Thermostats for Agriculture

I am often asked by growers and processors to recommend a thermostat for a greenhouse, cooler, or postharvest process use.  There are many to choose from and their specifications can be confusing. It is important to remember just what a thermostat does. It is essentially no different from the light switch on the wall with one very significant exception.  Instead of depending on a person to switch it from ON to OFF, we use a temperature measurement.  The accuracy of both the temperature setpoint (what you set) and the actual temperature (what the actual condition is) can be critical for production quality and energy efficiency. Continue reading Thermostats for Agriculture

Finish Surfaces for Produce and Food Areas

This cooler space was finished with Trusscore PVC panels resulting in a smooth, cleanable surface.

Download an updated PDF version of this information here!

Smooth and cleanable surfaces are an important aspect of areas where produce is washed, packed, stored and processed.  Many farms are investing in renovations and expansions of these areas and are seeking materials to meet this “finish surface” need regardless of specific regulation.  Meanwhile, food processing companies are often required to incorporate these materials due to regulation.  This is a summary of some of the finish surface materials that are available, their pros, cons and pricing at this time.

 

Notes:

A properly outfitted cooler results in a clean install both visually, and physically. Note the use of trim pieces to close gaps at corners.
  • These are not necessarily compliant for food contact surfaces; they are meant to be finish materials for areas where food is being washed, packed or stored.  The general guidance is “smooth and cleanable.” Check with the appropriate local and/or state enforcement agency to confirm applicability to your project.
  • The prices listed are material cost only. The products differ in with regard to installation labor.  For example, flexible sheathing like FRP will require some sort of rigid wall material to mount to where as rigid panels such as Trusscore, Extrutech and Utilite can be installed on top of furring strips.  No installation costs have been captured in the prices listed.
  • Links to manufacturer info are included.  Most manufacturers sell via distribution channels.  Check with your local building supply company for availability and current pricing. As with most materials, higher volume purchasing generally results in lower unit costs.
  • The pricing on these materials is quite variable depending on the source, when you obtain a quote, the quantity being ordered and how it is delivered. The listed price is the best information available at the time of writing.  Shop around and obtain quotes from several distributors.
    Several manufacturer’s use panel locking mechanisms such as the tongue and groove system found in Trusscore. This provides for a smooth finish and hides the fasteners.

    Common shapes of available trim options to cover and seal all edges and seams. This keeps water from seeping behind the finish surface and entering the walls which can lead to molds and mildews and structural damage.
  • Most manufacturer webpages include an easy to find, specific, installation guide for their product that will be helpful in guiding installation.
  • FRP panels use H or J channel trim between pieces and corners which are calked in place to ensure a moisture proof seam. Follow the manufactures installation procedures.

    Ribcore 3’ or 9’ rib pattern options for ceilings

 

New Crop Storage Planning Tool

DSCN1606I have been toying with an Excel-based crop storage planning tool for several years.  I finally have it at point where I want to make it available to others and start collecting feedback for improvement.  You can download the tool here, and instructions are available in the tool and at this page.  Enjoy and please be in touch with feedback.

 

Final Report – Increasing Supply and Quality of Local Storage Vegetables

We recently completed a project aimed at improving the ability of Vermont vegetable farms to store crops such as beets, carrots, parsnips, potatoes, onions, squash and sweet potatoes, all of which have unmet demand in late winter when local supplies run out.

Beets can be stored in bulk bins for months at the right conditions.
Beets can be stored in bulk bins for months at the right conditions.

The physiology of these crops allows them to be stored for many months after harvest if specific storage conditions are met. However, several distinct sets of conditions are optimal for different groups of crops, and achieving each condition requires careful control and monitoring of temperature and relative humidity in storage. Currently, Vermont’s commercial vegetable farms rarely achieve the optimal conditions due to lack of sufficiently separated storage compartments, and lack of modern environmental monitoring and control equipment.

Installing a remote monitoring system to keep track of temperature and humidity of a storage facility.
Installing a remote monitoring system to keep track of temperature and humidity of a storage facility.

