Category Archives: EUROSUN 2008

Objective and scope

The objective of this international collaboration is to develop advanced materials for compact storage systems. The systems will store thermal energy for renewable heating and cooling or for energy conservation.

The objective can be subdivided into six primary goals:

• to identify, develop, and test advanced materials for compact storage,

• to design and develop new materials or composites,

• to develop measuring and testing procedures to characterise these new materials reliably and reproducibly,

• to perform pre-standardisation work for advanced thermal energy storages,

• to develop and demonstrate novel compact thermal energy storage systems employing the advanced materials, and

• to disseminate the gained knowledge.

Because seasonal storage of solar heat is the main application for thermal energy storage, this field will have the focus of this task. However, other applications are also very important, including applications in both renewable energy and energy conservation technologies. Because of this, and because materials research is not and can not be limited to one application only, this task will not focus on only one application area. Other applications in renewable energy and energy conservation-such as solar cooling, micro-cogeneration, biomass, or heat pumps-will also be taken into account.

In terms of classes of materials, this task will be focused around three main classes:

• phase change materials, including micro — and macro-encapsulation and slurries,

• thermochemical materials, including sorption, and

• composite materials and nanostructures.

The latter category includes for instance the combination between zeolites and silicagels, and materials where the molecular and crystalline structure are engineered in detail. Production technologies, the ability to produce advanced materials on a large scale at reasonable costs, is an important aspect in all of these classes.

Main activities

The task will be organised around the following main activities:

• material engineering: analysis and engineering of advanced materials, synthesis of new materials and composites, and materials characterisation and testing;

• numerical modelling: numerical modelling of materials, including molecular interactions, mass and heat transport phenomena, and bulk behaviour;

• components and systems: development, numerical modelling, and testing of (prototypes of) thermal storage components and systems that use the materials developed in the other activities.

Collaboration

The main challenge for this task is to bring together material experts and application experts (particularly solar applications, given the primary scope of the task). The active participation of both groups of experts is essential for this task. However, these groups are traditionally organised in different Implementing Agreements. ECES, Energy Conservation through Energy Storage, has a strong tradition in material research, while SHC, Solar Heating and Cooling, has a strong tradition in solar applications.

Because of the particular nature of this task, a collaboration between these two Implementing Agreements is essential. Hence, this task will take the form of a Joint Task between these Implementing Agreements.

Progress

On October 5, 2007, a first expert meeting was held in Zurich, Switzerland, followed by a Task Defintion Workshop in Petten, The Netherlands, on April 10-11, 2008. Both meetings were very well attended, not only by researchers, but also by representatives of European industries, both large and small. Based on these meetings, the outline of the new task’s objective, scope, and main activities were defined. In a subsequent expert meeting in Bad Tolz, Germany, on June 4-6, 2008, the topics of the task were discussed in more detail.

One of the conclusions of these meetings wass that the main value in this task is to actively combine the knowledge of experts from materials science as well as from solar/renewable heating and energy conservation. Hence, a strong co-operation with other IEA Implementing Agreements is essential to the success of the task, as already mentioned above. Another noteworthy conclusion of the meeting was the strong interest of industry in this task: the industry representatives at the meeting all agreed on the importance of the development of new storage materials.

In their respective ExCo meetings in May and June 2008, the new task was officially approved by both the SHC and ECES Executive Committees. The task will be designated as Task 42 in SHC, and as Annex 24 in ECES. Both ExCos were very positive on the scope and topic of this task. Wim van Helden of ECN, the Netherlands, and Andreas Hauer of ZAE Bayern, Germany, were appointed as Operating Agents.

The official starting date for this four-year task is January 1, 2009. Although four years is relatively long compared to other tasks in either SHC or ECES, this is warranted by the fundamental nature of the work in this task. Because the work on advanced storage materials is still in a very fundamental stage, the trajectory towards applications is still relatively long. It takes several years to identify, characterise and optimise the right materials or composites, and again to develop the reactors, proof-of-principles, and prototypes of the advanced storages made of these materials.

Join the task

Experts from industries and research organisations worldwide, that are active in this field, are cordially invited to join this new task. If you are interested, please contact the authors of this paper or your national ECES or SHC ExCo member.

More information on this task can be found on the temporary task website at www. ecn. nl/ieamaterials.

