Journal of Scientific LIN & Industrial Research et al: FUZZY CONTROLLER FOR A FORCED CIRCULATION SOLAR WATER HEATER Vol. 69, July 2010, pp. 537-542
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Development and applications of a fuzzy controller for a forced circulation solar water heater system Chern-Sheng Lin1*, Mon-Lian Lin1, Shuo-Rong Liou1, Hung-Jung Shei2 and Wen-Pin Su1 1
Department of Automatic Control Engineering, Feng Chia University, Taichung, Taiwan 2
Department of Mechanical Engineering, China Institute of Technology, Taipei, Taiwan Received 09 March 2010; revised 18 April 2010; accepted 20 April 2010
This study presents fuzzy control to improve solar heat absorbance efficiency (SHAE) of a forced circulation solar water heater (FCSWH) system, and simplify user settings. FCSWH system controls startup and stopping of circulating pump based on temperature difference between outlet and inlet of collector. Fuzzy control adapts to uncertainty of sunshine duration and modulates temperature difference to maximize SHAE. According to results, water temperature in heat storage cylinder increased by 3~5°C under fuzzy control system, showing a clear advantage in SHAE over manual control system. Keywords: Fuzzy control, Maximum heat energy transfer, Solar heat
Introduction Solar water heater system can be divided into natural circulation and forced circulation systems. Natural circulation exchanges cold water inside heat storage cylinder with hot water inside collector to accumulate heat energy in heat storage cylinder based on thermosiphon theorem. Forced circulation exchanges hot water inside collector with cold water inside heat storage cylinder through circulating pump. Thus, forced circulation solar water heater (FCSWH) system has both flexibility and esthetics at installation site. However, in traditional control of circulating pump, control point to start/stop pump is temperature difference, which is a fixed value between outlet and inlet. When there is limited sunshine on rainy days and temperature difference for starting circulating pump is significant, number of circulations will be very few, sometimes without even one startup, and therefore there will be low solar heat absorbance efficiency (SHAE). On the contrary, if there is adequate sunshine on sunny days, and temperature difference for starting circulating pump is small, circulating pump will be started frequently, which may waste energy in pumping operation. This study used fuzzy control1-3 to adapt to sunshine duration, and modulate temperature difference of control point for starting and stopping pump to optimize SHAE and energy saving of the system4. *Author for correspondence E-mail:
[email protected]
Experimental Section Forced Circulation Solar Water Heater (FCSWH) System
FCSWH system is composed of a heat storage cylinder, a collector, a collector outlet thermometer Toutlet , a collector inlet thermometer Tinlet and a circulating pump. Control point of circulating pump startup5 is ∆THigh and control point of circulating pump stop is ∆TLow ; when Toutlet − Tinlet = ∆T ≥ ∆THigh , pump starts up circulation. Hot
water inside collector flows to heat storage cylinder through outlet6-8, and cold water inside heat storage cylinder flows to collector through inlet. When Toutlet − Tinlet = ∆T < ∆TLow , circulating pump stops (Fig. 1).
Fig. 1—Forced circulation solar water heater system
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Input variable, Temp.(oC) (a)
Input variable, Rate (min) (b)
Output variable, ∆ (c) Fig. 2—Membership function of: a) Tinlet ; b) Toutlet _ rate ; and c) ∆THigh Fuzzy Control Model Determination of Variables for Calculating ∆THigh
Let temperature Tinlet at inlet of collector is first input variable. Temperature change rate Toutlet _ rate = dToutlet / dt at outlet is second input variable9-11. ∆THigh is threshold temperature difference value for starting up circulating pump startup. Set up Rulebase
Rulebase of fuzzy control is set up according to physical system, and fuzzy set in domain is defined by LN [large negative (very small)], SN [small negative (small)], ZR [zero (moderate)], SP [small positive (large)] and LP [large positive (very large)]. For
collector inlet temperature Tinlet , subdivision to Tinlet variables Tinlet _ sub is defined as
Tinlet _ sub
LN temp ≡ Tinlet < 60°C SN temp ≡ 50°C < Tinlet < 70°C = ZRtemp ≡ 60°C < Tinlet < 80°C SP ≡ 70°C < T < 90°C inlet temp LPtemp ≡ Tinlet > 90°C …(1)
Here, triangular membership function for Tinlet and Toutlet _ rate was used (Fig. 2). For Toutlet _ rate , subdivision to Toutlet _ rate variables, Toutlet _ rate _ sub (oC/min) is defined as
LIN et al: FUZZY CONTROLLER FOR A FORCED CIRCULATION SOLAR WATER HEATER
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Fig. 3—Function surface of fuzzy control rulebase
Subdivision to ∆THigh variables ∆THigh _ sub is given as a)
∆THigh _ sub
LN ∆T ≡ ∆T < 6°C SN ≡ 4°C < ∆T < 8°C ∆T = ZR∆T ≡ 6°C < ∆T < 10°C SP ≡ 8°C < ∆T < 12°C ∆T LP∆T ≡ ∆T > 12°C …(3)
Practical FCSWH system defines fuzzy control Rulebase by membership function (Fig. 3) of input
b)
variable Tinlet , Toutlet _ rate and output variable ∆THigh (Table 1). Determination of ∆TLow Fig. 4—Schematic diagram of: a) Collector captures heat energy; b) Solar heat energy transfer
LNrate ≡ Toutlet _ rate < 1.5 SN ≡ 0.5 < T outlet_ rate < 2.5 rate ZR Toutlet _ rate _ sub = rate ≡ 1.5 < Toutlet_ rate < 3.5 (unit : °C / min) SPrate ≡ 2.5 < Toutlet _ rate < 4.5 LPrate ≡ Toutlet _ rate > 4.5
Sunshine duration is a time-varying function W (t ) , collector captures solar heat (Fig. 4a). So collector absorbs solar heat energy as
∫
H = W (t )dt
…(4)
Water inside collector may cause a temperature rise after absorbing heat energy as
…(2)
∆T =
H m*S
…(5)
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Time (h,min) Fig. 5—Schematic diagram of outlet/inlet temperature difference
where, m , quality of water inside collector; S , specific heat of water. In case of Toutlet − Tinlet = 0 , all absorbed solar heat energy is transmitted completely into heat storage cylinder (Fig. 4b). However, in an ideal situation, collector absorbs solar heat energy, so actual temperature on collector will be Toutlet − Tinlet ≥ 0 during circulation, and temperature difference will increase as collector area and sunshine duration increase, Toutlet − Tinlet > 0 . Traditional control mode sets a fixed ∆TLow as control cut-off point of pump circulation, which is unlikely to maximize heat energy transfer. If ∆THigh = 12°C and ∆TLow = 2°C , when Toutlet − Tinlet ≥ 12°C , circulating pump is started (Fig. 5). If collector area or sunshine duration is too large, there is still a temperature difference of Toutlet − Tinlet > 2°C = ∆TLow between collector outlet and inlet after a period of circulation. Thus, collector outlet temperature change rate Toutlet _ rate = dToutlet / dt after circulation start is taken as the basis of circulation cut-off of circulating pump to maximize heat energy transfer Toutlet _ rate = dToutlet / dt , and low set value of ∆TLow , which will result in continuous running of circulating pump, can be avoided. Heat transfer ratio rh can be expressed as
Fig. 6—Curve diagram of water temperature of traditional control solar water heater system Table 1—Fuzzy control rules LN temp
SN temp
ZRtemp
SPtemp
LN rate
LN ∆T
LN ∆T
SN ∆T
ZR ∆T
ZR ∆T
SN rate
LN ∆T
SN ∆T
ZR∆T
ZR ∆T
ZR ∆T
ZRrate
SN ∆T
ZR∆T
ZR∆T
ZR ∆T
ZR ∆T
SPrate
ZR∆T
ZR∆T
ZR∆T
ZR∆T
SP∆T
LPrate
ZR ∆T
ZR ∆T
ZR∆T
SP∆T
LP∆T
