Parabolic Trough Collectors (PTC) are one of the most widely used technology amongst the solar thermal
systems used by the power generation industry. In recent years, numerous scientific investigations have
focused on this topic to assess the thermal performance and to improve its thermal efficiency. The current
paper presents a short but concise review of the PTC system showing the recent and past studies in a
quest to improve and enhance the thermal and optical efficiencies. We discuss briefly the techniques used
for single and two-phase flow modelling, design variables and experimental processes. Furthermore,
studies investigating the enhancement of thermal performance are critically summarized such as: use
of nanofluids as a working fluid and passive heat transfer enhancement techniques (inserts for the solar
receiver).
Keywords: Nanofluids; Parabolic trough collector; Passive heat transfer enhancements; Solar thermal
energy
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.
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).
Professor, Chief Doctor, Director of Department of Pediatric Surgery, Associate Director of Department of Surgery, Doctoral Supervisor Tongji hospital, Tongji medical college, Huazhong University of Science and Technology
Senior Research Engineer and Professor, Center for Refining and Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia
Interim Dean, College of Education and Health Sciences, Director of Biomechanics Laboratory, Sport Science Innovation Program, Bridgewater State University