Optical and thermal optimisation of parabolic trough solar collectors for heating applications via a novel receiver tube

Optical and thermal optimisation of parabolic trough solar collectors for heating applications via a novel receiver tube

Non-renewable energy sources (fossil fuels) remain economically advantageous because of their abundance and capacity to generate a large amount of energy in single location. Pollution is a major issue of utilising this energy, while it is becoming more challenging to extract them. In the world in each second approximately 1.2 million kilograms of CO2 are released into the atmosphere; non-renewable resources contribute the majority of these emissions, because they are mostly responsible for energy generation. The resultant climate change increases the earth’s surface temperature, leading to loss of ice mass, elevation of sea level and flooding. Therefore, due to the increase in the world’s fossil fuel energy consumption, limit to its resources and growing worldwide pollution issues, there is an urgent need for environmentally-friendly and sustainable energy resources. Renewable energy resources are sustainable resources that produce zero greenhouse gas emissions while producing the energy. It should be noted that pollution is generated during the manufacture and decommissioning of renewable technology components. Solar energy is the most abundant and geographically widespread resource and it has tremendous advantages over other renewable energy resources. Concentrating solar power (CSP) is the main solar thermal technology for commercially converting solar energy into electricity, in addition to its ability to provide energy for heating applications.

Among all of the CSP technologies, Parabolic Trough Solar Collectors (PTSC) are the most mature, efficient and cost-effective technology. The main principle of PTSC operation is to reflect direct solar radiation from a parabolic mirror (the reflector) that focusses the radiation on the receiver tube to heat a Heat Transfer Fluid (HTF). The receiver tube is constructed by encasing the metal absorber tube (through which the HTF flows) with a glass envelope; the space between the absorber tube and glass envelope is evacuated in many existing designs.

There are some challenges with the operation of PTSC systems. PTSC optical efficiency is principally reduced by the reflected radiation deviating from the focal line due to geometric inaccuracies, such as collector and receiver tube misalignments, in addition to wind changes. Moreover, the HTF temperature can reach 400 °C, increasing the temperature of the glass cover surrounding the absorber tube. The resultant heat losses reduce the useful heat gain and decrease the PTSC thermal efficiency. In addition, the bottom portion of the receiver tube is normally exposed to concentrating sunlight, whereas the upper part of the receiver is subjected to direct normal solar radiation. As a results the heat flux distribution on the absorber tube receiver surface is highly non-uniform, which results in high temperature gradients. Consequently thermal deformation in the absorber tube occurs, could break the glass cover. The evacuated receiver tube is costly, amounting to 30% of the total initial material cost of the PTSC solar field; it is difficult to repair any faults, which results in a high replacement cost.

Therefore, the goal of this thesis is to design, model and investigate an alternative receiver tube for PTSC systems. This new design can reduce the optical and heat transfer losses in addition to improving (making more uniform) the solar heat flux and temperature distributions on the absorber tube. By achieving this goal there is an economic benefit due to improving PTSC performance (producing more heat), while simultaneously reducing the maintenance and replacement costs. This will result in greater acceptance of PTSC technology, improving its commercial market. To achieve this goal, optical and thermal enhancements have been implemented.

For the optical aspect, a PTSC heating system consisting of two identical collectors has been designed and manufactured; one collector was used as a control during each test. To confirm the suitability of the control, the collectors were setup identically; the tests confirmed that there was no bias in the system. The standard configuration (a parabolic collector and absorber tube) was designated 'Cp'. The other collector was modified by attaching a smaller, secondary parabola (SP) with a mirror sheet (SPM) on the opposite side of the absorber tube so that the primary collector and SP shared the same focal line. This configuration was called Cs. The purpose of the SPM is to reflect any deviated solar radiation towards the absorber tube, and also to potentially distribute some of the reflected heat flux onto the upper part of the absorber tube. The Cs was directly compared to Cp in some experiments to assess the potential improvements in efficiency of the SPM.

Because the SPM blocks some of the solar radiation from reaching the absorber tube or the primary mirror in Cs configuration, another SP was painted black (SPB). The SPB was setup in the same manner as the SPM in configuration 'Cb'. Therefore, the effect of the lost direct radiation was isolated by comparing Cs and Cb directly. Two types of experimental tests were performed for the two configuration comparisons: tracking tests, where the PTSC tracked the sun throughout the day from sunrise until sunset; and fixed tests, where the PTSC was aimed at the noon position of the sun (simulating the noon position of the sun throughout the year). The experimental results for the Cs and Cp overall average efficiencies during the tracking stage were 44.94% ± 0.04% and 44.55% ± 0.04% respectively (Cp efficiency equals 99% of the Cs efficiency), while their overall average annual noontime efficiencies during the fixed stage were 23.74% ± 0.02% and 20.55% ± 0.02% respectively (Cp efficiency equals 86% of the Cs efficiency). This means that the SPM improved the optical efficiency of the Cs collector, because 8.5% of its aperture area was blocked by the SPM aperture area. The mirror on the SP improved the thermal efficiency of the Cs when compared to the Cb efficiency. The experimental results for the Cs and Cb overall average efficiencies during the tracking stage were 42.92% ± 0.03% and 38.52% ± 0.03% respectively (Cb efficiency equals 86% of the Cs efficiency), while their overall average annual noontime efficiencies during the fixed stage were 18.43% ± 0.02% and 13.95 ± 0.02% respectively (Cb efficiency equals 75% of the Cs efficiency).

Improving the PTSC optical performance by using an SPM increased the HTF temperature. However, this increased fluid temperature could augment the receiver tube heat loss. Therefore, the blocked area that resulted due to utilising the SPM geometry (optical enhancement) was employed for receiver tube thermal enhancement by including thermal insulation in the receiver tube. The insulation layer that was placed on the inner surface of the upper part of the receiver glass envelope has the same shape and covered the same area as the SPM. This configuration was investigated using 3D Computational Fluid Dynamics (CFD). Due to the low thermal conductivity of the insulation layer, conduction heat transfer across the glass cover is reduced and therefore the glass cover outer surface temperature was reduced. This reduced the heat transfer loss between the glass outer surface and the atmosphere. In addition, the insulation layer also increased the air temperature between the absorber tube and the glass envelope for a non-evacuated receiver tube, especially in the upper portion of this region. This produced a better temperature distribution on the absorber tube outer surface, which would reduce its propensity for thermal deformation. Several designs of receiver tube cross-section were tested for various wind speeds. It was found that the insulation enhanced the collecting efficiency for each cross-section, and the efficiency of the circular receiver tube was the highest.

These results demonstrate that modification of the receiver tube is a viable possibility for a future design that improves the operating efficiency and working life of the receiver tube.