Design and principle of operation of a solar collector (heat-pipe)
Collector design:
The solar collector consists of vacuum tubes made of borosilicate glass. During production, an appropriate mixture of SiO2 and B2O3 oxides was used, resulting in a product with good chemical resistance as well as extraordinary purity and homogeneity. Borosilicate glass is environmentally friendly and can be recycled multiple times. A thermal tempering process was also applied. Combined with the low thermal expansion typical of borosilicate glass, this achieves a particularly high resistance to temperature changes compared to ordinary glass. The tubes are resistant to hail up to 25 mm in size. The use of tubes with diameters of 47 mm and 58 mm allows for concentric placement of one inside the other. The air between the tubes is pumped out, and the tubes are fused together. The vacuum between the two glass layers acts as an excellent insulator and prevents heat loss. In a triple magnetron metallization process, an absorber is applied (a compound that absorbs solar rays and converts them into thermal energy). The new special absorption layer, ALN/AIN-SS/CU with copper added, represents the next generation of absorption coatings. Succeeding the AL/N/AL layer, it features higher efficiency (by up to 12%) and excellent absorption properties for both direct and diffuse solar radiation. Additional absorber layers are designed to retain as much energy as possible inside the tubes and prevent heat loss through infrared radiation. The inside of the vacuum tube can heat up to as much as 300°C. A so-called "heat pipe" is installed inside the vacuum tubes. Aluminum fins located inside the vacuum tubes assist in transferring energy to the copper heat pipes. Following the principle that the boiling point drops as pressure decreases, the pressure inside the heat pipe was lowered by evacuating the air. The liquid inside the heat pipe exchanger boils at a temperature as low as 25°C. The copper used during the production of the heat pipe is oxygen-free, ensuring long and reliable operation.
The high efficiency of the collector results from its ability to absorb diffuse solar radiation (e.g., on cloudy days) and minimize heat loss. Energy is obtained not only from direct sunlight but also from reflected light. The collector's manifold is made of a copper pipe. Inside it, copper sleeves are installed into which the condenser of the heat pipe is inserted. To achieve better contact between the copper surfaces and, consequently, more efficient heat transfer, high-temperature thermal paste is applied to the contact points. The collector's manifold is thermally insulated with mineral wool. Although it has slightly worse insulating properties than polyurethane foam, it is a better solution in this case. Mineral wool does not oxidize and is more resistant to high temperatures that can occur, for example, when the fluid circulation in the system stops. There is also a dedicated spot in the manifold for mounting a temperature sensor. The manifold casing and its frame (bracket) are made of aluminum. The use of lightweight metals is quite important when installing collectors on building roofs.
Principle of operation:
Energy from solar radiation heats the inside of the vacuum tubes. Through the aluminum fins, heat from inside the tube is transferred to the heat pipes. After just a moment, at a temperature of 25°C, the liquid in the heat pipe begins to evaporate. The vapor rises to the top into the exchanger head (condenser) where, via the collector's manifold, it transfers heat and condenses again. It then flows back to the bottom of the heat pipe to repeat the entire process. The heating medium (e.g., glycol) flowing through the collector has no contact with the vacuum tubes or the absorber applied inside them; it only absorbs heat from the heat pipe's condenser. The connection between the heat pipes and the heat exchanger (in which the glycol flows) is a "dry" connection.
The simplest and cheapest system is a gravity-fed (thermosiphon) installation. The heating medium heated in the collector rises to the top of the storage tank without the use of a circulation pump; then, after releasing its heat in the tank, the cooled medium returns to the collector. In such a setup, it is necessary to place the storage tank above the collectors. In practice, this requires placing the collectors on frames on the ground and the tank on an upper floor inside the building.
The second solution used is a forced circulation installation. It does not have the drawbacks of a gravity installation, but it requires the use of a pump and an automatic control system. Usually, tanks equipped with two coils (bivalent tanks) are used in such a circuit. They allow cooperation with two heat sources. The solar installation is connected to the lower coil, and the heating boiler is connected to the upper one. When favorable conditions occur (the fluid temperature in the collector is 5 to 8 degrees Celsius higher than the water temperature in the tank), the circulation pump is automatically turned on, forcing the heated fluid from the collector into the coil inside the tank.
In case of damage to a vacuum tube, the whole system still works. Only the efficiency of the system decreases. There are no liquids inside the vacuum tubes, which means a tube can be dismantled at any time without having to empty the system.
For quick and easy connection of the collector to the accumulation tank, we recommend using twin pre-insulated pipes with synthetic rubber foam of increased thermal resistance. The pipes are made of stainless steel or soft copper. Their flexibility means that no additional fittings or connectors need to be used between the collector and the tank. They are also equipped with an integrated control cable (for the temperature sensor in the collector). In addition to maintaining the highest technical parameters to minimize energy loss, this system significantly shortens installation time and increases its reliability.
Advantages:
- Higher efficiency of the vacuum collector with the heat-pipe system (year-round operation).
- Possibility to select different collector sizes for various tank capacities.
- Damage to a vacuum tube with a heat pipe does not shut down the entire system, it only reduces the collector's efficiency.
- Lower probability of collector clogging compared to flat-plate collectors or those based on U-pipes.
- Possibility of integration with the central heating system to reduce energy expenses.