THERMAL PERFORMANCE OF EVACUATED TUBE AND FLAT PLATE SOLAR COLLECTORS IN NORDIC CLIMATE CONDITIONS

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 6, Issue 2, February (2015), pp IAEME: Journal Impact Factor (2015): (Calculated by GISI) IJMET I A E M E THERMAL PERFORMANCE OF EVACUATED TUBE AND FLAT PLATE SOLAR COLLECTORS IN NORDIC CLIMATE CONDITIONS Dmitri Loginov 1, Teet-Andrus Koiv 2, Mikk Maivel 3, Kalev Kalda 4 1,2,3,4 Department of Environmental Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia ABSTRACT In this paper the results of the experiments of measuring the performance of evacuated tube and flat plate type solar collectors in Nordic climate conditions are presented. The measurements of the collectors of a given and equal gross surface area were performed in the test installation environment. While the azimuth of the collectors was preserved constantly by 180, the vertical incline was varied in order to identify the most suitable value of the thermal performance. In addition, the comparison of the thermal performance of the two solar collector types under different climatic conditions was done. Also the year-round simulation of the collectors performance was possessed in PolySun environment. Based on the measurements and simulation results the cost benefit calculations of solar collectors were performed. Key-Words: Energy performance, Cold climate, Nearly zero-energy building, PolySun, Solar collectors 1 INTRODUCTION As the global energy consumption increases constantly and the fossil fuels are essentially nonrenewable and therefore will eventually run out, the importance of renewable energy sources, including solar energy is difficult to underestimate. The total rate of solar energy received by our planet is thousands times larger than the amount of energy actually consumable by the modern solar collectors technologies. Nevertheless, the Directive of Energy Performance of Buildings (2010/31/EU) demands that by the year 2020 all new buildings have to be nearly zero-energy. Without 81

2 the extensive usage of the renewable energy sources in buildings energy systems, such as solar panels and collectors, this target is hardly achievable. As buildings are responsible for 40% of energy consumption and 36% of CO2 emissions in the EU, the adaptation of the high-efficiency solar panels in buildings energy systems will help to minimize both of the affected values. This topic becomes important also in the northern regions of Northern hemisphere where the solar irradiation rates are comparably moderate. In order to define the most optimal solar collector type for the Nordic region, the comparison of the thermal performance of two solar collector types under different climatic conditions was possessed by conducting the field experiments and also by software simulation. The testing facility was actually located in Tallinn, Estonia, that is 80 km south from Helsinki, Finland. Thus the climatic data and conditions are very similar to those used in Nordic region. The average actinometrical solar resource in Estonia is 990 kwh/(m 2 a), with the bigger values on islands and coastal area (Fig. 1). The value of solar constant on the global level, on the other hand, varies between 1,360 1,367 kw/m 2 [1]. Several similar researches were conducted earlier. In [2], for example the functional differences between evacuated tube and flat plate type solar collectors are considered. In [3] authors also state that the performance of the flat plate solar collectors (FPC) is more affected by the environmental conditions comparing to the evacuated tube collectors (ETC). There also researches, which deals with an improved computational models of solar collectors. One of them is [4] where FPC and ETC collectors are experimentally investigated and a new dynamic collectors test method is proposed. This method, being more computationally complex that the existing ISO based static procedures, allows however more accurate collectors parameters evaluation and modelling. In [5] the exergy analysis for a single ended glass evacuated tube solar collector system was carried out. In this paper the technical details and results of the comparison of the thermal performance of evacuated tube and flat plate type solar collectors in Nordic climate conditions are presented. Figure 1. Annual sunshine durations (hours) in Estonia [6] 82

