Fig.4 shows the hydraulic schemes of the old and the new heating system with the heat meters which are red marked and named “Fdxx” and listed in Table 1.

to 10th October, Lisbon — Portugal *

Old heating system:	New solar combisystem:
—	Fd1: “Solar Gain”
—	Fd2: “Boiler”
Fd15: “Domestic Hot Water-Heating”	—
Fd16: “Space Heating”	Fd3: “Space Heating”
Fd18: “Domestic Hot Water-Circulation”	Fd5: “Domestic Hot Water-Circulation”
Fd19: “Domestic Hot Water-Consumption”	Fd4: “Domestic Hot Water-Consumption”
Gas meter: “Natural Gas Consumption”	Gas meter: “Natural Gas Consumption”
Electricity meter: “Electricity Consumption”	Electricity meter: “Electricity Consumption”
	Tc5 ~^ Collector —< Tc19

Table 1. List of energy meters in the old and the new heating system of the demonstration house.

In Table 2 the main measurement results of the two monitoring periods (without solar heating system in 2005 and with solar heating system in 2006/07) are presented. Solar gain of the collectors for one year is 2500 kWh or 370 kWh per m2 respectively. The solar fraction is SF=13.5%. The calculation of the fractional energy savings according to the definitions within IEA SHC Task 26 results in fsav, therm = 20% against the Task26 reference system. The FSC (Fractional Solar Consumption) can only be estimated based on weather data in Copenhagen resulting in about FSC = 0.25.

Space heating consumption (SH) in the second monitoring period is about 20% less compared to the first monitoring period. This fits quite well to the reduction of the heating degree days by 17% (3097 HDD to 2578 HDD) even though two additional rooms in the basement were heated in the second period.

After an intensive teaching process to the occupants of the house, how to use the thermostat valves at the radiators correctly, the measurements showed a quite big change: the temperature difference in the space heating loop increased significantly from 7.7 to 12 Kelvin on average. The occupants also reported that the indoor

Table 2. Monthly monitoring data and key numbers for one year periods for the old conventional heating system and the new solar combisystem. Ambient Temperature Average Source: DMI Natural Gas Consumption Low Heating Value = 10.67 kWh/m 3 Solar Gain Space Heating Space Heating Temperature Difference Monthly average Domestic Hot Water-Consumption Domestic Hot Water-Consumption Average daily consumption Domestic Hot Water Consumption Temperature Difference Monthly average Domestic Hot Water-Circulation (21/11-05 till 20/12-05; daily 6-10 and 17-20) Electricity consumption per day Total heating System (from 21/11-05) Boiler Efficiency (Boiler/Gas) Natural Gas — COP (DHW+SH)/(Gas) Circulation as Loss Solar Fraction (Solar)/(Solar+Boiler) Hydraulic Efficiency (DHW+SH)/(Gas*eta_boil+Solar) Circulation as Loss Ta Gas Solar SH DHW Circ Electr. eta_boil COP SF eta_hyd [°C] [kWh] [kWh] [kWh] [K] [kWh] [kWh/d] [K] [kWh] [kWh/d] [%] [%] [%] [%] Old conventional heating system 01.2005 2,6 3055 0 2322 8,2 315 10,2 55,8 0 — 88,8% 86,3% — 97,2% 02.2005 -0,3 3477 0 2665 10,3 350 12,5 57,1 0 — 89,1% 86,7% — 97,3% 03.2005 0,9 3602 0 2822 10,4 324 10,5 57,0 0 — 89,9% 87,3% — 97,1% 04.2005 7,8 2224 0 1685 6,6 292 9,7 54,6 0 — 92,5% 88,9% — 96,1% 05.2005 11,0 1655 0 1207 5,7 257 8,3 52,6 0 — 93,6% 88,5% — 94,5% 06.2005 14,1 1038 0 663 6,3 229 7,6 50,6 0 — 93,1% 85,9% — 92,2% 07.2005 17,2 400 0 15 13,8 220 7,1 48,1 0 — 83,8% 58,8% — 70,1% 08.2005 15,2 401 0 77 4,0 183 5,9 30,3 0 — 90,4% 64,9% — 71,8% 09.2005 13,9 808 0 424 3,4 241 8,0 54,5 0 — 94,5% 82,3% — 87,0% 10.2005 10,3 1890 0 1378 5,2 281 9,1 57,0 0 — 93,6% 87,8% — 93,8% 11.2005 5,2 2926 0 2124 7,1 292 9,7 59,2 91 3,5 89,3% 82,6% — 92,5% 12.2005 1,9 3654 0 2593 10,2 295 9,5 59,4 206 3,2 88,6% 79,0% — 89,2% 2005 8,3 25131 0 17974 7,7 3279 9,0 52,8 — — 90,4% 84,6% — 93,6% New solar combisystem. 10.2006 11,7 1266 143 1040 11,9 216 7,0 35,3 18 2,9 96,5% 99,2% 10,5% 92,0% 11.2006 7,4 2146 66 1830 12,7 238 7,9 37,4 14 3,1 99,1% 96,4% 3,0% 94,3% 12.2006 6,6 2244 27 1853 12,5 274 8,8 39,2 10 3,2 99,5% 94,8% 1,2% 94,1% 01.2007 4,3 2828 30 2421 12,3 214 6,9 39,6 14 3,2 97,8% 93,2% 1,1% 94,2% 02.2007 1,5 2953 59 2579 13,1 203 7,3 41,3 12 3,2 97,4% 94,2% 2,0% 94,8% 03.2007 6,2 2116 312 1999 9,6 227 7,3 41,6 15 3,3 97,3% 105,2% 13,2% 93,9% 04.2007 9,0 1166 472 1226 10,8 219 7,3 39,8 15 3,0 95,3% 123,9% 29,8% 91,2% 05.2007 12,3 601 363 648 16,9 181 5,8 35,6 14 2,2 91,6% 137,9% 39,7% 90,7% 06.2007 16,4 109 287 22 10,0 219 7,3 31,5 17 1,2 81,8% 221,1% 76,3% 64,1% 07.2007 15,8 139 274 0 251 8,1 25,2 15 1,1 85,8% 180,6% 69,7% 63,8% 08.2007 17,1 52 230 14 17,2 167 5,4 32,9 12 0,9 78,4% 348,1% 84,9% 66,8% 09.2007 12,9 975 237 816 12,0 215 7,2 34,8 13 2,5 89,4% 105,7% 21,4% 93,0% 2006/07 10,1 16595,5 2500 14448 12,0 2624 7,2 35,3 169 2,5 96,8% 102,9% 13,5% 92,0%

