STANDARD PROCEDURE OF A TIGHTNESS TEST OF A SOLAR COLLECTOR BOX. (EUROSUN 2000 SOLAR CONGRESS)

Transcription

1 STANDARD PROCEDURE OF A TIGHTNESS TEST OF A SOLAR COLLECTOR BOX. (EUROSUN 2000 SOLAR CONGRESS) Ole Holck and Svend Svendsen Department of Buildings and Energy, Technical University of Denmark Building 8, DK-2800 Lyngby, Denmark, Phone Number , Fax Number , address oh@ibe.dtu.dk Kenneth Möller and Bo Carlsson Swedish National Testing and Research Institute, SP. Brinellgatan 4, Box 857 S-50 5 Borås (Sweden) Phone Number , Fax Number , address Kenneth.möller@sp.se Stefan Brunold and Ueli Frei Solar Energy Testing and Research Group, ITR. Oberseestr. 0 CH-8640 Rapperswil,Switzerland Phone Number , Fax Number , address Stefan.Brunold@solarenergy.ch Michael Köhl and Markus Heck Fraunhofer Institute for Solar Energy Systems, ISE. Oltmannsstr. 5, 7900 Freiburg, Germany Phone Number +49 (76) , Fax Number +49 (76) , address Mike@ise.fhg.de Henk Oversloot Building and Construction Research, TNO. P.O.Box 49 visiting address: Schoemakerstraat VK Delft 2600 AA Delft (the Netherlands) Phone Number , Fax Number , address H.OVERSLOOT@Bouw.tno.nl Abstract The durability of ventilated solar-thermal collectors is influenced by the corrosivity of the microclimate inside the collector. The micro-climate depends on the design of the collector, the operation condition and the ambient climate. The ventilation rate, which means the airflow rate into the collector at a given pressuredifference between ambient and the air gap in the collector, describes the correlation between microclimate and ambient climate. Thermal buoyancy, wind or gusts might cause the pressure difference. This paper describes a standard test procedure of a tightness test of the collector box. This is needed for calculation of the ventilation rate in the collector, which is one of the major effects that influence the microclimate in the collector and thereby influence the loads for degradation of the component in the solar collector. Collector test methods of relevance for micro-climate in collectors have been studied as a part of the work going on within the working group "Materials in Solar Thermal Collectors" of the IEA Solar Heating and Cooling Program.. INTRODUCTION The correlation between microclimate and ambient climate is described by the ventilation rate, which means the airflow rate into the collector. The driven forces for ventilation of the collector is primarily an air pressure difference between the surroundings and the air gap. In the participating laboratories the air tightness of a reference collector was determined by applying a defined pressure difference. The tightness test gives the airflow through the collector box for different pressure differences between the inside of the collector and the ambient. The use of this test for the assessment of the ventilation rate has been found problematic because the pressure range of interest for this air flow is zero to only a few Pa. CFD calculations are made to see in detail the behaviour of the fluid dynamics in the collector. The results from the tightness test of the collector box are verified by calculations of the microclimate compared to measured microclimate data. 2. PROCEDURE FOR AIR TIGHTNESS TEST OF A SOLAR COLLECTOR BOX The tightness test of solar collector boxes is very central in order to make sure that an optimised microclimate is achieved. The tightness parameters are necessary for the calculation of the ventilation rate in the collector. The ventilation rate of collectors is important because it directly effects the microclimate in the collector box and can be an important factor on the degradation of the components in the solar collector.

2 2. Introduction/Objective The collector box is tightness tested to assess the ventilation of the collector. The procedure prescribes the use of controllable air source capable of over- and underpressure, an airflow meter and pressure difference equipment. The basic test procedure is to couple the air source to the collector box and measure the pressure differences between the surroundings and the collector box for a range of 6 setpoints of the airflow. The procedure leads to a set of 2 measurement series with over and under pressure of the box and the open. The result from the tightness test is needed to classify the microclimate in the collector. 2.2 Preparation of the collector. Connect a plastic hose to the collector with direct connection to the air. No supplementary leaks from the air gap to other parts of the collector must be introduced. The hose is sealed around the hole by silicone. The inner diameter of the hose is approximately 6 mm. The pressure sensor is mounted in a similar way. 2.3 Apparatus. The apparatus consists of a controllable air source, capable of supplying a value of positive or negative pressure at the collector for different collector leakage flow rate. An airflow meter with an accuracy of 2% is installed to measure the rate of leakage and a micromanometer with an accuracy of 2%, capable of measuring a gas pressure down to Pa, is installed to measure the difference in pressure inside and outside the collector. The air source, airflow meter and micro-manometer are connected to the collector. The tightness test is made in two steps. One with air sucked in (under pressure), and one with air blown out from the collector (over pressure). 2.4 Test procedure and instrumentation. A controllable air, capable of sucking air out from the collector at different flowrates. An airflow meter is installed to measure the rate of airflow. The air source and airflow meter is connected to the collector. Leaks between flow meter and collector are not allowed. First the collector is tested with overpressure in the collector box at 6 levels of air rate. In practise it is sometimes easier to set a pressure difference and then measure the flow rate, all depending on the equipment in use, but still the 6 levels of air rate have to be kept. The two lowest levels are optional if the range is out of practicability of the apparatus but at least 5 levels are taken with sufficient spread in the range of interest and practicability. Four readings should be taken of the pressure with a stabilised airflow at each level. Next the collector is tested in the same way, but with an underpressure in the collector box instead of overpressure. The levels needed for airflow range is about 50, 00, 200, 400, 600 and 000 l/h. If lower range is of practicability of the apparatus they are preferable. The procedure can be repeated with the ventilation openings of the collector closed or parts of the collector closed to assess the airways of the ventilation and the distribution of the leakiness of the collector around the outer surface. For that purpose plastic films tape or other means can be used. 2.5 Transferring results from test to the real situation. The test condition differs from the real situation where air is flowing in and out at different apertures at the same time. If we consider a chimney situation and if the leakages are divided equally around the top and the bottom of the solar collector the pressure difference for entering the gap in the bottom is equal to the pressure difference for the aperture in the top of the collector. The air stream had to overcome both the pressure differences on the way in at the bottom and on the way out at the top of the collector. The pressure difference for testing with underpressure in the collector is therefore added to the pressure difference for testing with overpressure. In the test all apertures are available for the stream for one way only. This is not the case in the reality were the half of the openings are used for streams coming in the opposite direction. That means that the measured volume flow is the double of the realistic flow rate. At the tightness tests the volume flow rate through the solar collector is measured at various pressure differences and the results are given by regression expressed in terms of approximated functions in the form of φ v = C δp n. In this relation the factor C describes the airflow in chosen units at Pa pressure difference. The factor n is dependent on the type of airflow through the openings. Assuming that in the pressure ranges for the test the exponent n is approximately ½ the change to realistic flow rate by using the half of the measured flow rate has the influence of changing the pressure difference by a quarter. The pressure difference needed is only a /4 /2 of the reality if the test is performed with a collector having an even distribution of leakage. The volume flow rate is hereafter found for a given pressure difference P, as the pressures from the functions by inserting the pressure divided by 8. φ v = C δ(p/8) n The solar collector tested must be examined for apertures so that the transferring of results from the test has the right meaning. Detecting the airways from air gap to the surroundings can be done by covering some parts of the collector with plastic foil during the test procedure. If no more precise assessments have been made the formula can be used with the factor n depending on the airflow through the openings set to one for the low pressure range in the real situation.

3 3. TEST OF THE PROCEDURE. In applying this proposal the procedure was tested in a round robin procedure. There were 4 laboratories participating in the tests: ISE, Germany IBE, Denmark SPF, Switzerland TNO, the Netherlands The collector tested was selected by ISE in Germany. The collector box was selected on basis of its full metal casing consisting of one piece without any seams. The main source of leakage is expected to be the edge around the glazing and the inlet and outlet connections. The box itself has some small 4 mm ventilation openings at the bottom of the collector. The collector was shipped in a heavy crate so that differences in measurement results due to transport damage could be ruled out. open ; overpressure pressure difference [Pa] flowrate [dm³/h] ibe ise spf tno Figure : Giving the difference between the ventilation characteristics in pressure and suction as measured by the various labs, overpressure. The results of the measurement series of the participating laboratories were collected. In order not to get a confusing range of closely packed lines the graphs are kept apart for each test. So we have set of 2 graphs (Figure to Figure 2) each giving the difference in overpressure and suction. In general the agreement between the measurements above 5 Pa is good enough for the purpose in question. In the low-pressure region below 5 Pa there seems to be a split between ISE + IBE and SPF and TNO. In the suction curve the ISE measurements in the range below 5 Pa are more clearly different from the 3 other labs. In this region it is difficult to measure the pressure difference accurately. The ISE curve seems to indicate the use of a different instrument above 500 l/h, as there is kink in the curve. The curve of SPF seems to be more or less a curve instead of a straight line. In general it can be assumed that the small disagreements originate from the calibration of the pressure indicating instruments. Possibly the instruments are used out of their optimal range due to the low pressures involved. With respect to the purpose of the Round Robin test we can conclude that the general agreement between the measurements is good.

4 open ; suction pressure difference [Pa] flowrate [dm³/h] ibe2 ise2 spf2 tno2 Figure 2: Giving the difference between the ventilation characteristics in pressure and suction as measured by the various labs, underpressure. The purpose of the Round Robin test was to test the procedure of a standard test method for measuring the tightness of collector boxes. The standard method prescribes as measurement of setting a flow rate and measuring the pressure difference. The results on the collector do not indicate significant differences due to the method followed. The calibration of the various pressures indicating instruments in the low-pressure range needs special care for measurements in this low range. 4. COMPARING RESULTS FROM THE TIGHTNESS TEST WITH CFD CALCULATION. We are able to express the volume flow rate as a function of the pressure difference for each leakage-tested solar collector. The relationship between the test and the real situation under working condition is a factor of 8 on the pressure. Therefore the function has to be scaled. In order to verify this adoption we will compare the function with calculations using a CFD model. Steady-state simulations for two specific sets of outdoor conditions, normal operating and stagnation conditions have been calculated with the CFD model. At (Figure 3) we have drawn the line between the two sets of outdoor conditions from the CFD calculation and the lines representing the leakage-test of a reference collector from Germany, the Netherlands, Switzerland and Denmark. It seems that from the CFD calculations a linear behaviour can be adopted in this low region of pressure differences. This assumption is made in the light of the high ratio of pressure difference for the normal operating condition. In addition to the scale factor we therefore make the assumption that the function is linear with a fixed airflow for the pressure difference Pa. In (Figure 4) the scaled and linear functions are showed together with the line illustrating the condition calculated with the CFD model. It seems that the results from the leakage test made in Denmark differ from the results made in the other countries. It has therefore been necessary to use a factor of 5.3 represented by the dashed line in (Figure 4).

5 Pressure difference [Pa] TNO Ibe Ise SPF CFD Ventilation [l/h] Figure 3 The lower line represents a leakage test of the reference collector. The upper lines represent calculations with computed fluid dynamic software. Pressure difference [Pa] 5 4,5 4 3,5 3 2,5 2,5 0, TNO_scaled IBE_scaled ISE_scaled SPF_scaled CFD IBE_factor 5.3 Ventilation [l/h] Figure 4 The lines originated from the tightness test of the collector are scaled by a factor of 8. The dashed line is representing the Danish collector scaled by a factor of 5.3 to match the group of other collectors.

6 REFERENCES Influence of wind on the ventilation of collectors, Nov. 97 ;TNO Numerical investigation into the pressure distribution of a solar collector on a pitched roof, (positioning of ventilation holes in a collector), 6 March 999 "procedure of tightness test of a solar collector box" of 6 October 998, IBE "Modelling of microclimate in collectors" project C2, March 999, IBE Svendsen, S.: 986, Moisture in Solar Collectors Van de Linden, J. : 992, Micro-climate in Solar Collectors, Task X. Holck, O.: 993, Forbedring af solfangeres langtidsholdbarhed.