Page 106 - Sustainable On-Site CHP Systems Design, Construction, and Operations

84 CHP B a s i c s

Generally for hydronic loops, all the devices should be plumbed in series where possible
and the CHP thermal device should be on the return side of the cooling or heating loop
before it enters the non-CHP systems. This is done for two reasons, firstly and most impor-
tantly, to ensure the highest load gets applied to the CHP system. The second reason is that
most of the thermally driven chilling equipment associated with CHP plants run at higher
efficiencies when operated at higher evaporator temperatures. For a distribution system
that supplies 42°F chilled water and receives 56°F return water, an absorber that supplies
50 percent of the load will have higher capacity and efficiency when reducing the 56°F
water to 49°F than if it were to provide 42°F chilled water. In line with this concept, the flow
through the CHP thermal system should be designed for the highest possible leaving water
temperature for chiller applications or the lowest temperature for heating applications.
Steam generation devices will generally be interconnected to the main steam header
and set at a higher pressure set point than the non-CHP boilers so that it is activated
first in order to apply the maximum load to the CHP system.
When applying dehumidification or hot or chilled water or steam coils to an air
handling system, the CHP-driven desiccant or coils should be placed in front of any
supplementary coils such that it pretreats the air before other coils are activated.
For applications where the CHP-generated heat is applied to domestic hot water
heating, it is recommended that the CHP heat exchanger be located before the fresh
water is introduced to the existing or backup heating system. This provides the maximum
load possible as well as the smallest heat exchanger possible. Domestic hot water heating
will generally require some hot water storage and the system should also be designed
to handle large flows for short periods.
As illustrated above, the CHP system can be independently controlled from
temperature or pressure to ensure that it is the “first call” system. When CHP compo-
nents are controlled by a comprehensive building management system, the same dispatch
rules apply with the CHP system generally being first call for heating or cooling as long
as the prime mover is operational.
Generally the electric power supply for the CHP-related cooling, heating, and
energy distributions should be tied into the building grid rather than the generator. For
many applications residual heat energy can be provided after the prime mover has
shutdown and in the case of chillers, continued operation of chilled water and con-
denser water pumps is required to avoid damage to the equipment.
When CHP is being designed as a retrofit to existing buildings, the existing heating,
cooling and power systems are left in place with the CHP system displacing output
from existing systems when operational. It must be remembered that CHP systems are
generally designed to offset power and fuel purchases as a way of reducing costs and
emissions. They are not designed to provide continuous duty over long periods of time
and are subject to shutdown for scheduled or unscheduled maintenance multiple times
per year in some cases. Existing systems need to be kept on standby for duty when the
CHP system is down. When CHP is designed into a new building and integrated into
the thermal equipment design, some redundancy can be provided by the CHP system
to help offset equipment capital costs.
In all cases CHP can best benefit an existing or new building when properly sized
to meet facility loads. A properly sized CHP system provides full load power and thermal
output at least 85 percent of the time on an annual basis resulting in high operational
efficiency, highest possible return on investment, and ensures that the plant can operate
over the intended life span of the system.