According to the International Energy Agency [11] the world is set to add as much renewable power over 2022-2027 as it did in the past 20 years. This section reviews the current availability and utilization of these resources, highlighting their significant contributions to global energy generation.

In 2022, solar PV generation increased by 270 TWh, representing a 26% rise and bringing total generation close to 1300 TWh [12,13], which marks the largest generation of all renewable technologies for that year. The trend continued into 2023, where solar PV alone accounted for three-quarters of the total renewable capacity addition worldwide, underscoring its critical role in the global energy transition [14]. Wind energy has also made significant strides, with its share of total utility-scale electricity generation increasing by 265 TWh, a 14% rise, culminating in over 2100 TWh of total generation. This growth positions wind energy as the second highest-growing renewable technology, following solar PV [15,16]. The complementary nature of solar and wind energy is particularly advantageous; while solar energy is predominantly available during the day, wind energy can be harnessed at night and during periods of reduced solar availability, thus enhancing the stability and reliability of the energy supply. In 2023, these two sources together accounted for a remarkable 96% of renewable capacity additions [17,18].

Renewable electricity of about 340 GW was added globally in 2022 [19]. This progress is essential for keeping the average global temperature rise below 1.5°C, as outlined in international climate agreements [20]. Moreover, the shift towards battery-less off-grid renewable energy systems is gaining momentum, providing sustainable energy solutions without the need for expensive battery storage [21]. This approach not only reduces costs but also enhances energy accessibility in remote and underserved areas, further promoting the use of clean energy globally [22].

This paper provides a comprehensive review of various alternatives to battery storage technologies. It emphasizes the practical application of these systems in off-grid scenarios, such as water pumping, desalination, and rural electrification, etc., where battery-based systems are often limited due to cost. The paper also introduces an integrated system combining solar PV, concentrated solar power (CSP), wind turbines, molten salt thermal storage, and hydrogen production, showcasing an innovative approach to energy management and long-term storage. Additionally, the paper identifies key challenges such as technological limitations, economic viability, and regulatory hurdles, setting the stage for further research in the development of sustainable, cost-effective energy solutions that can drive the global transition to clean energy.

This review employed a structured literature-search strategy to ensure comprehensive coverage of the field. Peer-reviewed journal articles, conference proceedings, technical reports, and relevant grey literature published between 2000 and 2026 were searched in Scopus, Web of Science, IEEE Xplore, ScienceDirect, and Google Scholar. The primary Boolean search string was: (“battery-less” OR “battery-free”) AND (“off-grid” OR “stand-alone” OR “remote”) AND (“renewable energy” OR “solar” OR “wind” OR “photovoltaic”). Additional technology-specific searches were conducted for each storage option considered in the review, using combinations such as “supercapacitor” AND “off-grid solar”, “hydrogen storage” AND “renewable energy system”, and “molten salt” AND “CSP” AND “off-grid”.

Studies were included if they: (i) focused on off-grid, stand-alone, or remote renewable energy systems; (ii) explicitly examined battery-less, battery-free, or reduced-battery configurations; and (iii) were published in English. Studies were excluded if they focused exclusively on grid-connected, utility-scale systems without clear relevance to off-grid applications, did not examine battery-less or reduced-battery configurations, or lacked sufficient technical information for inclusion in the review. Following the screening and eligibility assessment, 153 relevant references were retained and cited throughout the review. The selected literature is organized thematically according to storage technology type in Section 2, renewable energy system configuration in Section 3, application domain in Section 4, and integrated system design in Section 5.

