This fourth and final article in the series on long-term vegetable storage (the first three parts appearing in GFM February and May of 2020, and January 2021) focuses on climate control in storage facilities, including cooling, heating, humidification, and ventilation. As previously noted, high-quality produce at harvest time and a well-designed storage facility will prepare you to successfully store and sell fall-harvested vegetables through the winter. Properly controlling the climate inside your storage room(s) is the final step. You’ll need to control the temperature and humidity and ensure both adequate air flow and an influx of fresh air. You may also wish to heat adjoining spaces in addition to the storage room(s) to maintain comfortable working conditions. 

 

Cooling

The ability to cool a storage space to optimal temperatures prior to the arrival of cold weather is a major difference between modern and traditional root cellars. This ability is what allows commercial growers to maintain market-quality produce deep into winter and beyond. Options for cooling systems are many, but refrigeration can be broken into three major groups: residential air conditioning (A/C) units, self-contained refrigeration units (commonly used with walk-in coolers), and remote refrigeration systems. 

Both self-contained and remote refrigeration systems are commonly used with commercial coolers for restaurants and food storage. The difference is that self-contained systems have both the evaporator and condenser housed inside a single unit, while remote systems have the evaporator and condenser in separate locations. Self-contained systems are usually mounted through walls, like window A/C units. Remote systems usually have the evaporator inside the cooler/storage space and the condenser located outdoors with a series of hoses to move coolant between the evaporator and condenser. 

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A remote refrigeration unit at Vermont Valley Farm in Blue Mounds, Wisconsin. 

The evaporator (above) pulls heat from the storage room 

and exhausts it outdoors to a remote condenser (below).

 

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Both types of commercial refrigeration systems can be similarly priced for a given cooling power, but remote systems are often more expensive to install, fix, and operate. Because coolant lines must run between the evaporator and condenser, installation and repairs often require professional help. Additionally, the extra pumping power required to move coolant through longer lines requires more electricity to operate. That said, remote refrigeration systems give you more flexibility for exhausting excess heat and will have the most powerful cooling options (i.e., BTU/hour) of the three groups. 

A benefit of choosing either self-contained or remote refrigeration systems is that they are designed to operate at temperatures relevant to cold storage and usually function with their full advertised cooling power (BTU/hour).  

Residential air conditioners modified with CoolBot® controllers are often the least expensive option for small farms and are easy to install without professional help. Both window and mini-split A/C systems can function with CoolBot® controllers, with window units being less powerful but less expensive than mini-splits. CoolBot® systems, although attractive for many small farms, have important limitations. 

Despite their low cost and ease of use and installation, CoolBot® systems can become both impractical and expensive compared to conventional refrigeration for large cooling loads and low cooler temperatures. First, residential air conditioners do not perform with their advertised cooling power at low temperatures (32–50°F) compared to the warmer temperatures they were designed for. Results from a study run by the Practical Farmers of Iowa suggest that cooling powers for window A/C units may decrease around 90 percent from what’s advertised at a set temperature of 40°F. 

By my calculations, the cooling power of a residential A/C may be only 5 percent of what’s advertised at a set temperature of 34°F. This means an air conditioner rated for 18,000 BTU/hour may only have the cooling power equivalent to 900 BTU/hour at a set temperature of 34°F. Put another way, if your storage room needs 5,000 BTU/hour of actual cooling power at 34°F, you may need enough residential A/C units to equal 100,000 BTU/hour of advertised cooling power to adequately cool the space. The second drawback of CoolBot® systems is their temperature limitation. Store It Cold does not recommend using a CoolBot® system to cool a room below 34°F. 

Although the optimal temperature for long-term storage is usually 32–33°F, you may find workarounds or accept the limitations of CoolBot® systems. For example, the temperature limitation might not matter if you are able to harvest late enough to use cool, outdoor air to reduce your cold storage room temperatures to around 32°F as well as to pre-cool vegetables from the field. 

You may also want to consider your goals for vegetable storage life. If you don’t need October-harvested carrots to last until May, 38°F may be adequate for a month or two while ambient air temperatures are too warm to use for cooling a storage space to desired temperatures. However, if you need to store high-quality fall-harvested produce late into the spring, the temperature limitations of CoolBot® systems might not be acceptable.

