J0rgen M. Schultz, Simon Furbo. Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs. Lyngby, Denmark. Email: js@byg. dtu. dk Fax: +45 45 88 32 82

Introduction

Solar heating systems for combined domestic hot water and space heating has a large potential especially in low energy houses where it is possible to take full advantage of low temperature heating systems. If a building integrated heating system is used — e. g. floor heating — the supply temperature (and the the return temperature) would only be a few degrees above room temperature due to the very low heating demand and the large heat transfer surface area.

One of the objectives in a newly started IEA Task 32 project is to investigate and develop improved thermal storages for combined solar systems through further improvement of water based storages and in parallel to investigate the potential of using storage designs with phase change materials, PCM.

The advantage of phase change materials is that large amounts of energy can be stored without temperature increase when the material is going from solid to liquid form (Fig. 1). Keeping the temperature as low as possible is an efficient way to reduce the heat loss from the storage. Furthermore, the PCM storage might be smaller than the equivalent water storage as more energy can be stored per volume. If the PCM further has the possibility of a stable super cooling, i. e. the material is able to cool down below its freezing point (Tfusion) and still be liquid, the possibility exist for a storage with a very low heat loss. When energy is needed from the storage the solidification is activated and the temperature rises almost instantly to the melting point.

The work within the IEA Task 32 project focuses on the phase change material Sodium Acetate with xanthan rubber. This material melts at 58 °C, which means that low temperature heating systems could make full use of such a storage system. Energy to a large extent can be withdrawn even when the storage is in its super cooled phase without activation of the phase change.

This paper presents an initial simulation model of a PCM storage for implementation in TRNSYS 15 [1] as well as the first test results achieved with the model.

Sodium acetate with xanthan rubber

For the moment only one material, Sodium Acetate with Xanthan rubber, is considered for the PCM storage. Sodium Acetate has a melting point of 58 °C and a heat of fusion capacity of 265 kJ/kg. Addition of xanthan rubber to the hydrate makes it very stable when super cooled [2].

Fig. 1 shows that the PCM storage compared to water has a slightly lower storage capacity in the solid phase below the melting point of 58 °C, but when the sodium acetate begins to melt the heat storage capacity increases dramatically due to the heat of fusion. It is also seen that the amount of energy stored at a temperature of 58 °C is about twice the amount of stored energy in traditional water storage even if this was heated to near 100 °C. This shows one of the advantages of a PCM storage: A very large amount of energy can be stored at a moderate temperature.

Figure 1 also shows the advantage of super cooling as the storage can be allowed to cool down to room temperature and still contain large amounts of latent energy (the dotted thick line in figure 1). If the storage has reached a temperature equal to the room temperature no further heat losses occur before the phase change is activated. When the super cooled PCM is activated the temperature increases almost instantly to 58 °C. However, some of
the heat of fusion is used to heating up the PCM to the melting point as indicated with the dashed arrow in figure 1

One of the critical questions is how to activate the phase change in the super cooled material. One method is to make contact between the super cooled material and a solid crystal of the same material. This method is however not feasible in case of thermal storages. Other methods are to apply a sudden force on the solution e. g. mechanically or acoustically [3].

The question on how to activate the super cooled phase change material has not been considered so far in the project and for the energetic potential evaluation it is anticipated that the PCM can be activated on demand.

Description of the PCM storage model

The solar system under consideration is outlined in Fig. 2. The system consists of a solar collector, a domestic hot water tank and the PCM storage. The use of two separate storages is due to the idea of extensive use of the super cooling effect of the PCM storage, which would be impossible if a combined storage for domestic hot water and space heating is used. The system is designed to give priority to the domestic hot water tank.

SHAPE * MERGEFORMAT

The PCM storage design for the first investigation is made without any thoughts on economy or practical problems as the first objective is to evaluate the potential of using a PCM storage compared to traditional water storages. If full benefit of the super cooling effect with respect to reduced heat loss should be achieved a multi- sectioned storage design is needed. By sub-dividing the storage into many separate layers or sections it will be possible only to activate the phase change in the storage volume needed to match the energy demand, and this will be the only part of the storage that will be heated up to the PCM melting point. This has been the main idea behind the design outlined in figure 3.

A first draft of a TRNSYS type model has been developed. The model subdivides the simulation time step in smaller time steps in order to achieve a sufficient accuracy. The following takes place in each of the small time steps (Fig. 4).

