## 3 Results
### Project Definition  
This section describes the outcome of applying the first stage of the methodology on the selected case study. As the state and energy performance of the case study have been reported in [37], this section outlines the main findings in relation to building characteristics and defines the additionally studied energy-producing systems. The primary energy demand for the building in its existing state is found to be 129.6 kWh/m\({}^{2}\) year by the BE18 reference energy model. It considers energy demand for heating, cooling, ventilation, domestic hot water, and electricity for operation of the building. The energy model has been developed and validated by the energy audit described in depth in [37]. The total heated floor area of 5250 m\({}^{2}\) is divided equally between the apartments, 48 of which are oriented to the west and 18 are oriented to the south. Both buildings are three storeys in height with unheated attic, basement, and utility room. Thermal characteristics for the main building envelope elements are shown in Table 1.  
To achieve NZEB in the Danish context, the primary energy demand must be lower than the limit value of 27 kWh/m\({}^{2}\) year and partially covered by renewable production [39]. For this specific building, the primary energy demand needs a reduction of nearly 80%. Moreover, it has been reported that heating and domestic hot water demand is covered by district heating sub-station, which is common for both buildings [37]. The sub-station consists of two heat exchangers (one for heating and one for DHW), which are installed when the building was constructed. Ventilation commences naturally through windows and ventilation openings in the bathrooms. Considering that the existing district heating sub-station is quite outdated, it is worth considering its replacement. This can be either the re-establishment of a new substation or switching to a different energy system. This, combined with the requirement in Danish building regulations for a share of renewable energy production, can have a large effect on the economic and energy performance, as different systems bear distinct characteristics. Here, it is important to consider local conditions and possibilities, e.g., the availability of district heating or a gas network, space for vertical or horizontal ground-source heat exchangers, available roof area and orientation, etc. The energy source for a system is a determining factor for defining it as renewable. For example, district heating should not be considered renewable if the source is coal. On the \begin{table}
\begin{tabular}{c c} \hline \hline
**Building Element** & **U-Value [W/m\({}^{2}\) K]** \\ \hline External wall & 0.58 \\ Windows & 2.9 \\ Attic slab & 0.35 \\ Basement/ground floor slab & 1.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: U-values of building envelope elements in existing state prior renovation. Adapted from [37]. other hand, district heating plants operated on biomass, geothermal, or other renewable sources can be considered renewable. In fact, in Denmark, more than half of district heating plants are powered by renewable sources [44]. Some renewable energy-producing systems cannot deliver energy on demand, as there are reliant on fluctuating natural driving forces. Therefore, such are better suited to supplement a main energy system that is capable of delivering energy on demand. The produced energy by the secondary renewable system may be used to power the main system directly, stored, or sold to the grid. This study investigates four main energy supply systems for heating and domestic hot water and two secondary renewable energy systems. The main supply systems included in the analysis are a district heating sub-station, a natural gas boiler, a Water-to-Water (W/W) heat pump, and an Air-to-Water (A/W) heat pump. Investigated secondary supply systems are the addition of a Domestic Wind Turbine (DWT) and photovoltaic panels (PV).