Page 117 - A Comprehensive Guide to Solar Energy Systems

Chapter 6 • Solar Water Heaters 115

convection (passive system) through the collector to a coil heat exchanger at the bottom
of a cylinder tank (indirect system), where the heat is further transferred to a storage tank
or is used directly. The tank is usually insulated and may contain an auxiliary heater, for
example, electric immersion heater or conventional boiler for winter use [12].

6.3 The Classification of SWH Systems

6.3.1 Passive and Active Systems
Based on whether the SWH system requires pumps or not to function, SWHs are catego-
rized into two basic configurations: passive or active systems, as shown in Fig. 6.4 [13].
Passive systems transfer heat from the collector to the tank located above the collector by
natural circulation, which could supply hot water at a temperature of the order of 60°C,
and are the most commonly used solar water heaters for domestic applications [14]. Active
systems use an electric pump to circulate water through the collector. A check valve may
be required to prevent reverse water circulation [13].
The efficiency of an active SWH system is usually between 35% and 80% [15] while that
of the passive system is in the range of 30%–50% [14]. An advantage of the active system
is that the collector does not need to be close to the tank and hence can be used in mul-
tistory buildings. However, the drawbacks of the active system include: its dependent on
electricity and the fact that it is more complicated in nature and requires the need of ex-
perienced personnel to ensure optimal operation. This leads to the active system, having
much higher running costs than the passive system.
6.3.2 Direct and Indirect Systems

SWH systems, which do not include a heat exchanger, are called direct systems (shown in
Fig. 6.4) while the SWHs, which are fitted with heat exchangers, are called indirect systems

FIGURE 6.4 Schematic of the passive and active SWH systems [13].