- Email: [email protected]
Experimental testing of ceramic solar collectors
Solar Energy 146 (2017) 532–542
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Experimental testing of ceramic solar collectors Miroslaw Zukowski ⇑, Grzegorz Woroniak Department of HVAC Engineering, Faculty of Civil Engineering and Environmental Engineering, Bialystok University of Technology, 15-351 Bialystok, Poland
a r t i c l e
i n f o
Article history: Received 7 January 2017 Received in revised form 12 February 2017 Accepted 8 March 2017
Keywords: Ceramic solar collectors Thermal performance Efficiency of energy conversion
a b s t r a c t An alternative to traditional solar collectors with absorbers made of metal or plastic can be panels which have their whole structure made of ceramics. The paper presents the results of the testing of ceramic solar collectors. The experimental research was carried out in summer 2016 in normal operating conditions. Rise in temperature of the fluid flowing through the collector was measured. The maximum value of this parameter did not exceed 7.5 °C and the maximum power output extracted from 1 m2 of the ceramic solar collector under test was equal to 650 W/m2. The thermal characteristic of ceramic solar collectors was determined as the relationship between efficiency and the reduced temperature difference. The straight line corresponding to this characteristic is described by zero-loss collector efficiency coefficient equal to 0.8332 and heat loss coefficient equal to 16.332. Based on the analysis of the test results, it can be concluded that, in spite of some disadvantages, ceramic collectors can compete with traditional solar panels currently available on the market. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The continuous decrease in fossil fuel resources makes it necessary to increase the use of renewable energy sources. Solar radiation is used to a small degree. The main problem is the high cost of the equipment used to convert solar energy into heat. A few years ago, a solar collector made of ceramic was patented and implemented. Fig. 1 shows a single ceramic panel, and Figs. 2 and 3 present two sections: across (A-A) and along (B-B) the channels, respectively. Its efficiency is slightly lower in comparison to the solutions used and available up to now on the market. However, according to Yang et al. (2013a) it has many advantages, which include: Very long life (at least 100 years); Ability to integrate with facades or roofs of all types of buildings; Natural, widely available and inexpensive materials such as clay, quartz, feldspar, water, all of which the device can be made of; Constant optical characteristics of the absorber; Monolithic and thus simple structure; Several times lower cost of implementation as compared to currently used commercial solutions.
⇑ Corresponding author. E-mail address: [email protected] (M. Zukowski). http://dx.doi.org/10.1016/j.solener.2017.03.022 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
A very important feature of the absorbers of solar collectors that should be considered is the largest possible value of solar radiation absorption coefficient. Typically, manufacturers have indicated this value to be in the range from 0.9 to 0.98. A micro layer of black ceramic V-Ti is used with the ceramic collectors. It is generated as a waste material from vanadium extraction process (Yang et al., 2013c). Such layers are characterized by very good absorption of solar radiation, as their absorption coefficient is in the range from 0.93 to 0.97 (Yang et al., 2013b). Therefore, their optical properties are similar to a black body. Unfortunately, ceramic conducts heat decidedly worse compared to steel, aluminium and copper, that are used in traditional solar collectors. The heat conduction coefficient of this material, depending on the content of Al2O3, is 1.2–1.7 W/(m K). This is an unfavorable feature, which affects the efficiency of energy conversion. However, the wall thickness of the flow channels is not too large, and thus the thermal resistance of these elements can be small. So far, experimental tests have been carried out of two large ceramic solar systems under natural conditions. The first test system consisting of 36 m2 panels integrated into the roof surface is located in Jinan (Yang et al., 2013a). The research has shown that the efficiency of the entire DHW system is slightly higher than 50%. So it is a relatively good result. What is more, the water outlet temperature does not exceed 60 °C. The second research station is located in Fujian (Sun et al., 2014). The effective ceramic collector area is 43.2 m2. The test results have shown that the average efficiency of the system is 50.6%, and the temperature of the heated water does not exceed 56 °C.
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 1. Example of ceramic solar panel with the main dimensions (author’s photo).
The above-described experiments have focused only on the overall assessment of the efficiency of hot water heating systems. In the available literature, there are no studies of heat transfer and flow in the channels of ceramic solar collectors for direct heating of hot water. The available studies of heat and hydraulic characteristics include ceramic compact heat exchangers (Li et al., 2011), applicable in high temperature solar systems or in systems operating in highly corrosive environments. Micro ceramic heat exchangers applicable in electronics cooling have been analysed
533
(Sommers et al., 2010). The performance testing of micro exchangers Al2O3 conducted by researchers Alm et al. (2005), has been limited to the temperature of 1000 °C, the assumed flow of 12.4– 80.6 kg/h and the maximum pressure of 8 bar. Rapid prototyping technology has been used to create a heat exchanger, setting output parameters using CFD software. The heat transfer coefficient obtained on the basis of the studies ranges between 7 and 15 kW/m2 K, the pressure loss in the channels of 0.15 bar (for flow of 12.4 kg/h) to 6 bar (with a flow of 80 kg/h) (Alm et al., 2005). In the subsequent paper (Alm et al., 2008), the authors have reported being able to obtain the heat transfer coefficients up to 22 kW/ m2 K for counter-flow heat exchangers and efficiency of the order of 0.1–0.22. Researchers Velasco Gómez et al. (2005) have studied the liquid-gas heat exchanger that is constructed of ceramic tubes Al2O3 having the density of about 2.5 g/cm3, the porosity of 22– 25% and 9–10% water absorption, arranged in the bunch of 7 rows and 7 columns, used for heat recovery and the treatment of air blown into the room. In the analysed ceramic heat exchanger flows take place through the porous walls where either condensation or evaporation followed, in a stream of flowing air and water flowing through the pipes. The authors have pointed out that the use of porous exchanger tubes secured against the development of undesirable bacteria. Comparative tests on an aluminium plate heat exchanger have shown that the ceramic heat exchanger has a higher cooling capacity. When considering lower cost ceramic heat exchanger, the authors suggest the possibility of their use in buildings. The limitations of the use of this type of ceramic heat exchangers are non-pressure systems (Velasco Gómez et al., 2005). Similar conclusions have been reached by Rey Martínez et al. (2004) in their work. Bower et al. (2005) examining the heat exchangers made of silicon carbide (SiC) at a flow rate, have achieved heat dissipation at 100 W/cm2 for air, and 790 W/cm2 for water flow. Due to the bulk
Fig. 2. Cross-section (A-A – Fig. 1) of the ceramic solar panel (author’s photo).
Fig. 3. Along -section (B-B – Fig. 1) of the ceramic solar panel (author’s photo).
