Category Archives: Particle Image Velocimetry (PIV)

Experimental apparatus and measure instruments

The solar installation with special monitoring and controlling system is built in South-West University “N. Rilski” (SWU) — Blagoevgrad. The scheme and description of installation is presented in [6]. The main target of monitoring and controlling system is to ensure possibility to: work in direct and indirect regime with different heat exchange area and location of serpentine unit in the tank; change angle of collectors toward by horizon and azimuth; regulate some operating parameters (fluid flow rate); measure all important parameters, influenced the system performance.

The test tank is a vertical cylindrical tank made of stainless steal material. In the tank are built three copper serpentines along all the height of the tank. Serpentines are 10 meters each in length. They can be switched on or off as a heat exchange unit by system of valves. So the system can work with one, two or three serpentines situated in different regions of the tank. Installation can work also in direct regime, when all three serpentines are turned off.

Monitoring system includes 12 thermo sensors assembled in accumulation vessel, 6 thermo sensors in collector circle, one thermo sensor for measuring the ambient air temperature. Solar meter, located near the solar collectors measures solar radiation. The inflow rate, heat energy and heat power are measured by combined heatmeter. All observed parameters are registered by automatic monitoring system. It includes a special electronic module for converting the analog data from sensors to digital signals. Digital data from converting module is collected by computer system. After that, stored data can be used for detailed analysis of thermal and economical efficiency of system and preparing the statistical calculation for long-term analysis.

Measuring module includes also a control unit, which governs the pump performance. It starts pump, if the temperature difference between inlet and outlet temperature of working fluid is above preliminary defined value. Ordinarily, the systems work stable and efficiently with control temperature difference between 2 and 5oC.

Consumption of hot water is realized by simulation of a typical consumption regime for small restaurant hot water system and domestic (family) hot water system.

Evaluation of the System Configurations

The options of solar collector and heat pump connection for domestic heating systems are shown in Figure 1. Several examples of realised or theoretically investigated solar heat pump systems from literature are described above. It is evident, that the system configura­tions have diverse advantages and disadvantages and are thus often applicable only in particular areas or buildings. Table 2 gives an overview of the system approaches pre­sented, evaluates their characteristics (especially concerning their applicability in Central European climate and buildings) and describes the problems and challenges linked with each concept.

The parallel solar heat pump concepts with ground collector and air source are in principle good solutions for domestic heating, especially in comparison to fossil fuel fired systems. However, one problem is the high space demand for the collector and the effort for its in­stallation in the ground. The air source heat pump clearly suffers from the long winter peri­ods in Central Europe and the problems linked with it (e. g. icing of the evaporator).

The same problem occurs in non-storage solar heat pump systems with a direct heat pump-collector connection. Monovalent operation seems not to be feasible in Central European climates with long frost periods and for example snow lying on the absorber area, which means an additional heat source (e. g. furnace) is required.

The water storage systems on the one hand have the disadvantage of large storage vol­umes and therewith high space demand. On the other hand, they use an easy to handle and low-cost storage medium in contrary to the latent heat storage tanks working with par­affin or salt hydrate. These tanks also have the problem that the melting point of many PCMs is relatively high and that the heat exchanger design is complex.

The heat exchanger design is also a challenge for stores that use the water/ice transition. Nevertheless, a water/ice-latent heat storage tank has the most positive influence on the collector degree of utilisation due to the low melting point of water. While collector effi­ciency and degree of utilisation are increasing, the heat pump’s COP is decreasing, hence heat pump development to achieve an acceptable seasonal performance factor (SPF: ratio of heat delivered and total energy supplied over the season) is necessary. Furthermore, the storage volume can be reduced considerably in comparison to a sensible water store by the utilisation of the phase change.

Geometrical properties

Incremental concentration factor (fig. 6) Ci = f (i): N+^R, i є (1; n) is a series of functions converging to n ^ да for ©A = const. At ©A = 5°, the biggest contribution to concentration will have the second zone, the for higher zones the contribution falls down exponentially. It can be seen from graphs that CLON with n > 10 has no real sense. For higher acceptance angles the number of zones with meaningful contribution to the overall concentration will be even less, e. g. for ©A = 20° CLON should have not more than 6 zones.

Yet more interesting is to read the dependance of cummulative (or total) concentration factor (fig. 7) Ci = f (i). It can be seen that CLON’s maximal concentration factor for ©A = 5° is C = 5. No CLON of practical measure will have acceptance angle lower.

Conclusions

From mentioned above follows that CLON is expected to be much advantageous in comparison to the classical solution of CPC, which is assumed to be a practical realisation of so called ideal concentrator. But on the other hand it is necessary to note that CLON is advancing CPC only at level of practical realisation and usage of flat mirrors is possible only due to flat character of output area. Rays incoming at angle of incidence equal to ©A will be concentrated nearby foci and there will be then higher local irradiation than on the rest of receiver, what can never be reached by CLON. However, this is neither supposed to CLON nor CPC.

Further it is necessary to remark that CLON is not intended to be and is not an approximation of common CPC, so also not its modification. Its concept given by optical scheme is new and principially different from CPC.

Comparison with a flat-plate collector

Still considering the solar heating plant, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest (Vejen N. K., Furbo S., Shah L. J. (2004). ) Arcon HT collector. The collectors are facing south and tilted 45° in Copenhagen. In Ummannaq the collectors are facing south and tilted 60°.

It can be difficult to compare the thermal performances of flat-plate collectors and tubular collectors as the effective area of a flat plate collector typically is defined as the transparent area of the glass cover and the effective area of a tubular collector can be defined in many ways. In the present comparison, the tubes are placed close together so that there is no air-gap between the tubes and the outer tube cross­section area (=L-2-rc-N) directly corresponds to the transparent area of a flat-plate collector.

Fig. 11 shows the thermal performance per m2 collector as a function of the solar fraction of the solar heating plant for the two collector types. Here, the solar fraction is defined as:

Qauxiliar

Qto

First of all, the figure shows that the tubular collector has the highest thermal performance for both locations. Further, it can be seen that the Uummannaq curves decreases more rapidly with increasing solar fractions. This is due to the lower air temperature in Uummannaq.

The figure also shows that the ARCON HT collector has a better thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector.

Conclusions

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. The model is based on traditional flat plate collector theory, where the performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbour tube as a function of the solar azimuth and zenith. This makes it possible to calculate the energy from the beam radiation.

The thermal performance of an all glass tubular collector with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Calculations with the new model of the tubular collector vertically placed and tilted 45° is compared with measured results and a good degree of similarity between the measured and calculated results is found.

Further, the collector model is used in a model of a solar heating plant and a sensitivity analysis of the tube centre distance, collector tilt and orientation with respect the thermal performance per tube is investigated for the two locations Copenhagen (Denmark) and Uummannaq (Greenland). The results show that the optimum tilt and orientation is about 45° south for Copenhagen and about 60° south for Uummannaq.

Finally, the thermal performance of the evacuated tubular collector is compared to the thermal performance of the newest Arcon HT collector. Here, the results show that the tubular collector has the highest thermal performance for both Uummannaq and Copenhagen. This analysis also illustrates the differences in the thermal behaviour of the two collector types: The ARCON HT collector has a higher thermal performance in Copenhagen than in Uummannaq, whereas the tubular collector performs best in Uummannaq compared to Copenhagen. The main reason for the result is that there is much more solar radiation ‘‘from all directions’’ in Uummannaq and this radiation can better be utilized with the tubular collector than with the flat plate collector.

Simulation assessment

The effect of the significantly higher performance levels of COAX 390 (heat insulation and heat transfer ability) has been tested in a simulation in Polysun. The experiment has been carried out for a solar thermal system with COAX 390 as well as two or three flat collectors PLANO 21 (figure 4) in comparison to a conventional tank with the same collectors (cli­mate Wurzburg).

Table 1 shows the different parameters. The values for heat losses of the connected pipes are taken from /1/.

Parameter

Conventional tank

COAX 390

Thickness insulation

100 mm

125 mm

Thermal conductivity insulation

0.049 W/(m K) (PU-foam, soft)

0.033 W/(m K) (LE-EPS + air)

Losses connections boiler supply, boiler return, cold water

10 W/(50°C — 20°C) = 0.33 W/K

0,9 W / (50°C — 20 °C) = 0.03 W/K

Losses connection hot water

15 W / 30K = 0.5 W/K

0.9 W / 30K = 0.03

Without even considering the additional yield through stratification, the COAX-system of­fers with the two tested factors alone a higher solar yield of 8 % or 14 %, respectively.

2. Conclusion

The substantial performance improvements of COAX 390 (achieved by stratification, heat transfer values, insulation) in comparison to the market standard leads to the conclusion that — when using the same collector area — a significantly higher energy yield can be achieved.

On the other hand, the cost for this tank is comparable to high-quality yet technically con­ventional tanks, which should lead to an extremely good price/performance ratio for the complete system.

3. Literature

/1/ Meitner, R., Siegemund, A., Leibfried, U.: Mit Mehrtagesspeichern gegen Warmev — erluste, Moderne Gebaudetechnik 10/2001

Tr_COAX_14_SYMPOS_as. doc

Optical design

In a conventional Compound Parabolic Concentrator (CPC), the reflector formation criterion is that light tangential to the edge of the closest tube (the tube nearly touching the cusp) is reflected back in the same direction by the CPC reflector, so that light which is not tangential strikes the closest tube more directly or via the reflector on a second bounce. Light cannot be reflected from one reflector to another and still be collected.

Fig. 1. Three tube element of CPC array showing involved reflectors.

In Fig 1, two dotted tangent lines are shown. Light passing along these lines will be reflected straight back along the incident direction. Light more normal to the collector aperture will be collected. Between the left and the centre tube, the light ray is collected after a single bounce. Some light will be lost through the gaps between the absorber surface and the cover tube but about 98% is aimed at the tubes before reflection and absorption losses are accounted for.

In a CPC the curvature usually used is that of a mathematical involute, and the curve is truncated at the height approximately as shown in Fig. 1. The actual height of the reflector rim between the tubes depends upon the tube spacing. A full involute is not used; it would finish level with the top of the inner absorber tubes and would allow about 12.5 standard evacuated tubes having a 34.2 mm absorber tube diameter in a 1350mm wide panel. In this case the spacing between the tube centrelines is equal to the absorber tube circumference (107.4 mm), so that peak optical concentration is about 1. Such a "full” CPC would use much more reflector area than a practical design and the shape could not be as easily fabricated, so a 14 tube panel with a lower reflector is probably a more acceptable compromise these days, especially as tubes are relatively low cost.

Potential of the hybrid technology

In the last years an increasing interest on the PV/T hybrid collector has been reported and some important international projects have been launched, like, to name a few:

1) The PV-HYBRID-PAS Joule project. The main goals of the project were the development of Procedures for Overall Performance and the evaluation of Hybrid Photovoltaic Building Components;

2) The PV Bonus negli USA;

3) Activity 2.5 TASK VII within IEA.

The joint implementation of the two technologies could favour both their markets by:

• sharing the experience of each market characteristics; the high tech aspect for the photovoltaics could add more reliability and trust to the solar collector and the already existing commercial sectors, surely more developed for the thermal collectors, could be exploited by the PV,

• the potential of a large costs reduction for the mounting, installation and manufacturing whether combined,

• the prevention and possible elimination of competition for the availability of the surface on the roofs for the buildings.

The history of that technology has showed that there have been different levels of integration:

1) at first the two systems have been kept separated, working in parallel, in symbol PV + T. With that configuration it has been difficult to realize the potential advantages;

2) then the systems have been combined and the two technologies have begun to match each other, symbolically PV&T. The Photovoltaics part is only added on the collector. Some benefits of the implementation have been gained;

3) the next pace is the integrated systems. The two parts are intimately joined and projected together so optimising the whole component, in symbol PV/T.

Some interesting aspects for the future development are listed below.

