Limitations of water and land are leading to an increased demand for hydroponic greenhouses. These places use solar power to develop crops in an enclosed environment but there are different aspects to it too. The study discussed today will highlight the environmental and energy aspects of using photovoltaics for hydroponic greenhouses in Alborz province.
Greenhouses are popular around the world due to their performance and potential to adapt to different environments. Thus, due to the geographic conditions, and arid and semi-arid climate, the agricultural sector in Iran is focusing on greenhouse development.
In terms of water and fertilizer usage, hydroponic greenhouses are more efficient than traditional ones. Also, they produce higher yields, but they have relatively higher energy consumption and costs. This poses a major cultivation problem during the cold seasons.
In recent decades, several greenhouses have used non-renewable energy sources like natural gas and oil, leading to environmental pollution with GHG, global warming, and climate change.
Study Photovoltaics for Hydroponic Greenhouses
Researchers collected data from a 3000 m2 greenhouse. The total energy input in this greenhouse was around 8652.20 GJ ha−1, with natural gas and electricity being the main sources. The calculated energy productivity was 0.20 kg GJ−1, and the energy ratio was 0.03.
The Life Cycle Assessment (LCA) results show that the greatest impact was from natural gas. Solar cells enhance environmental indicators and energy in the greenhouse. For the greenhouse discussed in the research, about 120 m2 of solar cells are required that cover only 4% of the roof.
Aims of the Study Photovoltaics for Hydroponic Greenhouses
This research discussed here has the following aims:
- Investigating – Environmental effects and energy use patterns of strawberry production in the greenhouse during the cultivation period.
- Simulation of PV system – To calculate the accurate amount of surface required by panels for supplying electricity to the hydroponic greenhouse. It also considers the appropriate slope of the panels for the same.
- Comparative analysis – To analyze environmental loads in a hydroponic greenhouse with and without a solar system.
- The Water Productivity Index was also investigated in the study.
- Total Radiation on Inclined Surface: The monthly average daily radiation received on the inclined surfaces of panels was calculated. It was done by considering radiation received on a horizontal surface.
Details of the Region Studied
A 3000 m2 commercial hydroponic strawberry greenhouse was the study site for this research. It is in Alborz province in Iran. There the average annual rainfall is around 252 mm and the average temperature recorded is 14.1° C, annually. Moreover, this site is located at 1300 m above sea level, which helps in depicting the internal perspectives of the greenhouse for the study.
Energy Use Parameters Considered
Through direct visits and measurements of consumed inputs, collecting data for the year 2021-2022 production period became possible. Using input energy coefficients, total energy inputs were calculated. The following are the physical inputs and their energy equivalents from the study site.
- Human Labor: Their energy input was calculated by their working hours during the production period along with the relevant coefficients.
- Stationary Equipment: Their energy value was calculated by considering the weight of the equipment. The researchers used the mean value per kilogram and the year for stationary equipment (8–10 MJ kg−1 yr−1).
- Electricity and Natural Gas: The phase meter and gas meter helped in recording the total consumption of both sources for stationary equipment, lighting, and other purposes during the production period. Then using energy coefficients, researchers calculated the energy equivalent.
- Chemical fertilizers and biocides: Due to their disease-control power, the demand for these materials has increased. Total chemical fertilizers (primary and secondary nutrients) and biocides (fungicides and insecticides) used during the production period with their energy equivalent were calculated.
- Nylon: It is used to cover the greenhouse from the outside plus it is used inside too. Its weight and useful life were used to calculate the amount of nylon recorded (weight) for the production period. 60 MJ kg-1 was the energy coefficient used for calculation.
- Output: It was the amount of strawberry produced whose weight was considered along with the energy coefficient. This helped in calculating the amount of output energy.
- Water Productivity Index: Amount of water used per cubic meter for producing 5 kgs of hydroponic strawberry.
Highlights: Energy Use Pattern in Hydroponic Greenhouses in Iraq
- Human Labor: 19,200 h ha−1
- Natural Gas: 119,693 m3 ha−1
- Nitrogen Fertilizers: 423 kg ha−1
- Total Energy Required (for different processes inside the greenhouse): 8652 GJ ha−1 (for the whole production period)
- Average Strawberry Yield: 120 tons ha−1
- Total Output Energy: 228 GJ ha−1
- Total Electricity Consumed: 159,300 MJ (44,250 kWh)
- Input Energy Consumption (Natural Gas): 5925 GJ ha−1 (68% of the total input energy).
- Electricity: Each 10 units generated by panels require 1 unit of energy, 1 MJ (PV) = 0.1 MJ
Energy Indices (strawberry production) under new conditions are shown in the last column of the table below. It is evident that PV improves all energy indices in the greenhouse. The energy ratio in the new situation was 0.033 which increases by the use of solar panels.
| Energy Indices | Units | Without Solar System | With Solar System |
| ER | – | 0.026 | 0.033 |
| NE | GJ ha−1 | −8424.2 | −6717.6 |
| EP | kg GJ−1 | 13.87 | 17.28 |
| WP | kg m−3 | 5.13 | 5.13 |
Causes for Using Non-Renewable Energy in Greenhouses
In Iran, fuel prices are low, leading to their inefficient use in different industries. This is the main cause of high consumption of natural gas. The heating systems in the investigated greenhouse are of low efficiency. It is not based on an intelligent heating system and thus wastes large amounts of gas. Generally, greenhouse heating accounts for huge energy consumption.
