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Total energy system analysis of heating
Energy 25 (2000) 807–822 www.elsevier.com/locate/energy
Total energy system analysis of heating Zhi-Ping Song
*
Graduate School, North China Electric Power University, Qinghe, Beijing, China 100085 Received 15 December 1999
Abstract This paper provides an analysis of heating from the point of view of the total energy system from the very input of primary energy until it reaches the end-users. In order to minimize the personal judgment in cost allocation between heat and power, the methodology of exergy-based Specific Consumption Analysis proposed by the author is used to evaluate the real consumption of the unity end product in respect to both fuel and monetary cost. Following this methodology the key factors affecting the emission level of a heating system are revealed. It is found that in some applications a total energy system of heating with a specific primary energy consumption as low as 5–15 kg c.e./GJ may be attained. The concept of the ‘reversible mode of heating’ is introduced and is seen as the basis of ultra-low emission heating systems. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Protection of the global climate against the much-feared greenhouse effect caused by increasing amounts of CO2 and other trace gases thought to be detrimental to the climate is currently a topic of great concern. In line with the rising demand for energy sources nationwide, fossil fuel emissions are also likely to increase. In order to meet this challenge, a more intelligent use of energy is urgently needed. A breakdown of energy use shows that almost two thirds of the primary energy is end-used in the form of thermal energy, a very large proportion (up to 5/6) of which is used as low-temperature heating below 100°C. Evidently, lowering emissions caused by such heating systems will greatly promote sustainable development strategy. In recent years a workshop at the Graduate School of North China Electric Power University has undertaken a program focusing on research into a more environmentally friendly system for space heating. The program is supported by the National Committee of Science and Technology of the People’s Republic of China. This paper is the outgrowth of the first report submitted by the workshop concentrating on the theory-based perspective of an ultra-low emission heating system. * Fax: +86-10-6238-1094. E-mail address: [email protected] (Z.-P. Song).
0360-5442/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 0 ) 0 0 0 2 1 - 9
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Nomenclature A=[aij] the incident matrix with elements aij=1 (if stream j flows into subsystem i); aij=⫺1 (if stream j flows out of subsystem i); aij=0 (if stream j has no direct connection with subsystem i) b fuel specific consumption of end product fuel specific consumption accrual of subsystem I bI b specific fuel consumption accruals due to irreversibility in terms of column vector c monetary specific consumption i.e. specific cost cI specific cost accrual of subsystem I cFI specific cost accrual due to irreversibility of subsystem I the local fuel market price cf c specific cost accruals due to irreversibility in terms of column vector cZI cost accrual due to fixed charge for the unity product attributed to subsystem I D steam mass flow rate E the column vector of m stream exergies E exergy EECR=W⌺⫺W Equivalent Electricity Consumption Rate, which means the sacrifice of a CHP plant in supplied power due to generating heat [8] M mass P the total exergy amount of the end product, the pressure of medium Q heat T temperature V volume W the net power output of the CHP plant involved in a total heating system the net power output of the CHP plant involved in the total heating system while the heat W⌺ output drops to zero e a coefficient of specific cost taking into account the pollution tax eQ the exergy index of the heat generation subsystem h the exergy efficiency p the ratio of the pump work consumed by the heat grid and the thermal exergy fed by the heat generation subsystem [1]
Subscripts 0 b f hp
under the ambient condition, the item caused by factors not associated with individual subsystem boiler or fuel energy delivery subsystem fuel heat pump
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I=1, 2, J=1, 2, K L min Q q QD R r R0 US W WD
809
%, n the order number of the subsystems in question %, n the order number of the subsystems in question steam turbine condenser used as a heat grid heater the low temperature heat source of the heat pump theoretically minimum value heat generation subsystem with respect to heat supply heat distribution subsystem at the locality of the end of heat distribution subsystem at the location of users’ room at the locality of the heat generation subsystem the end-users’ subsystem power generation subsystem power transmission and distribution subsystem
Superscripts in min out t
inlet minimum outlet transpose
Emissions from a heating system are effected by a number of factors such as energy efficiency, pretreatment of the fuel, the use of trace gases cleaning technology and etc. So far as the greenhouse gas CO2 is concerned, the energy efficiency is directly linked with the emission level and is considered to play a dominant role at the moment. It is therefore necessary to first investigate the possible innovative measures offering significant contribution to energy efficiency of heating systems.
2. Generalized heating system 2.1. Current heating systems Current heating systems serving the general public in urban areas are mainly classified into four types, namely, boiler heating systems, CHP (Combined Heat and Power) systems, heat pump systems and electric heating (see Table 1). In China, a total of some 400,000 small/medium sized boilers are scattered over a vast area. Most of them are of low efficiency, ranging from 50 to 60%. The overall CHP capacity is about 20,000 MW, accounting for 12% of the overall installed power capacity. Heat pumps and electric heating do not yet have widespread acceptance except for individual areas and/or cases. 2.2. Generalized heating system In order to compare the performance of different types of heating system, we use a generalized total heating system model [1,2] as shown in Fig. 1. It consists of six subsystems including fuel energy delivery, power generation, power distribution, heat generation, heat distribution and the end-users subsystem.
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Table 1 Current heating systems serving the general public in urban areas Type of Heating System
System Arrangement
Legend
boiler
F
Fuel Energy Delivery Subsystem
CHP
PG
Power Generation Subsystem
Heat Pump
PD
Power Distribution Subsystem
Electric Heating
HG
Heat Generation Subsystem
HD U
Heat Distribution Subsystem Users’ Subsystem
Fig. 1. Generalized heating system.
The function of each subsystem is quite different in nature. Those subsystems which are indispensable to the goal of the total energy system, i.e. heat supply, are referred to as first class subsystems. Second class subsystems are those, which function to reduce the fuel and/or monetary consumption for the unity end product although they themselves expend a certain amount of fuel and monetary cost. All types of heating systems can be represented by the generalized heating system with the probable exception of some second class subsystems. In order to facilitate the following analysis and exposition, we use the terms ‘hot end ’ and ‘cold end’ of the generalized heating system. The ‘hot end’ consists of the fuel energy delivery, the power generation
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and the power distribution subsystem, while the heat distribution subsystem and the end-users subsystem are components of the ‘cold end’ of the generalized heating system. 2.3. Reversible mode of heating system Although all real heating systems are irreversible without exception, the extent of their irreversibility may differ a great deal. In order to identify the extent of their irreversibility, heating systems are classified into two modes: conventional mode and reversible mode [3]. The reversible mode of heating system is also known as a thermodynamic heating system. The reversible mode of heating system is defined as a total energy system of heating fed by a nonthermal primary energy, in the energy conversion process of which at least one kind of non-thermal energies (e.g. electrical, mechanical or chemical energy) appears and at expense of this non-thermal energy the end product (i.e. the supplied heat) is generated by making use of waste and/or ambient heat sources. Heat pump heating system is of the typical reversible mode of heating system. Another reversible mode of the heating system is the combined heat and power (CHP), which makes use of the waste heat by virtue of the power sacrifice due to cogenerating heat and hence is viewed as thermodynamically equivalent with the heat pump heating. The term ‘reversible heating’ was first introduced by Professor H.D. Baehr in his work ‘Thermodynamik’ [14], which has been translated into several languages. Since this term has, to a certain extent, been accepted in the field of thermodynamic analysis, the present author uses this term with a further modification. In terms of energy efficiency, the two modes of heating systems cannot be mentioned in the same breath. The potential for improvement by making use of the reversible mode of heating system is tremendous and is viewed as the basis for developing ultra-low emission heating systems.
