Fuel cells and photovoltaic systems produce direct current electricity, which can be fed directly into an inverter; however wind turbines, micro-hydro turbines and biomass generators often produce variable voltage and frequency AC electricity which must be rectified prior to being fed into an inverter. Currently off-grid inverters that have a rating of 10 kW or less are typically designed to accept an input voltage from a battery bank with a nominal voltage of 12, 24 or 48 volts and output a nominal grid voltage, which, depending on the region is 110-240 Vrms at 50/60 Hz.
The input voltage requirement is driven primarily due to the large presence of solar and wind energy systems that drive the off-grid inverter market, as they require battery storage in order to ensure energy availability 24/7. Table 1 shows the average DC voltage that is produced by each of the representative alternative energy systems (wind and hydro are often designed to output AC power directly, however the voltages chosen are those of systems that utilize inverters to produce AC power).
Table 1: Typical voltages generated by small scale alternative energy systems (<10kW)
|Fuel Cell||7.8-72 VDC|
In order to create, for example, 240 Vrms from 24 VDC a boost ratio fewer than 15 is required to produce a peak voltage of 330 V. This large increase in voltage that is required to produce grid rated AC voltage leads to large current draws from the low voltage battery system and consequently a large amount of heat generation in the inverter’s boosting semiconductors. The generation of heat from the semiconducting components in these inverters often leads to higher failure rates and system inefficiencies.
The primary driver for the low input voltage used in off-grid inverters is the nominal voltage of the battery bank that it is connected to. Battery banks are used since wind and solar alternative energy technologies are intermittent and in order to ensure power is available when needed a storage system is required.
Since fuel cells and hydro systems are much more reliable than solar and wind systems, battery storage is not necessary and the power produced by these systems could be directly coupled to an inverter system in order to provide reliable off-grid power. In order to increase efficiency, decrease heat generation and therefore failure, the next generation of alternative energy off-grid inverters should increase the allowable input voltage range to facilitate direct coupling of the more reliable alternative energy systems to the inverter. Additionally, as the dependency on battery storage can be removed from the overall alternative energy system in some instances, the cost of the overall installation would be decreased.
- Russel, “The Promise of Reliable Inverters for PV Systems: The Microinverter Solution,” Greenray Solar, 18 June 2010.
- A. Sankaran, F. Rees and C. Avant, “Electrolytic Capacitor Life Testing and Prediction,” in BrowseConference Publications > Industry Applications Confere … Help Electrolytic capacitor life testing and prediction This paper appears in: Industry Applications Conference, Dearborn, 1997.
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