Page 105 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Thermal Design for CHP 83

The same facility has a base electric load of close to 1 MW and a reciprocating engine
CHP system with a heating T/E ratio of 4 MBH/kW would necessitate the power output
be significantly reduced to meet the 1.5 MMBH 85 percent thermal base load assuming no
other addressable thermal loads are available. In order to maximize the potential for the
CHP system it is desirable to increase the thermal load to the 3 MMBH average but doing
so would mean that the system would have many hours where the thermal energy needed
to be dumped resulting in poor economic performance for much of the operating period.
Thermal energy storage provides a solution that not only allows for upscaling of the CHP
system to meet the electric base load but also allows for the system to meet the facility’s
entire hot water needs. During low thermal load hours the CHP system generates hot
water in excess of the load requirements. This excess hot water can be stored and later
retrieved to meet the peak loads in conjunction with the full CHP system thermal output.
In this way the system can actually supply the total hot water load without the need for a
supplemental boiler and so benefits from the cost offset for this boiler.
Thermal energy storage involves the transfer and retrieval of thermal energy to a
storage medium. It can take many forms with the most common being water or ice as
the medium contained in insulated tanks. While water and ice are common media,
other media such as rock, brick, thermal oils, or chemicals can be used to also store
thermal energy. Liquid desiccants provide an example of chemical storage where the
desiccant can be stored in plastic tanks. Thermal storage is particularly compatible with
systems that have constant energy input but varying loads as is the case with many
CHP applications and as exemplified above.
For general cooling applications, ice is sometimes the storage medium of choice due
to its lower space requirements versus chilled water. However, it must be remembered
that many of the thermally driven technologies used in CHP applications cannot produce
ice so water becomes the main choice for CHP-based cooling and heating applications.
The designer of the thermal energy storage system must calculate the facility’s load
profile, recognize the charge and discharge characteristics of the media selected, and
size the system to allow for recovery of excess output during low load periods as well
as meeting the high load periods in conjunction with the CHP system output. Consid-
eration also needs to be given to the temperature required to charge the storage medium
relative to the temperature required by the distribution system as this can impact the
volume of storage possible or even the type of CHP system selected.

Integration with Building Systems
Thermal design in the case of CHP systems involves one of the most important steps in
the entire design process and integrates the generation process with the facility. As
mentioned in the introduction to this chapter, the thermal component is key to achieving
overall success and is also the most difficult component to address. When integrating a
CHP system with a building or facility the thermal and electric outputs not only need
to match facility needs in terms of quantity and quality, but also need to be connected
to the distribution and control systems.
Integration with the building’s thermal distribution system(s) requires that there be
a hydronic or closed circuit heat transfer fluid loop, steam header, air duct, or other
method to transfer the recoverable waste heat energy from the CHP system to the
respective buildings’ thermal loads. Given that most CHP systems only generate a portion
of the thermal energy needs of the facility, the system needs to work in conjunction with
other chillers, boilers, steam generators, etc. of different types.