Frequently Asked Questions
What is a "climate battery"?
A climate battery is a system that pushes warm humid greenhouse air underground through buried tubing to transfer heat to the greenhouse soil, storing heat energy for times of needed heating. We refer to the system of tubing, risers, manifolds, fans, and the insulated mass of soil it interacts with all as the climate battery. It is referred to as a battery for its capacity to store energy. Also known to be called a subterranean heating and cooling system (SHCS), Ground to Air Heat Transfer System (GAHTTM), geo-air exchanger, or “low-grade” geothermal.
How does a climate battery work?
During the day when the greenhouse interior is being heated by the sun, the climate battery fans push this heated air from high in the greenhouse down through the underground heat exchange tubing. This warm, moist air cools as it runs through the tubing, depositing heat by conduction into the surrounding soil, and condensed water vapor with latent heat through perforations in the tubing. This cooled, dryer air returns to the greenhouse space, cooling and drying the greenhouse, and regaining its capacity to absorb moisture and heat from the greenhouse again. It is a simple form of the heat pump cycle, that takes advantage of the latent heat energy stored in water vapor, and the phenomenon of condensating said vapor by bringing the air temperature down to dew point through heat transfer to the cooler soil.
Why do you recommend short runs of heat exchange tubing (25-35’) vs. less longer runs? Could longer runs be used?
We have found from experience that 25-35’ runs work well for efficient air-to-soil heat exchange in these systems, but that is not to say that longer runs couldn’t work as well. The issue we have found with longer runs is that after ~35’ oftentimes the air has already reached soil temperature and is no longer transferring heat to the soil, thereby wasting the remaining tube length. This also creates uneven heating of the greenhouse soil, with soil near the climate battery intake warmer than near the exhaust. We suspect there are arrangements that could make longer runs work, such as increasing air speed, but we have not tested it out enough yet to confidently recommend it. Any shorter than 25’ of tube length and you risk not bringing the temperature of the air down to dew-point while in the tubes, missing the benefits of water-vapor phase-change.
Does the climate battery capacity increase by burying the tubing at greater depths?
Essentially, yes. But it becomes a matter of diminishing returns, for every bit deeper you install your climate battery, your costs for trenching/excavation and the depth you need to insulate increases. Also as you go deeper the benefits of soil heat radiated passively back into the space does not increase, only your capacity for drawing heat out of the soil via the climate battery and heat transfer tubes. We have seen great success in Zone 4 in Colorado with climate batteries installed to 3-4ft deep.
Is there a point at which “charging” the battery becomes useless?
Yes, when your greenhouse air temperature is the same as your climate battery soil. Heat transfer requires a temperature differential to occur. When you have raised your entire affected soil mass to the same temperature as your greenhouse air, then you will find that climate battery intake air temp (same as greenhouse air temp), and climate battery exhaust temp will be equal, and you are no longer storing heat. Your capacity to dehumidify air will be minimal too, for it takes a drop in temperature to induce water vapor condensate.
What does it mean to “properly control” a climate battery?
Good question. To function to your benefit, a climate battery should run only when there is “excess heat” in a greenhouse to “charge” the battery, and when the greenhouse air temperature is below the soil temperature for heating. Also, you do not want to deplete your climate battery of stored heat. This control can be accomplished with a two-stage thermostat (with low setting, neutral, and high), two separate thermostats, or an integrated controller system that controls other automated systems in the greenhouse.
What does it mean to “deplete” a climate battery in extremely cold weather?
During extended extreme cold events enough heat energy can be extracted from the climate battery soil to reduce the soil temperature to below the surrounding soil temperature (<50-55°F). At this point it is better to turn off the climate battery, and switch to backup heating.
Can I climate battery be used primarily for cooling in the summertime?
If your climate dictates that cooling is your primary concern, a climate battery can be used to so such effect. The first step is to design your greenhouse to reduce sunlight penetration to only what is needed for plant growth, to reduce the amount of solar heat gain. This can be accomplished with opaque siding/roofing and insulation on the north, northeast, and northwest surfaces. Secondly you will want to maximize ventilation of your greenhouse, releasing solar-heated air to the surroundings. Next, a high-pressure cooling mist pump can introduce water vapor into your greenhouse environment, effectively sucking up the heat energy in the air and venting to the outside or condensating in the climate battery.
