Latent heat storage applied to horticulture

Solar Energy Vol. 28, No. 4, pp. 313-321, 1982 Printed in Great Britain.

0038-092XI821040313-09~3.0010 © I982 Pergamon Press Ltd.

LATENT HEAT STORAGE APPLIED TO HORTICULTURE LA B A R O N N E SOLAR G R E E N H O U S E ANDRI~JAFFRINand PASCALCADIER Centre de Recherche B~timents Solaires, B.P. 21-06562 ValbonneCedex, France (Received 27 March 1981;revision accepted 5 August 1981) Abstraet--A new conceptof solar heat storage applied to horticulture has been tested througha full heating season in a professional greenhouse. This experiment is run in a 500m2 multi-span, single-glazedgreenhouseof traditional geometry devoted to rose production. The solar heat available inside the greenhouse is transferred and stored by recyclingthe air through an underground network of flat heat exchangers filled with a phase changematerial. In an attempt to reach quasi autonomy in a mild climate, the glass is doubled on the inside with an insulating polyethylene Air-Cap(R) film. The solar greenhouse compartment is compared with a traditional greenhouse compartment of identical geometrybearing the same plantation. The cover of this control compartmentis single glazed. Thermal and cultural performanceswere analysedfrom December 1979to April 1980.The solarcompartmentachieved80 per cent savings in propane gaz, compared with the control compartment run at the same temperature. Compared with a control compartment doubled with Air-Cap polyethylene, the savings would reduce to 60 per cent. The electrical consumption of the fans of the solar system amount to less than 10 per cent of the basic heating loads. The solar prototype cost twice as much as the control compartment which compares favorably with other solar prototypes known at this time.

INTRODUCTION

The concept of energy conservation is now being introduced in most human activities, even in those which do not use large fuel consumptions. Agriculture and horticulture provide a typical example: only 1100 ha (2700 acres) of greenhouses are heated in France, and they use 3.10Stons of fuel each year; but the ability of French producers to maintain their activities in spite of a strong foreign competition, counts much in terms of national trade deficit in that field. In a first stage, starting in 1973, most efforts have been devoted to reduce heat losses in existing greenhouses: this was done mainly by doubling the glass covers with polyethylene on PVC plastic films on the inside. These films reduce the heat loads for several reasons: air leaks are reduced, the heated volume can be restricted, convective and radiative losses now take place through a double barrier. (Even a polyethylene film acts as a radiative barrier thanks to the water condensation on its inner surface.) At the same time new materials were introduced in the construction of greenhouses: horticol glass is now available with a non emissive coating, double layer covers made of acrylic, PVC, or polycarbonate can be used in place of glass. New frames and new geometries were designed to take advantage of the large dimensions of such plastic covers. But all of these improvements only concern the reduction of heat losses in the case of artificial heating. None of these contribute to making a more efficient use of the solar radiation incident in the greenhouses. Greenhouses started being real solar collectors with the introduction of a thermal storage. Two groups of solar system emerge: liquid systems and air systems.

Liquid devices generally make use of semi transparent collectors; they take advantage of the large heat capacity of the storage tanks, and of the good heat exchange coefficients between liquids and solids, while air systems require no collector at all, but large surfaces of exchange and bulky storages; and therefore are often restricted to plant nurseries. The prototype solar greenhouse discussed in this paper belongs to the second group. It has the property of being adapted to plantations in deep earth because its thermal storage, using latent heat materials, occupies a restricted volume located underground. 1. THE ORIGIN OF THE PROJECT

