Introduction

Steam Turbines are one of the oldest machines used to convert heat energy into useful mechanical or electrical energy. At the same time cogeneration is also an age-old technique used by large-scale steam users such as the sugar industry. Simultaneous generation of two forms of energy (i.e. steam and power) is generally known as Cogeneration or Combined Heat and Power (CHP). This paper elaborates how Steam Turbine based Cogeneration can be successfully applied to small and  medium scale process industries.

What is Cogeneration?

Cogeneration is simultaneous generation of two forms of energy – Electrical Power and Steam from a primary fuel source. In case of steam turbine based cogeneration, steam from the boiler is first fed to a steam turbine generating electrical power, and then the same steam is fed to a process plant at required pressure.  It is explained schematically in Figure 1 (see page 2).

Why Cogenerate?

Centralized Thermal Power Plants convert just about  33% of the energy available into useful electrical energy. This is because nearly 67%of the primary energy is lost to the atmosphere in the form of heat, thus adding to global warming. It also leads to an increase in the cost of electricity generated (Figure 2). Process industries generate steam for their various processes and buy electrical power from the utility to meet their power demand. On buying power from a utility, we also pay for the 67% energy lost to the atmosphere. This is also not without incurring huge Transmission and Distribution (T&D) losses. However a cogeneration system can enhance the total system efficiency to as high as 85%. Such systems usually generate power virtually free of cost since the steam is used twice i.e. first to generate power and then to meet the thermal demand of the process plant.

How to Cogenerate successfully

For successful implementation of a cogeneration system, a detailed analysis and proper conceptualization of the Steam Turbine Generating Set is very essential. An expert needs to do this job. In addition, several issues need to be sorted out before a right system is put into place.

Cause & Effect

One cannot always generate the amount of power intended from the cogeneration route. It is very important to understand the “Cause & Effect” for any successful cogeneration. “Amount of Steam” is the “Cause” and the “Power Generated” is the “Effect”. Power generated from cogeneration is however “Incidental Power”, which is generated incidentally while meeting the process plant’s steam demand. Therefore, one cannot always optimize the power generation by using higher pressure steam boilers. However, this is possible only up to a certain point for small to medium needs.