SKN_N0394R0_DegradationCollectors_v10
Method for determination of degradation for collectors with a risk on degradation
Background:
A massive disadvantage was found during a testing for certification of a heat pipe collector in autumn 2017 with respect to „post exposure performance testing“. If the climate conditions are not perfect, this technology with its unique requirement of post exposure testing is facing a clear handicap due to a massive reduced testing period. As the climate conditions in August and September 2017 were more than poor, it was not possible to collect the required irradiance doses for climate class A right in time to start a regular performance and IAM determination. Due to the size and weight of the collector, an economic indoor testing for collecting the missing irradiance doses was not possible. After fulfilling the irradiance requirements, an adequate determination with high accuracy of these values was not possible any more.
Disadvantage against all other collector technologies
The main aspect is the time depending disadvantage and the resulting limited test season for collectors with heat pipes. As the thermal performance test and its determination of the incidence angle modifier is the only test, which is currently only possible under outdoor conditions under best irradiance conditions, this is a massive handicap.
The intention for this “post exposure test” of collectors using heat pipes was usually the known possibility for degradation during their first operating days. But now, the complete collector has to be exposed and so this is the only technology, where also a degradation of the reflectivity of mirrors or manifolds and insulation will be taken into account. A tubular collector using a direct flow tube and reflector could be performance tested as a new product after only five hours of preconditioning. This is also a clear disadvantage for heat pipe collectors.
Method to re-establish fair testing requirements for different technologies
If the duration for outdoor exposure should not be reduced or the requirements for the relevant climate class not be changed, a new method for the determination of degradation of performance is needed.
The idea is to perform the full thermal performance test including IAM determination as pre-exposure (after the obligate preconditioning) and to approve the degree of degradation at the conversion point of the power curve. Indoor as well as outdoor data could be used for this approval and if the collectors are using reflectors, it is possible to shade the reflectors only for this comparison of initial and final conversion point. With this method, the view on degradation is nearly limited to the performance of the heat pipes.
The resulting degree of degradation is defining the action to be taken:
A)Degradation below 1%-Point
Following the requirements for comparison of indoor and outdoor testing (EN ISO 9806:2017; Clause 19.2.2) and taking the resulting measurement uncertainties into account, there’s no need for any change of the results of the pre-exposure performance test.
B)Degradation between 1…3 %-Point
Still following the requirements from 19.2.2, here a correction of the collector efficiency factor (eta0,hemrespectively eta0,beam) is required.
C)Degradation above 3%-Point
Only if the degradation is higher than 3 %-Point, a complete second performance test as “Post Exposure Test” is required.
The method of pointing out the degree of degradation is closely linked to the judgement method used for PV panels according to IEC standards as well as the “old” AS/NZS 2712 method for stagnation testing.
If the production quality of the heat pipes is high, the manufacturer can nearly eliminate the disadvantages resulting from post exposure testing. Furthermore, the manufacturer can do an adequate stabilization in advance to reduce the risk for a complete new post exposure test.
It is still guaranteed that a major degradation on performance will be detected and the intentions of all aspects of EN ISO 9806:2017 will be fulfilled.
As not only heat pipe collectors are showing risks for degradation, this method could also applied on collectors using temperature limiter or switching absorber/ glass coatings or other new developments with a risk for degradation.
Version 01; 19 February 2018