Question

‏Solar water heating systems using air as a working medium are considered as passive systems as fans and blowers are required and not circulating pumps ،it is right or wrong?

Answer 1

Passive systems do not use a pump to circulate water from the collector to storage or other locations. They employ three means: gravity, the tendency of hot water to rise above cold water and water pressure. Passive solar water-heating systems can be categorized as either direct or indirect.

The passive solar water heaters are simple heating systems that utilize the free solar heat energy without the use of external energy to transfer hot and cold solar fluids. This is the most popular way of harnessing free solar energy as they are the least expensive, simple and reliable.

Such a system uses the principle of physics (heated water rises to the upper part of the tank) to transfer the heat from the solar collector to the storage tank. Passive systems can be used for direct domestic water heating or indirectly by circulating the solar heat transfer fluid through the heat exchanger.

Because of its simplicity, passive solar water heater systems are more vulnerable than other solar heating systems.

Types of passive solar water heater systems

Two passive solar water heater systems are the most popular today:

• Integral Collector Storage System
• Thermosyphon solar heater system

Passive Integral Collector Storage System - Batch Water Heater:

Integral Collector Storage systems are passive as they don't require pumps for the operation. They are also called batch water heaters, and they are the only solar heating systems that do not need the storage tank. They consist of the ICS collector and piping only.

The water storage tank is the solar collector at the same, has an inlet pipe connected to the bottom of the tank from the house plumbing and from the top of the collector hot water is usually connected to the backup storage heater. Whenever you open the hot tap, the pressure from the home plumbing moves the hot water from the top of the solar collector/tank as the cold water is pushed to the bottom. If the solar fluid from the ICS collector is hotter than the temperature setting on the backup unit than the heater will not activate.

These passive solar water heater systems are integral as the big cylindrical storage tank is used as the solar collector. People are using the batch water heater for hundreds of years, as they are simple to design and build. This passive batch system is using the south oriented insulated glazed box with a tank inside, filled with water.

This is also an open-loop system as the cold domestic water is heated directly. Due to its weight, special care should be taken if installing the batch water heater on the roof, which is the best spot.

Batch passive solar water heater is the best option for your money due to its simple and low-cost design, and if you live in the warmer areas. This passive system is very popular in the southern parts, tropical areas, vacation homes and recreational facilities where it is used only during the summer. During the rest of the year, especially cold weather, water should be drained.

Thermosyphon: Thermosyphon solar water heaters are probably the most popular solar heating systems.

Thermosyphon work is based on the principle of physics where heated water rises, so the solar storage tank has to be installed above the solar panels. The main components of the thermosyphon passive systems are the solar storage tank, panels, pipes, and valves.

If you live in the warmer area, you might want to consider direct, open-loop thermosiphon heating system, where domestic water is heated directly inside the flat plate solar collectors or panels. Usually, the bottom of the storage tank is connected to the bottom of the collector while the top of the collector is linked to the top of the collector, transferring the heated water to the tank and from there to the backup heater.

The main disadvantage of the passive thermosyphon systems is that they are vulnerable to conditions like hard water, as the flat plate collectors are constructed of the small pipes, which can easily become clogged. One of the solutions is to use softener.

If you are located in the colder areas, where there is a danger that water inside the collectors can freeze up, the recommendation is to use the passive indirect closed-loop system filled with antifreeze solution, usually propylene glycol. A heat exchanger, inside or outside the heater, is used to transfer the heat from the solar fluid to the domestic water. This is what eliminates the problem caused by hard water. A solar storage tank in thermosyphon systems has to be well insulated to reduce the standby heat losses, especially during the night.