Summer performance analysis of coal-based CCHP with new configurations comparing with separate system

Energy 143 (2018) 104e113

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Summer performance analysis of coal-based CCHP with new configurations comparing with separate system Maolin Wei a, Weixing Yuan b, *, Lin Fu c, Shigang Zhang a, Xiling Zhao c a

Dept.of Energy Planning & Design, Tsinghua Planning & Design Institute, Beijing, China Laboratory of Ergonomics and Environmental Control, School of Aeronautic Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, China c School of Architecture, Tsinghua University, Beijing, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2017 Received in revised form 28 September 2017 Accepted 20 October 2017 Available online 27 October 2017

Conventional coal-based CCHP system currently is believed to be less efficient than separate vapor compression cooling system in summer. In this paper, new configurations of CCHP system (N-CCHP) are proposed and studied which apply comprehensive methods to improve the system performance. In the N-CCHP, an absorption heat pump is applied in power plant to heat the primary water instead of a traditional heat exchanger. And, a small turbine is used to improve system performance with high bleeding steam pressure. In substation, the primary water is used to drive an absorption chiller and liquid desiccant equipment in series. The domestic water is also produced by the primary water after regeneration of liquid desiccant. Key influencing factors of the N-CCHP system have been fully discussed. In this way, a highly efficient N-CCHP system configuration is obtained and is analyzed to compare with the electricity driven vapor compression (VC) refrigeration system in summer. Results show that the N-CCHP is energy saving and own better performances when the primary water transmission distance is less than 60 km, or the COP of the vapor compression chiller is lower than 7. © 2017 Elsevier Ltd. All rights reserved.

Keywords: CCHP Steam turbine Liquid desiccant Absorption chiller Absorption heat pump

1. Introduction Coal-based cogeneration power plants occupy large amount of the power supply in China. Because of the simultaneous production of electricity and heat, the cogeneration is energy efficient and environmental friendly in winter [1,2]. In 2016 winter, an unique cogeneration system with ultra-long distance heating network of more than 40 km is completed and put into operation in Taiyuan, China, which is believed to achieve significant energy saving and much less air pollution for the city in winter. Yet in summer, the conclusion is uncertain e for the lack of heat demand, the cogeneration is always off design condition, which results in lower primary energy efficiency [3]. Besides, the steam turbine condensing pressure is high due to high environment temperature in summer days, which also leads to a reduction of turbine generation efficiency. However, because of additional consumption of airconditioning, the electricity demand is much larger in summer than in winter [4]. Here, the contradiction between low power

* Corresponding author. Tel.: þ86 10 82338878. E-mail address: [email protected] (W. Yuan). https://doi.org/10.1016/j.energy.2017.10.095 0360-5442/© 2017 Elsevier Ltd. All rights reserved.

efficiency and high electricity load makes the cogeneration poor economic. To mitigate the contradiction, CCHP (combined cooling, heating and power) system is considered, which applies a heat driven chiller for cooling, thus the heat load of the plant is expanded and the electricity demand for air-conditioning is reduced in summer. It also can help to balance the peak-valley power load caused by fluctuation of power demand in day and night [5]. Many researches on various types of CCHP systems have been brought out, the results indicate that the CCHP system is energy saving and economic when it is established onsite and waste heat such as flue gas or jacket water is used as driving heat resource for the chiller [6]. But when large-scale steam turbine is considered as the prime mover, the CCHP system is always less energy efficient than separate compression cooling system [7e10]. A typical conventional coal-based CCHP system consists of a steam turbine, a steam-water heat exchanger, and an absorption chiller, as shown in Fig. 1. Zhang [11,12] et al. analyzed the economic feasibility of a typical CCHP system. The results indicated that the system performance was limited by the lower primary energy efficiency, which was caused by irreversible losses in steam-water heat transfer process, poor COP of absorption chiller and excess transmission cost of hot

M. Wei et al. / Energy 143 (2018) 104e113

ah ac be c cb dw de e ec et h hc is os pw pws s sp tr

Nomenclature COP E EER EGE H L m

h n P Q T v ε

Coefficient of performance Electricity (MW) Energy consumption rate Equivalent generating efficiency Enthalpy (kJ/kg) Distance (m) Mass flow rate (kg/s) Efficiency Proportion Pressure (MPa) Heat quantity (MW) Temperature ( C) Value Error rate

Superscripts and subscripts af After

water. Li [13] et al. replaced the single-effect absorption chiller with a double-effect absorption chiller, which owned a much higher COP of 1.2, yet according to calculation, the conclusion was the same e the system is still not efficient in cooling mode. Zhang [14] et al. brought out a new coal-based system in which the extraction steam was firstly used to generate electricity through a small turbine, and then drive the absorption chiller. The new system achieved higher efficiency than the typical one because of significant reduction of irreversible losses in steam-water heat transfer process. However, the transmission cost was not considered, and for long-distance transmission, steam-driven absorption chiller was not appropriate. Different methods have been taken to improve the system efficiency in aforementioned researches, but the modified CCHP systems are still not comparable with separate compression cooling system. The existing system improvement measures such as using hybrid chiller, reducing extraction steam exergy losses or applying desiccant dehumidifier all focus on local optimization [15e18]. No optimization for the whole system has been conducted or proposed. To achieve better system overall performance, improvement methods should be applied both in the power plant and in the substation. In the power plant, the problem is how to make full use of the steam exergy via a proper heat transfer procedure. Typical system uses a valve to keep the steam pressure constant and applies a steam-water heat exchanger to produce hot water, both lead to huge irreversible losses. The valve can be replaced by a small turbine, thus the exergy losses caused by throttling process is reduced. The steam-heat exchanger also can be partly replaced by an

