The solar system went into operation in April 2000. Table 1 shows the heat balances for the years 2001 to 2003.

In the first winter (2000/2001) there were some start-up problems with the heat pump. For this reason it was hardly possible to discharge heat from the ATES. This led to a higher fraction of heat that had to be delivered by the gas condensing boiler. The solar fraction still reached 32 % in 2001 due to a high direct usage of solar heat and the winter season 2001/2002 where the heat pump worked more reliably.

The total heat demand for space heating and domestic hot water in the first regular year of operation 2002 was 597 MWh/a. This was 20 % more than calculated during design (497 MWh). The solar collectors delivered a usable heat input of 278 MWh/a; 119 MWh/a were used directly, 158 MWh/a were provided by way of the ATES which worked with an energy return-ratio of 64 %. The electricity demand of the heat pump was 44 MWh/a, the gas boiler delivered 322 MWh/a. Referred to end energy use the solar fraction resulted to

43 %.

Figure 4 shows the monthly heat balances for the years 2002 and 2003. Due to optimized hydraulic adjustments and some improvements in the control system it was possible to increase the solar fraction to 49 % in 2003. Especially in the summer month the direct usage of solar heat could be increased (Figure 4). The summer heat load can still not be covered completely by solar energy. This is mainly caused by the strictly seasonal operation of the aTeS (see also Figure 6). During summer the ATES is in charging mode; to avoid blockings of the well screens the flow direction should not be changed frequently to discharging mode and back. Therefore in the summer months only the storage volume of the buffer heat store can be used for a solar heat delivery during night or during days without sun.

110

100

90

80

70

60

50

40

30

20

10

The yearly heat balance is indicated in an energy flow (sankey) diagram based on the results of the year 2003 in Figure 5.

balances of the ATES for the years 2002 and 2003 are illustrated. The already mentioned seasonal operation in a summer (charging) and a winter (discharging) mode can clearly be seen. Also an ATES-typical temperature decrease in the discharging mode can be observed. Only in the beginning of the discharging period a direct usage of the heat is possible. Afterwards the heat is discharged via the heat pump which has very good operating conditions in the beginning with COPs between 6 and 7 decreasing to approx. 3.5 at the end of the discharging period. The yearly mean values for the COPs can be found in Table 1.

For monitoring reasons, seven additional boreholes have been drilled to be able to place more than 50 temperature sensors into the storage volume. Figure 7 shows ground temperatures of one complete storage cycle starting from the end of the discharging period in spring 2003 (01.03.2003) to the end of the charging period (01.10.2003) and the following discharging period until the end of 2003. A small part at the top of the aquifer layer can be observed, where temperature changes occur much faster then in the other parts. Obviously in this narrow part the hydraulic conductivity is much higher than in the rest of the aquifer layer and by this the groundwater exchange takes place preferably in this part. Other temperature values show that this effect is not symmetrical around the well but with a stronger tendency in the direction shown in Figure 7. The consequence of this is an irregular shape of the storage volume and a slightly lower efficiency due to higher losses to the surroundings because of a bigger surface. This has been investigated more detailed by GTN by calibrating a 3-dimensional finite-element model for coupled flow and heat transfer with the monitoring data /2/.

In February 2001 a breakthrough of groundwater to the ground surface occurred at the cold well while discharging the store with a high flow rate. An investigation of the well showed large blocked parts in the screen which caused a higher pressure in the well. Additional failures in the connection of the well piping caused the breakthrough to the surface. An operation with a by 20 % reduced flow rate was still possible until the problem was fixed in August 2002 by cleaning the screen and installing a new well piping inside the old one.

Wolfgang Streicher, Richard Heimrath
Institute of Thermal Engineering, Graz University of Technology
Inffeldgasse 25/B, A-8010 Graz
Tel: +43-316-873-7306, Fax: +43-316-873-7305,

E-Mail: streicher@iwt. tugraz. at, heimrath@iwt. tugraz. at

Introduction

Task 26 of the Implementing Agreement of Solar Heating and Cooling (SHC) of the International Energy Agency (IEA) was dealing with solar combisystems for domestic hot water and space heat demand. One of the targets of Task 26 was to compare different combisystem designs by means of annual system simulation in TRNSYS. nine different solar combisystems were analyzed in great detail using the same reference conditions for climate, heat load (space heating and domestic hot water), and target functions.

The systems were optimized using sensitivity analysis with three target functions based on fractional energy savings (see also Weiss, 2003):

• Fractional thermal energy savings (fsav, therm)

Saved conventional fuel input of the solar combisystem compared to a reference heating system (same building, gas boiler heating systems with only small DHW store).

• Extended fractional energy savings (fsav, ext)

Extension of fsav, therm by the electricity demand of the solar combisystem and the reference system.

• Fractional savings indicator (fsi)

Inclusion of penalty functions of not reaching the required domestic hot water or room temperatures.

For each system between 12 and 30 parameters were varied starting from a base case. These parameters covered climate, collector type, size, orientation, mass flows, store size, store geometries, size of heat exchangers, heights of inlets and outlets, insulation, control settings of thermostats and control strategy of the whole system. The following paper gives on overview over the general results.

The analyzed systems and the detailed results can be found in Streicher, Heimrath, 2004 and repectively in Bales, 2003, 2003a, Bony and Pittet, 2003, Cheze and Papillon, 2003, Ellehauge, 2003, Heimrath, 2003, Jaehnig, 2003, Peter, 2003, Shah, 2003.

All Reports of Task 26 are available from

• http://www. fys. uio. no/kierne/task26/downloads. html

• http://www. fys. uio. no/kierne/task26/handbook/tech reports. html

A similar study is presented in Streicher, 2003a, where also similar results were found.

Methodology

In order to summarize the general dependency of fsav, ext in solar combisystems on the various parameters analyzed in the system reports of Task 26 the following statistical approach was chosen.

The specific alteration of a parameter was defined as

(P — P )

equ. 1: APspec = ^ Pmn)

Pbase

with

APspec specific alteration of a parameter in the sensitivity analysis of one system related to the base value of this parameter Pmax maximum value of parameter in sensitivity analysis Pmin minimum value of parameter in sensitivity analysis Pbase Base value of system parameter

The base for the different parameters was chosen as follows

• mainly the value given for the base case of each system

• 90° for azimuth sensitivity

• 45° for slope sensitivity

• 1 for specific heights within storage

The specific alteration of fsav, ext between Pmax and Pmin was defines as

_ (fsav, ext, max fsav, ext, min)

spec

‘sav, ext, base

specific change of fsav, ext between Pmin and Pmax related to Pbase

fsav, ext with Pmax fsav, ext with Pmin fsav, ext with Pbase

The dependency of fsav, exton the change of the parameter is defined as

with

fsax, ext(P) dependency of fsav, ext on the change of the parameter

Using this definition, parameters with different units can be compared to each other. Of course the value chosen for the base-case and the shape of the dependency of fsav on one parameter are influencing this calculated dependency fsax, ext(P). Nevertheless the values of fsax, ext(P) give a good feeling how strong the parameter change influences fsav, ext.

In the following pictures the median of fsax, ext(P) (dependency of fsav, ext) for all combisystems that varied this parameters are shown. As this values sometimes differ strongly to each other because of different base values und variation range of the parameter, the standard variation of the values of the different systems is additionally shown.

In case that there a parameter has been only analyzed in one system the bar of the median has a light colour and the standard variation is zero. These values are not analyzed in the following, because a general dependency of fsax, ext of this parameter for different systems can not be defined.

Additionally, parameter variations, where the user demand was not fulfilled, were taken out of the analysis.

Results

In this work, a computer program, VIETSIM, was written specifically for Vietnamese conditions. This approach was taken for a number of reasons. First, although the TRNSYS program is well-known and used worldwide, there are problems in applying the package directly to Vietnamese conditions. The method for generating synthetic hourly solar radiation and ambient temperature sequences, which the TRNSYS Weather Generator Subroutine uses, is not likely to be suitable for tropical countries like Vietnam, as indicated in previous studies. In this work, the submodel for generating weather data uses a different approach, and can more accurately generate hourly solar radiation and ambient temperature sequences for Vietnam. Second, with the current version of TRNSYS 14.1, users still need to have a certain background knowledge of FORTRAN programming, as recommended in TRNSYS itself, which is not readily available in Vietnam. The VIETSIM program, in contrast, is more user
friendly, and will be interfaced into the windows environment for the convenience of users. Third, TRNSYS is a package capable of handling many different solar thermal systems, whereas VIETSIM is targeted at the single solar thermal application, namely water heating.

There are three main submodels in the VIETSIM program. The first is used to generate hourly solar radiation and ambient temperature data, the two main weather variables for SHWS simulation, from monthly mean daily solar radiation, or monthly mean sunshine hours, and monthly mean ambient temperature. The second is the program for simulating SHW systems; and the third is the submodel for undertaking an economic analysis of the particular application.

The quantity integrator function is used calculate and display hourly, daily, weekly, monthly and/or yearly values of system performance, as users desire. Table 1 shows an example of monthly outputs from this submodel. This submodel has been validated by comparing its results with those from TRNSYS. The weather file for Hanoi, generated from submodel 1 of VIETSIM, was used to run TRNSYS. There is a small difference in the total energy supplied by the tank between the two programs. This difference results from the use of different approaches to calculate the tank energy: TRNSYS used the plug flow algebraic model; whereas VIETSIM is based on the multinode differential equation model. However, the difference is small and can be ignored.

( All energy is in GJ )

An easy to use user interface in the Windows environment using Borland Delphi has also been developed. It is aimed at providing a user with a friendly environment for the selection of different simulation and design options. The user can use VIETSIM for both simulation and design purposes as desired. The weather data can be entered as hourly values or simply as monthly average values, or even monthly average sunshine duration. Furthermore, the user can directly use the solar contour maps by moving the cursor to the location needing to be investigated in the maps.

The program will pick up latitude, longitude, and monthly average daily solar radiation and then generate hourly solar radiation sequence in order to enter to VIETSIM as input data. The VIETSIM model calculates values for the key parameter required for system economic analysis, namely the solar fraction. Figure 7 shows the interface window of the overall VIETSIM program. To upgrade the interface, several in-built functions and graphical display features may be easily incorporated in the future.

The identification of the loss of performance due to low airflow velocities around the interface region brine/air, underlined the necessity to introduce modifications of the prototype. These are meant to increase air velocity as close to the water/air interface as possible. They are:

• adoption of a lowered evaporation channel, inducing higher airflow velocities around the interface brine/air;

• enhancement of chimney performance through the adoption of non-imaging optics, CPC type [4] , increasing the solar collection area and producing the heating of the whole chimney surface, augmenting the airflow driving force.

The introduction of such improvements to the ASD will predictably produce increased evaporation rates, which will be monitored next.

William S. Duff Klaus Vanoli

Colorado State University Institut fur Solarenergieforschungs

Department of Mechanical Engineering Hameln Germany

Fort Collins CO 80524 USA k. vanoli@isfh. de

bill@enar. colostate. edu

Research on Integrated Compound Parabolic Concentrator (ICPC) evacuated solar collectors has been going on for more than twenty years [1, 2]. University of Chicago and Colorado State University researchers developed a novel ICPC design in the early 1990s that can be easily manufactured and solves many inherent problems of previous ICPC designs [3, 4]. This ICPC evacuated collector operates nearly as efficiently at double effect (2E) absorption chiller temperatures (150C) as do more conventional collectors at much lower temperatures. With this collector, a 2E chiller can cool a building using a collector field that is about half the size of that required for a lower temperature (90C) ordinary single effect (1E) absorption chiller. In 1998 this novel solar collector and a solar driven 2E absorption chiller were demonstrated for the first time in an office building in Sacramento, California. This project has now been operating for over six years [5, 6, 7, 8, 9, 10, 11, 12, 13, 14].

In Germany in 1978 two different evacuated tubular collectors began providing 50C domestic hot water to the Solarhaus Freiburg apartments. One of these, the Corning evacuated collector, had been providing 90C hot water to a 1E absorption chiller at Colorado State University’s Solar House I since 1975. This collector was uninstalled in 1978 and shipped to Freiburg. In 1982 the Philips VTR261/Stiebel — Eltron heat pipe evacuated collector array replaced the Philips Mark IV evacuated collector that had been installed on the Solarhaus Freiburg in 1978.

