Study of water-side free cooling technology for power lithium battery plant

Power lithium battery production process needs to consume a lot of energy. In this paper, an energy consumption analysis of a representative company was conducted, the energy consumption of the facilities that provide support for the process is higher than the energy consumption of the process equipment itself, which greatly affects the cost of battery manufacturing. Air-conditioning system as the most important energy-consuming system of the facilities, accounting for 25% to 45% of the total electricity consumption. The energy consumption of the cooling source system dominates, even in winter its energy consumption can reach 14% of the total energy consumption. On this basis, this paper analyzes the chilled-water-use characteristics of the air-conditioning terminal in winter through theoretical analysis, the chilled water temperature is therefore optimized according to the real-time outdoor humidity. The chilled water temperature is linearly increased after the humidity ratio fall below 7.5g/kg(a), reaching a maximum value of 12.1°C at 6g/kg(a). In addition, the cooling tower free cooling technology based on the optimized chilled water temperature is innovatively proposed to fully explore the energy-saving potential of the technology to achieve the energy saving of the cooling source system. TRNSYS simulation results show that after optimizing the chilled water temperature, the time period for cooling tower individual cooling under winter operating condition is extended by 60%, and cooling source system energy saving rate can reach 49.0% and the cost saving rate 48.0%, both of which are higher than 15% compared to the un-optimized chilled water temperature.


Introduction
Lithium battery is characterized by high energy and power density and light weight [1,2], therefore, it is the first choice for electric vehicle power supply [3].The shipments of power lithium batteries have been growing at a high rate in the past decade, and are expected to reach 1690GWh by 2025 [4].
Lithium battery manufacturing cost can be divided into material cost and process cost [5], of which process costs account for 24%.Each process production link is accompanied by huge energy consumption, which can be divided into material upstream energy consumption, production process energy consumption and shell manufacturing energy consumption, accounting for 38%, 38% and 24%, respectively [6].Among them, the process production energy consumption corresponds to the energy consumed in the battery production plant, which can be further divided into the process energy consumption consumed by each process machine and the energy consumption of plant facilities [7].Because the battery material is easy to absorb moisture, the workshop needs to strictly control the environmental humidity, which leads to high energy consumption of air-conditioning system [8].Existing energy-saving studies have focused on rotary dehumidifier, which is the key equipment for creating a low-humidity environment in lithium production workshop [9,10], while ignore the energy-saving improvement for the cooling source system.This paper will propose an innovative application of a traditional energy-saving measure for the cooling source system to achieve energy saving.
This paper firstly introduces the main process of lithium battery production and the corresponding humidity requirement; then analyzes the energy consumption of the project and the chilled-water-use characteristics of terminals in winter.Due to the lower humidity in winter, the chilled water with too low temperature will exacerbate the cooling and heating offset at the rotor; through analyzing the dehumidification characteristics of the rotor and derives the optimized chilled water temperature according to the real-time fresh air humidity.Finally, free cooling technology based on optimized chilled water temperature is proposed and validates the energy-saving performance of the technology based on the modeling of TRNSYS.

Refrigeration system
The project is located in the hot-summer and cold-winter area, China.The main workshop includes pole manufacturing workshop, former process workshop, pole winding workshop, post process workshop, capacity testing workshop, power station and finished product warehouse, with a total of 14 sets of pole production lines, 6 cell production lines, and a planned production capacity of 20GWh per year.There are two sets of chilled water systems, which are installed in the power station and the finished product warehouse.There are 2 low-temperature units and 7 medium-temperature units in power station; and there are 2 medium-temperature units and 2 high-temperature units in finished product warehouse.Figure 1 provides an overview of the chilled water system.

The production process and the corresponding environmental requirement
The overall production can be divided into three processes: pole manufacturing, cell synthesizing, and capacity testing.Ternary lithium battery need to strictly control the humidity in the production environment.The specific process and humidity requirements are shown in Figure 2.

Pole Manufacturing
The raw and auxiliary materials used in the project are all purchased.In order to avoid mutual contamination of cathode and anode materials, the raw material storage and the pole manufacture are separated into individual room.The main processes are: mixing, coating, rolling and slitting.

Cell Synthesizing
It is essentially an assembly process to complete the manufacturing of the cell, specifically the orderly assembly of the poles, with the separator and electrolyte.The main processes are: winding, electrolyte filling, welding, etc.

Capacity testing
After the completion of each process, the battery cells are tested for capacity and K value (voltage drop per unit time).This process can activate the battery to form a finished product that is safe to use and stable in performance.The main processes are: capacity grading, testing and so on.

