Greenhouse of an underground heat accumulation system

A greenhouse of an underground heat accumulation system wherein the radiant energy of the sun or wasted thermal energy is accumulated in the soil below the floor of the greenhouse over a prolonged period of time, and spontaneous release of the accumulated energy into the interior of the greenhouse begins in the wintertime due to a time lag of heat transfer through the soil. The release of the accumulated energy lasts throughout the winter.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
This invention relates to a greenhouse of an underground heat accumulation 
system in which radiant energy of the sun or wasted thermal energy is 
stored under the ground for use in heating the interior of the 
green-house. 
Several different heating systems are available for greenhouses of the 
prior art, which systems include a stove system, a hot air system, a hot 
water system, a steam system and an electric heat system. They all depend 
on fossil fuel as the source of energy, and the amount of the thermal 
energy used for this purpose is constantly increasing with the development 
of cultivation of plants all the year round. Meanwhile the price of fossil 
fuel is also yearly increasing and this, combined with a shortage of oil, 
exerts serious influences on the greenhouse industry. Thus, the need to 
conserve energy by switching the source of energy elsewhere from the 
fossil fuel has been keenly felt. In view of this situation, there have 
recently been made various proposals to adopt a system of heating with 
accumulated thermal energy by utilizing the radiant energy of the sun or 
wasted thermal energy to meet the needs of the times. In this system, 
excess thermal energy that has not been consumed in the daytime is 
accumulated and released at night for heating purposes. Since this system 
is generally operated on day-to-day basis for accumulation and release of 
necessary thermal energy, the system suffers the disadvantage that it does 
not satisfactorily work when the amount of the accumulated thermal energy 
is small and inclement weather lasts in the wintertime in the absence of 
an ancillary thermal energy source. This would cause the temperature in 
the greenhouse to drop, thereby causing damage to the plants. 
SUMMARY OF THE INVENTION 
This invention has been developed for the purpose of obviating the 
aforesaid disadvantage of the prior art. Accordingly the invention has as 
its object the provision of a greenhouse of the underground heat 
accumulation system capable of maintaining the temperature in the 
greenhouse at a suitable level in the wintertime without relying on fossil 
fuel. 
The outstanding characteristic of the invention is that the radiant energy 
of the sun or wasted thermal energy is stored for a prolonged period of 
time under the ground on which the greenhouse stands, so that the thermal 
energy thus stored underground can begin its spontaneous release into the 
greenhouse when the cold season sets in due to a time lag in the transfer 
of heat through the soil and the release of the stored thermal energy can 
last throughout the winter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and, more particularly to FIGS. 1-2, a 
greenhouse main body 1 has embedded in the soil under its floor, at a 
depth of about 1.5 m, an underground radiator 2 which, combined with the 
soil beneath the greenhouse body 1, constitutes an underground heat 
accumulating section. The underground radiator 2 is connected to a heat 
collector 3 through a fluid machine 4, which may, for example, be a pump, 
and a receiver tank 5 so as to cause hot water heated by the radiant 
energy of the sun to circulate through the system. The underground 
radiator 2 comprises a serpentine tube of metal evenly arranged beneath 
the floor of the greenhouse body 1 which, in the illustrated embodiment, 
has an area of 24 m.sup.2. 
In this construction, the temperatures of the hot water flowing through the 
heat collector 3, as measured at its inlet and its outlet and the 
differential between the inlet and outlet temperatures, are sensed by 
sensing means. When the sun shines to a desired degree, the hot water 
flowing through the heat collector 3 shows a difference in temperature 
between the inlet and the outlet of the heat collector 3. When the 
difference is above a predetermined value and the outlet temperature is in 
the predetermined range of, for example, between 55.degree. C. and 
65.degree. C., a valve 6 is opened and a valve 7 is closed to actuate the 
fluid machine 4 so as to cause a circulating flow of the hot water through 
the system of the underground radiator 2. In the event that the outlet 
temperature is lower than the predetermined value range in spite of the 
fact that there is a temperature difference between the inlet and the 
outlet of the heat collector 3, the valve 6 is closed and the valve 7 is 
opened so that the hot water bypasses the underground radiator 2 as it 
flows in circulation without the hot water flowing from the heat collector 
3 to the underground radiator 2. When the difference between the inlet and 
outlet temperatures of the hot water at the heat collector 3 drops below 
the predetermined value due to a reduction in the radiation of the sun, 
the operation of the system is stopped. In this way, the radiant energy of 
the sun is gradually accumulated in the earth below the greenhouse body 1. 
