Energy sustainability performance of a sliding cover solar greenhouse: Solar energy capture aspects

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Research Paper

Energy sustainability performance of a sliding cover solar greenhouse: Solar energy capture aspects Xuejiao Tong a, Zhouping Sun a,*, Nick Sigrimis b, Tianlai Li a a

College of Horticulture, Shenyang Agricultural University, National & Local Joint Engineering Research Center of Northern Horticultural Facilities Design & Application Technology (Liaoning), Key Laboratory of Protected Horticulture, Ministry of Education, Shenyang 110866, China b Agricultural University of Athens, Geosmart Spinout, Iera Odos 75, Athens, 11855, Greece

article info

A new-type of Sliding cover Energy-saving solar Greenhouse (SEG) has been developed. It is

Article history:

addressing several structural problems of traditional solar greenhouses, mainly energy

Received 27 February 2018

capture and balance features. At northern latitudes, solar irradiance in wintertime is

Received in revised form

limited, which, along with extreme low outside temperatures, raises many energy sus-

24 September 2018

tainability issues. Capturing incident energy primarily depends on south-facing shape and

Accepted 12 October 2018

cover transmittance characteristics, and this paper sets out an analysis framework for heat gain, leaving out other heat balance conservation factors (insolation). A circular shape has been selected for the SEG and is compared to Liaoshen-type Solar Greenhouse (LSG) that

Keywords:

uses an elliptical south shape, used in traditional Chinese solar greenhouses. The SEG

Solar radiation

replaces passive north wall heat storage of LSG with active heat storage in water, thus

Aperture efficiency

creating an advantage in stored energy management. The shape factors and aperture ef-

Energy capture

ficiencies of the two designs were analysed and experimentally assessed during typical

Solar greenhouse

clear-sky days in four seasons. This provides a theoretical basis for structural and environmental optimisation. The design of SEG moved the ridge forward and increased the lighting roof angle, resulting in better solar radiation capture for winter sun altitudes. The newly-introduced SEG design exhibits better winter energy capture and less summer cooling load and features, by construction, and better performance in many other aspects, including potential automatic intelligent management actions. In addition to the SEG's energy capture analysis, along with briefing on light and temperature superiority, the design may also compensate for its higher constructional cost by superior automation capacity that calls for a combined holistic intelligent solution and offers many operational sustainability advantages. © 2018 IAgrE. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (Z. Sun). https://doi.org/10.1016/j.biosystemseng.2018.10.008 1537-5110/© 2018 IAgrE. Published by Elsevier Ltd. All rights reserved.

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Nomenclature In Idn Idb Aeff Aphy ea S H L V E Ej Eo eh es Z a b r lj N

beam radiant intensity diffuse radiation on the horizontal diffuse radiation on the surface of b effective aperture physical aperture aperture efficiency span of the greenhouse (m) ridge height of the greenhouse (m) length of the greenhouse (m) height of north wall (m) the mean solar radiation (W m
2) solar radiation at point j (W m
2) solar radiation outdoor (W m
2) Energy harvesting efficiency Energy storage efficiency aspect ratio half ellipse horizontal axis(m) half ellipse vertical axis(m) radius of cover surface (m) width of measured illuminated zones (m) number of measuring points

Greek symbols a solar altitude angle b inclination angle g azimuth angle of greenhouse ε azimuth angle of sun q incidence angle t cover transmittance u hour angle

1.

Introduction

The Energy World Scope: “Renewable Energy Harvesting” (EH) is a worldwide wave due to fuel costs, shortage and threat of future depletion of resources (Li, 2005; Raptis et al., 2017). In addition, the environmental impact on a climate-changing earth, worsened by more energy dependent living, is increasing the pressure to innovate on green energy and invent and track sustainability pathways. This has made “green energy” a top priority of all countries’ developmental issues and is the motive behind the present article. The renewable energy sector must deliver power without contributing to climate change and is addressing emissions in five key areas: electricity, transportation, agriculture, manufacturing, and buildings. What EH will be adopted in the mW range for MEMs, for example in wearable technologies, was set out for one case by an early pioneer (Gettens, Sigrimis, & &Scott, 1986, p. 4618861). More efficient harvesting of every day solar energy, and best conservation and management of it, is the most important renewable energy task (Hemming, 2011; Li et al., 2017). Energy and climate change megatrends of today are also pressing for optimal storage, which began from early wind-power stored in flywheels to most modern

f m h Ο

89

roof angle of greenhouse transmittance of greenhouse solar radiation uniformity polar angle

Subscripts c circular e ellipse j ordinal number of measuring points d diffuse radiation n incidence direction Abbreviations SG Solar Greenhouse LSG Liaoshen-type Solar Greenhouse SEG Sliding cover Energy-saving solar Greenhouse EH Energy Harvesting PFD power flux density Po Power received (captured) SF Shape Factor IE Incident Energy EI Energy Intercepted CE Captured Energy TE Transmitted Energy HE Harvested Energy SE Stored Energy Acronyms MEMs Micro-Electro-Mechanical systems IoT Internet of Things FAO Food and Agriculture Organization

super batteries and flash-(bus) charging technologies (ABB, 2017). Present innovation needs for “smart energy” are also looking to the agricultural industry, as it is becoming a major industry for electronics and computing applications (including IoT), as forecast by Sigrimis, Antsaklis, and Groumpos (2001). High tech in agriculture today allows the design of smart buildings (greenhouses), smart equipment (better efficiency and lower production cost of green energy) as well as more intelligent management of energy harvesting and utilisation (Li et al., 2017), admirably approaching Industry 4.0. Solar energy is the unique energy driver for the earth and has created all reserves we are using today when creating the greenhouse gases. The problem of energy capture optimisation for permanent installations is of paramount importance in today's “mandate for green energy” and must meet a number of restriction norms. A small improvement may accumulate big lifetime benefits and this has been extensively researched (Li, Li, Wang, & Sigrimis, 2016; Raptis et al., 2017; Sigrimis, Pasgianos, Arvanitis, & Ferentinos, 2002). The Solar Greenhouse (SG) historically: One of the early adopters of belief in Solar EH was greenhouse structures, which range from a small plastic cloche to a multi-hectare modern glasshouse. In low cost labour and low-price

