Use of Semi-Transparent Solar Panels in Greenhouse Food Production

Solar panels and plants harness solar radiation for different but useful purposes. PV panels collect sunlight to generate electricity, and plants produce food through photosynthesis.

Agrivoltaics has transformed agriculture and enriched the portfolio of farmers. It also helped mitigate risks associated with climate without compromising agricultural production.

Greenhouse farming is efficient in food production. However, 30% of its operating costs are usually associated with greenhouse lighting.

Now, researchers are combining semi-transparent solar panels with greenhouses to capture sunlight for energy generation. Building the panels into the glass roof of greenhouses has proven that a greenhouse can be completely self-sufficient.

Want to know more about semi-transparent solar panels and their greenhouse applications?

In this post, we discuss semi-transparent solar panels in greenhouse food production.

What Are Semi-Transparent Solar Panels?

Semi-transparent solar panels are a type of photovoltaic panel that combines the benefits of green energy and shading. These panels can generate electricity without harming plants.

Greenhouses fitted with semi-transparent solar panels can produce food while generating electricity.

In conventional solar panels, assembly is usually done by placing the solar cells on a panel of glass.

By adjusting the distance between the solar cells, it's possible to regulate shading and light transmission. When the cells are closely positioned together, they are semi-transparent. But when positioned widely apart, they become transparent.

How Semi-Transparent Solar Panels Work?

A semi-transparent solar panel is the most cutting-edge technology that promises to be a game-changer. This technology can generate electricity while providing shade to plants and even homes and offices.

In fact, semi-transparent solar panels can be easily deployed in skyscrapers with large windows. Basically, they can replace traditional glass windows in homes and offices.

The amount of light that strikes the Earth's surface in an hour is enough to power the world's energy needs for a year. Conventional solar panels convert sunlight into electricity.

When the semiconductor gets exposed to sunlight, it absorbs solar radiation.

The semiconductor transfers the energy to electrons. Once the electrons are free, they begin to flow through the semiconductor material as an electric current. This current is usually extracted through grid-like metal contacts before storage in batteries.

Silicon is the most common semiconductor material used in solar panels. In fact, 95% of solar modules sold today have silicon as a semiconductor. Silicon is the second most abundant material in the world after oxygen.

Crystalline silicon solar cells are usually made of silicon atoms. They are then connected forming a crystal lattice.

This lattice creates an organized structure allowing for the conversion of sunlight into energy. Solar panels made of silicon provide a low cost, high efficiency, and long lifetime combination.

In fact, the lifetime of a silicon-based solar panel is 25 years or more. After this time, silicon solar panels will continue to produce 80% of their original power.

An important property of semiconductors is the bandgap. This indicates what wavelengths of light the material can absorb and convert into electrical energy.

If the bandgap of the semiconductor matches the light's wavelength on the solar cell, it makes efficient use of the energy.

Semi-transparent solar panels harness a portion of the solar spectrum. They do so while allowing some of the visible light to pass through.

Basically, the semi-transparent solar panel can absorb about 60% of the sunlight it receives. Compared to conventional solar panels, the semi-transparent models have a lower efficiency of 7.2%. However, it's possible to increase electricity generation.

In greenhouses, the power generated by semi-transparent solar panels circulates heat using fans.

As the semi-transparent solar panels collect solar radiation, they also allow some of the visible light to filter through. Plants use this light in photosynthesis for food production.

A PV solar panel generates 10 to 35 kWh per square foot every year. A standard greenhouse uses 1 kWh of energy per square foot every year.

So, if the greenhouse takes 10,000 square feet of space, you will need 27 solar panels measuring 3 by 5 feet to heat the greenhouse.

Semi-Transparent Solar Panels Help Greenhouses Become Energy Neutral

In a 2021 study, researchers fitted several greenhouses with semi-transparent solar panels.

To determine if the panels affected food production in the greenhouse, the researchers tested groups of red leaf lettuce. They did so under different light wavelengths and glass types.

All other variables were the same. For example, temperature, water supply, fertilizer, and carbon dioxide concentration.

The team discovered that lettuce grown under semi-transparent panels exhibited no difference.

They even measured the following and found no difference.

