Device Performance Improvement of Double-Pass Wire Mesh Packed Solar Air Heaters under Recycling Operation Conditions

Abstract

The improvement of device performance of a recycling solar air heater featuring a wire mesh packing was investigated experimentally and theoretically. The application of the wire mesh packing and recycle-effect concept to the present study were proposed aiming to strengthen the convective heat-transfer coefficient due to increased turbulence. Comparisons were made among different designs, including the single-pass, flat-plate double-pass and recycling double-pass wire mesh packed operations. The collector efficiency of the recycling double-pass wire mesh packed solar air heater was much higher than that of the other configurations for various recycle ratios and mass flow rates scenarios. The power consumption increment due to implementing wire mesh in solar air heaters was also discussed considering the economic feasibility. A fairly good agreement between theoretical predictions and experimental measurements was achieved with an analyzed error of 1.07%–9.32%.

1. Introduction

Various solar air heater configurations have been implemented to enhance collector efficiency compared to a simple flat-plate device consisting of glass covers, an absorber plate and air flow channels. Razika et al. [1] and Al-Kayiem and Yassen [2] discussed the effect of the inclination angle on the absorption-convection heat transfer mechanism in a solar air collector. El-Sebaii and Al-Snani [3] investigated the effects of using absorber plates coated with various selective coating materials on the collector performance. Vaziri et al. [4] presented the collector performances of new solar air collector designs having different inner collector colors. Many improved devices were designed taking into account the design parameters for strengthening the convective heat-transfer coefficient [5], enlarging heat-transfer area [6], and increasing flow turbulence [7]. Several investigators have confirmed the technical feasibility of the recycle-effect application to heat transfer devices and reactors. Ho et al. [8,9] derived the theoretical formulations of the recycle-effect concept to heat and mass transfer through a parallel-plate channel with recycle. Fudholi et al. [10] developed the improvement potential of the solar collector based on energy and exergy analyses. Singh and Dhiman [11] proposed an analytical study to predict the thermal and thermohydraulic efficiencies of recycling double-pass packed bed solar air collectors. Garg et al. [12] extended solar air heater operation to multiple-pass operations. Enhancement of convective heat-transfer coefficients was achieved by increasing the intensity of the turbulence in recycling solar air collectors with double-pass [13] and multi-pass [14] operations. In this work a new solar air heater design adopting a wire mesh packed double-pass design under recycling operation, as shown in Figure 1 and Figure 2, is proposed and studied. There are two purposes in the present study: first, to obtain theoretical predictions and to obtain the experimental results for the recycling double-pass wire mesh packed solar air heater; and second, to study the effects of the recycle ratio and air mass flow rate on the heat-transfer efficiency enhancement and to make an economic consideration of both heat-transfer efficiency improvement and power consumption increment.

Figure 1. Configuration of a recycling double-pass solar air heater.

Figure 1. Configuration of a recycling double-pass solar air heater.

Figure 2. A double-pass solar air heater with internal attached wire mesh.

Figure 2. A double-pass solar air heater with internal attached wire mesh.

2. Temperature Distributions

Before entering the lower subchannel, the inlet air mass flow rate and inlet temperature of $\dot{m}$ and T_a,i is premixed with the recycling air flow $R\dot{m}$ exits from the lower subchannel with the outlet temperature T_a,L. A schematic configuration is depicted in Figure 1 while the air flow arrangement is shown in Figure 2.

By following the similar mathematical treatment and experimental studies performed in our previous work [15], except instead for the wire mesh packing, the analytical solutions of the temperature distributions of the flowing air in the lower and upper subchannels can be obtained in dimensionless form as:

$$T_a(\xi )=C_1{e}^{Y_1\xi }+C_2{e}^{Y_2\xi }+\frac{B_3{B}_4-B_1{B}_6}{B_1{B}_5-B_2{B}_4}+T_s$$

(1)

$$T_b(\xi )=\frac{Y_1-B_5}{B_4}{C}_1{e}^{Y_1\xi }+\frac{Y_2-B_5}{B_4}{C}_2{e}^{Y_2\xi }-\frac{B_5(B_3{B}_4-B_1{B}_6)}{B_4(B_1{B}_5-B_2{B}_4)}-\frac{B_6}{B_4}+T_s$$

(2)

For the definitions of B_i, G_i, M_i, Y_i, C_i, F_i and I_i readers are referred to the Appendix. The outlet temperature of T_b,0 can be calculated from Equation (2):

$$T_b(0)=T_{b,0}=\frac{Y_1-B_5}{B_4}{C}_1+\frac{Y_2-B_5}{B_4}{C}_2-\frac{B_5(B_3{B}_4-B_1{B}_6)}{B_4(B_1{B}_5-B_2{B}_4)}-\frac{B_6}{B_4}+T_s$$