Overall energy use status
Production process energy consumption of electricity, natural gas and water.Electricity consumption is divided into production electricity used directly by each process machines and facility electricity consumed by facility systems; natural gas is used for steam boiler to produce regenerative heat source for dehumidification rotor and heat transfer oil boiler to provide heat source for coating process; water is used to produce pure water and provide cooling make-up water.Monthly natural gas consumption ranges from 0.95Mm³ to 2.3Mm³ , monthly electricity consumption from 9.2MkWh to 22.1MkWh, and the fluctuation of the trend of gas and electricity consumption is more synchronized, both in November and July to achieve maximum value, while January achieve minimum value due to the holiday shutdown.Monthly water consumption is between 33000m³ ~79000m³ .Overall facility electricity use is higher than production process, and facility electricity use corresponding to unit production electricity use shows high in winter and low in summer.As in Figure 3(c), The power station and finished product warehouse produce chilled water, so the energy consumption ratio is high in summer and low in winter.The capacity testing workshop has an energy consumption ratio higher than 10% except for the commissioning months.The ratio and fluctuation of energy consumption in the former and post process workshop are similar, ranging from 10% to 25%.The ratio of the pole winding and pole manufacturing is lower than 10% monthly.

Facility Electricity Consumption Split
The five main energy-consuming facility systems are: air-conditioning, compressed air, thermal oil boiler, vacuum, and utility.Figure 3(d) illustrates the percentage of electricity.Air-conditioning system as the main facility energy-consuming system, monthly accounted for more than 55% of the total electricity consumption, in the winter due to the cooling load and cooling water temperature is low, accounting for a relatively low percentage of 55% to 60%, while in May to September accounted for more than 70%; compressed air system to provide air source for automated production lines, and produce nitrogen, as the second largest energy-consuming facility system accounted for 10% to 21%; the utility, heat transfer oil boiler, the vacuum accounted for less than 10% each month.

Energy consumption of air-conditioning system
The air-conditioning system can be categorized into cooling source and terminal air-conditioning unit.Except for the initial two commissioning months, the energy consumption of terminal unit is strongly correlated with the production consumption, the ratio of electricity consumption is basically stable at 0.2~0.3; the energy consumption of cooling source is affected by the cooling load and the outside temperature, and the ratio is more than 0.6 from April to November, and less than 0.4 from December to March.The air-conditioning system consumption accounts for up to 45% of the overall total electricity consumption of the project (June 2023).Therefore, the energy saving of cooling source system can be a key point to enhance the competitiveness of the product.

Research on the application of cooling tower free cooling technology
This section introduces the winter operating situation of air-conditioning system, theoretically analyzes the chilled-water-use characteristics of terminal equipment, optimizes the chilled water supply temperature, and explores energy-saving technology based on the optimized temperature.

Analysis of the current situation of winter cooling and equipment performance
The main supply terminal equipment for chilled water in the power station includes rotary dehumidifier (DHU double rotor/ SDHU single rotor), make-up air unit MAU, fan coiler FCU, process cooling water PCW, and air-handling unit AHU.
Chilled water is supplied throughout the day for production needs.In summer, the chilled water system is divided into low-temperature (3℃ supply) and medium-temperature (6℃ supply), and the low-temperature water is only supplied to the front and rear surface coolers of the DHU/SDHU in the winding workshop.In winter, the medium-and low-temperature chilled water pipelines are combined.The chilled water temperature is 10℃ when the humidity ratio of the fresh air is lower than 7g/kg(a); and the temperature is 6℃ when the humidity ratio of the fresh air is higher than 7g/kg(a).Neither the FCU nor the MAU operates in winter, i.e., the chilled water is only for DHU and PCW in winter.
Firstly, select the winding workshop DHU-WP2-04 operation data for analysis, time granularity of 5 minutes, screening time period in which fresh air humidity ratio is lower than 7g/kg(a).So fresh air surface cooler only plays a role in cooling but not dehumidifying during this period.Meanwhile, the air supply dew temperature set point, the air valve and fan frequency have not been adjusted during this period.Spearman's rank correlation test is performed on the parameters of fresh air humidity ratio and temperature, front rotor inlet air temperature and regeneration air temperature that may affect the supply air dew point and heat map is drawn as in Figure 5. From Figure 5 can be further deduced: when the fresh air humidity ratio below a certain value, the surface cooler only cools the new air, so that the lower rotor inlet air temperature strengthens the cooling and heating offset at the rotor, the rotor regeneration temperature instead of a certain degree of increase (weak negative correlation between rotor inlet air temperature and front rotor regeneration air temperature), but there is not a clear reduction in supply air dew temperature (inlet air temperature and the supply air dew temperature is not correlated).This indicates that the lower humidity ratio of fresh air in winter, to the extent that supply air dew temperature can be met, chilled water should be as high as possible to avoid the rotor inlet air temperature is too low to exacerbate the offset phenomenon.
The use of cooling tower free cooling can improve the offset phenomenon and reduce energy consumption of the refrigeration system, that is, in the case of low fresh air humidity ratio, dehumidifier demand for chilled water supply temperature is higher, and at this time the low fresh air humidity results in lower outlet water from cooling tower, which can be matched with the higher chilled water temperature.Therefore, the use of cooling tower in winter not only saves energy consumption of refrigeration system, but also alleviates the cooling and heating offset.

