (pressurized water) 120 °C Cooling water inlet temperatures 29.4 °C Chilled water outlet temperatures 7.2 °C Mass flow rate ratios in all the flow splits in the cycles In all the splitters present in the two studied chillers the mass flow rate ratio is 0.5, that is the solution goes half in one direction and half in the other
1162
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 5. COP of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
Fig. 6. Capacity of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
5. Conclusions Performance simulations have been carried out for a novel component, the Vapor Recompression Absorber (VRA). It has been integrated in a series reverse flow double-effect absorption chiller and a numerical simulation of the resulting cycle has been performed. It has been demonstrated that the VRA unit enhances double-effect chiller improving the COP and the capacity of the chiller by increasing the mass flow rate of the refrigerant in the refrigerant circuit of the chiller and by increasing the temperature of the solution entering the high stage generator.
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1163
Fig. 7. Comparison of the refrigerant mass flow rate of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
Fig. 8. Comparison of the high stage generator inlet temperature of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
References [1] Herold KE, Radermacher R, Klein SA. Absorption chillers and heat pumps. Boca Raton, FL: CRC Press, 1996. [2] McNeely LA. Thermodynamic properties of aqueous solution of lithium bromide. ASHRAE Transactions 1979;85(1):413–34. [3] Alefeld G, Radermacher R. Heat conversion systems. Boca Raton, FL: CRC Press, 1994. [4] Meckler M. Enhanced lithium bromide absorption cycle water vapor recompression absorber. US Patent No. 5,816,070. 1998. [5] Meckler M. Optimizing absorption chillers. Consulting-Specifying Engineer, Oak Brook, IL: January 1999, p. 40-46.
Enhancement of a double-effect absorption cooling system using a vapor recompression absorber William M. Worek a,∗, Daniele Ludovisi a, Milton Meckler b a
Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607-7022, USA b Design Build Systems, 10573 West Pico Blvd., Encino, CA 90064, USA Received 27 August 2002
Abstract One of the limitations of aqueous lithium bromide (LiBr/Water) single-effect absorption chillers is their inherently low COP and their inability to take advantage of the availability of high temperature heat to achieve higher COPs. Recent efforts to develop triple or even quadruple-effect absorption chillers appear unlikely to find adequate support in the current deregulated utility marketplace because of their first-cost premiums, corrosion problems associated with high temperature operation, and an oversize footprint. Double-effect cycles, on the other hand, are beginning to capture a significant portion of the heat activated cooling marketplace. This study investigates the effect of adding a Vapor Recompression Absorber (VRA) to a double-effect machine. The VRA, which can be retrofitted into an existing or constructed as part of a new double-effect absorption chiller, is an adiabatic component and uses heat released by condensation to evaporate more refrigerant. This causes an increase in the refrigerant flow rate in the refrigerant circuit, increasing the cooling capacity of the system. Hence, a system utilizing a VRA can achieve higher COPs. This paper presents methods to mathematically characterize the VRA and numerical simulations of a double-effect absorption system employing the VRA unit. The performance enhancement is investigated and the benefits in terms of improving COP and capacity are documented. Also, the impact of design parameters is documented. 2003 Published by Elsevier Ltd.
1. Introduction Recent efforts to develop triple-effect absorption chillers are unlikely to find support in the current deregulated utility marketplace due to first cost pressures. These machines require ∗
Corresponding author. Fax: +1-312-413-0447. E-mail address: [email protected] (W.M. Worek).
0360-5442/03/$ - see front matter 2003 Published by Elsevier Ltd. doi:10.1016/S0360-5442(03)00104-X
1152
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Nomenclature COP coefficient of performance h enthalpy in kJ/kg (if not differently specified) LMTD log mean temperature in K or °C M motor to power the pressure enhancer in the VRA unit m mass flow rate in kg/s N nozzle to spray the LiBr/water solution on the separator wall of the VRA Q rate of heat transfer in kJ/kg (if not differently specified) S separator wall in the VRA unit UA overall heat transfer coefficient in kW/K W rate of work in kW (if not differently specified) x concentration of desiccant in kg of desiccant per kg of solution ω averaging weight Subscripts e evaporator g,high high stage generator vapor—a water vapor going out of the desorption chamber of the VRA unit vapor—d water vapor going into the absorption chamber of the VRA unit
additional desorbers, condensers, solution pumps and heat exchangers at the high temperature levels resulting in first cost premiums and oversized footprints. The high activation temperatures required by these machines exceed 160 °C and accelerate corrosion [1]. Moreover, with additional components, the number of design choices increases and, in turn, increases the difficulty in arriving at an optimized design. Another difficulty in modeling multiple-effect absorption cycles is that the property correlations for working pairs do not encompass the necessary operating temperature range. Often, the water/lithium bromide property routines used for numerical simulations are based on the McNeely’s work [2]. These correlations are valid only up to 175 °C, but usually the temperatures in the high temperature components of a multiple-effect chiller go above this value. Double-effect cycles are significantly more efficient than single-effect machines and they are already commercialized and well established in the marketplace [3]. They are able to take advantage of high temperatures without needing to operate at temperatures as high as in triple- or quadruple-effect machines. This paper investigates methods to further improve the performance of the double-effect absorption chillers by utilizing a new system component, the Vapor Recompression Absorber (VRA) [4].
