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Thermal Transmission

Recap of interim

This subsection focusses on effectively transferring heat from the receiver to the TES with minimal power consumption. In the interim, water was selected at heat transfer fluid, pump-driven thermal transmission method was validated and heat loss was identified as limitation with polyurethane/polyisocyanurate foam (PUR/PIR) selected as optimal material.

In this subsection, the implementation and evaluation of insulation and thermal transmission methods are conducted, analysing their impact on the overall system performance.

Insulation

The PUR/PIR insulation selected in the interim is compared with the commercially available aircon insulation (polypropylene random polymer, PPR), and a translucent insulation based off the concept of transparent insulation material (TIM).

PUR/PIR insulation was selected in the interim, comparing the cost, thickness required and durability. For this experiment, spray PUR/PIR foam is used to cover the 9mm outer diameter (OD) hose with a 7mm diameter aluminium duct to encapsulate the insulation, with support used to centre the insulation. (see Appendix A for thickness calculation)

Aircon insulation is a common insulation that is evaluated due to its ease of implementation. Here, PPR of thickness 5mm and inner diameter 17mm is used with 9mm OD hose.

TIM prevents heat loss through suppression of convection in the air pockets (the different internal geometries are located in figure 28), while allowing irradiance to heat the absorber (hose with water) though the transparent material. [1] Using this concept, translucent corrugated plastic board is used to wrap around the 9mm OD hose, forming air gaps between each layer (each layer is 0.03mm plastic sheet and 3.24mm trapped air), forming a structure closely resembling a absorber parallel structure with air channels parallel to the hose. (see figure 6 for cross-section, figure 7 for translucent insulation used in experiment and Appendix A for thickness calculation) The end of the hose is taped shut to further reduce convection.

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Insulation Experiment

The insulation experiments were conducted under different conditions to assess their rate of heat loss and was repeated for validation. As the system is expected to function between 40-60ºC, the focus is on trends at these temperatures to support system requirement of maintaining water temperature above 40ºC.

In shade

Under shaded conditions, PUR/PIR insulation performed best, as it has the lowest thermal conductivity (0.023W/(m.k)) [3] and the thickest insulation thickness (30.5mm). However, aircon insulation has almost the same performance as PUR/PIR despite higher thermal conductivity (0.036W/(m.k)) [4] and lower thickness (5mm). This is because of the airgap between the insulation and hose which increases the thermal resistance. Since the gap is small, convection is limited and heat transfer is primarily via conduction, which is less efficient in heat transfer. Thus, the thermal resistance increases with the added airgap, leading to similar performances between aircon and PUR/PIR insulation.

Under indirect sunlight

Under indirect sunlight (cloudy), PUR/PIR insulation performs best, with similar findings for the indirect to shade test. This trend follows from the results obtained in the experiment under shade. However, under indirect sunlight for extended periods, the translucent insulation attained the highest temperature resulting from the absorption of energy by the water that is transmitted thorugh the layers of translucent plastic.

Under direct sunlight

Under direct sunlight, aircon insulation has lower net heat loss due to its higher solar absorptivity (~0.9). [5] This results in greater solar energy absorption, which partially offsets heat losses. In contrast, PUR/PIR insulation has a reflective aluminium surface with lower absorptivity (~0.15), reducing energy gain. [6] This is seen below from the greater temperature fluctuations in aircon than PUR/PIR insulation. Hence, the net heat loss of aircon insulation appears lower despite PUR/PIR having better insulation performance. Further test from direct sunlight to shade test below verifies the low net heat loss of aircon insulation allowing it to maintain water ≥40ºC the longest. The translucent insulation, similar to the indirect sunlight condition, maintains a higher temperature than PUR/PIR over extended periods, but does not outperform aircon insulation. This is because a portion of the solar irradiance is transmitted through the translucent material and the transparent hose, with only a fraction (~13%) absorbed by the water. [7] Hence, while additional heat gain is present, it is less effective than the absorption in the aircon insulation, leading to moderate overall performance.

Conclusion

The results concluded that PUR/PIR has best performance in shade and has lower heat loss rate at high temperatures under indirect sunlight. Translucent insulation performs best considering the equilibrium temperature over time under indirect sunlight. Aircon insulation performs best under direct sunlight, with lowest rate of heat loss and highest equilirbium temperature of the insulations. From this, the hypothesis of PUR/PIR having lowest heat loss rate is partially supported considering with the absorptivity differences accounting for differences under direct sunlight.

