- Email: [email protected]
Thermodynamic analysis of a solar–enhanced geothermal hybrid power plant using CO2 as working fluid
Accepted Manuscript Thermodynamic Analysis of a Solar–Enhanced Geothermal Hybrid Power Plant Using CO2 as Working Fluid Pei-Xue Jiang, Fu-Zhen Zhang, Rui-Na Xu PII: DOI: Reference:
S1359-4311(16)32936-2 http://dx.doi.org/10.1016/j.applthermaleng.2016.12.086 ATE 9706
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
31 October 2016 11 December 2016 20 December 2016
Please cite this article as: P-X. Jiang, F-Z. Zhang, R-N. Xu, Thermodynamic Analysis of a Solar–Enhanced Geothermal Hybrid Power Plant Using CO2 as Working Fluid, Applied Thermal Engineering (2016), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2016.12.086
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermodynamic Analysis of a Solar–Enhanced Geothermal Hybrid Power Plant Using CO2 as Working Fluid Pei-Xue Jianga,b,c , Fu-Zhen Zhanga,b,c, Rui-Na Xua,b,c,1 a b
Beijing Key Laboratory for CO2 Utilization and Reduction Technology
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education c
Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
Abstract: CO2-based Enhanced Geothermal Systems (EGS) and closed-loop supercritical CO2 Brayton cycles for solar thermal systems are both currently being developed for environmentally-friendly power plant systems. However, the recompressor needed in closed-loop supercritical CO2 Brayton cycles operates at high temperature and pressure with attendant higher manufacture and maintain cost. Here a solar thermal-EGS hybrid system is proposed where the geothermal power plant provides the base-load electrical power while the solar system increases the system capacity factor by generating additional electric power during peak demand hours. The two stand-alone systems and the hybrid system were modeled to predict the system efficiencies. It is found that comparison with the stand-alone CO2-EGS and CO2-solar thermal systems, the hybrid system has equal or even higher efficiency than the efficiency of the sum of two separate systems. Moreover, the operating pressure can be decreased and the recompression compressor can be removed in the hybrid system which reduces costs of the system installation and maintenance. Key word: solar thermal- EGS hybrid system; enhanced geothermal system; solar thermal power system; supercritical CO2 Brayton cycle
1
Corresponding author. Department of Thermal Engineering, Tsinghua University, Beijing 100084, PR China. Tel.: +086 10 62792294; fax: +086 10 62794664. E-mail address: [email protected]
Nomenclature EGS
Enhanced geothermal system
ORC
Organic Rankine Cycle
A
stimulated fracture area (m2)
cp
specific heat (kJ/(kg·K))
d
well diameter (m)
D
Darcy (1μm2≈10-12 m2)
f
Fanning friction factor
h
specific enthalpy ( kJ/kg)
H
reservoir thickness (m)
L
injector-producer distance (m)
m
CO2 mass flow rate (kg/s)
md
millidarcy (1md≈10 -15 m2)
p
pressure (MPa)
qwell
heat rate per unit depth (W/m)
Q
thermal energy (kJ)
Re
Reynolds number
s
specific entropy (kJ/(kg·K))
t
temperature (℃)
u
velocity (m/s)
v
specific volume (m3/kg)
W
half of reservoir width (m)
z
depth (m)
Greek symbol αv
volume expansion coefficient
Δ
change during a calculational step
ε
wellbore roughness (m)
ηt
thermal efficiency (%)
ηT
turbine efficiency (%)
ηC
compressor efficiency (%)
κ
reservoir permeability (m2)
μ
viscosity (Pa·s)
ρ
density (kg/m3)
Subscripts 1,…,10
cycle point
i
injection well
p
production well
res
reservoir
1. Introduction Economic development must be balanced with environmental protection for sustainable human development. Our world needs clean, affordable, reliable and sustainable energy systems with improved energy efficiencies. Renewable energy sources hold great promise but there are no renewable energy sources that can provide large scale, base-load electricity, with each renewable energy source having its own merits and drawbacks. Thermal energy from Enhanced (or engineered) Geothermal Systems (EGS) represents a large resource that can provide base-load electric power. The U.S. Department of Energy has broadly defined EGS as engineered reservoirs that have been created to extract economic amounts of heat from low permeability and/or porosity geothermal resources [1]. However, EGS systems are rather expensive, especially the well drilling and completion costs, which account for 60% or more of the total capital investment. In addition, the peak-valley difference in the electricity consumption in China is 30%-40%, so there is also a need for peak load systems. The drilling and reservoir stimulation costs will further increase if the electricity demand is fully provided by EGS power plants. Solar energy is widely available on the earth’s surface but electricity generation from solar power plants is subject to the daily fluctuations in the solar radiation, so in the development and implementation of concentrated solar power (CSP) plants, thermal energy storage is indispensable to accommodate the energy demand and to suitably integrate the generated electric power into the grid. Many areas that are suitable for solar power plants coincide with hot geothermal zones [2]. Thus, solar and EGS energy resources can be combined in hybrid systems to compensate for the drawbacks of each individual system [3]. Previous studies have presented a number of potential solar–geothermal hybrid configurations. The solar system can be incorporated into the geothermal system in four ways: (1) The solar source is employed in series with the geofluid to heat the ORC working fluid to increase the mass flow rate and/or maximum temperature [2]. (2) The solar energy is employed to increase the superheat of the ORC working fluid vapor after the vaporizer. (3) The solar energy is incorporated into the geothermal system to reheat the condensed fluid after the condenser [4]. (4) The solar system is used to boost the power plant output or decrease the geofluid flow rate. Mir et al. [5] considered two hybridization configurations of a solar trough system with a geothermal power plant. The first system had a fixed geothermal brine flow rate with a solar
system to boost the output of the hybrid power plant. The second system provided a fixed electricity generation rate with the solar system used to reduce the geothermal brine flow rate. Ghasemi et al. [3] developed a hybrid system with a higher second-law efficiency than the combined individual geothermal and solar systems with the advantage of one system being used to mitigate the weaknesses of the other system rather than using the solar energy as only an additional energy source. Zhou [6] studied a hybrid solar-geothermal plant with a supercritical ORC to evaluate its superiority over a subcritical hybrid plant. In this configuration, the solar energy increased the ORC working fluid superheat. Calise et al. [7] and Buonomano et al. [8] investigate the integration of solar and geothermal energy to produce simultaneously electricity, thermal energy and cooling energy. All these studies have indicated that solar energy combined with a hydrothermal geothermal plant has a thermodynamic advantage over stand-alone system, and also increases the efficient use of the low enthalpy geothermal energy. These hybrid geothermal-solar systems were all based on hydrothermal type geothermal resources using water as the heat transmission fluid. In EGS power plants, the geofluid should be re-injected directly into the injection well after discharging its energy to the energy conversion system. Steam or geothermal brine is produced in hydrothermal geothermal systems, so surface energy conversion systems are needed with dry steam, single-flash and double-flash units or binary units based on ORCs. Almost all attempts to develop EGS systems have used water as the heat transmission fluid, such as the Fenton Hill project in the U.S. [9], the Rosemanowes project in U.K. [10], the Hijiori and Ogachi project in Japan [11], and the Soultz project in Europe [12]. Water is a powerful solvent for many rock minerals, which may change the fracture permeability [13] and water losses to the reservoir are not desirable since makeup water may be expensive. Brown [14] proposed an EGS concept using CO 2 instead of water as the heat transmission fluid to reduce atmospheric CO2 emissions and use CO2 as useful resource. This proposal has attracted much attention by both academics and policy makers for its great potential to reduce CO 2 emissions. Pruess [15] focused on the mass flow and heat extraction rates of CO 2-EGS systems. Their results confirmed the advantages of CO 2 over water as the heat transmission fluid for EGS. Haghshenas Fard et al. [16] and Atrens et al. [17, 18] designed CO 2 thermosiphons that could operate without a pump due to the strong buoyancy leading to high self-driven flow rates. Zhang
et al. [19] analyzed the thermodynamic performance characteristics of CO 2-based EGS and water-based EGS systems and concluded that the CO 2-EGS is more appropriate for smaller reservoirs, while the water-EGS systems perform better for larger reservoirs for a given well diameter. The studies have confirmed that CO2 is not only a good heat transmission fluid but also an excellent working fluid for power cycles because of its unique properties including favorable transport properties, low salt solubility, high buoyancy forces and large compressibility [20-24]. Viete and Ranjith [25] investigated the storage capacity of sedimentary basins and identification of optimum injection conditions for geo-sequestration. Wang et al. [26] combined a Brayton cycle and a transcritical CO 2 refrigeration to produce cooling output, heating output and power output simultaneously to uses solar energy as the heat source to reduce fossil fuel consumption. Recent research [27, 28] has suggested that advanced Brayton cycles using supercritical CO2 as the working fluid for solar power plants may offer equivalent or higher cycle efficiencies than supercritical or superheated water steam cycles at temperatures appropriate for concentrated solar power (CSP) applications. A Brayton cycle designed to operate with a solar energy source can greatly increase the thermal to electric conversion efficiency, and will likely yield efficiencies in the vicinity of 50% [29]. Also, the systems are more compact, with less weight and smaller volumes, which will lower capital costs and may reduce system installation, maintenance and operating costs [30]. However, there have been no published investigations of hybrid solar and geothermal energy generation systems using CO2 as both the working fluid and the heat transmission fluid. The present study describes a hybrid solar and EGS system using a CO 2 EGS system and a supercritical CO2 Brayton cycle. This hybrid system uses geothermal energy as the primary energy source to provide the base-load electricity and the solar energy as a supplement to meet the peak electrical energy. This study also compares the potential of the hybrid system compared to the two systems used separately.
2. CO2-EGS Pruess [13] suggested that CO2 is superior to water in its ability to mine the heat stored in a geothermal reservoir. Zhang et al. [19] also indicated that CO 2-EGS systems produce more power than water-EGS systems for reservoirs with low recoverable thermal energies. These results are due to the CO2 thermophysical properties. The advantages of CO2 as a heat transmission fluid for
EGS and as a working fluid for turbines are summarized in thermodynamics terms as follows [13,19,20]: (1) Low dynamic viscosity and larger thermal expansion coefficient. The low dynamic viscosity reduces the frictional resistance along the well and in the reservoir, which reduces the power consumption for the fluid circulation system. The larger expansion coefficient generates much stronger buoyancy forces, which results in a net pressure increase between the injection and production wells, so the CO2 design can operate as a thermosiphon without a pump. (2)
Direct expansion in the turbine. This energy conversion system differs from conventional energy conversion systems using water as the heat transmission fluid usually with single-flash or double-flash units or binary units based on the organic Rankine cycle (ORCs). Flashing is an irreversible process, which is a pressure drop process without work output. For an ORC system in a low-grade geothermal heat energy conversion system, exergy losses will occur when the brine transfers thermal energy to the working fluid. With CO2 as the geofluid, the CO2 arrives at the plant as a supercritical fluid, so the CO 2 can be expanded directly in a supercritical turbine, which is similar to a dry stream geothermal power plant.
