Category Archives: EUROSUN 2008

Volume of the storage tank

The volume of the seasonal storage tank necessary for a maximum solar fraction is calculated from the energy density of magnesium sulphate (2.52 kWh. m-3) and the part of annual heat demand which is not provided by the solar energy in the reference case. But Fig. 1 shows that a large amount of heat collected in summer by the vacuum tubes is wasted. The seasonal TESS thus enables to complete or replace the thermal energy production of the auxiliary electric heater during the winter. The solar fraction is defined here as the part of the annual heat demand covered by the solar thermal energy. The storage volumes of material are presented in the Table 1.


Solar fraction




Achieved solar fraction with seasonal storage

Volume of



Ratio volume of MgSO4 / Living space (L/m2)

Mass of



Mass of




48.5 %

50.0 %





(zone H1)

57.3 %






83.2 %

90.0 %





(zone H3)

100 %





Table 1. Estimation of storage volumes

Storage volumes ranging from 0.2 to 0.9 m3 are expected, depending on the type of climate and the achieved solar fraction. The excess solar energy available during the summer, used for the regeneration of the material, limit the storage capacity ; the solar fraction is therefore limited to a value of 57.3 % in Paris, whereas 100 % solar fraction is achievable in the sunnier region of Marseille. The summer regeneration of the material will be studied in detail in future works. It will lead to the re-sizing of the solar collectors area.

The corresponding stoechiometric amounts of water vapour, which reacts with MgSO4 during the phase of heat production, vary from 0.5 to 2.5 tons. The reactor is an open system and the water is taken from the ambient spoilt air exiting the house, which could be completed by external humid air or by an extra external air humidifier if natural humid air is not sufficient. The model is being optimised in order to get more accurate and meaningful values of the solar collector area, the volume of the tank and the control parameters with respect to the climate and the targeted solar fraction.

Two parameters are important when designing a TESS : the quantity of heat that can be stored per given volume and mass (the energy density) and the rate at which this energy can be delivered (the power density). A very high energy density is of no use if the heat is released very slowly, resulting in very low temperature lifts and unworkable thermal energy. Despite a high theoretical energy density, the practical use of pure magnesium sulphate is quite difficult. Under real operating conditions, the theoretical energy density of the magnesium sulphate cannot be reached at usable power densities. The material needs to be dispersed to react at a suitable rate, which decreases the energy density. For instance, the storage volume increases up to 2 m3 if 50 % of the theoretical energy density of the dense salt is achieved. One of the purposes of the experiments is to find a proper porous matrix to disperse the magnesium sulphate in order to reach the maximum energy density of the salt.

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.

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.


[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.



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


♦ 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 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



Water 2.5 mm HDPE



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


* 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.