A Sustainable Agri-Photovoltaic Greenhouse for Lettuce Production in Qatar

Abstract

Qatar identified that food supply security, including self-sufficiency in vegetable production and increasing sustainable renewable energy generation, is important for increasing economic and environmental resiliency. Very favorable solar energy resources in Qatar suggest opportunities to simultaneously meet this goal by integrating solar energy generation and food production. This study examines the feasibility of developing a sustainable agri-photovoltaic (APV) greenhouse design. A comprehensive greenhouse with solar energy generation included is developed for year-round operation in Lusail, Qatar. The performance of the system is predicted by integrating meteorological data and MATLAB simulations of system components. Important design considerations included optimizing solar energy generation by fixed solar photovoltaic panels placed on the maximum available surface area of the greenhouse canopy, while balancing crop insolation and energy needs for greenhouse HVAC systems. Electrical energy is also stored in an industrial battery. Results suggest the APV greenhouse is technically and economically viable and that it could provide benefits, including enhancing food security, promoting renewable energy, and contributing to sustainable food and energy production in Qatar.

1. Introduction

This study focuses on the design of an agricultural greenhouse with integrated solar-photovoltaic energy generation; an agri-photovoltaic (APV) system. Most populations favor maximizing renewable energy implications by integrating PV with agriculture [1]. This is because the combination of photovoltaic (PV) generation and agriculture can have food, energy, and water supply benefits [2,3,4]. Under certain conditions and with proper plant selection, when shading of the ground is less than 50%, plant growth is stimulated in Asia, Europe, and North America [5]. Further, it was shown that APV can satisfy water requirements for plant growth and panel cleaning, with any excess clean water available to be sent to the public, in the United States (US), Chile, Malaysia, China, France, Germany, South Korea, Australia, and Japan [6]. In Japan, over 1000 agri-voltaic locations were assessed to improve output yields to almost 50% more than conventional methods, while land productivity and accrual land productivity increased by 35–73% and 60–70%, respectively, promoting food security [7]. An assessment of greenhouse roof transmittance being low (0.7) and high (0.8) for APV roof setups in seven of the Canary Islands showed that they can provide 8% and 31% of their regional electricity demand, respectively [8]. Using a higher transmittance of the PV modules coupled with the transmittance of the cover materials of the greenhouse roof facilitates increased PV production and higher energy demands that can be sent to the grid [8]. Even though solar panels have efficiencies of 20% and coal yields 40% efficiency, the power generated from APV can be up to 2000 MW based on an estimate in the US [9,10]. APV is an energy system using solar thermal energy conversions with a 4.8 MW solar farm based in Australia that reduces the carbon emissions by 47,000 tons annually [11,12]. This Australian orchard also mitigates diesel fuel reliance by 85%, while managing the orchard and 1.5 million trees in Australian horticulture regions [12]. This displacement indicates an opportunity to substitute the non-renewable energy sources with the APV. In Thailand, the inclusion of bok choy, a cruciferous, leafy green vegetable, in the greenhouses with PV modules increased the voltage and power by 0.09%, while the mean temperature decreased by 0.18 Celsius, which is advantageous in hot climates [7]. According to predictions and simulations, the use of APV systems in Croatia can be a viable sustainable alternative for potentially growing vegetables, orchards, olive groves, and vineyards on about 6% of Croatian land [13]. For example, the economic benefits from an APV hydroponics system amount to 50% of farmers’ annual income, while also minimizing water consumption compared to traditional methods in Croatia [14]. In countries such as the US, South Korea, India, and Germany, an APV system can be used to change microclimate conditions by altering the ground coverage ratio to reduce photosynthesis by 30%, which can affect crop growth and yields, varying within 2.7–11%, especially with warm and dry weather [15,16].

The literature suggests significant water savings and food production synergies in hot climates. This study examines the potential design and performance of a sustainable APV greenhouse for year-round operation in Lusail, Qatar (24.74° N 50.90° E). The scope of this design includes accounting for changes in incident solar radiation and evapotranspiration of lettuce in the design of a sustainable greenhouse in Qatar with PV generation, a battery, and a HVAC system. Timeseries meteorological data are used as the input to simulations implemented in MATLAB (The Mathworks, Inc.; Natick, MA, USA). Qatar had a 30% decrease in local food generation since 2008, invests millions of USD in solar energy to reach a 2030 goal of 20% electricity by renewable energy, and considering the poverty rate is 0.4%, there should be increased dedication to expand sustainable agriculture practices [17,18,19].

The energy utilization in Qatar is 14,300 kWh/day and growing: it increased by 11% per year between 1999 and 2012. In this context, the government of Qatar proposed to generate 20% of energy using solar, which would require a solar generation capacity of 1800 MW [20]. However, Qatar did not reach this goal and now proposes a shift to 30% environmentally sustainable electricity by 2030 [21].

In APV systems, the fraction of land covered by solar panels is a central design variable. In Spain, having less than 20% PV coverage for tomato and 22% for lettuce had insignificant adverse effects on both crops, while having an organized arrangement of mini-PV increased these crops’ productivity in summer and spring seasons [22,23]. In Spain, the range of power output can be 61 to 226 kWh/m²/year for PV coverage of the greenhouse from 25% to 100% [22]. In China, increasing the coverage ratio on a greenhouse roof by about 10% reduced the cooling load, while generation from the PV panels increased by 0.45–1.02 kWh (1.7–3.19 kWh/day), and the mean net photosynthesis is reduced [24]. In the Netherlands, one-third coverage of PV on greenhouses increased the average uniformity of global irradiance, as found by Gao et al. (2018), to be 9.80% [23].

