Le principe du système thermosiphon en jeu.

Le thermosiphonage , aussi appelé thermosiphonage , est considéré comme une technologie appropriée . Ce processus utilise des ressources naturelles renouvelables et les lois fondamentales de la thermodynamique pour créer le mouvement d'un apport d'air ou d'eau chauffé. La source d'énergie pour ce processus est le rayonnement solaire (ou toute autre source de chaleur). L'énergie du soleil est capturée dans un dispositif de collecte solaire et est transférée à l'air ou à l'eau par conduction. L'ensemble du processus peut s'expliquer par l' effet de thermosiphonnage : lorsque l'air ou l'eau est chauffé, il gagne en cinétiquel'énergie de la source de chauffage et devient excité. En conséquence, l'eau devient moins dense, se dilate et monte donc. En revanche, lorsque l'eau ou l'air est refroidi, l'énergie est extraite des molécules et l'eau devient moins active, plus dense et a tendance à « couler ». Le thermosiphon exploite les différences de densité naturelles entre les fluides froids et chauds et les contrôle dans un système qui produit un mouvement naturel des fluides. Plusieurs systèmes basés sur cette technologie sont actuellement disponibles et peuvent être lus plus en détail dans le texte suivant.

Le principe du système de thermosiphon est que l'eau froide a une densité (densité) plus élevée que l'eau chaude, et donc plus lourde coulera. Par conséquent, le collecteur est toujours monté sous le réservoir de stockage d'eau, de sorte que l'eau froide du réservoir atteigne le collecteur via une conduite d'eau descendante. Si le collecteur chauffe l'eau, l'eau remonte et atteint le réservoir par une conduite d'eau ascendante à l'extrémité supérieure du collecteur. Le cycle réservoir → conduite d'eau → collecteur assure le réchauffement de l'eau jusqu'à ce qu'elle atteigne une température d'équilibre. Le consommateur peut alors utiliser l'eau chaude du haut du réservoir, l'eau éventuellement utilisée étant remplacée par de l'eau froide du bas. Le collecteur réchauffe alors à nouveau l'eau froide. En raison des différences de température plus élevées à des irradiances solaires plus élevées, l'eau chaude monte plus vite qu'elle ne le fait à des éclairements plus faibles. Ainsi, la circulation de l'eau s'adapte presque parfaitement au niveau d'irradiance solaire. Le réservoir de stockage d'un système de thermosiphon doit être placé bien au-dessus du collecteur, sinon le cycle peut fonctionner à l'envers pendant la nuit et toute l'eau se refroidira. De plus, le cycle ne fonctionne pas correctement à de très petites différences de hauteur. Dans les régions à forte irradiation solaire et architecture à toit plat, les réservoirs de stockage sont généralement installés sur le toit. le cycle ne fonctionne pas correctement à de très petites différences de hauteur. Dans les régions à forte irradiation solaire et architecture à toit plat, les réservoirs de stockage sont généralement installés sur le toit. le cycle ne fonctionne pas correctement à de très petites différences de hauteur. Dans les régions à forte irradiation solaire et architecture à toit plat, les réservoirs de stockage sont généralement installés sur le toit.

Les systèmes à thermosiphon fonctionnent de manière très économique en tant que systèmes de chauffage d'eau domestique. Le principe est simple, ne nécessitant ni pompe ni commande. Cependant, les systèmes à thermosiphon ne conviennent généralement pas aux grands systèmes, c'est-à-dire ceux qui ont plus de 10 m² de surface collectrice. De plus, il est difficile de placer le réservoir au-dessus du capteur dans les bâtiments avec des toits en pente, et les systèmes de thermosiphon à circuit unique ne conviennent que pour les régions hors gel.

Underlying physics

Thermodynamics is the study of energy.

  • First law of thermodynamics - States that energy may be changed from one form to another, but cannot be created or destroyed. Energy is always conserved.

