Design of a Novel Hybrid Concentrated Photovoltaic–Thermal System Equipped with Energy Storages, Optimized for Use in Residential Contexts

Abstract

Concentrated photovoltaic (CPV) technology is based on the principle of concentrating direct sunlight onto small but very efficient photovoltaic (PV) cells. This approach allows the realization of PV modules with conversion efficiencies exceeding 30%, which is significantly higher than that of the flat panels. However, to achieve optimal performance, these modules must always be perpendicular to solar radiation; hence, they are mounted on high-precision solar trackers. This requirement has led to the predominant use of CPV technology in the construction of solar power plants in open and large fields for utility scale applications. In this paper, the authors present a novel approach allowing the use of this technology for residential installations, mounting the system both on flat and sloped roofs. Therefore, the main components of cell and primary lens have been chosen to contain the dimensions and, in particular, the thickness of the module. This paper describes the main design steps: thermal analysis allowed the housing construction material to be defined to contain cell working temperature, while with deep optical studies, experimentally validated main geometrical and functional characteristics of the CPV have been identified. The design of a whole CPV system includes thermal storage for domestic hot water and a 1 kWh electrical battery. The main design results indicate an estimated electrical conversion efficiency of 30%, based on a cell efficiency of approximately 42% under operational conditions and a measured optical efficiency of 74%. The CPV system has a nominal electric output of 550 W_p and can simultaneously generate 630 W of thermal power, resulting in an overall system efficiency of 65.5%. The system also boasts high optical acceptance angles (±0.6°) and broad assembly tolerances (±1 mm). Cost analysis reveals higher unit costs compared to conventional PV and CPV systems, but these become competitive when considering the benefit of excess thermal energy recovery and use by the end user.

1. Introduction

The increasing demand for renewable energy sources (RES) has driven innovation in photovoltaic (PV) technology. Among these, concentrated photovoltaic (CPV) systems have emerged as a promising solution due to their high efficiency in converting sunlight into electricity. However, CPV systems also generate significant amounts of heat, which, if not managed properly, can reduce their performance and lifespan. To address this, hybrid CPV–thermal (CPV-T) systems have been developed, integrating thermal management to utilize the waste heat for additional energy applications. Hybrid CPV-T systems combine the electrical generation capabilities of CPV with thermal energy recovery, enhancing overall energy efficiency. The CPV component focuses sunlight onto high-efficiency solar cells using lenses or mirrors, while the thermal component captures the heat generated during this process. This dual functionality not only improves the energy yield but also provides a versatile energy solution, particularly advantageous for residential settings where both electricity and heat are required.

This paper aims to describe the design of a novel CPV system specifically studied for integration in residential and industrial settings. CPV technologies represent an advanced method for solar energy conversion. Unlike traditional PV systems, CPV systems use optical devices such as lenses or mirrors to concentrate sunlight onto high-efficiency solar cells. This concentration significantly increases the electrical output from the cells compared to conventional flat-plate PV systems. CPV systems are particularly advantageous in areas with high direct normal irradiance (DNI), as they can achieve higher efficiencies and better land-use efficiency [1]. CPV employment allows also for a significant reduction in the amount of semiconductor material required compared to conventional PV systems while potentially increasing the overall efficiency. Between 2010 and 2016, CPV technology saw considerable development, driven by a series of installations especially in China, USA and South Africa. Companies producing PV cells for space adapted their products for terrestrial applications, supplying PV cells for the construction of high-efficiency CPV modules [2]. In fact, the continuous development of multi-junction cells led to devices with solar conversion efficiencies reaching nearly 48% [3], making them particularly suitable for CPV technology. In Italy, several companies have explored this market, attempting to create concentration systems. Among the most notable efforts are those by Pirelli Labs [4] and BECAR [5].

Over the past 15 years, flat PV technology has seen widespread global adoption. This has led to a significant reduction in costs, driven by large economies of scale and advancements in panel production technologies. Notably, increasingly efficient cells have been developed with minimal thickness, reducing material usage [6,7].

However, CPV technology has become less attractive compared to flat-panel PV systems due to several disadvantages. These include the requirement for precise tracking and the inability to adapt to rooftop installations [8].

Research into CPV is diverse, focusing on areas like system design, thermal management, hybrid systems, and new materials. In detail, many studies have been conducted in recent years to explore the features reported below [9].

CPV system design optical components: recent studies have been improving the design and efficiency of optical concentrators, such as Fresnel lenses and parabolic mirrors, to maximize light capture and concentration. The goal is to enhance uniformity and reduce optical losses, which are critical for high-efficiency performance [10].

High-efficiency PV cells: researchers are focusing on multi-junction cells that can achieve efficiencies exceeding 45%. These cells are tailored to capture different segments of the solar spectrum, thus maximizing the energy conversion from sunlight.

Another important challenge is the management of the high operating temperature, which impacts the performance of CPV systems. Elevated cell temperatures lead to a reduction in electrical conversion efficiency and shorten the cell’s lifespan if not managed properly. Therefore, effective cooling methods are essential in lowering the operating temperature [11]. The latest research also explores microchannel heat sinks, phase-change materials, and passive radiative cooling to effectively dissipate heat.

As mentioned above, researchers are investigating hybrid CPV–Thermal (CPV-T) systems that integrate CPV with thermal collectors. These systems not only convert sunlight into electricity but also harness the thermal energy generated, improving overall energy yield. The key challenge is to optimize the balance between electrical and thermal output, and also to reduce the operating temperature of the cells thanks to an active cooling system [12].

Moreover, new designs are being developed that combine CPV with solar thermal systems, where the heat generated by the CPV cells is used for processes like heating water. This hybrid approach can significantly increase the total energy efficiency.

Research is also being conducted on integrating CPV systems with energy storage solutions, such as batteries or thermal storage, to ensure a reliable energy supply even when sunlight is intermittent. These hybrid designs are crucial for making CPV systems more viable in various climates and locations.

Another focus is on improving the long-term durability and reliability of CPV systems, especially in harsh environmental conditions. Research is ongoing to develop materials and coatings that can withstand high temperatures and UV exposure over extended periods.

A significant challenge remains in reducing the overall cost of CPV systems, including the manufacturing of optical components and high-efficiency cells [13]. While CPV systems are highly efficient in theory, scaling up these systems for widespread commercial use remains a critical hurdle. Researchers are investigating ways to simplify manufacturing processes and enhance system integration to make CPV more competitive with conventional solar technologies [13].

To introduce the novelty of the proposed system, the authors first provide an overview of the typologies of hybrid systems currently available on the market.

Photovoltaic thermal energy storage (PVTES) systems combine PV panels with thermal energy storage (TES) technologies. These systems are designed to maximize the use of solar energy by capturing both electrical and thermal components, making them highly efficient and versatile for various applications.

