Chapter 57 Optimizing Solar Hot Water Systems (Closed Systems) for Air-Conditioning Cycles in TRNSYS

Transcription

1 Chapter 57 Optimizing Solar Hot Water Systems (Closed Systems) for Air-Conditioning Cycles in TRNSYS A Kaabi Nejadian, Ali Mohammadi and Behnoosh Bakhtiari Heleyleh Abstract Parallel and series solar hot water systems (closed systems) were simulated and optimized. The system is designed to supply the required heat for solar air-conditioning cycles such as heat generators in ejector cycle, absorption cycle generator, regenerator in liquid desiccant cycle and regenerating absorbent of the desiccant wheel, and solid desiccant cycle. A model of solar hot water system was simulated in TRNSYS and then, based on solar fraction, the components of the system were optimized. For optimization of the cycle, the dynamic performance of the system in supplying the required temperature for a cycle of Three Tons Refrigeration was analyzed under different situations. The results were compared with the experimental results to obtain consistency. The effects of the parameters of the system, supplied heat capacity, the energy used by the heater, and the effect of the environment were surveyed regarding feasibility of achieving higher solar energy. Finally, the solar hot water system was optimized for Bushehr city. The results showed that the optimization of the solar system is influenced by environmental parameters such as radiation intensity, dry bulb temperature, relative moisture; the way the system is utilized (residential and institutional); the term of operation of the system; and, most importantly, the set point of the auxiliary hot water system. The optimized system was featured as follows: solar fraction: 0.5, area of collector: 86 m 2, angle of collector: 31, set point of the auxiliary hot water system: 75 C, capacity of tank: 4 m 3, solar collector discharge: 0.25 kg/s, and mass discharge rate of the thermal converter: 0.2 kg/s. Keywords Solar hot water system Flat-plate collector Storage tank Optimization Solar fraction A. Kaabi Nejadian ( ) Renewable Energy Organization of Iran, Tehran, Iran Kaabi@iranenergy.org.ir A. Mohammadi B. Bakhtiari Heleyleh Department of Environmental and Energy Engineering Science and Research Branch, Islamic Azad University Tehran, Tehran, Iran ali_makran60@yahoo.com B. Bakhtiari Heleyleh b.bakhtiari.h@gmail.com Springer International Publishing Switzerland 2016 A. Sayigh (ed.), Renewable Energy in the Service of Mankind Vol II, DOI / _57 635

2 636 A. Kaabi Nejadian et al. Nomenclature Q load Thermal capacity of the solar hot water system [kw] T in Temperature of the fluid of the solar hot water system entering the exchanger [c] T out Temperature of the fluid of the solar hot water system exiting of the exchanger [c] T set Regulated temperature of the auxiliary hot water system [c] C pw Water special temperature [kj/kg] M w Mass flow of the fluid inside the solar hot water system [kj/kg] COP Coefficient of performance [-] SF Solar fraction [-] A Total collector array aperture or gross area [m 2 ] A a Aperture area of a single collector module [m 2 ] A r Absorber area of a single collector module [m 2 ] a 1 Negative of the first-order coefficient in the collector efficiency equation [kj/h m 2 k] a 2 Negative of the second-order coefficient in the collector efficiency equation [kj/h m 2 k] b 0 Negative of the first-order coefficient in the incident angle curve fit equation [-] b 1 Negative of the second-order coefficient in the Incidence Angle Modifier (IAM) curve fit equation [-] F R Overall collector heat removal efficiency factor [-] F av Modified value of Fr Overall collector heat removal efficiency factor when the efficiency is given in terms of T av, not T i [-] F o Modified value of Fr Overall collector heat removal efficiency factor when the efficiency is given in terms of T o, not T i [-] I Global (total) horizontal radiation [kj/h m 2 ] I d Diffuse horizontal radiation [kj/h m 2 ] I T Global radiation incident on the solar collector (Tilted Surface) [kj/h m 2 ] I bt Beam radiation incident on the solar collector [kj/h m 2 ] M Flow rate at use conditions [kg/h] m test Flow rate in test conditions [kg/h] N S Number of identical collectors in series [-] T a Ambient (air) temperature [c] T av Average collector fluid temperature [c] Inlet temperature of the fluid to the collector [c] T i T o Outlet temperature of the fluid from the collector [c] U L Overall thermal loss coefficient of the collector per unit area [kj/h m 2 k] U L/T Thermal loss coefficient dependency on T [kj/h m 2 k] α Short-wave absorptance of the absorber plate [-] β Collector slope above the horizontal plate [ ] θ Incidence angle for beam radiation [ ] ρ g Ground reflectance [-] Τ Short-wave transmittance of the collector cover [-] ( τα) Product of the cover transmittance and the absorber absorptance [-]

3 57 Optimizing Solar Hot Water Systems (Closed Systems) 637 ( τα) b For beam radiation (depends on the incidence angle θ) [-] ( τα) n At normal incidence [-] ( τα) s For sky diffuse radiation [-] ( τα) g For ground-reflected radiation [-] C p Specific heat of the fluid [kj/kg k] H a Height of the auxiliary heater above the bottom of the tank [m] H t Height of the tank [m] H th Height of the thermostat above the bottom of the tank [m] k Thermal conductivity [W/m 2 k] m h Mass flow rate of the hot stream entering the tank [kg/h] m L Mass flow rate of the load [kg/h] Q aux Rate of auxiliary energy input to the tank [kj/h] Q env Rate of energy loss from the tank [kj/h] Q he Maximum rate of energy input to the tank by auxiliary [kj/h] Q in Rate of energy input to the tank from the hot fluid stream [kj/h] Q sup Rate of energy supplied to the load by the tank [kj/h] r i Ratio of insulation thickness of the top to the sides of the upright tanks [-] T D Temperature of water delivered by the tank to the load [c] T env Environmental temperature for losses [c] T h Temperature of the hot fluid entering the tank [c] T I Temperature of its segment [c] T L Temperature of the load stream entering the tank [c] T R Temperature of the fluid return to the heat source [c] T set Thermostat set temperature [c] T Average tank temperature [c] V h Volume of the fluid entering the tank from the heat source over a time interval Δt [m 3 ] V L Volume of the fluid entering the tank from the load over a time interval Dt [m 3 ] V t Tank volume [m 3 ] ΔE Change in the internal energy storage [kg m 2 /s 2 ] UA Overall Heat Losses value of the tank [kj/h] Δt Simulation time step [s] Thermostat temperature dead band [s] ΔT db ρ Fluid density [kj/kg k] γ heater Optional control function input (0 to 1) that disables or enables the auxiliary heater [-] 57.1 Introduction One of the greatest challenges today is world energy supply. For that reason, great efforts have been made by a number of developed countries to rationalize energy use by means of research seeking more energy-efficient systems or by using other energy sources such as the sun, wind, biomass, and wave and tide. Due to the fast

