Page 310 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 283

direct-fired absorption system, heat is provided directly by hot exhaust gases from the
prime mover, while indirect-fired systems use either steam or hot water to power the
refrigeration cycle. If supplemental heat is needed, it can be provided by burning
auxiliary fuel in a duct heater placed in the exhaust gas stream.

Efficiency of Absorption Chiller
The efficiency of an absorption chiller is given by the coefficient of performance
(COP ) defined as
AbChiller
Q evap
COP = (17-33)
AbChiller Q
gen
where Q is the rate at which water is cooled by the evaporator; Q is the heat input
evap gen
equal to the rate of heat loss from the exhaust gas, steam or hot water as it passes
through the absorption unit’s generator to desorb the refrigerant from the weak solu-
tion; and W is the pump energy, which is small compared to Q and is ignored.
in evap
Here,
Q = m c T ( − T )
evap evap, wi , p w evap, w i , evap, w o ,
,
= v ρ c T ( − T ) (17-34)
,
evap, wi , evap, wi , p w evap, w i , evap, w,o o
where m = mass flow rate of chilled water into the evaporator
evap, wi ,
v evap, wi , = volumetric flow rate of chilled water entering the evaporator
ρ and c = density and specific heat of chilled water entering the evaporator,
evap,w,i p,w
respectively
T and T = evaporator entering and leaving chilled water temperatures
evap,w,i evap,w,o
For direct-fired absorption chillers:
Q = v ρ c T ( − T ) (17-35)
,
gen ex, i ex, i p ex ex, i ex, o
where ρ = density of the exhaust gases entering the absorption chiller
ex,i
v = volumetric flow rate of exhaust gases entering the chiller
ex, i
c = specific heat of the exhaust gases (evaluated at the average exhaust gas
p,ex
temperature in the chiller) ∗
T and T = exhaust gas entering and leaving temperatures, respectively
ex,i ex,o
For absorption chillers that use hot water from an HRU to generate the refrigerant:
Q = v ρ c T ( − T ) (17-36)
,
gen hotwater, i hotwater, i p hotwater hottwater,i hotwater,o

∗ The product of volumetric flow rate of the exhaust gas and its density, which is the mass flow rate, is
constant through the chiller during steady operation; therefore, v ρ = v ρ , and this quantity can
ex, i ex, i ex, o ex, o
be evaluated at either the inlet or exit conditions. We recommend evaluating c at the average of the
p,ex
inlet and outlet temperatures of the exhaust gas; however, the difference in the value of c evaluated
p,ex
at the inlet conditions and the outlet conditions will be less than about 8 percent for the exhaust gases
in most practical situations.