Enhancement Techniques of Parabolic Trough Collectors: A Review of Past and Recent Technologies

To tackle the climate change and global warming, the world needs to reduce its dependency on fossil fuels. In recent years clean, renewable and sustainable sources of energy such as solar, wind, tidal etc. have thus become widely popular. In particular solar thermal energy has emerged as a major contender in the quest to reduce CO2 emissions especially for regions with hot tropical climate. The light or solar energy/heat from the sun can be harnessed to produce electricity via Photovoltaic Devices (PV) or Concentrating Solar Power (CSP) plants. The CSP plants operate on Direct Normal Irradiance (DNI), which is defined as the amount of received solar energy per unit area on the surface held normal to the rays of the sun. Depending upon the methodology to capture the suns energy, the CSP technology can be categorized into several technologies, four of the most common ones being; parabolic trough collectors (PTC: which is our focus), linear Fresnel reflectors, parabolic dishes and solar towers, in Figure 1.


Introduction
To tackle the climate change and global warming, the world needs to reduce its dependency on fossil fuels. In recent years clean, renewable and sustainable sources of energy such as solar, wind, tidal etc. have thus become widely popular. In particular solar thermal energy has emerged as a major contender in the quest to reduce CO 2 emissions especially for regions with hot tropical climate. The light or solar energy/heat from the sun can be harnessed to produce electricity via Photovoltaic Devices (PV) or Concentrating Solar Power (CSP) plants. The CSP plants operate on Direct Normal Irradiance (DNI), which is defined as the amount of received solar energy per unit area on the surface held normal to the rays of the sun. Depending upon the methodology to capture the suns energy, the CSP technology can be categorized into several technologies, four of the most common ones being; parabolic trough collectors (PTC: which is our focus), linear Fresnel reflectors, parabolic dishes and solar towers, in Figure 1.

Thermal performance of PTCs
The absorber tube (also known as heat collection element (HCE)) is one of the most important elements in a PTC system; its thermal efficiency directly impacts not just the reliability of the plant but also the cost of energy production. Because of these reasons various methodologies of heat transfer enhancement are generally used within the absorber tube for the PTC system. The most commonly used techniques such as, changing the working fluid, use of nanoparticles and the use of inserts (swirl generators etc.), are reviewed below. A fourth methodology which is based on combination of nanoparticles with inserts is also becoming popular.

Thermal performance by changing working fluids
Majority of the solar thermal power plants (STPP) with PTC systems around the world which are currently operational use thermal oil as HTF with the maximum working temperature of 398 C. Low vapour pressure, affordable price, long lifetime and good thermal stability are the obvious reasons for using thermal oils in the STPP. However, this does not mean that thermal oils are the best working fluid; limitation of temperature (around 400 C), environmental toxicity and flammability are some of the key drawbacks when using thermal oils. Alternative HTFs that have been examined in the literature instead are; liquidwater/steam, pressurized gases and molten salts. Some of these investigations and their key findings highlighting the advantages and disadvantages compared to thermal oils typically used in the STPP are summarized in Table 1.

Ref.
Working Fluid Details of Findings [2] Syltherm 800 oil and water Thermal loss of the collector was lower when using water than those predicted by using Syltherm 800 oil.
[3] Therminol VP1, Xceltherm 600, Syltherm 800, 60-40 Salt, and Hitec XL Salt Effect of working fluid was smaller than other parameters. The maximum thermal efficiency was provided by Xceltherm 600 and Syltherm 800, but these fluids are relatively expensive.
[4] Molten salt, water, oil Better efficiency has been obtained by using water.
The Syltherm 800 can be operated at a temperature higher than 700K, while the working fluids marlotherm X and syltherm XLT can only be operated at a temperature less than 700K; whereas, others can operate between 650K and 750K.
The most appropriate choice was Syltherm 800 which provided the maximum range of (700-800) K. The highest cost when using Santotherm LT was 129US $/kW h/day. Moreover, the best HTF was Syltherm 800 from the thermal capacity point of view.
[7] Pressurized nitrogen and synthetic oil A slight difference in the net electrical power between fluids, only (-0.91%), while the gross electrical production per year was the same.
[8] Gas The highest temperature reached by the gas was 400 C which cannot be reached by the synthetic oil. [9] Molten salt compared with the results of PTR70 It was deduced that the heat loss of the examined tube using PTR70 is smaller than that of using molten salt.
[11] Thermal oil, water The performance obtained by water was better than that measured by oil.
The performance of liquids was higher than that of gases. The pressurized water is the most appropriate fluid for temperature up to 500K while sodium liquid is better for temperatures up to 1100K.

ACET.000563. 3(3).2019
Thermal performance by adding nanoparticles One of the most commonly used technique to improve the thermal performance in PTCs is to add metallic or non-metallic nanoparticles inside the base working fluid; the mixture then referred to as nanofluid. These nanoparticles having different thermal properties than that of the base fluid results in a more efficient nanofluid thereby improving the overall thermal performance of the absorber system. Besides this, the nanoparticles also help in the reduction of the thermal stresses inside the absorber tube. However, agglomeration of nanoparticles in certain parts of the system results in higher pressure drops with raised power pumping requirements. To overcome this problem, the volume fraction of nanoparticles needs to be optimized for efficient heat transfer augmentation. A summarized review of previous studies is shown in Table 2 illustrating the use of nanofluids in the PTCs. Numerical modelling approaches either treat the nanofluids as a single phase or a two-phase model; the latter being more accurate. However, regardless of the treatment, the selection of thermos-physical properties of the nanoparticles is of paramount importance. The thermal efficiency is higher in the case of dispersing only in water since the mixture of water-EG has a disadvantage of boiling and freezing temperature which is higher than those of pure water.
[21] At CR 6%, Heat transfer rate and the system thermal efficiency enhanced by 32% and 12.5% respectively, whereas, the entropy generation decreased up to 20-30%.

Effects of swirl generators on the thermal performance
The usage of swirl generators inside a receiver is a passive method that is used to enhance the convective heat transfer rate. These devices could be twisted tapes, fins, coils, wires and spiral grooved tubes etc. The flow in such devices has important features such as; intense mixing of the near-wall region flows with main-stream flow and reduction of the thermal boundary layer. Improved overall thermal efficiency of the PTC, cost minimization and improvement in the system reliability are added further benefits of such passive enhancers. A comprehensive summary of such inserts is presented in Table 3 including the enhancement of both thermal and optical performances.

Summary
To effectively enhance the optical and thermal efficiencies of PTCs, some possible solutions from the literature are summarized in this paper related to improvement of the thermal properties of HTF and manipulation of the optical design of HCE.