THE INFLUENCE OF TEMPERATURE ON GEOTHERMAL POWER PLANT PERFORMANCE BASED ON GEOLOGY IN HUANGSHADONG GEOTHERMAL FIELD, CHINA

The geological structure is rather complicated in Guangdong province, in China. The 10 deep fault belts are mainly oriented in NE. The NE fault belts are good channels for deep thermal energy upwelling, which can form a geothermal water reservoir zone. The heating of atmospheric precipitation and surface water by deep rocks is the primary formation mechanism for the hydrothermal resources in the Huangshadong geothermal field. The results show that the two-stage conversion system of flash-binary is more reasonable when the geofluid temperature is higher than 130 o C. With every 10 o C increment of geofluid temperature for flash-binary system, the power output and exergy efficiency increase by 21.6-38.7% and 6.0-13.1% respectively. The power output and exergy efficiency will decrease by about 20-40% when cooling temperature arising from 15 to 25 o C. The research will provide the basic data for the demonstration of geothermal resource exploitation.


Introduction
Geothermal energy can be used as a heat source for energy conversion.It is mainly distributed at the junction of plates.The geothermal heat potential stored within the upper 5 km of the earth has been estimated as 1.4×10 8 EJ.Less than 1% of this resource has been developed yet 78 countries report using a combined total of 4.3×10 5 TJ direct geothermal heat [1] .The geothermal resources with temperatures higher than 130 o C are suitable for geothermal power generation [2] .In China, the annual utilization energy quality of hydrothermal geothermal resources is about 1.9×10 9 tonnes of standard coal [3] .The geothermal resources are abundant in Guangdong Province because of the influence of the movement of the Eurasian, Indian and Philippine Sea plates.
Geothermal power generation was first tested in Larderello geothermal field, Italy.A capacity of 250kW geothermal power plant was successfully installed based on the test, which is the beginning of commercial geothermal power generation.And then, wet and dry steam geothermal power plants were built in Wailaki, New Zealand, and Geysers geothermal fields, United States respectively [4][5] .
Geothermal power plants have a positive effect on carbon emission reduction for carbon neutrality.In China, most of the geothermal resources belong to hydrothermal type and the temperature is below 150 o C. As a result, the available geothermal power technologies mainly include flash, Organic Rankine, Karina cycle and a combination of the above technologies.At the end of 2021, the installed capacity of geothermal power plants in the world had reached 15950MW, an increase of about 30% over 2015 [6] .In China, the installed capacity of geothermal power plants is about 47MW by the end of 2021, of which the installed capacity of Yangbajing and Yangyi power plants in Tibet is 43.08 MW, accounting for 91.5% [7][8] .Other power plants with small capacity are in trial operation, as shown in table 1.The theoretical research on geothermal power system mainly focuses on thermal performance and economic simulation.The thermodynamic and mathematical performance of the different cooling systems for geothermal power plants at various climatic conditions were analyzed in Turkey.the hybrid cooling systems can be determined at the operation periods for the geothermal power plants [9] .
The thermodynamic performance can be further improved by introducing geothermal energy for a coal-fired power generation system.It is found that the generated power of geothermal energy is increased by1272 kW, the power generation exergy and thermal efficiencies of geothermal energy are enhanced by 29.1%and 6.0% [10] .The effect of climatic environment on organic Rankine cycle (ORC) power system performance was evaluated, the water-cooling mode is preferred with better thermodynamic and economic performance for ORC [11] .Some research study a more efficient plant as well as multigeneration productions, the influence of geothermal water temperature, flash pressure, and ambient temperature on plant's performance are parametrically investigated.The integration of various systems with the geothermal power plant will be an increase in system performances [12] .A novel hybridization of a flash-binary geothermal cycle, a gas turbine cycle, and an organic flash is proposed.The net power of the proposed system at the base conditions is about 8016 kW and with 35.43% exergy efficiency.The air preheater effectiveness has the highest impact on system's performance from the view of heat transfer sensitivity analysis [13] .Four typical geothermal power system are proposed for low dryness hot dry rock geothermal fluid with 150-200 o C, the double stage flash combined organic Rankine cycle system has the best matching characteristics with geothermal fluids in terms of power generation performance [14] .Franco quantifies cost and efficiency factors and proposes a sustainability objective function, which will be applied in the design of geothermal powerplants [15] .
Most of the literature focus on high temperature geothermal resource.However, most geofluid temperature is less than 150 o C in China.The paper revealed the law between the distribution of geothermal hot spots and deep fault belts, and combined the subface feature with the surface utilization technology.The novelty of the paper is that it analyzed the effect of cooling water temperature on the thermodynamic performance of four geothermal power systems for typical hydrothermal resources in Huangshadong geothermal field, Guangdong Province, which provides the basic data for the demonstration of geothermal resource exploitation.