Supercapacitors are energy storage devices that utilize an electric field to store and release energy rapidly. This characteristic makes them a promising alternative to traditional batteries, especially in applications demanding high power density, fast charging/discharging rates, and a longer lifespan. As highlighted by Gurung et al. [30], supercapacitors are particularly suited for off-grid applications, such as solar-powered systems, where they can provide a continuous power supply for low-energy devices like sensors and small electronics. The key electrode materials enabling supercapacitor performance in off-grid contexts include activated carbon (high surface area, low cost), graphene-based composites (high conductivity, >1000 F/g theoretical), and MnO₂-based pseudocapacitive electrodes (high energy density). Separator materials (polymer membranes, cellulose) and electrolyte choice (aqueous KOH, organic acetonitrile) critically determine operational temperature range and cycle stability. Future materials challenges include improving energy density to bridge the gap with batteries while preserving cycle life advantages exceeding 500,000 cycles. Malek et al. [31] propose a battery-less power supply system that incorporates wireless power transfer (WPT) technology. While WPT offers flexibility in device placement, its efficiency is significantly influenced by the distance between the transmitter and receiver coils. This limitation can hinder practical applications requiring greater flexibility. A more recent study by Rahman et al. [32] presents a self-powered, battery-less wireless sensor mote powered by a micro-solar energy harvesting system. This system employs flexible photovoltaic cells to capture solar energy, which is then stored in supercapacitors for powering a microprocessor unit and wireless communication.

Supercapacitors offer a viable storage solution for off-grid applications, particularly when coupled with renewable energy sources like solar power. While challenges related to charging efficiency exist, ongoing research and technological advancements are addressing these limitations. By optimizing system design and incorporating solutions like WPT and efficient power management circuits, supercapacitors can play a crucial role in enabling sustainable and reliable power supply for a wide range of off-grid applications.

Mechanical Flywheels store kinetic energy by rotating a mass around an axis. Energy is stored when the flywheel spins; when energy is needed, the rotation is slowed, and the energy is converted into usable electricity. Ezhilarasan et al. [33] presented a battery-less power conditioning system utilizing a mechanical flywheel and solar photovoltaic (PV). The system leverages the kinetic energy stored in a mechanical flywheel to provide power during outages. Experimental testing has demonstrated the system’s effectiveness in maintaining energy storage during power interruptions. The flywheel’s ability to retain energy was validated under various conditions, including supporting a 1 kW load at a constant speed of 1500 RPM for up to 10 seconds. Flywheel performance is primarily governed by rotor material selection. Carbon fiber composite rotors achieve the highest specific energy (100-130 Wh/kg) compared to steel (5-30 Wh/kg), but at higher cost. Magnetic bearing materials (high-temperature superconductors) eliminate friction losses. Vacuum housing materials must balance thermal management and structural integrity under high rotational stress. For off-grid applications, the primary R&D challenge is reducing the cost of carbon fiber rotors and improving durability in dusty or humid environments. While flywheels offer high energy density, they are bulky and have lower energy density compared to batteries. However, their integration with advanced technologies like soft switching converters and DC motors can enhance their performance. By carefully considering its limitations and exploring potential enhancements, this technology can contribute to a more sustainable and reliable energy infrastructure.

In compressed air energy storage (CAES) systems, excess energy is used to compress air and store it in large underground caverns or above-ground tanks. When energy demand exceeds generation, the compressed air is released, driving a turbine to generate electricity. While CAES is less common in small-scale off-grid systems, it has shown promise in larger renewable energy projects. The integration of CAES with RES, however, is still in its early stages, with ongoing research to optimize its efficiency [34]. Findings in Thombre et al. [35] suggest that using compressed air could be a viable alternative to water for energy storage, addressing issues like evaporation and energy density limitations associated with water reservoirs. CAES systems face significant materials challenges including high-pressure vessel integrity (carbon-fiber-wound composite tanks for above-ground storage, or salt cavern lining materials), thermal insulation for adiabatic CAES systems, and heat exchanger materials that must withstand cyclic pressure and temperature loads. Compressor valve materials (titanium alloys) and turbine blade coatings are critical for efficiency. For off-grid small-scale CAES, advanced polymer composite pressure vessels and phase-change thermal storage integration represent key materials research directions.

