Introduction
The quality of life is becoming more important in modern society, which requires a comfortable indoor environment. In order to provide a comfortable indoor environment instead of a bad outdoor environment, relying on mechanical equipment means that large amounts of energy and resources will be consumed, and the emission of CO2 will further deteriorate the outdoor environment. It is harmful to sustainable development. A comfortable indoor environment is not only associated with natural resources themselves but also relates to the way natural resources are used. In fact, the proper factors regulating the outdoor environment under certain climatic conditions will largely satisfy the requirements of a comfortable indoor environment. This paper describes the distribution of natural resources and the application potentials in Chongqing, China.
Potential analysis of natural cold and hot resources in Chongqing
Located in the southwest Sichuan Basin, Chongqing is the transition zone between China’s Qingzang Plateau and the Yangtze River Plain. The terrain increases gradually from west to east, and the north-south direction is towards the Yangtze River valley. Mountain is the main feature of the city, which accounts for 63.3% of the whole area; hill accounts for 25.3% and plain for about 11.4%. The average temperature annually is 16-18°C; the average temperature in the hottest month is 268°C.-29°C and in the coldest month 48°C.-8°C. The average relative humidity is between 70% and 80% for each year, which belongings to high humid areas in China. The sunshine hours are 1000-1400 per year and the percentage of sunshine is merely 25-35%. Chongqing ranks among the least sunshine areas in the country, with sunshine only 35% in winter and spring. However, the summer sunshine is relatively strong, especially in July and August when it reaches about 40% and 50%, respectively. Total solar irradiance is 944.4 MJ/m
2 in July and August, accounting for 30.88% of one year. Chongqing’s major climatic feature can be summarized as hot summer, cold winter, warm spring, cool autumn, long frost-free period, weak solar radiation, and abundant rain. Figure 1 shows the average dry bulb temperature and average relative humidity per month. The variations of dry bulb temperature in the hottest month and the coldest month are also shown in Fig. 1 [
1].
From Fig. 1 we can conclude that Chongqing is in the typical hot summer and cold winter zone. High temperatures in summer last for about four months, and the period of low temperature in winter is more than two months. The annual relative humidity is high, and the relative humidity of an average month is higher than 70%. This is one of the main causes of cold winter and hot summer in Chongqing
It can be confirmed from Fig. 2 that the distribution of total solar radiation is uneven in Chongqing; there is weak solar radiation in winter, less than 100 MJ/m
2, but strong solar radiation in summer and the maximum value is beyond 500 MJ/m
2 [
1]. These distributions of solar energy pose difficulty on sun access in winter and solar radiation prevention in summer.
Based on the annual analysis of meteorological parameters [
1], we can gain the Wind Rose map of Chongqing as shown in Fig. 3. Annual wind direction is mainly from north to northwest, and annual wind speed is a relatively low speed, about 99.04% lower than 5 m/s in one year; the speed of 2-4 m/s accounts for 64.15% and the speed of lower than 1 m/s accounts for 34.88%. According to the architectural criteria [
2], Chongqing is in a perennial gentle breeze zone and it is necessary to intensify the use of wind potential.
The temperatures measured at different depths of underground in Chongqing are shown in Fig. 4, and the distribution of temperatures in depths of 0 m, -5 m, -10 m, and -15 m of one year in Chongqing are shown in Fig. 5. From Figs. 4 and 5, we can conclude that the temperature fluctuation is evidently on the surface, and the highest temperature is 63°C, the lowest temperature 32.7°C. With the increase of depth, however, underground temperature fluctuations become very small. When the depth is up to -5 m, the highest is 22.2°C and the lowest is 12.1°C, and there exists a remarkable time delay between different underground depths. When the underground depth is up to -15 m, the temperature fluctuations are basically non-existent, and its temperature distribution is between 19.9°C and 20.1°C. According to the temperature calculated at different depths, if the depth continues to be increased, the underground temperature has a rising trend. The temperature at -40 m is 20.8°C and at 80 m about 22.2°C. According to calculation, if air-conditioning condensing temperature decreases by 1°C in summer the energy consumption decreases by 1%-2%, and if the evaporation temperature rises by 1°C in winter the energy consumption decreases by 1%-2%. This shows that there is huge energy efficiency for condensation in summer and evaporation in winter when the air-conditioning systems use ground source as a heat sink.
In addition, as Chongqing is located in the intersection of Yangtze River and the Jialing River, it has many geographical advantages and the temperature variation in river is small. The water temperature is about 22°C-26°C in summer and 12°C-16°C in winter. Based on the above analysis, rationally using the river in the air-conditioning heat exchange system has enormous energy-saving and environmental benefits.
