Experimental study of R142b in low-temperature energy conversion system based on Organic Rankine Cycle

Wei Lili

(School of Civil and Transportation Engineering，Ningbo University of Technology，Ningbo 315016，China)

Experimental study of R142b in low-temperature energy conversion system based on Organic Rankine Cycle

Wei Lili

(School of Civil and Transportation Engineering，Ningbo University of Technology，Ningbo 315016，China)

Experimental investigation of a low-temperature energy-to-power conversion system was presented using R142b as the working fluid. Screw compressor worked in the reverse operation as the expander. The heat source in this experiment was lower than 80 ℃. The maximum energy conversion efficiency was 6% with the systematic efficiency up to 5.16%. Due to the high energy consumption in the transmission process，although the expansion efficiency was as high as 97% in maximum，the mechanical efficiency was only 48%-65%，which lead to a low systematic efficiency. The maximum exergy efficiency was 32%. Higher evaporating pressure and lower condensing pressure raised the power output，which followed the theoretical principle. Theoretically，the inlet of liquid had no advantage in the expansion. The qualitative experimental result shows that for R142b，inlet of liquid improves the expansion slightly. But this promotion comes mainly from the reduction of leakage，which means lower outlet temperature and pressure，larger enthalpy difference and higher expansion efficiency. The experiments prove the feasibility of this energy conversion concept. However，the systematic efficiency is still considerably low，and further optimization work is required.

Organic Rankine Cycle(ORC)；low-temperature heat；energy conversion；power；efficiency

1 Introduction

Fossil energy has been braced the global development over a long period of time，which is unsustainable. It also brings environmental problems，which is becoming one of the most important social problem in China[1]. The crisis of energy and environment asks for higher energy efficiency and rise of the market share of renewable energy.

Low-temperature heat-to power conversion system，which makes use of the low-grade waste heat and renewable energy，like solar energy and geothermal energy，has been widely used in metallurgy，cement，petroleum and other fields[2-4]. Rankine Cycle is conventionally used in power plant with water steam as the working fluid. For heat sources at moderate or low temperature，the best efficiency and highest power output is usually obtained by Organic Rankine Cycle(ORC)using a suitable organic working fluid. The use of ORC process for heat recovery has been applied for many years and the appearance is tested out on the market[5].

There are many efforts in this specific field，since it obviously is an efficient way to make use of low-temperature heat[6]. However，lots of the previous work focuses on the theoretical analysis and computational simulation，and the considered heat sources are generally at moderate temperature higher than 100 ℃. Its performance for heat source lower than 100℃ is still not fully understood in practical level.

In this study，experiments are carried out to investigate the energy conversion processes with R142b as the working fluid. Through the analysis of each process，the performance of each component is evaluated. The obtained encouraging results prove the feasibility of ORC on low-grade energy conversion. Further optimization is still needed to raise the efficiency.

2 Methods and materials

The layout of the system is shown in Fig.1. The working fluid，R142b，is evaporated by the thermal energy in the evaporator. The expander is driven by the vapor，converting thermal energy into mechanical energy，which is converted into electric energy by the generator. The exhaust vapor is directed to the condenser. The liquid-feeding pump compresses the sub-cooled liquid to the evaporator，and the thermodynamic cycle is repeated. InT-SandP-hdiagrams，the ideal cycle isa-b-c-d-e-aas in Fig.2.

Fig.1 Layout of an energy conversion system based on ORC

Fig.2 Low-temperature ORC cycle shown in t-S and P-h diagram

Experiments were carried out to investigate the expansion and operational principles. The energy converted during expansion is the enthalpy difference during expansion. For waste heat sources，the temperature of the organic vapor is strictly constricted，while saturated organic vapor has the highest pressure，and has more power output and higher energy conversion efficiency，as shown in Fig.3. Another reason is that the overheated vapor has lower density and smaller mass flow rate when the volume flow rate is constant. Experimental research also verifies the result. Preliminary experiments were carried out with plate heat exchanger. The overheat degree largely affect the power output，as shown in fig.4. Thus in the experimental device，saturated vapor is preferred. Flooded evaporator is designed accordingly. Since the organic liquid must be heated from the condensing temperature to the evaporating temperature in the evaporator，which is always over 50℃，a tube and shell heat exchanger is adopted to preheat the sub-cooled liquid before it enters the flooded evaporator. The pre-heater provides long flowing path for the liquid，and the large temperature rise can be completed. The superheat can be controlled through adjusting the liquid level and thus the heat exchange area for superheating. The areas of the pre-heater and the flooded evaporator are 4.7 and 11.43 m2respectively[7-8].

