Solar collectors: types, planning & performance

Solar collectors form the core of a solar thermal system. As their name suggests, they collect the sun's rays. This is then followed by conversion into usable heat, which can then be used to heat domestic hot water or as a central heating backup in the home. This helps you to save on energy costs and contribute to a reduction in CO₂ in the atmosphere through the burning of fossil fuels.

Basic principle and types

A few special technical solutions aside, the collectors primarily used in Germany are ones which contain a circulating heat transfer medium. This medium is generally composed of a mixture of water and glycol antifreeze. The medium is in a tube. Depending on how they are installed, a distinction can be made between tubular and flat-plate collectors. What they both have in common, however, is that an absorber converts solar radiation into heat. A heat transfer medium absorbs the heat and conveys it away from the collector. This process is the same in every collector.

Vacuum tube collectors –– the Thermos flask principle

In tube collectors, the absorber is set into a glass tube that is under vacuum pressure (evacuated), similar to a Thermos flask. The vacuum has very good thermal insulation properties and ensures reduced heat loss. This is particularly beneficial in the case of high collector temperatures, in other words specifically those operating conditions that are common for solar central heating backup.

In general, tube collectors can be differentiated according to their design: in vacuum tube collectors with direct flow, the heat transfer medium circulates through the absorber pipes that are arranged inside the tubes. In heat pipe systems, the heat transfer medium does not flow through the tubes. Instead, a medium (usually water) evaporates in the copper pipe below the absorber. The steam condenses in the aptly named condenser at the upper end of the tubes – this is where the energy is passed to the heat transfer medium in the collector. The heat pipe collectors have the advantage of reliable heat absorption.

Viessmann offers the following vacuum tube collectors based on the heat pipe principle:

Vitosol  300-TM
Vitosol  200-TM

Flat plate collectors –– meandering tubes

With flat-plate collectors, the absorber is usually protected from the elements by a casing made from coated sheet steel, aluminium or stainless steel and a front cover made from low-ferrous solar safety glass. An anti-reflective (AR) coating on the glass can further reduce reflection. Thermal insulation of the collector casing reduces heat loss.

The absorber pipe is laid out in a meander, which ensures a reliable flow pattern through the collector. The absorber pipe is also welded at the bends, ensuring optimal heat transfer right to the edges. The floor plate is connected all-round to the collector frame. The pane seal is seamless and made of flexible, weather and UV-resistant sealing material.

Viessmann offers the following products:

Proper planning and installation 

On account of their diverse designs, solar collectors can be installed in almost any building concept, in new build as well as in modernisation projects, either on the building or close by. They can be installed on pitched roofs, flat roofs and on walls, as well as freestanding on the ground, as required. In all instances, collector and mounting form a single static unit. Viessmann offers fully load-tested systems for all conventional roof types and suitable for all collectors as part of its standard product range – ensuring enhanced dependability and peace of mind at the planning and installation stages.

The inclination and orientation of the collectors are crucial

The amount of energy that is available for heat generation is greatest when the radiation hits the collector surface at a right angle. At our latitude, this cannot be achieved with a horizontal surface. The collector surface can, however, be tilted accordingly. In addition, the orientation also determines the correct use of solar energy. In the northern hemisphere, an orientation towards south is ideal.

Performance characteristics – what is important?

A key value that you need to consider before buying a solar thermal system is the efficiency of the collectors. This value represents the proportion of solar radiation that is converted into usable heat energy. This value is determined according to the European standard EN 12975 and you can find it in the datasheets for the appliances.

Calculating the efficiency of the solar thermal collectors also takes the energy flows and heat losses into account. This means that not all the light reaching the surfaces can be used to generate heat (optical losses). In addition, a small part of the heat generated by the collectors is also lost (thermal heat losses).

Energy flows in the collector:  A  Irradiation on collector  E  Absorber heated by radiation

Optical losses:  B  Reflections at the glass pane  C  Absorption at the glass pane  D  Reflection at the absorber

Thermal losses:  F  Thermal conduction of the collector material   G Heat radiation of the absorber  H  Convection

Overheating protection with ThermProtect automatic temperature-dependent shutdown

If no heat is drawn from the collector (because the pump is at a standstill and the heat transfer medium is not circulating), the collector heats up to the so-called stagnation temperature. The risk of overheating increases with increasing temperature differential to the surroundings. Stagnation temperatures of 200 degrees Celsius and more lead to undesirable effects. In that case, the solar medium will evaporate and expand rapidly and widely in the solar circuit. The resulting high thermal load on the components and the heat transfer medium itself will then cause damage.

Characteristic efficiencies: the risk of overheating increases as the temperature differential to the surroundings increases. This is where collectors with overheating protection come in.

ThermProtect and the heat pipe principle to guard against overheating

Viessmann counters this phenomenon with a special absorber coating –– ThermProtect. As part of the process, the absorber radiates more and more heat as it warms up. This increases the heat losses of the collector, while at the same time, the collector temperature rises only slightly and the stagnation temperature remains significantly below the usual values. How does that work exactly?

ThermProtect changes the crystal structure of the flat-plate collectors. The optical properties also change at a temperature of 75  degrees Celsius. This means that the internal temperatures of the collectors cannot rise above 145  degrees Celsius. When temperatures fall again, the crystal structure returns to its original state.

Conversely, with vacuum tube collectors, the heat pipe principle is used to protect the system from overheating. If solar radiation is too high and heat transfer begins to decrease, phased temperature-dependent shutdown kicks in. This blocks condensation at the heat exchanger. The heat transfer medium can no longer liquefy, and heat is no longer transported. Heat transfer only resumes when the temperature inside the solar circuit has fallen.