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Chapter 11: Problem 2

Develop the preliminary design of a thermal storage system that would recover automobile engine waste heat for later use in improving the engine cold-start performance. Among the specifications are: Reliable operation down to an ambient temperature of \(-30^{\circ} \mathrm{C}\), a storage duration of 16 hours, and no more than 15 minutes of urban driving to return the storage medium to its maximum temperature of \(200^{\circ} \mathrm{C}\). Specify the storage medium and determine whether the medium should be charged by the engine exhaust gases, the engine coolant, or some combination. Explain how the system would be configured and where it would be located in the automobile.

Short Answer

Expert verified
Use a phase-change material and combine engine exhaust and coolant for efficient heat recovery. Place the system near the exhaust and ensure it operates reliably at \(-30^{\circ} \mathrm{C}\).

Step by step solution

01

Understand Specifications

The thermal storage system must operate reliably at \(-30^{\circ} \mathrm{C}\), have a storage duration of 16 hours, and be recharged in no more than 15 minutes of urban driving to reach \(200^{\circ} \mathrm{C}\).
02

Choose Storage Medium

Select a storage medium with high specific heat capacity and good thermal conductivity. Phase-change materials (PCMs) such as paraffin or eutectic salts are suitable because they store significant amounts of heat during phase transitions.
03

Determine Heat Source

Decide between the engine exhaust gases, engine coolant, or a combination. Engine exhaust gases are at a higher temperature, making them more efficient for rapid charging, while engine coolant is safer and easier to manage.
04

Combine Heat Sources

Utilizing both the engine exhaust and engine coolant can offer efficient heat recovery and stable operation. The exhaust can provide quick heating, while the coolant can maintain and regulate the temperature.
05

Design System Configuration

Design the system with heat exchangers connected to the exhaust and coolant system. The storage medium should be placed in an insulated container with heat exchangers to optimize thermal transfer.
06

Select System Location

Locate the thermal storage system in the engine bay or under the car near the exhaust system for direct access to heat sources. Ensure the location allows for sufficient insulation and safety measures.
07

Review Heat Transfer Efficiency

Calculate the heat transfer rates to ensure the system can achieve the required recharge time of 15 minutes and maintain operation at \(-30^{\circ} \mathrm{C}\). Use the formula \Q = mc \Delta T\ for sensible heating and \Q = mL\ for latent heating with phase-change materials.
08

Ensure Reliability

Test the system's performance under various conditions to ensure reliability, especially in low ambient temperatures and during engine cold starts.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Thermal Conductivity
Thermal conductivity is a measure of a material's ability to conduct heat. In the context of a thermal storage system, higher thermal conductivity is desirable because it allows for faster heat transfer from the heat source to the storage medium and vice versa.
Materials with high thermal conductivity, such as metals, facilitate efficient heat exchange, which is crucial for the system to recharge quickly (within the specified 15 minutes of urban driving) and maintain the necessary storage temperatures.
  • Metals, especially aluminum and copper, are often used for heat exchangers due to their high thermal conductivity.
  • PCMs like paraffin can be combined with materials that have high thermal conductivity to enhance overall system performance.
By balancing thermal conductivity in the design, we ensure that the thermal storage system can effectively capture and release heat, improving its overall functionality and reliability.

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Most popular questions from this chapter

In the expression \((\partial u / \partial T)_{v}\), what does the subscript \(v\) signify?

Complete the following exercises dealing with slopes: (a) At the triple point of water, evaluate the ratio of the slope of the vaporization line to the slope of the sublimation line. Use steam table data to obtain a numerical value for the ratio. (b) Consider the superheated vapor region of a temperature-entropy diagram. Show that the slope of a constant specific volume line is greater than the slope of a constant pressure line through the same state. (c) An enthalpy-entropy diagram (Mollier diagram) is often used in analyzing steam turbines. Obtain an expression for the slope of a constant-pressure line on such a diagram in terms of \(p-v-T\) data only. (d) A pressure-enthalpy diagram is often used in the refrigeration industry. Obtain an expression for the slope of an isentropic line on such a diagram in terms of \(p-v-T\) data only.

Methane at \(27^{\circ} \mathrm{C}, 10\) MPa enters a turbine operating at steady state, expands adiabatically through a \(5: 1\) pressure ratio, and exits at \(-48^{\circ} \mathrm{C}\). Kinetic and potential energy effects are negligible. If \(\bar{c}_{p b}=35 \mathrm{~kJ} / \mathrm{kmol} \cdot \mathrm{K}\), determine the work developed per \(\mathrm{kg}\) of methane flowing through the turbine. Compare with the value obtained using the ideal gas model.

At certain states, the \(p-v-T\) data of a gas can be expressed as \(Z=1-A p / T^{4}\), where \(Z\) is the compressibility factor and \(A\) is a constant. (a) Obtain an expression for \((\partial p / \partial T)_{v}\) in terms of \(p, T, A\), and the gas constant \(R\). (b) Obtain an expression for the change in specific entropy, \(\left[s\left(p_{2}, T\right)-s\left(p_{1}, T\right)\right]\) (c) Obtain an expression for the change in specific enthalpy, \(\left[h\left(p_{2}, T\right)-h\left(p_{1}, T\right)\right]\)

Over limited intervals of temperature, the saturation pressure-temperature curve for two-phase liquid-vapor states can be represented by an equation of the form \(\ln p_{\text {sat }}=A-B / T\), where \(A\) and \(B\) are constants. Derive the following expression relating any three states on such a portion of the curve: $$ \frac{p_{\text {sat, } 3}}{p_{\text {sat, } 1}}=\left(\frac{p_{\text {sat, } 2}}{p_{\text {sat }, 1}}\right)^{\tau} $$ where \(\tau=T_{2}\left(T_{3}-T_{1}\right) / T_{3}\left(T_{2}-T_{1}\right)\).
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