Estimating your maximum cooling load will help you decide which type of cooling system is necessary for your needs and budget. The cooling system will need to remove field heat from harvested produce and compensate for heat gained through respiration, opening the cooler door, and heat gained through insulation and miscellaneous air leakage. The point at which conventional refrigeration systems become less expensive than CoolBot® systems will vary depending on your local climate, harvest timing, desired storage temperatures, and whether you’re buying new or used cooling equipment. 

As an example, conventional refrigeration and CoolBot® systems will both cost around $2,000 to $3,000 new to achieve an actual cooling power of 5,000 BTU/hour for a storage room held at 38°F. If you drop the set temperature to 34°F, the point at which conventional systems become less expensive is probably closer to 2,000 to 3,000 BTU/hour, largely because residential A/C units drop their effective cooling power as the set temperature decreases.

 

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I created two tables to help you estimate the cooling powers necessary for two situations common to long-term vegetable storage: (A) removing field heat and (B) set-temperature maintenance in a full cooler. Table A shows the cooling power necessary to remove field heat from different weights of produce in a variety of storage-room sizes. To achieve maximum storage life, it’s recommended to reduce the temperature of most harvested veggies (those that don’t require curing) to at least 40°F within 24 hours of harvest. The table shows the cooling power necessary to remove field heat from a single-day’s harvest within 24 hours, as well as maintain a target temperature of 38°F in the storage room. 

Table B shows the cooling power necessary to maintain a set temperature of 34°F (chosen to include CoolBot® systems) in a variety of room sizes over a range of stored-produce weight. Both tables assume the storage room is well-insulated, with walls and ceilings insulated to R-25 and the floor to R-12 (equivalent to six inches of concrete plus two inches of EPS foam board). The tables also assume light amounts of use with at least several complete air exchanges per day depending on the room size. 

To account for heat produced by respiration, the tables assume uniform heat production of 2,200 BTU per ton per day—a relatively conservative estimate for most vegetables stored below 38°F. Additionally, I’ve assumed that the ground temperature is 45°F and both the outdoor air and harvested vegetable temperatures are 60°F. Because the outdoor air temperature isn’t always 60°F, I’ve also included the change in minimum cooling power for every 10°F change in outdoor temperature (in parentheses). 

Thus, to find the minimum cooling power for outdoor air temperatures of 50°F, subtract the number in parentheses from the corresponding number in the table. Likewise, to find the cooling power needed for outdoor temperatures of 80°F, multiply the number in parentheses by two and add that to the corresponding number in the table. Lastly, the minimum cooling powers have been increased by 10 percent to account for defrost cycles that prevent refrigeration units from freezing up.

The values in these tables will not perfectly match the conditions unique to your farm, but they’re intended to give you ballpark estimates to use as starting points in your design. Additionally, these estimates may be conservative because the calculations are for entirely above-ground structures. Dug-out structures would have better thermal performance in both winter and summer. For example, a structure with full (but insulated) ground contact on 3-walls would have minimum cooling powers 10 to 20 percent lower than a similar, but fully aboveground structure. 

You also may want to use the tables in conjunction with one another to account for scenarios when maximum cooling powers are needed. One example could be near the end of harvesting when the storage facility is nearly full but vegetables are still coming in from the field. This situation likely will require the highest cooling demand from your refrigeration system to both maintain a set temperature in a room full of respiring vegetables and remove field heat from newly harvested produce. 

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Thermostat control for air intake fans at Food Farm in Wrenshall, Minnesota. 

The thermostat controller activates a fan to pull in cold outdoor air 

to maintain a storage room temperature of 32°F. The 4°F differential 

gives the system wiggle room so that fans are not constantly turning on and off.

 

Sum the cooling powers corresponding to your nearly full harvest weight and your expected daily harvest weight. For example, if there are 50,000 pounds of produce in the storage room on the last day of harvesting 2,500 pounds per day, you likely need 3,920 + 3,780 = 7,700 BTU/hour in a 24 foot by 12 foot storage room with 8 foot ceilings at the temperatures described.

When choosing a refrigeration system, pay special attention to the temperature drop across the evaporator. If the temperature difference between incoming and outgoing air flowing over the evaporator coils/fins is too large, you may experience problems with frozen produce and have difficulty maintaining high humidity in the storage room. For example, if the air temperature drops 5°F when traveling over the evaporator coils, you may experience frozen produce if the set temperature is lower than 36° to 37°F. The air leaving the evaporator coils will be at 31° to 32°F and might freeze any produce near the cooling system. Furthermore, you could have difficulty achieving a relative humidity higher than 81 percent in this scenario. To achieve 95 percent relative humidity (without constantly adding humidity) with a set temperature near 32°F, the temperature drop across the evaporator cannot be much larger than 1°F. Contact the manufacturer prior to purchasing a new refrigeration system to ensure the system can function with a low temperature drop across the evaporator. 

