1. Introduction
Climate separation zone usually means the transition region between two different climate zones. Miles of towering mountains stand and interdict the extending of one climate from one side of the mountain to the other side. Rich wind resources in these regions are available for further exploitation but never fully used. Moreover, natural wind, inside the extra-long tunnels located in climatic separation zone, shows more obvious effect on ventilation than in other regions [
1,
2] and its velocity in the tunnels can be 4–7 m/s [
3]. However, in the design phase, natural wind is assumed as resistance in the tunnel ventilation modes [
4]. Actually, the defect of the assumption is that natural wind sometimes can be assistance. If the traditional ventilation method is applied in the design of extra-long tunnel, the number of fans will be unreasonable. Consequently, traditional ventilation design is not suitable for tunnels in climatic separation zone. In these extra-long tunnels, the control strategy combining natural ventilation and mechanical ventilation together is of great significance.
According to our literature search, different from the abundant research of natural ventilation in building, the application of natural ventilation in long tunnels is rarely reported. Yoon et al. reported that the airflow caused by the natural ventilation pressure is large enough (up to 29.26% of mechanical ventilation) to increase the efficiency of the ventilation system in tunnels with shaft [
5]. Yan et al. analyzed the naturally ventilated urban vehicular tunnels with multiple roof openings and held the view that piston effect exists in such tunnels but is weak [
6]. Under traffic jam, thermal pressure is a main factor which influences inside flow field [
7]. Liu et al. simulated the piston effects in a subway system. He found that a larger sectional area will result in smaller air velocity in shaft and less effective air exchange between tunnel and outside ambient [
8].
Mechanical ventilation usually takes up huge space in tunnel for ventilation equipment and costs much for maintenance and electrical energy consumption in operation [
9]. In semi-transverse ventilation, the driving space is treated as an air delivery duct or air exhaust duct. If setting air delivery ducts and air exhaust ducts in full-transverse ventilation, no longitudinal wind flow happens inside the tunnel. Good ventilation results are obtained in extra-long tunnels after applying this ventilation mode, but the drawback is high construction costs due to large occupied space in the tunnel [
10]. Shafts or inclined shafts serve as air inlets or outlets in segmented longitudinal ventilation, the extra-long tunnel is divided into several segments [
11]. Construction expense will obviously decline compared to semi-transverse ventilation [
12].
How can sufficient airflow in tunnels be provided with minimum energy consumption? With the development of sensor technology, control strategies of intelligent ventilation were put forward [
13,
14]. Tunnel ventilation is optimized by controlling jet fans and dust collectors installed inside the tunnel [
15]. Jet fans blow polluted air outside the tunnel and sensors measure the contents of pollutants in the tunnel. By means of the approach, it was possible to reduce energy consumption while keeping the degree of pollution within allowable range. Modern control methods in tunnel ventilation include fuzzy control, neural networks, expert systems, etc. Several studies about the intelligent control on tunnel ventilation were reported [
16,
17,
18,
19,
20,
21,
22,
23,
24]. The development tendency of tunnel ventilation will be comprehensive, systematic, intelligent and energy-saving [
25].
The Nibashan tunnel is located in climatic separation zone of China. The law of natural wind velocity in the tunnel was studied in this paper. According to the meteorological parameters on both sides of the tunnel and the meteorological principles, the natural wind velocity inside the tunnel was calculated. Then, the idea of different ventilation modes employed in different periods was proposed. Based on the perennial monitoring, the change rules of natural wind in the tunnel were obtained. By adjusting the number of the working fans, the intelligent ventilation will obviously reduce construction investment and operation consumption.
3. Results and Discussion
The energy-saving network system applied in the Nibashan Tunnel, simple but effective, is similar to fuzzy logic control system. Researchers established a complete set of wind velocity calculation methods and changed the control strategy. For the fuzzy logic control system, the design wind velocity in the tunnel is determined by CO concentration generally. CO concentration is involved in many factors such as traffic volume, so the wind velocity in the tunnel will keep changing accordingly [
15,
32]. However, for the energy-saving network system in the paper, the design velocity in the tunnel is determined due to statistics of the meteorological data. The air quality in the tunnel can be guaranteed as long as the actual wind velocity in the tunnel is larger than the design velocity.
3.1. Design Velocity of Natural Wind
Chinese specifications for ventilation and lighting design of highway tunnel [
4] specify that the natural wind velocity can be determined by the meteorological data and the tunnel parameters. For most tunnels, the natural wind velocity is set as 2–3 m/s. As to the Zhongnan Mountains, there were three-day field tests for deciding the natural wind velocity [
33,
34]. However, at this was a short time for monitoring, the data are not abundant enough to reflect the wind velocity rules inside the tunnel, especially for tunnels located in the climate separation zones.
With the calculation correlation utilizing the natural wind velocity inside the Nibashan Tunnel, the annual patterns of the natural wind inside the tunnel were figured out (illustrated by the case of January). The main tunnel and shafts of the Nibashan Tunnel constitute a simple ventilation system.
Figure 6 shows how the wind velocity trend changes in the main tunnel on 30 December 2008. One may see that the trends in each section vary greatly. In the first section, the wind direction is negative and the wind velocity is large (the maximum velocity is 5 m/s). In the second and third section, the wind direction changes often and the wind velocity is relatively small. Therefore, the designed wind velocity of 2–3 m/s, which is recommended in the specification, does not agree with the wind velocity features of the Nibashan Tunnel.
