Four formulae for estimating the greenhouse heating degree-hour have been presented so far by three authors in this country. Every one is expressed as a function of only the difference between an inside temperature desired and the daily minimum outdoor temperature without any consideration for sunshine; two of them indeed exclude daytime from calculating the degree-hour.
In this paper, the daily mean of bright sunshine hours (
S) is introduced into equations for daily degree-hour (
DH) which are based on four factors; desired inside temperature (
TC), daily maximum temperature (
TH), daily mean temperature (
TM) and minimum temperature (
TL). For
TC≥
TH,
DH=24(T
C-T
M)-S(T
C-T
H+T
M/2)
and for
TC<
TH,
DH=24(T
H-T
M)(T
C-T
L/T
H-T
L)
2-S/2(T
C-T
M)
2/T
H-T
MThese formulae were derived on the assumptions that bright sunshine renders greenhouse heating unnecessary even when the outside temperature is lower than
TC and that the diurnal course of the monthly averaged outdoor temperature can be approximated by two straight lines connecting
TH and
TL.
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