INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2006; 30:1117–1129 Published online 16 May 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/er.1201
Study of solar energy powered transcritical cycle using supercritical carbon dioxide X. R. Zhang1,n,y, H. Yamaguchi1, K. Fujima2, M. Enomoto3 and N. Sawada4 1
Department of Mechanical Engineering, Doshisha University, Kyoto 630-0321, Japan Mayekawa MFG. Co., Ltd., 2000 Tatsuzawa Moriya-city, Ibaraki-Pref., 302-0118, Japan 3 Showa Denko K. K., 1-480, Inuzuka, Oyama-city, Tochigi 323-8679, Japan 4 Showa Tansan Co., Ltd., 7-1, Ogimachi, Kawasaki-Ku, Kawasaki-city, Kanagawa, 210-0867, Japan 2
SUMMARY In this paper, the performance of solar energy powered transcritical cycle using supercritical carbon dioxide for a combined electricity and heat generation, is studied experimentally. The experimental set-up consists of evacuated solar collectors, pressure relief valve, heat exchangers and CO2 feed pump. The pressure relief valve is used to simulate operation of a turbine and to complete the thermodynamic cycle. A complete effort was carried out to investigate the cycle performances not only in summer, but also in winter conditions. The results show that a reasonable thermodynamic efficiency can be obtained and COP for the overall outputs from the cycle is measured at 0.548 and 0.406, respectively, on a typical summer and winter day. The study shows the potential of the application of the solar energy powered cycle as a green power/ heat generation system. Copyright # 2006 John Wiley & Sons, Ltd. KEY WORDS:
solar energy; transcritical; carbon dioxide; thermodynamic cycle; power generation; heat collection
1. INTRODUCTION The expanding population and the energy crisis have brought serious problems to the world environment and sustainable development. During the last two decades a number of researchers have worked on developing new combined power/heat or power/refrigeration thermodynamic cycles or improving existing ones. For example, high-temperature solar thermal technologies (including single-axis and two-axis tracking technologies) have been studied by a number of research groups (Francia, 1968; Mills and Morrison, 1999; Schramek and Mills, 2000; Mills, 2004). In addition, in recent years, low-temperature technologies have also been developed and the process is more efficient than it used to be. For example, Kalina (1983) proposed the use of n
y
Correspondence to: X. R. Zhang, Department of Mechanical Engineering, Doshisha University, Kyoto 630-0321, Japan. E-mail:
[email protected]
Contract/grant sponsor: Ministry of Education, Culture, Sports, Science and Technology
Copyright # 2006 John Wiley & Sons, Ltd.
Received 5 August 2005 Revised 14 December 2005 Accepted 5 January 2006
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ammonia/water mixture as a working fluid to improve thermal power cycles. Goswami (1995, 1998) proposed a new thermodynamic cycle, which is an ammonia-based combined power/ refrigeration cycle. On the other hand, from the viewpoint of protecting the ozone layer and preventing global warming, now there is strong demand for technology based on ecologically safe ‘natural’ working fluids, i.e. fluids like water, air, noble gases, hydrocarbons, ammonia and carbon dioxide. The Kyoto protocol went into effect in the beginning of 2005. Based on the protocol, collective emissions of the greenhouse gases (an average over the five-year period of 2008–2012) will be reduced by 5.2% compared to the year 1990. It is a good way of reducing the emissions of greenhouse gases, such as CO2, methane, HFCs by recycling or recovering these discharged gases. So the interest in CO2 as working fluid increased considerably from 1990s, and a number of development and co-operation projects were initiated by the industry and the research sectors (Lorentzen, 1990; Lorentzen and Pettersen, 1992). CO2 is the non-flammable and non-toxic fluid and has no influence on personal safety (Kim et al., 2004). Its critical pressure and temperature are 7.38 MPa and 31.18C, respectively. Because of the low critical temperature, CO2 is suitable to be utilized as the working fluid in thermodynamic cycles with moderate temperature from about 30–2008C. In 2004, the thermodynamic cycle was proposed (Zhang et al., 2004)}solar energy powered thermodynamic cycle using supercritical CO2, in which both solar energy and CO2 are used in order to form a cogeneration system of heat and power with environmental preservation. Figure 1 shows a schematic diagram of the CO2-based thermodynamic cycle. Evacuated solar collector is used to heat CO2 contained in heating channels. The heating in the collector makes the temperature of supercritical CO2 to be high. (Figure 1, state 1). The high-temperature supercritical CO2 drives the engine of the thermodynamic system and output power is made available from the turbine generator. The lower-pressure CO2, which is expelled from the turbine, is cooled in the heat recovery system. At the outlet of the turbine, supercritical CO2 still has a higher temperature (Figure 1, state 2), which can be utilized as a heat source for absorption refrigerating machine, boiling water and other uses. These can be achieved in the
