An effective and favourite hybrid application is the use of a rock bed in conjunction with a passive solar building. In many applications the rock bed is located beneath the source of hot air and thus natural convection cannot be used to transfer the heat. In this case a fan is normally employed, resulting in that part of the system technically being an active element. Rock beds can be used effectively in situations where there is an excess of energy in the form of air that is heated above the comfort level. It is desirable to remove this overheated air for three reasons:
1 to reduce the air temperature in the space and improve thermal comfort;
2 to store the heat thus removed for later retrieval; and
3 to redistribute heat from the upper south part of the building where hot air tends to accumulate into the lower north part of the building that normally tends to run colder.
Imbalances in temperature in the building, which might be created by the passive elements operating alone, can be corrected.
It requires very careful design to remove heat from the rock bed in the form of warm air. This air is then blown into the space to be heated. The air temperatures that can be achieved are low, and the flow rates that would be required are therefore high. The effect of a high air-velocity at low temperatures may be cold and unpleasant. A much preferred approach is to remove the heat from the rock beds by means of radiation and convection from the rock bed container surface. In this case the rock bed is thermally coupled to the space that is to be heated, rather than being thermally isolated from it. A convenient approach that is often used is to place the rock bed underneath the floor of the building, although it would also be possible to place it behind one of the walls. Distribution of heat from the rock bed to the space is entirely passive. The floor temperature or wall surface temperature will only be a few degrees above the room temperature. If the installation is properly designed the net result will be a very comfortable situation, heating the house slowly from a large radiant panel.
Experience with under floor rock beds has been very favourable. Comfort is greatly improved by keeping the floor temperatures 3-6°C above what they normally would be. By increasing surface temperatures and thus increasing the mean radiant temperature within the space, the air temperatures can be reduced and energy savings that are even greater than the actual amount of heat released from the rock bed can be realized.
A technique that is suitable for residential applications is to divide the building into two thermal zones and accept fairly large temperature swings in one zone in order to stabilize temperatures in the other. In Zone 1, which is a direct-gain space, large temperature swings can be expected because there is a large excess of heat. Heat storage is in the mass separating the zones and in the floor of Zone 1. Depending on the size of Zone 1, its enclosing mass surface area and the glazing area, temperature swings of 12°C to 17°C can be anticipated. However, such swings can be completely acceptable (and perhaps even advantageous). Uses of such a space could be as a greenhouse, sun room, atrium, conservatory, transit area, vestibule or as an airlock entry. A principal advantage of this approach is the reduced temperature swings in Zone 2. This is a buffered space protected from the extremes of Zone 1 by the time delay and heat capacity effects of the mass wall. With a little care in the design, one can phase the time of heat arrival into Zone 2 so as to maintain an almost constant temperature. An example of the effective use of the two-zone approach is the house in Bariloche.
TIPS: ROCK BED SIZING
1a The rock bed volume should be 0.6 m3 m-2 of sun-orientated glazing.
1b The air flow rate through the rock bed should be 0.03 m3 s-1 for each m2 of sun-orientated glazing.
2a More than one-third of the net heat should not be transferred out of the space to the bed rock.
2b The working bed temperature drop should be one-half of the working air drop.
3 The air velocity should not exceed 3.5 m s-1 and 10 ac h-1.
4a The pressure drop across the rock bed should be in the region 40-75 Pa.
4b The pressure drop across the ductwork should be less than one-fifth of the rock bed pressure drop.
The newly installed rock bed at Fuentes House in Bariloche, showing the concrete walls of the bed and the heat supply and extract ducts.
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