Using renewable energies to heat drinking water

A diminishing supply of fossil fuels and a societal orientation toward more sustainability in general have paved the way for renewable energy sources in recent years. Solar energy, for instance, can be used for hot water generation in the home, among other applications. However, fluctuating temperatures in hot water storage tanks compromise drinking water hygiene. The members of the Blue Responsibility initiative know what is needed when using solar systems.

Higher energy standards for new buildings, which have come into effect following the second tier of the German Energy Saving Ordinance at the start of the year, motivate many building owners to use a solar thermal installation to complement their heating systems. This allows them to simply undercut their maximum permissible annual primary energy requirement. During the summer, for example, a solar thermal installation can handle the heating of all of the domestic water, while in the winter only some of the heating requirements can be met. “Solar drinking water systems pose a particular challenge because – dependent on the season and the weather – the hot water temperature requirements might not be met,” explains Stefan Pohl of Kemper. Following worksheet W 551 issued by the DVGW association, a hot water temperature of 60°C must be maintained at the outlet of hot water heaters in large installations. If the drinking water system is unable to guarantee this, drinking water hygiene is compromised. “Not observing this required minimum temperature may lead to massive growth of Legionella, because these feel most comfortable at temperatures of between 25 and 55 degrees,” explains Volker Galonske of Honeywell. Generally speaking, various systems are available to heat domestic water; however, not all of them are equally suitable for combination with solar thermal installations.

No combination of storage and storage tank charging systems with solar energy systems
Storage, instantaneous and storage tank charging systems are examples of commonly used systems in the hot water supply of a building, which differ significantly in particular in relation to drinking water hygiene: Storage systems heat up domestic water and store it until it is withdrawn, while instantaneous water heaters heat the water only at the point of withdrawal. Storage tank charging systems represent a combination of both methods, whereby there is at least one water storage tank without an integrated heat exchanger. “All domestic water systems have in common that they need to be constructed as small as possible and as large as necessary,” Volker Galonske explains.

The advantage of storage systems is that they can produce large volumes of water, which are available without delay, with a relatively small heat output. When demand is high, several storage systems can be connected together. However, hygiene and water experts do advise against storage systems that can result in too lengthy periods of stagnation. “From a hygiene perspective, the aim is to store domestic water in buildings for only short periods of time,” Stefan Pohl confirms. Solar energy installations should under no circumstances be connected in this type of system, because this will only increase the risk of contamination with bacteria. This is also specified in DIN standard 1988-200.

Storage tank charging systems are used primarily in larger properties: The storage tank generally caters for approximately ten minutes of peak demand while an external heat exchanger ensures continuous output. However, even if the quantities stored here are smaller than with storage systems, a combination with a solar system is not advisable. “When demand is low, hot water is supplied by an instantaneous heating system. In such cases, the extra hot water stored to satisfy higher demand is not needed, and is even stored for longer periods of time than with storage-only systems. Here, too, solar systems pose an additional risk for drinking water hygiene,” Stefan Pohl explains.

Solar systems and instantaneous systems: a good team
From a hygiene point of view, instantaneous hot water heaters that only heat the water when it is drawn are the preferred solution. Here, the energy for hot water generation is produced by a buffer tank on the heater end. In other words, no water is stored on the domestic water end. For instantaneous water heaters, it is possible to connect renewable energy sources, such as solar systems. To this aim, an intermediate medium must be used in accordance with DIN standard 1988-100, table 2. A solar separator station, which is inserted between the solar system and the buffer tank, is one such possible solution. It predominately consists of a separator heat exchanger, pumps and a control system. “The solar separator station prevents the water of the solar system, which contains a frost protection agent, coming into contact with the heated water,” Stefan Pohl explains.

Professional planning and consulting ensure drinking water hygiene
Expert planners should always be involved in the sustainable design of a hot water generation system. When connecting solar circuits, it is important to ensure that the required hot water temperature can be maintained, even when experiencing unfavourable weather conditions. This can be achieved, for example, by adding a conventional heating system as an additional support. “In water storage tanks storing solar energy, protection by means of a scald protection system is equally important,” Volker Galonske of Honeywell explains. In addition, DIN standard EN 805, section 5 stipulates that hot water heaters must be serviced in regular intervals: for hygienic reasons for one, and to ensure flawless functioning and efficient operation for another. In principle, the increasing utilisation of renewable energy promotes a sustainable approach to resources, which is why we should embrace it. However, drinking water hygiene must not be compromised when solar energy is used. “Drinking water hygiene is always our top priority,” Wolfgang Burchard of Blue Responsibility stresses. Whether we use renewable energy or not: The objective must always be to design the system in such a way that it works with as low a water volume as possible while still catering for peak demand. “Here, the high-quality manufacturers of Germany’s sanitary industry do not just offer innovative technology. They also offer in-depth expertise,” Burchard sums it up.

Further details regarding this topic can be obtained on the manufacturers' websites und