A Review of Geothermal–Solar Hybrid Power-Generation Systems

Abstract

Hybrid geothermal–solar systems leverage complementary resources to enhance efficiency, dispatchability, and low-carbon supply. This review compares mainstream configurations (solar-preheating configurations, solar-superheating configuration, and other emerging concepts) and reports typical performance gains—thermal efficiency of 5–80% and exergy efficiency up to ~60%—observed across resource contexts. Findings indicate that preheating routes are generally preferable under medium direct normal irradiance (DNI) and operation-and-maintenance (O&M)-constrained conditions, while superheating routes become attractive at high DNI with thermal storage; integrated multigeneration systems can deliver system-level benefits for multi-energy parks and district applications. In addition, this paper identifies technical bottlenecks—source matching, storage dependence, and the absence of a unified evaluation—and summarizes control/optimization strategies, including emerging advanced artificial-intelligence algorithms. In addition, the review highlights a standardized comprehensive performance evaluation framework, which covers thermal and exergy efficiency, net power output, complexity, the levelized cost of electricity (LCOE), reliability, and storage. Finally, according to the research status and findings, future research directions are proposed, which pave the way for more effective exploitation of geothermal and solar energy.

1. Introduction

As the global energy system continues to transition towards low-carbon and renewable sources, promoting the complementary coupling of renewable energy has emerged as a crucial pathway for tackling the twin challenges of energy security and climate change [1]. Geothermal energy, as a stable and clean resource, is scarcely influenced by seasonal or environmental fluctuations and exhibits a broad geographic distribution [2,3,4], often serving as the base-load power supply in the grid [5]. Calculations reported in Geo-Heat Cent Q Bull indicate that geothermal resources contained within the upper 10 km of the Earth’s crust amount to approximately 3.6 × 10¹⁴ GWh; at the 2012 global energy consumption level (≈1.7 × 10⁸ GWh/yr), this reserve could theoretically sustain worldwide demand for about 2.17 million years [2]. In contrast, solar energy boasts high quality, strong cleanliness, and a short construction period, making it particularly suitable for rapid load response during periods of ample sunlight, ideal for peak load compensation [6]. The CSP (concentrated solar power) systems may drop to very low temperatures at night or when the sun is not shining; they are capable of attaining daytime operating temperatures above 350 °C by employing parabolic trough, linear Fresnel, or tower configurations. Their diurnal/seasonal profiles and different exergy levels make them natural complements for hybridization. For example, solar energy can supplement the stability of geothermal energy via long-term borehole thermal-energy storage, enabling seasonal heat extraction on demand [7]. Moreover, the partial coincidence of solar and geothermal “sweet-spot” regions offers favorable conditions for integrating these two renewable resources [8]. Taken together, these two resources are natural complements.

In this context, energy-storage technologies play a pivotal role, especially in the operation of renewable-based power generation. Hybrid systems typically pair thermal-energy storage—sensible (water tanks, molten salts), latent (phase-change materials), or thermochemical—with subsurface seasonal storage to buffer solar variability, shift heat to preheating/superheating sections, and reduce the consumption of geothermal resources or increase the specific enthalpy of geothermal fluids. Electrical storage (batteries) and mechanical storage (e.g., compressed air) can smooth short-term dispatch and support black-start. The choice depends on the temperature level, response time, and cost, and it directly shapes the LCOE and reliability. Accordingly, under current technological conditions, batteries are not an ideal solution because of their restricted capacity and limited-service life. By contrast, hydrogen generation from renewable-sourced electricity via water electrolysis is also an important option to smooth out fluctuations in power outputs caused by solar fluctuations, as hydrogen can be stored for flexible use at any time, and hydrogen can be produced from all primary energy sources.

At the spatial scale, hybrid feasibility depends on the co-location of resource grade and demand. Geothermal resources in China account for approximately 7.9% of the global total [9]. Geographically, these resources are mainly concentrated in areas such as the North China Basin and the Southwest Thermal Storage Zone, with significant potential for large-scale development of medium-temperature geothermal reservoirs [10]; solar energy, by contrast, is widely distributed across Northwest China (such as Xinjiang, Qinghai, and Gansu) and North China (such as Hebei and Shanxi), featuring high annual radiation levels and a solid foundation for photovoltaic power generation [4]. In particular, regions like Tibet not only have high-quality geothermal resources that are shallowly buried and abundant but also boast high altitude, low humidity, and strong solar radiation, providing excellent natural conditions for the complementary coupling of these two energy sources [11]. Moreover, the compatibility between solar- and geothermal-energy resources is relatively high [5]. Given this co-location of resource grade and demand, hybrid geothermal–solar systems deliver (i) electricity for grids and islanded microgrids (firm/peaking support); (ii) heat for district heating and industrial process heat (food, chemicals, pulp and paper); (iii) cooling via absorption chillers for buildings and data centers; (iv) water (desalination/industrial water) via thermal-driven or electrically driven units; (v) hydrogen via electrolysis. This improves the overall efficiency of thermal systems [12] and plays a dual role in base-load power supply and load regulation in urban low-carbon parks or smart microgrids, significantly enhancing the operational flexibility and energy self-sufficiency of the system [13,14].

Nevertheless, prior studies [1,15,16,17] remain fragmented across configurations and metrics, and cross-study comparisons are hindered by the lack of a unified evaluation framework—creating uncertainty for scenario-specific design and investment decisions. In response, this study seeks to systematically examine the typical configurations and evolutionary paths of geothermal–solar hybrid power generation systems, and to summarize their core technological characteristics in terms of thermal-energy conversion mechanisms, system-operation modes, and energy-coordination mechanisms. Building on this synthesis, we propose a harmonized feasibility evaluation framework/template to standardize reporting (thermal efficiency, exergy efficiency, net power, LCOE, complexity, reliability, storage) so as to enable reproducible comparisons across studies and regions. In parallel, the paper examines the key challenges faced by current coupled systems—particularly in energy-storage configuration, resource adaptability, and scheduling control. Addressing these issues requires downsizing/retargeting storage where dispatchability can be maintained by source–load dynamic matching and control with advanced artificial-intelligence (AI) and machine-learning (ML) methods—such as surrogate-assisted multi-objective design optimization, the time-series forecasting of DNI/loads and reservoir temperature, model predictive control (MPC), and safe deep reinforcement learning (DRL) for real-time supervisory control—thereby outlining pathways to optimize thermodynamic efficiency, dispatchability, and the levelized cost of electricity (LCOE) for geothermal–solar hybrid systems under diverse resource conditions.

2. Literature Review on Geothermal–Solar Hybridization

2.1. Configuration Structure of Geothermal–Solar Hybrid Power Generation System

Driven by growing interest in commercial-scale renewable generation, the deployment of solar–geothermal hybrid systems has accelerated [18]. Depending on where solar heat is integrated, these hybrids are commonly grouped into superheating, preheating, and innovative coupling schemes [19]. In parallel, on the geothermal side, the prime movers comprise dry-steam, flash (single- and double-flash), and binary-ORC configurations; this review contrasts their resource windows, operating principles, and solar-integration routes to clarify scenario-specific choices. To link these two taxonomies, we map each geothermal prime mover to the solar-integration route(s) with which it is most compatible and use this mapping to structure the comparisons that follow.

Dry-steam plants directly utilize saturated steam for expansion, making them ideally suited for solar superheating to enhance efficiency [19]. Flash plants (single or double) separate steam from high-enthalpy brine; using solar preheating of the brine increases steam production and lowers silica-scaling risk, and direct solar-steam generation improves dryness [20]. Binary-ORC units convert medium-/low-temperature heat via organic working fluids; they couple flexibly at preheating/evaporation/superheating of the working fluid and tolerate broader resource grades [21]. Combined layouts (e.g., flash + ORC, dual-/supercritical ORC) further expand applicability in sites with wide temperature spreads. Building on this overview, the next subsections examine the three solar-integration routes in turn.

(1) Solar-superheating configurations

In this type of structure, solar energy primarily acts on the superheating section before geothermal steam enters the expander or turbine, increasing the steam temperature and dryness, thereby enhancing the expansion work output per unit of working fluid [22]. Cardemil et al. [23] designed a single-stage/double-stage flash–solar hybrid power generation system using geothermal brine as the circulating working fluid, with results showing that solar assistance can significantly improve steam quality and system thermal efficiency. But the double-flash layout adds a second separator for higher peak output at the expense of additional valving and control. Lentz and Almanza [15] designed a direct solar-steam generation (DSG) scheme referenced to the Cerro Prieto geothermal field in Mexico, in which parabolic-trough collectors were employed to improve steam quality, enabling up to a 10% rise in steam mass flow under high solar irradiance. This configuration is often combined with molten-salt storage systems to mitigate the volatility of solar energy. And appropriate superheating mitigates moisture-related losses in flash/dry-steam plants. The DSG system has low complexity, and the corresponding operation and control costs are relatively low. El Haj Assad [13] designed a hybrid system (Figure 1) consisting of a single flash-steam geothermal power plant and a solar-thermal system using a parabolic-trough collector (PTC); the calculations were carried out for the PTC on a specific day, time, and location, and the geothermal power-plant (GPP) performance was simulated using the system advisor model (SAM) platform. The results show that January yields the highest monthly output, about 15 GWh, owing to elevated electricity and district heating demand. Ghasemi [24] developed a new hybrid strategy that expands the ORC operating envelope via solar preheating/evaporation/superheating. Relative to a stand-alone geothermal ORC, the proposed hybridization markedly increases net power output by enabling more effective extraction of geothermal energy. Compared with stand-alone geothermal or solar configurations, the hybrid arrangement attains higher exergy (second-law) efficiency, with improvements of up to 3.4%. Drawing on the parallel hybrid layout proposed by Ghasemi et al., Ayub [25] analyzed a novel solar–binary geothermal system composed of an existing low-temperature geothermal ORC and a solar-storage subsystem. By optimizing the operation for maximum net power output, the study showed that the hybrid configuration could lower the LCOE by about 2% relative to a stand-alone geothermal plant, while the optimized constant-flow and variable-flow solar modes could increase the net power output by 5.5% and 6.3%, respectively, compared with the optimized geothermal ORC alone. Tranamil-Maripe Y [1] assessed the performance and LCOE of an existing geothermal plant upgraded with a concentrating solar-power (CSP) system equipped with thermal-energy storage to increase power generation. The findings indicate that such hybrid configurations can lower the LCOE of the stand-alone geothermal plant by roughly 10 USD/MWh, thereby enhancing its economic competitiveness. Nevertheless, the extent of this reduction is highly sensitive to the size of the solar field and the storage-tank capacity. To investigate the potential advantages arising from the synergistic use of solar and geothermal resources, Zhou [26] analyzed the power output of two ORC-based solar–geothermal hybrid schemes to evaluate how diurnal, ambient temperature fluctuations influence the performance of air-cooled condensers. The results show that mixing can achieve stable performance, increase power generation by approximately 29%, and increase the total system thermal efficiency by 16.6% during peak operating hours. Zhou [5] carried out a comprehensive evaluation of geothermal–solar hybrid power generation. The result indicates that the performance of hybrid power plants will be superior to that of independent geothermal and solar power plants under specific conditions, and the hybrid configuration can deliver 2–3% higher electricity output than stand-alone geothermal or solar plants, and it can also surpass geothermal units economically by lowering the LCOE by as much as 20%.

Song [27] combined the mathematical modeling and experimental research of the geothermal–CSP integrated system. From a technical analysis perspective, the hybrid system generates a better output than individual geothermal power plants. For binary cycles, solar can intervene at preheating–evaporation–superheating stages. For example, Zhou [17] further analyzed the operation sequence of a superheating-oriented hybrid configuration and investigated solar–geothermal integration in a supercritical ORC to harness the potential synergy of the combined system, comparing its performance with that of a subcritical hybrid plant and individual solar and geothermal units. Hu [28] investigated the off-design performance of an organic Rankine cycle-based hybrid system over a 30-year operational lifespan. The system operates without a thermal-storage unit, utilizing geothermal energy as the base-load power source. The results show that the analysis of typical years reported in the past significantly underestimated the LCOE by 18%. They argued that the CSP retrofitting should preferably be implemented between the fifth and sixth year of geothermal plant operation. The study also revealed that, relative to a 30-year lifetime assessment, a single-year analysis can overestimate the hybrid system’s power output by as much as a factor of seven. Keshvarparast [29] performed a thermodynamic evaluation of a solar–geothermal hybrid plant using an air-cooled condenser. Under the criteria of minimum fan power demand and minimum condenser heat-transfer area, the condenser energy use was reduced by 47.32% and 33.58%, respectively. Jiang [30] proposed a solar–thermal/EGS hybrid configuration in which the geothermal plant supplies the base-load electricity, while the solar subsystem raises the capacity factor by delivering additional power during peak periods. Separate stand-alone and hybrid models were developed to predict system performance. The study showed that, relative to the individual CO₂–EGS and CO₂–solar-thermal schemes, the hybrid system attains an efficiency comparable to or even exceeding the sum of the two single systems. In addition, the hybrid layout allows lowering the operating pressure and eliminating the recompression compressor, thereby cutting installation and maintenance costs. Tempesti [31] evaluated micro combined-heat-and-power (CHP) plants powered by renewable energy and operating with ORC technology under two alternative layouts. The results indicated that, for low-temperature applications, the conventional single-pressure, dual-circuit configuration outperformed the dual-pressure arrangement. Boghossian [32] conducted a thermodynamic assessment of a power system simultaneously coupled to low-temperature geothermal and high-temperature solar heat sources and compared it with the corresponding single-source systems to identify possible synergies. The analysis showed that the hybrid configuration produced 29% less net power than the output of the respective stand-alone systems. Samrat [33] studied the techno-economic potential of a geothermal power plant with solar-thermal collectors adjunct to it. The hybrid geothermal–solar plant produced a 7% increase in power output relative to a stand-alone geothermal plant, leading to a LCOE reduction between 0−10%.

In superheating structures, the molten-salt thermal-storage tank (molten salt tank) is the most common method of energy storage, used to maintain the stability of steam enthalpy [34] during intermittent sunlight. In the PTSC + SCO₂-ORC system studied by Cakici [35], solar heat is initially accumulated in the molten-salt storage tank and subsequently discharged in a controlled manner to the heat exchanger to sustain a continuous heat supply to the superheating section, effectively mitigating output disturbances caused by light fluctuations. This structure is equipped with a molten-salt-based thermal-energy storage system to regulate solar-radiation fluctuations. The system proposed by Bokelman [36] operates under a dual-source power generation scheme, utilizing both solar and geothermal energy during periods of abundant solar irradiance and high electricity demand, whereas under low-load conditions, the ORC functions independently using geothermal energy as the sole driving source. The thermal-storage system achieves time-shifting of cold and hot loads, enhancing the operational flexibility and thermal efficiency (over 30%) of the system. However, this structure (see Figure 2) and operation mode cause the output instability of the power generation system.

Together, these studies indicate that high-DNI + right-sized TES conditions enable higher specific work and potential LCOE reduction, whereas materials/controls at high turbine-inlet temperatures become the main operational constraints, and power output fluctuations in the generation system become the primary drawback.

(2) Solar-preheating configurations

Preheating structures are widely applied in medium- and low-temperature geothermal resource areas. By using solar collectors to preheat geothermal water or working fluids, the temperature before they enter the evaporator or flash unit is effectively increased, thereby enhancing the thermodynamic performance and system output power of subsequent power-generation processes. This configuration features a comparatively simple layout and reduced construction costs, rendering it well suited for areas with ample solar resources but relatively low geothermal temperatures (such as Northwest China and southern Iran) [12,19]. Lentz [16] investigated the feasibility of using a parabolic-trough solar field to increase the enthalpy of geothermal fluids or to augment the steam flow rate. Furthermore, the study highlighted the potential of this approach to mitigate silica deposition during geothermal operations. They concluded that the utilization of parabolic-trough solar fields can increase the flow enthalpy of geothermal Wells. By inserting parabolic-trough solar fields, the generation of steam can be increased, thereby enhancing the capacity coefficient of geothermal power plants. Bassetti [37] proposed an integrated system of “solar preheating–thermal storage–ORC power generation,” where solar energy is employed to upgrade the quality of geothermal fluids, and a thermal-storage unit facilitates energy transfer between day and night, maintaining high efficiency throughout the day, with an average annual thermal efficiency of 6.3%. Gong et al. [38] constructed a dual-pressure-level structure geothermal–solar hybrid power generation system. The low-pressure stage of the power generation system is powered by geothermal energy and supplies the base-load electricity; the high-pressure stage is driven by solar heating, enhancing the thermal utilization level of the system, achieving an overall average annual thermal efficiency of 12.19%. Li et al. [39] proposed introducing a solar parabolic-trough collector field on top of intermediate-enthalpy geothermal resources, which can increase the temperature at the evaporator end of the ORC system, improve steam quality, and thus enhance cycle efficiency. In this cascade system, the high-temperature stage is driven by solar energy, while the low-temperature stage utilizes geothermal heat and waste heat, illustrating the effective cascading of heat quality and the transfer of process control. The configuration of the system is relatively simple and suitable for areas rich in solar resources but with low geothermal grades, typically not involving complex thermal-storage modules, but its output is limited by changes in sunlight. Manente [40] performed thermodynamic and economic assessments for solar–geothermal hybrid power plants operating under two different strategies to maximize the power output of subcritical isobutane ORCs driven by both heat sources. The study concluded that such plants are unsuitable for regions with relatively poor solar resources, since the LCOE of the hybrid option (182–191 USD/kWh) remains higher than that of stand-alone geothermal plants.

