Literature of Distributed Generation Systems

Distributed energy resources (DER) or Distributed Generation (DG)

15Distributed energy resource (DER) refers to systems that are small-scale power generation technologies (typically in the range of 3 kW to 10,000 kW) used to provide an alternative to or an enhancement of the traditional electric power system. The common problem with distributed generators are their high costs. The one exception is probably micro-hydropower. A well-designed plant has nearly zero maintenance costs per kWh, and generates useful power for many years.

One favored source is solar panels on the roofs of buildings. The production cost is $0.99 to 2.00/W (2007) plus installation and supporting equipment unless the

installation is DIY bringing the cost to $6.50 to 7.50 (2007). This is comparable to coal power plant costs of $0.582 to 0.906/W (1979), adjusting for inflation. Nuclear power is higher at $2.2 to $6.00/W (2007). Most solar cells also have waste disposal issues, since solar cells often contain heavy-metal electronic wastes, (CdTe and CIGS), and need to be recycled. The plus side is that unlike coal and nuclear, there are no fuel costs, pollution, mining safety or operating safety issues. Solar also has a low duty cycle, producing peak power at local noon each day. Average duty cycle is typically 20%.

15 Source: Wikipedia & Public Utility Commission, Texas State (http://www.puc.state.tx.us)

Another favored source is small wind turbines. These have low maintenance, and low pollution. Construction costs are higher ($0.80/W, 2007) per watt than large power plants, except in very windy areas. Wind towers and generators have substantial insurable liabilities caused by high winds, but good operating safety. Wind also tends to be

complementary to solar; on days there is no sun there tends to be wind and vice versa.

Many distributed generation sites combine wind power and solar power such as Slippery Rock University, which can be monitored online.

Distributed cogeneration sources use natural gas-fired microturbines or

reciprocating engines to turn generators. The hot exhaust is then used for space or water heating, or to drive an absorptive chiller for air-conditioning. The clean fuel has only low pollution. Designs currently have uneven reliability, with some makes having excellent maintenance costs, and others being unacceptable.

Co-generators are also more expensive per watt than central generators. They find favor because most buildings already burn fuels, and the cogeneration can extract more value from the fuel.

Some larger installations utilize combined cycle generation. Usually this consists of a gas turbine whose exhaust boils water for a steam turbine in a Rankine cycle. The condenser of the steam cycle provides the heat for space heating or an absorptive chiller.

Combined cycle plants with cogeneration have the highest known thermal efficiencies, often exceeding 85%.

In countries with high pressure gas distribution, small turbines can be used to bring the gas pressure to domestic levels whilst extracting useful energy. If the UK were to implement this countrywide an additional 2-4 GWe would become available. (Note

that the energy is already being generated elsewhere to provide the high initial gas pressure - this method simply distributes the energy via a different route.)

DER/DG systems may include the following devices/technologies:

• Combined heat power (CHP)

• Fuel cells

• Micro combined heat and power (MicroCHP)

• Microturbines

• Photovoltaic Systems

• Reciprocating engines

• Small Wind power systems

• Stirling engines

Summary of DG Technologies

This Appendix provides brief descriptions of leading DG technologies. For context, it includes generic cost and performance information. Readers should note that for any given situation it is important to consult with vendors or their agents or dealers regarding actual price. To assist, this Appendix includes a list of links to World Wide Web sites for many leading DG equipment vendors.

Introduction

Distributed generation (DG) systems may be comprised of one or more primary technologies such as internal combustion engines, combustion turbines, photo-voltaics,

and batteries. Innumerable combinations of DG technology/fuel options are possible, to take advantage of synergies between individual technologies, making them as robust and/or cost-effective as possible.

Most DG systems operate on gaseous or liquid hydrocarbon fuel to produce electricity as needed; natural gas fuel is piped in; diesel fuel is stored on-site. Battery systems store electric energy from the grid for use when needed. Renewable energy DGs use solar or wind energy as fuel.

One important DG type category is the duty cycle for the DG is used:

1) for “peaking” duty cycle applications DGs only operate for a small portion of the year, usually between 50 – 600 hours annually, and

2) for “baseload” duty cycle DGs operate for many hours per year for.

Peaking duty distributed generation tends to have relatively low installed cost and can take on load in just a few minutes (or less). It tends to be relatively inefficient and have significant air emissions per hour operated. Peak duty cycle DGs usually operate for just a few hundred hours between overhauls. Typical installed costs range from about $200 –

$500/kW and non-fuel operating cost ranges from 1¢ - 5¢/kWh. Primary distributed generation technologies used for base load duty cycle (when compared to peaking duty cycle described above) tend to be fuel efficient, reliable, and clean burning combustion-based options. Typical installed costs range from about $400 – $800/kW and non-fuel operating cost ranges from ½¢ - 3¢/kWh. 4 Perhaps one of the best examples is an uninterruptible power supply (UPS) that can carry a facility’s load for several minutes combined with a diesel engine generator that takes a few minutes to come on line.

Batteries are an expensive way to store/provide a significant amount of electric energy.

So in this case the synergy is that once running, the diesel generator provides much lower cost energy.

Most types of distributed generation can provide useful and valuable thermal energy. To do so, additional equipment (e.g., pipes and pumps) is added to the generation system so that during electricity generation otherwise wasted heat energy is captured and used to heat water or air, or for processes. This concept is often referred to as combined heat and power (CHP) or cogeneration. Depending on type of generator used, existing thermal energy infrastructure in the facility, and many other project specific factors, equipment for CHP can add 25% - 100% to the installed cost for a generation-only system. Important “enabling” subsystems include: • power conditioning equipment such as electricity generator, transformer, and inverters

• controls

• communications

• fuel handling and/or fuel storage

• emission controls

• sound attenuation enclosures.

Internal Combustion/Reciprocating Engine Generators

An internal combustion reciprocating (piston-driven) engine generator set (genset) includes an internal combustion engine as prime mover coupled with an electric generator and often control and power conditioning subsystems. Sound attenuation enclosures may be also needed.

Most engines are one of two types:

1) compression ignition of fuel — the diesel cycle in which fuel combustion occurs as fuel is compressed causing heat leading to ignition.

2) “spark-ignited” combustion of fuel — the Otto cycle characterized by how fuel spark ignition of fuel (gasoline fueled automobile engines employ the Otto cycle).

These are described in more detail below.

Diesel Engine Generators

Diesel engine generator sets (gensets) consists of a diesel cycle reciprocating engine prime mover, burning diesel fuel, which is coupled to an electric generator. The diesel engine operates at a relatively high compression ratio and at relatively low rpm (compared to Otto cycle/spark engines and to combustion turbines described below).

Diesel engine gensets are very common, especially in areas where grid power is not available or is unreliable. They are manufactured in a wide range of sizes up to 15 MW;

however, for typical distributed energy applications multiple small units, rather than one large unit, are installed for added reliability. These power plants can be cycled frequently and operate as peak load power plants or as load-following plants. In some cases, usually at sites not connected to a power grid, diesel gensets are used for baseload operation (sometimes referred to as "village" power). Diesel gensets are proven, cost-effective, and extremely reliable, and should have a service life of 20 to 25 years if properly maintained.

Installed cost for diesel engines varies significantly. Used/refurbished models can cost as little as $200/kW and newer, more robust, more efficient machines costing $500/kW or more. Depending on duty cycle and engine design, non-fuel O&M for diesel gensets operating on diesel fuel can vary widely, typically ranging from 2.5¢/kWh - 4¢/kWh, with an allowance for overhauls. Frequent cycling increases O&M costs considerably.

Though fuel conversion efficiency for diesels engines can exceed 43% (fuel input of about 7,900 Btu/kWhe, HHV), typical heat rates range widely from 8,000 Btu/kWhe to 10,000 Btu/kWhe (HHV).

“Dual Fuel” Diesel Engine Generators

A dual-fuel engine is a diesel (cycle) engine modified to use mostly natural gas.

Diesel cycle engines cannot operate on natural gas alone because natural gas will not combust under pressure like diesel fuel does, so they must operate in what is called “dual fuel” mode. For that, natural gas is mixed with a small portion of diesel fuel so that the resulting fuel mixture (i.e., 5 – 10% diesel fuel) does combust under pressure. This requires de-rating of and modest modifications to a diesel cycle engine. (Note: for the same displacement a diesel engine operating on natural gas generates less power than the same sized engine operating on diesel fuel only).

Although diesel engines are common, dual fuel versions are not. But because the underlying technology is commercial and well known, in theory natural gas fired versions (for power generation) could become much more common in sizes ranging from

kilowatts to megawatts. For distributed energy systems small multiple unit systems would probably be installed, rather than one single large unit, to improve electric service

reliability. Dual fuel gensets can be cycled frequently to provide peaking power or “load-following” or they can be used for baseload or cogeneration applications. They employ mostly well-proven technology and are very reliable. Service life should be at least 20 to 25 years if properly maintained.

Non-fuel O&M cost is similar to that for diesel gensets. It typically ranges from 2 – 4

¢/kWh including allowance for overhauls. Typical heat rates (HHV) also have a wide range, from 8,200 Btu/kWhe to 10,000 Btu/kWhe.

Spark Ignited/Otto Cycle Engine Generators

Spark-ignited combustion (Otto cycle) reciprocating engines are very common.

They range in power output of less than a horsepower to megawatts. Perhaps the most familiar use for these engines is for automobiles. For stationary power applications including DG a system includes the engine, internal combustion engine as prime mover coupled with an electric generator. The engine prime mover is usually one of two types:

Although spark-ignition engines designed to use gasoline are common, natural gas fueled versions are not so common. However, because the underlying technology is commercial and well known, in theory, natural gas fired versions (for power generation) could become much more common for a variety of applications and load sizes. Natural gas-fueled reciprocating engine gensets can be cycled frequently to provide peaking power or

“load-following” or they can be used for baseload or cogeneration applications. They employ mostly well-proven technology and are very reliable. Service life should be at least 20 to 25 years if properly maintained. Installed cost tends to range between

$400/kW – $600/kW. O&M cost is similar to and possibly somewhat lower than that for diesel gensets. It typically ranges from 2¢/kWh –4.5¢/kWh. Typical heat rates (HHV) also have a wide range, from 8,800 to 10,500 Btu/kWh.

Combustion Turbines

Combustion turbines (also called gas turbines) burn gaseous or liquid fuel to produce electricity in a relatively efficient, reliable, cost-effective, and in some instances

clean manner. Generically, combustion turbines s are "expansion turbines" which derive their motive power from the expansion of hot gasses—heated with fuel—through a turbine with many blades. The resulting high-speed rotary motion is converted to electricity via a connected generator using the Brayton heat cycle. A full generation system consists of the turbine itself, a compressor, a combustor, power conditioning equipment (usually electricity generator and transformer), a fuel handling subsystem, and possibly other subsystems. They may also include a sound attenuation enclosure.

Combustion turbine generation systems are commonplace as electricity generators and are available in sizes from hundreds of kilowatts to very large units rated at hundreds of megawatts.

Combustion turbine systems have a moderate capital cost, but they often are used to burn relatively high cost distillate oil or natural gas. Combustion turbine generation systems should have a minimum service life of 25 - 30 years if properly maintained and depending on how and how often they are used. Depending on the size, type, and application, full-load heat rates (HHV) for commercial equipment can range from 8,000 Btu/kWh to 14,000 Btu/kWh. Non-fuel O&M costs are relatively low – typically ranging from ½ ¢/kWh - 5 ¢/kWh. Variation is a function of criteria such as turbine size, turbine age, turbine materials, turbine complexity/simplicity, reliability required, availability of components, and maintenance protocol/frequency. Combustion turbines can start and stop quickly and can respond to load changes rapidly making them ideal for peaking and load-following applications. In many industrial cogeneration applications they would also make excellent sources of baseload power, especially at sizes in the 5 to 50 MW range.

“Conventional” Combustion Turbine Generators

Conventional combustion turbine generators vary significantly in price, size, and are designed for a wide range of duty cycles. Typical sizes range from 1 to 300 MW.

Smaller turbines used for stationary power generation are often those developed for transportation applications, especially for marine vessels and airplanes. (Note that for those applications reliability and in some cases fuel efficiency are important performance criteria.) Installed costs range from as low as $300/kW for refurbished units and lighter duty machines to 700 - $800/kW for heavier duty/more efficient versions, with non-fuel O&M ranging from .75¢/kWh - 4¢/kWh depending in large part on the intended duty cycle and on maintenance practices.

Microturbine Generators

Microturbines are small versions of traditional gas turbines, with very similar operational characteristics. They are based on designs developed primarily for

transportation-related applications such as turbochargers and power generation in aircraft.

In general, electric generators using microturbines as the prime mover are designed to be very reliable with simple designs, some with only one moving part. Typical sizes are 20 to 300 kW. Microturbines are "near-commercial" with many demonstration and

evaluation units in the field. Several companies, some of which are very large, are committed to making these devices a viable, competitive generation option. One key characteristic of microturbines is that their simple design lends itself to mass

production—should significant demand materialize. For the most part, prices too are still being established. Possibly the key driver will be manufacturing scale. Installed price is

currently in the range of about $1,000/kW – 1,500/kW. Definitive data on reliability, durability, and non-fuel O&M costs are just being developed though based on simplicity and in some cases well-proven designs non-fuel O&M could be similar to that of

conventional combustion turbines. Fuel efficiency tends to be somewhat or even significantly lower than that of larger combustion turbines and internal combustion

reciprocating engines, ranging from 10,000 Btu/kWhe –15,000 Btu/kWhe. Note, however, that if microturbines are used in situations involving use of steam and/or hot water, then they can generate electricity and thermal energy (combined heat and power, CHP) cost-effectively due to a) the temperatures involved and b) the large amount of waste heat produced.

Advanced Turbine System (ATS) Generators

The Advanced Turbine System (ATS) was developed as a small, efficient, clean, lowcost, power generation prime mover by Solar Turbines in conjunction with the U.S.

Department of Energy. It employs the latest combustion turbine design philosophy and state-of-the-art materials. It generates 4.2 MW. Fuel requirements are about 8,800 – 9,000 Btu/kWh (LHV). Installed cost is expected to be about $400/kW, with non-fuel O&M expected to be below ½¢ per kWh generated.

Fuel Cells

Fuel cells are energy conversion devices that convert hydrogen (H2) or high-quality (hydrogen-rich) fuels like methane into electric current without combustion and with minimal environmental impact. Due in part to how fuel cells convert fuel to electricity (i.e., without combustion) conversion is relatively efficient and fuel cells'

emissions of key air pollutants are much lower than for combustion technologies, especially nitrogen oxides (NOx). Fuel cells are very modular (from a few watts to one MW). Fuel cells are often categorized by the type of electrolyte used. The most common electrolyte for fuel cells used for stationary power is phosphoric acid; others include solid oxide and molten carbonate. Another promising type of fuel cell utilizes a proton

exchange membrane, hence the name PEM fuel cell.

A fuel cell system consists of a fuel processor, the chemical conversion section (the fuel cell "stack"), and a power conditioning unit (PCU) to convert the direct current (DC) electricity from the fuel cell's stack into alternating current (AC) power for the grid or for loads and for supporting hardware such as gas purification systems. Unless

hydrogen is used as the fuel, prior to entering the fuel cell stack, the raw fuel (e.g., natural gas) must be dissociated into hydrogen and a supply of oxygen from air must be available.

Within the fuel cell stack, the hydrogen and oxygen react to produce a voltage across the electrodes, essentially the inverse of the process which occurs in a water electrolyzer.

There are hundreds of fuel cells in service worldwide and the number of units in service is growing rapidly. Advocates are awaiting expected manufacturing advances that will reduce fuel cells' equipment cost and improve its efficiency such that they produce very low cost energy. Typical plant unit sizes (which can be aggregated into any plant output rating needed) are expected to range widely from a few kW to 200 kW. Currently available fuel cells based on phosphoric-acid electrolytes have heat rates (HHV) of 9,500 Btu/kWhe – 10,000 Btu/kWhe and cost about $3000/kW installed. Nonfuel O&M for installed devices is about 2.5¢/kWh – 3¢/kWh.

Advanced fuel cells systems are expected to have efficiencies of ranging from 40% to perhaps as high as 55%. (6,300 Btu/kWhe - 8,500 Btu/kWhe) over the next 5 years and

ultimately to cost less than $1000/kW installed.

Energy Storage Systems

Energy storage systems used for DG applications include devices that store energy: a) electrochemically or b) as mechanical energy, and which “discharge”

electricity for use when needed. Battery energy storage systems consist of the battery and a power conditioning unit (PCU) sub-system to convert grid power from alternating current (AC) power to direct current (DC) power during battery charging, and to convert battery power from DC to AC power during battery discharge.

Most batteries can change their rate of discharge/storage in milliseconds. Note that there are two key elements to energy storage plant cost (unlike generators with just one). They are: 1) output rated in Watts (or Volt-amps) indicating the rate at which the system can “discharge: (i.e. provide energy to a load) and 2) the energy storage capacity, the amount of energy that can be stored (rated in kilowatt-hours). Storage is used for a variety of applications, such as:

• increase reliability—for longer duration power outages

• reduce impacts from an electric supply’s poor power quality—for shorter duration electric service disruptions

• to take advantage of “buy low-sell high” (energy cost reduction) opportunities or

of peak shaving (electric demand reduction) opportunities

• to reduce peak demand on a local electricity infrastructure

Electrochemical batteries are by far the most common type of battery, primarily these are the “lead-acid” type, though other types are emerging as competitive options.

Electrochemical batteries are by far the most common type of battery, primarily these are the “lead-acid” type, though other types are emerging as competitive options.