Distributed Generation Technologies

Anibal T de Almeida and Pedro S. Moura

5.1.1 Introduction

Distributed generation (DG) can be defined as a source of electric power connected to a distribution network or a customer site, representing an innovative and efficient way to both generate and deliver electricity, since it generates electricity right where it is going to be used. Technological improvements now allow power generation systems to be built in smaller sizes with high efficiency, low cost, and minimal environmental impact.

Distributed generation can serve as a supplement to electricity generated by huge power plants and delivered through the electric grid. Located at a customer's site, DG can be used to manage energy service needs or help meet increasingly rigorous requirements for power quality (PQ) and reliability.

Distributed generation has the potential to provide site-specific reliability improvement, as well as transmission and distribution (T&D) benefits including: shorter and less extensive outages, lower reserve

margin requirements, improved PQ, reduced lines losses, reactive power control, mitigation of transmission and distribution congestion, and increased system capacity with reduced T&D investment. Distributed generation also provides economic benefits because DG technologies are modular and provide location flexibility and redundancy as well as short lead times. Economic benefits can also be gained by using DG technologies for peak-shaving purposes, for combined heat and power (CHP) (cogeneration), and for standby power applications. In addition, many DG technologies provide environmental benefits including reduced land requirements, lower or no environmental emissions, and lower environmental compliance costs.

Distributed generation technologies can be divided into two different categories according to availability: firm and intermittent power. The firm power technologies are those that enable the power control of DG units that can be managed as a function of the load requirements. Firm DG plants can be utilized as backup, working only in situations of grid unavailability, in periods of high consumption (when the electricity is more expensive), working continuously, or dispatched to meet the variable load in an optimal manner.

The intermittent power technologies do not allow the management of the produced energy by themselves having a random generation character. Examples of this kind of technology are wind power or solar power that only produces energy when the wind or the sun is available. These technologies can be installed aggregated with energy storage that, by filtering the energy generation fluctuation, enables the management of the delivered energy by the combined system.

In DG applications, traditional technologies can be used, such as internal combustion engines, gas turbines, and, in large installations, steam turbines and combined-cycle turbines. Other kinds of technologies such as microturbines, Stirling engines, fuel cells, or renewable energies, including solar power, geothermal power, or wind power can also be utilized (Figure 5.1).

5.1.2 Gas Turbines

Gas turbines are an often-used electricity production technology. The first studies of gas utilization to actuate turbines started at the end of the nineteenth century; however, the first efficient gas turbines started to operate in 1930. A gas turbine consists of a compressor, a combustion chamber, and a turbine

Residential photovoltaic system (5 kW)

Office building with natural gas microturbines (0.5 MW)

FIGURE 5.1 Power system with multiple energy sources. 1: air intake section; 2: compression section; 3: combustion section; 4: turbine section; 5: exhaust section; 6: exhaust diffuser.

Residential photovoltaic system (5 kW)

Substation

Office building with natural gas microturbines (0.5 MW)

FIGURE 5.1 Power system with multiple energy sources. 1: air intake section; 2: compression section; 3: combustion section; 4: turbine section; 5: exhaust section; 6: exhaust diffuser.

http://www.siemens.com. With permission.)"/>
FIGURE 5.2 Combustion turbine. (From Siemens Corp., http://www.siemens.com. With permission.)

coupled to the generator. The turbines with only one axis have all the pieces associated with a continuous axis, all rotating at the same speed. This kind of architecture is used when variations in the turbine speed are not foreseeable. The rotor that drives the generator can be mechanically separated from the rotor driven by the combustion of gases, with more flexibility in the operation speed.

In contrast to internal combustion engines, the gas turbines work in continuous process and not in a repetition of a sequence of different operations. However, the operation can be viewed as a set of four stages similar to the four strokes of the internal combustion engines (Figure 5.2).

1. A compressor drives a rotor that directs the work fluid (air) to the combustion chamber, where the air is compressed, increasing the pressure up to 10 bar and the temperature to 300°C.

2. The compressed air is mixed with the burning fuel, achieving temperatures of 1250°C. This combustion occurs with controlled conditions to maximize the fuel efficiency and minimize the emissions.

3. The air, at high pressure, is passed through the turbine that converts the air energy into mechanical energy. Part of this energy is transmitted to the compressor and the remnant is used for electricity generation through a generator.