Zhang et al. [19] studied the performance of a CO2-EGS with a rectangular reservoir. Figure 1 shows a schematic of the CO2-based EGS with the injection process (1-2) in the injection well, heat extraction (2-3) in the reservoir and production (3-4) in the production well. The cycle thermodynamic processes are shown in the pressure-enthalpy (lgp-h) diagrams in Fig. 2 for the supercritical cycle with expansion in the turbine from 4 to 5. The cycles also have isobaric heat rejection (5-6) and adiabatic compression (6-1). The pressures at the turbine outlet (state 5) and the injection wellhead (state 1) are optimized to give the maximum power output for a given EGS and operating conditions. If the injection pressure is equal to the turbine outlet pressure, a pump is not needed to increase the CO2 pressure up to the injection pressure, so the cycle self-sustaining without a pump. In an actual reservoir, a portion of the injected CO 2 may be lost in the pores as the CO2 circulates through the reservoir and extracts heat from the rock. The CO 2 lost in the pores is sequestered in the subsurface with CO 2 added from storage to compensate for the sequestered
portion. As a result, this geothermal power plant simultaneously extracts heat energy from the hot dry rock and sequesters CO2 in the subsurface. 2.1 Physical model and calculational method An idealized fractured reservoir was considered, whose parameters were loosely patterned after the conditions of Luo et al. [31]. A five-spot well configuration with a basic pattern area of 600 m×600 m was modeled as shown in Fig. 3 (a) with a production–injection well distance of 424.2 m instead of an injector-producer doublet. CO2 was assumed to be injected at the central well with production of hot fluid at the corner wells. The CO 2 injected at the bottom of the injection well was assumed to be distributed equally throughout the stimulated reservoir. CO 2 is heated by the hot dry rock, then flows at equal rates into the four production wells. Thus, as shown in Fig. 3 (b), the computational region only represented 1/4 of the reservoir. Figure 3 (c) shows a typical computational control volume. Wellbore The thermophysical properties and the parameters such as the stimulated rock volume, the reservoir permeability, the wellbore parameters and the gas cooler outlet temperature are summarized in Table 1. The pressure and enthalpy changes in the wellbore were determined using [19]: u m /( d 2 / 4)
(1)
f 1.8log 6.9 / Re / 3.7 / d
1.11
2
p f , well f u 2 ( z / d ) / 2 f (8m2 z ) /( 2 d 5 )
p g z p f , well h qwell z u 2 / 2 g z
(2) (3) (4) (5)
where z is the wellbore depth, which varies from z = 0 m at the surface to the depth of the wellbore bottom. The distance intervals along the injection and production wells, ⊿z, in the FORTRAN model was set to 5.0 m. A constant heat rate along per meter pipe of 100 W/m, qwell, is assumed to exist between the reservoir rock and the injection well wall boundaries according to Zhang et al. [19] for the injection well. The flow in the production well is assumed to be adiabatic. Heat extraction in the reservoir
The average reservoir temperature drop for a period of 30 years before thermal breakthrough was calculated as in Luo et al. [31]. This study assumes that the heat energy in the reservoir is discharged at a uniform rate for 30 years. As the CO 2 flows through the fractures, energy is transferred from the rock to the CO2, which increases the CO2 enthalpy as: qres cV tres / RLT m(h3 h2 )
（6）
where qres is the recoverable thermal energy that can be extracted from a stimulated volume (V) with a temperature drop ( tres ), in the reservoir. ρ and c are the density and specific heat of the rock. RLT stands for the Reservoir Life Time in seconds. The CO 2 temperature difference between the reservoir outlet and the bottom of the production well is assumed to be 5℃. Pressure drop in the reservoir Luo et al. [31] and Zhang et al. [32] showed that the fractures around the wellbore had to be enhanced to reduce the pressure drop in the reservoir. This study considered two types of induced hydraulic fractures at different locations. The induced fractures in the reservoir region around the wellbores were modeled with a permeability of 100 md, while the reservoir far from the wellbores had a permeability of 35 md. The reservoir pressure drop was then modeled assuming the Darcy flow mood as: p m
dl dA
(7)
where dl, the distance intervals along the reservoir, was 0.5 m. 2.2 CO2-EGS performance A FORTRAN computer code was developed to simulate the performance of the CO 2-based EGS. The CO2 thermodynamic properties were evaluated using the NIST REFPROP code. The thermal efficiency is defined as:
t
Wnet qres
(8)
Where Wnet is the net work output of the cycle that is the difference between the pump work, Wp, and the turbine work, Wt. The CO2-EGS performance varies with the injection pressure for reservoirs at different depths are shown in Fig. 4. These results show that the production pressure (p4) is greater than the
injection pressure (p1) for these conditions. The CO2 pressure at the production well exit increases as the geothermal source temperature increases. Since the temperature and pressure at the production well outlet exceed the injection parameters, the EGS is not only a heat source but also a compressor that circulates the CO 2. The CO2 can then expand as the working fluid through the turbine. This characteristic makes CO2-EGS systems more attractive.
3. Advanced Brayton cycle using supercritical CO2 as the working fluid for solar power cycles Closed-loop supercritical CO2 Brayton cycles have been studied by Denholm et al. [27,28]. The thermal efficiency of a simple CO2 Brayton cycle is not very high, because the CO2 outlet temperatures from the CO2 turbine are still quite high that means relatively little thermal energy is being converted into work. The CO2 expands in the turbine with (T / p) s determined assuming an isentropic expansion and Maxwell’s equation: T T v T v v cp p s T p c p
(9)
Although CO2 has a relatively large volumetric expansion coefficient ( ) and a low specific v
heat (cp) the typical turbine expansion ratio of about 2.0 gives a relatively small temperature drop during the expansion process. The recuperator then recovers the sensible heat in the waste gas. Studies suggest that recompression and partial cooling may give higher cycle efficiencies of 50% [27-29]. The reason that the recompression and partial cooling is superior to the simple regenerative cycle may be understood from the heat transfer between the high-pressure and the low-pressure fluids in the heat exchanger. The specific heat of supercritical CO2 varies strongly with pressure and temperature as shown in Fig. 5. The specific heat of CO 2 at 15.0 MPa is much larger than at 8.0 MPa for temperatures of 70-200℃. In a simple regenerative cycle with a constant mass flow rate, the mc p of the CO2 on both sides of the heat transfer surfaces in the recuperator are not well matched since cp is sensitive to the pressure and temperature, so the heat transfer temperature difference is quite large, which increases the irreversibility. Recompression and partial cooling can overcome this shortcoming, ever though the compressors consume work. The CO2 temperature at the solar receiver outlet (tsolar) (the maximum cycle temperature) was assumed to range from 400℃ to 600℃ and the thermal energy provided by the solar collectors
(Qsolar) was assumed to be 3.0 MW. The thermophysical properties, initial conditions, solar receiver parameters and turbine and compressor efficiencies are also summarized in Table 1. The irreversible heat transfer losses in the recuperators are reduced by using three recompressors in the cycle as shown in Fig. 6. A FORTRAN computer code also was then developed to calculate the cycle performance. To more intuitively display the temperature distributions along the heat exchangers, the model assumes counter-flow, double-pipe heat exchangers. The heat exchangers were assumed to be made of copper for the inner tube and stainless steel for the outer tube. The hot CO 2 discharged by the turbine flowed in the inner tube, while the cold CO2 from the compressor flowed in the annulus in the opposite direction. The heat transfer coefficients in the low-pressure supercritical region were calculated using the heat transfer correlation of Dang-Hihara [33], while the heat transfer coefficients for the high-pressure CO 2 that was heated by the low-pressure CO2 flowing in the inner pipe was calculated using the Jackson and Hall correlation [34] with consideration of the variable thermophysical properties with temperature and pressure. The cycle thermal efficiency is defined as:
t
Wnet Qsolar
(10)
The cycle thermal efficiencies vary with the operating pressure (the pressure at which CO 2 flow through the solar collector), p1, and the turbine discharge pressure (sink pressure), p2, as shown in Fig. 7. Table 2 shows that the CO2 heated to state 10 with a temperature near the turbine outlet temperature gives the maximum solar energy exergy efficiency. Figure 7 shows p1 and p2 for the maximum efficiency. The optimal operating pressures exceed 20.0 MPa and the compressor C3 (as shown in Fig. 6) discharge temperature, t9, exceeds 200℃. The allowable system operating pressure is limited by the mechanical strength of the heat exchangers with higher operating pressures increasing the facility cost. The compressor is the key component in the supercritical CO2 recompression cycle and high pressure and temperature are a challenge for CO2 compressor design. Sandia National Laboratories is investigating fundamental issues relate to compressor flow efficiencies [35] and have greatly improved designs, but are still early in the test program.
4. Hybrid solar-EGS system using CO2 as the working fluid For CO2-EGS, one of key issue is the well diameter, which limits the mass flow rate and in turn limits the recoverable thermal energy from large reservoirs. A second issue is to slow the depletion of the thermal energy in the geothermal reservoir over time and to extend the geothermal reservoir life time. The energy removal can be slowed by using the CO 2-EGS power plant to provide the base-load electric power, while the solar energy supplies additional electric power to meet the peak demand. As stated in section 2, the EGS serves as not only the heat source but also as the compressor. The pressure and temperature at the production well outlet are much higher than the injection pressure and temperature. The CO 2 from the compressor can then be replaced with CO 2 from the EGS in the supercritical Brayton cycle for the solar recompression power cycle. This study presents a hybrid solar-EGS power cycle that does not need a recompressor as shown by the schematic diagram in Fig. 8. The hybrid power cycle simulation divided the reservoir into three layers at different depths. Three sets reservoir depths are referred to case 1, case 2 and case 3 to show the effects of well depth on the system performance. The CO 2 is injected at different injection pressures into three injection wells connected to the reservoir at these different depths. The CO2 extracts heat from the reservoir and then flows to the surface through three production wells. The CO2 streams at the production wells have different temperatures and pressures with deeper reservoir levels giving higher temperatures and pressures. The CO2 from the shallowest reservoir RⅠ at state 1p is split into two streams. One stream flows into CO2 turbine TⅠ and expands to the sink pressure (1p-a) to generate work. The other stream flows into the solar power system as the base fluid for the solar-EGS hybrid system. The CO2 from reservoir RⅡflows first into CO2 turbine TⅡ and expands. When the high pressure fluid has expanded to the same pressure as the CO2 from reservoir RⅠ(2p-6), a portion of the CO 2 is extracted from turbine TⅡ and mixed with the CO 2 heated by the low-pressure CO2 in heat exchanger HXⅠ while the rest CO 2 expands directly to the sink pressure (2p-b). The CO2 from the deepest reservoir RⅢ undergoes the same thermodynamic process as the CO2 from reservoir RⅡexcept that the CO2 extracted from high pressure turbine TⅢ mixes with the CO 2 from HXⅡ. Thus, the three CO2 streams with the same pressure and different temperatures replace the three streams from the three compressors shown in Fig. 6. The mixed CO2 passes into the last stage of
the regenerative heater and is heated by the turbine exhaust CO 2 from the CO2 turbine. At the end, the CO 2 flows into the solar receiver and is heated by the solar energy to a high pressure, high temperature working fluid. The CO 2 expands in turbine TⅣ to generate work. The hot gas leaving TⅣ is hotter than the feed fluid at state 1p so a regenerative cycle is used to recover the waste heat. At the same time, the exhaust CO2 from HXⅠat state 5 with the exhaust fluid from TⅠ, TⅡ and TⅢ is cooled in a gas cooler by the energy sink. The main cold stream is then split into three streams and is compressed to the desired injection pressures and then injected into the three injection wells to complete the cycle. A FORTRAN computer model was developed to analyze this hybrid cycle with the CO 2 thermal and hydrodynamic properties evaluated using the REFPROP model. The parameters are also summarized in Table 1. The simplest model of this system simulates the regenerative heaters as constant pressure heat exchangers and the compressors and turbines as adiabatic. The complete description of this cycle includes the heat energy provided by the solar collector (Qsolar) the solar receiver exit temperature (tsolar) the reservoir parameters, the compressor efficiency, the turbine efficiency, the sink pressure and the injection pressure. With this information, all the operating states and interactions can be evaluated. The injection pressure and the turbine discharge pressure were varied to obtain the maximum thermal efficiency and power. The optimized operating parameters and the system outputs are tabulated in Table 3 for three solar CO2 temperatures (tsolar). The overall performance is judged by the cycle efficiency and the net power output. The cycle efficiency defined as:
t
Wnet Qsolar qres
(11)
where qres is the total heat of the CO 2 extracted from the three-level reservoirs. The net power output of the hybrid solar-EGS system is compared in Fig. 9 with the total for the stand-alone solar and EGS systems. The total net power output of the two stand-alone (CO2-EGS and solar) systems is based on a three-level geothermal reservoir with the solar system having a fixed energy and temperature outputs. The energy balance results including the power consumption and production and the detailed energy inputs and outputs are listed in Table 4.
