Page 91 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Thermal Design for CHP 69

the CHP design, the cost of such a system often makes the project economically unfea-
sible (unless the thermal distribution systems are installed as part of new building or
facility). Therefore, together with identifying the actual facility loads, a determination
must be made on how those loads will be connected to the CHP system. Loads that can-
not be connected or are prohibitively expensive to connect cannot be considered in the
load evaluation. The connection point for the thermal distribution system will also be a
significant factor in the actual location of the CHP plant. If no room exists in or close to
the connection point then the costs to bring the thermal output to this location must also
be figured into the economic evaluation.

Heat Recovery Options and Design
The available options and design of the heat recovery system are dependent on two
factors: (1) the type and quality of thermal energy required to meet the facility needs
and (2) the type of power generator used. As with CHP design in general, the type and
quality of thermal energy used by the building should be considered first so that maxi-
mum load factor can be achieved. This is especially important when retrofitting existing
buildings as this factor is already predetermined. The types of thermal energy available
are typically high- or low-pressure steam, high or low temperature hot water, chilled
water, refrigeration, dehumidification, or hot air. The quality relates to the temperature
and pressure of the media being used by the facility.
The type of power generator used also impacts the design and options for heat
recovery. Reciprocating engines generate hot water as well as exhaust heat, while
fuel cells generate low volumes of high temperature exhaust. Nonrecuperated
combustion turbines generate high volumes of high temperature exhaust, while
recuperated turbines generate similar volumes of exhaust energy but at considerably
lower temperatures.
The total heat recoverable from an engine is a function of the media flow rate, its
specific gravity, specific heat, and the temperature differential across the heat recovery
device. This can be expressed as follows:

Heat recovered = fl ow rate × specifi c gravity × specifi c heat
× (inlet temp − outlet temp)

Generally, the media is either exhaust or water with no or various amounts of glycol
additive. The media, its flow, specific gravity, specific heat, and temperature to the heat
recovery device are all functions of the type and performance of the prime mover. The
outlet temperature from the heat recovery device is a function of its design as well as
the type and quality of heat product required by the facility.
For exhaust-driven heat recovery devices, the temperature of the thermal product
(i.e., steam or hot water) is inversely proportional to the amount of heat energy that can
be recovered. This means that as the product temperature increases, the outlet tempera-
ture of the heat recovery device increases resulting in a reduced temperature differen-
tial and therefore a lower amount of heat recovered. In general, the exit temperature
from the heat recovery device should be no less than 250°F and more typically should
be above 300°F to avoid condensation and the associated formation of acid in the
exhaust stack. The higher temperature design basis also allows more flexibility to oper-
ate the system at part load without forming condensate in the stack.