Constant evaporation doesn't only have a positive influence on energy consumption, but also on mushroom quality and production. Air pressure fluctuations, that can disrupt constant evaporation, can be corrected using fan control and the moisture content. Allowing the maximum CO₂-limit to vary depending on the moisture content can also prevent an oxygen deficit for the decomposition process.

The Netherlands has an average air pressure of 1013 mbar, based on 0 metres altitude or sea level. This value is used as the basis for all the current climate programs. Changing weather conditions are an influential factor on variations in air pressure. Depending on the weather, air pressure in the Netherlands can be anywhere between about 960 and 1045 mbar. This means a difference between the actual air pressure and the fixed value contained in the climate program. Measurements that will vary are air density (becomes lower at low air pressure and vice versa) and the moisture content of the air (becomes higher with low air pressure and vice versa). The heat content of the air doesn't depend on the air pressure, but on the moisture content and will also deviate.

Air density

As well as air pressure, changes in temperature and to a lesser extent changes in humidity, also have an influence on air density. As an indication: a 10 mbar lower air pressure, a 3 degrees higher temperature or a 16 gram higher moisture content will result in a 1 % lower air density.

The climate unit introduces a certain volume of climatised air into the growing room. The amount depends on the fan RPM or the frequency controller position. Although in practice a constant fan position or constant air volume in cubic metres per hour is used, the actually achieved volumes of air in kilos per hour vary, and it is this that is ultimately important for the effects of climate control. The disparity in air volume - and therefore in evaporation is caused by variations in air density. To keep the air volume constant it is possible, for example, to adjust the fan position based on variations in the density of the inlet air, relative to a reference value of the air density of 1.2 kilo per cubic metre. The adjustments can (with a frequency controller) be based with an acceptable accuracy on a linear relationship between fan position and air volume in cubic metres per hour.

Calculation example: in the air inlet duct an air density is measured of 1.8 kilo per cubic metre. This is 10% lower than the reference air density of 1.2 kilo per cubic metre. If the fan position (of a frequency- controlled fan) has been set to e.g. 50 %, a fan position of 50 + (0,1 * 50) = 55 % will be necessary in order to keep displacing the same amount of air in kilos per hour.

Moisture content

Air moisture content is usually calculated based on the dry and wet bulb temperature. This calculation is based on the previously mentioned average air pressure of 1013 mbar. With a deviating air pressure, the calculated moisture content will no longer be correct, leading to differences in the moisture content. In extreme circumstances these differences can be as high as 5 % or 0.6 gram per kilo air. This is a relatively large difference for evaporation. If control is purely based on the moisture content of the air, we strongly recommend calculating the moisture content based on the actually measured air pressure. This also supplies the correct air deficit and correct heat content. If the influence of air pressure on the moisture deficit is examined, the moisture deficit appears to be less susceptible to changes in air pressure than the moisture content. The explanation lies in the fact that the moisture deficit is a difference measurement of the moisture content and the saturation point, which any air pressure change will cause to increase or decrease equally. The mutual difference (= moisture deficit) therefore varies relatively slightly. The fact that the moisture deficit control is less sensitive to air pressure, partly explains why this type of control is such a success.

Oxygen

Mushroom growth may be restricted if fresh air supplies are limited. It is unknown if this restricted growth is caused by an excess of CO₂ or by a lack of oxygen. A link between the moisture content and the sum of the CO₂ and oxygen concentrations has been proved. This means the oxygen content can be calculated based on the moisture content and the measured CO₂. Allowing the maximum CO₂-limit to fluctuate depending on the moisture content, can help prevent oxygen concentrations dropping below a certain minimum value leading to a lack of oxygen for the decomposition process.

There is currently a strong belief in giving evaporation highest priority and that with correct evaporation the CO₂ value will have relatively wide limits. Expanding these CO₂-limits can save energy, but extremely high CO₂-limits also involve potential risks of oxygen shortages.

An oxygen meter is not precise or sensitive enough to be used as the basis for air inlet control. Calibrating the oxygen meter in particular for humid outside air is very inaccurate, as this fails to take air pressure and moisture content of the outside air into account. Additionally, the sensitivity of an oxygen meter is factor 1000 less than that of a CO₂ meter (0,1 % = 1000 ppm), which means air inlet control can be compared to a CO₂ meter which works in steps of 1000 ppm!

Far more accurate results are obtained by calculating the oxygen concentration using the measured CO₂. It is an established fact that with a biological or chemical process the sum of the CO₂ and O₂ concentration is nearly constantly 20,979 % of the dry air pressure. To calculate the oxygen concentration, the moisture content (air pressure corrected!) and the CO₂ concentration must be known. The existing installation just has to be fitted with an air pressure meter.  A simple formula to calculate the oxygen concentration can be derived from a number of intermediate steps and formulas:

% O₂ = (1 - 1/(1+18,016/(x*28,964))) * 20,979 - % CO₂ measurement.

Calculation example:

Moisture content growing room (x) = 11 g/kg = 0,011 kg/kg

CO₂ concentration growing room = 1800 ppm = 0,18 %

% O₂ = (1 - 1/(1+18,016/(0,011*28,964))) * 20,979 - 0,18 = 20,43 %

Variable maximum CO₂-limit

A lack of oxygen can arise if the air inlet is closed to the maximum CO₂-limit. The fact that the maximum CO₂-limit differs in the winter and in the summer, indicates that more than one factor is influential here: moisture content and oxygen concentration. With a constant CO₂ concentration, a rising moisture content will cause the oxygen concentration to drop and the reverse. Maintaining a fixed maximum CO₂-limit in situations with a high moisture content in the growing room air, risk creating too low oxygen concentrations, similar to those occurring with existing climate control systems. In order to maintain a constant minimum oxygen concentration, it's necessary to correct the maximum CO₂-limit for moisture content. The example in the calculation shows that the maximum CO₂-limit must be lowered by on average 326 ppm per gram of excessive moisture content and the reverse. The limit for minimum oxygen concentration is determined based on the set climate. (figure 3)

Air pressure and oxygen

To achieve energy efficient climate control in the Netherlands, four projects concentrating on energy efficiency have been developed in the framework of the Long-Term Energy Agreements for mushrooms. Three of these projects have already been run, where energy efficient moisture control based on moisture deficit, a measurement and information system for heat, moisture, CO₂ emission and a correction method for air pressure and oxygen have been developed.

This article explains the results of the air pressure and oxygen project. The project was trialled on the growing facility of J.Jacobs in Kessel in cooperation with climate computer supplier Fancom BV.  The results and findings from this project gave a number of interesting ideas for further optimisation of climate controls. Via a recently started plan of implementation, climate control computer suppliers will be approached to ask them to introduce these features in their systems

The final project, which is still on going, concerns an energy efficient air inlet and ventilation control based on the moisture content of the inlet air. The results are expected in the autumn.