Energy Recovery Performance Research Articles

Short-cycle borehole thermal energy storage: Impact of thermal cycle duration on overall performance

Borehole thermal energy storage arrays are a common form of seasonal underground energy storage which relies on the thermal mass of subsurface rock or sediment to store hot or cold thermal energy. However in practice, typical seasonal operation seldom leads to cyclic energy recovery factors greater than 50%–60% due to significant thermal losses during prolonged periods between consecutive charges. In this paper, the impact of short cycle operation (down to a few days) on energy recovery and overall performance is investigated. Analytical and numerical models are used to estimate energy recovery as a function of cycle length for a typical array and for two distinct charging and discharging scenarios (constant temperature or thermal charge load), which is novel and has not been done before. The results show that shorter cycle duration yields increased recovery factors and storage capacity, which is ascribed to a greater rate of charge to rate of thermal loss ratio. In particular, short-cycle operation at the scale of days unlocks recovery factors in excess of 85 %. The results, supported by a numerical subsurface analysis of the temperature distribution, also show that the recovery efficiency increases with time as losses approach, but never fully reach, a steady-state.

Tradeoff analysis of the energy-harvesting vehicle suspension system employing inerter element

This paper explores the vibration isolation performance and vibration energy recovery performance of an energy-harvesting vehicle suspension system employing inerter element. The study takes into account the structural changes in the suspension caused by introducing the inerter as a new type of vibration isolation element. According to the parallel-series combination of an inerter and a damper, two different structures of energy-harvesting suspension dynamic models are constructed. The mechanism of the suspension parameter uptake on vehicle ride comfort and energy-harvesting characteristics is analyzed. A multi-objective optimal design method for the energy-harvesting vehicle suspension system employing inerter element is proposed, which considers both vehicle ride comfort and energy-harvesting characteristics. A trade-off was found between suspension isolation performance and energy-harvesting efficiency. The results indicate that various structures of energy-harvesting vehicle suspension systems employing inerter element exhibit different vibration isolation performances. Compared with the conventional energy-harvesting suspension, the series structure reduces body acceleration by 15.9 %, and the root mean square (RMS) of energy-harvesting power of the suspension is 22.2 W. The parallel structure reduces the RMS of body acceleration by 14.3 % and the RMS of energy-harvesting efficiency is 56.3 %, with the RMS energy-harvesting power of 84.9 W. The parallel structure demonstrates superior overall performance.

Experimental investigation of the structure effects on the energy recovery and jet impinging process in a fast cooling Joule-Thomson cryocooler

Compared to other Joule-Thomson (J-T) refrigeration systems, open-cycle miniature J-T cryocoolers offer exceptional rapid cooling capabilities, making them ideal for applications such as infrared guidance in missiles. The energy recovery in the heat exchanger enables the refrigerant reach saturation temperature quickly and improve the jet liquefaction rate. The heat transfer intensity of the impinging jet determines the cooling rate of the target. Hence, heat recovery and the impact jet process are the primary factors behind this rapid cooling, with distinct roles that require separate consideration. The structural differences will directly affect the energy recovery and jet impact process. To investigate these affects, an experimental system for rapid cooling J-T cryocoolers was established and three distinct cryocoolers with substantial structural variations were designed. The important structures, including jet height, orifice diameter, enhanced heat transfer treatment of the cold plate, heat exchanger height, and heat exchanger cone angle, were closely studied. In the range of our experiments, it was found that larger heat exchanger cone angle leading better energy recovery performance, while the length of the heat exchanger is limited by the type of refrigerant. Longer heat exchanger actually introduce too much thermal mass for the refrigerant with better energy recovery performance. In the aspect of jet impingement, enhanced heat transfer treatment and larger jet height will improve the jet heat transfer intensity.

