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Experimental performance of evaporative cooling pad systems in greenhouses in humid subtropical climates
Applied Energy 138 (2015) 291–301
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Experimental performance of evaporative cooling pad systems in greenhouses in humid subtropical climates J. Xu a, Y. Li a, R.Z. Wang a,⇑, W. Liu a, P. Zhou b a b
Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s Experimental performance of evaporative cooling in humid climate is investigated. 5 working modes are studied in the greenhouse. Vertical and horizontal temperature and relative humidity variations are analysed. Indoor temperature can be kept in required level by proper working modes.
a r t i c l e
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Article history: Received 13 January 2014 Received in revised form 8 September 2014 Accepted 24 October 2014
Keywords: Humid subtropical climate Evaporative cooling Greenhouse energy conservation Temperature and humidity distribution Quantified analysis of energy
a b s t r a c t To solve the overheating problem caused by the solar radiation and to keep the indoor temperature and humidity at a proper level for plants or crops, cooling technologies play vital role in greenhouse industry, and among which evaporative cooling is one of the most commonly-used methods. However, the main challenge of the evaporative cooling is its suitability to local climatic and agronomic condition. In this study, the performance of evaporative cooling pads was investigated experimentally in a 2304-m2 glass multi-span greenhouse in Shanghai in the southeast of China. Temperature and humidity distributions were measured and reported for different working modes, including the use of evaporative cooling alone and the use of evaporative cooling with shading or ventilation. These experiments were conducted in humid subtropical climates where were considered unfavourable for evaporative cooling pad systems. Quantified analyses from the energy perspective are also made based on the experimental results and the evaporative cooling fan–pad system is demonstrated to be an effective option for greenhouse cooling even in the humid climate. Suggestions and possible solutions for further improving the performance of the system are proposed. The results of this work will be useful for the optimisation of the energy management of greenhouses in humid climates and for the validation of the mathematical model in future work. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction A greenhouse is a plastic or glass structure that can provide a suitable micro-climate for various crops and plants, protecting them from severe and variable outdoor weather conditions. During the daytime with intense solar radiation, the greenhouse effect caused by the absorption of solar radiation makes the temperature inside the greenhouse even higher than the ambient temperature; thus, plants and crops may suffer being dried due to high rate of evapotranspiration caused by the excessive heat, and their ⇑ Corresponding author at: 800 Dong Chuan Road, Shanghai 200240, China. Tel.: +86 21 34206548. E-mail address: [email protected] (R.Z. Wang). http://dx.doi.org/10.1016/j.apenergy.2014.10.061 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
production may decline. To provide a suitable growing environment, cooling technologies are essential and must be introduced. It plays a vital role in the greenhouse industry especially in hot weather although it is still important in any other climate conditions. The climate of Shanghai is humid subtropical and features hot, humid summers and chilly, damp winters. Table 1 shows the typical variation in climate conditions in Shanghai. Summer sees high temperature that plants and crops cannot endure; as a result, air cooling technology is urgently required. Over the past decades, many relevant studies and experiments of greenhouse cooling technologies have been conducted throughout the world [2–6]. The method of computational fluid dynamics (CFD) [7,8] and TRNSYS [9] are applied to study the energy
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Nomenclatures RH (%) Tamb. (°C) Tin (°C) TH (°C) TL(°C) Tdo (°C) Tdi (°C)
relative humidity ambient temperature temperature inside the greenhouse highest temperature lowest temperature dry bulb temperature of outside air dry bulb temperature of cooled air exiting the pad Two (°C) wet bulb temperature of the outside air g efficiency of the evaporative pad NV natural ventilation (with roof vents and doors widely open) EC evaporative cooling ES external shading IS internal thermal screen CF circulation fan SHo (kgH2O/kgair) sensible humidity (humidity ratio) of outside air SHi (kgH2O/kgair) sensible humidity (humidity ratio) of cooled air exiting the pad
management of greenhouse. Also, it is mentioned [4] that the most energy consuming sectors in the agricultural industry is the greenhouse, which implies that reducing the greenhouse energy consumption can contribute significantly to the sustainable energy management. Cooling technologies for agricultural greenhouse applications can be classified as ventilation (natural and forced), shading, evaporative cooling and composite systems (heat exchanger) [10]. Of these, evaporative cooling is considered to be the most efficient and affordable method [11]. When the outdoor hot air flows through the wet material, water evaporation takes place on the wet surface and removes the sensible heat from the air, thus lowering the temperature. Currently, there are three evaporative cooling technologies in use: fan–pad, fog/mist and roof evaporative cooling [10,12]. The fan–pad system consists of fans on one side of the greenhouse wall and porous pads on the other side. Outdoor air is forced through the wet evaporative cooling pads by the extracting fans, heat is absorbed from the air by the water vaporisation process, and the air enters the greenhouse at a lower temperature than the ambient value. Evaporative cooling pads have played an important role in thermal environmental control in both agriculture and the poultry industry. The performance of such systems in the regulation of the greenhouse temperature has been evaluated in certain studies [13,14]. Kittas et al. [15,16] investigated the efficiency of fan–pad systems (both shaded and unshaded) in a greenhouse covered with double inflated polyethylene films in Greece and suggested that shading is not necessary in dry climates, as the fan–pad system alone is capable of preventing the overheating of crops. Liao and Chiu [17] designed a small-scale wind tunnel prototype to simulate the performance of fan–pad systems with two alternative pad materials (coarse fabric PVC sponge mesh and fine fabric PVC sponge mesh) in Taiwan where the mean relative humidity (RH) is approximately 50%. Shukla et al. [18]
c (kJ/kg) V_ (m3/s) _ (kg/s) m Q (kW) q (kg/m3) COP c (kJ/kg K) s (s) m (kg) T (°C) I (W/m2)
latent heat for vaporisation of water (2430) volumetric flow rate mass flow rate cooling capacity density coefficient of performance specific heat time mass mean temperature mean value of solar radiation
Subscript d w a amb.
dry climate humid climate air ambient
tested the combination of an evaporative cooling system and an inner thermal curtain in a cascade greenhouse and concluded that evaporative cooling is effective in hot summers but the fan–pad system is not recommended in such a greenhouse structure because it would increase the interior air temperature of the second greenhouse due to addition of extra thermal energy from the adjacent one. Jain and Tiwari [19] presented a mathematical model to predict the thermal behaviour of a greenhouse equipped with an evaporative cooling system. However, earlier reports [2,20] mentioned that evaporative cooling pads are seldom suitable in humid tropical regions because of the high humidity levels. Temperature reduction of the air after evaporation process is limited due to the ambient high humidity, thus making the performance of such technology is not as satisfied as in the dry climate region. According to author’s knowledge, the existing studies [21,22] that intended to improve such problem utilised auxiliary equipments like desiccators for air pretreatment. Besides, systems such as air conditioner or earth-to-air heat exchanger (EAHES) are usually applied in the humid climate to lower the interior air temperature but with the major disadvantage of the high initial cost as well as the subsequent operation and maintenance expenses [4]. Air conditioning systems are also studied to control greenhouse microclimate [23–25]. In light of the growing market of commercial greenhouses for cultivation of rare/high-price plants or flowers recently, evaporative cooling system which is favoured over the other alternatives due to its simplicity, reliability and cost/benefit is worth being popularised, especially on a large scale application. In this study, as previously mentioned, because natural ventilation alone is far from sufficient in Shanghai, evaporative cooling pad systems are usually used to adjust the inside air temperature to a comfortable level. Regarding that the yearly average relative humidity in Shanghai is more than 70%; as a result, other methods such as the combination of ventilation with shadings may be
Table 1 Historical climate data for Shanghai [1]. Month
January
February
March
April
May
June
July
August
September
October
November
December
Record high (°C) Average high (°C) Average low (°C) Record low (°C) Relative humidity (%)
22 8.1 1.1 10 75
26 9.2 2.2 8 74
29 12.8 5.6 6 76
34 19.1 10.9 1 76
36 24.1 16.1 7 76
37 27.6 20.8 12 82
40 31.8 25 16 82
41 31.3 24.9 19 81
37 27.2 20.6 11 78
32 22.6 15.1 2 75
29 17 9 4 74
23 11.1 3 8 73
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Fig. 1. Photographic view of the greenhouse and main components (a) general view; (b) evaporative cooling pad; (c) extracting fan; (d) external shading net; (e) internal thermal screen; and (f) circulation fan.
necessary. Therefore, shadings, which are very common in greenhouse construction, other than additional mechanical equipments are used to improve the cooling effect in humid regions. Such system taking full use of the existing components in the greenhouse is easy to control and cheap. The purpose of this study was to evaluate the performance of evaporative cooling pad-fan system in greenhouses in a humid subtropical climate, extending the application from only in the dry climate to the regions with relatively high humidity. Experiments were made with the objective of evaluating the combined effects of the fan–pad system and other methods. A 2304-m2 glass multi-span greenhouse installed with an evaporative cooling pad was studied. The temperature, relative humidity and air velocity were measured at different locations in the greenhouse. The effects of evaporative cooling combined with other methods such as external shading nets, internal thermal screens and circulation fans were investigated. Based on the experimental results, the idea of using existing greenhouse components as assistants to improve the cooling effect of evaporative cooling fan–pad in subtropical area is feasible and it allows for less consumption in both energy and investment aspects compared with other options. 2. Materials and methods 2.1. The greenhouse The greenhouse was a 48-m-wide 48-m-long multi-span greenhouse located in Shanghai at a latitude 31°050 Nandlongitude 121°50 E. The greenhouse was a 5- 9.6-m-span structure covered with 4-mm float glasses. There are two entrance-exits along the fan–pad direction in the greenhouse. Fig. 2(a) shows a photo of the greenhouse and Table 2 lists its dimensions. The greenhouse was commercially operated for some parent plants cultivation, including chrysanthemum, lettuce and cucumber. The plantation lines are parallel to the air stream. 2.2. Evaporative cooling system Corrugated cellulose paper pads (Fig. 1(b)) that were 42.1 m long and 1.9 m wide were situated on the north wall. Water was
Fig. 2. Schematic of the location of circulation fans in the greenhouse (dimensions are in m). Red dotted line represents the plane where the temperature and relative humidity sensors were placed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Design detail of the greenhouse. Width of the greenhouse (m) Length of the greenhouse (m) Width of the greenhouse entrance-exit (m) Height of the greenhouse entrance-exit (m) Number of extracting fans Power of extracting fans (kW) Distance between two adjacent extracting fans (m) Height of cooling pads (m) Height of cooling pads from the ground (m) Number of circulation fans Power of circulation fans (W)
48 48 2.4 3 10 1.1 3.2 1.9 1.25 10 90
pumped upwards then released onto the top of the cooling pads. Ten 1.1-kW extracting fans were installed on the south wall of the greenhouse (Fig. 1(c)). Ambient air was dragged inwards by the extracting fans and flowed through the greenhouse after being humidified and cooled by the wet pads.
