The PV is sized according with the electric final energy demand, shown in Table 4. For each user/solar thermal scenario we have different electricity demand and consequently different PV areas. The reference PV system considered in this study has a total efficiency of 10.7%, with south facing panels, 35 ° inclination. The yearly production of the PV is 1363 Wh/Wp installed. The PV system is connected to the supply grid, without the support of any store devices. When there is a surplus on the production, the excess electricity is injected into the grid, if the electricity produced on-site is not enough to meet demand, electricity is purchased from the grid.

3. Results

Table 4 and 5 show the total energy electrical energy demand and the system characteristics for the twenty scenarios considered. As previous addressed in section 4, the difference between the groups BAU and BEST is electrical appliance efficiency and usage patterns.

	BEST	BAU
ST Area/Vr |m2]/[l]	Heating [kWh/yr]	Cooling [kWh/yrJ	Aux. Support [kWh/yr]	Elec. Appliances [kWh/yr]	Electric Final Energy [kWh/yr]	Aux. Support [kWh/yr]	Elec. Appliances [kWh/yr]	Electric Final Energy [kWh/yr]
4/300			71	1179	1982	54	2982	3767
5/400			25	1179	1936	18	2982	3732
6/500	654		10	1179	1921	6	2982	3720
7/600			4	1179	1914	2	2982	3716
8/700		78	2	1179	1912	2	2982	3715
4/300		604	1179	1861	568	2982	3628
5/400			409	1179	1665	379	2982	3439
6/500	—		284	1179	1541	260	2982	3320
7/500			219	1179	1476	200	2982	3260
8/700			146	1179	1403	129	2982	3189

Table 5. Minimum PV areas needed to supply the different electricity demands (Total columns in Table 3). ST Area is the solar thermal panels area and the Vr is the volume of the ST reservoir.

ST Area/Vr |m2]/[l]	BEST	BAU
PV Areas [m!|	kWp	PV Areas [m1]	kWp
4/300	10.1	1.45	19.1	2.76
5/400	9.8	1.42	18.9	2.74
6/500	9.7	1.41	18.9	2.73
7/600	9.7	1.40	18.8	2.73
8/700	9.7	1.40	18.8	2.73
4/300	9.4	1.37	18.4	2.66
5/400	8.4	1.22	17.4	2.52
6/500	7.8	1.13	16.8	2.44
7/500	7.5	1.08	16.5	2.39
8/700	7.1	1.03	16.2	2.34

As expected, when the ST heats the house, less PV is required, as shown in table 5.

4. Financial Analysis

To estimate the initial investment cost for renewable energy systems we considered a cost of 700€/m2 for solar thermal system and 5€/Wp for the PV [8]. In the BEST group case we also accounted for an extra cost of 250€ for each machine of class A, for total of 750€ for the three machines.

Renewables reduce the house final electricity net yearly demand and costs to zero. The 10 year simple net cost is based on a house with the same envelope, but without renewables or efficient appliances. The 10 years cumulative cost of the electricity is deducted from the initial investment cost, see table 7.

Cooling	Elec. Appliances	Heating	DHW	Electric Final Energy
[kWh/yr]	[kWh/yr]	[kWh/yr]	[kWh/yr]	[kWh/yr]
78	2982	654	1422	5136

For the 10 years cost two scenarios were considered: 0.1€/kWh that is the price of electricity in Portugal and 0.2€/kWh, a conservative estimate of the “real ” price of electricity if it had followed the price rises of oil during the last years [9].

Table 7. Initial investment costs, IC, and 10 years simple net cost for electricity prices of 0.1€/kWh and 0.2€/kWh. BEST BAU ST Area/Vr [m3]/|l| IC[€] 10 years Simple Net Cost ‘ [€] IC €] 10 years Simple Net Cost ‘ [€] 0.1 €/kWh 0.2 €/kWh 0.1 €/kWh 0.2 €/kWh 4/300 10820 5684 547 16620 11484 6348 5/400 11351 6215 1079 17189 12053 6917 6/500 11997 6861 1724 17847 12711 7574 7/600 12672 7536 2400 18530 13394 8258 8/700 13364 8228 3092 19229 14093 8957 4/300 10377 5240 104 16109 10973 5837 5/400 10359 5223 87 16117 10981 5845 6/500 10603 5467 331 16380 11244 6107 7/500 11063 5927 791 16858 11721 6585 8/700 11495 6359 1223 17299 12163 7026