4 638 A. Kaabi Nejadian et al. development of economics and the tremendous energy demand in Iran and other developing countries, solar energy has been looked at as an attractive substitute energy source for space heating in recent years. Riffat et al. [1] constructed a thin-membrane, flat-plate, heat-pipe solar collector and developed an analytical model that was used to simulate heat transfer processes occurring in the collector and calculated its efficiency. Bong et al. [2] presented a validated theoretical model to determine the efficiency, heat removal factor, and outlet water temperature of a single collector and an array of flat-plate heat-pipe collectors. Ezekwe [3] analyzed the thermal behavior of solar energy systems using heat-pipe absorbers and compared them with systems using conventional solar collectors. Bojic et al. [4] modeled and simulated the performance of a forced circulation solar water-heating (SWH) system using a time-marching model. Jones and Golshekan [5] studied the desertification of a packed bed consisting of river pebbles along with several additional properties, and the experimental data of this study were found to be mutually consistent and were in good agreement with a previously published work. Sorour investigated a small-sized sensible heat energy storage unit constructed out of gypsum rocks and indicated the optimal design and operating conditions for maximum thermal efficiency of this small unit [6]. Different computational tools have been developed to numerically evaluate the long-term performance of solar systems and to study the effect of design parameters. TRNSYS is an extensive software for transient simulation of solar systems (thermal or photovoltaic (PV)), low-energy solar multi-zone buildings, renewable energy systems, fuel cells, and their related equipment. This program has been widely used to study and optimize solar systems. There have been several studies that used TRNSYS to evaluate the effect of different design parameters and operating conditions on the performance of a thermosyphon solar water-heating (TSWH) system. TRNSYS is a complete and extensible simulation environment for the transient simulation of systems, including solar thermal systems. However, not much work has been done concerning the use of TRNSYS simulation for solar air heating. The present chapter has focused on the use of TRNSYS to analyze a solar air-based heating system for a public building in North China. An investigation was performed to observe the design parameters of the system and to determine their optimum values. The annual and monthly solar fractions (SFs) of the entire system were used as the optimization parameters. Buckles and Klein [7] compared the performance of different configurations of forced circulation systems. Michaelides and Wilson [8] optimized the design criteria of an active SWH system for hotel application. Wongsuwan and Kumar [9] studied the performance of forced circulation systems experimentally and numerically. The numerical simulations were conducted using TRNSYS and artificial neural network. The results from both numerical models were found to be in good agreement with the experimental values. For instance, Furbo et al. [10] recommended the design of a large SWH as a low-flow system with hot water tanks, external heat exchangers, and stratification inlet pipes; Nayak and Amer [11] evaluated nine dynamic test procedures for the evaluation of flat-plate collectors; Tsilingiris [12]

5 57 Optimizing Solar Hot Water Systems (Closed Systems) 639 developed a simple simulation model for large SWH systems; Kikas [13] studied laminar flow distribution of reverse and direct return circuits in solar collectors; Prapas et al. [14] studied the thermal behavior of large central SWH; Fanney and Klein [15] investigated the influence of flow rate and also the incorporation of an auxiliary heat exchanger on the performance; Chiou [16] developed a numerical method to determine the variation in performance due to nonuniform flow distribution; and Klein and Beckman [17] presented a general design method for closedloop SWH systems. The present work is focused on using TRNSYS to analyze a forced circulation SWH system for a single-family residential unit in Montreal, Canada. A comprehensive study has been conducted to study all design parameters of the system and to determine their optimum values. The monthly and annual SFs of the entire system are used as the optimization parameters. The objective of this chapter is to develop a TRNSYS simulation model for forced-circulation SWH systems with flat-plate tube collectors and validate the model using measured field performance data. The validated model will be useful for long-term performance simulation under different weather and operating conditions. The model could also be used for system optimization under different load profiles Solar Hot Water System As pictured in Fig. 57.1, the model comprises two loops. One includes a solar collector, a pump, and a tank; and another includes a tank, an auxiliary hot water system, a three-way valve, a pump, and a balance valve. Fig Solar hot water system model in TRNSYS. AUX Auxiliary; HEX Heat Exchanger; HWST Hot Water Storage Tank

6 640 A. Kaabi Nejadian et al. Throughout the first loop, the temperature of the fluid (water) rises by passing the solar collector before being stored in the tank. By losing its heat (supplying warm water), the water is pumped to the cycle. When the temperature of water at the outlet is higher than that of the inlet, the pump is turned off by the controller so that energy waste is prevented. The second loop is featured with water flow from the tank to the three-way valve and inflow to the auxiliary hot water system, when the temperature of the discharge water from the tank is less than the preset temperature of the auxiliary hot water system; it heats the water up to T set temperature. When the temperature of the outflow tank is less than T set, the auxiliary hot water system is turned off. Then, the water enters the shell and pipe convertor (to increase wet-bulb temperature) and is pumped into the circulation pipe after being cooled down. The discharged fluid from the pump enters the balance valve. If the temperature of the inflow fluid to the pipe is less than that of the tank s outlet, the fluid is led to path 2, otherwise to the tank Equations Flat-Plate Collector The energy collected by the collector is obtained from Hottel Whillier equations. The equations of the flat-plate collector are as follows: The amount of energy received by the collector: Q u Ns A = F, j I ( τα) U, j T, j T N ( ( )) R T L i a s j= 1 (57.1) F R, j NsmcC é æ pc FUL, ja ö ù = 1 exp. AU - - L, j NsmcC ê ç pc ë è øúû (57.2) Heat transfer coefficient: h W W w = + m k ( ) + 2 f = I 004. h h ( I 009. N ) w w G ( ) C = I b b 2. (57.3) (57.4) (57.5)

