Build a Combined Heat and Power Model from Scratch

Develop a thermodynamic combined heat and power (CHP) model to help optimize plant utility usage.

Combined heat and power (CHP) systems generate electricity or mechanical power and useful heat from a single source of energy. The system may take a variety of configurations that include gas-turbine generators (GTGs), heat-recovery steam generators (HRSGs), boilers, and steam-turbine generators (STGs).

▲Figure 1. (a) Gas-turbine combined heat and power (CHP) plants generate power and hot fluegas from combustion. The fluegas is used to produce steam in a heat-recovery steam generator (HRSG), which can be used for heating, to generate electricity, or to drive rotating equipment. (b) Boiler CHP plants produce steam primarily to drive turbines and produce power, but some steam may be extracted for process heating.

A gas-turbine CHP plant (Figure 1a) combusts a mixture of fuel gas and air in the combustion chamber of the turbine. The combustion product flows through a series of blades attached to a rotating shaft and generator, which generates both power and hot fluegas that exits through the exhaust system. The high-energy exhaust gas is captured in an HRSG to heat boiler feedwater and produce steam. The steam can be used to drive a STG and produce electricity, which is called a combined cycle; or, it can be sent to a process plant for heating in a heat exchanger or to drive rotating equipment, such as pumps and compressors, which is called cogeneration.

A boiler CHP plant (Figure 1b) is an alternative type of system, in which one or more boilers generates steam. The steam is used to drive STGs for power generation. In some configurations, steam is extracted from the STG for process heating via heat exchangers.

Most of Saudi Aramco’s facilities use a combination of gas turbine and boiler CHP systems. It is necessary to build models for these systems to:

• identify opportunities for cost reduction through efficiency improvements
• enable accurate energy cost accounting
• evaluate the energy cost impact of proposed process changes on the demand side
• compare various CHP options during the early stages of project design
• identify load-sharing strategies (e.g., switching between electric motors and steam turbine drivers)
• monitor existing assets.

This article explains how to develop a CHP model from scratch, using a combination of readily available mixed-integer linear programming (MILP) optimization solver engines, steam property tables, and Microsoft Excel.