Modeling Greenhouse Efficiency Virtually
Virtual Grower is a high-tech tool that allows growers to examine the performance of their greenhouse structures, coverings and heating systems.
May 25, 2010
When you're comparing structures and coverings, wouldn't it be great to know which styles and materials will perform best for you before you make a change or start to build? USDA's free Virtual Grower software program allows you to do that by analyzing the efficiencies of your current structures and constructing models for new ones - all from the comfort of your office.
The idea began in 2003, when plant physiologist Jonathan Frantz joined USDA's Agriculture Research Service at the University of Toledo in Ohio. His background was in vegetable crops and he started asking local flower growers how they make decisions related to the greenhouse environment - light, temperature and air movement.
With energy efficiency becoming a top concern, Frantz and programmers Bryon Hand and Lee Buckingham set out to develop computer-generated models that would help growers make informed decisions that directly impact energy consumption. The model allows growers to compare styles of structures, the materials they are built from, fuel sources and crop times based on temperature settings and regional climate.
In many ways, this software was developed by growers for growers. "More than 90 percent of what you see in the program has come from grower input and suggestions," Frantz says. "On the costs page, we have the metrics growers prefer, like cost per square foot and cost per month. They wanted maximum BTUs to help them size boilers appropriately. We're also working with engineers who have experience doing audits. We execute the ideas given to us."
In addition to working with the Toledo Area Flower and Vegetable Grower's Association, Maumee Valley Growers and OFA, Frantz has been collaborating with fellow researchers and faculty members at Michigan State University (MSU), North Carolina State University and Purdue University.
In the "Energy-Efficient Annuals" series we published in Greenhouse Grower, March 2009-February 2010, MSU researchers used Virtual Grower to estimate the cost to heat a half-acre greenhouse to produce each crop for different finish dates at different locations in the United States. This data showed sometimes it is more cost effective and energy efficient to grow plants at warmer temperatures and finish sooner, depending on the regional climate.
"One of the biggest users of Virtual Grower is Erik Runkle at Michigan State," Frantz says. "He has really done things I wouldn't think of at all and has probably used the program more than I have. He really pushed what it can do and what to add. He and his students did most of the modeling for different species."
How It Works
To get started, go to VirtualGrower.net, which is part of USDA's Agricultural Research Service website, and download the program. When the program is opened, the design page is displayed.
On Virtual Grower's design page, select your city and state from the drop-down menus. A historical database for weather at that location is loaded, providing typical values of temperature, light and wind speed over the course of 12 months. The weather database from National Renewable Energy Laboratory has information from 230 sites.
The next step is to start building your greenhouse by clicking the "Add New Greenhouse" button. You can then create a simple greenhouse by entering information into the main design page or click on "Advanced Design Options" to create a more detailed greenhouse. Here, you can change the greenhouse's various dimensions, roof shape and materials.
"You can enter in the new material and test it, or test a new heater or curtain that is not already in the program," Frantz says. "Once added, you can select the new item from the menu, run a heating schedule and see if the numbers go up or down."
Virtual Grower can help describe the air exchange rate (leakage or infiltration) for a greenhouse. This allows users to see how small changes in the greenhouse can impact their overall costs. The heating efficiency section allows users to describe overall heat system efficiency based on simple user input. Users can see how changes in the management of a heating system (such as maintenance frequency) can impact overall costs.
The program also allows growers to design complex and realistic heating schedules by clicking on the "Heating Schedules" tab. You can select your greenhouse and click on "Add Schedule" and then customize the schedule to match what you are modeling. Clicking the "Costs" tab will allow you to estimate your fuel costs based on location, fuel, heating schedule and greenhouse design.
The "Plant Growth" section allows growers to grow plants in their virtual greenhouses and see how different heating schedules will impact a plant's development. Plant size and the time it will take to flower are estimated using light, temperature and CO2 concentrations in the greenhouse.
"A grower can do what-if scenarios in a few minutes, instead of changing systems and then finding out over several weeks or even a whole growing cycle, did I save money or lose it?" Frantz says. "Some of the findings have surprised me, such as the difference between glass and corrugated polycarbonate. Polycarbonate is actually worse than glass for heat retention. And others are counterintuitive, that you can save money by heating up the greenhouses, as was found in some of Runkle's studies."
Frantz also has been testing tried-and-true industry standards and rules of thumb. "I tested the rule that for every one degree Fahrenheit you reduce the temperature, you save 3 percent on heat," he says. "For certain areas, it's right on. For others, it's higher or lower. It depends on the structure, too."
A Work In Progress
The current version or generation of Virtual Grower is 2.5, and 3.0 is on the way. "The next version will be substantially different in appearance and we're changing computer languages so Mac users and others can use it," Frantz says. "We're also going to have more graphics, graphs, interactive maps and pictures of greenhouse buildings, so as you're building, you can see how things change in real time. In the future, there will be more eye candy and it will be fun to use."
A Spanish version debuted in February. "This was targeted for the migrant labor force," Frantz says. "Often times, they are in the greenhouses more than the manager and might know the facility better or more accurately describe and put in realistic numbers. It's also a training tool for how a greenhouse works versus spending a whole season making mistakes."
Virtual Grower has been tested and will continue to be tested in a number of commercial greenhouse operations, large and small. One 2.5-acre greenhouse operation in Ohio used it to justify an upgrade to its heating system, increasing efficiency from 60 percent to 80 percent. The operation typically spent $34,000 on propane a year and Virtual Grower and an outside engineer independently estimated the savings to be about $8,000 a year.
Another owner of a half-acre greenhouse in Ohio looked at saving energy by installing inexpensive, temporary side curtains. Most of the facility had single-layered glass walls. Just adding a plastic sheet on the west glass wall to enhance insulation was projected to result in a savings of $300-$400 from $6,000 spent on natural gas.
Another Ohio grower was looking to replace 20 old gravity-vented unit heaters with high-efficiency power-vented models. Upgrading from 60 percent efficiency to 80 percent was projected to save $15,500 off a $60,000 heating bill.
One grower building a new 5-acre gutter-connected facility used the model to compare a biomass heating system with a conventional natural gas-fired system. The model predicted the cost of heating to be about $56,000 per year, compared to the actual operational cost of the biomass heating system over the last two years at $50,000 a year.
In each case study, independent estimates and actual costs were used to verify the Virtual Grower projections. The program proved to be a valuable tool for growers calculating returns on their investments in improvements. Frantz is looking for more grower input and feedback.