**2.2 Energy harvesting**

Titania is also used for harvesting photoelectric and photothermal energy. The following sections discuss in detail the use of titania in various energy harvesting applications.

## *2.2.1 Photoelectric energy harvesting*

Photoelectric energy is harvested by photovoltaic modules, and titania has also been used to fabricate PV modules for efficient electricity generation from solar energy. Dye-sensitized TiO2 is used in Grätzel photoelectric solar cells to generate electricity from solar energy. Titania is the essential element of these solar cells. These photovoltaic modules were invented by Grätzel [48] and known as dyesensitized solar cells (DSSC). The working mechanism of DSSC has been presented in **Figure 11**. These modules are cheaper and produce good efficiency. Initially, the

**Figure 9.** *Effect of Re and nanoparticle concentration on Nu at 35 W source power [37].*

**Figure 10.** *Reflectance of titania nanoparticles [44].*

efficiency of these modules was about 7% as reported by Kay and Grätzel [50]. Later on, the claim of achieving 7%, the photoelectric conversion efficiency was verified by Hagfeldt et al. [51]. The limited efficiency of these modules is attributed to very high resistance and losses due to the cells being in series and resulting in a lower fill factor. Kim et al. [52] conducted a detailed study on power conversion efficiency (PEC) evaluation of these cells and reported 9.4% PEC. Several studies have been conducted on these cells to analyze the performance and environmental and economic aspects [48, 53–56].

#### *2.2.2 Photothermal energy harvesting*

Photothermal energy harvesting is carried out by various types of solar collectors (**Figure 12**). The basic element of solar collectors is the receiver tube in which a fluid flows to capture the solar energy by absorbing solar radiations energy and converting it into heat energy through the photothermal conversion process. The

*Titanium Dioxide: Advancements and Thermal Applications DOI: http://dx.doi.org/10.5772/intechopen.101727*

**Figure 11.** *Schematic illustration of DSSC [49].*

photothermal efficiency of these collectors is mainly dependent on the material and geometrical characteristics of the receiver tube and the thermophysical and optical characteristics of the working fluid. Conventionally, water has been utilized as a working fluid in solar collectors; however, water has a very limited range of solar absorbance, which means the rest of the energy is wasted in the form of reflected energy and thermal losses.

In the latest studies, nanofluids have been tested for photothermal conversion since the presence of nanoparticles increases the range of solar absorption, which results in increased thermal energy output and increased efficiency [58]. Moreover, the two types of nanoparticles are also dispersed in the base fluid (called hybrid nanofluids) to tailor the fluid's absorption range through the synergistic effect of different nanoparticle types [59]. Greater thermal conductivity and heat transfer coefficient of nanofluids also intensify the efficiency of solar collectors such as parabolic trough collectors. Since TiO2 carries good thermal conductivity, titaniabased nanofluids have widely been tested for photothermal energy conversion and heat energy transportation in solar collectors [60–62]. Titania depicts higher reflectance and smaller spectral absorbance. However, titania nanoparticles are used to broaden the spectral absorption range of water and other base fluids. Moreover, the combined use of titania nanoparticles with other types of nanoparticles in the base fluid further broadens the solar spectral absorption range and thermal efficiency of solar collectors (**Figure 13**).

Kiliç et al. [63] studied the performance of TiO2/water nanofluid as a working fluid in a flat plate collector (FPSC). They obtained 48.67% topmost efficiency of the collector when using titania-water nanofluid, and the efficiency of the collector reached 36.20% in the case of water as working fluid. They attributed the efficiency increase to the increased heat transfer surface area and higher heat capacity of the nanofluid. Said et al. [64] also appraised the performance of FPSC using titaniawater nanofluid and reported a 76.6% increase in energy efficiency at a 0.5 kg/min flow rate of 0.1 vol.% TiO2/water nanofluid as compared with water. The topmost energy efficiency was observed to be 16.9%. They reported negligible enhancement in pumping power for nanofluid as compared with the base fluid. Tehrani et al. [65] analyzed the performance of ribbed FPSC using TiO2/water nanofluid. Ribs

#### **Figure 12.**

*Photothermal energy-generating solar collector [57].*

improved the performance of FPSC by 10%. They reported slight improvement with increasing nanoparticle concentration as well.

