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UVM Extension AgEngineering Blog

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

Biomass Heating in Vermont Greenhouses

Posted: May 13th, 2014 by Chris Callahan

Farm to Plate Energy Success Stories

Posted: March 11th, 2014 by Chris Callahan

Vermont’s Comprehensive Energy Plan calls for obtaining 90% of the state’s energy from renewable sources by 2050 and reduce greenhouse gas emissions 50% from a 1990 baseline. What role can Vermont’s food system play in advancing this goal?

The Energy Cross-cutting Team of the Farm to Plate Network has released seven Energy Success Stories that showcase farms, businesses, vendors, installers, and technical assistance providers that have made a difference with energy efficiency savings and renewable energy production.

The stories were prepared by JJ Vandette and staff at Efficiency Vermont, Chris Callahan from UVM Extension, Alex DePillis from the Agency of Agriculture, and Sarah Galbraith and Scott Sawyer at VSJF. Funding for the project was provided by the Northeast Dairy Sustainability Collaborative (Ben & Jerry’s, Cabot Creamery Cooperative, Organic Valley, Stonyfield, Vermont Agency of Agriculture, and the Sustainable Food Lab).

The seven Energy Success Stories are the first in a series of resources that will highlight farms and businesses throughout Vermont’s food system that have made significant progress in saving energy and producing renewable energy.

The first 7 Energy Success Stories can be found on the Atlas at these links:

They can also be downloaded as PDFs here:


DIY Auto-fill Humidifier

Posted: March 4th, 2014 by Chris Callahan

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.


Oilseed Economics Update 2014

Posted: March 4th, 2014 by Chris Callahan

Yesterday we held our annual Oilseed Producer’s Meeting.  At this meeting, I presented an economic overview of oilseeds in Vermont.  Ina nutshell, Vermont has an installed on-farm biodiesel capacity of 600,000 gal/yr (5 sites) with a normalized initial cost of $1/gal of capacity (better than national average). Fuel can be produced for an average cost of $2.13/gal, and meal can be produced at an average cost of $340/ton.  The greenhouse gas emissions associated with this model are 60-100% better than US avg oilseed production (net sink) while the average energy return on energy invested (EROEI) is 4 to 1 (i.e. 4 gallons produced for every gallon used in production.  The model is on-farm production for on-farm use; i.e. cost avoidance.

This study made use of the Vermont Oilseed Cost and Profit Calculator, a tool we have developed over the years to collect all the enterprise costs associated with an on-farm oilseed operation that may turn the crop into meal, oil, and/or biodiesel.  It helps growers and others interested in the topic arrive at specific product costs and compare those costs to market prices.  We also have summarized three different likely oilseed enterprise scenarios in e report titled Vermont On-Farm Oilseed Enterprises: Production Capacity and Breakeven Economics. This work has had strong support from the Vermont Bioenergy Initiative of the Vermont Sustainable Jobs Fund and has been accomplished in close cooperation with the UVM Extension Northwest Crops and Soils Program.

Small Scale Oilseed Press Evaulations

Posted: March 2nd, 2014 by Chris Callahan

We recently completed a series of small scale oilseed press evaluations related to on-farm oilseed processing.  The video below summarizes the different presses and a full report is available for download.

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