Solar energy 4
1 Evacuated solar collector
Turbine Feed pump
CO2
Electric power output
Heat recovery system
3
2
Thermal power output
Figure 1. Schematic diagram of solar energy powered thermodynamic cycle using CO2. Copyright # 2006 John Wiley & Sons, Ltd.
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heat recovery system. Alternatively, several heat recovery systems can be set up to achieve the heat collection at different temperatures simultaneously. The heat recovery system is actually a heat exchanger. After leaving the heat recovery system (Figure 1, state 3), CO2 is pumped by the feed pump into the higher-pressure condition (Figure 1, state 4), and then the cycle recommences. Based on the previous investigations, the basic performance of the CO2 cycle is still not known. So the objective of the present study is to investigate and understand the basic cycle performance by experimental work, not only in the summer, but also in the winter conditions.
2. EXPERIMENTAL RESEARCH Figure 2 shows a schematic diagram of the experimental facility designed, constructed and tested. The experimental facility is mainly comprised of solar collector arrays, a pressure relief valve, heat exchangers, liquid CO2 feed pump and cooling tower. In addition, a measurement and data-acquisition system is also included. The solar collector is the heart of the thermodynamic cycle. Its characteristics play an important role in the successful operation of such systems. In the present study, the all-glass evacuated solar collectors with a U-tube heat removal system are used. A sketch of the solar collectors is shown in Figure 3. These collectors consist of a glass envelope over a glass tube coated with a selective solar absorber coating. This coating with a high solar absorbance of 0.927 and a low emissivity of 0.193 is applied on the vacuum side of the inner glass tube. The absorbed heat is conducted through the inner glass tube wall and then removed by heat removal
Figure 2. Schematic diagram of the experimental set-up. Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 3. A sketch of the evacuated solar collector used in CO2-based cycle.
fluid in a metal U-tube inserted in the inner tube with a fin (thickness is 0.2 mm) connecting the outlet arm of the U-tube to the inner glass tube. Each U-shaped heat removal fluid tube used is 3.6 m long and 0.005 m internal diameter, as shown in Figure 3. Efficient area (the projection area for solar radiation, in this study, the efficient area equals to cross-sectional area of glass envelope) of the solar collectors used in the cycle is about 9.6 m2. Furthermore, the solar collector efficiency is defined to describe the performance of collecting heat using supercritical CO2 in the collector, which represents a ratio of the heat quantity absorbed into CO2 in the collector during a time period of t to the total solar energy striking the collector surface during the time Rt mC ðhso hsi Þ dt ð1Þ Zcollector ¼ 0 R t 0 IA dt where mC is the mass flow rate of CO2, hso ; hsi are the specific enthalpy value of CO2 at the outlet and inlet of the solar collector, I is the solar radiation striking the collector surface and A is the efficient area of the solar collector. Supercritical CO2 has physical properties somewhere between those of a liquid and a gas. So it is difficult to decide whether a turbine of a gas or a liquid type is used for the thermodynamic cycle. In other words, to date there is no turbine available for supercritical CO2. Therefore, in the experiment, the pressure relief valve was used, instead of a turbine, in order to study the cycle performance. The pressure relief valve can provide various extents of opening for the cycle loop in order to simulate the pressure drop occurring in realistic turbine condition, and consequently a thermodynamic cycle can be achieved. No electric power is produced in the test, but the basic cycle performance for the power/heat production can be known based on thermodynamic estimations, although we are aware that the outlet temperature and thermophysical properties are somewhat different between the true turbine condition and the present condition. In the present study, the following thermodynamic equation is used to Copyright # 2006 John Wiley & Sons, Ltd.