Melting of the material

Several sources indicate that MgSO4.7H2O melts at a temperature range of 50-52.5°C during dehydration (see for example Refs [2, 7]). Melting is a problem, since it reduces the bed porosity of the material, and consequently the vapor transport through the bed, which limits the ability of the material to take up water again. The TGA-DSC curve for heating rate at 1 K/min (Figure 1) shows that for MgSO4.7H2O is completely converted to MgSO4.6H2O at 50°C and therefore no melting occurs.

However, if the heating rate is increased, the dehydration peak is shifted to higher temperatures due to kinetics. Figure 3 illustrates the effect of the heating rate on the dehydration peak:

Fig. 3. Influence of heating rate on the dehydration

It means that for higher heating rates (> 1 K/min), MgSO4.7H2O may still be present at 50-52.5°C and starts to melt. The melting of MgSO4.7H2O was studied by a combined TGA-DSC measurement. This can be distinguished from dehydration since melting causes a heat uptake of the material without a change in mass as illustrated in Figure 4.

Fig. 4. Melting for large particles. The red circle denotes position of melting temperature of MgSO4.7H2O

The results of the combined TGA-DSC experiments indicated that melting of MgSO4.7H2O typically occurred for high heating rates, large sample masses and/or large particles. The experiments also indicated that only melting of MgSO4.7H2O occurs during dehydration of the material.

Background data (solar radiation, temperature and collector tilt analysis)

The analytical analysis presented in this manuscript was made assuming the location Vagos, Portugal (40°32’42,18"N, 8°42’56,72"O) because this is the site where the prototype will be installed. Climate variables (solar radiation and temperature profiles) were obtained through the

SolTerm data base [1]. In this region, the minimum average temperature is 9,0°C in December and 19,7°C of maximum average temperature in July. Global solar radiation in the horizontal plane varies from a minimum of 1,7 kWh/m2 in December to a maximum of 6,6 kWh/m2, registered in July. Figure 1 presents the solar radiation profile as function of the angle of the solar collector with the horizontal.

Fig. 1. Solar radiation profile as function of the solar panel tilt.

Radiation profile presented in Figure 1 was obtained by applying the correction factor only to the direct radiation in spite of the fact that the diffuse radiation, coming on days with clear sky, is not uniformly distributed over the hemisphere but concentrated in the direction of the sun. Correction factor R was defined as Rd = cos 0 / cos 0z, where 0 represents the angle of the solar radiation with the normal of the panel and 0z the angle of the solar radiation with the vertical of the location.

2. System analysis

Homogeneous structuring of the (Na/K)NO3 Eutectics/graphite composites

The FTIR spectral analysis did not prove generation of chemical products during crystallization process of the Composite 3, but two peaks on DSC solidification curve of the Composite 3 (Fig.1) would suggest that the inhomogeneous structure was formed during crystallization process. Additionally the DSC curve of the Composite 2 (Fig.1) showed bigger temperature hysteresis of 4 degrees, while the subcooling is of 3 degrees. DSC results merely defined needs of more detailed further research and analysis of the composite structure. Next correlation between thermal and structural properties initiated observations of inhomogeneous fragments of the novel salt/graphite composites structures, well exemplified with SEM images below. (Table 3.).

The inhomogeneous composite structure shows inhomogeneous salt crystal distribution, salt/graphite clustering, untypical salt crystal formation caused by uncontrolled nucleation frequency and crystallization growth rates of the NaNo3/KNO3eutectics — important factors responsible for structural stability of the composite

Inhomogeneous salt crystal distribution

Uncontrolled salt
crystallization

Salt/graphite clusters

Table 3. Inhomogeneous structural fragments of NaNO3/KNO3eutectics/graphite composites.

Heat exchanger test

1.2 Measurement set-up

On the basis of simulation results and available air heater capacities an experimental heat exchanger set-up with nominal heat flow of 15 kW, maximum air inflow temperature of 800 °C and dimensions of 0.5 m height,

0.05 m width and depth of 0.1 m was designed and manufactured. The sand inflow and outflow were extended by ducts of 0.5 m height to reduce air leakages. The crucial elements of the heat exchanger are two porous walls for filtration of air at the air entrance and exit.

Fig. 3 Experimental air-sand heat exchanger unit

The filtering walls have to meet various requirements including

• Abrasion resistance for continuous sand flow

• Thermal stability up to 800 °C

• Low pressure drop

In view of the two requirements mentioned first, recrystallized silicon carbide (RSiC) was chosen as wall material.

