1. Introduction
By the end of 2016 in north China, a total of 20.6 billion m
2 building floor areas had space heating, about 83% of space heating areas used coal boiler or coal CHP (combined heat and power), and the annual coal consumption was 400 million ton-coal-equivalent [
1]. A coal based heating system contributes significantly to winter air pollution [
2,
3]. There is thus an urgent need for clean energy building heating technology at the present stage. Geothermal energy has showed significant potential as a renewable energy resource because of its low environmental impact, low greenhouse gas emissions and its feasible technology [
4,
5,
6].
The energy reserves in hot dry rock (HDR) within subsurface of 3–10 km depths are more than 2.52 × 10
25 J in China [
1,
4]. To extract heat from HDR, the enhanced geothermal system (EGS) is proposed by creating artificial fractures in the hot rocks and injecting fluid into them [
7,
8,
9,
10]. At the present stage, the exploiting technology of EGS is immature. Generally speaking, exploiting an EGS is a huge capital investment. Moreover, the creation of the artificial fractures especially in the hard rock has a high risk and a high cost [
11,
12,
13,
14]. Hydrothermal resources in China are equivalent to 1.25 × 10
3 billion tons of standard coal, and its recoverable resources per year are equivalent to 1.9 billion tons of standard coal. However, the hydrothermal heating area in China was only 102 million m
2 by the end of 2015 [
1,
3]. One of the main causes for the large mismatch between potential estimates and developed hydrothermal resources is the lack of large and continuous geothermal reservoirs.
There are often lots of hydrothermal systems with a single well output of less than 30 m
3/h or even less due to low permeability, which leads to the poor economization of exploitation and utilization. One of the alternatives to utilizing low output geothermal wells is the technology of the deep borehole heat exchanger (DBHE), as shown in
Figure 1. Some researchers have focused on the utilization of geothermal energy from DBHE for building heating [
15,
16,
17,
18,
19,
20]. Bu et al. carried out experimental and simulation studies on DBHE for building heating, and concluded that the heat extraction from DBHE can hardly be further improved due to poor thermal conductivity of rocks [
21,
22]. To enhance the heat transfer between fluid and rocks, the artificial fractures are created in EGS by the technology of hydraulic fracturing in order to form the convection heat transfer. Based on this thought, if the shallow depth hydrothermal system with low permeability and low output can implement hydraulic fracturing like with EGS for building heating, this not only has a good use of structural fissures in hydrothermal system but also reduces the risk and investment costs of EGS. The shallow depth hydrothermal system has natural fractures interconnecting an injection well to a production well, and it is thus easy to conduct hydraulic fracturing and stimulation to create new fractures and highly conductive zones. In this study, a shallow depth enhanced geothermal system (SDEGS) is proposed, which is reformed from the hydrothermal system. The common problems in SDEGS and conventional EGS are basically the same, such as (1) the thermal breakthrough; (2) the risk of induced seismicity; (3) the detection of formation stress distribution; and (4) the control technology of artificial fractures orientation and the isolation technology to reduce the loss of circulating fluid. The achievements from SDEGS are expected to apply to conventional EGS, and thus to accelerate its development.
China has abundant solar energy resources and more than two-thirds of areas receive an annual total solar radiation that exceeds 5.9 GJ/m
2 with sunshine duration more than 2200 h [
23]. However, solar thermal has contributed little for space heating; only about 150,000 m
2 solar collectors were used for space heating in China in 2014 [
24]. The above fact is mainly attributed to the unstable and discontinuous energy supply of solar heat, thus back-up sources or seasonal storage equipment are required to ensure the smooth operation of solar space heating system, which leads to high engineering costs. To circumvent these problems, a hybrid geothermal and solar energy heating system is proposed. Storing heat from solar energy into DBHE is restricted due to rock having poor thermal conductivity. While for SDEGS, the heat convection is formed in the artificial reservoir, whose heat transfer coefficient is far greater than that of heat conduction. If combining SDEGS with solar energy for building heating, solar energy is able to store in geothermal reservoirs during the non-heating season, while during the heating season heat energy is also able to be extracted from geothermal reservoirs. Therefore, a hybrid of SDEGS and solar energy for building heating is recommended in this study. In the hybrid system, the problem of unstable and discontinuous energy supply for solar energy in the heating season can be solved by adjusting geothermal energy output; while in the non-heating season, the heat from solar energy can be stored in geothermal reservoirs to extend the service life of the hybrid heating system. The main aim of this study is to evaluate the performance of the hybrid SDEGS and solar system for building heating.
