## 2 Material and Methods  
### The Examined Heating/Cooling System  
The first step in this study is the presentation of the single-effect absorption heat pump which is given in Figure 1. This device operates with LiBr/water working pair and it is a single-stage machine. The heat input of high temperature (e.g., 110 \({}^{\circ}\)C) is given in the generator from the boiler and also heat input of low temperature is given from the evaporator. The other devices (condenser and absorber) reject heat from the system to the ambient or the building depending on the operating mode. During the summer, the evaporator absorbs energy from the building and put it in the device, while during the winter the evaporator absorbs heat from the ambient. More specifically, the heat input in the generator makes possible the high-pressure steam evaporation (state point 3) which feeds the condenser where heat is rejected and the outlet (state point 6) is a high-pressure saturated liquid. This quantity is expanded in the throttling valve without losing energy and it becomes a low-pressure liquid/vapor mixture (state point 7) that feeds the evaporator. Inside the evaporator device, saturated steam of low pressure is created (state point 8) with heat absorbance. This quantity goes into the absorber where there is a LiBr/water weak solution of low pressure. In this device, the steam is absorbed by the LiBr crystals, and heat is rejected. The weak solution (state point 1) leaves the absorber, then with a solution pump reaches high pressure (state point 12) and goes to the solution heat exchanger where it is warmed up (state point 2) before the inlet to the generator. On the other side of the circuit, the strong solution leaves the generator (state point 4), then goes to the heat exchanger where it rejects heat to the weak solution and finally it reaches a colder high-pressure strong solution (state point 45). This quantity is expanded without losing energy in a throttling valve (state point 5) and goes into the absorber in order to close the total cyclic process. Figure 2 depicts the suggested unit for heating and cooling with the utilization of biomass. There are three modes of the system, as shown in Figure 2. More specifically, Figure 2a shows the cooling mode where the evaporator of the heat pump feeds by cooling the building, while the biomass boiler feeds the generator with heat input. Thermal oil is used as the heat transfer fluid from the boiler to the generator due to the high-temperature levels. Specifically, the generator temperature is selected at 110 \({}^{\circ}\)C in order to be able the operation during the summer period, while heat rejection temperatures in the condenser and the absorber were set at 10 K over the ambient temperature. In this scenario, the evaporator temperature is set at 5 \({}^{\circ}\)C in order to be able to feed a cooling system with fan coils. Figure 1: The examined absorption heat pump with LiBr/water working pair. Figure 2: The examined system with the different operating modes. In the heating mode, there are two cases depending on the ambient temperature. More specifically, the heat pump absorbs heat from the ambient during the winter in order to transfer it, after the upgrade, in the building. However, the water in the cycle cannot go lower than 4 \({}^{\circ}\)C, due to freezing reasons. Thus, in low ambient temperatures, the absorption heat pump does not operate and then, the biomass boiler feeds directly the fan coils of the building. Assuming a pinch point temperature of 5 K [23], the critical ambient temperature that separates the heating operating modes is equal to T\({}_{\mathrm{am,cr}}\) = 9 \({}^{\circ}\)C. More specifically, when the T\({}_{\mathrm{am}}\geq\) 9 \({}^{\circ}\)C, then the heat pump operates and provides heating in the building from the condenser and the absorber. On the other hand, when T\({}_{\mathrm{am}}\) < 9 \({}^{\circ}\)C, the heat pump is not possible to operate and the boiler gives direct heat input in the building.