World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1815 Optimization of embedded Heat Exchanger in a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), with indirect heat withdrawal. Kostas Gertzos 1, Yannis Caouris 2, Thrasivoulos Panidis 3 Dept. of Mechanical Engineering. & Aeronautics, University of Patras, 265 00 Patras, GREECE 1 Tel: +30-2610-997-250, Fax: +30-2610-969-414, Email: gertzosk@mech.upatras.gr 2 Tel: +30-2610-969-410, Fax: +30-2610-996-129, Email: caouris@mech.upatras.gr 3 Tel: +30-2610-997-242, Fax: +30-2610-997-271, Email: panidis@mech.upatras.gr Abstract The influence of the diameter of an immersed tube heat exchanger (HX), in contact with the front and back walls of a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), to the service hot water (SHW) temperature is examined numerically. The storage water is not refreshed and is recirculated by a pump to intensify the heat transferred between the storage and the service water. Various 3D Computational Fluid Dynamics (CFD) models were developed using the FLUENT package. The optimal HX inner tube diameter that maximizes the SHW temperature was found to be 16 mm. 1 Introduction The most accepted solar systems for Domestic Hot Water production are the thermosiphonic units. The Integrated Collector Storage Solar Water Heaters (ICSSWH) use part of the hot water storage as solar collector. They have reduced cost because they integrate collector and heat storage in the same construction. They can be broken down into two basic categories. One is a direct configuration in which the storage medium serves also as the energy transfer medium and the second is an indirect configuration in which the energy storage medium and the energy transfer medium are separated usually by a heatexchanger (HX). Thus, the storage fluid is maintained in an unpressurized state. Many investigations have examined the thermal performance of several types of ICSSWH systems, aiming to improve their operation. In the existing literature there are works on systems with flat, cylindrical or other type of water storage tank. Smyth et al. [1] presented a survey and a history of the evolution of various types of integrated collector storage solar water heaters. All the reviewed ICSSWH are of the direct heating type. M. Souliotis et al. [2-3] studied experimentally integrated collector storage (ICS) systems of direct heating with single horizontal cylindrical storage tank placed in symmetric CPC type reflector trough. Systems with various storage tank diameters are studied, in order to improve their thermal performance. J. Davidson et al. [4] presented an experimental study of two indirect rectangular thermal storages during transient discharge one undivided and one equally divided into two compartments. They concluded that the divided storage provides higher energy delivery rates and higher heat exchanger outlet temperatures during most of the discharge processes. S. Arora et al. [5] studied the reduction of the overall solar water heating system efficiency due to usage of a heat exchanger, by deriving a simple "penalty factor". A. Kumar et al. [6] presented transient analyses of a collection-cum-storage water heater (i.e., built-in-storage/shallow solar pond) by incorporating the effect of a heat exchanger placed inside the system. Calculations have been made for typical days in Delhi. D. Henderson et al. [7] examined both experimentally and numerically by CFD analysis, the performance of a direct heating flat plate ICS at various inclinations.