Figure 9 shows that the net power output of the hybrid system is greater than for the two stand-alone systems at the low solar collector discharge temperature for cases 1 and 2. For case 3, the hybrid system is superior to the two stand-alone systems up to 600℃. The hybrid system gives higher performance for two reasons. First, the heat transfer processes in the heat exchangers more carefully match when using CO2 from the three reservoirs at different depths as the base fluid and/or as the supplementary fluids. This can be seen in the simulation results for the three regenerative heaters as shown in Fig. 10 where the axial temperature difference is almost uniform along the whole length of the heat exchanger for case 3. This occurs only when
mcP hot mcP coolant for a counter-flow heat exchanger with the two streams flowing in opposite directions. Thus, the mass flow rates of the base fluid and the supplementary fluids must be optimized according to the reservoir characteristics and the solar system parameters to match the capacity rates ( mcP ) of the two streams on both sides of the heat exchangers even though the cp of the two streams differ significantly. The optimized design minimizes the heat transfer temperature difference between the two streams. The hybrid systems for cases 1 and 2 are not better than the two stand-alone systems at high solar collector discharge temperatures due to the pressure and temperature at the production wellhead being too low to integrate well with the solar system to obtain a good thermal match between the hot low-pressure fluid and the cold high-pressure fluid. The second reason is that the hybrid system is supercritical so the CO 2 supplied to the hybrid system after it expands to the operating pressure will produce less shaft work if it directly expands to the sink pressure than if the cold CO 2 is compressed for the solar system as shown in Fig. 6. As shown in Table 4, the total power consumption of the three compressors is 732.7 kW (327.6/62.1/343.0) which is 44.5% of the work output (1648.1 kW) of turbine TⅣ. In the hybrid system, the EGS production fluid is used as both the base fluid and the supplementary fluid without the need for recompression by a compressor. The net power output of the solar-EGS peaking unit increases to 1574.8 kW while the net power output of the CO 2-EGS base-load power plant is reduced by 563.6 (4280.9/3717.3) kW. The net power output is then increased by 659.4 kW and the total net power is increased by 95.8 kW. Table 4 shows that the solar hybrid system output is about 30% of the total output with higher solar collector discharge temperatures giving
higher proportions of the output provided by the solar system. 5. Discussion CO2-EGS not only offers advantages for geothermal energy utilization but also provide achieve geologic sequestration of CO2 as an ancillary benefit. The lower specific heat of CO2 gives a large CO2 mass flow rate for a given reservoir. Increasing the wellbore diameter or the ratio of the number of production wells to the number of injection well will improve the CO 2-EGS performance but will increase the cost of drilling wells. The solar energy system can be integrated into the EGS to provide more efficient electric peaking load generation. The hybrid system can then be successfully implemented to give better efficiencies for the following reasons: (1) Fully utilizing the CO2-EGS characteristics. The strong buoyancy in the CO2-EGS leads to the production well pressure being much greater than the injection well pressure, so the EGS is not only a heat source but also a CO2 compressor with the production pressure and temperature of the CO2-EGS increasing as the reservoir temperature increases. Integrating the CO2 from the EGS into the solar power plant cycle at the proper pressure and temperature can simplify the solar power plant system and reduce the optimal operating pressure. At the same time, thermal energy storage is not needed for the solar power plants. (2) Multiple reservoirs levels. Multiple reservoirs levels will reduce drilling costs and increase the power output for a given target reservoir by utilizing more of the thermal energy. At the same time, the multiple reservoir levels provide the pressures and temperatures required by the solar power plant cycle to match the capacity rates of the CO2 discharged from the turbines at different temperatures. (3) The reservoir level depths can be optimized relative to the solar collector outlet temperature and the operating and sink pressures to give the best temperature ranges since the CO2 specific heat on the two sides of the heat exchanger vary strongly with temperature. For the stand-alone solar recompression cycle, the gas cooler exit temperature and the compressor performance determine the temperatures at states 7, 8, and 9, which limit the cycle efficiency. Thus, the hybrid system provides a more flexible temperature glide match for the regenerative heaters that improve the cycle efficiencies.
6. Conclusions A hybrid solar-EGS power cycle was designed with the primary energy source being an EGS with the solar source providing supplemental power. Simulation results indicate that: (1)
Integrating the EGS into the solar power plant reduces the operation pressure, which is a key parameter affecting the heat exchangers safety and the equipment costs. At the same time, the solar power plant is simplified by reducing the capacity of thermal energy storage.
(2)
The system does not need a recompression compressor and simplify the cycle.
(3)
The net output of the hybrid system is greater than the total of the two stand-alone systems when the operating parameters and the reservoir levels are optimized for the solar system parameters and the sink temperature.
(4)
The electric peaking load is supplied by the solar system so the system will draw less energy out of the geothermal reservoir and extend the reservoir life time.
Acknowledgements This project was supported by the national key Research and Development Plan (No. 2016YFB0600805). We thank Prof. David Christopher for editing the English.
References [1] An assessment by an MIT-led interdisciplinary panel (chaired by J.W. Tester), The Future of Geothermal Energy, Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century; 2006. [2] M. Astolfi, L. Xodo, M.C. Romano. E. Macchi, Technical and Economical Analysis of a solar–geothermal hybrid plant based on an Organic Rankine Cycle, Geothermics 40 (2011) 58–68. [3] H. Ghasemi, E. Sheu, A. Tizzanini, M. Paci, A. Mitsos, Hybrid solar–geothermal power generation: Optimal retrofitting, Appl Energy 131 (2014) 158–70. [4] A. Lentz, R. Almanza, Parabolic troughs to increase the geothermal wells flow enthalpy, Sol Energy 80 (2006) 1290–5. [5] I. Mir, R. Escobar, J. Vergara, J. Bertrand, Performance analysis of a hybrid solar–geothermal power plant in Northern Chile, In: World Renewable Energy Congress, Sweden, 2011, pp, 1281–8. [6] C. Zhou, Hybridisation of solar and geothermal energy in both subcritical and supercritical Organic Rankine Cycles, Energy Convers Manage 81 (2014) 72–82. [7] F. Calise, A. Cipollina, M. Dentice d’Accadia, A. Piacentino, A novel renewable polygeneration system for a small Mediterranean volcanic island for the combined production of energy and water: Dynamic simulation and economic assessment, Appl Energy 135 (2014)
675–93. [8] A. Buonomano, F. Calise, A. Palombo, M. Vicidomini, Energy and economic analysis of geothermal-solar trigeneration systems: A case study for a hotel building in Ischia, Appl Energy 138 (2015) 224–241. [9] D. Duchane, Geothermal Energy. Encyclopedia of Chemical Technology, New York: John Wiley & Sons, 1993, pp, 512-39. [10] A.S. Batchelor, Reservoir behaviour in a stimulated Hot Dry Rock system, Proc. 11th Annual Workshop on Geothermal Reservoir Engineering, Stanford University, Calif., USA, 1986, pp, 35–41. [11] Y. Hori, K. Kitano, H. Kaieda, K. Kiho, Present status of the Ogachi HDR Project, Japan, and future plans, Geothermics 28 (1999) 637-45. [12] A. Genter, H. Traineau, C. Dezayes, P. Elsass, B. Ledesert, A. Meunier, et al., Fracture analysis and reservoir characterization of the granitic basement in the HDR Soultz project (France), Geotherm. Sci. Tech. 4 (1995) 189-214. [13] K. Pruess, Enhanced geothermal systems (EGS) using CO 2 as working fluid-a novel approach for generating renewable energy with simultaneous sequestration of carbon, Geothermics 35 (2006) 351–67. [14] D. Brown, A Hot Dry Rock geothermal energy concept utilizing supercritical CO 2 instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering. Stanford University, 2000, pp, 233–8. [15] K. Pruess, On production behavior of enhanced geothermal systems with CO2 as working fluid, Appl. Therm. Eng. 49 (2008) 1446–54. [16] M. Haghshenas Fard, K. Hooman, H.T. Chua, Numerical simulation of a supercritical CO 2 geothermosiphon, Int. Com. Heat Mass Transfer 37 (2010) 1447–51. [17] A.D. Atrens, H. Gurgenci, V. Rudolph, CO2 thermosiphon for competitive geothermal power generation, Energy Fuels 23 (2009) 553–7. [18] A.D. Atrens, H. Gurgenci, V. Rudolph, Economic optimization of a CO 2-based EGS power plant, Energy Fuels 25 (2011)3765–75. [19] F.Z. Zhang, P.X. Jiang, R.N. Xu, System thermodynamic performance comparison of CO2-EGS and water-EGS systems, Appl. Therm. Eng. 61 (2013)236-44. [20] W. J. Cao, W. B. Huang, F.M. Jiang, Numerical study on variable thermophysical properties of heat transfer fluid affecting EGS heat extraction, Int. J. Heat Mass Trans. 92 (2016) 1205-1217. [21] J.Y. Wang, S.B. Hu, Z.H. Pang, et al. Estimate of Geothermal Resources Potential for Hot Dry Rock in the Continental Area of China. Science & Technology Review 2012; 30:25–31. [22] Y.J. Zhang, Z.W. Li, L.L. Guo, P. Gao, X.P. Jin, T.F. Xu, Electricity generation from enhanced geothermal systems by oilfield produced water circulating through reservoir stimulated by staged fracturing technology for horizontal wells: A case study in Xujiaweizi area in Daqing Oilfield, China. Energy 78 (2014) 788–805. [23] P. Olasolo, M.C. Juarez, J. Olasolo, M.P. Morales, D. Valdani, Economic analysis of
Enhanced Geothermal Systems (EGS). A review of software packages for estimating and simulating costs, Appl. Therm. Eng. 104 (2016) 647-658. [24] J.L. Zhu, K.Y. Hu, X.L. Lu, X.X. Huang, K.T. Liu, X.J.Wu, A review of geothermal energy resources, development, and applications in China: Current status and prospects, Energy 93 (2015) 466-483. [25] D.R. Viete, P.G. Ranjith, The effect of CO2 on the geomechanical and permeability behavior of brown coal: Implications for coal seam CO 2 sequestration, Int. J. Coal Geol. 66 (2006) 204–16. [26] J.F. Wang, P. Zhao, X.Q. Niu, Y.P. Dai, Parametric analysis of a new combined cooling, heating and power system with transcritical CO2 driven by solar energy, Appl Energy 94 (2012) 58–64. [27] P. Denholm, M. Mehos, Enabling greater penetration of solar power via the use of CSP with thermal energy storage. NREL, Golden, CO, 2011. [28] P. Denholm, Y.H. Wan, M. Hummon, M. Mehos, An analysis of concentrating solar power with thermal energy storage in a California 33% renewable scenario, NREL Report No. P-6A20-58186. Golden, CO: NREL, 2013. [29] B.D. Iverson, T.M. Conboy, J.J. Pasch, A.M..Kruizenga, Supercritical CO2 Brayton cycles for solar-thermal energy. Appl Energy 111 (2013) 957–70. [30] T. Neises, C. Turchi, A comparison of supercritical carbon dioxide power cycle configurations with an emphasis on CSP applications, Energy Procedia 49 (2014) 1187 – 96. [31] F. Luo, R.N. Xu, P.X. Jiang, Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO2 as the working fluid (CO2-EGS). Energy 64 (2014) 307-22. [32] F.Z. Zhang, R.N. Xu, P.X. Jiang, Thermodynamic analysis of enhanced geothermal systems using impure CO2 as the geofluid, Appl. Therm. Eng 99 (2016) 1277–1285 [33] C. Dang, E. Hihara, In-tube cooling heat transfer of supercritical carbon dioxide. Part 1. Experimental measurement, Int. J. Refrigeration 27 (2004) 736–47. [34] J.D. Jackson, Fluid flow and convective heat transfer to fluids at supercritical pressure, Nucl. Eng. Des. 264 (2013) 24–40. [35] S.A. Wright, R.F. Radel, M.E. Vernon, G.E. Rochau, P.S. Pickard, Operation and analysis of a supercritical CO2 Brayton Cycle. SAND2010-0171, Sandia National Laboratories, Albuquerque, NM, 2010.
CO2 Turbine
Gas Cooler
5
Cooling Tower
6
1
4
Production Well
Makeup CO2 Injection Well
Reservoir 3
2
Fig. 1 CO2-based EGS schematic
3
p, MPa
2
4(p4, t4)
1(p1, t1) 6
5 h, kJ/(kg·K)
Fig. 2 Pressure-enthalpy (lgp-h) diagram for a CO2-EGS cycle with a higher gas cooler outlet temperature than critical temperature
Injection Well
l
H
dl
(c)
(b) Production Well
(a) 2W
l
W l
dl
dl H
W
2W L 2L
Fig. 3 (a) Five-spot well pattern. CO2 is injected at the central well with production of hot fluid
at the corner wells, (b) 1/4 computational model and (c) a computational control volume of width dl
0
t4, C
210 180 150
p4, MPa
30
Reservoir: 600 m00 m 200 m pcooler=8.0 MPa
0 25 t =35 C cooler
20
4.0 km 5.0 km 6.0 km
15 24
, %
22 20 18 16
8
9
10
11 12 p1, MPa
13
14
15
Fig. 4 CO2-EGS operating performance for reservoirs at different depths with similar sizes and other operating conditions.
5
4
cP
p=15.0 MPa 3
2
p=20.0 MPa
p=8.0 MPa
1 0
100
200
0
300
400
500
t, C
Fig. 5 Supercritical CO2 specific heat vs. temperature for different pressures
Solar receiver
CO2 Turbine 1
2 (a)
C3
C2
C1
9
8
7
10
6
5 3 HXⅠ
HXⅢ
4
HXⅡ
1
700 (b)
2
t, ℃
10
9 3 8 4 7 35
5
6
s, kJ/(kg·K)
Fig. 6 (a) Supercritical CO2 recompression Brayton cycle and (b) its temperature-entropy (t-s) diagram
43.0
t, %
42.5 42.0
p2=8.2 MPa tcooler=35 C
41.5
t1=600 C
41.0
16
18
20
22
24
26
28 30 p1, MPa
43 p1=20.0 MPa
t, %
42
tcooler=35 C t1=600 C
41 40 7.5
8.0
8.5
9.0 p2, MPa
9.5
10.0
Fig.7 Thermal efficiencies of the supercritical CO2 Brayton cycle in a solar power cycle for various operating sink pressures
a
receiver
Solar
Gas cooler
（a）
1 TⅣ
TⅠ HXⅡ
2
4
3
8
HX Ⅲ
b
7
d
c TⅡ
TⅢ
HXⅠ 5
CⅢ CⅡ CⅠ
6
1p
2p
3i
3p
2i
1i
RⅠ Reservoir
RⅡ RⅢ
3b
50
3r
（b）
p, MPa
40 30
2b 2r
3p
1b
20
1r
2p
1p 10
3i 1i,2i,d
6
5 300
8
7
4
2
3 600
1
900
h, kJ/kg
Fig. 8 (a) Combined geothermal and solar power generation plant and (b) its pressure-enthalpy (lgp-h) diagram
5.6
Case 3
W, MW
5.4
5.2
Case 1 Case 2
5.0
the two stand-alone systems the hybrid solar-EGS system
4.8 400
450
500 0 tsolar, C
550
600
Fig. 9 Net power output variations for a hybrid system and the two stand-alone systems with the solar collector discharge temeperture
to
350
ti
300
0
t, C
250 200
HX II
HX III
HX I
150 100 50
L, m
Fig. 10 Temperature profiles along the heat exchangers for CO2 streams on both sides of the counter-flow heat exchangers where ti is the CO2 temperature flowing in the inner tube and to is the CO2 temperature in the annulus.
Table 1 Geothermal five-spot reservoir and solar system characteristics Stimulated volume Three layers at different depths (z/m)
Solar system
Length (2W)
600 m
Thickness (H)
200 m
Reservoir
RⅠ
RⅡ
RⅢ
Case1
1500
3500
6000
Case2
1500
4000
6000
Case3
2000
4000
6000
905.7 J/kg/K
Rock density (ρ)
2650 kg/m3
Reservoir Life Time (RLT)
30 years
Initial Temperature for case 3 (tres)
115℃/205℃/295℃
Reservoir average temperature drop before thermal breakthrough for case 3 (Δtres)
20℃/40℃/60℃
Permeability
Wellbore
600 m
Rock specific heat (c)
Reservoir EGS parameters
Width (2W)
l=0~8m
k=10-13m2
l=8~416.26m
k= 3.5×10-14m2
Temperature gradient per 100 m
4.5℃
Injector-producer distance (L)
424.26m
Well diameter (d)
0.23m
Heat rate per meter along wellbore (qwell)
100W/m
Outlet temperature at the solar collector ( tsolar)
400~600℃
Heat energy provided by solar collector (Qsolar)
3000 kW
Temperature at the gas cooler exit (tcooler) Turbine efficiency ( T )
35℃
Compressor efficiency ( C )
0.85
0.80
Table 2 Solar system operating parameters and outputs t1 ℃
p1 MPa
p2 MPa
t2 ℃
t6 ℃
t7 ℃
t8 ℃
t9 ℃
t10 ℃
m kg/s
W kW
ηt %
400
26.6
12.4
323.90
35
53.45
122.40
218.63
297.49
23.01
0.915
30.51
500
22.8
9.4
407.60
35
58.84
154.66
271.59
382.78
20.66
1.119
37.31
600
24.2
9.0
490.23
35
63.40
172.14
304.57
463.79
17.61
1.292
43.05
Table 3 The hybrid system operating parameters and outputs for case 3* Global parameters tsolar 400 500
pd 8.2
600
EGS operation parameters Injection well td
35
p1i
t1i
mr1
9.8
41.48
40.31
9.6
40.74
39.67
9.4
39.98
39.03
p2i=9.8 p3i =10.6 mr2=39.59 mr3=40.21
Production well p1p
t1p
13.24
75.58
13.12
75.41
12.99
75.24
p2p=19.98 t2p=147.91 p3p=27.94 t3p=226.39
Hybrid system operation parameters Temp. at turbine exit
Temp.
System performance
ta
tb
tc
t2
t6
t7
t8
η
W
42.69
75.52
120.74
353.43
113.81
159.96
326.28
21.24
5.29
42.99
75.52
120.74
450.85
113.04
159.16
421.91
21.93
5.46
43.30
75.52
120.74
548.19
112.26
158.35
518.54
22.44
5.58
*p is pressure (MPa); t is temperature (℃); m is mass flow rate (kg/s); η is thermal efficiency (%); W is work (MW).
Two stand-alone systems
Hybrid solar-EGS plant
Table 4 Energy systems for the two optimized stand-alone systems and the hybrid solar-EGS power plant (tsolar=400℃) Net solar Geothermal Total heat Compressor Work Turbine Net power Power plants heat input heat input energy input consumption work output output (kW) (kW) (kW) (kW) (kW) (kW) 3648.6 0 389.0 RⅠ:2.0 km CO2-EGS RⅡ:4.0 km 7297.2 21891.5 0 1342.3 4280.9 plant 10945.7 183.6 2733.2 RⅢ:6.0 km C1 C2 C3 Solar plant 3000 3000.0 1648.1 915.4 327.6 62.1 343.0 1716.0 RⅠ:2.0 km Peaking RⅡ:4.0 km 3000 1215.3 7864.3 0 1574.8 1574.8 unit 1933.0 RⅢ:6.0 km 1715.6 135.5 233.5 RⅠ:2.0 km Base-load 6081.8 17027.4 0 1220.0 3717.3 RⅡ:4.0 km plant 9229.8 183.6 2582.9 RⅢ:6.0 km
Total net power output (kW)
5196.3
5292.1
Highlights: 1. Hybrid solar thermal- EGS power cycle uses CO2 as heat transmission and working fluid. 2. Integrating the EGS into the solar power plant reduces the operation pressure. 3. Multiple reservoirs at different depths improve the performance. 4. Recompression compressors are not needed and the system is simplified.
