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       Chris Callahan
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       Rutland, VT 05701
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UVM Extension AgEngineering Blog

Update on Heating Greenhouses with Biomass

Posted: September 14th, 2015 by Chris Callahan

In the Northeast, early and late season production of food crops using greenhouses requires the addition of heat to maintain temperature and also to control humidity. The heating fuel used is generally propane or other fossil fuels.

Jericho Inside  - Manifold

This hot water distribution manifold allows heating of late and early season crops directly in the ground.

The use of greenhouses, and greenhouse heating, are on the increase in Vermont as growers respond to increased demand for local food throughout the year. Greenhouse production is also on the rise because it allows growers to protect against extreme weather events such as heavy rain or drought, and it affords better control of the growing environment, leading to improved yield and quality. However, using fossil fuels to control the growing environment is costly and these fuels also contribute to greenhouse gas emissions. Vermont greenhouse growers produce $24.5 million in crops using 2.6 million square feet of growing area at an estimated annual heating cost of $1.8 million. Many of these growers are interested in alternatives to fossil fuels for heating in order to improve their profitability and/or reduce their environmental impact.

Testing emissions from biomass boiler at Jericho Settlers Farm in Jericho, VT.

Testing emissions from biomass boiler at Jericho Settlers Farm in Jericho, VT.

This project demonstrated the use of biomass heating for greenhouse vegetable production at sites across Vermont. From 2008 through 2015, 25 growers received cost-share funds for greenhouse biomass heating systems. The total installed cost of these systems was $312,766; the average cost per system was $12,511 and the average cost-share (i.e. sponsor funding) on these projects was 44% of the total cost. The growers installed a variety of system types depending on desired fuel, heating load and method of heat distribution (hot air or hot water). The project started in 2008 and the systems have operated for the equivalent of 96 growing seasons in total with an average of 3.8 growing seasons per system, an average net fuel savings of $2,696 per system per year, and an average payback of 4.8 years (at full cost). From 2008 through 2015 a total of 15.3 trillion BTU of biomass energy was provided to these greenhouses, equivalent to 167,000 gallons of propane. The cumulative equivalent carbon dioxide emissions avoided by this substitution of fuel is estimated to be 2.14 million pounds. This is roughly equivalent to the annual emissions from 204 cars, or 2.3 million miles of car travel.

You can download the final report here.

Dave Marchant (Riverberry Farm, Fairfax, VT) explains some of the features of his biomass heating system during a twilight meeting.

Dave Marchant (Riverberry Farm, Fairfax, VT) explains some of the features of his biomass heating system during a twilight meeting.

Final Report – Increasing Supply and Quality of Local Storage Vegetables

Posted: September 14th, 2015 by Chris Callahan

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.

Doser for Small Scale Vegetable Washing with Sanitizer

Posted: June 16th, 2015 by Chris Callahan

I recently put together a simple doser for manually measuring accurate doses of sanitizer into wash water solutions.  It is really just a homemade burette. The process of mixing a treatment dose of santizer requires metering a specific dose of concentrate into a larger volume of water.  I have also created a calculator to help with that. The UVM Extension Produce Safety Program maintains a great set of resources for general guidance on use of sanitizers including this guide sheet.  It is important to always have a copy of the official product “label” (not necessarily the same thing as the label on the container).  For easy reference, labels for typical sanitizers are linked below. Please check with your supplier to be sure you have the most recent version for the product you are using and the intended application.

There are a number of options available to avoid actually pouring these chemicals when dosing a mix tank.  When pouring them, splashing and spills can occur which are best avoided due to the corrosive and hazardous nature of the chemicals at stored concentrations. Even when using enclosed dispensing options, wear proper personal protective equipment including goggles and resistant gloves in case there are unexpected leaks or spills.

2015-06-14 005

Some of the dispensing options available include:

  • Dosatron – $940-$1000 – Allows for injection of sanitizing chemical directly into the flow stream of water being used in the process.  Measurement is done by adjusting flow ratio similar to a fertigation system.
  • Goat Throat – $299 – GoatThroat 300 Pump with Viton seals. Allows a manual, enclosed pumping with integral valve.   No closed measurement.
  • EnviroSelect Dispensing Pump (BioSafe Safety Value Pack) – $75 – Allows a manual pumping of liquid directly from container without pouring.  No integral valve, and no closed measurement.

When I reviewed these options, I felt there was still a need for something at the lower end of use volume.  Something that would work for 30 to 300 gallon washing batches.  So that is why I put together the assembly that is posted on FarmHack with a parts cost of less than $50 and assembly time of less than 1 hour.  I think it may be helpful. Let me know what you think, and feel free to join in the design discussion on FarmHack.

