Notes on SGH Chart No.1:

There are two copies of the chart posted. This was not the intent and appears to be a problem with The Author’s computer network.

Only part of a day is shown (11:02 am to about 2 pm). This was done to show the fine structure of the parameter interdependencies.

The weather over the period shown was 100% overcast with rain. Note that, although the outside RH is 100% most of the time, the outside AH is lower than the inside AH everywhere. Note that the control system is driving the Inside AH almost all the way down to the outside AH values. The remaining difference may be due only to the hysteresis built into the control algorithm.

For the graph shown, the setpoints are:

Maximum Inside RH: 85%

Minimum Inside T: 60 deg F

The units of AH are grams / cubic meter.

The points are not equally spaced in time. This is due to the use of an event-driven sampling algorithm in the VBA software program: If any parameter is outside preset limits, a sample is taken for all parameters.

The nutrient solution pump state is plotted on the left-hand axis to avoid cluttering the graph. The pump “on” periods can be seen shortly after 11 am, 12 pm and 1 pm. (The pump is only “on” for about 5 minutes.)

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Solar Greenhouse (SGH) Performance Chart No.1

Control Laws:

Terms Used:

Tmax: The goal is to not exceed this temperature inside the SGH

Tmin: The goal is keep the inside temperature above this value

RHmax: Maximum desirable inside Relative Humidity

AHins: The inside Absolute Humidity (AH)

AHouts: The outside Absolute Humidity

The control system is of the type commonly referred to as a “bang-bang” control system (there is no proportional, integral or derivative feedback used). The Control Laws are quite simple:

1. If the inside temperature (T) is above Tmax, the ventilation system shall be “on” and the heaters shall be “off”.

2. If inside T is below Tmin, the ventilation system shall be “off” and the heaters shall be “on”.

3. If inside T is between Tmax and Tmin, the heating and ventilation systems control shall be a function of RHmax:

3a. If the inside RH is below RHmax, both the heating and the ventilation systems shall be “off”.

3b. If the inside RH is above RHmax, the heating and ventilation systems control shall be a function of the difference between AHins and AHouts:

3ba. If AHins is greater than AHouts the ventilation system shall be “on” and the heaters shall be “off”.

3bb. If AHins is less than AHouts, the ventilation system shall be “off” and the heaters shall be “on”.

These control laws cause temperature and relative humidity control to be interdependent. Heating to increase T will cause RH to decrease. This is almost always desirable in a vegetable greenhouse on the Pacific Northwest (PNW) coast. Ventilating to decrease T can, in theory, cause RH to increase. In practice however, even on the high rainfall PNW coast, outside AH is almost always lower than the inside AH of a vegetable-growing hydroponic greenhouse.

The most important trade-off in the control system design is the decrease in T when the ventilation system is turned “on” to reduce inside RH. This can, and often does, reduce T to below Tmin, causing Control Law 2 to take over.

In reviewing measured data, one sees oscillations due to the above-described interdependencies. For the parameters of the Research SGH, the oscillations observed to date are considered acceptable. This is based on what is suspected to be a nearly a worst-case situation of many consecutive days of 100 percent overcast and rain.

The operating philosophy used to date is that first priority is to keep the inside RH less than 100%. (Condensation on plant leaves is a well-known cause of plant disease.) Second priority is to keep T at or above 60 deg F (for good fruit-setting of tomato plants). The simple (thermostats only) control system has trouble achieving these priorities. To date, the computer-based control system is doing better.

A Brief Description of the SGH

The SGH is located on the Southern Oregon Coast, at Latitude 42.4 degrees, Longitude –124.4 degrees (approximately). The Structure is oriented with the plane of the South wall normal to the azimuth of the solar zenith.

The SGH has glazing on the South-facing wall only. The glazing material is single-pane 1/8 in. tempered window glass. The glazing is installed at an angle of 60 degrees from the horizontal. The remaining 3 walls are opaque and heavily insulated, as is the floor and the roof. The SGH floor surface is of masonry construction, ¾ in. thick.

A single row of fifty-five gallon flat black-painted plastic drums is located on the floor, closely-spaced, along the entire length of the North Wall. These drums are connected together as a single reservoir, to hold a water-based nutrient solution for hydroponic plant culture. The solution also acts as a heat storage medium, to moderate diurnal temperature variation.

Plants are grown in containers located on benches placed along the South wall. The planting medium is pea gravel (1/8 to 3/8 in. dia. “River Run”). The base of each container is plumbed to a solution distribution system. A timer-controlled pump (located in the reservoir) runs until the containers are nearly full. When the pump shuts off, the nutrient solution gravity-flows through the pump back into the reservoir. The containers thus fill from the bottom up, displacing air. The draining of the containers pulls air (and thus oxygen) into the root areas. This process is repeated a number of times during daylight hours.

Supplemental heating is provided by electric heaters. The ventilation system consists of a motor-driven set of air intake louvers on one end of the SGH and a motor-driven exhaust fan on the opposite end. The exhaust fan is never operated without the air intake being “open”. The air intake is only opened when running the exhaust fan. Separate small fans are operated to circulate inside air when the ventilation system is not running.

The SGH is instrumented with sensors to measure interior and exterior temperature and relative humidity (RH). There are also sensors to measure nutrient solution temperature. These sensors are connected to a network that is controlled by a dedicated computer. The computer has two functions: (1) Logging the measured data and (2) controlling the SGH Heating and Ventilation System.

There is a digital Input/Output (I/O) board located in the SGH and connected to the network. The I/O board is used to input heater and ventilation system status to the computer and to output commands to the heating and ventilation systems.

The SGH has two heating and ventilation control systems. One control system consists simply of a conventional electromechanical thermostat for the electric heaters and a separate thermostat for the ventilation system. This system operates independent of the computer, although the computer logs measurements when either control system is running.

The second control system is implemented using the computer and the networked sensors. The computer is located in a building separate from the SGH.

The computer is a conventional PC, running WINDOWS xp. All SGH sensors and the I/O board are purchased “1-wire” network devices. Data is transferred from the sensors and the I/O board to an EXCEL spreadsheet, using the DDE (Dynamic Data Exchange) function in WINDOWS. Two software packages are used to accomplish this: DDEView, a commercially-available program and a VBA (Visual Basic for Applications) program written by L. Vincent Nash.

Site Charter

The purpose of this site is to exchange technical information related to greenhouse design, construction and operation. Applications in the Pacific Northwest (PNW) environment will be of primary interest. Of particular interest will be the impact of the ocean coast environment on greenhouse operation.


Some Areas of Interest

Solar Greenhouse Design & Construction

Hydroponic Plant Culture (Emphasis on Vegetable Culture)

Greenhouse Control Systems Design, Construction and Test

Greenhouse Performance Measurement and Analysis


Site Assets

The author has constructed a Solar Greenhouse (SGH) on the Southern Oregon Coast. The SGH is presently in operation utilizing hydroponic growing techniques.

One goal of The SGH project is the automation of SGH operation. The SGH has a dedicated computer for measurement and control. Interior and exterior temperature and humidity are measured on a continuous basis, as well as the status of the heating and ventilation systems. Based on these measurements, the computer controls the heating and ventilation systems, using software and hardware developed specifically for this application.

SGH performance data are produced and archived continuously. The Author’s intends to post SGH performance data on this site.




Site Status

This site was created 1/18/06. The SGH has been in operation since April 2005 using simple thermostat controls. Closed-loop operation of the SGH under computer control was started 1/16/06.

No significant postings are planned (by the author) until Feb. 2006.