Performance of organic oils in solar thermal collection is limited due to their low thermal conductivity when they are compared to molten salt solutions. Extraction of organic oils from plants can be locally achieved. The purpose of this study was to investigate the effect of use of copper nanoparticles in some base local heat transfer fluids (HTFs). Addition of volume fraction of 1.2% of the copper nanoparticles to oil-based heat transfer fluids improved their thermal conductivity as deduced from the thermal heat they conducted from solar radiation. The oil-based copper nanofluids were obtained by preparation of a colloidal solution of the nanoparticles. Impurities were added to increase the boiling point of the nano-heat transfer fluids. Stabilizers were used to keep the particles suspended in the oil-based fluids. The power output of the oil-based copper nano-heat transfer fluids was in the range of 475.4 W to 1130 W. The heat capacity of the steam in the heat exchanger was 93.7% dry and had a thermal capacity of 5.71 × 103 kJ. The heat rate of flow of the oil-based copper nano-heat transfer fluids was an average of 72.7 Js−1·kg−1 to 89.1 Js−1·kg−1. The thermal efficiency for the oil-based copper nano-heat transfer fluids ranged from 0.85 to 0.91. The average solar thermal solar intensity was in the range 700 Wm−2 to 1180 Wm−2. The heat exchanger used in this study was operating at 4.15 × 103 kJ and a temperature of 500.0°C. The heat transfer fluids entered the exchanger at an average temperature of 381°C and exited at 96.3°C and their heat coefficient ranged between 290.1 Wm−2°C and 254.1 Wm−2°C. The average temperatures of operation ranged between 394.1°C and 219.7°C with respective temperature efficiencies ranging between 93.4% and 64.4%. It was established that utilization of copper nanoparticles to enhance heat transfer in oil-based local heat transfer fluids can mitigate energy demand for meeting the world’s increasing energy uses, especially for areas inaccessible due to poor land terrain.
Solar thermal collectors are used for solar thermal collection and some of their various applications include heating and cooling of houses, drying agriculture food materials, and water desalination processes. The heat collected by the oil-based nanofluids passing through the absorber was exchanged between the oil-based nano-HTFs and a secondary fluid in the heat exchanger. It was noted that when Darcy number and porosity diminishes, the crash between the fluid flow and the pores of the permeable screen increased [
Acronyms, abbreviations, and subscripts.
Viscosity ( | |
Volume flow rate ( | |
Temperature efficiency (%) | |
Energy efficiency (%) | |
Efficiency of nanofluid (%) | |
Useful heat (J) | |
Solar power intensity (Wm−2) | |
Minimum capacity heat rate | |
Maximum capacity heat rate | |
Prandtl number | |
Heat flux | |
Specific capacity at constant mass | |
UA | Overall conductance (W/m2°C) |
U | Overall coefficient of heat transfer (W/°C) |
HTF | Heat transfer fluid |
NTU | Number of heat transfer units |
Total transfer of heat surface area | |
Dimensionless heat capacity ratio | |
Area | |
Temperature | |
Length | |
Effectiveness | |
ht | Hot |
Outlet cold stream | |
Inlet hot stream | |
Inlet cold stream | |
Pressure, mass | |
2, | Stream 2, inlet |
2, | Stream 2, outlet |
1, | Stream 1, inlet |
cm | Value with constant property |
Direction |
The particles enhance certain properties of the fluid such as thermal conductivity and further study can reveal the nature of the observed enhanced conductivity. In this study, the copper nanoparticles in oil base gave a large volume-to-surface-area ratio for thermal transfer. The copper nanoparticles were prepared using chemical reduction procedure [
A nanoparticle suspension can also be made in a two-step process involving synthesis of the particle powder and the dispersal of the particles in a fluid base [
Adding of copper particles to heat transfer fluids improved their efficiency of heat conduction [
In a study of heat transfer fluids, Syltherm 800 was observed to undergo thermal losses due to low heat capacity [
The method used for producing the copper nanoparticles was the continuous flow microfluidic microreactor for synthesis of copper particles [
A volume of 2.0 × 10−1 m3 of the oil-based copper nano-heat transfer fluids was passed through the parabolic trough solar collector of area of 32 m2 which was connected to a heat exchanger [
Mass flow rate was measured by use of Transit Time Flow Meter at intervals of five minutes. The mass flow rate of heat exchanger was measured using digital flow meters and rheometers. The volume control valve was adjusted to allow maximum volume of 15 kgh−1 of water to be discharged during high demand and a minimum amount to flow of 5 kgh−1 of steam during off-peak demand.
The pressurized steam on the shell side of the heat exchanger was caused by the hot nano-heat transfer fluids. The viscosity was measured using a viscometer which was located at the entrance and exit of solar collector and the exchanger.
Viscosity,
The total amount of heat absorbed by the oil-based copper nano-heat transfer fluids was determined from mass flow rate at ambient temperature flow, temperature change, and specific heat capacities of the oil-based copper nano-heat transfer fluids. The measurement flow rate and temperature drop were measured using digital oil flow meters and thermocouples, respectively, located at the entry and exit of fluid to and from the heat exchanger.
Equations (
The mean temperature efficiency of oil-based copper nano-heat transfer fluids was determined by use of
To determine the energy efficiency,
The efficiency of conversion of solar power by the receiver in which the oil-based copper nano-heat transfer fluids flowed was determined by use of the following equation [
Addition of copper nanoparticles increased the efficiency of nanofluids by 10% compared to when the base fluid did not have the nanoparticles [
The size of heat exchanger corresponded to the number of transfer of heat units. The number of transfer of heat units was obtained using the following equation [
Fluid heat capacity ratio,
The magnitudes of
The overall heat transfer conductance, UA, was obtained by use of equation (
The effectiveness,
The steam formed exited from the shell side of exchanger through the side outlet of the turbine. Heat transfer coefficients for the oil-based copper nano-heat transfer fluids were determined using documented details [
The temperature efficiency of the water at ambient temperature was determined using the following equation [
Rate fluid hot fluid flux,
The oil-based copper nanofluids conducted solar thermal heat energy which was in turn conducted away by the water flowing through the pressurized side of the shell of exchanger. Copper nanoparticles impurities which were added increased boiling point of the fluids to between 363.2°C of nano-castor oil heat transfer fluid and 447.5°C for the unused copper nano-heat transfer fluid. The viscosity of the oil-based copper nanofluids is as shown in Figure
Viscosity of oil-based copper nano-heat transfer fluids.
Figure
Efficiency of the oil-based copper nano-heat transfer fluids.
Figure
Maximum power output for oil-based copper nano-heat transfer fluids.
The solar power intensity was in the range 938.4 Wm−2 to 1100.4 Wm−2. The water which flowed on the pressurized side of shell of exchanger conducted more heat as the pressure was increased and it turned into steam. Engine oil-based nano-copper fluids conducted more thermal heat compared to the plant-based copper nanofluids used in this study. This was because the engine oils were synthetic in nature and degraded at higher temperatures of operation. The unused engine showed presence of carbon particles at a temperature of 410°C. Oil-based copper nano-heat transfer fluids under pressure in collector conducted a higher amount of solar thermal heat as compared to when no pressurization of the collector was done. In this study, the heat transfer was enhanced by addition of nanoparticles to the base fluids while in other studies the heat transfer was enhanced by use of fins of different types with considerations of variable thermal physical and geometric parameters [
The heat conducted by the copper nanofluids was larger with respect to solar power intensity falling on the collector area as shown in Figure
Average heat conducted against average solar power intensities.
The effectiveness of each of the streams was an average of 0.87 for the oil-based copper nano-heat transfer fluids flow (tube side) and an average of 0.94 for the hot water under pressure.
The percentage heat losses for the hot oil-based copper nano-heat transfer fluids occurred at an average temperature of 273.8°C and at an average pressure of 1.2 × 105 Nm−2. The percentage losses are as shown in Table
Percentage heat losses for the oil-based copper nano-heat transfer fluids.
