Evaluation of the effect of nanofluid-based absorbers on direct solar collector

https://doi.org/10.1016/j.ijheatmasstransfer.2012.05.087Get rights and content

Abstract

As conventional energy sources like fossil fuels are getting rare, cost of energy production has become higher as well as the concern of environmental pollution by burning of fossil fuels among the developed and developing nations. Solar energy is the most vastly available energy and very effective in terms of energy conversion. The most common solar thermal collector used is the black surface as radiant absorber but the thermal energy efficiency is low. In this study, the effect of nanofluid has been analyzed by using as working fluid for direct solar collector. The extinction coefficient of water based aluminum nanofluid has been investigated and evaluated by varying nanoparticle size and volume fraction. The particle size has minimal influence on the optical properties of nanofluid. On the other hand, the extinction coefficient is linearly proportionate to volume fraction. The improvement is promising within 1.0% volume fraction and the nanofluid is almost opaque to light wave.

Introduction

Sustainable energy generation is one of the most important challenges faced by our society today. Electricity consumption is increasing year by year and electricity generation has become a more major issue in the industry. Electricity supplies an increasing share of the world’s total energy demand and is growing faster than liquid fuels, natural gas, and coal in all end-use sectors except transportation. Fossil fuels such as coal, petroleum and natural gas are used to steam generation in boilers of power plants and these plants stand a majority stack in the electricity power plant in the whole world. In addition, air pollution due to burning of fossil fuels has always been an issue for the governments, investors, environmentalists and researchers.

Solar thermal collectors are heat exchangers that are used to transform solar radiation energy to internal energy of the transport medium. Non-concentrating solar collectors can be used if a large amount of solar radiation is concentrated on a relatively small collecting area. Concentrating collectors exhibit certain advantages as compared with the non-concentrating collectors [1]. The most common system used in water heating system are the flat-plate, black-surface absorbers, which absorb solar energy through a solid surface [2]. The flat plate solar collector (FPC) has been built in a wide variety of designs with many different materials that are usually employed for low temperature applications up to 100 °C [3]. A theoretical method and novel computation algorithm were developed for the simulation of solar flat plate collectors with transparent insulation (TI) [4]. Another category of collectors is the uncovered or unglazed solar collector [5]. These collectors are usually the cheaper solution but still offer effective solar thermal energy in applications such as water preheating for domestic or industrial use, heating of swimming pools [6], space heating and air heating for industrial or agricultural applications. Conventional flat-plate solar collector is used in sunny and warm climates where the performance is greatly shrunk during cold, cloudy and windy days [3].

The idea of volumetric absorption using small particles has been proposed long before the ability to controllably synthesize nanoparticles was developed [7], [8], [9], [10]. Tyagi et al. [11] investigated the feasibility of using a non-concentrating direct absorption solar collector (DAC) and compared its performance with a typical flat-plate collector. Aluminum nanofluids were used and it was observed that the absorption of incident radiation is increased by 9 times compared to pure water. The efficiency of DAC by using nanofluid has been found to be up to 10% higher than that of flat-plate collector.

Solar-weighted absorption coefficient for fluid’s baseline capacity for absorbing solar energy has been investigated and it was found that water is the best absorber among the four tested liquids namely water, ethylene glycol, propylene glycol and therminol VP-1 [12]. However, it is still a weak absorber, only absorbing 13% of the energy. On the other hand, the addition of small particles causes scattering of the incident radiation allowing higher levels of absorption within the fluid, and hence an enhancement in collector efficiency [13]. The optical properties of the effective fluid are highly dependent on the shape and size of the particle and the optical properties of the base fluid and particles themselves [14]. The radiation absorption characteristics of a Ni nanoparticle suspension were investigated by spectroscopic transmission measurement [15]. Recently, carbon nanohorns (CNHs), as the latest to be discovered in the family of carbon-based nanostructured materials [16] with large surface area and large number of cavities [17] have been used as nanoparticles to improve optical properties of direct solar absorbers. The studies showed promising improvement of absorption and scattering properties for carbon nanohorns-based nanofluid compared with water and glycol as base fluid [18], [19]. The optical absorption of nanofluid also depends on the concentration or volume/weight fraction of nanoparticles added to the base fluid. The higher concentration nanofluid shifts the absorption spectra towards longer wavelength [20], [21]. Carbon nanotubes, graphite, and silver were used as nanoparticles for absorption mechanism in solar thermal collectors where efficiency improvements of up to 5% were observed [22]. Taylor et al. [23] also used two models to obtain the optical properties of nanofluids. The authors claimed that by using nanofluid in an absorber near 95% of sunlight could be absorbed. Based on the model validated and presented by Lenert and Wang [24] when integrated with a power cycle, volumetric nanofluid receivers can cause the efficiency to go higher than 35%. They also outlined that optical thickness and solar concentration play substantial role in the performance. In another study by Lu et al. [25], it was stated that mass concentration can remarkably affect the performance and heat transfer in a solar collector (evaporating heat transfer coefficient was augmented up to 30%). PH values of the nanofluid has also been indicated to be effective by Yousefi et al [26]. They investigated the impact of using water with carbon nanotubes on the efficiency of a flat-plate solar collector and concluded that an increase on the difference between PH of the nanofluid and that of the isoelectric point causes the increase in the efficiency. The study by Yousefi et al. [27] also for flat-plate collectors showed an increase of 28.3% in the efficiency of the collector when operating with Al2O3/water nanofluid instead of pure water. The addition of nanoparticles to a base fluid has been shown to improve thermal conductivity. Studies suggested that the thermal conductivity is enhanced due to dispersion of nanoparticles [28], [29], intensification of turbulence [29], Brownian motion [30], [31] and thermophoresis [32]. The efficiency of a solar thermal collector relies on the effectiveness of absorbing solar radiant power and heat transfer from the absorber to the carrier, which is normally fluid. The conversion of highly concentrated sunlight into thermal energy suffers from relatively low efficiencies of 50% to 60% [33]. The application of nanofluids as a working medium for solar collectors is a relatively new concept. Further study to improve the physical properties for enhancing direct solar collectors has to be carried out. With the higher energy form and availability of solar power, researchers are interested in further developing the various aspects to make use of this energy.

The purpose of this study is:

  • (1)

    To investigate the suitability of nanofluid as a volumetric absorber.

  • (2)

    To explore the radioactive properties namely the base fluid and the nanoparticle.

  • (3)

    To find the effect of nanoparticle sizes and volume fractions for nanofluid as well as comparing its transmissivity of light.

Section snippets

Methodology

In this section the working principle, the incident solar intensity and transmissivity of Direct Absorption Collector (DAC), and finally the governing equations are used for nanofluid to develop its optical properties.

Results and discussion

Aluminum is used as nanoparticle in this research as it is one of the most common and vastly available metals. Besides, the thermal properties of aluminum based nanofluid have been studied extensively over the past decades and have shown promising improvement in thermal properties. The effects of nanoparticles in enhancing the optical properties are explored with nanoparticle sizes and volume fraction as changing parameters. The volume fraction of nanoparticles in the nanofluid is also to be a

Conclusions

Solar energy is considered to be one of the best sources of renewable energy with the least environmental impact. Direct absorption solar collectors have been used for a variety of applications such as water heating; however the efficiency of these collectors is found to be limited by the absorption properties of the working fluid, which is very poor for typical fluids which are used in solar collectors. Direct solar collector uses nanofluids as volumetric absorber and is expected to provide

Acknowledgement

The authors would like to acknowledge the financial support from the High Impact Research Grant (HIRG) project no: UM.C/HIR/MOHE/ENG/40.

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