Combined Heat and Power system (CHP system) or a cogeneration system is the simultaneous generation of multiple forms of useful energy through sequential operation in a single, integrated system. Such a system consists of a number of individual components like prime-mover, generator, heat recovery and electrical interconnection all configured into a whole integrated system.
The prime mover dives the overall system – and typically a cogeneration system derives its identification by its type. Prime movers for cogeneration (or CHP) systems include reciprocating engines, gas turbines and steam turbines. These prime movers use fuels such as coal, oil, natural gas or alternative fuels and produce shaft power. The produced shaft power (or mechanical energy) is generally used to drive a generator (to produce electricity) – or sometimes used to drive compressors, pumps and fans. Heat energy from the systems can be used in direct process applications, or indirectly used to produce steam, hot water, hot air etc for the process.
What are the benefits?
i) In sugar industries, the bagasse, which often poses disposal problem, is used as fuel for cogeneration scheme. This increases cost effectiveness. This is true for many other cases like rice husk of rice mills, saw dust of saw mills, refinery gases of refineries etc.
ii) Lower emissions to the environment and neutral in carbon, sequestration.
iii) They offer large cost savings and make for additional competitiveness for industrial and commercial users.
iv) Cogeneration provides one of the most important vehicles for promoting liberalization in energy markets.
v) Cogeneration provides an opportunity to have more decentralised forms of electricity generation – in such cases the plants are designed to consume locally available waste material (Biomass or Refinery gases) as fuel, and generate electricity for the local needs avoiding transmission losses. Flexibility in the system use is increased – especially if natural gas is the energy carrier.
Types of cogeneration systems
a. Steam turbine cogeneration system:
The two types of steam turbines widely used are the back pressure (Fig.1) and the extraction condensing steam turbine (Fig.2) types. The choice mainly depends on the quantities of power and heat, quality of heat and economic factors. The thermodynamic cycle is the Rankine cycle.
b. Gas turbine cogeneration system:
Gas turbine systems operate on the thermodynamic cycle known as Brayton cycle. There are two types of systems namely open cycle gas turbine cogeneration system (Fig.3), and closed cycle gas turbine cogeneration system (Fig.4).
c. Reciprocating engine cogeneration system:
Reciprocating engines have the advantage of starting quickly, respond to the load well and have good part-load efficiencies. Hence, these find applications in the distributed generation of industrial, commercial and institutional facilities.
Exergetic evaluation of cogeneration plants
Cogeneration of heat and power means production of two different forms of energies. To determine the efficiency of any cogeneration plants or to evaluate its performance, a common platform needs to be defined because the forms of energies differ in their ‘grade.’ The first law of thermodynamics deals with only ‘quantitative’ aspect of energy. This can give thermal efficiencies of plants ă sometimes an energy utilisation factor (EUF) is used for the evaluation of performance. 
,Where
Fig. 1: Back Pressure Steam Turbine…
Fig. 2: Extraction Condensing Steam Turbine…
Fig.3: Open Cycle Gas Turbine Cogeneration System…
Fig.4: Closed Cycle Gas Turbine Cogeneration System…
But, qualitatively, is high grade energy while is low grade energy. Therefore, energy efficiency cannot be the entirely satisfactory criterion of performance of the plant – as it gives equal and same weightage to both heat and electricity. Hence, a thermodynamically more accurate method of evaluation can be based on ‘exergetic efficiency’. Exergy is the measure of energy ‘quality’ and the exergetic efficiency is a measure of perfectness of a thermal system. Thermodynamics suggests the use of exergetic factor, which exactly indicates the quality of heat in terms of its work potential. An even more correct performance value is obtained if the exergy content of the fuel is also taken in to account.
Through a study conducted by the present authors, it has been shown that there can be a remarkable difference between energy and exergy efficiency of the same system. This study was conducted on a heat matched, bagasse based cogeneration plant of a typical 2500 tcd sugar factory using back pressure and extracting condensing steam turbine. It has been shown that the boiler is the major component contributing most to the plant’s total inefficiency.
Even a modern boiler with current technology utilises only 37% of chemical exergy of the fuel in steam generation and 63% is lost in combustion irreversibilities associated with the boiler. This suggests that there is enough scope for improving exergetic efficiency of boiler. Back pressure steam turbine cogeneration plant is the most efficient configuration from the point of integrating process steam demand and incidental power generation. Extraction condensing steam turbine cogeneration plant is the highly efficient steam power cycle from the surplus power generation point of view. In general, bagasse based cogeneration plants in the Indian sugar industries are considered environmentally and economically attractive.
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