105

Absorption heat pump Absorption chiller Before Cooling Combined cooling heating and power system Domestic water Dehumidifier Electricity Electric chiller Isentropic heating Heat used for cooling Inlet steam Outlet steam Primary water Primary supply water Steam Separate cooling system Transmission

absorption heat pump. Driven by low-pressure steam, the absorption heat pump can supply high temperature water and recycle waste condensing heat simultaneously. This configuration has already been widely used in CHP power plants [19e21], yet not found in literature about CCHP systems. In the substation, the problem is how to improve the coefficient of refrigeration performance. Typically, single-effect absorption chillers are used in CCHP systems. Their performances are not comparable with vapor compression chillers. Using multi-effect absorption chiller or hybrid chiller can help to improve system refrigeration performances. For example, in some BCHP systems, dehumidifier integrated cooling system has been researched, due to application of independent humidity control strategy, the system performance is improved [22e24]. In this paper, a coal-based novel CCHP (N-CCHP) system that owns high system efficiency is proposed. The N-CCHP applies improvement measures both in the power plant and in the substation. In the power plant, a small turbine is used to keep a constant extraction steam pressure; an absorption heat pump is applied to generate hot water; the existing primary network is used to transport hot water. In substation, a liquid desiccant integrated hybrid cooling system is involved to achieve separate treatment of heat and humidity. Besides, domestic hot water is also produced. Performances of the N-CCHP in different configurations are discussed, key influencing factors such as extraction steam pressure, primary supply hot water temperature and transmission distance are all studied. Through simulation of applying the N-CCHP in an urban area during a whole summer, the N-CCHP is proven to be

Fig. 1. Flow chart of conventional coal-based CCHP system.

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more energy efficient than separate compression cooling system. 2. N-CCHP system description Fig. 2 shows the proposed N-CCHP system. The bleeding steam firstly flows into a small turbine, which is operated with a constant back pressure. After pressure reduction, the steam is then used to drive an absorption heat pump. The heat pump recycled part of the condensing heat of the power plant to heat the primary water. Through condensing heat recovery, the turbine exhaust steam pressure is reduced, which benefits its power generation efficiency. The primary water can be further heated through a steam-water heat exchanger if necessary. After heating the primary water, the condensate water returns to the deaerator, and then back to the boiler. The primary supply water is transported to substation through existing primary network. In the substation, the hot water is utilized in steps. Through controlling valves, different configurations of the cooling system can be converted. When V2, V3 are closed, the hot water is firstly used to drive the absorption chiller, and then supply domestic hot water. If both V2 and V4 are closed, the hot water is applied to drive absorption chiller, regenerate dilute solution and supply domestic hot water in series. If V3 remains open, independent humidity control strategy can be applied. Thus the chilled water temperature can increase to 17 C/22  C, much higher than the conventional 7 C/12  C, which is helpful to improve the refrigeration system performance. The absorption chiller in this system can be single-effect type or double-effect type. When the temperature is high, double-effect type absorption chiller which owns high COP can be applied, yet

the steam-water heat exchanger is used in this case, which is adverse to the system primary energy efficiency. The chosen of chiller type has various effects on system performance that need to be considered comprehensively. Liquid desiccant system is applied due to its advantage of utilizing low temperature heat. And through independent humidity control, the COP of the hybrid chiller can be improved [25]. An internally-cooled dehumidifier is used. The cooling water is supplied through a cooling tower, which also serves as heat sink of the absorption chiller. Fresh air firstly flows into the dehumidifier, and then enters a surface cooler, where it is cooled by the chilled water. The regeneration air takes away the water vapor generated during solution concentration process in the regenerator. A domestic water heat exchanger is used to provide domestic water and to further reduce the network return water temperature. Lower return water temperature is helpful to improve the COP of the absorption heat pump and reduce the primary water transmission cost.

3. Simulation method Generally, the cooling load in summer is much larger than heating load in winter for the same urban area in China. Since the proposed N-CCHP system uses the same primary network to transport heat, it can hardly fully meet the entire cooling demand for the same urban. Excess electricity is needed. Fig. 3 shows the difference of energy utilization methods between proposed NCCHP system and separate compression cooling system. In the N-CCHP system, a portion of the primary energy is converted to heat, and used for cooling. Thus the generated power of the turbine is reduced. To estimate whether it is worthy to utilizing CCHP system, a steam equivalent generating efficiency is defined as:

EGEs ¼

Ebe  Eaf Qs

(3-1)

Ebe and Eaf means the power generated by the turbine before and after steam extraction. Qs represents the heat contained in the bleeding steam. 0 In N-CCHP, part of the cooling load ðQc Þ is handled by heat 0 driven refrigeration system, the rest ðQc  Qc Þ is solved through compressor chiller. The required heat and electricity are: 0

Qhc;cb ¼

Qc

(3-2)

hhc;cb 0

Eec;cb ¼

Qc  Qc COPec;cb $he;tr

(3-3)

The domestic hot water is supplied through a heat exchanger, the required heat is:

Qh;dw ¼

Qdw

hdw;cb $hh;tr

(3-4)

So, the equivalent electricity consumption of the CCHP system can be calculated through Eqs. (3)e(5).