534
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
thermal conductivity close to the silicon, the tests have shown that the SiC is a suitable material for use in the microelectronics industry (Sommers et al., 2010). In their work, the authors present a number of empirical relationships (Li et al., 2011) describing the heat conduction and friction coefficient for different types of heat exchangers made of different materials with a limit value of the tests. None of them characterizes the ceramic solar collectors for the direct heating of the water or glycol solution passing through. Accordingly, the interest in testing such exchangers, which have already appeared on the market in China, appears to be wellfounded. The use of ceramic components opens up new possibilities in engineering applications – in particular, where high-temperature processes and the reactions of corrosive agents can occur, for which the use of metal or polymer materials is not recommended. Due to the limitations and difficulties in the construction, ceramic simulation studies are needed in order to avoid the high cost of testing. Complete simulations require large computing resources and the use of commercial CFD tools (Alm et al., 2008). Research has shown that for the water flow rate in the efficiency range of 20 kg/h, the counter-flow heat exchanger is 0.16–0.22 (with and without insulation), and for cross-flow is 0.18. The maximum efficiency determined empirically is 0.19. Experimentally employed heat transfer coefficient is in the range 7–22 kW/m2 K and has always been higher than it appeared from theoretical calculations. Of considerable importance is the manufacturing accuracy of the heat exchanger, as sometimes, when connecting, canals have been partially or even completely blocked by binder (Takeuchi et al., 2010). A few articles focus on ceramic micro-channel heat exchangers. Takeuchi et al. (2010) have developed and used three-dimensional models useful in the design of heat exchangers of silicon carbide for high temperature applications in nuclear reactors. Schulte-Fischedick et al. (2007) have designed and tested platefinned heat exchangers to convert biomass. Three-dimensional modelling has been used in the design and development of heat exchangers. The longitudinal heat conduction through the solid plays a particularly important role in the efficiency of the device in small high-counter-flow heat exchangers. More comprehensive analyses can be achieved via three-dimensional simulation of the fluid flow and conjugate heat transfer (Kee et al., 2011). Enlarged inlet and outlet channels allow to increase the amount of heat exchanging plates without excessive pressure drop during fluid flow. Simulations show the trade-off between performance heat conduction and production requirements. Additionally, the temperature field in the heat exchanger plates can be predicted through simulations. This is important, because the local temperature gradients cause local stress along the thermal expansion, and this may cause damage to the ceramic. The design is made on the basis of three-dimensional CFD simulations including the conjugate heat transfer between fluids and solid. The simulation has been carried out for the same flow of ‘‘hot” and ‘‘cold” inputs (respectively 500 °C and 30 °C), with the assumption of perfect isolation. Computer simulations also allow to determine the impact of channel geometry on the performance of the heat exchanger and its hydraulic resistance – and thus also its performance. Therefore, in the production process, a compromise between these values is needed. From the simulation investigation also shows that, in this type of heat exchanger, axial temperature variations are essentially one-dimensional. Computer analysis can also be made for places exposed to mechanical damage, which significantly reduces the cost of prototyping device, and, at a later stage, the cost of the mass production of the exchanger (Kee et al., 2011). High temperature heat exchangers (HTHE) constructed with ceramics can achieve higher temperatures of operation. Resistance
Table 1 Simplified property features of monolithic ceramic materials by Sommers et al. (2010). Property
Value
Density Thermal conductivity Temperature Chemical resistance Mechanical properties Shape and join Cost Main weaknesses
Moderate to high Low to high High (<1650 °C) Excellent Good Difficult Moderate to high Inherent brittleness
to oxidation is the great advantage of using ceramics for this application (de Mello et al., 2013). As pointed by McDonald (2003), the use of ceramics for heat exchanger construction should be considered as an option to settle this temperature limitation. The use of plate and fin heat exchangers configuration for the construction of ceramic HTHE (Schulte-Fischedick et al., 2007; Kee et al., 2011; de Mello et al., 2016) is feasible and provides high heat transfer area per volume of the heat exchanger. The thermal performance and pressure drop of this configuration could be predicted using the standard approach already used for compact heat exchangers (Kays and London, 1984). The technology needed for the construction of plate and fin ceramic heat exchangers has not been established yet. The mechanical integrity of ceramics is an area of intense development (Gomez-Martín et al., 2016; Stadelmann et al., 2015). Some successful attempts can be found in literature (Fend et al., 2011; Alm et al., 2008; Kee et al., 2011), that considered the use of ceramic heat exchangers for different applications. However, some aspects of thermal, fluid and structural responses are considered as open-issues. Ceramic heat exchangers have the undoubted advantage that is the lack of corrosion at high temperatures, however the disadvantage in most cases is low thermal conductivity. Heat exchangers for processing and energy recovery applications are very important in terms of performance, cost and system size. Most ceramic materials are hard, porous and fragile, so the use of ceramic material requires the use of methods for the treatment of ceramics. Another hazard of the use of ceramics can also be its porous structure. Microscopic pores can act as the concentrator of tensions and can weaken the entire structure. In non-crystalline ceramic materials viscous flow is the dominant source of plastic deformation, but this process is very slow (Sommers et al., 2010). Table 1 shows an overview of the typical properties of monolithic ceramics. The thermal resistance of the ceramic materials gives an advantage in applications above 600 °C, since their resistance is sufficient up to 1400 °C. Resistance to chemical corrosion and erosion is the second element, which gives the advantage to ceramic materials. Corrosion, which normally takes place in normal conditions, strongly increases at elevated temperatures, it is particularly evident in the case of the exhaust gas flow (Sommers et al., 2010). Based on the literature review, it can be concluded that the ceramic materials are frequently used in heat transfer applications, especially at high temperatures, the problem of using ceramics is the treatment of this type of material, in particular, suitable pressure sealing applications.
2. Description of the ceramic solar collectors under test The solar collector unit (SCU) under test consists of three ceramic panels joined in series. The rectangular casing, made of aluminium, has the following dimensions: height – 2300 mm, depth – 110 mm (without the channel connections), length – 760 mm.
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Top cover of the SCU is solar glass with the thickness of 3.2 mm. Mineral wool thermal insulation (thickness – 40 mm, thermal conductivity k = 0.045 W/m/K) is applied as the protection against heat loss. The SCU system is equipped with a rack with a 32 degrees slope as the optimal angle for the research period and location (Zukowski and Radzajewska, 2015). The main dimensions of the SCU are as follows: Gross area (calculated as the height multiplied by length) – 1.748 m2, Aperture area (calculated as the area of the glazing exposed to the Sun’s radiation) – 1.597 m2. Fig. 4 shows the ceramic solar collector in casing without solar glass. Two SCUs are set at ground level by the building of Faculty of Civil Engineering and Environmental Engineering - Bialystok University of Technology (Poland). They are connected with flexible stainless steel pipes with the devices included in the measuring station. The pipes are insulated with black EPDM foam rubber suitable for high temperature applications with maximum service temperature up to 150 oC. Solar units are set on the south side of the building in a place partly sheltered from the wind. For the purpose of the study, two solar units, connected in parallel to the tech-
535
nology circuit, are applied. Fig. 5 shows the tested SCUs. Condensation inside the units is also visible. 3. Experimental setup The test stand (Fig. 6), in addition to the SCUs (1), consists of two hot water storage tanks (2) 140 L each, one solar group with circulating pump (3), one microprocessor controller (4) and a hot water air heater with own circulating pump (5) to remove excess heat from storage tanks through the DHW connectors. The mixture of water and glycol at a concentration of 3% are applied as the heating medium. The flow rate through the SCU can be adjusted separately using the control valves (6). Two expansion tanks (7) for securing hydraulic circuits are applied in addition to the safety valves (8). The temperature of the flowing medium is measured by the PT1000 resistance temperature sensors. A multi-channel data recorder (9) is used to collect the results of the investigations with a one minute time interval. To register the amount of heat obtained from each SCU, accurate heat meters with ultrasonic flow sensors (10) are applied. For meteorological parameters monitoring, an automatic weather station is applied and integrated with wireless data logger inside the building. The station can measure environmental parameters such as outside air temperature, wind speed and direction, humidity, and solar radiation. The sampling frequency of the weather conditions is set to one minute. The test stand diagram is given in Fig. 6. The measurement stand is installed close to the solar units. Fig. 7 shows the major part of measurement stand without the hot water air heater exchanger. 4. Results of experiments and discussion
Fig. 4. Tested ceramic solar collector - three panels connected in series (author’s photo).
The main goal of the present research was to carry out a preliminary study to investigate the basic thermal parameters of relatively new renewable energy device for converting solar radiation into heat. In order to determine the thermal characteristics, a most commonly recommended method was applied (Osório and Carvalho, 2012), i.e. the steady-state test. The conditions under which experiments should be carried out are exactly specified in the latest standard (ISO 9806, 2013). Three series of tests were performed with different mass flow _ SC of the working fluid: 0.033 kg/s, 0.039 kg/s, 0.045 kg/s. rates m These values are equal to: 0.019 kg/s, 0.022 kg/s, 0.026 kg/s per 1 m2 of the gross surface area of the collector. It is usually assumed that 0.02 kg/s/m2 is the reference value. Experimental tests lasted from April 27 to October 18, 2016.
Fig. 5. The SCUs with ceramic solar collectors tested experimentally in normal operating conditions (author’s photo).
536
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 6. Diagram showing the measurement stand.
In the first stage of the experiment, it was decided to determine the temperature rise DTSC (Eq. (1)) of the fluid passing through the collector. This is an important parameter for the designer, which allows to determine how many collectors can be connected in series.
DT SC ¼ T OUT T IN :
ð1Þ
The difference between the outlet TOUT and inlet TIN temperature of the working fluid as a function of intensity of solar radiation GSOL is presented in Figs. 8–10. As can be observed in Figs. 8–10, the maximum temperature rise did not exceed 7.5 °C, that may be regarded as a typical result for flat plate solar collectors. As is widely known, the increase in fluid flow results in its temperature drop. At maximum flow rate DTSC did not exceed 5.5 °C for solar radiation equal to 1000 W/m2. The rise in the fluid temperature is a linear function of solar irradiance, thus the measurement results can be approximated with the use of the linear regression method. Eqs. (2)-(4) shows the relation between DTSC and GSOL for the subsequent increasing flow rates. Approximation results of all data sets are very precise because the correlation coefficient R-squared is equal: 0.9952 (0.033 kg/s), 0.9934 (0.039 kg/s), and 0.9965 (0.045 kg/s). Fig. 7. The measurement stand (author’s photo).