• Size
• Weight
• Carbon dioxide absorption
• Antioxidants

What they also found was that the panels actually regulated the temperature in the greenhouse. The panels used in the study were semi-transparent organic solar cells (STOSCs) rather than silicon-based panels.

The former offers flexibility, and you can adjust them to absorb different light wavelengths.

Greenhouses have several benefits in food production, but they require more energy to run.

The discovery of using STOSCs in greenhouses is promising for food production. For now, the semi-transparent solar cells do not have similar sunlight to electricity conversion rate.

Researchers have also found that using tinted semi-transparent solar panels boosts food production. Plants and solar panels need sunlight, but they have distinct requirements in light quantity and quality.

It's possible to customize the quality of light absorbed by semi-transparent solar cells. For plants, absorption spectra depend on the plants' photosynthetic pigments. Even when it comes to the quantity of light absorbed by solar panels and plants, it differs.

For solar panels, food production requires light energy. But it does not correlate linearly with the intensity of incident light. In this study, researchers used basil and spinach.

They found that the use of semi-transparent solar cells resulted in an estimated financial gross gain of up to +35% for spinach and +2.5% for basil.

Besides increased food production, they also discovered other positive benefits. The semi-transparent solar panels protect the plants by creating favorable microclimatic conditions.

Plants were grown under the solar canopy also benefit from:

• Temperature moderation
• Wind mitigation
• Soil humidity
• Water redistribution
• Reduced evapotranspiration
• Protection from hailstones
• Protection against climatic uncertainty.

Implementation of agrivoltaics on soil-less vertical farming could boost food production. This is because it avoids widespread natural ecosystem disruption common in conventional agriculture.

Several studies have also tested the effects of experiments on plant growth using tomatoes, wheat, onion, and pepper.

Semi-Transparent Solar Panels with Thermal Mirror Capability

It's difficult to make silicon-based solar cells transparent. Scientists have been exploring ways of making semi-transparent panels with a substitute material for glass.

One of the best materials that are easy to manufacture, cheap to produce, and readily available is perovskites.

This is a hybrid organic-inorganic halide-based material for photovoltaic cells. Compared to silicon-based solar cells, perovskites are far more efficient.

In South Korea, a team developed a semi-transparent solar cell using perovskites. Very effective and highly efficient, it works as a thermal mirror.

They also developed a top transparent electrode (TTE) that works well with the perovskites solar cells. The key to realizing semi-transparent solar cells is to create a TTE compatible with a specific photoactive material. This is the case for perovskites solar cells.

The TTE is currently based on a multilayer stack. This stack consists of a metal film sandwiched between an interfacial buffer and a high refractive index layer.

They placed a thin layer of silver between the top high index dielectric layer of zinc sulfide and the bottom layer of molybdenum trioxide. The tri-layer matches the technique used for anti-reflection coating on glasses, but in this case, it involves a metallic layer.

The silver layer is 12 to 24 nm – two or three times thicker than conventional metal films. As such, the film reflects more infrared light. The zinc sulfide layer keeps the visible light transmittance high.

Conventional transparent electrodes focus on transmitting visible light only. But the TTE plays a dual role. It reflects infrared rays while allowing the passage of visible light.

Tests show that the TTEs exhibit a power conversion efficiency of more than 13.3% with 85.5% infrared rejection. To confirm the solar cells' thermal mirror capability, the team carried out an experiment using a halogen lamp.

They illuminated an object for five minutes through a perovskites solar cell, automotive tinting film, and glass.

An infrared camera took images of the object. When exposed through the glass, the object's temperature rose to 36.8 Celsius degrees. Whereas, the semi-transparent solar cell and tint film allowed the object to maintain a temperature of 27 Celsius degrees.

The semi-transparent solar cell remained cool. This is because it rejects solar radiation by reflection. From the experiment, the team discovered that the total solar energy rejection rate of the solar cell was 89.6%.

The bandgap of perovskite is usually dictated by chloride, iodide, and bromide. As the radii of the halides decrease, the bandgap of the perovskite increases. This enables more visible light to pass through the perovskite solar cell.