3. Specific consumption of a heating system In this paper, the fuel and monetary consumption for the unity end product are referred to as ‘specific consumption’. Unless otherwise stated they are always defined with respect to the ‘total energy system’ from the very input of primary energy until it reaches the end-users. The primary energy is understood to be a fossil fuel. In accordance with the advanced exergo-economic approach, specific consumption, both the fuel b and the monetary c, is composed of the theoretically minimum specific consumption (bmin or cmin) and the specific consumption accruals (bI or cI), where subscript I=1,2, …, n is the order number of the subsystem in question. 3.1. Theoretically minimum specific consumption The theoretically minimum specific consumption bmin/cmin arise in a hypothetical ideal energy system which is conceived as an entirely reversible energy system with an infinite lifetime and without any fixed costs. It follows that the theoretically minimum specific consumption for a heating system can be shown as bmin q⫽34.12(1⫺T0/Tr) kg c.e./GJ
(1a)
cmin q⫽cfbmin q $/GJ
(1b)
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where the abbreviation c.e. denotes the ‘coal equivalent’ [11] and 1 kg c.e.=7000 kcal=1/34.12 GJ. Supposing the ambient temperature T0=270 K (⫺3°C), the required room temperature Tr=293 K (20°C) and the local fuel market price cf=$0.03/kg, we have bmin q=2.68 kg c.e./GJ and cmin q=$0.08/GJ. 3.2. Specific consumption accruals Any irreversibility results in a fuel accrual and a relevant accrual of specific consumption b and c. Likewise any subsystem involved in a real energy system causes a cost accrual for the unity end product due to its fixed charges including depreciation rate and expenses related to owning and operating it (e.g. maintenance, overheads as well as interest, etc.). Let us consider an energy system consisting of n subsystems and m streams. The specific consumption accruals in terms of a column vector due to irreversibility are b⫽[b1 b2 % bn]t⫽(bmin/P)AE
(2a)
c⫽[cF1 cF2 % cFn]t⫽(cfbmin/P)AE
(2b)
where P the total exergy amount of the end product E the column vector of m stream exergies A=[aij] the incident matrix with elements aij=1 (if stream j flows into subsystem i); aij=⫺1 (if stream j flows out of subsystem i); aij=0 (if stream j has no direct connection with subsystem i)
The specific consumption of the end product equals its theoretically minimum specific consumption plus the sum of the specific consumption accruals, i.e.
冘 n
b⫽bmin⫹
bI
(3a)
0
冉 冘冊 n
c⫽ cmin⫹
冘 n
cFI (1⫹e)⫹
0
cZI
(3b)
0
where cZI denotes the cost accrual due to fixed charges for the unity end product attributed to subsystem I and subscript 0 represents the item caused by factors not associated with the individual subsystem. The symbol e represents a coefficient taking into account the pollution tax. 3.3. Specific consumption of a heating system The methodology described in the foregoing two sections is known as ‘Specific Consumption Analysis’ [3] proposed by the author in 1992. Applying this approach to a heating system and assuming that the exergy at output of the Ith subsystem equals that at input of the (I+1)th subsystem we have respectively the specific consumption accruals in subsystem I (bIq and cIq) and the specific consumption of the end product (bq and cq):
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冉
813
冊
n
bIq⫽ bmin q/ ⌸ hJ (1/hI⫺1)
冉
J⫽I⫹1
n
(4a)
冊
cIq⫽ cfbmin q/ ⌸ hJ (1/hI⫺1) J⫽I⫹1
(4b)
n
bq⫽b0q⫹bmin q/ ⌸ hI
(5a)
I⫽1
冉
n
冊
冘 n
cq⫽ cF0q⫹cmin q/ ⌸ hI (1⫹e)⫹ I⫽1
cZIq
(5b)
0
in where hJ=Eout J /EJ denotes the exergy efficiency/index of subsystem J, which consists of the exergy efficiency of the fuel energy delivery subsystem hb, the power generation subsystem hW, the power distribution subsystem hWD, the exergy index of the heat generation subsystem eQ, the exergy efficiency of the heat distribution subsystem hQD and the end-users subsystem hUS. In accordance with expressions (4) and (5), we have the typical values of the fuel specific consumption b of currently used heating systems as shown in Table 2. It can be seen from Table 2 that the fuel specific consumption of heating systems of the reversible mode is by far lower than that of the conventional mode.
4. A sample application 4.1. Site description An energy center is located in the urban area of a city. It was originally a power station equipped with a number of small power units supplying electricity only and has been shut down for a long time. This station is now planned to be rebuilt into an energy center mainly meeting about one third of the heat demand of its surrounding district within 6 km for space heating. According to the heat load duration curve the annual heat demand amounts to 400,000 GJ with the maximum load of 60 MW(heat)=216 GJ/h. The required room temperature is 20°C in the heating season. Nowadays China is confronted with the environmental issue. In view of environmental protection, in this city coal fired boilers are not allowed to run in the urban area. Recently natural gas piping has run through the city. There is also plenty of underground water with a year round temperature above 15°C. In the vicinity of the energy center the housing density is high enough for a heat grid to be installed and there are several spots of waste water source with a Table 2 Typical values of fuel specific consumption bq in kg c.e./GJ (heat) Subsystems
Boiler heating system
Electric heating system
Heat pump heating system
CHP heating system
Fuel specific consumption b
52
105
35
22
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mean temperature around 30°C, which provide the possibility of making use of them to serve the heat pump units as their low temperature source. The rising living standard in this city calls for year round air conditioning with about 2500 h space heating and 1800 h cooling. 4.2. Different options In accordance with the site description there are several solutions to the heat load requirements. 앫 The simplest solution is to install a conventional heating system with a gas-fired heat-only boiler supplying 90/70°C water to the main. The thermal efficiency is assumed to be 86%. It is true that with a clean fuel like natural gas, the content of solid particles as well as NOX and SO2 can be reduced in comparison with coal-fired boilers provided adequate measures were taken. The supplied energy can, however, be expensive due to the fact that the local market price per GJ natural gas is about four times that of coal. On the other hand, from an overall viewpoint of the public energy supply, the CO2 emission is improved far below that attainable. 앫 In terms of the emission reduction, the heating system may be improved by introducing a reversible mode of heating. Naturally, in accordance with the previous analysis, preference should be given to a CHP system. In China the simplest way of implementation is to use small power units rating from 6 to 50 MW scattered over a large number of urban areas which are mostly out of service due to their high fuel specific consumption for power generation but are still capable of operating for years. On this site a 6 MW unit is available. It is considered to retrofit it into a cogeneration one with an enhanced condenser pressure corresponding to an outflow/return water temperature of 100/70°C. 앫 Moreover, enhancement of the ‘hot end’ reversibility may bring further improvement. To this end the customary practice is to superimpose a gas turbine unit upon a steam cycle to achieve an efficiency of the combined cycle of about 45%. This involves replacing the steam boiler with a gas turbine and a heat recovery steam generator (HRSG), matching the steam supply requirement of the 6 MW steam turbine unit. The steam turbine retains, in principle, its operation conditions as before. The new investment in gas turbine and HRSG is relatively low and brings not only an increased power generation and efficiency but also, in accordance with expression (5), a greatly reduced fuel specific consumption for heat supply. 앫 The combined cycle is considered as a most promising solution. However, up to now gas turbines have not yet been widely used in China due to the primary energy infrastructure. In order to make the solution more demonstrative and practicable for other sites, as the fourth solution, we may concentrate our efforts simply on enhancement of the ‘cold end’ reversibility, i.e. lowering the temperature level of the turbine condenser to about 60°C. This needs the consumer’s subsystem to be designed to accommodate a water supplying temperature of about 55°C and a water temperature drop of not less than 20°C. 앫 In view of the fact that the natural gas is available on the site in question, the ultra-low emission heating system concept may be the best solution. This concept is essentially the assembly of all the positive measures involved in the aforementioned solutions, i.e. a heating system with an integration of the use of CHP, an improved ‘cold end’ reversibility as well as a combined cycle to enhance the ‘hot end’ reversibility. 앫 The heat pump system is of the reversible mode of heating system. From the point of view of emission reduction, however, the CHP system is preferred unless the housing density is low or the consumers are far away from the energy center. In this area there do exist a number of remote consumers suitable for installing heat pump systems using the surplus electricity and the waste heat sources of 35°C with return temperature of 25°C. The underground water at 15°C can also be utilized as the heat pumps’ source with reinjection [4] back at 5°C to a distance well. The remaining power surplus or shortage is balanced by the connected power network.
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4.3. Evaluation The evaluation and comparison of the above-mentioned six solutions are summarized in Table 3. There is no intention of making a comprehensive evaluation of these solutions. Evaluations are limited to those concerning the fuel specific consumption and, in turn, the relative emission evoked by the unity end-use heat supply. Current heating systems, including conventional CHP at a space heating temperature level have usually a specific primary energy consumption ranging from 20 to 50 kg c.e./GJ. Table 3 states that in many applications a specific primary energy consumption as low as 5-15 kg c.e./GJ can be achieved by using the proven technology with profitable marginal costs. It is surprising to notice that a steam turbine CHP system with merely the ‘cold end’ improvement is expected to give a specific fuel consumption for heating which can match that attainable by a CHP equipped with a combined cycle without ‘cold end’ improvement.