Proposed Solutions
- To reduce natural gas consumption: High-efficiency heating systems should be used. They will also produce less pollution.
- To reduce energy consumption: It is recommended to use a thermal screen. By using these screens, the space around the plants inside the greenhouse will be reduced. Thus, less space needs to be heated at night. It also optimizes fuel use.
- To prevent heat loss and improve insulation: Materials with low heat transfer should be used while building greenhouse walls and roofs.
- To provide necessary heating: Solar thermal collectors are recommended due to their low-cost technology. Also, a ground air collector is the best and most cost-effective way to heat the greenhouse.
- To store heat: With phase change materials it is easy to store heat for the night.
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Simulation of Solar System on the Roof of Greenhouse
As mentioned earlier, it is possible to replace the total electricity used in strawberry production with renewable resources. According to the value of electricity consumption (44,250 kWh), the needed panel surface area can be calculated. It is done by using extraterrestrial radiation (Ho) and total radiation (H) on a horizontal surface, along with the equations mentioned.
The results:
- Average daily total radiation (horizontal surface): 10.9 MJ m−2
- Radiation (H) observed in June: about 29 MJ m−2 (highest)
- Radiation (H) observed in December: about 9 MJ m−2 (lowest)
- Annual average clearness index: 63%
- Depletion of solar radiation: 27%
Slope Needed
The last column shows the monthly average radiation incident per m2 of the installed solar cell in the studied greenhouse. It was at a slope of 35°48ˊ to the south. As per this. 7736 MJ solar energy can be annually received per square meter of the panel surface. With this slope, there is a chance of increasing received solar radiation by 12%.
Electricity Generation
As the study states, annual electricity consumption is 159,300 MJ. To produce this amount, approx. 120 m2 of panel surface is needed and it accounts for 4% of the roof. This is without compromising the lighting and heating inside the greenhouse.
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Various previous studies
Researchers have tried different methods and ways to investigate the energy performance of PV cells equipped greenhouses. Their conclusions are shared below:
Study #1
In Japan, researchers observed how shading from PV arrays affects the growth of Welsh onions. 2 types of solar panel arrangements were tested. They found that the checker board array arranged displayed no significant impact on the growth of the crop. Whereas the straight-line arrangement had a positive impact and provided for the power needs of the greenhouse through renewable resources.
Study #2
Here the researchers tried to find the impact of solar panels integrated greenhouses on crop production. The results were positive, demonstrating that PV panels not only supply the required amount of electricity but also reduce irrigation requirements for the crops.
Study #3
This research focused on plant growth and energy production efficiency of rooftop panels. Two types of installation modes were investigated: fixed installation and sun tracking. Results showed that sun-tracking mode generated more electricity than fixed mode. Thus, it can supply a good amount of energy to the greenhouse.
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Study #4
Another study from Canada studied the effect of semi-transparent PVs installed on greenhouse rooftops. The results showed that solar panels caused internal shading but also supplied 43.7% of the electricity needed by the greenhouse for lightning purposes.
Study #5
In one research, researchers used solar air collectors with phase change material (PCM) for heating the greenhouse. They concluded that the indoor temperature of greenhouses remained higher, especially during the night. It was more than a conventional solar heating system.
Study #6
In Shiraz, Iran, thermo-environomic conditions of a greenhouse equipped with solar panels were studied. Around 14 different array configurations were examined. It was concluded that there was no significant effect on the illumination of the greenhouse even when 19.2% of the roof was covered with panels. In fact, it decreased natural gas consumption, and CO2 emission to a great extent.
So, various pieces of research studying solar technologies focused on one aspect, how they perform and their impact on greenhouses. Mostly, the environmental impacts of these technologies were not paid much attention in these studies.
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Conclusion
The aim of this research work was to evaluate the energy consumption pattern along with an environmental analysis of the site studied. The hydroponic greenhouse in Alborz province showed the feasibility of using PV cells for generating electricity, instead of non-renewable resources. So, as per the results derived from the current study, researchers have drawn the following conclusions.
Most of the input energy was derived from natural gas and electricity.
- The energy indexes show that strawberry production in hydroponic greenhouse is energy intensive.
- LCA results show that damage to the ecosystem and human health is due to stationary equipment and electricity. Also, natural gas is the major contributor in the resource damage category.
- The feasibility results of using solar energy in the greenhouse showed that around 120 m2s of solar cells can supply the required amount of electricity. Also, installing them on roof will not hamper the illumination inside the greenhouse.
- With PV systems in greenhouses, energy consumption, and adverse environmental effects can be reduced altogether.
So, with PV technologies and energy optimization techniques, hydroponic greenhouses can become more energy efficient. Thus, this research to demonstrate effectiveness of photovoltaics for hydroponic greenhouses seems impactful.
Source: Using photovoltaic cells for a commercial hydroponic greenhouse