4. The exhaust gases are released to the atmosphere, or may be used for generation of process heat or to increase the electricity generated as described below.

Because gas turbines produce a large volume of exhaust gases at high temperatures, the energy of these gases can be utilized for steam production for industrial processes (cogeneration mode) or for electricity production through combined cycle.

In a combined cycle, the gas turbine is used as the first cycle, where the exhaust gases of the gas turbine are used to produce steam in a heat-recovery steam generator. This steam is then used to drive a steam turbine, increasing the electrical global efficiency of the system to values up to 60%.

In the Cheng cycle, the steam is injected in the expansion chamber of the gas turbine (superheated steam injection). In the expansion chamber, the steam is mixed with the gases of the combustion that expand and produce additional work, thereby increasing the electrical efficiency.

TABLE 5.1 General Characteristic of Gas Turbines

Commercial availability

Size range

Fuel

Efficiency

Environmental emissions

Very low when controls are used, high noise

High

0.5-250 MW

Natural gas, biogas, oil derivatives

In cogeneration appliances, the exhaust gases can be used to heat water for residential buildings or for steam production for industrial processes, etc. In some industrial applications of cogeneration, a global efficiency of 60% is reached.

The conversion of mechanical energy to electricity is made almost always through synchronous generators. In DG applications, the rotation speed of the turbines can be higher than the generator synchronous speed which requires a gearbox, reducing the conversion efficiency by about 3%. The generator also works as an auxiliary engine to start the turbine.

Natural gas is the fuel that enables the best efficiency in gas turbines. However, gas turbines can work with other fuels, like fuel oil, diesel, propane, J-5 (used in aeronautics), kerosene, methane, and biogas. The heavy oil utilization decreases the efficiency and the power of the turbine by 5%-8%. Because heavy oil is less expensive, it can decrease the electricity production costs, but the emissions are higher than with other fuels.

The gas turbine generators are available in a wide power range, corresponding at three types of generators:

These generators use gas turbines with the same working principle, but with different configurations and operation characteristics. The large turbines are not normally considered DG. Table 5.1 shows the general characteristics of gas turbines.

5.1.3 Microturbines

Microturbines are small combustion turbines that produce between 25 and 500 kW of power. Microturbines were derived from turbocharger technologies found in large trucks or in the turbines found in aircraft auxiliary power units. Only the largest class of gas turbine generators, those made for central station utility application, are designed specifically for electric power production. In recent years, due to the new market requirements, microturbines have undergone significant innovations, enabling the energy production with high quality and reliability, with low greenhouse gases emissions, and with moderate costs, thus becoming a competitive technology.

Microturbines have the same working principle as the gas turbines, with various modifications in the system configuration. One of the innovations consists in the adoption of a unique shaft, on which the compressor, the turbine, and the generator are assembled (Figure 5.3). These systems eliminate the gearbox, reducing the cost, and increasing the reliability, but with a reduced overall efficiency.

The rotor rotates at a very high speed (up to 100,000 rpm). Another innovation is utilization of air bearings, avoiding the need of a fluid for refrigeration and lubrication, because the unique utilized element is the air. The air can be continuously renovated and will never be contaminated by the materials wastage and by the combustion products.

One of the key characteristics of the microturbines is heat recovery, which utilizes the thermal energy of the exhaust gases (at high temperatures) for preheating the air supply to the compressor. The mechanical energy is converted to electrical energy by a permanent magnet AC generator, which includes a low inertia rotor rotating at the turbine speed.

Exhaust outlet

Compre

Exhaust outlet

Compre

Combustion chamber

Recuperator housing

Turbine

FIGURE 5.3 Microturbine. (From http://www.capstone.com. With permission.)

Combustion chamber

Recuperator housing

Turbine

FIGURE 5.3 Microturbine. (From http://www.capstone.com. With permission.)

Because of the high rotor speed, the AC output has a frequency of approximately 2 kHz. To connect the microturbine generator with a 50 or 60 Hz network in normal applications, the microturbine voltage output must be connected to an AC-DC-AC converter. In this converter, microturbine voltage output is rectified, filtered, and converted to an AC voltage through an inverter system synchronized with the 50 or 60 Hz supply.