An effective method we've found is by installing a "cooling battery" outside of the greenhouse footprint to the north. The heat exchange tubes in this tubing battery should be spaced further apart 2-3' plus, and preferably a solar arc food forest should be planted above it, creating a living shade blanket that helps to cool the ground. Mulch is very effective too. This cooling battery can be used to draw cooled, dehumidified air into the greenhouse space.
How important is insulating the foundation?
In short, very important. You wouldn’t build a solar home without insulation, so you hopefully won’t build a passive solar greenhouse without it either. Insulation around the greenhouse soil serves to hold in the heat stored by the climate battery, without it that heat energy will be transferred through conduction to the colder soil surrounding it. Soil insulation can be as simple as sheets of EPS rigid foam insulation buried around the greenhouse perimeter in a trench to the depth of the lowest layer of heat transfer tubes in the system. If pouring a continuous foundation, those same sheets of rigid insulation can be used in the form system and left in place. Shallow frost-protected foundation insulation systems work well too.
How important is using perforated pipe?
Very important. Climate batteries built without perforations in the heat exchange tubing will fill up with water from the condensate, and become "plugged". ADS drainage tubing is available with perforations, it is worth sourcing this material.
Would it help to insulate below the climate battery as well?
In theory, this could help, effectively isolating your climate battery soil from the surrounding the soil, limiting the capacity of your climate battery, but reducing heat losses to the earth below. It becomes a question of cost-benefit: insulating your 70°F greenhouse soil from the surrounding soil that may be at freezing down to your local frost depth greatly reduces heat transfer out of your climate battery. Insulating that 70°F soil from the 50-55°F soil below it will reduce heat transfer, but that transfer is already not so significant. Also, with optimum sunny days for climate battery “charging”, you can actually raise the surrounding soil temperature around your climate battery, increasing your battery of stored heat energy.
What type of soil do I need for a climate battery?
Sandy loamy soil is very effective, and is ideal for plant growth. The soil should be no more than 20-25% clay content, and not contain “pottery clay”. Soil with a lot of clay can form a shell around the heat exchange tubing after successive wetting and drying periods, effectively plugging the climate battery. If soil at the site of your climate battery is found to be greater than 25% clay content, it should be excavated and a different soil mix used for backfill, or the same soil can be amended to reduce the clay content.
How deep should the climate battery go?
A typical 3-layer, 4”ADS tubing climate battery can be buried to about 3’4” of depth. This allows for a deep layer at 3’4” deep, a middle layer at 2’4”, and a top layer at 1’ 4”, with 12” of soil above it for root zone and garden tools. This is just a basic case.
In extremely cold climates, a deeper climate battery is recommended, possibly with 4 layers, to interact with more soil. In a mild climate where a small climate modification is desired, you can go with less layers, and a shallower climate battery. Denser arrays of heat exchange tubing will exchange heat quickly, but have limited capacity. Less dense arrays will have greater heating & cooling capacity, but will be less effective at interacting with all of the soil. Heavy soils will store more heat, and allow for denser heat exchange tubing arrays, while lighter soils hold less heat, and require more spacing between tubes.
A few rules of thumb are:
1) allow 12”+ above the top tubing layer to surface to avoid damaging the climate battery when planting, digging, etc.
2) allow at least 9-12” between heat exchange tubes, otherwise your volume of soil affected per tube will be low.
3) insulate around your perimeter to the greatest depth of your climate battery, in order to keep that stored heat from migrating out to the cold surrounding soil.
4) any tubing deeper than 3’ will store heat below the root zone of plants, and won’t do much to passively heat the space through radiation. Deeper tubes can be considered as “backup” storage, but will begin to lose effectiveness.
What’s the best way to install a climate battery?
Sometimes you can install a climate battery by trenching 6” trenches where your tubes must go, and layering the tubes directly on top of each other. This will work for less dense arrays, where horizontal tube spacing is on the order of 2’, leaving enough soil between trenches that they do not collapse.
