We propose and analyze designs for stationary and compact optical concentrators. The designs are based on a catadioptric assembly with a linear focus line. They have a focal distance of around 10 to 15 cm with a concentration ratio (4.5 to 5.9 times). The concentrator employs an internal linear-tracking mechanism, making it suitable for rooftop solar applications. The optical performance of the collector has been simulated with ray tracing software (Zemax), and laser-based indoor experiments were carried out to validate this model. The results show that the system is capable of achieving an average optical efficiency of around 66% to 69% during the middle 6 (sunniest) h of the day. The design process and principles described in this work will help enable a new class of rooftop solar thermal concentrators.
Recent studies [1-3] have demonstrated that nanotechnology, in the form of nanoparticles suspended in water and organic liquids, can be employed to enhance solar collection via direct volumetric absorbers. However, current nanofluid solar collector experimental studies are either relevant to low-temperature flat plate solar collectors (<100 °C)  or higher temperature (>100 °C) indoor laboratory-scale concentrating solar collectors [1, 5]. Moreover, many of these studies involve in thermal properties of nanofluid (such as thermal conductivity) enhancement in solar collectors by using conventional selective coated steel/copper tube receivers , and no full-scale concentrating collector has been tested at outdoor condition by employing nanofluid absorber [2, 6]. Thus, there is a need of experimental researches to evaluate the exact performance of full-scale concentrating solar collector by employing nanofluids absorber at outdoor condition.<p> </p> As reported previously [7-9], a low profile (<10 cm height) solar thermal concentrating collector was designed and analysed which can potentially supply thermal energy in the 100-250 °C range (an application currently met by gas and electricity). The present study focuses on the design and experimental investigation of a nanofluid absorber employed in this newly designed collector. The nanofluid absorber consists of glass tubes used to contain chemically functionalized multi-walled carbon nanotubes (MWCNTs) dispersed in DI water. MWCNTs (average diameter of 6-13 nm and average length of 2.5-20 μm) were functionalized by potassium persulfate as an oxidant. The nanofluids were prepared with a MCWNT concentration of 50 ± 0.1 mg/L to form a balance between solar absorption depth and viscosity (e.g. pumping power). Moreover, experimentally comparison of the thermal efficiency between two receivers (a black chrome-coated copper tube versus a MWCNT nanofluid contained within a glass tubetube) is investigated. <p> </p>Thermal experimentation reveals that while the collector efficiency reduced from 73% to 54% when operating temperature increased from ambient to 80 °C by employing a MWCNT nanofluid receiver, the efficiency decreased from 85% to 68% with same operating temperature range by employing black chrome-coated copper tube receiver. This difference can mainly be explained by the reflection optical loss off and higher thermal emission heat loss the front surface of the glass tube, yielding a 90% of transmittance to the MWCNT fluid and a 0.9 emissivity of glass pipe. Overall, an experimental investigation of the performance of a low profile solar collector with a direct volumetric absorber and conventional surface absorber is presented. In order to bring nanotechnology into industrial and commercial heating applications,
In this paper, a new type of linear focus, linear-tracking, catadioptric concentrator system is proposed and analysed for roof-integrated solar thermal applications. The optical concentrator designs have a focal distance of less than 10cm and are analysed using optical simulation software (Zemax). The results show that a relatively high concentration ratio (4.5 ~ 5.9 times) can be obtained and that the concentrators are capable of achieving an average optical efficiency around 66 - 69% during the middle 6 hours of a sunny day (i.e. a day with ~1000W/m<sup>2</sup> global irradiance). Optical efficiency is analysed for perfect and non-ideal optical components to predict the collector performance under different ‘practical’ circumstances. Overall, we intend for this paper to catalyse the development of rooftop solar thermal concentrators with compact form factors, similar to PV panels.