4 season off-grid greenhouse Survives Coldest part of December in far North.

For Questions or comments please leave contact us at info@rainAmp.com we are making some of the custom electronic controllers we built for our use available for others.

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Innovative Greenhouse Solutions: Maximizing Efficiency with Solar Power and Thermal Storage update for Dec 2023

This update is part of a continuous series detailing our experiments with a solar-powered, off-grid greenhouse. Our objective is to prove the feasibility of year-round cultivation in North Eastern Washington, near the Canadian border, without relying on grid electricity or fuel-based heating.

This is a great time of year to illustrate the performance of our winterization strategies under extreme weather conditions. This document describes what we have done and some of the observations we have made that continue to influence our next round of improvements.

Success during challenging freezing conditions

Throughout this period, despite numerous freezing nights and prolonged cloudy weather, we successfully maintained internal temperatures above 32°F. On December 16th, we activated our geo-exchange loop, though currently, it only connects to one out of fifteen thermal storage barrels. Pending additional plumbing components, this connection has already positively impacted temperature stabilization across all storage units. This effect is likely because the geo-exchange barrel consistently exhibits higher temperatures compared to the others, thereby reducing their thermal depletion.

Temperature log for off grid 4 season greenhouse during coldest part of december


A critical innovation adding nighttime fan

On December 21st, we introduced a DC-powered fan, operating predominantly at night. This fan circulates air around the barrel connected to the geo-exchange system, significantly lowering air temperature fluctuations within the greenhouse.

Nighttime temperatures stabilizing near 40F.

One of the key improvements from these changes is that the nighttime air temp is staying above 40F most night and isn’t dipping much below 38F. I have about 15% of the insulated glazing panels installed. I am hoping that as I add more of these and finish hooking the geo-exchange system to more barrels that we see even stronger stabilization above 40F.

Thermal Banking seems to be working

An intriguing observation during the last three sunny days was the geo-exchange controller redirecting heat back into the loop. This process raises some concerns. I am contemplating modifying the system controller to halt pumping when the thermal storage’s temperature is higher than the geo-field’s until the thermal storage exceeds 63°F.

As we approach January, we anticipate colder temperatures but increased sunlight. If our predictions hold true, the greenhouse’s conditions in January should closely resemble those observed during these last three sunny days.

Note the trending up of storage temperature for the last 3 days where the outdoor temperature remained below 32F most of the time but we had some good sun (partly cloudy). It required 6,880BTU to increase our thermal storage by 1 degree F.

Innovative Greenhouse Solutions: Maximizing Efficiency with Solar Power and Thermal Storage

4 season off grid greenhouse in feezing conditions.

Thermal Storage and Air Movement Control: At the core of this system is the use of 6,880 pounds of water, allocated across 15 barrels, which serve as the primary medium for thermal storage. This considerable amount of water acts as a heat sink, effectively storing thermal energy.

Maximizing Thermal Gain through Air Circulation: In conjunction with the thermal storage, we have implemented an active air movement control system. This system is ingeniously designed to cycle warm air from the ceiling of the greenhouse down past the water barrels during the day. This process not only maximizes thermal gain but also ensures a more uniform distribution of heat within the greenhouse. The strategic movement of warm air enhances the efficiency of the thermal storage barrels, making the most of the natural heat accumulated during daylight hours.

Enhancing Thermal Storage with Active Air Movement: The integration of thermal storage barrels in our greenhouse is a key element of our design, but their effectiveness hinges on the use of active air movement for thermal recharge. Without this, the greenhouse would experience issues like stagnant cold air pooling at the bottom and overheating at the top during the day. To counter this, we employ an active air movement system that adapts to the varying levels of inbound solar energy. This approach not only prevents detrimental heat spikes that could harm plants but also maximizes the heat absorption capabilities of the barrels. By circulating warm air efficiently, we ensure a more consistent and beneficial temperature throughout the greenhouse.

