A review of hybrid renewable energy systems: Solar and wind-powered solutions: Challenges, opportunities, and policy implications

1. Introduction

The rapid depletion of fossil fuels and the growing concern over climate change have propelled the world towards a critical juncture in energy transition. Amidst this paradigm shift, hybrid renewable energy systems (HRES), particularly those incorporating solar and wind power technologies, have emerged as prominent solutions to address the challenges of energy sustainability [1,2]. Fig. 1 illustrates the remarkable evolution of global renewable energy adaptation from 2010 to 2020, highlighting the pivotal rolethat renewables have played in reshaping the energy mix, as sourced from the Energy Information Administration (EIA) [3]. The growth in renewable energy capacity over these years show 1240 TW h in 2010, the capacity steadily rises, reaching 2960 TW h in 2020. This remarkable increase reflects the global shift towards cleaner and more sustainable energy sources, driven by factors such as technological advancements, environmental concerns, and supportive policies.

However, such systems mitigate the intermittency issues inherent to individual renewable sources, enhancing the overall reliability and stability of energy generation. Solar power exhibits peak output during daylight hours, while wind power can be harnessed even during periods of reduced solar availability [4]. By integrating these sources, the energy supply becomes more consistent, reducing the risk of power shortages during adverse weather conditions. Additionally, energy storage technologies integrated into hybrid systems facilitate surplus energy storage during peak production periods, thereby enabling its use during low production phases, thus increasing overall system efficiency and reducing wastage [5]. Moreover, HRES have the potential to significantly contribute to grid stability. The intermittent nature of standalone renewable sources can strain existing power grids, causing frequency and voltage fluctuations [6]. By incorporating hybrid systems with energy storage capabilities, these fluctuations can be better managed, and surplus energy can be injected into the grid during peak demand periods. This not only enhances grid stability but also reduces grid congestion, enabling a smoother integration of renewable energy into existing energy infrastructures.

While HRES offer promising solutions, their deployment does not come without challenges [7,8]. Technical complexities, such as optimizing the integration of different sources and managing energy storage, require careful consideration. Economic viability, including initial setup costs and ongoing maintenance expenses, needs to be evaluated in the context of long-term benefits. Moreover, policy frameworks and regulations should be formulated to incentivize the adoption of hybrid systems and ensure a seamless transition towards cleaner energy. The integration of solar and wind power in HRES holds immense potential to reshape the global energy landscape. This review delves into the challenges, opportunities, and policy implications associated with these integrated systems, shedding light on their transformative capabilities.

1.1. Motivation of the study

The pressing challenge of climate change necessitates a rapid transition from fossil fuel-based energy systems to renewable energy solutions. While significant progress has been made in the development and deployment of renewable technologies such as solar and wind energy, these standalone systems come with their own set of limitations. Solar energy generation is contingent upon daylight and clear weather conditions, whereas wind energy is unpredictable, depending on fluctuating wind speeds. The intermittency and variability of these energy sources pose a challenge to the stability of the electricity grid, thereby affecting the wider adoption of renewable energy systems. Furthermore, the current policy frameworks and economic models often do not adequately support the seamless integration of these disparate renewable resources into a unified and efficient energy system.

This study is motivated by the urgent need to explore how HRES specifically those integrating solar and wind energy can address the limitations inherent in single-source systems. By delving into the technical challenges, economic considerations, and policy landscapes, this review aims to provide a comprehensive overview that can guide future research, investment, and policymaking in this domain. Moreover, the study seeks to identify the gaps in current research and policy that need to be addressed to accelerate the adoption of hybrid renewable energy systems. By synthesizing existing knowledge and providing actionable insights, this review aims to contribute to the advancement of HRES as a viable, sustainable, and efficient solution for mitigating the impacts of climate change and securing a more sustainable energy future.

2. Singel energy sources technologies

2.1. Solar photovoltaic power systems

Solar photovoltaic (PV) power systems are a cornerstone of renewable energy technology, converting sunlight into electrical energy through the PV effect. This process takes place in solar panels comprised of interconnected solar cells, usually made of silicon [9]. The PV effect can be described by the following:(1) $I = I_{P h} + I_{d}$ where I represent the current generated by the solar cell, Iph is the photocurrentproduced due to absorbed photons, and Id signifies the dark current. The amount of generated current I is proportional to the intensity of incident sunlight.

The power output of a solar cell can be calculated using the equation:(2) $P = I \cdot V$ where P is the power output, I is the current, and V is the voltage generated by the solar cell. The voltage (V) across the terminals of a solar cell can be estimated by the Shockley diode equation [10]:(3) $V = V_{o c} - I \cdot R_{s}$ where Voc is the open-circuit voltage of the solar cell and Rs is the series resistance.

The efficiency (ηPV) of a solar PV system, indicating the ratio of converted solar energy into electrical energy, can be calculated using equation [10]:(4) $η_{P V} = P_{\max} / P_{i n c}$ where Pmax is the maximum power output of the solar panel and Pinc is the incoming solar power. Efficiency can be influenced by factors like temperature, solar irradiance, and material properties.

The instantaneous power generated by a PV system in (kW) can be described as follows [11,12]:(5) $P_{P V, t} = C_{P V} η_{P V} \cdot (\frac{G_{T, t}}{G_{T, S T C}}) \cdot [1 + α_{P} (T_{C, t} - T_{C, S T C})]$ where CPV is rated capacity of the PV array (kW), $η_{P V}$ denotes the PV derating factor (%), GT,t is the incident solar radiation (kW/m2), GT,STC is the incident solar radiation (kW/m2) at Standard Temperature Conditions (STC), $α_{P}$ denotes the PV cell temperature coefficient of power (%/oC), TC,t is the temperature of the PV cell (oC) and TC,STC is the temperature of the PV cell (oC) at STC.

Solar PV power systems offer numerous advantages over time, but they also face challenges related to intermittency, upfront costs, and storage. Balancing these strengths and weaknesses is essential for maximizing the benefits of solar energy and addressing its limitations effectively as presented in Table 1.

Table 1. Solar PV power systems strengths and weaknesses [[13], [14], [15]].

Strengths	Weaknesses
1. Renewable energy source: solar PV systems tap into abundant sunlight, providing a consistent and renewable source of energy for power generation.	1. Intermittency: solar energy production is limited to daylight hours and can be affected by weather conditions, leading to variability in output.
2. Predictable daily pattern: daily solar energy patterns are relatively predictable, allowing for better energy generation forecasts and grid integration.	2. Nighttime generation: solar panels do not produce energy at night, necessitating energy storage or alternative power sources during dark hours.
3. Scalability: solar arrays can be expanded by adding more panels, increasing energy production to match growing demand.	3. Seasonal variations: solar energy output can vary with the changing angle of the sun throughout the year, affecting overall annual production.
4. Low operating costs: solar PV systems have minimal operating costs after installation, as they do not require fuel or ongoing resource inputs.	4. High initial costs: the upfront cost of solar panel installation and equipment can be relatively high, impacting initial return on investment.
5. Decentralized generation: solar panels can be installed on rooftops and distributed across various locations, reducing strain on centralized power infrastructure.	5. Shading impact: shading on even a small part of a solar panel can significantly reduce energy production from the entire panel or string.
6. Environmental benefits: solar power reduces greenhouse gas emissions and air pollution, contributing to a cleaner environment and mitigating climate change.	6. Limited energy generation in low light conditions: energy production decreases significantly in cloudy, rainy, or heavily shaded conditions.
7. Low maintenance: solar panels require minimal maintenance, with no moving parts, reducing operational complexities and costs.	7. Aesthetic considerations: the appearance of solar panels might not always align with architectural preferences or community aesthetics.
8. Technological advancements: ongoing advancements improve solar panel efficiency, enhancing energy capture and reducing overall costs.	8. Geographical limitations: solar energy generation is location-dependent, with higher efficiency in regions with more sunlight.
9. Grid support: solar power can contribute to grid stability by generating power close to demand centers, reducing transmission losses.	9. End-of-life management: proper disposal and recycling of solar panels present challenges in minimizing environmental impact.
10. Energy independence: solar PV systems help diversify the energy mix and reduce dependence on fossil fuels, promoting energy security.	10. Energy storage requirement: storing excess solar energy for use during non-sunny periods requires efficient and cost-effective BT technology.

2.2. Wind turbine power systems

Wind power systems harness the kinetic energy of moving air to generate electricity, offering a sustainable and renewable source of energy. Wind turbines (WT), the primary components of these systems, consist of blades that capture wind energy and spin a rotor connected to a generator, producing electrical power through electromagnetic induction. The power output of a WT can be calculated [16]:(6) $P_{W T, t} = 0.5 \cdot ρ \cdot A \cdot v^{3} \cdot C p$ Where PWT represents the power output, ρ is the air density, A is the swept area of the rotor, v is the wind speed, and Cp is the coefficient of performance that captures the efficiency of the turbine energy conversion.

Wind power systems benefit from several strengths, including their ability to produce clean energy, contribute to energy independence, and offer relatively low operational costs [17]. However, they face challenges such as intermittent wind patterns and potential visual and noise impacts on landscapes and communities. Table 2 outlining the strengths and weaknesses of WT power systems from the perspective of energy production.

Table 2. WT power systems strengths and weaknesses [[18], [19], [20]].

