J. Cent. South Univ. (2012) 19: 1590-1599

DOI: 10.1007/s11771-012-1181-4

Experimental assessment of two-phase bubble pump for solar water heating

CHUNG Han-shik¹, WOO Ju-sik², SHIN Yong-han², KIM Jun-hyo³, JEONG Hyo-min¹

1. Institute of Marine Industry, Department of Mechanical and Precision Engineering,

Gyeongsang National University, 445 Inpyeong Dong, Tongyeong, Gyeongsangnamdo, 650-160, Korea;

2. Department of Mechanical and Precision Engineering, Gyeongsang National University,

445 Inpyeong Dong, Tongyeong, Gyeongsangnamdo, 650-160, Korea;

3. Division of Marine Engineering, Mokpo National Maritime University, Haeyangdaehang-Ro 91,

Mokpo-si, Jeollanam-do, 530-729, Korea

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The research goal is to develop a new solar water heater system (SWHS) that uses a solar bubble pump instead of an electric pump. The pump is powered by the steam produced from an evacuated tube collector. Therefore, heat could be transferred downward from the collector to a hot water storage tank. The designed system consists of two sets of heat-pipe evacuated tube collectors, a solar bubble pump installed at an upper level and a water storage tank with a heat exchanger at a lower level. Discharge heads of 1 and 5 m were tested. The bubble pump could operate at the collector temperature of about 90-100 ℃ and vapor gage pressure of 80-90 kPa. It is found that water circulation within the SWHS depends on the incident solar intensity and system discharge head. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. The thermal performance of the proposed system is compared with conventional solar water heaters. The results show that the proposed system achieves system characteristic efficiency of 10% higher than that of the conventional systems using electric pump if taking the consumption of electric power into account. And the former is a zero carbon system.

Key words: two-phase bubble pump; solar water heater system (SWHS); zero carbon

1 Introduction

Nowadays, solar energy is becoming an important alternative resource of renewable energy, while consuming fossil fuel severely leads to the problem of global warming.

Solar water heater has been employed around the world in most recent years. A solar water heater system (SWHS) is an example using solar energy. The water circulations in the SWHS are divided into two methods [1-2]: a thermosyphon method (passive circulation) and a force circulation method (active circulation). The thermosyphon method [3-4] uses natural convection to circulate water between a solar collector and a storage tank (ST), which is placed higher than the collector. The second one is force circulation system, where a water ST is installed at a lower level and an electric pump is used to circulate water between a ST and a solar collector. The advantage of this system is that it adds less weight to a building roof. However, this system does not save electrical energy [5-7].

SUMATHY [8] did experimental studies on a solar thermal water pump, which was composed of a 1 m² solar collector and had an overall efficiency of 0.12%- 0.14% for a discharge head between 6 m and 10 m and performed 12-23 cycles/d. A water mass of 15 kg was lifted in one cycle. WONG and SUMATHY [9-10] presented performances of a solar water pump using ethyl ether as working fluid and a thermodynamic analysis, together with optimization of the solar thermal water pump. PICKEN et al [11] investigated development of a water piston solar powered steam pump. LIENGJINDATHAWORN et al [12] presented parametric studies of a pulsating-steam water pump. NATTHAPHON and PICHAI [13] proposed solar water pump SWHS using water as a working fluid. WONG and SUMATHY [14] reviewed more details of solar water pumps.

The goal of this work is to develop a new water circulation system by using a solar bubble pump, instead of an electric pump in a SWHS, in order to decrease the system weight on a roof and save electrical energy. An analysis of the economy was also performed.

2 Experimental setup

The new bubble pump SWHS consists of three main parts, as shown in Fig. 1.

Fig. 1 Bubble pump solar water heater system: 1-Bubble pump; 2-HEC; 3-Heat exchanger; 4-ST; 5-Warm water supplements; 6-Water valve; 7-Pipe

1) A heat pipe evacuated tube solar collector (HEC) (two sets) having a total area of 2.61 m² and 45° inclination.

2) A bubble pump (BP), made of stainless-steel and rubber, having a height of 1.0 m and a diameter of 0.2 m. It includes three main parts: a separator, a condenser and an expansion chamber. The condenser has four tubes with two connected to the HEC and the other two connected to the heat exchanger in the ST. The structure of bubble pump is shown in Fig. 2.

3) A hot water storage tank (ST), which is a plastic bucket having a volume of 0.2 m³ and covered with insulation material. There is a heat exchanger installed within it. It is placed at a lower level of 1 m and 5 m under the solar collector.

All equipments were connected to a pipe in which hot water could flow within a closed loop. It was concluded that the bubble pump help make a auto pumping and circulating SWHS. The experiment was tested for discharge heads of 1 m and 5 m. The discharge head is the difference between the average levels of HEC and ST. All data were collected at the Department of Mechanical and Precision Engineering, Gyeongsang National University Tongyeong campus, Korea.

Fig. 2 Schematic diagram of bubble pump: 1-Expansion chamber; 2-Separator; 3-Condenser; 4-Spring; 5-Expansion bag; 6-Tube connected to inlet of heat exchanger; 7-Tube connected to outlet of heat exchanger; 8-Tube connected to outlet of HEC; 9-Tube connected to inlet of HEC; 10-Valve; 11-Vapor transporter pipe

The pressure inside the bubble pump was measured by two pressure transducers (4045-A piezoresistive pressure sensors) with accuracy of ±0.05% connected to the separator and condenser. Seven sets of T-type thermocouple were used to measure temperature of the surrounding air, water, and vapor at the HEC, the bubble pump, and the ST with accuracy of ±0.5 ℃. A DAQ32 Yokogawa data acquisition device was used to collect the temperature and pressure data. OVAL MK2 CN006-type mass flow meter was installed at the inlet of heat exchanger to measure the mass flow rate in the system. Solar irradiation was measured with photograph of insolation sensor with accuracy of ±0.2%. The device of this work is shown in Fig. 3.

3 System operation

The bubble pump must give the desired pump discharge (mass flow rate of pumped liquid) at the rated heat input. The important geometrical parameters, which govern the bubble pump behavior, are the driving head, the pump lift and the pump tube diameter. The bubble pump operates most efficiently in the slug flow regime and should operate at its maximum liquid flow rate. If the liquid flow rate needs to increase or decrease, then the diameter and vapor flow rate of the pump will be chosen such that this liquid flow rate is the maximum.

An operation of the new bubble pump SWHS was explained here in detail. The general operation of this system can be explained in several periods of working fluid (water): heating and pumping, separating and condensing, heat exchanging, and reheating, as shown in Fig. 1 and Fig. 2.

Fig. 3 Photographs of experimental setup (a) and bubble pump (b)

3.1 Heating and pumping period

Water as a working fluid is charged into the whole system from the valve in bubble pump. Before heating, HEC, heat exchanger and ST are full of cold water and bubble pump is empty. The system is a close loop after turning off the charging valve. In this stage, water in the HEC is heated by solar energy. The heating period continues until the temperature in the collector is high enough to boil the water. The pumping period starts when enough bubbles are generated. At this time, the mixture of vapor bubbles and hot water together move up to the separator of bubble.

3.2 Separating and condensing period

When the mixture of bubbles and hot liquid water arrives at the separator, bubbles break to the upper part and the liquid flows to the lower part of the separator. With the accumulating of vapor in the separator, the connected expansion bag is blown up and the spring is compressed. Meanwhile, a portion of vapor is pressed through pipe 18 to the condenser which has a lower temperature and is condensed to liquid there.

3.3 Heating exchanging reheating period

Hot water in the separator moves downward through a connecting pipe to the heat exchanger inside of the ST and transfers heat to warm water in the tank. During this process, water in the heat exchanger tube can be pushed by the high pressure in the separator to the condenser in bubble pump. The water in condenser is then supplied to HEC passing through a pipe to be reheated by solar energy.