Table 3 Evaluation of different options of heating system Conventional Simple CHP heating system
(a) Data Temperature (°C) Ambient ⫺3° Room 20 Outflow/return 100/70 Supply to users 98/71.4 Exergy efficiency Fuel energy delivery 0.2212 Power generation Power distribution Heat generation Heat grid 0.8844 Users’ subsystem 0.3202 (b) Evaluation Theoretical fuel bmin (kg/GJ) Fuel accrual b1 b2 b3 b4 b5 b6 Spec. fuel consum. b=bmin+⌺b61 (kg/GJ) Overall exergy efficiency (%)
⫺3 20 100/70 98/71.4
CHP with improved cold end
CHP with combined cycle
Comb. cycle with improved CHP cold end
Surplus power fed to heat pump
⫺3 20 55/35 54/35.6
⫺3 20 100/70 98/71.4
⫺3 20 55/35 54/35.6
⫺3 20
0.4340 0.593
0.4340 0.593
0.7260* 0.620
0.7260* 0.620
1.81 0.8844 0.3202
2.55 0.9167 0.5219
1.81 0.8844 0.3202
2.55 0.9167 0.5219
2.68
2.68
2.68
2.68
2.68
33.31
11.5 3.59
4.83 1.51
3.18 3.21
1.34 1.35
1.09 5.69 42.78
⫺4.24 1.09 5.69 20.32
⫺3.4 0.47 2.46 8.55
⫺4.24 1.09 5.69 11.61
⫺3.4 0.47 2.46 4.88
14.7
6.26
13.2
31.4
23.1
54.8
15.5
0.92 0.45
2.68
1.65
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5. Remarks on potential emission reduction of heating systems 5.1. Assessment of potential emission reduction of heating systems One of the major findings of the evaluation in the previous section is that a real fuel specific consumption as low as 5–15 kg c.e./GJ (heat) is to be expected. These primary-energy-based values obtained with the help of Specific Consumption Analysis reflect the objective reality with least subjective judgments. In order to assess the real specific consumption of different types of heating system, it is imperative to work out a criterion on a common and possibly most justified basis. Up to now the layperson’s concept of ‘energy’ has been taken as the basis for costing heat, power as well as CHP cogenerated products regardless of the different qualities of theses forms of energy. As a result it is seemingly impossible for the fuel specific consumption to be lower than 34.12 kg/GJ (heat) which corresponds to a thermal efficiency of 100%. This criterion frequently causes confusion and overlooks the tremendous potential for improvement of heating systems. In accordance with the thermal efficiency in terms of the ratio of ‘the heat obtained by the consumer’ and ‘the primary energy consumed to this end’, the value of 5–15 kg c.e./GJ (heat) would correspond to a thermal efficiency of 682–227%. At first glance these values seem too big. However from the viewpoint of the Second Law Analysis, these values correspond just to an exergy efficiency ranging from 17 to 52% and by no means violate the thermodynamic principles. As a matter of fact a thermal efficiency in this sense is not unusual for a reversible mode of heating system to have a value greater than unity [5]. In order to avoid the possible confusion and controversy, Professor Baehr suggested instead of ‘thermal efficiency’ using the term ‘heat index’ (‘die Heizzahl’ in Germany) to denote the ratio of ‘the heat obtained by the consumer’ and ‘the primary energy consumed to this end’ as the criterion of assessing the extent of energy conservation of a heating system [2]. For the sake of proper assessment, a number of other excellent exergy-based proposals with various advantages have been formulated [6]. This paper proposes an alternative of assessment using ‘Specific Consumption Analysis’ based on an applied approach of exergo-economics. This methodology places its emphasis on finding the primarily ‘real’ specific fuel/monetary consumption of the end-use heat and tries to reduce the effect of differential personal judgments to minimum [7]. 5.2. The ‘hot end’ reversibility In contrast to the layman’s idea, the use of a high efficiency engine in a CHP system is as important as in a power-only system. In fact, the ‘hot end’ reversibility plays a significant role in cutting down the fuel specific consumption not only for power supply but also for heat supply. Making use of advanced power cycles and state-of-the-art heat engines in a reversible mode of heating system is the major measure for ‘hot end’ improvement. It is customary to investigate the exergy efficiency of the fuel energy delivery subsystem hb together with that of the power generation subsystem hW, the product of the both being the power plant efficiency representing a major index of the ‘hot end’ reversibility: hbhW⫽W⌺/(⌺Ef)
(6)
where W⌺ is the net power output of the power plant involved in the total heating system while the heat output drops to zero under a specified fuel exergy input ⌺Ef and specified operational conditions. The current power plant efficiency ranges roughly from 0.20 to 0.55 depending on the power cycle arrangement and the operational conditions. From the viewpoint of the Second Law Analysis and in accordance with expression (5), the value of the fuel specific consumption in a reversible mode of heating system
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is strongly affected by the power plant efficiency, as shown in Fig. 2. which was plotted under the following assumptions: for CHP system the fresh steam conditions are 35 bar, 435°C, the gas-fired boiler thermal efficiency is 0.9, the outflow/return temperatures are 100/70°C, the overall temperature loss in heat grid is 2+1.4=3.4°C and accordingly hWD=1, eQ=1.8, hQD=0.89, hUS=0.322; for heat pump system, COP=3, the supplying temperature is 45°C and accordingly hWD=0.92, eQ=0.45, hQD=1, hUS=0.52. It follows from Fig. 2 together with expression (5) that considerable potential improvement of a heating system is offered by power plant efficiency enhancement. 5.3. The exergy index of the heat generation subsystem The grid water heater in a CHP system, the heat pump unit in a heat pump heating system and the electric heater in an electric heating system are known as heat generation subsystems of the relevant total energy system of heating. The exergy index of the heat generation subsystem is defined as eQ⫽ER0/EECR
(7)
where in out in out in out in ER0⫽MR0(T out R0 ⫺T R0)[1⫺T0/(T R0 ⫺T R0)⫻ln(T R0 /T R0)]⫹VR0(PR0 ⫺PR0)
(8)
indicates the exergy supply of the heat generation subsystem [1]. The abbreviation EECR (Equivalent Electricity Consumption Rate) is defined as EECR⫽W⌺⫺W
(9)
where W is the power supplied by the power plant involved in the total energy system of heating and W⌺ represents a hypothetical power supplied by the same power plant under the same operational conditions except for the heat load being shut down. In other words EECR indicates the sacrifice of the supplied
Fig. 2. Influence of power plant efficiency on bq.
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power due to cogenerating heat. The concept of EECR has been successfully applied to analyzing desalination units [8]. The value of eQ differs greatly from system to system. In an electric heating system, the exergy index eQ=(1⫺T0/TR) where T0 is the ambient temperature in K and TR is the mean temperature in K of the heat medium of the electric heater. As electricity converts directly into the low-grade heat, the electric heating system is not of reversible mode. As a result, the value of eQ is quite small. In a heat pump heating system the exergy index eQ is dependent on the exergy efficiency of the heat pump unit hhp and the heat source temperature TL eQ⫽ER0/EECR⫽hhp(1⫺T0/TR)/(1⫺TL/TR)
(10)
here EECR is virtually the power input of the heat pump. It follows from expression (10) that exergy index eQ gets to 100% in the case of a reversible heat pump without any heat source other than ambient environment. The expression also shows that it is possible to substantially enhance the value of the exergy index by making use of any heat source with temperature TL⬎T0 as the heat source of a heat pump. In the case of a CHP with steam turbine, the heat generation subsystem is thought to be the grid water heater fed by the heating steam extracted from the turbine. The exergy index eQ is the ratio of the exergy supplied by the grid water heater and the power sacrifice EECR. Notice that this definition is somewhat different from that of the exergy efficiency of the grid water heater in which, instead of EECR, the denominator is the exergy fed by the heating steam. Since the heating steam exergy content is always larger than the power sacrifice, the value of the exergy index always exceeds that of the exergy efficiency. If the CHP heating system is realized by a simple gas turbine cycle with waste heat utilization, then the value of EECR is thought to be the bottom cycle output that might be obtained if a combined cycle were used. 5.4. The advantages of CHP over heat pump heating The typical data concerning the exergy index of the heat generation subsystem are shown in Table 4. Generally speaking, the value of exergy index of the thermodynamically equivalent heat pump is substantially smaller than that of the CHP grid water heater since the heat pump heating is less straightforward and needs a multiple conversion process for heating steam energy. For this reason, it is not surprising for eQ of the CHP grid water heater to be much greater than that of the heat pump. Moreover, it is not unusual for its value to be greater than unity. An exergy index with a value in excess of unity will result in a negative accrual of fuel specific consumption for the heat generation subsystem. This implies that the heat generation subsystem of CHP has smaller fuel accrual compared with that of an ideal heat pump system under the same outflow/return temperature and other operational conditions.