Strengths	Weaknesses
1. High energy yield: wind turbines can generate significant amounts of energy, especially in regions with consistent and strong wind resources.	1. Intermittency: wind energy production is variable due to fluctuations in wind speed, leading to inconsistent power output.
2. Predictable output: over the long term, wind patterns can be relatively predictable, enabling better energy production forecasts and grid integration.	2. Low energy production in calm conditions: wind turbines require a minimum wind speed (cut-in speed) to start generating power, leading to low energy production during calm conditions.
3. Scalability: wind farms can be expanded by adding more turbines, increasing energy production to meet growing demand.	3. Shutdown in high wind: turbines have a maximum wind speed (cut-out speed) at which they shut down to prevent damage, reducing energy production during strong winds.
4. Reduces fossil fuel dependence: wind power reduces the need for fossil fuel-based power generation, promoting energy security and reducing greenhouse gas emissions.	4. Noise and aesthetic concerns: noise generated by turbines and their visual impact can lead to community opposition, affecting the placement and operation of wind farms.
5. Low operating costs: once installed, wind turbines have relatively low operational costs compared to fuel-dependent power plants.	5. Land use considerations: wind farms require significant land area, which might compete with other land uses, such as agriculture or conservation.
6. Decentralized generation: wind farms can be distributed across different geographic locations, reducing strain on centralized power infrastructure.	6. Resource limitations: wind energy is location-specific, and not all areas have sufficient and consistent wind resources for reliable power generation.
7. Environmental benefits: wind power reduces air pollution, water usage, and greenhouse gas emissions, contributing to a cleaner environment.	7. Maintenance challenges: WT maintenance, especially for offshore installations, can be complex and require specialized equipment and personnel.
8. Grid stability: wind farms can provide grid support by helping to stabilize frequency and voltage fluctuations.	8. Visual impact: the visual presence of wind turbines in landscapes can lead to concerns about their impact on scenic views and tourism.

The WT power systems offer substantial energy production potential along with environmental and economic benefits. However, they must contend with variability in wind conditions, visual and noise concerns, and challenges related to maintenance and site selection. Careful planning and technological advancements are essential to maximize the strengths of wind energy production while mitigating its weaknesses.

3. Multi sources energy system

3.1. Solar and wind combined

Combining solar and wind energy into a hybrid renewable energy system can be done in various ways to optimize energy production, reliability, and efficiency. Below are some methods supported by references.

•
Co-Located installations: one straightforward approach is to install solar panels and wind turbines at the same location. The combined systems can feed into a single electrical grid, ensuring a more stable and constant energy supply. This is particularly useful in regions where solar and wind resources are complementary; for instance, sunny days with little wind and windy nights or cloudy days [21].
•Integrated controllers: advanced control systems can be used to optimize the performance of both solar and wind systems. These controllers can divert power from an over-performing system to charge batteries or meet immediate consumption needs, thus balancing the load [22].
•Microgrids: in isolated or remote areas, solar and wind systems can be combined into a microgrid, which can operate independently of a central grid. Such systems often include energy storage solutions like batteries, which store excess energy from either source for later use [23].
•Power Electronics: The use of sophisticated power electronic devices allows for more seamless integration of solar and wind power. These devices can adjust voltage and frequency parameters in real-time to ensure a stable and reliable power supply [24].
•Optimization algorithms: computational algorithms can be employed to determine the optimal mix of solar and wind resources for a given location and time, factoring in variables like weather conditions, electricity demand, and storage capacity [25].
•Policy integration: on a broader scale, combining solar and wind necessitates coordinated policy efforts that provide financial incentives, feed-in tariffs, or subsidies aimed explicitly at hybrid systems [26].
•Demand response systems: some advanced hybrid systems use demand response mechanisms to match supply with demand, automatically adjusting the contributions from solar and wind resources based on real-time consumption patterns [27].

3.2. The need for hybridization

The need for hybridization of renewable energy systems arises from the inherent challenges and limitations of individual renewable sources. While renewable sources like solar and wind power offer substantial benefits, they also exhibit intermittency and variability in their energy generation. HRES combine multiple sources, often including solar, wind, hydro, or even fossil fuel-based backup, to leverage the strengths of each and mitigate their weaknesses.

•Hybrid systems enhance reliability and stability: by combining complementary sources, such as solar and wind, which peak at different times, a consistent and stable power output can be achieved. This ensures a more reliable energy supply, reducing the risk of power shortages during periods of low sun or wind [28].
•Hybridization improves energy availability: many regions experience seasonal variations in renewable energy generation due to weather patterns. Hybrid systems that integrate different sources can provide a more consistent energy supply throughout the year, helping to meet continuous energy demands [29].
•Hybrid setups enhance efficiency: some renewable sources, like solar panels, might have excess energy production during certain periods. By integrating energy storage technologies, surplus energy can be stored and utilized when production is low, increasing overall system efficiency and reducing wastage.
•Hybrid systems contribute to grid stability: the intermittent nature of some renewable sources can strain power grids [30]. Hybrid systems equipped with energy storage can act as grid stabilizers by supplying power during peak demand times, reducing grid congestion and enhancing overall stability.
•Hybridization aids remote and off-grid areas. In locations where access to a single reliable renewable source is limited, combining various sources allows these areas to generate sufficient power without relying solely on expensive fuel-based generators [31,32].
•Hybrid systems provide a pathway to a cleaner energy transition.Integrating renewable sources with low-carbon backup options, like battery (BT) storage or cleaner fossil fuel technologies, can help balance energy supply and demand while gradually reducing dependence on fossil fuels [33].

While they offer numerous advantages, there are challenges associated with their hybridization. Table 3 listing HRES hybridization challenges.

Table 3. Challenges of hybridization HRES.

