Demand for on-farm cold storage of produce and other Vermont agricultural products is increasing as local markets for these goods expand. I receive many inquiries regarding CoolBotsTM, an adaptation of a window air-conditioner to make a cooler out of an insulated space. This article is intended to collect related resources in one place and to also highlight some considerations adopters of CoolBots should be aware of.
Plan for maintenance (cleaning the coil, off-season storage, protection from elements, etc.)
These systems utilize a commercially available controller ($299) to allow the AC unit to run with a lower temperature than normal. Store-It-Cold, The manufacturer’s website has excellent resources and FAQ’s. They include a list of AC units that they have had positive experiences using. They are also very clear about who should consider NOT using a CoolBot. Applications for which the CoolBot is not well suited, according to the manufacture, include;
rapid “pull down” of temperature (e.g. high levels of field heat or frequent exchanges of product)
freezers – CoolBots perform best above 36 °F.
sites with many door openings per day (e.g. > 6 times per hour)
running through the winter – not a show stopper, but you need to be more careful about which AC unit you choose
Other things to be very aware of, according to the CoolBot controller manufacturer, include
A report commissioned by NYSERDA summarizes the cost, energy efficiency, and greenhouse gas emission benefits of a CoolBot installation when compared to a conventional walk-in cooler system at certain conditions. The cost estimate of the CoolBot system (15,000 BTU/hr) is $750 installed compared to $4,400 for a conventional system (8’x10′ cooler box cost not included).
The authors conclude that a CoolBot system can result in approximately 230 kWhr/year of energy savings ($30/year at $0.13/kWhr VT average) when cooling 100 ft2 of cooler floor area to 35 °F (assumes Albany, NY conditions). It is important to note that this analysis highlights the main energy efficiency benefit of the CoolBot system comes from the reduced operating time of evaporator fans. High efficiency fans and improved controls exist for conventional walk-in systems and they are even supported by rebates from Efficiency Vermont. When the CoolBot system was compared with a conventional cooler that also had evaporator fan controls, the savings went the other way; i.e. the conventional walk-in system resulted in 74 kWhr/year savings.
A recent report from the USDA ERS sums it up this way, “use of energy along the food chain for food purchases by or for U.S. households increased between 1997 and 2002 at more than six times the rate of increase in totaldomestic energy use. … The use of more energy-intensive technologies throughout the U.S. food system accounted for half of this increase, with the remainder attributed to population growth and higher real (inflation-adjusted) per capita food expenditures.”
Some of the loss occurs in storage, and I think we all can agree that we can do better with our storage practices. Regardless of whether you are root cellaring, using a CoolBot(TM) or a commercial walk-in cooler, the principles remain the same. Some loss occurs in transport and distribution which speaks to the benefit of the broader food system considerations espoused by UVM’s Food Systems Spire and the Vermont Farm to Plate Initiative. Some, of course, occurs in the kitchen or in consumer storage and suggests we have some work to do with consumers as well.
As one grower recently said to me, “By the time we put food in our farm cooler, 99% of our cost is sunk into that product. We gotta pay attention to what goes on in there and make sure we get paid for it.“
As I’ve mentioned in several other posts, I think the continual monitoring of conditions in greenhouses and food storage spaces is incredibly important for quality and safety and insightful for any operation. There is a really clever design for a do-it-yourself temperature monitoring system called Fido, on the FarmHack site. It uses an Arduino control and electronics platform, a cheap cell phone, and a few other pretty inexpensive pieces to do the job.
“A farmer-built electronic tool that can monitor greenhouse temperature, record greenhouse data, and alert the farmer to problems in the greenhouse via cell phone text message. This tool will be much more affordable and useful than commercially available greenhouse alarms (which rely on landline connections or internet connections, which usually aren’t available in the greenhouse).“
I’ll be trying to add RH monitoring to this soon, and will update the post when that is complete.
The presentation slides are available at the VVBGA’s website. I’m still very grateful for the responses to the food storage survey, and we discussed these at the meeting. I also highlighted 5 things I think are critical considerations for VT growers storing vegetables and berries.