1) It is an added value for the building;

2) It could reach benefits following the boom for the building integration photovoltaics;

3) It is compatible with the thin film technology, particularly with the amorphous Si, that is more transparent to greater wavelengths and has more favourable temperature coefficients;

4) It fits well with the energy certification of buildings, compulsory for the next future;

5) It matches well to the new trends in the modern architecture (sustainability, compatibility, awareness)

Selection of existing combi-systems

There are two basic differences in the system configuration of solar combi-systems in a way how the boiler and the thermal solar collectors are integrated in the system with a lot of variations. With a serial integration of the thermal solar system as shown in figure 2 only the solar collectors charge the buffer store. If the temperature of the buffer store is too low the boiler delivers the remaining necessary heat.

With a parallel integration (see figure 3) the boiler is charging e. g. the upper third of the buffer store and keeps it on a certain temperature level if the solar collectors do not deliver enough heat. An advantage of the parallel integration is that the boiler does not start as often as in serial integration which improves its efficiency. However the higher temperatures in the buffer store can diminish the solar fraction and lead to higher heat losses of the buffer store.

Figure 3: Parallel integration of the thermal solar system

Which of the two systems is better depends on certain criteria, e. g. the heat load of the connected housing area. This will be considered in the project as well. Both serial and parallel integration can be found in the chosen systems. An overview of these systems with the main features is given in table 1.

Energy Payback Time of Solar Domestic Hot Water Systems

In the following, the methodology of determining the energy payback time is explained by an example of two thermal solar systems. Both systems are domestic hot water systems (SDHW-systems) with the same design parameters: 5 m2 collector area, 300 l total store volume including an auxiliary volume of 150 l. The two systems differ only in materials used for the collector and the supporting frame. The fractional energy savings are equal for both systems investigated.

2.1 Cumulative Energy Demand for Production

The cumulative energy demand (KEAp) comprises the energy required for the production of the goods at all phases, including extraction, mining of raw materials, semi­manufactured products and the production process itself. For all following calculations the values are taken from an extensive database called "Okoinventare fur Energiesysteme” from Switzerland.

In order to determine the cumulative energy demand for production (KEAp) it is suitable to divide the system into components (collectors, mounting frame, heat store, solar station and piping) and to identify the main materials used with their weight proportion. The cumulative energy demand is obtained by multiplication of the weight of the main materials with their respective primary energy demand values.

COLLECTOR

SYSTEM 1

SYSTEM 2

Material

Unit

Quan­

tity

KEA

[kWh/

unitl

KEA

[kWh]

Material

Unit

Quan­

tity

KEA

[kWh/

unitl

KEA

[kWh]

Absorber

copper

[kg]

16

26.83

429

copper

[kg]

16

26.83

429

coating

sputtered

[m2]

5

5.30

27

galvanic coating (black chrome)

[m2]

5

12.37

62

Casing

fibre glass

[kg]

7

29.73

208

aluminium

[kg]

20

42.14

843

acrylonitrile-

butadiene-

styrene

[kg]

13

31.67

412

Cover

glass

[kg]

46

3.69

170

glass

[kg]

46

3.69

170

glass

hardening

[m2]

5

5.50

28

glass

hardening

[m2]

5

5.50

28

Insulation

mineral wool

[kg]

10

4.97

50

mineral wool

[kg]

5

4.97

25

polyurethane

[kg]

5

27.88

139

silicone

[kg]

1

28.19

28

silicone

[kg]

1

28.19

28

SUM

1351

SUM

1724

SUPPORTING

FRAME

stainless steel

[kg]

16

26.82

429

aluminium

[kg]

16

42.14

674

SUM

429

SUM

674

Table 1: Impact of different materials on the cumulative energy demand

Table 1 shows the impact of the use of different materials for the collector on the cumulative energy demand. The basis is system 1 with a collector that has a low cumulative energy demand. System 2 varies only in some collector materials used so that the impact on the energy payback time can be shown. The two systems only differ in absorber coating technique, the casing and insulation of the collector and in the material of the supporting frame. It can be seen that the absorber coating technique has only a minor influence on the cumulative energy demand of the whole system. Concerning the cumulative energy demand the relevant components of the collector are the absorber and the casing material.

The cumulative energy demand has to be determined for each component of the thermal solar system as shown in Table 2. It has to be considered that the store volume of the conventional heating system is reduced by using a thermal solar system. Therefore both systems are credited with the cumulative energy demand of a conventional hot water store of 135 litres (store credit in Table 2). The collector of system 1 is integrated in the roof, saving a large number of rooftiles. The cumulative energy demand for the saved rooftiles is therefore also credited to the thermal solar system.

Unit

SYSTEM 1

SYSTEM 2

Collector

[kWh]

1351

1724

Rooftile credit for roof integrated mounting

[kWh]

-408

0

Supporting frame

[kWh]

429

674

Store

[kWh]

1521

1521

Store credit

[kWh]

-839

-839

Solar station

[kWh]

507

507

Piping

[kWh]

309

309

Sum

[kWh]

2871

3896

Transport

[kWh]

256

275

Transport credit for integrated mounting mode

[kWh]

-205

0

Sum materials and transport

[kWh]

2922

4171

Assembly and installation

[kWh]

292

417

CUMULATIVE ENERGY DEMAND FOR PRODUCTION KEAp

[kWh]

3214

4588

Table 2: Determination of the cumulative energy demand for production

In addition the cumulative energy demand of the transport of the thermal solar system from the manufacturer to the place of installation has to be considered. It was assumed that a distance of 300 km from the manufacturer to the wholesale dealer is covered with a truck and that a distance of 100 km from the wholesale dealer to the place of installation is covered by a delivery van. The cumulative energy demand for transportation is directly coupled with the total weight of the thermal solar system (including package).

With respect to the integrated mounting mode a credit for the rooftile transport has to be granted. With a general approach that the average transport distance is 400 km and that the transport is carried out by truck, the transport credit amounts to 205 kWh.

Concerning assembly and installation of the thermal solar system no general data base is available. Since the effort of installation varies depending on the kind of thermal solar system, it is calculated with a general approach of 10% of the cumulative energy demand for production of the materials and for the transport.

Table 3 shows the impact on the energy payback time. It can be seen that both systems only differ in the cumulative energy demand for the production that comprises materials used, transport, assembly and installation of the system. All other influences on the energy payback time such as cumulative energy demand for operation and for maintenance and the primary energy saved by the solar system are equal for both systems investigated.

System 1 with a minor cumulative energy demand for production has an energy payback time of 1.4 years. The cumulative energy demand for production of system 2 is 43 % above the value of system 1. This results in an increase of the energy payback time to 2.1 years.

Symbol

Unit

SYSTEM 1

SYSTEM 2

PRIMARY ENERGY EMBEDDED IN THE SYSTEM

Materials

[kWh]

2871

3896

Transport

[kWh]

51

275

Assembly and installation

[kWh]

292

417

Cumulative energy demand for production

KEAp

[kWh]

3214

4588

Cumulative energy demand for operation

KEAo

[kWh/a]

312

312

Cumulative energy demand for maintenance

KEAm

[kWh/a]

41

41

PRIMARY ENERGY SAVED

Yearly primary energy demand of a conventional system

Qconv, tot

[kWh/a]

4687

4687

Auxiliary heating demand

Qaux, tot

[kWh/a]

2109

2109

Primary energy saved

PEAsub

[kWh/a]

2578

2578

ENERGY PAYBACK TIME

AZ

[a]

1.4

2.1

Table 3: Determination of the energy payback time

1.2 Cumulative Energy Demand for Operation

The cumulative energy demand for operation includes the electrical power consumption of the solar loop pump and the electrical power consumption of the controller. The power consumption in [W] is multiplied by the respective operating hours of the pump and the controller. For the determination of the cumulative energy demand the resulting electrical power consumption has to be multiplied by the primary energy equivalent for electrical power.

System 1 + System 2

Power

Consumption

[W]

Operating Hours [h/a]

Primary Energy Equivalent

[kWhprimar/kWh]

Cumulative Energy Demand [kWh/a]

Pump

43

1500

3.80

245

Controller

2

8760

3.80

67

Total

Cumulative Energy Demand for Operation

312

Table 4: Determination of the cumulative energy demand of operation

01

MANTLE TANK DESIGN ANALYSIS

Calculations with MANTLSIM were carried out in order to investigate how the thermal performance of a small low flow SDHW system is influenced by the mantle tank design. The mantle tank design analysis is carried out with the mantle tank, Danlager 1000 marketed by Nilan A/S, as the standard reference tank. The design analysis is performed in such a way that only one parameter has been changed at a time in the calculation. Table 3 gives data for the standard reference system.

The circulation pump in the system is controlled by a differential thermostat, which measures the temperature difference between the outlet from the solar collector and the bottom of the mantle. The differential thermostat has start/stop set point at 10/2 K.

All the calculations in this chapter are carried out with weather data from the Danish Test Reference Year [11]. The daily hot water consumption is 0.100 m3 heated from 10°C to 50°C, which is tapped from the tank in three equally large parts at 7 am, 12 am and 7 pm. The yearly hot water consumption is 1674 kWh. The auxiliary energy supply system heats the top 0.082 m3 of the tank to 50.5°C and the indoor air temperature is 20°C.

The tank parameters that are investigated are the mantle inlet position, the mantle height, height/diameter-ratio of the tank, auxiliary volume and insulation of the tank.

Figs. 3-7 show calculated yearly net utilised solar energy of the system with the differently designed mantle tank. The standard reference system is marked in the figures.

Fig. 3 shows the calculated yearly net utilised solar energy of the system as a function of the mantle inlet position. The figure shows that the thermal performance of the system increases for the mantle inlet position moved down from the top of the mantle to a relative position of 0.35 from the mantle top, and that the thermal performance decreases if the inlet position is moved further down. The net utilised solar energy can be increased by 2.5% by moving the inlet port down to a relative position of 0.35. These results are in good agreement with the experimental results from the previous section.

Fig. 4 shows the net utilised solar energy as a function of the mantle height. The highest thermal performance is obtained with a mantle height of 0.25-0.30 m. The thermal performance can be increased by 5% by reducing the mantle from 0.43 m to 0.27 m. This is not in agreement with earlier theoretical investigations showing that the top of the mantle is best situated just below the level of the auxiliary volume, because this position maximises the heat exchange area without the auxiliary energy supply system heating the solar collector fluid in the mantle [8], [14]. If the top of the mantle is located above the level of the auxiliary energy supply system then the auxiliary energy supply system will heat up the mantle fluid and the thermal performance of the system will decrease.

The main reason for the new results is that the simulation model now takes the heat flow in the water in the inner tank into consideration. The heat flow in the water in the inner tank is caused by the upward fluid velocities along the tank wall during supply of heat from the collectors. Therefore the model calculates the thermal stratification which is built up in the hot water tank during periods with supply of heat from the collectors, not only in the mantle level of the tank, but also above the mantle. Another reason is that the mixing, occurring in
the mantle caused by differences between the temperature of the incoming solar collector fluid and the temperature of the solar collector fluid which is already in the mantle, now is taken in consideration by the simulation model. Therefore the simulation model now calculates the heat, which in periods with relatively low solar collector fluid inlet temperatures to the mantle is transferred downwards in the tank. This mixing will equalize temperature differences in the tank resulting in a decreased thermal performance of the system. With a reduced mantle height the influence of this mixing on the thermal performance of the system will be reduced.

The reasons for the increased thermal performance of the system by reducing the mantle height are a reduced tank heat loss due to the smaller mantle surface area and the increased insulation thickness, a decreased equalization of temperature differences in the tank in periods with relatively low solar collector fluid inlet temperatures to the mantle and the fact that the heat transfer area for the heat transfer from the solar collector fluid to the domestic water is not strongly decreased by reducing the mantle height. This is the case because heat by thermal conduction is transferred from the tank wall surrounded by the mantle to the tank wall above the mantle. Consequently, the heat transfer area used for transferring solar heat to the domestic water is a large part of the tank wall. However, the heat flow model used in MANTLSIM was developed based on CFD-calculations where the mantle covered either the lower half of the tank or all the tank height. The mantle height that gives the highest thermal performance in this study is when the mantle is covering less than one-third of the mantle height. There is a risk that when the mantle gets too small the model is not calculating the natural convection flow in the inner tank correctly and is over-predicting the effect of natural convection.

On the other hand, if these results are true, it opens a new perspective in the mantle tank design because less material can be used making the mantle tank cheaper, less heavy and therefore easier to install if only the bottom third of the tank should be covered by the mantle. There is therefore a need to verify by experiments that the small mantle height is able to create the degree of thermal stratification above the mantle calculated by the model.

Fig. 5 shows the net utilised solar energy as a function of the height/diameter ratio of the tank. It appears that the thermal performance increases with increasing height/diameter ratios. The net utilised solar energy increases by about 4% if the height/diameter ratio is increased from 2 to 5. When the height/diameter ratio is increased, the thermal stratification in the inner tank will also be increased, because the distance between top and bottom of the tank is increased and because the cross section area of the tank is decreased. The heat loss from the tank is also increased when the height/diameter ratio is

increased, but this is for height/diameter ratio between 2 and 5 overshadowed by advantage caused by the thermal stratification. If larger height/diameter ratios than 5 were investigated, the net utilised solar energy would at some point start to decrease because the increasing heat loss gets more and more dominating.

Fig. 6 shows the net utilised solar energy as a function of the auxiliary volume. It is seen that the thermal performance increases with decreasing auxiliary volumes. This result is expected as a larger auxiliary volume will increase both the auxiliary energy demand and the heat loss, and thus decrease the net utilised solar energy. The storage capacity for the solar energy is increased with decreasing auxiliary volume, and this also influences the result. The net utilised solar energy increases by 19% by decreasing the auxiliary volume from 82 l to 39 l. The auxiliary volume should be as small as possible while still meeting the demand of hot water.

Fig. 7 shows the net utilised solar energy a a of the insulation thickness of the sides of the tank. The figure shows that the thermal performance of the system increases with increasing insulation thickness at the sides.

A number of parameter variations have been carried out to reveal how the different tank parameters influence the thermal performance of low flow SDHW systems. In the following it will be elucidated how to improve the design of the Danlager 1000 mantle tank by relatively simple geometrical changes. The change of the design is made in such a way that one parameter is changed at a time in the calculations. Also here the data from Table 3 are used for the reference system in the calculations.

The following tank parameters are changed: height/diameter ratio of the tank, mantle height, insulation, thermal conductivity of the tank material and the wall thickness of tank and mantle.

Fig. 8 shows the net utilised solar energy and the solar fraction as a function of the different changes in the mantle tank design. The first column shows the thermal performance of the system with the Danlager 1000 heat storage with mantle inlet position at the top of the mantle.

The first change is the height/diameter ratio of the tank, which is changed from 2 to 4. The total volume and the ratio between the auxiliary volume and the total volume are kept constant. The second change is the mantle height, which is decreased from 0.72 m to 0.55 m. The third change is the insulation of the tank. It is assumed that the tank should fit into a cabinet with dimensions 0.6×0.6×2.0 m3, and by increasing the height/diameter ratio the outer diameter is reduced, and the side insulation is increased by further 0.05 m. The fourth change is the tank material, which is changed from normal steel to stainless steel. The fifth, and last, change is the wall thickness of tank and mantle, which is reduced from 3 mm to 2 mm.

The change concerning the insulation gives the most significant improvement, while the change of the wall thickness gives the smallest improvement. The net utilised solar energy is increased from 802 kWh/year to 1009 kWh/year by applying the mentioned changes in the design. It is an improvement of 26% of the net utilised solar energy.

2. CONCLUSIONS

An improved simulation model for low flow SDHW systems has been developed and validated by means of experiments.

Calculations with the model have shown that marketed mantle tanks can be strongly improved by relatively simple design changes: By increasing the height/diameter-ratio, by reducing the mantle height, by increasing the insulation thickness on the sides of the tank and by using stainless steel instead of steel as tank material.

The thermal advantage foreseen by the calculations by decreasing the mantle height is of great interest, since the cost of the mantle tank can be decreased by reducing the mantle height. However, it must be mentioned that the model is not validated for small mantle heights. It is therefore recommended to start investigations with the aim to elucidate if the model simulates the thermal behaviour of mantle tanks with small mantle heights in the right way.

[1]