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In considcring the following jroblems involving heat transfer in the natural environment (outdoors), recog. nize that solar radiation is comprised of long and shon wavelength components. If this radiation is incident on a semitrarsporent medium, such as water or glass, two things will happen to the noneflecied porticn of the radiation. The long wavelength component will be aboubed at the surface of the medium, whereas the short wavelength component will be transmitted by the surface. (a) The number of panes in a window can strongly inftuence the heat loss from a heated room to the outside ambient air, Compare the single- and doublepaned units shown by identifying relevant healt transfer processes for each case. (b) In a typical fat-plate solar collector, energy is collected by a working fluid thut is circulated through fubes that are in good contact with the hack face of an absorber plate. The back. face is insulated frum the surroundings, and the absuber plate receives solar radiation on its front face, which is typically covered by one or more transparent plates. Idensify the relevant heat transfer processes, first for the absorber plate with no cover plate and then for the absorber plate with a single cover plate. (c) The solar energy collector design stoun below has been used for agricultural applications. Air is blown through a long duct whone cross section is in the form of an equilateral triangle. One side of the triangle is comprised of a double-paned, semitransparent cover, while the ceher two sides are constructed from aluminum sheets painted flat black on the inside and covered on the outside with a Layer of styroform insulation. During sunny periods, air entering the system is heated for delivery to cither a greenhouse, grain drying unit, or a storage system. Identify all heat transfer processes associated with the cover plates, the absorter plate(s), and the air. (d) Evacuated-tube solar collectors are capable of improved performance relative to flat-plate collectors. The design consists of an inner tube enclosed in an outer tube that is transparent to solar radiation. The annular space between the tubes is evacuated. The ouler, opaque surface of the inner tube absorbs solar raciation, and a working fluid is passed through the tube to collect the solar energy. The collector design generally consists of a row of such tubes arranged in frunt of a reflecting panel. Identify all heat transfer processes relevant to the performance of this device.

Step by step solution

01

Step-by-Step Solution for Part (a)

**Single-Pane Window:** 1. **Conduction:** Heat is conducted through the single glass pane. 2. **Convection:** Heat is lost from the inner surface to the room air and from the outer surface to the outside air. 3. **Radiation:** The inner surface radiates heat into the room, while the outer surface radiates to the environment. **Double-Pane Window:** 1. **Conduction:** Heat is conducted through two glass panes and the air or gas layer in between. 2. **Convection:** Convection occurs within the air gap, but it is less effective due to the confined space. 3. **Radiation:** Both panes radiate heat, reducing net heat loss. The air gap minimizes conduction and convection.

02

Step-by-Step Solution for Part (b)

**Absorber Plate with No Cover:** 1. **Radiation Absorption:** Solar radiation is absorbed directly by the plate. 2. **Conduction:** Heat is conducted from the surface into the plate. 3. **Convection:** Heat is lost from the plate's surface to the air, reducing efficiency. **Absorber Plate with Single Cover Plate:** 1. **Radiation Transmission and Absorption:** The cover transmits shortwave radiation to the absorber, while reducing longwave reradiation. 2. **Conduction:** Reduced conduction losses due to cover. 3. **Convection:** The cover reduces convective losses by creating a barrier to airflow.

03

Step-by-Step Solution for Part (c)

**Double-Paned Cover Plates:** 1. **Radiation Transmission:** Shortwave solar radiation is transmitted through the double panes, while absorbing some longwave radiation. 2. **Conduction and Convection:** Reduced due to the air gap. **Black Aluminum Absorber Plates:** 1. **Radiation Absorption:** These absorb radiation. 2. **Conduction:** Heat is conducted along the plate. 3. **Convection:** Heated air inside the duct is convected to the air stream. **Air within the Duct:** 1. **Convection and Conduction:** Air is heated by convection from the plates and conduction through the duct walls, then delivered to various systems.

04

Step-by-Step Solution for Part (d)

**Inner Tube:** 1. **Radiation Absorption:** The tube absorbs and stores solar radiation. 2. **Conduction:** Heat is conducted from the outer surface to the fluid inside. **Outer Tube and Annular Space:** 1. **Vacuum Insulation:** Minimized heat loss by conduction and convection (space is evacuated). **Reflecting Panel:** 1. **Radiation Reflection:** Reflects additional solar rays to the tubes, enhancing absorption.

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Key Concepts

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

Solar Radiation

Solar radiation is a powerful source of energy that affects various heat transfer processes on Earth. This radiation is categorized into long wavelength and short wavelength components. Each type of wavelength interacts differently with materials like water or glass.
When solar radiation hits a semitransparent medium, such as glass, it can either be reflected, absorbed, or transmitted.

• Short wavelength radiation tends to be transmitted through the medium, allowing it to pass through and provide heat to objects beneath.
• Long wavelength radiation, on the other hand, is absorbed by the surface of the medium, where it increases the surface temperature. This absorbed energy can then undergo conduction into the material or be released as radiation.

This behavior of solar radiation is crucial in the designing of energy-efficient windows, solar collectors, and other heat management systems.

Conduction

Conduction is a process of heat transfer where energy moves through a material without the bulk movement of the material itself. It occurs due to the temperature difference between two points in a material.
This mechanism is significant in both single- and double-paned windows.

• In single-pane windows, heat readily conducts through the single layer of glass, directly transferring thermal energy from the warmer side to the cooler side.
• In double-pane windows, the presence of two panes with an air or gas layer in between slows down this transfer. The intermediate layer acts as an insulator, reducing the rate of conduction.

The efficiency of conduction can be influenced by the type of material, its thickness, and the temperature gradient across the material. It plays an essential role in heat retention and loss in building materials.

Convection

Convection involves the transfer of heat by the movement of fluids such as gases or liquids. In window and solar collector applications, convection serves as a pathway for heat loss or distribution.

• For single-pane windows, convection occurs from the inner surface to the room air and from the outer surface to the outside air. This enhances the heat loss compared to other methods.
• In double-pane windows, the confined space between panes limits convection, effectively reducing heat loss.
• In systems like solar collectors, convection is utilized to transfer energy from heated elements, such as absorber plates, to the circulating air or fluid, distributing the captured solar energy.

Understanding how convection operates allows us to better control heating efficiency and energy transfer in various applications.

Radiation Absorption

Radiation absorption is a vital component of how materials interact with solar energy. It determines how much solar radiation a material can capture and convert into heat.

• When solar radiation strikes an absorber plate, such as those in solar collectors, the plate absorbs the radiation, thus raising its temperature.
• The amount of radiation absorbed influences the efficiency and performance of the collector. Materials are often chosen for their high absorptivity to maximize energy capture.

In devices like evacuated-tube solar collectors, effective radiation absorption is combined with minimized heat losses to achieve superior performance. These absorptive characteristics enable more optimal utilization of solar energy for heating purposes.