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2/538 Engineered Systems June 2010

Cogeneration can be employed anywhere there is

a simultaneous need for electrical and thermal

energy, and it is often referred to as combined heat

and power (CHP). CHP is any number of applied

technologies that simultaneously produces two or

more forms of energy from a single fuel source.

Prior to the development of the electrical distribution grid, indus-trial concerns generated their power on-site and developed cogeneration

techniques and applications to utilize the resulting waste heat. As the util-

ity industry grew and took hold, their economies of scale and reliability

eventually made on-site generation of electricity uneconomical in many

cases and thereby significantly reduced the prevalence of cogenerated

power. But in 1978, the Public Utilities Regulatory Policy Act (PURPA)

required public utilities to purchase a portion of their electric generating

capacity from non-utility generators that use alternative energy sources

and cogeneration in order to reduce dependence on foreign oil.

In the 80s, lower natural gas rates allowed the economical produc-

tion of on-site power and helped to spur cogeneration projects. Growth

in this sector was then stalled due to higher gas prices. The recession

has kept gas prices depressed and discoveries in Texas Barnett shale

and Pennsylvanias Marcellus shale may keep these prices low. Present

concerns about global warming and the long-term availability of fossil

fuels have created an environment where re-exploring the application

of cogeneration makes good sense, since this process can result in less

fossil fuel consumption and fewer emissions than generating electrical

power and heat through separate processes.

CHP SYSTEMS

Most CHP systems use a topping cycle, in which the fuel source is

first used for generating electricity and then to recover the resultant

heat for thermal needs, such as space and domestic water heating

and cooling and the regeneration of desiccants used for dehu-

midification. A bottoming cycle uses fuel to drive industrial thermal

processes, with the exhaust gases then being used to produce power,

typically with a heat recovery steam generator in series with a steam

turbine. The topping cycle can use a variety of prime movers, such

as reciprocating engines, steam turbines, gas turbines, microtur-

bines, and fuel cells to generate electrical power.

A common prime mover for CHP applications is the recipro-

cating engine, which has high mechanical efficiency over a wide

BY CARL C. SCHULTZ, P.E., LEED AP

Hospitals, hospitality, municipalities, and industrial applications may face their best chance in years to

save money (and natural resources) with a combined heat and power (CHP) system. From steam turbines

that rely on boilers to gas turbines, on to reciprocating engines and fuel cells, the variety of options is aswide as the range of settings where it might make sense. Reacquaint yourself with CHP, including the

sidebars small example that can save over $100,000 per year.



Closing The Cogeneration Gap

range of loading. By adding a turbocharger, the capacity of an

engine can be increased by 30 to 40%. Four-stroke engines operat-

ing at 1,800 rpm are a typical configuration. Engines can be run

on natural gas, diesel, or heavy (residual) fuel oils. In addition

to the electrical power produced, thermal

energy can be recovered from radiation off

the engine block, exhaust gases, lubricating

oil, jacket water, and the engine after-cooler.

Jacket heat can be recovered either as hot

water or as low-pressure steam, either underpressure in a forced circulation system or an

ebullient (boiling water) system.

The gas or combustion turbine consists

of a compressor that takes air at atmospheric

pressure and increases its pressure for entry

into the combustor where it is combined with

fuel and burned. The exhaust gases in the

neighborhood of 2,300F are then delivered

to the turbine, where they are converted to

mechanical work. Up to 50% of the turbine

power can be used to drive the compressor,

so strategies to cool the inlet air and increase

overall efficiencies are often implemented.The turbine exhaust temperatures are typi-

cally less than half that of the temperature

entering the turbine and can be used in a heat

recovery steam generator (HRSG) to produce

steam for heating, or they can be used directly for a process that

includes a direct-fired absorption chiller. Microturbines operate on

the same principal as gas turbines but have the advantage of being

small and lightweight, therefore offering installation flexibility.