In the embodiment of the above construction, the hot water flows in 
circulation through the heat collector 3 and the radiator 2 connected in 
one system. It is to be understood that the invention is not limited to 
this specific form of medium for transferring thermal energy, and that 
heated air may be used in place of the hot water. 
Alternatively, the heat collector 3 and the underground radiator 2 may be 
arranged to form separate systems which are connected by a heat exchanger, 
to allow different forms of fluid to flow in circulation through the 
different systems. For example, a gas may be through the system of heat 
collector 3 and a liquid may be circulated through the system of 
underground radiator 2. 
Accumulation of thermal energy may be started in, for example, October. 
Then, in about a month, the input to the underground radiator 2 will reach 
a constant level of about 19.4 w/m.sup.2 per unit area. FIG. 3 provides an 
illustration of the distribution of temperature in the underground heat 
accumulating section in which a curve a.sub.1 shows the temperature at the 
time of initiation of heat accumulation, a curve a.sub.2 shows the 
temperature two weeks after heat accumulation has started, and a curve 
a.sub.3 shows the temperature one month after heat accumulation has 
started. It will be seen that in about one month, the temperature in a 
position 30 cm below the surface of the earth which lies immediately below 
the area in which the roots of the plants spread stands at 30.degree. C., 
and the temperature near the surface of the earth (3 cm) is 20.degree. C. 
on an average. These temperature remain substantially constant thereafter, 
with the atmospheric temperature standing at 10.degree. C. 
The depth of the underground heat accumulating section will be described. 
Assume that a change .theta..sub.o in the temperature in a certain plane 
under the ground can be expressed as .theta..sub.o =Asin .omega.t. Then a 
change .theta..sub.x in the temperature in a plane parallel to the certain 
plane and spaced apart therefrom by x m can be expressed as .theta..sub.x 
=Ae.sup.-Kx sin (.omega.t-Kx), where .alpha. is the thermal diffusivity of 
the soil and K=(.omega./2.alpha.).sup.-1/2. By substituting 
.alpha.=10.sup.-4 m.sup.2 /h, .omega.=2.pi./24.times.365 and x=1.5 m into 
this equation, a phase difference t at a point x is obtained by the 
following relationship: t=Kx/2.pi.=2.2 months. This relationship indicates 
that with the heat accumulating section having a depth of 1.5 m, the heat 
accumulated underground at a depth of 1.5 m from the ground level can 
reach the earth's surface on which the greenhouse stands in the coldest 
season, assuming that the accumulation of heat can be continued up to 
December. Also, by calculating the influences exerted by changes in the 
mean temperature of each month on the ground level on the temperature of 
the earth below the ground level, it is possible to obtain a reduction of 
31% at a depth 1.5 m below the ground level. Meanwhile the phase 
difference between the temperature on the ground level and the temperature 
below the ground level at the depth of 1.5 m is 2.2 months, so that the 
temperature of the soil in October in which heat accumulation is initiated 
reaches a maximum level in the year and provides an advantageous condition 
for starting the accumulation of heat. The amount of heat Q accumulated at 
this time can be expressed by the following equation: 
Q=1/2C.rho..DELTA..theta..sub.x, where C is the specific heat of the 
earth, .rho. is the density of the earth, .DELTA..theta. is the difference 
in temperature between the surface of the earth and a point which is 1.5 m 
below the ground level, and x is the depth of the heat accumulating 
section. In this equation, the amount of accumulated heat Q will be Q=23.5 
kwh/m.sup.2 per unit area when the specific heat C of the earth is 2.52 
kJ/Kg.degree. C., the density of the soil is 1500 kg/m.sup.3, and the 
floor space of the main body 1 of the greenhouse is 24 m.sup.2. 
As to the heat balance in the greenhouse, Generally, accumulation of the 
radiant energy of the sun in the surface layer of the earth in the 
greenhouse is carried out for about eight hours a day on an average of 45 
w/m.sup.2 in winter, so that the amount of heat accumulated per day is 360 
wh/m.sup.2 d. Also, release of heat from the earth into the greenhouse is 
carried out for sixteen hours a day on an average of 36 w/m.sup.2 in 
winter, so that the amount of heat dissipated per day is 576 kwh/m.sup.2. 
Thus, the amount of heat actually consumed is 216 wh/m.sup.2 d. By 
consuming heat at the aforesaid rate, the aforesaid amount of heat 
accumulated or 23.5 kwh/m.sup.2 would be consumed in 3.5 months. 
