During both trials, micro-climate measurements were taken with a sampling rate of one minute, with the sensors mounted at the sides just above the growing trays. The collected data capture most of the periods listed above except for a few days, which were lost due to the process of downloading data from the sensors and setting them up to collect the next batch. During the first trial, for example, from the 30th of January until the 23rd of March, 1092 hours of temperature and humidity data were recorded (45.5 days) while 145 hours (6 days) were lost. Fig. 4.9 shows the
Figure 4.4: Finished Impatiens divine before harvest.
Figure 4.6: Root growth of Impatiens divine.
Figure 4.8: Unfinished Begonia tuberhybrida.
daily temperature variations along the length of the layer where begonias were grown during the first trial (see Fig. 3.5 for exact location of the sensors). Here,Begonia semperflorens were grown in the second layer from the top, with the other layers all empty (and with the LED lights switched off). Sensor 2, which is located at the front end of the layer of interest, is the only location on which the photo-period has negligible effect. This was expected as the air exchanges are more frequent at that location. Sensors 7 and 12 deviate by about 1 ◦C from the set-point (dashed black trace) during photo-periods, while further along the layer, sensors 17 and 22 reach 23.5 to 24 ◦C. In a similar manner to these, the back end sensor (27) deviates by about 1.5 ◦C from the set-point during lighting periods. The unused layers below the begonias are hardly affected by to the photo-periods. On the contrary, the top layer, which is above the begonias, did present a trend similar to Fig. 4.9 but at a smaller magnitude and only towards the back end of the growing area.
Again, for the first trial, the mean temperature and humidity distributions along the length of the grow-cell during light and dark periods are illustrated in Fig. 4.10
and (Fig. 4.11) respectively. These figures are generated using spline interpolation from the point measurements in MATLAB. As noted above, the begonia layer and the one immediately above it yield temperature levels up to 24◦C, while the temperature in the lower layers remain close near the set-point at all times. Furthermore, as
Temperature ( ° C)
11 days (1min samples)
2 7 12 17 0 1 2 3 4 5 6 7 8 9 10 20 22 24 22 27
Figure 4.9: Selected temperature sensor readings from layer 2 during the first trial, January 30 to February 10. The numbers 2, 7, 12, 17, 22 and 27 refer to the sensor locations in Fig. 3.5.
previously seen in section 3.8, the temperature distribution is relatively uniform and close the set-point during dark hours but shows significant variation during the light periods. In a similar manner to the temperature, the humidity distribution (around the set-point of 95%RH) was uniform for the three layers below the growing layer. However, the growing layer humidity drops to as low as 85%RH during photo-periods and climbs to 92%RH during dark hours (Fig. 4.11) because of the plants. The overall deviation from the set-point lies in the range 11%RH and 5%RH.
Finally, Fig.4.12displays the average temperatures and humidities for each layer, together with set-point data, for the whole growing period of the second trial in three hour intervals. During the first twelve days with a continuous photo-period, the active growing layers are consistently warmer by 1 to 3 ◦C. This gradient increases throughout the remaining period due to the excess heat generated by the increasing the voltage to the lights. The conditions for the two inactive layers at the bottom of the grow-cell are shown by black traces. As expected, the inactive layer at the bottom yields conditions closest to the set-point while the upper one receives a significant amount of heat due to the lights operating above it. The humidity shows
Height (m) 0.3 1.5 2.7 3.9 5.1 6.3 Length (m) 2.05 1.7 1.35 1 0.65 21.5 21.6 21.7 21.8 21.9 22 22.1 22.2 22.3 22.4 0.3 1.5 2.7 3.9 5.1 6.3 2.05 1.7 1.35 1 0.65 21.5 22 22.5 23 23.5 24
Figure 4.10: Interpolated temperature spatial distribution during lighting (upper plot) and dark (lower) hours, January 30 to February 10.
less variation in general.
4.4
Conclusions
The present chapter has briefly discussed the growth trials that took place between January-March and June-July 2015 by a third party company, a nursery which grows edible and ornamental plants. The discussion has largely focused on practical issues such as the feed system and micro-climate spatial variability. In fact, the primary objective of the growth trials within the scope of the present thesis was to test the entire prototype grow–cell system in an illustrative practical situation, to evaluate reliability of the developed feed, LED and conveyor system, and the ability to grow ornamental plants in a general sense.
