SOLAR COLLECTORS
10
Chapter 3
Efficiency curves of MEGASUN solar collectors
ST 2000 Selective (2,10m2, 24ºC) ST 2500 Selective SUN-POWER Selective
a. Test Nr: 1021 b. Test Nr: 30.0012.0-3 c. Test Nr 28601068
d. Avis Technique 14/05-947
• Courve a:
Test by DEMOKRITOS (Grece) n= 0,85 - 5,44 T*
• Courve b:
Test by CENER (Spain) n= 0,767-0,37 (tm - ta)/G
• Courve c:
Test by TÜV Bayern (Germany) n=0,794-3.907 T*- 0,0157 (θm-θL)T*
• Courbe d:
Calculs CSTBat (France) y = -4,8648x + 0,7063
Procedure for the calculation of estimated energy output of collector
The collector instantaneous efficiency curve is expressed by the following relation in linear or second-order form:
where n is the collector instantaneous efficiency, Tm is the mean temperature of water inside the collector, in ºC,
Ta is the ambient air temperature, in ºC and G is the total solar radiation that falls in the collector, in W/m2. The
parameters of the above equations of the instantaneous efficiency curve n0 and U0 are determined by testing
according to the standards EN 12975-2 and ISO 9806-1.
The estimated energy output of the collector is calculated using the values of parameters n0 and U0, as these
have been determined by testing from several accredited laboratories of Europe, for a number of cities and under
the following conditions:
- solar radiation, ambient air temperature and temperature of cold water (average monthly values as given in the
tables of the following page)
- temperature of hot water delivered by the collector to the user equal to 45ºC and 40ºC.
For every day of the month the efficiency of the collector is calculated, where the maximum efficiency and the
heat losses of the collector are taken into account depending on the existing climatic conditions of the day and
the desired temperature of hot water delivered by the collector to the user. Also, the latitude of the area of instal-
lation and the slope of the collector are taken into account. Following this, the mean monthly output of the col-
lector is calculated using the climatic data of the month.
Finally, the sum of the mean monthly outputs of the collector gives the total annual output. It is noted that the val-
ues of the estimated energy output of the collector that are calculated and given in the next tables are the maxi-
mum estimated and therefore they are achieved only by the optimum design and installation of the solar collec-
tor and the solar system. This means that that there must not be any shading of the collector during the hours of
sunshine and operation of the system, any water penetration inside the collector from the rain, any accumulation
of water in the inside part of the collector cover, any accumulation of dust or other substances on the outside part
of the collector cover, any deformation of any part or area or material of the collector and system, any leakage in
the hydraulic connections in any part of the collector or system, bad or no insulation of the piping of the solar sys-
tem, bad operation of the valves of the solar system, non proper maintenance of the collector and the system and
problems caused by deposition of salts within the tubes of the collector by the usage water.
