Steps involved in setting up a hybrid system deserve extra attention because of its importance or uniqueness.
A hybrid power system has more than one type of generator-usually a gasoline or diesel-powered engine generator and a renewable energy source such as PV, wind, or hydropower system. For explanation, A W-engine hybrid is the only type considered in this report.
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 W-generator system is one in which the PV array and the generator each charge the batteries.
A block diagram is shown in the figure below. 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.
Sizing for an Hybrid System
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 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 below.
PV-Generator Mix Plot for Omaha
Source: Stand – alone Photovaltaic System, A Handbook of Recommended Design Practices, Sandia Laboratories.
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). 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. 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.
Life- Cycle Cost
Doing a life-cycle cost analysis (LCC) gives you the total cost of your PV system--including all expenses incurred over the life of the system. There are two reasons to do an LCC analysis:
1) To compare different power options, and
2) To determine the most cost-effective system designs.
For some applications there are no options to small PV systems so comparison of other power supplies is not an issue. The PV system produces power where there was no power before. For these applications the initial cost of the system is the main concern. However, even if PV power is the only option, a life-cycle cost (LCC) analysis can be helpful for comparing costs of different designs and/or determining whether a hybrid system would be a cost-effective option. An LCC analysis allows the designer to study the effect of using different components with different reliabilities and lifetimes.
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