thermoelectric ratio

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I. Origin of the thermoelectric ratio: In order to accurately reflect the economic operating efficiency of power plants, tap into the potential for saving coal, oil, and electricity, and standardize the calculation of coal consumption and plant electricity consumption rates across the country, the Ministry of Electric Power formulated the \"Regulations on Coal and Electricity Saving in Power Grids and Thermal Power Plants\" in 1979; these regulations, issued under document No. (79) Dian Sheng Zi No. 66, specified the methods for calculating coal consumption and plant electricity consumption rates. This document standardizes the calculation methods for statistical indicators of power enterprises, while also assessing the economic performance of these enterprises. Inter-plant competitions among plants of the same type served as a guide for corporate evaluation activities, and in 1993, the \"Method for Calculating Coal Consumption for Power Generation and Supply Based on the Amount of Coal Fed into the Furnace\" was issued. It specifies the methods for calculating the standard coal consumption for power generation and heat supply in thermal power plants, as well as their plant electricity consumption rates. Article 5 clearly outlines the method for calculating the total standard coal consumption for power generation and heat supply; whether using the forward balance or backward balance method, the first step is to determine the “daily total standard coal consumption B_day_consumption”. Article 7 specifies the method for calculating the standard coal consumption rate for heat supply plants, namely: the daily standard coal consumption rate for heat supply b_day_heat_supply = B_day_heat_supply / Q_day_heat_supply × 1000 (kilograms per million kcal). Here, B_day_heat_supply represents the standard amount of coal used for heat supply on that day (in tons), while Q_day_heat_supply refers to the total amount of heat generated and used for heat supply by the plant on that day (in million kcal). The ratio Q_day_heat_supply / B_day_consumption is referred to as the heat supply ratio; it indicates the proportion of heat supplied by the thermal power plant relative to the total heat consumed for both power generation and heat supply. By using this heat-based approach, it is possible to allocate the coal consumption between heat and electricity production, thereby allowing for the calculation of the standard coal consumption rates for power generation and heat supply, as well as their respective unit costs. Derived from the above formula, the share of heat energy used for power generation is given by Q_day_elec = Q_day_consumption – Q_day_heat (in million kcal). The ratio Q_day_elec/Q_day_heat is known as the electricity-to-heat ratio, and it is used to assess the economic operating efficiency and profitability of thermal power plants. After the 1980s, the cogeneration industry experienced rapid development; local thermal power plants, self-owned thermal power plants, and regional thermal power plants emerged in large numbers, breaking the monopoly of the electricity sector on power generation. During the supply shortages in the 1980s, cogeneration coexisted peacefully with large thermal power plants. In the second half of the 1990s, as construction of large-scale thermal power plants accelerated, the imbalance between electricity supply and demand was alleviated, and the supply capacity exceeded demand. This led to a competition between the electricity generated by cogeneration systems and that produced by large thermal power plants, resulting in conflicts of interest between the cogeneration sector and the electricity industry. In order to ensure that the standard coal consumption level for power generation across the country is in line with international standards, while at the same time safeguarding the healthy development of cogeneration projects, on November 21, 1997, the Planning Commission, in conjunction with the Economic and Trade Commission, the Ministry of Electric Power, and the Ministry of Construction, issued a draft for comments. In 1998, the “Several Provisions on the Development of Cogeneration” were promulgated in Document No. 220, stipulating that “cogeneration should meet the following criteria: (1) The annual average overall thermal efficiency must be greater than 45%” ； (2) Heat and power ratio of cogeneration: For units with a capacity of less than 50 MW, the annual average value should be greater than 100%; for units with a capacity of 50–200 MW, the annual average value should be greater than 50%. For units with a capacity of over 20 MW that can be used for both steam extraction and condensing heating, the heat and power ratio during the heating season should be greater than 50%. ”It was formally proposed to use the \"thermal-to-electric ratio\" and \"overall thermal efficiency\" as quantitative indicators for defining cogeneration, in order to distinguish between and evaluate the operational performance of cogeneration