Oftentimes the whole area of the climate battery will need to be excavated, especially if it is determined that a different growing medium will be used as backfill. Excavate to your greatest depth, situate all manifolds and risers so they will not move around during backfill, and start with your bottom layer of tubing. Backfill over each successive layer of tubing, being careful not to drive any heavy equipment over any of the tubes.
Does each tube need to be of equal length, or can some be longer/shorter?
It is best to keep all the tubes as close to equal length as possible, in order to keep airflow equal through all the tubes. Air flow will be greater through shorter tubes due to their lower friction losses, and the climate battery will lose efficiency from uneven heat transfer.
How big of a fan do you need?
The climate battery fan should have high enough airflow ratings to move the entire volume of the greenhouse space 5x per hour (5ACH). We are seeing the potential for higher air changes per hour, but we will wait for more evidence to confirm. Also, we try to maintain 5fps air speeds in the heat exchange tubing, and keep the air in the tubes for 3-5 seconds. These parameters allow the air to cool to dew point, releasing water vapor and its stored latent heat.
Do you vary the climate battery fan speed in order to tune the output of the system?
Yes, it is helpful to tune your climate battery with a fan speed controller or variable speed controller. The goal is to maximize your temperature and relative humidity difference between intake and exhaust. This should be done at least twice a year, at the beginning of the heating season, and at the beginning of the cooling season.
Have you considered hybrid systems with one climate battery buried 8’ or deeper (“Citrus in the Snow”, Russ Finch style), to serve as backup heat during extreme cold times?
We have not experimented with this arrangement, but thank you Tim at Threefold Farm for having the motivation to try it out. Intuitively it should work great, although depending on your local environment’s low temperature extremes and the level of insulation of your greenhouse, you may still need additional backup heat periodically. Also, the deeper you dig, the more it costs.
Have you heard of any systems using a greenhouse within a greenhouse in order to isolate the growing space? How is this best accomplished?
Yes, we’ve seen and recommended this arrangement with great success in many forms. We always recommend the use of a retractable “energy curtain” when the budget allows, effectively blanketing the greenhouse space at night to reduce heat loss, or deploying during hot afternoons for shading. These can be deployed low to create the “cold roof” effect. We’ve also designed “bed batteries”, small climate batteries within insulated raised beds complete with row covers, isolating these smaller growing spaces within a greenhouse for smaller-scale winter production. Simple row covers and cloches within a greenhouse work great too. Check out Eliot Coleman’s book, “Four-Season Harvest”, for detailed methods of simple season extension.
Why do you recommend a horizontal airflow fan? Can these overcome the backpressure in the climate battery?
We have found that horizontal airflow fans work very well, especially for the reduced cost. With large risers and manifolds, and enough heat exchange tubes, the static pressure in the climate battery is relatively low compared to standard HVAC ducting.
Initial tests with squirrel-cage blowers led to premature burn-out of their motors. When all vents are closed, the air circulating through the greenhouse and climate battery is essentially a closed system; the squirrel-cage fans would pressurize their intake air enough to speed up their own motors, drawing more current and leading to premature failure. They also were very noisy.
Inline duct boosters and inline axial fans work very well, but for higher flow-rates (above ~1000cfm), their cost increases dramatically. Make sure to get a fan rated for use in outdoor environments, easily found through a greenhouse supplier (we like J.D.Schaefer fans a lot).
Why is the air pushed into the tubing rather than pulled?
Pushing air creates positive pressure within the climate battery, helping to reduce infiltration of silt into the heat exchange tubes. Also, with the fan on the intake side, the exhaust riser is free for adding cooling mist rings and/or hydronic heat coils. Also we often locate our intake risers so they can pull from higher above the ground to pull from the warmest air, which in turn helps to protect the fans from irrigation water.
What is the purpose of the manifold? Can all of your heat exchange tubes connect directly to the riser? Could a plastic barrel work as a riser/manifold? Are there other cheap options for the riser/manifold?
The manifolds are an effective way of distributing the heat exchange tubes evenly across the floor area, and to maintain equal tube lengths. Also it is often difficult to fit all the heat exchange tubes a climate battery design calls for into the surface area of an intake riser alone. Small footprints can be covered without the use of a manifold by using a plastic barrel as an intake riser/manifold and snaking the tubes around to maintain equal length. Risers have been built out of plywood too, although obviously their lifespan will be greatly reduced when buried compared to plastic. We are very open and curious to other suggestions for materials/methods.