In terms of energy-storage configuration, preheating systems mostly use low-cost water-tank thermal-storage (hot-water tank) or geothermal heat-storage technology to maintain the basic heating function during nighttime or periods of low irradiance, enhancing the continuity and stability of system operation. Some small systems do not have energy-storage units, relying instead on stable geothermal heat sources for regulating the system’s thermal input. In recent years, research has gradually introduced low-temperature phase-change material (PCM) thermal-storage units to further improve solar-energy-utilization efficiency and thermal-storage density, enabling more flexible thermal management strategies.

Reported efficiency spans from 6.3% annual thermal efficiency for compact retrofits up to 12.19% for double-pressure variants, but the latter entails larger solar fields, tighter TES integration, and higher O&M exposure. Accordingly, hybrid systems with preheating configurations often maximize maintainability and availability where DNI is moderate and storage budgets are limited.

(3) Other innovative coupling structures

Recently, with the development of multi-energy complementarity and system integration technologies, the structural forms of geothermal–solar coupled power generation systems have become increasingly diverse, exhibiting significant features such as multi-cycle synergy, hierarchical heat-exchange utilization, and intelligent scheduling control. To enhance system flexibility and the matching capability of multiple heat sources, several innovative structures have emerged, including a two-stage organic Rankine cycle (TSORC), Kalina–ORC coupled systems, and photovoltaic–thermal–hydrothermal composite systems. Among these, the TSORC structure achieves efficient energy quality utilization through hierarchical heat sources: geothermal resources drive the primary low-temperature ORC, while solar energy provides high-grade heat to drive the secondary high-temperature ORC, significantly improving the thermal synergy efficiency between multiple heat sources and the overall system performance [10]. The cascade ORC is also a typical multi-stage energy-utilization structure. In the scheme proposed by Alibaba et al. [41], solar energy powers the high-temperature ORC loop for primary electricity generation, while geothermal water and its residual heat drive the low-temperature ORC circuit, achieving multi-stage cascade recovery and the conversion of thermal energy, enhancing the depth of heat-source utilization and the thermal efficiency of the system. The hybrid power plant integrating the steam Rankine cycle with the ORC is illustrated in Figure 3. The small-scale solar–geothermal-heat pump-integrated system developed by Nahavandinezhad et al. [42] can simultaneously meet multiple terminal-energy demands, including power output, heating, cooling, and hot water, demonstrating the high integration potential of multi-energy complementarity in building and community energy systems. Bonyadi et al. [43] proposed a new type of solar–geothermal hybrid power plant (SGHEPP) by adding the solar steam-Rankine top cycle based on the existing binary geothermal power plant. This hybrid arrangement operates without altering the binary bottoming cycle or deviating from its design conditions, while achieving a higher turbine-inlet temperature, which in turn improves solar-to-power conversion efficiency. Boretti et al. [44] reported that, relative to the ORC system, the CSP–TES–EGS hybrid configuration achieves an efficiency improvement exceeding 40%. They believe that efficiency can be improved by raising the temperature of the ground fluid from as low as 200 °C to as high as approximately 600 °C. McTigue [45] examined the technical performance of various geothermal–CSP hybrid configurations, with a focus on the cost impacts of thermal-storage integration and the identification of optimal storage conditions. The study showed that combining concentrated solar energy with geothermal energy and heat storage could become an economically effective strategy to add dispatchable renewable energy to the power grid. Bist [46] investigated an optimal retrofit of a solar system into an existing ORC-geothermal plant with a declining power output in India. They concluded that the system should maximize the low-cost geothermal source within its temperature range, using solar energy primarily to supplementarily elevate the temperature beyond this baseline. Cuce [47] suggested that the solar-chimney power plant (SCPP) could serve as a viable large-scale technology for cost-effective solar-electricity generation. Shumiye [48] performed an exergy assessment of a solar–geothermal hybrid power plant additionally configured for cooking and reverse-osmosis desalination. The proposed system targets high-efficiency power generation, effective boiling and advanced water purification, improved exergy performance, reduced waste heat, and economic viability. Under daytime operation, the plant supplies electricity while charging the thermal-energy storage unit, achieving a thermal efficiency of 51.64%. In the nighttime cycle, the system utilizes waste heat for boiling-water treatment and power generation, operating with an efficiency of 49.25%. In Senturk Acar’s work [49], energy and exergy analyses were conducted for an ORC system jointly driven by solar and geothermal sources, using the Simav geothermal field and integrating solar collectors to charge a thermal-storage tank. The findings showed that, although adding solar energy reduced the energy and exergy efficiencies of the geothermal-based ORC, it nonetheless resulted in an increase in the total net power output. Boukelia [50] compared the thermodynamic behavior of a novel solar-tower–geothermal hybrid plant with that of a conventional solar-tower plant. The proposed configuration improves dispatchability and annual thermodynamic performance by more than 30% relative to the traditional tower system. Furthermore, raising the geothermal production temperature to 95 °C further increases the dispatchable output of the hybrid plant to as high as 152 GWh per year.

In terms of energy-storage configuration, such innovative systems typically adopt a high–low temperature-staged thermal-storage architecture: the high-temperature section integrates a molten-salt thermal-storage system to smooth out the high-temperature thermal input from solar energy; the low-temperature section deploys hot-water tanks or geothermal-storage systems (BTES) to regulate fluctuations in heat supply on the geothermal side and achieve load response and peak shaving functions when connected to the grid. Wan et al. [22] proposed a “flash + two-stage ORC + molten-salt storage + cooling/hydrogen production coupled system (Figure 4),” which utilizes solar-thermal storage for hydrogen production during the day and releases heat to generate electricity at night, establishing an all-weather dynamic energy-coordination mechanism that effectively enhances the stability and multifunctional response capabilities of the system.

Thermoelectric conversion technologies and solar-integration options for hybrid systems are summarized in

Table 1. A summary of thermoelectric conversion technologies and solar-integration options for hybrid.

Technology Type	Resource	Cycle/Working Fluid	Preferred Solar-Integration Points	Primary Hybridization Goals	Complexity and O&M Notes	Storage Dependence
Dry steam	High temperature, steam-dominant reservoirs (≈≥220–250 °C), dry/saturated steam at wellhead	Direct steam turbine	Superheating of steam upstream of turbine	Raise specific work; reduce turbine moisture; improve peak output	Base-cycle simple	Moderate–High (buffer DNI swings for stable turbine-inlet temperature)
single-/Double-flash	High-enthalpy liquid-dominant (≈≥180–230 °C), flashing at separators	Flash separation-steam turbine; brine reinjection	Preheating brine; direct solar steam generation to raise dryness; superheating separated steam	Increase steam mass flow/dryness; improve annual net output	Medium complexity (separators/valves/two-phase control); silica-scaling management crucial	Moderate (short-term TES smooths solar-driven variations in flash conditions)
Binary ORC	Medium/low-temperature brine (≈80–200 °C)	Sub-/supercritical ORC (isobutane, pentane, R1233zd(E), R1234ze(Z), R245fa)	Preheater/evaporator/superheater of the ORC loop	Broaden operating envelope; raise net power; enable multigeneration couplings	Medium complexity (multiple component; working-fluid management); good controllability	Low–Moderate (flexible part-load; small TES or operational strategies often sufficient)
Other combined cycle	Broad, stratified resources with wide temperature difference	Mixed cascaded cycles (e.g., flash + ORC; dual-/supercritical ORC; Kalina + ORC)	Preheat brine, steam superheat, ORC superheat; TES coordinated across stages	Maximize use of full temperature span; higher annual capacity factor; potential LCOE reduction	High complexity; tighter integration and advanced control/MPC recommended	Moderate–High (coordinated TES often needed for cross-stage stability)

. The table contrasts solar-coupling points, applicable resources, performance gains, LCOE tendencies, and storage dependences. This enables a traceable link between qualitative mechanisms and quantitative techno-economic implications.