25
S1359-4311(16)32936-2 http://dx.doi.org/10.1016/j.applthermaleng.2016.12.086 ATE 9706
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
31 October 2016 11 December 2016 20 December 2016
Please cite this article as: P-X. Jiang, F-Z. Zhang, R-N. Xu, Thermodynamic Analysis of a Solar–Enhanced Geothermal Hybrid Power Plant Using CO2 as Working Fluid, Applied Thermal Engineering (2016), doi: http:// dx.doi.org/10.1016/j.applthermaleng.2016.12.086
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermodynamic Analysis of a Solar–Enhanced Geothermal Hybrid Power Plant Using CO2 as Working Fluid Pei-Xue Jianga,b,c , Fu-Zhen Zhanga,b,c, Rui-Na Xua,b,c,1 a b
Beijing Key Laboratory for CO2 Utilization and Reduction Technology
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education c
Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
Abstract: CO2-based Enhanced Geothermal Systems (EGS) and closed-loop supercritical CO2 Brayton cycles for solar thermal systems are both currently being developed for environmentally-friendly power plant systems. However, the recompressor needed in closed-loop supercritical CO2 Brayton cycles operates at high temperature and pressure with attendant higher manufacture and maintain cost. Here a solar thermal-EGS hybrid system is proposed where the geothermal power plant provides the base-load electrical power while the solar system increases the system capacity factor by generating additional electric power during peak demand hours. The two stand-alone systems and the hybrid system were modeled to predict the system efficiencies. It is found that comparison with the stand-alone CO2-EGS and CO2-solar thermal systems, the hybrid system has equal or even higher efficiency than the efficiency of the sum of two separate systems. Moreover, the operating pressure can be decreased and the recompression compressor can be removed in the hybrid system which reduces costs of the system installation and maintenance. Key word: solar thermal- EGS hybrid system; enhanced geothermal system; solar thermal power system; supercritical CO2 Brayton cycle
1
Corresponding author. Department of Thermal Engineering, Tsinghua University, Beijing 100084, PR China. Tel.: +086 10 62792294; fax: +086 10 62794664. E-mail address: [email protected]
Nomenclature EGS
Enhanced geothermal system
ORC
Organic Rankine Cycle
A
stimulated fracture area (m2)
cp
specific heat (kJ/(kg·K))
d
well diameter (m)
D
Darcy (1μm2≈10-12 m2)
f
Fanning friction factor
h
specific enthalpy ( kJ/kg)
H
reservoir thickness (m)
L
injector-producer distance (m)
m
CO2 mass flow rate (kg/s)
md
millidarcy (1md≈10 -15 m2)
p
pressure (MPa)
qwell
heat rate per unit depth (W/m)
Q
thermal energy (kJ)
Re
Reynolds number
s
specific entropy (kJ/(kg·K))
t
temperature (℃)
u
velocity (m/s)
v
specific volume (m3/kg)
W
half of reservoir width (m)
z
depth (m)
Greek symbol αv
volume expansion coefficient
Δ
change during a calculational step
ε
wellbore roughness (m)
ηt
thermal efficiency (%)
ηT
turbine efficiency (%)
ηC
compressor efficiency (%)
κ
reservoir permeability (m2)
μ
viscosity (Pa·s)
ρ
density (kg/m3)
Subscripts 1,…,10
cycle point
i
injection well
p
production well
res
reservoir
1. Introduction Economic development must be balanced with environmental protection for sustainable human development. Our world needs clean, affordable, reliable and sustainable energy systems with improved energy efficiencies. Renewable energy sources hold great promise but there are no renewable energy sources that can provide large scale, base-load electricity, with each renewable energy source having its own merits and drawbacks. Thermal energy from Enhanced (or engineered) Geothermal Systems (EGS) represents a large resource that can provide base-load electric power. The U.S. Department of Energy has broadly defined EGS as engineered reservoirs that have been created to extract economic amounts of heat from low permeability and/or porosity geothermal resources [1]. However, EGS systems are rather expensive, especially the well drilling and completion costs, which account for 60% or more of the total capital investment. In addition, the peak-valley difference in the electricity consumption in China is 30%-40%, so there is also a need for peak load systems. The drilling and reservoir stimulation costs will further increase if the electricity demand is fully provided by EGS power plants. Solar energy is widely available on the earth’s surface but electricity generation from solar power plants is subject to the daily fluctuations in the solar radiation, so in the development and implementation of concentrated solar power (CSP) plants, thermal energy storage is indispensable to accommodate the energy demand and to suitably integrate the generated electric power into the grid. Many areas that are suitable for solar power plants coincide with hot geothermal zones [2]. Thus, solar and EGS energy resources can be combined in hybrid systems to compensate for the drawbacks of each individual system [3]. Previous studies have presented a number of potential solar–geothermal hybrid configurations. The solar system can be incorporated into the geothermal system in four ways: (1) The solar source is employed in series with the geofluid to heat the ORC working fluid to increase the mass flow rate and/or maximum temperature [2]. (2) The solar energy is employed to increase the superheat of the ORC working fluid vapor after the vaporizer. (3) The solar energy is incorporated into the geothermal system to reheat the condensed fluid after the condenser [4]. (4) The solar system is used to boost the power plant output or decrease the geofluid flow rate. Mir et al. [5] considered two hybridization configurations of a solar trough system with a geothermal power plant. The first system had a fixed geothermal brine flow rate with a solar
system to boost the output of the hybrid power plant. The second system provided a fixed electricity generation rate with the solar system used to reduce the geothermal brine flow rate. Ghasemi et al. [3] developed a hybrid system with a higher second-law efficiency than the combined individual geothermal and solar systems with the advantage of one system being used to mitigate the weaknesses of the other system rather than using the solar energy as only an additional energy source. Zhou [6] studied a hybrid solar-geothermal plant with a supercritical ORC to evaluate its superiority over a subcritical hybrid plant. In this configuration, the solar energy increased the ORC working fluid superheat. Calise et al. [7] and Buonomano et al. [8] investigate the integration of solar and geothermal energy to produce simultaneously electricity, thermal energy and cooling energy. All these studies have indicated that solar energy combined with a hydrothermal geothermal plant has a thermodynamic advantage over stand-alone system, and also increases the efficient use of the low enthalpy geothermal energy. These hybrid geothermal-solar systems were all based on hydrothermal type geothermal resources using water as the heat transmission fluid. In EGS power plants, the geofluid should be re-injected directly into the injection well after discharging its energy to the energy conversion system. Steam or geothermal brine is produced in hydrothermal geothermal systems, so surface energy conversion systems are needed with dry steam, single-flash and double-flash units or binary units based on ORCs. Almost all attempts to develop EGS systems have used water as the heat transmission fluid, such as the Fenton Hill project in the U.S. [9], the Rosemanowes project in U.K. [10], the Hijiori and Ogachi project in Japan [11], and the Soultz project in Europe [12]. Water is a powerful solvent for many rock minerals, which may change the fracture permeability [13] and water losses to the reservoir are not desirable since makeup water may be expensive. Brown [14] proposed an EGS concept using CO 2 instead of water as the heat transmission fluid to reduce atmospheric CO2 emissions and use CO2 as useful resource. This proposal has attracted much attention by both academics and policy makers for its great potential to reduce CO 2 emissions. Pruess [15] focused on the mass flow and heat extraction rates of CO 2-EGS systems. Their results confirmed the advantages of CO 2 over water as the heat transmission fluid for EGS. Haghshenas Fard et al. [16] and Atrens et al. [17, 18] designed CO 2 thermosiphons that could operate without a pump due to the strong buoyancy leading to high self-driven flow rates. Zhang
et al. [19] analyzed the thermodynamic performance characteristics of CO 2-based EGS and water-based EGS systems and concluded that the CO 2-EGS is more appropriate for smaller reservoirs, while the water-EGS systems perform better for larger reservoirs for a given well diameter. The studies have confirmed that CO2 is not only a good heat transmission fluid but also an excellent working fluid for power cycles because of its unique properties including favorable transport properties, low salt solubility, high buoyancy forces and large compressibility [20-24]. Viete and Ranjith [25] investigated the storage capacity of sedimentary basins and identification of optimum injection conditions for geo-sequestration. Wang et al. [26] combined a Brayton cycle and a transcritical CO 2 refrigeration to produce cooling output, heating output and power output simultaneously to uses solar energy as the heat source to reduce fossil fuel consumption. Recent research [27, 28] has suggested that advanced Brayton cycles using supercritical CO2 as the working fluid for solar power plants may offer equivalent or higher cycle efficiencies than supercritical or superheated water steam cycles at temperatures appropriate for concentrated solar power (CSP) applications. A Brayton cycle designed to operate with a solar energy source can greatly increase the thermal to electric conversion efficiency, and will likely yield efficiencies in the vicinity of 50% [29]. Also, the systems are more compact, with less weight and smaller volumes, which will lower capital costs and may reduce system installation, maintenance and operating costs [30]. However, there have been no published investigations of hybrid solar and geothermal energy generation systems using CO2 as both the working fluid and the heat transmission fluid. The present study describes a hybrid solar and EGS system using a CO 2 EGS system and a supercritical CO2 Brayton cycle. This hybrid system uses geothermal energy as the primary energy source to provide the base-load electricity and the solar energy as a supplement to meet the peak electrical energy. This study also compares the potential of the hybrid system compared to the two systems used separately.
2. CO2-EGS Pruess [13] suggested that CO2 is superior to water in its ability to mine the heat stored in a geothermal reservoir. Zhang et al. [19] also indicated that CO 2-EGS systems produce more power than water-EGS systems for reservoirs with low recoverable thermal energies. These results are due to the CO2 thermophysical properties. The advantages of CO2 as a heat transmission fluid for
EGS and as a working fluid for turbines are summarized in thermodynamics terms as follows [13,19,20]: (1) Low dynamic viscosity and larger thermal expansion coefficient. The low dynamic viscosity reduces the frictional resistance along the well and in the reservoir, which reduces the power consumption for the fluid circulation system. The larger expansion coefficient generates much stronger buoyancy forces, which results in a net pressure increase between the injection and production wells, so the CO2 design can operate as a thermosiphon without a pump. (2)
Direct expansion in the turbine. This energy conversion system differs from conventional energy conversion systems using water as the heat transmission fluid usually with single-flash or double-flash units or binary units based on the organic Rankine cycle (ORCs). Flashing is an irreversible process, which is a pressure drop process without work output. For an ORC system in a low-grade geothermal heat energy conversion system, exergy losses will occur when the brine transfers thermal energy to the working fluid. With CO2 as the geofluid, the CO2 arrives at the plant as a supercritical fluid, so the CO 2 can be expanded directly in a supercritical turbine, which is similar to a dry stream geothermal power plant.
