## 2 Methods
### A Multisource Heat Pump System for a New School Building: Design Phase  
The authors contributed to the design, monitoring, and data analysis of the multisource absorption heat pump plant of a new school building in the town of Agordo (North Italy). In this site, the climate is a temperate one (Koppen climate classification, Cfb-Cfc) with cold winters (3376 heating degree days--HDD) and mild or cool summers. The main building features are: 5680 m\({}^{2}\) of heated area and 19,640 m\({}^{3}\) of enclosed gross heated volume (Figure 1).  
A simplified functional scheme of the plant reports the main hydraulic streams and energy flows (Figure 2). As the building is mainly closed or unattended in summertime, no cooling loads are present, and the plant was designed to serve for the sole purpose of heating and ventilation. The very reduced demand for hot tap water is fulfilled by means of electric water heaters.  
Figure 1: View of the school building in Agordo (from south).  
Figure 2: Reduced functional diagram of the HVAC plant. The main mass and energy flows (gas, ground, solar, and recovery) are shown.  
The HVAC plant is set up by two main sections, space heating (the left part of Figure 2) and ventilation (the right part). Both sections feature two ammonia-water absorption heat pumps (model Robur GAHP-W-LB, HP1-HP4 in), producing thermal energy at 45 and 55 \({}^{\circ}\)C, respectively. The geothermal heat pumps are modeled in Trnsys(r) by a self-developed type (type 701), based on Robur's operation data. The type allows different cold sources (ground, solar, heat recovery) and the daily alternative operation of the two heat pumps for each section to be managed. The geothermal exchanger (total length 960 m, 6 probes \(\times\) 160 m each in a parallel row for the heating section; total length 750 m, 6 probes \(\times\) 125 m each in a row for the ventilation section) is constituted by vertical double-U pipes, with an outer diameter of 32 mm and a thickness of 2.9 mm. It is simulated by type 131, a Trnsys(r) type developed during 1990s on the base of the Hellstrom and Eskilson model on vertical geothermal heat exchangers [21]. Moreover, the plant includes 50 m\({}^{2}\) of flat type solar thermal collectors (4 arrays in parallel, each of those made of 5 modules in series), simulated by Trnsys(r) type 73.  
The heat recuperators downstream of the AHU are a static cross flow type, with an efficiency of 50%. Run-around coils are installed at the outlet of two out of four of AHUs (the one of the laboratories and the one of the teaching rooms, for a global volume flow of 20,600 out of 25,000 m\({}^{3}\)/h) to further increase thermal recovery when the external temperature exceeded 0 \({}^{\circ}\)C. The run-around coils can operate on the exhaust flow of a heat exchanger; the heat recovered can be sent to the evaporator of the absorption equipment (see next section for the control logic). Both the heat recuperator and the run-around coil are modeled by self-made types (types 712 and 713, respectively): The former has the efficiency and the inlet air temperature and humidity as known inputs to calculate outlet air conditions; the latter allows to calculate the water outlet temperature by means of the heat exchange with air.  
The HVAC system in teaching rooms, laboratories, and offices provides space heating by means of a radiant floor and ventilation by means of three independent AHUs, each of those serving a single duct system. The auditorium is served by an all-air system. According to the UNI EN 12831:2006 standard, the maximum thermal load is 146 kW for space heating and 122 kW for ventilation, with the design indoor conditions set to 20 \({}^{\circ}\)C of air temperature and the ventilation being neutral (air supplied at 20 \({}^{\circ}\)C).  
Heat pumps are designed to cover the base load, whereas a condensing boiler supplements the peak load as well as it is a backup for an eventual fault of the heat pumps (Table 1).