Nearly-Zero Energy Building Stocks – Assessing Economic and Ecologic Limitations of Renewable Energy Concepts

National and international agreements aim to limit climate change and thus call for a reduction of greenhouse gas (GHG) emissions to nearly zero. A wide range of technologies promise to reduce the heat demand of buildings and also promote renewable energies. One of these technologies is the use of solid building structures as thermal storage, so called thermally activated building parts or TABS. Thermal simulations of such energy concept for a typical single-family house with 140 m² living space featuring a heat pump, a solar thermal collector and TABS show that the share of solar heat for heat supply can be increased, resulting in a decreased use of the heat pump and thus a lower demand of electric energy. This leads to reduced greenhouse gas emissions and lower operating costs. Furthermore, the simulations show that larger sizes of the TABS and the solar thermal collector lead to lower demand of electric energy. To secure a reduction of greenhouse gas emissions and costs over the whole lifecycle of a building also production and dismantling, disposal and recycling must be considered. A Life Cycle Cost (LCC) Analysis shows that TABS in combination with solar heat reduce LCC, expressed as present values, by app. 34%. The reductions are mainly due to the lower operating costs of the heating system. Increasing the size of south-facing solar collectors leads to asymptotically decreasing costs. For the less favourable orientations to the West and East, the optimum size of the collector is between 30 and 40 m², depending on the orientation and the size of the TABS. A minimum size of the TABS must be available, while additional TABS do not lead to further reductions. Also in an ecologic sense, the use of TABS in combination with solar heat is beneficial. The simulations in this research show that the greenhouse gas (GHG) emissions over the whole lifecycle can be reduced by 27%. Again, the reduction mainly results from the decreased demand of electric energy and only slightly higher GHG emissions from the production of the TABS. Larger collector sizes lead to asymptotically reduced GHG emissions, when south facing. In contrast, orientations to the East and West lead to increased GHG emissions as the size of the collector increases. Integrated systems of heat pumps, solar thermal collectors and TABS could also be considered for multi-family housing and other building types. Simulations of LCC and LCA offer a suited means for assessing economic and ecologic impacts of innovative buildings concepts and should be used on a wider scale, ideally in combination.