 Ecb ¼ EGEs $ Qhc;cb þ Qh;dw þ Eec;cb þ Etr Fig. 2. Flow chart of the proposed N-CCHP system.

(3-5)

In the separate systems, electricity is used to supply cooling and domestic hot water, the electricity consumption is:

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107

Fig. 3. Energy utilization methods of separate and combined systems.

Esp ¼

Qc

COPec;sp $he;tr

þ

Qdw hdw;sp $he;tr

(4) Neglecting influence of shaft steal leakage.

(3-6)

The combined system electricity consumption reduction proportion is:

Dne ¼

  Esp  Ecb ¼ F EGES ; Etr ; hhc;cb / Esp

(3-7)

If Dne is positive, it means the separate cooling system consumes more electricity, thus using the N-CCHP system is energy efficient. If Dne is negative, the conclusion is opposite. Obviously, Dne is affected by various factors, especially the equivalent electricity generating efficiency and the refrigeration system performance coefficient. 3.1. Calculation of equivalent electricity generating efficiency To obtain the equivalent generating efficiency (EGE), the turbine generate power changes along with the bleeding steam amount should be solved. Thus, a calculation model of the steam turbine is established [26]. Mass balance and heat balance equations for the cylinders, heaters and condensation section are all involved in the model. The extraction steam pressure and mass flow rate of each € stage are solved through Flugel formula as shown in Eqs. (3)e(8) [26]. The steam theoretical enthalpy is obtained by isentropic process, thus according to Eqs. (3)e(9), the actual values can be calculated.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 u P is;af  P 2os;af ms;af ¼t 2 ms;be P is;be  P 2os;be
 Hn ¼ Hn1  Hn1  Het;n $hn

A 300 MW air-cooling steam turbine is used to verify the calculation accuracy of the model. THA (acceptance of turbine thermal consumption rate) state is used as the reference state, the simulation results in different turbine modes are listed in Table 1. The error rate of the simulation value is calculated as:

(3-8)

(3-9)

In above equations, ms;af and ms;be means the steam mass flow rate in each stages of the turbine after and before steam extraction. Pis and Pos represent the pressure of the stage inlet and outlet steam. H means the stage outlet steam enthalpy. n here means the stage number. h means the stage internal efficiency. Het;n refers to the outlet steam enthalpy if isentropic process occurs in stage n. In order to simplify the calculation, following assumptions are made for the model: (1) Internal efficiencies of the stages of the steam turbine (include high-pressure cylinder, intermediate-pressure cylinder and low-pressure cylinder) keep unchanged (except the governing stage and the last stage). (2) Specific volume of water is not changed with temperature. (3) Transfer coefficients of the regenerative heaters remain unchanged.

ε¼

jv1  v0 j v0

(3-10)

Where v0 represents accurate value, and v1 refers to the simulation value. Table 1 shows that the simulation results are accurate enough with an error rate of less than 2%. Although the error rate of turbine exhaust steam pressure is relatively high, the absolute error is smaller than 1 KPa, its influence is small. Therefore, the model is verified and can be used for system performance analysis. 3.2. Evaluation of refrigeration performance coefficient The efficiency of the heat driven refrigeration system is defined as: 0

hhc;cb ¼

Qc

(3-11)

Qhc;cb

It is the ratio of the cooling load handled by the heat driven cooling system to the heat used for cooling in combined system. Since the bleeding steam is used to heat the primary water that serves as driven heat resource of the cooling system, the efficiency can be divided into two parts e the heating efficiency in the power plant and the cooling efficiency in the substation. As shown in Eqs. (3)e(12). 0

hhc;cb ¼

Qc Qhc;cb

0

¼

Qpw;c Qc $ ¼ COPc $COPh Qpw;c Qhc;cb $hh;tr

(3-12)

hh;tr is the heat transportation efficiency, due to heat loss, it is always less than 1. In the calculation, it is assumed to be 0.95 [27]. COPc and COPh refers to the cooling coefficient in substation and the heating coefficient in power plant. Both of the two efficiencies are connected with the performances of the applied components e absorption heat pump, absorption chiller, and desiccant equipment. With stable driving heat sources, the performance changes of these components are relatively small. So, in the system simulation, they are all simplified as components that keep a constant COP. For all key components that may be used in the N-CCHP system, Table 2 lists their coefficients and corresponding operating temperature ranges [28,29]. In the power plant, if the supply water temperature is below

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Table 1 Verification of the steam turbine model. Mode Simulation input parameters

Simulation output parameters

Main steam flow rate (t/h) Main steam pressure (MPa) Main steam temperature ( C) Reheat pressure (MPa) Reheat temperature ( C) Generated Power (KW)

Simulation value Accurate value Error rate % Simulation value Accurate value Error rate % Simulation value Accurate value Error rate % Simulation value Accurate value Error rate %

Thermal Consumption (kJ/kwh)

Exhaust Steam Flow Rate (t/h) Exhaust Steam Pressure (KPa)

