## 2 Review and Analysis
### Combined Solar Panels  
Let us consider combined solar panels [16; 27]. Different designs of solar panels are known (Figure 5) [16], which contain a protective coating, a heat-insulating layer, and a heat-conducting layer located between them [28; 29]. The advantages of such structures are that they are building elements; independently perceive and accumulate solar energy; and are reliable and simple in construction, installation, and maintenance. This significantly reduces their cost compared to solar panels [30; 31]. However, the small contact surface between the pipelines and the protective coating significantly reduces the heat-transfer coefficient between them, and the high metal capacity of the structure significantly increases the cost of the panel and reduces the efficiency of its use. The most effective is the solar panel, the protective coating of which performs the functions of a waterproofing layer and an absorption coating. This makes it possible to cover roofs as a whole and allows the maximum use of materials that are usually most commonly used for roofing and waterproofing roof coatings [16]. The solar panel (Figure 6) [27] consists of a protective coating (1), which serves as a waterproofing and absorption layer and is located on a heat-conducting layer (2). It acts as a thermal energy accumulator, in which corrugated pipelines (4) for the supply and removal of the heat carrier are placed. A heat-insulating layer (3) for protecting slabs of the floor against overheating is placed under the heat-conducting layer, on which the heat-reflecting screen (5) is placed. Since these systems are elements of buildings, their installation can be foreseen both in new construction and in the reconstruction of the building, which will increase the efficiency of the system as a whole and reduce the payback period. However, it is not known which design features of the solar panel are rational to ensure the maximum efficiency of its operation. Therefore, it is necessary to technically improve and experimentally investigate the solar panel, which will reduce the cost of the product and increase its efficiency. Figure 5: Combined solar panel: 1—front light-receiving panel, 2—back panel (polyester reinforced with glass fiber), 3—thermal insulation layer (polyurethane foam), 4—tubes, 5—channel (reccess), 6—heat-conducting sheet (in the form of an aluminum radiator grill), 7—body shape, 8—absorbent coating, 9—thermostatic probe. Summarizing, the following should be stated:  
1. Currently, there are various designs of solar panels that differ in technical and economic indicators. Flat solar panels are quite reliable, efficient, and easy to operate, with a cost that is 20-40% lower than high-cost vacuum panels. However, flat panels inefficiently use solar energy in the morning and evening hours. 2. Systems with a passive use of solar energy have a relatively low cost and are environmentally friendly, but it is difficult to control the parameters of the indoor air and carry out the necessary air exchange in the room. Systems with an active use of solar energy are expensive. It is advisable to use a solar heat-supply system with combined solar heaters (solar panels). 3. The advantage of such structures is their belonging to building structures. They independently perceive and accumulate solar energy and are reliable and simple in construction, installation, and maintenance, which significantly reduces their cost compared to solar panels. However, the small contact surface between the pipelines and the protective coating significantly reduces the intensity of heat exchange between them, and the high metal content of the structure significantly increases the cost of the panel and reduces the efficiency of its use. 4. The most effective is the solar panel, the protective coating of which performs the functions of a waterproofing layer and an absorption coating, which makes it possible to cover roofs as a whole and allows maximum use of materials that are usually most commonly used for roofing. 5.