Keeping the heat where you want it
Solar heat collectors are designed to collect as much solar radiation as possi-
ble. Not only do you want to maximize surface area, but you also want to orient
the collector in a way that maximizes the amount of solar radiation gathered
over the course of a day. Solar heat collectors also ought to do the following,
all of which affect what kind of materials go where in your solar collector:
✓ Convert that radiation into heat as efficiently as possible. This conver-
sion almost always entails the use of a black surface, which absorbs
radiation most efficiently.
✓ Transfer the heat into a usable medium. In most cases, you want to
heat water. If you collect the heat in a black tube, you can simply run
water through that tube, and the heat naturally transfers into the water.
Other fluids, such as antifreeze fluid (which doesn’t freeze up in winter)
may also be used.
To move heat effectively, a material needs to be a good conductor (or as
physicists like to say, it needs to have high thermal conductivity).
✓ Insulate to prevent heat loss. For insulators, you want to use materials
that make for poor conductors. Converting radiation into heat in the
wintertime is easy, but then the cold weather and wind simply steal the
heat right back. In order to prevent this loss, people insulate the collec-
tor from the outside world with materials, such as fiberglass insulation,
or double-pane glass windows.
✓ Store a sufficient amount of heat. Heat capacity is a measure of how
much heat energy (in British thermal units, or Btus) is required to
raise the temperature of a given volume of a material by one degree
Fahrenheit. To be effective at storing heat, a material needs a high heat
capacity. Table 4-1 shows some materials of interest.Looking into the Future
The solar PV business is constantly evolving, and a wide range of new tech-
nologies are being developed. Higher efficiencies are desirable, but because
cost is usually the driver, lower cost technologies are proliferating more
quickly than high-efficiency technologies.
Large scale solar farms, often located out in the middle of nowhere, use low-
cost thin-film technologies because they have no need for high-efficiency. In
this situation, cost is the ultimate driver, not minimization of surface area.
When utilities purchase a high-output solar farm, residential customers inevi-
tably benefit because of economies of scale. The more solar PV built in the
world, the lower the cost.
The development curve that PV manufacturing is following is similar to that of
the semiconductor industry back in the 1960s. Prices plummeted while quality
increased dramatically. A lot of the manufacturing equipment used to make PV
cells is similar to that used to make microprocessors, except on a much larger
scale.
The PV industry grew over 100 percent from 2007 to 2008, and projections
are for this growth rate to continue. Ultimately, economies of scale and
increased competition will force module prices down. But because many
factors affect the price of a solar PV system, the price of solar PV residential
systems may not fall in the same way.
Solar PV systems ultimately compete with the existing power grid. When
solar power is cheaper than grid power, customers install solar systems. As
grid power prices continue to rise, expect the price of solar power to also
rise because the solar power industry can charge more for its equipment and
still be competitive with grid power.Appreciating a solar home’s
increase in value
Suppose a homeowner decides to sell her home five years after installing a
solar water heater. Energy costs are rising, and monthly savings from the solar
water heating system are now $48 per month, or $600 per year. A homebuyer
will pay more for the home because of this built-in cost reduction.
How much more? Let’s say new systems cost $2,800, and tax rebates are a
thing of the past because everybody and their brother are now in the market
for solar. A lot of work is involved, and most buyers don’t want to do it.
They’d have to read some highly technical books, for example (unfortunately,
they didn’t know about this handy little guide).
Even more importantly, most homebuyers use mortgages, where balancing
monthly payments with a fixed income is the game. Forty-eight dollars per
month in cost-savings translate into $48 that can be spent somewhere else. A
buyer could get a larger mortgage, for example. For $50 a month, after taxes,
you can borrow $14,000. So although putting in their own solar system may be
an option (after the purchase of the home) for homebuyers, they also have a
strong incentive to purchase existing solar equipment with their mortgage.
In terms of appreciation, the homeowners are likely to get about 125 percent
of the price of new equipment. For the example, a 25-percent appreciation
would be $3,500 (that is, $2,800 times 125 percent). Suppose that with rising
energy costs, the original investment of $1,400 paid for itself in 44 months
(see the preceding section). After that, the cost savings were all pure profit.
At 60 months, the sellers get a further profit of $2,100 at the sale of the house.
To find out more about how a home appreciates with a solar system, consult a
realtor in your area who can give you an idea of how much more solar homes
are selling for than conventional home