Design and development of a novel Four-Links rotational hybrid Energy-Harvesting suspension system compatible with conventional suspension technologies

This study introduces a novel rotational Energy Harvesting Shock Absorber (EHSA), specifically designed for use alongside existing vehicle suspension systems. Addressing issues faced by standalone EHSAs, this design incorporates a four-bar mechanism coupled to a rotary energy recovery system. The proposed EHSA system, dubbed the FL-EHSA (Four Links Energy Harvesting Shock Absorber), is based on link connections and is highly versatile. It has the potential to significantly improve vehicle efficiency by harnessing energy usually wasted by traditional shock absorbers. Using a geometric definition process influenced by various factors such as vehicle suspension type and commercial availability of components, the FL-EHSA system was modeled, and bench-tested to validate the system’s energy recovery potential and performance. The movement of the mechanism was analyzed to achieve the target damping value matching the value of a conventional suspension and to maximize efficiency through DOE analysis and to avoid mechanical locking (singular positions). It explains the effect of backlash in mechanical components through simulation and highlights the importance of using dedicated gearboxes for energy recovery. In addition, this work highlights the untapped potential of four-bar mechanisms in automotive energy recovery applications. Moreover, the proposed design is easily integrated into any vehicle, making it a promising solution for EHSA systems.

Analysis of an efficient liquified natural gas utilization cascade Brayton cycle and an improved cold energy recovery evaluation criterion

The cold energy utilization of liquified natural gas is a promising solution for electricity generation systems to raise power output. In order to enhance cold energy recovery performance, here we propose a cascade Brayton cycle, which can efficiently utilize the waste heat from gas turbine and the cold energy of liquefied natural gas. The temperature pinch in natural gas evaporator is successfully reduced by adopting helium as the working fluid. The optimal working condition is determined through parametric sensitivity analysis, and the exergy and economic performance is assessed and compared. The design case outperforming high energy efficiency (68.61 %), competitive exergy efficiency (58.51 %) and excellent levelized cost of electricity (0.0481$·(kW·h)−1). Moreover, the exergy destruction in natural gas evaporator reduced by 4.05 MW, resulting in 23.2 % improvement compared to conventional systems. Simultaneously, a modified criterion of “real specific work contributed per kg/s liquified natural gas” is proposed, which provides a more accurate cold energy recovery evaluation method for cold energy utilization power systems. The comparison of reported cold energy utilization systems are conducted based on the modified criterion, and our model delivers the best result among them.

Effects of food wastes based on different components on digestibility and energy recovery in hydrogen and methane co-production

This study was conducted for four organic fractions (carbohydrates, proteins, cellulose, lipids) at an inoculum concentration of 30 % and a total solid (TS) of 8 % to investigate the effect of the main components of food waste on the performance of the two-stage anaerobic digestion. The results showed that the gas phase products were closely related to the composition of the substrate, with the carbohydrate and lipid groups showing the best hydrogen (154.91 ± 2.39mL/gVS) and methane (381.83 ± 12.691mL/gVS) production performance, respectively. However, the increased protein content predisposes the system to inhibition of gas production, which is mutually supported by changes in the activity of dehydrogenase and coenzyme F420. Butyric acid (53.19 %) dominated the liquid phase products in both stages, indicating that all four organic fractions were butyric acid-based fermentation and that the final soluble chemical oxygen demand degradation reached 72.97 %–82.86 %. The carbohydrate and cellulose groups achieved the best energy recovery performance, with conversion rates exceeding 65 %. The above results can provide a useful reference for the resource utilization of food waste.

