A review on optical properties and application of nanofluids in direct absorption solar collectors (DASCs)
Introduction
The rapid increase in energy demand over the past decades caused by global economic development and growing population, has led to the depletion of fossil fuels. The rapid consumption of fossil fuels has resulted in several environmental consequences such as air pollution, acid rain, ozone layer depletion, and global climate change. There is growing empirical evidence that increased extreme weather conditions (e.g. hurricanes, flooding, droughts, heat waves, etc.) frequency, is linked to global warming [1], [2].
Considering the implications of fossil reserves, it is now generally believed that renewable energy technologies can meet much of the growing energy demand without harming the environment [3]. Solar energy is the most abundant permanent energy resource on earth. The solar radiation reaching the earth’s surface in just one year is approximately 3,400,000 EJ; more than 7500 times the world’s total annual primary energy consumption of 450 EJ [4].
It is envisaged that solar energy will be sustainably utilized in the near future instead of other alternative energy forms owing to its unlimited availability and desirable environmental and safety aspects. Because of the tremendous scientific and technological advances made during the last century and the ongoing research and development, it is predicted that by 2100, solar technologies will supply about 70% of world energy consumption [4] .
There are two typical approaches in practical exploitation of solar energy: solar thermal and photovoltaic (PV) technologies. Solar thermal technologies are used to heat air, water or other fluids, depending on the applications. Non-concentering solar thermal technologies usually provide water and air heating, air-conditioning, and industrial process heating. While concentering solar thermal technologies are utilized for electrical power generation.
Solar PV technologies are used to directly convert sunlight to electricity. Although solar PVs may gain more recognition, but solar thermal technology still represents the majority (~70%) of the global installed capacity of solar energy (with 406 GWth of solar water heating and 4.4 GW concentrating solar thermal power of the total 587.4 GW installed capacity of solar energy at the end of 2014). For reference, the global installed capacity of PV systems at the same time was 177 GWe (~30% of the total installed capacity) [5].
The major part of a solar thermal system is the solar thermal collector; a particular heat exchanger device in which the photothermal conversion of the incident solar radiation takes place. Solar thermal collectors can be classified according to their operating temperatures into three categories. Low-temperature collectors such as flat-plate and evacuated-tube collectors which can provide fluids at temperatures up to ~ 85 °C and ~ 120 °C, respectively; medium-temperature collectors which include linear concentrators such as parabolic troughs that can reach up to ~ 400 °C; and high-temperature collectors including dish concentrators and power towers which can achieve even higher temperature levels (~ 1000 °C) [6].
Conventional low-temperature collectors, which are used to collect heat for space heating or hot water supply, consist of surface-based absorbers. The absorber has a solid surface of thermally stable polymers, aluminum, steel or copper, to which a matte black or wavelength-selective coating is applied. This solid surface is backed by a coil of fluid tubing placed in an insulated casing with a transparent glazing. The working fluid circulates through the tubing and absorbs heat indirectly from the absorber via conduction and convection and as a result, the absorber plate is the hottest component of the system. Hence, large amount of heat is lost to the ambient by radiation and convection and to other components by conduction [7]. Due to the existing thermal resistance in converting the incoming radiation into the internal energy of the transport medium; an alternative design idea is proposed in which the working fluid is directly exposed to the incident radiation and the heat is volumetrically absorbed within the transport medium instead within a thin surface layer. The schematic of both designs and their thermal resistance networks are depicted and compared in Fig. 1. As can be seen, the thermal resistance in converting solar energy into thermal energy of the working fluid is markedly reduced in a direct absorption system.
The concept of direct absorption solar collector was initially proposed by Minardi and Chuang in the 1970s who designed a collector in which the working fluid flowed in transparent tubing and directly absorbs solar energy [9]. To increase the absorption capability of the transport medium, the ethylene glycol–water transport fluid was seeded with India ink -a carbon black additive. Lower heat losses and improved thermal performance were reported compared to conventional surface-based absorbers. Huang et al. [10] extended this concept to a parabolic trough collector which used highly absorbent black dye liquid water flowing in a glass tube to directly absorb the concentrated solar radiation. Arai et al. [11] developed a volumetric solar collector in which three kinds of micro-particle semitransparent liquid suspensions: graphite (black), carborundum (gray) and silicon dioxide (white) in diethylphthalate were utilized. Measurements of the absorption coefficient of each suspension demonstrated the effectiveness of using fine-particle additives. Further investigations on high flux direct absorption collector using molten salt demonstrated the necessity of adding particulates to molten salt -which is a relatively weak absorber in the visible and near infrared regions of the solar spectrum; in order to increase its absorption ability [12], [13], [14]. Meanwhile, it was demonstrated that the addition of organic and inorganic micron-sized chromophores such as black dyes or carbon black in low-flux collectors; as well as black metal oxide particles to the salt eutectics in high-flux systems, caused thermal and photochemical degradation [15]; which can lead to several practical drawbacks such as reduced absorption, fouling and clogging of surfaces, erosion and abrasion of pipes and pumps over time [16].
As an innovative product of the emerging world of nanotechnology – homogenous colloidal suspensions of controllably disperse nanoparticles (particles with one dimension <100 nm in size) – termed as “nanofluids” have proven to improve the conductive and convective heat transfer properties of conventional transport mediums as well as limiting the adverse effects contributed to micron-level additives such as sedimentation and degradation. The high surface area to volume ratio of nanoparticles drastically enhances the heat capacity and energy conversion properties of the host medium.
As most commonly used heat transfer fluids (water, glycols, oils, etc.) are weak absorbers over the ultra-violet and visible ranges of the solar spectrum – absorbing only 13% of the incoming solar energy [17]; numerous recent studies have shown that adding very low nanoparticle loadings (<0.01% vol.) considerably improves the optical and photothermal properties of these transparent fluids. Optical properties of nanofluids are of great importance particularly for their potential use in direct absorption collectors in which the working fluid is directly hit by the incident radiation. In addition; the material, shape, size, and volume fraction of nanoparticles can be adjusted in a way that the working fluid absorbs most of the incident radiation in the electromagnetic spectrum regions where a large portion of the incident sunlight energy is present (i.e., ultraviolet, visible, and infrared).
Several recent review papers have been dedicated to the study of nanofluids preparation and characterization, their thermo-physical and radiative properties and applications in solar thermal collectors which are listed in Table 1. The former reviews on applications of nanofluids in solar energy systems are mainly related to their application in various types of solar thermal collectors mostly including surface based systems such as flat-plate and evacuated tube solar collectors.
The present review is intended to exclusively bring the most recent nanofluids optical properties studies together with their utilization in direct absorption solar collector systems obtained by the scientific community; in order to facilitate the researchers to update the recent trends and realize the potential research gap in the field of nanofluid-based DASC systems.
Initially, an overview on the nanofluids preparation and optical characterization techniques is presented. The related modeling and simulation techniques are then illustrated and the latest numerical and experimental works and recent developments in the field of direct absorption collectors are summarized and discussed. Eventually, the present challenges and difficulties in exploiting nanofluids in DASCs as well as future possible directions are outlined.
Section snippets
Nanoparticle materials and characterization methods
Carbon nanotubes and nanohorns, graphite, metallic, metal oxide, nitride, carbide, core-shell, etc. are typical nanoparticles in form of dry powders available off-the-shelf. Nanoparticles are synthesized by either physical or chemical methods. Metal nanoparticles can be synthetized by physical methods that involve the evaporation of the bulk metal precursor followed by its condensation. Chemical synthesis approaches include plasma synthesis of metal oxide, nitride, and carbide nanoparticles [33]
Theories for modeling optical properties
Optical characterization of a material involves the estimation of extinction, absorption, and scattering coefficients, refractive index, dielectric constant, etc. When an electromagnetic wave or a photon interacts with a medium containing small particles, the radiative intensity may be attenuated by absorption and scattering [45]. The models describing absorption and scattering of nanoparticles in a host medium depend upon several factors such as nanoparticle material, size, shape, base fluid
Nanofluid-based DASCs theoretical modeling
Any problem relating to nanofluid-based DASCs can be represented by two main equations: the radiative transport equation (RTE) and the energy equation. Once the radiative transfer equation in participating medium is solved, the energy balance on the solar collector is applied and the temperature profile within the collector is obtained.
Low-flux DASCs
Low-flux direct absorption collectors operate under non-concentrating solar radiation with water or water/glycol mixtures as their base fluids. Ethylene glycol is commonly used as an antifreeze agent in conventional solar thermal collector applications. Theoretical studies focusing on low-flux collectors utilize incident fluxes less than or equal to 1367 W/m2 which is essentially the same as the solar constant [115]. However, most studies used the value of 1000 W/m2 which is the approximate
Current challenges
Based on the aforementioned literature; the main challenges for the utilization of nanofluids in DASC systems are related to the following issues:
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Generally, the high cost of the nanoparticle and nanofluids in preparation limits their commercial utilization. However, studies have shown that only a very loading (<0.01% vol.) of nanoparticles are needed to achieve high absorption in DASC systems which implies low added costs from the base fluid in such applications.
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The nanofluids long-term
Conclusions
The present review aims to summarize the latest development and researches on the optical properties and application of nanofluids in direct-absorption solar thermal collectors. As commonly used heat transfer fluids are weak absorbers over the ultra-violet and visible ranges of the solar spectrum, the addition of nanoparticles has been proved to enhance the optical characteristics of the base medium even at very low nanoparticle loadings. Studies indicate that the performance of DASC systems is
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