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A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) – Thermodynamic modelling studies
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A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) e Thermodynamic modelling studies Hao Liu*, Yingjuan Shao, Jinxing Li Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
article info
abstract
Article history:
This paper presents the results of thermodynamics modelling studies of a 2 kW (e)
Received 11 December 2008
biomass-fired CHP system with organic Rankine cycle (ORC). Three environmentally
Received in revised form
friendly refrigerants, namely HFE7000, HFE7100 and n-pentane, have been selected as the
5 June 2011
ORC fluids. The thermodynamic properties of the selected ORC fluids which have been
Accepted 10 June 2011
predicted by commercial software (EES) are used to predict the thermal efficiency of ORC.
Available online 8 July 2011
The results of modelling show that under the simulated conditions (1) the ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle
Keywords:
efficiency; the ORC efficiency depends on not only the modelling conditions but also the
Biomass-fired
ORC fluid e the highest predicted ORC efficiency is 16.6%; the predicted ORC efficiency
Combined heat and power
follows the following order: n-pentane > HFE7000 > HFE7100 (2) both superheating and
Organic Rankine cycle
sub-cooling are detrimental to the ORC efficiency (3) the electrical efficiency of the CHP
Micro-scale
system with the selected ORC fluids is predicted to be within the range of 7.5%e13.5%,
CHP
mainly depending on the hot water temperature of the biomass boiler and the ORC
Thermodynamic modelling
condenser cooling water temperature as well as the ORC fluid, and corresponding to about 1.5 kW and 2.71 kW electricity output (4) the overall CHP efficiency of the CHP system is in the order of 80% for all three ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
It is now almost universally accepted that the greenhouse gases, particularly carbon dioxides (CO2), resulted from the continual utilisation of fossil fuels have caused the global warming observed over the past decades. Energy saving and renewable energy have now been promoted in many parts of the world via various incentives and legislations. Combined Heat and Power (CHP) Generation has been considered worldwide as the major alternative to traditional systems in terms of significant energy saving and environmental
conservation [1]. Energy use in various buildings accounts for nearly half of the UK’s delivered energy consumption and half of the UK’s CO2 emissions. Many believe that micro-scale CHP is the most effective way to satisfy the energy demands and to reduce CO2 emissions of domestic and light commercial buildings such as small office buildings. The heating and power demands of typical domestic buildings and light commercial buildings can be fully met by micro-scale CHP systems within the size range of 1e10 kW (e). Currently, micro-scale CHP systems within the size range of 1e10 kW (e) are undergoing rapid development, and are
* Corresponding author. Tel.: þ44 115 6764; fax: þ44 115 9513159. E-mail address: [email protected] (H. Liu). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.06.025
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Organic Rankine Cycle (ORC) is one of the power generation concepts which have recently been applied to biomass-fuelled CHP systems [9,10]. Biomass-fired CHP systems based on the ORC with the size in the range of 200 kWe1.5 MW (e) have been successfully demonstrated and now they are
Hot Water Cycle
Organic Working Fluid Cycle
Turbine
Generator
Hot water Pump
Condenser
G Heat exchanger
2. The proposed 2 kW (e) biomass-fired CHP system with organic Rankine cycle
commercially available from several manufacturers with typical electrical efficiency of in the order of 15e20% [9,10]. The principle of the ORC-based power generation is similar to that of the steam-driven Rankine turbine cycle, except that an organic working fluid which has favourable thermodynamic properties (the boiling point, the critical point, the latent heat, the slope of the saturation vapour T-s line, the maximum stability temperature, etc.), is used as the working medium for the turbine [11]. The ORC-based power generation has been widely applied to the power generation from low temperature heat sources, such as the recovery of industrial waste heat, geothermal heat and solar heat, with the size of the generator ranging from a fraction of 1 kW to over 1 MW (e) [11]. An ORC turbine is more economical than a steam-driven turbine in terms of capital and maintenance costs due to the use of non-eroding, non-corrosive and low temperature working fluid vapour. In addition, the ORC-based power generation offers advantages in electricity generation efficiency over the steam-driven Rankine cycle power generation at small- and micro-scale systems. Over the past years, the Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, UK, has carried out several research projects on the ORC-based micro-scale power generation using different heat sources, such as solar energy and a gas-fired boiler [12,13]. Results from these research projects prove that the ORC-based power generation can be successfully applied to 1 kW (e)-scale CHP systems using various heat sources including hot-water generated from a biomass boiler. The proposed 2 kW (e) biomass-fired CHP system is schematically shown in Fig. 1. It consists of two cycles: the water cycle and the organic Rankine cycle. The heat released from the combustion of biomass inside the biomass boiler is used to heat the water via the boiler heat exchangers, while the hot water is used as the heating source of the organic Rankine cycle. The ORC working fluid is closely circulating within the organic Rankine cycle: the condensed ORC working fluid is pumped through the evaporator where it is heated by the circulating hot water to generate organic working fluid vapour which expands in the turbine to generate electricity; the working fluid at the turbine exhaust is condensed in the condenser and flows back to the circulation pump to begin a new cycle. Depending on the ORC working fluid used, the
Evaporator
emerging on the market with promising prospects for the near future commercialisation [2,3]. The UK has been forecast to become one of the three largest markets for micro-scale CHP installations in Europe [4]. In recent years, several fossil fuelpowered micro-scale (1e10 kW (e)) CHP systems, such as the natural gas-fired, Stirling engine-based 1 kW (e) CHP systems of WisperGen [2] and Baxi Ecogen [3], the natural gas or LPGfired, internal combustion engine-based 5.5 kW (e) Baxi DACHS [3], and the natural gas-fuelled, PEM fuel cell-based 1.5 kW (e) Baxi BETA 1.5 PLUS [3], have been in demonstration/trial operations in the United Kingdom. Although these systems have demonstrated to be able to save a certain amount of primary energy and hence CO2 emissions comparing with conventional separate heating and power (e.g. a dedicated heat-supply boiler and grid electricity), they still emit significant amount of CO2 which contributes to the climate change e simply because they are fuelled by fossil fuels. Paepe et al. [5] showed that some fossil fuel-powered micro-scale CHP systems may only have marginal energy and environmental benefits over the separate heating and power by use of modern condensing boilers and grid electricity. To drastically reduce the CO2 emissions, the energy demands of the buildings have to be supplied by renewable energy-based systems, such as building integrated wind turbines, solar PV, solar thermal and biomass-fuelled microscale CHP systems. Generating electricity from biomass based on combustion combined with a steam Rankine turbine cycle is the most developed technology at the present time. However, the steam-driven Rankine turbine power generation is not suitable for biomass-fired CHP systems smaller than 100 kW (e) because of its inherent low electricity generation efficiency and high capital costs. Development of biomass-fuelled CHP systems with size in the order of 100 kW (e) based on other concepts of power generation, such as gasification combined with internal combustion engines or gas turbines, combustion combined with Stirling engines or indirectly-fired turbines, has been the focus of a number of R&D projects in Europe over the past decade [6e8]. The majority of these biomass-fuelled CHP systems are at under development stages and their electricity generation efficiencies and capital costs are likely to deteriorate significantly when scaled-down to 1e10 kW (e) which are the typical size range for building applications. The market demand for biomass-fuelled CHP systems with the size range of 1e10 kW (e) is expected to increase significantly in the future as the price increases sharply, while the supply becomes less secure, of fossil fuels, particularly natural gas and petroleum oil. However, there have been very few studies which have concentrated on the development of biomassfuelled CHP systems sized for building applications.
Heat exchanger
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Cooling water inlet
Cooling water outlet
ORC fluid Pump
Fig. 1 e Schematic of the proposed 2 kW (e) biomass-fired CHP system.
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Table 1 e Main thermodynamic properties of the selected ORC fluids. ORC fluid HFE7000 HFE7100 n-pentane
Molecular formula
Boiling point ( C)
Critical pressure (kPa)
Critical temperature ( C)
Critical density (kg m3)
Heat of vaporization (kJ kg1)
CH3OC3F7 C4F9OCH3 CH3(CH2)3CH3
34 61 36
2478 195.3 3364
165 2230 196.5
531 555 429.9
142 111.6 357
heat of ORC fluid condensation can be used for hot water supply or under-floor heating etc. Prior to the time-consuming and expensive experimental investigations, the thermodynamic modelling of the proposed system is carried out to predict the system performance (efficiencies and outputs) under various conditions.