Table 4 Typical data of the exergy index of the heat generation subsystem Water temperature outflow/return 100/70 85/55
Heat pump heating TL=T0=⫺3°C 0.30 0.32
Heat pump heating TL=30°C 0.48 0.58
CHP heating system hbhW=0.22 1.795 1.855
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5.5. The ‘cold end’ reversibility The ‘cold end’ performance of the total energy system of heating plays an important role in achieving an ultra-low specific emission heating system. In order to improve the ‘cold end’ reversibility, a technoeconomical trade-off should be made to determine the most justified dimension of the end-users’ radiator enabling use of a lower grade of energy supply. To the same end it is recommended to carefully investigate the possibility of using a direct heat grid rather than an indirect one. This is especially important in cases where decentralized energy system is under consideration. It is customary to investigate the exergy efficiency of the heat distribution subsystem hQD together with that of the consumers subsystem hUS, the product of both being the ratio of the exergy obtained by the consumers’ room Er and the exergy supply from the heat generation subsystem ER0, i.e. hQDhUS⫽Er/ER0⫽(Qr/QR0)⫻[(1⫺T0/Tr)/(1⫺T0/TR0)]/(1⫹pQD)
(11)
where T represents the mean temperature in K, subscript R0 indicates the location of the heat generation subsystem, subscript r stands for the consumers’ room and pQD is the ratio of the pump work consumed by the heat grid and the thermal exergy fed by the heat generation subsystem [1]. The value of hQDhUS is strongly dependent on the mean temperature level of the heat grid water as shown in Fig. 3. It follows from Fig. 3 that in order to have a higher exergy efficiency of the ‘cold end’, it is essential to use possibly lower mean temperature of the heat grid water. This requires a lower outflow temperature in of the heat generation subsystem and a higher ‘temperature range’ (T out R0 ⫺T R0) of the grid water heater. out The temperature level, especially the outflow temperature T R0 , also strongly affects the value of exergy index of the heat generation subsystem. Fig. 4 is plotted for a grid water heater of a medium sized Rankine cycle power plant with reheat. The improvement of the ‘cold end’ reversibility is one of the most effective measures for reducing the emission level of heating systems, since this improvement leads to substantial enhancement not only of the values of hQD and hUS but also that of eQ. This is just the reason why the fuel specific consumption of the end-use heat of CHP with simply improved ‘cold end’ turns out to be lower than that of CHP with a combined cycle as shown in Table 3.
Fig. 3. The value of hQDhUS versus TR0.
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Fig. 4.
eQ versus TR0out for a steam power unit with reheat.
5.6. Some feasibility considerations There are a number of technological, economical, as well as regulatory factors affecting the implementation of the ultra-low emission heating system. It is evident that as a prerequisite, the use of a low temperature CHP system needs more elaborate consumers’ heat radiators [9] of high exergy efficiency. The rapid expansion of air conditioning in some areas makes the use of low temperature economically viable, since heat radiators for air conditioning, which are usually of high exergy efficiency, may be used for both heating and cooling. Hence the marginal costs for low temperature heating are insignificant. Of course some modifications are needed for the current consumers’ radiators to meet the ‘cold end’ improvement requirements. For instance, China’s fan-assisted air conditioning radiators are normally designed for supplying a temperature of 7–10°C for cooling, 50– 70°C for heating both with a temperature drop of about 5°C. A detailed analysis shows that after modification to fit for an unfavorable supplying temperature of 50°C for heating with a water temperature drop of 20°C, the heat transfer surface has to be enlarged by 60% and the cost is increased by only 20%. Another important question to be further investigated is the use of a direct heat grid. Heat grids are classified into direct and indirect ones. Because of the absence of heat exchangers between mains and consumers the direct grid greatly helps enhancement of the ‘cold end’ reversibility. This sort of grid is usually used in the small grid of a decentralized energy center. In view of problems like pressure control and water loss, most large heating systems use indirect ones. But there is no lack of successful practice with a sizeable direct grid in some countries such as in Denmark [10,11]. It should be noticed that for the sake of improving the ‘cold end’ reversibility of CHP, the low temperature level has to be attained by means of lowering the pressure of the extracted heating steam and increasing the temperature range of the heat grid. When the outflow temperature is low enough, it is sometimes reasonable for small steam turbines to use the turbine condenser as the grid water heater. But if a sizable steam turbine with reheat originally designed for power-only operation is involved, it is necessary to check the steam volumetric flow rate decreased by the enhanced condenser pressure, which may cause harmful vibration of the turbine unit. This is another point to be investigated. A preliminary study on a 200 MW steam turbine with reheat shows that there exists a zone suitable for reliable operation, where an enhanced condensing temperature Tk corresponds to a minimum condensing flow rate Dmin (see Fig. 5). K In addition, it is sometimes desirable to take the advantage of CHP to implement district cooling. Since the exergy index of the CHP-based heat generation subsystem is much higher than that of a heat pump/refrigerator-based one, we can expect the viability of district cooling with an absorption chiller using
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Fig. 5. The safe and unsafe zones for a 200 MW unit with enhanced condensing temperature.
the cogenerated steam and the existing piping grid, provided there is in vicinity of the energy center an adequate housing density and the heating and cooling loads are matched well enough. These feasibility considerations are dealt with in detail in the sequent reports or in separate papers [12,13]. They also include topics like the innovative load response technique, system optimization, integration with pretreatment of the fuel, the use of trace gases cleaning technology as well as the impact of energy market liberalization and demand side management (DSM), etc.
6. Conclusions In order to investigate the performance of current heating systems on a common basis, a methodology based on a generalized heating system and the author proposed ‘Specific Consumption Analysis’ is presented. It is found that the emission for unity end-use heat caused by primary energy consumption can be substantially reduced. For heating systems at a space heating temperature level, an ultra-low emission heating system with a specific primary energy consumption as low as 5–15 kg c.e./GJ in the framework of proven technology is to be attained in many applications. The key factors affecting the emission level of a heating system are revealed. It is pointed out that for the sake of lower emission, it is necessary to use the ‘reversible mode of heating system’ which is also known as thermodynamic heating. As far as emission reduction is concerned, CHP rather than heat pump heating is preferred unless there are available adequate waste heat sources or favorable climate conditions under circumstances when housing density is low or the users are far away from the energy center. It follows from the analysis and evaluation that the potential emission reduction caused by lowering the temperature level of the ‘cold end’ of a reversible mode of heating system is worth further investigation in detail.
References [1] Song Z-P, Wang JX. Fundamentals of second law analysis and exergo-economics (in Chinese). Beijing: China Water Resourcs and Electric Power Press, 1985. [2] Baehr HD. Zur Thermodynamik des Heizens. BWK 1980;32(1):10–5.
