- The principle behind concrete core temperature control
- Heat sinks / Heat sources
- System variations
- Planning aspects
- Energy efficiency
- Costs and economic viability
- Further information
The analysis takes a look at the following aspects: How flexibly can the buildings be used when concrete core temperature control is deployed? How energy-efficient are the thermally inertial surface cooling and surface heating systems? What are the prerequisites for the concept to be economically viable? What level of comfort is actually achieved? Here, the analysis is based on scientific studies of various ENBau pilot projects in which the building performance was evaluated in detail during actual operation using long-term monitoring.
1. The principle behind concrete core temperature control
Regardless whether it is too warm or too cold – conventional room conditioning technology always requires active counteraction, i.e. cooling or heating. This is not the case with concrete core temperature control: here, the building structure is utilised for storing thermal energy in order to release it when required.
The floor slabs are charged with heating or cooling energy by means of hot or cold water circulating through the tube heat exchangers integrated in the building element. As the water flows through the tube system, it transfers heating capacity or cooling capacity to the slab, thus heating or cooling it, depending on the water temperature. The heat or cold stored in the concrete core is then transferred to the room during several hours – 60 % via radiation and 40 % via convection.
The large heat transferring surface of the slab makes it possible to transfer considerable amounts of heat to the room even with temperatures which are only slightly too high or low. Thus effective use can be made even of the relatively low differences between room temperature and the temperatures of natural heat sinks (summer) or heat sources (winter): ground, groundwater and outdoor air.
2. Heat sinks / Heat sources
Different types of heat sinks are available depending on the local conditions:
# Borehole heat exchangers
The usual type of heat sinks are borehole heat exchangers, which are inserted in 50 to 100 m deep boreholes.
# Energy piles
These are foundation piles for buildings, which are used as borehole heat exchangers.
# Foundation slab
To a limited extent, heat can also be dissipated to the ground using suitable piping in the foundation slab.
# Recooling systems
Heat can also be released to external air via a recooling system. Dry and wet cooling towers are suitable for this. In contrast to the ground, external air cannot be used as a heat source in winter.
3. System variations
Concrete core activation is mainly suitable for removing heat loads in summer (cooling, no dehumidification). It is also possible to heat with the system in winter; this particularly applies for buildings with a very low heating requirement. However, it is limited in terms of its controllability. Concrete core activation is only worthwhile in economic terms when it is also to be used for cooling.
Concrete core activation can be deployed in various configurations:
A) Solely for building element cooling, which is combined with a conventional heating system and supplemented with natural or mechanical ventilation.
B) Concrete core temperature control can provide the sole heating and cooling surface in a room or building, provided that the cooling and heat loads are limited correspondingly.
C) The concrete core activation provides the temperature control, i.e. it provides the base load when heating and cooling. However, it is supplemented with additional systems that enable individual and needs-oriented temperature control.
4. Planning aspects
# Planning tools
For the concept phase, there are indeed planning handbooks and simple dimensioning aids, but thermal simulation of the building and systems is the most important planning instrument. The standard calculation methods, hydraulic circuits and control concepts used for the layout of cooling ceilings and underfloor heating systems are inadequate for the dimensioning and operation of CCTC due to its inertia. On the basis of the load characteristics for heating and cooling, the necessary supply and return temperatures and the overall mass flow rates at the main CCTC distributors are obtained. Parallel to this, on the generating side, the heat sink / heat source, the system hydraulics, and a simple control concept are calculated. Finally, the simulation allows assessment and optimisation of the building's energy balance and energy consumption, and how this interacts with the heat sink / heat source with regard to different variations or specific planning decisions.
In order to control and manage the system, clear requirements must be specified for the entire building services technology – ranging from the solar shading to the pump control.
# Supplementary systems
If, in summer, the heat in individual rooms cannot be dissipated, supplementary measures such as quick-reacting ceiling radiant cooling panels which can be controlled in a decentralised manner, for example, are necessary.
# Occupant comfort
CCTC is only capable of meeting the general expectations placed on air conditioning systems to a limited extent. The contractor, or occupant, and planners must clearly define the thermal comfort requirements. To this end, the current standards (prEN 15251 or ASHRAE 55) provide appropriate criteria.
The building can be subdivided into zones in order to control the temperatures of different areas according to demand. The zones are established according to orientation, storey, use, or facade concept, and catered for with different supply temperatures and at different charging times, according to requirements. These provisions limit the extent to which the floor plans can be changed during a later conversion of the building.
