The process of using energy is actually the process of energy transfer or transformation, and energy can be transformed into various forms of energy that people need under certain conditions. For example, coal releases heat after burning, which can be used for boiling water, cooking and heating; It can also be used to generate steam, promote the conversion of steam engines into mechanical energy, or promote the conversion of turbine generators into electrical energy. Electric energy can be converted into mechanical energy, light energy or internal energy by motors, lamps or other electrical appliances.
Generally speaking, energy cannot be completely converted into the energy 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 consumed at the beginning. The calculation formula is: energy conversion efficiency = output useful energy/input total energy.
-In today's society, energy is in short supply. How to improve energy efficiency is a hot issue that we urgently need to solve. The calculation of energy conversion efficiency is very common in examination papers. Here are some examples: (* *10 question)
1. Electric kettle boiling water: 100W electric kettle works normally for 28 minutes, and can heat 4 kilograms of water from 20℃ to 100℃. How efficient is it?
When boiling water with an electric kettle, the water temperature rises and it needs to absorb heat. The increased internal energy of water is the energy we need, which belongs to useful energy, and the electric energy consumed by the electric kettle is the total energy input, so the efficiency of boiling water in this electric kettle is as follows:
η = Q absorption/w = cm δ t/pt = 4.2×103× 4× (100-20)/(100× 28× 60) = 80%.
2. Boiler boiling water: If a boiler heats 100Kg of water from 32℃ to 100℃, it needs to burn 3.36Kg of anthracite, with a calorific value of 3.4 × 107J/Kg. How efficient is it?
When using a boiler to boil water, the internal energy increased by water is useful energy, and the energy released by complete combustion of fuel (i.e. chemical energy of fuel) is the total energy input, so the efficiency of this boiler to boil water is as follows:
η = Q suction /Q discharge = cmδt/QM coal = 4.2×103×100× (100-32)/(3.4×107× 3.36) = 25%.
3. Solar water heater: If there is a water heater with a total heat collection area of 10h of 1.35m2, it can heat 100Kg of water from 20℃ to 80℃, and the solar energy received by the surface per m2 per hour is 3.6 × 106J. What is the efficiency?
When the solar water heater works, the added internal energy of water is useful energy, and the solar energy radiated to the collector tube is the total input energy. The efficiency of this solar water heater is as follows:
η = Q absorption/q sun = cm δ t/q sun = 4.2×103×100× (80-20)/(3.6×106×1.35×/kloc-)
4. Heat engine:
(1).S 195 diesel engine is marked with "0.27Kg/Kwh", that is, it can output 1Kwh of useful energy for every 0.27Kg diesel fuel consumed (diesel calorific value q = 3.3x107j/kg). How efficient is it?
Heat engine is a machine that converts internal energy into mechanical energy, in which the obtained mechanical energy belongs to useful energy, and the heat released by complete combustion of fuel is the total input energy. The efficiency of this diesel engine is as follows:
η = W/Q amplifier =1kwh/QM = 3.6×106/(3.3×107× 0.27) = 40.4%.
(2) The output power of the new automobile engine is 69Kw, and the fuel consumption per kloc-0/h is 20Kg (the calorific value of gasoline q = 4.6 × 107J/Kg). How efficient is it?
In this process, the useful energy output is calculated by W =P out t, and the efficiency of this automobile engine is:
η= W/Q = P/T/QM = 69× 103×3600/(4.6× 107×20)= 27%。
5. Motor: The internal resistance of the motor coil marked "6v3w" is 3 Ω. How high is the normal working efficiency regardless of friction?
When the motor works, electric energy is converted into mechanical energy and internal energy. If friction is excluded, this internal energy is the electric heat generated when the motor coil itself is electrified, so the obtained mechanical energy is equal to the consumed electric energy minus the generated electric heat.
When the motor works normally, the current I = P/U = 3w/6v = 0.5A, and the motor efficiency is:
η = W machine /W total =(W total -Q)/W total = (Pt-I2RT)/Pt = (P-I2R)/P = (3-0.52× 3)/3 = 75%.
