Energy Procedia 30 ( 2012 ) 1035 – 1041

1876-6102 © 2012 The Authors. Published by Elsevier Ltd.Selection and/or peer-review under responsibility of PSE AGdoi: 10.1016/j.egypro.2012.11.116

SHC 2012

Solar heating and cooling with transparent façade collectors in a demonstration building

Christoph Maurera, Thibault Pfluga, Paolo Di Lauroa, Joze Hafnerb, Friderik Knezb, Sabina Jordanb, Michael Hermanna, Tilmann E. Kuhna

aFraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany bSlovenian National Building and Civil Engineering Institute (ZAG), Dimičeva 12, SI-1000 Ljubljana, Slovenia

Abstract

Transparent façade collectors can provide solar heating and cooling and lead to primary energy savings even compared to an opaque wall. They can be simulated with a detailed physical model based on laboratory measurements. The demonstration installation in Ljubljana will provide extensive measurement data of transparent façade collectors in an outdoor environment. This allows a comparison with simulations performed with the model validated by laboratory measurements. This paper presents the demonstration installation and the measurement system, which are based on the new modelling abilities and focused on the outdoor validation of such models. © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of PSE AG Keywords: Transparent solar thermal collector (TSTC), transparent façade collector, demonstration building, adsorption, building-integrated solar thermal systems (BIST), variable g value, solar cooling, building-integrated solar systems (BISS)

1. Introduction

Building-integrated solar thermal systems (BIST) can provide solar heat with little extra costs compared to conventional building components. This opens the door to cost-effective primary energy savings. Within the European research project “Cost-Effective” [1], several building-integrated solar systems were developed. Another type of transparent façade collector is already commercially available [2], which indicates the need for scientific models to quantify the benefits of such components.

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Nomenclature

gross area of one collector [m2]

heat transfer to the interior [W/m2]

thermal resistance between the central glass pane and the glass pane facing

the interior [K/W]

temperature of the central glass pane [K]

temperature of the glass pane facing the interior [K]

2. BIST components

The following two BIST components from the European “Cost-Effective” project will be monitored at the demonstration site:

Fig. 1 presents a photo and a schematic drawing of air-heating collector tubes. Air is used as the heat transfer medium in the inner tube. The tubes have openings at both ends to allow for continuoushorizontal flow with low pressure drops. Further research is necessary to evacuate the space between the inner and outer tubes at reasonable costs. The air-heating collector tubes of the demonstration installation thus have argon between both tubes. The air-heating tubes are integrated into a glazing unit, which can easily be cleaned and fulfils all necessary safety standards.

Fig. 1. (a) Air-heating collector tubes (during installation). (b) Schematic-drawing with air as heat transfer fluid in the inner glasstube and glass panes facing the exterior and the interior.

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Fig. 2 presents a transparent solar thermal façade collector (TSTC) with water-based collector fluid. Openings combined with fixed, tilted slats in the absorber allow visual contact to the exterior for peopleinside the building, while direct solar radiation is simultaneously blocked. Due to small profile anglesduring winter, this collector can also provide seasonal shading.

Although both technologies combine visual transparency, solar control and renewable energy supply,this paper focuses on the water-based TSTC for which a detailed physical model based on laboratory measurements is already available for whole-year building simulations [3].

Fig. 2. (a) Transparent solar thermal collector (TSTC). (b) Schematic-drawing with absorber slats, piping and three glass panes.

3. Demonstration building

The Slovenian National Building and Civil Engineering Institute (ZAG) provided a building for thedemonstration installation in Ljubljana, Slovenia. Fig. 3 presents a photo of the demonstration buildingwith the positions of the BIST components. The air-heating collector tubes replace the balustrade on thefifth floor, while the TSTC are attached to the stairwell. Both collector areas face almost south, the TSTC15° towards east and the air-heating tubes 15° towards west. The components were developed to be a substitute for the building skin. Since it was not possible to replace walls and glazing in the demonstration building, the BIST components are not integrated into but attached onto the building, which still allowsall measurements needed to validate the models.