The steam turbine is the workhorse of the utility power indus-

try and can be used on a smaller scale for CHP applications. Fuel

is burned in a boiler to generate high-pressure steam, where it is

sent to a steam turbine to generate electrical power. Before return-

ing to the boiler, the turbine exhaust must be condensed, and thiscan be done by using the heat to drive thermal processes instead

of being rejected to a heat sink such as a river or cooling tower. A

steam turbine can also be used in combination with a combustion

turbine in what is known as a combined cycle, where the output

from the HRSG is used to drive the steam turbine for additional

electric power production. Unlike the combustion turbine, which

can be dispatched quickly, the steam turbine system may take sev-

eral hours for warm-up.

Fuel cells produce electric power without combustion by a direct

conversion of the fuel into electricity typically by stripping the hydro-

gen from natural gas, propane, or butane and combining it with

oxygen in air. The byproducts include water, direct-current electricity,

heat, and low levels of CO, CO2, and NOx. Electrolyte filled cells areplaced in series to produce the voltage potential and are arranged in

parallel to develop amperage capacity. An inverter is used to convert

the direct current power to alternating current power. Although the

quantity of waste heat is limited since much of it is used in the conver-

sion process, the usable temperature can be high and can develop hot

water as well as low- and high-pressure steam. Although it is viewed

as an emerging technology, a fuel cells low noise level is an advantage

compared to other types of CHP systems.

APPLICATIONS

A base-loaded system will operate so that the prime mover is generat-

ing at full capacity nearly year-round only being stopped for annual

maintenance. At times when the rejected heat cannot be fully utilized,it will be dumped to the environment. CHP prime movers can also be

FIGURE 1. Diagram of a small congeneration system with a reciprocating prime mover.

500KW RECIPROCATING

GENERATOR500KW

ELECTRICITY

NATURAL GAS

5.3 MMBTUH

5,300 FT /HR

EXHAUST

LUBE OIL

HEAT EXCHANGER

JACKET WATER

HEAT EXCHANGER

EXHAUST GAS

HEAT EXCHANGER

RETURNLP STEAM

2,700 PPH

Input 125 at www.esmagazine.com/instantproductinfo



operated to track either the electrical load

or thermal load requirements of the site, or

they can be used in a limited capacity in a

peak-shaving operation. For facilities that

have negotiated interruptible rates or are

located in areas where the utility has insti-

tuted real-time pricing for electricity, theCHP system can be operated in an economic

dispatch mode to produce power when it is

less to do so that purchasing the power from

the utility.

Since CHP systems use a single fuel

input to provide electrical power and ther-

mal energy, having a simultaneous need for

both is a prerequisite for implementation.

Leading examples of industrial applications

include chemical, pulp and paper, textile,

and ethanol production. The hospitality

industry, where large hotels, resorts, and

casinos have on-site laundry, shower, andother thermally intense needs, can find a

CHP system to be an attractive proposi-

tion. Corporate research campuses offer

opportunities when there is a steam distri-

bution system that requires thermal input

year-round.

Hospitals and other types of critical

facilities can obtain the added benefit of a

redundant power supply. Health care and

higher education campuses have led the way

with smaller CHP plants, but municipalities

have gotten into the act with installations at

airports and with municipal district heating

and cooling systems. The thermal energy

developed through cogeneration can be used

during the cooling season to operate steam

turbine-driven chillers and absorption chill-

ers as well as for the regeneration process

involved with desiccant dehumidifiers.

Municipalities have tied CHP opera-tions with their waste programs by utiliz-

ing digester and landfill gas, trash, and

other urban wastes. District Energy St.

Paul operates the largest wood chip power

plant in the United States. This combined

heat and power plant heats 185 buildings

and 300 single family homes (31.1 million

sq ft), cools more than 95 buildings (18.8

million sq ft) as well as generates 25 MW

of electricity. The plant uses 280,000 tons

of wood waste/yr per year from the citys

recycling center, which is supplemented by

natural gas, oil, and coal. Military installa-tions account for CHP applications, includ-

ing remote facilities such as radar sites in

Alaska. In this application cogeneration is

the obvious choice as there is no grid to tie

into and there is a need for simultaneous

thermal and electrical production.

