In this document, we explore the causes of condensation and outline various strategies for managing it on greenhouse glazing. We provide detailed methods to anticipate when condensation will occur and to estimate its extent. An important aspect we cover is why increasing the air temperature in a greenhouse can actually worsen condensation issues. We examine a range of solutions designed to regulate humidity and consequently reduce condensation. These include air exchange systems, clay-based desiccant systems, the addition of inner glazing, and electrically powered dehumidifiers. Additionally, we offer an in-depth analysis of the energy costs associated with each method, providing valuable insights for informed decision-making in effective greenhouse management.

Water condenses on greenhouse glazing whenever the interior surface of the glazing is colder than the dew point of the air in the greenhouse.

The only way to control or avoid condensation on greenhouse glazing is to either warm the glazing or remove water from the air.

Jose Ellsworth Oct-2023

Understanding temperatures when condensation occurs

Effectively managing condensation in a greenhouse begins with a thorough understanding of its internal environment. For instance, at a temperature of 75°F and a relative humidity of 65%, the dew point reaches 62.4°F. Condensation starts forming on the glazing surfaces when they cool down below this dew point.

A practical approach to monitor these conditions is using a sensor that can measure temperature, humidity, and dew point. Having access to this data simplifies the task of predicting when and where condensation might occur. These readings are also crucial for calculating the extent of condensation, a topic I delve into in a later section.

My preferred sensor for measuring temperature and humidity is the BME280. However, a quick and user-friendly option is the “Govee Bluetooth Hygrometer.” Priced at just $15, this sensor delivers temperature, humidity, and dew point readings directly to your phone from up to 200 feet away. By placing one of these sensors inside your greenhouse and another outside, you can accurately gauge when condensation is likely to start forming on the glazing. It’s also helpful in predicting the amount of condensate you might expect.


Estimating the amount of condensate on glazing.

When the air in your greenhouse is at 75F and 65% RH it contains 14.37 grams of water. If the glazing temp drops to 32F the air in contact with the glazing can only contain 1.29 grams per cubic meter. This means that 13 grams of water per cubic meter of air in the greenhouse can be expected to condense out on the glazing. This is in addition to any water added via evapotranspiration from the plants.

Our 8’X16′ greenhouse contains about 27 cubic meters of air and our interior glazing temp does approach freezing on very cold nights so I can expect about 27 * 13 = 351 grams (0.77 pounds) of water per night if the air starts at 75F @ 65% relative humidity.

  • An easy way to obtain the amount of water in your greenhouse air is using a ChatGPT query: “Tell me the grams of water per cubic meter of air for air at 50F and 80% relative humidity” You can assume that the colder air which has reached the dew point will be very near 100% relative humidity. Once you have the two numbers it is simple subtraction to obtain the grams that will condense out per cubic meter of air inside the greenhouse. You can multiply the length * width * height of your greenhouse to get total cubic meters.

Increased Greenhouse Temperatures Can Lead to More Condensation

When tackling the issue of condensation in greenhouses, a common initial thought is to simply increase the air temperature. This seems like a logical step towards creating a dry, warm environment conducive to plant growth. This method is generally not effective in managing condensation and can exacerbate the problem. Understanding the nuanced relationship between warmer air, humidity, and condensation is key to addressing this challenge effectively. The short explanation is that warm air will absorb more moisture from the soil and plants. This actually raises it’s dew point and the amount of water available to condense.

  1. Impact of Increased Heat on Air: When you increase the heat or air temperature inside the greenhouse, the air’s capacity to hold moisture (vapor pressure) also increases. Warmer air can contain more water vapor compared to cooler air.
  2. Additional Moisture Absorption: The heated air tends to absorb more moisture from sources within the greenhouse, such as the soil and through plant transpiration (evapotranspiration). This process raises the overall humidity level of the air inside the greenhouse.
  3. Rising Dew Point: As the humidity level increases, so does the dew point. The dew point is the temperature at which air becomes saturated with moisture and can no longer hold it in vapor form. When the dew point rises, it becomes closer to the actual air temperature.
  4. Earlier and Increased Condensation: If the glazing (the greenhouse’s walls and roof) temperature is below this higher dew point, condensation will occur more readily and in larger quantities. This is because there is now more moisture in the air that can condense.
  5. Enhanced Heat Transfer Through Glazing: Additionally, increasing the temperature inside the greenhouse can accelerate heat transfer through the glazing to the outside. This effect means more energy is required to maintain the higher internal temperature, leading to increased operational heating costs.