Rated Load

Maximum Heating

50% THA

1024.884 16.7 538 3.535 538 301105 300169 0.3 8698.9 8717.8 ¡0.2 679.580 674.655 0.7 31.7 32 ¡0.9

1024.884 16.7 538 3.476 538 229871 231238 ¡0.6 4791.3 4796.1 ¡0.1 116.214 116.601 ¡0.3 9.9 10 ¡1

462.219 11.59 538 1.697 538 150197 150039 0.11 8725.7 8829.5 1.1 332.186 336.466 1.2 15.2 15 1.3

Table 2 COP of different components that may be used in N-CCHP. Components

Heat resource

Output

COP

Steam-water heat exchanger Absorption heat pump Double-effect absorption chiller

Steam Steam 0.2e0.4 MPa Steam 0.3e0.8Mpa or hot water 110e170  C

Single-effect absorption chiller

Hot water 70e90  C

Liquid desiccant Domestic hot water heat exchanger Electric compression chiller

Hot water 55e90  C Hot water 50e60  C Electricity

Primary water 90e130  C Primary water 40e90  C Chilled water 7e12  C Chilled water 17e22  C Chilled water 7e12  C Chilled water 17e22  C Dehumidified air Domestic water 45e55  C Chilled water 7e12  C Chilled water 17e22  C

1 1.6e1.7 1.0e1.2 1.1e1.3 0.6e0.7 0.75 0.6e1.2 1 5 6

90  C, only absorption heat pump is used for heating. If the temperature reaches above 90  C, steam-water heat exchanger should be applied. For the cooling system, different types of absorption chiller can be used corresponding to different primary supply water temperatures e a higher water temperature more than 110  C enables application of a double-effect absorption chiller, which owns higher coefficient. Table 2 also clearly shows that when independent humidity control strategy is applied, the chiller efficiency is significantly improved due to higher chilled water temperature. According to Eqs. (3)e(12), the cooling efficiency is defined as:

0

Qc Qpw;c

COPc ¼

(3-13)

The cooling load that handled by heat-driven cooling system 0 ðQc Þ can be divided into two parts e the sensible load handled by 0 absorption chiller ðQc1 Þ and the latent load handled by liquid 0 dehumidifier ðQc2 Þ. Here, the cooling load and the cooling efficiency can be expressed as:

0

0

0

Qc ¼ Qc1 þ Qc2 ¼ Qpw;c $n$COPac þ Qpw;c $ð1  nÞ$COPde 0

COPc ¼

Qc ¼ n$COPac þ ð1  nÞ$COPde Qpw;c (3-14)

n represents the proportion of the heat utilized by absorption chiller in the whole heat used for cooling. The heating efficiency in Eqs. (3)e(12) is defined as:

COPh ¼

Qpw;c Qhc;cb $hh;tr

(3-15)

The primary water is heated in steps in the plant. It is firstly heated to 90  C through the steam-driven absorption heat pump, and then further heated to required temperature by the steamwater heat exchanger. Thus with the coefficients listed in Table 2, the heat of primary water for cooling and the heating efficiency can be calculated by Eqs. (3)e(16).


 Qpw;c ¼ hh;tr $ Qhc;cb $nh $COPah þ Qhc;cb $ð1  nh Þ COPh ¼

Qpw;c ¼ nh $COPah þ 1  nh Qhc;cb $hh;tr

(3-16)

Here, nh is the proportion of the heat used to drive the heat pump in the whole heat used for cooling, which is connected with the supply primary water temperature. Through Eqs. (3)e(14) and Eqs. (3)e(16), the heat-driven cooling system efficiency hhc;cb can be calculated. The absorption technology has the advantage of keeping a stable coefficient in a great range of operating conditions. Thus detailed models of the absorption heat pump, absorption chiller and other components are not established in details, and average values of the coefficients listed in Table 2 are used in the calculation. 3.3. Evaluation of other influencing factors Besides of the equivalent electricity generation efficiency and the refrigeration performance coefficient, there are other factors that also affect the system energy consumption. One of them is the

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transmission energy consumption in the N-CCHP system, which is believed to be one of the most important factors that lead to low system energy efficiency [30]. The energy consumption rate while transporting the primary water can be calculated by following equation [31].

P Etr 0:0056$ð14 þ a$ LÞ ¼ Qpw DTpw X 8 0:0115; L < 500m > > < X a ¼ 0:0092; 500m  L < 1000m > > X : 0:0069; L  1000m EERtr ¼

(3-17)

Here, L is the transmission distance. Qpw is the heat transported through the primary network, both heat for heat-driven cooling and domestic water should be involved with consideration of heat loss during transmission. Although the domestic water is heated by primary water through a heat exchanger, the heat originally comes from the extraction steam. Since the temperature of primary water that used here is low, this part of heat is regarded as supplied through absorption heat pump. Thus, the required steam heat is:

Qh;dw ¼

Qdw

h;dw;cb $hh;tr

¼

Qdw COPah $hh;tr

(3-18)

There are also transmission losses for electricity that used for compression chiller. The line loss rate is about 8%, which means the electricity transmission efficiency is 92% [27]. If the user load is known, the N-CCHP system electricity consumption reduction proportion defined in Eqs. (3)e(7) can be obtained through above calculation methods.