This paper presents an evaluation of the quality, reliability and performance of the ICPC, Corning and Philips VTR261 solar collectors over their respective operating periods.

Collectors

Novel ICPC

The novel ICPC evacuated solar collector tubes are 125 mm (5 inches) in diameter and 2.7 meters (9 feet) long, each having an effective aperture area of 0.317 m2. A cross­section of the collector tube illustrating the two orientations is shown in Figure 1. The aperture area is defined as W x L, where W is the outside diameter of the tube and L is
the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of soda-lime glass. Each evacuated tube contains a thin wedge shaped absorber, positioned horizontally in half the evacuated tubes produced and vertically in the other half. The bottom half of the glass tube is silvered to form the matching CPC reflector running the length of the tube. A small feeder pipe is placed inside the 12 mm pipe that has been bonded to the absorber to allow fluid to flow into and out of the evacuated tube. The module manifolds are a concentric pipe-inside-pipe design as well.

The 336 tube 106.5 m2 aperture area collector array at the Sacramento demonstration is made up of three banks with 112 evacuated tubes each bank. The evacuated tubes in the banks are plumbed in parallel in a reverse — return arrangement. The tubes are oriented with their long axis north-south at an angle of 10o from the horizontal.

The north bank consists of all horizontal fin tubes and the middle bank consists of all vertical fin evacuated tubes. The south bank includes an even mixture of the two types. The three banks are in­turn plumbed in parallel in a reverse — return arrangement.

Corning

The Corning evacuated solar collector tubes shown in figure 2 are 103 mm (4 inches) in diameter, have a center-to — center distance (pitch) when set into the array of 111 mm and are 2.44 meters (8 feet) in length. Each tube has an effective aperture area of 0.232 m2. The aperture area is defined according to the Figure 2: Corning Evacuated Tube IEA SHAC Program Task VI definition

[17] as W x L, where W is the pitch between tubes in the array and L is the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of borosilicate (Pyrex) glass and each evacuated tube contains a flat fin shaped absorber running the length of the tube. A pipe that has been bent into a U-shape is bonded to the absorber to allow fluid to flow into and out of the evacuated tube.

The 144 tube 33.3 m2 aperture area collector array at the Solarhaus Freiburg is made up of two banks with 12 evacuated tube modules each bank. The tubes are oriented with their long axis east-west at an angle of 55o from the horizontal. The six evacuated tubes in each module are plumbed in series with the modules in the banks plumbed in parallel in a reverse-return arrangement. The two banks are in-turn plumbed in parallel in a reverse — return configuration.

Philips VTR261

The Philips VTR261 evacuated solar collector tubes shown in figure 3 are 67 mm (2.64 inches) in diameter, have a center-to-center distance (pitch) when set into the array of 104 mm and are 1.75 meters (5.75 feet) in length. Each tube has an effective aperture area of 0.163 m2. The aperture area is defined according to the IEA SHAC Program Task VI definition [17] as W x L, where W is the pitch between tubes in the array and L is the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of soda-lime glass and each evacuated tube contains a flat fin shaped absorber running the length of the tube. An anodized aluminium ripple reflector is positioned behind the evacuated tubes. A heat-pipe is bonded to the absorber to deliver thermal energy to a condenser located external to the evacuated space.

_ 2 , Figure 3: Philips VTR261 Evacuated Tube

The 180 tube 29.5 m2 aperture area

collector array at the Solarhaus

Freiburg is made up of three rows with five 12 evacuated tube modules each row. The evacuated tube condensers in each row are plumbed in series with the rows plumbed in a reverse-return arrangement. The tubes are oriented with their long axis north-south at an angle of 55o from the horizontal.

It is clear from the plots shown in Figs. 5-9, that circulating water through the front surface of a PV module will lead to cooling of the module. The cooling effect was at maximum at noon, and appeared to be uniform between 6-8 °C, as shown in Fig. 5, at the flow rate (36 lt/hr) used. The drop in the operating temperature of the PV cells may also affect the electrical conversion efficiency that will be revealed in another publication.