7 57 Optimizing Solar Hot Water Systems (Closed Systems) 641 Transfer coefficient and the total absorption coefficient: I ( τα) = ( τα) + I 1+ cos β cos β ( τα ) ρ 1 I 2 2 ( τα ) g +. I bt b d s T I T (57.6) Outlet temperature of each component: T o, j AFR, j[ IT( τα) UL, j( Ti, j Ta )] = + Ti. NmC s c pc (57.7) The fluid flow is zero: T p IT ( τα) = + Ta. U L (57.8) The waste coefficient of the solar collector is obtained by highly complicated equations including those developed by Klein [18] Hot Water Storage Tank Figure 57.2 represents the idea of a plug-flow tank, which is divided into four sections with capacities of V i and T i [18]. The temperature order is not represented by temperature inversion. For a specific time period, heat is extracted from a specific volume of fluid ( V h ), which is equal to ( mh ) D t/ r at temperature T. h Assuming T h > T L, a new section is added to the top of the tank, and the profile is transferred to the new section. At the same time, the inflow volume from the section Load is equal to ( mh ) D t/ r at temperature T. When T < T, a section is L L 4 added to the bottom of the tank, and the profile is retransferred. The transfer of the profile network in the tank is equal to the difference between the total volume of the temperature generator and the volume of the consumed fluid, or ( m h m L) t / r. The mathematical equations of the plug flow of the storage tank are as follows: Average water temperature inside the tank to be used: T D VT h h + ( VL Vh) T1 =. V L (57.9) The overall mortality rate of tanks: N Q = ( UA) ( T T ). env i i env i=1 (57.10)

8 642 A. Kaabi Nejadian et al. Fig Theoretical model of a storage tank with fixed volume Energy input to the tank from the hot fluid flow: Q = m C ( T T ). in h p h R (57.11) Energy available for consumption: Q sup = m LCp( TD TL). (57.12) Tank internal energy changes: E = C TV TV. N N ρ p i i i i t= TIME 0 i= 1 i= 1 (57.13)

9 57 Optimizing Solar Hot Water Systems (Closed Systems) Definition A definition of SF is introduced before discussing the optimization problem Solar Fraction This value is defined as the contribution of collector and thermal storage tank in total energy needed by cooling system; notably, the electric power of the pump is omitted. In general, different works have introduced different definitions of SF. The definition proposed in this work is as follows: q Solar fraction = load q load aux q = m C ( T -T ) load w pw in out. q (57.14) (57.15) 57.5 Optimum Diagram of a Solar Hot Water System Structural Optimum (Series/Parallel) The optimization of a solar hot water system can be approached in two ways: (1) optimization of the structure of the system (type of tank; multi-layer, single-layer) and (2) optimization of other components and features such as fluid discharge, area of collector, and volume of storage tank (Fig. 57.3). The structure of the system was divided into five models for optimization. The basis of optimization is the SF. The five models are as follows: 1. Multi-layer tank with parallel auxiliary hot water system 2. Single-layer tank with series auxiliary hot water system 3. Full-mix tank with parallel auxiliary hot water system 4. Full-mix tank with series auxiliary hot water system 5. Multi-layer tank with inside auxiliary hot water system The optimum model is assumed to have minimum energy consumption by the auxiliary hot water system or maximum SF. Other details of the system (i.e., area of the collector, discharge rate, storage tank capacity, and capacity of the auxiliary hot water system) are assumed unchanged. Table 57.1 lists the results of the five models based on SF for a year in the city of Tehran.

10 644 A. Kaabi Nejadian et al. Fig Optimum diagram of a solar hot water system Table 57.1 Solar fraction for the different models Model Solar fraction As indicated in Table 57.1, maximum SF was observed in model 1 and with small difference in model 3. Model 1 was not a good option given that the cost of building or buying a multi-layer tank is much more than that of a single-layer tank or a full-mix tank. Furthermore, the difference of SF between the two models was trivial. Model 3 was adopted as the optimum structure of a solar hot water system (Fig. 57.1). Here, having the structure of the solar hot water system optimized, the area of the collector, the discharge rate, the capacity of the storage tank, and the set point temperature (TSP) of the auxiliary hot water system are optimized.

11 57 Optimizing Solar Hot Water Systems (Closed Systems) 645 Fig Absorbed heat based on the collector slope angle (Bushehr city) Optimization of the Components Optimization of the Collector Angle The slope angle of the collector was set based on the absorbed temperature by the collector throughout the cooling phase for the climate of Bushehr city in the latitude of 30 (Fig. 57.4). Based on the experiment, the slope angle of the collector ranges from 1 to 5 higher than the latitude of the site. Clearly, the optimum angle is 31, which is consistent with the above Optimization of the Pump Mass Discharge Rate Two parameters need to be adjusted for the optimization of the mass discharge rate: SF and regeneration. The discharge rate cannot be optimized by keeping one of the parameters fixed as one parameter considerably influences the other. Figure 57.5 shows the optimized mass discharge of the right loop. The mass discharge rate of the solar collector was optimized using SF measure, after the optimization of the discharge rate of the right loop comprising auxiliary hot water system, convertor, and storage tank. Figure 57.6 illustrates the variation in SF based on mass discharge rate. The maximum SF of the solar hot water system is obtained for the discharge rate of 0.4 kg/s. It is notable, however, that the left loop was optimized first.

12 646 A. Kaabi Nejadian et al. Fig Effect of mass discharge on regeneration temperature and solar fraction Fig Effect of mass discharge rate of the collector on solar fraction

13 57 Optimizing Solar Hot Water Systems (Closed Systems) 647 As shown in the figure, the variation in mass discharge rate is trivially effective on SF, while SF is the basis of optimization in the solar desiccant cycle Optimization of the Area of the Collector and the Capacity of the Storage Tank There are two ways to optimize the area of the collector and the capacity of the storage tank. First, both components are optimized at the same time, so that the area of the collector is set and then the SF is obtained based on the capacity of the tank. Afterward, the area is adjusted and the process is repeated. The area is adjusted until the minimum capacity of the SF tank is obtained. In the second method, which is more accurate, the area is adjusted based on SF (by assuming a capacity for the storage tanker). The area for which the maximum area of SF is obtained is recorded, and then the capacity of the storage tank is adjusted based on the obtained area. Afterward, the maximum SF of the storage tank is recorded, and then the capacity of the new storage obtained in the last stage is replaced with the new capacity of the tank, and the area is readjusted to obtain the maximum area of SF. Then, using the optimum area and adjusting the capacity of the storage tanker, the maximum SF is obtained. The process is repeated until the difference between the obtained capacity of the storage tanker of two consecutive turns is close to zero (Figs ). At this point, the obtained area and capacity are optimum. It is notable that we started by choosing a preliminary discharge rate, Fig Solar fraction based on the area of the collector

14 648 A. Kaabi Nejadian et al. Fig Solar fraction based on the capacity of the storage tank Fig Solar fraction based on the area of the collector

15 57 Optimizing Solar Hot Water Systems (Closed Systems) 649 Fig Solar fraction based on the volume of the storage tank Fig Solar fraction for the area of the collector