plants, assess their economic viability, and also serve as criteria for approving projects aimed at developing cogeneration and building power plants. II. The meaning of the thermoelectric ratio: To properly guide the development of the thermoelectric industry, it is necessary to thoroughly understand the meaning of the thermoelectric ratio, its calculation methods, and the range of values for different types of units, so as to accurately assess the performance of thermoelectric power plants. ?1. The thermoelectric ratio is a technical and economic indicator for cogeneration units: it reflects the operational level and management efficiency of thermal power plants, and it is one of the main technical and economic indicators for such plants. The standard coal consumption rates for power generation and heat supply refer to the calculation and evaluation of operational and management efficiency when electricity or heat is considered as a single product, whereas the heat-to-electricity ratio is used to measure the degree of heat utilization and energy-saving effects in thermal power units during operation, thereby reflecting a company’s level of development in cogeneration activities and its energy utilization efficiency. It is required that the thermoelectric ratio be increased as much as possible under rated operating conditions, so as to approach or reach the design standard value for the unit’s thermoelectric ratio. It is expressed by the following formulas: qd = [D×(ho-hgs) – Dr×hr + Dbs×hbs] ÷ W (kg/kW); bd = qd/29 kg/kW; Qr/Qd × 100%. Here, qd represents the thermal consumption of the turbine (kg/kW), W is the power generated (kW), and bd is the standard coal consumption rate for power generation (kg/kWh). D, Dr, and Dbs represent the steam inlet volume, industrial steam extraction volume, and make-up water volume respectively (kg/h), while ho, hgs, hr, and hbs represent the enthalpy of the incoming steam, feedwater, industrial steam extraction steam, and make-up water respectively (kg/kg). ard% is the thermoelectric ratio (%); Qr represents the heat supplied for make-up purposes (mg), and Qd represents the heat used for power generation corresponding to Qr (mg). From these formulas, it can be seen that in a thermal power plant, only when the industrial steam extraction volume (i.e., the heat supplied externally) is large can the thermal consumption for power generation be low, resulting in a lower standard coal consumption rate for power generation. In such cases, the thermoelectric ratio will necessarily be high. Taking the C12-4.9/0.98 type unit as an example, when the electrical load is at the design value of 12.2 MW, the rated steam supply amount is 50 t/h; under these conditions, the heat consumption is 8397 kg/kW, the heat-to-electricity ratio is 3.48 hours, and the standard coal consumption for power generation is 0.287 kg/kWh. When the extraction volume reaches its maximum value of 80 t/h, the unit’s heat consumption drops to 6741 kg/kW, the heat-to-electricity ratio increases to 5.66, and the standard coal consumption for power generation becomes 0.23 kg/kWh. In comparison, heat consumption decreases by 19.72%, the standard coal consumption for power generation decreases by 24.73%, while the heat-to-electricity ratio increases by 62.64%. These figures illustrate the thermal efficiency of the thermal power unit and the level of cogeneration, as well as the technical and economic performance of power generation and heat supply. 2. The thermoelectric ratio is a key indicator used to approve thermoelectric power generation projects: Article 39 of the Energy Conservation Law states: “**The development of the following general energy-saving technologies is encouraged: (1) Promoting cogeneration and centralized heating to improve the utilization rate of thermoelectric units... and to increase the overall efficiency of heat utilization. ”As a result, the cogeneration industry is bound to continue to develop and grow, with regional or local thermal power plants as well as self-owned thermal power plants replacing large-scale thermal power plants and being built on a wider scale. The thermoelectric ratio and overall thermal efficiency serve as two quantitative indicators for the approval of thermoelectric power generation projects, acting as benchmarks for control; they also provide technical criteria for selecting appropriate machine models and for the rational allocation of generating units. Since different types of generators have varying thermoelectric ratios, and even the same type of generator can exhibit different thermoelectric ratios under various design parameters and operating conditions, it is advantageous to use the thermoelectric ratio and overall thermal efficiency as criteria for determining whether thermoelectric generation is necessary and valuable, for selecting the appropriate type of generator, and for assessing thermal efficiency. For example, in a certain thermal power plant project, the designed heat load is 45–50 t/h. If the C6-4.9/0.98 model is chosen, the heat-electric ratio is 5.57; with the C12-4.9/0.98 model, the heat-electric ratio is 3.463. The heat-electric ratio of the B3-4.9/0.98 model can be as high as 11.3. By calculating and comparing these heat-electric ratios, and taking into account the specific conditions and actual requirements of the construction project, it is possible to determine the feasibility of such a project, thereby providing a basis and convenience for project approval. 