Tuesday, December 21, 2010

Hoophouse II - A Qualitative Investigation

Let's think about how a hoophouse works. This is the first step toward ideas to improve the design.

First, consider why there are the seasons. The reason is that the axis of the earth's rotation is inclined to the plane of the orbit around the sun. In our summer the northern hemisphere is more directly exposed to the sun; that is, the surface is more nearly perpendicular to a line between the center of the earth and the center of the sun. As a result each unit of surface area receives more solar radiation in summer than it does in winter when it is inclined to this line. In addition, the surface is exposed to the solar radiation a greater length of time each day. For both of these reasons the hemisphere that is in summer receives the greatest amount of solar energy per day.

There is an additional effect that contributes to the timing of the seasons. In the northern hemisphere the maximum solar energy is received around the summer solstice, about June 20. But the hottest days occur in July and August. In these months we receive less energy per day than at the end of June, but we still receive more energy than is being lost each day, consequently the temperature of the surface of the earth continues to rise. A similar effect occurs in winter. The winter solstice is around December 20, but the coldest days are typically in January or February. In this case the surface of the earth is receiving less energy than it is losing, stored heat continues to be lost, and the surface continues to cool. This lag in the maximum and minimum temperatures is a result of the earth storing thermal energy. This storage is often called the “thermal mass” of the system

What does this have to do with a hoophouse? An unheated hoophouse modifies the season to make winter in the hoophouse milder than winter outside. But it cannot increase the solar energy received, in fact it reduces it slightly. Therefore the effect must be a result of reducing the heat lost. The hoophouse reduces the heat loss and allows the stored thermal energy to maintain a higher internal temperature.

It may seem surprising that a few thousandths of an inch of plastic can provide sufficient insulation to change the climate in the hoophouse, but it can because the effect is accumulative over a long period. This is exactly the same reason that the slight increase in greenhouse gasses in the atmosphere can lead to global warming.

How can you use this information to improve the design of your hoophouse? The micro climate in the hoop house is a result of the solar energy received, the stored thermal energy, and the rate of heat loss. You need to decide which of these variables you can modify, the effect you may achieve, and the cost.

The hoophouse designer has very little control over the solar energy received, other than the transparency of the covering. Theoretically, one could collect solar energy from a larger area and transport it to the hoophouse, but in practice this would be more a method of heating than increasing the solar energy received at the hoophouse. The solar energy received is a variable the hoophouse designer cannot control for a given location.

In a similar manner the hoophouse designer has little control over the stored energy, or thermal mass of the system. It may be possible to have an effect with something like barrels of water in a very small hoophouse, but in any moderate sized system the thermal mass of the earth inside the hoophouse will be many times larger than any energy storage system that is practical to add. If you chose to grow hydroponically the nutrient solution will had some thermal mass, and you will probably need to heat it to prevent freezing. But again this is more an approach to heating the hoophouse or greenhouse than increasing thermal mass.

This leaves the insulation variable. If 6 mill polyethylene can create a more favorable micro climate, clearly better insulation would have a greater effect. The problem is that the hoophouse covering must be transparent. Transparent materials that are also good insulators are expensive. The way to avoid this dilemma is to look for areas and times the hoophouse does not need to be transparent.

During the night the hoophouse does not receive solar energy and the plants can be covered with an insulating blanket to retain more heat. The hoophouse side walls and end walls do not receive very much solar energy and they can be insulated with very little reduction in solar energy received. Night time insulation and insulation of the areas that receive little solar energy can be very cost effective, yet are rarely done. Of all the hoophouses I have seen, only one used side and end wall insulation and night time row covers.

If anyone is interested in this analysis continuing in a more qualitative manner, leave a comment or email.

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