ECE 7800: Renewable Energy SystemsTopic 5: Distributed Generation with

Fossil Fuels

Spring 2010

© Pritpal Singh, 2010

Distributed Generation With the restructuring of the electric

utility industry, the new model of power generation is that of distributed generation (DG).

DG allows for higher efficiency because locally generated electricity can be combined with district heating and cooling.

The scales of DG technologies are shown on the next slide.

Distributed Generation (cont’d)

kW

0.02

0.1 1

10100

1,000

10,000

100,000

Distributed Generation Technologies• Reciprocating engines

• Combined Heat and Power (CHP)

• Micro-Combined Heat and Power (Micro- CHP)

• Microturbines

• Fuel Cells

• Stirling engines

• Wind Generators

• Solar Electric systems

• Micro-hydroelectric systems

Use fossilfuels

Renewableenergytechnologies

Distributed Generation with Fossil Fuels

The most widespread distributed generation sources are gasoline and diesel generators (using IC engines) used for backup power in various applications, e.g. hospitals, first responders, data centers, etc.

High and Low Heating Values of Fuels An important parameter associated

with fuels is the heating value. When a fuel is burned, some of the heat goes into latent heat for vaporization of water into steam. There are two types of heating value:

1) High heating value – takes into account the heat given to latent heat

2) Low heating value – does not take into account the heat given to latent heat.

High and Low Heating Values of Fuels (cont’d)

The high and low heating values for some common fuels are given in the below table:

Energy Density of Batteries and H2 Energy density of lead acid batteries

= 40Wh/kg = 144,000 J/kg = 144 kJ/kg

Energy density of lithium ion batteries

= 460-720 kJ/kg

Energy density of H2 = 143,000 kJ/kg

(however, volumetric energy density is

much less than gasoline)

- 5.6 MJ/L (compressed H2) vs. 34.2MJ/L (gasoline)

Reciprocating Internal Combustion Engines

Distributed generation is dominated by reciprocating, i.e. piston-driven internal combustion engines (ICE’s) connected to constant-speed ac generators. They range in power from 0.5 kW to 6.5 MW with LHV efficiencies of 37-40%. They can run on diesel, gasoline, kerosene, natural gas, fuel oil, alcohol, waste-treatment plant digester gas, and hydrogen. They are the least expensive DG technology and with natural gas, one of the cleanest.

Reciprocating I C Engines (cont’d)

The engines may be four-stroke or two-stroke and may be spark-ignited or compression-ignited. Four-stroke engines are more efficient than two-stroke and are cleaner burning; therefore two-stroke are generally not used. Spark-ignition engines use easily ignitable fuels, e.g. natural gas, gasoline and propane. Compression-ignition engines require heavier petroleum distillates such as diesel or fuel oil. A supercharger, which compresses the air-fuel mixture prior to it entering the cylinder, can be used to improve the efficiency of engines.

Reciprocating I C Engines (cont’d)

The basic cycle of a four-stroke engine is shown below:

Reciprocating I C Engines (cont’d) Since emissions are a major constraint, most of the interest in reciprocating engines is in natural gas engines. The below diagram shows the heat balance for present and target reciprocating engines:

Reciprocating I C Engines (cont’d)

For combined heat and power applications, waste heat from IC engines can be extracted using water jackets around the engine. An example of such a system is shown below:

Commercial Natural Gas Generator•60kW, 80 kW, 100 kW, 150 kW and 200 kW sizes, synchronous or induction.•Units may be operated in parallel for larger installations up to 3,000 kW•High-efficiency dedicated natural gas industrial engines.•Solenoid-operated automatic inlet gas cut-off valve.•Unit-mounted natural gas pressure regulation system.•Solid-state automatic air/fuel ratio controller system.•Extended-life replaceable air filtration system.•Electronic engine speed governor system accurate to ± 0.25% steady-state.•Pressurized closed-loop cooling system with oversized heavy-duty radiator.•12 volt D.C. engine electrical system.•Solid-state 110V battery charging system.•Heavy steel base with built-in connection points and lifting sockets.•All components mounted on composite vibration isolators.•All-weather 14-gauge steel enclosure with locking doors.•Sound Attenuation packages available for commercial, residential or critical applications.•All-weather exhaust silencer system options available for various sound attenuation level requirements.•Microprocessor based, digital readout auto-start control panel with programmable set points available. •Double-sealed, permanently lubricated single-bearing reconnectable brushless generator with SAE housing and positive alignment flexible drive plate connection. Induction or Synchronous generators available.•Guaranteed to meet or exceed all United States air quality requirements.•Customized Cogeneration options available for hot water, heated air, process heat, chilled water and air conditioning, specifically designed for the facility served.•Remote PC operation options for pre-alarm, alarm and/or control functions.•Paralleling switchgear, intertie protection packages, automatic transfer switches and numerous other types of control/operation packages available.•Extended Warranty, Maintenance Plans and Refurbishment programs available.

http://www.genergypower.com/System%20Features.htm

Microturbines Rankine cycle and combined cycle gas

turbines (described earlier) have been used as peaking power generators at scales of few MW to hundreds of MW. More recently, a new generation of small gas turbines ranging from about 500W to several hundred kW have become available. These are referred to as microturbines. A schematic drawing of a microturbine is shown on the next slide.

Microturbines (cont’d)

in

Ref: Vanek and Albright,Energy System Engineering

Microturbines (cont’d) Incoming air is compressed to 3-4 atm.

pressure. This is sent to a heat exchanger (recuperator) where it is heated by hot exhaust gases. The hot compressed air is mixed with fuel in the combustion chamber and burned. The expanding hot gases spin the turbine and the generator and compressor which are co-located on the same shaft. The exhaust gases are cooled in the recuperator transferring heat to the incoming air.

Microturbines (cont’d) Some commercial microturbine specs.

are given below:

Capstone C60 Microturbine

http://capstoneturbine.com/prodsol/techtour/index.asp

Microturbines (cont’d)

See Table 4.3 Capstone C60 info.

Microturbines (cont’d)

Examples 4.1 and 4.2

Biomass for Electricity Biomass uses plant material

for electricity production which is derived from waste from agriculture, forestry and municipal waste.

World wide there are about 14GW of biomass plants with about half in the US. About 2/3 are co-generation plants. Almost all operate on a conventional steam-Rankine cycle.

The McNeil power station built in 1998 in Vermont is capable of generating 50 MW of power from local wood waste products.

Ref: http://www.ucsusa.org/clean_energy/technology_and_impacts/energy_technologies/how-biomass-energy-works.html

Biomass for Electricity (cont’d) Biomass plants tend to be lower

efficiency (~20%) than conventional power plants due to high water vapor content of waste and lower operating temperatures and pressures. This results in relatively expensive electricity (~9¢/kWh). Coal and biomass can be co-fired and this increases overall efficiency and reduced emissions compared to only coal-fired power plants.

Biomass for Electricity (Cont’d) A two-step process for gasifying

biomass fuels is shown below:

Biomass for Electricity (Cont’d)Step 1: Raw biomass fuel is heated, causing

it to undergo pyrolysis in which volatile components are vaporized. Moisture is driven off first then syngas comprising H2, CO, CH4, CO2 and N2 is produced. Char and ash are byproducts of pyrolysis.

Step 2: The char is heated to 700ºC reacts

with O2,H2 and steam to produce more syngas.

Carbon Emissions of Biomass 

Ref: http://www.ucsusa.org/clean_energy/technology_and_impacts/energy_technologies/how-biomass-energy-works.html

Riding the Carbon Cycle: The carbon cycle is nature's way of moving carbon around to support life on Earth. Carbon dioxide is the most common vehicle for carbon, where one carbon atom is bound to two oxygen atoms. Plant photosynthesis breaks the carbon dioxide in two, keeping the carbon to form the carbohydrates that make up the plant, and putting the oxygen into the air. When the plant dies or is burned, it gives its carbon back to the air, which is then reabsorbed by other plants.