The mass of circulating water and the glass jacket both reduced the intensity of insolation reaching the PV cells. This was reflected as a drop in the electrical energy collected by the hybrid module and plotted in Fig. 8. The loss in electrical energy, EL= Ec — Eh, was however well offset by a large gain, Qw, in thermal energy that was collected by the circulating water, as shown in Fig. 9.

Regarding energy extraction from solar insolation, off course the hybrid unit showed a much better performance. The advantage of using the hybrid system may be better displayed by plotting a factor, which may be termed as Energy Gain factor, EGF (Fig.10). The overall energy gain, EG, may be defined as

Eg = Qw — El (3)

And Energy Gain factor defined as

EGF = Eg /Ec (4)

The hybrid system has an advantage of increasing the energy collection, however it also suffers a disadvantage in terms of extra cost incurred by having the jacket. Nevertheless, the extra cost forms only a fraction of the module cost.

Many researchers have studied a similar experimental set-ups [12-15]. All however with a difference that the cooling operation was applied at the rear of the PV module. In one case [12], a forced water cooling of the modules from 60 oC down to 25 oC increased the output power by 23%, while the open-circuit voltage was reported to increase by 18%. Similarly the PV conversion efficiency improved by 3%. Some researchers [13], accept the new structures as an important surplus value in terms of an enhanced architectural aestetics. However the pay-back period was difficult to calculate since the thermal energy yield could not be directly utilized.

CONCLUSION

Partial absorption of solar insolation by the glass jacket and mass of water in circulation, lead to some drop in the total electrical energy conversion. However, measurements based on electrical characteristics and the rate of heat exchange in a hybrid module showed that the overall energy extraction from solar insolation could be improved.

The improvement was in terms of thermal energy gained by the circulating water which can be used as preheat for another application. The Energy Gain Factor, EGF, which reflects the ratio of total energy gain over the Control module reached the value of about 2.0 showing a 100% improvement in energy extraction, over the PV system alone. In addition, more modification to the system are needed to increase the temperature difference of the circulating water and to improve the efficiency of the hybrid system.

REFERENCES

1. Statistics. TRNC, Statistics Office, General Report, 1999.

2. Erdil, E.. Harvesting solar energy in North Cyprus, Proceedings, 11th E. C. Photovoltaic Solar Energy Conference, Montreux, Switzerland, 1992, 1, pp. 1523 -1525.

3. Activity Report. TRNC., Electricity Generating Authority Activity Report, 1999.

4. Erdil, E. Rooftop electricity generating capacity of PV systems in north Cyprus. Proceedings, 13th E. C. Photovoltaic Solar Energy Conference, Nice, France, 1995, 1, pp. 494 — 495.

5. Maycock, P The world PV market 2000. Renewable Energy World 2000, 3 (4), pp. 58 — 76

6. Brinkworth, B. J., Estimation of flow and heat transfer for the design of PV cooling ducts. Solar Energy, 2000, 69 (5), pp. 413 — 420.

7. Sorensen, B., PV power and heat production: an added value, 16th European Photovoltaic Solar Conference, Glasgow, UK, 2000.

8. Messenger M., Ventre J., Photovoltaic Systems Engineering, pp.48, 2000, CRC Press.

9. Rijnberg, E., Kroon, J., Wienke, J., Hinsch, A., Roosmalen, J., Sinke, W., Scholtens, B., Vries, J., Koster, C,. Duchateau, A., Maes, I., and Hendrickx, H., Long-term stability of nanocrystalline dye-sensitized solar cells, 2nd word Conference on PV Solar Enerby Consrvation, Vienna, Luxembourg, 1998.

10. Yamamoto, K., Yoshimi, M., Tawada, Y., Okamoto, Y, and Nakajima, A., Cost effective and high performance thin film Si solar cell towards the 21stcentury, Technical Digest of the international PVSEC-11, Sapporo, Tokyo, 1999, pp. 225-228.

11. Sorensen, B., Renewable Energy, 2nd Edition, 2000, pp. 912, Academic Press, London.

12. Klugmann E., et al. Influence of temperature on conversion efficiency of a solar module

workink in photovoltaic PV/T integrated system. 16th E. C. Photovoltaic Solar Energy

Conference, Glasgow, UK, May 2000.