For calculation of heat transfer, heat losses and blower energy demand volume flows, air pressure drop and air and sand temperatures were measured at different positions.

Practical Considerations for Closed-Loop Systems

v0§CR (t) = . ^stored (t) • 100%

I

^ delivered, MAX (t)

^deliveredMAX (t2 ) ^delivered, MAX (t1 )

deliveredMAX

(t) = ■

12 t1

I

d elivered, MAX

(t) Edelivered (t)

E

delivered

T — T

delivered 0 J

(t) V lnr T

delivered

T

T0 J

(7)

(7) (9)

Подпись: (7) (7) (9)

For practical closed-loop applications, special consideration must be given to the flow rate limitations of the system. This is to be expressed by an additional index, the Operational Exergy Charge Reponse (цо&я), whereby:

t

(10)

Подпись: (10)Edelivered (t) = Eavail(t) dt
0

Edelivererd is a representative of a given day’s energy profile. The maximum exergy associated with the energy profile is subject principally to an upper temperature limit, Tdelivered. This can be the desired delivery temperature of the TES, the system’s stagnation temperature, or the working fluid’s boiling temperature. By applying the цо^ск to the fully stratified TES, one may gather a better indication of the exergy potentially lost due to the system’s flow rate limitations.

Discharging the Ice Store

Complete external discharging takes about 5 to 9 hours, depending on discharging temperature and discharging flow. Choosing a flow temperature of 20°C, discharging is possible at an average of 3 kW. At the end there were no significant differences detected between the two types of heat exchangers (Figure 6).

2.2

System A

System B

Discharging

Temperature

[°C]

20

22

24

Internal

20

20

22

24

Internal

20

Average Mass Flow

[kg/s]

0,319

0,319

0,319

0,252

0,319

0,319

0,319

0,270

Average Mass Flow in Store

[kg/s]

0,084

0,95

0,104

0,085

**

**

Average Discharging Rate

[kW]

3,33

4,11

4,99

4,34

3,34

**

**

2,47

Effective Capacity

[kWh

]_

28,55

28,81

28,01

27,33

26,86

**

**

29,23

Period of Discharging

[min]

514

420

337

370

482

**

**

710

Подпись: System A System B Discharging Temperature [°C] 20 22 24 Internal 20 20 22 24 Internal 20 Average Mass Flow [kg/s] 0,319 0,319 0,319 0,252 0,319 0,319 0,319 0,270 Average Mass Flow in Store [kg/s] 0,084 0,95 0,104 - 0,085 ** ** - Average Discharging Rate [kW] 3,33 4,11 4,99 4,34 3,34 ** ** 2,47 Effective Capacity [kWh ]_ 28,55 28,81 28,01 27,33 26,86 ** ** 29,23 Period of Discharging [min] 514 420 337 370 482 ** ** 710

Heat losses

No significant losses of cooling energy were measured. At an ambient Temperature of 23°C, loss of cooling energy in 24 hours is about 1 kWh. This means an average rate of loss of about 42 W.

Sand storage concept for solar power towers

With an increasing share of power generated by renewable energy sources, fluctuations in grid power supply due to varying solar radiation or varying wind speeds are increasing, too. Unlike many other renewable energy technologies solar thermal power plants can supply power dependent on the demand using thermal energy storages.

In central receiver systems/solar power towers with open volumetric receiver air is sucked through a porous structure (absorber), which is heated by concentrated solar radiation. By this the air is heated up to 800 °C. In normal operation the air heat is used to run a steam cycle for power generation in a conventional steam turbine-generator system. During periods with excess of solar energy a part of the air current can be also used to charge a thermal storage.

For high-temperature storage up to approximately 800 °C systems using packed beds of ceramic bodies or ceramic bricks have been analysed so far. These storage systems are featured by a simple configuration, but involved with higher costs for the storage material.

In view of storage material costs the sand storage concept for solar power towers with open volumetric receiver has been developed, as shown in Fig. 1:

[12] Introduction

Phase change materials offer a great potential for energy saving in cooling applications as well as efficient energy storage at different temperature levels. Commercially available applications of PCM are e. g. passive solid materials which are used in building walls to shave temperature peaks in the afternoons and are recooled by outside air during the night [1]. In active cooling applications, the material is integrated in storages, e. g. encapsulated water (ice storage) or paraffin panels and the refrigerant medium circulates around it thus withdrawing or releasing energy [2, 3].

By contrast Phase Change Slurries are emulsions or suspensions of PCM which circulate in the heat/cold network. So the energy is taken directly to the consumer (e. g. chilled ceiling). This technology has the advantage that the problem of slow heat transmission due to the poor heat transfer coefficients and low thermal conductivity of pure macroencapsulated PCM is reduced significantly. The disadvantage is the higher viscosity which can cause problems with small pipe diameters.

Presently PCS are used on major scale only in Japan [4]. There mainly ice-slurries are employed — a suspension of liquid and frozen water with additives preventing the accumulation of ice particles. With

[14] During the cooling process of PCM the initiation of crystallization, usually requires temperatures well below the melting point. This temperature difference is called subcooling.

[15] Q = dt*m*cw =14 °C * 500kg * 1,16Wh/kg = 8,12kWh

[16] V. M. van Essen, H. A. Zondag, R. Schuitema, W. G.J. van Helden, C. C.M. Rindt (2008), Materials for thermochemical storage: characterization of magnesium sulfate, Proceedings Eurosun 2008.

[17] A. N. Kalbasenka (2008), Basis calculations of reactor concepts, ECN memo Acknowledgement

This project has received financial support from the Dutch Ministry of Economic Affairs by means of the EOS support scheme. The work on thermochemical heat storage is part of the long-term work at ECN on compact storage technologies.

[18] Introduction

A novel solar powered 10 kW absorption chiller was developed and tested at the ITW in recent years. To extend the hours of operation and to achieve a high chiller efficiency for air conditioning purposes, a small ice store was designed, constructed and experimentally investigated.

The ultimate aim of this project is to offer an efficient and commercially viable solution for solar driven cooling of small residential and commercial buildings such as single occupancy houses and small office buildings. In a first series of measurements several of the offices of the Institute of Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart are cooled by a

[21] A. Hauer, Beurteilung fester Adsorbentien in offenen Sorptionssytemen fur energetische Anwendungen, Doctoral Thesis, Technische Universitat Berlin, Fakultat III Prozesswissenschaften, 2002.

[22] E. Lavemann, A. Hublitz, M. Pelzer, Betriebsergebnisse einer solarunterstutzten Flussigssorptionsanlage in Singapur (German), Proceedings 4th Symposium „Solares Kuhlen in der Parxis“, Stuttgart, April 3-4, 2006.

[23] The term “total solar fraction” is defined by the following equation: SFtotal = 1 — Eaux/Edemand. where Eaux is the

energy supplied from the auxiliary heater and Edemand is the sum of the energy needed to meet the heating and

cooling thermal comfort in the building and the energy to heat up the domestic hot water.

[26] Floor space heating has proved to be much more effective (compared to fan-coils) in terms of solar system efficiency. Most probably the floor elements will prove to increase the system’s efficiency furthermore, if they are used for cooling as well. However, since “floor cooling”, in order to be effective, needs some dehumidification device (and, consequently, complicated controls and simulations), we have chosen to use conventional fan-coils for cooling in summer. The “floor cooling” configuration (perhaps with a limited number of fan-coils for dehumidification) seems very promising and is, at the moment, under investigation.

[27] Currently, for the simulations there is no possibility to bypass the SST and deliver energy directly from the collectors to the chiller. However, this bypass could increase system’s performance and should therefore be implemented as the optimization procedure evolves.

[28] In the framework of High-Combi, the use of low cost materials as additional insulation of the SST has been investigated by laboratory measurements. The experimental results will be available soon in the project’s web site.

[29] One may argue that some winters may be more severe (or have less solar radiation) than others and, therefore, an auxiliary boiler is needed. In order to handle such fluctuations the solution is a slight increase the collectors’ field area.

[30] By fully charging the DHW tank: i) some energy is taken away from the SST, and ii) there is a reduction of the collectors’ efficiency (compared with their efficiency when charging the SST), due to the higher operating temperatures. However, the amount of energy lost by the SST in order to achieve 100% solar fraction for DHW is about 720 kWh and may, therefore, be achieved with an addition of a few (about 2) m2 of solar collectors.