In the case of the Nibashan Tunnel, set the section from Lugu portal to the exhaust shaft #1 as Section 1; the section from supply shaft #1 to the exhaust #2 as Section 2; and the section from supply shaft #2 to Ya’an portal as Section 3. With calculation, it can be known that the main natural wind direction is from Lugu to Ya’an. The annual probabilities of each section are 63.2% for the first section, 60.8% for the second section, and 61.5% for the third section. The situation in the inclined shafts is: blowing-in from the inclined shaft at Ya’an portal, with annual probability of 62.0%; and blowing-in from the inclined shaft at Lugu portal, with annual probability of 53.9%. The natural wind direction inside the tunnel is shown in
Figure 7.
According to the meteorological data in the whole year, it can be concluded that: the main wind direction in the first section is negative and at a velocity of 2.4 m/s; the main wind direction in the second section is positive and at a velocity of 2.4 m/s; and the main wind direction in the third section is positive and at a velocity of 1.2 m/s, as shown in
Figure A4.
The concept of wind velocity with guarantee rate is employed in the study. It refers to the wind velocity ensuring a certain probability when natural wind is considered as resistance. When natural wind velocity is below the velocity with the guarantee rate, the ventilation system can meet the requirements of operation. To ensure that there is larger probability for the ventilation system to meet the operational needs, the authors take 98% and 95% as guarantee rates of design wind velocity, as depicted in
Table 4.
3.2. Division of Control Period
Period control is about dividing the whole year into different control periods. According to the natural wind characters, researchers obtained the most unfavorable conditions within control period through calculation. The better the period division is, the more precise the period control is, and the higher the energy efficiency is.
3.2.1. Daily Control Strategy
Since there are large temperature differences between day and night, the control periods can be different between day and night. In line with the natural wind velocity vs. time diagram, time is split by two lines, namely the design wind velocity and wind velocity that are equal to zero. The period statistics is made depending on the split. The control period is determined as day (from 07:00 to 19:00) and night (from 19:00 to 07:00) in accordance with statistics results.
Figure 8 shows the daily control strategies in different sections of the left tunnel in January.
According to the natural wind control modes, the ventilation mode of the left tunnel in January is determined as shown in
Table 5.
3.2.2. Hourly Control Strategy
Depending on the distribution of natural wind throughout the year, the hourly control strategies can be different in control duration. One hour, two hours, three hours, four hours, five hours and six hours may be feasible duration for the control strategies. Taking the control strategy of one hour operating condition as an example, the ventilation mode of the left line of the Nibashan Tunnel is obtained by analyzing various periods of natural wind throughout the year.
Figure 9 shows the wind velocity variation trend and design wind velocity of the left line.
3.3. Energy-Saving Analysis
For most short road tunnels, only few fans are installed in the tunnels and the mechanical ventilation is not used as much as long tunnels. Thus, ventilation system occupies a small proportion (10–35%) of the total energy consumption compared to the lighting system [
35]. However, for extra-long road tunnels, the effect of natural wind on tunnel ventilation is rarely sufficient for sustaining good air quality without jet fans of large power. For example, the Nibashan Tunnel in Daxiang Range is 10 km long. The installed power of its ventilation system is 6500 kW. The electricity costs will be 3.06 million dollars annually if the system operates eight hours a day, which takes up 65% of its operation energy consumption (only 25% for illumination).
During the tunnel operation, the ventilation costs account for a large proportion of the overall tunnel operation costs. A reasonable fan control strategy, which takes full advantages of the natural wind dynamics, will lower the operation costs greatly [
36,
37]. The strategy of combining natural ventilation with mechanical ventilation was implemented in the Yuanliangshan Tunnel empirically, which has halved the number of jet fans in the tunnel but maintained the same ventilation effect [
38].
3.3.1. Energy Saving at Different Wind Velocity
Based on the traffic volume of the left line in the Nibashan Tunnel in 2010, the power of fans at different natural wind velocities was calculated. As the results listed in
Table 6 show, the increase of the natural wind velocity will remarkably increase the energy consumption. As the wind resistance correlates with the square of the wind velocity, the wind resistance will obviously increase with the wind velocity growth. Power of jet fans rises with the increase of wind resistance accordingly.
Considering the natural wind velocity and direction, there were favorable and unfavorable conditions. If the natural wind can be exploited as power properly, much energy will be saved. Based on the traffic volume of the Nibashan Tunnel in 2010, the power of fans was calculated when natural wind is at 2.5 m/s and at a velocity with guarantee rat of 95%. The results are shown in
Table 7.
From the results illustrated in
Table 7, the installed power calculated by the wind velocity with the guarantee rate of 95% is larger than that calculated by 2–3 m/s (as recommended in specifications). However, setting the design wind velocity at the velocity with a guarantee probability needs fewer operating jet fans. The ventilation effect is also better than traditional ventilation with the recommended wind velocity in specifications. Assuming the electricity price is 0.18 dollar per kWh, 0.35 million dollars will be saved in tunnel ventilation every year.
3.3.2. Energy Saving with Different Control Strategies
Division of control period is vital for ventilation energy saving in extra-long tunnels. The shorter the control period is, the closer the natural wind velocity will be to the actual condition. Based on the daily control strategy and the wind velocity with a guarantee rate of 95%, the power of fans and the corresponding energy saving effect are given in
Table 8. The energy saving effect in July and August is the most obvious, totally 410.5 kW is saved in July and August, while the energy saving in January and December is quite little. It can be explained by the natural wind distribution of the Nibashan Tunnel in these months. The wind resource in summer is richer than in winter. Thus, more natural wind contributes to the tunnel ventilation, and the energy saving effect is better.
The average power of jet fans in each period and condition, and the power of fans when wind is at 2.5 m/s are given in
Table 9 for comparison.
Table 9 shows that the effect of hourly control strategy is even better than the daily control strategy. If hourly control strategy were taken, it would lead to frequent change in the operational status of fans, eventually shortening the life span of jet fans.