Microturbines can also operate with a wide variety of fuels, like natural gas (at high or low pressure), propane, diesel, gasoline, biogas (methane), or kerosene. The electrical efficiency of microturbines is between 20 and 30%, with heat-recovery utilization. If the heat recovery does not exist, this value can decrease to 15%. In cogeneration systems, the global efficiency can reach 85% (Table 5.2).

Microturbine generators can be divided into two general classes:

• Heat-recovery microturbines recover heat from the exhaust gas to boost the temperature of the air stream supplied to the combustion and increase the efficiency. Further exhaust heat recovery can be used in a cogeneration configuration.

• Microturbines, without heat recovery (or simple cycle) have lower efficiencies, but also have lower capital costs.

Other applications of microturbine technology include:

• Core power conversion element of vehicles, such as buses, trucks, helicopters, and so on. Automotive companies are interested in microturbines to provide a lightweight and efficient fuel-based energy source for hybrid electric vehicles.

TABLE 5.2 Microturbines Overview

Size range Fuel

Efficiency

Environmental emissions Other features Commercial status

Yes (only a few manufacturers) 25-500 kW

Natural gas, hydrogen, propane, diesel 20%-30% (recuperated) Low ( < 9-50 ppm) NOx Cogeneration (50°C-80°C water) Medium volume production

• Standby power, PQ, peak-shaving, and cogeneration applications. Some types of microturbines are well suited for small commercial building establishments such as: restaurants, hotels, small offices, retail stores, and many others.

• Utilization of by-products of processes in oil-processing, gas-transferring, petroleum production, industrial waste utilization for the purpose of optimizing the use of natural gas, associated gas, biogas, landfill gas, etc.

The improvement of microturbine design, resulting in lower costs and higher performance, makes microturbines a competitive DG product.

Development is ongoing in a variety of areas:

• Heat recovery/cogeneration

• Use of waste heat for absorption cooling

• Increase of the efficiency

• Fuel flexibility

• Hybrid systems (e.g., fuel cell/microturbine, flywheel/microturbine)

5.1.4 Internal Combustion Engines

Internal combustion (IC) engines were one of the first technologies that used fossil fuels for electricity generation. Developed more than a century ago, IC engines are the most common of all DG technologies. They are available from sizes of a few kilowatts for residential backup generation to generators on the order of 10 MW.

An IC engine uses the thermal energy of fuel combustion to move a piston inside a cylinder, converting the linear motion of the piston to rotary motion of a crankshaft and uses that rotation to turn an AC electric generator. Internal combustion engines are also called reciprocating engines because of the reciprocating linear motion of the pistons.

Internal combustion engines can be fueled with gasoline, natural gas, diesel fuel, heavy oil, biodiesel, or biogas. The two primary types of IC engines used for DG applications are:

• Four-cycle spark-ignited engines (Otto cycle) that use an electrical spark introduced into the cylinder. This explosion engine, or ignition by spark, that uses the Otto cycle was invented by Nikolaus August Otto in 1867. Fast-burning fuels, like gasoline and natural gas, are commonly used in these engines. Biofuels, such as alcohols and biogas, may also be used.

• In 1892, Rudolph Diesel, developed the diesel engine, in which the combustion is initiated by compression. The compression-ignited (diesel cycle) engines, in which compression of the fuel-air mixture inside the piston cylinder rises it to a temperature where it spontaneously ignites, work best with slow-burning fuels such as diesel. Biofuels, such as biodiesel, vegetable oils, etc., may also be used.

Distributed generation engines have efficiencies that range from 25 to 45% (Table 5.3). In general, diesel engines are more efficient than Otto engines because they operate at higher compression ratios. In the future, engine manufacturers are targeting lower fuel consumption and shaft efficiencies up to 50%-55% in large engines (greater than 1 MW) by 2010. Efficiencies of Otto engines using natural gas are expected to improve and approach those of diesel engines.

Internal combustion engine generators for distributed power applications, commonly called gensets, are found universally in sizes from less than 5 kW to over 10 MW. Gensets are frequently used as a backup power supply in residential, commercial, and industrial applications. When used in combination with a 1-5 min uninterruptible power supply (UPS), the system is able to supply seamless power during a utility outage. In addition, large IC engine generators may be used as base-load generation, grid support,

TABLE 5.3 Internal Combustion (IC) Engines Overview

Commercially Available

Yes

Size range

0.005-10 MW

Fuel

Natural gas, diesel, heavy fuel, biogas

Efficiency

25%-45%

Environmental

Emission controls required for NOx and CO

Other features

Cogeneration (some models)

Commercial status

Widely available

or peak-shaving devices. Internal combustion engine generators have start-up times ranging between 0.5 and 15 min, and a high tolerance for frequent starts and stops. The smaller engines, available in sizes as small as a few kilowatts, are intended for dispersed applications, such as individual homes and small businesses coping with power outages.