Table 1. PVTES typologies: an overview.

Photovoltaic Thermal Energy Storage (PVTES)
Typology	Overview	Example
Hybrid Photovoltaic-Thermal (PVT) Systems with TES	These systems integrate PV panels with thermal collectors to simultaneously generate electricity and heat. The heat collected is stored in a TES system for later use, such as heating water or air.	PVT with Phase Change Material (PCM): A hybrid system where the thermal energy collected from the PVT panels is stored in PCMs, which absorb and release heat at specific temperatures. This stored heat can be used during periods when sunlight is not available.
Concentrated Photovoltaic Thermal (CPVT) Systems with TES	CPVT systems use mirrors or lenses to concentrate sunlight onto high-efficiency PV cells and thermal collectors. The excess heat generated is stored in TES systems, which can be used for power generation or industrial processes.	Solergy CPVT System: This system uses concentrated solar power to generate electricity while simultaneously capturing thermal energy. The stored heat is used to power industrial processes, provide heating, or even generate additional electricity during periods of low sunlight.
Building-Integrated PVT (BIPVT) with TES	BIPVT systems are integrated into building structures, such as facades or rooftops. They generate electricity and collect thermal energy, which is then stored and used for space heating, water heating, or cooling within the building.	PVT Roof Tiles with TES: These tiles generate both electricity and thermal energy, which is stored in a TES system. The stored energy can be used to regulate building temperatures, enhancing energy efficiency.
Off-Grid PVTES Systems	Off-Grid PVTES systems are designed for remote locations where access to the grid is limited or unavailable. They combine PVT systems with TES to provide a reliable source of electricity and heat.	Standalone Solar PVT with TES: An off-grid setup that combines solar PVT panels with a thermal energy storage unit, often using water tanks or PCMs. This system provides both electricity and heating solutions for remote homes or facilities.
Industrial PVTES Systems	Industrial PVTES systems are scaled up for industrial applications, where large amounts of electricity and thermal energy are required. The thermal energy stored is often used for process heating or cooling.	Solar PVT Industrial Park with TES: A large-scale industrial park that uses a PVT system with TES to supply electricity and process heat to multiple industrial units. The TES might involve molten salts, PCMs, or other high-capacity storage media.

presents the main types of systems currently available on the market.

Additionally, the authors aim to highlight the differences between CPV, PV, and PVT systems to provide a clear understanding of the systems currently available on the market. The key differences are summarized in

Table 2. Comparison CPV, PV and PVT systems.

COMPARISON CPV, PV and PVT
	CPV	PV	PVT
EFFICIENCY	Using optical concentrators to focus sunlight on high-efficiency multi-junction solar cells, it is possible to achieve module efficiencies up to 40% under optimal conditions. They are effective in regions with high DNI, because the diffuse radiation is not converted in energy.	Typically have lower efficiency, around 15–22% for silicon-based modules. They perform well under a wide range of sunlight conditions, including diffuse light.	Combine PV and thermal technologies, generating both electricity and heat. The overall efficiency can exceed 70%, as they utilize the waste heat from the PV cells for thermal applications.
COMPLEXITY	Elevated, owing to the requirement for tracking systems to sustain sunlight concentration. Additionally, it demands precise alignment and advanced cooling mechanisms to control heat.	Simple, with no moving parts; easy to install and maintain. Installation requires a standard mounting system with minimal cooling requirements.	More complex than PV due to integration of thermal components. It requires additional plumbing or thermal management systems compared to standard PV.
COST	Generally, have higher initial costs due to the complexity of the concentrators and the need for precise tracking systems. However, in areas with high DNI, the levelized cost of electricity (LCOE) can be competitive.	The most cost-effective and widely deployed solar technology. The cost has dropped significantly due to mass production and economies of scale, making it a popular choice for residential and commercial installations.	More expensive than standard PV systems due to the integration of thermal components. The additional cost can be justified in applications where both electricity and heat are needed, such as in industrial processes or building heating.
THERMAL MANAGEMENT	Thermal management is critical in CPV systems because concentrated sunlight can cause significant heating, which can degrade the performance of the PV cells. Advanced cooling techniques, such as active cooling or hybrid systems using thermoelectric or thermochemical elements, are often employed.	Standard PV systems typically rely on passive cooling, which is usually sufficient given the lower heat generation. However, high temperatures can still reduce their efficiency.	PVT systems are designed to manage thermal energy by actively extracting heat from the PV modules, which not only cools the cells (improving their efficiency) but also provides usable thermal energy. This dual-purpose approach can lead to higher overall system efficiency.
ENVIRONMENTAL IMPACT	Request of materials for concentrators and tracking systems, which could have a higher environmental impact in terms of material use and land footprint. Their high efficiency can offset this by requiring less land area for the same energy output compared to standard PV systems.	Low environmental impact, especially as the industry has moved towards more sustainable manufacturing processes. The main concerns are related to the lifecycle of the materials used in panels and their recycling.	PVT systems offer environmental benefits by combining electricity and heat generation, which can reduce the need for separate heating systems (like gas boilers), thereby reducing overall carbon emissions.
SCALABILITY	Most effective in large-scale installations where high DNI is available. Their scalability is limited in regions with less direct sunlight or where land for tracking systems is not available.	High scalable, from small residential rooftops to large utility-scale solar farms. Their flexibility and adaptability to various environments make them the most widely deployed solar technology.	PVT systems are also scalable but are often used in specific applications where both electricity and thermal energy are required. Their use is more niche compared to standard PV systems.

.

In this framework, the authors present a unique design of a CPV system suitable for installation on both residential and commercial roofs. Moreover, the studied solutions will allow use not only on flat but also sloped roofs. Therefore, this aims to overcome one of the major limitations of CPV that has certainly limited its diffusion and use over time. The residential use of this system is completed by the presence of both thermal and electrical energy storage systems to better match the energy demand curve of the end user.

Moreover, innovation in renewable technologies is the primary catalyst for advancing the development of smart grids and energy communities [14]. CPV systems have potential applications within energy communities, which are collective energy-sharing arrangements where members produce, consume, and share energy among themselves. Since CPV systems generate more electricity per unit area, they could be an ideal solution for energy communities with limited land or roof space, allowing the efficient use of available space. Energy communities in regions with high direct sunlight (e.g., Southern Europe, parts of the U.S. Southwest, and Australia) could benefit significantly from CPV systems. These regions offer ideal conditions for CPV systems to operate at peak efficiency. By generating a significant portion of their energy needs through CPV, communities in these areas could reduce their reliance on the external electricity grid, leading to greater energy independence. Some innovative projects might combine CPV with other renewable technologies (like wind or PV) to create hybrid energy systems tailored for community use.