How should the heat exchange tubes connect to the manifold?
The simplest way to install the heat exchange tubes is by drilling or cutting a hole and stuffing the end of the tube into it. We use a 4 5/8” hole saw for 4” ADS tubing (4 ¾” for tubing with the nylon sock). With the sock, push the end of the sock into the tube before you push the tube into its hole, to try to keep it on until you backfill. These connections do not have to be air-tight, just tight enough to reduce soil infiltration during backfill. The use of a cheap screw can help to keep the tube in place until backfilled.
Is the tubing with the sock necessary?
The ADS tubing that comes with the sock on it is very helpful to prevent the intrusion of dirt/silt into the tubing, especially during installation and backfill. After some period of initial use, most the silt in the soil around the tubing will have settled into its final resting place anyways, but the sock will keep it from entering the tubing in the process. Some dirt and silt in the climate battery tubing is not a big deal, but too much can impede airflow and reduce the effectiveness of individual tubes.
Could convection tubing (inexpensive vented plastic) be used to direct intake/exhaust location?
It could possibly be used as an exhaust riser extension, but I don’t think it would stay open under negative pressure at the intake side. We recommend using mylar ducting, available from Grainger up to 18” diameter, for intake extensions to reach up towards the ceiling of tall greenhouses. On the exhaust side it is not necessary to raise the height, and actually can benefit plants with the air movement close to the soil.
Should I screen my intake/exhaust? If so, how?
Yes, as a precaution to keep critters out, but more commonly to keep tools and cell phones from falling in! What a pain! Make sure to use large-opening screen (1/4 or ½” or bigger galvanized hardware cloth) to avoid reducing airflow. Window and insect screening is too fine.
Can the soil be overheated?
Overheating is not typically a concern, as heat only flows from warmer to colder. Therefore to heat the soil over 90°F, you would need greater than 90°F greenhouse air for an extended period of time, in which case overheating from the hot air would be more of a concern. The climate battery will lose its effectiveness when the soil has heated up to over 75-80°F, as its capacity to cool will be diminished. For this reason, it is key to pair proper ventilation with the climate battery to dump unwanted daytime heat to the outside during long periods of warm weather. Venting should be controlled to keep the greenhouse space warm and humid during heating seasons, storing heat with the climate battery for nighttime heating. In the cooling seasons the venting can be used to release excess heat, saving the climate battery for a cooling source when the outside air is above 85-90°F.
Do you have problems with critters entering the climate battery?
It hasn’t been an issue, but we do recommend the screening to prevent animal intrusion. Insects and such don’t seem attracted to it, likely because of the near-constant airflow. We could imagine a rat or mouse wanting to make a home in a climate battery, but this hasn’t been the case. By keeping the level of the intake/exhaust risers 2’ + off the soil, animals are further discouraged from climbing up into a slippery-walled plastic pipe. Any closer to the ground and a screen should definitely be implemented.
Don't you get mold or mildew in a system like this?
No. For two reasons at least: 1) the air is flowing constantly enough to prevent any molds or mildews from establishing themselves. Molds & mildews seem to prefer stagnant, moist air to flowing air. 2) the perforated tubing is in intimate contact with the soil microbes around it, which are competing for the same territory to live in. Normal soil with active biota in it is too competitive for one organism to win out over the others, and take over.
Do you still need supplemental heat with a climate battery in place?
We often recommend a backup heater to be paired with a climate battery in cold climates. On very cold nights, unless a greenhouse is extremely well insulated, the heat loss to the outside environment can be greater than what the climate battery can provide, causing a steady drop in greenhouse interior temperature. Especially after several weeks of cold, cloudy weather, the climate battery will have used up much of its stored heat energy, and could potentially cool the soil to below its average temperature of 50-55°F. At this point, it is recommended to turn off your climate battery fans and run backup heating to heat the space.
For example, at CRMPI the climate batteries in the Phoenix greenhouse are sufficient to keep tropical plants alive through the majority of winter nights. But, when outside temperatures drop far below 0°F, to -15 or -20°F, the sauna is used as backup heat to keep the greenhouse air temperature from getting too cold, and to avoid cooling the climate battery soil significantly.
