Feasibility of nanofluid-based optical filters

In this article we report recent modeling and design work indicating that mixtures of nanoparticles in liquids can be used as an alternative to conventional optical filters. The major motivation for creating liquid optical filters is that they can be pumped in and out of a system to meet transient needs in an application. To demonstrate the versatility of this new class of filters, we present the design of nanofluids for use as long-pass, short-pass, and bandpass optical filters using a simple Monte Carlo optimization procedure. With relatively simple mixtures, we achieve filters with <15% mean-squared deviation in transmittance from conventional filters. We also discuss the current commercial feasibility of nanofluid-based optical filters by including an estimation of today's off-the-shelf cost of the materials. While the limited availability of quality commercial nanoparticles makes it hard to compete with conventional filters, new synthesis methods and economies of scale could enable nanofluid-based optical filters in the near future. As such, this study lays the groundwork for creating a new class of selective optical filters for a wide range of applications, namely communications, electronics, optical sensors, lighting, photography, medicine, and many more

A review of solar hybrid photovoltaic-thermal (PV-T) collectors and systems

In this paper, we provide a comprehensive overview of the state-of-the-art in hybrid PV-T collectors and the wider systems within which they can be implemented, and assess the worldwide energy and carbon mitigation potential of these systems. We cover both experimental and computational studies, identify opportunities for performance enhancement, pathways for collector innovation, and implications of their wider deployment at the solar-generation system level. First, we classify and review the main types of PV-T collectors, including air-based, liquid-based, dual air–water, heat-pipe, building integrated and concentrated PV-T collectors. This is followed by a presentation of performance enhancement opportunities and pathways for collector innovation. Here, we address state-of-the-art design modifications, next-generation PV cell technologies, selective coatings, spectral splitting and nanofluids. Beyond this, we address wider PV-T systems and their applications, comprising a thorough review of solar combined heat and power (S–CHP), solar cooling, solar combined cooling, heat and power (S–CCHP), solar desalination, solar drying and solar for hydrogen production systems. This includes a specific review of potential performance and cost improvements and opportunities at the solar-generation system level in thermal energy storage, control and demand-side management. Subsequently, a set of the most promising PV-T systems is assessed to analyse their carbon mitigation potential and how this technology might fit within pathways for global decarbonization. It is estimated that the REmap baseline emission curve can be reduced by more than 16% in 2030 if the uptake of solar PV-T technologies can be promoted. Finally, the review turns to a critical examination of key challenges for the adoption of PV-T technology and recommendations

Nanofluid-based optical filter optimization for PV/T systems

Optical filters are essential in a wide range of applications, including optical communications, electronics, lighting, optical sensors and photography. This article presents recent work which indicates that optical filters can be created from specialized nanoparticle suspensions. Specifically, this article describes a theoretical optimization process for designing nanofluid-based filters for hybrid solar photovoltaic/thermal (PV/T) applications. This particular application is suitable because nanofluids can be utilized as both volumetric solar absorbers and flowing heat transfer mediums. The nanofluid filters described in this work compare favorably with conventional optical filters for five photovoltaic (PV) cell alternatives: InGaP, CdTe, InGaAs, Si, and Ge. This study demonstrates that nanofluids make efficient, compact and potentially low-cost, spectrally selective optical filters

High temperature optical properties of nanoparticle suspensions for direct solar absorption

Novel approaches for solar energy conversion are increasingly garnering research and commercial interest, particularly as hybrid thermal and electrical energy sources. Additionally, the need for systems capable of producing thermal energy at temperatures up to 300°C is growing as a means to provide process heat to industry and distributed generation for small communities. One such concept under rapid development since 2008 is the use of nanoparticles suspended in a heat transfer fluid that is directly exposed to incoming solar irradiance. Such collectors, often referred to as direct absorption solar collectors or volumetric solar collectors, have primarily been investigated (experimentally and numerically) at low temperatures due to the challenge of creating long-term stable suspensions at temperatures above 100°C. Working fluids with boiling points well above 100°C are not well investigated for nanoparticle dispersion. The likely reason for this gap is that many high temperature fluids are non-polar, which makes the stability problem even more challenging. Additionally, most surfactants work best in water and break down above 100oC. Thus, even though many solar collectors operate above 100oC and many applications require >100oC heat, only a limited number of investigations have measured the optical properties of direct absorber liquids after exposure to these temperatures. To our knowledge no study has investigated nanofluid optical properties above 200°C. This represents critical missing data in the field, since high temperatures can affect particle stability, particle morphology, and plasmon resonance all which can lead to spectral changes in transmittance while changes in the fluid optical properties can occur at temperature. Additionally, the large coefficient of thermal expansion associated with common heat transfer fluids can result in significant changes to the working volume fraction leading to reduction in overall absorption magnitude. In this study, the optical properties of selected fluids in the solar spectrum were measured from room temperature up to 300°C. Glycol, silicon, hydrocarbon, diphenyl-oxide/biphenyl based fluids with nanotubes and indium tin oxide nanoparticles suspended with various surface treatments. As such, this paper provides a vital set of optical property data to enable further development of promising candidates for broadband spectrally selective nanofluid solar absorbers.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Nanofluid-based direct absorption solar collector

Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors

Tunable optical filtration using liquid nanofluids

Optical filters are used in a wide range of applications - optical communications, electronics, optical sensors, lighting, photography and many more. In this article we report recent modelling and design work which indicates that mixtures of nanoparticles in liquids can be used as a feasible alternative to conventional thin film optical filters. The major motivation for creating liquid filters is that they can be pumped in and out of a system to meet transient needs in an application. To demonstrate the versatility of this new class of filters, we present (by comparison with conventional thin film filters) the design of nanofluids for use as long pass, short pass, and band pass optical filters using a simple Monte Carlo optimization process. For each of these types of filters we achieve < 15% mean squared deviation of transmitance from conventional filters. This analysis also revealed that nanofluid-based filters can potentially be fabricated at low cost. This is due to the fact that only extremely low particle concentrations, < 0.01% by volume, are required for effective optical filtration. Thus, this study gives a first look at how nanofluids can be used to create spectrally selective filters over any optical wavelength

Envisioning advanced solar electricity generation: Parametric studies of CPV/T systems with spectral filtering and high temperature PV

Hybrid photovoltaic/thermal (PV/T) devices can simultaneously generate thermal and electrical energy, but have been limited to low temperature applications. This is done to avoid performance degradation of the PV cell at high temperatures. While the low temperature approach limits PV losses, it would be desirable to develop concentrating photovoltaic/thermal (CPV/T) systems which operate at higher temperatures where the thermal energy can be utilized for electricity production. In addition to using more of the sunlight, these systems may help realize low costs as well as dispatchability through thermal energy storage. Presented here are two primary configurations: the first where the PV cell and thermal system are decoupled, and the second where the PV cell acts as the high temperature absorber. The efficiency of these systems and their ratio of thermal to electrical energy produced are reported as a function of architecture, cell bandgap, and thermal system peak temperature. The studies provide a basis to understand and compare the performance of CPV/T at different operating conditions and architectures. The results indicate that the configurations that utilize recovery of waste heat off the PV cell can achieve the highest efficiency at low concentration ratios, but it remains to be seen if PV cells can survive the temperatures necessary. The thermally decoupled case is attractive in terms of efficiency and operating temperature of the PV cell, but requires significantly higher concentration ratios. Both configurations can achieve exergetic efficiencies exceeding 40% and greater than 50% of the solar energy converted to dispatchable thermal exergy

Limits of selectivity of direct volumetric solar absorption

Direct volumetric absorption of solar radiation is possible with fluids which have controlled optical properties. As with conventional surface absorbers, it is possible to make direct absorbing collectors 'selective' where short wavelength absorption is maximised, but long wavelength emission is minimised. This work investigates the fundamental limits of this concept as it pertains to nanofluid-based direct absorbing collectors. This is especially important at higher operating temperatures (100-600. °C) where radiative losses increase significantly.A study of optical parameters of collector components is conducted herein to investigate the best theoretically (and practically) achievable 'selective' nanofluid-based direct absorbing collectors. When the effect of the short wavelength optical properties was investigated, a short wavelength optical depth of 3 was found to be sufficient for efficient absorption of solar radiation while scattering is minimised. It is also advantageous to use a base fluid which shows weak absorption at long wavelengths to reduce emission losses.Overall, this study directs future research of direct absorption by underlying theoretical and real-world limitations of a selective direct absorbing collector - an emerging receiver technology that can be used for efficient solar thermal harvesting