Due to the much more constant room temperatures the occupants stopped to open and close the windows for controlling the room temperature. This behaviour typically increases the space heating consumption significantly and therefore it can be assumed that this change of behaviour most likely is the reason why the total space heating consumption followed the heating degree days, even though two more rooms were heated.

Domestic hot water consumption (DHW) decreased significantly by 20% from 9.0 to 7.2 kWh/day. The main reason for that might be the lower set temperature of 50°C instead of 70°C in the old system (due to the small 50 litre hot water tank) and the higher resistance of the flat plate heat exchanger which reduced the maximum flow rate.

A huge difference could be observed at domestic hot water circulation heat losses. In the old system the circulation pump was operating daily from 6 to 10 and 17 to 20 resulting in 297 kWh heat losses just within one month. This was more than the hot water consumption itself during most of the months. Therefore, the occupants switched off again the circulation pump at the end of the first monitoring period.

In the new solar combisystem a new control strategy was introduced, the so called “circulation on demand” (see Fig. 5) [6]. A

pump. After about 15 to 20 seconds hot water reaches the tap without wasting water and for example the shower can start with hot water at the tap immediately. Using this strategy the circulation heat losses reduced dramatically to 169 kWh within a full year, this is only about 7% of the domestic hot water consumption.

Fig. 5. Circulation on demand: short tapping at 06:25 starts the circulation pump (Pc6) and hot water is heated up (Tc12) from 30°C to about 53°C; at 06:32 the shower starts. (location of temperature sensors see Fig. 2)

The electricity consumption of the total heating system during the heating season Nov 05 to April 06 is constant at about 3.3 to 3.4 kWh/day. The electricity consumption of the new solar combisystem in the same period was somehow lower even though that some more pumps and valves are installed and in operation. A main advantage in the solar combisystem might be that the main pump for space heating and hot water preparation is speed controlled and therefore operating quite efficient.

The last four columns in Table 2 present some calculated key numbers for both systems.

The boiler efficiency (eta_boil) shows clearly the difference between the old non-condensing and the new condensing natural gas boiler. In summertime with very low heat load the efficiency decreases significantly in both cases down to 80%. Nevertheless the old boiler shows surprisingly high efficiency in spring and autumn. An explanation for that could be, that in December 2004 of this old boiler had to be repaired including the change of the internal heat exchanger.

The hydraulic efficiency (eta_hyd) is defined as the heat demand (DHW and SH) divided by heat production (solar gain and boiler output). In this case heat losses of domestic hot water circulation are calculated as system losses and not as demand (see Table 2). Comparing the two hydraulic schemes in Fig. 4 shows clearly, that a solar combisystem consists of more hydraulic components, internal piping and last but not least a much bigger heat store (in this case 360 litres compared to 50 litres) leading to higher heat losses due to larger surface. These higher heat losses typically are compensated by solar energy in summertime, but may lead to decreased performance of the complete system in winter time. Therefore, all measures to reduce heat losses (good insulation of tank and hydraulic components, short internal piping, etc.) are essential for high overall performance.

Comparison of the monitoring results in the period January to March shows a difference of the hydraulic efficiency of 3%-points (97% to 94%) even though the old system has no hot water circulation in operation but the new solar combisystem has. In November and December 2005 the huge influence of the hot water circulation losses in the old system is clear to see. The new solar combisystem in total reached a hydraulic efficiency including hot water circulation losses of 92%, which is quite a high value.