# Free slab surfaces
Suspended ceilings or ceiling claddings significantly reduce heat transfer by convection, as well as direct radiation exchange, and are therefore not to be combined with CCTC. Slab surfaces are to be kept as free as possible (exposed concrete). As uncovered concrete slabs are acoustically "hard", attention must be paid to the reverberation time; sound-absorbent constructions are usually necessary.
# Cooling capacity and dew point
The upper limit for the cooling capacity is determined by the indoor air temperature dew point, as otherwise condensation forms on the ceiling. For an indoor air temperature of 26 °C with 50 % relative air humidity, the dew point is around 15 °C. Thus, primarily the influx of solar loads must be reduced by means of effective sun protection. Due to the relatively "high" supply temperatures of natural heat sinks, the dew point is almost never fallen short of.
The actual concrete core temperature control system is generally installed after the lower reinforcement bars have been laid. To avoid leaks during the construction, the tubing is visually inspected and a pressure test conducted once the upper reinforcement bars have been laid. A second visual inspection and pressure test are carried out once the concrete has been poured. If leaks are found, the defective areas are sealed with tube fittings.
The drill depth for fixing interior fittings (partition walls, lighting, etc.) has to be limited in accordance with the tube system’s position within the concrete slab.
5. Energy efficiency
For: Since natural, renewable heat sources and heat sinks are used with temperatures that differ only slightly from the outdoor temperature, these are low-exergy systems that generally require considerably less primary energy for cooling and, under specific conditions, can also be used for energy-efficient heating. The thermal storage can also be used over a longer period of time and can be charged at night, which ensures that the low-exergy system is more efficient. In addition, the auxiliary energy used for the distribution of heat and cold is less in water-carrying systems than in air-carrying systems.
Against: The thermal inertia of the system can lead to losses in the heat distribution. As it is not possible to know a room's exact heating or cooling requirements in advance, the nature of this system causes charge reserves to arise, which entail higher energy consumption. CCTC should therefore be used in conjunction with a suitable storage management system so that as little heat as possible is wasted in the ceilings and overheating or undercooling of the rooms is avoided.
6. Costs and economic viability
The investment costs for concrete core activation are as little as 40 to 50 euros per square metre. However, the low temperature differences require large surfaces, which means that the performance-related costs of around 1 to 1.50 euros per watt are often higher than, for example, when using cooling panels. Therefore, CCTC does not always provide the cheapest solution. CCTC is particularly cost-effective, however, when it is the sole system used for heating and cooling. In this case, the effect is enhanced even more because the ventilation system is reduced to the hygienically required level, which makes it possible to use, for example, smaller channel sections with a consequential reduction in costs and energy.
Energy costs: The low energy input (see above) means that the energy costs are low. It is only possible, however, to allocate heating and cooling costs in accordance with consumption in the case of larger tenants with suitable zoning relating to the division of the office space, whereby the floor plans can no longer be changed. The zoning is literally poured in concrete.
In combination with natural heat sinks, the supply temperatures are between a minimum of 18 °C and a maximum of 29 °C, so that the slab surface temperatures are near to the room temperature. In the Energon building in the German city of Ulm, for example, the slab surface temperatures in 2005 fluctuated within a temperature range of 20.5 to 25.0 °C. Due to the low radiation asymmetry, the interior climate is fully within the comfort zone.
Detailed analyses of the Energon building in Ulm and the BOB in Aachen show that when cooling by means of concrete core temperature control, it is almost always possible to maintain the required room temperatures with respect to the occupancy behaviour provided that there is a consistent reduction of solar and internal thermal loads. Also in winter, concrete core temperature control can ensure thermal comfort in these buildings, without additional static heating surfaces.
As ventilation in conjunction with CCTC performs only a hygienic function, and not a conditioning function, the air volume can be limited to the hygienically required minimum air renewal. The low air velocities and reduced noise which result from the reduced air volume flow also increase the comfort.
Abandonment of active cooling in summer in favour of passive cooling is only possible if buildings are planned carefully so that architecture, structural design, occupants' requirements, and building services equipment are coordinated in an integral overall concept.
With buildings with increased (internal) heat loads, CCTC is frequently used to considerably improve the indoor environment in summer using cooling energy generated from renewable sources. The system is also suitable for heating and, if certain prerequisites are met, as the sole heating system. However, individual control of room temperatures is only possible to a very limited extent, and it is very difficult to allocate heating costs to individual office units. CCTC is not suitable for refurbishment projects. For buildings without cooling requirements there are much cheaper systems available, particularly for buildings with very good thermal insulation.
9. Further information