The activity described in this study was developed as part of the Italian project “SOLARGRID—Thermodynamic Solar and Photovoltaic Systems with Storage for Co-Generation and Grid Flexibility” (Ref. MIUR: ARS01_00532—Specialization Area “Energy” PNR 2015–2020—Grant Decree No. 657 of 13/05/2020) funded by the Ministry for Universities and Research (MUR), which involved nine industrial and institutional partners [15,16]. This project aims to develop innovative and improved solutions, in terms of energy performance and economic competitiveness, for components and systems related to concentrating solar power (CSP) and (CPV) technologies for distributed generation of electrical and thermal energy.

The novelty of this paper lies in the following:

• It introduces CPV systems’ applications in residential and commercial contexts, mounting the system both on flat and sloped roofs.
• It provides a deep analysis on design of the CPV system, concerning the selection of the PV receiver and the design of the lens–cell system.
• It focuses on tests in indoor and outdoor conditions and of course on the mechanical design of the complete system.
• Finally, a cost analysis and the novelty and originality of the system is presented.

In detail, the paper is structured as follows. Section 2 presents a detailed analysis of the CPV system, with a focus on PV receiver selection and lens cell system design; Section 3 discusses the main results of the validation tests in outdoor condition; Section 4 runs through the design of the system; Section 5 reports a basic cost analysis of the system; and Section 6 summarizes the conclusions and lessons learned from the study.

2. Design of the CPV System

The design process of the CPV system developed by ENEA is described below.

The main development steps will be detailed, in particular the following:

• The selection of the PV receiver, also in relation to the definition of the system’s geometric characteristics.
• The design of the lens–cell system, study of the optical model, and thermal verifications.

In the following chapter, results of the following will also be reported:

• Tests in indoor and outdoor conditions for the experimental validation of theoretical results.
• Mechanical design of the complete system.
• Cost analysis.

2.1. PV Receiver Selection

The first crucial step in designing a CPV system is identifying the PV receiver to be used, as it is the key component for the system’s operation. To create a module with compact dimensions suitable for architectural integration, particular attention was given to small receivers capable of operating at high solar concentrations.

However, market analysis revealed a significant reduction in the number of companies offering CPV cells compared to a few years ago. The main suppliers potentially capable of producing these components at a European/non-European level that collaborated scientifically in the development of these devices are the following:

-

Azur Space [17];

-

Spectrolab [18];

-

Suncore [19];

-

TaiCrystal [20].

Several technical evaluations were conducted based on commercial data sheets to select the product that best fit the CPV system concept being developed in the SOLARGRID project. Considering the performance characteristics, compact size, and the availability of a pre-assembled receiver with a secondary optical element that ensures good performance in terms of acceptance angles, products from Azur Space were chosen.

For determining the type and dimensions of the cell to choose, analyses were conducted considering the intended use of the CPV system under development. The initial specifications are geared towards a system suitable for building integration applications, primarily for installation on flat roofs but also on pitched roofs. Initially, the focus was on systems with compact dimensions, leading us towards selecting a particularly small cell size. After evaluating various options, Azur proposed an alternative solution involving a cell with an active area of 5.185 × 5.185 square mm mounted on a “DCB Board” substrate composed by a 0.38 mm Al₂O₃-Ceramic plate in the middle, covered by two 0.25 mm copper layers: the upper one is used to realize the electric contacts, while the bottom works as a heat spreader. On the cell surface is glued a pre-assembled refractive secondary receiver (Figure 1).

The secondary optical element has a semi-spherical cap shape and is made from a fused glass with high optical transparency qualities and a refractive index of 1.52. Its lower surface is bonded to the upper surface of the cell using a silicone adhesive.

For this device, optimal working conditions are achieved with an effective concentration of 500 suns. From this assumption, a theoretical lens area has been calculated below (Equation (1)):

Al = Ac × 500/0.85

(1)

where:

• Al represents the area of the primary lens;
• Ac is the area of the cell, equal to 26.88 mm² (5.185 × 5.185 mm²);
• The number 500 is the effective concentration factor to be achieved on the cell;
• The number 0.85 represents the expected optical efficiency of the system.

If the area of the cell is equal to 26.88 mm², the area of the lens Al will be 15,815 mm², which corresponds to a squared lens with a 125 mm side. Below, the main characteristics of the Azur Space receiver are described:

• Substrate Type: “DCB Board” (0.38 mm Al₂O₃-Ceramic with two 0.25 mm copper layers);
• Substrate Dimensions: 15.875 mm × 11.866 mm;
• Electrical Connections: Soldered electrical contacts;
• PV Cell Used: Triple junction cell;
• Cell Construction Material: GaInP/GaInAs/Ge on Ge substrate;
• Net Area of the Cell: 5.185 mm × 5.185 mm = 26.88 mm²;
• Anti-Reflective Coating: TiOx/AlOx;
• Pre-mounted bypass diode on the receiver;
• Number of Electrical Output Contacts: 3;
• Cell Contact Welding Technique: “Au–wire bonding”;
• Material of Electrical Contacts: SnAg 96.5/3.5 alloy.

Cell Characteristics:

• Electrical Conversion Efficiency @ 500 suns in Standard Test Conditions (STC): ~42%;
• Thermal Coefficient for Current: 0.08%/K;
• Thermal Coefficient for Voltage: −0.135%/K;
• Thermal Coefficient for Power: −0.106%(rel)/K;
• Efficiency Thermal Coefficient for Cell: −0.106%(rel)/K.

Efficiency curves are shown in Figure 2 and Figure 3, as a function of effective concentration and parameterized based on the operating temperature of the device. A maximum efficiency of approximately 42% is observed at up to 500 suns; efficiency decreases for higher concentrations. The external quantum efficiency (EQE) chart of the cell is displayed on the right side of Figure 2. EQE represents the ratio of charge carriers collected by the solar cell to the number of photons at a specific wavelength striking the cell (incident photons). This chart is experimental: for a triple-junction cell, the test is conducted by activating each sub-cell individually and measuring the EQE values for the three junctions separately. As a result, the graph displays three distinct curves in different colors. The red curve corresponds to the top cell (operating between 300 and 700 nm), the green curve to the middle cell (operating between 600 and 1000 nm), and the blue curve to the bottom cell (operating between 800 and 1800 nm). Due to the partial overlap in the operating ranges of the three sub-cells, some overlap between the curves is visible.

2.2. Lens–Cell System Design

To conduct preliminary quantitative evaluations, an initial geometry for the CPV module was proposed. The concept involves constructing a module consisting of parallel rows of troughs, each one with a row of photovoltaic cells at the bottom and a row of lenses at the top to concentrate the incident light radiation.

