Solar Water Heaters

General efficiency issues

Hot water at temperatures under 100 degrees Celsius is needed in many of our activities: domestic, industrial and commercial. These temperatures are easily attainable with flat solar collectors (without concentration of radiation), which are relatively simple to build and are very reliable.

Because water has a very large specific heat capacity and because large quantities of heated water are used, solar water heaters became the most common solar energy product, both for commercial and for domestic applications. As the energy needed to heat up the water accounts usually for about one third of the total energy consumed in a household, a significant saving can be achieved by using an economical solar water heater.

In order to achieve a real financial saving, one has to spend less on the solar water heater than on buying the quantity of conventional energy equal to the energy the water heater will produce during its service life.

Natural environment issues

It is now well known that conventional energy (like electricity, gas or liquid fuels) pollute and produce global warming. We can help save the natural environment by using an environmentally friendly solar water heater. We can achieve this, if less waste and greenhouse gases are produced when manufacturing the solar hot water system than would result by heating the water in a conventional way.

General technical issues

Solar energy is not uniform in time: there is the day-night cycle and also some days are cloudier or colder than others. Some commercial and industrial water heaters use the hot water immediately after it is produced, without storing it. But all domestic solar hot water systems have to store quite a large quantity of energy (in a proportionally large quantity of water) for the time when solar radiation is not available.

The main functions of a solar hot water system are:

  • to transform the solar radiation into heat
  • to store the heat for times when solar radiation is unavailable
  • to produce hot water at the consumer's demand using the stored heat

There are many technical solutions designed to perform the above functions, but practically all solar hot water systems can be grouped under three main categories:

  • the thermosiphon systems that rely on the fact that hot liquid is less dense than cold one; this property is used to transfer heat from the solar collector to the hot water tank that stores the thermal energy for when it is needed
  • the pump systems, that use a pump to transfer heat from the solar collector to the hot water tank that stores the thermal energy for when it is needed
  • the integral systems that store the thermal energy in the solar collector, therefore not needing a separate storage tank

Usually, to compensate for long periods of time without enough solar radiation, domestic solar hot water systems have installed inside a conventional energy booster. A better method to compensate for periods without enough solar radiation is to connect the outlet of the solar hot water system to the inlet of a conventional energy hot water system, as we recommend our clients do with our products.

The thermosiphon systems are now the most frequent type of solar water heaters because they have no moving parts, therefore they are more reliable than the pump systems. Also, they preserve the heat better than the previous types of integral systems.

Design versions are numerous, almost each manufacturer adding an improvement with their products. Basically (see figure - pressure resistant system), they all have a solar collector where a black coated plate heats up in the sun and therefore heats a liquid that is in contact with it. This hot liquid rises (because of the thermosiphon effect) and creates a circulation between the solar collector and the storage tank. A conventional energy booster may be provided to the storage tank to compensate for periods when there is not enough hot water stored. When the hot water is withdrawn through an outlet, fresh cold water is admitted inside the storage tank where it is heated.

The oldest version (see figure - pressure resistant system) is using a storage tank at water mains pressure, the consumption water being used also for circulating through the collector and as storage liquid. This version is still the most frequent because it is simple and cheap to manufacture, despite having the following disadvantages:

  • the storage tank is very heavy (it has thick walls to withstand the mains water pressure)
  • the tank has to be placed above the solar collector (usually on the roof)
  • the corrosion is difficult to control because different metals are in contact with each other
  • it cannot withstand freezing conditions.

Subsequent versions (see figure - jacket thermosyphon) have introduced another heat exchanger circuit filled with antifreeze solution between the collector plate and the storage tank. Together with the initial heat exchanger that is in contact with the absorber, there are three heat exchangers that are in a chain, one after the other in the functioning of such a heat transfer device. This impedes upon its thermal efficiency, as temperatures decrease and less heat is transferred to the consumer (see colour coding). Because of their complex construction, most of these versions of thermosiphon systems are expensive (therefore lose their financial justification) and tend to be detrimental to the natural environment.

The pump systems are sometimes called active systems because the circulation between the solar collector and the storage tank is forced with a pump against the thermosiphon effect. This way the storage tank can be placed on the ground, but the pump incurs a relatively small electricity consumption. They are more complicated than the thermosiphon systems (see simplified diagram - pump system) and therefore are less reliable and more expensive to purchase and to maintain. Only the very large pump systems are economical and can make a positive contribution towards saving the environment.

The integral systems have a simpler construction and succeed to better transfer the heat from the absorber to the storage tank that is actually built inside the solar collector.

In one version, the solar radiation is absorbed at the surface of at least one thick walled, black coated tank that is placed inside a thermally insulated box (see figure - pressurized integral collector). Water from the mains is admitted inside the pressure resistant tank where it is heated up and is withdrawn and directly used on demand.

Having a simpler construction, the integral systems can be significantly less expensive and therefore more efficient than the thermosiphon and pump systems. Unfortunately, the old types of integral systems tend to quickly lose the heat they gained.