The measurements started in August 2004. Not all the energy quantities mentioned in section 3.1 were measured during the whole measuring period. A constant energy quantity of 10.67 kWh per m3 natural gas is assumed.

The total heat demand of the house, that is: Space heating demand + domestic hot water consumption + heat loss from circulation pipe, and the natural gas consumption month by month appear from figure 5. Both the heat demand and the natural gas consumption are much lower after the installation of the solar heating system than before. The reasons for the reduced natural gas consumption are:

• The decreased heat demand due to warm weather in the autumn of 2006, in 2007 and in 2008.

• The new energy system with solar collectors and an improved natural gas boiler.

The difference between the natural gas consumption and the total heat demand in the winter months is lower after installation of the solar heating system than before. In the spring, summer and autumn the natural gas consumption is lower than the total heat demand after installation of the solar heating system. Before installation of the solar heating system the natural gas consumption is, for all months, higher than the total heat demand.

Figure 6 shows the measured utilization of the natural gas as a function of the total heat demand month by month. The utilization of the natural gas is defined as: (space heating demand + hot water consumption + heat loss from circulation pipe)/natural gas consumption. The utilization of natural gas is strongly increased by the solar heating system, especially for low heat demands. Further, the utilization of natural gas after installation of the solar heating system is much higher in the sunny spring than in the less sunny autumn.

Based on the total monthly measured heat demand and the curves of figure 6 it is possible to estimate the natural gas consumption for the house for all months without and with the solar heating system, if only the heat demands are measured. Consequently, the natural gas savings for the house achieved by the solar heating system can be determined for all months when the total heat demand is measured.

The measured heat demand, natural gas consumption and heat produced by the solar collectors are shown in table 1 for a year without the solar heating system installed, 2005, and for a year with the solar heating system installed, 2007.

Table 1. Measured energy quantities.

The yearly natural gas savings estimated month by month as described above are 3400 kWh for 2005 and 3800 kWh for 2007. Further, based on measurements, yearly electricity savings of 200 kWh are estimated. The yearly total energy savings are therefore estimated to 3600 kWh for 2005 and 4000 kWh for 2007, corresponding to yearly energy savings of 530-590 kWh per m2 solar collector.

Figure 7 shows for 2007 quantities for the monthly total heat demand, the natural gas savings due to the solar heating system and the solar heat produced by the solar collectors. The heat demand is high in the winter and low in the summer. The monthly natural gas savings and the solar heat production are higher in the spring months than in the other months of the year.

Fig. 7. Measured total heat demand, natural gas savings and solar heat for 2007.

The energy savings will vary from year to year. In years with a high heat demand and a high solar radiation, especially in the spring, the energy savings will be high. In years with a low heat demand and a low solar radiation, especially in the spring, the energy savings will be low. The yearly heat production of the solar collectors is in 2007 about 65% of the yearly energy savings.

2. Conclusion

Measurements of the heat demand and energy consumption in a demonstration house before and after installation of a new developed solar heating/natural gas heating system based on a condensing natural gas boiler and 6.75 m2 solar collectors have been carried out.

Based on the measurements the energy savings by the solar heating system are for two years estimated at 3600 kWh and 4000 kWh per year, corresponding to 530-590 kWh/m2 collector per year. The energy savings will vary from year to year. In years with a high heat demand and a high solar radiation, especially in the spring, the energy savings will be high. In years with a low heat demand and a low solar radiation, especially in the spring, the energy savings will be low. The yearly heat production of the solar collectors is in 2007 about 2/3 of the yearly energy savings.

References

[1] S. Furbo, J. M. Schultz & A. Thur (2007). Energy savings for a solar heating system in practice. ISES Solar World Congress Proceedings, Beijing, China.

[2] T. Larsson (2000). Enkatundersokning om energibesparing och drift med solfangare. Orebro University, Sweden, report 00:00.

[3] A. Thur, L. J. Shah & S. Furbo (2006). Energy savings for solar heating systems. Solar Energy Vol. 80, Issue 11, pp. 1463-1474.

[4] S. Furbo, L. J. Shah,. C. Holm Christiansen & K. Vinkler Frederiksen (2004). Kedeleffektiviteter for oliefyr og naturgaskedler i enfamiliehuse. Department of Civil Engineering, Technical University of Denmark, report R-072.

[5] A. Thur (2007). Compact Solar Combisystem. High Efficiency by Minimizing Temperatures. Department of Civil Engineering, Technical University of Denmark, report R-160.