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vegetable markets, the traditional SG for small farmers (noneconomical size today) has advanced and grown bigger with as yet little automation (Fig. 1). However, today's energy cost and environmental footprint of agricultural practices have created pressure to reconsider the greenhouse structural model for more energy independence. Many features are introduced in the classical SG, from insulated walls to automated night curtain, outside covers, shading screens and cooling pads for year-round energy independent operation. Heat Storage: The improvement of energy balance aspects of the SG encompasses the maximisation of energy capture and the minimisation of heat losses. To accomplish these operations, dynamic low-cost energy storage, increasing when possible also energy harvesting, is another necessity coupled to the radiant energy capture design, which is the main aspect of the present article. Water-filled transparent plastic tubes, laid between plants' rows, have been used for passive solar energy harvesting and storage, increasing the thermal mass of the greenhouse that leads to smaller dayenight temperature differences. A passive storage element in the SG system is also the soil and the north wall that is being heated during sunny days from direct or diffuse sunrays, and by convection of hot inside air. In other experiments, soil was also used for active storage using a pipe exchange system assisted by a fan circulator for storage (daytime) and retrieval (night-time) of energy. Active storage in the wall was also suggested by several investigators to increase the energy harvest by active heat exchange (Tong, Sun, Li, & Liu, 2016). Geothermal energy in various forms is also frequently used today worldwide for heating greenhouses and other buildings. Water as a heat transfer medium is also used nowadays for more active storage and longer time-averaging operations. Attar, Naili, Khalifa, and Farhat (2013) simulated the performance in a tomato crop of a heating system with water as the working fluid, and examined the size of the storage tank with a capillary polypropylene heat exchanger as a solar collector. The system reported in the results section of this paper is equipped with active heat storage in water, in order to verify on the expected energy capture determined in the analytical part of the paper.

Fig. 1 e Farmers' zero-energy input solar greenhouses of Liaoning China.

SG design aspects: traditional Chinese SGs (Fig. 2) are typical horticultural low-cost facilities without a heating system. In winter, off-season production accommodated in SGs can meet the demand for vegetables and offers good economic and social benefits (Li, 2005). Solar radiation is the dominant source for the light and temperature environment. The resulting growth factors of light, humidity, temperature and nutrition directly affect plant physiological processes, production and quality (Elsner et al., 2000; Max, Horst, Mutwiwa, & Tantau, 2009). Solar radiation intercepted by an SG, at a particular time and location, depends upon its shape and orientation. Therefore, it is important to analyse, and measure to validate, the temperature, light illuminance and distribution performance of the distinctive designs of SGs. The greenhouse environment is affected by many factors, including geographical location, building orientation, roof shape, skeleton structure and transparent material of the south face, among others. These factors directly affect the light transmittance of SGs and, in the design, the south shape and roof angle are key to how much energy and light can be captured. Extensive research on temperature and light environment performance of SGs had been completed by many Chinese scholars to maximise winter performance through energy independence (Xu, Zhang, Li, & Wang, 2017; Zhang, Zou, & Li, 2014). The materials of the SG surface, dirt accumulation on the cover, and the type and orientation of SG have a profound influence on captured solar radiation (Albright, Seginer, € ller, Cohen, Pirkner, Israeli, & Tanny, Marsh, & Oko, 1985; Mo 2010). The level of development of agriculture facilities in western countries is greater but for different design criteria and shape of greenhouses (C ¸ akır and S‚ahin, 2015; Sethi, 2009; Gupta, Tiwari, Kumar, & Gupta, 2012; , pp. 2178e2181). With the increased scale of China's greenhouse industry in recent years, light environment research has focused on diverse types, roof angles and roof shapes of SGs. Zhang et al. (2014) designed a SG with variable incidence angle (9m-span) and compared theoretically with normal SGs. The daily average increase of light irradiance was 41.75% and 25.05%, compared with fixed roof angle SG of 8m-span and 9m-span respectively. Captured solar energy and temperature showed a significant improvement from the increase of roof angle and ridge height. Wang, Shi, and Pei (2010) simulated and calculated the sum of sunlight permeation and the mechanical performance of the arch of 4 different curvilinear roofs, the thrice spline, circular arc, ellipse and parabola. The results indicated that the ‘thrice spline’ roof of SG has better light performance than the others while the ‘ellipse’ is mechanically best. Ma et al. (2013) simulated and evaluated the internal light environment of distinct design shapes, using a model of light environment of SGs, and selected an optimal design shape of improved performance. These studies on light and temperature performance have allowed a vast improvement in vegetable production in China, and most of them have focused on the traditional SGs. However, there are still many remaining problems regarding sustainable performance over a longer time horizon in a changing economy, society and agricultural labour. As a result, our

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Fig. 2 e Chinese solar greenhouse and its structure: (A) Photographic view; and (B) Sectional view.

research team has studied and developed the Sliding cover Energy-saving solar Greenhouse (SEG), aimed at resolving the following production, yield and quality, problems of traditional SGs: (i) uneven distribution of light and temperature, (ii) east and west gable shading, (iii) poor rain-proof, snow-proof, wind-proof and fire-proof capability, (iv) mechanics not allowing easy automatic control for critical processes (Sun et al., 2013). The primary objective of this study is to compare energy capture and distribution performance of SEG with that in the traditional SG. The comparison is made for typical clear-sky days at spring and fall equinoxes, and winter and summer solstices. In section 2 we set the definitions of solar energy flows to greenhouse storage and analyse the basis for energy capture and the role of intercepting shape. In section 3 we analyse the design factors affecting the radiation performance, set the terms and calculation approaches on energy capture and present the experimental facilities used to verify the analytical results. In section 4 we present the analytical details of the two comparison shapes with the analysis of expected performance and the verification experimental results. In section 5 we discuss the agreements and deviations of the analytical versus experimental results and conclude in section 6 on pros and cons of the two designs.

2.

Solar energy aspects of greenhouse

Energy performance of a sustainable horticultural facility includes energy harvesting, conserve ability in storage, and management to achieve both productivity of plants and preclude any risk for crop damage at a minimum monetary and environmental cost. The local weather statistics drive the structural design for energy harvesting and the economically justified types of storage and their size.

2.1. Nomenclature and definitions on stages of solar energy flow in greenhouses Incident Energy (IE) is solar energy, both direct and diffuse, upon its arrival on a surface i.e. of a solar collector. The sun is the source of solar energy which covers a wide spectrum. IE is a product of radiant intensity and the apparent surface area of the receiver. It changes through the day with sun position, azimuth and altitude, which alters the irradiated “projected area”, and is the maximum available energy for any type of receiver.