The real difference can be observed when looking at the utilization factor COP (hot water and space heating consumption in relation to gas consumption) which combines the boiler efficiency (eta_boil) and the hydraulic efficiency (eta_hyd) of the heating system, which includes heat losses of the heat storage and all the pipes and components inside the technical unit as well. In the old system in July 2005 only 59% of the consumed energy of natural gas finally was used as hot water, 41% are losses. This inefficiency of conventional hot water preparation is also the reason why the energy savings of solar heating systems in summertime are much higher in comparison to the heat load [7].

The solar combisystem reaches much higher COP values mainly due to two reasons: the boiler efficiency (eta_boil) of the new condensing natural gas boiler is on average about 6%-points higher compared to the old non-condensing natural gas boiler and of course the gained solar energy is counting as non-fossil heat input and therefore calculated for “free”.

Within this project it was possible to measure the heat flows in the same building before and after the installation of a solar combisystem. Therefore the energy savings can be calculated based on measurements and not based on assumptions for the case of the old heating system.

Based on the investigated natural gas utilization (COP) and the measured heat load of the old heating system a characteristic COP-curve can be generated. The theoretical natural gas consumption during the second monitoring period can be interpolated with this COP-curve and the new measured heat load.

Table 3 presents the calculated energy savings of this demonstration house. The total natural gas savings are equal to 3615 kWh or 18% respectively. In relation to the collector area of 6.75 m2 the energy savings are 536 kWh/m2, where the solar gain is 370 kWh/m2.

Table 3. Calculated energy savings of the new heating system compared to the old one. Month Heat Load (SH+DHW) COP old COP SCS Natural Gas Consumption old Natural Gas Consumption SCS Energy Savings Energy Savings Solar Gain [kWh] [%] [%] [kWh] [kWh] [kWh] [kWh/m2] [kWh/m2] 10.2006 1256 88,2 99,2 1424 1266 158 23 21 11.2006 2068 86,5 96,4 2388 2146 242 36 10 12.2006 2127 86,4 94,8 2461 2244 217 32 4 01.2007 2635 85,4 93,2 3086 2828 258 38 4 02.2007 2782 85,1 94,2 3268 2953 315 47 9 03.2007 2226 86,2 105,2 2582 2116 466 69 46 04.2007 1445 87,8 123,9 1646 1166 480 71 70 05.2007 829 84,1 138,0 986 601 385 57 54 06.2007 241 59,0 221,1 408 109 299 44 43 07.2007 252 62,0 181,3 406 139 267 40 41 08.2007 180 46,0 346,2 391 52 339 50 34 09.2007 1031 88,6 105,7 1164 975 189 28 35 Total 17071 84,5 102,9 20210 16595 3615 536 370

3. Conclusion

A new hydraulic and control concept for a solar combisystem was developed. A prototype for the laboratory was built and tested, and finally a demonstration system was installed and measured in practice. The main goal was to develop a concept in combination with a standard condensing natural gas boiler, which leads to an overall coefficient of performance as high as possible. Based on the fact that such condensing natural gas boilers are powerful enough for direct hot water preparation, the newly developed concept avoids heating up the auxiliary volume to high temperatures, which is normally done just to have a buffer for hot water preparation. The new strategy leads to a lower average temperature in the solar tank, and more importantly, in the piping within the system, which further on significantly reduces the system heat losses. The major problem, which had to be solved, was to ensure hot water preparation with constant tap temperature during the always changing operating conditions.

Long term monitoring results in a demonstration one family house showed after 20 month of operation that the system works in a stable and reliable way. The results showed high performance thanks to low system heat losses and comparable high boiler efficiency. Due to the system concept, also significant reductions on the heat demand side could be realized: 20% less domestic hot water consumption, 93% less hot water circulation losses based on hot water consumption and slightly less space heating consumption even though two more rooms were heated in the demonstration house.

References

[1] Furbo, S., et. al. (2007), „Competitive solar heating systems for residential buildings”, Final Report, Department of Civil Engineering — Technical University of Denmark, Kgs. Lyngby, Denmark, http://www. physics. uio. no/energy/rebus/downloads/REBUS_final_01-2007.pdf

[2] Thur A., Furbo S., Fiedler F., Bales C., “Development of a compact solar combisystem”, Proceedings EuroSun 2006 Congress, Glasgow, Scotland, 2006

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

[4] A. Thur, Compact Solar Combisystem — High Efficiency by Minimizing Temperature, PhD-Thesis, Rapport R-160, Department of Civil Engineering, Technical University of Denmark, 2007; www. aee- intec. at/0uploads/dateien437.pdf

[5] F. Fiedler, Combined Solar and Pellet Heating Systems — Studies of Energy Use and CO-Emissions, Doctoral Dissertation, Malerdalen University, Vasteras, Sweden, 2006

[6] J. Apel, Vergleich von Zirkulationsstrategien fur den Trinkwasserkreislauf in einem Solarkombisystem, Diplomarbeit, Fachhochschule fur Technik und Wirtschaft, Berlin, 2005

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