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1153
2. Description of the VRA A simplified but quite accurate view of a double-effect absorption system can be obtained by considering it as a three-pressure level device as shown in Fig. 1. This study assumes that pressure drops occur only in throttling valves and pressure increases occur only in pumps. These approximations are quite accurate because other design constraints require the machine to be designed to minimize other pressure drops. Referring to the Fig. 1, the high-stage condenser (C1) and the high-stage generator (G1) operate at the high pressure, the low-stage condenser (C2) and lowstage generator (G2) operate at the intermediate pressure, and the evaporator and the absorber operate at the lowest pressure in the cycle.
Fig. 1. Double-effect water/lithium bromide absorption chiller.
1154
W.M. Worek et al. / Energy 28 (2003) 1151–1163
In Fig. 2, the double-effect system is modified by adding the VRA. We can consider the VRA to be an adiabatic system to which work W is provided by means of a small electric motor M and into which two lithium bromide/water solutions enter (States 11 and 20) and two exit (States 12 and 22) [5]. The VRA unit consists of two separate chambers both at the intermediate pressure level of the cycle. The concentrated high-temperature, high-pressure solution (State 8), from the high-stage generator (G1), is partly delivered to the VRA-heat exchanger (State 9). Upon exiting (State 10), the solution passes through a throttling valve where the pressure is reduced from the high pressure level of the cycle to the intermediate pressure level. The solution then flows into the first chamber of the VRA (State 11). Then it is driven out of that chamber (State 12) at a lower concentration and temperature and it goes into the solution pump that increases the pressure level from the intermediate pressure level to the high pressure level of the cycle. Finally, the solution enters the VRA-heat exchanger (State 13) where it is heated and then it flows into the generator G1 (State 14). Meanwhile, the solution exiting the low-stage generator G2 (State 19) is mixed with the remaining solution exiting the high-stage generator (State 17) to give the solution at State 20. This solution, which is at the intermediate pressure of the cycle and at an intermediate temperature, is then introduced into the other chamber of the VRA. After passing through the VRA, it exits at a higher temperature and concentration, but at the same intermediate pressure (State 22). Therefore, inside the VRA a mass transfer of the refrigerant (water vapor) occurs between the State 11 and State 20, and the VRA solution pump, shown in the Fig. 3, is necessary to effect this mass exchange. The principle on which the VRA is based is the fact that the cooling capacity of the system depends primarily on the amount of refrigerant that is vaporized in the evaporator. By adding the VRA, we are able to retain a portion of the refrigerant that is otherwise underutilized if allowed to flow from the generator G1 to the absorber (i.e., see Figs. 1 and 2). Therefore using the VRA, we are able to send the refrigerant back to the high-stage generator with a diluted LiBr/water solution (State 14). A larger amount of refrigerant is now available in the generator G1 to be desorbed and now more refrigerant can be delivered to the evaporator. This enables an increase in system capacity and COP. The VRA unit doesn’t need other inputs but a minimum amount of power to run a blower (powered by the motor M). Since the VRA is an adiabatic device, the heat transfer from the stream at State 11 to the stream at State 20 promotes the endothermic desorption process of the refrigerant at State 20 and the exothermic absorption process of the refrigerant at State 11. In the Fig. 3, a cross-section of the VRA unit is shown. It consists of two chambers separated by a heat-transfer surface S and of a motorized pressure enhancer. This small pressure enhancer, powered by the electric motor M, connects the two chambers and drives the vapor from the desorption chamber into the absorption chamber. This pressure enhancer is a fan, which causes a slight increase in the pressure of the vapor. Therefore, the wall separating the two chambers can be made using a polymeric material because of the resulting low pressure differential. This should result in the VRA being relatively inexpensive to manufacture. A double-effect absorption system with a VRA unit enables better performance than conventional systems and a potential lower first cost per unit of cooling capacity and smaller footprint than triple- or quadruple-effect absorption chillers. This makes such a systems more suited for the US market. Moreover, the VRA unit can be retrofitted on existing chillers improving their capacity and performance, producing reasonable annual operating cost savings.
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 2. Double-effect water/lithium bromide absorption chiller employing a VRA unit.
1155
1156
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 3. Enlarged detailed diagram of the Vapor Recompression Absorber Unit.
The solution at State 11 is sprayed in absorption chamber over the inner side SI of the heat transfer surface S. Meanwhile the solution at State 20 goes into the desorption chamber of the VRA and then is sprayed over the outer side SO of the heat transfer surfaces by the VRA solution pump (Fig. 3). Refrigerant falling film evaporation occurs in this last chamber of the VRA because of the temperature difference between the two chambers. Therefore, as water vapor is evaporated in the desorption chamber, the remaining concentrated solution is collected underneath the unit (in the sump shown in Fig. 3) and then it is sent out of the VRA at a higher temperature and concentration and at intermediate pressure (State 22). The evaporated refrigerant vapor, at intermediate pressure, is drawn through the pressure enhancer and sent in the absorption chamber where it is forced into the inner surface SI and is absorbed by the sprayed concentrated solution at State 11. The solution coming out the absorption chamber is at an intermediate pressure level and at lower concentration and temperature (State 12). It is then sent to the solution pump to increase its pressure to the high pressure level of the cycle and from there it is sent to the VRA heat exchanger to be warmed up and finally sent again to the generator G1 (State 14). As it is shown in Fig. 3, there is a “falling film evaporation” side surface SO of the separator wall S between the two chambers. This is accomplished by a solution distribution system consisting of a weir or a series of nozzles for recirculating low temperature solution from the sump. In practice, large surface areas SO and SI are employed and they can be accomplished in various ways such as by using flat sheets of heat transfer material, tubular or concentric walls as shown. The circular configuration of the VRA is compact and highly advantageous for the pressure
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1157
enhancer adaptation and for the solution sump as well. Moreover, it provides a large heat transfer surface area that increases the heat transfer rate, increasing the production of refrigerant. Finally, it should be pointed out that the VRA operates essentially as a first-stage absorber/heat pump and so it works in a double-effect absorption system as a third stage. This allows doubleeffect chillers using the VRA to operate basically at the same pressures and temperatures as a conventional double-effect chillers, but with the advantage that a greater amount of the refrigerant is available in the evaporator and so higher COPs can be achieved. 3. Modeling of the VRA As was shown in the previous section, the VRA consists of two chambers and a pressure enhancer connects them. In the absorption chamber an absorption process occurs and in the desorption chamber a desorption process occurs. Fig. 4 shows how the VRA is modeled. We don’t show there the VRA-heat exchanger, throttling valve and solution pump (Fig. 3), because they are conventional devices and so already known, and moreover we don’t consider the VRA solution pump because it is useful only for recirculating the solution of the sump and we neglect the pressure drops due to operation of the spray nozzles. Referring to the Fig. 4, the solution at State 20 enters the desorption chamber and a more concentrated solution (State 22) and water vapor (vapor—d) exit the chamber. The vapor—d state enters the pressure enhancer at intermediate pressure and exits at a pressure slightly higher. However, we neglect the small pressure increase given by the pressure enhancer and we assume that
Fig. 4. VRA unit seen as a parallel flow heat exchanger.