Analysis of thermal comfort zone in Chongqing
According to Chongqing Municipal Meteorological statistics, temperature and humidity data at 12:00 of one year were selected and the number of data was 4380. These data were distributed in an enthalpy-humidity chart, as illustrated in Fig. 6. The blue area is the comfortable area to meet the requirements of the human body. According to statistical analysis, we can see that the dry bulb temperature below 12°C has 1215 data points, which accounts for 27.74% of the total number and the dry bulb temperature higher than 28°C has 517 data points, which accounts for 11.8%. The humidity higher than 80 has 2567 data points, which accounts for 58.61% of the total number. As can be seen from the chart, the municipal meteorological parameters of Chongqing are mostly out of the blue box, but the relative humidity is better than the temperature, and more humidity data are in the blue box. If the temperature is regarded as the limit, the data of dry bulb temperatures lower than 12°C and higher than 28°C account for less than 40%, but the data of relative humidity which are not in the limits account for more than 58%. Thus, we can conclude that the impact of relative humidity on indoor thermal comfort is mainly followed by dry bulb temperature. Therefore, in order to make the data point into the blue box, we should increase the dry bulb temperature in winter and change the relative humidity appropriately. From the data distribution in the chart, we can see that some points with high humidity and low temperature in winter result in the cold weather in Chongqing. In the transition seasons, the main problem is to reduce the relative humidity; for summer, it is important to balance the temperature and humidity. As can be seen from the chart, in summer not only the temperature but also some relative humidity cannot meet the comfortable requirement. So the key problem in hot summer is to deal with the high temperature and humidity in Chongqing. If natural resources can be made of full use, indoor thermal environment can be improved and building energy consumption can be reduced.
Role of natural resources in improving indoor thermal environment
In order to illustrate the role of natural resources in the indoor environment, a building with eight stories in Chongqing was analyzed, as shown in Fig. 7.
CFD was used in the analysis of wind speed distribution combined in the Wind Rose in Fig. 3. The boundary conditions of wind in the simulation were set as follows: the wind direction is northwest and the wind speed is 2 m/s. Two directions of building were considered: east to west and southeast to northwest. In each direction of building, two conditions of window and door were also considered: window opened and door closed, and window opened and door opened. The simulation results are shown in Fig. 8 according to the simulation.
Comparing Figs. 8(a) to (b), the indoor environment was more significantly affected by the outdoor wind in the direction of northwest to southeast than east to west with 45° to the wind direction. The same conclusion can be made from the comparison between Figs. 8(c) and (d). Comparing Figs. 8(c) and (d) with Figs. 8(a) and (b), we can conclude that the wind flow was better in internal building under the condition of door opened and the direction of building paralleled to wind direction.
Through building wind field analysis, we can come to the preliminary layout position results to enhance the indoor environment for the construction of natural ventilation, and the reference of the open architecture design layout. The architectural design should combine lighting, shading, and some other factors surrounding buildings.
In accordance with the basic principles of thermal pressure [
3], we calculated the thermal pressure for individual window and the entire building from top to bottom. According to the difference of air temperature and humidity between indoor and outdoor, the thermal pressure values are shown in the following table.
According to the theory of fluid mechanics on the local resistance [
4], we can get the ventilation quantity 5000 m
3/h when the windows and doors are all open. If the wind speed is 0.5 m/s, the local resistance is 0.1 Pa. From the results in Table 1, when indoor and outdoor temperatures match that in Table 1, for the thermal pressure produced by the temperature difference between indoor and outdoor can drive about 5000 m
3/h air flow out of rooms through all the windows. When the temperature difference between indoor and outdoor increases, the thermal pressure will become greater, and more air flow will be out of rooms through the open windows.
For the entire building, we can get the thermal pressure for entire building from top to bottom, which is shown in Table 2.
According to the calculated data, if the temperatures of indoor and outdoor match the data in Table 2, 13000 m3/h air will be out of windows under the thermal pressure caused by the temperature difference of 2°C between indoor and outdoor. We should consider the airflow resistance in the process of actual flow. Therefore, the actual airflow volume of the entire floor can be determined by careful calculation.
According to the analysis, we can achieve air circulation because of the indoor and outdoor temperature difference. Because of architectural diversity and complexity of the layout existing in Chongqing and some other factors in the actual process, the flow resistance is not clearly defined. The momentum generated by thermal pressure is limited; therefore, in architectural design, we should give full consideration to the application for the thermal pressure. The resource of natural ventilation is air-mobile force combined wind pressure. In order to ensure the ventilation effect, the resistance of airflow process should be calculated carefully to ensure the power to conquest the resistance.
Figure 9 is the annual sun track in Chongqing. The sun altitude is a little high in summer in Chongqing at about 60° to 80°, is lower in winter about 20° to 30° more or less, and 40° to 70° in spring and about 40° in autumn. Therefore, when considering high sun altitude in summer and low sun altitude in winter, summer solar energy into the rooms through windows can be controlled by means of shading, and at the same time solar access to the rooms in winter through windows.
