Category Archives: EUROSUN 2008

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]

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.

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

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.

Testing facility for Phase Change Slurries

Bjorn Nienborg[12]*, Stefan Gschwander1, Li Huang[13], Peter Schossig1
1 Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg, GERMANY
2 Fraunhofer Institute for Environmental-, Safety — and Energy Technologies, Osterfelder Str. 3, 46047

Oberhausen

* Bjoern. Nienborg@ise. fraunhofer. de

Phase change materials (PCM) offer a great potential for energy saving in heating and cooling applications as well as efficient energy storage. A series of solid materials has come into the market during the last years. Phase Change Slurries (PCS) are mixtures of a Phase Change Material and a carrier fluid so the material can be pumped. At Fraunhofer Institute for Solar Energy Systems (ISE) two types of PCS are investigated: emulsions and suspensions.

As PCS compete with water which is usually used as pumpable heat transfer medium, they need to meet a multitude of requirements in order to compensate the higher investment cost by lower operation expenses.

At our institute the materials can be analyzed on small scale in a laboratory. Once a promising product is detected, it is tested in a testing facility which reproduces reality-like operation. This work describes the testing facility and illustrates the parameters that can be measured. Subsequently results of an example measurement of a phase change emulsion for cooling applications are presented.

Keywords: phase change material, PCM, Phase Change Slurry, PCS, thermal energy storage

these ice-slurries a large amount of energy can be transferred at 0 °C. For many applications e. g. building climatization with comfort temperatures of 22 °C — 24 °C, working with temperatures around the freezing point has a great disadvantage as they unnecessarily reduce the efficiency of the chiller [5]. For this reason Fraunhofer ISE is working on PCS on basis of paraffin with the melting point close to the targeted working temperature.

There are two types of PCS being investigated:

— Emulsions with the dispersed paraffin mixed directly with water and emulsifiers preventing the accumulation of the paraffin drops.

— Suspensions of microencapsulated paraffin with the shell preventing an interaction of the paraffin with the water.

A testing facility for PCS has been set up at Fraunhofer ISE, which allows the analysis of their physical properties and the suitability for the designated use.

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.

Economical Possibilities

High Storage Capacity

First pilot projects [3], [5] in Germany reached storage capacities of about 130 kWh/m3. That means such a TES can store about 2-3 times more thermal energy than hot water storages. If higher temperature around 200 °C would be available, Zeolite storages could reach capacities of about 250 kWh/m3. However it has to be stated that a sorption TES consist of more components that the storage container itself, e. g. heat exchangers. In closed systems the desorbed water has to be kept inside the system, which reduces the storage capacity as well [2], [3]. Open systems need in most cases a humidifier for discharging. These additional components are usually not taken into account, which makes it difficult to compare different TES technologies.

Heating and Air-Conditioning

Open and closed sorption storages are able to provide thermal energy for heating purposes as well as for air conditioning of buildings. For this application cold can be delivered from the evaporator of a closed system. Using an open system the air dried by adsorption will be humidified, which leads to low temperatures (“desiccant cooling”).

The achievable Coefficients of Performance COPth are usually between 0.3 and 0.8. The open Zeolite system in Munich [5] reached a value COPth = 0.87 at a charging temperature of 80 °C in the experiments. The storage capacity was in the range of 100 kWh/m3.

In the case of liquid absorption systems only air conditioning is possible. Such an open storage system for solar air conditioning was installed in Singapore by the company L-DCS [8]. In the current demonstration project L-DCS Technology supplied a liquid desiccant air de-humidification system (11,000 m3/h) for a factory unit in Singapore, owned by JTC Corporation. A 550 m2 flat plate solar collector array drives the desiccant regeneration and 12 m3 desiccant energy storage covers the difference between the energy need for air conditioning by absorption and the solar energy supply for regeneration.

3. Economical Limits

The invest costs of sorption storage systems are quiet high. Therefore the number of operating hours, hence storage cycles, and the amount of thermal energy provided from the storage per time has to be high as well. This makes the operation as a seasonal storage system, used for 1 cycle per year, from the economical point of view impossible.

However a higher number of storage cycles by applying the system for heating and cooling purposes can lead to economical advantages. Furthermore the higher prices for air conditioning compared to plain heating could shorten the pay back time for such installations. The Zeolite storage in Munich reached a 50 % reduction of the payback time from around 14 years (for heating only) to about 7 years (for heating in winter and air conditioning in summer) [5].

4. Conclusion

Sorption storages are due to their thermodynamic possibilities — high storage capacity and variable temperatures — very interesting for solar thermal applications. How ever for economical reasons seasonal storage of solar heat by such systems is not an option. Most of the installed demonstration plants are looking for many storage cycles per year. This can be achieved by using the system for heating and cooling or by using smaller sorption systems in heat pump applications

[3].

With respect to solar applications sorption systems for solar air conditioning are most suitable, because for a high solar fraction the integrated storage effect is crucial.

The paper concludes with the remark that even the high storage capacities and the possibility of providing heat and cold of sorption storages does not solve all solar thermal storage problems. It is still necessary to find an appropriate application and to carefully check the relevant boundary conditions.

5. Literature

[1] R. Sizmann, Speicherung thermischer Energie — Eine Ubersicht, BMFT Statusseminar “Thermische Energiespeicherung” Stuttgart, 1989.

[2] D. Jaehnig, Thermo-Chemical Storage for Solar Space Heating in a Single-Family House, Proceedings of the International Conference on Thermal Energy Storage, Ecostock 2006, Stockton, New Jersey, USA, May 31 — July 2 2006.

[3] Thomas Nunez, A Small Capacity Adsorption System in a Heating and Cooling Application: The German Field-Test in the MODESTORE Project, International Conference Solar Air-Conditioning, Kloster Banz, Bad Staffelstein, Germany, October 6th/7th, 2005

[4] ZeoTech GmbH, Internet: http://www. zeo-tech. de/

[5] A. Hauer, Thermal Energy Storage with Zeolite for Heating and Cooling, Proceedings of the 7th International Sorption Heat Pump Conference ISHPC ’02, Shanghai, China, 24.-27. September 2002.

[6] H. Kerskes, K. Sommer, H. Muller-Steinhagen, An Effective Application of an Open Adsorption, Process for Solar Thermal Heat Storage, Proceedings of the EuroSun 2006, Glasgow, UK, June 27-30, 2006. [21] [22]