performance of a solar heat collection and release system for ...

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developed to increase the night air temperature during the winter in Chinese Solar ... Energy saving, Heat collection, Heat release, Heat storage, Thermal walls.
PERFORMANCE OF A SOLAR HEAT COLLECTION AND RELEASE SYSTEM FOR IMPROVING NIGHT TEMPERATURE IN A CHINESE SOLAR GREENHOUSE H. Fang, Q. Yang, Y. Zhang, W. Sun, W. Lu, Y. Tong, H. Liang

ABSTRACT. To increase the year-round greenhouse production in North China, a sustainable heating method should be developed to increase the night air temperature during the winter in Chinese Solar Greenhouses (CSGs). Solar heating is an inexpensive and effective way to heat greenhouses, and has been investigated by several previous studies. For the present study, a heat collection-heat release (HCHR) system that was attached to the north wall was developed for CSG night temperature improvement. Two experimental greenhouses were located in Beijing, China, with a floor area of 392 m2 each. Environmental parameters (temperature, humidity, heat flux) inside and outside the greenhouse were investigated, including the average solar collection efficiency of the heating system and the pump energy consumption rates. The results showed that the average solar collection efficiency of the system was 52%, which was 1.3 times greater than the reported value of a HCHR system installed in a small CSG. The effective collector absorptivity was 0.59 and heat transfer proved to be by natural convection. The night air temperature in the experimental CSG was increased by 5°C on average compared to the reference CSG. To meet the heating demand of the CSG during cold winter nights the release capacity must be increased by 40%. Pump capacity to circulate the water proved to be crucial for energy efficiency. Keywords. Energy saving, Heat collection, Heat release, Heat storage, Thermal walls.

T

he Chinese Solar Greenhouse (CSG) that is widely used in North China is characterized by a lean-to south-facing roof, a removable insulating blanket, and a solid north wall (Chai et al., 2007, 2012). The south facing roof structure and removable insulating blanket maximize the exposure top short-wave radiation during the day and minimize heat loss at night (Tong et al., 2003; Ma et al., 2008; Li et al., 2010). The utilization of the north facing wall is a new method for CSGs. On the one hand, the thick wall decreases heat loss; on the other hand, the capacity for sensible heat exchange is significant, trapping short-wave radiation during the day and releasing it during the night. Therefore, the CSG is a good way to reduce the energy usage from fossil fuels and increase the sustainability of protected horticulture. Meanwhile, solar energy is an attractive and less expensive

Submitted for review in June 2014 as manuscript number PAFS 10817; approved for publication by the Plant, Animal, and Facility Systems Community of ASABE in November 2014. The authors are Hui Fang, Assistant Professor, Yi Zhang, Assistant professor, Yuxin Tong, Assistant Professor, WeiTuo Sun, Graduate Student, Wei Lu, Doctoral Student, and Qichang Yang, Professor, Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing, China and Key Lab of Energy Conservation and Waster Treatment of Agricultural Structures, Ministry of Agriculture, Beijing, China; and Hao Liang, Institute of Agricultural Engineering, Agricultural Research Organization, Volcani Center, Israel. Corresponding author: Qichang Yang, Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China; phone: +86108210-5983; e-mail: [email protected].

substitute for enclosed heating applications (Fidaros et al., 2010; Bartzanas, et al., 2005). Compared with passive heating, active heating has an obvious advantage in collecting and managing the solar energy with the combination of a solar collector and a circulation fluid, especially if the ambient night temperature is extremely low (as shown by the thermal performances of active and passive water heating systems based on annual operation), because it provides a flexible way to utilize and distribute the heat when needed. An active heating system was designed and presented in other publications (Liang et al., 2013; Yang et al., 2013). The greenhouse thermal performance was described and a heat collection-heat release system (HCHR) was tested. However, the factors affecting the performance of the HCHR system itself and the dimensions of the HCHR system matching the greenhouse heating demand were still not clear. Thus, the objective of this research was to investigate: 1) the technical performance of the HCHR systems to heat CSGs in north China, 2) the main factors that affect the collection and release efficiency, and 3) the nighttime environmental conditions inside the greenhouse heated by the HCHR.