Challenge	Explanation
Technical Challenges
Integration Complexity	Different energy sources may require specific control and management systems to be integrated seamlessly.
Intermittency	Renewable sources like solar and wind are intermittent, making prediction and management more complex.
Infrastructure Development	Building new infrastructure or retrofitting existing infrastructure for HRES can be complicated.
Energy Storage	Choosing, integrating, and managing energy storage solutions to ensure energy reliability can be challenging.
Power Quality	Integrating multiple sources may affect power quality, requiring proper management to maintain stability.
Economic Challenges
High Initial Costs	Hybrid systems may have higher initial investment costs compared to single-source systems.
Return on Investment (ROI) Uncertainty	The variability of renewable energy can affect the predictability of returns on investment.
Market Maturity	Some technologies in HRES might not be mature, leading to economic uncertainties.
Environmental Challenges
Land Usage	Combining multiple energy sources may require more land or specific types of land, leading to environmental concerns.
Resource Assessment	Accurate assessment of renewable resources (e.g., wind speeds, solar irradiance) is crucial but can be challenging.
Regulatory & Policy Challenges
Inconsistent Policies	Different energy sources might be subjected to varying policies and regulations, complicating system design.
Grid Integration Policies	Integrating HRES into existing grids may face regulatory hurdles, especially if grid policies are not updated.
Licensing and Standards	There might be a lack of standardized regulations for HRES, leading to uncertainties in licensing and operation.

3.3. HRES with/without grid setup

The HRES can be broadly classified based on their grid connection status into three categories: on-grid, off-grid, and microgrid systems. This classification is visually represented in Fig. 2 and has distinct implications for the design, operation, and policy regulation of HRES.

1.
On-grid systems: In this category, the HRES is directly connected to the centralized electricity grid. The primary advantage of an on-grid system is the ability to feed surplus electricity back into the grid, often benefiting from feed-in tariffs or net metering policies [34]. This type of system is generally more straightforward to implement from a regulatory standpoint and can take advantage of existing grid infrastructure. However, it is heavily dependent on the grid stability and may be affected by grid failures. On-grid systems are well-suited for urban and suburban areas where grid connectivity is reliable and robust [35].
2.
Off-grid systems: These systems operate independently of the centralized electricity grid and are often used in remote or rural areas where grid connectivity is either unavailable or unreliable. Off-grid HRES usually require a form of energy storage, like batteries, to store excess energy for use when renewable sources are not generating electricity [36]. Although off-grid systems provide energy independence, they generally have higher initial costs due to the need for storage and more complex control systems [37].
3.
Microgrid Systems: Falling somewhere between on-grid and off-grid systems, a microgrid is a localized energy system that can operate independently or in conjunction with the central grid [38,39]. Microgrids often incorporate multiple types of renewable energy sources, and possibly some conventional ones, along with energy storage solutions. Microgrids offer the flexibility of being able to operate in tandem with the grid or independently, providing resilience during grid failures. They are especially useful in institutional setups like universities, military bases, or industrial parks.

The configurations of HRES depend on the type and number of renewable energy sources used. The most common configurations are solar-wind, wind-hydro, and solar-hydro combinations. The selection of the configuration depends on the availability and variability of the renewable energy sources, the power demand, and the geographical location of the system.

3.4. HRES without storage units

The HRES can be configured either with or without storage units, each having distinct advantages and limitations.

HRES with storage units.

1.
Advantages:
- •
  Energy reliability: storage units can store excess energy generated, offering a buffer during periods when the renewable sources are not generating power. This enhances the overall reliability of the energy system [40].
- •Grid stability: in on-grid configurations, energy storage can help in load leveling and peak shaving, thereby aiding in maintaining grid stability [41].
- •Optimized use of resources: advanced control systems can intelligently manage the energy storage to maximize the efficiency of different energy resources.
- •Microgrid capability: in microgrid systems, storage units can enable the system to operate independently of the central grid if needed.
2.
Limitations:
•Cost: adding storage significantly increases the initial setup cost of the system.
•Maintenance: storage units like batteries degrade over time, requiring periodic maintenance and eventual replacement.
•Efficiency loss: energy storage usually involves some conversion losses, slightly reducing the overall efficiency of the system.

HRES without storage units.

1.
Advantages:
•Lower cost: eliminating the storage component reduces the upfront cost of the system.
•Simplicity: Fewer components mean simpler control systems and easier maintenance.
•Direct Usage: Energy is used as it is generated, ensuring that there are no storage-related energy losses.
2.
Limitations:
- •Intermittency: without storage, the system becomes highly susceptible to the variability of renewable resources like wind and solar, affecting its reliability [42].
- •Grid dependence: for on-grid systems, a lack of storage makes the HRES heavily reliant on grid stability [43].
- •Wastage: excess energy generated during periods of low demand may go to waste if it cannot be stored or fed into the grid.

The choice between an HRES with or without storage units depends on various factors, including the specific characteristics of the renewable sources, energy consumption patterns, and the level of reliability required. HRES with storage units offer enhanced energy reliability, grid stability, and the ability to manage fluctuations in renewable energy generation. However, they may involve additional costs related to energy storage infrastructure. On the other hand, HRES without storage units are suitable when the energy generation closely matches demand patterns and energy storage is not a critical requirement.

