First experimental results from a prototype plant for the interseasonal storage of solar energy for...

Solar Enerm Vol. 62, No. 4. D!J. 281-290, 1998 _. 0 1998 Eis&ier Science Ltd Pergamon

PII: SO038092X(98)00011-5 All rights reserved. Printed in Great Britain 003&092X/98 $19.00+0.00

FIRST EXPERIMENTAL RESULTS FROM A PROTOTYPE PLANT FOR THE INTERSEASONAL STORAGE OF SOLAR ENERGY FOR THE

WINTER HEATING OF BUILDINGS

G. OLIVETIS, N. ARCURI and S. RUFFOLO Department of Mechanical Engineering, University of Calabria, 87030 Rende (Cosenza), Italy

Received 3 February 1997; revised version accepted 4 December 1997

Communicated by DOUG HITTLE

Abstract-The first experimental results are presented regarding the workings of a plant which uses interseasonal heat stored solely from the sun as an energy source. The results obtained over one year concern radiation collection, the behaviour of the storage system and the satisfaction of the thermal requirements of a building to be heated. The comparison of performance obtained with those predicted by a plant simulation model provided a confirmation of the reliability of the models used for collection and storage tank accumulation. For the latter, the validity of the method used for calculation of heat losses that is the basis for the design of this kind of plant is shown. 0 1998 Elsevier Science Ltd. All rights reserved.

1. INTRODUCTION

The energy requirements needed for the winter heating of buildings can be supplied in part or in whole by solar radiation, using plants based on interseasonal heat storage. Either a water mass or a volume of ground is usually used as the means of storage. Significant examples of plants which use a water mass contained in a tank as a storage system and supplied with an integrative energy source have been achieved in Sweden (Grabrielsson, 1988) and The Netherlands (Bokhoven and De Geus, 1996), and are at present being completed in Germany (Fisch and Dalenblick, 1996).

An experimental prototype plant for study of the interseasonal storage of solar energy has been built at the University of Calabria. The plant uses solar collectors to heat a thermal storage system using an underground water tank, which ensures the total energy supply needed for the winter heating of a building.

This article presents the experimental results relative to a year’s operation of the plant, regarding radiation collection, energy storage, the heating of the building, and moreover, the first comparisons with a simulation model of the plant.

More detailed information about the building of the plant may be found in a previous article (Oliveti and Arcuri, 1995), in which the plan- ning methodology used for the design is shown,

*ISES member.

along with plant performance evaluated by a simulation model, the measuring system installed, the data acquisition system to obtain instantaneous and overall balances, and the procedures to make operation automatic.

Essentially the plant is made up of 91.2 m2 of vacuum-type heat pipe solar collectors; a 500 m3 storage system, in cylindrical shape with a flat bottom and spherical dome, made of reinforced concrete and insulated on the interior surfaces with 0.20 m of foam glass; and a 17.50 m3 building given to offices, which requires 111 GJ for its winter heating. The collectors are linked through a primary loop to the storage system, from which a secondary loop serves to withdraw the energy to supply to the building. A simplified scheme of the plant is shown in Fig. 1. A water mass is used as a means of storage whose temperature varies in the operat- ing conditions foreseen for the plant between 80°C at the start of the heating period of the building and 40°C at the end.

Calculation of the storage volume and of the collector surfaces was made by means of the overall thermal balance equations of the storage tank written with reference to the charging period and the subsequent energy withdrawal period (15 November-31 March). During the charging period, energy introduced is in part stored and in part lost to the outside; during the withdrawal period, introduced energy sum- mated to energy supplied of the storage tank is in part transferred to the building and in part

281

282 G. Oliveti ef al.

- I- Building

I\, i r4v

C-- ‘Collecting Area 9 I .2 mz

Underground Water Tank

Storage Tank Volume 500 m3

Building Volume 1750 m3

Fig. 1. Simplified scheme of solar plant.

lost. Energy coming from the solar collectors and introduced to the storage tank was calculated starting from incident energy since the average collection efficiency is well-known. Energy lost from the storage tank was calculated by multiplying the storage tank’s loss factors by the reference energy Mc,(T,- T,), with M the water mass contained in the storage tank, cP average specific heat, T, storage tank reference temperature, and Tg undisturbed ground temperature. Evaluation of average collection efficiency and of storage tank loss factors was obtained by means of an iterative procedure. The various steps of the procedure and the calculus codes used were shown in previous articles (Cucumo et al., 1985; Oliveti and Arcuri, 1995).

Once the plant was designed, its performance was calculated with a simulation code called COSIMI (Cucumo et al., 1984, 1985).

This code outlines the behaviour of the different solar plant components and of the building, and allows verification of the correct plant design. It should be pointed out that in the mathematical model of the storage tank the water mass was taken to be completely mixed. This simplification was also adopted in the models used for the design of the collector’s surface and of the storage tank. The simulation of the plant created by using climatic data of the reference year and with the storage system in periodically stabilised conditions gave a 55% average yearly value for collector efficiency, a

47% average efficiency for the storage system and an overall plant efficiency of 25%.

The experimental data reported here was taken between 1 May 1995 and 30 April 1996 and subsequently elaborated on an hourly, daily and monthly basis to determine the performance carried out during collection, storage and supply of energy.

The main interest of the paper lies in the analysis of the experimental results obtained seen in the light of their time trend: the record- ing of the quantities inherent to the thermal balance of the different plant components and to their subsequent simulation by means of a calculation code allowed comparison of the time trends, both predicted and measured, of transferred thermal power and of accumulated energy. For the accumulator in particular, which certainly is the most interesting and least well- known component from the thermal point of view, verification of the thermal balance equation provided confirmation of the method used for the calculation of heat losses, that is basic to the plant design.

In the following paragraphs the experimental and numerical results are presented in detail.

2. RADIATION COLLECTION

The collectors are made from 40 panels laid out in three rows and connected by a single pipe network. Each panel is obtained by assem- bling 14 heat pipes in parallel. The flow is

Interseasonal storage of solar energy for the winter heating of buildings 283

withdrawn from the lower part of the storage tank and introduced into the upper part so that the thermal stratification effect favours energy collection. The procedure used for the automatic operation of the plant has been described in a previous paper (Oliveti and Arcuri, 1995).

The collectors are able to collect significant amounts of energy even in low it-radiance conditions. This ability led us to evaluate the duration of insolation on the collector’s surface starting from a threshold value of 10 W m-‘, set with reference to the measurement limit of the pyranometer used. Energy during collection was determined by considering incident energy during the on time of the pump.

Figure 2Fig. 3 show examples of measured

80

60

40

20

0

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 time (hours)

Fig. 2. Incident power, useful power, collection efficiency and outside air temperature on a clear day (24 May 1995).

A=91.2m2; Ti=19.7”C.

I P NJ’) ----------

80

60

40

20

0. I I I I 7 8 9 IO II 12 13 14 15 16 17 18

hours

Fig. 3. Incident power, useful power, collection efficiency and outside air temperature on a cloudy day (24 January

1996). A=91.2m2; Ti=37.1”C.

SE 62-4-B

quantities in the course of collection, incident solar power, useful power, collection efficiency, and outside air temperature. Figure 2 refers to a clear day (24 May 1995) throughout which performance remained noticeably constant at around 59% if the start-up phase is excluded. Figure 3 instead refers to a cloudy day (24 January 1996) in which, for greater clarity, average values calculated every 10 min are shown. The figure shows that collection stopped owing to insufficient irradiance, and as soon as favourable conditions occurred, it started up again.

Figure 4 shows daily values obtained in February for incident energy during collection, for useful energy and for average efficiency. The figure shows how collection did not activate on two days for lack of sufficient irradiance, for all other days average efficiency was slightly variable at around 56%.

The results obtained in collection for all the months of the year are summarised in Fig. 5, by incident energy values, by incident energy during collection, that is, collectable energy, and by useful energy. Moreover, insolation and collection duration is shown. Overall, in the 4003.5 h of insolation, the incident energy was 5.78 GJ rnm2, collection totalled 2921.5 h during which incident energy was 5.53 GJ rne2. Due to insufficient irradiance and to a lack of sophisti- cation in the primary loop control system, 4.4% of available energy was lost, Useful energy collected in the year was 2.95 GJ rnm2, with a 53.3% average collection efficiency. The average duration of daily collection was 8 h, with values ranging between 11.1 h in May and 3.35 h in

I 8 15 22 days 29

Fig. 4. Daily values of incident energy (Ed), useful energy (Eud) and efficiency in February 1996. A = 9 I .2 m2.