Pumped hydroelectric systems store potential energy of water by pumping water uphill to a reservoir when excess energy is available. During periods of high demand, the water is released to flow downhill, turning turbines to generate electricity. These systems are highly efficient and scalable but are generally more suited to larger, grid-connected systems due to their significant infrastructural requirements [36]. In remote, off-grid areas, smaller-scale versions of this approach are being explored. Pali et al. [37] present a sustainable power generation solution that combines solar photovoltaic (PV) technology with pumped hydro storage (PHS) to provide continuous electricity, particularly in rural and remote areas. The system, which was designed to produce a continuous power output of 750 W at 230 V, was found to be economically feasible compared to diesel and electric pumping systems, with an overall energy conversion efficiency of 7.11% and a PHS system efficiency of 63.69%. While the system offers several advantages, it also faces some challenges. The efficiency of energy storage can be impacted by low solar irradiance or heavy rainfall. Additionally, the initial costs associated with setting up such system can be significant, leading to a levelized cost of electricity (LCOE) that may not be competitive.

Hydrogen-based systems involve using excess renewable energy to power electrolysis, which splits water into hydrogen and oxygen. The hydrogen is stored and later used in fuel cells to generate electricity when required [38]. This method is particularly attractive due to its scalability and the fact that hydrogen can be stored for long periods. Meziane et al. [39] recommend the implementation of effective hydrogen storage solutions to ensure energy availability during low wind periods in a hybrid power system. Akarsu et al. [40] and Scamman et al. [41] have also highlighted the potential of hydrogen storage systems to significantly enhance the performance of hybrid power systems, particularly in off-grid and remote applications. By storing excess renewable energy as hydrogen, these systems can improve reliability, reduce wasted power, and increase battery efficiency. However, the integration of hydrogen storage also presents challenges, such as the safe handling and storage of hydrogen, increased system complexity, and efficiency losses during the conversion process. Despite these challenges, the benefits of hydrogen storage make it a promising technology for improving the sustainability and performance of hybrid power systems. Hydrogen storage tank materials for compressed gas (350-700 bar) include Type III (aluminum liner with carbon fiber) and Type IV (polymer liner with carbon fiber) vessels, where fiber-resin interface integrity under cycling is a critical durability challenge [42].

Molten salts, such as sodium nitrate-potassium nitrate mixtures, are used to store and release large amounts of thermal energy efficiently and cost-effectively [43]. Compared to battery-based storage, these systems can achieve higher energy densities and longer discharge durations, making them ideal for applications requiring sustained energy supply. Studies have demonstrated the potential of molten salt storage in various energy systems. For instance, Rahbari et al. [44] presented a novel approach to integrating real-time modeling and optimization with Molten Salt Energy Storage (MSES) and a Supercritical Steam Cycle. This integrated system demonstrated improved renewable energy utilization and uninterrupted energy generation, storage, and distribution. Batgi et al. [45] explored the use of thermal energy storage (TES) and a hydrogen subsystem to meet the fresh water and electricity demands of a community. The TES helped balance the supply-demand energy mismatch, resulting in a system that generated 74.90 GWh of electricity annually and produced enough fresh water to support 4,708 families. Furthermore, a standalone TES system was introduced by Ma et al. [46] as a key component in achieving a carbon-free energy future. The study highlighted the advantages of TES in terms of larger capacity, lower costs, and versatility in driving various thermal power cycles. While there are still technical challenges to be addressed, such as corrosion and thermal cycling, researchers are actively working on developing materials and system designs to mitigate these issues and improve the overall performance of molten salt storage systems. The primary materials challenges in molten salt TES include corrosion, thermal stability at high temperature, insulation breakdown etc. [47].

To assist industry experts and researchers in technology selection, Table 1 presents a consolidated performance comparison of the battery-less storage technologies reviewed in this paper. Values are drawn from the literature cited in the preceding subsections and represent typical or reported ranges rather than absolute specifications, as system performance varies substantially with design, scale, and operating conditions with the Technology Readiness Level (TRL) assessment. This TRL can guide technology selection. For instance, near-term deployment in remote off-grid systems, the combination of pumped micro-hydro (where geography permits), PEM hydrogen, and molten salt TES offers the best balance of maturity, economics, and performance, which motivates the integrated system proposed in Section 4.3.