(3)

Estimation of the useful energy gained by the flowing air was obtained from the energy balance on the lower subchannel, upper subchannel and whole solar air heater with the known inlet and outlet temperatures, respectively:

$$Q_u=\dot{m}(1+R)C_p(T_{a,L}-T_{a,0})+\dot{m}{C}_p(T_{b,0}-T_{b,L})$$

(4)

or:

$$Q_u=\dot{m}{C}_p(T_{a,L}-T_{a,i})+\dot{m}{C}_p(T_{b,0}-T_{b,L})=\dot{m}{C}_p(T_{b,0}-T_{a,i})$$

(5)

For simplicity, the collector efficiency η_W of the double-pass wire mesh packed solar air heater with external recycle was obtained from the actual useful energy gained by the airflow and the incident solar radiation as:

$$\begin{array}{ll}{\eta }_W& =\frac{Q_u\left(\text{Useful gain of energy carried away by air}\right)}{I_0{A}_c\left(\text{Total solar radiation incident}\right)}\\ & =\frac{\dot{m}{C}_p(T_{b,0}-T_{a,i})}{I_0{A}_c}=\frac{I_0{\tau }_g^2{\alpha }_p-U_L(T_{p,m}-T_s)}{I_0}\end{array}$$

(6)

Equating and rearranging the terms of Equation (6), the average absorber temperature was obtained as:

$$T_{p,m}=T_s+(I_0{\tau }_g^2{\alpha }_p/U_L)-\frac{\dot{m}{C}_p(T_{b,0}-T_{a,i})}{A_c{U}_L}=T_s+(I_0/U_L)({\tau }_g^2{\alpha }_p-{\eta }_W)$$

(7)

3. Experimental Studies

The experimental setup of the recycling double-pass wire mesh packed solar air heater with subchannel width, length and height of 0.3 m, 0.3 m and 0.05 m, respectively, is shown in Figure 3. One set of heat sources with on/off switch consisted of 14 energy supplies (110 V, 125 W) situated 0.15 m above the glass cover, and the insolation was measured and recorded with instantaneous solar radiation meter (Model No. 455, the Epply Laboratory Inc., St. Paul, MN, USA). Temperatures of flowing air in the interior and at the inlet and outlet of the collector and the air mass flow rate were measured while the ambient temperature was regulated using an air conditioner. Before entering the lower subchannel, the airflow with mass flow rate $\dot{m}$ and temperature T_a,i will premix the air flow exiting from the lower subchannel with $R\dot{m}$ and T_a,L which is regulated by means of a valve situated at the end of the lower subchannel. Twenty pieces of the wire mesh were welded into the lower subchannel using a mesh interval of 0.015 m and mesh pitch of 0.003 m. The experimental runs were carried out to supply the ambient air by a blower (Teco 3 Phase Induction Motor, Model BL model 552, Redmond Co, Owosso, MI, USA) which was measured by an anemometer (Kanmax Japan Inc., Osaka, Japan). By substituting the specified values into the appropriate equations, the theoretical predictions were obtained and also presented graphically in Figure 4 and Figure 5 for comparisons between different devices.

Figure 3. Schematic diagram of a double-pass solar air heater with artificial simulation.

Figure 3. Schematic diagram of a double-pass solar air heater with artificial simulation.

4. Results and Discussion

Moffat [16] determined the precision analysis of each individual measurement directly from the experimental run as follows:

$$S_{{\eta }_{\mathit{exp}}}={\left\{{\displaystyle \sum _{i=1}^{N_{\mathit{exp}}}\frac{{\left({\eta }_{\mathit{exp},i}-{\overline{\eta }}_{\mathit{exp},i}\right)}^2}{N-1}}\right\}}^{1/2}$$

(8)

and the mean value of resulting uncertainty of experimental runs was defined by:

$$S_{{\overline{\eta }}_{\mathit{exp}}}=\frac{S_{{\eta }_{\mathit{exp}}}}{\sqrt{N_{\mathit{exp}}}}$$

(9)

Estimations of the precision index for I₀ = 830 and I₀ = 1100 W/m² with three air mass flow rates were calculated. The mean precision index of the experimental measurements in Figure 4 and Figure 5 ranged between $2.80\times {10}^{-3}\le S_{{\overline{\eta }}_{\mathit{exp}}}\le 7.75\times {10}^{-3}$ . Meanwhile, deviations between the experimental results the theoretical predictions may be defined as:

$$E=\frac{1}{N_{\exp }}{\displaystyle \sum _{i=1}^{N_{\exp }}\frac{\left|{\eta }_{theo,i}-{\eta }_{\exp ,i}\right|}{{\eta }_{theo,i}}}\times 100\%$$

(10)

where N_exp, η_theo,i and η_exp,i are the number of experimental measurements, theoretical predicted and experimental data of collector efficiencies, respectively. Accuracy deviations were calculated and shown in Table 1 within 1.33 ≤ E ≤ 9.32 under two incident solar radiations I₀ for two configurations without (flat-plate type) and with attached wire mesh. It is seen that the experimental results fairly confirm the theoretical predictions, as indicated from Table 1. Comparisons between the theoretical predications and experimental results were achieved in good agreement, as observed from Figure 4 and Figure 5.