A climate battery is a system that pushes warm humid greenhouse air underground through buried tubing to transfer heat to the greenhouse soil, storing heat energy for times of needed heating. We refer to the system of tubing, risers, manifolds, fans, and the insulated mass of soil it interacts with all as the climate battery. It is referred to as a battery for its capacity to store energy. Also known to be called a subterranean heating and cooling system (SHCS), Ground to Air Heat Transfer System (GAHTTM), geo-air exchanger, or “low-grade” geothermal.
How does a climate battery work?
During the day when the greenhouse interior is being heated by the sun, the climate battery fans push this heated air from high in the greenhouse down through the underground heat exchange tubing. This warm, moist air cools as it runs through the tubing, depositing heat by conduction into the surrounding soil, and condensed water vapor with latent heat through perforations in the tubing. This cooled, dryer air returns to the greenhouse space, cooling and drying the greenhouse, and regaining its capacity to absorb moisture and heat from the greenhouse again. It is a simple form of the heat pump cycle, that takes advantage of the latent heat energy stored in water vapor, and the phenomenon of condensating said vapor by bringing the air temperature down to dew point through heat transfer to the cooler soil.
Why do you recommend short runs of heat exchange tubing (25-35’) vs. less longer runs? Could longer runs be used?
We have found from experience that 25-35’ runs work well for efficient air-to-soil heat exchange in these systems, but that is not to say that longer runs couldn’t work as well. The issue we have found with longer runs is that after ~35’ oftentimes the air has already reached soil temperature and is no longer transferring heat to the soil, thereby wasting the remaining tube length. This also creates uneven heating of the greenhouse soil, with soil near the climate battery intake warmer than near the exhaust. We suspect there are arrangements that could make longer runs work, such as increasing air speed, but we have not tested it out enough yet to confidently recommend it. Any shorter than 25’ of tube length and you risk not bringing the temperature of the air down to dew-point while in the tubes, missing the benefits of water-vapor phase-change.
Does the climate battery capacity increase by burying the tubing at greater depths?
Essentially, yes. But it becomes a matter of diminishing returns, for every bit deeper you install your climate battery, your costs for trenching/excavation and the depth you need to insulate increases. Also as you go deeper the benefits of soil heat radiated passively back into the space does not increase, only your capacity for drawing heat out of the soil via the climate battery and heat transfer tubes. We have seen great success in Zone 4 in Colorado with climate batteries installed to 3-4ft deep.
Is there a point at which “charging” the battery becomes useless?
Yes, when your greenhouse air temperature is the same as your climate battery soil. Heat transfer requires a temperature differential to occur. When you have raised your entire affected soil mass to the same temperature as your greenhouse air, then you will find that climate battery intake air temp (same as greenhouse air temp), and climate battery exhaust temp will be equal, and you are no longer storing heat. Your capacity to dehumidify air will be minimal too, for it takes a drop in temperature to induce water vapor condensate.
What does it mean to “properly control” a climate battery?
Good question. To function to your benefit, a climate battery should run only when there is “excess heat” in a greenhouse to “charge” the battery, and when the greenhouse air temperature is below the soil temperature for heating. Also, you do not want to deplete your climate battery of stored heat. This control can be accomplished with a two-stage thermostat (with low setting, neutral, and high), two separate thermostats, or an integrated controller system that controls other automated systems in the greenhouse.
What does it mean to “deplete” a climate battery in extremely cold weather?
During extended extreme cold events enough heat energy can be extracted from the climate battery soil to reduce the soil temperature to below the surrounding soil temperature (<50-55°F). At this point it is better to turn off the climate battery, and switch to backup heating.
Can I climate battery be used primarily for cooling in the summertime?
If your climate dictates that cooling is your primary concern, a climate battery can be used to so such effect. The first step is to design your greenhouse to reduce sunlight penetration to only what is needed for plant growth, to reduce the amount of solar heat gain. This can be accomplished with opaque siding/roofing and insulation on the north, northeast, and northwest surfaces. Secondly you will want to maximize ventilation of your greenhouse, releasing solar-heated air to the surroundings. Next, a high-pressure cooling mist pump can introduce water vapor into your greenhouse environment, effectively sucking up the heat energy in the air and venting to the outside or condensating in the climate battery.
