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Installation and piping of solar thermal systems

Solar panels in front of a blue sky

In the case of solar systems, a distinction is made between photovoltaic systems for electricity generation and solar systems for hot water generation.  

Solar systems for hot water generation are usually used to provide hot water in the household, for swimming pool heating, for heating support and for process heat generation. They thus offer a sensible alternative to conventional water heating. Today, two-circuit systems are predominantly installed.  

In the first circuit, a heat transfer medium (water-antifreeze mixture) is pumped through the collector and the absorbed heat is transferred to the drinking water via a heat exchanger in the hot water tank. In the second circuit, the heated drinking water is led to the consumer. 

In single-circuit systems, the water is fed directly through the collector and heated. This system is sometimes used to heat swimming pool water without the risk of frost. Single-circuit systems in countries where there is a risk of frost must be able to be shut off and drained by means of an appropriate control system.  

The correct dimensioning of the system is the requirement for satisfactory operation. For small systems in one/two-family houses, one can assume a water consumption of around 50 l of hot water per person per day. With a temperature specification of 50 °C and the necessary reserves for days with less radiation, the collector area should be about 2 m² per person.  

Materials for the piping 

Suitable materials for the pipes of the solar circuit offer: 

  • sufficient temperature resistance 
  • glycol resistance 
  • high pressure resistance 
  • the necessary weathering and corrosion resistance for outdoor use (no galvanised pipes). 

Hot water tank 

The hot water storage tank should have a volume of 1.5 to 2 times the daily consumption of hot water per person, i.e. about 100 litres per person, to store hot water for days with less radiation.  
Storage tanks should have a slim, cylindrical shape to ensure good temperature stratification. The storage tank should have at least 10 cm of close-fitting and gap-free thermal insulation to minimise heat loss.  

hot water generation

Solar circuit

The solar circuit serves to transport heat between the collector and the heat exchanger in the hot water tank. The circuit should be as short as possible; for systems in one/two-family houses, a pipe diameter of 15 mm or 18 mm is usually sufficient. The high temperatures of over 110 °C in the collector and in the collector circuit also require matched thermal insulation of the pipes.  Outdoors, the thermal insulation must also withstand UV radiation, weathering and bird damage. Therefore, UV-resistant and / or leaded materials with corresponding temperature resistance must be used there. Frequently used foam materials, which are only designed for heating systems up to 90 °C, often fail here after only a few days. 

Special foamed materials are available on the market for the requirements of the collector circuit. In addition, there is the possibility of thermal insulation using rock or glass wool as well as melamine resin. The pipes must be insulated against heat loss in accordance with the insulation thicknesses of the heating system ordinance. In smaller systems for one/two-family houses, the common flow rate is 30 to 50 litres per m² collector area. The pressure test must be carried out according to the solar system manufacturer's instructions.

Formation of steam during standstill

If the solar system is at a standstill - e.g. because the storage tank is already fully charged and no consumption is taking place or the collector circuit pump is not in operation - steam can form in the collectors if solar radiation continues. The steam space usually corresponds to the collector volume and a short section of the connection pipes.

In conventional flat-plate collectors, standstill temperatures of up to 250 °C are reached, in vacuum tube collectors even up to 350 °C. When the system is restarted after such a standstill phase, quite high temperatures can occur in the solar circuit over a very short period of time. Measurements by the Fraunhofer Institute for Solar Energy Systems (ISE) show that when the flat-plate collector is not filled - i.e. during the construction phase when it is exposed to solar radiation - approx. 160 °C can occur at the collector at standstill temperature (210 °C collector temperature). Depending on the weather, this may be the case for several months, depending on the progress of construction. EPDM sealing rings would already experience preliminary damage here - without the system even having gone into operation. 

Use of copper fittings 

The use of SANHA® copper press fittings (series 12000/13000) is therefore recommended as fittings for copper pipes. Alternatively, the SANHA® NiroSan® press system series 18000 (stainless steel) or NiroTherm® Industry (series 98000) are available. 

Both press systems (see also www.sanha.com/en/about-sanha/news-and-press-releases/news/5-reasons-to-use-press-fittings/) have a higher temperature resistance due to the FKM sealing ring and are suitable for water-glycol mixtures in all mixing ratios. However, at peak temperatures as described above, even FKM sealing rings reach their limits. Therefore, the piping system should not be connected directly to the collector surface, but a distance of at least 3 metres should be maintained.

In the case of vacuum tube collectors, it is recommended that the connections of the collector and solar circuit as well as the other connections in the solar circuit itself be made by means of hard soldering, since temperatures of up to 280 °C can occur at the transition points and at the other connections between the pipe and fittings when the system is restarted after standstill. 

solar lines on a roof

Pipe routing

The supply and return lines must be laid with a gradient so that the system can be drained if necessary. For the solar circuit, special attention must be paid to the change in length of the pipes. Due to the high temperature differences to be expected, the copper or stainless steel pipes expand several times compared to a conventional hot water installation. This expansion in length must be taken into account through appropriate fastening (compensators) and the installation of expansion bends or bendable joints in the pipe. Solar pipes are dimensioned in the same way as heating pipes.