3 2. OBJECTIVES AND METHODS The objectives of the current research were as follows. - To compare the thermal performance and efficiency of evacuated tube and flat plate type solar collectors under same working conditions by measuring the respective parameters in the real test application and to select the most appropriate solar collector for domestic hot water production. - To compare the capacity charts and rates of accumulated solar thermal energy for evacuated tube and flat plate type solar collectors under different weather conditions (sunny weather, intermittent cloudiness, cloudy weather). - With a help of the simulation software compare the annual thermal energy delivery for the evacuated tube and flat plate type solar collectors with identical gross surface area. In addition, to find out the most efficient vertical incline degree/angle for each month of the year, assuming the azimuth is equal to 180 (direction South). - With a help of the simulation software determine the amount of accumulated solar thermal energy for different collectors positioning management scenarios. That is: a) the vertical incline angle is constant and equals 45 and horizontal angle is adjusted according to the Sun position dynamically, b) the vertical incline angle is adjusted monthly and the azimuth is constant and equals 180, c) both vertical and horizontal angles adjusted automatically and dynamically with a help of solar tracker. - To compare the simulated net amount of accumulated solar thermal energy with results of the analytical equation given in the respective local regulation and to conduct the cost benefit calculations of solar collectors based on the measurements and simulation results. The method to conduct the study includes the following. At first, the measurements for evacuated tube and flat plate type solar collectors were performed. Solar collectors are located at zeroenergy test-house of Tallinn University of Technology (TUT) (see Fig. 2, 3). Geographical coordinates of the installation are 59 23N 24 39E. The measurements periods were and The parameters were measured with 15 min time intervals and included capacity (kw), flow of the heat carrier (l/s), amount of heat energy received (accumulated) (kwh), flow and return temperatures of the heat carrier ( C). Both collectors have a comparable gross surface area of ca 2,4 m 2. Since the solar collectors installed were fixed firmly to their bases (vertical angle is 55, azimuth 180 ), the simulation software was used to model different incline angles of collectors and respective influence on their thermal performance. Figure 2 Installation of the evacuated tube solar collectors at TUT 83

4 Figure 3 Installation of the flat plate solar collectors at TUT 2.1 Tools and technologies used Simulation software PolySun 5.5, evacuated tube collector Intelli-Heat AL 30, flat plate collector Logasol SKS 4.0 and required hydronic heating and measurement equipment. 3 RESULTS In this section the results of the measurements and simulations experiments of the solar collectors of two different types are discussed. 3.1 The amount of heat energy received/accumulated by solar collectors Heat energy data was collected and analysed for August 2013, September 2013, March 2014 and April 2014 time intervals. In this subsection, due to the paper content constrains we present only major conclusions for majority of evaluated time intervals and only for April 2014 are brought more detailed data August 2013 From the total data collected is seen that the maximum Solar Energy Accumulated (SEA) for the evacuated tube collector (ETC) was 19 kwh on 23.08, when the weather was sunny. Minimum SEA was only 3 kwh on and when it was cloudy and rainy. Average day SEA value for this period was 10,7 kwh/day. Maximum daily specific SEA for evacuated tube collector was 1,94 kwh/m2 and minimum 0,31 kwh/m2. Average daily specific SEA was 1,09 kwh/m2. For the flat plate type collector (FPC) there were the following data. Maximum SEA was 13 kwh on 23.08, when the weather was sunny. Minimum SEA was 1 kwh also on and when it was cloudy and rainy. Average day SEA value for this period was 5,9 kwh/day. Maximum daily specific SEA was 2,74 kwh/m2 and minimum 0,21 kwh/m2. Average daily specific SEA was 1,24 kwh/m2. 84