Adaptive Energy Management with Proprietary Controllers: To address the challenges posed by the variability of solar power, our design includes a set of proprietary controllers. These controllers are specifically engineered to manage and activate different blowers based on the available PV energy. This adaptive technology is crucial for maintaining an optimal greenhouse environment, as it allows for real-time adjustments in response to the fluctuating levels of inbound solar energy. This system ensures that the greenhouse remains efficiently heated, even amidst the unpredictable nature of solar power availability.

Twin Wall Polycarbonate Glazing and Interior Panels: A significant enhancement to our greenhouse’s thermal efficiency comes from the use of twin-wall polycarbonate glazing combined with interior glazing panels. This setup has effectively more than doubled our R-value, leading to a substantial reduction in thermal losses. The reduction in light transmission is minimal, at approximately 9%, which is a small trade-off for the substantial thermal benefits. Prior to installing these interior panels, we observed freezing of condensate inside the glazing – a clear indicator of significant heat loss. With the addition of these panels, such issues have been eliminated, and we now only observe a light misting of condensate. Furthermore, these panels have the added advantage of reducing thermal bridging, a common problem associated with aluminum frames in greenhouse constructions. This enhancement not only improves the overall temperature control within the greenhouse but also contributes to the longevity and structural integrity of the construction.

Current Solar Power Setup and Planned Expansion: The greenhouse is presently powered by a 300-watt solar panel system, which has been sufficient so far. However, I am considering a significant upgrade, aiming to triple the solar capacity to 900 watts. This expansion is primarily driven by the need for more power to recharge the batteries for our geo-exchange field pump, especially during prolonged periods of cloudy weather. Currently, on cloudy days, our solar panels yield only about 30 Watts. With the proposed increase to a total capacity of 900 watts, the goal is to secure at least 100 Watts per hour of usable energy. This boost is crucial for maintaining consistent battery charging even under heavy cloud cover.

Optimizing Battery Performance: To further enhance our energy storage efficiency, I’ve transitioned to a different battery chemistry, one that is better suited for high burst charging. This change is particularly beneficial for those brief periods when sunlight is available. By optimizing the battery’s ability to charge quickly, we can maximize the energy stored during these short sunny intervals. This upgrade means that on days with abundant sunshine, our energy capacity will significantly exceed our typical needs.

Managing Excess Solar Energy: The surplus power on sunny days is far from wasted. We have high-powered venting blowers in place, designed to activate when internal temperatures reach levels that could be harmful to plants. These blowers are particularly useful after our thermal barrels have reached their maximum heat absorption capacity. Additionally, I am planning to integrate a larger, quadruple-sized geo-exchange pump. This pump will be activated only on days with excess solar energy, allowing us to enhance the amount of thermal energy we can bank into the geo-exchange field. This proactive approach in managing thermal energy will enable us to efficiently utilize the excess solar power, thereby optimizing our overall greenhouse environment.


Advanced Geo-Exchange System in Greenhouse Operations

Installation and Function of the Geo-Exchange System: In a significant step towards sustainable temperature management, I have installed a geo-exchange system comprising 300 feet of 1-inch poly pipe, buried in a 100-foot trench that’s 6 1/2 feet deep. This system is designed to cycle the coldest water from the bottom of our thermal storage barrels through the geo-exchange field and back into the barrels. This process effectively turns the barrels into heat exchangers, an innovative approach to temperature regulation in the greenhouse.

Proprietary Controller for Optimized Pump Operation: The operation of this geo-exchange system requires active pumping, which led to the development of another proprietary controller. This controller is critical as it activates the pump at precisely the right times, ensuring that both the electricity and the heat energy are available from the geo-exchange loop. This careful coordination is key to maintaining an efficient and sustainable greenhouse environment.

Experimentation with Direct Thermal Banking: Currently, I am experimenting with a method called direct thermal banking. This involves continuously running the loop with one barrel, which allows us to either store heat in the geo-exchange field or draw heat from it, depending on the relative temperatures. So far, this method has shown promising results, with a net increase in the temperature of water returning from the field. However, this has been during a period of relatively mild weather. I am eager to observe how this system performs under the harsher conditions expected in January.