Energy Intercepted (EI) is the intercepted incident solar radiation. It is the insolation energy hitting an object per unit of time and unit area. The object may not have a flat surface and different insolation may appear at different spots. The total energy intercepted is the integral over the surface of the object and it is equal to the Incident Energy. The intercepting surface may not be the minimum plane and the energy concentration may not be the maximum (Incident). The specific receiving object shape is then characterised by the Shape Factor (SF1). Shape Factor (SF) of the receiving object is a value that is affected by the object's shape but is independent of its dimensions. In a sense it is the directional integral of the object's surface with regards to incident radiation angle (see Eq. (8)). Captured Energy (CE) is how much power is delivered past the receiving elements, i.e. how much power is transmitted through the greenhouse cover that can be captured by plants, soil and other materials of which a part is also converted to heat and reradiated. This is the most crucial factor in energy sustainability of solar greenhouses and is extensively analysed as the main focus of this paper. Harvested Energy (HE) is net used energy, and depends on well-tuned receiving south face or on interior reflectance and changes with plants and losses. HE ¼ CE eh

(1)

where eh is harvesting efficiency from captured energy (i.e. eh ¼ 1 for a black body after greenhouse cover) Stored Energy (SE) is a part of the Harvested Energy (HE) not immediately consumed (photosynthetic capture and heating the air and plant leaves and steel) but stored in longer time constant mass (on soil, wall, etc. to use later for balance when additional energy is needed, energy retrieval). We do not consider biomass heat storage and latent heat as stored energy as it is a fast exchange process (high convection factor per unit mass), not manipulable by regular means. SE ¼ HE es

(2)

where es is storage efficiency from harvested energy. Active Storage means are the actuators to direct the harvested energy to specific storage media (water, soil, wall) for which retrieval is also by controlled active means and the storage efficiency depends on insulation. Therefore, we can modify the thermal dynamics of the system, and in fact manipulate the storage efficiency es, to produce “useful” and “controlled-flow” energy. We manage the stored energy

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retrieval (release) based on forecast weather “anticipatory” management, that defines a “smart energy” based on securing plant health and maximal productivity over the longer production horizon.

power that is transmitted inside the greenhouse) when irradiated by a uniform power density PFD (W m
2), the antenna's (or greenhouse's) effective aperture Aeff (m
2) is given by:

2.2.

Greenhouse Shape Factor and aperture efficiency: In general, the aperture of a greenhouse is not uniquely related to its physical size but is changing through the day, depending on the beam incident angle (time u or solar azimuth ε and altitude a, Fig. 4). The effective aperture, Aeff, is always less than the physical area of the greenhouse or physical aperture, Aphys. Aperture efficiency, ea, is the measure of how effective the given shape is to capture incident radiation (includes the cover material and structural properties, glass or plastic of different transmissivity properties), and is defined as the ratio of these two areas (m2):

Greenhouse energy aperture

The greenhouse aperture is the energy-capturing shaped face of an irradiated receiver and is an integral of receiver collecting surfaces. This is the integral over the surfaces of the parts, with their inclinations that cause reflections (radiation losses) that lower the Captured Energy. Given a day's insolation the captured energy allocation depends on outside temperature at night. So, the daily fluctuation of inside temperature depends on the day's irradiance and outside temperature changes (Fig. 3). A general description of the solar greenhouse process, to secure a sustainable production of yield and quality, is that we try to manage the energy flows so as to “loosely” regulate the temperature within certain specified (for the plant) tolerance limits and change-rates (Li et al., 2017). Given the evolution in consumer price-quality demands and the variety/trait advances to satisfy them, with consequent amplified climatic requirements, we need to improve the performance of traditional solar greenhouses. This evolution will affect a big percentage (>60%) of the total of 4.7 million ha of protected horticulture of China.

2.2.1.

Aeff ¼ Po=PFD ¼ Po=In


ea ¼ Aeff Aphys

(3)

(4)

Aperture efficiency is factored to Shape Factor (SF) and cover transmittance t including optical properties, which maximises at perpendicular beam transfer ðq ¼ 0 Þ.

Shape factor and radiant energy capture

Aperture antenna theory and greenhouse aperture: In electromagnetics, an antenna captures radiant energy much as a solar collector (such as a greenhouse) does. Antenna aperture, effective area, or receiving cross section is a measure of how effective an antenna (or the solar collector) is at receiving the power of incident radiant waves. The aperture is defined as the equivalent “black body” area, oriented perpendicular to the incoming direct radiation, which would intercept the same amount of power. The radiant beam has an irradiance In, or power flux density (PFD), which is the amount of radiant power passing through unit area (1 m2). If an antenna or a solar collector delivers captured power (Po, W) to the connected load (i.e. the

Fig. 4 e Solar system defined and parameters changing the incidence angle q. P is the Poynting vector.

Fig. 3 e Solar radiation and air temperature outside in winter in Liaoning Shenyang. This location has advantage for smart specialization design as it has (A) very low night outside-temperature after high solar energy availability (sunny); and (B) : solar radiation RED; : air temperature high winter outside-temperature at low solar energy availability (cloudy). outside BLUE. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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ea ¼ ðSF tÞ

(5)

Po ¼ In Aeff ¼ In Aphys ðSF tÞ

(6)

where In is the beam radiant intensity. Some of the Energy Intercepted is captured, of which a proportion is retained (harvested), part of which is stored for use when needed. The Shape Factor of greenhouse Energy Intercept south is defined as follows: 9 8 Aphys A > > Z = < 1 Zphys   In da ¼ In Aphys cosqna da EI ¼ In SFn Aphys ¼ > >Aphys ; : 0

1 Aphys

Idb ¼ Idn ð1 þ cosbÞ

(7)

Z

cosqna da

(8)

0

for given incidence direction (n). Direct radiation interception depends on solar incidence direction (n), defined by the angle (q) with surface element (da), that is dependent on solar time (u), and all other solar-earth global positioning parameters (Fig. 4). The max SF is 1 and refers to the plane surface with inclination b ¼ p/2 e a, so that, q ¼ p/2 e a e b ¼ 0 (i.e. sun tracking). Given this SF definition and CE defined earlier, we derive the following CE calculation.   CE ¼ Po ¼ EI t ¼ In Aphys SF t ¼ In Aphys ea ¼ In Aeff

