Solar collector field

According to ref. [13], the solar collector efficiency n is defined as a function of the ratio of the difference between the average fluid temperature tv and the ambient air temperature Tmb to the incident global solar radiation on the collector ql as follows:

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(eq. 1) п = к(©)• c0 — c

 

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The constants Co, c1 and c2 are efficiency-parameter of the solar collector and are known from the

manufacture. Figure 3 shows the solar collector efficiency which depends on factor x for normal incident radiation based on measurement data from the solar cooling plant collected in the duration of 2003 to 2007.

Подпись: Figure 4 is a diagram of the daily energy work supplied/consumed by the solar cooling plant from May to October in 2006.
The mean efficiency value of the solar collector field is about 0.63 and remains constant over a wide value range of the factor x, which indicates the good performance of the solar collector. The average value of the collectors’ field efficiency for the five year period is 0.28. By visual inspection of the collectors’ glass tubes, it has been found a thin layer of fouling material of residual combustion gases and other materials over the tube surfaces. These fouling materials are not removable with heavy rains.

Fig. 4. Daily energy supply/consumption of the solar cooling plant.

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The fraction of the driving heat covered by the solar collector field is referred as solar heat fraction. Until June, the solar collector field cover the heat demand of the absorption chiller. From June on, auxiliary heat from the heating network is needed to drive the absorption chiller. The reasons for the additional heat demand are bad weather conditions and the present collector field size, which is too small to cover the driving heat demand of the chiller at full load with its present completion. The five — year average heat fraction in the duration from 2002 to 2007 is 60 %. In total 89.2 MWhth heat work were consumed to operate the absorption chiller. 53.9 MWhth were generated by the solar collector field and 35.25 MWhth were provided by the heating network of the institute. But in the same period of time, the solar collector field generates also 8.13 MWhth surplus heat work, which was provided to the heating system where it substituted conventionally produced heat energy.