Category Archives: EUROSUN 2008

Usable Temperature Level

The achievable storage capacity of a sorption TES is depending on the available charging temperature and the required discharging temperature of the actual application. This relation can be calculated on the basis of the sorption equilibrium of the sorbent material. Figure 8 shows how the storage capacity depends on the temperature lift for different charging temperatures (TDes) in an open adsorption TES system [7 Dr AH]. The diagram shows curves for Zeolite and Silicagel. In addition to that the curve for the storage capacity of a hot water TES can be seen in figure 8.

It is obvious that higher charging temperatures are leading to higher storage capacities.

Figure 8: Storage capacity depending on the temperature lift

In a real application e. g. the heating system requires a minimum temperature. Below a certain temperature lift the thermal energy can not be used anymore. The values for the achievable storage capacity are strongly depending on the temperature lift. They are falling drastically, when higher Temperature lifts are needed. The curves of Zeolite and Silicagel intersect at a temperature lift of about 20 K. If a lower lift is sufficient Silicagel can reach higher values, if higher lifts are required Zeolite is delivers higher capacities.

Description of main achievements and developments

Major developments of the projects are two pre-industrial prototypes hereafter referred as OPICS1 and OPICS2, see Figure 1. Main features of these prototypes are:

• Prototype OPICS1 : ICS with a TIM-cover, a water store of 75 l and with a modular rectangular shape (of about 1m x 0.75 m x 0.20 m) that facilitates its integration in buildings. Other technical parameters are:

• Maximum relative pressure acceptable in the tank is limited to 1 atm

• Use of a high selective coating in the absorbing surface

• The absorbing area is of 0.75 m2

• The daily solar efficiency after 24 hours of outdoors exposure without draw-off is in the range of 55 %.

• Prototype OPICS2: ICS with a TIM-cover, a PCM store with an internal heat exchanger that permits to transfer energy from the PCM to the thermal fluid, and with a modular rectangular shape (of about 1m x 0.75 m x 0.15 m) that permits the integration in buildings. Other technical parameters are:

• Maximum relative pressure acceptable in the thermal fluid is above 10 atm

• Use of a high selective coating in the absorbing surface

• The absorbing area is of 0.75 m2

• The daily solar efficiency after 24 hours of outdoors exposure without draw-off is in the range of 50 %.

Both pre-industrial prototypes have been modelled, constructed and tested in detail following ISO procedures [4,5]. The tests included measurement of the daily efficiency, measurement of the heat loss coefficient and measurement of the draw off process. The tests have also been modelled using simulation tools [2,3], and good agreement has been observed between the experimental and numerical results. This has proved the credibility of the numerical model.

The numerical model has also been used to estimate the yearly performance of the new prototypes when installed in different climates and for four different demands corresponding to four reference applications:

• Pool heating

• Domestic hot water

• Space heating

• Absorption cooling using a LiBr-H2O machine of simple effect.

Three climates have been considered which corresponded to the cities of

• Barcelona

• Almeria (South of Spain)

• Copenhagen.

In order to analyse the advantages of the new prototypes, simulations of the OPICS prototypes have been compared to simulations of two ICSs using standard materials (no TIM nor PCM) and with a water store equal to the water store of the prototype OPICS1. They are referred as refl and ref2. The reference ICS refl uses a black paint as selective coating (low-medium selective coating), while the ICS ref2 uses the same high selective coating that is used in the prototypes OPICS1 and OPICS2.

Results have shown that, as already known by the solar thermal community, it is very important to use high selective coatings, because the industrial cost can be similar to that of low or medium selective coatings and energy gains are drastically increased.

Results from the yearly performance estimation also show as in all climates and applications, the energy gains in the solar systems are higher when using the OPICS prototypes than when using the standard ICS with a high selective coating, ref2. For applications at low temperature levels as pool heating and domestic hot water, the increase in the energy gains is small, in the range from 0 to 5 %. The explanation is that the use of TIM mainly contributes to a reduction of the heat losses through the cover due to convection effects, and for low temperature levels, thermal losses through the cover are low. For applications at higher temperature level, space heating and absorption cooling with the LiBr-H2O machine of simple effect, as thermal losses through the cover are more important, the reduction of the convection effects leads to a drastic increase of the energy gains. For the space heating application, this increase is in the range from 35 to 50 %. In the absorption cooling application, the reference prototype ref2 is only able to have energy gains in very hot and insulated climates as in Almeria. In other colder climates, even in the Mediterranean climate of Barcelona, no energy gains can be obtained from the ICS ref2. On the other hand, the OPICS prototypes are able to have energy gains in all climates.

2. Conclusions

In the project OPICS Integrated Collector Storage (ICS) devices have been developed using Transparent Insulation Materials (TIM) and Phase Change Material (PCM). Main features of the prototypes are a compact design that facilitates their integration in facades and roofs of buildings and an improved efficiency with respect to the standard ICSs.

The research approach has been based on the combination of virtual prototyping techniques (numerical simulation) and the construction and measurement of experimental set-ups and prototypes following ISO procedures.

Pre-industrial prototypes of the OPICS-ICSs have been constructed, tested and modelled in detail.

Results have proved that improved efficiencies can be achieved with respect to standard ICSs, and

that they can be used in applications were standard ICSs are not able to give a reasonable

performance.

Acknowledgements

This work was funded in part by the European Commission under the Fifth Framework

Programme, Thematic Programme: Energy, Environment and Sustainable Development FP5-

EESD, Project CRAFT-1999-70604.

References

[1] J. Cadafalch, A Detailed Numerical Model for Flat Plate Solar Thermal Devices, Sol. Energy (2008).

[2] J. Cadafalch, R. Consul and A. Oliva, Detailed Model for the Virtual Prototyping of Flat Plate Solar Thermal Devices, Proceedings EUROSUN 2006, Glasgow.

[3] J. Cadafalch et al., Optimised Integrated Collector Storage: Low-Cost Solar Thermal Systems for Houses and Offices (OPICS). CRAFT Publishable Report, EU Contract CRAFT-1999-72476, 2005.

[4] ISO9459-2. Solar heating -Domestic water heating systems — Part 2: Outdoor test methods for system performance characterization and yearly performance prediction of solar-only systems, 1995. International Organization for Standarization, ISO 9459-2:1995(E), Switzerland.

[5] ISO9459-5. Solar heating -Domestic water heating systems — Part 5: System performance characterization by means of whole-system tests and computer simulation, 1996. International Organization for Standarization, Draft International Standard ISO/DIS 9459-5, Switzerland.

Correlation of the pressure and the temperature in the fixed bed

Fig. 3: Temperature T and pressure p measurement in function of time t at different positions z in the fixed bed of spherical zeolite 13X particles of dp=1.51mm average diameter. The arrows are indicating the increasing sensor signal of the temperature and pressure sensor located at the same position z in the fixed bed.

time t [s]

Подпись: Fig. 3: Temperature T and pressure p measurement in function of time t at different positions z in the fixed bed of spherical zeolite 13X particles of dp=1.51mm average diameter. The arrows are indicating the increasing sensor signal of the temperature and pressure sensor located at the same position z in the fixed bed.

The temperature level of a storage system determines the user temperature. For this reason the particle surface temperature should be known and it was measured at the same position z as the vapour pressure was measured in the fixed. The pressure p is a direct measure for the mass m of vapour which itself determines the heat released in the phase transition i. e. in the adsorption process. In Fig. 3 the temperature T and pressure p measurement in the fixed bed of spherical zeolite 13X particles of average diameter of dp=1.51mm are shown. The arrows in the diagram are pointing on the same sensor signal time t at which the pressure and temperature sensors at the same position z are showing an increasing signal. A direct correlation of the pressure p and temperature T development can be seen.

In a sorption storage system the reachable temperature T has a dominating focus of interest because it determines the succeeding use. Although this main parameter is determined by the dynamic behaviour in the adsorption process through the pressure p development in the fixed bed mainly temperature

measurements are shown in the following section and through the above mentioned correlation of T and p the interpretation of results holds for all experiments.

Purpose of the study

First generation of highly conductive graphite composites with high thermal storage capacity of the (Na/K)NO3 eutectics salts opened next problems for improvement. Thermal stability limitations of newly produced composites require further fundamental research and development. The leakage of the salt from a graphite matrix, double peaks on a DSC curve, melting enthalpy differences between composites produced with melted salt eutectics (infiltration method) and those produced with mechanical mixtures of the salt components (cold compressed method), focused our examination on the salt eutectics, solidified in a graphite composite. Segregation problems of molten salt eutectics during melting in the composite structure; contribution of a larger interfacial contact in the solid solution of salt/graphite composite (compared to the crystallites of the single pure component salts) decided research on the control of salt eutectics crystallization process to achieve good thermal stability as a precondition for a high thermal capacity of the composite heat storage material.