The portion of the light spectrum used by plants is termed photosynthetically active radiation (PAR) and consists of certain wavelengths within the range from 400 to 700 nm [24]. Different plants require distinct PARs as displayed in

Table 1. Five crops in Qatar that need production increases to reach national self-sufficiency, with additional data relevant to the design of the greenhouse.

Crop	Consumed Amount (Tons/Year) [25]	Self-Sufficiency (%) [25]	Water Requirement (L/Plant/Week)	Source(s)	Needed PAR (µMol/m2/S) [26]	Growing Temperature Range (°C)	Source	Maturation Time (Days) [27]
Pepper	13,472	9	31.5	[28]	800	18–27	[29]	60–90
Watermelon	24,416	10	60.5	[30]	100–700	18–35	[30]	70–100
Potato	58,880	0	25.3	[31]	100–700	10–24	[32]	70–121
Onions	84,662	3	38.1	[33]	100–250	6–35	[34]	100–175
Lettuce	6749	6	25	[35]	100–250	9–20	[36]	45–85

.

Polycrystalline, amorphous, and monocrystalline silicon are potential PV types. Monocrystalline silicon has the highest efficiency of 19.8% relative to the other types considered [22]. In India, tests of monocrystalline, amorphous, and polycrystalline silicon PV panels placed on an institution’s roof, experiencing a wet and dry climate, found performance ratios of 91%, 73%, and 81%, respectively [37]. The spectral range according to four amorphous and crystalline silicon is 14% 500–700 nm and 440–650 nm, respectively [38]. The area required per kW for crystalline and amorphous silicon is approximately 8 and 15 m², respectively [38]. Growing vines or grapes of “Flame Seedless” in China in a greenhouse constructed from bamboo and iron with a 0.08 mm polyolefin film, as opposed to open field, with the same number of branches and buds can have a 30.1% increase in yield and 40.2% significant increase [39]. Lettuce, tomato, strawberries, cucumber, and sweet pepper produced in a straight line-configured APV with a cover ratio of 25% exhibited average yield reductions of 8%, 20%, 2.5%, 25%, and 25%, respectively, when compared to a conventional greenhouse in many different locations [40]. After 140 days, pepper plant height was 125 cm in an APV system and 110 cm for corresponding outdoor plants [41]. The APV system had the maximum number of fruits per day, with the largest mass per day compared to the conventional method [41].

HVAC systems can be controlled optimally to ensure energy savings are 50% greater than average HVAC energy savings, which is approximately 1.5 MWh/day compared to the HVAC without control as shown by data-driven models for commercial buildings with a confidence interval of 95% [42]. Specifically, the fan speed and HVAC temperature control should be optimized to minimize energy consumption [42,43]. Hot climates require the greenhouse to have a HVAC control and an indoor temperature of 17–22 °C and 14.5–26.5 °C, respectively, to grow tomatoes [43]. Typically, industrial greenhouses have dimensions of >12 m width by 200 m length, with a floor area coverage of 1000 m² to 50,000 m² [44]. It is economically preferred to have a lower canopy height. The optimal greenhouse canopy heights to grow leafy greens, tomatoes, and peppers, are <0.8 m and 1.2–1.5 m [44,45].

Recycling of low-density polyethylene has the least global warming potential (GWP) (29 kg CO₂ eq/tonne) compared to several other greenhouse cover materials, rendering it the most environmentally sustainable cover material option [46]. The carbon footprint (CF) of conventional and APV greenhouse systems was identical per land coverage in a study by Leon A. and Ishihara K. N.; however, global land productivity for monosystem full density and half density can increase by 35–73%, respectively, while the economic value rises by 30% when using APVs as opposed to greenhouses [47,48,49]. According to the National Renewable Energy Laboratory (NREL), when comparing electricity generation using coal (~1000 g CO₂ eq. kWh⁻¹) to PVs (~40 g CO₂ eq. kWh⁻¹), PVs show greater environmental sustainability [50]. NREL articulates the most carbon footprint impacts asserted on the environment to occur during PV production, accounting for 60–70% of the cumulative impact, due to upstream material extraction, manufacturing, material production, installation, and plant construction [50]. These mentioned processes can be targeted to minimize the GWP from the end-of-life stage, especially considering that Qatar aims to reduce its CF from greenhouse gases by 25% near 2030, with an investment totaling CAD 170 million [51].

A sustainable APV greenhouse will be designed for Qatar with the following goals: enhance food security in Qatar while maximizing the sustainability goals of the state while simultaneously implementing an effective solar energy conversion system to replace non-renewable energy sources. The study will solely focus on the renewable energy requirements and energy systems as per the specified dimensions and design of the greenhouse.

There are two designs of solar greenhouses, specifically the Chinese- and Canadian-style greenhouses. Chinese greenhouses are typically arched at front-facing walls and have an arched south-facing roof with plastic film and thermal blankets for warmth during the night. While the Chinese design is used prominently at about 26% of protected cultivation locations in China, the use of a Canadian-style greenhouse is more effective for protected cultivation within Qatar [52]. A Canadian greenhouse that is perfectly insulated, with solar lenses on the roof of the greenhouse, was designed by the Qatar University, in cooperation with the Ministry of Municipality and Environment [53]. Evidently, different Canadian-designed greenhouses exist, including even span, uneven span, vinery, modified arch, flat roof, and Quonset [54,55]. The disadvantages of a flat-roof greenhouse are snow accumulation and poor drainage, which are not issues with Qatar’s climate. The implementation of a flat-roof greenhouse in Qatar is economically sustainable and will be used in this study [54,55].