Anticipated Performance and Future Enhancements: The telemetry data from the system on particularly cold nights (when outdoor temperatures drop below 20°F) indicates a heat gain of around 1660 BTUs per hour from the loop. With the temperature of the loop still on an upward trend, likely due to thermal banking, I am optimistic about its performance. I am awaiting the arrival of the last parts needed to connect the remaining barrels, which should be in early January. With these in place and some further optimization, I anticipate that we can significantly increase the BTU gain from the geo-exchange field, potentially exceeding 7,000 BTUs per hour under extreme weather conditions. This enhancement will mark a significant step forward in our efforts to create a more efficient and environmentally friendly greenhouse operation.


Enhanced Temperature Regulation with Low-Power Blowers

Implementing Efficient Blowers for Nighttime Temperature Management: A crucial enhancement to our greenhouse’s temperature control system is the introduction of low-power blowers. These blowers are designed to activate automatically when the indoor air temperature falls below a predetermined threshold at night. Their role is to augment the heat exchange process, going beyond what could be achieved through natural convective air movement. By circulating a substantial amount of air past the thermal storage barrels, these blowers effectively stabilize the nighttime temperature, maintaining it at a higher, more consistent level.

Balancing Thermal Storage and Continuous Heat Source: While the increased use of these blowers does raise the possibility of depleting our thermal storage reserves more quickly, this concern is mitigated by our geo-exchange field. The field provides a continuous supply of water at a steady 42.7°F, ensuring that there’s always a baseline level of thermal energy available. This constant source is especially beneficial during extended periods of cloud cover and freezing nights, reducing the risk of exhausting our thermal storage.

Adaptive Use of Blowers and Future Adjustments: The necessity for active air movement at night is particularly critical when outdoor temperatures plunge below 20°F, and there is a lack of inbound solar energy. However, as we progress with the installation of the remaining insulated glazing panels, we anticipate a decrease in the need for these blowers. The enhanced insulation from these panels will further stabilize the greenhouse’s internal temperature, reducing the reliance on mechanical air movement for heat distribution. This adjustment is expected to contribute to a more energy-efficient and self-regulating greenhouse environment.

Adapting to Variable Solar Energy in Greenhouse Operations

Managing Fluctuating Solar Energy in Northeastern Washington: In our greenhouse located in northeastern Washington, we face the unique challenge of dealing with highly variable solar energy. The region is known for its significant cloud cover and limited sun exposure, influencing the performance of our solar panels. On the brightest days, our 300-watt solar panels can produce up to 18 amps, but typically, the output fluctuates between 2 and 6 amps. There are also days when the production drops to as low as half an amp. This variability necessitates meticulous power management, which has become a cornerstone of our greenhouse design.

Strategic Use of Multiple Blowers and Power Reservation: To effectively handle this variability, we employ several blowers of different sizes. The activation of these blowers is carefully calibrated to match the amount of photovoltaic energy available at any given time. This approach ensures that we optimally utilize the available solar energy without overburdening the system. Additionally, we have implemented a strategy to manage our power consumption. This involves reserving sufficient energy to charge a 100 amp-hour battery, which is crucial for powering both the geo-exchange pump and the nighttime blowers. These components are vital for maintaining thermal regulation, especially under challenging weather conditions.

Leveraging Photovoltaic Energy for Maximized Heat Absorption: An intriguing aspect of our setup is the correlation between the peak photovoltaic energy and the maximum thermal gain within the greenhouse. We capitalize on this synchronicity by enhancing air movement during these periods of maximum solar energy. This strategy allows us to amplify heat absorption when it’s most needed, aligning the greenhouse’s thermal management with the natural rhythms of solar energy. This synergy between photovoltaic power and thermal gain is a critical element of our greenhouse’s efficiency, enabling us to maintain optimal conditions for plant growth despite the variable climate of our region.

See Also:


Some material sourced from my prior posts on Facebook forums

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