CO-GENERATION VS TRI-GENERATION

Energy Generation or “Power Generation” means the production of electric power using fuel oil and its derivatives, natural gas, renewable energy sources, or any other method [1]. A more simplified definition is the processes resulting in the production of electricity and other byproducts from fuels. There are three major types of energy/power generation characterized based on the number of by-products produced alongside electricity. 

Process

Power Station: Fossil fuel power plants burn coal or oil to create heat.

Turbine: The hot steam drives the turbine to generate electricity. Other factors that can make the turbine spin include wind, water and solar energy.

Generator: In combined cycle gas turbines (CCGT) plants, a steam generator is used to increase the amount of electricity produced.

Transformer: Specifically a step up transformer, is used to adjust the voltage of the electricity produced.

Energy Efficiency for Different Methods of Energy Generation

Coal Thermal Plants

32 - 40 %

Gas Turbine Power Plants

25 - 30%

Combined Cycle Power Plant

55 - 60%

Co-generation

60 - 92%

Tri-generation

86 - 93%

CO-GENERATION

Cogeneration is the simultaneous production of heat and electricity from the same fuel source. This is a more efficient method of power generation because the production of the two requires less energy when done simultaneously. It is also known as combined heat and power (CHP) system. The electricity production process is the same as the one described previously, heat is processed and released into the environment as exhaust. The heat released is then captured and used for other purposes.

Brief History

As early as 1880 - 1890, CHP systems were used in Europe and the US. Electricity was primarily generated in coal-operated power plants and used to power factories, mills and mines. The steam that was produced as a byproduct was used as thermal energy for various industrial processes or to heat the space [2]. The first cogeneration plant was designed and built in 1882 by Thomas Edison with some of its thermal byproducts used to heat nearby buildings.

Advantages

1. Energy Efficiency: CHP plants have an increased efficiency ranging from 60 - 90%. Single generation plants typically use 40% of the fuel source with the other 60% accounting as losses. CHP plants on the other hand only lose 8% of the fuel used in generation.

2. Fuel and Energy Cost: Energy costs i.e cost incurred in generating a specific quantity of energy. CHP plants require less fuels in producing heat and electricity. Other units will require twice the amount of fuels to generate the same amount of heat and electricity gotten from CHP plants.

3. Reduction in Emission: Some of the gasses emitted from conventional generation methods are carbon dioxide (CO2), sulfur oxide (SOx) and nitrogen oxide (NOx). This reduction is mostly because CHP requires less fuel to produce a given energy output. Less fuel combusted equals less GHG emissions.

4. Transmission and Distribution Losses: Transmission and distribution losses typically occur when electricity travels over power lines. CHP reduces the need for new transmission and distribution infrastructure, and eases grid congestion when demand for electricity is high.

5. Reliability: CHP is an on-site generation resource and can be designed to operate independently from the electric grid to enhance facility reliability [3]. These systems can anticipate, prepare for and recover from energy outages caused by adverse circumstances.

Disadvantages

1. Installation Cost: CHP systems require space for the energy center and a large diameter heavily insulated metal piping for hot water networks. Other costs associated with running CHP systems include the cost of heat lost to the ground while in process. It cost considerably more for installation and setup than other convention systems.

2. Extra Cost: Since CHP systems already provide heat alongside the electricity generated, a cooling system is needed. Heating is needed in modern residential facilities during the winter/fall while cooling will be needed for the summer/spring. Setting up these two systems separately adds up to the extra cost.

3. Loss of Heat: This cogeneration system produces heat and electricity simultaneously and continuously. During hotter weather the heat generated that would typically be used as central heating in residential settings goes to waste. Precautions can be made to channel the heat somewhere else but will be costly to set up. In conventional systems the boilers are just turned off during the summer.

4. Flexibility: It is not practical to flex the scale of CHP to meet changes in demand, or expansion of the network [4]. Plants made specifically for cogeneration can not easily increase or decrease their scale or be repurposed for single generation. Changes can be done but redesigns, repurposing and construction will be needed which costs a lot.

