introduction
"Exergy", as a parameter that evaluates the value of energy, regulates the "value" of energy from both "quantity" and "quality" and solves the problem that there is no single parameter in the thermodynamics that can evaluate the energy value alone The problem has changed people's traditional viewpoints on the nature of energy, the loss of energy and the conversion efficiency that can be achieved, providing the scientific basis for thermal analysis. At the same time, it also profoundly reveals the essence of deterioration and degeneration of energy in the process of conversion, indicating the direction for rational use of energy.
The function of a heat pump is to draw heat from the environment and pass it on to a heated object (a hot object). At present, foreign heat pump technology has been widely used, and is still growing. With the national emphasis on energy conservation and environmental protection, the development and promotion of heat pumps in our country have also been rapidly developed. In our field of HVAC, heat pumps, especially compression heat pump has a very wide range of applications. In this paper, the exergy analysis is applied to the application of compression heat pump in heating system.
1 (exergy) and energy
For a long time, people were accustomed to measuring the value of energy from the amount of energy, regardless of what energy it consumed. As we all know, different forms of energy, the value of their power use is not the same. Even in the same form of energy, under different conditions also have different abilities. Although "enthalpy" and "internal energy" have the meaning and dimension of "energy", they do not reflect the quality of energy. However, "entropy" is closely related to the "quality" of energy, but it can not reflect the "quantity" of energy and there is no direct "quality" of energy. In order to be able to use it economically, it is necessary to adopt the same measure that reflects both the quantity and the "qualitative" differences between the various energies. Exergy is just such a thermodynamic physical quantity that scientifically evaluates the value of energy.
1.1 (exergy) and (fire without) concept
The power of all forms of energy to convert to "advanced energy" is not the same. If you use this conversion capacity as a yardstick, you can evaluate the pros and cons of various forms of energy. However, the size of the conversion capacity is related to the environmental conditions and is also related to the degree of irreversibility of the conversion process. Therefore, in fact, under the given environmental conditions, the theoretically maximum conversion capacity can be used as a measure of energy taste, which is called Exergy. Its definition is as follows:
When the system reversibly changes from an arbitrary state to a state that is in balance with a given environment, it can in theory be infinitely converted into that portion of energy in any other form of energy, called exergy [1].
Since the most complete conversion is possible only in the reversible process, it can be assumed that (exergy) is the minimum useful work or theoretically minimum useful work theoretically done in a reversible process under given environmental conditions.
Correspondingly, everything that can not be converted to exergy is called Anergy.
Any energy E consists of (ex) and (fire) (An) composed of two parts, namely
E = Ex + An
1.2 energy conversion rules
From the (exergy) and (fire) point of view, the law of energy conversion can be summarized as follows:
(1) The exergy and the exergy remain the same, that is, the principle of conservation of energy that we often say.
(2) (fire no) can no longer be converted to (exergy), otherwise it will violate the second law of thermodynamics.
(3) reversible process does not appear devalued deterioration, so the (exergy) the total conservation.
(4) In all practical irreversible processes, inevitable devaluations can occur. Exergy will be partially "degenerated" into "extinction" and become exergy losses. Because this kind of degradation can not be compensated, the exergy loss is the real loss in energy conversion.
(5) The exergy value of isolated systems does not increase, but only decreases, at most, the same, that is, the exergy reduction principle of the isolated system. So (exergy), like entropy, can be used as a criterion for the natural process directionality.
1.3 heat (exergy)
If the temperature of a system is higher than the ambient temperature, when the system reversibly changes from any state to a state of equilibrium with the state of the environment (also known as "dead state"), the system releases heat Q and at the same time makes the most useful contribution to the outside world. This maximum useful work is called ExQ heat. If the heat Q is obtained from a constant temperature heat source whose thermodynamic temperature is T, the maximum work W max that can be obtained from the heat when the ambient temperature is T 0, that is, the exergy ExQ
Heat (exergy) has the following properties:
(1) Heat (Exergy) is the maximum useful energy that can be converted by the heat released by the system.
(2) The amount of heat (exergy) is not only related to the size of Q, but also to the system temperature T and the ambient temperature T0.
(3) The same quantity of Q, with different heat (exergy) at different temperatures T, the higher the T is, the greater the exergy is when the ambient temperature is determined.
(4) Heat (exergy) is the same amount of process as heat, not state quantity.
2 Exergy Balance and Exergy Analysis
When we analyze the energy of the thermal system, we hope to find out the parts and causes of the loss through the analysis of the process of energy morphological changes, the quantitative calculation of the effective use of energy and so on, in order to propose improvement measures and predict the improved results . We usually use the energy balance analysis is divided into heat balance (enthalpy balance) analysis and (exergy) balance analysis of two.
2.1 Exergy balance and exergy loss
Conservation of energy is a general rule that energy balances should be balanced. However, exergy is only an available part of energy, and its income and expenditure are generally unbalanced. During the actual conversion process, some of the available energy can be converted into unusable energy and the exergy will be reduced, which is called Exergy loss. This does not violate the law of conservation of energy, and exergy balance is the sum of exergy and exergy loss (unavailable energy).