3. The thermoelectric ratio is an indicator used to determine the power generation schedule for thermal power plants: it represents the economic characteristics of a thermal power unit, and each model has a thermoelectric ratio corresponding to its rated operating conditions at the time of design. When the grid supply is abundant and it is necessary to limit the amount of electricity generated by thermal power plants and to arrange the power generation plan based on steam availability, the thermal power ratio can serve as a metric for evaluation. For example, in the design of the C12-4.9/0.98 type unit, with a rated electrical load of 12 MW, its rated steam extraction rate is 50 t/h, and the corresponding heat-to-electricity ratio is 3.463. During actual operation, when the heating load is 40 MW/h, in order to achieve a higher heat-to-electricity ratio, the allowable power generation load is calculated as heating capacity divided by the heat-to-electricity ratio; the result of this calculation is 9500 KW. If the power generation plan is determined based on the quantitative targets set by the Second Department and Second Committee for thermal power plants (i.e., greater than 100%), then the minimum amount of steam required for heating to meet those quantitative targets is also calculated using the thermoelectric ratio formula. Taking the C12-4.9/0.98 model as an example, the minimum amount of steam needed to meet the standards is = (12000×3600×100%) / 2992.03 = 14.4 t/h. 4. The thermoelectric ratio is a key indicator for thermal power plants in developing cogeneration and expanding their heat networks; whenever a thermal power plant comes online, there is still a difference between the thermoelectric ratio under actual operating conditions and the value specified in the design. Especially in some newly built regional thermal power plants, the construction of the power plant takes place first, while the expansion of the number of heat users comes later. Therefore, each thermal power plant should calculate the high heat-electric ratio at its rated load, based on the type and configuration of the installed generating units, and use this value to develop plans for expanding cogeneration and increasing the number of heat users. This will enable the plant’s heat-electric ratio to increase over time, so that it meets the specified standards within the timeframe set out in Document No. 220. It also helps to assess the level of cogeneration and the overall efficiency of heat utilization in that plant, as well as its corresponding economic benefits. 5. The thermoelectric ratio is a metric used for calculating the load distribution among the units in a thermal power plant: thermal power plants always strive to generate as much electricity as possible under a given heating load, while also aiming for a high thermoelectric ratio. Thus, the thermoelectric ratio of various units becomes a calculation parameter for load distribution in power plants. For example, if a thermal power plant installs one unit of each type, C12-4.9/0.98 and B3-4.9/0.98, then when the heating load is 60 t/h, how should the load be distributed so as to ensure that the generator connected to it has a higher electrical load and a higher heat-to-electricity ratio? Through calculations, the thermoelectric ratio of the C12 model at a rated exhaust volume of 50 t/h is 3.463, while that of the B3 model at a rated exhaust volume of 40 t/h is 11.306. The designed overall thermoelectric ratio for this plant is 8.46. When the external steam demand is 60 t/h, there are two possible ways to distribute the load: either the C12 model operates at 12 MW to meet a steam demand of 20 t/h, with a thermoelectric ratio of 1.385, or the B3 model operates at 3 MW to meet a steam demand of 40 t/h, with a thermoelectric ratio of 11.306. In this case, the plant’s overall thermoelectric ratio would be 4.66, allowing it to handle an electrical load of 15 MW ； Steam load 60 t/h ； Another approach involves using C12 units with a capacity of 12 MW to meet a steam load of 40 t/h; their heat-to-electricity ratio is 2.77. In contrast, B3 units can only handle an electric load of 1.5 MW due to a steam load of only 20 t/h. Although their heat-to-electricity ratio remains at 11.306, the overall heat-to-electricity ratio for this plant is 3.718, with a total load of 13.5 MW. Both of these values are lower than those obtained with the previous distribution method. This indicates that in power plants using a combination of extraction and back-pressure types of units, it is advisable to ensure that B-type units operate at their full steam load, while C-type units are used to regulate the thermal load, thereby achieving the best heat-to-electricity ratio and higher thermal efficiency. III. Factors related to the thermoelectric ratio: The thermoelectric ratio is a technical and economic indicator for thermoelectric power generation units, and it is an indicator that changes as other factors change. Understanding its relevant factors is beneficial for guiding the development of the thermoelectric industry. 