# Assessment of the RACPC Performance under Diffuse Radiation for Use in BIPV System
## 4 Experimental Performance Analysis
### Experimental Setup and Results  
The following equipment was used: (1) an RACPC-PV device and a single PV cell device, where each device uses a 1 cm by 1 cm PV cell; (2) inclinometer; (3) two multimeters, and (4) a digital slope meter. The single cell and the concentrator were connected to different multimeters to get simultaneous readings. The temperature depends on the day on which the experiment was carried out. The sun angle was obtained from www.suncalc.org; and with a digital slope meter the ideal inclination towards the south was setup each day in order to maximise the electrical output. The outdoor experiment was carried out on the roof of the EA building of Glasgow Caledonian University (GCU) (N 55\({}^{\circ}\)51'57.71'', W 4\({}^{\circ}\)15'00.49''). This location was selected for two reasons: the first one has to do with the Sky View Factor (SVF), which is a dimensionless parameter and represents the fraction of the visible sky on a hemisphere that lay centred over the selected location. The SVF has values between zero and one; thus, an SVF of one means the complete sky is visible and neither reflected short-wave radiation or additional long-wave radiation is received [35]. The SVF value for the selected site is 0.774 and is shown in Figure 6.  
Figure 5: The RACPC-photovoltaic (PV) cell on the glass substrate and the Sylgard-184® solution in a beaker (left), and the Heraeus Vacuthertm vacuum chamber to eliminate the air bubble (right). The second reason is that the wireless weather station of the GCU is located in the same place. The Integrated Sensor Suite (ISS) and the Vantage Pro2 console are the two components of the weather station. The first one manages the external sensor array and the second one provides the user interface, data display, and calculation [36]. The weather station also includes the solar radiation sensor and the UV sensor, thus providing relevant information such as temperature, wind speed, and the global irradiance every 15 min, which are required in order to calculate the exact amount of diffuse and direct light when the sun angle is known. Therefore, the calculation of reflected irradiation will not take place as it was not considered necessary. The experiment was performed on the 15 th of March 2017 from 9:00 a.m. to 15:30 p.m. with a temperature of 11.9 \({}^{\circ}\)C; and an inclination of 31.96\({}^{\circ}\) facing south. The short circuit currents from the RACPC-PV device and the bare PV cell were measured every 15 min, and using the empirical functions shown below, the global irradiation was separated into direct and diffuse irradiation through the \(\mathrm{k_{T}}\) factor. This takes into account the global irradiance \(\mathrm{E_{G,hor}}\), the extra-terrestrial irradiance \(\mathrm{E_{0}}\) that depends on the distance between the sun and the earth and is the irradiance before reaching the atmosphere (the average value is 1361 W/m\({}^{2}\)) and the sun height \(\mathrm{\gamma_{s}}\)[37] for each specific time. \[k_{T}=\frac{E_{G,hor}}{E_{0}\times\sin\mathrm{\gamma_{S}}} \tag{2}\]  
where \(\mathrm{k_{T}}\)\(\rightarrow\) 0 means overcast and \(\mathrm{k_{T}}\)\(\rightarrow\) 1 means clear days. The \(\mathrm{k_{T}}\) value of the experiment was between 0.14 and 0.44 confirming that the diffuse light was high during the period of the experiment. The diffuse irradiance at the horizontal plane is calculated depending of the value obtained previously due to the fact that the equation varies according to it [37].