Solar water heater

Instead of using the electromagnetic properties of sunlight, a solar water heater (also known as solar boiler) makes use of the heat provided by the sun. To capture this heat, a solar collector (#1, usually a flat-plate solar collector) is mounted facing the sun. The captured heat is stored in tubes, which are filled with a special fluid of high thermal inertia. The heated fluid is transported (pumped, #4) through a submerged heat exchanger, which gives off the solar heat to the water in the reservoir (#2). When hot water is requested (#5), a series of sensors determines whether it is necessary to use conventional means (#3) to add some additional heat to the water. Using a residential solar water heater, you can potentially save up to half the energy (45% on average) you need to heat your water! This is not only friendly for your wallet, but also for the environment. Using a solar water heater, you can easily cut back your CO2 emissions by well over 250kg a year! On this page, we will explore the most common types of solar water heaters. Note that several of the components described above are optional, depending on the solar water heater you choose.

Active or passive: to pump or not to pump

So-called passive systems do not require a pump to transport the water. Instead, water (and thus: heat) is circulated through the natural process of convection. Passive systems usually feature a solar collector that is directly connected to a water reservoir. Such a so-called “open loop system” is cheap and reliable, but it does have its disadvantages. First of all, it is unusable in climates where temperatures drop below zero. Since freezing water expands in volume, parts of the solar collector holding water will burst. Secondly, a passive open-circuit system has the tendency to work in reverse in periods where the water in the collector is colder than the water in the reservoir (think night or cloudy days). In effect, heat from the reservoir will then be used to heat the water in the collector. Passive systems are generally only recommended in warmer climates.

In northern Europe, most systems are of the so-called active kind. Active systems are more efficient, but also significantly more expensive. They make use of a pump to transport heated fluid from the collector to the reservoir. Since this pump is only activated when the collector temperature is higher than the reservoir temperature, the system will not lose heat at night or on cloudy days. The fluid is often pumped through a closed circuit and is usually some form of antifreeze. The heat is given off This allows the system to operate in colder climates as well. The obvious disadvantage of this system, is the fact that the pump needs power. Active systems are therefore not carbon-neutral, unless solar photovoltaic energy is used to power the pump.


Circulation systems can be divided into two separate classes: open loop circulation systems and closed loop circulation systems. In an open loop, the water supply in the collector is directly linked to the water supply in the reservoir. The reservoir can be integrated into the collector or can be external. In the case of an external open-loop system, it is necessary to position the reservoir above the collector. Since hot water has the tendency to rise, this will cause a convective pattern in the system in which heated water is in the reservoir. Since a correctly insulated water reservoir can add significant weight, the strength of the roof needs to be taken into account when installing a pumpless open-loop system.

In a closed loop, the conduits in the solar collector are filled with an intermediary transport fluid. This fluid, which often has an antifreeze component, is heated in the solar collector and is transported to a heat exchanger in the reservoir. Closed loop systems usually require a pump, although constructions in which the reservoir is placed above the collector are theoretically feasible. Closed loop systems with an antifreeze fluid can be used in colder climates. In such a climate, a pumped system is strongly recommended, since it will prevent reverse operation during cold hours.

The backup system and central heating

Since the sun doesn’t shine all-year round (or all day for that matter), it is often necessary to have a back-up system. Such a system makes sure that water in the reservoir is kept at a minimum temperature of 60 °C (150 °F), to prevent the growth of dangerous legionella bacteria. Additionally, a backup system ensures that hot water is available during hours of no sunshine. A typical backup system is a normal water heater, that uses mains electricity or gas to heat the water. In most cases, your present water heater will suffice as a backup system, although there are situations in which it might be beneficial to have a new system installed. Consult your local installer for personal advice on your particular situation.

The heated water in the reservoir can also be used to feed central heating conduits. Use of such a system is however limited, since days of bright sunshine are generally days in which you require less heat generated. Unless you want to run your radiators at full power in the summer, it is far more beneficial to use the solar hot water for hygienic and cooking purposes.

Video: solar water heating

The video below gives a basic overview of the various solar water heating systems available.