5 3.1.2 September 2013 For the evacuated tube collector type collector there were the following data. Maximum SEA was 16 kwh on 7.09, when the weather was clear and sunny. Minimum SEA was 1 kwh on and when it was cloudy and rainy. Average day SEA value for this period was 7,0 kwh/day. Maximum daily specific SEA was 1, 63 kwh/m2 and minimum 0, 10 kwh/m2. Average daily specific SEA was 0, 71 kwh/m2. For the flat plate type collector there were the following data. Maximum SEA was 9 kwh on , when the weather was clear and sunny. Minimum SEA was 0 kwh on and when it was massively cloudy and rainy. Average day SEA value for this period was 3,9 kwh/day. Maximum daily specific SEA was 1,90 kwh/m2 and minimum 0 kwh/m2. Average daily specific SEA was 0,82 kwh/m2. These data shows that in the autumn, when weather is bad, the efficiency of the flat plate solar collectors is low, comparable with the evacuated tube ones. In Fig. 4 the SEA data for 7.09 is presented March 2014 For the evacuated tube collector type collector there were the following data. Maximum SEA was 13 kwh on and , when the weather was clear and sunny. Minimum SEA was 0 kwh at the beginning of March, when it was cloudy and rainy. Average day SEA value for this period was 5,7 kwh/day. Maximum daily specific SEA was 1,32 kwh/m2 and minimum 0 kwh/m2. Average daily specific SEA was 0,58 kwh/m2. For the flat plate type collector there were the following data. Maximum SEA was 9 kwh on 29.03, when the weather was clear and sunny. Minimum SEA was 0 kwh also in the beginning of the month when it was massively cloudy and rainy. Average day SEA value for this period was 3,4 kwh/day. Maximum daily specific SEA was 1,90 kwh/m2 and minimum 0 kwh/m2. Average daily specific SEA was 0,72 kwh/m2. The data collected a show that in days with clear and sunny weather the average thermal efficiency of FPC is 37% higher than of ETC and in days with an intermittent cloudiness is 23% higher. On the other hand when the weather is bad, the efficiency of ETC is 33% higher than of FPC April 2014 For the evacuated tube collector type collector there were the following data. Maximum SEA was 20 kwh. Minimum SEA was 0 kwh. Average day SEA value for this period was 12,7 kwh/day. Maximum daily specific SEA was 2, 04 kwh/m2 and minimum 0 kwh/m2. Average daily specific SEA was 1,3 kwh/m2. For the flat plate type collector there were the following data. Maximum SEA was 13 kwh. Minimum SEA was 0 kwh. Average day SEA value for this period was 7, 6 kwh/day. Maximum daily specific SEA was 2,74 kwh/m 2 and minimum 0 kwh/m2. Average daily specific SEA was 1,6 kwh/m2. See Fig. 5 and Table 1 for a more detailed data on this time period. 85

6 Figure 4 The amount of Solar Energy Accumulated (SEA) for ETC and FPC collectors on 7 September Figure 5 The amount of Solar Energy Accumulated (SEA) per 1m 2 surface area for ETC and FPC collectors (for April 2014). 86

7 Figure 6 ETC and FPC collectors annual energy delivery [kwh] and vertical incline angles [ ]. 3.2 Modelling the thermal energy delivery of ETC and FPC with simulation software PolySun 5.5 In this subsection the annual thermal energy delivery for solar collectors for different vertical incline angles, the influence of the various collectors positioning on SEA and its comparison with results of the analytical equation are presented based on the year-round simulations. Table 1 The amount of Solar Energy Accumulated (SEA) for ETC and FPC collectors (for April 2014). 87

8 Table 2 FPC collector energy delivery [kwh] and vertical incline angles. Table 3 ETC collector energy delivery [kwh] and vertical incline angles. Table 4 Most efficient vertical angles and increase of SEA for FPC and ETC collectors Annual thermal energy delivery and efficient vertical incline angles In Table 2 and 3 the year-round thermal energy delivery and respective vertical incline angles are presented. The last two columns represent data for fixed vertical angle 45 /dynamically adjusted horizontal angle azimuth and dynamically adjusted both vertical/horizontal angles. In Fig. 6 the respective data is shown in a chart form. 88

9 From these data the most efficient vertical angles for the specific month of the year are clearly seen, these are summed up in the table shown in Table 4. From these tables one may determine also the amount of accumulated solar thermal energy for different collectors positioning management scenarios Comparison of SEA with results of the analytical equation and the cost benefit calculations The simulated annual net amount of accumulated solar thermal energy was % larger, compared to results of the analytical equation given in the local energy efficiency calculation regulation (504 kwh). In Table 5 8 the results of cost benefit calculations and the simple payback period are shown for different types of solar collectors with a gross surface area of 2 m 2. In calculations the cost of the complete solar thermal energy system (including installation expenses) is taken into account, which includes solar collector, accumulation tank, pump, sensors and automation equipment, expansion vessel, fasteners, solar tracker. Table 5 Payback periods for ETC collectors with fixed position. Table 6 Payback periods for ETC collectors equipped with solar-tracker. Table 7 Payback periods for FPC collectors with fixed position. 89

