Although it isn't desirable to make a greenhouse completely air-tight, there is still much you can do to help decrease your energy costs.
Probably the most effective technique would be to insulate the north wall of your greenhouse. Since little light will come from that quarter, you can insulate by using a 1 inch board of foil-covered polyurathane foam. This has the insulating qualities of 3 1/2 times the double inflated polythene walls which no doubt you already have. By themselves the double layers of plastic can save you over 40% heat loss as compared to a single layer.
With some crops you can reduce the temperature inside your growing area. A five per cent temperature decrease gives anywhere from 20-35% savings in heat required. But you can't do this unless you maintain good practice in plant cultivation. Now's the time also to check your thermostats. Those which don't work properly can rob your heating/cooling apparatus by decreasing the overall efficiency. Thermostats should be accurately calibrated at least once a year. And they should be placed where they can do the most good.
A thermostat should not be in line with air flow from fans or heaters. They should also be covered to keep the sun from having any effect. The best location for a thermostat is usually at plant level in the center of the greenhouse. A thermostat should not be attached to a wall. Finally, if you're using an aspirate thermostat (one which pulls air over the sensor), you should place it where the air will be pulled over the sensor and not blown upon it.
Horizontal air flow is also important. Especially after the ventilating fans have been turned off. In order to prevent air stratification, it's smart to place fans at various locations around the greenhouse. They should all blow together and form a circular air movement around the greenhouse interior. The fan velocity should be at least 40 feet per minute. You can check how the air is moving by placing strips of paper on 2 foot stakes at certain intervals. When all the strips are fluttering in the breeze, you know you have the proper air movement.
For a 30 x 100 ft house place two fans (each capable of moving 700 cu ft/minute), one on each side of the house about midway and next to a wall. Each fan should be blowing in the opposite direction from its companion. This air circulation will mix the air inside the house and cut down the incidence of disease. Ventilation fans should be attached to the leeward side of the house. In this way they can get help from the prevailing winds. You must place them where they won't interfere with nearby structures, particularly other greenhouses.
Weatherstripping around doors can be very important. A 1/2 inch crack can cost over $100 a year! A good guide is this: to replace any lost carbon dioxide, a one inch square opening for every 2000 BTUs your inside furnace puts out will be sufficient. Again, make certain all harmful fumes are properly exhausted to the outside. One more thing: you might consider other kinds of fuel. Wood is supposed to be the cheapest and most cost-effective. But a lot of growers say it's too much mess and too much work. If you're using wood to fire a hot water heating system, you could get a full payback on the system within two years.
Blanketing: How to save energy costs in a very simple way:
The environmental factors necessary for plant growth are: the air temperature, its humidity,it velocity, its gaseous percentages. In addition to these we have to consider the media: its temperature, its humidity, its gas percentages, its chemical balances. Finally, both visible and invisible radiation must be considered.
Blankets or "energy curtains" are a method for controlling radiation and energy gain or loss. Although white washes can be used on the outside roof of the greenhouse, it is better to use fabric shading inside the greenhouse in order to control the amount of shading. By using light meters you won't make the mistake of too much or too little shading. You must select you shading fabric with care as different fabrics respond with different results. The amount of radiation from the sun which reaches the plant will depend not only on the greenhouse cover but also on the type of shading you use. A proper fabric blanket can reduce heat needs at night by at least 50%.
You use shading during daylight to cut down on excess sunshine, depending on the crop you're growing. You also use it at night. especially during the winter months, to cut down on heat loss. In either case, your blanket will be a thin fabric which can be pulled out over polypropolyne monofilament line. You can pull either from side to side or from one end of the greenhouse to the other. The fabric must be the kind which will fold easily upon itself when not in use. It can then be packed against the eave on one side or at the north end if used lengthwise in the greenhouse. Your apparatus for this should be a system where the fabric can be pulled into a horizontal position above the crop - usually at the height of the eave (or gutter) of the house. You can either move the blanket by hand or have it automated.
What type of fabric should you use?
That depends upon the crop. Generally porous materials are best because water doesn't collect or condense on top and make the blanket difficult to handle. However, solid fabrics give more insulating qualities and thus save more energy. In most cases the fabric should be porous, strong, easy to handle, low in cost, capable of good insulation and high light reflection. Many growers settle for a white porous fabric which shades by 50%. They do this during the summer because it makes the greenhouse climate easier for workers to work in. By using blanketing in this manner, some of your fans can be reduced in size and number - by 25-35%. But if you select the fabric based only on summer readings, you will lose some energy savings later on. A certain compromise must be made between summer and winter shadings.
We have seen some growers who use shade cloth on the outside roof of the greenhouse. THIS PRACTICE IS NOT RECOMMENDED because the fabric will collect dirt and cut down on the amount of light being transmitted to the crop. Also the fabric will weather and wear out much faster. No doubt the rising cost of fabric will slow down and eliminate this practice. Vertical curtains can be used - especially at night and during the winter months. These curtains can be either porous or solid but solid will give more protection from heat loss. Since the curtain is vertical, you don't have to worry about condensation. The curtains are placed vertically right next to the crop. Some growers will even move the crop close together on wintry nights and "box in" the crop with vertical curtains on all four sides. Then they will use the overhead blanket to prevent upward radiation.
by: Bob Saffell
How to save energy in the Greenhouse
2:03 PM
ThanateTan