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[3] Song ZP. Specific consumption analysis model for district heating system (in Chinese). J Engng Therm Energy Pow 1995;10(2):305–10. [4] Miles L. Heat pumps: theory and service. New York: Delmar Publishers Inc, 1994. [5] Diamant RME, Kut D. District heating and cooling for energy conservation. New York: John Wiley, 1981. [6] Gaggioli R. A critical review of second law costing method. J Energy Resource Technol 1989;111(3):1–15. [7] Song ZP, Zhang G. Critical remarks on personal judgments and objective realities in cost allocation for utilitybased cogeneration systems (in Chinese). J Chi Soc Elec Engng 1996;16(4):217–21. [8] Song ZP. Indigenous construction of sizable desalination units for dual-purpose power plant in China. Energy 1991;16(4):721–6. [9] Kraft G. Low temperature heating system (in Russian, translated from German). Moscow: Architecture Press, 1983. [10] Drysdale A et al. Consumer connection stations for low temperature district heating in Denmark. Euroheat Power 1996;3:141–5. [11] Orchard WRH. Combined heat and power—whole city heating. New York: John Wiley, 1980. [12] Song ZP. Rediscovering the combined heat and power in terms of the sustainable development (in Chinese). J Chi Soc Elect Engng 1999;18(4):225–30. [13] Song ZP. Improvements of utility-based CHP heating systems. In Cai RX, editor. International Symposium on Thermodynamic Analysis and Improvement of Energy Systems. Beijing: Beijing World Publishing Corporation, 1997, 69–72. [14] Baehr HD. Thermodynamik. Berlin: Springer Verlag, 1978.
Total energy system analysis of heating Zhi-Ping Song
*
Graduate School, North China Electric Power University, Qinghe, Beijing, China 100085 Received 15 December 1999
Abstract This paper provides an analysis of heating from the point of view of the total energy system from the very input of primary energy until it reaches the end-users. In order to minimize the personal judgment in cost allocation between heat and power, the methodology of exergy-based Specific Consumption Analysis proposed by the author is used to evaluate the real consumption of the unity end product in respect to both fuel and monetary cost. Following this methodology the key factors affecting the emission level of a heating system are revealed. It is found that in some applications a total energy system of heating with a specific primary energy consumption as low as 5–15 kg c.e./GJ may be attained. The concept of the ‘reversible mode of heating’ is introduced and is seen as the basis of ultra-low emission heating systems. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Protection of the global climate against the much-feared greenhouse effect caused by increasing amounts of CO2 and other trace gases thought to be detrimental to the climate is currently a topic of great concern. In line with the rising demand for energy sources nationwide, fossil fuel emissions are also likely to increase. In order to meet this challenge, a more intelligent use of energy is urgently needed. A breakdown of energy use shows that almost two thirds of the primary energy is end-used in the form of thermal energy, a very large proportion (up to 5/6) of which is used as low-temperature heating below 100°C. Evidently, lowering emissions caused by such heating systems will greatly promote sustainable development strategy. In recent years a workshop at the Graduate School of North China Electric Power University has undertaken a program focusing on research into a more environmentally friendly system for space heating. The program is supported by the National Committee of Science and Technology of the People’s Republic of China. This paper is the outgrowth of the first report submitted by the workshop concentrating on the theory-based perspective of an ultra-low emission heating system. * Fax: +86-10-6238-1094. E-mail address: [email protected] (Z.-P. Song).
0360-5442/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 0 0 ) 0 0 0 2 1 - 9
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Nomenclature A=[aij] the incident matrix with elements aij=1 (if stream j flows into subsystem i); aij=⫺1 (if stream j flows out of subsystem i); aij=0 (if stream j has no direct connection with subsystem i) b fuel specific consumption of end product fuel specific consumption accrual of subsystem I bI b specific fuel consumption accruals due to irreversibility in terms of column vector c monetary specific consumption i.e. specific cost cI specific cost accrual of subsystem I cFI specific cost accrual due to irreversibility of subsystem I the local fuel market price cf c specific cost accruals due to irreversibility in terms of column vector cZI cost accrual due to fixed charge for the unity product attributed to subsystem I D steam mass flow rate E the column vector of m stream exergies E exergy EECR=W⌺⫺W Equivalent Electricity Consumption Rate, which means the sacrifice of a CHP plant in supplied power due to generating heat [8] M mass P the total exergy amount of the end product, the pressure of medium Q heat T temperature V volume W the net power output of the CHP plant involved in a total heating system the net power output of the CHP plant involved in the total heating system while the heat W⌺ output drops to zero e a coefficient of specific cost taking into account the pollution tax eQ the exergy index of the heat generation subsystem h the exergy efficiency p the ratio of the pump work consumed by the heat grid and the thermal exergy fed by the heat generation subsystem [1]
Subscripts 0 b f hp
under the ambient condition, the item caused by factors not associated with individual subsystem boiler or fuel energy delivery subsystem fuel heat pump
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I=1, 2, J=1, 2, K L min Q q QD R r R0 US W WD
809
%, n the order number of the subsystems in question %, n the order number of the subsystems in question steam turbine condenser used as a heat grid heater the low temperature heat source of the heat pump theoretically minimum value heat generation subsystem with respect to heat supply heat distribution subsystem at the locality of the end of heat distribution subsystem at the location of users’ room at the locality of the heat generation subsystem the end-users’ subsystem power generation subsystem power transmission and distribution subsystem
Superscripts in min out t
inlet minimum outlet transpose
Emissions from a heating system are effected by a number of factors such as energy efficiency, pretreatment of the fuel, the use of trace gases cleaning technology and etc. So far as the greenhouse gas CO2 is concerned, the energy efficiency is directly linked with the emission level and is considered to play a dominant role at the moment. It is therefore necessary to first investigate the possible innovative measures offering significant contribution to energy efficiency of heating systems.
2. Generalized heating system 2.1. Current heating systems Current heating systems serving the general public in urban areas are mainly classified into four types, namely, boiler heating systems, CHP (Combined Heat and Power) systems, heat pump systems and electric heating (see Table 1). In China, a total of some 400,000 small/medium sized boilers are scattered over a vast area. Most of them are of low efficiency, ranging from 50 to 60%. The overall CHP capacity is about 20,000 MW, accounting for 12% of the overall installed power capacity. Heat pumps and electric heating do not yet have widespread acceptance except for individual areas and/or cases. 2.2. Generalized heating system In order to compare the performance of different types of heating system, we use a generalized total heating system model [1,2] as shown in Fig. 1. It consists of six subsystems including fuel energy delivery, power generation, power distribution, heat generation, heat distribution and the end-users subsystem.
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Table 1 Current heating systems serving the general public in urban areas Type of Heating System
System Arrangement
Legend
boiler
F
Fuel Energy Delivery Subsystem
CHP
PG
Power Generation Subsystem
Heat Pump
PD
Power Distribution Subsystem
Electric Heating
HG
Heat Generation Subsystem
HD U
Heat Distribution Subsystem Users’ Subsystem
Fig. 1. Generalized heating system.
The function of each subsystem is quite different in nature. Those subsystems which are indispensable to the goal of the total energy system, i.e. heat supply, are referred to as first class subsystems. Second class subsystems are those, which function to reduce the fuel and/or monetary consumption for the unity end product although they themselves expend a certain amount of fuel and monetary cost. All types of heating systems can be represented by the generalized heating system with the probable exception of some second class subsystems. In order to facilitate the following analysis and exposition, we use the terms ‘hot end ’ and ‘cold end’ of the generalized heating system. The ‘hot end’ consists of the fuel energy delivery, the power generation
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and the power distribution subsystem, while the heat distribution subsystem and the end-users subsystem are components of the ‘cold end’ of the generalized heating system. 2.3. Reversible mode of heating system Although all real heating systems are irreversible without exception, the extent of their irreversibility may differ a great deal. In order to identify the extent of their irreversibility, heating systems are classified into two modes: conventional mode and reversible mode [3]. The reversible mode of heating system is also known as a thermodynamic heating system. The reversible mode of heating system is defined as a total energy system of heating fed by a nonthermal primary energy, in the energy conversion process of which at least one kind of non-thermal energies (e.g. electrical, mechanical or chemical energy) appears and at expense of this non-thermal energy the end product (i.e. the supplied heat) is generated by making use of waste and/or ambient heat sources. Heat pump heating system is of the typical reversible mode of heating system. Another reversible mode of the heating system is the combined heat and power (CHP), which makes use of the waste heat by virtue of the power sacrifice due to cogenerating heat and hence is viewed as thermodynamically equivalent with the heat pump heating. The term ‘reversible heating’ was first introduced by Professor H.D. Baehr in his work ‘Thermodynamik’ [14], which has been translated into several languages. Since this term has, to a certain extent, been accepted in the field of thermodynamic analysis, the present author uses this term with a further modification. In terms of energy efficiency, the two modes of heating systems cannot be mentioned in the same breath. The potential for improvement by making use of the reversible mode of heating system is tremendous and is viewed as the basis for developing ultra-low emission heating systems.
