Ruo-xi Yuan, Gui-ling Wang, Feng Liu, Wei Zhang, Wan-li Wang*, Sheng-wei Cao

1 Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China.2 Technology Innovation Center for Geothermal ＆ Hot Dry Rock Exploration and Development, Ministry of Natural Resources, Shijiazhuang 050061, China.

Abstract: To discover the characteristics, distribution and potential of shallow geothermal energy in the Beijing-Tianjin-Hebei Plain area. This paper, based on a large amount of data collection and field investigations, evaluateed the shallow-layer geothermal energy in the study area through the analytic hierarchy process and comprehensive index method. Based on suitability zoning results superimposed with 1:100 000 land use data, the study area is divided into encouraged, controlled, restricted and prospective mining areas regarding the development of shallow geothermal energy, and the economic availability of shallow geothermal energy in the encouraged and controlled areas are evaluated. The results show that the shallow geothermal energy in the Beijing-Tianjin-Hebei Plain can meet the heating and cooling demand of 6×108 m2 of buildings, equivalent to 1.15×107 t of standard coal, thus reducing carbon dioxide emissions by 2.73×107 t and reducing sulfur dioxide emissions by 1.95×105 t. According to the development and utilization mode, the energy demand level and the Beijing-Tianjin-Hebei coordinated development plan, the development and utilization of geothermal resources in the plain area has two types: Urban concentrated mining areas and rural scattered mining areas. The scale and level of intensive utilization of regional geothermal resources are of great significance.

Keywords: Beijing-tianjin-hebei plain; Shallow geothermal energy resources; Land use; Suitability evaluation

Introduction

Shallow geothermal energy is a resource that participates in the heat cycle within a certain depth of the earth’s surface and can be used under certain technical and economic conditions. It is renewable,widely distributed, large in reserve, clean and environmentally friendly, and economical (Lin,2012a; Ma et al. 2015). Shallow geothermal energy is extracted and exchanged through heat pump units, which can replace the use of coal, oil and natural gas to heat and cool buildings (Wu, 2014).The development and utilization of shallow geothermal energy must be established on the basis of geological survey and evaluation work. Wang et al. (2017a) evaluated the shallow geothermal energy resources in 336 cities above the prefecture level across the country. The annual mining capacity of shallow geothermal energy in these cities is equivalent to 7×108t of standard coal, and the amount of coal that can be saved is 4.1×108t,which can provide heating and cooling for 3.20×1010m2of building area.

For a long time, the proportion of fossil energy in the energy consumption in the Beijing-Tianjin-Hebei region has been too high, and 90% of the total energy consumption is from coal accounts.The unbalanced energy structure and the consequent environmental pollution have been very prominent issues (Wang et al. 2017b). Some of the main targets of adjusting and optimizing the energy structure in Beijing, Tianjin and Hebei area are to accelerate green and low-carbon development, to achieve carbon peak and carbon neutral strategy, and to vigorously develop geothermal renewable energy. In the past decade, shallow geothermal energy has been gradually developed and utilized on a large scale in the North China Plain, northeast China and northwest China. The state and relevant ministries and commissions have also introduced a series of encouraging policies and promotional measures to facilitate it, but many regions still have problems such as delayed exploration, immature evaluation of suitability at the early stage and chaotic management (Zhu et al.2015). The Beijing-Tianjin-Hebei region has the best geothermal resources and the highest degree of development and utilization, which in turn provides the largest scale of heating and cooling capacity in the country. However, the utilization of geothermal resources only accounts for 5% of renewable resources in eastern China (Wang et al.2020). In recent years, there have been many evaluations on the suitability of the development and utilization of geothermal energy in the upper shallow layer of the region (Lin, 2012a; Long et al.2016; Xing et al. 2019; Yuan et al. 2020), and in the regional evaluation, part of farmland and water areas far from human activity areas were also counted in the calculation, which often resulted in high heat transfer cost, and the unplanned establishment of Ground-Source Heat Pumps system brought further economic losses to developers. Therefore, it is of great significance to carry out economic zoning and resource evaluation of shallow geothermal energy based on land use(urban and rural) in the Beijing-Tianjin-Hebei Plain for scientific and rational development and utilization of this valuable resource.

