By John Trotti

When all is said and done, two principal reasons for onsite power stand at the fore—reliability and efficiency—with the major focus quite understandably on the former.

Every year brings us a raft of horror stories in which disasters of different stripes lay waste to large chunks of regional infrastructure, including, invariably, the electric grid and all the services dependent on its uninterrupted operation. We all know what such calamitous events could do to us in our homes and businesses, and you’d think that this alone would be an inescapable call to action; yet most of us—even those with what might be considered a fiduciary responsibility to prepare for such contingencies—tend to fret for a while and then succumb to the “Mañana” principle.

So if reliability isn’t enough of a motivation for onsite power, how does efficiency fare? Offhand it wouldn’t seem to be much of a motivator compared to calamity; but whereas the memory of disaster is fleeting, our awareness of day-to-day operating costs is a living presence, so let’s take a look at one aspect of the situation: wasted heat.

The Value of Heat Recovery
If you can’t make use of recovered heat, then the value of this discussion will be purely academic, but for many operations heat itself is a valuable resource. So what overall efficiency increase can be expected where heat recovery is involved? While the answer depends on the amount of heat recovered, you can use the following set of equations for purposes of analysis.

Efficiency (%) = 3,413 ÷ heat rate
Waste heat = heat rate x [1 – efficiency (%)]
Recoverable heat = waste heat x heat recovery (%)
Overall efficiency = [heat rate x efficiency (%) + recoverable heat] ÷ heat rate

To illustrate the situation, consider the case of a gas turbine with a heat rate of 10,000 Btus per kilowatt-hour (Btus/kWh). Since there are 3,413 Btus/kWh, from the above equation this means that the subject unit has an overall efficiency of 34%. Thus, 6,587 Btus of the energy value of the fuel are lost in this scenario.

While it’s not possible to recover 100% of the waste heat, there are systems that allow you to recover about 75%, which could boost the overall efficiency by recovering 4,940 Btus/kWh, or 49% (66% x 75%), of the input fuel. Whether or not you actually recover this much of the fuel depends on local thermal needs, but this at least establishes a practical upper limit for recovery.

Since this heat recovery displaces heat that you'd otherwise have had to achieve through other means—perhaps an onsite boiler—you can assume that this would displace another combustion process with an assumed efficiency of about 90% of its purchased fuel.

A summary of the economic—and environmental—impact of this 10,000-Btus/kWh gas turbine with 75% heat recovery is thus:

Where Lies the Bottom Line?
Does this mean that the fuel efficiency achieved by installing a gas turbine with a heat-recovery system in an operation makes it economically superior to the use of grid power in conjunction with an onsite boiler? Probably not, but the comparison may not be so simple. Since it is, however, germane to the discussion, it is one we will look at in the next issue.

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DE - September/October 2007