Farm Building Plans

Posted: June 15th, 2015 by Chris Callahan

I sometimes receive requests for help designing barns, sheds and other structures. It is a bit out of my scope of practice, but there are loads of designs available from the Midwest Plan Service (at Iowa State University) including their free building plans section.  There are also other plans available from the Canadian Plan Service and North Dakota State University.


Calculating Greenhouse and High Tunnel Heat Loss

Posted: April 1st, 2015 by Chris Callahan

I am often asked by growers to help estimate what size heater is needed for a greenhouse or what minimum temperature their high tunnel will reach at a certain outside temperature.  Below are some tools to help you do this yourself.  I have presented them in a range of complexity depending on how much you really want to get into the math.  Enjoy.

1. SIMPLEST – Online greenhouse heat load calculator. http://www.greenhousemegastore.com/greenhouse_btu_calculator This online calculator allows you to enter the dimensions, construction material and temperatures you are interested in and it estimates the heat (and cooling) load.

2. LITTLE MORE COMPLEX – VirtualGrower – http://ars.usda.gov/services/software/download.htm?softwareid=309. This is a free software tool from USDA ARS that is a bit more complicated than the simple form above. But there is benefit to the complication. As with any analysis, the more you put into it, the more you get out of it. VirtualGrower allows easier management of multiple “what-if” scenarios, includes regional weather and light data automatically, and accounts for heating and ventilation systems. You may find it interesting and useful.

3. HEAVY LIFTING, but FULFILLING – Do the calculations yourself! The formulae behind all of the tools above are well described in “Greenhouse Engineering, NRAES-33” by R. A. Aldrich and J. W. Bartok. Available here as a PDF: http://host31.spidergraphics.com/nra/doc/Fair%20Use%20Web%20PDFs/NRAES-33_Web.pdf. See p. 65-71 specifically.

LED Lights – Status, Cost/Benefit and Pro’s and Cons

Posted: February 25th, 2015 by Chris Callahan

I have been receiving several inquiries recently on supplemental lighting for greenhouse production. The most common question is “Should I install LED lights to support growing?”
I have found one report to be the most complete and current on this topic and wanted to share it here.

Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures by Jacob A. Nelson and Bruce Bugbee. Published: June 6, 2014. DOI: 10.1371/journal.pone.0099010. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0099010. Erik Runkle at Michigan State University also summarizes some of this work in Greenhouse Product News here.

There are some industry responses to this including this one from Inda-Grow. And a recent USDA report is somewhat contradictory in its findings here.

There is a also a nice summary by Robert Morrow in Hort Science (HortScience December 2008 vol. 43 no. 7 1947-1950) available here.

Nelson and Bugbee conclude;

The most efficient HPS and LED fixtures have equal efficiencies, but the initial capital cost per photon delivered from LED fixtures is five to ten times higher than HPS fixtures. The high capital cost means that the five-year cost of LED fixtures is more than double that of HPS fixtures. If widely spaced benches are a necessary part of a production system, LED fixtures can provide precision delivery of photons and our data indicate that they can be a more cost effective option for supplemental greenhouse lighting.

Manufacturers are working to improve all types of lighting technologies and the cost per photon will likely continue to decrease as new technologies, reduced prices, and improved reliability become available.

My take-away from all of this; LED’s have a higher initial cost, can have lower recurring costs, can be more effective for specific physiological benefit, and can support certain production layouts.  But the cost/benefit does not seem to pencil out quite yet.

Hops Harvesting and Beyond

Posted: December 7th, 2014 by Chris Callahan

I presented an update on small-scale hops harvesters at the 2014 Northeast Hops Alliance (NEHA) Winter Meeting yesterday.  We also talked a bit about drying and pelletizing at small scale.  The presentation is available here.

A summary of harvesters is provided in the table below.

Hops Harvester Table

One of the really exciting things for me to see is the number of small-scale, mobile harvesters available to people is increasing.  Mendon Precision, LLC (HopsHarvester.com), Wolverine, and LaGasse Works all have produced harvesters that they intend to have available in serial production.These are in addition to the Bine Implement and Steenland harvesters noted previously

Mendon Precision, LLC - HopsHarvester.com harvester.

Mendon Precision, LLC – HopsHarvester.com harvester.

Wolverine Harvester

Wolverine Harvester

This past year also saw more grower builds using the UVM mobile platform design including Aroostook Hops in Westfield, ME.

A harvester based on the UVM Mobile design by Aroostook Hops.

A harvester based on the UVM Mobile design by Aroostook Hops.

Simple DIY Outside Air Exchange

Posted: November 2nd, 2014 by Chris Callahan

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
Inside Air
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).

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.

Processing Mature Green Tomatoes

Posted: July 2nd, 2014 by Chris Callahan

Solar Hot Water Heating in Vermont Greenhouses

Posted: May 14th, 2014 by Chris Callahan

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