3. Results and Discussion
In
Figure 3a, only geothermal energy is used to provide heat for buildings. For the first fourteen years, the annual mean
Pg ranges from 4772.73 to 4268.73 kW, with a percentage reduction of 10.56%. The annual mean
Pg decreases drastically after fourteen years, indicating that the service life for stable output is 14 years in terms of reservoir behavior and operation conditions. The average
Pg in the first fourteen years is 4521.1 kW, which can provide heat for 129,174.3 m
2 buildings with specific heat load of 35 W/m
2 and can reduce carbon dioxide emissions by 13,671.8 tons per heating season.
Figure 3b shows the performance of the hybrid geothermal and solar energy heating system. In the heating season, the daily mean
Ps is 897.15 kW for
η = 0.5981. In the non-heating season, the daily mean
Ps is 1486.80 kW for
η = 0.6797. It is obvious from
Figure 3b that the annual mean
Pg decreases gradually for the first twenty-four years, and then it decreases quickly after twenty-four years. The average
Pg in the first twenty-four years is 4752.19 kW. In the first heating season, the annual mean
Pg is 4772.73 kW, which is the same as that of only geothermal energy heating. From the second to twenty-fourth heating season, the annual mean
Pg ranges from 4839.50 to 4672.94 kW, with percentage reduction about 0.15% per year.
Pg for the first heating season is less than that for the second heating season due to having no heat from solar energy stored into geothermal reservoir in the first heating season. The average
Pg of 35 years of operation is 4653.78 kW for the hybrid heating system. The
Pg difference between the second and the thirty-fifth heating season is 620.63 kW, with a percentage reduction of 12.82%. The daily mean
Ps is 897.15 kW, and the average
Pt for 35 years of operation is 5550.93 kW, which can provide heat for 158,598.20 m
2 buildings with a specific heat load of 35 W/m
2. For the hybrid heating system, the annual mean
Pg is 4218.87 kW for the thirty-fifth heating season, which is much the same as that of the fourteenth heating season for only geothermal heating systems. To sum up, the service life for a hybrid geothermal and solar energy heating system is 35 years compared to only geothermal energy heating system due to the solar energy heat stored in a geothermal reservoir.
Figure 3c shows the variation of
Pg from the beginning to the end of each heating season for both geothermal and solar energy. From
Figure 3c there is an attenuation for
Pg from the beginning to the end of each heating season, and the attenuation degree becomes bigger with the increase in heating years. For the fifth heating season, the attenuation degree of
Pg is 5.27% from the beginning to the end of the heating season, while it is, respectively, 6.79%, 8.08%, 9.20%, 11.11%, 19.89% and 27.43% for the 10th, 15th, 20th, 25th, 30th and 35th heating seasons.
From
Figure 3b,c, it can be concluded that
Pg is unstable during same heating season and the different heating season at the fixed injection water temperature (5 °C) and volume flow rate (50 m
3/h). Moreover, the energy supply of solar heat is unstable and discontinuous. Furthermore, the building heating loads vary with outdoor air temperature. To keep the steady running of the heating system, geothermal energy should be able to adjust the instability and meet the requirements of building thermal loads.
From the viewpoint of heat transfer principles, two main methods can be used to adjust heat transfer between rocks and fluid in term of the existing geothermal reservoir. The first method is to change the heat transfer coefficient, which can be implemented by changing the volume flow rate. The second method is to adjust the heat transfer temperature difference, which can be implemented by changing the injection water temperature.
The symbols
Q and
Tin, respectively, represent the volume flow rate and injection water temperature. From
Figure 4, the annual mean
Pg is 6667.81 kW at
Q = 70 m
3/h and
Tin = 5 °C, and it becomes 3897.73 kW at
Q = 50 m
3/h and
Tin = 20 °C.
Figure 4 indicates that
Q and
Tin have an important effect on
Pg. As is evident in
Figure 4, a higher
Q will cause a higher convective heat transfer coefficient between geothermal artificial reservoir and fluid, thus leading to a greater
Pg. Generally, the pump power has almost a cubic relationship with flow velocity. As a result, an increase in
Q will cause a rapidly increase in pump power. In the actual project, the pump power and
Pg should thus be comprehensively considered in order to decide a reasonable
Q. In addition, a decrease in
Tin causes an increase in temperature difference between rock and fluid, thus resulting in an increase in
Pg. To sum up, it can be concluded from
Figure 4 that
Pg can be adjusted easily by changing the injection water temperature and volume flow rate.