TCM bulk flow reactor system

In a bulk flow reactor (see Fig. 7), the TCM powder flows by means of gravity along a number of vertical plate heat exchangers. This concept has the lowest auxiliary energy use and allows for long reaction times, while heat and vapour transport may be optimised by keeping the layers of active material sufficiently thin (having a sufficiently large heat exchange area). However, special provisions will be required for the vapour transport and there is some risk of the TCM sticking to the heat exchanger plates. In addition, the system will need an active means to transport the TCM powder to the top of the reactor.

Fig. 7. TCM bulk flow reactor

For this system, no reliable value for the heat transfer between powder and plates was found in the literature. Therefore, a value of 300 W/m2K was assumed, being the lower limit for an extruder reactor, with the reasoning that also in the bulk flow concept some mixing of the material occurs. For the case of dehydration of the TCM, assuming that the heat transport would be the limiting factor for the power, for a 3kW reactor, this would lead to a reactor of about 20 cm in length and 10 cm x 10 cm base area, in which 50% of the reactor volume would consist of 11 parallel heat exchanger plates and the remaining volume would be filled up with TCM.

5. Conclusion

The practical realization of a separate thermochemical reactor would increase the feasibility of seasonal storage of solar heat in thermochemical materials. The present paper focuses on the boundary conditions for such a reactor and on reactor concepts that could fulfil these boundary conditions. Three compact powder reactor concepts for dehydration are shown. Further research is necessary to establish the practical feasibility of these reactor concepts for thermochemical storage.

References

[1] Visscher, K., Veldhuis, JBJ., Oonk, H. A.J., Van Ekeren, P. J., Blok, J. G. (2004), Compacte chemische seizoensopslag van zonnewarmte, ECN report C04074

[2] H. A. Zondag, A. N. Kalbasenka, M. Bakker, R. Schuitema, V. M. van Essen, W. G.J. van Helden (2008),

A first study into design aspects for reactors for thermochemical storage of solar energy, Proceedings OTTI-Symposium Thermische Solarenergie. [16] [17]

Ice Store

For the purposes described above, the ice store has to comply with several requirements: manufactured from standard components, high capacity, low cost, high flexibility during charging and discharging performance and use of cheap, easy-to-handle and ecologically harmless phase change material. Based on these requirements a prototype was built at the beginning of 2007 and experimentally investigated. Possible heat exchanger materials are copper and polypropylene. These materials may offer satisfactory thermal conductivity. They also provide the possibility to operate the ice store as heat store, as well. This may be necessary if the chiller is used as a heat pump. The tank is a standard hot water heat store with a volume of approximately 500 litres. Especially simultaneous charging and discharging require external melting. For this purpose, the ice store is separated into two circuits. The first circuit is for the coolant and connected to the evaporator of the absorption chiller. The second circuit has two functions: 1) the water in the second circuit freezes at the surface of the heat exchanger and 2) water which does not freeze is used for air conditioning. Hence, the possibility of water flow into and through the tank has to be assured at any time. Therefore complete freezing of water in the tank is not possible.

Another possibility to discharge the ice store is internal melting. In this case the heat exchanger in the store is connected to the chilled ceiling circuit. The ice will melt from the inside to the outer boundaries of the ice layer. In this operational mode of the ice store simultaneous charging and discharging is not possible.

Dronninglund

Dronninglund Fjernvarme is a district heating company producing 40.000 MWh/year with natural gas fuelled CHP and boilers. Dronninglund has got support to design an energy system with app. 35.000 m2 solar collectors, 50.000 m3 pit heat seasonal storage and 3 MW compression heatpump (thermal output) covering 50% of the yearly consumption. The heat pump uses CO2 as medium and can produce hot water at 80 oC as needed for forward temperature in the district heating sys­tem.

The pit heat storage will be of same type as the 10.000 m3 storage in Marstal, but the floating cover construction will be changed to a solution where LECA is uses instead of mineral wool and EPS as insulation and where the cover can be parted in sections making it possible to construct large covers and reduce problems if the constructions is not tight

Fig. 2. Construction cross section, Marstal

Roof-foi hvt. Underlayment for roof-foi

SecuGrid 30 x 30 380-500 mm Leca

Geotextile kl. IV

2,5 mm HDPE Polymermenbrane BAM

3 Horizontal valleys and 4 toppoint

bottom of valley

Minimun 4 permille slope.

