Energetic behaviour of solar system based on facade-integrated collector has been investigated through a computer simulation. Simulation was aimed to characterise performance of facade solar system for hot water preparation in block of flats building and to obtain information on effect of facade collector on building performance.

Computer simulation was performed using Transient System Simulation Program — TRNSYS [6]. TRNSYS model for integrated facade collector was composed from a multizone model and the solar system model with thermal interconnection between them. Only a one zone in the central part of the considered building facade was modelled to investigate the block of flats building performance with facade collector. TRNSYS model used for simulations is shown in Figure 7. Facade construction was divided to two surfaces, one of them has been coupled to collector absorber (absorber temperature is identical with the last layer surface temperature).

Solar systems (facade, roof) were modelled as conventional ones — collector connected to storage tank with stratification. Facade solar collector with slope 90° was modelled as thermally coupled to building facade as described above. Roof solar collector was modelled separately with slope 45°. Thermal characteristics of the collectors were obtained from detailed simulation through KOLEKTOR model. Standard parameters of hot water were used (heating from 12 °C to 55 °C, max. temperature 85 °C). Solar systems have been modelled with high flow forced circulation (100 l/h/m2).

Two facade types were investigated for application of facade collector. First, middle-weight facade is common to panel block of flats buildings based on 27 cm ceramzit-concrete panels. These types of construction represent a wide range of buildings in large housing estates. Second, heavy-weight facade based on 45 cm brick represents the old buildings antecedent to the panel housing. Both types should be renovated with respect to construction problems, indoor comfort and energy consumption. In the model, applications with overall thermal resistance R =1,3 and 6 m2K/W for building envelope were considered. Windows with heat insulating glazing were used (U =1.7W/m2K). Overall surface area of zone facade is 9.0 m2, the window area is 3.2 m2, the wall is 6 m2. Splitting of the wall into two surfaces allows changing the collector / facade area ratio for parametric analysis.

Simulations were performed for different cases which can be considered in decision­
making for building renovation. Simulated cases were: panel wall — base case (envelope insulation with/without roof collector)

— integrated case (envelope insulation with facade collector) brick wall — base case (envelope insulation with/without roof collector)

— integrated case (envelope insulation with facade collector)

Parametric analysis for different facade construction resistances R, collector field surfaces Ac, required solar fractions and orientations were performed. Test reference year for Prague (TRY_Prague) was used as a climate database in the system and building simulations. Principal observed parameters for the building behaviour were energy consumption in winter season, overheating characteristics (inside temperatures, PPD values) in summer season and possible temperature-induced risk in construction. For the solar system, specific annual solar gains qs, u, solar fraction f system efficiency n and unutilizable energy qs, nu due to collector stagnation were the required outputs.

Results

Results of parametric simulations of solar fraction achieved by investigated solar systems (roof, facade) are shown in Figure 8. The solar fraction is plotted against the specific area of collector field Ac/Vaku. Interesting solar fraction values for facade collectors result from higher insulation levels (R = 3, 6 m2K/W). While the facade collector area should be increased by 30% compared to roof collector (45°) area to achieve 60 % solar fraction, for solar fraction above 70 %, the required facade collector area is comparable or lower.

However, roof collector gets to much more higher levels of stagnation conditions, which lead to possible operation problems and material degradation. Vertical position of the facade collector results in a well-balanced useful solar gains profile and very low level of unutilizable energy gains in comparison with the roof collector case.

In the Figure 8, the unutilizability factor bnu is introduced and plotted. Unutilizability factor is defined as a ratio of solar energy gains available from solar collector, but not used due to upper temperature limits (Tmax) in the storage tank, to total available solar energy gains Qs from collector field (solar system gains Qs, u utilized for water heating + unutilized Qs, nu).

b

Comparison of solar fraction f, specific solar system gains qs, u and solar system efficiency П, annual profiles for roof and facade solar system (R = 6 m2K/W) at annual solar fraction 60 and 70% is shown in Figure 9. Facade solar collector orientation impact on system gains and achievable solar fraction is shown in Figure 10 (compared with roof collector with south orientation).

Interaction of facade solar system with building has been investigated for winter (from October to April) and summer (from June to August) season. Performance analysis through collector-zone coupled modelling has been done for two usual building types, middle-weight (panel) and heavy-weight (brick). Results for winter season were put in the Table 1. With increasing heat insulation level, the heat gains caused by facade collector tends to be negligible.

South-oriented buildings often suffer with overheating problems in summer season. Temperature of the building envelope raises and considerable heat gains through the facade and windows could contribute to space overheating. Facade collector, due to good level of thermal insulation and absorber temperatures kept under 70 °C (low level of collector stagnation as resulted from system simulation), doesn’t cause notable temperature increase inside the building. Average temperature Tinside and PPD value (predicted percentage of dissatisfied) inside the zone adjacent to facade with collector were derived from frequency histograms for summer season and these are shown in Figure 11 in dependence on collector/facade area ratio (solar fraction 60 % and 70 %). Panel wall case, due to lower storage capability (middle-weight wall), results in higher average temperature values than brick wall case (heavy-weight wall). Application of facade collector thermally coupled to the wall moves the average temperatures no more than 1 K higher.

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Fig. 11 Average temperature Tinside and PPD in the zone with facade collector in summer

season (June to August)

Figure 12 shows the temperature profiles in the facade collector-building construction (middle-weight ceramzit panel) during the typical summer day from 8 am to 8 pm. There are individual modelled layers outlined in the figure. The solar system heats the storage tank and during the day the absorber temperature raises up to 70 °C. Thermal insulation layers are affected by absorber, the first layer extremely. It should be made of materials capable to withstand high temperatures (up to 150 °C), e. g. mineral wool. Next layers (ceramzit panel) are kept at moderate temperatures with minimal variation during the day. These layers are mainly affected by the temperature variation inside the building. Brick wall case behaves in similar way with lower variations in indoor temperatures with respect to higher inertia.