Page 88 - Sustainable On-Site CHP Systems Design, Construction, and Operations

66 CHP B a s i c s

Thermal
Revenue

Electrical

Thermal
Profit

Electrical

0% 20% 40% 60% 80% 100%

FIGURE 4-1 Thermal contribution to CHP economics. (Courtesy of Integrated CHP Systems
Corporation.)

Conventional wisdom would lead to selection of a generator based on electric load
profile, then adding thermal equipment based on maximizing system efficiency and
then trying to fit the thermal output to the building. This design approach has led to
many CHP systems that simply do not meet efficiency or economic expectations as the
thermal output is not properly synced with the building resulting in poor load factor.
CHP planning experience and design wisdom tells us that it is better to select the thermal
components of the system based on the building or facility’s thermal loads first, then to
select a generator based on thermal output and then to fit the electric output to the
building. This of course needs to be done with close regard to the building electric load
profile and the process is an iterative one that must result in high load factor for both
the electric and thermal outputs of the system.

Load Factor versus Efficiency
It is important to understand the difference between CHP system efficiency and specific
equipment efficiency. In most cases, CHP system efficiency is calculated based on the
sum of the generator electric output plus the useful thermal energy recovered from the
engine generator divided by the fuel input in consistent units:

CHP effi ciency = (power out + useful heat recovered)/fuel input
Therefore, the more thermal energy recovered, the higher the CHP system efficiency.
Additionally, the typical goal of CHP design is to meet the facility needs—not
exceed them. A CHP plant can have multiple options on thermal conversion equipment
offering multiple outputs for the same input. It is not always optimal to choose
equipment offering the highest possible output. For example, a CHP system can be
configured to provide either 75 tons from a single-stage absorber or 100 tons from a