Passive Solar Water Heating Systems

Passive solar water heating systems uses no electricity, instead relying on heat driven convection to circulate water or heating solution between a collector and an elevated storage tank located above the collector. The principle of passive solar heating involves the density of the fluid decreasing as it heats up. As the fluid gets lighter, it rises to the top of the collector where it is drawn to the storage tank. Fluid which has cooled down at the bottom of the storage tank flows back through the collector. These passive systems can be divided into two main categories, the thermosyphon and theintegrated collector storageorbatch SWH systems.

Thermosyphon

In the thermosyphon system (Fig ), water comes from the overhead tank to the bottom solar collector via natural circulation and water circulates from the collector to the storage tank as a result of the water getting heated in the collector. Cold water at the bottom of the storage tank flows into the collector and displaces the hot water which is forced inside the insulated hot water storage tank. This circulation is dependent on the solar radiation on the collector and in its absence will come to a stop. Thermosyphon systems require minimal maintenance due to the absence of controls and instrumentation and coupled with its simplicity makes its viable for residential customers. Factors such difference between the collector and ambient temperature and intensity of solar radiation are integral in enhancing its efficiency (Shukla et al.,)

Batch

Batch system (also known as integral collector storage systems)  are passive systems consisting of one or more tanks that combine both collection and storage functions. One of the simplest designs of a batch system consists of a simple tank placed in an insulated box with a glass cover facing the sun as seen in the figure below ( Fig). The fact that it has a glazed side facing the sun allows it to act as a collector as well. Depending on the nature of the system, batch systems are inexpensive and simple to run as it has very few components, requires minimal maintenance and offers few equipment failures . However one of its major drawbacks is that it suffers from significant heat loss mainly at night, as the side facing the sun is mostly uninsulated. Effective measures such type of insulating materials, surface coatings and additional glazed glass can be used to reduces heat loss in turn enhancing efficiency.