An effective method we've found is by installing a "cooling battery" outside of the greenhouse footprint to the north. The heat exchange tubes in this tubing battery should be spaced further apart 2-3' plus, and preferably a solar arc food forest should be planted above it, creating a living shade blanket that helps to cool the ground. Mulch is very effective too. This cooling battery can be used to draw cooled, dehumidified air into the greenhouse space.
How important is insulating the foundation?
In short, very important. You wouldn’t build a solar home without insulation, so you hopefully won’t build a passive solar greenhouse without it either. Insulation around the greenhouse soil serves to hold in the heat stored by the climate battery, without it that heat energy will be transferred through conduction to the colder soil surrounding it. Soil insulation can be as simple as sheets of EPS rigid foam insulation buried around the greenhouse perimeter in a trench to the depth of the lowest layer of heat transfer tubes in the system. If pouring a continuous foundation, those same sheets of rigid insulation can be used in the form system and left in place. Shallow frost-protected foundation insulation systems work well too.
How important is using perforated pipe?
Very important. Climate batteries built without perforations in the heat exchange tubing will fill up with water from the condensate, and become "plugged". ADS drainage tubing is available with perforations, it is worth sourcing this material.
Would it help to insulate below the climate battery as well?
In theory, this could help, effectively isolating your climate battery soil from the surrounding the soil, limiting the capacity of your climate battery, but reducing heat losses to the earth below. It becomes a question of cost-benefit: insulating your 70°F greenhouse soil from the surrounding soil that may be at freezing down to your local frost depth greatly reduces heat transfer out of your climate battery. Insulating that 70°F soil from the 50-55°F soil below it will reduce heat transfer, but that transfer is already not so significant. Also, with optimum sunny days for climate battery “charging”, you can actually raise the surrounding soil temperature around your climate battery, increasing your battery of stored heat energy.
What type of soil do I need for a climate battery?
Sandy loamy soil is very effective, and is ideal for plant growth. The soil should be no more than 20-25% clay content, and not contain “pottery clay”. Soil with a lot of clay can form a shell around the heat exchange tubing after successive wetting and drying periods, effectively plugging the climate battery. If soil at the site of your climate battery is found to be greater than 25% clay content, it should be excavated and a different soil mix used for backfill, or the same soil can be amended to reduce the clay content.
How deep should the climate battery go?
A typical 3-layer, 4”ADS tubing climate battery can be buried to about 3’4” of depth. This allows for a deep layer at 3’4” deep, a middle layer at 2’4”, and a top layer at 1’ 4”, with 12” of soil above it for root zone and garden tools. This is just a basic case.
In extremely cold climates, a deeper climate battery is recommended, possibly with 4 layers, to interact with more soil. In a mild climate where a small climate modification is desired, you can go with less layers, and a shallower climate battery. Denser arrays of heat exchange tubing will exchange heat quickly, but have limited capacity. Less dense arrays will have greater heating & cooling capacity, but will be less effective at interacting with all of the soil. Heavy soils will store more heat, and allow for denser heat exchange tubing arrays, while lighter soils hold less heat, and require more spacing between tubes.
A few rules of thumb are:
1) allow 12”+ above the top tubing layer to surface to avoid damaging the climate battery when planting, digging, etc.
2) allow at least 9-12” between heat exchange tubes, otherwise your volume of soil affected per tube will be low.
3) insulate around your perimeter to the greatest depth of your climate battery, in order to keep that stored heat from migrating out to the cold surrounding soil.
4) any tubing deeper than 3’ will store heat below the root zone of plants, and won’t do much to passively heat the space through radiation. Deeper tubes can be considered as “backup” storage, but will begin to lose effectiveness.
What’s the best way to install a climate battery?
Sometimes you can install a climate battery by trenching 6” trenches where your tubes must go, and layering the tubes directly on top of each other. This will work for less dense arrays, where horizontal tube spacing is on the order of 2’, leaving enough soil between trenches that they do not collapse.
Oftentimes the whole area of the climate battery will need to be excavated, especially if it is determined that a different growing medium will be used as backfill. Excavate to your greatest depth, situate all manifolds and risers so they will not move around during backfill, and start with your bottom layer of tubing. Backfill over each successive layer of tubing, being careful not to drive any heavy equipment over any of the tubes.
