Thursday, July 23, 2009

I Can't Afford Shade Curtains in my Greenhouse

Greenhouse fuel costs for heating are competing with labor costs for the number one expense of operating a greenhouse enterprise. Natural gas price increases are stimulating greenhouse owners to consider alternative fuel sources or conservation tactics. Many often think of alternative fuels will cut their expenses. Natural gas is clean, easy and available, propane requires on-site storage, wood and wood products require transport, storage and handling, and solar just isn't quite there yet.

So energy conservation may be the answer and it may be easier than you think.

The very first greenhouse that I worked in was at the University of Nebraska in Lincoln back in the late 1970s. It was a brand new IBG fiberglass reinforced plastic greenhouse with pad and fan cooling. It was beautiful. Yet, it needed to be shaded in the summer. So every spring, we attached shade cloth to the outside and removed it every fall. It was a pain, but it did keep the greenhouse cool.

So now let's jump to the 21st century and talk about retractable shade curtains. Retractable shade curtains in the greenhouse are not new, not innovative, and are really pretty common. But, do YOU have them in YOUR greenhouse? I'll bet not and you probably think that they are too expensive to install.

Yes, they can be a pain to install in an existing greenhouse as you will have to re-think all that stuff you have in the gable, but they are easy to install during new construction.

So why install retractable shade curtains?

Retractable shade curtains do effectively keep the summer temperatures down in your greenhouse. This extends your seasons very well (unless you are one of those growers who just takes the summer off for fishing). Retractable shade curtains also are very effective in cutting your fuel costs. Yet, why do not more growers use them? Are they really too expensive?

Up to 85% of the heat loss from a greenhouse occurs during the night. Thermal blankets can be a cost efficient investment. Retractable shade curtain manufacturers claim a return on investment at between 2 to 3 years. Installation makes sense to me.

About 10 years ago, at Colorado State University, we built some new greenhouse facilities to which we included retractable shade curtains. Each gutter connected greenhouse has a single layer polycarbonate roof with triple layer polycarbonate side and end walls. The heating system uses hot water circulated through the floor and through unit heaters. The environment is controlled by a Wadsworth EnviroSTEP controller. They are nice greenhouses.

So to determine what the actual savings in a greenhouse with retractable shade curtains, one of my graduate students, Drew Miller, began a project to evaluate heating efficiency during cold winter evenings. He used two identical interior greenhouse sections, to which he disabled the floor heating system. The hot water unit heaters were the only source of heat. Both greenhouses had retractable shade curtains.

To determine the energy demand in each greenhouse, we used the switch logs from the environmental control system, which recorded the on time of the unit heaters. The shade curtains on one house were closed at sunset and the other left open. This study was conducted over several evenings and the houses were alternated to disallow for any random differences between the environments.

At the conclusion of the study, we struggled with what to do with the data. We finally derived an evaluation to compare the two conditions, curtains closed and curtains open. We determined what is known as heating degree day hours, which we derived from heating degree days.

Heating degree days in a season are derived by summing the difference between the average outdoor temperatures above a base (e.g., 65°F) each 24 hours and the base temperature. Heating degree hours (equal to heating degree days x 24) are used in computing seasonal energy flows in a building due to both conduction and convection.

We then plotted the cummulative heater run time in hours against the cummulative heating degree hours for the individual study evenings.

When the shade curtains were left open, heating began at less than 25 heating degree hours.

And when the shade curtains were closed, heating began at around 285 heating degree hours.

So what is the savings? At 436 heating degree hours, the greenhouse with the shad curtains open required 2.69 hours of heater time and the greenhouse with shade curtains closed required 0.295 hours of heater time. This resulted in a savings of 2.39 hours of heater time.

The unit heaters in each greenhouse are designed to ouput 250,000 Btu/hr. With this assumption, the greenhouse with open curtains required 672,500 Btus of fuel per evening and the greenhouse with closed shade curtains required 73,750 Btus of fuel.

With natural gas fuel prices projected to be at $11.63 per dekatherm, this will calculate to be a cost savings of $6.96 each evening. And this was for a greenhouse section measuring only 240 square feet.

You can't afford shade curtains? You can not afford to not install shade curtains.