(9)

2.2.2. Shape factor in relation to direct and diffuse radiation e aspect ratio 1) For direct radiation In on a given surface point, the incident power is IE ¼ In cosq

(10)

and the transmitted or captured energy (TE ¼ CE) by a surface element rda: TE ¼ In cosq tq rda

(11)

where tq is the cover transmittance, a complex function of incident angle, q tq ¼ fðcosqÞ < 1; and t90 ¼ 0

meaning the diffused intensity on a horizontal surface (b ¼ 0 ) is twice of that on a vertical surface (b ¼ 90 ), Idn computed based on a clear sky model and the diffuse energy capture is: I

I Idb tdb rda ¼ Idn tdb ð1 þ cosbÞr d a I  ¼ Idn t0 ð1ef ðbÞÞ ð1 þ cosbÞrda  t0 Idn 2Aphys b¼0

I

4) Flat panel, with interception angle q ¼ 0 , has higher Aeff and Captured Energy of direct radiation In than same aperture with non-minimum surface Aphys. Nonminimum surface will also allow higher heat losses. In the case of an SG, we investigate the non-minimum surface loss/gain, where the gain is mainly the advantage of more airspace for a better regulated plant environment and more useful space for the crop vertical growth. 5) For a given ground surface S to service (Fig. 5) and sun altitude a, the maximum capture energy density with SF ¼ 1 is with a flat collector with V]S tanb, of surface Aphys ¼ S/sina, with incident q ¼ 0 . That is, inclination and aspect ratio V/S]S tanb/S ¼ tanb ¼ cota and CE¼In SFt0 Aphys ¼ ðt0 In ÞS/sina maximises ea ¼ t0 and captured energy density on surface Aphys (SF ¼ 1 & t0 ¼ tqmax). For latitudes above 60 , sun altitude a is very small and V/S becomes too large. In fact very small S or rather vertical collectors can house one vertical layer of plants. 6) Aspect ratio for lower solar altitudes in the North as per example and comparing flat panel and curved

(12)

 In cosq tq rdaq¼0 ¼ In t0

I rda ¼ In t0 A

(13)

for q ¼ 0 2) Beam energy through the whole cover of non-minimum surface A': ct'q < t0 . I CEn ¼

In cosq tq rda ¼ In t'q

I

cosq rda ¼ In t'q ðSF A'Þ ¼ In ea A'

 In A't0 (14) where A' ¼ Aphys

(16)

The optimal design of the energy-capturing south dome depends on the weather we consider (e.g. bright sky with more than 80% of solar irradiance as beam or cloudy weather when diffuse is higher and we consider more horizontal parts).

for a flat surface perpendicular to incoming radiation, q ¼ 0 , cosq ¼ 1: ct0 ¼ tmax , and SF0 ¼ 1. CEn ¼

(15)

0

Aphys def

3) For diffused radiation outside Id, the vector energy intensity depends on surface orientation such as:

CEd ¼

where qa is the interception angle of surface element a, and

SFnq ¼ SFn ¼

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Fig. 5 e Capture energy shapesdHigher Aspect Ratio in North for higher CE per unit ground.

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(elliptical) surface for diffuse radiation capture (Fig. 5): from above examples we can state the following: 6a) Highest Aperture Efficiency is achieved with flat panel with inclination for incident angle q ¼ 0 with solar beam. The same is explained by Wael, Teamah, and Tanaka (2015) who showed that high solar altitude a in Africa has maximum CE at low aspect ratio which makes incident angle near zero. 6b) Higher aspect ratio Z ¼ V/S increases Solar Aperture and IE at low solar altitudes a in Northern latitudes. This is what we pay in land space for capturing solar energy on a larger surface area and concentrating to a smaller covered area. It dictates the dead distance between greenhouses (Fig. 1), as the in-between area is shaded and cannot be productive. This is the reason single span is, energy wise, preferable to multi-span in northern latitudes. 6c) Higher aspect ratio also makes a significant contribution to diffuse radiation capture because of the bigger exchange surface per cultivation area, and an elliptical surface, having a greater horizontal portion, will capture more diffuse radiation (when cloudy). Therefore, maximum Incident Energy and Energy Intercepted on flat panel collectors depend mainly on incidence angle and sun tracking technology is answering this maximisation pursuit. But for a fixed object the optimisation of collecting surface depends on requirements set for energy independence and local weather conditions. For the south face of a tilted panel for q ¼ 0 (straight south surface-line in Fig. 5, which satisfies SF ¼ 1 with a sun follower) the curved south face reduces the aperture efficiency and depends on, and is a trade-off to satisfy, several other requirements, like the appearance of solar collectors on building roofs (Tanaka, El-Maghlany, & Teamah, 2015).

3.

Materials and methods

3.1. Verification experiments between most prominent designs In experiments we analysed the Shape-Curve aperture of the greenhouse for an aspect ratio Z ¼ 1 (circular), which is expected to capture more energy than traditional solar greenhouses that have Z < 1. This aperture of the SG for incident solar radiation depends on the average shape factor presented over the day-length and the optical properties of the cover (shape/roof-curve and optical transmissivity of the cover to compute the total permeability). The direct vs. diffuse transmittance, internal re-radiation of the canopy and the “greenhouse effect” of the cover are subjects that are not considered in the present analysis but are inherent in the measured variables which are related to Captured Energy (I-radiance measurements inside) and T-performance (Temperature measurements).

3.1.1.

Description of experimental SGs

In this study, SEG and LSG are located in Shenyang Agricultural University experimental field at latitude 41.8 N and

longitude 123.6 E. Their south roofs are clad with polyolefin films. The skeleton structure was semi-circular arc with rockwool filled colour plates as heat preservation top covering materials, including three sections W, Z and N (Fig. 6). The fixed unit (N ¼ 60 ) is located on the north face of the greenhouse and remains stationary. The movable units W and Z are opened along the slide on the skeleton during the daytime to keep sufficient illumination for 180e60 ¼ 120 open area and all units are closed tight at night to retain heat. The panels of the east and west gable can move to enlarge the edge lighting entrance of the greenhouse following the sun position and the actual seasonal demand. The LSG (Fig. 2) has a near elliptical south face and a layer of straw and quilt rolled on top as heat preservation cover: the north wall and gables are constructed of red brick. Dimensions of both SEG and LSG are given in Table 1. The SEG, as an innovative design of SG, has active heat storage with a solar collector and air-to-water heat exchange on the north side, which provides a better energy harvesting efficiency eh so that the SEG can achieve better energy balance and higher winter temperatures than LSG, to be proved in this paper. Furthermore, the capacity for better insulation of SEG provides a significant superiority on energy self-sufficiency. This includes a rather bright colour for the north internal energy harvesting surface of the solar collector (Fig. 6B), with reflectance of light that improves light illuminance and uniformity of photosynthetic radiation in winter.

