**1. Introduction**

One of the main ways to overcome the negative impacts of modern industrial civilization is the transition to maximum harmonization with nature, which is especially topical and noticeable in the countryside [1]. The energy supply based on the resources whose use does not spoil the noosphere and the creation of such energy supply of efficient equipment whose operation does not cause any damage to nature and man is one of the mechanisms of environmental restoration. The resource for this is, first of all, solar energy [2–6].

The transition to the use of renewable energy resources is the most obvious and justifiable in agriculture. Photovoltaic (PV) systems are one of the best equipment options for specified purposes, especially considering the development of building-integrated photovoltaics (BIPV) and agrivoltaics (AgriPV).

For agriculture and rural areas, the use of photovoltaic solar energy conversion is especially topical because of the following factors [7–9]:

**Citation:** Shepovalova, O.; Izmailov, A.; Lobachevsky, Y.; Dorokhov, A. High-Efficiency Photovoltaic Equipment for Agriculture Power Supply. *Agriculture* **2023**, *13*, 1234. https://doi.org/10.3390/ agriculture13061234

Academic Editor: Massimo Cecchini

Received: 19 December 2022 Revised: 22 May 2023 Accepted: 25 May 2023 Published: 12 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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At the same time, the power supply to agriculture imposes special requirements for PV equipment, namely requirements associated with the impact of aggressive environments, e.g., ammonia, thermal and humidity conditions, etc., as well as special requirements for safety, ecological compatibility, and aesthetics. Agriculture is more sensitive to changes (increases) in the equipment cost.

In order to implement power supply in rural areas, first of all, through the application of solar energy resources and expansion of the scale of agricultural power supply with the use of PV systems, it is necessary to increase the efficiency of PV systems and make them more attractive for consumers. At present, the solutions to this problem are as follows:


The overall purpose of our many years of research is to increase PV equipment efficiency at all levels [18], including PV cells, PV modules, and PV systems, thus considerably increasing the possibilities of using PV systems in agriculture.

At present, more than 90% of industrial-grade PV cells are PV cells fabricated on a silicon wafer with a *p*–*n* junction unit, which is due to their substantial advantages over any other types of PV cells (e.g., relatively low cost, accessibility and abundance of silicon, acceptable output characteristics and properties of such PV cells, and well-proven technology) [10,11,13,19].

Homogeneous semiconductor PV cells on the basis of just one *p–n* junction can only generate voltages limited by the potential barrier height on the junction. For silicon and gallium arsenide (i.e., the best technologically investigated PV materials), this voltage is approximately 0.6 V to 0.9 V under nonconcentrated solar radiation. At the same time, most typical electronic devices require considerably greater voltage levels for their operation. When using single-junction PV cells for obtaining the required values of voltage, one has to connect a sufficiently large number of cells/modules in series, which results in power loss due to contact resistance and non–uniformity in the parameters of individual cells/modules. Additionally, it leads to a reduction in efficiency and sturdiness against external factors. The typical planar crystalline silicon PV modules (60 mono-Si PV cells with single *p*–*n* junction and with a size of 156.75 × 156.75 mm) have an open-circuit voltage (*V*o.c) of 44 V and a short-circuit current (*I*s.c) of 9.5 A, and their voltage and current at the maximum power point are 36.3 V and 8.8 A, respectively. The required voltage and power values of a PV array based on traditional PV modules are usually provided at sufficiently high DC values. Reducing these values can significantly reduce ohmic losses and therefore will

both increase the efficiency and reliability of PV systems and reduce costs. Moreover, such PV modules occupy quite a great area [10–13,19–22]. However, in agriculture, limitations generally exist for the placement of PV modules and, as mentioned above, have more stringent requirements for immunity to environmental factors and cost.

Based on this, the first task of our research was to develop high-voltage silicon PV cells that would enable a significant increase in the output voltage of PV equipment and overcome other disadvantages of conventional PV modules that limit the PV equipment used for agriculture power supply.

Earlier on, research was pursued to develop high-voltage vertical multi-junction Si PV cells for space applications [23]. These cells had acceptable efficiency only when the concentration was 1000 suns or higher, and they were not convenient for terrestrial applications. More recent studies also refer to cells operating under high concentrations (efficiency of up to 19% under 2480 suns), which have also not been applied in terrestrial photovoltaics [24]. Terrestrial high-voltage vertical multi-junction Si PV cells suitable for mass application were, for the first time, proposed and developed in the course of the studies summarized in this paper.

