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Study on a novel co-operated heat and power system for improving energy efficiency and flexibility of cogeneration plants
Applied Thermal Engineering 163 (2019) 114429
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Study on a novel co-operated heat and power system for improving energy efficiency and flexibility of cogeneration plants
T
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Yanting Wua, Lin Fua, , Shigang Zhangb, Daoke Tanga a b
Dept. of Building Technology and Science, Tsinghua University, Beijing, China Dept. of Energy Planning & Design, Tsinghua Planning & Design Institute, Beijing, China
H I GH L IG H T S
new system to improve energy efficiency and flexibility of CHP plants is proposed. • ARatio output can be adjusted from 46.2% to 95.0% while fully recover the waste heat. • Thermalof Power efficiency is 87.3% and electric efficiency is 23.8–30.6%. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Cogeneration Power adjustment Energy storage Waste heat recovery District heating
The power adjustment capacity of cogeneration power plants decreases while heating, which severely curtails wind power generation in northern China during winter. This article describes a new system, called the cooperated heat and power system (CoHP), which greatly increases the adjustment capacity while maintaining the maximum heat output and high thermal efficiency of cogeneration plants. The system includes a low-temperature storage tank, high-temperature storage tank, and electric heat pumps. Surplus power during valley hours is used to produce and store high- and low-temperature water; the stored water is released to increase the power output of the generator during peak hours. A model is built for simulation and analysis. The CoHP system is applied to a 300-MW water cooled cogeneration plant. Results show that the system has a power adjustment capacity of 156 MW and storage efficiency of 60.2%. Waste heat from the exhausted steam continues to be fully recovered such that the thermal efficiency has a high value of 87.3%. The electric efficiency is 26.9%, which is lower than the efficiency of the benchmark system (29.2%). The effects of the heating network temperature, temperature difference of heat transfer, and adiabatic efficiency of the compressor are also analyzed. The use of a CoHP system is an effective way of increasing both the energy efficiency and flexibility of cogeneration plants and the system can be widely applied in China.
1. Background
winter. The total wind power abandoned in 2016 was 49.7 billion kWh, with more than 70% of wind power abandoned during winter [1]. In Fig. 1, the power output of a gas cogeneration power plant in deep winter hardly changes over the course of a day. The flexibility of the power supply during winter plunges, aggravating the problem of wind power curtailment [2]. In China, many cogeneration power plants are being built as heat sources to replace highly polluting coal-fired boilers. As a result, studies have been carried out to raise the energy efficiency of cogeneration plants. Using absorption heat pumps to recycle the heat of the exhausted steam can increase the heating capacity of a cogeneration plant by more than 30%. Double-backpressure, dual-rotor swap technology is another way to recycle waste heat. This method has a higher efficiency
Coal boilers remain the main heat sources in China, accounting for 32% of urban heat sources [1]. Meanwhile, coal-burning power plants exhaust heat. These power plants can therefore recover heat from the exhausted steam and become cogeneration plants that are clean heat sources with neither additional fossil fuel consumption nor carbon emissions. However, all cogeneration power plants in China follow the principle of determining power according to the heat demand during heating seasons. This approach prioritizes the local heat demand in the determination of the power generation capacity. The power adjustment capacity is much reduced during heating seasons. This is the main reason for severe wind power curtailment in northern China during
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Corresponding author. E-mail address: [email protected] (L. Fu).
https://doi.org/10.1016/j.applthermaleng.2019.114429 Received 12 February 2019; Received in revised form 16 August 2019; Accepted 21 September 2019 Available online 23 September 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 163 (2019) 114429
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Nomenclature COP E Q T
o pe s sys v e P q t L.P. H.P. HTST η abhp c dp
coefficient of performance quantity of electricity, MWh quantity of heat, MWh absolute temperature, K
Acronyms CoHP I.P LTST
co-operated heat and power system intermediate cylinder low-temperature storage tank
Greek symbols Ф Δ
lifting coefficient for an absorption heat pump difference
ec ex h i l net p pw sh t
Subscripts a b cod e ehp g hs int ls
absorber of a heat pump backwater condenser evaporator of a heat pump electric heat pump generator of a heat pump high-temperature storage tank internal efficiency low-temperature storage tank
and requires less investment. By applying absorption heat exchangers in
outflow primary energy consumption supply water system valley process electric power, MW pressure, kPa heating power, MW temperature, °C low-pressure cylinder high-pressure cylinder high-temperature storage tank efficiency absorption heat pump condenser of heat pump temperature decision point of high-temperature storage tank extracted steam exhaust steam higher temperature of stored water inlet lower temperature of stored water heating network peak process pumped-water power station steam heater turbine
substations, the return water of the primary heating network can be reduced to about 20 °C. In Fig. 2, the 120-°C supply water from the primary heating network flows into the generator of an absorption heat pump. The outflow water from the generator then passes into a heat exchanger to heat the water of the secondary heating network. The water then flows into the evaporator of the absorption heat pump and reaches a temperature of 20 °C. Water in the secondary heating network is heated by the condenser and the absorber of the absorption heat pump and the heat exchanger to 60 °C. The waste heat of cogeneration plants can be fully recycled applying this technology [3,4]. The heat capacity of the fully recycled CHP plant decreases with a decreasing flow rate of the main steam, and to maintain the heat output, the power adjustment capacity of the fully recycled CHP plant is reduced. Studies aimed at increasing the adjustment capacity of cogeneration plants have been carried out. When a cogeneration power plant adjusts its power output, the heat output also changes. A heat storage tank is introduced into the cogeneration power plant to store the heat and thus stabilize the heat output of the system. The optimization of the size of the storage tank and operation method of this type of system has been widely studied [5]. This approach is widely used in Denmark, with the 73,000-m3 heat storage tank at Power Station Fyn being a good example [6]. Further studies on the thermal inertia of a heating network and building showed that the variation range of room temperature was less than 1.5 °C when the supply water temperature increased from 60 to 90 °C[7]. However, the total daily heat output of the system was reduced. Other heat sources, such as coal-fired boilers, are required to meet the heat demand. Electric boilers are also used to consume surplus electricity during periods of low demand (i.e., valley hours). They have very low energy efficiency and have been found to require the support of low electricity prices, which are not sustainable [8]. It is better to use electric heat pumps and studies have thus looked at methods of using electric heat pumps to balance the grid. The operation time of the electric heat
Fig. 1. Power generated by gas cogeneration in the heating season. 2
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Fig. 2. Flowchart of the absorption heat exchange system at a substation.
2.1. Systematic process during peak hours
pumps is adjusted according to the indoor temperature requirements and the real-time electricity price [9–11]. Other studies have combined distributed heat pumps and a cogeneration district heating system. These studies showed that the energy consumption of the whole system could be reduced when the coefficient of performance (COP) of electric heat pumps was larger than 2 [12,13]. In summary, cogeneration power plants require a high heating capacity, high energy efficiency, and large power adjustment capacity. However, current methods only meet one or two of these demands. Under these circumstances, a new system, which we call the co-operated heat and power (CoHP) system, is developed in the present article. The new system recycles all the waste heat, maintains maximum heat output, and enlarges the adjustment range of the cogeneration plant.
During peak hours, more electricity is required, and steam extraction should be reduced to increase the power output. However, this increases the amount of exhausted steam, and the heat pumps lack the capacity to recover waste heat from this increased volume. The waste heat must be stored in a storage tank if it is to be fully recovered. In Fig. 4, the storage tank is full of low-temperature water. Water released from the tank recovers the waste heat and returns it to the tank, and the stored water temperature rises. At the same time, the temperature of the supply water decreases. To meet the demand of the heating network, another storage tank is required to store the outflow water from the heat pumps, while the high-temperature water stored in the tank is released. Steam extraction from the turbine can stop when the power output of the plant reaches a maximum. The outflow water from the condenser then flows into the high-temperature storage tank (HTST) directly to release the stored water as supplied water.
2. Co-operated heat and power system The CoHP system is based on waste heat recovery in a system that provides heat. In Fig. 3, the heating network backwater is heated through a condenser, heat recovery heat pump, and steam heater. Waste heat from the exhausted steam is fully recovered in this system. When the amount of main steam reduces, the heat output also reduces. Therefore, the main steam supply to the turbine cannot be reduced if the system maintains maximum heat output. The flow rate of extracted steam should be increased to reduce the power output of the plants. The power output cannot be reduced further when the extracted steam reaches the maximum flow rate.