3.1.2.

Experimental setup and measuring equipment

The parameters measured included external and internal solar radiation on four typical clear-sky days, for spring and fall equinoxes and summer and winter solstices. The outdoor observation point was located more than 2 m to the south of the SGs without shelter, with sensors situated at 1.5 m above the ground level. Five indoor observation points (A~E) were set up at the height of 1.5 m from south to north and spaced 2 m apart (Fig. 7). The average of the five observation points is used as representing the general solar radiation value of the time. Solar radiation was measured using Apogee Instrument MP-200 handheld Pyranometer, with measuring range 0e1999 W m
2, spectral range 280e1120 nm, calibration uncertainty ±5%, and response time <1 ms.

3.1.3. Calculation method for transmittance and uniformity of solar radiation The transmittance of different measuring points and solar greenhouses are calculated by Eq. (17) and Eq. (18). The ratio between the transmittance of SEG and LSG is given by mc/me where subscript c is circular (SEG) and e is elliptical (LSG). mj ¼

Ej Eo

(17)

PN m ¼

j¼1 Ej


 lj S

Eo

¼

l S

PN

j¼1 Ej

Eo

(18)

Solar radiation uniformity is calculated by Eq. (19), ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1, 0sP 2 N  j¼1 Ej eE A E h ¼ 1
@ N
1

(19)

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Fig. 6 e Schematic diagrams of Sliding cover Energy-saving solar Greenhouse. (A) Photographic view; and (B) Tri-Sectional sliders view. N is fixed unit, W and Z are movable unit of cover slides in SEG. Table 1 e Structural parameters of SEG and LSG. SGs

Span (S) (m)

Length (L) (m)

Ridge height (H) (m)

North wall height (V) (m)

South roof angle (f) ( )

SEG LSG

10.4 10

66 60

5.2 5.0

e 3.0

45 30

where mj is transmittance of measuring points; m is transmittance of solar greenhouses; h is solar radiation uniformity of solar greenhouses; E is the mean solar radiation in solar greenhouses (W m
2); Ej is solar radiation at measuring point j in solar greenhouses (W m
2); Eo is solar radiation outdoors (W m
2); lj is width of measured illuminated zones (m) (Fig. 7), all equal l ¼ 2 m; S is greenhouse span (m) and N is number of measuring points.

3.2.

Design for maximum energy capture

The SG design must meet certain criteria, which may reflect two different goals i.e. 1) Best winter-round energy harvest to achieve maximum economic effect on heating energy, depending also on expected supplementary input energy cost. 2) Maximum energy harvest, storage and management to meet the most demanding winter period with exclusively renewable energy input, to warrant a sustainable production facility with safety margins set based on solar energy only. This period of peak energy demand is based on design for zero-energy input (green energy greenhouse), as identified from local weather records, and is mainly concerned with the heat storage types and sizes designed. Both design goals call for maximum harvest at a certain time or period in winter. Such a defined time period

specifies solar diurnal altitude a and we must design the shape for best incident q, associated with maximum overall transmittance, for maximum “captured” energy. Such a design critical case identification requires heavy computations to optimise parameters of a design based on local weather records. To support this, we have constructed actual size prototypes and will verify simulated performance and extract the real performance characteristics, which will be a valuable input for real-world designer software. The present work at the analytical stage only compares the two designs at critical energy phases such as the winter solstice, verified by real-world weather performance experiments. To answer the question of which design, and in what matched conditions, is the most appropriate for a solar greenhouse, a short analysis is performed here regarding the shape factor and aperture efficiency of two competitive shapes. This is then verified with real experiments shown in the next section. Values of incident beam intensity and diurnal variation of solar altitude and other variables, based on earth location coordinates, allow instant, diurnal and seasonal energy capture integrals to be calculated. Similar processes are given by Tanaka et al. (2015) and Raptis et al. (2017). Comparison (Fig. 8) is based on aperture efficiency calculations of the different shapes and we assume a given In at solar altitude a, corresponding to the experimental site at noon (u ¼ 0 ). We analyse the two shapes at minimum winter altitude a ¼ 24 (Winter: winter solstice is the worst

Fig. 7 e Section drawing of solar radiation measuring points. (A) SEG; and (B) LSG.

96

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

2

3 p

CEc1

6 Z2þa 7 6 7 7 ¼6 I t cosq r dOc n1 n c c 6 7 4 5 0

g¼0

2 3 Z pþa 6 2 7 p 6 Z2þa cosqc cosqc rc dOc 7 6 7 6 0 7 ¼ In1 t0 rc dOc 6 7 Z p2þa 6 7 6 7 r dO 0 c c 4 5 0

g¼0

¼ ðIn1 t0 ÞAphys eac

Fig. 8 e Receiving apertures of SEG and LSG for different solar altitudes. KX and KX′ are the aperture of SEG and LSG; c (circular) and e (elliptical) represent SEG and LSG; 1 and 2 represent winter and summer.

case for high energy deficit with small daily irradiation and highest heat demand) and maximum a ¼ 71 (Summer: summer solstice is the worst for high daily irradiation and high daily heat excess for cooling). We also assume g ¼ 0 (south orientation) to examine the two shapes for the common installation case at maximum incident energy.

3.2.1.

