Photovoltaic systems

Karsten Voss and Christian Reise 14.1.1 Concept

High-performance houses need very little heat, but a considerable amount of electricity, which is all the more significant when considered in primary energy terms. In this chapter, we assume that 1 kWh of heat from natural gas requires 1.14 kWh of primary energy, while 1 kWh of electricity requires 2.35 kWh of primary energy to produce. For this reason, it is highly attractive to consider ways of producing electricity from a renewable source, onsite. A photovoltaic (PV) system is an expensive investment, but promises trouble-free electricity production over a long life time.

It can be useful to compare the cost of PV to the costs of energy saving measures within the framework of least-cost planning. As a result of the high primary energy equivalent of electricity in the majority of European countries, the electricity yield of a PV system has roughly the same value as the heat output from a solar thermal system. This is true both for substituted primary energy and prevented CO2 emissions (Voss et al, 2002).

As discussed above, the primary energy equivalent of the annual PV yield can offset some to all of the primary energy (fossil fuels and electricity) needed by a house. In the case of an all-electric house (for example, space heating and DHW supplied by a compression heat pump), the PV yield can be directly compared to the electricity consumption by these technical systems. A sophisticated energy-saving concept is a precondition for such a PV application. Figure 14.1.1 shows the relevance of the PV output for different high-performance housing concepts. The light grey arrows indicate primary energy delivered; the dark grey arrows point to the primary energy equivalent of the annual PV yield. The width of each arrow indicates the amount of energy. Except for the stand-alone case, all PV systems are grid connected.

Source: Karsten Voss and Christian Reise

Figure 14.1.1 The relevance of the photovoltaic (PV) energy yield in different housing concepts

Source: Karsten Voss and Christian Reise

Figure 14.1.1 The relevance of the photovoltaic (PV) energy yield in different housing concepts

While PV panels are expensive, the simplicity of such an all-electric system also has economies. For example, no large thermal storage tank is needed, as would be the case to achieve the same annual solar coverage by a solar thermal system. At the current level of PV building applications, the electricity grid serves as the seasonal 'storage unit'. In limited numbers, such houses have almost no effects on the grid's power quality, line loads or transformers. In large solar housing developments, these aspects must, however, be considered.

14.1.2 The performance of PV systems

Energy

Optimal systems in Central Europe achieve an annual yield between 800 and 900 kWh per kWp. The unit 'kWp' is the power output from a PV cell under standardized test conditions: temperature 25°C, solar radiation of 1 kW, and a solar spectrum equivalent to1.5 atmospheres. For the middle European solar radiation levels, a PV system with a design of 1 kWp will supply between 950 and 1025 kWh of electricity annually. This equates to 2200 to 2400 kWh of primary energy. Depending on the module efficiency, an area of 8 m2 to 10 m2 of crystalline silicon is required per kWp. About twice this area is needed for amorphous silicon. Glass/glass or insulating glass modules that allow for the penetration of daylight need still larger areas due to the space between the solar cells.

The yield primarily depends on the incident solar radiation and, thus, on the orientation of the collecting surface. More than 90 per cent of the maximum possible solar radiation can be obtained by a range of tilts and orientations. Vertical building façades, however, receive not more than 70 per cent of the maximum solar radiation, even if they are oriented to the south. In addition, there are also higher reflection losses from the PV panels and a higher probability of shading due to surrounding buildings than on roofs, so that usually only about 60 per cent of the optimal yield can be achieved. An exception is locations with extended periods of snow cover. This can substantially increase the solar irradiation on façades (for example, for mountain huts).

Figures 14.1.2(a), (b) and (c) show the relative annual total for radiation on surfaces for different orientations (180° = south) and tilt angles (0° = horizontal). Distributions were determined for Stockholm (Sweden), Zurich (Switzerland) and Milan (Italy) from top to bottom (see Table 14.1.1), and may be scaled with the global radiation total for the location in question in Central Europe. The isolines are plotted for 97.5 per cent, 95 per cent, 90 per cent, 85 per cent, 80 per cent, 75 per cent, 70 per cent, 60 per cent, and 50 per cent of the maximum irradiation.

Table 14.1.1 Annual solar irradiation data for three cities

Stockholm

Zurich

Milan

Latitude

° N

59.21

47.20

45.43

Longitude

° E

17.57

8.32

9.28

Ghor

kWh/m2a

952

1087

1272

Gopt

kWh/m2a

1199

1240

1437

á ,

45°

35°

35°

Gopt = global radiation for optimum inclination. à = angle of inclination for maximum radiation.

Notes: G. = global radiation on horizontal.

Gopt = global radiation for optimum inclination. à = angle of inclination for maximum radiation.

Notes: G. = global radiation on horizontal.