3. Thermal dynamic modelling of the proposed 2 kW (e) biomass-fired CHP system 3.1.
Selection of the ORC working fluid
Broadly speaking, a good ORC working fluid should have the following desirable characteristics: appropriate boiling temperature, low critical temperature and pressure, small specific volume, low viscosity and surface tension, high thermal conductivity, suitable thermal stability, non-corrosive, nontoxic, compatible with turbine material and environmentally friendly [14]. Previous investigations with ORC processes have used a wide range of working fluids including HCFCs, HFCs and Freon (R114, R113, R11) etc [14e17]. Yamamoto et al. [15] concluded that HCFC-123 has the best characteristics over other candidates such as water and methanol for their ORC waste heat recovery system. Mago et al. [16] investigated dry organic fluids (of R113, R245ca, R123 and iso-butane) with boiling points ranging from 12 C to 48 C to convert waste energy to power from low-grade heat sources. Wei et al. [17] presented the results of analysis and optimization of an ORC system, driven by exhaust heat, using HFC-245fa as the working fluid. Some of the organic working fluids used in previous investigations of ORC processes, such as Freons, CFCs, are
Fig. 2 e T-s diagram of ORC.
ozone depleting substances and hence detrimental to the environment. HCFCs and HFCs have great Global Warming Potentials (GWP) and are either phasing-out or facing the calls of phase-out. On Friday, September 21, 2007, representatives of 191 countries met in Montreal and agreed to accelerate phase-outs of HCFCs from 2009, with developed countries agreeing to reduce production and consumption 10 years earlier than previously promised, with final phase-out in 2020 [18]. Even though HFC refrigerants remain ‘the best option to meet many needs’, HFCs will face the calls for phase-out because of their great GWPs. There are many environment-friendly working fluids, such as hydrofluoroethers (HFEs) and n-pentane, which can be used in ORC-based processes. Tsai [19] did an environment risk assessment of hydrofluoroethers (HFEs) and indicated that some HFEs such as HFE-7000, HFE-7100, HFE-7200 and HEF7500 could be considered to replace HCFCs, and HFCs in order to reduce the greenhouse gases emission and become environmentally friendly. Organic working fluids can be classified, by the slope of its saturation vapour curve in T-s diagram, as dry fluid, wet fluid and isentropic fluid [20]. A dry fluid (e.g. n-pentane) has a positive slope whereas a wet fluid (e.g. water) has a negative slope and an isentropic fluid (e.g. R123) has an infinitely large slope. This feature affects the ORC fluid applicability, operation and cycle efficiency [20]. The proposed 2 kW (e) biomass-fired CHP system with ORC shown in Fig. 1 will only use environmentally friendly ORC fluids. The three ORC fluids to be investigated by thermodynamic modelling are HFE7000, HFE7100, and n-pentane and all of them are dry ORC fluids. The main thermodynamic properties of the selected ORC fluids are summarised in Table 1.
Fig. 3 e T-s diagram of Type A ORC.
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3.2.
Thermal efficiency of ORC
An ideal organic Rankine cycle can be depicted on a T-s diagram as Fig. 2. As it can be seen from Fig. 2, from state point 2 to state point 3, the heat gain of the working fluid is qin ¼ h3 h2
(1)
From state point 4 to state point 1 is a constant pressure process, the heat rejected from the working fluid is jq2 j ¼ h4 h1
(2)
During the adiabatic expansion process (from state point 3 to state point 4), the work that has been done by the working fluid in the turbine is ðWs;T Þ3e4 ¼ h3 h4
(3)
From state point 1 to state point 2 is an adiabatic compression process, the consumption of power by the pump is Ws;p
1e2
¼ h1 h2
(4)
In this cycle, the net work done by the working fluid should be equivalent to the power output from the turbine minuses the power consumption of the pump: Woutput ¼ ðWs;T Þ3e4 Ws;p
1e2
¼ ðh3 h4 Þ ðh2 h1 Þ
(5)
Therefore, the thermal efficiency of the ORC can be calculated as follows: hORC ¼
Woutput ðh3 h4 Þ ðh2 h1 Þ ¼ qin h3 h2
(6)
where h1, h2, h3 and h4 are the specific enthalpies of the working fluids. The thermal efficiency of a Carnot cycle operating between high temperature (Tmax) and low temperature (Tmin) reservoirs is given by hCar ¼ 1
Tmin Tmax
(7)
As none of the cycles operating between temperatures Tmax and Tmin can exceed the efficiency of a Carnot cycle, hORC Eq. (6) should always be smaller than hCar Eq. (7).
a set of algebraic equations. EES can efficiently solve hundreds of coupled non-linear algebraic equations. EES can also be used to solve initial value differential equations. A major difference between EES and existing equation solving programs is the many built-in mathematical and thermophysical property functions which EES provides. For example, the steam tables are implemented such that any thermodynamic property can be obtained from a built-in function call in terms of any two other properties. Similar capability is provided for many other fluids, e.g., ammonia, nitrogen, methane, propane, all common CFC refrigerants, R-134a and including all three ORC fluids of this study: HFE7100, HFE7000 and n-pentane. Hence, all thermodynamic properties of the ORC fluids in this study were obtained by use of EES [22].
3.4. Assumptions and conditions of the thermodynamic modelling The following assumptions are adopted with the thermodynamic modelling: (1) The nominal thermal input (Qboi) and the thermal efficiency (hboi) of the biomass boiler are 20 kW and 85% (based on the low heating value of the biomass fuel). These data are those of the actual biomass boiler acquired by the research group of the authors. It is expected that the boiler thermal efficiency will slightly decrease with an increase in the hot water temperature. However, this has been neglected with the modelling studies reported in this paper. (2) The thermal efficiency of evaporator (heva) is assumed as 96%. A well-insulated SWEP compact heat exchanger is used as the evaporator of the ORC fluid and hence its heat loss is expected to be small. (3) The isentropic turbine efficiency (hsT) is assumed as 85% following Saleh et al. [21]. (4) The condenser thermal efficiency (hcon) is assumed as 98%. Another well-insulated SWEP compact heat exchanger is used as the condenser and hence its heat loss is expected to be small. Because of its lower operation temperature than the evaporator, a higher efficiency for the condenser is assumed.
3.3. Simulations of the thermodynamic properties of ORC fluids In order to model the thermal efficiency of ORC, the thermodynamic properties of the ORC fluid at each state point (state point 1estate point 4, Fig. 2) of the cycle must be known. The thermodynamic properties of an ORC fluid can be accurately described by an equation of state and calculated by means of software packages which have been specifically developed to calculate thermodynamic properties of organic working fluids [21,22]. Saleh et al. [21] used BACKONE to simulate thermodynamic properties of more than 70 working fluids of alkanes, fluorinated alkanes, ethers and fluorinated ethers in their ORC simulations. EES [22] is an acronym for Engineering Equation Solver. The basic function provided by EES is the solution of
Fig. 4 e T-s diagram of Type B ORC.
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Fig. 5 e T-s diagram of Type C ORC.
(5) The isentropic pump efficiency (hsP) is assumed as 65% following Saleh et al. [21]. (6) The inlet temperature of the cooling water to the condenser is 10 C whereas the flow rate and the outlet temperature of the cooling water can be adjusted to achieve the specified minimum ORC fluid temperature of 20, 30, and 40 C. The maximum outlet temperature of the cooling water is assumed as 10 C below the condenser inlet temperature of the ORC fluid (i.e. the temperature at State point 4). Three types of organic Rankine cycle of the selected 3 ORC fluids which belong to the category of dry working fluids are modelled and detailed below: Type A cycle: This type of ORC is shown in the T-s diagram in Fig. 3. The working fluid leaves the condenser as saturated
liquid (state point 1). Then it is compressed in the pump to evaporator pressure (state point 2) with an isentropic pump efficiency hsP. After that, the fluid is heated at constant pressure and achieves the cycle maximum temperature (state point 3) till it becomes saturated vapour. Then it enters the turbine and expands to state point 4 where it reaches as superheated region with an isentropic turbine efficiency hsT. In the end of process, the working fluid enters the condenser and removes heat at constant pressure till it becomes a saturated liquid (state point 1). Type B cycle: This type of ORC is represented in the T-s diagram in Fig. 4. The state point 1, 2 and 4 are same as Type A cycle. The working fluid is heated from state point 2 till it becomes saturated vapour and thereafter it is superheated (state point 3). Type B cycle will be referred as ‘superheating’ because ‘state point 3’ is in superheated state. Type C cycle: This type of ORC is represented in the T-s diagram in Fig. 5. The process steps in this cycle are similar to Type A cycle with only difference that the state point 1 lies in the compressed liquid region. Type C cycle will be referred as ‘sub-cooling’ because the temperature of state point 1 is below the saturation temperature of the ORC liquid. The thermal efficiency of each type of ORC (Type A, B or C) can be calculated by Eq. (6) providing that the actual enthalpy of the ORC at each state point (state point 1 to state point 4) is used in the calculation.
3.5.
Results of thermodynamic modelling
3.5.1.
HFE7000
In total, 20 cases under different conditions have been modelled using HFE7000 as the ORC fluid and involving all three types of ORC (Type A, B, and C). The main variables are the maximum ORC temperature (100, 120 or 140 C) and the
Table 2 e Modelling results with HFE7000 as the ORC fluid.