4 System analysis

Thermal energy stored in the ST can be calculated as

                  (1)

where Q_s is the heat transferred from the heat exchanger to water in the ST, dt is time period, m_w,s is the mass of water in the ST, c_p,w is the water specific heat, and ?T_s is the rise in the water temperature in the ST. The daily system thermal efficiency η_t is defined as the ratio of daily thermal energy stored in the ST to the total solar irradiation incident on the collector:

                         (2)

or

                           (3)

where A_c is the collector area and I_T is the solar irradiance. H_tot is the total solar irradiation incident on the collector.

The daily pump efficiency is given by

                            (4)

where N is the number of water circulating cycles per day, and W_h is the required hydraulic energy per cycle, expressed by

W_h=V_cρ_wgh                                  (5)

where V_c is the volume of the pumped water per cycle, ρ_w is the water density, g is he acceleration of gravity, and h is the discharge head of the system.

The cumulative efficiency of the pump, η_pi, is defined as

                             (6)

where E_h is the cumulative hydraulic energy used by the pump and E_s the cumulative solar energy incident on the collector area [15-16].

Additionally, critical solar irradiance for the thermal water pump is introduced. The critical value is the solar energy threshold supplied to the pump so that the pump can start pumping water.

5 Results and discussion

In order to give the pertinence inspection of the performance of bubble pump, both short-term test and long-term test are analyzed. Due to the different working condition, this work also gives the comparison results of summer and winter. Necessarily, the comparison of bubble pump system and electric pump system will also be given in this section.

5.1 Short-term result and result analysis

Short-term analysis is specifically detecting a 29 h result from summer, from 2:00 am on 5th April 2007 until 7:00 am in the second day. The properties include the temperature of environment, bubble pump’s separator and water storage tank, because the environment temperature influences the performance of system and separator temperature is the first respond character when the solar energy is absorbed. The results are shown in Figs. 4-9. The pressures of separator and condenser are checked because they are the critical proprieties of bubble pump. Also, insolation and mass flow rate are detected. All the properties are recorded every 10 s. From Fig. 4, it can be seen that insolation starts to increase slowly from around 4:30 am but sharply increase at about 6:30 am when the sun-rise. The peak value is achieved at around 13:00 and after this it starts to decrease until a sharp decrease when the solar angle is 0° around 16:30. After the sun-set at around 17:30, the insolation is nearly zero. During the daytime, there are several low points as a result of the cloud. As the insolation increases, electromagnetic energy is absorbed by solar collector and transforms into the heat energy of water. This firstly increases the temperature of separator, which can be seen from Fig. 8, and the temperature increases from 20 ℃ to 100 ℃ within 1 h.

Fig. 4 Solar radiation intensity results of one day

Fig. 5 Temperature results from water storage tank and environment

Fig. 6 Temperatures from water storage tank and solar radiation intensity results

Fig. 7 Temperature results from separator storage tank and environment

Fig. 8 Temperature from separator and solar radiation intensity results

During this time, an over flow resulted from the volume increase of working fluid first happens. This spillage brings the initial circulation. As the continually absorbed energy, the temperature achieves boiling condition, then a higher frequency overflow in separator occurs, and the circulation tends to be stable. This state sustains around 5 h. During this period, temperature in separator is constant below 100 ℃ but with small fluctuation, as shown in Fig. 9.

Fig. 9 Temperature from separator and mass flow rate results

Along with the rise of solar radiation intensity, as shown in Fig. 5, the water temperature in storage tank increases from 25 ℃ to 60 ℃. With the decrease of solar radiation intensity, the water temperature decreases correspondingly, but the decline slope in graph depends on the adiabatic condition. The better the adiabatic condition of storage tank, the smaller the declined slope. Peak temperature happens together with the peak solar radiation intensity. Environment temperature sensor is located beside the storage tank. It has an approximate value with room temperature and city water temperature.

Three temperature values which are environment temperature, water storage tank temperature and separator temperature are compared in Fig. 7. After the temperature in separator becomes steady, water tank temperature starts to increase and the peak temperature of water tank occurs on the top of a steady condition in separator end. The peak value of water tank temperature will increase and delay a spell depending on the adiabatic status. During daytime, the separator temperature is higher than room environment temperature and water tank temperature, but at night it is about 5 ℃ lower since the separator is outside.