Fig.3 Theoretical association between degree of superheat and ideal converted energy(evaporating pressure and condensing pressure are 0.987 MPa and 0.289 MPa，volume flow rate of organic fluid is 1 m3/s)

Fig.4 Experimental influence of superheat degree on power output

During the expansion of saturated organic vapor，part of it can be condensed to liquid，which is strictly forbidden in speed type expander in case of wet stroke and its damage. Thus，a volume type screw expander is used in this experimental device. The expansion is acquired through the meshing of two gears and the speed is far lower compared to the traditional turbine. Liquid is allowed in its expansion process and it has long life.

The condenser provides low backpressure for the expansion，and condenses the outlet vapor into liquid for the circulating pump. The degree of sub-cooling should not be large incase it adds to the burden of the evaporator. Thus，the heat exchange area was properly designed for proper degree of super-cooling.

The organic fluid-feeding pump pressurizes the condensed liquid to repeat the thermodynamic cycle. Its selection basis is the pressure head and the volume flow rate.

Fig.5 presents a view of the energy conversion unit. The experiment is carried out in a national secondary class laboratory bench and the error is less than 3%.

Fig.5 view of energy conversion device

3 Thermodynamic analysis of the energy conversion processes

Through the evaporator，energy is transferred from the heat source to the organic fluid. The generated organic vapor，saturated or superheated，drives the expander，converting the thermal energy into mechanical energy. Part of the generated mechanical energy is consumed as the kinetic energy of the rotating components.

Accordingly，the efficiency of each step is defined asηh，ηexp，ηmandηgsuccessively. The aforesaid “energy output of the heat source” and the “thermal energy input of the organic fluid” are expressed respectively as equation(1)and equation(2). The efficiency of the heat exchange process isηh.

(1)

(2)

Wherehis henthalpy，kJ/kg；ηis efficiency；subscript o presents organic fluid；subscripts a and e present the status of the organic fluid.

The ideal energy converted in the expansion process is just the energy output of the organic vapor. It is expressed as

(3)

WhereWis power output，kW；subscript“ideal”presents the ideal process；subscript b present the status of the organic fluid.

However，it is impossible to be converted into mechanical energy 100%. The efficiency is the above-mentionedηexp. Thus，the mechanical energy is

Wexp=Widealηexp

(4)

Wheresubscriptexppresenttheexpansionprocess.

Therotatingpartsconsumepartofthemechanicalenergyanddecreasethemechanicalefficiencyintoηm. It varies with the rotating speed，the weight of the rotating parts and their weight distribution. The rest mechanical energy is converted by the generator with an efficiency ofηg，which is considered 90%，in accordance with the characteristic of the generator from the manufacture.

The energy conversion efficiency is one of the most important criteria in the system evaluation. Based on the first law of thermodynamics，it is expressed as

(6)

Where subscript con present the energy conversion；subscript g present the power generator；subscript m present mechanical.

All the energy consumed in the system is for the pump，to circulate the organic fluid. It is expressed asWp. Net power output is

Wnet=Wg-Wp

(7)

Astothesystematicefficiency，thewholesystemshouldbetakenintoconsideration.Thus，Wpshould also be included. Equation(8)shows the systematic efficiency，

(8)

Where subscript sys present systematic analysis.

Although the energy efficiencies provide evaluation of energy loss，the exergy loss，which is even more important in low-temperature thermal energy conversion，is neglected. Thus，efficiency based on the second law of thermodynamics is induced，as

(9)

WhereEis exergy，kW；Iis exergy loss，kW；subscript ex present exergy.

The goal of this energy conversion system is to explore as much the exergy in heat source，and convert it into useful high-quality energy. All the efficiencies are adopted to evaluate the performance of the system.

4 Results and discussion

Experimental results reveal the performance of the system. There are lots of factors that have impact on the energy conversion efficiency，theoretically and experimentally.

In chapter 2.2，the analysis shows saturated vapor performs the best and flooded evaporator is chosen to supply saturated vapor. The evaporating temperature affects the evaporating pressure，and thus the expansion process. Its influence based on a selected basic condensing temperature of 11℃ is shown in Fig.6. Higher evaporating temperature means more power output and larger energy conversion efficiency，theoretically and experimentally. The same trends are revealed in Table.1. On the other hand，it also confirms that the use of low-temperature energy is more difficult. The disparity between the ideal and experimental energy conversion efficiency comes from the irreversible loss and the energy consumption of the rotating components. To reduce the disparity and lower the energy loss，optimization of the system is needed.

