

Horticultural model

In colder regions where few vegitables are viable in open 
fields and transport costs are high, horticultural 
structures such as polytunnels are a possible response
which are often difficult to justify because of heating
costs. This model is derived from a 2013 project in
the North of Scotland assessing options to lengthen the growing 
season via an improved polytunnel skin in conjunction 
with waste heat from a local processing facility.

The idea was to use a double skin transparent facade (rather
than a single skin polytunnel) and introduce the heat via
piping at 0.5m below the growing surface. The question was whether
this would be viable if the waste heat was limited in capacity (in
the order of 100-150W^m2 of growing area)
and low temperature (in the order of 40-50C).

The approach taken was to create a model of a 7m wide polytunnel
which was 15m long separated into three growing zones with an 
access door at one end and an ventilation opening
at the other. The form of the polytunnel was initially defined 
with an end polygon oval which was then extruded to form a thermal 
zone which was then copied and transformed and joined together.

Several alternative growing medium compositions were 
suggested and included in the model databases to support
the creation of model variants. earthalt_0.5m is referenced
in the model.

The access door and vent opening areas can be varied to be substantially
closed if the middle of the polytunnel was below 12C and progressively
opened if temperatures went above 20C via two flow control laws. 
Air flows between the the growing zones are bidirectional based on 
temperature differences or pressure. The width of the bidirectional
openings between zones is 2m rather than the full width of the
tunnel to dampen flows slightly. Cracks are included along the base 
of the polytunnel. 

The idea was to place the waste heat piping roughly 0.5m
below the surface and assume that the heat introduced would be
sufficient to raise the growing surface to perhaps 20C while limiting
overheating of the lower roots. Heating densities between 100W/m^2 and
150W/m^2 were assessed as well as a 40C to 50C working temperature.

The approach taken was to slightly abstract the definition of
the heating regime in order to explore temperature and capacity
issues. The model uses a thin thermal zone 0.5m below
the polytunnel with an upper surface made up of the growing medium
and the base composed of 1m of earth. Heat would be injected into
the heater zone and transit to the growing space and into the
ground below. This preserved the time lags inherent in such massive
distribution system and identified how often the core temperature
was insufficient under different control logic.

Similation parameter sets for January-March and annual assessments
are included. The images folder shows the results of one assessment
with the fluid limited to 40C and 10KW of heating capacity. The
surface of the growing medium is roughly 18-20C and air temperatures
in the polytunnel only occassionally drop below 8C.
