Natol House, Austria. Storing energy with concrete

This detached house has a thermal envelope that encloses an area of 161 m2. It includes a ground-floor level (including living room, kitchen, toilet and service/storage room), a first-floor level (including parents’ bedroom, two bedrooms for children, bathroom, hallway and toilet) and a partial basement on the south side of the building (including wardrobe, stair to ground floor and one basement room). No full basement was built due to the steeply sloping terrain and for cost reasons.

Thermal envelope
External walls, load-bearing partitions and ceilings are made from concrete. The building features a wooden roof structure; its non-load-bearing partitions were built from plasterboard fitted with soundproofing. Major portions of the building are buried in the ground, whereas its complete south side and half of the east and west sides are exposed to ambient air. All components in contact with the ground have been insulated with 26 cm extruded-polystyrene panels (two layers). The visible ventilated façade was fitted with 28 cm thick expanded-polystyrene insulation panels whilst preventing thermal bridges. The ventilated façade is “suspended” on bespoke stainless-steel anchors and is free from thermal bridges according to the PHI test certificate.  The cellulose-fibre roof insulation is 43 cm thick. All glazing was mounted in front of the façade and has a U value of less than 0.85 W/m2K in installed condition.
 
Heating and ventilation system, concrete energy storage
The primary heating and hot-water “energy source” is the 23 m2 solar-thermal system designed to cover 70% of total demand. This high ratio can be achieved with a relatively small collector area when combined with concrete storage because very low flow temperatures result in good yields also in the winter season. Overall, about 28 m³ of concrete were thermally activated so as to ensure that all rooms can be conditioned via concrete storage, thus eliminating the need for dedicated heating surfaces. The residual amount of energy is supplied by a small tiled stove in the living room at ground-floor level. This stove can also heat the concrete storage because it includes a small solar absorber embedded in the storage mass. This is why it can release a certain portion of the stove heat into the house distribution network. The comfort ventilation feature exclusively includes ventilation (without heating option). For comfort reasons, inflow temperatures significantly exceed the required level of 17 °C since manifolds are routed on the conditioned concrete ceiling (heating up the concrete).
 
Initial experience

The owners moved into the house on 24 December 2009. From 1 January 2010, daily records have been maintained to document the amount of supplied energy. From 1 January to 30 April 2010, 220 kg of wood were used to generate the residual amount of energy (i.e. 880 kWh in a total of 22 heating intervals). In the same period, the solar-thermal system generated about 2,100 kWh, which corresponded to a share of 65% in total consumption during these four winter months. The above figures include heating and hot-water consumption.

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