Liquid hydrogen cold energy recovery to enhance sustainability: Optimal design of dual-stage power generation cycles

Liquid hydrogen (LH2) is a promising hydrogen carrier because of its high density. However, liquefying hydrogen requires considerable energy and expenses. To enhance the sustainability, this study focuses on recovering cold energy from LH2 to mitigate costs and carbon emissions in LH2 supply chain. Three power generation configurations are proposed, combining the Brayton and Rankine cycles to efficiently recover cold energy across a wide temperature range of LH2. These configurations are the series Brayton and Rankine (SBR), the parallel Brayton and Rankine (PBR), and the integrated Brayton and Rankine (IBR) processes. In the SBR process, the Brayton cycle recovers cold energy from LH2 in the cryogenic temperature, followed by the Rankine cycle in the low temperature. The PBR process involves the Rankine cycle recovering LH2 cold energy indirectly from the Brayton cycle. In the IBR process, the Rankine cycle recovers cold energy not only from LH2 in the low temperature but also from the Brayton cycle. This results in the highest cold energy recovery performance and net power output, leading to a 6.19% reduction in carbon emissions within the LH2 supply chain. Furthermore, despite its high initial investment, the IBR process demonstrates economic feasibility due to its superior energy performance.

Numerical simulation of gas production from Class III hydrate reservoirs using low-frequency electric heating-assisted depressurization with horizontal wells

The global clean energy shortage has become a significant concern along with environmental pollution caused by the burning of conventional fuels. Natural gas hydrates are considered to be a potential source of clean energy with vast resources widely distributed in permafrost and marine sediments. In order to improve the gas production efficiency from Class III hydrate reservoirs, an innovative production method using low-frequency electric heating-assisted depressurization with the dual-horizontal well mode is first proposed. This method takes advantage of the in-situ heat generated within the hydrate layer through a low-frequency electric field and the wide range of fluid flow and uniform heating of horizontal wells. To better understand the production mechanisms of the proposed method and evaluate its feasibility for increasing gas production, its production performance is analyzed in this work using numerical simulations. Based on the geological data of the Shenhu Area in the South China Sea, a numerical simulation model is first established. Then, the gas production and energy recovery performance are studied. Finally, the influence of production parameters on production performance is analyzed. The results show that additional heat input at the early stage of production can have a synergistic effect with depressurization in the gradient decreasing voltage heating mode, thus resulting in better production performance compared to the gradient increasing voltage and constant voltage modes. The maximum energy efficiency ratio can reach 17.65, which implies that the energy utilization efficiency of the proposed method has great potential for hydrate recovery. Horizontal wells have a larger wellbore contact area with the hydrate reservoir compared to vertical wells; therefore, they exhibit an expanded range of depressurization and hydrate dissociation. When the horizontal wells are positioned in the upper part of the hydrate layer, the negative impact caused by gravity can be reduced. Cumulative gas production increases with the initial voltage, but the energy efficiency ratio decreases, highlighting the need for optimizing the initial voltage. The proposed method and the findings of this work provide a useful reference for the efficient development of gas hydrate reservoirs.

Investigation of energy recovery performance and frost risk of membrane enthalpy exchanger applied in cold climates

The membrane enthalpy exchanger (MEE) is a viable energy recovery device to alleviate building energy consumption. In this paper, a quasi-counter-flow MEE's energy-saving effect and frost risk under severe cold conditions is investigated experimentally and numerically. The experimental results demonstrate that frost occurs when the outdoor air temperature falls below −14 °C. At an outdoor temperature of −26 °C, the enthalpy effectiveness decreases from 63.8% to 43.3% after one hour of frosting, accompanied by a significant increase in the exhaust air's pressure drop by 2.7 times. Furthermore, the simulation results reveal that the lower latent effectiveness compared to sensible effectiveness contributes to the highest risk of saturation near the exhaust air outlet. Moreover, this study analyzes the influence of MEE's geometrical and physical characteristics on its recovery effectiveness and frost risk. The findings indicate that increasing the membrane spacing can prolong MEE's operation duration at the expense of recovery effectiveness. Specifically, the MEE's total energy recovery during the whole heating season in Shenyang, China, increases by 18% when the membrane spacing is extended from 2 mm to 5 mm with a membrane thickness of 70 μm. This study provides insights into the MEE design.

Introducing hydrothermal carbonization to sewage sludge treatment systems—a way of improving energy recovery and economic performance?

