SELECTION AND SIZING OF BATTERY ASSISTED PV PUMPING SYSTEMS IN RWH

Transcription

1 Chapter 4 SELECTION AND SIZING OF BATTERY ASSISTED PV PUMPING SYSTEMS IN RWH 4.1 General The requirement of a percentage of harvested rain water pumped up to the elevated upper tank in the cascading multi tank rain water harvesting (CMTRWH) model is inevitable in order to fulfill the demand as discussed in Chapter 3. Hence, steps should be taken to select the most suitable type of pump for the general scenario and optimize the operating parameters in order to maximize the performance efficiencies. Generally, two main types of pumps are in use, the centrifugal and the positive displacement pump which display different operating characteristics. In this chapter the two types of pumps and their input/output characteristics will be studied in detail in order to identify the most suitable pump for model under discussion. Pumping operation in the CMTRWH model follows a batch transfer process where a set quantity of water is pumped up from the ground level main storage tank as and when required at a time with a variable head as shown in the Figure 4.1 This can be achieved by use of a floater switch arrangement fixed to upper tanks. Figure 4.1: Batch transfer pumping with variable head 124

2 The minimum head occurs when the upper and lower tanks are full while the maximum head occurs during a prolonged period of non-rainy days, when the lower tank runs empty. As the system is automated through a floater switch mechanism to operate when the water level in the upper tank is low, a batch transfer of harvested rain water occurs. Further, it can be shown that for a typical household of four people with a per capita water usage of 200 Litres, the quantity of harvested rain water to be pumped up is approximately 235 Litres using a CMTRWH model as discussed in Chapter 3. This is when 40% of total water demand is met by rain water and the water saving efficiencies of the lower and upper tanks are of 85% and 50%, respectively. As such, it is clear that the pumping required is either at a higher flow rate for a shorter duration (typical of centrifugal pumping) or a lower flow rate for a longer duration (typical of positive displacement pumping) for a given head (H). Due to the relevance of the model for non-grid connected sites, solar power is selected as the mode of power supply. In addition, the two technologies (i.e. the photo voltaic technology and the rain water harvesting technology) are well matched in the CMTRWH model since solar pumping is required only on non-rainy days. As the output is direct current (DC) in both direct coupled or battery connected PV pumping systems, pumps run by DC motors are selected to avoid energy losses in inverters (typical losses, 5% to 10%). Further, it is observed that separately excited (permanent magnet) DC motors are more desirable as no additional current is drawn to energize the motor field windings. Therefore, to promote the energy independence from the grid connected power supply, PV technology is considered as the power source and the behavior of each type of pump in combination with PV technology has been studied in depth. In addition, the performance and the system reliability of the power source have also been studied in depth and the behavior of the pumping system when operated with a backup storage accumulator (battery) was analyzed. Finally, the most suitable type of pump and the power source was identified for the RWH model under discussion to supplement the typical household water usage demand. 125

3 Once an appropriate type of pump is selected for the Stand Alone Photo Voltaic Pumping (SAPVP) system, with optimized output characteristics complying with Photo Voltaic (PV) output characteristics, proper sizing of the system has to be carried out to provide the optimum power output without compromising on system reliability. Taking into consideration the low wire to water energy conversion efficiency in PV systems and the temporal variations of solar radiation, a deeper understanding of the PV technology is needed to ascertain the system performance. This is more so, in situations where no grid power is available, and the performance of the load solely depending on the performance of the PV module and the associated storage battery capacity. Many SAPV situations require battery storage due to the need of stable supply of current, as well as to ensure a no load shedding scenario. Essentially, with any PV generator with a backup battery, the right sizing of the generator and storage capacities bear a greater impact on the system cost. In the tropical islands, meteorological parameters such as total cloud cover play a significant role in determining the amount of incident solar radiation at any location and the land mass is broadly demarcated into geographical regions based on annual rainfall figure. It is observed that due to well established seasonal rainfall patterns brought in by monsoons and convective clouds, the monthly average daily radiation values do not vary significantly within a said region. On the other hand, as the ambient temperatures remain stable with minimal fluctuations in the tropics, the effect of temperature variation on the performance of the battery is minimal. Hence, if a load independent graphical relationship can be developed between the capacities of the PV generator and the storage battery, for no load shedding scenarios, such can be used as a generic curve for a particular region defined by the rainfall pattern. Taking into account the percentage of power requirement generated from the PV module for the load and the variations in incident solar irradiation levels over a period of time, the need of a storage battery is discussed. Due to its cost effectiveness and wide usage in PV applications lead-acid batteries are identified as the storage battery in this study. 126

4 Power output characteristics of a lead acid accumulator (storage battery) is non linear and varies with the load characteristics. Besides the battery life is determined by the load as well as the duty cycle and the system has to be properly designed so as to match the PV generator and the accumulator with the load. This will ensure a non load shedding scenario indicating reliability in operation. Hence a generalized sizing curve is developed and validated for the wet and dry regions of Sri Lanka to facilitate the selection of a matching pair of generator and battery capacities for a non load shedding situation. Such a curve can be used as a useful design tool. 4.2 Types of pumps A pump is a contrivance which produces energy to a fluid in a fluid system. It assists to increase the pressure energy or kinetic energy or both of the fluid by converting the mechanical energy. Pumps can be broadly classified into two main types; Rotor dynamic pumps (Centrifugal pumps) Volumetric pumps (Positive displacement pumps) Both types of pumps, having different performance characteristics to each other, are widely used in water pumping scenarios and will be discussed for suitability in CMTRWH model operation Centrifugal pumps The centrifugal pump moves liquid by rotating one or more impellers inside a volute casing. The liquid is introduced through the casing inlet to the eye of the impeller where it is picked up by the impeller vanes. The rotation of the impeller at high speeds creates the centrifugal force that throws the liquid along the vanes, causing it to be discharged from its outside diameter at a higher velocity. This velocity energy is converted to pressure energy by the volute casing prior to discharging the liquid to the system 127

5 Centrifugal pumps, by far the most common type, is classified according to the direction of flow through the impeller, type of casing, working head, number of impellers per shaft, and number of entrances to the impeller and for case under review, a low lift (up to 15m head), single stage, single entry, radial flow, volute pump is considered. The above type is the least power consuming, least expensive and the most commonly available pump Hydraulic output of a centrifugal pump The hydraulic output of a centrifugal pump is given by; Where, P p = ρgqh mano {4.1] 1000 P p, hydraulic power out put of the pump in KW, H mano, the manometric head consisting of Static head (H s ), Friction loss head (H f ), and Output velocity head (V 2 /2g) in m. Q, Output flow rate (m 3 /s) ρ, Density of water, (1000 Kg/ m 3 ) g, Gravity (9.81 ms -2 ) Losses and efficiencies of a centrifugal pump When a centrifugal pump operates, various losses such as hydraulic losses (shock or eddy losses at the entrance to and exit from the impeller, losses due to friction in the impeller, friction and eddy losses in the guide vanes/diffuser and casing and friction and other minor losses in the suction and delivery pipes), mechanical losses (losses due to disc friction between the impeller and the liquid which fills the clearance spaces between the impeller and casing, losses pertaining to friction of the main bearing and glands) and leakage losses could occur affecting the overall efficiency of the pump. Therefore, the overall efficiency of the pump (η o ) is the product of the manometric efficiency (η m ), volumetric efficiency (η v ) and the mechanical efficiency (η m ) 128

6 Where, η m = manometric head Head imparted by impeller to liquid η v = Liquid discharged per second from the pump Quantity of liquid passing per second through the impeller η m = Power delivered by the impeller to the liquid Power input to the pump shaft (P s ) Thus, η o = η m. η v. η m = ρgqh mano [4.2] P s Where P s is the shaft power (input power from motor) In practice, the overall efficiency of a best designed centrifugal pump displays a value in the range of 70% to 75% while average pumps available for domestic use in Sri Lanka gives overall efficiencies of less than 40% in most cases. It can be shown that for a centrifugal pump, the overall efficiency increases with the increase of the diameter of the impeller and the pump speed, N, and that the highest efficiency occurs only when the pump operates at the rated Best Efficiency Point (BEP) for a given speed, N Advantages of centrifugal pumps over PD pumps The centrifugal pump claims the following advantages with reference to a positive displacement pump. Discharging capacity of a centrifugal pump is much greater than a PD pump Its performance characteristics are superior It can be operated at very high speeds without any danger of separation and cavitation It can be directly coupled to an electric motor The torque on the power source is uniform; so is the output from the pump 129

7 It is compact and has smaller size and weight for the same capacity and energy transfer The cost of a centrifugal pump is less as it has fewer parts Installation and maintenance are easier and cheaper Limitations of centrifugal pumps As discussed in this study, the output characteristics of centrifugal pumps depend on the motor speed and the impeller diameter. Hence, the overall efficiency (η o ) increases with the increase in both the above parameters producing a higher output flow rate. Thus, for pumps with smaller power ratings, lower heads are preferred, limiting the use of such pumps in multi-story building situations, unless multi-staged. It is noted however that as the pump delivers only after attaining a speed to overcome the static head, lower speeds can only be achieved by sacrificing the total lift Positive Displacement pumps Positive Displacement (PD) pumps operate with a series of working cycles where each cycle encloses a certain volume of fluid and moves it mechanically through the pump into the system. Depending on the type of pump and the liquid being handled, this happens with little influence from the back pressure on the pump. While the maximum pressure developed is limited only by the mechanical strength of the pump and system and the driving power available, the effect of that pressure is controlled by a pressure relief or safety valve. A major advantage of the PD pump is that it can deliver consistent capacities because the output is solely dependent on the basic design of the pump and the speed of the motor. This means that, if the liquid is not moving through the system at the required flow rate, it can always be corrected by changing one or both of these factors. 130