2.2. A Comprehensive Review on Geothermal–Solar Hybrid Multigeneration Applications

Building on a geothermal–solar hybrid power system, the integrated multigeneration configuration (including co-, tri-, and polygeneration) simultaneously delivers electricity, heating, cooling, hydrogen, and water. This system can achieve efficient synergy and diversified value output of renewable energy [51]. By leveraging the stable base-load characteristics of geothermal energy and the high-quality intermittent resources of solar energy, it not only raises the overall efficiency but also markedly increases operational flexibility and broadens the range of potential applications. Consequently, deploying multigeneration systems is highly cost-effective, as it eliminates the need for separate installations for heating, cooling, and electricity production [52].

Numerous studies then extend this premise that solar–geothermal configurations possess substantial potential for multigeneration applications, including cooling, hydrogen production, power, space heating, and desalinated water, among others [53]. The high-temperature cascade ORC system proposed by Raji et al. [54] integrates a solar-thermal collector field, liquid-hydrogen cooling system, and reverse-osmosis fresh-water unit, achieving, at optimal operating points, a peak power output of 14.73 MW together with 132 kg/h of liquid hydrogen and 770 kg/s of fresh water, thereby enabling a high level of energy-stream integration and cascade utilization. Cao [55] carried out a detailed thermodynamic–economic assessment and optimization of a novel polygeneration system harnessing both solar and geothermal energy. The analysis showed a thermal efficiency of 19.2%, a levelized product cost of 13.9 USD/GJ, and a payback period of 3.96 years. Because the LCOP aggregates multiple energy carriers, it is reported separately from electricity-only LCOE. In our comparative tables, we retain the LCOP and, where appropriate, also provide the USD/MWh-equivalent for the electricity portion to avoid metric conflation. Siddiqui [56] designed an integrated solar–geothermal system for polygeneration, producing power, fresh water, hydrogen, and cooling. Thermodynamic (energy and exergy) analysis showed overall efficiencies of 42.3% and 21.3%, respectively. Notably, the absorption chiller generator and geothermal flash chamber were identified as the primary sources of exergy destruction, at 2370.2 kW and 643.3 kW.

In developing regional energy systems, Farajollahi et al. [57] designed a building/community-scale scheme integrating geothermal and solar resources, and employed artificial neural networks combined with genetic algorithms for coordinated optimization. This achieves a thermal efficiency improvement of 30.47% and reduces the system leveling cost to 13.04 USD/GJ, demonstrating excellent economic viability and potential for distributed deployment. Such systems are suitable for scenarios with high population density and significant coupling of thermal and electrical demands, such as remote communities, industrial parks, and integrated demonstration bases for “source-grid-load-storage.” The proposed system demonstrates LCOE reduction via right-sized storage and source–load matching: daytime solar boosts power output and charges TES; nighttime geothermal maintains base load. Geng et al. [58], based on the EBSILON simulation platform, constructed a combined heat and power system that integrates solar and geothermal energy, verifying its good operational stability in multi-season environments. Especially during winter, the system effectively enhances heating capacity through dual-source integration, showcasing practical application value in comprehensive energy security in mid-to-high latitude regions.

The introduction of hydrogen-energy modules has further enhanced the system’s flexible regulation capabilities and its ability to decouple operation from time and space. Prajapati and Shah [14] systematically reviewed the geothermal–solar hydrogen production technology route, covering thermal electrolysis temperature control, ORC waste-heat recovery, and photothermal catalytic heat collection. Liu et al. [59] integrated hydrogen as an energy-storage medium into the electro-hydrogen conversion system, effectively alleviating the contradiction between the intermittency of solar-thermal resources and load fluctuations through a “daytime hydrogen production, nighttime power return” strategy, providing a viable approach for multi-energy complementarity and source–load balance. Wang [60] proposed a novel solar–geothermal polygeneration system employing both flat-plate and concentrated photovoltaic thermal collectors. Their analysis identified the solar components as the primary source of system irreversibility, accounting for 81.99% of the net exergy destruction. Optimization demonstrated a potential 10.8% increase in exergy efficiency and a best total-investment-cost rate of USD 1.07/GJ as determined by the VIKOR method. Qi [61] proposed a novel system that combined PTSC with a double-flash geothermal system, which can generate electricity and cool simultaneously. This system includes a solar-heat-storage subsystem for power generation at night. In Almehmadi’s work [62], an ORC powered jointly by solar and geothermal energy was coupled with direct-contact membrane distillation (DCMD) to minimize the levelized cost of energy (LCOE) while maximizing water production. The results indicated that, with R123 as the working fluid, the system can deliver about 480 MWh of electricity annually and produce 2837 m³ of water, achieving a minimum LCOE of 0.136 USD/kWh. Ismail [63] proposed a novel geothermal–solar multigeneration system for producing electricity, fresh water, and hydrogen. The system integrated a geothermal-driven ORC and an ejector refrigeration cycle (ERC) with concentrating solar-power (CSP) towers, multi-effect desalination (MED), humidification–dehumidification (HDH), and proton-exchange membrane (PEM) electrolyzers and was optimized using machine-learning algorithms. Yilmaz [64] employed AI-based and genetic algorithm optimization to assess the feasibility and economics of a geothermal–solar hybrid multigeneration plant. GA optimization markedly enhanced performance, raising the net power output from 2240 kW to 2760 kW and the cooling capacity from 2720 kW to 3061 kW. It also reduced the electricity cost by 12.1% and lowered the cooling cost by 41.9%. Sen [65] designed and investigated a novel multigeneration system integrating a binary geothermal plant, a parabolic-trough CSP unit, and a hydrogen-storage/utilization module. The thermodynamic performance was assessed using the geothermal and solar resources characteristic of Afyonkarahisar. Calise [66] compared two hybrid configurations integrating geothermal with either evacuated flat-plate solar collectors or photovoltaic panels, both capable of delivering electricity, heating, and cooling. Each system incorporated an ORC, an absorption–refrigeration cycle (ARC), a biomass auxiliary heater, and electric/thermal-storage units to mitigate solar intermittency. The results show that PV had a lower payback period than the evacuated flat-plate collectors. Siddiqui [51] developed a novel trigeneration system of renewable energy for power, H₂, and cooling production. A flash-steam geothermal unit is used to generate electricity, and a CuCl thermochemical loop is adopted for hydrogen production, while the loop’s waste heat is reclaimed in an absorption chiller to supply cooling. The trigeneration system achieves overall exergy and thermal efficiencies of 19.6% and 19.1%, respectively, and the CuCl hydrogen subsystem alone attains 35.3% exergy efficiency and 35.9% thermal efficiency. Atiz [67] conducted energy and exergy analyses on the geothermal–solar integrated system for electric and hydrogen production and analyzed system performance under various solar-collector types. Individual integration with the geothermal source achieved daily hydrogen productions of 2758.69 g for PTSCs, 1585.27 g for ETSCs, and 634.42 g for FPSCs. Ezzat [68] performed energy and exergy evaluations of a novel geothermal–solar system capable of delivering five products—industrial refrigeration, residential space heating, domestic hot water, food drying, and electricity. The system achieved an overall thermal efficiency of 69.6% and an exergy efficiency of 42.8%. A techno-economic assessment by Khalid [69] on a geothermal–solar multigeneration system capable of supplying buildings with electricity, cooling, heating, hydrogen, and hot water revealed the following optimized outcomes: an electrolyzer hydrogen output of 2.7 kg/h, a net present cost of USD 476,000, and a levelized cost of electricity of USD 0.089/kWh. Guler [70] evaluated the performance of a solar–geothermal multigeneration plant through three configurations. Model 1 employs a parabolic-trough collector for single-stage heat transfer, while Model 2 utilizes a two-stage transfer with the same collector. Model 3 incorporates a flat-plate collector for two-stage heat transfer. The study reported energy efficiencies of 32.1%, 32.4%, and 30.6%, respectively; hydrogen production costs of USD 1.585/kg, USD 1.551/kg, and USD 1.585/kg; and fuel cell electricity costs of USD 0.0792/kWh, USD 0.0781/kWh, and USD 0.0792/kWh.

Bicer [71] introduced a solar–geothermal combined system for the co-production of hydrogen, electricity, cooling, and heat, and assessed its practical viability. In this layout, PV/T modules supply heat and drive hydrogen generation, while geothermal energy is used for cooling, power production, and supplementary hydrogen output. The analysis indicated that at a geothermal fluid temperature of 210 °C, the system achieves an overall thermal efficiency of 10.8% and exergy efficiency of 46.3%. A multigeneration system comprising a double-flash geothermal plant, a solar power tower, an ORC, an alkaline electrolyzer, and an absorption chiller was analyzed by Javadi [72] through energy, exergy, and exergoeconomic methods. The integration of the solar tower aimed to enhance the dual-flash system’s performance. The full utilization of the geothermal resource enabled outputs of 43,640 kW electricity, 161,604 kg/h hydrogen, 1243 kW cooling, and 8870 kW hot water.

Takleh [73] introduced an efficient solar–geothermal hybrid system for combined cooling, heating, and power, with performance evaluated via energy, exergy, and thermoeconomic analyses. The results indicated that, under multi-objective optimization, the thermal efficiency improved by 13.53% compared with the baseline configuration. The system attained overall efficiencies of 44.02% (energy) and 7.389% (exergy). Musharavati [74] proposed a multigeneration system integrating absorption chillers, organic flash-evaporation cycles, concentrated photovoltaic modules, reverse-osmosis units and thermoelectric modules to generate cooling, heating, electricity, and fresh water. Thermodynamic analysis demonstrates superior performance over the conventional system, with gains of 5.46% in thermal efficiency, 20.16% in exergy efficiency, and 70.85 kW in net output power. Ma [75] performed a comprehensive thermodynamic-conceptual and exergoeconomic evaluation of a novel multigeneration system integrating the Sabalan geothermal power plant with a solar subsystem based on linear Fresnel reflectors. A two-objective optimization was subsequently conducted to enhance system performance. The proposed system co-generates power (4.1 MW), heating (1.67 MW), cooling (1.46 MW), and hydrogen (5.75 kg/h), with energy and exergy efficiencies of 34.2% and 66.3%, respectively. The optimization potential indicates a 9.75% improvement in thermal efficiency with optimal inputs. Bamisile [76] performed an integrated thermodynamic, economic, and environmental analysis of an innovative CO₂-based geothermal micro-multi-energy system supplying electricity, cooling, heating, hydrogen, and domestic hot water. Optimization for exergy efficiency yielded energy and exergy efficiencies of 51.76% and 95.08%, respectively. The corresponding levelized costs were 0.04529 USD/kWh for electricity, 0.004564 USD/kWh for cooling, and 28.86 USD/kg for hydrogen. Forghani [77] proposed a solar–geothermal-based multigeneration system coupled with multi-effect distillation to supply fresh water, heating, cooling, and electricity. In the optimization, the total annual cost (TAC), accounting for pollution-related expenses, was taken as the objective. The results showed that the “base + solar + geothermal” configuration achieved the minimum TAC of 5.1436 × 10⁵ USD/year. A novel multi-output system for zero-energy buildings was analyzed by Baniasadi [78] through energy, exergy, and exergoeconomic methods. The system achieved thermal and exergy efficiencies of 13.27% and 32.44% in cooling mode, and 17.25% and 42.4% in heating mode. The capital investment and O&M cost rate was USD 4.288/h, with a distilled water production cost of 67.63 c$/m³. Sohani [79] investigated the optimal operation of a solar–geothermal multigeneration system capable of concurrently supplying hydrogen, fresh water, electricity, and heat, while enabling energy storage, using a dynamic multi-objective optimization (DMOA) framework. The optimization, carried out with NSGA-II combined with TOPSIS and benchmarked against SMOA, demonstrated that DMOA markedly improved all objectives, increasing the annual outputs of electricity, heat, hydrogen, and fresh water by 14.4%, 16.1%, 13.5%, and 14.3%, respectively. The three-flash-evaporation-ORC system designed by Zhang et al. [34] effectively releases the latent heat of phase change from high-pressure geothermal fluids in high-temperature geothermal fields. Through multi-stage flash evaporation, it drives the ORC to achieve staged energy extraction and integrated power-generation–hydrogen-production co-production, with a comprehensive thermal efficiency of up to 18.86%. The integrated system designed by Mohammadi et al. [80] combines steam Rankine cycles, two-stage ORC, electric cooling devices, and reverse-osmosis desalination modules. As shown in Figure 1 by Mohammadi et al. [80], due to the need for multi-production, the system complexity has increased and the difficulty of operation and maintenance has also risen. Based on multi-objective optimization algorithms, they systematically improved the thermal economic performance, ultimately achieving an energy efficiency of 25.4% and a water production capacity of 3.84 kg/s, demonstrating the potential for collaborative optimization in multi-objective scenarios for combined heat, power, and water systems.

Across multigeneration cases, machine-learning (ML)-enabled co-optimization consistently delivers coupled benefits—higher net output and lower levelized costs—by jointly scheduling electricity/heat/cooling/hydrogen subsystems under variable irradiance and reservoir behavior. Representative implementations combine artificial neural network (ANN)/Gaussian process regression (GPR) surrogates with genetic algorithm (GA) for design, plus model predictive control (MPC)/deep reinforcement learning (DRL) for operations, yielding simultaneous gains in thermal efficiency and the LCOE. These outcomes generalize a recurring pattern: intelligence at both design time and run time compresses storage needs, reduces cycling losses, and shortens payback periods.

Using the multi-objective sorting genetic algorithm (NSGA-II), Assareh [81] optimized the energy efficiency and cost rate of the hybrid system. The results showed that the proposed system had an energy efficiency of 32.39% and a cost of 36.32 USD/GJ in the best state. Fan [82] modeled the proposed cogeneration system from the perspectives of energy and exergy, exergoeconomic and economic, and adopted the NSGA-II method to obtain the optimal performance of the system. Zhu [83] proposed a small-scale solar-hot-spring-geothermal brackish water membrane distillation (MD) system, and adopted improved NSGAII genetic algorithms for multi-objective optimization of the MD system. They found that AI algorithms were very good at optimizing and predicting system performance. Yilmaz [84] developed a geothermal and solar-powered system for multi-energy and hydrogen production using ANN and GA optimization. Economic analysis shows an LCOE of 0.011 USD/kWh and a hydrogen cost of 1.491 USD/kg. Kalan [85] conducted a thermodynamic analysis and performance enhancement of a solar–geothermal integrated power-generation system based on Grey-Wolf optimization and LSTM-based forecasting. Optimization using the Grey-Wolf algorithm enhances the energy efficiency by 21%, the exergy efficiency by 38%. LSTM forecasting of solar irradiance also demonstrates high reliability. Ben Slimene [86] conducted research on different design scenarios using a three-objective optimization framework assisted by machine learning. They concluded that the integration of surrogate modeling with genetic algorithms enabled rapid and accurate optimization for performance improvement.

Overall, these AI/ML-based studies span three complementary functions: (i) multi-objective design-space exploration (NSGA-II/GA/Grey Wolf) that exposes cost–efficiency trade-offs, (ii) surrogate-assisted optimization that preserves Pareto structure while cutting evaluation time, and (iii) data-driven forecasting (e.g., LSTM) that de-risks solar variability at run time. Artificial-intelligence tools are conducive to the engineering application of geothermal–solar hybrid power generation systems.

Some scholars [87,88,89,90] integrated wind energy, biomass energy, etc., into the geothermal–solar multigeneration system. They found that the system had better thermal performance and improved the overall economy.

At present, multigeneration systems powered by geothermal and solar energy generally covers a geothermal collection and reinjection loop, solar-thermal collectors and steam-generation units, multi-stage ORC power-conversion modules, seawater desalination and chemical-treatment units, hydrogen–oxygen separation electrolysis systems, LNG cold-energy utilization chains, and deep cryogenic storage for liquid hydrogen/nitrogen, and so on. It comprehensively demonstrates the synergistic mechanism of multi-energy integration and thermoelectric coupling, representing one of the key technological pathways for achieving low-carbon, intelligent, and regionally sustainable energy sources in the future.

3. Characteristics of Geothermal–Solar Hybrid Power Generation Systems

Geothermal–solar coupled power generation systems leverage the stability of geothermal resources and the high-quality characteristics of solar energy to construct complex multi-cycle thermoelectric conversion and multifunctional co-production platforms. This has become a key technological approach for multi-energy complementarity and green energy development. The system not only boasts technical advantages such as high thermal efficiency and strong energy quality matching but also faces challenges in actual operation, including significant fluctuations in power generation and high system complexity.