While research and projects are proposed for APV greenhouses in Qatar, there exists a research gap for flat-roof Canadian-designed APV greenhouse focusing on food supply security including self-sufficiency in vegetable production [56,57,58]. Research needs to be completed for sustainable designs due to rapid population growth sparking food demands in their recently expanding agriculture sector [59].

2. Materials and Methods

Designing an APV system requires consideration of solar irradiation, thermal dynamics, biological properties of plants, and more. Essential elements of an APV include the greenhouse infrastructure, solar panels, charge controller, battery storage, inverter, HVAC system, and an optional backup generator [60]. Considerations for the PV panel coverage and their arrangement on the roof, with respect to their densities of modules (ratio of placement on roof area) and reflectance or absorption of light, are influenced by solar irradiance, battery capacity, and other factors [61,62,63,64,65,66]. The height of the greenhouse infrastructure with PVs is important, with the trend being that heterogeneous radiation will increase if the height is in near proximity to the earth’s surface [61]. Decreasing the height of the entire infrastructure has sustainability implications, as fewer embodied emissions, costs, and societal problems related to greenhouse height will result from the lower height [67]. Single-span pitched-roof and single-span vaulted-roof greenhouses are the specific designs of interest. Furthermore, heat and mass transfer with solar irradiance implications are important for modelling the HVAC requirements, as illustrated by Ravishankar et al. [68], who show the complexity of modelling energy processes in a greenhouse.

Direct solar radiation can have deleterious effects on some crops, hindering their production in climates with higher solar irradiance [69]. Electricity generation for growing the crops can ensure they obtain a sustainable growing environment [69]. While the APV system aims to maximize power generation, it should also optimize crop insolation and production [69]. Optimal indoor air temperature is required in an APV to meet crop growing conditions, as solar radiation causes the temperature to fluctuate throughout the growing season [70]. The self-sufficiency percentages to reach the food security strategy goals in Qatar for growing pepper, watermelon, tomato, onions, and lettuce are 9%, 10%, 0%, 3%, and 6%, respectively [25]. Lettuce has the lowest water requirement, required PAR, and maturation time, and a reasonable growing temperature range relative to other plants in Table 1. Though Qatar has ongoing development of its APV greenhouse standards, a list of current standards, codes, and safety restrictions set by Qatar and global and international organizations is listed in

Table 2. Rules, regulations, and goals related to buildings, sustainability, materials, energy, and safety both internationally and in Qatar.

Greenhouse Infrastructure Rules and Regulations	Parameter	Value	Source
Global Sustainability Assessment System (GSAS) Section 7	Roof U Value	0.25 W/m2K	[71]
	External Wall	0.3 W/m2K	[71]
LEED Certification	Roof Assemblies	Minimum 100 mm	[71]
Qatar Commercial Development Manual	Height	15–56 m	[72]
IEC 63092-1 and ISO 29584	Must follow the standards		[73]
Photovoltaic System Rule and Regulations	Parameter	Value	Source
Qatar Commercial Development Manual	Glazing Walls/Roofs	<60%	[72]
	Glazing Thickness	Double glazed (12 mm)	[72]
Qatar Transmission Grid Code (ES-M4)	Synchronous/Nonsynchronous Generator Power Capacity	>10 MW	[74]
	Synchronous/Nonsynchronous Generator Connected Voltage	>11 kV	[74]
IEC TS 61724-3, Photovoltaic system performance	All Generators Power Capacity	2–10 MW	[74]
	All Generators Connected Voltage	11 kV	[74]
Greenhouse Interior	Parameter	Value	Source
ASHRAE Standard 90.1-2001, Energy Standard	Hydronic System Pump Power	10 hp	[75]
Agriculture Safety Requirements and Goals	Details	Source
Codex Alimentarius Standards-Law No.24 (2005)	Pesticides and contaminants regulations, QS 382/1996 and QS 383/1996 regulate pesticides	[76]
Domestic Self Sufficiency	Increase vegetable production by establishing a hydroponics greenhouse cluster to reach 70% self-sufficiency	[25]
Energy Requirements	Details	Source
Qatar Sustainability Report	Installing 2–4 gigawatts (GW) of Solar by 2030 to reduce CO2 emissions by 5 MPTA	[77]

.

Table 1 shows that lettuce is one of the five most consumed vegetables by Qatari citizens. The water requirements, shortest maturation time, and wide growing temperature range make lettuce favorable for greenhouse production. Therefore, the APV greenhouse will be designed for industrial-scale production of lettuce. Guidelines, sustainability goals, rules, regulations, and standards are provided in Table 2. Qatar’s sustainability objective is to have almost 110 ha of high-tech greenhouses by 2030, while they plan to increase self-sufficiency of renewable electricity up to 70% by 2050 [20,78,79]. While the self-sufficiency of crops grown in domestic greenhouses in Qatar increased to 46% on average in 2023 from 10% in 2017, there remains a 24% need for improvement in future years [78,79].

The required data and, solar panel technical data and solar panel material properties needed to simulate a potential APV greenhouse design are included in Table 1, Table 2,

Table 3. Average annual meteorological data for the location of 24.74° N 50.90° E for GHI, direct normal irradiance (DNI), diffuse horizontal irradiance (DHI), wind speed, temperature, pressure, CDD, and HDD.

Average Qatar Conditions
GHI [Wm−2]	254
DNI [Wm−2]	277.55
DHI [Wm−2]	64.60
Wind Speed [ms−1]	4.42
Temperature [°C]	27.50
Pressure [mbar]	1009.8
CDD	3730.9
HDD	10.21

,

Table 4. Solar Panel (monocrystalline 500 W) -Technical Data at Standard Test Conditions (STC) of 1000 W/m² and 25$℃$ [80].