The basic unit of the smart CPV module is illustrated in Figure 4. At the top, a Fresnel lens with an active area of 140 × 140 mm² is positioned: a lens model with a short focal distance was chosen in order to contain the module thickness. At the bottom, an Azur Space receiver with an active area of 5.185 × 5.185 mm², integrated with a refractive secondary concentrator as described earlier, is placed. A metallic tray is positioned between these two main components, and the receiver is mounted on an aluminum heat sink.

• Thermal verification

Based on this geometry, thermal simulations were performed using the Comsol Multiphysics software (version 5.6) to estimate the temperature of the cell under real working conditions and, consequently, to choose the material for the tray and the dimensions of the aluminum heat sink, which represent the elements that mostly affect the cell temperature. Worst environmental conditions were considered as boundary conditions for the simulation:

✓

DNI of 1000 W/m²;

✓

Optical efficiency of 85%;

✓

No ventilation outside the module (only natural convection and radiative heat dissipation were considered);

✓

Non-functional cell (all incident energy must be dissipated as heat).

Under these assumptions, the cell temperature was calculated both with an ambient temperature of 20 °C and 40 °C, using either a steel or an aluminum tray, and different configurations of the aluminum heat sink. Figure 5 shows the results of the thermal simulations with the steel case and the aluminum one, considering both the ambient temperature equal to 20 °C and 40 °C. As can be clearly seen in Figure 5a,c, the resulting temperature values with the steel case are extremely high, and not compatible with the integrity of the cell, which has a maximum operating temperature of about 115 °C, whereas the maximum cell temperatures obtained with the aluminum case (shown in Figure 5b,d) are well below the cell maximum operating temperature. Therefore, the initial choice represented by the aluminum case was confirmed. Figure 6 shows the results obtained from the thermal simulations realized to evaluate the effects of the variation of the orientation and the size of the aluminum heat sink on the cell temperature. In particular, Figure 6b shows the temperature field relative to the base case, Figure 6a refers to the case with the aluminum heat sink rotated of 90 degrees around the vertical axis (parallel to y axis) passing through the center of the cell, and Figure 6c is relative to the case with the aluminum heat sink 50% thicker in the y direction than the base case. It can be noticed that the differences between the cell maximum temperatures in the above three cases are negligible, confirming the effectiveness of the base case configuration.

2.3. Optical Design

The lens–cell system, as defined above, was also used to perform optical simulations in order to determine the geometry and tolerances of the system. This work was carried out using Trace Pro Expert software by Lambda Research Corporation (version 24.2.0). Once the basic components such as lens, cell, and secondary concentration system were modeled, a virtual model of the system was created in the Trace Pro environment

The primary optic is a Fresnel lens with an active area of 140 × 140 mm², a nominal focal length about of 154 mm, a thickness of 1.8 mm, and grooves with a constant pitch of 0.254 mm. The secondary optic is refractive and has a semi-spherical dome shape with a receiver at the bottom with an active area of 5.5 × 5.5 mm².

The secondary concentrator is integrated into the photovoltaic receiver purchased from Azur Space; therefore, there was no sense in analyzing components with different geometries. This study was focused on the type of primary lens that best suited the secondary one. The lens active area was chosen in order to make the cell work in the best conditions (under a real concentration factor varying in the range of 450–550 suns). Furthermore, several preliminary simulations were carried out to define the focal length. As mentioned, the aim was to obtain a system that is as compact as possible from a geometric point of view, to facilitate its integration in residential contexts. Therefore, the lowest focal length value that would allow for acceptable optical behavior was chosen. In fact, by considering optics with a longer focal length, higher optical efficiency values can be obtained, to the detriment of the compactness of the module. With lower focal length values, the system was significantly penalized both in terms of efficiency, but above all, in terms of optical acceptance. Therefore, the choice of a focal length of 154 mm was a compromise between optical efficiency requirements and system compactness.

These elements preliminarily modeled using a 3D CAD software (Autodesk Inventor Professional 2016) were subsequently imported into Trace Pro environment, where the optical characteristics of the construction materials were assigned. Polymethyl methacrylate (PMMA) with a refractive index of 1.49 was assigned to the lens, while a glass with good optical properties and a refractive index of 1.52 was allocated to the secondary concentrator. The absorption coefficients of the two components were considered practically negligible.

The cell, on the other hand, was modeled as a square with a 5.5 mm side, with the upper face (the one in contact with the secondary concentrator) being optically perfectly absorbing. The optical efficiency was calculated as the ratio between the power absorbed by this face and the power incident on the primary lens. The simulations aimed to achieve the following objectives:

✓

To define the focal distance;

✓

To determine the optical efficiency of the system;

✓

To characterize the behavior of the system under off-axis conditions;

✓

To define the alignment specifications for the system.

2.3.1. Definition of Focal Length and Theoretical Efficiency

To define the focal length and the related optical efficiency, numerous simulations were conducted by varying the distance between the primary lens and the cell and calculating the optical efficiency. Figure 7 shows the trend of the system’s optical efficiency as the distance between the outer surface of the primary lens and the output surface of the secondary, which corresponds to the absorbing cell surface. It is observed that there is an area, highlighted by the red circle in Figure 7, of high and stable efficiency for focal length values between 149 mm and 154 mm: therefore, a focal length value of 151.5 mm (indicated by the red arrow in Figure 7) was considered as a nominal value. The expected optical efficiency is about of 84.8%.

2.3.2. Characterization of System Behavior under Off-Axis Conditions

To evaluate how the system worked when misaligned with the direction of incoming solar radiation, such as due to a tracking error, a series of simulations were conducted by imposing increasing inclination angle between the incident light beam and the normal to the lens. Firstly, the effects of a simple rotations in a vertical plane orthogonal to the lens surface and parallel to one of its sides were considered. Then, combined rotations, consisting of two simple rotations in two vertical planes orthogonal to the lens surface and parallel to two orthogonal edges of the lens, were examined. Figure 8 illustrates the change in the optical efficiency of the system as simple misalignment increases. The irradiance values on the cells were normalized with respect to the one measured when the system was perfectly aligned. Optical acceptance is typically defined as the misalignment value at which the system achieves 90% of its nominal efficiency, which is approximately 0.43° in this case.

In contrast, Figure 9 presents a graph for compound misalignment: X and Y coordinates display the values of the two simple misalignments on the two vertical planes, as explained above. The corresponding Z coordinate indicates the relative optical efficiency for those configurations. As shown, the graph depicts a curve with declining values as misalignment increases.

For each pair of misalignment values, the overall inclination angle of the solar beam with respect to the normal was calculated. Specifically, by defining α_x and α_y as the two simple rotations of the solar rays’ direction relative to the lens perpendicular, the overall misalignment angle δ is determined using Equation (2):

$$\delta =\sqrt{({\alpha }_x^2+{\alpha }_y^2)}$$

(2)

With each δ value has been associated the relative optical efficiency calculated for the pairs of its coordinates α_x and α_y.