3.1. Advantages

Geothermal–solar coupling system, as an important path of clean energy development and utilization, has made remarkable progress in recent years in structural optimization, system efficiency improvement and application expansion. A considerable body of research has demonstrated that this kind of system has multiple advantages in resource complementarity, operation stability and economic adaptability, showing broad development potential.

(1) Significant improvement in power-generation efficiency: In a typical GPP + PTC configuration, solar energy is used to preheat geothermal steam, significantly increasing the dryness and enthalpy of the steam, thereby enhancing thermal efficiency by approximately 15–29% [91]. Multiple experimental and simulation studies have shown that after photothermal assistance, the system’s thermal efficiency can increase on average between 5.5% and 81.13%, with some configurations achieving up to 64.5% [1], far exceeding the performance ceiling of a single geothermal power system.

(2) Strengthened power-output stability: Geothermal energy serves as a stable base-load source, while solar energy is used to provide adjustable peak loads. The two complement each other well on the time scale. By configuring parabolic-trough collectors and molten-salt thermal-storage units, direct solar-thermal energy can be supplied during the day and stored or released at night, achieving continuous output capabilities throughout the day and significantly reducing the impact of fluctuations on grid-connected operations [61,92,93].

(3) Broadening application scenarios for medium- and low-temperature geothermal resources: The integration of solar energy effectively increases the preheating temperature of working fluids, making medium- and low-temperature geothermal resources, which previously had limited power-generation efficiency due to low temperatures, more adaptable. This feature is especially advantageous for areas rich in medium-temperature geothermal and high solar irradiance, such as the arid northwest of China, southern Iran, and the South American plateau [94,95], enhancing the technical feasibility of regional energy development.

(4) System adaptability: This system type is particularly well suited to remote regions with high solar irradiance, shallow geothermal resources, and weak transportation or power infrastructure, such as the East African Rift Valley in Africa, the Andes Plateau in South America, and high-altitude regions in western China [52]. Its high autonomy, reliability, and clean renewable characteristics provide a practical solution for building off-grid energy-security systems.

3.2. Drawbacks and Technical Constraints

Although geothermal–solar coupling systems show significant advantages in efficiency improvement, energy complementarity and regional adaptability, they still face a series of key problems and technical bottlenecks in practical engineering applications, which are mainly reflected in the following aspects:

(1) Thermal storage is highly dependent on external conditions, leading to high system costs. Due to the significant intermittency and diurnal fluctuations in solar energy, coupled systems must be equipped with stable and efficient energy-storage devices (such as molten-salt thermal-storage systems and hot-water tanks) to mitigate the instability of heat sources caused by light variations, ensuring continuous power generation and balanced output power [96]. However, the equipment investment and operation maintenance costs for thermal-storage systems are relatively high, especially for molten-salt storage, which can cost up to USD 40–USD 80 per MWh annually, becoming one of the economic constraints for system expansion and commercialization [1]. This cost intensity reframes the storage role from a default necessity to a design trade-off variable. Where dispatchability can be maintained by source–load dynamic matching and control (Section 5.1), designs with downsized storage may improve the LCOE despite lower peak efficiency, particularly in medium-DNI contexts. Conversely, in high-DNI, storage-friendly settings, larger storage can unlock higher-efficiency layouts without prohibitive cost penalties.

(2) Power output is affected by fluctuations in sunlight, making stable operation challenging [97,98]. The system’s power generation capacity is highly dependent on solar irradiance. In high-latitude regions, during winter, or under cloudy/rainy conditions, the instability of solar irradiance can still prevent hybrid geothermal–solar power systems from achieving stable or full-load outputs, increasing the uncertainty and technical barriers of grid-connected operations.

(3) The system structure is complex, increasing the difficulty of operation and maintenance [33]. To achieve coordinated multi-source integration of geothermal, solar, and energy storage, hybrid geothermal–solar power-generation systems typically require the introduction of dual-heat-source heat exchangers, multi-stage evaporators, and fine control-valve assemblies. This significantly increases the number of thermal flow paths and control loops, enhancing the complexity of system structure and operational control. During operation, issues such as heat exchanger scaling, silicon deposition, and unstable thermal-loss control can easily arise, increasing long-term maintenance costs and the risk of failures.

(4) The initial capital investment of geothermal–solar hybrid systems is relatively high, and large-scale engineering deployment remains constrained. Compared with single-source energy systems, hybrid configurations entail increased costs associated with equipment integration, heat-exchanger structure design, and control-system implementation. This is particularly evident for critical components such as dual-source preheaters, molten-salt thermal-storage units, and multi-loop organic Rankine-cycle modules. The initial investment can account for 30% to 45% of the total system construction cost, placing higher demands on the capital recovery period [98,99].

In summary, the geothermal–solar hybrid power generation system, in its journey towards engineering and large-scale application, needs to address key bottlenecks such as “high dependence on energy storage + intermittent instability + high initial construction costs + high operational complexity.” Future efforts should focus on optimizing thermal-source control strategies, reducing energy-storage costs, simplifying system structure, and introducing intelligent scheduling algorithms.

3.3. Key Technical Challenges

Although geothermal–solar coupled power generation systems show broad prospects in improving thermal efficiency and expanding the coordinated utilization of multiple energy sources, they still face several core technological challenges during practical engineering promotion and system optimization, mainly focusing on four aspects: heat-source matching, system control, cost modeling, and comprehensive evaluation.

Table 2. Current technical challenges of geothermal–solar coupled power generation systems.

Challenge Categories	Description	Literature Reference
Heat source coupling mismatch	Geothermal energy is stable, but the temperature is low, and solar energy is high, but the temperature fluctuates greatly. The thermal-mass matching between the two is difficult in the heat exchanger, which affects the overall thermal efficiency and system coupling stability.	Cakici [35] Erdogan [12]
Control policies are complex	The system usually involves three types of energy: geothermal, photothermal, and energy storage. It is difficult for traditional PID and other control methods to meet the requirements of rapid switching and dynamic response.	Tranamil-Maripe et al. [1] Yi [100]
Cost estimates are uncertain	The system’s LCOE is strongly contingent on the quality of the geothermal resource, solar-energy utilization rate, energy-storage configuration scheme and grid connection conditions. Currently, a standardized economic evaluation framework is still lacking.	Tranamil-Maripe et al. [1]
The standard of systematic evaluation is missing	At present, most assessments rely on single metrics—such as thermal efficiency, exergy efficiency, or net power output, and a multidimensional and scenario-based comprehensive evaluation framework for system performance has not been established.	Li et al. [19]

outlines the principal issues requiring urgent attention.

To break through the aforementioned bottlenecks, it is imperative to conduct in-depth research and technical breakthroughs in the following areas: First, develop hierarchical heat-exchanger and coupled heat–flow channel design models that accommodate different heat sources and their varying qualities, to achieve efficient and coordinated utilization of thermal energy. Second, optimize dynamic matching and control strategies between heat sources, incorporating intelligent scheduling algorithms based on artificial intelligence (such as reinforcement learning and neural network predictive control) to enhance the accuracy and response speed of system dynamic control. Third, establish a unified LCOE calculation and sensitivity analysis platform to support feasibility and investment decision-making assessments for multi-source collaborative systems across different resource regions. Fourth, promote the development of a comprehensive system evaluation index system tailored to practical application scenarios, achieving synergistic optimization of multiple objectives including thermal efficiency, environmental impact, economic benefits, and dispatch stability.

4. Evaluation of Geothermal–Solar Hybrid Power Generation Systems

Geothermal–solar hybrid power generation systems face significant challenges in performance evaluation due to their complex structure, diverse energy types, and multiple output targets. Currently, there is no unified and systematic evaluation standard in academia or engineering. Different studies often focus on single or partial indicators, making it difficult to compare and optimize configurations across systems. The main existing evaluation indicators include the following categories:

4.1. Thermal Efficiency

Thermal efficiency is the most basic index to evaluate the energy conversion capacity of a system. It quantifies the share of supplied heat that is effectively transformed into mechanical (or electrical) output. It is defined as follows:

$${\eta }_{th}={\displaystyle \frac{W_{net}}{Q_{in}}}$$

(1)

Among them, the system net power output is $W_{net}$ (including the pump power and auxiliary energy consumption); $Q_{in}$ is the total heat input received from geothermal and solar energy.