Cooling tower performance analysis
Figure 6 shows the thermal characteristic curve of the cooling tower.In order to make the same outlet temperature, a lower flow rate of inlet water can be realized at a higher wet bulb temperature; and in the case of the same amount flow rate of the inlet water, the smaller the cooling water temperature difference corresponds to a higher wet bulb temperature.Therefore, the application of smaller inlet flow rate and cooling water temperature differentials allows the cooling tower to operate at higher wet bulb temperatures, extending the free cooling hours [11].

PCW Performance analysis
PCW is set up in the former process and pole manufacturing workshop for rolling process cooling and solvent recovery.The secondary side water supply temperature is controlled by the opening of the primary side return water regulating valve.PCW parameters are shown in Table 1.In order to meet the cooling demand, there must exist an upper temperature limit for the inlet chilled water, and the following uses the ε-NTU method [12] to calculate this temperature limitation.
From equation (1), the former process and pole manufacturing heat exchanger(HX) effectiveness ε are 0.52 and 0.51, respectively, then the upper limit of the chilled water temperature can be obtained from equation (2).
The upper limit chilled water temperature for the PCW are 12.1°C and 12.2°C of the former process workshop and the pole manufacturing workshop, respectively, resulting in the upper chilled water temperature limit for PCW being 12.1°C.

Rotary dehumidifier performance analysis
Correlation analysis shows that, under the premise of meeting the supply air humidity requirement, to increase the rotor inlet air temperature is energy-saving.Dehumidifier regeneration temperature need to set the upper and lower limits, and front and rear rotor regeneration temperature upper limit is 80℃ in winter.The latter analysis is based on these two premises.
Dehumidifier is divided into fresh-air, mixed-air and return-air type.Fresh-air type DHU dehumidification load is the largest in the same amount of supply air volume.Due to the lower load in winter, fresh-air type DHU individually carries the room load, so only need to count in the fresh-air type DHU.From 2022.12.27 to 2023.1.17supply air daily average dew temperature of each room fresh-air type DHU is shown in Figure 7.It can be seen that the anode rolling room demand for supply air dew temperature is the lowest, -11℃, corresponding humidity ratio is 1.46g/kg(a), that the dehumidifier dehumidification demand is considered to be the highest, that is, the chilled water demand temperature is the lowest, Figure 8 demonstrates the corresponding air handling flow chart.Where T is the temperature in K;  is the humidity ratio in kg(H2O)/kg(a).
Schultz [14] proposed two effectiveness values ε1/ε2 to correcting model for non-ideal cases.
In Table 2, actual operating conditions were selected to verify the model accuracy, and all the selected conditions were maintained for at least 30 minutes.The errors of T and are all controlled within 10% to support the subsequent calculations.The rotor dehumidification characteristics are derived using the isopotential line model as shown in Figure 10.The dehumidifier front and rear rotor is the same, then the required supply air humidity can be deduced from the fresh air humidity.The rotor inlet air temperature corresponding to each humidity ratio can be obtained by making a reference line in Figure 10, as shown in Table 3.
The equation for εdry is Where, εdry is related to heat transfer coefficient U, the area Adry, the specific heat of the air flowing Cpa and the specific heat of the water flowing Cpw.Therefore, the surface cooler in the fixed air/water flowrate, εdry is identical.According to the actual dry working condition of the surface cooler, εdry=0.96.
Surface cooler air temperature and water temperature to meet the equation ( 9).
, , , , () In Figure 11, the optimized rotor inlet air temperature is the upper limit of the rotor inlet air temperature corresponding to a certain air humidity ratio, substituting into the equation ( 9) to obtain the required chilled water temperature under the theoretical highest fresh air temperature situation, i.e., the upper limit of the fresh air surface cooler water temperature, as shown in Table 4.
Figure 11.Outdoor air status and rotor inlet air temperature.As shown in Figure 11, when the humidity ratio is lower than 4.14g/kg(a), the fresh air temperature is lower than the optimized rotor inlet air temperature, which indicates that the fresh air does not need to be cooled to reach supply air dew temperature.And at the point (7.5g/kg(a), 10.1℃), the air relative humidity has reached the apparatus dew point, indicating that when the fresh air humidity ratio is higher than 7.5g/kg(a), the surface cooler needs to dehumidify so that supply air humidity requirement 1.46g/kg(a) can be achieved.When the fresh air humidity ratio is between 7.5 and 4.14g/kg(a), the rotor inlet air should be cooled to the optimized inlet air temperature at least.No cooling required Middle surface cooler needs to cool the air warmed up by the front rotor to the upper limit of the rotor inlet air temperature, and the effectiveness of the middle surface cooler εdry=0.87,and then obtain the upper limit of middle surface cooler chilled water temperature as Table 5. 28.4 16.5 In summary, there is a mapping relationship between the upper limit of the chilled water temperature and the fresh air humidity ratio, as shown in Figure 12.The fresh air humidity ratio is low so that the dehumidifier demand for chilled water temperature shows a certain rise.