Fig.6 Association between power output and evaporating temperature (condensing temperature is 11 ℃)

On the contrary，based on a certain evaporating temperature of 75℃，the influence of the condensing pressure is also investigated，as shown in Fig.7. Lower condensing temperature means lower expansion back pressure for the organic vapor，and larger enthalpy difference of the expansion process. Lowering the condensing temperature has the same consequence as increasing the evaporating temperature. Referring to Fig.2，it is the enthalpy difference between the expander inlet and outlet organic vapor that is converted to mechanical energy. Thus，the evaporating pressure and the condensing pressure together depends on the expansion process. Larger expansion pressure difference means larger enthalpy difference and more energy output，as Fig.1-2 and Fig.6-7 shows. Table 1 also elucidates the same trends clearly. The only difficulty is to find the cold enough coolant. The research of using liquefied natural gas(LNG)as the coolant is based on the same theoretical analysis[9-13]. The larger temperature difference between the evaporator and condenser is，the more power output and higher energy conversion efficiency is acquired.

Fig.7 Association between power output and the condensing temperature (evaporating temperature is 75 ℃)

Table 1 Analysis of overall energy conversion system

Referring to Table 1，the power output and the systematic efficiency increase as the evaporating temperature rises. Since the rotational kinetic energy is only determined by the mechanical parameters and rotating speed，which are relatively fixed，the mechanical efficiency is dependent on the generated mechanical energy. The part of the rotational energy decreases as the generated mechanical energy increases. For large energy conversion systems，the rotational kinetic energy takes only a small part，which is negligible，thus the mechanical efficiency tends to be 100%.

The expansion efficiency is affected by several factors，such as the pressure difference of the expansion，the leakage rate of the screw expander，the status of the organic vapor and the lubrication. It varies between 91%-97%. It is one of the most important factors in the energy conversion process.

The energy conversion efficiency evaluates the overall energy conversion process and is up to 6%. It is influenced by the systematic design，the characteristic of the expander，the inlet vapor status and so on. Taking the energy consumed by the pump into consideration，the systematic efficiency is 5.16% maximally，which is still considerably low. The promotion of the system is also constrained by low systematic efficiency.

The mass flow rate of the organic vapor is one of the critical factors. It is determined by the density and the volume flow rate together. Since screw expander is a volume-type machine，the volume flow rate of the organic vapor is determined by the mechanical parameters and the rotating speed. For a selected screw expander，to raise the rotating speed is one of the fundamental ways for more power output. For the expander in use，the volume flow rate changes linearly with the rotating speed，as shown in Fig.8. However，because of the leakage，the curve doesn’t pass through the origin. The leakage raises the backpressure， and reduces the energy output.

Fig.8 Relationship between rational speed of expander and flow rate of steam

Thus，to reduce the leakage is also an important issue in the mechanical and systematic research.

There is another aspect in this approach. Since the drive pulleys consume part of the generated mechanical energy，high rotating speed seems counteractive to the power output. Rotational kinetic energy increases exponentially with the rotating speed，as in Table 2. It varies with the weight and weight distribution of the rotating parts. Taking the two aspects into consideration，3 000 r/min is believed the most suitable rotating speed.

Table 2 Calculated kinetic energy of drive pulleys

Exergy efficiency takes not only the quantity of energy，but also the quality into consideration. In thermodynamics，exergy means the maximum useful work during a process that brings the system into equilibrium with a heat reservoir. Lower the temperature means less exergy. That is also a bottleneck of the low-temperature heat utilization. In this system，the exergetic efficiency is up to 32% as in Table 1. The exergy loss comes from the entropy increases and the exergy consumption of the rotational parts and the pump. Investigation of the exergy loss of each process and each component is one of the most efficient ways on optimization of the system. Further work needs to be done.