Fig. 4 presents a schematic drawing of the HVAC system. During the heating season, the BISTcomponents supply the thermally activated building system (TABS) of 100 m2 office space with fluid at temperatures above 35 °C. During the heating season, heat at temperatures above 65 °C is stored in a 3 m3

hot water tank to drive an adsorption chiller with a nominal cooling power of 8 kW. Excess heat isreleased by cooling towers and the cooling power is stored by a 1 m3 cold water tank to provide a fluid temperature of about 15 °C to the thermally activated building system. Conventional flat-plate collectorsare available as a backup heat source.

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Fig. 3. Photo of the demonstration building with the location of the air-heating collector tubes at the balustrade of the fifth floor (outlined in red) and the TSTC on the stairwell (outlined in green).

Fig. 4. Schematic drawing of the HVAC system (TSTC = transparent solar thermal collector, TABS = thermally activated building system).

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4. Measurement system

4.1. Solar radiation

Both the global and the diffuse irradiance on a horizontal plane are measured on-site. This allows the calculation of the irradiance for numerous directions of incidence [4-6]. An additional pyranometer measures the global irradiance on the collector plane for comparison to the calculated values.

4.2. External infrared heat transfer

The radiative heat exchange between the collector and its surroundings is measured with a pyrgeometer in the collector plane.

4.3. External convective heat transfer

The wind speed is measured by an anemometer and a meteorological station on-site measures the ambient temperature sensors. The temperature of the external collector surface is measured with locally shaded Pt100 sensors. The convective heat transfer coefficient between the external collector surface and the ambient air can then be calculated [7, 8] as well as the convective heat exchange between the collector and the ambient.

4.4. Collector gain

The mass flow of the collector fluid is measured for the whole collector field as well as the fluid inlet and outlet temperatures to calculate the collector gain.

4.5. Solar transmission

The demonstration installation does not allow measurements of the solar transmission with an accuracy comparable to laboratory measurements. A large number of small pyranometers covering the indoor-facing collector surface would be ideal to assess all possible shading configurations.

The current measurement concept includes two small, outward-facing pyranometers LI-200 on the indoor-facing collector surface: one aligned with the centre of a riser tube in the centre of the collector and the other aligned with the centre of a column of slatted openings in the centre of the collector. The data from these two sensors will enable the validation of daylighting simulations for the TSTC.

4.6. Internal heat transfer

The operative temperature of the building side of the TSTC is difficult to measure, because the TSTCs are attached to walls and the opening of the stairwells. Even if the temperature of the walls and the air were known, the convective internal heat transfer coefficient could only be determined by extensive measurements of the wind speed in combination with CFD simulations.

Therefore, the temperatures of the central glass pane Tcentre and of the glass pane facing the interior of the building Tback are measured. The thermal resistance Rcentre-back between the two glass panes can be simulated with two-dimensional thermal simulations. Additional temperature sensors are placed in the edge sealant to validate the thermal simulations. With the assumption of negligible solar absorption by the

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glass pane facing the interior, the heat transfer to the interior can be calculated with the collector gross area Acoll as

(1)

Even if the laboratory measurements should reveal significant solar absorption by the glass pane facingthe interior, the measured heat flux between the central and indoor glass panes can be compared to thesimulated heat flux between these panes to validate the simulation model.

5. Validation concept

Fig. 5 presents a schematic drawing of the validation concept. TSTC test models are measured in thelaboratory and modelled on the basis of the laboratory measurement results. The meteorologicalmeasurements at the demonstration site enable simulations of the demonstration installation with thisTSTC model. The results of the simulations of the demonstration installation can then be compared to theTSTC measurements at the demonstration site.

Fig. 5. Schematic drawing of the validation concept.

6. Conclusions

The demonstration installation offers the opportunity to evaluate the heating and cooling of transparent façade collectors in an outdoor environment. TSTC test models will be measured in the laboratory andmodelled based on these lab measurements. The demonstration installation can then be simulated andmeasured, and the results compared to assess the accuracy of models based on laboratory measurements.

Acknowledgements

The authors would like to thank ZAG and HIDRIA for the demonstration installation at their institute and IPB for designing the HVAC system. We also thank PERMASTEELISA, KOLLEKTORFABRIK and INTERPANE for the production of the demonstration collectors.

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