The two secondary schools at Enkplatz (8.800m²) and the gym needed to be renovated. It was planned to raise the ambitions for the refurbishment and to plan the gym as a “Nearly Zero-Energy Gym” or even as a positive energy building using solar energy, making it a real lighthouse building for the district. The gym was deliberately taken on board as an opportunity to enlarge the scope and reach-out of the project to more target groups in the district (students, teachers, parents and neighbours). ICT-based central monitoring and energy management for controlling the solar heat energy flow into the secondary district heating grid will be established.
1) Reconstruction instead of Refurbishment
The Reconstruction of the gym instead of an economically not feasible refurbishment offered the possibility to fulfil also the spatial needs of the two secondary schools. The construction of an extension to the existing building made improvements possible:
▪ Reintegration of the 4 classrooms currently located in a temporary building at the school site
▪ 15 additional classrooms, 4 leisure classes, 3 technical rooms and ancillary rooms
▪ Construction of 4 divisible standard gyms for the two schools and for external usage
2) The Energy Concept: Zero-Energy-Gym
The objective of the project was to reduce the energy requirement of the sports section to the level of "zero energy buildings". The goal was not to operate the sports section in an energy-independent way, but to find a balance between the shell of the building and renewable energy sources in order to achieve at least a zero-energy balance for the sports sector over the year.
For the replacement of the gymnasium, the project management commissioned a project study including a concept for heating, air conditioning, aeration and sanitary installations (DE: “HKLS-Konzept”), which provides heat supply to the building primarily via heat pumps using ambient heat and solar thermic installations. The electricity demand of the building was supposed to take place mainly through a photovoltaic system, so that a zero-energy balance for the new building could be achieved as a planning target.
In view of an integral planning, a rough building technology concept was already outlined before the call for tenders in co-ordination with Wien Energie as a partner for the solar feed-in, as intended in the objectives of the project-proposal. The aim was to develop a concept that best meets the project goals and the conditions on site. For this purpose, a rough dimensioning of the trades was carried out based on a coordinated building technology and hydraulic concept.
In the preliminary dialogue-oriented evaluations, the limited roof surface and its proportional occupancy with solar heat collectors or photovoltaic modules proved to be the most critical point for achieving the project goals. For this reason, a minimum installation area for the solar heating system with a size of 680 m² was specified in the documents for the architectural competition. There are three different sources of renewable energy supply for the building:
2-1. Geothermal Energy (with sol)/ water heat pump
The underfloor heating in the gym and the heating register of the ventilation system is done by geothermal energy. Heat is extracted from the earth via 8 depth drillings (depth approx. 150m) and lifted to a usable temperature level by a heat pump. A buffer tank and an infinitely variable capacity control of the refrigeration circuit are installed to reduce the start of the device. To regenerate the underground that surrounds the piles and to cool the room, heat from the room cooling system is fed into the ground via a separate heat exchanger in the summer. Since the existing heating installations are used for this, the performance is significantly lower than with a designated refrigeration system, but therefore extremely cost-effective.Since it is unlikely that the heat output from the room cooling in summer will be sufficient to completely regenerate the piles, it is planned to feed heat from solar thermal energy into the ground from about September. Cooling energy is no longer available from the beginning of this forced regeneration until next summer.
2-2. Solar Thermal System and Feed-in into the District Heating Grid
On the roof of the new building, Wien Energie builds a solar thermal system. It supplies a buffer storage for heat that will be used on site in the school on one hand and on the other hand to feed thermal energy into the district heating network. In the detailed planning, the possibility of using solar heat in the building was limited compared to the feasibility study. Originally, additionally to the support of space heating, the use of solar heat was intended for the regeneration of the geothermal piles over the entire solar season, as well as for the support of the hot water production. In coordination with the planner for the technical building services, Wien Energie's solar system was adapted to the new supply and hydraulic concept. Consequently, most of the solar heat is fed into the supply of the district heating network at around 70 °C during most of the solar season. In the building, primarily the space heating at a low temperature level will be supported. In addition, at the end of the cooling season, the possibility of regeneration of the geothermal probes is foreseen as an option. Simulations without the use of solar heat for regeneration resulted in significantly higher gains when using vacuum tube collectors instead of the mostly used flat-plate collectors. The economic optimum was achieved with the maximum possible utilisation of the provided roof area, so that a total of 320 m² gross collector area of the vacuum tube collectors in a horizontal design were planned. The heating system of the school is designed to consume first the heat from the buffer tanks and only then starts the heat pump system or, if necessary, obtains heat from the district heating as a backup.
2-3. Photovoltaic System:
In addition to the solar thermal system, a photovoltaic system will be erected on the extensively greened flat roof of the extension building. With an output of 67 kWp over the year, this covers the consumption of the school sports halls. The direct use rate is forecasted at approx. 75%; considering the entire location, 100% direct use is to be assumed. Electricity that is not consumed directly in the property (mainly during the holidays) is fed into the electricity grid.
District heating is available all year round. It supplies the old building, the water heating and the above-ground storeys of the new building, when there is not enough solar heat available. As a backup in the event of failure of the heat pump system, the low-temperature distribution system of the gym can also be supplied with heat via a heat exchanger.
4) Building Envelope
Transmission heat losses are greatly minimised by the underground construction of the gym. However, the underground construction method is only possible in a few exceptional cases for living areas. Office space or classrooms would be difficult to implement in this design, as the necessary natural solar exposure is difficult to ensure. The school building was designed in such a way that it has significantly better energy figures than required by law. The building achieves the national standard "Lowest Energy Building 2020, Class A ++" due to the low energy efficiency factor. When considering the necessary heating energy requirements, only class B would be achieved. When fulfilling the heating requirement class A, insulation thicknesses of more than 40cm and high-quality passive house windows would have to be used. Increasing the heat protection qualities would result in a significant increase in the production costs. The calculated key figures for the heating energy demand of the project in question ensure a cost optimality. With a heating demand of class A, the cost optimality would no longer be reached.
5) Ventilation system with heat recovery
A mechanical ventilation system with rotary heat exchanger with efficiency of 85-90% is planned for the gymnasium. This not only serves to recover heat but can also, to a certain extent, recycle moisture into the air. This results in a higher comfort for the users by having a more pleasant room climate, especially in winter months.