Arbejdsrapport fra Miljøstyrelsen 19/2007 – Varmeakkumulering i beton – Vurdering af betons termiske masse i relation til bygningsreglementet og energiberegninger

Varmeakkumulering i beton

Summary and conclusions

Introduction

The Danish building regulations which came into force in 2006 stipulate that the heating and cooling requirements for new buildings are to be calculated.

It is possible to utilize the thermal mass in solid concrete constructions to reduce the variation of room temperatures and thereby create a more uniform indoor climate and to reduce the requirements for heating and cooling.

Aim

On this background this project aims to:

- perform calculations which quantify the effect of the utilisation of the concrete’s thermal mass for accumulation of heat in building surfaces.

- analyse the effect of using concrete in relation to the energy rules in the building regulations. The energy and comfort-related performance of the different choices of building materials are compared. The calculation principles which correspond to the new building regulations are applied by using software developed by SBi (Danish Building Research Institute).

- show examples of how the material data on concrete can be used in connection with the new building regulations.

- provide a guide on how it is possible to calculate heat accumulation in a construction.

- participate with calculation examples based on the Danish calculation method in a European Round Robin investigation performed by CEMBUREAU, the representative organisation of the cement industry in Europe.

Projects

An overview of recent projects in Denmark shows that comprehensive work has been done on development of systems which enable concrete structures to store heat or remain cool. It is assessed that systems that are heated or cooled actively, e.g. by tubes integrated in the construction, have a large potential.

Heat Capacity

To fulfil the stipulations in the Danish building regulations for new buildings it is necessary to have a limited heating requirement. The heating requirement is calculated on the basis of a number of energy-related parameters. One of these parameters is the heat capacity of the building. In the present project three different methods for determination of the heat capacity of the building have been assessed: tabulated values in connection with the Danish calculation method, a simplified CEN method and active heat capacity.

To estimate the active heat capacity an analysis was made where primarily concrete surfaces were exposed to a diurnal variation of the room temperature. The heat transmitted into the surface during a 12 hour period with positive heat flux, and which is stored in the material, is called the ability to store heat.

The calculations show that the ability to store heat increases with increasing density of the material. Among the materials investigated, concrete has the largest ability to store and release heat.

The results show that the ability to store heat is in general increased by increasing thickness of the materials. The increase is largest the first few centimetres. A further increase of the thickness will result in a smaller increase of the heat stored. The effect of increasing the thickness from 5 to 10 cm is relatively limited.

Only a part of the heat capacity of a material can be used for storing heat. The part of the heat capacity which is utilized is called the active heat capacity. The reason why only a part of the heat capacity is utilized is that there is a resistance to heat conduction in the material itself and a resistance to heat flow at the surface of the material.

The part of the thermal capacity which is utilized to store heat may, from an overall point of view, be considered to be more dependent on the thickness of the material rather than on the thermal properties of the material. It is the ability to store heat per surface area which will be the single most important parameter to take into consideration when the performance of a material is assessed in relation to the storage of heat.

The size of the utilized thermal capacity is relative uniform for all the assessed materials. The consequence of this is that the differences in the ability of storing heat will be reflected correspondingly for the different materials.

It is possible to use the data curves from the study to characterize other materials than those investigated. This can be done by using the thermal effusivity as the input parameter.

The input parameter used in the Danish calculation method for calculation of the heating requirement of a building is the heat capacity per m² of heated floor area.

It is illustrated that the Danish tabulated values correspond approximately to what can be achieved by the simplified CEN method for large thicknesses of the solid materials. For lightweight materials the simplified method will provide values less than estimated by the Danish tabulated values.

Using the active heat capacity will give a considerably smaller heat capacity of the building than according to the Danish tabulated values for large thicknesses of materials. If small thicknesses of the materials are applied the Danish tabulated values will also be expected to provide considerably smaller heat capacity than if the assessment is done by a more detailed technique.

It is possible to calculate the total heat capacity of a building by adding the heat capacity of the single surfaces.

The investigation shows that to obtain a large storage of heat it is necessary to ensure the lowest possible internal surface resistance. The performance will improve if the amount of insulating coatings, floorings and furniture in close connection with the heat-storing surfaces is limited.

Surfaces exposed to solar radiation will have an improved performance if these can be directly exposed to solar radiation. This is due to a large absorption of heat and the lack of surface resistance for transfer of heat to the surface.

Analysis of the Heating Requirement

Calculations were performed in which the heating requirement of different building models was calculated according to the method used for assessing the heating requirement in the Danish building regulations.

Calculations were performed on an office building and on a single-family house. The results show that a large reduction of the heating requirement can be achieved when a large thermal mass is used. The difference in heating requirement between the models with the lowest and largest thermal capacity is between 4 % and 13 %.

Even if some of the building components have a large thermal mass exposed to the interior environment it will still be possible to obtain a reduction of the heating requirement by adding building components with a large thermal mass.

The heat which has to be removed due to overheating will also be reduced by increased thermal mass.

Conclusions

In Denmark a number of research and development projects have been executed where it has been demonstrated that concrete is well suited for enabling building constructions to both store heat and remain cool.

The calculations performed where heat is stored in concrete show that a large part of the heat capacity can be used for storing heat. The part of the concrete closest to the surface stores more heat than the part further away from the surface.

It is demonstrated that if the guidelines for assessment of heating requirements in relation to the Danish building regulations are followed, there will be a low heating requirement due to storing of heat in the building construction. In the calculated examples the reductions of the heating requirements are between 4 % and 13 % when the constructions are made of solid concrete instead of a lightweight construction.

Similar results have been obtained in connection with the results elaborated for the examples made in the Round Robin investigation arranged by the representative organisation of the cement industry in Europe CEMBUREAU.

The tendency of overheating will be reduced when heavy constructions are used instead of lightweight constructions.

In the present report shows how it is possible to calculate the heat capacity for a certain type of concrete. It demonstrates how the heat capacity for the single surfaces can be added in order to calculate the heat capacity per m² heated floor area, which is used as one of the input values in the Danish calculation programme. It is normally not expected that a detailed calculation will provide a larger thermal capacity than when the Danish tabulated values are used. This is the case for both lightweight and solid buildings. There might be a need for assessment of the Danish tabulated values to check whether the level is appropriate.

Using concrete results in energy and comfort-related performance benefits no matter which method for calculation of input data is applied.