Performance of a vapour absorption heat transformer operating with ionic liquids and ammonia

Accepted Manuscript Performance of a vapor absorption heat transformer operating with ionic liquids and ammonia

I. Sujatha, G. Venkatarathnam PII:

S0360-5442(17)31672-9

DOI:

10.1016/j.energy.2017.10.002

Reference:

EGY 11644

To appear in:

Energy

Received Date:

06 May 2017

Revised Date:

22 September 2017

Accepted Date:

01 October 2017

Please cite this article as: I. Sujatha, G. Venkatarathnam, Performance of a vapor absorption heat transformer operating with ionic liquids and ammonia, Energy (2017), doi: 10.1016/j.energy. 2017.10.002

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Ionic liquids are green designer salts that are liquid at room temperature Heat transformers operating with NH3-ionic liquids as refrigerant-absorbent studied An exergy efficiency of about 50% can be achieved at a GTL of 30-35 K with IL-NH3 combination Results show that IL-NH3 working pairs can be considered as possible alternative to conventional fluids

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PERFORMANCE OF A VAPOR ABSORPTION HEAT TRANSFORMER OPERATING WITH IONIC LIQUIDS AND AMMONIA I. Sujatha G. Venkatarathnam1 Refrigeration and Air Conditioning Laboratory Department of Mechanical Engineering Indian Institute of Technology Madras Chennai 600036, India.

ABSTRACT The performance of a single stage vapour absorption heat transformer operating with five different imidazolium based ionic liquids as the absorbent and ammonia as the working fluid has been studied in this work. The properties of pure components and their mixtures were calculated using the PC–SAFT equation of state. The performance of a heat transformer with different working fluid combinations is compared using performance parameters such as coefficient of performance (COP), second law efficiency (ηII), circulation ratio (f), molar circulation ratio (f’) and gross temperature lift (GTL). Exergy analysis was performed to quantify the losses occurring in different components. The results of our study shows that a second law efficiency of 50%, a GTL of about 30 - 35 K can be obtained with a heat transformer operating with ionic liquids [emim][AC] and [emim][SCN] as the absorbent and ammonia as the refrigerant. Ionic liquids and ammonia can be considered as possible alternative to the conventional working fluids for medium temperature lift applications in heat transformers. Key words:

Heat transformer, Ionic liquid, Ammonia, PC–SAFT equation of state,

performance, second law efficiency.

Corresponding author Email: [email protected] Phone:+91 44 2257 4685 Fax:+91 44 2257 0509 1

1

Nomenclature aij , bij

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Energy parameters of PC–SAFT model

f

Circulation ratio (kg/kg)

f'

Molar circulation ratio (mol/mol)

h

Enthalpy (kJ/kg)

I

Exergy loss (kW)

kij

Binary interaction parameter

𝑚

mass flow rate (kg/s)

m

number of spherical segments

𝑛

mol flow rate (mol/s)

𝑄

heat load (kW)

T

Temperature (K)

t

Temperature (°C)

𝑤𝑃1 Solution pump work (kW) 𝑤𝑃2 Refrigerant pump work (kW) x

Mass fraction of absorbent in the solution

y

Mol fraction of absorbent in the solution

Greek symbols ηad

Adiabatic efficiency of the pump

σ

Segment diameter

εAB

Association energy

ε/kB

Segment energy parameter

κAB

Association volume 2

ηII

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Subscripts 1, 2, 3... State points A

Absorber

C

Condenser

E

Evaporator

G

Generator

P1

Pump-1

P2

Pump-2

SHX

Solution heat exchanger

SV

Solution valve

o

Ambient conditions

ref

refrigerant

s

Strong solution in ionic liquid

w

Weak solution in ionic liquid

Abbreviations AAD Average absolute deviation COP

Coefficient of performance

Ex

Exergy

GTL

Gross temperature lift

PC–SAFT VLE

Perturbed chain statistical associating fluid theory

Vapour liquid equilibrium

[emim][AC]

1–ethyl–3–methylimidazolium acetate

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[emim][ETSO4] 1–ethyl–3–methylimidazolium ethylsulfate [emim][SCN] [emim][TF2N] [hmim][Cl]

1–ethyl–3–methylimidazolium thiocyanate 1–ethyl–3–methylimidazolium bis(trifluoromethylsulfonyl)imide 1–hexyl–3–methylimidazolium chloride

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1.

INTRODUCTION

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Heat transformers are used to increase the temperature of waste heat to higher levels required in different applications. A heat transformer (Fig. 1) essentially consists of a combination of a heat engine operating between a temperature level of TM and TL (TM > TL) and a heat pump working between a temperature level of TM and TH (TH > TM > TL) [1]. A part of heat supplied to the heat engine (QE) and heat pump (QG) is delivered at a high temperature after rejecting some amount of heat (QC) at low temperature as shown in Fig. 1. Vapour absorption based heat transformers have been studied in the literature with a variety of absorbents and refrigerant combinations. A summary of these studies is presented in Table 1. Figure 2 shows a single stage vapour absorption heat transformer. Heat is supplied at the evaporator (𝑄𝐸) and generator (𝑄𝐺) at the heat source temperature. A solution of ionic liquid and ammonia (stream 1 in Fig. 2) enters the generator, where it is heated to the source temperature. The increase in temperature results in a decrease in the solubility of ammonia in the ionic liquid. The separated vapour ammonia is sent to the condenser (stream 7), where it is condensed completely (stream 8) to a saturated/sub cooled liquid. The liquid ammonia is pumped using a pump to a higher pressure and evaporated in the evaporator to the heat source temperature. Ammonia leaves the evaporator in a saturated or superheated vapour state (stream 10). The ionic liquid partially depleted of ammonia in the generator (desorber) (stream 6) is pumped to a higher pressure using a pump, and heated using an internal heat exchanger (also known as the solution heat exchanger) before it reaches the absorber (stream 4). The ammonia leaving the evaporator is absorbed by this ionic liquid solution (strong solution) entering the absorber, resulting in a decrease of the ionic liquid concentration. Stream 3 (weak solution) leaving the absorber is heated in an internal heat exchanger before it reaches the generator. The absorption process results in an increase in temperature, enabling delivery of heat at a temperature higher than the source temperature. 5