The Portuguese micro-generation law [10] gives the possibility to sell PV electricity at a subsidized price. The difference between the purchase and the subsidized price of electricity works as a financial incentive to the implementation of micro-generation. To estimate the 10 years simple net cost for the subsidized scenario we deducted from the initial investment cost, the cost of electricity that we had to buy during those 10 years, if the house didn’t have any renewable sources available, see Table 6 (price of 0.1€/kWh). In addition we also deducted the PV electricity that was not consumed in the house at the price of the difference between the purchase and the subsidized price (0.1-0.5€/kWh).

Table 8. 10 years simple net cost for purchased electricity (0.1€/kWh) and subsidized sale price(0.5€/kWh). ST Area/Vr [m2]/[l] BEST BAU Sale/Buy [kWh/yr] Subsidized lOyears [€] Subsidized Payback [yr] Sale/Buy [kWh/yrJ Subsidized lOyears [€J Subsidized Payback [yrj 4/300 1725 -1218 9.0 3374 -2013 8.9 5/400 1690 -546 9.5 3353 -1358 9.3 6/500 1679 145 10.1 3344 -664 9.6 7/600 1673 843 10.7 3339 36 10.0 8/700 1728 1315 11.1 3364 637 10.3 4/300 1619 -1234 8.9 3192 -1795 9.0 5/400 1470 -656 9.4 3060 -1259 9.3 6/500 1375 -31 10.0 2978 -668 9.6 7/500 1321 644 10.6 2929 5 10.0 8/700 1260 1321 11.3 2873 669 10.4

5. Conclusions

As can be seen in Table 3 from energy saving and financial perspective the best option is a house heated by the ST system with 5m2 of panel and a reservoir of 400l. Since the electrical appliances represent by far the biggest share of final energy use, for the same solar thermal configuration the BEST group always performed considerably better.

The cost and performance of the NZEB system shows low sensitivity to the size of the ST, whenever solar hot water is used to its maximum, with the best cases occurring in a wide range of panel areas: 4-8m2. In this case, the total panel (ST+PV) area is approximately constant (15m2). Clearly the infinite reservoir that the PV system has (the grid) in conjunction with the mismatch between demand and supply of the ST systems (less heat provided in winter) even the “competition” between the two systems in our NZEB scenario.

However in the subsidized micro generation scenario significant changes occur; the best scenarios are the ones where more final energy is used. This is a direct consequence of the NZEB approach, where increased energy consumption leads to more PV, and consequently more electricity sold to the grid. Clearly, if feasible, the micro generation scheme should have a variable rate, with higher rewards for the most energy efficient consumers (in our case the “BEST” consumer scenario).

Overall the results are encouraging, it is possible to make an NZEB single family house with an investment payback of ten years or less. The initial investment is approximately 100€/m2 (5-10% of the typical total house price).

6. References

[1] Green paper — Facts sheet: A European Strategy for Sustainable, Competitive and Secure Energy, Commission of the European Communities, Brussels, 8.3.2006

[2] European Energy and Transports — trends to 2030, European Commission, Office for Official Publications of the European Communities, Luxembourg, 2003

[3] Karsten Voss; keynote: “ Net Zero Energy Buildings”; University Wuppertal; Building Physics and Technical Building Service; Zero Energy Buildings — IEA SHCP task definition Workshop, Washington DC, USA, January 2008.

[4] Paredes, P. — "Aplicagao a uma casa unifamiliar", Optimizagao Energetica nos edificios, 2006 , Portugal.

[5] Carrilho da Graga, G.; Lerer, Maria M. and Paredes, Pedro C. — WP 2, System and Component Characterisation REPORT — Passive House, NaturalWorks, 2006, Portugal.

[6] EnergyPlus, Version 2.1, October 2007.

[7] Report: “Eficiencia energetica em equipamentos e sistemas electricos no sector residential”; Portuguese ministry of economy, April 2004. [13] [14] [15]