In this way, a two-dimensional correlation, shown in Figure 9, between the overall misalignment angle δ and the system relative optical efficiency was found.

The results are intriguing, as they indicate that the trend of this parameter closely mirrors that observed in the case of simple rotations (see Figure 10). The angular acceptance value in this instance is approximately 0.42°, nearly identical to the value calculated for simple misalignments.

Therefore, the optical behavior of the system does not seem to depend on the direction of the misalignment but essentially on the magnitude of the angle between the normal to the lens and the direction of incidence of the solar beam, regardless of the plane in which these two directions lie.

Further simulations were conducted to verify the lens–cell mounting tolerance: in ideal conditions, the centers of the lens and of the cell are perfectly aligned along a perpendicular to the lens surface. In reality, due to the imprecisions of the module assembly process, this alignment is not perfect, and a gap between the projections of the lens and cell center on the focal plane can be revealed. To simulate this situation, optical performance of the lens–cell system was calculated by shifting the lens with respect to the cell. In other words, the nominal optical yield was calculated assuming that the center of the lens and the center of the cell were perfectly aligned with the direction of incidence of the solar rays. From this starting position, various conditions of cell displacement in the focal plane were simulated to calculate the optical yield using as moving directions the two sides of the primary lens (indicated as X and Y). For each pair of displacements along X and Y (said d_x and d_y), a value of relative optical efficiency was calculated: the 3D surface in Figure 11 shows these values calculated by considering the displacements in X and Y varying from 0 to 3 mm.

The overall displacement value d for each pair of displacements in the focal plane (d_x and d_y) was calculated using Equation (3):

$$d=\sqrt{(d_x^2+d_y^2)}$$

(3)

For each overall misalignment value, the corresponding relative optical yield was determined, resulting in the following graphs (Figure 11 and Figure 12).

The result shows that, regardless of the displacement components in the plane (d_x and d_y), the optical behavior of the system depends on the overall displacement value, i.e., d. As seen, for displacement magnitudes less than 1 mm, optical losses are practically negligible. For larger misalignments, the increase in losses is still limited, reaching 10% for large displacements (2.7 mm).

2.3.3. Analysis of Energy Flux Distribution on the Cell

An analysis of the distribution of concentrated light flux on the cell was performed based on the results of the optical simulations. Figure 13 shows a top view of the flux distribution over the cell’s active area. The simulation assumed an incoming radiation density of 1000 W/m² on the primary lens, corresponding to an incident energy of 19.6 W on the lens area (0.14 × 0.14 m²).

Under optimal alignment conditions, 16.621 W was directed onto the cell, resulting in a maximum optical efficiency of 84.801%.

As shown more clearly in the 3D graph in Figure 14, concentration peaks of around 2700 kW/m² are reached, but the actual average concentration is equal to 618 kW/m². This value is compatible with the efficient operation of the chosen solar cell, which has efficiency values around 41% for these concentrations. It is also worth noting that these maximum values were calculated with a DNI of 1000 W/m², conditions that are rarely sustained for long operational periods. Normalizing the result to a DNI of 850 W/m² yields concentration values around 520 kW/m², at which the cell operates with efficiencies close to its maximum, around 42%.

The spatial distribution reveals isolated peaks where maximum concentration values occur, potentially negatively affecting the cell’s performance. It is important to note that this distribution is influenced by the idealized geometric models used, which have sharp lens edges. In contrast, real structures exhibit rounded edges and minor, unavoidable geometric imperfections. These real-world characteristics will likely result in a smoother actual distribution and significantly lower real peaks than those calculated.

3. Validation Tests in Outdoor Condition

A prototype of the lens–cell system was tested outdoors to validate the simulation results and determine the manufacturing tolerances required. The tests were conducted using a double-axis solar tracker with a tracking accuracy about of ±0.05°, at the outdoor test station at CR ENEA in Portici (Italy) (Figure 15).

This setup included a mockup with a Fresnel lens and a PV receiver, both used in the optical simulation. Specifically, the primary lens was fixed on the top plate of the tracker structure with a specific holder (see Figure 15, right side). The Azur solar receiver was mounted on a stage fixed on the bottom plate of the structure that allowed for positional adjustments in three directions thanks to three micrometer actuators having an accuracy of 0.01 mm, enabling tests on variations in focal length and shifts of the lens center relative to the cell center.

The tracker was instrumented with a calibrated pyrheliometer integral to the tracker structure (see Figure 15, left side) to continuously measure the direct solar radiation incident on the lens, while a k-type thermocouple was placed underneath the cell to measure the temperature in operating conditions.

The cell was instrumented to measure the short circuit current produced by an electronic power consisting of a 375 W Keithley DC Power Supplies.

During the tests, we observed a DNI ranging from 800 to 850 W/m², with an ambient temperature of approximately 26 °C. The diffuse radiation was around 60 W/m², and wind speed was nearly negligible. These measurements were collected using the ENEA weather station, located about 100 m from the testing site. The first operation carried out was to perfectly center the cell at the lens’s focus, moving the receiver with the stages until the current produced was maximized. During this process, direct radiation remained practically constant. In this aligned condition, the focal distance was measured and found to be 152.8 mm.

All the measured short-circuit current values were normalized to a working cell temperature of 25 °C, using the thermal I_SC coefficient of the cell, which is −0.08%/K. This normalization considers the experimentally measured cell temperature and is calculated using Equation (4):

$$I_{SC\_25}=I_{S{C}_{MEAS}}+I_{S{C}_{MEAS}}\left(T_{cell}-25\right)\ast {\displaystyle \frac{-0.08}{100}}$$

(4)

where:

• $I_{SC\_25}$ is the short-circuit current normalized at 25 °C;
• $I_{MEAS}$ is the measured short-circuit current;
• $T_{cell}$ is the measured cell temperature;
• −0.08%/K is the thermal I_SC coefficient of the cell.

Subsequently, the obtained value was normalized to the irradiance of 1000 W/sqm by Equation (5):

$$I_{SC\_25\_1000}={\displaystyle \frac{I_{SC\_25}\ast 1000}{{DNI}_{MEAS}}}$$

(5)

where:

• $I_{SC\_25\_1000}$ is the short-circuit current normalized at 25 °C and 1000 W/sqm of DNI;
• $I_{SC\_25}$ is the short-circuit current normalized at 25 °C;
• ${DNI}_{MEAS}$ is the current value of measured DNI during the tests.

The optical efficiency of the system was calculated considering that the normalized I_SC value in the condition of perfect alignment corresponds to a real concentration on the cell of 480 suns. Because the geometrical system concentration is about of 650×, the optical efficiency was calculated as the ratio between the real and the nominal concentration ratio: it corresponds to 74%. The measured focal distance closely matched the simulation result of 151.5 mm. However, a significant discrepancy in optical efficiency was noted, likely due to the primary lens used for the mockup being a commercial component not specifically developed for optical applications, thus possessing poor optical quality.

