Passive Solar Heating

Solar Water Heating Systems

Passive solar heating relies on gravity or natural convection to circulate heated household water or an antifreeze solution (propylene glycol) through the system without any pumps.

Characteristics of a passive solar water heater:

• Not recommended in climates with severe freezing.

• More reliable – no pumps or motors and other mechanical parts.

• Easier to maintain.

• Often less expensive than active systems.

• Not as efficient as active systems.

• Difficult to retrofit – storage tanks must be located above and close to the solar panel

First Some Definations:

Passive systems rely on gravity or natural convection to circulate fluids through the system.

Direct (open loop) systems circulate household or potable water through the solar panels. The heated water from the solar panel feeds directly into the storage tank.

Indirect (closed loop) systems use a heat transfer fluid; water or diluted antifreeze. The fluid is heated in the solar panel. A heat exchanger transfers the heat from the heat transfer fluid to the household or potable water.

Passive Solar Water Heater

Thermosiphon Solar Water Heating Systems

The Passive Direct Thermosiphon Systems always have the storage tank above the solar panel and rely on natural convection to circulate water through solar panels.

Freeze Protection: None

Hard Water Tolerance: minimal

Pros: Simple, no electricity is needed to operate the system. Ideal where hot water needs occur in the afternoon and evenings. Low maintenance

Cons: Solar panel must be located below the tank, this system is not suited for use with hard water (open loop system)

How the System Works: The storage tank is located above the solar heating panel or collector. When solar energy heats the water in the solar panel it becomes lighter and naturally rises into the storage tank.

This cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system.

At the same time, cooler water in the tank sinks to the bottom of the collector, causing circulation throughout the system. The storage tank is attached to the top of collector so the thermosiphoning can occur.

Passive Indirect Thermosiyphon Heating Systems use a heat exchanger located within the storage tank above the solar heating pane.

Freeze Protection: moderate

Hard Water Tolerance: good

Pros: Simple design, uses no electricity to operate

Cons: Solar panel must be located below the storage tank

How the System Works: Passive Solar Heating: Thermosiphon Passive Indirect Solar Heating Systems relies on heated water rising naturally to circulate water through the system.

A heat exchanger location inside the storage tank uses propylene glycol or other non-freezing fluids for freeze protection. The storage tank sits higher than the solar panels. The tank is located on the outside of the roof or inside the attic space above the solar panels. Other systems have tank located inside attic spaces above the collectors.

Application: They are commonly used for swimming pool heater. They also are ideal for summer homes or camps where the system would be drained and not function during the cold months. Freeze Protection: moderate, infrequent or light freezing

Hard Water Tolerance: minimal

Pros: Simple, no moving parts

Cons: Inefficient in cold climates, small systems only

How the System Works: Integrated Collector Storage (ICS) or Batch Systems are very simple solar heating panels made up of one or more black storage tanks in an insulated glazed box. The tank and solar panel are combined. Potable water is heated and stored in the ICS collector. As hot water is used, cold water fills the solar panel from the bottom.

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