Fluid | Heat (KJ) | Percentage loss (%) |
---|---|---|
Castor oil | 16697.3 | 5.85 |
Sunflower oil | 16498.5 | 5.6 |
Coconut oil | 16991.3 | 5.95 |
Used engine oil | 172744.7 | 6.1 |
Unused engine oil | 17769.6 | 6.4 |
Table
Temperature efficiencies of nanofluids at average operational temperatures.
Oil-based copper nano-heat transfer fluids fluid | Average temperature of operation (°C) | Temperature efficiency (%) |
---|---|---|
Unused engine oil | 394.1 | 93.4 |
Used engine oil | 365.6 | 83.9 |
Coconut oil | 297.9 | 79.6 |
Sunflower oil | 275.4 | 70.6 |
Castor oil | 219.7 | 64.4 |
The temperature efficiency of the oil-based copper nano-heat transfer fluids increased with the temperatures of operation. Increase in solar power intensity led to increase in the operational temperatures of the oil-based copper nano-heat transfer fluids. Temperature efficiencies were an indicator of performance of the oil-based copper nano-heat transfer and hence unused engine oil-based nanofluid performed the best while the castor-oil-based nanofluid performed the worst out of the five studied copper particles oil-based nanofluids. The heat coefficient for the oil-based copper nano-heat transfer fluids and their corresponding steam flow rates are as shown in Table
Average variation of heat coefficient with steam flow rate.
Fluid | Heat coefficient (W/m2°C) (average) | Steam flow rate (kgs−1) (average) |
---|---|---|
Unused engine oil | 290.1 | 7.49 |
Used engine oil | 284.4 | 6.53 |
Coconut oil | 279.6 | 5.61 |
Sunflower oil | 267.1 | 4.67 |
Castor oil | 254.5 | 3.69 |
The heat loss coefficient increased with amount of heat conducted by the oil-based copper nano-heat transfer fluids. The solar power intensity was an average solar power intensity of 953.7 Wm−2. The heat coefficient was proportional to the solar power intensity. The steam flow rate increased with the heat coefficient of the oil-based copper nanofluids. During the cloudy and diffuse solar radiation periods, there was reduced mass flow rate of the steam and hence lower heat coefficients for the oil-based nano-heat transfer fluids.
Figure
Heat conducted by nanofluids against solar power intensity.
Figure
Mass flow rate of steam against pressures of operation.
Solar thermal collection by use of local copper nanofluids can be used to provide power for off-grid areas with solar power intensities of between 900 Wm−2 and 1100 Wm−2. The 1.2% concertation of the copper nanoparticles increased the thermal conductivity significantly as was seen from the amounts of heat conducted by the oil-based nano-heat transfer fluids. The power produced was an average of 1130 W. The steam flow rates realized in this study were between 7.49 kg/s and 3.69 kg/s that can be scaled up and be utilized in process heat industries, where agricultural food processing, mining, power generation, etc. are in high demand. The colloidal suspensions of local oil-based copper nanofluids conducted solar thermal heat from solar thermal radiation in the range of 20 Kj/s to 80 Kj/s during clear sky conditions. The performance of the oil-based fluids was enhanced threefold by use of copper nanoparticles. The impurities were used to increase the boiling points of the nanofluids. Stabilizing the heat transfer fluids at temperatures beyond 380°C was a challenge due to degradation of the oil-based fluids. Industrialization of third-world countries without environmental pollution requires enhancement of nano-heat transfer fluids that operate at higher temperatures with minimal degradation. Significant long-term oil-based nanofluid settlement and the decreased rates of flow require effective surfactants to minimize blockage of system passages. The use of effective nanofluid stabilizers is recommended without compromising the thermal conductivity of the HTF. The atomic level interactions between the base fluid and the nanoparticles which leads to enhanced thermal conductivity of the oil-based heat transfer fluids require further investigation.
Data are available from the author upon request.
The author declares no conflicts of interest.
University of Embu is appreciated for providing the laboratory space, equipment, and apparatus for the study.