The main disadvantages of solar shingles today are their many wire connections and their low efficiency, which is made worse by the high temperature that develops when the insolation is high. For example, the efficiency of the DOE/NREL design shown in Figure 2 is 4% to 5%. Higher efficiency designs are evolving. The efficiency of Dow Chemical's thin-film, copper-indium-gallium-deselenide (CIGS) design is 10% to 12%. The traditional PV collector efficiencies range from 20% to 30%, and nano-solar designs also hold promise.
The installed cost of solar singles is two to three times that of regular asphalt shingles, or about $500 per sq. meter. The yearly electricity consumption of the average home is about 15,000 to 20,000 kWh, and at $0.2/kWh the yearly electricity bill today ranges from $3,000 to $4,000.
To generate this amount of electricity during the time when solar electricity is available (when the sun is out) requires a system size of over 5 kW. The yearly insolation in Arizona is about 2,500 kWh/m2/yr. Therefore, at 10% efficiency, 60 to 80 square meters of southern exposure roof area needs to be covered by solar shingles to provide 5 kW. At a unit cost of $500/m2, the installed cost is $30,000 to $40,000. In the New York area, both the area requirement and the investment cost would at least be doubled because the insolation is about half that of Arizona, and the sunny days are also fewer.
In the above example for Arizona, the system cost per watt was about $6 to $8 ($30,000 to $40,000 for a 5-kW system). This cost can be reduced if rebate programs exist in the state. For example, the rebate in New York State is $1.75/W or about 25%. In the case of large rooftop installations on commercial warehouses, the payback period is likely to be further reduced. For example, LPS industries in Moonachie, N.J., installed a 704-kW system on its 16,000 sq m rooftop for $5.7 million and, with the 30% government grant, found the payback period to be about five years.
A key component of the control and optimization of energy-free housing is the by-directional electric meter that connects the home to the grid. These bi-directional electric meters can measure complex rates, and their soft switches allow the user to make functional changes without the need for changing any hardware. They can also be provided with recording, totalizing and a variety of logic and peak-shedding functions. In the future, the software in the intelligent electric meter might also be able to perform other control tasks, such as automatically charging the batteries of electric car(s) when the electricity is inexpensive (at night), and maximizing the amount of electricity sent to the grid (by temporarily turning off optional users) during periods when electricity rates are at a peak.
In the next installment of this series, I will discuss the control software requirements of both the grid-connected and the totally self-sufficient, distributed and wireless energy supplies to transportation, housing and general industry, plus the methods for providing "grid-less" local back-up and energy storage.
Béla Lipták, (firstname.lastname@example.org. ) process control consultant, is the editor of the Instrument Engineers Handbook.