3.1. System Behavior by Changing the Focal Distance

Starting from the nominal position, the receiver was moved using a stage that allowed for the focal length to be increased or decreased while maintaining perfect alignment between the lens and the cell. Movements were made in 0.25 mm increments, with short-circuit current, temperature, and direct radiation intensity recorded at each step. This data set enabled the above-described normalization process of all current measurements to a cell operating temperature of 25 °C and an irradiance of 1000 W/m², ensuring comparability across all measurements. The maximum current value was recorded at the nominal position, with lower current values observed when the cell was moved either closer to or further from the lens. For each position, the relative optical efficiency of the system was calculated as the ratio of the current produced at that position to the current produced in the nominal condition. The following graph illustrates these results (Figure 16):

The results indicate that increasing variations in the focal distance led to a slight decrease in efficiency, while the system was more sensitive to decreases in the focal length. However, for variations of ±1 mm, the efficiency losses are minimal, around 2%, which aligns well with optical simulations.

3.2. System Behavior by Moving the Receiver in the Focal Plane

Starting from the system’s nominal position, the cell was moved in the focal plane parallel to the two sides of the lens (referred to as X and Y displacements) in 0.25 mm increments. At each step, the short-circuit current, temperature, and intensity of direct radiation were recorded. Using the same procedure as described previously, the optical performance of the system was calculated at each positioning step, resulting in the following graphs (Figure 17 and Figure 18).

The graphs show that the following:

-

Moving the cell in the focal plane results in a decrease in efficiency that is not perfectly symmetrical with respect to the direction of the movement.

-

The optical system appears to have good tolerance, with misalignments of ±0.5 mm, while losses of around 2–3% are recorded.

3.3. Measurement of Optical Acceptance

The procedure followed to measure the optical acceptance of the system involves the following steps:

• The cell–lens system in the nominal position is tracked on the heliostat, and the short-circuit current produced is measured.
• The tracker is stopped, allowing the natural rotation of the sun to cause a misalignment between the direction of the rays and the perpendicular to the primary lens.
• At regular intervals of 30 s, the short-circuit current produced by the lens is recorded, along with the cell temperature and direct irradiance values.
• For each measurement step, the current produced is normalized to a temperature of 25 °C and a direct irradiance value of 1000 W/m² for comparability.
• The optical efficiency of the system is calculated for each step as the ratio between the short-circuit current and the current produced under perfect pointing conditions.
• The angular misalignment value for each step is calculated using trigonometric formulas that consider the coordinates of the location, the moment the tracker was stopped, and the stop time interval.

It is evident that the system achieves 90% of its nominal optical efficiency with an angular misalignment of 0.6°, as show in Figure 19, which can be regarded as the system’s angular acceptance. Throughout these tests, the cell’s temperature was continuously monitored and never surpassed 90 °C, despite the cell being mounted on a simple aluminum block of the handling stage rather than a proper dissipation system.

4. Design of the System

The CPV system consisted of 10 parallel linear trays (Figure 20—arrow (1)) made of aluminum sheets, each one containing a row of nine lens–cell systems. The lens array was fixed to the upper surface of the trays, while the receivers were located on the bottom plate. The linear modules were precisely sealed, and a breathing valve compensated for pressure changes caused by heating and cooling cycles of the internal air. These 10 trays were mounted on a press-bent steel sheet frame with overall dimensions of 2.9 m × 1.5 m × 0.2 m (Figure 20 and Figure 21 arrow (2)), with a pitch of 290 mm between them. This pitch was chosen based on a thorough analysis to find the best balance between minimizing component size and reducing self-shading among the linear modules.

The trays rotated synchronously around their longitudinal axes, driven by a motor connected to the central tray and a leverage system linking all the trays (Figure 20 and Figure 21—arrow (3)). The entire frame could rotate around an axis through its center using a slewing drive connected to an electric brushless motor (Figure 20 and Figure 21—arrow (4)). This combination of rotational movements enabled the system to track the sun even if it is mounted on sloping roofs.

The same system can be utilized on a flat roof by employing a supporting structure designed as a trolley (Figure 21—arrow (5)). This trolley can accommodate all auxiliary components of the system, including the movement control box (Figure 21—arrow (6)), energy conversion systems, and electrical and thermal storage units (see Figure 22 and Figure 23).

The system has an electrical nominal power of approximately 550 W_p, generated by 90 photovoltaic receivers connected to a microinverter through a parallel configuration of five series, each consisting of 18 cells. These receivers are actively cooled by a water circuit that flows along the bottom surface of the receiver supports and feeds a 95 L tank for domestic hot water use.

The simulations carried out indicate that each receiver recovers approximately 7 thermal watts under nominal conditions, resulting in a total of 630 thermal watts collected per system. Given that the total active area of the lenses is 1.8 m², the total incident power (P_in) with an irradiation of 1000 W/m² is 1800 W. The overall energy efficiency of the system (η_tot), calculated as the ratio between the sum of the electrical and thermal output power and the incident power, is as follows (Equation (6)):

$${\eta }_{tot}={\displaystyle \frac{P_{el}+P_{th}}{P_{in}}}={\displaystyle \frac{550+630}{1800}}=65.5\%$$

(6)

where:

-

η_tot is the overall efficiency value;

-

P_el is the electrical power output in nominal condition equal to 550 W;

-

P_th is the thermal power output in nominal condition equal to 630 W;

-

P_in is the radiating power hitting all the aperture area of the system in nominal condition equal to 1800 W.

This calculation can be repeated by converting the thermal power generated into equivalent electrical power. For this purpose, if the system operates continuously for one hour under nominal conditions, based on the power data provided above, the corresponding energy values can be derived:

-

E_el, as the electrical energy output, is equal to 550 Wh;

-

E_th, as the thermal energy output, is equal to 630 Wh;

-

E_in, as the radiating energy hitting all the aperture area of the system, is equal to 1800 Wh.

Given that most residential users rely on natural gas boilers to produce domestic hot water, the authors considered it appropriate to use this generation system for making the necessary equivalences. Therefore, assuming an average conversion efficiency of 85% for domestic hot water production and a lower calorific value of methane at 9.27 kWh/Smc, the following amount of gas is required to generate E_th (Equation (7)):

$$Q_{gas}={\displaystyle \frac{E_{th}}{0.85\ast LCV}}={\displaystyle \frac{630}{0.85\ast 9270}}=0.08 \mathrm{Smc}$$

(7)

where:

-

E_th is the thermal energy output equal to 630 Wh;

-

0.85 is the considered conversion efficiency value to produce domestic hot water;

-

LCV is the lower calorific value of methane equal to 9.27 kWh/Smc.

