SOLAR AIR HEATERS

1. INTRODUCTION

Solar air heaters are most often used for space heating during cold seasons, but can also be used for drying purposes (e.g., drying agricultural products) (Sukhatme and Nayak, 2017). Due to the simplicity of their design and their low operating temperature (well below 100°C), they can be manufactured locally and are relatively inexpensive (compared to solar water heaters) to purchase and operate. Moreover, compared to solar water heaters, the lower thermal properties (specific heat and thermal conductivity) of air mean that the flow path (hydraulic diameter) inside solar air heaters needs to be relatively large.

2. GENERAL DESCRIPTION

A typical solar air heater is shown in Fig. 1. The main feature of the solar air heater is the black absorber plate. This plate is generally a thin plate made of conducting metal (most often copper or aluminum) (Duffie and Beckman, 2013). The function of this plate is to absorb the solar irradiance, and then to transfer the heat to the air flowing directly below it. In order to maximize solar energy absorption, the top surface of this metallic plate can be coated with a special coating known as the “solar selective coating” (Kalogirou, 2014). This coating serves a dual role. Firstly, due to its high absorptivity in the solar spectrum (which consist of shortwave electromagnetic radiation, in the range of 0.3–3 μm), it is highly absorptive (around 90–95%). The remaining portion (5–10%) is lost as reflection losses. Secondly, once the plate heats up, depending on the location weather conditions and operational conditions, it will reradiate some of the energy back to the sky. However, a selective coating would have very low emissivity (and similarly low absorptivity) in the infrared radiation range (approximately in the wavelength of about 5–20 μm), which covers most of the black body spectrum of the plate. Thus, a selective absorber plate is able to operate at a higher thermal efficiency as compared to ordinary black surfaces.

Moreover, in order to prevent convective losses from the system, a transparent glass cover is placed on top of the absorber plate, as shown in Fig. 1. This creates a thermal barrier of stagnant air between the cold ambient air (which may be blowing) and the hot absorber plate. Another key function of the glass cover is to allow the solar irradiance to pass through it, with limited reflection and absorption; thus, it is usually prepared from high purity glass. At the bottom of the system, a thick insulation is provided, which prevents conduction and convection losses from the bottom and sides of the system.

3. OPERATION OF THE SOLAR AIR HEATER

As seen in Fig. 1, air enters the system from one side and flows below the absorber plate for some distance, during which time it heats up and then exits from the other side. The system is designed in such a way that maximum possible heat transfer is achieved between the hot absorber plate, and the cold air flowing beneath it, transferring as much thermal energy as possible from the plate to the air.

There are several challenges that are faced during this process. Since the thermal conductivity of air is extremely low [0.028 W/m·K at 60°C (Cengel, 2000)] and its specific heat is also low [1.007 kJ/kg·K (Cengel, 2000)], a large temperature difference is required to transfer heat from a metallic plate (absorber plate) to the air.

In addition to that, pressure losses due to skin-friction drag can hurt the overall efficiency if a blower is needed to maintain the flow of air, which may also consume electricity. When employed, a blower pulls the cold air from surroundings (or in the case of room heating, from the room) across the absorber. Depending on whether the heated air is returned back to the heater or not, the system can be operated in either closed- or open-cycle configurations.

4. CLASSIFICATIONS OF SOLAR AIR HEATER

Solar air heaters can be classified in different ways. One standard way is to differentiate between passive and active type, as well as between air heaters that use a porous absorbing plate compared to those that use a nonporous absorbing plate. Furthermore, they can have different numbers and types of glass plates on top of them, in which case they can be unglazed (having no glass cover, which is highly unlikely), single glazed (having a single glass cover), double glazed (having two glass covers), and so on. The absorber can be made of different types of materials (metals, nonmetals, composites) and can have different shapes (flat, with slots, with fins, or with ribs). Moreover, the solar air heater can be operated in different configurations, as shown in Fig. 2.

