Basic overview of cogeneration

Introduction

Cogeneration, also known as combined heat and power (CHP), is a highly efficient system that generates both electricity and useful thermal energy from a single fuel source. In this process, steam turbine generators not only produce electrical power but also supply steam for heating purposes to industrial or residential users. Thermal power plants operating in cogeneration mode are designed to maximize energy efficiency by utilizing the waste heat that would otherwise be lost. The steam used for external heating typically comes from the exhaust of extraction steam turbines or back-pressure steam turbines. These steam pressures are generally categorized into two ranges: 0.78 to 1.28 MPa for industrial applications and 0.12 to 0.25 MPa for residential heating. Since cogeneration eliminates cold source losses, it can achieve a thermal efficiency of up to 85%, significantly higher than conventional condensing power plants, which usually operate at around 40% efficiency. This increased efficiency leads to substantial energy savings, reduced emissions, and improved environmental quality, ultimately enhancing the living standards of communities.

However, cogeneration requires careful planning and integration between power generation and heating systems. It reduces the operational flexibility of power plants and increases capital investment. Therefore, it is most effective when implemented in urban areas with centralized heating systems, where there is a consistent and sufficient heat demand. A well-designed comprehensive plan must be developed under conditions of guaranteed heat load to ensure optimal economic and environmental benefits.

Specific Requirements

For cogeneration to be successful, thermal power stations must be located near industrial zones and urban residential areas to maximize energy efficiency and minimize transmission losses. In many Western and Eastern European countries, cogeneration has been widely adopted, with thermal power plants accounting for about 30% of total electricity generation. These systems are primarily used for industrial production and district heating. Industries such as paper, steel, and chemicals (including petrochemicals) are among the largest users of cogeneration. They not only consume large amounts of electricity but also generate waste gases like blast furnace gas, which can be utilized as fuel for cogeneration units. Urban industrial zones and densely populated areas are also ideal locations for cogeneration projects, but it's crucial to analyze local heat demand carefully. The general heating coefficient should be at least 0.5, meaning that industrial heat loads should have an annual utilization of over 3,500 hours, while residential heating should last no less than three months during winter. Additionally, the distance between a thermal power plant and its heat users should not exceed 5 to 8 kilometers.

Cogeneration systems require high-quality fuels with low sulfur and phosphorus content. The location of the plant should also be chosen strategically, preferably downwind of the city’s prevailing winds, to reduce environmental pollution. When there is excess steam available during cogeneration, absorption chillers can be used to cool water for air conditioning or process cooling. These chillers use steam as their working fluid, producing chilled water at temperatures between 6°C and 8°C. Steam generated by boilers can also be directed to back-pressure or extraction steam turbines, providing both electricity and heat for various applications.

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