5.1.5 Stirling Engines

Stirling engines (Figure 5.4) are a class of reciprocating piston engines and are classed as external combustion engines, invented in 1816 by Robert Stirling. They constitute an efficient thermodynamic machine for the direct conversion of heat into mechanical work with a theoretical efficiency of 40%. Stirling engines were commonly used prior to beginning of the twentieth century. As steam engines improved and the competing compact Otto cycle engine was invented, Stirling engines lost favor. Recent developments in DG and solar thermal power have revived interest in Stirling engines. As a result, research and development efforts in this area have increased in recent years. The principles of Stirling engines are described in another chapter and will not be covered here.

The operation is reversible, i.e., by supplying thermal energy, mechanical energy is produced, and by supplying mechanical energy, thermal energy is produced. Stirling engines can be fueled by any source of

http://www.ent.ohiou.edu/~urieli/stirling/engines/engines.html)"/>
FIGURE 5.4 Stirling engine. (From http://www.ent.ohiou.edu/~urieli/stirling/engines/engines.html)

TABLE 5.4 Stirling Engine Overview

Commercially available

Size range

Fuel

Efficiency Environmental Other features Commercial status

On a limited scale < 1-25 kW

Fuel flexibility—fossil or renewable heat is possible

Potential for low emissions Cogeneration (some models) Availability for specialized applications heat (fossil fuel or renewable) and some models have possibilities to perform cogeneration. In a Stirling engine, the continuous combustion process, which is easier to optimize and control, results in lower emissions compared to the intermittent explosions of fuel air mixtures in IC Otto and diesel engines.

Stirling engines are commercially available for some marine applications and have been available for several years. Trials in domestic CHP are occurring in several countries, on the scale of hundreds of units. Usually Stirling engines are found in sizes from 1 to 25 kW and are currently being produced in small quantities for specialized applications (Table 5.4).

Large 25-kW Stirling motors have an electrical efficiency of approximately 30%, although the goal is to increase this efficiency to greater than 34% with more development. Stirling engines are ideally suited for solar thermal power. Using a gas as an operating fluid, there is no practical limit placed on the solar unit's upper temperature due to its operating fluid. Maximum temperature would be limited only by the materials used in its construction.

Recent Stirling engine developments have been directed at a wide range of applications, including:

• Small scale: residential or portable power generation.

• Solar dish applications: heat reflected from concentrating dish reflectors is used to drive the Stirling engine; several research programs are aimed at enhancing this application.

• Vehicles: auto manufacturers have investigated utilizing Stirling engines in vehicles to improve the fuel economy.

• Refrigeration: Stirling engines are being developed to provide cooling for applications, such as microprocessors and superconductors.

• Aircraft: Stirling engines could provide a quieter-operating engine for small aircraft.

• Space: Power generation units aboard space ships and vehicles.

The primary challenges faced by Stirling engines over the last two decades have been their long-term durability/reliability and their relatively high cost.

Fuel cells are a technology for power generation that is quiet and highly efficient with no moving parts. Fuel cells generate electricity through an electrochemical process in which the energy stored in a fuel is converted directly into DC electricity and thermal energy. The chemical energy normally comes from hydrogen contained in various types of fuels (hydrocarbon fuels, such as natural gas, methanol, ethanol, biogas, etc.), including pure hydrogen. Fuel cells are described in detail in another chapter.

Several hundred phosphoric acid fuel cell (PAFC) demonstration and test plants have been built in the mid-1990s to early twenty-first century, mostly with 200-kW capacity appropriate for DG applications, in many commercial buildings to provide premium PQ for demanding loads. The operating temperature is about 200°C, which is suitable for cogeneration applications in buildings and in small industrial plants. They do not offer opportunity of self-reforming and they require platinum for their catalyst. PAFCs' efficiency and peak output capability deteriorate by about 2% per year.

