This article is reprinted with permission from GM Pro magazine, where it originally appeared in the February 2000 issue on page 48.  For other informative articles on issues that concern the greenhouse industry, go to GM Pro’s web site by clicking on the GM Pro logo above.

By Jeff Woolsey

Integration basically means to make into a whole by bringing all the parts together which is the strength of a computer-controlled system.  >>

Integrated temperature control
Greenhouses have an unusual set of challenges that must be met to control their environmental temperature.  These challenges include rapid, widely varying changes in solar radiation, preheating delay of large hot-water-heating systems, adjusting to unknowns that affect heat loss or gain like shading, and equipment malfunctions and evaporative cooling effects of plant transpiration.

To make matters worse, different greenhouse structures and designs have their own ‘personality’ when it comes to how they react to ambient temperature and light conditions.

Glasshouses collect more light and heat and lose more heat than poly houses under any set of conditions.  >>

Different designs of a house’s heating (i.e. unit heaters vs. hot water) and cooling (i.e. passive vs. active) systems cause them to be more or less aggressive in their effect on temperature.

These factors, and many others, make no two house designs the same in their reaction to ambient conditions.  This disparity is precisely what makes a well-integrated control program so effective when it comes to environmental temperature control.

By monitoring climate conditions, including outdoor temperature and relative humidity, ambient light levels and wind speeds, the computer can make ‘intelligent’ choices, and even more importantly, anticipate future environmental changes.  This leads us to the second advantage of computer-controlled systems: anticipation.

Impact of greenhouse design
Heating and cooling is often an all-or-nothing occurrence in greenhouses.  For example, your cooling system may be running all day, and then as the sun starts to set, the cooling system shuts down.  Within 20 minutes the heating system turns on.  In contrast, under the same ambient conditions your home or office may be able to maintain a fairly stable temperature for several hours after sunset.

The reason for the environmental difference is design.  Greenhouses are designed to provide maximum light transmission as a primary objective.  In your home or office, personal comfort, energy efficiency and appearance are generally more important considerations.  Light is a minor factor compared to a greenhouse.

Because of their designs, greenhouses collect light energy in much greater quantities than conventional buildings.  Along with the visible light that is collected are other forms of radiation, including ultraviolet and infrared.  >>

Infrared radiation causes the heating effect you observe on a bright sunny day, similar to parking your car in the sun with the windows rolled up.  However, when the light is gone, the process is reversed.  The transparent nature of the greenhouse allows much of the heat to escape in the form of infrared radiation.  This is one of the reasons that energy curtains are installed to save heat.  The curtains reflect much of the radiating heat back into the greenhouse so it cannot escape.

Because of the greenhouse’s solar gain/heat loss characteristics, temperature conditions can change very quickly, even more quickly than the heating and cooling systems can react.  For example, if sunlight is present in great quantities and suddenly disappears (as when a cloud passes overhead), the effect on the temperature within the zone can be very rapid, and quite drastic.  Thus the need to anticipate heating and cooling loads within the zone before conditions change.  This is where computer-controlled integration can be very important.

Anticipating heating needs
Consider this typical greenhouse situation: your hot-water-heating system is operating properly when suddenly the outdoor temperature begins to drop as a strong cold front passes through the area.  Since a hot water heating system has a lot of water in it, it takes time to add more heat to the water to enable the system to react to the outdoor temperature change.  This resistance to temperature change, both in the water and greenhouse air is referred to as thermal inertia.  >>

Resistance to temperature change
A hot-water system is not the only one that can benefit from anticipatory control, although the effect is often most pronounced within this type of system.

Greenhouses, simply because of their sheer volume and size, have their own thermal inertia or resistance to temperature change that has to be overcome.  In addition, heating and cooling systems have some inertia (both thermal and mechanical), which is usually less than that of a hot or chilled water system.  >>

Think of this thermal or mechanical inertia as a large boulder that you’re trying to move.  It’s hard to get it rolling, but once you do, you must be careful or it might roll right over you.  This is why anticipatory algorithms were developed.

Anticipatory algorithms
An algorithm is a step-by-step problem-solving formula widely used by computer programmers and engineers.  An anticipatory algorithm integrates all of the environmental and ambient conditions that are known to affect your heating and cooling programs, and makes changes to the system before the boulder rolls you over, so to speak.  >>

How does it work?
If you go back to the example of the hot water heating system and the falling outdoor temperatures, see the chart below to see an example of an anticipatory response to these conditions:

An environmental computer that anticipates greenhouse heating/cooling needs ensures no drastic changes occur to the zone air temperature.

In the graph, that the hot water heating target for this zone is almost a mirror image of the outdoor temperature, although it rises when the outdoor temperature falls, and vice-versa.  This happens instantaneously as the outdoor temperature changes.  Because accompanying changes are being anticipated, there is virtually no change at the zone’s air temperature probe (represented by the black line). 

Without anticipatory heating and cooling, there would be more exaggerated peaks and valleys in the zone’s air temperature, as the system tries to catch-up to the changing ambient conditions.  >>

The graph only shows one of the factors being calculated in the anticipatory algorithm.  At the same time, it is also checking wind speed, which raises the heat loss of a structure, and the ambient light level, which lowers heat load.

For anticipatory cooling, ambient light and outdoor temperature and humidity, which affects the performance of evaporative cooling systems, are generally monitored.

The computer will also make appropriate adjustments even if a vent is leaking, a heat curtain is torn or an exhaust fan loses a belt.  The zone’s temperature will be maintained even in an imperfect world.

Is this what you need?
Do you really need such precise temperature control?  Only you can best answer that question.  With an anticipatory program you can precisely control virtually any greenhouse heating and cooling system with a minimal energy outlay.

Here are some of the benefits of anticipatory heating and cooling:

• Crop quality is improved because plants are exposed to very similar, predictable, temperature conditions throughout the production cycle, and from season-to-season.

• Graphical tracking is more precise because average daily temperatures and DIF (difference in day and night) temperatures are maintained within a tighter margin.  >>

• Improved energy efficiency is achieved by eliminating over-heating and -cooling that is often caused by some control programs’ tendency to over compensate after the system catches up.

• There is less wear and tear on equipment since the program tends to find the ideal stage or target right away, and makes adjustments only as conditions warrant.

• Eliminates lag times on hot-water-heating systems.

• Minimizes thermal shock to boilers by adding heat load more gradually because the program immediately anticipates needed heat as it is happening.

Won’t overcome deficiencies
There is one big caution – an anticipatory program or any other computer program cannot overcome mechanical or engineering deficiencies in your heating and cooling systems.  If your heating or cooling systems are undersized, these programs cannot fix that.  They can only do a better job within the physical capabilities of your system.