Hydrothermal carbonization (HTC) can mitigate the disposal costs of sewage sludge in a wastewater treatment plant. This study analyzes the impact of integrating HTC with anaerobic digestion (AD) and combustion from a combined energy and economic performance perspective. Net energy balance and investment opportunity are investigated for a number of technical scenarios considering i) different combinations of the technologies: AD + HTC, AD + thermal dryer + combustion, and AD + HTC + combustion, ii) different options for HTC process water treatment: wet oxidation (WO) + AD, and direct return to AD, and iii) different products: heat-only, heat and electricity, hydrochar, and phosphorus.The results show trade-offs between investment cost, self-supplement of heat, and output electricity when WO is used. In AD + HTC, net heat output decreases compared to the reference plant, but avoided disposal costs and hydrochar revenue result in profitable investment when the process water is directly returned to the AD. Although HTC has a lower heat demand than the thermal dryer, replacing the thermal dryer with HTC is only possible when AD, HTC, and combustion are connected, or when WO covers HTC’s heat demand. HTC may impair the electricity production because of the necessity for a high-temperature heat source, whereas the thermal dryer can utilize a low-temperature heat source. In conclusion, energy advantages of HTC in AD + HTC + combustion are insufficient to provide a promising investment opportunity due to high investment costs of HTC. The investment opportunity improves by co-combustion of hydrochar and external sludge.

Investigation of a sole gas expander for gas pressure regulation and energy recovery

To recover pressure energy effectively during the gas pressure regulation, innovatively, this work proposed a sole expander for gas pressure regulation and energy recovery: By detecting the outlet gas pressure, the PID controller to control the speed of the expander through the magnetic brake, which adjusts the gas flow, and the outlet gas pressure of the expander reaches stable in this way. By establishing an experimental platform, the pressure regulation and energy recovery performance was investigated in detail. Results showed that three PID parameters P, I and D exerted different influence on pressure regulation, and an optimal pressure regulation accuracy of 1.0% was observed at the optimal PID parameters (P = 15%, I = 50 and D = 2). The gas flow has a certain influence on the pressure regulation while the target pressure has no obvious impact on pressure regulation. The pressure regulation accuracy at maximum flow and minimum flow is limited to the allowable speed and inter leakage of expander. The output power was influenced greatly by gas flow, and a maximum output power 145 W was obtained. The method proposed in this work is proved to be feasible and reliable and has good prospect in gas pressure regulation and energy recovery area.

Ligand Substitution: An Effective Way for Tuning Structures of ZIF-7 Nanoparticles (NPs) and Improving Energy Recovery Performance of ZIF/PA TFN Membranes.

It is an important initiative to reduce the building energy consumption using energy recovering ventilation (ERV) systems. The application of ERV systems is hindered by the low CO2 barrier performance of commercial total heat exchange membranes (THEMs) that lead to unsatisfactory indoor air refreshing rate, and there is an urgent need for THEMs that have improved CO2 barrier properties and effective energy recovery efficiencies. Here, we report the formation of novel ZIF/PA TFN THEMs based on ZIF-7-X nanoparticles (NPs) with "core-shell" structures and tunable particle sizes, formed from benzimidazole (BIM) ligands and BIM substituted by -NH2, -CH3, -C2H5, and -C3H7 functional groups. The NPs were mixed with pyr omellitic triformyl chloride (TMC) in the organic phase during the interface polymerization process to form ZIF/PA TFN membranes. The total heat exchange performance of ZIF/PA TFN membranes could be effectively modified by the type and quantity of ZIF-7-X NPs added. The CO2 barrier properties and water vapor permeability of ZIF/PA TFN membranes could be improved by the addition of optimal levels of ZIF-7-X NPs, showing low CO2 permeance of 7.76 GPU, high H2O permeance of 663.8 GPU, and excellent enthalpy exchange efficiency of 72.1%. This work provided an effective strategy for tuning not only the nanostructures of ZIF-7 fillers but also the CO2 barrier properties of the formed ZIF/PA TFN membranes.

