In general, solar thermal energy is an excellent solution to limit the environmental impact of hot water production.
When one analyzes the environmental impact of a solar thermal system during its entire life cycle according to an LCA method, it appears to be very small and depends mainly on the type of post-heating used. The most important indicators for estimating the impact are the primary energy consumption and the CO 2 emissions from the production of hot water.
A solar water heater that covers 50% of the domestic hot water needs, with 4.4 m² flat-plate collectors under glass, a load-bearing structure, a 300 liter steel storage vessel , copper solar pipes (10 m), a control system, an expansion tank, a solar pump and additional gas heating would have an energy recovery time in our regions in the order of 1.3 to 2.3 years.
For combined solar systems (COMBI), the energy recovery time is slightly higher, usually 2 to 4.3 years (Energy Payback Time – A Key Number for the Assessment of Thermal Solar Systems, Proceedings Euro sun, 2004).
In a system that is dimensioned to obtain a solar fraction of 55%, the consumption of an additional energy source (gas, electricity, fuel oil, …) represents more than 80% of the impact of the system on the consumption of primary energy and the associated CO 2 emissions, compared to a system with identical references but without solar energy.
A decisive impact of certain components on the energy recovery time
The components of the solar system with the greatest impact on primary energy consumption and CO 2 emissions are:
The thermal solar collectors, due to the use of aluminum for the manufacture of the profiles
The storage vessel, due to the amount of steel used for its manufacture
The copper solar conductors.
The dossier The life cycle of materials describes the methods for the analysis of the life cycle of materials and is a valuable tool in their choice.
Impact of the solar water heater on health
Broadly speaking, under normal conditions of use, a thermal solar system has no impact on skin health. Flat-plate collectors under glass must be handled by at least two experienced technicians, especially on roofs, where a significant wind load occurs during assembly.
Ideally, the rinsing and filling of the collectors take place outside the periods in which the collectors collect sun. In the other case, they are covered with an opaque tarpaulin, in order to prevent the engineers from burning themselves or causing the liquid in the collectors to evaporate.
Only the heat-conducting liquid can have health consequences if it is accidentally swallowed.
In the case of a solar water heater with an idle system, the heat-conducting liquid can, in theory, be neutral distilled water. In practice, the use of glycol with corrosion inhibitors is recommended for most systems sold as a kit. The most commonly used liquids are antifreeze mixtures of water/propylene glycol, to which the manufacturers add certain corrosion inhibitors to protect the components of the primary circuit against corrosion, aging and deposits.
In the case of an installation under permanent pressure, the use of glycol liquid is always mandatory. This propylene glycol-based liquid is usually enriched with corrosion inhibitors. As a result of the high temperatures to which it is exposed, the liquid will slowly decompose, forming corrosive products.
Most manufacturers recommend factory-produced heat-conducting liquids that meet the specific requirements of solar systems. These liquids are not suitable for consumption. They are usually classified as class 3 fluids (food quality), which corresponds to an LD50 toxicity level of more than 2000 mg/kg body weight. Some manufacturers offer heat-conducting liquids from renewable sources (for example, Tycofor L-eco, opposite).
In accordance with the regulation of toxic products, it is strictly forbidden to pour the liquid into the sewer. All waste from the installation must be recovered and taken to a container park.
Liquid toxicity class and mandatory network protection
For a class 3 liquid, the installation of a decoupler with differential pressure zone (CA) on the cold water supply of the network is no longer necessary.