6. Solar cell: In a solar car, the total radiation power of sunlight on the panel of the solar car is 8× 103W. In clear weather, the voltage generated by the panel facing the sun is 160v, which provides a current of 10A to the motor on the car. How efficient is it?
Solar cells are devices that use solar energy to obtain electrical energy. The generated electric energy belongs to useful energy, while the consumed solar energy is the total input energy. The efficiency of this solar cell is:
η = W power /Q power = UIt/P power t = UI/P power =160×10/(8×103) = 20%.
7. Incandescent lamp: A 40W incandescent lamp works normally 1 s, generating about 8J of light energy. How efficient is it?
When incandescent lamps work normally, electric energy is converted into light energy and internal energy, in which the obtained light energy is useful energy and the consumed electric energy is the total input energy. The luminous efficiency of this incandescent lamp is:
η = W light /W electricity = W light /Pt = 8 /(40 × 1)= 20%,
8. Thermal power generation: A power plant can generate 92Kwh by burning 1t anthracite. How efficient is its power generation?
Thermal power generation is to convert the chemical energy of fuel into electric energy, the obtained electric energy is useful energy, and the consumed chemical energy of fuel is the total input energy. Its power generation efficiency is:
η = W power /Q amplifier = 92kwh/QM = 92x3.6x106/(3.4x3107x3) =10%.
9. High-voltage transmission: There is a 1 10Kv, 22Mw high-voltage transmission equipment with a total resistance of 50Ω. How is its transmission efficiency?
In high-voltage transmission, the electric energy at the output end (for users) is useful energy, and the electric energy at the input end is the total input energy, and the difference between them is the electric energy consumed by the transmission line itself (that is, electric heating).
In the process of high voltage transmission, the current I = p/u = 22w×106/(10/0v×103) = 200a, and its transmission efficiency is:
η= W with/W total =(Pt-I2r line T)/Pt =(P-I2r line)/P =(22× 106-2002×50)/(22× 106)= 90.9%。
10, electric vehicle: 160V 10A electric vehicle, driving at a constant speed on a flat road, with a ground resistance of 288N, 15Km for 0h, how efficient is it?
When an electric vehicle is running, it converts electric energy into mechanical energy, and the obtained mechanical energy overcomes friction to make the vehicle move forward, so the work it does to overcome friction is useful energy, and the consumed electric energy is the total input energy. The efficiency of this electric car is:
η= W You/W Total = f . S/UIT = 288× 15× 103/( 160× 10×3600)= 75%。
—— The above are just common types of calculating energy conversion efficiency, and there are still many problems about energy conversion efficiency in real life. The heat of combustion of fuel can be expressed by HHV (high calorific value) or LHV (low calorific value). The heat of combustion with high calorific value is the heat of combustion when the water vapor of the product after combustion has condensed into liquid, so the latent heat of water condensation is added. Low calorific value combustion heat refers to the combustion heat when the water vapor of the product remains gaseous after combustion, regardless of the latent heat of water condensation.
The choice of fuel calorific value will affect the calculation of its energy conversion efficiency. In Europe, the energy produced by a fuel is expressed by its low calorific value. If the latent heat of water is not considered, the "thermal efficiency" of the condensing boiler can be calculated in this way, and its value may exceed 100%, because its working principle will use the latent heat of some water when it condenses, but this part is not considered when calculating the input energy, which does not violate the first law of thermodynamics. In countries outside Europe, the energy produced by a fuel is expressed by its high calorific value. Considering the latent heat of water condensation, the energy conversion efficiency cannot exceed 100%.
Energy conversion mode Energy efficiency of internal combustion engine and external combustion engine 10% ~ 50% gas turbine engine can reach 40% gas turbine engine plus steam turbine engine (combined cycle) can reach 60% hydraulic engine can reach 90% wind engine can reach 59% (theoretical upper limit) solar battery can reach 6% ~ 40%.
