## 4 Conclusions  
As a conclusion from the first case-study, the authors' analysis revealed that not only a suitable design of the plant and its control logic is necessary, but an in-depth energy analysis based on data monitoring during the operation is a key factor to keep the energy efficiency at or even above the designed value. This is especially true in a multisource heat pump plant that often appears redundant to the service: if some components fail, they will not be soon replaced when this failure does not produce a plant stop. The integration of different heat sources remarkably increases the efficiency of the system in terms of primary energy consumption. This happens because the control logic allows a high exploitation of the solar heat as of a meaningful fraction to the space heating need and as heat source to the evaporator of the heat pumps. Moreover, it allows a large share of the evaporators' energy need for the heat pumps to be supplied by the ventilation recovery. In the second case study, a suitable control logic developed by the authors for a dual source (ground + glazed PVT) heat pump system allows the ground regeneration by cooling the PVT, thus keeping the electric efficiency of the cells close to the peak value. Furthermore, it allows the heat sources to decouple from the heat pump by a suitable disposition of storage tanks. This permits the efficient use of the heat pump as a chiller with useful heat recovery to face the contemporaneous heating and cooling building loads. The design of the plant by means of dynamic simulation revealed the best alternative and the advantage of increasing the solar field to a certain extent to reduce the ground field extension with better performance and lower costs. As a further result, the plant is more than self-sufficient for electricity on a yearly basis. Figure 13: Annual values of the thermal, electric, and total efficiency of the PVT plant, PER of the whole plant, and the specific non-renewable primary energy consumption. Comparing the two case studies, very high primary energy ratios (around 1.4 based on measured data, and 4.7 based on simulated data) were determined for the first and the second case, respectively. As a consequence, a primary energy consumption of 37 and 3.9 kWh m\({}^{-2}\) y\({}^{-1}\) was deduced respectively. In terms of CO\({}_{2}\) emission, 7.4 and 1.2 kgCO\({}_{2}\) m\({}^{-2}\) y\({}^{-1}\) can be calculated by considering a specific emission factor of 0.201 and 0.308 kgCO\({}_{2}\) kWh\({}^{-1}\) for the natural gas combustion and the Italian electricity mix from the grid respectively. Conceptualization, F.B., R.L., and M.N. ; methodology, F.B., R.L., and M.N. ; software, F.B. and M.N. ; validation, F.B., R.L., and M.N. ; investigation, F.B., R.L., and M.N. ; data curation, F.B., R.L., and M.N. ; writing--original draft preparation, M.N. ; writing--review and editing, F.B., R.L., and M.N. All authors have read and agreed to the published version of the manuscript. This work was founded by the Italian Ministry of Education, University and Research (MIUR) through PRIN 2017 FlexHeat 2017 KAAECT project. Not applicable. Authors would like to kindly thank [PERSON] (Areatecnica), [PERSON], and [PERSON] (General Service Dept of the Belluno Province Administration) for the data provided. The authors declare no conflict of interest. * [PERSON] et al. (2017) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] A novel PV /T-air dual source heat pump water heater system: Dynamic simulation and performance characterization. _Energy Convers. Manag._**2017**, _148_, 635-645. [CrossRef]
* [PERSON] et al. (2017) [PERSON]; [PERSON]; [PERSON]; [PERSON] Operation characteristics of a novel dual source multi-functional heat pump system under various working modes. _Appl. Energy_**2017**, _194_, 236-246. [CrossRef]
* [PERSON] and [PERSON] (2017) [PERSON]; [PERSON] Determining of the optimal design of a closed loop solar dual source heat pump system coupled with a residential building application. _Energy Convers. Manag._**2017**, _147_, 40-54.