1. Introduction

China has abundant coal resources and is one of the world’s major coal mining countries. During coal mining, washing, and processing, a large amount of coal gangue is produced. According to statistics, the coal gangue yield is 5–10% [1], which is the largest industrial solid waste in China [2]. Due to the low utilization rate of coal gangue resource utilization, it has accumulated over time and formed gangue hills. Currently, the amount of accumulated coal gangue exceeds 7 billion tons, and there are about 2600 large-scale gangue hills, occupying more than 220,000 hectares of land [3,4]. In addition, the quantity of coal gangue is still increasing at a rate of 500 million to 800 million tons per year [5]. Although it is currently required that newly mined coal gangue not be brought to the surface, old coal mines are still in their original state.

A large amount of coal gangue will cause huge environmental hazards, including land acquisition, geological desertification, ecological damage, and other issues [6]. Currently, the gangue hills are basically treated by “closed coverage and greening”. Sealing refers to blocking the contact between air and coal gangue to prevent spontaneous combustion. Covering refers to using soil to cover the sealed ground, making the soil thicker until the height of the soil can meet the requirements of plant growth, generally 0.5–2 m according to the type of vegetation planted. Greening is the last step, which is to plant on abandoned gangue hills. In China, the management of coal gangue hills follows the principle of “who mines, who manages, and who benefits”, aiming to encourage coal mining enterprises to take active management measures. For coal mining enterprises, the promotion of coal gangue hill ecological restoration benefits from government funding and self-raising, and is a hard requirement of environmental protection inspections. Ecological restoration is not a one-time job and requires long-term maintenance and management. Once government support and enterprise funding are lost, the ecological restoration of coal gangue hills will face major problems. According to research, during the time period from the “Eleventh Five-Year Plan” to the “Thirteenth Five-Year Plan”, a total of about 23,000 coal mines were closed nationwide [7]. The “Guiding Opinions on the High-Quality Development of the Coal Industry in the “14th Five-Year Plan” period proposed that the domestic coal output be controlled at around 4.1 billion tons, and the number of coal mines nationwide be controlled within 4000 [8]. These mines all face the problem of continuing to bear the cost of ecological restoration. Therefore, finding ways to transform coal gangue hills into economic benefits is a crucial task.

1.1. Condition of SanKuang

SanKuang is a large coal mining area located in the mining district of Yangquan City, Shanxi Province, China. Situated in the urban core area of Yangquan City, it has multiple pits and was once known as “Asia’s No.1 Mine” in the 1990s. However, due to the depletion of coal resources, it was closed in 2019.

The SanKuang area consists of six coal gangue hills, covering an area of approximately 1.3 square kilometers. These hills are SanKuang Beitouzui Gangue Hill, New Waste Gangue Hill, DaNaoliang Gangue Hill, 280 Gangue Hill, Qichi Coal East Gangue Hill, and Qichi Coal West Gangue Hill. Since 2015, the cumulative investment in the restoration of coal gangue hills has exceeded 77 million CNY. The emissions of coal gangue are generally located in the concave between two hills, mainly dumped from top to bottom along the hill (Figure 1), and only the 280 Gangue Hill is piled on flat ground.

Of the six coal gangue hills in the SanKuang area (Figure 2), five are already in a maintenance state, except for SanKuang Beitouzui Coal Gangue Hill, which is still in use and is preparing to be closed for restoration.

The maintenance of coal gangue hills can be divided into three levels based on the years of closure of the gangue hill, the severity of rain erosion, the survival rate of vegetation, the drainage conditions, and other infrastructure conditions. The first level refers to the most severe conditions, with erosion depth greater than 50 cm or vegetation coverage less than 70%. The second level refers to conditions where erosion depth of the veins is 20 to 50 cm or vegetation coverage is 70 to 80%. The third level is the best state, with vegetation coverage exceeding 90%. The maintenance expenses after restoration mainly include manual inspection and temperature measurement, covering and repairing gangue hills, and vegetation maintenance. The maintenance cost of coal gangue hills in SanKuang area is about CNY 9 million.

SanKuang Beitouzui Coal Gangue Hill is a large gangue hill and is currently in the process of restoration and emission, as shown in Table 1. According to the average cost of covering and ecological restoration of coal gangue hills of 15.5 CNY/m2, the annual ecological restoration cost of SanKuang Beitouzui Coal Gangue Hill is about CNY 6.67 million.

The restoration of coal gangue hills is a challenging task as it faces problems such as high energy consumption, high costs, and low efficiency, especially for closed mines. These mines have limited power and can only wait for instructions from higher authorities, so they do not have strong competitiveness in the restoration of coal gangue hills. To address these issues and promote the governance of coal gangue hills, deeper reforms are needed.