The Energy Recovery Effect of Wave Energy Utilization System under the Rolling Motion of Ships

In order to realize the utilization of wave energy on ships, this study proposes a wave energy utilization system mounted on ships. The wave energy utilization system can transform wave energy into electric energy. The working principle and installation mode of the system are introduced in detail, and the motion model of the system on the ship is established. The motion equation of the system is solved by the Newmark-β method, and the influence of different slider masses (150–500 kg) and different radius of sliding rail (7–15 m) on the energy recovery performance of the system is studied. The data simulation shows that under a certain radius of the sliding rail, there is a slider mass that makes the system achieve the best energy collection effect. With the increase of the radius of the sliding rail, the slider mass corresponding to the best energy recovery performance of the system also increases. In addition, the energy recovery performance of the system under actual sea conditions is simulated, and the research shows that the system can continuously output power.

Energy saving analysis and thermal performance evaluation of a hydrogen-enriched natural gas-fired condensing boiler

Hydrogen-enriched natural gas (HENG) has attracted widespread attention due to its lower pollutant emissions and industrial decarbonization in the past decades. HENG combustion boosts the water content in the flue gas, which is highly favorable for condensing boilers to recover additional latent heat. The energy saving and thermal performance of a condensing boiler burning HENG were evaluated at a constant heat load of 2.8 MW in this study. The variations in combustion products and boiler efficiency were investigated based on the material balance and energy conservation. The heat transfer calculations were employed to evaluate the thermal performance of boiler heating surfaces. The energy recovery performance of the condenser was assessed via a thermal design method. Results show that H2 enrichment enhances the radiation intensity of the flame due to the incremental triatomic gases with higher emissivity in the furnace. The heat absorption ratio increases with H2 enrichment in the radiative heating surface, while it shows a reverse tendency in the convective heating surface. The condensing boiler efficiency based on lower heating value increases from 101.83% to 110.60%, the total heat transfer rate of the condenser increases from 2.77 × 105 W to 4.61 × 105 W, and the total area required decreases from 46.45 m2 to 42.16 m2, as the H2 enriches from 0 to 100% under the exhaust flue gas temperature of 318 K. Although the amount of recoverable heat in the exhaust flue gas increases considerably after H2 blending, the original condenser with natural gas as the designed fuel could meet the requirements of the heat recovery for HENG without increasing the extra heating surface. When the H2 fraction is enriched from 0 to 100%, CO2 emission intensity drops from 6.05 × 10−8 kg J−1 to 0. This work may offer some theoretical references for the application and generalization of HENG condensing boilers.

A Superstructure Mixed-Integer Nonlinear Programming Optimization for the Optimal Processing Pathway Selection of Sludge-to-Energy Technologies

The perception of sewage sludge has increasingly changed from being a waste, which is a burden to the environment and society, to a useful resource of materials and renewable energy. There are several available technologies at different stages of maturity that aim to convert sludge to energy in the form of electricity and/or fuels. In this paper, a decision-making support tool is proposed to help in choosing the optimal pathway for the sludge-to-energy conversion from a techno-economic perspective. The conversion technologies under study are: (1) anaerobic digestion, (2) pyrolysis, (3) gasification, (4) incineration, (5) supercritical water oxidation, (6) supercritical water gasification, as well as the corresponding dewatering and drying methods for each technology. Different synergies between the available technologies are compared by the formulation of a superstructure optimization problem expressed in a mixed-integer non-linear program (MINLP) model. The applicability of the proposed model is explored via a case study for a hypothetical sludge treatment plant with a capacity of 100 tons of dry solids (tDS) per day. The model is solved via the BARON solver using GAMS software within a reasonable processing time. According to the obtained results, the fast pyrolysis technology, coupled with filter press dewatering and thermal drying as pre-treatment steps, show the most promising outcomes with the minimum treatment cost of USD 180/tDS. Fast pyrolysis converts the sludge to bio-oil, which can be used as an alternative fuel after further refining, and biochar, which can be used for soil amendment or adsorption purposes. The model parameters are subject to uncertainty that is addressed in the sensitivity analysis section of this paper. Moreover, the pyrolysis pathway shows a high degree of robustness in most of the sensitivity analysis scenarios. Meanwhile, anaerobic digestion coupled with fast pyrolysis demonstrates the best energy recovery performance upon increasing electricity prices.