3. Specific consumption of a heating system In this paper, the fuel and monetary consumption for the unity end product are referred to as ‘specific consumption’. Unless otherwise stated they are always defined with respect to the ‘total energy system’ from the very input of primary energy until it reaches the end-users. The primary energy is understood to be a fossil fuel. In accordance with the advanced exergo-economic approach, specific consumption, both the fuel b and the monetary c, is composed of the theoretically minimum specific consumption (bmin or cmin) and the specific consumption accruals (bI or cI), where subscript I=1,2, …, n is the order number of the subsystem in question. 3.1. Theoretically minimum specific consumption The theoretically minimum specific consumption bmin/cmin arise in a hypothetical ideal energy system which is conceived as an entirely reversible energy system with an infinite lifetime and without any fixed costs. It follows that the theoretically minimum specific consumption for a heating system can be shown as bmin q⫽34.12(1⫺T0/Tr) kg c.e./GJ
(1a)
cmin q⫽cfbmin q $/GJ
(1b)
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where the abbreviation c.e. denotes the ‘coal equivalent’ [11] and 1 kg c.e.=7000 kcal=1/34.12 GJ. Supposing the ambient temperature T0=270 K (⫺3°C), the required room temperature Tr=293 K (20°C) and the local fuel market price cf=$0.03/kg, we have bmin q=2.68 kg c.e./GJ and cmin q=$0.08/GJ. 3.2. Specific consumption accruals Any irreversibility results in a fuel accrual and a relevant accrual of specific consumption b and c. Likewise any subsystem involved in a real energy system causes a cost accrual for the unity end product due to its fixed charges including depreciation rate and expenses related to owning and operating it (e.g. maintenance, overheads as well as interest, etc.). Let us consider an energy system consisting of n subsystems and m streams. The specific consumption accruals in terms of a column vector due to irreversibility are b⫽[b1 b2 % bn]t⫽(bmin/P)AE
(2a)
c⫽[cF1 cF2 % cFn]t⫽(cfbmin/P)AE
(2b)
where P the total exergy amount of the end product E the column vector of m stream exergies A=[aij] the incident matrix with elements aij=1 (if stream j flows into subsystem i); aij=⫺1 (if stream j flows out of subsystem i); aij=0 (if stream j has no direct connection with subsystem i)
The specific consumption of the end product equals its theoretically minimum specific consumption plus the sum of the specific consumption accruals, i.e.
冘 n
b⫽bmin⫹
bI
(3a)
0
冉 冘冊 n
c⫽ cmin⫹
冘 n
cFI (1⫹e)⫹
0
cZI
(3b)
0
where cZI denotes the cost accrual due to fixed charges for the unity end product attributed to subsystem I and subscript 0 represents the item caused by factors not associated with the individual subsystem. The symbol e represents a coefficient taking into account the pollution tax. 3.3. Specific consumption of a heating system The methodology described in the foregoing two sections is known as ‘Specific Consumption Analysis’ [3] proposed by the author in 1992. Applying this approach to a heating system and assuming that the exergy at output of the Ith subsystem equals that at input of the (I+1)th subsystem we have respectively the specific consumption accruals in subsystem I (bIq and cIq) and the specific consumption of the end product (bq and cq):
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冉
813
冊
n
bIq⫽ bmin q/ ⌸ hJ (1/hI⫺1)
冉
J⫽I⫹1
n
(4a)
冊
cIq⫽ cfbmin q/ ⌸ hJ (1/hI⫺1) J⫽I⫹1
(4b)
n
bq⫽b0q⫹bmin q/ ⌸ hI
(5a)
I⫽1
冉
n
冊
冘 n
cq⫽ cF0q⫹cmin q/ ⌸ hI (1⫹e)⫹ I⫽1
cZIq
(5b)
0
in where hJ=Eout J /EJ denotes the exergy efficiency/index of subsystem J, which consists of the exergy efficiency of the fuel energy delivery subsystem hb, the power generation subsystem hW, the power distribution subsystem hWD, the exergy index of the heat generation subsystem eQ, the exergy efficiency of the heat distribution subsystem hQD and the end-users subsystem hUS. In accordance with expressions (4) and (5), we have the typical values of the fuel specific consumption b of currently used heating systems as shown in Table 2. It can be seen from Table 2 that the fuel specific consumption of heating systems of the reversible mode is by far lower than that of the conventional mode.
4. A sample application 4.1. Site description An energy center is located in the urban area of a city. It was originally a power station equipped with a number of small power units supplying electricity only and has been shut down for a long time. This station is now planned to be rebuilt into an energy center mainly meeting about one third of the heat demand of its surrounding district within 6 km for space heating. According to the heat load duration curve the annual heat demand amounts to 400,000 GJ with the maximum load of 60 MW(heat)=216 GJ/h. The required room temperature is 20°C in the heating season. Nowadays China is confronted with the environmental issue. In view of environmental protection, in this city coal fired boilers are not allowed to run in the urban area. Recently natural gas piping has run through the city. There is also plenty of underground water with a year round temperature above 15°C. In the vicinity of the energy center the housing density is high enough for a heat grid to be installed and there are several spots of waste water source with a Table 2 Typical values of fuel specific consumption bq in kg c.e./GJ (heat) Subsystems
Boiler heating system
Electric heating system
Heat pump heating system
CHP heating system
Fuel specific consumption b
52
105
35
22
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mean temperature around 30°C, which provide the possibility of making use of them to serve the heat pump units as their low temperature source. The rising living standard in this city calls for year round air conditioning with about 2500 h space heating and 1800 h cooling. 4.2. Different options In accordance with the site description there are several solutions to the heat load requirements. 앫 The simplest solution is to install a conventional heating system with a gas-fired heat-only boiler supplying 90/70°C water to the main. The thermal efficiency is assumed to be 86%. It is true that with a clean fuel like natural gas, the content of solid particles as well as NOX and SO2 can be reduced in comparison with coal-fired boilers provided adequate measures were taken. The supplied energy can, however, be expensive due to the fact that the local market price per GJ natural gas is about four times that of coal. On the other hand, from an overall viewpoint of the public energy supply, the CO2 emission is improved far below that attainable. 앫 In terms of the emission reduction, the heating system may be improved by introducing a reversible mode of heating. Naturally, in accordance with the previous analysis, preference should be given to a CHP system. In China the simplest way of implementation is to use small power units rating from 6 to 50 MW scattered over a large number of urban areas which are mostly out of service due to their high fuel specific consumption for power generation but are still capable of operating for years. On this site a 6 MW unit is available. It is considered to retrofit it into a cogeneration one with an enhanced condenser pressure corresponding to an outflow/return water temperature of 100/70°C. 앫 Moreover, enhancement of the ‘hot end’ reversibility may bring further improvement. To this end the customary practice is to superimpose a gas turbine unit upon a steam cycle to achieve an efficiency of the combined cycle of about 45%. This involves replacing the steam boiler with a gas turbine and a heat recovery steam generator (HRSG), matching the steam supply requirement of the 6 MW steam turbine unit. The steam turbine retains, in principle, its operation conditions as before. The new investment in gas turbine and HRSG is relatively low and brings not only an increased power generation and efficiency but also, in accordance with expression (5), a greatly reduced fuel specific consumption for heat supply. 앫 The combined cycle is considered as a most promising solution. However, up to now gas turbines have not yet been widely used in China due to the primary energy infrastructure. In order to make the solution more demonstrative and practicable for other sites, as the fourth solution, we may concentrate our efforts simply on enhancement of the ‘cold end’ reversibility, i.e. lowering the temperature level of the turbine condenser to about 60°C. This needs the consumer’s subsystem to be designed to accommodate a water supplying temperature of about 55°C and a water temperature drop of not less than 20°C. 앫 In view of the fact that the natural gas is available on the site in question, the ultra-low emission heating system concept may be the best solution. This concept is essentially the assembly of all the positive measures involved in the aforementioned solutions, i.e. a heating system with an integration of the use of CHP, an improved ‘cold end’ reversibility as well as a combined cycle to enhance the ‘hot end’ reversibility. 앫 The heat pump system is of the reversible mode of heating system. From the point of view of emission reduction, however, the CHP system is preferred unless the housing density is low or the consumers are far away from the energy center. In this area there do exist a number of remote consumers suitable for installing heat pump systems using the surplus electricity and the waste heat sources of 35°C with return temperature of 25°C. The underground water at 15°C can also be utilized as the heat pumps’ source with reinjection [4] back at 5°C to a distance well. The remaining power surplus or shortage is balanced by the connected power network.
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4.3. Evaluation The evaluation and comparison of the above-mentioned six solutions are summarized in Table 3. There is no intention of making a comprehensive evaluation of these solutions. Evaluations are limited to those concerning the fuel specific consumption and, in turn, the relative emission evoked by the unity end-use heat supply. Current heating systems, including conventional CHP at a space heating temperature level have usually a specific primary energy consumption ranging from 20 to 50 kg c.e./GJ. Table 3 states that in many applications a specific primary energy consumption as low as 5-15 kg c.e./GJ can be achieved by using the proven technology with profitable marginal costs. It is surprising to notice that a steam turbine CHP system with merely the ‘cold end’ improvement is expected to give a specific fuel consumption for heating which can match that attainable by a CHP equipped with a combined cycle without ‘cold end’ improvement.
Table 3 Evaluation of different options of heating system Conventional Simple CHP heating system
(a) Data Temperature (°C) Ambient ⫺3° Room 20 Outflow/return 100/70 Supply to users 98/71.4 Exergy efficiency Fuel energy delivery 0.2212 Power generation Power distribution Heat generation Heat grid 0.8844 Users’ subsystem 0.3202 (b) Evaluation Theoretical fuel bmin (kg/GJ) Fuel accrual b1 b2 b3 b4 b5 b6 Spec. fuel consum. b=bmin+⌺b61 (kg/GJ) Overall exergy efficiency (%)
⫺3 20 100/70 98/71.4
CHP with improved cold end
CHP with combined cycle
Comb. cycle with improved CHP cold end
Surplus power fed to heat pump
⫺3 20 55/35 54/35.6
⫺3 20 100/70 98/71.4
⫺3 20 55/35 54/35.6
⫺3 20
0.4340 0.593
0.4340 0.593
0.7260* 0.620
0.7260* 0.620
1.81 0.8844 0.3202
2.55 0.9167 0.5219
1.81 0.8844 0.3202
2.55 0.9167 0.5219
2.68
2.68
2.68
2.68
2.68
33.31
11.5 3.59
4.83 1.51
3.18 3.21
1.34 1.35
1.09 5.69 42.78
⫺4.24 1.09 5.69 20.32
⫺3.4 0.47 2.46 8.55
⫺4.24 1.09 5.69 11.61
⫺3.4 0.47 2.46 4.88
14.7
6.26
13.2
31.4
23.1
54.8
15.5
0.92 0.45
2.68
1.65
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5. Remarks on potential emission reduction of heating systems 5.1. Assessment of potential emission reduction of heating systems One of the major findings of the evaluation in the previous section is that a real fuel specific consumption as low as 5–15 kg c.e./GJ (heat) is to be expected. These primary-energy-based values obtained with the help of Specific Consumption Analysis reflect the objective reality with least subjective judgments. In order to assess the real specific consumption of different types of heating system, it is imperative to work out a criterion on a common and possibly most justified basis. Up to now the layperson’s concept of ‘energy’ has been taken as the basis for costing heat, power as well as CHP cogenerated products regardless of the different qualities of theses forms of energy. As a result it is seemingly impossible for the fuel specific consumption to be lower than 34.12 kg/GJ (heat) which corresponds to a thermal efficiency of 100%. This criterion frequently causes confusion and overlooks the tremendous potential for improvement of heating systems. In accordance with the thermal efficiency in terms of the ratio of ‘the heat obtained by the consumer’ and ‘the primary energy consumed to this end’, the value of 5–15 kg c.e./GJ (heat) would correspond to a thermal efficiency of 682–227%. At first glance these values seem too big. However from the viewpoint of the Second Law Analysis, these values correspond just to an exergy efficiency ranging from 17 to 52% and by no means violate the thermodynamic principles. As a matter of fact a thermal efficiency in this sense is not unusual for a reversible mode of heating system to have a value greater than unity [5]. In order to avoid the possible confusion and controversy, Professor Baehr suggested instead of ‘thermal efficiency’ using the term ‘heat index’ (‘die Heizzahl’ in Germany) to denote the ratio of ‘the heat obtained by the consumer’ and ‘the primary energy consumed to this end’ as the criterion of assessing the extent of energy conservation of a heating system [2]. For the sake of proper assessment, a number of other excellent exergy-based proposals with various advantages have been formulated [6]. This paper proposes an alternative of assessment using ‘Specific Consumption Analysis’ based on an applied approach of exergo-economics. This methodology places its emphasis on finding the primarily ‘real’ specific fuel/monetary consumption of the end-use heat and tries to reduce the effect of differential personal judgments to minimum [7]. 5.2. The ‘hot end’ reversibility In contrast to the layman’s idea, the use of a high efficiency engine in a CHP system is as important as in a power-only system. In fact, the ‘hot end’ reversibility plays a significant role in cutting down the fuel specific consumption not only for power supply but also for heat supply. Making use of advanced power cycles and state-of-the-art heat engines in a reversible mode of heating system is the major measure for ‘hot end’ improvement. It is customary to investigate the exergy efficiency of the fuel energy delivery subsystem hb together with that of the power generation subsystem hW, the product of the both being the power plant efficiency representing a major index of the ‘hot end’ reversibility: hbhW⫽W⌺/(⌺Ef)
(6)
where W⌺ is the net power output of the power plant involved in the total heating system while the heat output drops to zero under a specified fuel exergy input ⌺Ef and specified operational conditions. The current power plant efficiency ranges roughly from 0.20 to 0.55 depending on the power cycle arrangement and the operational conditions. From the viewpoint of the Second Law Analysis and in accordance with expression (5), the value of the fuel specific consumption in a reversible mode of heating system
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is strongly affected by the power plant efficiency, as shown in Fig. 2. which was plotted under the following assumptions: for CHP system the fresh steam conditions are 35 bar, 435°C, the gas-fired boiler thermal efficiency is 0.9, the outflow/return temperatures are 100/70°C, the overall temperature loss in heat grid is 2+1.4=3.4°C and accordingly hWD=1, eQ=1.8, hQD=0.89, hUS=0.322; for heat pump system, COP=3, the supplying temperature is 45°C and accordingly hWD=0.92, eQ=0.45, hQD=1, hUS=0.52. It follows from Fig. 2 together with expression (5) that considerable potential improvement of a heating system is offered by power plant efficiency enhancement. 5.3. The exergy index of the heat generation subsystem The grid water heater in a CHP system, the heat pump unit in a heat pump heating system and the electric heater in an electric heating system are known as heat generation subsystems of the relevant total energy system of heating. The exergy index of the heat generation subsystem is defined as eQ⫽ER0/EECR
(7)
where in out in out in out in ER0⫽MR0(T out R0 ⫺T R0)[1⫺T0/(T R0 ⫺T R0)⫻ln(T R0 /T R0)]⫹VR0(PR0 ⫺PR0)
(8)
indicates the exergy supply of the heat generation subsystem [1]. The abbreviation EECR (Equivalent Electricity Consumption Rate) is defined as EECR⫽W⌺⫺W
(9)
where W is the power supplied by the power plant involved in the total energy system of heating and W⌺ represents a hypothetical power supplied by the same power plant under the same operational conditions except for the heat load being shut down. In other words EECR indicates the sacrifice of the supplied
Fig. 2. Influence of power plant efficiency on bq.