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calculate the power generation from the cycle Wpower ¼ mC Zgen ðhti hto Þ
ð2Þ
where hti ; hto are the specific enthalpy value of CO2 at the inlet and outlet of the pressure relief valve and Zgen is the generator efficiency. It should be mentioned here that the pressure relief valve and feed pump affect the CO2 mass flow rate. Using the pressure relief valve artificially reduces the CO2 flow rate in the loop. And the feed pump can be adjusted to increase the flow rate. So the CO2 flow rate under the turbine condition should be larger than that of the present condition. The power generation and other useful outputs in the true turbine condition would be larger than the values estimated in the present analysis. The two shell and tube heat exchangers are used for achieving simultaneous heat recovery of both higher and lower temperatures, with tube side of CO2 and shell side of water. The higherand lower-temperature waters are provided to the heat exchangers 1 and 2, respectively, to simulate the heat recovery processes. The total heat exchanger area utilized is about 0.76 m2. In the higher-temperature heat recovery system, a mechanical-draft water-cooling tower with a cooling capacity of 22 kW is used as a heat sink, dissipating the heat recovered from the cycle to the ambient. The following efficiencies are defined to describe the performance of the cycle: COPpower ¼
COPheat ¼
Wpower Qsolar
ð3Þ
Qrec Qsolar
ð4Þ
where COPpower is the power generation efficiency, COPheat is the heat recovery efficiency, Qrec is the heat quantity recovered from the cycle and Qsolar is the solar energy striking the collector surface. The heat quantity recovered from the cycle can be calculated as Qrec ¼ mC ðhto hpi Þ
ð5Þ
where hpi is the specific enthalpy value of CO2 at the pump inlet. A high-accuracy measurement and data-acquisition system is used to achieve real-time data measurement, data acquisition, and processing. Main measurements and accuracies are listed in Table I. Meteorological data, such as solar radiation, atmospheric temperature values, can be acquired through the meteorological instruments installed in the experiment system, as shown in Figure 2. Accuracies of Sun radiation sensor and air temperature gauge are 0.3% and 0:15 þ 0:0002jtj8C; respectively. In addition, five thermocouples and five pressure transmitters are mounted in the CO2 loop to measure the CO2 temperatures and pressures, with accuracy of 0.18C for the temperature measurements and 0.2% for the pressure measurements. Five platinum resistor temperature sensors are utilized to measure the water temperatures at the inlets and outlets of the heat exchangers with accuracy of 0:15 þ 0:0002jtj8C. The CO2 mass flow meter is mounted at the downstream of pump exit with an accuracy of 0.1% and two water flow meters are mounted in the heat recovery systems with an accuracy of 0.5%. Therefore, the accuracies of the cycle parameters defined in the above equations Copyright # 2006 John Wiley & Sons, Ltd.