Stockholm/

45 Tilt Angle

Zurich

45 90 135 180 225 270 315 360 Azimuth angle

45 90 135 180 225 270 315 360 Azimuth angle

45 90 135 180 225 270 315 360 Azimuth angle

0 45 90 135 180 225 270 315 360 Azimuth angle

Source: Karsten Voss and Christian Reise (based on TRY data)

Figure 14.1.2 (a) Relative annual radiation on surfaces with different orientations and tilts: Stockholm, Zurich and Milan

45 90 135 180 225 270 315 360 Azimuth angle

Economics

The cost of a system decreases with increasing size. For example, in Germany during 2004, a small PV system with standard modules cost about €6500/kWp; the price decreases to about €5000/kWp for large systems. System costs with PV roofing tiles are 2 per cent to 30 per cent higher. Modules installed as overhead glazing or as a component of functional insulating glass units cost substantially more, but such applications have other values, as well. In the best case of a large system with standard modules, the cost per substituted unit of primary energy is about €0.20/kWh. In the case of a small unit on a house, the equivalent cost is about €0.25/kWh (basis: 25 years' lifetime; 2 per cent maintenance and insurance costs; 4 per cent real interest rate, no subsidies).

Embodied energy

The primary energy needed today to produce a PV system is between 8000 to 11000 kWh/kWp for standard modules with crystalline silicon solar cells. For an optimally orientated system, this energy is amortized in three to five years (Möller et al, 1998). If amorphous silicon is used, the production energy is about 5000 kWh/kWp, so that amortization times of less than two years can be achieved. In all cases, it should be considered that the embodied energy needed for the production of a component can decrease with improvements of the production processes and the plant utilization.

When life-cycle energy is considered, a PV system with the highest possible yield should be selected. By contrast, as the heating demand of a high-performance house is decreased, producing this heat becomes increasingly expensive and the energy amortization period grows. With increasing PV energy production when the electricity is fed into the grid, the amortization time decreases.

14.1.3 Photovoltaic systems for high-performance housing

Residential PV systems are typically sized between 1 and 3 kWp. Assuming the primary energy consumption of a standard house with a floor area of 150 m2 (33,000 kWh/a), this private power supply meets between 8 per cent and 24 per cent of the total primary energy demand for heating, ventilation and DHW. In high-performance housing, it provides a notable 20 per cent to 60 per cent. Combining PV together with extensive conservation measures can easily lead to a 'zero primary energy balance' house.

Figure 14.1.3 shows such a demonstration house by a German manufacturer of prefabricated houses (architecture: Seifert und Stockmann, Frankfurt; Energy concept and monitoring: Fraunhofer ISE, Freiburg). The PV system on the roof has an area of 27 m2 and is rated at 3 kWp. Heat is supplied by a condensing gas boiler combined with 8 m2 solar collectors. During the season 2001/2002, energy consumption of the house and the contribution of the PV system were monitored (see Figure 14.1.4). After subtracting the electricity consumption for the technical systems serving the heating, ventilation and DHW systems, 19.8 kWh/m2a were fed to the grid. On a primary energy basis, after the 21 kWh/m2a of natural gas needed for heating and DHW, there is still a surplus of 34 kWh/m2a. This surplus almost completely covers the electricity consumed by the household appliances. Figure 14.1.4 shows electricity consumption for the building services, as well as all household appliances. All energy data refer to primary energy. The energy balance curve results from balancing demand and PV yield. The curve demonstrates the seasonal mismatch of demand and supply and, therefore, the function of the grid as seasonal storage.

Source: Fraunhofer ISE

Figure 14.1.3 'Zero-energy balance' house in Emmendingen, Germany

Source: Karsten Voss and Christian Reise

Figure 14.1.4 Monitored energy consumption in 2001/2002for the demonstration house in Emmendingen, Germany

Source: Fraunhofer ISE

Figure 14.1.3 'Zero-energy balance' house in Emmendingen, Germany

Source: Karsten Voss and Christian Reise

Figure 14.1.4 Monitored energy consumption in 2001/2002for the demonstration house in Emmendingen, Germany

14.1.4 Conclusions

When installed in high-performance houses, PV systems can compensate for a significant share of the primary energy consumed for heating, ventilation and DHW at realistic investment costs. The same PV system in a conventional house could not begin to cover even a significant fraction of the energy consumed. In order to be economical and ecological, a high system yield with a favourable orientation and tilt are essential. Not all systems that are attractive to the eye fulfil these criteria. The task is thus to strive for an architecture which succeeds in combining the building and its energy technology within the context of sustainable construction and costs. High-performance houses need highperformance PV systems! PV in high-performance housing has an assured future.

References

Goetzberger, A., Stahl, W., Bopp, G., Heinzel, A. and Voss, K. (1994) 'The self-sufficient solar house

Freiburg', Advances in Solar Energy, vol 9, pp1-70 Möller, J., Heinemann, D. and Wolters, D. (1998) Ecological Assessment of PV Technologies, Proceedings of the Second World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, pp2279-2282

Voss, K., Kiefer, K., Reise, C. and Meyer, T. (2002) 'Building energy concepts with photovoltaics -concept and examples from Germany', Advances in Solar Energy, vol 15, American Solar Energy Society, Boulder, US

Websites

International Energy Agency www.iea-pvps.org

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