Figures 10, 11 and 12 indicate the pressure change during one day. The pressure in the bubble pump at separator and condenser part is shown in Fig. 10. Both separator and condenser pressures are waving in a small domain of 81-99 kPa and 74-88 kPa, respectively.

The pressure increases as the energy is absorbed in collector, and there is an overshot when the boiling  starts. After an upward shot, the working fluid starts to circulate and the expansibility inside bubble pump is abated by the drawing back cold water from storage tank. From this time, the pumping action intermittently happens steadily. At around 13:00, there is a rock bottom of pressure, which is resulted from the cloudy reduce of solar radiation intensity. On account of lower energy input, water temperature in collector decreases, and since a sealed loop for the medium, the pressure decreases at this time accordingly. There is no larger fluctuant at night. Comparing the waving domain with separator and condenser, pressure difference of them has a relatively smaller range of 6-11 kPa, as shown in Fig. 12.

Fig. 10 Pressure results from separator and condenser of bubble pump (p_a is atmospheric pressure, 101 325 Pa)

Fig. 11 Solar radiation intensity and pressure difference results

This parameter is directly related with the solar radiation intensity, as shown in Fig. 11 and Fig. 12. It inspects a whole day process of circulation in the close loop. When the solar radiation intensity increases in the morning, pressure difference fluctuates between two parallel values around 8 and 95 kPa. During this time, every single fluctuation represents one time circulation, the rebound in fluctuate is resulted from the energy input by sun and after a water drawback it reduces again. The appearance of pressure difference is different from morning to afternoon. The value also fluctuates but with a mean value decreasing. In every single fluctuate, the pressure difference rebounds to relatively lower level with a decreasing energy input which is enough for boiling but not enough to achieve the last status since the solar radiation intensity reduces.

Fig. 12 Pressure difference and mass flow rate results

These phenomena sustain until the circulation stops and another rebound of pressure difference occurs since there is still a solar energy input. The temperature still increases at a low level in separator. After the energy input is zero, pressure difference keeps rebounding lentamente until the water temperatures in collector and bubble pump are equal. Pressure difference only depends on the location difference of them, mainly the structure of bubble pump and the initial system setup.

Anyhow, the pressure difference together with the potential energy difference provides the power to circulate the water in this close loop, and performs the heat transfer between collector at a high level and heat exchanger locates lower without any force from electric device. The intention of applied bubble pump in solar indirect active hot water system is fulfilled.

Mass flow rate, as an inspect parameter of working medium circulation status, is given in Figs. 12-14. The value of flow rate based on time is quite fluctuating. Every single circulation between bubble pump and solar heat exchanger brings a bubble in the conduit between them and induces a small negative value, but it is helpful to the entire heat transfer because the convertion of flow direction provides a second chance for the heat exchange. The average value of flow rate should be calculated with the absolute value from experiment and it is about   0.55 kg/min, as shown in Fig. 13. When the solar radiation intensity increases, the boiling happens more frequently and it results in the more frequent circulation of water, which indicates the increase of mass flow rate. After comparing Fig. 12 with Fig. 13, the response gradation of every parameter should be noticed. After the increase of solar radiation intensity, firstly the temperature of separator increases and the pressure increases at the meantime. When the temperature in separator arrives a required level, the circulation starts and the mass flow rate displays a non-zero result. At last, the temperature of water tank starts to increase. Each reaction occurs at different moments and the time slot shows the energy transfer efficiency.

Fig. 13 Mean value of mass flow rate

Fig. 14 Solar radiation intensity and mass flow results

The temperature difference indicates the energy absorbency of the system, as shown in Fig. 15. Every vertical line between two temperature lines represents a saved energy at that time. Before the peak point, the length of line increases along the time and decrease after the peak, and the decreasing rate depends on the adiabatic condition. Under an ideal adiabatic state, the peak temperature will be high and will delay until the mass flow rate equals zero. However, it should be aware that the circulation stops earlier, then the vertical line between two temperatures from Fig. 15 equals zero, and certainly will reflect an energy lose. Moreover, this is restricted by the inherent property of bubble pump system and it is avoided less. Nevertheless, compared with the whole day’s condition, this amount is limited, and with a good heat preservation instrument, this limited value can still be reduced. This short-term result describes one-day-running detail and the similar circulation happens on other days with similar intraday weather.

