## Exeruses

4.1 For seven collectors in series, each 1.2 m2 in area, FRiULi = 7.5 W/m2-°C, and Fr1(toi)i = 0.79 at a flow rate of 0.015 kg/s-m2, estimate the useful energy collected if water is circulated through the collectors, the available solar radiation is 800 W/m2, and the AT (=T — Ta) is equal to 5°C.

4.2 Repeat Example 4.2 for September 15 considering that the weather conditions are the same.

4.3 Find the Fr(ta)„ and FRUL for a collector 2.6 m2 in area with the following hour-long test results.

 Qu (MJ) It (MJ/m2) T (°C) Ta (°C) 6.05 2.95 15.4 14.5 1.35 3.05 82.4 15.5

4.4 For a collector with Fr(to>)„ = 0.82 and FRUL = 6.05 W/m2-°C, find the instantaneous efficiency when T = Ta. If the instantaneous efficiency is equal to 0, Ta = 25 °C, and T = 90 °C, what is the value of solar radiation falling on the collector?

4.5 The data from an actual collector test are shown in the following table. If the collector area is 1.95 m2 and the test flow rate is 0.03 kg/s, find the collector characteristics FR(ja)n and FRUL.

 Number Gt (W/m2) Ta (°C) T (°C) To (°C) 1 851.2 24.2 89.1 93.0 2 850.5 24.2 89.8 93.5 3 849.1 24.1 89.5 93.3 4 855.9 23.9 78.2 83.1 5 830.6 24.8 77.9 82.9 6 849.5 24.5 77.5 82.5 7 853.3 23.9 43.8 52.1 8 860.0 24.3 44.2 52.4 9 858.6 24.5 44.0 51.9

4.6 For a 5.6 m2 collector with F' = 0.893, UL = 3.85 W/m2-°C, (ro)av = 0.79, and flow rate = 0.015 kg/m2-s, find FR, Qu, and efficiency when water enters at 35°C, the ambient temperature is 14.2°C, and It for the hour is 2.49 MJ/m2.

4.7 The characteristics of a 2 m2 water-heating collector are FR(ja)n = 0.79 and FrUl = 5.05 W/m2-°C. If the test flow rate is 0.015 kg/m2-s, find the corrected collector characteristics when the flow rate through the collector is halved.

 Passive systems Active systems Thermosiphon (direct and indirect) Integrated collector storage Direct circulation (or open loop active) systems Indirect circulation (or closed loop active) systems, internal and external heat exchanger Air systems Heat pump systems Pool heating systems

Natural circulation occurs by natural convection (thermosiphoning), whereas forced circulation systems use pumps or fans to circulate the heat transfer fluid through the collector. Except for thermosiphon and integrated collector storage systems, which need no control, solar domestic and service hot water systems are controlled using differential thermostats. Some systems also use a load-side heat exchanger between the potable water stream and the hot water tank.

Seven types of solar energy systems can be used to heat domestic and service hot water, as shown in Table 5.1. Thermosiphon and integrated collector storage systems are called passive systems because no pump is employed, whereas the others are called active systems because a pump or fan is employed to circulate the fluid. For freeze protection, recirculation and drain-down are used for direct solar water heating systems and drain-back is used for indirect water heating systems.

A wide range of collectors have been used for solar water heating systems, such as flat plate, evacuated tube, and compound parabolic. In addition to these types of collectors, bigger systems can use more advanced types, such as the parabolic trough.

The amount of hot water produced by a solar water heater depends on the type and size of the system, the amount of sunshine available at the site, and the seasonal hot water demand pattern.