MATERIALS AND METHODS GREENHOUSES AND HCHR This experiment was carried out in two identical CSGs located in the Changping District of Beijing (latitude 39°54′ N, longitude 116°24′ E). Each greenhouse is 49 m long and 8 m wide with a 3.7 m ridge height and a 2.5 m

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© 2015 American Society of Agricultural and Biological Engineers ISSN 0883-8542 DOI 10.13031/aea.31.10817

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height of the north wall. The greenhouses were east-west (ridge) oriented and covered with a single layer of polyvinyl chloride (PVC) film with a thickness of 0.08 mm. The north wall and sidewalls were made of red brick and polystyrene board (12 cm thickness of the inner layer of red brick, 10 cm thickness of the middle layer of polystyrene insulation board made from beads, and 24 cm thickness of the outer layer of red brick), which are widely used by Chinese growers. The roof walls are made of concrete brick and polystyrene board (10 cm thickness of the outer layer of polystyrene insulation board and 10 cm thickness of the inner layer of concrete brick). One greenhouse was outfitted with the HCHR system, and the other greenhouse was not. There was 8 m of separation in north-south direction between the two greenhouses. The HCHR system is composed of three parts (fig. 1): 1) a heat collection-heat release plate that consists of one layer of 20 mm polystyrene insulation board, two layers of 0.2 mm black plastic film at a distance of 3 mm; 2) plumbing for the water flow; and 3) a heat storage tank, composed of an inner layer of red brick (thickness of 12 cm) and an outer layer of polyester insulation board (thickness of 10 cm). Two water pumps, each consuming 750 W of electrical power, were linked to the HCHR system, with a resulting water flow rate through each of the plates of 0.48m3/h. In one greenhouse, 29 HCHR plates, each with a height of 200 cm and a width of 135 cm (total HCHR area is 78.3 m2, total wall area is 122.5 m2; wall coverage: 64%), were installed inside the greenhouse along the north wall with their base 40 cm above ground level. The heat storage tank, with a volume of 10 m3, was buried underground in the middle of the greenhouse. MEASUREMENTS Temperatures were measured using copper-constantan thermocouples (manufacturer-stated accuracy was ±0.2°C). Three uniformly spaced air temperature sensors (thermocouples) were placed in the greenhouse, each at 1.5 m above the ground and 4 m from the north wall. Two other air temperature sensors, each at a height of 1.5 m in the middle of the greenhouse, were set up at distances of 2 and 6 m from the north wall. All of the air temperature Inlet North wall

Inlet Water flow

North wall

Water flow

sensors were shielded and aspirated. Three water temperature sensors were placed in the heat storage tank, one in the middle, one at the inlet, and one at the outlet of the tank. Two temperature sensors were mounted on the HCHR system, one of which at the front surface and one between the two layers of the black plastic film in the middle of the plates. Solar radiation was measured by a pyranometer (model CMP3, Kipp & Zonen, Delft, The Netherlands; measurement range from 0 to 2000 Wm-2; spectral range from 300 to 2800 nm), mounted vertically at the surface of the north wall at a height of 1.5 m above the ground and a distance of 24 m from the west wall. All data from the sensors (fig. 2) were automatically recorded using a data collection system averaging the data at a time interval of 10 min (CR1000, Campbell Scientific, Inc., North Logan, Utah). The air temperature outside the greenhouses was measured using another temperature sensor (test174T, Testo GM Company, Germany; accuracy: temperature ±0.5°C). SENSOR CALIBRATION The copper-constantan thermocouples were calibrated by using a low temperature tank (DC-300, NINGBO TIANHENG Instrument Factory, Ningbo, China) at oil temperatures of 0°C, 10°C, 20°C, 30°C, and 40°C. The outside air temperature sensor was calibrated using an Assmann-aspirated psychrometer (Y-5011, Yoshino, Co., Tokyo, Japan) at air temperatures of -20°C, -10°C, 0°C, 10°C, 20°C, and 30°C.