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9 Case10 Case11 Case12 Case13 Case14 Case15 Case16 Case17 Case18 Case19 Case20
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
hele,CHPa (%)
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
100 100 100 120 120 120 140 140 140 120 120 120 140 140 140 140 140 140 100 100
20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30
662.3 662.3 662.3 662.3 662.3 662.3 662.3 662.3 662.3 1042 1042 1042 1042 1042 1042 1573 1573 1573 662.3 662.3
57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 119.7 119.7
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
13.4 12.0 10.4 13.0 11.6 10.0 12.6 11.2 9.7 14.9 13.5 12.3 14.6 13.2 11.9 16.0 14.6 13.3 9.2 9.8
21.4 18.8 16.1 25.4 22.9 20.3 29.0 26.6 24.2 25.4 22.9 20.3 29.0 26.6 24.2 29.0 26.6 24.2 21.4 18.8
10.9 9.8 8.5 10.6 9.5 8.2 10.3 9.1 7.9 12.2 11.0 10.0 11.9 10.8 9.7 13.1 11.9 10.9 7.5 8.0
2.19 1.96 1.70 2.12 1.89 1.63 2.06 1.83 1.58 2.43 2.20 2.01 2.38 2.15 1.94 2.61 2.38 2.17 1.50 1.60
13.85 14.07 14.33 13.91 14.14 14.39 13.98 14.20 14.44 13.61 13.83 14.03 13.66 13.88 14.09 13.43 13.66 13.87 14.52 14.43
50.9 56.1 61.2 71.7 77.0 82.0 92.2 97.4 103.3 61.1 66.3 71.3 82.5 87.8 92.9 70.3 75.6 80.6 61.2 61.2
A A A B B B B B B A A A B B B A A A C C
a Electrical efficiency of the CHP system: hele,CHP ¼ hORC hboi heva.
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Table 3 e Modelling results with HFE7100 as the ORC fluid.
Case21 Case22 Case23 Case24 Case25 Case26 Case27 Case28 Case29 Case30 Case31 Case32 Case33 Case34 Case35 Case36 Case37 Case38
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
100 100 100 120 120 120 140 140 140 120 120 120 140 140 140 140 140 140
20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40
314 314 314 314 314 314 314 314 314 504.2 504.2 504.2 504.2 504.2 504.2 775.8 775.8 775.8
22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48
Type A Type A Type A Type B Type B Type B Type B Type B Type B Type A Type A Type A Type B Type B Type B Type A Type A Type A
12.9 11.6 10.0 12.3 11.0 9.5 12.0 10.6 9.2 14.3 13.1 11.9 13.8 12.6 11.4 15.4 14.3 13.1
21.4 18.8 16.1 25.4 22.9 20.3 29.0 26.6 24.2 25.4 22.9 20.3 29.0 26.6 24.2 29.0 26.6 24.2
hele, (%)
CHP
10.5 9.5 8.2 10.0 9.0 7.8 9.8 8.6 7.5 11.7 10.7 9.7 11.3 10.3 9.3 12.6 11.7 10.7
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
2.11 1.89 1.63 2.01 1.80 1.55 1.96 1.73 1.50 2.33 2.14 1.94 2.25 2.06 1.86 2.51 2.33 2.14
13.93 14.14 14.39 14.03 14.23 14.47 14.07 14.30 14.52 13.71 13.90 14.09 13.79 13.98 14.17 13.53 13.71 13.90
58.2 62.7 67.0 78.7 83.3 87.7 98.4 103.3 107.5 71.8 75.6 79.8 91.4 96.0 100.4 83.5 88.2 92.5
minimum ORC temperature (20, 30 or 40 C). The results of the 20 cases with HFE7000 as the ORC fluid are summarised in Table 2. Table 2 shows that when HFE7000 is used as the ORC fluid, under the simulated conditions, the organic Rankine cycle efficiency is within the range of 9.2%e16.0%, whereas the electrical efficiency of the proposed CHP system ranges from 7.5% to 13.1% with the corresponding electricity output of 1.5 kW (Case 19, Type C cycle) to 2.61 kW (Case 16, Type A cycle).
3.6.
Discussion of thermodynamic modelling results
3.5.2.
3.6.1.
Comparisons of 3 organic working fluids
HFE7100
18 cases using HFE7100 as the ORC fluid have been modelled under different conditions. Similar to the modelling with HFE7000, the main variables are the maximum ORC temperature (100, 120 or 140 C) and the minimum ORC temperature (20, 30 or 40 C) but only two ORC types (Type A and Type B) have been modelled with HFE7100. The results of modelling with HFE7100 are summarised in Table 3. Table 3 shows that when HFE7100 is used as the ORC fluid, under the simulated conditions, the organic Rankine cycle efficiency is within the range of 9.2%e15.4%, whereas the electrical efficiency of the proposed CHP system ranges from 7.5% to 12.6% with the corresponding electricity output of 1.5 kW (Case 29, Type B cycle) to 2.51 kW (Case 36, Type A cycle).
3.5.3.
n-Pentane
Only Type A cycle is modelled when n-pentane is used as the ORC fluid. The maximum ORC temperature is chosen as either 120 C or 140 C and the minimum ORC temperature is set as
either 30 C or 40 C. The results of the modelling with npentane are summarised in Table 4. Table 4 shows that when n-pentane is used as the ORC fluid, the maximum ORC efficiency is 16.6%, whereas the maximum electrical efficiency of the proposed CHP system is 13.5% with corresponding electricity output of 2.71 kW (Case41, Type A cycle).
The comparison of the predicted ORC efficiency with the Carnot cycle efficiency is shown in Fig. 6 (a) with HFE7000 as the ORC fluid and in Fig. 6 (b) with HFE7100 as the ORC fluid. Fig. 6 clearly shows that the predicted ORC efficiency is well below the Carnot cycle efficiency for either of the ORC fluids, with the former being about 60% of the latter. The dependence of the ORC efficiency on the ratio between the minimum ORC fluid temperature and the maximum ORC fluid temperature can be seen clearly from Fig. 6. However, unlike the Carnot cycle efficiency, the dependence of the ORC efficiency is not linear to the ratio. The non-isentropic processes involved in the ORC are the main cause of the non-linearity predicted with each of the ORC fluids. The comparison of the predicted ORC efficiencies of different ORC fluids is shown in Fig. 7 for Type A cycle. The ORC efficiency depends on not only the modelling conditions but also the ORC fluid e with Type A cycle, the highest predicted ORC efficiency is 16.6%. Under the same modelling
Table 4 e Modelling results with n-pentane as the ORC fluid.
Case39 Case40 Case41 Case42
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
120 120 140 140
30 40 30 40
904.3 904.3 1329 1329
82.62 116.3 82.62 116.3
Type Type Type Type
14.9 13.3 16.6 15.2
22.9 20.3 26.6 24.2
A A A A
hele, (%)
CHP
12.2 10.9 13.5 12.4
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
2.43 2.17 2.71 2.48
13.61 13.87 13.34 13.56
55.1 61.7 63.0 69.5
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a
b
Fig. 7 e Comparison of ORC efficiency between selected ORC fluids e Type A cycle.
with the conclusions obtained with Type A cycle described above. For either HFE7000 or HFE7100, the ORC efficiency with superheating from 100 C to 140 C is worse than that with superheating from 100 C to 120 C. The effect of superheating on the ORC efficiency will be further analysed below.
3.6.2.
Fig. 6 e Comparison of ORC efficiency with Carnot cycle efficiency (a) HFE7000 as the ORC fluid, (b) HFE7100 as the ORC fluid. conditions, the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100. It is expected that HFE7000 has a higher ORC efficiency than HFE7100 because the boiling point of HFE7000 is much lower than that of HFE7100 (34 C and 61 C respectively, Table 1). HFE7000 and npentane have similar boiling points (34 C and 36 C, respectively, Table 1). However, n-pentane appears to be a much better ORC fluid than HFE7000 in terms of the ORC efficiency (Fig. 7). The main reason for this observation is due to the difference in the heat of vaporisation between two ORC fluids: 357 kJ kg1 vs 142 kJ kg1 (Table 1). The much higher heat of vaporisation of n-pentane makes it a better ORC fluid in terms of achieving higher ORC efficiency. The comparison of the predicted ORC efficiencies of HFE7000 and HFE7100 is shown in Fig. 8 for Type B cycle, i.e. with superheating. Fig. 8 shows that the ORC efficiency increases with the pressure ratio of the turbine for both HFE7000 and HFE7100. This is expected as the turbine output power increase with the pressure ratio. Fig. 8 also shows that the ORC efficiency of HFE7000 is always higher than that of HFE7000 under the same conditions and this is in agreement
Effect of superheating on ORC efficiency
Figs. 9 and 10 show the comparisons of the ORC efficiencies between Type A cycle (without superheating) and Type B cycle (with superheating) for HFE7000 and HFE7100, respectively. Figs. 9 and 10 clearly show that when HFE7000 or HFE7100 is used as the ORC fluid, superheating is detrimental to the ORC efficiency. This is in agreement with Saleh et al. [21] who found a decrease of the thermal efficiency by superheating for dry fluids. The effect of superheating can be explained as follows: with superheating, the state point 3 is moved from the saturate vapour line (Type A Cycle, Fig. 3) to the
Fig. 8 e Comparison of ORC efficiency between HFE7000 and HFE7100 e Type B cycle.