3.5. Forms of energy storage unites use in HRES

There are different types of storage units to store the energy used in HRES. Here are some of the most commonly used storage units in HRES.

1.
Batteries: are the most commonly used storage units in HRES. They store excess energy generated from renewable sources and release it when the demand is high [44]. There are different types of batteries such as lead-acid, lithium-ion, and flow batteries, each with its own advantages and disadvantages [45].
2.
Supercapacitors and ultra-supercapacitors: are a type of capacitor that can store much more energy than traditional capacitors. Supercapacitors have a higher energy density and can store more energy per unit of weight or volume than conventional capacitors [46]. They can be used to supplement or replace batteries in a HRES, providing a high-power output when needed, but with a lower energy density than batteries. In a HRES, supercapacitors can be used to balance power and energy demand, which can be used to provide short-term power demands, such as during peak load periods or when a sudden gust of wind generates an unexpected surge in power [47].
3.
Pumped hydro storage: is an energy storage system that utilizes two reservoirs located at different elevations. During times of low energy demand, excess energy is used to pump water from the lower reservoir to the upper reservoir [48]. When energy demand is high, the water is released from the upper reservoir to generate electricity.
4.
Compressed Air Energy Storage (CAES): is an energy storage system that stores compressed air in underground caverns. When energy demand is high, the compressed air is released to power turbines and generate electricity [49].
5.
Flywheels: are energy storage devices that store kinetic energy. They consist of a spinning rotor that rotates at a high speed, which stores energy [50]. When the demand for energy is high, the rotor releases its stored energy to power turbines and generate electricity.
6.
Thermal Energy Storage: is an energy storage system that stores excess heat generated from renewable sources such as solar energy. The stored heat is used to generate steam, which powers turbines and generates electricity when energy demand is high [51].
7.
Hydrogen and Fuel Cell: hydrogen can be produced from excess renewable energy and stored for later use in fuel cells. The fuel cells can then convert the stored hydrogen into electricity when needed [52]. This process creates a clean and efficient way to store and use renewable energy, as hydrogen produces only water as a by-product.
8.
Gravitricity energy storage: is a type of energy storage system that has the potential to be used in HRES. It works by using the force of gravity to store and release energy. In this energy storage system, heavy weights are lifted up and down within a deep shaft, using excess electricity generated from renewable sources such as wind or solar. When there is excess energy, the heavy weights are lifted to the top of the shaft. When energy is required, the weights are released and descend to the bottom of the shaft, generating electricity through a generator. The speed of descent can be controlled to adjust the power output, and the process can be repeated as required. Gravitricity energy storage is still a relatively new technology, it shows promise as a potential energy storage solution for HRES. Its fast response time, compact size, and ability to be used in combination with other storage systems make it a valuable addition to the suite of energy storage options available [53,54].

4. Scenarios of HRES: on/off grid models based on PV and WT with BT and ultrasupercapcitor (USC) storage systems

The on/off-grid HRES models embody the forward-thinking approach necessary for a sustainable energy future. By combining renewable energy and energy storage solutions, these systems provide adaptable and resilient energy options for both connected grid environments and isolated off-grid locations [55]. The section dedicated to reviewing both on-grid and off-grid HRES models exemplifies the versatility and adaptability of integrating various renewable energy sources to cater to a wide array of energy scenarios. Fig. 3, show the HRES scenarios has been reviewed in this study.

4.1. Photovoltaic + battery

Whether connected to the grid or operating independently, this model offers a balanced combination of solar power generation and BT storage. On the grid, the BT can contribute to load leveling, while off the grid, it ensures a stable energy supply during periods without sun [56,57]. Fig. 4 succinctly illustrates the dual capabilities of the PV + BT system, showcasing its adaptability to different energy contexts. This scheme emphasizes the role of energy storage in enhancing the stability, reliability, and autonomy of renewable energy systems, irrespective of their connection to the grid.

Combining a BT and a PV system for energy storage in both on-grid and off-grid scenarios involves a set of equations for modeling the system. These equations describe the balance of energy flow, power conversions, state-of-charge (SOC) of the battery, and interaction with the grid or load. Below is a simplified framework for modeling such a system:

Energy balance for off-grid scenario:(7) $P_{P V, t} + P_{B T, t} = P_{l o a d}$

Energy balance for on-grid scenario:(8) $P_{P V, t} + P_{B T, t} + P_{g r i d} = P_{l o a d}$

SOC of the battery:(9) $S O C (t) = S O C (t - Δ t) + P_{B T} \cdot \frac{Δ t}{E_{B T, \max}}$ where EBT, max is the maximum energy storage capacity of the battery.

Power charged/discharged by the BT for off-grid:(10) $P_{B T} = η_{B T} \cdot η_{I n v} \cdot (P_{P V} - P_{l o a d})$

Energy balance for on-grid scenario:(11) $P_{B T} = η_{B T} \cdot η_{I n v} \cdot (P_{P V} + P_{g r i d} - P_{l o a d})$

Power from/to the grid for on-grid:(12) $P_{g r i d} = P_{l o a d} - P_{P V} - P_{B T}$ when Pgrid>0, the system is importing power from the grid.