Heat Collecting Efficiency The total amount of heat collected and released and the associated total energy was calculated with equations 1-4: Qc ,τ = ρ w v w C w ( To ,τ − Ti ,τ )

Ec =

Qc,τ ⋅ τ

(1) (2)

τ

Q r ,τ = ρ w v w C w ( Ti ,τ − To ,τ )

Er =

Qr ,τ ⋅ τ

(3) (4)

τ

where Qc,τ and Qr,τ are the instantaneous collected and released heat flows during time τ, respectively, kW; ρw is the density of the circulating water, kg m-3; CW is the specific heat of the circulating water, J kg-1 K-1; To ,τ and

Ti ,τ are the mean water temperatures at the outlet and the Heat collect

Heat release

Outle t

Outlet

Water tank

Insulation layer

Water tank

Insulation layer

inlet of the HCHR system during time τ, respectively, K; Ec and Er are the total collected and released energy quantities, respectively, J; τ is the 10 min time interval used for the measurements. The heat collecting efficiency is typically applied as an indicator for assessing the performance of a solar collector in industrial buildings. In this study, the efficiency of heat collection by the HCHR was estimated with equation 5:

Figure 1. Schematic diagram of the heat collection-heat release (HCHR) system used for increasing the air temperature in a Chinese solar greenhouse at night.

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• : temperature measurement point; ■: solar radiation measurement point Figure 2. Schematic diagram of the heat collection-heat release (HCHR) system installed in a Chinese solar greenhouse.

ηc =

Qc ,τ A c Ic ,τ

=

ρ w v w C w ( To ,τ − Ti ,τ ) A C Ic ,τ

, 0 ≤ ηc ≤ 1

(5)

where ηc is the heat collection efficiency of the HCHR system in heat collection mode, dimensionless; Ac is the effective heat collecting area, m2; and Ic is the solar radiation intensity on the surface of the greenhouse north wall, W m-2. It can also be stated that the collecting efficiency equals the collected irradiation minus the heat losses from the plates over the available solar energy for the plates (Goldberg et al., 1980): ηc =

(

aA c Ic⋅τ − kA c T plate,τ − Tg ,τ

=a−

(

A c Ic⋅τ

k T plate,τ − Tg ,τ

) (6)

)

where a is the absorptivity of the plates, Tg ,τ is the mean air temperature inside the experimental greenhouse during time τ, K; T plate,τ is the mean plate temperature during time τ, K; From equation 6 we can analyze the collection performance of the plates. In addition to this efficiency over a short time interval, we can define the efficiency over a whole day with equation 7:

τ

Ec

A c Ic ,τ ⋅ τ

E wp = 2 × Wwp × h × 3600

(8)

where Ewp is the energy consumption of the water pump, J; Wwp is the electric power of the water pump, W; h is the water pump total run time, h. The energy consumption reduction rate was calculated with equation 9:

Ewp

(9)

Er

where R is the energy consumption reduction rate of the HCHR system, dimensionless, and with Er determined for the same run time. However, in equation 9, electric energy is compared to conventional energy. Taking into account that electricity generation has an efficiency of approximately 40% (Liu, 2013), the overall energy consumption reduction rate Roverall can be expressed as:

R overall = 2.5

Ewp Er

(10)

(7)

where Es is the total solar radiation quantities on the HCHR system over a whole day, J.

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The energy saving performance of the HCHR system was analyzed and compared with the situation in an identical greenhouse having the same energy input to be covered conventionally. The energy consumption of the greenhouse with HCHR was determined from the energy consumption of the two water pumps. Therefore it was calculated with equation 8:

R=

I c⋅ τ

E ηc ,tot = c = Es

Energy Saving Performance Evaluation of the HCHR System

RESULTS AND DISCUSSION ENVIRONMENTAL CONDITIONS The experiments were performed from 15 December 2012 through 15 January 2013. From this period 3 days

285

January 9/10, 2013

January 8/9, 2013

December 11/12, 2012

35

horizontal surface. In both greenhouses the insulating blanket was removed at 8:40 a.m. and put back in place at 16:00 p.m. At night, the air temperature inside both experimental greenhouses was higher than the outside air temperature. During the day, temperatures in both greenhouses were higher than outdoor due to the incoming solar radiation, and higher at 9 and 10 January due to the higher amount of direct radiation. The daytime variation is due to the varying ventilation set points implemented by the grower. During the night of 12 December, the HCHR system was switched on for heating (1:00 a.m.) and air temperature (with HCHR) increased between 1:00 a.m. and 2:00 a.m. by 2.5°C and then remained stable. In the reference greenhouse air temperature continued to decrease. The resulting air temperature in the HCHR greenhouse was 10.2°C, which was 2.4°C higher than in the reference greenhouse and 14.7°C higher than the outdoor temperature. When the insulating blanket was opened at 8:30 a.m., 12 December, a decrease in greenhouse air temperatures can be observed, most striking in the reference greenhouse. This is due to the fact that solar irradiation was very low then because of snowfall. During the relatively cold nights of 9 and 10 January with outdoor mean air temperature of approximately -13°C, the inside air temperature of the HCHR greenhouse also increased after starting operation of the HCHR system and was realized at about 12°C, which was 6°C higher than in the reference greenhouse and 25°C higher than the outdoor