Figure 2 shows the four configurations (among many) in which the solar heater can be operated. In the first configuration, shown in Fig. 2(a), the air flows above the black absorber plate, which itself is supported on top of the insulation material. The advantage of this configuration is its simple design and layout. In the second configuration, as shown in Fig. 2(b), the air flows below the black absorber plate which is placed above the insulation at some gap. Furthermore, there is some gap between the black absorber plate and the glass cover as well. In the third configuration, shown in Fig. 2(c), air flows in both the channels, above as well as below the absorber plate. Here, the plate is able to transfer its heat to the fluid on both sides. Finally, as shown in Fig. 2(d), the fourth configuration has the air enter on the top side (between the glass cover and the absorber plate), and then the same air is returned back from the bottom channel after changing its direction toward one end. This allows the temperature of the air to rise to a much greater extent, leading to higher ratings.

5. IMPORTANCE OF HEAT TRANSFER ENHANCEMENT BETWEEN THE AIR AND ABSORBER PLATE

In most practical applications of solar air heaters, the black absorber plate is often artificially roughened in order to improve the heat transfer between the plate and the air flowing above and/or below it. This is needed mainly due to the low thermal properties (thermal conductivity and specific heat) as well as low convective heat transfer coefficient (h) between the two. Creating such roughness is quite an economical way to achieve higher thermal performance of the solar air heaters.

The presence of such roughness elements disrupts the laminar sublayer that flows over the plate. An uninterrupted laminar sublayer tends to act as an insulator and prevents further heat transfer between the hot plate and the cold air. The removal of this barrier is expected to augment the overall heat transfer process. The amount of increase in the heat transfer rate depends on several factors and can be quantified in terms of Nusselt number (Nu), which is a non-dimensionalized form of the convective heat transfer coefficient (h), as follows:

Nu = hLc/ka

(1)

where L_c stands for the characteristic length, and in this case is equal to the length of the plate, and k_a denotes the thermal conductivity of air, which has a value equal to 0.0255 W/m·K at 25°C (Cengel, 2000). The improvement in heat transfer can hence directly be gauged by comparing values of Nu for different flow configurations. It is known to mainly depend on factors such as flow speed (which can be quantified in terms of Reynolds number) and other geometrical parameters of the roughness-causing elements, such as relative height of roughness, angle of attack, α, and periodicity of such obstacles, p (Hans et al., 2009). Some examples of such roughened plates are shown in Fig. 3.

6. COMPARISON BETWEEN SOLAR WATER HEATER AND SOLAR AIR HEATERS

A brief comparison of the various features of the solar water heaters are compared to solar air heaters and is shown in Table 1. The main differences are due to the relatively low values of specific heat, thermal conductivity, and overall heat transfer coefficient (h) for air as compared to water.

7. SOLAR AIR HEATER CONFIGURATIONS

Figure 4 shows the typical layout of the solar air heater which is often used for space heating. As can be seen in Fig. 4, the solar air heater is directly placed on top of the house, above the roof where it is able to absorb the sunlight during most portions of the day, and as a result the air gets heated. This stream of hot air enters the air handling unit (AHU) with the aid of a blower or fan, which circulates and guides the flow of air. Some portion of the outdoor air (fresh air) is also continuously mixed within the AHU. The addition of a small amount of fresh air ensures that indoor air gets slowly circulated and prevents dampness, smell, and an unhealthy accumulation of contaminants within the house.

Figure 5 shows a schematic of a typical solar air heater that is used for drying (mostly agricultural products). In the configuration shown in Fig. 5, the design is very simple and does not require any moving parts. The sunlight is incident on the system and is predominantly absorbed by the absorber plate, which causes the air to heat. As a consequence, the hot air rises and is guided through the vertical channel downstream. Such a motion of the hot air automatically pulls cold air from the downstream so that natural circulation of air takes place. The duration of the drying product can be adjusted in order to obtain the desired amount of drying. There could also be advanced versions of the same configuration where some additional ducts can be installed to further control the mixing rate of cold and hot air. For scaling this system, taller chimneys can be used along with the installation of air blower which would create higher flow rates.

8. CONCLUSION

Solar air heaters have unique features and operating conditions. They are a very viable solution for heating indoor space during cold weather conditions. In addition, they can be used for many purposes such as for drying, cooking, and timber seasoning. Solar-heated homes are gaining popularity due to the simple and cost-effective functioning of solar air heaters in small- to medium-sized residences. They can also be coupled with packed bed storage for improved performance.