This law may be applied to the movement of water in thermosiphoning system: Energy from the sun is directed and transferred (via conduction and convection) to either water, air, or another medium of choice. This natural process of heating eliminates the need for external energy sources such as fossil fuels or electricity.

  • Second law of thermodynamics - States that in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state. The net return of a system is always less than that of which was initially put in.

Energy is always conserved, however energy (or heat in this case) may often be lost in a given system (thermosiphoning) as heat. Adding insulation with appropriate R values to the system and its plumbing may greatly reduce heat loss, and thus increase efficiency.

  • Planck's Law - The wavelength of radiation emitted from a surface is proportional to the temperature of the surface. Energy transferred as a result of temperature differences between two objects. Dark objects absorb heat, while light objects reflect.

Darkly colored collection plates within the solar collector will aid in increasing solar absorption, thus increasing the amount of heat available to heat water or air in thermosiphoning. In contrast, reflective or lightly colored piping and storage tanks should be utilized as the light colors will help to reduce heat radiation out of the system.

Passive water heating

The passive thermosiphoning of water is the process of heating and moving water within a system without the need or use of electricity. This process functions by utilizing natual phenomena such as solar energy, gravity, and an available water source. A solar collector, piping, and a water tank are materials required for the heating process. The flow of water is distributed into, within, and out of the solar collector. Cool water enters the bottom of the solar collector where it is then heated via convection by solar radiation. When water is heated it becomes less dense than cooler water, expands, and then rises (flows) through the piping. The heated water exits the top of the solar collector naturally. The cooler and more dense water sinks and remains within the solar collector until it is heated. As the cool water is heated, it expands, rises, is pushed out of the top of solar collector, allowing cool water to flow into the solar collector. This process continues naturally until the temperature of the water reaches an equilibrium with solar radiation input.

Two types of thermosiphon water exchange systems are currently available: the close-coupled system, and the gravity-feed system.

Close-coupled system

Schematics

Close-coupled systems function on the same principles of passive thermosiphoning mentioned above. The storage tank of these systems must be placed above the solar collector to utilize the water circulation driven by the passive thermosiphoning process.

Materials

  • Solar Energy
  • Solar Collector
  • Piping
  • Insulation
  • Water
  • Storage Tank
  • Strong roof or other support system

Cost

  • 2007 research suggests that passive thermosiphon water heaters may range from $500 to $6,500. Pricing may vary due to tank size, solar exposure, and geographical location
  • Many countries, states, and utility services provide incentives for renewable energy participation

Pros

  • Non-polluting
  • Energy Savings - No electricity needed for passive thermosiphoning
  • Cost Effective
  • Space saving - (i.e. indoors)

Cons

  • Tank exposure to external environmental condition may reduce efficiency, depending on geographical location
  • Aesthetics - May be considered visually unpleasing
  • Strong support structure needed (i.e. roof)
  • Not suitable for extremely cold climates
  • Location - must be positioned in an area with suitable solar exposure (i.e. south side of desired area)

Gravity-feed system

Schematics

Gravity-feed systems utilize the same principals of passive thermosiphoning as does the close-coupled system, however placement of the tank differs. Tanks are installed horizontally into a roof, which is often located directly above the solar collector. Once needed, the heated water within the storage tank takes the path of least resistance and moves via gravity down into the desired location. Gravity-feed systems require more piping/plumbing to distribute the heated water, and this factor should be taken into consideration when installing or purchasing a thermosiphoning system.