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power due to cogenerating heat. The concept of EECR has been successfully applied to analyzing desalination units [8]. The value of eQ differs greatly from system to system. In an electric heating system, the exergy index eQ=(1⫺T0/TR) where T0 is the ambient temperature in K and TR is the mean temperature in K of the heat medium of the electric heater. As electricity converts directly into the low-grade heat, the electric heating system is not of reversible mode. As a result, the value of eQ is quite small. In a heat pump heating system the exergy index eQ is dependent on the exergy efficiency of the heat pump unit hhp and the heat source temperature TL eQ⫽ER0/EECR⫽hhp(1⫺T0/TR)/(1⫺TL/TR)
(10)
here EECR is virtually the power input of the heat pump. It follows from expression (10) that exergy index eQ gets to 100% in the case of a reversible heat pump without any heat source other than ambient environment. The expression also shows that it is possible to substantially enhance the value of the exergy index by making use of any heat source with temperature TL⬎T0 as the heat source of a heat pump. In the case of a CHP with steam turbine, the heat generation subsystem is thought to be the grid water heater fed by the heating steam extracted from the turbine. The exergy index eQ is the ratio of the exergy supplied by the grid water heater and the power sacrifice EECR. Notice that this definition is somewhat different from that of the exergy efficiency of the grid water heater in which, instead of EECR, the denominator is the exergy fed by the heating steam. Since the heating steam exergy content is always larger than the power sacrifice, the value of the exergy index always exceeds that of the exergy efficiency. If the CHP heating system is realized by a simple gas turbine cycle with waste heat utilization, then the value of EECR is thought to be the bottom cycle output that might be obtained if a combined cycle were used. 5.4. The advantages of CHP over heat pump heating The typical data concerning the exergy index of the heat generation subsystem are shown in Table 4. Generally speaking, the value of exergy index of the thermodynamically equivalent heat pump is substantially smaller than that of the CHP grid water heater since the heat pump heating is less straightforward and needs a multiple conversion process for heating steam energy. For this reason, it is not surprising for eQ of the CHP grid water heater to be much greater than that of the heat pump. Moreover, it is not unusual for its value to be greater than unity. An exergy index with a value in excess of unity will result in a negative accrual of fuel specific consumption for the heat generation subsystem. This implies that the heat generation subsystem of CHP has smaller fuel accrual compared with that of an ideal heat pump system under the same outflow/return temperature and other operational conditions.
Table 4 Typical data of the exergy index of the heat generation subsystem Water temperature outflow/return 100/70 85/55
Heat pump heating TL=T0=⫺3°C 0.30 0.32
Heat pump heating TL=30°C 0.48 0.58
CHP heating system hbhW=0.22 1.795 1.855
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5.5. The ‘cold end’ reversibility The ‘cold end’ performance of the total energy system of heating plays an important role in achieving an ultra-low specific emission heating system. In order to improve the ‘cold end’ reversibility, a technoeconomical trade-off should be made to determine the most justified dimension of the end-users’ radiator enabling use of a lower grade of energy supply. To the same end it is recommended to carefully investigate the possibility of using a direct heat grid rather than an indirect one. This is especially important in cases where decentralized energy system is under consideration. It is customary to investigate the exergy efficiency of the heat distribution subsystem hQD together with that of the consumers subsystem hUS, the product of both being the ratio of the exergy obtained by the consumers’ room Er and the exergy supply from the heat generation subsystem ER0, i.e. hQDhUS⫽Er/ER0⫽(Qr/QR0)⫻[(1⫺T0/Tr)/(1⫺T0/TR0)]/(1⫹pQD)
(11)
where T represents the mean temperature in K, subscript R0 indicates the location of the heat generation subsystem, subscript r stands for the consumers’ room and pQD is the ratio of the pump work consumed by the heat grid and the thermal exergy fed by the heat generation subsystem [1]. The value of hQDhUS is strongly dependent on the mean temperature level of the heat grid water as shown in Fig. 3. It follows from Fig. 3 that in order to have a higher exergy efficiency of the ‘cold end’, it is essential to use possibly lower mean temperature of the heat grid water. This requires a lower outflow temperature in of the heat generation subsystem and a higher ‘temperature range’ (T out R0 ⫺T R0) of the grid water heater. out The temperature level, especially the outflow temperature T R0 , also strongly affects the value of exergy index of the heat generation subsystem. Fig. 4 is plotted for a grid water heater of a medium sized Rankine cycle power plant with reheat. The improvement of the ‘cold end’ reversibility is one of the most effective measures for reducing the emission level of heating systems, since this improvement leads to substantial enhancement not only of the values of hQD and hUS but also that of eQ. This is just the reason why the fuel specific consumption of the end-use heat of CHP with simply improved ‘cold end’ turns out to be lower than that of CHP with a combined cycle as shown in Table 3.
Fig. 3. The value of hQDhUS versus TR0.
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Fig. 4.
eQ versus TR0out for a steam power unit with reheat.
5.6. Some feasibility considerations There are a number of technological, economical, as well as regulatory factors affecting the implementation of the ultra-low emission heating system. It is evident that as a prerequisite, the use of a low temperature CHP system needs more elaborate consumers’ heat radiators [9] of high exergy efficiency. The rapid expansion of air conditioning in some areas makes the use of low temperature economically viable, since heat radiators for air conditioning, which are usually of high exergy efficiency, may be used for both heating and cooling. Hence the marginal costs for low temperature heating are insignificant. Of course some modifications are needed for the current consumers’ radiators to meet the ‘cold end’ improvement requirements. For instance, China’s fan-assisted air conditioning radiators are normally designed for supplying a temperature of 7–10°C for cooling, 50– 70°C for heating both with a temperature drop of about 5°C. A detailed analysis shows that after modification to fit for an unfavorable supplying temperature of 50°C for heating with a water temperature drop of 20°C, the heat transfer surface has to be enlarged by 60% and the cost is increased by only 20%. Another important question to be further investigated is the use of a direct heat grid. Heat grids are classified into direct and indirect ones. Because of the absence of heat exchangers between mains and consumers the direct grid greatly helps enhancement of the ‘cold end’ reversibility. This sort of grid is usually used in the small grid of a decentralized energy center. In view of problems like pressure control and water loss, most large heating systems use indirect ones. But there is no lack of successful practice with a sizeable direct grid in some countries such as in Denmark [10,11]. It should be noticed that for the sake of improving the ‘cold end’ reversibility of CHP, the low temperature level has to be attained by means of lowering the pressure of the extracted heating steam and increasing the temperature range of the heat grid. When the outflow temperature is low enough, it is sometimes reasonable for small steam turbines to use the turbine condenser as the grid water heater. But if a sizable steam turbine with reheat originally designed for power-only operation is involved, it is necessary to check the steam volumetric flow rate decreased by the enhanced condenser pressure, which may cause harmful vibration of the turbine unit. This is another point to be investigated. A preliminary study on a 200 MW steam turbine with reheat shows that there exists a zone suitable for reliable operation, where an enhanced condensing temperature Tk corresponds to a minimum condensing flow rate Dmin (see Fig. 5). K In addition, it is sometimes desirable to take the advantage of CHP to implement district cooling. Since the exergy index of the CHP-based heat generation subsystem is much higher than that of a heat pump/refrigerator-based one, we can expect the viability of district cooling with an absorption chiller using
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Fig. 5. The safe and unsafe zones for a 200 MW unit with enhanced condensing temperature.
the cogenerated steam and the existing piping grid, provided there is in vicinity of the energy center an adequate housing density and the heating and cooling loads are matched well enough. These feasibility considerations are dealt with in detail in the sequent reports or in separate papers [12,13]. They also include topics like the innovative load response technique, system optimization, integration with pretreatment of the fuel, the use of trace gases cleaning technology as well as the impact of energy market liberalization and demand side management (DSM), etc.
6. Conclusions In order to investigate the performance of current heating systems on a common basis, a methodology based on a generalized heating system and the author proposed ‘Specific Consumption Analysis’ is presented. It is found that the emission for unity end-use heat caused by primary energy consumption can be substantially reduced. For heating systems at a space heating temperature level, an ultra-low emission heating system with a specific primary energy consumption as low as 5–15 kg c.e./GJ in the framework of proven technology is to be attained in many applications. The key factors affecting the emission level of a heating system are revealed. It is pointed out that for the sake of lower emission, it is necessary to use the ‘reversible mode of heating system’ which is also known as thermodynamic heating. As far as emission reduction is concerned, CHP rather than heat pump heating is preferred unless there are available adequate waste heat sources or favorable climate conditions under circumstances when housing density is low or the users are far away from the energy center. It follows from the analysis and evaluation that the potential emission reduction caused by lowering the temperature level of the ‘cold end’ of a reversible mode of heating system is worth further investigation in detail.
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