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The efficiency of the evaporative pad is calculated by Eq. (1) [26]:
g¼
T do T di 100% T do T wo
ð1Þ
in which Tdo is the dry bulb temperature of the outside air, Tdi is the dry bulb temperature of cooled air exiting the evaporative pad, and Two is the wet bulb temperature of the ambient air. 2.3. Shadings Flexible shadings, including both internal thermal screens and external shadings nets, were installed throughout the greenhouse to keep out the excessive radiation during the summer time. The black external shading nets (Fig. 1(d)) were made of highdensity UV polyester strips that provided 70% shading in our experimental greenhouse. They were used to reduce the amount of incoming solar radiation when necessary. Internal thermal screens (Fig. 1(e)) were made of 50% shadereflective aluminium fibre. The properties of these materials allow the thermal screens to both prevent radiation from entering the lower part of the greenhouse during the hot summer and retain heat inside the greenhouse during the winter time. 2.4. Circulation fans Circulation fans (Fig. 1(f)) were installed to provide stable and moderate air flow to enhance the heat exchange between the air and the plants. By increasing the effect of plant transpiration, plant temperatures can be reduced. Fig. 2 shows the placement of circulation fans that were installed 2.5 m above the ground. The air flow occurs in a continuous pattern. The red dotted line marks the plane where the temperature and humidity sensors were placed. The power of each circulation fan was 90 W. 2.5. Measurement and data collection system The temperature and relative humidity inside the greenhouse were measured at various positions (shown in Fig. 3) by thermistor thermometer and relative humidity probes (WTHOT1, Wangyunshan, China) with respective accuracies of ±0.5 °C and ±5% RH. The purpose of such an arrangement was to investigate the vertical and horizontal temperature and relative humidity distribution inside the greenhouse. The data were automatically recorded at one-minute intervals. The temperature of the air flowing in and out of the evaporative cooling pads was measured manually with two wet and dry bulb thermometers (272-A, Huachen, China) with the accuracy of ±0.5 °C. The velocity of the air flowing through the greenhouse was measured with a hot-wire anemometer (A531, Kanomax, Japan) with an accuracy of ±2%. Hot wire anemometers use a wire electrically heated up to some temperature above the
ambient and after the air flowing past the wire, a cooling effect occurs on the wire. Since the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed. Because the air has different psychrometric properties under different working modes in the greenhouse, the half of the green house nearest the cooling pad was named First Half Area and the half near the extracting fans was called Second Half Area, as shown in Fig. 3. At the same time, outside climate variables were recorded every 60 s with a weather station (Vantage Pro2, Davis, USA) that included a rain collector, an anemometer, and temperature, humidity and solar radiation sensors. The ambient temperature on August 14 and 15 of 2012 were recorded every hour. 3. Results The experiments were carried out on August 4, 5, 14, and 15 of 2012. The weather conditions of these days are listed in Table 3. The relative humidity and solar radiation listed are the mean values during the daytime (from 8 am to 6 pm). TH and TL refer to the highest and lowest temperatures of the daytime period, respectively. According to the standard engineering data from ASAE [27], the temperature inside the greenhouse should be lower than 30–32 °C. However, the measured ambient temperature (Table 3) was higher than this value and the inside temperature would have been even higher if active cooling had not been applied. Hence, the evaporative cooling pads and fans were used when the indoor temperature was higher than 30 °C; otherwise, natural ventilation is capable. Given the measured wet and dry bulb temperature on both sides of the evaporative pad (Tdo = 32.7 °C, Tdi = 28.3 °C, Two = 26.9 °C), the efficiency of the wet pad was approximately 75.9%. After humidification, the air could be cooled to 27–29 °C under humid conditions. The measured average velocity of the air stream in front of the pad was 1.1 m/s. Five major working modes that combine different cooling/temperature control methods were implemented to investigate their effects on the thermal environment of the greenhouse. Table 4 gives a detailed description of the working modes. In the natural ventilation mode, the roof vents and doors of the greenhouse were left wide open.
Table 3 Weather conditions during the daytime (8:00–18:00). Date August August August August
4 5 14 15
TH (°C)
TL (°C)
Mean RH (%)
Mean solar radiation (W/m2)
32.3 33.8 34 35
29.4 29.8 29.4 28
74.3 66.7 72.3 74.1
472.1 603.3 598 570.7
Fig. 3. Measurement positions in the central section of the greenhouse (dimensions are in m).
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Table 4 Operating modes for the control of the thermal environment in the greenhouse. Mode
Description
NV EC EC + ES EC + ES + IS
Natural ventilation (with roofs and doors wide open) Evaporative cooling pad Evaporative cooling pad combined with external shading Evaporative cooling pad combined with external shading and internal thermal screens Circulation fans with other methods
EC (. . .) + CF
The influence of each working mode on the temperature and relative humidity distribution in the greenhouse was investigated and reported accordingly.
3.1. Influence of different working modes on air temperature control inside the greenhouse
Fig. 5. Temperature and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 5th, 2012).
In this subsection, the temperature variations under different operating conditions were studied. The measurement point (T2b in Fig. 3) 12 m from the evaporative cooling pad at a height of 1.5 m was selected as a representative point to analyse the variations in temperature. Fig. 4 shows the variation in solar radiation, ambient temperature and indoor temperature of the greenhouse on August 4th, 2012. Three working modes were applied: (1) natural ventilation (NV), (2) evaporative cooling (EC), and (3) evaporative cooling combined with external shading nets (EC + ES). From 8:00 to 8:30 am, the roof vents and entrance/exit doors were left open; the temperature inside the greenhouse ranged from 30 to 31 °C, which was 1–2 °C higher than the ambient temperature. From 8:30 onwards, the solar irradiation rose to more than 500 W/m2, thus leading to a significant increase in indoor temperature. The difference between the indoor and ambient temperature exceeded 4 °C, and the temperature inside the greenhouse reached 34 °C, which is harmful to the plants and crops. At 9:50 am, the fan–pad system was turned on to cool the greenhouse and the roof vents were closed. Initially, the air temperature in the greenhouse decreased significantly due to the evaporative cooling pad-fan system. Then, the inside air temperature increased slightly while Tamb increased. At midday (from 12:30 to 13:40), the greenhouse switched to the working mode, where both evaporative cooling and external shading (EC + ES) were used concurrently. The effect of the external shadings was immediate, as the indoor temperature decreased to 2–3 °C lower than the ambient temperature. Subsequently, when Tamb decreased to approximately 31 °C, the external shading was removed to allow the incoming solar
radiation to penetrate into the greenhouse. From 16:15 to 18:00, there was almost no difference in temperature between the indoor and the ambient environment due to the low solar radiation of approaching dusk. When comparing the variations in solar radiation and the greenhouse temperature in Fig. 4, it is obvious that solar radiation is the key factor that affects the greenhouse temperature. With the help of external shadings, the influence of solar radiation can be reduced. Fig. 5 illustrates the indoor temperature profile measured on August 5th, 2012 under the operation mode consisting of the evaporative cooling pad system combined with both external and internal shadings (EC + ES + IS). As previously described, external shading nets can prevent part of the solar radiation from entering the greenhouse, which can decrease the indoor heat load and heat disturbance of the air. From 9:40 am onwards (Fig. 5), evaporative cooling pads and external shading nets were used; Tin dropped immediately by 7 °C and remained 2–4 °C lower than the ambient temperature. Starting from 11:05 am, internal thermal screens were spread partially throughout the greenhouse (approximately 50% of the area was shaded with internal thermal screens) to further regulate the micro-climate; the indoor temperature Tin decreased by another 2 °C and stayed 4 °C lower than Tamb, even under 1000 W/m2 outside solar radiation at midday. Circulation fans were used along with the other methods in the experiment on August 14th (Fig. 6). The influences of different working modes on temperature were similar to the results obtained in Figs. 5 and 6. The largest temperature difference
Fig. 4. Temperature (T2b) and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 4th, 2012).
Fig. 6. Temperature and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 14th, 2012).
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between Tin and Tamb. (in the range of 2–4 °C) was achieved with the EC + ES + CF working mode. No significant improvement in the results described in Figs. 5 and 6 was observed with the addition of circulation fans.