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Heat transformers are called ‘future technology which will be important for energy utilization in the 21st century’ by the IEA (International Energy Agency) [2]. Theoretical and experimental studies on heat transformers reported in the past few decades are summarised in the literature [3,4]. The applications of heat transformers include desalination and/or purification of water [5,6], recovery of waste heat from synthetic rubber plant [7], recovery of energy from distillation column of butane, pentane [8] etc. Conventional vapour absorption heat transformers working with lithium bromide as the absorbent and water as the refrigerant combination have inherent drawbacks such as crystallization of the solution, corrosion of metals, and operation at sub atmospheric working pressures resulting in large volume of equipment (cost) etc., while systems operating with water as the absorbent and ammonia as the refrigerant require a rectification column to separate ammonia from water. The concerns for global warming have renewed the interest in the development of new working fluid combinations and cycles for heat transformers. Ionic liquids are a new class of designer salts that are usually liquid at room temperature. Different combinations of organic cation and organic/inorganic anion can be chosen to meet the requirements of absorption systems such as negligible vapour pressure, high thermal stability, low toxicity, non-flammability and the ability to dissolve many gases including ammonia, carbon dioxide, water, hydrofluorocarbon, hydrocarbon, etc. [9]. A number of researchers have recently studied the performance of ionic liquid as absorbents with different refrigerants such as water, ammonia and fluorocarbon fluids in vapour absorption refrigeration systems. Zhang and Hu [10] simulated the performance of an absorption chiller using the ionic liquid [emim][DMP] as absorbent and water as refrigerant. Kim et.al [11] studied the performance of a vapour absorption refrigerator using various ionic liquids as absorbents and HFC refrigerants. Ionic liquids have also been studied as working fluids in heat transformers by a few authors recently.

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Zhang and Hu [12] simulated the performance of a single stage absorption heat transformer with ionic liquid [emim][DMP] and water combination. Their simulations predicted a COP of 0.481 and an exergy efficiency of 62% for a heat source temperature of 90 °C, a sink temperature of 35 °C and an absorber temperature of 130 °C, which is only slightly less than the performance predicted with a combination of LiBr–H2O under the same operating conditions. Ayou et al. [13] simulated the performance of single stage and double effect absorption heat transformer using the following imidazolium cation based ionic liquids: [emim][BF4] and [bmim][BF4] as the absorbent (solvent) and 2,2,2–trifluoroethanol (TFE) as the refrigerant. A gross temperature lift (GTL), defined as the difference between the internal sink and source temperatures, of 41.8 K was predicted with a combination of TFE and [bmim][BF4]. The highest exergy efficiency was predicted to be 0.669 with a COP of 0.368 and a circulation ratio of 11.26 (kg/kg) at a source temperature of 70 °C, a sink temperature of 25 °C and a concentration difference between strong and weak solution of 5 mass %. While water is an excellent refrigerant, water condenses at sub atmospheric pressures at ambient temperature, resulting in large equipment sizes and consequently high capital cost. Many of the disadvantages with water can be overcome by using ammonia in the place of water in heat transformers, as in vapour absorption refrigerators [14,15]. Off–the shelf ammonia equipment such as condensers, evaporators, expansion valves etc. used in vapour compression systems can be used in heat transformers operating with ammonia as the refrigerant, resulting in lower capital cost. To the best of our knowledge, no literature is available on the performance study of heat transformers with ionic liquid and ammonia combination. In this work the performance of a single stage heat transformer with five different imidazolium based ionic liquids such as [emim][ETSO4], [hmim][Cl], [emim][AC], [emim][SCN] and [emim][TF2N] as absorbents and ammonia as refrigerant has been compared.

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2.

MODELING

Table 2 shows the governing equations of the different equipment of a single stage vapour absorption heat transformer obtained from an energy balance, overall mass balance and ionic liquid component mass balance across each equipment in the system. The governing equations of all the equipment are solved together for a unit mass flow rate of the refrigerant circulating through the condenser and evaporator to determine the performance of the heat transformer at specified heat source temperature (temperature of the evaporator and the generator) and ammonia condensing temperature. In this work the governing equations of different equipment of the system have been solved using the sequential modular approach, similar to that followed in an earlier paper on vapour absorption refrigerator from our group [15], using a commercial process simulator Aspen Plus version 9 [16]. The equilibrium concentration of ionic liquid (absorbent) and ammonia (refrigerant) is estimated using the Perturbed Chain Statistical Associating fluid theory (PC–SAFT) equation of state. The thermodynamic properties of ionic liquid–ammonia solution as well as that of pure ammonia have been estimated using the PC–SAFT equation of state. The PC–SAFT equation of state is based on the SAFT equation of state with modifications to the residual Helmholtz energy expression to account for the dispersion forces. Its applicability includes systems of small or large molecules over a wide range of temperature and pressure. The PC–SAFT equation of state [17,18] has been used by other researchers to determine the properties of ionic liquids [19,20], and has been adopted in this work. The pure fluid parameters of the PC–SAFT equation of state such as the number of spherical segments forming the chain (m), hard sphere segment diameter (σ), segment energy parameter

