Page 87 - Sustainable On-Site CHP Systems Design, Construction, and Operations

CHAPTER 4

Thermal Design for CHP

Gearoid Foley

his chapter discusses the issues surrounding the thermal aspects of CHP plan-
ning and design, waste heat recovery, and the application of the resultant recov-
Tered energy to meet facility requirements. The profitable use of the thermal
output from a CHP plant is essential to realizing its economic potential. As illustrated in
Fig. 4-1, while the revenue from offset electric purchases typically represents 80 percent
of the revenue stream, the thermal output typically represents the entire profit
margin. This advantage cannot be fully realized unless the CHP system’s thermal
component is properly designed and its output matches the facility needs 12 months of
the year. This chapter will help the reader understand the steps involved in optimizing
CHP design to meet a facility’s energy needs and therefore maintain high load factor.

Thermal Design for CHP Systems
To implement sustainable CHP, the planning and design process needs to work to
maximize the operational savings of the system while minimizing the cost of the project.
A good way to attain both these objectives is to design for the maximum electric and
thermal load factor achievable. This means that the resultant CHP system will fully
utilize the purchased equipment to offset energy from other sources and it also means
that costs were not incurred for equipment that is not used. In order to achieve maxi-
mum load factor the design process should emphasize matching the facility thermal
loads first and then selecting a prime mover (e.g., engine generator or combustion turbine)
to meet these requirements. In this way, a thermal first design approach leads to high
load factor which in turn leads to a successful project.
This philosophy is based on a couple of basic principles as follows; firstly, it is rela-
tively easy to apply electricity to a facility as it is generally all at a common frequency
(e.g., 60 Hz) alternating current; however, the thermal energy used by a building or
facility takes many forms including steam at various pressures, high temperature hot
water, low temperature hot water, chilled water, refrigeration, hot air, and so on. In
many cases, the design will need to incorporate more than one form of thermal energy
from the same system. Secondly, load factor is typically more important than efficiency
as spending capital resources on high efficiency equipment when the load is not avail-
able to meet the equipment output is usually wasted money.

65