8 There are a number of different types of PD pumps, namely the reciprocating pump (piston pump), gear and lobe pump, screw pump and progressive cavity pump, but for simplicity in calculations and for comparison with centrifugal pumps, only the reciprocating pump type is discussed. The reciprocating pump is a positive displacement (PD) pump as it sucks and raises the liquid by actually displacing it with a piston/plunger that executes a reciprocating motion in a closely fitting cylinder. The amount of liquid pumped is equal to the volume displaced by the piston. The pumps designed with disk pistons create pressures up to 25 bar and the plunger pumps build up still higher pressures. Discharge from these pumps is almost wholly dependent on the pump speed. The total efficiency of a reciprocating pump is about 10% to 20% higher than a comparable centrifugal pump. For this research, only the single acting, single cylinder pump is analyzed from among the various other types of piston pumps Hydraulic output of a PD pump The hydraulic output of a positive displacement pump is given by the following: P p = ρgq(h s +h d ) [4.3] 1000 Where, P p, hydraulic power out put of the pump in KW,. Q, Output flow rate (m 3 /s), Q = ALN/60 Where, A cross sectional area of the piston L Length of the stroke N Speed of the crank, rpm h s and h d are suction and delivery heads ρ, Density of water, (1000 Kg/ m 3 ) g, Gravity (9.81 ms -2 ) 131

9 Losses and efficiencies In a reciprocating pump, the actual discharge, (Q act ) is always slightly different from the theoretical discharge (Q th ) due to; Leakage through valves, glands and piston packing Imperfect operation of the valves (suction and discharge) Partial filling of the cylinder by liquid The coefficient of discharge (C d ) of the PD pump is denoted by, C d = Actual discharge = Q act [4.4] Theoretical Discharge Q th The volumetric efficiency of the PD pump is the coefficient of discharge (C d ) expressed as a percentage. Volumetric efficiency of a PD pump, which can also be considered as the pump overall efficiency, depends upon the dimensions of the pump and its value ranges from 85% to 98% Advantages of PD pumps over centrifugal pumps Unlike in centrifugal pumps where a number of variables dictate the overall efficiency, only the volumetric efficiency (which is governed by the cylinder-piston assembly design), indicate the overall efficiency of a PD pump. As such PD pumps are typically 15% to 20% more efficient than a centrifugal pump of similar power rating. The capability of delivering at low pump speeds is one of the major advantages displayed by the PD pump. This is in contrast to a centrifugal pump which needs a minimum rotational speed for the impeller output head to overcome the static head. Therefore, PD pumps can be used in low power input situations as well as when the power input is variable. As such delivery reliability of PD pumps are much higher in Photo Voltaic (PV) pumping situations. 132

10 Although centrifugal pumps are superior in higher output flow rates, PD pumps are much superior in achieving higher delivery heads at low output rates. Therefore, in overcoming higher heads, such as that found in multi-story building situations, multi-staging is not required compared to the case of using centrifugal pumps Limitations of PD pumps Apart from high capital and maintenance costs incurred in employing PD pumps, the risk of separation at higher speeds, limits the maximum operating speed of the motor, limiting the delivery output flow rate. As the piston accelerates at the beginning of the suction stroke and at the end of the delivery stroke, the absolute pressure heads created at such points should not fall below the vapor pressure to avoid separation and loss of efficiency. As acceleration is a function of motor speed, the maximum speed of a PD pimp is limited. 4.3 Photo Voltaic (PV) pumping Power source for all types of pumps can be grid power or any renewable energy source when grid power is unavailable. However, for reasons discussed above, employing solar power through PV technology and the use of Direct Current (DC) motors to run the pump will be analyzed Characteristics of photo voltaic pumping systems Many researchers have studied the performance of photovoltaic powered water pumping systems (PVPS). The results of several experimental studies and theoretical analyses of PVPS have been published. Normally, the solar water pumping system consists of three components: the PV array, the direct current (DC) motor and the pump. Each component has its own operating characteristics. They can be identified as the I-V characteristics for the PV array and the DC motor, and the torque-speed characteristics for the motor and pump. The DC motor drives the pump whose torque requirements vary with the speed at which it is driven. The 133

11 DC motor is operated by the power generated from the PV array whose I-V characteristics depend non-linearly on the solar radiation variations and on the current drawn by the DC motor. For the efficient operation of the system, the two sets of PV output and DC motor input characteristics should be matched. Electronic matching devices known as Maximum Power point Trackers (MPPT) allow solar pumps to start and run under low-light conditions. This permits direct use of sun s power without bothersome storage batteries. Figure 4.2: Components of a PV pumping system Bany and Appelbaum (1979) analyzed a direct coupled PV pumping system under steady state conditions. The starting characteristics of a DC motor and pump powered by a PV array without maximum power point tracker (MPPT) have been examined by Bany and Appelbaum (1979). The solar cell modules can only provide maximum power at specific voltage and current levels. So, for the PV array, there is a unique point on its I-V curve at which the power is at its maximum value, and for optimum utilization, the equilibrium operating point of the PV array should coincide with this point. However, since the maximum power point varies with radiation and temperature, it is difficult to maintain optimum matching at all radiation levels, except for a specially designed DC motor. In order to improve the performance of a PV pumping system, a DC-DC converter known as a maximum power point tracker (MPPT) is used to match continuously the output characteristics of a PV array to the input characteristics of a DC motor. The MPPT normally consists of a power electronic circuit controlled by a signal circuit, which drives 134

