Three different performance indicators defined initially in Task 26 and adapted to Task 32 are shown in the Excel tool developed to evaluate the simulation results. They are compared with a reference system which has no collectors and all the energy is provided by fossil energy [6]. The three performance indicators evaluated are the thermal energy savings (fsav, therm), the extended thermal energy savings (fsav, ext) and the fractional solar savings indicator (fsi) and are defined in Eq. 1 to Eq.3.

The fractional thermal energy savings (fsav, therm) are a measure of the percentage the auxiliary (non­solar) energy input for heating can be reduced with the solar system, this term does not account for electricity use unless it is used directly for heating and it is defined as:

Qboiler + Qel, heater

П boiler, ref

The extended fractional energy savings (fsav, ext) are defined in a similar way than the fractional thermal energy savings (fsav, therm), but they also include electricity use for pumps, valves, controllers etc.

Qboiler. Qel, heater + Wpar

Finally, it is theoretically possible to achieve a high fsav, therm and at the same time not meeting the comfort criteria for space heating or warm-water production. Therefore the fractional solar savings indicator (fsi) includes a penalty term that compensates and even punishes for not meeting comfort criteria and it is defined as:

nboiler, ref n el

where:

Qboiler [kWh]: Energy delivered to the system by the auxiliary boiler (energy balance on the water-side of the boiler)

Qboiler, ref [kWh]: Energy delivered to the reference system (system without solar) by the boiler in the reference system (energy balance on the water-side of the boiler)

Qel, heater [kWh]: Energy delivered to the system by an electrical heater

nboiler [-]: Efficiency of auxiliary boiler of the solar system (dependent on the overall system) nboiler, ref [-]: Efficiency of auxiliary boiler of the reference system (= 0.85)

nel [-]: Efficiency of electricity production and transport to the site of use (= 0.4 independent of the source)

Wpar [kWh]: Electricity used for pumps, valves, etc. as well as for direct heating or driving a heat pump Wpar, ref [kWh]: Electricity used for pumps, valves, etc. of the reference system Qpenalty [kWh]: Penalty for the solar system for not meeting comfort criteria Qpenalty, ref [kWh]: Penalty for the reference system for not meeting comfort criteria

Two different kinds of penalties were considered, a penalty function for not meeting the required tapping water temperature of 45 °C (Qpen45), and a penalty function for not meeting comfort criteria (Qpen20) [5]. The penalties evaluation is also an important parameter whose influence is reflected in the fractional solar savings indicator (fsi) as shown in Eq. 3 where Qpenalty are the total penalties (DHW plus heating).

2. Simulation results

For the first set of simulations, where the aim was to observe the influence of some parameters in the result of the simulations, the most influencing parameter was the position of the outlet to the space heating (Z_SbB). This parameter had a strong influence on the three different performance indicators. On the other hand, the position of the auxiliary temperature sensors that operates the auxiliary system shows a slight influence on the results. Analyzing the influence of the position of Z_SbB (Table 1), it was easy to conclude that the lower the relative position of the outlet to the space heating, the better the value of the performance indicators fsav, therm and fsav, ext. However, the value of the fsi performance indicator was worst because of high penalty values, taking into account that Qpenalty, ref (Eq. 3) has always the same value. It was also seen that the fossil fuel used decreased with a lower position for the outlet to the space heating. When the outlet to the space heating was placed at a higher position, the value of the penalties was lower and the fsi indicator was also better.

Table 1. Variations on the position of the outlet to the space heating (Z_SbB) Modification fsav, therm f -Lsav, ext fsi Qburn kWh Qpen45 Qpen20 Z_SbB=0.8 0.506 0.435 -0.059 3411.99 2947.15 240.68 Z_SbB=0.74 0.647 0.548 -0.146 2191.72 113.62 5058.91

A lower position of the space heating outlet reduces the penalties regarding the DHW demand at 45 °С. This is because the water placed above the Z_SbB outlet is at a higher temperature and therefore, it is easier to provide water at the set point temperature of 45 °С. However, the water temperature going to the space heating system is at a lower temperature, thus it has less energy to release to the ambient and it is more difficult to reach the comfort temperature of at least 19.5°C inside the building.

A higher position of the space heating outlet benefits the heating system. The operation time of the space heating system is lower because the water going to the heating is at a higher temperature and more energy is released to the ambient reaching easier the comfort temperature. However, this is a drawback for the DHW demand and the penalties for it increase.

For the second set of simulations the aim was to check the influence of the PCM modules characterization. That is the placement of the PCM modules, the length of the modules and the amount of PCM. Several simulations were performed with variations of the length and diameter of the modules, which involved a variation of the number of PCM modules, and as a consequence, a variation of the PCM volume into the store (Table 2). Only slight improvements up to 2% in the performance indicators were obtained, when compared with the reference simulation without PCM in the tank. However, this small advantage is within the numerical uncertainties in the calculations [7]. Also the amount of energy provided by the auxiliary system had very few variations. Some more simulations without the stratifier device were carried out with and without PCM but the performance indicators showed no difference. The water store resulted to be at least as good as a water-PCM store.