The solar–geothermal–ORC coupling system constructed by Bassetti et al. [37] has an annual thermal efficiency of 6.3% with fewer components and control loops, implying lower structural/controls complexity and maintenance burden; the double-pressure system proposed by Gong et al. [38] has a thermal efficiency of 12.19% but at the expense of larger solar fields, tighter integration with thermal storage, and increased complexity—thereby elevating CAPEX and O&M exposure. Consequently, the cost–efficiency frontier shifts across resource contexts: under medium-DNI and constrained O&M, preheating routes often dominate in cost–performance; under high DNI with robust financing and maintenance capacity, efficiency-maximizing routes become attractive.

4.2. Exergy Efficiency

Thermal efficiency does not reflect the difference in thermal-energy quality, but exergy efficiency makes up for this deficiency and is used to measure the system’s ability to use “available energy”, defined as follows:

$${\eta }_{ex}={\displaystyle \frac{W_{net}}{{m_{gf }e}_{gf+}{E}_{sol}}}$$

(2)

Among them, $m_{gf}$represents geothermal fluid mass flow rate and $e_{gf}$the specific exergy of the geothermal fluid, and $E_{sol}$is the exergy of solar irradiance received by the system. Bokelman et al. [36] measured the exergy efficiency of more than 30% in their system with a molten-salt heat-storage module, which is significantly better than the conventional single-heat-source system.

4.3. Net Power Output

Net power generation is a direct indicator of the energy output capacity of the system. The calculation formula is as follows:

$$W_{net}=W_{turbine}-W_{pump}$$

(3)

This index is often used in multi-cycle configurations, transient simulations, and component comparison research, which can reveal the improvement in power-generation performance brought by specific structure optimization.

4.4. Economic Evaluation

Although geothermal–solar hybrid power-generation systems can offer long-term savings, their initial installation costs may be quite high. Enhancing the appeal of hybrid power-generation systems to investors and stakeholders requires economic analysis.

The LCOE is a comprehensive consideration of construction, operation, and generation over the lifecycle of a system for economic evaluation. It is defined as

$$\mathrm{LCOE}={\displaystyle \frac{\sum _{t=1}^n(C_t+O_t)/(1+r{)}^t}{{\sum }_{t=1}^n{E}_t/(1+r{)}^t}}$$

(4)

Among them, $C_t$is the capital expenditure, $t$ is the year, $O_t$ is the operation and maintenance expenditure, $E_t$ is the power generation of the year, and $r$ is the discount rate.

AI-driven optimization links directly to economics through better sizing and dispatch: a surrogate-assisted multi-objective search jointly tunes solar field area, storage capacity, and ORC setpoints to minimize the LCOE subject to reliability constraints. Farajollahi et al. [57], based on the artificial neural network optimization of the solar–geothermal system, reduced its LCOE to 13.04 USD/GJ, showing strong cost-control potential. This evidence also motivates treating AI/ML not merely as a control add-on but as a core lever in techno-economic design.

The LCOP aggregates multiple energy carriers, and it is reported separately from electricity-only LCOE.

$$\mathrm{LCOP}={\displaystyle \frac{Z_{investment}+Z_{fuel}}{t·E_p}}$$

(5)

Among them, $Z_{investment}$ is investment cost, $Z_{fuel}$ is fuel cost, and $E_p$ is the exergy rate of products.

For cross-comparison, reported USD/GJ values are converted to USD/kWh (1000 kWh = 3.6 GJ). We adopt USD/kWh as the unified LCOE unit in figures and tables hereafter to avoid unit-driven bias in economic comparisons.

Economic analysis can indicate that the geothermal–solar integrated power-generation system exhibits a superior LOCE compared with stand-alone geothermal or solar power stations.

Table 3. Summary of power and multigeneration schemes based on geothermal–solar hybrid systems.

Author, Reference No, /Year	Power Generation/Multigeneration Systems	Metric-Scope/Value	Payback Years	Environmental Analysis
Cao et al. [55] (2022)	polygeneration system	LCOP 0.05 USD/kWh	3.96	-
Ding et al. [90] (2022)	polygeneration system	LCOE 0.098 USD/kWh	-	-
Javadi et al. [72] (2021)	multigeneration system	the cost of hydrogen and electricity production 0.15 USD/kWh	-	-
Sohani et al. [79] (2022)	multigeneration system	-	4.4–5.6	-
Farajollahi et al. [57] (2024)	multigeneration system	LCOP 0.04 USD/kWh	-	-
Mohammadi et al. [80] (2023)	multigeneration system	the total unit cost of products 0.12 USD/kWh	-	-
Zhou [5] (2013)	power generation system	LCOE 0.225 USD/kWh	-	-
Tranamil-Maripe Y [1] (2022)	power generation system	LCOE 0.081 USD/kWh	-	-
Farayi Musharavati [74] (2021)	multigeneration system	the electricity cost rate 108.4 USD/h	-	-
Raji Asadabadi [54] (2025)	multigeneration system	total cost rate for the entire system 541.53 USD/h	-	Lowers CO2 emissions 0.181 kg/kWh
Bozgeyik, A et al. [89] (2023)	multigeneration system	The overall unit product cost 0.078 USD/kWh	-	Social ecologic factor 1.37
Yilmaz, C et al. [64] (2024)	multigeneration system	the cost of electricity 0.0145 USD/kWh	-	-
Gong, L et al. [38] (2021)	power generation system	LOCE 0.063 USD/kWh	-	-
Ismail, M.A et al. [63] (2025)	multigeneration system	-	5.73–5.07	-
Zhang, L. et al. [34] (2023)	power/hydrogen generation system	-	4.82	-
Astolfi, M et al. [6] (2011)	power generation system	LOCE 0.157–0.302 USD/kWh	-	-
Wang, W. et al. [60] (2023)	polygeneration system	total investment cost rate 1.01 USD/h	-	-
Hu, S. et al. [28] (2022)	power generation system	LCOE 0.188 USD/kWh	-	-
Khalid, F et al. [69] (2017)	multigeneration system	LCOE 0.089 USD/kWh	-	-
Bonyadi, N et al. [43] (2018)	power generation system	LCOE 0.163–0.172 USD/kWh	-	-
Bamisile, O et al. [76] (2023)	multigeneration system	LCOE 0.04529 USD/kWh	-	-

consolidates reported economic evaluation indicators from the literature reviewed here.

Economically, hybrid systems with solar preheating configurations can reach a lower LCOE where high-DNI and TES-friendly conditions apply (e.g., ~0.081 USD/kWh), while some storage-light cases remain higher (e.g., ~0.225 USD/kWh). Some solar-preheating configurations or ORC retrofits cluster at a moderate LCOE when O&M and integration costs dominate (e.g., ~0.063–0.19 USD/kWh). Multigeneration studies often report LCOP or mixed cost rates; converted electricity-only figures indicate competitive ranges when co-product value offsets CAPEX. In addition, it can be concluded from Table 3 that the environmental assessment of geothermal–solar hybrid power-generation systems has received comparatively limited attention. The environmental assessment metrics for hybrid power-generation systems have not yet been standardized. CO₂ emission reduction is still the primary indicator used to characterize the environmental performance of geothermal–solar hybrid systems.

4.5. Multi-Objective Composite Indicators (Multi-Objective Performance Evaluation)

As the system expands from single power generation to multi-product production, research gradually introduces multiple objective performance indicators (such as thermal efficiency + cooling output + cost + response speed) for collaborative evaluation. Eslami et al. [101] developed a composite assessment framework integrating electricity, cooling, and hydrogen outputs, thereby markedly improving both the degree of system integration and the model’s discrimination capacity.

In summary, the current evaluation methods for hybrid geothermal–solar coupling systems still suffer from issues such as scattered indicators, lack of standards, and poor comparability. Future recommendations: establish a unified multidimensional performance indicator system that covers thermal efficiency, environmental impact, economic viability, and load adaptability; promote the standardization of simulation platforms and evaluation methods to enhance comparability between different coupling structures; and introduce data-driven evaluation and multi-objective optimization methods to improve model predictive capabilities and engineering deployment adaptability.

To move beyond indicator enumeration and enable apples-to-apples comparisons, we conduct a critical comparative analysis of hybrid configurations—which allows us to articulate explicit trade-offs (cost–efficiency; complexity–reliability; storage-dependence–dispatchability) rather than listing indicators in isolation. The results of the comparative evaluation are shown in

Table 4. Critical comparative analysis results for geothermal–solar hybrid configurations.