The problem of low temperature heat storage has been the main concern of our CNRS solar energy laboratory since its creation in 1977. A promising direction had already been taken with the use of stabilised calcium chloride hexahydrate[1] as a storage medium for solar heated home spaces. It was a challenging idea to extend this technique to other domains, like agriculture, where large glazed areas cannot efficiently collect solar energy for lack of low temperature heat storage. Conditions for a full scale experiment experiment were met when the research and development center (CREAT) of the Departmental Chamber of Agriculture in Nice decided to build a new facility in La Baronne. PIRDES, the solar interdisciplinary branch of French CNRS agreed to finance the solar overcost of a 500 m2 prototype greenhouse to be compared with a traditional greenhouse of same size and situation. All physicists and students from the Solar Energy Laboratory helped on the project and the experiment was ready to start in 313

A. JAFFRINand P. CAI)IER

314

winter 1979-80 with a new plantation of young Meilland Sonia roses. A large body of experimental results and technical remarks is now available on this innovative solar greenhouse after one year of CNRS-CREAT collaboration [2-6]. 7,. GENERAL DKqCRII~ON OF THE SOLAR GREENHOUSE

The 2000 m 2 ridge furrow greenhouse constructed by the Departmental Chamber of Agriculture in La Baronne, 15kin away from Nice, is part of a larger facility designed to promote new cultural technics among professionals. This rectangular greenhouse, shown in Fig. 1, makes use of traditional materials: galvanized iron frame and ordinary horticoi glass of 0.61 m width. Its orientation, characterized by ridges pointing east to west, contrary to the tradition, optimizes the optical transmission of the glass during the cold season. Fig. 2 shows the result of a computer simulation of the solar radiation transmitted through a glass cover having a 21° tilt from the horizontal. In addition, this orientation is consistent with a minimum cross section of the metal profiles in the directions corresponding to maximum solar energy in winter. (i.e. about 8 per cent of the total area). As a result this

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greenhouse transmitts, between November and February, 10-20 per cent more energy than it would do with the perpendicular orientation often chosen by professionals on the grounds of light uniformity. The whole surface is divided into four identical compartments by means of vertical polyethylene films, so that each compartment contains an identical succession of ridges and furrows from south to north. The solar and control compartments are the ones situated in the middle. The east and west compartments were not studied in the present experiment, but they were kept at the same temperature conditions. Each compartment is equipped with its own propane auxiliary heating system. 3. DESCRIFrlON OF THE SOLAR COMPARTMENT

3.1 Thermal insulation An ideal goal for a solar system is to achieve selfsufficiency on sunny days in winter. This implies that the heat losses through the transparent cover of the greenhouse and by air leaks, should not exceed normal solar gains. Among various solutions tested before the experimentation, we chose to use a double polyethylene film of type Air-Cap D120 (R), which involves encapsulated 12mm thick air cushions. The heat loss coefficient is an average 2.8 W/m2°K, compared with an average 5.5 W/m'°K for a single glazing: this is partly due to the convective barrier between the polyethylene layers, and partly to the supplementary infra-red radiative barrier (the inside plastic sheet is permanently covered with water condensation). In conclusion the solar compartment heat requirements are reduced by half, compared with the control compartment, for the same temperature. The doubling has its draw-back on the optical characteristic of the cover: laboratory measurements on already aged materials showed that the light transmission coefficient drops down from 85 per cent for a single glazing to roughly 72 per cent for one glass and a double polyethylene layer, in case of dry surfaces. The water condensation is responsible for additional optical reflections and a lower transmission at large incident angles. But, as it was said before, orienting the roof slopes to the south minimizes the negative impact of the doubling in winter. 3.2 Thermal storage The thermal storage created inside the volume of the solar greenhouse must be able to accumulate and return a quantity of heat of the order of 1 kWh/m 2 (here a total of 500 kWh). This is achieved partly (60 per cent) through phase change processes taking place in 13.5 tons of calcium chloride hydrate, and partly through the heat capacity of the soil. In order to be able to maintain acceptable temperatures in the closed greenhouse during sunny days, the original calcium chloride hexahydrate, chliarolithe[l], melting at 28°C, was modified by a well defined excess of water so that the phase change takes place between 25°C (complete fusion) and 15°C (halfsolidification). The latent and sensible heat of the material add to a total of 37.103 kcal/m 3.