Zhang et al. [19] studied the performance of a CO2-EGS with a rectangular reservoir. Figure 1 shows a schematic of the CO2-based EGS with the injection process (1-2) in the injection well, heat extraction (2-3) in the reservoir and production (3-4) in the production well. The cycle thermodynamic processes are shown in the pressure-enthalpy (lgp-h) diagrams in Fig. 2 for the supercritical cycle with expansion in the turbine from 4 to 5. The cycles also have isobaric heat rejection (5-6) and adiabatic compression (6-1). The pressures at the turbine outlet (state 5) and the injection wellhead (state 1) are optimized to give the maximum power output for a given EGS and operating conditions. If the injection pressure is equal to the turbine outlet pressure, a pump is not needed to increase the CO2 pressure up to the injection pressure, so the cycle self-sustaining without a pump. In an actual reservoir, a portion of the injected CO 2 may be lost in the pores as the CO2 circulates through the reservoir and extracts heat from the rock. The CO 2 lost in the pores is sequestered in the subsurface with CO 2 added from storage to compensate for the sequestered
portion. As a result, this geothermal power plant simultaneously extracts heat energy from the hot dry rock and sequesters CO2 in the subsurface. 2.1 Physical model and calculational method An idealized fractured reservoir was considered, whose parameters were loosely patterned after the conditions of Luo et al. [31]. A five-spot well configuration with a basic pattern area of 600 m×600 m was modeled as shown in Fig. 3 (a) with a production–injection well distance of 424.2 m instead of an injector-producer doublet. CO2 was assumed to be injected at the central well with production of hot fluid at the corner wells. The CO 2 injected at the bottom of the injection well was assumed to be distributed equally throughout the stimulated reservoir. CO 2 is heated by the hot dry rock, then flows at equal rates into the four production wells. Thus, as shown in Fig. 3 (b), the computational region only represented 1/4 of the reservoir. Figure 3 (c) shows a typical computational control volume. Wellbore The thermophysical properties and the parameters such as the stimulated rock volume, the reservoir permeability, the wellbore parameters and the gas cooler outlet temperature are summarized in Table 1. The pressure and enthalpy changes in the wellbore were determined using [19]: u m /( d 2 / 4)
(1)
f 1.8log 6.9 / Re / 3.7 / d
1.11
2
p f , well f u 2 ( z / d ) / 2 f (8m2 z ) /( 2 d 5 )
p g z p f , well h qwell z u 2 / 2 g z
(2) (3) (4) (5)
where z is the wellbore depth, which varies from z = 0 m at the surface to the depth of the wellbore bottom. The distance intervals along the injection and production wells, ⊿z, in the FORTRAN model was set to 5.0 m. A constant heat rate along per meter pipe of 100 W/m, qwell, is assumed to exist between the reservoir rock and the injection well wall boundaries according to Zhang et al. [19] for the injection well. The flow in the production well is assumed to be adiabatic. Heat extraction in the reservoir
The average reservoir temperature drop for a period of 30 years before thermal breakthrough was calculated as in Luo et al. [31]. This study assumes that the heat energy in the reservoir is discharged at a uniform rate for 30 years. As the CO 2 flows through the fractures, energy is transferred from the rock to the CO2, which increases the CO2 enthalpy as: qres cV tres / RLT m(h3 h2 )
（6）
where qres is the recoverable thermal energy that can be extracted from a stimulated volume (V) with a temperature drop ( tres ), in the reservoir. ρ and c are the density and specific heat of the rock. RLT stands for the Reservoir Life Time in seconds. The CO 2 temperature difference between the reservoir outlet and the bottom of the production well is assumed to be 5℃. Pressure drop in the reservoir Luo et al. [31] and Zhang et al. [32] showed that the fractures around the wellbore had to be enhanced to reduce the pressure drop in the reservoir. This study considered two types of induced hydraulic fractures at different locations. The induced fractures in the reservoir region around the wellbores were modeled with a permeability of 100 md, while the reservoir far from the wellbores had a permeability of 35 md. The reservoir pressure drop was then modeled assuming the Darcy flow mood as: p m
dl dA
(7)
where dl, the distance intervals along the reservoir, was 0.5 m. 2.2 CO2-EGS performance A FORTRAN computer code was developed to simulate the performance of the CO 2-based EGS. The CO2 thermodynamic properties were evaluated using the NIST REFPROP code. The thermal efficiency is defined as:
t
Wnet qres
(8)
Where Wnet is the net work output of the cycle that is the difference between the pump work, Wp, and the turbine work, Wt. The CO2-EGS performance varies with the injection pressure for reservoirs at different depths are shown in Fig. 4. These results show that the production pressure (p4) is greater than the
injection pressure (p1) for these conditions. The CO2 pressure at the production well exit increases as the geothermal source temperature increases. Since the temperature and pressure at the production well outlet exceed the injection parameters, the EGS is not only a heat source but also a compressor that circulates the CO 2. The CO2 can then expand as the working fluid through the turbine. This characteristic makes CO2-EGS systems more attractive.
3. Advanced Brayton cycle using supercritical CO2 as the working fluid for solar power cycles Closed-loop supercritical CO2 Brayton cycles have been studied by Denholm et al. [27,28]. The thermal efficiency of a simple CO2 Brayton cycle is not very high, because the CO2 outlet temperatures from the CO2 turbine are still quite high that means relatively little thermal energy is being converted into work. The CO2 expands in the turbine with (T / p) s determined assuming an isentropic expansion and Maxwell’s equation: T T v T v v cp p s T p c p
(9)
Although CO2 has a relatively large volumetric expansion coefficient ( ) and a low specific v
heat (cp) the typical turbine expansion ratio of about 2.0 gives a relatively small temperature drop during the expansion process. The recuperator then recovers the sensible heat in the waste gas. Studies suggest that recompression and partial cooling may give higher cycle efficiencies of 50% [27-29]. The reason that the recompression and partial cooling is superior to the simple regenerative cycle may be understood from the heat transfer between the high-pressure and the low-pressure fluids in the heat exchanger. The specific heat of supercritical CO2 varies strongly with pressure and temperature as shown in Fig. 5. The specific heat of CO 2 at 15.0 MPa is much larger than at 8.0 MPa for temperatures of 70-200℃. In a simple regenerative cycle with a constant mass flow rate, the mc p of the CO2 on both sides of the heat transfer surfaces in the recuperator are not well matched since cp is sensitive to the pressure and temperature, so the heat transfer temperature difference is quite large, which increases the irreversibility. Recompression and partial cooling can overcome this shortcoming, ever though the compressors consume work. The CO2 temperature at the solar receiver outlet (tsolar) (the maximum cycle temperature) was assumed to range from 400℃ to 600℃ and the thermal energy provided by the solar collectors
(Qsolar) was assumed to be 3.0 MW. The thermophysical properties, initial conditions, solar receiver parameters and turbine and compressor efficiencies are also summarized in Table 1. The irreversible heat transfer losses in the recuperators are reduced by using three recompressors in the cycle as shown in Fig. 6. A FORTRAN computer code also was then developed to calculate the cycle performance. To more intuitively display the temperature distributions along the heat exchangers, the model assumes counter-flow, double-pipe heat exchangers. The heat exchangers were assumed to be made of copper for the inner tube and stainless steel for the outer tube. The hot CO 2 discharged by the turbine flowed in the inner tube, while the cold CO2 from the compressor flowed in the annulus in the opposite direction. The heat transfer coefficients in the low-pressure supercritical region were calculated using the heat transfer correlation of Dang-Hihara [33], while the heat transfer coefficients for the high-pressure CO 2 that was heated by the low-pressure CO2 flowing in the inner pipe was calculated using the Jackson and Hall correlation [34] with consideration of the variable thermophysical properties with temperature and pressure. The cycle thermal efficiency is defined as:
t
Wnet Qsolar
(10)
The cycle thermal efficiencies vary with the operating pressure (the pressure at which CO 2 flow through the solar collector), p1, and the turbine discharge pressure (sink pressure), p2, as shown in Fig. 7. Table 2 shows that the CO2 heated to state 10 with a temperature near the turbine outlet temperature gives the maximum solar energy exergy efficiency. Figure 7 shows p1 and p2 for the maximum efficiency. The optimal operating pressures exceed 20.0 MPa and the compressor C3 (as shown in Fig. 6) discharge temperature, t9, exceeds 200℃. The allowable system operating pressure is limited by the mechanical strength of the heat exchangers with higher operating pressures increasing the facility cost. The compressor is the key component in the supercritical CO2 recompression cycle and high pressure and temperature are a challenge for CO2 compressor design. Sandia National Laboratories is investigating fundamental issues relate to compressor flow efficiencies [35] and have greatly improved designs, but are still early in the test program.
4. Hybrid solar-EGS system using CO2 as the working fluid For CO2-EGS, one of key issue is the well diameter, which limits the mass flow rate and in turn limits the recoverable thermal energy from large reservoirs. A second issue is to slow the depletion of the thermal energy in the geothermal reservoir over time and to extend the geothermal reservoir life time. The energy removal can be slowed by using the CO 2-EGS power plant to provide the base-load electric power, while the solar energy supplies additional electric power to meet the peak demand. As stated in section 2, the EGS serves as not only the heat source but also as the compressor. The pressure and temperature at the production well outlet are much higher than the injection pressure and temperature. The CO 2 from the compressor can then be replaced with CO 2 from the EGS in the supercritical Brayton cycle for the solar recompression power cycle. This study presents a hybrid solar-EGS power cycle that does not need a recompressor as shown by the schematic diagram in Fig. 8. The hybrid power cycle simulation divided the reservoir into three layers at different depths. Three sets reservoir depths are referred to case 1, case 2 and case 3 to show the effects of well depth on the system performance. The CO 2 is injected at different injection pressures into three injection wells connected to the reservoir at these different depths. The CO2 extracts heat from the reservoir and then flows to the surface through three production wells. The CO2 streams at the production wells have different temperatures and pressures with deeper reservoir levels giving higher temperatures and pressures. The CO2 from the shallowest reservoir RⅠ at state 1p is split into two streams. One stream flows into CO2 turbine TⅠ and expands to the sink pressure (1p-a) to generate work. The other stream flows into the solar power system as the base fluid for the solar-EGS hybrid system. The CO2 from reservoir RⅡflows first into CO2 turbine TⅡ and expands. When the high pressure fluid has expanded to the same pressure as the CO2 from reservoir RⅠ(2p-6), a portion of the CO 2 is extracted from turbine TⅡ and mixed with the CO 2 heated by the low-pressure CO2 in heat exchanger HXⅠ while the rest CO 2 expands directly to the sink pressure (2p-b). The CO2 from the deepest reservoir RⅢ undergoes the same thermodynamic process as the CO2 from reservoir RⅡexcept that the CO2 extracted from high pressure turbine TⅢ mixes with the CO 2 from HXⅡ. Thus, the three CO2 streams with the same pressure and different temperatures replace the three streams from the three compressors shown in Fig. 6. The mixed CO2 passes into the last stage of
the regenerative heater and is heated by the turbine exhaust CO 2 from the CO2 turbine. At the end, the CO 2 flows into the solar receiver and is heated by the solar energy to a high pressure, high temperature working fluid. The CO 2 expands in turbine TⅣ to generate work. The hot gas leaving TⅣ is hotter than the feed fluid at state 1p so a regenerative cycle is used to recover the waste heat. At the same time, the exhaust CO2 from HXⅠat state 5 with the exhaust fluid from TⅠ, TⅡ and TⅢ is cooled in a gas cooler by the energy sink. The main cold stream is then split into three streams and is compressed to the desired injection pressures and then injected into the three injection wells to complete the cycle. A FORTRAN computer model was developed to analyze this hybrid cycle with the CO 2 thermal and hydrodynamic properties evaluated using the REFPROP model. The parameters are also summarized in Table 1. The simplest model of this system simulates the regenerative heaters as constant pressure heat exchangers and the compressors and turbines as adiabatic. The complete description of this cycle includes the heat energy provided by the solar collector (Qsolar) the solar receiver exit temperature (tsolar) the reservoir parameters, the compressor efficiency, the turbine efficiency, the sink pressure and the injection pressure. With this information, all the operating states and interactions can be evaluated. The injection pressure and the turbine discharge pressure were varied to obtain the maximum thermal efficiency and power. The optimized operating parameters and the system outputs are tabulated in Table 3 for three solar CO2 temperatures (tsolar). The overall performance is judged by the cycle efficiency and the net power output. The cycle efficiency defined as:
t
Wnet Qsolar qres
(11)
where qres is the total heat of the CO 2 extracted from the three-level reservoirs. The net power output of the hybrid solar-EGS system is compared in Fig. 9 with the total for the stand-alone solar and EGS systems. The total net power output of the two stand-alone (CO2-EGS and solar) systems is based on a three-level geothermal reservoir with the solar system having a fixed energy and temperature outputs. The energy balance results including the power consumption and production and the detailed energy inputs and outputs are listed in Table 4.