Boosting wastewater bioelectricity recovery via solvent mediation and zinc fencing: Dual regulation for catalyst spatial structure and active sites

Bioelectrochemical systems (BESs) are promising in self-sustaining energy recovery of wastewater. Cathode catalysts are crucial to enhancing the oxygen reduction reaction (ORR) process in BESs. In this study, a dual regulation strategy including solvent mediation and zinc fencing was proposed for regulation of catalyst spatial structure and active sites. The particle size of catalysts derived from bimetallic zeolitic imidazolate frameworks (ZIFs) was tuned over a wide range via solvent mediation while the dispersion of cobalt active sites was achieved via zinc fencing. As-optimized cobalt active sites embedded nitrogen-doped porous carbon with a Co/Zn molar ratio of 1/20 (Co1/20@NPC) directed a four-electron transfer pathway in ORR process. The BES employing Co1/20@NPC as cathode catalyst produced a maximum power density of 2057 ± 21 mW m−2, 40% higher than the BES using Pt/C as cathode catalyst. Co1/20@NPC BES displayed fast organic elimination and efficient energy recovery with a coulombic efficiency of 73 ± 1% and a normalized energy recovery of 0.361 ± 0.006 kWh m−3. The outstanding ORR activity, high bioelectricity generation and operation stability made Co1/20@NPC a prospective cathode catalyst. This dual regulation strategy provides implications for controllable synthesis of ZIF-derived catalysts with tailored structures and characteristics, and demonstrates potential in improving wastewater energy recovery performance in BESs.

Insight into the Impacts and Removal Pathways of Perfluorooctanoic Acid (PFOA) in Anaerobic Digestion

Perfluorooctanoic acid (PFOA) that accumulates in wastewater and excess sludge interact with the anaerobes and deteriorate the energy recovery and pollutants removal performance in the anaerobic digestion (AD) system. However, the interaction between PFOA and microbial metabolism in the AD systems remains unclear. This study aimed to clarify the effects and mechanism of PFOA on the AD process as well as the removal pathways of PFOA in an AD system. The results showed that the methane recovery efficiency was inhibited by 7.6–19.7% with the increased PFOA concentration of 0.5–3.0 mg/L, and the specific methanogenesis activity (SMA) was inhibited by 8.6–22.3%. The electron transfer system (ETS) was inhibited by 22.1–37.3% in the PFOA-containing groups. However, extracellular polymeric substance (EPS) gradually increased due to the toxicity of PFOA, and the ratio of protein to polysaccharide shows an upward trend, which led to the formation of sludge aggregates and resistance to the toxic of PFOA. The PFOA mass balance analysis indicated that 64.2–71.6% of PFOA was removed in the AD system, and sludge adsorption was the main removal pathway, accounting for 36.1–61.2% of the removed PFOA. In addition, the anaerobes are proposed to have the potential to reduce PFOA through biochemical degradation since 10.4–28.2% of PFOA was missing in the AD system. This study provides a significant reference for the treatment of high-strength PFOA-containing wastes.