2.2. Systematic process during valley hours In the peak process described above, the system requires a tank storing low-temperature water and another tank storing high-temperature water. These two tanks should be filled during valley hours. There are two typical ways of heating or cooling water for these tanks. One way is to increase the extracted steam flow rate. In Fig. 5, the flow rate of exhausted steam decreases when the flow rate of extracted steam rises. The amount of heat needed to be exchanged in the condenser is reduced, such that part of the low-temperature backwater can be stored directly in the low-temperature storage tank (LTST). The extra
Fig. 3. Flowchart of the waste heat recovery and heating system of a cogeneration plant. 3
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Fig. 4. Peak process of the CoHP system.
temperature of water flowing into the HTST is determined by the water source. In Fig. 7, the water can be from the condenser, absorption heat pump, or steam heater. We use the outflow temperature tabhp, o of the ABHP as the decision point tdp . The amount of heat released from the HTST with a temperature range higher than tdp is
extracted steam is used to heat the water in the HTST. In this way, the power output of the plant reaches the floor when the flow rate of extracted steam reaches its maximum. Another way of filling the two tanks is to use electric heat pumps. In Fig. 6, electric heat pumps are used to transfer heat from the LTST to the HTST. Electricity is consumed by the heat pumps, such that the power output of the system can be further reduced. The different processes running during peak and valley hours form a cycle. The main steam supply to the turbine remains stable during this cycle and waste heat from the exhausted steam is fully recovered. The heat output is thus stable throughout the cycle, while the power output can be adjusted over a large range. As discussed above, there are two typical approaches of storing valley heat. The following discusses how to operate the two approaches together and how the electric heat pump approach is applied differently depending on the ABHP outflow temperature. In the peak process, the temperature of water flowing into the LTST is determined by the condenser outflow temperature. This temperature is stable when the pressure of the exhausted steam does not change. The
qhs, h = cp, water Gnet (ths, h − tdp)
(1)
Under this condition, the released heat replaces the heat from the steam heater to reduce the amount of extracted steam. The heat of the reduced extracted steam equals Qhs, h . However, when the stored water has a temperature range lower than the decision point, the heat released from the HTST with a temperature range lower than tdp replaces the heat from the absorption heat pump. This heat contains heat from the extracted steam and exhausted steam. The heat of the reduced extracted steam is therefore
ΔQec, abhp =
cp, water Ghs, l (tdp − ths, l ) Qhs, l = COPabhp COPabhp
(2)
In the first valley process, additional extracted steam can heat water
Fig. 5. Valley process when increasing the flow rate of extracted steam. 4
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Fig. 6. Valley process when using an electric heat pump.
Fig. 7. Peak process when the HTST is divided into two temperature ranges.
heat pump. When the temperature of the outflow water from the condenser of the electric heat pumps ths, h is lower than tdp , the valley process is the same as that in Fig. 6, and the COP of this low-temperature electric heat pump is higher. However, heat produced by the low-temperature electric heat pump can only replace 1/ COPabhp of the heat of the extracted steam, which is replaced by the high-temperature electric heat pump. Adopting all three processes concurrently, the system can maximize
at the higher temperature range. The temperature range for heating by additional extracted steam should thus be the highest one. The second approach uses electric heat pumps. The process is shown in Fig. 6. The COP of this high-temperature electric heat pump is low when the temperature of water from the HTST ths, l is higher than tdp . An optimized process is shown in Fig. 8. The outflow water from the ABHP and the stored water from the HTST meet and flow into the condenser of the electric heat pump together. The temperature of the outflow water of the condenser is lower and thus raises the COP of the electric 5
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Fig. 8. Optimized valley process with a high-temperature electric heat pump.
Fig. 9. Valley process with maximum adjustment capacity. 6
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its power adjustment capacity. Fig. 9 is a flowchart of the valley process. 3. Model for the CoHP system 3.1. Time distribution of the power output A simplified power output is shown in Fig. 10. The orange dotted line shows the power output of the waste heat recovery system, which is set as the benchmark process. The blue1 line shows the simplified power output. One process runs during valley hours and the other runs during peak hours. In this study, we assume that the numbers of valley and peak hours are equal. Fig. 10. Simplified power output for one day.
3.2. Volume balance for stored water
conditions changes little, and the COP of the ABHP in this article is thus set to a constant value of 1.7 [14,15].
When the valley process starts, the starting temperature of all stored water in the LTST is tls, h , while that in the HTST is ths, l . These two temperatures are determined by the peak process described in the following chapter. During the valley process, stored water is released, and the same volume of low/high-temperature water is stored. The level of low/high-temperature water in the storage tank rises and reaches 100% at the end of the valley process. The process reverses during peak hours. At the end of peak hours, the temperature of all stored water returns to the starting temperature. The total volume of water flowing through the storage tank during valley hours should equal the flow volume during peak hours. The valley and peak flow rates are also equal because we assume that the valley and peak times are equal.
3.3.3. Model of the electric heat pump The electric heat pump is divided into five stages. Flow directions of water through the evaporator and condenser are set as a counter heat exchanger. The following assumptions are made for each stage. (1) The overheating of the evaporator and overcooling of the condenser can be neglected. (2) The adiabatic efficiency of the compressor is a constant value of 0.8. (3) The refrigerant of heat pumps having a condenser temperature lower than 90 °C is R134a while the refrigerant is R245fa for higher temperatures.
3.3. Models of major equipment
3.3.4. Evaluation indicators The power adjustment capacity is an indicator used to evaluate the enlarged range of the power output of the cogeneration plant. The waste heat recovery system is taken as a benchmark process. The power output reduction Δe v is the difference in power output between the benchmark process ebase and valley process e v . The power output increase Δep is the difference in power output between the benchmark process ebase and peak process ep . The power adjustment capacity is defined as the sum of the reduction and increase, which is the total adjustment range of the CoHP system during a running cycle:
3.3.1. Model of the turbine To solve various plant parameters, such as flow rates of the main steam supply, extracted steam, and exhausted steam and plant power generation, a turbine model is established under the following assumptions. (1) The heat transfer coefficient of each heat exchange equipment is constant. (2) The internal efficiency of each cylinder is constant. (3) The effect of shaft seal leakage is neglected.
Δeall = Δe v + Δep.
The extracted steam pressure is calculated using the Flügel formula:
Gt Pt2, i − Pt2, o
The storage efficiency of a pumped-water power station is defined as the ratio of electricity released to electricity stored. Similarly, the storage efficiency of the CoHP system ηsys is defined as the ratio of electricity reduction during valley hours to the electricity increase during peak hours. Because the peak and valley times are equal, ηsys can also be defined as the ratio of the power output increase Δep to the power output reduction Δe v :
= constant. (3)
The actual extracted steam enthalpy is calculated according to
ηint =
ht , i − ht , o . ht , i − ht , o, ise
(5)
(4)
ηsys =
3.3.2. Model of the absorption heat pump Absorption heat pumps used in waste heat recovery systems are mainly of the single-effect type. The ABHP has a LiBr–H2O working pair. Fig. 11 is a flowchart of an ABHP. A flow of extracted steam drives the ABHP to recover heat from a flow of exhausted steam and to heat the water of the heating network. The COP represents the ratio of the heat that goes into the heating network and the heat from the extracted steam. In this article, the temperature of the evaporator ranges from 20 to 70 °C and the temperature of the condenser and absorber ranges from 40 to 90 °C. The COP of the absorption heat pump running under these working
ΔEp ΔEv
=
Δep Δe v
.
(6)
The storage efficiency represents the performance difference between the CoHP system and benchmark system. However, the performance of the benchmark system is also affected by external parameters, such as the heating network temperature. Therefore, the total thermal efficiency and electric efficiency of the benchmark system and CoHP system are used to compare the different systems. The total thermal efficiency is defined as the ratio of the output electricity plus the heating output during a day to the primary energy consumption:
ηthermal =
Eall + Qall . Qpe
(7)
The electric efficiency is defined as the ratio of the output electricity during a day to the primary consumption:
1 For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
7
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Fig. 11. Flowchart of a single-effect absorption heat pump.
Fig. 12. Flowchart of the CoHP system applied to a 300-MW cogeneration plant. Table 1 Design parameters of the heating network in the benchmark process. Node number
Flow rate t/h
Temperature °C
H1 H2 H3 H4 H5
4209 4209 4209 4209 4209
20 45 90 90 120
ηelectric =
Eall . Qpe
(8)
4. Simulation and analysis of the CoHP system 4.1. Design operation of the CoHP system The system evaluation in this study is based on a 300-MW watercooled steam turbine of model N320-16.7/537/537. A CoHP system with maximum power adjustment capacity is applied to the turbine. 8
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the default backwater temperature is set to 20 °C. (2) The temperature difference of heat transfer is defined for each heat exchanger in the system, being 3 °C for the condenser of the turbine and 2 °C for the evaporators and condensers of the heat pumps. (3) The adiabatic efficiency of the compressor is set to a constant value of 0.8. (4) The COP of the ABHP is set to a constant value of 1.7.
