TEXT 7. Distributed Utilities

Distributed utilities (sometimes referred to as DU) is the current term used in describing distributed generation and storage devices operating separately and in parallel with the utility grid. In most cases, these devices are small in comparison to traditional utility base or peaking generation, but can range up to several megawatts. For the purposes of this section, DU will be limited to devices 5 MW and below applied at either the secondary voltage level, 120 V single phase to 480 V three phase, and at the medium voltage level, 2.4 kV to 25 kV, although many of the issues discussed would apply to the larger units as well.

Figure 1. Distributed generation technology chart.

Fig. 1 is a listing of different technologies, their size ranges, fuel sources, and AC interface type, and most likely applications.

Some of the technologies are listed below. All of the energy storage devices and many of the small emerging generation devices are inverter/converter based.

 

Fuel Cells

Fuel cell technology has been around since its invention by William Grove in 1839. From the 1960s to the present, fuel cells have been the power source used for space flight missions. Most of the present technologies have a fuel reformer or processor that can take most hydrocarbon-based fuels, separate out the hydrogen, and produce high-quality power with negligible emissions. This would include gasoline, natural gas, coal, methanol, light oil, or even landfill gas. In addition, fuel cells can be more efficient than conventional generators. Theoretically they can obtain efficiencies as high as 85% when the excess heat produced in the reaction is used in a combined cycle mode. These features, along with relative size and weight, have also made the fuel cell attractive to the automotive industry as an alternative to battery power for electric vehicles.

Fuel cell power plants can come in sizes ranging from a few watts to several megawatts with stacking.

The main disadvantage to the fuel cell is the initial high cost of installation.

Microturbines

Microturbines are typically defined as systems with an output power rating of between 10 kW up to a few hundred kilowatts. These systems are usually a single-shaft design with compressor, turbine, and generator all on the common shaft, although some companies are engineering dual-shaft systems. Like the large combustion turbines, the microturbines are Brayton Cycle systems, and will usually have a recuperator in the system. A recuperated Brayton Cycle microturbine can operate at efficiencies of approximately 30%.

Another requirement of microturbine systems is that the shaft must spin at very high speeds, in excess of 50,000 RPM and in some cases doubling that rate, due to the low inertia of the shaft and connected components. Since the turbine requires extremely high speeds for optimal performance, the generator cannot operate as a synchronous generator. Typical microturbines have a permanent magnet motor/generator incorporated onto the shaft of the system. The high rotational speed gives an AC output in excess of 1000 Hz, depending on the number of poles and actual rotational speed of the microturbine. This high-frequency AC source is rectified, forming a common DC bus voltage that is then converted to a 60-Hz AC output by an onboard inverter.

The microturbine family has a very good environmental rating, due to natural gas being a primary choice for fuel and the inherent operating characteristics, which puts these units at an advantage over diesel generation systems.

 

Storage Technologies

Storage technologies include batteries, flywheels, ultra-capacitors, and to some extent photovoltaics. Most of these technologies are best suited for power quality and reliability enhancement applications, due to their relative energy storage capabilities and power density characteristics, although some large battery installations could be used for peak shaving. All of the storage technologies have a power electronic converter interface and can be used in conjunction with other DU technologies to provide “seamless” transitions when power quality is a requirement.

 

Combustion Turbines

There are two basic types of combustion turbines (CTs) other than the microturbines: the heavy frame industrial turbines and the aeroderivative turbines. The heavy frame systems are derived from similar models that were steam turbine designs. As can be identified from the name, they are of very heavy construction. The aeroderivative systems have a design history from the air flight industry, and are of a much lighter and higher speed design. These types of turbines, although similar in operation, do have some significant design differences in areas other than physical size. These include areas such as turbine design, combustion areas, rotational speed, and air flows.

The combustion turbine unit consists of three major mechanical components: a compressor, a combustor, and a turbine. The compressor takes the input air and compresses it, which will increase the temperature and decrease the volume per the Brayton Cycle. The fuel is then added and the combustion takes place in the combustor, which increases both the temperature and volume of the gaseous mixture, but leaves the pressure as a constant. This gas is then expanded through the turbine where the power is extracted through the decrease in pressure and temperature and the increase in volume.

1. Read and translate the text.

2. Read and decide if the following statements are true (T) or false (F).

1) Distributed utilities operate within the utility grid.

2) Fuel cells have been used for space flight to the present day.

3) Fuel cell power plants output can be no more than a few watts.

4) For optimal operation the turbine needs intensely high speed.

5) The storage technologies can be used together with other distributed utilities.

6) The heavy frame industrial turbines are of higher speed design than the aeroderivative systems.

 

3. Find English equivalents of the following phrases in the text

Местная энергосеть, скорость вращения, избыточное тепло, первоначальная стоимость, достигать нескольких мегаватт, величина выходной мощности, космический полет, повышение надёжности, цикл Брейтона, система производства электроэнергии.

4. Match the words from the text with their corresponding definitions.

1) cell	a) a rod forming part of a machine, that turns in order to pass power on to the machine
2) combustion	b) a change, or a process of change, that improves something or increases its value
3) shaft	c) a device that produces a continuous electric current directly from the oxidation of a fuel, as that of hydrogen by oxygen.
4) enhancement	d) The energy, power, or work produced by a system or device
5) output	e) any process in which a substance reacts with oxygen to produce asignificant rise in temperature and the emission of light

 

5. Complete the sentences from the text

1) The microturbine family has a very good environmental rating, due to …

2) Theoretically fuel cells can obtain efficiencies as high as 85% when…

3) The generator cannot operate as a synchronous generator since…

4) Most of the storage technologies are best suited for power quality and reliability enhancement applications, due to …

5) The high rotational speed gives an AC output in excess of 1000 Hz, depending on …

6. Make a written summary (abstract, annotation) to the text.