Heat losses investigation

1.1. Problem description

When solar radiation come to an absorber layer, the spectral response of the ideal material should allow an absorbance on the bandwidth of incoming radiation and an emittance completely shifted in a different bandwidth. In this case all incoming radiation would be transferred into thermal radiation. Part of this radiation is lost under the form of emittance of the absorbing layer to the environment, part becomes thermal energy and is transferred to the below layers. The structure of the solar tube avoids conduction and convection losses to the environment due to the evacuated area between two concentric glass cylinders inside which area the absorbing material is deposited, usually on the external surface of the inner tube. The consequence is that, all conduction and convection losses concentrate in the various layers from the absorber to the vector fluid. The investigation of the problem starts with the evaluation of the efficiency for the analysed solar collector as certified according to EU certification EN.12975-2:2006. The solar collector efficiency is the ratio between the energy Q (energy density) absorbed from the vector fluid and the energy (solar energy density) incident on its external surface.

image231"(1)

and similarly, as usually reported, from the relation:

(2)

where tjq is the solar collector efficiency at TM = 0, ai and a2 are heat transfer coefficients, G is the total solar radiation and:

image232"(3)

where tm is the panel average temperature and is the ambient temperature. The relation (2) is

the main instrument to evaluate the quality of a solar collector, and is within the certification reports. The efficiency of the solar collector investigated in the actual work can be viewed in the Fig. 1. It starts at 71,8 % at ambient temperature and it’s referred to an incident radiation of 1000 W/m2. In the same graph are represented the various heat losses calculated from optical and geometric factors, tube materials, reflector characteristics and cermet thermal properties. "Other losses", the orange area, resume some other factors, the most relevant of whose is the thermal resistance between the cermet layer and the vector fluid.

Подпись: Efficiency and thermal loss on a vacuum solar collectorПодпись: ■ Other Losses ■ Conduction Loss to External ■ CPC Concentrator ■ Cermet Emissivity ■ Borosilicate Glass ■ Collector Efficiency Подпись: 0 0.025 0.075 0.125 0.175 0.275 tm-ta / G image233100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

Fig. 1. Evaluation of the thermal losses on the real solar collector.

The Fig. 2 represents a section of the Evacuated Solar Tube (EST), selected for the modelling and used in the experimental prototype.

Temp

Parabolic

Concentrator

(W)*

Borosilicate Glass (W)

Cermet

(W)

100

7.6

12.8

8.3

150

7.6

12.8

38.1

200

7.6

12.8

68.5

300

7.6

12.8

120.2

Table 1. Main contribution to radiation heat losses due to different optical behaviour and properties of the evacuated solar tube.

* surface reflectance = 0.9

Подпись: Fig. 2. Sectional view of the analysed evacuated solar tube (EST)

From external to internal the different layers of the EST are:

• borosilicate glass 3.3 tube — Glass TubeEXT ^EXT = 58mm; tb = 0.92, p = 0.062, a = є = 0.018 );

• vacuum (pINT~10-7 Torr, фЕХ1- = 56.4mm, фют = 47mm);

• cermet layer (a > 0.93);

• borosilicate glass tube — GlassTubeINT (фЕХЇ = 47mm, 5 = 1.5mm);

• computed air layer for materials tolerance between GTINT and aluminium profile (5 = 0.7mm);

• aluminium foil;

• copper tube (фЕХт = 7mm, фШт = 6mm);

• water (v = 0.556 m/s).

The problem of the evaluation of the heat loss and the development of a modified object with higher thermal efficiency is the first step to begin the research for a new device able to produce electrical, thermal and cooling power, with an efficiency higher than 70%. The first step is to build an evacuated solar tube with the capability to work at 250°C and more, without loosing thermal energy with consequences on its efficiency. The actual evacuated solar tube, at a vector fluid temperature of 523 K, has a thermal efficiency n = 0.23.