1. Relationship between thermoelectric ratio and unit type: Thermal power units can be classified into four major categories, comprising nearly 300 different models and specifications. Specifically, they are divided into backpressure machines (Type B), extraction-backpressure machines (Type CB), double-extraction machines (Type CC), and extraction-condensing machines (Type C). Through calculations of the thermoelectric ratio for 300 different models and specifications, it was found that there are significant differences in the thermoelectric ratios among these different types; their ranges are shown in Table 1:

| Item | Machine Type | B Type | CB Type | CC Type | C Type |
|------|--------------|--------|---------|---------|-------|
| Number of Models | 134 | 67 | 37 | 61 |
| Parameters | 2.35–8.83 MPa, 390–535°C | 2.35–8.83 MPa, 390–535°C | 3.43–8.83 MPa, 435–535°C | 1.28–16.7 MPa, 340–537°C |
| Capacity | 0.1–50 MW | 1–25 MW | 12–140 MW | 1.5–220 MW |
| Thermoelectric Ratio | 8–22.7 | 8–19.6 | 1.08–6.8 | 1.3–6.7 |

Table 1 shows the ranges of thermoelectric ratios for various machine types. As can be seen from Table 1, the thermoelectric ratio of backpressure machines is the highest, ranging from 8 to 23, while that of extraction-condensing machines is the lowest, at around 1.3 to 6.7. When comparing different models with the same capacity and parameters, the differences can be seen (see Table 2). The minimum value for condensing units is 3.46, while the maximum value for back-pressure units is 10.5; the difference between these two values is 7.04. The reason for this is that the steam extraction volume of back-pressure units is much higher than that of condensing units. Model? Items: B12-4.9/0.98, CB12-4.9/0.98/0.49, CC12-4.9/0.98/0.49, C12-4.9/0.98. Parameters: 4.9–MPa/470°C; 0.98/268, 4.9/450; 0.98/268, 0.49/199, 4.9/470; 0.98/300, 0.49/280, 4.9/470; 0.98/300. Capacity: 12 MW, 12, 12, 12. Steam extraction rate: 152 t/h, 132, 70, 50. Thermal efficiency: 10.5, 8.87, 4.37, 3.46. Table 2 shows the thermal efficiencies of different models with the same capacity and parameters. Through calculation and comparison, this table provides a range of options for determining the appropriate unit type, selecting equipment, and setting thermal efficiency requirements when constructing thermal power plants. It is also stated that each model has a thermoelectric ratio, along with a variable range. 2. Relationship between the thermoelectric ratio and heating parameters: Heating parameters refer to pressure, temperature, and flow rate. From the formula for the thermoelectric ratio, ard = 1000×Drhr/3600×W, it can be seen that hr represents the enthalpy of the heating steam; this value increases as temperature rises and decreases as pressure increases. In the case of heat engine systems, hr increases as both temperature and pressure increase. When Dr/W is at the rated value of the unit, the thermoelectric ratio for different model specifications increases as their heating steam extraction parameters (pressure, temperature) increase. When the unit is operating under its rated electrical load, the thermoelectric ratio ard? increases as the amount of steam extracted for heat generation rises, following a linear relationship. The slope of this relationship is tgδ = hr/W × 1000/3600 = 0.278 hr/W; therefore, ard? = 0.278 hr/W · Dr. This is typical of a extraction turbine unit. For back-pressure units, electricity generation is determined by steam consumption; the load carried by the unit is related to the heat output at that time, as expressed by the formula W=Dr/dr (in kw), where Dr represents the heat output in kg/h and dr represents the steam consumption rate of the back-pressure unit in kg/kWh. Substituting these values into the formula gives ard?=1000/3600×Dr/hr/(Dr/dr)=0.278 hr·dr. It can be seen that the heat-to-electricity ratio of a back-pressure unit depends on its steam consumption rate as well as the enthalpy value corresponding to the exhaust steam parameters (pressure and temperature). Therefore, the heat-to-electricity ratios for different models of back-pressure units form a horizontal line that does not change as their heat output varies (Figure 1). When ard?=1, that is, when the steam extraction unit meets the **specified quantitative criteria**, there exists a critical amount of steam extraction heat relative to the rated electrical power. This critical value can be calculated using the formula Dr1?