8

(ε/kB), association energy

ACCEPTED MANUSCRIPT and association volume (κAB) for the components used in the

(εAB),

present study and adopted from published literature are presented in Table 3. The heat capacity of ideal gas as a function of temperature is required to estimate enthalpy of a mixture using any equation of state. This data is not known for the ionic liquids studied in this work. Kim and Kohl [21], Yokozeki and Shiflett etc. [22] used the predictive method of Harrison and Seaton [23] to estimate the heat capacity of ideal gas of different ionic liquids while estimating the performance of a vapour absorption refrigeration system operating with ionic liquids and different refrigerants. Yokozeki and Shiflett [22] also validated the applicability of this method. Following Kim and Kohl [21], Yokozeki and Shiflett etc. [22], the predictive method of Harrison and Seaton [23] has been adopted in this work to determine the ideal gas heat capacity. The critical pressure and critical temperature values are taken from reference [24]. Studies carried out by us show that a small variation of critical pressure and critical temperature data has negligible effect on the performance parameters such as COP, second law efficiency etc. Vapour liquid equilibrium (VLE) of ionic liquid–ammonia mixtures has been predicted using the PC–SAFT equation of state. The binary interaction parameters for different ionic liquids and ammonia (kij) studied in this work have been regressed from the experimental VLE data available in the literature and are presented in Table 4. The simulations have been carried out with the following assumptions. 1.

Generator temperature and evaporator temperature are taken as same

2.

The solution is a saturated liquid at the absorber exit.

3.

Ammonia leaves the condenser as a saturated liquid, and leaves the evaporator as a saturated vapour.

4.

Pressure drop and heat losses in all components is zero.

5.

The adiabatic efficiency of the two pumps (ηad) is 50%. 9

6.

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Solution heat exchanger effectiveness, defined as the ratio of actual heat transfer to the maximum possible heat transfer,

𝑡4 ‒ 𝑡5 𝑡3 ‒ 𝑡5

7.

Ambient temperature is 20 °C

2.1

PERFORMANCE PARAMETERS

is taken as 1.

The performance of the vapour absorption heat transformers operating with different working fluids, at different operating conditions such as heat source and heat sink temperature can be compared using the following performance parameters: (a) coefficient of performance (COP) (b) second law or exergy efficiency of the system (ηII) (c) circulation ratio (f) (d) molar circulation ratio (f‘) and (e) gross temperature lift (GTL). Traditionally the coefficient of performance (COP) of a heat transformer is defined as the ratio of the rate of heat output from the absorber at the heat delivery temperature to the total energy input to the system as follows. 𝐶𝑂𝑃 =

𝑄𝐴

(1)

𝑄𝐺 + 𝑄𝐸

In this work the second law efficiency (ηII) is defined as the ratio of exergy output to exergy input as follows:

( ) ( ) ( ) 𝑄𝐴 1 -

𝜂𝐼𝐼 =

𝑄𝐺 1 -

𝑇𝑜

𝑇𝐺

+ 𝑄𝐸 1 -

𝑇𝑜

𝑇𝐴

𝑇𝑜

𝑇𝐸

(2) + 𝑤𝑝1 + 𝑤𝑝2

Traditionally the second law efficiency (ηII) is defined as the ratio of COP to Carnot COP [25]. The pump work is neglected in the estimation of both COP and second law efficiency. Ionic liquids are more viscous than conventional working pairs such as LiBr–H2O or ammonia–water. It is therefore prudent to include the pump work in the estimation of second law efficiency.

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The gross temperature lift (GTL) is the difference between absorber temperature and generator temperature and is defined as follows: GTL = TA - TG

(3)

The circulation ratio (f) is defined as the ratio of solution pumped from the generator to the flow rate of refrigerant vapour and can be estimated from the equilibrium concentration of the strong and weak solutions as follows: (4)

f=

𝑚𝑠 𝑚𝑟𝑒𝑓

=

𝑥𝑤 𝑥𝑠 ‒ 𝑥𝑤

where x is the mass fraction of the ionic liquid in the ionic liquid-refrigerant solution. Similarly, the molar circulation ratio f' is defined as the mole flow rate of strong solution leaving the generator to the mole flow rate of the refrigerant circulating through the evaporator as follows:

f' =

𝑛𝑠 𝑛𝑟𝑒𝑓

=

𝑦𝑤

(7)

𝑦𝑠 ‒ 𝑦𝑤

where y is the mole concentration of the ionic liquid in the ionic liquid-refrigerant solution. 2.2

MODEL VALIDATION

In order to validate the thermodynamic model used in the present work, studies were conducted with ammonia-water as the working pair and compared with the simulation results of Best et.al [26]. Table 5 shows the comparison between present work and published work of Best et al. at tC=30 °C, tG=tE=60 °C, tA=70 °C without using a solution heat exchanger. The results show a good agreement with the published work, with the deviation between the published results and that in our model being less than 4.5% for COP and 3.4% for circulation ratio. The difference between the concentration of the strong and weak solution between our predictions and that in the literature is 2.3% and 0.36% respectively. 11

3.

