Pilot greenhouse for the utilization of low-temperature waters

Geothermics (i97o) - SPECIALISSUE 2 U N. Symposiumon the Developmentand Utilization of G~thermal Resources, Pisa '97o- Vol. 2, Part j

Pilot Greenhouse-for the Utilization of Low-Temperature Waters G. DRAGONE ** ~D O. RUMI *

ABSTRACT A pilot installation for the heating of a greenhouse with low temperature waters (30 to 70 °C) is described. The heating system combines space with soil heating. As different combinations are possible (only space or soil heating or both, etc) the optimum utilization conditions can be determined for the different temperatures of the water.

Introduction The use of geothermal waters for the heating of greenhouses has been of ever increasing importance for a long time. Iceland, which can be considered a pioneer in this field, began the exploitation of natural hot waters for this purpose in the early 1920's. Today the area covered by glass-houses is more than 110,000 m z, one third of which is used for flowers and two thirds are used for tomatoes, cucumbers, lettuce, etc (LINDAL 1961, BIORNSSON 1966). In the past years other countries have developed this type of utilization: among them, Hungary .and the USSR have largely extended in few years the total covered ground Q). In Italy the utilization of natural hot waters in agriculture was never seriously considered (2) on a large scale; the only consistent example is that of Castelnuovo, near Larderello, where a total covered area of about 3000 m 2 is heated by means of natural steam (BENvENUTI 1961). This plant, started in 1956 when the steam field was exploited by Larderello Co., could not have further development because in 1961, with the nationalization of the energy sources, ENEL, the National Electric Energy Agency, had to drop all those activities not strictly related to the production of electrical energy. Purpose of a pilot greenhouse When a natural heat source is available, no relevant problems are encountered in heating a greenhouse if * lstituto Internazionale per le Ricerche Geotermiche del C.N.R., Pisa, Italy. ** Istituto di Fisiea Teenica del Politecnico di Milano Italy. (1) In 1963 the total area covered by greenhouses in USSR was 20,000,000 m2 (A~'ro~tm 1963). (~) The main reason for this low interest is the mild climate of Italy; in many sites ,~here hot water is available, the need of a heated glasshouse is reduced to very short periods of time. 918

the thermal content of the source is high. The same cannot be true when low temperature water, i.e. water at a temperature from 30°C to 70°C, is available. In this case the heating system should be carefully designed, so as to extract all the possible h e a t from the source: to achieve this result, soil heating can be coupled to space heating. As far as we know, only in the USSR has attention been paid to heating with low temperature waters (ANTONIOK 1963). It seems however that only particular cases have been considered and that no attempt has been made to generalize the study of the subject. In fact the number of the sources with water at a temperature not really high, but still not so low as to be lost away, is tremendous all over the world. It is thus worth-while studying which are the possible conditions that can be obtained at different temperatures of the heating water and with different combinations of heating systems. The aim is that of determining the best working conditions for a given temperature of the available source, and the limits within which the. different cultivations are possible. This should lead to the construction of low-cost modular greenhouses.

Description of the heating system Figures 1 and 2 show the scheme of the heating system of the greenhouse and the layout of the underground pipes. The elements of interest are: Heat exchanger (S). Natural steam is used to heat the water (in a closed loop) at the different desired temperatures. m Unit heaters (A). Three of these units are employed for space heating, they have been considered to allow the possibility of varying the room conditions within large limits; in future modular greenhouses these units will be replaced by finned tubes. Polyethylene pipes (UP). Two series of these pipes are used for the underground heating of the soil. The pipes can be connected in series (with the possibility of having different temperatures in two parts of the ground at the same time) or in parallel (same temperature all over the ground). There is also the possi-

TRS

"

it Steam inlet

c

s

I t

Water

', t-A

outlet

__d

1---%

A =Unit

Heaters

t

C =Water

Meters

G -Float

Valve

P =Circulating S =Heat

Pump

Exchanger

=

Thermometer

T = Temperature TRS= T h e r m o s t a t

Transducer

UP = U n d e r g r o u n d Polyethylene Pipes VE = Expansion Tank

VEC=Motor

Operated

Valve

FIG. I -- Pilot greenhouse. H o o k u p of the heating system.

bility of totally or partially excluding the heating system of the soil. - - E x p a n s i o n t a n k (VE). The water coming from the underground pipes is collected in this tank. A float valve G, connected to a water inlet, secures that a minimum quantity of water is always flowing in the system.

(TRS). It allows the automatic control of the water temperature at the outlet of the unit heaters. --Thermostate

- - C i r c u l a t i n g p u m p (P). It gives the necessary head for the water circulation. After the pump, a bypass allows the recirculation of part of the water; in this way it is possible to modify the temperature of the water entering the underground pipes.

(VEC). This valve is automatically operated by the thermostat; by increasing or decreasing the flowing quantity of heating steam, the water temperature is changed. --

Motor operated valve

(C). These allow the metering of the water flowing in the main system and in the by-pass. --

Water meters

- - M u l t i c h a n n e l r e c o r d e r . This allows the automatic recording of the temperatures of interest (room, external, soil, etc.). - - V a l v e s . These are gate valves distributed in the system for flow regulation.

Outline of the p r o g r a m m e d m e a s u r e s As said in the second paragraph, the purpose of a pilot greenhouse is that of obtaining a number of results to be used for the construction df low-cost modular units. Particular attention will be paid to the fdllowing points: - - importance of the soil heating to the greenhouse efficiency, - - heat exchange between polyethylene pipes and soil, - - r e l a t i o n between soil mean temperature and minimum and maximum water temperatures that allow an efficient heating of the greenhouse, 919

21.. 20

l,.I0

20. I0

r-~e

water feed

IL

i

71

I{

'I

j1

I U

potyethylene pipes

Sect.

FIG. 2

~

8B

Pilot greenhouse. Layout o/ the underground heating pipes.

amount of heat to be given directly to the soil for the different greenhouse cultivations, limit soil .temperatures that allow the best efficiency of the greenhouse,

REFERENCES ANTOr~UK A. A. 1963 -- Use of geothermal waters in agriculture (in Russian). USSR Acad. Sci., Moscow. B e N v ~ t r n A. 1961 w U n esernpio di utilizzazione di energia termica di netura endogena nel campo agrario: I'Azienda Orticola delia Soc. Larderello (Pisa). L'Agricoltura Its.

liana, 6.

minimum temperature and minimum water flow rate necessary for heating a greenhouse that can be compared with the conventionally heated houses.

920

BJ6~SSON

Rev.

S. 1966 -- Hot springs and thermal energy. Iceland

LI~AL B. 1961 - - Green-houses by geothermal heating in Iceland. U.N. Conf. New Sources Energy, Rome.