Solar Panel -YS500M-96	Specifications
Rated Maximum Power at STC	500 W
Maximum Power Voltage (Vmp)	48.4 V
Maximum Power Current (Imp)	10.33 A
Open Circuit Voltage (Voc)	59.3 V
Short Circuit Current (Isc)	10.54 A
Module Efficiency	19.51%
STC	Mechanical Properties
Cell Type	Mono-crystalline 156 × 156 mm (6 inch)
No. of Cells	96 (8 × 12)
Dimension	1956 × 1310 × 45 mm
Glass	3.2 mm, High Transmission, Low Iron, Tempered Glass
Output Cable	Section Size: 4 mm2, Length: 900 mm
Frame	Anodized Aluminum Alloy
Working Conditions and Temperature Coefficient
Maximum System Voltage	DC 1000 V(IEC)/1500 V(IEC)/1000 V
Operating Temperature	−40 °C~+85 °C
Maximum Series Fuse	20 A
NOCT	45 ± 2 °C
Temperature Coefficient of Pmax	−0.40%/°C
Temperature Coefficient of Voc	−0.30%/°C
Maximum Series Fuse	20 A
Cost *	0.47 CAD/W

* The cost of this type of solar panel is 0.47 CAD/W with a warranty of 25 years, 96 cells, and an ISO certification. Additionally, this solar panel is designed to maximize power output in Qatar.

and

Table 5. Selected vegetable (lettuce) and the Penman–Monteith–related data specific to the Qatar location from the TMY data and sources [81,82,83,84,85,86,87].

Lettuce Evapotranspiration	Quantity
Net Radiation at Surface [W m−2]	254
Soil Heat Flux Density [Wm−2]	2.5
Air Temperature at 2 m [$℃$]	20
Wind Speed at 2 m [ms−1]	3.79
Saturation Vapour Pressure [kPa]	2.33
Dew Point Temperature [$℃$]	19
Relative Humidity [%]	60.9
Actual Vapour Pressure [kPa]	0.32
Saturation Vapour Pressure Deficit [kPa]	2.0128
Slope Vapour Pressure Curve [kPa $℃$−1]	0.123
Psychrometric constant [kPa °C−1]	0.0013

, respectively. Table 5 consists of relevant information for calculating lettuce water requirements. Table 3 shows the average annual meteorological conditions at 24.74° N 50.90° E based on the TMY data [73]. Specific solar panel details are outlined in Table 4.

The average, maximum, and minimum temperatures for this location are 27.50 °C, 47 °C, and 7 °C, respectively, as shown in Figure 1 for the average daily air temperature throughout the year. Generally, the panel temperature was higher than the temperature of the air, reaching extremes of 78–79 $℃$, while the air temperature reached 47–48 $℃$, as shown in Figure 2. When the temperature of PV panels increases, their efficiencies decrease since the voltage drop leads to a power drop. Figure 1 and Figure 2 were from the TMY data within the study, Figure 3 and Figure 4 are the GHI over 365 days and the proposed APV design respectively, while Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 were modelled by implementing Equations (1)–(18) with MATLAB [88,89,90]. Instead of a glass or transparent roof, PV panels line the roof (~52%) of this design and generate energy over their large surface area. This energy is used within the greenhouse to grow crops with the irrigation system and LED lights, and overall, sufficiently supplies the energy requirements of the greenhouse. For example, in Europe, a greenhouse had 50% PV roof coverage without LEDs, while a single-slope Canadian greenhouse with a PV roof cover of 100% in Sardinia, Italy had exceptional energy production and related profits [91]. The yields from 100% PV roof cover in Spain was less than that of a conventional greenhouse and vertical farm integrated in closed APV greenhouse system [91]. The renewable energy generated from the 100% PV roof cover design significantly reduces the environmental footprint using non-renewable forms of energy for the systems energy requirements and specific material properties of the panel [92,93]. This study proposes a flat-roof APV greenhouse suitable for Qatar’s climate with about 52% cover ratio, close to the European APV design cover ratio. Additionally, this Qatar study can be scaled to other countries with a similar climate including, but not limited, to Kuwait, Jordan, Saudi Arabia, and Oman [94].

The daily total global horizontal irradiance (GHI) over the year is given in Figure 3. The minimum and maximum daily total GHI during the year is 2.2 and 8.0 kWh·m⁻², respectively. The annual average wind speed at this location in Qatar is approximately 4.42 m/s. This location experiences about 11 heating degree days (HDD) and circa 3731 cooling degree days (CDD).

The APV greenhouse energy budget is modelled by implementing solar engineering models retrieved from the text by Duffie and Beckman [95]. The earth’s orbit is eccentric, thus causing the solar constant on earth to vary in Equation (1):

$${\mathrm{G}}_{\mathrm{o}}={\mathrm{G}}_{\mathrm{c}} (1+0.033\left({\displaystyle \frac{360n}{365}}\right))$$

(1)

$${\mathrm{G}}_{\mathrm{c}}: 1367 {\displaystyle \frac{+}{-}} 1\% {\mathrm{W}/\mathrm{m}}^2,$$

where n is the Julian Day (n is 1 on 1 January, and 365 on 31 December).