Does each tube need to be of equal length, or can some be longer/shorter?
It is best to keep all the tubes as close to equal length as possible, in order to keep airflow equal through all the tubes. Air flow will be greater through shorter tubes due to their lower friction losses, and the climate battery will lose efficiency from uneven heat transfer.
How big of a fan do you need?
The climate battery fan should have high enough airflow ratings to move the entire volume of the greenhouse space 5x per hour (5ACH). We are seeing the potential for higher air changes per hour, but we will wait for more evidence to confirm. Also, we try to maintain 5fps air speeds in the heat exchange tubing, and keep the air in the tubes for 3-5 seconds. These parameters allow the air to cool to dew point, releasing water vapor and its stored latent heat.
Do you vary the climate battery fan speed in order to tune the output of the system?
Yes, it is helpful to tune your climate battery with a fan speed controller or variable speed controller. The goal is to maximize your temperature and relative humidity difference between intake and exhaust. This should be done at least twice a year, at the beginning of the heating season, and at the beginning of the cooling season.
Have you considered hybrid systems with one climate battery buried 8’ or deeper (“Citrus in the Snow”, Russ Finch style), to serve as backup heat during extreme cold times?
We have not experimented with this arrangement, but thank you Tim at Threefold Farm for having the motivation to try it out. Intuitively it should work great, although depending on your local environment’s low temperature extremes and the level of insulation of your greenhouse, you may still need additional backup heat periodically. Also, the deeper you dig, the more it costs.
Have you heard of any systems using a greenhouse within a greenhouse in order to isolate the growing space? How is this best accomplished?
Yes, we’ve seen and recommended this arrangement with great success in many forms. We always recommend the use of a retractable “energy curtain” when the budget allows, effectively blanketing the greenhouse space at night to reduce heat loss, or deploying during hot afternoons for shading. These can be deployed low to create the “cold roof” effect. We’ve also designed “bed batteries”, small climate batteries within insulated raised beds complete with row covers, isolating these smaller growing spaces within a greenhouse for smaller-scale winter production. Simple row covers and cloches within a greenhouse work great too. Check out Eliot Coleman’s book, “Four-Season Harvest”, for detailed methods of simple season extension.
Why do you recommend a horizontal airflow fan? Can these overcome the backpressure in the climate battery?
We have found that horizontal airflow fans work very well, especially for the reduced cost. With large risers and manifolds, and enough heat exchange tubes, the static pressure in the climate battery is relatively low compared to standard HVAC ducting.
Initial tests with squirrel-cage blowers led to premature burn-out of their motors. When all vents are closed, the air circulating through the greenhouse and climate battery is essentially a closed system; the squirrel-cage fans would pressurize their intake air enough to speed up their own motors, drawing more current and leading to premature failure. They also were very noisy.
Inline duct boosters and inline axial fans work very well, but for higher flow-rates (above ~1000cfm), their cost increases dramatically. Make sure to get a fan rated for use in outdoor environments, easily found through a greenhouse supplier (we like J.D.Schaefer fans a lot).
Why is the air pushed into the tubing rather than pulled?
Pushing air creates positive pressure within the climate battery, helping to reduce infiltration of silt into the heat exchange tubes. Also, with the fan on the intake side, the exhaust riser is free for adding cooling mist rings and/or hydronic heat coils. Also we often locate our intake risers so they can pull from higher above the ground to pull from the warmest air, which in turn helps to protect the fans from irrigation water.
What is the purpose of the manifold? Can all of your heat exchange tubes connect directly to the riser? Could a plastic barrel work as a riser/manifold? Are there other cheap options for the riser/manifold?
The manifolds are an effective way of distributing the heat exchange tubes evenly across the floor area, and to maintain equal tube lengths. Also it is often difficult to fit all the heat exchange tubes a climate battery design calls for into the surface area of an intake riser alone. Small footprints can be covered without the use of a manifold by using a plastic barrel as an intake riser/manifold and snaking the tubes around to maintain equal length. Risers have been built out of plywood too, although obviously their lifespan will be greatly reduced when buried compared to plastic. We are very open and curious to other suggestions for materials/methods.