Latent heat storage applied to horticulture

minute during the 1979-80 winter. These parameters are:

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The latent heat storage consists of 9000 flat bags of I dm 3 capacity each, weighing 1.5 kg. These bags, made of a polyester-aluminum-polyethylene complex are sealed to be air-tight. They are distributed on 500 m2 of horizontal asbestos-cement shelves, piled up on five levels in semi cylindrical concrete tunnels, buried in the ground (Fig. 3). The tunnels are covered with reinforced concrete slabs which constitute convenient parallel paths between the rose lines. On the whole, 60 tons of concrete, and up to 75 tons of surrounding soil add their contribution to the latent heat material to take care of the daily incident solar radiation in winter. 3.3 Heat trans[er Warm and humid air, resulting from radiation on the ground and on the plants is extracted during the day from the top of the ridges in the completely closed greenhouse and run through galvanized iron ducts to the storage tunnels. Each of the 8 centrifugal fans absorbs 500Watt, and blows 1500 m3/hr of air under a head loss of 200 Pascal, to 4 storage tunnels, each 8 m long. Altogether they achieve an internal circulation rate close to 8 volumes of air per hour. The heat exchange from the air to the storage takes place on both sides of the bags set on the shelves, and in addition on the inner surface of the concrete tunnels. The inner surface of the insolated horizontal slabs, on the contrary, tends to heat up the air along its path. After having gone over 4 × 4 4 m 2 of storage exchange surface, this stream of air is re-injected to the greenhouse volume through grids, and undergoes a new cycle. As the sun sets down, or when the internal temperature drops down, a thermostat switches off the fans. Then the slabs, heated by natural convection from the PCM bags, radiate and convect part of the heat to the greenhouse volume, while another part is conducted through the soil, raising the ground temperature around the roots. This is a passive way of heating the greenhouse; an active mode can be selected by operating the fans during the night, in order to force the air through the storage: the rate of heat transfer is then multiplied by a large factor. 4. INSTRUMENTATION

Many physica] parameters were recorded minute by

--various air temperatures (4 probes in the solar compartment +3 probes in the control compartment); --ground temperatures (4 + 3 probes); --storage temperatures (3 probes); --outside temperature (1 probe); --solar radiation inside the solar compartment on a horizontal plane. Outside radiation measurements are available from a meteorological station near the greenhouse. In addition, episodic measurements of glass and plastic temperatures, and of humidity content of the air, at the input and the output of the storage tunnels, were performed. The culture itself is run with the help of mechanical temperatures and humidity recorders distributed in each compartment. 5. GLOBAL RESULTS

The heated period started only in late 1979, because of delays in the construction of the gas heating system. The experiment started simultaneously in both compartments with night temperatures gradually rising to 15°C. All heating systems were stopped in early April; therefore the present experimental campaign refers to a heating season shorter than usual: 100 days instead of 145 days. 5.1 Meteorological data January 1980 was characterized by temperatures close to normal, but 15 per cent less solar radiation than the last decade average: February received I 1 per cent more solar radiation than average and night temperatures were 1-2°C higher than usual. Conditions in March were close to the average. As a whole, the 197%80 winter season appears to have been relatively mild. Figure 4 shows the weather conditions (mean night temperature and daily solar radiation) and the average temperature recorded at night in both solar and test compartments during a typical winter period. Figure 5 shows the gas (burners) and electricity (solar fans) consumptions during that same period. The day-by-day record is shown in Fig. 6 in terms of percentage of economy, for the complete experimental period: --The percentage of economy increases gradually during the first month, and levels off afterwards; --the electric consumption of the solar compartment is small at the beginning, but ceases to be negligible in February and March. 5.2 Savings in heating costs As a whole, an 80 per cent savings in gas was achieved during that shortened season, and 75 per cent in terms of total energy (gas + electricity). During the first month, the savings did not exceed 70 per cent because of special circumstances: the soil was still cold in the first operating days, and part of the energy transferred from the greenhouse volume to the underground storage was used to heat up the deep soil, thus was not immediately available at night; this transient period lasted about 2 weeks, in the absence of