World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1816 S. Knudsen et al. [8] investigated both experimentally and numerically the inlet positions of vertical mantle heat-exchangers for solar domestic hot-water (SDHW) systems. None of the above works examined mechanisms of heat transfer intensification. This was firstly done by Gertzos et al. [9], who validated a numerical 3-D Computational Fluid Dynamics (CFD) model of the flow field inside a flat plate storage with recirculation, using experimental data taken by a Laser Doppler Velocimetry (LDV) system. The standard k-ω turbulence model was selected as the most appropriate. Pnevmatikakis et al. [10] examined the thermal behavior of a similar flat plate ICSSWH, with and without recirculation experimentally. Furthermore, Gertzos et al. [11] presented the optimal arrangement of the position and size of the recirculation ports of a same ICSSWH. The development of the present numerical models is based on derived conclusions from these studies. The configuration under examination shown in Fig. 1, is of indirect heating type. It uses a pump (3) that recirculates the storage tank water, whenever there is a service water (SW) demand, keeping the heat transfer between the storage and the service water as high as possible. The additional cost of the HX and the circulating pump is much overbalanced by the relaxation of the anticorrosion and pressure durability requirements. The inner dimensions of the studied ICSSWH are 81 x 135.5 x 10.2 cm. The HX consists of six tubes in contact to the front surface of the storage, plus six tubes in contact to the back surface. The effective HX length is 16.26 m Fig. 1. Schematic diagram of the examined flat plate ICSSWH. (1) Storage tank, (2) tube heat exchanger (3) circulating pump, (4) Service water inlet, (5) Service water outlet. The heat transfer that occurs across the HX is determined by both tube-side and storage-side convection coefficients and heat transfer surface area. The storage-side convection process has been extensively examined by previous investigations [9-11], leading to an optimal arrangement and size of the recirculation ports. The optimal HX tube diameter is examined here, for the maximization of the heat transfer between the two fluid circuits and consequently the maximization of SHW temperature too. Various CFD simulations in steady state were carried out, to estimate the optimal HX diameter. Two more transient simulations were performed, in order to estimate the settling time and to simulate the withdrawal behavior. 2 CFD Analysis In previous works [9,10] CFD models were validated by experimental measurements of velocities and temperatures. The optimal inlet and outlet port diameter examined by K. Gertzos et al. [11] was found to be 8 mm. In addition, the optimal placement of the inlet recirculation port was found to be on the right surface (viewed from top) of the ICSSWH, at a distance of 100 mm from the top surface.. In the present investigation the influence of the HX inside diameter to the temperature of the service water is examined. To estimate this influence, four cases of HX tubes are considered: 12x1 mm, 15x1 mm, 18x1 mm and 22x1 mm, where the first number represents the outer tube diameter and the second the tube thickness. The inside diameters of the previous cases are 10, 13, 16 and 20 mm respectively. For each one of the above cases, 3D CFD models were developed, using the pre-processor GAMBIT. The following materials were considered. a) steel of 1 mm thickness for the storage tank walls, with properties ρ = 8030 kg m -3, c p = 502.48 J kg -1 K -1, k = 16.27 W m -1 K -1, b) copper for the HX tubes with properties ρ = 8978 kg m -3, c p = 381 J kg -1 K -1, k= 387.6 W m -1 K -1, c) water with the following properties and correlations as functions of temperature:
World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1817 ρ = 1.3187x10 T + 1.8447x10 T + -7 4-4 3-2 2 9.9428x10 T 23.82T 1113.5-11 4-8 3 µ = 3.533x10 T 4.8141x10 T -5 2 + 2.4637x10 T 0.0056188T + 0.48281 cp = 3.321729x10 T 4.459811x10 T 2 2 + 2.248733T 5.041488x10 T 4 + 4.654524x10-6 4-3 3-10 4-7 3 k = 6.2068x10 T 8.0897x10 T -4 2-2 + 3.8437x10 T 7.7569x10 T + 6.1019 where T in K. (a) (b) Fig. 2 (a) Computational grid for the model with HX inlet diameter 13mm (b) details near the inlet recirculation port. The computational grid shown in Fig. 2a is mainly composed of hexahedral cells, except the region near the inlet and outlet recirculation ports, where tetrahedral cells are used (Fig 2b). The number of cells for the case presented in Fig. 2 arises to 188.598 (144.566 for the tank water + 44.032 for the tube water). As the optimization concerns the HX diameter that maximizes the service water outlet temperature, all other quantities must be kept constant during the numerical simulations. Thus, the Navier-Stokes and the energy equations are solved in steady state conditions. Τhe Reynolds number inside the ΗΧ tubes is for all cases (inside diameter d in = 10-16 mm and service water flow rate Q tu = 300-700 l/h) far greater than 2300, denoting a turbulent flow. The Reynolds number for tank water based on the vessel depth and the tank water mean velocity is for all HX diameters much greater than 2300. Thus the flow inside the tank is also turbulent. The turbulence intensity I on the recirculation input port was computed by the relation I 0.16 ( Re) 1/8 = [12]. The boundary conditions are defined as: a) mass flow inlet, for the tank and tube inlet port, with mass flow rate computed by the 6 relation m& = ρ ( Q / 3.6x10 ) ; b) outflow, for the outlet ports, with flow rate weighting by the relation FRWi = m& i /( m& tu + m & ta ) i = ta, tu where the indices ta and tu denote the tank and tube circuit; c) wall, for all other surfaces. The shell conduction option was enabled to compute heat conduction within the wall in addition to conduction across the wall. The velocity-pressure coupling was treated using the SIMPLE algorithm and the Second Order Upwind scheme was used for Momentum, Turbulent Kinetic Energy and Turbulence Dissipation Ratio. The k-ω turbulence model was used [9]. For the accuracy of solutions a value of 10-4 was used for all residual terms, except for the Energy term which was set to 10-6. The Volume Weighted Average tank water velocity was monitored and computed by the relation: U 1 n ta UiVi V 1 =, where U i and V i are the velocity and volume of the ith cell and V the total tank volume. The outlet temperature was computed as the Area Weighted Average Temperature at the outlet port surface, i.e.: T 1 n out Ti Ai A 1 =, where T i and A i are the temperature and surface area of the ith cell and A the total outlet port cross-sectional area. The storage is considered to be perfectly insulated.
