Catalyzing Cooling Tower Efficiency: A Novel Energy Performance Indicator and Functional Unit including Climate and Cooling Demand Normalization
Abstract
:1. Introduction
1.1. Previous Literature and Indicators
Efficiency Indicator | Cost Definition (Effort) | Benefit Definition (Use) | Reference | |
---|---|---|---|---|
Energy Management | Energy Efficiency | input of energy | output of performance, service, goods, commodities, or energy | ISO 50001 [8] (p. 7) |
Best Available Techniques | Specific Direct Energy Consumption | energy consumed by all energy-consuming equipment | dissipated MWth | [18] (p. 4) |
Efficiency of Buildings | Specific Electricity Consumption | electricity consumption (kWhel) | thermal load (MWth) 2219 operation hours [h] | DIN V 18599-7 [19] (p. 64) |
EER | Energy Efficiency Ratio | ‘effective power input […] to the device’ (W) | ‘total cooling capacity’, ‘at any given set of rating conditions’ (W) | ISO 13253 [21] |
LCA Eco-Efficiency | Environmental Impacts Per Functional Unit | environmental impacts | functional unit: ‘1 kWh of electricity produced by the plant, […] referred to the operating period of 1 year’ | ISO 14040 [23] ISO 14044 [31], [32] (p. 1079) |
environmental impacts | ‘provision of 1-megawatt heat rejection (cooling) capacity (MWth) for a period of 1 year’ | [33] (p. 50) | ||
environmental impacts | ‘cooling of 1 kg water from 35 to 28 °C in Germany for the overall usage time’ | [34] (p. 140) | ||
environmental impacts | ‘cooling throughout 2019 with 2,450,000 m3 of circulating water cooled from 21.21 to 14.87 °C […] in Stuttgart-Vaihingen’ | [35] (p. 3) | ||
Exergy Analysis | Exergy Efficiency | = exergy output / exergy input = 1 − exergy destruction / exergy input | [24] (p. 190), [27] (p. 504) | |
= change in product exergy / change in supply exergy | [24] (p. 191) | |||
= (air exergy output + input) / (water exergy input − output) | [26] (p. 2797) |
1.2. Objectives of This Work
2. Materials and Methods
2.1. Efficiency Evaluation
2.1.1. The Quantified Benefit of Cooling Towers
- The cooling tower operates at pure counterflow.
- The air outlet temperature equals the coolant inlet temperature: .
- The cooling tower uses evaporative cooling.
- The outlet air is 100% saturated: .
- Nevertheless, the ambient wet-bulb temperature (WBT) must be less than the required cooling temperature, .
2.1.2. The Effectiveness Indicator
2.1.3. The Airflow Performance Indicator (AirPI)
2.2. System and Data
- Dry cooling towers;
- Wet cooling towers with open circuits (direct);
- Wet cooling towers with closed circuits (indirect);
- Hybrid cooling towers with direct wetting;
- Hybrid cooling towers with spraying devices;
- Hybrid cooling towers with wetting mats.
2.2.1. System Definition
2.2.2. Dataset of Cooling Tower Models
2.2.3. Case Study Data
3. Results and Discussion
3.1. Efficiency Evaluation of Different Cooling Tower Models
3.2. Efficiency Evaluation across Varying Environmental Conditions
3.3. Discussion on Indicator Feasibility
3.4. Implications for the SDGs
4. Conclusions
- Regarding the model dataset, the AirPI quantitatively confirms that wet cooling towers are more energy-efficient than dry cooling towers: dry cooling towers consume 2.3 kW/kg/s at the median, whereas wet cooling has a median of 1.0 kW/kg/s and hybrid cooling with wetting mats or spraying devices 1.3 and 1.8 kW/kg/s, respectively.
- Furthermore, the minimum airflow underscores the energy-saving potential of evaporative cooling. Regarding the median of dry cooling towers, approximately 7.8 times more airflow is theoretically needed if evaporative cooling is not implemented, directly correlating to the required fan power.