Key observations from this comparison: Supercapacitors and flywheels excel in power density and cycle life, making them ideal for short-duration, high-frequency applications (e.g., power quality correction, transient buffering). Pumped hydro and molten salt TES offer the lowest cost-per-kWh for large-scale, long-duration storage subject to specific geographical or infrastructure conditions. Hydrogen storage offers the highest energy density by mass and the longest storage duration, making it uniquely suited to seasonal storage and multi-day backup, but suffers from the lowest round-trip electrical efficiency (25-45%), which substantially affects system economics. These trade-offs motivate the hybrid approach proposed in Section 4.3.

Single-source battery-less renewable energy systems rely on a singular renewable resource for power generation. They offer significant benefits for off-grid applications, especially in rural and remote areas. These systems are often cost-effective and straightforward to implement, especially in regions with an abundance of a specific resource. However, they suffer from variability of renewable energy resources. Table 2 presents a detailed summary of selected single-source renewable energy systems, their applications, inherent challenges, and the ongoing research efforts to mitigate the challenges.

Dual RES systems typically combine two renewable energy sources to provide a more stable and reliable energy supply, taking advantage of the complementary availability of energy resources which reduce the need for expanded energy storage unit [72,73]. Table 3 presents a summary of the review of recent studies on dual battery-less RES with the aim of highlighting key trends, common challenges, and optimization strategies. This trend underscores a shift towards more sustainable and multifunctional energy solutions that can adapt to varying demands and resource availability.

Despite the promising combinations, several common challenges persist across the studies. The inherent variability and intermittency of renewable energy resources [78], need of advanced control strategies to maintain stability and performance [81], and initial high investment costs for such hybrid systems pose significant barriers to implementation, particularly in economically constrained regions. Real-time monitoring and energy management systems allow for better adaptation to changing energy availability, improving overall system reliability [85].

These systems, often referred to as multi-source RES or multi-hybrid systems, combine three or more sources such as solar, wind, hydropower, or biomass to create a more balanced and stable power supply [87,88,89]. One of the primary drivers behind multi-source RES is the ability to harness energy from multiple sources that peak at different times. While multi-source RES provide unmatched reliability, they come with higher complexity in terms of system management and design. The need to balance the power input from three or more sources requires advanced control systems that can adjust energy production and distribution dynamically [90]. Additionally, the initial costs of implementing such systems are often higher due to the infrastructure required to accommodate multiple energy sources. Despite these challenges, the long-term benefits of hybrid systems such as improved energy reliability and reduced operational costs make them an attractive option for regions with highly variable renewable resources [91]. Table 4 compares battery-less and reduced-battery renewable energy configurations against conventional battery-dominant systems across key performance metrics.

Battery-less renewable energy systems are transforming how off-grid regions and industries meet their energy needs. By reducing reliance on conventional batteries, these systems provide a more sustainable and cost-effective approach to harnessing renewable energy sources. 5 highlights key applications where battery-less systems have been successfully implemented, focusing on areas such as water desalination, pumping water, off-grid electrification, and small-scale industrial applications. Each deployment is evaluated based on its operating principles, ideal locations, associated studies, and benefits. A detailed summary of these application areas, along with associated studies and benefits, is presented in Table 5.

In these systems, energy produced by say, solar photovoltaic (PV) panels or wind turbines is used for various applications, from powering homes and irrigation systems to driving industrial processes without immediate storage. For example, solar PV-powered water pumping systems have been widely implemented in rural agricultural settings [1]. By aligning energy generation with real-time consumption needs, these systems eliminate the need for expensive battery storage and reduce system complexity [52].

Although directly coupled renewable energy systems can operate effectively when generation coincides with demand, energy storage is required when renewable generation and load demand do not occur simultaneously. Hydrogen has emerged as a promising long-duration storage option for off-grid renewable energy systems and can complement or reduce dependence on conventional battery storage [28,41,42]. Stored hydrogen may be used directly as a transportation fuel in remote poly-generation systems [79] or reconverted into electricity through a fuel cell when renewable generation is insufficient [39,41,42]. In a hydrogen fuel cell, hydrogen reacts electrochemically with oxygen to produce electricity and heat, with water as the principal reaction product [106].