[ W / m 2 ]; K C , shelter factor. Results and Discussion Traditional Control System
Traditional control system uses fixed ∆THigh and ∆TLow as control points of circulating pump to start and stop. In this paper, a SE-DTCR-01 type differential temperature controller is used for comparison. Practical set parameters are ∆THigh = 12°C and ∆TLow = 2°C . In an example of actual measured temperature data (Fig. 6), lower curve is temperature of heat storage cylinder and upper curve is collector outlet temperature. Middle curve is collector inlet temperature. Fuzzy Control System
t2
rh = (T final − Tinitial ) * C * Vtan k /[ K C * ∑ IR _ RAD (t )] t1
…(6) where T , initial temperature of tank[°C]; T final , initial final temperature of tank [°C]; C , specific heat capacity of water; V tan k , capacity of tank; t1 , t 2 , start and ending time; IR _ RAD (t ) , radiation intensity of sun light
LPtemp
Control point ∆THigh of circulating pump starting in fuzzy control system is determined by fuzzy control rulebase (Table 1). Control point of circulating pump stop is defined by dToutlet / dt ≤ 0.5°C / 10 sec . In actual measured temperature data (Fig. 7), blue curve is temperature of heat storage cylinder, pink curve is collector outlet temperature and yellow curve is collector inlet temperature. As shown by arrow headed
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Temp., °C
Heat transfer ratio, %
LIN et al: FUZZY CONTROLLER FOR A FORCED CIRCULATION SOLAR WATER HEATER
Time (h, min)
Initial temp., °C Fig. 7—Heat transfer rate of fuzzy control solar water heater system
Fig. 8—Water temperature in heat storage cylinders of fuzzy control and traditional control systems (2009/12/06)
Table 2—Meteorological data date
∆THigh [°C] #1
Cycle times of pump #1
∆THigh [°C] #2
Cycle times of pump #2
2009/12/02
2009/12/06
2009/12/16
4
12
10
19
14
16
4.7~8
8~12
4.68~9.08
15
12
14
light blue curve, black clouds result in a large decrease in sunshine duration, so increased temperature at hot water tap slows. However, fuzzy control modulates ∆THigh to capture heat energy in heat storage cylinder, and ∆THigh is adjusted when black clouds dissipate and sunshine duration increases.
storage cylinder of FCSWH system applying fuzzy control was increased by 14°C , whereas temperature of traditional control system was increased by 10°C. If capacity of heat storage cylinder of this system is 200 l, about 3344K J energy can be saved, equaling energy generated using a 4KW electric heater to keep heating for about 14 min.
Comparison of Traditional and Fuzzy Systems
Traditional system uses special controller of FCSWH system. Its control parameters are ∆THigh = 12°C and ∆TLow = 2°C . When temperature difference between collector outlet thermometer Toutlet and collector inlet thermometer Tinlet is Toutlet − Tinlet ≥ 12°C , pump begins circulation, and pump stops circulation when Toutlet − Tinlet ≤ 2°C . Value of pump starting circulation in fuzzy control system is determined by fuzzy control Rulebase (Table 1). Control point of pump stopping circulation is defined by dToutlet / dt ≤ 0.5°C / 10 sec . Meteorological data is shown in Table 2. Fuzzy control is adaptable to weather change (Fig. 7) when applied to solar heat energy capture, and fuzzy control applied systems (Fig. 8) have clear advantages in SHAE (increased water temperature in heat storage cylinder by 3~5°C). As for experiment dates, both were performed on cloudy days. Finally, temperature of heat
Conclusions Traditional control mode was replaced by fuzzy control so that system adjusted control parameters of circulating pump more efficiently according to weather conditions. Circulating pump circulated maximum heat energy to heat storage cylinder, and maximized service efficiency of pump. In addition, fuzzy control greatly simplified setting input of controller, providing good operational practicability. Acknowledgment This work was sponsored by National Science Council under Grant no. NSC 96-2221-E-035-028-MY3. References 1
Passino K M & Yurkovich S, Fuzzy Control (Addison Wesley Longman Inc., California, USA) 1998, 23-101.
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