1.2. Photovoltaic Conditions in Yangquan

Yangquan is located in the middle and east of Shanxi Province, on the eastern edge of the Loess Plateau, mainly composed of hills. Yangquan belongs to the temperate semi-humid continental monsoon climate zone. The solar radiation in Yangquan is relatively abundant throughout the year. The range of solar energy resources can be between 1500 and 1600 kWh/m2, as shown in Figure 3. For solar photovoltaic systems in the most inclusive countries, the annual power generation hours can reach 1250 to 1500 h. According to the “Total Radiation Level of Solar Resources” (GB/T3155-2014), Yangquan belongs to the B category (very rich).

Therefore, in recent years, the country has vigorously developed various forms of “photovoltaic+” industries, and policies and technologies have continued to improve and mature. Yangquan Sanmine has suitable lighting conditions, and the reasonable use of coal gangue hills as mining waste land to develop into photovoltaic farms is a trend.

1.3. Development of the Combination of Mining Waste Land and Photovoltaics

In 2019 and 2020, the National Development and Reform Commission issued multiple documents advocating the development of green industries related to energy conservation, environmental protection, clean production, and clean energy technologies [9,10,11]. Solar photovoltaic technology refers to the use of photovoltaic effects to convert solar energy into electrical energy. The main components of the solar photovoltaic system are solar panels. When radiation intensity reaches the critical value, the solar panel will generate electric current, and the current is collected through the controller to form a power generation device [12].

As an emerging power generation technology, solar photovoltaics has the characteristics of pollution-free, noise-free, and easy maintenance, showing tremendous development space and application prospects. However, solar photovoltaic technology has a low energy density, which means that more land is needed to produce electricity [13].

To protect land resources, China often combines photovoltaic power generation with desert or mining waste land management. A large number of practices have proved that the combination of desert control and solar photovoltaics can complement each other. By analyzing ecological hydrological parameters, including regional precipitation, evaporation, solar radiation, wind speed, temperature, land use, and vegetation coverage, the array layout, height, and installation angle of photovoltaic panels can be optimized. In this way, the climate, soil, and vegetation conditions of the region can be improved. In Qinghai [14], Gansu [15], and Inner Mongolia [16], vegetation on the desert has grown again, and the desert has been successfully transformed into an oasis [17]. This combination emphasizes ecological benefits while attaching importance to economic benefits of land use [18], making it an industry that can achieve both economic output and ecological protection, which is beneficial to protecting the “red line” of national farmland.

Research on photovoltaic energy in mining waste land is mostly focused on subsidence and mine pits, mostly combining “photovoltaic + coal mining subsidence area” and “photovoltaic + mine pit”, including integrated development [19], stability evaluation [20], benefit analysis [21], development strategies [22], paths [23], and development and utilization modes [24]. Regarding the combination of coal gangue hills and photovoltaic power generation, it mainly discusses the potential and prospects of developing photovoltaic power plants on coal gangue hills, proposes solutions and suggestions for solving problems, and proposes targeted layout and design principles for photovoltaic power plants. In recent years, there have been increasing practical cases of photovoltaic utilization in mining waste land. In 2016, the State Power Investment Corporation’s Datong leader 100,000-kilowatt photovoltaic project solved the problem of subsidence land use [25]. In 2017, the first floating photovoltaic power station was built by utilizing the water surface of Pan Yang coal mining subsidence in Huainan City, Anhui Province [26]. Subsequently, photovoltaic projects in mining waste land have emerged in areas such as Fengfeng Mining Area in Hebei, Inner Mongolia [27], and Shanxi [28].

At present, for the combination of coal gangue hills and photovoltaics, practical cases have been carried out in Datong, Yangquan, Jixi, etc., focusing on using the top platform of coal gangue hills to carry out photovoltaics and form a combination of photovoltaics and ecological restoration [25]. For gangue hills, the platform is where the vegetation cluster organization formed by ecological restoration is most abundant, while the slope is mainly composed of shrubs and grasses. For slopes, there are ideas for transforming tailings ponds [29], slag hills, and landfills into photovoltaics, and proposing an overall solution. Therefore, based on the above research, this article proposes the following ideas: (1) Use the slope of the gangue hill to form a photovoltaic base, and combine photovoltaics with ecological restoration. Reserve the platform of the coal gangue hill as a multi-species settlement. (2) Combine the treatment and maintenance of the gangue hill slope with the photovoltaic support to provide technical feasibility for photovoltaics. (3) Combined with the improvement plan, better promote the joint operation of multiple companies, not only achieving better income economically but also making the restoration of gangue hills a sustainable development industry.

