Archimedes Solar Energy Laboratory (ASEL) Optimization of Thermosiphon Solar Water Heating systems working in a Mediterranean Environment of Cyprus 7-8 October, Nicosia Cyprus Rafaela A. Agathokleous
Solar water heating systems worldwide Solar power systems are experiencing a rapid growth worldwide and mostly in the countries with high amount of solar radiation Studies show that the worldwide leader country for the use of solar water heating systems per capita is Cyprus
Solar water heating systems worldwide Between the EU countries, in 2011 Cyprus had the highest installed collector capacity with 541.2 kw th /1000 inhabitants and then Austria and Greece with 355.7 kw th /1000 inhabitants and 268.2 kw th /1000 inhabitants, respectively UK Turkey Germ Austra Greece Barba Israel Austria Cyprus Collector capacity in operation in 2011 0 200 400 600 kw th / 1000 inhabitants
Types of solar water heating systems There are two types of solar thermal systems: The forced circulation systems (active systems) The thermosiphon systems (passive systems)
Solar water heating in Cyprus The majority of the systems installed in Cyprus are of the thermosiphon type Characteristics of a flat plate solar collector Parameter Characteristics Riser pipe diameter 15 mm Riser tubes material copper Number of riser pipes 10 Header pipe diameter 28 mm Absorber plate thickness 0.5 mm Glass type 4 mm low iron glass Fiberglass 30 mm Collector insulation sides Fiberglass 50 mm back Glazing Low iron glass External casing material Galvanized sheet Typical thermosiphon system specifications Description Value Total aperture area 3.0-3.8 m 2 Storage tank capacity 160-180 liters Auxiliary heater 3 kw electric element Flow rate per unit area at test 0.02 kg/s m 2 conditions Intercept efficiency 0.79 Slope of performance curve 5.5-7.0 m 2 C /W (F R U L ) Collector slope angle 35-45
Solar water heating in Cyprus The majority of the systems installed in Cyprus are of the thermosiphon type Characteristics of a flat plate solar collector Typical thermosiphon system specifications
This Study The aim was to investigate the design parameters that affect the system s performance of the SWH systems operating thermosiphonically in order to find the optimum design to increase the system s efficiency Parameters examined: The riser and header tubes diameters The number of riser pipes The slope of the collector The height difference between the collector output and the storage tank output The orientation of the tank, horizontal or vertical The shape of the collector and length of the risers and header pipes The considerations of the parameters that affect the performance of the thermosiphon solar water heater have been made using TRNSYS simulation program through the use of component Type 45 for the TMY of Nicosia, Cyprus
Orientation of the storage tank In the past years, the storage tank of the thermosiphonic systems used to be vertically installed while in the recent years is being installed horizontally for the purpose of reducing the overall height of the system
The riser and header tubes diameters & the number of risers The various configurations evaluated, concerning header and riser pipes diameters: Header diameter (mm) Riser diameters (mm) 15 6, 8, 10, 12 22 6, 8, 10, 12, 15 28 6, 8, 10, 12, 15, 22 35 6, 8, 10, 12, 15, 22, 28 Although all the parameters were simulated using TRNSYS, there was a need to define some other parameters needed for the program for each case.
The riser and header tubes diameters & the number of risers Equations from theoretical analysis of the system were used to estimate: The heat removal factor (F R ) The overall heat loss coefficient (U L ) The transmittance - absorptance product (τα) The slope of performance curve (F R U L ) The intercept of the efficiency curve (F R (τα)) Riser Diameter Number of riser (mm) tubes F R (τα) F R U L (kj/hr m 2 o C) 6 29 0,836 25,263 8 20 0,825 24,94 10 16 0,817 24,685 12 13 0,807 24,395 15 10 0,792 23,94 22 7 0,765 23,13 28 5 0,731 22,103
Thermal Efficiency of the system % The riser and header tubes diameters & the number of risers A small riser diameter of 8 mm selected as the optimum 20 pipes of 8 mm The smallest pipe of 6 mm is not selected because the systems employed in Cyprus are usually open, circulating the water supplied to the user directly into the collector, which may create scaling problems. A header diameter of 22 mm is selected in terms of performance because this is the same diameter as the connecting pipes between the collector and storage tank, which makes the installation easier. 42.5 42 41.5 41 40.5 40 39.5 39 38.5 38 Header Diameter=15mm Header Diameter=22mm Header Diameter=28mm Header Diameter=35mm Optimum system Optimum selected system Typical system 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Riser tube diameter (mm)
Thermal efficiency of the system % Thermal efficiency of the system % The slope of the collector The slope of the collector is very important for the system s performance so the system was simulated and tested in slopes from 20-90 at steps of 5 System with vertical storage tank 42 41 40 39 38 37 36 35 34 33 32 0 10 20 30 40 50 60 70 80 90 100 Slope of the collector (degrees º) System with horizontal storage tank 44 42 40 38 36 34 32 30 0 10 20 30 40 50 60 70 80 90 100 Slope of the collector (degrees º)
Thermal efficiency of the system % The height difference between the collector and the storage tank Six cases of this height difference were investigated, for vertical distance between the top of the collector and bottom of the tank -30 cm, -20 cm, -10 cm, 0 cm, 10 cm and 20 cm System with vertical storage tank 42 0 m 40 38 36 34 Hv= - 0.30 m Hv= - 0.20 m Hv= - 0.10 m Hv= 0 m Hv= 0.10 m Hv= 0.20 m 32 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Slope of the collector (degrees º)
Thermal efficiency of the system % The height difference between the collector and the storage tank Six cases of this height difference were investigated, for vertical distance between the top of the collector and bottom of the tank -30 cm, -20 cm, -10 cm, 0 cm, 10 cm and 20 cm System with horizontal storage tank 44 42 40 0 m 38 36 34 32 30 Hv= - 0.30 m Hv= - 0.20 m Hv= - 0.10 m Hv= 0 m Hv= 0.10 m Hv= 0.20 m 28 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 Slope of the collector (degrees º)
System's Efficiency % The shape of the collector and length of the riser and header pipes Four cases of different shapes of collector have been investigated in this study as follow: 1. Length= 1.5 m Width= 1 m Risers= 10 (typical) 2. Length= 1.15 m Width= 1.30 m Risers=13 3. Length= 1.875 m Width= 0.8 m Risers=8 4. Length= 1 m Width= 1.5 m Risers=15 40.4 40.2 40 39.8 39.6 39.4 39.2 Typical system 1 2 3 4
System's Performance (%) General Outcomes In the typical form, the system with vertical tank is more efficient than the system with the horizontal tank By considering all changes, the system with the horizontal tank is more efficient than the system with the vertical tank 44 43 42 41 40 39 38 37 Vertical tank Horizontal tank Typical system Change in collector design Slope and height of the tank In total it is achieved to improve the efficiency of the system with vertical tank by 2.4% and the system with horizontal tank by 4.4%.
E-mail: rafaela.agathokleous@cut.ac.cy