Page 95 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Thermal Design for CHP 73

Thermal Technologies
For many CHP designs, there is a need to provide some form of thermal energy other
than heating, in order to maintain high annual load factor or to meet process needs. For
the purposes of this discussion, thermal technologies are considered to be devices that
are driven by heat and used to provide some form of useful thermal energy other than
heat. Thermal technologies generally use the products of the heat recovery system but
in some cases they can use the thermal energy generated by the prime mover directly
and thereby combining the heat recovery function into the thermal technology. These
thermal technologies can be broken into various equipment groups as follows:

• Absorption chillers
• Adsorption chillers
• Steam turbine–driven chillers
• Desiccant dehumidifiers

We will review each of these technologies briefly with respect to their applicability to
CHP design. Further information on the operation and particular characteristics for
each technology is available from a variety of sources.

Absorption Chillers
There are multiple forms of absorption chillers available using a variety of refrigerants
and configurations. In order to understand the basics we will take the most common
form of absorber available—the hot water–fired single-stage lithium bromide absorption
(LiBr) chiller.
The operating principle is based on the relationship between the absolute pres-
sure and boiling point of water. At atmospheric pressure (14.7 psia) pure water boils
at 212°F. As the pressure increases the boiling point is raised and as the pressure
reduces the boiling point is lowered. In a LiBr absorber, water is used as the refrigerant
and is sprayed on the tube bundles which contain building chilled water at 54°F, for
example. The absolute pressure in the absorbers evaporator section is reduced to
approximately 0.1 psia. At this pressure, water will boil at approximately 40°F. The
building chilled water inside the evaporator tubes provides sufficient heat to cause
the refrigerant water to boil. This change in phase of the refrigerant requires energy
which is supplied by the building chilled water. The energy given up by the building
chilled water is in the form of sensible energy resulting in a drop in temperature, thus
providing a cooling effect.
A water/lithium bromide solution is used in conjunction with the refrigerant
water due to its hygroscopic properties and high boiling point. Once the refrigerant
vaporizes to cause the cooling effect it will begin to increase the pressure within the
system and must be removed in order to continue the process. LiBr is sprayed in the
“absorber section” of the chiller (hence the name) where it absorbs the refrigerant
(water) vapor to prevent an increase in pressure. The resultant dilute (or weak) LiBr
is then directed to the chiller’s generator section where the weak solution is heated
using thermal energy such as hot water. The absolute pressure in the generator is
considerably higher than in the evaporator to increase the boiling point and
allow higher temperatures in this area. The water vapor is driven off to regenerate or