[31] Higher solar fraction

Most Danish natural gas fuelled CHP-plants are now operating in a free electricity marked, where one of the main sources of income comes from regulation of electricity production. The regulation is needed because of a large fraction of windmill produced electricity in the Danish system.

That means normally less running hours for the engines and 25-50% of the heat produced in a natural gas boiler. Natural gas is an expensive fuel that very often means production prices of more than 60 €/MWh.

Therefore solar heat is an attractive alternative, but if the solar fraction exceeds app. 10%, the ac­cumulation tank is too small for electricity regulation. The CHP-plants are thus eager to find solu­tions making it possible to increase the solar fraction and at the same time keep the regulation pos­sibility. In the following 4 new projects with higher solar fraction are described.

Measuring of a Phase Change Slurry

A series of measurements has been carried out with water as reference. Subsequently an emulsion produced by Fraunhofer Institute for Environmental-, Safety — and Energy Technologies (UMSICHT) was tested. This emulsion consists of 30% PCM (RT20) and 70% water. The melting enthalpy is about 60 kJ/kg over a temperature range of approximately 10 K (between 12 °C and 22 °C). Related to the melting temperature range this corresponds to a heat capacity of only 6 kJ/kgK which is only 1,43 times the value of water.

Fig. 2: The integrated heat flux curve for the pure RT20 and the utilized emulsion with 30% of the material during cooling and heating; measured with a Calvet-DSC

An important figure to evaluate the practicability of a PCS is its viscosity and the resulting pressure drops over different components. If the viscosity is much higher than the one of water, the advantage of higher energy density of PCS may be compensated by the higher hydraulic power necessary for pumping it. Also higher viscosities lead to reduced heat transfer in heat exchangers as the flow tends to laminar regime. Therefore the pressure drop of the phase change emulsion at the hot side heat exchanger was measured and compared to water as shown in Figure 3. The PCS was tested at two temperatures: at 5 °C as it is considered that all the PCM is in solid state and at 25 °C for molten PCM. As expected the viscosity is higher when the PCM is frozen, approximately by 20%. At low flow rates the relative pressure difference compared to water is high; at flow rates with practical relevance (above 300l/h) it takes values between 1,2 and 2. In a real application the PCM will melt or freeze within the heat exchanger, so the real pressure difference will lie between the obtained results.

The following measurement was performed simulating a cooling application with 22 °C supply temperature and 26 °C return temperature, so the entire melting range of the material could be used. The storage was cooled down to 10 °C and the chiller then switched off. The heater was set to maintain the 26 °C. For the flow rate in the heating (load) circuit the following profile was defined: 300 l/h — 500 l/h — 1000 l/h — 500 l/h for an hour each, starting at 20:00 (equivalent to approx. 1390 W — 2320 W — 4640 W — 2320 W). The slurry pump was then controlled in such way that the 22 °C supply temperature was maintained independently of the flow rate in the heating circuit.

time [day of measurement H:M]

Fig. 4: Measured values at the hot side heat exchanger: V_sl_h — slurry flow rate, V_h — heater flow rate, hex- coefficient — heat transfer coefficient, T_WThp_in/out — in-/ outlet temperature of the heater side of the heat exchanger, T_WThs_in/out — in-/ outlet temperature of the slurry side of the heat exchanger

Figure 4 shows that during the first 3 hours the supply temperature (T_WThp_out) can be maintained constant with low slurry flow rates which means the heat can well be transferred to the slurry. A little before 23:00 the slurry temperature (T_WThs_in) starts to rise, so the controller increases the flow rate in the slurry circuit. The heat transfer coefficient is proportional to the volume flow rate, so the set temperature can be maintained although the temperature difference decreases. Shortly before 24:00 the

storage is entirely discharged, which the controller tries to compensate by setting the pump to full speed.

25

Подпись:

5

Подпись:

10

18:00

Подпись:

10 10 10 20:00 21:00 22:00

time [day of measurement H:M]

Подпись:

14

12

10

8

6

4

Подпись:

2

Подпись:

Fig. 5: Process of charging the storage down to 8 °C and discharging to 22 °C; TS_8_0 to TS_123_0 are the temperatures in the centre of the storage at heights from 8cm to 123cm; P_h is the heating power, P_c the cooling

power and Q_h the integrated heat

Подпись:

11

00:00

Подпись: 11 00:00 лі

>s

о

аз. с

ЛІ о

CL

In Figure 5 one can observe the charging and discharging process of the storage. It is cooled down (charged with cold) between 18:00 and 19:30. As a consequence of stratification the bottom part first reaches the set temperature. At 19:30 the discharging commences and the storage is heated up from the top downwards. The stratification is maintained stable as long as the slurry flow rate remains low. Higher flow rates lead to a mixing within the storage. During the entire discharging process 10,4kWh heat are absorbed by the PCS. Due to the large temperature spread (from 8 °C to 22 °C) this is only 1,3 times the amount of energy which could have been stored in water[15]. The same experiment was carried out with the storage being cooled down to different temperatures. By cooling it down to 14 °C and discharging it under the same conditions as before close to 7 kWh can be stored by the PCS. Water could have stored about 4,6 kWh in the same temperature range, so the PCS has a energy density about

1,5 times higher than water in this case. The same factor was obtained by cooling the storage down to 16 °C.

Figure 6 shows the result of the storage being cooled down to 18 °C. By heating it up to 22 °C roughly 4 kWh can be stored in it. The factor compared to water — which could store about 2,3 kWh in this temperature range — is 1,7

28

27

26

25

24

23

22

Q.

1st International Congress on Heating, Cooling, and Buildings — 7th to 10th October, Lisbon — Portugal /

21

8

00:00

05:00

06:00

time [H:M]

2^

> — t—■

О

(0

Cl

<D

Fig. 6: Process of charging the storage down to 18 °C and discharging to 22 °C, in order to cool the heating

18

16

14

12

10

circuit from 26 °C to 22 °C.

This fact implies that the use of PCS is most advisable for small temperature differences. Else the unbeatably high sensible heat capacity of water compensates the advantage of the high latent heat capacity in a narrow temperature band of the PCS.

3. Conclusion

The testing facility which is operated at Fraunhofer ISE allows the comprehensive analysis and characterization of Phase Change Slurries. Not only can the PCS be evaluated on laboratory scale or under stationary conditions — the testing facility with 500 l storage also facilitates the reproduction of reality-like applications on small scale.

The measurement of one Phase Change Emulsion demonstrates the range of parameters that can be analyzed by this facility. Also the applicability of the material under varying operation conditions is manifested. The relatively low increment in heat capacity at the large temperature band of the presented experiment confirms once more that the relative low fraction of latent storable heat argues for applications where even with water only a small temperature difference can be used. The objective for further research is therefore the development of PCS with higher melting enthalpies in a smaller temperature range. With a high AT water as a very cheap heat-carrier-fluid has clear advantages in comparison to PCS.

Acknowledgement

This paper is based on a project supported by the German Federal Ministry of Economics and Technology. Partners within the project were Fraunhofer Institute for Environmental-, Safety — and Energy Technologies (UMSICHT), IoLiTec GmbH & Co. KG and RubiTherm GmbH.

References

[1] P. Schossig, H.-M. Henning, S. Gschwander, T.. Haussmann, Micro-Encapsulated phase — change Materials integrated into constructions materials, Solar Energy Materials & Solar Cells, 2005, Vol. 89, p. 297-306, 2005

[2] Cristopia Energy Systems, Thermal Energy Storage, Product brochure, 2004

[3] Rubitherm Technologies GmbH, Rubitherm FB, Data sheet, 2006

[3] Y. Tsubota, Y. Okamoto, Prospects of Ice Slurry Systems, including PCM slurries, in Japan, IEA/ECES Annex 18 First Workshop, Tokyo, Japan, 2006

[4] H. Recknagel, E. Sprenger, Taschenbuch fur Heizung — und Klimatechnik 2005/06, Oldenbourg R. Verlag GmbH, 72nd Edition, MUnchen, Germany 2004

[5] S. Gschwander, P. Schossig; Paraffin Phase Change Slurries; 7th Conference on Phase Change Materials and Slurries for Refrigeration and Air Conditioning, Dinan, France 2006

Discussion of Results

As described previously, the storage tanks were seen to stratify in a sequential manner as well as individually during the charge sequences. During periods of declining charge temperatures, there was some evidence of mixing, resulting in a slight temperature drop in the upper sections of the storage tanks. As well, it was noted that, during the intervals corresponding to low charge temperatures, the storage tanks appear to be slowly dropping in temperature. This effect is a result of a number of factors including: standby heat losses from the tank walls to the surrounding environment; heat losses from the natural convection loop; and reverse thermosyphoning caused by discharge or carry-over of heat from a high temperature storage to a lower temperature storage. A careful inspection of Figs. 8, 11 and 14 during these intervals show that the bottoms of the tanks are actually increasing in temperature as the upper sections are dropping.

Time (hour)

Fig. 16. Case B, Series: Temperature profile of storage tanks during charging for 2-day high/low input power test.

 

Time (hour)

Fig. 18. Case B, Parallel: Temperature profile of storage tanks during charging for 2-day high/low input power test.

 

Fig. 17. Case B, Series: Individual charge rates across each heat exchanger for 2-day high/low input power test.

 

Fig. 19. Case B, Parallel: Individual charge rates across each heat exchanger for 2-day high/low input power test.

 

SHAPE * MERGEFORMAT

Figure 20 shows the effect of the collector loop flowrate on the magnitude of the heat transfer rate for Day 1. This figure indicates that at lower charge loop flowrates, higher degrees of stratification are obtained in the storage tanks. That is, at lower flowrates, more energy is transferred in Tank 1, driving it to a higher temperature. At higher charge loop flowrates, less energy is transferred to Tank 1 and more is carried over to Tanks 2 and 3. This result is expected as the temperatures in the storages will tend towards a uniform temperature at higher flowrates. In the extreme case, the storages will behave as though the tanks were connected in parallel. This conclusion is supported by results shown in Figs. 16 to 19

1st International Congress on Heating, Cooling, and Buildings, 7th to 10th October, Lisbon — Portugal /

0 2 4 6 8 10

Time (hour)

Подпись: 0 2 4 6 8 10 Time (hour)

Fig. 20. Individual charge rates across each heat exchanger for variable input power tests at various charge flowrates.

3. Conclusion

Laboratory tests were conducted on a series-connected modular thermal storage to measure the unit’s thermal performance and temperature profiles under specified charge conditions. In particular, tests were performed to study the interaction of the individual sequentially-connected tanks and to investigate the effects of rising and falling charge loop temperatures on temperature profiles in the storage tanks. Preliminary results indicate that sequential stratification was observed for the charge profiles studied however a small amount of mixing was observed in the upper section of the storage tanks due to falling charge temperatures in the afternoon period. In addition, results indicate that a small amount of heat was also carried over from the high temperature storage to the lower temperature, downstream storages during charging. Furthermore, the dependence of the heat transfer rate on the magnitude of the charge loop flowrate was also shown. Finally as predicted by theory, as collector loop flow rate is increased, the distinction between the series and parallel connected storages disappears.

Acknowledgements

This study was supported by the Canadian Solar Buildings Research Network, the Natural Science and Engineering Research Council of Canada and EnerWorks Inc.

References

[1] Mather, D. W., Hollands, K. G. T. and Wright, J. L. 2002. Single — and Multi-tank Energy Storage for Solar Heating Systems: Fundamentals, Solar Energy 73, pp. 3-13.

[2] Cruickshank, C. A. and Harrison, S. J., 2006. Experimental Characterization of a Natural Convection Heat Exchanger for Solar Domestic Hot Water Systems, Proceedings of ISEC 2006, International Solar Energy Conference, Denver, Colorado, USA.

[3] Cruickshank, C. A. and Harrison, S. J. 2006. Simulation and Testing of Stratified Multi-tank, Thermal Storages for Solar Heating Systems, Proceedings of the Eurosun 2006 Conference. Glasgow, Scotland.

[4] Cruickshank, C. A. and Harrison, S. J. 2006. An Experimental Test Apparatus for the Evaluation of Multi­Tank Thermal Storage Systems, Proceedings of the Joint Conference of the Canadian Solar Buildings Research Network and Solar Energy Society of Canada Inc. (SESCI), Montreal, Quebec.

[5] Cruickshank, C. A. and Harrison, S. J. 2008. Experimental Evaluation of a Multi-tank Thermal Storage Under Variable Charge Conditions, Accepted for Publication in the Proceedings of the Joint Conference of the Canadian Solar Buildings Research Network and Solar Energy Society of Canada Inc. (SESCI). Fredericton, New Brunswick.