Figure 9 shows that the net power output of the hybrid system is greater than for the two stand-alone systems at the low solar collector discharge temperature for cases 1 and 2. For case 3, the hybrid system is superior to the two stand-alone systems up to 600℃. The hybrid system gives higher performance for two reasons. First, the heat transfer processes in the heat exchangers more carefully match when using CO2 from the three reservoirs at different depths as the base fluid and/or as the supplementary fluids. This can be seen in the simulation results for the three regenerative heaters as shown in Fig. 10 where the axial temperature difference is almost uniform along the whole length of the heat exchanger for case 3. This occurs only when
mcP hot mcP coolant for a counter-flow heat exchanger with the two streams flowing in opposite directions. Thus, the mass flow rates of the base fluid and the supplementary fluids must be optimized according to the reservoir characteristics and the solar system parameters to match the capacity rates ( mcP ) of the two streams on both sides of the heat exchangers even though the cp of the two streams differ significantly. The optimized design minimizes the heat transfer temperature difference between the two streams. The hybrid systems for cases 1 and 2 are not better than the two stand-alone systems at high solar collector discharge temperatures due to the pressure and temperature at the production wellhead being too low to integrate well with the solar system to obtain a good thermal match between the hot low-pressure fluid and the cold high-pressure fluid. The second reason is that the hybrid system is supercritical so the CO 2 supplied to the hybrid system after it expands to the operating pressure will produce less shaft work if it directly expands to the sink pressure than if the cold CO 2 is compressed for the solar system as shown in Fig. 6. As shown in Table 4, the total power consumption of the three compressors is 732.7 kW (327.6/62.1/343.0) which is 44.5% of the work output (1648.1 kW) of turbine TⅣ. In the hybrid system, the EGS production fluid is used as both the base fluid and the supplementary fluid without the need for recompression by a compressor. The net power output of the solar-EGS peaking unit increases to 1574.8 kW while the net power output of the CO 2-EGS base-load power plant is reduced by 563.6 (4280.9/3717.3) kW. The net power output is then increased by 659.4 kW and the total net power is increased by 95.8 kW. Table 4 shows that the solar hybrid system output is about 30% of the total output with higher solar collector discharge temperatures giving
higher proportions of the output provided by the solar system. 5. Discussion CO2-EGS not only offers advantages for geothermal energy utilization but also provide achieve geologic sequestration of CO2 as an ancillary benefit. The lower specific heat of CO2 gives a large CO2 mass flow rate for a given reservoir. Increasing the wellbore diameter or the ratio of the number of production wells to the number of injection well will improve the CO 2-EGS performance but will increase the cost of drilling wells. The solar energy system can be integrated into the EGS to provide more efficient electric peaking load generation. The hybrid system can then be successfully implemented to give better efficiencies for the following reasons: (1) Fully utilizing the CO2-EGS characteristics. The strong buoyancy in the CO2-EGS leads to the production well pressure being much greater than the injection well pressure, so the EGS is not only a heat source but also a CO2 compressor with the production pressure and temperature of the CO2-EGS increasing as the reservoir temperature increases. Integrating the CO2 from the EGS into the solar power plant cycle at the proper pressure and temperature can simplify the solar power plant system and reduce the optimal operating pressure. At the same time, thermal energy storage is not needed for the solar power plants. (2) Multiple reservoirs levels. Multiple reservoirs levels will reduce drilling costs and increase the power output for a given target reservoir by utilizing more of the thermal energy. At the same time, the multiple reservoir levels provide the pressures and temperatures required by the solar power plant cycle to match the capacity rates of the CO2 discharged from the turbines at different temperatures. (3) The reservoir level depths can be optimized relative to the solar collector outlet temperature and the operating and sink pressures to give the best temperature ranges since the CO2 specific heat on the two sides of the heat exchanger vary strongly with temperature. For the stand-alone solar recompression cycle, the gas cooler exit temperature and the compressor performance determine the temperatures at states 7, 8, and 9, which limit the cycle efficiency. Thus, the hybrid system provides a more flexible temperature glide match for the regenerative heaters that improve the cycle efficiencies.
6. Conclusions A hybrid solar-EGS power cycle was designed with the primary energy source being an EGS with the solar source providing supplemental power. Simulation results indicate that: (1)
Integrating the EGS into the solar power plant reduces the operation pressure, which is a key parameter affecting the heat exchangers safety and the equipment costs. At the same time, the solar power plant is simplified by reducing the capacity of thermal energy storage.
(2)
The system does not need a recompression compressor and simplify the cycle.
(3)
The net output of the hybrid system is greater than the total of the two stand-alone systems when the operating parameters and the reservoir levels are optimized for the solar system parameters and the sink temperature.
(4)
The electric peaking load is supplied by the solar system so the system will draw less energy out of the geothermal reservoir and extend the reservoir life time.
Acknowledgements This project was supported by the national key Research and Development Plan (No. 2016YFB0600805). We thank Prof. David Christopher for editing the English.
References [1] An assessment by an MIT-led interdisciplinary panel (chaired by J.W. Tester), The Future of Geothermal Energy, Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century; 2006. [2] M. Astolfi, L. Xodo, M.C. Romano. E. Macchi, Technical and Economical Analysis of a solar–geothermal hybrid plant based on an Organic Rankine Cycle, Geothermics 40 (2011) 58–68. [3] H. Ghasemi, E. Sheu, A. Tizzanini, M. Paci, A. Mitsos, Hybrid solar–geothermal power generation: Optimal retrofitting, Appl Energy 131 (2014) 158–70. [4] A. Lentz, R. Almanza, Parabolic troughs to increase the geothermal wells flow enthalpy, Sol Energy 80 (2006) 1290–5. [5] I. Mir, R. Escobar, J. Vergara, J. Bertrand, Performance analysis of a hybrid solar–geothermal power plant in Northern Chile, In: World Renewable Energy Congress, Sweden, 2011, pp, 1281–8. [6] C. Zhou, Hybridisation of solar and geothermal energy in both subcritical and supercritical Organic Rankine Cycles, Energy Convers Manage 81 (2014) 72–82. [7] F. Calise, A. Cipollina, M. Dentice d’Accadia, A. Piacentino, A novel renewable polygeneration system for a small Mediterranean volcanic island for the combined production of energy and water: Dynamic simulation and economic assessment, Appl Energy 135 (2014)
675–93. [8] A. Buonomano, F. Calise, A. Palombo, M. Vicidomini, Energy and economic analysis of geothermal-solar trigeneration systems: A case study for a hotel building in Ischia, Appl Energy 138 (2015) 224–241. [9] D. Duchane, Geothermal Energy. Encyclopedia of Chemical Technology, New York: John Wiley & Sons, 1993, pp, 512-39. [10] A.S. Batchelor, Reservoir behaviour in a stimulated Hot Dry Rock system, Proc. 11th Annual Workshop on Geothermal Reservoir Engineering, Stanford University, Calif., USA, 1986, pp, 35–41. [11] Y. Hori, K. Kitano, H. Kaieda, K. Kiho, Present status of the Ogachi HDR Project, Japan, and future plans, Geothermics 28 (1999) 637-45. [12] A. Genter, H. Traineau, C. Dezayes, P. Elsass, B. Ledesert, A. Meunier, et al., Fracture analysis and reservoir characterization of the granitic basement in the HDR Soultz project (France), Geotherm. Sci. Tech. 4 (1995) 189-214. [13] K. Pruess, Enhanced geothermal systems (EGS) using CO 2 as working fluid-a novel approach for generating renewable energy with simultaneous sequestration of carbon, Geothermics 35 (2006) 351–67. [14] D. Brown, A Hot Dry Rock geothermal energy concept utilizing supercritical CO 2 instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering. Stanford University, 2000, pp, 233–8. [15] K. Pruess, On production behavior of enhanced geothermal systems with CO2 as working fluid, Appl. Therm. Eng. 49 (2008) 1446–54. [16] M. Haghshenas Fard, K. Hooman, H.T. Chua, Numerical simulation of a supercritical CO 2 geothermosiphon, Int. Com. Heat Mass Transfer 37 (2010) 1447–51. [17] A.D. Atrens, H. Gurgenci, V. Rudolph, CO2 thermosiphon for competitive geothermal power generation, Energy Fuels 23 (2009) 553–7. [18] A.D. Atrens, H. Gurgenci, V. Rudolph, Economic optimization of a CO 2-based EGS power plant, Energy Fuels 25 (2011)3765–75. [19] F.Z. Zhang, P.X. Jiang, R.N. Xu, System thermodynamic performance comparison of CO2-EGS and water-EGS systems, Appl. Therm. Eng. 61 (2013)236-44. [20] W. J. Cao, W. B. Huang, F.M. Jiang, Numerical study on variable thermophysical properties of heat transfer fluid affecting EGS heat extraction, Int. J. Heat Mass Trans. 92 (2016) 1205-1217. [21] J.Y. Wang, S.B. Hu, Z.H. Pang, et al. Estimate of Geothermal Resources Potential for Hot Dry Rock in the Continental Area of China. Science & Technology Review 2012; 30:25–31. [22] Y.J. Zhang, Z.W. Li, L.L. Guo, P. Gao, X.P. Jin, T.F. Xu, Electricity generation from enhanced geothermal systems by oilfield produced water circulating through reservoir stimulated by staged fracturing technology for horizontal wells: A case study in Xujiaweizi area in Daqing Oilfield, China. Energy 78 (2014) 788–805. [23] P. Olasolo, M.C. Juarez, J. Olasolo, M.P. Morales, D. Valdani, Economic analysis of
Enhanced Geothermal Systems (EGS). A review of software packages for estimating and simulating costs, Appl. Therm. Eng. 104 (2016) 647-658. [24] J.L. Zhu, K.Y. Hu, X.L. Lu, X.X. Huang, K.T. Liu, X.J.Wu, A review of geothermal energy resources, development, and applications in China: Current status and prospects, Energy 93 (2015) 466-483. [25] D.R. Viete, P.G. Ranjith, The effect of CO2 on the geomechanical and permeability behavior of brown coal: Implications for coal seam CO 2 sequestration, Int. J. Coal Geol. 66 (2006) 204–16. [26] J.F. Wang, P. Zhao, X.Q. Niu, Y.P. Dai, Parametric analysis of a new combined cooling, heating and power system with transcritical CO2 driven by solar energy, Appl Energy 94 (2012) 58–64. [27] P. Denholm, M. Mehos, Enabling greater penetration of solar power via the use of CSP with thermal energy storage. NREL, Golden, CO, 2011. [28] P. Denholm, Y.H. Wan, M. Hummon, M. Mehos, An analysis of concentrating solar power with thermal energy storage in a California 33% renewable scenario, NREL Report No. P-6A20-58186. Golden, CO: NREL, 2013. [29] B.D. Iverson, T.M. Conboy, J.J. Pasch, A.M..Kruizenga, Supercritical CO2 Brayton cycles for solar-thermal energy. Appl Energy 111 (2013) 957–70. [30] T. Neises, C. Turchi, A comparison of supercritical carbon dioxide power cycle configurations with an emphasis on CSP applications, Energy Procedia 49 (2014) 1187 – 96. [31] F. Luo, R.N. Xu, P.X. Jiang, Numerical investigation of fluid flow and heat transfer in a doublet enhanced geothermal system with CO2 as the working fluid (CO2-EGS). Energy 64 (2014) 307-22. [32] F.Z. Zhang, R.N. Xu, P.X. Jiang, Thermodynamic analysis of enhanced geothermal systems using impure CO2 as the geofluid, Appl. Therm. Eng 99 (2016) 1277–1285 [33] C. Dang, E. Hihara, In-tube cooling heat transfer of supercritical carbon dioxide. Part 1. Experimental measurement, Int. J. Refrigeration 27 (2004) 736–47. [34] J.D. Jackson, Fluid flow and convective heat transfer to fluids at supercritical pressure, Nucl. Eng. Des. 264 (2013) 24–40. [35] S.A. Wright, R.F. Radel, M.E. Vernon, G.E. Rochau, P.S. Pickard, Operation and analysis of a supercritical CO2 Brayton Cycle. SAND2010-0171, Sandia National Laboratories, Albuquerque, NM, 2010.
