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14 solar thermal collectors on the rooftop
at Williams Selyem Estate Winery face due
south on tilt racking to catch maximum
solar exposure. Inside the collectors is copper
piping filled with glycol to collect the
heat, and send it down into the building’s
hot water system.
heat-transport fluid (glycol) flowing
through copper tubes to accept
heat from the absorber, and 4) a heat
insulating backing.
The collector consists of a thin
sheet of thermally stable copper to
which a black or selective coating
is applied, and a grid or coil of copper
tubing held in an insulated casing
with a glass or polycarbonate
cover. Fluid is circulated through
the tubing to transfer heat from the
absorber to an insulated water tank.
Heat transfer to the hot water tank
may be achieved directly, or though
a heat exchanger. In the latter system,
pumps circulate fluid through
the copper tubes inside the collectors.
This heated fluid then moves
through a heat exchanger (either
internal or external) to an insulated
tank, where the fluid’s heat is transferred
to potable water. This solar
pre-heated water is then fed into
the building’s regular water heater,
reducing the amount of fuel (natural
gas, electricity, or propane) typically
used to heat water.
Some advantages of solar thermal
systems are:
- Very high liquid temperatures
may be achieved;
- By concentrating sunlight, these
systems can get better energy efficiency
than simple solar PV cells;
- During cloudy conditions and
overnight, insulated tanks can store
fluids heated by the collectors;
- Materials and installation time
are less expensive than solar PV;
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olar hot water systems are
a relatively low-cost way for
wineries to use the power of
the sun to reduce energy costs.
Kunde Family Estate (Kenwood,
CA) installed a solar thermal system
in 2010, and Williams Selyem Estate
Winery (Healdsburg, CA) built a
new facility with barrel storage, a
bottling line, and visitor center with
a solar photovoltaic (PV) energygenerating
system for electricity,
and a solar thermal system for hot
water generation.
Solar thermal systems
Solar photovoltaic (PV) panels
(see PWV,
January/February
2004, July/August 2005, January/
February 2008) collect energy from
direct daylight ultraviolet rays and
send it to an inverter, which converts
the energy to DC current that
powers winery and vineyard electrical
needs. Collected energy is most
often stored in the existing utility
energy grid until it is needed.
Solar thermal collectors are different
from solar PV collectors in that
they do not generate electricity —
they transfer the sun’s energy into
heated liquid (glycol) that is then
used to heat water. Solar thermal
collectors are much less costly to
install (on residential systems, about
half the cost of solar PV), and they
are four to five times as efficient
— an 80% efficiency rating versus
about 15% for photovoltaic.
50 flat-plate solar thermal collectors on
Kunde Family Estate’s south-facing roof
send hot glycol to a 1,040-gallon water
storage tank inside the winery, heating
enough water to cut the winery’s annual
bill for natural gas by half.
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The efficiency of solar thermal collectors
varies using this equation:
Temperature of liquid in collector
– temperature of ambient air
= efficiency of the collector
The greater the difference between
the two temperatures, the lower the
efficiency of the collector.
Flat-plate solar thermal collectors
(such as the ones installed at
Kunde and Williams Selyem) consist
of: 1) a dark flat-plate absorber of
solar energy,
2) a transparent cover
that allows solar energy to pass
through but reduces heat losses, 3)
a
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