Thermal Transmission

Thermal transmission transfers thermal energy from the receiver to the thermal energy storage (TES) and includes:

• Heat transfer fluid (HTF)
• Fluid motion
• Insulation

The design specifications are summarised below.

Heat transfer fluid

Existing Methods: Heat transfer fluid

Heat transfer fluid (HTF) transfers thermal energy from the receiver to the TES. HTM effectiveness depends on these properties:

• Heat transfer coefficient $\mathrm{HTC}=\sqrt{\frac{k\rho C_{\mathrm{p}}}{t}}$
• Specific heat capacity
• Thermal conductivity
• Density: mass per unit volume
• Viscosity
• Thermal stability

The system’s operating temperature and storage requirements are important as different HTF have different properties. [64] Furthermore, as large volumes of HTM are required, cost and performance needs to be balanced. [65]

This summary consists of the most common and major categories of HTMs. These are:

1. Molten salt
2. Liquid metal
3. Thermal oils
4. Gaseous type
5. Water/steam
6. Nanofluids
7. Silicon-based fluids

The common HTFs are molten salt and thermal oils, in high-temperature and moderate-temperature CSPs respectively. [64] Nanofluids and silicon-based fluids are new emerging materials. The comparison of each HTF are summarised below.

Heat transfer fluid selection

The HTFs are selected based on the maximum temperature of our CSP system (67ºC, solar tower configuration). We set the operating temperature range from room temperature to 100ºC, accounting for improvements in the light subsystem.

The HTFs are compared based on these:

1. Safety
2. Cost
3. Heat capacity
4. Thermal conductivity
5. Density
6. Dynamic viscosity

(see table 19 for comparison)

The HTFs are compared using the decision matrix below.

From the decision matrix, water is the best option (with it being readily available) and water-ethanol solution (10-50%) is the alternative. Water is thus chosen and is already being used in the light subsystem, with it proving to work well within the temperature range of the existing system.

However, if temperature rises above 100ºC, water can be switched to water-ethanol solution or the other HTF options as they have a higher maximum operating temperature, up to 785ºC.

Fluid motion

Existing Methods: Fluid motion

CSPs generally uses pumps, selected based on system requirements (material compatibility, operating temperature, flow rate, pressure requirements). [71] Pump companies like Ruhrpumpen and Sulzer provides pumping solutions. [72][73]

Alternatively, thermosiphon can drive flow by gravity and natural circulation. (see figure 29)

It is mainly used in solar water heaters under low volume and temperature. [76] In CSPs, its operational range and performance depends on HTF properties and the pressure from HTF expansion must be maintained for safe operations. [76] They are used in low to medium-temperature small-scale or prototype CSPs (see figure 30) and are not used commercially. [77]

Two-phase (loop) thermosiphons is another option, driving flow by evaporation and condensation which requires the fluid to operate near its boiling point. (see figure 31)

It is tested in prototype CSP systems, like the linear collector CSP below, and is not used commercially. [63] Due to vaporisation, pressure must be controlled for safe operations. [63]

Fluid motion mechanism selection

Assessing the feasibility of each mechanism:

• Thermosiphons requires the TES to be above the receiver, blocking the light rays.


• Two-phase loop thermosiphons can function at 0m height difference between the evaporator and condensor. [42] However, it requires the HTF to function near its boiling point, which our current system cannot reach. Should our system enable the HTF temperature to reach its boiling point, this method can be tested for its feasibility.


• Pumps are chosen due to reliability and flexibility in meeting system requirments.

Proof of concept: Can pumps drive flow through the receiver?

Experiment 1: Flowing open system

This experiment is conducted to observe if flow can be achieved using pumps.

Results and analysis:

The outlet temperature is 17.5ºC higher than the inlet, indicating water flow through the receiver.

The power absorbed by the water is higher for the flowing (142.3W) than static system (16.01W). The forced convection from the pump replaces hotter fluid with cooler fluid faster, increasing temperature gradient ($\Delta T$) and convective coefficient between the fluid and receiver. This causes the higher power absorbed by the convection heat transfer equation, $\dot{Q}=hA(T_{\mathrm{coil}}-T_{\mathrm{fluid}})$.

Experiment 2: Flowing closed (circulating) system

This experiment is conducted to observe the maximum temperature of the system.

Results and analysis:

The temperature stabilises at 57.5ºC, lower than the static system (64ºC), due to losses between the receiver and container. This is used to verify the TES (TES subsection).

Overall shortcomings and limitations of both experiments

The pipes between the containers and receiver are not insulated, causing higher heat loss and lower temperature measured (to be addressed later).

Selection of pump

Selection of temperature range between inlet and outlet of receiver

The choice of temperature difference affects:

1. Heat transfer rate between HTF and TES
2. Structural integrity

Considering a high temperature rise, it increases the heat transfer rate between the HTF and TES, $\dot{Q}=\mathrm{hA}(T_{\mathrm{HTF}}-T_{\mathrm{TES}})=\mathrm{hA}\Delta T$, improving efficiency of TES charging. However, it causes failures in the structure from thermal expansion. [61]

Hence, 5-10ºC change is selected.

Pump requirements

The system is assumed to be as below.

For the pump requirements (see table 29), we calculate:

• Flow rate (see table 28)
• $Q_{\mathrm{required}}={\mathrm{mass}}_{\mathrm{HTF}}{C}_{\mathrm{p,\; HTF}}\Delta T$
• ${\mathrm{time}}_{\mathrm{required}}=\frac{Q_{\mathrm{required}}}{P_{\mathrm{absorbed\; by\; HTF}}}$
• $\mathrm{Velocity}=\frac{{\mathrm{Length}}_{\mathrm{receiver}}}{{\mathrm{time}}_{\mathrm{required}}}$
• ${\mathrm{Flow\; rate}}_{\mathrm{l/min}}=\mathrm{Velocity}{\mathrm{Area}}_{\mathrm{cross-sectional}}$
• Total dynamic head (TDH)
• $\mathrm{TDH}=\mathrm{HS}+\mathrm{HF}+\mathrm{HP}$
• Net positive suction head available (NPSHa)
• ${\mathrm{NPSH}}_{\mathrm{a}}=h_{\mathrm{a}}-h_{\mathrm{vpa}}+h_{\mathrm{st}}-h_{\mathrm{f}}$
• Differential pressure ($\Delta P$)
• $\Delta P=\mathrm{0.0981}\times \mathrm{TDH}\times \mathrm{SG}$
• (see MATLAB flowrate_receiver and pump_requirements)

Pump selection

The pumps are chosen based on the pump requirements above except for flow rate, which is not followed closely as the temperature range is assumed. The selected pumps are below.

To pumps are compared based on these:

1. Temperature rise, $\Delta T$
• ${\mathrm{time}}_{\mathrm{through\; receiver}}=\frac{{\mathrm{Volume}}_{\mathrm{HTF,\; receiver}}}{{\mathrm{<a\; href="\#TT-theories-concepts">Flow\; rate</a>}}_{\mathrm{m/s}}}$


• $Q_{\mathrm{absorbed\; by\; HTF}}=P_{\mathrm{absorbed\; by\; HTF}}\times {\mathrm{time}}_{\mathrm{through\; receiver}}$


• $\Delta T=\frac{Q_{\mathrm{absorbed\; by\; HTF}}}{{\mathrm{mass}}_{\mathrm{HTF,\; receiver}}{C}_{\mathrm{p,\; HTF}}}$


• (see MATLAB flowrate_to_temp)


2. Power consumption: Powered using solar panels


1. Power formula: P = IV


2. For AC (Grothen G328A):


• ${\mathrm{<a\; href="\#TT-theories-concepts">I</a>}}_{\mathrm{AC}}=\frac{P_{\mathrm{DC}}}{\mathrm{PF}\dot{}\eta \dot{}{V}_{\mathrm{AC}}}$


• Assuming:


• Grothen G328A uses SMPS. [47]
• Power factor (PF) = 0.6, assuming worst case (without PFC). [48]
• Electric efficiency = 90%. [49]
• See Appendix X for calculation


3. Cost

(see table 31 for comparison)

The pumps are compared using the decision matrix below.

As such, Grothen G328A is selected as the best option for the current non-optimised system, with the Grothen G928 as an alternative. In the next semester, the variable flow rate capability of the Grothen G328A will be utilised to test and verify the optimal flow rate which maximises the TES charging efficiency, provided that its flow rate range remains sufficient after system optimisation.

Pipe insulation: Design Specifications

Pipe insulation selection

The intermediate pipe used is PVC hose for flexibility, durability and low thermal conductivity. The maximum allowable heat loss is set at 5% of power received (7.11W). Insulating materials with lowest thermal conductivity and those commonly used (eg. polyethelyne foam for air conditioner) are selected.

To compare them, we consider:

1. Thermal conductivity
2. Cost
3. Durability

Accounting for heat loss requirement, we consider:

1. Cost per unit length to meet the thickness required


• Calculated using cost, size and thickness required (see MATLAB cost_insulation and cost_per_length)


2. Thickness required


• Calculated using:


• ${\dot{Q}}_{\mathrm{HTF\; to\; atm}}=\frac{T_{\mathrm{HTF}}-T_{\mathrm{atm}}}{R_{\mathrm{total}}}$


• $R_{\mathrm{total}}=R_{\mathrm{HTF,\; conv}}+R_{\mathrm{pipe,\; cond}}+R_{\mathrm{insulation,\; cond}}+R_{\mathrm{air,\; conv}}$


• where $R_{\mathrm{conv}}=\frac{1}{\mathrm{hA}}$ and $R_{\mathrm{cond}}=\frac{\ln \left(\frac{{\mathrm{radius}}_{\mathrm{outer}}}{{\mathrm{radius}}_{\mathrm{inner}}}\right)}{\mathrm{kA}}$ for pipes.


• (see MATLAB insulation_thickness)


3. Durability

(see table 33 for comparison)

A decision matrix is used to select the best option.

The best option is PIR/PUR foam, which will be tested in the next semester. PE foam will be kept as an alternative.