12 the power electronic circuit to force the PV array to operate at its maximum power point. Under such conditions, the MPPT will improve the efficiency of a PVPS. Any off-the-shelf water pump allows itself to be powered by Photo Voltaic panels in some way or other and turned into a solar water pumping system. The most common pumps used for this purpose are centrifugal, positive displacement and Helical Rotor pumps. Some are matched with AC, others with DC motors. If a pump has an alternating current (AC) motor, an inverter would be required to convert the DC electricity produced by the solar panels to AC electricity. Due to the increased complexity and cost, and the reduced efficiency of an AC system, most solar-powered pumps have DC motors. Solar modules, usually the greatest expense item in any solar design, convert sunlight into electricity quite inefficiently by 14% on average. A highly productive, cost effective solar water pumping system therefore, will require careful matching of all component parts Direct coupled PV pumping Direct coupled PV pumping where the pump motor is directly coupled to the PV module is employed when only day time pumping is required. Since the output of the pump depends on the variation of meteorological and geographical parameters, the performance of the pump with regard to the output characteristics of the PV module should be investigated. Chart 4.1 gives the output characteristics of a PV module. 135

13 Chart 4.1: Output characteristics of a PV module As both output voltage (V) and current (I) vary non-linearly with the variation of solar radiation, the resultant motor speed fluctuates with time Direct coupled centrifugal pumps Centrifugal pumps are conventional, faster and deliver higher quantities, but needs to operate at higher speeds and at low Total Dynamic Heads (TDH). Smaller, stand alone, PV pumping units of less than 500W do not usually employ centrifugal pumps, except for at the lowest heads. For any significant lift, multi staging is required and even then the low specific speed required and the size of impeller, inevitably lead to efficiencies of 25% 30% (wire to water) compared with 65% 70% for the best positive displacement units. Centrifugal pumps are of surface mounted and submersible types. Run at variable speed from a permanent magnet DC motor, the centrifugal pump does have one particular advantage, in that its power demand curve matches well to the I-V characteristic of the PV array (Roger, 1979, Akababa, 1998). With increasing power from the array, current and voltage will increase until there is just enough voltage (hence rotational speed) for the 136

14 pump to overcome the static lift and start to deliver water. As can be seen in the Chart 4.2 flow output is rapidly reduced, for a given voltage, as head is applied (Bany & Applebaum, 1979) Chart 4.2: Characteristic curves for centrifugal pumps One of the disadvantages of a centrifugal pump is that it has to operate at a high enough rpm to push the water all the way out of the well/tank. If it is cloudy and the solar array is not producing enough power, the pump/motor may be turning but not fast enough to do this. Use of a tracker is highly recommended with a centrifugal pump since it increases the solar array s power output over a longer period of time which increases the daily volume of water delivered. Centrifugal pumps do not work efficiently below 25 L/min, but their performance drops off disproportionately at reduced speeds (under low light conditions). Also, conventional pumps use AC motors that do not work at reduced voltage. One solution to these problems involves the use of storage batteries and a conventional AC 137

15 pump. Energy accumulates over time in the batteries and is discharged quickly to run the pump for short periods. A battery system complicates the installation, operation and maintenance of a system and loses 20% of the stored energy. Operation of the AC pumps with DC power requires an inverter. The inverter adds cost and complexity and increases energy requirements by an additional 10%. The most efficient low volume, non battery systems use positive displacement DC Pumps Direct coupled Positive Displacement pumps Unlike the centrifugal pump, the positive displacement pump will produce discharge flow whenever it is rotating. Positive Displacement pumps are of several different types, namely; Diaphragm, Rotary Vane Piston and Jack pumps. They are available in a wide range of sizes from 1 HP down to an incredible 0.1 HP. The low power pumps offer cost savings due to smaller PV arrays and reduced pipe size (Pipe size is minimized by low rate pumping). Positive displacement pumps however require higher starting torque (current) and are usually coupled to the PV array through a MPPT. MPPT or Linear Current Boosters (Solar pump controllers) deliver high current even in low light conditions by increasing the current at the expense of lower voltage. This allows pump operation throughout the solar day, however slowly, even in moderately cloudy conditions. As long as sufficient current is available to overcome the torque required to start the pump, water will be discharged even at very low irradiance such as early in the morning or under cloud cover. However, it is worth noting that, for a particular irradiance, increasing the head from Low Head (LH) to a High Head (HH), leads to an increase in current being drawn, resulting in the pump running at high voltage, with consequence danger of over speed, particularly ay lower heads as shown in Chart