2. Literature Review

2.1. Ecological Restoration of Coal Gangue Hills

There are several aspects related to the ecological restoration of coal gangue hills:

The overall goal of ecological restoration summarizes the advantages and disadvantages of domestic and foreign ecological restoration methods for coal gangue hills [30], and discusses the current situation and future prospects of coal gangue hill governance. It also analyzes and summarizes the relevant theories and technologies for the ecological restoration of coal gangue hills. Based on engineering practice, it proposes that the key to ecological restoration and development of coal gangue hills is the construction of mining ecological systems and optimization of reclamation and utilization structures [31].

The development direction of ecological restoration can be applied to forestry, animal husbandry, and landscaping [31]. There is more research on landscaping, which proposes to use the theories of restoration ecology, landscape ecology, and landscape design; consider methods to increase landscape heterogeneity through aesthetic perception and functional analysis; create new landscape patterns [32]; and classify the site types for ecological restoration and landscape reconstruction [33]. It also proposes the determination of the best use of the landscape and reasonable spatial distribution patterns [34]. Specific transformation plans for coal gangue hills include converting them into parks and rest areas together with mining parks. From the perspective of landscape architecture, it is pointed out that the landscape regeneration of mine waste land has multiple benefits such as ecology, environmental protection, leisure, entertainment, economy, and culture, and requires interdisciplinary cooperation. There is less research on the development direction of pure agriculture, mainly focused on specific planting techniques, and it is pointed out that at least 60 cm of soil cover is required for agricultural development [35]. Currently, there are many practical cases of planting fruit trees on gangue hills, such as planting peaches and plum trees on the Guozhuang village north gangue hill in Pingdingshan, and planting vegetables and fruits on the Tai’an coal mine waste dump.

The micro-level of ecological restoration development of agriculture involves the change of microbial soil [36]. Using modern testing technology, new soil construction technology for vegetation restoration on coal gangue hills has been studied [37], in addition to the soil microorganisms, enzyme activity, and influencing factors of different reclamation vegetation types on coal gangue hills [38].

In summary, the research on the ecological restoration of coal gangue hills in China is mainly focused on exploring aspects such as the ecological environment and landscape reconstruction [39]. However, research on the direction of agriculture is mostly on the level of planting techniques and soil communities.

2.2. Slope Treatment of Coal Gangue Hills

Research on the slope treatment of coal gangue hills is an important research topic in the field of ecological restoration. Currently, there are relevant specifications that clearly indicate the guiding opinions of slope treatment [40]. The research mainly includes:

The application direction of slope stability: The research points out that the difficulty in landscape reconstruction of coal gangue hill slopes lies in their structural instability and poor water retention [41]. It proposes that the landscape reconstruction of gangue hills should consider the green covering of slopes and their humanistic characteristics.

Chang et al. [41] conducted a stability evaluation of coal gangue hills, preliminary construction of influencing factors [42]. They used software such as slope stability analysis and finite element simulation to evaluate and simulate.

Zhao et al. [43] used slope treatment technology for two types of coal gangue hills. For the “top-down” natural stacking of coal gangue hills, corresponding treatment engineering techniques such as shaping and compacting of soil covering are proposed to eliminate hidden dangers of slopes [44]. Currently, newer technologies adopt the method of “mining while reclaiming” to stack the gangue hills [45]. The discussion of setting the optimal slope for coal gangue hills according to different types of ecological restoration directions has also been proposed.

Zhao et al. [46] cover specific technologies for slope stability, mainly introducing shallow jet grouting technology and customer soil spraying technology [47]. It proposes a targeted injection plan for hole conversion [48].

In conclusion, research on slope treatment of coal gangue hills mainly focuses on the stability evaluation and specific technologies for slope stability, as well as their application directions.

3. Methodology and Data

3.1. Research Methods

3.1.1. Field Research

The object of the field research is the mining area of Yangquan City, located on the west side of the middle section of the Taihang Hills in the central east of Shanxi Province, between 112°54?~114°04? E longitude and 37°40?~38°31? N latitude [49]. The narrowest east–west distance of the mining area is only 32 km, the widest east–west distance is 82 km, the north–south length is 106 km, and the total area is 4452 square kilometers, accounting for 3% of the total area of the province [50]. The coal seams in Yangquan City are distributed in the northeast corner of the Qinshui coalfield, and the coal mining area is located in the southwest of the jurisdiction of the mining area. The excavation, stripping, and waste stacking in the mining process have caused changes in the surface topography, and 26 waste rock hills have been accumulated by mining activities. Considering that the distribution of mining waste coal and rock hills in this area is relatively scattered and fragmented, combined with the actual conditions of coal mine distribution, topography, hydrological characteristics, etc., data collection, discussions, interviews, on-site survey, photography, etc., were conducted through a survey, and relevant materials were summarized, sorted, and analyzed.