5. Suitability: Full CHP systems are generally only suitable for sites where there is a constant load requirement for space heating and hot water demand with on-site power in order to maximize the cogeneration cost [5]. Larger scale systems heat and power need to remain consistent for maximum efficiency.

Uses

1. Sugar Plants.

2. Chemical, paper, tyre, food and wood processing industries.

3. Distillery, pharmaceutical, textile and fertilizer units.

4. Refineries.

Types

1. Steam turbine based cogeneration system.

2. Gas turbine based cogeneration system.

3. Engine based cogeneration system.

TRI-GENERATION

Tri-Generation also known as Combined Cooling Heat and Power (CCHP) refers to the production of three forms of energy from a single fuel input. The produced energy from trigeneration units are heat, cooling and electricity. Trigeneration systems typically have a cogeneration unit.

Just like how CHP systems repurpose the waste product (heat) from the energy generation process, CCHP systems use that heat to produce cooling that is used for air conditioning or refrigeration.  In a CCHP plant, some or all of this heat is then used in an absorption chiller or absorption refrigeration system to produce chilled water or sub-zero refrigeration [6].   

Advantages

1. Energy Efficiency: Using the same rationale as in cogeneration systems, more energy is generated with less fuel. CCHP systems generate the amount of energy typically obtained from three different systems. Hence the energy efficiency in CCHP systems which is typically 86 - 93% is more than that of CHP systems.

2. Fuel Cost: Also as a result of the high energy efficiency, less fuel is used when the three types of energy are generated simultaneously compared to if three different generation methods were used. 

3. Low Electricity Usage: Specifically during the summer, the amount of electricity used to power fans, air conditioning and other cooling appliances is reduced because cool air is generated and vented out.

4. Reduction in GHGs: Less fuel burnt equals less gaseous byproducts released into the atmosphere. This difference in GHG emissions is massive compared to conventional/single generation systems. Currently it is estimated that over 50% of GHG emissions in buildings are from the cooling and heating loads.

5. Recycling Heat: The heat generated by the engine in an engine based system is repurposed and used to produce steam of hot water for onsite use. A similarity shared with CHP systems that are engine based. 

Disadvantages

1. Feasibility: Applicability differs with each project; detailed feasibility studies are required for each new project [7]. This reinforces the reason why CCHP systems are only used in large scale projects where intense planning and research is required.

2. Capital Intensive: Even though the startup costs associated with CCHP systems are made up for by the reduced fuel and energy cost, it is still worth mentioning. These costs include the manpower and expertise that goes into designing and building this sophisticated system.

3. Sustainability: Any energy produced from fossil fuels is put under scrutiny for many reasons especially because of its lack of sustainability. Merging the idea of trigeneration with sustainable energy generation methods is a welcome idea.

4. Quantity of Energy: As stated earlier, energy efficiency is one of the selling points for CCHP systems. While stating this fact we sometimes overestimate the amount of energy produced. The most important energy type generated is electricity which is why it is in the highest quantity. Heat and cooling are by-products of the electricity generation process hence are produced in smaller amounts.

Uses

1. Industry and agriculture.

2. Hospitals, health and social care facilities.

3. Schools, hotels and sports centers.

CONCLUSION

Research on more innovative and sustainable energy generation methods is ongoing and evolving. Cogeneration and trigeneration are the most sophisticated due to improvements made on conventional/single generation systems used in the past. Trigeneration systems are more energy efficient, require less fuel, reduce energy costs and produce less GHG emissions. Initial start-up cost may hinder the more sophisticated and efficient system (CCHP) from being used more frequently, in this case cogeneration systems can be used instead.

REFERENCES

[1] https://www.lawinsider.com/dictionary/energy-generation 

[2] https://www.vistaprojects.com/blog/what-is-a-cogeneration-plant/ 

[3] https://www.epa.gov/chp/chp-benefits 

[4] https://www.benuk.net/Advantages-Disadvantages-Combined-Heat-Power.html 

[5] https://helec.co.uk/why-chp/combined-heat-power-advantages-disadvantages/ 

[6] https://brdgstn.com/trigeneration/ 

[7] https://nzebnew.pivotaldesign.biz/knowledge-centre/hvac-2/tri-generation/