This project installed environmental monitoring equipment to improve storage conditions and ultimately the quality of 1,736 tons of winter storage crops at 9 farms throughout Vermont .  The cumulative market value of these storage crops produced during the 2012-2014 growing seasons was $3.5 million.   Improved storage monitoring led to better control of storage conditions, in part through automated notification to farmers when abnormal conditions were occurring. This allowed for prompt correction of problems such as open doors and failing or inoperative cooling equipment. Losses of storage crops (cull rates) were reduced from ~15% to ~5% of stored volume. Sixty-six  energy efficiency measures were also implemented at 5 of these farms, saving a total of 40,269 kWh of electricity and $5,800 annually.  The systems deployed have increased the confidence of growers to expand their winter storage of Vermont-grown vegetables, leading to an increased supply of local produce outside of the traditional growing and marketing season.

You can download the complete report here.

Simple DIY Outside Air Exchange

In colder climates winter storage crops have been stored in passive root cellars for centuries, striking the balance between outside conditions and storage conditions. Although modern refrigeration systems are generally used for large scale, longer-term storage of these crops some enterprises seek lower cost and high energy efficiency options. Outside air exchange systems use an exchange fan to draw colder outside air into a storage room to maintain a depressed temperature. The control of this requires monitoring outside temperature and inside temperature and only allowing air exchange when the outside air temperature is low enough to cool the room, but also only when the inside room requires cooling.

This control can be accomplished with two thermostats wired in series; one setup for heating (outside/colder air) and one setup for cooling (inside/room/warmer air). All thermostats are essentially a switch whose state (on or off) is controlled by a temperature sensor and a setpoint. They are either purchased or configurable for heating (turn on the load at or below a setpoint temperature) or cooling (turn on the load at or above a certain load). In our case, we are using a heating thermostat “cascaded” to a cooling thermostat so that our “load” (fan) only comes on at or below a certain oustide temperature and at or above a certain inside room temperature.

Outside Air Exchange Overview Pic
An overall view of a mocked up air exchange system. This article doesn’t address routing or the air ducting. When using a bathroom exhaust fan, the inlet to the fan is the grill shown and would generally be mounted high to exhaust the warmer air. The outlet should be ducted outside. A separate inlet duct should be routed from outside to supply make-up (exchange / cold) air.

In this system (a mockup used for cold storage workshop instruction) I used the following items:
QTY 1 – Standard Light Switch – $0.69
QTY 1 – Switch Box – $0.91
QTY 2 – Johnson Control A419 Thermostats w/ 6.5 ft probe (one setup for cooling, one setup for heating) – $58.95 each ($117.90 total)
QTY 1 – 70 CFM Bathroom exhaust fan – $29.97
Misc 3 and 4 conductor 14 AWG solid wire.

Total parts: $149.47.
Total time about 2 hours for first build.

 

Outside Air Exchange Schematic and Wiring Diagram
Schematic and wiring diagram for the system shown.

A note on thermostats. I like a digital thermostat with a remote probe and I tend to use the Johnson Control A419. It has a 1 degF setpoint resolution and down to 1 degF differential. Each one can be setup for heating or cooling by making the proper adjustment of dip switches inside the box (see p.7 of the manual). Ranco make a similar thermostat in their ETC line. I’m still on the lookout for a good, inexpensive delta thermostat (a thermostat that controls a load based on a true delta-T, or temperature difference, between two locations.) Most are designed for solar hot water systems and don’t seem to allow for control at lower temperatures. And they are fairly expensive.

I ran mainly 3 conductor wire in this setup, but did find the 4 conductor wire to be a clean way of getting an additional wire from the first thermostat to the second. This allows for both thermostats to be powered regardless of the output state of the first.  This lets the user see both measured temperatures and make setpoint adjustments since both thermostats are always powered whenever the main power switch is on.

Outside Air Exchange Wiring Pic
Detail showing the wiring at each thermostat. Thermostat probe wiring and jumper setting not shown, refer to manual for your thermostat.