The average
Pt of 5550.93 kW from geothermal (4653.78 kW) and solar energy (897.15 kW) is used as the designed heating load, and it can provide heat for 158,598.20 m
2 buildings with a specific heat load of 35 W/m
2. The coldest weather comes in January (in the middle of heating season, China), and thus the actual heating load in January is generally greater than the designed heating load. The actual heating load in the middle of heating season is assumed to be 20% greater than the designed heating load. Similarly, the actual heating load is less than the designed heating load at the beginning and end of each heating season. Furthermore, the actual heating load is assumed to be 20% less than the designed heating load at the beginning and end of each heating season. The hybrid heating system will be safe if it can meet the heating load requirement in the 35th heating season due to having minimum
Pg. In the middle of the 35th heating season, the actual heating load is 6661.12 kW, and it is 4440.74 kW at the beginning and end of heating season. From
Figure 3c, at
Q = 50 m
3/h and
Tin = 5 °C,
Pg is, respectively, 4196.54 and 3553.61 kW at the middle and at the beginning and end of the 35th heating season, therefore, the difference between the actual heating load and
Pg is, respectively, 2464.58 and 887.13 kW. From
Figure 4, the difference of 887.13 kW at the end of heating season is easy to adjust by changing
Q and
Tin. In the middle of heating season with the coldest weather, only geothermal energy should be able to provide heat for all buildings if solar collectors do not work. From
Figure 4, at
Q = 70 m
3/h and
Tin = 5 °C, the annual mean of
Pg is 6667.81 kW, which is greater than the maximum value of actual heating load of 6661.12 kW, indicating that the design for the hybrid geothermal and solar energy heating system is reasonable. That is to say, the maximum heat loads requirement can be met by adjusting injection water temperature and volume flow rate even though solar collectors do not work.
The performance of DBHE in winter mode, without and with solar heat storage, is shown in
Figure 5. The area of solar collectors is 270 m
2 in
Figure 5. DBHE is a closed loop system and circulating fluid is not directly in contact with surrounding rocks, avoiding the problem of corrosion, scale formation and fluid losses occurring frequently in the open-loop system. However, in the closed loop system of DBHE, the heat conduction through rocks is the main heat transfer pattern, limiting its performance improvement due to the poor thermal conductivity of rocks. Likewise, storing heat from solar energy into DBHE is also restricted due to rock having poor thermal conductivity. We calculate several different solar collector areas and conclude that solar collectors with an area of 270 m
2 is appropriate and matched to DBHE with depth of 2600 m (the other parameters are the same as that of SDEGS, as described in
Section 2.1). While for SDEGS, the heat convection is formed in the artificial reservoir, whose heat transfer coefficient is far greater than that of heat conduction. For SDEGS, solar energy is able to store in geothermal reservoirs during the non-heating season, while during the heating season heat energy is also able to be extracted from geothermal reservoirs. Thus, a larger area of solar collectors (10,000 m
2) is needed for SDEGS.
4. Conclusions
Although China has rich heat energy reserves in hot dry rock, the technology used to exploit this at the present stage is immature. In addition, hydrothermal geothermal resources in China are also rich, however, the developed resources are far below the potential estimates due to scattered resources and low output of a single well. To efficiently utilize a low output geothermal well, a shallow depth enhanced geothermal system (SDEGS) is proposed, which can be implemented by hydraulic fracturing technology to transform the shallow depth hydrothermal system into an EGS. The shallow depth hydrothermal system has natural fractures, and it is easy to conduct hydraulic fracturing to create new fractures and highly conductive zones, and thus leading to the reduction of the hydraulic fracturing risk.
Only SDEGS with an artificial reservoir volume of 3.93 × 106 m3 provides heat for buildings, the service life for almost stable output is 14 years with the average Pg of 4521.10 kW. To extend the service life of geothermal reservoir, the hybrid SDEGS and solar energy heating system is proposed, which installs 10,000 m2 solar collectors by the roof-integrated technology to store heat in geothermal reservoirs. For the hybrid heating system, the average Pg for the 35 years of operation is 4653.78 kW at Q = 50 m3/h and Tin = 5 °C, and it can provide heat for building areas of 158,598.20 m2 plus solar energy. In the middle of heating season with the coldest weather, the maximum value of actual heating load with building areas of 158,598.20 m2 is 6661.12 kW, only geothermal energy can provide heat output of 6667.81 kW at Q = 70 m3/h and Tin = 5 °C even if solar collectors do not work. The hybrid SDEGS and solar energy heating system can extend the service life of SDEGS by storing solar energy heat in geothermal reservoirs and solve the problem of unstable and discontinuous energy supply for solar energy, as well as adjust the geothermal thermal output easily to meet the requirements of building thermal loads varying with outdoor air temperature. As a result, the smog problem in China caused by winter heating using fossil energy can be effectively solved by applied SDEGS technology and the hybrid SDEGS and solar energy heating technology due to having many shallow depth hydrothermal systems in China.
Compared with DBHE, the cost is one of the main factors obstructing the installation of SDEGS. Therefore, the effect of cost on SDEGS should be considered in future studies.