3.1.2. The Net Present Value Model

For coal gangue hill p, the yearly solar electricity output (

E
p

) depends on the cumulative installed solar capacity (

I

C
p

) in MW, the local annual peak solar hours (

T

p
s
,
p

) in hours, the overall performance ratio (

e

), and the degradation rate (

?

) in %. n is the year. The overall performance ratio is 80% for LSPV, and the degradation rate of the PV module introduces only a small change of around 0.7%/year [51].

E

p
,
n

=
I

C
p

·

T

p
s
,
p

·
e
·

1
?
?

n

In this paper, net present value (NPV) was employed to evaluate the economic performance of solar PV stations. NPV is an indicator representing the profitability of a project, which reveals all the cash flows during its lifetime. When the NPV is larger than zero, the project is worth pursuing.

p

s
e
l
l
,
p

refers to the selling price of electricity in CNY/kWh, which is the source of the cash inflow. The cash outflows (

C

F

o
u
t
,
p

) include the maintenance and operation (M&O) cost (

O
p
E

x

D
S

), the replacement cost of the inverter (

R
e

), and the initial investment costs (

C

D
S
,
p

). n is the lifetime of PV modules, which was set to be 30 years [52], and r is the discount rate.

N
P

V

L
S
,
p

=
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p

+

?

n
=
1

30

C

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p

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p

1
+
r

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1

30

E
p

·

p

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p

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O
p
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x

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A
L

R
p

1
+
r

n

?
R
e
 

The weighted average cost of capital is adopted as the discount rate, with E, D, and V as equity value, debt value, and the total value, respectively. re is equity cost, which was set as 6.4%, and rd is the debt cost, which is 4.9% [53], following the latest loan prime rate issued by The People’s Bank of China. For LSPV, the proportion of debt was set as 75%.

r
=

E
V

·

r
e

+

D
V

·

r
d

 

In terms of environmental performance, the United Nations Framework Convention on Climate Change has proposed a tool to calculate the emission factor for an electricity system (Tool) for the Clean Development Mechanism, which has also been adopted by the National Development and Reform Commission (NDRC) to calculate the baseline emission factor of the regional power grid in China. In the Tool, three emission factors are introduced: operating margin (OM), build margin (BM), and combined margin (CM). Since the low-cost/must-run resources constitute less than 50% of the total provincial grid generation, the OM was applied [54]. Instead of focusing on the regional grid level as the NDRC have [55], we calculated the local province/municipality level operational margin factor (

E
F

) (t CO2/MWh), with

F

C
j

 

as the amount of fuel type

j

,

N
C

V
j

 

as the net calorific value of fuel type

j

,

E

F

CO
2

,
j

 

as the emission factor of fuel type j (t CO2/GJ), and

E
G

as the electricity generated and delivered to the grid.

E
F
=

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j

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C
j

·
N
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j

·
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F

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2
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j

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Therefore, the CO2 emission reduction derived from the installed solar PV systems can be calculated as follows, with TP as the local proportion of thermal power output to the total power output.

CO

2
r
e
d
u

=

E
p

·
E
F
·
T

P
P

By employing the shadow price of CO2 (

S

P
p

), the environmental benefit (

E
B

) can be monetarily quantified. Due to the improvements in emission reduction of thermal power plants and the increasing proportion of renewable energy power,

?

is introduced to represent the annual CO2 emission reduction which equals 4%. The discount rate was set at 10% [56], reflecting the uncertainty of CO2 shadow price and time value.

E
B
=

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n
=
1

30

CO

2
r
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d
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p

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3.2. Data

3.2.1. Slope Shaping

In the Sankuang area, the soil covering and greening of coal gangue hill adopts the method of anti-slope terraces, mainly divided into three parts: platform, slope, and horse road. Coal gangue hills are generally layered. As shown in Figure 4, each layer is about 6 m to 15 m high, with a slope less than 1:1.75. The first step of restoration is to compact the coal gangue onto the slope and then cover it with a layer of soil. Then, the slope needs to be further compacted to reduce the pore space between the coal gangue. Two layers of cover are required for greening slopes. The first layer is a barrier layer, covering a thickness of 0.2 m to 0.45 m of loess. The loess should be compacted to form an impermeable layer to prevent rainwater from seeping into the gangue pile. The second layer is a planting layer, covered with stored natural mature soil, with a thickness generally ranging from 0.3 to 0.9 m, depending on the type of vegetation.