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RESULTS AND DISCUSSIONS

Figure 3 shows the variation of coefficient of performance (COP) with absorber temperature at condensing temperatures of 25 and 35 °C and a source temperature of 70 °C. It can be seen that the COP varies only by a small value with absorber temperature for all the ionic liquid–ammonia combinations studied. It can also be seen that the COP changes only slightly with an increase in the condensing temperature. Figures 4a and 4b show the variation of the power input to the pump per unit heat input to the system with the absorber (heat rejection) temperatures for [emim][AC]–ammonia combination. It can be observed that the dimensionless power input to the refrigerant pump is very small, and also one order less than that to the solution pump at all conditions. The power input to the solution pump is less than 3–4% of the heat input to the system at normal operating conditions (TA < 105 °C) because of their high viscosity. However, the pump work required can be higher than 10% of the total heat input to the system at high pressures. In exergy terms, this is a non–negligible value, and should therefore be accounted in the calculation of the second law efficiency (Eq. No. (2)). Hence the pump work has been included in the exergy calculation in this work to give a true performance of the system, unlike other authors [12,13] who have neglected pump work in estimating both COP and second law efficiency. The second law efficiency as defined in this work is therefore a better performance indicator than the traditional COP. The variation of the second law efficiency, (ηII) with absorber and source temperature are shown in Figs. 5a and 5b respectively at condensing temperatures of 25 and 35 °C. The temperature at which heat is added in the generator and the evaporator is assumed to be same throughout the study, and is called source temperature. It can be seen from Fig. 5a that the second law efficiency increases marginally with an increase in the absorber temperature and decreases thereafter with an increase of the absorber temperature. On the other hand, there is a marked change in the second 12

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law efficiency with generator temperature at low values while the variation is small at higher generator temperatures. The reason for this variation can be understood by examining the exergy loss in each component of the system at different absorber and generator temperatures. Figure 6 shows the utilisation of input exergy with absorber temperature in the entire system at a condensing temperature of 25 °C, source temperature of 70 °C and a solution heat exchanger effectiveness of 100%. The exergy loss in any equipment is the difference between the input and output exergy. The detailed expressions for the calculation of exergy losses in each component are given in Table 6. It can be seen from Fig. 6 that the exergy loss in the absorber, generator, condenser and evaporator decreases while that in the solution pump and valve increases with an increase in the absorber temperature. The reasons for the increase/decrease of exergy losses with an increase/decrease of generator temperature can be understood by examining the performance of individual equipment at different operating temperatures. Consider the absorber. An increase in absorber temperature (TA) results in a decrease of solubility of ammonia in the ionic liquid resulting in a lower heat of absorption (𝑄𝐴). The difference in concentration of ionic liquid in the strong and weak solutions decreases with an increase in the absorber temperature, and ultimately becomes zero beyond a certain absorber temperature, as shown in Fig. 7a. The temperature at which the concentration difference becomes zero is also a function of pressure in the absorber. Since the pressure in the absorber is the same as the vapour pressure of ammonia in the evaporator, the source (generator, evaporator) temperature has a strong influence on the absorber temperature (see Fig. 8) as well as heat rejected in the absorber. A decrease in the concentration difference between the strong and weak solution with absorber temperature results in a lower exergy loss due to mixing of the strong solution and the refrigerant (ammonia) in the absorber. The equilibrium concentration of the strong solution is essentially controlled by the temperature and pressure of strong solution leaving the generator (stream 6). Since the pressure of the stream 6 13

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is the same as the condensing pressure of ammonia in the condenser, the condensing temperature also has a strong influence on the temperature at which the system ceases to operate (∆𝑋→0) as shown in Fig. 7a. An increase in the absorber temperature and a consequent decrease in the difference in concentration of ammonia in the strong and weak solutions results in a smaller flow rate of ammonia per unit flow rate of the weak solution and thereby results in a decrease in the heat transfer rate across the condenser, evaporator and generator. A decrease in the flow rate of refrigerant for a fixed solution flow rate with an increase of absorber temperature also results in a decrease in the fraction of exergy loss in the condenser, evaporator and generator as shown in Fig. 6. The pump work and exergy loss across the valve essentially remain the same with an increase in the absorber temperature. However, as the rate of heat rejection in the absorber comes down with an increase in absorber temperature, the fraction of exergy loss in the pump and the valve increases with an increase of absorber temperature as shown in Fig. 6. The variation of second law efficiency with absorber temperature at a given condenser temperature shown in Fig. 5a is controlled by the rate of decrease of exergy losses in the absorber, generator, condenser and evaporator and the rate of increase of exergy losses in the valve and the solution pump (pump-1) with absorber temperature, leading to the parabolic variation of second law efficiency with absorber temperature seen in Fig. 5a. The large drop in second law efficiency in Fig. 5a beyond a temperature of 120 °C at a condenser temperature of 25 °C and beyond 110 °C at a condenser temperature of 35 °C is due to the difference in equilibrium concentration of strong and weak solution tending to near zero value at these temperatures, as shown in Fig. 7a. Similarly the decrease in second law efficiency with source temperature (Fig. 5b) below a temperature of 64 °C at a condenser temperature of 35 °C and below 58 °C at a condenser temperature of 25 °C can be understood with the help of input exergy utilisation across different equipment shown in Fig. 9. At any given generator pressure, a minimum source temperature is required to generate vapour, and for the concentration difference 14

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between the strong and weak ionic liquids solution (∆x) to be greater than zero as shown in Fig. 7b. Since the generator pressure is essentially the same as the vapour pressure of ammonia at the condenser pressure, the condensing temperature has a strong influence on the temperature at which the generator starts to function (∆x>0). A decrease in the solubility of ammonia in the ionic liquid with an increase of the absorber temperature and a decrease of the source temperature essentially results in a large increase in circulation ratio (f) as shown in Figs. 10a and 10b respectively. The circulation ratio is typically less than 10 in NH3–H2O system and LiBr–H2O system. A large circulation ratio results in large heat transfer equipment and higher capital cost. It is evident from Figs. 10a and 10b that a heat transformer working with ionic liquid–ammonia combination needs to be operated below an absorber temperature of about 105–120 °C (depending on the condensing temperature) and above a source temperature of 58–64 °C to limit the circulation ratio below 20. The solubility of ammonia at a given pressure and temperature is different for different ionic liquid–ammonia combinations studied in this work. Consequently the circulation ratio is also different when different ionic liquid–ammonia combinations are used. The circulation ratio is the lowest with [emim][AC] and the highest when [emim][TF2N] is used. (f with [emim][AC] < [emim][SCN] < [hmim][Cl] < [emim][ETSO4] < [emim][TF2N]). It can be seen from Figs. 5a and 5b that there is no marked difference in the second law efficiency when different ionic liquids are used. The second law efficiency of a system essentially controls the operating cost, and the circulation ratio controls the capital cost. A heat transformer operating with [emim][AC]–NH3 working pair shows the highest second law efficiency as well as the lowest circulation ratio and can be considered as a the best absorbent to be used with ammonia among the ionic liquid–ammonia combinations studied in this work. Thiocyanate based ionic liquid [emim][SCN] can be considered as the next best absorbent to be used along with ammonia in a heat transformer. 15