Study on water spraying distribution to improve the energy recovery performance of indirect evaporative coolers with nozzle arrangement optimization

In response to the worldwide call for energy conservation and carbon emission reduction, indirect evaporative cooling as a green and environmentally friendly technology has been extensively researched and developed. In the practical application of this technology, the spray nozzle arrangement results in different wettability factors of the wet channel surface, which accordingly has a significant impact on its cooling efficiency. Existing research has discussed indirect evaporative coolers (IECs) in various external shapes, internal configurations, and materials. However, the IEC performance improvement was rarely analyzed from the perspective of the spray nozzle arrangement. This paper developed and validated a numerical model to predict the spray water density distribution of the solid cone nozzles on the impact surface with uniformly divided square grids. The actual water spray density obtained from this model could then be used to correct the wetting factor in the existing IEC numerical model. The control variable method was applied to compare the effect of inclined angle and distance between the nozzles. According to the uniformity coefficient as well as coverage ratio of the water spray, the optimal arrangement scheme, i.e. the nozzles mounted along the centerline with the distance of 160 mm, was determined. Results showed that a uniform coefficient of 0.74 can be obtained in the optimal distribution of the full cone nozzle. Greater air cooling and dehumidification were achieved by the optimized nozzle arrangement scheme compared with the original single-line arrangement scheme of the nozzles. The coefficient of performance (COP) of the IEC system could be increased by 16% under the same operating conditions when the optimized spray nozzle arrangement was adopted, which indicated the benefit and importance of the IEC spray nozzle arrangement optimization.

Low-Cost Sensors for Indoor PV Energy Harvesting Estimation Based on Machine Learning

With the number of communicating sensors linked to the Internet of Things (IoT) ecosystem increasing dramatically, well-designed indoor light energy harvesting solutions are needed. A first step in this direction would be to be able to accurately estimate the harvestable energy in a specific light environment. However, inside, this energy varies in spectral composition and intensity, depending on the emission source as well as the time of day. These challenging conditions mean that it has become necessary to obtain accurate information about these variations and determine their impact on energy recovery performance. In this context, this manuscript presented a method to apply an innovative energy harvesting estimation method to obtain practical and accurate insight for the design of energy harvesting systems in indoor environments. It used a very low-cost device to obtain spectral information and fed it to supervised machine learning classification methods to recognize light sources. From the recognized light source, a model developed for flexible GaAs solar cells was able to estimate the harvestable energy. To validate this method in real indoor conditions, the estimates were compared to the energy harvested by an energy harvesting prototype. The mean absolute error percentage between estimates and the experimental measurements was less than 5% after more than 2 weeks of observation. This demonstrated the potential of this low-cost estimation system to obtain reliable information to design energetically autonomous devices.

Sensitivity of Reservoir and Operational Parameters on the Energy Extraction Performance of Combined CO2-EGR–CPG Systems

There is a potential for synergy effects in utilizing CO2 for both enhanced gas recovery (EGR) and geothermal energy extraction (CO2-plume geothermal, CPG) from natural gas reservoirs. In this study, we carried out reservoir simulations using TOUGH2 to evaluate the sensitivity of natural gas recovery, pressure buildup, and geothermal power generation performance of the combined CO2-EGR–CPG system to key reservoir and operational parameters. The reservoir parameters included horizontal permeability, permeability anisotropy, reservoir temperature, and pore-size-distribution index; while the operational parameters included wellbore diameter and ambient surface temperature. Using an example of a natural gas reservoir model, we also investigated the effects of different strategies of transitioning from the CO2-EGR stage to the CPG stage on the energy-recovery performance metrics and on the two-phase fluid-flow regime in the production well. The simulation results showed that overlapping the CO2-EGR and CPG stages, and having a relatively brief period of CO2 injection, but no production (which we called the CO2-plume establishment stage) achieved the best overall energy (natural gas and geothermal) recovery performance. Permeability anisotropy and reservoir temperature were the parameters that the natural gas recovery performance of the combined system was most sensitive to. The geothermal power generation performance was most sensitive to the reservoir temperature and the production wellbore diameter. The results of this study pave the way for future CPG-based geothermal power-generation optimization studies. For a CO2-EGR–CPG project, the results can be a guide in terms of the required accuracy of the reservoir parameters during exploration and data acquisition.