3.4. Load simulation through DesT DesT is a mature software that can accurately calculate the hourly energy demands of the buildings [32,33]. In this section, a simple model of an urban area that comprised by commercial buildings and civil buildings is established, and its energy demands both in winter and summer are estimated. The urban is assumed to be heated by a cogeneration power plant that owns a 300 MW aircooling steam-turbine in winter. The total area of the urban is about 8 million square meters. Since the buildings in the area are assumed to be independent, several buildings that hold about 10% of the total area are chosen as representative to be modeled in DesT. The Table 3 General information of the model established in DesT. Location Building area Type Function People Electric equipment Lights Setting temperature Type Function People Electric equipment Lights Setting temperature

Jinan, China 800,000m2 Commercial buildings Office room, guest room, toilet, dining hall, shopping market 0.5 people/m2 (office room); 2 for everyday (guest room); 0.2people/m2 (shopping market) 20 W/m2 (office room); 200 W (guest room); 10 W/m2 (shopping market) 10 W/m2 22e26  C, 60% RH in a year Civil buildings Living room, bedroom, toilet, dining room 4 people for everyday 6 W/m2 4 W/m2 16e18  C in winter; 25e26  C, 60% RH in summer

Fig. 4. Simulation results of the load during a whole year.

whole area energy demands are obtained through expanding the calculation results in proportion. General information of the buildings is presented in Table 3. The results calculated through DesT are demonstrated in Fig. 4. Both of the energy demand in summer and winter are obtained. In summer, the latent load is also simulated, which occupies about 50% of the total cooling load. The peak value of the cooling load is about 550 MW, and the heating load is about 380 MW. The energy needed for domestic hot water is calculated. In the simulation, the domestic water load is assumed to be connected with building area, which means when the area keeps unchanged, the load will be constant. It is a calculation method that recommended in DesT [32]. The simulation results are then used in the following parts to estimate the N-CCHP system performance via load changes during a whole summer season. The energy saving of the N-CCHP is also analyzed. 4. Analysis and discussion In this section, the proposed N-CCHP system is supposed to be applied in the aforementioned urban area. In winter, the primary water temperature is assumed to be 120/60  C, thus the primary water mass flow rate can be calculated. It is also the mass flow rate in summer. With the simulation methods introduced before, the system performance is calculated and analyzed and optimal system configuration is established. The comparison of the combined system with the separated system is also conducted. 4.1. Effects of bleeding steam pressure The bleeding steam pressure affects its equivalent electricity generation efficiency significantly. Generally, it is kept constant through valve control. In the proposed N-CCHP system, a small turbine is used instead to recover work and provide constant pressure steam. The internal efficiency of the small turbine is assumed as same as the low-pressure cylinder of the turbine. The absorption heat pump can utilize steam pressure ranges from 0.2 MPa to 0.4 MPa. The equivalent generating efficiencies of the extraction steam changes along with the extraction steam pressure and the heat quantity are shown in Fig. 5. Supplying water at 90  C and 120  C are chosen as two cases here. 90  C means only absorption heat pump is used in the plant to heat the primary water, and in case of 120  C, not only the absorption heat pump, but also a steam-water heat exchanger is

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applied. For both of the two cases, the efficiency decreases with the reduction of steam pressure. Low-pressure steam owns relatively poor power capacity, thus its effects on the turbine power generation is small. Lower the bleeding steam pressure is benefit to the system performance. Fig. 5 also shows that with the output heat increases, the efficiency firstly increases and then decreases. This is because when the extraction steam mass flow rate increases, the steam volume flow rate in the turbine last stage is affected. Thus the internal efficiency of the stage is changed. Fig. 6 shows the characteristics of the last stage internal efficiency and the steam EGE in different heat quantities. It shows that as the extraction steam heat quantity increases, the internal efficiency of the turbine last stage decreases rapidly at the beginning, means the impact of the extraction steam on the turbine power generation is huge. Thus the EGE increases at first. Then the drop rate of the last stage internal efficiency slows down. However, the heat released by per unit extraction steam remains almost unchanged, thus the equivalent generation efficiency turns to decrease and almost be constant when the output heat quantity is more than 500 MW. Fig. 5 also shows that supply water temperature almost has no effects on EGEs when the heat is constant. Different supply water temperature just means different heating methods that applied in power plant. Although supply water at 90  C makes the absorption heat pump recycle more condensate heat than at 120  C, which is beneficial to the steam turbine, but the effect is really small, and when the extraction steam heat is huge, this effect is negligible. The influences of supply water temperature should be estimated under the view of the overall system. In the following parts, the steam pressure is kept constant at 0.2 MPa.

4.2. Optimal configurations of the N-CCHP system Various system configurations have been described before, and in different configurations, the refrigeration system is different. Application of the system configuration is greatly influenced by the supply water temperature. Generally, the system can be operated in four cases, as listed in Table 4. The energy consumptions of the N-CCHP system in different cases are calculated. Fig. 7 is the system equivalent power consumption with the increment of supply water temperature. In the calculation, the total cooling load is assumed to be 400 MW, a

Fig. 5. Efficiency changes with steam heat under different Pex and Tpws values.