The separation between one slope and another depends on the horse road, which divides the slope into a step-like structure. The horse road allows vehicles to pass and can be used for rescue and maintenance. The width of the horse road is usually 3 to 6 m. A platform is set on the top of the gangue hill for compacting the top layer of coal gangue, forming a scaled greening platform. The height of the gangue hill is generally within 60 m, but it mainly depends on the height and stability evaluation of the surrounding hills, so that the gangue hill can blend in with the surrounding hills.

3.2.2. Vegetation Selection

According to years of practical experience in Yangquan City, the selection of tree species for coal gangue hills in the Sanmei area follows the following steps. First, cover the top platform and slope of the gangue hill with loess and apply appropriate agricultural fertilizers to improve soil fertility for greening. The slope is selected in the form of a grass–shrub combination. Alfalfa is the most popular grass species. It is a shallow-rooted plant with a critical temperature tolerance of up to 41 degrees and a high survival rate. For shrub species, it is required that the plants have deep roots, developed root systems, small crowns, and dwarfed scales to reduce the impact of trees on slope stability. The purple clover shrub is mostly selected. Its critical temperature tolerance is 39 degrees, and its survival rate is very high.

The top platform of the coal gangue hill is also in the form of a grass–shrub–tree combination. Trees and shrubs on the top platform are planted in pits.

To enhance the ornamental value of shrubs, hill walnuts, peaches, elm leaves, and crabapples are planted. In the trees, deciduous trees such as locust and torch are mainly planted, and evergreen trees such as pine and cypress are mainly selected. Figure 5 shows apple trees, peach trees, and vegetable greenhouses planted on the platform.

3.2.3. Slope Stability

The gangue hill rock mass is not as dense as natural rock masses and other natural hills. Although the gangue hill rock mass has been naturally accumulated for a long time, its interior is still relatively loose, with many voids. Shallow sliding of gangue and natural subsidence due to down-shift can easily occur, and water erosion and gully formation can appear [57]. In the slope engineering treatment of Yangquan mining area, the main method currently used is to form a concrete slope frame. Concrete is made into a mesh-like slope protection structure on the slope surface, and a 40 cm wide concrete grid is directly poured on the slope protection site to form a grid structure. Herbaceous plants are planted in the grid. The grid size is generally a 4 m × 4 m mesh. (Figure 6)

3.2.4. Cost Decomposition of Solar PV Stations

Table 2 shows the detailed initial investment decomposition [58,59]. The electricity prices, including benchmark electricity prices for desulfurization coal (BEDC) and retail tariffs, were collected manually from the provincial Development and Reform Commissions website. The yearly insolation data in Yangquan were collected from the National Aeronautics and Space Administration (NASA) [60].

The amount of fuel consumption for thermal power plants’ production in Shanxi and the net calorific value of various fuels are from the China Energy Statistical Yearbook 2017. The carbon dioxide emission factor is from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories Volume 2 Energy, which remains unchanged in the 2019 refinement version. The shadow prices of CO2 are from Ref. [9].

4. Results

4.1. Coal Gangue Hill Restoration Design

4.1.1. Suitability Exploration

Firstly, the construction of solar photovoltaic power plants will occupy a large amount of land resources, which are both precious and limited. Combining ecological restoration of coal gangue hills with photovoltaics can make full use of land resources. Secondly, the ecological restoration of coal gangue hills has a strong compatibility with photovoltaic power generation. The restored coal gangue hills have a stepped terrain, which is flat and regular, and is conducive to laying photovoltaics. However, not all coal gangue hills are suitable for the development of photovoltaics. The main influencing factors are the distance from residential areas, the orientation and stability of the coal gangue mountain, and the transportation convenience. According to existing research, the 280 Gangue Hill has returned to nature and there is no trace of the gangue hill. Moreover, it is not suitable for the development of photovoltaics due to the large slope caused by the flat ground stacking. The Qichi Coal East and Qichi Coal West gangue hills are closer to residential areas and are more suitable for developing social service functions, so they are not suitable for developing the photovoltaic industry.