Based on the project of “Beijing, Tianjin and Shijiazhuang Geothermal Resources Survey” and“ National Geothermal Flow Measurement and Target Selection” organized by China Geological Survey, this paper identified the occurrence characteristics and suitable economic zones of shallow geothermal energy in the Beijing-Tianjin-Hebei region through field investigation and experimental analysis. With the combination of 1:100 000 land use data (2018) of Beijing-Tianjin-Hebei region, the district of Beijing, Tianjin, three counties of Langfang city in the north, the Xiong’an New Area and some other towns and villages were divided into Groundwater Heat Pumps(GWHP) and Ground-Coupled Heat Pumps(GCHP) development and utilization economic zones. Economic evaluations of availability were carried out, providing reference for the reasonable planning of shallow geothermal resources development and utilization in Beijing-Tianjin-Hebei region.

1 Research area and research ideas

1.1 Overview of the research area

The Beijing-Tianjin-Hebei region is located in North China, including three administrative regions of Beijing, Tianjin and Hebei Province,which is an important political, cultural and industrial center in China. The region falls in a warm-temperate monsoon climate zone with four distinct seasons. The annual precipitation is in the range of 50-900 mm. The topography of the region is higher in the northwest and lower in the southeast. The study area is located in the Beijing-Tianjin-Hebei Plain (Fig. 1), with a total area of 9.05×104km2from the foothills to the coast of the Bohai Sea, which can be divided into piedmont alluvial-diluvial inclined plain, the central alluvial lacustrine plain, and the eastern alluvial marine coastal plain. The Quaternary deposits in this area often reach hundreds of meters in thickness. The lithology is mainly unconsolidated or semiconsolidated gravel, sand and clay. Atmospheric precipitation, mountainous underground runoff and surface runoff are the main sources of groundwater replenishment. The buried depth of the constant temperature zone in this area is basically 15-20 m,and the geothermal gradient is 2-4℃/100 m (Wei et al. 2010; Wang, 2018; China Geological Survey,2012).

Fig. 1 Scope of the study area

1.2 Data source and processing

The 1:100 000 land use data (2018) was generated by Beijing Digital Space Technology Co. Ltd.,based on Landsat remote sensing images. The hydrogeological data comes from the field survey of the geological survey project “Beijing, Tianjin and Shijiazhuang Geothermal Resources Survey”.The basic map of the evaluation is partly derived from the revision of the Atlas of Sustainable Groundwater Utilization in the North China Plain(2009) (Zhang et al. 2009). This study uses the MapGIS6.7 and ArcGIS10.6 working platforms for data processing and spatial analysis. The topographic map of the study area is used as the base map, and other maps are used for spatial coordinate registration and format conversion to obtain attribute data for evaluation index and carry out evaluation.

1.3 Research framework and process

The development and utilization of shallow geothermal energy mainly uses Ground-Source Heat Pumps (GSHP) technology to heat and cool buildings. The main methods are GWHP and GCHP, the two of which vary in suitability conditions (Luo et al. 2016; Lin et al. 2012b). The GWHP conducts capacity exchange by extracting groundwater at the same temperature as the formation. When using GWHP, the availability of groundwater with abundant quantity and required quality is necessary to meet the water source requirements for heat pump air-conditioning, while the tail water can be recharged after use. The original aquifer does not have a significant impact on the geological environmental conditions. The GCHP transfers the cold and heat to the demand side of the building via the buried circulation pipe system and heat pump equipment. The use of GCHP should be based on drilling technical conditions and economic costs. The underground rock and soil layer has a sustainable cyclic heat transfer capability and will not have a significant impact on the underground temperature field(Wang et al. 2017; Somogyi et al. 2017). The research ideas adopted in this paper are shown in Fig. 2:

Fig. 2 Flow chart of shallow geothermal resources evaluation in Beijing-Tianjin-Hebei Plain

2 Results and discussion

2.1 Evaluation of suitability for geothermal development and utilization

2.1.1 Evaluation method

The suitability evaluation for geothermal development and utilization was carried out in the Beijing-Tianjin-Hebei plain area. Field surveys,hydrogeological tests, and thermal response tests have been used to obtain relevant parameters.Analytic Hierarchy Process (AHP) and comprehensive index methods are adopted to carry out the evaluation (Francesco et al. 2018; Saaty, 1990).For GWHP, the suitability zoning mainly considers the influence of geological and hydrogeological conditions, hydrodynamic field, hydrochemical characteristics and geological environment, which demarcates the important water source protection areas and severe land subsidence areas into poor suitability areas. For GCHP, the main consideration is the influence of geological and hydrogeological conditions, thermophysical characteristics,stratum lithology and economic rationality (Wang et al. 2017; Guan et al. 2014; Zhu et al. 2018). The evaluation indicators for different types of GSHP are shown in Table 1 and Table 2:

Table 1 GWHP suitability evaluation index classification quantitative standard

2.1.2 Evaluation results

(1) GWHP Results show that the areas with high suitability index in the study area are mainly distributed in the piedmont plains, including most of Beijing,especially the top of several alluvial and diluvial fans, and the western area of Shijiazhuang,Tangshan, Baoding, Xingtai and Handan in Hebei Province, with a total area of 2.78×104km2,accounting for 31.6% of the plain area. The lithology of the aquifer is mainly gravel and medium coarse sand, and the salinity is less than 1 g/L. The unit groundwater yield of a single well is generally 30-50 m3/h·m, and the maximum yield is 80-120 m3/h·m.

Areas of medium suitability is mainly located in the central plain, distributed in strip, including the northern part of Tianjin, the southwest of Langfang, Hebei Province, the south of Tangshan,the east of Baoding, the southeast of Shijiazhuang,the east of Hengshui, the east of Xingtai and some parts of Handan, with a total area of 4.56×104km2,accounting for 51.8% of the plain area. The lithology of the aquifer in this area is mainly medium-coarse sand, with a salinity between 1-3 g/L. The unit groundwater yield per well is generally 20-30 m3/h·m and the maximum can reach 50 m3/h·m.Lower yield such as 5-10 m3/h·m may occur locally.

Table 2 GCHP suitability evaluation index classification quantitative standard

The areas with poor suitability are mainly located in the coastal plain, including most areas of Tianjin and Cangzhou, with a total area of 1.47×104km2, accounting for 16.6% of the plain area. These areas have low groundwater availability. The lithology is mainly fine sand and silt sand, which has low hydraulic conductivity to effectively promote groundwater recharge. The salinity is generally between 2-3 g/L and greater than 3 g/L in some areas. The groundwater yield is generally 5-20 m3/h·m and may be less than 5 m3/h·m in some areas (Fig. 3).

(2) GCHP

In the Beijing-Tianjin-Hebei plain, areas with good suitability for GCHP are mainly distributed in the central plains and coastal plains, including central Tianjin, Tangshan, Cangzhou, western Langfang,western Hengshui, eastern Xingtai, and eastern Handan. The total area is 2.82×104km2, accounting for 32% of the plain area. The thickness of the Quaternary deposits in this area is between 300-600 m. The major lithology of the stratum is silty fine sand and clay, which has high drillability, good storage and heat transfer capacity,and good groundwater runoff conditions.

Fig. 3 Suitability zoning of GWHP

Area of medium suitability is mostly distributed in the lower part of alluvial-diluvial fan and alluvial-diluvial plain in the Beijing plain, with a total area of 4.93×104km2, accounting for 56% of the plain. This area is a multi-layer sand zone with high drillability and economic rationality. The southern part of Tianjin also falls in the zone with medium suitability, which has good conditions of storage and heat transfer capacity.

