The collectors

Site-built solar air collectors

Due to their simplicity, air collectors are often built onsite using semi-finished products. In most cases, these systems have a low efficiency because of:

• flow distribution inside the collector; and

• poor design of the heat transfer coefficients.

Therefore, site-built collectors are not recommended for high-performance housing.

Factory-built solar air collectors

Over the years, collectors have been engineered to improve their performance. The heat transfer can be improved by increasing the absorber surface area in contact with the air stream. This is done by:

• channelling the air flow first over the absorber surface, then beneath it;

• using an absorber with fins on the underside where the air flows; and

• using a black textile absorber through which the air flows.

The heat transfer between the absorber and the air stream can be increased by:

• creating obstructions in the air flow to cause turbulence and to destroy the insulating boundary layer of air at the absorber surface; and

• making a porous absorber.

To improve winter performance, selective absorber coatings are recommended for high-performance houses. The given figures of performance assume selective surfaces. Typical technical data as a guide for selecting a collector product:

Efficiency Curves for different Solar Air Collectors mass flow 40 kgh'W Surrounding air speed 3 ms 1

Efficiency Curves for different Solar Air Collectors mass flow 40 kgh'W Surrounding air speed 3 ms 1

(To-TlVO

Collectors with:

1 underflow, finned absorber, selective surface;

2 black textile absorber;

4 plane black absorber, underflow;

5 rippled absorber flow on both sides;

6 black painted façade element flow on both sides;

7 site built, trapezoid absorber underflow;

8 black façade element underflow.

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 12.1.3 Efficiency curves of different air collector types

Figure 12.1.3 Efficiency curves of different air collector types

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 12.1.4 Efficiency versus mass flow rate

Different collector types perform in different ways. For example, plane absorbers transfer less heat to the air stream. Collectors where air flows between the glazing and absorber have higher heat losses. The choice of collector depends on the desired air outlet temperature and the volume flow. Figure 12.1.3 shows the efficiency curves of different collector types. For a detailed examination, see Hastings and Mork (2000).

Due to the high sensitivity of the heat transfer, the flow rate of the collector strongly affects the efficiency, as seen in Figure 12.1.4.

The flow rate of an air collector influences not only the flow resistance and the heat transfer, but also the air outlet temperature. These, in turn, affect collector efficiency. A high flow rate increases the efficiency, but also the pressure drop, so more electricity is consumed to supply the needed fan power. Therefore, a compromise between high thermal efficiency and high electricity demand for the fan has to be found. Figure 12.1.5 gives an impression of the total efficiency.

To reduce the problem of high pressure losses or high flow rates inside the collector, individual collectors should be connected in parallel. The parallel collector array, however, increases the cost for manifolds and air ducts. Therefore, a mix of parallel and serial-connected collectors is needed.

Source: Morhenne Ingenieure GbR, Wuppertal

Figure 12.1.5 Collector efficiency: A comparison between thermal efficiency and efficiency considering fan electricity

Air collectors suitable for high-performance houses are characterized by a high efficiency and low pressure losses. In solar-assisted ventilation systems, the pressure loss is crucial if the existing fans of the ventilation system serve the solar collector. The collector configuration, in this case, should accept a wide range of flow rates.

Building integration

Collectors can be built into the roof or façade, providing the weather skin and part of the insulation of the building envelope. This helps to reduce costs, in theory; but the detailing must be carefully done to ensure rain and wind tightness. Moisture dissipation from the building into the collector is, in most cases, not critical because the collector is vented during operation. In addition, high-performance housing has to have a tight membrane in the wall or roof construction. Systems solutions are available for roof and façade integration.

Source: Andreas Gutermann, AMENA, Winterthur

Figure 12.1.6 A Swiss apartment building with integrated solar air collectors

Source: Andreas Gutermann, AMENA, Winterthur

Figure 12.1.6 A Swiss apartment building with integrated solar air collectors

12.1.6 Construction details: Dos and don'ts

The collectors can be integrated within the roof or the façade. In high-performance houses, hot water heating offers the greatest potential to save energy using solar energy. Therefore, integrating a bypass with an air-to-water heat exchanger is recommended. The collector area and performance should be determined by the space heating or ventilation heating demand. Accordingly, the system may even be over-dimensioned for summer water heating. The optimum collector tilt, in this case, is the latitude of the location when facing south. This is, however, a flat optimum. More important for the system performance is a short distance and a direct duct from to the collector to the heat exchanger, hypocausts or ventilation system. Air ducts have a large surface area to lose heat, so duct runs should be as short as possible.

Dos and don'ts

• Insulate air ducts facing to the ambient (still better, avoid having them).

• Avoid changes of air velocity inside the air system.

• The air velocity should not exceed 2 m/s to 3 m/s.

• Use air-tight dampers and test them to be sure that they are air tight.

• Ensure that ducts and junction boxes are air tight (leaks reduce performance).

• Use special sealing, collars, etc where air ducts penetrate the vapour and wind barriers of the building (this is especially critical in high-performance housing).

References

Fechner, H. and Bucek, O. (1999) 'Vergleichende Untersuchungen an Serien-Luftkollektoren im Rahmen des IEA Tasks 19', 9. OTTI-Symposium Thermische Solarenergie, S.91-95 OTTI Energie Kolleg, Wernerwerkstr. 4, D-93049 Regensburg, www.otti.de Hastings, S. R. and Mork, O. (eds) (2000) Solar Air Systems: A Design Handbook, James and James

(Science Publishers) Ltd, London, www.jxj.com Morhenne, J. (2002) Solare Luftsysteme: Themeninfo II/02, Bine Informationsdienst Fachinformationszentrum Karlsruhe, www.bine.info

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