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forced ventilation at night. As soon as the soil temperature exceeded the air temperature, a larger fraction of heat stored in the ground was able to be returned. The relative importance of electric consumption increased as the days increased in length, and when the night heat loads decreased: this becomes obvious after 12 February, as a semi-active restitution mode was

tested by an intermittent storage ventilation at night. It must be mentioned, though, that the electrical consumption of the solar fans is partly compensated by a reduced consumption of the fans of the auxiliary heating system. As it is mentioned above, the heat loss coefficient of the glass + Air Cap film cover is roughly half that of a single glass cover. Therefore the savings performed by the

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solar greenhouse, compared with a similarly insulated one amounts to 60 per cent in terms of gas alone, 50 per cent in terms of gas + electricity. Another way of presenting the efficiency of the solar system is to calculate a coefficient of performance of the fans, considered as heat pumps: the resulting COP is 7.5. 6. DAILY ENERGY BALANCE

The processes by which the solar energy is collected and stored, then recovered, are analysed in details from the temperatures and radiation recordings. A few typical days will be examined. 6.1 24 December 1979; sunny day The heating period had just started a few days before and the ground is still relatively cold in both compart-

ments. Figure 7 shows that the temperature of the solar compartment remains lower than that of the test compartment, in spite of its lower air renewal rate; this results from the heat transfer to the cold storage. The air blown into the storage at 18-22°C warms up the PCM bags and the concrete sides initially at 14°C, and escapes from the tunnels 3-5°C colder; it loses sensible heat and part of its water content as long as the initial relative humidity stays above 75%. During the night, the auxiliary air heaters operate intermittently to maintain the temperature above a preset value (13°C). They create periodic heat pulses of large amplitude. As a consequence of the heat flux gradually emitted from the PCM storage and from the ground to the greenhouse volume, the auxiliary heater of the solar compartment requires only

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20kg (=250kWh) of propane, compared to 69kg (= 860kWh) in the control one; thus the savings in gas amounts to 71 per cent for that day (50 per cent due to the polyethylene insulation, 21 per cent due to proper solar energy). The electric consumption of the solar fans is 25 kWh, but 8 kWh were saved by a shorter operating time of the auxiliary air heater• The energy balance of that particular day is shown on Table 1. The difference between the solar and electric gains and the heat losses during day time gives the energy stored: 250kWh (0.5kWh/m2). The temperature

recordings show that 200 kWh are stored in the storage medium (PCM+concrete tunnels), and 50kWh are stored into the ground, either by direct radiation on the soil surface, or by conduction from the tunnels. The night time heat load (450 kWh) is partly taken in charge by the auxiliary heater (56 per cent) and partly by the storage return (44 per cent). In addition, the storage retained an extra 60 kWh at the end of that 24hr period, enabling the average temperature of the PCM and of the ground to slowly warm up during this initial sequence of days.

Table 1. Energy balance of the 500 m2 solar greenhouse. 24--25December 1979 Useful solar energy

425

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Latent heat storage applied to horticulture 6.2 18 February 1980, sunny day Starting on 12 February, we tested an active way of heat return by an episodic ventilation at night, with the purpose of accelerating the heat transfer from the storage to the heated volume. Figure 8 shows the air temperature recorded in the solar greenhouse and in the control, Fig. 9 shows the temperatures in the ground and Table 2 gives the energy balance for that same day. The storage medium returned 60kWh more than it stored; this is due to the night ventilation: the average status of

the storage tends to a lower value, which allows a higher heat transfer during the day-time ventilation, and increases the storage capacity. Since the heat load was quite low during that particular day, the electricity consumption was relatively high, so the savings, in regard with the test greenhouse, does not exceed 75 per cent, although the propane burners did not operate. As the days became milder, the fans were run later in the morning to prevent storing more energy than needed for the night.