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Figures 11a and 11b show the variation of molar circulation ratio (f’), defined as the ratio of mole flow rate of strong solution to the mole flow rate of refrigerant, as a function of absorber temperature and generator temperature at different condensing temperatures. It can be seen that the molar circulation ratio is typically less than 2 at absorber temperatures less than 100 °C at both condensing temperature 25 °C and 35 °C and generator temperature of 70 °C. The typical molar circulation ratio for LiBr–H2O and NH3–H2O combinations is about 5 and 10 respectively. Thus the molar circulation ratio with ionic liquid–ammonia combination is less than those with traditional absorbent–refrigerant combinations used in a heat transformer. The reason for the large circulation ratio on mass basis when ionic liquid–ammonia combinations are used is due to the larger molecular weights of ionic liquids (typically greater than 150) compared to the absorbents H2O and LiBr (18 and 86.8 respectively). Table 7 shows the comparison of the equilibrium concentrations of ammonia and ionic liquid combinations with the conventional working pair such as NH3–H2O on mass and mole basis at a source temperature of 70 °C, condenser temperature of 25 °C, absorber temperature of 100 °C It is evident that, on a molar basis the solubility of ammonia in ionic liquids is as good as the solubility of ammonia in water. However, because of large difference in molecular weights of ionic liquids, the equilibrium concentration on mass basis is much smaller. One way of decreasing the mass circulation ratio is to use fluids with much larger molecular weights such as butanes, pentanes or fluorinated hydrocarbons such as tetrafluoropropylene (R1234ze). Figure 12 shows the variation of the gross temperature lift (difference between heat rejection and heat source temperature) and circulation ratio (f) with the difference in mass concentration of strong and weak solutions (∆x) at a condenser temperature of 25 °C for [emim][AC] and [emim][SCN] combinations. A GTL of 35 K and a circulation ratio 10 can be obtained with both [emim][SCN] and [emim][AC] with ammonia at a concentration difference of about 8 (mass%) at a condensing temperature of 25 °C. In order to limit the size and art of the system to a 16

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reasonable value, the circulation ratio should be limited normally about 20. The GTL value becomes maximum when the concentration difference approaches a minimum value, but at those values the operation of the heat transformer requires a large circulation ratio. Table 8 shows the comparison of performance of ionic liquid-ammonia working pairs with conventional working pairs like ammonia–water and lithium bromide-water at tC=25 °C, tG=tE=70 °C and tA=100 °C. The COP of the heat transformer with LiBr-H2O is higher than that with all the working pairs studied in this work as well as that of conventional working pair of NH3-H2O. The COP and second law efficiency of ionic liquid-ammonia working pairs is close to NH3-H2O working pair and circulation ratio is below 10 for [emim][AC]-NH3 and [emim][SCN]-NH3 working pair. The circulation ratio is also smaller in the case of LiBr-H2O. However, the working pressures are subatmospheric when water is used as the refrigerant. The large specific volume of water at subatmospheric pressures results in large equipment size, and thereby cost when LiBrH2O pair is used as the working fluids. This disadvantage, however, does not exist when ammonia is used as the refrigerant. Conventional vapour compression refrigeration system components such as ammonia condenser and evaporator can be used off-the-shelf when ammonia is used as the refrigerant, irrespective of the absorbent used. However, the requirement of a distillation column makes the NH3-H2O based systems expensive. Since ionic liquids are salts, they have very little vapour pressure, and a distillation column is not required when they are used as the absorbent as in the case of LiBr water systems. Ionic liquid-ammonia working pairs thus combine the lower cost of ammonia system with the absence of distillation column of LiBr system, and would be financially competitive with LiBr-H2O system despite a slightly lower cost. Additionally, ionic liquids are non-corrosive, while LiBr is very corrosive. 4.

CONCLUSIONS

A maximum second law efficiency close to 50% can be obtained with two ionic liquids, [emim][AC] and [emim][SCN] with ammonia as the refrigerant at a condensing temperature of 17

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25 °C, and a heat source temperature of 70 °C and a heat rejection temperature of 105 °C (or a GTL of 35 °C). The circulation ratio at these conditions with both the above ionic liquids is of the order of 10 only. Because of the favourable performance, [emim][AC] and [emim][SCN] are therefore recommended for use in single stage heat transformers operating with ammonia as the working fluid in medium temperature lift applications. ACKNOWLEDGEMENTS The authors are grateful to the Department of science and Technology of India and the Indian Institute of Technology Madras for financial support.