Solutions to eliminate condensation on greenhouse glazing

Using an Inner Glazing Layer to Minimize Condensation.

Adding an inner layer of glazing to a greenhouse is an effective strategy to increase the temperature of the inner surface of the greenhouse’s glazing, thereby reducing condensation. Here’s a detailed explanation of how this works:

  1. Thermal Insulation Principle: Adding an additional layer of glazing creates an insulating air gap between the two layers. This gap acts as a thermal barrier, reducing the amount of heat lost from the inside of the greenhouse to the outside environment.
  2. Air Gap Functionality: The trapped air in the gap has low thermal conductivity compared to solid materials. Since air is a poor conductor of heat, the gap minimizes the heat transfer from the warmer interior to the colder exterior glazing layer.
  3. Increasing Inner Surface Temperature: By reducing heat loss, the inner surface of the glazing (the side facing the greenhouse interior) remains warmer. When the internal greenhouse air, which may be moist, comes into contact with this warmer surface, it is less likely to cool down to its dew point.
  4. Dew Point Dynamics: Condensation occurs when moist air cools down to its dew point, the temperature at which air becomes saturated with moisture and can no longer hold it in vapor form. By keeping the inner glazing surface temperature above the dew point of the interior air, condensation on the glazing is significantly reduced.
  5. Enhanced Greenhouse Environment: By maintaining a warmer inner surface, the overall temperature stability within the greenhouse improves. This leads to a more controlled and consistent growing environment, which is beneficial for plant health and growth.
  6. Energy Efficiency: This method is energy-efficient as it relies on passive insulation principles. The additional glazing layer helps in retaining heat without the need for extra heating sources, thus saving on energy costs.
  7. Preventing Moisture-Related Issues: Reducing condensation is crucial in a greenhouse setting, as excessive moisture can lead to problems such as mold growth, plant diseases, and poor growth conditions.

The addition of an inner glazing layer in a greenhouse creates an insulating air gap that helps maintain a warmer temperature on the inner surface of the greenhouse’s glazing. This increase in temperature reduces the likelihood of condensation by keeping the surface temperature above the dew point of the interior air, thereby enhancing the overall greenhouse environment.


Air Mixing with Thermal enhancements to control condensation

To control condensation effectively in greenhouses, a combination of air mixing, thermal scavenging, and solar thermal air heating is utilized. Each method contributes uniquely to managing humidity and temperature.

The process begins by introducing drier outdoor air into the greenhouse. This simple yet efficient air mixing technique helps in reducing indoor humidity, which is a key factor in condensation formation.

  1. Principle: Air mixing involves introducing outdoor air into the greenhouse to reduce indoor humidity levels. This is effective when the outdoor air is drier than the indoor air.
  2. Process: By mixing the drier outside air with the more humid inside air, the overall humidity level inside the greenhouse is reduced, thus lowering the potential for condensation.
  3. Control Mechanism: Ventilation systems, fans, or natural airflow can be used to facilitate this mixing. The process is typically regulated by hygrometers and controlled automatically by greenhouse management systems.

Enhancing Efficiency with Solar Thermal Collectors

To optimize this method, a solar thermal collector is used. This device pre-heats the incoming outdoor air, enhancing the efficiency of the air mixing process. By warming the outdoor air before it enters the greenhouse, we can reduce heat losses from the outdoor air while maintaining a consistent internal temperature. Since heat can be expensive to replace this saves money.

  1. Solar Panels: These panels capture solar energy and convert it into heat, which is then used to warm the incoming air.
  2. Integration with Ventilation: The heated air from the solar panels can be directed into the greenhouse, either mixing with the internal air or directly replacing it.
  3. Reduced Energy Costs: By using solar energy, these systems can significantly reduce the reliance on electrical or fossil fuel-based heating systems, lowering operational costs.

Thermal Scavenging for Energy Conservation

In conjunction with the solar collector, a thermal scavenging heat exchanger plays a crucial role. It recycles heat from the outgoing air to warm the incoming air. The re-use of existing thermal energy reduces the overall heating costs and is particularly effective in conserving energy.

  1. Principle: Thermal scavenging involves reclaiming heat from the air being expelled from the greenhouse and using it to warm the incoming air.
  2. Heat Exchangers: This is often achieved using a heat exchanger system. The outgoing warm, humid air passes through the exchanger, transferring its heat to the colder incoming air but without mixing the two air streams.
  3. Energy Efficiency: This process recovers heat that would otherwise be lost, making it an energy-efficient way to pre-heat incoming air, especially in colder climates.

Geo-Thermal / Geo-Exchange Pre-heating

When using air mixing to control greenhouse condensation we need to understand the volume of air that must be mixed and how it will affect energy demands. In an 8’x16′ greenhouse, reducing humidity from 75°F at 65% RH to levels that prevent condensation on 36°F glazing necessitates mixing in an air volume equivalent to 15 times the indoor air volume, assuming the outdoor air is at 32°F. This translates to moving 14,295 cubic feet of air (953 cubic feet x 15) over the coldest 11 hours, equating to an airflow requirement of about 21 CFM. To offset the resultant heat loss from this air exchange, approximately 11,068 BTUs are needed.

Geo-Exchange Air Pre-heat (Not Recommended)

One method to reduce heating costs while introducing outdoor air is to use a buried pipe to pre-heat the incoming air using ground heat.

Pipe Specifications and Airflow Calculations:

  • Typical setups use a 3″ to 4″ pipe, at least 250 feet long, such as 4″ perforated drain tile pipe with a sock.
  • To achieve the necessary 21 CFM airflow for our greenhouse, a high pressure centrifugal blower is used to push 21CFM through 250 foot of 4″ ribbed pipe. Energy demands for centrifuge blowers vary but a blower drawing 25 to 40 watts is a good starting assumption.

Disadvantages:

  • Potential for mold and mildew growth within the buried pipe, posing risks due to limited serviceability and access. If the buried air pipe is used I would suggest including a Air/Ozone generator that can deliver enough ozone to sterilize the entire 250 foot of pipe be priced into the system.

Geo-Exchange Fluid Loop (Preferred Option)

The geo-exchange fluid loop is an effective solution for offsetting the BTU loss incurred through air mixing. Its efficiency in heat exchange, coupled with lower costs and low energy consumption, makes it a good choice. This system avoids the risks of mold and mildew associated with air pipes. It uses a small fan to introduce outdoor air or cools indoor air to outdoor ambient temperatures via a heat exchanger for dehumidification. The core advantage lies in the geo-exchange system’s ability to transfer heat from the ground loop into the building by heating our thermal storage barrels. This approach not only mitigates the energy-intensive nature of air mixing but also aligns with the goals of sustainable greenhouse management by offering a balanced solution in terms of both energy efficiency and capital costs.

Energy Calculations and Fluid Dynamics:

  • To replace the 11,068 BTU heat loss due to air mixing, a flow rate of about 0.251 gallons per minute is needed, calculated as (0.606 gallons * 24.85 cycles per hour hour ) / 60 minutes.
  • A 12V, 5-watt pump moving more than this amount of water draws about 0.5 amps, necessitating a 5.5 amp-hour battery capacity.
  • Additional tubing and pump energy should be considered for compensating for thermal losses through glazing and other structural areas.

Pipe Requirements:

  • The required length of the pipe for our geo-exchange system is calculated based on the need to offset a heat loss of 11,068 BTUs. With an estimated heat exchange rate of 50 BTU per hour for each foot of 1″ PEX pipe, the basic calculation suggests a need for 20 feet of pipe. However, the typical heat exchange rate for PEX pipe is often rated between 150 and 300 watts. Given that our fluid flow volumes are relatively low and the initial temperature difference (TDelta) is also lower, we’ve adjusted this estimate downward to better fit our specific conditions.