The TMY file reports data are based on regular local clock time t_c, while solar engineering calculations are based on solar time t_s, which varies from the clock time. Solar time can be calculated based on a known clock time as:

t_s = t_c + 4 Lst − Lloc + E

(2)

E = 229.2(0.000075 + 0.001868 cos B − 0.032077 sin B − 0.014615 cos 2 B − 0.04089 sin 2 B)

(3)

$$B=\left(n-1\right) \left({\displaystyle \frac{360}{365}}\right).$$

(4)

In this case, Lst is the longitudinal center of the local time zone, and Lloc is the longitude of the greenhouse site.

The hour angle (the solar time of day in angular units) ($\omega$) can be determined based on the solar time:

$$\omega =\left({\displaystyle \frac{15°}{h}}\right)({\mathrm{t}}_{\mathrm{s}}-12 \mathrm{n}\mathrm{o}\mathrm{o}\mathrm{n}).$$

(5)

The declination angle (δ) measured relative to the equatorial plane is the measure of the sun at solar noon:

$$\mathsf{\delta }=23.45° \mathrm{s}\mathrm{i}\mathrm{n} \left(360\right({\displaystyle \frac{\left(284+n\right)}{365}}\left)\right).$$

(6)

The incidence angle (θ) can then be found:

cos θ = sinδsinφcos β − sinδcosφsinβcosγ + cosδcosφcosβcosω + cosδsinφsinβcosγcosω + cosδsinβsinγsinω

(7)

where β is the surface slope, and φ is the site latitude. The solar altitude (α_s) is:

cos θ = sin α_s = sin δ sin φ cos β + cos δ cos φ cos β cos ω.

(8)

The sun azimuth angle (γ_s) and α_s are needed to identify the position of the sun in the sky:

$$\gamma \mathrm{s}=sign\left(\mathsf{\omega }\right)\left|{\cos }^{-1}{\displaystyle \frac{cos{\theta }_{z}sin\varphi -sin\delta }{sin{\theta }_{z}cos\varphi }}\right|.$$

(9)

This study uses the TMY file with data for the direct normal ($I_{BN})$, diffuse horizontal ($I_{DH}),$ and global horizontal ($I_{GH})$ irradiances. The total radiation is a summation of all terms to calculate the total radiation otherwise refered to as total radiation ($\mathrm{I}\mathrm{T}$). This is calculated on an hourly basis as shown in Equation (10):

$$\mathrm{I}\mathrm{T}=I_{BN}+I_{DH}+I_{GH}.$$

(10)

$P_{mpp}$ is the maximum powerpoint power at standard test conditions (STC), ${\mu }_{mpp}$ is the maximum powerpoint temperature coefficient at STC, $T_c$ is the hourly temperature of the solar panel, and $\mathrm{I}\mathrm{T}$ is the total irradiance. The power output of the panel (PPV) can be found by Equation (11):

$$\mathrm{PPV}=\left(P_{mpp}\left(1+{\mu }_{mpp}\left(T_c-25℃\right)\right)\right)\left({\displaystyle \frac{\mathrm{I}\mathrm{T}}{1000 \frac{w}{m^2}}}\right)$$

(11)

The energy in a battery, or its state of charge (SOC), can be approximated by the hourly load subtracted from the harnessed PV energy. The SOC is approximately 50% (or divided by 2) of the battery charge to avoid permanent damage to the storage bank caused from excessive discharge [96]. Given that the initial battery capacity is 15,000 W and the SOC is half of this while the hourly load (Load) is 990 W, the hourly PV energy (PPV) has the load subtracted from it as in Equations (12) and (13):

$$\mathrm{SOC}={\displaystyle \frac{Battery Capacity}{2}}$$

(12)

SOC = PPV – Load.

(13)

Calculating the plant evapotranspiration for the plant water requirements can be completed using the Penman–Monteith Equation given in Equation (14) [49]. The equation and variables are explained in detail by Talbot, M. H., and Monfet, D. [97]. The values of input variables are defined in Table 5.

$$E{T}_o={\displaystyle \frac{0.408\left(R_n-G\right)+\gamma \frac{900}{T+273}{U}_2\left(e_s-e_a\right)}{∆+\gamma \left(1+0.34U_2\right)}}$$

(14)

$E{T}_o$ is the reference evapotranspiration [mm day⁻¹]$, R_n$ is the net radiation at the crop surface [MJ m⁻² day⁻¹], G is the soil heat flux density [MJ m⁻² day⁻¹], T is the air temperature at 2 m height [°C], and $U_2$ is the wind speed at 2 m height [m s⁻¹]. Next, e_s is the saturation vapour pressure [kPa], e_a is the actual vapour pressure [kPa], e_s − e_a is the saturation vapour pressure deficit [kPa], ∆ is the slope vapour pressure curve [kPa °C⁻¹], and γ is the psychrometric constant [kPa °C⁻¹]. The following variables are quantitatively and qualitatively provided in Table 5.

The HVAC system will be designed to sustain the heat load in the greenhouse, which is required to have an optimal temperature range within the greenhouse. Assuming steady state, heat can enter or exit the system from solar radiation, ventilation, conduction, equipment and crops, and thermal radiation. The energy balance for heat is given by Equation (15) [81,95,97]:

$$Q_{load}=Q_{conduction}+Q_{ventilation}+Q_{rops}$$

(15)

$$Q_{conduction}=uA(T_1-T_2).$$

(16)