1 — 4 drain pipes

A II TOM

Water

Water 2.5 mm HDPE

30324

Подпись:Polymermenbrane

Geotexti e к. V

20000 85000.

Fig. 3. Contraction croos section, Dronninglund

The heat production price is calculated to 70 €/MWh (annuity 0,1). Investment costs are app. 11. mio. €. The Danish Energy Agency is applied for support.

Air-sand heat exchanger

The heat exchanger should show the following properties:

• Good heat transfer, heat losses < 20 %, temperature difference between sand outlet and air inlet temperature < 10 %

• Low pressure loss ApAir < 5000 Pa

• Compact design

To meet these requirements a concept has been chosen, which is featured by:

• Cross flow arrangement of heat transfer media air and sand

• Low width design for reduction of pressure drop in the moving packed bed

The hot air enters through a porous wall, passes through the moving bed of sand, and flows out at lower temperature through another porous wall, see Fig. 3.

The requirements to be met by the porous walls include:

• Thermal strength

• Abrasion resistance

• Low pressure drop

• Mechanical strength

The Drake Landing Solar Community

The Drake Landing Solar Community (DLSC) consists of a small suburb of 52 homes, where at least 90% of the space heating load is to be provided from solar thermal energy within five years of its operation. A description of the DLSC’s operation is provided elsewhere [3]. In use at the DLSC are two short-term thermal stores (STTSs) which interact with the various thermal systems at the site. The STTSs are typical liquid thermal energy stores, albeit large. Their configuration is illustrated in Figure 1.

1.1.

EUROSUN 2008

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

t ank Dimensions.

• ~11.5 m (length)

• ~3 m (diameter)

separating flow within each tank

Operating Flow Rates:

• 3.35 L/s to 14.9 L/s (Re ~ 100,000 ^ 450,000)

Connector Pipe:

• 0150 mm

Tank Insulation:

• R-20 along tank shell

• Adiabatic conditions assumed during charging/discharging

Three Operating Loops:

Solar Collector Loop (0100 mm) District Heating Loop (075 mm) Seasonal Storage Loop (050 mm)

Figure 1. The Short-Term Thermal Stores at the Drake Landing Solar Community


Prior Work: A CFD Model of the Drake Landing STTSs

In a previous study by the authors [4], an effective CFD model of the STTSs was developed using the FLUENT 6.3.26 commercial software package. The model was validated against real performance data recorded at the Drake Landing site in October 2007. The modelling procedure developed in the previous work is applied similarly in this study, and is listed below in Table 1. Further information is provided in the original paper.

♦ PISO Pressure-Velocity coupling with neighbour correction

Подпись:

♦ Double-precision, ♦

segregated 1st-order implicit unsteady solver

♦ k-epsilon Realizable turbulent model

♦ Default simulation convergence criteria

Polynomial correlation to define temperature dependent material properties (i. e., density). Thermodynamic data: [5]

Pressure under-relaxation of 0.9 Momentum under-relaxation of 0.2 Default values for all other components

Second Order Upwind ♦

discretization of momentum, turbulence, and energy. Body Force Weighted discretization of pressure

Подпись: Polynomial correlation to define temperature dependent material properties (i.e., density). Thermodynamic data: [5]

Table 1. Modelling parameters applied to the CFD simulation of the STTSs

Experimental study of composite sorbents

2.1. Experimental setup

The chemical storage is designed to undergo the endothermic heat storage reaction during the summer, using heat obtained from tube solar collectors, and release the heat stored by using the exothermic reverse reaction during the winter. The hydration/dehydration cycle of magnesium sulphate has previously been identified as particularly promising [3]. An experimental study has been performed on this material to describe and understand its behaviour during its hydration by water vapour. The principal thermal properties of the MgSO4 / MgSO4.7H2O system are presented in Table 2.

Reaction

MgSO4 + 7 H2O ^ MgSO4.7H2O hydration, exothermic — dehydration, endothermic

Enthalpy of hydration

— 410 kJ. mol-1

Temperature of dehydration

122°C

Density of the dense salt

MgSO4

2660 kg. m-3

MgSO4.7H2O

1680 kg. m-3

Energy density of the dense salt

MgSO4

9.06 GJ. m-3 / 2.52 MWh. m-3

MgSO4.7H2O

2.80 GJ. m-3 / 0.78 MWh. m-3

Table 2. Properties of the MgSO4 / MgSO4.7H2O system [4]

By dispersing the magnesium sulphate over a larger exchange surface, the thermal power released during hydration is greatly increased. Composite materials made of a sorption material and magnesium sulphate were prepared in such a way to preserve the porosity of the sorption material. This feature is essential in order to allow the water vapour to react with both the salt and the matrix. Sorption materials, such as zeolites and silica gels, were tested as host materials for the salt composites.

The samples were hydrated in a thermally insulated (non-isothermal) open reactor using a flow of moist air at a relative humidity close to 100% at ambient temperature.

2.2. Results

For all of the hydration tests, the rate of water uptake (grams of water per gram of material per hour) is calculated from the exit air humidity and temperature measurements. These values qualify the thermal characteristics of the system : the water uptake is proportional to the heat produced and the rate of water uptake is proportional to the power delivered by the reaction (Fig. 6).

Fig. 6. Rate of water uptake during hydration

High rates of water uptake are obtained for a much longer period with the zeolite/MgSO4 composite ZM15 compared to the analogous silica composite SM16 and the reference pure materials (Fig. 6). This results in higher temperature lifts for the zeolite composite (Fig. 7). Maintaining the high rates of reaction over a long time will ensure that a maximum amount of usable high-grade heat is produced.

Fig. 7. Temperature lifts and peaks of power density during hydration

These peaks of temperature lift are associated with peaks of power of 28.4, 20.1, 20.7 and 15.6 mW per gram of material for the hydration of ZM15, zeolite, SM16 and silica gel respectively. The zeolite/ magnesium sulphate composite proves more favourable than the choice of a silica gel matrix. Other materials, such as polymer binders, are currently being investigated to create a porous expanded structure for pure magnesium sulphate.

4. Conclusion

A high performance long-term heat storage system is needed to meet the huge heat demand of the building sector and the seasonal variations in the availability of the solar resource. Thermochemical

energy storage proves to be a relevant solution for this purpose since it does not loose energy with time and can provide the high energy densities necessary for compact storage.

Modelling and experimental studies have been performed on the hydration/dehydration cycle of magnesium sulphate composites to design an innovative system of seasonal storage of thermal energy.

A reference solar combisystem, without any long-term heat storage, has been simulated. The size of the required heat storage material tank has been calculated for a 191 m2 single-family house, with a space heating demand of 37.2 kWh. m-2 (53.0 kWh. m"2 including domestic hot water), under the Parisian climate. The results predict a volume of 0.2 to 0.9 m3 of magnesium sulphate (i. e. 1.0 to 4.8 m3 per m2 of living space) to reach a solar fraction ranging from 50 to 57 %. For the same house located in Marseille, with a space heating demand of 15.4 kWh. m-2 (31.1 kWh. m-2 including domestic hot water), a 100% solar fraction is achievable with a volume of 0.7 m3 of magnesium sulphate.

The limited performances of pure magnesium sulphate powder in real conditions have led to the investigation of porous composite materials, such as zeolite/salt and silica gel/salt composites. Thus, for example, the design of a composite material with 50% of the theoretical energy density of the dense salt would be enough to reach an acceptable storage volume of 2 m3. Temperature lifts around 30°C and maximal power of 28 mW. g-1 have been obtained during the hydration of the zeolite/MgSO4 composite ; lower values were obtained with pure zeolite, silica, or silica/MgSO4 composite tested under the same conditions. Further experiments are in progress to test the stability of these materials after several cycles and to find other appropriate composites to improve the performances. Summer regeneration of the material along with complete modelling of the chemical heat store will also be investigated.

References

[1] « Factor 4 : Doubling wealth — halving resource use, A report to the Club of Rome », Earthscan Publications Ltd., London, 1997.

[2] TRNSYS : http://sel. me. wisc. edu/trnsys/

[3] K. Visscher and J. B.J. Veldhuis , “Comparison of candidate materials for seasonal storage of solar heat through dynamic simulation of building and renewable energy systems”, paper presented at the Buildings Simulations 2005, in Montreal, Canada, August 15-18, 2005.

[4] J. Van Berkel, “Storage of solar energy in chemical reactions”, Thermal energy storage for solar and low energy buildings, IEA SHC Task 32, JC Hadorn (Edt), ISBN 84-8409-877-X, 2005.