- The case study demonstrates that the indicator serves to determine the efficiency of cooling towers in operation instantaneously or integrated over time, for example, as seasonal AirPI. However, the operation differs significantly from the nominal heat transfer. Thus, we recommend assessing the effectiveness simultaneously. The effectiveness, quantified as the ratio between minimum airflow of actual to nominal benefit, highlights the necessity of additional cooling devices.
- Thanks to the normalization of outside conditions, the case study results are comparable to the dataset of 6575 cooling tower models in nominal operating points. The investigated wet cooling system turns out to be more efficient than the median of the manufacturer’s nominal data of wet cooling towers. However, at high ambient temperatures, the efficiency decreases, and the effectiveness is low. Further investigations must include the entire cooling and chilling system.
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Nomenclature
Abbreviations | |
COP | coefficient of performance |
DBT | dry-bulb temperature |
EER | energy efficiency ratio |
EnPI | energy performance indicator |
HLRS | High-Performance Computing Centre of the University of Stuttgart |
IQR | interquartile range |
LCA | life cycle assessment |
SDG | sustainable development goal |
WBT | wet-bulb temperature |
Symbols | |
AirPI | airflow performance indicator |
c | heat capacity in kJ/kg/K |
EnPI | energy performance indicator |
H | enthalpy in J; enthalpy flow rate in W |
h | enthalpy per mass in J/kg (enthalpy of humid air refers to the mass of dry air) |
m | mass in kg; mass flow rate in kg/s |
P | power in W |
p | pressure in Pa |
Q | heat in J; heat flow rate in W |
T | temperature in K |
V | volume in m³; volume flow rate in m³/s |
X | absolute humidity in kgw/kgda |
ϑ | temperature in °C |
φ | relative humidity in % |
η | efficiency in % |
… flow rate […/s] | |
Subscripts | |
‘’ | saturated |
0.25 | 25% quartile |
0.75 | 75% quartile |
a | air |
ct | cooling tower |
da | dry air |
el | related to electricity |
h | heating element |
ha | humid air |
i | input, inlet |
min | minimum |
n | nominal |
o | output, outlet |
p | pump or isobaric |
r | real, actual |
th | thermal |
v | vapor |
w | water or coolant |
wt | water treatment |
Appendix A
Type | Description |
---|---|
dry cooling towers | conductive heat transfer to dissipate heat to the surrounding air with a heat exchanger and forced ventilation |
wet cooling towers with open circuit | heat dissipation by evaporation as the coolant trickles down fillers in direct contact with the ambient air |
wet cooling towers with closed circuit | evaporative heat transfer but with a coolant-air heat exchanger and a separate circulating coolant system where the coolant trickles down to facilitate evaporation |
hybrid cooling towers with direct wetting | hybrid cooling is a combination of dry and indirect wet cooling methods with seasonal shift, while directly wetted hybrid cooling towers involves a wetting circuit comprising an auxiliary water pump, water trickling down the heat exchanger, and a water collection tank |
hybrid cooling towers with spraying device | the spraying system saturates the ambient air to enhance subsequent cooling in the heat exchanger |
hybrid cooling towers with wetting mats | wetting-mat systems facilitate the water distribution by additional water trickling down the mats, enabling evaporation into the ambient air that subsequently passes the heat exchanger |
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Wenzel, P.M.; Fensterle, E.; Radgen, P. Catalyzing Cooling Tower Efficiency: A Novel Energy Performance Indicator and Functional Unit including Climate and Cooling Demand Normalization. Sustainability 2023, 15, 15454. https://doi.org/10.3390/su152115454
Wenzel PM, Fensterle E, Radgen P. Catalyzing Cooling Tower Efficiency: A Novel Energy Performance Indicator and Functional Unit including Climate and Cooling Demand Normalization. Sustainability. 2023; 15(21):15454. https://doi.org/10.3390/su152115454
Chicago/Turabian StyleWenzel, Paula M., Eva Fensterle, and Peter Radgen. 2023. "Catalyzing Cooling Tower Efficiency: A Novel Energy Performance Indicator and Functional Unit including Climate and Cooling Demand Normalization" Sustainability 15, no. 21: 15454. https://doi.org/10.3390/su152115454