The collector heat loss coefficient UL was determined with the ventilation flap in operation for the experimental set-up B and data of Sept. 9 and Sept. 14. The dimensions of the slit aperture, the flap’s sensitivity setting and the collector tilt angle are given in Fig. 7. Fig. 7 shows that UL

increases for larger AT from the interval 5.4 — 6.8 W/(m2 K) to the interval 8.2 — 9.3 W/(m2 K) due to the self-triggered opening of the ventilation flap. Here, a constant collector heat capacity of Ce=5 kJ/(m2 K) was chosen. The experimental set-ups A and B have different dimensions for the thermal insulation and the distance between collector cover and absorber, hence the values for UL and Ce from set-up A an B cannot be directly compared.

The trend of UL for September 9 and 14 in Fig. 7 shows that the heat loss coefficient increases steeply when the flap is opened and converges then to a final value. This is due to the dimensions of the flap opening and slit aperture. At first the dimensions are sufficient to provide sufficient cooling. With increasing AT an asymptotic limit is reached where the geometric and aerodynamic limit is so that the maximum heat loss of the collector through passive ventilation is reached.

For Sept. 14 the initial and the final value for UL are larger than for Sept. 9. This is caused by the enlargement of the slit aperture at the bottom of the collector from 15 cm to 20 cm and the changed flap’s sensitivity settings. Due to the larger slit aperture (bottom) the heat loss is already larger from the beginning (Sept. 14). Further, due to the higher sensitivity of the flap setting the flat opens wider for certain AT, which results in a higher heat loss coefficient.

9	Sept. 9, 2006	9	Sept. 14, 2006
8	,»• *	8	• ♦ ♦♦
7	t	7	. /
6	* . « . *	6	. *
5		5
4		4
3		3	Tilt angle: 60%
2	Tilt angle: 42%	2	Flap setting: 6 mm
	Flap setting: 8 mm	1	Slit aperture (bottom): 20 mm
	Slit aperture (bottom): 15 mm	0	———— 1———— 1———— 1———— 1———— 1———— 1———— 1————

= (Tbs

— Ta )

^T = (Tbs

— Ta )

0

Fig. 7. Heat loss coefficient UL as a function of the temperature difference AT = (Tabs, avg — Ta) with ventilation
flap in operation; measurements for different tilt angles, flap settings and bottom slit apertures [set-up B];

2. Summary

Passive ventilation has been investigated as a method to limit overheating in polymeric collectors during thermal stagnation. Measurements were performed outdoors at two experimental set-ups, which consisted respectively of a ventilated and a non-ventilated (reference) collector.

For set-up A, a maximum temperature reduction up to 30 K between ventilated — and reference collector was measured. Corresponding results were obtained in [1,2] by modelling and in [3] by measurements; for the latter the ventilation was performed on the backside of the absorber.

In the present studies the temperature in the solar collector was reduced from above 145 °C to less than 120 °C. The temperature reduction was obtained with a distance of 28 mm between the absorber surface and the collector cover. The slit aperture openings were varied between 10­20 mm. This changed the temperature reduction with 5-10 K, with largest cooling effect for the large slit aperture. The temperature difference between the reference and the ventilated collector increased with increasing tilt angle.

In set-up B, the slit aperture in the bottom of the collector frame was constant while an adjustable, temperature-triggered flap determined the slit aperture in the top. It was shown that the longitudinal thermal expansion of the polymeric absorber is sufficient to trigger the ventilation flap in the top and initiate the ventilation. The ventilation flap concept seems to work effectively.

A simple method to determine the heat capacity of glazed polymeric solar collectors by means of stagnation temperature measurements has been investigated. The advantage of the method is that the collector efficiency can be obtained from the collector’s stagnation temperature and solar irradiation measurements only. The disadvantage is that additional information about optical and heat removal properties is required. This method is a valuable tool to determine the heat loss coefficient and the heat capacity (and hence the collector efficiency) of building integrated, polymeric collectors.

References

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[2] J. P. Roberts, M. J. Brandemuehl, J. D. Burch, K. M. Gawlik, Overheat protection for passive solar water heating systems using natural convection loops. Madison: Proc. of ASES Annual Conference Solar 2000, Wisconsin, 2000.

[3] S. J. Harrison, Q. Lin, L. C.S. Mesquita, Integral stagnation temperature control for solar collectors. SESCI 2004 Conference University of Waterloo, Ontario, Canada, 2004.

[4] Wavin B. V. Solar collector with plastic tubes for transfer medium — and heat-exchangeable overheat protector to open housing wall for convection cooling. Patent, Publication number: NL8105671, Publication date: 1983-07-18.

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[10] J. Gjessing, Ventilering som metode for a redusere stagnasjonstemperatur i solfangere. Master thesis at the University of Oslo, November 2006.

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[19] J. Gjessing, J. Rekstad, M. Meir. A method to determine the u-value and the heat capacity of glazed polymeric solar collectors. Manuscript prepared for submission for Buildings and Energy; status: September