3.2. Influence of different working modes on air temperature variation in the horizontal direction To investigate the influence of different working modes on the horizontal air temperature throughout the greenhouse, the temperature was measured at 4 m, 12 m, 28 m, 36 m and 44 m from the pad at a height of 1.5 m above the ground. The horizontal temperature variations from the evaporative pad under different working modes are shown in Fig. 7. When the fan–pad system was used alone, a distinct temperature gradient was formed throughout the greenhouse in the fan–pad direction (Fig. 7(a)). Temperatures at 4 m and 12 m away from the pad were 29 and 31 °C, respectively, which were lower than the ambient temperatures. At 12 m from the pad, the temperature varied within the range of 31–33 °C, which would barely allow the plants to survive. Beyond 12 m, satisfactory cooling could not be achieved. The temperature difference between the two ends of the green house (4 m and 44 m from the pad) was approximately 9 °C. The temperature variations under EC + ES mode and EC + ES + IS mode are shown in Fig. 7(b) and (c), respectively. When external shading nets and evaporative cooling (EC + ES) were used, almost all of the measurement positions recorded temperatures lower than the ambient temperature, even near the extracting fans
at hot noontime. The temperature difference between the two ends was narrowed to 3–4 °C and could be further reduced to 2–3 °C with partial shading using the flexible internal thermal screens. The overall indoor temperature was kept 2–4 °C lower than the outside temperature, and no significant fluctuations in temperature were observed. Fig. 7(d) shows the results of adding circulation fans. The temperature difference between the positions 4 m and 44 m away from the wet pad was 3–4 °C, which was almost the same as the difference obtained without additional circulation fans. This indicates that circulation fans do not contribute to the equalisation of the horizontal air temperature gradient.
3.3. Influence of different working modes on the variation of vertical air temperature To evaluate the influence of different working modes on the vertical air temperature gradient in the greenhouse, measurements taken 1.5 m, 3 m, and 4 m above the ground were analysed. The measurement sites 1.5 m and 3 m above the ground were within the range of the cooled air stream of the evaporative pads, while the measurement position 4 m above the ground was above the cooled air stream. Measurement points 28 m away from the pad (refer to T3 in Fig. 3) were selected, as the air flow was stable at that position. When using the evaporative cooling pad alone (Fig. 8(a)), temperatures inside the greenhouse were greatly influenced by solar radiation. Because of the buoyancy effect, the temperature in the top area of the greenhouse envelope (4 m above the ground)
Fig. 7. Influence of working modes on horizontal air temperature (s 4 m; 4 – 12 m; } – 28 m; 5 – 36 m; – 44 m; j – Amb.): (a) EC; (b) EC + ES; (c) EC + ES + IS; and (d) EC + ES + CF ((a and b) on August 4th; (c) on August 5th; and (d) on August 14th).
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Fig. 8. Vertical air temperature variation 28 m away from the wet pad (T3): (a) EC; (b) EC + ES; (c) EC + ES + IS; and (d) EC + ES + CF ((a) on August 4th; (b and c) on August 5th; and (d) on August 14th, 2012) (s – 1.5 m, – 3 m; 4 – 4 m).
was approximately 4 °C higher than in the lower part (1.5 and 3 m from the ground). As seen in Fig. 8(b), a 0.5 °C temperature difference between the 1.5-m and 3-m positions was observed as a result of the use of external shadings. At the same time, the difference in temperature between positions at 3 m and 4 m was slightly reduced to 3 °C. When the partially-spread internal thermal screen was used (Fig. 8(c)), the vertical air temperature difference became more apparent. The temperature at 4 m (just below the internal screens) remained the same, while the temperature at lower positions decreased instantly due to the reduction in solar radiation (in comparison with Fig. 8(b)). In this working mode, the temperature differences between measurement positions at 1.5 m and 3 m/3 m and 4 m were 1.5 and 4 °C, respectively. Fig. 8(d) shows the vertical air temperature profiles under the EC + ES + CF operating mode. It appears that a quite different effect on the vertical distribution of temperature was produced with circulation fans and shading methods: the temperature difference increased at the lower position (between 3 m and 1.5 m) and decreased at the higher position (between 3 m and 4 m). The vertical air temperatures 12 m away from the wet pad (refer to measurement T2 in Fig. 3) are given in Fig. 9.Combinedwith the results obtained 28 m away (Fig. 8(b) and (c)), it can be found that the temperature gap between the ends of the lower part (1.5 m and 3 m) gradually decreased with increasing distance from the evaporative pad. For example, at 12 m, the temperature difference in the lower part was 2.5 °C; at 28 m, the gap was reduced to 0.5 °C in the EC + ES working mode, which means that the distribution in air temperature in the second half (near the fan) of the
greenhouse is even more pronounced than in the first half (near the pad). 3.4. Influence of different working modes on air relative humidity in the greenhouse The variations in RH both inside and outside the greenhouse under four different operating conditions on August 4th, 5th,
Fig. 9. Vertical air temperature variation at 12 m from wet pad (T2) on August 5th (s – 1.5 m, – 3 m; 4 – 4 m).
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Fig. 11. Temperature variation inside the greenhouse (conducted on August 15th) (s – 4 m; 4 – 12 m; } – 28 m; – 44 m; j – Amb.).
Fig. 10. Variation of relative humidity at different distances from the evaporative pad under four different operating conditions conducted on two days (s – 4 m; – 20 m;5 – 44 m; j – Amb.): (a) NV, EC + ES & EC + ES + IS on August 5th and (b) EC on August 4th. Fig. 12. Comparison of the air psychrometric conditions under dry/humid climate.
2012 are shown in Fig. 10. The ambient RH was in the range of 60– 70% throughout the experimental period. When the evaporative cooling pads were not working (NV in Fig. 10(a)), the RH inside the greenhouse was at 40–55% due to the high indoor temperature. When the fan–pad system was used in combination with external shading nets, the indoor RH increased instantly from 50% to 70– 80%. Subsequently, the slight increase in RH (approximately 5%) confirmed that the internal thermal screens can keep the air inside the greenhouse enclosure at a high relative humidity conditions. The distribution of RH is shown in Fig. 10(b) for another operating condition in which the fan–pad system was used without shadings. The difference in RH between the two ends of the greenhouse (4 m and 44 m) reached 10%. Moreover, the position near the aisle in the middle of the greenhouse had a lower plant density, resulting in less of a transpiration effect; as a result, the measurement point 20 m away from the cooling pad had the lowest relative humidity. 3.5. Influence of air tightness on air temperature In the experiment conducted on August 15th, a short test period was conducted (⁄ in Fig. 11) when the roof vent was partially opened. In the first phase, external shading nets were applied to assist the fan–pad system and Tin was controlled at an appropriate level. Starting from 9:28 am, the roof vent was opened slightly; the temperature increased immediately, thus dramatically reducing the effect of evaporative cooling pads. During this short period of 20 min, the temperature in the greenhouse rose by 3 °C, reaching 34 °C. At 9:48 am, the roof vents were closed again and the air temperature returned to its normal value.
4. Discussion The experimental results show that the temperature and relative humidity inside the greenhouse vary rapidly in response to changes of outside environmental conditions. Solar radiation is one of the key factors that influence the internal micro-climate. Fig. 12 presents a psychrometric chart of the typical measured states of the experimental results; point 1 denotes the outdoor climate, point 2 refers to the air flowing out from the pad after being humidified, and point 3 represents the measurement point 44 m away from the pad that can be considered as the point with the highest temperature along the pad-fan direction. For the points with the subscript ‘‘w’’, the data were collected from present experiments. The numbers with apostrophe marks represent the results obtained on August 5th. In conditions of high humidity, the air temperature cannot be reduced as much as in a dry climate [16] (shown as the process 1d to 2d) after the air flows through the cooling pad. Under conditions of high RH (approximately 70%) such as those seen in Shanghai, ambient air (state 1w) can be cooled down to approximately 28 °C (state 2w) and the humidity ratio can be increased from 0.0202 to 0.0236 kg/kg after the air flows through the evaporative cooling pads and is humidified (The slight increment of enthalpy during the process 1w to 2w could be explained by the measurement errors, and the intrinsic enthalpy the water is carrying before in its liquid state, right before it is evaporated). Temperature measured 44 m away from the pad under the EC + ES mode was successfully kept under the ambient
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temperature, and slight increase in humidity ratio (0.0002 kg/kg (EC + ES, 2w–3w) was observed. 4.1. System analysis from temperature/humidity perspective When evaporative cooling alone was used in a 48-m-long multi-span greenhouse, no cooling effect was seen in the half area near the fans (Fig. 7(a)) where the temperature reached 37 °C (44 m away from the fans); however, as shown in Fig. 5, the temperature in the first half of the greenhouse nearing the cooling pad is appropriate for plant growth. At positions far away from the cooling pad, the high incoming solar radiation results in an even worse performance of the fan–pad system alone compared to natural ventilation. Therefore, evaporative cooling should be complemented with other methods under conditions of high humidity. The combination of evaporative cooling, external shading nets and internal thermal screens were able to keep the indoor air temperature at an acceptable level (Figs. 4 and 5) and narrow the horizontal air temperature gradient by reducing the amount of incoming solar radiation. As shown in Fig. 7, the temperature gap between the positions 4 m and 44 m away from the pad was lowered by 6 °C and 7 °C with the use of external and internal shadings, respectively. External shadings are installed outside the greenhouse and only influence the solar radiation, while internal thermal screens also reduce the volume of indoor air that needs to be cooled, thus leading to a better performance of the evaporative cooling pad system. Because solar radiation is a critical factor for photosynthesis, only half of the area of the internal thermal screens was used to provide as much sunshine as possible to the plants. Although the internal thermal screens were not fully used, satisfactory results were nevertheless obtained. The temperature near the extracting fans and evaporative cooling pads was 1–2 °C and 3–4 °C lower, respectively, than the ambient temperature, even under intense radiation of 1000 W/m2. The temperature gradient in the horizontal direction can be reduced to less than 2 °C. Generally speaking, internal shading is more effective than external shading when used in combination