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Table I. Main measurements and accuracies. Measurements
Accuracies
Sun radiation sensor Air temperature gauge Thermal couple (Measuring the CO2 temperatures) Pressure transmitters (Measuring the CO2 pressures) Platinum resistor temperature sensors (Measuring the water temperatures) CO2 mass flow meter Water flow meter
0.3% 0.15+0.0002|t|8C 0.18C 0.2% 0.15+0.0002|t|8C 0.1% 0.5%
(a)
40 35
0.6
30
0.4
25 20
0.2 0.0 5:00
15 7:00
9:00
10 11:00 13:00 15:00 17:00 19:00 35
Solar radiation (kW/m2)
1.0
30
0.8
Air temperature
0.6 Solar radiation
25 20 15
0.4
10 0.2 0.0 5:00
Air temperature (°c)
0.8
45 Air temperature Solar radiation
Air temperature (°c)
Solar radiation (kW/m2)
1.0
5 7:00
(b)
9:00
0 11:00 13:00 15:00 17:00 19:00 Time (hh:mm)
Figure 4. Variations of solar radiation and air temperature measured with time: (a) summer; and (b) winter.
are calculated to be less than 1.0%. The measuring points of all the sensors are shown in Figure 2. During a typical experiment, first the water pumps are turned on and the rates of incoming water flows in the heat recovery systems are adjusted. The CO2 pump is switched on and the Copyright # 2006 John Wiley & Sons, Ltd.
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opening of pressure relief valve is adjusted to the expected extent. When finishing the test, the CO2 pump is turned off only after the pressure relief valve is adjusted to a full open state. Finally, the water pumps are turned off. During the test, temperatures, pressures, etc. attached to the thermodynamic cycle are measured and transported through the computerized dataacquisition equipment to record data as a function of time. Thermal insulation coating is installed for CO2 and water loops to reduce heat losses from piping.
3. RESULTS AND DISCUSSION A total amount of about 6.0 kg of CO2 was charged into the CO2 loop of the experimental facility. During the test time, the inlet water temperature and flow rate are, respectively, controlled at 30.08C and 830.0 l h1 (0.230 kg s1) for the heat exchanger 1 and at 9.08C and 200.0 l h1 (0.056 kg s1) for the heat exchanger 2.
1.2 Collector outlet temperature Relief valve outlet temperature
1.0
150 0.8 100
0.6
CO2 flow rate Pump inlet temperature Pump outlet temperature
0.4
50
CO2 flow rate (kg/min)
CO2 temperature (˚c)
200
0.2 0 10:00
11:00
12:00
13:00
14:00
15:00
16:00
0.0 17:00
200
1.2
150
1.0 Collector outlet temperature Relief valve outlet temperature 0.8 0.6
100
0 9:00
0.4
CO2 flow rate Pump inlet temperature Pump outlet temperature
50
10:00
(b)
11:00
12:00 13:00 Time (hh:mm)
0.2
14:00
15:00
CO2 flow rate (kg/min)
CO2 temperature (˚c)
(a)
0.0 16:00
Figure 5. The measured CO2 temperatures and flow rate in the thermodynamic cycle loop: (a) summer; and (b) winter. Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 4 shows the solar radiation and air temperature measured as a function of time. Summer and winter weathers were selected, both of which are not so sunny, not so cloudy, and represent typical summer and winter weather conditions. The experimental test was carried out, respectively, in the summer and winter. It can be seen from Figure 4, that the solar radiation increases with time until it reaches a maximum value, and then, the solar radiation begins to decrease with time. The time-averaged solar radiation and air temperature are about 0.58 kW m2 and 36.38C in the summer, and 0.30 kW m2 and about 17.08C in the winter, respectively. Figure 5 shows the measured CO2 temperatures in the thermodynamic cycle loop, in which the measured CO2 flow rate is also included. During the test hours, the pressure relief valve was adjusted to a state of half-close. It can be seen that the CO2 flow rate achieved in the cycle loop is about 0.7 and 0.5 kg min1, in the summer and winter conditions, respectively, which is kept throughout most of the daylight time. Almost throughout the test there is no abrupt change in the CO2 flow rate with the solar radiation variation. It can also be seen that the CO2 temperature at the outlet of the solar collector increases with elapsed time, and at noon, the CO2 temperature reaches up to about 180.0 and 140.08C in the summer and winter days, respectively. After that, the temperature tends to drop gradually with time, which may be due to the natural decrease of the solar radiation and air temperature. The averaged temperature at the collector outlet is about 160.08C for the summer day and 120.08C for the winter day. The result is encouraging since such a high temperature achieved even in the winter day makes it easy to
3.500 Solar radiation per hour
3.000
Heat quantity collected per hour
MJ/m2h
2.500 2.000 1.500 1.000 0.500
MJ/m2h
(a)
(b)
0.000 2.000 1.800 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000
10:00
11:00
12:00
13:00
14:00
15:00
16:00 17:00
Solar radiation per hour Heat quantity collected per hour
9:00
10:00 11:00 12:00 13:00 14:00 15:00 16:00 Time (hh:mm)
Figure 6. The amount of solar radiation and heat quantity collected in the solar collector per area and per hour: (a) summer; and (b) winter. Copyright # 2006 John Wiley & Sons, Ltd.
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collect thermal energy from the cycle and achieve power generation in the turbine. Furthermore, the time-averaged CO2 temperature at the outlet of the pressure relief valve is about 135.0 and 98.08C, respectively, a relatively high temperature, and such a heat source can be utilized for boiling water, air conditioning, etc. Figure 6 shows the calculated solar energy striking the collector and the heat quantity absorbed by CO2 in the collector per collector area and per hour based on the measured data. The thermodynamic and transport properties of CO2 are calculated based on the measured temperatures and pressures using a Program Package for Thermophysical Properties of Fluids database version 12.1 (PROPATH 12.1). It can be seen that the variation of the heat quantity absorbed by CO2 in the collector with time is similar to the variation of the amount of solar radiation. During the time period, the averaged amount of solar radiation is about 1:98 MJ m2 h1 ; the heat quantity collected in the collector is about 1:40 MJ m2 h1 for the summer day and 1.17 and 0.71 MJ m2 h1 for the winter day. Average collector efficiency is estimated at 63.0 and 52.0%, respectively. The result shows that even in the winter day, the collector used is also effective in collecting heat using supercritical CO2, which may explain to a certain extent why a relatively high temperature of 160.0 and 120.08C are achieved at the collector outlet for the summer and winter days, respectively. 10 Collector outlet pressure(state 1) Pump outlet pressure(state 4)
CO2 pressure (MPa)
9 8 7 Relief valve outlet pressure(state 2)
6 Pump inlet pressure(state 3)
(a)
5 10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
CO2 pressure (MPa)
8
7
Collector outlet pressure(state 1) Pump outlet pressure(state 4)
6
Relief valve outlet pressure(state 2)
5 Pump inlet pressure(state 3)
4 9:00
10:00
(b)
11:00
12:00 13:00 Time (hh:mm)
14:00
15:00
16:00
Figure 7. Variations of CO2 pressures measured with time in the cycle loop: (a) summer; and (b) winter. Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 7 shows the CO2 pressures measured during the test hours in the cycle loop. It can be seen that there appears a large pressure difference (about 2.0 MPa) between states 1, 4 and states 2, 3. The supercritical high-pressure side and subcritical low-pressure side of the CO2-based cycle can be obviously observed in this figure. The high pressures reach up to 8.0 MPa and low pressures are about 6.0 MPa. The results show that almost throughout the test, the CO2-based prototype works in a transcritical cycle. Furthermore, the useful energy outputs and the efficiencies are shown in Figure 8. The thermodynamic cycles are shown in Figure 9 in the p–h diagram, in which the time-averaged CO2 temperatures and pressures during the test are calculated and the thermodynamic cycles based on the average levels of the temperatures and pressures are drawn. From the results, it can be seen that Wpower and Qrecovery obtained from the CO2-based cycle are found to be relatively stable throughout the test hours. No abrupt variations of the useful outputs with time occur even in the cloudy time, which is very different from the output of solar cell. The time-averaged Wpower is estimated at 0.29 and 0.18 kW for the summer and winter day, respectively. The time-averaged Qrecovery is estimated at 4.16 and 1.23 kW, respectively. The time-averaged COPpower is found to be 0.063 and 0.053, and COPheat is 0.485 and 0.353. The total efficiency is estimated to be about 0.548 and 0.406, respectively. It is mentioned here that
6
1.0 Heat recovery
Heat recovery efficiency
0.8 0.6
3 0.4 2 1
Power generation Power generation efficiency
0 (a) 10:00
11:00
12:00
13:00
14:00
0.2
15:00
16:00
0.0 17:00 0.8
3 Heat recovery efficiency
Useful output(kW)
Efficiency
4
2
0.6
Heat recovery
0.4
Efficiency
Useful output (W)
5
1 Power generation
0 10:00
11:00
12:00
(b)
Power generation efficiency
13:00 14:00 Time (hh:mm)
15:00
0.2
16:00
0.0 17:00
Figure 8. Variations of the useful cycle outputs and efficiencies: (a) summer; and (b) winter. Copyright # 2006 John Wiley & Sons, Ltd.