Fig. 15 Temperature from storage tank and environment

5.2 Long-term result and analysis of system

Since the winter running performance in the testing area is concerned more than other season, the long-term analysis will be based on the experiment results from winter. Figure 16 shows one-month radiation intensity results. The average value is about 0.4 kW/m² at daytime. But from Fig. 16, it can be seen that the storage tank temperature can achieve 50 ℃ if the bubble pump is running well. This proves the ability of bubble pump solar hot water system applied in experimental location.

From Fig. 16, it can be concluded that the separator temperature is not only related with solar radiation intensity. There is an influence from another factor, which can be found from Fig. 17.

Nevertheless, it should be noticed that from Fig. 16 and Fig. 17, when outdoor temperature is below zero, the solar radiation intensity is still normal, and there is no circulation in the close loop because the water in connecting tube between heat exchanger and bubble pump is frozen.

Fig. 16 Contrast of solar radiation intensity and separator temperature: (a) Radiation intensity; (b) Separator temperature

Fig. 17 Pressure fluctuation for long term experiment in winter: 1-Seperater pressure; 2-Condensor pressure; 3-Inlet temperature of heatexchanger; 4-Zero Celsius line

This detail can be found from Fig. 18. At this time, pressure sensor shows an over scale value of -3.337 because the circulation is choked by the frozen ice inside of tube. The temperature in separator is very low, and since the heat exchanger, solar collector together with bubble pump compose a close loop, the pressure will decline together with temperature. When it is lower than the minimum scale, a minus error value will display.

Fig. 18 Magnify of pressure over shoot detail: 1-Separater pressure; 2-Condensor pressure; 3-Inlet temperature of heatexchanger; 4-Zero Celsius line

Surveying the whole winter results, it can be concluded that both the temperature and solar radiation intensity influence the performance of bubble pump. Compared with summer, the influence from outdoor temperature in winter takes more account. However, this influence can be reduced by the improved heat preservation method. And since the lowest outdoor temperature is only around -2 ℃ and rarely happens, it is absolutely can be redeemed by a good heat preservation measure.

5.3 Comparison of summer and winter running condition

Bubble pump solar water heater works based on the sun. Naturally, the season difference of processing exists. The figures show that in summer the solar radiation intensity is higher than that in autumn and winter. Correspondingly, the temperature gained is higher. In summer, the water tank temperature can arrive 80 ℃, but during winter it is only 50 ℃. Autumn is better than winter, and the highest temperature is around 60 ℃. Surely, the influence of surrounding temperature should not be neglected.

5.4 Comparison of bubble pump system and electric pump system

The experiment result from bubble pump solar hot water system is compared with an electric system result. The comparison is based on semi-experiment and semi-calculation. The results are not meticulous but have large significance.

The electric pump system has the same component with bubble pump system besides an electric pump and control panel. Control panel powered by electricity has two temperature sensors located on the outlet and inlet of heat exchanger. If temperature of inlet is higher than that of outlet, then the electric pump runs. If not, the pump stops.

The research imposes two values, solar radiation intensity and temperature difference between inlet and outlet of heat exchanger, to simulate the processing of electric pump. No matter a bubble pump system or an electric pump system, the final determination is energy source of sun. The control panel in an electric pump can be replaced by an insolation meter in theory in any means, but the different thing is that there is a delay in the temperature increase from insolation, and the temperature difference resulted from bubble pump system has a small difference because some absorbed energy is used to circulate working fluid. So, here two items are used together to simulate electric pump, as shown in Fig. 19.

But since the performance of bubble pump is not all well on the whole testing time, the simulation based on solar radiation is more accurate.

The capacity of electric pump for domestic hot water is usually 600-100 W, with similar collector size, and in the same working duration with bubble pump,

Fig. 19 Calculating result with solar radiation intensity and temperature: (a), (b) Temperature; (c), (d) Solar radiation intensity

electric pump needs to work for around 80-90 h in 22 d. It requires about 480-900 kW·h electric power. If using bubble pump system, this energy is from solar and no extra energy is required, as listed in Table 1.

Table 1 Efficiency analysis of bubble pump solar water heating system

6 Conclusions

1) The performance of the bubble pump is tightly associated with the weather including solar radiation intensity and surrounding temperature. The flow rate is fluctuant compared with the reference. The mass flow rate and pressure difference inside the bubble pump which serves as circulation head of the system are unattached to solar radiation intensity.

2) Long-term result shows that bubble pump can be applied to indirect active solar water heating system to replace the electric pump in the above-zero zone. If the temperature is below zero, enhanced incubation or changed working fluid is necessary.

3) The involvement of bubble pump extends the application scope of solar water heater. Bubble pump can be applied to indirect (close loop) active solar water heating system instead of the electric pump.

References

[1] LEE D W, SHARMA A. Thermal performances of the active and passive water heating systems based on annual operation [J]. Sol Energy, 2007, 81: 207-215.

[2] KALOGIROU S A. Long term performance prediction of forced circulation solar domestic water heating systems using artificial neural networks [J]. Applied Energy, 2000, 66: 63-74.

[3] JOSHI S V, BOKIL R S, NAYAK J K. Test standards for thermosyphon-type solar domestic hot water system: Review and experimental evaluation [J]. Sol Energy, 2005, 78: 781-798.

[4] BELESSIOTIS, MATHIOULAKIS, BELESSIOTIS V, MATHIOULAKIS E. Analytical approach of thermosyphon solar domestic hot water system performance [J]. Solar Energy, 2002: 307-315.

[5] SUMATHY K, VENKATESH A, SRIRAMULU V. The importance of the condenser in a solar water pump [J]. Energy Conversion and Management, 1995: 1167-1173.

[6] SUMATHY K, VENKATESH A, SRIRAMULU V. A solar thermal water pump [J]. Applied Energy, 1996: 235-243.

[7] SUMATHY K, VENKATESH A, SRIRAMULU V. Heat-transfer analysis of a flat-plate collector in a solar thermal pump [J]. Energy, 1994: 983-991.

[8] SUMATHY K. Experimental studies on a solar thermal water pump [J]. Appl Therm Eng, 1999, 19: 449-459.

[9] WONG Y W, SUMATHY K. Performance of solar water pumps with ethyl-ether as working fluid [J]. Renew Energy, 2001, 22: 389-394.

[10] WONG Y W, SUMATHY K. Thermodynamic analysis and optimization of solar thermal water pump [J]. Appl Therm Eng, 2001, 21: 613-627.

[11] PICKEN D J, SEARE K D R, GOTO F. Design and development of a water piston solar powered steam pump [J]. Sol Energy, 1997, 61(3): 219-227.

[12] LIENGJINDATHAWORN S, KIRTIKARA K, NAMPRAKAI P, KIATSIRIROAT T. Parametric studies of a pulsating-steam water pump [J]. Ambient Energy, 2002, 23(1): 37-46.

[13] NATTAPHON R, PICHAI N, NARIS P. Experimental studies of a new solar water heater system using a solar water pump [J]. Energy, 2008, 33 : 639-646.

[14] WONG Y W, SUMATHY K. Solar thermal water pumping systems: A review [J]. Renew Sustain Energy Rev, 1999, 3(2/3): 185-217.

[15] NORTON B, PROBERT S D. Recent advances in circulation solar energy water heater design [J]. Appl Energy, 1983, 15(1): 15-42.

[16] ZERROUKI A, BOUMEDIEN A, BOUHADEF K. The natural circulation solar water heater model with linear temperature distribution [J]. Renewable Energy, 2002, 26(4): 549-559.

(Edited by HE Yun-bin)

Foundation item: Project(2011-0021376) supported by Basic Science Program through the National Research Foundation (NRF) Funded by the Ministry of Education, Science and Technology of Korea

Received date: 2011-09-06; Accepted date: 2011-12-26

Corresponding author: JEONG Hyo-min, Tel: +82-10-9548-3184; Fax: +82-55-772-9119; E-mail: hmjeong@gnu.ac.kr