The highest variation (2%) is observed in the fsi indicator, which is the one that considers the penalties when the demand, DHW or space heating, is not fulfilled. Concerning a PCM-water store, penalties are always smaller compared with a water store (Table 2). They can be even completely avoided in a PCM-water store with the same characteristics of a water tank. Therefore, the introduction of PCM helps to decrease the penalties for not reaching the specified conditions.

Table 2. Variations of the PCM modules geometry Modification fsav, therm f -Lsav, ext fsi Qburn (kWh/a) Qpen45 (kWh/a) Qpen20 (kWh/a) PCM vol. (%) Water 0.515 0.495 0.322 3784.92 46.81 0 1 PCM at the top 0.25 P a area 0.516 0.495 0.327 3776.14 0 0 2.22 (DHW) 0.5 Parea 0.519 0.497 0.329 3757.25 0 0 4.45 Z_SbB=0.9125 0.75 Parea 0.518 0.497 0.329 3760.55 0 0 6.67 1 PCM at the top (DHW) and 1 in the middle 0.25 P a area 0.517 0.496 0.328 3769.29 0.52 0 8.25 0.5 Parea 0.517 0.495 0.327 3771.27 0.68 0 16.5 (space heating) 0.75 Parea 0.514 0.494 0.324 3791.09 14.26 0 24.77 Water 0.535 0.508 0.324 3632.39 91.62 68.91 1 PCM at the top 0.25 P a area 0.531 0.505 0.326 3660.51 55.21 54.70 3.17 (DHW) 0.5 Parea 0.532 0.505 0.330 3655.81 2.25 61.07 6.35 Z_SbB=0.82 0.75 Parea 0.531 0.505 0.329 3657.53 33.36 50.07 9.52

2. Conclusions

Advantages offered by PCM have been already tested theoretically and experimentally in DHW installations. Several simulations were performed to check its suitability in a DHW and space heating demand system. A complete and powerful tool with Trnsys was developed in the framework of Task 32 of the International Energy Agency (IEA) to perform simulations with this system.

Two different set of simulations were carried out. For the first one, where the aim was to observe the influence of some parameters in the result of the simulations (three performance indicators: fsav, fsav, ext, fsi), the most influencing parameter was the position of the outlet to the space heating. The lower the relative position of the outlet to the space heating, the better the value of the performance indicators fsav, therm and fsav, ext but the worst the value of fsi because of high penalty values. This parameter has a strong influence on the performance of the system since the outlet position benefits one of the demands but affects negatively the other one. Another important conclusion is that the fossil fuel used decreased with a lower position for the outlet to the space heating.

For the second set of simulations the aim was to check the influence of the PCM modules characterization. New placement and modules configuration was tested. Only slight improvements up to 2% in the performance indicators were obtained when compared with the reference simulation without PCM in the tank. Even some simulations without the stratifier device were carried out but no differences were observed concerning the performance indicators. A water store was at least as efficient as a PCM-water store. The highest variation (2%) is observed in the fsi indicator, which is the one that considers the penalties when the demand, DHW or space heating, is not fulfilled. DHW penalties are always smaller in the PCM water store than in the water store with the same characteristics and they can be even completely avoided in a PCM-water store. Therefore, the introduction of PCM helps to decrease the penalties for not reaching the comfort conditions in the demand.

With the system designed as it is and the control applied (typical differential control for simple water tank), only slightly better results were obtained for a PCM-water store compared to a water store regarding the performance indicators used. However, this system is not the commonly used only-water store, it is a PCM-water store so the differential control applied could be no the suitable for this application. A new control strategy taken into account the PCM should be applied. Another possibility could be a new composition of the system, this is for example placing the auxiliary system out of the store.

Acknowledgements

The work was partially funded with the project ENE2005-08256-C02-01/ALT and 2005-SGR-00324. Dr. Marc Medrano would like to thank the Spanish Ministry of Education and Science for his Ramon y Cajal research appointment.

References

[1] M. Nogues, L. F. Cabeza, J. Roca, J. Illa, B. Zalba, J. M. Marin, S. Hiebler, H. Mehling, Efecto de la Insercion de un Modulo de PCM en un Deposito de ACS. Anales de la Ingenieria Mecanica, vol 1 (2002) 398-402.

[2] L. F. Cabeza, M. Ibanez, C. Sole, J. Roca, M. Nogues, Experimentation with a water tank including a PCM module, Solar Energy Materials and Solar Cells, 90 (2006) 1273-1282.

[3] M. Ibanez, L. F. Cabeza, C. Sole, J. Roca, M. Nogues, Modelization of a water tank including a PCM module, Applied Thermal Engineering, 26 (2006), 1328-1333.

[4] C. Sole, M. Medrano, A. Castell, M. Nogues, H. Mehling and L. F. Cabeza, Energetic and exergetic analysis of a domestic water tank with phase change material, International Journal of Energy Research, 32 (2008) 204-214.

[5] R. Heimrath and M. Haller (2007). Project Report A2 of Subtask A: The Reference Heating System, the Template Solar System.

[6] W. Weiss, (2003). Solar Heating Systems for Houses. A design handbook for solar combisystems, JamesXJames, London (United Kingdom).

[7] E. Talmatsky and A. Kribus, PCM storage for solar DHW: An unfulfilled promise?, Solar Energy, in press.