In addition to mechanical refrigeration, use outside air to cool your storage room when outdoor temperatures are at or below storage temperatures.

You’ll need a thermostat controller that monitors both indoor and outdoor air temperature. When outdoor air temperature is below the storage room temperature, and when the storage temperature is becoming too warm (for example, 35°F in a room set to 32°F), you want the fans to engage and bring in outdoor air. Likewise, fans should turn off before the storage temperature gets too cold. This is a great way to reduce the necessary size of your cooling system and reduce energy consumption. 

I did this cheaply on my old farm in the Upper Peninsula of Michigan, Root Cellar Farm, by connecting two 110 V thermostat receptacles (cost $35 each) in series with one another. The thermostat connected to the wall receptacle was in “cooling mode,” activating when room temperatures got above 36°F and deactivating when temperatures dropped below 33°F. The second thermostat was plugged into the first and was set to “heating mode,” activating when outdoor temperatures were above 0°F and deactivating when outdoor temperatures rose above 36°F. 

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Cold storage is possible at many scales. The photo shows the small space heater 

and circulation fan that prevented vegetables from freezing inside the storage room 

at Root Cellar Farm in Toivola, Michigan. The small 1,500 W heater (~5,000 BTU/hour) 

is plugged into a thermostat receptacle, and the temperature probe for the receptacle 

is near the floor of the two-tiered storage room.

 

Lastly, a 6-inch duct fan was plugged into the second thermostat. The setup was similar to a nested if-then statement in Microsoft Excel: IF indoor temperatures were higher than 33°F AND if outdoor temperatures were below 36°F, then the fan was turned on. If either weren’t true, the fan was off. Of course, this can be achieved with more expensive (and probably more reliable) thermostat controllers.

 

Heating

Depending on your local climate, the amount of produce you’re storing, and how your storage facility was constructed and insulated, you may need to add supplemental heat to your storage room to keep vegetables from freezing. Whether natural gas or electric, you’ll want a forced air, or convection, type heater. Radiant heaters feel good, but they will heat produce unevenly leading to problems with spoilage or freezing. Forced-air type heaters should be coupled with circulation fans to ensure that warmed air is distributed evenly throughout a storage room. 

It may be advantageous to directly heat an adjoining wash/pack room and mechanically move warm air into the storage room through vent fans. These fans could be controlled by thermostats in a similar manner to those used for cooling a storage space with outdoor air (discussed above). Doing this would allow you to purchase a single heater to warm multiple sections of a storage facility. Be sure to add passive vents to allow pressure to equalize between rooms. This technique may not be suitable for those trying to maintain high humidity in the main storage room, but that depends on the desired humidity level and the necessary heating load.  

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(Above) Two types of stacked bins at Food Farm in Wrenshall, Minnesota, 

covered with polyethylene bin liners to create humid microenvironments inside the bins. Photo by the author. 

(Below) A winter-CSA box in January at Root Cellar Farm in Toivola, Michigan. 

The box includes squash, beets, potatoes, onions, garlic, Jerusalem artichokes, 

carrots, and cabbage, all harvested in the fall and stored.

 

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Humidity and air exchange

Humidity is one of the most important factors to successfully storing produce long-term. As discussed in the second article in this series, in the May 2020 GFM, most winter storage crops require relative humidity over 95 percent in their storage environment. Winter squash, garlic, and onions are notable exceptions and prefer relative humidity ranging from 60 to 70 percent. In addition to losing marketable weight and becoming soft and floppy, low humidity can lead to increased respiration rates and faster spoilage in long-term storage. However, high relative humidity can lead to condensation and dripping from a storage room’s ceiling and walls, which could also cause problems with early spoilage if not carefully monitored.

For this reason some farms, such as Food Farm in Wrenshall, Minnesota, maintain the 32°F storage room near 70 to 80 percent relative humidity but store sensitive crops inside closed plastic bin liners. For crops that don’t produce much ethylene or are not ethylene-sensitive, storing them inside plastic bin liners traps moisture in a humid microenvironment without overly compromising storage life or quality.

