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Field Test Best Practices: A Resource for Practical Residential Building Science

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Solar Water Heaters

Solar water heaters collect heat from the sun to heat water for domestic water use. There are a wide range of different types of residential solar water heaters, ranging from simple to very complex. Usually, a solar water heater consists of a collector that sits on the roof or on the ground near the house and a tank for holding the newly hot water. In most climates, backup heating is required to supplement the solar water heating in the event of cloudy days. Backup heating can be provided directly in the storage tank (either electric resistance elements or a gas burner) or could be found in a separate water heater. In warm climates, water is warmed in the solar collectors and fed back into the home's domestic hot water system. In colder climates where freezing is likely, a fluid such as glycol is heated in the collector instead and passes through a heat exchanger to transfer the heat to the domestic water system. This arrangement prevents freezing and increases the longevity of the system.

A solar water heater consisting of many vertical evacuated tubes is shown. The evacuated tubes prevent heat loss from the enclosed water pipes via convenction.

Solar thermal water heater
with evacuated glass tubes
around copper pipes act
as the solar collector
Credit: NREL PIX 09501

Some common configurations for the solar collector are shown in the figures on this page. One of the most efficient, but most expensive, types of collectors includes evacuated glass tubes around copper water/liquid pipes painted black. The evacuated tubes prevent convective losses from the copper pipes, but the assembly is expensive and the vacuum can be lost in time, reducing the efficiency. The glass and copper assembly is also very heavy, making shipping costs higher and requiring more support if placed on a roof. A simpler version of this system involves black-painted copper tubes inside a flat collector with a glass outer surface. This is a more common setup that helps to reduce the convection from the copper tubes without the added cost of the evacuated tubes.

Another arrangement for the collectors is to have an integrated collector and storage unit, as shown below. This can be as simple as a smaller tank painted with an absorptive paint or a hybrid between that and the typical flat plate collectors. The system shown in the following figure has several copper storage cylinders sitting below a glass cover. Water (or water and antifreeze) flows through all the storage cylinders when there is demand, otherwise water sits in the cylinders and warms up.

An integrated collector storage solar thermal water heater is shown. This type of water heater holds a larger volume of water in the collector and heats it until hot water is called for in the house.

Integrated collector storage solar water heater
Credit: NREL PIX 19617

Other arrangements aimed at lower costs do not include any glass or metal, which are both heavy and expensive, and are made entirely of polymers. These systems are less efficient but the reduction in cost is significant and makes up for the reduced efficiency.

Recommended quantities to monitor in a field test of a solar hot water heater: