Thermal Collectors - Amplifying Solar Heat

The sun regularly heats every part of the earth. This considerable energy can be recovered, multiplied, and distributed both easily and efficiently on a small scale.
Simple thermal collectors are sufficient to produce pollution-free heating and hot water in each home free of charge (except for equipment and installation costs).

An Omnipresent Heat Source

A thermal collector is an appliance that collects heat from the sun's rays and transfers it to a fluid, which in turn transports it to the areas to be heated. This method is one of the most widely used for this huge energy source. The sun's rays propagate passively via the architecture of buildings (glazed surfaces, etc.).
Thermal collectors amplify this effect. For example, active solar systems are used to heat housing, domestic water, swimming pools, etc.

The advantages of this energy source are:

   • The fact that it is renewable
   • The absence of pollution
   • The fact that it is free (excluding installation and maintenance costs).

However, it is intermittent (the sun goes down at night) and its intensity can be restricted by cloud cover. Moreover, even though the sun shines everywhere, not all regions get the same amount of sunshine - or solar flux - depending on latitude and the season.

There is such a thing as solar-heated showers.

True. Thermal collectors embedded in house roofs alongside tiles are used to heat water, which is stored at 45°C, hot enough for showers and bathing children.

A Thermal Collector for Every Use

The principle of thermal solar collectors is to convert solar energy into heat. They have three basic elements:

   • A flat surface - usually glazed - through which the sun heats a heat-transfer fluid (that can circulate its heat through the distribution circuit - water, air, etc.).
  
   • A circulation system that transfers this heated fluid to a place where it can be stored or used. It includes a pump or a thermosiphon. A thermosiphon uses the properties of hot water, which is lighter than cold water; in a circuit of vertical pipes, hot water rises while cold water falls.

   • A regulation system maintains the temperature at the required setting. It is fitted with temperature probes and a thermostat that controls when the circulation pump starts and stops.

There are 3 types of collectors, depending on the temperature required and on how the energy is to be used.

  • Flat-plate collectors

These consist of an insulated box with a glass or plastic cover. Inside, a dark metal plate absorbs the heat from the sun trapped inside the box. This heat is transmitted to air, water or any other heat-transfer fluid not subject to freezing. The fluid circulates freely or inside pipes to the point of use. The temperature difference with ambient air can be as high as 70°C. This system is ideal for producing domestic hot water or for heating buildings.
Glazed flat-plate collectors accounted for 89% of all collectors in the European Union in 2006, i.e. 3 million m².

   • Unglazed collectors

These are long black plastic or metal tubes (black is best for trapping heat) in which water circulates. Since they are not insulated, the temperature difference obtained is low - 20°C compared to ambient air temperature. These collectors are often used to heat swimming pools.
Unglazed collectors accounted for 3% of all collectors in the European Union in 2006.

The temperature difference compared to ambient air can be as high as 70°C.

  
   • Vacuum collectors

These are panels with rows of transparent glass tubes with their air removed - a vacuum being one of the best thermal insulators. Inside each tube, an absorber traps solar heat and an exchanger transmits it to a heat-transfer fluid. As with the other collectors, the fluid circulates to the point of use. Because heat loss is minimal, the temperature can reach 100-140°C.
These collectors are suited for industrial applications requiring high temperatures (cleaning abattoirs, pasteurizing canned food, etc.).

Vacuum collectors accounted for 8% of all collectors in the European Union in 2006.

France has a plan to use thermal collectors for all its heating.

True. At the instigation of its Environment and Energy Management Agency (ADEME), France significantly increased the number of its thermal solar plants between 1999 and 2009. Its Solar Energy Plan subsidized the installation of thermal collectors on 715,000 housing units in mainland France and overseas departments and territories, representing a surface area of 2.22 million m² and installed capacity of 1550 MWth. French authorities continue to support the goal of extensive solar energy use for hot water production by 2020.

 

For individuals, solar collectors are used to produce hot water at 50-60°C or to provide underfloor heating to 25°C. To obtain hot water at France's latitude, 3-5m2 of collectors is enough for a household of 4 people. For underfloor heating, 1m2 of collectors is required to heat a surface area of 10m².