3.1.1 Proposed Improvements from Semester 01

The table provides a summary of the future work proposed in Semester 01.

3.1.2 Implementation of Proposed Improvements

3.1.2.1 Explore methods to make the wok surface reflective, smooth and concentrate light better

The wok was primarily used as a reflector due to its parabolic shape, which enables light concentration. However, its surface is not sufficiently reflective for this application. Therefore, methods to improve the surface reflectivity were explored. Previous tests showed that both reflectivity and surface smoothness are critical factors in effectively concentrating light.

Several approaches were considered to enhance reflectivity while maintaining a smooth surface:

• Paste a thin reflective film on the wok surface which is able to cover the whole surface area to avoid jagged edges.
• Chrome mirror vinyl film wrap
• One-way mirror window privacy film
• Polish the wok surface to make it reflective.

Although chrome mirror vinyl film and one-way mirror films are more effective at reflecting visible light, these films are typically designed for indoor use, are not weather-resistant, and may peel off when exposed to environmental conditions such as rain. In contrast, polished metal surfaces are commonly used in concentrated solar power parabolic trough configurations to reflect light [1].

Based on these considerations, the wok surface was polished to achieve a smooth and reflective finish, enabling more effective light concentration.

Results & Analysis

The polished wok achieved a higher static temperature of 74 °C across repeated experiments, compared to the mirrored wok, which reached a maximum of only 63 °C. Although the polished surface is less reflective than the mirror tiles, it likely enabled better light concentration due to reduced scattering from its smoother surface. Therefore, the polished wok was selected for subsequent experiments.

3.1.2.2 Integration of the solar tower and parabolic dish

Results & Analysis

Figure 5: Results after integration

For most of the time, the water at the bottom of the receiver was hotter than at the top. This is likely due to the contribution of the parabolic dish beneath the receiver, with the average temperature difference between top and bottom around 8.5 °C.

However, the water temperature never reached 82 °C, the highest temperature ever recorded for the solar tower. Despite integrating both the solar tower and the parabolic dish, we had expected a higher static temperature, but this was not achieved.

3.1.2.2.1 Potential reasons for lower static temperature:

• Gaps in the receiver result in the loss of concentrated light.
• An unshielded receiver exposed to wind experiences convective heat losses to the surrounding.
• Concentrated light from the wok that does not reach the receiver leads to a loss in reflector system efficiency. Currently, the receiver is positioned above the wok’s focal point. As seen in the image, the insulation near the focal point has discolored, indicating that the light is not being properly concentrated onto the receiver.

AS shown in the image below, the receiver is placed above the wok’s focal point because positioning it at the focal point (red dot) would require the curved solar tower reflectors to be nearly vertical to reflect light onto the receiver. This would reduce their ability to capture maximum sunlight during midday, when irradiance is highest.

In the current setup, the solar tower configuration is prioritised because it offers higher reflectivity (due to the acrylic mirrors) and a larger effective surface area compared to the wok. However, due to the wok’s geometry (0.4 m radius), the focal point of the curved reflectors (located 0.3 m from the surface, shown as the yellow dot) does not align with the receiver. The reflectors also cannot be positioned closer because of physical interference from the wok.

As a result, the reflected rays still reach the receiver, but only after diverging, meaning the energy is not optimally concentrated.

3.1.2.2.1.1 Eliminating gaps in the receiver

Results & Analysis

The maximum static temperature exceeded the previously recorded value of 72 °C. The static water temperature at the bottom of the receiver was higher than at the top. A positive correlation was observed between irradiance and static water temperature. However, on certain days, high irradiance did not correspond to equally high temperatures, likely due to heat losses caused by wind.

3.1.2.3 Explore methods to reduce convective heat losses from the receiver due to wind

Results & Analysis

Covering the receiver and reducing convection losses increased the static water temperature to up to 100 °C, except on 4 ^th May, which was a rainy day. The bottom temperature readings exceeding 100 °C are likely due to evaporation, once the water reaches its boiling point, it evaporates, and the sensor may instead reflect the temperature of the inner copper surface. This evaporation occurs because the receiver is sealed using two rubber caps that are not completely airtight. This design was intentionally chosen to prevent pressure buildup within the system.

The higher bottom water temperature on 4th May can be explained by the irradiance trend. As shown in the graph, irradiance plateaus around midday, when the wok captures and concentrates the most sunlight at the bottom of the receiver. This results in greater heating at the bottom compared to the top, highlighting the contribution of the wok despite incomplete light concentration.

3.1.2.3.1 Enclosing the receiver to block wind

Based on the data gathered from the previous experiment, it was deduced that the receiver needs to be covered to minimise convection losses. A cover was 3D printed using PC-ABS filament which has a glass transition temperature of 115°C [2]. The clear panels are made out of the same polyethylene film that we used to cover the receiver before.

A wind sensor was installed to measure the wind speed to understand what kind of a wind speed would cause convection heat losses in the receiver. The average wind speed and maximum wind speed was recorded across 16 days, and the average speed on a typical day was around 1.5 m/s with peak speeds reaching up to 7m/s.

3.1.3 Compliance with Design Specifications

Access the evaluation of the final design’s (light subsystem) compliance with the design specifications here.

3.1.4 Further Improvements

1. Redesign the reflectors such that their focal points are aligned with the receiver, thereby minimizing losses in concentrated light.
• Increase the diameter of the curved solar tower reflectors, or reduce its depth to achieve a greater focal length.
• Increase the diameter of the wok to 1 m (in line with the 1 m² rig), or reduce its depth to achieve a greater focal length.

Figure 15: Focal point - parabolic dish [3]

2. Construct a frame for the receiver using a more transparent material to avoid blockage of light.

The frame shown in the image was constructed to test different materials, allowing the panel inserts to be easily swapped. This enables evaluation of materials that can:

• Withstand the high temperatures of the CSP system
• Effectively transmit light to the receiver
• Reduce convective heat loss to the environment
• Endure varying weather conditions

Once a suitable material is identified, the frame can be removed, and the cover can be redesigned to minimize blockage of sunlight, as seen in the image. Although not tested, a potential candidate is borosilicate glass which is used in lab equipment to heat chemicals, as it satisfies the criteria outlined above.

3. Apply an anti-corrosive, hydrophobic coating on to the metal surface

Although the polished wok surface has not rusted yet, stainless steel is highly corrosion-resistant but can still corrode over time, especially in coastal areas. Therefore, the parabolic dish should be coated with an anti-corrosive layer, ideally with hydrophobic properties. This will prevent water stains, reduce cleaning requirements, and help maintain the surface’s reflectivity for a longer period.

4. Implementation of a sunlight tracker

Figure 18: Sunlight tracker [4]

Implementing a sunlight tracker can increase the amount of solar energy captured by actively adjusting the reflector to follow the sun. To maximize sunlight, the receiver would also need to move, meaning both the reflector and receiver must track the sun, which increases system complexity and cost. An evaluation is therefore needed to determine whether the additional expense is justified by the potential gain in captured sunlight.

Appendix A: