District Energy - 2005 Third Quarter

to supplement emergency power, but only if when the emergency power is up and running first.

In the case of UW, steam is delivered at 175 psig campus-wide and reduced to 10 psig within the buildings. Several large-scale buildings exist on campus and are planned for the future that have year-round steam loads – like hospital and research facilities – and are prime candidates for STGs. One particular candidate is a research building currently under design that is more than 1 million sq ft and has an estimated peak load of more than 140,000 lb/hr. It is estimated that the base load on the facility would fall between 10,000 lb/hr and 15,000 lb/hr.

This simplified analysis looked at STG

capacities within this range as well as one size smaller and larger and the available operating hours based on an estimated annual load profile. Table 1 summarizes the results of the analysis, and figure 1 is a graphical representation of paybacks based on electric rates and operating hours.

As seen in table 1, the paybacks are very favorable and actually accelerate as size increases due to the economies of scale. Figure 1 shows even faster paybacks when using higher electric rates. In closing, this building and others may be candidates for an STG if they have high-pressure steam (>75 psig) available and a minimum 5,000 lb/hr base load of low-pressure steam operating for more than half the available hours per year.

Author’s Note: The author thanks Rob Otremba of Affiliated Engineers Inc. and the friendly folks at Turbosteam for assistance with this article. Some recommended IDEA resources for further reading: ( 1) “Free Electricity From Steam Turbine-Generators: A system level economic analysis” by Sean Casten, Turbosteam Corp., and Thomas O’Brien, Integrated Process Solution, District Energy magazine, first quarter 2003, and ( 2) “Backpressure Steam Turbines Used as Steam Pressure-Reducing Stations,” Bruce McLay and Krishnan Ramesh, P.E., Affiliated Engineers Inc., Proceedings from the 2003 IDEA Campus Energy Conference, Austin, Texas. (Available free of charge to IDEA members in IDEA’s Web archives.)

Source: Affiliated Engineers Inc., cost of STG per Turbosteam.

Table 1. Example of Induction Steam Turbine Generator Simple Payback.

STG size Output
(lb/hr) (kW)

5,000

10,000

15,000

20,000

*Estimate includes material and installation labor, design fees, permits and annual operating costs.

86

198

299

387

Blended Estimated
Electric Rate Hours of
Used Operation

$0.053 8,500

$0.053 7,000

$0.053 6,000

Estimated Cost Annual Simple
Cost* per k W Savings Payback
(Years)

4. 6

3.0

2. 8

$180,000

$266,000

$311,000

$348,500

$1,950 $39,200

$1,250 $89,900

$950 $112,000

$820 $124,000

Steve Tredinnick, P.E., is a
project engineer for Affiliated
Engineers in Madison, Wis.,
with more than 20 years’
experience related to building
HVAC systems. The past 10
years of his work have been
focused on district energy
systems. Tredinnick is a graduate of Pennsylvania
State University with a degree in architectural
engineering. He is a member of IDEA and
ASHRAE and is currently vice chair of ASHRAE
TC 6. 2 District Energy. Tredinnick may be
reached at stredinnick@aeieng.com.

Column also available at
www.districtenergy.org/de_magazine.htm

Two Great IDEA Workshops Together!

At the marketing workshop you will discuss strategy, messages, materials, methods and the myriad opportunities to win the confidence of your key customers. Connect with your colleagues. Exchange ideas. RECHARGE! We’re using ideas from the annual conference forum meetings to determine presentations and panel topics.

At the distribution workshop, discussions will focus on the challenges of building and maintaining safe, reliable thermal piping networks – steam, hot water, chilled water. What works… what doesn’t…what’s new?

18th Marketing Workshop and 17th Annual Distribution Workshop

Nov. 2-4, 2005

Doubletree Paradise Valley Resort

Scottsdale, Ariz.

Hosted by Northwind Phoenix

Distribution and marketing professionals mark your calendars! Don’t miss your forum workshop this fall in Scottsdale, Ariz.! IDEA is offering two separate but simultaneous distribution and marketing workshops Nov. 2-4, 2005.