By taking into consideration the thermal diffusivity of the soil, the 
amount of heat accumulated underground and the heat balance of the 
greenhouse, the optimum depth at which the underground radiator 2 is 
embedded below the surface of the earth will be about 1.5 m. A depth over 
1 m from the surface of the earth will be an effective depth depending on 
the time at which heat accumulation is started, the heat accumulating 
rate, and other factors. The underground radiator 2 may be constructed 
such that it is surrounded by latent heat accumulating material and has 
soil placed thereon in layers. In this heat accumulating construction, it 
is possible to increase the amount of heat accumulated per unit area and 
to reduce the area of the underground radiator or the length of the 
serpentine tube. 
The area required for the heat collector 3 will be described. The area 
required for this purpose can be obtained from the relation between the 
amount of heat input of 19.4 w/m.sup.2 per unit area referred to 
hereinabove and an average amount of heat collected by the heat collector 
a day ("average radiation from the sun".times."average time from sunrise 
to sunset".times."heat collecting efficiency"). Thus, the area would be 
1/6 the floor space of the greenhouse when the average radiation for 
October, the time from sunrise to sunset and the heat collecting 
efficiency are 814 w/m.sup.2, 6 h and 0.6, respectively. By starting heat 
accumulation earlier than October, the amount of accumulated heat can be 
increased and the required area of the heat collector 3 can be 
correspondingly reduced. Thus, the area of the heat collector 3 would be 
advantageously over 1/6 the floor space of the greenhouse. 
The underground radiator 2 will now be described. Let the radius, the 
surface area and the spacing interval of parallel portions of the heat 
dissipating serpentine tube of the underground radiator 2 and the floor 
space of the greenhouse be denoted by r, S.sub.p, p and S.sub.G, 
respectively. Then the ratio of the surface area S.sub.p of the heat 
dissipating tube to the floor space P.sub.G S.sub.p /S.sub.G has the 
following relationship: 
EQU S.sub.p /S.sub.G =2.pi./(p/r). 
In the embodiment described hereinabove in which the underground radiator 2 
is disposed about 1.5 m from the surface of the earth, the amount of heat 
transferred from the underground radiator 2 to the surface of the earth on 
which the greenhouse stands is about 0.7 w/m.sup.2. .degree.C. when the 
ratio S.sub.p /S.sub.G is 1.0 (or p/r=2.pi.). This value is 0.6 w/m.sup.2. 
.degree.C. when S.sub.p /S.sub.G is 0.1 (or p/r=20.pi.) and 0.54 
w/m.sup.2. .degree.C. when S.sub.p /S.sub.G is 0.05 (or p/r=40.pi.). Thus, 
a large variation in the area ratio S.sub.p /S.sub.G causes no great 
change in the amount of transferred heat. For practical purposes, any 
value above 0.05 would be enough for the area ratio S.sub.p /S.sub.G. 
The temperature at which the underground radiator 2 is heated should be 
limited to 70.degree. C. at maximum by taking into consideration the 
effects exerted by heat on the ecology of microorganisms in the soil. 
Also, the underground radiator 2 may be of any construction as desired so 
long as its heat transfer area with respect to the earth is over about 
0.05 time the floor space of the main body of the greenhouse. For example, 
the underground radiator 2 may consist of the heat dissipating tube alone, 
metal plates may be attached close to the surface of the heat dissipating 
tube or heat-transfer fins may be connected to the heat dissipating tube. 
The heat dissipating tube may be formed of a synthetic resinous material 
of a thermal conductivity equal to or higher than that of the soil. The 
heat dissipating tube may have an inner wall surface which is not circular 
but elliptic or any other irregular shape in cross section. 
Experiments were carried out on heating the interior of the greenhouse by 
spontaneous heat release into the greenhouse from the heat source 
consisting of the underground accumulated heat stemming from the radiant 
energy of the sun collected and stored underground over a prolonged period 
of time, at midnight in early January when the outdoor temperature was 
-0.7.degree. C. The results obtained show that the temperature stands at 
6.7.degree. C. in the center of the greenhouse and at 10.degree. C. near 
the floor where the plants rooted in the soil grow, and that it is 
possible to keep the difference between the outdoor temperature and the 
indoor temperature at over 7.degree. C. It will be seen that the condition 
is favorable for cultivation of tomatoes, strawberries, pumpkins, 
eggplants and cucumbers which are said to require a minimum temperature of 
below 10.degree. C. at night. 
