Monitoring results

Подпись: Fig. 2. REBUS system schematic with sensor positions for the REBUS controller (blue with subscript c) and the monitoring system (red with subscript d).

In Figure 3 the heating energy supply for the building is presented from January 2006 to June 2007. The data before the installation of the REBUS system are calculated from the readings of the house owner from the main electrical meter and his records of wood use. The electricity for heating has been calculated based on measured household electricity in 2007. It can be seen that almost all bought electricity for heating was replaced by pellet and solar energy. About 200 kWh electricity were used by the electrical back up heater in the standby store in October 2006. The pellet stove was installed in the middle of October and not enough solar radiation was available, which explains the use of the electrical heater that month. Some electricity was also used in December 2007. Reason here was that the 80 litre standby volume heated by the pellet stove was too little to cover the peak heat demand so that the electrical heater in the standby store was backing up the heating.

Подпись: Fig. 3. Monthly heat supply for space heating and domestic hot water, *Electricity SH + DHW is the calculated electricity use for SH + DHW before the detailed monitoring.

This has also caused a high number of starts and stops with stove run times often not more than one hour. Consequently in the end of December 2006, the hydraulics have been modified so that also the upper third of the solar store, which is connected in serial with the standby store, is heated by the boiler.

This increases the standby volume to in total 200 litres. As a result of this modification it can be seen that the number of starts and stops have decreased drastically (Figure 4). The average run time increased to about 3 to 4 hours per start. Due to the longer run time of the stove also the proportion of the useful heat delivered from the stove to the water jacket has increased from 61% in December to 67% in January. This is still lower than the average 80% for the combustion range that is has been measured for the stove at the Austrian test institute BfL [1].

3000

Подпись:Подпись:Подпись:Подпись: 2Подпись: 174Подпись: 291Подпись: okt-06 nov-06 dec-06 jan-07 feb-07 mar-07 apr-07 maj-07 jun-07Подпись: Fig. 4. Monthly heat delivery of the pellet stove and number of starts and stops.image0732500

Подпись:
In figure 5 the energy use for the monitored time period is presented. The monthly hot water demand varies between 240 and 400 kWh. The annual space heating demand is about 7400 kWh, which is relatively low for a building with a heated area of approximately 130 m2 and indicates a good thermal insulation.

During one year the solar collectors have delivered about 2529 kWh which is a reasonable value considering the small solar store volume, the non-optimal orientation of the solar collectors and the low heat demand. In terms of bought energy this gives a solar fraction of 19%.

Table 1: Annual heat supply and useful heat

Annual heat supply (kWh)

Annual useful heat

(kWh)

Pellet

9319

Space heating

7611

Wood

1434

Hot water

3673

Solar

2543

Electricity

278

Total

13574

11284

2. Discussion and Conclusion

The newly developed solar combisystem has been monitored for one year. For the most part the system is working as expected. All loops and components are properly controlled by the new controller software. Only some fine adjustments of some parameters were necessary. Most adjustments were necessary for the settings of the pellet stove controller to ensure a proper interaction with the main system. A modification of the hydraulic connections was done to increase

the buffer volume of the pellet stove. Using only the 80 litre volume of the standby store led to very short run times and many starts of the stove. After the change the run times of the stove increased and the number of starts and stops decreased drastically. Similar findings have been reported also in other studies (3; 5).

From the annual energy values in Table 1 the system efficiency can be calculated using the following equation:

_ annual useful heat

sys annualsupplied primary energy for heating

With a primary energy factor of 0.4 for electricity the total system efficiency is 0.83. Including also the parasitic electricity consumption of pumps, valves etc. decreases the system efficiency to 0.74. The parasitic electricity of 680 kWh accounts for almost 5 % or the total energy input to the system. It is obvious that the parasitic electricity consumption should be reduced e. g. by the use of more efficient pumps which stand for the main part of the parasitic consumption.

3. Acknowledgement

We are grateful to the Nordic Energy Research and the Dalarna University College for their financial support for this work within the REBUS project.

4. References

[1] BfL, "Prufbericht Pelletskaminofen EVO AQUA." BLT Aktzahl: 053/04, Bundesanstalt fur Landtechnik, Wieselburg, Austria. 2003.

[2] F. Fiedler, "Combined solar and pellet heating systems — Study of energy use and CO-emissions," PhD thesis, Malardalen University, Vasteras. 2006.

[3] F. Fiedler, C. Bales, and T. Persson, "Optimisation Method for Solar Heating Systems in Combination with Pellet Boilers/Stoves." International Journal of Green Energy, 4[3], 325 — 337. 2007.

[4] S. Furbo, A. Thur, C. Bales, F. Fiedler, J. Rekstad, M. Meir, D. Blumberga, C. Rochas, T. Schifter — Holm, and K. Lorenz, "Competitive Solar Heating Systems for Residential Buildings (REBUS)." Byg, DTU, Copenhagen, Denmark. 2006.

[5] A. Heinz, "Fortschrittliche Warmespeicher zur Erhohung von solarem Deckungsgrad und Kesselnutzungsgrad sowie Emissionsverringerung durch verringertes Takten." Technische Universitat Graz, Institut fur Warmetechnik, Graz, Austria. 2006.

[6] IEA, "International Energy Agency — Solar Heating and Cooling Program." IEA-Task 26. 2002.