(The general efficiency is about 15%, and the theoretical upper limit is 85% ~ 90%, depending on the technology adopted. ) Firearms ~ 30% (0.300 inch bullets) [0.3 inch ≈ 7.62mm] Fuel cells can electrolyze 85% water 50% ~ 70% (theoretical upper limit is 80% ~ 94%).
Motor with power between 10W and 200W: 50% ~ 90%;
Motors with power exceeding 200 watts: more than 99%. The low-level system of household refrigerator is about 20%, the high-level system is about 40-50% of light bulb, 5- 10% of light emitting diode, 35% of fluorescent lamp at most, 28% of sodium lamp, 40.5% of metal halide lamp and 24% of switching power supply. The practical application can reach 95% of electric water heater, 90%-95% of electric heater and 95% of electric heater. Such as electrical energy and mechanical energy.
A: The main method is media transformation. For example, through the medium of water, water is first turned into high-temperature and high-pressure steam, then it is used to drive a steam turbine or steam engine into mechanical energy, and finally the steam turbine drives a generator to convert it into electrical energy. Various heat engines (gasoline engine, diesel engine and gas turbine) can also be used to convert thermal energy into mechanical energy through the medium of gas. If mechanical energy is further used to drive the generator, it can naturally be converted into electrical energy.
There is also thermoelectric power generation, which is directly converted into electric energy through similar thermocouples, without medium, but it is inefficient and cannot be applied on a large scale.
"Easy to use" is the first to promote electric energy, which can be easily converted into mechanical energy by motor and into heat energy and light energy by electric heating device.
The "convenient form of stored energy" should be mainly chemical energy in the form of batteries. Energy consumption is a very hot topic, so energy conversion is more important. Electronic equipment has become an indispensable part of our daily life, and it will be of great significance to reduce the energy consumption of these equipment. The new IC (integrated circuit) technology can not only save energy, but also maintain the required functions and performance at low cost.
Suppose there is an existing generator, which is driven by electricity and generates electricity. Now please plug in the power supply, start the generator, and then store all the generated electricity. After the generator runs for a period of time, the electricity meter shows * * * consumption 10 kWh, but the stored electricity is only 9 kWh, so the energy conversion efficiency of the motor generator is 9/ 10, that is, 90%. Of course, in the real world, it is impossible to drive a generator with electricity. Here, for the convenience of explanation, it is just an example. Beijing, July 25, 2008: A research team composed of Osaka University in Japan and Ohio State University in the United States has successfully doubled the energy conversion rate of thermoelectric materials.
On July 25th, 2008, the electronic edition of American Science Journal published relevant papers.
According to a report by Kyodo News Agency (July 25th, 2008), thermoelectric materials are semiconductors that can convert thermal energy into electrical energy, and the energy conversion rate is the highest in high-temperature working environments such as automobile engines. Because the engine will emit a lot of heat, covering the engine with this material can convert heat energy into electric energy and use it effectively.
Kenji Kurosaka, an assistant professor at Osaka University, said: "This technology used to be inefficient and even failed to reach the practical level. ..... Now, with the maturity of this technology, it can be applied to fields such as environmentally friendly cars. "The research team successfully developed a new material by adding thallium to a substance called lead tellurium. Sodium was added before, but after thallium was used, the electronic structure changed and the energy conversion rate doubled. What needs to be solved in the future is the high cost of thallium and the safety of lead. According to Kurosaki, researchers are also considering using new thermoelectric materials as power sources for space probes. In an ecosystem, energy exists in all trophic levels of the food chain. In the process of continuous flow and transformation, the energy or assimilation amount absorbed by a certain trophic level organism accounts for the biomass percentage of the previous trophic level biotransformation or energy. 1942 was put forward by Lin Deman. He believes that if the energy conversion rate from one trophic level to another is 10%, then the production efficiency will gradually decrease along the trophic level, that is, each trophic level will reduce energy by 90%. If this proportion is unbalanced, it means that the quantitative balance between organisms in the ecosystem is destroyed. In other words, the impact of energy conversion efficiency on ecology can not be ignored.