16 Chart 4.3: Characteristic curves for positive displacement pumps System sizing in PV pumping The hydraulic energy required (kwh/day) = volume required (m 3 /day) x head (m) x water density x gravity/ (3.6x10 6 ) = x volume (m 3 /day) x head (m) [4.5] The solar array power required (kw p ) = Hydraulic energy required (kwh/day) [4.6] Av. daily solar irradiation (kwh/m 2 /day x F x E) Where F = array mismatch factor = 0.85 on average and E = daily subsystem efficiency = (typically) 139

17 4.3.4 Performance comparison of pumps in PV pumping From the above analysis it can be seen that the centrifugal pump at low radiation levels and the PD pump at higher radiation levels are not reliable in operation. As the incident radiation levels fluctuates, the operating efficiency of the pump will widely differ from the Best Efficient Point (BEP) and the overall efficiency of the pumping system will be much less. At the extreme scenarios, the pump may not have the sufficient impeller speed to overcome the static head resulting in non-delivery. Due to the inherent low efficiency of the centrifugal pump, even the low heads therefore will not be serviceable at low radiation levels. On the other hand, once the starting torque is overcome, PD pumps operate even at low radiation levels as the pump discharge is solely depend on the cylinder dimensions and the motor rpm. PD pumps tend to display separation problems at higher speeds as discussed above causing loss of efficiency and pump damage. 4.4 Battery assisted PV pumping To prevent the output fluctuations and the resultant motor speed variations, the excess output from the PV module is stored in an accumulator (battery) which can be used to supply the pump in low radiation situations. 140

18 Figure 4.3: Direct and In-direct solar pumping Lead-acid batteries with a nominal voltage of 12V are typically employed while higher voltages can be achieved by series coupling of several batteries. A charge regulator is required to cut-off charging and discharging at specific voltages in order to prevent overcharging and subsequent gassing and to prevent draw-off below maximum Depth of Discharge (DOD), both conditions shortening the battery life rapidly. The battery capacity is given in Ampere-hours (Ah) and the battery connected PV system sizing for service reliability will be discussed in detail. As the centrifugal pump requires a higher operating speed to overcome the static lift, such pumps draw-off a higher current from the battery in addition to the current drawn-off to overcome the starting torque load of the motor. It is observed that rapid discharging causes the battery internal resistance to increase and the resultant voltage drop prolongs the duty cycle of the battery. Moreover, as the charging current from the PV module is low and variable over time, recovery after a deep discharge is low, thus shortening the life span of the battery. 141

19 PD pumps on the other hand can be operated on a lower current, though at a lower output rate, over a much longer period of time while maintaining a lower depth of discharge prolonging the battery life. 4.5 Components of a SAPVP system Components of a Stand Alone Photo Voltaic Pumping (SAPVP) system comprise mainly of the PV module or array, the charge accumulator, motor and pump as indicated in Figure 4.4. In direct coupled PV pumping situations, the battery is eliminated and the system operates as long as solar irradiation is available. Figure 4.4: PVP system: B-Battery, M-Motor, P- Pump PV generator The solar module or the PV part is the heart of the whole PV system. The solar module is composed of several individual PV cells connected in series or parallel. Primarily the number of individual cells within a module and the arrangement of these cells in the module influence the energy produced by a PV module. The cells can be arranged in a module to produce a specific voltage and current to meet the particular electrical requirements. Similarly the PV modules can be arranged to form a solar array to produce a specific voltage and current. 142

20 4.5.2 Charge accumulator (Battery) Direct coupling of the array and the load is certainly the cheapest of all PV systems. A directly coupled PV pumping system is composed of a PV array directly connected to a DC motor driving a pump. Thus, such a system is simple, reliable, and low cost because it does not include battery storage or a battery voltage regulator. However, Anis et.al (1994) reported that a load composed of a DC motor driving a centrifugal pump represents a non-matched load to the PV array. This is because the motor driving a volumetric pump requires an almost constant current, for a given head, apart from the starting current that tends to be higher. This condition does not match the PV array characteristics where the current varies almost linearly with solar irradiance. On the other hand, the high cost of PV energy dictates that even a small percentage increase in total system efficiency will result in a significant economic impact. Batteries are used in most PV systems to perform two essential functions. i.e. as a power buffer between the array and loads, and as an energy storage bank. In addition, batteries provide a more stable system voltage (Perez, 2000) Battery selection is often a trade-off between price and the desired performance. Whilst tubular-plate, flooded-electrolyte batteries generally give the best performance in terms of service life, they may not always be affordable to the user. Thus, flat-plate, lead-acid batteries are often used Performance characteristics of a lead-acid battery Common use of the word, "battery" in electrical terms, is limited to an electrochemical device that converts chemical energy into electricity, by a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes of different metals or metal compounds (an anode and a cathode) and an electrolyte (usually acid, but some are alkaline) solution. A "Battery" is two or more of those cells in series, although many types of single cells are usually referred to as batteries - such as flashlight batteries. 143

21 A typical lead-acid battery consists of metal electrodes and the electrolyte which in chemical reaction produces an electric current. Practically all batteries used in PV and all but the smallest backup systems are Lead- Acid type batteries. Even after over a century of use, they still offer the best price to power ratio. A few systems use NiCad, but these types are recommended in cases where extremely cold temperatures (-27 0 C or less) are common. They are expensive to buy, and very expensive to dispose of due the hazardous nature of Cadmium. Battery capacity typically given in Ampere-hours (Ah), is reduced as temperature goes down, and increased as temperature goes up. If the battery undergoes a significant temperature drop, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at 25 0 C. At approximately C, battery Ah capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures, for example at C battery capacity would be about 12% higher. Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at C to 2.3 volts per cell (13.8 volts) at 50 0 C for a six cell battery. Therefore, temperature compensation should be considered when charging batteries, subject to wide temperature variations. Some charge controls have temperature compensation built in, which works fine if the controller is subject to the same temperatures as the batteries. Another complication is that large battery banks make up a large thermal mass. Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 0 C over 24 hours internally, even though the air temperature varies from 20 to 70 0 C. For this reason, external (add-on) temperature sensors should be attached to one of the positive plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature. Even though battery capacity at high temperatures is higher, battery life, defined as the number of duty cycles, is shortened. Battery capacity is reduced by 50% at C - but 144

22 battery life increases by about 60%. Battery life is reduced at higher temperatures - for every 10 0 C over 25 0 C, battery life is cut in half. This holds true for any type of Lead- Acid battery, whether sealed, gelled, or industrial. It should be noted that the specific gravity of the electrolyte, as does the voltage, increases with the decrease of temperature and vice versa Cycles vs. Life A battery cycle is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% to 20%, and then back to 100%. However, there are often ratings for other depth of discharge cycles, the most common ones are 10%, 20%, and 50%. It is important to note that, life of a battery is inversely related to its depth of discharge (DOD). Therefore, when DOD is low, a battery can undergo increased number of cycles and vice-versa. In other words, battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. It is observed that the most practical number to use is 50% DOD on a regular basis. Therefore in designing a system, it is desirable to have a DOD of average 50%, though occasionally the load can draw-off up to a higher DOD of the battery capacity. Also, there is an upper limit - a battery that is continually cycled 5% or less will usually not last as long as one cycled down 10%. This happens because at very shallow cycles, the Lead Dioxide tends to build up in clumps on the positive plates rather in an even film. Chart 4.4 shows how lifespan is affected by depth of discharge. The chart is for a Concorde Lifeline battery, but all lead-acid batteries will be similar in the shape of the curve, although the number of cycles will vary. 145

23 Cycle No. Depth of Discharge % (DOD) Chart 4.4: Battery life in No. of Cycles vs. Depth of Discharge (DOD%) Chart 4.5:12V Lead Acid Battery SOC vs. Voltage while under discharge 146

24 Battery failure Batteries self-discharge faster at higher temperatures. Lifespan can also be seriously reduced at higher temperatures - most manufacturers state this as a 50% loss in life for every 15 0 C over a 25 0 cell temperature. Lifespan is increased at the same rate if below 25 0 C, but capacity is reduced. This tends to even out in most systems, if they will spend part of their life at higher temperatures, and part at lower. At higher temperatures, increased number of cycles accelerates the change in chemical balances in the cell, thus reducing the internal chemical balance shortening the battery life and vice-versa Motor and pump In selecting a suitable pump/motor sub system for PV pumping, consideration should be given to the matching of input characteristics of the motor to that of output characteristics of the PV generator/battery. Typically, a permanent magnet DC motor is used in PV pumping situations where no excitation current is needed for start up. Accordingly, the energy generated by the PV array is partially saved. Since the torque exerted on the motor shaft is uniform, the centrifugal pump displays superior output characteristics. However, considering the variation in daily solar radiation incident on an array and the requirement of lower current required, positive displacement (PD) pumps are used in many PV pumping situations. Besides, as the efficiency of a PD pump mainly depends on its volumetric efficiency, they display greater efficiency than centrifugal pumps Inverter An inverter is used in PV pumping systems where the motor runs on alternative current (AC). The efficiency shown by typical inverters are in the order of 80% - 90%. 147

25 4.5.5 Charge controller A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types of regulators. 4.6 SAPV system sizing As more attention is given to PV systems as energy converters in place of fossil fuel, the need to optimize such systems in terms of performance and cost arises. This is more so in case of stand-alone PV systems where optimum system reliability is required at the lowest cost due to higher percentage of cost is incurred on the PV modules. The accurate sizing is an essential step to achieve these aims since it will ensure that demand is met and costs to be cut, thereby lending greater credibility to the practical use of PV systems in the renewable energies market SAPV sizing for Sri Lanka Based on the numerical-analytical approach described in literature review by Fragarki (2007), an attempt is made to derive a set of values for Sri Lanka, validating the isoreliability curves developed, so that such curves can be used for sizing SAPV battery charging systems independent of the load. As the demand in domestic SAPV systems can be safely considered a constant for any time t, the focus will be on the distribution of the minimum system capacity. By developing a sizing model using system variables in graphical form for a constant load for a given location, where the monthly mean global radiation is known, it can be used as a valuable design tool in optimum system designing. For Sri Lanka, though the geographical parameters vary insignificantly due to the small land mass covered, it is recognized that the meteorological parameters vary for different 148