1158
W.M. Worek et al. / Energy 28 (2003) 1151–1163
the vapor exits at an intermediate pressure level (vapor—a, Fig. 4). Then the vapor enters the absorption chamber where it is absorbed by the solution entering at State 11. Therefore, a more diluted solution (State 12) than the solution at State 11 leaves the VRA unit and comes back to the high stage generator G1 (Fig. 2). Only a thin wall separates the two chambers and so a heat transfer occurs from the absorption to the desorption chamber. Since the VRA is adiabatic, this heat transfer from one chamber to the other is the only heat that activates the absorption and desorption processes. 3.1. VRA analysis The numerical system simulation of the VRA performed in this study is based on the assumptions stated above. To completely describe the VRA by equations we have to describe separately the three principal components that constituted the VRA: the desorption chamber, the pressure enhancer and the absorption chamber. Each component is treated as a control volume with their respective inputs and outputs, and the First Law Analysis is applied for each. The VRA must be described also as a single device. This requires that we need linking equations to express the VRA as a single device. 3.1.1. Component models 3.1.1.1. Desorption chamber The pressure level in this chamber is maintained at the intermediate pressure level of the double-effect cycle. By adding the thermal energy to the entering stream 20, a vapor stream, vapor—d, is generated while any remaining liquid leaves as stream 22. It is assumed that the vapor leaving the desorber is in equilibrium with the incoming liquid stream. However, the validity of this assumption depends strongly on the design of the actual heat exchange surface and flow regime. The entering solution may be at the saturated state or at vapor-liquid phase and in this last case it is allowed to reach an equilibrium state (State 21), that is the saturation state. The governing equations for this component are: m20 ⫽ mvapord ⫹ m22
Total mass balance Absorbent mass balance Energy balance
m20x20 ⫽ m22x22
mvapordhvapord ⫹ m22h22⫺m20h20⫺QVRA ⫽ 0
(1) (2) (3)
Equilibrium Condition (State 21): The equilibrium temperature (State 21) in the desorption chamber is given by the temperature of saturated LiBr/Water solution at the intermediate pressure and at the concentration of the liquid phase of the entering solution (State 20). 3.1.1.2. Pressure enhancer Compressors, as well as pumps and fans are devices used to increase pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. Because we employ this component only for circulating the water vapor from the desorption chamber into the absorption chamber, we neglect the pressure increase given by this component and then it results that the pressure inside the absorption chamber is at intermediate level and the work required is zero. The governing equations for this component are:
W.M. Worek et al. / Energy 28 (2003) 1151–1163
hvapora ⫽ hvapord
Energy balance
1159
(4)
3.1.1.3. Absorption chamber As was stated above, the pressure level in this chamber is assumed to be at the intermediate pressure level of the cycle. By extracting the thermal energy from the entering stream 11 and from the vapor—a, the vapor stream vapor—a is absorbed by the solution at State 11, and the diluted solution (State 12) exits the chamber, that is the VRA. Because the VRA is an adiabatic system the heat extracted from this chamber is provided in the desorption chamber to activate the desorption process. The governing equations for this component are: Total mass balance Absorbent mass balance
m11 ⫹ mvapora ⫽ m12
(5)
m11x11 ⫽ m12x12
(6)
m11h11 ⫹ mvaporahvapora⫺m12h12⫺QVRA ⫽ 0
Energy balance
(7)
where QVRA is the same as in the Eq. (3) and mvapora = mvapord. 3.1.2. Linking equations To complete the simulation of the VRA unit, connections between the desorption chamber, the pressure enhancer and the absorption chamber need to be further described. These connections represent the conductive heat transfer through the VRA internal separator wall and the effect of the pressure enhancer assembly on the refrigerant temperature. Typically, the heat transfer rate is described as QVRA ⫽ (UA)VRALMTDVRA
(8)
where (UA)VRA is the product of the overall heat transfer coefficient U times the area A available for the heat transfer process in the VRA, and LMTDVRA is the log mean temperature difference of the streams in the VRA unit. The VRA unit can be modeled as a parallel flow heat exchanger device. Fig. 4 shows a schematic of the VRA used for modeling. The dotted line encloses the control volume of the equivalent parallel flow heat exchanger and the two dashed lines represents the two flows passing through this heat exchanger. We could consider as entering streams the solution 11 and 20, and as exiting streams, respectively, the solution 12 and 22. By representing the VRA in this way, the expression for the LMTDVRA would be the following: LMTDVRA ⫽
(T12⫺T22)⫺(T11⫺T20) T12⫺T22 ln T11⫺T20
冉
冊
(9)
Here we are neglecting the flow vapor—d coming out the desorption chamber and the flow vapor—a coming into the absorption chamber. Because the temperatures of the streams vapor— d and vapor—a are different respectively from the temperatures of the streams 22 and 11, we need to decide to use Tvapordinstead of T22 or to use Tvaporainstead of T11. To avoid this difficulty, we calculate for both these two pairs of streams the mean temperature,