In view of the highlighted benefits as well as challenges associated with multiple RES systems, a conceptual framework for the integration of three renewable sources, namely, Photovoltaic (PV), Concentrated Solar Power (CSP), and Wind Turbine (WT) is presented. The proposed off grid microgrid system aims to establish a robust renewable energy infrastructure that delivers continuous, stable, and reliable power to meet load demands. Figure 2 shows the conceptual diagram of the proposed off-grid battery-less renewable energy system. Unlike previous studies, this system not only generates electricity for various applications but also produces green hydrogen which can be utilized to fuel small equipment and vehicles, particularly in agricultural settings [149,150,151].

The design and implementation of this system aim to address the challenges faced by battery-operated renewable energy systems, such as the lack of affordable battery recycling technologies, safety concerns, and the complexities that batteries introduce to the design, operation, and maintenance of off-grid systems [152,153,154,155]. The proposed integrated energy system represents a significant advancement in renewable energy technology, offering a sustainable solution that enhances energy reliability, reduces environmental impact, and meets diverse energy needs without the reliance on traditional battery storage. The Figure 3 presents the proposed detailed System Schematic battery-less off-grid hybrid renewable energy system in which three generation sources — Solar PV (~10 kWp via MPPT), a Wind Turbine (5-7.5 kW), and a CSP turbine (2.5-10 kW) drawing on molten salt thermal storage converge onto a shared DC bus (200-400 V) managed by an EMS/BMS for intelligent mode switching and priority dispatch. Rather than relying on conventional batteries, the system stores energy through complementary technologies: molten salt TES (380-565°C, η_thermal ≈ 93-99%, 4-10 hr discharge) for short-to-medium duration buffering, and a PEM electrolyzer coupled with a compressed hydrogen tank (350-700 bar, >20 yr lifespan) and fuel cell stack (1-4 kW, η ≈ 40-60%) for long-duration storage and reconversion. Generated power serves community AC loads, DC water pumps, and a SCADA/IoT monitoring platform, while the stored hydrogen uniquely serves a dual role — as a backup electricity source via the fuel cell and as a direct fuel for agricultural vehicles and equipment, replacing diesel and representing the system’s key novel contribution.

The configuration proposed in this work brings together several elements that have not previously been combined in a battery-less, off-grid context. Specifically, the system integrates: (i) CSP with molten salt thermal storage, (ii) PEM electrolysis for green hydrogen production, (iii) solar PV and wind turbines, and (iv) complete elimination of battery storage in an off-grid setting with dual-hydrogen usage (electricity reconversion + direct agricultural fuel), represents a configuration not previously reported in the battery-less off-grid literature to the authors’ knowledge. The central innovation is the dual-pathway of hydrogen utilization for long-duration electricity backup and as a direct fuel substitute which enhances. Table 7 summarizes the differentiation of the proposed system from prior art.

The system operates across four distinct modes depending on the availability of solar irradiance, wind speed, and stored energy levels:

Mode 1 — Surplus Generation: When combined PV and WT generation exceeds the instantaneous load demand (P_PV + P_WT > P_load), the surplus power is directed in priority order to: (i) the CSP molten salt heat exchanger via electric heaters to top up thermal storage, and (ii) the PEM electrolyzer for hydrogen production. This prioritization maximizes the utilization of dispatchable thermal storage while producing hydrogen as a secondary long-term energy carrier.

Mode 2 — Balanced Generation: When PV and WT generation approximately meets load demand (P_PV + P_WT ≈ P_load), the system operates in direct-dispatch mode. The DC bus controller maintains bus voltage regulation through a DC-DC boost converter. No thermal or hydrogen charging occurs in this mode.

Mode 3 — Deficit Generation — Short-Term: During short-term shortfalls (e.g., transient cloud cover, wind lulls), the CSP turbine is activated to draw from molten salt thermal storage to supplement electrical generation. The thermal storage system can provide dispatchable output for 6-12 hours depending on the storage tank sizing.