4.1.2. Design Framework

To ensure the ecological benefits of coal gangue hills and maximize the economic benefits of coal gangue hills, the design idea of combining ecological restoration of coal gangue hills with photovoltaics is: (1) Decompose the coal gangue hills into platforms, slopes, and horse roads, so that each part can play a different role in the combination of ecological restoration and photovoltaics. (2) For the coal gangue hill platform, cover it with soil and plant it, and only lay solar panels at the edge of the platform. The platform maintains its vegetation diversity and is composed of trees, shrubs, and grasslands, which can better develop the agricultural economy. This will not affect the growth of vegetation, and the shade of trees will not affect the operation of photovoltaic panels. (3) The slope of the coal gangue hill is the main area for the construction of solar photovoltaic power plants. The construction can be carried out by combining slope stability treatment with fixed solar photovoltaic brackets. The slope of the coal gangue hill and the inclination angle of the photovoltaic panel are basically the same, so the slope protection project and the construction of solar photovoltaics can form a better combination. According to Table 2, the total slope area of the Yangquan Sanjing gangue hills, except for the Sanjingbeitousui gangue hill, is 746,876 square meters, and the platform area is 343,166 square meters. The area of the slope is twice as large as that of the platform, so the advantage of the large slope area can be fully utilized. In addition, the vegetation on the slope of the coal gangue hill is planted in the form of shrubs and grasses, and the vegetation is generally low, so the height requirements for photovoltaic brackets are not high and can be set at 1200 mm. Vegetation that does not require high illumination can be planted under the photovoltaic panel. This can not only ensure the requirement of a large area of solar photovoltaics, but also better combine slope protection and ecological restoration of the coal gangue hill. (4) The horse road of the coal gangue hill can be used as an internal road between solar photovoltaic stations, and trees can also be planted to form an ecological restoration combining trees, shrubs, and grasses.

4.2. Design of Photovoltaic Brackets Combined with Slope

In response to the New Waste Coal Gangue Hill and Danaoliang Coal Gangue Hill of Yangquan Sanjing, which are in a first-level maintenance state, and Sankuangbeitouzui Coal Gangue Hill, which is in an emission control state, a design plan combining photovoltaic brackets with slopes is proposed.

4.2.1. Design of Photovoltaic Panels and Brackets

The design of the coal gangue hill photovoltaic panel laying should not only meet the requirements of ecological restoration but also optimize the photovoltaic laying scheme with the least cost, using existing conditions to strive for economic benefits. We propose setting up photovoltaic panels on slopes below 35 degrees, arranged horizontally and vertically in a single continuous row. The photovoltaic panel size is designed to be 10,000 × 3200, composed of 20 photovoltaic panels. The size of the photovoltaic panel component is 1580 mm (length) × 880 mm (width) × 35 mm (thickness), with a horizontal pillar spacing of 2000 and a vertical pillar spacing of 4000, as shown in Figure 7 and Figure 8. The foundation of the photovoltaic bracket adopts a strip foundation with a width of 400 and a height of 500, C20 fine stone concrete, and a beam with a height of 400 in between.

4.2.2. Integration of Slope Stability and Photovoltaic Brackets

A grid-like slope protection is made on the slope of the coal gangue hill, and a grid structure is formed by directly pouring concrete on the slope protection site to form a grid with a size of 4 m × 4 m. The cross-sectional size of the grid structure is 400 mm (width) × 400 mm (height). Where photovoltaic brackets are needed, the grid size is adjusted to 2000 horizontally and 4000 vertically, and the cross-sectional size of the grid structure is adjusted to 400 width × 500 height.

4.3. The Area Photovoltaic Panel Layout

Most of the coal gangue hills in the Sanjing area are facing south, southeast, and southwest, which is conducive to the solar photovoltaic system’s power generation. In addition, the slope of the gangue hill is low, and about 80% of the slope is at 30 degrees or below. As shown in Table 3, the total slope area of the coal gangue hills is about 964,876 m2.

4.4. Economic Benefits of Coal Gangue Hill Restoration

This section takes the New Waste Coal Gangue Hill as an example to analyze the economic benefits of building a solar photovoltaic power plant. The New Waste Coal Gangue Hill has a total of 19 slopes with a total area of 132,511 square meters. As shown in Figure 4, the average height of the slope is 10 m, and the slope ratio is 1:1.8. Therefore, the length of the slope is about 20 m.