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21

LIST OF FIGURES

ACCEPTED MANUSCRIPT

Fig. 1 Temperature levels in a vapour absorption heat transformer Fig. 2 Schematic representation of a vapour absorption heat transformer. Fig. 3 Variation of COP with absorber temperature at different condensing temperature. (tG=tE=70 °C) Fig. 4 Effect of Absorber temperature on Pump work/heat input for (a) Pump–1, (b) Pump–2 at different condenser temperature for [emim][AC]–NH3 working pair. (tG=tE=70 °C) Fig. 5 Variation of Second law efficiency (ηII) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 30 °C. Fig. 6 Utilisation of input exergy at different absorber temperature for the working pair [emim][ETSO4]/NH3 at condenser temperature tC=25 °C and source temperature tG=70 °C. Fig. 7 Variation of difference in concentration of ionic liquid in the strong solution and weak solution with (a) absorber temperature (b) source temperature. Fig. 8 Variation of mass concentration difference with absorber temperature at different source temperatures at tG=60 °C and 70 °C. Fig. 9 Utilisation of input exergy at different generator temperature for the working pair [emim][ETSO4]–NH3 at condenser temperature tc=25 °C and absorber temperature tA=90 °C. Fig. 10 Variation of Circulation ratio, f (kg/kg) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 35 °C. Fig. 11 Variation of molar circulation ratio f’, (mol/mol) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 30 °C. Fig. 12 Variation of gross temperature lift (GTL) and circulation ratio (f) with mass concentration difference in strong and weak solutions at a source temperature (tG=tE) of 70 °C and a condenser temperature of tC=25 °C.

22

ACCEPTED MANUSCRIPT TH QA

P QG

TM

TM

QE E E

QC

TL

Fig. 1 Temperature levels in a vapour absorption heat transformer.

23

ACCEPTED MANUSCRIPT Heat Rejection (QA)

Heat Absorption (QE)

.

10

mref

Evaporator

Absorber 3

4

9

SHX

Weak IL Solution

Pump-2 8

Strong IL Solution 2

Solution valve 1

Condensor

5

Pump-1 6

Generator 7

Heat Rejection (QC)

Heat Absorption (QG)

Fig. 2 Schematic representation of a vapour absorption heat transformer.

24

ACCEPTED MANUSCRIPT

Coefficient of performance, COP

0.45 TG=TE=70 °C

0.44 0.43

TC=25 °C

0.42 0.41 0.4 0.39

TC= 35 °C

0.38 0.37

[emim][ETSO4]

[hmim][Cl]

0.36

[emim][SCN]

[emim][TF2N]

[emim][AC]

0.35 80

85

90

95

100

105

110

115

120

Absorber temperature, TA (°C)

Fig. 3 Variation of COP with absorber temperature at different condensing temperature. (tG=tE=70 °C).

25

ACCEPTED MANUSCRIPT 0.2 (a)

tG=tE=70 °C

0.18

WP1/(heat input)

0.16 0.14 0.12 0.1 tC=35 °C

0.08

tC=30 °C

[emim][AC]

0.06 0.04

tC=25 °C

0.02 0 80

90

100

110

120

Absorber temperature, tA (°C)

(b)

0.005 tG=tE=70 °C

0.0049

WP2/(heat input)

0.0048 tC=25 °C

0.0047 0.0046

tC=30 °C

0.0045 0.0044

tC=35 °C

0.0043 0.0042 0.0041 [emim][AC]

0.004 80

90

100

110

Absorber temperature, tA (°C)

120

Fig. 4 Effect of Absorber temperature on Pump work/heat input for (a) Pump–1, (b) Pump–2 at different condenser temperature for [emim][AC]–NH3 working pair. (tG=tE=70 °C)

26

ACCEPTED MANUSCRIPT 0.6

tG=tE=70 °C

(a)

Second law efficiency, ηII

0.5 tC=25 °C

0.4 [emim][ETSO4]

0.3

tC=35 °C

[hmim][Cl]

0.2

[emim][SCN]

0.1

[emim][Tf2N] [emim][AC]

0 80

90

100

110

120

130

Absorber temperature, tA (°C)

140

0.6

tA=90 °C

(b)

Second law efficiency, ηII

0.5

0.4

tC=25 °C

tC=35 °C

0.3

[emim][ETSO4]

0.2

[hmim][Cl] [emim][SCN]

0.1

[emim][Tf2N] [emim][AC]

0

52

56

60

64

68

72

Source temperature, tG,tE (°C) Fig. 5 Variation of Second law efficiency (ηII) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 30 °C.

27

ACCEPTED MANUSCRIPT

100 valve loss Pump-1 loss Pump-2 loss SHX loss Codenser loss Evaporator loss Absorber loss Generator loss useful effect

Utilisation of input exergy, (%)

90 80 70 60 50 40 30 20 10 0 80

90

100

Absorber temperature, tA (°C)

120

Fig. 6 Utilisation of input exergy at different absorber temperature for the working pair [emim][ETSO4]/NH3 at condenser temperature tC=25 °C and source temperature tG=70 °C.

28

ACCEPTED MANUSCRIPT 25

tG=70°C

(a)

[emim][ETSO4] [hmim][Cl]

∆x (%)

20

[emim][SCN]

15

[emim][Tf2N]

tC=25 °C

[emim][AC]

10 tC=35 °C

5 0 80

90

100 110 120 Absorber temperature, tA (°C)

130

20

∆x (%)

tA= 90 °C

[emim][ETSO4]

(b) 18 16

[hmim][Cl]

14

[emim][SCN]

12

[emim][Tf2N]

10

[emim][AC]

140

tC=25 °C

8 6 tC=35 °C

4 2 0 55

58

61

64

Source temperature, tG,tE (°C)

67

70

Fig. 7 Variation of difference in concentration of ionic liquid in strong solution and weak solution with (a) absorber temperature (b) source temperature.

29

ACCEPTED MANUSCRIPT

25

tC=25 °C

[emim][ETSO4] [hmim][Cl]

20

∆x (%)

[emim][SCN] [emim][TF2N]

15

[emim][AC] 10 tG=70 °C

5 tG=60 °C 0 80

90

100

110

120

130

140

Absorber temperature, tA (°C) Fig. 8 Variation of mass concentration difference with absorber temperature at different source temperatures at tG=60 °C and 70 °C.