FIG. 4 is a diagram showing changes in temperature in various sections of 
the greenhouse of this embodiment occurring for five consecutive days in 
the wintertime. The weather was cloudy on the first, second and third days 
and fine on the fourth and fifth days. In the diagram, a curve b.sub.1 
represents the outdoor temperature, a curve b.sub.2 represents the 
temperature of the soil 25 cm below the surface outside the greenhouse, 
curves b.sub.3, b.sub.4 and b.sub.5 represent the temperatures of the soil 
3, at 27 and 147 cm below the surface, respectively, of the heat 
accumulating section, and a curve b.sub.6 represents the indoor 
temperature of the greenhouse. In the diagram of FIG. 4, it will be seen 
that the indoor temperature shows little change regardless of the weather, 
and that, although the temperature of the soil at a point of 25 cm from 
the surface outside the greenhouse is about 7.5.degree. C., the 
temperature of the soil at a point of 3 cm from the surface inside the 
greenhouse is 18.degree. C. and rises in going further deeper into the 
earth. In view of the fact that minimum, soil temperature at which 
cultivation of vegetables can be carried out for practical purposes is 
about 18.degree. C., this soil temperature would be considered optimum for 
raising the aforesaid vegetables. At this time, the temperature of the 
soil at a point 1.5 m from the surface is 45.degree. C. which has a 
difference of about 25.degree. C. with respect to the temperature of the 
soil near the surface, and release of heat to the surface of the earth 
continues. 
FIG. 5 is a diagrammatic representation of the results of tests in which 
changes in the temperature in the greenhouse of the system of underground 
storing of heat over a prolonged period of time according to the invention 
are compared with those in the greenhouses of the system of vinyl sheet 
house and the system of underground storing of heat for a short period of 
time by selecting days of similar weather conditions. In the diagram of 
FIG. 5, curves C.sub.1, C.sub.2 and C.sub.3 represent temperatures in the 
greenhouses of the vinyl sheet house system, short period heat storing 
system, and long period heat storing system respectively, and a curve 
C.sub.4 indicates outdoor temperatures. It will be seen in the diagram 
that the system of long period heat storing shows a better performance 
than the system of short period heat storing, much less the vinyl sheet 
house system which lags far behind the system of long period heat storing 
system. An additional advantage of the system of long period heat storing 
over that of short period heat storing is that the former is capable of 
keeping the temperature in the greenhouse at an optimum level for several 
days even if inclement weather persists as compared with the latter whose 
performance may vary depending on whether it is fine or cloudy in the 
daytime. 
From the foregoing description, it will be appreciated that the system of 
storing heat underground over a prolonged period according to the 
invention which accumulates below the surface of the earth the radiant 
energy of the sun at least in autumn and releases the stored energy by 
spontaneous release to heat the greenhouse is capable of maintaining the 
interior of the greenhouse at an optimum temperature level for the 
cultivation of vegetables throughout the period of severe cold and of 
achieving better results in heating the greenhouse than the system of 
short period heat storing. In the foregoing description, the radiant 
energy of the sun has been described as being used as a source of thermal 
energy to be stored in the underground heat accumulating section. However, 
it is to be understood that the invention is not limited to this specific 
form of thermal energy and that wasted thermal energy may be used as a 
source of thermal energy to be stored in the underground heat accumulating 
section. 
Cultivation of plants was carried out by usual means in the greenhouse 
according to the invention in which spring-sown vegetables were sown in 
December and harvested at the beginning of March. This is attributed to 
not only the high temperature in the greenhouse but also the high 
temperature of the soil which promotes the growth of the roots. 
Thus, the invention enables the greenhouse to be kept at a room temperature 
of over 6.degree. C. and the soil at a temperature of over 16.degree. C. 
by utilizing the radiant energy of the sun or wasted thermal energy 
without using fossil fuel, thereby making it possible to cultivate spring 
sown vegetables in winter. 
Generally a greenhouse consumes about 2 liter/m.sup.2 of fossil fuel for 
heating purposes in one winter. If the heating of the greenhouse can be 
effected by utilizing the radiant energy of the sun alone, it would be 
possible for a greenhouse of 1000 m.sup.2 to conserve 2000 liters of oil. 
Moreover, since the radiant energy of the sun is free from the danger of 
polluting the air and its supply is inexhaustible, the utilization thereof 
offers advantages besides being able to conserve energy stemming from 
fossil fuel. 
Thus, the invention enables the temperature in a greenhouse at a suitable 
level in winter without using fossil fuel.