26 climatic zones indicating different incident global radiation levels in each zone. Therefore, if two load independent sizing curves can be developed for the wet and dry regions of Sri Lanka it could be used reliably to estimate system capacities for optimum performance and at minimum cost for any given load Deriving SAPV sizing curves for Sri Lanka In deriving Stand Alone Photo Voltaic (SAPV) sizing curves for Sri Lanka, the generalized Battery Open Circuit Voltage (OCV) versus Battery State of Charge percentage (SOC %) for deep cycle lead acid batteries in discharge is used. (Chart 4.3, Perez, 2000) In the chart, C x denotes the magnitude of the discharging current in relation to the battery capacity. For example, for a battery capacity of 100 Ah, C 10 means a current of 10 Amperes (100/10) is drawn in discharging the battery. Hence the appropriate curve can be selected to determine the SOC% for a given measured battery voltage when the drawn discharge current is known Selection of system components A 75 W p, 12 V PV module with exposure area of 0.25 m 2 is selected which could capture on average 120 Wh per day. The peak current produced is 6.25 A (i.e. 75 W/12 V) which is used to charge the battery. As the charging rate is compatible with C 10 for a 100 Ah battery, C 10 is selected instead of C 5 as charging current fluctuates at lower level than 6.25 A due to change in incident radiation. A constant load of 80W is selected to run for 1.3 hours matching the array with a ratio of 1.00 (i.e. C A =1.00). The current drawn by the load can be calculated as 6.66 A (i.e.80w/12v) which is the discharging current for the battery and is approximately same as the charging current. A 12V lead-acid deep cycle battery of capacity 100Ah is selected as storage. Hence discharging is also on C 10 curve. Selecting the battery size for the prototype is done considering the number of cycles a battery of 50% SOC could support a given load. For tropical Sri Lanka, the maximum number of days for which lower radiation levels prevail, due to cloud cover, can be taken as approximately 5 (The 149

27 Department of Meteorology, Sri Lanka) on the basis of historical radiation and rainfall data of 10 years. For the prototype model set up in the dry region of Sri Lanka, a larger load was required to match the average annual generator capacity due to higher level of global radiation prevailing compared to the wet region. All the other parameters can be taken as same. Since the average annual generator capacity is 131 Wh/day, the constant was taken as 80W operating in 90 minutes. It is noted that C 10 curve is ideal due to lower disturbances to internal resistance of the battery as well as non-gassing while charging. Since charging is also on C 10 curve charging and discharging do not result in widely different internal resistances. Therefore selection of a 100Ah battery is justified for the research while the battery being of deep cycle type is suitable to supply the load down to 10% of SOC (90% DOD) Methodology A 12 V, 75 W p stand-alone PV module of exposure area of 0.25 m 2 (i.e. 0.5 mx0.5 m) with manufacturer specified conversion efficiency (η) of 10% is connected to a 100 Ah, 12V deep cycle lead-acid battery via a 10 Amp charge regulator. The battery is allowed to charge when solar radiation is available during the day and after allowing for a settling period of 1 hour for the battery to establish its chemical balance, a load of 115 Wh is connected to let the battery discharge. Before the load is connected, the open circuit voltage (OCV) of the battery at rest is measured and recorded using a digital voltmeter and after the regulated discharge, OCV is measured and recorded again at rest. The load is calculated to match the average annual generator capacity for the location (η*a*g h ) so that C A = ηag h /L is approximately G h, the average daily global radiation values for one year are calculated by taking hourly global solar radiation values using a Solarimeter at the two sites in Colombo and Anuradhapura in The hourly radiation values thus obtained are used to calculate the average daily solar radiation. The daily solar radiation values from the two sites are given in Appendix 3.2. In this case, the load equivalent to 115 Wh is taken as 4 numbers 20 W CFL bulbs running for 1.3 hours. The PV module is placed horizontally to minimize the tilt effect 150

28 hence to avoid monthly variations on incident global radiations. The measurements were taken for one full year to encompass all seasonal variations. Using the recorded values of Open Circuit Voltage (OCV), the SOC% of the battery as a percentage is derived from the generalized curves (Perez) for deep cycle lead-acid batteries. The SOC% is taken to the nearest round figure from which the maximum battery usage, in days, is calculated (C S = C U /L). In calculating C S, first the energy balance of the battery in Wh is calculated taking SOC% of 100 as the full battery capacity. Then, to make the reading independent of the load, energy balance is divided by the load thus obtaining the available battery capacity in days for the given load. The available battery capacity to support the given load for non load shedding situation in number of days, C S, is calculated by deducting the energy balance per unit load from the corresponding energy balance when the battery is at its full capacity. From all C S values calculated for the year, the maximum value for a given C A (in this case C A = 1.00) is selected and plotted against C A. Therefore, in effect C S is the minimum number of days a battery should support the load for a given generator capacity for a particular location. The exercise is carried out simultaneously for C A = 1.25, 1.5 and 2.00 with increased number of modules connected in series and the corresponding maximum C S values rounded off nearest to the next whole numbers are plotted against C A values. For C A = 2.00, 2 numbers of 75W p modules are used with a combined exposure area (A) of 1.00 m 2 and for C A = 1.25 and 1.5 the module surface is partially covered accordingly. Calculated values of C S for given C A values based on incident global solar radiation in Colombo and Anuradhapura from 1 st January to 31 st December 2009 is given in Appendix 3.2 Daily global radiation values are obtained using a pyronometer (solarimeter) equipped with PV technology to measure and digitally display the data. The meter was calibrated using measured daily global solar radiation data from the Colombo station of the Meteorological Department of Sri Lanka. All collected SR data are validated using SWERA data base which gives monthly average global solar radiation data for selected cities in Sri Lanka, including Colombo. Special precautions were taken with a blackened metal cover around the measuring equipment to prevent ground reflected SR affecting the final global SR values. However, it can be seen that SAPV sizing curve (C A vs. C S Chart) developed using SR values calculated from 151

29 rainfall correlation (Chapter 5) for calculating C A (Chart 4.7) closely resembling Chart 4.6 thus allowing development of sizing curves for any given location in tropical islands. Chart 4.6: SAPV sizing curves for the wet and dry regions of Sri Lanka CA Wet Region Cs (Days) Chart 4.7: SAPV sizing curve for the wet region of Sri Lanka when SR is calculated from Rainfall (RF) correlation 152

30 4.6.3 Developing C A vs. C S curves for Central Hills of Sri Lanka In the case of central hills of Sri Lanka, where the altitude is more than 750 m, two factors will have to be taken in to account. They are the higher percentage of cloud cover and lower temperatures. Though broadly categorized as located in the wet region, cloud formation towards the central hills is more due to evapo-transpiration from vegetation and lower wind speeds due to hilly terrain thus lowering the incident radiation levels marginally. The effect is marginal due to reduction in direct radiation compensated by diffuse radiation. Lower temperatures, particularly in the night, increases the internal resistance of the battery thus increasing the charging voltage while depressing the values recorded in the discharging OCV, lowering the SOC% values. Therefore, the calculated C S values could be marginally higher than that of the wet region. As the weather parameters such as the relative humidity, ambient temperature and wind speed change more frequently in that region, a longer series of data are required to accurately estimate C S values for C A = However, when C A 1.25, the corresponding values should coincide with that of wet region due to higher generator capacity compensating for lower K T values. To ascertain optimized values for C S for C A = 1.00, it is clear that data collected over long period of time is necessary. This time period should be a minimum of 12 years to cover the periodic climatic changes associated with solar flair-ups which are estimated to occur in 11 year cycles. 4.7 Summary Once rain water harvesting (RWH) system is integrated in to a building, a suitable pump, which will consume the least amount of energy accomplishing the given task, has to be selected in order to deliver the collected rain water to user points. Of the various pump designs, the two most commonly used types are the centrifugal and positive displacement pumps, having different operating characteristics. It is observed that, while positive displacement pumps are more suitable for low discharge rate, high head situations, centrifugal pumps are capable of pumping high volumes at low heads. Therefore, in order to achieve higher heads, such as in multi storey situations, multi 153

31 staging is required if centrifugal pumps are to be utilized. When the comparative pump efficiencies are considered, it is clear that positive displacement pumps are far superior due to limited number of moving parts incorporated in the operation, with the energy losses due to turbulence and friction minimized. Further, since the minimum discharge rate from a positive displacement pump is not limited by the speed of the motor, unlike in the centrifugal pumps, low flow rates can be achieved, ideal for household pumping situations where per capita daily service water requirement is found to be approximately same for a given set of consumers. In Sri Lanka this amount is found to be approximately 200 L per person per day. However, when the power source of the pump is solar energy through a photo voltaic (PV) module, it is observed that the current voltage (I-V) characteristics of a centrifugal pump is well matched with the output characteristics of the PV module. Further, the danger of pump over-speeding, resulting in formation of vacuums is not observed in centrifugal pumps at higher input current levels. However, one salient feature stands out in positive displacement pumps, their capacity to operate even at low light conditions, such as at sun rise/sun set and in cloudy conditions, due to non-requirement of a specific rotational speed to start pumping like in centrifugal pumps. Nevertheless, when the pumping is of bulk transfer nature at high discharge rates with additional retention volumes to buffer for low radiation periods, then the use of centrifugal pumps are more advantageous to maximize the usage of available solar radiation at a given situation. Therefore, selecting the pump in PV pumping should be carefully done so as to optimize system parameters. However, when the PV system is battery assisted, a regular power supply to the pump can be maintained and the pump with the highest efficiency should be selected. Since small scale, compact and highly efficient positive displacement pumps are available they can be selected over centrifugal pumps for household service water pumping at low discharge rates. Besides, since the current drawn from the battery is low and at a lower discharge rate, the life of the battery can be expected to be prolonged when positive displacement pumps are used in battery assisted PV pumping scenarios. 154