Conventional solar systems are designed to maximise annual solar gains. The basic principle to achieve this is proper orientation and slope of collector field. In the central Europe conditions, maximum annual irradiation is received with a surface with south orientation and slope between 35° and 45°. In the case of facade collectors with slope 90°, the reduction in annual irradiation sum is around 70 %. Figure 2 shows annual profile of daily solar irradiation for roof (45°) and facade (90°) collector based on test reference year for Prague (TRY_Prague). Comparison shows a large difference between summer peak and cold season values for roof collector and relatively uniform profile for facade collector which corresponds closely to hot water demand profile (approx. constant with decrease in summer season). This feature allows the design of solar systems with a high solar fraction (above 50 %) without extremely increased periods of collector stagnation in summer as it appears in roof mounted systems with the same solar fraction.

Solar collector performance generally depends on optical and thermal losses which determine the efficiency of solar energy conversion in the collector. A detailed mathematical model KOLEKTOR was used for an investigation of solar collector thermal performance based on the knowledge of thermal processes in the individual parts of collector. Model consists of absorber outer energy balance (heat transfer through glazing, air gap, frame and absorber surface) and absorber inner energy balance (heat transfer within the absorber fins with solar radiation and piping). Absorber outer energy balance determines the temperature dependent overall U-value of the collector. Absorber inner energy balance obtains the collector performance factors dependent on the absorber material and geometry (F, FR). In the model, the temperature distribution in the collector is solved in an iterative loop from the input parameters. Input parameters are solar collector properties (dimension, physical properties of individual parts), climate data and operation parameters (input temperature, mass flow). Useful gain, efficiency of solar energy conversion and temperature of heat transfer fluid leaving the collector are the outputs from the model. The model was created in Excell sheet processor with use of Visual Basic programming.

The mathematical model was experimentally validated in the research of solar collectors with different covers (single, multiple, transparent insulations) and absorbers (non­selective, selective) [1]. For a specified set of operation conditions, a collector standard efficiency n can be determined in dependence on reduced temperature difference (Tm — Ta)/G. Model is feasible for sensitivity analysis in solar collector research. It has been used for an estimation of facade collector performance compared to roof collector. Further model description can be found in [1,2].

Facade integrated collector, when compared with collector located on the flat roof
(collector slope optimal 45°), shows considerably reduced heat transfer coefficients, especially for:

■ natural convection in the gap between the absorber and glazing

■ wind-related convection

■ back and edge frame heat loss coefficient

Due to vertical orientation of the air gap between absorber and glazing, heat transfer due natural convection is reduced in comparison with the gap under 45°slope to approximately 80 %. In Figure 3, Nusselt number in dependence on the slope of air layer according to different authors and the correlation obtained with statistical methods from the published experimental results is shown [1]. The correlation was used in the solar collector model. Since the air gap is a critical part in the single glazed selective collector, this reduction is reflected in overall collector heat loss coefficient. Standard solar collector efficiency curves determined for different slope angles (20°, 45°, 70°, 90°) are compared in Figure 4. Curves were calculated with use of mathematical model KOLEKTOR for standard weather conditions (ambient temperature Ta = 20 °C, incident solar radiation G = 800W/m2, wind velocity w =4 m/s). From the comparison, the slope impact on collector performance has been shown significant, especially for higher temperatures.

Fig. 3 Comparison of Nusselt number for Fig. 4 Solar collector efficiency curves for inclined air layer based on experiments by different slope angles

different authors

Calculation of wind-related forced convection heat transfer coefficients for solar collectors is not a distinct problem. There is a large number of models, which gives completely different transfer coefficient values in dependence on wind velocity. Some of them result from very detailed wind tunnel measurements, the other from measurements in real turbulent wind, but only for specified conditions and collector-building configuration. In solar engineering, McAdams’s [3] simple linear model

hw = 5.6 + 3.8 w

is regarded as reliable for usual heat transfer coefficient calculation. Sparrow et al. [4, 5] carried out a number of experiments to investigate the local heat transfer coefficients on the heated plates (e. g. solar collectors) under airflow at different conditions (angles of inclination of the plate relative to oncoming airstream, different velocities, framing surfaces). It was realised that average convection heat transfer coefficients are practically independent of the incident angle of airstream. From the airflow patterns on the plate, a considerable difference between the local heat transfer coefficients in the center of plate and on the edges was found. Higher velocity on the edge leads to higher local heat transfer coefficient, while the coefficients near the center are lower due the airflow stagnation. Consequently, the average convection heat transfer coefficients can be
substantially reduced when thermally active surface (solar collector) is framed by another thermally inactive surface (facade surface). Sparrow gives an equation to obtain the rate of reduction of the average heat transfer coefficient [5]

h/h* = (Lc/Lf)1/2 where h and h* respectively denote the coefficients in the presence and in the absence of the frame. The hydrodynamic dimensions Lc (collector) and Lf (framing surface) are determined as characteristic lengths from

L = 2L1La/(L1 + L>)

In the case of flat roof located solar collector and facade collector integrated in the building envelope, different values of wind-related heat transfer coefficients will be achieved. While at the glazing surface of roof collector, the average heat transfer coefficient corresponding to wind velocity is present, the average heat transfer coefficient at the surface of facade collector is lower due to framing effect (see Figure 5). For an investigated common block of flats case, the wind-related coefficient can be reduced to 60-80 % of the value for roof located collector.

Back and edge heat loss coefficient is reduced to minimum in dependence on thermal resistance of adjacent facade construction and "outer" collector frame temperature at value 20 °C (room temperature).

Synergetic impact of these individual heat transfer reductions is shown in the Figure 6 through the standard efficiency curves comparison for the roof and the facade collector with adjacent construction thermal resistance R =1,3 and 6 m2K/W. Facade integration brings qualitative improvement in solar energy conversion efficiency and better thermal performance especially for increased collector-ambient temperature differences.