Zoned Storage – While many are zoning (or grouping) their stored products based on optimal temperature and relative humidity (RH), it is also important to consider a zone for pre-cooling product as it comes into storage. The sudden addition of product with field heat and elevated respiration can contribute significantly to the cooling load in the room and could lead to slightly warming other crops already in storage. Additionally, we talked about the need to consider ethylene production of crops and also their sensitivity to it; sometimes requiring outside air exchange to remove the ethylene. Most are familiar with ethylene production from apples, but even common vegetable crops also produce some. Storage conditions for main crops as well as respiration rates and ethylene emission rates can all be found in USDA Handbook 66.
Measurement and Monitoring – It is understandable that one should expect a cooler to be at the temperature you set on the thermostat. But I’m a believer in secondary, accurate measurement to confirm storage conditions. This means both temperature and RH. I urge growers to check it regularly (daily), and to keep track in some sort of log so that trends are captured. This can take the form of an advanced remote data monitoring system, but it can also take the form of a simple clipboard or notebook. The important thing is that the conditions are actually measured with an accurate device such as a certified and calibrated thermohygrometer or sling psychrometer and be recorded. Here’s a video showing how to use a sling psychrometer (equally useful in a greenhouse or cooler, although I recommend “slinging” for 1 minute or more, taking 3 readings to check for stability, and using a psychrometric calculator to determine RH as the slide calculator on the device is not terribly accurate.)
Scouting – Despite all the best intentions; zoning your storage and confirming the conditions, sometimes you still run into problems. There are varietal differences in storage and many other factors that will influence how the crops keep in storage. So it is important to “scout” the storage as well. This can be daunting with bins and boxes piled high, but catching a problem early could help prevent a major loss. It is possible, as well, that you have to deviate from the published references for storage conditions for a certain crop. The verification of the storage conditions is the measurement step above, but the validation is the crop quality. The proof is always in the pudding.
Cooler Audits – It is hard to make time to stop and smell the roses, and it is hard to take time to stop and audit your cooler. But there are things you can do on a routine basis that take little to no additional time.
Check Door Seals – Walk inside the cooler, shut off the lights and look around the door for daylight. If you find spots with light shining through look more closely at the seal in that area, it may need repair or replacement. Look also for frost (on freezers) or condensation (on coolers) which can also be signs of air leakage.
Door Closure Tightness – Even if your seals are in good condition, the door must shut snugly to have them work. Most commercial cooler doors have adjustable latches. Make sure there is no play in the latch when the door is closed, and adjust as needed so it closes tightly.
Mold, Condensation – Keep an eye out for mold and/or water condensation, this may point to air circulation issues or dead spots of air flow that need to be addressed.
Noise – Noise is energy, and if you get to know the typical “hum” of your compressor and fans, you’ll be able to tell when something is amiss. New noises or more frequent operation of the compressor can signal a significant change in the refrigeration system (a higher than normal load, or heavier work than normal.) Keep an ear out for new noises and do a complete walk around on a regular basis to catch maintenance issues early.
Coil Cleaning – The air coils are the lungs of the system, and they need to be clear of debris. Regular coil cleaning should be added to any preventative maintenance or seasonal job list. If your system can’t reject heat (either inside the box or outside the box), you’re not cooling as effectively as you could. This definitely means reduced efficiency and increased energy use, but it could also mean reduced storage efficacy and premature spoilage.
Mechanical Maintenance – A trained mechanical contractor should inspect your system on a regular basis (yearly prior to your main storage season). This will help minimize the chances of system failures and (worse) crop loss.
Technical Resources – There are several excellent resources available on crop storage. The New England Vegetable Guide is an excellent overall crop guide that includes basic storage information. To dive a bit deeper, look at the USDA Handbook 66, note that the online edition has increased detail than the last print edition. I also recommend the UC Davis Postharvest Technology site which has a wide array of searchable resources, many of which are crop specific. If you get real deeply involved in environmental control (temperature and humidity), you might want to learn more about psychrometric charts and calculators. These allow you to very accurately understand the relationship of water vapor and air and are especially useful when used with a sling psychrometer.
As an engineer, I love data. It turns out farmers do also. At least, Pete Johnson and Isaac Jacobs at Pete’s Greens in Craftsbury, VT do. “Is it working yet?” Isaac asks as I put the finishing touches on the remote data monitoring system we have been installing in the four zone drive-in cooler. “Just about… I think.” I say with trepidation. Isaac has been up and down in a scissor lift several times at placing and removing a sensor that was being difficult. And I’ve been wrestling with a data station to make it communicate over the wireless network so that we can actually see the data being collected by the new remote monitoring system.
