Making Hennepin a Cool County
Minnesota’s Hennepin County is committed to reducing greenhouse gas
emissions from county operations by 80 percent by 2050 as part of the Cool
County Initiative – a coalition of counties across the United States that are
taking action to eliminate the causes of global climate change. Hennepin
County is a founding member of the coalition.
equipment to be installed and securing
necessary permits and approvals were
necessary for the work to be done under
Seventh Street. Modifications at HERC
were also designed and constructed to
avoid any interference with the existing
setup and operation of the facility.
The county will reduce emissions by assessing current greenhouse gas output,
implementing mitigation programs and policies, coordinating energy efficiency
efforts with Minnesota state departments and supporting federal emission
The county plans to reach the emission reduction goal by
; increasing energy efficiency in buildings,
; incorporating green building design,
; using renewable energy,
; increasing fuel efficiency in the county fleet and using alternative fuels,
; leading efforts to improve transit and develop livable communities, and
; engaging employees to reduce their environmental impact.
system includes 6 miles of steam piping
and 4 miles of chilled-water piping.
Establishing a Partnership
The first step in developing the steam
line was for Hennepin County and NRG to
establish a contract laying out construction plans and the terms for ownership,
operation and maintenance, and pricing
for the line. The contract included the
; Construction – NRG acted as general
contractor for the steam line construction. Hennepin County and NRG selected
NewMech to perform construction. The
county covered the costs of all work
done on the HERC site and split the costs
with NRG for all work done off the site.
The steam line cost $3.4 million, with
the county covering $2.9 million. The
county’s share was larger because it
included the equipment installed at
HERC. Payback for the project is expected
to be about five years.
; Ownership – Hennepin County owns
everything on the HERC property and
about half the steam line, from where it
leaves the HERC site and runs over the
railroad. NRG owns the other half of the
line beyond the bridge over the railroad
to where the steam line connects with
NRG’s downtown district heating system.
; Operation and maintenance – Hennepin
County is responsible for the maintenance of everything on the HERC site,
with the county covering those costs.
NRG maintains everything off the HERC
site with the county and NRG sharing
; Pricing – Hennepin County bills NRG
for the steam purchased. The price is
based on local natural gas prices.
NRG agreed to purchase two-thirds
of the pipeline’s steam from HERC and
deliver the other one-third to the county’s
district energy system at no charge. Steam
is delivered from the NRG system to the
county district energy system at the
Hennepin County Government Center.
The steam delivered to the county system
will reduce the amount of steam that the
Hennepin County Energy Center is required
to produce. This allows the county to
generate steam at a lower cost and use
less fossil fuel.
Hennepin County also needed to get
approval from Covanta Energy, the contractor that operates HERC, to install the
steam line. By diverting steam, both
Hennepin County and Covanta would lose
some revenues from the sale of electricity
but would gain revenues from selling
steam to NRG. This required an amendment to the service agreement between
the county and Covanta, with the two
parties agreeing to share revenues from
The bridges on Seventh Street had
been designed for a 12-inch steam line
that could carry 100,000 to 120,000
lb/hr of steam. The installed steam line
can carry up to 100,000 lb/hr of steam.
Several approvals and permits had
to be secured before work could be done
on Seventh Street. NewMech obtained
approval from the railroad authority to
install the steam line on the bridge over
the railroad tracks; air rights over the
tracks had to be clarified before work
could begin. NewMech also secured permits
from the Minnesota Department of Transportation for work to be done on the
bridge over Interstate 394. This part of
the project was completed without closing the interstate.
Constructing the Line
Design plans for the steam line started
in 2007, a contractor was chosen in April
2008, and construction began shortly
thereafter. Determining the size of the
The necessary modifications at HERC
were designed and constructed to facilitate
the ongoing plant operation:
; Diverting steam – The diverted steam
is taken off the main steam header
before it enters the turbine. A T-joint
and valve were installed off the main
steam line during a regularly scheduled
maintenance outage at HERC, allowing
the steam line to be added without
shutting down the facility’s operation.
Pipes for the steam line were designed
to be routed around existing HERC
structures and operations.
; Reducing temperature and pressure –
Because high-pressure steam is diverted
into the steam line, the pressure and
temperature of the steam need to be
reduced before it can be incorporated
into the downtown district heating
system. Pressure-reducing valves were
installed to lower steam pressure from
600 psi to 250 psi in summer and 150
psi in winter. An attemperation system
was also installed that sprays water into
the steam to lower the temperature from
750 degrees F to about 400 F.
Although attemperation is not the
preferred operation method, it allowed
for system startup without taking down
the boiler. It also allows the plant to burn