When Pgrid<0, the system is exporting power to the grid.

Additional constraints:

0≤ SOC ≤1.

PBT must be within the BT max/min charge and discharge rates.

PPV should consider environmental factors like irradiance and temperature

Where:

PPV: power output from the PV array.

Pload: power demand by the load.

PBT: power charged/discharged by the battery.

Pgrid: power imported/exported from/to the grid.

SOC: state-of-charge of the battery.

EBT: energy stored in the battery.

ΗBT: efficiency of the battery

ηinv: efficiency of the inverter

T: time.

Δt: time step.

Fig. 5 Show the global installed capacity of on-grid PV systems, this growth was driven by falling costs of solar panels and increasing government incentives and regulations promoting renewable energy [58,59]. One of the major developments in on-grid PV systems during this period was the increasing use of energy storage systems, which allow users to store excess energy generated during the day for use at night. This technology has made on/off-grid PV systems more attractive for homeowners and businesses looking to offset their energy usage.

The studies indicate that PV + BT energy systems, both on and off the grid, have seen substantial progress in terms of efficiency and value for money. A detailed techno-economic examination of PV-BT systems in Switzerland was carried out by Han et al. [61]. This study delved into the practicality and economic advantage of merging PV panels with BT storage for home energy use. It scrutinized different system dimensions, BT storage capabilities, and patterns of energy use. The results underscored the possible perks of these setups, like decreased grid dependence and potential monetary savings. However, the cost-effectiveness differed depending on aspects like the system scale, BT performance, and prevailing energy costs. Another study by Wu et al. [62] delves into the ideal BT storage size for PV systems connected to the grid, keeping in mind BT wear and tear. This research utilized a dual-tier optimization method grounded in mixed-integer nonlinear programming (MINLP) to curtail life cycle expenses (LCC). The study probed the effects of BT wear on the best BT size, self-use ratios (SCR), and overall LCC. It also assessed the impact of varied tariff setups, input restrictions, and PV wear on optimizing the system. The conclusions revealed that BT wear might amplify operating expenses, leading to bigger ideal BT sizes and elevated LCC. Moreover, the study provides insights into refining PV-BT setups considering BT wear and tariff plans, thereby providing crucial advice for eco-friendly energy alternatives.

The article by Khezri et al. [63] offers an overview of optimal planning approaches for solar PV and BT storage systems in grid-connected residential settings. The study delves into the challenges and emerging perspectives associated with the integration of these systems. It explores various methodologies for optimizing PV and BT systems, considering factors such as energy generation, consumption patterns, and economic feasibility. The outcomes emphasize the importance of addressing technical, economic, and regulatory challenges in implementing these systems. The study by Zhang et al. [64] introduces a techno-economic approach for sizing grid-connected household PV/BT systems. The research focuses on assessing the optimal system size considering technical and economic factors. It aims to strike a balance between energy generation, consumption patterns, and economic feasibility. The study explores a wide range of cost factors, ranging from 0.373 to 0.628 CNY/kWh, showcasing the potential for partial grid parity in Shanghai. Mulleriyawage and Shen [65] investigate the optimal sizing of BT energy storagecapacity in residential PV- BT systems. Through operational optimization, the research focuses on a case study of Australian households with an 8 kWp PV system. The study reveals that by employing operational optimization, the optimal BT energy storage capacity is determined to be 3.45 kW h at an installed cost of AU$800/kWh. However, when considering the self-consumption maximization (SCM) approach, the optimal BT capacity reduces significantly to 1.49 kW h. Hlal et al. [66] focuses on determining the optimum BT depth of discharge (DOD) for an off-grid solar PV-BT system. The research investigates various DOD values and their impact on system performance. Through analysis, the study identifies that the optimal DOD value for the investigated solar PV system is found to be 70 %. At this DOD value, the system achieves a low levelized loss of power (LLP) of 0 % and a competitive cost of energy of 0.20594 USD/kWh.

Ashtiani et al. [67] conducts a techno-economic analysis of a grid-connected PV/BT system utilizing the teaching-learning-based optimization algorithm. The research evaluates the economic viability and efficiency of the system compared to a non-renewable alternative. The findings indicate that the on-grid PV- BT system exhibits improved economic performance. Specifically, when compared to the non-renewable case, the on-grid PV- BT system demonstrates a 15.6 % reduction in net present cost and a 16.8 % decrease in the cost of energy. Zou et al. [68] conduct a comparative study on the operation strategies for grid-connected PV- BT systems in office buildings. The investigation focuses on two strategies: time-of-use (TOU) and minimum state of charge (MSC). The study examines their economic and BT performance implications. The findings reveal that the TOU strategy exhibits better economic performance compared to the MSC strategy, particularly when BT costs are relatively low (<1600 CNY/kWh). However, it is noted that the TOU strategy leads to more BT aging. Rezk et al. [69] conduct a performance evaluation and optimal design of a stand-alone solar PV- BT system for irrigation in isolated regions, focusing on a case study in Al Minya, Egypt. The research aims to determine the economic feasibility and efficiency of the system. The outcomes reveal that the system achieves a net present cost of $109,856 and an energy cost of $0.059 per unit. The cost of energy is notably lower compared to previously reported values due to careful selection of PV size, type, and location. The study by Tostado-Véliz et al. [70] introduces a novel methodology for optimizing the sizing of PV - BT systems in smart homes. The study takes into account factors such as grid outages and demand response capabilities. The findings emphasize the importance of considering both grid reliability and demand response potential when sizing PV - BT systems.

