Rising power costs are making many building owners and operators look again at distributed generation to cut electric bills. Many devils are in the details of siting such units, with problems related to fuel delivery and storage, fire safety, noise, emissions permits, flue gas exhaust, etc. Wouldn't it be great to find a generation source that avoided nearly all of those problems while producing cost-effective power?

As a result of new steam turbine technology supported by a major hvac vendor, buildings that receive utility or district steam may now have that option.

What is District Steam?

Buildings in over 100 major U.S. cities receive high-pressure (usually 125 psi) steam from district-wide systems through underground pipelines. Many private district steam systems are also installed at colleges, medical centers, prisons, military bases, office parks, etc. The International District Energy Association (www.districtenergy.org) lists the cities having such systems, which exist in 38 of the 50 states.

Turning Heat Into Torque

Steam generated at high pressure must be stepped down for most final uses (e.g., in radiators for space heating). Most facilities use pressure-reducing valves (PRV) that waste a portion of the steam's energy in the process. Industrial plants often create that pressure drop by running high-pressure steam through reciprocating or turbine engines that drive motors or small electric generators.

Steam-driven reciprocating machines are generically called spilling engines and have been used for over 50 years. While not very efficient, these workhorses are adept at using steam generated from high-pressure boilers to run large fans, generators, and other consumers of torque. For more information about them, go to EPC Energy (www.epcenergy.com) which is a major distributor in North America.

In cities where high electric prices are common, some central plants employ single-stage turbines to run pumps and fans. While using steam provided by the plant's own boilers, the turbines offer a low-cost source of hp; such units may have a long payback period at typical utility steam and electric rates. One major vendor of such products is Turbosteam Corporation (www.turbosteam.com).

Douglas Energy and Carrier Step Up

Under a DOE grant, Douglas Energy (www.douglasenergy.com) developed and is installing prototypes of a new two-stage steam turbine in California and New York. Its system is being offered by the Carrier Corp. as the 150-kW Microsteam(tm) Turbine Generator. Boasting nearly twice the thermal efficiency of single-stage turbines (70% against a single stager's 40%), this unit uses most of the enthalpy that would be lost during pressure reduction, thus providing nearly free electric power.

What, that's not good enough for you? How about a state grant to cover 50% (or more, in some cases) of the total cost to put one in your building? Some state energy agencies (such as the New York State Energy Research and Development Authority, www.nyserda.org) are offering subsidies to demonstrate this technology, especially where power grids are already strained.

Such agencies also know that adopting this technology where utility steam systems exist cuts NOx and CO2 emissions for 50% or more while supplying a quick way to site much-needed generation.

Since no on-site combustion is involved, siting such units does not require any emission permits, exhaust stacks, fuel delivery/storage, or additional fire safety systems. It's small enough to fit in small mechanical rooms and its noise level (below 85 dB) is tolerable in enclosed spaces.

More Details

The 150-kW unit is 6-ft tall, 28-in. wide, 26-in. deep and includes a turbine generator, frequency converter, control panel, and an optional stepdown transformer. As an induction generator, it is not intended to provide stand-alone or emergency power by itself.

Running on inlet steam as low as 65 psi, it reduces pressure at about a 3:1 (or greater) ratio. With sufficient utility steam pressure, its output can run a two-stage absorption chiller.

Designed to operate between 4,000 and 10,000 lb/hr, a single unit is well sized for the steam flow of a 200,000-sq-ft commercial building in a moderate climate.

Douglas Energy is looking at a design life of 20 years (running 8,760 hr/yr), with regular maintenance every 3 to 5 years. Simple payback should fall in the 2 to 6 year range, depending on relative pricing for utility steam and electricity.

The Mannhattan demo unit is being installed at the Rolex Realty Building on Fifth Avenue, while another is going into a California State University facility. ES