Table 2 Design parameters of steam flow in the benchmark process. Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 430 159 180 191 142
537 310 537 346 346 346 346 48 48
16.7 3.3 3 0.8 0.8 0.8 0.8 0.011 0.011
3394 3012 3539 3154 3154 3154 3154 2530 2530
In the benchmark process, valves V1 to V8 are closed while valves V9 to V11 are open. Heating network backwater from H1 flows into 1 (see Fig. 12) to recover part of the waste heat from exhausted steam and then flows into 2 to recover the rest of the waste heat using the ABHP, which is driven by extracted steam from S4. The main nominal para-
Fig. 13. Flowchart of the example peak process.
meters of the heating network water flow nodes and steam flow nodes are given in Tables 1 and 2. The nominal backwater temperature is lowered to 20 °C by applying absorption heat exchangers in substations as shown in Fig. 2. In the benchmark process, the total flow rate of extracted steam is 339 t/h and the power generated by the plant is 245 MW. The heat supplied is 489 MW. In the peak process, valves V1, V2, V5, and V6 are open while valves V3, V4, V7, V8, V9, V10, and V11 are closed. Fig. 13 is a flowchart of the peak process. The main steam of the turbine is unchanged. The flow rate of
Fig. 12 is a flowchart of the system. Default values of parameters are described below. (1) The temperatures of the supply water and backwater need to be set in the heating network. In China, the designed temperature of supply water for the primary heating network is always within the range of 120–130 °C. Here, the default supply water temperature is set to 120 °C. The temperature of the backwater is 40 °C in a typical primary network and can be lowered to 20 °C using absorption heat exchangers. Lowering the temperature of the backwater makes it easier to fully recover the waste heat in a cogeneration plant. Here, 9
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discussed in the following section.
Table 3 Design parameters of water flow in the peak process. Node number
Flow rate t/h
Temperature °C
Heating network water flow H1 H2 H3 H4 H5
4209 14,773 – – 4209
20 45 – – 123.8
CoHP system water flow C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
10,564 – 10,564 4209 – 4209 – – – –
20 – 45 45 – 123.8 – – – –
4.2. Analysis of main factors External parameters and the characteristics of heat pumps and heat exchangers affect the performance of the system. Temperatures of the supply water and backwater of the heating network decide the process of the waste heat recovery system, which is the benchmark process, and thus affect the performance of the CoHP system. Moreover, the temperature difference of heat transfer can be reduced or increased by respectively increasing or reducing the heat change area to affect the efficiency of the CoHP system. Additionally, increasing the adiabatic efficiency of the compressors in the electric heat pumps raises the COP of the heat pumps and thus improves the efficiency of the CoHP system. Quantitative analysis is carried out in the following section to present the effects of these parameters
4.2.1. Heating network temperature Heating network temperatures of the supply water and backwater are the most important parameters affecting the performance of the CoHP system. In China, the designed supply water temperature ranges from 120 to 130 °C. However, the highest temperatures for practically supplying water range from 90 to 110 °C throughout the low heating season. Additionally, the backwater temperature was set to be 20 °C in Section 4.1. However, in practical projects, not all substations can apply absorption heat exchangers. The return water from unreformed substations is 50 °C or higher. The total return water temperature can therefore be varied from 20 to 40 °C. Changes in the temperatures of the supply water and backwater greatly affect the performances of the benchmark process and CoHP process. The thermal efficiency and electric efficiency of the benchmark process and CoHP process are given in Table 8. After applying the CoHP system, the waste heat from the exhausted steam continues to be fully recovered such that the thermal efficiency of the system remains at 87.3%. However, only part of the electricity consumed during valley hours can be released, such that the total electricity generated during a day decreases and the electric efficiency during an operation cycle is lower than that for the benchmark process. The electric efficiency increases with decreasing temperature of the supply water. The difference between the electric efficiency of the benchmark process and the CoHP process also becomes less. The reason is that a lower supply temperature reduces the capacity of high-temperature heat pumps, and the COP of the high-temperature heat pumps increases. This is also represented by the system storage efficiency. As the backwater temperature increases, the turbine needs more extracted steam to fully recover waste heat, and the electric efficiency of the system thus decreases. Moreover, as shown in Table 8, after the backwater temperature increases to 40 °C, waste heat from the exhausted steam cannot be fully recovered when the supply water temperature is higher than 107.7 °C. The thermal efficiency of the benchmark process then decreases. However, by applying the CoHP system, surplus waste heat can be stored in the LTST and fully recovered by electric heat pumps in valley hours, such that the thermal efficiency of the CoHP system remains high when the backwater temperature is high. Moreover, the storage efficiency is presented when the thermal efficiency of the benchmark system is the same as that of the CoHP system. A comparison of ηsys at different backwater temperatures reveals that a higher backwater temperature increases the storage efficiency. The reason is that the higher backwater temperature raises the evaporator temperature of electric heat pumps, such that the COP increases. However, the electric efficiency of the benchmark is low and the electric efficiency of the CoHP system with a higher backwater temperature therefore remains lower than that of the system with lower backwater temperature.
Table 4 Design parameters of steam flow in the peak process. Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 769 – – 672 –
537 310 537 346 346 – – 48 –
16.7 3.3 3 0.8 0.8 – – 0.011 –
3394 3012 3539 3154 3154 – – 2530 –
extracted steam is reduced to zero to generate maximum power. Surplus waste heat from the additional exhausted steam is stored in the LTST. High-temperature water stored in the HTST is released and supplied to the heating network. The designed parameters of the heating network water flow, CoHP system water flow, and steam flow are given in Tables 3 and 4. In the peak process, steam extraction from the turbine stops and the power generation of the plant rises to 304 MW, which is 59 MW higher than that of the benchmark process. The heat output is 508 MW. In the valley process, valves V1, V2, V3, V4, V6, V7, V8, V10, and V11 are open while valves V5 and V9 are closed. Fig. 14 is a flowchart of the peak process. The main steam of the turbine remains unchanged. The total flow rate of extracted steam is 400 t/h, which is the maximum value. The two sets of electric heat pumps are driven by surplus power generated by the plant to recover stored waste heat in the LTST. The net power output of the system equals the power generated minus the power consumed by the heat pumps. The designed parameters of the heating network water flow, CoHP system water flow, and steam flow are given in Tables 5 and 6. The power output of the systems in the valley process is given in Table 7. The heat output is 508 MW, which is the same as that for the peak process. In summary, by applying the CoHP system, the power output of the cogeneration plant can be adjusted from 46.2% to 95.0% of the rated power generation. The total adjustment capacity of the system is 156 MW while the storage efficiency is 60.2%. The power output of the system during a day is shown in Fig. 15. This section presents an example of a CoHP system. Performance indices of the system, such as the total thermal efficiency and electric efficiency of the system, are affected by external parameters, which are
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Fig. 14. Flowchart of the example valley process.
Table 5 Design parameters of water flow in the valley process. Node number
Table 6 Design parameters of steam flow in the valley process.
Flow rate t/h
Temperature °C
Heating network water flow H1 H2 H3 H4 H5
4209 2860 4209 8418 4209
20 45 90 103.8 123.9
CoHP system water flow C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
1349 9200 1349 – 4209 4209 4209 8418 9200 4209
20 20 45 – 45 123.9 90 103.8 36.0 90
Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 369 159 241 130 142
537 310 537 346 346 346 346 48 48
16.7 3.3 3 0.8 0.8 0.8 0.8 0.011 0.011
3394 3012 3539 3154 3154 3154 3154 2530 2530
model, and dT is thus decided by the heat transfer area. As the heat transfer area decreases, dT of the turbine condenser increases, tcod, o decreases, and more extracted steam is needed to fully recover the waste heat. Therefore, the electric efficiency decreases. The electric efficiency reduces by 0.9% (from 27.3% to 26.4%) when dT increases from 1 to 5 °C as shown in Fig. 16(a). The value of dT of heat exchangers of the electric heat pumps affects the COP. A larger temperature difference means a higher condenser temperature and lower evaporator temperature, which reduces the COP of the heat pumps. The electric efficiency therefore decreases by 1.5% (from 27.2% to 25.7%) when dT increases from 1 to 5 °C as shown in
4.2.2. Heat-transfer temperature difference The heat-transfer temperature difference (ΔT) is determined by the heat transfer coefficient and heat transfer area. As mentioned in Section 3.3.1, the heat transfer coefficient is set to be constant in the simulation 11
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Table 7 Power output in the valley process.