5.1.6 Fuel Cells

One of the most promising developments is the design of hybrid systems. Solid oxide fuel cells (SOFCs) promise top efficiency, particularly in a combined cycle operation mode, in which they can surpass conventional combined-cycle gas turbine plants. High efficiencies under part-load operation also result in high overall efficiency.

For applications in industrial heat-power cogeneration and public electricity supply, high-temperature fuel cells (SOFCs and molten carbonate fuel cells [MCFCs]) are most suitable. Both systems are still in an early stage of development, but permit the use of a wide range of fuels. Such fuel cell systems compete in the lower rating range with gas turbines and motor cogeneration plants, and in the upper rating range with combined gas and steam turbine power plants. Conventional plants have a clear advantage in terms of practical experience and in terms of comparatively low capital costs compared with fuel cell plants. High-temperature fuel cells are likely to gain market penetration due to a decrease in specific need for primary fuels and also due to a sharp decrease of specific pollutant emissions in comparison to conventional generation. This last factor is a key advantage in DG applications in urban areas.

5.1.7 Solar Photovoltaic

Photovoltaic (PV) cells, or solar cells, convert sunlight directly into electricity. Photovoltaic technology has several applications, including:

• Grid attached residential and commercial buildings

• Remote communication systems

• Central power plants (above 1 MW)

Traditionally, PV cells have been used to power structures such as individual homes in locations where it is expensive or impossible to send electricity through power lines. Solar power has traditionally been used in remote areas where the grid is not available; such systems store electricity in batteries for use when the sun is not shining and are called stand-alone power systems. Currently, PV-generated power is less expensive than conventional power where the load is small and the area is too difficult to serve by electric utilities. However, solar power is now appearing more in urban areas due to innovative policy mechanisms (rebates, feed-in tariffs) to promote PV generation. Here, the surplus solar electricity is injected into the grid. These are called grid-connected solar systems because the owner has the security of the grid available.

In the decade 1995-2004, average annual growth has been 20% with global sales reaching over 1000 MW per year at the end of that period. Photovoltaic is the most modular and operationally simple of the clean, distributed power technologies, with benefits that include the ability to provide power during summer peak periods, distribution congestion benefits, environmental benefits, reduced fuel price risk, and local economic development. As a result of private and government research, PV systems are becoming more efficient and affordable.

Distributed PV systems that provide electricity at the point of use are reaching widespread commercialization. Chief among these distributed applications are PV power systems for individual buildings. Interest in the building integration of photovoltaics, where the PV elements actually become an integral part of the building, often serving as the facade or exterior weather skin, is growing worldwide. Photovoltaic specialists and innovative designers in Europe, Japan, and in the U.S. are now exploring creative ways of incorporating solar electricity into their work. A building integrated photovoltaics (BIPV) system consists of integrating photovoltaics modules into the building envelope, such as the roof or the facade (Figure 5.5). By simultaneously serving as building envelope material and power generator, BIPV systems can provide savings in materials and in electricity costs.

Photovoltaics may be integrated into many different assemblies within a building envelope:

• Incorporated into the facade of a building, complementing or replacing traditional glass.

• Incorporated in the external layers of the wall of a building facade.

FIGURE 5.5 Building integrated photovoltaics in a facade and in a roof.

• Use in roofing systems providing a direct replacement for different types of roofing material.

• Incorporated in skylight systems in which part of the solar light is transmitted to the inside of the building and the other part is converted into electricity.

5.1.8 Wind Power

Wind energy became a significant research area in the 1970s during the energy crisis and the resulting search for potential renewable energy sources. Modern wind turbine technology has made significant advances over the last 25 years. Today, wind-power technology is available as a mature, environmentally sound, and convenient alternative. Generally, individual wind turbines are grouped into wind farms containing several turbines. Many wind farms are megawatt scale, ranging from 1 MW to tens of megawatts. Wind turbines may be connected directly to utility distribution systems. The larger wind farms are often connected to transmission lines.

The land can still be used for animal grazing and some agriculture operations. The small-scale wind farms and individual units are typically defined as DG. Residential systems (1-15 kW) are available (Figure 5.6). However, they are generally not suitable for urban or small-lot suburban homes due to large space requirements.

http://www.oksolar.com/wind/. With permission.)"/>
FIGURE 5.6 Building-integrated wind power. (From http://www.oksolar.com/wind/. With permission.)

