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Hybrid Power Systems
These designs are printed in the "Stand-Alone Photovoltaic Systems - a Handbook of Recomended Design Practices" available from Sandia. General information is provided in this section. For additional information select from the menu below.
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A hybrid power system has more than one type of generator - usually a fueled-engine generator and a renewable energy source such as PV, wind, or hydropower. A PV-engine hybrid is the only type considered here. A hybrid system is most often used for larger applications such as village power; residential systems where generators already exist; and in applications like telecommunications where availability requirements are near 100 percent. Almost all PV-generator hybrid systems include batteries for storage.
The most common configuration for a PV-generator system is one in which the PV array and the generator each charge the batteries. This configuration is intended to optimize the use of both power sources during normal operation. In many systems, the photovoltaic array is sized to supply power to the load during normal conditions. The generator is used only if solar radiation is low for several days in a row, or if load demand is unusually high. The generator is run for a short period of time near its optimum operating point, typically at 80 to 90 percent of rated power. This kind of operation reduces generator maintenance and fuel costs and prolongs the useful life of the generator.
Other advantages of using a hybrid system are
Improved Economics - A large part of the cost of PV stand-alone systems results from the need to size the array and batteries to support the load under worst-case weather conditions. In many applications, this marginal power may be less expensive if provided by a generator. In regions with variable climate, where average daily insolation in winter is two or three times less than in summer, the use of a hybrid system may be a good option. Figure 20 demonstrates how the marginal cost of photovoltaic systems changes relative to power availability. This plot indicates that a PV system providing 90 percent of the load will cost about $3,600 but the cost rapidly goes past $8,000 before an availability of 98 percent is reached. It may be more economical to provide some of this power with a generator. However, maintenance, logistics, and fuel costs can be quite expensive for generators operating in remote areas. These factors must be considered in any cost estimate of the hybrid system. Lower Initial Cost - An engine generator costs less than a PV system of equal size. Increased Reliability - The two independent power systems provide redundancy and possibly greater overall reliability if the hybrid system is properly maintained and controlled. Design Flexibility - The design of a hybrid system depends on the load mix between the engine generator and the PV system. As the size of the PV array increases the operating time of the generator goes down. This saves fuel, lowers maintenance, and prolongs generator life but the initial cost will be higher than a power system with a smaller PV array. For a hybrid system the size of the battery bank is usually smaller than for a stand-alone PV system designed for the same application. This is because the fueled generator will be available to keep the battery state-of-charge above the recommended limit. When sizing the batteries, be sure the generator charging current does not exceed the recommended charge rate for the battery (usually less than C/3).
Improved Economics - A large part of the cost of PV stand-alone systems results from the need to size the array and batteries to support the load under worst-case weather conditions. In many applications, this marginal power may be less expensive if provided by a generator. In regions with variable climate, where average daily insolation in winter is two or three times less than in summer, the use of a hybrid system may be a good option. Figure 20 demonstrates how the marginal cost of photovoltaic systems changes relative to power availability. This plot indicates that a PV system providing 90 percent of the load will cost about $3,600 but the cost rapidly goes past $8,000 before an availability of 98 percent is reached. It may be more economical to provide some of this power with a generator. However, maintenance, logistics, and fuel costs can be quite expensive for generators operating in remote areas. These factors must be considered in any cost estimate of the hybrid system.
Lower Initial Cost - An engine generator costs less than a PV system of equal size.
Increased Reliability - The two independent power systems provide redundancy and possibly greater overall reliability if the hybrid system is properly maintained and controlled.
Design Flexibility - The design of a hybrid system depends on the load mix between the engine generator and the PV system. As the size of the PV array increases the operating time of the generator goes down. This saves fuel, lowers maintenance, and prolongs generator life but the initial cost will be higher than a power system with a smaller PV array. For a hybrid system the size of the battery bank is usually smaller than for a stand-alone PV system designed for the same application. This is because the fueled generator will be available to keep the battery state-of-charge above the recommended limit. When sizing the batteries, be sure the generator charging current does not exceed the recommended charge rate for the battery (usually less than C/3).
Sizing
Two hybrid worksheets, HY 1 shown in the inset, and HY 2 are provided in Appendix B. The key factors to be determined are
the load mix between PV and generator, the size and type of generator, and the battery size.
the load mix between PV and generator,
the size and type of generator, and
the battery size.
The sizing method assumes that a stand-alone PV system has already been considered--the load has been estimated and the solar radiation at the site is known. The primary decision is the load mix between generators. Selecting the mix is simplified by using the graph given in Figure 21.
The designer selects a hybrid array to load ratio for the system realizing that the higher up the curve, the higher the percentage of load supplied by the PV array. The load mix will be a key determinant in the type and size of the generator and the battery. The most cost-effective system is obtained by selecting a point on or slightly below the knee of the curve. For example, a hybrid array/load ratio of 0.25 should give a hybrid system design where the PV array supplied 90 percent of the annual load demand. An array/load ratio of 0.15 would give a system with lower initial cost because the amount of load provided by the PV array would be about 57 percent. The generator would operate more in this latter design with corresponding increases in fuel cost and maintenance. If the generator is in a remote location the cost of this maintenance may be exorbitant. These are the design tradeoffs that must be made.
If high reliability is required, the system should be designed for 90 to 95 percent PV contribution. The generator is used only for back-up during worst-case conditions, typically in the winter months when it is most difficult to get a generator started. Therefore, having two power sources at an unattended site does not, in itself, guarantee 100 percent reliability. The control system must be properly designed for fail-safe operation and regular maintenance performed, particularly on the generator. Also, the control system for a hybrid system is more complex because the regulation of the batteries and load must be maintained under all operating conditions.
All generators require periodic routine maintenance (i.e., oil change, engine tune-up, and eventually engine rebuilding). The designer should always look carefully at the generator service requirements, see Table 7, which depend on the run time and thus the generator's electrical power contribution to the hybrid system. At a remote unmanned microwave relay site, the desired generator maintenance interval for oil change and engine tune-up may be only once a year. In contrast, the owner of a hybrid home power system is often willing to perform this routine maintenance monthly. The type of generator and the percentage of load demand met by the generator depend on these issues.
With a generator available for back-up power, the battery size in the hybrid system may be decreased without lowering system availability. However, the battery must be carefully matched to the loads and power sources. To extend battery life, the designer must use a reliable controller to protect the smaller battery and prevent frequent cycling or excessive depth of discharge. The batteries must have sufficient capacity to provide the maximum peak power required by the load and to accept the maximum charge current provided by the generator.
The discharge capability of the battery is a function of the battery size and state of charge. Batteries that are discharged quickly will drop in voltage, and may shut down the inverters and/or loads. A discharge factor of 5 or greater is recommended. Like the charge factor, this number is given relative to the rated capacity, C, of the battery, i.e., and a 100-ampere-hour battery should not be discharged at more than a 20 ampere rate for a long period.
Conversely, the batteries must have sufficient capacity to accept the maximum charge current from the generator/charger and the PV system. If not, the battery may be damaged by the high current. Few batteries can withstand a charge rate greater than C/3 amperes.
Generation Selection
The choice of the size and type of generator is critical to successful hybrid design. Several types of generators, their size range, applications, and approximate cost/watt are given in Table 8. The portable, light-duty generator is the least expensive option for a small intermittent load where reliability is not a major factor. For industrial systems with high reliability requirements, a stationary heavy-duty generator is recommended. Important considerations in choosing the type of generator are
Size and Nature of the Load - Consider the size of the load, the starting requirements, and running time. Fuel Type - Consider fuel availability, handling and storage requirements, and environmental factors, such as temperature and likelihood of contamination. Propane or LPG fuel is an excellent choice for many remote homes because it is readily available in most parts of the U.S., requires no handling on the part of the homeowner, is easily stored, and is excellent for cold weather starting. Although diesel fuel is widely available, contamination can occur and lead to difficulties in cold weather starting. Generator Running Speed - Choose a generator running speed suitable for the expected run time. If the generator is only used occasionally to charge a battery bank, a 3,600 rpm unit may suffice. If the generator will be used over 400 hours per year, a unit with a lower running speed, 1,800 rpm, is recommended. Compatibility with Controls - Check the generator specifications for details on operational control and whether the generator can be integrated into a central control system. Larger generators often have built-in control systems to prevent the generator from starting or operating when engine failure might occur; i.e., when oil pressure is low.
Size and Nature of the Load - Consider the size of the load, the starting requirements, and running time.
Fuel Type - Consider fuel availability, handling and storage requirements, and environmental factors, such as temperature and likelihood of contamination. Propane or LPG fuel is an excellent choice for many remote homes because it is readily available in most parts of the U.S., requires no handling on the part of the homeowner, is easily stored, and is excellent for cold weather starting. Although diesel fuel is widely available, contamination can occur and lead to difficulties in cold weather starting.
Generator Running Speed - Choose a generator running speed suitable for the expected run time. If the generator is only used occasionally to charge a battery bank, a 3,600 rpm unit may suffice. If the generator will be used over 400 hours per year, a unit with a lower running speed, 1,800 rpm, is recommended.
Compatibility with Controls - Check the generator specifications for details on operational control and whether the generator can be integrated into a central control system. Larger generators often have built-in control systems to prevent the generator from starting or operating when engine failure might occur; i.e., when oil pressure is low.
When the generator size is calculated, the main consideration is operating efficiency. Generators operate most efficiently when running near their rated output power. Efficiency can drop by 50 percent or more when operating at low loads. This will result in greater maintenance costs and shorter generator lifetime. Size the generator to provide the current needed to operate the loads and charge the battery efficiently. Power losses in the battery charger and those losses due to environmental conditions and fuel type must be accounted for. The generator's capacity to supply power under the system's actual operating conditions depends on the current required to start the load, the duration of generator run time, fuel consumption at the desired running efficiency, and maintenance requirements under real conditions (i.e., considering temperature, altitude, dust, moisture, and contamination). This information is provided in the generator and battery charger specifications.
Control
Integration of a generator into a PV system requires a more sophisticated control strategy. Most controllers are custom designed by an experienced electronic engineer/technician. Controls for PV- generator systems perform two main functions--battery regulation and subsystem management. Battery regulation is the same as the control process in a stand-alone PV system where batteries must be protected against excessive charging and discharging. Subsystem management of the generator, photovoltaic array, and load requires starting or stopping the generator, and connecting or disconnecting the loads or portions of the PV array. Finally, it may be desired to actuate alarms, either on-site or via telephone link, in the event of system malfunction or to automatically provide an equalization charge to the batteries. Remember, the more one requires of the control system, the higher the price and the higher the chance of failure.
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