# Assessment of the RACPC Performance under Diffuse Radiation for Use in BIPV System
## 1 Introduction  
A significant amount (around 40%) of the energy usage worldwide is consumed by buildings. The necessity of transforming buildings from energy users to energy producers has motivated an increase in renewable energy's share of global energy consumption [1; 2; 3; 4; 5; 6]. Building integrated photovoltaic(BIPV) systems integrate photovoltaic panels into buildings as windows, facades or roofs by replacing conventional materials and producing renewable electricity onsite. The cost-effectiveness of the system comes from the considerable savings in terms of material costs, electricity, and improvements to the buildings' energy performance [7; 8; 9]. Figure 1 shows some of the BIPV implementations. BIPV systems have been implemented in different places around the world proving their benefits. This has motivated projects developers, investors and architects to include this type of system in their building planning. In Mexico City a BIPV system was implemented with a photovoltaic (PV) skylight in a shopping centre, which prevented the release of 233 tons of CO\({}_{2}\) into the atmosphere and with a payback of less than a year. In Honolulu there is another example where a photovoltaic canopy was implemented into a building generating 58,000 kWh per year, this being enough power to feed 3300 lights in the building and prevent the release of 38 tons of CO\({}_{2}\) into the atmosphere [12]. Therefore, different targets regarding building energy efficiency have been created. In the United Kingdom, the \"Nearly Zero-Energy Buildings\" of the Energy Performance of Building Directive (EPBD) [13] seeks to achieve that all new domestic buildings meet a Zero Carbon Standard. In the USA the \"Net Zero Energy Building\" introduces the BIPV as an ambitious system which needs to be implemented in all new buildings with the aim of fulfilling the set standards for every building of energy neutrality [4]. A Nearly (or Net) Zero Energy Building is a building with a high energy performance. It can generate its own energy on-site or nearby--typically via renewable sources--to meet all its annual energy requirements. A major issue for the PV industry and hence to the BIPV sector, is to offer customised products at a competitive price. Certainly, the most expensive part of a solar PV system is the PV cells, which represent between 40% and 50% of the total cost [14]. Therefore, the PV and BIPV industries seek to achieve an increase in the efficiency of the system at an affordable price. They seek to improve existing technologies and bring the development of new technologies together with conventional building materials to serve as an intelligent, multifunctional compound [4]. The concentrating photovoltaic (CPV) system is one of the most recent technological advances proposed to achieve this goal, since it replaces the solar cell material with a cheaper material of which the optical concentrator is made. To date, several types of CPV systems have been investigated incorporating many concentrator designs such as variations of compound parabolic concentrators (CPCs) [15; 16; 17; 18; 19], variations of dielectric totally internally reflecting concentrators (DTIRCs) [20; 21], luminescent solar concentrators (LSCs) [22; 23], etc. [PERSON] et al. [24] studied the performance of an asymmetric CPC-PV and tested it indoor. He found that his concentrating module could achieve a maximum electrical conversion efficiency  
Figure 1: Building integrated photovoltaic (BIPV) integration in (**a**) facade; (**b**) rooftop; (**c**) spandrels and parapets; (**d**) sun shading. Reproduced from [8; 10; 11]. of 12.1%. In terms of cost per unit power output, the CPV module could achieve a 20% reduction when compared with a non-concentrating counterpart. [PERSON] and [PERSON] [25] tested the prototype of a solar concentrator module consisting of a different asymmetric CPC design for building facade integration.