What Is Energy Conversion Efficiency?

Energy conversion efficiency refers to the ratio of the available energy output by an energy conversion device to its input energy. The output available energy may be electrical energy, mechanical work or heat. There is no consistent definition of energy conversion efficiency, which is mainly related to the extent to which the output energy is available.

Energy conversion efficiency

Common types of energy conversion efficiency calculations

When it comes to efficiency, students generally think of the mechanical efficiency of simple machines, that is, the ratio of useful work to total work. In fact, efficiency has a wide range of applications in energy transfer or conversion.

The process of using energy is actually a process of energy transfer or conversion. Under certain conditions, energy can be converted into various forms of energy that people need. For example, coal emits heat after it is burned, which can be used to boil water, cook food, and heat; it can also be used to produce steam, and promote the conversion of steam engines into mechanical energy, or the conversion of turbine generators into electrical energy. Electric energy can be converted into mechanical energy, light energy or internal energy by electric motors, lamps or other electrical appliances.

In general, energy cannot be completely converted into the energy that people need. The so-called energy conversion efficiency is the ratio of the energy that people need to obtain (that is, useful energy) to the total energy that was originally consumed. The calculation formula is: energy conversion efficiency = output useful energy / The total energy input . ^[1]

In today's society, there is a shortage of energy. How to improve energy efficiency is a hot issue that we urgently need to solve. The calculation of energy conversion efficiency is common on examination papers. The following are examples: (a total of 10 questions)

1. [Electric kettle boiling water]: If an electric kettle with a power of 100W works normally for 28 minutes, it can heat 4Kg of water from 20 ° C to 100 ° C. How effective is it?

When using an electric kettle to boil water, the temperature of the water needs to absorb heat. The increased internal energy of the water is the energy we need and belongs to the useful energy . The electric energy consumed by the electric kettle is the total input energy , so the efficiency of this electric kettle for boiling water for:

= Q _suction / W = cmt / Pt = 4.2 × 10 ³ × 4 × (100-20) / (100 × 28 × 60) = 80%

2. [Boiler water]: If a boiler heats 100Kg of water from 32 ° C to 100 ° C, it needs to burn 3.36Kg of anthracite with a heating value of 3.4 × 10 ⁷ J / Kg. How effective is it?

When using a boiler to boil water, the increased internal energy of the water is useful energy, and the energy released by the complete combustion of the fuel (that is, the chemical energy of the fuel) is the total input energy, so the efficiency of this boiler for water burning is

= Q _suction / Q _discharge = cmt / qm _coal = 4.2 × 10 ³ × 100 × (100-32) / (3.4 × 10 ⁷ × 3.36) = 25%

3. [Solar water heater]: If a water heater with a total heat collection area of 1.35m ² can heat 100Kg of water from 20 to 80 for 10h, and the solar energy received by the earth's surface per m ² per hour is 3.6 × 10 ⁶ J, How effective is it?

When the solar water heater works, the internal energy increased by the water is useful energy. The solar energy radiated to the heat collecting tube is the total input energy. The efficiency of this solar water heater to burn water is:

= Q _suction / Q _sun = cmt / Q _sun = 4.2 × 10 ³ × 100 × (80-20) / (3.6 × 10 ⁶ × 1.35 × 10) = 51.85% ^[1]

4. [Heat engine]:

(1). The S195 diesel engine is marked with "0.27Kg / Kwh", that is, it can output 1Kwh of useful energy for every 0.27Kg of diesel consumed (the diesel heating value is q = 3.3 × 10 ⁷ J / Kg).

A heat engine is a machine that converts internal energy into mechanical energy. The mechanical energy obtained is useful energy, and the heat emitted by the complete combustion of the fuel is the total input energy. The efficiency of this diesel engine is:

= W _with / Q _put = 1Kwh / qm = 3.6 × 10 ^{6 / (} 3.3 × 10 ⁷ × 0.27) = 40.4%

(2). The output power of a new car engine is 69Kw, and the fuel consumption is 20Kg in 1h (the calorific value of gasoline is q = 4.6 × 10 ⁷ J / Kg). How effective is it?

In this process, the useful energy output is calculated with W = Pout · t. The efficiency of this automobile engine is:

= W _with / Q _put = P output · t / qm = 69 × 10 ³ × 3600 / (4.6 × 10 ⁷ × 20) = 27%

5. [Motor]: The internal resistance of the motor coil marked "6v3w" is 3. How effective is it to work normally without friction?

When the motor is operating, the electrical energy is converted into mechanical energy and internal energy. If friction is not taken into account, this internal energy is the electrical heat generated when the motor coil itself is energized. The mechanical energy obtained is equal to the electrical energy consumed minus the electrical heat generated.

When the motor is working normally, the current I = P / U = 3w / 6v = 0.5A, the efficiency of this motor is:

= W _machine / W _total = (W _total- Q) / W _total = (Pt-I ² Rt) / Pt = (P-I ² R) / P = (3-0.5 ² × 3) / 3 = 75 %

6. [Solar battery]: A solar car, the total radiation power of the solar panel is 8 × 10 ³ W. In sunny weather, the voltage generated by the panel facing the sun is 160v. The motor on the car provides 10A, how efficient is it?

A solar cell is a device that uses solar energy to obtain electrical energy. The generated electrical energy is useful energy, and the consumed solar energy is the total input energy. The efficiency of this solar cell is:

= W / Q _too = UIt / P _too t = UI / P _too = 160 × 10 / (8 × 10 ³ ) = 20% ^[1]

7. [Incandescent lamp]: A 40W incandescent lamp normally produces about 8J of light energy for 1 second. How effective is it?

When the incandescent lamp works normally, the electric energy is converted into light energy and internal energy. The obtained light energy is useful energy, and the electric energy it consumes is the total input energy. The efficiency of this incandescent lamp is:

= W _light / W _electricity = W _light / Pt = 8 / (40 × 1) = 20%,

8. [Thermal power generation]: A power plant burns 1t of anthracite to generate 92Kwh. What is the power generation efficiency?

Thermal power is the conversion of fuel's chemical energy into electrical energy. The obtained electrical energy is the useful energy, and the chemical energy of the consumed fuel is the total input energy. Its power generation efficiency is:

= W _power / Q _discharge = 92 Kwh / qm = 92 × 3.6 × 10 ^{6 / (} 3.4 × 10 ⁷ × 10 ³ ) = 10%

9. [High-voltage power transmission]: There is a 110Kv, 22Mw high-voltage power transmission equipment. The total resistance of the transmission line is 50. What is the transmission efficiency?

In high-voltage transmission, the energy at the output (provided to the user) is useful energy, and the energy input at the input is the total input energy. The difference between the two is the power consumed by the transmission line itself (ie, electric heating).

During high-voltage transmission, the current I = P / U = 22w × 10 ⁶ / (110v × 10 ³ ) = 200A, and its transmission efficiency is:

= W _available / W _total = (Pt-I ² R _line t) / Pt = (P-I ² R _line ) / P = (22 × 10 6-200 ² × 50) / (22 × 10 ⁶ ) = 90.9%

10. [Electric vehicle]: 160V 10A electric vehicle runs at a constant speed on a straight road, the ground resistance is 288N, and it travels 15Km in 1h. How effective is it?

The electric vehicle converts electric energy into mechanical energy when it is running. The obtained mechanical energy overcomes friction to do work and advances the car, so the work done to overcome friction is useful energy, and the consumed electric energy is the total input energy. The efficiency of this electric vehicle is: ^[1]

= W Yes / W _Total = fS / UIt = 288 × 15 × 10 ³ / (160 × 10 × 3600) = 75%

[ The above are just common types of calculating energy conversion efficiency. There are many questions about energy conversion efficiency in real life.

Energy conversion efficiency Fuel calorific value and efficiency

The heat of combustion of a fuel can be measured by its HHV ( high heating value ) or LHV (low calorific value) indicates that the high calorific value combustion heat is the combustion heat when the water vapor of the product has condensed into a liquid state after combustion, so the latent heat when water condenses is added. The heat of combustion with low calorific value is the heat of combustion when the water vapor of the product remains in the gaseous state after combustion, without considering the latent heat of water condensation.

The choice of fuel heating value will affect the calculation of its energy conversion efficiency . In Europe, the energy that a fuel can produce is represented by its low heating value. The latent heat when water is condensed is not considered, and the "thermal efficiency" of a condensing boiler is calculated in this way. Its value may exceed 100% because of its working principle Some of the latent heat during the condensation of water will be used, but the calculation of the input energy does not take into account this part, and does not violate the first law of thermodynamics. In countries outside Europe, the energy that a fuel can produce is expressed by its high calorific value, and the latent heat when water condenses has been taken into account. Based on this calculation of energy conversion efficiency, its number cannot exceed 100%.

Secondary energy efficiency map for coal conversion

Efficiency of different energy conversion methods

Energy conversion method	Energy efficiency
Internal combustion engine and external combustion engine	10% 50%
Gas turbine engine	Up to 40%
Gas turbine engine plus steam turbine engine ( combined cycle )	Up to 60%
Water engine	Up to 90%
Wind engine	Up to 59% (theoretical upper limit)
Solar battery	6% 40% (Depending on the technology used, the general efficiency is about 15%, and the theoretical upper limit is 85% to 90%)
firearms	30% (.300 inch bullet) [0.3 inch 7.62 mm]
The fuel cell	Up to 85%
Electrolysis of water	50% to 70% (theoretical upper limit is 80% to 94%)
Photosynthesis	Up to 6%
muscle	14% 27%
electric motor	Small motors with power less than 10 watts: 30% to 60%; Motors with power between 10 and 200 watts: 50% to 90%; Motors with a power of more than 200 watts: more than 99%.
Household refrigerator	Low-order system is about 20%, high-order system is about 40-50%
Bulb	5% 10%
led	Up to 35%
Fluorescent light	28%
sodium lamp	40.5%
Metal halide lamp	twenty four%
Switching power supply	Practical applications can reach 95%
Electric water heater	90% 95%
Heater	About 95%

Energy conversion efficiency How can thermal energy be converted into other energy?

Question: How can thermal energy be converted into other forms of energy that are more convenient to use or store ? Such as electrical energy, mechanical energy.

Answer: It is mainly based on media conversion . For example, through the medium of water, the water is first turned into high-temperature and high-pressure water vapor, and then it is used to drive a steam turbine or steam engine to become mechanical energy, and finally the steam turbine drives a generator to convert it into electricity. It is also possible to use various kinds of heat engines (gasoline engines, diesel engines, gas turbines) to convert thermal energy into mechanical energy through the medium of gas. If the mechanical energy is used to drive the generator, it can also be converted into electrical energy.

There is also thermoelectric power generation using a thermocouple that is directly converted into electrical energy without using a medium, but the efficiency is low and cannot be applied on a large scale.

"Easy to use" is the first to introduce electrical energy, which can be easily converted into mechanical energy by an electric motor, and easily converted into thermal energy, light energy, etc. by an electric heating device.

The "convenient form of energy storage" should be chemical energy in the form of batteries.

The core of the problem of improving energy conversion efficiency

Improve energy conversion efficiency, that is, make energy as little as possible into other energy forms, and more simply into the required energy form. You should know that the more direct the energy conversion process, the higher the conversion efficiency (the simpler the path, the simpler the process, the higher the conversion efficiency). For example, electric vehicles are more efficient than gasoline vehicles, because electricity can be directly connected to the motor through wires, and energy is not lost too much due to resistance. Most of them are converted into mechanical energy, while gasoline engines need gas to expand after burning To push the piston, and there are a bunch of bearings connected to the gear. The more parts connected and the more steps required, the more energy will be lost and the efficiency will naturally decrease.

Energy conversion efficiency

Energy consumption is a very hot topic, so energy conversion is even more important. Electronic equipment has become an essential part of our daily life, and reducing the energy consumption of these devices will be of great significance. The new IC (Integrated Circuit) technology can not only achieve the purpose of energy saving, but also maintain the required functions and performance at low cost. ^[2]

Suppose there is an existing generator that is driven by electricity and produces electricity. Please plug it in, start the generator, and store all the electrical energy produced. When this generator was running for a period of time, the meter showed a total of 10 watts of electricity consumed, but the stored electricity was only 9 degrees, so the energy conversion efficiency of this electric-driven generator is 9/10, that is, 90%. Of course, in the real world, it is impossible to use electric power to drive generators, here is just an example for convenience of illustration.

Energy conversion efficiency doubles for new thermoelectric materials

China News Service, July 25, 2008: A research team composed of Osaka University in Japan and Ohio State University has successfully doubled the energy conversion rate of "thermoelectric materials".

The relevant article was published in the electronic edition of the "Science" magazine on July 25, 2008.

According to Japan s Kyodo News Agency (July 2008) 25, thermoelectric materials are a type of semiconductor that can convert thermal energy into electrical energy. It has the highest energy conversion rate in hundreds of degrees of high-temperature operating environments such as automobile engines. Since the engine emits a lot of heat, covering the engine with this material can convert heat energy into electrical energy for efficient use.

Ken Kurosaki, an assistant professor at Osaka University, said: "This technology was previously inefficient and could not even reach a practical level .... Now, as the technology matures, it can be applied to fields such as environmentally friendly cars." Adding thorium to a substance called lead tellurium successfully developed a new material. Previously, sodium was added, but the electronic structure changed after using rhenium, and the energy conversion rate doubled. What needs to be addressed in the future is the high cost of thorium and ensuring the safety of lead. According to Kurosaki, researchers are also considering using new thermoelectric materials as a power source for space probes. ^[3]

Energy conversion efficiency Energy conversion in ecosystems and natural systems

In the ecosystem, energy exists between the various trophic levels of the food chain. In the process of continuous flow and transformation, the amount of energy or assimilation taken by a certain trophic-grade organism accounts for the biomass percentage of the previous trophic-grade bioequivalent or energy. Proposed by Lindemann in 1942, he believed that the energy conversion rate from one nutritional level to another nutritional level was 10%, and the production efficiency gradually decreased along with the nutritional level, that is, each time a nutritional level was passed, the energy was reduced by 90%. If this value is out of proportion, it means that the quantitative balance between the organisms in the ecosystem is disrupted. In other words, the efficiency of energy conversion on the ecology cannot be ignored.

In natural systems, energy exists mainly in the form of thermal energy, electrical energy, internal energy, light energy, acoustic energy, chemical energy, mechanical energy, electromagnetic energy, atomic energy, and biological energy. They are mainly carried out through some machinery The transformation from "this kind of energy" to "other kind of energy".

Five major types of energy conversion and interaction diagrams (incomplete diagrams)

Energy Conversion Efficiency Transfer and Utilization of Biological Energy

Energy transfer efficiency: The energy is gradually reduced in the process of flowing along the food chain. If the trophic level is taken as a unit, the energy transfer efficiency between two adjacent trophic levels is 10% to 20%.

It can be represented by an energy pyramid, and the calculation formula is: energy transfer efficiency = assimilation amount of the previous trophic level / assimilation amount of the next trophic level × 100%.

Calculation of energy transfer efficiency:

Energy transfer efficiency = amount of assimilation in the next trophic level / amount of assimilation in the current level;

For simple ecosystems, the energy transfer efficiency is generally between 10% and 20%;

For complex ecosystems, the energy transfer efficiency is generally less than 10% (eg primary succession, secondary succession). ^[4]

Energy utilization efficiency: usually the ratio of energy flowing into humans to the energy of producers, or the ratio of the highest nutritional energy to the energy of producers. Or consider the involvement of the resolver to achieve multi-level utilization of energy. In an ecosystem, the shorter the food chain, the higher the energy efficiency. At the same time, the more biological species in the ecosystem and the more complex the nutritional structure, the higher the energy utilization rate.

Analysis from the research object: The energy transfer efficiency is based on the trophic level, while the energy utilization efficiency is based on the highest trophic level or human.

The concept of biological assimilation:

Refers to all chemical energy obtained from the external environment by a trophic level. It can be expressed as: this nutritional level of respiratory consumption, this nutritional level flows to the next

Assimilation diagram ^[5]

Nutritional energy, energy from this nutritional level to the decomposer, unutilized amount of this nutritional level.

1. For producers (generally green plants), it refers to the solar energy fixed in photosynthesis, which is the total primary production (GP).

2. For consumers (generally animals), the amount of assimilation represents the energy absorbed by the digestive tract (the food eaten may not be absorbed, so it is not the energy of eating). Feces are not included in the amount of assimilation, but the respiratory consumption Energy count.

3. For decomposers (generally saprophytic organisms), it refers to the absorbed energy outside the cells. ^[4]

Basic calculation of biological assimilation amount:

Assimilation amount = Intake of energy from previous nutritional level-Energy in stool

Assimilation volume = self-growth, development and reproduction volume + respiratory digestion volume

Amount of assimilation = energy consumed by breathing in the form of thermal energy + energy flowing to the next trophic level + energy flowing to the decomposer + unused energy ^[4]

Energy conversion efficiency energy quality

There is not only the quantity of energy, but also the quality. It is also because of the high and low quality of energy that the directionality of the process and the second law of thermodynamics are available. Electrical energy and mechanical energy can be completely converted into mechanical work, which belongs to a higher quality energy; only a part of thermal energy can be converted into mechanical work, and the energy quality is low. With the transmission of energy, the amount of energy may not change, but the energy quality can only decrease. Under the limit conditions, the quality does not change. This is called the principle of energy depreciation, which is a more general and generalized statement of the heat law.

There are high and low energy qualities, which can be viewed from the value they can be used: The energy stored in coal, oil, natural gas and other energy sources is of high quality, because the energy they contain is highly useful and can be converted into mechanical energy, electrical energy, etc For human use. When high-quality energy is dissipated, it is downgraded to a less usable form, such as internal energy. Therefore, although energy dissipation will not reduce the total amount of energy, energy will decrease, so we must save energy. ^[5]

Energy conversion efficiency exists between energy conversions, and this is related to the level of energy quality. For example, electrical energy has a very high energy quality, and it can achieve high conversion efficiency when it is converted into any form of energy. And if you use superconductors to transmit electrical energy, you can even achieve 100% energy conversion.

Others, such as thermal energy, cannot achieve 100% conversion efficiency when converted into mechanical energy or electrical energy, because the second law of thermodynamics limits its conversion efficiency (heat cannot be converted to work 100%). Thermal power plants generate only about 45% of their thermoelectric conversion efficiency. On average, this is equivalent to nearly two-thirds of the energy lost. Therefore, the energy quality of thermal energy is naturally lower than electrical energy.

In the absence of other changes, the energy conversion efficiency will not exceed 100%. But in some special environments, fuel cells can break 100%.