Performance Improvement of the Parabolic Trough Solar Collector Using Different Types of Fluids with Numerical Simulation

Solar concentrators are an important facility to utilize the solar energy. There are many kinds of solar concentrators. In this work an experimental has been implemented to improve the thermal performance of Parabolic Trough Solar Collector (PTSC) using three different fluids as a working fluid (water, nanoparticles of CuO mixed with distilled water nanoparticles of Al2O3 mixed with distilled water) with concentration ratio 0.01% and mass flow rate 20Lt/hr without tracking system. The experimental tests have been carried out in electro-mechanical engineering department at university of technology in Baghdad city during October 2017 and daytime between (9am -15pm) hours. The obtained results for three different fluids are as follows: Using (CuO + distilled water) as a working fluid increases the average of the output temperatures by 10.4%, the average of useful heat gains increases by 11% and the average of the collector efficiencies increases by15%. Using (Al2O3+distilled water) as a working fluid increased the average of output temperatures by 4%, the average of useful heat gains is increased by 6.5% and the average of collector efficiencies is increased by 8.2%.


Introduction
The sun is the best energy source in this contextually and is almost a wellspring.Energy efficiency and solar technology are very important parameters to the building or community design.The radiation of the sun can be converted into other forms of energy by direct or indirect way, for example heat and electricity, which can be utilized by human.The researches and developments which started before 1970 are carried out in few countries to utilize the thermal energy in best manner, but most of these researches remained academic [1].
There are many regions and countries suitable to use the solar energy that is depending on their climate, Iraq is one of these countries because the solar rays that received with approximate time of 4000 hours per year in convenient places of solar energy [2].The Concentrated Solar Power (CSP) is the best technology in consuming solar radiation, which bestirs the way to produce energy.Different types of CSP are used in crafting for generation of power.The Parabolic Solar Collectors (PSC) are the best technology proliferate in power plants.Efficiency of the (PSC) and heat losses during storage times are two main drags while utilizing the PSC in solar energy.Recent finds suggested that adding nano-particles to the heating fluid improve the thermal conductivity, thermo-physical and heat transfer properties of the fluid [3].Nano-fluid will have a favorable effect on the performance of (PSC).[4] Described the heat transport enhancement by means of nano-powder.Altered nanofluids including  2  3 (20 nm), CuO (50 nm), and Cu (25 nm) nanoparticles in water were considered in their work and has been conducted in the course of laminar flow with heat transfer development through the circular tube.They specified their results to identify the improvement of heat transfer with the increase in concentration of nanoparticle.They had taken the optimum concentration value of Cu nanoparticles to be 2% of volume fraction and the other nanoparticles  2  3 and CuO as 2.5% of volume fraction.The results obtained from their experimental work showed that the metallic nanoparticles had improved the heat transport coefficient as compare to that of the oxide particles.[5] Investigated theoretically thermal efficiency of a nano-fluid based direct absorbition solar parabolic trough collector, and used aluminium nanoparticles at the volume fraction of 0.05% suspended in the base fluid Therminol-VP1.Their results showed that thermal efficiency increases compared to a conventional PTC by 10 % at low temperatures and by 5% at high temperatures.[6] Presented an attempt to increase the efficiency of parabolic solar collector by using copper oxide and alumina nanofluid with concentration 0.01% and the average size of nanoparticle is 20-30 nm.The maximum instantaneous efficiency was 39.4% at flow rate 60 Lit/hr with copper oxide.[7] Studied the performance of solar nanofluid heating system using (Cu (30nm) +DW) and (Ti 2 (50) + ) as a nanofluids.Also he used four particles concentration ratios (0, 1, 3 and 5 % vol), mass flow rate (30, 60 and 90 lit/hr m²) and the distilled water as working fluid.The efficiency of collector for nanofluid (Cu (30 nm) +DW) was more than nanofluid (Ti 2 (50 nm) +DW) because the particle size of copper was small as compared with titanium oxide.

Theory:
The solar collector is formed in the shape of parabola, which is usually a mirror, or anodized Aluminum sheet depends on the applications to concentrate the radiation rays of the sun on the receiver tube that located in the focal line of the collector.The material of the absorber tube most be mild steel or copper and it is painted with black paint to improve the performance of it.The absorber tube transform the radiations into thermal energy which is carried by the working fluid that pass through the tube and use it in require application.This type of collectors can reach to 400ºC, depending upon the material that is used as a reflecting surface, absorber tube materials and heat transfer fluid.The concentrating collectors has an important factor called concentration ratio which is defined as the ratio of the aperture area of collector to the area of the absorber tube and it ranges are (20-70).