16 650 A. Kaabi Nejadian et al. Fig Optimization flowchart of the area of the collector and the capacity of the tank based on solar fraction which is almost relative to the discharge rate of the fluid on the desiccant cycle, and SF is set to 0.5. First, the optimum area is obtained for a storage capacity of 2 m 3. With an area of 100 m 2, the optimum capacity for the tank is obtained (Fig. 57.8). The area is recalculated for a capacity of 3 m 3 (Fig. 57.9). In Fig , the capacity of the tank is obtained for a collector area of 85 m 2. For the third turn, the area is obtained for a capacity of 4 m 3 (Fig ). Thus, the optimum area and capacity are 84 m 2 and 4 m 3, respectively, as shown in Fig Optimization of TSP of the Auxiliary Hot Water System Optimizing TSP is one of the critical and most effective steps in the optimization of solar hot water systems. Taking into consideration that economic optimization is not a concern here, SF was set to 0.5, based on which the temperature of the auxiliary hot water system (Fig ) was obtained. However, the designer cannot make an accurate optimization regarding regeneration temperature as there are cycles with nonsolar regeneration, and there is always a fixed temperature. The generator of heat is either gas or an electric torch, which enables the designer to set the best temperature. On the other hand, the solar temperature generator varies throughout the day and the only way of modification by the designer is to set the TSP, which is the minimum required temperature. Another serious challenge ahead of the designer is the high cost of a solar hot water system.

17 57 Optimizing Solar Hot Water Systems (Closed Systems) 651 Fig Optimization of the set point temperature of the auxiliary hot water system based on solar fraction (SF) 57.6 Simulation Results A solar hot water system was simulated and analyzed regarding dynamic and timeto-time performance in TRNSYS. As mentioned above, the optimization of the system was discussed. Here, the performance of different components of the solar hot water system for an apartment located in Bushehr city with an area of 115 m 2 for 48 h in August is analyzed. The performance of the cycle after optimization is as follows: Area of collector: 86 m 2 Angle of collector: 31 Water discharge rate of the right loop: 0.2 kg/s Water discharge rate of the left loop: 0.25 kg/s Capacity of the storage tank: 4 m 3 Auxiliary hot water system temperature ( T setpoint ): 75 C References 1. Riffat SB, Zhao X, Doherty PS (2005) Developing a theoretical model to investigate thermal performance of a thin membrane heat-pipe solar collector. Appl Therm Eng 25(5e6):899e Bong TY, Ng KC, Bao H (1993) Thermal performance of a flat-plate heat-pipe collector array. Sol Energy 50(6):491e498

18 652 A. Kaabi Nejadian et al. 3. Ezekwe CI (1990) Thermal performance of heat pipe solar energy systems. Sol Wind Technol 7(4):349e Bojic M, Kalogirou S, Petronijevi K (2002) Simulation of a solar domestic water heating system using a time marching model. Renew Energy 27(3):441e Jones BW, Golshekan M (1989) Destratification and other properties of a packed bed heat store. Int J Heat Mass Transf 32(2): Sorour MM (1988) Performance of a small sensible heat energy storage unit. Energy Convers Manage 28(3): Buckles WE, Klein SA (1980) Analysis of solar domestic hot water heaters. Sol Energy 25(5): Michaelides IM, Wilson DR (1997) Simulation studies of the position of the auxiliary heater in thermosyphon solar water heating systems. Renew Energy 10(1): Wongsuwan W, Kumar S (2005) Forced circulation solar water heater performance prediction by TRNSYS and ANN. Int J Sustain Energy 24(2): Furbo S, Vejan NK, Shah LJ (2005) Thermal performance of a large low flow solar heating system with a highly thermally stratified tank. ASME J Sol Energy Eng 127: Nayak JK, Amer EH (2000) Experimental and theoretical evaluation of dynamic test procedures for solar flat-plate collectors. Sol Energy 69(5): Tsilingiris PT (1996) Solar water-heating design a new simplified dynamic approach. Sol Energy 57(1): Kikas NP (1995) Laminar flow distribution in solar systems. Sol Energy 54(4): Prapas DE, Veliannis I, Evangelopoulos A, Sotiropoulos BA (1995) Large DHW solar system with distributed storage tanks. Sol Energy 55(3): Fanney AH, Klein SA (1988) Thermal performance comparisons for solar hot water systems subjected to various collector and heat exchanger flow rates. Sol Energy 40(1): Chiou JP (1982) The effect of non-uniform fluid flow distribution on the thermal performance of solar collectors. Sol Energy 29(6): Klein SA, Beckman WA (1979) A general design method for closed loop solar energy systems. Sol Energy 22: Klein SA et al (2004) TRNSYS Version. 16, Solar Energy Laboratory, University of Wisconsin-Madison

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