Power (MW)
Generated by the plant
Consumed by the high-temperature heat pumps
Consumed by the low-temperature heat pumps
Net power output of the system
235
39
48
148
Fig. 15. Power output of the CoHP system during a day. Table 8 Thermal efficiency and electric efficiency of the benchmark process and CoHP process. Supply water temperature (°C)
Benchmark process
CoHP process
ηthermal
ηelectric
Δeall (MW)
ηthermal
ηelectric
ηsys
Backwater temperature 20 °C 120.0 87.3% 115.0 87.3% 110.0 87.3% 105.0 87.3% 103.0 87.3% 100.0 87.3% 95.0 87.3% 90.0 87.3%
29.2% 29.5% 29.9% 30.3% 30.4% 30.7% 31.2% 31.7%
156 145 134 125 121 114 103 94
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
26.9% 27.6% 28.2% 28.7% 29.0% 29.4% 30.1% 30.6%
60.2% 63.1% 65.2% 66.2% 66.2% 68.1% 69.1% 66.7%
Backwater temperature 30 °C 120.0 87.3% 115.0 87.3% 110.0 87.3% 105.0 87.3% 100.0 87.3% 98.7 87.3% 95.0 87.3% 90.0 87.3%
28.3% 28.6% 29.0% 29.4% 29.8% 29.9% 30.3% 30.9%
169 157 146 136 126 124 114 104
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
26.1% 26.9% 27.5% 28.1% 28.7% 28.8% 29.4% 30.0%
64.4% 68.2% 71.2% 73.1% 73.7% 73.5% 75.7% 75.3%
Backwater temperature 40 °C 120.0 82.9% 115.0 84.5% 110.0 86.4% 107.7 87.3% 105.0 87.3% 100.0 87.3% 95.0 87.3% 94.3 87.3% 90.0 87.3%
27.9% 27.9% 27.9% 27.9% 28.1% 28.6% 29.1% 29.2% 29.7%
206 180 161 153 149 139 130 129 117
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
23.9% 25.4% 26.6% 27.1% 27.3% 27.9% 28.5% 28.5% 29.2%
– – – 83.0% 82.6% 84.7% 84.9% 84.7% 87.4%
Fig. 16. Electric efficiency of the CoHP system at different temperature differences of the heat transfer.
4.2.3. Adiabatic efficiency of the compressors of electric heat pumps The adiabatic efficiency of the compressors is set at 0.8 by default. This parameter strongly affects the COP of the electric heat pumps and further affects the performance of the system. Fig. 17 shows that the COPs of both the high-temperature heat pump and low-temperature heat pump are lower when the adiabatic efficiency of the compressor is reduced. A lower COP means more electricity consumption during valley hours when producing the same amounts of high- and low-temperature stored water. The volume and temperature of stored water remain unchanged, and the power output during peak hours thus also remains unchanged. The electric efficiency of the CoHP system is thus reduced. The electric efficiency of the CoHP system is lowered by 2.9% (from 27.6% to 24.7%) when the adiabatic efficiency reduces from 0.9 to 0.6. 5. Conclusions A new system named CoHP was developed. The system has two water storage tanks with different storage temperatures and electric heat pumps. By storing surplus waste heat during peak hours and recovering it using electric heat pumps, which consume surplus electricity during valley hours, the system retains high thermal efficiency while the net power output can be adjusted in a large range. Simulation results of a CoHP system applied to a 300-MW water cooled power plant show that the total power adjustment capacity of the system reaches 156 MW. The net power output of the system can be adjusted from 148 to 304 MW. The storage efficiency of the system is 60.2%. Because part of the consumed valley electricity cannot be released during peak
Fig. 16(b). Results show that the electric efficiency of the system is increased more by reducing dT of heat pumps than reducing dT of the turbine condenser. Therefore, an additional heat transfer area should be first applied to the heat pumps. The results can be used in economic analysis to optimize heat transfer areas of heat exchangers. The thermal efficiency of the system remains unchanged because the waste heat remains fully recovered under the conditions described above. A lower temperature difference can therefore increase the electric efficiency of the system. 12
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adiabatic efficiency of the compressor and reduction in the temperature difference of the condenser and evaporator of the electric heat pumps increase the COP of the heat pumps and thus improve the system performance. In summary, China currently has inadequate heating sources and insufficient power adjustment capacity. In addressing these problems, the use of CoHP technology has the potential to be a widely applicable and effective way of increasing both the energy efficiency and flexibility of cogeneration plants. Acknowledgments This research is supported by National Natural Science Foundation of China (Grant No. 51706115). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114429. References [1] Department of Energy of China, Wind power grid operation in 2016 [EB/OL]. (2017-1-26) [2018-9-2]. http://www.nea.gov.cn/2017-01/26/c_136014615.htm. [2] Yi Wang, Yongfeng Xue, Nan Deng, Study on heat-load-based peak regulation for cogeneration units, Electr. Power 46 (3) (2013) 59–62. [3] Li Yan, Fu Lin, Zhang Shigang, et al., A new type of district heating method with cogeneration based on absorption heat exchange (co-ah cycle), Energy Convers. Manage. 52 (2) (2011) 1200–1207. [4] Wentao Li, Jingquan Zhao, Lin Fu, et al., Energy efficiency analysis of condensed waste heat recovery ways in cogeneration plant, Energy Convers. Manage. 101 (2015) 616–625. [5] Stjepko Katulić, Mislav Čehil, Željko Bogdan, A novel method for finding the optimal heat storage tank capacity for a cogeneration power plant, Appl. Therm. Eng. 65 (1–2) (2014) 530–538. [6] Mogens Kjær Petersen, Jørgen Aagaard, Heat accumulators, News from DBDH, vol. 1, 2004. http://dbdh.dk/images/uploads/presentations2011/heat %20accumulators%20lille.pdf. [7] Bing Qin, Lin Fu, Yi Jiang, Electric peak shaving for CHP plant by using thermal inertia of heat supply system, Gas Heat 25 (10) (2005) 6–8. [8] Jianlin Li, Zhijia Xie, Dexin Li, Chunguang Tian, Research on key technologies of electric boiler with thermal energy storage in facilitating wind power accommodation capability, Electr. Energy Manage. Technol. 01 (2018) 1–7. [9] K.M. Nielsen, T.S. Pedersen, P. Andersen, Heat pumps in private residences used for grid balancing by demand desponse methods, Transmission and Distribution Conference and Exposition (T&D), IEEE PES, 2012, pp. 1–6. [10] Georgios Papaefthymiou, Bernhard Hasche, Christian Nabe, Potential of heat pumps for demand side management and wind power integration in the German electricity market, IEEE Trans. Sustain. Energy 3 (4) (2012). [11] T.S. Pedersen, P. Andersen, K.M. Nielsen, H.L. Stærmose, P.D. Pedersen, Using heat pump energy storages in the power grid, 2011 IEEE International Conference on Control Applications (CCA). Part of 2011 IEEE Multi-Conference on Systems and Control. September 28–30, (2011). [12] Hongyu Long, Ruilin Xu, Jianjun He, et al., Incorporating the variability of wind power with electric heat pumps, Energies 4 (10) (2011) 1748–1762. [13] Hongyu Long, Jianwei Ma, Kai Wu, et al., Energy conservation dispatch of power grid with mass cogeneration and wind turbines, Electr. Power Automat. Equip. 31 (11) (2011) 18–22. [14] Xiaoyun Xie, Yi Jiang, An ideal model of absorption heat pump with ideal solution circulation, J. Refrig. 36 (1) (2015) 1–12. [15] Xiaoyun Xie, Yi Jiang, The ideal process model for absorption heat pumps with real solution, J. Refrig. 36 (1) (2015) 13–23.