The poor suitability areas are primarily located in the piedmont plains and the top areas of alluvial fans, including Tangshan, Baoding, Shijiazhuang,Xingtai and northern Tianjin, with a total area of 1.06×104km2, accounting for 12% of the plain.According to the thermophysical properties, the thermal conductivity of the bedrock is higher, and its heat exchange per linear meter is larger, but the drillability of the bedrock is poor, and the increase in drilling costs will inevitably result in a huge initial investment (Fig. 4).

Fig. 4 Suitability zoning of GCHP

2.2 Development and utilization of economic zoning

2.2.1 Land use data

Beijing-Tianjin-Hebei Land Use Data (2018) is 1:100 000 land use data generated from Landsat remote sensing images, including 6 first-level categories, i.e. cultivated land, woodland, grassland,water area, construction land and unused land, and 25 second-level categories, i.e. woodland, shrub,sparse woodland and other woodland. The mountainous area of Beijing-Tianjin-Hebei is mainly composed of woodland and grassland, with diverse landform types from densely covered hills to interlocked basins. Small basins and valleys are scattered among mountains and valleys. The land use types in plain areas are basically paddy fields and dry land. Fig. 5 shows the land use types in the study area, in which the arable land accounts for 47.4% of the total area, forest land 21%, grassland 15.6%, urban land and rural residential land 4%and 7.1%, respectively.

Fig. 5 Land use pattern in Beijing-Tianjin-Hebei region (2018)

2.2.2 Principles of economic zoning

According to the results of suitability evaluation,combined with the scope and planning of the current urban construction area, and with the superimposition of land use map in the Beijing-Tianjin-Hebei region, the Ground-Source Heat Pumps development and utilization area in the study area can be divided into restricted mining area, controlled mining area, encouraged mining area and prospective mining area. The division principle is as follows:

(1) According to the suitability of different types of heat pumps, areas with good suitability are defined as encouraged mining areas, middle suitability areas are controlled mining areas, and poor suitability areas are restricted mining areas.

(2) On the basis of the above regional division,considering that the current development and utilization of shallow geothermal energy resources are all on-site development, and only have practical value in urban and rural construction land, the following divisions can be made:

① The urban and rural construction land in the plain area is divided into restricted mining area,controlled mining area and encouraged mining area with poor, medium and good suitability, respectively. The farmland area is designated as prospective mining area as it is temporarily unable to be developed and utilized. Groundwater Heat pumps in mountainous urban and rural construction land are also demarcated as prospective mining areas, and Ground-Coupled Heat Pumps are demarcated as controlled mining areas.

② Beijing, Tianjin, Xiongan New Area and north Langfang County are the key urban areas in the Beijing-Tianjin-Hebei region. These areas have a high degree of urbanization, and the urban construction areas may be expanded in a large scale in the future. Therefore, economic zoning for development and utilization of all administrative areas in these areas is carried out.

2.2.3 Economic zoning results

(1) GWHP The whole district of Beijing has been demarcated as encouraged and controlled exploitation areas for GWHP, among which the former accounts for 44%and the latter accounts for 56%. These areas have good groundwater yield, strong recharge capacity,and excellent hydrogeological conditions.

The controlled mining area for GWHP in Tianjin is concentrated in the northern area,accounting for 22% of the plain area. Compared with Beijing, the suitable area to develop GWHP in Tianjin is smaller. The restricted mining area for GWHP is located in the central and eastern areas of Tianjin.

The encouraged mining areas for GWHP in Hebei Province are mainly located in Shijiazhuang,northern Tangshan, Baoding and most areas of Xingtai; the controlled mining areas are located in the Xiong’ an New Area, central and southern Langfang, Hengshui and most areas of Handan(Fig. 6).

(2) GCHP

The lower part of alluvial and diluvial fan and alluvial plain in Beijing plain area are demarcated as the controlled mining areas for GCHP,accounting for 62%. The lithology of upper layer of alluvial and diluvial fan is mostly uniform sandy gravels, which is suitable for burying pipes,therefore it is marked as restricted mining areas.