284 G. Oliveti et al.

- F(h) ?? Em (MJm-*)

800 . . . ..____._..__.... Fc (h) fi!i Ecm (MJme2)

600

Fig. 5. Monthly values of incident energy (E,,,), collectable (EC,) and useful (.E,,,J, of insolation (F) and collection

(F,)

December. On 20 days energy collection did not take place.

Figure 6 shows the average monthly values in the form of average available, collectable and useful power, and moreover, average efficiency values and flow temperature at the collector’s inlet. The available average yearly irradiance was 401 Wm-‘, irradiance during collection 526 Wm-‘, with variable values between 622 W mm2 in October and 408 W m-2 in December. Useful monthly power was 280 W m-‘, with values ranging from 310 to 209 W m-‘, respectively obtained in October

600 60

400 40

200 20

0 0

ti z L:

Fig. 6. Avcragc monthly values of incident power (P,), col- lcctablc power (PC,) and useful power (P,,). of inlet flow

tcmpcraturc CT,,,,) and of cllicicncy (q,,,).

and December. Average monthly efficiency attained was variable between 59.0% in May and 47.2% in November. These extreme efficiency values were obtained by supplying the collectors with average temperatures, respectively of 18.1”C and 62.O”C.

3. ENERGY STORAGE AND SUPPLY

The 450,000 kg water mass contained in the storage system feeds the primary loop of the collectors and the secondary energy supply loop without the use of heat exchangers.

Heating of the storage tank began on 1 May 1995 with a starting temperature of 11.7”C, and energy withdrawal for the heating of the building began on 15 November 1995 with the storage tank at an average temperature of 71°C. Energy supply continued until 5 March 1996 when the average temperature of the storage tank had dropped to 30.7”C. The lack of heating for the remaining days of March was caused by the lower starting energy available in the storage tank, since the period 1 May 1995 to 14 November 1995 was too short to raise the initial water mass temperature from 11.7”C to the necessary level of 80°C.

Figure 7 shows the average temperature history of the storage tank. The highest value reached was 71.6”C, obtained on 31 October and the minimum on 5 March. At the end of the first working year (30 April 1996) storage tank temperature was 45.5”C; in the second year by 31 July it had reached 69.3”C, higher than the 55.4”C reached the previous July.

Thermal stratification phenomena in the stor-

Fig. 7. Yearly trend of storage tank tcmpcrature at two dilrcrcnt lcvcls and of average tcmpcraturc.


age system should not be overlooked. A probe with instrument placed on the accumulator axis allows water temperature measurement at five different levels. The internal energy of the storage tank and its average temperature was calculated by subdividing the volume occupied by the water into five partial volumes. For each volume, the relative mass was supposed to be at even temperature and the same as that supplied by the corresponding heat sensor. Figure 7 also shows the temperature history recorded on the storage tank at 5.32 m and 0.07 m from the bottom. The greatest temperature difference produced by stratification was 15.2”C which was found on 15 October, while the minimum at the end of withdrawal was less than 5°C.

Temperature measurement at different levels in the storage system and calculation of energy introduced and withdrawn enabled a definition of the monthly storage tank energy balance. Figure 8 shows a comparison for all months of energy introduced, energy withdrawn, variation of internal energy and energy lost. This loss was calculated indirectly as the difference between net entering energy and internal energy variation of the stored mass. The same figure also shows the average temperature record for the storage tank. Monthly losses from the storage tank have a trend similar to that of the average temperature: increasing up to a maximum in November at 17.67 GJ, subsequently reducing

30

0

II II

u II I

Fig. 8. Monthly storage tank balance: introduced energy (E,,), withdrawn energy (E,,), lost energy (E,,,,), variation of internal energy ((I- (I,),, average temperature trend

(T,).

to 4.74 GJ in March, when it begins to increase again.

