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.

Figure 1:Current CSP types, Philibert and Frankl [1].

The PTC system consists mainly of three important sub-systems; the solar field, the storage system and the power block. The solar field can be categorized as a type of a large heat exchanger with the main components being the solar collector and the reflector surface. The reflector surface is generally made up of a series of mirrors that directs the solar energy to the solar collector. The solar collector then converts the absorbed incident solar radiation into thermal energy which is carried through the collector via the Heat Transfer Fluid (HTF). Within the solar collector, an absorber tube is generally made from a metal which is coated with black color to achieve larger solar absorbance and to reduce the thermal emittance. The absorber tube is encased within a glass envelope which is itself covered with an anti-reflective coating to reduce the heat losses by convection.

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.

Table 1:Effects of changing Heat Transfer Fluid (HTF) on the thermal performance.

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.

Table 2:Effects of nanoparticles Concentration Ratio (CR) on the thermal performance of Parabolic Trough Collector (PTC).

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 mainstream 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.

Table 3:Effects of insert types

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.

The authors would like to thank the UK’s Department of Business, Energy and Industrial Strategy for the financial support through Newton institutional links fund (Engineering Sustainable Solar Energy and Thermocline Alternatives-ESSEnTiAl, Grant ID 332271136).

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