with evaporative cooling pads system because of the smaller air volume that need to be cooled with internal shading. However, considering the importance of photosynthesis, adequate solar radiation should be provided to the plants and crops. The use of screens needs to be carefully managed to insure that plants can grow healthily. It is suggested that the internal thermal screens be applied only at peak times. It is also recommended that shadings be used in conditions of intense radiation, such as at midday, in humid regions. Circulation fans are usually adopted to remove air stratification, but the results of this study indicate that circulation fans do not significantly improve the uniformity of the horizontal air temperature (Fig. 7(d)). Circulation fans can reduce the air temperature in the higher parts of the greenhouse (4 m above the ground) (Fig. 8(d)); however, because most plants and crops are cultivated in the bottom part of the greenhouse, the vertical temperature distribution is not that important. Hence, circulation fans are not recommended for use with cooling pad systems. The effectiveness of the humidification of the evaporative cooling pad has been demonstrated in the experiments and the internal thermal screen scan help to conserve high relative humidity conditions of the greenhouse air (Fig. 10). The average relative humidity can be maintained stable at approximately 80%, which is 15% higher than the outdoor value. Air tightness is a key factor for the adequate performance of a fan–pad cooling system. Although hot air is easily accumulated in the top space of the greenhouse due to its buoyant force, leaving a leak in the ridge to remove the heat is not helpful. The result shown in Fig. 11 proved that even small amounts of turbulence will strongly disturb the air flow in the horizontal direction of the lower
part area, which leads to an increment in the air temperature. As a result, the greenhouse needs to be carefully sealed when evaporative cooling pads system is in place, especially in the vertical direction. It is also noteworthy that the fan-system should be turned on prior to closing the windows. When the windows are closed, the hot air that had accumulated in the top of the greenhouse cannot be released, thus leading to a sharp increase in temperature inside the space, which is very unfavourable for the plants or crops. Moreover, considering that the temperature uniformity of the air flow cannot be achieved because solar radiation will heat the air throughout the enclosed space, and because plant photosynthesis is extremely vital, it is useful to strategically place the plants in accordance with their characteristics. For example, hightemperature-resistant plants, such as soya bean and cucumber [22], can be placed near the extracting fans side, while plants that cannot withstand high temperatures, like lettuce [22], should be placed near the cooling pads. 4.2. System analysis from energy perspective The above sections mainly deal with the temperature/relative humidity variation/gradient of air, which indicates the applicability of the evaporative cooling pad system even in the climate with high humidity, while in the following part, system analysis from energy saving perspective is focused. To offer a convincing explanation about the energy conserving merit of the evaporative cooling fan–pad system, a comparison between the afore-presented system and the conventional air conditioner is made. The cooling capacity of the evaporative system is calculated by Eq. (2) with the experimental data (inlet and outlet air temperature of the evaporative cooling pad) which is shown in Fig. 12. The power that needs to be consumed by the air conditioner to provide the same amount of cooling capacity is calculated by Eq. (3).
_ a c ¼ ðSHi SHo Þ ðqa V_ a Þ c Q_ ¼ DSH m
ð2Þ
in which c and V_ a refer to the latent heat of water vaporisation and the volumetric flow rate of the air, respectively; while 4SH represents the difference of air specific humidity (also called humidity ratio) between the outlet and the inlet at the evaporative cooling pad.
_ ¼ Q_ =COP W
ð3Þ
where 7 is assumed as the COP (coefficient of performance) value of the conditioner. The calculation result indicates that evaporative cooling fan–pad system can save more than 9 times electricity energy consumption than the air conditioner system based on our experimental result because the electricity is only consumed by the extracting fans and circulating pump in the former option. In fact, the value set for the COP here (7) is a comparatively ideal value for an air conditioning system to achieve, which implies the evaporative cooling fan–pad system may have even better energy-saving performance in real operation condition. Energetic quantities (as tabulated in Table 4) extracted from the greenhouse in each working mode are calculated to evaluate the contribution they made to the overall cooling effect. The air inside the greenhouse is viewed as a control volume and steady state in a short period is assumed. In addition, to make these
Table 5 Ambient parameters chosen for the calculation. Mode
NV
EC
EC + ES
EC + ES + IS
Standard Deviation
T amb: (°C) I (W/m2)
31.8 789.7
31.4 871.3
32.2 865.1
32.7 834.8
0.5 37.3
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Fig. 13. Comparison of the temperature and equivalent cooling effect under different operating modes.
quantified results comparable, the durations with the similar average ambient temperature and solar radiation input are selected as shown in Table 5. The air temperature inside the greenhouse is determined by the mean value measured at 12 m and 36 m from the pad. Thus, the equivalent cooling effects brought by the listed cooling modes can be estimated by Eq. (4).
ca ma ðT a T amb: Þ Q_ eq ¼
s
2. The indoor temperature and humidity of the greenhouse responds rapidly to the changing operation modes. 3. Solar irradiation is the key factor that influences the indoor temperature of the greenhouse. Shading is an effective way to partially isolate unwanted incoming solar energy so that the heat load inside the enclosed space can be reduced and the uniformity of the air temperature can be achieved. According to the experimental results, the internal air can be kept 2–3 °C below the ambient temperature at a relative humidity of 80% using both evaporative cooling pad and shadings. 4. Circulation fans do not significantly improve the temperature in comparison to shadings when evaporative cooling pads are used. 5. When the evaporative cooling system is turned on, the greenhouse must be carefully closed/sealed to avoid disturbances from unnecessary air flows. 6. Although for a greenhouse in the subtropical climate, using evaporative cooling pan and fan system is not the most satisfactory way because it is not as efficient as it in the dry climate, it has the advantage of the energy-saving and easily-adjust over the other alternatives, such as air conditioning and composite system. When the fan–pad alone is not sufficient, proper use of shadings is necessary and effective, as demonstrated by the experimental results. At the same time, to help the photosynthesis process of the plants, we can adjust the strategy of shading-use based on the real situation, such as plant species and their growing conditions and real-time solar radiation values.
ð4Þ
in which s refers to the duration chosen for the calculation whereas T a and T amb: represent the mean temperature of the interior air and the ambient during the chosen period. Fig. 13 illustrates the comparison of the temperature and equivalent cooling effect under different operating modes. The negative value of Q_ eq indicates in which working mode, the average interior air temperature is higher the ambient value. As shown in Fig. 13, by means of NV, the air temperature in the greenhouse exceeded 36.5 °C even when the solar radiation is about 10% lower compared to that in the EC mode. Although the Q_ eq for the EC mode is 9 kW, remarkable cooling effect can still be observed compared to NV, which indicates that even in the humid climates, EC can partially cover the cooling load. To keep the average air temperature under the required level (32 °C as mentioned before) under this operating mode (EC alone), additional 5.5 kW cooling effect should be provided by other methods. When the EC + ES and the EC + ES + IS modes are applied, there is no need for the other cooling method in these cases. Furthermore, it should be mentioned that although the additional IS can further improve the cooling effect on the basis of EC + ES mode, the concurrent side effect such as the negative one on photosynthesis should be paid attention to. 5. Conclusions This paper describes experiments studying the cooling of a greenhouse with a fan–pad system in Shanghai in a humid subtropical climate. Five working modes of cooling consisting of the combination of evaporative cooling with other methods (internal thermal screens, external shading nets and circulation fans) were investigated. The results indicate that, with proper modes, the performance of the cooling system is acceptable, even under conditions of high temperature and high solar intensities. The main conclusions of the study are listed below: 1. Under high RH condition such as in Shanghai, air can be cooled to approximately 27–29 °C after air flows through and is humidified by the evaporative cooling pads.
In conclusion, the evaporative cooling pad system is an efficient way to prevent greenhouse overheating in the summer which is worth being popularised. Acknowledgements This work was supported by the Key project of the Natural Science Foundation of China for international academic exchanges under the contract No. 51020105010 and POREEN-Partnering Opportunities between Europe and China in the Renewable Energies and Environmental industries, People Marie Curie Actions International Research Staff Exchange Scheme, (FP7-PEOPLE2012-IRSES). References [1] China Meteorological Administration.. [2] Kumar KS, Tiwari KN, Jha MK. Design and technology for greenhouse cooling in tropical and subtropical regions: a review. Energy Build 2009;41(12):1269–75. [3] Panwar NL, Kaushik SC, Kothari S. Solar greenhouse an option for renewable and sustainable farming. Renew Sustain Energy Rev 2011;15(8):3934–45. [4] Vadiee A, Martin V. Energy management in horticultural applications through the closed greenhouse concept, state of the art. Renew Sustain Energy Rev 2012;16(7):5087–100. [5] Vadiee A, Martin V. Energy management strategies for commercial greenhouses. Appl Energy 2014;114:880–8. [6] Yang S-H, Rhee JY. Utilization and performance evaluation of a surplus air heat pump system for greenhouse cooling and heating. Appl Energy 2013;105: 244–51. [7] Lee I-B, Bitog JPP, Hong S-W, Seo I-H, Kwon K-S, Bartzanas T, et al. The past, present and future of CFD for agro-environmental applications. Comput Electron Agric 2013;93:168–83. [8] Campen JB, Bot GPA. Determination of greenhouse-specific aspects of ventilation using three-dimensional computational fluid dynamics. Biosyst Eng 2003;84(1):69–77. [9] Vadiee A, Martin V. Energy analysis and thermoeconomic assessment of the closed greenhouse – the largest commercial solar building. Appl Energy 2013;102:1256–66. [10] Sethi VP, Sharma SK. Survey of cooling technologies for worldwide agricultural greenhouse applications. Sol Energy 2007;81(12):1447–59. [11] Chen Q, Pan N, Guo Z-Y. A new approach to analysis and optimization of evaporative cooling system II: Applications. Energy 2011;36(5):2890–8. [12] Al-Jamal K. Greenhouse cooling in hot countries. Energy 1994;19(11): 1187–92.
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[21] Lychnos G, Davies PA. Modelling and experimental verification of a solarpowered liquid desiccant cooling system for greenhouse food production in hot climates. Energy 2012;40(1):116–30. [22] Davies PA. A solar cooling system for greenhouse food production in hot climates. Sol Energy 2005;79(6):661–8. [23] Yang Ming-Ta, Chen Cheng-Chuan, Kuo Yen-Long. Implementation of intelligent air conditioner for fine agriculture. Energy Build 2013;60: 364–71. [24] Attara I, Nailia N, Khalifab N, Hazamia M, Lazaara M, Farhata A. Experimental study of an air conditioning system to control a greenhouse microclimate. Energy Convers Manage 2014;79:543–53. [25] Lazaar Mariem, Bouadila Salwa, Kooli Sami, Farhat Abdelhamid. Conditioning of the tunnel greenhouse in the north of Tunisia using a calcium chloride hexahydrate integrated in polypropylene heat exchanger. Appl Therm Eng 2014;68(1–2):62–8. [26] ASHRAE. Evaporative air-cooling equipment. Equipment Handbook. American Society of Heating, Refrigeration, and Air-Conditioning Engineers. Atlanta, Georgia; 1983. [27] ASAE. Standard engineering practice data. ASAE Standards. 42nd ed.; 1995.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Experimental performance of evaporative cooling pad systems in greenhouses in humid subtropical climates J. Xu a, Y. Li a, R.Z. Wang a,⇑, W. Liu a, P. Zhou b a b
Institute of Refrigeration and Cryogenics, Key Laboratory of Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, China
h i g h l i g h t s Experimental performance of evaporative cooling in humid climate is investigated. 5 working modes are studied in the greenhouse. Vertical and horizontal temperature and relative humidity variations are analysed. Indoor temperature can be kept in required level by proper working modes.
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 8 September 2014 Accepted 24 October 2014
Keywords: Humid subtropical climate Evaporative cooling Greenhouse energy conservation Temperature and humidity distribution Quantified analysis of energy
a b s t r a c t To solve the overheating problem caused by the solar radiation and to keep the indoor temperature and humidity at a proper level for plants or crops, cooling technologies play vital role in greenhouse industry, and among which evaporative cooling is one of the most commonly-used methods. However, the main challenge of the evaporative cooling is its suitability to local climatic and agronomic condition. In this study, the performance of evaporative cooling pads was investigated experimentally in a 2304-m2 glass multi-span greenhouse in Shanghai in the southeast of China. Temperature and humidity distributions were measured and reported for different working modes, including the use of evaporative cooling alone and the use of evaporative cooling with shading or ventilation. These experiments were conducted in humid subtropical climates where were considered unfavourable for evaporative cooling pad systems. Quantified analyses from the energy perspective are also made based on the experimental results and the evaporative cooling fan–pad system is demonstrated to be an effective option for greenhouse cooling even in the humid climate. Suggestions and possible solutions for further improving the performance of the system are proposed. The results of this work will be useful for the optimisation of the energy management of greenhouses in humid climates and for the validation of the mathematical model in future work. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction A greenhouse is a plastic or glass structure that can provide a suitable micro-climate for various crops and plants, protecting them from severe and variable outdoor weather conditions. During the daytime with intense solar radiation, the greenhouse effect caused by the absorption of solar radiation makes the temperature inside the greenhouse even higher than the ambient temperature; thus, plants and crops may suffer being dried due to high rate of evapotranspiration caused by the excessive heat, and their ⇑ Corresponding author at: 800 Dong Chuan Road, Shanghai 200240, China. Tel.: +86 21 34206548. E-mail address: [email protected] (R.Z. Wang). http://dx.doi.org/10.1016/j.apenergy.2014.10.061 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
production may decline. To provide a suitable growing environment, cooling technologies are essential and must be introduced. It plays a vital role in the greenhouse industry especially in hot weather although it is still important in any other climate conditions. The climate of Shanghai is humid subtropical and features hot, humid summers and chilly, damp winters. Table 1 shows the typical variation in climate conditions in Shanghai. Summer sees high temperature that plants and crops cannot endure; as a result, air cooling technology is urgently required. Over the past decades, many relevant studies and experiments of greenhouse cooling technologies have been conducted throughout the world [2–6]. The method of computational fluid dynamics (CFD) [7,8] and TRNSYS [9] are applied to study the energy
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Nomenclatures RH (%) Tamb. (°C) Tin (°C) TH (°C) TL(°C) Tdo (°C) Tdi (°C)
relative humidity ambient temperature temperature inside the greenhouse highest temperature lowest temperature dry bulb temperature of outside air dry bulb temperature of cooled air exiting the pad Two (°C) wet bulb temperature of the outside air g efficiency of the evaporative pad NV natural ventilation (with roof vents and doors widely open) EC evaporative cooling ES external shading IS internal thermal screen CF circulation fan SHo (kgH2O/kgair) sensible humidity (humidity ratio) of outside air SHi (kgH2O/kgair) sensible humidity (humidity ratio) of cooled air exiting the pad
management of greenhouse. Also, it is mentioned [4] that the most energy consuming sectors in the agricultural industry is the greenhouse, which implies that reducing the greenhouse energy consumption can contribute significantly to the sustainable energy management. Cooling technologies for agricultural greenhouse applications can be classified as ventilation (natural and forced), shading, evaporative cooling and composite systems (heat exchanger) [10]. Of these, evaporative cooling is considered to be the most efficient and affordable method [11]. When the outdoor hot air flows through the wet material, water evaporation takes place on the wet surface and removes the sensible heat from the air, thus lowering the temperature. Currently, there are three evaporative cooling technologies in use: fan–pad, fog/mist and roof evaporative cooling [10,12]. The fan–pad system consists of fans on one side of the greenhouse wall and porous pads on the other side. Outdoor air is forced through the wet evaporative cooling pads by the extracting fans, heat is absorbed from the air by the water vaporisation process, and the air enters the greenhouse at a lower temperature than the ambient value. Evaporative cooling pads have played an important role in thermal environmental control in both agriculture and the poultry industry. The performance of such systems in the regulation of the greenhouse temperature has been evaluated in certain studies [13,14]. Kittas et al. [15,16] investigated the efficiency of fan–pad systems (both shaded and unshaded) in a greenhouse covered with double inflated polyethylene films in Greece and suggested that shading is not necessary in dry climates, as the fan–pad system alone is capable of preventing the overheating of crops. Liao and Chiu [17] designed a small-scale wind tunnel prototype to simulate the performance of fan–pad systems with two alternative pad materials (coarse fabric PVC sponge mesh and fine fabric PVC sponge mesh) in Taiwan where the mean relative humidity (RH) is approximately 50%. Shukla et al. [18]
c (kJ/kg) V_ (m3/s) _ (kg/s) m Q (kW) q (kg/m3) COP c (kJ/kg K) s (s) m (kg) T (°C) I (W/m2)
latent heat for vaporisation of water (2430) volumetric flow rate mass flow rate cooling capacity density coefficient of performance specific heat time mass mean temperature mean value of solar radiation
Subscript d w a amb.
dry climate humid climate air ambient
tested the combination of an evaporative cooling system and an inner thermal curtain in a cascade greenhouse and concluded that evaporative cooling is effective in hot summers but the fan–pad system is not recommended in such a greenhouse structure because it would increase the interior air temperature of the second greenhouse due to addition of extra thermal energy from the adjacent one. Jain and Tiwari [19] presented a mathematical model to predict the thermal behaviour of a greenhouse equipped with an evaporative cooling system. However, earlier reports [2,20] mentioned that evaporative cooling pads are seldom suitable in humid tropical regions because of the high humidity levels. Temperature reduction of the air after evaporation process is limited due to the ambient high humidity, thus making the performance of such technology is not as satisfied as in the dry climate region. According to author’s knowledge, the existing studies [21,22] that intended to improve such problem utilised auxiliary equipments like desiccators for air pretreatment. Besides, systems such as air conditioner or earth-to-air heat exchanger (EAHES) are usually applied in the humid climate to lower the interior air temperature but with the major disadvantage of the high initial cost as well as the subsequent operation and maintenance expenses [4]. Air conditioning systems are also studied to control greenhouse microclimate [23–25]. In light of the growing market of commercial greenhouses for cultivation of rare/high-price plants or flowers recently, evaporative cooling system which is favoured over the other alternatives due to its simplicity, reliability and cost/benefit is worth being popularised, especially on a large scale application. In this study, as previously mentioned, because natural ventilation alone is far from sufficient in Shanghai, evaporative cooling pad systems are usually used to adjust the inside air temperature to a comfortable level. Regarding that the yearly average relative humidity in Shanghai is more than 70%; as a result, other methods such as the combination of ventilation with shadings may be
Table 1 Historical climate data for Shanghai [1]. Month
January
February
March
April
May
June
July
August
September
October
November
December
Record high (°C) Average high (°C) Average low (°C) Record low (°C) Relative humidity (%)
22 8.1 1.1 10 75
26 9.2 2.2 8 74
29 12.8 5.6 6 76
34 19.1 10.9 1 76
36 24.1 16.1 7 76
37 27.6 20.8 12 82
40 31.8 25 16 82
41 31.3 24.9 19 81
37 27.2 20.6 11 78
32 22.6 15.1 2 75
29 17 9 4 74
23 11.1 3 8 73
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Fig. 1. Photographic view of the greenhouse and main components (a) general view; (b) evaporative cooling pad; (c) extracting fan; (d) external shading net; (e) internal thermal screen; and (f) circulation fan.
necessary. Therefore, shadings, which are very common in greenhouse construction, other than additional mechanical equipments are used to improve the cooling effect in humid regions. Such system taking full use of the existing components in the greenhouse is easy to control and cheap. The purpose of this study was to evaluate the performance of evaporative cooling pad-fan system in greenhouses in a humid subtropical climate, extending the application from only in the dry climate to the regions with relatively high humidity. Experiments were made with the objective of evaluating the combined effects of the fan–pad system and other methods. A 2304-m2 glass multi-span greenhouse installed with an evaporative cooling pad was studied. The temperature, relative humidity and air velocity were measured at different locations in the greenhouse. The effects of evaporative cooling combined with other methods such as external shading nets, internal thermal screens and circulation fans were investigated. Based on the experimental results, the idea of using existing greenhouse components as assistants to improve the cooling effect of evaporative cooling fan–pad in subtropical area is feasible and it allows for less consumption in both energy and investment aspects compared with other options. 2. Materials and methods 2.1. The greenhouse The greenhouse was a 48-m-wide 48-m-long multi-span greenhouse located in Shanghai at a latitude 31°050 Nandlongitude 121°50 E. The greenhouse was a 5- 9.6-m-span structure covered with 4-mm float glasses. There are two entrance-exits along the fan–pad direction in the greenhouse. Fig. 2(a) shows a photo of the greenhouse and Table 2 lists its dimensions. The greenhouse was commercially operated for some parent plants cultivation, including chrysanthemum, lettuce and cucumber. The plantation lines are parallel to the air stream. 2.2. Evaporative cooling system Corrugated cellulose paper pads (Fig. 1(b)) that were 42.1 m long and 1.9 m wide were situated on the north wall. Water was
Fig. 2. Schematic of the location of circulation fans in the greenhouse (dimensions are in m). Red dotted line represents the plane where the temperature and relative humidity sensors were placed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 2 Design detail of the greenhouse. Width of the greenhouse (m) Length of the greenhouse (m) Width of the greenhouse entrance-exit (m) Height of the greenhouse entrance-exit (m) Number of extracting fans Power of extracting fans (kW) Distance between two adjacent extracting fans (m) Height of cooling pads (m) Height of cooling pads from the ground (m) Number of circulation fans Power of circulation fans (W)
48 48 2.4 3 10 1.1 3.2 1.9 1.25 10 90
pumped upwards then released onto the top of the cooling pads. Ten 1.1-kW extracting fans were installed on the south wall of the greenhouse (Fig. 1(c)). Ambient air was dragged inwards by the extracting fans and flowed through the greenhouse after being humidified and cooled by the wet pads.
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The efficiency of the evaporative pad is calculated by Eq. (1) [26]:
g¼
T do T di 100% T do T wo
ð1Þ
in which Tdo is the dry bulb temperature of the outside air, Tdi is the dry bulb temperature of cooled air exiting the evaporative pad, and Two is the wet bulb temperature of the ambient air. 2.3. Shadings Flexible shadings, including both internal thermal screens and external shadings nets, were installed throughout the greenhouse to keep out the excessive radiation during the summer time. The black external shading nets (Fig. 1(d)) were made of highdensity UV polyester strips that provided 70% shading in our experimental greenhouse. They were used to reduce the amount of incoming solar radiation when necessary. Internal thermal screens (Fig. 1(e)) were made of 50% shadereflective aluminium fibre. The properties of these materials allow the thermal screens to both prevent radiation from entering the lower part of the greenhouse during the hot summer and retain heat inside the greenhouse during the winter time. 2.4. Circulation fans Circulation fans (Fig. 1(f)) were installed to provide stable and moderate air flow to enhance the heat exchange between the air and the plants. By increasing the effect of plant transpiration, plant temperatures can be reduced. Fig. 2 shows the placement of circulation fans that were installed 2.5 m above the ground. The air flow occurs in a continuous pattern. The red dotted line marks the plane where the temperature and humidity sensors were placed. The power of each circulation fan was 90 W. 2.5. Measurement and data collection system The temperature and relative humidity inside the greenhouse were measured at various positions (shown in Fig. 3) by thermistor thermometer and relative humidity probes (WTHOT1, Wangyunshan, China) with respective accuracies of ±0.5 °C and ±5% RH. The purpose of such an arrangement was to investigate the vertical and horizontal temperature and relative humidity distribution inside the greenhouse. The data were automatically recorded at one-minute intervals. The temperature of the air flowing in and out of the evaporative cooling pads was measured manually with two wet and dry bulb thermometers (272-A, Huachen, China) with the accuracy of ±0.5 °C. The velocity of the air flowing through the greenhouse was measured with a hot-wire anemometer (A531, Kanomax, Japan) with an accuracy of ±2%. Hot wire anemometers use a wire electrically heated up to some temperature above the
ambient and after the air flowing past the wire, a cooling effect occurs on the wire. Since the electrical resistance of most metals is dependent upon the temperature of the metal, a relationship can be obtained between the resistance of the wire and the flow speed. Because the air has different psychrometric properties under different working modes in the greenhouse, the half of the green house nearest the cooling pad was named First Half Area and the half near the extracting fans was called Second Half Area, as shown in Fig. 3. At the same time, outside climate variables were recorded every 60 s with a weather station (Vantage Pro2, Davis, USA) that included a rain collector, an anemometer, and temperature, humidity and solar radiation sensors. The ambient temperature on August 14 and 15 of 2012 were recorded every hour. 3. Results The experiments were carried out on August 4, 5, 14, and 15 of 2012. The weather conditions of these days are listed in Table 3. The relative humidity and solar radiation listed are the mean values during the daytime (from 8 am to 6 pm). TH and TL refer to the highest and lowest temperatures of the daytime period, respectively. According to the standard engineering data from ASAE [27], the temperature inside the greenhouse should be lower than 30–32 °C. However, the measured ambient temperature (Table 3) was higher than this value and the inside temperature would have been even higher if active cooling had not been applied. Hence, the evaporative cooling pads and fans were used when the indoor temperature was higher than 30 °C; otherwise, natural ventilation is capable. Given the measured wet and dry bulb temperature on both sides of the evaporative pad (Tdo = 32.7 °C, Tdi = 28.3 °C, Two = 26.9 °C), the efficiency of the wet pad was approximately 75.9%. After humidification, the air could be cooled to 27–29 °C under humid conditions. The measured average velocity of the air stream in front of the pad was 1.1 m/s. Five major working modes that combine different cooling/temperature control methods were implemented to investigate their effects on the thermal environment of the greenhouse. Table 4 gives a detailed description of the working modes. In the natural ventilation mode, the roof vents and doors of the greenhouse were left wide open.
Table 3 Weather conditions during the daytime (8:00–18:00). Date August August August August
4 5 14 15
TH (°C)
TL (°C)
Mean RH (%)
Mean solar radiation (W/m2)
32.3 33.8 34 35
29.4 29.8 29.4 28
74.3 66.7 72.3 74.1
472.1 603.3 598 570.7
Fig. 3. Measurement positions in the central section of the greenhouse (dimensions are in m).
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Table 4 Operating modes for the control of the thermal environment in the greenhouse. Mode
Description
NV EC EC + ES EC + ES + IS
Natural ventilation (with roofs and doors wide open) Evaporative cooling pad Evaporative cooling pad combined with external shading Evaporative cooling pad combined with external shading and internal thermal screens Circulation fans with other methods
EC (. . .) + CF
The influence of each working mode on the temperature and relative humidity distribution in the greenhouse was investigated and reported accordingly.
3.1. Influence of different working modes on air temperature control inside the greenhouse
Fig. 5. Temperature and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 5th, 2012).
In this subsection, the temperature variations under different operating conditions were studied. The measurement point (T2b in Fig. 3) 12 m from the evaporative cooling pad at a height of 1.5 m was selected as a representative point to analyse the variations in temperature. Fig. 4 shows the variation in solar radiation, ambient temperature and indoor temperature of the greenhouse on August 4th, 2012. Three working modes were applied: (1) natural ventilation (NV), (2) evaporative cooling (EC), and (3) evaporative cooling combined with external shading nets (EC + ES). From 8:00 to 8:30 am, the roof vents and entrance/exit doors were left open; the temperature inside the greenhouse ranged from 30 to 31 °C, which was 1–2 °C higher than the ambient temperature. From 8:30 onwards, the solar irradiation rose to more than 500 W/m2, thus leading to a significant increase in indoor temperature. The difference between the indoor and ambient temperature exceeded 4 °C, and the temperature inside the greenhouse reached 34 °C, which is harmful to the plants and crops. At 9:50 am, the fan–pad system was turned on to cool the greenhouse and the roof vents were closed. Initially, the air temperature in the greenhouse decreased significantly due to the evaporative cooling pad-fan system. Then, the inside air temperature increased slightly while Tamb increased. At midday (from 12:30 to 13:40), the greenhouse switched to the working mode, where both evaporative cooling and external shading (EC + ES) were used concurrently. The effect of the external shadings was immediate, as the indoor temperature decreased to 2–3 °C lower than the ambient temperature. Subsequently, when Tamb decreased to approximately 31 °C, the external shading was removed to allow the incoming solar
radiation to penetrate into the greenhouse. From 16:15 to 18:00, there was almost no difference in temperature between the indoor and the ambient environment due to the low solar radiation of approaching dusk. When comparing the variations in solar radiation and the greenhouse temperature in Fig. 4, it is obvious that solar radiation is the key factor that affects the greenhouse temperature. With the help of external shadings, the influence of solar radiation can be reduced. Fig. 5 illustrates the indoor temperature profile measured on August 5th, 2012 under the operation mode consisting of the evaporative cooling pad system combined with both external and internal shadings (EC + ES + IS). As previously described, external shading nets can prevent part of the solar radiation from entering the greenhouse, which can decrease the indoor heat load and heat disturbance of the air. From 9:40 am onwards (Fig. 5), evaporative cooling pads and external shading nets were used; Tin dropped immediately by 7 °C and remained 2–4 °C lower than the ambient temperature. Starting from 11:05 am, internal thermal screens were spread partially throughout the greenhouse (approximately 50% of the area was shaded with internal thermal screens) to further regulate the micro-climate; the indoor temperature Tin decreased by another 2 °C and stayed 4 °C lower than Tamb, even under 1000 W/m2 outside solar radiation at midday. Circulation fans were used along with the other methods in the experiment on August 14th (Fig. 6). The influences of different working modes on temperature were similar to the results obtained in Figs. 5 and 6. The largest temperature difference
Fig. 4. Temperature (T2b) and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 4th, 2012).
Fig. 6. Temperature and solar radiation variation (4 – T2b (12 m); j – T(Amb.)) (August 14th, 2012).
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between Tin and Tamb. (in the range of 2–4 °C) was achieved with the EC + ES + CF working mode. No significant improvement in the results described in Figs. 5 and 6 was observed with the addition of circulation fans.
3.2. Influence of different working modes on air temperature variation in the horizontal direction To investigate the influence of different working modes on the horizontal air temperature throughout the greenhouse, the temperature was measured at 4 m, 12 m, 28 m, 36 m and 44 m from the pad at a height of 1.5 m above the ground. The horizontal temperature variations from the evaporative pad under different working modes are shown in Fig. 7. When the fan–pad system was used alone, a distinct temperature gradient was formed throughout the greenhouse in the fan–pad direction (Fig. 7(a)). Temperatures at 4 m and 12 m away from the pad were 29 and 31 °C, respectively, which were lower than the ambient temperatures. At 12 m from the pad, the temperature varied within the range of 31–33 °C, which would barely allow the plants to survive. Beyond 12 m, satisfactory cooling could not be achieved. The temperature difference between the two ends of the green house (4 m and 44 m from the pad) was approximately 9 °C. The temperature variations under EC + ES mode and EC + ES + IS mode are shown in Fig. 7(b) and (c), respectively. When external shading nets and evaporative cooling (EC + ES) were used, almost all of the measurement positions recorded temperatures lower than the ambient temperature, even near the extracting fans
at hot noontime. The temperature difference between the two ends was narrowed to 3–4 °C and could be further reduced to 2–3 °C with partial shading using the flexible internal thermal screens. The overall indoor temperature was kept 2–4 °C lower than the outside temperature, and no significant fluctuations in temperature were observed. Fig. 7(d) shows the results of adding circulation fans. The temperature difference between the positions 4 m and 44 m away from the wet pad was 3–4 °C, which was almost the same as the difference obtained without additional circulation fans. This indicates that circulation fans do not contribute to the equalisation of the horizontal air temperature gradient.
3.3. Influence of different working modes on the variation of vertical air temperature To evaluate the influence of different working modes on the vertical air temperature gradient in the greenhouse, measurements taken 1.5 m, 3 m, and 4 m above the ground were analysed. The measurement sites 1.5 m and 3 m above the ground were within the range of the cooled air stream of the evaporative pads, while the measurement position 4 m above the ground was above the cooled air stream. Measurement points 28 m away from the pad (refer to T3 in Fig. 3) were selected, as the air flow was stable at that position. When using the evaporative cooling pad alone (Fig. 8(a)), temperatures inside the greenhouse were greatly influenced by solar radiation. Because of the buoyancy effect, the temperature in the top area of the greenhouse envelope (4 m above the ground)
Fig. 7. Influence of working modes on horizontal air temperature (s 4 m; 4 – 12 m; } – 28 m; 5 – 36 m; – 44 m; j – Amb.): (a) EC; (b) EC + ES; (c) EC + ES + IS; and (d) EC + ES + CF ((a and b) on August 4th; (c) on August 5th; and (d) on August 14th).
J. Xu et al. / Applied Energy 138 (2015) 291–301
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Fig. 8. Vertical air temperature variation 28 m away from the wet pad (T3): (a) EC; (b) EC + ES; (c) EC + ES + IS; and (d) EC + ES + CF ((a) on August 4th; (b and c) on August 5th; and (d) on August 14th, 2012) (s – 1.5 m, – 3 m; 4 – 4 m).
was approximately 4 °C higher than in the lower part (1.5 and 3 m from the ground). As seen in Fig. 8(b), a 0.5 °C temperature difference between the 1.5-m and 3-m positions was observed as a result of the use of external shadings. At the same time, the difference in temperature between positions at 3 m and 4 m was slightly reduced to 3 °C. When the partially-spread internal thermal screen was used (Fig. 8(c)), the vertical air temperature difference became more apparent. The temperature at 4 m (just below the internal screens) remained the same, while the temperature at lower positions decreased instantly due to the reduction in solar radiation (in comparison with Fig. 8(b)). In this working mode, the temperature differences between measurement positions at 1.5 m and 3 m/3 m and 4 m were 1.5 and 4 °C, respectively. Fig. 8(d) shows the vertical air temperature profiles under the EC + ES + CF operating mode. It appears that a quite different effect on the vertical distribution of temperature was produced with circulation fans and shading methods: the temperature difference increased at the lower position (between 3 m and 1.5 m) and decreased at the higher position (between 3 m and 4 m). The vertical air temperatures 12 m away from the wet pad (refer to measurement T2 in Fig. 3) are given in Fig. 9.Combinedwith the results obtained 28 m away (Fig. 8(b) and (c)), it can be found that the temperature gap between the ends of the lower part (1.5 m and 3 m) gradually decreased with increasing distance from the evaporative pad. For example, at 12 m, the temperature difference in the lower part was 2.5 °C; at 28 m, the gap was reduced to 0.5 °C in the EC + ES working mode, which means that the distribution in air temperature in the second half (near the fan) of the
greenhouse is even more pronounced than in the first half (near the pad). 3.4. Influence of different working modes on air relative humidity in the greenhouse The variations in RH both inside and outside the greenhouse under four different operating conditions on August 4th, 5th,
Fig. 9. Vertical air temperature variation at 12 m from wet pad (T2) on August 5th (s – 1.5 m, – 3 m; 4 – 4 m).
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Fig. 11. Temperature variation inside the greenhouse (conducted on August 15th) (s – 4 m; 4 – 12 m; } – 28 m; – 44 m; j – Amb.).
Fig. 10. Variation of relative humidity at different distances from the evaporative pad under four different operating conditions conducted on two days (s – 4 m; – 20 m;5 – 44 m; j – Amb.): (a) NV, EC + ES & EC + ES + IS on August 5th and (b) EC on August 4th. Fig. 12. Comparison of the air psychrometric conditions under dry/humid climate.
2012 are shown in Fig. 10. The ambient RH was in the range of 60– 70% throughout the experimental period. When the evaporative cooling pads were not working (NV in Fig. 10(a)), the RH inside the greenhouse was at 40–55% due to the high indoor temperature. When the fan–pad system was used in combination with external shading nets, the indoor RH increased instantly from 50% to 70– 80%. Subsequently, the slight increase in RH (approximately 5%) confirmed that the internal thermal screens can keep the air inside the greenhouse enclosure at a high relative humidity conditions. The distribution of RH is shown in Fig. 10(b) for another operating condition in which the fan–pad system was used without shadings. The difference in RH between the two ends of the greenhouse (4 m and 44 m) reached 10%. Moreover, the position near the aisle in the middle of the greenhouse had a lower plant density, resulting in less of a transpiration effect; as a result, the measurement point 20 m away from the cooling pad had the lowest relative humidity. 3.5. Influence of air tightness on air temperature In the experiment conducted on August 15th, a short test period was conducted (⁄ in Fig. 11) when the roof vent was partially opened. In the first phase, external shading nets were applied to assist the fan–pad system and Tin was controlled at an appropriate level. Starting from 9:28 am, the roof vent was opened slightly; the temperature increased immediately, thus dramatically reducing the effect of evaporative cooling pads. During this short period of 20 min, the temperature in the greenhouse rose by 3 °C, reaching 34 °C. At 9:48 am, the roof vents were closed again and the air temperature returned to its normal value.
4. Discussion The experimental results show that the temperature and relative humidity inside the greenhouse vary rapidly in response to changes of outside environmental conditions. Solar radiation is one of the key factors that influence the internal micro-climate. Fig. 12 presents a psychrometric chart of the typical measured states of the experimental results; point 1 denotes the outdoor climate, point 2 refers to the air flowing out from the pad after being humidified, and point 3 represents the measurement point 44 m away from the pad that can be considered as the point with the highest temperature along the pad-fan direction. For the points with the subscript ‘‘w’’, the data were collected from present experiments. The numbers with apostrophe marks represent the results obtained on August 5th. In conditions of high humidity, the air temperature cannot be reduced as much as in a dry climate [16] (shown as the process 1d to 2d) after the air flows through the cooling pad. Under conditions of high RH (approximately 70%) such as those seen in Shanghai, ambient air (state 1w) can be cooled down to approximately 28 °C (state 2w) and the humidity ratio can be increased from 0.0202 to 0.0236 kg/kg after the air flows through the evaporative cooling pads and is humidified (The slight increment of enthalpy during the process 1w to 2w could be explained by the measurement errors, and the intrinsic enthalpy the water is carrying before in its liquid state, right before it is evaporated). Temperature measured 44 m away from the pad under the EC + ES mode was successfully kept under the ambient
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temperature, and slight increase in humidity ratio (0.0002 kg/kg (EC + ES, 2w–3w) was observed. 4.1. System analysis from temperature/humidity perspective When evaporative cooling alone was used in a 48-m-long multi-span greenhouse, no cooling effect was seen in the half area near the fans (Fig. 7(a)) where the temperature reached 37 °C (44 m away from the fans); however, as shown in Fig. 5, the temperature in the first half of the greenhouse nearing the cooling pad is appropriate for plant growth. At positions far away from the cooling pad, the high incoming solar radiation results in an even worse performance of the fan–pad system alone compared to natural ventilation. Therefore, evaporative cooling should be complemented with other methods under conditions of high humidity. The combination of evaporative cooling, external shading nets and internal thermal screens were able to keep the indoor air temperature at an acceptable level (Figs. 4 and 5) and narrow the horizontal air temperature gradient by reducing the amount of incoming solar radiation. As shown in Fig. 7, the temperature gap between the positions 4 m and 44 m away from the pad was lowered by 6 °C and 7 °C with the use of external and internal shadings, respectively. External shadings are installed outside the greenhouse and only influence the solar radiation, while internal thermal screens also reduce the volume of indoor air that needs to be cooled, thus leading to a better performance of the evaporative cooling pad system. Because solar radiation is a critical factor for photosynthesis, only half of the area of the internal thermal screens was used to provide as much sunshine as possible to the plants. Although the internal thermal screens were not fully used, satisfactory results were nevertheless obtained. The temperature near the extracting fans and evaporative cooling pads was 1–2 °C and 3–4 °C lower, respectively, than the ambient temperature, even under intense radiation of 1000 W/m2. The temperature gradient in the horizontal direction can be reduced to less than 2 °C. Generally speaking, internal shading is more effective than external shading when used in combination with evaporative cooling pads system because of the smaller air volume that need to be cooled with internal shading. However, considering the importance of photosynthesis, adequate solar radiation should be provided to the plants and crops. The use of screens needs to be carefully managed to insure that plants can grow healthily. It is suggested that the internal thermal screens be applied only at peak times. It is also recommended that shadings be used in conditions of intense radiation, such as at midday, in humid regions. Circulation fans are usually adopted to remove air stratification, but the results of this study indicate that circulation fans do not significantly improve the uniformity of the horizontal air temperature (Fig. 7(d)). Circulation fans can reduce the air temperature in the higher parts of the greenhouse (4 m above the ground) (Fig. 8(d)); however, because most plants and crops are cultivated in the bottom part of the greenhouse, the vertical temperature distribution is not that important. Hence, circulation fans are not recommended for use with cooling pad systems. The effectiveness of the humidification of the evaporative cooling pad has been demonstrated in the experiments and the internal thermal screen scan help to conserve high relative humidity conditions of the greenhouse air (Fig. 10). The average relative humidity can be maintained stable at approximately 80%, which is 15% higher than the outdoor value. Air tightness is a key factor for the adequate performance of a fan–pad cooling system. Although hot air is easily accumulated in the top space of the greenhouse due to its buoyant force, leaving a leak in the ridge to remove the heat is not helpful. The result shown in Fig. 11 proved that even small amounts of turbulence will strongly disturb the air flow in the horizontal direction of the lower
part area, which leads to an increment in the air temperature. As a result, the greenhouse needs to be carefully sealed when evaporative cooling pads system is in place, especially in the vertical direction. It is also noteworthy that the fan-system should be turned on prior to closing the windows. When the windows are closed, the hot air that had accumulated in the top of the greenhouse cannot be released, thus leading to a sharp increase in temperature inside the space, which is very unfavourable for the plants or crops. Moreover, considering that the temperature uniformity of the air flow cannot be achieved because solar radiation will heat the air throughout the enclosed space, and because plant photosynthesis is extremely vital, it is useful to strategically place the plants in accordance with their characteristics. For example, hightemperature-resistant plants, such as soya bean and cucumber [22], can be placed near the extracting fans side, while plants that cannot withstand high temperatures, like lettuce [22], should be placed near the cooling pads. 4.2. System analysis from energy perspective The above sections mainly deal with the temperature/relative humidity variation/gradient of air, which indicates the applicability of the evaporative cooling pad system even in the climate with high humidity, while in the following part, system analysis from energy saving perspective is focused. To offer a convincing explanation about the energy conserving merit of the evaporative cooling fan–pad system, a comparison between the afore-presented system and the conventional air conditioner is made. The cooling capacity of the evaporative system is calculated by Eq. (2) with the experimental data (inlet and outlet air temperature of the evaporative cooling pad) which is shown in Fig. 12. The power that needs to be consumed by the air conditioner to provide the same amount of cooling capacity is calculated by Eq. (3).
_ a c ¼ ðSHi SHo Þ ðqa V_ a Þ c Q_ ¼ DSH m
ð2Þ
in which c and V_ a refer to the latent heat of water vaporisation and the volumetric flow rate of the air, respectively; while 4SH represents the difference of air specific humidity (also called humidity ratio) between the outlet and the inlet at the evaporative cooling pad.
_ ¼ Q_ =COP W
ð3Þ
where 7 is assumed as the COP (coefficient of performance) value of the conditioner. The calculation result indicates that evaporative cooling fan–pad system can save more than 9 times electricity energy consumption than the air conditioner system based on our experimental result because the electricity is only consumed by the extracting fans and circulating pump in the former option. In fact, the value set for the COP here (7) is a comparatively ideal value for an air conditioning system to achieve, which implies the evaporative cooling fan–pad system may have even better energy-saving performance in real operation condition. Energetic quantities (as tabulated in Table 4) extracted from the greenhouse in each working mode are calculated to evaluate the contribution they made to the overall cooling effect. The air inside the greenhouse is viewed as a control volume and steady state in a short period is assumed. In addition, to make these
Table 5 Ambient parameters chosen for the calculation. Mode
NV
EC
EC + ES
EC + ES + IS
Standard Deviation
T amb: (°C) I (W/m2)
31.8 789.7
31.4 871.3
32.2 865.1
32.7 834.8
0.5 37.3
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Fig. 13. Comparison of the temperature and equivalent cooling effect under different operating modes.
quantified results comparable, the durations with the similar average ambient temperature and solar radiation input are selected as shown in Table 5. The air temperature inside the greenhouse is determined by the mean value measured at 12 m and 36 m from the pad. Thus, the equivalent cooling effects brought by the listed cooling modes can be estimated by Eq. (4).
ca ma ðT a T amb: Þ Q_ eq ¼
s
2. The indoor temperature and humidity of the greenhouse responds rapidly to the changing operation modes. 3. Solar irradiation is the key factor that influences the indoor temperature of the greenhouse. Shading is an effective way to partially isolate unwanted incoming solar energy so that the heat load inside the enclosed space can be reduced and the uniformity of the air temperature can be achieved. According to the experimental results, the internal air can be kept 2–3 °C below the ambient temperature at a relative humidity of 80% using both evaporative cooling pad and shadings. 4. Circulation fans do not significantly improve the temperature in comparison to shadings when evaporative cooling pads are used. 5. When the evaporative cooling system is turned on, the greenhouse must be carefully closed/sealed to avoid disturbances from unnecessary air flows. 6. Although for a greenhouse in the subtropical climate, using evaporative cooling pan and fan system is not the most satisfactory way because it is not as efficient as it in the dry climate, it has the advantage of the energy-saving and easily-adjust over the other alternatives, such as air conditioning and composite system. When the fan–pad alone is not sufficient, proper use of shadings is necessary and effective, as demonstrated by the experimental results. At the same time, to help the photosynthesis process of the plants, we can adjust the strategy of shading-use based on the real situation, such as plant species and their growing conditions and real-time solar radiation values.
ð4Þ
in which s refers to the duration chosen for the calculation whereas T a and T amb: represent the mean temperature of the interior air and the ambient during the chosen period. Fig. 13 illustrates the comparison of the temperature and equivalent cooling effect under different operating modes. The negative value of Q_ eq indicates in which working mode, the average interior air temperature is higher the ambient value. As shown in Fig. 13, by means of NV, the air temperature in the greenhouse exceeded 36.5 °C even when the solar radiation is about 10% lower compared to that in the EC mode. Although the Q_ eq for the EC mode is 9 kW, remarkable cooling effect can still be observed compared to NV, which indicates that even in the humid climates, EC can partially cover the cooling load. To keep the average air temperature under the required level (32 °C as mentioned before) under this operating mode (EC alone), additional 5.5 kW cooling effect should be provided by other methods. When the EC + ES and the EC + ES + IS modes are applied, there is no need for the other cooling method in these cases. Furthermore, it should be mentioned that although the additional IS can further improve the cooling effect on the basis of EC + ES mode, the concurrent side effect such as the negative one on photosynthesis should be paid attention to. 5. Conclusions This paper describes experiments studying the cooling of a greenhouse with a fan–pad system in Shanghai in a humid subtropical climate. Five working modes of cooling consisting of the combination of evaporative cooling with other methods (internal thermal screens, external shading nets and circulation fans) were investigated. The results indicate that, with proper modes, the performance of the cooling system is acceptable, even under conditions of high temperature and high solar intensities. The main conclusions of the study are listed below: 1. Under high RH condition such as in Shanghai, air can be cooled to approximately 27–29 °C after air flows through and is humidified by the evaporative cooling pads.
In conclusion, the evaporative cooling pad system is an efficient way to prevent greenhouse overheating in the summer which is worth being popularised. Acknowledgements This work was supported by the Key project of the Natural Science Foundation of China for international academic exchanges under the contract No. 51020105010 and POREEN-Partnering Opportunities between Europe and China in the Renewable Energies and Environmental industries, People Marie Curie Actions International Research Staff Exchange Scheme, (FP7-PEOPLE2012-IRSES). References [1] China Meteorological Administration.
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