Conventional air conditioning has one major mode of heat transfer: Forced Convection. It needs the air in the room as heat transfer fluid. When the air comes into contact with bodies, conduction starts to play a role at the surfaces.
Both the above actions have a linear relationship of heat transfer based on the temperature difference between two bodies and is affected by any sort of thermal insulation - which in some cases is wanted and in other cases not..
Radiant heat on the other hand, needs no heat transfer fluid. It is transmitted over an infinite distance in space, but with a temperature relationship between two bodies of T14-T24. It also has a strong relationship with the distance between two bodies due to the radial nature of radiation.
Hence, with radiant heat only a small difference in temperature between two bodies creates a strong path for heat transfer.
The human body in fact prefers radiant heat transfer to regulate body temperature since it is adapted to control blood flow to the skin very accurately. If it needs to loose some heat, it will direct more blood flow to the skin and aid the action with evaporative cooling by sweating.
Building structures absorbs and release heat during the course of a day. The envelope of the building in particular is a very active thermal element being exposed to the sun and the wind. The envelope also has windows that allows through radiant heat from the sun that is then absorbed by all the bodies touched by the light.
Because radiant heat is driven so strongly by temperature difference, a human in close contact with any surface that is hotter or colder than the skin, will feel the effect. Think about driving in your car on a very cold morning. Even with the heater on full blast, your cheek will feel quite cold where it is next to the freezing window. The same goes for a hot day.
If this absorbed heat from a building structure has to be removed by forced convection, it can take a very long time as it is a more indirect mode of transfer.
However, it is possible to remove the heat from building elements - like the slabs - directly.
By casting in a network of water pipes into the slab, the entire slab becomes an active heat exchanger. Removing the heat this way is very efficient, as it is now done via direct conduction in the slab. The same goes for heating it up in winter.
A further advantage is that the building element becomes an active thermal storage where energy can be stored in a direction of your choice. If you want cooling energy stored, make it on average colder than the typical day. If you want heating energy stored, make it warmer.
In a building where slabs are not active storage, it becomes a matter of the tail wagging the dog. The air conditioning has to respond to the slab, not the other way around with an activated slab where the slab has to respond directly to the energy input.
This is a very powerful way of controlling energy into a building.
The biggest advantage is the much improved levels of comfort experienced by building occupants. Unlike convective systems where air temperatures can change quickly - something that the human body finds hard to del with - thermally activated surfaces are very stable and gives a near total natural swing in indoor temperature in relation to outdoor temperature.
One does not experience the typical thermal shock entering and leaving an activated building. You just walk in and all you experience in summer is that it is not hot, and in winter that it is not cold. It is just damn nice.
At Sol Plaatje University where this technology was implemented on large scale - one of the largest if not the largest in the world - the result were impressive.
Energy input for cooling into the building came a peak of less than 15W/m2, because it can run 24 hours a day. This has enormous advantages for electrical power management where the climate control system now becomes a base load and not a highly variable load.
Secondly, thermally activated buildings maintains comfort through electrical load shedding periods. Again a huge advantage.
The systems are so stable, that no electronic temperature control is needed apart from keeping the supply water temperature constant.
System advantages:
- Huge thermal energy storage. Electrical load shedding is hardly noticed.
- Low maximum energy input. The system is a base load.
- Extremely space efficient compared to ducted systems.
- In most spaces, no air ducts are needed. Natural ventilation can be done.
- When ducts are needed, it will only be for fresh air to areas like lecture halls. Size is reduced to 40% that of a typical all-air system.
- No to little visual impact. The system is literally casted in concrete.
- Very little maintenance due to large elimination of ducted systems with fans and filters.
- No controls are needed.
- Changeover from cooling to heating happens twice a year.
- Buildings needs to be well insulated.
- Buildings has not carpets or ceilings - less maintenance.
- It can use multiple sources of heating, including waste burners and solar water heating.
System Disadvantages
- Architects cannot design building envelopes with unlimited amounts of glass - which is after all only responsible.
- Surface finishes are limited.
- No carpets or ceilings requires some measures for acoustic control in areas like lecture halls.