Know your renewables – Solar hot water system basics

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Solar hot water systems are steadily becoming more popular in Australia. Lance Turner explains the types and how they work.

Solar water heaters have been around in their modern form for almost 100 years. However, there is a lot of confusion between solar water heaters and solar photovoltaics, the common ‘solar panels’ that generate electricity directly.

Solar hot water (SHW) systems are what’s known as a solar thermal technology. They use the sun’s heat to heat water, either directly or indirectly. There is generally no electricity involved, except for the use of circulation pumps and backup boosting in some systems.

The basic design is that a flat panel that contains tubes for the water to flow through is connected to a storage tank. Water flows from the tank, is heated in the panel by the heat of the sun and flows back to the tank as heated water. However, there are a number of different configurations of tank and panels, and each has a different method of getting the water to the panels and back to the tank.

The simplest type is the close-coupled direct heating system. In this, the solar collector is mounted on the house roof with the water tank mounted directly above it. Water flows from the tank into the collector where it is heated by the sun. As warm water is less dense than cold water, the warm water rises up through the collector tubes and flows back to the tank as heated water, drawing colder water from the tank into the bottom of the collector for heating. This system is called thermosiphoning and is the most reliable and simple of the solar thermal water heating systems.

The other common system usually has the tank mounted at ground level, either inside or outside the house. A pump circulates water from the tank up to the collector, where it is heated and then flows back to the tank. A pump is needed in such systems as thermosiphoning only works when the tank is mounted above the panels. The pump is controlled by a special controller that has multiple temperature sensors in the tank and the collector.

This type of system is known as a remote-coupled or split system. It is more complex than a close-coupled system due to the added complexity of the pump and controller.

Panel types

While there are two main types of systems, there are also two main types of solar collectors. The first is the flat-plate collector, which is a flat, insulated box containing an array of pipes connected to a metal sheet, all painted black. The metal sheet absorbs incoming solar heat and transfers it to the attached pipes and hence the water inside them.

Read the full article in ReNew 125

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