Fig. 17. System performance for different adiabatic efficiencies of the compressor.
hours, the electric efficiency of the system is 26.9%, which is lower than that of the benchmark system (29.2%). In addition, waste heat from exhausted steam continues to be fully recovered throughout the operation cycle, such that the total thermal efficiency of the system remains 87.3%. Simulation results of system performance at different heating network temperatures show that reducing the supply water temperature or reducing the backwater temperature increases the electric efficiency of the system. Despite heat transportation energy consumption, a lower network temperature improves the performance of the CoHP system. By lowering the heating network temperature from 120 °C/40 °C to 90 °C/ 20 °C, the electric efficiency of the system rises from 23.9% to 30.6%. Additionally, at a certain network temperature, the waste heat recovery system cannot fully recover the waste heat from the exhausted steam. Heat supplied by electric heat pumps during valley hours replaces the heat supplied by the ABHP and the steam heater in peak hours. Therefore, to increase the same amount of electricity in peak hours, the system with a higher COP of electric heat pumps consumes less valley electricity and has a higher storage efficiency. An increase in the heat transfer area reduces the temperature difference of heat exchangers and thus increases the COP of heat pumps. Additionally, an increase in the
13
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Study on a novel co-operated heat and power system for improving energy efficiency and flexibility of cogeneration plants
T
⁎
Yanting Wua, Lin Fua, , Shigang Zhangb, Daoke Tanga a b
Dept. of Building Technology and Science, Tsinghua University, Beijing, China Dept. of Energy Planning & Design, Tsinghua Planning & Design Institute, Beijing, China
H I GH L IG H T S
new system to improve energy efficiency and flexibility of CHP plants is proposed. • ARatio output can be adjusted from 46.2% to 95.0% while fully recover the waste heat. • Thermalof Power efficiency is 87.3% and electric efficiency is 23.8–30.6%. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Cogeneration Power adjustment Energy storage Waste heat recovery District heating
The power adjustment capacity of cogeneration power plants decreases while heating, which severely curtails wind power generation in northern China during winter. This article describes a new system, called the cooperated heat and power system (CoHP), which greatly increases the adjustment capacity while maintaining the maximum heat output and high thermal efficiency of cogeneration plants. The system includes a low-temperature storage tank, high-temperature storage tank, and electric heat pumps. Surplus power during valley hours is used to produce and store high- and low-temperature water; the stored water is released to increase the power output of the generator during peak hours. A model is built for simulation and analysis. The CoHP system is applied to a 300-MW water cooled cogeneration plant. Results show that the system has a power adjustment capacity of 156 MW and storage efficiency of 60.2%. Waste heat from the exhausted steam continues to be fully recovered such that the thermal efficiency has a high value of 87.3%. The electric efficiency is 26.9%, which is lower than the efficiency of the benchmark system (29.2%). The effects of the heating network temperature, temperature difference of heat transfer, and adiabatic efficiency of the compressor are also analyzed. The use of a CoHP system is an effective way of increasing both the energy efficiency and flexibility of cogeneration plants and the system can be widely applied in China.
1. Background
winter. The total wind power abandoned in 2016 was 49.7 billion kWh, with more than 70% of wind power abandoned during winter [1]. In Fig. 1, the power output of a gas cogeneration power plant in deep winter hardly changes over the course of a day. The flexibility of the power supply during winter plunges, aggravating the problem of wind power curtailment [2]. In China, many cogeneration power plants are being built as heat sources to replace highly polluting coal-fired boilers. As a result, studies have been carried out to raise the energy efficiency of cogeneration plants. Using absorption heat pumps to recycle the heat of the exhausted steam can increase the heating capacity of a cogeneration plant by more than 30%. Double-backpressure, dual-rotor swap technology is another way to recycle waste heat. This method has a higher efficiency
Coal boilers remain the main heat sources in China, accounting for 32% of urban heat sources [1]. Meanwhile, coal-burning power plants exhaust heat. These power plants can therefore recover heat from the exhausted steam and become cogeneration plants that are clean heat sources with neither additional fossil fuel consumption nor carbon emissions. However, all cogeneration power plants in China follow the principle of determining power according to the heat demand during heating seasons. This approach prioritizes the local heat demand in the determination of the power generation capacity. The power adjustment capacity is much reduced during heating seasons. This is the main reason for severe wind power curtailment in northern China during
⁎
Corresponding author. E-mail address: [email protected] (L. Fu).
https://doi.org/10.1016/j.applthermaleng.2019.114429 Received 12 February 2019; Received in revised form 16 August 2019; Accepted 21 September 2019 Available online 23 September 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature COP E Q T
o pe s sys v e P q t L.P. H.P. HTST η abhp c dp
coefficient of performance quantity of electricity, MWh quantity of heat, MWh absolute temperature, K
Acronyms CoHP I.P LTST
co-operated heat and power system intermediate cylinder low-temperature storage tank
Greek symbols Ф Δ
lifting coefficient for an absorption heat pump difference
ec ex h i l net p pw sh t
Subscripts a b cod e ehp g hs int ls
absorber of a heat pump backwater condenser evaporator of a heat pump electric heat pump generator of a heat pump high-temperature storage tank internal efficiency low-temperature storage tank
and requires less investment. By applying absorption heat exchangers in
outflow primary energy consumption supply water system valley process electric power, MW pressure, kPa heating power, MW temperature, °C low-pressure cylinder high-pressure cylinder high-temperature storage tank efficiency absorption heat pump condenser of heat pump temperature decision point of high-temperature storage tank extracted steam exhaust steam higher temperature of stored water inlet lower temperature of stored water heating network peak process pumped-water power station steam heater turbine
substations, the return water of the primary heating network can be reduced to about 20 °C. In Fig. 2, the 120-°C supply water from the primary heating network flows into the generator of an absorption heat pump. The outflow water from the generator then passes into a heat exchanger to heat the water of the secondary heating network. The water then flows into the evaporator of the absorption heat pump and reaches a temperature of 20 °C. Water in the secondary heating network is heated by the condenser and the absorber of the absorption heat pump and the heat exchanger to 60 °C. The waste heat of cogeneration plants can be fully recycled applying this technology [3,4]. The heat capacity of the fully recycled CHP plant decreases with a decreasing flow rate of the main steam, and to maintain the heat output, the power adjustment capacity of the fully recycled CHP plant is reduced. Studies aimed at increasing the adjustment capacity of cogeneration plants have been carried out. When a cogeneration power plant adjusts its power output, the heat output also changes. A heat storage tank is introduced into the cogeneration power plant to store the heat and thus stabilize the heat output of the system. The optimization of the size of the storage tank and operation method of this type of system has been widely studied [5]. This approach is widely used in Denmark, with the 73,000-m3 heat storage tank at Power Station Fyn being a good example [6]. Further studies on the thermal inertia of a heating network and building showed that the variation range of room temperature was less than 1.5 °C when the supply water temperature increased from 60 to 90 °C[7]. However, the total daily heat output of the system was reduced. Other heat sources, such as coal-fired boilers, are required to meet the heat demand. Electric boilers are also used to consume surplus electricity during periods of low demand (i.e., valley hours). They have very low energy efficiency and have been found to require the support of low electricity prices, which are not sustainable [8]. It is better to use electric heat pumps and studies have thus looked at methods of using electric heat pumps to balance the grid. The operation time of the electric heat
Fig. 1. Power generated by gas cogeneration in the heating season. 2
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Fig. 2. Flowchart of the absorption heat exchange system at a substation.
2.1. Systematic process during peak hours
pumps is adjusted according to the indoor temperature requirements and the real-time electricity price [9–11]. Other studies have combined distributed heat pumps and a cogeneration district heating system. These studies showed that the energy consumption of the whole system could be reduced when the coefficient of performance (COP) of electric heat pumps was larger than 2 [12,13]. In summary, cogeneration power plants require a high heating capacity, high energy efficiency, and large power adjustment capacity. However, current methods only meet one or two of these demands. Under these circumstances, a new system, which we call the co-operated heat and power (CoHP) system, is developed in the present article. The new system recycles all the waste heat, maintains maximum heat output, and enlarges the adjustment range of the cogeneration plant.
During peak hours, more electricity is required, and steam extraction should be reduced to increase the power output. However, this increases the amount of exhausted steam, and the heat pumps lack the capacity to recover waste heat from this increased volume. The waste heat must be stored in a storage tank if it is to be fully recovered. In Fig. 4, the storage tank is full of low-temperature water. Water released from the tank recovers the waste heat and returns it to the tank, and the stored water temperature rises. At the same time, the temperature of the supply water decreases. To meet the demand of the heating network, another storage tank is required to store the outflow water from the heat pumps, while the high-temperature water stored in the tank is released. Steam extraction from the turbine can stop when the power output of the plant reaches a maximum. The outflow water from the condenser then flows into the high-temperature storage tank (HTST) directly to release the stored water as supplied water.
2. Co-operated heat and power system The CoHP system is based on waste heat recovery in a system that provides heat. In Fig. 3, the heating network backwater is heated through a condenser, heat recovery heat pump, and steam heater. Waste heat from the exhausted steam is fully recovered in this system. When the amount of main steam reduces, the heat output also reduces. Therefore, the main steam supply to the turbine cannot be reduced if the system maintains maximum heat output. The flow rate of extracted steam should be increased to reduce the power output of the plants. The power output cannot be reduced further when the extracted steam reaches the maximum flow rate.