Supposing that the exergy input is Exin, the exergy output is Exout, the internal exergy loss is Ii and the external work is W, then their equilibrium relationship is
ΣExin + W = ΣExout + ΣIi
Exergy balance takes into account not only the amount of energy, but also the quality of the energy. In considering the exergy balance, the key is the need to write down the exergy losses in order to maintain balance. Among them, the internal irreversible (exergy) loss term is not reflected in the heat balance. Therefore, there is a qualitative difference between the two methods of analysis. However, there is an intrinsic link between the two and the exergy balance is based on heat balance.
2.2 Exergy Analysis and Exergy Efficiency
The usual caloric balance and energy conversion efficiency do not reflect the extent of exergy utilization, so we introduced the concept of exergy efficiency. Exergy efficiency and energy conversion efficiency are defined similarly, except that exergy efficiency is the ratio of earnings (exergy) to payments (exergy).
With the concept of exergy efficiency, we can set up an exergy balance for a given thermodynamic system and perform an exergy analysis of it to achieve the following:
(1) Quantitative calculation of various expenditures, utilization and losses of energy (exergy). Revenue and expenditure balance is the basis, the flow of energy to include revenue items and a variety of loss items, according to the proportion of the distribution can be divided primary and secondary.
(2) Through the calculation of efficiency, to determine the effect of energy conversion and the degree of effective use.
(3) Analyze the rationality of energy utilization, analyze various loss size and influencing factors, propose the possibility of improvement and ways to improve, and predict the improved energy-saving effect.
3 compression heat pump (exergy) analysis
A "heat pump" is an energy-using device that transfers heat from a heating object to a hot object. Proper use of heat pumps can turn those heating heat that can not be directly used into useful heat, thereby increasing heat utilization and saving a lot of fuel. Not only that, with the help of heat pumps, it is also possible to harness the inexhaustible low-heat sources of the atmosphere, oceans, rivers and earth. Although the heat pump itself is not a natural energy source, it does indeed play an "energy" role in terms of its ability to output usable energy, so it is called "special energy source" [2].
3.1 compression heat pump working principle
The working principle of the heat pump is the same as that of the refrigeration device, and the reverse cycle is also adopted. However, the purpose is not to cool but to heat, that is, the range of working temperature is different from that of a refrigerator. It has two types: compression and absorption. The following brief introduction to the working principle of compression heat pump.
Compression heat pumps heat at the cost of consuming a portion of high energy (mechanical or electrical energy), as shown in Figure 3-1. Low-boiling point refrigerant through the compressor, the external power consumption W, the working fluid pressure and temperature. Due to its temperature above the temperature required for heating TH, let it through the condenser to the indoor heat supply Q1 itself is condensed. Then through the expansion valve throttle buck, while the temperature is also reduced. Since its temperature will be lower than the temperature TL of the heat source (usually the ambient temperature T0), the heat is absorbed by the evaporator Q2 and evaporates. Vapor back to the compressor to continue to compress, to complete a cycle.
The measure of the performance of a compression heat pump is the "heating factor," or "coefficient of performance," COP (Coefficient of Performance). It refers to the heat users get the ratio of heat and power consumption, that is
If the heat pump is completely reversible, that is, according to the reverse Carnot cycle 1-2-3-4-1, as shown in Figure 3-2, then the maximum heating coefficient.
In fact, due to heat transfer there must be a temperature difference, the working fluid to the indoor heat when the condensation temperature T1 is higher than TH, from the heating heat source when the heat absorption temperature T2 below TL. If the working temperature range of the actual working temperature (T1-T2) to calculate the maximum thermal coefficient.
As can be seen from the above equation, the smaller (T1-T2), or T2 / T1, the greater. Always greater than 1. When T2 / T1 approaches 1, it tends to infinity. This shows that the amount of heat that the heat pump can provide is in excess of the work consumed. And, the smaller the difference in heat transfer temperature, the greater its effectiveness. In this regard, the use of heat pump heating is the most suitable.
In addition to the actual heat pump heat transfer irreversible loss, due to the compressor and the expansion valve there is also an irreversible loss, so the actual heating coefficient will be less than the theoretical value of the heat pump in determining the working fluid, the thermal cycle parameters and the compressor After efficiency, the thermodynamic properties of the working fluid can be used to calculate the actual pyro-thermal coefficient
3.2 compression heat pump (exergy) analysis
The exergy analysis of the heat pump is shown in Figure 3-3. The hatched area in the figure represents the exergy stream and the rest is the ((firefree)) stream. If the temperature TL of the cold source is higher than the ambient temperature T0, the quantity of heat expended by the heat pump Q2 is small (exergy) and its exergy value is
ExQ, L = Q2 = (Q1-W)
The amount of heat expended by the heat pump for the indoor Q1 is (exergy)
ExQ, H = Q1
A is the sum of the exergy losses in the heat pump. B is the exergy loss caused by the temperature difference (T1-TH) when the working medium is transferred to the room. The total exergy loss is ΣIi.
