# Use of Exergy Analysis to Quantify the Effect of Lithium Bromide Concentration in an Absorption Chiller

^{*}

## Abstract

**:**

## 1. Introduction

_{2}emissions showing that as long as the Coefficient of performance (COP) of an electric chiller was greater than 6.1 the compression based cooling produced less CO

_{2}emissions and was more energy efficient than absorption cooling [9].

## 2. Methodology

#### 2.1. Exergy

_{b}is the system boundary absolute temperature where the heat transfer is occurring:

_{0}, h

_{0}and s

_{0}are the dead state temperature, enthalpy and entropy respectively. The energy efficiency (η

_{en}) of the chillers considered here is a measure of useful energy is produced from a given input and can be expressed in Equation (6); for cooling cycles this is often represented as the COP. For absorption chillers, the COP is defined based on the heat transfers associated with the evaporator and the generator [16].

_{cw}and ṁ

_{st}respectively. Δh is the enthalpy change associated with the chilled water and steam. Ẇ

_{p}is the electrical work supplied to the solution pump. Exergetic efficiency (η

_{x}) is defined in for an absorption chiller in general terms in Equation (7):

#### 2.2. Scope Definition

## 3. Case Study: University of Idaho Energy Plant

^{3}of water. This tank is charged each night and discharged during the peak heating demands of the day to reduce the less efficient usage of the electric chillers. The electric chillers operate mostly during the cool night and early morning hours to assist the absorption chiller in charging the storage tank. The chillers are staged to allow the absorption chiller to run at a full and constant capacity throughout the cooling season. When the load is greater than what is able to be provided by the absorption chiller, additional electric chillers as well as the TES are used. When the campus cooling load is less than the full capacity of the absorption chiller, the TES is used and recharged with the absorption chiller to facilitate the fully loaded efficiencies of the chiller.

## 4. Analysis

## 5. Results and Discussion

## 6. Validation

## 7. Cost Savings

## 8. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## Nomenclature

CFCs | Chlorofluorocarbon refrigerants |

COP | Coefficient of performance |

DE | District energy |

EES | Engineering Equation Solver |

GHGs | Greenhouse gases |

LiBr | Lithium Bromide |

TES | Thermal energy storage |

UI | University of Idaho |

Ψ | Specific exergy (kJ/kg) |

h | Specific enthalpy (kJ/kg) |

$\dot{m}$ | Mass flow rate (kg/s) |

P | Pressure (kPa) |

$\dot{Q}$ | Heat transfer rate (kW) |

s | Specific entropy (kJ/kg·K) |

SHX | Solution Heat Exchanger |

T | Temperature (°C or K) |

V | Velocity (m/s) |

$\dot{X}$ | Exergy rate (kJ/kg) |

Z | Height (m) |

ƞ | Efficiency |

## Subscripts

0 | Reference Property |

abs | absorber |

b | boundary |

cond | Condenser |

cw | Chilled Water |

des | Destroyed |

en | Energy |

evap | Evaporator |

g | gravity |

gen | Generator |

f | Flow |

p | Pump |

Q | heat transfer |

st | Steam |

sys | system |

w | work |

x | Exergy |

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**Figure 2.**Single effect absorption chiller diagram (gray area from Figure 1).

Parameter | Value |
---|---|

Boiler (10.43 MW) | |

Steam flow rate | 1.3 kg/s |

Steam Pressure | 194.8 kPa |

Steam Temperature | 408.7 K |

Cooling Tower (2391 kW) | |

Cooling tower water flow rate | 116.6 kg/s |

Temperature from condenser to cooling tower | 314.8 K |

Temperature from cooling tower to absorber | 304.3 K |

Absorption Chiller (2208 kW) | |

Temperature from absorber to condenser | 310.1 K |

Chilled water flow rate | 81.0 kg/s |

Chilled water entering temperature | 285.9 K |

Chilled water exiting temperature | 279.4 K |

Weak Solution Concentration | 56% |

**Table 2.**Data for flows and conditions for the absorption chiller when T

_{0}= 300.0 K, P

_{0}= 101.7 kPa. State points refer to Figure 2.

Point | ṁ (kg/s) | T (K) | P (kPa) | h (kJ/kg) | s (kJ/kg·K) | ψ (kJ/kg) |
---|---|---|---|---|---|---|