The width of a solar panel is about 3 m per kilowatt, and there should be a gap between solar panels set to 8 m, as shown in Figure 8. Therefore, the slope can be divided into 736 smaller units, and the area of each unit is about 180 m2. The available area for building a solar photovoltaic power station is about 94,208 square meters, so a total of 9.42 MW of solar photovoltaic power generation system may be in a stagnant state. The annual power generation can reach 12,130.32 MWh. The Shanxi desulfurized coal’s on-grid electricity price is 0.332 CNY/kWh, so the annual cash flow of the solar photovoltaic power plant can reach CNY 4027.27 million. Compared with the annual maintenance and repair cost of CNY 4.516 million, the economic benefits of the solar photovoltaic power plant are considerable. In terms of net present value, the total net present value of the solar photovoltaic power plant can reach CNY 4663.19 million. In addition, by reducing the consumption of thermal power, the annual carbon emissions can reach 11.75 tco2e. In fact, with the establishment of the carbon trading plan, these carbon emission reduction credits can bring additional economic returns.

4.5. Ecological Benefits of Integrating Solar PV stations with Coal Gangue Hills

The platform can mainly exert ecological effects, adopting a combination of trees, shrubs, and grasses for planting to ensure its main ecological benefits. A slope covered by photovoltaics can make rainwater fall on the photovoltaic panel and then fall down along the inclined plate to the ground below the photovoltaic panel, so that the soil moisture content within 80 cm wide below the photovoltaic panel can be increased by 30 to 60%. That is, the photovoltaic panel forms a rainwater collection surface [61]. The vegetation density under the lower edge of the photovoltaic panel on the slope is high, and the vegetation diversity has also increased significantly [62]. We propose planting local shrubs and grasses, such as black locusts and wolf tails, in the middle area of the slope photovoltaic panel. Experiments have shown that the shear strength of the black locust root composite soil with a 7-year growth is the largest, which improves the stability of the loess slope and makes the slope less likely to slide [63]. This way, the slope can not only achieve economic benefits but also ecological benefits.

5. Discussion

Firstly, from an economic perspective, the ecological restoration cost of coal gangue hills mainly comes from government financial allocation and self-raised funds by enterprises. Although enterprises have the responsibility to complete the ecological restoration of coal gangue hills, it still adds to the burden of enterprises. On the one hand, ecological restoration is not an overnight task, and long-term maintenance and management are required in the later stage, which requires long-term expenditure by enterprises. Considering the current financial situation of Yangquan Coal Group, the Three Mines area will face a funding gap of CNY 9 million in the next two years for processing and maintenance (without considering the increase in maintenance costs and unit prices). On the other hand, for closed mines, once the labor force is transferred elsewhere and the economic cycle is lost, the operating and management expenses of coal gangue hills will be even greater funding gaps that cannot be sustained. Thirdly, due to the immaturity of technology in the early stage, the survival rate of plants in coal gangue hills is extremely low, and there will still be phenomena of exposure and pungent odors. With the continuous upgrading of technology and the economic conditions permitting, the ecological sustainable development of coal gangue hills can be achieved. Therefore, the development of coal gangue hills urgently needs sustained financial support.

Secondly, this article changes the design of the solar photovoltaic system of coal gangue hills, which was mostly limited to the platform, to a combination with the slope of the gangue hill. From an ecological perspective, the top of coal gangue hills is more suitable for plant growth, and the slope is relatively weak, which is more conducive to ecological restoration. From the perspective of functionality, it can achieve the decomposition of the functions of the coal gangue hill platform and slope. From the perspective of government managers, it can improve existing land use policies and management methods. The platform can be handed over to agricultural and forestry departments, and the slope can be managed by local land and resources departments. This can change the previous method of environmental protection department acceptance, which is still under the management of coal enterprises. From the perspective of investment promotion, different social capital can be introduced to the platform and slope, operated in a market-oriented manner, and generate revenue.

Thirdly, based on the current situation, the combination of solar photovoltaic systems and coal gangue hills is still in the exploration stage. The rational combination of ecology and photovoltaics, considering the ecological benefits, has more significant benefits. The combination of slope stability and photovoltaic brackets requires better and more convenient construction, so the standardization of components and the convenience of construction need to be studied in follow-up research.

6. Conclusions and Recommendations

This paper discusses the feasibility of developing photovoltaic and ecological restoration in coal gangue hills from ecological, economic, design, and management perspectives. The main conclusions are as follows:

Firstly, based on the combination of the unique conditions of the coal gangue hill itself and the refinement design after suitability evaluation, the photovoltaic and ecological restoration can be realized. The advantages of platform planting can be fully utilized to maximize ecological benefits, and the advantages of the large area and unobstructed slope of the gangue hill can be used to develop photovoltaics. The optimization of the gangue hill at the optimal point can be achieved to develop photovoltaics and ecological restoration, achieving maximum economic benefits.