Materials

  • Solar Energy
  • Solar Collector
  • Piping
  • Insulation
  • Water
  • Storage Tank
  • Strong roof or other support system

Cost

  • Gravity-feed systems are typically the least expensive passive thermosiphoning water heaters
  • 2007 research suggests that the cost may range from $400 to $5,500 (not including the cost -if applicable- of installation). Pricing may vary due to tank size, solar exposure, and geographical location
  • Many countries, states, and utility services provide incentives for renewable energy participation

Pros

  • Non-polluting
  • Energy Savings - No electricity needed for passive thermosiphoning
  • Cost Effective
  • Space savings - (i.e. indoors)
  • Aesthetics - (Horizontal tank placement)

Cons

  • Plumbing and piping add additional costs to the system
  • Aesthetics - May be considered visually unpleasing
  • Strong support structure needed (i.e. roof)
  • Not suitable for extremely cold climates
  • Location - must be positioned in an area with suitable solar exposure (i.e. south side of desired area)

Active water heating

Schematics

Active solar heating systems, also known as pump systems or split systems, function on the same basis of the thermosiphoning effect, however active systems utilize an energy source other than solar energy to help drive the process. This system installs only the solar collector on the roof, while the storage tank is installed on the ground or anywhere else below. These active water heating units require some external form of energy to pump the water throughout the system. By utilizing additional energy, these active systems are less cost efficient than passive systems.

Materials

  • Solar Energy
  • Solar Collector
  • Electrical energy
  • Electrical pump
  • Additional piping
  • Insulation
  • Water
  • Storage Tank

Cost

  • 2007 research suggests that active thermosiphon water heaters may range from $1,200 to $10,500. Pricing may vary due to tank size, internal piping requirements, solar exposure, and geographical location
  • Many countries, states, and utility services provide incentives for renewable energy participation

Pros

  • Money Savings
  • Cost Effective
  • Aesthetics - Storage tank not placed on the roof
  • Greenhouse gas reduction - If insulated properly, it has the potential of polluting as little as passive systems.

Cons

  • Uses more energy than a passive system
  • Requires more maintenance than a passive system
  • Heat loss - during the transfer from the solar collector to the storage tank below
  • Pollutes some - from the electrical usage
  • Location - must be positioned in an area with suitable solar exposure (i.e. south side of desired area)

Passive air exchange

Schematics

An example of a passive solar thermal heating system method is Thermosiphon Heat Exchange. It is based on the principle of natural convection, in which air or water is circulated in a vertical closed-looped circuit without using a pump. Cool air indoors travels through a vent and is directed into an opening in the bottom of a solar collector. The air contained within the solar collector is then heated by the sun via solar radiation. Cool air is dense and will sink, while warm air is less dense and will rise. As the air heats up within the solar collector, it becomes less dense than the cooler air and rises. The warm air rises out of a vent in the top opening of the solar collector, moves into the desired area (i.e. indoors), and is replaced by cooler air. This air exchange process will continue until the indoor air temperature reaches an equilibrium with the temperature outdoors.

Materials

  • Solar collector - The bigger the solar collector, the better.
  • Frame
    • 6 vertical 2-by-6-inch boards - Sideboards
    • 2-by-6, and a 2-by-8 boards - Top sill
    • Lag screws - Recommended, but not necessary for attachment
  • Glaze
    • Corrugated polycarbonate panels
    • 10 panels - 26 in wide by 8 ft high
    • Pairs of panels overlapped over 1-by-1-in vertical wood strip - Makes 4-foot-wide panels for each bay
    • Ultraviolet-resistant coating - Apply to sun-facing side to extend longevity
  • Solar absorption plate
    • 2 layers black metal window screen - Attached across the top and bottom of bays
  • Vents
    • Holes cut through building's siding - Plastic flaps will prevent back flow of air through upper vents at night.

Cost

  • 2007 research suggests that passive heat exchangers may range from $55.00 to $400. Pricing may vary due to size of collector/s, insulation of area to be heated, solar exposure, and geographical location.
  • Many countries, states, and utility services provide incentives for renewable energy participation

Pros

  • Low cost
  • Energy saver
  • Pollution reduction
  • May be used to cool electronics

Cons

  • Increased maintainence - (i.e. covering during times of low solar radiation)
  • Geographical location may alter effectiveness
  • Requires manual closing of back draft dampers at night
  • South facing installments preferred

Related projects

References

External links

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