The cogeneration plant

Nowadays the human being is encountering the large stress of energy and environment. Fossil fuel is the main existing energy resources which includes oil, coal, and natural gas.

The extensive utilization of fossil energy has caused a lot of problems especially endangering environmental situations, such as global warming, destruction of carbon balance in the ecological circle, damage to ozonosphere, emission of dangerous substances, and acid rain, etc.

Electricity generation from renewable resources is considered as one of the most encouraging solutions against global warming and reduction of fossil sources, while its application on a large-scale is still limited because of economical reasons. In comparison with plants which provide power and heat separately, cogeneration plants which produce power and heat together, have important advantages on energy utilization and are greatly utilized now.

What is cogeneration?

Cogeneration is defined as the sequential generation of two forms of useful energy from a single primary energy source; typical two forms of energies are mechanical energy and thermal energy. Mechanical energy may be used to either to drive an alternator to produce electricity or  torque to devices like motor, compressor, pump or fans etc., for delivering different services. Thermal energy may be used directly for the process for heating purpose or indirectly to produce the steam generation, hot water or hot air for dryer and chilled water generation for process cooling. 

In this paper the main forms of energy to take into account will be the electric energy and the thermal energy, this configuration is called combined heat and power.

 

Why is cogeneration needed?

Thermal power plants are major sources of electricity in the world. The conventional method of power generation and supply to the customer is wasteful in the sense that only about a third of the primary energy fed into the power plant is actually made to available to the user in the form of

electricity. In conventional power plant, efficiency is only 33% and remaining 65% of energy is lost. The main loss in the conversion process is the heat rejected to surrounding water or air due to the inherent constraints of the different thermodynamic cycles employed in power generation. Besides of further losses of around 10%-15% are associated with the transmission and distribution of electricity in the electrical grid.

Through the utilization of the heat, the efficiency of the cogeneration plant can reach 90% or more. In addition, the electricity generated by the cogeneration plant is normally used locally, and then transmission and distribution losses will be inconsiderable. Cogeneration therefore offers energy savings ranging between 15%-40% when compared against the supply of electricity and heat from the power stations and boilers.

What are the benefits of cogeneration?

Actually, technologies like micro turbine and internal combustion engine, have already been used well for cogeneration of heat and electricity, which may bring considerable energy saving and economic benefits with respect to the separate production  of the same energy vectors in the centralized power system and in local boilers.

A cogeneration system for heat and power using a micro gas turbine may have several advantages as follows.

·         Increase the rate of energy utilization.

·         Good supply reliability and security.

·         Flexibility in supplying the energy.

As a result of their increased energy performance, cogeneration systems can also present important benefits in terms of reduction in greenhouse gas emission with respect to the separate production.

How can use the cogeneration?

There are several ways to utilize a cogeneration (combined heat and power) system and each of them has its own advantages and drawbacks.

·         Electrical Dispatch.

 One way to operate cogeneration is that the amount of required electricity takes the first priority and heat produce has the second priority.

In this case, capacity magnitude of cogeneration system power produce is determined as maximum required electricity load.

·         Thermal Dispatch.

This approach the first priority is required heat production and power production takes the second priority. In this method, the output power of cogeneration is regulated in a way that meets the local thermal need.

·         Hybrid Strategy.

In this case, the best state of operation cogeneration production is done according to that choice. In this algorithm, the rated power is chosen as the start point. Then, considering the generated heat and comparing to the heat demand at that time, it is determined that whether boiler be operated or not. If the generated power could not supply need of consumer, the remaining power is purchased from the network.

What is the best strategy for cogeneration?

Cogeneration generation is different in various strategies. So, it is expected that the amount of CO2 production and also the overall thermal efficiency and the overall energy saving are different. Therefore, considering the goal and purpose of combined heat and power system utilization as well as available economical limitations, the operator should choose the appropriate strategy and utilize the system. For example the electrical dispatch has the largest amount of CO2 production because a lot of unusable heat is generated in this situation while the hybrid dispatch has the smallest amount of CO2 generation among different strategies.

Considering the increase in fuel price and available concerns about environmental pollutants currently the usage of cogeneration is greatly growing. The utilization of this system is done by different strategies. Each of these strategies has its own advantage and drawbacks and the operator of cogeneration system has to choose the appropriate strategy. The hybrid dispatch strategy has the best performance and is the best choice in most cases. Therefore, the operation of cogeneration in hybrid dispatch strategy is suitable for both from the view point of the operator and the society.