The trick is to put an absorbent, but not messy material in the bottom of the bin prior to loading with veggies. Food Farm uses peat moss covered with burlap. I’ve used several materials including wood shavings and paper towels but landed upon several sheets of brown paper as my favorite. If you’re washing crops prior to storage, place them into bins (with the absorbent material already inside) wet, and allow veggies to dry with the bag open overnight before closing the bag. 

If you’re not washing produce prior to storage, lightly dampen the material prior to loading veggies into bags. If the material is dry, it will pull moisture from nearby veggies, desiccating them. Loosely close the bag by pulling excess flaps over one another. You want there to be some air exchange, but produce near the bag openings will dry out if too much air is allowed in. 

To control overall storage-room humidity, use a humidifier controlled by a humidistat. Small, residential humidifiers can work well for small storage rooms but require constant refilling. Small centrifugal humidifiers are available in the output range of 0.4 to 2 gallons per hour, as well as larger, more expensive centrifugal humidifiers. “On-Farm Cold Storage” by Scott Sanford and John Hendrickson at the University of Wisconsin Extension, recommends at least 1 gallon per hour of added humidity for every 1,000 cubic feet per minute of air exchange or for every 1,500 cubic feet of storage room volume. 

You want to find the “sweet spot” for humidity in your storage room, i.e., the highest possible level that does not produce condensation on the walls and ceiling. Harkening back to the previous article in this series in the January 2021 GFM, careful application of insulation to avoid thermal bridging and subsequent cold spots on walls and ceilings can reduce problems with condensation.

It’s important to bring fresh air into the storage room to remove excess carbon dioxide and any ethylene that’s present. Stored vegetables are alive and respiring, bringing in oxygen and exhausting carbon dioxide while maintaining a baseline metabolism. If oxygen levels get too low, some vegetable tissues may begin to die. In potatoes, for example, this manifests as black heart. That said, mildly low oxygen concentrations—above about 5 percent—don’t seem to harm most storage crops and may even slow their metabolism for longer storage lives. Carbon dioxide buildup may also harm certain crops. Carrots, for example, show increased spoilage when stored with carbon dioxide concentrations higher than 5 percent (50,000 ppm). 

There’s little consensus on precise air-exchange rates for winter vegetable storage, but you should plan for at least several full air exchanges in your storage room each day, exhausting stale air out while bringing fresh air inside. If in doubt, you could purchase a carbon dioxide meter (usually less than $50) and make ventilation decisions based on the readings from your storage room and containers. 

Air exchange may occur naturally through opening and closing the storage room doors, but you may want to plan for additional air exchange to account for times of light use. A simple solution is to install passive or forced ventilation for air exchange with the outdoors. This ventilation method is inexpensive and easy to install but may incur heating and cooling penalties over time. It may cost an extra $20 to $60 per year to heat and cool air deliberately brought in for air exchange. 

Another option is to use a ductless energy-recovery ventilation (ERV) unit. Similar to heat-recovery ventilation (HRV), ERVs allow for heat exchange between outgoing and incoming air to pre-condition incoming fresh air and reduce overall heating and cooling costs. ERVs, unlike HRVs, also conserve humidity. Stand-alone, ductless ERV units can cost several hundred dollars, but they will improve the air quality in your storage room plus reduce heating, cooling, and humidification costs usually caused by air exchange. Your local climate and air-exchange rate will determine how long it takes for this option to pay for itself.

 

Monitoring

If you’re like me, your new storage facility likely will house stored produce during its first winter after construction or modification. Thus, a chunk of your livelihood will depend on conditions within the untested storage room(s) to maintain vegetable quality. Obviously, this can be a nerve-wracking experience. There are tools, though, to give you peace of mind that a power outage in the middle of the night won’t ruin your entire inventory. I highly recommend purchasing a wireless thermometer/hygrometer that allows you to monitor storage-room temperature and humidity remotely. There are models that communicate with smart phones to log past and current data for under $50. 

Make sure there is an alarm system in place that notifies you if the temperature or humidity goes outside an acceptable range. Another safeguard is to have a generator onsite to maintain heat and cooling during power outages. Think of a small gasoline-powered generator as an insurance policy. These safeguards will help ensure your storage facility maintains the vegetable quality you’ve worked so hard to achieve and keep your customers happy through the winter and spring.

 

Sam Knapp has experience farming in Alaska, Sweden, and Wisconsin, and spent the last three years running a farm in Michigan’s Upper Peninsula focused on long-term storage and winter sales. Sam is currently moving to Fairbanks, Alaska, where he plans to start a new farm business centered on winter vegetable storage.