DT SC ¼ 0:008 GSOL 0; 5488:
ð2Þ
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
_ SC = 0.033 kg/s. Fig. 8. Dependence between fluid temperature rise and solar radiation intensity for m
_ SC = 0.039 kg/s. Fig. 9. Dependence between fluid temperature rise and solar radiation intensity for m
_ SC = 0.045 kg/s. Fig. 10. Dependence between fluid temperature rise and solar radiation intensity for m
537
538
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
DT SC ¼ 0:0068 GSOL 0; 4350:
ð3Þ
DT SC ¼ 0:0061 GSOL 0; 4123:
ð4Þ
Fig. 11 shows the dependence of solar radiation intensity on fluid temperature increase as the result of flow through the panel, for all considered cases. Knowing the value of fluid temperature increase we can estimate the actual useful power extracted, using the following relation:
qSC ¼
_ SC cp DT SC m ; AA
ð5Þ
The dependence of power output extracted from 1m2 of the solar collector on intensity of solar radiation is shown in Fig. 12. As can be seen, the maximum qSC reaches a value of 650 W/m2. At solar radiation flux density of 1000 W/m2 gives a good efficiency of energy conversion equal to 65%. The most important parameter of thermal performance is the efficiency of a solar collector g defined by Eq. (6) as the ratio of the useful energy gain QU at a certain time period to the solar energy incident on an exposed collector surface A over the same time interval (Duffie and Beckman, 2013).
RQ
g ¼ RUG A
where
dt
SOL
dt
:
ð6Þ
Under steady state conditions, the instantaneous efficiency gA related to the aperture area AA can be expressed as:
_ SC ¼ V a qf ; m Va – volume flow rate [m3/s], qf – working fluid density [kg/m3], cp – heat capacity of the working fluid [J/kg/K], AA – aperture area [m2].
gA ¼
_ SC cp DT SC m : AA GSOL
Fig. 11. Dependence between fluid temperature rise and solar radiation intensity.
Fig. 12. The influence of solar radiation on energy intercepted by the collector under test.
ð7Þ
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
The Hottel-Whillier-Bliss model (Hottel and Whillier, 1958) is often employed to determine the thermal characteristic of solar collectors. Reduced temperature difference T m (Eq. (8)) is applied in this method:
T m ¼
Tm Ta ; GSOL
ð8Þ
where
T m ¼ T IN þ
DT SC ; 2
Ta – ambient air temperature [°C]. The relationship between efficiency and reduced temperature difference can be plotted in the form of a straight line:
gA ¼ a0 a1 T m :
ð9Þ
The a0 point, where the line crosses the y-axis, corresponds to zero-loss collector efficiency gA,0. Coefficient a1 is the slope of the line. This value depends on the energy loss of the solar collector. Fig. 13 shows the result of the approximation of the selected experimental data sets under stationary and clear-sky conditions. For the ceramic solar collector operating under steady-state conditions (solar irradiance greater than 800 W/m2, diffuse solar irradiance is less than 20%, the average value of wind speed between 2 and 4 m/s, constant fluid flow rate equal to 0.039 kg/ s ± 0.0008 kg/s) zero-loss coefficient gA,0(a0) is equal 0.8332 and heat loss coefficient a1 is equal 16.332. It should be noted that the data sets presented in Fig. 13 are the mean value for both collectors. Thus it can be concluded that the ceramic solar collector zero loss efficiency is significantly better than the average value. According to the statistical analysis of the test results obtained by National Center for Quality Supervision and Testing of Solar Heating Systems the average value of gA,0 was 0.75 (Sun et al., 2007). As it turned out, the disadvantage of this collector is comparatively high heat loss through the cover systems, and especially through the rear side. This is evidenced by the value of the solar collector heat loss coefficient a1, which is higher than the
539
7.6 W/(oCm2), i.e. average value for flat plate collector specified in the (Sun et al., 2007). It should be noted that the total heat loss included a heat loss of pipes (length of 2 2.5 m DN18) that connected collectors to hydraulic system inside the laboratory. Temperature sensors were placed in the manifold located near the entrance of the pipes to the lab room. However, the value of the additional loss was not too high (about 8% of the total energy loss), because they were insulated with 20 mm EPDM foam rubber. In addition, the increase in energy loss was affected by lack of tightness of the frame or leaking internal joints of the ceramic panels, or due to both reasons. It was resulted in water vapor condensation on the inside of the glazing when there was low temperature of the surrounding air (Fig. 14). The negative role that condensation plays on the flat plate collector performance was developed by Anderson (2014). The next two figures shows plots of daily variation of solar radiation and power output of the solar collector (Fig. 15), and the working fluid temperature at the inlet and outlet of the ceramic collectors and surrounding air temperature (Fig. 16) for a clear sky day before noon and increasing cloudiness in the afternoon.
Fig. 14. Moisture condensation inside the solar collector under test.
Fig. 13. The instantaneous efficiency of the collector versus reduced temperature difference.
540
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 15. Daily variation of solar radiation and power output extracted from 1m2 of the solar collector (August 5, 2016).
Fig. 16. Daily variation of ambient air temperature and temperature at the inlet and outlet of the collectors (August 5, 2016).
As can be observed, ceramic collectors, due to relatively high thermal capacity, smooth even rapid fluctuations in solar radiation. As the result, useful energy gains supplied to the system are more uniform. Response time for solar system is important parameters, which determine its performance in transient conditions. The time constant sC, that depends on the thermal inertia of the collector, can be determined theoretically based on the following formula:
C
sC ¼ _ ; mSC cp
ð10Þ
where
C ¼n
4 X pi mi ci ; i
n – number of panels, pi - weighting factor given in ISO 9806 (absorber – 1, insulation – 0.5, heat transfer medium – 1, and external glazing 0,01a1) [–], mi – mass of each constituent element of the collector [kg], ci – specific heat of each element of the collector [J/kg/K].
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
The calculation results showed that the solar collector unit under test, has a relatively long time constant, which is equal to 626 s, 529 s, 459 s for 30 °C and mass flow rates of the working fluid: 0.033 kg/s, 0.039 kg/s, 0.045 kg/s, respectively. The dynamic properties of two flat plate solar collectors and vacuum tube liquid-vapor (heat-pipe) collector have been analysed by Obstawski et al. (2016). They have determined the time constants based on the Eq. (9), and technical documentation of those solar panels. The following values have been obtained: flat plate solar collector with harp coil arrangement – sC = 67.6 s, flat plate solar collector with meandering coil – sC = 69.8 s, and heat pipe collector – sC = 56.6 s. So it should be noted that the ceramic collectors are characterized by a significantly higher (almost ten times) thermal inertia compared to traditional collectors. This feature causes the ceramic collector starts to produce useful energy with a time lag, because he must warm up. On the other hand, high thermal capacity reduces outlet temperature fluctuations, which are caused by variations in the intensity of solar radiation. Uncertainty in estimating the collector thermal efficiency was determined based on the root-sum-square method. The combined standard uncertainty can be calculated according to the following relation:
(
uðgA Þ ¼
uðV a Þ
@ gA @V a
2
2 2 @g @ gA þ uðqf Þ A þ uðDT SC Þ @ qa @ DT SC )12
@ gA þ uðGSOL Þ @GSOL
2
;
ð11Þ
where
@ gA qf cp DT SC ¼ ; @V a AA GSOL @ gA V a cp DT SC ¼ ; @ qf AA GSOL V a qf cp @ gA ¼ ; @ DT SC AA GSOL V a qf cp DT SC @ gA ¼ : @GSOL AA G2SOL The relative uncertainty r(gc) can be estimated through the Eq. (11), as the result of dividing the absolute uncertainty by thermal efficiency.