ECONOMICS

The average coal-fired power plant in the

U.S. operates at an efficiency of about 32%,

which is primarily due to the 40% efficiency

of the typical steam turbine in converting

mechanical power to electrical power. Con-

Closing The Cogeneration Gap


Cogeneration Economics: An Example A research campus has minimum low-pressure steam consumption of 2,700 lb/hr in the summer at night just to offset losses in the distribution system and has

decided to install a CHP plant using a 500-kW natural gas reciprocating generatorin a base loaded configuration, which will operate 8,600 hrs/yr. The facility has anaverage purchased-power cost of $0.07/kWh and a gas cost of $5/MMBtuh. Theplant assumptions are an electrical conversion efficiency of 32% and a heat recoveryeffectiveness of 70%, for an overall efficiency of roughly 80%.

The plant will consume 46,000 MMBtuh of natural gas annually x $5 = $230,000/yr The plant will produce 8,600 hr/yr x 500 kWh = 4,300,000 kWh for an avoided cost

of 4,300,000 kWh x $0.07/kWh = $301,000 Annual maintenance is assumed at $0.01/kWh production or $43,000 The annual heat recovery is estimated at 22,500 MMBtuh, which would require a

boiler input of 27,500 MMBtuh assuming an 80% efficient boiler, for an avoidedcost of 27,500 MMBtuh x $5 = $137,500/yr

Total electric and gas avoided cost is $301,000 + $137,500 = $438,500.

The avoided cost of $438,500 minus the gas consumption of $230,000 yields a$208,500 annual utility cost savings. Subtracting out the $43,000 maintenance costresults in a $165,500 net annual savings.

At an installed cost of $1,100 per kWh or $550,000, the CHP plant has a simplepayback of $550,000/$165,500 = 3.3 years.


5/5www.esmagazine.com 43

sequently, two-thirds of the prime energy

input is wasted as heat to the atmosphere.

Because of this, CHP systems can be viable

in applications where there is a large and

continuous demand for thermal energy in

close proximity to the cogeneration plant.To be economical, this thermal load should

be year-round and not just seasonal in nature.

CHP systems obtain most of their econom-

ic benefit from the production of electrical

power where large installations may sell excess

power to the electric utility, while smaller

systems will typically use the power on-site to

offset purchased power from the grid. CHP

economics fare better where they have a high

generating efficiency, so a microturbine-based

system with a 24% efficiency may not be as

attractive as one with a reciprocating engine

with a 36% efficiency.An example of a small CHP application

is shown in the sidebar, using a gas recipro-

cating engine that has an electric efficiency

of 32% that coincidently matches the U.S.

average of coal-fired plants.

CONCLUSION

The ultimate appeal of the concept of cogen-

eration depends on ones vantage point, with

technicians marveling at its sheer efficiency,

the environmentally minded at its promise

for the economical use of the Earths remain-

ing fossil fuels, and with the plant managerlooking to reduce operational costs.

Most of us should by now have the sense

that the cost of fossil fuels will rise over time

in one of three possible scenarios: 1) slowly,

as technological advances in exploration and

discovery help offset increases in consump-

tion; 2) rapidly, as we emerge from economic

recession or are faced with carbon caps or

taxes; or 3) catastrophically, as we head off a

production cliff. Whatever your profession,

political tendencies, or social disposition,

you should applaud advances in the devel-

opment of cogeneration technologies, sincethey will help slow the use of finite resources,

reduce pollution, and help us compete in a

competitive global economy. ES

Schultz is a vice president with Advanced

Engineering Consultants in its Columbus,

Ohio office. He is a graduate of the Ohio State

University with a BSME and has 20 years of

experience designing mechanical systems for

commercial and institutional facilities. In addi-

tion, he has extensive design experience with

central steam, high temperature hot water,

and chilled water plants. He is a registered

professional engineer in over a dozen states

and is the author of many technical articles

related to HVAC system design and commis-

sioning. Contact him at [email protected].

Founded in 1998, Advanced Engineering

Consultants (AEC) provides a wide varietyof mechanical and electrical engineering ser-

vices to support the commercial, industrial,

and institutional facilities sectors. AEC is a

minority-owned firm

committed to offering

the latest in sustain-

able design practices.

For more information,

visit the companywebsite at www.aec-

mep.com.

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