ABSTRACT 2. REVIEW OF PREVIOUS STUDIES. 2.1 Types of Solar Thermal Collectors

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1 AN EMPIRICALLY DERIVED MODEL REGARDING THE PERFORMANCE OF FLAT PLATE SOLAR THERMAL COLLECTORS BASED ON VARYING RATIOS OF DIRECT TO DIFFUSE SOLAR RADIATION T. Landon Abernethy Department of Technology and Environmental Design Appalachian State University Katherine Harper Hall Boone, NC Brian W. Raichle Department of Technology and Environmental Design Appalachian State University Katherine Harper Hall Boone, NC ABSTRACT The increasing availability of solar thermal collectors with non-flat-plate geometries suggests that a static, direct-beambased performance testing methodology may not provide comparable results. Standardized testing under controlled conditions, while well established and reproducible, requires a narrow range of conditions that may not reflect conditions under which a collector operates. Further, certain collectors may respond differently to diffuse-beam radiation or short duration variations in radiation. An analysis which accounts for varying radiation quality and collector conditions, while difficult, may reveal indicators that better represent collector performance. This project has tested three thermal collectors of very different geometries; a flat plate, an evacuated tube heat pipe, and a CPC, under naturally varying conditions. An analysis methodology will be proposed, and benchmarking to SRCC efficiency parameters will be reported. Preliminary results for the flat plate collector show that the slope and y- intercept both increase with increasing direct beam fraction.. Economic implications for a variety of locations will be discussed. This study was undertaken at the Appalachian State University Solar Research Laboratory, Boone, NC. 1. INTRODUCTION This study attempts to isolate the effects of varying fractions of direct and diffuse solar radiation on the performance of flat plate solar thermal collectors (FPC). The study was undertaken with the goal of characterizing FPC performance in varying radiative conditions and developing a performance characterization methodology that could be applied to other types of collectors including evacuated tube collectors (ETC), and compound parabolic collectors (CPC). The study provides a basis for further research regarding the relative abilities of the different types of collectors to utilize varying ratios of direct and diffuse solar radiation in real world climatic and operative conditions. This stands in contrast to the way in which solar thermal collectors are typically tested and theoretically modeled to predict performance. 2. REVIEW OF PREVIOUS STUDIES 2.1 Types of Solar Thermal Collectors This study is concerned with the performance of collectors most commonly installed in Solar Domestic Hot Water (SDHW) systems, i.e. stationary collector designs. Three geometries of stationary solar thermal collectors are currently commercially marketed for domestic hot water production in the United States: flat plate collectors (FPC), evacuated tube collectors (ETC), and non-evacuated compound parabolic collectors (CPC). Each of these collectors has intrinsic costs and benefits associated with their designs. The flat plate collector has been the historic worldwide industry standard; but evacuated tube collectors are rapidly gaining market share driven by widespread adoption and manufacturing in China (1). Flat plate collectors are currently the most common type of solar thermal collectors installed in the United States and are typically employed in low temperature situations where the temperature of water produced is less than 100 C. FPCs, as defined for the purposes of this study, are non-evacuated panels in which a highly absorptive coating on a flat absorber and a glass glazing are utilized to transfer direct and diffuse radiation to a working fluid running through the 1

2 panel. One generally identified limitation of FPCs is their relative inability to absorb solar radiation from large incidence angles encountered as the sun moves across the sky due to reflection from the glazing (2). This would imply that FPCs are also limited in their ability to utilize diffuse radiation throughout the day. Evacuated tube collectors are defined by their use of evacuated glass tubes. According to Kalogiru (3), the ETC is capable of collecting direct and diffuse radiation like a FPC, but their efficiency is higher than FPCs at extreme incidence angles. It has been argued that ETCs are likely to perform relatively better in conditions with large proportions of diffuse radiation compared to FPCs due to their geometry (2). Compound parabolic concentrators, as defined for this project, are non-evacuated collector panels that combine a flat-plate style absorber and glazing with compound parabolic, non-imaging optics to focus direct and diffuse solar radiation onto an absorber tube containing a thermal working fluid (4). Pramuang and Exell (5) note that CPCs are capable of utilizing solar radiation throughout relatively long periods of the day and of concentrating diffuse radiation, which is not possible with a non-stationary or imaging collector. While the ability of CPCs to utilize diffuse radiation in comparison to non-stationary collectors is apparent, it is not clear how they perform compared to FPCs and ETCs in the same regard. The ability of a solar thermal collector to harvest nonnormal incident rays is generally characterized by an incident angle modifier (6). These parameters likely predict a collector s performance in diffuse conditions. 2.2 Direct and Diffuse Radiation The earth intercepts approximately 1.8 x kw of solar radiation, of which about 60% reaches the surface (7). Total global irradiance is affected by scattering, absorption, and reflection as it passes through the atmosphere, leading to the beam, diffuse, and reflected components of global irradiance (8). Beam radiation refers to the irradiance component that is unaffected in its direction by atmospheric or ground effects, the diffuse component of irradiance is the radiation impacted by atmospheric scattering, and reflected radiation is the portion of global irradiance affected by ground reflectance (8). The spatial distribution of diffuse radiation is frequently described by one of three physical models: the isotropic model radiation is scattered uniformly from the entire sky dome, circumsolar radiation is preferentially scattered from near the sun disk, and horizon brightening radiation is preferentially scattered from near the horizon (2). The circumsolar radiation component is technically diffuse, but in practice is accounted for as beam radiation (2). This study is concerned with measurement of the direct and diffuse components of global irradiance. According to (2), it is very difficult to measure the true diffuse radiation, but a pyrheliometer can be used to make a good direct beam measurement. 2.3 Factors that Affect Thermal Collector Performance The efficiency and energy production of a solar thermal collector is impacted to some degree by all factors associated with ambient conditions, but mainly by the solar irradiance incident on the collector and ambient air temperature (2). The rate at which a solar thermal collector is able to produce useful thermal energy is directly related to the rate of solar radiation incident on the surface of the collector. Solar thermal collectors are most efficient when the temperature of the working fluid is closest to the ambient air temperature due to lower heat loss (9). 2.4 Thermal Performance Testing of Collectors There are multiple certification agencies that perform standardized performance testing on solar thermal collectors around the world. Important solar thermal testing standards include the EN 12975, ANSI/ASHRAE and the ISO 9806 standards. Regardless of the agency providing certification, or the test standard on which a particular certification agency bases its certification, the underlying purpose for collector performance testing is to provide a means to compare the performance of various collectors and to provide collector parameters to be used in models to predict the long-term performance of solar thermal systems (10). There are two types of testing protocols that underlie the certification testing standards: steady-state and quasidynamic. All common testing methodologies parameterize thermal collector performance as the slope and y-intercept of an efficiency vs. T i -T a /I graph, where T i and T a are collector inlet and ambient air temperature, and I is solar irradiance. 2.5 Steady-state Testing Protocol Steady-state testing implies that thermal performance testing is undertaken under a highly regulated set of ambient and performance conditions. The steady-state approach was the original solar thermal certification test standard, and is still the most commonly used today (8). Steady-state testing typically requires strict control of total irradiance, diffuse fraction, incidence angle, ambient temperature, and inlet temperature during testing (10). One common criticism of the steady-state testing method is that it does not properly 2

3 account for the specific optical characteristics of collectors other than FPCs (8). 2.6 Quasi-dynamic Test Protocol The quasi-dynamic test standard was developed largely due to perceived limitations in the steady-state testing protocol. The quasi-dynamic method allows both for more variable testing conditions and more realistic performance characterization of solar thermal collectors (11). Rojas et al. (10) note that the major difference in the quasi-dynamic test method is that the energy gain is measured over short intervals while solar irradiance and ambient temperature are allowed to vary. According to Horta (8), the quasi-dynamic test allows for the effects of different radiative components to be accounted for; however, more experience is needed with the test methodology, especially in relating its results to long-term model predictions. The quasi-dynamic method attempts to account for a collector s ability direct and diffuse radiation; however, no attempt is made to empirically test the effects of varying fractions of direct and diffuse radiation. At this time, the SRCC had tested only one collector under the quasi-dynamic testing method, and it was a tracking concentrating collector. 2.7 Limitations and Comparison The testing and certification of solar thermal collectors is a crucial component of determining the relative performance of various collectors and as a means of comparing collectors without side by side empirical testing. It was determined by Rosas et al. (10) that the collector parameters obtained by steady-state and quasi-dynamic tests on flat plate collectors are very similar. Zambolin and Del Col (12) note that limited results are available regarding quasi-dynamic testing of ETCs, and there is a similar void in the literature regarding CPCs. So while the quasi-dynamic testing method may better characterize the effects of different radiative fractions, its use in practice is still to be determined. Furthermore, according to Zambolin and Del Col (12), neither the steady-state nor the quasi-dynamic test method necessarily represents usual operating conditions or actual performance of the collector over the full day. Sideby-side testing of commercially available solar thermal collectors of varying geometry under real world operation conditions is needed to characterize the ability of various collector types to utilize solar radiation in actual working conditions. 3. RESEARCH METHODOLOGY 3.1 The Thermal System One FPC, considered to be representative of type, was installed at latitude tilt (36⁰) and 180 azimuth. The FPC, an Alternate Energy Technologies Morningstar MSC-32, was part of an active-indirect system with a 50/50 Dowfrost/water mixture circulating through an 80 gallon tank with an internal heat exchanger. Flow was controlled by a differential controller with a turn on set point of 6⁰C. The tank was actively cooled. Flow was measured with a Seametrics SB-050 paddle type flow meter. Inlet and outlet thermal fluid temperatures were measured at the collector with Campbell Scientific 108 thermistors. 3.2 Meteorological Measurements Direct normal irradiance (DNI) was measured with a Hukseflux DR-1 Class 1 pyrheliometer mounted on a Minitrak II Solar Tracker. Global diffuse radiation (GDIFF) was measured with a Hukseflux SR-11 Class 1 pyranometer. Total radiation in the plane of aperture of the solar thermal collectors (POA) was measured by a Hukseflux LP02 pyranometer. Wind speed and wind direction was measured directly adjacent to the solar thermal collectors with a Met b wind set. Ambient temperature and humidity at the site were measured with a Campbell Scientific HMP 50 temperature and humidity sensor. Precipitation was measured with a Texas Electronics 525 tipping rain bucket. 3.3 Data Collection Procedures The system was allowed to run unencumbered from October 24, 2011 to November 29, 2011 at the Appalachian State University Solar Research and Teaching Lab. Data were measured every 15 seconds and averaged into 1 minute data sets with a Campbell Scientific CR1000 data logger. Flow was set to the ASHRAE specified 0.02 kg/hr/m 2 of collector area. Measurements were averaged on a five-minute basis. 3.4 Data Validation Five-minute averages between 4 PM and 10 AM were excluded, as were minutes with a measured flow that deviated from the design flow by more than ± 10%. Data with negative heat loss were excluded, as these correspond to periods of cooling. To reduce scatter due to collector thermal capacitance effects, only five-minute intervals in which POA varied by less than 10% from the previous minute were analyzed (as described in the following section). The resulting data set contained 8,068 five-minute average values. 4. ANALYSIS 4.1 Collector Performance Calculations Instantaneous collector efficiency was investigated as a function of (T i -T a )/I. 3

4 Instantaneous efficiency delivered by the FPC was calculated using where the total power incident on the collector is taken to be the irradiance measured in the plane of aperture (POA). The parameter T i -T a /I was calculated using measured temperatures and measured POA irradiance. To investigate collector performance as a function of radiative quality, the direct beam fraction was calculated by dividing the component of DNI normal to the collector aperture by the measured POA, as changed significantly. If the irradiance increased significantly the collector efficiency was calculated too low; a significant decrease in irradiance resulted in too high of an efficiency. In other words, real world testing is sensitive to collector thermal inertia. To remove this effect, 5-minute data for which the POA irradiance varied by more than 10% from the previous 5- minute average were removed from the analysis. The resulting graph is shown in Figure PRELIMINARY RESULTS 5.1 The Effect on Efficiency of Short Term POA Variability and Global Irradiance The objective of this analysis is to produce an efficiency slope and y-intercept according to the ASHRAE standard, and as used by the SRCC. That is, the slope and y-intercept of an efficiency vs. T i -T a /I graph, so that the y-intercept is the collector s optical (maximum) efficiency and the slope (always negative) describes the increased rate of heat loss as the collector operates hotter than the ambient air temperature. Often when testing under real world conditions, independent variables are identified that effect the results but aren t of interest. These variables must be accounted for in some way. Figure 1 shows a scatter plot of measured collector efficiency versus all values of T i -T a /I. Fig. 2: A scatter plot of measured efficiency vs. T i -T a /I of the AET MSC-32 FPC with POA variation less than 10%. The data were then binned according to POA irradiance values. POA values below 300 W/m 2 were excluded. Figure 3 graphs the average efficiency vs. T i -T a /I. The calculated y-intercept of 0.66 and slope of W/m 2 /⁰C can be compared with SRCC values of and W/m 2 /⁰C respectively. Fig. 3: Efficiency values averaged in T i -T a /I bins. The fit line is linear. Fig. 1: A scatter plot of measured efficiency vs. T i -T a /I of the AET MSC-32 FPC for all POA values. Little correlation is evident. After closer scrutiny it was discovered that considerable scatter in efficiency values resulted from consecutive minutes over which the irradiance 5.2 The Effect of Differing Ratios of Direct and Diffuse Radiation on Efficiency Slope and Y-Intercept Collector performance was also investigated binned in direct beam fraction of POA irradiance, as described in the previous section. While this is not the most commonly used 4

5 irradiance value, it is the direct beam fraction most representative of the solar power delivered to the collector. Figure 5 shows how values of DNI, POA (called POA direct), POA and GHI (global horizontal irradiance) evolve throughout a sunny day. POA is noticeable lower early and late in the day. Fig 4. Irradiance components on a representative clear day. Slope and y-intercept of collector efficiency vs. T i -T a /I was calculated in bins of DBF, and are shown in Figure 5. DBF bins greater than 50% are presented. collector experiences a relatively equal drop in efficiency. However, the lowest two DBF bins cause a flattening of the efficiency slope that results in a cross over at (T i -T a )/I of around This would indicate that at any point beyond that crossover, the FPC is working at a higher efficiency under DBF conditions lower than 70% than it would at direct beam radiative conditions greater than 70%. This also implies that the efficiency penalty caused byincreasing (T i T a ) decreases as conditions approach the cross-over point. Implications of these general trends are offered in Table 1. The table shows the expected performance of the FPC under a typical warm climate system and a typical cool climate system based on the measured performance data. These cases show modeled efficiency at various direct beam fractions under the same POA irradiance. The values illustrate the trend of a greater efficiency penalty from difference in radiative conditions in cool climates than warm climates. That is not to say that the trend is limited to climate. It would also manifest in situations where the collector is working at a higher or lower temperature than a typical SDHW system, which would also increase or decrease the difference between inlet and ambient temperatures. TABLE 1: Range of Modeled Efficiencies Based on DBF for Typical Warm and Cool Climate Systems. Fig. 5: Slope and y-intercept values binned in DBF. It is apparent that the y-intercept of the FPC decreases as the direct beam fraction decreases. Therefore, the collector s maximum efficiency has decreased from 69% under % DBF to below 55% under 50-60% DBF conditions. The data show that for a DBF greater than 70% the collector efficiency drops by small proportions. Below 70% DBF conditions the y-intercept of the collector drops drastically and the y-intercept stays virtually the same for the 50-60% and the 60-70% direct beam fraction bins. The statistical uncertainty on the y-intercepts show that the y-intercepts are statistically separate for the three high direct beam fraction bins and statistically inseparable for the two lower bins. The slope of the efficiency curve is more enigmatic. Slopes behave predictably for the three highest DBF bins. A statistical uncertainty analysis shows that the three slopes are separable; however, they are very close and never cross over each other. This indicates that for increased T i - T a, the 5.3 Regarding the Impact of Diffuse Radiation as a Performance Indicator for Adequate Performance Characterization in Standardized Testing It is clear from the data presented that ignoring the effects of collector performance under diffuse radiative conditions inaccurately characterizes collector performance. As seen in Figure 5, the FPC has varying efficiencies according to DBF. It was observed during the relatively small test 5

6 window that values of between 50% and 70% DBF occur over a wide range of POA irradiances ranging from 500 to over 1100 w/m 2. High diffuse radiative conditions do not necessarily translate to low total irradiance in the plane of aperture. Without understanding how a collector would perform under these conditions, it is impossible to accurately predict how a collector would perform in a region that is marked by high occurrences of similar radiative conditions. Even at the higher direct beam fractions, it may be important to account for the efficiency decline that results from a relatively small decrease in direct beam fraction to make meaningful estimations of system energy production. 6. SUMMARY AND CONCLUSIONS This research illuminated some interesting points in regards to the performance of solar thermal collectors under different ratios of direct beam to diffuse irradiance. The research serves the larger goal of acting as a stepping off point for a comparison of the major types of solar thermal collectors as well as standing on its own as a proof of concept for the general research methodology This research conclusively shows that the FPC performance characteristics change under varying ratios of direct beam to diffuse radiation. The research indicates that those performance characteristics change minimally between 100% and 70% direct beam fraction, and then change dramatically below 70% direct beam fraction. It follows that the standard method of disregarding the effects of diffuse radiation on the FPC leads to an insufficient characterization of collector performance. In order to make meaningful estimates of energy that will be produced by a solar thermal system, it is important to have performance information across a wide range of climactic and radiative conditions. While highly controlled, standardized testing will likely remain the only way to cheaply and efficiently test collectors for comparison, at least a general understanding of how the basic geometries of collectors perform under varying radiative conditions is needed. More information yields better planning and estimation tools and promotes the maturation of the industry as a whole. 7. REFERENCES (1) International Energy Agency. (2009). Renewable Energy Essentials: Solar Heating and Cooling. Retrieved fromhttp:// heating_cooling.pdf (2) Andersen, E., & Furbo, S. (2009). Theoretical variations of the thermal performance of different solar collectors and solar combi systems as function of the varying yearly weather conditions in Denmark. Solar Energy, 83(4), doi: /j.solener (3) Kalogirou, S. (2004). Solar thermal collectors and applications. Progress in Energy and Combustion Science, 30(3), doi: / j.pecs (4) Solargenix Energy, LLC. (n.d.) Technical Data for Solar Collector Type: Winston Series CPC. Raleigh, North Carolina. Retrieved April 1, 2011, from Technicalmanual.pdf (5) Pramuang, S., & Exell, R. (2005). Transient test of a solar air heater with a compound parabolic concentrator. Renewable Energy, 30(5), doi: / j.renene (6) Solar Rating and Certification Corporation. (2011). SRCC Document RM-1: Methodology for Determining the Thermal Performance Rating for Solar Collectors. Cocoa, Florida. Retrieved October 3, 2011 from certification/ogdocuments/rm-1.pdf (7) Thirugnanasambandam, M., Iniyan, S., & Goic, R. (2010). A review of solar thermal technologies. Renewable and Sustainable Energy Reviews, 14(1), doi: /j.rser (8) Horta, P., Carvalho, M., Pereira, M., & Carbajal, W. (2008). Long-term performance calculations based on steady-state efficiency test results: Analysis of optical effects affecting beam, diffuse and reflected radiation. Solar Energy, 82(11), doi: / j.solener (9) Ramlow, B. (2006). Solar water heating : a comprehensive guide to solar water and space heating systems. Gabriola Island BC: New Society Publishers. (10) Rojas, D., Beermann, J., Klein, S., & Reindl, D. (2008). Thermal performance testing of flat-plate collectors. Solar Energy, 82(8), doi: /j.solener (11) Fischer, S. (2004). Collector test method under quasidynamic conditions according to the European Standard EN Solar Energy, 76(1-3), doi: /j.solener (12) Zambolin, E., & Del Col, D. (2010). Experimental analysis of thermal performance of flat plate and evacuated tube solar collectors in stationary standard and daily conditions. Solar Energy, 84(8), doi: /j.solener