Geology Background
The development of rock fault structures and neotectonic activities in Guangdong province are continuously intense due to the influence of plate movement, especially against the background of repeated activity of deep and major faults, which provide favorable external environmental conditions for the formation of Guangdong geothermal fields.Figure 1 shows the deep and major faults of Guangdong Province.The 10 deep fault belts are mainly oriented in NE, and the 11 major fault belts are mainly oriented in EW and NW.The scales of NE and EW-trending fault belts are large, which constitute the main structural framework in the area; The scale of NW-trending faults is relatively small, the formation of faults is generally late and they often cut NE and EW-trending faults [16] .
The NE-trending deep fault belts mainly include Wuchuan-Sihui, Enping-Xinfeng and Lianhuashan fault belts.These deep and major fault belts are composed of large-scale ductile shear zones, dynamic thermal metamorphic zones and a series of brittle faults trending NE.The characteristic of these NE-trending faults is compressive and torsional first and then extensional.They are regional active major faults deep in the crust to the mantle and are good channels for deep thermal energy upwelling.They can form a geothermal water reservoir zone with certain width at the fault belt, which is a regional thermal control fracture.The formation of the geothermal field may be influenced by the Early Cretaceous and Late Jurassic intrusive rocks, and the geothermal spots are more active in the Huangshadong geothermal field where the Late Jurassic intrusive rocks are distributed.There is obvious correlation between the geothermal reservoir and the intrusive rocks.the geothermal reservoir belongs to convective geothermal field.The EW-trending deep fault belts mainly include Fogang-Fengliang, Gaoyao-Huilai and Suixi fault belts.Many hot spots arranged in NE direction show intense geological activity.
The exploitable amount of geofluid (down to 1000 m) in Guangdong Province is about 642,000 m 3 •d -1 .There are 320 natural hot spots with temperatures higher than 30 o C in Guangdong Province.There are 66 geothermal fields (20.7%) with temperatures between 30 and 40 o C; 158 geothermal fields (49.5%) with temperatures between 40 and 60 o C; 84 geothermal fields (26.6%) with temperatures between 60 and 90 o C; 12 geothermal fields (3.8%) with temperatures higher than 90 o C. The hot spots are presented in Figure 1.

Figure 1 The deep and major faults of Guangdong Province
The hot spots are arranged along the deep regional faults, and about seventy-five percentage of the hot spots are arranged on the NE or EW trending faults, in the vicinity of the deep fault belts.The hot spots occurring in groups are always found at the junction of faults and the hot spots are distributed linearly.Table 2 lists the locations of geothermal water above 90 o C in Guangdong Province (blue dots in figure 1).Huangshadong geothermal field in Huizhou is a naturally exposed hot spot group, located on the southwest edge of Lianhua Mountain Range.Huangshadong geothermal field is in hilly terrain with high terrain in the north and low terrain in the south.The geofluid in Huangshadong is of hydrothermal characteristic, and mainly distributed in the contact belt between a magmatic rock body and the surrounding rock.The geothermal system is controlled by faults and late volcanic and magmatic activities.The formation mechanism of hydrothermal resources for the Huangshadong geothermal field can be described as follow: atmospheric precipitation, surface water or normal temperature groundwater infiltrates through faults and weathering fissures, and then absorbs normal or high rock heat in the deep circulation process of fault belts, and then rises and is naturally expose or is revealed by artificial engineering exposure.The geothermal resources in research area are affected by the orogenic belt-type heat control tectonic system, which belongs to the plate boundary and the orogenic belt and the internal orogenic belt of the plate.Most of them are medium-high temperature geothermal resources, and the geothermal flow value is high, the average value is about 90-150 mW•m -2 .The boundary conditions of the Huangshadong geothermal field are delineated based on water and heat-conducting structures and combination with topography and isotope geochemical characteristics, as shown in Figure 2.
The temperature of hydrothermal water in the Huangshadong geothermal field is relatively high (see below), the pH value of geothermal water is 6.92 ~ 8.95, and the total soluble solids content is 847 ~ 1037 mg•L -1 .The water chemistry belongs to the HCO 3 -Na type.There source temperature estimated by the potassium and magnesium temperature scale method is 105.8 ~ 116.8 o C, with an average of 111.3 o C. The temperature of thermal storage estimated by the silica temperature scale calculation method is 155.3 ~ 158.9 o C, with an average of 157.1 o C. The circulation depth of hydrothermal geothermal water is about 1000m.
In the next section, a thermodynamic sensitivity analysis of the power system options for the Huangshadong geothermal field is presented.The following initial conditions of the hydrothermal geothermal water are estimated based on the geothermal well "Huire No 1" as shown in figure 2. (1) the depth of the production well is 1000 meters, (2) the temperature of geothermal water is 130 o C, (3)  the production rate of hot water is 150 t•h -1 .

System Description
The four physical models are single flash, double flash and Organic Rankine Cycle (ORC) and flash-binary systems.Figure 3 shows the four physical models of the proposed systems.
For single and double flash system, the geothermal water is extracted from the production well, and then flows in to de separator.The geofluid is separated into saturated steam and water.The steam flows into the turbine and the saturated water is pumped to the rejection well.
For organic Rankine cycle (ORC).the geothermal water is an intermediate fluid in the cycle.The organic fluid exchange heat energy with geofluid in the vaporizer and preheater.The gas of organic fluid flows into the turbine and then cooled in the condenser.The geothermal water flows into the injection well.The system was adopted indirect water-cooling method.A recuperator was used to improve the efficiency in the cycle.
For flash-binary system.It combined single flash and ORC.The process was the same as single flash system and ORC.However, the recuperator was not adopted in the ORC for the reason of simplification.
The optimization is a multi-objective, multi-variable problem.The initial parameters related to the system performance had to be defined, i.e. the heat source temperature and the mass flow rate.The optimum flash and evaporator pressure will be defined by maximizing the specific net power output (Ne).
Table 3 gives the related equations of the four systems.EXCEL was used to simulate the performance of the four systems.