However, in the intended application, the insulation will be largely underneath the reflectors (see below) and will be shaded. Under these conditions, the PUR/PIR insulation demonstrates better thermal performance.

Testing methods of thermal transmission

In the interim, pump-driven circulation was validated as method of thermal transmission. In this section, thermosyphons are evaluated at small-scale to assess their feasibility as a passive alternative. Thermosyphons works by utilising gravity and density differences between hot and cold fluid to generate natural convection to circulate fluid. [8] Several tests were conducted to assess the performance of thermosyphons under different conditions. Flow and circulation are assessed using an acrylic-based pigment ink as tracer, which is introduced after the system has been operating for a period of time to observe established flow.

Baseline test of standard thermosyphon

A standard thermosyphon configuration was constucted to establish a baseline (see diagram below).

Figure 17: Video of baseline thermosyphon

Clear convective circulation patterns can be observed at the outlet (top pipe), confirming working of the thermosyphon.

Horizontal configuration test

The setup is repeated with the pipes arranged horizontally, with a 0m height difference between the tank (inlet and outlet pipe) and the heated section (see below).

Figure 19: Video of horizontal thermosyphon

In the horizontal configuration, no sustained circulation was observed as the flow did not consistently flow in one direction through the pipe. Instead, the flow was intermitten and reversed direction over time. This is because thermosyphon flow is driven by buoyancy forces from density differences between hot and cold fluid. Without a vertical height difference, the pressure gradient required to drive flow is absent. Hence, only local convection occurs, significantly reducing heat transfer effectiveness.

Test with constraints as in integrated system

The third test is conducted with the heated section below the tank (as in baseline test), but replicates the constraints of the integrated system, where drilling the sides of the container would affect its insulation. The heated section was also replaced with a coil to simulate the receiver. This tests if thermosyphon operation is affected by complexity of the pipes. The rough layout is in the diagram below.

Figure 21: Video of constrained thermosyphon

Although flow was observed in the constrained configuration, the delayed onset and weak circulation indicate limited driving pressure. The effectiveness of heat transfer depends on the rate at which fluid circulated, and a higher flow rate allows more heated fluid to be transported to the TES. When compared to pump-driven flow, the thermosyphon provides significantly lower flow rates which is insufficient for effective heat transfer in the full-scale system, where just the receiver length is 10.3m.

Future work

Future work should include quantifying flow rates under different height differences and temperature gradients to determine the minimum viable operating conditions.

When testing on integrated rig, the implementation requires the TES to be located outside of the 1x1m rig, exceeding the intended footprint of the system. Hence, future work also includes exploring alternate TES configurations to maintain height difference within the TES and receiver and maintain 1x1m configuration.

Based on the results, thermosyphons are not yet suitable for the current system configuration due to insufficient flow rate and spatial constraints.

Appendix A: Insulation experiment

Minimum insulation for PUR/PIR and translucent insulation

For the PUR/PIR insulation and translucent insulation, the thickness required can be calculated to ensure minimum thickness is met to satisfy a 2.37W/m heat loss.

For PUR/PIR, the results is based on this equation:

q^{'} = \frac{2 π L (T_{i} - T_{o})}{\frac{1}{r_{1} h_{i}} + \frac{\ln (\frac{r_{2}}{r_{1}})}{k_{hose}} + \frac{\ln (\frac{r_{3}}{r_{2}})}{k_{PUR/PIR}} + \frac{1}{r_{3} h_{c}}}

$q^{'}$ : Heat loss per unit length (W/m)

$L$ : Length of PUR/PIR (m)

$T_{i}$ : Water temperature (°C)

$T_{o}$ : Ambient temperature (°C)

$r_{1}$ : Inner radius of the hose (m)

$r_{2}$ : Outer radius of the hose/Inner boundary of PUR/PIR (m)

$r_{3}$ : Outer radius of PUR/PIR (m)

$h_{i}$ : Convective heat transfer coefficient of water (W/m²·K) – Inside pipe

$h_{c}$ : Convective heat transfer coefficient of air (W/m²·K) – Outside insulation

$k_{hose}$ : Thermal conductivity of the hose (W/m·K)

$k_{PUR/PIR}$ : Thermal conductivity of PUR/PIR (W/m·K)

Based on the MATLAB script pipe_insulation_main.m, the thickness required is 27.982177mm. 70mm pipe is used for 30.5mm thickness of PUR/PIR as it meets the minimum thickness required and was the closest size available to allow the insulation to be built with minimum thickness.