The study focuses on the optimal energy management of a grid-connected PV- BT system presented by Chakir et al. [71]. The research investigates strategies to efficiently control the flow of energy between the PV system and the BT within a grid-connected context. The findings contribute to enhancing the overall performance and economic viability of such systems. Bandyopadhyay et al. [72] presents a techno-economic model for optimal sizing of PV- BT systems in microgrids. The research utilizes a comprehensive approach that considers both technical and economic aspects to determine the optimal sizing of the system components. The study contributes valuable insights into optimizing the design of PV- BT systems for microgrids. Ge et al. [73] introduces a novel hybrid BT-Fuzzy controller based maximum power point tracking (MPPT) technique for grid-connected PV- BT systems. The research focuses on optimizing the power output of the PV system through advanced control strategies. By combining BAT (Bat Algorithm) and Fuzzy logic, the proposed method enhances the efficiency of the MPPT process. The study provides insights into advanced control techniques for improving the performance of grid-connected PV- BT systems. The study by Mosavi et al. [74] presents a fractional-order fuzzy control approach for PV- BT systems that addresses challenges such as unknown dynamics, variable irradiation, and temperature fluctuations. The study focuses on developing an advanced control strategy that adapts to varying environmental conditions and system dynamics. The proposed fractional-order fuzzy control technique offers an effective solution to optimize the operation of PV- BT systems under changing parameters. Chaianong et al. [75] focuses on analyzing the customer economics of residential PV- BT systems in Thailand. The research assesses the economic viability of these systems by considering factors such as investment costs, electricity savings, and incentives. The findings provide insights into the financial benefits of adopting PV- BT systems for residential energy generation and consumption in the Thai context. The study contributes to understanding the economic considerations associated with such systems and their potential contribution to renewable energy adoption.

Ridha et al. [76] presents a comprehensive approach for sizing and implementing off-grid stand-alone PV - BT systems. The research combines multi-objective optimization and techno-economic analysis to determine the optimal system size that considers technical efficiency and economic viability. The findings offer insights into achieving an effective balance between system performance and costs. The proposed methodology, referred to as Multi-Objective Optimization and Techno-Economic (MADE) analysis, contributes to designing and implementing efficient and cost-effective off-grid PV-BT systems. Tian et al. [77] introduces a nonisolated symmetric bipolar output four-port converter designed to interface with a PV- BT system. The research focuses on developing an innovative converter that facilitates efficient energy exchange between the PV system and the BT. The findings demonstrate the feasibility and effectiveness of this converter design in enhancing the integration and operation of PV- BT systems. The study contributes insights into advanced converter technologies for optimizing energy transfer and utilization within PV- BT systems.

Table 4 highlights recent studies exploring the integration of PV systems with BT technology across a range of aspects. These investigations, encompass diverse objectives and system targets, representing both residential and larger-scale applications. These studies collectively contribute insights into the multifaceted implications and benefits of PV and BT integration in various energy applications.

Table 4. Recent literature investigated PV + BT as several aspects.