Thermodynamic Performance Analysis and Discussion
Figure 4 gives the influence of T heat,source on maximum Ne and η 2 with different cooling water temperatures with single flash system.The maximum Ne and η 2 increase with the rising of T heat,source .
However, the slope of the curve is increasing in Figure5(a), that is, the increment ratio between power output per unit geofluid and geofluid temperature is rising with the rising of T heat,source , which means the Ne is more sensitive to geofluid temperature.The slope of the curve is decreasing in Figure 5(b), that is, the increment ratio between exergy efficiency and geofluid temperature decreases with the rising of T heat,source , which means the η 2 is less sensitive to geofluid temperature.At the condition of cooling water temperature T c of 25 o C, when the geofluid temperature rises from 80 to 90 o C, the increment of Ne and η 2 are 0.58 kWh•t -1 and 2.90 % respectively.When the geofluid temperature rises from 140 to 150 o C, the increment of Ne and η 2 are 1.34 kWh•t -1 and 1.36% respectively.
The smaller the cooling water temperature is, the better are the maximum Ne and η 2 .Under the condition of T c of 15 o C, and with the T heat,source are 100, 130 and 150 o C, the maximum Ne are 3.183, 6.535 and 9.405 kWh•t -1 respectively, which are increased by 40.7%, 26.6% and 21.8% respectively compared with those for cooling water temperature of 25 o C, The maximum η 2 are 25.9%, 30.49% and 32.78% respectively, which are increased by 41.0%, 26.7% and 21.8% respectively compared with those for cooling water temperature of 25 o C. Figure 6 gives the influence of T heat,source on maximum Ne and η 2 with different cooling water temperatures for double flash power system.The maximum Ne and η 2 increase with the rising T heat,source .Similar to the characteristic of the single flash system, the Ne is more sensitive to T heat,source , and η 2 is less sensitive to T heat,source .For a cooling water temperature of 25 o C, when the T heat,source rises from 80 to 90 o C, the increment of Ne and η 2 are 0.76 kWh•t -1 and 3.77 % respectively.When the T heat,source rises from 140 to 150 o C, the increment of Ne and η 2 are 1.71 kWh•t -1 and 1.64% respectively.
The smaller the cooling water temperature is, the better are the maximum Ne and η 2 .For a condition of cooling water temperature of 15 o C, and with the T heat,source are 100, 130 and 150 o C, the maximum Ne are 4.155, 8.452 and 12.09 kWh•t -1 respectively, which are increased by 40.4%, 26.2% and 21.4% respectively compared with those for a cooling water temperature of 25 o C, and the maximum η 2 are 33.8%,39.4% and 42.1% respectively, which are increased by 40.4%, 26.2% and 21.3% respectively compared with those for a cooling water temperature of 25 o C.
Figure 7 gives the influence of vaporizer temperature T 5 on Ne and η 2 for the ORC.Under the condition of constant T heat,source , the Ne and η 2 rise first and then decline with the ascendant of T 5 .When the Ne and η 2 get the maximum value, the T 5 is of a best value.With the T heat,source is 130 o C, the best value of T 5 is 86.02 o C, the Ne and η 2 get the maximum value of 5.205 kWh•t -1 and 24.14% respectively.The larger T heat,source is, the better are the Ne and η 2 .
(a) Ne (b) η 2 Figure 7 The influence of T 5 on Ne and η 2 of ORC Figure 8 gives the effect of geofluid temperature on maximum Ne, η 1 and η 2 with different cooling water temperatures for binary power system.The maximum Ne, η 1 and η 2 increase with the rising of T heat,source .Similar to the characteristic of the single flash system, the Ne is more sensitive to geofluid temperature, and η 1 and η 2 are less sensitive to geofluid temperature.Under the condition of cooling water temperature of 25 o C, with the geofluid temperature rises from 80 to 90 o C, the increment of Ne and η 2 are 0.56 kWh•t -1 and 3.34 % respectively.When the geofluid temperature rises from 140 to 150 o C, the increment of Ne and η 2 are 1.61 kWh•t -1 and 2.49 % respectively.
The smaller the cooling water temperature is, the better are the maximum Ne and η 2 .Under the condition of cooling water temperature of 15 o C, and with the T heat,source are 100, 130 and 150 o C, the maximum values of Ne are 2.749, 6.433 and 10.01 kWh•t -1 respectively, which are increased by 34.0%, 23.6% and 19.6% respectively compared with that under the condition of cooling water temperature of 25 o C, and the maximum η 2 are 22.1 %, 29.9 % and 34.7 % respectively, which are increased by 34.0%, 23.8% and 17.9% respectively compared with that under the condition of cooling water temperature of 25 o C. rise of geofluid temperature.Similar to the characteristic of the single flash system, the Ne is more sensitive to geofluid temperature, and η 2 is less sensitive to geofluid temperature.For a cooling water temperature of 25 o C, when the geofluid temperature rises from 100 to 110 o C, the increment of Ne and η 2 are 1.07 kWh  9 The effect of T heat,source on maximum Ne and η 2 of flash-binary power system with different cooling water temperature

The proposal of power plant construction
It is suitable to adopt the flash-binary power system in Huangshadong geothermal field.The indirect cooling water system should be used for the power system.The installed capacity of the power system is about 1MW.The mass flow rate and temperature of geofluid are 150 t•h -1 and 130 o C respectively.The mass flow rate and temperature of cooling water are 605 t•h -1 and 25 o C respectively.The sub-flash and rejection temperature of power system are 100.3 and 70.6 o C respectively.The power output per unit geofluid is 6.5 kWh•t -1 .The thermal and exergy efficiency are 9.4% and 30.5% respectively.

