FiGURE 9.13 Schematic diagram of a battery.
Other parameters related to batteries are the charge or discharge regime and the lifetime of the battery. The charge (or discharge) regime, expressed in hours, is the parameter that reflects the relationship between the nominal capacity of a battery and the current at which it is charged (or discharged)—for example, a discharge regime is 40 h for a battery with nominal capacity of 200 Ah that is discharged at 5 A. The lifetime of the battery is the number of charge-discharge cycles the battery can sustain before losing 20% of its nominal capacity.
In general, the battery can be viewed as a voltage source, E, in series with an internal resistance, Ro, as shown in Figure 9.13. In this case, the terminal voltage, V, is given by
An inverter is used to convert the direct current into alternating current electricity. The output of the inverter can be single or three phase. Inverters are rated by the total power capacity, which ranges from hundreds of watts to megawatts. Some inverters have good surge capacity for starting motors, others have limited surge capacity. The designer should specify both the type and size of the load the inverter is intended to service.
The inverter is characterized by a power-dependent efficiency, r|inv. Besides changing the DC into AC, the main function of the inverter is to keep a constant voltage on the AC side and convert the input power, Pin, into the output power, Pout, with the highest possible efficiency, given by
Pout _ Kc 4ccosM Pin Vdc ^dc
1dc = current required by the inverter from the DC side, i.e., controller (A). Vdc = input voltage for the inverter from the DC side, i.e., controller (V).
Numerous types of inverters are available, but not all are suitable for use when feeding power back into the mains supply.
9.3.3 Charge Controllers
Controllers regulate the power from PV modules to prevent the batteries from overcharging. The controller can be a shunt type or series type and also function as a low-battery voltage disconnect to prevent the battery from overdischarge. The controller is chosen for the correct capacity and desired features (ASHRAE, 2004).
Normally, controllers allow the battery voltage to determine the operating voltage of a PV system. However, the battery voltage may not be the optimum PV operating voltage. Some controllers can optimize the operating voltage of the PV modules independently of the battery voltage so that the PV operates at its maximum power point.
Any power system includes a controller and a control strategy, which describes the interactions between its components. In PV systems, the use of batteries as a storage medium implies the use of a charge controller. This is used to manage the flow of energy from PV system to batteries and load by using the battery voltage and its acceptable maximum and minimum values. Most controllers have two main modes of operation:
1. Normal operating condition, where the battery voltage varies between the acceptable maximum and minimum values.
2. Overcharge or over-discharge condition, which occurs when the battery voltage reaches a critical value.
The second mode of operation is obtained by using a switch with a hysteresis cycle, such as electromechanical or solid-state devices. The operation of this switch is shown in Figure 9.14.
As shown in Figure 9.14a, when the terminal voltage increases above a certain threshold, Vmax,off, and when the current required by the load is less than the current supplied by the PV array, the batteries are protected from excessive charge by disconnecting the PV array. The PV array is connected again when the terminal voltage decreases below a certain value, Vmax,on (Hansen et al., 2000).
Similarly, as shown in Figure 9.14b, when the current required by the load is bigger than the current delivered by the PV array, to protect the battery against excessive discharge the load is disconnected when the terminal voltage falls below
FiGuRE 9.14 Operating principle of overcharge and overdischarge protection. (a) Overcharge. (b) Over-discharge.
FiGuRE 9.14 Operating principle of overcharge and overdischarge protection. (a) Overcharge. (b) Over-discharge.
a certain threshold, Vmin, off. The load is connected to the system again when the terminal voltage is above a certain threshold, Vmin, on (Hansen et al., 2000).
As was seen before, PV cells have a single operating point where the values of the current (I) and voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I, as specified by Ohm's law. A PV cell has an exponential relationship between current and voltage, and there is only one optimum operating point, also called a maximum power point (MPP), on the power-voltage (or current) curve, as shown in Figure 9.8. MPP changes according to the radiation intensity and the cell temperature, as shown in Figure 9.9. Maximum power point trackers (MPPTs) utilize some type of control circuit or logic to search for this point and, thus, allow the converter circuit to extract the maximum power available from a cell. In fact, peak-power trackers optimize the operating voltage of a PV system to maximize the current. Typically, the PV system voltage is charged automatically. Simple peak-power trackers may have fixed operator-selected set points.
The MPPT is a method to let the controller operate at the optimum operating point. A maximum power point tracker is a specific kind of charge controller that utilizes the solar panel to its maximum potential. The MPPT compensates for the changing voltage against current characteristic of a solar cell. The MPPT monitors the output voltage and current from the solar panel and determines the operating point that will deliver that maximum amount of power available to the batteries or load.
A maximum power point tracker is a high-efficiency DC-to-DC converter that functions as an optimal electrical load for a solar panel or array and converts the power to a voltage or current level that is more suitable to whatever load the system is designed to drive.
MPPT charge controllers are desirable for off-grid power systems, to make the best use of all the energy generated by the panels. MPPT charge controllers are quickly becoming more affordable and more common than ever before. The benefits of MPPT regulators are greatest during cold weather, on cloudy or hazy days, or when the battery is deeply discharged. Peak-power trackers can be purchased separately or specified as an option with battery charge controllers or inverters. In all cases, however, the cost and complexity of adding a peak-power tracker should be balanced against the expected power gain and the impact on system reliability.
PV modules are designed for outdoor use under harsh conditions, such as marine, tropic, arctic, and desert environments. The PV array consists of a number of individual photovoltaic modules connected together to give a suitable current and voltage output. Common power modules have a rated power output of around 50-180 W each. As an example, a small system of 1.5-2 kWp
FiGuRE 9.16 Schematic diagram of a direct coupled PV system.
FiGuRE 9.16 Schematic diagram of a direct coupled PV system.
may therefore comprise some 10-30 modules covering an area of around 1525 m2, depending on the technology used and the orientation of the array with respect to the sun.
Most power modules deliver direct current electricity at 12 V, whereas most common household appliances and industrial processes operate with alternating current at 240 or 415 V (120 V in the United States). Therefore, an inverter is used to convert the low-voltage DC to higher-voltage AC.
Other components in a typical PV system are the array mounting structure and various cables and switches needed to ensure that the PV generator can be isolated.
The basic principle of a PV system is shown in Figure 9.15. As can be seen, the PV array produces electricity, which can be directed from the controller to either battery storage or a load. Whenever there is no sunshine, the battery can supply power to the load if it has a satisfactory capacity.
In a direct coupled PV system, the PV array is connected directly to the load. Therefore, the load can operate only whenever there is solar radiation, so such a system has very limited applications. The schematic diagram of such a system is shown in Figure 9.16. A typical application of this type of system is for water pumping, i.e., the system operates as long as sunshine is available, and instead of storing electrical energy, water is usually stored.
Stand-alone PV systems are used in areas that are not easily accessible or have no access to an electric grid. A stand-alone system is independent of the electricity grid, with the energy produced normally being stored in batteries. A typical stand-alone system would consist of a PV module or modules, batteries, and a charge controller. An inverter may also be included in the system to convert the direct current generated by the PV modules to the alternating current form required by normal appliances. A schematic diagram of a stand-alone
system is shown in Figure 9.17. As can be seen, the system can satisfy both DC and AC loads simultaneously.
Nowadays, it is usual practice to connect PV systems to the local electricity network. This means that, during the day, the electricity generated by the PV system can either be used immediately (which is normal for systems installed in offices, other commercial buildings, and industrial applications) or be sold to one of the electricity supply companies (which is more common for domestic systems, where the occupier may be out during the day). In the evening, when the solar system is unable to provide the electricity required, power can be bought back from the network. In effect, the grid is acting as an energy storage system, which means the PV system does not need to include battery storage. A schematic diagram of a grid-connected system is shown in Figure 9.18.
In the hybrid-connected system, more than one type of electricity generator is employed. The second type of electricity generator can be renewable, such as a wind turbine, or conventional, such as a diesel engine generator or the utility grid. The diesel engine generator can also be a renewable source of electricity when the diesel engine is fed with biofuels. A schematic diagram of a hybrid-connected system is shown in Figure 9.19. Again, in this system, both DC and AC loads can be satisfied simultaneously.
9.4.5 Types of Applications
These are some of the most common PV applications:
• Remote site electrification. Photovoltaic systems can provide long-term power at sites far from utility grids. The loads include lighting, small appliances, water pumps (including small circulators of solar water heating systems), and communications equipment. In these applications, the load demand can vary from a few watts to tens of kilowatts. Usually, PV systems are preferred to fuel generators, since they do not depend on a fuel supply, which can be problematic, and they do avoid maintenance and environmental pollution problems.
• Communications. Photovoltaics can provide reliable power for communication systems, especially in remote locations, away from the utility grid. Examples include communication relay towers, travelers' information transmitters, cellular telephone transmitters, radio relay stations, emergency call units, and military communication facilities. Such systems range in size from a few watts for callbox systems to several kilowatts for relay stations. Obviously, these systems are stand-alone units in which PV-charged batteries provide a stable DC voltage that meets the varying current demand. Practice has shown that such PV power systems can operate reliably for a long time with little maintenance.
• Remote monitoring. Because of their simplicity, reliability, and capacity for unattended operation, photovoltaic modules are preferred in providing power at remote sites to sensors, data loggers, and associated meteorological monitoring transmitters, irrigation control, and monitoring highway traffic. Most of these applications require less than 150 W and can be powered by a single photovoltaic module. The batteries required are often located in the same weather-resistant enclosure as the data acquisition or monitoring equipment. Vandalism may be a problem in some cases; however, mounting the modules on a tall pole may solve the problem and avoid damage from other causes.
• Water pumping. Stand-alone photovoltaic systems can meet the need for small to intermediate-size water-pumping applications. These include irrigation, domestic use, village water supply, and livestock watering. Advantages of using water pumps powered by photovoltaic systems include low maintenance, ease of installation, and reliability. Most pumping systems do not use batteries but store the pumped water in holding tanks.
• Building-integrated photovoltaics. Building-integrated photovoltaics (BIPV) is a special application in which PVs are installed either in the façade or roof of a building and are an integral part of the building structure, replacing in each case the particular building component. To avoid an increase in the thermal load of the building, usually a gap is created between the PV and the building element (brick, slab, etc.), which is behind the PV, and in this gap, ambient air is circulated so as to remove the produced heat. During wintertime, this air is directed into the building to cover part of the building load; during summer, it is just rejected back to ambient at a higher temperature. A common example where these systems are installed is what is called zero-energy houses, where the building is an energy-producing unit that satisfies all its own energy needs. In another application related to buildings, PVs can be used as effective shading devices.
• Charging vehicle batteries. When they are not used, vehicle batteries self-discharge over time. This is a major problem for organizations that maintain a fleet of vehicles, such as the fire-fighting services. Photovoltaics battery chargers can help solve this problem by keeping the battery at a high state of charge by providing a trickle charging current. In this application, the modules can be installed on the roof of a building or car park (also providing shading) or on the vehicle itself. Another important application in this area is the use of PV modules to charge the batteries of electric vehicles.
The electrical power output from a PV panel depends on the incident radiation, the cell temperature, the solar incidence angle, and the load resistance. In this section, a method to design a PV system is presented and all these parameters are analyzed. Initially, a method to estimate the electrical load of an application is presented, followed by the estimation of the absorbed solar radiation from a PV panel and a description of the method for sizing PV systems.
As is already indicated, a PV system size may vary from a few watts to hundreds of kilowatts. In grid-connected systems, the installed power is not so important because the produced power, if not consumed, is fed into the grid. In stand-alone systems, however, the only source of electrical power is the PV system; therefore, it is very important at the initial stages of the system design to assess the electrical loads the system will cover. This is especially important in emergency warning systems. The main considerations that a PV system designer needs to address from the very beginning are:
1. According to the type of loads that the PV system will meet, which is the more important, the total daily energy output or the average or peak power?
2. At what voltage will the power be delivered, and is it AC or DC?
3. Is a backup energy source needed?
Usually the first things the designer has to estimate are the load and the load profile that the PV system will meet. It is very important to be able to estimate precisely the loads and their profiles (time when each load occurs). Due to the initial expenditure needed, the system is sized at the minimum required to satisfy the specific demand. If, for example, three appliances exist, requiring 500 W, 1000 W, and 1500 W, respectively; each appliance is to operate for 1 h; and only one appliance is on at a time, then the PV system must have an installed peak power of 1500 W and 3000 Wh of energy requirement. If possible, when using a PV system, the loads should be intentionally spread over a period of time to keep the system small and thus cost effective. Generally, the peak power is estimated by the value of the highest power occurring at any particular time, whereas the energy requirement is obtained by multiplying the wattage of each appliance by the operating hours and summing the energy requirements of all appliances connected to the PV system. The maximum power can easily be estimated with the use of a time-schedule diagram, as shown in the following example.
Estimate the daily load and the peak power required by a PV system that has three appliances connected to it with the following characteristics:
1. Appliance 1, 20 W operated for 3 h (10 am-1 pm).
The daily energy use is equal to
To find the peak power, a time schedule diagram is required (see Figure 9.20). 40 -
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