Stirling Engines An alternative approach to IC engines

is external combustion engines, i.e. the heat source is located outside the engine (rather inside the engine as in an ICE). A Stirling engine is an example of a reciprocating engine that uses an external combustion source.

It can run on any almost any fuel or other source of high temperature heat, e.g. concentrated sunlight on a black absorber plate.

Stirling Engines (cont’d) The basic operation of a Stirling

engine is shown in the below figure:

Stirling Engines (cont’d) The engine comprises two pistons in

the same chamber. One is on the hot side and the other on the cold side. They are separated by a short-term thermal storage device called a regenerator. This is usually a wire or ceramic mesh that allows gas to flow in either direction. As the gas passes through the regenerator it either heats up or cools down depending on the direction it is traveling.

Stirling Engines (cont’d)There are four states of operation:

1)->2) The hot piston is stationary while the cold one moves to the left, compressing the gas while transferring heat to the cold sink.

2)->3) Both pistons move simultaneously to the left. The gas heats up as it passes through the regenerator, increasing temperature and pressure while its volume remains constant.

Stirling Engines (cont’d)3)->4) The gas in the hot space absorbs

energy from the hot source and expands, pushing the hot piston to the left. This is the power stroke.

4)->1) Both pistons move simultaneously to the right. The gas passing through the regenerator into the cold space drops in temperature and pressure.

The piston movement can be controlled by a rotating crankshaft. When this is tied to a generator, electricity can be generated.

Stirling Engines (cont’d) The pressure-volume diagram for a

Stirling engine is shown below:

Stirling Engines (cont’d)Animation of Stirling engine:

http://en.wikipedia.org/wiki/Stirling_engine

Stirling Engines (cont’d) Stirling engines range in size from

< 1kW to about 25 kW. Their efficiencies are relatively low (< 30%). However, progress is being made in improving efficiency and the external combustion nature makes them relatively quiet engines when compared to IC engines.

Stirling Engines (cont’d)•The AC WhisperGenTM is set to revolutionise the home energy market. It is an innovative system developed to provide central heating, water heating and electricity in your home.

•The WhisperGenTM is a co-generation (heat and power) system based on a small four cylinder Stirling Engine. The AC WhisperGen is a unique micro combined heat and power (microCHP) system providing efficient, low maintenance generation of heat and electricity. It operates as a floor mounted, fully automatic boiler, providing up to 8.0kW of thermal energy for hot water and space heating while generating a max. 1.2kW 230V AC power output.

•Designed and styled as a whiteware appliance with minimal noise and vibration, the WhisperGenTM replaces the traditional domestic boiler, and supplements the commercial electricity supply to your home.

•Running on natural gas or LPG, the AC WhisperGenTM efficiently uses over 90% of the fuel energy resulting in a cleaner and more cost effective alternative to traditional electricity generation. Electricity generated can be fed back into the electricity grid or used in the home, reducing electricity costs even further.

•The WhisperGenTM is currently being evaluated internationally with systems operating in several countries, including the UK, Netherlands, Germany and France.

http://www.whispergen.com/main/residential/

(Based in Christchurch, New Zealand)

Combined Heat and Power Some DG technologies produce waste heat

(e.g. combustion turbines, fuel cells) and others do not (e.g. solar panels, wind turbines). By utilizing this waste heat, both the energy efficiency and the economic value of the energy generator can be greatly enhanced.

High temperature waste heat can be used for process steam, absorption cooling, and space heating while low temperature heat is normally used for water heating.