2.2. Systematic process during valley hours In the peak process described above, the system requires a tank storing low-temperature water and another tank storing high-temperature water. These two tanks should be filled during valley hours. There are two typical ways of heating or cooling water for these tanks. One way is to increase the extracted steam flow rate. In Fig. 5, the flow rate of exhausted steam decreases when the flow rate of extracted steam rises. The amount of heat needed to be exchanged in the condenser is reduced, such that part of the low-temperature backwater can be stored directly in the low-temperature storage tank (LTST). The extra
Fig. 3. Flowchart of the waste heat recovery and heating system of a cogeneration plant. 3
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Fig. 4. Peak process of the CoHP system.
temperature of water flowing into the HTST is determined by the water source. In Fig. 7, the water can be from the condenser, absorption heat pump, or steam heater. We use the outflow temperature tabhp, o of the ABHP as the decision point tdp . The amount of heat released from the HTST with a temperature range higher than tdp is
extracted steam is used to heat the water in the HTST. In this way, the power output of the plant reaches the floor when the flow rate of extracted steam reaches its maximum. Another way of filling the two tanks is to use electric heat pumps. In Fig. 6, electric heat pumps are used to transfer heat from the LTST to the HTST. Electricity is consumed by the heat pumps, such that the power output of the system can be further reduced. The different processes running during peak and valley hours form a cycle. The main steam supply to the turbine remains stable during this cycle and waste heat from the exhausted steam is fully recovered. The heat output is thus stable throughout the cycle, while the power output can be adjusted over a large range. As discussed above, there are two typical approaches of storing valley heat. The following discusses how to operate the two approaches together and how the electric heat pump approach is applied differently depending on the ABHP outflow temperature. In the peak process, the temperature of water flowing into the LTST is determined by the condenser outflow temperature. This temperature is stable when the pressure of the exhausted steam does not change. The
qhs, h = cp, water Gnet (ths, h − tdp)
(1)
Under this condition, the released heat replaces the heat from the steam heater to reduce the amount of extracted steam. The heat of the reduced extracted steam equals Qhs, h . However, when the stored water has a temperature range lower than the decision point, the heat released from the HTST with a temperature range lower than tdp replaces the heat from the absorption heat pump. This heat contains heat from the extracted steam and exhausted steam. The heat of the reduced extracted steam is therefore
ΔQec, abhp =
cp, water Ghs, l (tdp − ths, l ) Qhs, l = COPabhp COPabhp
(2)
In the first valley process, additional extracted steam can heat water
Fig. 5. Valley process when increasing the flow rate of extracted steam. 4
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Fig. 6. Valley process when using an electric heat pump.
Fig. 7. Peak process when the HTST is divided into two temperature ranges.
heat pump. When the temperature of the outflow water from the condenser of the electric heat pumps ths, h is lower than tdp , the valley process is the same as that in Fig. 6, and the COP of this low-temperature electric heat pump is higher. However, heat produced by the low-temperature electric heat pump can only replace 1/ COPabhp of the heat of the extracted steam, which is replaced by the high-temperature electric heat pump. Adopting all three processes concurrently, the system can maximize
at the higher temperature range. The temperature range for heating by additional extracted steam should thus be the highest one. The second approach uses electric heat pumps. The process is shown in Fig. 6. The COP of this high-temperature electric heat pump is low when the temperature of water from the HTST ths, l is higher than tdp . An optimized process is shown in Fig. 8. The outflow water from the ABHP and the stored water from the HTST meet and flow into the condenser of the electric heat pump together. The temperature of the outflow water of the condenser is lower and thus raises the COP of the electric 5
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Fig. 8. Optimized valley process with a high-temperature electric heat pump.
Fig. 9. Valley process with maximum adjustment capacity. 6
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its power adjustment capacity. Fig. 9 is a flowchart of the valley process. 3. Model for the CoHP system 3.1. Time distribution of the power output A simplified power output is shown in Fig. 10. The orange dotted line shows the power output of the waste heat recovery system, which is set as the benchmark process. The blue1 line shows the simplified power output. One process runs during valley hours and the other runs during peak hours. In this study, we assume that the numbers of valley and peak hours are equal. Fig. 10. Simplified power output for one day.
3.2. Volume balance for stored water
conditions changes little, and the COP of the ABHP in this article is thus set to a constant value of 1.7 [14,15].
When the valley process starts, the starting temperature of all stored water in the LTST is tls, h , while that in the HTST is ths, l . These two temperatures are determined by the peak process described in the following chapter. During the valley process, stored water is released, and the same volume of low/high-temperature water is stored. The level of low/high-temperature water in the storage tank rises and reaches 100% at the end of the valley process. The process reverses during peak hours. At the end of peak hours, the temperature of all stored water returns to the starting temperature. The total volume of water flowing through the storage tank during valley hours should equal the flow volume during peak hours. The valley and peak flow rates are also equal because we assume that the valley and peak times are equal.
3.3.3. Model of the electric heat pump The electric heat pump is divided into five stages. Flow directions of water through the evaporator and condenser are set as a counter heat exchanger. The following assumptions are made for each stage. (1) The overheating of the evaporator and overcooling of the condenser can be neglected. (2) The adiabatic efficiency of the compressor is a constant value of 0.8. (3) The refrigerant of heat pumps having a condenser temperature lower than 90 °C is R134a while the refrigerant is R245fa for higher temperatures.
3.3. Models of major equipment
3.3.4. Evaluation indicators The power adjustment capacity is an indicator used to evaluate the enlarged range of the power output of the cogeneration plant. The waste heat recovery system is taken as a benchmark process. The power output reduction Δe v is the difference in power output between the benchmark process ebase and valley process e v . The power output increase Δep is the difference in power output between the benchmark process ebase and peak process ep . The power adjustment capacity is defined as the sum of the reduction and increase, which is the total adjustment range of the CoHP system during a running cycle:
3.3.1. Model of the turbine To solve various plant parameters, such as flow rates of the main steam supply, extracted steam, and exhausted steam and plant power generation, a turbine model is established under the following assumptions. (1) The heat transfer coefficient of each heat exchange equipment is constant. (2) The internal efficiency of each cylinder is constant. (3) The effect of shaft seal leakage is neglected.
Δeall = Δe v + Δep.
The extracted steam pressure is calculated using the Flügel formula:
Gt Pt2, i − Pt2, o
The storage efficiency of a pumped-water power station is defined as the ratio of electricity released to electricity stored. Similarly, the storage efficiency of the CoHP system ηsys is defined as the ratio of electricity reduction during valley hours to the electricity increase during peak hours. Because the peak and valley times are equal, ηsys can also be defined as the ratio of the power output increase Δep to the power output reduction Δe v :
= constant. (3)
The actual extracted steam enthalpy is calculated according to
ηint =
ht , i − ht , o . ht , i − ht , o, ise
(5)
(4)
ηsys =
3.3.2. Model of the absorption heat pump Absorption heat pumps used in waste heat recovery systems are mainly of the single-effect type. The ABHP has a LiBr–H2O working pair. Fig. 11 is a flowchart of an ABHP. A flow of extracted steam drives the ABHP to recover heat from a flow of exhausted steam and to heat the water of the heating network. The COP represents the ratio of the heat that goes into the heating network and the heat from the extracted steam. In this article, the temperature of the evaporator ranges from 20 to 70 °C and the temperature of the condenser and absorber ranges from 40 to 90 °C. The COP of the absorption heat pump running under these working
ΔEp ΔEv
=
Δep Δe v
.
(6)
The storage efficiency represents the performance difference between the CoHP system and benchmark system. However, the performance of the benchmark system is also affected by external parameters, such as the heating network temperature. Therefore, the total thermal efficiency and electric efficiency of the benchmark system and CoHP system are used to compare the different systems. The total thermal efficiency is defined as the ratio of the output electricity plus the heating output during a day to the primary energy consumption:
ηthermal =
Eall + Qall . Qpe
(7)
The electric efficiency is defined as the ratio of the output electricity during a day to the primary consumption:
1 For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
7
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Fig. 11. Flowchart of a single-effect absorption heat pump.
Fig. 12. Flowchart of the CoHP system applied to a 300-MW cogeneration plant. Table 1 Design parameters of the heating network in the benchmark process. Node number
Flow rate t/h
Temperature °C
H1 H2 H3 H4 H5
4209 4209 4209 4209 4209
20 45 90 90 120
ηelectric =
Eall . Qpe
(8)
4. Simulation and analysis of the CoHP system 4.1. Design operation of the CoHP system The system evaluation in this study is based on a 300-MW watercooled steam turbine of model N320-16.7/537/537. A CoHP system with maximum power adjustment capacity is applied to the turbine. 8
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the default backwater temperature is set to 20 °C. (2) The temperature difference of heat transfer is defined for each heat exchanger in the system, being 3 °C for the condenser of the turbine and 2 °C for the evaporators and condensers of the heat pumps. (3) The adiabatic efficiency of the compressor is set to a constant value of 0.8. (4) The COP of the ABHP is set to a constant value of 1.7.
