Calculating Energy Savings

There’s a new energy in town and it comes from saving energy. And not just saving, but knowing exactly how much you’re saving, where and what pays off when.

Greenhouse businesses have long been focused on reducing fuel and other energy costs. Far before sustainability became the moment’s hottest buzzword, growers knew energy was their highest cost besides labor. Saving energy would benefit not only their businesses, but their customers and, eventually, consumers.

Knowing how and where to start saving is quite another story. And knowing whether investments like energy curtains make sense in your specific greenhouse – not a generic average operation – is difficult to determine. But with a quick and simple payback calculator, growers can assess how quickly investments like energy curtains pay off.

Getting Started

For many growers, knowing where to start saving is as difficult as actually making the changes to save. So what do you look at first? Think of a pyramid. Start at the bottom and work your way to the top to determine the best way to manage and save energy at your operation. At the bottom of the pyramid are items that require little investment but generate quick payback. As you work your way to the top, time and investment increase and payback stretches out.

The first step is a simple Energy Audit. For a very small fee, an engineer will analyze an operation’s energy use, then provide a detailed report outlining specific ways to save and conserve. Some recommendations are as simple as tightening a loose vent. Others are more involved, like upgrading fans. Either way, you’ll have a blueprint for exactly how and where energy is being used at your greenhouse.

Next, modify your behavior for energy conservation. This is where change really starts to impact. For example, consolidate or co-locate early crops with warmer temperature needs to avoid heating or overheating more space than is necessary. Grow the same plants in a different way. It may cost a little more upfront, but the payback will far exceed it.
Then, really focus on increasing your energy efficiency. Grow at the same temperatures in the greenhouse with more efficiency, such as upgrading to more efficient motors or lamps, setting computers to control temperatures at set targets and installing energy curtains.

When you’ve got the equipment online for energy savings, take a look at time of use management. Can you adjust by taking advantage of energy savings opportunities? For example, many power companies offer vast savings for off-peak versus peak-time electrical power. Or, warm hot water off peak and store in insulated tanks to use at night. Take advantage of variable rates and reap the rewards.

One of the last steps will be examining major infrastructure changes, for several reasons. Changes like these – incorporating renewable energy like wind, solar or biofuels – take a major investment, so you’ll want to have all of your other energy conservation efforts in place before you move on them. Know exactly what your energy use is so you don’t invest too heavily or overcompensate.

Save Heat, Save Energy

Heat escapes from a greenhouse in every corner and any way it can. How fast it escapes depends on a host of different factors, including temperature fluctuation, wind speed, greenhouse sealing, radiation and conduction properties, cloud cover and greenhouse construction.

That’s why it’s essential you identify and block every possible opportunity for heat loss. For example, creating a greenhouse attic with an energy curtain is a good heat loss barrier but makes a well-sealed perimeter critical for energy efficiency.
Screens work by doing just that, slowing the heat loss rate by blocking outgoing radiation and trapping the insulating cold air layer above the screen in the attic. By separating the warm, moist air of the growing space from the cool, dry air in the roof space, you minimize heat loss and maximize energy efficiency. But exactly how much do curtains save?

Extensive testing in two greenhouses with artificial crops reveals these energy savings numbers. One has a horizontal screen; one does not. We count heat loss only through the roof and maintain identical temperature control to within +/- 1°C. We select a percentage relevant for all glazing types and test for all heat transfer, using computer-generated mathematical models for validation.

Then we look at approximate installation costs. The base cost for flat, sliding, cable-driven Firebreak curtains installed on a 1-acre range is $1.25-1.50 per square foot. Prices increase 12 to 14 percent for slope-flat-slope, 10 to 12 percent for suspended, 10 cents per square foot for push-pull, and 20 cents per square foot for blackout systems. Smaller, multiple zone, occupied or busy greenhouses cost more. Replacement costs run about two-thirds the cost of the original.

Knowing that information, we can calculate the typical energy savings with different types of screens:

• An all-clear curtain, such as the XLS10 Firebreak with no aluminum, functions just like glass or poly, allowing a great deal of light in (85 percent direct light transmission), and provides 47 percent energy savings.

• A screen made with 50 percent aluminum, such as the XLS15 Firebreak, increases the energy savings to 57 percent by blocking more light (46 percent direct transmission).

• Using a curtain with open space between the fibers, like the XLS15 F Firebreak, allows 50 percent direct transmission and provides 20 percent energy savings.

• And putting in a two-layer blackout curtain, such as the XLS Obscura A & B, offers a whopping 75 percent energy savings. The combination is at the same time reflective to prevent overheating and nearly opaque with less than 0.1 percent direct transmission to intercept any light that leaks through.

The bottom line is energy savings pay back; you just need to figure out which options, from energy curtains to renewable energy, make the most sense for your business.

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