Configuration	Resource	Evaluation Indicators	Complexity	Reliability	Storage Dependence	Judgment
Solar-preheating configurations	Medium DNI, low-medium temperature geothermal	ηth: 6.3% [37], 16.6 [26], over 30% [36] ηex: 42.8% [34], 22.7% [26] Net power: 1–11% boost [24], 6.3% boost [25], 7% more [33], 4% improvement [4]	2–3 (few stages/circuits)	Simple structure and relatively friendly operation and maintenance	Low-medium (may weaken energy storage)	Highly cost-effective in scenarios with limited DNI and limited O&M resources; good cost–efficiency balance
Solar-superheating configurations + complex emerging concepts	High DNI, medium-high temperature geothermal	ηth: 12.19% [38], 50% [44] ηex: 51.64% [48], 15.2% improvement [39] Net power: 27% improvement [39], 60% more electricity [43], 9% improvement [45]	4–5 (significant increase in components and control loops)	Scaling/deposition and heat-loss control become difficult	Medium-high (strong reliance on energy storage)	High DNI/ sufficient funding and O&M can lead to higher efficiency; however, the risk of increased cost and complexity

.

Compared across a unified set of metrics (thermal/exergy efficiency, LCOE, structural/controls complexity, reliability proxies, and storage dependence), we observe clear trade-offs among mainstream configurations. For instance, solar superheating combined with dual-pressure or supercritical layouts tends to deliver higher efficiencies yet requires larger solar fields and tighter integration with thermal storage, escalating both CAPEX and O&M burden. By contrast, solar-assisted preheating of geothermal brine achieves moderate efficiency gains with fewer components and control loops, which improves maintainability and availability under resource-constrained operation. Given the documented high-cost intensity of thermal storage, strategies that prioritize source–load dynamic matching and downsized storage can improve cost–performance balance in medium-DNI regions. Overall, selecting among these options should be scenario-driven: in high-DNI, well-resourced contexts, the efficiency-maximizing routes are attractive; in low-to-medium-DNI or operation-constrained settings, lower-complexity, storage-light layouts can yield more robust techno-economics.

5. Discussion and Suggestions

Hybrid geothermal–solar power generation systems, as typical representatives of renewable energy synergistic utilization, show significant potential in enhancing the comprehensive efficiency of renewable energy and promoting the diversified development of clean energy. However, current systems still face numerous constraints in cost control, resource matching, and operational stability. Based on the analysis presented earlier, this paper proposes the following three key recommendations for future research and engineering design reference.

5.1. Promoting Optimization of Dynamic Matching Between the Heat Sources and Reducing the Dependence on High-Cost Energy-Storage Systems

Future research should prioritize dynamic source–load matching, time-of-use operating strategies, and adaptive control algorithms to deliver stable operation with minimal—or even no—dedicated energy storage. At present, most hybrid plants rely on large, high-cost storage units (e.g., molten-salt tanks, hot-water tanks, and phase-change materials) to buffer solar intermittency. A storage-light operating policy would instead be dispatched by availability: when insolation is high, meet demand primarily with solar; when insolation is low, shift the lead role to geothermal and, if present, draw only limited support from thermal storage to smooth ramps. Such coordinated dispatch reduces power fluctuations and, by allowing substantial downsizing of storage capacity, lowers the capital cost of storage modules.

5.2. Establishing a Multi-Heat-Source Precision-Matching Mechanism

Owing to the significant differences between geothermal and solar energy in resource grade, thermal quality level, and response speed, future system designs should no longer adopt the “equivalent superposition” approach but instead focus on energy quality synergy for tiered heat-exchanger design, thermal process–zoning coupling, and dynamic energy flow allocation mechanisms. For example, low-temperature geothermal sources can be used to drive low-pressure ORC at base load, while solar energy can be utilized for preheating, superheating, or hydrogen production in the high-temperature section, achieving differentiated positioning and complementary synergy among different heat sources in the thermal process. This mechanism helps enhance the overall thermal efficiency and cycle flexibility of the system.

5.3. Building a Unified Performance Evaluation System and Regional Adaptation Framework

We operationalize the evaluation system by publishing a reproducible template: a feasibility table (configuration–thermal efficiency/exergy efficiency–net power–LCOE–complexity–reliability–storage–judgment). This initiative enables the consolidation of performance data and reduces cross-study ambiguity, thereby addressing the fundamental challenge posed by the current lack of unified evaluation standards across scenarios and configurations, which otherwise severely hinders horizontal technical comparisons and engineering deployment.

5.4. AI/ML-Enabled Optimization and Operational Intelligence

AI/ML can enhance both the planning and operation of hybrid geothermal–solar systems. At design time, surrogate models (ANN, GPR) combined with Bayesian search or non-dominated sorting genetic algorithm II (NSGA-II) speed up the multi-objective sizing of heat exchangers, ORC parameters, and storage under uncertainty. At run time, short-term forecasts (DNI, loads, reservoir temperature) feed MPC to coordinate solar input, storage charge/discharge, and geothermal extraction; safe DRL complements MPC when model mismatch is large, cutting curtailment and cycling while respecting temperature/pressure limits. For reliability, anomaly detection and digital twins support condition-based maintenance and online recalibration. Key gaps remain in data sparsity and cross-site transferability, interpretability/safety under constraints, and the lack of common benchmarks; we encourage releasing anonymized datasets and unified tasks (forecast–dispatch–LCOE) for fair comparison.

In summary, the future development direction of geothermal–solar hybrid power generation systems should shift from “technology stacking” to a systematic optimization path that integrates energy quality coordination, intelligent operation, and regional customization. Only by achieving breakthroughs in key areas such as reasonable matching between the heat sources, integration of control strategies, and unified evaluation mechanisms can we truly advance these systems from experimental demonstrations to large-scale engineering implementation.

6. Conclusions

Geothermal–solar hybrid power-generation systems for multi-energy utilization have received widespread attention and in-depth research in recent years. This paper systematically reviews the structural types, performance advantages, key technical challenges, and future development trends of such systems. It focuses on the evolution of system configurations, multi-product pathways, complex cycle integration, evaluation methods, and optimization strategies, leading to the following main conclusions:

(1) The system structures are diverse, with considerable potential for complementarity between sources. Preheating routes (solar-preheating configurations) are generally the best performers under medium-DNI and O&M-constrained conditions, delivering cost-effective gains with simpler layouts and fewer control loops. Under high-DNI and storage-friendly settings, solar-superheating routes become attractive by maximizing cycle efficiency, albeit with tighter integration and higher complexity. For multi-energy parks and district applications, the multigeneration systems may offer the highest system-level benefits when evaluated against reliability and lifecycle cost.

(2) Compared with a single geothermal system, the geothermal–solar hybrid power generation systems can increase the thermal efficiency by 5–80%, and the exergy efficiency is up to 60%. The hybrid system is suitable for areas with abundant medium-low-temperature geothermal resources and strong solar radiation, especially in remote plateau and dry hot areas.

(3) Energy-storage systems are constrained and influenced by many factors, resulting in high operation costs and scheduling complexity. At present, most systems rely on molten salt, PCM, or alternative storage media to mitigate solar-induced fluctuations, but this causes problems such as cost increase, complex structure, and difficult grid coordination, which could be the obstacles restricting their large-scale application. Addressing these hurdles requires downsizing/retargeting storage where dispatchability can be maintained by source–load dynamic matching and control with advanced artificial-intelligence algorithms, plus simplification of thermal paths and smarter coordination.

(4) The evaluation system lacks uniformity, making it difficult to support system optimization and comparison. Although a single indicator such as thermal efficiency, exergy efficiency, and the LCOE can be used for system evaluation, it cannot comprehensively reflect the energy-quality coordination, operational stability, and multi-output integration capabilities of a hybrid system. There is an urgent need to establish a multidimensional, standardized comprehensive performance evaluation framework, which encompasses thermal and exergy efficiencies, net power output, the LCOE, system complexity, reliability, and storage capacity.

(5) Target front-end support to de-risk geothermal exploration and storage, where justified by resource and demand; adopt a standardized LCOE evaluation method with scenario-based sensitivity (resource grade, storage sizing, complexity, reliability) to guide procurement; and encourage data sharing using a reproducible feasibility table to reduce ambiguity in techno-economic comparisons. These measures directly address the observed LCOE sensitivity to resource grade, solar utilization, and storage configuration.

(6) Future research should focus on intelligent cooperative control, energy quality hierarchical utilization, and regional adaptation path. Applying an operation mechanism without using energy storage, carrying out the optimization of a system with multiple heat sources, and utilizing advanced artificial-intelligence tools will become the key breakthrough for the engineering implementation of geothermal–solar hybrid systems.