Nano-fluid
A nano-fluid is a fluid containing nano-meter sized particles, named nanoparticles.These fluids are engineered colloidally suspensions of nano-particles in the base fluid.The nano-particles used in nano-fluid are normally made of metals, oxides, carbides, or carbon nano-tubes.Nano particles have high thermal conductivity and convective heat transfer coefficient.Knowledge of the rheological behaviour of nanofluids is found to be very critical in deciding their suitability for convective heat transfer applications.

Concentration Ratio of Nanofluid
It presents the ratio of nanoparticles in the base fluid and it is calculated from [8]:

Nanofluid Density
The model that is used by assuming an equipoise state between particles and fluid is obtained as follows [8]: Where,  = subscripts, =stand for fluid and nano-particle respectively, = Concentration of nano-particles in nano-fluid.

Nano-fluid Specific heat capacity
The nano-fluid specific heat capacity is evaluated from equation [8]:

Nano-fluid viscosity.
Different equations had been used to evaluate viscosity of nano-fluids.In this work Einstein model [9] is used:

Nano-fluids thermal-conductivity
It is the most critical factor for analyzing heat transfer of nano-fluids.Multi models were found; in this work, Maxwell model [9] is used:

Efficiency Evaluation:
The actual useful energy obtained from parabolic trough solar collector is given as [10]: The thermal theoretical efficiency is written as [10]: The thermal experimental efficiency is written as [10]:

Preparation technique of Nano-particles
The mixing of the nanoparticles with the water is carried out in the University of Technology in the corrosion laboratory at the materials engineering department by ultrasonic cleaner device that is shown in figure (2).The particle size of CuO and  2  3 is (30-40) nm and the concentration ratio is 0.01%

Results and Discussion
The results show the performance parameters of parabolic collector (output temperature, useful heat gain and efficiency).The values of solar radiation were calculated by MATLAB program .These experiments were performed in sunny days during (9 am-15 pm) of three fluids of parabolic trough solar collector.

Temperature versus time
There is a gradual increasing of output temperature of the working fluid as a result of beam solar radiation increasing.The averages values of outlet temperatures are (47.1,52, 49) ºC for water, water based CuO and water based  2  3 respectively.We note that the outlet temperature using water based CuO as a working fluid is more than the water and water based  2  3 by (10.4,6.1)% respectively.

Heat gain versus time
There is a gradual increasing in useful heating gain because of solar radiation increasing.The averages values of useful heat gains from the solar collector are (335, 372, 357) watt for water, water based CuO and water based  2  3 respectively.We note that the useful heat gain using water based CuO as a working fluid is more than the water and water based  2  3 by (11, 4.2)% respectively.

Efficiency versus time
There is a gradual increasing in the efficiency of the solar collector as a result of solar radiation increasing.The averages values of efficiencies from the solar collector are 40%, 46.2% and 43.3 for water, water based CuO and water based  2  3 respectively.We note that the efficiency of the solar collector when used water based CuO as a working fluid is more than the water and water based  2  3 by (15.5, 6.7) % respectively.

Comparative between experimental and numerical results
The numerical simulation was taken for four hours only (9am, 11am, 13pm and 15pm), therefore it is results should be compared with the same flow rate and time.There are differences between experimental and numerical results.
The average of numerical output temperature is more than the experimental by (9.6, 12.1, 11)% for water, water based CuO and water based Al 2 O 3 respectively.

Conclusion
The main goal of this work is to improve the performance of the Parabolic Trough Solar Collector by using water, distilled water based copper oxide CuO and distilled water based Aluminum oxide  2  3 with concentration ratio 0.01% and with mass flow rate 20 Lt/hr without tracking system.The experimental results are shows that: 1-By using water based CuO as a working fluid and compare the results with results of water: The output temperature is improved by 10.4%, useful heat gain is improved by 11% and the collector efficiency is improved by 15.5%.2-By using water based  2  3 as a working fluid and compare the results with results of water: The output temperature is improved by 4%, useful heat gain is improved by 6.5% and the collector efficiency is improved by 8.2%.