In the natural system, the existing forms of energy mainly include: heat energy, electric energy, internal energy, light energy, acoustic energy, chemical energy, mechanical energy, electromagnetic energy, atomic energy, biological energy and other concentrated forms, which are mainly transformed from "one kind of energy" to "another kind of energy" through some mechanical equipment. Energy transfer efficiency: refers to the gradual decrease of energy in the process of flowing along the food chain. The energy transfer efficiency between two adjacent trophic levels is 10% ~ 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 = assimilation amount in the next trophic level/assimilation amount in this level;
For a simple ecosystem, the energy transfer efficiency is generally between 10%-20%;
For complex ecosystems, the energy transfer efficiency is generally less than 10% (such as primary succession and secondary succession).
Energy utilization efficiency: it is usually the ratio of energy flowing into human body to producer energy, or the ratio of the highest trophic level energy to producer energy. Or consider the participation of decomposers to realize multi-level utilization of energy. In an ecosystem, the shorter the food chain, the higher the energy utilization rate. At the same time, the more species in the ecosystem, the more complex the nutritional structure and the higher the energy utilization rate.
From the analysis of 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 beings.
The concept of biological assimilation amount;
Refers to the total chemical energy obtained by a certain trophic level from the external environment. It can be shown as: respiratory consumption of this trophic level, energy flowing from this trophic level to the next trophic level, energy flowing from this trophic level to decomposers, and unused amount of this trophic level.
1, for producers (usually green plants), refers to the fixed solar energy in photosynthesis, that is, the total primary production (GP).
2. For consumers (usually animals), assimilation amount is talking about the energy absorbed by the digestive tract (the food eaten may not be absorbed, so it is not eating energy). Feces are not counted in the assimilation amount, but the energy consumed by breathing is counted.
3. For decomposers (usually saprophytes), it refers to the energy absorbed outside the cell.
Basic calculation of biological assimilation amount;
Assimilation amount = energy intake of previous trophic level-energy in feces.
Assimilation amount = self-growth, development and reproduction+respiration and digestion.
Assimilation amount = energy lost by respiration in the form of heat energy+energy flowing to the next trophic level+energy flowing to decomposers+unused energy. There is not only a problem of quantity, but also a problem of quality. It is precisely because of the quality of energy that the directionality of the process and the second law of thermodynamics are obtained. Electric energy and mechanical energy can be completely converted into mechanical work, which belongs to high quality energy; Only a part of heat energy can be converted into mechanical work, and the energy quality is low. With the conduction of energy, the quantity of energy may not change, but the quality of energy can only decrease. Under extreme conditions, the mass remains the same. This is the so-called energy depreciation principle, which is a more general and generalized expression of the laws of thermodynamics.
The quality of energy is high or low. From its available value, we can see that 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 and electric energy for human use. When high-quality energy is dissipated, it will degenerate into less useful forms, such as internal energy. Therefore, although energy dissipation will not reduce the total amount of energy, it will reduce energy, so we must save energy.
Energy conversion efficiency exists between energy conversion and is related to the quality of energy. For example, electric energy has a very high energy quality and can achieve high conversion efficiency when converted into any form of energy. And if superconductor is used to transmit electric energy, it can even achieve 100% energy conversion.
Others, such as heat energy, can't reach the conversion efficiency of 100% when they are converted into mechanical energy or electric energy, because the second law of thermodynamics limits its conversion efficiency (heat can't be converted into work 100%). The thermoelectric conversion efficiency of thermal power plants is only about 45%. On average, this is equivalent to losing nearly two-thirds of the energy. Therefore, the energy quality of thermal energy is naturally lower than that of electric energy.
Without other changes, the energy conversion efficiency will not exceed 100%. But in some special cases, the fuel cell can exceed 100%.