Page 317 - Sustainable On-Site CHP Systems Design, Construction, and Operations

290 Op erations

Efficiency/
Effectiveness
Component Purpose Relation Variables
Cooling towers Cool chiller (T CT, ,wi −T CT, ,wo ) η = cooling tower efficiency
CT
condenser water η = (effectiveness)
CT
(T −T )
via evaporation CT, ,wi wb η = cooling tower electric
CT,elec
and sensible Q utilization efficiency
heat transfer to η CT,elec = W CT,th T = inlet temperature of
CT,w,i
ambient air CT,elec condenser water to the tower
T = outlet temperature
CT,w,o
of condenser water from the
tower
T = wet-bulb temperature of
wb
the ambient air
Q = rate of heat loss by
CT,th
the cooling water as it passes
through the cooling tower
W = electric power use
CT,elec
by the cooling tower fans and
pumps
Desiccant Remove moisture η = Q d η = desiccant system
D
systems from air with D Q efficiency
the desiccant d,input Q = rate of moisture
d
regenerated using removal from the air stream
waste heat (dehumidification load)
Q = rate of heat input for
d,input
desiccant generation
Pumps Create a pressure W Pump η Pump = pump efficiency
difference in liquid η Pump = W W = mechanical power
Pump
to instigate flow Pump,elec output from the pump to the
using an electric liquid
motor as a source W = electric power input
Pump,elec
of mechanical to the pump motor
rotational energy
Fans Create a pressure η = W Fan η = overall efficiency of the fan
Fan
difference in air to Fan W W = useful power output from
Fan
support flow, using Fan,elec the fan
an electric motor W Fan,elec = electric power input
as the source to the fan motor
of mechanical
rotational energy

TABLE 17-1 Summary of Expressions for CHP Component Efficiencies (Continued)
where Q is the rate of fuel energy input at point j in the system (e.g., to the prime
Fuel,j
mover or for supplemental heating of exhaust gases before entering the heat recovery
unit) with the sum being over all fuel inputs to the CHP system. These fuel inputs may
include the same fuel (e.g., natural gas) introduced at several different points in the
system or may be different fuels (e.g., diesel fuel for a reciprocating engine prime mover