According to Austin[14]and Smith[15]et al.，wet inlet organic vapor raises the energy conversion efficiency. A dryness of 10%-15% is considered the best. However，according to the ideal analysis of the authors，the inlet liquid brings only extra sensible energy，with the volume flow rate of the vapor slightly decreased. According to its characteristic，the vapor of R142b keeps saturated during the expansion. Only the inlet sensible heat vaporizes a small amount of liquid. The qualitative influence of the dryness on the energy conversion efficiency is shown in Fig.9. According to the experiment，when a small quantity of liquid is imported into expander，the power output increases slightly. But the increase can not last. It falls down as the quantity of the liquid increases instantly. Since there is no lubrication in the experimental system，the imported liquid plays a role in sealing，which reduced the mechanical leakage，and the power efficiency is increased. Also，the reduction of leakage lowers the temperature of the exhaust vapor. Meanwhile the volume flow rate is also reduced. However，as the liquid continuously atlends the expansion，there are far more than the required sealing liquid，and the conversion efficiency falls accordingly because the liquid takes part of the volume flow rate. More research is needed for accurate quantitative results. Moreover，lubrication system should be added for the sake of higher expansion efficiency and lower mechanical wear. Furthermore，the sealing effect of the inlet liquid can be excluded，and more accurate results are expectable.

Fig.9 Qualitative influence of inlet dryness on energy conversion efficiency for R142b(evaporating temperature and condensing temperature is 70 ℃ and 20 ℃ respectively)

5 Conclusion

The experimental study of the low-temperature energy conversion system with R142b as the working organic fluid was conducted. Analysis reveals that a rotating speed of 3 000 r/min is most suitable for the expander. A conversion efficiency of 6.00% and a systematic efficiency of 5.16% are obtained. One of the most critical factors that lower the systematic efficiency is the mechanical efficiency，which is only between 48%-65%. The result is caused by the large mechanical energy loss，which is consumed by the rotational transmission. For large systems，the rotational kinetic energy is negligible and the mechanical energy can be substantially increased. The expansion efficiency is up to 97%，which also varies with the status of the inlet vapor，the mechanical precision of the screws，the leakage rate and the lubrication system and so on. The exergetic efficiency is also elucidated，since it is one of the most important criteria to evaluate the energy conversion system. It is up to 32%，and is closely related to the evaporating temperature，following the second law of thermodynamics.

The experimental results follow the theoretical principles well. Through analysis，saturated vapor performs best，thus the combination of pre-heater and flooded evaporator is adopted to supply saturated organic vapor for expansion. Higher evaporating pressure and lower condensing pressure raise the power output consistently. The total flow concept is also tested qualitatively. However，different principles are drawn. When a small amount of liquid is added，the leakage of the expander is reduced and the expansion efficiency is raised instantly. However，there is no visible contribution to the expansion. Proper lubrication system can also lower the leakage，and the effect of liquid can be examined alone. More work should be done on this aspect for more quantitative results.

The experiments give good guideline on the low-temperature thermal energy conversion system. The systematic efficiency is considerable，especially for large systems. This technology is feasible on utilization of low-grade waste heat and renewable solar and geothermal energy，with varied efficiency based on the quality of the energy sources. It also contributes to lower the environmental pollution. More work should be done on the optimization of the overall system to raise the energy conversion efficiency. The impact of wet expansion also needs to be studied furthermore.

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2016-01-23；

2016-07-02

魏莉莉，女，34歲，博士、講師。Wei Lili，female，34 years old，doctor and university lecturer in School of Civil and Transportation Engineering，Ningbo University of Technology in China.

TB653

A

1000-6516(2016)04-0061-08

以R142b為工質(zhì)的有機朗肯循環(huán)低溫發(fā)電系統(tǒng)研究

魏莉莉

(寧波工程學院建筑與交通工程學院 寧波 315016)

研究了以R142b為循環(huán)介質(zhì)、采用螺桿式膨脹機的低溫熱能有機朗肯循環(huán)發(fā)電系統(tǒng)。在熱源溫度低于80 ℃的條件下，膨脹機最大能量轉(zhuǎn)化效率為6%，系統(tǒng)總效率5.16%。在系統(tǒng)膨脹效率達97%的情況下，傳輸能耗導致機械效率僅有48%—65%，因此系統(tǒng)總效率較低，但最大效率為32%。提高蒸發(fā)壓力、降低冷凝壓力是提升能量轉(zhuǎn)化效率的根本途徑。實驗研究表明，降低膨脹機入口蒸汽干度對膨脹效率略有促進，主要由于少量液體參與膨脹減少了膨脹環(huán)節(jié)的滲漏，提高了膨脹效率。實驗表明該低溫熱能發(fā)電系統(tǒng)可行，但系統(tǒng)效率較低，有待進一步優(yōu)化提高。

有機朗肯循環(huán) 低溫熱源 能量轉(zhuǎn)化 發(fā)電 效率