Fig. 6. Efficiency changes with extraction steam heat (Pex ¼ 0.2 MPa, Tpws ¼ 90  C).

normal value chosen from the load simulation results, in which the latent load proportion is 0.5. The primary water mass flow rates are equal. The primary water transmission distance is assumed to be 10 km, with this, the transmission energy consumption is involved. Besides, in order to fully meet the cooling load, additional compression chiller is used. And for the compression chiller, it is assumed that independent humidity control strategy is also applied. When handling the surplus latent cooling load, the COP of the VC chiller is set to be 5, and it turns to be 6 while serving for the sensitive cooling load. In case 1 and case 2, a same system composition is applied, yet the strategy of heat utilization is different. In case 1, the heat is mainly used for removing the moisture load, after it is fully handled, the surplus heat is used to drive the absorption chiller. In case 2, the strategy is opposite, the heat is mainly used to drive the absorption chiller. Since water below 70  C cannot serve as heat resource of absorption chiller, the system energy costs in case 1 and case 2 are equal when the supply water temperature is low. When it exceeds 70  C, system in case 1 achieves less cost than in case 2. It means utilizing heat to handle latent load is more efficient. System in case 3 only consists of absorption chiller, and its equivalent power consumption is the largest among all cases, which means only apply absorption chiller is not suitable. In case 4, major heat is used for moisture removal. Thus the power consumption keeps consistent with that of case 1 at first. However, as the supply water temperature increases, the heat transported by primary network also increases. When the heat amount exceeds the require quantity, the return water temperature will increase. In this condition, more steam is used through steam-water heat exchanger, which leads to system efficiency reduction. This is why the equivalent power consumption of the system in case 4 increases at high supply water temperature. The system energy consumption in Fig. 7 changes in a complicate way with the increment of supply water temperature for these four cases. This is mainly caused by the changes of the bleeding steam equivalent generation efficiency. Take case 1 as example. Fig. 8 shows the bleeding steam mass flow rate and the corresponding steam equivalent generation efficiency changes along with the supply water temperature improves. Firstly, it should be made clear that in case 1, the CCHP system consumption decreases with the increment of supply water temperature, indicates that its refrigeration performance is better than compression cooling. In Fig. 8, although the bleeding steam mass flow rate increases linearly with the supply water temperature, the steam equivalent generation efficiency increases first and then

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Table 4 N-CCHP refrigeration system in different cases. Case number

Description

Case 1

Composition: Liquid desiccant equipment, single-effect absorption chiller, double-effect absorption chiller, domestic water heat exchanger; Operation: Heat is mainly used for moisture removal; Composition: As same as case 1; Operation: Heat is mainly used for driving absorption chiller; Composition: Single-effect absorption chiller, double-effect absorption chiller, domestic water heat exchanger; Operation: Heat is used to drive absorption chiller and supply domestic hot water; Composition: Liquid desiccant equipment, domestic water heat exchanger; Operation: All heat is used for moisture removal and supply domestic hot water;

Case 2 Case 3 Case 4

generation efficiency and the application of double-effect absorption chiller. From Fig. 7, it can be concluded that in the CCHP system, both of double-effect absorption chiller and liquid desiccant equipment should be involved to achieve higher system efficiency. The return water temperature should be low and the temperature difference between supply and return water should be as large as possible. The heat that supplied by primary water should be mainly used to handle moisture load, and the surplus heat is considered to drive absorption chiller. The CCHP system in case 1 is applied in the following discussions. The latent load proportion also influences the CCHP system performance. Fig. 9 shows the CCHP system equivalent power consumptions in different latent load proportions. It shows that with the increment of latent load proportion, the CCHP system equivalent power consumption keeps decreasing. The moisture load can be removed more efficient through heat dehumidification system. Therefore, the proposed N-CCHP system is more suitable to be applied in high humidity areas. Fig. 7. System equivalent power consumption in different cases.

4.3. Comparison with separated compression system turns to decrease. Before the efficiency reaches the peak value, the changes of system equivalent power consumption became more and more small. When the efficiency comes to decrease, the system power consumption turns to reduce rapidly. Fig. 7 indicates that enlarging the temperature difference between supply and return water is benefit to system efficiency. Since the primary water mass flow rate keeps constant, larger temperature difference means larger plant output heat. In this condition, although the steam-water heat exchanger is used, its adverse effect is completely offset by the reduction of steam equivalent

In this section, the electricity consumptions of separate compression cooling system and the proposed N-CCHP system with case 1 are compared. The two systems are assumed to serve for the same urban area described before. The cooling and heating load of the urban area are demonstrated in Fig. 4. The transmission distance is set to be 10 km. Fig. 10 is the electricity consumption of the two systems. During a whole summer, the N-CCHP system electricity cost is about 61GW$h, the

Fig. 8. Steam mass flow rate and equivalent efficiency in different supply water temperatures.

Fig. 9. Changes of System equivalent power consumption in different latent load proportions.

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Fig. 10. System consumption comparison during a whole season.

