Page 419 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Eco-Footprint of On-Site CHP versus EPGS Systems 391

steam (LPS). The LPS is then used to make heating hot water (HHW) for distribution to
the campus. Any energy not utilized by the plant is rejected to a dump condenser to be
rejected to atmosphere by either a cooling tower or radiator. The balance of heating and
cooling loads that are not served by the cogeneration plant are served with gas-fired
boilers and electric driven-centrifugal chillers.

ICHP/CGS Plant
The inherently self-regulating ICHP/GCS, as shown in Fig. 24-2, met the nominal 1040-ton
(3658-kW) cooling requirement of our 3.5-MW campus project by employing more effi-
cient, commercially available low-mass hybrid steam generators and utilizing a com-
mercially available, nominal 1040-ton (3658-kW) adapted two-stage high-temperature
heat transfer fluid (HTHTF) heated absorption chiller with an assumed heat rate of
10,600 Btu/h/ton (COP = 1.13). The ICHP/GCS plant can be functionally integrated with
controls, plate-and-frame heat exchangers, turbine inlet cooling coil, pumps, intercon-
necting piping, and CGT waste heat extraction coil and prefabricated (for minimal on-site
erection) water type absorption chiller. The ICHP/GCS plant uses an exhaust-to-HTHTF
heat exchanger (HEX) to recover the exhaust heat by heating the HTHTF from approxi-
mately 250°F to as high as 600°F (316°C). The HTHTF can first supply a hybrid HEX to
produce LPS. The LPS can be used to drive a single-stage absorption chiller.
The HTHTF is then used to drive a two-stage absorption chiller followed by a
plate-and-frame HEX to produce HHW. Note that domestic hot water (DHW) can
also be produced to further utilize the recovered heat. However, in the specific case
analyzed here, the majority of recovered heat was utilized for campus heating and
cooling demands, and dumping of recovered heat was minimal. The thermal utiliza-
tion is arranged in this order due to the heat temperature and quality requirements of
the various system components. For example, the two-stage absorption chiller has a
maximum HTHTF inlet temperature of 425°F (218°C). Therefore, some of the recov-
ered heat may need to be utilized prior to the two-stage absorption chiller depending
on the HTHTF supply temperature. Though the most efficient way to use heat would
be to produce HHW prior to the two-stage absorption chiller, the coincident campus
cooling and heating loads are not such that the HHW HEX would always reduce the
HTHTF below 425°F (218°C). Since the HHW HEX requires lower-temperature HTHTF
than the two-stage absorption chiller, the HEX was placed downstream of the chiller.
Like the conventional plant, the balance of heating and cooling loads that are not
served by the cogeneration plant are served with gas-fired boilers and electric-driven
centrifugal chillers.

Direct Turbine Exhaust-Fired Two-Stage LiBr-Water Chiller Plant
This direct turbine exhaust-fired two-stage LiBr-water chiller plant, as shown in
Fig. 24-3, includes an absorption chiller capable of producing both chilled and hot
water, which is directly coupled to the CGT exhaust stream. The subject absorption
chiller can produce 1740 tons of cooling (at 0 percent heating) and approximately 17 × 106
Btu/h of heating (at 0 percent cooling). It incorporates an integral heat recovery chiller
therefore an HRSG is not required. Note that the cooling load must be at least 30 percent
of the heating load in order to allow simultaneous heating and cooling. Therefore, it was
assumed that whenever the cooling load was below 30 percent, the absorption chiller
would operate in heating mode.