Based on the input parameters the most favourable section of the storage is chosen for storage of solar energy or — in case of a space heating demand — heating of the solar fluid to cover the space heating demand. The strategy is always to minimise the storage mean temperature and to avoid activation of phase change in a super cooled section as long as possible.

In each time step the transfer of energy between the solar fluid and the PCM storage is calculated. Next the heat loss to the surroundings is calculated and the final temperature of each section is found. This first model does not take internal heat exchange between the different storage sections into account.

The inputs, parameters and outputs are shown in table 1.

Each section is simulated as a lumped model, i. e. the section is supposed to have a uniform temperature. Figure 5 shows the model of one section.

Of special interest is the STATUS parameter, which is the measure of the state of the PCM material. If the storage section is liquid STATUS equals 1 and if the storage section is solid STATUS equals 0. When a fully solid PCM layer reaches the melting point continuous supply of energy will make the PCM begin to melt and a mixture of solid and liquid PCM material will be present. In the simulation model this is registered in the STATUS parameter, which increases proportional to the fraction of melted PCM from a value of 0 to the value of 1, when an energy amount equal to the heat of fusion has been supplied to the storage section.

Fig. 5 Lumped model of one section of the PCM storage

Preliminary simulation results

The model has been implemented in TRNSYS 15. Data for the main components is shown in table 2. For the reason of simplicity and the fact that the model is purely theoretical no pipes have been modelled, i. e. no pipe heat loss. The flow in the solar collector loop is constant all year round. If the outlet temperature from the solar collector is lower than the supply temperature to the collector + 10 K the controlled valve bypasses the solar collector. If the fluid temperature is higher than the bottom temperature of the domestic hot water tank the fluid is lead through the DHW tank before it reaches the PCM storage. Hourly values of the space heating demand is read from a file generated with a detailed building simulation program tsbi3 [4]. The building is a single-family low energy house with an annual space heating demand of approximately 15 kWh/m2 (2000 kWh/year) corresponding to the Passive House concept.

Figure 6 shows the temperature evaluation and the status of the PCM storage for a theoretical example with almost no heat loss from the storage (U = 0.001 W/m2K). The example has been chosen for illustrative purpose only.

It is clearly seen that during the late winter and early spring the storage is charged and discharged several times without fully melting of the PCM material (the temperature does not exceed the melting point of 58 °C).

When the space heating demand becomes close to zero in the late spring the storage is charged and the status becomes equal to 1 meaning that the storage is fully liquid. In the autumn the storage is discharged and the status drops from 1 to zero with only a small plateau at 58 °C.

Fig. 7 shows the same system but with a U-value for PCM storage insulation of 0.66 W/m2K. The system is not supposed to be optimised. The annual results are shown in table 3

Conclusion

Phase Change Materials (PCM) for heat storage in combined solar systems offers the possibility of reducing the storage size compared to traditional water storages. Of special interest is the use in combination with low energy houses with low temperature heating systems. If the PCM further allows for stable super cooling the possibility exists for a storage with very low heat losses.

As part of the IEA Task 32 project "Advanced storage concepts for solar thermal systems in low energy buildings” a first draft of a TRNSYS type model of a PCM storage has been developed.

The first simulations with the model have proven the functionality of the model and reasonable overall results are obtained. On a detailed scale some irregularities are observed with respect to unexpected temperature drops (1 — 2 K) during periods with no space heating demand.

The future work will be concentrated on more detailed evaluations of the model and afterwards use of the model for detailed parametric studies in order to evaluate in detail the energetic potential of PCM storages compared to traditional water tanks.

References

[1] "TRNSYS 15, User Manual”. The University of Wisconsin. Madison, USA.

[2] "Report on heat storage in a solar heating system using salt hydrates”. S. Furbo & S. Svendsen. Report No. 70, Thermal Insulation Laboratory, Technical University of Denmark, July 1977.

[3] "Triggering crystallization in supercooled fluids. B. Sandnes. Department of Physics, University of Oslo, Norway. 2004.

[4] "tsbi3 User Manual”. Danish Building Research Institute, 1993.

[5] "Design Reference Year — A new Danish reference year”. J. M. Jensen & H. Lund. Report No. 281, Thermal Insulation Laboratory, Technical University of Denmark, 1995.