We Have Met the Future and It Is Green - Sustainable Design: The Science of Sustainability and Green Engineering - page 315

Figure 8.8 Photovoltaic system employed at the Home Depot
Smart Home at Duke University. The system converts radiant
energy from the sun to electricity. Radiant energy passes
through a glass cover with a nonreflective coating onto a silicon
(Si) “sandwich.” The Si atoms are arranged in a cubic matrix. The
n-layer has excess electrons that will exit, whereas the p-layer is
missing electrons (i.e., it has electron holes). Thus, each PV cell is
configured like a battery, with a positive and a negative side,
separated by a permanent electrical field (the junction). The
electrons flow from p to n exclusively. A photon that hits the
n-layer releases an electron that remains in the n-layer.
Conversely, a photon that hits the p-layer also releases an
electron, but it moves easily to the n-layer. The excess electrons
that accumulate in the n-layer are allowed to exit via a
conducting wire. Thus, a direct current is generated as electrons
flow from the negative side to the positive side, as long as there
is sunlight to produce radiant energy and its photons, there will
be excess electrons flowing (i.e., there will be a continuous
electrical current). The current is delivered to a load (depicted
here as a light bulb). The system can be even more efficient if
excess electrons not used by the load are stored in a battery.
Thus, PV systems are improving commensurately with advances
in materials (e.g., glass, coatings, sandwiches) and batteries.

load

current

sunlight

n-type
silicon

junction

p-type
silicon

photons

electron flow

“hole” flow

up to 150 ◦ F, voltage output in the panels can drop as much as 20%, and that in
a typical solar panel, only 14% of the sunlight is converted to electricity, 7% is
reflected, and 79% is converted to heat.
Because of the heat gain, PV panels are mounted above the roof surface
to provide air circulation below the panel. The proposal laminates PVs to a
corrugated panel through which water may flow, simultaneously cooling the
panels and heating the water. The heated water is stored in a hot tank for use in
a dishwasher, for washing clothes, and so on. The laminating of the PVs to the
corrugated surface also makes it possible for the surface to move and “track” the
path of the sun to maximize the collection period each day. Hunter's innovation
proposal suggests that the potential to capture this waste heat could eliminate or
at least offset some of the household energy used to heat water, this energy use
on average typically being 30% of the total household energy consumption.
Harvesting rainwater from the surface of the panels provides a portion of the
water used to cool the panels and feeds a second storage tank, the cool tank .
The final aspect of the proposal is the use of rainwater that has been harvested