In Fig. 10, the transmission distance is 10 km. When the distance increases, the transmission consumption also increases, which will worsen the N-CCHP system performance. Fig. 11 shows the energy consumption reduction proportion of the N-CCHP system changes with the increment of transmission distance. It shows that the advantage of the N-CCHP system is weakened when the power plant is far away from the consumer area. If the distance increases to 40 km, the consumption reduction proportion drops to 11%. If the distance is over 60 km, no energy saving effect is observed. Another factor that owns a great impact on the comparison results is the COP of the compression chiller. In the above discussions, the COP is 6 when the chiller is used to handle sensitive load, while used for moisture removal, the COP is set as 5. If the chiller COP increases, the separate system performance will be improved, thus using the N-CCHP system may not efficient. Fig. 12 shows Dne changes when the COP of compression chiller increases. The COP here refers to the coefficient of the chiller when latent load is handled. When the sensitive load is processed, the COP of the chiller is simply assumed to be equal to the previous value added by 1. Fig. 12 indicates that when the transmission distance is below 20 km, the N-CCHP system is more efficient than the compression cooling system even the compression chiller coefficient is as high as 7. If the distance is enlarged to 40 km, the CCHP system achieves energy saving only when the COP of the used compression chiller is below 6. Generally, the COP of the residential air conditioner is less than 5. Thus in most cases, the CCHP system can be considered as more efficient than separate cooling system. 4.4. Economic analysis of the N-CCHP

Fig. 11. CCHP consumption changes in different transmission distances.

separate system consumption is about 83GW$h. The N-CCHP system costs 25.7% less electricity than the separated compression system.

When serving for the same urban area, the N-CCHP system needs more investment since the absorption chiller is more expensive than the compression chiller, and extra absorption heat pump is needed. Table 5 lists the economic comparison between the N-CCHP system and compression cooling system. The heat transmission distance is 10 km, and the COP of the compression chiller is 6. In Table 5, the equipment investment is the investment of absorption heat pump/chiller, desiccant system and compression chiller for the N-CCHP, yet for the compression cooling system, it only refers to the investment of compression chiller. The investment for each component is estimated according to its installed capacity [34,35]. For the compression chiller, it is about 0.4RMB/W, for the absorption heat pump/chiller, it is about 0.6RMB/W, and for the desiccant system, it is about 0.5RMB/W. the installation cost is assumed as equal to 25% of the equipment investment. Table 5 shows that the total investment of the N-CCHP is 52.5 million RMB higher than the compression cooling system. However, the power consumption is 22 GW$h smaller. The power price in Jinan is about 0.55RMB/ ðKW$hÞ. So the cost saving of the NCCHP during a whole summer season is about 12.1 million RMB. The system static payback period is 4.3 years. Table 5 shows that the N-CCHP owns good economic performance. 5. Conclusion

Fig. 12. Dne changes in different chiller COP.

In this paper, a coal-based N-CCHP system is proposed. The system applies a small turbine to reduce the utilized steam pressure, thus its adverse impact on the turbine generation capacity is weakened. Absorption heat is used to help to heat the primary water. Single-effect absorption chiller, double-effect absorption chiller and liquid desiccant equipment are also used alone or in combination in different system configurations. Through the analysis and discussion, following conclusions can be obtained.

M. Wei et al. / Energy 143 (2018) 104e113 Table 5 Economic analysis of the N-CCHP system.

Investment of equipment (Million RMB) Installation cost (Million RMB) Total investment (Million RMB) Power consumption (GW$h) Operating cost (Million RMB) Static payback period (Year)

N-CCHP system

Compression cooling system

242

200

60.5 302.5 61 33.55 4.3

50 250 83 45.65 e

1. For the CCHP system, lower the bleeding steam pressure can help to improve the overall system performance. Enlarge the difference of supply water temperature and return water temperature also benefits the system efficiency. 2. The liquid desiccant equipment should be involved in the CCHP system, thus independent humidity control strategy can be used. Thus the return water temperature is lowered, and the refrigeration efficiency is improved. The heat transferred by primary water should be mainly used for moisture removal. The CCHP system is more suitable to be used in high humidity areas. While applying liquid desiccant system, its drawbacks such as air steam pollution or possible sorbent carryover should be considered. Effective mist eliminator must be used. 3. Compared with the separate compression cooling system, the proposed N-CCHP system achieves 25% less energy consumption when the transmission distance is 10 km and the COP of the compression chiller is set as 5. And the system investment static payback period is about 4.3 years in this condition, which shows good economic performance. The N-CCHP system energy consumption increases with the increment of the transmission distance. If the distance is enlarged to 60 km, the CCHP system shows no energy saving advantage. The COP of the compression chiller also affects the comparison results. If it is as high as 7, the CCHP system is efficient only when the transmission distance is less than 20 km. Acknowledge This work was supported by the National Twelfth Five-year Plan of China. NO: 2011BAJ07B00. References [1] Xu J, Sui J, Li B, Yang M. Research, development and the prospect of combined cooling, heating, and power systems. Energy 2010;35(11):4361e7. [2] d'Accadia MD, Sasso M, Sibilio S, et al. Micro-combined heat and power in residential and light commercial applications. Appl Therm Eng 2003;23(10): 1247e59. [3] Li M, Jiang XZ, Zheng D, et al. Thermodynamic boundaries of energy saving in conventional CCHP (Combined Cooling, Heating and Power) systems. Energy 2016;94:243e9. [4] Wang J, Sui J, Jin H. An improved operation strategy of combined cooling heating and power system following electrical load. Energy 2015;85:654e66. [5] Schicktanz MD, Wapler J, Henning HM. Primary energy and economic analysis of combined heating, cooling and power systems. Energy 2011;36(1):575e85. [6] Popli S, Rodgers P, Eveloy V. Trigeneration scheme for energy efficiency enhancement in a natural gas processing plant through turbine exhaust gas waste heat utilization. Appl energy 2012;93:624e36. [7] Wu DW, Wang RZ. Combined cooling, heating and power: a review. Prog energy Combust Sci 2006;32(5):459e95. [8] Liu M, Shi Y, Fang F. Combined cooling, heating and power systems: a survey.

