Page 168 - Biosystems Engineering

Models for Heat Transfer in Heated Substrates 147

requirements at each developmental stage. The systems should
also be capable of providing uniform temperatures in time by miti-
gating in as much as possible the effects of daily and seasonal ther-
mal fluctuation.
The high cost of the energy supply required for adequate heating
demands maximum energy efficiency of the heating system. We must
consider that the heat losses from the greenhouse are dependent on
several factors such as wind speed, difference between indoor and
outdoor temperatures, air temperature, type of cover, and greenhouse
design (Sturrock 1989). In addition, heat consumption can be mod-
eled as a linear function of the outdoor temperature (Strøm and
Amsen 1981). However, heat consumption can also be estimated as a
function of temperature exposure, which represents the difference
between the temperatures inside and outside the greenhouse, and
accumulated over a certain period. Temperature exposure can be
determined from monthly means of daily maximum and minimum
temperatures (Seginer and Jenkins 1987).
Any heating system used for greenhouse climate control must
meet three basic conditions: safety, reliability, and flexibility. The sys-
tems used for heat transfer by conduction under the surface of green-
house soils, or in greenhouse benches, show good prospects for use
because they meet the three conditions mentioned. From a physics
perspective, such systems enable the use of the air-exchange surface
provided by the substrate, which is larger than the exchange surface
of the traditional aerial systems (Feuilloley and Baille 1992) and make
it possible to reduce greenhouse air temperature below the values
considered as optimal without affecting production or quality, thus
resulting in potential energy savings (Boulard and Baille 1984a;
Boulard and Baille 1984b; Gosselin and Trudel 1984). However, such
a temperature reduction may cause problems for the crop if the rela-
tionship between substrate temperature and greenhouse air tempera-
ture is not properly established (Jones et al. 1978).
As compared to other heating systems, use of substrate-heating
systems shows additional advantages from the perspective of energy:
(1) heat transfer occurs at low temperatures, which allows for correct
vertical distribution of temperatures inside the greenhouse (Kurpaska
and Slipek 2000); (2) substrate temperature provides sufficient heat in
the root zone at low overall temperatures (Jones et al. 1978; Zeroni et al.
1984); (3) the high thermal storage capacity of the soil minimizes the
effect of fluctuations in temperature supply; (4) the differential tem-
perature between the soil surface layer and the air around it produces
heat transfer into the aerial greenhouse environment; (5) use of low
operating temperatures allows for the use of clean energies, such as
geothermal and solar power; and (6) the system shows large thermal
inertia, which means that a time lag is required before the substrate
temperature increases. Similarly, the temperature decreases slowly,
which is an important advantage in case of system breakdown.