Thursday, December 23, 2010

Passive Heating And Cooling


Passive systems can be defined as those, in which the control of flow of thermal energy is by natural means.
The following 5 types of passive heating and cooling systems are common.
1. Direct gain
2. Thermal storage wall
3. Solar greenhouse
4. Roof pond
5. Convection loop

1. Direct gain
A large exposure of vertical glass window in the south admits the suns rays allowing it to strike the masonry floor and wall, which acts as thermal storage. The thermal energy stored is distributed in the room by convection and conduction. Temperature swings up to 10 degree Celsius can be achieved.
The system can be controlled:
External reflectors increase solar gain,
Movable insulated shutters used at night reduce heat loss,
External blinds and overhangs can reduce overheating,
The vertical glazing allows lower winter sun to strike at right angles whilst reflecting the high glancing summer rays.

Heating Mechanism
Short wave radiation from sun enters the room and strike the thermal mass. Heat is distributed from this mass by long wave radiation (whish cannot pass through glass), by convection of room over the slab, and by conduction (if one sits or stands on the thermal mass). Movable insulation keeps the heat in at night.

2. Thermal storage wall:
In this method solar radiation is collected in or blocked outside a living space by creating a thermally massive wall between it and the sun. Heat is stored in this thermal mass and is redistributed to the living space by re-radiation through the back of the wall and or by convection of cool room air passing the warm face of the mass by natural thermosyphoning. This thermosyphoning effect is created on the sunny face of the wall by placing a glazed screen about 50 mm away from the face wall, punching holes in the top and buttom of the wall and letting the room air naturally thermosyphon and become warm. Such walls are called trombe walls. When horizontally stacked drums of water are used as thermal mass it is called drum wall, if vertical drums used it is called a water wall.

Heating Mechanism
Short radiation from the sun strikes the thermal mass. During a winter day cool room air cycles up between the glass and the warmed mass, re-entering the room space at the top. During a winter night, heat that has diffused through the wall is radiated into the living space while room air is also heated by convection.
Control of this heating mode is done by:
The size of thermal mass is optimized to ensure a sufficient time lag before re-radiation occurs.
External blinds and overhangs reduce summer overheating.
Manually operated dampers can control the direction and volume of the thermosyphoning air.

3. Solar greenhouse
This heating mode is development of trombe wall. The only difference is that the space between the wall and glass id widened to make a green house. The thermal movement as well as control is similar. The greenhouse also serves as usable living space and provides opportunity for growth of vegetation. The solar greenhouse can be made with glazed wall between greenhouse and living space or a massive wall between the two.

4. Roof Pond
This heating mode transfers the thermal storage to the roof, consists of plastic bags of water that are supported on a steel deck roof or RCC construction on the roof to form a pond with proper water treatment avoiding leakage.
Control is done by positioning movable insulating shutters.
The shutters are left open during day and closed during night in summer. In winters the vice-versa.

6. Convection Loop
In this system an angular solar collector heats a transport fluid and by a thermosyphoning loop allows it to rise to a thermal store located above the collector.
Control is affected first by the correct sizing of the thermal store and collector and second by the use of dampers to regulate the direction and volume of air flow.

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