Table 1 gives a summary of results obtained in the three periods: loading only (1 May to 14 November), loading and withdrawal (15 November to 5 March) and for the whole year (1 May to 30 April).

Energy introduced up to 14 November was 175.6 GJ, the amount stored was 64%, and the remaining 36% was lost. In the withdrawal period, energy supplied to the building was 78.3 GJ and energy lost 41.0 GJ, for a total 119.3 GJ. Of this energy 35.5% came from the collectors and the remainder of 64.5% from internal energy variation. Finally, the yearly storage tank energy balance is defined thus: energy introduced 257.2 GJ, fraction used 30.4%, lost 44.6%, stored 25.0%.

As far as heating of the building is concerned, Fig. 9 shows an example of the hourly history for power and energy supplied obtained on 11 January 1996. The heating began at 7:45 and continued until 18:30. The same figure shows the hourly outside and internal air temperatures. It should be pointed out that in the first 15 min of heating the storage system provided notably higher power than that needed in other hours of the day, with a rapid increase in inside air temperature. Figure 10 shows the histogram of daily energy supplied in February 1996 with relative values of average outside air temperature during the hours of heating.

4. YEARLY ENERGY BALANCE OF THE

PLANT

Experimental results determined of the energy balance of the system, solar collectors-storage tank-building, after the first year of operation; see Table 2. In outlining this balance, the energy required for the working of the pumps was considered negligible, and moreover, losses along the secondary loop were incorporated into the energy supplied to the building. The initial internal energy of the storage tank was calculated at 11.7”C. Incident energy during collection was assumed as reference energy in the balances.

Since the storage system does not return to starting temperature at the end of the period under consideration, but offers a greater amount of stored energy, the average yearly efficiency of the storage tank was calculated with the


Table 1. Storage system balance

1 May 95-14 November 95 15 November 95-5 March 96 1 May 95-30 April 96

(GJ 1 (GJ 1 (GJ) G

Energy introduced 175.6 42.4 257.2 Energy withdrawn 78.3 78.3 30.4 Energy lost 63.1 41.0 114.7 44.6 Variation of internal energy 112.5 -76.9 64.2 25.0

150

Pb, E

125

100 . . . . . . . . . . .._.._.__ Tai 18

7s

16 SO

25 14

0 12

8 9 IO I1 I2 I3 14 IS I6 17 I8 hours

Fig. 9. Hourly trend of average power (Pb) and energy (E,,) supplied to the building. Hourly profiles of inside air temperature (rai) and outside air temperature (T,,). 11

January 1996.

2 ?? 20

GJ) - Ebd

. CC)

1.6- - Taod

-16

1.2 12

0.8 8

0.4 4

0 0

I 3 5 7 9 11 I3 I5 17 I9 21 23 25 27 29 days

Fig. 10. Daily energy supplied (E,,,,) for heating of the building and average temperature of outside air (Taod). February

1996.

relation:

~=~E,+(u-u~)+~E,

The numerator represents useful energy, the sum of withdrawn energy CE,, and internal energy variation (U- U,); the denominator rep-

resents energy introduced, obtained by summat- ing lost energy CE, to the two previous contributors. The overall efficiency of the plant, calculated as the ratio of useful energy and available energy during collection, was 28.2%. It can be assumed that, because of the increase in the thermal level of the storage tank in the second year, overall efficiency will decrease owing to the greater losses from the storage tank and the collection system.

5. COMPARISON WITH THE SIMULATION MODEL

The results obtained in this first year of plant operation were compared with those supplied by the simulation code COSIMI.

The COSIMI code resolves a system of equations, written in implicit finite difference form, of the instantaneous balance of the collectors, of the primary loop, of the secondary loop, of the air contained inside the building and of its outer walls (Cucumo et al., 1985). Collector inertia was overlooked for the balance equation of the collectors. The system of equations was solved by taking into account the conditions imposed for controlling the primary and secondary loops (Oliveti and Arcuri, 1995).