Based on the running track, the different directions were analyzed through the building model. The building direction in Fig. 10(a) is northwest to southeast, in Fig. 10(b) is east to west, and in Fig. 10(c) is southwest to northeast. It can be seen in the three figures as follows according to the sun track that the two main facades with windows can receive sunlight in summer. In the winter, however, one of the two facades can accept less sunlight in Figs. 10(a) and (c). In Fig. 10(b), in either winter or summer the two main facades can get good sunshine. But this raises another issue. We can see from Fig. 10(b) that due to the direction being east to west, they can receive adequate sunlight in winter. In summer, however, solar radiation through windows into the rooms will be very strong, and it will increase the air-conditioning load and cause the problem of energy consumption. Taking into account the summer sun altitude and the height of the building, shadings of different widths can be installed outside the window to block solar radiation according to different floors. Meanwhile, under the combined plan, we can find a balance between Figs. 10(a) and (b), to minimize solar radiation in the direction of northwest (see Fig. 10(a)) in summer afternoons and enhance solar radiation in the northwest (Fig. 10(a)) in winter.
For the single building, the optimum direction is between southeast and east and if the direction closes to east, more light and solar access are available. For the best direction of the building, it should be based on the layout of the building surroundings and balance of the solar radiation and lighting, and take into account the wind direction to enhance indoor ventilation.
According to the solar radiation statistical data in Chongqing Municipality [
1,
5], we can get the annual typical distribution of solar radiation on the surface in different directions, as shown in Fig. 11. From curves shown in Fig. 11, solar radiation is weak in autumn and winter in Chongqing and strong in summer; the maximum available value is 800 w/m
2 and the daily average value reached 350-500 W/m
2; there is big variation of solar radiation in spring, which is in relation to climate change in spring in Chongqing. Its average value is still higher than that in autumn and winter and the inconvenience is caused in the practical application because of big variations.
Figure 12 shows the average trend curves of solar radiation on slope surfaces for different angles per year. The solar radiation is concentrated in June, July, August, and September in summer. From Fig. 13 we can see that in these months the sun altitude angle is high at noon, which corresponds to the strongest solar radiation, reaching 70°-80°. Through above comprehensive analysis, we believe that the best slope angle of solar energy using, water heating, and photovoltaic panels, is 20° for the whole year in Chongqing .
The ground source heat pump technology has been launched in a series of nationwide application. Many local governments are actively promoting the development of ground source heat pump technology. Just in the past three to five years, a 3000-MW ground source heat pump has been installed in China, exceeding the total capacity of photovoltaic and wind power in the past 20 years [
6]. The application of ground source heat pump technology is moving fast in Beijing, Shenyang, Qingdao, Dalian, Weihai, Tianjin, Ningbo, Guangzhou, Chongqing and other cities. In addition to the original underground pipe laying and underground water system, the water source heat pump system, which uses surface water as a heat exchanger and heat resource, is given more attention. All the systems promote the technology development of ground source heat pump in China.
Taking the ground source heat pump system of underground pipe as an example, in Chongqing, the underground depth of 0-100 m is mainly sandstone. The thermal physical properties of soil can be obtained from the geological histogram analysis. In Chongqing, according to the analysis of soil at the underground depth of 0-100 m, we can obtain the following results: thermal conductivity=1.98 w/(m·°C), density=2207 kg(/m
3·°C), specific heat=1003.47 J/kg·°C. Table 3 shows the unit heat exchange based on the calculated method in Ref. [
7].
According to the chart on the temperature distribution above, as compared with the outdoor climate analysis figure in Chongqing, we can see that underground temperature is about 8-15°C lower than the outdoor temperature in summer and underground temperature is about 10-20°C higher than the outdoor temperature in winter. Generally speaking, the temperature of ground source heat pump is relatively stable throughout the year. It is higher than the ambient air temperature in winter and lower in summer. According to the principle that if the evaporation temperature increases or if condensation temperature decreases, the system energy consumption decreases, and the annual energy saving potential is more than 30-40%. This shows that the shallow geothermal resource used in air-conditioning systems has great potential for energy efficiency and adaptability in Chongqing.
In addition, making full use of water circulation of underground pipes in the transition seasons in order to achieve the heat exchange between water and soil can satisfy the need by rising or reducing the water temperature and improving the indoor thermal comfort in the transition seasons.
As the ground source heat pump is still in the exploration phase, some technological problems need to be pointed out in the practical projects: refill of sand in the vertical pipe, the depth of vertical pipe, material choice of pipe, spatial distance between pipes, heat balance in different seasons, energy consumption of water transportation for water source heat pump, water quality and environmental issues of water source heat pump [
8,
9].
Conclusions
Through above analysis, the application principles and potentials of natural resources used in Chongqing are clarified, but a lot of problems still exist in practical projects. The application of natural resources in building needs to consider the architectural composition such as building position, building form, building function etc, which can make out the real technological system for the building energy saving and indoor thermal environment.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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