Table 1. Deviations between the experimental results and theoretical predictions.

	$E=\frac{1}{N_{\exp }}{\displaystyle \sum _{i=1}^{N_{\exp }}\frac{\left|{\eta }_{theo,i}-{\eta }_{\exp ,i}\right|}{{\eta }_{theo,i}}}\times 100\%$
	Flat-Plate Type		Wire Mesh Packed
	I0 (W/m2)		I0 (W/m2)
$\dot{m}$ (kg/h)	830	1100	830	1100
38.52	7.17	9.32	1.65	1.07
57.96	4.70	2.82	7.44	6.87
77.04	3.25	1.33	7.08	5.54

Table 1. Deviations between the experimental results and theoretical predictions.

	$E=\frac{1}{N_{\exp }}{\displaystyle \sum _{i=1}^{N_{\exp }}\frac{\left|{\eta }_{theo,i}-{\eta }_{\exp ,i}\right|}{{\eta }_{theo,i}}}\times 100\%$
	Flat-Plate Type		Wire Mesh Packed
	I0 (W/m2)		I0 (W/m2)
$\dot{m}$ (kg/h)	830	1100	830	1100
38.52	7.17	9.32	1.65	1.07
57.96	4.70	2.82	7.44	6.87
77.04	3.25	1.33	7.08	5.54

Figure 4. Effects of recycle ratio and air mass flow rate on collector efficiencies (I₀ = 830 W/m²).

Figure 4. Effects of recycle ratio and air mass flow rate on collector efficiencies (I₀ = 830 W/m²).

Figure 5. Effects of recycle ratio and air mass flow rate on collector efficiencies (I₀ = 1100 W/m²).

Figure 5. Effects of recycle ratio and air mass flow rate on collector efficiencies (I₀ = 1100 W/m²).

Effects of recycle ratio and air mass flow rate on collector efficiencies, η_D and η_W, for both flat-plate type and wire mesh packed devices, respectively, are presented in Figure 4 and Figure 5. Both collector efficiencies, η_D and η_W, increase with increasing recycle ratios and air mass flow rates owing to the air velocity enlargement, and thus, a higher convective heat transfer coefficient is achieved.

Collector performance improvements of I_D and I_W, for the flat-plate and wire mesh packed double-pass devices, respectively, under the same operating conditions with various incident solar radiations, air mass flow rates and recycle ratios as parameters are defined by the percentage increase in the collector efficiency compared to that in the downward single-pass device as follows:

$$I_D=\frac{{\eta }_D-{\eta }_S}{{\eta }_S}\times 100\%$$

(11)

$$I_W=\frac{{\eta }_W-{\eta }_S}{{\eta }_S}\times 100\%$$

(12)

Some calculated results are presented in Table 2.

Table 2. Theoretical predictions of heat-transfer efficiency improvement of I_D and I_W.

m (kg/h)	R	I0 = 830 (W/m2)		I0 = 1100 (W/m2)
		Flat-Plate	Wire Mesh	Flat-Plate	Wire Mesh
		ID (%)	IW (%)	ID (%)	IW (%)
38.52	0.25	38.19	62.46	39.16	63.11
	0.5	45.63	76.38	46.60	77.67
	0.75	51.78	87.06	52.75	89.00
	1	56.96	95.15	57.93	98.06
	1.25	61.49	101.94	62.46	105.18
	1.5	65.37	107.12	66.34	110.68
	1.75	68.93	111.97	69.90	115.53
	2	72.17	115.86	72.82	119.42
57.96	0.25	29.73	62.43	31.08	64.59
	0.5	35.68	71.62	37.03	74.59
	0.75	40.81	78.65	41.89	81.89
	1	44.86	83.78	45.95	87.57
	1.25	48.38	88.38	49.46	91.89
	1.5	51.35	90.81	52.43	95.41
	1.75	54.05	94.05	55.14	98.11
	2	56.22	96.22	57.30	100.27
77.04	0.25	23.74	59.95	24.94	61.87
	0.5	28.78	66.67	29.98	69.31
	0.75	32.85	71.46	34.05	74.34
	1	36.45	75.06	37.41	78.18
	1.25	39.09	77.94	40.29	81.29
	1.5	41.73	79.86	42.69	83.45
	1.75	43.88	81.53	44.60	85.13
	2	45.56	82.97	46.52	86.57

Table 2. Theoretical predictions of heat-transfer efficiency improvement of I_D and I_W.

m (kg/h)	R	I0 = 830 (W/m2)		I0 = 1100 (W/m2)
		Flat-Plate	Wire Mesh	Flat-Plate	Wire Mesh
		ID (%)	IW (%)	ID (%)	IW (%)
38.52	0.25	38.19	62.46	39.16	63.11
	0.5	45.63	76.38	46.60	77.67
	0.75	51.78	87.06	52.75	89.00
	1	56.96	95.15	57.93	98.06
	1.25	61.49	101.94	62.46	105.18
	1.5	65.37	107.12	66.34	110.68
	1.75	68.93	111.97	69.90	115.53
	2	72.17	115.86	72.82	119.42
57.96	0.25	29.73	62.43	31.08	64.59
	0.5	35.68	71.62	37.03	74.59
	0.75	40.81	78.65	41.89	81.89
	1	44.86	83.78	45.95	87.57
	1.25	48.38	88.38	49.46	91.89
	1.5	51.35	90.81	52.43	95.41
	1.75	54.05	94.05	55.14	98.11
	2	56.22	96.22	57.30	100.27
77.04	0.25	23.74	59.95	24.94	61.87
	0.5	28.78	66.67	29.98	69.31
	0.75	32.85	71.46	34.05	74.34
	1	36.45	75.06	37.41	78.18
	1.25	39.09	77.94	40.29	81.29
	1.5	41.73	79.86	42.69	83.45
	1.75	43.88	81.53	44.60	85.13
	2	45.56	82.97	46.52	86.57

The collector efficiency improvements increase with increasing recycle ratio but to a more remarkable extent with decreasing air mass flow rate. However, there is no significant influence of the incident solar radiation on the collector efficiency improvement. It is seen from Table 2 that the heat-transfer efficiency improvement of the wire mesh packed solar air heater is higher than that of the flat-plate type without attaching the wire mesh. The present work is actually the extension of the previous work [17] except for the different type of the external recycle. The graphical representation for comparisons with some experimental results and theoretical predictions obtained in [17] under the same design and operating parameters were illustrated to explain how the present device is preferred to be employed, as confirmed by Figure 6. This is the value and originality of the present device.

Figure 6. Comparisons of collector efficiency between the present device and the previous work (I₀ = 830 W/m²).

Figure 6. Comparisons of collector efficiency between the present device and the previous work (I₀ = 830 W/m²).

The power consumption increment for the wire mesh packed devices I_P (P_S and P_W single-pass and double-pass devices, respectively) is defined by the percentage increase in the power consumption compared to that in the downward single-pass device [18,19]:

$$I_P=\frac{P_W-P_S}{P_S}\text{ for wire mesh packed solar air heaters}$$

(13)

The ratio I_W/I_P of both the collector efficiency improvement I_W and the power consumption increment I_P is illustrated to take into account both effect indexes for economic considerations in obtaining the suitable selections of the operating parameters. The results indicate the optimal ratio of I_W/I_P occurs at R = 0.5~1.0 for various mass flow rates, as indicated in Figure 7. It is of practical importance that applications of recycling operation and wire mesh packing for enhancing device performance of wire mesh packed solar air heaters is technically and economically feasible.

Figure 7. Effects of recycle ratio and air mass flow rate on the value of I_W/I_P.

Figure 7. Effects of recycle ratio and air mass flow rate on the value of I_W/I_P.

5. Conclusions

The collector efficiency improvement in recycling double-pass solar air heaters with wire mesh packing have been developed analytically and experimentally. The comparisons of double-pass configurations with and without attaching a wire mesh were made to investigate the device performance improvement. The new design in the present study provides better collector efficiency due to the convective heat-transfer coefficient enhancement which the turbulent intensity created with the wire mesh attachment. The forgoing results can be summarized as follows: (1) With recycling operations and inserting a wire mesh, the collector efficiency increases with increasing recycle ratio R, incident solar radiation and air mass flow rates; (2) Collector performance improvements increase with increasing recycle ratio but with decreasing air mass flow rate, and no significant influence is found with respect to the incident solar radiation; (3) The optimal operating conditions for economic considerations of collector efficiency improvements with a relative small compensation of hydraulic dissipated energy increment were found at R = 0.5~1.0 for various mass flow rates; (4) the advantages of the present device are evident, and they will be an important contribution to the design of any particular application coupling external recycle and packing materials.