( 2 2 uðqf Þ @ gA 2 uðV a Þ @ gA uðDT SC Þ @ gA rðgA Þ ¼ þ þ gA @V a gA @ qa gA @ DT SC 1 ) 2 2 uðGSOL Þ @ gA : þ gA @GSOL
ð12Þ
Uncertainties in measurement of solar radiation and flow rate were approximately equal to ±20 W/m2 and ±0.0028 m3/h, respectively. Total temperature measurement uncertainty was estimated to be within ±0.2 °C. For these parameters the maximum uncertainty in the ceramic collector efficiency did not exceed 4.75% and relative error ranged from 0.062 to 0.068. Thus it can be concluded that the accuracy of the experimental results obtained in the present study should be considered as high. 5. Conclusions The main goal of this research project was to determine the thermal characteristic of the ceramic solar collectors in normal
541
operating conditions. The fluid temperature rise DTSC inside the collectors was measured at various flow rates and solar radiation intensity. As it turned out, the maximum value of DTSC did not exceed 7.5 °C. This parameter can be considered as normal for the typical flat plate solar collector. The actual useful power and energy efficiency can be determined based on increase in fluid temperature. It was found that the maximum power output extracted from 1 m2 of the solar collector at solar radiation intensity of 1000 W/m2 was equal 650 W/m2. In the next step of this study, the efficiency for the conversion of solar radiation into heat energy as the function of reduced temperature difference was estimated. As indicated by the calculation results, zero-loss collector efficiency (the ambient temperature and the mean fluid temperature are the same) for ceramic solar collector was equal to 0.83. That may be classified as a very good parameter. Unfortunately, it should be noted that, the second parameter - heat loss coefficient - was less than 16. This indicates that the collectors under tests are characterized by high heat loss. In the paragraph discussing the results of the measurements reasons that could have an impact on this negative result were mentioned. However, this disadvantage can be easily eliminated by increasing the thickness of the thermal insulation and the use of better sealing material. In conclusion, we can say that the main advantage of ceramic collectors compared to conventional flat plate collectors is a very long service life of the monolithic absorber - estimated at 100 years. It is usually assumed that the service life of the conventional structure i.e. pipes attached to the absorber plate does not exceed 20–25 years. Constant optical properties of a micro layer of black ceramic V-Ti are another advantages of the ceramic panels. While it is also known that, the traditional selective absorber coatings can change original absorption and emission characteristics during the operation. Besides, ceramic absorbers (without casing) can be an integral part of the roof or balcony. Widely available and natural materials used to make absorbers is a particular advantage at a time when sustainable development is preferred in every sphere. But we should not forget about the shortcomings of this device. First of all, the weight and the water capacity of the ceramic panels is several times higher than typical flat-plate collectors. This is related to the fact that these devices have a high thermal inertia. As demonstrated by this study, ceramic panels have a slightly lower operating efficiency in comparison to the conventional solar modules, but we should not forget that this is a fairly new design which is in the process of developing and improving. However, in our opinion, the ceramic collectors will be able to compete with traditional flat plate solar panels in the near future. In the second stage of this project, which will be carried out during the summer season of 2017, the original rubber joints connecting the ceramic panels to the hydraulic loops will be changed, and the temperature will be measured directly at the inlet and outlet of the solar collectors. The authors also plan to investigate the thermal characteristic of a single panel without casing as this type of ceramic panels can be used as balcony railings or roof coverings. Acknowledgments This work was performed within the framework of a Grant of Bialystok University of Technology – Poland (Grant No. S/ WBIIS/4/2014). References Alm, B., Knitter, R., Haußelt, J., 2005. Development of a ceramic micro heat exchanger design, construction, and testing. Chem. Eng. Technol. 28, 1554– 1560. Alm, B., Imke, U., Knitter, R., Schygulla, U., Zimmermann, S., 2008. Testing and simulation of ceramic micro heat exchangers. Chem. Eng. J. 135, 179–184.
542
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Anderson., T., 2014. Condensation in Glazed Flat Plate Solar Collectors. In: Conference Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne, May 2014. Bower, C., Ortega, A., Skandakumaran, P., Vaidyanathan, R., Phillips, T., 2005. Heat transfer in water-cooled silicon carbide milli-channel heat sinks for high power electronic applications. J. Heat Transf. 127, 59–65. Duffie, J.A., Beckman, W.A., 2013. Solar Engineering of Thermal Processes. John Wiley & Sons Inc, Hoboken, New Jersey. Fend, T., Völkerb, W., Miebach, R., Smirnova, O., Gonsior, D., Schllgen, D., Rietbrock, P., 2011. Experimental investigation of compact silicon carbide heat exchangers for high temperatures. Int. J. Heat Mass Transf. 54, 4175–4181. Gomez-Martín, A., Orihuela, M.P., Becerra, J.A., Martínez-Fernandez, J., RamírezRico, J., 2016. Permeability and mechanical integrity of porous biomorphic SiC ceramics for application as hot-gas filters. Mater. Des. 107, 450–460. Hottel, H.C., Whillier, A., 1958. Evaluation of flat-plate solar collector performance. In: Carpenter, E.F. (Ed.), Transactions of the Conference on Use of Solar Energy, Vol. 2. University of Arizona Press, Tucson, Arizona, pp. 74–104. ISO 9806: 2013. Solar energy - Solar thermal collectors - Test methods. Kays, W.M., London, A.L., 1984. Compact Heat Exchangers. McGraw-Hill, New York, NY. Kee, R.J., Almand, B.B., Blasi, J.M., Rosen, B.L., Hartmann, M., Sullivan, N.P., 2011. The design, fabrication, and evaluation of a ceramic counter-flow microchannel heat exchanger. Appl. Therm. Eng. 31, 2004–2012. Li, Q., Flamant, G., Yuan, X., Neveu, P., Luo, L., 2011. Compact heat exchangers: a review and future applications for a new generation of high temperature solar receivers. Renew. Sustain. Energy Rev. 15, 4855–4875. McDonald, C.F., 2003. Recuperator considerations for future higher efficiency microturbines. Appl. Therm. Eng. 23, 1463–1487. de Mello, P.E.B., Villanueva, H.H.S., Scuotto, S., Donato, G.H.B., Ortega, F., 2016. Heat transfer, pressure drop and structural analysis of a finned plate ceramic heat exchanger, Energy, (in press) doi: 10.1016/j.energy.2016.11.113. de Mello P.E.B., Scuotto, S., Ortega, F., Donato, G.H.B., 2013. Heat transfer and pressure drop in a plate and fin ceramic heat exchanger. In: Conference Proceedings of the 8th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, 2013, 16–20. Obstawski, P., Bakon, T., Tulej, M., 2016. Analysis of dynamic properties of flat plate and heat pipe solar collectors. Measur. Autom. Monitor. 03 (62), 105–108.
Osório, T., Carvalho, M.J., 2012. Testing of solar thermal collectors under transient conditions. Energy Proc. 30, 1344–1353. Rey Martínez, F.J., Velasco, Gómez.E., Herrero, Martín.R., Martínez, Gutiérrez.J., Varela, Diez.F., 2004. Comparative study of two different evaporative systems: an indirect evaporative cooler and a semi-indirect ceramic evaporative cooler. Energy Build. 36, 696–708. Sommers, A., Wang, Q., Han, X., T’Joen, C., Park, Y., Jacobi, A., 2010. Ceramics and ceramic matrix composites for heat exchangers in advanced thermal systems-a review. Appl. Therm. Eng. 30, 1277–1291. Schulte-Fischedick, J., Dreißigacker, V., Tamme, R., 2007. An innovative ceramic high temperature plate-fin heat exchanger for EFCC processes. Appl. Therm. Eng. 27, 1285–1294. Stadelmann, R., Lugovy, M., Orlovskaya, N., Mchaffey, P., Radovic, M., Sglavo, V.M., Grasso, S., Reece, M.J., 2015. Mechanical properties and residual stresses in ZrB2-SiC spark plasma sintered ceramic composites. J. Europ. Ceram. Soc. 36, 1527–1537. Sun, X.-Y., Sun, X.-D., Li, X.-G., Wang, Z.-Q., He, J., Wang, B.-S., 2014. Performance and building integration of all-ceramic solar collectors. Energy Build. 75, 176–180. Sun, Z., Lu, B., He, T., Li, Z., Feng, A., Zhang L., Huang, Z., Deng, Y., 2007. Studies on test results for thermal performance of solar collectors in China, In: Conference Proceedings of the ISES World Congress 2007 ‘‘Solar Energy and Human Settlement” (Vol. 1-Vol.5), Beijing, China 18–21 September 2007, 716–720. Takeuchi, Y., Park, C., Noborio, K., Yamamoto, Y., Konishia, S., 2010. Heat transfer in SiC compact heat exchanger. Fusion Eng. Des. 85, 1266–1270. Velasco Gómez, E., Rey Martínez, F.J., Varela Diez, F., Molina Leyva, M.J., Herrero Martín, R., 2005. Description and experimental results of a semi-indirect ceramic evaporative cooler. Int. J. Refrig. 28, 654–662. Yang, Y., Wang, Q., Xiu, D., Zhao, Z., Sun, Q., 2013a. A building integrated solar collector: all-ceramic solar collector. Energy Build. 62, 15–17. Yang, Y., Cao, S., Xua, J., Cai, B., 2013b. All-ceramic solar collectors. Ceram. Int. 39, 6009–6012. Yang, Y.G., Xu, J.H., Cai, B., Wang, Q.C., Xiu, D.P., Zhao, Z.B., Sun, Q.Z., Cao, S.L., 2013c. Synthesis and applications of black ceramic from recycled industrial wastes. Adv. Appl. Ceram. 112, 146–148. Zukowski, M., Radzajewska, P., 2015. The optimal tilt angle of solar collectors for the territory of Poland. Cieplownictwo, Ogrzewnictwo, Wentylacja 4, 138–142 (in Polish with English abstract).
