Development of high concentrating photovoltaic/ thermal system

Alamri, Yassir Ali S (2022). Development of high concentrating photovoltaic/ thermal system. University of Birmingham. Ph.D.

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Abstract

Sustainable energy technologies have gained significant interest around the world as countries look for ways to reduce their carbon footprint and reach net zero emissions. Even though, some milestones have been reached by developed countries, there is still a long way to go to reach net zero emissions for both developed and developing countries. Solar energy is a prime example of a renewable and sustainable form of energy with huge potential that is yet to be reached. The annual global electricity consumption can be fulfilled by one hours’ worth of incident solar irradiance on Earth. There are several technologies that have been developed over the years to harness the incident solar energy, the most prominent of which are photovoltaic (PV) panels, however, the recent developments in the solar industry have provided more efficient and practical solutions in the form of Multi-Junction Solar Cell (MJSC). The MJSC are capable of absorbing a significantly greater amount of radiation when compared to PV panels and producing a greater amount of electricity, while drastically reducing the required active receiver area.
Moreover, the MJSC can be incorporated into concentrated photovoltaic (CPV) systems, which integrate ‘cheaper’ optical elements capable of multiplying the incident solar radiation onto the cells by up to 2000 suns. These systems are still in the early stages of development and have gained a lot of interest recently as a potential application to harness the huge amounts of solar energy available. The CPV system has also been developed further to incorporate a thermal aspect such as cooling channels to absorb the excess thermal energy in the CPV system for use in further thermal applications. These systems have become known as CPV/T.
The aim of this work is to develop a CPV/T system that is efficient, compact and cost-effective. This was done through initially developing the optical elements and utilising ray-tracing simulation available in COMSOL Multiphysics to characterise the system’s optical performance. Subsequently, two cooling channels were developed to evaluate and compare the thermal aspect of the system and develop a study into the use of multiple units in series. This study was carried out using the coupled optical and thermal simulation method in COMSOL Multiphysics. The final part of the work involved constructing the developed CPV/T system and carry out outdoor experimental testing to evaluate its optical, electrical and thermal performance.
The developed CPV/T system included a Fresnel lens as the primary optical element, a Plano-concave lens and multi-leg homogeniser (MLH) as the secondary optical elements, four multi-junction solar cells and a cooling channel placed under the cells. The idea behind this configuration was to allow for the incident solar irradiance to be evenly distributed by the MLH onto the four MJSCs, which allows for a compact system. The parameters investigated include the MLH height (20 to 60 mm), geometric concentration ratio (250 to 1000 GCR), Fresnel lens F-number (1 to 2) and acceptance angle (0 to 4º). The optical characterisation of the system showed that the best configuration in terms of optical efficiency was using a geometric concentration ratio (GCR) of 400X, F-number= 1.12, and Plano-concave lens height (H)= 50 mm, achieving an optical efficiency of 79.6% and uniformity of 56%. The acceptance angle (2θ) was also found to be 1.04°. Additionally, the application of an anti-reflective coating to the secondary optical elements showed that the optical efficiency increased to 87.48%. The thermal evaluation of the CPV/T system showed that the only configuration that allows 8 units in series to operate below the 110 ºC limit, is using cooling channel-2 (76.2 mm X 25.4 mm X 3.25 mm) with water flow velocity of 0.005 m/s. Using Cooling channel-1 (70 mm X 10 mm X 2 mm) with flow velocity 0.01 m/s allows for the use of 6 units which represents the next best configuration. Several scenarios were also studied to evaluate the CPV/T systems performance when integrated with a small-scale ORC system to produce 10 kW. Scenario 6 with velocity=0.005 m/s, 8 units and 17 channels was found to be the optimal solution. The outlet water temperature was equal to 61.44 °C, maximum cell temperatures=101.3 °C, total electrical power= 6.7 kW and electrical efficiency= 39.1%.
An experimental test rig was then constructed to house the developed CPV/T system to test outdoors in Birmingham, UK. Both reflective and refractive MLH were tested. The outdoor experiment showed good agreement with the modelling results and showed that as direct normal irradiance (DNI) increased with constant cooling water flow rate, the short circuit current increased, whereas open circuit voltage decreased but at a relatively much smaller rate. In the refractive case, the Isc increased from 2 A at 408 W/m2 to 3.6 A at 720 W/m2, whereas the voltage decreased by only 0.1 V from 2.89 V to 2.79 V as irradiance increased from 408 W/m2 to 720 W/m2. Furthermore, the electrical efficiency of the system decreased with an increase in solar intensity. The highest experimental electrical efficiency value in the refractive case was 31.57% at 408 W/m2, whereas the lowest value was 30.46 % at 720 W/m2. The highest experimental thermal efficiency in the refractive case which occurred was 40.72% at peak irradiance of 720 W/m2. The thermal efficiency increased with solar intensity.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):