Cogeneration

Overview

Cogeneration or combined heat and power (CHP) is the use of a heat engine to simultaneously generate both electricity and useful heat.

Thermal power plants and heat engines in general, do not convert all of the available primary energy into electricity. In most heat engines, more than 50% of the primary energy is wasted as excess heat. By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an aggregate efficiency (electric and thermal) of 80% up to 95%, compared with at most 40% for the conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy, or - as well - less pollution is produced for a given economic benefit. As there is such high potential in energy efficiency, cogeneration is considered to be an important contributor to climate change mitigation while offering economical as well as reliability benefits on energy supply. 

Applications

Cogeneration technology is applicable in a wide range of sector segments as shown in the figure below. For instance, cogeneration plants are commonly found in central heating systems for hospitals, hotels and industrial plants with large heating needs adding to their electricity demand.

Another field of application with high potential is the use within the powering agriculture context: By employing cogeneration technology, food processing plants can utilize biomass by-products to generate heat and power, which in return can be used for the production process.  

Obviously, cogeneration plants are ideally suited for on-site generation and thus can be an important building block for the establishment of an energy system based on distributed generation. System capacities depend on the field of application, but usually range between a few kilowatts electric (kW_el) up to 20 megawatts electric (MW_el) when regarding projects related to international development cooperation.

Technology

Capacity ranges and common technologies

Cogen encompasses a wide range of proven technologies. Combined heat and power technologies can be divided into small scale applications, starting from some 1 kWel engines that are usually focused on thermal energy supply, up to large-scale powerplants mainly focused on electric power production. These types of cogen plants can be differentiated by technology and feasible capacity, as illustrated below.

Topping cycle and bottoming cycle

In general, one can differentiate between two basic principles: 

1. Topping cycle - produces electricity first, and then the exhausted steam is used for heating. The hot water from condensed steam is well-suited for space and water heating.
   
2. Bottoming cycle - produces high heats for an industrial process, and then a waste heat recovery boiler feeds an electrical plant. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing hence rare.
   

Fuels

As possible plant sizes vary depending on the technology, so does the choice of fuel.Waste heat recovery and thus cogeneration can be operated in all types of combustionprocesses and fuels.The choice of fuel is subject to fuel availabilityand costsas well as technological and economic optimisation. Common types of fuels are:

• Coal
  
• Natural gas
  
• Heating oil
  
• Vegetable oil
  
• Bioethanol
  
• Biogas
  
• Biomass
  
• Municipal waste
  

Coal is the dominant fuel for large-scale cogen (> 50 MWel) in the power sector, whereasnatural gas and biomass are common in smaller-scale applications.

Not listed above for lack of a real fuel, cogeneration can also be applied to steam turbines being part of a concentrated solar power (CSP) plant.

Efficiency

The efficiency of cogeneration can vary with technology, dimensions, boundary conditions and fuels, but high efficiency gains of 30% and more compared to separate generation of heat and power are common. It is important to mention that this efficiency relates to the aggregate - electric an thermal - efficiency. 

The influence of boundary conditions on system efficiency can be illustrated by a simple example:  CHP is most efficient when the heat can be used on-site or very close to it. Overall efficiency is reduced when the heat must be transported over longer distances. This requires heavily insulated pipes, which are expensive and inefficient.

Benefits and Relevance

Cogeneration and trigeneration (also known as Combined (Cooling) Heat and Power (C(C)HP)) are key technologies to unlock energy efficiency potential in many sectors such as commerce and industry, buildings and other public infrastructure.

The convincing advantage of this technology are high efficiency gains of 30% and more resulting from one single generation process of power, heat and – for trigeneration – also cooling in comparison to separate electricity generation usually wasting vast amounts of heat. Additional strong benefits comprise CO₂ and NO_x emission reductions, GHG neutral cooling, as well as cost reductions based on better fuel efficiency and flexible, decentralized on-site plant operation, which helps to avoid energy losses and expensive grid upgrades over long distances.

Further Information

• Guide: Cogeneration & Trigeneration - How to produce energy efficiently (GIZ, 2016)
  
• wikipedia.org: Cogeneration
• cogen.org: what is CHP
  
• localpower.org