# Possibilities of Powering Military Equipment Based on Renewable Energy Sources
## 2 Literature Review  
### The Use of Photovoltaic Cells  
Obtaining electricity from sunlight is an attractive way of powering electronic devices in areas without energy infrastructure and with limitations in securing the logistic of troops [1; 2; 3]. Photovoltaic cells would usually act as a backup power source or support the operation of diesel-electric generators. At command posts, they can support the work of a field energy system. The amount of energy obtained from photovoltaic panels depends on the area, the efficiency of panels and the amount of sunlight [4; 5]. Such power systems can only be operated during the daytime, which requires the accumulation of electricity. Batteries with different parameters are used to power devices that soldiers are equipped with (e.g., radio stations, night vision devices, GPS devices). Their working time is limited by the capacity of the batteries and the temperature at which the battery operates [6; 7; 8]. The easiest way to extend the run-time of your devices while completing tasks is to use additional power sources. However, it increases the weight of the soldier's equipment. An alternative solution to the problem is to equip troops with portable charging devices powered by photovoltaic panels. Their use in various militaries, however, requires meeting additional expectations in terms of masking, resistance to mechanical damage, resistance to weather conditions, low weight, and small geometric dimensions [1; 2; 6]. Photovoltaic panels consist of single cells connected in various configurations [3; 9; 10] and currently manufactured panels can be grouped according to the material they are made of and their crystal structure. Most often they are manufactured on silicon basis with a crystal and amorphous structure, less often they are made of semiconductor compounds such as halides, GaAs, InP, as well as dye and organic materials [11]. Currently conducted research is aimed at obtaining the highest possible ratio of the power recovered in a PV cell to the power of solar radiation falling on the surface of a PV cell [10; 11; 12; 13; 14]. The manufacturing technology of PV cells can be divided into three generations. The first generation consists of PV cells based on crystalline or polycrystalline silicon. The second-generation PV cells are made of thin-film PV cells based on amorphous silicon, polycrystalline CIS (CuInSe2), CIGS (CuInGaSe2) layers. The third generation of PV cells is manufactured in a multi-layer technique from materials with different energy potentials [11; 13; 14]. Due to their surface shape, thermal properties, and color, PV panels may have features that reveal the positions of troops. Dye-sensitized solar cells (DSSC) belonging to the third generation of PV cells can eliminate that disadvantage. The efficiency of PV DSSC cells is 15% [12], and they cannot compete with silicon cells in terms of efficiency. PV DSSC cells, due to the possibility of producing them in any colors and shapes, as well as the high durability of the cells and operation at low light intensity, can be used in military applications. When considering the use of PV DSSC cells, their sensitivity to both too low and too high temperatures should be taken into account [12]. The conducted research on flexible PV cells technology is focused on thin-film cells with increased efficiency and greater structure flexibility. This allows them to be integrated with soldier's personal equipment (such as a uniform, backpack, bulletproof vest, helmet, etc.). The following solution enables maintenance-free powering of soldier's life monitoring systems and charging batteries used in low-power radio stations, GPS receivers and light sources. Flexible PV panels are mostly produced with the use of PV cells made in the amorphous silicon technology as multi-junction structures or with organic technology [3; 9; 13; 15]. The largest producers of flexible PV cells are American companies such as Global Solar, Uni-Solar, and Power Film, which also specialize in military production [15; 16; 17; 18; 19; 20].