Secondly, from the perspective of combining slope stability and photovoltaic economic development, the 8 × 8 enclosure design is proposed to design the layout of the photovoltaic panel, considering the optimal combination of ecology and photovoltaics, and expanding the area of photovoltaic installation. The combination of slope stability and the foundation of photovoltaic brackets can not only ensure the long-term avoidance of landslide and soil cracking of the gangue hill but also become a good foundation for photovoltaic support.

Thirdly, through economic calculations, the income of the photovoltaic industry can effectively alleviate the maintenance cost of coal gangue hills and can be combined with surrounding hills to further expand the development of the photovoltaic industry, achieving higher economic benefits.

Fourthly, for coal resource-based cities such as Yangquan, facing problems such as production capacity withdrawal and transformation and development, the road of combining ecological restoration of coal gangue hills with industrial development can be explored to achieve coordinated development of ecological construction and clean energy industries, realizing a win–win model of ecological construction and economic development.

Author Contributions

Conceptualization, F.S. and X.L.; Methodology, F.S., Y.C. and B.B.; Validation, Y.C.; Investigation, Y.C. and B.B.; Data curation, F.S.; Writing—original draft, F.S. and B.B.; Writing—review and editing, X.L.; Supervision, X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Beijing Normal University Young Talent Research Start-up Funding grant number 310432106.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1.
The schematic diagram of a coal gangue dump.

Figure 1.
The schematic diagram of a coal gangue dump.

Figure 2.
The distribution of coal gangue hills in Sankuang.

Figure 2.
The distribution of coal gangue hills in Sankuang.

Figure 3.
Solar radiation in Shanxi Province. (A) The total annual radiation on horizontal surface; (B) the annual equivalent utilization hours.

Figure 3.
Solar radiation in Shanxi Province. (A) The total annual radiation on horizontal surface; (B) the annual equivalent utilization hours.

Figure 4.
Coal gangue hill slope map.

Figure 4.
Coal gangue hill slope map.

Figure 5.
Fruit trees and vegetable greenhouses planted on the platform of a coal gangue hill.

Figure 5.
Fruit trees and vegetable greenhouses planted on the platform of a coal gangue hill.

Figure 6.
Slope protection diagram.

Figure 6.
Slope protection diagram.

Figure 7.
Coal gangue hill photovoltaic laying schematic diagram.

Figure 7.
Coal gangue hill photovoltaic laying schematic diagram.

Figure 8.
Coal gangue hills laid pv diagram.

Figure 8.
Coal gangue hills laid pv diagram.

Table 1.
The main characteristics of coal gangue hills in Sankuang.

Table 1.
The main characteristics of coal gangue hills in Sankuang.

No.NameStatusArea (m2)Others1280 Coal Gangue HillClosed46,620.69
(2 slopes, 21,563.75; 1 platform, 17,543.68; 1 bridle path, 7513.26)Restored with luxuriant green plants2New Waste Coal Gangue HillClosed262,595.35
(19 slopes, 132,511.12; 1 platform, 70,868.31; 18 bridle paths, 58,488.91)Connected with Yikuang district3Qichimei East Coal Gangue HillClosed88,950.96
(6 slopes, 46,587; 4 platforms, 33,938.56; 3 bridle paths, 8425.4)Two plant areas are still idle4Qichimei West Coal Gangue HillClosed70,559.50
(6 slopes, 383,489; 4 platforms, 24,907.9 m2; 3 bridle paths, 7302.6)Restored as a park5Danaoliang Coal Gangue HillClosed402,264.37
(8 slopes, 165,724.84; 2 platforms, 195,907.71; 8 bridle paths, 40,631.83The ratio of living trees is low

Table 2.
Initial investment decomposition of DSPV for the residential sector.

Table 2.
Initial investment decomposition of DSPV for the residential sector.

ItemsPrice (CNY/kW)PV module1930PV support250Inverter300Cable200Confluence box and distribution box400Monitoring system150Labor costs (design, installation, debugging)400 (on average)Others (insurance, etc.)400 (on average)

Table 3.
The feasibility of developing the solar photovoltaic system.

Table 3.
The feasibility of developing the solar photovoltaic system.

No.NameArea of Slope
(m2)OrientationSlope GradientBuilding Solar Photovoltaics or Not1280 Coal Gangue Hill21,564Whole hill body25No.
It is a park already.2New Waste Coal Gangue Hill132,511Southwest30Yes.3Qichimei East Coal Gangue Hill46,587Southeast30No.
It is a park already.4Qichimei West Coal Gangue Hill383,489Southwest30No.
It is already well restored.5Danaoliang Coal Gangue Hill165,724.84West25Yes.6Sankuangbeitouzui Coal Gangue HillAbout 215,000Southwest30Yes.

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