10 Table 8 Payback periods for FPC collectors equipped with solar-tracker. 3.3 Further research The intended future research directions are related with the solar cogeneration technology [7].Such integration of photovoltaic (PV) and solar hot water (SHW) technology into one system makes solar cogeneration the most cost-effective solar energy solution available for commercial and industrial scale customers [8], [9]. Thus, the future research directions may be as follows. 1. Simulation of the combined solar PV&SHW panels/collectors thermal performance. 2. The measurements of the combined solar PV&SHW panels/collectors thermal performance and comparison with the simulated data. 4 CONCLUSION This paper describes the results of the experiments of measuring the performance of evacuated tube (ETC) and flat plate (FPC) solar collectors in Nordic climate conditions. The testing of solar collectors was held from August to September in 2013 and from March to April in 2014 in TUT. According to the results, during sunny days FPC produced 21-37% more thermal energy than ETC. On partly cloudy days when the weather was changing FPC produced 8-23% more thermal energy. During rainy and cloudy days ETC produced 21-64% more. To sum the testing results up, FPC are more efficient in Nordic climate for heating water for daily use. Comparing FPC and ETC with simulation software PolySun showed that FPC produce 25% more heat than ETC for the same gross area. Using solar-trackers increases productivity of FPC collectors by 48% and 41% of ETC. Solartrackers are more efficient with FPC because they produce more heat with direct solar radiation. The payback period for installing complete solar heating system with a 2 m 2 FPC is 13,2 years and 17,8 years with ETC. Using solar-trackers the payback period cuts down to 10,3 and 14,5 years respectively. By installing more collectors the payback period can be shortened, e.g. with three 2 m 2 flat plate collector and solar trackers the payback period is approximately 4,6 years and 6,9 years with a evacuated collector. 5 ACKNOWLEDGEMENTS The research was supported by the Estonian Research Council, with Institutional research funding grant IUT1-15, and by the project Civil and Environmental Engineering PhD School, DAR9085, financed by SA Archimedes. REFERENCES 1. Kopp. G, Lean. J. L, A new, lower value of total solar irradiance: Evidence and climate significance, Geophysical Research Letters, Vol.38, Heliodyne. Solar flat plate vs. evacuated tube collectors, professional s/downloads/evacuated%20tube%20comp.pdf,

11 3. Sassan Mohasseb, Ali Kasaeian, Comparing the Performance of Flat Plate Collector and Evacuated Tube Collector for Building and Industrial Usage in Hot and Cold Climate in Iran with TRNSYS Software, in Proc. 9th International Conference on Engineering Computational Technology, At Napoli, Italy, 2014, pp W. Kong, Z. Wang, J. Fan, B. Perersb, Z. Chen, S. Furbo, E. Andersen, Investigation of thermal performance of flat plate and evacuated tubular solar collectors according to a new dynamic test method, Journal of Energy Procedia, No. 30, 2012, pp Hamza Al-Tahaineh1, Rebhi Damseh, Exergy Analysis Of A Single-Ended Glass Direct Flow Evacuated Tube Solar Collector, International Journal Of Advanced Research In Engineering And Technology, 4(7), 2013, pp Russak, V.; Kallis, A. Eesti kiirguskliima teatmik, Tallinn, 2003, p. 54, (Estonia directory of irradiance levels, in Estonian). 7. Sorensen H., Munro D., Hybrid PV/Thermal Collectors, In Proceedings from the 2 nd World Solar Electric Buildings Conference, 8-10 March, Sydney, Mohamed Ali Darwish and Sayeed Mohammed, Solar cogeneration power desalting plant with assisted fuel, Qatar Foundation Annual Research Forum Proceedings, Vol., EEP58, Anderson, T., Bura, S., Duke, M., Carson, J. & Lay, M, Development of a building integrated photovoltaic/thermal solar energy cogeneration system, in Proc. of 3rd International Conference on Sustainability Engineering and Science: Blueprints for Sustainable Infrastructure, 9-12, December, Auckland, New Zealand., 2008, pp Yogesh C. Dhote & Dr. S.B. Thombre, Parametric Study on the Thermal Performance of the Solar Air Heater with Energy Storage International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 1, 2012, pp , ISSN Print: , ISSN Online: Ajeet Kumar Rai, Pratap Singh, Vivek Sachan and Nripendra Bhaskar, Design, Fabrication and Testing of A Modified Single Slope Solar Still International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 4, 2013, pp. 8-14, ISSN Print: , ISSN Online: Prof. Sunil Kumar and Prof. (Dr.) S.K.Singh, Comparative Analysis of Performance of Salt Gradient Solar Pond with Conventional (Flat) and Corrugated Bottom International Journal of Mechanical Engineering & Technology (IJMET), Volume 5, Issue 4, 2014, pp , ISSN Print: , ISSN Online: