Solar thermal collectors and applications

Thermosyphon systems (passive)

Thermosyphon systems, shown schematically in Fig. 24, heat potable water or heat transfer fluid and use natural convection to transport it from the collector to storage. The water in the collector expands becoming less dense as the sun heats it and rises through the collector into the top of the storage tank. There it is replaced by the cooler water that has sunk to the bottom of the tank, from which it flows down the collector. The circulation continuous as long as there is sunshine. Since the driving force is only a small density difference larger than normal pipe sizes must be used to minimise pipe friction. Connecting lines must be well insulated to prevent heat losses and sloped to prevent formation of air pockets which would stop circulation. At night, or whenever the collector is cooler than the water in the tank the direction of the thermosyphon flow will reverse, thus cooling the stored water. One way to prevent this is to place the top of the collector well below (about 30 cm) the bottom of the storage tank.

The main disadvantage of thermosyphon systems is the fact that they are comparatively tall units, which makes them not very attractive aesthetically. Usually, a cold water storage tank is installed on top of the solar collector, supplying both the hot water cylinder and the cold water needs of the house, thus making the collector unit taller and even less attractive. Additionally, extremely hard or acidic water can cause scale deposits that clog or corrode the absorber fluid passages. For direct systems, pressure - reducing valves are required when the city water is used

directly (no cold water storage tank) and pressure is greater than the working pressure of the collectors.

There have been extensive analyses of the performance of thermosyphon SWH, both experimentally and analyti­cally by numerous researchers. Some of the most important are shown here.

Gupta and Garg [129] developed one of the first models for thermal performance of a natural circulation SWH with no load. They represented solar radiation and ambient temperature by Fourier series, and were able to predict a day’s performance in a manner that agreed substantially with experiments.

Ong performed two studies [130,131] to evaluate the thermal performance of a SWH. He instrumented a relatively small system with five thermocouples on the bottom surface of the water tubes and six thermocouples on the bottom surface of the collector plate. A total of six thermocouples were inserted into the storage tank and a dye tracer mass flow meter was employed. Ong’s studies appear to be the first detailed ones on a thermosyphonic system.

Kudish et al. [132] in their study measured the thermosyphon flow rate directly by adapting a simple and well-known laboratory technique, a constant level device, to a solar collector in the thermosyphon mode. The thermo­syphon flow data gathered were utilised to construct a standard efficiency test curve, thus showing that this technique can be applied for testing collectors in the thermosyphon mode. Also, they determined the instan­taneous collector efficiency as a function of time of day.

Morrison and Braun [133] have studied system model­ling and operation characteristics of thermosyphon SWH with vertical or horizontal storage tank. They found that the system performance is maximised when the daily collector volume flow is approximately equal to the daily load flow, and the system with horizontal tank did not perform as well as a vertical one.

Hobson and Norton [134] in their study developed a characteristic curve for an individual directly heated thermosyphon solar energy water heater obtained from data of a 30 days tests. Using such a curve, the calculated annual solar fraction agreed well with the corresponding value computed from the numerical simulation. Further­more, the analysis was extended, and they produced a simple but relatively accurate design method for direct thermosyphon solar energy water heaters.

Shariah and Shalabi [135] have studied optimisation of design parameters for a thermosyphon SWH for two regions in Jordan represented by two cities, namely Amman and Aqaba through the use of TRNSYS simulation program. Their results indicate that the solar fraction of the system can be improved by 10-25% when each studied parameter is chosen properly. It was also found that the solar fraction of a system installed in Aqaba (hot climate) is less sensitive to some parameters than the solar fraction of a similar system installed in Amman (mild climate).

Solar thermal collectors and applications

Collector thermal efficiency

In reality the heat loss coefficient UL in Eqs (2) and (42) is not constant but is a function of collector inlet and ambient temperatures. Therefore: TOC o "1-5" h …

Global climate change

The term greenhouse effect has generally been used for the role of the whole atmosphere (mainly water vapour and clouds) in keeping the surface of the earth warm. Recently however, …

Limitations of simulations

Simulations are powerful tools for process design offering a number of advantages as outlined in the previous sections. However, there are limits to their use. For example, it is easy …

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