Increasing demand for uninterrupted electric power, a desire to integrate renewable energy sources such as wind and solar power and the integration of unconventional power generation with conventional sources of energy have resulted in the evolution of hybrid energy systems. Hybrid energy systems take many forms and have been around for quite some time.
Increasing demand for uninterrupted electric power, a desire to integrate renewable energy sources such as wind and solar power and the integration of unconventional power generation with conventional sources of energy have resulted in the evolution of hybrid energy systems.
Hybrid energy systems take many forms and have been around for quite some time. In fact, the familiar uninterruptible power supply, as shown in Figure 1 (opposite page), is an example of a simple hybrid energy system, with two power sources: utility power and precharged battery or flywheel. The utility power acts as the primary source while the battery acts as the backup. And the rectifier/inverter makes it possible to provide 100% availability of power to the load for a period, depending upon the size of the battery.
A hybrid automobile is another simple example. Figure 2 is an elementary drawing, but it makes the basic point. The two sources are a precharged battery and the conventional gasoline engine. The battery acts as the primary source while the engine acts as the backup source. A direct-current motor, powered by the battery through an electronic regulator, provides the propulsion. When the battery voltage falls below a certain level, the engine-driven generator does two things:
It supplies the propulsion motor, and at the same time, charges up the battery.
But keep in mind that the battery—the kind used in a UPS or automobile—is not really an independent source of energy but an energy storage device. A true hybrid energy system should consist of three basic components:
Two or more independent sources of energy
Energy storage devices
Independent sources of energy are
Utility power (generated by nuclear, oil-fired, coal-fired, gas turbine or hydroelectric power plants)
On-site diesel generators acting either as emergency or standby or cogeneration sources
Photovoltaic (PV) solar panels
Energy storage devices, such as batteries, flywheels, pumped storage systems and electrolysis systems that generate and store hydrogen, are required to store excess energy when available and to help in the transfer of load from one source to the other.
Electronic controllers are required to ensure proper operation of the hybrid system. The most common types of controllers are DC regulators and DC/AC inverters.
Power from the utility—and from the diesel generator—is 60-Hz, 3-phase or 1-phase at the consumer voltage (480-volt, 3-phase or 120-volt, 1-phase). These sources need no converters or controllers to connect to the load. On the other hand, fuel cells and PV panels generate only DC power and require DC/AC inverters. A microturbine is like a jet engine driving a permanent-magnet AC generator. It generates high frequency because of its high speed, typically 30,000 to 40,000 rpm. This source requires AC/DC and DC/AC converters before connecting to the load.
Wind-turbine generators are either AC induction generators connected to feed power directly into the utility or stand-alone variable frequency generators requiring AC/DC and DC/AC converters.
There are several concepts for configuring hybrid systems. Much depends upon the design goal and the availability of energy sources. One concept involves the use of renewable energy sources and fossil fuel sources. In Figure 3, for example, the renewable energy acts as primary source, and the fossil energy acts as back up. This system has been used in remote stations for weather monitoring and other types of data collection and research. The PV panels and the wind-turbine generator together act as primary source, one complementing the other, because the output of one is usually high when the output of the other is low. The engine-generator acts as the standby source, starting up when the battery voltage is too low to generate the required voltage. The DC regulator controls the charging of the battery from the PV source. The DC/AC inverter is bi-directional and acts as a rectifier to charge the battery when the engine-generator is in operation.
A second increasingly popular concept is the one using fuel cells as primary sources and fossil fuel energy for back up shown in Figure 4 (p.27). In this configuration, the automatic transfer switch starts the diesel generator and switches over the load in the event of a problem in the fuel cell source. Several individual fuel cells are stacked and connected in cascade to produce the required voltage and power. The result is that the fuel-cell stack generates a continuous DC voltage as long as there is a supply of hydrogen and oxygen. This type of hybrid system requires no storage batteries. The standby diesel-generator is required only when the fuel-cell stack is down for maintenance or repair—the ideal system for supplying several hundred kilowatts of power to a large industrial user.
Several other configurations are not only possible, but also technically viable so that individual consumers need not rely solely on the utility power. After all, utility power is not always as reliable and as "clean" as is normally assumed. A hybrid system would provide a reliable and a clean source of power. While the types of hybrid systems are many, they have one thing in common: potential power quality issues.
PQ in Hybrids
In all types of hybrid energy systems, there are electronic converters and controllers which are known sources of power quality problems. The severity of the problems depends upon the particular configuration of the hybrid system.
For example, in the PV-and-wind system of Figure 2, voltage sags and surges due to rapid variations of the sunlight or the wind are absorbed by the battery. However, the DC/AC inverter does generate harmonics and spikes on the AC side.
A similar situation exists in the hybrid system using the fuel cells and the engine-generator. The fuel cells themselves produce relatively smooth DC voltage but the inverter, usually a pulse-width-modulated electronic circuit using insulated gate bipolar transistors (IGBTs) is a source of harmonics and spikes because of the switching of the electronic devices.
Except in low-power applications, DC/AC inverters generally produce three-phase AC. Pulse-width modulation is a technique for switching the IGBTs on and off at a high frequency (typically two to five kHz) so as to produce an approximately sinusoidal output voltage. As the output voltage is not perfectly sinusoidal, it consists of the usual 5th, 7th, 11th, 13th, etc. harmonics in addition to a high-frequency component. Multi-phase inversion using 12-pulse or 18-pulse 3-phase inverters would produce a cleaner AC voltage. Passive and active harmonic filters can also be used depending upon the severity of the problem. Generally, most industrial loads are more tolerant of power quality variations than electronic instruments and other devices.
Therefore, in order for a hybrid system to work satisfactorily, it is necessary to evaluate the power quality issues carefully at the design stage and devise suitable mitigation means. Hybrids can thrive with the right kind of care.
Promising Developments in Fuel Cell Technology
Four types of fuels cells are in advanced stages of development and application. Phosphoric acid fuel cells (PAFCs) are currently available in the 200-kW range. Molten carbonate fuel cells (MCFCs) are being developed in megawatt ranges that can be used in parallel with the utility power. Solid oxide fuel cells (SOFCs), and proton exchange membrane fuel cells (PEMs) will soon be available in commercial kW ranges. Efficiencies of 40% to 60% are possible. When used as sources of heat and electricity, efficiency greater than 85% can be realized, particularly with the high temperature MCFCs and the SOFCs. A hybrid source using SOFCs and microturbines capable of generating 190 kW—enough to supply approximately 200 homes—has been tested in California. Test data show an electrical efficiency of about 53%. The table gives an overview of current status of the development of fuel cells.
Status of Fuel Cell Development
100 %%MDASSML%% 200 kW
1 kW %%MDASSML%% 10 MW
250 kW %%MDASSML%% 10 MW
3 %%MDASSML%% 250 kW
Natural gas, landfill gas, digester gas, propane
Natural gas, hydrogen, landfill gas, fuel oil
Natural gas, hydrogen
Natural gas, hydrogen, propane, diesel
Some commercially available
Likely commercialization in 2004
Some commercially available
Likely commercialization in 2003/2004