=W·ard1/h(t/h), where ard1 represents the quantitative value of the unit’s thermoelectric ratio; for units with a capacity of 25MW or less, this value is 1. It can be seen that the critical steam extraction amount Dr1? depends on the unit’s capacity as well as the enthalpy values related to the steam extraction parameters. For example, the critical steam extraction rate for the C12-4.9/0.98 type unit is 14.093 t/h, while that for the C25-8.83/0.98 type unit is 29.4 t/h. Similarly, both CB and CC type units have a minimum (critical) steam extraction rate at which the heat-to-electricity ratio reaches a specified value; it is thus possible to create curves showing the critical steam extraction rates for various unit types and different single-unit capacities (Figure 2). ?3. Relationship between the thermoelectric ratio and the rated operating conditions of the unit: For extraction-type units (types C, CB, CC), the manufacturer provides various design operating conditions, such as the rated electrical load condition, the rated extraction condition, the maximum extraction condition, the condition of maximum electrical power output, and so on. Taking the C12-4.9/0.98 model from Nanjing Turbine Factory as an example, the manufacturer specifies 13 rated operating conditions, each corresponding to a specific thermoelectric ratio; the corresponding relationships are shown in Table 3: Sequence, Operating Condition Name, Heat Consumption, Thermoelectric Ratio, Overall Thermal Efficiency, Electrical Power, Design Value of Extraction Load, Guaranteed Value, Design Value, Guaranteed Value. 1. Rated Electrical Power, Extraction Load Condition: KW?12257, MPa t/h?0.79/5, 0 kcal/kw?1926.1, 1983.9, 3.43%, 3.43%; Guaranteed Value: 67.55, 65.592. 12213, 0.98/502008: 2068.3, 3.48%, 3.48%; Guaranteed Values: 66.52, 64.583. 122161.28/50: 2124, 2188, 3.52%, 3.52%; Guaranteed Values: 65.01, 63.124. Rated Electrical Power, Maximum Extraction Load Condition: 12284, 0.79/80: 1516, 1561.55, 5.47%, 78.75%, 76.465; Guaranteed Values: 11988, 0.98/80: 1612, 1660.4, 5.66%, 5.66%; Guaranteed Values: 77.69, 75.436. 12270, 1.28/80: 1780, 1833.4, 5.60%, 5.60%; Guaranteed Values: 75.37, 73.177. Maximum Electrical Power, Rated Extraction Load Condition: 15550, 0.79/50: 2016, 2076.4, 2.71%, 2.71%; Guaranteed Values: 62.93, 61.108. 14958, 0.98/50: 2068, 2130, 2.85%, 2.85%; Guaranteed Values: 62.71, 60.889. 15340, 1.28/50: 2193, 2259, 2.82%, 2.82%; Guaranteed Values: 62.62, 60.810. Maximum Steam Inlet Volume, Electrical Power Condition: 15086, 0.98/75: 1809, 1863, 4.21%, 4.21%; Guaranteed Values: 71.82, 69.7311. Maximum Steam Inlet Volume, Extraction Load Condition: 14091, 0.98/80: 1707, 1758, 4.8%, 4.8%; Guaranteed Values: 77.01, 74.7712. Pure Condensation Rated Operating Condition: 12401, 0, 2683, 2763, 0, 0, 32.05%, 31.1213. Pure Condensation Maximum Power Operating Condition: 14030, 0, 2727, 2809, 0, 0, 32.05%, 31.12. Table 3 shows the relationship between thermoelectric ratios and the rated operating conditions of the unit. Four conclusions can be drawn from this table: (1) This unit has 11 extraction load operating conditions, corresponding to 11 different thermoelectric ratio values, which range from 2.17 to 5.66. (2) At the rated electrical power, whether under the rated steam extraction condition or the maximum steam extraction condition, the value of the thermoelectric ratio increases or decreases in sync with its steam extraction parameters. (3) Under the same extraction conditions, the thermoelectric ratio decreases as the motor power increases. (4) Each type of unit has a nameplate thermoelectric ratio, which is calculated based on the rated electrical power and the rated steam extraction conditions. For example, if a unit belongs to the C12-4.9/0.98 model, its nameplate thermoelectric ratio is 3.48, with a comprehensive thermal efficiency of 64.58%; this nameplate thermoelectric ratio is used as the reference value for evaluating the unit’s thermoelectric performance. 4. Relationship between the thermoelectric ratio and electrical load: The relationship between the thermoelectric ratio and electrical load has two meanings. First is the relationship between the change in the capacity of individual units and the heat-to-electricity ratio ； Second is the relationship between the heat-to-electricity ratio and the unit’s electrical load for the same type of unit with a fixed capacity, under the same condensate extraction conditions. When designing the units, manufacturers take into account the increase in initial steam parameters; as a result, although both the electrical power output of the unit and the amount of steam extracted increase simultaneously, the rate of increase in electrical power is greater than that of the steam extraction capacity. From the formula, we have a_rd1 = Dr1/W1. When the capacity of a single unit increases to W2, its steam extraction volume also increases accordingly, from Dr1 to Dr2. At this point, the thermoelectric ratio becomes a_rd2 = Dr2/W2 = (Dr1 + △Dr)/(W1 + △W1). Let δDr1 = △Dr1/Dr1 and δW1 = △W1/W1. Statistical analysis shows that △Dr1/Dr1 ranges from 0.7 to 0.9, while △W1/W1 increases, taking values such as 1.5, 3, 6, 12, 25, 50… MW. Therefore, △W1/W1 > △Dr1/Dr1, and thus a_rd2…