## 3 Results and Discussion
### System Results  
The performance of the boiler-driven absorption heat pump is given in Figure 7. This figure shows its performance for the different operating conditions and more specifically there are three operating modes, two for heating and one for cooling. In this simplistic presentation, it is assumed that the cooling mode starts for ambient temperatures over 20 \({}^{\circ}\)C and the cooling performance has a decreasing rate with the ambient temperature increase. The range of the cooling performance is from 0.62 to 0.78. It is significant to state that the increase in the ambient temperature over 35 \({}^{\circ}\)C leads to an important reduction in the performance because the system faces difficulties to produce cooling on very hot days. The heating modes are separated by the critical ambient temperature of 9 \({}^{\circ}\)C. In lower ambient temperatures, only the boiler operates with an efficiency of 90%, while in higher temperatures the heat pump operates. The performance of the heat pump ranges from 1.49 to 1.63 and it has an increasing trend with the ambient temperature increase. \begin{table}
\begin{tabular}{c c c} \hline
**Month** & **Heating (kWh)** & **Cooling (kWh)** \\ \hline January & 2712 & 0 \\ February & 2193 & 0 \\ March & 1597 & 0 \\ April & 186 & 0 \\ May & 5 & 151 \\ June & 0 & 1412 \\ July & 0 & 2751 \\ August & 0 & 2669 \\ September & 0 & 1148 \\ October & 0 & 37 \\ November & 487 & 0 \\ December & 1956 & 0 \\ \hline Year & 9136 & 8168 \\ \hline \end{tabular}
\end{table} 
Table 4: Monthly thermal loads for the examined building in Athens. Figure 6: Monthly thermal loads (heating and cooling) for the examined building in Athens. Figure 8 shows the instantaneous demand for biomass heat and Figure 9 shows the respective instantaneous demand in fuel mass flow rate. There is a great variation in the demand due to (i) the variation of the load, (ii) the variation of the efficiency of the absorption heat pump and ii) the different operating modes of the system. The maximum demand for the heating mode was found to be 9.28 kW with a respective fuel mass flow rate of around 0.5 g/s. In the cooling mode, the maximum demand is 12.83 kW with a respective fuel demand of around 0.7 g/s. The difference between the maximum demands is justified by the reduced efficiency of the system during the cooling period. Figure 8: Heat demand of the boiler during the year period, both for covering heating and cooling. Figure 7: Performance of the absorption heat pump in different weather conditions. Figures 9 and 10 depict the cumulative energy demand curves for the fuel energy and the fuel mass consumption. In total, the heat input demand from the biomass was calculated at 17859 kWh and it is separated into two quantities one for heating and one for cooling. The fuel energy demand for cooling was calculated at 10870 kWh (61% of the total demand), while the heating demand is 6989 kWh (39% of the total demand), significantly lower due to the higher heating performance. It is useful to state that the respective mass of the fuel, according to Figure 11, for the heating production is 1644 kg, while for cooling is significantly higher at 2119 kg, while the total consumption during the year is 3763 kg. More specifically, Figure 12 makes clear that the efficiency during the winter is higher compared to the summer period. It was calculated that the mean heating energy performance is 1.307, while the respective cooling efficiency is 0.751. Figure 13 shows that the heating performance is characterized by higher exergy efficiency values compared to the cooling period. It is important to state that the mean heating mode exergy efficiency is 14.48%, while for the cooling mode is only 6.31%, respectively. The main reason for this result is the reduced energy performance during the summer which leads to higher fuel consumption in this period. Figure 9: Consumption of biomass during the year period, both for covering heating and cooling. Figure 11: The cumulative biomass consumption for heating, cooling, and total.