Question

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

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

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 \(200^{\circ} \mathrm{C}\).

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.

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.

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.

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.

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.

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.

Ensure Reliability

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

Key concepts

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.