Considering the fact that mass technologies are oriented toward the production of planar PV modules and also that the components of PV systems (mass design solutions), such as PV plants, BIPV, and AgriPV, are designed on the basis of the conventional planar PV modules, planar high-voltage silicon PV cells are required. Making such cells was a new idea. Our ideas and theoretical studies were presented in [25–30]. Besides increasing the output voltage, these cells had to accomplish the objective of creating high-voltage PV modules with dimensions matching the standard dimensions of the mass-produced crystalline Si PV modules. Additionally, such modules should be manufactured with the maximum use of existing equipment.

The higher the concentration of the solar energy arriving at radiation receivers (RRs) is, the higher the photoelectric conversion efficiency is. The use of concentrators enables an increase in the irradiance of the active surface of PV cells and therefore increases the efficiency of PV modules/PV systems [11,14,31,32]. A review of the current state of concentrator applications in photovoltaics is presented in [32–36]. Two types of solar radiation concentrators were of interest in terms of developing highly efficient concentrator photovoltaic (CPV) modules: focon-type concentrators and concentrators containing a symmetrical reflecting surface (RS) fabricated as a chute (compound parabolic concentrators). Concentrators of these types have the following important advantages, compared with other types of concentrators: (1) a long daily operation period within the parametric angle of the concentrator without tracking the sun; (2) the possibility of using diffuse radiation, thus increasing the amount of the total irradiance coming to the concentrator aperture, by up to 20%. The mirror RS on the current technological horizon may have a reflection coefficient of 99% or more, which sets this mirror concentrator apart from lens-based ones.

The main disadvantage of concentrators manufactured in the form of solids of revolution, including focon types, and other toroids, as well as concentrators based on Fresnel lenses, is a nonuniform distribution of irradiance intensity over the active surface of RRs. This results in a decrease in efficiency because PV converters are most efficient when irradiance is uniform. Compound parabolic concentrators have uniform radiation density in the rectangular focal region. The main problem of such concentrators is that the rays reflected from the part of RS located close to the focal plane fall on the active surface of RRs at high angles. This results in a sharp increase in the coefficient of radiation reflected from the active surface of RRs (Fresnel losses) [14,36–39].

Moreover, all of the operated efficient PV systems based on CPV modules are bulky and have incomparably larger dimensions and weights than those based on traditional planar PV modules. They require complex and insufficiently reliable tracking systems. This, to a large extent, brings to naught their advantage of a higher performance factor. In addition, naturally, this makes them hardly applicable in agricultural power supply systems.

Free from these defects are CPV modules based on microconcentrators whose dimensions are comparable to those of conventional planar PV modules. The best results in implementing microconcentrators are presented in [40–42]. There is a considerable problem associated with the unevenness of RR illumination while using mirror concentrators, which may be reduced by optimizing the form of RS, and there is also a problem of inherent losses caused by absorption when Fresnel lenses are used [36,39].

In certain situations, the use of toroidal concentrators is the most effective. In [43], we presented a series of newly developed semitoroidal concentrators.

Another task of our research was to remove/minimize the previously mentioned defects of focon-type concentrators and compound parabolic concentrators and to design efficient CPV modules on the basis of the developed concentrators and high-voltage multijunction cells.

The aim of this article was to present the final generic research results that yielded real practical outcomes, starting from scientific ideas to operating devices and installations. Schematically, the reviewed studies are outlined in Figure 1. The results presented in the paper include two types of high-voltage PV cells (Section 2), optimized solar concentrator structures (Section 3), and four types of PV modules on their basis (Sections 2 and 3). As well, this article presents research on the development of more efficient PV systems by increasing the level of systematization and expanding the boundaries of PV systems. The final efficiency of such systems is evaluated by the degree of consumer need satisfaction (Section 4). To date, the results of our theoretical studies related to PV cells and concentrators have mainly been published. The research results of fabricated PV cells, concentrators, and modules on their basis, are published for the first time in this article. For the time being, quite a high correlation between theoretical calculations and the operating characteristics of real instruments was achieved, which enabled us to present the relevant results in this manuscript.

When proposing the ideas related to the development of PV devices (Sections 2 and 3), along with efficiency improvement, a task was set to minimize the costs at the stage of the introduction of devices into production (by minimizing changes in the manufacturing technology) and at the stage of the creation of PV systems based on the developed PV and CPV modules (by minimizing changes in the technology and design of PV systems and mating structures). These aspects were indispensably taken into account when analyzing and choosing the optimal options. The solutions to this task included the choice of silicon as the basic material for PV cells, the creation of planar high-voltage cells whose dimensions correspond to those of conventional crystalline silicon PV cells, and CPV modules whose dimensions correspond to those of traditional planar PV modules.

**Figure 1.** General scheme of research (numbers correspond to the section/subsection number in this article).