0 | - | 300.0 | 101.7 | 112.7 | 0.39 | - |

1 | 1.3 | 408.7 | 194.8 | 2739.0 | 7.22 | 598.5 |

2 | 1.3 | 362.9 | 194.8 | 376.1 | 1.19 | 26.8 |

3 | 1.3 | 362.9 | 101.7 | 376.1 | 1.19 | 26.7 |

4 | 116.6 | 304.3 | 101.7 | 130.4 | 0.10 | 104.7 |

5 | 116.6 | 310.1 | 101.7 | 154.7 | 0.18 | 105.5 |

6 | 116.6 | 314.8 | 101.7 | 174.5 | 0.24 | 106.5 |

7 | 81.1 | 285.9 | 101.7 | 46.5 | −0.19 | 105.5 |

8 | 81.1 | 279.4 | 101.7 | 26.4 | −0.26 | 106.6 |

9 | 0.9 | 348.6 | 7.4 | 2641.0 | 8.11 | 236.7 |

10 | 0.9 | 313.2 | 7.4 | 167.5 | 0.22 | 106.1 |

11 | 0.9 | 275.9 | 0.8 | 167.5 | 0.26 | 95.7 |

12 | 0.9 | 275.9 | 0.8 | 2506.0 | 8.73 | −83.4 |

13 | 8.8 | 305.9 | 0.8 | 82.4 | 0.06 | 0.2 |

14 | 8.9 | 305.9 | 7.4 | 82.37 | 0.06 | 0.2 |

15 | 8.9 | 335.7 | 7.4 | 143.3 | 0.25 | 4.7 |

16 | 7.9 | 363.2 | 7.4 | 223.3 | 0.37 | 11.8 |

17 | 7.9 | 326.5 | 7.4 | 155.2 | 0.18 | 2.5 |

18 | 7.9 | 308.8 | 0.8 | 155.2 | 0.18 | 1.2 |

Component | Equation | Exergy Destruction Rate (kW) |
---|---|---|

Condenser | ${X}_{des,Cond}={\dot{Q}}_{cond.}\left(1-\frac{To}{{T}_{10}}\right)+{\dot{m}}_{9}\left({\psi}_{9}-{\psi}_{10}\right)$ | 240.5 |

Refrigerant Valve | ${X}_{des,Rvalve}={\dot{m}}_{10}\left({\psi}_{10}-{\psi}_{11}\right)$ | 9.7 |

Evaporator | ${X}_{des,Evap}={\dot{m}}_{11}\left({\psi}_{11}-{\psi}_{12}\right)+{\dot{Q}}_{Evap.}\left(1-\frac{To}{{T}_{12}}\right)$ | 41.7 |

Absorber | ${X}_{des,Abs}={\dot{m}}_{12}{\psi}_{12}+{\dot{Q}}_{abs.}\left(1-\frac{To}{{T}_{13}}\right)+{m}_{18}{\psi}_{18}-{m}_{13}{\psi}_{13}$ | 265.1 |

Pump | ${X}_{des,P}={\dot{m}}_{13}\left({\psi}_{13}-{\psi}_{14}\right)+{\dot{W}}_{p,x}$ | 1.6 |

Solution Heat Exchanger | ${X}_{des,SHX}={\dot{m}}_{14}\left({\psi}_{14}-{\psi}_{15}\right)+{\dot{m}}_{16}\left({\psi}_{16}-{\psi}_{17}\right)$ | 34.8 |

Generator | ${X}_{des,Gen}={\dot{m}}_{15}{\psi}_{15}+{\dot{Q}}_{gen.}\left(1-\frac{To}{{T}_{16}}\right)-{m}_{16}{\psi}_{16}-{m}_{9}{\psi}_{9}$ | 266.4 |

Solution Valve | ${X}_{des,Svalve}={\dot{m}}_{17}\left({\psi}_{17}-{\psi}_{18}\right)$ | 10.4 |

Point | Location | Temperature Increase (°C) |
---|---|---|

9 | Leaving the generator (Refrigerant) | 5.9 |

15 | Entering the SHX (Weak Solution) | 3.7 |

16 | Entering the generator (Weak Solution | 7.0 |

17 | Leaving the generator (Strong Solution) | 2.5 |

18 | Leaving the SHX (Strong Solution) | 5.5 |

Component | Exergy Destruction Rate (kW) | Percent Difference (%) |
---|---|---|

Condenser | 235.3 | −0.48 |

Refrigerant Valve | 9.4 | −0.03 |

Evaporator | 40.4 | −0.13 |

Absorber | 219.8 | −5.13 |

Pump | 1.6 | 0.00 |

Solution Heat Exchanger | 41.5 | +0.78 |

Generator | 308.4 | +4.94 |

Solution Valve | 10.8 | +0.06 |

Quantity | Measured | Model | Error |
---|---|---|---|

Boiler | |||

Steam flow rate | 1.3 kg/s | 1.26 kg/s | 3.46% |

Cooling Tower | |||

Cooling tower water flow rate | 116.6 kg/s | 113.2 kg/s | 2.92% |

Absorption Chiller | |||

Temperature from absorber to condenser | 310.1 K | 310 K | 0.03% |

Chilled water flow rate | 81 kg/s | 81.1 kg/s | 0.12% |

Chilled water entering temperature | 285.9 K | 285 K | 0.31% |

Chilled water exiting temperature | 279.4 K | 279 K | 0.14% |

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**MDPI and ACS Style**

Lake, A.; Rezaie, B.; Beyerlein, S.
Use of Exergy Analysis to Quantify the Effect of Lithium Bromide Concentration in an Absorption Chiller. *Entropy* **2017**, *19*, 156.
https://doi.org/10.3390/e19040156

**AMA Style**

Lake A, Rezaie B, Beyerlein S.
Use of Exergy Analysis to Quantify the Effect of Lithium Bromide Concentration in an Absorption Chiller. *Entropy*. 2017; 19(4):156.
https://doi.org/10.3390/e19040156

**Chicago/Turabian Style**

Lake, Andrew, Behanz Rezaie, and Steven Beyerlein.
2017. "Use of Exergy Analysis to Quantify the Effect of Lithium Bromide Concentration in an Absorption Chiller" *Entropy* 19, no. 4: 156.
https://doi.org/10.3390/e19040156