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Experimental testing of ceramic solar collectors Miroslaw Zukowski ⇑, Grzegorz Woroniak Department of HVAC Engineering, Faculty of Civil Engineering and Environmental Engineering, Bialystok University of Technology, 15-351 Bialystok, Poland
a r t i c l e
i n f o
Article history: Received 7 January 2017 Received in revised form 12 February 2017 Accepted 8 March 2017
Keywords: Ceramic solar collectors Thermal performance Efficiency of energy conversion
a b s t r a c t An alternative to traditional solar collectors with absorbers made of metal or plastic can be panels which have their whole structure made of ceramics. The paper presents the results of the testing of ceramic solar collectors. The experimental research was carried out in summer 2016 in normal operating conditions. Rise in temperature of the fluid flowing through the collector was measured. The maximum value of this parameter did not exceed 7.5 °C and the maximum power output extracted from 1 m2 of the ceramic solar collector under test was equal to 650 W/m2. The thermal characteristic of ceramic solar collectors was determined as the relationship between efficiency and the reduced temperature difference. The straight line corresponding to this characteristic is described by zero-loss collector efficiency coefficient equal to 0.8332 and heat loss coefficient equal to 16.332. Based on the analysis of the test results, it can be concluded that, in spite of some disadvantages, ceramic collectors can compete with traditional solar panels currently available on the market. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The continuous decrease in fossil fuel resources makes it necessary to increase the use of renewable energy sources. Solar radiation is used to a small degree. The main problem is the high cost of the equipment used to convert solar energy into heat. A few years ago, a solar collector made of ceramic was patented and implemented. Fig. 1 shows a single ceramic panel, and Figs. 2 and 3 present two sections: across (A-A) and along (B-B) the channels, respectively. Its efficiency is slightly lower in comparison to the solutions used and available up to now on the market. However, according to Yang et al. (2013a) it has many advantages, which include: Very long life (at least 100 years); Ability to integrate with facades or roofs of all types of buildings; Natural, widely available and inexpensive materials such as clay, quartz, feldspar, water, all of which the device can be made of; Constant optical characteristics of the absorber; Monolithic and thus simple structure; Several times lower cost of implementation as compared to currently used commercial solutions.
⇑ Corresponding author. E-mail address: [email protected] (M. Zukowski). http://dx.doi.org/10.1016/j.solener.2017.03.022 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
A very important feature of the absorbers of solar collectors that should be considered is the largest possible value of solar radiation absorption coefficient. Typically, manufacturers have indicated this value to be in the range from 0.9 to 0.98. A micro layer of black ceramic V-Ti is used with the ceramic collectors. It is generated as a waste material from vanadium extraction process (Yang et al., 2013c). Such layers are characterized by very good absorption of solar radiation, as their absorption coefficient is in the range from 0.93 to 0.97 (Yang et al., 2013b). Therefore, their optical properties are similar to a black body. Unfortunately, ceramic conducts heat decidedly worse compared to steel, aluminium and copper, that are used in traditional solar collectors. The heat conduction coefficient of this material, depending on the content of Al2O3, is 1.2–1.7 W/(m K). This is an unfavorable feature, which affects the efficiency of energy conversion. However, the wall thickness of the flow channels is not too large, and thus the thermal resistance of these elements can be small. So far, experimental tests have been carried out of two large ceramic solar systems under natural conditions. The first test system consisting of 36 m2 panels integrated into the roof surface is located in Jinan (Yang et al., 2013a). The research has shown that the efficiency of the entire DHW system is slightly higher than 50%. So it is a relatively good result. What is more, the water outlet temperature does not exceed 60 °C. The second research station is located in Fujian (Sun et al., 2014). The effective ceramic collector area is 43.2 m2. The test results have shown that the average efficiency of the system is 50.6%, and the temperature of the heated water does not exceed 56 °C.
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 1. Example of ceramic solar panel with the main dimensions (author’s photo).
The above-described experiments have focused only on the overall assessment of the efficiency of hot water heating systems. In the available literature, there are no studies of heat transfer and flow in the channels of ceramic solar collectors for direct heating of hot water. The available studies of heat and hydraulic characteristics include ceramic compact heat exchangers (Li et al., 2011), applicable in high temperature solar systems or in systems operating in highly corrosive environments. Micro ceramic heat exchangers applicable in electronics cooling have been analysed
533
(Sommers et al., 2010). The performance testing of micro exchangers Al2O3 conducted by researchers Alm et al. (2005), has been limited to the temperature of 1000 °C, the assumed flow of 12.4– 80.6 kg/h and the maximum pressure of 8 bar. Rapid prototyping technology has been used to create a heat exchanger, setting output parameters using CFD software. The heat transfer coefficient obtained on the basis of the studies ranges between 7 and 15 kW/m2 K, the pressure loss in the channels of 0.15 bar (for flow of 12.4 kg/h) to 6 bar (with a flow of 80 kg/h) (Alm et al., 2005). In the subsequent paper (Alm et al., 2008), the authors have reported being able to obtain the heat transfer coefficients up to 22 kW/ m2 K for counter-flow heat exchangers and efficiency of the order of 0.1–0.22. Researchers Velasco Gómez et al. (2005) have studied the liquid-gas heat exchanger that is constructed of ceramic tubes Al2O3 having the density of about 2.5 g/cm3, the porosity of 22– 25% and 9–10% water absorption, arranged in the bunch of 7 rows and 7 columns, used for heat recovery and the treatment of air blown into the room. In the analysed ceramic heat exchanger flows take place through the porous walls where either condensation or evaporation followed, in a stream of flowing air and water flowing through the pipes. The authors have pointed out that the use of porous exchanger tubes secured against the development of undesirable bacteria. Comparative tests on an aluminium plate heat exchanger have shown that the ceramic heat exchanger has a higher cooling capacity. When considering lower cost ceramic heat exchanger, the authors suggest the possibility of their use in buildings. The limitations of the use of this type of ceramic heat exchangers are non-pressure systems (Velasco Gómez et al., 2005). Similar conclusions have been reached by Rey Martínez et al. (2004) in their work. Bower et al. (2005) examining the heat exchangers made of silicon carbide (SiC) at a flow rate, have achieved heat dissipation at 100 W/cm2 for air, and 790 W/cm2 for water flow. Due to the bulk
Fig. 2. Cross-section (A-A – Fig. 1) of the ceramic solar panel (author’s photo).
Fig. 3. Along -section (B-B – Fig. 1) of the ceramic solar panel (author’s photo).
534
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
thermal conductivity close to the silicon, the tests have shown that the SiC is a suitable material for use in the microelectronics industry (Sommers et al., 2010). In their work, the authors present a number of empirical relationships (Li et al., 2011) describing the heat conduction and friction coefficient for different types of heat exchangers made of different materials with a limit value of the tests. None of them characterizes the ceramic solar collectors for the direct heating of the water or glycol solution passing through. Accordingly, the interest in testing such exchangers, which have already appeared on the market in China, appears to be wellfounded. The use of ceramic components opens up new possibilities in engineering applications – in particular, where high-temperature processes and the reactions of corrosive agents can occur, for which the use of metal or polymer materials is not recommended. Due to the limitations and difficulties in the construction, ceramic simulation studies are needed in order to avoid the high cost of testing. Complete simulations require large computing resources and the use of commercial CFD tools (Alm et al., 2008). Research has shown that for the water flow rate in the efficiency range of 20 kg/h, the counter-flow heat exchanger is 0.16–0.22 (with and without insulation), and for cross-flow is 0.18. The maximum efficiency determined empirically is 0.19. Experimentally employed heat transfer coefficient is in the range 7–22 kW/m2 K and has always been higher than it appeared from theoretical calculations. Of considerable importance is the manufacturing accuracy of the heat exchanger, as sometimes, when connecting, canals have been partially or even completely blocked by binder (Takeuchi et al., 2010). A few articles focus on ceramic micro-channel heat exchangers. Takeuchi et al. (2010) have developed and used three-dimensional models useful in the design of heat exchangers of silicon carbide for high temperature applications in nuclear reactors. Schulte-Fischedick et al. (2007) have designed and tested platefinned heat exchangers to convert biomass. Three-dimensional modelling has been used in the design and development of heat exchangers. The longitudinal heat conduction through the solid plays a particularly important role in the efficiency of the device in small high-counter-flow heat exchangers. More comprehensive analyses can be achieved via three-dimensional simulation of the fluid flow and conjugate heat transfer (Kee et al., 2011). Enlarged inlet and outlet channels allow to increase the amount of heat exchanging plates without excessive pressure drop during fluid flow. Simulations show the trade-off between performance heat conduction and production requirements. Additionally, the temperature field in the heat exchanger plates can be predicted through simulations. This is important, because the local temperature gradients cause local stress along the thermal expansion, and this may cause damage to the ceramic. The design is made on the basis of three-dimensional CFD simulations including the conjugate heat transfer between fluids and solid. The simulation has been carried out for the same flow of ‘‘hot” and ‘‘cold” inputs (respectively 500 °C and 30 °C), with the assumption of perfect isolation. Computer simulations also allow to determine the impact of channel geometry on the performance of the heat exchanger and its hydraulic resistance – and thus also its performance. Therefore, in the production process, a compromise between these values is needed. From the simulation investigation also shows that, in this type of heat exchanger, axial temperature variations are essentially one-dimensional. Computer analysis can also be made for places exposed to mechanical damage, which significantly reduces the cost of prototyping device, and, at a later stage, the cost of the mass production of the exchanger (Kee et al., 2011). High temperature heat exchangers (HTHE) constructed with ceramics can achieve higher temperatures of operation. Resistance
Table 1 Simplified property features of monolithic ceramic materials by Sommers et al. (2010). Property
Value
Density Thermal conductivity Temperature Chemical resistance Mechanical properties Shape and join Cost Main weaknesses
Moderate to high Low to high High (<1650 °C) Excellent Good Difficult Moderate to high Inherent brittleness
to oxidation is the great advantage of using ceramics for this application (de Mello et al., 2013). As pointed by McDonald (2003), the use of ceramics for heat exchanger construction should be considered as an option to settle this temperature limitation. The use of plate and fin heat exchangers configuration for the construction of ceramic HTHE (Schulte-Fischedick et al., 2007; Kee et al., 2011; de Mello et al., 2016) is feasible and provides high heat transfer area per volume of the heat exchanger. The thermal performance and pressure drop of this configuration could be predicted using the standard approach already used for compact heat exchangers (Kays and London, 1984). The technology needed for the construction of plate and fin ceramic heat exchangers has not been established yet. The mechanical integrity of ceramics is an area of intense development (Gomez-Martín et al., 2016; Stadelmann et al., 2015). Some successful attempts can be found in literature (Fend et al., 2011; Alm et al., 2008; Kee et al., 2011), that considered the use of ceramic heat exchangers for different applications. However, some aspects of thermal, fluid and structural responses are considered as open-issues. Ceramic heat exchangers have the undoubted advantage that is the lack of corrosion at high temperatures, however the disadvantage in most cases is low thermal conductivity. Heat exchangers for processing and energy recovery applications are very important in terms of performance, cost and system size. Most ceramic materials are hard, porous and fragile, so the use of ceramic material requires the use of methods for the treatment of ceramics. Another hazard of the use of ceramics can also be its porous structure. Microscopic pores can act as the concentrator of tensions and can weaken the entire structure. In non-crystalline ceramic materials viscous flow is the dominant source of plastic deformation, but this process is very slow (Sommers et al., 2010). Table 1 shows an overview of the typical properties of monolithic ceramics. The thermal resistance of the ceramic materials gives an advantage in applications above 600 °C, since their resistance is sufficient up to 1400 °C. Resistance to chemical corrosion and erosion is the second element, which gives the advantage to ceramic materials. Corrosion, which normally takes place in normal conditions, strongly increases at elevated temperatures, it is particularly evident in the case of the exhaust gas flow (Sommers et al., 2010). Based on the literature review, it can be concluded that the ceramic materials are frequently used in heat transfer applications, especially at high temperatures, the problem of using ceramics is the treatment of this type of material, in particular, suitable pressure sealing applications.
2. Description of the ceramic solar collectors under test The solar collector unit (SCU) under test consists of three ceramic panels joined in series. The rectangular casing, made of aluminium, has the following dimensions: height – 2300 mm, depth – 110 mm (without the channel connections), length – 760 mm.
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Top cover of the SCU is solar glass with the thickness of 3.2 mm. Mineral wool thermal insulation (thickness – 40 mm, thermal conductivity k = 0.045 W/m/K) is applied as the protection against heat loss. The SCU system is equipped with a rack with a 32 degrees slope as the optimal angle for the research period and location (Zukowski and Radzajewska, 2015). The main dimensions of the SCU are as follows: Gross area (calculated as the height multiplied by length) – 1.748 m2, Aperture area (calculated as the area of the glazing exposed to the Sun’s radiation) – 1.597 m2. Fig. 4 shows the ceramic solar collector in casing without solar glass. Two SCUs are set at ground level by the building of Faculty of Civil Engineering and Environmental Engineering - Bialystok University of Technology (Poland). They are connected with flexible stainless steel pipes with the devices included in the measuring station. The pipes are insulated with black EPDM foam rubber suitable for high temperature applications with maximum service temperature up to 150 oC. Solar units are set on the south side of the building in a place partly sheltered from the wind. For the purpose of the study, two solar units, connected in parallel to the tech-
535
nology circuit, are applied. Fig. 5 shows the tested SCUs. Condensation inside the units is also visible. 3. Experimental setup The test stand (Fig. 6), in addition to the SCUs (1), consists of two hot water storage tanks (2) 140 L each, one solar group with circulating pump (3), one microprocessor controller (4) and a hot water air heater with own circulating pump (5) to remove excess heat from storage tanks through the DHW connectors. The mixture of water and glycol at a concentration of 3% are applied as the heating medium. The flow rate through the SCU can be adjusted separately using the control valves (6). Two expansion tanks (7) for securing hydraulic circuits are applied in addition to the safety valves (8). The temperature of the flowing medium is measured by the PT1000 resistance temperature sensors. A multi-channel data recorder (9) is used to collect the results of the investigations with a one minute time interval. To register the amount of heat obtained from each SCU, accurate heat meters with ultrasonic flow sensors (10) are applied. For meteorological parameters monitoring, an automatic weather station is applied and integrated with wireless data logger inside the building. The station can measure environmental parameters such as outside air temperature, wind speed and direction, humidity, and solar radiation. The sampling frequency of the weather conditions is set to one minute. The test stand diagram is given in Fig. 6. The measurement stand is installed close to the solar units. Fig. 7 shows the major part of measurement stand without the hot water air heater exchanger. 4. Results of experiments and discussion
Fig. 4. Tested ceramic solar collector - three panels connected in series (author’s photo).
The main goal of the present research was to carry out a preliminary study to investigate the basic thermal parameters of relatively new renewable energy device for converting solar radiation into heat. In order to determine the thermal characteristics, a most commonly recommended method was applied (Osório and Carvalho, 2012), i.e. the steady-state test. The conditions under which experiments should be carried out are exactly specified in the latest standard (ISO 9806, 2013). Three series of tests were performed with different mass flow _ SC of the working fluid: 0.033 kg/s, 0.039 kg/s, 0.045 kg/s. rates m These values are equal to: 0.019 kg/s, 0.022 kg/s, 0.026 kg/s per 1 m2 of the gross surface area of the collector. It is usually assumed that 0.02 kg/s/m2 is the reference value. Experimental tests lasted from April 27 to October 18, 2016.
Fig. 5. The SCUs with ceramic solar collectors tested experimentally in normal operating conditions (author’s photo).
536
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 6. Diagram showing the measurement stand.
In the first stage of the experiment, it was decided to determine the temperature rise DTSC (Eq. (1)) of the fluid passing through the collector. This is an important parameter for the designer, which allows to determine how many collectors can be connected in series.
DT SC ¼ T OUT T IN :
ð1Þ
The difference between the outlet TOUT and inlet TIN temperature of the working fluid as a function of intensity of solar radiation GSOL is presented in Figs. 8–10. As can be observed in Figs. 8–10, the maximum temperature rise did not exceed 7.5 °C, that may be regarded as a typical result for flat plate solar collectors. As is widely known, the increase in fluid flow results in its temperature drop. At maximum flow rate DTSC did not exceed 5.5 °C for solar radiation equal to 1000 W/m2. The rise in the fluid temperature is a linear function of solar irradiance, thus the measurement results can be approximated with the use of the linear regression method. Eqs. (2)-(4) shows the relation between DTSC and GSOL for the subsequent increasing flow rates. Approximation results of all data sets are very precise because the correlation coefficient R-squared is equal: 0.9952 (0.033 kg/s), 0.9934 (0.039 kg/s), and 0.9965 (0.045 kg/s). Fig. 7. The measurement stand (author’s photo).
DT SC ¼ 0:008 GSOL 0; 5488:
ð2Þ
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
_ SC = 0.033 kg/s. Fig. 8. Dependence between fluid temperature rise and solar radiation intensity for m
_ SC = 0.039 kg/s. Fig. 9. Dependence between fluid temperature rise and solar radiation intensity for m
_ SC = 0.045 kg/s. Fig. 10. Dependence between fluid temperature rise and solar radiation intensity for m
537
538
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
DT SC ¼ 0:0068 GSOL 0; 4350:
ð3Þ
DT SC ¼ 0:0061 GSOL 0; 4123:
ð4Þ
Fig. 11 shows the dependence of solar radiation intensity on fluid temperature increase as the result of flow through the panel, for all considered cases. Knowing the value of fluid temperature increase we can estimate the actual useful power extracted, using the following relation:
qSC ¼
_ SC cp DT SC m ; AA
ð5Þ
The dependence of power output extracted from 1m2 of the solar collector on intensity of solar radiation is shown in Fig. 12. As can be seen, the maximum qSC reaches a value of 650 W/m2. At solar radiation flux density of 1000 W/m2 gives a good efficiency of energy conversion equal to 65%. The most important parameter of thermal performance is the efficiency of a solar collector g defined by Eq. (6) as the ratio of the useful energy gain QU at a certain time period to the solar energy incident on an exposed collector surface A over the same time interval (Duffie and Beckman, 2013).
RQ
g ¼ RUG A
where
dt
SOL
dt
:
ð6Þ
Under steady state conditions, the instantaneous efficiency gA related to the aperture area AA can be expressed as:
_ SC ¼ V a qf ; m Va – volume flow rate [m3/s], qf – working fluid density [kg/m3], cp – heat capacity of the working fluid [J/kg/K], AA – aperture area [m2].
gA ¼
_ SC cp DT SC m : AA GSOL
Fig. 11. Dependence between fluid temperature rise and solar radiation intensity.
Fig. 12. The influence of solar radiation on energy intercepted by the collector under test.
ð7Þ
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
The Hottel-Whillier-Bliss model (Hottel and Whillier, 1958) is often employed to determine the thermal characteristic of solar collectors. Reduced temperature difference T m (Eq. (8)) is applied in this method:
T m ¼
Tm Ta ; GSOL
ð8Þ
where
T m ¼ T IN þ
DT SC ; 2
Ta – ambient air temperature [°C]. The relationship between efficiency and reduced temperature difference can be plotted in the form of a straight line:
gA ¼ a0 a1 T m :
ð9Þ
The a0 point, where the line crosses the y-axis, corresponds to zero-loss collector efficiency gA,0. Coefficient a1 is the slope of the line. This value depends on the energy loss of the solar collector. Fig. 13 shows the result of the approximation of the selected experimental data sets under stationary and clear-sky conditions. For the ceramic solar collector operating under steady-state conditions (solar irradiance greater than 800 W/m2, diffuse solar irradiance is less than 20%, the average value of wind speed between 2 and 4 m/s, constant fluid flow rate equal to 0.039 kg/ s ± 0.0008 kg/s) zero-loss coefficient gA,0(a0) is equal 0.8332 and heat loss coefficient a1 is equal 16.332. It should be noted that the data sets presented in Fig. 13 are the mean value for both collectors. Thus it can be concluded that the ceramic solar collector zero loss efficiency is significantly better than the average value. According to the statistical analysis of the test results obtained by National Center for Quality Supervision and Testing of Solar Heating Systems the average value of gA,0 was 0.75 (Sun et al., 2007). As it turned out, the disadvantage of this collector is comparatively high heat loss through the cover systems, and especially through the rear side. This is evidenced by the value of the solar collector heat loss coefficient a1, which is higher than the
539
7.6 W/(oCm2), i.e. average value for flat plate collector specified in the (Sun et al., 2007). It should be noted that the total heat loss included a heat loss of pipes (length of 2 2.5 m DN18) that connected collectors to hydraulic system inside the laboratory. Temperature sensors were placed in the manifold located near the entrance of the pipes to the lab room. However, the value of the additional loss was not too high (about 8% of the total energy loss), because they were insulated with 20 mm EPDM foam rubber. In addition, the increase in energy loss was affected by lack of tightness of the frame or leaking internal joints of the ceramic panels, or due to both reasons. It was resulted in water vapor condensation on the inside of the glazing when there was low temperature of the surrounding air (Fig. 14). The negative role that condensation plays on the flat plate collector performance was developed by Anderson (2014). The next two figures shows plots of daily variation of solar radiation and power output of the solar collector (Fig. 15), and the working fluid temperature at the inlet and outlet of the ceramic collectors and surrounding air temperature (Fig. 16) for a clear sky day before noon and increasing cloudiness in the afternoon.
Fig. 14. Moisture condensation inside the solar collector under test.
Fig. 13. The instantaneous efficiency of the collector versus reduced temperature difference.
540
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Fig. 15. Daily variation of solar radiation and power output extracted from 1m2 of the solar collector (August 5, 2016).
Fig. 16. Daily variation of ambient air temperature and temperature at the inlet and outlet of the collectors (August 5, 2016).
As can be observed, ceramic collectors, due to relatively high thermal capacity, smooth even rapid fluctuations in solar radiation. As the result, useful energy gains supplied to the system are more uniform. Response time for solar system is important parameters, which determine its performance in transient conditions. The time constant sC, that depends on the thermal inertia of the collector, can be determined theoretically based on the following formula:
C
sC ¼ _ ; mSC cp
ð10Þ
where
C ¼n
4 X pi mi ci ; i
n – number of panels, pi - weighting factor given in ISO 9806 (absorber – 1, insulation – 0.5, heat transfer medium – 1, and external glazing 0,01a1) [–], mi – mass of each constituent element of the collector [kg], ci – specific heat of each element of the collector [J/kg/K].
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
The calculation results showed that the solar collector unit under test, has a relatively long time constant, which is equal to 626 s, 529 s, 459 s for 30 °C and mass flow rates of the working fluid: 0.033 kg/s, 0.039 kg/s, 0.045 kg/s, respectively. The dynamic properties of two flat plate solar collectors and vacuum tube liquid-vapor (heat-pipe) collector have been analysed by Obstawski et al. (2016). They have determined the time constants based on the Eq. (9), and technical documentation of those solar panels. The following values have been obtained: flat plate solar collector with harp coil arrangement – sC = 67.6 s, flat plate solar collector with meandering coil – sC = 69.8 s, and heat pipe collector – sC = 56.6 s. So it should be noted that the ceramic collectors are characterized by a significantly higher (almost ten times) thermal inertia compared to traditional collectors. This feature causes the ceramic collector starts to produce useful energy with a time lag, because he must warm up. On the other hand, high thermal capacity reduces outlet temperature fluctuations, which are caused by variations in the intensity of solar radiation. Uncertainty in estimating the collector thermal efficiency was determined based on the root-sum-square method. The combined standard uncertainty can be calculated according to the following relation:
(
uðgA Þ ¼
uðV a Þ
@ gA @V a
2
2 2 @g @ gA þ uðqf Þ A þ uðDT SC Þ @ qa @ DT SC )12
@ gA þ uðGSOL Þ @GSOL
2
;
ð11Þ
where
@ gA qf cp DT SC ¼ ; @V a AA GSOL @ gA V a cp DT SC ¼ ; @ qf AA GSOL V a qf cp @ gA ¼ ; @ DT SC AA GSOL V a qf cp DT SC @ gA ¼ : @GSOL AA G2SOL The relative uncertainty r(gc) can be estimated through the Eq. (11), as the result of dividing the absolute uncertainty by thermal efficiency.
( 2 2 uðqf Þ @ gA 2 uðV a Þ @ gA uðDT SC Þ @ gA rðgA Þ ¼ þ þ gA @V a gA @ qa gA @ DT SC 1 ) 2 2 uðGSOL Þ @ gA : þ gA @GSOL
ð12Þ
Uncertainties in measurement of solar radiation and flow rate were approximately equal to ±20 W/m2 and ±0.0028 m3/h, respectively. Total temperature measurement uncertainty was estimated to be within ±0.2 °C. For these parameters the maximum uncertainty in the ceramic collector efficiency did not exceed 4.75% and relative error ranged from 0.062 to 0.068. Thus it can be concluded that the accuracy of the experimental results obtained in the present study should be considered as high. 5. Conclusions The main goal of this research project was to determine the thermal characteristic of the ceramic solar collectors in normal
541
operating conditions. The fluid temperature rise DTSC inside the collectors was measured at various flow rates and solar radiation intensity. As it turned out, the maximum value of DTSC did not exceed 7.5 °C. This parameter can be considered as normal for the typical flat plate solar collector. The actual useful power and energy efficiency can be determined based on increase in fluid temperature. It was found that the maximum power output extracted from 1 m2 of the solar collector at solar radiation intensity of 1000 W/m2 was equal 650 W/m2. In the next step of this study, the efficiency for the conversion of solar radiation into heat energy as the function of reduced temperature difference was estimated. As indicated by the calculation results, zero-loss collector efficiency (the ambient temperature and the mean fluid temperature are the same) for ceramic solar collector was equal to 0.83. That may be classified as a very good parameter. Unfortunately, it should be noted that, the second parameter - heat loss coefficient - was less than 16. This indicates that the collectors under tests are characterized by high heat loss. In the paragraph discussing the results of the measurements reasons that could have an impact on this negative result were mentioned. However, this disadvantage can be easily eliminated by increasing the thickness of the thermal insulation and the use of better sealing material. In conclusion, we can say that the main advantage of ceramic collectors compared to conventional flat plate collectors is a very long service life of the monolithic absorber - estimated at 100 years. It is usually assumed that the service life of the conventional structure i.e. pipes attached to the absorber plate does not exceed 20–25 years. Constant optical properties of a micro layer of black ceramic V-Ti are another advantages of the ceramic panels. While it is also known that, the traditional selective absorber coatings can change original absorption and emission characteristics during the operation. Besides, ceramic absorbers (without casing) can be an integral part of the roof or balcony. Widely available and natural materials used to make absorbers is a particular advantage at a time when sustainable development is preferred in every sphere. But we should not forget about the shortcomings of this device. First of all, the weight and the water capacity of the ceramic panels is several times higher than typical flat-plate collectors. This is related to the fact that these devices have a high thermal inertia. As demonstrated by this study, ceramic panels have a slightly lower operating efficiency in comparison to the conventional solar modules, but we should not forget that this is a fairly new design which is in the process of developing and improving. However, in our opinion, the ceramic collectors will be able to compete with traditional flat plate solar panels in the near future. In the second stage of this project, which will be carried out during the summer season of 2017, the original rubber joints connecting the ceramic panels to the hydraulic loops will be changed, and the temperature will be measured directly at the inlet and outlet of the solar collectors. The authors also plan to investigate the thermal characteristic of a single panel without casing as this type of ceramic panels can be used as balcony railings or roof coverings. Acknowledgments This work was performed within the framework of a Grant of Bialystok University of Technology – Poland (Grant No. S/ WBIIS/4/2014). References Alm, B., Knitter, R., Haußelt, J., 2005. Development of a ceramic micro heat exchanger design, construction, and testing. Chem. Eng. Technol. 28, 1554– 1560. Alm, B., Imke, U., Knitter, R., Schygulla, U., Zimmermann, S., 2008. Testing and simulation of ceramic micro heat exchangers. Chem. Eng. J. 135, 179–184.
542
M. Zukowski, G. Woroniak / Solar Energy 146 (2017) 532–542
Anderson., T., 2014. Condensation in Glazed Flat Plate Solar Collectors. In: Conference Proceedings of the 52nd Annual Conference, Australian Solar Energy Society (Australian Solar Council) Melbourne, May 2014. Bower, C., Ortega, A., Skandakumaran, P., Vaidyanathan, R., Phillips, T., 2005. Heat transfer in water-cooled silicon carbide milli-channel heat sinks for high power electronic applications. J. Heat Transf. 127, 59–65. Duffie, J.A., Beckman, W.A., 2013. Solar Engineering of Thermal Processes. John Wiley & Sons Inc, Hoboken, New Jersey. Fend, T., Völkerb, W., Miebach, R., Smirnova, O., Gonsior, D., Schllgen, D., Rietbrock, P., 2011. Experimental investigation of compact silicon carbide heat exchangers for high temperatures. Int. J. Heat Mass Transf. 54, 4175–4181. Gomez-Martín, A., Orihuela, M.P., Becerra, J.A., Martínez-Fernandez, J., RamírezRico, J., 2016. Permeability and mechanical integrity of porous biomorphic SiC ceramics for application as hot-gas filters. Mater. Des. 107, 450–460. Hottel, H.C., Whillier, A., 1958. Evaluation of flat-plate solar collector performance. In: Carpenter, E.F. (Ed.), Transactions of the Conference on Use of Solar Energy, Vol. 2. University of Arizona Press, Tucson, Arizona, pp. 74–104. ISO 9806: 2013. Solar energy - Solar thermal collectors - Test methods. Kays, W.M., London, A.L., 1984. Compact Heat Exchangers. McGraw-Hill, New York, NY. Kee, R.J., Almand, B.B., Blasi, J.M., Rosen, B.L., Hartmann, M., Sullivan, N.P., 2011. The design, fabrication, and evaluation of a ceramic counter-flow microchannel heat exchanger. Appl. Therm. Eng. 31, 2004–2012. Li, Q., Flamant, G., Yuan, X., Neveu, P., Luo, L., 2011. Compact heat exchangers: a review and future applications for a new generation of high temperature solar receivers. Renew. Sustain. Energy Rev. 15, 4855–4875. McDonald, C.F., 2003. Recuperator considerations for future higher efficiency microturbines. Appl. Therm. Eng. 23, 1463–1487. de Mello, P.E.B., Villanueva, H.H.S., Scuotto, S., Donato, G.H.B., Ortega, F., 2016. Heat transfer, pressure drop and structural analysis of a finned plate ceramic heat exchanger, Energy, (in press) doi: 10.1016/j.energy.2016.11.113. de Mello P.E.B., Scuotto, S., Ortega, F., Donato, G.H.B., 2013. Heat transfer and pressure drop in a plate and fin ceramic heat exchanger. In: Conference Proceedings of the 8th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, 2013, 16–20. Obstawski, P., Bakon, T., Tulej, M., 2016. Analysis of dynamic properties of flat plate and heat pipe solar collectors. Measur. Autom. Monitor. 03 (62), 105–108.
Osório, T., Carvalho, M.J., 2012. Testing of solar thermal collectors under transient conditions. Energy Proc. 30, 1344–1353. Rey Martínez, F.J., Velasco, Gómez.E., Herrero, Martín.R., Martínez, Gutiérrez.J., Varela, Diez.F., 2004. Comparative study of two different evaporative systems: an indirect evaporative cooler and a semi-indirect ceramic evaporative cooler. Energy Build. 36, 696–708. Sommers, A., Wang, Q., Han, X., T’Joen, C., Park, Y., Jacobi, A., 2010. Ceramics and ceramic matrix composites for heat exchangers in advanced thermal systems-a review. Appl. Therm. Eng. 30, 1277–1291. Schulte-Fischedick, J., Dreißigacker, V., Tamme, R., 2007. An innovative ceramic high temperature plate-fin heat exchanger for EFCC processes. Appl. Therm. Eng. 27, 1285–1294. Stadelmann, R., Lugovy, M., Orlovskaya, N., Mchaffey, P., Radovic, M., Sglavo, V.M., Grasso, S., Reece, M.J., 2015. Mechanical properties and residual stresses in ZrB2-SiC spark plasma sintered ceramic composites. J. Europ. Ceram. Soc. 36, 1527–1537. Sun, X.-Y., Sun, X.-D., Li, X.-G., Wang, Z.-Q., He, J., Wang, B.-S., 2014. Performance and building integration of all-ceramic solar collectors. Energy Build. 75, 176–180. Sun, Z., Lu, B., He, T., Li, Z., Feng, A., Zhang L., Huang, Z., Deng, Y., 2007. Studies on test results for thermal performance of solar collectors in China, In: Conference Proceedings of the ISES World Congress 2007 ‘‘Solar Energy and Human Settlement” (Vol. 1-Vol.5), Beijing, China 18–21 September 2007, 716–720. Takeuchi, Y., Park, C., Noborio, K., Yamamoto, Y., Konishia, S., 2010. Heat transfer in SiC compact heat exchanger. Fusion Eng. Des. 85, 1266–1270. Velasco Gómez, E., Rey Martínez, F.J., Varela Diez, F., Molina Leyva, M.J., Herrero Martín, R., 2005. Description and experimental results of a semi-indirect ceramic evaporative cooler. Int. J. Refrig. 28, 654–662. Yang, Y., Wang, Q., Xiu, D., Zhao, Z., Sun, Q., 2013a. A building integrated solar collector: all-ceramic solar collector. Energy Build. 62, 15–17. Yang, Y., Cao, S., Xua, J., Cai, B., 2013b. All-ceramic solar collectors. Ceram. Int. 39, 6009–6012. Yang, Y.G., Xu, J.H., Cai, B., Wang, Q.C., Xiu, D.P., Zhao, Z.B., Sun, Q.Z., Cao, S.L., 2013c. Synthesis and applications of black ceramic from recycled industrial wastes. Adv. Appl. Ceram. 112, 146–148. Zukowski, M., Radzajewska, P., 2015. The optimal tilt angle of solar collectors for the territory of Poland. Cieplownictwo, Ogrzewnictwo, Wentylacja 4, 138–142 (in Polish with English abstract).