Page 202 - Sustainable On-Site CHP Systems Design, Construction, and Operations

The Engineering Pr ocess 175

• Desiccant dehumidification
• Product drying
• Process heating
Additional power can be produced, for example, by using steam generated in a HRSG
to drive a steam turbine generator. The steam turbine generator can either be a con-
densing type or backpressure turbine type that reduces steam pressure to a pressure
required by another system (e.g., from 250 to 15 psig required by single-effect absorption
chillers). Space heating, for example, can be provided with steam or hot water (HW) coils,
and hot water can be generated either from steam in a steam-to–hot water heat exchanger
or from a JW-to-HW heat exchanger. At some facilities, high temperature hot water
(HTHW) is generated at the CHP plant and circulated to facility buildings, where the
temperature is stepped down at a building in a HTHW-to-HW heat exchanger. However,
generally low temperature HW (LTHW) systems are preferred as they are safer, have less
thermal expansion challenges, and work well with IC reciprocating engines.
Chilled water for space cooling can be generated by a number of methods as
described in Chap. 4 including: single- or double-effect absorption chillers using lithium
bromide or ammonia; adsorption chillers; and steam turbine–driven centrifugal chillers.
Also, as discussed in Chap. 4, a single-effect absorption chiller is capable of being fired
with low temperature HW (less than 250°F) but typically requires temperatures above
200°F to be cost-effective. Double-effect absorption chillers typically require 125-psig
steam or equivalent high temperature fluid and therefore are only used with CTG CHP
systems. Likewise, steam turbine–driven centrifugal chillers require medium-pressure
steam and therefore are also only used with CTG CHP systems.
Providing a variety of thermal uses with a correctly sized CHP system will enable
the CHP plant operators to maximize the use of available heat recovered. The CHP
plant operators will also sometimes be required to make equipment operations choices,
for example, would it better to produce additional power or to produce additional cool-
ing or to use the heat instead for space heating. Later chapters and case studies describe
valuing the CHP products and costs and determining which thermal output has the
greatest value. Of course having to make a choice where to put the heat is vastly superior
to not having a choice and being forced to dump heat or to turn down or to shut down
the CHP plant altogether.

Electrical Interconnection and Protections
The proper electrical interconnection, generator and facility electrical system pro-
tections and safeties, system grounding, as well as generator paralleling controls
(assuming electric utility interconnection) that meet all code and utility interconnection
requirements are key for successful CHP operation, and these issues are discussed in
detail in Chap. 11. As with the mechanical and thermal elements, the type of CHP sys-
tem has an effect on the CHP electrical design itself. The generator type must be com-
patible with the existing electrical system and transformers and switchgear are typically
required. The generator voltage usually matches the highest system voltage needed at
the facility to minimize electrical resistance losses. Often, the local utility will require a
detailed interconnection agreement that will, for example, specify minimum import,
system protections, and grounding requirements. Protections must also be in place, for
example, to account for voltage spikes and sags, as well as a loss of power.