Control strategies and modeling of cooling tower free cooling technology
The main control strategy includes cooling mode switching control, number of chiller control, number of cooling tower control and cooling tower outlet temperature control.The low-temperature units need to be activated when high temperatures occur in the workshop, so the low-temperature refrigeration units are not involved in technology application.Figure 13 shows the schematic, and the main equipment of the medium-temperature units is shown in Table 6.

Cooling mode switching control
There are three cooling modes: cooling tower individual cooling, combined cooling, and chiller individual cooling.According to the HX cooling tower outlet water temperature Thxcts, terminal return water temperature Ttr, HX secondary side outlet water temperature Thxs and chilled water supply temperature Tts to determine the cooling mode.The temperature difference between the primary side inlet water and the secondary side outlet water of the heat exchange is taken as 1.5℃.

Number of chiller control
When cooling mode is cooling tower individual cooling the number of chiller in operation Nch=0。 When the cooling mode is combined cooling or chiller individual cooling, Nch is calculated from equation (10).

Number of cooling tower control
Cooling tower for chiller, as originally controlled, with a group of 3 cooling towers per chiller that is Nchct=3Nch.
Cooling tower for HX need to limit the flowrate into the tower and the temperature difference between the water in and out, limit the former is to prevent freezing when the temperature is too low, this paper limits the cooling tower flowrate of 50% design flow rate; limit the latter because the smaller the cooling water temperature difference the longer the free cooling time, but it leads to an increase in the energy consumption of the cooling water pumps and a decrease in the overall energy efficiency.Adopting a cooling tower temperature difference of 2~3℃ is energy efficient [15], therefore, the temperature difference hxct T  taken as 3℃ when calculating the required number of units.FLOOR function represents downward rounding to a multiple of 3, that is, to obtain the number of cooling tower enabled units, calculated as equation (11).
In summary, the number of cooling tower controller will output 3Nch+Nhxct when the cooling mode is cooling tower individual cooling or combined cooling.The cooling tower for chiller, the lower the cooling water outlet temperature the higher the COP of the chiller, but in order to ensure a certain amount of cooling capacity, need to limit the condensing pressure, i.e., the cooling water outlet temperature can not be too low.Under the requirements of the chiller manufacturer, the chiller can be utilized as a minimum condenser water inlet of 18 ℃.
The cooling tower for HX, following the principle of cooling tower priority cooling, should increase the fan speed as much as possible to maximize the use of natural cooling source.However, when the outdoor wet bulb temperature is very low and the cooling mode is cooling tower individual cooling, the water temperature controller needs to control the RFS according to the terminal demand.
In summary, each key control logic and TRNSYS simulation model are obtained as follows.