In this context, the growth trials were regarded as a success by the industry partner and collaborating nursery. For Impatiens divine and Begonia semperflorens, the uniformity of the crop was reported as satisfactory, mainly due to light consistency, although plants were relatively smaller at the outer edges where the light intensity
Height (m) 0.3 1.5 2.7 3.9 5.1 6.3 Length (m) 2.05 1.7 1.35 1 0.65 90 91 92 93 94 95 96 97 0.3 1.5 2.7 3.9 5.1 6.3 2.05 1.7 1.35 1 0.65 84 86 88 90 92 94 96
Figure 4.11: Interpolated humidity spatial distribution during lighting (upper plot) and dark (lower) hours, January 30 to February 10.
50 100 150 200 250 20 22 24 26 28 Temperature ( ° C) 50 100 150 200 250
3 hour interval data 50 60 70 80 90 100 Humidity (%RH)
Figure 4.12: Temperature and humidity responses throughout the five week period of the second trial, plotted against sample number (3 hour samples). The set- points, active and empty layers are indicated by blue, red and black coloured traces respectively.
is lowest. Survival rate of plants was not formally quantified but is reported to be close to 100%. Growth of Begonia tuberhybrida was less successful and it is believed this relates to the soil used, although this requires further research.
More systematic experiments that use the grow-cell to rigorously investigate plant development, plant quality and whole system operating costs is an essential next step for future research. Indeed, this is the motivation for the development of the prototype in the first place. In this regard, the lessons learnt and some preliminary cost considerations are discussed in the following Chapter 5.
Chapter 5
Discussion and Conclusions
Based on the literature review, design considerations and laboratory work from Chap- ters 2 and 3, and the growth trial results from Chapter 4, a number of observations and recommendations can be made regarding the grow-cell prototype, particularly in relation to the control of micro-climatic variables within the airspace, subsystem de- sign issues, energy consumption and operating costs. These are discussed in sections 5.1 through to 5.6 below, followed in section 5.7 by a brief summary and conclusion. In this manner, the present chapter provides the conclusions and suggestions for further research that are connected to Part A of the thesis.
5.1
Freight container and air-conditioning unit
The development of the test grow-cell was based on a freight container, for which the original Heating-Ventilation and Air-Conditioning (HVAC) unit had been been selected and optimized for food storage rather than plant growth. Hence, it has an energy cumbersome heating and cooling capacity to begin with. Partitioning the airspace into two sections changed the whole structure of the air-flow supply system, while the incorporation of lights added heat within the environment that is clearly not fully compensated for by the HVAC unit. This is evident in, for example, Fig. 3.12, Fig. 3.13, and Fig. 3.15. Added to the above is the empty space that the air has to travel before reaching the entrance of the growing area, at which point its flow reduces significantly. Despite these limitations, which will primarily impact upon the energy consumption costs, Chapter 4 showed that the unit was generally
sufficient in terms of providing the conditions required for plants to grow.
Two very simple future modifications, apart from re-sizing the HVAC’s heating and cooling capacity, relate to the distribution of the airflow and the incorporation of sensors in the growing area. For the former, a horizontal airflow supply scheme, by utilising louvres and dampers at the entrance of the growing area would need less power to operate (in practice, plants need a ventilation rate as low as 1 ms−1). With
the current configuration, an unnecessary (by default) 10 ms−1 was used, decreasing to about 1.5 ms−1 at the entrance of the growing area. Furthermore, by utilising
temperature, humidity and airflow sensors along the growing area, the HVAC unit would have more information in order to compensate for the heat gain induced by photo-periods and the humidity gain associated with this and the irrigation cycles. Control systems should be developed that utilise this information and so improve the homogeneity of the growing volume by acting upon the magnitude of the air-flow, dampers and louvres in an appropriate manner. Such a scheme would also reduce the conveyor system’s energy usage as it would not need to carry out extra circulations to compensate for an irregular micro-climatic distribution; it would be used instead only for irrigation, inspection of the crop, and pre/post harvesting tasks.