Page 98 - Sustainable On-Site CHP Systems Design, Construction, and Operations

76 CHP B a s i c s

This means that a 100 nominal ton chiller will only provide 70 tons at 210°F inlet hot
water temperature. Therefore in order to meet the full cooling capacity from a system
capable of producing 100 tons the system will require a chiller sized at 100 tons
divided by 70 percent or 143 nominal tons. This adds to the cost, size, and weight of the
chiller and needs to be factored into the economic and engineering evaluation. As
the inlet temperature to the absorber (outlet temperature from the heat recovery device)
goes below 200°F the capacity loss becomes too large for many applications to remain
viable. Note that the efficiency remains fairly consistent through most of the temperature
range and is much less of a concern.
Many absorption manufacturers now provide “low temperature” versions of their
single-stage design to specifically address this issue. These low temperature hot water
absorbers provide enhanced tube surface, higher heat transfer surface area, multiple
pass circuits for longer residence time and other alterations to improve capacity resulting
in a more cost-effective absorption chiller that will also provide improved performance
at part load conditions. It should be noted that these low temperature chillers do cost
more per ton and are larger and heavier than “standard” design chillers.
If cooling from an internal combustion (IC) engine–based CHP system is a critical
factor, then it is recommended to select an engine that can offer the highest hot water
temperatures within the size range required to help reduce the cost impact of the
cooling system.
Hybrid single-/two-stage absorbers are also available for reciprocating engine–
based CHP systems. These chillers take the exhaust energy directly into the first stage
of the absorption generator and the jacket coolant loop into the second stage of the
generator. The system can generate cooling in the first stage at a COP of 1.2 and in the
second stage at 0.7 for a combined COP of approximately 0.95 assuming a generator
with 50 percent of its thermal output in the form of exhaust heat and 50 percent in the
jacket coolant loop. The hybrid chiller offers a good option for larger reciprocating
engine–based systems where the additional cooling output gain over single-stage units
can be base loaded. Hybrid absorption chillers also incorporate the heat recovery device
into the chiller for reduced heat recovery space and cost requirements. However, hybrid
chillers are more expensive and require more sophisticated control and maintenance
than either the single- or two-stage “indirect-fired” absorption systems.
While single-stage absorbers have a lower efficiency than either the two-stage or
hybrid chillers, they are also cheaper and simpler to operate and maintain and are less
sensitive to variations in system operation. This can be a significant advantage when
applying CHP systems to facilities that do not have full-time qualified facility mainte-
nance personnel.

Adsorption Chillers
Adsorbers combine absorption and desiccant systems. They employ water as the refrig-
erant using the same pressure/temperature relationship described for absorption. Dur-
ing the cooling process, 54°F return chilled water from the building passes through
tubes in the evaporator section, which is held at an absolute pressure of approximately
0.1 psia. Refrigerant water is sprayed on surface of these tubes and is evaporated resulting
in a drop in temperature of the building chilled water inside the tubes. Instead of using
a chemical absorption reaction as with lithium bromide, the adsorber uses a solid
desiccant such as silica gel, which adsorbs the water vapor to allow the cooling process
to continue.