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Experimental performance of perforated glazed solar air heaters and unglazed transpired solar air heater
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ScienceDirect Solar Energy 119 (2015) 251–260 www.elsevier.com/locate/solener
Experimental performance of perforated glazed solar air heaters and unglazed transpired solar air heater Roozbeh Vaziri a,⇑, M. Ilkan b, F. Egelioglu a a b
Mechanical Engineering Department, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey School of Computing and Technology, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey Received 5 December 2014; received in revised form 18 May 2015; accepted 15 June 2015
Communicated by: Associate Editor Ruzhu Wang
Abstract In this study, the thermal performances of perforated glazed solar air heaters (PGSAHs) having different inner collector colors and a black colored unglazed transpired solar air heater (UTSAH) were investigated experimentally. Two PGSAHs having perforated plexiglas glazing and different inside bottom colors were constructed. The third solar air heater was a UTSAH where the top cover was black colored perforated sheet metal. The diameters of the holes on the plexiglas covers and sheet metal cover were 3 mm and the pitch distance was 30 mm. No absorber plates were used in PGSAHs, where perforated metal cover is the absorber plate in the UTSAH. Added fans draw ambient air into the collectors through the perforated plexiglas in the PGSAHs. Similarly an added fan draws ambient air into the collector through the perforated metal in the UTSAH. The air mass flow rate was varied between 0.017 kg/s and 0.036 kg/s. The highest efficiencies were achieved at mass flow rate of 0.036 kg/s. The highest values of efficiency for black, green, blue, red, violet, light yellow and white PGSAHs were, 85%, 84%, 76%, 65%, 61%, 54%, and 55% respectively, while at the same mass flow rate the maximum value of efficiency for the UTSAH was 50%. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Perforated glazed solar air heater; Transpired solar air heater; Thermal efficiency; Collector colors
1. Introduction Depending on the absorber plate, the collectors can be classified as transpired or untranspired collector. The unglazed transpired solar collector (UTC) concept was developed in the late 1980s. UTCs consist of dark colored perforated absorber sheets which are attached to a building’s south facing wall to preheat ventilation air. Air is drawn through the solar heated perforated absorber plate into the building by a fan. UTC has no glazing thus, its structure is simple and it costs less compared to glazed collectors. There is no reflection or absorption due to glazing ⇑ Corresponding author. Tel.: +90 5338322329.
E-mail address: [email protected] (R. Vaziri). http://dx.doi.org/10.1016/j.solener.2015.06.043 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
as there is no glass or plastic cover in the UTCs. Typically, UTCs have about 70% efficiency (Cali et al., 1999; Wang et al., 2006). UTC is the product of a private solar heating and energy conservation company and National Renewable Energy Laboratory (NREL) in the USA. NREL (2006) indicated that UTCs can preheat the ambient air by as much as 22 °C. Cali et al. (1999) described four UTCs and their installations around the world in detail. Similar detailed descriptions and installations of the UTCs can be found in (SolarWall, 2008; NREL, 1998). (Dymond and Kutscher, 1997) developed a computer model for the analysis of transpired collectors. Van Decker et al. (2001) presented the measurements of heat exchange effectiveness for UTCs. Hollick (1998) monitored the existing UTCs and indicated that UTCs
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Nomenclature Ac c D h L m_ Q r t I Tout Tin Tair V
area of the collector (m2) specific heat (kJ/kg K) diameter (m) coefficient of heat transfer (W/m2 K) length (m) air mass flow rate (kg/s) volume flow rate (m3/s) radius (m) time (s) solar radiation intensity (W/m2) outlet temperature ð CÞ inlet temperature ð CÞ temperature of air ð CÞ velocity (m/s)
q g
density of air (kg/m3) efficiency of the mC p ðT out T in Þ g¼ IAc
solar
collector
Sub-scripts A average f fluid i inlet o outlet Abbreviation PGSAHs perforated glazed solar air heaters UTSAH unglazed transpired solar air heater
Greek symbols DT temperature difference ðT out T in Þð CÞ l viscosity of air (N s/m2)
are able to save the premises energy consumption up to 1 MW h/m2/year. UTC is suitable for preheating ventilation air in commercial and industrial buildings which have large ventilation requirements and where heating seasons are long. UTCs’ applications also include agricultural drying and curing, preheating combustion air for industrial furnaces, etc. The most widely used solar air heaters are glazed and untranspired. The conventional solar air heaters mainly consist of a panel, insulated hot air ducts and air blowers if it is an active system. The panel consists of an absorber plate thermally insulated from the bottom, the sides are also insulated, and a glass or plastic cover fixed above the absorber plate to form a passage for air flow. The working fluid, air, gains heat from the absorber plate. The coefficient of convective heat transfer between the air and the absorber plate is low. The temperature of the absorber plate is high due to the low heat transfer rate to air. Thus, radiation loss is high and another major heat loss is through the top cover. Heat losses through the bottom and the sides of the collector are negligible as they are very well insulated. In conventional solar air heaters, heat losses and heat transfer coefficient inside the solar collector are the two important parameters affecting the efficiency of the collector. In order to improve the efficiency of conventional solar air heaters, researchers have suggested improvements to decrease heat losses from the collector and increase the convection coefficient between air and the absorber plate. In order to minimize the heat losses, double glazing was suggested by some researchers (Martin and Fjeld, 1975; Prasad et al., 2009). Double pass channel was suggested by several researchers where the air flows from above and then below the absorber plate (Sopian et al., 1999, 2009; Paisarn, 2005a,b; Ho et al.,
2005; Lertsatitthanakorn et al., 2008; Yeh et al., 2002; Esen, 2008; Ozgen et al., 2009). Yeh et al. (2002) designed and constructed double-flow solar air heaters with fins attached and achieved considerable improvement in collector efficiency of solar air heaters. Ho et al. (2005) conducted experiments to study a device for inserting an absorbing plate into the double-pass channel in a flat-plate solar air heater with recycle. Their results indicated that by utilizing recycle type double-pass devices instead of conventional double-pass heater, significant enhancement in the efficiency of collector could be obtained. In order to improve the heat transfer coefficient inside the collector, different modifications are suggested and applied. These modifications include using finned absorber plates (Paisarn, 2005a,b; Lertsatitthanakorn et al., 2008; Yeh et al., 2002; Esen, 2008; Ozgen et al., 2009; Yeh and Ho, 2009) or using porous material inside the collector like wire mesh screen (Qenawy and Mohamad, 2007; Mohamad, 1977; Thakur et al., 2003) limestone and gravels (Ramadan et al., 2007) limestone, gravels and iron scraps (El-Sebaii et al., 2007). Pottler et al. (1999) suggested an optimization method for solar air heaters with the flow behind the absorber plate. Their results showed that using fins with optimal distance between them of about 5–10 mm has provided the highest energy gain. On the other hand, they pointed out that finned absorbers operate more efficiently. Mittal and Varshney (2006) conducted thermo hydraulic investigations on black wire mesh solar air heater and showed that the thermal gain and effective efficiency of these novel collectors are much higher than conventional flat plate collectors. El-khawajah et al. (2011) presented an experimental investigation on double pass solar air heater with 2, 4, and 6 fins attached which used wire mesh
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layers as an absorber plate. They reported that there is a considerable enhancement in the thermal efficiency by increasing air mass flow rate and number of fins. The use of porous media tends to increase the ratio of surface area to unit volume substantially and has significantly improved the thermal efficiency of heaters. The effect of different shapes and orientation of roughness elements and size of artificial roughness on the performance of solar air heater is investigated throughout the literature. The significant enhancement in thermal efficiency and effective efficiency for roughened solar air heater as compared to solar air heater having smooth absorber plate has been obtained by Brij and Ranjit (2012). Singh et al. (2014) have experimentally investigated of heat transfer and friction characteristics of a rectangular channel having multiple arc-shaped roughness elements on the absorber plate of solar air heater. The maximum heat transfer and friction factor were found at an angle of attack of 60 . Maximum enhancement of Nusselt number and friction factor were found to be 5.07 and 3.71 times respectively over smooth one. Exergetic performance of solar air heater having arc shape oriented protrusions as roughness element has evaluated by Sanjay et al. (2014). It is found that the arc shaped protruded roughened solar air heater is more efficient than the smooth conventional flat-plate solar air heater for Reynolds number less than 20,000. At present, glazed untranspired solar air heaters are the most widely used solar air heaters but the UTC is now a well-recognized solar air heater for preheating ventilation air. Glazing is used to reduce the absorber plate’s radiant and convective heat losses. As Glazing is not required in the UTC, the air continuously drawn through the perforated plate almost eliminates the convective loss (Arulanandam et al., 1997). Furthermore, Arulanandam et al. (1997) mentioned that the radiant heat loss is minimized as the sucked air keeps the plate temperature low. Omojaro and Aldabbagh (2010) experimentally studied the single and double pass solar air heaters. They used
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seven steel wire mesh layers as an absorber plate and installed longitudinal fins along the lower and upper pass of solar air heater with fins. The maximum value of efficiency achieved for the double pass solar air heater was 63.74% for the mass flow rate of 0.038 kg/s. Deniz et al. (2010) carried comparative study of various types of different types of flat-plate solar air heaters for various mass flow rates, tilt angle and temperature conditions versus time. The results indicated that efficiencies of finned double glass collector were the highest. In order to improve the thermal performance of a solar air heater, Krishnananth and Murugavel (2013) integrated thermal energy storage with double pass solar air heater. Solar collectors usually have black colored absorber plates and in general black colored solar collectors are not compatible aesthetically with the color of building facades and roofs. Cyprus is one of the leading countries employing solar water heating but solar air heating is not a common practice, one of the reasons is the architectural unattractiveness of solar air heaters to dwellings. Several studies have been done using different absorber plate colors that are compatible with the color of buildings. Tripanagnostopoulos et al. (2000) constructed and tested three flat plate solar water collectors having black, blue, red and brown absorbers with and without glazing. The results indicated that the efficiencies of collectors having blue, red and brown absorber plates are close to the black colored absorber plate and therefore, they can be more acceptable in integration to the buildings. Anderson et al. (2010) presented the effect of color on the thermal performance of the building integrated solar collectors. This paper presents the performances of a new type solar air heater (i.e., PGSAH) and UTSAH. The aim is to construct a glazed transpired solar air heater with different collector colors and experimentally investigate its performance. It is also aimed to compare the performances of colored PGSAHs with UTSAH. The effects of color on the performance of the PGSAH’s were investigated. Using different colored panels are more applicable to adjust
Fig. 1. Schematic of perforated plexi glass solar air heater.
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Fig. 2. Two different perforated glazed solar air heaters & transpired solar air heater simultaneously.
with the appearance and integration into the buildings and could provide a substantial fraction of the heating load. 2. Experimental set up As mentioned earlier, three solar air heaters were constructed; two PGSAHs and a UTSAH and experimentally investigated at a geographic location of Cyprus in the city of Famagusta. Experiments on the SAH’s were performed in the clear days of December and November 2012. Fig. 1
shows the schematic view of the PGSAH. The UTSAH has the same dimensions of PGSAHs. The length and width of the collectors were 0.90 m each. The Plexiglas covers of PGSAHs were 3 mm thick and perforated by drilling 3 mm diameter holes. The center-to-center distance between holes in the perforated Plexiglas (i.e., pitch) were 3 cm (Fig. 3). The black perforated sheet metal cover of UTSAH was drilled similarly. The distance between the covers and the bottom of the collectors which creates airspace were 30 cm in each collector. The radial fans were mounted under each box. The fan of each collector creates a negative pressure in the box and sucks the heated air into the box through the holes in the cover plate (Fig. 5.). In order to minimize heat loss from the solar air heaters the sides and bottom of the collectors were insulated by 30 mm polystyrene. The air velocity was measured by using the Extech 407112 Vane Anemometer. The accuracy of reading is ±(2% + 0.2 m/s) and the resolution is 0.01 m/s for the range 0.4–10 m/s. The outlet temperature, Tout, and the inlet temperature, Tin, were measured by using T-type thermocouples. Two thermocouples were fixed inside the pipe before the fan to measure the outlet temperature of the air. Two thermocouples were placed underneath the solar collector to measure the ambient air temperature (i.e., Tin). The temperature readings were recorded by Ten-channel Digital Thermometer (MDSSi8 Series digital, Omega) ±0.5 °C accuracy (Fig. 4b). The global solar radiation incident on an inclined surface was measured using an Eppley Radiometer Pyranometer (PSP)
Fig. 3. (a) Plexiglas cover of PGSAH. (b) Dimmer. (c) Ten-channel Digital Thermometer. (d) The radial fan.
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Fig. 4. Solar intensity versus different standard local time of days for different PGSAH’s and transpired SAH on three different days.
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Fig. 6. Outlet and inlet air temperature difference versus time of the day for red PGSAH, violet PGSAH and transpired SAH.
3. Uncertainty analysis Uncertainties due to the air mass flow rate and the thermal efficiency are demonstrated in this section. _ is a function of three indepenThe mass flow rate ðmÞ dent variables (i.e., m_ ¼ f ðq; V ; AÞ) where, q is the density of air, V is the outlet air velocity and A is the cross sectional area of the exit. The mass flow rate is calculated as: m_ ¼ qVA
ð1Þ
where the fractional uncertainty for the mass flow rate, is calculated according to Holman (1989):
xm_ , m_
Fig. 5. Outlet and inlet air temperature difference versus time of the day for black PGSAH, green PGSAH and blue PGSAH.
coupled to an instantaneous solar radiation meter model HHM1A digital, Omega 0.25% basic dc accuracy and a resolution of ±0.5% from 0 to 2800 Wm2. The pyranometer was fixed beside the Plexiglas cover of the collector. In order to maximize the solar radiation incident on the glazing cover, the GTC was oriented facing south and tilted with an angle of 36° with respect to the horizontal (Tiwari, 2002). The air is circulated for 30 min prior to the period in which the data were taken. The variables measured and recorded were the inlet and outlet temperatures of the air circulating through the collector, vane anemometer reading, and the solar radiation. All tests started at 9 am and ended at 3 pm.
Fig. 7. Outlet and inlet air temperature difference versus time of the day for white PGSAH, yellow PGSAH and transpired SAH.
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Fig. 8. Outlet and inlet air temperature difference versus time of the day for blue PGSAH, green PGSAH and transpired SAH.
xm_ ¼ m_
" # 2 x 2 1=2 xT air xV 2 r þ þ4 T air V r
Fig. 10. Outlet and inlet air temperature difference versus time of the day for white PGSAH, yellow PGSAH and transpired SAH.
ð2Þ
where Tair is the film air temperature between the outlet x and inlet, r is radius of the pipe, TTairair is the fractional uncertainty for the film air temperature, xVV is the fractional uncertainty for the outlet air velocity, xrr is the fractional uncertainty for the radius of the pipe, xDTDT is the fractional uncertainty for the DT ; xII is the fractional uncertainty for the solar intensity. The ratio of energy gain to solar radiation incident on the collector plane is the efficiency of solar collector, g and is calculated as:
Fig. 11. Efficiency of black PGSAH, green PGSAH and blue SAH versus time of the day.
g¼
_ P ðT out T in Þ mC IAc
ð3Þ
where I is the solar intensity, Cp is the specific heat of the air and Ac is the area of the collector. The fractional uncerx tainty of efficiency, gg is a function of DT ; m_ and I. The fractional uncertainty of efficiency is: 1=2 xg xm_ 2 xDT 2 xI 2 ¼ þ þ ð4Þ m_ g DT I Fig. 9. Outlet and inlet air temperature difference versus time of the day for red PGSAH, violet PGSAH and transpired SAH.
The fractional uncertainties of mass flow rate, xm_m_ and efficiency are calculated by utilizing the uncertainties
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Famagusta weather conditions. Famagusta is a city in North Cyprus that is located on 35.125° N and 33.95° E longitude. During the experiments, the average value of wind measured was approximately 5.34 m/s. Three solar air heaters were tested simultaneously, two colored perforated glazed solar air heaters and one unglazed transpired solar air heater. Fig. 4, shows solar intensity versus standard local time of the day for all the days the experiments were done. As expected the maximum value of solar intensity was at noon time. The mean value of solar intensity is calculated for each day. The maximum value of solar radiation intensity recorded during tests, was 914.2857 W/m2 at 12:30 h and the average value of solar intensity through
Fig. 12. Efficiency of red PGSAH, violet PGSAH and transpired SAH versus time of the day.
associated with the measurements of the parameters. The fractional uncertainties of Tair, V, r, DT and I, are 0.0016, 0.0021, 0.005, 0.035 and 6.55 106, respectively. The calculations show that percent uncertainties in the mass flow rate and the efficiency for maximum air mass flow rate of (0.036 kg/s) are found to be 1.05% and 3.65%, respectively.
4. Results and discussion The performance of PGSAHs and UTSAH were studied experimentally between 12.11.2012 and 31.12.2012, under
Fig. 14. Efficiency of blue PGSAH, green PGSAH and transpired SAH versus time of the day.
Fig. 13. Efficiency of white PGSAH, yellow PGSAH and transpired SAH versus time of the day.
Fig. 15. Efficiency of red PGSAH, violet PGSAH and transpired SAH versus time of the day.
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that particular day was 756.71 W/m2. According to data, ambient temperature enhances from morning till afternoon but because of wind some fluctuations were observed. A photograph of the experimental set up is shown in Figs. 1 and 2. The components of apparatus are illustrated in Fig. 3. The performances of PGSAHs were studied by changing the color of the collectors and varying the air mass flow rate from 0.017 to 0.036 kg/s. In general, DT reduces with increasing mass flow rate. In addition, DT is increasing to maximum at noon time and then DT decreases in the afternoon. Figs. 5–10 show the temperature differences DT = Tout Tin versus time of day for different colored PGSAHs and UTSAH at m_ ¼ 0:036 kg=s and m_ ¼ 0:024 kg=s. For black, green and blue PGSAHs, the maximum values of temperature differences, at the mass flow rate of 0.024 kg/s, were 17.9 °C, 17.3 °C and 15.4 °C at 13:00 h. The results indicate that black PGSAH has the highest DT and dark colored PGSAHs such as green, blue and red had higher temperature differences compared with the white and yellow colored PGSAHs. The minimum value of temperature difference obtained from this work with yellow PGSAH was 9.4 °C with mass flow rate of 0.036 kg/s at 13:00 h (Fig. 7). The minimum value of temperature difference achieved with UTSAH for same mass flow rate was 9.2 °C at 13:00 h. It is clear from Figs. 6, 8 and 9 that temperature difference achieved in darker colored PGSAHs are higher than the DTs obtained from UTSAH. The temperature differences achieved in white and light yellow colored PGSAHs were almost the same compared with UTSAH’s DTs (see Figs. 7 and 10). Figs. 11–16, illustrate the efficiency versus standard local time of the day for different colored PGSAHs and UTSAH at mass flow rate of 0.036 kg/s and 0.024 kg/s. It is found
Fig. 16. Efficiency of white PGSAH, yellow PGSAH and transpired SAH versus time of the day.
Fig. 17. Efficiency of blue PGSAH, green PGSAH and transpired SAH versus time of the day.
that the thermal efficiency is dependent on the ambient temperature and with higher ambient temperature, the thermal efficiency is increased. As the ambient temperature in the morning was low, the heat losses were higher from the solar air heaters to the environment in the morning in comparison with afternoon. The thermal efficiency trends upward with increasing the air mass flow rate. The thermal efficiency of black, green and blue PGSAHs are each higher than that of light colored PGSAH and UTSAH for the same mass flow rate. The maximum value of thermal efficiency achieved in the black colored PGSAH was 85% between 12:00 and 13:00 h at the maximum mass flow rate (i.e., 0.036 kg/s). The maximum values of thermal
Fig. 18. Efficiency of black PGSAH versus time of the day for different mass flow rates.
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PGSAH’s has increased. Fig. 20, demonstrates efficiency versus temperature rise parameter at different mass flow rates for blue PGSAH, green PGSAH and UTSAH. It is found that the efficiency of green PGSAH has the second highest value at maximum mass flow rate compared with other solar air heaters and increases at higher DT/I ratio. 5. Conclusions In the current study, experimental results regarding the performance of perforated glazed solar air heaters (PGSAHs) having different inner collector colors and a black colored unglazed transpired solar air heater (UTSAH) are presented and analyzed. The main conclusions drawn from the results of this study are as follows:
Fig. 19. Efficiency versus temperature rise parameter at different mass flow rates for black PGSAH.
1. The solar collector efficiency is enhanced substantially by utilizing PGSAH. 2. The thermal efficiencies of PGSAHs (black, green, blue, red and violet) are each significantly better than that of UTSAH. 3. The thermal efficiencies of green and blue PGSAHs are close to that of black PGSAH, which has maximum efficiency. 4. For PGSAHs in which light colors were used (yellow and white), thermal efficiencies were almost close to the efficiency of UTSAH for the same mass flow rates. 5. The higher mass flow rates increase the efficiency but decrease the temperature difference. 6. The results indicated that the thermal efficiency of dark colored PGSAHs were better than the light colored ones. This research is concerned with main point of view that PGSAHs having different inner collector colors without any absorber plate compared the UTSAH illustrate preferable and acceptable thermal performance. As mentioned earlier, by using different colored PGSAHs the aesthetics categories of building facades and roofs can be improved.
Fig. 20. Efficiency versus temperature rise parameter at different mass flow rates for blue PGSAH, green PGSAH and transpired SAH.
efficiencies for green PGSAH and blue PGSAH are calculated as 84% and 76%, at m_ ¼ 0:036 kg=s, respectively. According to the obtained result, the average thermal efficiency of light colored PGSAHs and UTSAH were compatible. The maximum values of thermal efficiencies of the yellow, white PGSAHs and UTSAH at the mass flow rate of 0.036 kg/s are found to be 54%, 52% and 55% at noon time respectively. (see Fig. 13). Figs. 17 and 18, show the thermal efficiency increases by higher value of mass flow rate for different colored PGSAHs. From Fig. 19, it is evident that the slopes of the efficiency curves enhance with increase of mass flow rate and by increasing DT/I ratio, the thermal efficiency of black
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ScienceDirect Solar Energy 119 (2015) 251–260 www.elsevier.com/locate/solener
Experimental performance of perforated glazed solar air heaters and unglazed transpired solar air heater Roozbeh Vaziri a,⇑, M. Ilkan b, F. Egelioglu a a b
Mechanical Engineering Department, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey School of Computing and Technology, Eastern Mediterranean University, Famagusta, North Cyprus, via Mersin 10, Turkey Received 5 December 2014; received in revised form 18 May 2015; accepted 15 June 2015
Communicated by: Associate Editor Ruzhu Wang
Abstract In this study, the thermal performances of perforated glazed solar air heaters (PGSAHs) having different inner collector colors and a black colored unglazed transpired solar air heater (UTSAH) were investigated experimentally. Two PGSAHs having perforated plexiglas glazing and different inside bottom colors were constructed. The third solar air heater was a UTSAH where the top cover was black colored perforated sheet metal. The diameters of the holes on the plexiglas covers and sheet metal cover were 3 mm and the pitch distance was 30 mm. No absorber plates were used in PGSAHs, where perforated metal cover is the absorber plate in the UTSAH. Added fans draw ambient air into the collectors through the perforated plexiglas in the PGSAHs. Similarly an added fan draws ambient air into the collector through the perforated metal in the UTSAH. The air mass flow rate was varied between 0.017 kg/s and 0.036 kg/s. The highest efficiencies were achieved at mass flow rate of 0.036 kg/s. The highest values of efficiency for black, green, blue, red, violet, light yellow and white PGSAHs were, 85%, 84%, 76%, 65%, 61%, 54%, and 55% respectively, while at the same mass flow rate the maximum value of efficiency for the UTSAH was 50%. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Perforated glazed solar air heater; Transpired solar air heater; Thermal efficiency; Collector colors
1. Introduction Depending on the absorber plate, the collectors can be classified as transpired or untranspired collector. The unglazed transpired solar collector (UTC) concept was developed in the late 1980s. UTCs consist of dark colored perforated absorber sheets which are attached to a building’s south facing wall to preheat ventilation air. Air is drawn through the solar heated perforated absorber plate into the building by a fan. UTC has no glazing thus, its structure is simple and it costs less compared to glazed collectors. There is no reflection or absorption due to glazing ⇑ Corresponding author. Tel.: +90 5338322329.
E-mail address: [email protected] (R. Vaziri). http://dx.doi.org/10.1016/j.solener.2015.06.043 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
as there is no glass or plastic cover in the UTCs. Typically, UTCs have about 70% efficiency (Cali et al., 1999; Wang et al., 2006). UTC is the product of a private solar heating and energy conservation company and National Renewable Energy Laboratory (NREL) in the USA. NREL (2006) indicated that UTCs can preheat the ambient air by as much as 22 °C. Cali et al. (1999) described four UTCs and their installations around the world in detail. Similar detailed descriptions and installations of the UTCs can be found in (SolarWall, 2008; NREL, 1998). (Dymond and Kutscher, 1997) developed a computer model for the analysis of transpired collectors. Van Decker et al. (2001) presented the measurements of heat exchange effectiveness for UTCs. Hollick (1998) monitored the existing UTCs and indicated that UTCs
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Nomenclature Ac c D h L m_ Q r t I Tout Tin Tair V
area of the collector (m2) specific heat (kJ/kg K) diameter (m) coefficient of heat transfer (W/m2 K) length (m) air mass flow rate (kg/s) volume flow rate (m3/s) radius (m) time (s) solar radiation intensity (W/m2) outlet temperature ð CÞ inlet temperature ð CÞ temperature of air ð CÞ velocity (m/s)
q g
density of air (kg/m3) efficiency of the mC p ðT out T in Þ g¼ IAc
solar
collector
Sub-scripts A average f fluid i inlet o outlet Abbreviation PGSAHs perforated glazed solar air heaters UTSAH unglazed transpired solar air heater
Greek symbols DT temperature difference ðT out T in Þð CÞ l viscosity of air (N s/m2)
are able to save the premises energy consumption up to 1 MW h/m2/year. UTC is suitable for preheating ventilation air in commercial and industrial buildings which have large ventilation requirements and where heating seasons are long. UTCs’ applications also include agricultural drying and curing, preheating combustion air for industrial furnaces, etc. The most widely used solar air heaters are glazed and untranspired. The conventional solar air heaters mainly consist of a panel, insulated hot air ducts and air blowers if it is an active system. The panel consists of an absorber plate thermally insulated from the bottom, the sides are also insulated, and a glass or plastic cover fixed above the absorber plate to form a passage for air flow. The working fluid, air, gains heat from the absorber plate. The coefficient of convective heat transfer between the air and the absorber plate is low. The temperature of the absorber plate is high due to the low heat transfer rate to air. Thus, radiation loss is high and another major heat loss is through the top cover. Heat losses through the bottom and the sides of the collector are negligible as they are very well insulated. In conventional solar air heaters, heat losses and heat transfer coefficient inside the solar collector are the two important parameters affecting the efficiency of the collector. In order to improve the efficiency of conventional solar air heaters, researchers have suggested improvements to decrease heat losses from the collector and increase the convection coefficient between air and the absorber plate. In order to minimize the heat losses, double glazing was suggested by some researchers (Martin and Fjeld, 1975; Prasad et al., 2009). Double pass channel was suggested by several researchers where the air flows from above and then below the absorber plate (Sopian et al., 1999, 2009; Paisarn, 2005a,b; Ho et al.,
2005; Lertsatitthanakorn et al., 2008; Yeh et al., 2002; Esen, 2008; Ozgen et al., 2009). Yeh et al. (2002) designed and constructed double-flow solar air heaters with fins attached and achieved considerable improvement in collector efficiency of solar air heaters. Ho et al. (2005) conducted experiments to study a device for inserting an absorbing plate into the double-pass channel in a flat-plate solar air heater with recycle. Their results indicated that by utilizing recycle type double-pass devices instead of conventional double-pass heater, significant enhancement in the efficiency of collector could be obtained. In order to improve the heat transfer coefficient inside the collector, different modifications are suggested and applied. These modifications include using finned absorber plates (Paisarn, 2005a,b; Lertsatitthanakorn et al., 2008; Yeh et al., 2002; Esen, 2008; Ozgen et al., 2009; Yeh and Ho, 2009) or using porous material inside the collector like wire mesh screen (Qenawy and Mohamad, 2007; Mohamad, 1977; Thakur et al., 2003) limestone and gravels (Ramadan et al., 2007) limestone, gravels and iron scraps (El-Sebaii et al., 2007). Pottler et al. (1999) suggested an optimization method for solar air heaters with the flow behind the absorber plate. Their results showed that using fins with optimal distance between them of about 5–10 mm has provided the highest energy gain. On the other hand, they pointed out that finned absorbers operate more efficiently. Mittal and Varshney (2006) conducted thermo hydraulic investigations on black wire mesh solar air heater and showed that the thermal gain and effective efficiency of these novel collectors are much higher than conventional flat plate collectors. El-khawajah et al. (2011) presented an experimental investigation on double pass solar air heater with 2, 4, and 6 fins attached which used wire mesh
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layers as an absorber plate. They reported that there is a considerable enhancement in the thermal efficiency by increasing air mass flow rate and number of fins. The use of porous media tends to increase the ratio of surface area to unit volume substantially and has significantly improved the thermal efficiency of heaters. The effect of different shapes and orientation of roughness elements and size of artificial roughness on the performance of solar air heater is investigated throughout the literature. The significant enhancement in thermal efficiency and effective efficiency for roughened solar air heater as compared to solar air heater having smooth absorber plate has been obtained by Brij and Ranjit (2012). Singh et al. (2014) have experimentally investigated of heat transfer and friction characteristics of a rectangular channel having multiple arc-shaped roughness elements on the absorber plate of solar air heater. The maximum heat transfer and friction factor were found at an angle of attack of 60 . Maximum enhancement of Nusselt number and friction factor were found to be 5.07 and 3.71 times respectively over smooth one. Exergetic performance of solar air heater having arc shape oriented protrusions as roughness element has evaluated by Sanjay et al. (2014). It is found that the arc shaped protruded roughened solar air heater is more efficient than the smooth conventional flat-plate solar air heater for Reynolds number less than 20,000. At present, glazed untranspired solar air heaters are the most widely used solar air heaters but the UTC is now a well-recognized solar air heater for preheating ventilation air. Glazing is used to reduce the absorber plate’s radiant and convective heat losses. As Glazing is not required in the UTC, the air continuously drawn through the perforated plate almost eliminates the convective loss (Arulanandam et al., 1997). Furthermore, Arulanandam et al. (1997) mentioned that the radiant heat loss is minimized as the sucked air keeps the plate temperature low. Omojaro and Aldabbagh (2010) experimentally studied the single and double pass solar air heaters. They used
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seven steel wire mesh layers as an absorber plate and installed longitudinal fins along the lower and upper pass of solar air heater with fins. The maximum value of efficiency achieved for the double pass solar air heater was 63.74% for the mass flow rate of 0.038 kg/s. Deniz et al. (2010) carried comparative study of various types of different types of flat-plate solar air heaters for various mass flow rates, tilt angle and temperature conditions versus time. The results indicated that efficiencies of finned double glass collector were the highest. In order to improve the thermal performance of a solar air heater, Krishnananth and Murugavel (2013) integrated thermal energy storage with double pass solar air heater. Solar collectors usually have black colored absorber plates and in general black colored solar collectors are not compatible aesthetically with the color of building facades and roofs. Cyprus is one of the leading countries employing solar water heating but solar air heating is not a common practice, one of the reasons is the architectural unattractiveness of solar air heaters to dwellings. Several studies have been done using different absorber plate colors that are compatible with the color of buildings. Tripanagnostopoulos et al. (2000) constructed and tested three flat plate solar water collectors having black, blue, red and brown absorbers with and without glazing. The results indicated that the efficiencies of collectors having blue, red and brown absorber plates are close to the black colored absorber plate and therefore, they can be more acceptable in integration to the buildings. Anderson et al. (2010) presented the effect of color on the thermal performance of the building integrated solar collectors. This paper presents the performances of a new type solar air heater (i.e., PGSAH) and UTSAH. The aim is to construct a glazed transpired solar air heater with different collector colors and experimentally investigate its performance. It is also aimed to compare the performances of colored PGSAHs with UTSAH. The effects of color on the performance of the PGSAH’s were investigated. Using different colored panels are more applicable to adjust
Fig. 1. Schematic of perforated plexi glass solar air heater.
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Fig. 2. Two different perforated glazed solar air heaters & transpired solar air heater simultaneously.
with the appearance and integration into the buildings and could provide a substantial fraction of the heating load. 2. Experimental set up As mentioned earlier, three solar air heaters were constructed; two PGSAHs and a UTSAH and experimentally investigated at a geographic location of Cyprus in the city of Famagusta. Experiments on the SAH’s were performed in the clear days of December and November 2012. Fig. 1
shows the schematic view of the PGSAH. The UTSAH has the same dimensions of PGSAHs. The length and width of the collectors were 0.90 m each. The Plexiglas covers of PGSAHs were 3 mm thick and perforated by drilling 3 mm diameter holes. The center-to-center distance between holes in the perforated Plexiglas (i.e., pitch) were 3 cm (Fig. 3). The black perforated sheet metal cover of UTSAH was drilled similarly. The distance between the covers and the bottom of the collectors which creates airspace were 30 cm in each collector. The radial fans were mounted under each box. The fan of each collector creates a negative pressure in the box and sucks the heated air into the box through the holes in the cover plate (Fig. 5.). In order to minimize heat loss from the solar air heaters the sides and bottom of the collectors were insulated by 30 mm polystyrene. The air velocity was measured by using the Extech 407112 Vane Anemometer. The accuracy of reading is ±(2% + 0.2 m/s) and the resolution is 0.01 m/s for the range 0.4–10 m/s. The outlet temperature, Tout, and the inlet temperature, Tin, were measured by using T-type thermocouples. Two thermocouples were fixed inside the pipe before the fan to measure the outlet temperature of the air. Two thermocouples were placed underneath the solar collector to measure the ambient air temperature (i.e., Tin). The temperature readings were recorded by Ten-channel Digital Thermometer (MDSSi8 Series digital, Omega) ±0.5 °C accuracy (Fig. 4b). The global solar radiation incident on an inclined surface was measured using an Eppley Radiometer Pyranometer (PSP)
Fig. 3. (a) Plexiglas cover of PGSAH. (b) Dimmer. (c) Ten-channel Digital Thermometer. (d) The radial fan.
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Fig. 4. Solar intensity versus different standard local time of days for different PGSAH’s and transpired SAH on three different days.
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Fig. 6. Outlet and inlet air temperature difference versus time of the day for red PGSAH, violet PGSAH and transpired SAH.
3. Uncertainty analysis Uncertainties due to the air mass flow rate and the thermal efficiency are demonstrated in this section. _ is a function of three indepenThe mass flow rate ðmÞ dent variables (i.e., m_ ¼ f ðq; V ; AÞ) where, q is the density of air, V is the outlet air velocity and A is the cross sectional area of the exit. The mass flow rate is calculated as: m_ ¼ qVA
ð1Þ
where the fractional uncertainty for the mass flow rate, is calculated according to Holman (1989):
xm_ , m_
Fig. 5. Outlet and inlet air temperature difference versus time of the day for black PGSAH, green PGSAH and blue PGSAH.
coupled to an instantaneous solar radiation meter model HHM1A digital, Omega 0.25% basic dc accuracy and a resolution of ±0.5% from 0 to 2800 Wm2. The pyranometer was fixed beside the Plexiglas cover of the collector. In order to maximize the solar radiation incident on the glazing cover, the GTC was oriented facing south and tilted with an angle of 36° with respect to the horizontal (Tiwari, 2002). The air is circulated for 30 min prior to the period in which the data were taken. The variables measured and recorded were the inlet and outlet temperatures of the air circulating through the collector, vane anemometer reading, and the solar radiation. All tests started at 9 am and ended at 3 pm.
Fig. 7. Outlet and inlet air temperature difference versus time of the day for white PGSAH, yellow PGSAH and transpired SAH.
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Fig. 8. Outlet and inlet air temperature difference versus time of the day for blue PGSAH, green PGSAH and transpired SAH.
xm_ ¼ m_
" # 2 x 2 1=2 xT air xV 2 r þ þ4 T air V r
Fig. 10. Outlet and inlet air temperature difference versus time of the day for white PGSAH, yellow PGSAH and transpired SAH.
ð2Þ
where Tair is the film air temperature between the outlet x and inlet, r is radius of the pipe, TTairair is the fractional uncertainty for the film air temperature, xVV is the fractional uncertainty for the outlet air velocity, xrr is the fractional uncertainty for the radius of the pipe, xDTDT is the fractional uncertainty for the DT ; xII is the fractional uncertainty for the solar intensity. The ratio of energy gain to solar radiation incident on the collector plane is the efficiency of solar collector, g and is calculated as:
Fig. 11. Efficiency of black PGSAH, green PGSAH and blue SAH versus time of the day.
g¼
_ P ðT out T in Þ mC IAc
ð3Þ
where I is the solar intensity, Cp is the specific heat of the air and Ac is the area of the collector. The fractional uncerx tainty of efficiency, gg is a function of DT ; m_ and I. The fractional uncertainty of efficiency is: 1=2 xg xm_ 2 xDT 2 xI 2 ¼ þ þ ð4Þ m_ g DT I Fig. 9. Outlet and inlet air temperature difference versus time of the day for red PGSAH, violet PGSAH and transpired SAH.
The fractional uncertainties of mass flow rate, xm_m_ and efficiency are calculated by utilizing the uncertainties
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Famagusta weather conditions. Famagusta is a city in North Cyprus that is located on 35.125° N and 33.95° E longitude. During the experiments, the average value of wind measured was approximately 5.34 m/s. Three solar air heaters were tested simultaneously, two colored perforated glazed solar air heaters and one unglazed transpired solar air heater. Fig. 4, shows solar intensity versus standard local time of the day for all the days the experiments were done. As expected the maximum value of solar intensity was at noon time. The mean value of solar intensity is calculated for each day. The maximum value of solar radiation intensity recorded during tests, was 914.2857 W/m2 at 12:30 h and the average value of solar intensity through
Fig. 12. Efficiency of red PGSAH, violet PGSAH and transpired SAH versus time of the day.
associated with the measurements of the parameters. The fractional uncertainties of Tair, V, r, DT and I, are 0.0016, 0.0021, 0.005, 0.035 and 6.55 106, respectively. The calculations show that percent uncertainties in the mass flow rate and the efficiency for maximum air mass flow rate of (0.036 kg/s) are found to be 1.05% and 3.65%, respectively.
4. Results and discussion The performance of PGSAHs and UTSAH were studied experimentally between 12.11.2012 and 31.12.2012, under
Fig. 14. Efficiency of blue PGSAH, green PGSAH and transpired SAH versus time of the day.
Fig. 13. Efficiency of white PGSAH, yellow PGSAH and transpired SAH versus time of the day.
Fig. 15. Efficiency of red PGSAH, violet PGSAH and transpired SAH versus time of the day.
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that particular day was 756.71 W/m2. According to data, ambient temperature enhances from morning till afternoon but because of wind some fluctuations were observed. A photograph of the experimental set up is shown in Figs. 1 and 2. The components of apparatus are illustrated in Fig. 3. The performances of PGSAHs were studied by changing the color of the collectors and varying the air mass flow rate from 0.017 to 0.036 kg/s. In general, DT reduces with increasing mass flow rate. In addition, DT is increasing to maximum at noon time and then DT decreases in the afternoon. Figs. 5–10 show the temperature differences DT = Tout Tin versus time of day for different colored PGSAHs and UTSAH at m_ ¼ 0:036 kg=s and m_ ¼ 0:024 kg=s. For black, green and blue PGSAHs, the maximum values of temperature differences, at the mass flow rate of 0.024 kg/s, were 17.9 °C, 17.3 °C and 15.4 °C at 13:00 h. The results indicate that black PGSAH has the highest DT and dark colored PGSAHs such as green, blue and red had higher temperature differences compared with the white and yellow colored PGSAHs. The minimum value of temperature difference obtained from this work with yellow PGSAH was 9.4 °C with mass flow rate of 0.036 kg/s at 13:00 h (Fig. 7). The minimum value of temperature difference achieved with UTSAH for same mass flow rate was 9.2 °C at 13:00 h. It is clear from Figs. 6, 8 and 9 that temperature difference achieved in darker colored PGSAHs are higher than the DTs obtained from UTSAH. The temperature differences achieved in white and light yellow colored PGSAHs were almost the same compared with UTSAH’s DTs (see Figs. 7 and 10). Figs. 11–16, illustrate the efficiency versus standard local time of the day for different colored PGSAHs and UTSAH at mass flow rate of 0.036 kg/s and 0.024 kg/s. It is found
Fig. 16. Efficiency of white PGSAH, yellow PGSAH and transpired SAH versus time of the day.
Fig. 17. Efficiency of blue PGSAH, green PGSAH and transpired SAH versus time of the day.
that the thermal efficiency is dependent on the ambient temperature and with higher ambient temperature, the thermal efficiency is increased. As the ambient temperature in the morning was low, the heat losses were higher from the solar air heaters to the environment in the morning in comparison with afternoon. The thermal efficiency trends upward with increasing the air mass flow rate. The thermal efficiency of black, green and blue PGSAHs are each higher than that of light colored PGSAH and UTSAH for the same mass flow rate. The maximum value of thermal efficiency achieved in the black colored PGSAH was 85% between 12:00 and 13:00 h at the maximum mass flow rate (i.e., 0.036 kg/s). The maximum values of thermal
Fig. 18. Efficiency of black PGSAH versus time of the day for different mass flow rates.
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PGSAH’s has increased. Fig. 20, demonstrates efficiency versus temperature rise parameter at different mass flow rates for blue PGSAH, green PGSAH and UTSAH. It is found that the efficiency of green PGSAH has the second highest value at maximum mass flow rate compared with other solar air heaters and increases at higher DT/I ratio. 5. Conclusions In the current study, experimental results regarding the performance of perforated glazed solar air heaters (PGSAHs) having different inner collector colors and a black colored unglazed transpired solar air heater (UTSAH) are presented and analyzed. The main conclusions drawn from the results of this study are as follows:
Fig. 19. Efficiency versus temperature rise parameter at different mass flow rates for black PGSAH.
1. The solar collector efficiency is enhanced substantially by utilizing PGSAH. 2. The thermal efficiencies of PGSAHs (black, green, blue, red and violet) are each significantly better than that of UTSAH. 3. The thermal efficiencies of green and blue PGSAHs are close to that of black PGSAH, which has maximum efficiency. 4. For PGSAHs in which light colors were used (yellow and white), thermal efficiencies were almost close to the efficiency of UTSAH for the same mass flow rates. 5. The higher mass flow rates increase the efficiency but decrease the temperature difference. 6. The results indicated that the thermal efficiency of dark colored PGSAHs were better than the light colored ones. This research is concerned with main point of view that PGSAHs having different inner collector colors without any absorber plate compared the UTSAH illustrate preferable and acceptable thermal performance. As mentioned earlier, by using different colored PGSAHs the aesthetics categories of building facades and roofs can be improved.
Fig. 20. Efficiency versus temperature rise parameter at different mass flow rates for blue PGSAH, green PGSAH and transpired SAH.
efficiencies for green PGSAH and blue PGSAH are calculated as 84% and 76%, at m_ ¼ 0:036 kg=s, respectively. According to the obtained result, the average thermal efficiency of light colored PGSAHs and UTSAH were compatible. The maximum values of thermal efficiencies of the yellow, white PGSAHs and UTSAH at the mass flow rate of 0.036 kg/s are found to be 54%, 52% and 55% at noon time respectively. (see Fig. 13). Figs. 17 and 18, show the thermal efficiency increases by higher value of mass flow rate for different colored PGSAHs. From Fig. 19, it is evident that the slopes of the efficiency curves enhance with increase of mass flow rate and by increasing DT/I ratio, the thermal efficiency of black
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