Fagade integration of polymeric solar collectors with coloured absorbers

M. Meirf, J. Rekstadf, E. Svasand*

fUniversity of Oslo, Department of Physics, P. O. Box 1048, Blindern, N-0316 Oslo, Norway Tel.: +47 228-56469, Fax: +47 228-56422, E-Mail: mmeir@fys. uio. no *Agricultural University of Norway, Department of Mathematical Sciences and Technology, P. O. Box 5003, 1432 As, Norway

Abstract — The present paper studies the performance of fagade integrated solar collectors with absorbers of polymeric materials. Absorbers with different colours have been integrated in a timber-frame wall with and without ventilated cavity. The temperature and the relative humidity have been measured in several layers of the wall construction behind the integrated collectors. The objective was to determine whether non-ventilated integration is possible in a wooden construction under Nordic climate. It was found that omitting the ventilated cavity did not represent a risk for high relative humidity and condensation inside the wall construction. The integrated collector fagade improved the U-value by more than 15% without that additional thermal insulation was added to the wall. Simulations have shown that fagade integration can be a way to overcome the mismatch between the availability of solar energy and the heating demand.

1. Introduction

Traditionally, solar collectors have been mounted on roofs. In the last decade, a considerable market growth was observed for combined solar systems for domestic hot water preparation and space heating (solar combisystems) in the middle and north of Europe. Requiring larger collector areas, building-integrated collector installations become a natural choice for solar combisystems. At high latitudes, as in Norway, the integration into the fagade represents an obvious alternative due to the low declination of the sun during the heating season from the middle of September to the end of April. Further, by introducing coloured absorbers, the fagade integration opens new opportunities for building planners and architects. Coloured fagade collectors can be seen as multi­functional building modules, providing energy, new possibilities of fagade design and surface protection for the building. Currently, considerable attention is given to fagade integration of solar collectors in several European countries, where metal absorbers, building physics and coloured coatings with spectral selectivity are subject of investigation (Tripanagnostopoulos, 2000; Bergmann and WeiR, 2002; Bergman and Muller, 2003; WeiR, 2003). The present work studies non-selective, polymeric solar collectors produced by the Norwegian company Solarnor. The study was carried out within the REBUS project (Competitive Solar Heating Systems for Residential Buildings, http://energi. fysikk. uio. no/rebus), a collaboration between Danish, Latvian, Swedish and Norwegian research institutes and companies, partially financed by Nordic Energy Research.

Examples of realized and planned collector fagades in Sweden and Norway (Source: Solarnor).

The temperature (T) and relative humidity (RH) in the layers of the collector fapade were measured since June 2003. The sensors were placed behind the collector field with — and without ventilated cavity at two different vertical positions (15 cm/ 95 cm and 140 cm measured from the lower end of the collector field) and in two different layers of the wall (see Fig. 2b and c). The temperature and the RH-signals were recorded by two temperature and voltage loggers, NI 4351 by National Instruments, and LabVIEW software. The instrument has ±0.12 K RTD-accuracy. The temperatures were measured by Pt-100 sensors and the relative humidity by absorption based humidity sensors from Honeywell, series HIH3610, with an accuracy of ±2%.

The layers of the standard timber-frame wall and the collector wall without ventilated cavity are compared in Table 1 for the present test house. The static U-value is 0.31 W/(m2K) for the regular timber-frame wall and 0.26 W/(m2K) for the collector wall without ventilated cavity. This represents an improvement of 16% without adding any additional thermal insulation.

The collector fapade will during cold periods have a higher temperature than a wall with standard exterior cladding (passive solar heating during solar system standstill). The temperature gradient will therefore be smaller than for a regular wall and provide a further reduction of the building’s thermal losses.

Table 1. Thermal resistance and U-values of the walls in the test house (source: Edvardsen and Torjussen, 2000)

Timber-frame wall with ventilated cavity (Fig. 2a)

Solar collector wall without ventilated cavity (Fig. 2c)

Layer

Thermal

resistance

[m2K/W]

Layer

Thermal

resistance

[m2K/W]

interior surface

0.13

interior surface

0.13

15 mm interior covering

0.12

15 mm interior covering

0.12

0.5 mm vapour barrier, polyethylene

0.05

0.5 mm vapour barrier, polyethylene

0.05

100 mm mineral wool

2.78

100 mm mineral wool

2.78

0.5 mm wind barrier

0.03

12 mm wind barrier (Fig. 2)

0.24

10 mm ventilated cavity

0.07

10 mm absorber

0.05

18 mm exterior wood cladding

10 mm cavity in collector

0.15

exterior surface Rse

10 mm collector cover, PC exterior surface Rse

0.32

0.04

Rtota [m2K/W]

3.18

Rtota [m2K/W]

3.88

static U-value [W/(m2K)]

0.31

static U-value [W/(m2K)]

0.26