This is the first time we’ve installed this kind of system which consists of a “base station” and “remote sensors”. The remote sensors in this case measure temperature and relative humidity (RH), both parameters which can drastically influence the storage life, quality and food safety of produce. In fact, the motivation for multiple storage “zones” in the cooler is to provide each group of vegetables their desired set of conditions; e.g. potatoes at 38 °F and 90-95% RH, onions at 32 °F and 65-70% RH, cabbage at 32 °F and 99% RH and squash at 55 °F and 70% RH.
“What are those spikes in temperature?” Pete asks as we glance over the first set of data that pops onto the screen. There are spikes in temperature every 8 hours in the potato room that last about one hour. Not big spikes (2 °F above the nominal), but they stand out. And perhaps as important, relative humidity drops by 2% when the temperature goes up.
Later, when Isaac and I are looking at the electrical wiring to see where we can plug in additional sensors we note that evaporator defrost system in that room is on an 8 hour timer; that’s the source of the heat. “Well, they don’t even need to be on right now, that room isn’t even being cooled.” The circuit breakers for the evaporator heaters (intended to defrost the evaporator when it freezes up) are shut off, reducing the farm’s electric bill slightly over the next few months. And this is only two hours after we started getting data.
The power of data is three-fold. First it is inspired by inquisition. It then raises additional questions and with further review, it should answer questions and improve life. The principle behind this project is to allow for easier access to process data for Vermont’s growers and to demonstrate this type of system. As farms push the seasonal envelope in response to increased local demand, year-round production and long term storage of fruits and vegetables will be increasingly important. I plan to use this system at multiple locations in Vermont to collect data on refrigerated storage, greenhouse and high tunnel production, and whatever else comes along that is interesting and makes sense.
One question that seems to come up regularly is “how big a cooler do I need?” I’d recommend grower’s review Table 7 in Heatcraft’s Engineering Manual which includes typical “product loading densities” in pounds per cubic foot. Based on your typical yields and acres in production you can use this to estimate a reasonably sized storage space.
“The three main objectives of applying postharvest technology to harvested fruits and vegetables are:
to maintain quality (appearance, texture, flavor and nutritive value)
to protect food safety, and
to reduce losses between harvest and consumption.
Effective management during the postharvest period, rather than the level of sophistication of any given technology, is the key in reaching the desired objectives. While large scale operations may benefit from investing in costly handling machinery and high-tech postharvest treatments, often these options are not practical for small-scale handlers. Instead, simple, low cost technologies often can be more appropriate for small volume, limited resource commercial operations, farmers involved in direct marketing, as well as for suppliers to exporters in developing countries.”
“VPR looks at the economic impact of food production in Vermont in the special series The Business of Food.
All this week during Morning Edition, we’re looking at the expansion of farmer’s markets from a summer calendar schedule to a year-round operation, how community planners in southeastern Vermont say it’s time to go beyond co-ops and plan for a “post-oil” world, how “farm to plate”initiatives have resulted in good social policy and how downtown planners are using food co-ops as a lure to enliven central business districts.
Listen this week at 7:50 a.m. during Morning Edition.”
Thanks to all of those who took the time to respond to the food storage survey several weeks back. The information you provided has been really helpful. A clear desire for additional programming in this area was expressed, and I’ll be working to develop that.
Here are the survey result highlights:
82% of respondents expressed interest in a formalized course on food storage topics with the overwhelming majority preferring classroom / workshop format and/or a webinar.
66% of respondents have plans to expand your food storage capacity in the next 24 months.
While most respondents expressed concern about the utility costs of their storage systems, relatively few (37%) currently know those costs.
60% are storing in multiple zones (Temp and RH), but the overall knowledge of optimal storage conditions is relatively low (see related post on USDA Handbook 66).
Winter markets included (% reporting sales to each)
Farmer’s Markets (41%)
Winter CSA (38%)
The most common systems in use are (% reporting use):
Walkin cooler (57%)
Chest freezers (55%)
Heated Winter Storage (39%)
Root Cellar / Other Cool Storage (not refrig’d) (31%)