Page 167 - Biosystems Engineering

146 Cha pte r F o u r

of soil temperature has had recent applications. Two approaches that
allow for soil temperature prediction have been developed by George
(2001) and Mihalakokou (2002). The latter obtained good results for
prediction of daily and annual variations in soil surface temperature.
Statistical Methods and Combinations
A number of methods have been used to model soil temperatures.
Statistical regression models were used by Dwyer et al. (1988, 1990),
Pikul (1991), and Kluender et al. (1993). Other models (Gupta et al.
1981, 1984; Sharratt et al. 1992) have been developed by combining
statistical methods for predicting surface temperature based on air
temperature with the solution of the heat-conduction equation for
determining temperature at depth. Geostatistics has incorporated
new engineering and research tools, such as digital elevation models
(DEM) for developing temperature-prediction models, such as the
model developed by Kang et al. (2000).

4.4 Greenhouse Substrate Heating
Greenhouse substrate-heating systems are essentially made up of the
following components:
1. Heating elements, which can be divided into three main catego-
ries: heating pipes with circulating warm water (Ahmed et al.
1983), heating pipes with circulating warm air (Boulard et al.
1989; Kurpaska and Slipek 1996), and electrical heating cables
(Rikbost et al. 1975).
2. Support material (pearlstone, sand, peat or a sand-peat mixture,
among others), inside which the heating elements are buried.
The support material acts as a substrate and as a heat storage
element.
3. Thermal insulation to minimize heat losses.
4. Metal plate with high thermal conductivity often placed immedi-
ately above the heating elements in order to achieve uniform
heat distribution throughout the entire mass of the support
material.

Protected cultivation has experienced many changes in the last
few years, including the development of strategies that improve facil-
ities and lower production costs without changing the crop growth
cycle or the cultural techniques used. Such strategies have introduced
technology in greenhouse production. This development, together
with the evolution toward soilless culture techniques, has contrib-
uted to the application of substrate-heating systems.
Substrate-heating systems should be capable of guaranteeing
the preset temperatures for each substrate layer according to crop