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Figure 9. The CO2-based thermodynamic cycle based on the time-averaged parameters measured during the test: (a) summer; and (b) winter.
the useful outputs estimated in the present study represent the minimal values, because the CO2 flow rate in the true turbine condition is larger than that in the present condition and more heat can be collected in the collector. In Figure 9, the p–h diagrams clearly show that the transcritical CO2 cycles were achieved in the test. It is also noted that the power consumption of the liquid CO2 feed pump is not considered in the above efficiencies, because the pump used is originally designed for water. If the power consumption is considered, COPpower would drop further. Therefore, a higher efficiency of CO2 feed pump or solar energy powered pump will be considered to reduce the power consumption. It is also seen from Figure 8 that COPheat is much higher than COPpower, and therefore, to a certain extent, the benefit obtained from the Copyright # 2006 John Wiley & Sons, Ltd.
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CO2-based cycle also largely stems from the reasonable utilizations of the recovered heat quantity. 4. CONCLUDING REMARKS Experimental investigations have been carried out for the solar energy powered transcritical cycle using supercritical CO2. A pressure relief valve was used to complete the thermodynamic cycle instead of a turbine. The results show that supercritical CO2 can effectively collect heat in the evacuated solar collector and even in the winter day the CO2 temperature in the collector can reach about 140.08C. And the CO2-based thermodynamic cycle works in the transcritical region throughout most of the test hours. The thermodynamic analyses based on the measured data show that the CO2-based cycle can achieve stable outputs of power and heat with a reasonable thermodynamic efficiency of COPpower ¼ 0:053 and COPheat ¼ 0:353 for the winter day. The study shows the potential of the application of the CO2-based cycle powered by solar energy. A further investigation is needed to understand the natures for the flow and heat transfer of supercritical CO2 in the evacuated solar collector, and optimize the cycle for maximum electricity, heat power outputs, etc. And a turbine condition is needed to further investigate the CO2-based cycle. NOMENCLATURE A COPpower COPheat h I m Qrec Qsolar t Wpower
=efficient area of the solar collector (m2) =power generation efficiency =heat recovery efficiency =specific enthalpy value of CO2 (kJ kg1) =solar radiation striking the collector surface (kW m2) =mass flow rate of CO2 (kg s1) =heat quantity recovered from the cycle (W) =solar energy striking the collector surface, Qsolar ¼ IA (W) =time (s) =power generation (W)
Greek letters Zcollector Zgen
=collector efficiency =generator efficiency, a value of 0.95 is used in the paper
Subscripts C pi si so ti to
=carbon dioxide =pump inlet =collector inlet =collector outlet =inlet of the pressure relief valve =outlet of the pressure relief valve
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ACKNOWLEDGEMENTS
This study was supported by the Academic Frontier Research Project on ‘Next Generation Zero-Emission Energy Conversion System’ of Ministry of Education, Culture, Sports, Science and Technology, Japan.
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