How Solar Thermal Works

One small way to incorporate the use of renewable energy in a home is to install a solar water heating system.

Water heating is one of the largest consumers of energy in your home. Use of a solar water heater, with a backup electric or gas water heater, can cut those costs in half, according to ENERGY STAR.

Instead of using sunlight to make electricity, a solar thermal water heating system captures sunlight to use as a heat source. A solar water heater makes this possible by using a solar collector and a storage tank.

These systems almost always require a conventional storage water heater as a backup system for cloudy days and times of increased demand.

System Components

A flat plate collector.

The most common solar collection option is a flat-plate collector, which is basically an insulated box covered with tempered glass. Inside the box is a series of parallel copper tubes that serve as passageways for water flow. Flat-plate collectors resemble standard solar panels and two are usually enough to service a family of four.

There is a more efficient alternative called an evacuated tube collector, but it can cost twice as much per square foot. An evacuated tube collector is not flat and has a row of large glass tubes that act like thermoses. Within each large tube is a smaller glass or metal tube used to warm the water. This type of collector is suitable for regions that experience temperatures below freezing.

This sums up the basic structure of a solar thermal heating system. However, the way a system transports the heated water can be described as either active or passive and there are several options available under these two distinctions.

An active system uses electric pumps and controls to transport water from the solar collector to the storage tank. A passive solar thermal heating system has no pumps. Instead, it simply uses forces of nature to transport the water.

An example of a batch system.

These are the two types of passive systems:

  • A batch system, also called an Integrated Collector-Storage system, is as uncomplicated as it gets. It consists ofone or more dark colored water tanks within an insulated box that acts as a solar collector. The water warms up inside the water tank(s) and is transported to a home’s pipes by gravity or natural convection (the tendency of hot water to rise). This system is not recommended for cold climates.
  • thermosyphon system uses a flat-plate or evacuated tube solar collector to heat water. Once the water is heated, natural convection pumps the hot water into a storage tank separate from the collector. Then, the water moves from the tank into the home’s pipes.

There are three types of active systems:

  • direct system uses electric pumps and controls to move heated water from a solar collector straight to a storage tank or a tankless water heater. This system only works well in climates where it rarely freezes.
  • An in-direct system uses a solar collector to heat non-toxic antifreeze, rather than water. The antifreeze then flows into a heat exchanger, which is surrounded by water in a storage tank. The fluid in the exchanger heats the water, which is then pumped into a storage tank. The antifreeze then flows back to the collector. This system works in freezing climates.
  • drainback system is like an indirect system except it uses distilled water instead of antifreeze. The system has a separate “drainback” tank for the distilled water. When the water is heated in the solar collector, it moves to the drainback tank until it is needed to warm the water in the main storage tank. Once it warms the storage tank, it can move back to the collectors or the drainkback tank if the collectors are too hot.

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Got questions? Let us help.

We are here to help homeowners and businesses along the process of going solar. If you have any questions throughout the process, please call our office or send us an email.

If you are interested in solar, fill out an application and we will distribute it to three local solar installers who will contact you to provide you a quote.

How much does a solar thermal system cost?

The size and type of system you choose will play a big roll in how much the system will cost.

The CPS Energy Solar Water Heater rebate can only be used if the solar heater is replacing an electric water heater.The estimated number of kWh saved annually will be multiplied by a rate of $0.60. Rebates for solar water heating systems will have a maximum allowable rebate amount of $2,000.

The Federal Solar Tax Credit offers 30% tax credit for the cost of your system, with no upper limit.You simply reduce the federal taxes you owe by 30% of the cost of your system after any rebates are received. Any unused tax credit can be carried over to the following year. Further information can be found at: or from your tax professional.

How much space does the system need?

The space required for a solar thermal system will vary depending on the type of system you use and  your water usage needs. The installer will be able to customize a plan that is suitable for the space available.

Do the panels need to face south?

Unlike photovoltaic panels, solar hot water panels do not have to face directly due south. Panels can be mounted up to 30 degrees off due south.

What happens on a cloudy day?

Even though the heat output of the solar collector is reduced on cloudy days it should still provide heating. If it is an especially cloudy or rainy day, conventional heating may be required to maintain water at the required temperature. Usually systems can be automated so you don’t have to worry about running out of hot water on a rainy day.

How much will the energy saved be worth?

The average solar thermal system in San Antonio will save approximately 2,450 kWh of electricity per year. At the current energy rates, the energy saved will equal to about $220 per year.

How long will a solar water heating system last?

If properly designed and installed correctly, a solar water heating systems can last 20 years or more. Most come with a warranty of five to 10 years.

Do I need to install a new roof before the solar panels are installed?

If your roof is older and needs to be replaced in the near future, you will want to replace it at the time that the solar thermal system is installed to avoid the cost of removing and reinstalling your solar thermal system.

Does the solar thermal system need to be mounted on my roof?

Panels are often mounted on the roof, but can also be mounted on the ground if the space is available. Keep in mind, wherever the panels are located needs to be free of shade. Roof mount systems are most common in residential installations.