Energy Efficiency Measures of CHP

Energy Efficiency Measures of CHP (cont’d)

One approach to comparing systems with and without CHP is to compare their thermal efficiencies:

Overall thermal = = 78%

Efficiency (w/CHP)

Overall thermal = = 52%

Efficiency (w/o CHP)

%100100

4830x

%1006090

4830x

Energy Efficiency Measures of CHP (cont’d)

For an industrial facility, it needs heat anyway for process heat. An alternative measure that considers how much extra heat is needed to generate electricity is the energy-chargeable-to-power (ECP) given by:

total thermal - displaced thermal

ECP = input input

Electrical output

Units: Btu/kWh or kJ/kWh

Energy Efficiency Measures of CHP (cont’d)

Example 5.13

CHP Economics The ECP depends on the amount of

usable heat recoverable from the boiler or furnace and the efficiency of the process. When ECP is modified to account for the cost of the fuel, a measure of the added cost for electricity generation is the operating cost chargeable to power (CCP) given by:

CCP = ECP x unit cost of energy

Units: $/kWh

CHP Economics (cont’d)

Example 5.14

Design of CHP System The design of CHP systems is

complex since it involves balancing the amount of heat and electrical power that the CHP system delivers.

The power-to-heat (P/H) ratio may be constant for an industrial plant but vary significantly in a residential building (see figure on next slide).

Residential Thermal and Electrical Loads

Smoothing the P/H Ratio As seen in fig. 5.11, the P/H ratio can

vary widely throughout the year due to high demand for electricity for air conditioning in the summer and high demand for space heat in the winter.

There are several approaches that can be considered to smooth out this P/H ratio throughout the year.

Smoothing the P/H Ratio (cont’d)Some examples are:

• Heat pumps can displace heat with electricity in the winter

• Absorption cooling systems can displace electricity with heat in the summer.

Another strategy (that does not affect the P/H ratio) is:

• Ice making for cooling water for chillers in the summer displaces the time of day that the electricity is used.

(see section 5.6 textbook)

Distributed Generation-Related Standards

An important issue related to distributed generation is interconnection standards. The utilities have put up several roadblocks related to interconnecting distributed generation sources to the grid. These revolve around maintaining power quality, e.g. harmonics injected onto the grid, constant frequency, safety for maintenance crew, etc.

Efforts are underway at the federal level

and within individual states to develop fair and uniform interconnection standards to help facilitate the deployment of distributed generation.

Distributed Generation-Related Standards The National Electric Code has

procedures and practices related to installation of commercial and residential Photovoltaic Systems.

The IEEE has a working group (P1547) developing standards for interconnection of DG equipment to the electrical grid.

Distributed Generation-Related StandardsThe IEEE Standards are shown below:

http://grouper.ieee.org/groups/scc21/dr_shared/

Distributed Generation-Related Standards Interconnection standards for the

state of Ohio are given at this weblink:

http://www.puco.ohio.gov/emplibrary/files/smed/Technical_Requirements.pdf

Distributed Generation-Related Standards Example: IEEE 929-2000 for Photovoltaic Systems

IEEE 929-2000: Safety

Utilities and the PV community now have an approved interconnection standard that ensures the safe operation of a PV system connected to a utility grid. Safety for the utility lineman, for the utility equipment, and for the customer was the primary concern throughout the development of the interconnection standard. The IEEE 929-2000 standard includes tightly-defined specifications that require the PV system inverter to cease to energize the utility line for specific out-of-tolerance conditions such as voltage and frequency trip settings when values are outside of acceptable limits. These inverters also include sophisticated and reliable anti-islanding protocols that include active detection functions to ensure that the inverter does not deliver power to the utility system when utility power is cut off or disconnected from the inverter. Additionally, detection functions ensure that the inverter will cease to energize the utility line when an excess of dc current is present at the ac interface.

IEEE 929-2000: Power Quality

The quality of power provided by the PV system must meet specifications for voltage, flicker, frequency, and distortion. Out-of-bounds conditions for any of these variables require the inverter to cease to energize the utility line. Voltage and frequency set points for systems larger than 10 kW may be altered by the utility to accommodate system-specific needs.

http://www.solarelectricpower.org/interconnection/position_statement.cfm