A solar water heater is a device that captures sunlight to heat water. It can be an economical way to generate hot water for your family (for shower and bath).

A solar heater not only enables substantial energy savings as solar power is free in contrast to natural gas or fuel oil. Moreover, it is a way to produce hot water for sanitary use throughout the year without emitting any CO₂.

1m² of roof = 100m³ of natural gas/year

In our latitudes, the sun shining on 1m² of roof replaces 100 l of heating oil or 100 m³ of natural gas (approximately 1 000 kWh) a year. Generally speaking, it is possible to heat 50 to 70 % of the water used in the kitchen and the bathroom in this way. However, a back-up heating system is required for times when there is insufficient luminosity.

Supplying a washing machine or dishwasher with this hot water also cuts out the electricity used by these appliances to heat the water and shortens the washing cycles, as long as certain precautionary measures are taken.

Hot water all year round

The efficiency of the collectors is at its highest at midday, in summer, when the sky is cloudless, and when the collectors face south.

However, the collectors also work well in other seasons, when the sky is cloudy, for a good part of the day, even if they face east or west.

Likewise, the efficiency is best at a gradient of 35° to the horizon, but good results can be achieved with collectors fitted vertically to a façade.

An investment of € 4,500 to € 6,000

A basic solar water heater (i.e. for a family of 4 people, demanding 200 to 300 litres, with 3 to 5 m² of solar collectors) costs on average € 4,500 to € 6,000, including VAT and installation. To help you with this investment, the Brussels Region gives you an energy grant, which is supplemented by certain communes.

How does a solar water heater work?

1. Light is absorbed by solar collector

The solar water heater absorbs light by means of a collector placed on the roof and converts it into heat.

2. Heat transferred to water tank

The heat is then passed to a water tank by means of a circulating pump. This exchange is triggered by the thermal regulator, but exclusively when the collector is hotter than the water in the tank. This not only prevents the circulating pumps from needlessly using electricity but conversely, also avoids overheating.

3. Insufficient sunlight? Back-up heating system

When there is insufficient sunlight, the water is preheated and a back-up system takes over to bring the water to the required temperature. This system can therefore be used at a constant temperature throughout the year.

Solar Water Heaters

olar water heaters — also called solar domestic hot water systems — can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use — sunshine — is free.

HOW THEY WORK

Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don’t.

Active Solar Water Heating Systems

There are two types of active solar water heating systems:

• Direct circulation systems
  Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.
• Indirect circulation systems
  Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.

Passive Solar Water Heating Systems

Passive solar water heating systems are typically less expensive than active systems, but they’re usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems:

• Integral collector-storage passive systems
  These work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs.
• Thermosyphon systems
  Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. They are usually more expensive than integral collector-storage passive systems.

STORAGE TANKS AND SOLAR COLLECTORS

Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.

Three types of solar collectors are used for residential applications:

• Flat-plate collector
  Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors — typically used for solar pool heating — have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
• Integral collector-storage systems
  Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
• Evacuated-tube solar collectors
  They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin’s coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.

Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a tankless or demand-type water heater for backup.

SELECTING A SOLAR WATER HEATER

Before you purchase and install a solar water heating system, you want to do the following:

• Estimate the cost and energy efficiency of a solar water heating system
• Evaluate your site’s solar resource
• Determine the correct system size
• Investigate local codes, covenants, and regulations.

Also understand the various components needed for solar water heating systems, including the following:

• Heat exchangers for solar water heating systems
• Heat-transfer fluids for solar water heating systems

INSTALLING AND MAINTAINING THE SYSTEM

The proper installation of solar water heaters depends on many factors. These factors include solar resource, climate, local building code requirements, and safety issues; therefore, it’s best to have a qualified solar thermal systems contractor install your system.

After installation, properly maintaining your system will keep it running smoothly. Passive systems don’t require much maintenance. For active systems, discuss the maintenance requirements with your system provider, and consult the system’s owner’s manual. Plumbing and other conventional water heating components require the same maintenance as conventional systems. Glazing may need to be cleaned in dry climates where rainwater doesn’t provide a natural rinse.

Regular maintenance on simple systems can be as infrequent as every 3–5 years, preferably by a solar contractor. Systems with electrical components usually require a replacement part or two after 10 years. Learn more about solar water heating system maintenance and repair.

When screening potential contractors for installation and/or maintenance, ask the following questions:

• Does your company have experience installing and maintaining solar water heating systems?
  Choose a company that has experience installing the type of system you want and servicing the applications you select.
• How many years of experience does your company have with solar heating installation and maintenance?
  The more experience the better. Request a list of past customers who can provide references.
• Is your company licensed or certified?
  Having a valid plumber’s and/or solar contractor’s license is required in some states. Contact your city and county for more information. Confirm licensing with your state’s contractor licensing board. The licensing board can also tell you about any complaints against state-licensed contractors.

IMPROVING ENERGY EFFICIENCY

After your water heater is properly installed and maintained, try some additional energy-saving strategies to help lower your water heating bills, especially if you require a back-up system. Some energy-saving devices and systems are more cost-effective to install with the water heater.

 

How It Works — Solar Water Heaters

Solar water heaters come in a wide variety of designs, all including a collector and storage tank, and all using the sun’s thermal energy to heat water.

Solar water heaters are typically described according to the type of collector and the circulation system

.Batch collectors, also called Integrated Collector-Storage (ICS) systems, heat water in dark tanks or tubes within an insulated box, storing water until drawn. Water can remain in the collector for long periods of time if household demand is low, making it very hot. A tempering valve is your protection from scalding at the tap. The tempering valve mixes in cold water to decrease the water’s temperature before it’s delivered to the tap. Batch collectors are incompatible with closed-loop circulation systems. Thus, they are generally not recommended for cold climates.

Flat-plate collectors typically consist of copper tubes fitted to flat absorber plates. The most common configuration is a series of parallel tubes connected at each end by two pipes, the inlet and outlet manifolds. The flat plate assembly is contained within an insulated box, and covered with tempered glass.

Flat plate collectors are typically sized to contain 40 gallons of water. Two collectors provide roughly half of the hot water needed to serve a family of four.

 

Evacuated tube collectors are the most efficient collectors available. Each evacuated tube is similar to a thermos in principle. A glass or metal tube containing the water or heat transfer fluid is surrounded by a larger glass tube. The space between them is a vacuum, so very little heat is lost from the fluid.

These collectors can even work well in overcast conditions and operate in temperatures as low as -40°F. Individual tubes are replaced as needed. Evacuated tube collectors can cost twice as much per square foot as flat plate collectors.

Circulation Systems
Direct systems circulate water through solar collectors where it is heated by the sun. The heated water is then stored in a tank, sent to a tankless water heater, or used directly. These systems are preferable in climates where it rarely freezes. Freeze protection is necessary in cold climates.
Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The sun’s thermal energy heats the fluid in the solar collectors. Then, this fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.
Active, or forced-circulation, systems use electric pumps, valves and controllers to move water from the collectors to the storage tank. These are common in the U.S.
Passive systems require no pumps. Natural convection moves water from the collectors to the storage tank as it heats up.

Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The sun’s thermal energy heats the fluid in the solar collectors. Then, this fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.