Except for the northern part, the rest of Tianjin are regarded as encouraged and controlled mining areas for GCHP, accounting for 91%. These areas have better conditions such as the good storage and heat transfer capacity of the stratum, and they are more suitable for GCHP projects than GWHP projects.

When Tsar Dolmat saw Tsarevitch Ivan riding on the false Horse with the Golden Mane he rejoiced exceedingly. He came out, embraced Ivan in the wide courtyard and kissed him on the mouth, and taking his right hand, led him into his splendid rooms. He made a great festival, and they sat at oak tables covered with embroidered40 cloths and for two days ate, drank and made merry. On the third day the Tsar gave to Tsarevitch Ivan the Fire Bird in its golden cage. Ivan took it, went to the green lawn where he had left Helen the Beautiful, mounted the real Horse with the Golden Mane, set the Tsarevna on the saddle before him, and together they rode away across the three times nine lands towards his native country, the Tsardom of Tsar Vyslav.

The encouraged mining areas for GCHP in Hebei Province are located in the northeast of Shijiazhuang, southern Baoding, western Handan and most areas of Cangzhou; the controlled mining areas are located in most areas of Xiong’an New Area, Hengshui, Tangshan, and Xingtai (Fig. 7).

Fig. 6 Protection zoning of GWHP

Fig. 7 Protection zoning of GCHP

2.3 Evaluation of shallow geothermal energy resource potential

2.3.1 Evaluation method

Because the prospective mining areas are far from the residential area and the suitability of development in the restricted mining area is poor, both of which are not cost-effective to be developed,therefore mining in these areas are not recommended. The heating (cooling) area of buildings is used as a standard to indicate the economically available resource potential of shallow geothermal energy in the controlled mining area and encouraged mining area in Beijing-Tianjin-Hebei plain area.The evaluation method can be referred to the shallow geothermal energy exploration and evaluation specification (DZ/T0225-2009). The heating(cooling) area of the Ground-Source Heat Pumps is the ratio of the heat transfer power of the encouraged and controlled area in the Beijing-Tianjin-Hebei Plain to the heating load in winter (60 W/m2) and the cooling load in summer (90 W/m2).

2.3.2 Evaluation results

(1) Resource evaluation of GWHP

The GWHP in the encouraged and controlled area of the Beijing-Tianjin-Hebei Plain has a cooling area of 1.89×109m2in summer and a heating area of 1.20×109m2in winter, equivalent to 2.02×107t of standard coal. It can reduce sulfur dioxide emissions by 3.5×105t, nitrogen oxidation emissions by 1.2×105t, and carbon dioxide emissions by 4.8×107t. The encouraged and controlled area of Beijing Plain District has a cooling area of 5.63×108m2in summer and a heating area of 3.58×108m2in winter. The encouraged and controlled area of Tianjin City has a cooling area of 2.57×108m2in summer, and a heating area of 1.63×108m2in winter.

(2) Resource evaluation of GCHP

(3) Comprehensive resource evaluation

At present, the economically exploitable amount of shallow geothermal energy in the encouraged and controlled areas of Beijing-Tianjin-Hebei Plain is as follows: cooling area of 4.43×109m2in summer,heating area of 2.44×109m2in winter, equivalent to 4.6×107t of standard coal, which can reduce sulfur dioxide emissions by 7.8×105t, reducing nitrogen oxide emissions by 2.7×105t, and reducing carbon dioxide emissions by 1.09×108t.An usability coefficient of 25% is applied to the exploitable amount to evaluate the economically usable shallow geothermal energy resource for heating (cooling) of buildings in the Beijing-Tianjin-Hebei region (Table 3). Among them,GSHP in the encouraged and controlled area has a cooling area of 1.11× 109m2in summer and a heating area of 6.12×108m2in winter, equivalent to 1.15×107t of standard coal, reducing sulfur dioxide emissions by 1.95×105t, reducing nitrogen oxide emissions by 0.68×105t, and reducing carbon dioxide emissions by 2.73×107t.

2.4 Zoning based on development and utilization

Beijing, Tianjin, three counties of in the northernLangfang, Xiong’an New Area and other urban areas in the Beijing-Tianjin-Hebei region is demarcated as centralized mining areas for shallow geothermal energy. These areas are densely populated, and urban construction has occupied a large amount of underground space, therefore they are suitable for centralized use of shallow geothermal energy for heating and cooling. The rural residential areas out of cities and towns are demarcated as scattered exploitation areas.Population in these areas are sparsely distributed and the main demand for geothermal resources is scattered heating. Therefore, it is of great significance to develop shallow geothermal energy for scattered heating and cooling.

Table 3 Heating (cooling) areas of economically usable amount of shallow geothermal energy in Beijing-Tianjin-Hebei

A zoning map of the comprehensive development of shallow geothermal energy has been created based on the coordinated development zones in the Beijing-Tianjin-Hebei region.The central core functional area and the eastern coastal development area have dense populations and large-scale populated areas such as cities and industrial parks, which provides good conditions for centralized mining. It is recommended that centralized mining should be developed in Beijing,Tianjin, Xiong’ an New Area, three counties in northern Langfang, Tianjin Binhai New District,Cangzhou Bohai New District, Tangshan Caofeidian New District and other areas. Scattered mining should be properly developed in rural areas. Rural areas are relatively large In the southern functional zone and the northwest ecological zone which are mostly rural areas,scattered shallow geothermal heating (cooling) can replace part of coal consumption. Regional nodal cities, towns, and new village areas can also use centralized mining to extract shallow geothermal energy (Fig. 8).

3 Conclusions

(1) 31.6% of the area in the Beijing-Tianjin-Hebei Plain has a good suitability for GWHP, mainly distributed in the western and northern piedmont plains. 51.8% of the area has moderate suitability for GWHP, mainly distributed in the central plain area. 16.6% has poor suitability for GWHP, mainly distributed in the coastal plain area. The encouraged and controlled mining area account for 25%of the plain area. 32% of the Beijing-Tianjin-Hebei plain area has a good suitability for GCHP, mainly distributed in the eastern plains. 56% has moderate suitability for GCHP, mainly distributed in the central plains. 12% has poor suitability for GCHP,mainly distributed in the piedmont plains, and the encouraged and controlled mining areas account for 31%.

Fig. 8 Development and utilization zoning for shallow geothermal energy in Beijing-Tianjin-Hebei region

(2) In the Beijing-Tianjin-Hebei Plain, the economically usable shallow geothermal energy in the encouraged and controlled mining areas can meet the heating (cooling) needs for 6.1×108m2buildings, including 1.11×109m2for cooling in summer and 6.12×108m2for heating in winter. It is equivalent to 1.15×107tons of standard coal,reducing sulfur dioxide emissions by 1.95×105t,reducing nitrogen oxide emissions by 0.68×105tons, and reducing carbon dioxide emissions by 2.73×107t. It is suggested that shallow geothermal energy should be developed and utilized in encouraged and controlled mining areas, while restricted mining areas have poor suitability and are not recommended to be developed. The exploitation cost of the prospective mining area is too high, so it is recommended not to develop before it is changed to urban construction area.

(3) Shallow geothermal energy should be actively developed for heating and cooling in the Beijing-Tianjin-Hebei region. Combining the resource advantages and economic development plans of different areas, development and utilization models and management methods can be formulated by functional zoning. In areas with high population density and high urbanization,central heating (cooling) by shallow geothermal energy should be developed in combination with the economic zoning of ground source heat pumps.In the vast rural areas, scattered utilization of shallow geothermal energy should be adopted to replace the consumption of coal for heating. The development should also consider the protection of geological resources and groundwater environment,to support and serve the Beijing-Tianjin-Hebei coordinated development in the national strategy,and to solve the regional haze pollution in winter.

Acknowledgements

This study was supported by Geological survey project of China (No. DD20190128 and No.DD20160190).