13. Leenders F., et al. Technology review on PV/Thermal concepts. 16th E. C. Photovoltaic Solar Energy Conference, Glasgow, UK, May 2000.

14. Fujisava T., and Tani T,. Optimum design for photovoltaic-thermal binary utilization system by minimizing auxiliary energy. Electrical Engineering in Japan, V:137, N1,2001, pp. 28-35.

15. Affolter P, et al. Absorption and high temperature behaviour evaluation of amorphous modules. 16th E. C. Photovoltaic Solar Energy Conference, Glasgow, UK, May 2000.

With equation (9) and (10) we can calculate the normalized efficiency curve.

EN 12975 [ 1 ] defines the following conditions for that curve:

• beam radiation: 680 W/m2 (85% of the global radiation)

• diffuse radiation: 120 W/m2 (15% of the global radiation)

• global radiation: 800 W/m2

• Incidence angle: 15°

Vnorm =Ъ. !AMdir_15 + G. MGdJ ; IAMdir_e = 1 — b0 • ^—S — — ij (9)

The uncertainty of each point of the normalized efficiency curve is calculated with equation (11). This equation is comparable to equation (7). With the collector coefficients determined by the quasi-dynamic test, it is also possible to calculate the equivalent normalized efficiency curve of a steady state test.

The apparatus consists of a collector mounted on a suitable stand, storage tank, pump and insulated pipes. Copper-constantan (Type T) thermocouples are used for the measurements of the inlet, outlet and ambient temperatures. A turbine flowmeter
and a SP1110 pyranometer are used for measuring of the flowrate and solar radiation respectively. The data from the seven thermocouples, the pyranometer and flow meter are been recorded by a commercial, mains powered datalogger (using Matlab, via National Instruments datalogger).

The ISO 9806-1, ASHRAE Standard 93-86 and SRCC document RM-1 provide the standard test methods for flat-plate solar collectors. The general test procedure is to operate the collector in a test facility under nearly steady conditions and measure the data that are needed for analysis. Although details differ, the essential features of all of the procedures can be summarized as below:

1. Solar radiation is measured by a pyranometer in the plane of the collector.

2. Flow rate of working fluid, inlet and outlet fluid temperatures, ambient temperature, and wind speed) are measured.

3. Tests are made over a range of inlet temperatures.

4. The inlet pressure and pressure drop in the collector are measured.

Information available from the test is data on the thermal input, data on the thermal output, and data on the ambient conditions. These data characterize a collector by parameters, FR (та), and FRUL that indicate absorption of solar energy and energy loss from the collector. Instantaneous efficiencies can be determined from:

Пі = Qu/AcGr = mCp (To — Ti )

Ap Gr (5.1)

Where ro is the exit temperature of the working fluid. With the test data over a range of inlet temperatures, the instantaneous efficiency can be plotted as a function of (ri-ra)IGr.

The second important aspect of collector testing is the determination of effects of incident angle of the solar radiation. The standard test methods include experimental estimation of this effect and require a clear test day so that the experimental value of (та) is essentially the same as (та )b. ASHRAE Standard 93-86, recommends that experimental determination of Kt a be done with the incidence angles of beam radiation of 0, 30, 45, 60o. For flat-plate solar collector Souka and Safwat have suggested an expression for angular dependence of KTa as

KTa = 1 + bo( Cos &1- 1) (5.2)

3 CONCLUSION

A mathematical model of the honeycombed collector has been developed. It estimates the net solar energy collected per unit area of the collector. This system consists of a flat plate collector, with a triple walled extruded polycarbonate substituting for the glass cover and absorber plate. The utilisation of the polycarbonate in the solar collector has the advantage of reducing the weight by more than half in comparison with a traditional collector using essentially metals with similar performances [7].

This program was designed to study the properties of a polycarbonate solar collector. The model also facilitates changes to the collector physical properties such as dimensions of the channels, ambient temperature, flowrate, selective and non­selective absorbers, material thermal properties, collector and system design optimisation.

The results from the program will allow a full parametric study of different collector design criteria, with this polycarbonate structure. The results will be compared to a standard flat plate collector design, to see if this polycarbonate flat plate collector is a more effective design. The simulation results are being validated with current experimental testing. ISO 9806-2 standards are being used to validate the results, for the parametric study in the lab, under steady state conditions. The final optimum design will then be tested outdoors using the quasi-dynamic conditions set out by the European Standard EN 12975-2. Weather data, obtained from the weather station set up at CIT, will be used as the input for the weather conditions for out door testing. Following the testing, long-term prediction of this type of collector performance will be looked into.

In the following, the disagreement between the corresponding effective capacities of the

J.3-procedure and of a charging process of the storage tank is demonstrated by an illustrative example. (The first hint for this inconsistency was given in [1].)

Typical values of the physical thermal capacities Cphys of the components of a dewar-type vacuum tube collector are given in table 1.

When a step change of irradiance is applied, increasing G from zero to 1000 W/m2, and starting from Tabs = TF = Ta, then ATF « 8 K, and ATabs « 30 K (this corresponds to habsF = 30 W/irFK and a thermal power of about 660 W/m2, in agreement with a typical conversion factor ^0 = 0.66). The calculation for the resulting thermal capacity CJ3 is given in table 2. The result is CJ3 = 21.4 kJ/irFK.

These results are now applied for the calculation of a storage charge period. On a sunny day, it takes about 4 hours to heat up a 300 litre storage tank by 20 K (4 m2 collector, mean irradiance 800 W/irF, mean efficiency 0.6).

The energy needed to load the capacities of the collector during this period is calculated as follows. As discussed in section 5, the amplitude ATabs is smaller or approximately equal to the amplitude ATF. For the sake of simplicity it is assumed here that both components are heated up by 20 K. In reality, the physical capacities Cphys of the components are heated (and not any model capacities). So the result is AEcol, phys = 20 K ■ (4.5+4.5)kJ/m2K = 180 kJ/irF. In contrast to this, a simulation model that uses the capacity CJ3 calculates AEcol, J3 = 20 K ■ 21.4 kJ/mFK = 428 kJ/mF. By this, the energy that loads the capacities of the components is strongly overestimated. The difference AEcol, J3 — AEcol, phys = 248 kJ/mF of energies stored in the capacities corresponds to an extra time that the simulated collector needs to achieve the temperature rise of 20 K.

With a collector thermal power of 0.6 ■ 800W/m[2] this time delay is 517 s (approximately

8.5 minutes). So the collector with CJ3 needs 3.6 % longer to increase the system temperatures by 20 K than the realistic one.

Furthermore, it has to be kept in mind that the measured J.3-capacity of this collector was even higher than in our example above (40 instead of 21.4 kJ/m2K). Here the error of the energetic description of the storage charging process amounts to 620 kJ/m2, which corresponds to a delay of 1292 s. Hence the collector gain in the period under consideration is underestimated by 9%.

Moreover, it has to be kept in mind that the process of charging the storage is a significant and typical one, since it is a real everyday process for solar thermal systems.

An investments analysis for the different configurations of the system and for all the localities we have considered has been made by using the economic index NPW to estimate the better investment first of all and then the value of the other indices.

The economic indices taken in consideration are:

whereas REc is the amount of annual energy supplied by the plant. It has been assumed an economic lifesoan of the plant of 20 years, and Italian economic market be characterized by the following rates: g=2.5 %, e=8%, d=4%. In Italy methane costs 0.0187 €/MJ, diesel oil does 0.0244 €/MJ and the LPG 0.0369 €/MJ.

and system efficiency for the heating period. Supplied energy for domestic hot water

SHAPE * MERGEFORMAT

Fig 6 — Milan. Solar fraction and system efficiency in relation to the variation of collectors surface for the different volumes in the period of heating building.

Our analysis does not consider at least at first government financial supports and only methane, which is the cheapest fuel oil has been considered for feeding an integration boiler. Afterwards advantages coming from other fuel have been evidenced, so as government financial supports. In order to estimate the better investment on the considered period, the best NPW value of the three localities has been considered (fig. 8 Cosenza and Rome have quite the same values while for Milan lower values have been obtained). The system configuration with collectors’ surface of 12 m2 and a tank’s storage volume of 1 m3, provides the greatest value of NPW. For Cosenza a value is obtained, which is next to NPW, even with 8 m2. Figure 9 shows profit index (PI) for the towns here considered; As a result PI value is always above zero tending to fall with the increasing of collectors’ surface. While Cosenza and Rome both share almost the same values, those ones concerning Milan are lower. Profit index also show that volumes of 0.5 m3for low surfaces are cheaper, while for surfaces grater than 8 m2 the index of 1 m3 provides best results. It values 1.8 for Cosenza and Rome for a surface of 12 m2 and a volume of 3 m3, it assumes the value of 1.5 for Milan. The growth of payback time is directly proportional to increasing of collectors’ surface and of tank’s storage volume (figure 10). For the optimal configuration obtained it amounts to 12 years for Cosenza and Rome and 14 years for Milan. The cost of energy produced by solar plant "cep" (figure 11) is kept below the cost of methane (0.0187 € / MJ) for those surfaces until 20 m2; it sometimes excedees in such a value for 28 m2. In particular for a surface of 12 m2 and a volume of 1 m3, the "cep” was equivalent to 0.0124 € / MJ for Cosenza, 0.0121 € / MJ for Rome and of 0.0139 € / MJ for

Milan. All values being below the cost of methane with a reduction between 20% and 33%. If integrating feeding with diesel oil or the LPG is taken in consideration the best values produced from the economic indices can be seen in table VI. Payback time are reduced of approximately 2 years with the diesel oil and of approximately 5 with the LPG. The NPW value improves of approximately 70% with the diesel oil while with the LPG it is more tripled.

Fig. 8 — NPW: Net Present Worth Figura 9 — PI: Profit Index for the three

for the three localities. localities.

Table VI — Economic indices for a collector’s surface of 12 m2 and a storage volume of 1 m3 in absence of financial supports.

Figura 11: Cost of energy produced for the three localities.

In case a capital account financing of 30% is considered, as provided for by Italian public bands, an improvement of all the economic indices and the cep takes place as shown in the table VII.

Table VII — Economic indices for a collector’s surface of 12 m2 and a storage volume of 1 m3 with financial supports.

2. CONCLUSIONS

In this work the possibility has been analyzed to heat residential buildings using solar energy. Instead of traditional heaters radiant floor has been chosen, for its bigger suitability, for its thermic capacity and because it can be supplied even at low temperatures. That allows solar collectors to work more efficiently: It also provides

thermic energy in the tank to be used to very low temperatures. Simulation code has made possible to determine for three localities of Italian territory thermic and economic performances of the system, to the variation of collectors’ surface and tank’s storage volume. The fraction of thermic requirements both for building heating and domestic hot water, supplied by solar energy, as a result is deeply dependent on collectors’ surface. For a surface of 4 m2 a solar fraction of approximately 30% for Cosenza has been obtained, being instead of 25% for Rome and 7% for Milan; eventually to get a fraction of 72% for Cosenza, 68 % for Rome and 41% for Milan for a surface 28 m2 and a volume of 3 m3. System efficiency decreases with increasing of collector’s surface and with the falling of tank’s storage volume, above all because of a raising in temperature in the tank providing low collection performances. Among the three localities considered there is not a great difference in seasonal energy supplying for building heating, because a great amount of monthly incident solar energy, which characterizes harsh areas, is balanced with a longer period of heating. With a collectors’ surface of 28 m2 and a tank’s storage volume of 2 m3, seasonal energy supplied for building heating, was of 16.7 GJ for Rome, 15.5 GJ for Cosenza and 13.8 GJ for Milan. The economic analysis has shown that the system with collectors’ surface of 12 m2 and a tank’s storage volume of 1 m3 is the more suitable one. For such a configuration economic indices obtained for three different types of fuel, being financed or not, have shown the advantage of such an investment. In particular, in such bad conditions, as lack of government supports and using methane as integration fuel, the cost of the energy produced by the plant in a 20 years long lasting lifespan resulted below a percentage between 20% and 33%.

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