World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1818 3 Results The heat transfer that occurs across the HX is determined by both tube-side and storage-side convection coefficients and heat transfer surface area. The HX tube diameter influences all the above parameters. For given recirculations and SW flow rates, the decreas of the HX tube diameter, results to greater velocities inside the tubes, but also outside them (of the recirculated water velocities, as the resistance to flow drops down) and consequently to greater convection coefficients. Fig. 3 Service water outlet temperature versus HX inside diameter for SW flow rates 300, 500 and 700 l/h and SW inlet temperatures 15, 20 C. Fig. 4 Mean tank water velocity and Reynolds number versus HX inside diameter for recirculation flow rate 810 l/h and tank water temperature 60 C. Fig. 5 Predicted flow path lines and velocity magnitude for recirculation flow rate 810 l/h. On the other hand the heat transfer surface decreases. The aim of this analysis is to determine the optimal diameter which maximizes the heat transferred between the two water circuits. Results for twenty four steady state numerical simulations were extracted, for HX inside diameters of 10, 13, 16 and 20 mm, for SW flow rates 300, 500 and 700 l/h, recirculation flow rate 810 l/h and SW inlet temperatures 15 and 20 C. The tank water temperature was considered constant at 60 C. The SW outlet temperature and the mean tank water velocity were computed. Fig.3 shows outlet temperatures versus HX inlet diameter. It is clear that maximum SHW temperatures occur for HX tube 18x1 mm, with inner diameter of 16 mm. This maximum temperature appears for all the SW flow rates and SW inlet temperatures. Fig. 4 depicts the mean water tank velocity versus the HX inner diameter. The mean velocity varies from 0.103 m/s to 0.076 m/s for diameters of 10 mm and 20 mm respectively. The velocity seems to be stabilized for diameters greater than 16 mm. The predicted flow path lines on the middle plane of the storage are shown in Fig. 5, for steady state conditions. This situation (the flow field to be fully developed) is reached in a certain time interval (settling time) after the recirculation startup. A transient simulation was performed, with a time step of 1s. Fig. 6 shows the predicted path lines at time instances 5, 10, 15, 20, 25, 30 s after the recirculation startup, with HX optimal diameter, while Fig. 7 shows the mean velocity distribution versus
World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1819 time for the cases of HX inner diameter 13 and 16 mm. The settling time is practically 20 s, less than the draw off time of the comprised volume of hot water inside the HX tubes. The draw off time for a HX with inner diameter 16 mm, varies from 43 s to 19 s for SW flow rates 300 l/h and 700 l/h respectively. As the compised water is initially already heated the temperature time profile becomes smooth. Fig. 6 Predicted path lines at time steps 5, 10, 15, 20, 25, 30 s after the recirculation startup. Fig. 8 Service water outlet temperature versus mean tank water temperature for T in = 10, 20, 25 C and Q tu = 300, 500, 700 l/h. Fig. 7 Predicted storage tank water mean velocity versus time after the recirculation startup. Twenty four more simulations were repeated for the optimal HX diameter with mean tank temperature 40, 60, 80 C, SW inlet temperatures 10, 20, 25 C and flow rates 300, 500 and 700 l/h. Fig. 8 represents these results. The presence of the HX reduces the service hot water outlet temperatures. The difference between the storage tank temperature depends on the SW flow rate, the storage temperature, the SW inlet temperature and