Page 120 - A Comprehensive Guide to Solar Energy Systems

118 A COMPrEHEnSIvE GUIdE TO SOlAr EnErGy SySTEMS

Table 6.1 Comparison of the Evacuated-Tube and Flat-Plate Collectors [13]
Evacuated-Tube Collector Flat-Plate Collector
Heat production Rapid Slow
Heat losses during daytime Negligible High
Influence of the sunrays’ incidence Maximum solar absorption through Maximum solar absorption at noon
angle the day
Cold weather operation Satisfactory performance Limiting effect, freezing risk
Maximum operating temperature Above 95°C Up to 80°C
range
Cost-effective Advanced technology at competitive Old technology at higher price
price
Hot water availability Greater number of days throughout Lesser number of days throughout
the year the year
Position of the collector Assembled onto the roof surface Preassembled

6.4 Most Advanced Technologies of SWHs

Three kinds of the most advanced technologies of SWH systems: (1) SWHs with PCMs, (2)
SWHs with lHPs, and (3) SWHs with MCHPs.

6.4.1 SWHs With Phase Change Materials
PCM, also called latent heat storage material, has a high capability of storing and releasing
large amount of heat within a constant or a narrow temperature range [1,18,19]. Two prop-
erties that make PCMs attractive in SWH systems are: their compactness and also their
small volume change during a phase change [20–22]. The SWH systems, which include
PCMs, can be divided into two types: those where the PCM is directly linked to the solar
collector and those where the storage unit is filled with the PCM.
A schematic diagram of a flat-plate solar collector involving a PCM is given in Fig. 6.7.
The impure PCM surrounds the solar collector tubes and is covered with black absorber
[23]. Typically the system can maintain an operating temperature of the collector of under
40°C for 80 min with a constant solar radiation of 1000 Wm [22]. Such systems have been
−2
shown to have efficiencies of between 42% and 55% higher than that of conventional SWH
systems [24].
A refined version of this type of SWH system was investigated by Chen et al. [25]
(Fig. 6.8) and has the tubes of the flat-plate solar collector embedded within a high porous
aluminum foam incorporating paraffin. They found that the performance of the system
was improved significantly compared with the paraffin system without aluminum foam as
can be seen in Figs. 6.9 and 6.10 [25].
SWH systems involving PCMs in the storage unit have been investigated by Tarhan
et al. [26]. The PCMs they used were lauric acid and myristic acid. The results showed that
the lauric acid storage could retain a stable water temperature, and the myristic acid stor-
age could reduce the heat losses during night.