Table 2 Design parameters of steam flow in the benchmark process. Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 430 159 180 191 142
537 310 537 346 346 346 346 48 48
16.7 3.3 3 0.8 0.8 0.8 0.8 0.011 0.011
3394 3012 3539 3154 3154 3154 3154 2530 2530
In the benchmark process, valves V1 to V8 are closed while valves V9 to V11 are open. Heating network backwater from H1 flows into 1 (see Fig. 12) to recover part of the waste heat from exhausted steam and then flows into 2 to recover the rest of the waste heat using the ABHP, which is driven by extracted steam from S4. The main nominal para-
Fig. 13. Flowchart of the example peak process.
meters of the heating network water flow nodes and steam flow nodes are given in Tables 1 and 2. The nominal backwater temperature is lowered to 20 °C by applying absorption heat exchangers in substations as shown in Fig. 2. In the benchmark process, the total flow rate of extracted steam is 339 t/h and the power generated by the plant is 245 MW. The heat supplied is 489 MW. In the peak process, valves V1, V2, V5, and V6 are open while valves V3, V4, V7, V8, V9, V10, and V11 are closed. Fig. 13 is a flowchart of the peak process. The main steam of the turbine is unchanged. The flow rate of
Fig. 12 is a flowchart of the system. Default values of parameters are described below. (1) The temperatures of the supply water and backwater need to be set in the heating network. In China, the designed temperature of supply water for the primary heating network is always within the range of 120–130 °C. Here, the default supply water temperature is set to 120 °C. The temperature of the backwater is 40 °C in a typical primary network and can be lowered to 20 °C using absorption heat exchangers. Lowering the temperature of the backwater makes it easier to fully recover the waste heat in a cogeneration plant. Here, 9
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discussed in the following section.
Table 3 Design parameters of water flow in the peak process. Node number
Flow rate t/h
Temperature °C
Heating network water flow H1 H2 H3 H4 H5
4209 14,773 – – 4209
20 45 – – 123.8
CoHP system water flow C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
10,564 – 10,564 4209 – 4209 – – – –
20 – 45 45 – 123.8 – – – –
4.2. Analysis of main factors External parameters and the characteristics of heat pumps and heat exchangers affect the performance of the system. Temperatures of the supply water and backwater of the heating network decide the process of the waste heat recovery system, which is the benchmark process, and thus affect the performance of the CoHP system. Moreover, the temperature difference of heat transfer can be reduced or increased by respectively increasing or reducing the heat change area to affect the efficiency of the CoHP system. Additionally, increasing the adiabatic efficiency of the compressors in the electric heat pumps raises the COP of the heat pumps and thus improves the efficiency of the CoHP system. Quantitative analysis is carried out in the following section to present the effects of these parameters
4.2.1. Heating network temperature Heating network temperatures of the supply water and backwater are the most important parameters affecting the performance of the CoHP system. In China, the designed supply water temperature ranges from 120 to 130 °C. However, the highest temperatures for practically supplying water range from 90 to 110 °C throughout the low heating season. Additionally, the backwater temperature was set to be 20 °C in Section 4.1. However, in practical projects, not all substations can apply absorption heat exchangers. The return water from unreformed substations is 50 °C or higher. The total return water temperature can therefore be varied from 20 to 40 °C. Changes in the temperatures of the supply water and backwater greatly affect the performances of the benchmark process and CoHP process. The thermal efficiency and electric efficiency of the benchmark process and CoHP process are given in Table 8. After applying the CoHP system, the waste heat from the exhausted steam continues to be fully recovered such that the thermal efficiency of the system remains at 87.3%. However, only part of the electricity consumed during valley hours can be released, such that the total electricity generated during a day decreases and the electric efficiency during an operation cycle is lower than that for the benchmark process. The electric efficiency increases with decreasing temperature of the supply water. The difference between the electric efficiency of the benchmark process and the CoHP process also becomes less. The reason is that a lower supply temperature reduces the capacity of high-temperature heat pumps, and the COP of the high-temperature heat pumps increases. This is also represented by the system storage efficiency. As the backwater temperature increases, the turbine needs more extracted steam to fully recover waste heat, and the electric efficiency of the system thus decreases. Moreover, as shown in Table 8, after the backwater temperature increases to 40 °C, waste heat from the exhausted steam cannot be fully recovered when the supply water temperature is higher than 107.7 °C. The thermal efficiency of the benchmark process then decreases. However, by applying the CoHP system, surplus waste heat can be stored in the LTST and fully recovered by electric heat pumps in valley hours, such that the thermal efficiency of the CoHP system remains high when the backwater temperature is high. Moreover, the storage efficiency is presented when the thermal efficiency of the benchmark system is the same as that of the CoHP system. A comparison of ηsys at different backwater temperatures reveals that a higher backwater temperature increases the storage efficiency. The reason is that the higher backwater temperature raises the evaporator temperature of electric heat pumps, such that the COP increases. However, the electric efficiency of the benchmark is low and the electric efficiency of the CoHP system with a higher backwater temperature therefore remains lower than that of the system with lower backwater temperature.
Table 4 Design parameters of steam flow in the peak process. Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 769 – – 672 –
537 310 537 346 346 – – 48 –
16.7 3.3 3 0.8 0.8 – – 0.011 –
3394 3012 3539 3154 3154 – – 2530 –
extracted steam is reduced to zero to generate maximum power. Surplus waste heat from the additional exhausted steam is stored in the LTST. High-temperature water stored in the HTST is released and supplied to the heating network. The designed parameters of the heating network water flow, CoHP system water flow, and steam flow are given in Tables 3 and 4. In the peak process, steam extraction from the turbine stops and the power generation of the plant rises to 304 MW, which is 59 MW higher than that of the benchmark process. The heat output is 508 MW. In the valley process, valves V1, V2, V3, V4, V6, V7, V8, V10, and V11 are open while valves V5 and V9 are closed. Fig. 14 is a flowchart of the peak process. The main steam of the turbine remains unchanged. The total flow rate of extracted steam is 400 t/h, which is the maximum value. The two sets of electric heat pumps are driven by surplus power generated by the plant to recover stored waste heat in the LTST. The net power output of the system equals the power generated minus the power consumed by the heat pumps. The designed parameters of the heating network water flow, CoHP system water flow, and steam flow are given in Tables 5 and 6. The power output of the systems in the valley process is given in Table 7. The heat output is 508 MW, which is the same as that for the peak process. In summary, by applying the CoHP system, the power output of the cogeneration plant can be adjusted from 46.2% to 95.0% of the rated power generation. The total adjustment capacity of the system is 156 MW while the storage efficiency is 60.2%. The power output of the system during a day is shown in Fig. 15. This section presents an example of a CoHP system. Performance indices of the system, such as the total thermal efficiency and electric efficiency of the system, are affected by external parameters, which are
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Fig. 14. Flowchart of the example valley process.
Table 5 Design parameters of water flow in the valley process. Node number
Table 6 Design parameters of steam flow in the valley process.
Flow rate t/h
Temperature °C
Heating network water flow H1 H2 H3 H4 H5
4209 2860 4209 8418 4209
20 45 90 103.8 123.9
CoHP system water flow C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
1349 9200 1349 – 4209 4209 4209 8418 9200 4209
20 20 45 – 45 123.9 90 103.8 36.0 90
Node number
Flow rate t/h
Temperature °C
Pressure MPa
Enthalpy kJ/kg
S1 S2 S3 S4 S5 S6 S7 S8 S9
972 822 822 769 369 159 241 130 142
537 310 537 346 346 346 346 48 48
16.7 3.3 3 0.8 0.8 0.8 0.8 0.011 0.011
3394 3012 3539 3154 3154 3154 3154 2530 2530
model, and dT is thus decided by the heat transfer area. As the heat transfer area decreases, dT of the turbine condenser increases, tcod, o decreases, and more extracted steam is needed to fully recover the waste heat. Therefore, the electric efficiency decreases. The electric efficiency reduces by 0.9% (from 27.3% to 26.4%) when dT increases from 1 to 5 °C as shown in Fig. 16(a). The value of dT of heat exchangers of the electric heat pumps affects the COP. A larger temperature difference means a higher condenser temperature and lower evaporator temperature, which reduces the COP of the heat pumps. The electric efficiency therefore decreases by 1.5% (from 27.2% to 25.7%) when dT increases from 1 to 5 °C as shown in
4.2.2. Heat-transfer temperature difference The heat-transfer temperature difference (ΔT) is determined by the heat transfer coefficient and heat transfer area. As mentioned in Section 3.3.1, the heat transfer coefficient is set to be constant in the simulation 11
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Table 7 Power output in the valley process.