Mode 4 — Deficit Generation — Extended: During extended low-generation periods (e.g., multi-day low irradiance and wind), stored hydrogen can be converted back to electricity via a fuel cell stack, providing a final layer of energy security. In the proposed agricultural application, hydrogen is also available directly for fueling farm equipment, eliminating the need for diesel.

At any time t, the instantaneous electrical power balance of the hybrid renewable-energy system is expressed as

where P_PV(t) and P_WT(t) denote the instantaneous electrical power generated by the photovoltaic array and wind turbines, respectively; P_CSP(t) represents the dispatchable electrical power supplied by the concentrating solar power plant, including power generated using thermal energy discharged from the molten-salt storage system; and P_FC(t) is the electrical power generated by the hydrogen fuel cell. The demand-side terms P_load(t), P_EL(t), and P_TC(t) represent the instantaneous electrical load demand, the power consumed by the proton-exchange-membrane electrolyzer, and the electrical power consumed by the heater used to charge the molten-salt thermal-energy storage system, respectively. The term P_loss(t) accounts for conversion, storage, transmission, and distribution losses within the system.

Equation (1) represents an instantaneous power balance, and all terms should therefore be expressed in consistent power units, such as kW or MW. The corresponding daily energy balance is obtained by integrating the power terms over a daily operating period T_d, typically 24 h:

where the daily energy associated with each system component x is defined as

For a discrete-time simulation, Equation (3) can be written as

where Δt is the simulation time-step duration and N_d is the total number of time steps within the daily operating period. The energy quantities in Equations (2)-(4) should be expressed in consistent units, such as kWh or MWh. The term P_TC(t) is included in the electrical power balance only when the molten-salt storage system is charged using an electrically powered heater. Thermal energy transferred directly from the CSP solar field to the molten-salt storage system should be represented separately in the CSP thermal-energy balance [156]. This framework provides a simplified representation of the system’s energy flows. A full simulation using HOMER Pro or an equivalent tool, incorporating hourly meteorological data for a representative off-grid site, is identified as the next step and will be presented in a follow-on publication.

The practical deployment of the proposed battery-less PV-WT-CSP-hydrogen system requires coordinated consideration of economic, operational, scalability, and reliability constraints.

Economic feasibility: The system requires substantial capital investment in PV and WT generation, the CSP receiver and power block, molten-salt TES, PEM electrolyzer, water-treatment unit, hydrogen compressor and storage tank, fuel cell, power converters, and supervisory control. Operating costs include CSP cleaning, salt circulation and heat tracing, water purification, compressor servicing, hydrogen-system inspection, and electrolyzer and fuel-cell stack replacement. Feasibility should therefore be evaluated using net present cost, levelized cost of energy, and levelized cost of hydrogen, with sensitivity to equipment cost, discount rate, renewable-resource availability, hydrogen demand, diesel prices, and policy incentives [110,113,114,117,157]. Dual hydrogen use may improve equipment utilization, but this benefit must be quantified through system-level techno-economic analysis.

System control and coordination: The energy-management system must coordinate variable PV and WT outputs, CSP-TES charging and discharging, electrolyzer and fuel-cell operating limits, hydrogen-reserve allocation, and DC-bus and AC-load regulation. Because the system excludes batteries, converter coordination, short-term power balancing, black-start capability, load shedding, and renewable-generation forecasting are particularly important [81,85,90,125].

Scalability: Deployment is constrained by site-specific solar irradiance, direct normal irradiance, wind availability, land and water requirements, CSP plant scale, hydrogen-storage and dispensing infrastructure, and access to technical support. A staged transition from pilot demonstration to community-scale deployment using measured resources and load data is therefore required [122,147,158,159].

Maintenance and reliability: Major concerns include PV soiling, WT mechanical wear, CSP mirror and receiver degradation, molten-salt corrosion or freezing, electrolyzer and fuel-cell aging, compressor wear, hydrogen leakage, and pressure-vessel inspection [117,122,151,160]. SCADA-based condition monitoring, hydrogen-leak detection, freeze protection, fault isolation, preventive maintenance, and scheduled component replacement are consequently essential for reliable long-term operation.