Analysis of energy-saving potential
This paper adopts the ratio of free cooling RFC to evaluate the energy saving effect of free cooling technology, which is more accurate compared to the free cooling hours.
Where QFC is the cooling capacity provided by the cooling tower, Qt is the total terminal cooling load.
Based on this index, this paper introduces the evaluation period, according to the DHU winter operating condition to determine the evaluation period from December 1 to March 15(the following year), the average RFC is obtained by integrating and averaging over time.
Firstly, adjust the cooling mode switching controller so that the cooling mode is always chiller individual cooling, and obtain the simulated energy consumption of the power station chilled water system in the evaluation period, as shown in Table 7.The overall energy consumption of the model deviates slightly from the actual, indicating that the simulation platform is able to accurately identify the system operating characteristics.Base on the TRNSYS model, simulate and compare the energy consumption difference of the free cooling technology before and after chilled water temperature optimization, as shown in Figure 15.The enhancement of cooling tower individual cooling hours is significantly stronger than the enhancement of overall free cooling hours, and the former have been extended by 40%~110%, which indicates that the optimized chilled water temperature has converted part of the combined cooling hours into individual cooling hours, resulting in the significant improvement of the RFC.The enhancement of individual cooling hours is the largest in March therefore the RFC enhancement rate was up to 49%, and optimizing the water supply temperature Optimization of water supply temperature achieves the expected effect.The lower temperature in January made the RFC reach the highest value of 89%, and the total free cooling hours reached 706h; in February, due to the long precipitation period, the fresh air humidity ratio was too high, which made the proportion of individual cooling reduced, and the RFC decreased; in the initial 15 days of March, the RFC was lower than that of January, but due to the higher terminal load, the energy saving rate and the cost saving rate reached the highest values of 52.8% and 51.6%.Compared with the system without free cooling, the technology can achieve energy saving rate and cost saving rate of 25%~34% in each month of winter, and after optimizing the chilled water temperature is the two saving rates are increased by more than 15%.Overall, the cooling tower free cooling technology with optimized chilled water temperature saved 49.0% of the refrigeration system energy consumption during the evaluation period, corresponding to an electricity saving of 1.689 million kWh and a cost saving of 854 thousand yuan.

Conclusion
This paper focuses on the winter chilled-water-use characteristics of lithium battery plant, optimizes the chilled water temperature according to the fresh air humidity ratio, and introduces the cooling tower free cooling technology to further exploit the energy-saving potential based on the optimized chilled water temperature.The main conclusions are as follows.
1) Lithium plant air-conditioning system is the largest proportion of energy-consuming facility systems, monthly electricity consumption accounted for more than 55% of the facility consumption, and accounting for the highest proportion of total electricity consumption up to 50%.The energy consumption of the cooling source is dominant, except in January, the other months are higher than the terminal air conditioning unit, which is even twice as high in the summer months, so the application of the cooling source energy-saving technologies for the manufacture cost is very critical.
2) In the lithium manufacture project application of free cooling technology should be necessary to optimize the chilled water temperature, in order to maximize the use of natural cooling source.Winter fresh air humidity ratio is lower, on the one hand, the DHU dehumidification load decreases, during when chilled water temperature should be appropriate upward adjustment; on the other hand, making the cooling tower outlet water temperature lower.Therefore, using the cooling tower as the cooling source in winter becomes practical and advantageous.According to the former analysis, when the humidity ratio is below 7.5g/kg(a), rotor dehumidifier dehumidification needs is reduce so that the chilled water temperature can be gradually adjusted upward, and the maximum value of 12.1℃ is achieved in 6g/kg(a); when the humidity ratio below 6g/kg(a), due to the need to ensure that the PCW cooling needs, the chilled water temperature can not be further increased to maintain at 12.1℃.Different from the fixed chilled water temperature, according to the humidity ratio optimization of chilled water temperature, can increase the cooling tower individual cooling time 60%, and achieve the cooling system for higher than 45% of the energy-saving rate and cost-saving rate during winter condition.

Figure 1 .
Figure 1.Axonometric drawing of the chilled water system.

Figure 2 .
Figure 2. Lithium battery production process and corresponding humidity requirements.

Figure 3 (
a)(b) shows the overall energy use of the project from August 2022 to July 2023.

Figure 3 .
Figure 3. Overall energy consumption of the project.Monthly natural gas consumption ranges from 0.95Mm³ to 2.3Mm³ , monthly electricity consumption from 9.2MkWh to 22.1MkWh, and the fluctuation of the trend of gas and electricity consumption is more synchronized, both in November and July to achieve maximum value, while

Figure 4 .
Figure 4. Electricity consumption relationship between air-conditioning system and production.