The mathematical model of the storage tank models the water mass with a single node set at average temperature and the surrounding ground as an infinite conductive medium in both axial and radial directions. The storage tank model does not take into account thermal stratification phenomena of the water. In the constructed plant, the condition of the infinite ground around the storage tank was not obtained, since the water mass loses heat to the outside air through the spherical covering dome of the tank. This element of heat exchange was counteracted by placing another layer of insulation on the spherical dome. For building details of the system see (Cucumo et al., 1985; Oliveti and Arcuri, 1995).


Table 2. Yearly energy balance of the plant. Energy measured and calculated and average efficiency

Collection Incident energy Incident during collection energy Collected energy

Storage tank Energy introduced Energy lost Energy withdrawn Variation of internal energy Useful energy

Total losses

Division of losses Collection Primary loop Storage tank

Yearly average ejficiency Solar collectors Primary loop Storage tank Overall efficiency

Experimental Simulated

(GJ) (o/o) (GJ) (%I

521.5 527.5 504.5 512.8 268.9 287.5

257.2 275.1 114.7 114.7 78.3 78.3 64.2 82.1

142.4 28.2 160.4 31.3

362.1 71.8 352.4 68.7

235.6 46.7 225.2 43.9 11.7 2.3 12.4 2.4

114.8 22.7 114.7 22.4

53.3 56.1 95.6 95.7 55.4 58.3 28.2 31.3

As input the code requires data regarding the geometry of the various plant components, the thermophysical properties of the different layers, the convection and convection-radiation heat exchange coefficients, the efficiency curves of the solar collectors, the working flow in the two loops, the average monthly hour values of beam and diffuse solar irradiation, and of the outside air temperature. Moreover, the initial node temperatures are needed, and the numerical vector that identifies the period and the daily duration of the heating of the building. For the ground surrounding the storage system, a thermal conductivity of 2.593 W m-‘°C-’ is assumed, a density of 2500 kg rnm3, corresponding to damp ground, and a starting temperature of 10.6”C. For insulation of the storage system, a thermal conductivity of 0.058 W m-‘“C-r was assumed.

Testing of the balances with the simulation code was done so that the energy needed for the heating of the building was the same as that measured experimentally. In this way the comparison of the calculated and measured perfor- mances was achieved at the same incident energy on the collectors and at the same energy supplied to the building, and the comparison was therefore limited to the solar collector-storage tank system.

In Fig. 11 the monthly collected energy was

Fig. 11. Monthly irradiation collection. Comparison of measured and calculated energy.

compared to that calculated by the simulation code. For the whole year the model totalled 2841 collection hours with 3.15 GJ me2 of useful energy collected, 6.9% greater than the value obtained experimentally, and an average yearly collection efficiency of 56.1% against the measured value of 53.3%. The lower energy collected by the plant can be ascribed to lower efficiency values than those of the efficiency


curve supplied by the construction firm. This has already been pointed out in a previous paper (Arcuri et al., 1995). Moreover, the collection model adopted is stationary and does not take inertia phenomena into consideration, which can cause reduction in collected energy especially in the winter months (Oliveti and Arcuri, 1997).

400

(GJJ 350 -

300 -

250 -

200 -

The comparison of average collection efficiency during the loading period and the subsequent unloading period of the storage tank gave the following results: loaded, 56.8% calculated and 52.7% measured; unloaded, 52.5% calculated and 53.2% measured.

Collected energy differs from energy introduced into the storage tank because of heat losses along the primary loop. These losses were about 4% of collected energy both in the experimental loop and in the numerical simulation.

0 so 100 I so 200 250 300 350 400 450 day

Fig. 12. Storage tank energy balance. Comparison of measured and calculated accumulated energy: Z&, ZE,, introduced energy; ZE,,, LE,, energy lost; CE, withdrawn.

Variation in internal energy: (U- U,),, (U- U,),. The experimental and numerical trends of the

totalled values for energy introduced into the storage system, for energy withdrawn, for lost energy and for internal energy variations, are shown in Fig. 13. The coincidence of trends for lost energy is due to the fact that in the numerical calculation of losses a compensation was made between the lesser losses recorded by the storage tank model and the increase in them produced by the higher average temperature of the storage tank, owing to greater energy coming from the collectors.

80

60

40

The experimental and numerical results of the accumulated values of energy introduced into the storage system, energy withdrawn, energy lost and variations of internal energy, are shown in Fig. 12. The average temperature of the storage tank experimentally obtained (curve a) and numerically obtained (curve b) are shown in Fig. 13. After a year the average temperature was 45.5”C, while the calculation code provided 55.3”C.

2Ir , (:I cy,ated,at yity :’ ~ introduced and withdrawn energy

0 so 100 IS0 200 250 300 350 400 450 days

Fig. 13. Average storage tank temperature trend. Comparison of measured profile and two numerically calcu-

lated profiles.

The parity of storage tank heat loss in Fig. 12 to non-coinciding temperature profiles necessi- tates research into the reliability of the model used to describe storage tank behaviour.

Figure 13 also shows average temperatures, numerically obtained, under the hypothesis that energy coming into a storage tank is identical to that measured in the plant (curve c).

end of the first year, reduction with respect to the experimental result was 10.4%, with a conse- quent increase in the average temperature of the storage tank from 45.5”C to 51.5”C. The average yearly thermal flux lost by the storage tank assessed experimentally was 10.6 W m-*, compared with a calculated value of 9.6 W m-*.

The comparison of the experimental profile The lower losses provided by the simulation (a) with the numerical profile (c) of storage model can be mainly put down to the modelling tank temperature, at the same energy received adopted for the ground surrounding the storage from the collector loop and at the same energy system to the purely conductive model of heat supplied to the building, shows that the storage transmission in the ground, to uncertainties in tank model underestimates energy loss. At the the estimation of thermophysical parameters of


the ground and of the thermal conductivity sponding figures were f,,=O.355 and of the insulation in use. f,=O.328.

The influence of this latter quantity is rele- vant. If in plant simulation energy coming into and going out of the storage tank is still kept the same as the measured values, and the value of the thermal conductivity of the insulation is increased by 10% (k=0.064 W m-‘“C-‘), after a year, a storage tank temperature of 49.4”C is obtained, with a difference of 3.9”C with respect to the experimental value.

6. CONCLUSIONS

Definitively, the raising of the temperature profile obtained in plant simulation in the recorded working conditions is due to the lower storage tank losses and to the higher energy coming from the collector field. The coincidence of energy losses in Fig. 12 is due to the effects produced by these two causes.

The experimental results obtained in the operation of the plant have shown that the collectors and the procedure adopted for control of the loop have enabled the collection of 53.3% of the incident energy, with an average daily collection duration of 8 h. The storage system was capable of utilising 55.4% of introduced energy, of which 30.4% was used to heat the building and the remaining 25.0% stored. The overall efficiency of the plant was 28.2%. The greatest losses were in collection, at 46.7%, whereas 22.7% occurred in storage.

In Table 2 the numerically obtained yearly balance was compared to experimental results. In percentage terms, the greater efficiency obtained numerically in irradiance collection and in energy storage leads to a 31.3% overall efficiency against an experimental value of 28.2%.

Simulation of the plant behaviour showed that collected energy, evaluated with collector efficiency curves, leads to an overestimation of 6.9% of measured energy, and that the storage tank model underestimates the heat losses: at the same introduced and withdrawn energy through the two loops, the reduction is 10.4%.

To better point out the thermal behaviour of the storage tank, Fig. 14 shows the experimental and numerical records of monthly calculated loss factors as a ratio of energy lost from the storage tank and reference energy Mc,(T,- T,), with T,=8O”C and Tg= 10.6”C. The comparison shows the different calculation of monthly losses. For the year under consideration, the average loss factor regarding the loading phase evaluated with experimental data was f,,=O.582 against the calculated figure of f’rc =0.609; for the unloading phase the corre-

The simulation model of the collector field- storage tank system supplied a heat loss overall, after a year, due to the two preceding causes, substantially the same as that obtained from the experimental plant. The loss coefficients during the charging and withdrawing phases are also in very good agreement with the values supplied by the plant.

Greater prediction accuracy is possible by considering the effective efficiency curve in the collector model, and a thermal conductivity value for the insulation layer, determined with reference to working conditions, in the storage tank model.

0.04

Y _ ;” _ “. . ‘d.

numerical 0.02

experlmenrai 0.00 I , , , , , , , , , , , , , ,

I 2 3 3 5 6 7 8 9 IO II 12 13 14 IS month\

Fig. 14. Comparison of measured and calculated values of monthly storage tank loss factors.

Records at the start of the second year show, on 31 July 1996, an average storage tank temperature of 69.3”C, sufficient to predict a final value around 80°C at the start of building heating. With this storage tank heat level, predictions show that the plant will be capable in its second year of supplying useful energy very close to the required energy needs.

It should be pointed out that a reduction in collecting and storage efficiency occurs at higher storage tank heat levels because of an increase in losses, and this implies a reduction in plant efficiency.

Definitively, the experimental results obtained in the first year quite completely characterise the thermal behaviour of this plant. The good agreement, capable of further improvement,


with the numerical model predictions provides a significant verification of the methodology used for the calculation of the storage tank heat loss factors, that are basic to the storage tank- collector field system design. More years of observation will allow climatic variations to be better considered.

Interseasonal storage for the heating of buildings is one of the most interesting applications of solar energy. The spread of these non-conven- tional plants is bound up with the economic results obtainable, that are largely dependent on the energy requirements of the building, on the availability of solar energy, and on tradi- tional energy source costs.

A 5

E EL. EC E, 4 EO E” fl

f2

F FC M P

Pb PC P”

T T.

T,i T a0 T,

NOMENCLATURE

Area of collectors (m’) Average specific heat of storage tank water (J kg-’ K-l) Incident energy on collectors (MJ) Energy supplied to building (h4J) Collectable energy (MJ m-*) Energy introduceh to the storage tank (GJ) Energy lost from storage tank (GJ) Energy withdrawn from storage tank (GJ) Useful energy supplied by collectors (MJ) Average storage tank loss factors in the loading period Average storage tank loss factors in the unloading period Insolation duration (h) Collection duration (h) Storage tank water mass (kg) Incident solar power (W m-‘) Power supplied to building (kW) Collectable solar power ( W m -‘j Useful solar Dower (W rn-? Average temperature of storage tank (“C) Average reference temperature of storage tank (“C) Inside air temperature (“C) Outside air temperature (“C) Undisturbed ground temperature (“C)

Ti Inlet flow temperature at the collectors (“C) (V- uO) Storage tank internal energy variation (GJ)

z Measurement level of storage tank temperature (m)

Greek letters

q Collector’s efficiency

Subscripts

c Calculated d Daily e Experimental

m Monthly y Yearly

REFERENCES

Arcuri N., Cucumo M. and Oliveti G. (1995). Determinazi- one sperimentale dell’efficienza istantanea di un campo di collettori solari a tubi di calore, In Condizionamento dell’aria, Riscaldamento e Rejiiigerazione. Luglio, pp. 70777 15.

Bokhoven T. P. and De Geus A. C. (1996) Large solar heating system with seasonal storage for bulb drying in Lisse, The Netherlands. Euro&n’96 1, 77-82.

Cucumo M., Marinelli V., Oliveti G. and Sabato A. (1984). Long-term solar heat storage through underground water tanks for the heating of housing, In Solar Energy Applications to Dwellings, Series A, Vol. 4. Reidel, Dor- drecht, pp. 460-468.

Cucumo M., Marinelli V., Oliveti G. and Sabato A. (1985). Long-term solar heat storage through underground water for the heating of housing. Final Report EUR 9774 EN.

Fisch M. N. and Dalenback J. 0. (1996) Large-scale solar district heating-status and future in Europe. Euro- Sun’96 1, 1566170.

Grabrielsson E. (1988) Seasonal storage of thermal energy-Swedish experience. Trans. ASME J. Solar Eng 110, 203-207.

Oliveti G. and Arcuri N. (1995) Prototype experimental plan for the interseasonal storage of solar energy for the winter heating of buildings: description of plant and its function. Solar Energy 54, 85597.

Oliveti G. and Arcuri N. (1997) Solar radiation utilizability method in heat pipe panels. Solar Energy 57, 3455360.

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