Vergara - 1976 - Physiological and morphological adaptability of ri

PHYSICS or (TONTROLLED ENVIRONMENT mo) PLANT oaovttrrr 145
VARIABILITY
The next matter to be considered is variability of the external environment and
hence variability of the operating load conditions for the tw"o types of installations.
For the naturally lit installation three types of variability have t0 be considered.
Each will vary substantially according to location throughout the world.
The first consideration is seasonal variability in daily irradianec and in ambient
temperature between summer and winter. The variation in daily itradiance
is generally accepted as an experimental cost of the naturally lit design. Additionally’.
the drop i11 outside air temperature which occurs in many locations in
the winter increases heating costs in naturally lit space.
The second is daily variability. The dominant day to dag.’ variability in most
geographical areas is generally in irradiance. Data for New Zealand indicate
this has a coefficient of variation of 30% and up to 40% per month.
Data Gaastra quotes for the Netherlands give equivalent results (Gaastra.
1964).
A third consideration is the substantial variance, in many parts of the world,
during the day in irradiance intensity for short-term periods of half an hour or
less. Data indicate that for New Zealand, and also for the Netherlands, the
coefficient of variance in this instance is 30-40%. This particular variability’
places a major additional requirement on the complexity of the control systems.
and the size of the control surfaces for temperature removal, if they are to respond
to that order of variance in the operating heat load. Further. and more
importantly for plant experimental purposes. this variability imposes considerable
restrictions on humidity conditions. The cooler the surfaces carrying out
the heat extraction and the larger their area, the more they inevitably’ remove an
appreciable proportion of the heat extracted from the room by condensing the
‘water vapor in the air. i.e. as latent heat. This adds little to the reduction of
sensible air temperature which is being sought and adds to the total refrigeration
capacity required.
Equally. the condensation of water vapor of those cooling sttrfaces and on the
cool surfaces of the glass during cold nights. places substantial operating restrictions
on the ranges of humidity which can be maintained effectively in these
naturally lit spaces. The higher the ambient temperatures of these spaces the
greater the restrictions become where there is a major heat load also to be disposed
of from the rooms. This has been generally’ accepted as an operating fact
of life for naturally lit spaces. We note increasing evidence that humidity is an
experimental variable of prime significance in the interpretation of interactions
of plants with their field environment.
Overall, it follows that much more stiphisticated control systems, and hence
a much wider combination of operating conditions, can be installed in artificially’
lit rooms without raising their costs relative to naturally’ lit rooms.