Natasha Hjerrild

Boosting solar energy conversion with nanofluids

Suspensions of metallic nanoparticles can harvest valuable heat from sunlight that would otherwise go to waste in a photovoltaic cell.

Desing and on-sun testing of a hybrid PVT prototype using a nanofluid-based selective absorption filter

Hybrid photovoltaic/thermal (PVT) solar collectors represent a promising approach to generate electrical and thermal energy from the same compact package. Moreover, beam splitting can be incorporated in these collectors to physical decouple the receivers to optimize their efficiency. In such systems, the PV cells are illuminated only with the region of the solar spectrum that matches well with their spectral response curves, while the separate thermal receiver can be operated to produce valuable heat (i.e. a high temperature output). In this work, the design of a 8x suns concentrated PVT prototype using a liquid (nanofluid) selective absorption filter is presented. Finally, actual transmittance curves of different nanofluids are presented which demonstrate that they are excellent candidates to be used as selective band-pass filters in PVT collectors using beam splitting. Preliminary experimental results of this research are also reported - in which the hybrid output is compared with the electrical output of the same PV cell array without spectral splitting.

Experimental investigation of a nanofluid absorber employed in a low-profile, concentrated solar thermal collector

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) [4] 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 [6], 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. 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. 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,

Spectrum splitting using gold and silver nanofluids for photovoltaic/thermal collectors

The thermal yield of hybrid photovoltaic/ thermal (PV/T) collectors is presently limited to low temperatures to prevent degradation of PV efficiency and thermal damage to the cells. This work reports a nanofluid optical filter, which transmits only the portion of the solar spectrum which is most efficiently converted to electricity by the underlying solar cell. This is achieved by suspending both core-shell silver-silica nanodiscs (Ag-SiO2 NDs) and gold nanorods (AuNRs) in an aqueous base fluid to absorb visible light and infrared wavelengths, respectively. The transmission spectrum of each nanofluid can be tailored according to PV cell spectral response and to accommodate for electricity and gas price fluctuations.