Conclusions
The formation mechanism of hydrothermal resources for the Huangshadong geothermal field is analyzed based on the geological background.Four geothermal power systems are considered for the Huangshadong geothermal field.The conclusions are as follow: (1) There are mainly 10 deep fracture zones and 11 large fracture zones in Huizhou city, Guangdong province, China.The deep fracture structures distributed in NE, and the large fracture structures distributed in EW and NW.The hot spots occurring in groups are found always at the junction of faults and the hot spots distributed along with deep fracture.The heating of atmospheric precipitation and surface water by deep rocks is the primary formation mechanism for the hydrothermal resources in the Huangshadong geothermal field.(2) The thermodynamic performance of two-stage conversion systems is better than that of ORC and single flash systems.The two-stage conversion system of flash-binary is more reasonable when the geofluid temperature is higher than 130 o C.
(3) The geofluid temperature has greater effect on power output compared with exergy efficiency.With every 10 o C increment of geofluid temperature for flash-binary system, the power output and exergy efficiency increase by 21.6-38.7%and 6.0-13.1% respectively.
(4) The cooling water temperature has an important influence on system performance.The power output and exergy efficiency will decrease by about 20-40% when cooling temperature arising from 15 to 25 o C.

Figure 2
Figure 2 Huangshadong geothermal field boundary range map

(a) Ne (b) η 2 Figure 4
The effect of geofluid temperature on maximum Ne and η 2 of single system with different cooling water temperature Figure5gives the influence of high-pressure separator temperature T 2 and low-pressure separator temperature T 8 on Ne with double flash system with T heat,source are 100, 130, and 150 o C the condition of constant T 2 , the Ne rises first and then declines with rising T 8 .Under the condition of constant T 8 , the Ne rises first and then declines with rising T 2 .The calculated data explanate that with the T heat,source are 100, 130 and 150 o C, the maximum Ne are 2.96, 6.695 and 9.958 kWh•t -1 respectively ， the maximum η 2 are 24.08%,31.25% and 34.72% respectively, the optimum T 2 are 76.8, 100 and 108.7 o C respectively, the optimum T 8 are 54.6, 70.56 and 69.5 o C respectively.With the condition of T heat,source is 130 o C, the pressure value of high-pressure separator is great than 1atm, the volume of high-pressure separator will be reduced.(a) T heat,source =100 o C (b) T heat,source =130 o C (c) T heat,source =150 o C Figure 5 the relationship between Ne and separator temperature for double flash system

(a) Ne (b) η 2 Figure 6
The effect of geofluid temperature on maximum Ne and η 2 of double system with different cooling

(a) Ne (b) η 2 Figure 8
Figure 9 gives the influence of geofluid temperature on maximum Ne and η 2 with different cooling water temperatures for flash-binary power system.The maximum Ne and η 2 increase with the

Table 3 The mathematical model of components
•t -1 and 2.96 % respectively.When the geofluid temperature rises from 140 to 150 o C, the increment of Ne and η 2 are 1.76 kWh•t -1 and 1.97 % respectively.The smaller the cooling water temperature is, the better are the maximum Ne and η 2 .Under the condition of cooling water temperature of 15 o C, and with the T heat,source are 100, 130 and 150 o C, the maximum value of Ne are 3.922, 8.243 and 11.99 kWh•t -1 respectively, which are increased by 42.1%, 27.2% and 22.1% respectively compared with that under the condition of cooling water temperature of 25 o C, and the maximum η 2 are 32.1 %, 38.8 % and 42.2 % respectively, which are increased by 42.1%, 26.9% and 21.8% respectively compared with those for a cooling water temperature of 25 o C.