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Cogeneration And Trigeneration Methods Engineering Essay

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Published: Mon, 5 Dec 2016

Cogeneration and trigeneration are methods used for producing more than one useful form of energy from only one energy source. These two methods in today’s world are very important because efficiency, fuel prices and emissions are a great a concern. Both methods give the opportunity to increase efficiency drastically up to 60% to 80% which is much more than the conventional power plant which has an efficiency of about 30%. [1] 

The 60% to 80% efficiencies that both methods present have a great effect on cost savings, reduced air pollution, greenhouse gas emissions, increased power reliability and quality, reduced grid congestion and avoided distribution losses. [2] All these factors mentioned, as discussed before are all important and cogeneration and trigeneration help achieve this.

Both systems are normally combined but I will explain both cogeneration and trigeneration separately and then give examples were these systems are used in real life.

Cogeneration

Cogeneration is also known as combined heat and power (CHP). Plants using a cogeneration system use the exhaust of for example a reciprocating engine to heat the premises. This means that instead of “throwing away” the exhaust, it is being reused but this time as heat energy. This is done by using a heat exchanger to transfer heat from the exhaust gas to process heat. Cogeneration produces a given amount of electric power and process heat with 10% to 30% less fuel than it takes to produce the electricity and process heat separately. [3] 

Cogeneration – Combined Heat and Power

Figure 1: Cogeneration diesel engine generator plant [4] 

Figure 1 above shows a diesel engine driving a generator. Fuel in the form of natural gas, bio gas or bio diesel is used to run the engine. The engine is coupled to a generator which produces electricity. The plant above shows two heat exchangers, one for cooling the engine and raising the used cold water coming from process heat and the other is an exhaust gas heat exchanger only to raise again the temperature of the water for process heat. The water cooling the engine which is normally at around 90oC enters the heat exchanger (bottom one) which by heat transfers and the second law of thermodynamics is cooled but the cold water’s temperature is raised. In the exhaust gas heat exchanger the temperature of the water already heated in the bottom heat exchanger is heated again and sent for process heat.

A simple example is, imagine a hotel having a generator to produce electricity and at the moment its full of guests and the ambient temperature outside is around 2oC below zero. This means that the central heating needs to be running constantly to keep the guests in a comfortable environment. Therefore a lot of fuel is being used to heat up the boilers to produce steam for central heating. Imagine the hotel employs the plant shown in figure 1. No fuel will be used for heating because the fuel used to run the engine is also being used (indirectly) for process heat.

There exist two main types of cogeneration techniques which are topping cycle and bottoming cycle plants.

Topping cycle cogeneration plant

Topping cycle plants generate electricity or mechanical power first. Some facilities may generate the electricity for themselves only and some may even sell any power that is not being used. There exist four types of topping cycle cogeneration plants.

Combined cycle topping system

The first type burns fuel in a gas turbine as shown in figure 2 or diesel engine to produce electrical or mechanical power. Process heat, or steam that then drives the secondary steam turbine is produced by the exhaust entering a heat recovery boiler. [5] 

Figure 2: Combined cycle topping system [6] 

Steam turbine topping system

The second type of system burns fuel which can be any type to produce high-pressure steam that then passes through a steam turbine to produce power. The exhaust provides low-pressure process steam as shown in figure 3. [7] 

Figure 3: Steam turbine topping system [8] 

Heat recovery topping system

This type burns a fuel such as natural gas, diesel etc. The cooling system (engine coolant) goes to a heat recovery boiler, where it is converted to process steam and hot water for space heating.

Figure 4: Heat recovery topping system

Therefore this type produces electricity, process steam, hot water supply and heating as shown above in figure 4. [9] 

Gas turbine topping system

This type of system uses a natural gas turbine to drive a generator. The exhaust gas goes to a heat recovery boiler that makes process steam and process heat as shown in figure 5. [10] 

Figure 5: Gas turbine topping system [11] 

Bottoming cycle plants

These type of plants are less common then topping cycle plants. Normally this type of plant is used in heavy industries where high temperature furnaces are used.

http://4.bp.blogspot.com/-MV_kG3RuIEY/T9moeWntcgI/AAAAAAAAAg4/XDZ1gNqAUJE/s1600/bottoming-cycle-system-cogeneration.jpg

Figure 6: Bottoming cycle system [12] 

After heating the furnace or any manufacturing heating process the waste heat is then passed through a recovery boiler. The waste heat is used to produce heat which then drives a steam turbine to produce electricity.

Trigeneration

Trigeneration also referred to combined heating, cooling and power (CHCP) is a system where this time three types of energies are produced from one energy source. The difference between cogeneration and trigeneration is that in trigeneration chilled water for air conditioning or process use is produced. This is done by using an absorption or adsorption chiller.

Just as a cogeneration power plant captures and makes use of the waste heat, absorption or adsorption chillers capture the waste (or rejected) heat and produce chilled water. [13] 

Therefore the major advantage over cogeneration is that now if a plant works using trigeneration, hot water, air conditioning (using chilled water) and power generation. Therefore an industry having this system will spend less money due to having a more efficient plant when compared to cogeneration and especially when compared to conventional plants.

Adsorption Chiller

An adsorption chiller works purely using hot water. It uses the principle of using solid sorption materials such as silica gel and zeolites.

http://www.raee.org/climatisationsolaire/graph/new/adsorption_sh.gif

Figure 7: Schematic for an adsorption chiller [14] 

These type of chillers have a strong structure and are easy to install. There are no possibilities of crystallization, corrosion, hazardous leaks, and the electricity consumption is minimal [15] . They are commonly used in a commercial environment.

Absorption Chiller

An absorption chiller works by using hot water, steam or combustion. The solution used contains water and lithium bromide salt to absorb heat from the surroundings. No refrigerant is used which means no harm is done to the environment.

http://www.raee.org/climatisationsolaire/graph/new/absorption.gif

Figure 8: Schematic for an absorption chiller [16] 

Absorption chillers are the most distributed worldwide and they are more efficient. Their lifetime is much longer than adsorption chillers. Moreover they have high maintenance time and low corrosion protection. [17] 

Say something small about trigenerators and put schematic of plant.


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