Now, it is possible to convert this amount of natural gas first into tons of oil equivalent (TOE) and then transform this into equivalent electrical energy as follows (Equation (8)):

$$TOE=Q_{gas}\ast f_{TOE}=0.08\ast 0.000836=6.69·{10}^{-5}$$

(8)

where:

-

Q_gas is the amount of natural gas needed to produce E_th, that is equal to 0.08 Smc, as above calculated.

-

f_TOE is the conversion factor between Smc of natural gas and tons of oil equivalent that is equal to 0.000836.

TOE can be converted into equivalent electrical energy (E_el,eq) using the following method (Equation (9)):

$$E_{el,eq}=TOE\ast f_{TOE-EE}=6.69·{10}^{-5}\ast \mathrm{5,347,600}=357 \mathrm{Whe}$$

(9)

where:

-

TOE is the number of tons of oil equivalent to Q_gas, that is equal to 6.69 × 10⁻⁵, as calculated above.

-

f_TOE−EE is the conversion factor between TOE and equivalent electrical energy that is equal to 5,347,600.

To summarize, in one hour under nominal irradiation conditions, the system produced 550 Wh of actual electrical energy and 375 Wh of equivalent electrical energy. Therefore, the overall equivalent electrical efficiency of the system (η_elect,eq) can be calculated as the ratio of the sum of the actual and equivalent electrical energy to the total incident energy (Equation (10)).

$${\eta }_{elect,eq}={\displaystyle \frac{E_{el}+E_{el,eq}}{E_{in}}}={\displaystyle \frac{550+375}{1800}}=51.3\%$$

(10)

where:

-

E_el is the real electrical energy output, equal to 550 Wh;

-

E_el,eq is the electrical equivalent energy output, equal to 375 Wh;

-

E_in is the radiating energy hitting all the aperture area of the system, equal to 1800 Wh.

On the electrical side, the system includes a 1 kWh electrical storage unit and can be directly connected to the electrical grid through a standard domestic socket.

Novelty and Originality of the System

The key feature of the described CPV system is its design for integration into residential and/or commercial environments, addressing the limitations of traditional CPV systems, which are typically designed for ground-based power plants. In the configuration shown in Figure 21, the system can be installed directly on flat roofs, and the inclination of the tray support frame can be adjusted relative to the horizontal plane. This adjustment helps minimize optical losses caused by self-shading between trays, which would be at their maximum if the frame were placed horizontally. The optimal frame inclination depends on the installation site’s latitude.

In the configuration depicted in Figure 20, the frame can be mounted directly onto a sloped roof, parallel to it, using an interface support between the roof structure and the plate that secures the worm gear (marked #4 in Figure 20 and Figure 21). Solar tracking is enabled by a custom control system that accounts for both the roof’s inclination relative to the horizontal and its orientation with respect to the South. These values are automatically detected by the system: the inclination is measured by an inclinometer, while the orientation is determined by a solar sensor, which also provides feedback to the control system. Given the type of motors used, angular backlash, reduction factors, and the developed control system’s feedback sensor, an accuracy of approximately +/− 0.1° is expected.

The potential to install these systems in urban areas is crucial for their widespread adoption, as the energy produced could be used directly at the site, eliminating the need for users to purchase equivalent energy from the grid. In this self-consumption mode, the energy generated and consumed on-site offers a benefit equivalent to the user’s purchase cost, which is considerably higher than the price of selling energy from a CPV plant in land-based installations. This results in a shorter return on investment compared to traditional CPV systems. The addition of an electrical storage system further enhances this advantage, as it allows the stored energy to be utilized during periods of low or no sunlight. The storage system typically consists of a lithium battery with a nominal charge–discharge efficiency of 90%.

The decision to include an active cooling system not only reduces the operating temperature of the cells but also enables the low-temperature waste heat (40 °C–45 °C) to be used for domestic hot water production.

Another distinctive construction feature of this CPV system is the receiver mounting system. Specifically, all receivers are screwed onto a single aluminum profile using custom-designed spacers and bases (see Figure 24). This approach allows for easy replacement or repair of a damaged receiver via lateral inspection windows (see Figure 25). By contrast, the conventional method of gluing receivers would make dismantling them nearly impossible without causing damage.

5. Cost Estimation

An estimate of the costs for the proposed CPV system has been conducted, using market prices and literature analyses of similar components, considering economies of scale for a production and installation volume of at least 10 MW/year.

Table 3. Cost of the system.

Items	Cost (€)
CPV module fabrication	700
Tracker	440
Inverter	85
Installation	150
1 kWh electric storage	300
95 L thermal storage system	600
Total cost	2275

presents the costs of the main components for a single system with an electrical power output of 550 We and a thermal output of 630 W.

The calculation of the cost per watt peak can be approached in various ways. If we consider as output only the electrical power of 550 W_p, the unit cost C_u,1 is (Equation (11)):

$${\mathit{C}}_{\mathit{u},\mathbf{1}}={\displaystyle \frac{Total cost}{Electric Power}}={\displaystyle \frac{2275 \mathrm{€}}{550 {\mathrm{W}}_{\mathrm{p}}}}=4.13 \mathrm{€}/{\mathrm{W}}_{\mathrm{p}}$$

(11)

Taking into account the benefit of heat recovery, estimated to be equivalent to 375 electrical watts (as detailed in Section 4), the unit cost C_u,2 becomes (Equation (12)):

$${\mathit{C}}_{\mathit{u},\mathbf{2}}={\displaystyle \frac{\mathit{T}\mathit{o}\mathit{t}\mathit{a}\mathit{l}\mathit{c} \mathit{o}\mathit{s}\mathit{t}}{\mathit{E}\mathit{l}\mathit{e}\mathit{c}\mathit{t}\mathit{r}\mathit{i}\mathit{c} \mathit{P}\mathit{o}\mathit{w}\mathit{e}\mathit{r}+\mathit{E}\mathit{q}\mathit{u}\mathit{i}\mathit{v}\mathit{a}\mathit{l}\mathit{e}\mathit{n}\mathit{t} \mathit{e}\mathit{l}\mathit{e}\mathit{c}\mathit{t}\mathit{r}\mathit{i}\mathit{c} \mathit{p}\mathit{o}\mathit{w}\mathit{e}\mathit{r}}}={\displaystyle \frac{\mathbf{2275} \mathrm{€}}{\mathbf{550} {\mathbf{W}}_{\mathbf{p}}+\mathbf{375} {\mathbf{W}}_{\mathbf{p}}}}=2.45 \mathrm{€}/{\mathrm{W}}_{\mathrm{p}}$$

(12)

Finally, to compare the cost with that of conventional CPV systems, which, according to the literature, is approximately EUR 2/W_p [21], we must exclude the costs of both electrical and thermal storage, as these are not provided with conventional systems. Under these conditions, the total cost is EUR 1375, resulting in a unit cost C_u,3 of (Equation (13)):

$$C_{u,3}={\displaystyle \frac{Total cost (without storage)}{Electric Power}}={\displaystyle \frac{1375 \mathrm{€}}{550 {\mathrm{W}}_{\mathrm{p}}}}=2.5 \mathrm{€}/{\mathrm{W}}_{\mathrm{p}}$$

(13)

This value is higher than that of conventional systems for three reasons:

• Tracker Costs: The cost of the tracker is higher because each 550 W_p system requires a small tracker, which must be fully equipped with actuators and control systems. In contrast, conventional CPV systems use a single large tracker for systems of 10 kW_p or more, resulting in a lower tracker cost per unit.
• Receiver Mounting: The system’s modules use a screwing method to fix the receivers, allowing for precise alignment and easier maintenance. This method incurs higher costs compared to the standard procedure of gluing the components.
• Heat Recovery System: The modules are designed for excess heat recovery using an active cooling system, which is more expensive than the passive cooling systems typically used in conventional CPV modules.

Despite the 25% higher cost, this system’s ability to be used in residential contexts, which conventional systems cannot accommodate, provides significant advantages. The electricity produced is used directly by the end user, avoiding grid purchases and thus being valued at the final purchase cost, which is usually nearly double the selling price of conventional PV energy. Consequently, the higher initial investment is largely offset over time.

A final cost consideration involves comparing this CPV hybrid system to an equivalent setup consisting of separate photovoltaic and solar thermal sections. For a small residential PV system, the cost is approximately EUR 1.4/W [22]. Therefore, for a 550 W system, the estimated cost is around EUR 770. Additionally, a 1 kWh electric storage system connected to the PV plant costs about EUR 300.

A small solar thermal system with similar thermal performance to the CPV hybrid system would include a 1 sqm collector and a 95 L storage tank. The estimated cost for the components and materials is about EUR 850, with an additional EUR 600 for installation. Thus, the total estimated cost for the equivalent system is EUR 2520, which is higher than the cost of the CPV hybrid system. This higher cost is primarily due to the expenses associated with the solar thermal collector and its installation, which can be avoided by using the CPV hybrid system instead of maintaining two separate systems.

6. Conclusions

The presented work outlines the main design steps of a novel CPV system intended for residential applications, suitable for installation on both flat and sloped roofs. The lens–cell system has been extensively analyzed from both thermal and optical perspectives. The results indicate that to keep the cell’s operating temperature below 100 °C, aluminum sheet must be used as the housing material. The optical system features a short focal length of approximately 152 mm and a wide angular acceptance of ±0.6°, as experimentally measured. This characteristic allows the use of a solar tracker system that does not require extreme precision. Mounting tolerances have been established through theoretical studies and experimental tests, with the system accommodating a wide tolerance for the focal distance (±1 mm) and good tolerance for lens–cell misalignment (±0.5 mm). Outdoor tests demonstrated an optical efficiency of 74%, which is lower than the theoretical value, primarily due to the suboptimal optical quality of the primary lens used.

The system design has been presented: it consists of an array of 10 parallel linear modules that rotate around their longitudinal axis, all mounted on a common frame that can rotate around an axis passing through its geometrical center. A specialized control system, currently under development, will enable tracking of the modules on both flat and sloped roofs. To achieve this, the system will be equipped with an inclinometer to measure the roof slope and a solar sensor to calculate the orientation of the mounting surface. These data will be used to accurately determine the rotation angles for the two actuators, allowing the system to track the sun throughout the day.

Specifically, to improve the reliability and precision of the calculations, the help of machine learning and data-driven techniques for control could be adopted [23].

The system design features a nominal electrical power of 550 W_p with an estimated electrical conversion efficiency of 30%. The CPV system simultaneously produces a thermal power of 630 W. This allows us to calculate an overall system efficiency of 65.5% and a net conversion efficiency (considering the electrical power equivalent to the thermal power) of approximately 51%.

The heat removed from the cells using an active water system will be recovered and used for domestic applications, supported by a 95 L storage tank. Additionally, a 1 kWh electric storage unit is included in the design. These two storage systems are integrated into the version designed for flat roofs, effectively transforming the CPV system into a “small energy station” capable of providing both thermal and electric energy even after daylight hours. Furthermore, the system is designed as a modular entity, allowing it to function both as a standalone generator and as part of a more complex PV system.

In the coming months, the first module prototypes will be ready, and an intensive testing campaign will be conducted to verify the design results and measure the main electrical characteristics at the module level, such as short-circuit current, open-circuit voltage, efficiency, and fill factor.

This study represents an attempt to adapt a technology typically used for utility-scale applications to residential purposes, potentially promoting its wider adoption by increasing economic benefits through self-consumption of the produced energy. Currently, CPV applications are not cost-competitive with traditional PV systems.

The higher costs associated with CPV systems can be attributed to several factors compared to traditional PV systems, including the following:

(a) The inability to utilize scattered radiation; (b) increased sensitivity of the cells to the spectral composition of incident light; (c) the requirement for highly accurate tracking, which increases costs; (d) greater need for module cleaning; (e) challenges in bankability due to CPV being considered a new and immature technology; (f) notable optical losses due to high concentration factors (up to 1000×); and (g) delays in technology standardization.

Despite these drawbacks, CPV systems offer significant advantages over traditional PV systems, such as the following:

(a) Higher conversion efficiencies, which can exceed 30% and even 40% in experimental systems [24], leading to reduced land use for the same power output; (b) lower temperature coefficients compared to flat modules (typically 0.1%/K for CPV cells versus 0.25–0.30%/K for traditional modules); (c) the potential for utilizing additional waste heat in systems with active cooling; (d) increased and stable energy production throughout the day due to (two-axis) tracking; (e) reduced energy payback time; (f) limited impact from potential increases in semiconductor prices due to the small size of the cells; and (g) greater potential for future efficiency improvements compared to single-junction flat-plate systems.

This project could become a viable option if the production costs of concentrated multijunction receivers, the core technology, are reduced. Although a basic cost analysis shows that the unit costs of this system are higher than those of conventional PV and CPV systems, it is cost-competitive compared to a system using standard PV and separate solar thermal plants for equivalent electrical and thermal energy production.

The design of a novel hybrid CPV-T system equipped with integrated energy storages represents a significant advancement in solar energy technology, particularly suited for residential applications. By combining high-efficiency CPV cells, effective thermal management, and robust energy storage, this system promises to deliver a reliable, cost-effective, and sustainable energy solution for modern homes. As the demand for clean energy continues to grow, such innovative systems will play a crucial role in shaping the future of residential energy consumption.