the recirculation rate. For a SW inlet temperature 20 C and storage water temperature 60 C this decrease varies from 2.2 to 9.8 C, for SW flow rates 300 l/h and 700 l/h respectively. The decrease becomes higher as the SW inlet temperature falls and even higher as the storage temperature increases. Another transient simulation (for 1 hour period) was developed, for SW inlet temperature 20 C, SW flow rate 500 l/h and initial storage water temperature 60 C. Fig. 9 shows the predicted mean storage water temperature, SW outlet temperature and SW mean temperature during the one hour withdrawal. The SW outlet temperature
World Renewable Energy Congress (WRECX) Editor A. Sayigh 2008 WREC. All rights reserved. 1820 appears a decrease of 6.1 C from tank water temperature at the beginning, falls to 3.9 C after 20 min and to 1.6 C after 40 min. Fig. 9 Predicted temperatures versus time. 4 Conclusions The characteristics of the flow field inside the storage of a flat plate ICSSWH with immersed HX, have been investigated for a range of operating conditions and inner HX diameters, leading to the following conclusions. The mean tank fluid velocity is reduced as the HX diameter increases and varies between 0.103 m/s to 0.076 m/s for inner HX diameter 10 mm and 20 mm respectively. The service water outlet temperature is maximized for a HX inside tube diameter of 16 mm, appearing a decrease from the tank water temperature, varying from 2.2 C to 9.8 C, for SW flow rates from 300 l/h to 700 l/h respectively. 5 References [1] M. Smyth, P.C. Eames, B. Norton, Integrated collector storage solar water heaters, Renewable and Sustainable Energy Reviews 10 2006, 503 538 [2] M. Souliotis, Y. Tripanagnostopoulos, Experimental study of CPC type ICS solar systems, Solar Energy, 76(4) 2004, 389-408 [3] M. Souliotis, Y. Tripanagnostopoulos, Study of the distribution of the absorbed solar radiation on the performance of a CPC-type ICS water heater. Renewable Energy 33 2008, 846 858. [4] J.H. Davidson and V. Ragoonanan, Divided storage in an indirect integral collector storage with immersed heat exchanger. Proceedings of ISEC2005, 2005 International Solar Energy Conference, August 6-12, 2005, Orlando, Florida. [5] S. Arora, J. Davidson, J. Burch, S. Mantell, Thermal Penalty of an Immersed Heat Exchanger in Integral Collector Storage Systems, Journal of Solar Energy Engineering 121(3) 2001, 180-186 [6] A. Kumar and G.N. Tiwari, Transient analysis of collection-cum-storage water heater integrated with heat exchanger. Energy Convers. Mgmt 28(3) 1988, 201-206. [7] D. Henderson, H. Junaidi, T. Muneer, T. Grassie, J. Currie, Experimental and CFD investigation of an ICSSWH at various inclinations. Renewable and Sustainable Energy Reviews 11 2007, 1087 1116. [8] S. Knudsen, S. Furbo, Thermal stratification in vertical mantle heatexchangers with application to solar domestic hot-water systems, Applied Energy 78 2004, 257-272 [9] K.P. Gertzos, Y.G. Caouris, Experimental and computational study of the developed flow field in a flat plate integrated collector storage (ICS) solar device with recirculation, Experimental Thermal and Fluid Science 31(8) 2007, 1133-1145. [10] S.E. Pnevmatikakis, Y.G. Caouris, K.P. Gertzos, Development of an Integrated flat plate Collector Storage ICS Solar Thermosiphon, Proc. 7nth Nat. Congr. on Renewable Energy sources, 6-8 Nov. 2002, Patras, Greece, Vol. 1, pp. 451-460, ISSN 1108 3603. [11] K. P. Gertzos, Y. G. Caouris, Optimal arrangement of constructive parts of a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), Experimental Thermal and Fluid Science 32(5) 2008, 1105-1117. [12] Fluent, Fluent 6.3 User s Guide, Lebanon, New Hampshire (USA) Fluent Inc.; (2006)