Utility-scale turbines range in size from 50 to 5000 kW. Single, small turbines, below 50 kW, are used for homes, telecommunications stations, or for water pumping. A detailed description of wind-power technology is given in a different chapter.

5.1.9 Cogeneration

Combined heat and power, also known as cogeneration, is an efficient, clean, and reliable approach to producing both electricity and usable thermal energy (heating and/or cooling) at high efficiency and near the point of use, from a single fuel source. Because CHP is highly efficient, it reduces traditional air pollutants and carbon dioxide emissions, the leading greenhouse gas associated with climate change.

Combined heat and power can use a variety of technologies to meet an energy users needs. The range of technologies available allows the design of cogeneration facilities to meet specific onsite heat and electrical requirements. Combined heat and power systems consist of a number of individual components—prime mover, generator, heat recovery, and electrical interconnection—configured into an integrated system. The type of equipment that drives the overall system (i.e., the prime mover) typically identifies the CHP system.

Typical CHP prime movers include:

• Combustion turbines

• Reciprocating engines

• Boilers with steam turbines

• Microturbines

Combined heat and power may be used in a variety of applications ranging from small 10-kW systems to very large utility-scale applications approaching 1000 MW. The first step in assessing which CHP application is right for a particular facility is to identify whether there is coincident demand of electrical and thermal energy at the host site. The CHP project will be most economically viable when the system

Heat losses

Trigeneration

Heat losses

Trigeneration

Line losses

FIGURE 5.7 Trigeneration technology.

provides the maximum amount of energy that can be used. Therefore, CHP project development begins with an analysis of site electrical and thermal load profiles. Based on these profiles, the type of CHP technology which most closely matches the facility's power and demand will be chosen.

In developed countries, about 10% of all electricity is generated in CHP plants, leading to huge primary energy savings and reduction of emissions. Combined heat and power in some industries, such as the pulp and paper industry, uses biomass by-product as fuel input.

Trigeneration can provide even greater efficiency than cogeneration. Trigeneration is the conversion of a single fuel source into three useful energy products: electricity, steam or hot water, and chilled water (Figure 5.7). Trigeneration converts and distributes up to 90% of the energy contained in the fuel burned in a turbine or engine into usable energy. Introducing an absorption chiller into a cogeneration system means that the site is able to increase the operational hours of the plant with an increased utilization of heat, particularly in summer periods. Trigeneration has been applied with very positive results in buildings such as hotels and hospitals, which feature a large space conditioning load during most of the year.

Electric-drive vehicles (EDVs) include battery electric vehicles, hybrid vehicles, and fuel cell vehicles running on gasoline, natural gas, or hydrogen. These vehicles have gained attention in the past few years due to growing public concerns about urban air pollution and other environmental and resource problems. All these vehicles have within them power electronics that generate AC power at power levels from 10 to 100 kW; this power, with suitable electronics, can be fed into the electric grid (Figure 5.8). It allows such vehicles to use their installed power to help balance load in localized grid segments during peak load periods. This concept of bidirectional grid interface is known as vehicle-to-grid power or V2G. Sharing power assets between transportation and power generation functions can accelerate commercialization of battery electric vehicles, hybrid vehicles, and fuel cell vehicles.

These vehicles can be recharged during off-peak hours at cheaper rates while helping to absorb excess nighttime generation. There is a potential to supply extra power during peak demand if electric-drive vehicles are grid-connected to allow discharge from their batteries, or run their onboard generators.

To work in vehicle-to-grid power systems, each vehicle must have three required elements:

• A connection to the grid for electrical energy flow.

• Control or logical connection necessary for communication with the grid operator.

• Controls and metering onboard the vehicle.

5.1.10 Vehicle-to-Grid

For fueled vehicles (fuel cell and hybrid), a fourth element, a connection for gaseous fuel (natural gas or hydrogen), could be added so that onboard fuel is not depleted.

DC power 300-450 V 0-50 A

Power electronics unit (inverter)

FIGURE 5.8 Vehicle-to-grid enabling technology. (From AC Propulsion. Vehicle-to-grid demonstration project: Grid regulation ancillary service with a battery electric vehicle (December 3-10, 2002, http://www.acpropulsion.com/reports/V2G%20Final%20Report%20R5.pdf. With permission.)