As such, it's possible to change the transparency of the perovskite film. All that needs to change is the bromide to iodide ratio. As the bromide ratio increases, the average transparency of the film also increases.

How Do I Calculate How Many Solar Panels I Need?

Before calculating how many solar panels you need, you must know the following:

• How much energy does your greenhouse uses
• Usable greenhouse roof surface area
• Peak sunlight in your area
• Climate
• Semi-transparent solar panels wattage

To know how many solar panels you need, multiply your greenhouse hourly energy requirement by the peak sunlight hours in your area. Divide this by the semi-transparent solar panel's wattage.

We recommend looking at your greenhouse electricity usage to determine kilowatt-hours used. For your calculations, you need daily and hourly usage. However, you can use the monthly or annual average.

Peak sunlight hours in your location have a direct impact on the energy produced. If living in Seattle, expect fewer peak sunlight hours than someone living in Phoenix.

This does not mean that someone in Seattle cannot power his greenhouse using semi-transparent solar panels. It just means the individual will need more solar panels.

To convert your hourly power generation, multiply your hourly usage by 1,000. Then divide your average hourly wattage requirement by the number of daily peak sunlight hours.

This will give you the amount of electricity your panels will produce per hour. From analysis, a typical greenhouse uses 1 to 2 kWh of electricity per square foot per year.

How Is Solar Panel Efficiency Measured?

A solar panel's efficiency is usually measured by multiplying the amount of sunlight in your area by the panel's area. Then multiply by 100%. In a conventional solar panel, if it has 20% efficiency, it means 20% of all light that hits the solar cells gets translated into energy.

A solar panel with a higher efficiency rating will convert more sunlight into electricity. Typical solar panels have an efficiency rating of 15% to 18%.

With organic or carbon-based semi-transparent solar panels, researchers achieved 8.1% to 9.8% efficiency. This is still a far cry from silicon-based panels that cost less to manufacture and have a higher efficiency of 15% or more.

The use of perovskite in the manufacture of semi-transparent solar cells has revealed exciting results. Unlike organic semi-transparent solar cells, perovskite panels have an efficiency of 20%. This is slightly more than silicon-based panels with an efficiency of 15% or more.

How Does Local Climate Impact Semi-Transparent Solar Panels Sizing?

Semi-transparent solar panels perform better in sunny climates. This is true in regions where they get more sun hours per day. This gives the PV system the ability to collect more sunlight for generating electricity.

In the US, most regions average 3 to 6 hours of sun every day. If you build your semi-transparent system in Seattle or Wisconsin, it needs to be twice as large as a system built in New Mexico or Arizona. This will enable you to produce the same amount of electricity.

During the Spring Equinox, the system can produce its maximum instantaneous output per week. This is because the air is clear, and the angle of the solar panels is perfect. However, in the Fall Equinox, the output will not match the Spring.

By Fall, the air is thick with debris from harvests. In fact, the debris is full of spores, insects and others scattering the sunlight. By doing so, they reduce its intensity which results in lower output.

How Long Do Semi-Transparent Solar Panels Last?

Conventional solar panels have a lifespan of 25 years under warranty. When the warranty is up, the panels do not break down. They do continue generating electricity up to 80% of their original output.

For example, a silicon-based solar panel rated 300 watts will still produce 240 watts after the 25-year mark.

Studies show that over 75% of silicon-based panels outperformed their warranty. However, parts such as batteries and inverters have a shorter lifespan. Inverters have a 10 to 20-year lifespan which means you need to replace them once or twice a year during the system's lifespan.

Lead batteries have a 3 to 7 years lifespan, while lithium batteries have a lifespan of 10 to 15 years. Remember, this depends on how well you maintain your batteries and inverters.

Semi-transparent organic solar cells have shown great promise, but scientists have yet to reveal how long they will last. With the advancement in technology, we expect semi-transparent solar panels to have an expected lifespan of 40 to 50 years.

Final Thoughts

Advances in semi-transparent solar panels mean that we can grow more plants other than basil and lettuce. Studies show that semi-transparent solar cells do not deprive plants of vital sunlight.

Since they are more flexible than other PV technologies, it's possible to adjust the wavelength of light they collect. This increases power output even in areas that experience few sun hours.