30

ACCEPTED MANUSCRIPT

100 valve loss Pump-1 loss Pump-2 loss SHX loss Codenser loss Evaporator loss Absorber loss Generator loss useful effect

Utilisation of input Exergy, (%)

90 80 70 60 50 40 30 20 10 0

54

58

60

70

Generator temperature, TG (°C)

Fig. 9 Utilisation of input exergy at different generator temperature for the working pair [emim][ETSO4]–NH3 at condenser temperature tc=25 °C and absorber temperature tA=90 °C.

31

ACCEPTED MANUSCRIPT 100

(a)

Circulation ratio, f (kg/kg)

TG=TE=70 °C

[emim][ETSO4]

90 80

[hmim][Cl]

70

[emim][SCN]

60

[emim][Tf2N]

50

TC=35 °C

[emim][AC]

TC=25 °C

40 30 20 10 0 80

90

100

110

120

130

Absorber temperature, TA (°C)

Circulation ratio, f (kg/kg)

100 (b) 90

TA=90 °C

[emim][ETSO4] [hmim][Cl]

80

[emim][SCN]

70

[emim][TF2N]

60

[emim][AC]

50

TC=25 °C

TC=35 °C

40 30 20 10 0 54

56

58

60

62

64

66

68

70

72

Source temperature, TG (°C) Fig. 10 Variation of Circulation ratio, f (kg/kg) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 35 °C.

32

ACCEPTED MANUSCRIPT

Molar circulation ratio, f' (mol/mol)

(a)

20

TG=TE=70 °C

[emim][ETSO4]

18

[hmim][Cl]

16

[emim][SCN]

14

[emim][TF2N]

12

[emim][AC]

10

TC=35 °C

8 TC=25 °C

6 4 2 0 80

90

100

110

Absorber temperature, TA (°C)

120

130

` 20

Molar circulation ratio, f' (mol/mol)

(b)

[emim][ETSO4] [hmim][Cl] [emim][SCN] [emim][TF2N] [emim][AC]

TA=90 °C

18 16 14 12

TC=35 °C

10

TC=25 °C

8 6 4 2 0 54

56

58

60

62

64

66

Source temperature, TG (°C)

68

70

72

Fig. 11 Variation of molar circulation ratio f’, (mol/mol) with (a) absorber temperature, (b) source temperature at condenser temperature tC=25 °C and 30 °C.

33

ACCEPTED MANUSCRIPT

60

Gross temperature lift, GTL

100

TG=70 °C TC=25 °C

50

f=20

40

f=10

10

30 20

Circulation ratio, ( f )

70

[emim][SCN]

10

[emim][AC]

0 0

5

1 10

15

20

Concentration difference, xs-xw (mass %)

25

Fig. 12 Variation of gross temperature lift (GTL) and circulation ratio (f) with mass concentration difference in strong and weak solutions at a source temperature (tG=tE) of 70 °C and a condenser temperature of tC=25 °C.

34

ACCEPTED MANUSCRIPT

Table 1 Summary of single stage vapour absorption heat transformers studied in the literature

Working fluid combination (Refrigerant– Absorbent) R21–DMF, R22–DMF, R22–DMETEG, H2O–LiBr TFE–PYR, H2O–LiBr

Operating conditions

Reference

Source temperature 50–70 °C, Sink temperature 15–40 °C. Source temperature 40–110 °C, cooling water temperature 4–5°C NH3–H2O, H2O–LiBr Source temperature 70–90 °C, sink temperature 30 °C H2O– LiBr+LiI+LiNO3+LiC1, Source temperature 60–80 °C, sink temperature 10 – 40 °C H2O–LiBr H2O–Carrol (LiBr–ethylene Source temperature 60–90 °C, sink glycol 1:4.5 by mass), temperature 30 °C. H2O–LiBr R22–DMF,DMA,NMP, Source temperature 50–77 °C, sink DMEDEG,DMETEG, temperature 20–50 °C. DMETrEG H2O–H2SO4 Source temperature 80 °C, sink temperature 15 °C H2O–Alkitrate, Source temperature 105 °C, Condenser temperature 50 °C, Evaporator temperature H2O–LiBr 100 °C TFE–NMP,TFE–E181, Generator temperature 50–70 °C, Condenser TFE–PYR, H2O–LiBr temperature 10–40 °C.

[27] [28] [29] [5] [30]

[31] [32] [33] [25]

H2O–[emim][DMP], H2O–LiBr, TFE–E181

Generator temperature 90 °C, Condenser temperature 35 °C.

[12]

TFE–[emim][BF4], TFE–[bmim][BF4], TFE–TEGDME, H2O–LiBr H2O–(LiCl+LiNO3), H2O–LiBr,H2O–LiI, H2O–(LiBr+LiNO3), H2O–(LiBr+ZnBr2+LiCl2) H2O–(LiCl+CaCl2+Zn(NO3)2) NH3–1–4butanediol, NH3–2–3butanediol, NH3–TEG,DME SO2–DMA, NH3–H2O

Source temperature temperature 20–40 °C.

60–80

[13]

Source temperature temperature 20–40 °C.

60–80

°C, °C,

sink sink [34]

Source temperature 50, 60 °C, Condenser temperature 25, 30 °C.