Figure 5 .
Figure 5. Correlation coefficients between winter operating parameters of DHU-WP2-04.From Figure5can be further deduced: when the fresh air humidity ratio below a certain value, the surface cooler only cools the new air, so that the lower rotor inlet air temperature strengthens the cooling and heating offset at the rotor, the rotor regeneration temperature instead of a certain degree of increase (weak negative correlation between rotor inlet air temperature and front rotor regeneration air temperature), but there is not a clear reduction in supply air dew temperature (inlet air temperature and the supply air dew temperature is not correlated).This indicates that the lower humidity ratio of fresh air in winter, to the extent that supply air dew temperature can be met, chilled water should be as high as possible to avoid the rotor inlet air temperature is too low to exacerbate the offset phenomenon.The use of cooling tower free cooling can improve the offset phenomenon and reduce energy consumption of the refrigeration system, that is, in the case of low fresh air humidity ratio, dehumidifier demand for chilled water supply temperature is higher, and at this time the low fresh air humidity results in lower outlet water from cooling tower, which can be matched with the higher chilled water temperature.Therefore, the use of cooling tower in winter not only saves energy consumption of refrigeration system, but also alleviates the cooling and heating offset.

Figure 6 .
Figure 6.Thermal characteristic curve of cross-flow cooling tower.

Figure 7 .
Figure 7. Supply air dew temperature for different rooms.It can be seen that the anode rolling room demand for supply air dew temperature is the lowest, -11℃, corresponding humidity ratio is 1.46g/kg(a), that the dehumidifier dehumidification demand is considered to be the highest, that is, the chilled water demand temperature is the lowest, Figure8demonstrates the corresponding air handling flow chart.

Figure 8 .
Figure 8. DHU-GY-01 air handling flow chart.Jurinak[13] developed isopotential line model for silicone rotors to analyze dehumidification characteristics.The air state points are solved using isopotential lines F1 and F2 as in equation (3).

Figure 10 .
Figure 10.Characteristic curve of rotary dehumidification.The dehumidifier front and rear rotor is the same, then the required supply air humidity can be deduced from the fresh air humidity.The rotor inlet air temperature corresponding to each humidity ratio can be obtained by making a reference line in Figure10, as shown in Table3.

Figure 12 .
Figure 12.Optimized chilled water temperature and rotor inlet air temperature.

4 .
Cooling tower outlet temperature controlCooling tower outlet water temperature control are realized by PID, by controlling the relative fan speed RFS so that the cooling tower outlet temperature meet the requirement.

Figure 15 .
Figure 15.Analysis of the energy saving potential of free cooling technology.The enhancement of cooling tower individual cooling hours is significantly stronger than the enhancement of overall free cooling hours, and the former have been extended by 40%~110%, which indicates that the optimized chilled water temperature has converted part of the combined cooling hours into individual cooling hours, resulting in the significant improvement of the RFC.The enhancement of individual cooling hours is the largest in March therefore the RFC enhancement rate was up to 49%, and optimizing the water supply temperature Optimization of water supply temperature achieves the expected effect.The lower temperature in January made the RFC reach the highest value of 89%, and the total free cooling hours reached 706h; in February, due to the long precipitation period, the fresh air humidity ratio was too high, which made the proportion of individual cooling reduced, and the RFC decreased; in the initial 15 days of March, the RFC was lower than that of January, but due to the higher terminal load, the energy saving rate and the cost saving rate reached the highest values of 52.8% and 51.6%.Compared with the system without free cooling, the technology can achieve energy saving rate and cost saving rate of 25%~34% in each month of winter, and after optimizing the chilled water temperature is the two saving rates are increased by more than 15%.Overall, the cooling tower free cooling technology with optimized chilled water temperature saved 49.0% of the refrigeration system energy consumption during the evaluation period, corresponding to an electricity saving of 1.689 million kWh and a cost saving of 854 thousand yuan.

Table 3 .
Upper limit inlet air temperature humidity ratio.

Table 4 .
Optimized rotor inlet air temperature and upper limit of fresh air surface cooler water temperature.

Table 5 .
Upper limit of middle surface cooler water temperature.

Table 6 .
Main equipment of the medium-temperature unit parameters.Equipment type and parameters Rated input power/kW Chiller, rated cooling capacity is 10200kW 1937 Chilled water pump, rated flowrate is 1930m³ /h, head is 55.5m 400 Cooling water pump, rated flowrate is 2280m³ /h, head is 25m 200 Cooling tower, rated flowrate is 820m³ /h 30

Table 7 .
Simulated and actual energy consumption.