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6.3 19 February 1980; rainy day During a rainy day, the fans do not operate, and the storage is not ventilated. We see, though, that the gas savings is still high on 19 February: 80 per cent. The PCM storage alone cannot provide that extra heat restitution. In fact the temperatures recorded in the ground (Fig. 10) show that it cools down slowly by an average 2°C during the night, contributing to about 60 per cent of the heat requirements in the solar compartment. This phenomenon proves that the energy stored in the PCM and in the soil can easily exceed the requirements of a single night, and that the greenhouse is in fact equipped with a two-day-storage. It is legitimate to try to separate the contribution of the PCM material from the contribution of the ground. This require to analyse the recorded data by means of a 2-dimensional model representing the heat conduction in the ground. This was done on a computer with a finite

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Latent heat storage applied to horticulture the relevant boundaries were taken from the data recorded on 18 February. The resulting ground heat content as a function of time is shown in Fig. I I for both compartments. It is clear that twice more heat is stored and returned in and from the ground of the solar compartment, although the available ground area was reduced. This leaves room for simpler types of heat storage, based on the sensible heat of the ground itself, making difference equation method, both for the geometry of the solar compartment (presence of tunnels) and for the simpler geometry of the control compartment (semiinfinite medium). The temperature conditions applied to use of well designed air to ground heat exchangers. Such a system is now under study in a greenhouse used by a producer to grow vegetables.

CONCLUSION La Baronne solar greenhouse is a unique example of flower producing greenhouse making an extensive use of phase change materials as a heat storage. The results obtained during a full heating season proved the efficiency of this concise underground thermal storage and showed that a properly insulated greenhouse can reach autonomy in the mild climate of southern France. Little effect on the flower output was noticed in spite of the slightly peculiar cultural conditions. The experiment is now being pursued on a long term basis to verify the stability of thermal performances over successive years, and to analyse the potential dangers for the culture of the specific conditions which favors the heat transfers: nearly saturated atmosphere, wet and

conductive soil, large temperature swing between day and night. With a 100 per cent solar overcost, the economical aspect of this prototype solar greenhouse is not yet very attractive for professional producers. Furthermore, the PCM bags developped for this experiment are not commercially available at this time, because they do not represent the optimum combination of latent heat content and fusion temperature. Efforts are now being made to find some better adapted phase change material with a lower fusion point. REFERENCES

1. ANVAR-CNRS patent No. 7913296. M. Schneider, J.-D. Sylvain, A. Jaffrin, X. Berger and L. Bourdeau, Mat6riau accumulateur de calories A temp6rature constante et applications de ce mat6riau (1977). 2. X. Berger, L. Bourdeau, P. Cadier, A. Jaffrin, P. Bergeaudand P. Guglielmi Prototype de serre horticole solaire. Proc. Int. Solar Congress, Nice, France, 24--26Oct. 1979. 3. P. Cadier, L. Bourdeau, X. Berger and A. Jaffrin, Agricultural greenhouse using latent heat storage. Proc. lind Int. Syrup. Solar Energy, Fundamentals and Applications. Izmir, Turkey, 6-8 Aug. 1979. 4. P. Cadier, A. Jaffrin, P. Bergeaud and P. Guglielmi, Performance of La Baronne solar greenhouse. Proc. 3rd Int. Solar Forum, Hamburg, Germany, 24-27June 1980. 5. P. Cadier and A. Jaffrin, Winter performance of "La Baronne" latent heat solar greenhouse. Proc. PCL Int. Seminar on The Use of Solar Energy in Agriculture. London, 18 Sept. 1980. 6. P. Cadier, R6alisation, exp6rimentation et mod61isation du chauffage solaire d'une serre horticole :~stockagepar chaleur latente. Th6se de 3 6me cycle. University Pierre et Marie Curie. Paris (1980).