CO2 Turbine
Gas Cooler
5
Cooling Tower
6
1
4
Production Well
Makeup CO2 Injection Well
Reservoir 3
2
Fig. 1 CO2-based EGS schematic
3
p, MPa
2
4(p4, t4)
1(p1, t1) 6
5 h, kJ/(kg·K)
Fig. 2 Pressure-enthalpy (lgp-h) diagram for a CO2-EGS cycle with a higher gas cooler outlet temperature than critical temperature
Injection Well
l
H
dl
(c)
(b) Production Well
(a) 2W
l
W l
dl
dl H
W
2W L 2L
Fig. 3 (a) Five-spot well pattern. CO2 is injected at the central well with production of hot fluid
at the corner wells, (b) 1/4 computational model and (c) a computational control volume of width dl
0
t4, C
210 180 150
p4, MPa
30
Reservoir: 600 m00 m 200 m pcooler=8.0 MPa
0 25 t =35 C cooler
20
4.0 km 5.0 km 6.0 km
15 24
, %
22 20 18 16
8
9
10
11 12 p1, MPa
13
14
15
Fig. 4 CO2-EGS operating performance for reservoirs at different depths with similar sizes and other operating conditions.
5
4
cP
p=15.0 MPa 3
2
p=20.0 MPa
p=8.0 MPa
1 0
100
200
0
300
400
500
t, C
Fig. 5 Supercritical CO2 specific heat vs. temperature for different pressures
Solar receiver
CO2 Turbine 1
2 (a)
C3
C2
C1
9
8
7
10
6
5 3 HXⅠ
HXⅢ
4
HXⅡ
1
700 (b)
2
t, ℃
10
9 3 8 4 7 35
5
6
s, kJ/(kg·K)
Fig. 6 (a) Supercritical CO2 recompression Brayton cycle and (b) its temperature-entropy (t-s) diagram
43.0
t, %
42.5 42.0
p2=8.2 MPa tcooler=35 C
41.5
t1=600 C
41.0
16
18
20
22
24
26
28 30 p1, MPa
43 p1=20.0 MPa
t, %
42
tcooler=35 C t1=600 C
41 40 7.5
8.0
8.5
9.0 p2, MPa
9.5
10.0
Fig.7 Thermal efficiencies of the supercritical CO2 Brayton cycle in a solar power cycle for various operating sink pressures
a
receiver
Solar
Gas cooler
（a）
1 TⅣ
TⅠ HXⅡ
2
4
3
8
HX Ⅲ
b
7
d
c TⅡ
TⅢ
HXⅠ 5
CⅢ CⅡ CⅠ
6
1p
2p
3i
3p
2i
1i
RⅠ Reservoir
RⅡ RⅢ
3b
50
3r
（b）
p, MPa
40 30
2b 2r
3p
1b
20
1r
2p
1p 10
3i 1i,2i,d
6
5 300
8
7
4
2
3 600
1
900
h, kJ/kg
Fig. 8 (a) Combined geothermal and solar power generation plant and (b) its pressure-enthalpy (lgp-h) diagram
5.6
Case 3
W, MW
5.4
5.2
Case 1 Case 2
5.0
the two stand-alone systems the hybrid solar-EGS system
4.8 400
450
500 0 tsolar, C
550
600
Fig. 9 Net power output variations for a hybrid system and the two stand-alone systems with the solar collector discharge temeperture
to
350
ti
300
0
t, C
250 200
HX II
HX III
HX I
150 100 50
L, m
Fig. 10 Temperature profiles along the heat exchangers for CO2 streams on both sides of the counter-flow heat exchangers where ti is the CO2 temperature flowing in the inner tube and to is the CO2 temperature in the annulus.
Table 1 Geothermal five-spot reservoir and solar system characteristics Stimulated volume Three layers at different depths (z/m)
Solar system
Length (2W)
600 m
Thickness (H)
200 m
Reservoir
RⅠ
RⅡ
RⅢ
Case1
1500
3500
6000
Case2
1500
4000
6000
Case3
2000
4000
6000
905.7 J/kg/K
Rock density (ρ)
2650 kg/m3
Reservoir Life Time (RLT)
30 years
Initial Temperature for case 3 (tres)
115℃/205℃/295℃
Reservoir average temperature drop before thermal breakthrough for case 3 (Δtres)
20℃/40℃/60℃
Permeability
Wellbore
600 m
Rock specific heat (c)
Reservoir EGS parameters
Width (2W)
l=0~8m
k=10-13m2
l=8~416.26m
k= 3.5×10-14m2
Temperature gradient per 100 m
4.5℃
Injector-producer distance (L)
424.26m
Well diameter (d)
0.23m
Heat rate per meter along wellbore (qwell)
100W/m
Outlet temperature at the solar collector ( tsolar)
400~600℃
Heat energy provided by solar collector (Qsolar)
3000 kW
Temperature at the gas cooler exit (tcooler) Turbine efficiency ( T )
35℃
Compressor efficiency ( C )
0.85
0.80
Table 2 Solar system operating parameters and outputs t1 ℃
p1 MPa
p2 MPa
t2 ℃
t6 ℃
t7 ℃
t8 ℃
t9 ℃
t10 ℃
m kg/s
W kW
ηt %
400
26.6
12.4
323.90
35
53.45
122.40
218.63
297.49
23.01
0.915
30.51
500
22.8
9.4
407.60
35
58.84
154.66
271.59
382.78
20.66
1.119
37.31
600
24.2
9.0
490.23
35
63.40
172.14
304.57
463.79
17.61
1.292
43.05
Table 3 The hybrid system operating parameters and outputs for case 3* Global parameters tsolar 400 500
pd 8.2
600
EGS operation parameters Injection well td
35
p1i
t1i
mr1
9.8
41.48
40.31
9.6
40.74
39.67
9.4
39.98
39.03
p2i=9.8 p3i =10.6 mr2=39.59 mr3=40.21
Production well p1p
t1p
13.24
75.58
13.12
75.41
12.99
75.24
p2p=19.98 t2p=147.91 p3p=27.94 t3p=226.39
Hybrid system operation parameters Temp. at turbine exit
Temp.
System performance
ta
tb
tc
t2
t6
t7
t8
η
W
42.69
75.52
120.74
353.43
113.81
159.96
326.28
21.24
5.29
42.99
75.52
120.74
450.85
113.04
159.16
421.91
21.93
5.46
43.30
75.52
120.74
548.19
112.26
158.35
518.54
22.44
5.58
*p is pressure (MPa); t is temperature (℃); m is mass flow rate (kg/s); η is thermal efficiency (%); W is work (MW).
Two stand-alone systems
Hybrid solar-EGS plant
Table 4 Energy systems for the two optimized stand-alone systems and the hybrid solar-EGS power plant (tsolar=400℃) Net solar Geothermal Total heat Compressor Work Turbine Net power Power plants heat input heat input energy input consumption work output output (kW) (kW) (kW) (kW) (kW) (kW) 3648.6 0 389.0 RⅠ:2.0 km CO2-EGS RⅡ:4.0 km 7297.2 21891.5 0 1342.3 4280.9 plant 10945.7 183.6 2733.2 RⅢ:6.0 km C1 C2 C3 Solar plant 3000 3000.0 1648.1 915.4 327.6 62.1 343.0 1716.0 RⅠ:2.0 km Peaking RⅡ:4.0 km 3000 1215.3 7864.3 0 1574.8 1574.8 unit 1933.0 RⅢ:6.0 km 1715.6 135.5 233.5 RⅠ:2.0 km Base-load 6081.8 17027.4 0 1220.0 3717.3 RⅡ:4.0 km plant 9229.8 183.6 2582.9 RⅢ:6.0 km
Total net power output (kW)
5196.3
5292.1
Highlights: 1. Hybrid solar thermal- EGS power cycle uses CO2 as heat transmission and working fluid. 2. Integrating the EGS into the solar power plant reduces the operation pressure. 3. Multiple reservoirs at different depths improve the performance. 4. Recompression compressors are not needed and the system is simplified.
25