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0
Actual Organic Rankine Cycle Efficiency, %
Pmax=662.3 kPa (saturated pressure @100 C) 11
0
Pmin=119.7 kPa (saturated pressure @40 C)
10
9
8
7
Type C
Type C
Type A
20
30
40
6 0
Working fluid temperature after the condenser, C Fig. 9 e Comparison of ORC efficiency between Type A and Type B cycles e HFE7000.
superheated region (Type B cycle, Fig. 4) which leads to an increase in thermal input (Eq. (1)), whereas the power output from the turbine is almost identical with and without superheating as the pressure ratio (Pmax/Pmin) of the turbine is identical; therefore the ORC efficiency with superheating is lower than that without superheating. It is worth noting that in practical applications, a slight superheating can be beneficial to the operation of the turbine: superheating can guarantee that only ORC vapour, not a mixture of vapour and liquid, enters the turbine. Mixed ORC vapour and liquid can cause damage to the turbine blades and shorten the operational life of the turbine.
3.6.3.
Effect of sub-cooling on ORC efficiency
Fig. 11 shows the comparison of the ORC efficiency between Type A and Type C cycles. In a Type C cycle, the ORC fluid is
0
Actual organic Rankine cycle efficiency, %
No superheating (Tmax=100 C), Type A cycle 0
Superheating to Tmax=120 C, Type B cycle
14
0
Superheating to Tmax=140 C, Type B cycle
13 0
Tmin=30 C
12
0
Tmin=20 C
11 10 0
Pmax - saturated pressure@100 C
9 0
0
Tmin=40 C
Pmin - saturated pressure @20, 30, 40 C
8 6
8
10
12
14
Pmax/Pmin
Fig. 10 e Comparison of ORC efficiency between Type A and Type B cycles e HFE7100
Fig. 11 e Comparison of ORC efficiency between Type A and Type C cycles e HFE7000
not only condensed to liquid in the condenser but also further cooled down to a temperature below the saturation temperature of the condenser, i.e. sub-cooling. The right column of Fig. 11 corresponds to the ORC efficiency of Type A cycle (no sub-cooling), while the left and the centre columns correspond to the ORC efficiencies of sub-cooling, from 40 to 20 C and from 40 to 30 C, respectively. From Fig. 11 we can see clearly that sub-cooling is detrimental to the ORC efficiency and should be minimised if possible. When the ORC fluid is sub-cooled in the condenser, more thermal input is needed to raise the ORC fluid to the state point 3 (Fig. 5), whereas the power output from the turbine is the same as the case of no sub-cooling, therefore a lower ORC efficiency is expected.
3.6.4. Electrical efficiency and overall thermal efficiency of the proposed CHP system With the assumptions described in Section 3.4, the electrical efficiency of the proposed CHP system is predicted to be between 7.5% and 13.5%, depending on the modelling conditions, the type of ORC (Type A, B, or C) and the ORC fluid (Tables 2e4), which correspond to 1.5e2.71 kW electricity output. The overall thermal efficiency of the proposed CHP system is predicted to be about 80%, which has little dependence on the ORC fluid or the modelling conditions. However, modelling results shown in Tables 2e4 do indicate the amount and quality of heating depend on the modelling conditions and the ORC fluid. As expected, the conditions and ORC fluids which are good for the electricity output would be bad for the heating both in terms of quantity and quality. Tables 2e4 show that the maximum hot water temperature for heating, which is assumed as 10 C below the inlet temperature of the ORC fluid to the condenser, varies greatly under the simulated conditions, ranging from 50.9 to 107.5 C. When the maximum hot water temperature is low (e.g. in order of 50 C), the heating may only be suitable for underfloor heating, whereas when the maximum hot water temperature is above 70 C, the heating can be carried out by
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 8 5 e3 9 9 4
traditional radiators. In addition, heat recuperating within the ORC should be considered if the expected maximum hot water temperature is higher than that required by the heating application. Heat recuperating within the ORC will improve the ORC efficiency and hence the electrical efficiency of the CHP system. The effect of heat recuperating within the ORC will be investigated in the near future via thermodynamic modelling and experimental investigation [23].
4.
Conclusions and future work
A 2 kW (e) biomass-fired CHP system with organic Rankine cycle has been proposed and thermodynamically modelled with three selected dry organic working fluids, namely HFE7000, HFE7100 and n-pentane. The following conclusions can be drawn from the results of thermodynamic modelling: under the simulated conditions, The ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle efficiency; The ORC efficiency depends on not only the modelling conditions but also the ORC fluid e the highest predicted ORC efficiency is 16.6% and the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100; Both superheating and sub-cooling are detrimental to the ORC efficiency; The electrical efficiency of the proposed 2 kW (e) biomassfired CHP system with the selected ORC fluids is predicted to be within the range of 7.5%e13.5%, mainly depending on the hot water temperature of the biomass boiler and the ORC condenser cooling water temperature as well as the ORC fluid, and corresponding to 1.5 kW and 2.71 kW electricity output; And the overall CHP efficiency of the proposed 2 kW (e) biomass-fired CHP system is in the order of 80% for all ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions. In order to improve the ORC efficiency, heat recuperating within the ORC should be considered, particularly when the inlet temperature of the ORC fluid to the condenser is much higher than that required by the heating application. Further thermodynamic modelling of the proposed system with heat recuperating within the ORC is planned for the near future [23]. Experimental investigation with the proposed CHP system is on going and the feasibility of the proposed CHP system has been experimentally proved [24]. Quantitative experimental results will be presented in the future and the thermodynamic modelling results reported in this paper will be compared with the experimental results.
Acknowledgement The authors wish to acknowledge the financial support from the UK EPSRC (EP/E020062/1) via ERA-NET Bioenergy and the
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UK Technology Strategy Board (TPQ3082A) for their research on ‘The Development of Biomass-Fired Micro-scale CHP’ at the Department of Architecture and Built Environment of the University of Nottingham. F-Chart Software (http:// www.fchart.com/) is acknowledged for the provision of EES and Dr. Guoquan Qiu is acknowledged for helping with the calculations of thermodynamic properties by means of EES.
references
[1] Denntice d’Accacia M, Sasso M, Sibilio S, Vanoli L. MicroCombined heat and power in residential and light commercial applications. Appl Therm Eng 2003;23:1247e59. [2] http://www.whispergen.com/, [accessed 05.11]. [3] http://www.baxi.co.uk/products/combinedheatandpower. htm, [accessed 05.11]. [4] Huhn K. Cogeneration in Europe e heading for applications below 10 kW. Cogeneration and ON-Site power production; JulyeAugust 2001. 55e59. [5] Paepe MD, Herdt PD, Mertens D. Micro-CHP systems for residential applications. Energ Convers Manag 2006;47: 3435e46. [6] Karellasa S, Karl J, Kakaras E. An innovative biomass gasification process and its coupling with microturbine and fuel cell systems. Energy 2008;33:284e91. [7] Kautz M, Hansen U. The externally-fired gas-turbine (EFGTCycle) for decentralized use of biomass. Appl Energ 2007;84: 795e805. [8] Dong L, Liu H, Riffat SB. Development of small-scale and micro-scale biomass-fuelled CHP systems e A literature review. Appl Therm Eng 2009;29:2119e26. [9] http://www.turboden.it/en/products_categoria.asp? categoria¼CHP, [accessed 06.11]. [10] http://www.adoratec.com/productnav.html, [accessed 06. 11]. [11] Hung TC, Shai TY, Wang SK. A review of organic Rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 1997;22:661e7. [12] Nguyen VM, Doherty PS, Riffat SB. Development of a prototype low-temperature Rankine cycle electricity generation system. Appl Therm Eng 2001;21:169e81. [13] Yagoub W, Doherty PS, Riffat SB. Solar energy-gas driven micro-CHP system for an office building. Appl Therm Eng 2006;26:1604e10. [14] Maizza V, Maizza A. Unconventional working fluids in organic Rankine-cycles for waste energy recovery systems. Appl Therm Eng 2001;21:381e90. [15] Yamamoto T, Furuhata T, Arai N, Mori K. Design and testing of the organic Rankine cycle. Energy 2001;26:239e51. [16] Mago PJ, Chamra LM, Srinivasan K, Somayaji C. An examination of regenerative Organic Rankine Cycles using dry fluids. Appl Therm Eng 2008;28:998e1007. [17] Wei D, Lu X, Lu Z, Gu J. Performance Analysis and optimization of Organic Rankine Cycle (ORC) for waste heat recovery. Energ Convers Manag 2007;48:1113e9. [18] http://www.acr-news.com/news/news.asp?id¼429&;title¼ Montrealþprotocol’sþ birthdayþdealþquickensþHCFCþ phase-out, [accessed 06.11]. [19] Tsai W-T. Environmental risk assessment of hydrofluoroethers (HFEs). J Hazard Mater 2005;A199:69e78. [20] Liu B-T, Chien K-H, Wang C-C. Effect of Working fluids on Organic Rankine Cycle for waste heat recovery. Energy 2004; 29:1207e17.