    To maximize efficiency and cost-effectiveness, especially considering the expenses associated with bringing an excavator on site, we’ve decided to triple the length of the pipe. This brings the total to approximately 60 feet. Installing extra pipe length in this manner ensures better heat exchange capacity while also making the most of the excavation process.”
  • This pipe length is in addition to what’s required for compensating heat loss through other parts of the structure.

Advantages and Practical Considerations

  • Cost-Effectiveness: This integrated approach is cost-effective, providing a good return on investment by minimizing the need for additional heating and dehumidifying equipment.
  • Energy Efficiency: Both thermal scavenging and solar thermal heating reduce the need for additional heating sources, leading to energy savings.
  • Environmental Sustainability: Utilizing solar energy and recovering waste heat are sustainable practices that reduce the greenhouse’s carbon footprint.
  • Improved Air Quality: Regular air exchange helps to maintain a healthy environment for plants by reducing excess humidity and providing fresh air.

Limitations

  • Dependence on External Conditions: The effectiveness of air mixing is contingent on the relative humidity and temperature of the outdoor air. In climates where the outdoor air is often humid or very cold, this method may be less effective. The effectiveness of air mixing is contingent on the outdoor air being drier than the indoor air. In regions with high outdoor humidity, this method may be less effective.
  • Solar Reliance: The efficiency of the solar thermal collector depends on adequate sunlight. On overcast days, its effectiveness can be reduced. Solar thermal panels depend on sufficient sunlight. On cloudy days or during extended periods of poor weather, their effectiveness diminishes.
  • Thermal Scavenging Efficiency: The efficiency of heat recovery in thermal scavenging systems can vary. In some designs, if the temperature differential between incoming and outgoing air is low, the amount of heat recovered may be limited.
  • Initial Setup and Maintenance Costs: Installing solar panels, heat exchangers, and adequate ventilation systems can involve significant upfront costs and require regular maintenance.
  • Space Requirements: Solar panels and heat exchangers require additional space. In smaller greenhouses or where space is at a premium, accommodating these systems can be challenging.

Air Mixing for Greenhouse Condensation Control can be energy intensive.

Lets examine the energy required to adjust the dew point below the glazing temperature using Air Mixing. The unfortunate conclusion is that unless your heat is free it may by quite expensive. Using the solar heat collectors and thermal scavenging can help mitigate the energy cost.

Greenhouse Conditions: At 75°F with 65% relative humidity, the indoor air of the greenhouse holds substantially more moisture than cooler outdoor air.

Dew Point Calculation: The dew point under these conditions is approximately 62.4°F, indicating the temperature at which condensation starts forming.

Glazing Temperature Goal: Aiming to maintain a glazing temperature of 36°F poses a challenge, as it is significantly lower than the dew point.

Air Mixing Requirement: Reducing the indoor air’s dew point below the glazing temperature of 36°F is essential to prevent condensation. This requires significant air changes with colder outdoor air.

Moisture Content and Air Changes: The indoor air at 75°F and 65% RH contains about 14.37 times more water vapor than air at 32°F. Consequently, around 15 air changes might be necessary to effectively reduce humidity and eliminate condensation on the glazing.

Energy Calculations:

  • Baseline Energy Requirement: It consumes 0.018 BTU to heat one cubic foot of air by one degree Fahrenheit. We must heat each cubic foot of air brought in from the outside back to the target greenhouse temperature of 75F. This means for each cubic foot exchanged we will consume 0.774 BTU.
  • One Air Change Energy: To heat the greenhouse’s air volume of 953.34 cubic feet (equivalent to 27 cubic meters) from 32°F to 75°F (a 43°F increase), approximately 737.89 BTUs are needed for one air change.
  • Total Energy for Multiple Air Changes: For 15 air changes, the total energy requirement is about 11,068.28 BTUs.

Operational Considerations:

  • The significant energy demand for heating highlights the challenges of using air mixing as a primary method for condensation control, especially in regions with large temperature differentials.
  • Greenhouse operators need to consider this energy requirement in their operational planning and budgeting.

Seeking Efficient Alternatives:

  • Given the high energy consumption of air mixing, exploring alternatives like desiccant systems for humidity control or thermal energy recovery strategies might be more efficient.

