Solar thermal collectors and applications

. Evacuated tube collectors

Conventional simple flat-plate solar collectors were developed for use in sunny and warm climates. Their benefits however are greatly reduced when conditions become unfavourable during cold, cloudy and windy days. Furthermore, weathering influences such as condensation and moisture will 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 Fig. 5.

ETC have demonstrated that the combination of a selective surface and an effective convection suppressor can result in good performance at high temperatures [21]. The vacuum envelope reduces convection and conduction losses, so the collectors can operate at higher temperatures than FPC. Like FPC, they collect both direct and diffuse radiation. However, their efficiency is higher at low incidence angles. This effect tends to give ETC an advantage over FPC in day-long performance.

ETC use liquid-vapour 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 vapour travels to the heat sink region where it condenses and releases its latent heat. The condensed fluid return back to the solar collector and the process is repeated. When these tubes are mounted, the metal tips up, into a heat exchanger (manifold) as shown in Fig. 5. Water, or 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 to water that is stored in a solar 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 self­limiting temperature control is a unique feature of the evacuated heat pipe collector.

ETC basically consist of a heat pipe inside a vacuum - sealed tube. A large number of variations of the absorber shape of ETC are on the market [55]. Evacuated tubes with CPC-reflectors are also commercialised by several manu­facturers. One manufacturer recently presented an all-glass ETC, which may be an important step to cost reduction and increase of lifetime. Another variation of this type of collector is what is called Dewar tubes. In this two concentric glass tubes are used and the space in between the tubes is evacuated (vacuum jacket). The advantage of this design is that it is made entirely of glass and it is not necessary to penetrate the glass envelope in order to extract

Table 4

Characteristics of a typical ETC system

Parameter

Value

Glass tube diameter

65 mm

Glass thickness

1.6 mm

Collector length

1965 mm

Absorber plate

Copper

Coating

Selective

Absorber area for each collector

0.1 m2

Efficiency mode

nvs(Ti - Ta)/G

Gtest : flow rate per unit area at test conditions (kg/s m2)

0.014

c0 : intercept efficiency

0.82

c1 : negative of the first-order coefficient of the efficiency (W/m2 °C)

2.19

b0 : incidence angle modifier constant

0.2

Collector slope angle

Latitude + 5 to 10°

heat from the tube thus leakage losses are not present and it is also less expensive than the single envelope system [56]. The characteristics of a typical ETC are shown in Table 4.

Another type of collector developed recently is the integrated compound parabolic collector (ICPC). This is an ETC in which at the bottom part of the glass tube a reflective material is fixed [57]. The collector combines the vacuum insulation and non-imaging stationary concentration into a single unit. In another design a tracking ICPC is developed which is suitable for high temperature applications [58].

Solar thermal collectors and applications

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