One of the questions on many aspects of solar and renewable energy is the optimum scale of the device or system. We have seen in Figures 5.4 and 5.5 concentrating solar thermal installations on the scale of 11 MW and 25 kW, respectively, with a good indication that the present efficiency of the smaller installation is nearly twice that of the large installation. (It is not clear what the cost in $/W comparison is, but it probably favors the large system.) It has been argued above that much higher efficiency than the present 15% should be available in the solar tower systems with advances in engineering and in materials. We will see later that arrays of small concentrating solar cells built into panels are commercially available using small plastic Fresnel lenses (as suggested in Figure 5.1) and in a miniature form of the parabolic reflector as shown in Figure 5.5. A long-standing suggestion has been for concentrating arrays above the earth in space, with transmission ofpower to earth by microwaves. The cost has so far been prohibitive. A large but small mass parabolic mirror in space might be constructed cheaply from aluminum foil, or aluminized mylar, to be unfurled after reaching orbit. The positioning of such power satellites above large cities would avoid the difficulty of building new power lines on earth. In principle, an array of power satellites, following the lead of the GPS satellites, might provide power 24 h. Many large cities are coastal or on large lakes, where the receiving units could be located on barge arrays with underwater DC power lines to land.
The City of New York has recently carried out a careful mapping assessment of the solar power capacity of its one-million-plus rooftops , concluding that 66.4% of the buildings of the city had roof space suitable for solar panels. It was concluded that 5.847 GW could be generated putting solar panels on hundreds of thousands of buildings. It was concluded that 49.7% of the city’s peak power usage could be generated from solar power and 14.7% of the city’s annual electricity use, taking into account typical weather conditions. David Bragdon, director of the Mayor’s Office of Long-Term Planning and Sustainability said the city could realistically add “thousands of megawatts” in solar power. The city would likely establish a uniform approach and presumably would negotiate with a company like First Solar to do the installations. The budget of the city is $65.7 billion for 2012. Nominally, the installation would cost around $5.8 billion that over 10 years would be 0.9% per year of the city budget.
Such a project would be a large-scale deployment of small installations, and would be in the power of the city to mandate. The article states that U. S. nationwide installed solar capacity is 2.3 GW, less than half the rooftop potential of New York City. We will return in Chapter 11 to some of the issues regarding the deployment of renewable energy resources, which on the national level is made difficult by the sway of vested fossil fuel interests over the Congress. The existence of the Mayor’s Office of LongTerm Planning and Sustainability, able to obtain city funding of $450 000 for the aerial survey, is in itself encouraging, and suggests that large cities may be an avenue toward rational implementation of renewable energy systems.
126 I 5 Introduction to Solar Energy Conversion