AC power 100-250 V 50-60 Hz 0-80 A

re a

FIGURE 5.8 Vehicle-to-grid enabling technology. (From AC Propulsion. Vehicle-to-grid demonstration project: Grid regulation ancillary service with a battery electric vehicle (December 3-10, 2002, http://www.acpropulsion.com/reports/V2G%20Final%20Report%20R5.pdf. With permission.)

Vehicles with significant energy stored in batteries could perform as uninterruptible power systems for whole houses and support the grid exceptionally well by providing any of a number of functions known collectively as ancillary services. These services are vital to the smooth and efficient operation of the power grid. These vehicles could provide:

• Extra power during demand peaks

• Spinning reserve

• Grid regulation (automatic generation control (AGC))

• Uninterruptible power source for businesses and homes

• Active stability control of transmission lines

Hybrid vehicles with IC engines show the potential for power generation at specific emissions levels in some cases better than the best new large power plants. A continuous source of fuel for hybrid or fuel cell vehicles can be provided with a connection to low-pressure natural gas or biogas at compatible parking locations. Over just a decade or two, V2G could revolutionize the ancillary services market, improve grid stability and reliability, and support increased generation from intermittent renewables.

One conceptual barrier is an initial belief that their power would be unpredictable or unavailable because the vehicles would be on the road. Although availability of any one vehicle is unpredictable, the availability of an average number of vehicles is highly predictable and can be estimated from traffic and road-use data.

From the societal point of view, the large-scale application of V2G can lead to a much more cost-effective allocation of resources to provide highly reliable electricity to a wide range of resources.

5.1.11 Conclusions

The utilization of DG technologies enables the creation of a power system with multiple energy sources, allowing the integration conventional central power plants with dispersed DG fossil fuel-based generation, as well as with dispersed DG renewable generation. It is anticipated that DG growth and its large-scale application may lead to an improvement in reliability, to improved security of supply, to the decrease of power costs, and to the minimization of environmental impacts. In Figure 5.9 and Table 5.5, a characterization of the most important DG technologies is presented showing typical parameters associated with each technology.

High temnerature

High temnerature

High temperature FC

High temperature FC

Combined cycle

Combined cycle

Microturbines

Microturbines

1 10 100 1000 System size (MW)

FIGURE 5.9 Distributed generation technologies comparative efficiency range.

TABLE 5.5 Distributed Generation (DG) Technologies Characterization

Internal Combustion Engines

Gas Turbines

Microturbines

Fuel Cell

Wind Power

Photovoltaic (PV)

Stirling Engine

Power range (kW)

5-50,000

500-250,000

20-500

1-10,000

0.3-5000

0.07-1000

Up to 25

Electric efficiency (%) (LHV)

25—45

25-45

20-30

30-70

25-40

5-15

12-30

Efficiency with partial load

Reasonable until

Reasonable until 40%

Bad below 40% of

Good/reasonable until

a

a

a

35%—40% of the

of the rated load

the rated load

35%—40% of the

rated load

rated load

Load following capacity

Very good

Goodb

Reasonable/lowc

Very good

a

a

a

Start time

10 s to 15 min

2 min to 1 h

60s

a

a

NA

a

Availability (%)

90-98

90-98

90-98

90-95

10—40d

5—25d

High6

Interval between the

0.5-2

30

5-8

10^0

4

NA

a

maintenance stops

( X1000 h)

Useful life time (years)

15-20

20-25

10

20e

20

20-30

Long e

Fuel flexibility

Good

Good

Good

Good

NA

Good

Excellent

Noise

High

Moderate to high

Moderate

Low

Moderate

NA

Low

Acquisition costs (€/kW)

300-900

300-1000

700-1100

>4000

700-1300

4000

2000-50,000

O&M costs (€/kWh)

0.005-0.015

0.004-0.010

0.005—0.016e

0.005-0.030

a

a

a

Energy costs (€/kWh)

0.07-0.15

0.05-0.15

0.05-0.15

>0.15

0.03-0.20

>0.20

a

Commercial availability

High

High

Moderate

Low

High

High

Very low

a a Dependent upon conditions.

Installation costs of the system boiler/turbine. c Despite present a high potential to load folowing, the actual models do not present a good fulfillment at this level.

Depending on the climate condition at the location. e Estimated value. NA, not applicable.

ui cn

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