In Equation (16), $u$ is the overall heat transfer coefficient [3.69${\displaystyle \frac{W}{m^{2}k}}$] and represents the low-density polyethylene film cover used in the greenhouse, with justifications provided in Table 6.$A$ is the area of the greenhouse [2068 m²], $T_1$ is the optimal temperature in the greenhouse to grow a specific crop [in this case: 277.03–293.15 k], and $T_2$ is the temperature outside of the greenhouse [the average of site: 301.34 k]. Further, $V$ is the volume of the greenhouse [2719.81 m³], $C_p$ is the specific heat capacity of air [700 J/kg/k], and $\rho$ is the density of air [1.18 kgm⁻³]. Equations (17) and (18) show the calculation for this:

$$Q_{ventilation}=\left({\displaystyle \frac{ACH}{3600\frac{s}{h}}}\right)V{C}_p\rho \left(T_{IN}-T_{OUT}\right)$$

(17)

$$Q_{crops}=A\left(125{\displaystyle \frac{W}{m^2}}\right).$$

(18)

This analysis utilized meteorological data from a typical meteorological year (TMY) for the location of 24.74° N 50.90° E [90]. MATLAB will simulate Equation (1) through Equation (18) to assess outputs, including but not limited to, the annual and average total irradiance, average PV power output, total annual PV energy generation, PV capacity factor, battery capacity, and electrical load. The electrical load will primarily be based on requirements for other APV greenhouses within similar climates as Qatar. The capital cost for this design will be estimated following a thorough discussion of the design. Necessary data required for the successful completion of the project are comprised in this study. MATLAB code was generated with reliance on the TMY file for Lusail, Qatar, for modelling [89], since the proposed APV greenhouse is precisely located at 24.74° N 50.90° E within Lusail, Qatar. The capital costs associated with various components of the design are calculated in this study. The MATLAB code provides several graphical representations of the meteorological, solar, and technological conditions at the site; for example, the temperature, irradiances, PV energy, battery state of charge, capacity factor, heat transfer, electrical load, and indicates if the battery is empty. This APV greenhouse design ensures the battery is never empty.

3. Results

Simulations were conducted for a flat roof south-facing greenhouse that measures 91.44 m in length, 6.1 m in width, and 4.88 m in height, with a β of 45° from the horizontal roof surface. There are approximately 290 m² of solar panels, equivalent to about 116 solar panels, assumed to be evenly distributed across the greenhouse roof. The PV panels cover the entire roof of the APV greenhouse and span a total of 290 m² at a 100% cover ratio as shown in Figure 4. Lettuce requires LEDs powered by PV energy in this closed loop system, as research indicates the LEDs generate greater fresh and dry mass than those grown under fluorescent lamps with similar growth and quality as lettuce grown from natural light [98]. With a cover ratio higher than 50%, optimal plant growth requires supplementary lighting powered by PVs to offset the shading effect on crops [99]. Annually, the panels remain at the optimal surface slope of 45$°$, providing the same shading, while the potential shading these panels have on surrounding land is outside of the study bounds. Figure 4 shows the design of the 2068 m² total surface area APV greenhouse. The battery capacity needed to support the greenhouse electrical requirement of 8668,440 Wh/year is 15,000 Wh/h and the system will have a 21.6 kWh battery since it is the most economically sustainable option [100]. One 21.6 MWh battery is incorporated in the design sustains the electrical requirement for annual greenhouse operation [101]. In Figure 4, the dimensions and specifications of a three-dimensional view (A), top view (B), and a side view (C) are modelled using SolidWorks [102].

Lettuce has an evapotranspiration or plant water requirement of about 0.932 mm per day. The optimal canopy height for energy efficiency and lettuce growth is 0.8 m [45]. The theoretical heat loss in the greenhouse from Equations (15) to (18) and MATLAB has maximum, minimum, and total of 3.$9499\times {10}^5$ W, $-8.7675\times {10}^5$ W, and $1.3508\times {10}^9$ W, respectively.

Figure 5 shows the total heat loss per day throughout the year in the greenhouse, which illustrates that the system requires significantly more cooling than heating utilizing the HVAC system. As shown in Figure 5, the heat transfer in the greenhouse (in purple) fluctuates every day.

The hourly electrical requirement for this greenhouse size that includes the water requirements is 990 W [103,104,105]. The greenhouse cover utilized in the design is transparent low-density polyethylene with the transmittance being 0.72 and other related details in Table 6.

Table 6. Selected greenhouse cover film based on least environmental impact.

Low Density Polyethylene Film [106,107]
Standard Met	ASTM D2103 [108]
Transmittance	0.72
Size/Piece (Width*Length)	32′ × 100′
U-Value [W/m2K]	3.69

Table 6. Selected greenhouse cover film based on least environmental impact.

Low Density Polyethylene Film [106,107]
Standard Met	ASTM D2103 [108]
Transmittance	0.72
Size/Piece (Width*Length)	32′ × 100′
U-Value [W/m2K]	3.69

Lettuce will be grown in the greenhouse since it has the least water requirement, low existing production within Qatar, a shorter maturation time, and requires lower photosynthetic active radiation (PAR) and growing temperatures relative to the other crops in Table 1. The greenhouse requires LED lighting, a fan releasing heat, a vent for constant ventilation, and an irrigation system to grow the lettuce crop. With the optimal surface slope, the annual average and annual total incident solar energy were 254 W/m² and 2.23 MWh/m², respectively, as shown in

Table 7. Data included in MATLAB simulations reflecting location-specific solar conditions, requirements, and plants grown; total irradiance (IT), photovoltaic (PV).

Parameter	Value
Optimal Surface Slope that Maximizes IT ($°$)	45
Annual Average IT (W/m2)	254.1
Annual Total GHI (MWh/m2)	2.23
Annual Total IT (MWh/m2)	2.23
Annual Total IT on Solar Panels (MW)	645.14
Annual Total PV Power Output (MW)	304.35
PV capacity factor (unitless)	0.6952
Battery capacity (Wh/h)	15,000
Electrical load (Wh/h)	990
Hour’s battery empty (hr)	0

. The average and annual total photovoltaic (PV) energy and power outputs were approximately 2,224,600 kWh and 304.35 MW, respectively. The resulting PV capacity factor is approximately 69.5%.