(23)

Energy incident on the circular shape for winter altitude of a ¼ 24 is from Oc ¼ 0 to 90þa ¼ 114 but for the summer a ¼ 71 the beam is tangent at 90 þ71 ¼ 161 . However the cover slides obscure 180/3 ¼ 60 and so uncovered angle for incident radiation: 180 e 60 ¼ 120 . Therefore, the summer analysis is integrating from 0 to 120 {min (161,120)}. 2

2

3

6 Z3p2 7 6 7 6 7 EIc2 ¼ 6 In2 cosqc rc dOc 7 6 7 40 5 g¼0

3

6Z 3p2 7 6 7 6 7 cosq r dO c c c 6 0 7 7 ¼ In2 6 6 Z 3p2 7 6 7 6 rc dOc 7 4 5 0

3p

Z2

rc dOc 0

g¼0

¼ In2 Aphys SFc (24) 2

3

6Z 7 6 7 6 7 CEc2 ¼ 6 In2 tn cosqc rc dOc 7 6 7 40 5 3p 2

Design and calculation details of SEG 

SEG Circular: rc ¼ 5.20 m, roof angle fc ¼ 45 , sliding cover retracted angle is p/3 (point H in Fig. 8). Analytical process description: polar angle Oc: integrate radiation incident from Oc ¼ 0 to Oc ¼ p/2þa, Oc ¼ qcþa, aþqcþbc ¼ p/2, tilt (inclination) angle bc ¼ p/2eOc, and assume simplistic tnq ¼ t0cosqc. IE, EI and CE are computed per unit length of the greenhouses. Winter:

2

3

6Z 3p2 7 6 7 3p 6 7 Z2 cosq cosq r dO c c c c 6 0 7 6 7 ¼ ðIn2 t0 ÞAphys eac ¼ In2 t0 rc dOc 6 Z 3p2 7 6 7 0 6 7 rc dOc 4 5 0

IEc1 ¼ In1 Aperture ¼ In1 KF ¼ In1 KF cosðIn1 ; KFÞ ¼ In1 ð2  5:2sin57 Þcosð33 Þ ¼ 7:31In1

(25)

(20)

Summer:

3.2.2.

IEc2 ¼ In2 Aperture ¼ In2 KH ¼ In2 KH cosðIn2 ; KHÞ ¼ In2 ð2  5:2sin60 Þcosð11 Þ ¼ 8:83In2

(21)

3

2 p

7 6 Z2þa 7 6 7 EIc1 ¼ 6 I cosq r dO n1 c c c7 6 5 4 0

g¼0

2 3 Z pþa 6 2 7 6 cosqc rc dOc 7 6 7 6 0 7 ¼ In1 6 Z pþa 7 2 6 7 6 rc dOc 7 4 5 0

g¼0

p

Z2þa rc dOc ¼ In1 Aphys SFc 0

(22)

Design and calculation details of LSG

As shown in Fig. 8, the LSG with the given dimensions H ¼ 5 m/ S ¼ 10m has a bigger aperture than SEG but SEG has a bigger roof angle and we expect to have better winter effective aperture and better captured energy performance and even better harvest energy because of more effective active storage. LSG ellipse: approximate elliptical south face, we specify time instant of comparison at noon (u ¼ 0 ) and the two edge solar altitudes at solstices. The roof angle fe ¼ 30 , a ¼ 8.66m, b ¼ 5m, qe ¼ p/2eaebe.   be ¼ arctan Z2e cotOe

(26)

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 re ¼ b= Z2e cos2 Oe þsin Oe

(27)

where Ze ¼ b=a. For a simplified: tnq ¼ t0 cosqe

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

97

Winter: IEe1 ¼ In1 KL cosð36 Þ ¼ 8:09 In1 =per unit length of the greenhouse (28) Summer: IEe2 ¼ In2 KL cosð11 Þ ¼ 9:82 In2 =per unit length of the greenhouse (29) 2 3 Z p 6 2 7 p 6 Z2 cosqe re dOe 7 6 7 6 0 7 ¼ In re dOe 6 Z p 7 2 6 7 6 7 r dO 0 e e 4 5

3

2 p

7 6 Z2 7 6 7 EIe ¼ 6 I cosq r dO n e e e 7 6 5 4 0

0

g¼0

g¼0

¼ In Aphys SFe (30)

Fig. 10 e Polar Angle is the front free (uncovered) arc starting from zero (fully closed cover). SF, ea, EI and CE of SEG (c ¼ circular) BLUE and LSG (e ¼ elliptical) RED at winter a ¼ 71 . SF is Shape factor, ea is aperture efficiency, EI is Energy Intercepted and CE is Captured Energy. ─: SFc; ─ · · ─ · · ─: SFe; B: eac; :: eae; ═: EIc; ═ ═ ═: EIe; ─ · ─ · ─: CEc; ·····: CEe.

3

2 p

7 6 Z2 7 6 7 CEe ¼ 6 t I cosq r dO n n e e e 7 6 5 4 0

g¼0

2 3 Z p 6 2 7 p 6 Z2 cosqe cosqe re dOe 7 6 7 6 0 7 ¼ t0 In re dOe 6 7 Z p 2 6 7 6 7 r dO 0 e e 4 5

¼ ðt0 In ÞAphys eae

0

g¼0

(31)

4.

Results and analysis

4.1.

Analytical results of two designs

Figures 9 and 10 shows the progress of SF and ea as the slides of SEG and the night curtain of LSG start from fully closed (0 at x-axis) to fully open at the end of x-axis. They also show EI and CE of both greenhouses. There is considerable differentiation

Fig. 9 e Polar Angle is the front free (uncovered) arc starting from zero (fully closed cover). SF, ea, EI and CE of SEG (c ¼ circular) BLUE and LSG (e ¼ elliptical) RED at winter a ¼ 24 . SF is Shape factor, ea is aperture efficiency, EI is Energy Intercepted and CE is Captured Energy. ─: SFc; ─ · · ─ · · ─: SFe; B: eac; :: eae; ═: EIc; ═ ═ ═: EIe; ─ · ─ · ─: CEc; ·····: CEe.

after rolling up halfway at winter a ¼ 24 (Fig. 9) and they have a notable difference at summer a ¼ 71 (Fig. 10). This latter difference with LSG higher is actually a disadvantage as it brings higher cooling requirements in summer. The circular shape has been supported by many researchers but, as found in the analysis above, it performs slightly less well than LSG because the latter has a construction size that provides a bigger apparent size even at winter a ¼ 24 . From Fig. 8 schematic and the conducted analysis and Figs. 9 and 10, we computed that the Energy Intercept is bigger for LSG at winter a ¼ 24 (EIe > EIc, Table 2). Therefore, SEG was found to have little better Aperture Efficiency at noon at a ¼ 24 eac > eae but much bigger on daily sum in winter Avg (a1) although LSG captured 4.1% more energy CEc ¼ 5.559 EIc. However, the SEG is better on the winter daily average for all factors except the EI, and this last difference is because LSG has a bigger apparent aperture by design (see Fig. 8, KX' > KX).

4.2.

Experimental results

4.2.1.

Temporal and spatial variation of solar radiation

Figure 11 presents diurnal variation of solar radiation at 4 different seasons. Solar radiation decreases gradually as sun altitude decreases to lowest value at winter solstice. The daily solar radiation sum in the winter time (a1 ¼ 24 ), computed from data of Fig. 11, was 11.6% higher in SEG, that is, SEG was able to harvest more energy over the day hours. In all other seasons SEG presents timely shift to smaller shape factor (SF) with a significantly lower SF and effective aperture in the summer season a2 ¼ 71 . So SEG needs less ventilation and cooling in the summer. The transmittance ratio mc/me of SEG/ LSG (Fig. 11) during the day is greater than 1, so SEG receives more solar radiation than LSG and verifies the analytical results (Table 2). For winter, the average solar radiation in the LSG was between 300 and 400 W m
2 from 10:20 to 12:50, while in the SEG this level was maintained from 10:15 to 13:15, 30 min longer (Fig. 12). This superiority of SEG verifies the analytical result

98

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

Table 2 e SEG (c ¼ circular) and LSG (e ¼ elliptical) performance Radiation receiving factors at noon (a1 ¼ 24 & a2 ¼ 71 ) and daily sum at winter (Avg (a1)). Parameters 

a1 ¼ 24 Avg (a1) a2 ¼ 71

SFc

SFe

eac

eae

EIc

EIe

CEc

CEe

0.711 0.690 0.810

0.744 0.653 0.896

0.597 0.570 0.690

0.589 0.489 0.830

7.356 6.627 8.826

8.121 7.135 9.789

5.559 4.934 6.766

5.786 4.810 8.155

Fig. 11 e Diurnal variation of solar radiation and transmittance ratios mc/me under different seasons. : solar radiation : solar radiation intensity in LSG RED; £ : solar radiation intensity outdoor; C: transmittance ratio intensity in SEG BLUE; mc/me (right axis). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) that the circular SEG has bigger SF and aperture efficiency than LSG at lower solar altitudes. The radiation distribution in the solar greenhouses is gradually decreased along the south-north direction. In winter for example (Fig. 13), the north point E in LSG receives much less radiation and this leads to non-uniform plant growth (Sun et al., 2013). From the measured data, it was found that solar radiation intensity of measuring point E was higher in SEG. This was because the Water-cycling Heat Storage System (N slider, Fig. 6), installed on the north side of the SEG to replace the heat storage function of north wall in LSG, was light coloured to increase reflected light and brightness at the north side. The solar radiation uniformity along the span of two solar greenhouses was calculated according to Eq. (19), and the results are shown in Fig. 13. The variation of solar radiation uniformity through the day in the same solar greenhouse was not significant except in the morning and summer time when the North is exposed to less radiation due to roof shading. Comparing the two solar greenhouses, the average radiation uniformity of the SEG was 0.89 compared to 0.79 of LSG, in addition to the better north-south distribution of the SEG.

In summer, the radiation distribution at the 5 selected points was even in both solar greenhouses except at measuring point E. As depicted in Fig. 14, point E of SEG receives all day diffuse radiation (shade of retracted sliders in SEG). It is shown as a low transmittance of outside radiation, compared to the high direct radiation in LSG, where point E is reaching the other points in summer.

4.2.2. Daily accumulated solar energy intercepted by greenhouses The daily solar energy intercepted by SGs (Fig. 15) was notably reduced in winter because the number of hours and the altitude of the sun in winter solstice are very low. As expected from Table 2 and Fig. 11 LSG is superior in intercept and capture in summer (a2 ¼ 71 ). At the winter solstice for the 6-hour measurement period presented in Fig. 15, the daily integral of solar energy captured by SEG, calculated based on measured values, was 6.15 MJ m
2, while at summer solstice for the presented period of 9 h it was 13.85 MJ m
2. The LSG respectively captured 11.6% less in winter and 21.9% more in summer, which is inferior performance in both cases.

5.

Fig. 12 e Diurnal variation of solar radiation intensity : 0~100 W m¡2; : under different SGs in winter. ¡2 : 200~300 W m¡2; : 300~400 W m¡2. 100~200 W m ;

Discussion

The heating of solar greenhouses is based on intercepting the incident solar radiation, and the angle and shape of the south roof are the most crucial factors that affect captured energy and radiation performance of solar greenhouses. In this paper, the skeleton structure of SEG is a semi-circular arc, with ridge moved forward to the middle of the greenhouse, and the south roof angle is 45 , far greater than that of the LSG, together with a reflecting surface at inside north wall to make more uniform lighting. At the same time, east and west side gable of SEG can

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

99

Fig. 13 e Solar radiation transmittance of five different points and solar radiation uniformity in winter. -: measuring point A; A: measuring point B; :: measuring point C; C: measuring point D; £ : measuring point E; - - -: solar radiation uniformity h (Right axis).

Fig. 14 e Solar radiation transmittance of five different points and solar radiation uniformity in summer. -: measuring point A; A: measuring point B; :: measuring point C; C: measuring point D; £ : measuring point E; - - -: solar radiation uniformity h (Right axis). with the technological expectations in the light of Sustainable Crop Production Intensification (FAO).

5.1.

Fig. 15 e Solar energy intercepted by SGs. : solar energy : solar energy accumulated in accumulated in SEG BLUE; LSG RED; C: winter; △: summer.

be opened automatically in the morning and afternoon, respectively. This solves the problem of east and west gable shading, and effectively improves solar energy interception, and land use efficiency of SEG in terms of required separation distance in multi-lot setup. Automation capacity of SEG is in line with holistic precision management and is harmonised

Roof angle of solar greenhouses

Performance of the SGs is affected by the roof angle, which refers to the angle between light transmitting surface of solar greenhouse and the ground plane (f, Fig.2 & Fig.6). The roof angle determines the transmittance of direct radiation to a large extent, and it is the key to the design and construction of energy-saving solar greenhouse (Liu, Zheng, Hu, Shi, & Teng, 2007). Further, the daily variation of incident angle under different roof angles at the winter solstice has been presented by Ding et al. (1998). According to the low-cost design of roof angle of 24 , it achieved incident angle (45 ) at 10:40 in LSG, and good lighting duration was 2.5 h. When the roof angle increased to 40 , it achieved 45 incident angle at 09:00, and good lighting duration was extended to 6 h. Therefore, to improve the light and heat performance at low solar altitudes, it is best to increase roof angle of the solar greenhouses and this is more efficient than both LED lighting and supplemental heating. For an optimal solution on adequate illuminance in winter with an economical structural improvement of solar greenhouses, it provides an optimal solution of ridge height and span for this locality/latitude. This

100

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

determines the optimum roof angle for lighting duration to be more than 4 h every day during winter time. Experimental measurements of inside illuminance, as shown in data files attached to this paper, have shown that the light transmittance has the same variational patterns as the radiation transmittance and as such SEG has shown again superiority for light transmittance in winter (SEG ¼ 0.77, LSG ¼ 0.59). Experimental measurements confirmed an advantage of SEG on daily duration of lighting and energy capture (as shown in Figs. 16 and 17). Circle, ellipse, parabola and other mathematical roof expressions have been used by Chinese researchers to study the roof shape. Li, Zhang, and Wang (2001) showed that radiation interception efficiency increased with the ratio of the height to span of SG and decreased for more northern latitudes. Radiation interception efficiency of circular arc shape was the highest and parabolic surface was lowest. Chen, Zheng, Zhang, and Qiu (1992) calculated and compared the daily amount of direct radiation in four types of greenhouses in Beijing. They found, for the period October to April, that the total amount of direct radiation is best on compound circularparabolic type greenhouse and worst on elliptical. Compared to the elliptical surface, circular arc was better in winter, but

the result was reversed in spring and summer. This was supported by our analysis and experimental results.

5.2.

Solar energy captured by different curved surfaces

Wael et al. (2015) showed that the ellipse aspect ratio had a significant effect on the captured solar energy. As the aspect ratio (Z ¼ 0.25e4) increased, the captured energy per m2 of land was increased. If we regard the lighting surface of SEG and LSG as the analysed surface with the aspect ratio for 1 and 0.6 respectively, the total solar energy intercepted per cultivated m2 of SEG is about 10% higher than LSG, which is verified by our measurements (Fig. 16).  nchez et al. (2014) estimated total incident Rodrı´guez-Sa energy of 6 GJ m
2 over the year, by establishing mathematical model at 40.3 N with azimuth angle at 0 and found the energy intercept by SEG was approximately 4 GJ m
2, or an average aperture efficiency of 70%. When the ridge height of LSG increased to 5.5m, the roof angle reached 33 , and solar energy intercepted increased by 26.5%, which is 18% more than SEG in winter. Compared to our analytical and experimental results, as well as other research findings mentioned above, we could state overall: 1) Energy capture of the two shapes was computed giving a small difference in winter and a big one in summer that makes another shortcoming for the LSG. Furthermore, the east and west gables and the ability to use water heat storage in SEG, have the potential to realise better active energy harvest; a light colouring on the inside North stationary unit, in contrast with the dark passive wall in LSG, provides higher light level and uniformity. 2) Overall the management of SEG, with the control possibilities offered, can be based on intelligent use of forecast weather, and this is expected to play a major role in the future sustainability of protected and the open field agriculture. This will further boost crop productivity of SEG, and also achieve energy “self-sufficiency”.

Fig. 16 e Solar energy intercepted by LSG with different : solar energy accumulated with roof roof angle. : solar energy accumulated with roof angle ¼ 30 BLUE; angle ¼ 33 RED. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 17 e Variation air temperature in the greenhouses.

5.3.

A holistic position in solar greenhouse industry

We computed the daily average of all parameters, showing that SEG has better capture at lower solar altitude, and found

: air temperature in SEG BLUE;

: air temperature in LSG RED.

b i o s y s t e m s e n g i n e e r i n g 1 7 6 ( 2 0 1 8 ) 8 8 e1 0 2

superiority on computed energy capturing parameters ea and CE (Table 2). This advantage for circular SEG provides final superiority of SEG on winter energy balance, as shown by the verification measurements (Fig. 17) of inside temperature. At first estimate, this temperature gain is attributed to both better energy capture efficacy as well as better exploitation capacity of stored energy by active retrieval compared to passive storage and retrieval in LSG. The contribution of each depends on the weather events, such as a long medium lowlevel night temperature or a fast-ultra-low night chilling wind, that will be a subject of deeper investigation for proactive energy management. Overall, the robust controllability of SEG construction provides powerful automatic actions for energy harvesting, energy saving, summer cooling and controlling the plant environment. Given also the lower summer cooling load (Fig. 10 and Table 2 a2 ¼ 71 ) it may prove a better choice for year-round sustainable production operation. A number of design and operational aspects need further research on optimal design and intelligent management to prove overall sustainability of SEG.

6.

Conclusions

The performance of SEG and LSG in solar energy capture and inside temporal and spatial uniformity, on typical clear-sky days of four seasons, was compared. The results indicate that the roof angle and shape of the transparent south surface of SGs have substantial effects on total solar energy capture and distribution. The design of SEG moved the ridge forward and increased the lighting roof angle by 15 , which improves the aspect ratio for better winter energy gain and temperature performance. It increased, compared to LSG, the daily energy capture in winter by at least 3% while increasing the effective aperture by 16%, decreasing the cooling load in summer by 15%, and improving distribution uniformity from north to south. It also eliminated the shading of East and West gable walls by altering the traditional structure of SG of three wall sides to just one side wall. The full capability of the SEG, with active heat storage and retrieval facility, can manage the captured energy more effectively and showed a considerable increase of minimum winter night temperature by about 3e5  C. The SEG has combined the advantages of Chinese traditional SG and the operability of large modern greenhouses. The new greenhouse has adopted a form of sliding cover with thermal insulation steel plate instead of the rolling thick night blanket, and this improves the ability for dust removal for a steady cover transmissivity and the operational needs for rain events. Further the SEG has the capability for automatic control and precision operations, pro-active energy management and timely allocation (ongoing research) and provides a new effective way for the intelligent management and modernisation of SGs in China.

Acknowledgements The support of this work by the National Key Research and Development Program of China (No. 2016YFD0201004) and the

101

China Agriculture Research System (Grant No. CARS-25) are gratefully acknowledged. Also, EU-CHINA TEAP project regards the sustainability and early warning systems of greenhouse production in WP2, led by Prof. Sigrimis, which allowed many exchanges and demo trials on the energy aspects of solar greenhouses of Northern China.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.biosystemseng.2018.10.008.

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