This is the most expensive energy form and it also consumes the most primary energy to produce. Accordingly, it is highly desirable to generate it from the solar radiation striking the building envelope. Photovoltaic conversion of light to electricity is a proven technology, has no moving parts, produces no noise and no emissions, and is reliable over decades. The only limitation is its cost. The investment needed to produce a photovoltaic kWh must be compared to the investment needed to save a kWh through insulation, air tightness, high quality windows and ventilation with heat recovery - even considering the primary energy factors.

To increase the amount of energy that can be won by a photovoltaic system and to improve its economics), one approach is to capture some of the 'waste' heat generated by the panels. Given that a photovoltaic panel at best converts up to 15 per cent of the solar resource to electricity, 85 per cent is lost by either reflection or as heat. The goal of hybrid PV systems is to capture and transport some of

Source: Wagner & Co, Coelbe, www.wagner-solar.com

Figure I.4 A solar water storage 'tank in tank'

this waste heat - for example, to temper ventilation air. The solutions must compete economically with other heat sources (such as heat recovery from a heat exchanger).

Finally, the most economical and environmental kWh of electricity is that which did not need to be produced. It therefore makes sense to select electrical appliances (and the technical systems for the heating, ventilation and water heating equipment) that need as little electricity as possible. An investment in a more efficient washing machine can save more energy than the marginal investment in another conservation technology. However, the lifetime of the component must also be considered. Investing in a highly insulated fa├žade provides energy savings for the lifetime of the structure. Once built, it is difficult and likely uneconomical to make later improvements.

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