Page 90 - Sustainable On-Site CHP Systems Design, Construction, and Operations

68 CHP B a s i c s

The cooling T/E ratio (T/E ) for a CHP system is expressed in terms of the cooling
C
output in refrigeration tons divided by the power output in kilowatts as follows:
T/E = thermal output (tons)/power output (kW)
C
For a CHP system with an electric output of 1000 kW that can also provide 250 tons of
cooling, the T/E ratio is 0.25 ton/kW.
C
The above examples are typical for a reciprocating engine–based CHP system with
jacket and exhaust heat recovered to provide water heating which can be used to drive
a single-stage absorption chiller for cooling. If we need to alter the T/E ratio, it is
possible to configure the same engine to recover exhaust only and use a higher effi-
ciency two-stage chiller for cooling. This configuration would still provide 1000 kW, but
would have a T/E of 2 MBH/kW and a T/E of 0.2 ton/kW. The same 1000 kW engine
H C
can also be configured to recover the jacket heat only and use a single-stage absorber for
cooling to provide a T/E of 2 MBH/kW and a T/E of 0.13 ton/kW.
H C
As discussed in Chap. 3, combustion turbine–based CHP systems that have more of
the fuel energy converted to thermal output will have higher heating and cooling T/E
ratios. In addition, the thermal output can be recovered as high-pressure steam
(e.g., 125 psig) allowing the use of higher efficiency two-stage absorbers or steam
turbine chillers to further boost the cooling T/E ratio. For a typical 5-MW combustion
turbine recovering the exhaust to generate steam for heating or to drive a high
efficiency chiller, the T/E is 5.5 MBH/kW and the T/E is 0.6 ton/kW. The same CHP
H C
configuration using a duct burner to double thermal output will have a T/E of
H
11 MBH/kW and the T/E of 1.2 tons/kW. If we replace the high efficiency chillers with
C
a single-stage steam absorption chiller, the system will have a T/E of 5.5 MBH/kW
H
and a T/E of 0.35 ton/kW.
C
These examples serve to illustrate that defining a specific thermal load does not
necessarily restrict either the choice or size or the prime mover. This is also a key ele-
ment in properly applying the thermal first design approach.
Building Loads
For maximum load factor, the CHP system should in general be designed to address the
facility minimum or base thermal and electric loads. It will often be necessary to com-
bine more than one thermal load in order to maximize the system load factor such that
the CHP system will be able to provide heating in winter and cooling in summer. The
system should also be able to provide part heating and part cooling load simultane-
ously so that high load factor can be maintained through the shoulder seasons (spring
and fall). Based on the above discussion on thermal electric ratios, it is apparent that we
can vary the heating and cooling T/E ratios independently without having to alter the
power output. Assessing all the building thermal needs as well as the power needs will
help provide us with the optimal configuration for the CHP system without necessarily
restricting choice of the power generator size or technology. In fact, it is essential to
properly assess the year round thermal needs of the facility in order to be able to
optimize the CHP system design.
A highly significant factor in determining and serving the available facility loads to
be addressed by a CHP system is the availability of a thermal distribution system. In
order for the thermal energy to be “useful” to the facility, the thermal energy must be
capable of being distributed. While a new distribution system can be incorporated into