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Light

This section focuses on the capturing of the sun's energy by reflecting and concentrating sunlight. Main components of this section include:

Sunlight

1.1 Solar Spectrum

The solar radiation spectrum consists of:

1.2 Irradiance (W/m2)

Power of electromagnetic radiation (solar spectrum) received per unit area [11].

  • Direct Normal Irradiance (DNI) – Solar radiation received directly from the sun, measured on a surface perpendicular to the sun’s rays.
  • Diffuse Horizontal Irradiance (DHI) – Solar radiation scattered by the atmosphere and reaching the surface from all directions.
  • Global Horizontal Irradiance (GHI) – Total solar radiation on a horizontal surface, combining both direct and diffuse components:
    • GHI=DHI+DNI⋅cos(θ)

1.3 Sunlight hours

Number of hours where solar irradiation is the strongest during the day (5.5hrs).

1.4 Zenith Angle (θ)

Angle between the sun and the vertical.

The zenith angle near the equator changes very little during the day. In Singapore (1.35°N), Solcast data shows it varies only between 0.3° and 0.9° during sunlight hours.

Hence, a sunlight tracking system is unnecessary, as the small angle variation doesn’t justify the added cost.

Literature Review of Existing Concentrated Solar Power Methods

2.1 Reflector

Below is a summary of existing reflector designs:

From these, we will focus on the Solar Tower and Parabolic Dish methods, as they offer the highest efficiency.

2.2 Receiver

2.2.1 Receiver Designs

Since both Solar Tower and Parabolic Dish configurations use compact receiver designs instead of absorber tubes, as seen from the images above, this section will explore the receiver designs considered for these systems.

2.2.2 Receiver Materials

Desirable properties for materials include:

Most commonly used receiver materials:

From these materials, Inconel 740H provides the best lifetime and cost/power performance [20].

Design Specifications

To capture the maximum amount of sunlight and effectively concentrate it onto the receiver, the following design specifications are considered.

Design

3.1 Concept selection

Below is the morphological chart and concept scoring table, which outline various design combinations considered for the light subsystem based on the design specifications.

Based on the concept scoring, we have decided to select option C as the initial concept.

3.2 Reflector

An 80 cm stainless steel wok was selected for constructing the parabolic dish, as it was the largest commercially available option that could fit within the 1 m2 solar tower rig, which could maximise the reflective surface area. Mirror tiles were applied to the wok’s surface to enhance its reflectivity.

Based on formula (f = D2 / (16d))[21], SolTrace and Tonatiuh simulations, and experiments the focal point of the parabolic dish was determined to be 17.17 cm from the base.

3.3 Receiver

3.3.1 Receiver Design

Based on existing receiver designs, after evaluating optical and thermal efficiency, fabrication and maintenance complexity, and operating temperature range, we chose a hybrid design combining external tubular and cavity receivers.

According to an analysis done on cavity receivers, the Cylinder-conical receiver performs the best, as shown in the graph.

To maximise the receiver’s available surface area for absorbing heat from sunlight, the receiver’s exterior will remain uninsulated, unlike conventional cavity receivers. Thermal losses will be minimal since our system operates at a lower temperature than large-scale CSP systems [22],[23].

The receiver dimensions from the paper were scaled to match the receiver from the Solar Tower CSP rig for comparable results and fabricated using a 3D-printed mould.

Receiver Mould

3.3.2 Receiver Material

As existing receiver materials are expensive and less accessible, alternatives were considered and copper was selected from the table below.

An experiment was conducted to determine if thinner copper tubes transfer heat more efficiently than thicker ones, which is crucial given the limited sunlight hours. Based on the results, thin coils were selected.

Results & Analysis

The thinner coil shows a faster temperature rise than the thicker coil, indicating a higher rate of heat transfer. Since it contains less water, it gains and loses heat more quickly, leading to a lower thermal equilibrium temperature. However, this will not affect our system as the Heat Transfer Fluid circulates in an enclosed loop.

Another experiment was conducted to determine if black paint increased the thermal energy absorption. Based on the results obtained, the receiver was painted black.

Results & Analysis

The painted surface shows a higher heat transfer rate to water, indicated by its steeper temperature gradient, and reaches a stable water temperature of 53°C, 16°C higher than the unpainted setup. Therefore, the black-painted surface demonstrates superior energy absorption and heat transfer and will be used in subsequent experiments.

Based on the experiments and analysis conducted, the receiver was constructed using thin (6.35mm) copper tubes and was painted black to maximise the absorption of heat.

Testing

4.1 Artificial Lighting

Since natural sunlight is not always available, artificial lighting was used to simulate the solar spectrum, with LEDs for visible light and halogen lamps for infrared. An irradiance of 1000 W/m2 will be supplied, as standard solar tests are conducted under this condition [28].

Limitations of using artifical lighting:

The following rig was designed to place the light during testing:

The height of the light from the ground was adjusted to simulate 1000W/m2 as shown based on:

I ∝ (1/d)2 [32]

4.2 Experiment

4.2.1 Effectiveness of parabolic dish with mirrors

An experiment was conducted to evaluate the effectiveness of the parabolic dish (wok) with mirrors.

Results & Analysis

Limitations

The smaller temperature difference observed between the copper surface and water in the non-mirrored wok experiment, compared to both the no-wok and mirrored-wok setups, may be due to the thermocouple probe making contact with the inner copper surface of the receiver. To ensure a more accurate measurement of the water temperature, this experiment will be repeated using an end-cap to properly secure the probe.

Based on the data, the wok with mirrors heats both the water and copper to higher steady-state temperatures. However, the water and receiver surface temperatures remain similar for both mirrored and non-mirrored wok experiments.

This may be due to poor light concentration caused by the uneven surface resulting from pasting mirrors on the wok. This theory was verified by measuring the irradiance along the central axis for the parabolic dishes with and without mirrors.

In addition, multiple reflections of the light ray within the wok before reaching the receiver may have also reduced the intensity of the absorbed light.

4.2.2 Static experiment in sunlight

An experiment was conducted to evaluate the maximum temperature achievable by the CSP system in heating the water within the receiver under natural solar conditions.

Results & Analysis

The maximum temperature reached under natural sunlight within sunlight hours is 61.6°C.

Under sunlight, the Solar Tower configuration achieved higher temperatures than the Parabolic Dish. Even when compared to the Parabolic Dish with LED under constant irradiance, the Solar Tower still reached higher temperatures, likely due to its larger reflective surface area.

Future Improvements

Appendix

Calculation of thermal energy absorption by light subsystem

Equations



Next subsection: Thermal Transmission