Reference	Year	Off/on grid	Analysis objective	Target of the system	Outcomes
Bhayo et al. [78]	2019	Off-grid	Assessment of standalone system	Electricity generation and water pumping	Analysis of system feasibility and power management
Al Essa [79]	2019	On-grid	Home energy management	Residential electricity management	Study of home energy management with thermostatically controlled loads
Al-Soeidat et al. [80]	2019	On-grid	Reconfigurable DC-DC converter design	Improve PV- BT integration	Development of a converter for efficient energy exchange
Aghamohamadi et al. [81]	2020	On-grid	Two-stage robust sizing and operation	Residential PV- BT systems	Robust optimization considering PV generation and load uncertainty
Liu et al. [82]	2019	On-grid	Optimal design and operation	Heat pumps interdependency	Integration of heat pumps in PV- BT systems
Li [83]	2019	On-grid	Optimal sizing	Residential houses	Optimal sizing for grid-connected residential systems
Belkaid et al. [84]	2019	On-grid	Converter control design	PV-BT system	Evaluation of a controlled converter in PV- BT systems
Bhayo et al. [85]	2020	Off-grid	Power management optimization	Standalone electricity generation	Power management optimization for hybrid systems
Benavente et al. [86]	2019	Off-grid	PV- BT system sizing	Rural electrification	Sizing study for rural electrification with demand considerations
Ganiyu et al. [87]	2019	On-grid	Green energy for wastewater treatment	Wastewater treatment	Application of PV- BT system for wastewater treatment
Mazzeo [88]	2019	On-grid	3 E analysis	Electric vehicle charging	3 E analysis of energy, economic, and environmental impacts
Cai et al. [89]	2020	Off-grid	Optimal sizing and location based on economic parameters	Hybrid system with PV, BT, and diesel	Economic-based optimization for off-grid hybrid systems
Babatunde et al. [90]	2020	Off-grid	Feasibility analysis	Farm facility	Study of feasibility for off-grid system at a farm facility
Tsianikas et al. [91]	2019	Off-grid	Economic trends and comparisons	Grid-outage resilience	Analysis of economic trends for grid-outage resilience
Sandelic et al. [92]	2019	Both	Reliability evaluation	PV systems with integrated batteries	Study of reliability for PV- BT systems
Shivam et al. [93]	2021	On-grid	Multi-objective predictive energy management	Residential grid-connected PV- BT systems	Energy management strategy using machine learning
Vega-Garita et al. [94]	2019	Both	BT technology selection	PV- BT integrated module	Study of BT technology selection for PV integration
Alramlawi & Li [95]	2020	On-grid	Design optimization with BT lifetime estimation	Residential PV- BT microgrid	Detailed optimization based on BT lifetime estimation
Pena-Bello et al. [96]	2019	Both	Optimization for combining applications	PV-coupled BT systems	Investigation of BT technology and geographical impact
Akeyo et al. [97]	2020	On-grid	Design and analysis of large solar PV farms	Large solar PV farms with DC-connected batteries	Analysis of large PV farm configurations with batteries
Schleifer et al. [98]	2021	On-grid	Evolving energy and capacity values	Utility-scale PV-plus- BT systems	Analysis of energy and capacity values over time
Dufo-López et al. [99]	2021	Off-grid	BT lifetime prediction models	Stand-alone PV systems	Comparative study of BT lifetime prediction models
Rezk et al. [100]	2020	Off-grid	Optimization and energy management	Hybrid PV- BT system	Optimization of hybrid system for water pumping and desalination
Huang & Wang [101]	2020	On-grid	Capacity scheduling based on DRL	PV- BT storage system	Application of deep reinforcement learning for capacity scheduling
Agyekum [102]	2021	Both	Techno-economic comparative analysis	PV power systems with and without storage	Comparative techno-economic analysis of PV systems
Al-Khori et al. [103]	2021	Both	Comparative techno-economic assessment	Integrated PV-SOFC and PV- BT hybrid systems	Analysis of integrated hybrid system with different technologies
Coppitters et al. [104]	2020	On-grid	Robust design optimization and stochastic performance analysis	PV system with BT and hydrogen storage	Robust design optimization and performance analysis
Bonkile & Ramadesigan [105]	2019	Off-grid	Power management control strategy	Standalone PV- BT hybrid systems	Power management strategy using physics-based BT models
Angenendt et al. [106]	2019	On-grid	Optimization and operation	Integrated homes with PV- BT storage systems	Optimization of homes with PV BT systems and power-to-heat coupling
Li et al. [107]	2019	Both	Stratified optimization strategy	Restoration using PV- BT as black-start resources	Strategy for restoration using PV- BT systems as black-start resource

Fig. 6 presents the growing deployment of PV and BT energy systems in various countries from 2015 to 2022. Germany has been leading the trend, with its capacity increasing from 4500 MW in 2015 to an impressive 7500 MW in 2022. Australia closely follows, growing from 3800 MW in 2015 to 7000 MW in 2022. The US and Japan also show robust expansion, with the U.S. moving from 2500 MW to 5500 MW and Japan from 2000 MW to 3680 MW over the specified time frame. China, South Korea, Italy, France, the United Kingdom, and Spain are also making notable contributions, albeit at a smaller scale, to this global shift toward renewable and sustainable energy systems. This data underscores the increasing commitment of countries around the world to adopt cleaner, renewable energy solutions, with a marked emphasis in more developed economies.

4.2. Photovoltaic + ultracapacitor

The integration of PV and USC energy systems offers a versatile solution for both on-grid and off-grid energy applications. PV panels convert sunlight into electricity, providing a clean and renewable source of power. However, PV systems can be intermittent due to fluctuating weather conditions. This is where USC come into play. Unlike traditional batteries, USC can rapidly store and discharge energy, making them ideal for smoothing out the short-term fluctuations in electricity generation from the PV panels. In an on-grid scenario, this PV + USC system can work in conjunction with the main electrical grid to supply a stable and reliable flow of electricity. It can quickly respond to sudden changes in demand or supply, making the overall grid more resilient. In an off-grid setup, the system provides a standalone power source that can operate independently of any centralized grid, offering a reliable power supply even in remote or inaccessible locations. The quick charging and discharging capabilities of USC can be particularly useful for high-demand applications like water pumping or electric vehicle charging in these off-grid setups. Fig. 7illustrates the configuration of a PV + USC energy system in both on-grid (a) and off-grid (b) scenarios.

Modeling the combination of a PV system and an USC for energy storage in both on-grid and off-grid applications involves several equations to describe the energy flow, state of charge, and constraints. Below are the equations that describe such a system: