## 2 Experimental Setup
### Test Procedure  
Following the successful integration of the heat exchanger into the facility, the heat exchanger performance can be evaluated by means of two types of tests, namely, the steady-state test and the on-sun test. In the steady-state test, the T100 unit serves as a heat source since once the T100 has started it can be operated independently, without any help from the heliostat field, by burning diesel fuel. However, to start up the T100 unit, the temperature of air entering the combustion chamber must be around 300 \({}^{\circ}\)C to sustain the combustor flame. To achieve that, the particle temperature inside the heat exchanger must be above 300 \({}^{\circ}\)C to preheat the air. The particle temperature is brought to such temperature by running the facility through a process called the startup/preheating process where particles are circulated through the PHR loop. The particles are allowed to fall through the PHR and get irradiated by the concentrated sunlight reflected from the heliostat field. The particle preheating process continues until the particle temperature inside the heat exchanger gets higher than 300 \({}^{\circ}\)C. By then, the T100 unit can be started. During the T100's startup, the air leaving the recuperator has a temperature less than a 100 \({}^{\circ}\)C, so to reduce the possibility of the heat exchanger thermal shock, the air inlet pipe is wrapped with an electric coil heater (4 kW) to preheat air before entering the heat exchanger. In the same way, the air outlet pipe is wrapped with another coil heater (4 kW) to further heat the air before entering the T100's combustor. Once the T100 unit succeeds to run, the heliostat field and the coil heaters are turned off. The particles are then diverted to the bypass line and kept circulated through the bypass loop. Figure 4 shows the air and particle temperatures at the inlet and outlet of the heat exchanger, in addition to the particle temperature at the PHR's inlet, during the preheating/startup process until steady-state condition is reached. As shown in the figure, the preheating process takes around two hours.  
The control system of the T100 unit allows the turbine outlet (flue gases) to be maintained at a fixed temperature. Consequently, the compressed air leaving the T100's recuperator goes to the heat exchanger with constant conditions (i.e., constant temperature, constant pressure, and constant flow rate). The temperature of the recuperated air can go up to 550 \({}^{\circ}\)C. This hot air is used to heat the particles circulated in the heat exchanger's tubes. The particle heating process in the heat exchanger continues until the whole facility reaches a steady-state situation, where the air and particle temperatures reach a plateau as seen in Figure 4. Several hours (around 8 h, as seen in Figure 4) are needed to reach the steady state due to the large thermal inertia of the facility. The steady-state experimental data are then collected to evaluate the heat exchanger performance.  
In the on-sun test, the hot particles (with a temperature usually around 500 \({}^{\circ}\)C) are diverted to the PHR, get irradiated by the heliostat field. The further-heated particles then flow inside the PBHX's tubes to heat the compressed air needed to operate the T100 unit. The test is usually conducted after the facility reaches a steady-state condition. The test really depends on the weather conditions and cannot be performed unless the direct normal irradiance (DNI) is suitable (usually above 700 W/m\({}^{2}\)). This test is intended to investigate the percentage of the captured solar energy (in the PHR) which can be transferred to the compressed air of the T100 unit. This percentage is called the solar contribution. The solar contribution depends on the effectiveness of the heat exchanger. The higher the effectiveness, the more thermal energy is transferred from particles to the air of the T100 unit. More elaboration on the on-sun operation can be found in [14].