113

Renew Sustain Energy Rev 2014;35:1e22. [9] Cho H, Smith AD, Mago P. Combined cooling, heating and power: a review of performance improvement and optimization. Appl Energy 2014;136:168e85. [10] Al Moussawi H, Fardoun F, Louahlia-Gualous H. Review of tri-generation technologies: design evaluation, optimization, decision-making, and selection approach. Energy Convers Manag 2016;120:157e96. [11] Zhang QL, Fu L, Di HF. Analysis of energy efficiency and economy of city distribute cooling in summer. Acta Energiae Solaris Sin 2011;32(6):855e61 [in Chinese]. [12] Zhang Q, Fu L, Li L, et al. The energy efficiency and economic feasibility analysis of the distributed absorption cooling combined with district heating System. In: ASME 2009 3rd international conference on energy sustainability collocated with the heat transfer and InterPACK09 conferences. American Society of Mechanical Engineers; 2009. p. 49e55. [13] Li H, Fu L, Geng K, et al. Energy utilization evaluation of CCHP systems. Energy Build 2006;38(3):253e7. [14] Tiantian Z, Yufei T, Li B. Numerical simulation of a new district cooling system in cogeneration plants. Energy Procedia 2012;14:855e60. [15] Liu M, Shi Y, Fang F. A new operation strategy for CCHP systems with hybrid chillers. Appl energy 2012;95:164e73. [16] Rosen MA, Le MN, Dincer I. Efficiency analysis of a cogeneration and district energy system. Appl Therm Eng 2005;25(1):147e59. [17] Daou K, Wang RZ, Xia ZZ. Desiccant cooling air conditioning: a review. Renew Sustain Energy Rev 2006;10(2):55e77. [18] Yang M, Lee SY, Chung JT, et al. High efficiency H2O/LiBr double effect absorption cycles with multi-heat sources for tri-generation application. Appl Energy 2017;187:243e54. [19] Li Y, Fu L, Zhang S, et al. A new type of district heating method with cogeneration based on absorption heat exchange (co-ah cycle). Energy Convers Manag 2011;52(2):1200e7. [20] Li Y, Fu L, Zhang S. Technology application of district heating system with Cogeneration based on absorption heat exchange. Energy 2015;90:663e70. [21] Li Y, Chang S, Fu L, et al. A technology review on recovering waste heat from the condensers of large turbine units in China. Renew Sustain Energy Rev 2016;58:287e96. [22] Fu L, Zhao XL, Zhang SG, et al. Laboratory research on combined cooling, heating and power (CCHP) systems. Energy Convers Manag 2009;50(4): 977e82. [23] Henning HM, Pagano T, Mola S, et al. Micro tri-generation system for indoor air conditioning in the Mediterranean climate. Appl Therm Eng 2007;27(13): 2188e94. [24] Nayak SM, Hwang Y, Radermacher R. Performance characterization of gas engine generator integrated with a liquid desiccant dehumidification system. Appl Therm Eng 2009;29(2):479e90. [25] Liu X, Li Z, Jiang Y, et al. Annual performance of liquid desiccant based independent humidity control HVAC system. Appl Therm Eng 2006;26(11): 1198e207. [26] Li W, Zhao J, Fu L, et al. Energy efficiency analysis of condensed waste heat recovery ways in cogeneration plant. Energy Convers Manag 2015;101: 616e25. [27] Wang JJ, Jing YY, Zhang CF, et al. Performance comparison of combined cooling heating and power system in different operation modes. Appl Energy 2011;88(12):4621e31. [28] Li S, Wu JY. Theoretical research of a silica gelewater adsorption chiller in a micro combined cooling, heating and power (CCHP) system. Appl Energy 2009;86(6):958e67. [29] Deng J, Wang RZ, Han GY. A review of thermally activated cooling technologies for combined cooling, heating and power systems. Prog Energy Combust Sci 2011;37(2):172e203. [30] Badami M, Portoraro A. Performance analysis of an innovative small-scale trigeneration plant with liquid desiccant cooling system. Energy Build 2009;41(11):1195e204. [31] Lin F, Yi J, Weixing Y, et al. Influence of supply and return water temperatures on the energy consumption of a district cooling system. Appl Therm Eng 2001;21(4):511e21. [32] DeST development group in Tsinghua University, building environmental system simulation and analysis-DeST. Beijing, China: Architecture & Building Press; 2006 [in Chinese]. [33] Liu XH, Geng KC, Lin BR, et al. Combined cogeneration and liquid-desiccant system applied in a demonstration building. Energy Build 2004;36(9): 945e53. [34] Liu X, Li Z, Jiang Y, et al. Annual performance of liquid desiccant based independent humidity control HVAC system. Appl Therm Eng 2006;26(11): 1198e207. [35] Mone CD, Chau DS, Phelan PE. Economic feasibility of combined heat and power and absorption refrigeration with commercially available gas turbines. Energy Convers Manag 2001;42(13):1559e73.