According to the previous exergy balance relation
W = ExQ, H-ExQ, L + ΣIi
Put the relationship between the previous formula into the formula after finishing
W = Q1
As can be seen from the above equation, the actual coefficient of thermal deviations from the reversible Carnot heat pump ideal heating coefficient depends on the size of the heat pump (exergy) loss coefficient and. (Exergy) the greater the loss, the actual value of the pyroelectric coefficient is lower.
Heat pump exergy efficiency e, H can be expressed as
From the above equation, we can see that the heat pump exergy e, H is the effective coefficient including the heat transfer temperature difference (exergy loss), e, H will be less than the heat pump effective coefficient.
4 Exergy analysis of heat pumps used in heating systems
Heating with a heat pump is to heat the room from outside (air, water, earth, etc.) to heat the room. It can provide more heat than is consumed. Compared to other heating methods, heat pump heating there is a big advantage.
4.1 save energy
Our normal winter heating requirements at room temperature maintained at about 20 ℃. If the direct use of electric heater heating, energy consumption is the biggest waste. Because the electric heater can convert all the electric energy into heat energy, 1kW · h can only generate 3600kJ heat. With an electric heat pump, several times the amount of electricity is supplied to the room because of its much greater heating coefficient than 1. For example, if the room temperature TH is 20 ℃, the outdoor temperature T0 is -5 ℃, heat transfer temperature difference between them and the working fluid is 5 ℃, the heat pump effective coefficient is assumed to be 0.6, the actual heating coefficient
This shows that it can provide more than five times as much power. In other words, heating with a heat pump saves 80% of the electricity while providing the same amount of heat.
From the perspective of the rational use of energy, the exergy exergy given to the room after the consumed electric energy W is converted into heat energy has an exergy efficiency e, H of
e, H == 1- = 1- = 0.085
It can be seen from the above equation that 91.5% of the advanced electric energy is converted to ((firefree)) during the electric heating process. The heat pump (exergy) efficiency e, H is
e, H === 0.43
5.1 times for electric heating. Heat pump to the environment a large number of ((no fire)) transferred to the indoor use, so reduce the (exergy) consumption, the rational use of energy.
4.2 increase the utilization of heating waste heat
The effective utilization of waste heat is related to its temperature level, and a large amount of waste heat can not be directly utilized due to the low temperature level. The heat pump can increase the temperature of the heat energy. If the waste heat source is used as the heating heat source of the heat pump, the heat pump heats out the heat of the higher temperature from the heat source to meet the needs of the user so that the heating heat energy can be effectively utilized. Because the heating residual heat source temperature is higher than the ambient temperature, reducing the heat pump temperature rise (TH-TL), thereby increasing the heat pump heat coefficient, and ultimately save energy consumption. Literature [3] also elaborates on the exergy and thermal economics.
4.3 other
In addition to low-temperature waste heat, solar energy, geothermal energy and groundwater can also be used as low-temperature heat sources for heat pumps to form a system of total energy consumption. It is one of the promising measures for effectively utilizing natural renewable energy and improving human living environment . In addition, heat pump heating and boiler heating compared to, but also save fuel, the specific reference can be found in the literature [2].
5 concluding remarks
As can be seen from the above, exergy analysis can help us find a better and more reasonable heating method. In general, exergy analysis is more capable of analyzing the essence of things than energy analysis, and has the same measurement standard for different qualities. (Exergy) analysis of the role of three main points: â‘ a reasonable assessment of the effective use of energy; â‘¡ scientifically diagnose the size and proportion of energy losses; â‘¢ direct the correct direction of energy-saving. With the development of exergy analysis, a new discipline is being formed - "thermal economics", which provides a new way to design the optimal system.
In addition, we can see that the heat pump can make the effective use of heating energy, to save energy and improve energy efficiency purposes, we should be highly valued. At present the heat pump is not only in the heating, but also in the drying, distillation, evaporation and so has been widely used, and achieved good economic benefits. China is a country that is not rich in energy and has a vast heating area. At the same time, there is a large number of low-grade heat sources available for utilization. Development of heat pump technology To solve the industrial and civil needs for energy below 100 ℃, saving high energy consumption, there will be a broad prospect [4].
references
[1] Zhu Mingshan, Linzhao Zhuang, Liu Ying, Peng Xiaofeng. Engineering Thermodynamics [M]. Beijing: Tsinghua University Press, 1995
[2] Tang Xuezhong editor. Heat Conversion and Utilization (Second Edition) [M]. Beijing: Metallurgical Industry Press, 2002
[3] Cai Xiangxing, Yang Donghua. Exergy analysis and thermal economics analysis of heat pump system [A]. Ministry of Education, Engineering Thermophysics Coordination Group compiled. Engineering Thermophysics Proceedings [C]. Beijing: Science Press, 1988, 45-50
[4] Xu Bangyu, Lu Yajun, Ma Liang series. Heat pump [M]. Beijing: China Building Industry Press, 1988
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