1160
W.M. Worek et al. / Energy 28 (2003) 1151–1163
weighted according the amount of energy that they carry. In particular, the equations to calculate the mean temperature are the following: Weight22
w22 ⫽
Weightvapord
wvd ⫽
Weight11
w11 ⫽
Weightvapora
wva ⫽
m22h22 mvapordhvapord ⫹ m22h22 mvapordhvapord mvapordhvapord ⫹ m22h22 m11h11 mvaporahvapora ⫹ m11h11 mvaporahvapora mvaporahvapora ⫹ m11h11
(10) (11) (12) (13)
Mean Temperature between stream 22 and vapord T22/vapord ⫽ w22T22 ⫹ wvdTvapord
(14)
Mean Temperature between stream 11 and vapora T11/vapora ⫽ w11T11 ⫹ wvaTvapora
(15)
With these definitions we can redefine more properly the log mean temperature to use in the Eq. (10) as follows: LMTDVRA ⫽
(T12⫺T22/vapord)⫺(T11/vapora⫺T20)
冉
ln
T12⫺T22/vapord
冊
(16)
T11/vapora⫺T20
This formula, taking into account the mean temperature of the mixed stream 11 and vapor— a and of the separated stream 22 and vapor—d, results to be more precisely than the formula (11) in describing the heat transfer process inside the VRA unit. However, further experimental investigations should be addressed in finding a more fundamental definition to determine the LMTDVRA. 4. Performance of the cycle employing the VRA unit 4.1. Variable definitions The COP for the absorption system in Fig. 1 is defined as follows: COP ⫽
m20(h21⫺h21) Qe ⫽ Qg,high m22(h22⫺h23)
(17)
while the COP for the VRA enhanced double-effect absorption system shown in Fig. 2 is defined as follows:
W.M. Worek et al. / Energy 28 (2003) 1151–1163
COP ⫽
Qe m30(h31⫺h32) ⫽ Qg,high m32(h32⫺h33)
1161
(18)
The energy term “W” defined earlier for the pressure enhancer, which would normally have been added to the denominator of the Eq. (18), is negligible and therefore ignored. 4.2. Simulation results discussion Using the design and operating conditions given in Table 1, for a reverse series flow doubleeffect cycle with and without a VRA unit, the performance of the two systems were compared. The value of the parameter UAVRA is basically a measure of the size of the VRA. Fig. 5 shows the variation of the coefficient of performance (COP) of the chiller using a VRA unit (Fig. 2) as compared with the performance of the same chiller without a VRA (Fig. 1). It is possible to see that the COP of the system with the VRA is 6.5% higher that the system without, and it slightly increases when increasing values of the UAVRA. Furthermore the capacity of the system with the VRA also increases by as much as 7.5% as compared to the system without VRA (Fig. 6) and it increases when the UAVRA increases. Figs. 7 and 8 show that the benefit of using the VRA unit consists of increasing the mass flow rate of the refrigerant and of increasing the temperature of the solution going into the high stage generator (State 7, Fig. 2). This reduces the heat input. These two effects improve the performance of the chiller.
Table 1 Inputs for the reverse series flow double-effect cycle model with and without VRA unit Heat transfer characteristics (UA) Absorber 6.06 kW/K Condensers 2 17.74 kW/K Condensers 1 and Generator 2 3.37 kW/K Evaporator 11.84 kW/K Generators 1 8.42 kW/K Solution Heat Exchangers 1 & 2 2.01 kW/K VRA Unit Heat Exchanger 0.5 kW/K Mass flow rates Cooling water to the absorber and then the condenser 2 3.62 kg/s Heat source generator 1 3.12 kg/s Chilled water from the evaporator 2.25 kg/s Weak solution from absorber 0.45 kg/s Operating temperatures Heat source temperatures (pressurized water) 120 °C Cooling water inlet temperatures 29.4 °C Chilled water outlet temperatures 7.2 °C Mass flow rate ratios in all the flow splits in the cycles In all the splitters present in the two studied chillers the mass flow rate ratio is 0.5, that is the solution goes half in one direction and half in the other
1162
W.M. Worek et al. / Energy 28 (2003) 1151–1163
Fig. 5. COP of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
Fig. 6. Capacity of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
5. Conclusions Performance simulations have been carried out for a novel component, the Vapor Recompression Absorber (VRA). It has been integrated in a series reverse flow double-effect absorption chiller and a numerical simulation of the resulting cycle has been performed. It has been demonstrated that the VRA unit enhances double-effect chiller improving the COP and the capacity of the chiller by increasing the mass flow rate of the refrigerant in the refrigerant circuit of the chiller and by increasing the temperature of the solution entering the high stage generator.
W.M. Worek et al. / Energy 28 (2003) 1151–1163
1163
Fig. 7. Comparison of the refrigerant mass flow rate of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
Fig. 8. Comparison of the high stage generator inlet temperature of double-effect absorption chillers with and without VRA unit with different values of the UAvra coefficient.
References [1] Herold KE, Radermacher R, Klein SA. Absorption chillers and heat pumps. Boca Raton, FL: CRC Press, 1996. [2] McNeely LA. Thermodynamic properties of aqueous solution of lithium bromide. ASHRAE Transactions 1979;85(1):413–34. [3] Alefeld G, Radermacher R. Heat conversion systems. Boca Raton, FL: CRC Press, 1994. [4] Meckler M. Enhanced lithium bromide absorption cycle water vapor recompression absorber. US Patent No. 5,816,070. 1998. [5] Meckler M. Optimizing absorption chillers. Consulting-Specifying Engineer, Oak Brook, IL: January 1999, p. 40-46.