For translucent insulation, each layer consists of a 0.03 mm plastic sheet and a 3.24 mm stagnant air gap, giving a total thickness of 3.27 mm per layer. The calculation is based off this:

q^{'} = \frac{2 π L (T_{i} - T_{o})}{\frac{1}{r_{1} h_{i}} + \frac{\ln (\frac{r_{2}}{r_{1}})}{k_{hose}} + \frac{\ln (\frac{r_{3}}{r_{2}})}{k_{air}} + \frac{\ln (\frac{r_{4}}{r_{3}})}{k_{plastic}} + \frac{1}{r_{4} h_{c}}}

$q^{'}$ : Heat loss per unit length (W/m)

$L$ : Length of PUR/PIR (m)

$T_{i}$ : Water temperature (°C)

$T_{o}$ : Ambient temperature (°C)

$r_{1}$ : Inner radius of the hose (m)

$r_{2}$ : Outer radius of the hose/Inner radius of air gap (m)

$r_{3}$ : Outer radius of air gap/Inner boundary of plastic (m)

$r_{4}$ : Outer radius of plastic (m)

$h_{i}$ : Convective heat transfer coefficient of water (W/m²·K) – Inside pipe

$h_{c}$ : Convective heat transfer coefficient of air (W/m²·K) – Outside insulation

$k_{hose}$ : Thermal conductivity of the hose (W/m·K)

$k_{air}$ : Thermal conductivity of stagnant air (W/m·K)

$k_{plastic}$ : Thermal conductivity of plastic (W/m·K)

Based on the MATLAB script insulation_twinwall_layers_main.m, a total of 10 layers are used, giving an overall thickness of approximately 32.7 mm.

Next subsection: Thermal Energy Storage

References

[1] A. Paneri, I. L. Wong, and S. Burek, “Transparent insulation materials: An overview on past, present and future developments,” Solar Energy, vol. 184, pp. 59–83, May 2019, doi: https://doi.org/10.1016/j.solener.2019.03.091.
[2] B. Happold, “Transparent insulation,” www.designingbuildings.co.uk. https://www.designingbuildings.co.uk/wiki/Transparent_insulation
[3] "Insulation materials and their thermal properties," greenspec. 2011. http://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/
[4] Zhejiang Hailiang Co., Ltd., “Hailiang Custom Fireproof Black Rubber Thermal Insulation Pipe,” Made-in-China.com. https://hailiang1.en.made-in-china.com/product/QZNaBgfYaJcW/China-Hailiang-Custom-Fireproof-Black-Rubber-Thermal-Insulation-Pipe.html?acc=5494762105-lxy&cpn=21714728580-&tgt=&net=x&dev=c-&gid=CjwKCAjwspPOBhB9EiwATFbi5LcM3xtO6p1yNPq527jXaa6mTHovafQ-oeqxcfyLIuN9QMxQ_--SJxoCaNsQAvD_BwE&kwd=&mtp=&loc=9062542-&gad_source=1&gad_campaignid=21721137545&gbraid=0AAAAA-M7K1iP3cuouqCv8a6A5c06mehHi&gclid=CjwKCAjwspPOBhB9EiwATFbi5LcM3xtO6p1yNPq527jXaa6mTHovafQ-oeqxcfyLIuN9QMxQ_--SJxoCaNsQAvD_BwE
[5] Engineering toolbox, “Absorbed Solar Radiation,” Engineeringtoolbox.com, 2009. https://www.engineeringtoolbox.com/solar-radiation-absorbed-materials-d_1568.html
[6] “The Solar-AC FAQ: Table of absorptivity and emissivity of common materials and coatings,” www.solarmirror.com. http://www.solarmirror.com/fom/fom-serve/cache/43.html
[7] T. P. Otanicar, P. E. Phelan, and J. S. Golden, “Optical properties of liquids for direct absorption solar thermal energy systems,” Solar Energy, vol. 83, no. 7, pp. 969–977, Jul. 2009, doi: https://doi.org/10.1016/j.solener.2008.12.009.
[8] A. Jamar, Z. A. A. Majid, W. H. Azmi, M. Norhafana, and A. A. Razak, “A review of water heating system for solar energy applications,” International Communications in Heat and Mass Transfer, vol. 76, pp. 178–187, Aug. 2016, doi: https://doi.org/10.1016/j.icheatmasstransfer.2016.05.028.