Gan et al. [66] appraised the performance of evacuated tube solar collectors ETSC using TiO2/water nanofluid. They recorded a 16.5% increase in thermal efficiency of the collector by using optimized titania-water nanofluid as compared with the efficiency of the water-based collector. The obtained collector's thermal efficiency was 44.85%. An increase in nanoparticle concentration improved the thermal conductivity of the nanofluid, which resulted in higher thermal efficiency. Hosseini and Dehaj [67] also tested the performance of U-shaped ETSC using titania-water nanofluid. They evaluated the effect of titania particle shape on the collector's efficiency. TiO2 nanowires (NWs) and nanoparticles (NPs) dispersed in

*Titanium Dioxide: Advancements and Thermal Applications DOI: http://dx.doi.org/10.5772/intechopen.101727*

**Figure 13.** *Absorptance of titania, silver, and combined titania-silver [60].*

the base fluid. Efficiency improvement of 21.1% for TiO2 NWs nanofluid and 12.2% for TiO2 NPs nanofluid was recorded. However, NWs nanofluid depicted a higher pressure drop.

#### *2.2.3 Thermal energy storage*

Titanium holds porous properties and a good ability to stay stable when impregnated in some chemical. Therefore, titania is extensively used in thermal energy storage applications. Thermal energy-storing phase change materials (PCMs) have good thermal storage capacity, but due to their small thermal conductivity, the rate of thermal storage is much slower, which causes them to store a small amount of thermal energy. The addition of titania nanoparticles improves their thermal conductivity, and therefore, the capacity to store thermal energy becomes much higher. Titania also provides chemical stability.

Fikri et al. [68] added titania nano-powder in paraffin wax (PW) PCM to overcome the challenge of low thermal conductivity and small spectral absorption. They also reported the thermal and optical advantages of adding titania in PCMs. Sun et al. [69] reported a 26% elevation in thermal conductivity of polyethylene glycol (PEG) PCM by adding titania nanoparticles.

The challenge associated with nano-enhanced PCMs is the deterioration of thermal conductivity and heat storage capacity due to the thermal cycles. Sami and Etesami [70] reported that the thermal conductivity of nano-titania enhanced PW-PCM dropped below the thermal conductivity of pure PW after several thermal cycles. Deterioration of characteristics takes place due to structural and chemical degradation. Chemically stable nano-PCMs are direly needed for the successful operation of PCMs. Deka et al. [71] impregnated TiO2 powder in 1-tetradecanoic acid to prepare chemically stable composite-PCM. They observed good chemical stability through rigorous testing techniques. Due to the inclusion of titania, the thermal conductivity of PCM increased by 188% and latent heat storage (LHS) of 97.75 J/g at 52.04°C melting temperature was recorded of composite-PCM. However, after thermal cycling, the LHS declined by only 16.65%.

Another important aspect of PCMs is the time taken for melting and solidification. Common PCMs such as PW take very long to melt and solidify, which reduces

#### *Titanium Dioxide - Advances and Applications*

the heat storage rate. Studies have suggested the combined use of metallic heat sinks and nanofluids with PCMs to increase the rate of the phase change process. Ding et al. [72] reported a 32.90% reduction in melting time and a 22.57% decrease in solidification time of PW PCM by incorporating a microchannel heat sink with 1.0 wt.% TiO2/water nanofluid. An increase in nanoparticle concentration increased the rate of melting and solidification due to an increased rate of heat transfer. **Figure 14** represents the effect of varying nanoparticle concentration on Nu and melting and solidification time of PCM.

#### **Figure 14.**

*Nu variation (a) during the melting process, (b) during the solidification process of PCM for different samples of nanofluid, and (c) time of melting and solidification of PCM [72].*
