## 3 Results
### Single Actions
#### 3.2.3 Global Cost and Energy Performance of Main and Secondary Energy Systems  
Figure 4 shows primary energy demand and global cost for the main and secondary supply options. Global cost is presented using the NPV indicator for the applied calculation period of 50 years, while primary energy demand is found using the compliance tool BE18. Primary energy and NPV for the reference case are represented by a vertical and a horizontal dashed line, respectively (\(\sim\)130 kWh/m\({}^{2}\) year and \(-\)18.22 million DKK). With that representation, cheaper solutions than the reference appear above the horizontal reference line. Consequently, solutions with lower energy demand are located to the left of the vertical reference line. The limit values for reaching NZEB, as well as voluntary renovation classes 1 and 2, are denoted by three additional vertical dashed lines. Renovation  
Figure 3: The cost-effectiveness parameter (CEP) for renewable energy-producing technologies. classes 1 and 2 are stated in Danish building regulations and correspond to 52.8 and 70.4 kWh/m2 year, respectively [39]. As defined in Section 2.1, reaching NZEB in a Danish concept is equivalent to 27 kWh/m2 year, which is marked by the green vertical dashed line. This representation is applied to all following figures comparing NPV and energy demand. The different scenarios of main energy supply are first computed without any application of secondary renewable energy production. Then, each renewable system is added in a one-at-a-time manner. Figure 4 shows that the cheapest option is to run the building on natural gas; however, using gas results in higher primary energy than the reference case. As the required heat demand for district heating and gas scenarios is equal, the increase of primary energy is solely due to the applied national primary energy factors. A factor of one is applied for natural gas compared to 0.85 for district heating and 1.9 for electricity. As a consequence, in cases where the target is reaching NZEB, the natural gas case is not favored in the Danish context. Switching from district heating to either a Water-to-Water (W/W) or Air-to-Water (A/W) heat pump results in lower primary energy demand. Given that the primary energy factor for electricity is higher than this for district heating and gas, the reduction is a consequence of the \"free\" energy produced by the heat pump's high Coefficient of Performance (COP). The COP of the investigated W/W heat pump is higher (4.2) than that of the A/W heat pump (3.65); thereby, the W/W heat pump realizes higher primary energy savings. Although the W/W heat pump is approximately 55% more expensive than A/W  
Figure 4: Comparison of the four studied main supply options, combined with secondary renewable energy-producing systems. No energy-saving measures applied. in regard to acquisition and replacement cost, its higher efficiency brings 17% lower supply demand and 6.5% lower global cost compared to A/W. In spite of their differences, the two heat pump types are comparable. Both scenarios yield lower primary energy demand and higher NPV in comparison to the reference. Nonetheless, the cost increase is relatively small related to the achieved primary energy savings; hence, heat pumps are favored as the main heating supply solution for NZEB renovation. Figure 4 shows that the addition of renewable electricity production results in a reduction of cost and energy demand, regardless of the system type and size. This can be explained by the assumption that all produced energy is used on site. This simplification is made, as the goal of the analysis is to identify the potential of each system rather than find the exact amount energy used on-site. Produced on-site electricity is modeled to cover the necessary demand for building operation and common spaces first, while excess production is assumed to be utilized by private apartments. For all scenarios including DWT, increasing the system size results in a reduction of primary energy and NPV.