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[21] Saleh B, Koglbauer G, Wendland M, Fischer J. Working fluids for low-temperature Organic Rankine Cycles. Energy 2007;32: 1210e21. [22] Klein SA. Engineering equation solver (EES) for microsoft windows operating systems, Academic Commercial V8.400, 2009, Madison USA, WI: F-Chart software (available at http:// www.fchart.com/, [accessed 06.11].
[23] Shao Y. Development of a biomass-fired micro-scale CHP with ORC. PhD thesis. University of Nottingham. 2011 (under preparation). [24] Liu H, Qiu GQ, Shao YJ, Daminabo F, Riffat SB. Preliminary experimental investigations of a biomass-fired micro-scale CHP with organic Rankine cycle. Int J Low Carbon Technol 2010;5:81e7.
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
A biomass-fired micro-scale CHP system with organic Rankine cycle (ORC) e Thermodynamic modelling studies Hao Liu*, Yingjuan Shao, Jinxing Li Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
article info
abstract
Article history:
This paper presents the results of thermodynamics modelling studies of a 2 kW (e)
Received 11 December 2008
biomass-fired CHP system with organic Rankine cycle (ORC). Three environmentally
Received in revised form
friendly refrigerants, namely HFE7000, HFE7100 and n-pentane, have been selected as the
5 June 2011
ORC fluids. The thermodynamic properties of the selected ORC fluids which have been
Accepted 10 June 2011
predicted by commercial software (EES) are used to predict the thermal efficiency of ORC.
Available online 8 July 2011
The results of modelling show that under the simulated conditions (1) the ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle
Keywords:
efficiency; the ORC efficiency depends on not only the modelling conditions but also the
Biomass-fired
ORC fluid e the highest predicted ORC efficiency is 16.6%; the predicted ORC efficiency
Combined heat and power
follows the following order: n-pentane > HFE7000 > HFE7100 (2) both superheating and
Organic Rankine cycle
sub-cooling are detrimental to the ORC efficiency (3) the electrical efficiency of the CHP
Micro-scale
system with the selected ORC fluids is predicted to be within the range of 7.5%e13.5%,
CHP
mainly depending on the hot water temperature of the biomass boiler and the ORC
Thermodynamic modelling
condenser cooling water temperature as well as the ORC fluid, and corresponding to about 1.5 kW and 2.71 kW electricity output (4) the overall CHP efficiency of the CHP system is in the order of 80% for all three ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
It is now almost universally accepted that the greenhouse gases, particularly carbon dioxides (CO2), resulted from the continual utilisation of fossil fuels have caused the global warming observed over the past decades. Energy saving and renewable energy have now been promoted in many parts of the world via various incentives and legislations. Combined Heat and Power (CHP) Generation has been considered worldwide as the major alternative to traditional systems in terms of significant energy saving and environmental
conservation [1]. Energy use in various buildings accounts for nearly half of the UK’s delivered energy consumption and half of the UK’s CO2 emissions. Many believe that micro-scale CHP is the most effective way to satisfy the energy demands and to reduce CO2 emissions of domestic and light commercial buildings such as small office buildings. The heating and power demands of typical domestic buildings and light commercial buildings can be fully met by micro-scale CHP systems within the size range of 1e10 kW (e). Currently, micro-scale CHP systems within the size range of 1e10 kW (e) are undergoing rapid development, and are
* Corresponding author. Tel.: þ44 115 6764; fax: þ44 115 9513159. E-mail address: [email protected] (H. Liu). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.06.025
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Organic Rankine Cycle (ORC) is one of the power generation concepts which have recently been applied to biomass-fuelled CHP systems [9,10]. Biomass-fired CHP systems based on the ORC with the size in the range of 200 kWe1.5 MW (e) have been successfully demonstrated and now they are
Hot Water Cycle
Organic Working Fluid Cycle
Turbine
Generator
Hot water Pump
Condenser
G Heat exchanger
2. The proposed 2 kW (e) biomass-fired CHP system with organic Rankine cycle
commercially available from several manufacturers with typical electrical efficiency of in the order of 15e20% [9,10]. The principle of the ORC-based power generation is similar to that of the steam-driven Rankine turbine cycle, except that an organic working fluid which has favourable thermodynamic properties (the boiling point, the critical point, the latent heat, the slope of the saturation vapour T-s line, the maximum stability temperature, etc.), is used as the working medium for the turbine [11]. The ORC-based power generation has been widely applied to the power generation from low temperature heat sources, such as the recovery of industrial waste heat, geothermal heat and solar heat, with the size of the generator ranging from a fraction of 1 kW to over 1 MW (e) [11]. An ORC turbine is more economical than a steam-driven turbine in terms of capital and maintenance costs due to the use of non-eroding, non-corrosive and low temperature working fluid vapour. In addition, the ORC-based power generation offers advantages in electricity generation efficiency over the steam-driven Rankine cycle power generation at small- and micro-scale systems. Over the past years, the Department of Architecture and Built Environment, Faculty of Engineering, University of Nottingham, UK, has carried out several research projects on the ORC-based micro-scale power generation using different heat sources, such as solar energy and a gas-fired boiler [12,13]. Results from these research projects prove that the ORC-based power generation can be successfully applied to 1 kW (e)-scale CHP systems using various heat sources including hot-water generated from a biomass boiler. The proposed 2 kW (e) biomass-fired CHP system is schematically shown in Fig. 1. It consists of two cycles: the water cycle and the organic Rankine cycle. The heat released from the combustion of biomass inside the biomass boiler is used to heat the water via the boiler heat exchangers, while the hot water is used as the heating source of the organic Rankine cycle. The ORC working fluid is closely circulating within the organic Rankine cycle: the condensed ORC working fluid is pumped through the evaporator where it is heated by the circulating hot water to generate organic working fluid vapour which expands in the turbine to generate electricity; the working fluid at the turbine exhaust is condensed in the condenser and flows back to the circulation pump to begin a new cycle. Depending on the ORC working fluid used, the
Evaporator
emerging on the market with promising prospects for the near future commercialisation [2,3]. The UK has been forecast to become one of the three largest markets for micro-scale CHP installations in Europe [4]. In recent years, several fossil fuelpowered micro-scale (1e10 kW (e)) CHP systems, such as the natural gas-fired, Stirling engine-based 1 kW (e) CHP systems of WisperGen [2] and Baxi Ecogen [3], the natural gas or LPGfired, internal combustion engine-based 5.5 kW (e) Baxi DACHS [3], and the natural gas-fuelled, PEM fuel cell-based 1.5 kW (e) Baxi BETA 1.5 PLUS [3], have been in demonstration/trial operations in the United Kingdom. Although these systems have demonstrated to be able to save a certain amount of primary energy and hence CO2 emissions comparing with conventional separate heating and power (e.g. a dedicated heat-supply boiler and grid electricity), they still emit significant amount of CO2 which contributes to the climate change e simply because they are fuelled by fossil fuels. Paepe et al. [5] showed that some fossil fuel-powered micro-scale CHP systems may only have marginal energy and environmental benefits over the separate heating and power by use of modern condensing boilers and grid electricity. To drastically reduce the CO2 emissions, the energy demands of the buildings have to be supplied by renewable energy-based systems, such as building integrated wind turbines, solar PV, solar thermal and biomass-fuelled microscale CHP systems. Generating electricity from biomass based on combustion combined with a steam Rankine turbine cycle is the most developed technology at the present time. However, the steam-driven Rankine turbine power generation is not suitable for biomass-fired CHP systems smaller than 100 kW (e) because of its inherent low electricity generation efficiency and high capital costs. Development of biomass-fuelled CHP systems with size in the order of 100 kW (e) based on other concepts of power generation, such as gasification combined with internal combustion engines or gas turbines, combustion combined with Stirling engines or indirectly-fired turbines, has been the focus of a number of R&D projects in Europe over the past decade [6e8]. The majority of these biomass-fuelled CHP systems are at under development stages and their electricity generation efficiencies and capital costs are likely to deteriorate significantly when scaled-down to 1e10 kW (e) which are the typical size range for building applications. The market demand for biomass-fuelled CHP systems with the size range of 1e10 kW (e) is expected to increase significantly in the future as the price increases sharply, while the supply becomes less secure, of fossil fuels, particularly natural gas and petroleum oil. However, there have been very few studies which have concentrated on the development of biomassfuelled CHP systems sized for building applications.
Heat exchanger
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Cooling water inlet
Cooling water outlet
ORC fluid Pump
Fig. 1 e Schematic of the proposed 2 kW (e) biomass-fired CHP system.
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Table 1 e Main thermodynamic properties of the selected ORC fluids. ORC fluid HFE7000 HFE7100 n-pentane
Molecular formula
Boiling point ( C)
Critical pressure (kPa)
Critical temperature ( C)
Critical density (kg m3)
Heat of vaporization (kJ kg1)
CH3OC3F7 C4F9OCH3 CH3(CH2)3CH3
34 61 36
2478 195.3 3364
165 2230 196.5
531 555 429.9
142 111.6 357
heat of ORC fluid condensation can be used for hot water supply or under-floor heating etc. Prior to the time-consuming and expensive experimental investigations, the thermodynamic modelling of the proposed system is carried out to predict the system performance (efficiencies and outputs) under various conditions.
3. Thermal dynamic modelling of the proposed 2 kW (e) biomass-fired CHP system 3.1.
Selection of the ORC working fluid
Broadly speaking, a good ORC working fluid should have the following desirable characteristics: appropriate boiling temperature, low critical temperature and pressure, small specific volume, low viscosity and surface tension, high thermal conductivity, suitable thermal stability, non-corrosive, nontoxic, compatible with turbine material and environmentally friendly [14]. Previous investigations with ORC processes have used a wide range of working fluids including HCFCs, HFCs and Freon (R114, R113, R11) etc [14e17]. Yamamoto et al. [15] concluded that HCFC-123 has the best characteristics over other candidates such as water and methanol for their ORC waste heat recovery system. Mago et al. [16] investigated dry organic fluids (of R113, R245ca, R123 and iso-butane) with boiling points ranging from 12 C to 48 C to convert waste energy to power from low-grade heat sources. Wei et al. [17] presented the results of analysis and optimization of an ORC system, driven by exhaust heat, using HFC-245fa as the working fluid. Some of the organic working fluids used in previous investigations of ORC processes, such as Freons, CFCs, are
Fig. 2 e T-s diagram of ORC.
ozone depleting substances and hence detrimental to the environment. HCFCs and HFCs have great Global Warming Potentials (GWP) and are either phasing-out or facing the calls of phase-out. On Friday, September 21, 2007, representatives of 191 countries met in Montreal and agreed to accelerate phase-outs of HCFCs from 2009, with developed countries agreeing to reduce production and consumption 10 years earlier than previously promised, with final phase-out in 2020 [18]. Even though HFC refrigerants remain ‘the best option to meet many needs’, HFCs will face the calls for phase-out because of their great GWPs. There are many environment-friendly working fluids, such as hydrofluoroethers (HFEs) and n-pentane, which can be used in ORC-based processes. Tsai [19] did an environment risk assessment of hydrofluoroethers (HFEs) and indicated that some HFEs such as HFE-7000, HFE-7100, HFE-7200 and HEF7500 could be considered to replace HCFCs, and HFCs in order to reduce the greenhouse gases emission and become environmentally friendly. Organic working fluids can be classified, by the slope of its saturation vapour curve in T-s diagram, as dry fluid, wet fluid and isentropic fluid [20]. A dry fluid (e.g. n-pentane) has a positive slope whereas a wet fluid (e.g. water) has a negative slope and an isentropic fluid (e.g. R123) has an infinitely large slope. This feature affects the ORC fluid applicability, operation and cycle efficiency [20]. The proposed 2 kW (e) biomass-fired CHP system with ORC shown in Fig. 1 will only use environmentally friendly ORC fluids. The three ORC fluids to be investigated by thermodynamic modelling are HFE7000, HFE7100, and n-pentane and all of them are dry ORC fluids. The main thermodynamic properties of the selected ORC fluids are summarised in Table 1.
Fig. 3 e T-s diagram of Type A ORC.
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3.2.
Thermal efficiency of ORC
An ideal organic Rankine cycle can be depicted on a T-s diagram as Fig. 2. As it can be seen from Fig. 2, from state point 2 to state point 3, the heat gain of the working fluid is qin ¼ h3 h2
(1)
From state point 4 to state point 1 is a constant pressure process, the heat rejected from the working fluid is jq2 j ¼ h4 h1
(2)
During the adiabatic expansion process (from state point 3 to state point 4), the work that has been done by the working fluid in the turbine is ðWs;T Þ3e4 ¼ h3 h4
(3)
From state point 1 to state point 2 is an adiabatic compression process, the consumption of power by the pump is Ws;p
1e2
¼ h1 h2
(4)
In this cycle, the net work done by the working fluid should be equivalent to the power output from the turbine minuses the power consumption of the pump: Woutput ¼ ðWs;T Þ3e4 Ws;p
1e2
¼ ðh3 h4 Þ ðh2 h1 Þ
(5)
Therefore, the thermal efficiency of the ORC can be calculated as follows: hORC ¼
Woutput ðh3 h4 Þ ðh2 h1 Þ ¼ qin h3 h2
(6)
where h1, h2, h3 and h4 are the specific enthalpies of the working fluids. The thermal efficiency of a Carnot cycle operating between high temperature (Tmax) and low temperature (Tmin) reservoirs is given by hCar ¼ 1
Tmin Tmax
(7)
As none of the cycles operating between temperatures Tmax and Tmin can exceed the efficiency of a Carnot cycle, hORC Eq. (6) should always be smaller than hCar Eq. (7).
a set of algebraic equations. EES can efficiently solve hundreds of coupled non-linear algebraic equations. EES can also be used to solve initial value differential equations. A major difference between EES and existing equation solving programs is the many built-in mathematical and thermophysical property functions which EES provides. For example, the steam tables are implemented such that any thermodynamic property can be obtained from a built-in function call in terms of any two other properties. Similar capability is provided for many other fluids, e.g., ammonia, nitrogen, methane, propane, all common CFC refrigerants, R-134a and including all three ORC fluids of this study: HFE7100, HFE7000 and n-pentane. Hence, all thermodynamic properties of the ORC fluids in this study were obtained by use of EES [22].
3.4. Assumptions and conditions of the thermodynamic modelling The following assumptions are adopted with the thermodynamic modelling: (1) The nominal thermal input (Qboi) and the thermal efficiency (hboi) of the biomass boiler are 20 kW and 85% (based on the low heating value of the biomass fuel). These data are those of the actual biomass boiler acquired by the research group of the authors. It is expected that the boiler thermal efficiency will slightly decrease with an increase in the hot water temperature. However, this has been neglected with the modelling studies reported in this paper. (2) The thermal efficiency of evaporator (heva) is assumed as 96%. A well-insulated SWEP compact heat exchanger is used as the evaporator of the ORC fluid and hence its heat loss is expected to be small. (3) The isentropic turbine efficiency (hsT) is assumed as 85% following Saleh et al. [21]. (4) The condenser thermal efficiency (hcon) is assumed as 98%. Another well-insulated SWEP compact heat exchanger is used as the condenser and hence its heat loss is expected to be small. Because of its lower operation temperature than the evaporator, a higher efficiency for the condenser is assumed.
3.3. Simulations of the thermodynamic properties of ORC fluids In order to model the thermal efficiency of ORC, the thermodynamic properties of the ORC fluid at each state point (state point 1estate point 4, Fig. 2) of the cycle must be known. The thermodynamic properties of an ORC fluid can be accurately described by an equation of state and calculated by means of software packages which have been specifically developed to calculate thermodynamic properties of organic working fluids [21,22]. Saleh et al. [21] used BACKONE to simulate thermodynamic properties of more than 70 working fluids of alkanes, fluorinated alkanes, ethers and fluorinated ethers in their ORC simulations. EES [22] is an acronym for Engineering Equation Solver. The basic function provided by EES is the solution of
Fig. 4 e T-s diagram of Type B ORC.
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Fig. 5 e T-s diagram of Type C ORC.
(5) The isentropic pump efficiency (hsP) is assumed as 65% following Saleh et al. [21]. (6) The inlet temperature of the cooling water to the condenser is 10 C whereas the flow rate and the outlet temperature of the cooling water can be adjusted to achieve the specified minimum ORC fluid temperature of 20, 30, and 40 C. The maximum outlet temperature of the cooling water is assumed as 10 C below the condenser inlet temperature of the ORC fluid (i.e. the temperature at State point 4). Three types of organic Rankine cycle of the selected 3 ORC fluids which belong to the category of dry working fluids are modelled and detailed below: Type A cycle: This type of ORC is shown in the T-s diagram in Fig. 3. The working fluid leaves the condenser as saturated
liquid (state point 1). Then it is compressed in the pump to evaporator pressure (state point 2) with an isentropic pump efficiency hsP. After that, the fluid is heated at constant pressure and achieves the cycle maximum temperature (state point 3) till it becomes saturated vapour. Then it enters the turbine and expands to state point 4 where it reaches as superheated region with an isentropic turbine efficiency hsT. In the end of process, the working fluid enters the condenser and removes heat at constant pressure till it becomes a saturated liquid (state point 1). Type B cycle: This type of ORC is represented in the T-s diagram in Fig. 4. The state point 1, 2 and 4 are same as Type A cycle. The working fluid is heated from state point 2 till it becomes saturated vapour and thereafter it is superheated (state point 3). Type B cycle will be referred as ‘superheating’ because ‘state point 3’ is in superheated state. Type C cycle: This type of ORC is represented in the T-s diagram in Fig. 5. The process steps in this cycle are similar to Type A cycle with only difference that the state point 1 lies in the compressed liquid region. Type C cycle will be referred as ‘sub-cooling’ because the temperature of state point 1 is below the saturation temperature of the ORC liquid. The thermal efficiency of each type of ORC (Type A, B or C) can be calculated by Eq. (6) providing that the actual enthalpy of the ORC at each state point (state point 1 to state point 4) is used in the calculation.
3.5.
Results of thermodynamic modelling
3.5.1.
HFE7000
In total, 20 cases under different conditions have been modelled using HFE7000 as the ORC fluid and involving all three types of ORC (Type A, B, and C). The main variables are the maximum ORC temperature (100, 120 or 140 C) and the
Table 2 e Modelling results with HFE7000 as the ORC fluid.
Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9 Case10 Case11 Case12 Case13 Case14 Case15 Case16 Case17 Case18 Case19 Case20
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
hele,CHPa (%)
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
100 100 100 120 120 120 140 140 140 120 120 120 140 140 140 140 140 140 100 100
20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30
662.3 662.3 662.3 662.3 662.3 662.3 662.3 662.3 662.3 1042 1042 1042 1042 1042 1042 1573 1573 1573 662.3 662.3
57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 57.16 83.82 119.7 119.7 119.7
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
13.4 12.0 10.4 13.0 11.6 10.0 12.6 11.2 9.7 14.9 13.5 12.3 14.6 13.2 11.9 16.0 14.6 13.3 9.2 9.8
21.4 18.8 16.1 25.4 22.9 20.3 29.0 26.6 24.2 25.4 22.9 20.3 29.0 26.6 24.2 29.0 26.6 24.2 21.4 18.8
10.9 9.8 8.5 10.6 9.5 8.2 10.3 9.1 7.9 12.2 11.0 10.0 11.9 10.8 9.7 13.1 11.9 10.9 7.5 8.0
2.19 1.96 1.70 2.12 1.89 1.63 2.06 1.83 1.58 2.43 2.20 2.01 2.38 2.15 1.94 2.61 2.38 2.17 1.50 1.60
13.85 14.07 14.33 13.91 14.14 14.39 13.98 14.20 14.44 13.61 13.83 14.03 13.66 13.88 14.09 13.43 13.66 13.87 14.52 14.43
50.9 56.1 61.2 71.7 77.0 82.0 92.2 97.4 103.3 61.1 66.3 71.3 82.5 87.8 92.9 70.3 75.6 80.6 61.2 61.2
A A A B B B B B B A A A B B B A A A C C
a Electrical efficiency of the CHP system: hele,CHP ¼ hORC hboi heva.
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Table 3 e Modelling results with HFE7100 as the ORC fluid.
Case21 Case22 Case23 Case24 Case25 Case26 Case27 Case28 Case29 Case30 Case31 Case32 Case33 Case34 Case35 Case36 Case37 Case38
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
100 100 100 120 120 120 140 140 140 120 120 120 140 140 140 140 140 140
20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40
314 314 314 314 314 314 314 314 314 504.2 504.2 504.2 504.2 504.2 504.2 775.8 775.8 775.8
22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48 22.33 34.15 50.48
Type A Type A Type A Type B Type B Type B Type B Type B Type B Type A Type A Type A Type B Type B Type B Type A Type A Type A
12.9 11.6 10.0 12.3 11.0 9.5 12.0 10.6 9.2 14.3 13.1 11.9 13.8 12.6 11.4 15.4 14.3 13.1
21.4 18.8 16.1 25.4 22.9 20.3 29.0 26.6 24.2 25.4 22.9 20.3 29.0 26.6 24.2 29.0 26.6 24.2
hele, (%)
CHP
10.5 9.5 8.2 10.0 9.0 7.8 9.8 8.6 7.5 11.7 10.7 9.7 11.3 10.3 9.3 12.6 11.7 10.7
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
2.11 1.89 1.63 2.01 1.80 1.55 1.96 1.73 1.50 2.33 2.14 1.94 2.25 2.06 1.86 2.51 2.33 2.14
13.93 14.14 14.39 14.03 14.23 14.47 14.07 14.30 14.52 13.71 13.90 14.09 13.79 13.98 14.17 13.53 13.71 13.90
58.2 62.7 67.0 78.7 83.3 87.7 98.4 103.3 107.5 71.8 75.6 79.8 91.4 96.0 100.4 83.5 88.2 92.5
minimum ORC temperature (20, 30 or 40 C). The results of the 20 cases with HFE7000 as the ORC fluid are summarised in Table 2. Table 2 shows that when HFE7000 is used as the ORC fluid, under the simulated conditions, the organic Rankine cycle efficiency is within the range of 9.2%e16.0%, whereas the electrical efficiency of the proposed CHP system ranges from 7.5% to 13.1% with the corresponding electricity output of 1.5 kW (Case 19, Type C cycle) to 2.61 kW (Case 16, Type A cycle).
3.6.
Discussion of thermodynamic modelling results
3.5.2.
3.6.1.
Comparisons of 3 organic working fluids
HFE7100
18 cases using HFE7100 as the ORC fluid have been modelled under different conditions. Similar to the modelling with HFE7000, the main variables are the maximum ORC temperature (100, 120 or 140 C) and the minimum ORC temperature (20, 30 or 40 C) but only two ORC types (Type A and Type B) have been modelled with HFE7100. The results of modelling with HFE7100 are summarised in Table 3. Table 3 shows that when HFE7100 is used as the ORC fluid, under the simulated conditions, the organic Rankine cycle efficiency is within the range of 9.2%e15.4%, whereas the electrical efficiency of the proposed CHP system ranges from 7.5% to 12.6% with the corresponding electricity output of 1.5 kW (Case 29, Type B cycle) to 2.51 kW (Case 36, Type A cycle).
3.5.3.
n-Pentane
Only Type A cycle is modelled when n-pentane is used as the ORC fluid. The maximum ORC temperature is chosen as either 120 C or 140 C and the minimum ORC temperature is set as
either 30 C or 40 C. The results of the modelling with npentane are summarised in Table 4. Table 4 shows that when n-pentane is used as the ORC fluid, the maximum ORC efficiency is 16.6%, whereas the maximum electrical efficiency of the proposed CHP system is 13.5% with corresponding electricity output of 2.71 kW (Case41, Type A cycle).
The comparison of the predicted ORC efficiency with the Carnot cycle efficiency is shown in Fig. 6 (a) with HFE7000 as the ORC fluid and in Fig. 6 (b) with HFE7100 as the ORC fluid. Fig. 6 clearly shows that the predicted ORC efficiency is well below the Carnot cycle efficiency for either of the ORC fluids, with the former being about 60% of the latter. The dependence of the ORC efficiency on the ratio between the minimum ORC fluid temperature and the maximum ORC fluid temperature can be seen clearly from Fig. 6. However, unlike the Carnot cycle efficiency, the dependence of the ORC efficiency is not linear to the ratio. The non-isentropic processes involved in the ORC are the main cause of the non-linearity predicted with each of the ORC fluids. The comparison of the predicted ORC efficiencies of different ORC fluids is shown in Fig. 7 for Type A cycle. The ORC efficiency depends on not only the modelling conditions but also the ORC fluid e with Type A cycle, the highest predicted ORC efficiency is 16.6%. Under the same modelling
Table 4 e Modelling results with n-pentane as the ORC fluid.
Case39 Case40 Case41 Case42
Tmax ( C)
Tmin ( C)
Pmax (kPa)
Pmin (kPa)
Type of ORC
hORC (%)
hCar (%)
120 120 140 140
30 40 30 40
904.3 904.3 1329 1329
82.62 116.3 82.62 116.3
Type Type Type Type
14.9 13.3 16.6 15.2
22.9 20.3 26.6 24.2
A A A A
hele, (%)
CHP
12.2 10.9 13.5 12.4
Electricity Output (kW)
Heat Output (kW)
Maximum Hot Water T ( C)
2.43 2.17 2.71 2.48
13.61 13.87 13.34 13.56
55.1 61.7 63.0 69.5
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a
b
Fig. 7 e Comparison of ORC efficiency between selected ORC fluids e Type A cycle.
with the conclusions obtained with Type A cycle described above. For either HFE7000 or HFE7100, the ORC efficiency with superheating from 100 C to 140 C is worse than that with superheating from 100 C to 120 C. The effect of superheating on the ORC efficiency will be further analysed below.