Figure 6 shows the global horizontal irradiance (GHI) versus the incident solar radiation (IT) for each hour of the year, while Figure 2 depicts the panel temperature versus the air temperature.

In Figure 7a,b, the hourly PV energy for a ~290 m² surface (blue) and the hourly total irradiance (red) per square meter, respectively, are depicted. This APV greenhouse utilizes approximately 290 m², or circa 116 PV panels, so in Figure 7a, the PV hourly energy throughout the year is quantitatively higher with respect to the hourly total irradiance per m² in Figure 7b. Finally, Figure 8 shows the PV energy and IT for a surface of 290 m² demonstrating the IT to be almost double the PV power, as the average annual IT and PV power are 73,680 W and 34,759 W, respectively.

In Figure 9, the state of charge of the battery is shown, indicating that there are no hours throughout the year when the battery is empty. The total annual battery capacity/charge with the SOC set to 50% of 15,000 W hourly is 1.026 $\times {10}^8$ Wh, the power from the solar panel is 3.0435 $\times {10}^8$ W, and the annual greenhouse load is 8,668,440 Wh.

Sources of cost estimates are included in Table 8 and Table 9. The capital expenditure for the design of the greenhouse infrastructure, excluding interior design, or maintenance is computed in Table 9 to be circa 213,412.59 CAD per greenhouse APV, with Table 8 serving as the basis for the cost estimations. Currently, there is a research gap associated with the capital cost of greenhouses in Qatar, serving as an inconvenience for comparison validation purposes. The costs for structures and hardware of mounted PV panels over 0.15 ha in Italy and Germany range from 8379 to 26,460 CAD [15,109,110]. The solar panel installation cost is an underlying factor for the higher capital cost since it is based on North American data, which might vary significantly in other geographic regions.

Additionally, there is a research gap for capital costs of greenhouse APV systems that do not directly attach the PV to the greenhouse canopy but instead mount these structures in fields for agricultural purposes of crops, grazing, apiculture, and insect habitat [111,112]. However, greenhouse rooftops can be more favorable than ground-mounted APVs since greenhouses require high loads of energy [99,113,114,115]. The assumed lifespan of this system is 25 years. Since the system generates approximately 398 MWh of additional energy annually, assuming a rate of 0.095 CAD/kWh, passive possible income can total 37,800 CAD/year or 945,250 CAD/lifespan [116]. Assuming the lettuce is grown and distributed at 2 CAD per head, the total income from lettuce production would be 2,200,000 CAD/lifespan.

Table 8. Proposed capital costs associated with the design of a sufficient greenhouse APV system.

Greenhouse APV Components	Quantity	Unit	Source(s)
Household-Business Electricity	0.0432–0.049	CAD/kWh	[116]
Metal Frame	0.36	CAD/m2	[117]
Wood Frame	0.13	CAD/m2	[117]
Polycarbonate Sheet Covering	1.26–2.52	CAD/m2	[118]
Glass Sliding	0.36	CAD/m2	[119]
Greenhouse Construction	3.14	CAD/m2	[119]
Thermostat	1269.52	CAD	[120]
Installation Irrigation	1.51	CAD/m2	[121]
Irrigation System	608.99–844.46	CAD	[122]
LED Lights	0.18	CAD/kWh/light	[123]
HVAC System Installation and Cost	6935.59	CAD	[124,125]
Water	3.71	CAD/m3	[126]
Electrical Wiring for Solar System	0.84	CAD/m	[119,127]
Inverter/Charge Collector	2000	CAD Total	[128]
Solar Panel Installation	~2.5	CAD/W	[129]
Solar Panel Cost	~0.48	CAD/W	[80]
21.6 kWh Battery	23,750	CAD/Piece	[100]

Table 8. Proposed capital costs associated with the design of a sufficient greenhouse APV system.

Greenhouse APV Components	Quantity	Unit	Source(s)
Household-Business Electricity	0.0432–0.049	CAD/kWh	[116]
Metal Frame	0.36	CAD/m2	[117]
Wood Frame	0.13	CAD/m2	[117]
Polycarbonate Sheet Covering	1.26–2.52	CAD/m2	[118]
Glass Sliding	0.36	CAD/m2	[119]
Greenhouse Construction	3.14	CAD/m2	[119]
Thermostat	1269.52	CAD	[120]
Installation Irrigation	1.51	CAD/m2	[121]
Irrigation System	608.99–844.46	CAD	[122]
LED Lights	0.18	CAD/kWh/light	[123]
HVAC System Installation and Cost	6935.59	CAD	[124,125]
Water	3.71	CAD/m3	[126]
Electrical Wiring for Solar System	0.84	CAD/m	[119,127]
Inverter/Charge Collector	2000	CAD Total	[128]
Solar Panel Installation	~2.5	CAD/W	[129]
Solar Panel Cost	~0.48	CAD/W	[80]
21.6 kWh Battery	23,750	CAD/Piece	[100]

Table 9. The capital costs associated with the proposed greenhouse infrastructure covering 0.15 ha utilizing.