Power (MW)
Generated by the plant
Consumed by the high-temperature heat pumps
Consumed by the low-temperature heat pumps
Net power output of the system
235
39
48
148
Fig. 15. Power output of the CoHP system during a day. Table 8 Thermal efficiency and electric efficiency of the benchmark process and CoHP process. Supply water temperature (°C)
Benchmark process
CoHP process
ηthermal
ηelectric
Δeall (MW)
ηthermal
ηelectric
ηsys
Backwater temperature 20 °C 120.0 87.3% 115.0 87.3% 110.0 87.3% 105.0 87.3% 103.0 87.3% 100.0 87.3% 95.0 87.3% 90.0 87.3%
29.2% 29.5% 29.9% 30.3% 30.4% 30.7% 31.2% 31.7%
156 145 134 125 121 114 103 94
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
26.9% 27.6% 28.2% 28.7% 29.0% 29.4% 30.1% 30.6%
60.2% 63.1% 65.2% 66.2% 66.2% 68.1% 69.1% 66.7%
Backwater temperature 30 °C 120.0 87.3% 115.0 87.3% 110.0 87.3% 105.0 87.3% 100.0 87.3% 98.7 87.3% 95.0 87.3% 90.0 87.3%
28.3% 28.6% 29.0% 29.4% 29.8% 29.9% 30.3% 30.9%
169 157 146 136 126 124 114 104
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
26.1% 26.9% 27.5% 28.1% 28.7% 28.8% 29.4% 30.0%
64.4% 68.2% 71.2% 73.1% 73.7% 73.5% 75.7% 75.3%
Backwater temperature 40 °C 120.0 82.9% 115.0 84.5% 110.0 86.4% 107.7 87.3% 105.0 87.3% 100.0 87.3% 95.0 87.3% 94.3 87.3% 90.0 87.3%
27.9% 27.9% 27.9% 27.9% 28.1% 28.6% 29.1% 29.2% 29.7%
206 180 161 153 149 139 130 129 117
87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3% 87.3%
23.9% 25.4% 26.6% 27.1% 27.3% 27.9% 28.5% 28.5% 29.2%
– – – 83.0% 82.6% 84.7% 84.9% 84.7% 87.4%
Fig. 16. Electric efficiency of the CoHP system at different temperature differences of the heat transfer.
4.2.3. Adiabatic efficiency of the compressors of electric heat pumps The adiabatic efficiency of the compressors is set at 0.8 by default. This parameter strongly affects the COP of the electric heat pumps and further affects the performance of the system. Fig. 17 shows that the COPs of both the high-temperature heat pump and low-temperature heat pump are lower when the adiabatic efficiency of the compressor is reduced. A lower COP means more electricity consumption during valley hours when producing the same amounts of high- and low-temperature stored water. The volume and temperature of stored water remain unchanged, and the power output during peak hours thus also remains unchanged. The electric efficiency of the CoHP system is thus reduced. The electric efficiency of the CoHP system is lowered by 2.9% (from 27.6% to 24.7%) when the adiabatic efficiency reduces from 0.9 to 0.6. 5. Conclusions A new system named CoHP was developed. The system has two water storage tanks with different storage temperatures and electric heat pumps. By storing surplus waste heat during peak hours and recovering it using electric heat pumps, which consume surplus electricity during valley hours, the system retains high thermal efficiency while the net power output can be adjusted in a large range. Simulation results of a CoHP system applied to a 300-MW water cooled power plant show that the total power adjustment capacity of the system reaches 156 MW. The net power output of the system can be adjusted from 148 to 304 MW. The storage efficiency of the system is 60.2%. Because part of the consumed valley electricity cannot be released during peak
Fig. 16(b). Results show that the electric efficiency of the system is increased more by reducing dT of heat pumps than reducing dT of the turbine condenser. Therefore, an additional heat transfer area should be first applied to the heat pumps. The results can be used in economic analysis to optimize heat transfer areas of heat exchangers. The thermal efficiency of the system remains unchanged because the waste heat remains fully recovered under the conditions described above. A lower temperature difference can therefore increase the electric efficiency of the system. 12
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adiabatic efficiency of the compressor and reduction in the temperature difference of the condenser and evaporator of the electric heat pumps increase the COP of the heat pumps and thus improve the system performance. In summary, China currently has inadequate heating sources and insufficient power adjustment capacity. In addressing these problems, the use of CoHP technology has the potential to be a widely applicable and effective way of increasing both the energy efficiency and flexibility of cogeneration plants. Acknowledgments This research is supported by National Natural Science Foundation of China (Grant No. 51706115). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114429. References [1] Department of Energy of China, Wind power grid operation in 2016 [EB/OL]. (2017-1-26) [2018-9-2]. http://www.nea.gov.cn/2017-01/26/c_136014615.htm. [2] Yi Wang, Yongfeng Xue, Nan Deng, Study on heat-load-based peak regulation for cogeneration units, Electr. Power 46 (3) (2013) 59–62. [3] Li Yan, Fu Lin, Zhang Shigang, et al., A new type of district heating method with cogeneration based on absorption heat exchange (co-ah cycle), Energy Convers. Manage. 52 (2) (2011) 1200–1207. [4] Wentao Li, Jingquan Zhao, Lin Fu, et al., Energy efficiency analysis of condensed waste heat recovery ways in cogeneration plant, Energy Convers. Manage. 101 (2015) 616–625. [5] Stjepko Katulić, Mislav Čehil, Željko Bogdan, A novel method for finding the optimal heat storage tank capacity for a cogeneration power plant, Appl. Therm. Eng. 65 (1–2) (2014) 530–538. [6] Mogens Kjær Petersen, Jørgen Aagaard, Heat accumulators, News from DBDH, vol. 1, 2004. http://dbdh.dk/images/uploads/presentations2011/heat %20accumulators%20lille.pdf. [7] Bing Qin, Lin Fu, Yi Jiang, Electric peak shaving for CHP plant by using thermal inertia of heat supply system, Gas Heat 25 (10) (2005) 6–8. [8] Jianlin Li, Zhijia Xie, Dexin Li, Chunguang Tian, Research on key technologies of electric boiler with thermal energy storage in facilitating wind power accommodation capability, Electr. Energy Manage. Technol. 01 (2018) 1–7. [9] K.M. Nielsen, T.S. Pedersen, P. Andersen, Heat pumps in private residences used for grid balancing by demand desponse methods, Transmission and Distribution Conference and Exposition (T&D), IEEE PES, 2012, pp. 1–6. [10] Georgios Papaefthymiou, Bernhard Hasche, Christian Nabe, Potential of heat pumps for demand side management and wind power integration in the German electricity market, IEEE Trans. Sustain. Energy 3 (4) (2012). [11] T.S. Pedersen, P. Andersen, K.M. Nielsen, H.L. Stærmose, P.D. Pedersen, Using heat pump energy storages in the power grid, 2011 IEEE International Conference on Control Applications (CCA). Part of 2011 IEEE Multi-Conference on Systems and Control. September 28–30, (2011). [12] Hongyu Long, Ruilin Xu, Jianjun He, et al., Incorporating the variability of wind power with electric heat pumps, Energies 4 (10) (2011) 1748–1762. [13] Hongyu Long, Jianwei Ma, Kai Wu, et al., Energy conservation dispatch of power grid with mass cogeneration and wind turbines, Electr. Power Automat. Equip. 31 (11) (2011) 18–22. [14] Xiaoyun Xie, Yi Jiang, An ideal model of absorption heat pump with ideal solution circulation, J. Refrig. 36 (1) (2015) 1–12. [15] Xiaoyun Xie, Yi Jiang, The ideal process model for absorption heat pumps with real solution, J. Refrig. 36 (1) (2015) 13–23.
Fig. 17. System performance for different adiabatic efficiencies of the compressor.
hours, the electric efficiency of the system is 26.9%, which is lower than that of the benchmark system (29.2%). In addition, waste heat from exhausted steam continues to be fully recovered throughout the operation cycle, such that the total thermal efficiency of the system remains 87.3%. Simulation results of system performance at different heating network temperatures show that reducing the supply water temperature or reducing the backwater temperature increases the electric efficiency of the system. Despite heat transportation energy consumption, a lower network temperature improves the performance of the CoHP system. By lowering the heating network temperature from 120 °C/40 °C to 90 °C/ 20 °C, the electric efficiency of the system rises from 23.9% to 30.6%. Additionally, at a certain network temperature, the waste heat recovery system cannot fully recover the waste heat from the exhausted steam. Heat supplied by electric heat pumps during valley hours replaces the heat supplied by the ABHP and the steam heater in peak hours. Therefore, to increase the same amount of electricity in peak hours, the system with a higher COP of electric heat pumps consumes less valley electricity and has a higher storage efficiency. An increase in the heat transfer area reduces the temperature difference of heat exchangers and thus increases the COP of heat pumps. Additionally, an increase in the
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