Cpc Evacuated Tube

Manifold

Evacuated tube Absorber plate

Heat pipe evaporator

Evacuated tube Absorber plate

Heat pipe evaporator

Cross-sectional detail

FIGURE 3.7 schematic diagram of an evacuated tube collector.

influences, such as condensation and moisture, cause early deterioration of internal materials, resulting in reduced performance and system failure. Evacuated heat pipe solar collectors (tubes) operate differently than the other collectors available on the market. These solar collectors consist of a heat pipe inside a vacuum-sealed tube, as shown in Figure 3.7. In an actual installation, many tubes are connected to the same manifold as shown in Figure 3.8.

Evacuated tube collectors (ETCs) have demonstrated that the combination of a selective surface and an effective convection suppressor can result in good performance at high temperatures. The vacuum envelope reduces convection and conduction losses, so the collectors can operate at higher temperatures than flat-plate collectors. Like flat-plate collectors, they collect both direct and diffuse radiation. However, their efficiency is higher at low incidence angles. This effect tends to give evacuated tube collectors an advantage over flat-plate collectors in terms of daylong performance.

Evacuated tube collectors use liquid-vapor phase change materials to transfer heat at high efficiency. These collectors feature a heat pipe (a highly efficient thermal conductor) placed inside a vacuum-sealed tube. The pipe, which is a sealed copper pipe, is then attached to a black copper fin that fills the tube (absorber plate). Protruding from the top of each tube is a metal tip attached to the sealed pipe (condenser). The heat pipe contains a small amount of fluid (e.g., methanol) that undergoes an evaporating-condensing cycle. In this cycle, solar heat evaporates the liquid and the vapor travels to the heat sink region, where it condenses and releases its latent heat. The condensed fluid returns to the solar collector and the process is repeated. When these tubes are mounted, the metal tips project into a heat exchanger (manifold), as shown in Figure 3.7. Water or

Heat Storage Latent Tank
FIGURE 3.8 Actual ETC installation.

glycol flows through the manifold and picks up the heat from the tubes. The heated liquid circulates through another heat exchanger and gives off its heat to a process or water stored in a solar storage tank. Another possibility is to use the ETC connected directly to a hot water storage tank.

Because no evaporation or condensation above the phase-change temperature is possible, the heat pipe offers inherent protection from freezing and overheating. This self-limiting temperature control is a unique feature of the evacuated heat pipe collector.

Evacuated tube collectors consist of a heat pipe inside a vacuum-sealed tube. The characteristics of a typical collector are shown in Table 3.2. Evacuated tube collectors on the market exhibit many variations in absorber shape. Evacuated tubes with CPC reflectors are also commercialized by several manufacturers. One such design, presented recently in an attempt to reduce cost and increase lifetime, consists of an all-glass Dewar type evacuated tube collector. This uses two concentric glass tubes, and the space in between the tubes is evacuated, creating a vacuum jacket. In this type of ETC, the selective coating is deposited onto

Table 3.2 Characteristics of Typical Evacuated Tube Collector System

Parameter

Value

Glass tube diameter

65 mm

Glass thickness

1.6 mm

Collector length

1965 mm

Absorber plate material

Copper

Coating

Selective

Absorber area

0.1 m2

FIGURE 3.9 Evacuated tube collectors array with reflectors. (a) Flat diffuse reflector. (b) CPC reflector.

the outside surface of a glass tube domed at one end. This tube is then inserted into a second, larger-diameter domed glass tube and the tubes are joined at the open end. The advantage of this design is that it is made entirely of glass and it is not necessary to penetrate the glass envelope to extract heat from the tube, eliminating leakage losses and keeping it cheaper than the single-envelope system. However, these are suitable only for low-pressure systems and have the disadvantages that the tubes cannot be drained; if one tube breaks, all the working fluid may be lost (Morrison, 2001). This is also called a wet tube ETC. A variation of the wet tube ETC is a normal single-glass ETC in which water (or any other fluid) flows through the collector in either a U tube or coaxial pipe.

As ETCs are relatively expensive. The cost effectiveness of these collectors can be improved by reducing the number of tubes and using reflectors to concentrate the solar radiation onto the tubes. A diffuse reflector (reflectivity, p = 0.6) mounted behind the tubes, spaced one tube diameter apart, as shown in Figure 3.9a, increases the absorbed energy in each tube by more than 25% for normal incidence. This system also presents a 10% increase in energy collection over a full day because of incidence angle effects. Greater enhancement per tube can be achieved by using CPC-type reflectors, as shown in Figure 3.9b. Evacuated tube arrays with stationary concentrators may have stagnation temperatures exceeding 300°C.

Another type of collector developed recently is the integrated compound parabolic collector (ICPC). This is an evacuated tube collector in which, at the bottom part of the glass tube, a reflective material is fixed (Winston et al., 1999). In this case, either a CPC reflector, Figure 3.10a, or a cylindrical reflector,

(a) Solar radiation (b) Solar radiation

(a) Solar radiation (b) Solar radiation

Non Tracking Solar Concentrators

tor with finned absorber.

Figure 3.10b, is used. The latter does not achieve the concentration of the shaped reflector but has a very low manufacturing cost. In this way, the collector combines into a single unit the advantages of vacuum insulation and non-imaging stationary concentration. In another design, a tracking ICPC is developed that is suitable for high-temperature applications (Grass et al., 2000).

Evacuated tube collectors are produced in a variety of sizes, with outer diameters ranging from 30 mm to about 100 mm. The usual length of these collectors is about 2 m.

tor with finned absorber.

Figure 3.10b, is used. The latter does not achieve the concentration of the shaped reflector but has a very low manufacturing cost. In this way, the collector combines into a single unit the advantages of vacuum insulation and non-imaging stationary concentration. In another design, a tracking ICPC is developed that is suitable for high-temperature applications (Grass et al., 2000).

Evacuated tube collectors are produced in a variety of sizes, with outer diameters ranging from 30 mm to about 100 mm. The usual length of these collectors is about 2 m.

Was this article helpful?

+1 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment