Cogeneration and Conservation of Energy

In this article we will discuss about cogeneration and conservation of energy.

Cogeneration of Energy:

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

All thermal power plants emit a certain amount of heat during electricity generation. This heat can be released into the natural environment through cooling towers, flue gases or by some other means. Alternatively, this waste heat may be used for some useful purpose such as heating or producing steam. Thus, a thermal power plant may use fuel energy to generate electrical energy only or generate electrical energy along with the production of steam for industrial purposes.

The industries in which the steam is used for different processing purposes are:

(i) Paper mills,

(ii) Textile mills;

(iii) Sugar mills and refineries;

(iv) Chemical works;

(v) Breweries;

(vi) Plastic manufacturing;

(vii) Food processing;

(viii) Laundries and hotels,

(ix) Metallurgy, and

(x) Places that use district heating.

In generation of electrical energy along with production of steam for industrial purposes, known as cogeneration, steam for a certain section of the turbine is extracted for the industrial purposes and the remaining steam is allowed to expand in the turbine or the exhaust steam may be used for the purpose of process steam.

A cogeneration system can be either an inplant power generation system or a reject heat utilisation system. In inplant power generation, the steam is produced at a higher temperature and pressure than required for manufacturing purposes. The steam so produced is used in a turbine-generator set to generate electrical energy and the exhaust steam from turbine is used for manufacturing purpose. The inplant power generation is employed in industries.

In reject heat utilisation system, which is used in power plants, some heat is extracted from the turbine (at a suitable temperature and pressure) and supplied to an adjacent industry for manufacturing process.

There are two main categories of cogeneration viz., topping cycle and bottoming cycle.

In a topping cycle, the fuel energy is used to generate steam at high temperature and at high pressure and electrical energy is generated in the usual manner. Depending upon the requirements, the process steam at low temperature and pressure is either extracted from the turbine at an intermediate stage (similar to feed water heating) or taken at the turbine exhaust. In a bottoming cycle, the steam produced in a boiler at high temperature and pressure is directly used for the manufacturing process and the reject heat from the process is used for generation of electrical energy, obviously at low efficiency.

The combined efficiency of the bottoming cycle will most certainly be less than that for separate generation of process steam and electrical energy. Thus, the bottoming cycle is of little thermodynamic or economic interest. Generally the steam required for industrial processes is at low temperature whereas high temperature steam is required for generation of electrical energy, only the topping cycle, therefore, is suitable for cogeneration. The overall efficiency of a topping cycle is about 75% while the combined efficiency of two separate system (for generating electrical energy and steam) is about 55%.

Advantages of Cogeneration:

Advantages of cogeneration are as follows:

1. Fuel Economy:

There is a considerable economy in the consumption of primary fuels (coal, oil or gas) in cogeneration. This is because of higher thermodynamic efficiency of cogeneration system as compared to separate power producing and heat producing systems. Moreover, the extra fuel required to generate electrical energy for the same quantity of steam produced for process needs, is only around 10%.

2. Lower Capital Cost:

Capital cost for setting up facilities reduces with cogeneration. It is observed that an industry requiring steam for processing has to invest in boilers. The additional investment required to upgrade boiler so as to generate electrical energy is very small as compared to the cost of the boiler. It has been found that additional investment in cogeneration system is only around 50% of the investment required by an electric utility to supply the same power to the industry. Thus, there is a large saving in capital costs if cogeneration is adopted.

3. Smaller Gestation Period:

For a utility to set up a generation unit, about 6 years are required. But installation of cogeneration system in an industry requires 3 years. Because of smaller gestation period, there is saving in interest, early utilization of facility, early return on investment and lesser chances of cost escalation of the project.

4. Saving to Industry from Power Cuts and Power Interruption:

The power cuts and power supply interruption cause huge losses to industries. In case, an industry opts for cogeneration to meet its power needs, it will not face any power cuts or power interruption. Thus, it is an additional advantage of cogeneration.

Cogeneration Technologies:

The commonly used cogeneration technologies are:

1. Steam turbine system,

2. Gas turbine systems,

3. Combined cycle system, and

4. Diesel engine system.

All of these cogeneration technologies are presently employed for topping cycles.

These cogeneration technologies are briefly described as follows:

1. Steam Turbine System:

This system uses boiler, steam turbine, generator and other auxiliaries, as in case of thermal power plant. The fuel used may be natural gas, oil, coal, wood or other similar material. The high pressure steam, which is generated by boiler, is used to drive a turbine coupled to an electric generator. The low pressure of the steam turbine is employed for industrial applications, such as, space heating etc.

Back pressure steam turbines are usually preferred for this system as they exhaust low pressure steam and are more efficient. The introduction of this system leads to increase in pollution (nitrogen oxides, particulates and fly ash) and needs special pollution control measures. There is an energy saving up to 15% in this system of cogeneration.

2. Gas Turbine System:

This system consists of a compressor, combustion chamber, gas turbine and electric generator. The air is sucked in by the compressor from the atmosphere through the filter and is compressed and finally supplied to the combustion chamber. Fuel is burnt in the combustion chamber and is used for heating the compressed gas. The products of combustion comprising of mixture of gases at high temperature and pressure are passed through the gas turbine.

These gases expand in the turbine which drives the electric generator (or alternator). The exhaust from the gas turbine is used as process heat. If required, the exhaust from gas turbine can be used to generate steam in a heat recovery boiler. The advantage of this technology is that energy saving around 25% can be achieved. The disadvantage is that due to scarcity and high cost of petroleum products, cost of electrical energy generated by this process is high. Also, the maintenance requirements of gas turbines are as compared to those in case of steam turbines.

3. Combined Cycle System:

This system is a combination of gas turbine and steam turbine system.

4. Diesel Engine System:

In this system, diesel engine obtains energy from a liquid, usually called the diesel oil, and drives an electric generator (an alternator or dc generator). The engine exhaust and jacket cooling are employed to raise steam in a waste heat recovery boiler where the waste heat is transferred to water.

This system has higher electricity to thermal ratio than that needed for most industries. So, this system can be adopted only if electrical energy can be sold. The generation cost per unit of electrical energy is higher than in other systems. The advantage of this system is that the environmental pollution is minimum. Saving is around 26%. 

Conservation of Energy:

Energy conservation refers to the methods of reduction in energy consumption by way of elimination of wastage and promotion of efficiency. The energy consumption can be reduced by adopting various ways of energy conservation which includes efficient use of technologies and avoiding energy wastages. Energy conservation is the key to hold off climate change/global warming, as one unit of electricity saved is equal to 3 or 4 units of electricity generated.

Largely enhanced energy efficiency provides both the best short-term opportunity for addressing the major energy challenges and an essential component of a long-term strategy. The technology pathways for efficiency involve buildings vehicles and industrial processes. Around 2/3rd electrical energy is used for residential and commercial buildings.

Improved lighting, HAVC appliances [energy efficient household appliances such as ACs (air-conditioners), heaters, dishwashers, lights etc., and also office equipment, active energy managements, cogeneration and energy efficient designs could dramatically reduce our energy needs.

Also, new approaches, such as passive ventilation and day-lighting can both reduce energy consumption and improve comfort. In addition, new designs (greener) for the residential and commercial buildings and for the coming “giga cities” can minimize both energy consumption and pollution. Options include advanced engine design integrated with new approaches to fuel utilization, hybrids and plug-in hybrids, light weighing, hydrogen and fuel cells.

Methods of Saving Energy/Reducing Energy Costs:

1. By Using Energy Efficient Products:

In India, to ensure efficient consumer appliances, Ministry of Power has set up an agency- Bureau of Energy Efficiency (BEE) to provide energy efficiency labels (Green Labels). About 90% of companies manufacturing tube lights, 60% of refrigerators and 50% of air-conditioners have voluntarily opted for this label. BEE hopes to make it mandatory soon. According to BEE calculations, the country could prevent waste of as many as 18 billion units of electricity annually just by saving this end- use energy.

Next on Agenda-Green inverters, diesel pumps, transform­ers and consumer electronics.

Treadle water pump has been developed by the International Development Enterprises (India) (IDE-I). It uses human power to pump water from wells, streams and lakes up to the fields. A study by The Energy Resources Institute (TERI) clearly indicates that these devices can be used in place of diesel pumps in households. It reduces diesel fuel use by 0.45 litre per hour on average and saving 0.65 T of CO₂ per year.

2. For Lighting:

(i) One of the best energy saving devices is the light switch. Turn-off lights when not required.

(ii) Replace 40 W tubes light with 36 W tubes light with electronic ballast.

(iii) As far as possible, use task lighting which concentrates light where it is required.

(iv) Clean tube lights and bulbs regularly so as to increase their luminous efficiency.

(v) Use Compact Fluorescent Light (CFL) in place of incandescent lamps, the General Lighting System, GLS. A CFL converts electricity to light up to 5 times more efficiently than ordinary lamps. A 15 W CFL is equivalent to 60 W ordinary bulbs and saves up to 80% electricity.

India uses 18,000 MW of electric power for lighting purposes. By switching over to CFLs, there can be saving of 10,000 MW electric power and CO₂ emissions can be reduced by 55 million tonnes. Other advantages of CFLs are- Less chances of getting fused, Daylight brightness, weather friendly (disposes no heat), and no fire risk. However, biggest problem of a CFL is its safe disposal. Ministry of Energy is formulating a policy regarding its safe disposal.

Another option is use of LEDs (Light Emitting Diodes). At present, they have limited use because of their low energy intensity. They use 40% less energy than CFL. Life is 10 years. Even though, they have quite high initial cost, but they are most eco-friendly and energy saving man made lights. Energy efficient LEDs are popping up in public places around the world from parks in U.S. to streets in China.

Although LEDs can run at triple the cost of conventional light, they consume 30 to 90% less power and can last 20 to 30 years without being replaced or even wiped clean. So, these are cost savings in the long run. A comparison of LED, CFL and incandescent light is given below by “Lucifier lights”.

Total saving over an incandescent light = 97,087.50 – 1,950 = Rs 95,137.50

3. For Room Air-Conditioners:

(i) Use ACs as sparingly as possible.

(ii) Trees and shrubs around your house keep the hottest sun off. Also, shade your windows by curtains. These steps can enhance the efficiency of ACs by 40%.

(iii) Each degree above 22°C increases efficiency of AC by about 1 %. So, set the AC thermostat at 25° for best comfort at lowest cost.

(iv) Clean filter of the AC regularly. A clogged filter restricts air flow and can damage the unit.

4. For Refrigerators:

(i) Refrigerator motors and compressors produce heat, so allow enough space around the refrigerator so that there may be continuous flow of air.

(ii) Assure adequate air circulation inside the refrigerator.

(iii) When it’s dark, place a lit flashlight inside the refrigerator and close the door. If light around the door is seen, get the seals replaced.

(iv) Keep the refrigerator away from all sources of heat, including direct sunlight, radiators and appliances like the oven and cooking range.

(v) Allow hot and warm foods to cool and cover them well before putting them in refrigerator. This will reduce energy consumption along with condensation.

(vi) Keep the frequency of opening refrigerator door to minimum.

5. For Water Heaters:

(i) Use solar water heater for heating of water.

(ii) By reducing the temperature setting of water heater from 600°C to 500°C, there will be a saving of 18% of the energy consumed at higher setting.

(iii) Insulate hot water pipes, especially where they are run through unheated areas so as to reduce heat loss. But never insulate plastic pipes.

6. For Microwave Ovens:

(i) Use of microwave oven saves up to 50% on your cooking energy costs for a conventional oven, especially for small quantities of food.

(ii) Microwave ovens cook food from the outside edge toward the centre of the disk, so if you are cooking more than one item, place larger and thicker item on the outside.

7. For Electric Kettles:

(i) Electric kettle is more efficient than an electric cook top element, so prefer use of electric kettle for water heating.

(ii) While buying a new electric kettle, choose one with automatic shut off button and heat resistant handle.

(iii) Regularly clean your electric kettle by combining boiling water and vinegar so as to remove mineral deposits. It will save energy as dirty kettle draws more energy for heating.

8. For Computers:

(i) Turn off your home/office equipment when not in use. A computer that runs for 24 hours a day, for instance, consumes more power than an energy efficient refrigerator.

(ii) If your computer must be left on, turn off the monitor; this device alone consumes more than half the system’s energy.

(iii) Set computers, monitors, and copiers to sleep-mode when not in use. This will reduce energy costs by approximately 40%.

9. Unplug Your TV Home Entertainment:

These systems are big energy hogs to begin with. But many of them have a feature that eats even more power. They remain in standby mode after you shut them off, continuing to draw electricity. Standby power is responsible for 5% to 10% of total electricity consumption in most homes and accounts for roughly 1% of global CO₂ emissions, according to International Energy Agency. “Many people don’t realize that they spend more money to power DVD players when they are turned off than when they are in use.”

Nstar suggests that customers connect their electronic devices to power strips and turn off the strips when devices are not in use—which guarantees the gadgets won’t go into standby mode. You could also simply unplug your devices after turning them off.

10. Do not run kitchen and bathroom exhaust fan for long periods.

11. Conduct preventive maintenance checks regularly.

12. Ensure proper input voltage.

13. Use BIS approved pumps and motors.

14. Stop idle running of vehicle at traffic lights.

The energy conservation is the best way toward power sector reforms and to provide energy security to the country. The power can be saved by monitoring of all the commercial buildings, industries, municipal corporations/ committees, agricultural machinery, transport etc., by conducting regular energy audits and national level awareness through media, newspaper, distributing pamphlets, conducting workshops/ seminars. The Bureau of Energy Efficiency has been established by Govt., of India under Ministry of Power to coordinate, regulate and enforce the Energy Conservations Act, 2001.

Commercial and residential buildings account for more than 30% of the electricity consumption in India. We add more than 40 million m² of commercial and residential space annually and this result in the additional burden of ground 5.5 billion units of electricity.

However, it is possible to cut this figure down by 2 billion units through the adoption of energy efficiency measures. Energy efficiency can be achieved through an even mix of architectural techniques combined with technological interventions. The applications of these techniques would bring down energy consumption and demand. Renewable energy technique such as solar and wind power can be used to supplement the use of grid power.