Page 312 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 285

The value of η only indicates how well the cooling tower cools the condenser
CT
water in terms of how close the water temperature approaches the limiting wet-bulb
temperature of the ambient air. It does not indicate how the cooling was achieved or
how much external electrical energy input was used to achieve this reduction in tem-
perature. For example, if the cooling tower medium becomes fouled, increasing resis-
tance to air flow and inhibiting heat transfer, the cooling tower fans might run longer or
at a higher speed (for variable speed fans) to achieve the same temperature drop for the
water that was accomplished with less fan energy when the medium was not fouled.
In addition, electric power is used to pump the condenser water to and from the cooling
tower. To provide a metric for how efficiently electricity is used in this process, we
define a cooling tower electric utilization efficiency (η ) ∗ as
CT,elec
Q
η = CT,th (17-39)
CT,elec
W
CT,elec
where Q is the rate of heat rejection from the cooling tower and is equal to the heat
CT,th
loss by the condenser water in passing through the cooling tower and W is the
CT,elec
electric power use by the cooling tower fans and pumps. The electric power use is the
sum of the electric power used by all the individual pumps and fans, that is,
CT,elec ∑
=
W W CT,elec, j (17-40)
j
where W is the electric power use by the jth pump or fan, and the summation is
CT,elec,j
over all pumps and fans.
The rate of heat loss from the condenser water can be determined from measure-
ments of the entering water temperature (T ), the exiting water temperature
CT,w,i
(T ), and the volumetric flow rate of water through the cooling tower ( v )
CT,w,o CT, w
using the relation
Q = ρ v c T ( − T ) (17-41)
CT,th w CT, w p w CT, w i , CT, w o ,
,
where ρ and c are the density and specific heat of liquid water, respectively.
w p,w
Combining Eqs. (17-39) through (17-41), the cooling tower electric utilization effi-
ciency can be expressed as

ρ v c T ( − T )
η = w CT,w p w, CT,w i, CT,w o, (17-42)
CT,elec ∑ W
j CT,ellec,j

Cooling Tower Performance Calculation
The algorithms used to calculate the performance of the cooling tower are based on
Eqs. (17-38) and (17-42). The density and specific heat of liquid water are assumed to be
constant across the cooling tower. They can be evaluated at the average of the water
inlet and outlet temperatures to improve accuracy.

∗ Including condenser water system pumps—it may be considered as condenser water system efficiency.