The energy storage potential of concrete is often left unexploited by designers. But the latest range of off-the-shelf, precast concrete floor units are set to make summer heat gain a thing of the past

For architects and building services engineers looking to exploit the energy storage potential of concrete, an answer could soon be rolling off the production line. Tarmac Precast has combined forces with Structural Conditioning (a division of Warmafloor) to launch Thermocast, a range of pre-cast, coffered concrete floor units that are designed to absorb, store and remove heat gains in occupied spaces.

While bespoke precast coffered units are nothing new, Tarmac has standardised the profile of the coffer in the Thermocast range, enabling the same set of moulds to be used for production and allowing costs to be reduced. In addition to providing off the shelf, fair-faced finished units that can be directly decorated, the Thermocast slabs are embedded with a matrix of polybutylene pipework that provides an active means of supplementing their thermal mass.

With its higher thermal mass and embedded water cooled pipe lattice, Thermocast uses a high proportion of radiative heat absorption or emission to control temperatures – 60%. Tarmac claims that the ability to channel temperature controlled water through the slabs boosts cooling capacity enough to handle internal heat gains of up to 80 W/m2.

The Thermocast system is designed to run in conjunction with either a natural ventilation strategy or with mechanical ventilation (designed to temper the air during winter and meet the latent cooling loads during the summer months), preferably delivered through a raised access floor. Water flows in the region of 13°C should provide sufficient cooling for most commercial applications, making it suitable for groundwater cooling sources, canals and cooling ponds, as well as chillers, dry air coolers and energy piles.

The system uses two water circulation circuits to overcome any issues with cross contamination. The primary circuit flows through the precast concrete elements and is connected via a plate heat exchanger to a secondary circuit. This secondary circuit acts as the mains distribution pipework, incorporating the flow and return valves. The units can be configured to suit the building layout and zoned to accommodate variations in cooling and heating load resulting from orientation, occupancy and internal gains.

It is anticipated that Thermocast will primarily run in cooling mode. After the building has been completed and the system has achieved its set point of 18-20°C, the time lag between circulating water through the pipes and the effect being discernable at the surface will be about 30 minutes. Should there be variations to internal conditions, the inherent passive capacity of the slabs should act as a buffer for long enough for the water circulation to effect energy exchange.

Control of the Thermocast panels will rely on the building management system providing either individual control, zone control, north and south orientation or overall building control. This strategy would be in the form of on/off line valves to account for local variations around the building, working in conjunction with an overall compensated supply strategy. The control strategy allows for the slabs to be continuously charged in proportion to the rate of cooling required at the surface. This can be fine tuned locally by in-room thermostats, pulsating the water circulation through on/off line valves.

The control strategy will also need to take account of the risk of the slab surface temperature falling below the local dew point and causing moisture to condense on the soffit. A monitoring device – consisting of a chrome bracket attached to the surface of a unit with a dew point sensor – looks for misting and prevents the surface temperature from dropping to such a level. However, care will need to be taken not to activate the device at a level that will significantly decrease the cooling performance of the system.

The profile of the Thermocast slabs is designed to optimise their thermal performance as well as their generic acoustic and lighting performance. Buro Happold carried out wide ranging computational modelling and analysis of the Thermocast system in three building applications: a typical mechanically ventilated office, a naturally ventilated space and a four bed hospital ward. Results showed that heat transfer by convection for the Thermocast system was about 15% greater than that for a flat slab – as a result of increased convection from the coffered section – and that there was a 60% reduction in convection if the cooled water pipework was removed from the coffered section.

Acoustical analysis concluded that there was no fundamental reason that the use of precast concrete ceiling elements in offices, schools and hospitals should not be possible; based on the essential inclusion of absorption panels, elsewhere in the room. However, the report did raise an issue with the use of ceiling elements in open plan offices concerning the reflection of sound by the exposed concrete ceiling. Additional downstands in the coffered cross section, particularly if these include absorptive material, might provide a typical solution.

Installation

The pre-cast structural slabs have been designed to be installed into steel, insitu or precast concrete framed structures in clear spans of up to 16 m. Depending on the span and loading, the 2.4 m wide units will be supplied in 450 mm or 600 mm depths, and can be provided without the requirement for structural screeding to accommodate raised floor construction.

Termination of the embedded pipe coils can be in any plane. Generally this will be beneath a raised access floor, however pipework can also terminate below the Thermocast units and then be formed in a builder’s work detail, encasing the pipework at high level. Fittings are available to then connect the flow and return pipework to the distribution system using either fusion weldable sockets, push-fit connectors or compression couplings.

Tarmac also believes that the floor units should enable contractors to reap benefits in other areas, such as reduced build programmes and shorter secondary mechanical and electrical fix. While other buildings have already exploited the principles behind thermal mass, Thermocast should provide the opportunity to do so on a much larger scale.

Why polybutylene?

Polybutylene pipe has been widely used for heating and plumbing applications in everything from domestic to commercial and industrial buildings since the 1970s. In addition to being non-corrosive and flexible, polybutylene can be recycled. Unlike alternative materials, such as cross-linked polyethylene, the molecular structure is not altered in the production process, therefore the pipe can be converted back into granular form ready for re-use in the production of other plastic-based products.

Polybutylene can also be incinerated (its recoverable heat energy is identical to that of heating oil) without producing toxic or corrosive gases.

While pipes for hot and cold water services are normally single layer extrusions, for central and underfloor heating/cooling applications and embedded coil technology applications the pipes need to incorporate an internal oxygen barrier. The purpose of this barrier is to prevent oxygen entering the system and contributing to corrosion within associated plant such as boilers or control manifolds. Ideally this layer should be fully protected between the inner and outer pipe walls as it may be affected by moisture.

Thermal mass in existing buildings

A challenge increasingly faced by engineers is how to improve comfort levels in existing buildings using the building’s inherent thermal mass.

Exposing soffits to provide a direct thermal link between the slab and the space is one answer, but many existing buildings have false ceilings and floors that thermally isolate slabs from the conditioned spaces. In a bid to overcome this, engineering consultant Faber Maunsell has developed CoolDeck; this is a system that provides a means of storing the cooling from night-time ventilation in the building’s structure so it can then be used to condition the space the following day.

The systems consists of sheet metal elements that are attached to the slab in the void. Air is circulated through the narrow path created between the slab and the metal sheet and the turbulent flow used to enhance the heat transfer between the concrete and the air.

The system has been used in the refurbishment of Stevenage council’s offices. Originally naturally ventilated, the offices had been suffering from summertime overheating stemming from an increased use of IT. Rather than resort to a fully air-conditioned solution, the council decided to implement passive measures. However, directly exposing the structure to the conditioned spaces caused aesthetic and acoustics problems as well as concern about the impact on the existing heating system.

As a result, CoolDeck was adopted. On summer nights, cool outside air is introduced into the space by window fans. The CoolDeck fans operate in parallel at the same rate to store the coolth in the slab and during the day the CoolDeck fans run to release the coolth from the slab.

Outside air ventilation is provided naturally during the day by manual windows under occupant control. In mid season the CoolDeck fans can also be used in a heat recovery mode to store any excess heat gains during the dayMonitoring has shown reductions in internal temperatures in the region of 5°C – about 1-2°C is attributed to the solar blinds, 3-4°C to the CoolDeck system.

Source

Building Sustainable Design