Proposed Greenhouse Design	Value	Units	Cost (CAD)/0.15 ha
Width	6.1	m	-
Length	91.44	m	-
Height	4.88	m	-
Canopy	0.84	m	-
Total Film Used	596.19	m2	1126.80
Solar Panel	0.47	CAD/W	27260
Solar Panel Installation	2.5	CAD/W	145,000
HVAC System Combined	6935.59	CAD/HVAC System and Installation	6935.59
Metal Beams	0.36	CAD/m2	198.46
Battery	23,750	CAD/21.6 kWh Battery	23,750
Electrical Wiring	0.84	CAD/m	503.05
Construction	3.14	CAD/m2	1885.68
Door (Glass Sliding)	0.36	CAD/m2	0.76
LED Lights	0.18	CAD/Light	1036.36
Thermostat	2029.91	CAD/Thermostat	2029.91
Installation Irrigation	1.51	CAD/m2	841.52
Irrigation System	844.46	CAD/Irrigation System	844.46
Inverter/Charge Collector	2000	CAD/Charge Controller	2000
Total Cost (CAD)	213,412.59

Table 9. The capital costs associated with the proposed greenhouse infrastructure covering 0.15 ha utilizing.

Proposed Greenhouse Design	Value	Units	Cost (CAD)/0.15 ha
Width	6.1	m	-
Length	91.44	m	-
Height	4.88	m	-
Canopy	0.84	m	-
Total Film Used	596.19	m2	1126.80
Solar Panel	0.47	CAD/W	27260
Solar Panel Installation	2.5	CAD/W	145,000
HVAC System Combined	6935.59	CAD/HVAC System and Installation	6935.59
Metal Beams	0.36	CAD/m2	198.46
Battery	23,750	CAD/21.6 kWh Battery	23,750
Electrical Wiring	0.84	CAD/m	503.05
Construction	3.14	CAD/m2	1885.68
Door (Glass Sliding)	0.36	CAD/m2	0.76
LED Lights	0.18	CAD/Light	1036.36
Thermostat	2029.91	CAD/Thermostat	2029.91
Installation Irrigation	1.51	CAD/m2	841.52
Irrigation System	844.46	CAD/Irrigation System	844.46
Inverter/Charge Collector	2000	CAD/Charge Controller	2000
Total Cost (CAD)	213,412.59

4. Discussion

The use of photovoltaic panels can be a sustainable method to capture solar energy and convert it to energy that can optimize agricultural activities. Not only has this study modelled an APV system that generates sufficient energy for the growth of lettuce, but the energy sufficient to sustain the greenhouse design based in Qatar.

The self-sufficiency of vegetable production within Qatar could be enhanced by implementing this greenhouse photovoltaic system. Moreover, modelling of the total heat transfer showed there was a deficit (negative) heat transfer throughout the year relative to the total positive heat transfer throughout the year in this design, which is justifiable considering the hot average climate in Qatar. This heat transfer from the greenhouse presents an option to employ other technologies that can capture this heat and convert it to useful forms of energy, such as thermoelectric generators, heat pumps, and phase change materials [130,131,132]. Efficiently regulating the heat transfer in this system can reduce the costs by alleviating the scales of battery capacity and energy requirements. The objective is to have an APV system operating year-round, and while considerable solar energy fundamentals were explored in this investigation, future simulations and solutions to the heat transfer in the greenhouse would be an asset.

Months having lower minimum extreme GHIs in Qatar at this specific location were January, February, April, and December. According to the meteorological data, the annual average GHI at the site was 254 Wm⁻², which is moderately consistent with the findings of Perez D. and Bachour D. of 225.2 Wm⁻² [133]. While this design focuses on the solar panels, solar conversion system, physical greenhouse structure, and heat transfer within the facility, there remains comprehensive elements relevant to implications of cost expenditures and interior designing.

According to the Intergovernmental Panel for Climate Change, the safest energy conversion system analyzed was renewable energy rather than non-renewables, with solar energy posing the fewest human deaths relative to wind and hydropower [134]. This safe system is economically sustainable because it reduces operating costs, has less environmental impact than non-renewables, provides a long-term investment with a lifespan of about 25 years (prior to major maintenance), and fosters technological advancements, improving Qatar’s renewable energy efficiency [135,136,137]. According to Shubbar et al., non-renewable energy forms such as natural gas produce 0.475 kg CO_2eq/kWh of electricity annually, while solar energy only generates 0.0189 kg CO_2eq/kWh of electricity generated [138]. Adopting APV systems in Qatar could help mitigate the currently overwhelming carbon footprint [139].

5. Conclusions

In conclusion, implementation of a sustainable greenhouse APV system in Lusail, Qatar, delivers many advantages. The optimizing of agricultural crop output and reduction in possible reliance on non-renewable energy sources means that the system greatly enhances food self-reliance in Qatar with the ability to be scaled in other climatic locations similar to Qatar.

Incorporating solar conversion technology that is known to have minimal societal impact compared to other energy sources gives way to achieving Qatar’s sustainability objectives and enhance global efforts to combat climate change if implemented on a global scale. From the results, it is imperative that the system contributes to environmental sustainability. Lastly, despite initial investment costs, the long-term benefits of the APV greenhouse, such as reduced operating expenses and increased agricultural productivity, coupled with potential revenue streams from excess electricity, make it financially viable for this design to be sustainably incorporated.

The paper suggested that the development of APV systems in Qatar is a possible solution to vegetation self-reliance and promoting solar energy. This could reduce the carbon footprint generated from the agriculture sector globally. This paper provides opportunity for future expansion and detail with innovative heat to electricity conversion systems, specific pump selection and power, and environmental emissions assessment within a life cycle assessment. In conclusion, the paper improved the APV research, as their currently exists a research gap for this system within Qatar, forming a basis for sustainable development.