Energy

Important considerations for solar water heating system storage cylinders include:

• cylinder size
• cylinder location
• control of the boosting element
• position of the heating elements.

They must also:

• be able to handle expected temperatures – stainless steel and copper cylinders are frequently used
• have sufficient capacity to meet both peak loads and overall daily demand without requiring the use of a booster heating system.

Cylinder location

The cylinder location depends on whether heat is transferred by thermo-siphon or pump and whether an open loop (direct) or closed loop (indirect) system is used.

For thermo-siphon systems, relative heights of the cylinder and the panel must be carefully designed to allow a natural thermo-siphon to occur.

Some open loop (direct) solar water heating systems incorporate a solar hot water cylinder, while others can be used with a remote or existing hot water storage cylinder. The storage cylinder may be on the roof just above the solar panels or separate, e.g. inside the roof space or fixed to the exterior of the building.

The weight of the water must be considered when sizing framing for roof-mounted or ceiling space cylinders, and they must have earthquake restraints.

In buildings with low pitched roofs or limited attic space and in retrofit situations, the cylinder may need to mounted outside on the roof, which means there may be increased heat losses from the cylinder.

Allow for maintenance access to the cylinder and other components of the water heating system.

Cylinder size

The storage cylinder should be larger than for a single energy source cylinder such as gas or electric, so that when there is good solar gain, the cylinder can store the heat that is available.

Control of the boosting element

It is very important the any electric element inside a solar water heater is controlled so that it doesn’t simply come on first thing in the morning when people have a shower and there is only a small amount of solar energy available to heat the water. This controller should include a timer to allow the element to be turned off in the mornings. Note the requirements for G12/AS2 for Legionella control.

Position of heating elements

Positioning a second heating element in the top of the cylinder allows only the water about the heating element to be heating when boosting is required.