500

T(℃)

30 25

400

20

300

15

200

10 100

5 0

Solar radiation intensity/W.m-2

were chosen as illustrative for the behavior of the system. During this period the water volume in the 10 m3 tank was approximately 5.5 m3. For evaluating the performance of the system, the daytime radiation and the period to release this heat are relevant. Therefore, the days were defined from 8:30 a.m. until 8:30 a.m. the following day. One day was hazy (1/12 December), one day was sunny with relatively high temperatures (8/9 January), and one day was sunny with low temperatures (9/10 January), which is normal for the Beijing area this time of the year. The variations of outdoor solar irradiation and outdoor air temperature during these days are shown in figure 3. The solar irradiation maxima were 407, 492, and 495 W.m-2, respectively, for the three days. Solar irradiation at the vertical back wall and air temperature in the greenhouse with and without the HCHR system are shown in figure 4. The solar irradiation maxima at solar noon are 299, 478, and 513 W.m-2, respectively, for the three days. Compared to the outdoor solar irradiation this is much lower at 11 December and about equal at 8 and 9 January. 11 December was a hazy day with a high amount of diffuse radiation and 8 and 9 January were clear days with a high amount of direct radiation. Chinese Solar Greenhouses with south facing transparent covers have a high transmissivity for direct radiation but due to the north wall and the roof have low transmissivity for diffuse radiation (Gao et al., 2003). At the low solar altitude during this time of the year (in Beijing at 10 January at solar noon: 28°), irradiation on the vertical surface is higher than on the

0 8

12

16

20

0

4

8 8

12

16

20

0

4

8 8

12

16

20

0

4

8

Time of day (h) Figure 4. Time courses of solar radiation at vertical back wall and greenhouse air temperature with and without HCHR. The data were obtained on 11/12 December 2012, 8/9 January 2013, and 9/10 January 2013. : solar radiation intensity at vertical back wall; : greenhouse air temperature with HCHR system; : greenhouse air temperature without HCHR system.

January 8/9, 2013

January 9/10, 2013 500

0 T(℃)

400 300

-5

200 -10

100

-15

Solar radiation intensity/W.m-2

December 11/12, 2012

5

0 8

12

16

20

0

4

8

8

12

16

20

0

4

8 8

12

16

20

0

4

8

Time of day (h) Figure 3. Time courses of outdoor solar radiation intensity and outdoor air temperature on 11/12 December 2012, 8/9 January 2013, and 9/10 January 2013. : solar radiation intensity (on horizontal surface); : outdoor air temperature.

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PERFORMANCE OF THE HCHR The performance of the HCHR system can be analyzed via the collected and released heat. The variations of the total heat collected (Ec) and released (Er) by the HCHR system during the day/night periods investigated are presented in table 1, together with the radiation sum Es at the back wall over the total area of the HCHR. The system collected much more energy during sunny days with an average solar radiation of 318 W.m-2 (8-9 January 2013) and 337 W.m-2 (9-10 January 2013) than during cloudy days with an average solar radiation of 165 W.m-2 (11-12 December 2012), as shown in table 1. The utilization ratio of the collected heat of the HCHR, expressed as Er/Ec, was calculated to be between 71% and 79%. So the collected heat was not utilised completely during the night as some heat was lost during transport and storage. The heat collecting efficiency ηc,tot (eq. 7) for these days was calculated as 54%, 57%, and 56%, respectively, which is fairly constant. The heat collection efficiencies as function of the ratio of the temperature difference between collecting plates and inside air over the available solar radiation (eq. 6) for the 10 min measurement periods (total number of measurements are 97) during the three separate days are shown in figure 5. During the heat collection period, the water temperature increased from 15°C to 22.7°C on 11 December from 17 to 32°C on 8 January, and from 19°C to 36°C on 9 January, Table 1. Overview of total solar radiation quantities, collected, and released energy at different stages and dates. Es Ec (Ec/ Es) Er (Er/Ec) Date (MJ) (MJ) (MJ) 11-12 December 2012 334 182 (0.54) 143 (0.79) 8-9 January 2013 630 358 (0.57) 277 (0.77) 9-10 January 2013 670 374 (0.56) 265 (0.71)

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1

Heat collection efficiency

temperature. Despite of the fact that the reference greenhouse has no heating system, mean air temperature was still approximately 19°C higher than outdoor temperature due to good insulation of the enclosure and heat released from the north wall. After starting HCHR system operation, during the relatively warm night of 12 December, the HCHR system was able to compensate for the heat losses, resulting in a relatively stable greenhouse temperature. During the colder nights of 9 and 10 January, greenhouse air temperature still decreased during operation of the HCHR system. So for these conditions, the HCHR system capacity is too low. The heat losses during the night of 12 December occurred at a temperature difference between the indoor and outdoor of 18°C. During the nights of 9 and 10 January, the temperature difference was 25°C. So to compensate for the losses during these nights, the heat release capacity has to be increased proportional to the temperature difference, and therefore should be approximately 1.4 times higher. Obviously, this is linked to the heat collection and storage system and better insulation could further improve the system. However, CSGs are already heavily insulated during the night.

0.8

y = -8.2609x + 0.5878 R² = 0.87

0.6 0.4 0.2 0

-0.06

-0.04

-0.02

0 0.02 (Tplate-Tg)/Ic

0.04

0.06

Figure 5. Average heat collection efficiency (eq. 6) of the HCHR system as affected by the ration of the temperature difference between plates and greenhouse air over the solar radiation. ▲: the data were obtained on 11 December 2012; •: the data were obtained on 8 January 2013; ♦: the data were obtained on 9 January 2013. Each data point represents a 10 min average.

respectively. The heat collection efficiency decreased with the increase in temperature difference between water flow and inside air. This temperature difference ranged between -2 and +6°C. When the temperature difference is 0°C, we can derive from regression function and equation 6 that the heat collection efficiency was 0.59. This means that the effective collector absorptivity a equals 0.59. This can be improved by choosing a material with higher absorptivity. With positive temperature differences, heat losses increase linearly (fig. 5). According to equation 6, the negative slope of the line represents the heat transfer coefficient k, which equals 8.3 Wm-2K-1. This value can be expected for heat transfer in a greenhouse with small temperature differences between air and cover (Bot, 1983). For small or even negative temperature differences collector efficiency is highest. Thus HCHR system can be improved by applying a heat pump decreasing the plates temperature and thereby collector efficiency. Over the entire measurement period, the heat collection efficiency of the HCHR system ranged from 15% to 70% with an average of 52%, which was 1.3 times greater than the reported value of the HCHR system installed in a small CSG (Wang, 2006). After collection during the day, the stored heat was released at night. The heat release rate Qr,τ per m2 of plate surface was established by measuring the inlet/outlet water temperature and volume flux over the 10 min periods of the presented days (eq. 3) and is shown in figure 6 as dependent on the difference between plates and greenhouse air temperatures (total number of measurements are 150). Most striking in the figure is that there seem to be two data sets, the left small line is collected in the night of 12 December and the other longer line is collected in the nights of 9 and 10 January. There was no difference in the system operation during those nights so the difference can only be explained by measurement errors linked to this type

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CONCLUSIONS

250

Heat release rate (Wm-2)

200 150 100 50 0 0

5

10 15 Tplate-Tg (℃)

20

25

Figure 6. Average heat release rate per m2 plates of the HCHR as affected by the temperature difference between plate and greenhouse air. ▲: the data were obtained on 12 December 2012; •: the data were obtained on 9 January 2013; ♦: the data were obtained on 10 January 2013. Each data point represents a 10 min average. : subscript analogous to all measurements added plotted natural convection heat transfer.

of experiment. For small temperature differences there are no data, however if we compare them to the heat collecting data during the daytime, the first night data correspond to the constant heat transfer coefficient of 8.3 Wm-2K-1. For the larger temperature difference appearing during the night natural convection will obviously appear. In figure 6 the released heat is plotted as Q r ,τ = 5.0 ( Tplate,τ − Tg ,τ )

5

4

which covers the data reasonable. So the resulting heat transfer coefficient then equals k = 5.0 ( Tplate,τ − Tg ,τ )

1

4

which is in line with natural convection heat transfer. The reduction in energy consumption of the HCHR system (eq. 10) indicates how much energy is needed for the two water pumps compared to the energy released to the greenhouse (that without an HCHR system has to be provided by a conventional heating system). The total heat released by the HCHR system was 1.43 × 108 J on 12 December 2012, 2.77 × 108 J on 9 January 2013, and 2.65 × 108 J on 10 January 2013, respectively. The run time of the water pumps was 43.5 h so a total of 2.3 × 108 J of electric energy was consumed. This equates to 0.77 × 108 J consumed each day, needing 2.5 times more primary energy, and therefore resulting in 1.925 × 108 J. The increase or reduction in energy consumption was 135%, 69%, and 73% during the three days, respectively. This means that the energy consumption by the water pump is crucial for the efficiency of the system. This efficiency is linked to the plumbing of the system which was done here for a research greenhouse with low attention to the pressure losses. As a result, in practical application this must be designed carefully.

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During this study, the heat collection-heat release (HCHR) system was used to store energy during the daytime and heat a Chinese solar greenhouse at night during the winter. The technical and environmental analyses of this system have contributed to the following conclusions. The experimental Chinese solar greenhouse maintained higher temperatures during the night compared to the greenhouse without an HCHR system. The inside air temperature difference between the two greenhouses ranged from 0.3°C to 6.7°C with an average of 5.0°C. HCHR systems installed in greenhouses can supply a significant part of the heating requirements. At an indooroutdoor temperature difference of 18°C, the HCHR system was able to compensate for heat losses. For colder nights, the indoor temperature still decreased, although less so compared to the reference greenhouse. These colder nights with indoor-outdoor temperature difference of approximately 25°C are common across the Beijing area. To meet these conditions, the described HCHR system capacity should be increased by 40%. The energy collection efficiency during the daytime decreased sharply with declining plate-air temperature differences. To have high energy collection efficiencies, plate-air temperature differences must be kept high and this can be achieved by applying a heat pump to reduce the circulating water temperature and transfer the energy to another water tank. The effective collector absorptivity was found to be 0.59. Therefore, another improvement could be the application of black materials with a higher absorptivity. During the nighttime, the heat transfer coefficient of HCHR was found to be dependent on the plate-air temperature difference by 5.0 ( Tplate ,τ − Tg ,τ )

1

4

, which is

in line with natural convective heat transfer. In addition, the recorded heat transfer during the daytime, which corresponded with low plate-air temperature differences, followed this relationship. The increase or decrease in energy consumption was found to be 135%, 69% and 73% during the three days, respectively. This can be improved by proper design of the water flow system. During our experiment, no attention was paid to this aspect. So the use of the HCHR for heating the greenhouse at night during the winter can improve the environmental conditions inside Chinese solar greenhouses for crop production, achieving high energy collection efficiency and a reduction in energy consumption. To do so, the discussed improvements are recommended. ACKNOWLEDGEMENTS This study was sponsored by the National High Technology Research and Development Program (863 program) about key technologies of energy-saving equipment and intelligence for protected plants in tropical greenhouses, project #2013AA102407, Ministry of Science and Technology, P.R. China.

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ABBREVIATIONS HCHR heat collection-heat release system wp water pump PVC polyvinyl chloride

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VARIABLES Q heat flux (kW) v volumetric flow rate of water (m3 s-1) C specific heat of water (kJ kg-1 K-1) T temperature (K) E energy (kJ or kWh) η heat collecting efficiency (%) A heat collection area (m2) I solar radiation intensity (W m-2) h time (h) W electric power of water pump (kW) ρ density of water (kg m-3) R energy consumption reduction rate (%) α solar radiation absorptivity (%) SUBSCRIPTS τ time w water o outlet i inlet r release c collection wp water pump tot total g experimental greenhouse s solar energy plate heat collection-heat release plate

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