How should the heat exchange tubes connect to the manifold?
The simplest way to install the heat exchange tubes is by drilling or cutting a hole and stuffing the end of the tube into it. We use a 4 5/8” hole saw for 4” ADS tubing (4 ¾” for tubing with the nylon sock). With the sock, push the end of the sock into the tube before you push the tube into its hole, to try to keep it on until you backfill. These connections do not have to be air-tight, just tight enough to reduce soil infiltration during backfill. The use of a cheap screw can help to keep the tube in place until backfilled.
Is the tubing with the sock necessary?
The ADS tubing that comes with the sock on it is very helpful to prevent the intrusion of dirt/silt into the tubing, especially during installation and backfill. After some period of initial use, most the silt in the soil around the tubing will have settled into its final resting place anyways, but the sock will keep it from entering the tubing in the process. Some dirt and silt in the climate battery tubing is not a big deal, but too much can impede airflow and reduce the effectiveness of individual tubes.
Could convection tubing (inexpensive vented plastic) be used to direct intake/exhaust location?
It could possibly be used as an exhaust riser extension, but I don’t think it would stay open under negative pressure at the intake side. We recommend using mylar ducting, available from Grainger up to 18” diameter, for intake extensions to reach up towards the ceiling of tall greenhouses. On the exhaust side it is not necessary to raise the height, and actually can benefit plants with the air movement close to the soil.
Should I screen my intake/exhaust? If so, how?
Yes, as a precaution to keep critters out, but more commonly to keep tools and cell phones from falling in! What a pain! Make sure to use large-opening screen (1/4 or ½” or bigger galvanized hardware cloth) to avoid reducing airflow. Window and insect screening is too fine.
Can the soil be overheated?
Overheating is not typically a concern, as heat only flows from warmer to colder. Therefore to heat the soil over 90°F, you would need greater than 90°F greenhouse air for an extended period of time, in which case overheating from the hot air would be more of a concern. The climate battery will lose its effectiveness when the soil has heated up to over 75-80°F, as its capacity to cool will be diminished. For this reason, it is key to pair proper ventilation with the climate battery to dump unwanted daytime heat to the outside during long periods of warm weather. Venting should be controlled to keep the greenhouse space warm and humid during heating seasons, storing heat with the climate battery for nighttime heating. In the cooling seasons the venting can be used to release excess heat, saving the climate battery for a cooling source when the outside air is above 85-90°F.
Do you have problems with critters entering the climate battery?
It hasn’t been an issue, but we do recommend the screening to prevent animal intrusion. Insects and such don’t seem attracted to it, likely because of the near-constant airflow. We could imagine a rat or mouse wanting to make a home in a climate battery, but this hasn’t been the case. By keeping the level of the intake/exhaust risers 2’ + off the soil, animals are further discouraged from climbing up into a slippery-walled plastic pipe. Any closer to the ground and a screen should definitely be implemented.
Don't you get mold or mildew in a system like this?
No. For two reasons at least: 1) the air is flowing constantly enough to prevent any molds or mildews from establishing themselves. Molds & mildews seem to prefer stagnant, moist air to flowing air. 2) the perforated tubing is in intimate contact with the soil microbes around it, which are competing for the same territory to live in. Normal soil with active biota in it is too competitive for one organism to win out over the others, and take over.
Do you still need supplemental heat with a climate battery in place?
We often recommend a backup heater to be paired with a climate battery in cold climates. On very cold nights, unless a greenhouse is extremely well insulated, the heat loss to the outside environment can be greater than what the climate battery can provide, causing a steady drop in greenhouse interior temperature. Especially after several weeks of cold, cloudy weather, the climate battery will have used up much of its stored heat energy, and could potentially cool the soil to below its average temperature of 50-55°F. At this point, it is recommended to turn off your climate battery fans and run backup heating to heat the space.
For example, at CRMPI the climate batteries in the Phoenix greenhouse are sufficient to keep tropical plants alive through the majority of winter nights. But, when outside temperatures drop far below 0°F, to -15 or -20°F, the sauna is used as backup heat to keep the greenhouse air temperature from getting too cold, and to avoid cooling the climate battery soil significantly.