35

[35]

ACCEPTED MANUSCRIPT

Table 2: Governing equations of different equipment of the vapour absorption heat transformer obtained by energy, overall mass and ionic liquid component balance across each equipment Equipment

Governing equations 𝑄𝐺 = 𝑚𝑠ℎ6 + 𝑚𝑟𝑒𝑓ℎ7 ‒ 𝑚𝑤ℎ1

Generator

𝑚1 = 𝑚6 + 𝑚7 𝑚1𝑥𝑤 = 𝑚6𝑥𝑠 𝑄𝐴 = 𝑚𝑠ℎ4 + 𝑚𝑟𝑒𝑓ℎ10 ‒ 𝑚3ℎ3

Absorber

𝑚3 = 𝑚4 + 𝑚10 𝑚3𝑥𝑤 = 𝑚4𝑥𝑠 𝑄𝐶 = 𝑚𝑟𝑒𝑓(ℎ7 ‒ ℎ8)

Condenser

𝑚7 = 𝑚8 = 𝑚𝑟𝑒𝑓 𝑄𝐸 = 𝑚𝑟𝑒𝑓(ℎ10 ‒ ℎ9)

Evaporator

𝑚9 = 𝑚10 = 𝑚𝑟𝑒𝑓 𝑤𝑃1 = 𝑚𝑠(ℎ5 ‒ ℎ6)

Pump–1

𝑚5 = 𝑚6 = 𝑚𝑠 𝑤𝑃2 = 𝑚𝑟𝑒𝑓(ℎ9 ‒ ℎ8)

Pump–2

Solution valve Solution heat

𝑚8 = 𝑚9 = 𝑚𝑟𝑒𝑓 ℎ1 = ℎ2 𝑚1 = 𝑚2 = 𝑚𝑤 𝑚𝑤(ℎ3 ‒ ℎ2)=𝑚𝑠(ℎ4 ‒ ℎ5)

exchanger

36

ACCEPTED MANUSCRIPT

Table 3 PC–SAFT Equation of state pure fluid parameters used in this work

[emim][ETSO4]

236.29

4.22

419

3.94

.00225

3450

Data source [19]

[hmim][Cl]

202.37

4.33

407.8

3.72

.00225

3450

[19]

[emim][AC]

170.21

4.23

405.7

3.13

.00225

3450

[19]

[emim][TF2N]

391.32

4.18

360.5

5.38

.00225

3450

[19]

[emim][SCN]

169.25

4.22

383.8

3.05

.00225

3450

[19]

NH3

17.03

2.40

187.7

2.94

0

0

[36]

Mw(g.mol–1)

Component

σ (A°)

ε/kB(K)

κAB

m

εAB/kB (K)

Table 4 Binary interaction parameters (kij = aij + bij/Tr) regressed in this work

aij

bij

AAD %

[hmim][Cl]–NH3

.10352

0.214

1.41

Data source [22]

[emim][AC] NH3

.01213

0.0995

0.23

[37]

[emim][ETSO4]–NH3

.07102

0.0591

0.16

[37]

[emim][SCN]–NH3

.02906

0.0774

2.27

[37]

0.02862

0.20

[22]

Mixture

[emim][TF2N]–NH3

.08412 Where kij = aij + bij/Tr and Tr = T/298.15

Table 5 The comparison of results using present model with that of published data of Best et.al [26] Performance parameter

Best et. al. 81.14

Present model 81.44

% deviation 0.36

Concentration of solution from absorber Concentration of solution from generator

55.91

54.61

2.3

Coefficient of Performance (COP(-))

0.443

0.42

4.5

Circulation ratio (f (-))

1.75

1.7

3.4

37

ACCEPTED MANUSCRIPT Table 6 Equations for the calculation of exergy loss in various components [38]

( ) +Ex -Ex +𝑄 (1 ‒ ) 𝑇0

IG=Ex1- Ex7- Ex6+𝑄𝐺 1 ‒ 𝑇 IA=Ex10

𝐺

𝑇0

4

3

𝐴

𝑇𝐴

IC=Ex7 - Ex8

(

𝑇0

IE=Ex9 - Ex10 +𝑄𝐸 1 ‒ 𝑇

)

𝐸

IP1=Ex6 - Ex5 +WP1 IP2=Ex8 - Ex9 +WP2 ISV=Ex2 - Ex1 ISHX=Ex3+ Ex5 - Ex4 - Ex2

(

𝑇0

)

(

𝑇0

)

Input exergy = 𝑄𝐺 1 - 𝑇 + 𝑄𝐸 1 - 𝑇 +𝑤𝑃1+𝑤𝑃2 𝐺 𝐸

(

𝑇0

Useful effect = 𝑄𝐴 1 ‒ 𝑇 𝐴

)

Table 7 Concentration of strong solution and weak solution in mass and mol % of different working fluids at a generator temperature of 70 °C, absorber temperature of 100 °C and condenser temperature of 25 °C. Working fluid [emim][ETSO4]–NH3

Concentration of strong solution xs (mass %) ys (mol %) 94.9 57.5

Concentration of weak solution xw (mass %) yw (mol %) 87.7 33.9

[hmim][Cl]–NH3

93.2

53.6

84.5

31.4

[emim][SCN]–NH3

92.1

54.1

85.8

37.9

[emim][TF2N]–NH3

95.8

50.4

89.5

27.1

[emim][AC]–NH3

91.8

52.9

81.8

31.0

38

ACCEPTED MANUSCRIPT 61.8

H2O–NH3

60.3

52

50.6

Table 8 Performance comparison of working pairs at tC=25 °C, tG=tE=70 °C and tA=100 °C Performance parameter COP(-)

NH3- LiBrH2O H2O 0.402 0.50

[emim][SCN]NH3 0.41

[hmim][Cl]NH3 0.42

[emim][ETSO4] [emim][TF2N] -NH3 -NH3 0.406 0.41

[emim][AC] -NH3 0.409

f (-)

3.24 0.56

9.45 0.52

10.7 10.51

13.13 0.49

9.18 0.513

𝜂𝐼𝐼

5.1 0.73

39

15.0 0.5