The basic settings for each thermostat in this setup were:

Outside Air
Thermostat
Inside Air
Thermostat
Jumper 1 JUMPED (Heating) OPEN (Cooling)
Jumper 2 OPEN (Cut In at SP) OPEN (Cut in at SP)
SP (Set Point)* 35 degF 40 degF
Diff (Differential) 1 degF 1 degF
ASD (Anti Short Cyle Delay)
1 min 1 min
OFS (Offset for BIN) 0 – Not used 0 – Not used
SF (Sensor Failure Operation) 0 (De-energize) 0 (De-energize)

* – Note the outside air thermostat should always have a lower setpoint (SP) compared to the inside air (room) setpoint.  This is what ensures the fan only comes on when the outside air will actually provide cooling.

A note on sizing the fan. Moving air works out to about 1 BTU/hr per degF per CFM (or ft3/min).

Why?
Cp_air = 0.24 BTU/lb/F
Rho_air = 0.07 lb/ft3…

Q_dot {BTU/hr} = V_dot {ft3/min} x Rho_air {lb/ft3} x Cp_air {BTU/lb/F} x (Tin – Tout) {F} x 60 {min/hr}

which all works out to

Q {BTU/hr} = 1.008 {BTU/hr/CFM/F} x V_dot {ft3/min} x (Tin – Tout) {F}.

So with a 75 CFM fan and a 10 degF temperature difference, this system can cool at a rate of 750 BTU/hr. Large fans or multiple stages are even possible. Watch the amperage on the load compared to the thermostats, though. You may need an intermediate relay to handle the larger current in bigger systems.

Be careful of drying out whatever you’re cooling. When you bring air in from outside, especially when cold out, it will generally be more dry (lower relative humidity). So you’ll want to keep track of humidity and may need to add some to the air.

Cross posted on FarmHack.net, join the discussion. Download a fact sheet on this tool.

DIY Auto-fill Humidifier

I recently built a humidifier I think others might find useful.  This could be useful for cheese aging, meat curing, and storage of winter crops all of which require maintaining specific temperature and humidity. The details are available on the Tool Wiki at FarmHack.  Take a look, and let me know if you find ways to improve it or if you have any questions.  As part of this project I also developed this handy spreadsheet that calculates relative humidity based on dry-bulb and wet-bulb temperature and/or calculates wet-bulb temperature based on dry-bulb temperature and relative humidity.  The spreadsheet also helps you tailor the humidifier design to your needs by estimating humidification capacity in gallons of water evaporated per 24 hours.

The idea was to turn a 5 gallon bucket into a high capacity (4 gal/day), automatic fill humidifier. The bucket serves as a reservoir for the water and also as a mounting platform for the parts required to operate the humidifier. Water heated to a know temperature will transfer a predictable amount of water vapor to an air stream of a known temperature and humidity (wet bulb temperature). We use this property to develop a highly controlled humidifier using a temperature control to sense water temperature and control the heater, tank deicer for heat, and a CPU fan for air flow. We also add a toilet fill valve to the assembly to allow for automatic fill of the humidifier.

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Food Storage Workshops

veg in storage 3UVM Extension with support from the USDA’s Northeast Sustainable Agriculture Research and Education program will offer five one-day workshops on crop storage throughout Vermont this fall. Workshops will focus on long-term storage of crops for sale through the winter and into early spring, but will be relevant to many agricultural and food storage needs.

Topics include:

  • the growing importance of long-term crop storage
  • principles of energy and heat transfer
  • basic heating and refrigeration
  • construction for utility and efficiency
  • maintaining temperature, airflow and humidity
  • biological processes of crops in storage
  • storage characteristics of various crops, and
  • sizing and design of storage systems.

Workshop schedule:

  • Brattleboro – 9/17
  • Rutland – 9/19
  • White River Junction – 10/9
  • St. Johnsbury – 10/10
  • Shelburne – 10/16

Workshop fee is $20 and registration is online. Visit workshop webpage for more information.

CoolBots(TM): Inexpensive Cold Storage

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.

In a nutshell:

A farmer-built cooler (photo from storeitcold.com).

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 CoolBot installation (photo from storeitcold.com)

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.

FIDO – A Do-it-yourself Temperature Monitor with Text Message Alerts

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.