Conclusion: The Cost of Air Exchange:

While air mixing can be effective in managing greenhouse condensation, it is accompanied by considerable energy demands. These demands must be balanced against the benefits of condensation control. Exploring more energy-efficient strategies is crucial for maintaining a sustainable greenhouse environment.


Solar Clay based condensation control system

A solar thermal powered desiccant clay-based system for controlling greenhouse glazing condensation is a sophisticated and environmentally friendly solution, particularly suitable for off-grid greenhouses. Here’s a detailed explanation of how it works and its advantages:

How the System Works

  1. Desiccant Material: The system utilizes clay as a desiccant material. Clay has a natural ability to absorb moisture from the air, making it an effective tool for controlling humidity levels inside a greenhouse.
  2. Solar Thermal Power: The system harnesses solar energy to regenerate the desiccant. Solar collectors capture solar radiation and convert it into heat. This heat is then used to dry the clay, rejuvenating its moisture-absorbing capabilities.
  3. Daytime Drying Cycle: During the day, when solar energy is abundant, the clay desiccant is heated by the solar thermal system. The heat drives the moisture out of the clay, effectively ‘recharging’ its desiccant properties.
  4. Nighttime Moisture Absorption: At night, when temperatures drop and condensation risk increases, the dried clay is exposed to the greenhouse air. It absorbs moisture, reducing the overall humidity level and thereby reducing the potential for condensation on the greenhouse glazing.
  5. Air Circulation: Fans or natural convection can be used to circulate air through the clay, enhancing its effectiveness in moisture absorption. Properly designed airflow ensures that moist air passes over the desiccant material and drier air is redistributed throughout the greenhouse.

Advantages for Off-Grid Greenhouses

  1. Energy Efficiency and Sustainability: The system primarily relies on solar energy, making it highly energy-efficient and sustainable. This is particularly advantageous for off-grid greenhouses, where energy resources are limited and conservation is paramount.
  2. Reduced Dependence on External Power Sources: Since the system uses solar energy to regenerate the desiccant, it minimizes the need for electricity or other external power sources. This feature makes it ideal for off-grid applications where electricity availability is a constraint.
  3. Effective Humidity Control: By actively removing moisture from the air, the system can maintain optimal humidity levels inside the greenhouse, directly addressing the primary cause of condensation.
  4. Low Maintenance and Operational Cost: Clay is a low-cost, readily available material. The system does not require complex machinery or expensive maintenance, making it a cost-effective solution for humidity and condensation control.
  5. Environmental Benefits: Using natural and renewable resources like clay and solar energy, the system has a minimal environmental footprint. It aligns with sustainable agriculture practices and reduces the greenhouse’s carbon footprint.
  6. Adaptability to Varying Climate Conditions: The system’s effectiveness is not heavily dependent on external weather conditions. It can maintain consistent humidity control regardless of external temperature fluctuations, which is crucial in variable climates.
  7. Prevention of Moisture-Related Issues: By maintaining optimal humidity levels, the system helps prevent mold growth, plant diseases, and other moisture-related issues in the greenhouse, ensuring healthier plant growth and yield.

A solar thermal powered desiccant clay-based system for controlling greenhouse glazing condensation is particularly effective for off-grid greenhouses due to its reliance on solar energy, low operational costs, and effective humidity control. This system aligns with sustainable practices and addresses the key challenge of condensation in a greenhouse environment.

Our cutting-edge clay-based desiccant system offers a sustainable and efficient solution to greenhouse humidity control. Utilizing solar energy, the system dries out clay during the day, preparing it to absorb moisture at night effectively. The system is designed to maximize the clay’s surface area, ensuring optimal moisture absorption. A key challenge we’ve overcome is generating a consistent flow of 150°F air for recharging the desiccant, achieved with a highly efficient solar thermal collector. This innovative system not only regulates humidity but does so with minimal energy usage, making it ideal for eco-conscious greenhouse operators.

Are you interested in revolutionizing your greenhouse’s humidity control with our clay-based system? Contact us at info@rainamp.com for purchase details or to contribute to its ongoing development.

Please Contact me if you are interested in purchasing or helping with design of the clay based condensate removal system. info@rainamp.com


Traditional dehumidifiers, An energy intensive option to control condensation on greenhouse glazing

An old fashion electric dehumidifier can work but they can be expensive to run due to their energy consumption. They can not be used in off grid greenhouses without a huge investment in solar panels and inverters. They also tend to freeze up and fail at temperatures below 45F.

Most electric dehumidifiers work on the principle of refrigeration. They cool a surface to a temperature below the dew point of the surrounding air. Moisture in the air condenses on this cool surface and is collected as water. This process requires a significant amount of electrical energy to power the compressor, the fan, and other components.

Incompatibility with Low-Temperature Environments: Most standard electric dehumidifiers are not designed for low-temperature operation. In cooler environments, their performance drops significantly, making them unsuitable for use in unheated or poorly insulated spaces.

Thermoelectric (TEC) and refrigerant-based dehumidifiers use different technologies to remove moisture from the air. Each has its own mechanism, advantages, and limitations. Let’s explore the differences between the two:

Thermoelectric (TEC) Dehumidifiers

  1. Working Principle: TEC dehumidifiers use the Peltier effect to create a temperature difference. They consist of a Peltier module, which, when electric current is passed through it, creates a temperature differential across two sides of the module – one side gets hot while the other gets cold.
  2. Condensation Process: Air is passed over the cold side of the Peltier module. As the air cools, moisture condenses and is collected in a reservoir.
  3. Energy Efficiency: Generally, TEC dehumidifiers are less energy-efficient compared to refrigerant models, especially at higher humidity levels. However, they are more efficient in smaller spaces or for less demanding dehumidification tasks.
  4. Size and Portability: They are typically smaller, lighter, and quieter, making them suitable for small spaces like closets, RVs, or small rooms.
  5. Temperature Range: TEC dehumidifiers are less sensitive to low temperatures compared to refrigerant models and don’t freeze up as easily, making them usable in cooler environments.
  6. Maintenance and Durability: They have fewer moving parts, which can translate to lower maintenance needs and longer durability.

Refrigerant-Based Dehumidifiers

  1. Working Principle: These dehumidifiers work like a refrigerator. They use a compressor to circulate refrigerant through a coil system, cooling the coils to a temperature below the dew point of the surrounding air.
  2. Condensation Process: As moist air passes over the cold coils, moisture condenses out of the air and drips into a collection tank.
  3. Energy Efficiency: Refrigerant dehumidifiers are generally more energy-efficient than TEC models, especially in environments with high humidity levels. They are capable of removing more moisture from the air over a shorter period.
  4. Size and Use Cases: They are usually larger and noisier but are more effective for larger spaces and heavy-duty dehumidification needs.
  5. Temperature Sensitivity: They tend to struggle in colder environments (below 45°F). The coils can freeze, reducing efficiency and potentially causing damage.
  6. Maintenance: The presence of a compressor and refrigerant system can mean more maintenance compared to TEC models, and potential for refrigerant leaks.

If you are trying to use an electric dehumidifier in the winter then consider a TEC based unit rather than a refrigerant pump unit. The TEC unit will build up ice on the condenser but it will simply melt off the next time the greenhouse is warm. The same condition for most refrigeration based units can ruin the compressor. The TEC based units are readily available and can normally run on DC power and come with a AC power adapter.

Be cautious about published energy consumption numbers

While dehumidifiers are often marketed with specific energy efficiency estimates such as 3Kwh per gallon removed. These assumptions do not hold true when using for condensation control in winter greenhouses because we are operating them outside their intended design parameters. With this in mind approach vendor published figures with caution, especially when using these units in winter greenhouse environments. The energy efficiency of dehumidifiers can significantly decline when tasked with reducing humidity to levels low enough to prevent condensation on cold greenhouse glazing. In winter conditions, dehumidifiers have to work much harder and longer to achieve the desired dew point leading to substantially higher energy consumption than advertised. We have observed conditions where these units either fail all together or consume 10 to 20X the energy they are rated to remove. In some instances we have watched them just cycle and never reach a low enough temperature to actually condense anything.


Wrap-up

I hope the information included in “Strategies for managing condensation in Greenhouses” have been useful or at least entertaining. Please let me know how we can improve the content.

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One Comment

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