3.6.2.
Fig. 6 e Comparison of ORC efficiency with Carnot cycle efficiency (a) HFE7000 as the ORC fluid, (b) HFE7100 as the ORC fluid. conditions, the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100. It is expected that HFE7000 has a higher ORC efficiency than HFE7100 because the boiling point of HFE7000 is much lower than that of HFE7100 (34 C and 61 C respectively, Table 1). HFE7000 and npentane have similar boiling points (34 C and 36 C, respectively, Table 1). However, n-pentane appears to be a much better ORC fluid than HFE7000 in terms of the ORC efficiency (Fig. 7). The main reason for this observation is due to the difference in the heat of vaporisation between two ORC fluids: 357 kJ kg1 vs 142 kJ kg1 (Table 1). The much higher heat of vaporisation of n-pentane makes it a better ORC fluid in terms of achieving higher ORC efficiency. The comparison of the predicted ORC efficiencies of HFE7000 and HFE7100 is shown in Fig. 8 for Type B cycle, i.e. with superheating. Fig. 8 shows that the ORC efficiency increases with the pressure ratio of the turbine for both HFE7000 and HFE7100. This is expected as the turbine output power increase with the pressure ratio. Fig. 8 also shows that the ORC efficiency of HFE7000 is always higher than that of HFE7000 under the same conditions and this is in agreement
Effect of superheating on ORC efficiency
Figs. 9 and 10 show the comparisons of the ORC efficiencies between Type A cycle (without superheating) and Type B cycle (with superheating) for HFE7000 and HFE7100, respectively. Figs. 9 and 10 clearly show that when HFE7000 or HFE7100 is used as the ORC fluid, superheating is detrimental to the ORC efficiency. This is in agreement with Saleh et al. [21] who found a decrease of the thermal efficiency by superheating for dry fluids. The effect of superheating can be explained as follows: with superheating, the state point 3 is moved from the saturate vapour line (Type A Cycle, Fig. 3) to the
Fig. 8 e Comparison of ORC efficiency between HFE7000 and HFE7100 e Type B cycle.
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0
Actual Organic Rankine Cycle Efficiency, %
Pmax=662.3 kPa (saturated pressure @100 C) 11
0
Pmin=119.7 kPa (saturated pressure @40 C)
10
9
8
7
Type C
Type C
Type A
20
30
40
6 0
Working fluid temperature after the condenser, C Fig. 9 e Comparison of ORC efficiency between Type A and Type B cycles e HFE7000.
superheated region (Type B cycle, Fig. 4) which leads to an increase in thermal input (Eq. (1)), whereas the power output from the turbine is almost identical with and without superheating as the pressure ratio (Pmax/Pmin) of the turbine is identical; therefore the ORC efficiency with superheating is lower than that without superheating. It is worth noting that in practical applications, a slight superheating can be beneficial to the operation of the turbine: superheating can guarantee that only ORC vapour, not a mixture of vapour and liquid, enters the turbine. Mixed ORC vapour and liquid can cause damage to the turbine blades and shorten the operational life of the turbine.
3.6.3.
Effect of sub-cooling on ORC efficiency
Fig. 11 shows the comparison of the ORC efficiency between Type A and Type C cycles. In a Type C cycle, the ORC fluid is
0
Actual organic Rankine cycle efficiency, %
No superheating (Tmax=100 C), Type A cycle 0
Superheating to Tmax=120 C, Type B cycle
14
0
Superheating to Tmax=140 C, Type B cycle
13 0
Tmin=30 C
12
0
Tmin=20 C
11 10 0
Pmax - saturated pressure@100 C
9 0
0
Tmin=40 C
Pmin - saturated pressure @20, 30, 40 C
8 6
8
10
12
14
Pmax/Pmin
Fig. 10 e Comparison of ORC efficiency between Type A and Type B cycles e HFE7100
Fig. 11 e Comparison of ORC efficiency between Type A and Type C cycles e HFE7000
not only condensed to liquid in the condenser but also further cooled down to a temperature below the saturation temperature of the condenser, i.e. sub-cooling. The right column of Fig. 11 corresponds to the ORC efficiency of Type A cycle (no sub-cooling), while the left and the centre columns correspond to the ORC efficiencies of sub-cooling, from 40 to 20 C and from 40 to 30 C, respectively. From Fig. 11 we can see clearly that sub-cooling is detrimental to the ORC efficiency and should be minimised if possible. When the ORC fluid is sub-cooled in the condenser, more thermal input is needed to raise the ORC fluid to the state point 3 (Fig. 5), whereas the power output from the turbine is the same as the case of no sub-cooling, therefore a lower ORC efficiency is expected.
3.6.4. Electrical efficiency and overall thermal efficiency of the proposed CHP system With the assumptions described in Section 3.4, the electrical efficiency of the proposed CHP system is predicted to be between 7.5% and 13.5%, depending on the modelling conditions, the type of ORC (Type A, B, or C) and the ORC fluid (Tables 2e4), which correspond to 1.5e2.71 kW electricity output. The overall thermal efficiency of the proposed CHP system is predicted to be about 80%, which has little dependence on the ORC fluid or the modelling conditions. However, modelling results shown in Tables 2e4 do indicate the amount and quality of heating depend on the modelling conditions and the ORC fluid. As expected, the conditions and ORC fluids which are good for the electricity output would be bad for the heating both in terms of quantity and quality. Tables 2e4 show that the maximum hot water temperature for heating, which is assumed as 10 C below the inlet temperature of the ORC fluid to the condenser, varies greatly under the simulated conditions, ranging from 50.9 to 107.5 C. When the maximum hot water temperature is low (e.g. in order of 50 C), the heating may only be suitable for underfloor heating, whereas when the maximum hot water temperature is above 70 C, the heating can be carried out by
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 9 8 5 e3 9 9 4
traditional radiators. In addition, heat recuperating within the ORC should be considered if the expected maximum hot water temperature is higher than that required by the heating application. Heat recuperating within the ORC will improve the ORC efficiency and hence the electrical efficiency of the CHP system. The effect of heat recuperating within the ORC will be investigated in the near future via thermodynamic modelling and experimental investigation [23].
4.
Conclusions and future work
A 2 kW (e) biomass-fired CHP system with organic Rankine cycle has been proposed and thermodynamically modelled with three selected dry organic working fluids, namely HFE7000, HFE7100 and n-pentane. The following conclusions can be drawn from the results of thermodynamic modelling: under the simulated conditions, The ORC thermal efficiency with any selected ORC fluid is well below (roughly about 60% of) the Carnot cycle efficiency; The ORC efficiency depends on not only the modelling conditions but also the ORC fluid e the highest predicted ORC efficiency is 16.6% and the predicted ORC efficiency follows the following order: n-pentane > HFE7000 > HFE7100; Both superheating and sub-cooling are detrimental to the ORC efficiency; The electrical efficiency of the proposed 2 kW (e) biomassfired CHP system with the selected ORC fluids is predicted to be within the range of 7.5%e13.5%, mainly depending on the hot water temperature of the biomass boiler and the ORC condenser cooling water temperature as well as the ORC fluid, and corresponding to 1.5 kW and 2.71 kW electricity output; And the overall CHP efficiency of the proposed 2 kW (e) biomass-fired CHP system is in the order of 80% for all ORC fluids although the amount and quality of heating supplied by the CHP system depend on the ORC fluid selected and the modelling conditions. In order to improve the ORC efficiency, heat recuperating within the ORC should be considered, particularly when the inlet temperature of the ORC fluid to the condenser is much higher than that required by the heating application. Further thermodynamic modelling of the proposed system with heat recuperating within the ORC is planned for the near future [23]. Experimental investigation with the proposed CHP system is on going and the feasibility of the proposed CHP system has been experimentally proved [24]. Quantitative experimental results will be presented in the future and the thermodynamic modelling results reported in this paper will be compared with the experimental results.
Acknowledgement The authors wish to acknowledge the financial support from the UK EPSRC (EP/E020062/1) via ERA-NET Bioenergy and the
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UK Technology Strategy Board (TPQ3082A) for their research on ‘The Development of Biomass-Fired Micro-scale CHP’ at the Department of Architecture and Built Environment of the University of Nottingham. F-Chart Software (http:// www.fchart.com/) is acknowledged for the provision of EES and Dr. Guoquan Qiu is acknowledged for helping with the calculations of thermodynamic properties by means of EES.
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